Wall Thickness Principles for Injection Moulding Tool Design
Uniform wall thickness design balances polymer flow, prevents shrink defects, cuts cycle times, and reduces landed part cost in injection moulding tools.
Differential Shrink
Meaning
Non-uniform volumetric contraction across different regions of a moulded part generates internal stresses during cooling. Variations in cooling rates, wall thickness, or fiber orientation cause differential shrink, resulting in part warpage, twisting, and dimensional instability after ejection. The phenomenon stops causing shape distortion once the component reaches ambient thermal equilibrium and internal stresses drop below the material yield strength.
Material datasheets report linear mold shrinkage measured on standard test bars, which fails to capture the complex directional variations occurring in complex three dimensional geometries. Controlling mold wall temperatures and packing profile balances volumetric changes across thick and thin features.
Volumetric Contraction
Thick wall sections store heat longer than thin ribs, maintaining liquid core states while outer skins freeze. Polymer molecules in slower cooling zones undergo greater volumetric contraction as crystalline structures pack tightly together. Unequal volume reduction pulls frozen surface layers out of nominal alignment.
Fiber Orientation
Glass fibers align parallel to the melt flow direction during cavity filling, restricting longitudinal thermal contraction. Transverse shrinkage remains high because polymer chains between fibers contract freely during solidification. Anisotropic shrinkage causes flat panels to bow or twist along the flow axis.
Geometrical Distortion
Uneven contraction forces planar surfaces to warp away from flat tool faces after ejection from the cavity. Post-moulding fixtures restrain warm parts during cooling to minimize final distortion in critical assembly areas.