Shrinkage

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Precision injection molded components measured for dimensional accuracy

Definition of Shrinkage

Shrinkage is the volumetric contraction of plastic as it cools from melt temperature to ambient. Amorphous and semi-crystalline materials shrink differently, and fiber or glass fill can make shrinkage directional rather than uniform.

Mold cavities are oversized by a shrink factor so the final part meets print dimensions. Process variables (pack pressure, cooling rate, gate freeze time, and mold temperature) all influence actual shrinkage on the production floor.

Why It Matters for Plastic Injection Molding

Dimensional tolerance

Predicting shrinkage wrong means parts fail fit checks or assembly. AMP uses material data sheets, simulation, and T-zero sampling to lock shrink allowances before steel is cut.

Warpage and assembly risk

Uneven shrinkage across thick and thin sections causes warpage or internal stress. Rib-to-wall ratios, gate location, and cooling layout are tuned together with shrink models.

Related Terms

Hold PressureCavityMelt Index
Process tuning (especially pack and hold and mold temperature) can pull dimensions toward spec within a limited window. Large or persistent deviation usually requires cavity steel adjustment or gate changes.