10 Injection Molding Design Rules That Prevent Costly Tooling Revisions

Injection molding tooling is expensive — $10,000 to $100,000+ depending on part complexity. Every design revision after steel is cut costs time and money. The best way to protect your investment is to get the design right before the mold is built. Here are ten rules that experienced molders wish every engineer followed.

1. Keep Wall Thickness Uniform

This is rule number one for a reason. Thick sections cool slower than thin sections, and the resulting differential shrinkage causes warping, sink marks, and internal voids. Target 1.5–3 mm for most engineering plastics. Where transitions are necessary, taper gradually at a 3:1 ratio — never use abrupt steps.

2. Apply Draft to Every Vertical Surface

Draft — a slight taper on surfaces parallel to the mold opening — lets the part release cleanly during ejection. Without it, parts stick, scuff, or crack. Use 1° minimum per side; textured surfaces need 1.5–3° depending on texture depth. It's a trivial geometry change that prevents major production issues.

3. Use Ribs, Not Thick Walls, for Stiffness

When you need rigidity, add ribs instead of increasing wall thickness. Ribs provide structural performance at a fraction of the material and cycle time cost. Design rules: rib thickness = 50–70% of the adjoining wall, height ≤ 3× wall thickness, minimum 0.5° draft on rib sides.

4. Round All Internal Corners

Sharp corners concentrate stress, restrict flow, and create hot spots during cooling. Add internal radii of at least 0.5× wall thickness (0.75× is better). External radii should equal the internal radius plus wall thickness. This simple practice improves both moldability and part durability.

5. Place Gates at the Thickest Section

The gate determines how plastic fills the cavity. Placing it at the thickest section ensures material flows from thick to thin, maintaining positive packing pressure throughout. Keep gates away from cosmetic surfaces and high-stress areas. Each gate creates a witness mark and a potential weld line — plan accordingly.

6. Eliminate Undercuts Where Possible

Undercuts require side actions, lifters, or collapsing cores in the mold — all of which add cost and maintenance complexity. Redesign features to pull straight from the mold whenever possible. When undercuts are unavoidable, keep them simple and accessible for reliable mold action over production volumes.

7. Design for Shrinkage

All plastics shrink during cooling (0.4–2% depending on material). Semi-crystalline materials like nylon and POM shrink more than amorphous materials like ABS and polycarbonate. Non-uniform shrinkage = warping. The remedy: consistent walls, adequate packing, and proper shrinkage compensation in the mold steel.

8. Size Bosses Correctly

Bosses for screw fastening should have an OD of 2–2.5× the screw diameter, with wall thickness at 50–75% of the nominal part wall. Connect bosses to adjacent walls via ribs rather than thickening the boss. Oversized freestanding bosses create visible sink marks on the opposite surface.

9. Plan Ejection from Day One

Every molded part must be pushed out of the tool without damage. That means designing ejector pin pads on non-cosmetic surfaces, ensuring adequate draft on deep cores, and considering stripper plates for large flat parts. Addressing ejection during part design avoids expensive mold modifications after first trials.

10. Prevent Jetting Defects

Jetting happens when plastic squirts through the gate into an open cavity as a stream instead of a spreading flow front, leaving snake-like lines on the surface. The fix: orient gates so material hits a wall or pin immediately upon entry. Wider gates, slower initial injection speeds, and higher mold temperatures also promote laminar flow and eliminate jetting.

The Bottom Line

These rules aren't theoretical — they're the distilled experience of thousands of mold trials. Apply them during CAD design and you'll compress your development timeline, reduce tooling iterations, and reach production faster. If you're outsourcing injection molding overseas, having this knowledge also makes you a better buyer — you'll spot potential issues before they become expensive problems.