Optimize Wall Thickness
Keep it uniform: Uneven wall thickness is the leading cause of defects. Thick sections cool slower than thin ones, leading to sink marks (surface depressions) and internal voids.
Avoid abrupt changes: Use gradual transitions between different wall thicknesses to ensure smooth plastic flow and consistent cooling.
Incorporate Draft Angles
Facilitate ejection: Draft angles are the slight tapers applied to vertical faces of the part. Without them, the part creates a vacuum and drags against the mold steel during ejection, causing scuff marks, stress whitening, or even tearing.
General rule: A minimum of 1° to 2° draft is recommended for most surfaces, with more required for textured surfaces.
Design Ribs and Bosses Carefully
Rib thickness: Ribs are used to add strength without increasing overall wall thickness. To prevent sink marks on the opposite cosmetic surface, the rib's base thickness should be 50% to 60% of the nominal wall thickness.
Boss design: Screw bosses should be designed with gussets (triangular supports) for strength and must have adequate draft on both the inner and outer diameters.
Use Radii (Rounding) Instead of Sharp Corners
Reduce stress: Sharp internal corners create stress concentration points, making the part prone to cracking. They also restrict plastic flow.
Improve mold life: Sharp corners in the mold cavity act as stress risers for the steel, leading to premature cracking or chipping of the mold. Always use generous radii (fillets).
Manage Undercuts
Simplify the mold: An undercut is any feature that prevents the part from being ejected straight out of the mold (e.g., side holes, recesses). These require complex, expensive mechanisms like side-actions (sliders) or lifters.
Design alternative: Try to redesign the part to eliminate undercuts if possible, or "shut off" areas to allow the steel to touch, simplifying the mold structure.
Account for Shrinkage and Tolerances
Material selection: Different plastics shrink at different rates (e.g., ABS shrinks less than Polypropylene). The mold must be machined oversized to compensate for this.
Realistic tolerances: Do not specify tight tolerances (±0.05mm) unless absolutely necessary for function. Tight tolerances increase mold cost and production difficulty.
Plan for Gate and Ejector Locations
Gate location: Discuss with your mold maker where the plastic will enter the part. The gate location affects the weld lines (where flow fronts meet) and aesthetic appearance.
Ejector pins: Ensure there are flat surfaces or ribs where ejector pins can push the part out without leaving visible marks on cosmetic surfaces.
