Liquid Composite Molding (LCM) is a method in which a liquid polymer is injected into a closed mold cavity covered with a fiber preform, or a resin film placed in the mold cavity is heated to melt it. At the same time, the fiber is impregnated at the same time as the mold, and the composite material preparation technology is formed into the product through the curing process. Compared with other traditional composite material molding processes, LCM has closed mold molding environment-friendly, low process cost, smooth surface and high quality, and a wide range of parts: it can be impregnated with cores, reinforcements, embedded parts, etc. Large components. Product defects that are prone to occur in the LCM process include air bubbles in the resin infiltration phase, dry spots during resin collection and curing, and residual stress. The corresponding defects can be effectively reduced by controlling the relevant process parameters of the impregnation and curing process. At present, LCM technology has been widely used in aerospace, automotive parts, high-speed rail, shipbuilding and other manufacturing industries, such as Russian United Aviation Manufacturing Group (UAC) on its MS-21 single-aisle aircraft using a liquid forming process to manufacture the wing structure. In the autoclave process, the liquid molding process can reduce weight by 10%, reduce costs by 30%, reduce energy consumption by 50%, and reduce man-hours by 30%. Although the above data is to be confirmed, the cost advantage of the liquid molding process is undoubted. With the country's strong support for energy saving and emission reduction and new energy vehicles, and the current domestic technology gap in the composite material molding process relative to European and American countries, LCM technology with many advantages has huge market potential and prospects in the field of domestic automotive lightweighting. broad.
Safran 3D woven RTM composite aviation turbofan engine blade
BMW i3 composite body
1. RTM (Resin Transfer Molding)
RTM is a liquid thermosetting resin and curing agent are extracted from the storage barrel by the metering equipment, mixed by a static mixer, and injected into a closed mold pre-fiber-reinforced by a injection gun. After filling, curing, and demolding Processed into products.
VARTM (Vacuum Assisted RTM)
VARTM, as its name implies, is based on the RTM process, firstly evacuating the vacuum in the closed mold, and then injecting the resin, or the resin is injected only by the pressure difference between the internal and external pressure caused by the vacuum in the mold cavity. Significant effect. The comparison of RTM and VARTM process flow is shown in Figure 3.
2. Light-RTM
The Light-RTM process is based on the VARTM process technology, using semi-rigid molds and rigid molds to form the cavity of the part, and using vacuum sealing technology, the mold can be used repeatedly, which greatly reduces the cost of the mold and the manufacturing cost of the part; At the same time, a ring-like channel is used to inject glue from the periphery of the mold and the collection cup collection technology, which greatly improves the flow filling of the resin and improves the production efficiency. Light-RTM mold schematic is shown in Figure 4.
In the resin filling process, unlike the RTM technology, which diffuses from the injection port to the surroundings, Light-RTM uses a peripheral injection method, as shown in Figure 5.
3. SCRIMP (Seemann Composite Resin Infusion Molding Process)
Seemann is the name of the patented technology registration company. The SCRIMP process is based on the addition of VARTM. The resin is injected into the mold cavity through a specially designed resin distribution system under vacuum. Among them, according to the different resin distribution system, it can be further subdivided into high permeability medium SCRIMP and drainage trough type SCRIMP process.
4. RFI
In the RFI process, a pre-prepared resin film or thick resin block is placed on the bottom of a mold, and the upper layer is covered with a preform. Then, the cavity is sealed according to the points of the vacuum bag molding process, and the resin film (block) is melted by heating The process of vacuum sucking the resin from the bottom to the top to fill the entire blank (that is, the preform) space, and the process of obtaining the composite material after curing and molding.
5. HP-RTM (High Pressure-RTM)
High-pressure RTM technology is currently a relatively new RTM process technology suitable for mass production of high-performance thermoset composite parts. It uses preforms, steel molds, vacuum-assisted exhaust, high-pressure mixed injection, and resin impregnation and curing of fibers under high pressure. Compared with the traditional RTM, the HP-RTM process has many advantages: (1) fast mold filling, good wetting effect, significantly reducing air bubbles and lowering porosity; (2) using high-activity, low-viscosity resin, shortening production cycle , High process stability and repeatability; (3) the use of internal release agent and self-cleaning system, the surface of the product is excellent, thickness and shape deviation is small. Can achieve low cost, short cycle (high volume), high quality production. The author has exchanged this technology with Professor Bickerton, who is responsible for the research on composite molding of BMW i3 and i8 series composite car body. Professor Bickerton confirmed that BMW has widely adopted this technology more than ten years ago to achieve rapid prototyping of composite automotive parts. . Fully automatic HP-RTM process center jointly developed by Dieffenbacher and KraussMaffei, Germany, including manufacturing of prefabricated parts (prepreg cutting, placement, etc.), pressure and injection units (high-pressure injection molding of parts and Fast curing), complete unit (demoulding and self-cleaning) three main parts, to achieve full automation of the HP-RTM process, currently only one private compound material company in China imported two of this equipment.
6. HP-CRTM (High Pressure- compression RTM)
HP-CRTM is to increase the gap of the sealed mold cavity and the resin injection channel before resin injection in the HP-RTM process. After the injection is completed, the mold is completely closed under high pressure. The resin system flows with the closing pressure to fill and mold. The pressure is relatively low, which avoids the impulse of the fiber, improves the resin's permeability and flow distance, increases the fiber's impregnation speed, effectively avoids the occurrence of fiber dry spots, and shortens the molding cycle of the product.
Summarizing the entire LCM technology, no matter which specific molding process method is used, the resin impregnation process is an indispensable process in all methods, and the main defects in the LCM process such as bubbles are also generated during the impregnation process. Therefore, the research on the resin impregnation characteristics in LCM process molding is of great significance to improve the manufacturing quality of composite parts and optimize the process parameters.