Proper draft angles and fillets are two of the most effective design decisions for extending the service life of aluminum die casting molds, especially those made from H13 tool steel. These features reduce mechanical wear, thermal stress, and ejection force—three primary causes of premature mold failure.
Draft Angles: The Key to Smooth Ejection
Aluminum shrinks as it solidifies. External walls shrink away from the cavity, while internal features shrink tightly onto cores. Without adequate draft, the part grips the steel, creating high friction and drag marks that accelerate wear on the mold surface and ejector system.
Industry guidelines recommend:
- 1–2° on external (cavity) surfaces
- 2–3° (or roughly twice the external value) on internal walls and cores
- 3° or more for deep ribs, bosses, textured surfaces, or deep pockets
Adequate draft dramatically lowers ejection force, minimizes surface abrasion, and helps the mold maintain dimensional accuracy over tens of thousands of shots.
Fillets: Eliminating Stress Concentrations
Sharp internal corners act as stress risers. During every thermal cycle, these points experience extreme temperature gradients that initiate heat checking—the network of fine cracks that eventually forces mold repair or replacement.
Best practice is to apply generous fillets:
- At wall junctions: radius equal to 1–1.25 times the thinnest wall thickness
- Minimum internal radius of approximately 0.5–1.5 mm (larger for deeper features)
- Smooth transitions at the base of ribs and bosses
Fillets also improve molten metal flow, reduce turbulence and erosion, and distribute thermal load more evenly across the cavity surface.
When draft angles and fillets are designed together as part of a thoughtful DFM process, mold life can be extended significantly—often moving a standard H13 tool from the lower end of the 80,000–150,000-shot range toward the higher end or beyond. The result is lower tooling cost per part, fewer production interruptions, and more predictable long-term performance.