Understanding Injection Moulding Defects and How to Resolve Them

Injection moulding is a widely used manufacturing process for producing plastic parts with high precision and repeatability. Despite its efficiency, defects can occur that affect the quality, functionality, and appearance of the final product. Identifying the causes of these defects and applying effective solutions is essential for maintaining production standards and reducing waste.

Surface defects on injection moulded plastic part

Common Injection Moulding Defects

Injection moulding defects vary in type and severity. Some of the most frequent issues include:

• Short Shots: Incomplete filling of the mould cavity, resulting in missing sections.

• Flash: Excess thin layer of plastic that leaks outside the mould cavity.

• Sink Marks: Depressions or dimples on the surface caused by uneven cooling.

• Warping: Distortion or bending of the part after moulding.

• Burn Marks: Dark discolorations caused by trapped air or overheating.

• Voids: Internal cavities or bubbles within the part.

• Jetting: Wavy or snake-like patterns on the surface due to high injection speed.

  
  

Understanding these defects helps in diagnosing the root causes and applying targeted fixes.

Causes of Injection Moulding Defects

Each defect arises from specific issues in the moulding process, material, or equipment. Here are some common causes:

Material-Related Causes

• Moisture in Resin: Plastic pellets absorbing moisture can cause bubbles or voids.

• Incorrect Material Grade: Using a resin not suited for the part design or processing conditions.

• Degradation: Overheating or prolonged exposure to heat can degrade the polymer.

  
  

Process-Related Causes

• Injection Speed and Pressure: Too low speed or pressure can cause short shots, while too high can cause jetting or flash.

• Mould Temperature: Uneven or incorrect mould temperature leads to sink marks and warping.

• Cooling Time: Insufficient cooling causes deformation and internal stresses.

• Holding Pressure: Low holding pressure can result in voids and sink marks.

  
  

Equipment and Mould Issues

• Worn or Damaged Moulds: Imperfections in the mould surface cause defects like flash.

• Poor Venting: Trapped air causes burn marks and incomplete filling.

• Inadequate Clamping Force: Insufficient force allows plastic to leak, creating flash.

  
  

How to Fix Injection Moulding Defects

Addressing defects requires a systematic approach, starting with identifying the defect type and then adjusting process parameters or equipment.

Fixing Short Shots

• Increase injection pressure and speed to ensure complete cavity filling.

• Check for blockages or damage in the mould channels.

• Raise mould temperature slightly to improve flow.

  
  

Eliminating Flash

• Increase clamping force to prevent mould separation.

• Inspect mould for wear or damage and repair if needed.

• Reduce injection pressure if excessive.

  
  

Reducing Sink Marks

• Increase holding pressure and extend holding time to pack material properly.

• Optimize cooling time to allow uniform solidification.

• Adjust wall thickness in the design to avoid thick sections.

  
  

Preventing Warping

• Balance cooling channels to ensure even temperature distribution.

• Use materials with lower shrinkage rates.

• Modify part design to reduce stress concentrations.

  
  

Avoiding Burn Marks

• Improve mould venting to release trapped air.

• Lower injection speed and temperature to reduce overheating.

• Clean mould regularly to prevent residue buildup.

  
  

Minimizing Voids

• Dry resin thoroughly before processing.

• Increase holding pressure to pack material tightly.

• Adjust injection speed to avoid trapping air.

  
  

Controlling Jetting

• Reduce injection speed to allow smooth flow.

• Modify gate design to direct flow evenly.

• Increase melt temperature for better fluidity.

  
  

Practical Tips for Quality Improvement

• Regular Maintenance: Keep moulds clean and inspect for wear to prevent defects.

• Material Handling: Store resins properly to avoid moisture absorption.

• Process Monitoring: Use sensors and control systems to maintain consistent parameters.

• Trial Runs: Conduct test runs when changing materials or moulds to fine-tune settings.

• Training Operators: Skilled operators can detect early signs of defects and adjust processes promptly.

  
  

Final Thoughts on Injection Moulding Defects