Silicone Seal Ring Trial Mold Too Large? Here’s How We Fixed It

A real case study in custom silicone molding—and why mold design, material weight, and press pressure all matter.

A long-time customer placed an order for two large silicone seal rings. Both had an outside diameter of around 150 mm. The mold is a 4-cavity tool. Our mold maker calculated the mold dimensions using standard shrinkage rates. (Chicarubber)

With silicone, shrinkage changes with hardness. Higher hardness means less shrinkage. For 50 Shore A silicone, our shrinkage rate is 1.028. In normal molding, the finished part comes out smaller than the mold. So to get a target product size, you divide the target size by the shrinkage rate to calculate the mold size.

On the first trial, the parts measured more than 2 mm oversized.

The customer’s feedback was clear: the parts could be undersize, but they could not be oversize. So we went back to the mold.

We adjusted the mold parameters and added overflow and venting grooves. These grooves let excess material flow out instead of staying in the cavity and pushing the dimensions over. On the second trial, the dimensions measured within spec.

What Controls the Final Size of a Silicone Part?

The mold is the number-one factor. Molding conditions can fine-tune the size a little, but they cannot fix a large dimensional error.

Here is what we focus on:

  • Mold dimensions and shrinkage: Get the shrinkage rate right for the specific silicone hardness.
  • Overflow and venting: Give excess material a place to go.
  • Material charge weight: Use the minimum required weight. Weigh it accurately—not too much, not too little.
  • Post-deflashing weight: After removing the flash, weigh the part to confirm it is correct.
  • Press pressure: Increase pressure during molding. For this type of sampling, we typically use a 250-ton press with about 180 tons of clamping force.

The Takeaway

If your custom silicone O-ring or seal ring comes out oversized on the first trial, don’t just adjust the press. Check the mold, the shrinkage calculation, the venting, and the material charge. A small change in the mold can make a big difference in the final part.(Chicarubber)

We work with custom silicone seals, large O-rings, and compression-molded rubber parts for industrial and consumer applications. If you need help with a tight-tolerance silicone project, contact us. We can review your dimensions, material, and mold design before the first trial.

Silicone Seal Ring Trial Mold Too Large? Here’s How We Fixed It

If you need to customize silicone products, please contact us at chicarubber