Building by Bonding: BMW, the i3 and Carbon Fiber
It’s not obvious from the outside of BMW’s vehicle production facility in Leipzig, Germany, that there is something special going on here. The rain doesn’t exactly paint the scene with bright colors either, as the bus stops and we scurry inside one of several rectangular buildings. The main building was designed by the renowned architect Zaha Hadid, but we enter another, where the work is being done. We go in to see the very first production i3 electric vehicle (EV) roll off the line.
BMW’s revolutionary achievement with its first EV is the widespread use of carbon fiber parts in a mass production vehicle. While carbon fiber bodies have long been the purview of Formula One cars, BMW is bringing the technology to, if not the masses, at least the mid-market. The i3 EV will retail for a little more than $42,000 for the base model.
This is a facility where conventional stamping and welding have given way to molding and adhesive bonding as the company produces its purpose-built i3 EV. Remarkably, whereas assembly plants are often noisy, the production of the i3 is so quiet that the rain outside provides the sole source of ambient noise.
Years in the Making
Because i3 production requires fewer body parts and leverages parallel assembly lines—one that builds the carbon fiber body and another that builds the aluminum frame—it takes about 20 hours to build the EV, or half the amount of time it takes BMW to build a conventional 3 Series car. A specially developed adhesive bonds the body and frame together on the final assembly line. The result is an innovative, lightweight (2,635 lb. including the 450-lb. battery) EV intended for city driving with a range of 100 miles and the ability to accelerate from 0-60 in about seven seconds.
To produce the i3, BMW engineers spent a decade refining and automating the carbon fiber production process. While it has been producing roof panels from carbon fiber reinforced plastic (CFRP) for its M3 CSL sports sedan since 2003, the i3 (and the sportier i8*, built mostly in BMW’s Landshut plant) takes the use of carbon fiber to new levels.
“There’s been a maturation process,” says Daniel Schaefer, manager of the i3 production process. “We’ve reduced cycle times significantly, by 50%, and material production cost by 30-40%.”
A big part of keeping material costs in check is having “control” of carbon fiber production, Schaefer says. To gain this control, BMW formed a joint venture with SGL Group, a producer of carbon-based products, including carbon fibers. The joint venture, SGL Automotive Carbon Fibers, opened a production facility in Moses Lake, WA, in 2011. Now, two production lines there produce 3,000 tons of carbon fiber annually to meet BMW production requirements.
This is vertical integration on a scale analogous to Ford making its own steel in the last century.
Because the BMW i program is about sustainability, the SGL plant is located near a hydroelectric power source, providing readily accessible renewable energy to fuel the complex and energy-intensive process of converting the precursor material, polyacrylonitrile (PAN), into carbon fibers. Each fiber is extremely thin, measuring seven microns (0.007mm), so approximately 50,000 of them are bundled into “rovings” and wound onto reels for transport to Germany.
In an SGL facility in Wackersdorf, Germany, the rovings are processed into laminates. These laminates, which have different fiber alignments, are cut to shape and stacked for shipping to the i3 plant. Again for purposes of sustainability, scrap CFRP resulting, for example, from cutting the shapes is recycled and ultimately used to make components including the roof panel for the i3.
Forming and bonding
In the press shop at the Leipzig plant the CFRP laminates are placed in one of three preforming machines where, through a combination of heat and pressure, they are formed to a three-dimensional shape before the next step, resin transfer molding (RTM). This process injects liquid resin into the preform pieces under high pressure. When the fibers and resin bond are hardened, the parts become rigid. The Leipzig press shop has seven RTM machines, which make 150 CFRP parts for the i3. In the past, these parts would be hardened in an oven—a time-consuming process. To speed up the hardening, BMW engineers developed automated presses with specific time, pressure and temperature parameters to ensure the RTM process takes minutes rather than hours.
Next, in the body shop, the CFRP parts are bonded together to form the body in a 100% automated process. All of the bonding is done with a single type of glue that took three years to develop. The key was finding an adhesive that was “strong yet elastic.”
“The main connection is the glue,” notes Helmut Schramm, the plant manager for electric vehicle production. “The whole construction is very tough. The components are positioned precisely at a defined bond gap in order to ensure the joint is as strong as possible.”
The bond begins to take hold in 90 seconds and fully hardens in 90 minutes if left alone. However, to speed the hardening, BMW engineers developed a supplementary thermal process. By heating specific points on the parts with infrared light, the hardening is reduced to a mere 10 minutes.
Temperature control is essential, Schramm explains. Digital thermometers are visible above each automated bonding station. “We have special sensors that control the temperature on each station,” he says.
In the “Technology Exterior Components Building,” there are the molding machines for the exterior plastic parts, including the door panels and bumpers. Here, workers use one of three processes to mold the plastic, depending on the part.
A standard injection molding process is used for the simplest parts. Others are produced with a “twin” process wherein the outer skin and substructure are injection molded and then bonded in separate, successive stages. The door panels are made with a third process, “bonding via injection molding.” This involves injection molding the outer skin and substructure in parallel, and joining them together within the same automated process.
Once the plastic parts are molded, they move to the paint shop (located inside the same building) where the outer skin parts are coated to protect against stone chipping and sunlight exposure.
The paint shop doesn’t resemble others used to paint conventional steel-bodied BMWs because the plastic doesn’t require primer, which alone reduces the mass of the vehicle by 10 kg (BMW figures that the plastic outer is about half the weight of a similar steel assembly—with the added advantages of being corrosion free and more resistant to minor damage). What’s more, the overall paint shop has a smaller footprint.
Inside the main assembly building, the aluminum chassis, or “drive module” in BMW parlance, is fitted with a lithium-ion battery and motor/transmission. Both the chassis and battery are produced at BMW’s Dingolfing, Germany, plant, while the motor/transmission unit comes from the company’s Landshut plant. BMW is offering an optional range extender—a 650-cc, two-cylinder engine—that is also fitted on this line.
Once the chassis and body are ready, they meet at the marriage station. Here the body is glued onto the chassis and bolted at four points. The bonding agent is applied to the chassis by two robots, then lifted and centered before the body is lowered by another robot. The joining process is initiated by the body’s weight. Once complete, the i3 is fitted with its plastic exterior skin.
A sign hanging in the plant says: “Bringing a Revolution to Life”—and it appears this revolution is very much alive.
*The i8, a 2+2 plug-in hybrid sports car with an MSRP of $135,700, is built in a BMW plant in Landshut, Germany.
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