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Comparison between the vacuum resin infusion process and the hand lay-up process. The advantages and disadvantages of both are compared as follows: The hand lay-up process is an open-mold process; currently, it accounts for 65% of the applications involving glass fiber-reinforced polyester composites. Its advantages include great flexibility in changing the shape of the mold, low mold costs, strong adaptability, market recognition for the product’s performance, and low investment requirements. So it is particularly suitable for small companies, as well as for the shipbuilding and aerospace industries, where large components are usually used once. However, this process also has a series of problems, such as excessive emissions of volatile organic compounds (VOCs), significant impacts on the health of operators, high staff turnover, many restrictions on allowable materials, low product performance, resin waste, and high consumption of resin. In particular, the product quality is unstable; factors such as the ratio of glass fiber to resin in the products, component thickness, the manufacturing speed of layers, and the uniformity of layers are all affected by the operators, which requires them to possess good skills, experience, and competence. The resin content in hand-poured products is generally around 50%-70%. The VOC emissions from the mold opening process exceed 500 PPm, with the volatilization rate of styrene reaching 35%-45% of its usage amount. And the regulations in various countries are all between 50-100PPm. Currently, abroad, cyclopentadiene (DCPD) or other resins with low styrene release are mostly used, but there is still no good substitute for styrene as a monomer. The vacuum resin infusion process is a low-cost manufacturing technique that has been developed over the past 20 years, and it is particularly suitable for the production of large-scale products. Its advantages are as follows: (1) The product has excellent performance and a high yield rate. With the same raw materials, compared to hand-laid components, the strength, stiffness, and other physical properties of components formed using the vacuum resin infusion process can be increased by 30%-50% or more (Table 1). Once the process is stabilized, the yield can approach 100%. Table 1 Comparison of Properties of Typical Polyester Fiber-Reinforced Plastic Materials Reinforcement material: Untwisted roving fabric, biaxially oriented fabric; Untwisted roving fabric, biaxially oriented fabric Molding process: Hand lay-up, hand lay-up, vacuum resin infusion, vacuum resin infusion Fiber content: 45, 50, 60, 65 Tensile strength (MPa): 273.2, 389, 383.5, 480 Tensile modulus (GPa): 13.5, 18.5, 17.9, 21.9 Compressive strength (MPa): 200.4, 247, 215.2, 258 Compressive modulus (GPa): 13.4, 21.3, 15.6, 23.6 Flexural strength (MPa): 230.3, 321, 325.7, 385 Flexural modulus (GPa): 13.4, 17, 16.1, 18.5 Interlaminar shear strength (MPa): 20, 30.7, 35, 37.8 Transverse shear strength (MPa): 48.88, 52.17 Transverse shear modulus (GPa): 1.62, 1.84 (2) The product quality is stable with good repeatability. Product quality is little affected by operators, with a high degree of consistency both within the same component and among different components. The amount of fiber in the product is placed in the mold in the specified quantity before resin is injected, resulting in a relatively constant resin content in the component, typically ranging from 30% to 45%. As a result, the consistency and repeatability of the product’s properties are much better than those of products made using the hand-laying process, with far fewer defects. (3) Fatigue resistance is improved, which can reduce the structural weight. Due to the high fiber content, low porosity, and excellent product properties, especially the improved interlayer strength, **the fatigue resistance of the product is enhanced. When the requirements for strength or stiffness are the same, products manufactured using the vacuum infusion process can reduce the structural weight. (4) Environmentally friendly. The vacuum resin infusion process is a closed-mold process in which volatile organic compounds and toxic air pollutants are confined within a vacuum bag. Only trace amounts of volatiles are released when the vacuum pump exhausts (which can be filtered) and when the resin tank is opened. The VOC emission standard does not exceed 5PPm. This also **improved the working conditions for operators, stabilized the workforce, and expanded the range of available materials. (5) The product has good overall integrity. The vacuum resin infusion process enables the simultaneous molding of reinforcement ribs, sandwich structures, and other inserts, improving the integrity of the product; thus, it can be used to manufacture large-scale components such as fan shrouds, ship hulls, and superstructures. (6) Reduce the use of raw materials and labor. With the same layup, the resin usage is reduced by 30%. There is minimal waste, with a resin loss rate of less than 5%. It features high labor productivity, saving over 50% of the labor force compared to the hand-laying method. Especially when manufacturing core and stiffened structural components with large, complex geometries, the savings in materials and labor are even more significant. For example, in the manufacturing of vertical control surfaces in the aviation industry, reducing the number of fasteners by 365 results in a 75% cost reduction compared to traditional methods, while the product weight remains unchanged and its performance improves. (7) The products have high precision. The dimensional accuracy (thickness) of products manufactured using the vacuum resin infusion process is superior to that of hand-laid products. With the same lay-up, the thickness of products manufactured using conventional vacuum resin infusion techniques is 2/3 that of hand-laid products. The product thickness deviation is approximately ±10%, while the hand-laying process typically results in a deviation of ±20%. The surface flatness of the product is better than that of hand-laid products. The inner wall of the hood product produced using the vacuum resin infusion process is smooth, with a resin-rich layer forming naturally on its surface, eliminating the need for an additional top coat. It reduces the labor and materials required for grinding and painting processes. Of course, the current vacuum resin infusion process also has certain drawbacks: (1) The preparation procedures are time-consuming and relatively complex. Proper lamination, placement of the guiding medium, use of guide tubes, and effective vacuum sealing are required. Therefore, for small-sized products, the processing time is actually longer than that of the hand-laying process. (2) The production cost is high, and a large amount of waste is generated. Auxiliary materials such as vacuum bag films, flow-guiding media, demolding cloths, and flow-guiding tubes are all used once only, and many of them currently rely on imports; therefore, the production cost is higher than that of the hand-laying process. But the larger the product, the smaller this difference becomes. As auxiliary materials become domestically produced, this cost difference is also decreasing. The use of reusable auxiliary materials in current research is a development direction for this process. (3) The manufacturing process involves certain risks. Especially for large and complex structural products, a failure during resin pouring can easily result in the product being scrapped. Therefore, good preliminary research, strict process control, and effective remedial measures are necessary to ensure the success of the process. 