Thermo101 Allpages

[This is one in a series of articles introducing general concepts in thermoforming.]

. 101 Articles Index

The collection of articles
in this book is listed in
chronological order with a
summary of each one for
easy reference. The first
article appeared in the fall
of 1998 and they continued
in every Thermoforming
Quarterly until the third
quarter 2007.

17-3 Thermoformable Polymers

This article features a short description of the two basic categories of polymers
(thermoplastic and thermoset) and the two types of polymers (amorphous and crystalline).
An explanation of the “thermoforming window” or temperature range in which a polymer
can be formed is also discussed.

17-4 Polymer Properties

This is a more technical article covering material modulus, stress, strain and melt index in a
theoretical discussion of the characteristics of the material to be thermoformed.

18-1 Polymer Properties II

This is a continuation of the previous article but deals more with the thermal properties of
polymers and measurement of “heat capacity” or “specific heat.” It gets into more advanced
concepts of thermal conductivity and thermal diffusion. These 2 articles are for those who
need to understand the theory of polymers’ reaction to heat.

18-2 Basic Heat Transfer

A basic description of the three modes of heat transfers: conduction, convection and
radiation. Includes discussion of where these heat sources occur in thermoforming.

18-3 Mold Materials

In this article the reader will learn about the types of materials used in thermoform molds,
their qualities and when to use them.

18-4 Heaters

Types of heat sources are described with an explanation of how they transfer energy.

19-1 Oven Design

The role of the oven, the various oven designs and types available as well as the controls
used to regulate oven temperature.

19-2 The Forming Temperature

This is a key article on the subject of the right temperature to get optimum forming results.
Upper and lower sheet temperature limits and heating the inner thickness of the sheet is
discussed.

19-3 Stretching the Sheet – Part I

This is the first of three parts dealing with sheet behavior during heating and forming. This
part focuses on sheet behavior while it is still in the oven.

19-4 Stretching the Sheet – Part II

This is the second of a series of three parts on stretching the sheet. This part deals with the
draw-down of the hot sheet into a mold and its relationship to the stress-strain behavior of
the plastic.

20-1 Stretching the Sheet – Part III

The final part of this three-part series deals with the pre-stretching of the sheet
pneumatically or mechanically (blowing a bubble or assisting with a plug).

20-2 Cooling the Formed Part

Cooling the formed part while on the mold is second only to heating the sheet in getting
quality parts. Maintaining mold temperature and additional cooling techniques are
discussed.

20-3 Trimming – Part I: General Comments

This article is the first of three parts on trimming. Because trimming of thin- and heavy-
gauge parts covers a wide range of techniques and tools, they are dealt with in separate
articles. The first deals with some generalities related to how plastics can be trimmed and
what needs to be considered prior to deciding how to trim the part out of the formed sheet.

20-4 Trimming – Part II: Thin-Gauge

The second of three parts on trimming deals with processes used to trim thin-gauge parts
and the types of cutting tools used.

21-1 Trimming – Part III: Thick-Gauge

Finally, the trimming of thick-gauge parts from the formed sheet is the last in this series.
Various types of trim tools and techniques are discussed.

21-2 Collecting Thin-Gauge Parts

Thin-gauge parts must be stripped from the sheet by hand or by mechanical means. This
article describes the equipment and methods by which this is done.

21-3 What Part of “Regrind” Don’t You Understand?

This is an in-depth look at an intrinsic characteristic of all thermoforming: the material that
surrounds the formed and trimmed part that must be ground and reprocessed into sheet or
other products.

21-4 How to Interpret Technical Articles

The author of most of the 101 articles, Dr. Jim Throne, added this discussion about the need
to understand more complex, technical aspects of thermoforming. It is an advanced view of
why technical articles must contain the theoretical information to provide the credibility for
what have become accepted practices in our industry.

22-1 In the Beginning

This article is an excellent summary of the very early forms of thermoforming and how they
progressed. Statistics related to our industry and the process and market categories generally
related to it are listed to give the reader a good overview.

22-2 Square One: Polymer Selection

Additional material characteristics of those discussed in earlier articles are needed to
produce quality thermoformed parts. This article plus the next two go into more detail with
respect to the various characteristics of polymers. Extrusion basics such as melt viscosity and
orientation are introduced in this article.

22-3 Square One: Polymer Selection-Orientation

This is a continuation of the previous article on the extrusion process with attention given
to orientation of the sheet, testing for orientation and the comparison of orientation and
shrinkage.

22-4 Square One: Observe Your Sheet as it Heats

This final article of three discusses how we can observe the effects of orientation as the sheet
is being heated and cooled during extrusion and thermoforming.

23-1 Recrystallization – What Does That Mean?

This is a study of the crystalline and semi-crystalline materials such as PP, CPET and HDPE and
what happens to these materials as they are heated.

23-2 Alphabet Soup

This title is misleading in that this article and the two following explain the meaning of
the acronyms used in thermoforming. Tg, Tm and DSC refer to glass transition, melting
temperature and differential scanning calorimeter, respectively. It is quite advanced in terms
of chemical terminology but does relate to thermoformable polymers.

23-3 ABCs of Alphabet Soup

The second in a three-part series, this is a very technical article related to IR and FTIR or
infrared energy and Fourier transformation respectively, as they relate to heat in the
thermoforming process.

23-4 XYZs of Alphabet Soup

Continuing from the previous two articles, this explains the meaning of HDT, DTA and DTMA
(or, heat deflection temperature, differential thermal analysis and differential thermal
mechanical analysis, respectively).

24-1 Why Part Design is Important

This article covers the many do’s and don’ts of part design. It is of prime importance for
thermoformers to know the limitations of the process.

24-2 Comparing Concept to Reality

The limitations of thermoforming are dealt with in more detail. Also it discusses how to deal
with customers from the most knowledgeable to the technically naïve. Working with the
customer to determine the right material and design for his product as well as educating him
on the advantages and limitations are covered.

24-3 Understanding How a Sheet Stretches

Assuming we have just received an order for a new job, this article deals with the decision to
design the tooling as female or male. Also, the question of running the mold on top or on
the bottom is addressed.

24-4 The Ubiquitous Draw Ratio

“Draw ratio” is a term in thermoforming commonly used to describe the depth of a female
cavity to its width at the narrowest point or the height of a protrusion on a male part in
relation to another protrusion close to it. We learn about several methods of measuring
draw ratio here.

25-1 Draft Angles

Because material shrinks during the cooling process, draft angles are important in the part
design. Here we learn about the draft angles required in various situations.

25-2 Corners

Corners in a thermoformed part should be designed to avoid material thinning (in a female
part). This sometimes conflicts with the design requirements or volume calculations of a
container. This is discussed in general terms here.

25-3 The Cutting Edge

The edge of the part directly after trimming can be less than clean. This article deals with
what trimming methods produce the best results and how to finish the part after trimming
to produce a better edge.

25-4 The Rim

This article covers the design of the periphery of the part commonly referred to as the rim.
The rim rolling process, detents for lid locking and de-nesting features are discussed as are
the “dam” design, the hidden trim line and what happens to a textured sheet in the rim
area.

26-1 Process – Cycle Time

Improvements in cycle time can have a big impact on your costs. This article deals with the
different segments of the forming process and how these segments can be shortened.

26-2 Down Gauging – It’s a Good Thing

This is a look at why we quote starting gauge and how we could sell differently by agreeing
with the customer on the wall thickness requirements of his part.

26-3 The Impossible Draw Ratio

Difficult draw ratios can be overcome by designing tooling with pre-draw boxes and plug
assists. This article details an example of a difficult part and suggests a tool design.

A TECHNICAL ARTICLE 1998 VOLUME 17, #3

Thermoformable Polymers

Although we generally consider the words
“plastics” and ”polymers” interchangeable,
the term “plastics” refers to the product
delivered as resin pellets or sheet. Nearly all
plastics contain “polymers,” the pure long-chain
hydrocarbons, but they also contain shopping
lists of additives such as thermal stabilizers,

antioxidants, color correcting dyes,
internal and external processing
aids, as well as product-specific
additives such as fire retardants,
colorants, UV stabilizers and fillers.
However, because the term “plastic”
connotes cheapness and poor quality,
the industry is now calling all polymeric
materials “polymers.”

There are two general categories
of polymers. When the polymer can
be heated and shaped many times
without substantial change in its
characteristic, it is a “thermoplastic.”
When the polymer cannot be
reshaped after being heated and
shaped the first time, it is a “thermoset.”
Thermoforming is primarily
concerned with thermoplastics.

Thermoformers use two general
types of polymers. When a polymer
is heated from very low temperature,
it undergoes a transition from
its glassy state to a rubbery state.
Although this transition occurs
over several degrees of temperature,
usually only one temperature value
is reported as the “glass transition
temperature.” Polymers that only
have a glass transition temperature
are called “amorphous polymers.”
Polystyrene, ABS, PVC and polycarbonate
are examples of amorphous
polymers. 80% of all polymers thermoformed
are amorphous polymers.
80% of all amorphous polymers are
styrenic, that is, polystyrene, impact
polystyrene, ABS, and other similar
polymers.

Certain polymers exhibit a second
transition, from the rubbery state to a
molten or melt state. Again, this transition
occurs over several degrees of
temperature, and again usually only
one temperature value is reported as
the “melt temperature.” Polymers
that have both glass transition and
melt temperatures are called “crystalline
polymers.” Polyethylene and
polypropylene are examples of crystalline
polymers.

If only one polymer is used in a
given plastic recipe, the polymer is
called a “homopolymer.” Examples
of homopolymers include general
purpose polystyrene [GPPS or sometimes
called “crystal polystyrene”
because parts made of the unpigmented
water-white polymer have
the appearance of fine crystal], low-
density polyethylene or LDPE and
polyethylene terephthalate or PET. If
one polymer is reacted with another,
the polymer is called a “copolymer.”
Impact polystyrene or HIPS is an
example of polystyrene reacted with
a rubber such as butadiene. Many
copolymers are used in thermoforming,
including polypropylene-polyethylene
and PVC-PMMA.

If three polymers are reacted
together,
the
polymer
is called a
“terpolymer.” The
classic terpolymer is
ABS, which is a reacted
product of Acrylonitrile,
Butadiene and Styrene.

Occasionally, two polymers
are extrusion- or melt-
blended together to make
a plastic recipe. The classic
blended polymer is modified polyphenylene
oxide or mPPO, which
is a near-equal blend of polystyrene
and polyphenylene oxide. mPPO is
desired for its good impact resistance
and fire retardancy.

The “thermoforming window” is
the temperature range over which
the polymer is sufficiently subtle or
deformable for stretching and shaping
into the desired shape. Typically,
amorphous polymers have broader
thermoforming windows than
crystalline polymers. Polystyrene,
for example, can be formed from
around 260°F or about 50°F above
its glass transition temperature to
about 360°F or only a few degrees
below the temperature where it is
injection moldable. Polypropylene
homopolymer, on the other hand, is
so fluid above its melting temperature
of 330°F that its thermoforming
window may be no more than
one degree or so. As a result, it is
frequently formed just below its
melting temperature. Even then,
its thermoforming window
may be only two or three
degrees. ¦

Keywords: amorphous,
crystalline, glass
transition temperature,
melting temperature,
homopolymer,
copolymer, terpolymer,
blend, thermoforming
window

[This is one in a series of articles introducing general concepts in thermoforming.]

A TECHNICAL ARTICLE 1998 VOLUME 17, #4

Polymer Properties

Thermoforming involves stretching of rubbery
solid plastic sheet. When force is applied
to any material, it stretches or elongates. The
amount that it stretches depends on the amount
of force per unit area, or “stress,” applied to the
sheet, the nature of the material and its temperature.
The amount that the material stretches

is elongation or “strain.” For most
metals, ceramics and many polymers
below their glass transition temperatures,
the amount of strain in the material
is proportional to the amount
of stress applied to the material. The
proportionality is referred to as the
material “modulus.” The modulus
of a given polymer depends on the
molecular make-up of the polymer,
the nature and level of the additives
in the polymer and the temperature of
the polymer. For example, according
to Modern Plastics Encyclopedia, the
ASTM D638 range in modulus of PS
at room temperature [77°F or 25°C] is
330,000 to 475,000 psi.

For many polymers, the stress-
strain curve is not linear, but is
curved. The room temperature modulus
for LDPE, for example, is oven
in Modern Plastics Encyclopedia as
25,000 to 41,000 psi. But at room
temperature, LDPE is far above its
glass transition temperature of -25°C.
Therefore, reported modulus is the
slope of the stress-strain curve at zero
strain. Furthermore, as the polymer
is heated above its glass transition
temperature, the stress-strain curve
remains curved but flattens. The
modulus, being the slope of the curve
at zero strain, also decreases with
increasing temperature. In addition,
the elongation at break increases with
increasing temperature.

The decreasing modulus, the flattening
of the stress-strain curve, and
the increasing elongation at break of
a given polymer or polymer recipe
with increasing temperature are all
important in thermoforming, because
the sheet must be stretched into the
deepest recesses of a mold. Two other
aspects of the stress-strain characteristics
of a given polymer are also
important, however. If the softening
range of the polymer is too narrow,
that is, the polymer goes from being
very stiff to extremely soft over a
very narrow temperature range, the
thermoforming window will be very
narrow. This is the case with most
grades of nylon 6, for example. And if
the stress or force per unit area needed
to stretch the polymer is always very
high, regardless of the polymer temperature,
traditional vacuum forming
and even normal pressure forming
pressures may be insufficient to
stretch the polymeric sheet to the farthest
reaches of the mold. This is the
case for many classes of highly filled
and fiber-reinforced polymers.

Again, thermoforming focuses on
the solid properties of a polymer, such
as stress-strain. Nevertheless the fluid
properties of the polymer are important
as well. After all, the polymer
must be extruded into sheet. Fluid
properties of polymers are related
to the polymer liquid resistance to
applied stress. The polymer liquid
resistance is given as “rate of strain”
and “viscosity” is the slope of the

stress-rate of strain curve. As with
solid polymer stress-strain
curves, liquid
polymer
stress-rate
of strain
curves are temperature
dependent,
with polymer viscosity
decreasing with
increasing temperature.
Very high viscosity, being
a measure of the polymer
liquid resistance to applied
stress, can lead to

sheet extrusion problems. So can
very low viscosity. Unwanted
orientation and internal stresses in
sheet can be traced back to the viscosity
of the polymer at the time of
extrusion.

Frequently thermoformers are told
to use a polymer with a given “melt
index.” The melt index test was established
years ago as a quick check of
the flowability of polyethylene melt.
Basically it is the amount of molten
plastic, in grams, at a prescribed
temperature that can be squeezed
through a hole of a given diameter
in ten minutes. Ten grams of a polyethylene
with a melt index of 10, say,
will extrude through the hole in ten
minutes, whereas only 1 gram of a
polyethylene with a melt index of 1
will extrude through the same hole
in the same period of time. Thus, the
polymer with the greater melt index
value will flow more rapidly at the
same stress level and therefore, will
have a lower viscosity. For a given
type of polymer, a lower viscosity
usually means a lower molecular
weight. Extruders prefer polymers
with relatively high melt indexes.
Keep in mind, however, that melt
index gives very little information
about temperature- and
shear rate-dependent nature
of the viscosity of a given
polymer. And extending
the concept of melt index
beyond polyethylenes and
polypropylenes is risky, at
best. ¦

Keywords: modulus,
viscosity, stress, strain,
stress-strain curve,
stress-rate of strain
curve, melt index

A TECHNICAL ARTICLE 1999 VOLUME 18, #1

Polymer Properties II

In addition to stress-strain and stress-rate
of strain characteristics of polymers, thermo-
formers need to know about the thermal properties
of polymers.

“Heat capacity” is a measure of the amount
of energy required to elevate the polymer temperature.
Heat capacity is sometimes called

“specific heat.” The field of study
that focuses on energy uptake of materials
is called “Thermodynamics.”
In thermodynamics, one of the fundamental
measures of energy uptake
is “enthalpy.” Enthalpy increases
with increasing temperature. When
a material goes through a characteristic
change such as melting, the
temperature-dependent enthalpic
curve changes dramatically. When
a material goes through a characteristic
change such as glass-to-rubber
transition, the temperature-dependent
enthalpic curve changes subtly
if at all. As expected, it takes far more
energy to heat a crystalline polymer
from room temperature, say, to
above its melt temperature than to
heat an amorphous polymer from
room temperature to the same temperature.
For example, it takes more
than twice as much energy to heat
polyethylene, a crystalline polymer,
to 360°F than it does to heat polystyrene
to the same temperature.
And since the formed shape must be
cooled, twice as much energy must
be removed to cool polyethylene
to a given temperature than to cool
polystyrene to the same temperature.
A single value of specific heat is
frequently given for a specific polymer.
These values are determined
by dividing the enthalpy difference
by the temperature difference. Such
values are acceptable for amorphous
polymers but care must be taken
with a crystalline polymer, since the
slope of the temperature-dependent
enthalpy curve, and hence the specific
heat, changes dramatically as
the temperature approaches the melt
temperature of the polymer.

“Thermal conductivity” is the
measure of energy transmission
through a material. The thermal
conductivity values of organics, in
general, are substantially lower, by
orders of magnitude, than, say,
metals. In other words, polymers
are thermal insulators. As an example,
the thermal conductivity
of aluminum, a common metal for
thermoforming molds, is one-thousand
times greater than the thermal
conductivity of, say, polystyrene.
During thermoforming, thermal
conductivity is a measure of energy
transmission through the polymer
sheet. Even though the thermal
conductivities of polymers are low,
there are differences in values among
polymers. For instance, the thermal
conductivity of HDPE is about four
times higher than polystyrene or
ABS. Thermal conductivity and its
companion property, thermal diffusivity,
discussed below, are quite
important when forming very thick
sheets, because the rate of energy
transfer into the sheet governs,
to a large
extent, the
formability of
the sheet. Although
thermal conductivity
typically decreases slightly
with increasing temperature,
for all intents, the value can be
considered constant.

Polymer density decreases and
its reciprocal, “specific volume,”
increases with increasing temperature.
In the vicinity of the glass transition
temperature, the slope of the
temperature-dependent specific volume
curve changes perceptively. In
the vicinity of the melt temperature,
the slope changes dramatically. Typically,
the density of an amorphous
polymer at its forming temperature
is about 10% to 15% less than that
at room temperature. The density
of a crystalline polymer at its forming
temperature may be as much
as 25% less than that at room temperature.
Obviously as the polymer
cools from its forming temperature,
its density will increase, its volume
will decrease, the final part dimensions
will decrease and the part will
exhibit shrinkage. This point will be
amplified in later articles.

“Thermal diffusivity” is a polymer
property that is a combination
of other polymer properties.
Thermal diffusivity is divided by
its density and specific heat, and is
the fundamental polymer property
in time-dependent heat transfer to
materials. Because of the unique
bundling of temperature-
dependent characteristics
of the polymer
properties, thermal
diffusivity is nearly
independent of temperature
for nearly
all polymers.

¦

Keywords:
Heat capacity,
specific heat,
enthalpy, thermal
conductivity,
specific volume,
thermal diffusivity

A TECHNICAL ARTICLE 1999 VOLUME 18, #2

Basic Heat Transfer

Thermoforming involves first
adding energy to plastic sheet to
elevate its temperature to a forming
temperature, then forming the
sheet against a mold, then cooling
the formed sheet to a temperature
where the part retains the shape of
the mold. There are three modes of
heat transfer which are important
during the heating and cooling of
the plastic.

“Conduction” is energy transmission
through solid objects. In
thermoforming, energy is conducted
from the surface of the polymer
sheet to its interior during heating,
and from its interior to its surface
during cooling. As noted in an earlier
article, the thermal conductivity
or more properly, thermal diffusivity
of the polymer is the fundamental
property in determining the rate
of energy transfer through the solid
or rubbery polymer. The higher the
thermal diffusivity of the polymer,
the more rapidly energy is transferred
and the more uniform is the
temperature through the polymer.

“Convection” is energy transmission
between solid objects and
fluids. In thermoforming, air is
the fluid surrounding the sheet in
the oven and typically in contact
with the free surface of the formed
part on the mold surface. Convective
energy transmission depends
strongly on the flow rate of the
fluid. The greater the flow rate, the
greater the rate of energy transfer.
The proportionality is called the
“heat transfer coefficient.”
Convective heat
transfer is also important
during the cooling
of the plastic part on
the mold surface, since
the coolant running
through the mold piping
is a fluid. Typically,
the cooling efficiency
of liquids is greater, by
an order of magnitude, than that
for air. For example, cooling water
is about one hundred times more
effective in cooling than fan-blown
air.

“Radiation” is energy interchange
between two solid objects
having different temperatures.
Unlike conduction, which requires
direct contact between solid objects,
and convention, which requires
direct contact between fluids and
solid objects, radiation is electromagnetic
energy transfer, requiring
no contact. However, radiation
energy transfer requires that the
two objects “see” each other. In
thermoforming, radiation energy
transfer occurs in the oven between
the heater surfaces and the sheet
surface. It also occurs between heater
surfaces and oven walls, clamp
frames, and the outside world if
the oven is open. Radiation energy
transfer also occurs when the
sheet is removed from the
oven, since
the sheet
is hotter
than its surroundings.
However,
the amount of
energy transfer is a
function of the fourth
power of the temperature
of the solid object and
so radiant energy transmission
in the thermoforming
oven is far more significant
than anywhere else in the process.
Radiant energy intensity is usually
identified in terms of object temperature
or wavelength. Traditional
thermoforming heaters operate
between about 100°F and 1500°F,
and have peak wavelengths of 2.5
to 9 microns. This range is referred
to as “far intrared.”

