Meaning
Dimensional behavior where a polymer part reduces in size by different magnitudes along different axes as it cools from a molten state. Designers account for anisotropic contraction to ensure that high-tolerance components meet assembly requirements after ejection from the tool. This phenomenon occurs most prominently in semi-crystalline resins and fiber-reinforced materials where molecular or structural orientation aligns with the flow direction.
Orientation Effect
Flow direction often exhibits lower shrinkage than the transverse direction because polymer chains or glass fibers resist compression along their length. Engineers manage anisotropic contraction by adjusting gate locations to control the internal stress distribution. This variance leads to warping if the part geometry cannot accommodate the uneven internal forces.
Mechanical Compensation
Steel dimensions are cut larger than the target part to compensate for expected volume loss during the cooling phase. Predicting anisotropic contraction accurately requires mold flow simulation that considers local fiber orientation, cooling rates, packing pressure and gate location. Toolmakers often use different shrinkage factors for length and width.
Accurate predictions allow the moulder to hit tight tolerances on the first trial.
Quality Impact
Precision parts suffer from dimensional drift when processing temperatures or packing pressures fluctuate. Inconsistent anisotropic contraction creates ovality in cylindrical features or bowing in flat surfaces. Monitoring melt temperature helps maintain stable shrinkage values.
