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Thermoforming Process Optimization for Reduction of Inter-Ply Friction of Out-of-Autoclave Material



Thermoforming of composite materials includes deforming the flat sheets of prepreg laminates into complex shaped composite parts. This deformation of flat laminates into complex shapes can cause wrinkles in the final part. External processing conditions of forming process must be chosen in a way to minimize wrinkles. Taguchi method is the statistical way to optimize the processing conditions and improve the quality of final product. In this paper, a numerical approach is described to find the optimum processing parameters in thermoforming of composites. The three main processing parameters are temperature, normal pressure and forming rate. Three levels were selected for each processing parameter. A test rig was designed and manufactured to perform experiments. This test rig can simulate the conditions of different temperatures and normal pressures. It was installed on a tensile testing machine which can simulate different forming rates by pulling the prepreg plies at different speeds. Load cell mounted on the tensile testing machine was used to measure frictional force between plies. An appropriate orthogonal array was selected and experiments were performed according to the combinations of process parameters in the array. After performing the experiments, inter-ply friction was analyzed. Using smaller-the-better formula Signal to Noise ratio was calculated. Analysis of Variance was used to find the contribution of each parameter. It was observed that optimum processing conditions for lowest inter-ply friction are high temperature, low normal pressure and low forming rate. Temperature appeared as the most influencing processing parameter.

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