First, it is important to realize
that all three modes of heat transfer
– conduction, convection and radiation
– are important in the heating
of thermoformable polymer sheets.
The primary mode of energy transfer
varies depending primarily but
not exclusively on the thickness
of the polymer sheet. Very thick
sheets, 13 mm or 0.5 inch in thickness
or thicker, can be heated rather
efficiently in “pizza oven” heaters,
where hot air is convected or blown
around the sheet that is supported
on all sides to allow for uniform circulation.
Even though the air might
be heated by being blown across
hot panels, the primary modes of
heat transfer are convection to the
sheet surface and conduction of the
energy into the volume of the sheet.
At the other extreme, thin sheet,
0.75 mm or 0.030 inch in thickness,
can be heated extremely rapidly
with very intensive radiant heat,
since conduction through thin sheet
is very rapid.

All commercial energy
sources used in thermoforming
today produce
heat both by convection
[hot air moving across the
heater surface] and radiation
heater surface temperatures
greater than sheet
surface temperatures].

¦

Keywords: conduction,
convection, radiation, far
infrared

A TECHNICAL ARTICLE 1999 VOLUME 18, #3

Mold Materials

Most commercial thermoforming
molds are made from aluminum.
Aluminum is used because it is light,
it is easily worked, is relatively inexpensive
and has a very high thermal
conductivity. It is also used because
the forming forces against the finished
mold are low when compared with,
say, injection molding.

Larger commercial molds are usually
cast from the melt. In addition
to the common atmospheric casting,
molds can be made by vacuum casting
and pressure casting. Smaller molds
are frequently machined from plate.
Computer-controlled machining
stations have made manufacture of
many-cavity molds quite competitive
with other means of manufacture.

For the most part, thermoforming
molds are single-surfaced. That is, one
surface of the plastic sheet is forced
against the mold surface, while the
other surface is “free” or untouched
by another mold surface. In certain
instances, such as foam and composite
forming, the sheet is so stiff at
the forming temperature that it must
be pressed between two “matched
mold” surfaces in order to accurately
form the part.

In large, cast molds, water lines are
typically attached to the reverse sides
by soldering or secondary casting. In
smaller, machined molds, cooling is
frequently done through flood plates
attached to the rear of the molds.
When water lines are needed, for
deeply drawn parts, they are gun-
bore drilled in, in much the same
manner as water lines are drilled in
injection molds.

In certain instances, other metals
are used for molds. For composites,
for example, temperature and pressure
requirements may preclude the
use of aluminum. Steel, particularly
chrome-plated steel, and stainless
steel are good alternatives. Steel has
about one-third the thermal conductivity
of aluminum and about twice
the modulus. Stainless steel has about
one-fifth the thermal conductivity of
aluminum and about 50% greater
modulus.

Because thermoforming pressures
are relatively low, usually not exceeding
100 psi, many other materials
can be used for molds. Although
electroformed nickel is much more
expensive than other metals, it is used
when extremely high detail is needed
or when a very intricate pattern must
be replicated. Very large parts, such as
exterior door panels, have been made
on electroformed nickel tools. Usually
nickel is electroformed onto a pattern,
water lines are placed against the
nickel shell, then the nickel is backed
with a cheaper white metal.

Sprayed metal is also used for
prototyping and limited production.
Molten white metal such as
zinc is atomized and atmospherically
sprayed against a pattern in
a fashion similar to paint spraying
or polyester spraying. A reasonably
thick layer of metal can be sprayed in
a reasonably short time. Water lines
are placed against the metal shell and
sprayed in place. This is
then backed wither with
metal-filled epoxy or pot
metal. Many sprayed
metal applications have
been taken over by computer-
driven machining,
and so typical sprayed
metal molds today
are small and highly
detailed.

There
are even
more
materials
available for
straight vacuum
formed prototype

parts. Wood is an
obvious choice,
with ash and hard
maple offering the
best balance of properties such as
compression strength, shaping and
sanding quality and resistance to
splitting, checking, and warping.
Hydrocal is a dense industrial plaster
that makes a high quality mold. Plaster
mold fabrication is quick, with the
primary drawbacks being the messy
nature of plastic casting, including
plastic dust, weight [compared with
wood], and brittleness.

More recently, medium density
fiberboard or MDF has found extensive
use, primarily for shallow
draw and male molds, since it can
be quickly worked with traditional
woodworking tools and has no grain
and no propensity to warp, split or
check. It is relatively expensive and
restricted in thickness. Syntactic
epoxy or polyester foam was
originally developed as a plug assist
material but is now computer-driven
machined into smaller molds. It can
be expensive, particularly if a substantial
amount of the initial billet must be
machined away to make the mold.

¦

Keywords: cast aluminum,
machined aluminum, computer-
driven machining, chrome-plated
steel, stainless steel, electroformed
nickel, hydrocal, medium density
fiberboard, syntactic foam, sprayed
metal

A TECHNICAL ARTICLE 1999 VOLUME 18, #4

Heaters

There are three primary energy sources for heating plastic
sheet in thermoforming. Electric heat is used more than
gas heat or hot fluid heat. Some common heating sources
include hot air, hot water or steam, sun lamps, nichrome
spiral wire or toaster wire, steel rod heaters, steel or nichrome
tape, tungsten and halogen tube heaters, quartz
tube heaters with nichrome or tungsten wire or tape, steel
plates with embedded resistance wire, ceramic plates with
embedded resistance wire, ceramic bricks with embedded
resistance wire, ceramic bricks with embedded resistance
wire, steel plates that reradiate combustion energy from gas
flame, indirect gas combustion on catalytic beds and direct
gas combustion energy. Keep in mind that all hot surfaces
transfer energy by conduction, convection and radiation.

Hot Fluid Heating

Recirculating hot air or forced convection ovens are used
when heating times are not critical or when sheet is very
thick, usually greater than 0.500 inches. There are several
oven designs in use. Air is blown across metal coils and then
across the sheet in indirect electric ovens. Electric panels,
usually in the top of the oven, are combined with fan-circulated
air in direct electric ovens. Architectural products
such as commercial or industrial skylights, soaking tubs
and whirlpools are frequently made using these methods
of heating. Direct gas-fired heaters similar to those used
in rotational molding ovens, are used to heat plastics such
as polyethylene that are not easily oxidized or chemically
attacked by combustion products.

Direct Contact Heating

Direct contact heating is used extensively for very thin
sheet or thermally sensitive polymers. For a very short
time, the sheet is brought in contact with a heated PTFE-
coated metal plate. It is then quickly formed against the
mold. Direct contact heating is a common heating method
in form, fill and seal (FFS) machines, where the sheet may
be heated sequentially on both sides, by running it against
heated rolls. Oriented Pet such as Mylar™, oriented polystyrene
(OPS), nylon 6, 66 and 11, some calendered PVC,
and cast polyimide such as Kapton™ are heated using
direct contact heating.

Electric Heaters

Electric heaters can be categorized as round heaters, such
as wire, rod or quartz heaters, and flat heaters such as panel
heaters. Metal rod heaters have long heat-up times, tend
to age quickly, have poor temperature control, cannot be
easily zoned, but are extremely rugged and relatively inexpensive.
Quartz and halogen heaters are basically nichrome
or tungsten wires in quartz glass tubes. These heaters are
known for their very short heat-up times, excellent temperature
control, and very high temperature capability, but
they are very fragile, the glass is easily etched, and they
are very expensive. Panel heaters include coated metal
plates that reradiate heat from nichrome wires embedded
in ceramic, quartz glass and quartz cloth plates that
transmit heat from similarly embedded nichrome wires.
Panel heaters have moderately long heat-up times, good
temperature control, and excellent longevity, but they
are difficult to zone effectively. Ceramic bricks that have
embedded heating wires are reasonably rugged, have
moderate heat-up times, excellent temperature control and
moderate longevity, but they are fragile and it is difficult
to determine burn-out.

Combustion Heating

The “2000 Years of Thermoforming” cartoon on the 1996
SPE Thermoforming Division tee-shirt depicted a caveman
stomping on a sheet of plastic suspended over a roaring
fire. Direct gas heating using natural gas or propane rather
than wood is a viable way of heating plastic. However the
energy output from direct combustion is very high and
sheet scorching or ignition is always a concern. Indirect
catalytic heaters provide a more uniform energy source,
although energy output is admittedly less than that for
electric heaters, and, until recently, temperature control
was “on-off.” Installation cost is higher than that for electric
heaters, but energy costs are as low as 20% of that for
equivalent output electric heaters. Catalyst longevity was
problematical early on, but fourth generation catalysts appear
to have minimized loss in efficiency and formation of
hot spots. High pressure indirect gas combustors known
as ported surface burners are currently being tested as an
alternative to the high-energy electric heaters.

Selection of the “Correct” Heater

There is no “correct” heater. Heater
selection depends on many intrinsic and
extrinsic factors including retrofitting,
sheet geometric characteristics
including
thickness, polymer
thermal sensitivity,
day-to-day running costs,
maintenance costs, initial installation
cost, versatility of the
heater, and the inherent design
of the thermoforming machine
and its surroundings. ¦

Keywords: Electric heat, gas heat,
fluid heat, direct contact heat,
nichrome, tungsten, quartz, halogen,
catalytic heater

A TECHNICAL ARTICLE 2000 VOLUME 19, #1

Oven Design

THERMOFORMING

101

Various types of heaters were de-
scribed in an earlier tutorial. In
this lesson, the effect of the heat produced
by these heaters is considered.
The focus is on non-contact heating.

The Role of The Oven

The oven serves several purposes.
It holds the sheet while it is being
heated. For the most part, it isolates the
heating sheet from the environment
outside the oven. It provides a rigid
structure for the heaters. It provides
a way for the sheet to enter the oven
and a way for it to exit the oven. And
it provides fixed spacing between
the sheet and the heaters. Most
importantly, the oven must protect
both the sheet and the heaters from
thermal or mechanical damage, if
something goes wrong.

The simplest oven consists of a
single heater bank suspended over a
clamped sheet, with no provision for
isolating the sheet from the outside
environment. This heating method is
sometimes used, even though the heat
transmission from the heater to the
sheet is quite poor. Heater efficiency
is improved by shrouding the heater
in sheet metal and covering that with
several inches of fiberglass. At the
other extreme, ovens are available
that actively clamp, top and bottom,
against the sheet, completely isolating
it from the environment. These ovens
are very energy efficient and can be
quite expensive.

Regardless of the type of heater
used, the heaters are usually held in
planes above and below the sheet
plane. One thermoforming machinery
manufacturer uses curved rod heaters,
with the heaters curved downward
near the sheet edges. This helps
minimize heater inefficiency along
sheet edges. Usually the heaters are
fixed in position relative to the sheet
plane. However, machines have been
built in which the lower heaters travel
downward as the heating sheet sags.

Most electric ovens operate on
480V/3Ø. Heaters are usually rated in
“watt density,” with W/in2 being the
standard U.S. dimension. Rod, panel,
and ceramic heater watt densities are
up to about 40 W/in2. Quartz heaters
are available to 60 W/in2. Gas-fired
heaters operate at gas pressures of
about 5 to 10 ozs., although new
ported burners require up to 5 lb/in2
gas pressure. All catalytic gas heaters
also require electric preheaters, which
can operate at 240V1Ø but are more
efficient at 480V/3Ø. Catalytic gas
heaters have equivalent watt densities
of up to about 30 W/in2. Ported
burners and free-surface burners have
equivalent watt densities of about 500
W/in2.

Just as there is no ideal heater,
there is no ideal oven design. An
optimum oven design sufficiently isolates
the sheet from the outside oven
environment to minimize drafts and
energy loss. But the design should
allow for relatively easy means of
transferring the sheet into and out of
the oven. And it should allow for any
sheet movement during heating, such
as sag, by spacing bottom heaters
away from the final sheet shape.

Controls

Oven controls range from simple
on-off electrical switches to temperature-
sensed proportional and
proportional-integral-derivative or
PID controllers. Oven heaters can be
ganged and operated from a single
controller or individually connected
to separate controls. Early ceramic
heaters were sometimes individually
connected to individual ten-turn
potentiometers or pots, yielding a
“B movie” science-fiction-like wall of
hundreds of adjustable dials. Now,
most small heaters are “clustered”
into a more manageable number of
controllable “zones” which are then
adjustable through a digital or even
touch-screen monitor at the process
control station. PID controllers are
a must for heaters that require rapid
changes in power level. These controllers
minimize power overshoot.
This minimizes sheet overheating
and extends the lifetime of the heater
element

Burning or decomposing plastic
is always a concern in thermoform-
ing. Ovens are usually designed
with both passive and active means
of minimizing damage due to a
dropped sheet. At the very least, the
bottom heater is protected with easily
removed, inexpensive chicken-wire
screen suspended above it. In certain
instances, quartz plates are placed
above the bottom heater. When pristine,
quartz is transparent in infrared
energy and so does not affect the
heater efficiency. Since the quartz
does not heat, the dropped sheet is
quickly frozen by the cold plate, this
facilitating sheet removal and minimizing
damage to the lower heater.
Sag monitors, really photoelectric
sensors, detect excessive sag and can
sound a klaxon, shut down the heaters,
drop baffles in place, or activate
oven pull-back or fly-open controls.
In some oven designs, high-volume
blowers, activated either by sag monitors
or infrared sensors, blow room
temperature air over the overheated
sheet, either before it drops onto
the heater or while it resides on the
chicken-wire screen. In other designs,
the oven cabin is flooded with carbon
dioxide whenever fire is detected.
Mechanically, ovens can be equipped
with baffles or doors that isolate the
heaters from the sheet, or the ovens
may be clam-shell opened or horizontally
retracted to remove the heat
source from the sheet. ¦

Keywords: Oven design, power
requirements, watt density, fire
control, heater control, on-off
control, proportional control, PID
control

A TECHNICAL ARTICLE 2000 VOLUME 19, #2

The Forming Temperature

Is the sheet ready to be formed?”
This is the most difficult question
in all of thermoforming. Part of the
difficulty lies in the broad spectrum of
polymers and part designs. And part
lies in the difficulty in determining
what measurable physical property
in the polymer best characterizes the
polymer formability. As discussed
in an earlier lesson, thermoforming
is best described as a rubbery sheet
stretching process. As a result, the
elastic character of the polymer, reflected
in its temperature-dependent
tensile strength and modulus, should
give a strong clue. Methods of measuring
and interpreting the elastic
character are discussed in another
lesson.

The Calibrated
Eyeball

But first, what about other
methods of determining
formability? As noted in an
earlier lesson, sheet sag is a
manifestation of lowering
tensile strength. And sag is
used by the experienced operator
to gauge when a sheet
is hot enough to be formed.

All things equal, the sheet should
sag the same amount at the same
temperature, time in and time out.
Sheet “thumping” or the manual
manipulation of the sheet during the
later heating stages is also a gauge.
A screwdriver or key thrust into the
corner of the sheet will also yield a
“calibrated eyeball” assessment of
formability. Other indicators are the
change in gloss of the sheet surface
and the nature of the “smoke”1 being
evolved from the sheet. Of course,
the trained observer must then correlate
these experiential judgments
with the extrinsic nature of the part
that is being formed. In other words,
deeply drawn parts probably require
hotter sheet, which is then translated
by the operator into greater
sag or more loss in gloss, and so on.

Upper and Lower
Forming Temperatures

Most references list upper and
lower forming temperatures for
generic polymers. Polystyrene, for
example, has a lower forming temperature
of 260°F and an upper forming
temperature of 360°F. Compare
this with polystyrene glass transition
temperature of 210°F and its normal
injection molding temperature of
425°F. Is it really true that PS has a
100°F thermoforming window? No. In
normal practice, the thermoforming
window for PS being stretched into
a specific mold shape may be 10°F or
less. The practical forming window
for PP may be one or two degrees, at
best. Some polymers, such as many
nylons, have no practical forming
windows.

So, why list these temperatures?
The lower forming temperature represents
the very lowest temperature
at which the plastic can be bent or
twisted from its flat sheet shape.
Mechanical forming and certain very
thin-gauge shallow-draw package
forming can take place at or slightly
above this temperature. The upper
forming temperature represents the
very highest temperature at which
the plastic remains a sheet. Above this
temperature, the sheet will probably
drip into the heater, smoke vigorously,
ignite, and/or turn to charcoal. Don’t
go there!

Normal Forming
Temperature

Many references also list normal
forming temperatures for many generic
plastic types. This temperature,
too, is a guide to good forming, for it
represents a reasonable starting temperature
target. For example, polystyrene
has a 300°F normal forming
temperature. Keep in mind, though,
that only the surface of the sheet is
measured with infrared thermometers.
The centerline temperatures of
very thick sheets may be substantially
below surface temperatures. For very
thin sheets, infrared thermometers
that measure at 3.5 microns must be

used to prevent
also measuring
heater temperatures
on the
other sides of
the sheets. More
about temperature
measuring
in a later article.

How to Establish An
Initial Temperature
Protocol

First, sheet temperature, not heater
temperature, is the key to successful
forming. Then, an initially uniform
surface temperature across the sheet
should be obtained, through adjustment
of individual heaters. The
normal forming temperature for the
plastic should be the initial sheet surface
temperature target. And finally,
individual heaters should be adjusted
to achieve pattern or zonal heating,
where certain locations on the sheet
are deliberately made hotter or colder
than the rest of the sheet. ¦

Keywords: forming temperature,
normal forming temperature, upper
forming temperature, lower forming
temperature, sheet temperature
protocol

THERMOFORMING

101

1 In truth, the “smoke” is probably not polymer decomposition products being evolved but volatile additives such as internal or external
lubricants or processing aids.

A TECHNICAL ARTICLE 2000 VOLUME 19, #3

Stretching The Sheet – I

This is a three-part tutorial in sheet behavior
during heating and forming. This part
focuses on sheet behavior while it is still in
the oven. The second part considers pre-
stretching. And the third part considers
draw-down into or onto the mold.

Sheet Behavior In The
Oven

It is common for a sheet to exhibit
periodic shape changes, including
waffling or swimming, tautness and
sag as it is being heated to its forming
temperature. In many cases, the
sheet is relieving stresses1 that were
imparted during the cooling portion
of the extrusion process. As might
be expected, shape changes that
occur early in the heating
process are the result of conditions
imposed late in the
extrusion process. Shape
changes occurring late in
the heating process are the
result of changes imposed
in the roll stack portion
of the extrusion process.
Orientation or stresses
that are frozen in during
the extrusion of the sheet
are typically relieved
relatively late in the heating process
when the sheet is becoming quite soft.
Annealing of this residual orientation
causes the sheet to contract. The
effect is seen as a tightening of the
sheet between the clamps. Excessive
orientation can cause the sheet to pull
free of the clamps.

The Nature of Sag

In addition to relaxation of imposed
stresses, the heating sheet is also
experiencing a rapid reduction in
physical properties, such as modulus
and tensile strength. As the sheet approaches
its transition temperature,
the polymer is no longer strong
enough to support its own weight.
The sheet begins to sag or droop
under its own weight. As expected,
the extent of the sag increases with
increasing sheet temperature. For all
but the very earliest sag, the sheet is
being stretched in tension. Therefore,
the hot tensile strength of the polymer
is very important in determining the
extent of sag. However, the viscous
character of the polymer is now considered
to be a contributing factor to
the rate at which the sheet sags.

Although sag is an anticipated aspect
of sheet heating, it is difficult to
deal with. Sag can cause nonuniform
thinning in the sheet prior to forming.
As the sheet sags, it becomes
“salad bowl”-like. As a result, the
local heating efficiencies, top and bottom,
are altered, although the effect
is apparently not as dramatic as one
might expect2. The sagged sheet may
rub against the lower oven wall as it
exits, although ovens are designed
with drop sides to accommodate the
sag. And certain polymers simply tear
away during sagging. Technically,
substantial strides are being made in
mathematically modeling sag using
finite element analysis and linear viscoelastic
models for the polymer.

Heating Rate

Many things influence the rate at
which a sheet heats to its forming temperature.
Certainly the dominant factors
include heater temperature, the
thermal properties of the sheet and
its thickness. Other factors include
the efficiency of heat transfer between
the heaters and the sheet, the energy
absorption characteristics of both the
sheet and the heaters (better known
as their emissivities), air temperature
and movement around the sheet while
it is heating, and the sheet dimensions
relative to the heater dimensions.
More subtle factors include the color,
texture, and transparency of the sheet.
Shiny or polished sheet is thought to
reflect more energy than roughened
or matte sheet. The technical aspects
of this effect may focus on the spectral
rather than diffuse nature of the
reflection of incoming rays of energy.
Dark sheets are thought to heat more
rapidly than white sheets. This may
be due to the absorbing characteristics
of the pigments near the sheet surface.
Sheet transparency refers to transparency
in the
far infrared
region. Thin
polyethylene
sheet
is nearly
transparent
to IR energy
and so heats
quite slowly.
Thin PTFE
sheet on the
other hand
is nearly
opaque in the
IR region and so heats quite rapidly.

Newer heating technologies use
short IR wavelength energy. According
to ported gas burner and halogen
heater manufacturers, the high heater
temperature generates short wavelength
energy that is absorbed in the
volume of the sheet rather than just
at the surface, as is the case for far
infrared radiative heaters. This provides
for more uniform heat, lower
sheet surface temperature, and more
rapid heating rates. This technology
appears most effective for thick sheets
with relatively low pigmentation
levels. ¦

Keywords: sag, finite element
analysis, viscoelasticity, tensile
strength, extrusion process, heating
rate, infrared region

THERMOFORMING

101

1 Residual stress, orientation and shrinkage are addressed in a future tutorial.

2 A. Buckel, “Comparison of American and European Heavy-Gauge Thermoforming Machines”, TFQ 18:3, 1999, p. 13.

A TECHNICAL ARTICLE 2000 VOLUME 19, #4

Stretching The Sheet – II

This is a three-part tutorial in sheet behavior
during heating and forming. The
first part focused on sheet behavior while
it is still in the oven. This part considers
draw-down into or onto the mold. The last
part considers pre-stretching.

