Meaning
Physical property of molded plastic involves a difference in the rate of contraction between the flow direction and the cross-flow direction. Shrinkage anisotropy occurs when polymer chains or fibers align during the injection process, leading to unequal dimensional changes as the part cools. This phenomenon is a primary cause of warping and internal stress in precision components.
Directional Variance
Molecular orientation in the melt creates a structure that resists contraction along the axis of the flow but allows it perpendicular to the flow. When shrinkage anisotropy is high, the final dimensions of a part will not match the mold cavity, leading to assembly failures or functional defects. Engineers must account for this behavior when designing the tool and selecting the gate locations.
Tooling Impact
Mold designers use compensation factors to adjust the size of the cavity based on the expected shrinkage of the resin. If shrinkage anisotropy is not correctly predicted, the uneven pulling forces will cause the part to bow or twist. High-performance polymers with fiber reinforcement are particularly prone to this issue and require careful thermal management during the cooling phase.
Dimensional Stability
Process variables such as injection speed and hold pressure influence the degree of orientation in the part. Reducing shrinkage anisotropy involves balancing the cooling rates and adjusting the processing parameters to minimize molecular alignment. Achieving uniform dimensions across a production run depends on the tight control of these variables.
