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

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.

Injection moulding troubleshooting FAQ

Injection moulding: what causes short shot (incomplete fill), and how do I fix it?

Symptoms: Part not fully formed, missing features at the flow ends. Likely causes: Insufficient material/shot size; Low injection pressure/speed; Cold melt or mould; Blocked gate/vents. Fixes: Increase shot size/cushion; check for screw slippage. Raise injection pressure and speed within window. Raise barrel/mould temperature to spec. Clear the gate; improve venting to release trapped gas. Prevention: Process validation, check non-return valve, maintain temperatures and venting.

Injection moulding: what causes flash (excess material at parting line), and how do I fix it?

Symptoms: Thin fins of plastic at the parting line or vents. Likely causes: Clamp force too low; Over-packing / high pressure; Worn or dirty mould faces; Melt too hot. Fixes: Increase clamp tonnage; verify it's adequate for projected area. Reduce injection/hold pressure and speed. Clean and inspect parting line; repair damaged steel. Lower barrel temperature to reduce flow into the parting line. Prevention: Clamp-force checks, process control, mould face maintenance.

Injection moulding: what causes sink marks, and how do I fix it?

Symptoms: Depressions over thick sections/ribs. Likely causes: Insufficient hold pressure/time; Cooling too short; Thick section design. Fixes: Increase pack/hold pressure and time. Extend cooling; check mould temperature uniformity. Core out thick areas (design); for now optimise pack/cool. Prevention: Optimised pack/hold, adequate cooling, design for uniform wall.

Injection moulding: what causes part sticking in the mould, and how do I fix it?

Symptoms: Ejection problems, part deforms or stays in cavity, cycle stops. Likely causes: Over-packing; Insufficient draft / mould damage; Inadequate release / cooling. Fixes: Reduce injection/hold pressure. Inspect for undercuts/scoring; polish/repair; verify draft. Adjust cooling; apply approved release agent if permitted. Prevention: Process within window, mould polishing/repair, correct ejection setup.

Injection moulding: what causes warping / dimensional instability, and how do I fix it?

Symptoms: Parts bow or twist, out of tolerance after ejection. Likely causes: Uneven cooling; Residual stress (pack/hold); Ejection too early / hot. Fixes: Balance mould cooling; check water circuits and temperatures. Optimise pack/hold and cooling time; reduce over-packing. Extend cooling; adjust ejection. Prevention: Balanced cooling, validated pack/hold, adequate cooling time.

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: EU-built, so your production data stays in EU jurisdiction, with computer-vision true-cause of micro-stops, a closed loop from PLC-read OEE to an auto-routed work order, and ISO 27001 / 20000-1 / 9001 (supports audit-readiness).

See how Fabrico finds root cause

The directory stays free.

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