AUG 2018


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AUGUST 2018 46 CompositesWorld FOCUS ON DESIGN Nastran [MSC Software, Newport Beach, CA, US] for static analysis, Abaqus [Dassault Systèmes, Waltham, MA, US] for nonlinear static analysis, Fibersim [Siemens PLC, Waltham, MA, US] for laminate draping simulation and HyperWorks [Altair Engi- neering, Troy, MI, US] for TO, generating finite-element models toward analyzing different load cases," says Cordoba. Multiple analyses iteratively refined the modeling of the subframe for different loads, boundary conditions and materials. Krull notes that many composite materials were evaluated. "We began to look at compression molding as a manufacturing process that could provide high-volume production," he adds. "SMC was a good fit for making this part, and we did have in-house devel- opment of carbon fiber SMC, so that is how we moved." e next step was the part envelope. "e TO resulted in a box section as the best solution," recalls Cordoba. "As we looked at the SMC compression molding process, the design that emerged was to use two parts to achieve this." us, the frame comprises sepa- rately molded top and bottom halves, which are joined with poly- urethane structural adhesive (Ashland, Columbus, OH, US) and rivets. Co-molded dual SMC Magna developed the carbon fiber SMC using its glass fiber SMC experience and a pilot line that it had acquired. "We developed proprietary technology for how we process the carbon fiber for this SMC and found we could also run NCF on that line," Krull explains. e material is similar to prepreg in that it is impregnated before molding, but unique in that this is done on the SMC line with the same resin and, thus, termed NCF SMC. "As we refined the subframe design analyses, we used properties testing results from these materials." e EpicBlend SMC is compounded by Magna using chopped 50K carbon fiber tow from Zoltek (Bridgeton, MO, US) and Ashland vinyl ester resin modified in-house. e vinyl ester gives good adhesion to, and wet-out of, the carbon fiber. is would be locally reinforced and co-molded with six plies of 0°/90° NCF SMC, made by Magna with the same vinyl ester resin and NCF fabric from Zoltek, and cut into patches. Combining the short- and long-fiber SMC was key to the design, but also a real challenge. e short-fiber SMC enables the molding of complex geometry and overmolding of the steel inserts for engine and steering mounts while the NCF patches resist high loads where the engine and lower control arms are attached. e combination of the two SMC materials cuts 9.3 kg vs. the stamped steel subframe. "It took quite a bit of work to develop the co-molding of the 0°/90° SMC patches," Krull recalls, noting issues with how to achieve flow of the chopped fiber SMC during molding to get the NCF patches integrated without dry spots or other quality issues. Bolted connections Bolted connections were also an issue. "e point loads into the composite are high — 80-100 kN — where the control arm and steering get bolted to the subframe," says Wagner. ere are four Read this article online | Molded "box section," top and bottom The hollow box design, prescribed by topology optimization to ensure subframe stiffness using minimal material, was realized by molding separate top and bottom halves, which were joined by adhesive and rivets.

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