Experimental Study of Unstable Flow of Polymer Melts from Several Aspects

Abstract

Two kinds of polyethylene, which have a discontinuous point on their flow curves, and one kind of polypropylene, which has no discontinuous point, were used.
From the isochromatic patterns measured by the flow birefringence method, strong stress concentration developing near the edge of entrance in nozzles with an entrance angle of 180° were observed to decrease considerably through tapered nozzles or curved nozzles, and in some cases slip fracture and inner fracture were almost suppressed.
The variation of temperature rise was correspondent with onset of surface fracture, slip fracture and inner fracture.
Slip fracture caused much more temperature rise than did surface and inner fracture.
The yield stress of the cooled extrudates did not change over the shear rate for the various nozzles.
Measurement of molecular orientation by X-ray diffraction and polarized fluorescent light intensity showed clearly the difference in micro-structure between smooth and deformed extrudates.