How to reduce sinks in the plastic injection moulding process

The plastic injection moulding process is a refined procedure, which offers up a number of potential challenges that have to be overcome to ensure the finished product is perfect.

One factor that has to be considered is the avoidance of sink marks in any part we produce. A sink mark is a small depression in the surface of a plastic part that resembles a dimple or dip that is typically caused by excessive shrinkage in a localised area of a part as it cools.

While sinks may not affect the structural strength or resilience of a part, they reduce the aesthetic quality of the product and are regarded as a sign of poor craftsmanship.

There are a number of factors that can cause sinks and therefore a number of steps that have to be taken to reduce them. They include:

Insufficient cooling time

The cooling phase of the moulding cycle is the time when the part is held under pressure as it cools. The cooling period allows a strong skin to form on the resin, so if insufficient cooling time is allowed the surface of the resin will not have sufficient strength or thickness to hold strong and sinks may appear.

We have tried and tested formulas for the cooling time we allow for certain resins depending on the thickness of a part, to reduce the appearance of sinks.

Not enough injection pressure

We must make sure we use the correct amount of pressure when injecting the molten matter, so it fills all areas of the cavity of the mould. It must then be held there for the right amount of time under the correct pressure, so all the molecules are of the same size – ensuring sinks will not emerge.