Numerical analysis of flow mark surface defects in injection molding flow

A.M. Grillet, A.C.B. Bogaerds, G.W.M. Peters, M.J.H. Bulters, F.P.T. Baaijens

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In order to elucidate the mechanism of flow mark surface defects, the stability of injection moldingflow is investigated numerically using a transient finite element method. Experiments performed bySchepens and Bulters @Bulters, M., and A. Schepens, ¿¿The origin of the surface defect ¿slip-stick¿on injection moulded products,¿¿ Paper IL-3-2, in Proceedings of the 16th Annual Meeting of thePolymer Processing Society, Shenghai, China, 2000a, pp. 144¿145# using a novel two colorinjection molding technique are summarized and they indicate that surface defects are caused by aflow instability near the free surface during filling of the mold. Steady finite element calculations ofa model injection molding flow using a single mode, exponential Phan-Thien¿Tanner constitutiveequation supply information about the base state streamlines and polymer stresses. By varying theparameters of the model, the degree of strain hardening in the extensional viscosity can becontrolled. Then a linear stability analysis is used to determine the most unstable eigenmode of theflow and the dependence on the extensional properties of the polymer. For strain softeningmaterials, the injection molding flow is predicted to be stable up to a Weissenberg number of five.However, the most unstable disturbance is consistent with the swirling flow near the interfaceobserved experimentally. For strain hardening rheologies, an instability is observed in the channelflow far from the interface, in agreement with calculations performed by Grillet et al. @Grillet, A.M., A. C. B. Bogaerds, G. W. M. Peters, and F. P. T. Baaijens, ¿¿Stability analysis of constitutiveequations for polymer melts in viscometric flows,¿¿ J. Non-Newt. Fluid Mech. ~accepted, 2001!# onplanar Poiseuille flow of a Phan-Thien¿Tanner fluid.
Original languageEnglish
Pages (from-to)651-669
JournalJournal of Rheology
Issue number3
Publication statusPublished - 2002


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