Effects of process parameters and mold design on resin transfer molded composites.

Abstract

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. 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. Increased injection rate is found to reduce both the strength and stiffness of the molded parts due to the formation of more voids behind the faster moving fluid front. At higher packing pressures, fewer voids and improved strength and stiffness are observed. Mechanical properties are correlated with the total void fraction for disks molded at different packing pressures. A 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 about 15% decrease in strength and stiffness.

Channels are observed to significantly alter the flow pattern, by allowing the fluid front to reach the mold edge early in the filling process for longer channel lengths. (Abstract shortened by UMI.)

An experimental study is performed to characterize the effect of the thickness of random preforms on injection pressure and mechanical properties of 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 volume fractions of 9.8, 21.2, and 33.1%. 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 fractions. However, at the same fiber content, parts molded with thin mats require significantly lower injection pressures compared to parts containing thick mats. The final phase of the presented research goes beyond traditional RTM. An enhancement to the molding process is presented which is expected to yield substantial reduction in molding pressure. Small flow channels are machined in mold walls, resulting in low resistance flow paths for resin flow during impregnation. Experiments are performed to evaluate the effects of channels on molding process. Disk-shaped parts are fabricated via RTM utilizing molds modified with either 1 or 2 flow channels on the top mold wall. Channel length is varied to 33%, 50%, 67%, and 100% of the mold cavity to further characterize the process. Molded parts are visually observed for void content and dry spots to verify that pressure reduction does not occur at the expense of overall part quality. Transient pressure measurements are compared for each configuration.