Analysis of fiber mat preforming in liquid composite molding

This work is an analysis of the preforming process of a thermoformable fiber mat. Important processing variables associated with the preforming operations were extracted from stamping experiments. From the study of deformation modes, we may conclude that the defects found in fiber mat preforming are closely related to these deformation modes. Stretching causes fiber thinning and orientation. Bending results in mat springback and thickness reduction, while in-plane compression induces wrinkles through buckling. Each defect may be reduced by specific actions. These actions, however, tend to interact with each other, which makes the preforming control a very demanding task.;A production-scaled preforming and molding operation for producing automotive body panels was also conducted. From this operation, phenomena such as flow front lead-lag were observed. Surface digitization and model construction were conducted to simulate the resin flow in the mold cavity. In a complicated mold geometry, the preforming induced deformation altered the properties of the fiber reinforcement. The changes in fiber content, fiber orientation, and bending induced channeling accounted for the observed irregular flow patterns encountered, which have the potential to form voids in the composite parts.;A fiber network model based on deformed fiber loops was established in order to consider the orientation effect. The permeability derived from this model was a function of two factors: porosity factor and orientation. The predicted permeabilities were compared to experimental data and found very good agreement.;The last effort was to establish a tool for predicting the deformed configuration of a preform made from fiber mats. This tool incorporated the lattice model and geometric modeling techniques. A major contribution of this effort was the incorporation of the edge clamping effect. The simulation was used to help visualize the shear angle distribution and the fiber content distribution in a stamped preform.;The experimental techniques developed in this work have provided means to quantify the deformation modes and to establish the relationship between forming conditions and defects. The permeability model and study of deformation modes constitute a bridge linking the preforming and the molding processes. The data may be used to set up guidelines for the preforming process.