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Resin transfer molding speeds composite making.

RTM fabrication is faster and less labor-intensive than conventional composite making. RTM parts are lighter than metals and can be formulated to have superior durability. But like all composite parts, they are expensive and are made in limited runs.

The first sign that resin transfer molding (RTM) composite parts may be cracking the U.S. industrial marketplace has appeared. Chrysler Corp. (Bloomfield Hills, Mich.) is making most of the exterior of its 1992 limited-edition Dodge Viper RT/10 high-performance sports car from RTM components.

While the Viper is hailed as a step in the right direction by RTM processors, who compete with traditional composite makers in niche applications, they acknowledge that it would take a major commercial application to bring RTM into the realm of composites. RTM parts are lighter than metals and can be formulated to have superior durability. But like all composite parts, RTM components are expensive and are made in limited runs.

Speeding Cycle Times

In addition to being used on high-performance sporting equipment-- golf clubs, ski poles, and ice hockey sticks--RTM processing, as well as traditional composite techniques, is used by companies such as Brunswick Composites, a subsidiary of the Brunswick Corp. (Skokie, Ill.), to manufacture composite aerospace parts. Among those parts are the wing flaps for McDonnell Douglas C-17 aircraft and radar housings for military jets, including the F-15 and F-111. James Cecere, a technical associate at Brunswick Composites, said that the major advantage of RTM fabrication is that it is faster and less labor-intensive than conventional composite making. "Autoclave composite manufacturing involves heating a resin until it is a liquid and using it to impregnate a fabric form called a prepreg," said Cecere. A number of these prepregs are then stacked, or layed up, to the required thickness of the part and are placed in an autoclave for curing.

Cecere described vacuum bag processing as similar to autoclave processing, with the additional step of placing the laid-up prepregs in a plastic membrane from which air is evacuated during autoclave curing. This forces the composite material against the tool to give the part its final form.

Filament winding involves coating a fiber with resin and weaving the wet fiber into the desired shape before curing in an autoclave. "Sheet-molded compounding requires that a mixture of resin and usually chopped fibers be compression-molded into shape," said Cecere.

In RTM processing, a single three-dimensional fabric preform is woven in the shape of the finished part and placed in a mold. A vacuum is induced before the liquid resin is injected into the mold to help draw the resin through all the spaces of the preform. The part is either cured in the mold or removed to. an autoclave for final curing.

"Using a woven preform lowers labor costs, facilitates automation of the RTM process to speed cycle times, and reinforces the three-dimensional strength of the finished part," said Cecere. He noted that cycle times for conventional composite part manufacture are as long as 8 hours, while RTM processing can take as little as 30 minutes.

Hitching a Ride on the Viper

The Viper represents the first sign that U.S. automakers are considering RTM parts for their vehicles. The Viper is the first domestically produced car on which the hood, front fenders, rear quarter panels, decklid, and roof support are produced by RTM. Body panels formed with RTM represent a 40 percent savings in weight compared to sheet metal, a significant reduction in a high-performance sports car. RTM also reduces machining time.

"We will be able to control the panel-forming process so precisely that when each piece comes out of the mold, it will require only 10 to 15 minutes of hand finishing before reaching a 'Class A' level of appearance," said Russell Spencer, Viper technology development executive. "With sheet-molded compound processing, you may have to do hours of hand finishing to achieve that."

A limited production run of about 3000 units is projected for the Viper in each subsequent model year. Chrysler management described the car as combining the style of 1960s roadsters like the Shelby Cobra with the latest manufacturing techniques. Some European sports cars, including the BMW Z1, Alfa Romeo SZ, Lotus Elan, and Lotus Esprit, have already incorporated RTM panels.

Weaving Aerospace Parts

Currently, the aerospace industry is a major user of RTM components. Fiber Innovations Inc. (Norwood, Mass.), a composite part manufacturer, for example, performs each stage of the RTM process mostly for aerospace clients. "We work with the engineers of our client companies to design the composite part. We then braid the 3-D fiber preform and mold the preform using the RTM process," said Garrett Sharpless, president of Fiber Innovations.

