Recent trends in CFRP development: Increased usage in European vehicles
BMW shows new technology, German companies jointly develop new plastic forming processes
- CFRP monocoque body of BMW i3/i8 Series
- Carbon fiber body is 39% lighter than steel body
- CFRP life module forming technology
- Latest technologies in Resin Transfer Molding (RTM)
- Overview of lightweight molding technologies in Europe
- Evolution of thermosetting and thermoplastic resin molding technologies
- Germany's strength in developing automobile weight reduction technologies
- Expanded use of CFRP in all-new BMW 7 Series
An increasing number of European automakers are applying Carbon Fiber Reinforced Plastics (CFRP) to their production vehicles. In 2013, BMW launched the i3 Series of vehicles, followed by the i8 Series in 2014, as the first production models that featured the carbon fiber monocoque body. The company has also announced plans to apply carbon fiber as well as aluminum to the basic structure of its core model, the 7 Series, slated for launch in 2016. Advanced development is also underway at Mercedes-Benz and Audi toward the adoption of CFRP in their production vehicles as the pillar of weight reducing technologies.
Reported below is the interview on recent trends of CFRP in Germany with Mr. Yasujiro Uemura who represents GSI Creos Corporation and is well-versed in CFRP production engineering. GSI Creos sells high-performance press machine imported from Dieffenbacher GmbH, a leading German manufacturer of equipment, and resin molding machines imported from KraussMaffei. GSI Creos is in the course of exploring CFRP businesses.
Technologies from BMW’s EfficientDynamics strategy highlight company direction (May 2015)
JSAE Exposition 2015: Mazda2 features latest body structural technologies (Jun. 2015)
Automotive Weight Reduction Expo 2015 (Mar. 2015)
International Plastic Fair Japan 2014 (Nov. 2014)
CFRP monocoque body of BMW i3/i8 Series
|BMW i3 body structure CFRP life module shown over aluminum drive module Source: BMW||BMW i8 body structure CFRP life module shown over aluminum drive module Source: BMW|
|Source: GSI Creos|
The life module forming the cabin of the BMW i3 body features a CFRP monocoque structure. Another module, the drive module that packages the powertrain, chassis and battery, is made up of aluminum. The two modular structures are integrated by an innovative method.
The carbon fiber is supplied by Mitsubishi Rayon Co., Ltd. BMW sold 16,000 units of BMW i3 by May 2015, a year and half after the launch in 2013. The use of CFRP structure had been limited to high-end supercars and the BMW i3 is the first production car in which massive CFRP is used.
As shown in the weight composition pie chart, CFRP accounts for 68.5kg, nearly a half of the total weight of the BMW i3 body structure. The carbon fiber monocoque structure of the cabin weighs approximately 101.3kg including 13.3kg of thermoplastic resin and 19.5kg of adhesives, foams, etc., other than the CFRP.
The remaining 36.8kg is the weight of the body structure other than the chassis, powertrain and battery in the drive module located under the floor. The underfloor structure weighs 36.8kg consisting of 15.9kg of aluminum sheet, 5.4kg of extruded aluminum, 5.4kg of forged aluminum, and 10.1kg of steel.
Carbon fiber body is 39% lighter than steel body
|BMW i3 life module||Mazda2 body|
The body weight of the BMW i3 is compared above with that of the Mazda2. The Mazda2 was developed with the sheer goal of achieving maximum weight reduction using only steel and without new materials such as aluminum or CFRP. Toward that goal, Mazda engineers sought expanded use of ultra-high and high tensile steel sheet. In addition, they analyzed the physical properties of the high tensile steel sheet and devised a surface shape that best retained the original strength of the thin sheet. As a result, they achieved weight reduction of 10.2kg, an equivalent of 35.6kg weight reduction taking into consideration the additional weight of 25.4kg resulting from the larger vehicle size and improved specifications (see Note). Its vehicle performance is highly rated and the Mazda2 seems to provide a fair comparison with the BMW i3. Their overall length is about the same and the wheelbase having a major impact on the vehicle weight is identical. The BMW i3 is slightly wider and higher than the Mazda2. To make up for the dimensional differences, we used 228.1kg as the body weight of the Mazda2 for comparison purpose.
