Application of CFRP Forming Technology in Automobile Lightweighting

 

 

At present, composite materials for automotive parts can be divided into two categories: thermosetting and thermoplastic. The raw materials are mostly glass fiber. The main production processes are SMC, LFT and GMT processes. They are mainly used in roofs, rear doors and side doors, covers, engine hoods and front fenders, upper and middle spoilers, front bumpers, fenders, dashboard frames and front brackets.

 

The main advantages of carbon fiber composite materials (CFRP) for automobiles are high specific strength and specific stiffness. The specific gravity is less than 1/4 of steel, and its tensile strength is generally above 3.5 GPa, which is 7 to 9 times that of steel, and the tensile modulus is 230 to 430 GPa. CFRP has high corrosion resistance and its service life is significantly longer than that of metal materials, without expensive anti-corrosion protection measures. Experiments have shown that replacing the current steel body with CFRP structure can reduce weight by up to 60% on a large scale, thereby improving fuel efficiency by more than 30%.

 

In order to meet the needs of lightweight automobiles and ensure the level of comprehensive performance indicators of automobiles, in recent years, raw material companies, composite material production and equipment companies and automobile companies have focused on developing low-cost, high-performance carbon fiber composite material molding processes and equipment that meet the requirements of automobile production rhythm.

 

1. Resin Transfer Molding Process

 

The Schematic Diagram of the Resin Transfer Molding Process (RTM-Resin Transfer Molding) is shown in Figure 1. The preform is laid in the mold, and pressure can be applied in advance to make the fabric fit the mold shape as much as possible, or the fabric layer can be fixed by bonding; then, the upper mold and the lower mold are closed, and the resin is injected into the mold cavity. After the fiber is impregnated, the resin introduction is stopped, and the composite material is demolded after solidification. Resin injection and curing can be carried out at room temperature or under heating conditions.

 

 

RTM has been widely used in the engine hood, fenders, body, chassis and other parts of some sports cars, and has achieved mass production. For example, the engine hood, fenders, front and rear bumpers of Aston Martin Aniquish, as shown in the left figure of Figure 2, are the body side panels, with an annual output of 400 pieces. The right figure of Figure 2 shows the engine hood, fenders and roof of BMW M3CSL prepared by RTM with the assistance of 1800t press using 5 layers of carbon fiber and epoxy resin, with an annual output of 100 pieces.

 

2. High-Pressure Resin Transfer Molding Process

 

The traditional RTM process can form complex products with high volume content and two-sided light. During the molding process, the general injection pressure is 6~15bar, and the maximum does not exceed 20bar. The time for resin injection and infiltration of the reinforcement is generally long, and the process cycle sometimes takes several hours, which cannot meet the needs of automobile production rhythm. At present, the BMW i3 carbon fiber composite material adopts high-pressure resin transfer molding (HP-RTM). The HP-RTM process is shown in Figure 3, which mainly includes preforming, pressing process, injection molding process and curing.

 

 

Compared with traditional RTM, HP-RTM process has the characteristics of fast mold filling, fast molding and excellent product performance. HP-RTM process has fast mold filling and good impregnation effect, which significantly reduces bubbles and porosity; using highly active resin, the production cycle is shortened to several minutes, and the process stability and repeatability are high; using internal release agent and self-cleaning system, the surface effect of the product is excellent, and the thickness and shape deviation are small. HP-RTM process ensures low cost, short cycle (large batch) and high quality production.

 

In order to meet the requirements of rapid prototyping, the process time required for carbon fiber composite materials from preform preparation, resin injection, curing to demolding can be reduced from the original 160min to 30min, and the curing time for HP-RTM resin is less than 3min.

 

At present, foreign companies that provide HP-RTM equipment mainly include Germany’s Dieffenbacher, KraussMaffei, Hennecke, Fulimo and Italy’s Cannon Company.

 

 

The passenger compartment of the BMW i3 all-electric vehicle adopts a carbon fiber composite structure, as shown in the upper left of Figure 4, which is prepared by KraussMaffei injection molding machines and HP-RTM machines. The polyurethane matrix resin LoctteMAX3 developed by Henkel is used to produce the Roding Roadster R1 roof cover through KraussMaffei’s HP-RTM technology, as shown in the upper right of Figure 4. In addition, the Volvo XC90 spring leaf, as shown in the lower Figure 4, is made of polyurethane and glass fiber through HP-RTM.

