Joining of Polymer Films with High-Intensity Focused Ultrasound: Investigation on the Physical Principles

Previous studies showed that using a 1 MHz solid waveguide transducer based on High-Intensity Focused Ultrasound (HIFU)enables 320 μm thick low density polyethylene (LDPE)films heating above the melting temperature within 200 ms. However, the physical mechanisms causing the heating of such thin polymer films are not fully understood. It is not clear in detail, how the waves are transmitted and concentrated. Therefore, in this contribution, the physical principles responsible for polymer films heating are investigated using sound field, vibrometry (sLDV)and infrared measurements. The sound field distribution measured does not look like the typical focused sound field for an unbounded wave propagation. When the gap between waveguide and counterpart is small, the waves cannot propagate to focus and generate an energy concentration in the gap. Depending on the gap size, an irregular superposed wave field occurs. A correlation between the acoustic particle velocity at the waveguide tip and the heating inside the polymer is found. From these findings, it can be concluded that the heating in LDPE-films is caused by waveguide tip hammering on the films leading to intermolecular frictions rather than by the absorption of the focused ultrasonic wave inside the thin films.