Shrinkage Compensation Calculator

Every injection-molded part shrinks as it cools from melt temperature down to room temperature, so a mold cut to the exact final dimensions would produce an undersized part. This calculator inverts that relationship: for each axis it computes moldDimension = nominalDimension / (1 − shrinkageRate), oversizing the cavity by exactly enough that the part lands on its nominal size once it has finished shrinking. It runs the calculation independently for the flow direction, the cross-flow direction, and the thickness direction, because shrinkage is not uniform — polymer chains and any fiber reinforcement tend to align with melt flow, so shrinkage across the flow path is typically 10-50% higher than shrinkage along it.

That mismatch is captured in the anisotropy ratio (cross-flow shrinkage divided by flow shrinkage): ratios above roughly 1.3 flag a real risk of warpage, since the two directions are pulling the part out of flat by different amounts as they cool. The tool also runs a rough tolerance-feasibility check, assuming shrinkage itself varies by about 15% of its nominal value from shot to shot (due to pressure, temperature, and hold-time variation) and comparing that expected swing against your stated part tolerance. The biggest thing to keep in mind: these are nominal shrinkage rates for unfilled resin at typical processing conditions — glass fill, wall thickness, gate location, and packing pressure all shift the real number, so treat the mold dimensions here as a strong starting point for tooling, not a substitute for a shrinkage study on your actual material and part geometry.