The BMW i3 has the body weight of 138.1kg, which is 90kg (39%) lighter than that of a Mazda2-based steel-made vehicle of the same size. Considering the fact that the efforts put into the Mazda2 in reducing weight by using the best steel technologies have led to 35.6kg weight reduction, the body structure of the BMW i3 made of CFRP and aluminum clearly proves its weight reducing effect.
(Note) See "JSAE Exposition 2015: Mazda2 features latest body structural technologies"
|Comparison of body weight between the BMW i3 and a steel body|
|Volume: cubic meters||11.033||10.323||Length x width x height|
|Body weight: kg||138.1||213.4|
|Body weight (equivalent condition) : kg||138.1||228.1||Mazda2's weight converted to the same volume as BMW i3|
Comparison of body weight between BMW i3 and a steel body
CFRP life module forming technology
|CFRP manufacturing process of BMW i3 Source: BMW|
Process engineering was the main factor that enabled application of CFRP to volume production cars like the BMW i3 at an affordable price of around JPY 5 million. The conventional autoclave method takes several hours to form a structure of a supercar. The process time was reduced to a matter of several minutes by a high-pressure resin injection method called Resin Transfer Molding (RTM) to form the carbon fiber life module of the BMW i3.
In the RTM process, several laminated carbon fiber sheets of different fiber orientations are stacked and cut to the contour of the finished product. The formed stack is heated into a stable three-dimensional shape. The preformed "blank" is set in the mold and resin is injected under pressure to join fiber and resin. The carbon fiber compound is pressed under heat in the final stage of the process to produce a product having high rigidity.
The carbon fiber compound of the BMW i3 has many carbon fiber parts that are joined together to form the monocoque structure of its life module. Compounds are joined together by an advanced, fully automated bonding process after accurately defining the gaps between joining surfaces. The curing time is shortened by post-heating. The BMW i3 has a 20mm wide joining surface that extends 160 meters in total.
Latest technologies in Resin Transfer Molding (RTM)
The CFRP part manufacturing process of the BMW i3 consists of forming equipment supplied by Dieffenbacher GmbH, a leading manufacturer in Germany, and KraussMaffei. It also consists of the latest facilities reflecting the outcome of technical tie-up and joint development with Fraunhofer ICT, a research institute. The CFRP body structure that forms the cabin of the BMW i3 is a thermoset mold manufactured by RTM process. The latest version of the RTM process reflects ever-evolving variations. Three such variations are used in making the carbon fiber components of the BMW i3.
1) High-Pressure Resin Transfer Molding (HP-RTM)
Thirteen carbon fiber parts of the BMW i3 are made by the HP-RTM process.
The process involves cutting the carbon fiber material to length and incorporating the cuts in a machine press (preforming). The preform is set in an RTM machine and the mold is evacuated. Then, resin is injected under pressure and the material is cured under heat in the mold. The parts are cured under heat and ejected from the mold.
|HP-RTM process Source: GSI Creos|
|HP-RTM system Source: GSI Creos|
|HP-RTM resin injector by KraussMaffei, a German company Source: GSI Creos||High-performance press machine by Dieffenbacher Source: GSI Creos|
2) Wet Molding Process
Nineteen carbon fiber parts of the BMW i3 are made by the thermosetting wet molding process which is an evolved variation of the HP-RTM process. The process involves cutting the material as in the HP-RTM process but resin is injected before the material is set in the mold. The pile of resin-injected carbon fiber material is transferred to the RTM station where the mold is closed and pressurized for impregnating the fiber with resin, followed by curing under heat. This process can be used with recycled fiber sheets as well.
|Wet Molding process Source: GSI Creos|
|Wet Molding system Source: GSI Creos|
3) HP-RTM Braided/W Core forming
Two carbon fiber parts of the BMW i3 are made by the HP-RTM Braided/W Core forming process.
The HP-RTM Braided process starts with carbon fiber being pulled in from a circumferential feeder and braided into a hose as if making a weaved rubber hose. The preformed material is resin-injected in the RTM machine and cured under heat into tubular CFRP components. This method is used for making tubular frames and reinforcing beams.
Overview of lightweight molding technologies in Europe
Dieffenbacher GmbH is a German company engaged in developing resin injection molding (RTM) and other plastic forming processes jointly with several partners as shown below.
The first is the resin injection molding (RTM) process. As stated earlier, the Wet Molding has been developed as a variation of the HP-RTM.
