What is a Mold Shrinkage Calculator?
The mold shrinkage calculator applies material-specific shrinkage rates to back-calculate the mold dimensions needed to produce a target part size.
How it's calculated
Mold dimension = part dimension × (1 + shrinkage rate)
Shrinkage varies by material: PP/PE are typically 1.5–2.5%, ABS is typically 0.4–0.7%.
Variable definitions
| Variable | Meaning | Unit |
|---|---|---|
| Shrinkage rate | Volume reduction after cooling | % |
| Part dimension | Target finished part size | mm |
| Mold dimension | Actual cavity size to machine | mm |
Assumptions
This simplification assumes shrinkage happens uniformly in every direction. In reality, shrinkage differs along versus across the resin's flow direction (anisotropy), so precision parts need direction-specific shrinkage values.
Worked example
Target part length 100mm, using PP resin (2% shrinkage)
- Mold dimension = 100 × (1+0.02) = 102mm
- Machining the mold cavity to 102mm yields a finished part of exactly 100mm after cooling.
Why does plastic shrink?
Injection molding forces molten resin into a mold and then cools it. Material that was expanded while hot contracts as it cools, so a finished part is always smaller than the mold cavity that made it — which is why molds are deliberately made larger than the target part.
Crystalline resins (PP, PE, nylon, POM) form ordered molecular structures as they cool and shrink more (1.5–3%), while amorphous resins (ABS, PC, PS, acrylic) cool into a disordered structure and shrink less (0.4–0.8%).
Good to know
- Shrinkage direction matters — it differs along versus across the flow direction of the resin, which is why thin, long parts are prone to warping.
- Thicker sections cool more slowly and shrink more, creating a sink mark on the surface — which is why designers aim for uniform wall thickness.
- Shrinkage rate is not a fixed number — mold temperature, packing pressure, and injection speed all affect it, so datasheet values are only a starting point refined by test shots.
- Molds are typically cut slightly undersized and enlarged later, since machining metal away is easy but adding it back is not.
- Adding glass fiber (GF) reinforcement significantly reduces shrinkage — 30% GF-reinforced nylon can shrink to less than half the rate of the unreinforced material.
Frequently asked questions
Q. Which shrinkage value should I use?
Start with the material manufacturer's datasheet value, then refine based on actual test-shot results, since real conditions affect the final rate.
Q. Why does PP shrink so much more than ABS?
PP is a crystalline resin, meaning its molecules arrange into an ordered structure as it cools, causing significant volume reduction. ABS is amorphous and shrinks much less.
Q. How do I reduce warping?
Design for uniform wall thickness, and adjust gate location and cooling channel layout so shrinkage occurs evenly.