| Part size | Pattern size | Difference |
|---|---|---|
| 100.00 mm (3.94 inch) | 101.01 mm (3.98 inch) | +1.01 mm |
| 50.00 mm (1.97 inch) | 50.51 mm (1.99 inch) | +0.51 mm |
Linear shrinkage only. Add machining allowance and draft separately.
Why patterns and moulds are made bigger
Metal and plastic both contract as they cool from the molten state. A pattern or mould cavity is therefore made slightly larger than the finished part, by the material's shrinkage, so the part comes out at the right size.
- Tool size = part size ÷ (1 − shrinkage)
- Part size = tool size × (1 − shrinkage)
- Pattern makers use a contraction (shrink) rule marked with the same allowance
- Machining allowance and draft are added on top of shrinkage
Casting shrinkage allowances
| Material | Typical range | Common value |
|---|---|---|
| Grey cast iron (FG 200–300) | 0.8–1% | 1% |
| SG / ductile iron | 0.5–1% | 0.8% |
| White cast iron | 1.6–2.1% | 2% |
| Carbon and low-alloy steel | 1.5–2.2% | 2% |
| Stainless steel | 2–2.6% | 2.2% |
| Aluminium alloys (LM6, LM25…) | 1–1.6% | 1.3% |
| Brass | 1.3–1.6% | 1.5% |
| Bronze / gunmetal | 1–2% | 1.5% |
| Copper | 1.6–2% | 1.8% |
| Zinc alloys (Zamak) | 0.5–0.7% | 0.6% |
| Magnesium alloys | 1–1.4% | 1.3% |
Plastic mould shrinkage
| Material | Typical range | Common value |
|---|---|---|
| ABS | 0.4–0.7% | 0.5% |
| PP (polypropylene) | 1–2.5% | 1.6% |
| HDPE | 1.5–3% | 2% |
| LDPE | 1.5–3% | 2% |
| Nylon PA6 | 0.7–1.5% | 1.2% |
| Nylon PA66 | 1–2% | 1.5% |
| Nylon PA66 + 30% glass fibre | 0.3–0.8% | 0.5% |
| POM (acetal, Delrin) | 1.8–2.5% | 2% |
| PC (polycarbonate) | 0.5–0.7% | 0.6% |
| PMMA (acrylic) | 0.2–0.8% | 0.5% |
| PS / HIPS | 0.4–0.7% | 0.5% |
| Rigid PVC | 0.2–0.5% | 0.4% |
| PBT | 1.5–2% | 1.8% |
| PET | 1.2–2% | 1.5% |
| TPU | 0.8–1.5% | 1% |
Questions
- How do you calculate pattern size with shrinkage?
- Pattern size = casting size ÷ (1 − shrinkage). For a 200 mm (7.87 inch) aluminium casting at 1.3% shrinkage: 200 ÷ 0.987 = 202.6 mm (7.98 inch). The quick rule, casting size × (1 + shrinkage), gives almost the same answer at these small percentages.
- What is the shrinkage allowance for cast iron?
- Grey cast iron is usually given about 1% (some foundries use 0.8%), SG iron about 0.5–1%, and steel castings about 2%. Large, heavy sections often shrink a little less than thin ones.
- How much does ABS shrink in a mould?
- Typically about 0.4–0.7%. Semi-crystalline plastics such as PP, POM and nylon shrink much more (about 1–2.5%), and glass fibre reduces shrinkage.
- Is shrinkage the same in every direction?
- Not always. Glass-filled plastics shrink less along the flow than across it, and castings that are held by cores or features shrink less than free ones. For tight parts, mould makers allow steel to adjust after the first trial.
Planning the tool? See sand casting, gravity die casting and injection mould making.
Have something that needs making?
Post your requirement free. Workshops that take your kind of work see it and get in touch. You only talk to the ones that fit.
Get startedShrinkage values are typical. Actual shrinkage depends on the part shape, section thickness, process and grade; confirm with your foundry or moulder for close-tolerance parts.