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| 2013 BMW's Megacity EV concept |
New manufacturing tech for Megacity EV puts BMW at the forefront of Carbonfibre making.
Carbonfibre has long held the status of wonder-material. Exceptionally light and strong, it has been the preserve of the motor racing industry and, more recently, commercial aircraft manufacturers. But the car industry has never taken it seriously as a potential material for mass-production use. Until now.
Unlike steel and aluminium, which can be stamped and
pressed into shape in a matter of seconds, carbon fibre panels are built
up from layers of fabric, which have to be carefully impregnated with
resin and then heated and pressurised until cured. But BMW has long been
convinced it’s worth the effort. And the 2013 Megacity electric car
project will be the first fruit of two decades of research and
development.
BMW says carbonfibre reinforced polymer (CFRP)
is “extremely corrosion-resistant”, giving it a “far longer lifespan
than metal”. And importantly, the firm adds, “it retains its integrity
under all climatic conditions”. Another major advantage is that CFRP
panels can be optimised to be thicker where greater strength is needed
and thinner where it’s not. This breakthrough in CFRP production
technology will allow it to use CFRP for the upcoming megacity, but it
could roll out its use in other future models.
Carbonfibre sheets start life as individual fibres just 0.0007mm thick – many times smaller than a human hair. These individual filaments are wrapped into fibre bundles of 50,000 or so filaments. They are then made into sheets, with all the fibres running in the same direction. These unidirectional carbon fibre sheets can also be oriented layer by layer to make the final panel particularly stiff in one direction. The upshot, according to the early Megacity crash test results, in an exceptionally strong structure. “While CFRP is at least as strong as steel, it is approximately 50 per cent lighter,” says BMW. “Aluminium, on the other hand, would save ‘only’ 30 per cent in terms of weight over steel. That makes CFRP the lightest material that can be used in body construction without compromising safety.
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| Research and Development on CFRP at BMW |
The key innovations in making high-volume CFRP panels are heated moulds and a sophisticated resin injection technology. First, the carbonfibre sheets are cut to shape, stacked on top of each other and gently heated so they start to reflect the shape of the lower mould. The resin that will hold the individual fibres and sheets in shape is then rejected into the layers of fabric at high pressure. The resin has to penetrate all the layers completely and evenly. The upper half of the mould is then lowered and the wet panel is cured through heat and pressure, a process that takes around 20 minutes. These cured panels are then trimmed and mounting points cut out with high-pressure water jets. Finally, complete CFRP sub-sections are bonded together to build the finished monocoque.
In future, this CFRP technology could be used to make
anything from whole cars to key sub-assemblies such as side pressings.
Whether it will make a big difference to BMW’s market performance
remains to be seen, but in getting even this far, the company has stolen
a huge march on the rest of the industry.
Source: Autocar India
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