Ultrasonic preheating and embossing of polymers
Tan, Jun Liang
Date of Issue2016
School of Mechanical and Aerospace Engineering
Singapore Institute of Manufacturing Technology
Energy conservation, greenhouse gas emissions and energy consumption reduction in manufacturing processes are important considerations with the depletion of fossil fuels. Process improvement, appropriate machine design and waste heat recovery are some of the steps for reducing energy consumption. Energy consumed by a conventional hot embossing process can be considerable. In contrast, using ultrasonic vibrations for polymer embossing could result in a rapid, yet low energy consumption process; this is because of its ability to localise energy at a specific region resulting in a small effective thermal mass. Although ultrasonic embossing is rapid, it has inherent difficulties to be addressed. Commercial ultrasonic systems are configured for welding and cutting and are not for embossing process. The required embossing precision in the micrometre range cannot be achieved. In this investigation, a compact machine frame was designed specifically for the ultrasonic embossing process. Alignment between the ultrasonic actuator, the mold and the embossed polymer substrate is achieved with an innovative adjustable pretension passive alignment module, without the need of complex active stages. This allows the incorporation of a commercial ultrasonic module into the machine, but with the required precision. Through the development of a proper control strategy, it is shown that ultrasonic vibration alone can achieve the required preheating of the polymer substrate without having any additional heating and cooling peripherals. This is an important achievement. It results in significant reduction in energy usage, and defect free feature with more consistent dimensions can be embossed. Ultrasonic embossing displaces material to form the feature without any material removal. Shoulder formation is thus inherent in an embossing process. Shoulder formation is generally regarded as undesirable but cannot be totally eliminated as mass conservation has to be satisfied. Through ultrasonic preheating, shoulder formation has been significantly reduced. A further innovation of this investigation is to utilise this unavoidable shoulder formation as energy directors for the subsequent ultrasonic bonding process to form the encased channels; as such, the shoulder is no longer a defect or an undesirable feature. This bonding process, can be achieved with the same developed system. Energy benchmarking conducted indicates that the energy consumed by the ultrasonic embossing system is 6 times less than a conventional hot embossing system. This low energy usage can be attributed to the localised heating characteristic of ultrasonic embossing resulting in a small effective thermal mass. This was verified by cross sectioning the formed feature, where a small region of heat processed zone could be observed at the vicinity of the embossed feature.