MIM Shrinkage and Tooling Scale Factor Calculator
In metal injection molding the molded part is oversize by the volume of binder that is removed and the porosity that sinters out. This calculator derives linear shrinkage and the tooling scale factor directly from powder loading and sintered density, so you can size a cavity before the first trial.
Your numbers
The finished dimension on the drawing.
Volume percent powder in the feedstock. Many MIM feedstocks fall between 58 and 65 volume percent.
Typical MIM sintered parts reach 95 to 99 percent of theoretical density.
MIM commonly holds about +/-0.3 to 0.5 percent as-sintered.
Observed spread of linear shrinkage across feedstock lots and furnace runs.
316L about 7.9, 17-4 PH about 7.8, Fe-2Ni about 7.85.
Wax and polymer binder blends are usually 0.9 to 1.1.
Your results
Assumes isotropic shrinkage, no residual porosity beyond the stated final density and complete binder removal. Real tools are corrected after first-article measurement.
Shrinkage and binder mass share vs powder loading
Sweeps Feedstock powder loading from 50 to 68, holding your other inputs fixed.
Track tooling and shrink data in one place
Optional: a short review of linking MIM tool revisions and lot shrinkage data to ERP and quality records.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the finished dimension from the drawing, the feedstock powder loading in volume percent (ask the feedstock supplier) and the sintered relative density you expect. The calculator returns linear shrinkage and the scale factor to apply to every sintered dimension to obtain cavity dimensions. Then enter your drawing tolerance and the shrinkage variation you see across lots to judge whether the feature can hold tolerance as-sintered or needs sizing, coining or machining.
- Use the volume percent loading, not weight percent.
- Apply the scale factor to all dimensions, then adjust the critical ones after trials.
- Ask for sintered density measurements from the same furnace profile.
The physics behind it
Debinding removes only binder, so the powder skeleton keeps its volume until sintering densifies it. If powder fills a fraction f of the green volume and the part sinters to relative density d, the volume ratio sintered / green is f / d. Linear ratio is its cube root, assuming isotropic shrinkage. For 60 percent loading and 97 percent density the linear ratio is about 0.852, giving roughly 14.8 percent shrinkage and a scale factor near 1.174. Higher loading shrinks less and is more dimensionally stable.
Reading the result
The tolerance-to-spread ratio tells you whether the process can hold the feature without extra operations. A ratio below about 2 means normal lot variation consumes most of the tolerance. Long dimensions suffer most because shrinkage variation scales with size. Green mass per part and binder share help check shot size and debinding load: a higher binder fraction means longer debinding and a higher risk of slump and distortion on thin or unsupported features.
Common mistakes
Using one uniform factor for every dimension ignores anisotropy: shrinkage along the flow direction often differs from across it, and gravity sag during sintering distorts long unsupported spans. Gating position and setters change results. Also do not assume final density is 100 percent; entering 100 percent understates the factor. Always cut the cavity steel-safe, with a slightly smaller scale, and open it up after measuring first-article parts.
Frequently Asked Questions
What is a typical MIM shrinkage?
Most MIM parts shrink about 12 to 20 percent linearly from molded to sintered state, which corresponds to scale factors of roughly 1.14 to 1.25. The exact value depends on powder loading and final density.
Why is shrinkage not the same in every direction?
Powder orientation from flow in the mold, gating location, gravity during sintering and support geometry produce directional differences. Tool makers apply a base factor and then correct individual dimensions after first-article measurements.
Does binder density change shrinkage?
Not the linear shrinkage, which depends on powder volume fraction and final density. Binder density matters for molded shot mass and binder share, which affect shot size, debinding time and feedstock cost estimates.
What tolerance can MIM hold?
A common rule is about +/-0.3 to 0.5 percent of the dimension as-sintered, with tighter values possible on stable features after sizing or secondary machining. Treat it as a starting point and confirm with process capability data.
If you want MIM tooling revisions, shrink factors and lot data tracked in your ERP and quality system, we can build that traceability layer.
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