At present, there are many manufacturing processes for composite material structures, which can be applied to the production and manufacturing of different structures. However, considering the industrial production efficiency and production costs of the aviation industry, especially civil aircraft, it is urgent to improve the curing process to reduce time and costs. Rapid Prototyping is a new manufacturing method based on the principles of discrete and stacked forming, which is a low-cost rapid prototyping technology. Common technologies include compression molding, liquid forming, and thermoplastic composite material forming.
1. Mold pressing rapid prototyping technology
The rapid prototyping technology of molding is a process that places pre laid prepreg blanks in the molding mold, and after the mold is closed, the blanks are compacted and solidified through heating and pressure. The molding speed is fast, the product size is accurate, and the molding quality is stable and uniform. Combined with automation technology, it can achieve mass production, automation, and low-cost manufacturing of carbon fiber composite structural components in the field of civil aviation.
Molding steps:
① Obtain a high-strength metal mold that matches the dimensions of the required parts for production, and then install the mold in a press and heat it.
② Preform the required composite materials into the shape of the mold. Preforming is a crucial step that helps improve the performance of finished parts.
③ Insert the preformed parts into the heated mold. Then compress the mold at a very high pressure, typically ranging from 800psi to 2000psi (depending on the thickness of the part and the type of material used).
④ After releasing the pressure, remove the part from the mold and remove any burrs.
Advantages of molding:
For various reasons, molding is a popular technology. Part of the reason why it is popular is because it uses advanced composite materials. Compared to metal parts, these materials are often stronger, lighter, and more corrosion-resistant, resulting in objects with better mechanical properties.
Another advantage of molding is its ability to manufacture very complex parts. Although this technology cannot fully achieve the production speed of plastic injection molding, it does provide more geometric shapes compared to typical laminated composite materials. Compared to plastic injection molding, it also allows for longer fibers, making the material stronger. Therefore, molding can be seen as the middle ground between plastic injection molding and laminated composite material manufacturing.
1.1 SMC Forming Process
SMC is the abbreviation for sheet metal forming composite materials, that is, sheet metal forming composite materials. The main raw materials are composed of SMC special yarn, unsaturated resin, low shrinkage additives, fillers, and various additives. In the early 1960s, it first appeared in Europe. Around 1965, the United States and Japan successively developed this technology. In the late 1980s, China introduced advanced SMC production lines and processes from abroad. SMC has advantages such as superior electrical performance, corrosion resistance, light weight, and simple and flexible engineering design. Its mechanical properties can be comparable to certain metal materials, so it is widely used in industries such as transportation, construction, electronics, and electrical engineering.
1.2 BMC Forming Process
In 1961, the unsaturated resin sheet molding compound (SMC) developed by Bayer AG in Germany was launched. In the 1960s, Bulk Molding Compound (BMC) began to be promoted, also known as DMC (Dough Molding Compound) in Europe, which was not thickened in its early stages (1950s); According to the American definition, BMC is a thickened BMC. After accepting European technology, Japan has made significant achievements in the application and development of BMC, and by the 1980s, the technology had become very mature. So far, the matrix used in BMC has been unsaturated polyester resin.
BMC belongs to thermosetting plastics. Based on material characteristics, the temperature of the material barrel of the injection molding machine should not be too high to facilitate material flow. Therefore, in the injection molding process of BMC, controlling the temperature of the material barrel is very important, and a control system must be in place to ensure the suitability of the temperature, in order to achieve the optimal temperature from the feeding section to the nozzle.
1.3 Polycyclopentadiene (PDCPD) molding
Polycyclopentadiene (PDCPD) molding is mostly a pure matrix rather than reinforced plastic. The PDCPD molding process principle, which emerged in 1984, belongs to the same category as polyurethane (PU) molding, and was first developed by the United States and Japan.
Telene, a subsidiary of Japanese company Zeon Corporation (located in Bondues, France), has achieved great success in the research and development of PDCPD and its commercial operations.
The RIM molding process itself is easier to automate and has lower labor costs compared to processes such as FRP spraying, RTM, or SMC. The mold cost used by PDCPD RIM is much lower than that of SMC. For example, the engine hood mold of Kenworth W900L uses a nickel shell and cast aluminum core, with a low density resin with a specific gravity of only 1.03, which not only reduces costs but also reduces weight.
