2010 BMW Group Innovation Days Mobility of the Future - CFRP A Material for the Future.

SEE ALSO: 2010 BMW Group Innovation Days Mobility of the Future
Complete Report:
Chapter 1. Why Electromobility?
Chapter 2. Project i.
Chapter 3. The Electric Drivetrain.
Chapter 4. Lightweight design and the LifeDrive concept.
Chapter 5. CFRP - A Material for the Future.

Chapter 5. CFRP - a material for the future

The aerospace industry and the world of motorsport already rely heavily on carbon fibre-reinforced plastic (CFRP). This material has many advantages, which automotive engineers are increasingly exploiting wherever components need to combine high stress resistance with light weight, high stiffness and high strength. The experts at the Landshut Innovation and Technology Centre (LITZ) have been intensively investigating this high-tech material for over ten years.

This long-running development work, and a successful transition into volume production, means the BMW Group now has know-how which is unrivalled in the motoring industry about processes, tools and manufacturing techniques. It has also achieved high levels of mass production. But what is so special about this material?

Incredibly light and as strong as steel.
Carbon fibre-reinforced plastic (CFRP) is a composite material consisting of carbon fibres surrounded by a plastic matrix (resin). Very few other materials can compare with CFRP, which exhibits a unique combination of positive properties. First and foremost, CFRP is extremely rigid and strong – yet at the same time extremely lightweight. For equivalent or even better functionality, CFRP is approximately 50% lighter than steel and 30% lighter than aluminium. It is also resistant to corrosion, acid and organic solvents, giving it a much longer useful life than metal. In addition, CFRP remains stable under all climatic conditions, showing very little change in shape even when exposed to large temperature fluctuations.

Light weight involves no compromises on safety.
The high rigidity of this material is also accompanied by excellent damping qualities and high impact strength. CFRP has remarkably good energy-absorbing capabilities, making it very damage-tolerant. As a result, body components made of CFRP are not only very light, they also demonstrate outstanding crash performance. In fact, CFRP is the lightest material that can be used in vehicle body manufacturing without concessions on safety. That said, CFRP’s stress tolerance is not unlimited. When its load capabilities are exceeded, the fibre composite structure breaks down, in a controlled manner, into its individual components.

“The forces (or accelerations) needed to destroy CFRP are far higher than might be assumed.” (Bernhard Dressler)

The right strength in the right places.
The secret of this high-strength material lies in the carbon fibres. In contrast to quasi-isotropic metals like aluminium or steel, which have equal strength in all directions, CFRP is anisotropic. This gives it very high strength, like a rod, in one direction, namely along the tensile/compression axis. This is its key advantage. Since an individual component is never simultaneously exposed at all points to loads in all directions, this special quality means that the stress resistance of components can be optimally matched to the loads they will actually encounter in practice. As in nature, where bones or plants use thicker structures only where really necessary, so the BMW Group engineers likewise tailor the thickness and fibre alignment of CFRP components to meet actual requirements, varying the quantity of fibres used and aligning them in the direction or directions along which loads will be exerted in the future product. These precisely gauged component parameters also help to minimise weight.

“CFRP allows material to be used efficiently, combining optimal strength and functionality with minimal weight.” (Bernhard Dressler)

Using CFRP is more than simply a straight substitution, like using aluminium in place of steel. With its special properties, this high-tech material also opens the door to completely new approaches and design concepts. Electric mobility is a case in point, where CFRP offers great potential as a material for vehicle body components since its lightweight properties result in a higher power-to-weight ratio, and therefore an extended driving range. Provided that this material is properly understood, it can be strategically deployed to achieve vast improvements in a wide range of lightweight products.

“Lightweight design requires a proper understanding of the properties of CFRP.” (Jochen T�pker)

Technological expertise in the BMW Group.
There are a number of reasons why, in the past, use of CFRP was confined to small-batch or prototype production. For the most part, components using this still relatively new material were built by means of a manual, highly time-consuming production process. This resulted in high costs and lengthy cycle times, which posed a major obstacle to mass production. Then, in 2003, the BMW Group took the plunge and began volume production of CFRP components. It has been ramping up output ever since. Today the Landshut plant is even mass-producing roofs (for the BMW M3 and M6 models) and bumper supports (for the M6) in CFRP.

