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This is a standard method of making small quantities of polymer parts. Any rapid prototyping-generated part can be used as a
pattern to make silicone rubber tooling. These tools can be used to mold small to medium quantities of parts in a large variety
of urethane, epoxy or other polymers. Some of these polymers have properties which emulate particular engineering thermoplastics,
and it's also possible to fill them for added strength. The method doesn't produce a part which is identical to an injection
molded part, however, because the conditions of manufacture aren't the same. Injection molded parts may have functionally
important anisotropic mechanical properties that depend on how the material flows in the mold and cools, for example.
Nevertheless, silicone rubber tooling is inexpensive, offers good accuracy and finish, and the parts produced are often adequate
for prototypes or small production runs. The materials are often used in a natural state, but painting and other secondary
operations can result in parts that are very attractive.
Silicone tools can typically be used to mold several parts before it becomes necessary to replace them. The number depends on
accuracy and finish requirements and the specific geometry of the item produced. It may be possible to make many dozens of
simple, or non-critical parts from a single silicone rubber mold, but ten to twenty is typical if the parts are more complex.
Wear of the mold occurs due to the exothermic and reactive nature of the polymers, and because of the necessity to mechanically
deform the mold to remove the part. It may often be necessary to replace the RP-generated pattern as well, depending on the
number of molds to be made and similar accuracy and geometric considerations.
The process is carried out by placing the RP-generated pattern in a frame, usually made of wood. The pattern itself usually
must undergo secondary operations to bring it to the desired state of accuracy and finish before it can be used. See the section
on RP-generated patterns. Silicone rubber room temperature vulcanizing (RTV) molding compound is then poured around the pattern.
It may be necessary to apply a vacuum to the assembly to pull air bubbles out of the rubber and insure fidelity to the pattern.
Once the rubber has solidified, the pattern is removed and the mold is ready to be used.
With this method it's even possible to produce parts with undercuts, since the part is able to be removed through distortion of
the flexible moldparts, which springs back to its original shape if the distortion is not too severe.
Silicone rubber tooling is most often used in manual casting processes, but in recent years more automated technologies have
appeared like vacuum casting, when a vacuum chamber is used.
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