High-Performance Composites

MAR 2013

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

Issue link: https://hpc.epubxp.com/i/110847

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Page 40 of 67

glass instead of carbon, in a 400 g/m² (1.31 oz/ft²) 2x2 twill weave. A key element of the CBS system is the highly filled surfacing film/primer, which provides a resin-rich shell that blocks fiber read-through and enhances paint adhesion. Developed inhouse, the film typically is 700 g/m2 (2.29 oz/ft2) by weight and features a solvent-free, hot-melt epoxy film sandwiched between fiberglass scrim and proprietary, finely woven thermoplastic scrim layers. Because it is flexible and slightly tacky, the film layer goes into the tool first and, during cure, forms an opaque, homogeneous surface. Gurit attributes fiber read-through to a coefficient of linear thermal expansion (CLTE) mismatch between the resin and the reinforcement. The film, therefore, is formulated to have a CLTE intermediate between standard epoxy and carbon fiber. After it is cured, the panel is sandable and can be prepared and painted like metal or sheet molding compound (SMC). Because there's no solvent in the film that can be trapped during cure, voids and paint pops are greatly reduced. As a result, Gurit says, less finish work is needed and the film's surface quality is a key driver for use of this technology. The fiber layers — whether carbon alone or carbon and glass — are singlesided semipregs that are produced, much like the surface film, by extruding a mix of epoxy and catalyst as a film and then laying woven fiber on top. Featuring carbon on the part's A surface and glass on the B surface, the thin laminate offers stiffness comparable to steel and aluminum but is less prone to twisting than an unbalanced carbon/carbon laminate with a resin-rich surface, says Gurit. The partially impregnated stack and central syntactic foam layer are reportedly less expensive and lighter than many competitive systems and offer good conformability/drapability. Indeed, laminate edges typically are rolled over a loose edge tooling bar to mimic the rolled edge that is common in stamped steel and aluminum. This eliminates postmold edge trimming. Although the new product is based on previous SPRINT CBS technology that uses epoxy-based resin and a similar structure/ply schedule, the chemistry has changed throughout the laminate (surface film, semipregs and syntactic core), and scrims have been eliminated from the surface films, says Gurit's managing director Martin Starkey. The new system Source: Gurit Automotive Low-pressure, short-cycle press technology To complement CBS 200, Gurit developed a new press-based molding process that uses matched-metal dies and relatively low pressure to achieve a 10-minute cycle time, a significant improvement compared to Gurit's previous vac-bag process, which took a full hour to form and cure parts. reportedly provides better fiber wetout, greater thermal stability/higher glasstransition temperature (Tg), faster cure and a very low, controllable exotherm — properties usually not found together. Although other epoxy producers offer high-Tg grades (typically for aerospace use), the cycle time usually goes up with the Tg. Gurit's new proprietary formulation delivers high Tg with shorter cycles. Starkey also says that its epoxide expertise and long experience molding thick laminates for wind turbine blades and boat hulls helped Gurit control exotherm while significantly reducing cure time. The previous epoxy cured in 60 minutes, offered a Tg of 110°C/230°F, and could withstand extreme ambient temperatures (e.g., those likely in a black car sitting outside on a 30°C/86°F day). The new technology cures in six minutes, offers a Tg of 200°C/392°F and, although it loses some modulus above its Tg, it can endure 10 minutes in the E-Coat oven — another step toward making CFRP panels easier to use on auto assembly lines. Starkey notes that because it does not produce its own fiber, Gurit has the flexibility to try many fiber sources and types (e.g., nonwovens and crimps). "Fiber choice can be optimized in our process," he points out, "so we tend to source in, and that gives us a lot of flexibility to go with whatever the end-component's mechanical performance requires." In the fiber segment, he notes, "that's a benefit, because there are more interesting products and players every day." New press process To the new laminate technology, Gurit has added a new press process to form the panels. Previously laminates were molded in a vacuum-bag/out-ofautoclave setup that accepted a variety of tooling types (carbon composite, Source: Gurit Automotive Flexible material, faster manufacturing Because it features semipreg and a flexible syntactic core, the CBS 200 laminate is reportedly more flexible and, therefore, has better drapeability/conformability than most prepreg laminates, so it can better conform to complex tool geometries without dry spots and bridging. The new process is expected by Gurit to produce up to 40,000 parts per year on a single tool set. MArch 2013 | 39

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