RTM Process to Reduce Costs

 

Airbus Bremen aims to replace dozens of prepreg parts and assembly operations with a single-piece, multi-spar composite flap.

 

 

Reducing manufacturing costs through RTM: The Composite Multi-Flap (CMF) project at Airbus Bremen simplifies the production of 7.4-meter-long outboard flaps for narrow-body aircraft by integrating 26 individual carbon prepreg parts (see Figure 1, below) into a single-shot, unitized structure made through resin transfer molding (RTM).

 

Figure 1 – Conventional flap structure: The 26 prepreg parts used in a conventional CFRP flap must be individually layered, autoclaved, machined and non-destructively inspected before being shipped to Bremen for multiple assembly operations.

 

 

Figure 2 – Interim CMF design changes: As shown in these photos, the initial CMF design replaced stringers and ribs with four to five braided boxes. This was subsequently modified to five “double-T” (I-beam) beams (shown on the left) using 5-satin to improve fiber orientation accuracy and process scalability.

 

 

Step 1: Full-scale demonstrator fabrication begins with cutting dry fabric (or prepreg for SQRTM parts).

 

 

Step 2: Use a laser projection system to precisely stack the cut pieces on the transport board

 

 

Step 3: The curved skin preforms, including the bottom skin, leading edge and top skin, are stacked on a sheet fixture and unpacked using a reusable vacuum bag.

 

 

Step 4: Similarly, the plies of the beam precast are laid out into a “double T” mandrel and broken down.

 

 

Step 5a: Here the lower RTM tool is shown prepared and ready for the preform layup.

 

 

Step 5b: Load the “Double T” spar preform into the lower tool.

 

 

Step 5c: The curved skin preforms (bottom skin, leading edge and top skin) are then loaded into the lower tool.

 

 

Step 6: The RTM tool is closed and then heated to 100°C before being injected with HexFlow RTM 6 resin.

 

 

Step 7: Heat the injected part to 180°C, cure for 2 hours, then demould while hot (see next photo for results)

 

 

Cost savings without trade-offs: The finished CMF, as shown, untrimmed (front), trimmed, painted and fully equipped (back), has been proven (TRL 6) to operate without the added weight of a fully equipped flap. In this case, parts logistics and assembly operations are significantly reduced, with the potential to save more than 20% of costs.

 

The goal of the CMF-Composite Multispar Flap project, led by Airbus Bremen (Bremen, Germany), is to simplify the production of complex 7.4-meter-long outer wing flaps for narrow-body commercial aircraft. Airbus Bremen is responsible for the design and manufacture of flaps, spoilers and other high-lift systems, which are movable wing components that optimize takeoff and landing.

 

The CMF project was funded in 2005 and a multifunctional team led by Airbus engineer Dr. York Roth, working closely with partners Radius Engineering (Salt Lake City, Utah, USA) and Faserinstituit Bremen (FIBRE Bremen, Germany), began design studies and feasibility test. The team targeted the A320 wing’s outer flap, which in the current production version consists of 26 individual carbon fiber reinforced polymer (CFRP) parts, including two skins, leading edge parts, and multiple ribs and spars. .

 

All these prepreg parts must be individually layered, autoclaved, machined and non-destructively inspected before being shipped to Bremen for an equally intensive assembly process (see Figure 1, left). The metal end ribs and load frames – the latter enabling attachment and load transfer to the wing – must be installed in the assembly jig, followed by installation of the ribs and skin stringer panels. Technicians drill holes for the rivets, then take apart the composite and aluminum elements, clear the drilled holes of debris and replace all parts into fixtures. Multiple drilling and riveting steps followed, followed by manual installation of the composite leading edge and metal trailing edge components.

 

The CMF alternative is envisioned as a multi-section torsion box that integrates all 26 prepreg components, including the leading edge, into a unitized structure made using out-of-autoclave (OOA) resin transfer molding (RTM) technology middle. Dr. Stefan Bauer, architect of industrial activities at Airbus Bremen, said: “CMF can not only eliminate a large number of assembly operations, but also the high lead times and complex process chains of many pre-preg components.” The challenge will be how to design the parts and pass them through A single injection and cure creates this now closed structure with acceptable porosity while maintaining current procedural tolerances.

 

“The final geometry had to be precise because of the need to match the carrier to the flap track, to connect to the wing, and to meet aerodynamic requirements,” explains Mohamed Attia, manufacturing engineer at Airbus Bremen. Airbus Bremen accepted this challenge due to the potential cost savings of more than 20% without increasing the weight of the entire painted and equipped flap.

 

Refining the Design

 

“The original idea was to replace all the prepreg stringers and ribs with four or five braided boxes, and then the upper and lower skins,” Bauer recalls.

