Design for Injection Molding: 10 Rules That Prevent Expensive Tool Changes

Changing a part in CAD costs minutes. Changing it after the steel is cut costs weeks and a tooling invoice. Most mold modifications trace back to a handful of design issues that a DFM review would have caught. These ten rules cover the most common ones. Values are typical rules of thumb; resin data sheets and your molder’s experience take precedence.

The 10 rules

1. Keep walls uniform. Thick sections cool slowly, causing sink marks, voids and warp. Most housings use walls of roughly 1–4 mm depending on resin and size; transition gradually where thickness must change.
2. Add draft to every wall in the pull direction. About 1° is a common starting point; textured surfaces need more — a frequently used rule is about 1° extra per 0.025 mm of texture depth.
3. Make ribs thinner than the wall. Ribs around 50–60% of the adjoining wall thickness avoid sink on the cosmetic side; use more ribs rather than taller ones.
4. Design bosses correctly. Core them out, keep their walls thinner than the main wall, connect them to walls with ribs, and follow the screw manufacturer’s hole sizes for plastics. Cracked bosses are a classic failure.
5. Round the corners. Sharp internal corners concentrate stress and disturb flow. Inside radius at least about half the wall thickness, outside radius = inside radius + wall.
6. Avoid undercuts — or design them deliberately. Each undercut needs a slide or lifter, adding cost and maintenance. Pass-through cores, snap features on the parting line or relocated holes often remove them.
7. Agree the gate location and expect weld lines. Where flow fronts meet, weld lines form — visible and weaker. Keep them away from cosmetic and highly loaded areas.
8. Use realistic tolerances. Plastics shrink and move with temperature and moisture. Hold tight tolerances only on critical features, and expect a tuning loop after T1 samples.
9. Plan textures and cosmetics early. Define cosmetic surfaces, texture standard (e.g. VDI or MT references) and gloss; textures need draft and are hard to add later.
10. Design snap fits within the material’s strain limits. Use the resin supplier’s allowable strain data and add lead-in angles; brittle grades and glass-filled resins tolerate little deflection.

Before you release for tooling

• Ask for a mold-flow or DFM report covering fill, sink, warp and gate location
• Confirm the exact resin grade, color and flame rating (see enclosure plastics)
• Mark cosmetic surfaces and where ejector pin marks are acceptable
• Decide which dimensions are critical and how they will be measured on T1 samples
• Agree how many tool modifications and sample rounds the quotation includes

Design decisions drive both tooling and part cost — see injection mold cost explained. If your volume is still uncertain, consider bridge tooling. Or send your STEP file for a DFM review before you commit to steel.