Stress distribution around capillary die exit: an interpretation of the onset of sharkskin defect

In order to understand the possible initiating mechanism of the sharkskin d efect, a numerical study of flow conditions at the exit of an axisymmetric die has been carried out. Using the finite element method, the flow of a vi scoelastic fluid along the reservoir, the convergent and the die land, and the free surface of the swelling extrudate, have been computed. The constit utive equation is a multimode Phan-Thien and Tanner model with five relaxat ion times, whose parameters were derived from rheological measurements in s hear and elongation. The tangential stresses and deformations supported by the molten polymer flowing along peripheral streamlines close to the free s urface have been analyzed. The results put in evidence the existence of a s mall traction zone, located at the periphery of the free surface of the ext rudate, close to the die exit. With an increase of flow rate, the dimension s and the intensity of this peripheral tensile area grows. Whatever the flo w rate, the depth of this zone remains limited, and of the order of magnitu de of sharkskin amplitude. If a critical value of tensile stress is assumed , the numerical results can support the interpretation of a rupture of the extrudate skin. On the other hand, whereas differences on sharkskin amplitu de and periodicity are relevant between a long die and an orifice die, neit her the area nor the intensity of the peripheral traction zone varies with die geometry. As a consequence, tangential stress can not be the unique par ameter able to explain the dynamic of the surface defect. The history of de formation experienced by the polymer before the die exit seems to be also i mplied in the mechanism of sharkskin formation. (C) 2000 Elsevier Science B .V. All rights reserved.