Molding design rules that protect the tool + the part.
These injection molding design guidelines are the rules of thumb our engineers use when they review your part before tooling is cut. Limits vary with resin, part size and texture, and we confirm them for yours at quote.
Design breaks a rule?
Send it anyway. Undercuts, inserts and thick sections aren’t disqualifiers. Our engineers review every part before tooling starts, flag what drives tool cost, and tell you what a side action or lifter would add.
Quality + Compliance
AS9100D Compliant · ISO 9001:2015 · ISO 13485 Compliant · ITAR Registered · NIST 800-171 Compliant
Walls + draft
Where most molding defects start.
- Wall thickness
- 0.040" to 0.150" for most resins.
- Uniform walls
- Keep variation within about 25%.
- Transitions
- Taper thick to thin over 3x the change.
- Draft
- 1° to 2° minimum on pulled faces.
- Textured faces
- Add about 1° per 0.001" of texture.
- Inside corners
- Radius at least 0.5x the wall.
- Outside corners
- Inside radius plus the wall.
Ribs, bosses + tooling
Where tool cost gets decided.
- Ribs
- 50% to 60% of the adjoining wall.
- Rib height
- 3x wall thickness or less.
- Bosses
- Wall about 60% of nominal, cored out.
- Undercuts
- Side actions or lifters add tool cost.
- Gates + ejectors
- Keep them off cosmetic faces.
- Knit lines
- Plan where flow fronts meet.
Why it matters
Thick sections cool slower than thin ones, which is where sink and warp come from. Missing draft drags parts on ejection and wears the tool. Every side action adds steel, cycle time and a chance for flash. The cheapest fix is the one made before the tool is cut.
Injection molding design questions
0.040″ to 0.150″ works for most resins. Keep walls as uniform as you can, within about 25%, and taper transitions over 3x the change. Thick sections are where sink and warp start.
Plan on 1° to 2° on every face that pulls from the mold, plus about 1° per 0.001″ of texture depth. Without draft, parts drag on ejection and the tool wears faster.
Yes. Undercuts need side actions or lifters, which add steel and cycle time. They aren’t disqualifiers. We tell you what each one adds to tool cost before the tool is cut.
Send it anyway. Our engineers review every part before tooling starts, flag what drives tool cost, and tell you what a change would save.
