The i3 carbon-fiber body: from filament to finished product

  • 28-Oct-2013 08:10 EDT
polyacrylonitrile.jpg

Polyacrylonitrile tows consist of 100,000 white filaments each and are good for the production of two carbon-fiber tows of 50,000 filaments each.

The first production all-electric i3 rolled off the line at BMW’s plant in Leipzig, Germany, on Sept. 18 (click here to watch an AEI video on the i3 carbon-fiber body). The compact hatchback all-electric EV is the first of several models planned for the company’s relatively new i sub-brand devoted to vehicles with electrified powertrains. Next in line is the i8 plug-in hybrid sports car, which will be assembled at a different plant in Germany.

The starting point for one of the i3’s most advanced features—its carbon-fiber-reinforced-plastic (CFRP) body—is the United States.

At a plant co-owned by BMW and SGL Group in Moses Lake, WA, temperature and pressure are used to convert a polyacrylonitrile-based thermoplastic textile fiber precursor into a web of extremely long and narrow (0.007 mm/0.0003 in) carbon-fiber (CF) filaments. They then undergo oxidation and carbonization.

The CF filaments are spun together in tows and further treated. Tows have different filament counts (in the many thousands). At the end of the process in Moses Lake, the tows are wound onto spools and prepared for shipping.

SGL Automotive Carbon Fibers’ plant gets all of its power from hydroelectric sources on the Columbia River.

The spools are shipped to BMW’s plant in Wackersdorf, Germany, where they are weaved into mats of differing CF alignments. The mats are then shipped to CFRP press shops at the Leipzig plant and at the Landshut plant, the latter of which is where the i8 will be built. Each runs three production lines for CFRP parts.

A pressing matter

The mats of different size and orientation for different parts are cut and then pre-formed using heat and a plastic powder binder in a large press. At the Leipzig plant, where AEI was invited for a plant tour as part of the Sept. 18 start-of-production ceremony, there are three identical pre-forming presses. The pre-forming step gives the mat stacks a shape similar to their final one and prevents tearing that otherwise might occur if the mats went directly to final resin transfer molding (RTM).

In RTM, resin is injected at high pressure and temperature into the CFRP material in one of seven smaller, identical presses at Leipzig. The company says this is an advance over the conventional, time-consuming oven-hardening process associated with typical CFRP production.

BMW has refined its expertise in CFRP parts production at its Landshut plant in Germany over the past 10 years, and over about 13 years companywide, according to Daniel Schaefer, Head of Production Concept for BMW i. In those 13 years, engineers have cut cycle time by more than 50%, and production and material costs by 30-40%.

Engineers already have some ideas for additional cycle time improvement in CFRP parts production, which is the bottleneck in i3 production, Schaefer said. “And then, of course, as soon as you start series production, your learning curve rises significantly.”

BMW notes that its RTM process allows for many smaller elements that make up a larger part, such as a body side, to be integrated in the mold. This, according to the company, reduces overall part count to about a third the number used in a conventional steel body.

The size of the CFRP parts made in Leipzig is unmatched by any competitor, said Helmut Schramm, Head of Production, Electric Vehicles, BMW. The company invested €400 million total in the Leipzig plant for the i3, and about €600 million at other BMW plants for the i sub-brand overall. The less complicated CFRP parts for the i3 are made at Landshut and shipped to Leipzig.

Waterjet cutting is used to insert holes and trim the parts following the RTM process. Next, they are glued together in a new, completely automated body shop at Leipzig. The surfaces are first roughened for better adhesion.

Secret-formula glue

Glue with a special composition developed by BMW together with a “major chemical company” for fast hardening is used to join body parts. Within a matter of seconds the glue reaches a point of functionality that allows the securely joined part to move to the next joining station even though the glue is not completely cured. Spot infrared heating helps enable the fast hardening time in key locations in the part.

The total length of bonded joints in the i3 body is 160 m (525 ft).

Aside from the roof, which is made of recycled CFRP, the i3’s outer skin is plastic—a BMW first. The company says the plastic panels are half the weight of equivalent steel ones. About a quarter of the plastic skin is from renewable or recycled material.

In a different part of the Leipzig plant, the plastic panels are made in one of three different thermoplastic injection molding processes, depending on the part. The most advanced of the three is one in which the outer skin and substructure are injection molded and bonded in the same process.

Painting of the panels is done piece by piece. Some parts require just a top-coat and clear-coat while others also require a primer. Using a conventional cataphoretic dip priming process would have added 10 kg (22 lb) to the car, BMW claims. Lack of a conventional paint shop is the main reason the Leipzig plant uses 50% less energy and 70% less water than typical.

All of the energy needed for i3 production at Leipzig is generated by on-site wind turbines.

The large lithium-ion battery and the aluminum load-bearing structure (called the Drive Module) of the i3 are produced at BMW’s plant in Dingolfing. The structure consists of 160 welded parts. The battery consists of 96 hard prismatic cells from Samsung. They are clamped into eight separate modules, bonded, and welded in a fully automated process involving 20 robots. The modules are then placed in an aluminum casing that serves a positive role in vehicle body rigidity, and manually connected in series.

The BMW-designed 125-kW·h electric motor is built at the Landshut plant. The stator has about 2 km (1.2 mi) of copper wire wound in a special way to save weight.

Also built at Landshut is the instrument panel.

The body (called the Life Module) and the Drive Module (consisting of load-bearing structure, battery pack, electric motor, optional internal-combustion engine, suspension, and related hardware) are built up on separate, parallel lines at Leipzig. After fitment of equipment in each, they are “married.” Schramm noted that 10 metal fasteners are used to help join the Life and Drive Modules, but that the special BMW glue is the main ingredient in the permanent union.

At 20 h, total processing time in the body shop and in the final assembly shop is only half of what would be required in a conventional process, BMW claims.

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