 

3. Prepreg Compression Molding Process

 

Prepreg Compression Molding (PCM-Prepreg Compression Molding) is a process for rapidly preparing carbon fiber reinforced composite materials by superimposing fast-curing prepreg sheet layers for hot preforming and then curing them through compression molding, as shown in Figure 5.

 

 

The surface finish of the products produced by PCM process is very close to that of Class A surface SMC, which can basically meet the surface requirements of the car’s outer surround.

 

Mitsubishi Rayon’s fast-curing prepreg for PCM is made of TR50S carbon fiber (tensile strength 4900MPa, modulus 240GPa, elongation at break 2.0%) and bisphenol A epoxy resin, and there are two types: R02 and RO3. It can be made into unidirectional prepreg or fabric prepreg (plain, twill and satin). R02 prepreg cures for about 5 minutes at 140°C; R03 has a shorter curing cycle, curing for only 3 minutes at 140°C. When using PCM to make the engine cover, the outer panel of the engine cover (600mm×600mm×1.1mm) is formed with carbon fiber prepreg, and the inner panel with a more complex structure (600mm×600mm×1.5mm) is formed with carbon fiber SMC. The entire engine cover is made of two plates, the inner and outer plates. Due to the use of carbon fiber composite materials, the weight of the engine cover is reduced by 63%, weighing 5.3kg.

 

The body size of Mitsubishi MOMA (CLO1-M) is 4450mm long × 1890mm wide × 1230mm high, and the dry weight is 1030kg. The molding method adopts prepreg sheet compression molding, as shown in Figure 6, the CF woven sheet is impregnated with fast-hardening resin and then placed in the mold for compression molding, and the molding time is about 10 minutes. Currently, a 200t punching machine is used, and a 100t punching machine is planned to be introduced in the near future.

 

 

Compared with the resin transfer molding (RTM) and vacuum assisted resin transfer molding (VARTM) processes, PCM has a shorter curing molding cycle, significantly improved molding efficiency, and can achieve mass production. Due to the use of filament prepreg, PCM has higher mechanical properties than the sheet molding process (SMC) using chopped fibers. Due to the easy molding, high production efficiency, and excellent mechanical properties of the PCM process, it has greater advantages over other molding processes. With further optimization of the process, PCM is expected to achieve industrial mass production of CFRP, providing more possibilities for the application of CFRP in the automotive industry.

 

4. Wet Molding Process

 

During the wet molding process, the dry fiber preform is first placed in the mold, then sprayed with resin, and then the mold is transferred to the press, and the resin is filled with the fabric surface and solidified by the applied force. The process flow is shown in Figure 7. During the whole process, the resin evenly covers the preform without resin flow. It takes 15 to 20 seconds to spray the resin on the preform during the molding process, and the entire manufacturing time does not exceed 60 seconds. Both Dow and Huntsman own patents for related resin systems and process methods.

 

 

During the wet compression process, the resin infusion and die punching processes are omitted, and the molding stage is a simple curing stage with a fast curing speed. With RTM, the curing time is reduced to 60s, while the wet process can reduce the curing time to 30s. The faster mold heating rate can further shorten the curing time. In production, complex parts are often manufactured by RTM, while simple parts can be manufactured by wet compression.

 

5. Winding Molding Process

 

The winding process is a process in which continuous fibers or cloth tapes soaked in resin glue are wound onto a core mold or lining of the corresponding product cavity size according to a certain rule under the conditions of controlling fiber tension and predetermined linear shape, and then cured and demolded to become reinforced plastic products. This process can design the winding rule according to the stress condition of the product, so that the strength of the fiber can be fully utilized, and the specific strength is high. At the same time, fiber winding products are easy to realize mechanized and automated production. After the process conditions are determined, the quality of the wound products is stable and accurate. In addition, mechanized or automated production requires fewer operators and a fast winding speed (240m/min), so the productivity is high.

 

The winding process is mainly suitable for preparing tubular parts such as automobile transmission shafts and drive shafts. The carbon fiber composite transmission shaft used in the BMW M3 is 40% lighter than the pure steel structure, as shown in the left figure of Figure 8. The TORQLine carbon fiber transmission shaft used in the Mitsubishi VOX has successfully passed the high-speed and high-torque tests, as shown in the right figure of Figure 8.