High-performance fiber-reinforced advanced resin-based composite materials, such as fibers, are widely used in aerospace structures because of their outstanding strength and specific modulus, low coefficient of thermal expansion, good designability, and ease of integral molding. The application has become one of the four major structural materials for aerospace.
It is worth noting that the manufacturing technology of composite products is different from conventional metal materials. The formation of composite materials is usually done in a mold. While the new material is being molded, the final structure (blank) is also completed. The mold determines the geometric boundary of the product, clearly defines the relationship with other components, and influences the internal quality and surface state of the product to a great extent. These factors determine that the mold plays a decisive role in the manufacturing process of the composite product.
However, there are many kinds of manufacturing processes for composite materials. Commonly used are: vacuum bag molding, autoclave molding, compression molding, winding molding, pultrusion molding, flexible film expansion molding, injection molding, electron beam curing, and infiltration molding ( Different molding methods, such as RTM, have different requirements for mold materials and structural forms, and at the same time promote continuous development of mold technology.
1, metal frame mold
Frame-type molds generally consist of a formwork and a support structure. The template uses sheet metal, stamping and other processes to form the desired profile and requires a high degree of finish and sealing. The support structure is generally made of metal profiles (sometimes even wood) for supporting and fixing the stencils on top. Its internal hollow structure, small heat capacity, easy to heat conduction, and lighter weight, easy to transport. This structure is suitable for a variety of processes such as autoclave molding, vacuum bag molding, and vacuum induction molding.
2, metal single mode or combination mode
In addition to some thin products or simple parts using single-mode, molding, pultrusion, RTM and other processes used in the mold, mostly for the mold and other combinations. The various parts of the combined mold need to be precisely matched, run in place, and have accurate positioning. Therefore, high stiffness, strength, surface hardness, and positional accuracy are required, and carbon steel or aluminum alloy materials are generally used.
Although the surface of steel and aluminum is smooth, dense, hard and easy to release, it is not easy to damage when cleaning the mold, and the temperature resistance is good, but there is a problem that the coefficient of thermal expansion of the composite material does not match (The expansion coefficient of steel is about 12×10. At -6/°C, the expansion coefficient of aluminum is about 24×10-6/°C, and the expansion coefficient of carbon fiber composites is generally lower than 3.5×10-6/°C, which results in the high precision of the part's profile and large dimensional errors. Curing stress is greater. The coefficient of thermal expansion of Invar's material can reach 2×10-6/°C, which can match the linear expansion coefficient of the composite material, but the machining cost of Invar's molds is high.
3, expansion or contraction mode
The bulge is mainly made of elastic material, and the arbitrarily shaped characteristics of the elastic material are very convenient for the molding of a complex profile or a closed cavity structure. The expansion die utilizes the volumetric expansion characteristics of the elastic material during the heating process to provide the pressure required for curing the composite material. The material used for the expansion mold is silicon-containing or non-silicon-containing rubber. Currently, a typical material that is relatively mature and commercially available is silicone rubber. In the use of rubber molds, it is generally designed as the cavity of the mold, the exterior of which is a closed rigid structure, and sometimes rubber is used as the core mold.
Shrinking molds mainly include shrink tube and shrink tape. They are mostly used for the molding of composite rods or long shafts. The material of shrink tube or shrink tape is heat-shrink plastic, which is made according to the “elastic memory effect†produced by irradiation treatment of some polymers. Currently, there are (modified) polyolefins and poly-perfluoroethylene-propylene. , fluorine rubber and so on. However, there are some deficiencies in the heat shrinking die, mainly because the accuracy of the control of the solidification pressure is low and the flatness of the surface of the product is poor, so that mechanical processing is required, which greatly affects the quality of the product.
4, composite material mold
In foreign aerospace products, the application of composite molds has become quite common. Domestic aviation system units have studied earlier on composite molds and produced relatively large quantities, while other units, including the aerospace sector, have used less.
Most of the composite material molds are made of carbon fiber or (and) glass fiber composite materials, and can be further repaired, so that the mold can be made very accurately. As the mold material and the product are generally the same type of material, the thermal expansion coefficient matching problem of the mold and the composite material is effectively solved, and the product size and the surface accuracy are excellently ensured. This is the main trend of the development of the composite material forming mold.
This kind of mold has been widely used in composite products with high dimensional accuracy and shape accuracy or large size. Because the mold itself is also a composite material, its manufacturing process has certain complexity and unknowability.
At present, the main problems with composite molds are as follows:
(1) The manufacturing process of the composite material mold is complex, the process control requirements are strict, and there are relatively large differences in the quality of different process methods and even different batches of products.
(2) The surface of the composite material mold is poorly sealed, especially when there are manufacturing defects (such as high porosity), vacuum leakage is very likely to occur.
(3) The surface hardness of the composite material mold is low, and it is easy to produce mechanical damage, and the gel coat or the plating layer is easy to fall off. In addition, problems such as delamination, dregs, and deformation are easily caused in the process of lifting and handling, which may affect normal use.
(4) Compared with metal molds whose service life is more than a thousand times, the service life of composite molds is relatively short, generally only about tens of times. Of course, there are examples of quality composite molds that have been used for nearly a thousand times in foreign countries.
(5) The manufacturing cost of composite molds is much higher than that of ordinary metal molds.
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