Common injection moulding defects

Use this guide to organize first checks when a defect appears. The actual cause depends on the resin, mould, machine and process window.

Short shot

Possible causes: Insufficient filling, restricted flow, low melt or mould temperature, inadequate venting.

First checks: Check material feed and drying, gate and vent condition, fill profile and shot size.

Sink marks or voids

Possible causes: Uneven or insufficient packing, thick sections, early gate freeze or uneven cooling.

First checks: Check packing/holding pressure and time, gate freeze, cooling balance and wall thickness.

Flash

Possible causes: Excessive cavity pressure, poor parting-line contact, damaged shutoff or insufficient clamping.

First checks: Inspect the parting line and shutoffs; verify clamp force and fill/packing conditions.

Burn marks

Possible causes: Trapped gas, poor venting or excessive local shear and temperature.

First checks: Inspect vents and gas traps; review fill speed and material temperature within the supplier’s limits.

Splay or silver streaks

Possible causes: Moisture, contamination, degradation or air in the melt.

First checks: Verify drying conditions, material handling, purge quality and residence time.

Weld lines

Possible causes: Flow fronts meet after cooling or around an obstruction; venting or temperature may be inadequate.

First checks: Review gate position, fill balance, melt/mould temperature and venting.

Warpage

Possible causes: Uneven shrinkage or cooling, orientation, residual stress or unbalanced filling.

First checks: Check cooling balance, packing, gate layout and part geometry.

Dimensional variation

Possible causes: Unstable material, process variation, temperature drift or inconsistent measurement.

First checks: Stabilize the process, confirm material lot and measuring method, then check cavity-to-cavity variation.

Note: These are diagnostic starting points, not universal machine settings. Change one factor at a time and stay within the resin supplier’s and machine manufacturer’s limits.

Read the step-by-step injection moulding troubleshooting guide