Controlling Anisotropic Shrinkage in Reinforced Polyamide Injection Moulding
Controlling anisotropic shrinkage in reinforced polyamides requires directional shrinkage tool scaling, steel-safe cavity design, and optimized hold pressure profiles.
Parallel Shrinkage
Meaning
Dimensional contraction of a plastic part along the direction of the melt flow during the cooling phase defines the primary axis of material reduction. This parallel shrinkage is measured relative to the corresponding dimensions of the mould cavity under specified processing conditions. The measurement is distinct from contraction occurring across the flow direction, which is influenced by different molecular dynamics.
Flow Influence
Shear forces during the injection phase stretch the polymer chains and align them in the direction of the flow. As the polymer cools within the cavity, these oriented chains attempt to recoil and return to a random state, which increases the amount of parallel shrinkage. This effect is most pronounced in semi-crystalline resins where polymer crystals grow preferentially along the aligned chains.
Warpage Consequence
Discrepancy between the contraction along the flow and across the flow is the main cause of part warpage and internal stress. When parallel shrinkage is significantly higher or lower than perpendicular shrinkage, the part will twist or bow after ejection to relieve the unbalanced internal forces. Designers must use mold-filling simulation software to anticipate these directional differences and adjust gate locations accordingly.
Material Variation
Amorphous resins generally exhibit isotropic contraction with minimal differences between flow and cross-flow directions. In contrast, fiber-reinforced compounds show much lower parallel shrinkage because the rigid fibers do not contract and are aligned along the flow direction. This structural difference means that the presence of fillers dramatically reduces contraction along the flow axis while leaving the perpendicular direction relatively unaffected.
The compounder must therefore optimize the fiber loading and coupling agents to balance these directional changes and prevent part distortion.