Plastics, rubber and composites
Rotational moulding: what goes wrong and where to inspect
Large, stress-free hollow parts such as water tanks, chemical containers and kayaks are made by tumbling polyethylene powder inside a heated, rotating mould.
01How the process works
Powder is loaded into a mould that rotates on two axes in an oven. The powder melts onto the mould wall, building a uniform layer, then the mould is cooled while still rotating and the part removed.
The cure state is critical: undercooked parts are porous and weak, overcooked parts are brittle. Internal air temperature monitoring and impact tests confirm the cycle.
- Powder loading. Measured charge placed in mould.
- Heating and rotation. Biaxial rotation in the oven.
- Cooling. Rotation continues while cooling.
- Demoulding. Part removed.
- Finishing and testing. Fittings, wall and impact tests.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Undercure and pinholes | Short cycle, low temperature | Visual inside, impact test |
| Overcure and brittleness | Excess heat | Impact test, colour |
| Wall thickness variation | Rotation ratio | Ultrasonic gauge |
| Warpage | Uneven cooling | Visual, measurement |
| Voids at corners | Mould design | Visual |
| Fitting leaks | Insert moulding | Leak test |
03Where an inspector should look
Before production
- Material grade and UV stabilisation
- Cycle and internal temperature
During production
- Cycle records
- Shot weight
After production
- Wall thickness
- Impact test sampling
- Leak test of tanks
04Standards and references
ARM low temperature impact test guidance, EN 13341 (static thermoplastic tanks).
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.