Resin Transfer Molding: A New Direction for Low-Cost Manufacturing Of Aviation Composites

 

 

The aviation industry continues to be optimistic about the next generation of narrow-body aircraft, electric aircraft and flying car markets, and its enthusiasm for high-speed, low-cost out-of-autoclave (OOA) manufacturing technology for composite primary structures continues to rise. Therefore, the resin transfer molding (RTM) process will undoubtedly be more widely used in the future. Airbus has even used RTM to make a 7-meter-long integral multi-beam flap prototype for the Airbus A320. However, the disadvantages of traditional RTM are also very obvious – the speed is too slow, and the resin injection and curing time of typical frames, beams, ribs and other parts are measured in hours.

 

On the other hand, as more and more cars begin to use composite materials and choose the RTM process, this long production cycle is unacceptable to manufacturers such as BMW. To this end, the industry has successively developed processes such as high-pressure RTM and compression RTM (HP-RTM/C-RTM), which have compressed the injection and curing time from several hours to several minutes.

Characteristics Of High-Pressure And Compression RTM

 

1.HP-RTM: High Pressure and Resin Mixing For the injection and curing speed of resin in the RTM process, pressure is a key factor. Conventional RTM usually uses an injection pressure of 10-20 bar, while HP-RTM can reach 150 bar in the injection head and 30-120 bar in the mold, which can at least double the injection and curing speed.

 

2.C-RTM: Gap Injection and Low Pressure C-RTM uses a resin injection method called gap injection, in which the mold is initially partially closed, leaving a gap between the preform and the upper surface of the mold, and then a vacuum is drawn and a precise dose of resin is injected. The resin first floats on the preform, and then a compression stroke is performed – the molding machine quickly closes the gap in the mold, forcing the resin to quickly penetrate and wet the entire preform in the Z direction under the action of mechanical compression force. Gap injection was funded by the Air Force Research Laboratory in 1990, but its subsequent application in the automotive industry has been more successful.

 

In contrast to HP-RTM, the interstitial injection of the C-RTM process can inject high-viscosity resins at low pressure. Due to the additional compression stroke of the molding press, the penetration speed of the resin in the preform is much faster than that of traditional RTM, even within 5 minutes. At the same time, there is no need for a large clamping force to prevent resin flushing, and the mold pressure is only 6 bars, so a low-tonnage molding press can be used, thereby reducing investment costs.

Progress In Aviation Research And Development Of The Two RTM Processes

 

The European Union and Airbus have always been active advocates of OOA technology in the civil aviation field. They have also funded and implemented several HP-RTM and C-RTM research and development projects for aviation main structures, committed to improving the technical/manufacturing maturity of the entire process.

 

The “Spartan” project produced 20 demonstration parts. The curing time of these door frames is driven by the resin formulation and the Airbus-certified process. It is still long, but the entire molding cycle is far less than 4 hours. Preforms can now be removed after an initial cure of only 30 minutes at 180°C and additional post-cure under vacuum to eliminate thermal stress and ensure mechanical properties.

 

Airbus CTC developed a 1.5-meter-long and 0.5-meter-wide wing rib using the HP-RTM process and exhibited it in 2018, with a cure time of 15 minutes and a forming cycle of 20 minutes. CTC found that if it is only for better mixing of highly reactive resins, the mold pressure does not need to be as high as in the automotive industry, because automotive parts are often formed within 2 minutes, requiring the resin to quickly penetrate the entire preform, so in addition to high injection pressure, high mold pressure is also required.

 

In 2016, the EU Clean Sky 2 program launched the “Optimized Composite Structures for Small Aircraft” project in the “More Affordable Composite Structures” work package. One of the goals is to reduce the production cost of small aircraft such as regional aircraft through integrated structures and automated liquid forming methods. The project developed a three-dimensional preforming unit, which increased speed, improved process repeatability and quality, and reduced costs.

 

In terms of resin, for a 0.7m long and 0.2m wide wing rib, the injection time is reduced from 40 minutes to 5 minutes; for an I-shaped longitudinal beam of 0.9m long and 0.15m high, the injection time is reduced from 1 hour to less than 5 minutes. The reduction in injection time will be more obvious for large parts (such as complete wing skins or helicopter rotor blades). The curing time is 90 minutes, which is the longest link in the entire production. Compared with CTC’s HP-RTM wing ribs, the forming cycle of C-RTM wings is much longer.

 

Future Application Prospects

HP-RTM‘s rapid injection and curing, C-RTM‘s rapid and low-cost injection, and the automation of the entire process including preforming have greatly compressed the forming cycle, and therefore have been favored by the automotive and aviation industries. At present, it seems that the aviation industry is starting with frame, beam, and rib structures with a scale of 1 to 2 meters, and exploring their introduction into the manufacturing of composite main structures.

 

For the next generation of narrow-body aircraft and electric aircraft, the large-scale use of composite materials is almost inevitable, which gives HP-RTM and C-RTM a broader market prospect. In particular, the mature application of the automotive industry means that future products such as helicopters, tilt-rotor aircraft, flying cars and vertical take-off and landing flying electric vehicles with similar or even the same frame structure will be more likely to adopt these two processes, thereby greatly expanding their application.