Polypropylene (PP) is typically solid phase thermoformed at temperatures close to its crystalline melting point, usually in the 150° to 160° Celsius range. In such conditions the mechanical properties of the material rapidly decline with temperature and these large changes in properties make Polypropylene one of the more difficult materials to process by thermoforming. This paper presents the findings of a study into the thermoforming behaviour of an industrial thermoforming grade of Polypropylene. Practical tests were performed using custom built thermoforming equipment at Queen's University Belfast. Numerical simulations of these processes were similarly constructed to replicate thermoforming processes using industry standard Finite Element Analysis software.
Polypropylene (PP), a semi-crystalline material, is typically solid phase thermoformed at temperatures associated with crystalline melting, generally in the 150° to 160° Celsius range. In this very narrow thermoforming window the mechanical properties of the material rapidly decline with increasing temperature and these large changes in properties make Polypropylene one of the more difficult materials to process by thermoforming. Measurement of the deformation behaviour of a material under processing conditions is particularly important for accurate numerical modelling of thermoforming processes. This paper presents the findings of a study into the physical behaviour of industrial thermoforming grades of Polypropylene. Practical tests were performed using custom built materials testing machines and thermoforming equipment at Queen's University Belfast. Numerical simulations of these processes were constructed to replicate thermoforming conditions using industry standard Finite Element Analysis software, namely ABAQUS and custom built user material model subroutines. Several variant constitutive models were used to represent the behaviour of the Polypropylene materials during processing. This included a range of phenomenological, rheological and blended constitutive models. The paper discusses approaches to modelling industrial plug-assisted thermoforming operations using Finite Element Analysis techniques and the range of material models constructed and investigated. It directly compares practical results to numerical predictions. The paper culminates discussing the learning points from using Finite Element Methods to simulate the plug-assisted thermoforming of Polypropylene, which presents complex contact, thermal, friction and material modelling challenges.The paper makes recommendations as to the relative importance of these inputs in general terms with regard to correlating to experimentally gathered data. The paper also presents recommendations as to the approaches to be taken to secure simulation predictions of improved accuracy.
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.
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