Transient thermal analysis of close pressure thermoforming process

The processing of polymer sheets above the glass transition makes the thermoforming process temperature sensitive. The temperature distribution of the sheet has a significant impact on the mechanical behavior of the raw sheet during the process of transformation into the final product. It governs the aesthetic characteristics of the product formed, as well as its functionality. Thus, it is imperative to investigate the change in the thermal state of the sheet during processing. Various devices are available to monitor the temperature distribution of sheet with the requirement of direct access to the sheet surface, which not possible in the case of a closed thermoforming setup. Hence, in this study, the thermal state of the PMMA sheet during conversion into a hemispherical dome using a closed pressure thermoforming setup is investigated numerically. To incorporate the temperature dependence, temperature-dependent material properties, along with the necessary contact conditions, were used. It was found that radiation is the dominating mode of heat transfer, and for a most accurate result, realistic contact conditions must be used. Moreover, an experimental investigation was performed, and it was found that numerically predicted sheet temperature has good agreement with the measured sheet temperature. We proposed numerical simulation as a tool to monitor sheet temperature for the closed thermoforming setup where thermal monitoring is not possible without degrading the process output.