Ultrasonic thermoforming of a large thermoplastic polyurethane film with the aid of infrared heating

This study concerns ultrasonic thermoforming of Thermoplastic polyurethane (TPU) film for the fabrication of a television speaker diaphragm. The speaker diaphragm has a convex-protruded feature with a rounded-rectangular shape on which a number of micro-corrugations are formed for sound quality improvement. This diaphragm has generally been manufactured out of thin TPU film using a thermoforming process, which required cycle time as long as several minutes for proper heating and cooling of the polymer film and mold. In this study, an ultrasonic thermoforming process was introduced to reduce the cycle time to fabricate the diaphragm by taking advantage of the rapid and localized heating capability of ultrasonic vibration energy. To improve the forming quality of a large diaphragm for a television speaker, infrared heating was added to the process to preheat the TPU film before the forming stage. Various processing parameters including ultrasonic thermoforming and infrared heating conditions were investigated in relation to the forming quality. As a consequence, the diaphragm could be fabricated with acceptable forming ratios (> 95 % in the band region and > 70 % in the micro-corrugations) and short cycle time (12 s). This means that the proposed process is superior to conventional thermoforming processes that require long cycle time (> 200 s).