A Finite Element Study of Viscoelastic Effects in Double-Layer Coextrusion of Polymer Melts* X.-L. Luo and E. Mitsoulis Department of Chemical Engineering University of Ottawa Ottawa, Ontario, Canada KIN 9B4 ABSTRACT A finite element simulation of viscoelasticflows in double-layer coextrusion of polymer melts has been performed. The constitutive equation used is a realistic integral model of the K-BKZ type which can describe the viscoelastic memory characteristics of polymer melts. Viscosity and first normal stress data of typical PS and HDPE melts at 200°C are employed along with afeed ratio used in actual experiments found in the literature. Our results reveal important viscoelustic effects that could not be predicted by previous inelastic simulations. In particular, when the outer layer is more elastic than the inner layer, an enhanced extrudate swell is observed relative to the opposite eonfiguration at the same flow rate, even if the outer layer is less viscous than the inner layer. In terms of a dimensionless stress ratio SR measured on the die wall, the more elastic material at the outer layer results in a higher SR value than that of the opposite configuration at the same flow rate. On the other hand, when the outer layer is less elastic than the inner layer, a reduced extrudate swell is observed relative to the opposite configuration at the same flow rate even if the outer layer is more viscous than the inner layer. When compared at the same SR value, however, our results show that the inelastic swell mechanism proposed by Tanner still applies qualitatively, i.e., a more viscous outer layer will result in larger extrudate swell than the opposite configuration. INTRODUCTION In recent years coextrusion is commonly practiced by the polymer processing industry for the production of multilayered films, fibers, pipes, wire coatings, etc. Up to 11 layers can be used in many combinations.' The *Financial assistance from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. Advances in Polymer Technology, Vol. 10, No. 1, 47-54 (1990) 0 1990 by John Wiley & Sons, Inc. industrial design of coextrusion dies has been so far based on trial-and-emor practical experience. Much progress has been made and new feedblock technology can pro- vide capabilities to coextrude materials with viscosity ratios up to 40: 1. It is also possible to coextrude materials with melt temperature differences as high as 80°C without causing damage or thermal degradationto a heat-sensitive material. The layer thickness can also be adjusted so as CCC 0730-6679/90/010047-08$04.00