What is Sink?

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Sink is a phenomenon which occurs mostly in injection moulded parts but possibly in metal castings and other moulding processes. 

As a part cools down, thicker areas will cool and shrink at different rates and amounts than thinner areas. 
Sink can make the part look bad, so if it is an aesthetic or presentation part, it’s best to avoid it. However, if it’s not visible, you may not need to worry. 

A general rule of thumb for the design of injection moulded parts is to keep the “walls” of your part with a uniform thickness. 

There isn’t a lot that can be done to improve the moulding of a poorly designed part although a mould maker can add cooling channels to help manage the results. But this can be a costly addition. 

Coring it Out. Instead of having one thick mass of material, you can design the part with a hollow “core”. This can result in a part that has the main function and shape you need but also has a uniform wall thickness for good moulding results. 


Sink can be a problem when making a moulded component. Depending on your use, you may need to consider designing the part to avoid sink. There is some finesse to designing a good injection moulded part and there are many books and reference guides available. We will cover more in the future. 

For a professional result, you may need to hire a trained industrial designer who specializes in the design of injection moulded components.