Prediction of optimal flow front velocity to minimize void formation in dual scale fibrous reinforcements

Liquid Composite Molding (LCM) is an increasingly used class of processes to manufacture high performance composites. Engineering fabrics commonly used in LCM generally have a dual scale architecture in terms of porosity: microscopic pores exist between the filaments in the fiber tows, while macroscopic pores appear between the tows. Capillary flows in fiber tows play a major role on the quality of composites made by resin injection through fibrous reinforcements. This paper reports on an investigation on fabric imbibition characterization and subsequent evaluation of the optimal flow front velocity during resin injection through fibrous reinforcements. The goal is to devise more robust LCM processes and improve part quality. In order to evaluate a priori the injection conditions that minimize void formation, an impregnation model is developed based on imbibition characterization. This approach allows predicting the optimal front velocity without having to model complex dual scale flows through fibrous reinforcements and without performing expensive and time-consuming fabrication tests. After a summary of previous imbibition results obtained with a probe fluid, the optimal modified capillary numbers are computed by the new predictive model and the values are compared with results reported in the literature on void formation in LCM processes. Afterwards, capillary rise measurements are carried out with four infiltration fluids in order to evaluate the range of optimal flow front velocity that minimizes void formation. This characterization is implemented with vinyl ester resin, epoxy anhydride resin, styrene and anhydride. Finally, the optimal flow front velocity is evaluated for several fabric configurations.