Rotational Moulding Machine Troubleshooting: Causes & Fixes

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Problems on this page:

Bubbles in the part wall Parts warping Parts under-cured or over-cured Uneven wall thickness Part sticking in the mould Mould vent problems Arm rotation faults Oven not reaching temperature Flash at the parting line

Bubbles in the part wall

Hits OEE: Quality

Symptoms: Cut sections show bubbles in the wall, and impact strength is low.

Part not cooked long enough

Extend the oven time or check the inside air temperature reached in the cycle.

Moisture in the powder

Store powder dry and check it before use.

Powder particle size wrong

Check the powder against its specification.

Mould vent blocked

Clean the vent so air can escape during heating.

Prevention: Wall section checked on the first part of each run.

Parts warping

Hits OEE: Quality

Symptoms: Parts come out bowed or twisted.

Cooling uneven

Check the fans and water sprays in the cooling station.

Part released too early

Leave the part to cool longer before demoulding.

Part pulling away from the mould wall unevenly

Check release agent coverage.

Mould design with large flat areas

Add support fixtures for cooling.

Prevention: Part dimensions checked on samples.

Parts under-cured or over-cured

Hits OEE: Quality

Symptoms: Parts are weak, or yellow and brittle on the inside.

Oven temperature or time wrong

Check oven settings against the recipe.

Inside air temperature not monitored

Use internal air temperature monitoring where available.

Mould wall thicker than the last one

Set oven times per mould, not only per product.

Burner output varying

Have the burner and its gas supply checked.

Prevention: Impact test on samples.

Uneven wall thickness

Hits OEE: Quality

Symptoms: Some areas of the part are thin and others thick.

Rotation ratio wrong for the part shape

Set the major and minor arm speeds for the mould shape.

Mould heating unevenly

Check mould wall thickness and use shielding or vents where some areas heat too fast.

Powder not flowing freely

Check the powder flow properties and particle size.

Mould mounted off centre on the arm

Check the mould mounting position on the spider.

Prevention: Wall thickness measured on samples.

Part sticking in the mould

Hits OEE: Availability

Symptoms: Parts are hard to remove from the mould.

Release agent worn off

Reapply release agent as the supplier specifies.

Mould surface damaged

Have scratches and dents in the mould surface repaired.

Undercuts in the mould

Review the part and mould design for draft angles.

Part not cooled enough before demoulding

Extend cooling so the part shrinks away from the mould.

Prevention: Release agent applied on a schedule.

Mould vent problems

Hits OEE: Quality

Symptoms: Parts show blowholes near the vent or powder escapes.

Vent blocked with plastic

Clean the vent at every mould change.

Vent too small for the mould volume

Check the vent size with the mould maker.

Vent packing missing

Pack the vent with the material that keeps powder in and lets air out.

Vent tube damaged

Replace damaged vent tubes.

Prevention: Vents checked at each mould change.

Arm rotation faults

Hits OEE: Availability

Symptoms: An arm stops rotating or rotates unevenly.

Drive chain worn or slack

Check chain tension and wear and lubricate as the maker sets.

Rotary joint worn

Have the rotary joint inspected for wear and leaks.

Drive or motor fault

Read the drive fault log and check the motor.

Arm bearings worn

Check the arm bearings for play and noise.

Prevention: Arm drives on the maintenance plan.

Oven not reaching temperature

Hits OEE: Availability

Symptoms: The oven heats slowly.

Burner fault

Have the burner serviced and its flame checked.

Oven door seals leaking

Replace worn door seals.

Recirculation fan fault

Check the fan and its drive, since heat must reach every side of the mould.

Gas supply pressure low

Check the gas supply pressure at the burner.

Prevention: Oven temperature logged; burner work done by qualified gas engineers.

Flash at the parting line

Hits OEE: Quality

Symptoms: Material leaks out at the mould parting line.

Mould clamps loose

Tighten and check the mould clamps before each cycle.

Parting line worn

Have the mould parting line repaired.

Mould distorted by heat

Check the parting line flatness.

Too much powder for the mould

Check the shot weight against the part weight.

Prevention: Parting lines inspected.

Guidance only. Always follow lockout/tagout and your site's safe-work procedures, and verify against OEM manuals before acting.

Related equipment guides: blow moulding machine · injection moulding · granulator · extruder · industrial oven

Stop the same fault coming back

Recurring rotational moulding machine stops usually trace to a cause you cannot see by hand. The partner we recommend is Fabrico: it tracks OEE and downtime live from PLCs, IoT sensors or AI cameras; where cameras are fitted, the video of a stop sits next to its machine data so the cause can be reviewed; and a technician opens the work order from the stop in the same system. Data is hosted in the AWS EU region, and Fabrico and the company behind it hold ISO/IEC 27001, ISO/IEC 20000-1 and ISO 9001.

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Methods that cut recurring stops: the six big losses · root cause analysis · preventive vs predictive maintenance · TPM · SMED & changeover