Measuring melt density in polymer extrusion processes using shear ultrasound waves

Ultrasound shear waves propagating at a frequency of 2.25 MHz are used to measure the density of polymer melt in real time during extrusion process. The acoustic impedance of the polymer melt is calculated using the measured reflection coefficient off the polymer melt interface boundary. A normalisation procedure was developed to normalise for the effect of other process and material parameters not pertinent to the melt density. The ultrasound measurement is independent of the attenuation in the polymer melt and the thickness of the melt stream, which makes this technique highly desirable for heavily loaded polymers and large extrusion dies. The system was evaluated using polyvinyl chloride compounds containing various levels of foaming agent and process aid at fixed processing conditions. The system measurements of the melt density are compared to the weight of the extrudate measured by a laboratory scale. The correlation coefficient between the two measurements is approximately 96%.