To compensate the shrinkage, it is necessary to inject more material after 100% filling of the part.
The length of packing time (green line in the time-pressure diagram below) must be selected in such a way that the gate freeze time is reached.
A short packing time leads to a higher risk of sink marks and voids.
Similar things will happen, if there is still molten material within the part but the gate is already frozen.

Packing Time in Plastic Industry

Injection Molding Process: The packing time is the time between 100% filling and gate freeze time.

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