2 Requirements for raw materials in the vacuum resin infusion process. Requirements for the resin used in engine cowls produced via the vacuum infusion process: (I) Low viscosity. Generally, it’s around 100–400 mPa·s. It is best to keep it at no more than 200 mPa·s; (2) an appropriate exothermic peak temperature, generally not exceeding 80℃ ； (3) The fiberglass layer retains appropriate strength before the operating temperature reaches 60°C ； (4) It maintains a good bonding strength with the selected glass cloth even when exposed to a humid environment (relative humidity of 95%): (5) It can cure at room temperature ； (6) It has a sufficiently long gel time to ensure the completion of the process, with complete curing at the end ； (7) Good weather resistance ； (8) Good oil resistance ； (9) Good flame retardancy ； (10) Low price ； (11) Low curing shrinkage, etc. For the various components of the resin system, such as the resin, curing agent, accelerator, polymerization inhibitor, color paste, and fillers, studies on their fluidity, viscosity, and curing reaction kinetics must be conducted to ensure the reliability of the manufacturing process (Figure 3). Its research methods include DSC, DTA, dynamic viscometers, etc. Generally, various forms of reinforcing materials such as chopped fiber mats, filament mats, untwisted roving fabrics (tartan fabrics), twisted fabrics, sewn fabrics, and core materials (foam, balsa wood, and honeycomb) can be used, with the fabric surface density reaching up to 87 kg/M2. It should be noted, however, that different fabrics have a significant impact on the vacuum impregnation process; it is advisable to use fabrics with high permeability and good resin wettability. When using a core material, a GPS core material must be employed. 3 Research on vacuum resin infusion process. 3.1 Study on the fluidity of resin In the vacuum resin infusion process, Darcy’s Law (Equation (l)) is primarily used to describe the flow of resin through the preform. Among these, u: resin flow velocity; K: permeability of the preform; u: resin viscosity; △P/△x: pressure gradient. In Darcy’s law, the resin is considered to be an incompressible Newtonian fluid whose viscosity is not affected by the shear rate. In experiments, other liquids can be used as substitutes for resin, such as syrups, glycerin, and aqueous cellulose solutions; this can **reduce experimental costs and increase the speed of testing. Fabric preforms are regarded as porous media, whose properties can be characterized by porosity and permeability. These properties influence the direction and speed of resin flow within the preform, thereby determining key parameters such as the vacuum pressure required during composite molding, the filling time, and the flow path. They also affect the design of key structures such as the resin inlets, outlets, and flow channels, ensuring that the resin fills the preform before it gels. The flow of resin can be divided into two categories: (Figure 3) infiltration or macroscopic flow determined by a pressure gradient, which is the flow rate between the bundles of yarns. (Macroscopic flow). The flow rate of penetration or microflow (within the yarn bundle), determined by fiber capillary pressure and surface tension. (Microcosmic flow) The factors that affect flow velocity and flow channels include raw materials, guiding media, layering, and vacuum level. The two speeds must be comparable; once the flow fronts meet, it becomes difficult to expel the trapped gas. At the microscopic level, the expulsion of gas is influenced by the viscosity of the resin and the surface tension around the fiber bundles. Studies have found that the use of high-permeability guiding media **reduces the molding time**, as the resin flows more rapidly in these guiding media than in the preform; however, the difference between the two remains constant. The molding time is solely a function of the permeability of the guiding media, with little influence from the permeability of the preform. The use of a flow guide medium reduces the molding time by 50-80%. In the manufacturing process, it is necessary to prevent the \"cutline effect\" caused by improper layering and similar issues; in these areas of low resistance, the flow rate of the resin increases by 10 to 100 times, which prevents the process from proceeding as intended. Currently, there are quite a number of software tools available to simulate the flow processes in the vacuum infusion process, including the position and pattern of the resin flow front. These tools allow potential problems in the process to be identified in advance, enabling the process to be optimized. 3. 2 Study on the compression behavior of preforms In the vacuum infusion process, it is also necessary to know the thickness and fiber content of the final product. Since the vacuum bag is flexible, it is not possible to directly control the thickness of the product. The product thickness is related to the fiber content and the compression behavior of the preform, including the compression and relaxation behavior of the fibers under pressure, as well as the interaction between the fibers and the resin. Tests show that the product thickness changes depending on the flow direction of the resin; the farther from the vacuum source, the higher the resin content and the lower the fiber content (resulting in a thicker product). In the VARIM process, the external pressure acting on the preform is atmospheric pressure (Patm), which is composed of resin pressure (Pr) and fiber structure support (Pf) (Equation 2). Patm = Pr + Pf (2). The pressure of the resin at the inlet is 1 atmosphere, while the pressure at the flow front is zero. As one moves from the outlet to the inlet, the resin pressure increases from zero to 1 atmosphere; the farther away from the outlet, the higher the resin pressure. Consequently, the pressure exerted on the preform is lower, the compression on the fibers is less, and the thickness is greater (Figure 4). After the resin reaches the outlet, the resin inlet is closed while the vacuum outlet remains open, allowing the resin pressure to decrease steadily and thus further compressing the preform, which helps to reduce uneven thicknesses.