Even with a well-optimized injection moulding process, defects can still occur. Understanding common defects and how to prevent them is key to producing high-quality parts.
1. Short Shots
What it is: The moulded part is incomplete because the plastic didn’t fill the entire cavity.
Causes & Solutions:
✔ Low Injection Pressure or Speed → Increase pressure and injection speed.
✔ Insufficient Material Flow → Ensure the plastic is fully melted and flows properly.
✔ Blocked or Undersized Gates → Enlarge the gate or redesign its position.
✔ Air Traps → Improve venting or adjust mould temperature.
2. Flash (Burrs or Excess Material)
What it is: Thin extra plastic around the edges of the part, usually at the mould parting line.
Causes & Solutions:
✔ Excessive Injection Pressure → Reduce pressure or clamping force.
✔ Worn or Poorly Fitting Mould → Ensure proper mould maintenance and alignment.
✔ Too Much Material in the Mould → Adjust shot size to prevent overflow.
3. Warping (Part Distortion)
What it is: The part bends or twists after cooling.
Causes & Solutions:
✔ Uneven Cooling → Optimize cooling channels to ensure even temperature distribution.
✔ High Residual Stress → Adjust mould temperature and injection speed.
✔ Incorrect Material Choice → Some plastics shrink more than others; choose a material with lower shrinkage.
4. Sink Marks (Small Dents or Depressions on the Surface)
What it is: Indentations on the surface of thick areas where the plastic shrinks as it cools.
Causes & Solutions:
✔ Thick Wall Sections → Avoid uneven thickness by redesigning the part.
✔ Fast Cooling Rate → Slow down cooling to allow proper solidification.
✔ Low Packing Pressure → Increase packing pressure to compensate for shrinkage.
5. Weld Lines (Visible Seams or Weak Joints)
What it is: Thin lines where two flow fronts of plastic meet, often creating a weak spot.
Causes & Solutions:
✔ Low Melt Temperature → Increase temperature to ensure better flow.
✔ Poor Gate Positioning → Adjust the gate location to avoid multiple flow fronts.
✔ Slow Injection Speed → Increase injection speed to allow better bonding.
6. Bubbles or Voids (Air Pockets Inside the Part)
What it is: Small trapped air pockets that weaken the part.
Causes & Solutions:
✔ High Moisture Content in Material → Dry plastic before moulding.
✔ Insufficient Packing Pressure → Increase pressure to eliminate air pockets.
✔ Fast Injection Speed → Slow down the speed to allow air to escape.
7. Burn Marks (Dark Spots, Streaks, or Discoloration)
What it is: Black or brown marks caused by overheating or trapped air.
Causes & Solutions:
✔ Excessive Residence Time in Barrel → Reduce cycle time and prevent overheating.
✔ Poor Venting → Improve venting in the mould to let trapped gases escape.
✔ High Injection Speed → Reduce speed to avoid excessive shear heating.
Most defects can be avoided by optimizing machine settings, mould design, and material handling. Proper mould maintenance and regular quality checks are also crucial to ensuring consistent part quality.