Large deformation finite element analysis for polymer forming processes

Abstract

<p>The object of this work is to explore in depth a variety of polymer forming processes and to examine the behavior of polymers during processing by developing large deformation finite element models and computer tools for the numerical simulations of the processes. Thermoforming, one of the major forming processes dealt with in this thesis, is the fabrication of numerous plastic products by use of heat, pressure and mold. Thermoforming is penetrating existing and new product categories due to ease of production, low costs, and the high performance of final products. Axisymmetrical finite element models have been developed that include large deformation, large strain, moving boundaries, contact of polymer with rigid mold, deformation-dependent loading, free surface evolution, and material nonlinearities. The thermoplastics in the forming processes are considered as incompressible, hyperelastic materials, since there is little time for viscous dissipation. The incompressibility condition is accurately incorporated by employing the penalty method. No restriction of sheet thickness is made in the models for thermoforming so that thermoforming of single layer or multilayer composite sheets of finite thickness can be dealt with, as well as plug-assist forming of thick sheet. A simple, efficient method for passing the limit point is established in the finite element formulations, and for the first time the limit point in thermoforming is successfully simulated using this method. Numerical simulations compare well with analytical solutions for simple geometries and experiments for thermoforming, plug-assist forming, combined plug-assist pre-stretching and vacuum forming, and compression forming. The important parameters in the processes which influence processing and products are studied, including the effects of material constants, boundary conditions, and processing sequence. For these processes, information of deformation and stress can be obtained by the developed computer models, such as deformed profile, thickness variation, the relationship between applied loading and deformation, and stress and strain variations at critical areas, which are important in process optimization and damage analysis. The computer modeling and analysis provide a comprehensive understanding of these forming processes and could be valuable for industrial designers in the process of reducing trial-and-error procedures, optimizing their designs, minimizing material and cost, and maximizing product performance.</p>