Injection Moulding Troubleshooting: Causes & Fixes

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Problems on this page:

Short shot (incomplete fill)Flash (excess material at parting line)Sink marks Part sticking in the mould Warping / dimensional instability Burn marks (dieseling)Splay or silver streaks Visible or weak weld lines Black specks or contamination in parts

Short shot (incomplete fill)

Hits OEE: Quality

Symptoms: Part not fully formed, missing features at the flow ends.

Insufficient material/shot size

Increase shot size/cushion; check for screw slippage.

Low injection pressure/speed

Raise injection pressure and speed within window.

Cold melt or mould

Raise barrel/mould temperature to spec.

Blocked gate/vents

Clear the gate; improve venting to release trapped gas.

Prevention: Process validation, check non-return valve, maintain temperatures and venting.

Flash (excess material at parting line)

Hits OEE: Quality

Symptoms: Thin fins of plastic at the parting line or vents.

Clamp force too low

Increase clamp tonnage; verify it's adequate for projected area.

Over-packing / high pressure

Reduce injection/hold pressure and speed.

Worn or dirty mould faces

Clean and inspect parting line; repair damaged steel.

Melt too hot

Lower barrel temperature to reduce flow into the parting line.

Prevention: Clamp-force checks, process control, mould face maintenance.

Sink marks

Hits OEE: Quality

Symptoms: Depressions over thick sections/ribs.

Insufficient hold pressure/time

Increase pack/hold pressure and time.

Cooling too short

Extend cooling; check mould temperature uniformity.

Thick section design

Core out thick areas (design); for now optimise pack/cool.

Prevention: Optimised pack/hold, adequate cooling, design for uniform wall.

Part sticking in the mould

Hits OEE: Availability

Symptoms: Ejection problems, part deforms or stays in cavity, cycle stops.

Over-packing

Reduce injection/hold pressure.

Insufficient draft / mould damage

Inspect for undercuts/scoring; polish/repair; verify draft.

Inadequate release / cooling

Adjust cooling; apply approved release agent if permitted.

Prevention: Process within window, mould polishing/repair, correct ejection setup.

Warping / dimensional instability

Hits OEE: Quality

Symptoms: Parts bow or twist, out of tolerance after ejection.

Uneven cooling

Balance mould cooling; check water circuits and temperatures.

Residual stress (pack/hold)

Optimise pack/hold and cooling time; reduce over-packing.

Ejection too early / hot

Extend cooling; adjust ejection.

Prevention: Balanced cooling, validated pack/hold, adequate cooling time.

Burn marks (dieseling)

Hits OEE: Quality

Symptoms: Black or brown marks at the end of fill or next to the vents.

Trapped air compressed at the end of fill

Clean the vents or add venting, and slow the last stage of injection.

Melt too hot or held too long

Lower barrel temperatures within the material's window and check residence time.

Too much shear at the gate

Review gate size and injection speed.

Prevention: Vent cleaning in every mould maintenance.

Splay or silver streaks

Hits OEE: Quality

Symptoms: Silvery streaks on the part surface running in the flow direction.

Moisture in the resin

Dry the material as the resin datasheet says and check the dryer is working.

Overheated or degraded material

Reduce melt temperature or residence time.

Air drawn in at decompression

Reduce suck-back.

Prevention: Dryer checks at every material change.

Visible or weak weld lines

Hits OEE: Quality

Symptoms: Lines where two flow fronts meet, sometimes cracking there.

Melt or mould too cold

Raise melt and mould temperature within the material's window.

Injection too slow

Increase injection speed.

Poor venting at the weld

Vent the area where the fronts meet.

Gate position

Move the gate or add an overflow, a tooling change.

Prevention: Process windows recorded for each mould and material.

Black specks or contamination in parts

Hits OEE: Quality

Symptoms: Dark specks or foreign particles in the part.

Degraded material caught in the screw, barrel or nozzle

Purge, and inspect the screw, check ring and nozzle.

Contaminated regrind or material handling

Control regrind and clean hoppers and loaders.

Worn screw or barrel

Inspect and repair the screw and barrel.

Prevention: Purging at material changes and clean material handling.

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

Stop the same fault coming back

Recurring injection moulding 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