The BMW Group (Munich, Germany) on Sept. 19 formally began manufacture and assembly of its new all-electric, composites-intensive i3 car in Germany. BMW says that with the car's groundbreaking architecture, the concept calls for the use of carbon fiber composites and injection molded plastics, as well as innovative production processes.
BMW says the i3 is the first vehicle project for which sustainability objectives were agreed and subsequently pursued with the same vigour as cost, weight and quality objectives. The aim is also to reduce the environmental impact of production as much as possible, focusing on aspects such as energy supply and water consumption, solvent emissions and waste treatment. It is an objective to which all the locations in the BMW i production network are committed – including Moses Lake, Wash., USA (carbon fiber production) and Wackersdorf, Germany (processing into carbon fiber laminates). Both these facilities are operated by SGL Automotive Carbon Fibers (ACF), a joint venture set up by the BMW Group and the SGL Group. They are joined by the BMW Group’s own plants in Dingolfing, Landshut and Leipzig, Germany.
The architecture of the BMW i3 comprises two elements: the aluminium Drive module – which incorporates the powertrain, chassis, battery, and structural and crash functions – and the Life module or passenger cell, made from carbon fiber-reinforced plastic (CFRP). The LifeDrive concept and use of CFRP allows production times to be cut by half compared to those required for an equivalent car built along conventional lines. The process is less investment-intensive as the high costs required for a conventional press shop and paintshop are no longer an issue and the Life and Drive modules can be manufactured alongside one another.
The SGL ACF factory in Moses Lake produces carbon fibers from a polyacrylonitrile (PAN)-based thermoplastic textile fiber precursor. All of the energy used in carbon fiber production in Moses Lake is obtained from renewable, locally generated hydroelectric power and is therefore completely CO2-free. Moses Lake has been producing carbon fiber since the end of 2011. Two production lines with a total capacity of 3,000 tonnes per year supply the required quantities. The two parent companies – the BMW Group and the SGL Group – have invested €72 million ($100 million) in the Moses Lake production facility to date and created 80 new jobs.
The rovings produced in Moses Lake are sent to the joint venture’s second site, at the Wackersdorf Innovation Park, Germany, for industrial processing into lightweight carbon fiber laminates. Following an investment of €20 million and the creation of around 150 new jobs, today several thousand tonnes of carbon fiber laminates can be manufactured annually at the Wackersdorf site.
Carbon fiber laminates with different fiber alignments are then arranged into stacks made up of several layers and following various lines, before being cut to shape. These stacks form the base material for the production of CFRP parts and components at the BMW facilities in Landshut and Leipzig. Scrap CFRP is recycled in Wackersdorf and subsequently channelled back into use – in roof of the BMW i models, for example. Currently, around 10 percent of the carbon fibere used in the BMW i3 is recycled material.
The stacks supplied by Wackersdorf are turned into body components for the BMW i3 and BMW i8 at the innovation and production facilities at BMW’s Landshut and Leipzig, Germany, plants, both of which run three production lines for CFRP body components. For more than 10 years, the BMW Group’s specialists have steadily refined and automated the CFRP production process at the Landshut plant so that, for the first time, it is now possible to volume-produce CFRP body components cost-efficiently, to a high quality and with high process stability. In doing this, the manufacturing costs for CFRP body components over this period have been cut by around 50 percent.
A heat source is used to give the preformed carbon fibre stacks a stable, three-dimensional form. Several of these preformed blanks can then be joined to form a larger component. In this way CFRP can be used, for example, to produce body components with a large surface area that would be difficult – or significantly more expensive – to manufacture from aluminium or sheet steel. Preforming and preform joining are followed by the next stage in the process: high-pressure resin injection using resin transfer molding (RTM).
The CFRP process is no longer comparable with conventional sheet steel manufacturing. This industrialized manufacture of CFRP is extremely economical and makes the production of large CFRP composite components for the automotive industry a feasible proposition for the first time.
Even complex assemblies with many structural elements already integrated, such as an entire side frame for the BMW i3 Life module, are produced at the facility with a high level of automation. Additional processing stages include the finishing work, such as precise contour cutting and the insertion of remaining openings. This work is performed using a special waterjet cutting system and the bonding surfaces are then sandblasted before further processing. A conventional sheet steel side frame, by contrast, would have to be built up successively from several different inner and outer components.
The CFRP composite components are bonded together in the new bodyshop in Leipzig. This is where the basic structure of the Life module takes shape. A high level of geometric integration means that the CFRP structure requires only a third of the number of body components used in a conventional steel body; the Life module’s basic CFRP structure comprises around 150 CFRP parts in total.
