Shrinkage Calculator
Calculate injection mold cavity dimensions from part size and plastic shrinkage rate using cavity = part × (1 + s), with typical shrinkage ranges by material.
Introduction
Thermoplastics contract as they cool from melt temperature to room temperature, so an injection mold cavity must be cut larger than the finished part. The Shrinkage Calculator sizes that cavity for you: pick a resin, enter the target part dimension and the shrinkage rate from your material datasheet, and it returns the required mold dimension to three decimal places. It also displays the typical published shrinkage range for the selected resin — PP runs 1.0–2.5%, ABS 0.4–0.7%, PC 0.5–0.8%, PE 1.5–4.0%, POM 1.8–2.5%, PS 0.3–0.6%, PMMA 0.2–0.6%, and PA6 0.8–1.5% — so you can immediately spot a datasheet value that looks suspicious.
How it Works
The calculation is the standard linear compensation formula: mold dimension = part dimension × (1 + s), where s is the shrinkage rate expressed as a decimal. For a 100 mm part molded in PP at 2% shrinkage, the cavity must be 100 × 1.02 = 102.000 mm. Alongside the result, the tool looks up the min–max shrinkage band for your resin so you can bracket the cavity size for steel-safe machining: cutting toward the low end of the band leaves stock to re-cut if the first shots come out small.
Usage Scenarios
- Sizing a new cavity insert for a PP living-hinge container where the datasheet quotes 1.8% shrinkage and you need the steel dimension before sending the insert out for hard milling.
- Checking a mold maker's quoted cavity dimensions against the resin supplier's shrinkage range before approving the tooling drawing.
- Troubleshooting undersized parts after a resin substitution — switching from ABS (0.4–0.7%) to POM (1.8–2.5%) without recutting the cavity guarantees out-of-tolerance parts.
- Estimating how much a critical 250 mm rail dimension will move if the molder raises pack pressure and the effective shrinkage drops from 2.2% to 1.9%.
- Bracketing steel-safe dimensions: machine the cavity at the low end of the shrinkage band, sample, then re-cut toward nominal.
FAQ
Why does the calculator show a shrinkage range as well as my entered rate?
The range is the typical published band for the selected resin family. If your entered rate falls outside it, double-check the datasheet — filled grades, wall thickness, and pack pressure all shift actual shrinkage.
Is linear compensation accurate for glass-filled resins?
Filled grades shrink anisotropically — less along fiber orientation, more across it. Use the flow-direction and cross-flow rates from the supplier separately, and run the calculator once for each axis.
Should I use the minimum or maximum of the range when cutting steel?
Cut steel-safe: size the cavity using the low end of the range so the first articles come out slightly small on the steel, leaving material to re-cut after sampling.
Does the formula account for sink or warpage?
No. The (1 + s) compensation handles bulk linear shrinkage only. Sink marks and warp are local effects driven by wall-thickness ratios, gate position, and cooling balance.