Numerical simulations of the coextrusion process used in producing multi-layer plastics products have been carried out. The isothermal multi-layer axisymmetric coextrusion flow was analyzed with a streamline finite element method. Extrudate swell from an annular die was predicted and the position of the multi-layer fluid interface was also determined. The K-BKZ integral type viscoelastic constitutive equation was employed to provide good descriptions of rheological properties for polymer melts. A double nodes technique at the interfaces was used to capture pressure discontinuity and to ensure continuity of total stress and velocity. Numerical case studies were performed to examine the behaviors of two or three layers extruded from a straight type annular die. Numerical results show that even at the same flow rate, variations of the layer configurations produce changes in the extrudate swell. For Newtonian fluids, the less viscous outer layer material reduces the extrudate swell relative to the mono-layer configuration. On the other hand, for viscoelastic fluids, the more elastic outer layer material enhances the extrudate swell even if the outer layer is less viscous than the inner layer. These are in general agreement with the numerical results found in the literatures. The numerical predictions for the extrudate swell of a practical problem were quantitatively examined with experimental data.

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A new decoupled finite element method has been applied to solving the extrudate flow of viscoelastic (single mode Giesekus) fluids, where non-consistent streamline upwinding and a 2×2 subelement were adopted for the constitutive equation. The elements were chosen to be 9-node quadrilaterals with biquadratic and bilinear interpolation functions for velocity components and pressures, respectively, while bilinear in each subelement for stress components. Additional subjects we examined in this study were: (1) substituting method of stress in momentum equation and (2) consistent interpolation of variables between constitutive and momentum equations. It turned out that the convergence behavior is greatly affected by a change in each treatment of the above subjects; and a recommended scheme has been identified for improving the convergence behavior of decoupled method, which is able to predict the flow of viscoelastic fluids up to a Weissenberg number of 63.

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