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# # # 1. Molding

Compression molding is a traditional material processing and molding method. Compression molding of composite materials is similar to compression molding of metal materials, and raw materials are heated and pressurized by a male mold and a female die. The molding process flow is as follows: according to the specific process requirements, powder materials, fiber resin or prepreg are laid in the mold cavity, and certain pressure and temperature are applied to them by hydraulic equipment. During the heating process, the resin becomes a flowable state, which fully soaks the fiber and fills the entire mold cavity. Finally, under the action of temperature and pressure, it is cured and molded for a period of time to obtain the corresponding products.

Compression molding has the characteristics of less loss of raw materials, high dimensional accuracy of products, low internal stress, small warping deformation, smooth surface without secondary modification, small wear of mold cavity, low maintenance cost, high production efficiency, and easy realization of specialization and automatic production. However, its mold manufacturing is complicated and the cost is large; Limited by the model, most small and medium-sized products are mass-produced; For products with high dimensional accuracy, this process is in short supply; After molding, the flash of the product is thick, and the workload of removing flash is large; It is not suitable for molding complex products with depressions, side slopes or small holes.

# # # 2. Vacuum bag compression molding

The vacuum bag compression molding process is to lay the fiber fabric (preform) on the surface of the mold according to the actual process requirements, and then lay the release cloth, the guide net, the guide pipe and the vacuum film in turn, and place the resin guide pipe and the vacuum suction pipe at both ends respectively. It is sealed by sealant, and the internal air is exhausted by vacuum pump, so that a "vacuum cavity" is formed between the vacuum film and the mold. Then, the resin is introduced by atmospheric pressure to infiltrate the material until it is completely infiltrated. Finally, the vacuum bag is kept sealed, so that it is always in a vacuum state and the curing is completed.

The vacuum bag compression molding process has low requirements for molds, only rigid molds and vacuum films are needed; The infiltration effect on fiber is good. Under negative pressure, the resin fully flows to improve the infiltration degree of fiber. In addition, it belongs to the closed mold forming process, which limits the volatilization of harmful substances in the process of curing resin and can reduce the pollution to the environment. However, this method has high technical requirements for operators, and the control of resin mixing and resin content depends largely on the proficiency of operators and the low fiber content of the molded skin.

# # # 3. Autoclave molding

Autoclave molding is a method of heating and pressurizing molding by using high temperature and high pressure air in the tank. Because the temperature and pressure of the air in the tank are uniform, it ensures the uniform heating and compression during the molding process, which is helpful to improve the consistency of product quality. Autoclave molding has the characteristics of uniform heating, wide application range and relatively simple mold requirements, and is suitable for the molding of skin, wallboard and fuselage with large area and complex profile. The temperature and pressure in the autoclave can meet the molding process requirements of almost all polymer-based composites. The molding process is stable and reliable. However, this method takes a long time to form and consumes high energy, and the auxiliary materials used in the curing process are expensive.

In addition to the above-mentioned molding process, the aircraft skin with specific materials may also involve other specific manufacturing processes, such as using double five-axis mirror milling technology for processing. This technology is suitable for skin processing of large-size, flexible and ultra-thin surfaces, and can achieve high precision and high efficiency.

To sum up, there are various manufacturing processes of aircraft skin, and the specific choice of which process depends on the material, shape, size and production requirements of the skin.