Thermo101 Impossibleratio
. Thermoforming 101
The Impossible
Draw Ratio
A Technical Article 2006 Volume 26, #3
. By Barry Shepherd
(Technical Editors 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 cant 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.
Lets 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 ratios 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.
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