↔ % Casting shrinkage allowance
Casting shrinkage allowance · Fundición (ISO 10135) · ISO 1101
Example in plan
How to interpret it
On the drawing, “1.5% shrinkage, gray iron, on pattern” states a casting shrinkage allowance requirement.
What controls
Percentage by which the metal contracts as it cools from solidification to room temperature, which the patternmaker compensates for by making the pattern larger than the final part. The drawing always dimensions the cold, finished part: shrinkage is a tooling parameter, not a tolerance for the shop to apply. Its value depends on the alloy and also on the shape of the part, because a section restrained by cores contracts less than a free one.
How to verify
Verified indirectly, by measuring several parts from the lot and comparing them with the CAD model (calipers and micrometers for individual dimensions, scanner or portable measuring arm for the whole) and looking for systematic, not random, deviations. On large parts, measure with the part stabilized at room temperature, because a freshly shaken-out casting is still moving. Records from several pours are what make it possible to distinguish misjudged shrinkage from normal process scatter.
Workshop guideline values
- General
- On the order of 1% for gray iron, the typical value the patternmaker applies to the pattern.
- Accuracy
- On the order of 1.5% to 2% for steels and many aluminum alloys.
- High precision
- Values adjusted by area and validated with the first pour when the part is critical.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Applying the shrinkage factor to the drawing dimensions: the drawing already describes the cold part.
- Confusing poorly compensated shrinkage (systematic deviation across the lot) with process scatter (random deviation).
- Measuring parts that are still hot and drawing conclusions about the tooling.
How to get it in the workshop / if it comes out of tolerance
Never correct a dimension by multiplying by the shrinkage factor: the drawing is already at cold size. What does matter on the shop floor is the effect of a misjudged shrinkage allowance on the pattern, which shows up as a systematic deviation of every part in the lot in the same direction. If you detect that, it is not a problem with your setup: notify the foundry with data from several parts.
All parts short by the same proportion: the shrinkage factor applied to the pattern is too low and the tooling must be corrected. Deviations that differ by area: shrinkage is being restrained by the cores or by the geometry, and the solution lies in mold design. From the machining side, while the pattern is being corrected, the way forward is to increase the machining allowance and distribute it by probing.
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