Design for Die Casting — 8 Rules That Reduce Cost and Improve Quality

Kinaguiden is a precision aluminium die casting company based in Stockholm, Sweden, specialising in custom mould design, HPDC, re-engineering, and global logistics.

Most quality problems in aluminium die casting — porosity, sink marks, warping, dimensional rejects — can be traced back to the original part design. Die casting has specific requirements that differ from machined parts, plastic injection moulding, or gravity casting. Understanding these rules early in the design process avoids expensive mould modifications and delays later.

These eight design for manufacturing (DFM) principles are the most important to get right from the start.

1. Add Draft Angles to All Vertical Walls

Draft angle is the taper applied to surfaces parallel to the direction of mould opening. Without sufficient draft, the solidified part cannot be ejected cleanly — it sticks in the mould, causing damage to the part surface and the mould itself.

As a rule of thumb: exterior walls need a minimum of 1° draft, and interior walls (cores) need 2° or more because the shrinking aluminium grips them tightly. Complex textured surfaces may need 3–5°. Parts with zero-draft walls require side actions in the mould, which adds cost.

2. Keep Wall Thickness Uniform

Sudden changes in wall thickness cause the metal to cool at different rates, creating thermal stress, sink marks, and porosity at the transition. Design for consistent wall thickness throughout the part — ideally between 1.5 mm and 4 mm for HPDC. Where thickness must change, use gradual tapered transitions rather than abrupt steps.

3. Add Generous Fillets and Radii

Sharp internal corners are stress concentration points that can crack under load. They also cause turbulence in the metal flow during casting, increasing porosity risk. All internal corners should have a minimum radius of 1 mm — ideally 2–3 mm or more. External corners can be sharp if required for fit or aesthetics.

4. Position Ribs Correctly

Ribs are an excellent way to add stiffness without adding mass. However, ribs that are too thick create localised heat buildup and sink marks on the opposite surface. The standard rule is: rib thickness should be 60–70% of the adjoining wall thickness. Ribs should also have draft angles and generous root radii.

5. Minimise Undercuts

Undercuts are features that prevent the part from being pulled straight out of the mould. They require moving mould elements called side actions or lifters — which add significant tooling cost and complexity. Where possible, redesign features to eliminate undercuts. Holes that would create undercuts can often be replaced with open slots or relocated to a position accessible from the parting direction.

6. Design the Parting Line Early

The parting line is where the two halves of the mould meet. Its position affects how features are formed, where parting flash appears, and how the part is ejected. A poorly chosen parting line can make certain features impossible to achieve without side actions. Discuss the parting line with your die casting supplier early — it is easier to adjust the design than to change it after tooling is made.

7. Specify Tolerances Realistically

As-cast HPDC can hold tolerances of approximately ±0.1–0.2 mm on general dimensions. Tighter tolerances require CNC machining after casting, which adds cost. Review your drawing tolerances critically — apply tight tolerances only to features where they are functionally necessary, and use general tolerances (ISO 8062 or similar) for non-critical dimensions.

8. Consider Porosity for Pressure-Tight Parts

HPDC parts inherently contain some gas porosity from the high-speed injection process. For most structural and cosmetic applications this is acceptable. However, parts that must be pressure-tight — hydraulic valve bodies, pneumatic housings — require either low-porosity casting techniques (vacuum-assisted HPDC, squeeze casting) or post-cast impregnation to seal any porosity. Specify this requirement early so the process and mould design can accommodate it.