How do I compensate for resin shrinkage?

Make the mold cavity larger than the finished size you want. Because the part shrinks by a fixed percentage as it cures, the mold dimension equals the finished dimension divided by (1 minus shrinkage over 100). For 1% shrinkage you scale every dimension up by about 1.0101.

How much does resin shrink?

Linear shrinkage is typically 0.1% to 3% depending on the resin type and chemistry. Polyester and some casting resins shrink more, while many epoxy and urethane resins shrink less. Always check the datasheet or measure a test cast for your exact product.

Why is the volume factor larger than the linear factor?

Volume scales with the cube of the linear factor. A 1% linear shrinkage means the part shrinks roughly 3% by volume, so multiply your finished-volume resin estimate by the volume factor to know how much resin the oversized mold actually needs.

Does part thickness affect shrinkage?

Yes. Thicker sections cure hotter and can shrink more, and uneven thickness can cause warping rather than uniform shrinkage. For critical fits, cast a test piece of similar thickness and measure it before committing to a final mold.

Should I scale all axes equally?

Resin shrinkage is generally isotropic, so the same percentage applies to every axis and the tool scales length, width and height by the same factor. If you notice direction-dependent shrinkage in tests, adjust the worst axis individually.

Resin Casting Shrinkage & Mold Size Calculator

When resin cures it shrinks slightly, so a mold built to the exact finished size produces a part that comes out a little too small. For parts that must fit together or match a spec, you compensate by building the mold cavity oversized. This calculator works out exactly how much oversize each dimension needs.

How it works

If a part shrinks by a linear percentage s as it cures, the finished dimension F relates to the mold cavity dimension M by:

F = M × (1 − s ÷ 100)

Solving for the mold cavity you need to build:

M = F ÷ (1 − s ÷ 100)

The tool applies this to length, width and height and reports the per-axis oversize, the linear scale-up factor, and the volume factor (the linear factor cubed) so you can estimate how much extra resin the larger mold consumes.

Example

You want a finished block of 50 × 30 × 20 mm from a resin with 1% linear shrinkage. The scale factor is 1 / (1 − 0.01) ≈ 1.0101, so the mold cavity must be about 50.51 × 30.30 × 20.20 mm. The volume factor is roughly 1.0306, meaning the mold holds about 3% more resin than the finished volume.

Notes

Shrinkage varies with resin brand, mixing ratio, temperature and part thickness, so the percentage on the datasheet is a starting point. Always cast a test piece of similar geometry, measure it, and back-calculate the real shrinkage before cutting an expensive master mold. All calculations run locally in your browser.