An experimental study is performed to characterize the effect of the thickness of random preforms on injection pressure and mechanical properties of resin transfer molded (RTM) parts. Center-gated, disk-shaped parts are molded using two different chopped-strand glass fiber preforms. Both preforms have random microstructure but different planar densities (i.e., different uncompressed layer thicknesses). Tensile strength, short-beam shear strength, and elastic modulus are measured for parts molded with each preform type at three different fiber volume fractions of 6.84, 15.55, and 24.83%. Although mechanical properties are found to increase linearly with volume fraction, significant difference is not observed between disks containing thick and thin mats at equivalent fiber volume fraction.
However at the same fiber content, parts molded with thin mats require significantly lower injection pressures compared to parts containing thick mats. To characterize this phenomenon, a pressure-matching method to determine planar permeability is presented. Permeability values for each preform would provide a quantitative description of the required injection pressure due to changes in preform thickness, with lower permeabilities resulting in higher injection pressure. Transient pressure data is collected at a fixed radial location within the mold cavity during filling for both preforms at three different volume fractions. Permeability is obtained by fitting the theoretical pressure equation derived from Darcy’s law to the transient pressure data. Permeability values are found to be as much as 227% higher for thin mats compared with thick mats at the same fiber content. The results demonstrate that equivalent mechanical properties can be obtained at lower injection pressures by using thinner random mats.
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