Pressure Oscillations during Capillary Extrusion of High Density Polyethylene

Abstract This paper demonstrates the usefulness of frequency analysis in monitoring melt flow instabilities. The flow behavior of a film-blowing grade of high density polyethylene has been studied using a capillary rheometer at constant piston velocities. The pressure in the reservoir was measured by a transducer fitted to the driving end of the piston. The signal was amplified and processed in a Fast Fourier Transform (FFT) frequency analyzer. The pressure fluctuations (noise) have a very low intensity at low flow rates in the sharkskin region. The noise signal increases dramatically at a critical shear stress of about 1.5 × 10 5 Pa, where the flow assumes a pulsating character (spurt defect). Above the pressure oscillation region the melt goes through a pseudostable region, followed by a secondary pressure oscillation region, eventually reaching a chaotic stage. The noise spectrum intensity seems to be related to the severity of the surface disturbances of the extrudate in this latter branch of the flow curve.