Numerical simulation of injection/compression liquid composite molding. Part 2: preform compression

In the injection/compression liquid composite molding process (I/C-LCM), a liquid polymer resin is injected into a partially open mold, which contains a preform of reinforcing fibers. After some or all of the resin has been i njected, the mold is closed, compressing the preform and causing additional resin flow. This paper addresses compression of the preform, with particul ar emphasis on modeling three-dimensional mold geometries and multi-layer p reforms in which the layers have different mechanical responses. First, a n ew constitutive relation is developed to model the mechanical response of f iber mats during compression. We introduce a new form of nonlinear elastici ty for transversely isotropic materials. A special case of this form is cho sen that includes the compressive stress generated by changes in mat thickn ess, but suppresses all other responses. This avoids the need to model slip of the preform along the mold surface. Second, a finite element method, ba sed on the principle of virtual displacement, is developed to solve for the deformation of the preform at any stage of mold closing. The formulation i ncludes both geometric and material nonlinearities, and uses a full Newton- Raphson iteration in the solution. An open gap above the preform can be inc orporated by treating the gap as a distinct material layer with a very smal l stiffness. Examples show that this approach successfully predicts compres sion in dry preforms for three-dimensional I/C-LCM molds. (C) 2001 Elsevier Science Ltd. All rights reserved.