Simple Draw-Down

As noted earlier, thermoforming is
technically deformation of a rubbery
mostly-elastic membrane. In simple
terms, we are stretching the plastic as
if it is a rubber sheet. The stretching
mechanism is quite easy to
explain. Imagine a simple
drinking cup female
mold. The hot plastic
first contacts the rim of
the cup, then sags uniformly
into the cup.
Vacuum is applied to
the cup cavity and the
sheet begins to stretch
into the cavity1, forming
a dome. Then a
portion of the plastic
contacts the cup edge. For all intents,
the friction between the hot sheet and
the mold surface holds that portion
against the mold throughout the rest
of the draw-down. As vacuum continues,
an additional portion of the
plastic contacts an additional portion
of the cup wall. This plastic is also immobilized
against or “stuck on” the
mold wall. Since some of the original
plastic is already on the mold wall,
this additional plastic must come
from the dome that is still free of the
mold surface. And since some of the
original plastic is already on the mold
wall, it is only logical that the additional
plastic must be thinner than
the original plastic. As draw-down
or stretching continues, more and
more plastic is drawn from the hot
plastic dome that is free of the mold
surface and is deposited on the mold
wall. And it is apparent that both the
thickness of the plastic in the dome
and that of the plastic just deposited
on the mold wall must decrease as
draw-down continues.

In other words, the wall of the cup
gets progressively thinner toward the
bottom of the cup. And as expected, as
the plastic draws into the last portion
of the cup mold, the corner, it becomes
even thinner.

Stress-Strain Related To Draw-
Down

In an earlier tutorial, it was stated
that:

When force is applied to any material, it
stretches or elongates. The amount that it
stretches depends on the amount of force
per unit area, or “stress,” applied to the
sheet, the nature of the material and its
temperature. The amount that the material
stretches is elongation or “strain.”

We can now relate the material
behavior to applied load, or stress-
strain, to the draw-down of a plastic
sheet into the cup mold. The shape
and magnitude of the stress-strain
curve of any polymer depends on
the nature of the polymer and its
temperature. Typically, in the forming
temperature region, the polymer
stretching initially increases slowly
with increasing stress, then increases
more rapidly as the applied stress
increases. Typically, at low temperatures,
the polymer stretches a
relatively small amount before rupturing.
As the polymer temperature
increases, the polymer “elongation
at break” or its ability to stretch further
and further without breaking,
increases dramatically. At very high
temperatures, this stretching limit
begins to drop abruptly, indicating
that the polymer molecular structure
is too weak to support load.

Initially, the sheet sags into the mold
without the application of vacuum2.
The stress being applied to the sheet
is just its own weight per unit area. As
vacuum is applied or the stress on the
sheet increases, the sheet elongates.
This is recognized as “thinning.” So
long as the applied stress increases,
the sheet will
stretch and thin
as it is deposited
against the cup
mold wall.

There are many
reasons why a
sheet may not
fully stretch into
the farthest corner
of the mold.
The sheet may
quickly cool as it is being stretched.
As a result, the amount of stress or
equivalently, the applied force, may
not be enough to stretch the sheet
beyond a certain point. The initial
sheet temperature may be too low,
and the sheet resistance may be too
high to allow the sheet to fill the cavity.
Certain plastics “strain harden,”
that is, beyond a certain strain level,
the force needed to stretch the plastic
further may quickly increase. If the
force is not enough, the plastic stops
stretching. Crosslinked polyethylene
is an example of such a plastic.
For filled and short-fiber reinforced
plastics, the force required to stretch
the sheet even a modest amount may
be so high that forming may require
pressures higher than those used in
simple vacuum forming. Pressure
forming will be considered in a subsequent
tutorial. ¦

Keywords: stress, strain, differential
pressure, elongation, elongation at
break, thinning, strain hardening

THERMOFORMING

101

1 The pressure difference between atmospheric pressure on one side of the sheet and vacuum on the other is referred to as “differential
pressure.”

2 The shape of the sheet is similar to the shape taken by a freely hanging chain or rope held by both ends.

A TECHNICAL ARTICLE 2001 VOLUME 20, #1

Stretching The Sheet – III

This is a three-part tutorial in sheet
behavior during heating and forming.
The first part focused on sheet behavior
while it is still in the oven. The second
part considered draw-down into a mold
and its relationship to the stress-strain
behavior of the plastic. This part considers
pre-stretching.

Why Pre-Stretch?

As we saw in Stretching The Sheet-
II, the sheet gets progressively thinner
as it is stretched deeper into the
mold. For large draw-ratio parts, such
as drink cups and refrigerator
liners, the sheet may be
thinned so much at the
bottom of the part that
the part may fail there.
Redistribution of sheet
from thicker regions to
thinner regions must be
done to provide useful,
functional parts in both
thin-gauge and thick-
gage thermoforming.
This redistribution is
called “pre-stretching.” There are
two general ways to do this – stretching
the sheet with air pressure and
stretching the sheet with mechanical
means. These are considered here.

Pneumatic Pre-Stretching

This is a technical way of saying that
the sheet is pre-stretched using differential
air pressure. One way is to
clamp the sheet over a “blow box”
and blow low-pressure air into the
box. Air pressure of 3 to perhaps 10
psi is usually sufficient to “inflate” the
sheet into a dome. The mold is then
raised into the inflated sheet. Pneumatic
pre-stretching is used mostly in
thick-gauge thermoforming. Another
way is to clamp the sheet over a “draw
box” and apply a vacuum to the draw
box. Again, a soft vacuum of 5 to 15
inch Hg is usually sufficient to “draw”
the sheet into a dome. The mold is
then immersed in the drawn sheet.
Regardless of whether the sheet is
blown into a bubble with air pressure
or drawn into a dome with vacuum,
the forces acting to pre-stretch the
sheet are differential forces, due entirely
to the unbalanced air pressure
across the sheet. Vacuum pre-stretching
is used in both thin- and thick-
gauge thermoforming.

As experienced thermoformers know,
there needs to be a careful balance
between the stretching characteristics
of the plastic, the sheet temperature,
the extent of differential pressure,
the rate of pre-stretching, the extent
of pre-stretching, and the timing between
inflation and mold immersion.
For example, ABS and PMMA can be
greatly pre-stretched, even into hemispheres.
PS and PC are more difficult
to pre-stretch extensively. RPVC and
PET are quite resistant to extensive
pre-stretching. Further, RPVC will
pull apart and PET will locally draw if
pre-stretched too quickly. The sheet is
nearly uniformly stretched across its
surface in pneumatic pre-stretching.
Stretching in mostly one direction occurs
only where the sheet is clamped
to the frame.

Mechanical Pre-Stretching

Mechanical pre-stretching relies on a
solid object called a plug or a pusher.
This device is mechanically or pneumatically
driven into the heated sheet
before it touches the mold cavity.
Plugs are used extensively in both
thin- and thick-gauge thermoform-
ing. In general, two things happen
when the solid object contacts the
sheet. The first is that the sheet is
impaled on or sticks to that portion of
the plug that contacts the sheet. As a
result, that portion doesn’t stretch and
it cools by transferring its energy to
the cooler plug. This can lead to objectionable
“plug markoff” on the part.
And then stretching takes place in
the sheet free of the mold surface and
between the edge of the plug and the
edge of the mold.
This can lead to
an objectionable
ridge or “witness
line” at the edge
of the plug. Unlike
pneumatic
pre-stretching,
plug stretching
is primarily in
one direction,
between the edge of the plug and the
edge of the mold.

As with pneumatic pre-stretching, in
plug assist pre-stretching, there needs
to be a balance between polymer
stretching properties, sheet temperature,
rate of stretching, and extent of
stretching. As with pneumatic pre-
stretching, polymers such as ABS and
PMMA are easily pre-stretched with
plugs and PVC and PET are more difficult
to pre-stretch with plugs.

Plugs are more versatile than air for
redistributing plastic across a mold
surface, particularly as the part becomes
more complex. But plug design
and shape remain mostly trial-and-
error. ¦

Keywords: pre-stretching, blow box,
draw box, plug assist

THERMOFORMING

101

A TECHNICAL ARTICLE 2001 VOLUME 20, #2

Cooling the Formed Part

So far, we have heated the sheet
and stretched it. The sheet is now
against the cooler mold surface. This
part considers how the sheet cools.

Sheet Characteristics on the
Mold

As discussed earlier, the sheet
stretches differentially against the
mold surface. That is, the sheet that
touches the mold first yields the thickest
portion of the formed part. The
sheet that touches the mold last is usually
the thinnest portion of the formed
part. Further, the sheet that touches
the mold first is cooled longer than
the sheet that touches the mold last.
The difference in thickness, cooling
rate, and cooling time across
the part surface may lead to
different thermal stresses
in the final part. And these
different thermal stresses,
together with the different
degrees of stretching in
the part during forming,
can lead to part problems
such as warping, uneven
shrinkage, and part distortion.
These problems are not restricted
to general part size or initial sheet
thickness or nature of the polymer,
but can occur in thin-gauge and
heavy-gauge parts.

Energy From the Sheet to
the Mold

We discussed mold materials in
an earlier tutorial. Here we discuss
how the mold removes heat from the
sheet. First, not all molds are actively
cooled. Prototype tooling usually has
no cooling channels. As a result, the
heat extracted from the sheet by the
cooler mold simply goes to heat the
mold. As more and more parts are
produced, the mold simply continues
to heat, albeit at a slower and slower
rate, since some heat is always lost
to the room. This means that parts
produced at the beginning of the run
will have different levels of stress than
parts produced at the end of the run.
With “hand samples” or “show-and-
tell” parts, this is rarely a problem.

Production molds are almost always
actively cooled. For most commercially
thermoformed polymers,
water is the cooling medium. The
cooling water is circulated through
water channels drilled into or added
to the back of the mold. The cooling
water is either recirculated through
a chiller and back into the mold or is
exhausted to the drains. As we will
see later in our discussions on process
control, incoming and outgoing water
temperature should be monitored to
maintain uniform mold temperature
across the entire mold. By the way,
there are certain thin-gauge applications
where chilled or refrigerated
water is used as the cooling medium.
And certain high-temperature applications
where either steam or heated
oil is used.

The energy is removed from the
thermoformed sheet through the surface
in contact with the mold surface
by conduction. That energy is then
conducted through the mold metal
to the cooling channel, where it is removed
from the mold by convection.
Conduction depends on the thermal
conductivity and the thickness of the
mold material. Convection depends
on the rate of flow and the chemical
nature of the coolant through
the cooling channel. The greater the
distance between the plastic surface
and the cooling channel is, the
longer it takes to cool the plastic. Low
thermal conductivity mold materials,
such as stainless steel, will conduct
heat slower than higher thermal
conductivity mold materials, such as
aluminum. The farther the coolant
channel is from the mold surface, the
slower the part will cool. The greater
the coolant flow rate, the more rapidly
the part will cool. And water is a
more effective cooling medium than
oil, and steam is much more effective
than water.

Energy From the Sheet to
the Air

For all but matched die forming,
the free part surface, or the part
surface away
from the mold
surface, is exposed
to ambient
mold conditions.
For thin-
gauge parts
that are pressure
formed,
the free surface
environment is
static or quiescent air. Air has notoriously
poor heat removal characteristics,
so the free surface cools primarily
by conduction through the side of the
sheet that is in contact with the mold
surface. For heavy-gauge parts, fans
are usually used to remove heat from
the free surface. The more rapidly
the air is circulated against the free
surface, the more rapidly the part
will cool. In certain applications,
humidified air or air containing water
microdroplets is used to further
enhance the rate of cooling from the
free surface. The technical reason for
trying to quickly cool the free surface
is that, if both sides of the sheet are
cooled equally, the sheet cools four
times faster than if only one side is
cooled. ¦

Keywords: coolant channel, active
cooling, conduction, convection, free
surface

THERMOFORMING

101

A TECHNICAL ARTICLE 2001 VOLUME 20, #3

Trimming – i – general comments1

So far, we’ve defined the polymer
characteristics, then we’ve heated
it, stretched it, and cooled it on the
mold surface. It is now necessary to
remove the formed part from the sheet
around it. The polymer material that
is not a portion of the formed part(s)
is known in the industry as trim, web,
or skeleton. It is not known as scrap,
since this material is destined to be
reground and reprocessed into sheet
or used in non-thermo-
forming applications. This
discussion is part of three
parts on trimming1.

What Exactly is
Trimming?

Trimming is usually
the mechanical separation
of the formed part
from the unformed sheet. Mechanical
separation is a kind way of saying that
we break or fracture or sever the part
from the web. Technically, we begin
with a single structure containing
both part(s) and non-part(s) and end
up with at least one part here and one
non-part there.

As we noted in the heating and
stretching articles, we can treat the
technical aspects of the process without
primary regard for gauge thickness.
That is, breaking is breaking,
whether the sheet is 10 mils thick or
half-inch thick. It is the gauge of the
sheet that dictates the way in which
we break the sheet.

To reinforce this, consider trimming
of thin-gauge sheet. Steel-rule
die cutting is the common method
of trimming. As we see below, this
is done by forcing a sharpened steel
blade perpendicularly into the sheet,
fracturing or breaking the sheet into
two pieces. Now consider trimming
of heavy-gauge sheet. Routing and
drilling are the common methods of
trimming. As we will see later, this is
done by pressing a rotating toothed
bit against the sheet, forcing the teeth
to fracture or break the sheet into
two pieces by splitting out smaller
pieces.

Do All Polymers Trim in the
Same Fashion?

No. There are several polymer
material considerations that must
be considered when trimming parts
from sheet. Typically brittle plastics
such as acrylic and polystyrene break
easily. As a result, trimming tends to
be easy and trimming forces tend to
be low. However, the breaking process
can yield jagged edges and local
microcracking. And substantial trim
dust which can be quite tenacious.
On the other hand, tougher polymers,
such as ABS, rigid vinyl, NorylR and
polycarbonate can require substantial
trimming forces. The fracture
surface is usually less jagged than
brittle plastics. Surprisingly, very
soft polymers, such as polyolefins,
flexible vinyls and thermoplastic elastomers,
are frequently more difficult
to trim than tough polymers. Soft
polymers “flow,” can stick to cutting
tools and drill bits, and the cut edge
is frequently quite irregular. When
cutting any polymer, sheet thickness
and the modulus of the plastic at the
trimming temperature usually dictate
the type and the cutting speed of the
trimming device.

Is Trimming Speed a
Factor?

In thin-gauge trimming, the rate at
which the steel rule die is pressed into
the plastic is guided somewhat by the
nature of the plastic. Brittle materials
can be “snap-cut,” that is, cut at
a high rate. Tough plastics do better
when the steel
rule die speed
is slowed. However,
in general,
trimming speed
is a minor factor
in thin-gauge
thermoforming.

Very frequently,
trimming
speed is
an important economic factor when
trimming heavy-gauge parts. From a
processing view, one should always
strive for parity between the forming
time and the trimming time. That is,
ideally it should take no longer to trim
a part than to form it. The greater the
length of the cutter path relative to the
surface area of the part and the thicker
the part, the more the per-part trimming
cost will be and the farther from
parity the trimming/forming ratio
will be. In practical terms, this means
that either there will be more trimming
presses than forming presses, or
the forming presses will be sitting idle
for a portion of the trimming time.
From a technical point, the objective
is to maximize the rate at which the
volume of plastic in the cutter kerf
is removed. Trim speed that is too
low may cause the cutter head to
overheat, which in turn, may cause
plastic fragments to momentarily
stick, which in turn, may cause cutter
head chatter. ¦

Keywords: Fracture, steel rule die,
router, cutter speed

THERMOFORMING

101

1Ed. Note. This issue contains two other
articles on trimming. The Moskala-Barr
technical article focuses on steel rule die
cutting of thin-gauge PET, PETG, and
OPS. The Van Niser Industry Practice
article focuses on router cutting of heavy-
gauge plastics.

A TECHNICAL ARTICLE 2001 VOLUME 20, #4

Trimming – iI – THIN-GAUGE1

What is Used to Trim
Thin Sheet?

The steel rule die is the most common
method of trimming thin-gauge
sheet. The steel rule die is basically a
special-grade steel strip that has been
sharpened on one edge. The strip is
bent to the contours of the trim line on
the part. It is then mounted in a base
plate. This assembly is then attached
to the trim platen. The specific
details about the steel
rule die, the base plate
and the assembly depend
strongly on the trimming
equipment. For high production,
punch-and-die
or matched metal die
assemblies are used. For
these assemblies, sharpened
machined or forged
hardened steel dies are used.

Thin-Gauge Trim
Techniques

The simplest thin-gauge trimming
machine consists of a horizontal motor-
driven roller and a rigid table. The
gap between the roller and the table
is manually adjustable. The sheet
containing the formed parts is placed
in a fixture and a base plate, usually
of plywood, containing the steel rule
die, is placed atop the plastic and
fixture. The entire assembly is then
hand-fed through the roller. The nip
pressure forces the steel rule die into
the plastic and the parts are cut free.
This technique is ideal for prototype
operation.

At the other end of the spectrum,
trimming presses are employed. The
sheet containing the formed parts is
fed continuously between reciprocating
platens. A steel cutting die is
forced against the trim die line. If the
press employs matched metal dies,
the cutting die edge squeezes the
sheet against a steel backing plate
until it is cut through. The steel backing
plate is usually spring-loaded
so that the cutting die edge is not
striking an immovable surface. If
the press employs a punch-and-die
arrangement, the punching die edge
essentially pinches the plastic against
an immovable die. As the plastic is cut
through, the punch passes inside the
immovable die.

There are horizontal trim presses,
where the plastic sheet is fed vertically
into the horizontally reciprocating
platens, and vertical trim presses,
where the plastic is fed horizontally
into vertically reciprocating platens.
For parts where holes and slots are
needed, multiple trim presses are
used. The first press cuts the slots or
holes and the second press trims the
part from the web.

There are many other trim techniques
that fall between the hand-operated
trim press and the automated
in-line multiple trim press. For example,
in-mold or in-situ trimming
has become popular, following its
acceptance in Europe more than a
decade ago. Here, the trim die is part
of the mold. As the sheet is stationed
over the mold cavity, the trim die
moves against it, pinning it to the
mold cavity. In this fashion it acts
as a hold-down fixture, cavity isolator,
or grid. Once the part has been
formed, the trim die continues into
the plastic, separating the part from
the web. With micrometer gapping on
new presses, millions of cuts without
replacing the dies are possible. Steel
rule dies and forged dies are used in
in-mold trimming.

Successful Thin-Gauge
Trimming

Typical thin-
gauge trimming
problems are angel
hair or very
fine fibers, fuzz,
dust, and edge
microcracks.
These are usually
related to
a mismatch between
the nature
of the cutting edge of the trim die and
the cutting characteristics of the polymer.
Cutting edge sharpness is always
critical, but so is the edge bevel. And
the rigidity and planarity of the trim
die is also important, particularly for
deep and very long, linear cuts. As
we discussed in the first part, soft,
gummy plastics tend to “flow” away
from the cutting edge, whereas brittle
plastics tend to form dust and edge
cracks. Certain polymers, such as PET
and PETG, benefit by being cut hotter,
but that is not always possible.

Registration problems can be severe
if the plastic has significant shrinkage
and orientation after leaving
the forming press. PP and CPET are
classic examples. Even with multiple
molded-in registration posts, substantial
set-up time may be needed
to correctly position the in-line trim
dies. Small processing changes, such
as sheet temperature, forming time,
and mold temperature, may lead to
mis-registration. ¦

Keywords: steel rule die, forged
die, machined die, trim press, trim
problems

THERMOFORMING

101

1Ed. Note: In the first part of this three-part
series, we defined trimming as the means
of separating the formed plastic part from
the web, skeleton, or unformed sheet
surrounding it. In this part, we consider
methods of trimming thin-gauge parts.

A TECHNICAL ARTICLE 2002 VOLUME 21, #1

Trimming – iII – THICK-GAUGE1

As defined earlier, heavy-gauge or
thick-gauge forming refers to
parts formed from sheet having thicknesses
greater than about 3 mm, 120
mils or 1/8-inch. Typically, heavy-
gauge parts are formed from cut
sheet, with the part and the unformed
sheet around it being removed from
the machine clamp frame to an off-
line trimming station. The method of
trimming depends on several factors
including the number of parts that are
to be trimmed, the accuracy and finish
of the trimmed edge, the planarity of
the trim line, and the extent of secondary
cutting required.

Hand Trimming

Trimming using a handheld router
was at one time the primary way
of separating the product from the
unformed sheet. Although largely
supplanted by numerically controlled
routers, hand trimming still has its
place in prototyping or when a few
parts are needed. Guides and tracking
grooves improve the accuracy of
the trim line.

Planar Trimming

For many parts, from refrigerator
liners to garden ponds to skylights
to tote boxes, the trim line is planar
or linear. As a result, trimming is
usually accomplished with fixed,
horizontally mounted rotary saws,
vertically mounted band saws, and
even guillotines. For saw cutting, the
part is usually manually moved into
the saw. For guillotines, which are
basically sheet cutting or shearing
devices, the products to be trimmed
are frequently robotically moved
between the knives. In certain heavy-
gauge forming operations, the trim
device may be incorporated as part of
the mold assembly, much like that in
thin-gauge in-mold trimming.

