Meaning
Physical reduction in material volume occurs naturally as a polymer cools from its elevated processing melt temperature down to ambient room temperature. This thermal contraction drives the overall shrinkage of injection-moulded components, requiring toolmakers to scale up cavity steel dimensions relative to nominal part drawing specifications. The extent of volume reduction depends directly on the polymer’s thermal expansion coefficient, molecular structure, and cooling rate under holding pressure.
The process ends once the component reaches thermal equilibrium with the ambient environment, after which subsequent dimensional changes are driven by post-moulding crystallization or moisture absorption rather than thermal cooling.
Volumetric Reduction
Cooling polymer chains lose kinetic energy, reducing intermolecular distances and increasing material density as heat dissipates into the mould steel. Semi-crystalline thermoplastics like high-density polyethylene undergo severe thermal contraction due to the phase change from a disordered melt into a densely packed crystalline lattice, shrinking up to three percent volumetrically. Amorphous polymers like polystyrene experience far less volume loss during cooling because their randomly entangled molecular structure resists close packing.
Tooling designers apply empirical shrinkage factors to CAD models to compensate for this natural volumetric contraction during cavity machining.
Shrinkage Gradient
Differences in cooling rates across varying wall thicknesses create non-uniform thermal contraction profiles throughout the part geometry. Fast-cooling thin sections experience rapid thermal contraction that freezes early, while thick sections cool slowly and continue to contract over extended periods. This imbalance induces internal residual stresses that manifest as post-ejection part warpage, bowing, or aesthetic sink marks.
Process technicians optimize cooling circuit layouts and adjust packing pressures to equalize cooling rates and minimize localized shrinkage differentials across the part.
Cavity Sizing
Toolmakers machine injection mould cavities oversize based on historical shrinkage data derived from specific resin grades and processing conditions. If processing conditions drift, altering melt or mould temperatures, the actual thermal contraction deviates from the predicted steel-safe allowance, throwing part dimensions out of tolerance. Fiber-filled compounds exhibit anisotropic contraction, shrinking significantly less along the fiber alignment axis than in the perpendicular transverse direction.
Accurately predicting thermal volume changes ensures final components meet strict drawing dimensions upon demoulding and cooling.
