Mold Shrinkage Calculator

Calculate nominal cavity size from part dimension and linear shrinkage. This practical tool is an engineering estimate—validate decisions against machine, material, mold, and process data.

Formula and method

Cavity dimension = target molded part dimension ÷ (1 − linear shrinkage).

Inputs and units

Enter measured or specified values in the displayed units. Keep units consistent; results are rounded for practical planning. This first release uses the displayed engineering units rather than a unit-switcher.

Worked example

100 mm ÷ (1 − 0.015) = 101.52 mm cavity dimension.

Get shrinkage from the relevant grade and direction

Record resin grade, filler level, flow direction, test or processing basis, and the molded dimension being controlled. A generic polymer-family value is not a substitute for grade-specific data or trial results.

Use the result as a nominal cavity target

Apply the result to early cavity planning, then validate dimensions with the chosen process window and material condition. Adjustments belong in the controlled tooling and trial record.

Stop using one value when behavior is directional

Fiber-filled or orientation-sensitive parts can shrink differently by direction. Use direction-specific evidence rather than applying one isotropic allowance across the drawing.

Interpretation, assumptions, and limitations

This tool provides an estimate for planning and education. It does not replace machine maker limits, material data sheets, mold trials, cavity-pressure data, process validation, or qualified engineering review.

FAQ

Does this replace a mold trial?

No. It provides a first-pass cavity allowance. Actual shrinkage depends on the specified grade, geometry, packing, temperatures, and flow direction.

Check geometry before applying a shrinkage allowance

Use the Part Design & DFM workflow to document wall, rib, boss and draft geometry. Shrinkage remains material-, direction-, geometry-, gate- and process-dependent.