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Audi, Hexcel and Secar Technologie have developed a composite engine bay brace for the Audi R8 using an overmoulding process to produce a hybrid structure combining Secar pullwound carbon fibre sections and Hexcel’s HexMC-i 2000 carbon fibre/epoxy moulding compound.
Hexcel’s Automotive Business Unit approached Audi’s composites development team in early 2016 with the aim of qualifying HexMC-i into a production ready manufacturing process. This approach coincided with Audi conducting its own market evaluation of carbon fibre sheet moulding compound materials and led to the selection of a suitable component.
The cross-shaped component braces the R8’s mid-mounted V10 engine and provides increased torsional stiffness, enhancing the driving dynamics. The project’s aim was to produce a composite version of the existing aluminium part that would offer significant lightweighting as well as enhancing the visual appearance of the part by providing a more organic moulded shape. The process is patented by Audi.
Snap cure press moulding
HexMC-i 2000 is a fast curing high performance moulding material. Using high strength carbon fibre and the snap curing HexPly M77 prepreg system, randomly orientated rectangular prepreg ‘chips’ are assembled into a 2000 gsm material that is designed for compression moulding processes.
For the R8 X-Brace, foam filled pullwinding carbon fibre tubes were produced and then overmoulded with HexMC-i 2000 to produce the central node and tube end terminations for direct mounting to the car.
One of the toughest challenges in the X-Brace development was consolidating the moulding compound and ensuring the strongest adhesive bond with the thin walled (<1 mm wall thickness) carbon tubes without crushing these pre-cured elements. The high adhesive bond strength of Hexcel’s HexPly M77 resin system produced a strong durable bond with no over-compression of the tubes.
Secar and Hexcel were also able to optimise tool loading and press cure cycles to provide the optimum processing parameters for HexMC-i with the M77 snap cure epoxy resin allowing a weight reduction of 15% over the previous aluminium version.
All metallic inserts for mounting the X-Brace were moulded directly into the part during production, with the demoulded part requiring minimal finishing before installation into the R8’s engine bay.
One further benefit of HexMC-i, due to the precise chopping of the prepreg ‘chips’ and their random orientation in the material, is the distinctive and attractive visual surface finish that was another key requirement from Audi.
Following the production of the first prototype parts at Secar, Audi took the testing process back in-house and subjected the new components to a rigorous programme of static and dynamic load testing under room temperature as well as hot wet conditions. One of the customer’s key concerns was being able to achieve a sufficiently strong bond to the pullwound tubes. Hexcel reports all test requirements were satisfied, with no adhesive film or additional bonding material used between the tubes and the HexMC-i moulding compound.
The joint development programme between Hexcel and Secar validated the lightweighting opportunity and one-shot moulding process for the new composite X-Brace and succeeded in qualifying HexMC-i with Audi.
“We were delighted with the success of the HexMC-i overmoulding process in such a highly visible finished part and are looking forward to supporting further developments in the very near future,” comments Achim Fischereder, Director Sales and Marketing, Automotive, Hexcel.
“We see a lot of applications where we can combine the impressive characteristics of HexMC-i with our innovative and high performance pullwinding and pullbraiding technologies,” adds Werner Stoeger, Sales and Marketing Director at Secar Technologie. “This is a unique combination and a good answer to increased performance requirements in the automotive market.”
Image provided by Hexcel
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