Injection moulding: how it works, costs and design rules

Molten plastic is injected into a steel or aluminium mould. After the tooling is paid for, parts cost cents and come out in seconds. It is the standard for plastic parts in the thousands to millions.

Typical tolerance

±0.1 mm for small parts, depends on material shrinkage

Quantities

1,000 to millions

Lead time

4–10 weeks for the mould, then days

Materials

ABS, PC, PP, PA, POM, PBT, TPE and many more

How it works

Plastic pellets are melted and injected at high pressure into a closed mould. The part cools, the mould opens and ejector pins push it out. Cycle times are typically 15–60 seconds.

Benefits

• Lowest cost per part at volume.
• Excellent repeatability.
• Fine details, textures and colours in one step.

Limitations

• Mould costs from a few thousand euros (aluminium) to tens of thousands (hardened steel).
• Design changes after the mould is made are expensive.
• Strict rules for wall thickness and draft.

What drives the cost

• Mould complexity: slides and lifters for undercuts, number of cavities.
• Cycle time, driven by the thickest wall.
• Material price.

Design rules

• Keep wall thickness uniform, typically 1.5–3 mm.
• Add draft of at least 1° on vertical faces (more for textured faces).
• Make ribs 50–60% of the wall thickness to avoid sink marks.
• Avoid undercuts or plan for slides.

Preparing your STEP file

• Send a clean solid with draft already applied.
• State the material grade and texture (for example VDI 3400 grades) on the drawing.
• Many moulders 3D print or machine prototypes first from the same STEP.

See also the full STEP export checklist and the comparison of all processes.