Carbon composites fly high: improved techniques of resin transfer molding are being used to...

Improved techniques of resin transfer molding are being used to fabricate flight-critical carbon-composite structures for aircraft and jet engines.

Hand lay-up methods have been the traditional means to fabricate fiber-reinforced resin-composite parts. The procedure typically involves laying

Only in the last few years has resin transfer molding (RTM)--a family of processes in which resin is injected into fiber preforms enclosed in heated mold cavities--emerged as a viable alternative for producing composite parts. RTM can often speed processing because it performs the shaping and curing functions in one step. The method also features the ability (in principle) to achieve precise control of the placement, orientation, and quantity of reinforcing fibers in the formed structure. Thus, RTM lends itself well to the fabrication of highly complex structural shapes that usually pose a challenge to the lay-up method.

Premium products such as high-tech sporting equipment and mostly nonstructural aerospace components have been made using the RTM process. Despite offering high strength-to-weight ratios and other desirable qualities, however, RTM composites have often failed to gain greater application because of high-priced "touch labor" and the costly tooling needed to produce them. Another issue is a general lack of consistency from part to part in dimensional tolerances and mechanical properties, which results from variances in raw materials and processing conditions. Flaws can also occur when using matched metal molds, because the bulk or residual thickness of the stacked fabric plies often resolves itself into wrinkles or voids when the tools are closed. These problems can lead to high rejection and scrap rates.

Not satisfied with the limitations of standard RTM technology, engineers at Dow-United Technologies, a maker of composite parts based in Wallingford, Conn., have spent the last few years working to make the promising manufacturing technique suitable for flight-critical aircraft structures. Dow-United Technologies--a joint venture of the Dow Chemical Co. in Midland, Mich., and United Technologies Corp. (UT) in Hartford, Conn.--has about 250 employees working at the Connecticut facility; an additional 450 operate another plant in Tallassee, Ala.

Dow-UT's process, advanced RTM, differs from the base RTM methods in several ways, said Lawrence N. Varholak, vice president for engineering and technology. Efforts were made to automate the fabrication process where it made sense, he said. For example, "Dow-UT's advanced RTM features our tackifier technology, which allows you to take dry (untreated) fabric plies and form them into very complex shapes while they're still pliable." The process, he added, enables the resulting shaped carbon-fiber preforms to be assembled into a complex part that approximates a complicated forging. Standard RTM procedures have difficulty producing complex shapes with the reinforcements in the desired locations.

Varholak also said that proprietary injection and precision tooling technology, "developed with a lot of research and development money," permits faster fabrication with better repeatability. "For example, we can fabricate as many as 24 parts in one injection cycle."

The resulting carbon-composite parts--which can feature dimensional tolerances from [+ or -] 0.005 to [+ or -] 0.05 inch, depending on the application--are replacing some titanium and aluminum in high-performance aerospace components. "We are quite competitive with titanium on a cost/weight savings basis," Varholak said, "although we're typically not as competitive with aluminum."

While the use of RTM structural aerospace components is still in its early stages, Dow-UT's list of customers for these parts is impressive. Boeing and Lockheed-Martin have contracted with the firm for fracture-critical airframe components for the US. Air Force's F-22 Raptor air superiority fighter. The company is making portions of helicopter airframes for Sikorsky. General Electric, Allison Engine Co., and Pratt & Whitney incorporate Dow-UT parts in their latest turbine-engine designs. Meanwhile, Motorola's first Iridium communications satellite, built by Lockheed-Martin, was recently sent into orbit with an advanced-RTM outer structure.

This industry trend should continue, said Bruce E. Alspach, Dow-UT's president and chief executive officer. "I think we're on the verge of a period of significant out-sourcing from the aerospace industry."

PROCESS DETAILS

Advanced-RTM parts typically have a higher fiber content than traditional RTM components--greater than 58 percent by volume versus 30 to 50 percent, according to Shari Tidrick, Dow-UT's manager of material and process technology. "The greater fiber percentage allows higher strength levels for a specific weight," she said.

Company engineers usually use carbon fiber in composite parts to take advantage of that material's high tensile strength, low mass, high tensile moduli, and low coefficient of thermal expansion, Tidrick said. The carbon-fiber grades typically used in advanced-RTM parts include AS4 for lower-cost applications, IM7 for higher-performance uses, and T1000 for components that require ultrahigh strength. Thermoset resin systems are either epoxy, which cures (solidifies) at 350 [degrees] F, or temperature-resistant bismaleimide, which has a 440 [degrees] F cure temperature.

The parts are designed using Catia software from Dassault Systemes in Suresnes, France. The as-formed shape of the plies are predicted using FiberSim, a package from Composite Design Technologies Inc. in Wellesley, Mass. Another useful software package, developed in collaboration with Dow Chemical and Ohio State University in Columbus, is PreFlow, which is used to predict the details of the resin injection process. Varholak expected that PreFlow will become available to the rest of industry in the near future.

