Hexcel's HexMC-i 2000 carbon/epoxy molding compound has been used in the new overmolding process to produce a hybrid structure of Secar pullwound carbon fiber sections and Hexcel highly structured, fast-curing HexMC-i.
Based on the success of previous development projects, Hexcel's automotive business unit approached Audi's composites development team in early 2016 with the goal of aligning the HexMC-i with the production-prepared manufacturing process. This approach coincides with Audi's own market assessment of carbon fiber sheet molding compound (C-SMC) materials and leads to the rapid selection of the right components.
Audi, Hexcel and Secar Technologie of Hönigsberg, Austria, teamed up to develop the high-performance Audi R8 composite engine bracket. This cross-shaped part supports the R8's mid-mounted V10 engine and increases torsional stiffness for enhanced driving dynamics. The goal of the project is to produce a composite version of existing aluminum alloy parts that provide significant weight savings and enhance the visual appearance of the part by providing a more organic molded shape.
Quick cure molding at the touch of a button
For R8 X-Brace, a foam-filled drawn-wound carbon fiber tube was produced and then overmolded with HexMC-i 2000 carbon fiber/epoxy molding compound to produce a central node and tube end for direct mounting to a car terminal.
One of the toughest challenges in X-Brace development is to cure the molding compound and ensure the strongest adhesion to thin-walled (<1mm wall thickness) carbon tubes without damaging these pre-cured components. According to the manufacturer, Hexcel's HexPly M77 resin system has a very high bond strength that produces a strong, durable bond without over-compressing the tube.
Secar and Hexcel also optimize tool loading and press cure cycles, providing the best process parameters for HexMC-i for M16 fast cure epoxy, which is 15% lighter than previous aluminum versions. All metal inserts for X-Brace are molded directly onto the part during production and require minimal finishing before being installed in the R8 engine compartment.
Another advantage of the HexMC-i is that it is another key requirement for Audi development engineers due to the precise cutting of the prepreg "chip" and its random orientation in the material - a unique and attractive visual surface finish.
Bond strength test
After Secar produced the first prototype parts, Audi returned the test process to the interior and performed rigorous static and dynamic load tests on the new components at room temperature and hot and humid conditions. A key issue for customers is the ability to achieve a sufficiently strong bond with the drawn tube. Hexcel is pleased to see that all test requirements are met as planned, without the use of adhesive films or additional bonding materials between the tube and the HexMC-i molding compound.
Successful qualification
The joint development plan of Hexcel and Secar clearly validated the lightweight opportunity and one-shot molding process of the new composite X-Brace, and successfully qualified Audi as the OEM for HexMC-i.
“We are pleased that the HexMC-i overmolding process has been successful in this highly visible finished part and looks forward to supporting further development in the near future,” commented Achim Fischereder, director of automotive sales and marketing at Hexcel.
“We see a lot of applications, we can combine the impressive features of the HexMC-I with our innovative high-performance Pullwinding and Pulllbraiding technologies. This is a unique combination of high performance requirements and a good solution for the automotive market. “Secar Technologie Sales and Marketing Director Werner Stoeger said.
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