1、 The RTM process includes resin filling flow, heat transfer and curing reaction. Among them, heat transfer and curing reaction are common to other composite processes, and the key point of process control is mold filling flow. In RTM filling process, the mold cavity is filled with fiber preform, which can be called fiber bed, which contains solid phase - fiber and mobile phase - air. The filling process of resin is to ensure that the resin flows through these irregular pores and displaces the air to make the resin full of pores. The flow of resin in these irregular pores is very complex, and there are two types of flow at the same time, one is the macro flow between fiber bundles, the other is the micro flow within fiber bundles. These two flows compete with each other in the filling process, which may lead to poor infiltration or bubble inclusion. When the injection pressure is low, the capillary force in the fiber bundle plays a major role. The flow front shape of the fluid is shown in figure (a). The flow front in the fiber bundle is ahead of the flow front between the fiber bundles. When the leading fluid front flows and merges along the transverse direction, the air that is not discharged in the fiber bundle is wrapped to form a large bubble between the fiber bundles. On the contrary, when the injection pressure is high When the pressure is high, the capillary pressure has less effect than the dynamic pressure, so the flow front of the fluid in the fiber bundle lags behind the flow front between the fiber bundles, as shown in figure (b). When the flow front converges laterally, small bubbles are formed in the fiber bundle. Bubble content is one of the important indexes of composite quality. The existence of air bubbles leads to low degree of fiber infiltration, poor adhesion, inconsistent mechanical strength and poor surface quality of composite components; at the same time, it is easy to cause stress concentration, crack generation, and reduce the durability, fatigue resistance and weather resistance of composite. In the process of flow, the formation of bubbles forms the micro defects of the products, which mainly exist between the fiber bundles or between the monofilaments in the fiber bundles, while the formation of dry spots is the macro defects of the products. In the treatment of fiber preforms, there are many problems, such as bending of preforms, loose edge of fabrics and change of permeability of partial preforms. When cutting the preform, it is difficult to achieve accurate size accuracy. The looseness of the fiber bundle will reduce the fiber volume content at the edge. In addition, when the mold design is not reasonable or the mold closing is not right, it is easy to create a gap between the preform and the cavity wall and the corner, and this gap forms the priority flow channel of resin. The flow of resin in this channel is ahead of the flow in the preform, which destroys the normal flow front mode. This effect is called "edge effect" or "flow channel effect". Because of the shear, compression or compression effect of the preform, especially the edge effect, the change of the performance of the reinforcement material has an important influence on the filling process, which is easy to cause the problems of inadequate fiber impregnation and dry spots. Generally, the channel effect area is millimeter level. In some cases, the static flow area of 1-2mm will also have a considerable impact on the filling process, resulting in a large area of dry spots. 2、 Based on the theory of fluid flowing through porous media, Darcy's law is adopted as the momentum control method. Darcy's law was proposed by Henry Darcy in the middle of the 20th century based on a series of experimental results, which has been widely used in soil science. Darcy's law describes that the velocity of fluid flowing through porous media is directly proportional to the pressure gradient applied, and inversely proportional to the viscosity of the fluid. Darcy's law is a special form of momentum balance equation, which can be directly substituted into continuous equation, and the governing equation of pressure can be obtained. For horizontal flow, the Darcy's law can be expressed as follows: u (M / s) is the velocity vector of the fluid, K (M2) is the permeability tensor of the preform, is the internal property of the preform, μ (PA? S) is the viscosity of the fluid, and P (PA / M) is the pressure gradient. It can be seen that Darcy's law is an average description of volume, and all complex interactions between fluid and fiber preform are reflected by permeability tensor. Permeability is the key parameter to describe the flow characteristics of fluid in the filling process. Obtaining accurate permeability data plays an important role in obtaining the simulation results consistent with the actual production. For anisotropic porous media, the permeability tensor can be expressed as follows: for a special orthotropic isotropic preform, when the local coordinates are consistent with the main direction of the fiber preform, the permeability tensor can be described as: wherein, K 11, K 22 and K 33 are the permeability values of each main direction respectively. For isotropic preforms, the permeability tensor is simplified as a scalar, that is, RTM process is mainly used to manufacture shell components, and the resin flow is mainly in-plane flow of the preform, and the flow along the thickness direction is very weak, which can be ignored. Therefore, the permeability along the thickness direction can be ignored, so in this case, the permeability can be simplified as: the permeability of fiber preform in RTM application is usually obtained by three ways: experimental measurement, analytical solution and numerical estimation. 3、 The flow simulation of RTM process can qualitatively predict the resin flow process. If the input parameters are reasonable and reliable, injection pressure, flow rate and flow state can be accurately predicted by simulation. The core problem of establishing analytical technology is how to obtain the input important parameter data, such as permeability, resin chemical rheological properties, thermal diffusion and edge flow. These parameters are closely related to resin system, reinforcement material system, mold and process of composite. The finite element simulation theory of liquid flow process is the difficulty and hot spot of simulation technology. At present, the finite element simulation software of resin flow has become mature, such as RTM Worx developed by polyworx company in the Netherlands, pam-rtm developed by ESI company in France and Moldflow developed by Moldflow company in the United States. The application of these flow simulation systems can effectively simulate the production process of products and guide the production of actual products. Through the research on the application of flow simulation technology in the actual production process, the general process of engineering application of flow simulation technology can be obtained: (1) computer geometric modeling of parts, and finite element subdivision of the model with the help of finite element method; (2) selection of several feasible resin injection methods according to the geometry of parts; (3) selection and determination of resin All parameters needed for flow simulation, including injection pressure, permeability and fiber volume content of preform, resin viscosity, etc.; (4) simulation of resin flow process of various injection modes; (5) comprehensive consideration of various factors such as resin flow time and final filling position, as well as the difficulty of process operation, to select the optimal resin injection mode and guide Actual production. The following is an example of using RTM Worx software to simulate the filling process of radome RTM process. The geometry and dimension of the part are shown in the figure below: geometry and dimension of the part (dimension unit: mm)
In the simulation, the injection pressure is set as 0.5MPa. It is assumed that the preform of the reinforcement material is isotropic, the permeability is 1 × 10-10m2, and the distribution is uniform. The volume content of the fiber is 40%. The thickness of the spherical cap and the working area is set as 14mm, the thickness of the ring is set as 20mm, and the viscosity of the resin is set as 0.4pa? S. The simulation results are shown in the following figure: the flow front diagram and pressure distribution of the simulated resin
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