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Abstract

In this paper, the unsaturated flow encountered in the woven or stitched fiber mats used in RTM is studied for the case of constant rate injection into a 1-D mold. Such fiber mats are characterized as a dual scale porous medium and a two-layer model, based on the difference in the length scales of the intra-tow and inter-tow spaces, is proposed. The mass balance is derived from first principles for an idealized representation of such type of porous media and incorporation of a sink term in the macroscopic equation of continuity is established. First a simple sink function, the constant sink, is studied and the downwardly drooping profile of the injection pressure, reported in previous experiments, is shown to be a natural consequence of the absorption of resin by the tows. Next an On/Off type constant sink function is proposed; critical parameters of unsaturated flows such as pore volume ratio and sink strength are introduced. Then the two-layer model with rectangular cross section is extended to two other types of cylindrical cross sections encountered in woven mats. A numerical scheme to solve the governing equations of sink models is discussed and results of simple analytical models and the two-layer models are presented. These models classify the flow through such woven mats as either a developing or a developed flow depending upon whether all or some tows behind the resin front are unsaturated. Parametric studies to identify important parameters affecting the length of the partially saturated regions near the front are presented. The simple On/Off type sink model also predicts the slopes of the injection pressure generated by the two-layer models for developed flow with good accuracy. The inlet pressure curve of the two layer models reproduce the characteristic drooping trend. Finally, the effect of different cross sections of the two-layer model upon the inlet pressure history is studied.

Keywords

RTM resin transfer molding flow modeling flow through porous media woven fiber mats permeability unsaturated flow

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A Model for Unsaturated Flow in Woven Fiber Preforms during Mold Filling in Resin Transfer Molding

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