Finite element modelling the forming of components for portable electronic devices
Due to the growing popularity of portable electronic devices, a drive for their continuous development and improvement is ever-present. Micro-speaker components are no different, with new materials and configurations proposed to fulfil these increasing performance requirements. The favourable inherent mechanical properties possessed by poly(ether-ether-ketone) (PEEK) suggest its inclusion as a critical component within a global material assembly for the next evolution in micro-speaker component membranes. The industrial practice of thermoforming is widely used for the large-scale manufacture of lightweight, thin-walled polymeric parts with the advantage of producing repeatable, complex products, paired with relatively cheap production costs. Desire to accurately predict the deformation behaviour throughout the forming process has led to the development of mathematical models representative of material behaviour, with the ultimate aim of embedding these relationships into forming simulations enabling process and product optimization. Herein lies a methodology to effectively characterise and model the constitutive thermomechanical response of thin PEEK films when subject to loading histories characteristic to those experienced during the thermoforming process. Following design and manufacture of a custom rig chosen to replicate the multi-axial state of stress induced associate with the forming process, initial load controlled bulge testing experiments revealed non-linear strain paths in principal directions yielding average and maximum strain rate ranges of 2.5–5 s-1 and 5–18 s-1 respectively, across the forming temperature range of 130–155 ⁰C. Extensive in-plane biaxial testing used to uncover the specific contributions of external variables on the observed stress-strain behaviour highlighted the anisotropic non-linear viscoelastic behaviour of the films – with strong dependence of the yield and strain hardening behaviour on the temperature and strain history at conditions equivalent to the forming process. Upon knowledge of the thermomechanical stress-strain behaviour, a step-by-step calibration of a suitable viscoelastic material model was performed. Modifications were performed accounting for the inherent anisotropy and time-temperature dependency of yielding behaviour observed at temperatures above and below the glass transition. The modelled stress-strain results showed the revised material model to be in good quantitative agreement with the biaxial deformation behaviour of PEEK films exhibited during extensive displacement-controlled experimental tests. A finite element model replicating the loading and geometry associated with load-controlled biaxial tests was constructed in order to validate the previous calibration of the modified material model. Comparison between the material behaviour generated from the simulations and those recorded experimentally showed stress and strain plots, along with shape evolution, were seen to be accurately reproduced for the chosen forming conditions across the temperature range. In summary, this thesis addresses key limitations concerning the multi-axial characterisation, modelling and simulation PEEK films intended for micro-speaker component diaphragm applications. The presented results may be used to aid in the future optimisation of the forming of polymeric micro-speaker membranes of uniform thickness.
poly(ether-ether-ketone), thermoforming, constitutive modelling, biaxial stretching, finite element model, simulation
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