Stability analysis of polymer melt injection molding flows
Gerrit Peters, Arjen Bogaerds, Martien Hulsen, Frank Baaijens
Eindhoven University of Technology
The Netherlands

Keywords: stability analysis, viscoelastic, injection molding

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Unstable flow conditions during injection molding of polymer products can result in severe restrictions on the process or the final product properties. Specifically, this instability can cause non-uniform surface reflectivity characterized by alternating shiny bands perpendicular to the main flow direction. This phenomenon is often referred to as `flow marks', `tiger stripes' or `ice-lines' and is observed for many polymer and polymer blend systems including PP, LDPE, ASA and PC/ABS blends. In order to be able to elucidate the mechanism of instability in this and other processing flows we first need to the apply a stable numerical algorithm. This linear stability analysis includes transient finite element calculations combined with a perturbation technique that incorporates the dynamic response of the free surface near the flow front to a disturbance in the flow. As the occurrence of the instability is largely dependent on the fluid rheology, the constitutive model that is used needs to be able to predict the necessary physics of the polymer melt. Here, we will apply the recently proposed eXtended Pom-Pom model to this flow. This model has shown to be able to accurately describe a number of polymer melts but it also allows us to independently alter the configuration of the representative Pom-Pom molecule and study its effect on the stability behavior of the
flow.