1.4 Direct Online Forming of Fiber Reinforced Thermoplastic Composite Materials (LFT-D)
Around 1990, LFT (Long Fiber Reinforced Thermoplastics Direct) was introduced to the market in Europe and America. CPI Company in the United States is the world's first company to develop direct in line composite long fiber reinforced thermoplastic molding equipment and corresponding technology (LFT-D, Direct In Line Mixing). It entered commercial operation in 1991 and is a global leader in this field. Diffenbarcher, a German company, has been researching LFT-D technology since 1989. Currently, there are mainly LFT D, Tailored LFT (which can achieve local reinforcement based on structural stress), and Advanced Surface LFT-D (visible surface, high surface quality) technologies. From the perspective of the production line, the level of Diffenbarcher's press is very high. The D-LFT extrusion system of German Coperation company is in a leading position internationally.
1.5 Mouldless Casting Manufacturing Technology (PCM)
PCM (Pattern less Casting Manufacturing) is developed by the Laser Rapid Prototyping Center of Tsinghua University. The rapid prototyping technology should be applied to traditional resin sand casting processes. Firstly, obtain the casting CAD model from the part CAD model. The STL file of the casting CAD model is layered to obtain cross-sectional profile information, which is then used to generate control information. During the molding process, the first nozzle accurately sprays the adhesive onto each layer of sand by computer control, while the second nozzle sprays the catalyst along the same path. The two undergo a bonding reaction, solidifying the sand layer by layer and forming a pile. The sand in the area where the adhesive and catalyst work together is solidified together, while the sand in other areas remains in a granular state. After curing one layer, the next layer is bonded, and after all layers are bonded, a spatial entity is obtained. The original sand is still dry sand in areas where the adhesive is not sprayed, making it easier to remove. By cleaning out the uncured dry sand in the middle, a casting mold with a certain wall thickness can be obtained. After applying or impregnating paint on the inner surface of the sand mold, it can be used for pouring metal.
The curing temperature point of PCM process is usually around 170 ℃. The actual cold laying and cold stripping used in PCM process is different from molding. Cold laying and cold stripping involves gradually laying the prepreg on the mold according to the product structure requirements when the mold is at the cold end, and then closing the mold with the forming press after the laying is completed to provide a certain pressure. At this time, the mold is heated up using a mold temperature machine, The usual process is to raise the temperature from room temperature to 170 ℃, and the heating rate needs to be adjusted according to different products. Most of them are made of this plastic. When the mold temperature reaches the set temperature, insulation and pressure preservation are carried out to cure the product at high temperature. After curing is completed, it is also necessary to use a mold temperature machine to cool the mold temperature to normal temperature, and the heating rate is also set at 3-5 ℃/min, Then proceed with mold opening and part extraction.
2. Liquid forming technology
Liquid forming technology (LCM) refers to a series of composite material forming technologies that first place dry fiber preforms in a closed mold cavity, then inject liquid resin into the mold cavity after mold closure. Under pressure, the resin flows and soaks the fibers. Compared to the hot pressing can forming process, LCM has many advantages, such as being suitable for manufacturing parts with high dimensional accuracy and complex appearance; Low manufacturing cost and simple operation.
Especially the high-pressure RTM process developed in recent years, HP-RTM (High Pressure Resin Transfer Molding), abbreviated as HP-RTM molding process. It refers to the molding process of using high-pressure pressure to mix and inject resin into a vacuum sealed mold pre laid with fiber reinforced materials and pre embedded components, and then obtaining composite material products through resin flow filling, impregnation, curing, and demolding. By reducing injection time, it is expected to control the manufacturing time of aviation structural components within tens of minutes, achieving high fiber content and high-performance parts manufacturing.
The HP-RTM forming process is one of the composite material forming processes widely used in multiple industries. Its advantages lie in the possibility of achieving low-cost, short cycle, mass production, and high-quality production (with good surface quality) compared to traditional RTM processes. It is widely used in various industries such as automotive manufacturing, shipbuilding, aircraft manufacturing, agricultural machinery, railway transportation, wind power generation, sports goods, etc.
3. Thermoplastic composite material forming technology
In recent years, thermoplastic composite materials have become a research hotspot in the field of composite material manufacturing both domestically and internationally, due to their advantages of high impact resistance, high toughness, high damage tolerance, and good heat resistance. Welding with thermoplastic composite materials can significantly reduce the number of rivet and bolt connections in aircraft structures, greatly improving production efficiency and reducing production costs. According to Airframe Collins Aerospace, a first-class supplier of aircraft structures, non hot pressed can formed weldable thermoplastic structures have the potential to shorten the manufacturing cycle by 80% compared to metal and thermosetting composite components.
The use of the most suitable amount of materials, the selection of the most economical process, the use of products in the appropriate parts, the achievement of predetermined design goals, and the achievement of the ideal performance cost ratio of products have always been the direction of efforts for composite material practitioners. I believe that more molding processes will be developed in the future to meet production design needs.
Post time: Nov-21-2023
