Thermo101 Stretch

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.

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