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The Art of Aerospace Composites PUBLIC ACCESS

Keeping the Pressure on the Resin Transfer molding Process for Manufacturing Flight-critical Carbon Parts.

[+] Author Notes

Associate Editor

Mechanical Engineering 121(04), 58-61 (Apr 01, 1999) (4 pages) doi:10.1115/1.1999-APR-4

This article reviews the advanced resin transfer molding (RTM) process of GKN Westland Aerospace. This process is refined enough, with customized equipment and a proprietary resin binding material, so that hundreds of different aircraft parts that would otherwise be heavier (made of titanium) are being produced for customers that include GE, Pratt & Whitney, Lockheed Martin, and Boeing. GKN is making five-axis, hollow vein, and integrated attachment nodes. It has produced carbon-fiber and resin components as thick as 3½ inches, and designs can combine what were many parts. Depending on the part and desired strength (in the desired directions), the fiber tow is woven in a variety of ways. For strength in mainly one direction, the engineers specify that 75 percent of the tow runs in one direction and just 25 percent of it is used to weave across it, for example. The next step in GKN’s advanced RTM evolution is a unihybrid composite that takes great loads in just one direction and can be made very thick, up to 3½ inches. A slightly less rigorous process has already been licensed, to a company in Mexico that produces a component for the Dodge Viper sports car.

GKN Westland Aerospace, when it was still the Dow-United Technologies composite products joint venture, had a fastidious, production-capable resin transfer molding (R TM) process 10 years ago. And R TM parts for everything from portable toilets to golf clubs have been around even longer.

What's different now-very different-is the artisan-like approach GKN has been able to hold onto. It produces very complex parts with exact repeatability and precision, and with the strength needed for helicopter rotors and jet engine fans. The company has dubbed it "advanced R TM," and the process is refined enough, with customized equipment and a proprietary resin binding material, so that hundreds of different aircraft parts that would otherwise be heavier (made of titanium) are being produced for customers that include GE, Pratt & Whitney, Lockheed Martin, and Boeing.

The former Dow-UT Composite Products is owned by GKN, a British industrial holding company. With its new British parentage, GKN Westland hopes to be even more likely to win new Airbus Industrie business, and get closer to engine maker Rolls-Royce.

Patents continue to come in. One of the most recent was for a manufacturing cell able to process many different parts, so that the company doesn't have to re-equip for each part it runs. GKN says it will continue to replace titanium parts, and go beyond the shapes and configurations allowable with titanium. GKN is making five-axis, hollow vein, and integrated attachment nodes. It has produced carbon-fiber and resin components as thick as 3Y2 inches, and designs can combine what were many parts, according to John P. Gendreau, director of engineering research and development at the Wallingford, Conn., company.

A walk through the process 'at the manufacturing operation brings home the craftsman analogy. GKN technicians watch over the intricate and sometimes lengthy processes throughout, and there seems to be much more direct human involvement than one sees in computer chip or camera manufacturing, for instance.

The flow starts with braiding, or weaving, when the carbon fabric is made. The carbon fiber is wound fr0111 spools of carbon tow, which looks from afar just like a spool of graphite-colored thick thread. The tow is aerospace- grade-made to either the AS4 or IM7 specifications- and GKN buys it primarily from Hexcel Inc. of Salt Lake City.

Here is just one part of the process that brings the word "craftsmanship" to mind. The tow is fed into retrofitted, 1940s-era cloth looming equipment, which is programmed by, of all anachronisms, punch cards. Such looms are "much more robust because the graphite dust [the particles that rub off during looming] is not in the least bit friendly to electronics," Gendreau said. The punch cards are generated elsewhere by desktop computer, but otherwise the looms are kept all-mechanical to avoid the hassle and expense of building systems that could withstand graphite infiltration.

As a technology demonstration, GKN Westland Aerospace produced this engine fan exit case using its advanced resin transfer molding, or RTM, process. Ten of these sections complete a circle of spacers for a large bypass turbofan engine. The company uses composites to make primary structures for aircraft.

Grahic Jump LocationAs a technology demonstration, GKN Westland Aerospace produced this engine fan exit case using its advanced resin transfer molding, or RTM, process. Ten of these sections complete a circle of spacers for a large bypass turbofan engine. The company uses composites to make primary structures for aircraft.

The "pick and place" machinery manipulates carbon fiber plies and stacks them in the layers that form the underlying structure that is later surrounded by resin. The process is key in maintaining the sequence and alignment.

Grahic Jump LocationThe "pick and place" machinery manipulates carbon fiber plies and stacks them in the layers that form the underlying structure that is later surrounded by resin. The process is key in maintaining the sequence and alignment.

Depending on the part and desired strength (in the desired directions), the fiber tow is woven in a variety of ways. For strength in mainly one direction, the engineers specify that 75 percent of the tow runs in one direction and just 25 percent of it is used to weave across it, for example. (The thread of your broadcloth shirt is run 50/50.) If it's 75125, "it's twice as thick and you have to cut it half as many times," Gendreau explained.

The fabric of graphite tow can be woven with a ridge running perpendicular to the first ply to eventually make a vein structure that a part might require. For other shapes and configurations, the tow is actually braided on a broad n1.achine about 10 times the size of the loom.

For one application, the tow is braided around what look like metal vertebrae. The spine-like shape is sinusoidal, or in waves, because the eventual parts, thrust reverser cascades for jet engines, require the change in plane for greater integrity. The alloy vertebrae, more properly termed mandrels, give the part that shape, and they stay within the graphite fabric until the resin is completely set. After resin injection and curing, when the fabric is a solid part (like fiberglass), the metal within is dissolved in a chemical bath that leaves only the carbon-fiber part. It dissolves because it is a eutectic fusible alloy, which decomposes under the right conditions to form two solids that can be remade into the alloy.

