Effects of material properties and numerical simulation on thermoforming acrylic sheets

Thermoforming in manufacturing plastics is considered as a “secondary” forming process in which a previously extruded or cast polymeric sheet is reheated and “inflated” using vacuum or pressure into a mould cavity. Material properties of thermoforming polymeric materials such as elongational mode, strain hardening and/or softening, glass transition temperature and rubberlike properties are worthwhile to be investigated as the preliminary stage of numerical modelling. Finite element simulation of thermoforming process aims at obtaining more useful features by tracking the polymeric materials from final configuration back to the original sheet and analysing the thickness distribution of the final products. Thus the process parameters such as forming temperature, forming pressure, heating time and plug speed etc. can be correctly selected. Meanwhile optimal mould geometry design and good quality of final products using minimum materials are achieved as well. In this study, “Shinkolite” acrylic sheets (opaque and transparent) are chosen as the research materials, which are now widely used in manufacturing bathwares and kitchen appliances. As thermoforming of acrylic sheets is carried out above their glass transition temperature, acrylic sheets under large deformation and large strain demonstrate the viscoelastic behaviours and temperature-dependent mechanical properties. Due to the consideration of isotropic or anisotropic properties of acrylic sheets during the first forming process, shrinkage test is exclusively executed. Fundamental study begins with the investigation of basic mechanical properties of acrylic sheet by conducting uniaxial tensile tests at elevated temperatures (150 C to 190 C ) and with various crosshead speeds (50 mm/min to 500 mm/min). Thermal and viscoelastic properties and analysis of glass transition temperature of acrylic sheets are also taken into account by performing DMTA (Dynamic Mechanical Thermal Analysis) and DSC (Differential Scanning Calorimetry) tests respectively. FEM (Finite Element Method) is introduced with the assumption of 2-D membrane approximation, which neglects the influence of the non-uniform temperature, in-plane bending and shear stress. By reviewing the theoretical models of thermoforming simulation, Mooney-Rivlin model and Ogden model prone to hyperelastic materials are employed with the least-square method to fit the uniaxial tensile test curves. Computer simulation of the thermoforming process is carried out with a commercial dynamic explicit code Pam-Form TM incorporating the obtained material properties under the isothermal condition. During Pam-Form TM simulation, adaptive mesh refinement is automatically programmed to improve the performance of final simulation geometry results. Furthermore in the initial uniaxial tensile test simulation, algorithm validity by comparing with the experimental data and Mooney-Rivlin model analytical solution is well constructed. Satisfactory agreement with the classical theoretical solution is also obtained for the bubble inflation and cup-forming simulation. At the final stage, a rectangular container with the complex shape is simulated with the implementation of adaptive mesh refinement and nonlinear contact algorithm to study the potential of dynamic explicit scheme dealing with the complicated models.

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