Die Casting Design Guide
Design die cast parts that fill right, eject cleanly, and last
Die casting produces complex near-net-shape parts at high volumes, but the process imposes strict design constraints. Poor draft angles, uneven wall thickness, or bad gate placement can cause porosity, sink marks, hot tears, and short shots. This guide gives you the rules that matter most.
Core Design Principles
Uniform Wall Thickness
Uneven walls cause differential cooling — leading to porosity, warping, and sink marks. Keep walls as uniform as possible.
Draft Everything
Every surface parallel to the die pull direction needs draft. Zero draft = stuck parts = damaged tooling.
Flow-Friendly Geometry
Molten metal must fill every corner of the cavity. Avoid thin sections that freeze before filling.
Design for Ejection
Parts must release cleanly from both die halves. Undercuts require slides or complex tooling.
DFM Rules & Guidelines
Wall Thickness
Walls that are too thin may not fill; walls too thick cause porosity and long cycle times.
Aluminum: 1.5–4.0 mm (ideal 2.0–3.0 mm). Zinc: 0.75–2.5 mm. Keep variation under 2:1 ratio across the part.
Walls thinner than 1.0 mm for aluminum. Thick sections (> 6 mm) without coring.
Draft Angles
Draft allows the part to eject from the die without scraping or sticking. More draft = longer tool life.
1–3° on external surfaces. 2–5° on internal surfaces (core pins). 0.5° minimum with EDM-textured surfaces.
Zero draft on any surface. Textured surfaces with insufficient draft (texture depth × 1.5° minimum).
Ribs & Bosses
Ribs add strength without increasing wall thickness. Bosses provide mounting points. Both must follow proportion rules.
Rib thickness: 50–70% of adjoining wall. Rib height: max 5× thickness. Boss OD: 2–3× hole ID. Fillet all rib bases (R ≥ 0.5 mm).
Ribs thicker than the wall they adjoin. Bosses without adequate draft or fillet radii.
Parting Line & Undercuts
The parting line determines where the die splits. Undercuts perpendicular to the pull direction require slides, adding tooling cost.
Keep parting line on a flat plane if possible. Minimize number of slides. Design undercuts that can use standard slide mechanisms.
Complex 3D parting lines. More than 2–3 slides per die. Undercuts that prevent core pull.
Fillets & Radii
Sharp corners are stress concentrators and impede metal flow. Generous radii improve fill and part life.
Internal fillet radius ≥ 1.0 mm (larger is better). External radius ≥ 0.5 mm. Transition radii at wall thickness changes.
Sharp internal corners. Fillets less than 0.5 mm on stressed features.
Flatness & Tolerances
Die castings warp during cooling. As-cast tolerances are wider than machined tolerances.
Linear: ±0.1 mm for first 25 mm, +0.04 mm per additional 25 mm. Flatness: 0.1 mm per 25 mm. Machine only critical surfaces.
Expecting CNC-grade tolerances from as-cast features. Large flat surfaces without stiffening ribs.
⚠️ Common Design Mistakes
- ⚠ Designing thick bosses without coring — creates shrink porosity that weakens the part under load.
- ⚠ Forgetting draft on textured surfaces — the texture acts like a barb, locking the part in the die.
- ⚠ Placing gates at thin sections — the metal freezes before it reaches the far end of the cavity.
- ⚠ Specifying pressure-tight castings without designing for it — porosity is inherent in die casting; plan for impregnation if needed.
- ⚠ Not accounting for die thermal expansion — die steel grows during production, affecting dimensions on long runs.
💡 Pro Tips
- ▸ Discuss gate location with your die caster early — it affects surface finish, porosity distribution, and trim cost.
- ▸ Use cored holes instead of drilling — saves machining cost and is achievable with core pins down to ~Ø3 mm.
- ▸ Add overflow wells opposite the gate — they capture cold-flow metal and improve internal quality.
- ▸ Vacuum-assist die casting can achieve much lower porosity for structural or pressure-tight applications.
- ▸ If you need both die cast and machined features, provide a single drawing with clear "as-cast" vs. "machined" callouts.
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