Meaning
Contraction of molten polymer during cooling and phase change from liquid to solid state reduces physical dimensions within an injection mold cavity. Mold designers calculate volumetric shrink to determine mold cavity dimensions required to achieve final part drawing tolerances. Thermal expansion coefficient, melt density change, and polymer crystallization drive volume reductions as polymer cools from melt temperature to ambient room temperature.
Crystallization Impact
Semicrystalline polymers like polypropylene undergo substantial volumetric contraction during polymer chain folding into ordered crystal lattices. Amorphous resins exhibit lower, predictable contraction because polymer chains lack long-range crystalline ordering upon cooling. Processing conditions that increase crystallinity, such as high mold temperatures, increase total volumetric contraction.
Packing Effects
Mold cavity packing pressure forces additional polymer melt into the cavity during cooling to compensate for density changes. Inadequate packing pressure or premature gate freeze yields hollow sink marks, internal voids, and dimensional under-sizing. Proper packing profile adjustments reduce volumetric density changes across thick part sections.
Dimensional Control
Volumetric contraction variation across part geometries creates differential shrinkage, inducing internal stress and part warpage. Mold cooling channel design must ensure uniform heat extraction to balance contraction rates throughout complex part walls. Post-molding dimensional stability checks confirm part tolerance compliance before assembly operations.
