CompositesWorld
Issue link: https://cw.epubxp.com/i/442058
JANUARY 2015 26 CompositesWorld » Autoclave cure produces composite parts of the highest quality and lowest void content. All other things being equal, it's still the best insurance against part rejection. But today, all things aren't equal. Te world in which part performance is everything and cost is no object is passing away, even in previously "cost-plus" defense and space-exploration sectors. Demand for faster production, particularly in the part-per-minute world of automotive composites, has exposed autoclave cure, with its lengthy temperature ramps, to the charge of "production bottleneck." Part consolidation and elimination of labor-intensive assembly steps — strategies made possible by the nature of composites and which justify their higher cost — have progressively increased the size of composite parts. Corresponding increases in autoclave volume have caused the autoclave's purchase price and operational costs to skyrocket out of reach for all but the biggest, most well-funded fabricators (see "Learn More" p. 27). Further, regard- less of part size, the autoclave can make pre-production development cost-prohibitive and can discourage prototyping of tooling and parts, especially in prospective and/or one-of or few-of-a-kind programs. Tese realities have motivated the development of out-of-auto- clave (OOA) epoxy prepreg (epoxy prepreg is the material form of choice for most autoclaved parts) and bismaleimide (BMI)-based alternatives that can be oven-cured. OOA epoxies and BMIs have seen signifcant development in tooling applications, but a relative Benzoxazines for OOA tooling Can infusable formulations of this newer resin system produce aerospace-capable tools? newcomer, benzoxazine, continues to gain ground since structural composites-capable formulations were introduced in 2008 (see "Learn More"). Hot/wet performer Benzoxazine boasts continuous hot/wet service temperatures, a key parameter in tooling applications, as high as or greater than epoxy's maximum hot/wet service temperature and also poses less risk of runaway exotherm that could contribute to tool distortion. Further, it can match BMI's performance in tools (e,g., <1% springback) and has the practical advantage of a signifcantly longer outlife — 6 months at room temperature (see "Learn More"). Tese factors were the motivation for recent tests on nonproduc- tion tooling manufactured from carbon fber and benzoxazine resin via vacuum infusion. Toolmaker Leading Edge Aerospace (LEA, Wichita, KS, US) partnered on the R&D; project with an aerospace parts manufacturer to examine the viability of the infusion process as a substitute for prototype and production tooling typically made from prepreg cured in an autoclave. LEA had previously produced tooling via vacuum infusion for other customers. A notable instance involved autoclave-capable infused tools for the wingskin, spars and other composite components that were incorporated into the Scorpion, a tactical fghter jet concept indepen- dently developed by Textron Airland LLC (Wichita, KS, US; see "Learn More"). Te Scorpion tools combined a dry layup of 12K carbon fber 2x2 twill fabric and an infusion- optimized epoxy resin, followed by a 12-hour cure under vacuum and a postcure. Te use of benzoxazine for this new project enabled LEA and its partner to evaluate the performance of a tooling material that promised increased toughness and better high-temperature perfor- mance, compared with epoxy systems. A female master model was CNC-machined from a 254-mm by 1,016-mm by 1,219-mm block of monolithic graphite. Next, approxi- mately 12 plies of 2x2, 12K carbon fber fabric were layed up on the master, bagged and vacuum infused with Beta Benzoxazine resin, Monolithic Master For its recent tests on non- production tooling manufactured from carbon fber and benzoxazine resin via vacuum infusion, Leading Edge Aerospace (Wichita, KS, US) CNC-machined this female master mold from a block of monolithic graphite. Source / Leading Edge Aerospace ... the world in which part performance is everything and cost is no object is passing away. WORK IN PROGRESS