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 were
making. And second, there really
isnt 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 Hell 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
dont 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. Thats 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 wont 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, well
take a look at some simple issues
such as female or negative molding
and male or positive forming. Well
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 well 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.