Design for Manufacturability (DFM) is the practice of designing products so they can be manufactured efficiently, reliably, and cost-effectively. Ignoring DFM principles is one of the most expensive mistakes product designers make.
Rule 1: Maintain uniform wall thickness. Uneven walls cause warping, sink marks, and longer cooling times in injection molding. Aim for 1.5-3mm for most plastics, with gradual transitions where thickness changes are necessary.
Rule 2: Add draft angles. Any surface perpendicular to the mold opening direction needs a minimum 1-2 degree draft angle. Without it, parts stick in the mold, causing damage and increasing cycle times.
Rule 3: Round all edges. Sharp internal corners are stress concentrators that weaken parts and cause flow problems in molding. Use a minimum radius of 0.5mm on internal edges and 1mm on external edges.
Rule 4: Minimize undercuts. Undercuts require side actions or lifters in the mold, significantly increasing tooling cost. Redesign features to pull straight out of the mold whenever possible.
Rule 5: Design appropriate rib-to-wall ratios. Ribs should be 50-60% of the nominal wall thickness to avoid sink marks on the opposite surface. Rib height should not exceed 3x the wall thickness without adding supporting gussets.
Rule 6: Consider parting line placement. The parting line (where mold halves meet) leaves a visible witness line. Place it on edges or non-cosmetic surfaces. Design the part geometry to create a natural parting line location.
Rule 7: Account for tolerances. Standard injection molding holds ±0.1mm for most features. If tighter tolerances are needed, secondary machining may be required — adding cost. Design to the loosest tolerance that meets functional requirements.
Rule 8: Design snap-fits correctly. Snap-fit features are cost-effective assembly methods but must be designed with proper cantilever geometry, strain limits (2% for ABS, 4% for PE), and assembly/disassembly angles.
Rule 9: Plan for material shrinkage. Different plastics shrink at different rates (0.4-2.0%). Your CAD model should account for shrinkage in critical dimensions. Your manufacturer will adjust the mold, but understanding shrinkage prevents design surprises.