A Study of the Density of Polymer Melts During Extrusion

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Abstract
Measurements have been made of the apparent density of polymer melts extruding from a reservoir into a capillary orifice. The technique involves simultaneous measurement of extrudate mass and the corresponding decrease in volume of polymer in the reservoir. Data at low shear rates agree closely with density values from other experimental methods. Sequential density readings fluctuate because variations in speed of the driving piston are damped by successive compressions and expansions of the polymer melt. A linear polyethylene was shown to suffer a significant density decrease as the shear rate was increased. This dilation is due to the triaxial action of normal stresses and can be used, with an appropriate model, to estimate the primary normal stress difference and recoverable shear in the flowing melt.

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