Performance And Molding Process Of Carbon Fiber Composite Materials For Automobiles

 

Judging from the current status and development trends of lightweight materials at home and abroad, although high-strength steel is still the most commonly used and mature automotive lightweight material at this stage, it will not be replaced for the time being. Aluminum alloys, magnesium alloys, engineering plastics and GFRP The applications of CFRP are also showing a gradual growth trend, but CFRP has significant advantages in terms of light weight, high strength and high modulus, impact resistance, shock absorption and sound insulation performance, corrosion resistance, etc. that are unmatched by other materials.

 

At present, the CFRP molding processes with the most application potential in the automotive industry include rapid RTM molding process, prepreg compression molding (PCM), sheet molding compound (SMC) and long fiber reinforcement Thermoplastic resin composite materials (long fiberreinforced thermoplastics, LFT), etc.

 

1. Rapid RTM Molding Process

 

The RTM molding process is the most important liquid molding technology. It has a short molding cycle, high fiber content in the product, good surface finish, and high dimensional accuracy. Since there is no need to use prepregs and autoclaves, the cost of the RTM process is relatively low. In the aviation industry, it is widely used to produce large structural parts.

 

However, the traditional RTM process takes more than 2 hours from fiber laying, resin injection, impregnation, curing to final demoulding, which is difficult to meet the needs of the modern automotive industry for rapid manufacturing technology. Therefore, rapid RTM technology is not only the current first choice for integrated molding of large and complex structural CFRP parts, but also the development direction of future automotive CFRP molding processes.

 

High pressure RTM (HP-RTM) is an effective method to increase injection speed by increasing injection pressure. Using this process, the injection pressure can reach several thousand megapascals, ensuring high mold closing and pressing speeds, greatly shortening component molding time, and improving process efficiency.

 

At the same time, increasing pressure can prompt resin to quickly fill the mold cavity, increase fiber resin infiltration, reduce the number of resin injections, promote air discharge, and reduce the porosity of the finished product, thereby achieving excellent surface properties. If you choose to inject a low-viscosity resin system or a low-viscosity reactive mixture system at the same time, the injection speed can be further increased; the injection time can also be shortened by accurately measuring the reaction materials through high-pressure metering technology.

 

In addition, due to the strong designability of the structure and performance of CFRP products, the advantages of HP-RTM are even more obvious when applied to the manufacturing of large and complex structural components. Not only can the integrated molding of components be achieved within 5 minutes, but also the number of parts and components can be significantly reduced. The number of firmware simplifies connection and assembly, greatly reducing energy consumption in the production process and lowering production costs.

 

A large number of CFRP parts for the BMW i3 body are produced using HP-RTM technology. BMW’s Leipzig and Landshut factories are equipped with two HP-RTM injection units for each 3000t hydraulic press. When the automated production line accurately places the carbon fiber prefabricated parts into the steel mold After the mold is closed, the HP-RTM unit can inject resin into the mold with the help of high pressure and complete the curing of the epoxy resin within 5 minutes. The use of HP-RTM technology reduces the number of CFRP parts in the BMW i3 by 2/3 compared to traditional metal parts, to only about 150.

 

2. PCM Molding Process

 

Compression molding is a molding method in which the stamped CFRP semi-finished product is placed in a mold in advance, and then heated and pressed to form and solidify. Among them, the forming blank before hot pressing is the key to achieving rapid manufacturing.

In recent years, prepregs have become more and more widely used due to their precise fiber and resin ratio. The PCM molding process is an ideal hot pressing process outside the CFRP tank.

 

It can not only greatly shorten the molding cycle and improve production efficiency, but also has the characteristics of high dimensional accuracy, good surface finish, relatively low production cost, and easy one-time molding of complex structural parts.

 

At the same time, due to the good fiber orientation in the product, the strength and stiffness of the product are relatively high, and it has become an important molding process for automotive CFRP.

The rapid curing PCM molding process launched by Mitsubishi Corporation of Japan in 2012 uses 60kP330 and 50kWCF two types of large-tow carbon fiber prepregs, hoping to obtain good processability, excellent mechanical properties and high productivity similar to small-tow CFRP.

 

In 2014, Mitsubishi Rayon applied the PCM process to the manufacturing of Nissan Nismo version GT-R trunk door. The weight is only 1/2 of the aluminum alloy product, and the molding cycle is shortened to about 10 minutes. It can be used for CFRP automotive parts. mass production.

 

The thermoplastic CFRP prepreg rapid hot pressing molding process developed by Ningbo Materials realizes the continuous operation of the continuous fiber yarn/fabric film laminated melt prepreg process. It is used in the mass production of bumpers for a certain Chery Automobile model. The molding efficiency reaches 8 pieces per hour, product quality meets safety collision standards.

 

4. SMC Molding Process

 

SMC is a sheet-shaped molded material made of fibers or chopped fiber mats impregnated with resin paste and covered with polyethylene films on both sides. It belongs to the range of prepreg felts. SMC has high molding efficiency, good surface finish, good dimensional stability, short molding cycle, and low cost. It is suitable for mass production and the production of thin-walled products with little change in cross-section. It has been used in the field of GFRP automotive parts production. Widely used.

 

Currently, in the automotive CFRP molding process, SMC is mainly used for the production of sheet-like chopped fiber composite materials. Due to the discontinuity of the fibers, the strength of the products is not high, and the strength has in-plane isotropic characteristics. The wettability of carbon fiber in resin paste is an important issue faced by the SMC process. By performing necessary surface treatment on carbon fiber and using appropriate wetting and dispersing agents, the wettability and uniformity of carbon fiber in resin paste can be effectively improved. . Carbon fiber SMC has also found many applications in the automotive industry.

 

5. The 2003 Dodge Viper (Viper) is the first model to use continuous carbon fiber and glass fiber hybrid reinforced vinyl resin SMC parts in batches. CFRP is mainly used in the manufacturing of door and windshield structures.

 

The windshield strength of this model is greatly improved compared to the original model. The new door has increased strength while reducing weight, and the door sag is well controlled.

 

In 2012, Japan’s Asahi Organic Materials Industry Co., Ltd. took advantage of the conductivity of carbon fiber and used the SMC process to produce components for absorbing radio waves for electric vehicles. The components were used for both electromagnetic shielding and structural materials.

 

In 2013, auto parts supplier Magna and large-tow carbon fiber manufacturer Zoltek combined PANEX35 carbon fiber with Magna’s EpicBlendSMC formula and technology to jointly develop low-cost carbon fiber SMC technology for vehicles. and products, greatly improving production efficiency.

 

In addition, domestic car companies, such as SAIC, BAIC, Chery, FAW, etc., have also carried out a lot of research on SMC and applied CFRP to tailgates, new energy vehicle battery box covers, engine hoods, rear roof covers, front cabin covers, etc. On the car exterior panels.

 

4. LFT Injection Molding Process

 

In addition to thermosetting resins, carbon fiber fabrics, and continuous fibers, thermoplastic resins and discontinuous carbon fibers also have many applications in the automotive field.

 

The LFT molding process has excellent molding processability, high molding rate, high yield, relatively simple equipment, and low process cost. The long fiber length forms a skeleton structure inside the product, giving the product good impact resistance and stiffness. , so LFT products can be used for vehicle body parts that are subject to greater stress.

 

LFT has been widely used in automobile bodies and is also a molding process with great application potential. Compared with aluminum alloy and high-strength steel, carbon fiber-reinforced nylon 6 LFT composite materials have the same specific modulus and 50% to 250% higher specific strength. They are quite competitive in the manufacturing of automotive secondary load-bearing structural parts.