The advanced-RTM process starts by weaving fabric from carbon fiber on various looms in the plan, said John Gendreau, director of development engineering. "Seventy-five percent of the tows are in the warp direction and 25 percent are in the fill direction, which provides more uniform packing compared with bias-weave fabric."

Dow-UT operates several looms adopted from the textile industry to produce fiber-reinforcement fabric in a range of widths: two Iwer rigid rapier looms, built in the 1960s; a Compton & Knowles flying-shuttle loom from the 1940s; and a custom-built narrow fabric loom that can perform polar weaving, a weave with reinforcements in the third dimension for greater structural integrity. "The narrow fabric loom uses a unique take-up system that moves the cloth in such a way that it weaves a helix in the third dimension," Gendreau said. Varholak added that polar weaving is particularly important in the production of energy-storage flywheels and reaction wheels that serve as gyroscopes and power-storage devices in satellites.

The fabric then goes to the aforementioned tackifier machine, which handles binder-deposition duties. At this station, a powdered binder material is applied and sintered to the fabric in preparation for the preforming process, Gendreau said. The tackifier material behaves like a thermoplastic while a heating operation sticks the tows together into fiber preforms. "It's like starching a shirt," he said. "Once the treated fabric is heated and pressed, it stays in shape. It also maintains its drapability."

According to Gendreau, the current tackifier is chemically compatible with thermoset resin, meaning that it becomes part of the matrix and does not affect the final part properties. The company started out mechanically stitching preforms together; this approach eventually evolved into using a parasitic binder that degraded mechanical properties, which was in turn replaced by the compatible material.

The next station is a pick-and-place machine built by American GFM Corp. in Chesapeake, Va., which features a special end effector to move successive fabric plies into position, stacking them in the proper location relative to one another. The system also features a numerically controlled ultrasonic knife that cuts the ply patterns. "The ultrasonic knife works better on dry fabric than a conventional knife," Gendreau said.

The consolidated preforms are then combined with dimensionally matched preform tooling or a precision RTM mandrel made of Invar, or a eutectic lead alloy if the form is to melt out during processing. In the early stages of the RTM-process development, the molding presses would bring parts into final registration; now, registration is accomplished with freestanding pins and bolts on the tooling. "This gives us greater control over the process," Gendreau said, "as well as the ability to inspect at the sub-component stage, prior to injection."

At this point, the tooling assembly is deposited into a mold frame, which is integrally heated and instrumented with extensive sensor telemetry apparatus. The entire apparatus is then heated and the resin is injected at several hundred pounds of pressure. "We do brute-force injection," Tidrick said. The injection step is followed by a cure cycle and high pressure. After cooling, the part is demolded, cleaned, and trimmed. Dow-UT uses several huge presses, with capacities ranging from 100 to 2,000 tons. An ultrasonic device inspects all parts for defects (voids, porosity, and foreign objects).

AIRFRAME APPLICATIONS

One of the more-impressive part families produced through advanced RTM is sine-wave wing spars, which are the main structural support framework of the wing of the F-22 fighter. The new fighter aircraft is being built by Lockheed-Martin Aeronautical Systems and the Boeing Defense & Space Group in Seattle. The sine-wave spar is part of F-22 contracts totaling $20.3 million. Other Dow-UT composites on the F-22 include forward fuselage parts and vertical tall ribs.

"We've designed what we call a sine-wave spar that is lightweight and structurally robust," said Geary Long, the F-22 internal-spar integrated-product-team leader for Boeing, "but we needed a process that would yield higher-quality parts again and again, throughout the life of the program. That's when we turned to resin transfer molding.... Few RTM parts have been this highly loaded."

Unlike a conventional I-beam, which has a straight central member, a sine-wave spar has an undulating sine-wave-shaped support connecting the two sides of the beam. Pound for pound, the sine-wave design is stronger than traditional I-beam spars, according to Boeing. The advanced-RTM composite spar reduced the cost of building these key components by 20 percent and has cut the number of reinforcement parts in half, Long said. Each F-22 wing contains 46 composite spars.

Long said that the sine-wave spars were originally to be built using prepreg composite fabrics. But after evaluating various competing fabrication processes both in-house and from outside suppliers, engineers decided that Dow-UT's RTM process had the potential to cut recurring manufacturing costs significantly. "Although the RTM has high initial capital costs, we felt that there was a big `cost carrot' downstream that we might take advantage of," he said. "The process does require a higher up-front investment for tools and development than other composite manufacturing methods. However, over the entire production run, the investment will pay for itself and save us money.

