Most of a casting's cost and most of its defect risk are decided on the drawing, long before metal is poured. These eight guidelines help engineers design grey and SG iron parts that are easier to make well.
1. Keep wall thickness as uniform as possible
Thick and thin sections cool at different rates. Large differences cause shrinkage porosity in heavy sections and hard, chilled edges in thin ones. Where thickness must change, make the transition gradual.
2. Use generous fillets and radii
Sharp internal corners concentrate stress in service and create hot spots during solidification. Generous fillets improve metal flow, reduce cracking risk and make the pattern stronger.
3. Avoid isolated heavy sections
A boss or pad that is much thicker than the walls around it becomes a hot spot that is hard to feed. Core it out, connect it to a feeder path, or reduce it where function allows.
4. Allow draft on vertical faces
Draft lets the pattern draw cleanly from the sand. Your foundry will advise the angle for the moulding process; adding draft early avoids redesign later.
5. Think about the parting line
Where the mould splits affects flash, mismatch and tolerances across the joint. Keep critical dimensions on one side of the parting line when you can, and ask the foundry where it would place it.
6. Design cores for support and venting
Internal passages need cores, which need core prints to locate them and a path for gases to escape. Very small or long, unsupported cores raise cost and scrap. Machining a small hole is often cheaper than coring it.
7. Specify machining allowance and datums deliberately
Agree machining allowances and casting datums with the foundry. When the same supplier casts and machines, as we do, datums can be designed into the pattern for repeatable fixturing.
8. Involve the foundry early
A short manufacturability review before tooling can combine parts, remove cores, improve yield and prevent defects. Our in-house pattern design team reviews every new drawing with these points in mind.
Use general casting tolerances where you can. Reference a recognised casting tolerance standard such as ISO 8062 for non-critical dimensions, and tighten only the features that matter. Over-tight tolerances on as-cast faces add cost without adding function.
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