Measurement of temperature and pressure distribution during ultrasonic processes by sensor foils from polyvinylidene fluoride

Micro and nano structures and systems are generated in polymer surfaces with cycle times of a few seconds by ultrasonic processes such as ultrasonic hot embossing, welding and thermoforming paving the way for a variety of new products. However, measuring temperature and pressure during these processes is very difficult because the polymer is enclosed between an anvil and a sonotrode. Temperature and pressure distribution during ultrasonic processing now have been measured inside of a stack of thermoplastic polymer layers by sensor foils from polyvinylidene fluoride, 55 µm in thickness. The measurements are based on the piezoelectric and pyroelectric effect of polyvinylidene fluoride allowing to achieve resolutions in temperature and pressure of up to ± 1 °C and ± 0.5 kPa, respectively. The achieved resolutions in time and in normal and lateral direction are approximately 1 µs, 60 µm and 1 cm, respectively. The maximum temperature inside a foils stack that could be measured was 73 °C because the sensor foils lost sensitivity when heated up more. Every single oscillation of the polymer was measured as a pressure change. The difference in temperature change and ultrasonic pressure amplitude measured in lateral direction below a sonotrode with outer dimensions of 8 × 12 cm are approximately 12 °C and 4 kPa, 25 and 60%, respectively, indicating the width required for process windows of ultrasonic processing. Moreover, phase shifts are measurable and thus analysis of oscillation characteristics of sonotrodes were investigated.