Mechanical Trimming

The advent of the computer-programmed
robotic trimming station
has revolutionized heavy-gauge
trimming technology in the past decades.
The computer imparts speed,
accuracy and reliability to the trimming
process. Compared with hand
routing, robotic trimming initially
requires much greater technical skills
to create the operational trim path, but
very little additional labor thereafter.
However, it must be kept in mind that
robotic trimmers are very expensive,
particularly when compared with
handheld trimming devices. As a result,
robotic trimming yields financial
rewards usually when many identical
parts are needed.

There are several variations on the
computer-numerically-controlled or
CNC router. Multiaxis stations include
two- and three-axis machining
stations, five-axis routing stations,
and even linear motor-driven six-axis
robots. All these devices require that
the cutter path be preprogrammed
in a language special to the device or
class of devices. And care is required
during setup to ensure that the device
is indeed following the desired cutter
path. Disaster can occur if the machine
incorrectly interprets the code.

Drilling and Slotting –
Secondary Cutting

In addition to separating the part
from its trim, very frequently, holes
must be cut in the part. Handheld
drills, routers, and hole saws have
performed these functions for decades.
And frequently they still do.
And now CNC devices are used. As
with hand operations, the CNC devices
frequently require tool changes
to accomplish all trimming functions.
These changes are automatically programmed
into the computers.

Tooling for Trimming2

The nature of the polymer frequently
dictates the type of tool to be used
for trimming. For example, care must
be taken to prevent microcracking
when drilling or saw-cutting brittle
polymers such as acrylics and styrenics.
And when soft, easily flowed
polymers such as polyethylene are
trimmed, the cutter must move a hot
sticky chip quickly away from the
cut area to prevent it from rewelding
itself. In addition, it is important to
keep in mind the relationship between
linear speed of travel, usually
in inches/min, and cutter speed, usually
in revolutions/minute. Excess in
either of these variables can lead to
poor quality cut surfaces. But going
too slow can also lead to problems in
poor efficiency and burned plastic. In
short, it is always wise to work with
companies that specialize in cutters
specifically designed to cleanly cut a
given type of plastic.

General Comments About
Trimming

It was noted at the beginning of
this three-part series that trimming
involves mechanical fracture of plastic.
Further, trimming has become
an integral, if not formidable, part of
the thermoforming process. It should
now be noted that there is a dearth of
technical information but a substantial
plethora of widely held beliefs
on trimming methodologies. Perhaps
an industry focus will aid trimming
technology in a manner similar to that
on heating technology. ¦

Keywords: Router, multiaxis,
trimming, secondary cutting

1 Ed. Note: In the first part of this three-part
series, we defined trimming as the means of
separating the formed plastic part from the web,
skeleton, or unformed sheet surrounding it. In
the second part, we considered methods of
trimming thin-gauge parts, including nip rolling,
matched die cutting and punch and die cutting.
Some concepts of successful thin-gauge trimming
were included. In this section, we consider
the more popular ways of trimming thick- or
heavy-gauge parts.

2 Many Industry Practice articles on cutters are
found in back issues of TFQ.

A TECHNICAL ARTICLE 2002 VOLUME 21, #2

COLLECTING THIN-GAUGE PARTS

Thin-gauge forming operations
generate many, many parts per
hour. And these parts need to be
rapidly and accurately collected or
collated. It is here that the mechanical
engineer or technologist shines.
It is difficult in this short tutorial to
discuss all the collection methods
currently in vogue. Instead a simple
cataloguing is in order.

Parts Separated from the
Web on the Mold

Trim-in-place was discussed
in the trimming
tutorial. Basically, the
part is held against the
mold surface during
forming by the trimming
knife. When the
part is fully formed and
rigidified, the trimming
knife severs the part
from the web. The severing can be
of two types – complete, so that the
part is free from the web, or partial,
with several tabs holding the part to
the web.

If the part is completely separated
from the web, it must somehow be
removed from the mold cavity before
the next forming step can initiate. One
technique involves a robotic “picker”
that shuttles into the mold cavity. The
picker typically may have fingers with
vacuum tips that secure the formed
parts to the fingers. Another design
uses a mold that rotates to dump the
parts into bins below the sheet plane.
Air blow-back is sometimes used in
conjunction with the rotating mold
to ensure that the parts are blown free
of the individual mold cavities. Although
rocker or “to-and-fro” molds
have been used, three- and four-sided
rotary molds seem to offer the fastest
dump time. The key to quality part
collection lies in successful emptying
of all mold cavities, each time, every
time.

Tabbed Parts Removed in
Stacker

If the parts have been “tabbed” and
remain with the web, a second station,
usually called a stacker, is needed to
push the part away from the web.
While both “up-stackers,” meaning
that the parts are pushed into
collectors from below, and “down-
stackers,” meaning that the parts are
pushed into collectors from above, are
used, up-stackers are easier to manually
unload and so are more popular.
The key to rapid and accurate stacking
is the strength of the tab. If the tab
is too strong, the pusher can damage
the part before the part separates
from the web. If the tab is not strong
enough, the part may be hanging free
of the web and the pusher can damage
the part this way.

Stackers and the ancillary collection
sleeves or channels work best if the
parts have ample draft so that they
nest easily in the collection devices.

Parts Removed From
Flat-Bed In-Line Trim
Presses

Parts that are trimmed when lying
in the horizontal plane need to be
collected either in the “up” direction
or “down” direction. Since these
trim presses tend to be massive forging-
type presses, collection can be
difficult.

Parts Removed From
Canopy Trim Presses

In canopy presses, the trimming
step is usually followed immediately
by the separation step. The pusher
forces the trimmed part from the web
onto a horizontal or slightly inclined
collection table. Each subsequent part
pushes the previous parts across the
table. Counting and collecting are
easier with canopy trimming operations
than with most of the other
techniques. In fact,
these tasks are often
done manually.
The key to quality
trimming with
canopy presses is
the positive push
of the part into the
collection devices.

The Effect of Part
Geometry on Trimming
Methods

For deeply drawn cylindrical parts
with substantial draft, such as cups,
in-press up-stackers or the rotating
mold dumping techniques with ancillary
cup orienters offer advantages
over other methods. For shallow draft
parts such as plates, down-stacking
has an advantage. Rectangular and
odd-shaped parts can be easily collected
on near-horizontal tables from
canopy trim presses. Lidded containers,
where the lid is of shallow draw
and the container has a deep draw,
are always difficult to collect and
collate. Horizontal tables seem to be
favored. The robotic shuttle offers
good versatility for trim sets with
several different shapes and depths
of draw. In many instances, with
complex trim sets, hand sorting and
stacking from a catch bin may be the
only solution.

¦

Keywords: Tabbing, rotating mold,
up-stack, down-stack, collection
table

THERMOFORMING

101

A TECHNICAL ARTICLE 2002 VOLUME 21, #3

What part of “regrind” don’t you understand?1

Thermoforming is burdened by
two serious economic albatrosses.
First, thermoforming is considered a
secondary process. That is, it is a process
that takes place after the primary
process of extrusion, used to produce
the sheet that represents our incoming
material. And second, thermoforming
never, ever uses the entire sheet
that we purchase from the extrusion
house. In fact, it is strongly believed
that thermoforming cannot exist as
a major, growing process without
extensive methods of recycling its
non-product, called web, skeleton,
edge trim or selvage (but never, ever
scrap!). This tutorial looks at that part
of the sheet that never, ever produces
money for the thermoformer. And in
fact, costs the thermoformer dearly.
The subject is regrind, that is, taking
the non-product, chopping it or chipping
it, and feeding it to the hopper
of the extruder, along with a proper
amount of virgin polymer.

Let’s bound the problem first. For
the most part, heavy-gauge thermo-
formers use cut sheet. The mold cavity
is smaller than the mold frame, so
a portion of the sheet resides on the
mold frame and not in the mold cavity.
Then the edges of the sheet must be
held in a clamping fixture. This portion
of the sheet also does not participate in
the final part. Because the heavy-gauge
thermoformer can get sheet cut to size,
his/her trim is usually around 20%
of the original sheet for single mold
cavities. But, if the mold contains more
than one cavity, the plastic between the
cavities adds to the trim fraction. Usually
the trim sections are large enough
to be reground and reprocessed into
sheet. But if the formed part requires
machining, routing, or drilling, the
polymer that is cut away may simply
be dust or shavings. This trim is not
normally reprocessed. The trim for
a complex part with many slots and
cutout holes may be as much as 40%
of the original sheet.

The trim in thin-gauge forming is
usually greater than that for heavy-
gauge forming. This is particularly
true for axisymmetric or round parts
such as cups. Keep in mind that the
thin-gauge former must also hold
the sheet and must also provide
mold metal around the cavities. The
plastic in these regions becomes trim.
In most thin-gauge operations, the
mold cavity layout is rectangular. In
technical vernacular, this is called a
square pitch. This allows the maximum
number of cavities on a rectangular
mold frame, such as 6 across by 8
deep. Surprisingly, the square pitch
does not yield the minimum amount
of trim. An equilateral triangular pitch
yields the minimum amount of trim.
But a triangular pitch requires a parallelepiped
mold frame. And asking for
that mold frame will cause your local
mold maker, your setup man, and the
guy doing make-ready on the trim
press to question your sanity. So thin-
gauge thermoformers live with trim or
skeleton or web, up to 65% or more.

So what is the big problem with
reprocessing trim? Very little if a few
cardinal rules are followed. First, the
amount of trim needs to be determined.
Relatively accurately. Then
the allowable amount of reground
trim in the incoming sheet needs to be
determined. This time, very accurately.
Obviously if the maximum allowable
amount of regrind is determined to be
20%, say, and the thermoformer generates
30%, say, something must be done
with the rest. The extruder and former
must agree on the amount of regrind
to be used, to plus-or-minus 5%, say.
And this agreement must remain in
place, regardless of the ebb and flow
of regrind availability.

And second, the thermoformers
biggest concern when dealing with the
regrind stream is contamination. Contamination
from the original extrusion
process, including black specks and
gel, from the thermoforming process,
including oil and grease, from the
regrind process, including cross-contamination
from other polymers, and
from the handling process in general.
Production facilities, shipping, warehousing,
all generate detritus. And
most polymers are easily statically
charged, thereby attracting airborne
“stuff.” And moisture is readily absorbed
[meaning the water resides on
the surface] or absorbed [meaning that
water is drawn completely into the
polymer]. If molded parts are rejected
for contamination, they must never be
tossed into the regrind stream. Doing
so will ensure an accumulation of contamination
and hence an ever-increasing
fraction of rejected parts.

Most reprocessing operations are
steady state. That is, the regrind is
mixed with virgin polymer at the
extruder in a constant ratio, say, 50%,
for this example. Consider the implications
of steady-state reprocessing.
A virgin polymer molecule has a 50%
chance of becoming regrind. And a
25% chance of becoming regrind a
second time, 12% a third time, 6% a
fourth time, 3% a fifth time. In fact,
it has a small but finite chance of going
around for years. Consider what
happens to the molecule if it loses
10% of its strength, say, each time. It
maintains only about half its strength
on the fifth pass. In fact, for this example,
the entire polymer sheet at
steady-state processing, as delivered
to your thermoforming machine, has
only 80% of the strength of the virgin
polymer. Similar analyses are available
for color change, fire retardancy, even
fiber length. Physical property loss
is an important consideration when
designing plastic parts that contain
regrind.

Certain polymers can be reprocessed
many times without apparent property
loss. This is true for most polyethylenes.
Polypropylene on the other hand
loses some important additives such
as odor suppressants, antioxidants,
and crystallizing enhancers. Funky
smell and increased haze may follow.
PVC exhibits color deterioration and
increased flow resistance. Flexible
PVC may lose plasticizers, leading to
loss of flexibility and decreased texture
retention. PET is extremely moisture
sensitive and even when carefully
dried will lose molecular weight. This
can lead to haze generation and loss
in impact strength. ABS and HIPS
will yellow after many recycles. Most
polymer suppliers have run extensive
recycle tests on their thermoformable
polymers. It is your obligation to
exploit the results of these tests. ¦

Keywords: regrind, contamination,
steady-state, scrap

1Readers please note that there is an Industrial
Practice article on regrind, by Bill McConnell,
also in this issue.

A TECHNICAL ARTICLE 2002 VOLUME 21, #4

How To Interpret Technical Articles

We have now completed our first
pass through thermoforming. There
is obviously much more to cover. Design
of parts, for one. But perhaps it’s time to
pause to contemplate what all this verbiage
is all about. So, in this tutorial, we pause to
examine perhaps the thorniest issues confronting
even the smartest thermoformer.
First, why on earth do we need these
abstruse technical articles, anyway? And
second, is there really something important
among all the graphs and equations?

Why Do We Need Technical
Articles?

And perhaps more importantly, why
are they featured so prominently in the
Quarterly? And who on earth decides
which technical articles to feature? The
answer to the last question first. The SPE
Annual Technical Conference or ANTEC
is the primary supply of technical efforts
in our industry. Each year in early May,
half-a-dozen learned works are presented
at ANTEC. These papers are usually
generated from academic or advanced
industrial research programs. As we all
know, there are only a handful of universities
worldwide financed sufficiently to do
research in thermoforming. Who decides
which articles to feature? Since I have been
reviewing the ANTEC papers for dozens
of years, I think I have a good idea what
work should be of interest to thermoformers
in general. Why put them in TFQ? In
reality, there is no other forum that brings
together technical and practical aspects
of thermoforming. We all need to realize
that visionaries are working on solutions
to the myriad technical problems that we
face daily. And that their results can find
an appropriate repository. Researchers
love to be appreciated. It’s second only to
having their work financially supported. If
I had my druthers, I would publish all the
ANTEC technical articles. As it is, I try to
pick out those that seem to focus on meaningful
problems in our industry.

You Mean There Really Are
Important Results Buried
in All Those Equations and
Graphs?

Yep. The biggest challenge for each
of us is to find out those results. In this
tutorial, I’ll try to give you a synopsis of
a typical technical paper1, and one way of
understanding it. The paper usually begins
with the Abstract or summary of the work.
For you business types, this is akin to the
Executive Summary of a report. By reading
this, you’ll quickly determine if the paper
fits in your general area. If it doesn’t, go
on to something else. If it does, you’ll need
to read further. With any proper paper, an
Introduction section follows. In this section,
the authors usually identify the problems
they are solving. They also identify other
technical works that are relevant to the
problems. These references are important
to people who are doing similar research.
Once the problems have been posed, the
authors present the methods and materials
used in attempting to solve the problem.
Polymers and additives are usually described
in detail. Procedures are also detailed,
with specific pieces of equipment or
computer programs carefully documented.
Again, this is important to others working
in this field, since others need to know if, in
order to repeat the experiments, the materials
must be compounded or the equipment
needs to be constructed or the computer
programs are not commercially available.
The next section usually features the step-
by-step procedures needed to solve the
problems. Good researchers know that this
section must be methodical so that knowledgeable
readers will believe their results.
In short, you cannot say, “We put the sheet
in the oven for a while” or “We mixed some
ABS with some HIPS and just extruded it.”
Credibility is the key to acceptance.

On occasion, the researchers need to
mathematically generate or analyze their
results. Here’s where nearly everyone’s
eyes glaze over. Math really isn‘t the
thermoformers’ forte now, is it? But again,
think, “Credibility is the key to acceptance.”
A good researcher knows that his audience
may insist on mathematically reproducing
the final result. Unfortunately, there never
is enough room to fully detail the math. So
we only get the highlights.

And now we get to the Results. The
results are normally presented as tables
or graphs. Then in a section usually called
Discussion, they are interpreted by the
author. The reader should always keep
an open mind at this point, since the most
important parts of the results are, in fact,
the tables and graphs. It is not uncommon
for another researcher to use these results
to form entirely different opinions. And in
fact, you, as an astute reader, should also
be able to form your own opinions.

And …?

So there you have it. The skeleton and
meat of a technical article. If you have
decided that this paper is of interest in
your work, you will have tried to follow
the author’s work with some diligence. At
this point, you must sit back and ponder
the work. Did the author, in fact, solve
the posed problem? Are the materials and
methods appropriate for the solution? Is
the work complete or just an outline? Do
the results make sense? And finally, do you
agree with the author’s conclusions?

Perhaps the most difficult thing for all
of us lies in deciding how significant the
work is to our work. If the materials are
not easily available, if the methods employ
exotic or expensive equipment, if the math
is beyond our capabilities, we may decide
not to try to extrapolate the work to our
problems. Or we may decide to contract
the work to the author’s laboratory.

Probably a more difficult aspect of this
is determining whether, in fact, the work
can be extrapolated. If the work is done for
XXX polymer, for instance, can we apply
the results to YYY polymer? If the work
was done on ABC machine, is it applicable
to work done on XYZ machine? For computer
modeling, for example, can we find
all the necessary physical properties for
our polymer? And so on.

And Finally …

After reading and trying to digest many
technical treatises, you will become sensitive
to probably the most important feature
of all. Is there something in the author’s
work that triggers new questions or offers
insights to old problems? Things that the
author never saw or pointed out. In other
words, is there a new invention hidden in
the work? Or another way of solving an
entirely different problem? Or a hidden
clue showing why a specific problem has
never been solved?

For those of you who thrive on this aspect
of our industry, I offer the following
challenge. Carefully reread either A.C.
Mack, Quality Management in An In-Line
Thermoforming Operation, TFQ 19:1, pp.
5-10, or M.J. Stephenson, A Snapshot of the
Quality and Variability of Continuous Cut
Sheet Thermoforming Operations, TFQ 17:4,
pp. 9-17, together with C.-H. Wang and
H.F. Neid, Solution of Inverse Thermoforming
Problems Using Finite Element Simulation,
TFQ 21:1, pp. 5-10.

Do you now understand a little of the
process control problems we all face? You
still don’t? Really? ¦

Keywords: ANTEC, technical articles,
technical interpretation

THERMOFORMING

101

1For this discussion, I’ll focus on the typical
ANTEC paper format. Most other technical
papers follow a similar format.

A TECHNICAL ARTICLE 2003 VOLUME 22, #1

In The Beginning

Introduction

The thermoforming cognoscenti
among you know that this series has
focused on some of the general concepts
in thermoforming. We began
with brief descriptions of polymers,
then discussed heat transfer, mold
materials, heaters, oven design, forming
temperatures, sheet stretching and
cooling, trimming, and ended with
regrind. A complete list of topics appears
at the end of this article.

In truth, the series was to have
ended in the last issue. But, after reviewing
the 18 “lessons,” it became
apparent that there were
some monstrous holes,
the most obvious of
which was the lack of
substantial discussion
on product design. So,
consider the next few
articles to be “hole
pluggers.”

Just What is
Thermoforming?

Thermoforming is the manufacture
of useful articles of commerce by
heating, shaping, cooling, and
trimming thermoplastic sheet.

Where Did It Come From?

Although historians consider the
forming of tortoise shell, tree bark,
and horn to be the earliest forms of
thermoforming, purposeful manufacture
of products from semi-manmade
thermoplastic sheet began in the
mid-1800s, with the commercialization
of polymerized cellulose nitrate.
The production of thermoformed
household items such as hairbrush
backs, mirror cases, baby rattles, and
piano keys was a reasonably large
business by the turn of the twentieth
century. With the invention of Bakelite,
a completely synthetic thermosetting
polymer in 1909, the emphasis on
product development moved quickly
to compression molding. The commercialization
of new thermoplastics such
as polystyrene, polymethyl methacrylate
(acrylic), and cellulose acetate in
the 1930s spurred the development of
thermoforming, as did WWII. Then,
drape forming over male molds and
free-blow forming were the common
ways of forming heavy-gauge sheet.
Vacuum forming into female molds
was the common way of forming thin-
gauge sheet.

How Big is
Thermoforming?

In 1960, U.S. thermoforming produced
about 100 million pounds of
product. In 2000, that number was approaching
5,000 million pounds. This
is a sustained growth
rate of 6% per year over
forty years. Thermoforming amounts
to about 5% to 6% of the total U.S.
plastics consumption. Conservatively
there are about 500 U.S. heavy-gauge
thermoformers and 125 U.S. thin-
gauge thermoformers.

Is All Thermoforming the
Same?

No. Thermoforming is usually
(loosely) categorized in several ways.

Probably the most apparent way
is in terms of sheet thickness. Simply
put, thin sheet is provided to the
thermoformer as a continuous roll.
Thick sheet cannot be rolled and is
supplied as palletized cut sheet. Thin
sheet forming is frequently called thin-
gauge forming. Thick sheet forming
is called heavy-gauge or thick-gauge
forming.

Here is one way of categorization:

Foil (very Less than 0.010 in., 10

thin sheet) mils, or 250 microns in
thickness

Thin-gauge Less than 0.060-in., 60
mils, or 1.5 mm in thickness

Mid-range 0.060-0.120-in., 60-120
mils, or 1.5-3.00 mm in
thickness

Heavy-gauge Greater than 0.120-in.,
120 mils, or 3.00 mm in
thickness

Plate Greater than 0.500-in.,
500 mils, or 13 mm in
thickness

Keep in mind that “foil” may be
used for any thin-gauge sheet in
Europe. Another way of categorization
is:

Roll-fed Sheet provided to the
thermoformer in a roll

Cut sheet Sheet provided to the
thermoformer on a pallet

This category is useful for determining
the type of machine to be used to
form the products. Another:

Packaging Usually considered as
thin-gauge sheet products

Industrial or Usually considered as

structural heavy- gauge sheet
products

And another:

Disposable Usually considered as
thin-gauge sheet products

Permanent Usually considered

as heavy-gauge sheet
products

And finally one more:

Vacuum Draw-down by evacu-

forming ating the space between
the sheet and the mold

Pressure Application of air

forming pressure in excess of one
atmosphere

Be careful of using one of these ways
as shorthand in formal communication.
Always define the terms you use to
avoid misinterpretation. For example,
even though low-density polystyrene
foam can range in thickness up to 0.250-
inch (250 mils, 6.4 mm), it is delivered
to the thermoformer in rolls. ¦

Keywords: history, categorization,
thin-gauge, heavy-gauge, market size

THERMOFORMING

101

A TECHNICAL ARTICLE 2003 VOLUME 22, #2

Square One – Polymer Selection1

Bill McConnell is fond of saying
that polymer problems account for
more than three-quarters of processing
troubles. In short, there would be
no thermoforming without thermo-
formable polymers and without thermoformable
polymers in sheet form.
Earlier, we discussed the general characteristics
of polymers. In the next set
of lessons, we consider additional characteristics
that are needed to produce
quality thermoformed products.

