Plastic and Injection Molding Industry Blog


Explaining Shrink in Injection Molding

Posted by Corie Yodis on July 30, 2018


Shrink is a common issue in all injection molding processes, both silicone and plastic. As long as you are working with a skilled manufacturer, who has ample knowledge in material science, they should know how to account for the shrink in your design. Silicone shrinks more predictably than plastic, so it is easier to account for. All plastic resins have difference shrink rates, and they are much larger than the shrink rates of silicone. Plastic shrink can also result in sink, which can be a large problem atheistically.

Shrink is always accounted for in mold design. The manufacturer increases the cavity to be 1-4% larger than the desired part, based on the material data sheets and the shrink rate. When the product cools or cures in the mold, it will begin shrinking. Shrinking will continue to happen to the part for the next 24 hours. If you are ever doing measurements on critical dimensions of a part, it should wait until 24 hours after molding.

Plastic Shrink & Sink

Plastic resins can shrink anywhere from 1-4% of its size after molding. Therefore, it is important to build the shrink into the mold design. Different wall thicknesses will shrink at different rates, and this is where sink come in to the equation. Sink happens on a plastic part when the part is too thick in areas. It is important to thin these areas out and create equal wall thickness throughout the part. Sink will look like a divot in the part on the thick areas.

 Sink marks silicone injection molding

Shrink happens while the part is cooling, and when the area is very thick, it takes a much longer time to cool, causing excessive shrink to happen in that area. Using an experienced molder, they will be able to flag situations like this and help you design for manufacturability. 

Silicone Shrink

All silicone shrinks at approximately the same rate; therefore, it can be more predictable. The shrink is simply built into the mold design. Even though sink is not much of a problem, we still must watch out for thick wall parts in silicone. Thick wall parts can result is under-cured parts, deforming or backrind. 

  • Undercured: Thick silicone parts will take a long time to cure, so when it is removed from the mold, it may still be soft and uncured on the inside. This will result in post baking.
  • Deformed: While the thick part is still uncured on the inside, it may become deformed during removal from the tool, or while the part cured outside of the mold.
  • Backrind: If we attempt to increase the cure time inside the tool to finish the cure instead of post baking, the silicone part will begin to expand under the heat. This causes the material to press up against the sides of the tools and rip at the parting line. This is call backrind. This can be a difficult idea to grasp, but one that any silicone molder is well versed in.

These common shrink issues are avoidable by partnering with a skilled manufacturer who are ample knowledge in material science. Partnering with the right manufacturer will help your components turn out right the first time, reducing production times and costs. If you’re looking for a manufacturer for your injection molded components, contact us today.

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Tags: Molding, Injection Molding, Product Design, Silicone Shrink