Compensating Thermoset Composite Panel Deformation using Corrective Molding

Abstract

Open mold composite (OMC) body panels, comprising three thermosetting layers, warp significantly after removal from the mold. Due to an asymmetric layup of these laminates, a residual stress develops due to chemical shrinkage of the gel coat resin during cure. The resulting bending moment distorts the panel. This work first develops a finite element model (FEM) to calculate panel deformation based on the chemical shrinkage and mechanical properties of the laminated panel. The FEM is then validated by a direct comparison with the measured distortion of door-outer panels produced using a conventional panel-shaped mold. This FEM is then integrated into an algorithm to calculate a mold shape that corrects for this distortion. Starting with a mold duplicating the desired panel shape, the initial panel deformation is calculated using the FEM. Trial molds are then generated by partial inversion of the deformed panel's deviation from the desired shape. The algorithm is iterated until it converges to a corrective mesh that deforms to the mesh of the desired shape of the panel. A corrective mold for a door-outer panel is fabricated and the effectiveness of the corrective mold is evaluated by measuring the distortion of the door-outer panels it produces. Compared to the panels produced by the conventional mold, the corrective mold reduces panel distortion by 45%.