Calculate plastic mold shrinkage to find cavity size, final part size, or measured shrinkage from cavity and molded part dimensions.
Related Calculators
- Conduit Shrink Calculator
- 1:10 Taper Angle Calculator
- Hole Size Calculator
- Laser Cutting Time Calculator
- All Construction Calculators
Mold Shrinkage Formula
The mold shrinkage calculation uses the expected percentage reduction from the mold cavity dimension to the cooled part dimension. The calculator has three modes: find the required cavity size, find the final part size, or measure shrinkage from an actual molded part.
Required cavity size
C = P/(1 - S/100)
Final part size
P = C*(1 - S/100)
Measured shrinkage
S = ((C - P)/C)*100
- C = mold cavity dimension
- P = molded part dimension after cooling
- S = mold shrinkage percentage
For cavity size, enter the desired finished part dimension and the material shrinkage. The calculator increases the cavity dimension enough so the cooled part should land near the target size.
For part size, enter the existing cavity dimension and the material shrinkage. The calculator estimates the finished molded dimension after shrinkage.
For measured shrinkage, enter the cavity dimension and the actual molded part dimension. The calculator returns the shrinkage percentage. If the part dimension is larger than the cavity dimension, the result is negative, which usually means the units or measurements should be checked.
Typical Plastic Mold Shrinkage Values
Use the material table as a starting point only. Actual shrinkage depends on resin grade, filler content, mold temperature, part thickness, gate location, packing pressure, and flow direction.
| Material | Typical shrinkage | General range |
|---|---|---|
| ABS | 0.5% | Low |
| Polycarbonate, PC | 0.7% | Low to moderate |
| Polypropylene, PP | 1.8% | High |
| Polyethylene, PE | 2.0% | High |
| Nylon, PA | 1.5% | Moderate to high |
| Acetal, POM | 2.0% | High |
| PVC | 0.4% | Low |
| PET | 0.3% | Low |
Example Mold Shrinkage Calculations
Example 1: Find the required cavity size
You want a finished ABS part dimension of 50 mm. The shrinkage is 0.5%.
C = 50/(1 - 0.5/100)
C = 50.2513 mm
The mold cavity should be about 50.2513 mm for that dimension.
Example 2: Measure shrinkage from a molded part
A cavity measures 50.25 mm. The molded part measures 50.00 mm.
S = ((50.25 - 50.00)/50.25)*100
S = 0.4975%
The measured shrinkage is about 0.498%.
FAQ
Is mold shrinkage based on the cavity size or the part size?
In this calculator, shrinkage is based on the cavity size. The measured shrinkage formula is cavity minus part, divided by cavity, then multiplied by 100. This matches the formulas used to move between cavity size and final part size.
Why does the required cavity size use division instead of just adding the shrinkage percent?
The cavity is the original size before shrinkage, so the finished part is a percentage of the cavity. For example, with 2% shrinkage, the part is 98% of the cavity. To find the cavity from the desired part, divide by 0.98. Simply adding 2% to the part gives a close estimate for small shrinkage values, but it is not the exact inverse calculation.
Why can actual shrinkage differ from the material preset?
The preset is only a typical value. Real shrinkage can change with resin grade, fillers, moisture, processing temperature, mold temperature, packing pressure, cooling time, wall thickness, and the direction of material flow. For tight tolerances, use measured shrinkage from trial parts or the material supplier's data for the specific grade.
