Meaning
Polymer volume reduction during cooling varies depending on the direction of molecular alignment or fiber orientation. This differential contraction, known as anisotropic polymer shrinkage, occurs when polymer chains or reinforcing fibers align along the flow path during injection molding, causing greater shrinkage perpendicular to the flow than parallel to it. This behavior governs the final part dimensions and limits the use of uniform scaling factors in mold design.
Material Deformation
Molecular alignment during the molding cycle creates directional stresses that resolve during cooling. When anisotropic polymer shrinkage is uncorrected, parts warp and twist as they release from the tool. The shrinkage rate is influenced by cooling velocity and gate placement.
Production Run
Tooling designers must evaluate fill patterns to map the orientation of the polymer chains. When transition to high-rate production begins, the discrepancy between the supplier’s mold trial and the actual production rate becomes apparent. Accelerated cycle times in production increase the directional temperature gradients, which amplifies anisotropic polymer shrinkage and drives parts out of specification.
Tolerance Limit
Measuring parts after a standardized cooling period reveals the stable geometry. Anisotropic polymer shrinkage dictates the allowable part tolerances and requires directional offsets in the mold cavity.
