Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
A finite element simulation of viscoelastic flows 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 a feed ratio used in actual experiments found in the literature. Our results reveal important viscoelastic 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 configuration 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.
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