Characterization and modeling of race-tracking in liquidcomposite molding processes

Abstract

Liquid composite molding (LCM) processes require the impregnation of a polymeric resin through a porous preform, usually composed of glass, carbon, or Kevlar fibers. Numerical mold-filling simulations are currently being developed to predict the flow through LCM mold cavities and are a powerful design tool. The accuracy of such simulations are very sensitive to the permeability components, input provided by the user to represent the resistance to flow provided by the porous preform. Air channels can be present within a mold cavity, either unintentionally formed, or intentionally placed to enhance the mold filling process. Such channels provide paths of relatively low flow resistance, and can drastically alter flow front advancement and injection pressures, this effect being commonly referred to as ‘race-tracking'. Race-tracking must be considered in numerical mold-filling simulations, one common approach being the application of ‘equivalent permeabilities' to finite elements within a model that physically represent the air channels present. A planar rectangular mold cavity is studied here, having a single air cavity of known dimensions running along one side of the mold. This geometry provides the simplest flow configuration, all Darcy velocity components being in-plane. In this paper, the equivalent permeability magnitudes are based upon steady-state, fully developed flow through a rectangular duct. Detailed flow visualization experiments have been completed, recording flow-front advancement and injection-pressure histories, for two different preform types, and a variety of mold cavity thicknesses, preform volume fractions air-channel widths. Significant effort was made to provide exact comparisons of both flow front advancement and injection pressure, between the experiments and simulations based on the equivalent permeability approach. The comparisons are very good, the magnitude of the deviations being close to what should be expected from natural permeability variations from preform to preform. Sources for experimental error have been examined, the major limitation of these experiments being caused by the complex interaction of the acrylic mold, and stiffening bars used. The equivalent permeability approach has been shown to model flow front advancement and injection pressures very well within a Darcy's law based mold filling simulation, for the volume fractions studied (0.50 to 0.16). This study has been limited to low volume fractions, and care should be taken when extending the equivalent permeability approach to woven and stitched preform styles. An alternative numerical method was investigated, modeling flow in the preform by using Darcy's law, and one-dimensional Stokes flow in the air channel, with a variable source term to model the transverse flow into the preform. The results from the two simulations are in excellent agreement, demonstrating that the equivalent permeability approach employing Darcy's law over the entire domain, models the transverse flow into the preform from the air channel accurately.