In Pursuit Of The Perfect Injection Moulded Part 5 – Sink Marks

Sink marks

Sink marks are small recesses or depressions in an otherwise flat and consistent surface of a moulded part. These can occur when the inner part of a moulded component shrinks, pulling material from the outside inward.

Causes and prevention of sink marks

In Pursuit Of The Perfect Injection Moulded Part - Sink Marks

Sink marks are similar to vacuum voids but are reversed in cause and effect.  Rather than the material cooling too rapidly near the exterior of the part, the material cools too slowly. The resulting shrinkage pulls the outside material inward before it’s had a chance to adequately cool, leading to a depression. As with voids, sink marks are more likely to occur in thicker parts of a component. Here are some steps you can take to prevent this defect:

  • Increase holding pressure and time to allow the material near the part’s surface to cool
  • Increase cooling time to limit shrinkage
  • Design your mould with thinner component walls to allow for faster cooling near the surface

Patterson & Rothwell are a Plastic Injection Moulding Company based in the North West of England, UK producing goods for many business sectors. Our ethos is to offer our customers a professional yet flexible approach and of course provide attention to detail at every stage. Please contact us if you wish to discuss any Plastic Injection Moulding Project.