Numerical Simulation Of Resin Transfer Molding Processes

It has a great realistic significance to develop the investigation of the advanced manufacturing technology for composite materials, to achieve the high performance, low defect and low cost for the large aerobus project in Medium and Long term Development for China's Science and Technology and in Eleventh Five-Year Plan. RTM processes used for manufacturing the advanced composites has been applied more and more extensively in aerospace, automotive and architecture industries due to its significant advantages, such as low cost, fast molding rate, high production efficiency and little pollution. It has been one of the most quickly growing and extensively developing directions in the field of the advanced composites manufacturing at home and abroad.One of the major obstacles to an even larger scale application of RTM processes is the presence of defects in the parts such as dry spots, voids and poor wetting. Therefore, the resin flow behavior in the intra-tow and inter-tow spaces and the effects of the edge effect on the filling process are investigated deeply. It may help reveal the influence factors of the fiber wetting and the formation of defect in materials and implement the optimization design of RTM processes. Then the performance of the composites is improved and the cost of manufacture is reduced.In this paper, the flows in the global cavity are divided into the flows in the fiber preform which include intra-tow flow and inter-tow flow and the edge flow when small clearances exist between the fiber preform and the mold edges. The influence factors of the fiber wetting and the formation of defect in materials are analyzed quantitatively through the numerical simulation of these flows.According to the theory analysis of formation mechanisms of macrovoid between the fiber tows and microvoid inside the fiber tows, considering the effect of additional pressure induced by surface tension on flow, the mathematical and physical model of fluid flow in the inter-tow space is constructed. The technical route of the numerical simulation is confirmed. The transient flow process of resin between the fiber tows is simulated. The influences of such process parameters as temperature, inter-tow dimension and filling velocity on fluid pressure and flow front are revealed. But the acknowledgement of void formation mechanisms can't only base on one or several models or all influence factors of void formation which obtained through the numerical simulation of inter-tow flow, it is necessary to further study the interaction between the flow in the gaps around the fiber tows and the flow inside the fiber tows.Local volume averaging method is adopted in order to obtain the mathematical description of resin flow through the fiber preform. The volume averaged equations of continuity and momentum considering the inertia and viscous terms are derived. The application of the volume averaged momentum equations is more extensive than that of the Darcy law which describes the transient flow of RTM mold filling process. The resin flow through the fiber preform is modeled as a two-phase (resin and air) fluid flow through porous media. Momentum equations considering the inertia and viscous terms are discretized by Finite Volume Method. Combining the FVM and the Volume of Fluid/Piecewise Linear Interface Construction approach, a numerical simulation algorithm is developed to solve the transient flow problem in the fiber preform. Based on the algorithms described above, a numerical simulation program of RTM mold filling process is composed, and three cases are numerically simulated. The resin flow front locations and shapes at any instant in time and the curve between flow front location and filling time are obtained. Good agreement is found between the numerical results and analytical solutions as well as experimental results, thus the validity and reliability of the numerical simulation algorithm proposed in this paper and the simulation program is validated. The software platform is provided for the numerical analysis of the interaction between inter-tow flow and intra-tow flow and the edge effect.The flow problems in intra-tow and inter-tow regions and in fiber preform and edge channel are essentially the flow problem in porous media/homogenous fluid coupled areas. In order to avoid the explicit formulation of the boundary conditions at the porous media/homogenous fluid interface, the single approach is adopted, the region is considered as a continuum region in this paper, therefore, the resin flow behavior in porous media/homogenous fluid coupled areas can be described by one set of general governing equations.In order to investigate deeply the influence factors of void formation and fiber wetting in the fiber preform, numerical analysis of the interaction between the flow in the gaps around the fiber tows and the flow inside the fiber tows is carried out. The resin flow inside the fiber tows is considered as the flow through porous media, it is formulated using Brinkman's equation; the inter-tow space is considered as free fiber region, the resin flow in the open space around the fiber tows is formulated by Stokes' equation. Thereout the mathematical and physical model is developed to study the resin flow in the inter-tow and intra-tow spaces. A numerical simulation algorithm in the cylindrical coordinate system is developed to analyze the flow problem in porous media/homogenous fluid coupled areas. A numerical simulation program of interaction between inter-tow flow and intra-tow flow is composed on the basis of the software platform described above. The mesoscopic flow behavior at constant flow rate injection and at constant pressure injection is investigated separately. The effects of resin viscosity, inter-tow dimension and intra-tow permeability on flow are revealed. The potential factors of void formation inside fiber tows are analyzed. The studies demonstrate that the flow between the fiber tows is always ahead of the flow within the fiber tows, which increases the probability of microvoid formation inside the fiber tows, and it is impossible to wet all the individual fibers only by the hydrodynamic pressure.In RTM processes, small clearances exist between the fiber preform and the mold edges, which result in a preferential resin flow in the edge channel and then disrupt the flow patterns during the mold filling stage. A numerical simulation algorithm in the rectangular coordinate system is developed to analyze the flow problem in porous media/homogenous fluid coupled areas. Two mathematical models are successively constructed. In the first model, the flow behavior in the edge channel is formulated using full two-dimensional Navier-Stokes equations, while that in the fiber preform is formulated by momentum equation considering inertia and viscous terms. When the dimensions of the cavity thickness and the edge channel width are similar, the simulated results show that the effect of cavity thickness on resin flow cannot be omitted in the edge channel. In the second model, based on the mathematical analysis of governing equations of fully developed flow in a rectangular duct and the formulation of the equivalent permeability, comparing with three-dimensional Navier-Stokes equations, the modified governing equations are obtained in the edge channel. Numerical simulations of the edge effect at constant flow rate injection and at constant pressure injection are carried out separately using the second model. The effects of edge channel width, cavity thickness and resin viscosity on flow are investigated. The studies demonstrate that on one hand the edge effect may lead to the unwanted effect on resin flow fields to format dry spots, on another hand it can enhance the mold filling efficiency, reducing the required injection pressure at constant flow rate injection, and reducing the filling times at constant pressure injection.