Carbon fibers will not make it into the next generation of single aisle planes—unless manufacturers learn from “the painful lessons the automotive industry suffered from,” Andreas Wüllner, chairman, business unit composites–fibers and materials at SGL Group, said at this year’s AeroDef show in Texas.
“The usage of carbon fibers in the next generation of single aisle planes, which may enter into service during the second half of the next decade, will heavily depend on two factors,” he added. “First, is there a viable business case for the use of CFRP [carbon-fiber-reinforced plastic] in comparison to other materials? And, second, will the required low takt times be achieved with capable manufacturing technologies? Looking at the state-of-the-art autoclave production value chain, I’m skeptical that this will be the path forward.”
Takt time is the average time recorded between the start of production of one unit and the start of production of the next unit, when these production starts are set to match the rate of customer demand.
Highly automated, out-of-the-autoclave technologies must be implemented to achieve low takt times, said Wüllner, who has worked at the SGL Group longer than 20 years.
That’s because build rates of the single-aisle airliners will grow going forward: “Think about 10 to 12 twin aisles, A350 or B787, aircraft per month. Doing the math with 20 average workdays results in .5 aircraft per day or a takt time of something like 2880 minutes. Compare this to a takt of approximately five minutes, which comes close to the BMW 7-series production!”
Building the Airbus 350 or Boeing 787 requires a production rate of one wing per day by 2020, he said.
“This is doable with autoclave technologies. But, imagine if six wings per day for the single aisle planes, A320 and B737, had to be manufactured with CFRP. How should this be achieved? Who could and would afford such investments?”
In automotive, carbon fiber composites evolved from carbon fiber prepreg monocoques—vehicle structures in which the chassis is integral with the body—for racing and super sports cars cured in autoclaves in the 1980s. “This is still the prevailing technology for expensive, low-volume cars,” Wüllner said.
“Then, roofs and hang-on parts found their way into larger production volumes, up to a few thousand vehicles per year. By the end of the 1990s, BMW started its CFRP in-house development.”
BMW gained “a lot of experience in small series applications, such as CFRP roofs or bumper structures for the famous M models,” he said.
When BMW in 2008 began investigating “the future of mobility with a team called project I, one of the objectives was to develop a vehicle for the world’s megacities,” Wüllner said. A year later, the automaker had to choose between an aluminum chassis and a carbon fiber passenger compartment mounted on an aluminum frame.
“As you may know, they decided in favor of the carbon fiber solution and started to develop the underlying product and production technologies for the car that became the BMW i3 battery electric vehicle.”
The passenger compartment is made from several plies of 50k carbon fiber unidirectional non-crimped-fabric of different area weights and orientations, he added. “Hence, the designers had full flexibility to add plies where needed, without inflating the number of base materials.
The stacking of such plies is performed in fully automated production lines. Molding and infiltration with epoxy resin is done by high-pressure resin transfer molding of the preform or by compression molding, followed by oven curing.”
Several hurdles had to be overcome for setting up the technology for a capacity of more than 30,000 units per year. But “these learnings were the foundation for the next development step, the so-called carbon core of the recent BMW 7-series,” Wüllner said. “This set of different technologies allows for the combination of several materials, conventional steel, press-hardened steel, aluminum, and CFRP in the vehicle chassis.”
In 2009, SGL Group began to set up “the unique supply chain for BMW’s i3, i8, and 7-series models,” he said. “The 50k polyacrylonitrile precursor is spun in our Japanese joint venture in Otake. From there it is being shipped in boxes to our state-of-the-art carbon fiber plant in Moses Lake, WA. Further production steps are being made in Germany. Non-crimped-fabrics and nonwovens made from recycled production off-cuts are being manufactured in Wackersdorf. Stacking of the non-crimped-fabric plies is done in Wackersdorf by BMW.
The German firm has three US sites: Hitco Carbon Composites in Arkadelphia, AR, producing fabricated insulations for the aerospace industry; Hitco Carbon Composites in Gardena, CA, serving automotive, aerospace and industrial applications; and SGL’s American hub for high tow count carbon fiber production, in Moses Lake, WA.
Wüllner called being involved in designing and building up the dedicated global supply chain for BMW, stretching from carbon fiber precursor to textile materials, a “once in a lifetime opportunity.”
