Design for Manufacturing (DFM) Guide: Optimizing Draft Angles & Wall Thickness

Designing metal components for die casting requires a thorough appreciation of how molten metal solidifies inside a rigid steel cavity. Adhering to fundamental Design for Manufacturability (DFM) guidelines upfront reduces tool modification loops, shortens lead times, and prevents costly quality rejections.

1. Wall Thickness: Consistency is King

The number one cause of internal shrinkage porosity in aluminium die castings is isolated thick sections. As the perimeter cools rapidly against the steel tool, the molten core of an oversized boss shrinks by 6% in volume without access to liquid feed metal, resulting in internal voids.

• **Rule of Thumb:** Maintain uniform wall thickness across 85%+ of the part.
• **Coring Out:** Core out heavy bosses and structural pillars from the non-aesthetic side.
• **Transitions:** Use gradual tapers (blended at a 1:3 ratio) rather than abrupt 90-degree step changes.

2. Draft Angles for Clean Demolding

Because aluminium shrinks onto the die cores as it solidifies, sufficient draft angle is essential to prevent drag marks, galling, and ejector pin punch-through.

• **Inside Walls & Cores:** Minimum 1.0° to 2.0° (since the metal contracts *onto* these steel features).
• **Outside Walls:** Minimum 0.5° to 1.0° (as metal shrinks *away* from outer cavity walls).
• **Zinc Alloys:** Zinc contracts less and exhibits higher lubricity; draft angles as low as 0.25° to 0.5° are often acceptable.

3. Radii & Fillets: Eliminating Stress Concentrations

Sharp internal corners create severe stress concentrations both in the cast part under load and in the tool steel during high-velocity thermal cycling, triggering premature die heat checking.

• **Minimum Internal Radius:** $R \ge 1.0$ mm (ideally equal to 50% of the adjacent wall thickness).
• **External Corners:** Radiused at $R \ge 1.5$ mm to ensure smooth molten metal wavefront propagation without oxide skin folding.