Extrusion
Basics

Thermoforming
is considered a secondary
process,
since it begins with
sheet. Extrusion is
the primary process.
The most common
form for an extruder is a single
auger-like screw turning in a horizontal
heated, steel barrel. Polymer, in the
form of powder or pellets, is fed into
the extruder through a hopper. The solid
polymer is conveyed down the barrel
length where it is heated and melted
or plasticated. The plasticated melt is
pressure-metered through the end of
the barrel into a shaping or slot die. The
molten extruded sheet is then laid onto
a rotating, cooled cylindrical steel roll.
In some instances, the molten extruded
sheet is “nipped” or squeezed between
this roll and another “kiss” or “gauge
control” roll. It is then conveyed from
this cooled roll to another cooled roll,
where it is further cooled. These three
rolls are usually called the “chill roll
stack.” The roll stack acts to size and
cool the extruded sheet.

Somewhere along the cooling path,
the sheet is trimmed to the purchase
order-specified width. Depending on
the material specifications, the trim
may be ground and returned immediately
to the extruder hopper.

Further cooling can be achieved,
either in ambient air or in a cooling
tunnel. If heavy-gauge sheet is being
extruded, the sheet is either saw-cut or
guillotined into appropriate lengths,
and stacked and
palletized. If thin-
gauge sheet is being extruded, the
sheet is fed to a takeup roll.

Polymer Characteristics
in Extrusion

Extrusion is a high-shear, high-temperature
process. In general, extrusion
plants wish to maximize throughput.
That is, they try to minimize the cost
needed to produce a pound or kilogram
of sheet. For a given polymer,
throughput is increased by increasing
temperature and shear rate.

Thermally sensitive polymers such
as rigid PVC and polyethylene terephthalate
or PET may suffer thermal
damage during extrusion. Certain
polymers such as olefinics may form
gel particles during extrusion. Gels
or “fish-eyes” are usually partially
crosslinked particles. Some polymers
that contain rubber such as impact
polystyrene and ABS may generate
black specks. Many extrusion-grade
polymers are provided with antioxidant
packages to minimize damage
from high-temperature oxygen in the
air in the early portions of the extrusion
process.

Selecting a polymer with low melt
viscosity and elasticity will also increase
throughput without necessarily
increasing temperature and shear
rate. However, we know in thermo-
forming that lower melt viscosity usually
means greater sag in the sheet as it
is being heated. And low melt elasticity
can mean difficulty in plug-assist
stretching into deep
cavities.

Another factor of
great importance to
thermoformers is orientation
in the sheet.
Polymer molecules are
stretched during extrusion
through the die. If
the polymer is cooled
before the molecules are allowed to
fully recover, the sheet will have orientation
in the extrusion direction or
the MD or “machine direction.” If the
extruded sheet is squeezed between
the first two chill rolls such that the
polymer is forced outward toward
the roll edges, the sheet will have
orientation in the cross-machine or
TD direction. It is usually the case in
very wide sheet that both MD and TD
orientations will vary in degree from
the center of the sheet to its edges.

MD and TD orientations are really
“frozen-in strain.” When the sheet is
reheated in the thermoforming oven,
this strain is relieved. If the frozen-in
strain is great, the sheet will distort
and may pull from the clamp frame
or pin chain.

To achieve the lowest levels of MD
and TD orientations, the sheet should
be extruded slowly and at moderately
low temperatures. Of course, these
conditions are not conducive to produce
the highest throughput possible.
And so compromises are needed. ¦

Key words: extrusion, plasticated,
MD orientation, TD orientation

THERMOFORMING

101

1 Thermoforming 101 is designed to be a tutorial
on the basic building blocks of the thermoforming
industry. The first series of lessons concluded in
TFQ 21:3, 2002. This is the second in the second
series of lessons that have as their objective to fill
in the gaps from the first series of lessons.

A TECHNICAL ARTICLE 2003 VOLUME 22, #3

Square One – Polymer Selection-Orientation1,2

In the last TF101, we discussed some
of the aspects of extrusion that are of
importance to the thermoformer. We
began by summarizing the extrusion
process, then focused on polymer characteristics
that influence the extrusion
process. In this lesson, we continue our
investigation of the extrusion process.

Orientation

Orientation is by definition, frozen-in
stretching or elongation of the polymer
molecules. If the sheet is extruded, this
stretching occurs during the polymer
journey from the extrusion die to the
windup or cutoff area of the extrusion
process. Unless carefully controlled,
extrusion can induce substantial orientation
in both the machine direction
(MD) and cross-machine direction (TD).
Orientation is usually less in calendered
sheet and there is rarely any orientation
in cast sheet. Although there is no general
rule, polymers that have extensive
side branches or bulky side branches
along the polymer backbone tend to be
more susceptible to frozen-in elongation
than polymers that have little side
chains. Polymers that are quite rubbery
or elastic tend to be less susceptible to
frozen-in orientation than polymers that
have little elasticity.

If the polymer crystallizes, the desired
crystalline state shows ball-like crystallites
called spherulites. Polymers that
crystallize quickly, like high-density
polyethylene, tend to have higher levels
of spherulites and thus lower levels of
orientation than polymers that crystallize
slowly, like polypropylene. In a
word, the slowly crystallizing polymer
is frozen into an oriented pattern before
it can fully crystallize into the spherulitic
state. The crystallites are then in an “extended
chain” state. When the polymer
is reheated in the thermoforming oven,
the crystallites reform into the sphere-
like state. This causes the sheet to distort
and shrink, with the results ranging
from uneven part wall thickness to sheet
pulling from the clamping grips.

Testing for Orientation

Although many tests have been devised
to determine orientation in sheet,
the so-called Chrysler test is
still preferred by both practitioners and
researchers, alike. There are many variations
of this test. In one version, 1-in. x
10-in. strips are cut from a test sheet. It
is always recommended that several sections
be cut with some having the length
in the machine direction and others in
the cross direction. For wide thin-gauge
sheet, it is also recommended that strips
be cut at the edges and the middle of the
sheet, to determine local orientation.
These strips are then placed in an oven
at about the normal thermoforming
temperature for the polymer. After an
appropriate length of time, dictated by
the thickness of the sheet, the strips are
again measured. The greater the difference
in the “before” and “after” lengths,
the greater the orientation. In some extreme
instances, strips may actually curl,
indicating extensive orientation.

In certain instances, for transparent
polymers such as polystyrene, acrylic,
polyethylene terephthalate, and in
some polyvinyl chlorides, orientation
can be observed by passing the sheet
between polarized film. Orientation
will appear as rainbow patterns across
the sheet. The narrower the color bands
become, the greater will be the local
orientation.

Orientation in
Thermoforming

When we stretch a plastic in the forming
press, we orient the molecules. When
the plastic is pressed against the cool
mold, we freeze this orientation. Simply
put, our plastic part is now oriented. The
level of orientation is a function of the
extent of stretching needed to push the
part into the various corners of the part3.
Importantly here is that the nature of the
orientation of a polymer is affected by the
rate of cooling of the plastic against the
mold surface. This is particularly true
for slowly crystallizing polymers such
as polyethylene terephthalate and polypropylene.
The levels of orientation can
be reduced by reducing the cooling rate
even for amorphous polymers such as
polystyrene and ABS. This is sometimes
called “annealing.”

Orientation v. Shrinkage

As a thermoformed part is cooled in
the mold, it appears to “shrink” away
from female portions and onto male
portions of the mold. This dimensional
change is codified according to whether
the polymer is amorphous or crystalline.
Amorphous polymers always show lower
dimensional changes than crystalline
ones. However, we must distinguish between
dimensional change that is due to
relaxation of orientation and dimensional
change that is inherent in density increase
due to cooling. Technically, “shrinkage”
is temperature-dependent volume
change. When polystyrene, for example,
is slowly cooled from 300°F to room
temperature, its density changes from
0.99 spgr4 to 1.05 spgr. In other words, it
shrinks 6% volumetrically. On the other
hand, the density of PP changes from
0.77 spgr at 330°F to 0.92 spgr at room
temperature. This is a 19% volumetric
change. Cooling the part too quickly will
prevent the polymer from reaching its final
density. Reheating the part sometime
later will allow the plastic to continue its
densifying. This may result in warpage
and distortion.

So, to get the true “mold shrinkage,”
as commonly used, we need to add the
effect of relaxation of orientation, as measured
by the Chrysler test or some other
test, to the natural polymer dimensional
change values. ¦

Keywords: orientation, Chrysler
test, shrinkage, spherulite

THERMOFORMING

101

1 Thermoforming 101 is designed to be a tutorial
on the basic building blocks of the thermoforming
industry. The first series of lessons concluded in
TFQ 21:3, 2002. This is the third in the second
series of lessons that have as their objective to fill
in the gaps from the first series of lessons.

2 Readers should note that C. Rauwendaal’s
book, Polymer Extrusion, is reviewed in this
issue.

3 In a future lesson on part design, we will deal
with orientation and shrinkage and their influence
on part performance.

4 Spgr is specific gravity, in grams per cubic centimeter.
Multiply by 62.4 to get lbs. per cu. ft.

A TECHNICAL ARTICLE 2003 VOLUME 22, #4

Square One – Observe Your Sheet As It Heats1

In the last TF101, we discussed the
difference between orientation and
shrinkage. Here we continue a portion
of this discussion by considering how
we can observe the effects of orientation
or frozen-in stretching as the sheet
is being heated.

Mirror-Image

Consider this thought experiment.
Stretch a rubber band and cool it in liquid
nitrogen while it is stretched. The
rubber band orientation is now frozen
in. Now place the rubber band on a
table and watch as it slowly reheats.
Ultimately all the frozen-in stretch is
relieved and the rubber band returns
to its original length.

A long time ago, we said that thermoforming
was basically an elastic
process. The plastic is heated until
it is pliable. It is then stretched and
“frozen” against a cool mold. If the
formed part is reheated to the forming
temperature, most or nearly all of
it slowly returns to a flat sheet.

Now if we accept this premise, then
we should be able to observe any orientation
that has been frozen in during
the extrusion process. And in fact, we
can, as we shall see.

To do this mirror-image thing, we
begin at the extruder die exit and follow
the thermal history of the sheet,
step by step, until it arrives at the thermoformer.
At each step, we consider
where in the thermoforming process
the sheet sees that temperature.

Just Beyond the
Extruder Die

The sheet is extruded from the
die and is laid onto the middle roll
of the roll stack. The top roll may
press against the sheet to calibrate its
thickness. The molten plastic may be
squeezed in the cross-machine direction
to achieve a specific final sheet
width. The molten plastic is cooling
and some of the extrusion stresses are
relaxing. Where might this occur in
the thermoforming process? The sheet
termperature is hottest just as it exits
the oven. So sheet sag may be related
to the sheet conditions between the
extruder die and the roll stack.

As the Sheet Cools

on the Rolls

The underside of the sheet is cooled
by direct contact with the middle roll
of the roll stack. The top surface is
only cooled with room air. The uneven
cooling can freeze in
stresses on only one side of the sheet.
Where might these stresses be relieved
in the thermoforming process? Since
the sheet is now colder than it was a
few moments ago, we would expect
that this would occur before the sheet
exhibited substantial sag. And it would
be manifested as a tightening of the
sheet as the stresses relieved.

As the Sheet Temperature
Approaches a Transition
Temperature

Amorphous polymers, such as polycarbonate
and polystyrene, go from
being rubbery to glassy at their glass
transition temperatures. Crystalline
polymers, such as polyethylene and
polypropylene, go from being nearly
fluid to rigid at their melting temperatures2.
In the extrusion process, this
occurs on the rolls for thin to moderately
thick sheet. Transitions usually
entail density increases. If the sheet is
confined, internal stresses occur. These
are frozen in by the transition. What
would we expect to see as we reheat
the sheet in thermoforming” Certainly
during reheating, the polymer density
decreases. Because the stresses are not
locked in uniformly through the sheet
thickness, we see the sheet ripple or
“swim.”

Heat Retention

In extrusion, the sheet is never allowed
to cool to room temperature
before being cut and stacked on pallets
or wound onto rolls. As a result, the
rolls or pallets retain heat for extended
periods of time. This retained energy
can often provide some mild annealing
or help relieve some of the locked-in
stress. Regardless of extent to which
this happens, the concern is that the
thermal history of the sheet on the bottom
of the pallet is different than one
in the middle. And that one is different
than the one on the top. The same
analysis holds for rolled goods. The
extent of this stress relief is observed
in the initial tightening of a sheet in
the very early heating times. Certainly
if this tightening varies throughout
the production run, the temperature
control of the sheet suffers.

Smoking

Plastics are filled with many small
molecule additives – internal and external
lubricants, antiblocking agents,
UV absorbers, organic dyes and colorants,
and so on. Some of these migrate
to the sheet surface and some are volatile.
In extrusion, the sheet may off-gas
or smoke as it leaves the extruder and
as it forms over the middle chill roll.
In thermoforming, at some place in the
oven after the initial sheet tightening,
the sheet may smoke.

Moisture

Regardless of how well thin-gauge
rolls are wound or heavy-gauge sheet
is palletized, air diffuses between the
sheet plies in storage. And with air
comes moisture. For some polymers
such as polycarbonate and polyethylene
terephthalate (PET), the moisture
is absorbed into the sheet. For others,
such as polyethylene, the moisture is
simply adsorbed on the surface of the
sheet. In thermoforming, where does
this moisture exit? In the very early
stages of heating, we might actually
see the sheet steaming. Keep in mind
that steaming is not smoking. These
effects occur at different times in the
heating process. ¦

Keywords: Rippling, tightening,
off-gas, stress relief, moisture

THERMOFORMING

101

1 Thermoforming 101 is designed to be a tutorial
on the basic building blocks of the thermoforming
industry. The first series of lessons concluded in
TFQ 21:3, 2002. This is the fourth in the second
series of lessons that have as their objective to fill
in the gaps from the first series of lessons.

2 Really at their recrystallization temperatures.
Perhaps we will consider this concept in a later
lesson.

COMMON SHEET MATERIALS & SUPPLIERS

Prepared by Thermoforming Division Materials Committee

Mono or coextruded

Type of sheet company phone web site

ABS Allen Extruders (888) 833-1305 allenx.com

Alltrista Industrial Plastics (812) 479-5960 plasticssolutions.com

Bunzl/Southern (804) 346-2400 bunzlextrusion.com

Empire Plastics (740) 498-5900 empireplastics.com

Futurex Industries (800) 541-2353 futurexind.com

Gage Industries (800) 443-4243 gageindustries.com

Premier Material Concepts (419) 429-0042

Primex Plastics (800) 222-5116 primexplastics.com

Quality Plastic Sheet (574) 293-2752

Select Plastics (877) 501-2530

Senoplast USA (636) 922-3874 senoplastusa.com

Spartech Plastics (314) 721-4242 spartech.com

Acrylic Aristech Acrylics (859) 283-1501 thedreamhome.com

Atofina Chemical (215) 419-7000 atofinachemicals.com

Bunzl/Southern (803) 796-0600 bunzlextrusion.com

Cyro (973) 442-6000 cyro.com

Empire Plastics (740) 498-5900 empireplastics.com

Goex (608) 754-3303 goex.com

Ineos (901) 381-2000 ineosacrylics.com

Kleerdex (803) 642-6864 kydex.com

Pawling (800) 431-0101 pawling.com/extrusion

Plaskolite (614) 294-3281 plaskolite.com

Spartech Plastics (314) 721-4242 spartech.com

Barex Goex (608) 754-3303 goex.com

Klockner Pentaplast (540) 832-3600 klockner.com

Mullinex (219) 747-3149

EDS HMS Compounds (817) 468-3099 hmscompounds.com

Noryl Allen Extruders (800) 833-1305 allenx.com

Bunzl/Southern (804) 346-2400 bunzlextrusions.com

Primex Plastics (800) 222-5116 primexplastics.com

Spartech Plastics (800) 721-4242 spartech.com

Westlake Plastics (610) 459-1000 westlakeplastics.com

Polycarbonate Bunker Plastics (972) 245-9656 bunkerplastics.com

Film Specialists

Fox Lite (937) 864-1966 foxlite.com

GE Plastics (800) 451-3147 genstructuredproducts.com

Goex (608) 754-3303 goex.com

Rowland Technologies (203) 269-1437 rowlandtechnologies.com

Sheffield Plastics (413) 229-8711 sheffieldplastics.com

Spartech Plastics (314) 721-4242 spartech.com

Tekra (262) 784-5533 tekra.com

Westlake Plastics (610) 459-1000 westlakeplastics.com

Polyester Alcoa Kama (800) 628-7598 kamacorp.com

Allen Extruders (800) 833 1305 allenx.com
Alphatec (920) 748-7421 alphatecextrusion.com

Goex (608) 754-3303 goex.com

Klockner Pentaplast (540) 832-3600 klockner.com

Pacur (920) 236-2888 pacur.com

PETCO/Lavergne (514) 354-5757 lavergne@lavergne.ca

Plaskolite (614) 294-3281 plaskolite.com

Primex Plastics (800) 222-5116 primexplastics.com

Sheffield Plastics (413) 229-8711 sheffieldplasticsinc.com

Spartech Plastics (314) 721-4242 spartech.com

VPI (920) 458-4664 vpicorp.com

Polyethylene Alcoa Kama (800) 628-7598 kamacorp.com

Allied Extruders (718) 729 5500 alliedextruders.com

COMMON SHEET MATERIALS & SUPPLIERS

Mono or coextruded

Type of sheet company phone web site

Polyethylene (cont’d.) B & F Plastics (800) 562-8365 bfplastics.com

Bunzl/Southern (804) 346-2400 bunzlextrusion.com

Conplex (904) 824-0422 conplex.com

Futurex Industries (800) 541-2353 futurexind.com

Gage Industries (800) 443-4243 gageindustries.com

Goex (608) 754-3303 goex.com

HPG (732) 271-1300 hpg-intl.com

Primex Plastics (800) 222-5116 primexplastics.com

Quality Plastic Sheet (574) 293-2752

Spartech Plastics (314) 721-4242 spartech.com

VPI (920) 458-4664 vpicorp.com

Polypropylene Alcoa Kama (800) 628-7598 kamacorp.com

B & F Plastics (800) 562-8365 bfplastics.com

Conplex (904) 824-0422 conplex.com

Ex-Tech Plastics (815) 678-2131 extechplastics.com

Goex (608) 754-3303 goex.com

HPG (732) 271-1300 hpg-intl.com

Interplast Group (973) 994-8000

Pacur (920) 236-2888 pacur.com

Premier Material Concepts (419) 429-0042

Primex Plastics (800) 222-5116 primexplastics.com

Spartech Plastics (314) 721-4242 spartech.com

VPI (920) 458-4664 vpicorp.com

Witt Plastics (937) 548-7272 wittplastics.com

Polystyrene Alcoa Kama (800) 628-7598 kamacorp.com

Allen Extruders (888) 833-1305 allenx.com

Alltrista Industrial Plastics (812) 479-5960 plasticssolutions.com

Bunzl/Southern (804) 346-2400 bunzlextrusion.com

Farber (516) 378-4860

Flock Tex (800) 556-7286

Futurex Industries (800) 541-2353 futurexind.com

Goex (608) 754-3303 goex.com

Impact Plastics (860) 828-6396

Joe’s Plastic (562) 949-3619

New Hampshire Plastics (800) 258-3036 nhplastx@usa.pipeline.com

Plaskolite (614) 294-3281 plaskolite.com

Primex Plastics (800) 222-5116 primexplastics.com

Spartech Plastics (314) 721-4242 spartech.com

VPI (920) 458-4664 vpicorp.com

PVC B & F Plastics (800) 562-8365 bfplastics.com

Empire Plastics (740) 498-5900 empireplastics.com

Ex-Tech Plastics (815) 678-2131 extechplastics.com

Goex (608) 754-3303 goex.com

Kleerdex Company (803) 642-6864 kydex.com

Klockner Pentaplast (540) 832-3600 klockner.com

Nan Ya (409) 532-5494

Naugahyde Div. of Uniroyal (574) 733-5983

Poly One (540) 667-6666

Spartech Plastics (314) 721-4242 spartech.com

VPI (920) 458-4664 vpicorp.com

TirePlast B & F Plastics (800) 562-8365 bfplastics.com

TPO Alltrista Industrial Plastics (812) 479-5960 plasticssolutions.com

B & F Plastics (800) 562-8365 bfplastics.com

Primex Plastics (800) 222-5116 primexplastics.com

Spartech Plastics (314) 721-4242 spartech.com

TPR

Premier Material Concepts (419) 429-0042

WoodPlast B & F Plastics (800) 562-8365 bfplastics.com

XT Goex (608) 754-3303 goex.com

A TECHNICAL ARTICLE 2004 VOLUME 23, #1

Recrystallization – What Does That Mean?1

In the last TF101, I mentioned recrys-
tallization. In this tutorial, I will
explain what it is and why it is important
in thermoforming.

