Differential Shrinkage

Fiber Alignment

Fiber-reinforced resins contract less along the direction of the glass fibers because the stiff fibers oppose the movement. This anisotropy produces differential shrinkage between the parallel and perpendicular flow directions. Unreinforced resins, by contrast, shrink more uniformly but are still subject to flow-induced molecular orientation.

Cooling Gradient

Variations in wall thickness across the molded part create temperature differences during cooling. Thicker areas cool slower than thinner areas and undergo greater volumetric reduction, which generates differential shrinkage. Moulders can minimize this effect by maintaining uniform wall thicknesses and optimizing the cooling channel layout in the mold tool.

Part Warpage

Internal stresses build up when different regions of the part try to contract by different amounts. If these stresses exceed the structural strength of the polymer, the part warps or twists after ejection.