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Designing for Injection Moulding to Prevent Surface Defects

When reviewing the finish look and appearance of a product you may face difficulty due to everyone’s own opinions and what’s appealing. In the same way that art is subjective, so it the finish of an injection moulding part. You can improve the appearance within the designing stage through the consideration of material choice.


Voids are air bubbles that form within the material due to extreme shrinkage, resulting in product weakness and surface defects whilst injection moulding. This occurs in thicker sections and are usually visible on clear materials, but when particularly bad they can break through to the service. We can avoid this issue through reducing thick sections and considering the injection moulding materials in use. Some polymers are more prone to voids than others.

Sink Marks

Sink marks are shallow depressions or dimples on the surface of a finished product, usually found over projections such as ribs or bosses. These are created through shrinkage as a result of localised thickening of the sections or by a low fill of the cavity. You can avoid this through creating thinner sections whilst injection moulding and keeping the thickness of ribs and bosses to no more than half of the main product wall. It’s possible to hide these marks by incorporating a textured surface.


Burn spots can be identified as small dark brown or black discolouration on the surface of a moulded product. These burn spots are usually located at the last place to fill within the cavities. These are caused by trapped air in the cavities of the injection moulding, when there is inadequate venting at the edge of the moulding. These can be prevented through smoother transitions to replace sharp corners or steps within the product.

Our injection moulding team have years of experience designing to minimise surface defects and can work with our tooling team to further reduce the likelihood of undesirable product finishes. Speak to a member of our friendly injection moulding team today and see how we can help you to improve product finish and avoid surface defects.


Get in touch today and talk to the specialists, click here.

Our team will be more than happy to help, advise and quote for your requirements.

Looking for more information? Check out our blog, News & Views for useful articles, tips and tricks on plastic injection moulding.