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01/10/2006
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A combination of advanced polymers, 3D modelling and the pressure of new legislation are bringing about a revolution in the design of automotive front ends, writes Lou Reade.
Front end modules are usually made from a combination of sheet metal and glass-reinforced plastics components. These ‘hybrid’ technologies have been developed, and patented, by the plastics companies. Dow, Ticona, Rhodia, BASF, Lanxess and GE Plastics have all supplied hybrid technologies to major automotive manufacturers.
Now two of them, Rhodia and Lanxess, have taken the concept a stage further and plan to dispense with the metal component.
Jean-Claude Steinmetz, president of Rhodia Polyamide, quoted a weight saving of 20% for its redesigned front end, saying: “The new, pure plastics structures are coming. The first will be commercial within six months.”
He added that validation programmes are running at several vehicle manufacturers and Tier One suppliers. Rhodia is giving away few details of how it has achieved this but confirms that the new structure is made from a high fluidity grade of its Technyl Force polyamide 6, reinforced with long glass fibres (LGFs) which improve the stiffness and strength of the final part.
He said the hybrid part weight was 3,668g, while the LGF polyamide module weighs just 2,919g.
For its part, Lanxess replaces the sheet metal component with an ‘organic sheet’ of fibres and fabric in a thermoplastic matrix such as polyamide.
“What we have is a hybrid part made completely of plastic,” said Ulrich Dajek, a Lanxess specialist. “It is lighter, stiffer and stronger. Nor does it need any protection from corrosion.”
Once the sheet has been shaped it is placed into an injection mould and overmoulded. While the organic sheet – developed by German firm Bond-Laminates – is more expensive than its steel equivalent, the tooling is far cheaper. In the future, the organic sheet might be shaped within the injection mould, reducing costs still further.
Because the sheet is heated before overmoulding, it forms a ‘frictional connection’ which leads to a stronger bond and better mechanical properties.
“The overmoulded structure is ‘welded’ to the organic sheet,” said Dajek, who added that cost savings are most likely to be seen for production runs of fewer than 30,000 parts. The technology is still in pre-development.
Reducing pedestrian injuries
BASF has helped car manufacturer Adam Opel redesign the front end in line with pedestrian protection directives. A plastic foam core helps the bumper absorb more energy in a collision, which reduces the force on the pedestrian’s leg. And a lower bumper stiffener (LBS) has been developed which spreads the load during a collision, and prevents severe twisting of the knee.
A key factor in developing the LBS was BASF’s ability to model the behaviour of the material. “We focus on a numerical description of the polymer materials,” said Andreas Wüst, senior engineer at BASF’s applications engineering structural parts division. “We have software that can look at the distribution of fibres and calculate mechanical data out of that.”
The part must be designed to fit in with the existing structure, so must not add excessive weight. It also had to be designed to meet the insurance classification test. This means it has to fail in the event of a heavier impact, such as with another vehicle, and is sacrificed to so that more expensive parts to not have to be replaced. “We don’t want to prevent failure, we want to control it,” said Wüst.
The LBS is a roughly C-shaped part, with a number of moulded-in ribs to add stiffness. Each side of the part has three weak positions for controlled failure.
The glass fibre-filled part is highly anisotropic – it has different physical characteristics in different directions. This is very difficult to model. At the same time, it must take account of non-linearity in stress-strain and other factors. BASF did this by developing a program that is integrated into finite element analysis software. This is later fed into LS-Dyna which makes crash calculations.
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Author
Lou Reade
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