Numerical Simulation of Push-Pull Injection-Moulding

Controlled freezing of plastic melt flow offers a possibility to develop structure in plastics and therefore properties of plastic parts. This can be done by supplying an advanced injection moulding technique called push-pull injection moulding, a specialized method of injection moulding. The difference in that process is, that during push-pull-processing the melt will remain moving after the mould-filling stage in contrary to conventional injection moulding. Describing the flow of a polymer melts is a complex problem with many coupled parameters. Polymer melts exhibit viscoelastic non-Newtonian flow behaviour. The shear flow is strongly manipulating rheological behaviour as it aligns molecules parallel in the flow direction and suppresses physical entangling of molecule loops and segments that gives rise to shear thinning and normal stress differences. Temperature and morphology of the material, two parameters that strongly depend on each other additionally influence the viscosity. This is especially valid for semi-crystalline polymers that change their flow behaviour during crystallisation. This phase transition is a source for latent heat in addition to the viscous heat set free in all sheared polymers caused by friction. On the other hand nucleation and crystal growth are influenced by shear. Analogous particles in the melt, like reinforcing short fibres are oriented by the flow but influence the flow themselves. All these parameters that occur during PPP are time-dependent, since the process is discontinuous. Various parameters - as mentioned in the previous paragraph - influence the process. Thus the considerations for designing moulds and selecting the settings for the injection moulding machine is very complex. Computer simulation is a powerfull tool in order to get a more profound knowledge of the happenings in the mould during manufacturing. Simulation with special regards to this kind of process has to be developped. Simulation shall help to improve the process itself and it shall supply users of the technique with the necessary know-how to attain the advantages of the process. Supervisor: Jesper deClaville Christiansen. (Karel Waschitschek