Thermo101 Cycletime
A TECHNICAL ARTICLE 2007 VOLUME 26, #1
Process Cycle Time
(Editors 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 lets 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. Lets 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 dont 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 its 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. Its 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