Common injection moulding defects and how to avoid them

The most common process used in manufacturing plastic components is injection moulding. This worldwide process uses specialised machines that melt, inject, and shape thermoplastics into specific shapes and products. As with any manufacturing solution, it’s prone to potential defects or low-quality outcomes if not properly managed and corrected.

Any successful injection moulding project must consider a number of factors to ensure that the outcomes are correct and no further issues are raised. By addressing these common defects through careful design, material selection, and process optimisation, manufacturers can improve the quality and consistency of injection moulded parts.

These are depressions or indentations on the surface of the moulded part caused by shrinkage.

To avoid sink marks, ensure the uniform wall thickness and proper gate design. Adjusting injection parameters such as pressure and temperature can also be beneficial.

Flash occurs when excess material escapes from the mould cavity and forms thin fins or protrusions along the parting line.

To prevent flash, optimise clamping force, use appropriate mould materials, and maintain proper mould alignment.

Warping is the distortion of the moulded part due to non-uniform cooling or excessive internal stresses. To minimise warping, ensure uniform cooling, maintain consistent wall thickness, and use proper material selection. Additionally, consider incorporating cooling channels in the mould design.

Short shots happen when the mould does not fill with molten material, resulting in incomplete parts. To prevent short shots from occurring, optimise injection pressure, temperature, and speed.

Additionally, ensure proper venting and gate design to facilitate material flow.

Burning occurs when the material overheats during the moulding process, leading to discolouration or degradation. To avoid burning, manufacturers should maintain proper barrel temperatures, optimise cycle times, and use materials with suitable heat stability. Regular maintenance of the moulding machine can also help prevent overheating.

Jetting is the formation of streaks or lines on the surface of the part caused by high-speed molten material shooting through the mould cavity. To mitigate jetting, make sure to optimise gate design to control material flow, reduce injection speed, and increase mould temperature. Adjusting material viscosity and pressure can also help.

Knit lines occur when molten material flows around obstacles or converges from different directions, resulting in weak bond lines. To prevent knit lines, optimise gate placement and design, increase injection pressure, and use materials with good flow properties. Increasing mould temperature can also improve material flow and reduce knit lines.

Voids are air pockets or gaps within the moulded part caused by insufficient material flow or trapped air during the injection process. To minimise voids, optimise venting, increase injection pressure, and use materials with good flow characteristics. Proper mould design and maintenance are also essential to prevent air entrapment.

Moisture or gas coming from the product is injected into the mould cavity. If embedded inside the melt, these can appear as bubbles. To avoid this issue, dry the material to suggested moisture levels, increase back pressure and reduce the melt temperature.

With over 20 years in the plastic injection moulding industry, we take pride in manufacturing moulds to the highest possible standards. As part of our customer service, we also offer full mould trial services and undertake full production runs if required.

We operate across the UK including Nottingham, Derby and Leicester so wherever you are, give us a call or use our contact form today

If you would like to talk about your injection moulding needs then please call us on 01623 904 417 and a member of our technical team will be happy to help. Alternatively, you can email us using our contact page.