Dynamics of binder displacement in liquid molding

This paper provides experimental data and an analytical description for mass transfer of soluble thermoplastic binder from a fiber preform to the resin during nonisothermal filling in liquid molding processes. The variation of binder concentration in resin during fill and the viscosity variation must be determined and controlled. This is because of the consequences for wet-out, preform deformation, development of ''fingering'' flow patterns and nonuniformities in molded parts. Experimental binder washout curves have been obtained from random continuous glass mat preforms with a range of flow lengths, into a vinyl ester resin. An experimental method developed in this work was then used to estimate from such washout curves the binder concentration profiles within resin at the end of fill for the entire preform. Thus binder concentrations in the resin at the end of fill are seen to vary from 1 to 5 wt%. This range of binder concentrations yields a viscosity increase of more than two-fold within the mold. A model has been developed and tested for the resin viscosity that takes into account three factors-binder dissolution, temperature variations, and a small amount of cure. This relation has been incorporated into a process model for one-dimensional flow in a heated mold. Two parameters for the mass transfer coefficient relation were estimated from a run at a reference mold wall temperature. The model then provided good estimates over a range of mold wall temperatures, for binder concentration profiles as well as temperature profiles during filling, that match with experimental data to within 10%. Hence the model describes the interaction between nonisothermal effects and binder dissolution effects well. Calculations with this model thus provide quick estimates of viscosity variation under various process conditions. Design guidelines based on the predicted viscosity variation have also been developed for operating conditions in resin transfer molding.