Composites engineers are expanding their craft to build more complex, durable parts at higher production volumes. One way they are achieving this objective is by using infusion-molding processes based on Resin Transfer Molding (RTM) and Vacuum Assisted Resin Transfer Molding (VARTM). Parts made with these processes are replacing both metal parts and parts made with more traditional composite methods, such as pre-impregnated fabrics that are cut, shaped, and cured in an autoclave.
Infusion-molding processes use a mold or a tool draped with dry reinforcement, preforms, or core materials. RTM uses a two-sided tool with resin pumped in, while VARTM typically uses only one tool, and the resin is drawn in. In VARTM, a vacuum bag surrounds the entire assembly of an open tool filled with reinforcement—the bag pressure forms the part on the side opposite the tool.
The F/A-22 Raptor program was the first aerospace project to make extensive use of RTM in an aero structure, says David Ledbetter, senior manager of manufacturing research and integration at Lockheed Martin (Marietta, GA). He points out that development of the basic process engineering for the F/A-22 began in the late 1980s. Ledbetter observes that the current F/A-22 is composed of about 24% composite parts. Components made using RTM replaced more than 400 parts made using metal or pre-impregnated materials.
A distinct advantage of RTM over hand layup is precision. In conventional layup, according to Ledbetter, the thickness of the resulting part cannot be precisely controlled, because shape is controlled only on the mold side. In RTM, all surfaces are controlled with a closed mold, so tolerances can be maintained in any direction or thickness. This capability limits overall tolerance buildup in the resulting structure. “In many cases, the difference [tolerance] is controlled within a few thousandths of an inch. The closed mold eliminates buildup of tolerances, and makes a big difference in assembly fit,” he explains.
Eliminating emissions is another plus for infusion-molding processes, both for safety and cost, says Scott Lewit, president of Structural Composites (W. Melbourne, FL), a boat and marine structures manufacturer. Gregg Ferrell a director of GKN Aerospace (Tallassee, AL) echoes this observation: “Infusion processes are inherently safer for workers because they are not exposed to resins as in hand layup.” Also, the economics of tooling cost dictates longer production runs. “In aerospace production, RTM is considered a high-volume process,” says Ferrell.
One restriction for high-strength applications of RTM and VARTM is that the fiber volume is typically lower than that employed in pre-impregnated laminate processes, according to Ledbetter. “Our biggest challenge in the F/A-22 program was getting enough fiber density into the infusion processes,” he says.
“Typically, for VARTM systems, resin systems must be less than 300 centipoise in viscosity,” says Garrett Sharpless, general manager of EDO Corp.’s Fiber Innovations business unit (Walpole, MA.) “VARTM, because it uses only vacuum, makes it more difficult to pull a higher-viscosity material through the reinforcement.” RTM resin systems can have higher viscosity, in the range of 800 centipoise, says Sharpless. This is because the pressure required to pump in the resin for RTM can be up to 300 psi (2.1 MPa), therefore allowing a higher-viscosity material to be used. “The real key for manufacturing aerospace composites is the qualification of material systems for use in infusion-molding processes,” states Sharpless.
Creating complex shapes from composite materials is an area where liquid infusion molding processes may be uniquely well-suited. EDO has evolved a process to create parts of complex shape in high-volume production. Atriaxial braiding machine lays down dry fiber reinforcement over a complex mandrel to create a net-shape braided preform. This preform is then infused with resin using either the RTM or VARTM process.
“Braiding is known in the textile industry for making any type of protective fiber reinforcement,” explains Sharpless. “We feel the biggest benefit we created was to couple this triaxial braided preform process with an infusion-molding process to set up a tool string, similar to an automotive manufacturing operation.”
A benefit of combining the braiding and RTM techniques is the ability to achieve high fiber volume when compared to other RTM methods. “In many cases,” Sharpless observes, “composites made with braided preforms and RTM can match the fiber volume of the traditional pre-impregnated fabric layup and autoclave-cure processes.”
Manufacturing engineers at EDO use their braided preforming process to mass-produce parts in low, medium, and high-volume production. Production quantities range from 20 to 50 parts for satellite structures, over 800 fuselage and wing structures for the Joint Air-to-Surface Standoff Missile (JASSM), and up to 55,000 launch tubes for a Marine Corps’ shoulder-launched weapon program (SMAW-HEDP). The JASSM program will eventually include at least 4900 missiles.
As an example of the savings possible over hand lay-up with pre-impregnated materials, it’s estimated that EDO’s processing methods will save the JASSM program $19M over the production life of the missile program.
Innovations in qualified resin-and-material systems could make liquid-molding-infusion processes even more desirable in the future. “There are more resin systems becoming available, and formulations that have lower viscosities and longer processing windows for these applications,” notes Sharpless.
Another process challenge for defense aerospace applications is that most qualified resin systems are toughened systems, which are thick and viscous compared to the more fluid resin systems typical for infusion processes, according to Todd Szallay, project lead, Manufacturing Technology Development, Northrop Grumman (El Segundo, CA).
“Using the toughened resin systems, smaller parts can be done—such as composite fairings and spars, which are usually limited to dimensions no more than 5 x 2′ [1.5 x 0.6 m]—but it’s difficult to use infusion processes such as RTM in larger structures like skins and fuselage parts. The resin just can’t flow, and the tooling is too expensive,” explains Szallay. “In the aerospace industry, the movement in composites is towards large integrated structures. Not just individual composite skins, but building unitized composite assemblies with structure and skins together.” The expensive match-mold tools required to meet dimensional requirements are another problem with infusion processes.
