Calculation of residual stress development in injection moulding using a nonlinear viscoelastic model

The residual stress development in amorphous polymers is calculated for rapid cooling under pressure during injection moulding. This is done primarily to predict the warpage and shrinkage behaviour of components. A nonlinear viscoelastic model based on the free-volume concept is used as a constitutive equation to describe polymer behaviour. The cooling process in injection moulding can be divided into two stages: until ejection, the part is clamped into the mould, while the remaining cooling down to room temperature happens free of constraints. Both steps are calculated viscoelastically in the simulation procedure. First, the stress profiles are determined in one material point of a plate-like component. In these calculations, the influence of different processing conditions, such as ejection time, packing pressure, or different wall temperatures, is investigated. Next, to be able to take more complex geometries into account, the nonlinear constitutive equation is used in a commercially available finite-element program. This is done by programming user-defined subroutines. The solution of the coupled thermal-mechanical problem allows the prediction of the residual stresses as well as the warpage and shrinkage behaviour for components with arbitrary geometry. Finally, the calculated warpage is compared with the behaviour of an experimental component. (Example material: PES.)