CJ Tool and Mouldings Logo

Designing for Injection Moulding to Prevent Surface Defects

It can be difficult when reviewing the finished look and appearance of a product, because everyone finds different things appealing – in the same way art is subjective, so is the finish and design of an injection moulded part. That said, a lot can be done when designing to improve the appearance of a product. We would also suggest when designing to consider material choice, as this can affect the finished look of a product.


Designing to Prevent Surface Defects

Below we’ve identified several surface defects that can occur on a finished injection moulded product and how they can be prevented through design. In this article, we’ll also explain why they happen and how to design to prevent such defects and improve the overall finished look.



What are they?

Voids are air bubbles that form within the material due to extreme shrinkage, resulting in product weakness and surface defects.

Why do they happen?

Voids typically occur in thicker sections and are usually only visible on clear materials, but when particularly bad can break through to the surface.

How to prevent them?

When designing we would avoid overly thick sections and consider different materials, as some polymers are more prone to voids than others.


Sink Marks

What are they?

Sink Marks are shallow depressions or dimples on the surface of a finished product, usually found over projections such as ribs or bosses.  

Why do they happen?

Sink Marks are again created by extreme shrinkage as a result of localized thickening of the sections or by a low fill of the cavity.

How to prevent them?

As with voids, when designing we would avoid overly thick sections to minimise the chances of shrinkage and keep the thickness of ribs and bosses to no more than half of the main product wall. It is possible to hide sink marks by incorporating a textured surface finish into the design and our designers would best advise you on this approach.



What are they?

Burning, refers to burn spots that 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.

Why do they happen?

Burn spots are caused by trapped air in the cavities of the injection mould, when there is inadequate venting at the edge of the moulding.


How to prevent them?

Burn spots can be prevented by designing smoother transitions to replace sharp corners or steps within the product. It is sometimes necessary to add venting, perhaps around ejector pins to reduce the amount of trapped air in the cavities. Our team will ensure the design of the product does not encourage or allow air to become trapped in the mould cavities, thus reducing the chances of burning.


Weld Lines

What are they?

Weld Lines appears as cracks or lines on the surface of a finished moulded product, causing component weakening or breaking.

Why do they happen?

Weld lines happen where melted material flows together during the moulding phase.

How to prevent them?

Weld lines are perhaps the most common injection moulding defect faced by manufactures. When designing it is important to consider wall thickness because non-uniformed wall thickness can vary the flow and cause weld lines. Other considerations are whether the design has holes or depressions, which can split the melt flow and cause weld lines.

Good tool design can be central in reducing weld lines, our designers would work closely with our tooling team to ensure the best solution is executed.


In Summary

Our design team have years of experience designing to minimse surface defects and can work with our tooling team to further reduce the likelihood of undesirable product finishes. The surface defects listed above are the most common problems faced, however there are other issues which we have not mentioned in this article that our design team can help you with. Ensuring your design is correct from the beginning, taking into consideration the points raised above, means you’ll less likely to face problems and incur costs further down the line.

Speak to a member of our friendly team today and see how we can help you design 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.