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/332723
APPLICATIONS J U L Y 2 0 1 4 | 4 7 APPLICATIONS Low-rate layup tools made cost-effective with high-density polyurethane foam blocks Nemesis Air Racing (Oro Valley, Ariz.) is an air racing team and kit-plane company owned and operated by Jon and Patricia Sharp. Formerly a composites engineer at Lockheed Martin Skunk Works (Palm- dale, Calif.), Jon Sharp is a legendary air racer: He won the Reno Air Racing Na- tional Championships nine consecutive times in a tiny International Formula 1 racing aircraft, dubbed Nemesis, which he and his team developed and fabricated in the early 1990s. The most successful aircraft in air-racing history, the original Nemesis is on display in the Smithsonian National Air and Space Museum (Wash- ington, D.C.). When Sharp decided to build a larger composite racer, the Neme- sis NXT, an unexpected rule change man- dated that each participating aircraft had to be available for purchase as a kit. For- tunately, the Nemesis name ensured that the required fve kits were quickly sold. Sharp and his team were then faced with producing the carbon fber/epoxy com- posite kit elements from CAD fle data. Based on his extensive experience at Skunk Works, Sharp understood the benefts of soft tooling for prototyping and limited production: "We were drawn to high-density polyurethane because of the greater level of design fexibility that was available. The inevitable design changes were much less of a headache than with metal tools." Coastal Enterprises Co. (Orange, Calif.) supplied Sharp with Precision Board Plus, a 20 lb/ft 3 , closed-cell, polyurethane foam in customized, pre- bonded shapes, to create the molds for the NXT's horizontal and vertical tail, leading edges, interior components and the craft's 90-gal gas tank. The pre- bonded shapes saved time and labor during CNC-machining with standard cutting tools, because less material had to be machined to achieve the fnal mold shapes. As Sharp told Coastal Enterprises, "The foam molds gave us our most accurate airframe ever pro- duced. The tight cell structure and ease of machining enabled us to bring our frst computer- designed aircraft to life." "If you make the decision to buy metal tooling, you're constrained by a lot of fac- tors — higher cost, size and weight and overall lead time for the tool build," contends Coastal Enterprises presi- dent Chuck Miller. "High- density urethane foam offers much more fexibility and speed, and the coeffcient of thermal expansion can be easily compensated for." Pre- cision Board Plus reportedly is suitable for tooling appli- cations up to 300°F/149°C, is available in densities from 4 lb/ft 3 up to 75 lb/ft 3 , and can be autoclave-cured, says Miller. He also explains that Coastal's abil- ity to bond polyurethane foam blocks together in the approximate shape of a fnal tool (see photo, lower left) prevents bondline voids, which can cause "chunk- ing" of the material during machining (custom mandrels in any size are also on offer): "We formulate our own EP76 epoxy adhesive to match the density of the material," says Miller, "which helps reduce any witness lines in the part." Miller adds that tooling developers must remember that polyurethane board is not steel, aluminum or even epoxy, and that precise planning is needed to ensure complete cure of the epoxy pre- preg. "Any out-gassing from the tool during heating, which can come from sealers or fllers, can react with the epoxy resin." But, cure problems might also occur because high-density polyure- thane is a good insulator, and not a good heat conductor. "Typical temperature ramping time is two minutes per degree Fahrenheit going up, and one minute per degree coming down," he explains. "This is critical to allow for thermal expansion, without causing stress risers and possi- ble cracking of the tool." The ramping also ensures adequate heat soak for complete prepreg cure. Source: Air-C-Race.de Source: Coastal Enterprises 0714HPC Application-OK.indd 47 6/17/2014 11:02:23 AM