Plastic injection moulding is one of the most versatile and widely used manufacturing processes in the world. From automotive parts to medical devices and household goods, the technique delivers precision, speed, and repeatability. However, like any complex process, it comes with its share of challenges. Even a small error in design, mould, or processing parameters can result in costly defects and production delays.
In this article, we’ll explore the Top 10 most common problems in plastic injection moulding—and how to solve them effectively.
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1. Short Shots
Problem: The mould cavity isn’t completely filled, leaving voids or incomplete parts.
Causes: Low injection pressure, insufficient material flow, poor venting.
Solution:
Increase injection pressure or temperature.
Improve venting to allow trapped air to escape.
Ensure the gate and runner system are properly sized for material flow.
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2. Flash
Problem: Excess plastic seeps out along the parting line, forming thin unwanted layers.
Causes: Worn moulds, excessive injection pressure, poor clamping force.
Solution:
Repair or replace worn mould surfaces.
Adjust clamping pressure to hold mould halves tightly.
Reduce injection speed and pressure if excessive.
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3. Sink Marks
Problem: Small depressions or dimples on thick sections of a moulded part.
Causes: Uneven cooling, thick wall sections, insufficient packing pressure.
Solution:
Optimize cooling time and uniformity.
Increase packing/holding pressure.
Redesign parts to maintain consistent wall thickness.
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4. Weld Lines (Knit Lines)
Problem: Visible lines where two flow fronts meet, often weakening the part.
Causes: Low melt temperature, poor venting, complex flow paths.
Solution:
Raise melt and mould temperature.
Adjust gate locations to improve flow.
Add vents at weld line locations to prevent weak bonding.
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5. Warpage
Problem: Parts deform or twist after ejection, making them unusable.
Causes: Uneven cooling rates, poor part design, residual stress.
Solution:
Improve cooling channel design for uniform cooling.
Use materials with lower shrinkage.
Adjust packing and holding parameters to balance stress.
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6. Burn Marks
Problem: Dark brown or black streaks appear on the part surface.
Causes: Trapped air or excessive melt temperature.
Solution:
Improve venting in mould cavities.
Reduce injection speed.
Lower melt and barrel temperature to prevent overheating.
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7. Voids & Bubbles
Problem: Internal air pockets or bubbles weaken the part.
Causes: Trapped gas, moisture in material, improper cooling.
Solution:
Pre-dry raw material to remove moisture.
Increase holding pressure to compress trapped air.
Optimize cooling cycle to eliminate void formation.
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8. Jetting
Problem: Wavy, snake-like flow patterns on part surface.
Causes: High injection speed, improper gate design.
Solution:
Reduce injection speed for smoother flow.
Increase mould and melt temperature to improve filling.
Modify gate design for gradual material entry.
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9. Discoloration
Problem: Streaks or uneven coloration appear on the part.
Causes: Contaminated resin, improper colorant mixing, high residence time in barrel.
Solution:
Clean barrel and hopper to avoid contamination.
Use masterbatch or liquid colorants with proper mixing.
Reduce material residence time by adjusting shot size.
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10. Delamination
Problem: Thin surface layers peel or separate from the part.
Causes: Moisture contamination, incompatible materials, poor bonding.
Solution:
Thoroughly dry material before moulding.
Ensure only compatible resins are used.
Increase mould temperature for stronger bonding.
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