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Mold Concerns
Draft—providing angles or tapers on product features such as walls, ribs, posts, and bosses that lie parallel to the direction of release from the mold—eases part ejection.
How a specific feature is formed in the mold determines the type of draft needed. Features formed by blind holes or pockets—such as most bosses, ribs and posts—should taper thinner as they extend into the mold. Surfaces formed by slides may not need draft if the steel separates from the surface before ejection. Other rules of thumb for designing draft include:
- Draft all surfaces parallel to the direction of steel separation;
- Angle walls and other features that are formed in both mold halves to facilitate ejection and maintain uniform wall thickness;
- Use the standard one degree of draft plus one additional degree of draft for every 0.001 inch of texture depth as a rule of thumb; and
- Use a draft angle of at least one-half degree for most materials. Design permitting; use one degree of draft for easy part ejection. Apec resins typically require five degrees of draft.
Generally, polished mold surfaces require less draft than surfaces with machined finishes. An exception is thermoplastic polyurethane resin, which tends to eject easier from frosted mold surfaces.
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