Effect of Nozzle Structure on Unstable Flow of Polymer Melts

Abstract

Onset of unstable flow of polymer melts, which is sometimes called melt fracture, was studied in relation to nozzle structure. Nozzle structure was changed in length, entrance angle and diameter of curvature at entrance. Two kinds of polyethylene samples and one polypropylene sample were used.
The flow curves of polyethylene were discontinuous, but that of polypropylene was continuous. Three types of unstable flow, namely surface fracture, slip fracture and inner fracture, were observed. The two polyethylene samples and the polypropylene sample showed slip fracture alone, both surface and slip fracture, and both surface and inner fracture, respectively.
Though the discontinuity of polyethylene did not disappear, tapered nozzles made the extrudate deformation by slip fracture very much smaller, and curved nozzles like a quadrant nozzle almost suppressed the deformation by slip and inner fracture. On the contrary, surface fracture occurred at lower shear rate through curved nozzles.