 

“One of the main challenges was that the section was over seven meters long,” Bauer notes. “We had never worked with RTM on such a large scale before. However, Radius Engineering was an excellent partner. We started with small parts and worked our way up.” A 1.5-meter section was built in 2007 and statically tested to demonstrate the design and process, achieving a Technology Readiness Level (TRL) of 4.

 

By 2010, the braided box idea had been abandoned. Bauer explains that the braided “socks” used for the flap preform are of constant diameter, but the width of the flap is not. Instead, they are highly tapered, and so, too, are the box beams. This means that the ±45° fibers in the braid are only optimally oriented at the braid diameter, which is roughly half the length of the box beam. At the wide (≈200 mm) and narrow (≈80 mm) ends, the fiber orientation will not actually be ±45°.

 

According to Attia, this less-than-ideal fiber orientation requires a greater reduction in performance, and therefore increased weight, to meet stress requirements. “Dry braids can easily go askew,” he adds. This further complicates the optimal fiber alignment. The scalability of the manufacturing process to industrial production rates also needs to be considered. For braided box girders, original tooling is required early in the process to provide the forming mandrel for the preform.

 

If a “double-T” (also known as an I-beam) spar is used, a pseudo-mandrel can be used for preforming, and the RTM mandrel is only required during the part injection and curing cycle. With the above in mind, the design was changed to an I-beam spar with a conventional 5-wire satin fabric.

 

Making a Full-Size Flap

 

The new I-beam-based design was used to produce a full-size demonstrator in 2010 and eventually reached TRL5. Because a rectangular shape is easier to manufacture than an actual tapered flap, this iteration of CMF was made using a 7.8-meter-long aluminum mandrel and RTM tooling, then machined to its final dimensions. Both the mandrel and the outer surface tooling required extensive engineering. The mandrel is divided into three parts for easy disassembly. “For thick parts, RTM makes it easy to meet thickness tolerances,” Bauer said. But CMF is relatively thin, 2 mm to 5 mm in the reinforced areas. Typically, thin parts like this are made using prepreg and vacuum bagging, where, as Bauer explains, “you’re mostly subject to tolerances on the raw material resin content and some due to a single tool face.” But changing CMF to RTM resulted in part thickness determined entirely by the two milled tool faces.

 

“Tooling accuracy has a twofold impact on part thickness,” Bauer notes, adding, “Tooling has to be very accurate to ensure that fiber volume content is within design tolerance.” Because part thickness changes resin content, he notes, “In some areas, if your tooling is even less than 0.2 mm off, you’re already out of tolerance on fiber volume.” Part production begins with the fabrication of the skin preform, which is made by laying down dry fabric on a transport table using a laser projection system (steps 1 and 2, left).

 

This 5-satin fabric is made with dry carbon fiber from Hexcel (Stamford, CT, USA). The curved skin preform, which includes the bottom skin, leading edge, and top skin, is then stacked on a sheet fixture, and the spar preform is similarly stacked in a “double-T” mandrel (steps 3 and 4).

 

In both cases, a reusable vacuum bag is used to break apart and form the preform stack. Bauer acknowledges that there are a lot of handling and preforming steps, the latter of which requires a lot of attention to accuracy. “The edge positioning of the ply is critical to achieving tolerance,” he notes. “But you never know if the preform is going to be exactly where it is inside the tool. Is it within tolerance? So we developed an innovative concept and engineering design to deal with this.” The skin and I-beam spar preforms are then paired, placed and encapsulated in the RTM tool designed and built by Radius Engineering (steps 5-7).

 

The tool, preform and resin are preheated to about 100°C, the resin is injected, and the composite is then brought up to a cure temperature of 180°C. Hexcel’s HexFlow RTM6 resin was used because it is the primary RTM epoxy currently qualified for Airbus structures. “This part is so complex that we didn’t want to control the flow of the resin like an infusion,” Bauer says. The process had to be stable and reliable. Porosity issues were addressed by controlling vacuum and pressure.

 

“Of course, the mold design is important,” he notes. “The temperature is chosen so that no outgassing occurs during processing, which could cause porosity.” Notably, only one injection point and one exit port were used for such a large part. This is unusual, as the typical setup for many RTM parts employs multiple injection and exit points. “But the problem with that is that you have to control them,” counters Bauer. “The more points you have to control, the greater the risk, due to leaks and flow issues.” With CMF, he adds, injection occurs at just one point, but then moves in a straight line within the tool.

 

“It’s a very simple but robust process,” says Bauer. “It has to be, because if we lose a part, it’s not just one rib, it’s a whole 7-meter-long flap.” After a 2-hour cure cycle, the finished part is hot-demolded while still above 100°C, as cooling on the tool can cause part removal issues. Using aluminum tools, especially in the reinforced load introduction areas of the part, can result in undercut damage unless it is demolded before the metal shrinks.

