JSAE Exposition 2016: CFRP application trends in Europe
Cutting-edge RTM and CFRP / metal composites
Floor tunnel cover on BMW7 Series
Carbon Fiber Reinforced Plastic (CFRP) is said to be at the cutting edge in automotive weight reduction technology. Its use was limited to small-lot production vehicles but in recent years utilization has seen gradually increasing in Europe. The following report summarizes exhibits and lectures at workshops during the JSAE Exposition 2016 held in Yokohama. The latest developments in CFRP technology will also be introduced.
Various CFRP molding methods have been developed and are still evolving. Among them, resin transfer molding (RTM) is finding increasing use in the automotive industry, and the molding processes of HP-RTM (High Pressure RTM), compression RTM, and wet-RTM will be analyzed as representative examples, along with the products they are used to make. Also CFRP-metal composite, in which CFRP is integrated with metals such as steel or aluminum, will be introduced. The tailored rolled blank B-pillar reinforced with a CFRP stiffener that was developed by Mubea Carbo Tech, and a new composite material called KRAIBON developed by KRAIBURG, will also be examined.
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HP-RTM（High Pressure Resin Transfer Molding）
A practical example of CFRP material RTM
HP-RTM, or high pressure resin transfer molding, is a process that is currently used to make the carbon monocoque body of the BMW i3 and i8. The process is described in the process-data diagram above, which was provided by the German resin molding machinery manufacturer Krauss Maffei. First woven CFRP material is unrolled and cut (steps 1 to 3), and preformed under a press (4). The preformed parts are set in an RTM molding machine (5) and the mold is evacuated (6). Resin is then injected under pressure (7). The parts are then cured in the mold and the process is completed (8). Parts for racing cars and other small-lot super cars are manufactured in an autoclave and require several hours to form. The HP-RTM process drastically reduces the manufacturing time to just a few minutes. Thirteen parts used in the BMW i3 are manufactured with HP-RTM.
This process can be used to make large parts with complex geometries and can even form hollow parts.
Mass production of wheels with CFRP rims (an application of HP-RTM by Mubea Carbo Tech）
Mubea Carbo Tech exhibited a wheel with a CFRP rim and forged aluminum spokes using HP-RTM. Mubea Carbo Tech’s wheel has spokes that are fastened from inside the rim with titanium bolts so that they are not exposed. A wheel with a CFRP rim that has the same structure is also offered as an option for the BMW M4 GTS.
A molding process for rims that uses HP-RTM takes around 20 minutes for curing (from resin injection through curing).
Weight comparison of wheels manufactured by different processes Source: Mubea Carbo Tech
The image above compares the weights of 7.5Jx20" wheels made with different materials in the case of a load of 500 kg per wheel. It indicates that a wheel with a CFRP rim and forged aluminum spokes can reduce weight by 23 percent compared to a forged aluminum wheel. According to Mubea Carbo Tech, a full-CFRP wheel currently being developed, and consisting of a CFRP rim and CFRP spokes, can reduce weight by 30 percent.
VW XL-1 monocoque body (Mubea Carbo Tech’s application of HP-RTM)
In 2013, Volkswagen (VW) launched the VW XL-1, which was based on a concept car developed as a super fuel-efficient vehicle. The company produced and sold around 1,000 units during the year. The monocoque body that forms the cabin was manufactured by Mubea Carbo Tech. Setting the material in the mold, injecting epoxy resin, and curing under pressure and heat (curing time was 20 to 30 minutes) takes around 2 hours.
The image above states that if the entire body is made of aluminum instead of steel, weight can be reduced from 350 kg to 200 kg. A hybrid of aluminum and CFRP reduces weight by 20 percent, and a full CFRP body weighs 50 percent less than aluminum. Other than the VW XL-1, Mubea Carbo Tech has supplied CFRP bodies for the McLaren MP4-12C and Porsche 918.