Thanks to intensive material and process engineering efforts over the past ten years, the BMW Group has built up a high level of expertise in CFRP-specific production processes, efficient tooling solutions and optimised cycle times. The BMW Group’s CFRP 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 mass-produce CFRP body components cost-efficiently and to a high quality standard. The process engineers at the Landshut Innovation and Technology Centre (LITZ) have thereby removed one of the main hurdles to increased use of carbon fibre components in vehicle body manufacture. With these pioneering advances, the BMW Group is helping to ensure that CFRP’s potential is even better utilised in the future.

State-of-the-art production in the BMW Group.
The still widespread production technique whereby prepregs – fibres pre-impregnated with resin – are processed and then cured in an autoclave (a kind of giant oven) is not compatible with mass production of automobiles. As early as 2003, the BMW Group therefore introduced a next-generation production system geared to high-quality volume production of CFRP parts. This state-of-the-art process has very short cycle times. What’s more, it is not tied exclusively to the Landshut plant but could theoretically be used at any BMW plant in the world, subject to certain basic requirements. So how exactly does this BMW production process for CFRP components work?

From fibre to fabric.
The starting point for CFRP production is the so-called precursor. This thermoplastic polyacrylonitrile fibre is the raw material for carbon fibre manufacture. In a complex multi-stage process, conducted under varying temperature and pressure conditions, the various constituent elements of the fibre are removed one by one, by gasification, eventually leaving a fibre that consists of virtually pure carbon, with a stable graphite structure. The resulting carbon fibres are just seven microns (0.007 millimetres) thick. A human hair, by comparison, has a diameter of 50 microns. For automotive application, approximately 50,000 of these individual filaments are bundled into so-called rovings or heavy tows and wound, prior to further processing. In addition to automotive applications, fibre bundles of this thickness are also used, for example, in large rotor blades for wind turbines.

In the next stage in the process, the fibre bundles are processed into non-woven fabrics. In contrast to a woven fabric, in these fabrics the fibres are not placed at right angles to one another and interwoven, but are all aligned in the same plane. Weaving would kink the fibres and detract from their special properties. The alignment of the fibres in the fabric is crucial to achieving optimal quality in a CFRP component.

Preforming and preform joining – a component takes shape.
At the so-called “preforming” stage, the cut but still flat fabric begins to acquire a shape. During this process a heat source is used to give a fabric stack a stable, three-dimensional contour. The final shape of the component is already clearly visible. Several of these preformed stacks can then be joined to form a larger component. In this way CFRP can be used, for example, to produce highly integrated components with a large surface area, which would be extremely cumbersome to manufacture from aluminium or sheet steel. This has major benefits for vehicle body design and manufacture. For example, mounting parts or other features can be integrated directly into the component. Also, complex structural components and entire body modules with varying wall thicknesses can be produced in a single moulding tool.

At both process stages – preforming and preform joining – the big challenge lies in ensuring good production processability of the flexible fabric so that the preforms will maintain a stable shape and can be joined with maximum precision. Here, too, the BMW Group has acquired valuable expertise over the years.

High-pressure resin injection with Resin Transfer Moulding (RTM).
The joined preforms are now ready for the next stage in the process: resin injection. This second major component in the composite structure – the resin – ensures that the preformed stacks permanently maintain their preconfigured shape. The resin transfer moulding (RTM) process involves high-pressure injection of resin into the preforms. Firm bonding between the fibres and resin, and the subsequent hardening process, give the material the rigidity which is key to its outstanding qualities.