 

A final challenge is that the flap structure is a closed box. This raises the question of how to inspect the final structure, explains Bauer. The CMF team began working with Bremen-based thyssenkrupp System Engineering to inspect the removed unitized parts using a phased array ultrasonic (UT-ultrasonic) nondestructive testing system that uses a single-sided channel and a water film as a coupling agent to the part surface. thyssenkrupp helped develop the inspection technology and patented nondestructive testing equipment, which uses spring loading to help focus the phased array ultrasonics on the contours of the part and look inward at the spar under the skin and the radius between the spar and the skin.

 

In fact, the system uses two heads—one that moves along the spar and the other that checks the radius between the spar and the skin—to speed things up and ensure a thorough inspection. “We also use different end effectors to look and inspect these areas in one step,” Bauer explains. The team also discussed repair methods for flaps that have been damaged in service, including inspection, damage removal, and patching techniques.

 

The second issue raised by the closed-box structure is how to achieve the connection and load transfer of the wing. Previously, this was achieved using metal load frames and end ribs. Although the new combined structure significantly reduces mechanical fasteners, some fasteners are still required for the metal load transfer components and metal trailing edge. Attia points out that the CMF’s lower fastener count not only reduces fatigue risk, “it also reduces fiber damage caused by machining.” He adds, “Drilling holes in highly optimized composite laminates is very inefficient in terms of design and manufacturing.” This is one reason why the Bremen R&D team continues to push for greater part integration.

 

Alternative designs for load attachment points use carbon fiber reinforcements, enabling integration and conversion from metal to composite. The originally separate metal trailing edge has also been integrated, converted to a sandwich structure using Evonik (Essen, Germany) closed-cell foam and carbon fiber skins, layered and RTM molded with the rest of the CMF.

 

“The original CMF design was not fully optimized, as it could have been in a completely new aircraft design,” explains Attia. “This is because the solution was developed as a retrofit design for an in-service A320 with design constraints, especially in the load introduction area.” The team believes that it can increase the cost savings of the CMF to 30% using these integrated CFRP trailing edges and load transfer components.

 

Ready for Industrialization

 

The full-scale demonstrator was a success and was shown at the 2013 JEC Europe show in Paris, France. Bauer cautions that the manufacturing steps, shown here, have not yet been industrialized. Once they are, many of them—for example, fabric layup, unpacking, placing the tool into the RTM mold—will be automated using robots.

 

When the process goes into production, will there be potential bonding issues when so many previously separate parts are co-cured together in one RTM cycle? Bauer says no. “The problem with this delamination is typical of prepreg,” he says. “The problem with RTM is porosity.” As is typical for most aerospace structures, the composite wing flap must have a porosity of less than 1 percent. With CMF, the risk is the large area of ​​the part and the porosity that can exist within the radius between the skin and the spar, Bauer says.

 

“We have achieved good lamination quality along the entire length of the mast, as validated by NDI,” he said. “In terms of the radius, this is not easy to check for porosity, but we deal with this in the inspection methods we have developed in combination with the knockdown factors used in the design.” “The CMF technology is ready for industrialization,” said Bauer. “A year ago, it passed the TRL 6 review, which means it has moved out of the R&D phase and is ready for adoption on future platforms.”

 

He added that the new method could also be used for other aircraft components. Looking at the production floor at Airbus Bremen, and all the parts in the current assembly process for the A320 flap, what will happen to all these operations? Bauer replied that they will be reduced and mainly replaced by lay-up preforming. Is that the goal? Both Bauer and Attia responded that the only future for commercial aircraft composite production is to become increasingly efficient.

 

CMF project’s process combines prepreg with RTMTo achieve its goal of streamlining the production of narrowbody aircraft outboard wing flaps, the Composite Multi-Flap (CMF) project, led by Airbus Bremen (Bremen, Germany), demonstrated the integration of 26 carbon fiber reinforced polymer (CFRP) components into a one-piece structure that was injected and cured using a single shot resin transfer molding (RTM) process.

 

In addition to demonstrating the production of the new CMF using a conventional RTM process using dry fabric and liquid resin, the Airbus Bremen team also demonstrated the manufacture of the part using the Same Qualified Resin Transfer Molding (SQRTM) process developed by Radius Engineering.

 

SQRTM uses a prepreg layup instead of a dry fabric preform, and the RTM process injects the same resin used in the prepreg, but in liquid form. The benefit of this approach is that it avoids any need to qualify a new material. Why SQRTM? Because the CMF concept can be applied to other structures, such as the inboard wing flaps.

 

However, as Dr. Stefan Bauer, Industrial Activities Architect at Airbus Bremen, explains, the inboard flaps have to withstand the impact of runway debris and therefore require a toughened resin system. “We did not have a qualified toughened resin system for RTM,” he adds, “so the feasibility of SQRTM was also demonstrated on a 7.8-meter full-scale component in the CMF project with good results. The concept is basically the same.” Bauer says both the dry fabric RTM and SQRTM process variants are ready for industrialization.