The second is the Sheet Molding Compound (SMC) process in which thermosetting resin sheets containing discontinuous carbon fiber are set and compressed in a mold and cured under heat. An advanced variation is the D-SMC process in which the material is introduced directly before the forming stage in a production line. This process is ideal for making sheets of higher quality at lower cost.
The third is the Long Fiber Thermoplastics Direct (LFT-D) process which is a production line for long-fiber-reinforced thermoplastics. An advanced variation, Tailored LFT-D, has been developed for integrating additional reinforcements at the same time.
The fourth is the Glass Mat Reinforced Thermoplastics (GMT) process in which unwoven glass fiber mat is impregnated with thermoplastic resin for forming. An advanced variation, Lightweight Reinforced Thermoplastics (LWRT) process, forms carbon fiber and other fiber reinforcements over the conventional glass fiber.
All of above processes are the outcome of joint development efforts with Fraunhofer ICT, a partner that plays a key role in the development of lightweight construction technologies in Europe.
Overview of lightweight molding technologies in Europe (courtesy of Dieffenbacher)Source: GSI Creos
D-LFT/Tailored D-LFT forming technology
|Tailored D-LFT process Source: GSI Creos|
Direct Long Fiber Thermoplastics (D-LFT) is a molding process for thermoplastic material containing long fiber compounds. The entire production process is located directly on the same line from polymer melting to molding. A distinctive feature of the process is the inclusion of carbon fiber. This enables manufacturing highly heat-resistant products of complex shape, high dimensional accuracy and tensile strength. D-LFT process is used to manufacture a broad range of products including the frame of the front end module, underfloor panel and spare tire panel.
In the Tailored D-LFT process, additional reinforcing materials are introduced, in combination with the D-LFT material, as required to increase rigidity, strength and energy absorption capacity. Dieffenbacher has been developing these processes jointly with Fraunhofer ICT and DSM. Together, they are supplying equipment for making a wide range of automotive parts.
Evolution of thermosetting and thermoplastic resin molding technologies
|Compound forming technologies using high-performance press machine Source: GSI Creos|
Technologies for forming compounds using high-performance press may be grouped as shown above. Thermosetting resin has been the chief material for manufacturing CFRP components but the use of thermoplastic resin is increasing. RTM technology is also evolving and new variations are being developed. In the case of HP-RTM process for manufacturing thermosetting resin parts, for instance, thermoplastic resin compounds may be formed by HP-T-RTM process in the CFRP preforming stage.
Germany's strength in developing automobile weight reduction technologies
|Fraunhofer ICT Source: GSI Creos|
Joint development projects are encouraged in Germany among industrial sectors such as automakers, suppliers and equipment manufacturers, universities and research institutes. A scheme for joint research and development by industrial, government and academic sectors already exists in Germany and they are all contributing their expertise toward common goals. This also presents an excellent case of human resource development in that those representing universities and research institutes in joint projects are sometimes hired by industrial sectors.
One of Dieffenbacher's partners, Fraunhofer, has research functions in more than 60 locations in Germany. Approximately 20,000 researchers are working on applied studies toward commercialization within three to five years.
One of its research functions, Fraunhofer ICT (Institute for Chemical Technology), is engaged in the development of lightweight construction technologies. They are developing CFRP products jointly with automakers and equipment manufacturers. Fraunhofer ICT has annual research budget of approximately EUR 40 million with 550 researchers consisting of 300 from the defense and 150 from automotive and other industrial sectors.
Expanded use of CFRP in all-new BMW 7 Series
BMW has announced that the all-new BMW 7 Series slated for launch in 2016 will have CFRP components. The car will have a body structure with CFRP reinforcing parts made by RTM process. Unlike the BMW i Series cars whose cabin is formed entirely by a CFRP monocoque structure, the all-new BMW 7 Series will have aluminum and CFRP parts integrated in a steel body structure to achieve weight reduction. Audi and Mercedes-Benz are also planning to use CFRP components in their new models. Efforts to develop technologies to use CFRP on production cars are accelerating in Europe.
|Body structure of the all-new BMW 7 Series Source: BMW|
|Center tunnel reinforcement Source: BMW||Rear pillar reinforcement Source: BMW|
|CFRP used in center pillars and side sills Source: BMW|
|Roof bow and roof side rail Source: BMW||Roof side rail Source: BMW|
|Roof side rails Source: BMW|
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