# Material Characterization for Reliable Resin Transfer Molding Process Simulation
## 1 Introduction  
Fiber-reinforced composite materials are widely used in several lightweight applications in the nautical, aerospace, and automotive sectors due to high strength-to-density ratios. More extensive applications are limited by materials and technology costs. Even if the cost of carbon and glass fibers follows a decreasing trend, the manufacturing costs will still be almost constant if conventional high-performance technologies are used (i.e., autoclave manufacturing). Moreover, several composite technologies cannot provide a component cost reduction by increasing the production volume. To achieve a reliable component cost reduction, new out-of-autoclave technologies (like infusion technologies, prepreg compression technologies, or short-fiber compound processing) have been recently optimized for industrial use. Among the others, resin transfer molding (also known as light RTM), and its variants vacuum-assisted resin infusion (VARI) and high-pressure resin transfer molding (HP-RTM), are promising technologies for producing complex components with high performance at a higher volume rate. Moreover, these processes are economically advantageous with respect to the others, such as autoclave curing of pre-impregnated layers, due to cheaper materials and shorter processing times. RTM technologies consist of placing a dry three-dimensional preform into a mold cavity and injecting resin-hardener mixture into the closed mold. The injection pressure (assisted by a vacuum for VARI or high pressure in HP-RTM processing), together with fiber permeability, is able to produce components with 50% and higher fiber volume fraction, thus providing excellent mechanical properties [1; 2; 3]. Nevertheless, in the light RTM process, the low flow rate of resin and low injectionpressure result in a long resin injection time, thus hampering the use of fast-cure resins and, consequently, the overall productivity. As a consequence, the light RTM process is mainly limited to the low-volume manufacturing capability and, only to a minor extent, to slightly lower mechanical properties related to the lower fiber volume fraction. High-volume manufacturing with RTM is possible only if the process cycle time is significantly reduced. VARI and HP-RTM variants are improvements in this direction. Due to the differential pressure between inlet and outlet flows (VARI) or to higher flow rates (HP-RTM), together with the choice of adequate resin and hardener, production time can be reduced. A critical point of RTM technologies is the assurance of a full impregnation of the component, which is influenced by reinforcement types, the number of layers, resin viscosity, position of resin injection (ports), and vacuum extraction (vents). As molds can reach very high manufacturing costs when wide components are involved or when metal molds are required, tools supporting the design and critical process selection are needed in order to reduce uncertainties already in the preliminary phase of component development. In this direction, Finite Element Method (FEM) software has been developed in the last few years to assist in component design and processing, predicting issues such as resin-rich areas, air bubbles, dry spots, zones of high porosity, etc., and for optimization of mold geometry and process parameters, such as port and vent locations [4; 5; 6; 7]. In the case of critical molds, therefore, it is advantageous to perform a preliminary study by means of numerical simulation in order to avoid expensive experimental setups and prototypes. In this context, the ESI group developed a PAM-RTM module, which can solve liquid resin infusion processes, like light and high-pressure RTM, compression resin transfer molding (C-RTM), and VARI. ESI's approach, resulting from several years of collaboration with academics [8; 9; 10], consists of a finite element solver based on the coupling of resin flow (governed by [PERSON]'s law) and the preform behavior (considered as a porous medium undergoing deformations).