Depending on the variation of density between processing and ambient temperature in injection molding shrinkage might appear.
Shrinkage is divided into two fields: processing and post processing shrinkage.
Processing shrinkage occurs in the mold due to volumetric contraction, mold restrains, crystallinity and orientation.
Post processing shrinkage is measured usually after 24 to 48 hours after the production.
Shrinkage happens mainly due to the relaxation of residual stresses.

Shrinkage in Plastic Industry