Plastics, rubber and composites
Plastic injection moulding: what goes wrong and where to inspect
The dominant process for plastic parts, from small clips to large housings. Quality depends on the mould, the dried material and a stable process window.
01How the process works
Plastic granules are dried, melted by a rotating screw and injected into a closed steel mould. The melt is held under pressure while it cools, then the mould opens and ejectors push the part out. Cycles take seconds to minutes; multi-cavity moulds produce several parts per shot.
Most defects are explained by four factors: mould design and condition, material and drying, machine set-up, and the process window (temperatures, pressures, times). Dimensional stability also depends on conditioning, because many plastics such as polyamide change size as they absorb moisture.
- Material drying. Granules dried to the supplier's moisture limit.
- Plasticising. Screw melts and doses the material.
- Injection and hold. Melt fills the cavity; holding pressure packs it.
- Cooling and ejection. Part solidifies and is ejected.
- Post processing. Degating, inspection, packing.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Short shot | Low injection pressure, cold mould, blocked vents | Visual, weight check |
| Sink marks and voids | Thick sections, low hold pressure | Visual, sectioning |
| Warpage | Uneven cooling, fibre orientation | Fixture, CMM |
| Flash | Worn parting line, excess pressure | Visual |
| Weld lines | Flow fronts meeting, low temperature | Visual, strength test |
| Splay and silver streaks | Wet material | Visual, moisture analysis |
| Burn marks | Trapped air, overheating | Visual |
| Brittle parts | Degraded material, excess regrind | Impact or snap test |
03Where an inspector should look
Before production
- Mould approval and cavity identification
- Material grade, colour and regrind policy
- Dryer settings and moisture limit
- Process window documented
During production
- First off approval per cavity
- Process parameter monitoring
- Part weight trend
- Cavity balance
After production
- Dimensions after conditioning
- Visual against limit samples
- Functional and assembly checks
- Packaging to avoid deformation
04Standards and references
ISO 20457 (tolerances for moulded plastic parts), ISO 294 (test specimens), material datasheets for drying and processing.
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.
05How we support buyers of injection moulding parts
Frequently asked questions
How many parts should be measured at first article?
At least one part from every cavity of the mould, measured completely against the drawing after the conditioning time required by the material. Cavity numbers must be traceable on the parts.
Why do dimensions change after moulding?
Plastics continue to shrink while they cool and, for hygroscopic materials such as PA6 and PA66, change size when they absorb moisture. Measuring too early or without conditioning gives misleading results.