Consider when thermoplastic melt in a mould cavity cools from processing temperature to room temperature. Density increases and volume contracts by around 9 %, equivalent to 3 % shrinkage on linear dimensions. With semi-crystalline thermoplastics, because of the additional increase in density due to crystallisation, the volume shrinkage can be as much as 17 % (6 % linear).
So why do materials suppliers and toolmakers quote values for linear shrinkage in the range 0.5 – 1.0 % for amorphous thermoplastics and 1.2 – 2.3 % for semi-crystalline?
The explanation for the discrepancy involves two factors. Firstly, the melt would have been injected under high pressure. Depressurisation during cooling compensates for some of the thermal shrinkage. However, taking depressurisation into consideration would only reduce linear mould shrinkage values to the order of 1 % and 4 % respectively. In practice, after the mould cavity has been filled under pressure, it is possible to pump more melt into the molten core. This compensates for some of the thermal and crystallisation shrinkage.
To allow for mould shrinkage toolmakers traditionally used an educated guess to deliberately design an oversize cavity. However, this would involve a number of assumptions regarding melt temperature, injection pressure and the level of compensation during the pressure hold phase before the gate freezes off. To make matters worse, some thermoplastics show significant differences in shrinkage between the direction of flow and the transverse direction.
There are so many variables related to materials, processing conditions, mould geometry and feed systems. So, it is not surprising that customers sometimes end up with mouldings which are not the intended dimensions. The worst news a toolmaker can hear? A late change in material for a project, particularly a change from an amorphous to a semi-crystalline thermoplastic.
Investing in moulding simulation analysis before cutting steel can be money well spent.
Finally, more information on this topic may be available on the excellent British Plastics Federation website.