Effect of Fiber Content on Void Morphology in Resin Transfer Molded E-Glass/Epoxy Composites

Effect of fiber volume fraction on occurrence, morphology, and spatial distribution of microvoids in resin transfer molded E-glass/epoxy composites is investigated. Three disk-shaped center-gated composite parts containing 8, 12, and 16 layers of randomly-oriented, E-glass fiber perform are molded, yielding 13.5, 20.5 and 27.5% fiber volume fractions, respectively. Voids throughout these disk-shaped composites are evaluated by microscopic image analysis of samples obtained along the radius. Each identified void’s equivalent radius, area, and shape are determined at 200x magnification. Number of voids is found to decrease moderately with increasing fiber content. Void areal density decreased from 10.5 to 9.5 voids/mm² as fiber content is increased from 13.5 to 27.5% fiber content. Similarly, void volume fraction decreased from 3.1 to 2.5%. Average void size is observed to remain similar at 53 to 55 μm when the fiber content is increased from 13.5 to 27.5%. Increasing fiber volume fraction from 13.5 to 27.5% lowers the contribution of irregularly-shaped voids from 40% of total voids down to 22.4%. Along the radial direction, combined effects of void formation by mechanical entrapment and void mobility are shown to yield a spatially complex void distribution. However, increasing fiber content is observed to affect the void formation mechanisms as more voids are able to move towards the exit vents during molding. These findings are believed to be applicable not only to resin transfer molding, but generally to liquid composite molding processes.