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The most common moulding issues and how to solve them:

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When looking at common moulding issues, there are generally four main areas in which plastic injection moulded parts can become poor in quality. These four areas are; part design, tool design, process set up and material quality.

Mouldability of a part is dictated by part design. It’s important that a product designer must design a part for the manufacturing process that is fit for purpose in order to reduce common issues of faulty parts. Below are listed the ten most common issues that can arise.


Short “shot”

This is the complete filling of a mould cavity, this results in the production of an incomplete part. If a part short shots, then the plastic will not fill the cavity.

This can be resolved through aiding material flow by increasing the thickness of the sections. As well as this, it’s important to check venting and increase where necessary. You should also look to increase the polymer melt temperature and increase the shot size.


Sink Marks


This can be identified through a dimple or groove. This is defined as a depression and is caused by excessive localised shrinking of the material after the part has cooled.

To help prevent this from occurring, ensure one of the walls is between 60% and 70% of the mating wall thickness. As well as this you can reduce the thickness where the sink occurs and increase the size of the runner system. In terms of the process setup, reduce the melt temperature, increase the shot size and reduce the injection speed.




This is a deformation that occurs when there is an uneven shrinkage in the different parts of the moulded component. You’ll see unintended results of twisted, uneven or bent shaped areas within the part.

This is commonly due to non-uniform cooling of the mould material. Different cooling rates across multiple areas of the part cause the plastic to cool differently and create internal stress, this then leads to warping when released.

To avoid this, you should ensure that the cooling time is sufficiently long and is slow enough to prevent the development of residual stresses being locked into the part. The design should also have uniform wall thickness as this allows the plastic to flow in a single direction.  It’s also important to consider the material, you should select materials that are less likely to shrink and deform.

For more information when it comes to plastic injection moulding please contact our specialists today! We can also provide further details into what you need to know about designing for mouldability, view our Design Guidelines.

Added: 12 Jun 2018 10:36

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