Sensitivity of thermoforming to friction and heat transfer: Simulation and experimental validation

Thermoforming is a widely used manufacturing technology for producing thin-walled thermoplastic components, ranging from packaging and travel trolleys to automotive parts. Despite its broad applications, numerical simulation of the process remains challenging, particularly due to the complex surface interactions between sheet and mold, such as friction and heat transfer, which are not yet fully understood. This study presents a coupled thermal-structural simulation of the forming step, with a particular focus on the mold-sheet interface. The interfacial behavior between sheet and mold is strongly temperature-dependent. To characterize this behavior, the coefficient of friction at elevated temperatures is measured using a modified standard friction tester, while the heat transfer coefficient is estimated from in-situ measurements. A nonlinear viscoelastic–viscoplastic model, calibrated using shear and elongational rheological data, is applied to high-impact polystyrene in its rubbery state. For validation, a mold with a high draw ratio and a sharp negative edge was designed. The results show that, in addition to accurately predicting thickness distribution, the model successfully captures defects such as shock marks and loss of detail on sharp edges. Furthermore, the model enables an investigation of how variations in interfacial parameters influence the process outcome. The findings confirm that thermoforming is highly sensitive to both friction and heat transfer. However, since heat transfer between sheet and mold is relatively high, the interfacial temperature can be assumed to remain constant.