A casting that ignores DFM rules invites porosity, cold shuts, warping, sink marks, and ejection cracks—and forces expensive secondary machining or tool rework. Getting these six parameters right reduces scrap, shortens lead time, and lowers piece price. For background on the defects DFM prevents, see our casting defects guide; for the processes themselves, the aluminum casting guide. Need hands-on review? Our DFM support service checks your model before tooling release.
Draft is the slight taper on walls parallel to the die-pull or pattern-draw direction. Without it, the part drags, scuffs, or sticks during ejection.
Uniform, moderate walls are the single biggest lever on casting quality. Thick sections cool slowly and shrink (porosity, sink); abrupt thickness changes trap stress and cause cracks.
Sharp internal corners cause turbulence, hot spots, and stress concentrations. Rounding them improves metal flow and part strength.
Surfaces that must be precise are cast oversize, then machined to final dimension. Too little stock risks cleaning up into porosity; too much wastes metal and machining time.
See our tolerance standards for how cast and machined tolerances combine.
Tolerance capability differs sharply by process; specifying tighter than the process can hold forces machining (and cost) you may not need.
| Process | Typical linear tolerance (as-cast) |
|---|---|
| High-pressure die casting | ~±0.1–0.2 mm on the first inch (tightest) |
| Gravity / permanent mold | Moderate, looser than die |
| Sand casting | ~±0.5–1.5 mm and up (loosest) |
Apply tight tolerances only to functional features, and reach them by machining the cast blank where needed. Compare grades and properties in our material comparison guide.
The parting line is where the two die halves (or mold halves) meet. Its placement drives tooling cost, flash, and where witness marks land.
Uniform wall thickness, adequate draft for ejection, generous fillets and radii, sensible machining allowance, realistic tolerances matched to the process, and a well-planned parting line. Wall thickness uniformity has the biggest impact on quality.
Typically 1 to 3 degrees on most faces, with about 1.5 to 2 degrees on internal walls. Deep or textured surfaces need more; shallow cosmetic faces can use less. Sand and gravity casting generally need larger draft.
For die casting, structural walls are usually 2 to 4.5 mm, with thin features down to about 0.5 to 1 mm on small parts. Sand and gravity castings need thicker minimums, commonly 3 to 5 mm and up.
Die castings need little, about 0.5 to 1.5 mm. Sand castings need more, often 1.5 to 6 mm depending on size and surface orientation. Add stock only where a feature must be machined to final size.
High-pressure die casting holds the tightest as-cast tolerances (around plus or minus 0.1 to 0.2 mm on the first inch), while sand casting is much looser. Specifying tighter than the process allows forces extra machining and cost.
Send your drawing or 3D model and our engineers will flag draft, wall, fillet, and tolerance issues before tooling—saving you scrap and rework. Matson Alucasting supports die, sand, and gravity casting in aluminum.