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Mold Shrinkage Calculator: Cavity Dimension from Part Dimension

Plastic parts come out of the mold smaller than the cavity because the resin shrinks as it cools. This calculator turns a nominal part dimension and a shrinkage rate into the cavity dimension to machine, and shows how much of your tolerance the shrink variation uses up.

Your numbers

Nominal part dimension

The finished dimension on the part drawing.

Resin shrinkage rate

1.5 %

From the resin datasheet or your own molded data. Unfilled semi-crystalline resins are usually higher than amorphous or glass-filled grades.

Shrinkage variation (plus or minus)

Lot to lot and process variation you expect around the mean shrink rate.

Part tolerance (plus or minus)

Half of the total tolerance band on the drawing.

Your results

Cavity dimension

121.8274

Dimension to cut in steel so the molded part reaches nominal after shrinkage (exact form).

Tolerance consumed by shrink variation

80%

Over 100 means shrink variation alone exceeds the tolerance.

Shrink allowance

1.8274

Extra steel added to the part dimension, in mm.

Simple method cavity size

121.8

Part x (1 + S). Common shortcut, slightly smaller than the exact form.

Difference between methods

0.0274

Matters on large parts or high shrink resins.

Dimensional swing from shrink variation

0.24

Plus or minus mm caused by variation alone.

Shrinkage depends on gate location, wall thickness, packing, mold temperature and fiber orientation. Use as a starting point and confirm with sample parts.

Save results:

Shrink allowance (mm) vs shrink rate

Sweeps Resin shrinkage rate from 0.2 to 3, holding your other inputs fixed.

Capture measured shrink data from your ERP

Optional: we can review how your inspection results are stored and show how to build a shrink history per mold and resin lot.

Optional. The tool and every export work without this. No spam.

How to use it

Enter the finished dimension from the drawing, the shrinkage rate from the resin datasheet (or from your own measured parts), the variation you expect around that rate, and the plus or minus tolerance. The cavity dimension is the main result. Run it per critical dimension, because shrink differs in the flow direction and across it, and a long dimension sees more absolute shrink than a short one.

Use the mean shrink rate for the molding conditions you will run in production
Enter tolerance as one side only, for example 0.3 for a 0.6 mm band
Check the tolerance consumed figure before committing to steel

The formula

Shrinkage S is defined against the cavity: S = (cavity - part) / cavity. Solving for the cavity gives cavity = part / (1 - S). Many shops use the shortcut cavity = part x (1 + S), which is fine for small S and short dimensions but understates the cavity slightly. At 1.5 percent shrink on 120 mm the exact value is 121.83 mm and the shortcut gives 121.80 mm. Mold designers also cut critical features steel-safe so they can be opened up after the first samples.

Reading the tolerance figure

Shrink variation is a process-driven error that tooling cannot remove. If a dimension swings by 0.24 mm from shrink alone and your tolerance is 0.3 mm, 80 percent of the band is gone before measurement error, tool wear and warpage are counted. When the consumed figure approaches or exceeds 100 percent, options are a lower-shrink or filled resin, tighter process control, a different gate location, or a conversation about the tolerance.

Common mistakes

Using the datasheet shrink for a different wall thickness is the usual error, because thick sections shrink more. Others are ignoring post-mold shrinkage in nylons that absorb moisture, applying one rate to both flow and cross-flow directions in glass-filled resins, and forgetting that the shrink allowance applies to hole and boss dimensions in the opposite sense: pins and cores are cut smaller for holes to end up at size.

Measure parts 24 hours after molding for stable readings
Record the shrink actually achieved in your ERP item master notes
Re-run when the resin grade or supplier changes

Frequently Asked Questions

What is a typical shrinkage rate for injection molded plastics?

Amorphous resins such as ABS and polycarbonate are often around 0.4 to 0.8 percent. Semi-crystalline resins such as polypropylene and polyethylene are commonly 1 to 2.5 percent. Glass fiber filling usually lowers it. Always confirm with the datasheet for your exact grade.

Why is the exact formula different from part times one plus shrink?

Shrinkage is defined as the loss relative to the cavity size, not the part size. Dividing by one minus S accounts for that. The gap is tiny at low shrink rates and short dimensions but grows with larger parts and higher shrink resins.

Does shrinkage differ in flow and cross-flow directions?

Yes, especially in fiber filled or highly oriented resins. Fibers align with flow and restrict shrink in that direction, so cross-flow shrink is often higher. Mold designers use separate rates per direction where the datasheet supplies them.

Can I use this for rubber or die casting?

The same relationship applies to any process where the part shrinks from a cavity, but the shrink rates, temperatures and units are different. Enter the rate for your material and treat the result as a starting point.

If your molding data lives in an ERP or MES, Netray can pull measured part dimensions and lot data into a shrink history so the next tool is cut on your own numbers.

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