Liquid molding of textile reinforcements: Analysis of flow-induced voids and effect of powder coating on preforming and moldability

The use of textile reinforcements is becoming increasingly popular for fabricating near-net-shaped parts from liquid composite molding processes such as resin transfer molding (RTM) and structural reaction injection molding (SRIM). This work addresses two major issues in molding parts with textile reinforcements: fiber wetting and void formation during mold filling, and use of a tackifier to control consolidation of bulky preforms.; A 2D flow visualization technique was developed to observe the microflow behavior during impregnation of unidirectional stitched fiberglass mats. From flow visualization experiments, mechanisms of formation of macro and microvoids were proposed and correlated to the relevant processing variables (surface tension and viscosity of the liquid, injection flow rate, and liquid/fiber contact angle) by the dimensionless modified capillary number. Depending on the magnitude of the modified capillary number and on the flow pattern, large voids or macrovoids were formed between the fiber tows, while smaller voids or microvoids were formed within the fiber tows. The effect of voids on microcrack formation and dynamic mechanical properties of aged composite samples were also investigated.; Consolidation behavior of 4 HS woven preforms was studied using a reactive BMI and a non-reactive PMMA powder. The processing variables that were observed to affect consolidation and springback were powder concentration, application technique (powder vs. solvent), location of application (interlayer vs. intralayer), and debulking temperature. A two level consolidation theory was proposed to explain the mechanism of fiber springback, viz., interlayer consolidation or compression of the gaps between the fiber tows, and intralayer consolidation or compression of gaps within the fiber tows. It is hypothesized that fiber springback will occur when the force exerted by the elastic fibers is greater than the "holding" force provided by the tackifier. The latter depends upon the tackifier modulus, and the wetted surface area of the preform which can be either inter or intralayer depending upon the rheological characteristics of the polymer melt. Other issues investigated were permeability and wettability of tackified preforms, and the effect of tackifier cure on flexural properties of molded composites and cast resin plaques.