Ferrell of GKN Aerospace also notes that the trend in aerospace is the use of composites in larger structures, including primary load-bearing structures. An economical way of building such large parts is to escape from the need for autoclaves, the so-called out-of-autoclave movement. He notes that infusion processes, such as RTM and VARTM, can help reduce the need for autoclaves because, typically, the resin used does not need to be cured under high pressure. “However,” he cautions, “it must be noted that RTM is a closed-mold process, and the molds limit the size of the part just as an autoclave does.”
To overcome some of the issues of tooling cost, material qualifications, and strength, composite engineers have devised innovative variants of infusion molding processes.
Radius Engineering (Salt Lake City, UT) has developed an approach using currently specified pre-impregnated fabrics to produce net-shaped composite components. Prior to this development, such highly unified structures were thought to require the classic RTM method, according to Steve McMahon, engineering director for Radius Engineering. “There are fairly large costs associated with qualifying RTM products for aerospace applications, either the dry fiber or the resins,” he says. “We developed the Same Qualified Resin Transfer Molding Process [SQRTM] to use existing qualified prepreg in a closed mold under high pressure to avoid these costs.”
Although it uses pre-impregnated materials, resin is injected into the mold just as in a traditional RTM process. “The resin that is injected provides hydrostatic pressure on the prepreg in the mold just as an autoclave would,” explains McMahon. The mold is heated to cure the part, achieving the dimensional control and repeatability of RTM processes.
Getting rid of the autoclave was a major motivation of Radius Engineering’s SQRTM hydrostatic-pressure process. “You use an autoclave to apply a lot of backpressure on the part to keep volatiles in solution so they don’t create voids, while applying heat to cure the part. That is what the SQRTM process does without an autoclave,” says McMahon. He reports production of parts as long as 22′ (6.7-m), and does not believe there are any scalability issues—even larger parts might be possible.
Eliminating the autoclave directly reduces production costs, and decreases cycle time. “A problem with autoclaves in a production cycle is that they tend to heat up slowly,” says McMahon. “They heat up at 1–2°F per minute [0.56–1.12°C/min].” Therefore, for a 350°F [177°C] cure, heating up the autoclave requires 380–390 min. The SQRTM tooling can be heated up more quickly, in some instances as fast as 5°F [2.78°C] per minute. “We can save about 120 min out of the cycle time, about a 30% savings.”
Radius provides expertise and equipment to companies that produce composite parts. Vought Aircraft Industries (Dallas) is using SQRTM in production to build the next-generation Global Hawk wingtip extension, a part that is about 12′ (3.7-m) long.
Employing a different approach to overcome some limitations, Structural Composites is using an infusion molding process called Recirculation Molding to build a large matched-tool part for the Navy—a Composite Twisted Rudder (CTR) for the DDG 51 class of ships. The rudder is 14′ (4.6 m) at the base by 13.75′ (4.2-m) tall, and is constructed of composite material over a HY-80 steel frame. Layers of unidirectional material are laid up to create a quasi-isotropic laminate. Scott Lewit reports that the infusion process takes a few hours to complete using this approach. The rudder is then cured at room temperature.
Lewit calls recirculation a brute-force approach to RTM or VARTM. Forced out of the vacuum ports, the resin is recirculated after collection in the traps. Air pockets are removed as the part is constantly flushed with resin. This process is more forgiving than RTM or VARTM, and results in excellent wet-out, according to Lewit.
Even these variants on infusion processes have their limitations. Structural Composites and its partner Compsys Inc. (W. Melbourne, FL) want to provide a unitized build structure for the boating industry. They do this via a patented closedmolding process called Prisma that can create preform reinforcements for use in a unitized structure.
Preforms with a foam core surrounded by dry reinforcing fabric are made in closed molds. These preforms, in the form of stringers, bulkheads, and frames, are then placed into a boat mold and saturated with resin in either an open-molding or closed-molding process.
The preform frames enable boat builders to move from sandwich construction to simpler single-skin construction, according to Lewit. “By replacing the old stringer and bulkhead system with these new structural preforms, we are now taking 10–20 man-hr down to 1–3 man-hr. On cockpit soles, we are taking builders back to single-skin construction. This saves cost, reduces warranty claims, and improves toughness,” asserts Lewit.
One example of combined innovation in materials and resins is the Priform system from Cytec Engineered Materials (Tempe, AZ). They rethought the infusion process, and added toughness in the form of soluble fibers into the dry preform fabric, rather than as an additive in the resin. (As mentioned earlier, toughness makes resin systems more viscous.)
This processing concept resolves the difficulties in finding a balance between toughening properties and the viscosity requirements of a closed mold process. When the dry preform is infused with their Priform 977-20 resin system and heat cured, the toughening fibers dissolve into the resin.
This system is based on their 977-2 resin system, which is qualified for use in numerous primary structure aerospace applications where damage resistance is required. Fisher Advanced Components AG (Ried, Austria) uses these materials in an RTM process to create a composite spoiler hinge assembly for Airbus Industrie’s A330-300 and A340-500/600. The hinge assembly is noted for both its structural application and the complexity of the part. Cytec reports that the Priform system is also being qualified using RTM, VARTM, and other resin infusion processes on additional primary structure applications for commercial aerospace programs.
It would seem that hand layup and pre-impregnated fabrics will remain a mainstay of making composite parts, but innovations like lower tooling costs will advance the uses of infusion processes. “I think the aerospace industry recognizes the advantages of using liquid infusion molding processes like RTM and VARTM,” says EDO’s Garrett Sharpless.
This article was first published in the April 2007 edition of Manufacturing Engineering magazine.
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