The weaving yields the ply that will go to "tackifying." Gendreau uses the analogy of starching a shirt to explain this step in the R TM process. The plies of woven carbon are infused with a binder material (like starch in a collar) at a high temperature. Only normal atmospheric pressure is required. This binder material is a key to the structural integrity GKN needs to get from these components to make them "primary structures," the ones flight depends on in aircraft.

The binder is a powder, which GKN holds proprietary, and it is chemically compatible with the resin in the next step of the process, when it is injected into a metal mold, or "tooling," that contains the carbon fiber plies. The binder "inherently becomes part of the resin," Gendreau said.

The tackified fabric must be made, by "cutting and kitting," into exact preforms that will be placed in the mold and resin-injected. This is a significant part of what GKN had to do to make the process repeatable and guaranteed to meet the requirements of the Department of Defense and the Federal Aviation Administration. If the fabric is not handled exactly right, the weave can be distorted.

A numerically controlled ultrasonic knife cuts each ply. Then one is placed on top of another with a custommade pick-and-place machine the size of a large truck bed. (This machine is not to be confused with the pick-and-place machines that assemble printed circuit boards.) Depending on the size, the ply may also be cut with a steel rule die, almost like a cookie cutter, where an imprint is pushed through the fabric.

The pick-and-place machine assures that, for instance, the 10 or 12 plies to be stacked into a highly loaded part are placed exactly as they should be to make proper preforms. Next, the fabric is pressurized at 15 psi and 200°F. "At this point, it stops being a ply," Gendreau noted.

The day that we visited, GKN was producing, among other parts, fan spacers for jet engines. The spacers hold the fan blades apart, as they take the load of a central hub spinning at 3,000 rpm, and also help to protect the engine when birds find their way into the fan, Gendreau said.

The stiffened preforms are placed in the tooling where the resin will be injected, in one of the most standard of all the steps GKN performs to make the components. Three basic types of resin are used, depending on what the part must be able to withstand. The first is a basic resin system procured from Minneapolis-based 3M Corp.; it can withstand in-service temperatures up to 300°F. The second type is more robust, a bismaleimide resin graded up to 375°F, which GKN obtains from Cytec Fiberite Inc., of Newark, Del.

The third variety is an experimental class of resin that GKN is developing on its own; Gendreau said the resin withstands operating environments as hot as 650°F. This third resin would be suitable for engine components up to stator 3 under the DOD's standards. All the resins are the consistency of kerosene when they are injected. They are much less viscous than a plastic, Gendreau said.

Finally, the hardened composites head to machining, where diamond-encrusted grinding tools finish the edges and contours down to their specifications. From there, they head to quality assurance, or perhaps to GKN's analysis lab. The lab will give the parts a once-over with spectrometers and chromatographs. At quality assurance, a number of specifications are double-checked. There is a numerically controlled ultrasonic sound wave check for material integrity which is done under water. QA also checks balancing, which must be within 0.3-ounce of the spec.

Preforms of "tackified" carbon plies are ready to head into resin injection molding. They are the fiber architecture of the components. The material lends itself to a variety of shapes and contours.

Grahic Jump LocationPreforms of "tackified" carbon plies are ready to head into resin injection molding. They are the fiber architecture of the components. The material lends itself to a variety of shapes and contours.

The carbon tow is braided around a series of eutectic alloy mandrels to produce a sinusoidal airplane wing spar for the Lockheed Martin Corp

Grahic Jump LocationThe carbon tow is braided around a series of eutectic alloy mandrels to produce a sinusoidal airplane wing spar for the Lockheed Martin Corp

GKN devised the patented flexible molding cell for this process. The automated molding system is one series of equipment that can mold many parts without any need for retooling. This enables GKN to achieve the 126 components per quarter it supplies to Lockheed Martin's F-22 program.

Grahic Jump LocationGKN devised the patented flexible molding cell for this process. The automated molding system is one series of equipment that can mold many parts without any need for retooling. This enables GKN to achieve the 126 components per quarter it supplies to Lockheed Martin's F-22 program.

Mitsui Denki produced a graphite machining system that is used to finish the resined parts.

Grahic Jump LocationMitsui Denki produced a graphite machining system that is used to finish the resined parts.

The analysis lab, where chemical and mechanical inspection chores are completed, is housed adjacent to the manufacturing department at GKN's Wallingford, Conn., plant.

Grahic Jump LocationThe analysis lab, where chemical and mechanical inspection chores are completed, is housed adjacent to the manufacturing department at GKN's Wallingford, Conn., plant.

A worker examines finished fan spacers for a jet engine, which is one of three options in Boeing's latest 777 airplanes.

Grahic Jump LocationA worker examines finished fan spacers for a jet engine, which is one of three options in Boeing's latest 777 airplanes.

"In the 1980s, a lot of RTM parts were not suitable for flight-critical parts, for primary structures," Gendreau said. Of course, "flight-critical" means critical for the air-. plane or helicopter to stay airborne. Gendreau explained the difference between the requirements of lower tech parts like golf clubs and those of GKN's aerospace components. The lower tech parts might be 25 percent fiber by volume; GKN's are 55 to 60 percent fiber.

The next step in GKN's advanced R TM evolution is a unihybrid composite that takes great loads in just one direction and can be made very thick, up to 312 inches. A slightly less rigorous process has already been licensed, to a company in Mexico that produces a component for the Dodge Viper sports car.

Copyright © 1999 by ASME
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