Uniform wall thickness in injection molding: what is the rule?

Keep walls uniform, typically 1–4 mm, and change thickness gradually. Thick sections shrink and leave sink marks and voids.

Draft in injection molding: what is the rule?

At least 1° per side on vertical walls; 2–3° where the surface is textured. Without draft the part sticks in the mold and scrapes.

Ribs in injection molding: what is the rule?

Rib thickness about 50–60% of the wall, height up to about 3× the wall. Thick ribs show as sink marks on the other side.

Radii in injection molding: what is the rule?

Inside radius at least about 0.5× the wall thickness. Sharp corners concentrate stress and block flow.

Undercuts in injection molding: what is the rule?

Avoid undercuts, or accept side actions that add mold cost. Undercuts need moving parts in the mold.

Injection molding: design rules

Forcing molten plastic into a steel or aluminium mold; cheap per part at volume, expensive to start.

Typical tolerance

Typical: about ±0.1–0.25 mm on small dimensions, wider for large parts; depends on the plastic.

Files to send

STEP of the part, with notes on material, colour, texture and critical dimensions.

Common materials

ABS, polypropylene, polycarbonate, nylon, POM, TPU and many more.

The rules, with reasons

Uniform wall thickness. Keep walls uniform, typically 1–4 mm, and change thickness gradually. Why: Thick sections shrink and leave sink marks and voids.
Draft. At least 1° per side on vertical walls; 2–3° where the surface is textured. Why: Without draft the part sticks in the mold and scrapes.
Ribs. Rib thickness about 50–60% of the wall, height up to about 3× the wall. Why: Thick ribs show as sink marks on the other side.
Radii. Inside radius at least about 0.5× the wall thickness. Why: Sharp corners concentrate stress and block flow.
Undercuts. Avoid undercuts, or accept side actions that add mold cost. Why: Undercuts need moving parts in the mold.
Tooling and volume. Tooling usually costs thousands to tens of thousands; use 3D printing or CNC until the design is stable. Why: Changing a steel mold is expensive.

Before you ask for a quote

Material and colour
Annual and first-order quantity
STEP file
Texture and tolerance notes
Tool type (aluminium for hundreds, steel for many thousands)
Certifications (food, medical, UL)

Common mistakes

Uneven wall thickness
No draft
Ordering a mold before the design is proven
Ignoring where the gate and ejector marks go

Agents and scripts can check a design automatically with the check_design MCP tool or POST /api/design-rules. How to connect.

Request a quote Find manufacturers

Typical rules of thumb, not a guarantee. The manufacturer's own design guidelines override them: ask for them and check your design against both before ordering.