Hexcel’s HexMC®-i 2000 carbon fiber/epoxy molding compound has been used in a novel overmolding process to produce a hybrid structure combining Secar pullwound carbon fiber sections and Hexcel’s highly structural, rapid curing HexMC®-i.
Building on the success of previous development projects, 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. With perfect timing, this approach coincided with Audi conducting their own market evaluation of carbon fiber sheet molding compound (C-SMC) materials and led to the rapid selection of a suitable component.
Audi, Hexcel and Secar Technologie GmbH, Hönigsberg, AUSTRIA joined forces to develop a composite engine bay brace for the high-performance Audi R8. This 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 aluminum part that would offer significant light weighting as well as enhancing the visual appearance of the part by providing a more organic molded shape. The process is patented by Audi.
For the R8 X-Brace, foam filled pullwinding carbon fiber tubes were produced and then overmolded with HexMC®-i 2000 carbon fiber/epoxy molding compound 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 molding compound and ensuring the strongest adhesive bond with the thin walled (<1mm wall thickness) carbon tubes without crushing these pre-cured elements. Fortunately, the very 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 optimize 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 percent over the previous aluminum version.
All metallic inserts for mounting the X-Brace were molded directly into the part during production, with the demolded 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’s development engineers.
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 program 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 was pleased to see that all test requirements were satisfied as planned, with no adhesive film or additional bonding material used between the tubes and the HexMC®-i molding compound.