Why Silicone Parts Shrink After Vulcanization

Silicone parts shrink after molding. The silicone is cured under heat, and as the part cools and the material finishes curing, it becomes slightly smaller than the mold cavity. If the mold is cut to the exact size on the drawing, the finished part will come out undersize.

What affects shrinkage

• Material and hardness. Different compounds shrink by different amounts.
• Part shape and wall thickness. Thick parts and thin parts do not shrink in the same way.
• Curing temperature and time. Post-curing in an oven can cause a little more shrinkage.
• Inserts. A metal or plastic insert holds the rubber and changes how it shrinks.

How to avoid size problems

The mold maker adds a shrinkage allowance to the cavity based on the chosen material. For silicone this is often a few percent, but the right value depends on the compound and the part. Tell us the material, hardness and critical dimensions at the start. We will confirm the allowance and check the first samples against your drawing before mass production.

If a dimension is critical, mark it on the drawing and tell us the tolerance. That lets us decide where to focus the sample check.