The reduction of the specific volume of the part is called volumetric shrinkage.
It takes place inside the mold between injection and ejection.
The thermal contraction is the main reason for the volumetric shrinkage.
For semi-crystalline polymers the crystallization is another cause.
High differences in wall thickness and inadequate packing lead to a high volumetric shrinkage.
This results in a high warpage, sink marks and voids.

Volumetric Shrinkage in Plastic Industry

Plastic: In the red areas the volumetric shrinkage is higher than in the blue areas

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.