The impregnation of the fibres by the resin is a highly complex process full of conflicting requirements. For example, on the one hand the resin must reach every area of the material with minimal delay, impregnating every fibre right down to microscopic level. This means the resin must have as low a viscosity as possible, so that it can flow freely enough to be dispersed quickly throughout the fabric. On the other hand, as soon as it has impregnated all the material, the resin needs to harden as quickly as possible. Thirdly, a release agent is required that will allow the resinated components to be parted from the moulding tools without the components being damaged – yet without affecting the bonding between the fibre and the resin. Resolving all these conflicting requirements simultaneously is a highly complex task. The BMW Group has developed its own process, moulding tools and production equipment to resolve these conflicts and to ensure high productivity combined with very high quality.

During resin impregnation of the fibres, some ten different substances and materials must be bonded together, but under no circumstances must they react with one another. It is also important to ensure full bonding throughout the composite structure – between the carbon fibre fabric, the resin, the hardening agent, the binder, the yarn, the release agent and other materials – both at macroscopic and at microscopic level. This is a major challenge when working with fibre composites, because the material is always only as good as the bond between resin and fibre.

Final processing – a water jet cutter applies the finishing touches.
After resin injection and hardening, the production process is almost complete. All that remains is the finishing work such as precise contour cutting and the insertion of any further openings that may still be required. At BMW Group plants this finishing work is performed by a water jet cutting machine. Since the finished CFRP component is already, following resination, very stiff and robust, ordinary milling heads would quickly run into wear and tear problems and would require frequent replacement. Water jet cutting and drilling on the other hand are wear-free. For CFRP applications, this technique does require certain modifications – which the BMW Group has already introduced.

“Our matured production processes enable us to produce components precisely in line with the engineers’ specifications and in accordance with the product’s function.” (Andreas Reinhardt)

Recycling – offcuts make a comeback as new structural components.
The BMW Group’s CFRP strategy extends throughout and beyond the life cycle of the product. Over the course of time, a variety of concepts have been developed and evaluated for recycling this high-grade material, but it was not until recently that a solution was found for optimal recycling of production waste. Now the BMW Group has developed a concept for recycling segregated production waste into commercial-quality raw material. This system, the first of its kind in the world, allows a substantial proportion of carbon fibre waste to be returned to the production process. Thanks to a special refining procedure, the resulting material can even be used as a substitute for primary fabric. After all, the benefits of recycling are twofold: cutting waste reduces not only environmental impacts but consumption of new material as well.

“Our aim was to reuse offcuts from the production processes in high-grade applications for our own products.” (Andreas Reinhardt)

Of course, BMW’s CFRP strategy is ecologically sustainable not just with regard to recycling but also in terms of production. For example, the Group is committed to ensuring that the new plant in Moses Lake, USA, operated by the joint venture with SGL ACF (Automotive Carbon Fibers), obtains its energy exclusively from renewable sources. The plant will also set standards for energy efficiency.

Cradle-to-grave approach for optimal results.
The developers and CFRP experts have continuously improved all processes, materials, production equipment and tools over the past ten years, thereby ensuring a successful ramp-up to mass production. Throughout, the improvements were focused on the complete process and value chain. The BMW Group is currently in a unique position in that it is able to influence all processes in this chain, from fibre production to recycling. That means it can ensure that progress at particular points in the chain quickly feeds through to the system as a whole.

Mass production was always the aim.
With the steady ramping up of output, and the development of innovative processes, the BMW Group has now accumulated a vast amount of in-house expertise and experience. This know-how is spread across its workforce, its production equipment and its processes. It was only possible to achieve such a high level of expertise thanks to the unwavering focus on one overriding goal: mass production of CFRP components. That’s because the BMW Group sees CFRP not simply as a niche application for specific vehicles, but as a pioneering technology for automotive design in general. That is why, from the outset, the company has invested heavily in acquiring and continuously developing in-house competences – whether process capabilities or employee skills. This high level of self-sufficiency and in-house expertise throughout the production process has also served to make the BMW Group largely independent of and free from external constraints. The result is a production process whose maturity is reflected above all in high component quality.

“The key to successful mass production of CFRP components is a focus on creating quality in-process rather than through end-of-process checking.” (Jochen T�pker)