Compression RTM（Resin Transfer Molding）
Compression RTM (C-RTM) was developed primarily for parts that have flatter shapes rather than three-dimensional geometries. The process is described in the process-data diagram provided by Krauss Maffei above. The mold is partially open and resin is injected quickly through a 0.3 mm gap. Resin is injected from the mold center and hardened quickly through a chemical reaction. Compared to HP-RTM, which requires 20 to 30 minutes of curing, molding with C-RTM requires no more than 5 minutes.
An application of the C-RTM process co-developed with Alpex and Roding is shown above (roof panel for a Hyundai concept car).
Wet-RTM（Resin Transfer Molding）
Wet-RTM, also called Wet Molding, is an evolution of HP-RTM conceived to reduce cycle time. It follows the same steps as HP-RTM from unrolling through cutting but does not require preforming. Resin is poured over the CFRP core before it is set in the mold. The resin and CFRP core are transported to the RTM station, where the mold is closed and pressurized, and resin is infused into it. It is then cured again to complete the part. Removing the performing process leads to a cycle time of 120 to 180 seconds.
An application of wet-RTM
BMW 7 Series floor tunnel reinforcement
Source: Krauss Maffei
An application of wet-RTM
BMW 7 Series roof beam
Source: Krauss Maffei
Many CFRP parts are used in the BMW 7 Series that was launched last year. The center console reinforcement, room beams and other parts are made with wet-RTM.
A layout example of an automated Wet-RTM system
Source: Krauss Maffei
The Wet-RTM process can be fully automated.
CFRP and steel composite structures
Various studies are being undertaken for components that protect passengers in the event of a crash in the manner that the B-pillar does, and therefore cannot have their shape changed. What Mubea is proposing is consideration of a lightweight structure that also has firm rigidity and strength through the fusion of a tailor rolled blanking (TRB) steel B-pillar base with a box-shaped CFRP cross-section. In the example given above, the panel forming the TRB material B-pillar (on the left in the diagram) is combined with a CFRP stiffener (center), as well as a locking plate (right) with a constant thickness that was formed by rolling or hot stamping, to reduce the weight of the highly rigid and strong structure. Currently, B-pillars are made with several methods including hot stamping and TRB. It is to be expected that there will be further efforts to develop lightweight yet highly stiff and strong structures.
KRAIBON: A new composite material containing soft material
KRAIBON composite material Source: KRAIBURG
KRAIBON is a new industrial material in which rubber that has high damping properties is integrated with composite fiber. As illustrated in the photo to the upper-left, sandwiching KRAIBON between CFRP layers results in outstanding vibration damping characteristics.
Fig. 1 shows a comparison of the vibration damping effect of KRAIBON-integrated material (blue) with material that does not use it (red). The graph clearly indicates that KRAIBON has a high vibration damping effect.
Figs. 2 ~ 4 show the loss factor (a factor that indicates the vibration damping effect) of various materials in terms of the frequency of vibration along the horizontal axis and the ambient temperature along the vertical axis. The loss factor change is indicated by colors from blue (low loss factor) to yellow (high loss factor) and red (higher loss factor). Fig. 2 shows the loss factor of material entirely made of CFRP. The graph is completely blue, which indicates that the material has high stiffness and low damping effect. It is the lightest of all materials compared with an area weight of 3.5kg/m2. Fig. 3 shows the loss factor of CFRP-aluminum composite. Its loss factor is higher than that of CFRP but not particularly large. It is the heaviest of all materials compared with an area weight of 5.9kg/m2. Fig. 4 shows the loss factor of KRAIBON composite. It has a high loss factor and high damping effect. It is heavier than CFRP but lighter than the CFRP and aluminum composite.
KRAIBON is far lighter than the aluminum composite and its damping performance (loss factor to weight) is 375 percent higher. KRAIBON is integrated directly into the CFRP that is applied to metals such as steel and aluminum. According to KRAIBURG, KRAIBON provides high vibration damping properties to the CFRP composite, along with high energy absorption in an impact. The overall weight and cost of producing modern vehicles keep rising due to the need to use highly rigid parts and damping materials to block the vibrations and sounds from road surfaces, the engine, and the powertrain. It appears that the availability of parts with high vibration damping performance like KRAIBON will play a key role in industry-wide efforts to reduce vehicle weight.
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