Real-time monitoring of fabrication process of microfluidic plastic devices using miniature ultrasonic sensors

Miniature ultrasonic sensors have been developed for real-time process monitoring of a microfluidic device fabrication using a polymer injection molding machine. A thick piezoelectric PZT film as an ultrasonic transducer was fabricated onto one end of 4 mm-diameter and 12 mm-long steel buffer rods using a sol gel spray technique. A signal-to-noise ratio of more than 30 dB for ultrasonic signals reflected at the probing end has been achieved. The center frequency and 6 dB bandwidth were 17 MHz and 14 MHz, respectively. The sensor can operate continuously at 200°C without ultrasonic couplant and cooling. Clear ultrasonic signals have been obtained during injection molding of a 1 mm- thick part having test patterns on its surface. Shrinkage of the molded part and the part detachment from the mold were successfully monitored. Surface imperfection of the molded parts due to the lack of the holding pressure was discussed.