There is no noise from bolting or riveting and no sparks from welding in the manufacturing process for a CFRP body. Instead, only the latest bonding technology is used, which is 100 per cent automated. In this BMW-developed assembly process, the individual components are positioned at a precisely defined bond line gap in order to ensure the resulting joint is as strong as possible. The bonded joints of each BMW i3 measure a total of 160m/525 ft in length.
In order to minimise hardening times for volume production of the BMW i3, BMW has greatly accelerated the hardening process. Significant advances in the development of the adhesive mean it is now workable for only 90 seconds after being applied to a component and before adhesion begins. An hour and a half later it has fully hardened and achieved its full strength. This represents a 10-fold acceleration of conventional adhesive hardening times. In order to further reduce the hardening time to below 10 minutes, BMW has developed a supplementary thermal process. This involves heating specific points on the CFRP parts which are to be bonded, thereby accelerating the hardening process even further.
The BMW i3 is the first ever BMW with an outer skin made entirely of thermoplastic. The only exception is the roof, which is made of recycled CFRP gathered from Moses Lake and the Germany plants. The weight of the plastic parts is around half that of sheet steel parts. Plastic also provides corrosion-free outer protection and requires less energy to manufacture, as well as being resistant to minor damage. The thermoplastic outer skin is produced using 25 percent recycled or renewable material.
The entire outer skin of the BMW i3 is produced at BMW’s Leipzig plant. Like the front and rear aprons on conventional BMW models, the plastic parts for the BMW i3 are produced using one of three thermoplastic injection molding processes, depending on the part in question. These three techniques comprise: a standard process; a twin injection molding process, where the outer skin and substructure are injection molded then bonded in separate, successive stages; and, third, a bonding via injection molding process where the outer skin and substructure, which are injection molded in parallel, are also joined together within one and the same automated process.
The final painting process gives the outer skin parts their sheen while also protecting against the effects of environmental exposure, for example due to stone chipping or sunlight. The new paintshop in Leipzig uses dry overspray separation and is therefore completely wastewater-free, while its energy consumption is just a quarter that of a conventional paintshop. At the same time water consumption is reduced by 70 percent because the process does not involve priming, painting and drying of the complete body, as with conventional models. Instead, the bumpers, front, rear and side parts of the BMW i3 can simply be painted individually, which conserves resources. Dispensing with conventional cataphoretic dip priming reduces the weight of the vehicle by 10 kg. Approximately 300 employees work on the production of plastic outer skin components for the BMW i3 in Leipzig.
Unlike vehicles with integral body and frame construction, the BMW i3 has a horizontally split LifeDrive architecture consisting of two separate, independent modules. As a result, the Leipzig assembly shop is the first in the history of BMW to feature two separate, parallel production lines – one for the Life module and one for the Drive module. This has led to significant advances in terms of the ergonomic design of the workstations, which provide optimal accessibility for all assembly operations.
During assembly of the Drive module in Leipzig, the aluminium chassis is fitted with the battery and the motor/transmission unit. First of all, the high-voltage battery, weighing 230 kg/507 lb, is installed in the floorpan and bolted to the basic Drive module structure. Integrating the high-voltage battery into the floor of the aluminium chassis results in optimal weight distribution, which also makes for excellent driving dynamics. The Drive module is then fitted with the motor/transmission unit supplied by the Landshut plant, which is likewise bolted in place. Optionally, a range extender (a twin-cylinder petrol engine) is also available, which increases the vehicle range to 300 km/185 miles. Once the front axle subframe – preassembled in Dingolfing – and further structural parts have been fitted, the BMW i3 Drive module is ready to move on to the final assembly stage.
Meanwhile, the CFRP passenger cell makes its way from the bodyshop to the assembly shop, where, on the Life line, its customer-specific equipment is fitted. This is the final step before the marriage with the aluminium Drive module, during which the CFRP passenger cell and the aluminium chassis are bonded together. The two units are also bolted together at four points. The result is optimal stiffness and strength. After the bonding agent has been applied to the Drive module by two robots, the Drive module is conveyed to the marriage station, lifted and centred. A robot gripper then lowers the Life module onto the Drive module and the joining process is launched by the body’s own weight. Only at this stage is the BMW i3 fitted with its outer plastic skin.
At 20 hours, the total processing time in the bodyshop and on the assembly line is only half of what would be required in a conventional production process. This is due to the parallel assembly processes and the fact that the CFRP structure comprises fewer parts.
In total, €400 million has been invested in the Leipzig plant for production of the BMW i and 800 new jobs have been created. The US Green Building Council has already awarded the new buildings in Leipzig an LEED (Leadership in Energy and Environmental Design) gold certification for their sustainable design.
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