Understanding a frequency shifting phenomenon interfering with in-line acoustic monitoring of an extrusion compounding process for polymer composites

This study investigated an anomalous frequency shift observed in collected spectra from an inline monitoring system based on guided ultrasonic waves, with the changing flow rate of an extrusion compounding process for fiber-reinforced thermoplastics. Three possible process parameters to explain the ultrasonic peak shifting, namely melt temperature, velocity, and fiber length were evaluated. The unlikely potential of a doppler moment due to melt velocity, was readily dismissed in the analysis since fluid flow through the die was too slow and while resonance frequency variation may be possible from the related fiber damage associated with increasing flow rates, there was insufficient physical evidence of this anticipated effect in this study. Melt temperature variation associated with viscous dissipation was concluded to be the dominant cause for the frequency shifting noted in the acoustic spectra. The changes in material temperature through which the sound travelled were varying the extent and frequency of the dispersion modes in the polymer melt. These findings are new guidance to processors on setting up a system using active ultrasonics for in-line monitoring in the polymer composites industry.