Amorphous and Crystalline Plastics
– A Brief Review

In a very early lesson, we learned
that there are two general classes
of plastics used in thermoforming.
Plastics such as polystyrene, ABS,
polycarbonate and even rigid PVC are
considered amorphous. That is, they
are glassy-brittle at room temperature.
When they are heated to a general
temperature range called the glass
transition temperature, they become
rubbery. If we continue to heat them,
they become less and less rubbery
and more and more fluid-like. When
we cool these polymers to their glass
transition temperature, they immediately
become glassy-brittle again.
Amorphous polymers represent the
majority of plastics thermoformed
today.

But as we learned in that early
lesson, thermoformers are bent on
forming crystalline – or more correctly,
semicrystalline – polymers such as
polyethylene, polypropylene and PET.
For many years, polyethylene was the
only semi-crystalline plastic that was
widely thermoformed. PET is usually
formed in the amorphous state (as
APET). Special processes are needed to
produce crystalline PET structures.

Melt Forming PE

High-density PE has exceptional
hot melt strength above its melt temperature
of about 275°F (135°C). That’s
why the blow molder can extrude a
tube of polyethylene, then capture it
in a clamshell mold to make a bottle.
Thermoformers also rely on the hot
melt strength of it in sheet form. We
heat the sheet above its melt temperature
just prior to forming it, as a melt.
Polyethylenes are 50-80% crystalline
and the sleek shape of the molecule
allows very rapid crystallinity once
the formed part is cooled below its
melt temperature. As a result, HDPE is
the most successful semicrystalline
polymer thermoformed.

Solid State Forming of PP

Until recently, polypropylene recipes
did not have sufficient hot strength to
remain sheets in the thermoforming
ovens. As a result, PP was thermoformed
in the solid state. What this means is the
PP sheet was (and is) heated to just
below its melting temperature range,
which is about 330°F (165°C) for
homopolymer polypropylene
(homoPP). PP becomes rubbery in a
very narrow temperature range just
below the melting temperature.

There are two reasons for this
rubberiness. First, the crystallinity of
PP is about 50%, meaning that about
half the PP is not locked in crystallites,
but is instead in an amorphous state.
The glass transition temperature of
homoPP is about 15°F ( -10°C). So
when homoPP sheet is at a forming
temperature of about 320°F (160°C),
say, the amorphous portion of the
sheet is 305°F (170°C) above its Tg2.
Secondly, imperfect crystallites
tend to melt below the stated melt
temperature. This means that more
polymer is added to the amorphous
side of the equation, making the sheet
even more rubbery.

With sufficient pressure then, we can
squeeze, push and otherwise press PP
against the mold. Pressures of 50 to 100
psi have been used to do just this.

So, what is the problem with solid
state forming of PP? Really, nothing.
It just requires higher forming pressures
than what would be used for, say,
PS. Oh, and the product is not water
white but instead, about as translucent
as the original sheet. This is because
we don’t melt out the crystallites and
the crystallites are of sufficient size to
interfere with visible light (0.4 to 0.7
microns).

Melt Forming of PP

Copolymerization of polyethylene
in PP and now, short- and long-chain
branching of PP has greatly improved
PP hot strength. This lets us to thermoform
PP in the melt state, or the
state where all the crystallites are fully
melted. Copolymer PP or coPP usually
melts around 310-320°F (155-160°C).

But melt forming PP is not like
melt forming polyethylene. We run
into a very difficult problem. PP
recrystallizes at a much slower rate
than polyethylene. Even when coPP
is cooling at 9°F/minute (5°C/minute)
3, it recrystallizes around 210°F
(100°C), or about 100°F (60°C) below
its melt temperature. Small amounts
of recrystallization rate enhancers such
as sorbitols can increase the recrystallization
temperature by about 20°F
(10°C). While this may shorten the
hold time on the mold, we still need
to hold coPP on the mold longer than
we might think.

And more importantly, we need to
be concerned about recrystallization
that might continue long after we
remove the formed coPP part from
the mold surface. When the formed
part isn’t constrained, different areas
of the part can crystallize at different
rates and to different crystallinity levels.
Distortion, warping, cupping, and
general mayhem can occur long after
the part is formed. ¦

Keywords: Recrystallization, solid
state forming, melt forming, hot melt
strength

THERMOFORMING

101

1 Thermoforming 101 is designed to be a
tutorial on the basic building blocks of the
thermoforming industry. The first series of
lessons concluded in TFQ 21:3, 2002. This
is another in the second series of lessons
that have as their objective to fill in the gaps
from the first series of lessons.

2 Compare this with PS at its thermoforming
temperature of 350°F (175°C), or 140°F
(75°C) above its Tg of 210°F (100°C).

3 In practice, we would never cool PP this
slowly. The test equipment used to measure
melting and recrystallizing temperatures
operates at about this cooling rate.

A TECHNICAL ARTICLE 2004 VOLUME 23, #2

Alphabet Soup?

It seems that plastics people never
tire of their alphabet soup – ABS,
PTFE, PVC, PUR, and on and on. The
soup continues when we consider
evaluating the characteristics of plastics.
In this short series, we consider a
few of the letters in this soup.

Tg, Tm and DSC

We’ve already discussed the first
two. Tg is glass transition temperature,
or the temperature above which
polymers become rubbery rather than
glassy. Tg’s for polystyrene and acrylic
are around 210°F (100°C). Tg for
rigid PVC is around 185°F (85°C) but
can be as low as -25°F (-30°C) when
highly plasticized. The glass transition
temperature for nylon 6 is only
122°F (50°C). The Tg for polyethylene
is around -125°F (-90°C) and that for
homopolymer PP is 15°F (-10°C).

You’ll recall that Tm is the melting
temperature for crystalline polymers
such as polyethylene, polypropylene
and nylon. The melting temperature
for HDPE is around 275°F (135°C). The
melting temperature for homopolymers
PP is 330°F (165°C), and that for
nylon 6 is 430°F (220°C).

One popular method for measuring
Tg and Tm is with DSC. So, what is
DSC? Differential scanning calorimetry.
Consider heating a substance from
room temperature, say, to a specific
processing temperature. Let’s use water
as an example. It takes exactly one
calorie of energy to heat one gram of
water one degree Centigrade. In British
units, it takes one British Thermal
Unit of energy to heat one pound of
water one degree Fahrenheit. This rule
works until water reaches its boiling
point of 212°F (100°C). At the boiling
point, the temperature remains constant
even though substantial energy
is inputted to the water. If we were
to compare the energy uptake of a
substance that did not boil with that
of water, we would see that the temperature
of the non-boiling substance
would continue to climb while that of
water would remain constant until all
the water had evaporated.

Conversely, if we cool water from
room temperature to 32°F (0°C), it
freezes. At the freezing point, the
temperature remains constant even
though substantial energy is removed
from the water.

Physical changes that take up
energy with little or no temperature
change, such as boiling or melting, are
called endothermic changes. Physical
changes that give off energy with
little or no temperature change, such
as freezing or crystallizing, are called
exothermic changes.

We can build a device that compensates
for these temperature differentials.
The device uses a well-characterized
substance as its reference. The
substance to be tested is then heated
at the same rate as the reference substance.
This is done by carefully controlling
the energy ratio between the
reference substance and the test substance.
Since the device is measuring
calories or units of energy, it is called a
calorimeter. Since the device measures
the temperature difference between
two substances as they heat, it is a
scanning device. And since the device
is looking at the difference between
the two substances, it is a differential
device. It we put this all together we
see that the device is a differential
scanning calorimeter, or DSC!

What Can We Learn From
DSC?

First, we must realize that the DSC
can be used either in a heating mode or
in a cooling mode. Samples are usually
heated beginning at room temperature
and they are usually heated at a fixed
temperature rate such as 10°C/minute.
The temperature range and energy
requirements of transitions are the
primary information gathered from
heating DSCs. The most common
transitions are the glass transition
temperature and the melting temperature,
if any.

Between transitions, the DSC provides
relative energy uptake by the
test substance. This is directly related
to that for water, as specific heat or
heat capacity. As we saw above, the
amount of energy absorbed by water
is 1 cal/gm°C or 1 Btu/lb°F. So its heat
capacity is 1.0. It takes 100 cal/gm°C or
180 Btu/lb°F to heat water from 32°F
to 212°F. We find that polystyrene has
about 55% of the heat capacity of water
and that for PVC has about 37% of that
of water. PP has about 85% of the heat
capacity of water and that for LDPE
is about the same as that of water.
Remember, now that these values are
between transitions.

DSC is important when trying to
determine the extent of crystallization
of a polymer. Consider the case for a
100% crystalline polymer that requires
100 cal/g to melt. If that polymer is
cooled from the melt and DSC determines
that only 50 cal/g was liberated
during recrystallization, it is safe to say
that the polymer at room temperature
is only 50% crystalline.

In the last lesson, we learned that
coPP melts around 155°C but recrystallizes
at around 100°C. How did we
know that? From DSC, of course.

The DSC can teach us another aspect
to polymer characterization. As
we increase the cooling rate for some
crystalline polymers, we retard the
temperature at which recrystallization
begins. And we reduce the final
level of crystallinity. How do we know
this? Consider PET. It has a melting
temperature of 510°F (265°C). If we
cool PET very slowly, we find that it
recrystallizes at around 250°C to about
40-45%. If we cool PET very rapidly,
we find that there is no recrystallization
region. PET remains amorphous
at room temperature and beyond. DSC
is therefore a tool for determining how
rapidly a plastic crystallizes. ¦

Keywords: glass transition, melting,
recrystallization, calorimeter,
endothermic, exothermic

THERMOFORMING

101

A TECHNICAL ARTICLE 2004 VOLUME 23, #3

ABCs of Alphabet Soup?

In our last lesson, we learned about
Tg, Tm, and DSC. These are important
letters in our alphabet soup. In this
lesson, we look at some new letters.

IR and FTIR

IR means infrared and FTIR means
Fourier-transform infrared. We usually
heat our plastic sheet with radiant
heaters. These heaters emit infrared
energy or IR. Infrared energy is part
of the electromagnetic spectrum of energy.
The spectrum is usually defined
in terms of the length of the emitting
rays. And the length is usually given
in microns. The length is identified
by the symbol .m. Radio waves are
long-length waves, in the range of 107
to 1010 .m. They reside near one end of
the electromagnetic spectrum. Gamma
rays are short-length waves, in the
range of 10-4 to 10-7 .m. They reside
near the other end of the electromagnetic
spectrum.

In contrast, visible light has the
wavelength range of 0.4 to 0.7 .m. It
is about in the middle of the electromagnetic
spectrum. Energy of shorter
wavelengths, between 0.4 .m and
about 10-2 .m, is ultraviolet energy. Energy
of longer wavelengths, between
0.7 .m and about 103 .m, is called
infrared energy. The infrared energy
wavelength range is usually separated
into near infrared energy, having wavelengths
between 0.7 .m and about 2.5
.m, and far infrared energy, having
wavelengths between about 2.5 .m
and 103 .m.

Our thermoforming radiant heaters
typically operate in the wavelength
range of about 3.5 .m to about 20 .m or
so. Or in the far infrared energy wavelength
range. As you might expect, as
the temperature of the radiating body
increases, the peak wavelength shifts
to short and shorter wavelengths. And
as the radiating body temperature increases,
the amount of energy emitted
increases as well.

For example, if your heater temperature
is 400°F (~200°C), the peak wavelength
is 6.06 .m, and the maximum
amount of energy emitted at this wavelength
is 2.94 units1. If you raise the
heater temperature to 600°F (~315°C),
the peak wavelength is 4.92 .m, and
the maximum amount of energy emitted
is 6.78 units. If you raise the heater
temperature to, say, 900°F (~480°C), the
peak wavelength is 3.83 .m, and the
maximum amount of energy emitted
is 17.3 units.

A couple of reference points, please!
Certainly! The sun radiates at 12,000°F
(~6500°C), its peak wavelength is 0.43
.m, and the maximum amount of energy
it emits is nearly 120,000 units2!
What about us! Because radiation is
electromagnetic energy interchange,
we radiate back to the sun at 98.6°F
(37°C). Our peak wavelength is 9.35
.m, and we emit about 0.52 units.
We’re pretty feeble radiators!

Okay, what about the sheet we’re
trying to heat? Well, as the sheet
heats, the amount of energy it radiates
increases. Suppose the heater temperature
is 600°F. If the sheet reaches 400°F,
it radiates a maximum of 2.94/6.78
= 43% of the energy it gets from the
heater back to the heater! Really! Isn’t
infrared energy fun?

Okay, what is FTIR? Keep in mind
that heaters emit and sheet absorbs
far infrared energy. However, plastics
do not absorb energy uniformly.
The amount of energy absorbed at
any given wavelength depends on
the type of plastic being heated. Very
few plastics uniformly absorb radiant
energy only on their surfaces. When
the radiant energy is not entirely absorbed
in the surface layer, some of it
is transmitted into the plastic. If the
sheet is thick enough, all the radiant
energy that impinges the sheet surface
is absorbed in the polymer. Plastics
such as polyethylene are notoriously
poor at absorbing energy on the sheet
surfaces. Others, such as PVC, absorb
a substantial portion of the incoming
energy on the sheet surface.

But how can we tell whether a sheet
of plastic is absorbing most of its energy
on its surface or inside the sheet?
That’s where FTIR comes in. For the
moment, ignore the “FT” part of the
alphabet soup. I take a thin film of my
plastic, place it in an infrared or IR
scanner, and pass a monochromatic3
infrared beam through it. I measure the
decrease in energy transmitted through
the plastic film at that wavelength. I
change the wavelength of the infrared
beam and again measure the transmitted
energy. If I scan the film with an
infrared beam of wavelength range of
2 .m, say, to 20 .m, I will have covered
the majority of the wavelength range
of our heaters.

Now, I double the film thickness and
repeat the scan. This tells me how deep
the infrared energy penetrates into my
film. I continue doubling the film thickness
until none of the infrared energy
is transmitted through the film.

The IR scanner is mostly used in an
analytical polymer laboratory, where
polymer chemists determine the general
composition of the polymer and
its additive packages. The various
peaks that are generated at specific
wavelengths are directly related to the
molecular confirmation of the polymer.
For example, the carbon-hydrogen
bond is stretched at a frequency of
about 3.5 .m. As a result, all polymers
containing C-H bonds absorb 100% of
the 3.5 .m infrared energy. PP does.
PE does. PTFE doesn’t because it has
no C-H bonds.

What about the “FT” portion of
FTIR? The polymer chemist obtains
infrared scans of various recipes in order
to compare the ingredients. These
scans are then mathematically encoded
so that the polymer chemist (or his
computer software) can arithmetically
subtract the infrared spectrum of the
polymer, say, from the spectrum of the
recipe. What’s left must be the additive
package. By subtracting the infrared
spectrum from a known additive package,
for example, the polymer chemist
can determine the composition of any
unknown in the recipe. The mathematical
encoding is called Fourier Transformation
or “FT.” And we’ve just put the
“FT” back into FTIR.

Do we care what the recipe of our
plastic is? Not really. We just need to
know how much infrared energy our
plastic absorbs. So we just “piggyback”
on the polymer chemist’s FTIR
device. ¦

Keywords: Infrared, far infrared,
electromagnetic energy, wavelength,
Fourier Transform

THERMOFORMING

101

1 The units are kW/m2 – .m.

2 Think Clearwater Beach, Florida at 1400
hours on July Fourth!

3 Monochromatic: Of or composed of radiation
of only one wavelength.

A TECHNICAL ARTICLE 2004 VOLUME 23, #4

XYZs of Alphabet Soup?

This is the third in a series on plas-
tics alphabet soup. So far, we’ve
tackled Tg and Tm. And we spent time
with DSC and IR and FTIR. Are there
more acronyms1 that we should know
about? Sure. Lots. In this discourse, we
look at four more – HDT, DTA, DMA,
and DTMA.

What is HDT?

HDT stands for heat deflection temperature2.
It is an ASTM test (D648) and
an ISO test (75-1, 75-2)3. The test focuses
on the three-point deflection of a
plastic bar of very specific dimensions4.
The bar is placed in an oil bath. A dead
weight is placed in the center5. The oil
bath is heated at a very specific rate6.
As the plastic increases in temperature,
it softens. HDT is the temperature at
which it sags a fixed amount7. This
test, without ASTM or ISO numbers,
is more than 60 years old.

HDT is often used to sort or rank
plastics. And it is often used for
quality control. In thermoforming, it
provides a crude, early estimate of
the lower temperature for forming.
Unfortunately, it is not a very good
test. For example, it has no value for
plastics that are relatively soft at room
temperature, such as some plasticized
PVCs and TPOs. The test yields a
single data point that should not be
used to predict long-term behavior.
The test is often tricked by residual
stress in the test specimen. And it is
tricked if the polymer has very low
thermal conductivity or if the oil bath
is not vigorously stirred.

Here is another way the data can
mislead. Consider HDT values for
polycarbonate and nylon 6 for two
loads:

Plastic HDT @ HDT @

66 psi 264 psi

Polycarbonate 290°F 275°F

Nylon 6 370°F 150°F

What is the real HDT value for
nylon 6?

And one more reason to avoid HDT
values. Consider glass-reinforced polycarbonate
and nylon 6 HDT values:

Plastic Unrein- Reinforced

forced @ HDT @

264 psi 264 psi

Polycarbonate 275°F 295°F

Nylon 150°F 420°F

Again, what is the HDT of nylon 6?
Nuff, said!

DTA, DMA, and DTMA.
Are These the Same?

First, DTA. DTA stands for differential
thermal analysis. Remember our
discussion on DSC, differential scanning
calorimetry? Well, DTA uses the
same equipment and the same analysis
as that for DSC. The difference is that
the data are interpreted differently for
DTA. For example, DTA yields specific
heat, or the amount of heat absorbed
by the plastic as a function of temperature.
Time-dependent changes such
as rate of crystallization are obtained
by running the DSC/DTA at different
heating (or cooling) rates.

DMA is the acronym for differential
mechanical analysis and DTMA is the
acronym for differential thermal mechanical
analysis. The earliest device is
described in ASTM D 2236. A review of
various mechanical testing techniques
is described in ASTM D 4065. In general,
a solid plastic test bar is subjected
to torsional or flexural oscillation.

Usually, if the device is operated at
a fixed temperature and the frequency
of oscillation is varied, the test is
called DMA. If the device is operated
at a fixed oscillation frequency and
the temperature is varied, the test is
called DTMA. There is an imperfect
correlation between these two testing
procedures.

What is DMA/DTMA used for?
If the plastic is elastic, its resistance
matches that of the oscillating device.
If the plastic is fluid or viscous, its
resistance is out-of-phase with that of
the oscillating device. As the temperature
or frequency is changed, device
detects shifts from elastic to viscous
or viscous to elastic character in the
polymer. As a result, the data will yield
the glass transition temperature of the
polymer. More importantly, however,
the test will show plastic resistance to
applied load over a temperature range
up to its melting temperature. This is
important for thermoformers, since
we are stretching the plastic while it
is primarily in its rubbery solid state.
For example, we can quickly assess
the forming temperature range of the
plastic. And within a given polymer
family, we can determine which polymer
recipe yields the broadest forming
window. DMA/DTMA will probably
be the subject for a TF 101 lesson.

DMA/DTMA data obviate single-
point values such as those obtained
with HDT/DTUL devices. ¦

Keywords: mechanical analysis,
heat deflection, heat distortion,
oscillation frequency

THERMOFORMING

101

1 Acronym: a word formed from the initial
letters of a multi-word name.

2 HDT was originally called heat distortion
temperature.

3 Another acronym used for the ASTM test
is DTUL, meaning deflection temperature
under load.

4 5 inches long by 1/2-inch thick by a width
not to exceed 1/2-inch. ISO specifications
are similar but in metric units.

5 The weight is equivalent to either 66 or
264 lb/in2 fiber stress. ISO specifications
are similar but in metric units.

6 2 +/- 0.2°C/min.

7 0.010 inches. ISO specifications are
similar but in metric units.

A TECHNICAL ARTICLE 2005 VOLUME 24, #1

Why is Part Design Important?

Throughout this series of
tutorials, we have assiduously1
avoided the issue of part design. And
for good reasons. First and foremost,
technologists – of which I am one
– are normally not good designers.
We tend to get hung up on the nuts-
and-bolts of problem solving rather
than the esthetics of the thing we’re
making. And second, there really
isn’t a good way of categorizing part
design, particularly when there are
so many applications and variants
on the process.

Having cited these caveats, perhaps
it is time to review at least
some of the generic aspects about
thermoformed part design. We try to
do this in the next series of lessons.
And we begin by considering some
of the limitations to the thermoforming
process.

Can You Make the Part
the Customer Wants at the
Price He’ll Pay (and Still
Make a Profit)?

There are some fundamental reasons
for not quoting on a job, even
though it appears “doable” and the
potential profit is substantial. Some
of these are obvious, to wit:

• The parts are too large for the
available equipment

• The parts are too small for the
available equipment

• Too few parts are needed

• Too many parts are needed

Others depend on the nature of the
plastic needed for the job. Consider
these limitations:

• The polymer cannot be ex-
truded into sheet

• The polymer cannot be drawn
to the requisite depth

• The polymer needs to be drawn
to near its extensional limit

• The polymer cannot be reground
or reprocessed economically

• The design requires high-performance
plastics

• The design requires highly filled
or reinforced plastics

Some depend on the match between
the part requirements and
your forming abilities:

• The design requires complex
forming techniques that you
don’t have

• It is more exotic than your current
skills

• The design accuracy is greater
than your current abilities

• You cannot trim to the required
accuracy

• Your workers do not have the
skills to repeatedly form quality
parts

• You do not have in-house ability
to test product serviceability

• You cannot prototype to determine
part acceptability

And still others depend on the
characteristics of the design, such
as:

• The forces required to achieve
the final shape are too high for
the available equipment

• The design requires excessive
web or trim

• Part tolerances, draft angles are
unachievable in thermoforming

• Part design requires uniform
wall thickness

• Part design requires stepped
wall thicknesses

And finally, the coup de grace2
– Competitive processes are more
competitive! This one is probably
the most difficult design limitation,
simply because companies
using competitive processes are
now recognizing the capabilities of
thermoforming and now are either
altering their technologies to compete
more effectively or are deciding
to enter the thermoforming field.

