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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 × 105 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.

This paper demonstrates the usefulness of frequency analysis in monitoring flow behavior in a single-screw extruder fitted with a capillary die with a length to diameter aspect ratio of 20. The melt pressure in the barrel and die was measured by a transducer, and the pressure fluctuations processed in a Fast Fourier Transform (FFT) frequency analyzer. The pressure fluctuations were studied under various processing conditions for high density polyethylene, linear low density polyethylene and polypropylene. In general, stable extrudate flow generates a noise spectrum with low intensity, the intensity increasing with the level of instability. The machine vibrations appeared to have a direct effect on flow stability. Also, unstable melting of the resin along the screw resulted in an increase in noise level. It seems probable that the frequency of the surface distortions can be related to the shape of the noise spectrum. In general, although more complex in nature, the noise results related to melt flow in the extruder were in agreement with data reported for capillary flow using a piston rheometer.

The results of an investigation of the flow behaviour of hard-metal carbide powder compounds in a capillary rheometer operating at constant piston speed are presented. The suspensions, consisting of a polymeric binder (PE + paraffin + copolymer EBA, EVA or EAA) and carbide powder, were prepared by melt mixing using a Brabender Plasticorder. The relations of Maron-Pierce, Quemada and Mooney were applied to calculate maximum loading values for the compounds used. Close to the maximum loading the character of the flow changed from pseudoplastic to plug flow. This was accompanied by the appearance of flow instabilities similar to pressure oscillations observed with unfilled polymers. The calculations of the average thickness of the polymer layer bound to the surface of the carbide particles were in fair agreement with microscopic observations.

A fundamental investigation of the influence of fibre loading, fibre length and matrix properties on shear viscosity, complex viscosity and the first normal stress coefficient of filled polyethylene melts prepared by different compounding techniques was carried out. Carbon fibre was compounded into two different types of low-density polyethylene (PEA and PEB) at concentrations up to about 20% by volume using different compounding techniques: a single screw extruder, and a twin screw extruder. The main difference in flow behaviours for the two kinds of carbon fibre-filled systems (CF/PEA and CF/PEB) is not due to differences between the matrix polymers, but due to the difference in fibre length or its distribution caused by the difference in compounding technique. Increasing fibre loading and fibre length increases the shear viscosity and the normal stress coefficient, with the highest increase observed at low shear rates. Especially for longer fibre-filled melts (CF/PEA series) one can estimate the yield stress in the very low shear rate region from the steady shear and dynamic flow data. In the moderate and high shear rate regions the viscosity and normal stress coefficient are influenced greatly by shear flow. Data on the relative viscosities ηr, the relative complex viscosities |η*|r and the relative normal stress coefficients ψr of the two filled systems in the moderate and high shear rate regions were compared with the empirical equation derived previously for ηr for short fibre filled polymer melts. It was found that a form of the expression similar to that obtained previously could describe ηr, |η*|r, and ψr for relatively long carbon fibre filled systems.