A die casting is shaped by how molten metal flows and how it solidifies. Designs that respect both are cheap and reliable to produce. Designs that ignore them produce porosity, distortion and scrap that no amount of process tuning fully fixes.
1. Keep wall thickness uniform
This is the single most important rule. Thick sections solidify last and shrink most, and because the surrounding thin material has already frozen, there is no liquid metal left to feed that shrinkage. The result is internal porosity or a sink mark on the surface.
Aim for a consistent wall throughout. Where a transition is unavoidable, taper it gradually rather than stepping abruptly.
2. Use ribs instead of thickness
If a part needs to be stiffer, the instinct is to make it thicker. In casting, that is usually the wrong move. Ribs give far more stiffness per unit of metal and solidify without creating a hot spot.
- Rib thickness: roughly 60 to 80 percent of the wall it sits on
- Include draft on rib sides
- Fillet the rib root, never leave a sharp junction
3. Always add draft
Every surface parallel to the direction of die opening needs draft, or the part will drag and scuff on ejection. As a general guide, allow around 1 to 2 degrees on external surfaces and 2 to 3 degrees on internal walls, with deeper features requiring more.
If a surface genuinely cannot be drafted, it can be machined afterwards, but that adds cost, so it is worth designing around.
4. Radius every corner
Sharp internal corners concentrate stress in the finished part and cause turbulence and hot spots during filling. A generous fillet radius improves metal flow, extends die life and strengthens the casting. Sharp external corners are equally difficult to fill.
5. Design bosses properly
A solid boss is a thick section by another name. Core out the centre so the boss becomes a uniform-wall tube, blend it to the wall with a fillet, and support it with ribs if it needs to be tall.
6. Think about the parting line early
The parting line is where the two die halves meet, and a small amount of flash is unavoidable there. Place it where flash will not affect function or appearance, and away from sealing faces. Deciding this at design stage, rather than letting the tool maker decide later, prevents unpleasant surprises.
7. Specify realistic tolerances
Over-specifying tolerances is one of the most common and expensive mistakes on a casting drawing. Every tight tolerance either requires a secondary machining operation or increases inspection and scrap.
| Feature | Realistic approach |
|---|---|
| General as-cast dimensions | Standard casting tolerance, no machining |
| Critical mating faces | Machine after casting |
| Bores and bearing seats | Machine, to ± 0.05 mm where required |
| Across the parting line | Allow additional tolerance for die closure |
| Cosmetic surfaces | Specify a finish standard, not a dimension |
Identify the handful of dimensions that are genuinely critical to function, and let the rest run to standard casting tolerance. It reduces cost, speeds up inspection, and focuses attention where it actually matters.
8. Allow for machining stock where needed
Where a face will be machined, leave enough material to clean up reliably, but no more than necessary. Too little and the machined surface may not clean up across the whole face; too much and you are paying to cut away metal you paid to cast.
Send it early, not late
The cheapest time to change a casting design is before the die is cut. We review drawings and 3D data as part of quotation and will flag anything that will cause trouble in production, uneven sections, missing draft, unrealistic tolerances, awkward parting lines.
You can see the tolerances and wall thicknesses we work to on our capabilities page.
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