CompositesWorld
Issue link: https://cw.epubxp.com/i/532917
JULY 2015 20 CompositesWorld WORK IN PROGRESS open injection mold, closing the mold to thermoform the blank to shape and then injection overmolding with fber-flled melt to provide the fnished three-dimensional surface. As presented by Alan Wood at the 2 nd International Conference and Exhibi- tion on Termoplastic Composites (ITHEC 2014, Oct. 27-28, Bremen, Germany), Victrex had to work through an array of issues, including support of the substrate during preheating, control of fber orientation during forming, potential restriction to a uniform blank thickness to ensure matrix melting, and the development of polymers that would indeed fuse together during overmolding. "We could mold PEEK over PEEK, but it doesn't fuse together," says Herr. Fusion of the matrices requires a substrate with a lower melt temperature than the overmold. "If you don't fuse the two, you don't consolidate the part," Herr points out. "So, we needed a low- temperature processing PAEK that has PEEK properties." Tink of PAEK as the polymer family — Victrex can make diferent formu- lations, each with a specifc molecular weight, melt tempera- ture and T g — with PEEK being one species. Tus, Victrex developed the PAEK A250 polymer, with a melt temperature of 305°C (see Table 1, p. 21). "We have enabled not only overmolding with PEEK," claims Herr, "but also the ability to achieve PEEK-type properties at processing temperatures similar to PPS or PEI with current equipment." Victrex also proved fber orientation can be maintained during forming and that there is the potential for variations in substrate laminate thickness, which enables local tailoring of stifness in the fnal part. Te process requires less energy than anticipated, with substrate preheating typically maintained at 200°C. Overmolding ofers aerocomposites molders the advantages inherent in both molding "worlds": Complex aerospace thermoset composites can provide desired functionality and strength at low weight but "require hand layup and auto- clave processing, resulting in parts production that takes hours to days," comments Kneath. In contrast, injection molding of neat thermoplastics produces complex components in minutes, but limits the molder's ability to optimize part functionality. "What's great about this hybrid process," Kneath points out, "is that you can put the material where you need it, vs. standard injection molding, where you can't really selectively reinforce parts easily." Because the substrate blanks are multi-ply composite layups, they are easily tailored for specifc properties and/or load handling. For example, Tri-Mack has the ability to place and orient UD tapes in the blanks, using in-house automated molding cells. In fact, the company's Advanced Composites Center, adjacent to its main plant, was designed specifcally to enable automated layup, consol- idation and thermoforming of composite materials in conjunc- tion with its legacy injection molding capabilities, to replace both metals and thermoset composites in aerospace applications. Building a better bracket To demonstrate the process, Tri-Mack needed a part that would deliver the message. "Tere are so many brackets used in aero- space," says Kneath, "and we've made a simple bracket with ther- moplastic composites, so we decided to design a complex bracket shape that would also bear signifcant load." Te design is loaded in three axes and incorporates four mounting points that rely on co-molded metal inserts in the PEEK overmold, the latter a typical feature of Tri-Mack's unreinforced thermoplastic brackets. "We load the aerospace-grade, carbon fber-reinforced PAEK substrate and four metal inserts into a standard injection mold," describes Kneath. "We use a standard process to inject VICTREX PEEK 150CA30" — 30% carbon fber-reinforced compound — "over the substrate, and minutes later, we have an integrated assembly that marries the performance of PEEK composites with the cost efciency of injec- tion molding." (See photo above.) Initial testing shows a very robust bond between the PAEK and PEEK matrices. "Tis will build conf- dence in the engineering community as they start to design parts exploiting this technology," Kneath asserts. "Te design fexibility is huge," he adds, noting the same bracket made with forged metal would require a signifcant amount of machining. "Tat's easy to beat with a 2-3 minute injection Best performance at least weight Two views of a high-performance loaded bracket that demonstrates the new hybrid molding process, which uses a fat, lower temperature VICTREX PAEK composite substrate overmolded by short fber-reinforced PEEK with co-molded metal inserts. The resulting fnished bracket is as much as 60% lighter than comparable metal compo- nents. Source | Tri-Mack Read this article online | short. compositesworld.com/PEEKover Read online about Daher-Socata's use of PEEK in thermoplastic composite fuselage clips in "Inside a thermoplastic composites hotbed" | short.compositesworld.com/TCHotbed Read online about Premium Aerotec's automation of fuselage clip production in "Thermoplastic composites 'clip' time, labor on small but crucial parts" | short.compositesworld.com/RTPClips Read online about injection overmolding in the auto industry in "CAMISMA's car seat back: Hybrid composite for high volume" | short.compositesworld.com/Camisma