That allowed the production of the BMW i3, a purpose-designed battery electric vehicle, starting in 2013.
“Since then, we came a long way until the launch of the carbon core BMW 7-series model in 2015.”
It was his education “regarding series manufacturing with carbon fibers in the automotive industry.”
Wüllner asked the AeroDef crowd, “Wouldn’t it be great to build the next generation of single-aisle commercial aircraft in the second half of the 2020s with a significant percentage of carbon fiber composites?”
Boeing started using carbon fiber-reinforced plastics in commercial airplanes in the late 1960s, with its 737 and 747 models. Airbus followed with the 300. “However, the percentage of the airframe weight remained in the single-digit order of magnitude. CFRP utilization in new commercial aircraft programs showed a steady increase from about 5% to about 15% between 1970 and 2000. In those early years, CFRP was mainly used in secondary structures.”
Since 2000, driven by the new A380, B787, and A350 programs, the CFRP content in the aircraft structure surged to above 50% in the Dreamliner and A350, he said. Now, CFRP is also extensively used in primary aircraft structures with the highest performance requirements.
But, carbon fiber composites remain expensive and the manufacturing process is more complicated and typically takes longer compared with other materials.
The aerospace and defense industry was always at the forefront of the technology development for carbon-fiber-based parts. “Numerous types of high-performance carbon fibers, materials, manufacturing and curing methods have been developed over time,” Wüllner said. “However, the aerospace and defense industry typically doesn’t have to care about takt time as the auto industry has to.”
Thousands of cars are built each month. But only tens of commercial aircraft are built each month. “The existing manufacturing technologies are not adequate for further market penetration in aerospace and defense.”
While CFRP adoption of up to 50% of the airframe weight in Boeing’s and Airbus’ twin-aisle models is remarkable, “this development took four waves and began more than 40 years ago,” he said. “We expect the usage of carbon fiber in commercial aircraft to reach a plateau around 2020. Until then, build rates will go up and the 777 X will have entered into service. However, due to the longevity of the supply contracts, I don’t see major changes in the supply base.”
On top of that, other sub-suppliers are converting carbon fibers into braided preforms and prepreg patches.
In automotive, “besides developing the products and the underlying production technologies we had to build the plants from scratch,” he said. “The carbon fiber plant in Moses Lake was erected between 2010 and 2014…. The oxidation process is fueled with electric energy generated from hydropower. All carbon fiber lines are based on the same design. The first lines built have been retrofitted to resemble the newer lines benefitting from the learnings made over time.
“Hence, in Moses Lake we have a plant with the most modern abatement technology to comply with the stringent environmental standards,” he added. “Statistical process control and traceability from polymer production in Otake to part manufacture in Germany are core principals of our production system.”
Wackersdorf, with its “unique unidirectional fabrics tailor-made for the requirements of large volume series production,” is the largest carbon fiber textiles plant in the world, Wüllner said.
“Exchangeability is key to our supply chain. Therefore, any part can be made from any combination of spinning line, carbon fiber line, or warp-knitting machine of our qualified supply chain. This allows for immediate response to changes in demand.
“The plants in Moses Lake and Wackersdorf are connected to a common manufacturing and quality software system.”
He showed the Aerodef audience a slide with the 34 different CFRP parts of the BMW i3.
“The victory tour of carbon fiber composites in automotive and aerospace depends on material utilization,” Wüllner said. “Therefore, waste reduction in combination with low takt times is crucial.”
Towpregs and braided parts offer “excellent material utilization but still suffer from long cycle times,” he said. “Those must be reduced to become successful. Otherwise high capital expenditure and operating cost will lead to negative business cases.”
Aerospace and defense firms can learn these things from the automotive industry’s experience with large series CFRP production at low takt times, Wüllner said in summary:
The design process has to be adopted to series production. In addition, the tool design for such processes should incorporate solutions for minute variations in part geometry.
To reduce cost along the value chain, define and implement standard materials, “as we did with our 50k carbon fiber, which was optimized for minimum variation of properties and for the downstream textile manufacturing steps.”
Adopt automated manufacturing processes with short cycle times to minimize CapEx.
Focus on waste reduction, statistical process control and inline quality inspection.