What Not To Do

In most cases, we know the limitations
of our equipment and ourselves.
So we quote on parts we know we
can mold. In some cases, however,
the thrill of “taking a chance” is too
much to pass by. That’s when the
thin-gauge part must be molded
diagonally with the mold ends extending
beyond the platen. Or when
we try to “pressure form” in a press
without a proper clamping system,
hoping that the press won’t open until
the part has completely form. Or
when the depth of draw of the part
is so great that we need to heat the
sheet until it sags to the point where
it drags across the tooling. Or when
… Well, you get the idea.

So, What Lessons Will
We Learn?

In this series-within-a-series, we’ll
take a look at some simple issues
such as female or negative molding
and male or positive forming. We’ll
consider design aspects such as corners
and chamfers, vent hole locations,
and lip and edge formation.
And surface texture, draft angles,
and more. It should be fun. And
maybe we’ll all learn something on
the way. ¦

Keywords: Design, formability,
dimensional tolerance, draft angle

THERMOFORMING

101

1 Assiduously: Unceasingly; persistently.

2 Coup de grace: A decisive, finishing
stroke.

A TECHNICAL ARTICLE 2005 VOLUME 24, #2

Comparing Concept to Reality1

We began our discussion of part
design by reviewing why we
might not want to quote on a job.
If we are serious about fabricating
the customer’s concept, we need
to understand the methodology in
reducing a concept to reality.

Naiveté v. Experience

Before we consider developing a
hard cost for a given project, we need
to ascertain the technical level the
customer brings to the design. Most
of us have dealt with customers of at
least one of the following levels:

• Expert Customer. Fully cognizant
of the advantages and
limitations of thermoforming
in general, conversant of the
plastics characteristics, and
having a complete understanding
in the myriad ways
of fabricating his design, in
particular.

• Experienced Customer. Has
designed certain parts in thermoforming
in the past but
is not up-to-date, vis-a vis2,
newer processing techniques,
mold materials, polymers, and
so on.

• A Non-Thermoforming Technical
Customer. Has extensive
experience in blow molding,
rotational molding, or injection
molding, but has no
knowledge of the differences
between these techniques and
thermoforming.

• A Technically Naïve Customer.
Knows little about plastics and
nothing about thermoforming.
Has always purchased his
plastic products to either mate
with or package his non-plastic
products.

• The Totally Naïve Customer.
Has a great idea worked out
on the back of a Burger King
napkin, has no funding, no
customer, and no idea how to
reduce his idea to reality.

We all agree that it is very difficult
to treat each of these in the same
fashion. In other words, a checklist of
things necessary to reconcile prior to
quotation might be too technical for
the naïve customer and an insult to
the experienced one. Nevertheless,
we should all keep in mind before
every take-off and landing, the pilot
and copilot are required to complete
an extensive checklist, regardless
of their years of experience and the
number of times they had flown the
specific airplane. So let’s take a look
at a typical design checklist.

General Advantages
and Limitations of
Thermoforming

We all know the advantages and
limitations of our skills. But the
customer may not. So tell him/her.
Some advantages:

• Lower tooling costs

• Quicker design-to-prototype
time

• Quicker prototype-to-production
time

• Relatively wide selection of
polymers, grades

• Large surface area per unit
thickness

• Economic production of a few
pieces (heavy gauge) or many,
many pieces (thin gauge)

Some limitations:

• Non-uniform wall thickness

• Single-surface molds

• Hollow parts difficult

• Sheet cost

• Extensive trimming, recycling
needed

• Mostly neat plastics (few
reinforced and highly filled
plastics)

• Wide forming windows desired
(needed)

The Material Issue

We, along with the astute customer,
need to discuss material choices in
some detail. It is not enough for the
customer to specify “general purpose
polystyrene.” He/she needs to work
with us to develop a list of property
requirements. In other words, what
are the elements of the environment
in which the product must perform?
Some examples are:

• Environmental temperatures
(high and low)

• Corrosive/erosive conditions

• Static/dynamic loading conditions

• Impact conditions

• Surface quality

• Product lifetime

• Assembly restrictions (if any)

And we must all be aware that
some of these conditions are compound.
For example, the product
may need to withstand dynamic
loading at high temperature in a
corrosive environment. And the
customer must understand that
not all grades of plastics that meet
the desired criteria are available in
sheet form.

Before we can discuss design concepts
with our customer, we need to
review them ourselves. We’ll continue
this litany after our review. ¦

Keywords: advantages,
limitations, material choice,
experienced customer, naïve
customer

THERMOFORMING

101

1 This is the second in a series that
focuses on part design

2 vis-à-vis, French for face-to-face, with the
usual meaning being “as compared with”
or “in relation to.”

A TECHNICAL ARTICLE 2005 VOLUME 24, #3

Understanding How a Sheet Stretches1

We began our discussion of part
design by reviewing why we
might not want to quote on a job. But
let’s suppose that we did quote on the
job. And we got it. Now what?

Forming into a Mold v.
Forming onto a Mold

In the not-so-politically-correct jargon
of the day, if we form into a mold
cavity, the mold is called a “female
cavity.” A better PC2 phrase is “negative
mold.” If we form onto a mold,
the mold is called a “male mold.” The
proper PC phrase is “positive mold.”
Is there a difference in forming “into”
v. forming “onto”? Of course. Let’s
consider for the moment, forming a
very simple truncated cone. If we use
a mold cavity, the sheet first drapes
into the open cavity, then stretches
into the cavity with the sheet progressively
laying on the mold surface.
Keep in mind that the sheet that contacts
the mold surface usually doesn’t
stretch any further. As a result, the
sheet that is free of the mold becomes
thinner and thinner as it is stretched
to the bottom of the mold. The wall of
the resulting part is thickest at the rim
and thinnest at the bottom. The thinnest
region of the part is in the corner
where the wall meets the bottom. We
can show arithmetically that if the
wall makes a 60-degree angle with
the horizontal rim, the wall thickness
decreases linearly from the rim to the
corner. If the wall makes a 90-degree
angle [think soup can], the wall thickness
decreases exponentially.

Now consider using a truncated
cone male mold. The sheet first touches
the mold at the bottom of the part
being formed. As the mold pushes into
the sheet, the sheet stretches between
the clamp and the bottom of the mold.
If the sheet doesn’t touch the sides of
the mold until the mold is completely
immersed in the sheet, the sheet thickness
is usually quite uniform. If the
sheet progressively touches the sides of
the mold as the mold is being pushed
into the sheet, the wall of the resulting
part will be thickest at the bottom and
thinnest at the rim.

Does it make a difference whether
we form into a cavity or over a mold?
If part performance is important,
probably not, if the part draw ratio3 is
very low [think picnic plate or aircraft
engine cover]. As the draw ratio increases,
however, the thinnest sections
of the part begin to control the performance
of the part. Several other factors
can influence our decision, such as:

• Is it easier to prestretch the sheet
when forming into a cavity or over
a mold?

• Is it easier to machine a cavity or
a male mold?

• Is the rim thickness important, as
in the case of thin-gauge containers?

• And does the customer need the
inside or the outside of the part to
be the positive surface4?

Usually – but not always – mechanical
plugs are more effective in stretching
sheet into a cavity, female molds
are easier to fabricate than male molds,
and rim thickness is better controlled
with female molds. We’ll revisit some
of these factors later.

Forming “Up” v. Forming
“Down”

What does this mean? If the mold
is placed above the sheet, the mold is
immersed in the sheet and the part is
formed up onto or into the tool. If the
mold is placed below the sheet, the
sheet sags into or onto the mold and
the part is formed down onto or into
the tool. Why is this an issue? In thin-
gauge thermoforming, forming up has
advantages with female molds. Gravity
helps when releasing parts from
multi-cavity tooling. And the parts are
properly oriented for in-line trimming.
Having said that, keep in mind that it
is easier to mechanically prestretch the
sheet into female cavities if the molds
are below the sheet.

Although the mold weight may
prevent mounting the mold over the
sheet in heavy-gauge forming, there
are some advantages here too. For example,
when a male mold is mounted
over the sheet plane, sheet sag acts to
prestretch the sheet prior to the mold
immersion. The sheet is formed down
for most heavy-gauge forming into
female molds. Again, sheet sag acts to
prestretch the sheet prior to forming.
And certainly, it is easier to activate
and maintain mechanical plugs if they
reside above the sheet rather than
below.

Mating Parts

It should be apparent that the part
side against the mold maintains a more
accurate dimension than the other
side. The mold side is chosen whenever
the part is to mate with another
dimensioned part. For example, for
an integral-lid container to be liquid
tight, the outside of one half must mate
with the inside of the other. This may
require that one half is formed into a
female mold while the other is formed
on a male tool.

An Observation

When quoting on a job, it is always
advisable to keep in mind the capability
of your equipment to form the part
in the most efficacious5 and least costly
manner. If you can’t form up, don’t
quote on a job that is best produced
in this fashion. The more tortuous
the path to perfect parts, the greater
the degree of difficulty. And surely
the greater the chance for quality issues.
¦

Keywords: positive mold, male mold,
negative mold, female mold, draw
ratio, forming up, forming down, sag

THERMOFORMING

101

1 This is the third in a series that focuses
on part design.

2 PC. Politically correct.

3 We discuss draw ratio in the next lesson.

4 By “positive surface,” we mean that surface
that the customer considers to be the
more important one. Usually the surface
against the mold is considered the positive
surface, but not always.

5 Efficacious: Producing or capable of producing
a desired effect.

A TECHNICAL ARTICLE 2005 VOLUME 24, #4

The Ubiquitous1 Draw Ratio

Probably the first thing a
novice hears in thermo-
forming after he/she learns to
spell “thermoforming,” is the
phrase, “Draw Ratio.” So, this
lesson focuses on the concept of
draw ratio.

Is There More Than One
Definition?

Unfortunately, yes. There are
at least three definitions. Let’s
define the common ones.

Areal Draw Ratio, often given
the symbol RA, is the ratio of the
area of the part being formed
to the area of the sheet needed
to make the part. Although I
promised not to use equations
in our TF 101 lessons, some
simple ones here won’t hurt all
that much:

RA = AreaPart/AreaSheet

A simple example, please?
Consider a cylinder one unit in
diameter by one unit high. The
area of the cylinder is (.+./4)
= 5..4. The area of the sheet
used to form the cylinder is
./4. Therefore the areal draw
ratio, RA, is 5. As an interesting
aside, the reciprocal of the
areal draw ratio is the average
reduced thickness of the formed
part, being 1/5 = 0.20. In other
words, the original sheet thickness
has been reduced by 80%,
on the average.

Linear Draw Ratio, often
given the symbol RL, is the ratio
of the length of a line scribed
on the part surface to the original
length of the line. Again, in
equation form:

RL = LinePart/LineSheet

For the same example, the length
of the line on the cylinder is
(1+1+1) = 3. The original length
of the line is 1. Therefore, the
linear draw ratio, RL, is 3. The
linear draw ratio is akin to
the way in which the plastic
is stretched in a tensile test
machine,

Height-to-Diameter Ratio,
often written as H:D, is the
height of the cylinder (1), to the
diameter of the cylinder (1). Or
H:D = 1. H:D is used primarily
for axisymmetric2 parts such as
cones or cylinders, such as drink
cups.

In summary, for the cylinder
described above, RA=5, RL = 3,
and H:D = 1. So you see, there
is no agreement between these
definitions.

Are Draw Ratios of
Use? Importance?

So, which one do we use?
Depends. First, we need to
determine whether draw ratio is
a useful concept.

Let’s focus on areal draw
ratio to determine its utility.
As we have already learned,
the reciprocal of RA is the average
reduced thickness. But
where is this reduced thickness?
Somewhere down the side of
the formed part. In fact, there
is probably a line around the
periphery of the part where
the part thickness is exactly the
average reduced thickness. So,
what does this tell us about
the uniformity of the part wall
thickness? Or the degree of difficulty
in forming the part? Or
whether webs are formed somewhere
in the part? Or what the
plug needs to look like? Or …?
Really, nothing.

Having said that, areal draw
ratio is perhaps the easiest
concept to understand. Linear
draw ratio, as noted, is often
compared with extension limits
determined from tensile
testing equipment. And H:D is
often used in Europe to describe
formability of plastics for cup
applications.

At best, draw ratios represent
bragging rights rather than information
about the degree of
difficulty in forming the parts.
Many formers will tell you that
parts that have very small draw
ratios are much more difficult
to form reliably than parts with
large draw ratios. And parts
with many compartments are
far more difficult to form than
parts with single compartments,
even when the draw ratios of the
two types are identical.

[See? Those equations didn’t
hurt at all, now, did they?] ¦

Keywords: Areal draw ratio,
linear draw ratio, H:D

THERMOFORMING

101

1 Ubiquitous: Being present everywhere
at once.

2 Axisymmetric: Having symmetry around
an axis.

A TECHNICAL ARTICLE 2006 VOLUME 25, #1

Draft Angles

Some time ago, we discussed
shrinkage and warpage. At that
time, we pointed out that plastic, like
most other materials, increases in
volume when heated and decreases
in volume when cooled. And we
said that to form the desired shape,
the hot plastic is pushed against a
cool mold surface. It follows that as
the plastic cools, it shrinks. But the
mold doesn’t change in dimension.
If the mold is male or positive, or if
even a portion of the mold is male or
positive, the plastic will shrink onto
the mold surface. And if the mold is
not properly designed, we will have
a devil of a time getting the part off
it. Thus we face the subject of draft
angles.

Draft Angles – Defined

The best definition of a draft angle
is the angle the mold wall makes
with the vertical. If the mold wall is
vertical, the draft angle is zero. Recall
that most thermoforming molds are
single-surfaced. That is, the sheet
is pulled into or over a single mold
surface. For draw-down into a
female or negative mold, the sheet
is constrained on its outer surface
by the mold. As a result, when the
sheet cools, it tends to shrink away
from the mold surface. As a result,
it is entirely feasible to thermoform
into a female mold having zero draft
angles. Most part designers prefer a
slight draft angle, say 0° to 2°, “just
in case.” The average is generally
1/2° to 1°.

On the other hand, when the sheet
is drawn over a male or positive mold,
it is constrained on its inner surface
by the mold. As a result, when the
sheet cools, it tends to shrink onto
the mold surface. To release the part
from the mold, it is necessary to
provide a draft angle on the vertical
mold surfaces. The amount of draft
depends strongly on the volumetric
change in the polymer. If the polymer
is amorphous – PS, PVC, PC – the
draft angle may be no more than
2° to 3°. If the polymer is crystalline
– PE, PE – the draft angle may be in
excess of 5°. The average is generally
4° but the designer must be alert to
effects of temperature variation and
recrystallization rates.

A textured surface requires
an increase in draft angle. It is
recommended that the draft angle be
increased at least 1° for every 0.2 mils
[0.0002 in or 5 microns] in texture
depth. Keep in mind that increasing
applied pressure, sheet temperature,
and mold temperature will result in
greater penetration of the sheet into
the texture.

What About Parts
With Male and Female
Components?

Multiple-compartment trays and
pallets1 can pose series drafting
issues. Consider a female cavity
bordered by two male segments. The
sheet will attempt to shrink away
from the female mold surface but
onto the male segments. Excessive
draft on the male segments may
allow the sheet to release from the
female mold surface before the sheet
has replicated the mold surface. On
the other hand, inadequate draft
on the male segments may allow
the sheet to satisfactorily form the
female mold surface, but the sheet
may “lock” onto the male segments.
The problem is exacerbated2 when
molding compartment trays where
the male portions are interrupted.
Essentially interrupted walls in
the molded part. In addition to the
shrinkage issues, interrupted male
segments may also be sources of
internal webbing3.

How Serious is the Draft
Problem?

The draft angle can lead to serious
dimensional changes in the formed
part. Consider a simple example, a
10-inch male mold. The vertical wall
is 1 inch wide at the top. Consider
a draft angle of 5°. The width at
the bottom of the vertical wall is
determined as follows:

The increased width on one side is
10 x tan 5° = 0.875 in. The total width
at the bottom is then 1 + 2 x 0.875 =
2.75 in.

This is a substantial dimensional
change in the thickness of the vertical
wall.

When is the Draft Angle Not a
Draft Angle at All?

When it is used for something
else. The classic example is the drink
cup. The sidewalls are tapered as
much as 20° for stacking purposes,
not shrinkage. In multi-compart-
ment parts, care must be taken in
the design to accommodate both the
draft angle required for shrinkage
and the necessary stacking taper.
Stacking lugs, stand-offs, or rings
are often designed into complex
parts, simply because it is not always
possible to predict the exact local
shrinkage. ¦

Keywords: draft angle, taper,
shrinkage

THERMOFORMING

101

1 These parts are sometimes called androgynous,
meaning that they have both
female and male characteristics.

2 Exacerbate: To aggravate.

3 Webbing will be discussed in a later
lesson.

A TECHNICAL ARTICLE 2006 VOLUME 25, #2

CORNERS

Most plastic parts have corners.
And most corners are radiused.
Designers often seek sharp corners or
more properly, corners with very
small radii. Aesthetics is often cited
as the reason for this. But aesthetics
is not the only reason. Often the
container must contain material
of a specific volume. For a given
dimensioned container, the internal
volume decreases with increasing
corner radii. Con-versely, for a given
volume, the overall dimensions of
the container (and thus the amount of
plastic needed to make the container)
increase with increasing corner
radii. In this lesson, we consider the
concept of the corner.

Can a Part Have More
Than One Type of Corner?

Of course. Consider the simplest
type of corner, being the place where
two planes intersect. Picture the
bottom edge of an axisymmetric part
as a drink cup or a can, for instance.
The vertical or near-vertical side of
the container intersects the bottom of
the container at a right or near-right
angle, thus forming the corner, in
this case, a bottom two-dimensional
or 2D corner. Of course, any good
thermoformer worth his or her
salt would not make a sharp angle
at the intersection. The reason for
this is intuitively obvious but will
be explained in a little more detail
later.

Is there more than one type of
corner on a five-sided box? Sure.
There’s the intersection between
the vertical wall and the bottom.
And the intersection between one
vertical wall and another. And what
about the intersection between two
vertical walls and the bottom? So we
have bottom two-dimensional or 2D
corners, vertical 2D corners, and in
the last case, three-dimensional or 3D
corners. And, as with the cup or can
example, corners should have radii.

We must keep in mind that the
plastic stretches from the sheet that
is not contacting the mold surface. As
more and more of the plastic sheet
contact the mold surface, the sheet
not contacting the mold becomes
thinner and thinner. For a part such
as a cup or can, the plastic stretches
into the bottom 2D corner last. As
a result, the material in the corner
is usually the thinnest. Although
mechanical and pneu-matic assists
help redistribute the sheet during
stretching, the part wall is usually
thin in the corners. And smaller
corner radii usually lead to thinner
part walls. In other words, sharp
corners lead to thin-walled parts in
corners.

Wall Thickness in
2D Corners

The wall thickness in the bottom
2D corner of a five-sided box is
proportional to the corner radius to
about the 0.4-power. If the design
calls for a radius in one area of the
bottom of the part that is 50% of that
in another area of the bottom of the
part, the part thickness in that area
will have about 75% of the thickness
of the other area. If the design radius
is 25%, the thickness in that area will
be about 55% of that of the other
area.

Interestingly enough, wall
thickness in vertical 2D corners is
about equal to wall thickness of
surfaces adjacent to the corners. This
is probably because the part walls
in the vertical corners are formed
at the same time the part walls of
adjacent surfaces are formed and not
afterwards, as is the case with bottom
2D corners.

Wall Thickness in
3D Corners

The wall thickness in the 3D corner
of a five-sided box decreases in
proportion to the corner radius to the
1.0-power. If a design calls for a 3D
radius in one corner of the part that
is 50% of that in another corner of the
part, the part thickness in that corner
will have 50% of the thickness in the
other corner. If the corner design
radius is 25%, the part thickness will
be 25% of that in the other corner.

Why are we concerned about
part wall thickness in 3D corners?
Because many of our parts are similar
to the five-sided box we’ve used as
an example. And five-sided boxes
are often filled and handled during
shipping, installation, and use. And
3D corners of five-sided boxes are
most susceptible to impacting. In
an earlier lesson we discussed that
when we stretched a sheet, we
thinned it. We needed greater forces
to stretch the sheet to greater and
greater extent. And when we cooled
the sheet we locked in the stresses
we used to stretch the sheet. So
when we impact the 3D corner of
the formed part, we are applying
stress on top of those already frozen
into the corner. On top of this, the 3D
corner is very thin. In short, sharply-
radiused corners are often desired
by designers but of great concern
to thermoformers. As a result, the
designer must often accept greater
radiuses than he/she desires.

In a subsequent lesson, we consider
alternative designs for corners, as
well as other product features. ¦

Keywords: vertical 2D corner,
bottom 2D corner, 3D corner,
corner radius

THERMOFORMING

101

A TECHNICAL ARTICLE 2006 VOLUME 25, #3

The Cutting Edge

For those of you who came in late,
we have been examining the various
aspects of part design. In this lesson, we
focus on the edge or periphery of the
part. The first thing we need to realize
is that the part we’ve just thermoformed
is still attached to the plastic that held
it in the clamp frame while it was
being formed. This is true whether the
entire assembly, formed part and edge
material, is removed to a separate fixture
or whether the formed part is punched
from the trim material immediately after
forming. We’ve discussed trimming
in earlier tutorials. In this tutorial, we
discuss the characteristics of the edge
itself.

