Rotational Molding worked example

Part Shrinkage Allowance with oversized cavity dimension of 63 units: a worked example

Here is what the math looks like when conditions slip. We hold every other input steady and drop oversized cavity dimension to 63 units, then walk the calculation through step by step. 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.

The inputs for this scenario

  • Oversized cavity dimension: 63 units (the input this scenario stresses; the baseline uses 125)
  • Target finished part dimension: 100 units (held at the documented default)
  • Reference (nominal) dimension: 100 units (held at the documented default)

Working through the calculation

  • The calculation starts from the formula this tool documents: Part Shrinkage Allowance margin = available value - required value.
  • Margin works out to -37 % at these inputs, and this is the headline figure for the scenario.
  • Absolute margin works out to -37 value at these inputs.
  • Available amount works out to 63 value at these inputs.
  • Required amount works out to 100 value at these inputs.

How this compares with the baseline

  • Against the tool's baseline example, where oversized cavity dimension sits at 125 units and the headline result is 25 %, this scenario comes in 248% below the baseline at -37 %.
  • The practical read: the gap between this scenario and the baseline is entirely attributable to oversized cavity dimension, so recovering it is worth quantifying in dollars before considering equipment or staffing changes. 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.

Results at a glance

  • Margin: -37 % (headline result)
  • Absolute margin: -37 value
  • Available amount: 63 value
  • Required amount: 100 value

Run it with your numbers

  • To rerun this with your own numbers, open the live Part Shrinkage Allowance calculator, set oversized cavity dimension to your actual value, and adjust the remaining inputs to match your operation.

Last reviewed 2026-05-12.