In-line density monitoring of rigid PVC foam during extrusion process

An ultrasound-based sensor has been investigated to monitor the density of rigid PVC foam, in-line, during the extrusion process. A 2.25 MHz ultrasoun d transducer was used to measure the acoustic properties: i.e. speed of sou nd, acoustic impedance, and attenuation, in the polymer melt prior to exili ng the extrusion die. A special transducer adapter and,wave reflector were incorporated in the design of the extrusion die. The density of PVC foam wa s varied by varying the blowing agent content in the resin compound prior t o extrusion. A mathematic al derivation of the behavior of ultrasound waves in the melt is presented and used to calculate the attenuation coefficient of the polymer melt at various foam densities. An empirical relation was d erived between the calculated attenuation coefficient of the polymer melt a nd the measured density of the foam extrudate. This relation can be used to estimate the foam density based on the acoustic measurement of the melt in the extrusion die. Further work is proposed to normalize for the effect od variations in the melt temperature and pressure on the ultrasound measurem ents.