The flow of molten polymers in converging and diverging channels has been studied by rheo-optical techniques including flow birefringence and flow velocity measurements. The flow patterns of HDPE, LDPE, PS, PP, PVC and copolymers of PAN were analysed. The results show that, unlike the stress distribution, the velocity profile is unaffected by the viscoelastic properties of the melt when the converging flow is forced or when the flow occurs in a divergent geometry. The size of the secondary flow which can be observed in a sharp-edged slit die entrance, varies with the flow rate and the rheological parameters of the melt. On the contrary, when the flow is developed in a 180° slit die exit, the secondary flow becomes independant of these parameters and is volumetrically constant. These concluding remarks and the study of the stress distribution in the converging flow permit to correlate the shape of the secondary flow, and particularly the natural converging angle, to the viscoelastic properties of the melt. Indeed the natural converging angle can be related not to the Weissenberg number, but to the ratio of tensile stress to shear stress.
KeywordsViscoelastic Property Secondary Flow Weissenberg Number Half Angle Molten Polymer
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