DFM for Injection Molding: 12 Rules to Reduce Cost and Lead Time
Good DFM (Design for Manufacturing) is the difference between a smooth project and endless revisions. Here are 12 injection molding DFM rules that most sourcing teams use to reduce tooling complexity, prevent defects, and shorten lead time.
1) Keep wall thickness uniform
Uneven thickness leads to sink marks and warpage. If thicker areas are needed, use ribs and gussets instead of solid plastic.
2) Add draft on all vertical walls
Draft helps ejection and protects surface finish.
- Smooth surface: 1°–2°
- Textured surface: 2°–5° (or more depending on texture depth)
3) Use ribs for strength, not thick walls
A common guideline:
- Rib thickness = 40%–60% of nominal wall
- Rib height ≤ 3× wall thickness
4) Avoid sharp corners
Sharp corners create stress concentration and poor flow. Add fillets:
- Inside radius: 0.5× to 1× wall thickness
- Outside radius = inside radius + wall thickness
5) Design bosses carefully
Bosses are often the source of sinks and cracks.
- Boss wall thickness should be reduced
- Support with ribs
- Keep boss away from outer cosmetic surfaces if possible
6) Simplify undercuts
Undercuts require slides/lifters, increasing mold cost and risk.
If possible:
- Replace undercuts with snap features
- Modify geometry to avoid side actions
7) Choose a smart parting line
A clean parting line improves appearance and reduces flash risk. It also affects ejection and gating.
8) Gate location affects quality
Gate should allow balanced filling and reduce weld lines.
For cosmetic surfaces, consider hidden gates (sub-gate/tunnel gate).
9) Add proper venting areas
Poor venting causes burns, short shots, and inconsistent filling. Mold design should include vents at end-of-fill zones.
10) Be realistic with tolerance
Over-tight tolerances increase cost and rejection rate. Identify only the truly critical dimensions.
11) Consider shrinkage early
Different resins shrink differently, and glass-filled materials behave differently along flow direction. Confirm material and shrinkage before finalizing critical fits.
12) Plan for assembly
If the part must be assembled:
- Confirm screw type / insert type
- Confirm snap engagement and fatigue life
- Avoid interference due to warpage
A simple DFM workflow that saves time
- Share 3D model + usage environment
- Supplier provides DFM report + mold flow suggestions (if needed)
- Confirm gate/ejection/parting line
- Freeze design → start tooling
If you want, send your CAD file and we can provide a practical DFM review with cost-saving recommendations.