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, e ach having corners of different radii (from 0.06 to 8.0 in.). The primary g oal of this work has been to determine whether corners in LCM molds signifi cantly affect the filling process, by altering the structure of the preform locally in such regions. Consistent trends were found for each series of e xperiments completed in the mold, for which the same preform type, and numb er of layers were used. For constant Row rate injection, the required injec tion pressures to fill the two molds with tighter radii was significantly i ncreased as compared to the other molds. Composite parts were manufactured in these molds, measurements made on these parts revealing design flaws, th e cavity thicknesses not being equal in all sections of the molds. Several numerical simulations are presented in this paper, the goal being to separa te any effects due to the varying thicknesses from effects due to the corne rs present. Careful simulations have been completed, taking into account th e actual thicknesses in each mold, and the resulting preform volume fractio n. Experimentally measured permeability data was employed, and the predicte d injection pressures match very well for all four molds studied, seeming t o indicate that the corners present have not affected the filling. A simple model for preform compression in corners has been developed, which predict s local permeability modifications due to in-plane compression of the fabri c layers. These predictions have been employed in conjunction with an exist ing tool to analytically predict the permeability components of a preform i n a Rat cavity. This code requires from the user a geometrical model of a p reform 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 on ly 30-60%. Sensitivity analyses have demonstrated the strong relationship b etween permeability and the details of the preform cell. Two numerical stud ies 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 significan tly affected, while the permeabilities in this region were reduced up to 10 0 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 exis tence of any strong behavior related to mold radii. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.