How to optimize every component for injection moulding  

By Lars Butenschon 

It is a designer’s worst-case scenario: in designing and manufacturing an injection mould, days and weeks go by. When production finally begins and the first component falls out of the mould, a nasty surprise arises. The surface of the moulding has dents that not only look unsightly but also impair the function. 

What is this? 

The phenomenon is called shrinkage. Hot plastic expands during injection moulding. During cooling, the material shrinks in the mould – the shrink factor is between 0.2 and 2 per cent of the part volume. That is physics and it’s unavoidable. 

The problem: the component does not shrink evenly. While external surfaces cool down quickly, the process takes longer in places with lots of material. This causes stresses in the component, causing parts of the outer surface to cave in and openings to appear. In addition, there is a risk of cavities forming on the inside – micro-cellular and bubble-shaped cavities that reduce stability. Tolerances are difficult to control for both deformations and cavities. 

How to avoid variable wall thicknesses in injection moulding 

To keep the phenomenon of shrinkage from occurring during the design phase, we can avoid thick walls to excess and mass accumulations. The ideal wall thickness – which, of course, depends on the material – is between 0.5 and 2 millimetres. If thicknesses in corners lead to accumulations of material, it is advisable to place these bodies free-standing and secure them with a thin wall or supports to prevent lateral deformation. 

How can you further reduce the wall thickness of your injection-moulded component, and save on cost in mould production?