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In a past OMNIview, we featured an
article on how we used stereolithography parts (click
to read the article) to construct complex appearance models. Another
use for those accurate (but not very robust) plastic parts is for
producing original patterns when building silicone-rubber molds, also
called "soft tools". We use soft tooling for small parts and
larger desktop-sized cases when we need multiples of less than twenty or
thirty.
The advantages of silicone molds over
other tooling methods are low cost, speed, versatility, and accuracy.
Costs for the molding materials and the
stereolithography model/ pattern are usually measured in hundreds, rather
than thousands of dollars. After the molds are built, duplicates can be
cast in a matter of days. The flexible nature of a soft tool also allows
the engineer to test different casting materials without changing drafts.
Best of all, the resultant prototype
parts are accurate within a few thousandths of an inch.
We build molds that range from
pea-sized to some that are as large as a toaster oven.
First a mold container (MDF or acrylic) is constructed, then the original
part is suspended within. See picture at left.
(The yellow clay was inserted so that the original
part would not be completely encased in the cured silicone mold. Before
the top half of the mold is poured, the clay will be removed. This way,
the two mold halves can be separated after they're cured. )
Liquid silicone is poured
into the container, covering and completely encasing the original part. See
picture at right. In a few hours, the silicone cures (this process
can be accelerated with heat) and the hardened rubber block is removed
from the mold box.
 The
next challenge is to extract the original encased part without damaging
either it or the new "soft tool". An experienced and steady hand
is needed to cut cured silicone in the right places so that the original
model can be safely removed. The resulting flexible mold can be bent open
to remove the original part. See picture at left
which shows half of a two-piece mold. If the part is a complicated
one, it may need to be sectioned.
 After
re-assembly, the void where the original stereolithography part was
originally located is filled (cast) with the material of choice, usually a
two-component epoxy or urethane. The fill material is cured, and the new
cast replica is removed from the soft tool. Depending on the material
used, we can make a score of duplicates from one mold. Picture
at left shows both mold halves with an actual cast part after it was
removed. A close look at the
picture reveals "sprues" (they look like stalagmites) on
the cast part. They are the result of holes that were added to the mold to
inhibit bubble formation in the finished part. The sprues are removed
during final finishing.
We
used this prototyping process for the Hampton lock housing project (see
OMNIview 17); one that required a lot of testing with snap-fits, crush
ribs, and material selection. The accuracy of the resultant parts and the
ability to test different production materials significantly impacted
development time. It was the “soft-tooling” that made it possible for
us to meet our customer's time line.
Read
about our full time model maker, Mike Ammerman, and his amazing modelmaking
abilities.
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