Experimental study of the volume defects in polystyrene extrusion

This paper presents an experimental characterization of the volume defects in polystyrene extrusion. In a first step, these defects are quantified in capillary rheometry. In a second step, a transparent slit die, fed by an extruder, is used to perform birefringence experiments and to visualize stress transients during the instability. Isochromatic fringe oscillations are observed in the reservoir at the onset of the defect and during its development. Their frequency has been estimated and appears to be very sensitive to flow conditions (temperature, convergent geometry), but not to apparent shear rate. Using small entry angles allows to postpone the defect occurrence. Furthermore, the use of a smooth convergent leads to the suppression of the instability, at least until an apparent shear rate of 390 s(-1). The possible origin of the instability is discussed. (C) 2004 Elsevier B.V. All rights reserved.