Injection molding defects can cause scrap, increase costs, and delay production. Knowing the causes and corrective actions allows faster troubleshooting, better yields, and more reliable parts.
Short Shots / Incomplete Fill
Symptoms: Part is missing material or cavity is not fully filled.
Causes:
- Low injection pressure or speed
- Insufficient melt temperature
- Blocked or small gate
- Cold material or moisture
Solutions:
- Increase injection pressure and speed
- Raise barrel temperature
- Inspect gates and runners
- Dry material properly before molding
Warping / Part Distortion
Symptoms: Part bends or twists after ejection.
Causes:
- Uneven cooling
- High shrinkage material
- Inconsistent mold temperature
- Excessive packing pressure
Solutions:
- Balance cooling channels
- Reduce packing/holding pressure
- Adjust mold temperature for uniform cooling
- Consider design changes (ribbing or thicker sections)
Sink Marks / Surface Depressions
Symptoms: Small depressions on the part surface, especially thick sections.
Causes:
- Uneven cooling or thick sections
- Insufficient packing
- Low mold temperature
Solutions:
- Increase packing/holding time
- Add cooling near thick sections
- Modify part design to reduce wall thickness
Flash / Excess Material
Symptoms: Thin layers of material beyond cavity edges.
Causes:
- Worn or damaged mold
- Excess injection pressure
- Mold clamping force insufficient
Solutions:
- Inspect and repair mold
- Reduce injection pressure
- Increase clamping force or tighten mold
Burn Marks
Symptoms: Dark or black marks on the part, often near gates.
Causes:
- Trapped air
- High injection speed
- Overheated material
Solutions:
- Improve venting in mold
- Reduce injection speed
- Reduce melt temperature
Voids / Air Pockets
Symptoms: Hollow spots inside thick sections
Causes:
- Improper packing
- Poor venting
- High shrinkage material
Solutions:
- Increase packing pressure and time
- Improve mold venting
- Reduce wall thickness if possible
Surface Blemishes / Flow Lines
Symptoms: Visible lines or patterns on part surface
Causes:
- Inconsistent melt temperature
- Incorrect injection speed
- Poor material mixing
Solutions:
- Stabilize barrel temperature
- Adjust injection speed and pressure
- Ensure proper material drying and mixing
Quick Tips for Troubleshooting
- Always check material dryness and storage.
- Run sample cycles before full production.
- Monitor mold temperatures carefully.
- Document every change for reproducibility.
- Consider CNC and 3D printed mold inserts for trial runs.
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