Cavitation in thermoplastic melts: New insights into ultrasound-assisted fibre-impregnation

The impregnation of continuous carbon fibre roving with thermoplastic melt is a challenging task due to the high viscosity and surface tension of the melt. A new technique is under development utilizing ultrasonic oscillations within the thermoplastic melt which encloses a fibre roving, to achieve fibre impregnation. Despite ultrasonic processing being very efficient, the specific conditions created in the thermoplastic melt have never been studied before. This study investigated whether cavitation effects could be present during ultrasound-assisted fibre impregnation. The observed acoustic effects allowed us to suggest the possible underlying mechanisms. For the purpose of the study a melt-bath impregnation setup with polylactide was built. To detect the cavitation effects and acoustic parameters a calibrated high-temperature cavitometer was used. The results showed the formation of small cavitation zones in the direct vicinity to the sonotrode tip where the fibre roving would be positioned. Therefore, the occurrence of cavitation was established, and induced effects like shock waves, microjets and microstreaming should be further considered for detailed investigation of the ultrasound-assisted impregnation mechanism.