Measurement & Modelling of Slip during Plug-Assisted Thermoforming

Abstract

Plugs are a common feature of most deep-draw thermoforming processes and are used to ensure that the wall thickness distribution in the final product is controlled and balanced. Through contact with a moving mechanical plug, the heated sheet is locally captured and protected from excessive deformation and thinning. Previous work has clearly demonstrated that slip plays a critical role during this process and that its magnitude is determined by frictional properties that are strongly dependent on temperature. Work to discover the appropriate friction relationships has been very limited to date and this has greatly hampered the progress towards effective thermoforming process simulations. In this paper the magnitude of slip that occurs during the plugging stage of the thermoforming process was experimentally investigated. Preform shapes were created by pushing a specially designed plug into a heated sheet and then freezing it at the end of the plug displacement. A variety of processing parameters such as the plug and sheet materials, the temperature and plug displacement were evaluated. The results show that large variations in slip occur when different combinations of plug and sheet materials are employed and these are most affected by the contact temperature. A finite element based simulation of the plugging process is currently being constructed and it will be used to investigate different friction relationships and compare their performance with the experimental behaviour.