Isotropic Linear Shrinkage

Shrinkage Physics

Molten polymer chains lose thermal energy and contract into denser, low-energy configurations during solid-state cooling. Unfilled amorphous polymers such as polycarbonate and polymethyl methacrylate exhibit near-uniform dimensional contraction because random polymer coil entanglements lack crystalline lattice ordering. Linear shrinkage formulas scale cavity dimensions by adding the shrinkage percentage directly to target part geometry.

Toolmakers cut cavity steel oversized using these baseline shrinkage rates to yield nominal part drawings upon cooling.

Orientation Distortion

Processing conditions introduce shear stresses that stretch molecular chains along the flow path, creating slight orientation differences. When cooling is rapid or injection pressure is unbalanced, even unfilled resins deviate from pure uniform contraction, producing minor directional shrinkage deltas. Semi-crystalline resins like polypropylene exhibit high volumetric shrinkage that varies with wall thickness and cooling speed, departing significantly from isotropic behaviour.

Moulders must balance pack pressure across all cavity zones to maintain uniform density throughout the part.

Tolerance Verification

Quality control protocols measure moulded parts twenty-four and forty-eight hours post-moulding to account for secondary relaxation. Regrind additions alter the molecular weight distribution, slightly modifying linear contraction compared to virgin resin datasheets. Component designers define critical tolerances based on verified resin shrinkage rather than generalized handbook values.

Moulders adjust holding pressure and coolant flow rates to keep finished dimensions within drawing limits without redesigning hard steel cavities.