RJG Blog // How to Troubleshoot 9 Common Defects in Injection Moulded Parts
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Identifying and troubleshooting plastic injection moulding defects is essential for maintaining product quality, reducing waste, and optimising production efficiency. In this blog, RJG discusses 9 common injection moulding defects and provides practical solutions for addressing them. The most accurate way to predict and determine injection moulded defects is through the use of cavity pressure sensors and a process control system, but here are some techniques to try regardless of your available equipment.
*Please note, these are not exhaustive lists, as there are countless things that could cause any one of these, but it’s a starting point.
1. Sink Marks
Sink marks are depressions or indentations on the surface of moulded parts, typically occurring in thicker sections.
Causes:
- Insufficient cooling time
- Inadequate packing pressure
- Improper gate design
- Poor part design
Solutions:
- Increase cooling time to ensure complete solidification.
- Adjust packing pressure to ensure material fills voids during cooling.
- Redesign the gate to improve material flow and pressure distribution.
- Change part design.
2. Warping
Warping is the distortion or bending of a part, leading to an uneven or twisted shape.
Causes:
- Uneven cooling rates
- Mould temperature imbalance
- Material shrinkage
- Non-uniform part thickness
Solutions:
- Ensure uniform cooling by optimising the cooling channel design.
- Balance mould temperatures to prevent differential cooling.
- Use materials with lower shrinkage rates or adjust processing parameters to control shrinkage.
3. Flash
Flash is excess material that extends from the edges of a part, often along the parting line of the mould.
Causes:
- Filling the mould too full during fill
- Worn or damaged mould parts
- Improper clamping force
Solutions:
- Decouple the process to ensure the mould is not being over-filled during filling.
- Regularly maintain and inspect moulds to prevent wear and damage.
- Adjust clamping force to ensure the mould halves are properly aligned and sealed.
4. Short Shots
Short shots occur when the mould cavity is not filled, resulting in incomplete parts.
Causes:
- Insufficient material supply
- Low injection pressure or speed
- Blockages in the gate or runner
Solutions:
- Ensure adequate material supply and proper feeding.
- Increase injection pressure and speed to improve flow.
- Clear any blockages in the gate or runner and ensure they are properly sized.
5. Weld Lines
Weld lines are visible lines on the surface of parts where two flow fronts meet and fail to bond properly.
Causes:
- Low melt temperature
- Slow injection speed
- Poor venting in the mould
- Gate location and part design
Solutions:
- Increase melt temperature to improve flow front bonding.
- Increase injection speed to reduce cooling before meeting.
- Improve mould venting to allow gases to escape and prevent weak bonds.
6. Air Traps
Air traps are pockets of air that get trapped within the part, leading to voids or bubbles.
Causes:
- Inadequate venting in the mould
- High injection speed
- Complex part geometry
Solutions:
- Improve mould venting to allow air to escape during injection.
- Reduce injection speed to prevent air entrapment.
- Redesign part geometry to minimise areas where air can become trapped.
7. Burn Marks
Burn marks are dark discolorations or blackened areas on the part, typically caused by trapped gases.
Causes:
- High injection speed
- Excessive melt temperature
- Poor venting in the mould
Solutions:
- Reduce injection speed to prevent overheating.
- Lower melt temperature to avoid material degradation.
- Enhance mould venting to release trapped gases and prevent burning.
8. Surface Delamination
Surface delamination is the separation of the surface layer from the underlying material, often appearing as flakes or layers.
Causes:
- Contaminated material
- Incompatible material blends
- Improper material handling
Solutions:
- Ensure material purity and proper handling to avoid contamination.
- Use compatible material blends and verify supplier quality.
- Implement thorough material inspection and handling procedures.
9. Voids
Voids are internal pockets caused by the material shrinking away from itself, leading to structural weaknesses.
Causes:
- Insufficient packing pressure
- Uneven cooling
- High melt temperature
Solutions:
- Increase packing pressure to eliminate voids during solidification.
- Ensure uniform cooling to prevent differential shrinkage.
- Lower melt temperature to reduce gas formation within the material.
Conclusion
Troubleshooting defects in injection moulded parts requires a comprehensive understanding of the process and careful analysis of potential causes. By systematically identifying and addressing the root causes of defects, you can enhance product quality, reduce waste, and improve overall production efficiency. Taking RJG’s Fundamentals of Systematic Moulding course equips you with the knowledge and skills to methodically approach troubleshooting, ensuring more effective problem-solving and long-term process stability. Implementing regular maintenance, optimising process parameters, and leveraging advanced tools, like CoPilot, are essential strategies for achieving consistent, high-quality injection mouded parts.
Read more from RJG here.
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