Simulation of the plug-assisted thermoforming of polypropylene using a large strain thermally coupled constitutive model

Thermoforming is widely employed in industry for the manufacture of lightweight, thin-walled products from pre-extruded plastic sheet and its largest application is in packaging. Over many years attempts have been made to simulate the process and thereby exploit modern computational tools for process optimisation. However, progress in this area has been greatly hampered by insufficient knowledge of the response of polymer materials under thermoforming conditions and an inability to measure this and other processing phenomena accurately. In recent years some address has been made to these problems through advances in measurement technologies, and in particular, the development of high speed, high strain, biaxial testing machines that are designed to replicate the conditions in thermoforming processes. In this work the development of an advanced finite element-based thermoforming process simulation is presented. At its heart is a sophisticated large strain thermally coupled (LSTC) material model for polypropylene, which has been developed after several years of research and is founded directly on biaxial test results at elevated temperatures. This material model has been demonstrated to provide an excellent fit to the biaxial data and to offer a very stable computational platform for the process simulation. The performance of the working simulation was validated through comparison with matching experimental test results, and this enabled investigation of the sensitivity of the process output (in the form of part wall thickness distribution) to changes in a range of other processing parameters. This work confirmed that the process is most sensitive to the parameters controlling plug/sheet contact friction. Heat transfer parameters were also shown to be significant and the requirement for the model to be fully thermo-mechanically coupled has been clearly established.