The effects of flow rate and post-fill cure pressure, i.e., packing pressure, on the mechanical properties of resin transfer molded disks are experimentally investigated. An experimental molding setup is constructed to fabricate fiber-reinforced, center-gated, disk-shaped composite parts. Disks are molded at different flow rates and packing pressures in order to observe the effects of these parameters on the mechanical properties and void content of the final parts. Specimens are cut from three different locations in the molded disks for testing. Specimens from the first two locations are tensile tested to obtain strength and stiffness properties, and the third location is used for microscopic analysis to determine void content and void properties. Increased injection rate is found to reduce both the strength and stiffness of the molded parts due to more voids induced by the faster moving fluid front. Packing pressure is also found to have a significant effect on specimen properties. At higher packing pressures fewer voids and improved strength and stiffness are observed. Mechanical properties are correlated with total void fraction for disks molded at different packing pressures. Exponential decrease in both tensile strength and elastic modulus is observed with increasing void fraction. Doubling the void volume fraction from 0.35 to 0.72% results in a 15% decrease in strength and a 14% decrease in stiffness. The results demonstrate that selection of suitable injection rate and addition of packing pressure to resin transfer molding (RTM) process can improve mechanical properties of molded parts considerably.

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.

ABSTRACT:The effect of gravity on free surface shape during the filling of a disk-shaped cavity is experimentally studied. A disk-shaped mold cavity is constructed to measure spreading of the fluid front due to gravity. Fluid front sensors are mounted on the top and bottom mold walls at three radial locations. The time the fluid front reaches these sensor locations is recorded. This data is used to calculate the radial distance between the top and bottom of the fluid front at each sensor location. The important parameters that govern the fluid front dynamics are identified as the Reynolds, Bond, and Capillary numbers. Spreading is found to be mainly dependent on the Bond number, such that a large change in spreading is observed due to a small change in Bond number. Spreading increases with decreasing Capillary number and remains nearly constant over the range of Reynolds numbers studied.

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ABSTRACT:The effect of gravity on free surface shape during the filling of a disk-shaped cavity is experimentally studied. A disk-shaped mold cavity is constructed to measure spreading of the fluid front due to gravity. Fluid front sensors are mounted on the top and bottom mold walls at three radial locations. The time the fluid front reaches these sensor locations is recorded. This data is used to calculate the radial distance between the top and bottom of the fluid front at each sensor location. The important parameters that govern the fluid front dynamics are identified as the Reynolds, Bond, and Capillary numbers. Spreading is found to be mainly dependent on the Bond number, such that a large change in spreading is observed due to a small change in Bond number. Spreading increases with decreasing Capillary number and remains nearly constant over the range of Reynolds numbers studied.