Plastic shrinkage calculator.
The most comprehensive free plastic shrinkage calculator on the web. Search 86 materials, calculate mold dimensions, and compare shrinkage rates instantly. ASTM D955 compliant data trusted by injection molding professionals.
Shrinkage Database & Mold Dimension Calculator
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Mold Dimension Calculator
Calculate required mold cavity size from desired part dimension and shrinkage rate.
Mold Dimension = Part Dimension × (1 + Shrinkage% / 100)
Shrinkage Rate Calculator
Calculate actual shrinkage rate from mold and part measurements.
S (%) = (Mold Dim − Part Dim) / Mold Dim × 100
First Material
Second Material
Shrinkage Rate Comparison (Max %)
Predicting shrinkage before
you cut steel.
When molten plastic cools from roughly 200 to 300°C to room temperature, the polymer chains contract. That shrinkage must be compensated in mold design, using ASTM D955 standard data.
Semi-Crystalline
Materials like PP, PE, PA, and POM have ordered molecular regions that pack tightly during cooling. Crystallization causes higher shrinkage (typically 1.0 to 3.0%) and greater sensitivity to processing conditions.
Amorphous
Materials like ABS, PC, PS, and PMMA have random molecular arrangements. They exhibit lower, more predictable shrinkage (typically 0.3 to 0.8%) and better dimensional stability for tight-tolerance parts.
Mold shrinkage rate formulas
Mold cavity dimension: Dm = Dp × (1 + S/100), where Dm is the mold cavity dimension, Dp the desired part dimension, and S the shrinkage rate in percent. To achieve a 100mm part with 1.5% shrinkage you need a 101.5mm cavity.
Actual shrinkage rate: S (%) = (Dm − Dp) / Dm × 100. This reverse calculation verifies real shrinkage from measured parts versus mold dimensions. The data follows the ASTM D955 standard.
What moves actual shrinkage off the datasheet
- Wall thickness: thicker sections cool slower and shrink more. Keep walls uniform to prevent warpage and differential shrinkage.
- Mold temperature: higher mold temperatures increase shrinkage but improve surface finish; lower temperatures reduce shrinkage but can add stress.
- Packing pressure: higher packing pressure forces more material into the cavity, reducing shrinkage. Use our clamp force calculator to ensure adequate tonnage.
- Glass fiber content: glass fiber cuts shrinkage sharply (often a 50 to 70% reduction) but causes anisotropic shrinkage: different rates in flow versus cross-flow directions.
For tight-tolerance parts in plastic part design, amorphous materials often give better dimensional control, while semi-crystalline materials offer superior chemical resistance and fatigue properties. For tool steel selection, see our Brinell to Rockwell hardness calculator, and for build oversight, our tooling project management service.
Shrinkage rate questions, answered.
How accurate is this plastic shrinkage calculator?
It uses ASTM D955 standard values, which represent typical shrinkage ranges under standard processing conditions. Actual shrinkage may vary plus or minus 10 to 20% based on wall thickness, gate location, mold temperature, and packing pressure. For critical tolerances, run test shots with your specific material grade and processing parameters.
Why does my plastic shrink more in one direction than another?
That is anisotropic shrinkage, especially common with glass-filled materials. Polymer chains and fibers align with flow direction, causing lower shrinkage parallel to flow and higher shrinkage perpendicular to it. Filled materials may require flow-direction-specific values from your material supplier.
What is the best plastic for low shrinkage parts?
For minimum shrinkage, consider amorphous materials like PC (0.5 to 0.7%), PMMA (0.2 to 0.8%), or glass-filled grades. LCP (Liquid Crystal Polymer) offers extremely low shrinkage (0.1 to 0.5%) for precision applications. Use the comparison tool above to evaluate options.
How do I compensate for shrinkage in mold design?
Scale cavity dimensions with Mold Dimension = Part Dimension x (1 + Shrinkage%/100). For a 100mm part with 2% shrinkage, design a 102mm cavity. Size the cavity for the low end of the shrinkage tolerance and the core for the high end: steel is easier to remove than to add.
Does this plastic shrinkage calculator work for all materials?
The database covers 86 common thermoplastics used in injection molding, including all standard grades of ABS, PC, PP, PE, PA, POM, and engineering resins. Values vary by manufacturer and grade. For specialty compounds, thermosets, or elastomers, consult your material supplier's technical data sheet.
Need help with material selection
or mold design?
The calculator is just the beginning. MoldMinds provides comprehensive injection molding consulting, from material selection and shrinkage optimization to complete offshore tooling project management.