Fabric structure and mold curvature effects on preform permeability and mold filling in the RTM process.: Part II.: Predictions and comparisons with experiments

In Part I, an experimental study was completed in a series of five molds, each having corners of different radii (from 0.06 to 8.0 in.). The primary goal of this work has been to determine whether corners in LCM molds significantly affect the filling process, by altering the structure of the preform locally in such regions. Consistent trends were found for each series of experiments completed in the mold, for which the same preform type, and number of layers were used. For constant Row rate injection, the required injection pressures to fill the two molds with tighter radii was significantly increased as compared to the other molds. Composite parts were manufactured in these molds, measurements made on these parts revealing design flaws, the cavity thicknesses not being equal in all sections of the molds. Several numerical simulations are presented in this paper, the goal being to separate any effects due to the varying thicknesses from effects due to the corners present. Careful simulations have been completed, taking into account the actual thicknesses in each mold, and the resulting preform volume fraction. Experimentally measured permeability data was employed, and the predicted injection pressures match very well for all four molds studied, seeming to indicate that the corners present have not affected the filling. A simple model for preform compression in corners has been developed, which predicts local permeability modifications due to in-plane compression of the fabric layers. These predictions have been employed in conjunction with an existing tool to analytically predict the permeability components of a preform in a Rat cavity. This code requires from the user a geometrical model of a preform unit cell, this data being measured from samples cut from the parts manufactured. The resulting predictions for injection pressure are good for an entirely predictive approach, underpredicting experimental values by only 30-60%. Sensitivity analyses have demonstrated the strong relationship between permeability and the details of the preform cell. Two numerical studies were completed to determine how sensitive the injection pressure curves are to reduced permeabilities in the corner regions. For the two injection schemes having two different gate locations, pressures were not significantly affected, while the permeabilities in this region were reduced up to 100 times. Though the molds used were not ideal for isolating effects on mold filling due to corner radii, the evidence presented does not show the existence of any strong behavior related to mold radii. (C) 2000 Elsevier Science Ltd. All rights reserved.