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Resin volumetric changes and surface finish characterization of composite automotive panels

McGill University
Publication Date
  • Engineering - Materials Science


The automotive industry is increasingly using resin transfer moulding (RTM) to produce composite body panels at high volumes and low costs. However, one of the recurring issues is the control of the parts surface finish, which is closely linked to the resin volumetric changes during cure. These volumetric changes are caused by the resin cure shrinkage and thermal expansion which depend on material and processing parameters. Low profile additives (LPA) are thermoplastic particles incorporated to unsaturated polyester (UP) resin to compensate for cure shrinkage. The effect of LPA content and degree of cure on the volumetric cure shrinkage and the coefficient of thermal expansion (CTE) were investigated with a modified rheology procedure and thermo-mechanical analysis (TMA). The resin glass transition temperature (Tg) was also evaluated from the TMA results. The resin characterization efforts were validated through the moulding of Class A F3P glass preform composite panels under optimized processing conditions in order to obtain the best surface finish. The average roughness (Ra) and the average waviness (Wa) were measured by profilometry. To observe the effect of the painting process on the surface quality of the panels, the latter were submitted to low-temperature and high-temperature painting cycles according to the industry standards. Their surface finish was then measured again by profilometry and related to the as-moulded results. The results of the cure shrinkage and thermal expansion characterization were finally used to predict the dimensional changes of a panel during typical manufacturing by RTM. The surface quality was improved after the low-temperature painting cycle. There was however a decrease of quality after the high-temperature cycle. The dimensional changes predictions of typical UP/fibreglass panels during RTM were found to be in accordance with surface finish measurements.

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