Injection Molding Shrinkage Calculator
Estimate a starting mold cavity dimension from the required final part dimension and a specified linear shrinkage rate.
Use the material supplier’s shrinkage data and toolmaker/process knowledge. Actual molding shrinkage can vary with resin grade, fillers, flow direction, wall thickness, gate location, packing pressure, cooling and mold temperature. This is a starting dimensional estimate, not a tooling release value.
Why the exact formula differs from simply adding the percentage
If shrinkage is defined relative to the mold dimension, the exact inverse relation is part ÷ (1 − shrinkage). Multiplying the final part dimension by (1 + shrinkage) is only an approximation, although the difference is small at low shrinkage percentages.
For production tooling, shrinkage should be reviewed directionally and against the actual resin datasheet and molding conditions.