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Common Injection Moulding Defects and How to Avoid Them

Injection moulding is a widely used manufacturing process that facilitates the production of intricate plastic components with remarkable efficiency. However, like any manufacturing technique, it's not without its challenges. Even minor defects in injection moulded parts can have significant consequences, ranging from compromised product quality to increased production costs. To ensure the success of your injection moulding projects, it's crucial to be aware of common defects and how to prevent them.

Let's explore seven of these defects and strategies to avoid them.

Common Plastics Injection Moulding Defects Splay, Jetting, Warping, Flow Lines, Sink Marks, Burn Marks, Flash, Short Shots, Surface Delamination, Vacuum Void

Flow Lines

Flow lines, also known as weld lines or knit lines, occur when molten plastic flows around an obstacle and meets itself on the other side. This can result in weak points in the part, affecting its structural integrity. To minimise flow lines:

  • Optimise Mould Design: Ensure that the mould design facilitates smooth and uniform material flow.

  • Control Processing Parameters: Adjust injection speed, temperature, and pressure to promote even material distribution.

Sink Marks

Sink marks appear as depressions or dimples on the surface of an injection moulded part, often caused by uneven cooling rates. To prevent sink marks:

  • Gate Placement: Strategically place gates to facilitate uniform material flow and minimise cooling differentials.

  • Enhance Cooling: Employ techniques such as conformal cooling or advanced cooling channels in the mould design to promote uniform cooling.


Warpage refers to the deformation or distortion of a part from its intended shape, typically caused by non-uniform cooling or residual stress within the material. To mitigate warpage:

  • Uniform Wall Thickness: Design parts with consistent wall thickness to promote uniform cooling.

  • Optimise Cooling Rate: Adjust mould temperature and cooling time to minimise residual stress and ensure even cooling.


Short Shots

Short shots occur when the mould fails to fill completely, resulting in incomplete parts with missing sections. Common causes of short shots include inadequate injection pressure, improper venting, or insufficient material. To avoid short shots:

  • Optimise Injection Pressure: Adjust injection pressure to ensure complete cavity filling.

  • Verify Venting: Ensure proper venting to allow air to escape from the mould cavity during injection.


Flash occurs when excess material escapes from the mould cavity and forms thin, unwanted protrusions along the parting line. This defect is often caused by excessive injection pressure, worn moulds, or improper clamping force. To prevent flash:

  • Maintain Mould Integrity: Regularly inspect and maintain moulds to prevent wear and damage.

  • Optimise Clamping Force: Adjust clamping force to secure the mould properly and prevent material leakage.


Splay appears as streaks or lines on the surface of injection moulded parts, caused by moisture or volatile substances in the resin reacting with heat. To minimise splay:

  • Material Drying: Ensure that resin pellets are adequately dried before processing to remove moisture.

  • Optimise Processing Conditions: Adjust processing parameters such as temperature and pressure to prevent resin degradation.


Jetting occurs when high-speed molten plastic shoots into the mould cavity, leaving behind a visible mark or line. This defect is typically caused by insufficient injection pressure or improper gate design. To address jetting:

  • Gate Design: Opt for gate designs that promote smooth material flow and minimise turbulence.

  • Increase Injection Pressure: Adjust injection pressure to ensure proper cavity filling without excessive velocity.

In conclusion, understanding and addressing common defects in injection moulding are essential for achieving high-quality, defect-free parts. By implementing proper design considerations, process optimisation, and quality control measures, manufacturers can minimise defects and maximise the efficiency and reliability of their injection moulding processes.

We hope this exploration of common defects in injection moulding proves useful for your manufacturing endeavours. If you're considering plastic injection moulding for your component parts and want to ensure flawless production, why not schedule a free consultation with us? Our team is ready to provide expert guidance and assistance tailored to your specific needs, ensuring smooth and efficient manufacturing processes.

To arrange your consultation, call us at 01453 833 388 or email us at


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