Registration

Trimming devices need to trim the part
where the designer wanted it trimmed.
This means that the trim line and the
trim device must register. The accuracy
of registration is a design issue. In heavy
gauge forming, it is impractical to ask a
trim device to trim within thousandths
of the design trim line everywhere along
the trim line. Heavy-gauge parts may
be fixtured between the time they are
formed and the time they are trimmed.
Fixturing allows for some residual stress
relaxation and often improves the trim
registry. In thin-gauge forming, the trim
device should be able to trim very close
to the design trim line. Because many
thin gauge parts are axisymmetric,
meaning that the trim line is round,
registration focuses on the degree of
ovality of the formed trim line prior
to the trimming step. Thin-gauge parts
are often trimmed within minutes of
being formed. Certain polymers such
as polypropylene continue to crystallize
after forming. As a result, the design trim
line and the final part edge peripheral
location may be quite different.

Heavy-Gauge Cut Edge

The nature of the final cut edge
depends strongly on the trimming device.
In many robotic trimming steps, the edge
is rough-cut initially. This edge finish
may be adequate if the cut edge of the
part is completely hidden in the final
assembly. Polycarbonate skylights that
are edged in aluminum are examples.
Often the product requires a smoother
edge. For robotic trimming to achieve
the desired edge, the rough-cut edge
is routed a second time while the part
remains on the trim fixture.

In some applications, the edge must be
as smooth as the overall plastic surface.
Here are some ways of achieving a very
smooth, even polished edge.

• Fine grit sanding followed by Crocus
cloth or 1200-grit polishing

• The above method, followed by
pumice polishing

• For certain plastics, a light wipe
with a mild solvent will smooth
trim cuts. Care must be taken to
minimize the amount of solvent that
is absorbed into the polymer.

• Flame-polishing is popular with
transparent amorphous plastics
such as acrylics and polycarbonates.
Flame-polishing is not recommended
with plastics such as PVC.

• Laser cutting. The laser is a high-
intensity beam that cuts plastic by
melting and vaporizing it. The cut
line is usually very smooth.

Thin-Gauge Cut Edge

Thin-gauge trimming is substantially
simpler than heavy-gauge trimming.
Nevertheless, the trim edge charac-
teristics can be quite important to the
customer. There are three major issues
with the cut edges of thin-gauge parts:

• Trim dust and fibers, known as angel
hair and fuzz.

• Microcracks that can grow into the
formed part as it is flexed

• Jagged edges that can cut or abrade
the user

Edge and surface contamination are
often the results of problems in the
trimming step. But not always. It is very
difficult to trim polystyrene without
generating very tenacious trim dust. It
is often difficult to trim polypropylene
or PET without generating fibers and
fuzz. Adding antistatic agents to PS,
either as an additive that is compounded
into the polymer or as a topical coat
to the sheet prior to forming, helps the
trim dust problem. If fuzz and fibers are
objectionable to the customer, they are
often minimized by passing the container
edges through a hot air knife. The heat
shrivels the fibers to microscopic size.

Microcracks and jagged edges can
also be “healed” by heating the edges
with hot air. One approach is to collect
and nest a stacked, counted number of
parts and pass the stack though a hot air
tunnel prior to packaging or boxing for
shipment. ¦

Keywords: registration, flame
polishing, laser cutting, trim dust,
microcracks

THERMOFORMING

101

A TECHNICAL ARTICLE 2006 VOLUME 25, #4

The Rim

So, we know about draft angles
and corners and wall thickness
variation and on and on. But
what about the rim? You know,
the region of the formed part that
forms the periphery of the part.
This lesson focuses on some of the
important issues dealing with the
rim. In the next lesson, we’ll look at
the characteristics of the trimmed
edge itself.

Does the Rim Have a
Function in the Part?

Other than just being the edge
of the part, let’s say. In thin-gauge
forming of axisymmetric parts
such as cups, the trimmed-out
rim is usually manipulated in a
post-molding operation known
are rim-rolling. Here, the cup is
rotated along its axis as the rim is
heated and softened. The rotating
action forces the soften rim against
a shaping ring that effectively
rolls the rim into an annulus. The
rolled rim provides great stiffness
to an otherwise flimsy thin-walled
container.

Staying with thin-gauge products
for a moment, the rim design for
lidded containers often requires
interlocks and detents that must be
quite precise. In certain instances,
the container rim may include
denesting features that allow
stacked containers to be readily
separated by the customer.

What about the rim on a heavy-
gauge part? Often the rim is the
finished edge of the part. The rim
may be very simple, such as the
trimmed end of a flat surface. Or it
may be very complex, with radii,
chamfers, and ridges. The rim
may be designed to fit
into or over another
part, Or it may be
trimmed to accept
secondary assembly
features. The part
design may require
the trim line to
be “hidden,” so that the rim is
U-shaped with appropriate radii
or chamfers.

Can We Get the
Formed Part Off the
Mold?

Before we contemplate this
question in detail, remember that
thermoformed parts shrink as they
cool. So they shrink away from the
sides of a female or negative mold
cavity and onto the sides of a male
or positive mold cavity. If we build a
simple cup mold, for example, and
design the rim so that the plastic is
formed over a ring at the mold top,
we need to provide adequate draft
to get the thing off the mold. In
other words, the rim will not have
right-angled sides. Does this affect
the design? By the way, this design
is often called a “dam” design. This
design minimizes excess plastic
from being drawn over the edge of
the mold and into the mold cavity.
Frequently a trim line concentric
to the dam will also be molded in.
This is often called a “moat.”

We discussed the hidden trim line
a minute ago. How are we going to
get the part off the mold? Flip-up
sections? Removable sections? It is
very difficult to get moving mold
sections to seat without a gap
between mating parts. As a result,
we may wind up with a “witness
line” right at the most cosmetic
portion of the part. And keep in
mind that, without plug assist,
parts really thin rapidly when
vacuum- or pressure-drawn into
parallel-walled mold sections.

What About Texture?

Whenever you draw textured
sheet, the texture flattens. In grained
sheet, the effect is called “grain
wash.” The typical rule of thumb is
that texture flattening is acceptable
if the local draw ratio is less than
about two or the local thickness is
more than half the original sheet
thickness. The real problem occurs
in the rim area where the sheet is
often drawn into sharp corner radii.
One design method is to chamfer
the rim. A second is to facet the
surface design. A third is to use
a series of steps. In each of these
cases, the objective is to trick the
eye into seeing local architecture
rather than texture.

The alternative to drawing
textured sheet is to texture the
mold. However, as any mold maker
will tell you, it is very difficult to
build uniform texture into very
sharply radiused corners.

You should never fall into the
habit of leaving rim design to the
end of product design. ¦

Keywords: rolled rim, moat,
dam, hidden trim line,
textured sheet

THERMOFORMING

101

This is the last TF 101 column written by
Throne. The new TFQ editorial staff will determine
whether the column will continue.

A TECHNICAL ARTICLE 2007 VOLUME 26, #1

Process – Cycle Time

(Editor’s Note: This is the first Thermoforming
101 article written by your new technical editor.
Dr. Throne wrote 34 articles that date back to
1998, Volume 17, Number 3. He had originally
intended to write a series of 18 general interest
articles but the 101 series has become a
mainstay of the Quarterly. The year-end booklet
that contains every 101 article to date is a great
reference source for thermoforming practitioners.
This technical editor has every intention of
maintaining the series and the booklet which is
becoming the perfect reading material for people
entering the industry or seasoned personnel
who need help on a specific problem. Jim wrote
4 articles last year that dealt with part design. I
hope he will forgive me for not continuing with
the “Trimmed Edge” topic he suggested for this
lesson. I will deal with this topic when we take a
closer look at the subject of “Die-Cutting.” This
Thermoforming 101 article deals with a subject
about which we should all be more diligent.
Foreign competition has forced us to maximize
efficiency and become more competitive. So
let us review the basic factors that determine
cycle time.)

General Assumptions

We all should be aware that if we let the
operator determine when a machine cycles,
our production rate will suffer. Running
thermoforming machines on manual mode is
necessary for set up and of course if all you have
is a simple shuttle machine with rudimentary
controls you have no other choice. So let’s just
deal with thermoforming in automatic mode.
We will only deal with the forming part of
the process. Trimming of heavy gauge parts
is another topic. Also for this purpose we will
assume that when thinking roll-fed, we are
using a machine with in-line die-cutting.

The Basic Concept

If we take all the segments of the rotary or in-
line thermoforming process: heating. indexing
the sheet, closing the press, forming the part,
cooling the part, opening the press, trimming
and stacking (if in-line), the cycle time is
dictated solely by the slowest segment of the
process. Most people looking at our process for
the first time will say it has to be the heating
segment that is the slowest part of the process.
This is not necessarily so.

Roll-Fed

It is especially not so with roll-fed machines
that usually are designed to have 4 indexes in
the ovens. For example if the maximum mold
size in the index direction is 36″. The oven
length will be roughly 4 times 36″ or 12 feet
long. So if you are running .020 PVC which
would normally be in the oven for 20 seconds
to get up to forming temperature, your cycle
time, based on a 4 index oven, is 5 seconds
(20 divided by 4) or 12 cycles per minute. This
is not bad for running smaller and medium
size quantities but it can be a lot better. I will
explain later.

Sheet-Fed

OK, so what about heavy-gauge, sheet-
fed forming? The same principle applies. In
North America the machinery manufacturers
recognized early on that they must do something
about the length of time it takes to heat the sheet
evenly and thoroughly. So the 4 station rotary
machine was designed which cut heating time
dramatically by using 2 heater banks through
which the sheet travels on its way to the mold.
So why not build a 5 station rotary with 3 heater
banks and really cut heating time? The answer
is, there would be no point unless the part could
be formed and cooled in a time less than one
third the heating time. In fact the cooling of
some materials is so difficult that one heater
bank on a 4 station would have to be shut off
or set at a lower temperature to allow time
for proper cooling. So if we can do things to
speed up the heating of the sheet, what can
we do to cool the part quicker? This is where
it gets tricky.

The Forming Segment of the Cycle

On roll-fed machines, unless you are dealing
with super fast lines, you can forget about the
trimming and stacking segments of the cycle
when looking for what is slowing you down.
Concentrate on the forming segment from the
time the sheet leaves the heaters to the time the
formed part leaves the form station. Let’s break
down the actions that take place.

Index speed is the speed that the sheet
travels from the heaters to the form station.
Roll-fed pin chains can travel up to 95 inches
per second. A rotary turntable moves a lot
slower. On both roll-fed and sheet-fed lines
the stopping and starting actions can become
too violent if the index speed is too fast which
may cause the hot sheet to move as the mold
closes on it. Move the sheet as fast as possible
but make sure that the drape is stationary when
the mold closes.

Shut height or platen travel is the distance
the form platens must travel from the open
position to the closed position. All too often
set-up people will not take the time to reduce
the shut height to optimum levels. I have seen
a roll-fed job running very shallow pill blisters
with a female tool on the bottom and the
plugs on the top showing 3 inches of daylight
between the plugs and the sheet line because
the operator did not lower the shut height of the
top press. This added at least 1 second to the
cycle time and over a 30 hour run at 15 cycles
per minute added over 2 hours of unnecessary
labor and machine time. If you don’t have shut
height adjustment on your form press the only
way to do this is to add build ups behind the
tooling. Fortunately the new machines have
electric presses which make setting the shut
height so much easier.

Press speed affects the length of the cycle
time but sometimes it is necessary to slow the
press closing speed to accommodate plug or
assist action. If you are having difficulty with
de-molding you may need to slow the opening
speed. Other than these conditions, you can
move the platens as fast as you want. Third
motion tooling or independent plug control
with individual cavity clamping can greatly
improve cycle time but this is getting beyond
the scope of a 101 article.

Cooling time is by far the most important
factor in achieving a fast cycle time. In my very
early days of thermoforming we tried running
an epoxy mold on a modern in line machine.
Even with a water cooled base under the mold
the best we could do is 2 cycles per minute
simply because the mold never got a chance
to cool down. Using an aluminum mold on
a water cooled base allows you to run most
jobs at reasonable speeds as long as the height
(or depth if it’s a female) of the mold is no
more than say 2 inches. To achieve maximum
efficiency and reduce cooling time the mold
must be kept at the target temperature as
specified by the material supplier. Hot material
at 350 degrees F hitting the metal mold requires
a very efficient cooling system to maintain that
mold temperature that may have to run at 200
degrees F constantly to run fast cycles. The
only way to do this is to run cooling lines in the
mold itself usually no more than 2″ to 3″ apart
depending on the size and configuration of the
mold. Cast-in lines are the norm for aluminum
cast molds and machined in lines are the norm
for machined aluminum molds.

Cooling time on sheet-fed rotary machines
running thick HDPE can be improved by using
external fans, water mist or cold air directed
onto the part but care must be taken not to
form in stresses. A well built water cooled
mold is still necessary for the most significant
improvement in cycle time.

So how do some roll-fed thermoformers get
50,000 parts per hour? This will be the subject
of technical articles in the future. It’s not a
subject for the 101 series but here is a hint: third
motion tools, cavity clamping, pre-heaters and
great cooling in the molds.

Cycle time is just one way to make our
operations lean and more competitive. Other
ways will be discussed in future Thermoforming
101 articles. ¦

THERMOFORMING

101

A TECHNICAL ARTICLE 2007 VOLUME 26, #2

“Down Gauging” – It’s a Good Thing

How many of us quote jobs and
specify the starting gauge? I would
suggest that the majority of custom
thermoformers are accustomed to quoting
this way. In the case of proprietary
thermoformers producing such things
as food service items, although the
starting gauge is a major factor in the
costing of the parts, it is of little interest
to the customer because he or she only
cares about how well the part performs
which would relate only to the thickness
of the finished part. So why do custom
thermoformers continue to state starting
gauge on their proposals?

My point is this. Why should any
customer buying thermoformed parts,
care about what gauge of raw material
we start with. He should only care about
how well his part performs. We sell to a
wide variety of customers, from the very
knowledgeable to the –well lets be kind
and say, technically challenged. With
the latter we have a duty to explain the
process of thermoforming and how the
plastic thins during heating and forming.
Those who already understand may
need to be reminded and shown where
the thinning will be most prevalent.
In all cases however, we must educate
our customers and work with them
to determine what the minimum wall
thickness should be and in some cases
specify the thickness in various places on
the part. Once he or she has established
these thickness criteria, it should become
the specification with no mention of the
starting gauge.

Unless we are dealing with a seasoned
customer who has already considered
wall thickness requirements, most of the
time customers will indicate the need
for a specific starting gauge. This may
be because he or she has a competitive
proposal that specifies a starting gauge or
the part is existing and because the spec
calls for a starting gauge he or she simply
assumes that it should continue to be that
gauge. If we are not given a material
gauge requirement in the RFQ, many of
us will be unsure what gauge should be
used so we will quote 2 or more. Would
it not be more professional to do some
homework and quote the part stating a
minimum thickness or perhaps even a
range of wall thickness measurements
throughout the part?

When I bring up this point with those
in the industry I am told that the “good”
thermoformers do not quote starting
gauge. This leads me to believe that what
separates the “good” thermoformers
from the “not so good” is know-how.
That is, knowing how to produce the
part with the specified minimum wall
thickness requirements using the thinnest
possible starting gauge. What do we
need to know to be able to produce the
part with minimum wall thickness specs
and do so with a thinner material than
our competition? Are the lights going
on yet?

Dare I say that, “down gauging”
has some negative connotations that
relate to using a material gauge that
is less than what was quoted because
the term “down gauging” is sometimes
used when competitive pressures force
a need to reduce costs. However if
starting gauge is never specified, then
we would eliminate any possibility of
being accused of such a practice. It is
a win – win situation for supplier and
customer. The real competitive edge
goes to the thermoforming supplier with
the most know-how and there-in lies the
moral of the story.

We in the SPE are trying to educate
thermoformers to be more competitive,
more innovative and more successful.
Those who work to that end will
ultimately prevail. Having the know-
how to be able to guarantee a minimum
wall thickness with a thinner starting
gauge is indeed, a superior way to sell.
By using better material, better part
design, better tooling or better equipment
than our competition, we will get the
job and have a much better chance of
keeping the job if it gets shopped around
by the customer.

One way to produce a thermoformed
part in a thinner gauge, while still
maintaining a minimum wall thickness is
to look at some of the different forming
techniques available to us. Many of
us have listened to seminars by Bill
McConnell or Art Buckel that show
techniques such as billow forming or
snap-back forming. These methods are
designed primarily to provide better
material distribution which of course
relates to improved wall thickness in the
critical areas on the part. Of course in
order to utilize these techniques we must
build special tooling, have the equipment
that allows the extra step in the process
and we may have to extend the cycle
time a little. However it could result
in getting the job because of a more
uniform part, a significant reduction
in starting gauge and consequently a
reduction in material costs.

It is one thing to know what techniques
and tooling will improve material
distribution and another to predict wall
thickness accurately. An experienced
tool designer who has the benefit of
many years in the job will be able to do
so fairly well; however, these people are
scarce. There are computer simulation
programs available that can assist with
this and make the predictions within
a few thousandths of an inch. One of
these programs could become your best
sales tool.

Like most practices that have
become routine, modifying our quoting
procedures to reflect minimum wall
thickness instead of starting gauge will
take some effort. It will require us to take
more time with the customer to agree on
the specs. It will require knowledgeable
engineering personnel to determine tool
design, process techniques and what
gauge material to use. But in my opinion
it will make us better thermoformers
by putting the responsibility on our
engineers to find ways to down gauge
while maintaining wall thickness
requirements. ¦

THERMOFORMING

101

(Technical Editor’s Note: Thermoforming 101
articles are intended not only to educate but also
to generate interest in making improvements
in our industry and our businesses. I welcome
any feedback, positive, negative or otherwise.
If I have provoked some dialogue and thought
by writing an article like this it is for the good of
the industry.)

. Thermoforming 101

The Impossible

Draw Ratio

A Technical Article – 2006 Volume 26, #3

. By Barry Shepherd

(Technical Editor’s Note: Looking back over the last 9 years at 36
Thermoforming 101 articles, which are all presented in our annually
updated booklet, it is the most comprehensive collection of
basic technical information one could find on our process. For this
issue I have chosen to talk about a subject that my predecessor Jim
Throne wrote about in 2000 and 2001 – pre-stretching the sheet.
But this time I want to discuss what type of pre-stretching should
be used in a very difficult application.)

Knowing It Can Be Done

The customer knows what he wants and you want to give
him a part that will do what he wants but in the back of
your mind you are thinking, “I should be telling him this
is impossible.” However, you know it is possible with the
right tooling.

The main ingredient in getting hot plastic to form tight
over a mold is vacuum. Air pressure and other various
forms of assist tools make vacuum forming, thermoforming.
The trick is to decide what tooling options to use to
give the customer what he wants without creating problems
for your production department, while staying within
the customers tooling budget.

Back in the days when we used to say thermoforming is
half art, half science we would make a mold, put it into the
press and see what happens. Then start adding pieces of
wood we called web stretchers and if we had a top press
at that time we could build a pusher to assist the plastic
into a problem area. OK, so maybe some of us still do this
in prototyping but the ultimate aim for all of us is to build
production tooling that will go into the machine and start
forming good parts on the first shot.

Part Design/Tool Design

You can’t design a thermoformed part unless you have a
full understanding of tool design and what capabilities you
have in your equipment. This seems obvious but when the
part has extreme draw ratios and wall thickness requirements
that must be met, it is imperative.

Let’s take a heavy gauge part that has towers that defy
all principles of thermoforming, 8” high, only about 2”
diameter at the top and only 6” between towers and it
must be polyethylene which makes matters worse. The
configuration of the part is such that the tall sections are at
the perimeter. In other words, this is a job that would seem
impossible. But the customer is faced with having to build
these parts on a limited budget and other processes are
too expensive. The designer must make a decision knowing
that he has a number of tooling options available.

Pre-Stretch Tooling

The main problems that must be addressed in designing
the tooling for this part is a) how to pre-stretch the material
so that there is enough material in the areas around
the towers and b) how to get the material down into the
valleys between the towers without webbing or bridging.

Pre-stretching the material can be done by forming a seal
on the material around the edge of a box and drawing
a vacuum to pull the sheet into a bubble. This is called a
pre-draw box and this is done on the opposite platen to
the mold platen. So now we have stretched the material
to give us enough surface area to cover the towers without
getting too thin. Now how do we get all that material
down to the bottom of the valleys?

Plugging or Pushing

This is where a newer technique of plug assist can be used
effectively. Visualize the material in a bubble hanging
below the mold in the clamp frames. It has been pulled
down by the pre-draw box. With an independently acting
air cylinder inside the pre-draw box, a plug or pusher tool
can be mounted and used to push the pre-stretched material
into the valleys. Obviously you must have this capability
built into your machine and the timing must be such that
the mold, vacuum and pusher are activated in the right
sequence.

If the machine does not have the capability to have this
third motion tool then it may be possible to mount a fixed
pusher inside the pre-draw box. However this means the
material must then drape around the pusher during the
pre-stretching and this could mean that the material cools
in these areas causing other forming problems. Pusher
shape and heating then becomes critical.

Impossible No More

We see thermoformed parts now that once would be impossible
to thermoform – especially in roll-fed, thin gauge
applications. Third motion tooling, improved materials and
plug assist design has made severe draw ratio’s common
place in the packaging and drinking cup sector. The same
principals can be used in heavy gauge, sheet-fed thermo-
forming to form large heavy parts.

.

. Notes

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