Casting and foundry: design rules
Pouring molten metal into a mold: sand, investment (lost wax) or gravity casting.
Typical tolerance
Sand casting is rough (typically ±1 mm or more); investment casting is tighter (typically ±0.3–0.5 mm on small parts). Machine critical surfaces.
Files to send
STEP and a drawing marking machined surfaces; the foundry adds shrinkage and draft.
Common materials
Aluminium, iron, steel, bronze, brass, stainless.
The rules, with reasons
- Wall thickness. Sand casting about 3 mm and up in aluminium, investment casting about 1.5 mm and up; keep walls uniform. Why: Thin walls do not fill; thick junctions create shrinkage voids.
- Draft. About 1–3° on sand-cast walls. Why: The pattern must lift out of the sand.
- Machining allowance. Plan about 1.5–3 mm on sand-cast faces and about 0.5–1 mm on investment castings. Why: Cast surfaces are rough.
- Hot spots. Avoid thick junctions where walls meet. Why: They solidify last and shrink.
- Method. Sand for large, cheap, rough parts; investment for small, detailed parts. Why: The method sets surface finish and tolerance.
Before you ask for a quote
- Alloy
- Quantity
- STEP and drawing
- Machined surfaces
- Testing (pressure, X-ray, material certificate)
Common mistakes
- No machining allowance
- Uneven wall thickness
- Expecting machined tolerances from a casting
Agents and scripts can check a design automatically with the check_design MCP tool or POST /api/design-rules. How to connect.
Request a quote Find manufacturers
Typical rules of thumb, not a guarantee. The manufacturer's own design guidelines override them: ask for them and check your design against both before ordering.