Thermo101 Design

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.

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