Meaning
Volumetric contraction of a thermoplastic material occurs inside the mold cavity as the molten polymer cools from its melt temperature to its solid state under active injection and holding pressure. Controlling dynamic shrinkage during the molding cycle determines the final dimensions of the part and prevents molded defects like sink marks or warpage.
Process Variable
Packing pressure and melt temperature directly dictate how much a polymer compresses before the gate freezes. Molders adjust these parameters to compensate for viscosity changes.
Tooling Offset
Toolmakers cut the steel mold cavities larger than the nominal part design to account for the expected material contraction during cooling. This calculated offset is based on datasheet values, but actual contractions vary depending on the direction of fiber orientation in reinforced plastics. Precision parts often require a steel-safe tooling design that can be modified after initial trial runs.
Material Specification
Different resin families exhibit highly diverse contraction behaviors, with semi-crystalline polymers like nylon contracting far more than amorphous ones like polycarbonate. Virgin resins consistently yield predictable contraction rates, while regrind blends introduce variability due to thermal degradation and chain scission from previous heat cycles. This variance forces molders to segregate regrind percentages to maintain dimensional consistency.
