For the same part, how well it is designed directly determines whether it can be mass-produced smoothly, how high the yield is, and how easy the mold is to build. Injection parts follow a few DFM (Design for Manufacturing) rules; break them and at best you get sink and distortion, at worst the part won't fill or won't eject. This article spells out the most commonly tripped points so your drawing hits fewer landmines before tooling.
01Keep wall thickness uniform (the most important rule)
Uneven wall thickness is an injection part's worst enemy. Thick areas cool slowly, leaving sink marks on the surface and internal stress; thin areas may not fill. The design principle is simple: keep wall thickness as uniform as possible; when you need a solid feel or strength, use ribs rather than thickening the whole block.
02Always provide draft angle
For a part to eject from the mold, faces perpendicular to the draw direction must have draft, commonly 0.5–2°; a textured surface needs a bit more. Without draft, at best you get scratching and stress whitening on ejection, at worst the part won't eject at all.
03Use ribs, not solid thickening
For strength, add ribs, don't thicken the wall. Rib thickness is best at 50–60% of the main wall — a rib that is too thick will likewise leave a sink mark on the opposite face, defeating the purpose.
04Fillet the corners
Round both inside and outside corners where possible. Fillets reduce stress concentration and aid plastic flow; sharp corners are often the origin of cracks and flow marks.
05Design habits that avoid sink and distortion
- Uniform wall thickness (worth repeating — it's fundamental)
- Core out thick areas and switch to ribs
- Symmetrical design to reduce warpage from uneven cooling
- Mind the gate position; it affects flow direction and weld-line location
- Keep ejection locations off cosmetic surfaces
06Bring DFM in early
A good OEM does a DFM review before tooling, flagging these potential issues at the drawing stage rather than discovering them when T1 parts come out. Reviewing one step earlier often saves a round of mold rework in time and cost.
If the material isn't settled yet, first see the Plastic Injection Material Selection Guide; to understand the full process and defect remedies, see the Plastic Injection Molding Complete Process Guide and Injection Molding Defects Troubleshooting Guide. To have a drawing reviewed for DFM, send it via online inquiry and we'll check it before tooling.