Flow Monitoring and Permeability Measurements in LCM Processes by the Means of a Dielectric Sensor

Liquid Composite Molding (LCM) processes, widely used to manufacture thermosetting matrix composite materials, are characterized by the impregnation of a dry fibrous perform, by the means of injection or infusion of the catalyzed resin. The increasing industrial application of LCM processes is due to the demand for high performances materials and constant quality productions, combined with the need to reduce human intervention and costs due manufacturing inefficiency. The opportune planning of LCM processes results, however, very complex, being the process characterized by non-stationary multiphase flows in a three dimensional porous domain with anisotropic permeability, by the cure reaction, influencing the temperature, the degree of cure, and the viscosity of the processing resin, and by the elastic deformation of the fiber bundle due to the applied pressure, which affects significantly preform properties. Nowadays, process planning and optimization is mainly based on trial and error procedures or on computational simulations. Although the existing simulation packages, developed thanks to the efforts spent by several research groups, led to a better understanding and more effective application of LCM processes, on line monitoring of resin flow is very desirable to account for unpredicted variations of processing conditions. Moreover, an accurate experimental evaluation of fiber preform properties is crucial for a reliable process simulation. In this paper, a dielectric capacitive system has been designed, realized, and applied to monitor the position of the saturated as well as the unsaturated flow fronts and to evaluate in plane bulk permeability and tow permeability of dual scale fibrous porous media, typically used in LCM processes. The used sensors, analysed and optimized by computational simulations, have been embedded into opportunely designed rigid dies. Several preform impregnation tests have been performed. Good agreement has been found between results provided by the used system and data obtained using conventional techniques, evidencing the capability of proposed method for process monitoring, as well as for material properties evaluation.