Mold Shrinkage Factor Calculator
Scale factors for the cavity and core at their operating temperatures, corrected for mold thermal expansion.
Inputs
Defaults: cavity 75 °C, core 45 °C. Thermal expansion is referenced to 20 °C.
Results
Unrounded values
How this calculator works
PDCPD parts shrink as they cool after curing, typically around 0.9% to 1.0% depending on the resin grade. At the same time the mold itself expands at its operating temperature. The cavity and core usually run at different temperatures and may be made of different metals, so each half needs its own scale factor.
This calculator combines the resin shrinkage with the thermal expansion of the mold material, referenced to 20 °C, and returns a multiplier for the cavity and for the core.
Formulas
- Cavity scale factor
1000 / [(1 − resin shrinkage) × (1 + cavity coefficient × (cavity temp − 20))] × 0.001- Core scale factor
1000 / [(1 − resin shrinkage) × (1 + core coefficient × (core temp − 20))] × 0.001- Resin shrinkage
Options: 0.009, 0.0095, 0.01- Expansion coefficients
Aluminium 0.000023; Steel 0.000012; Copper 0.000017
The result is a multiplier (≈ 1.008), not a percentage. Multiply part dimensions by it to get mold dimensions.
Worked example
Resin shrinkage 0.9%, steel cavity at 75 °C, aluminium core at 45 °C:
| Resin shrinkage | 0.009 (0.9%) |
|---|---|
| Cavity material | Steel |
| Cavity temperature | 75 °C |
| Core material | Aluminium |
| Core temperature | 45 °C |
| Cavity scale factor | 1.008416 × |
|---|---|
| Core scale factor | 1.008502 × |
| Resin shrinkage | 0.0090 |
| Cavity expansion coefficient | 0.000012 /°C |
| Core expansion coefficient | 0.000023 /°C |
When to use it
- Scaling part CAD data to cavity and core dimensions
- Comparing aluminium, steel and copper inserts
- Checking the effect of a change in mold temperature
Frequently asked questions
What shrinkage should I use for PDCPD?
Typical PDCPD resin shrinkage is between 0.9% and 1.0%. The calculator offers 0.009, 0.0095 and 0.01; use the value from your resin supplier.
Why do the cavity and core get different scale factors?
They run at different temperatures and may be different metals. A steel cavity at 75 °C expands differently from an aluminium core at 45 °C, so with 0.9% resin shrinkage the factors are 1.008416 and 1.008502.
How do I use the scale factor?
Multiply the part dimension by the factor to get the mold dimension at 20 °C. A factor of 1.0084 means a 500 mm part feature is cut at about 504.2 mm.
Which thermal expansion coefficients are used?
Aluminium 0.000023 /°C, steel 0.000012 /°C and copper 0.000017 /°C, referenced to 20 °C.
Related calculators
Results are estimates from empirical rules used in PDCPD mold development. Verify critical dimensions before cutting metal.