NUMERICAL METHOD TO PREDICT VOID FORMATION DURING THE LIQUID COMPOSITE MOLDING PROCESS Zuzana Dimitrovová 1 and Suresh G. Advani 2 1 Researcher of IDMEC / IST and Invited Auxiliary Professor of DEM / ISEL Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugal: zdimitro@dem.ist.utl.pt 2 Professor, Department of Mechanical Engineering and Center for Composite Materials University of Delaware, Newark, DE 19716: advani@udel.edu Corresponding author’s email: zdimitro@dem.ist.utl.pt SUMMARY: Void formation during the injection phase of the liquid composite molding process can be explained as a consequence of the non-uniformity of the flow front progression. This is due to the dual porosity within the fiber perform (spacing between the fiber tows is much larger than between the fibers within in a tow) and therefore the best explanation can be provided by a mesolevel analysis, where the characteristic dimension is given by the fiber tow diameter of the order of millimeters. In mesolevel analysis, liquid impregnation along two different scales; inside fiber tows and within the open spaces between the fiber tows must be considered and the coupling between the flow regimes must be addressed. In such cases, it is extremely important to account correctly for the surface tension effects, which can be modeled as capillary pressure applied at the flow front. Numerical implementation of such boundary conditions leads to ill- posing of the problem, in terms of the weak classical as well as stabilized formulation. As a consequence, there is an error in mass conservation accumulated especially along the free flow front. A numerical procedure was formulated and is implemented in an existing Free Boundary Program to reduce this error significantly. KEYWORDS: void formation, surface tension, capillary pressure, mass conservation, free boundary flow, mesolevel analysis, dual porosity. INTRODUCTION Liquid Composite Molding is a composite manufacturing process in which fiber preforms consisting of stitched or woven bundles of fibers, known as fiber tows, are stacked in a closed mold and a polymeric resin is injected to impregnate all the empty spaces between the fibers. Fiber tows are usually millimeters in diameter and consist of bundles of 2000 to 5000 fibers [1]. An important step is to ensure saturation of all the fiber tows and regions in between them in order to avoid voids formation. Due to the dual porosity in woven fiber preforms, resin progression is not uniform, and a transition region where the flow has not yet stabilized and saturated, is formed along the macroscopic flow front. This region is very sensitive to voids formation. The best way to analyze this flow is at the mesolevel, i.e at the scale of fiber tows. FPCM-7 (2004) The 7 th International Conference on Flow Processes in Composite Materials Newark, Delaware, USA