3 Answers to your Silicone Shrinkage Questions

By: Kevin Franzino

Does the shrink for medical silicone values change with materials and with different medical silicone manufactures?

Yes, the percent shrink will vary from one silicone manufacturer to another and from material to material. However, the variance is not typically very much. Silicone material tends to shrink as a result of the molding process from between 1% to 4%, the best way to know what the linear shrink value is for a specific silicone is to contact your silicone material supplier.

How is shrink accounted for?

When designing a mold the part itself will be compensated for the percentage of shrink prior to cutting a cavity. While it is not quite

as easy as it sounds, if a part is going to shrink 3% of its original size the mold cavity will actually be 103% of its desired size. For simple parts a scaling process can be applied, but for overmolded components and parts with complex geometries the scaling process is less straight forward.

If you compensate for shrink, and wish to change durometers, materials, suppliers will the parts still be the right size?

Probably not, but it depends on your overall part size and tolerances. Since the percentage of shrink is such a small number in microscopic molded parts it rarely plays a role, and a change from a material with 2.05% shrink to 2.6% shrink is extremely difficult to measure, let alone notice. The difference would only be noticed on much larger parts, 2% shrink on a 10 inch would alter your part 0.200" while 4% shrink would change the same part 0.400", depending on what your tolerance is, this case may cause your parts to stop being in spec. Fortunately, most similar materials tend to shrink similarly, and a subtle change in durometer (40 -> 60) won't typically affect the shrink much, so most of the time you will be okay. The best way to be certain is to work closely with your silicone molder and silicone material provider to make sure that everyone is on the same page about shrink comp.