"We've been working in concert with Dow-UT process engineers from the beginning so that their process plans and our part designs are always consistent," Long added.

Working concurrently, researchers began to improve process control, including fiber alignment (within a few degrees), injection temperature control (the tools act as large heat sinks), and the sensory feedback system. Boeing, which designed the parts, likewise designed and built the needed tooling. Dow-UT made an $8 million to $10 million investment in the capital equipment needed to produce the fighter parts. "We built a hundred test bars to ensure we got it right," Long said. Not everything went perfectly, however: "Some of the first parts weren't coming out dimensionally correct, for example. We eventually found out that the large presses were deforming the Invar tools. We solved the problem by tweaking the pressure schedule. [As a result of the concurrent engineering program] we've scrapped zero tools due to redesigned parts."

The result of the Joint effort seem to be promising. Because of Dow-UT's quality, Long expected that Boeing's scrap and rejection rate would be near zero. "These parts have been virtually perfect." Indeed, the U.S. Air Force F-22 System Program Office recently cited Dow-UT for "most effectively managing cost, schedule, and performance" and for demonstrating "excellence in continuous improvement."

ENGINE PARTS

Turbine-engine components are another good example of a successful advanced-RTM application. Pratt & Whitney in West Palm Beach, Fla., for example, has developed a large composite fan inlet case that should significantly reduce the weight and cost of military aircraft engines. A fan inlet case supports the shape bearing to the engine case. It comprises an inner bearing hub, many airfoil shaped structures, and an outer support ring and mounting lugs--all molded as an integral unit.

The big composite part will replace the titanium assembly currently used on the F 119 engine, which Pratt & Whitney is building to power the F-22. Compared with the titanium assembly, the composite case saves about 15 pounds. Furthermore, the molded integral component costs considerably less than the titanium, which includes many machined details that are attached at a later stage. The full-scale case has been statically tested to worst-case load conditions. Advanced RTM allows all external air-passage surfaces of the fan inlet case to be molded to smooth final dimensions without secondary machining.

"There's no question that Dow-UT's process enables significant weight and cost savings," said Michael Givers, team leader of composites design at Pratt & Whitney's Compression System Component Center in West Palm Beach. "The ability to reduce the overall part count and eliminate many labor-intensive assembly operations makes this composite part an important improvement over titanium."

In May, Dow-UT won a five-year, $9.7 million contract to manufacture fan platform parts for the Cincinnati-based General Electric Aircraft Engines Division's GE90 axial-flow fan jet engine, which powers Boeing 777 commercial airliners. The fan platforms, which provide flow-path surface between the fan-blade airfoils, were redesigned to reduce the weight of the GE90 engines. Twenty-two composite components are used in each engine.

The composite-part maker has also signed a $6 million to $8 million contract to build bypass vanes for Allison Engine Co. in Indianapolis. The carbon-composite bypass vanes, each about 7 inches long and about 2 inches wide, will funnel air into the front section of the Allison AE3007 engine, which powers the Cessna Citation X, the Embracer 145, and other general-aviation craft and regional airliners. The five-year contract calls for between 10,000 and 15,000 vanes to be delivered beginning in late 1997.

Another prime market for composite parts is satellite structures, which until recently were fabricated one at a time rather than on a production line. Dow-UT, in collaboration with Lockheed-Martin Missiles and Space in Sunnyvale, Calif., is changing that paradigm with a $6 million effort to build 80 satellite housings for the Motorola-designed Iridium advanced telecommunications satellites. The 14-foot-long, 3-foot-wide Iridium exterior structure, which contains all the electronics, is being constructed using a prepreg molding process at Dow-UT's Alabama facilities.

FUTURE PROSPECTS

Still in question are the prospects for the future use of RTM technology in industries other than aerospace. While Dow-UT is relatively confident about aircraft and spacecraft uses, the chances of application growth in other industries is somewhat problematic. Tidrick pointed to marine products, biomedical products, and large civil infrastructure products (such as offshore drilling rigs, bridges, and buildings) as promising. She cautioned, however, that "high RTM tooling costs are a big barrier, as is RTM's limited production rate." The resin systems take a certain time to cure, so it's hard for a manufacturer to build a 1,000 units per day, as would be needed in the automotive industry, for example.

Many engineers agreed with Robert Stratton, senior engineer and program manager for F-22 RTM at Lockheed-Martin, who said that RTM-composites use should grow as young engineers who were exposed to the technology in school attain high-level, decision-making positions in future engineering projects. Another factor is increased access to accepted engineering data on composite materials. "It's hard to do initial sizing without good property numbers," Stratton said.

Dow-UT plans to address that problem by releasing its standardized mechanical-properties database for composites so engineers can better design with the nonmetallic materials. As Varholak said, "We think it's time to take information out of the black environment so people can use it."