In the braiding process, carbon, graphite, glass, or silicon-carbide fibers are interlaced over a mandrel supplied by the client or designed and procured by Fiber Innovations, which virtually duplicates the shape of the final part.

Fiber Innovations uses triaxial weaving to braid its preforms. That is, the fiber architecture consists of two fiber systems oriented in the bias direction, i.e., two at [+ or -]45 degrees and the third at zero degrees. The desired thickness is achieved by braiding a predetermined number of layers atop each other on the mandrel to create a multi-ply construction. The company has achieved fiber content as high as 65 percent for its composite parts. These levels of fiber loading are necessary to fabricate high-performance composites for primary aircraft frame structures.

The braided preform is positioned in a metal clamshell mold, usually made of aluminum, having a cavity machined to the net dimensions of the finished part. After the mold is closed and sealed, the tool is injected with resin using positive displacement pumps made by Liquid Control Corp. (North Canton, Ohio). The tool can be heated to aid in resin impregnation and curing. Programmable feedback loop systems control any ramped (incremental) heating cure profiles.

A vacuum system connected to the tool evacuates it prior to injection, to help draw the resin through the voids in the preform. The Fiber Innovations RTM process has achieved void contents of less than 1 percent in its composite parts, according to Sharpless. The part is generally removed from the mold and postcured, followed by trimming or machining if necessary.

Sharpless said that most of his company's RTM composites business involves replacing high-performance alloys in aerospace applications. "We primarily use carbon fibers within an epoxy matrix," Sharpless said. The carbon provides a high specific strength and stiffness and the thermal properties to control and reduce the thermal coefficient of expansion of the composite.

These qualities were essential to the aircraft engine actuator developed by Fiber Innovations for Bendix Engine Controls, a division of Allied Signal Corp. (Morristown, N.J.). "This actuator is being tested for a U.S. Air Force project to develop an improved generic engine for military aircraft for the year 2000," said John Powers, market development engineer at Allied Signal.

The actuator is approximately 12 inches long and 1 inch in diameter for most of its length, until it flares out to 2 inches in diameter to accommodate the piston head. Fiber Innovations engineers used braided carbon fibers to fabricate the preform. They used Allied Signal's own Primaset resin to impregnate the preform. This cyanate ester resin was chosen because its 600[degrees]F service temperature would enable it to withstand being exposed to engine heat and hot oil.

Fiber Innovations is making larger aerospace components through RTM, including subscale composite versions of aluminum J-frames, which are the main components of an aircraft's structural frame. These 8-foot-long models are being made for the Boeing Co. (Seattle). The aircraft manufacturer is interested in a composite J-frame that can be assembled more quickly, last longer, and weigh less than the aluminum frames now in service.

Fiber Innovations engineers are making the composite J-frames out of AS4 carbon fibers and RSL 1895 epoxy made by Shell Chemical Co. (Houston). "Many composites are susceptible to moisture. This material's high moisture resistance should enable it to survive in an aircraft service environment," explained Sharpless.

In addition to choosing a moisture-resistant epoxy, Fiber Innovations engineers used a clamshell mold and mandrel of Envar, a high-nickel steel with a coefficient of thermal expansion similar to the composite material and a much lower coefficient of thermal expansion than steel or aluminum tooling. "The geometry of the J-frame is such that cooling from the cure temperature of 350*F to room temperature would create enough stress to crush the composite frame," Sharpless said.

Turning Bumpy Bicyclists Into Easy Riders

RTM processing has also helped commercialize some composite parts. A case in point is the composite bicycle seat beams made by Softride Inc. (Bellingham, Wash.) for its Softride rear-suspension system. This part replaces the seat post, attached to the bike frame, which turns the rider into a shock absorber and causes back pain. The patented new seat beam eliminates the seat post and absorbs shocks by suspending the rider, not the bike. Both the Canadian and Irish Olympic racing teams chose the Softride system at the Olympic trials and games last summer in Barcelona, as did Carl Sundquist, the American Individual Pursuit racer at the Barcelona games.

