How do you calculate shrinkage allowance in rotational molding?

Subtract the target part dimension from the oversized cavity dimension to get the absolute margin, then divide by the reference dimension. A 125 cavity and 100 target over a 100 reference gives a 25-unit margin, or 25%.

What is a typical shrinkage rate for rotomolded polyethylene?

Most rotomolding-grade polyethylenes shrink roughly 2-4% linearly, with cross-flow shrinkage often higher than in-flow. The 25% in the default example is an illustrative reference figure, not a real PE shrink rate.

Why does rotomolded part shrinkage matter for mold design?

The cavity must be cut larger than the finished part by exactly the shrinkage amount so the cooled part lands on nominal. Miscalculating it means an undersized part and a costly tool rework.

What affects rotomolding shrinkage the most?

Resin grade and density, wall thickness, cooling rate, and part geometry all matter. Slow, uniform cooling and thicker walls generally increase shrinkage, while forced cooling can reduce it but risks warpage.

Is rotomolding shrinkage the same in every direction?

No, it is anisotropic. Shrinkage across the mold and in unrestrained areas is typically higher than in directions constrained by geometry or inserts, which is why flat panels warp.

How do I compensate for shrinkage in a rotomold?

Cut the cavity oversize by the expected shrink percentage, add draft, and validate with a first-article measurement. Adjust the tool or process (cooling rate, resin) if the measured part misses target.

Rotational Molding calculator

Part Shrinkage Allowance Calculator

Part shrinkage allowance is the amount a rotomolded part contracts as it cools from melt temperature to room temperature, expressed as a percentage of the target dimension. Because polyethylene and other rotomolding resins shrink 2-4% (sometimes more across the mold parting versus the flow direction), toolmakers must cut the cavity oversize so the finished part lands on-spec. Mold designers and process engineers use this figure to size the aluminum or steel cavity and to explain why a part comes out undersized. Getting it wrong means scrapping a tool or living with out-of-tolerance parts.

What this calculator does

Part shrinkage allowance is the amount a rotomolded part contracts as it cools from melt temperature to room temperature, expressed as a percentage of the target dimension.
Use it when part shrinkage allowance in rotational molding needs a clean margin number for a rotational molding go / no-go review.
It computes the shrinkage margin between the oversized cavity dimension and the target part dimension as a percentage of a reference dimension.

Formula used

Part Shrinkage Allowance margin = available value - required value
Margin percent = margin ÷ reference value

Inputs explained

Oversized cavity dimension:
Target finished part dimension:
Reference (nominal) dimension:

How to use the result

Use it during mold design to set cavity oversize, or during troubleshooting when finished parts miss their target dimensions.
Rotomolding shrinkage is anisotropic and varies with wall thickness, resin grade, and cooling rate, so a single percentage is an approximation that should be validated on a first-article part.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 319.371 (BLS, May 2026), up 19.5% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate shrinkage allowance in rotational molding? Subtract the target part dimension from the oversized cavity dimension to get the absolute margin, then divide by the reference dimension. A 125 cavity and 100 target over a 100 reference gives a 25-unit margin, or 25%.
What is a typical shrinkage rate for rotomolded polyethylene? Most rotomolding-grade polyethylenes shrink roughly 2-4% linearly, with cross-flow shrinkage often higher than in-flow. The 25% in the default example is an illustrative reference figure, not a real PE shrink rate.
Why does rotomolded part shrinkage matter for mold design? The cavity must be cut larger than the finished part by exactly the shrinkage amount so the cooled part lands on nominal. Miscalculating it means an undersized part and a costly tool rework.
What affects rotomolding shrinkage the most? Resin grade and density, wall thickness, cooling rate, and part geometry all matter. Slow, uniform cooling and thicker walls generally increase shrinkage, while forced cooling can reduce it but risks warpage.
Is rotomolding shrinkage the same in every direction? No, it is anisotropic. Shrinkage across the mold and in unrestrained areas is typically higher than in directions constrained by geometry or inserts, which is why flat panels warp.

Last reviewed 2026-05-12.