Measurement of Heat Transfer for Thermoforming Simulations

Thermoforming is a polymer processing technique in which an extruded sheet is heated to its softening temperature and then deformed through the application of mechanical stretching and/or pressure into a final shape. The stretching operation is often performed by the movement of a mechanical plug, which contacts some areas of the sheet. During contact it is known that conductive heat transfer between the plug and sheet materials is an important factor in determining the process output. Attempts are currently being made to build realistic simulations of the thermoforming process and it is therefore extremely important that these heat transfer effects are included. However, the measurement of thermal contact conductance (TCC) between polymer pairs is extremely difficult in practice and there are no published values in literature. In this study, an axial conductive heat flow test rig has been developed and used to measure the TCC between contacting polymer pairs. Preliminary results between PVC and PTFE have shown that the value of thermal conductance is very small compared to published values for polymer/metal interfaces. Tests have also been carried out on Hytac®-B1X and PP. The TCC values were found to lie between 28.2-34.4 W/m 2 -K for average interface temperature between 45-75 °C. There was a slight increase in TCC with increasing interface temperature up to a temperature of about 70 °C. However, more experiments will be required to ascertain this. Further tests are being carried out to measure the TCC between different polymer pairs, and to assess the effects of variables such as surface roughness and contact pressure. These results will then be used to develop realistic models for contact heat transfer in thermoforming simulations.