Injection Molding Design Guidelines for Production Parts

Short answer: most molding problems trace back to four decisions made long before a tool is cut — wall thickness, draft, rib design, and radii. Get these right in the model and the rest of the tool build gets easier.

Wall thickness

Aim for uniform wall thickness throughout the part. Thick sections cool more slowly than thin ones, and that mismatch is what causes sink marks, warp, and longer cycle times. Where a thickness transition is unavoidable, make it gradual rather than abrupt — a sharp step from thick to thin concentrates stress and often shows up as a visible sink line on the opposite face.

As a starting point, most engineering resins mold well in the 0.040–0.140 in (1–3.5 mm) range, but the right number for your part depends on the specific resin, the flow length from the gate, and any structural requirement. Confirm target wall thickness with your molder's DFM review rather than defaulting to a number from an unrelated project.

Draft angle

Every wall that runs parallel to the direction the tool opens needs draft — a slight taper that lets the part release from the cavity or core without dragging or scuffing. Without it, ejection either damages the part or overstresses the tool.

A reasonable starting point is 1–2° of draft per side for smooth surfaces, more for textured surfaces (texture depth adds to the effective draft needed — as a rule of thumb, add roughly 1° of draft for every 0.001 in of texture depth, though your molder should confirm this against the specific texture spec). Deeper features generally need more draft, not less, since a tall wall with the same low draft angle wears more on the tool over many cycles.

Ribs and bosses

Ribs add stiffness without adding a full wall thickness of material — but a rib that's too thick relative to the wall it's attached to will sink on the opposite (cosmetic) face as it cools. A common guideline is to keep rib thickness at roughly 50–60% of the adjoining nominal wall thickness, with a base radius rather than a sharp corner where the rib meets the wall.

Bosses (for screws or standoffs) carry the same sink risk, especially where a boss sits directly opposite a cosmetic surface. Coring out the boss to leave a more uniform wall thickness at its base is usually the fix.

Radii

Sharp internal corners are two problems in one: they concentrate stress in the finished part, and they create fill and cooling irregularities during molding. Adding a radius — even a modest one — at internal and external corners improves both structural performance and moldability. As a starting point, an internal radius of at least 25% of the wall thickness (and an external radius equal to the internal radius plus the wall thickness) keeps stress concentration and cosmetic sink risk in check.

Where this fits into quoting

None of these are rules to apply mechanically without context — the right answer depends on your resin, your tool budget, and what the part actually needs to do. That's what DFM review is for: we check wall thickness, draft, ribs, and radii against your specific material and geometry before tooling is quoted, and tell you the tradeoff wherever a design choice will affect cost, cycle time, or cosmetic risk.

See Injection Molding DFM for how we run that review, or request a quote to get feedback on your specific part.