Leading from the front

Engineering plastics are helping automotive manufacturers to lighten the load at the front of the car, reports Lou Reade



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.

Some plastics companies are slashing the weight of front ends by banishing metals completely. Others have helped to design components that minimise injury to pedestrians - in direct response to European legislation.

Automotive front end modules - which include the front bumper - 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.

Reducing weight at the front of the car is of particular importance, because it already accommodates the heaviest single component - the engine.

Jean-Claude Steinmetz, president of Rhodia Polyamide, quoted a weight saving of 20% for its redesigned front end.

"The new, pure plastics structures are coming," he said. "The first will be commercial within six months."

He adds that validation programmes are running at several vehicle manufacturers and Tier 1 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 that the hybrid part weight was 3,668g, while the LGF polyamide module weighs just 2,919g.

"There was also a 15% cost reduction," says Steinmetz. "We saved on the metal insert, the cycle time and had a lower tooling cost."

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," says Ulrich Dajek, an specialist in hybrid technology at Lanxess. "It is lighter, and has higher stiffness and strength. It does not need any protection from corrosion, which is an additional cost with metal."

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 higher mechanical properties.

"The overmoulded structure is 'welded' to the organic sheet," says Dajek.

Dajek estimates 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, so is not likely to become commercial for three or four years.

Helping pedestrians
Another recent plastics front-end development has helped a carmaker to comply with the EU's 'pedestrian protection' legislation - which insists that cars are designed to cause minimal injury to pedestrians in accidents.

All cars built since October 2005 must abide by these rules according to Andreas Wüst, a senior engineer at BASF's applications engineering structural parts division. More stringent requirements are likely to be introduced by 2010.

BASF has helped car manufacturer Adam Opel to redesign the front end in line with the legislation. A plastic foam core helps the bumper to absorb more energy in a collision - which reduces the force on the pedestrian's leg. At the same time, a new component called a lower bumper stiffener (LBS) has been developed. This 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," says Wüst. "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 had to fail in the event of a more serious crash - such as with another vehicle.

"We don't want to prevent failure - we want to control it," says Wüst. "For hybrid structures of metal and plastic, it's important to know the exact point when the plastics will fail. It's more sophisticated to calculate the whole failure rather than build a part so that it will not fail."

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. This leads to controlled failure under the insurance classification test. In effect, the part is 'sacrificed' so that more expensive components do not have to be replaced.

The glass fibre-filled part is highly anisotropic, meaning that 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.

"This is a plastics application where such a detailed modelling of the material is crucial factor in a proper description of its behaviour in a crash," says Wüst.

Wüst calls the technique 'integrative simulation', as it combines several different models - including one for the fibres and one for the material itself.

"We do not describe the part as having a single stiffness and elastic modulus," says Wüst. "We describe each piece of the part - each finite element - with a different mechanical model.

"This was a big step forward in being able to characterise plastic as it works."

Pointers

* Two plastics suppliers are developing structures that banish sheet metal parts from automotive front-end modules

* Sophisticated modelling software helped BASF to design a new part into an Opel front end

Author
Tom Shelley

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