During the research and development of the part, the Softride beam was made using compression molding, vacuum bag, and wet lay-up methods. The company was able to make about 50 individual beams a day, and about 20 percent were rejected. Cycle times for completing the beams were as long as four hours, largely due to machining and final assembly.

"We reduced labor costs, cycle time, and reject rates using RTM to make the beams," said David Carlson, product development supervisor at Softride. Carlson said that by using RTM processing, the company can now produce 100 beams a day-- less than 5 percent of which are rejected--in cycle times as short as 30 minutes per part.

The first stage of the Softride RTM process requires workers to create the beam frame. They use the H-2000 reaction-injection-molding machine made by the Gusmer division of PMC Inc. (Sun Valley, Calif.) to inject rigid urethane foam into the beam frame mold. Completed beam flames are sent to a preform area, where the flames are run through a braiding machine that weaves patterns of glass fibers and carbon fibers around them, converting the flames into RTM preforms.

The Softride preforms are placed into four-cavity resin molds made of aluminum that form two upper and lower beam sections. Company technicians inject Tactix resin from Dow Chemical Co. (Midland, Mich.) into the molds with a Venus-Gusmer (Kent, Wash.) EPO1 RTM machine. This device heats the low-viscosity resin to 150[degrees]F before injecting it into the mold, which is heated to 180[degrees]F. A vacuum line connected to the opposite end of the mold helps draw the resin into the preform. All of the Softride molding machines are automated with General Electric Co. (Fairfield, Conn.), Texas Instruments Inc. (Dallas), and Siemens AG (Erlangen, Germany) programmable-logic controllers.

When removed from the mold, the finished beam sections are jigged together. An automated meter mix dispenser made by Sealant Equipment & Engineering Inc. (Plymouth, Mich.) injects a thin coating of elastomeric urethane between the sections. The smooth finish that RTM processing imparts means that the sections can be joined without further machining or assembly.

Teaching Old Resins New Tricks

Not all RTM processing requires injecting a liquid resin into a preform contained in a closed mold. One composite maker, Hercules Inc. (Magna, Utah), developed a proprietary film application technique that is being used to fabricate aerospace stiffenets, including J-, I-, and C-frames as part of NASA's Advanced Composite Technology Program.

The Hercules RTM process grew out of the company's determination to use the resins it originally developed for traditional composite making to satisfy RTM applications. In the aerospace project, Hercules engineers chose the firm's 3501-6 epoxy. "This material has been used commercially for nearly 20 years, so there is a lot of data available on its capabilities that an end user would want to know, such as tensile compression," said Mark Wheeler, composite structure technology supervisor.

Wheeler said the Hercules RTM process applies heat and pressure to make a film of the epoxy in an open mold within an autoclave. A fiber preform of the part is brought into the autoclave. The Hercules production staff uses a combination of heat, pressure, and vacuum to draw the epoxy through the preform and make the part within the autoclave. In addition to its film process, Hercules has developed proprietary hardware to perform closed-mold RTM.

Waiting for Detroit

If RTM is proving itself technologically, the economics of the process are keeping it from being widely commercialized. RTM parts are costly, especially on a per-unit basis, given their limited production runs. Wheeler of Hercules said that since the making of the preform is the most expensive phase of RTM processing, more automated preforming techniques could lower the cost of the parts. He also said that it would take the economies of scale resulting from larger production numbers to pay for that automation.

Sharpless of Fiber Innovations agreed that the economies of scale from replacing metal automotive parts with RTM composites should be attractive to the industrial marketplace. "In addition to their light weight, which would reduce fuel consumption, RTM composites may reduce tooling costs incurred by traditional steel stamping," he said.
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Author:Valenti, Michael
Publication:Mechanical Engineering-CIME
Date:Nov 1, 1992
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