Troubleshooting Common Issues in Plastic Injection Moulding

Plastic injection moulding is a widely used manufacturing process, valued for its efficiency and ability to produce intricate parts at scale. However, like any industrial process, it comes with its challenges. Identifying and troubleshooting common issues can save time, reduce waste, and ensure high-quality output. Here's a guide to some prevalent matters in plastic injection moulding and how to resolve them effectively.

1. Short Shots

What It Is: A short shot occurs when the mould cavity isn’t filled with plastic, resulting in an incomplete product.

Causes:

• Insufficient material injection.

• Low melt temperature.

• Poor venting or air trapped in the mould.

• Inadequate injection pressure or speed.

Solutions:

• Increase the injection pressure and speed.

• Raise the melt and mould temperature.

• Check and clean the vents to ensure proper airflow.

• Verify that the shot size matches the mould’s requirements.

2. Flash

What It Is: Flash refers to excess plastic that leaks out of the mould cavity, forming thin protrusions around the part.

Causes:

• Excessive injection pressure.

• Mould misalignment or damage.

• Worn-out mould components.

• Low clamping force.

Solutions:

• Reduce injection pressure.

• Inspect the mould for alignment and wear; repair or replace damaged parts.

• Ensure that the clamping force is adequate for the material being used.

3. Warping

What It Is: Warping occurs when a moulded part twists or deforms as it cools, often due to uneven shrinkage.

Causes:

• Uneven cooling rates.

• Material stress from improper mould design.

• Inconsistent wall thickness in the design.

Solutions:

• Optimize the cooling system to ensure uniform cooling.

• Use a material with lower shrinkage properties.

• Redesign the part to maintain uniform wall thickness.

4. Sink Marks

What It Is: Sink marks are depressions or indentations on the surface of a moulded part, often near thick sections.

Causes:

• Uneven cooling or excessive material shrinkage.

• Inadequate holding pressure or time.

• Part design with thick sections.

Solutions:

• Increase the holding pressure and extend the holding time.

• Redesign the part to avoid thick sections.

• Improve cooling system efficiency to achieve uniform cooling.

5. Burn Marks

What It Is: Burn marks are discoloured or charred areas on the surface of a moulded part, caused by overheating or trapped air.

Causes:

• Trapped air or gas in the mould.

• Excessive injection speed.

• High melt temperature.

Solutions:

• Improve venting in the mould to allow trapped air to escape.

• Reduce injection speed and melt temperature.

• Inspect and maintain the mould regularly to prevent blockages.

6. Flow Marks

What It Is: Flow marks appear as wavy patterns or lines on the surface of a part, indicating inconsistencies in material flow.

Causes:

• Low melt or mould temperature.

• Slow injection speed.

• Poor material-mould compatibility.

Solutions:

• Increase the melt and mould temperature to improve material flow.

• Adjust the injection speed to achieve a consistent fill.

• Use a material better suited for the mould and part design.

7. Jetting

What It Is: Jetting occurs when the molten plastic squirts into the mould cavity in a jet-like manner, causing surface defects and weak spots.

Causes:

• High injection speed.

• Improper gate design.

• Incorrect melt temperature.

Solutions:

• Reduce the injection speed to allow smoother flow.

• Optimize gate design to reduce turbulence.

• Adjust the melt temperature to ensure consistent material viscosity.

8. Brittle Parts

What It Is: Brittle parts are weak and prone to cracking under stress.

Causes:

• Excessive moisture in the plastic resin.

• Inadequate drying of the material.

• Incorrect melt temperature.

Solutions:

• Ensure proper drying of the plastic resin before moulding.

• Adjust the melt temperature according to material specifications.

• Verify the material grade and compatibility with the moulding process.

9. Surface Defects

What It Is: Surface defects like roughness, blemishes, or matte finishes detract from a part’s appearance.

Causes:

• Impurities in the resin.

• Poor mould surface finish.

• Incorrect mould or melt temperature.

Solutions:

• Use high-quality, clean resin.

• Polish the mould to improve the surface finish.

• Adjust mould and melt temperatures to the ideal range.

Preventative Maintenance for Long-Term Success

• Regular Mould Maintenance: Clean, inspect, and repair moulds periodically to prevent recurring issues.

• Material Quality Checks: Ensure plastic resins are properly stored and free from contaminants.

• Process Optimization: Continuously monitor and refine the injection moulding parameters to maintain consistency.

• Training Operators: Skilled technicians can identify and resolve issues more efficiently, minimizing downtime.

Conclusion

By understanding and addressing these common issues, manufacturers can significantly enhance the quality and efficiency of their plastic injection moulding operations. Regular maintenance, careful material selection, and precise control over processing parameters are key to minimizing defects and maximizing productivity. Troubleshooting isn’t just about fixing problems—it’s about building a foundation for sustainable, high-quality production.