About ten years ago, when resin transfer molding (RTM) began to transition to high-pressure RTM (hp-rtm), it shortened the production cycle of composite parts from several hours to less than 2 minutes, which was highly praised in the automotive industry. Traditional low-pressure RTM (lp-rtm) usually uses 10-20 bar injection pressure, while hp-rtm uses 30-120 bar injection pressure. The aviation industry has a long history of using traditional RTM, including the containment case used to produce thousands of carbon fiber reinforced plastic (CFRP) fan blades and commercial aircraft engines. Airbus has even made a 7m long integral multi beam flap prototype for the Airbus A320 with RTM. However, it is rarely heard that hp-rtm will be used in the aviation industry. Can we convert the process that takes hours to the process that can fully auto form composite parts in minutes? Several major participants said it was possible.
ALPEX Technology Co., Ltd. of A330 spoiler hinge RTM parts in tooling is an innovative tooling system supplier for serialization production of composite parts. Ten years ago, an EU project funded ALPEX to use RTM to make fuselage frames. At that time, it was predicted that many parts of the A350 would be made of RTM, but this was not achieved. ALPEX, however, has been working on the technology and, as a supplier to the automotive and aerospace industries, has begun to study how to combine the processes used in the two industries. This concept will use Aerospace certified Hershey rtm6 epoxy, plus best practices in automotive tooling and automated injection systems to shorten production cycles. In 2013-2015, the Austrian government funded the "take off" program, with partners ALPEX, Airbus helicopter company, equipment supplier Klaus mafi, aerospace and advanced composite materials (AAC) and resin supplier Hirsch as partners. The program demonstrated that hp-rtm reduced the cost of A350 door frame by 30%. The program developed technologies including the use of two-component (2k) resin systems, intelligent injection and cure monitoring via in mold sensors, and the ability to further reduce part production cycles. ALPEX found that hp-rtm technology has the potential to help solve current CFRP production problems.
This view is also supported by the composite technology center (CTC) of Airbus operations limited, which has been developing automated RTM for ten years, first using lp-rtm and then transitioning to hp-rtm about five years ago. Hp-rtm's high-speed mixing process can use high reactive active resin to solve the bottleneck of injection and curing. CTC has developed a process that can produce complex shape parts with the size of 1-2m in 20 minutes to meet the requirements of Aerospace (for example, the volume content of fiber is 60%, and the porosity is less than 2%). In 2018, Airbus plans to transition many parts to hp-rtm, so as to increase the production speed of A320 aircraft. ALPEX is developing specific processes for these products, and is trying to use them for mass production.
1、 Hp-rtm A350 door frame passed the "Sparta" project under the "take off" plan. Airbus helicopter selected A350 door frame and let ALPEX complete the development. When opening the hatch to enter A350, you will see this kind of frame, which is about 2m high, 200-250mm wide and 8-10mm thick. It has complex shape and design features, and can meet various requirements of hatch assembly as the main structure. A350 hatch consists of 14 parts made by Airbus helicopter using lp-rtm. The box selected for "Sparta" project is a very challenging hatch part, and also a good demonstration part, because hp-rtm can be directly compared with standard aviation lp-rtm.
Tooling verification part of "Sparta" project
ALPEX has designed and manufactured lp-rtm tooling for A350 cabin door frame production. For the design and construction of hp-rtm tooling, it will adopt a more automotive way of thinking. ALPEX's goal is to automate the production of aerospace composite parts in the form of automobiles, which will be necessary if Airbus intends to manufacture a new generation of A320 series with more composite parts at the rate of 60 to 100 aircraft per month.
ALPEX has been actively trying to simplify the parts and process of the mould to reduce the production cycle since the production of the mould for BMW, while Airbus's problem comes from the complexity in the previous aviation industry. For example, when the project starts, Airbus helicopters are concerned about how many parts this tooling will have, while the current lp-rtm hatch products ensure that complex sets of tooling are required. However, the hp-rtm tooling developed by ALPEX will only have one upper mold and one lower mold. So ALPEX started with a simple mold design, and then added features to make the process more powerful and flexible - for example, a sealing system for mass production of automobiles, and the integration of all sensors into the mold.
2、 The preforms and processes currently used for A350 hatch frames use woven materials and require multiple compaction steps, so a three-day production cycle is required. ALPEX decided to switch to NCF, which greatly reduced the price and production cycle. The project finally demonstrated some parts made of Woven Preforms certified by hatch, but it can be seen that the composite industry outside the aerospace industry has changed, and SGL and other companies have moved from woven fabrics to banded and low-cost forms (such as NCF). With regard to injection point and fiber scour, hp-rtm tool has only one injection point, but there is no fiber scour, because the preform is fixed in the mold and is very stable.
Another key point is that ALPEX did not choose the single component (IK) rtm6 currently used by Airbus helicopters, but chose the 2K resin system, believing that the latter can have higher reactivity, more suitable for hp-rtm, and can save costs, because it is not a premixed and unnecessary dangerous material shipment. However, it does require a higher temperature of 180 ° C, which is the most challenging part, because in the past, it was more used to RTM process at 120 ° C.
Klaus mafi installed the completed tooling in the rimstar compaction hp-rtm system of its Munich lightweight technology center, which has the functions of automation, high pressure (up to 80 bar / 1100psi) mixing, metering and injection. Then, 20 demonstration parts are made with the system. The baseline process includes: injection at 80 ° C, heating up to 120 ° C, holding for one hour, heating up to 180 ° C and holding for 90 min curing time, cooling down (speed 2 ° C / min) and demoulding. Among them, injection takes only 20 seconds, but the whole forming cycle is about 4 hours.
Impregnation and injection tooling of MS-21 wing
3、 The curing time of the door frame in the production cycle and cost "Sparta" project is still very long, which is driven by Hershey rtm6 resin formula and Airbus certified process. ALPEX can achieve a curing time of 30min at 180 ° C, and then conduct additional post curing under vacuum to eliminate thermal stress and ensure mechanical properties. The performance is actually quite good, equivalent to lp-rtm parts.
The project proves that hp-rtm process can be used to realize the automation production of higher parts batch. The process can inject resin into the mold and cure it, and then post cure it in the molding machine, which requires additional tooling combination, but only one molding and injection unit. With this system, the cost of parts can be reduced by 700 euros, which makes it easy to purchase equipment, and the production speed of 500-1000 parts / year can be achieved, which is not very high.
4、 In addition to the tooling and process, the hp-rtm hatch project also includes the innovative use of fiber forms. CTC has achieved great success in using the woven fabrics certified by Airbus, and continues to use NCF and more innovative fabrics. The latter includes the placement of dry fibers and the insertion of stretched tow layers to increase toughness, but there are problems such as tight accumulation of fibers and plies, poor permeability and resin penetration. The project also developed solutions to this problem, such as NCF using special fibers to improve permeability.
As for the pressure of hp-rtm, it should be said that the process is driven more by the mixing technology of polyurethane industry than by the higher process pressure. Hp-rtm is only used to mix high active resin well, and the pressure in the mold may be higher, but this is secondary to the effect of rapid injection. For the automobile parts with a production cycle of 2 min, resin must penetrate into the dry preform faster than the aviation parts. The 15 minute curing time of this project provides more time for resin penetration, which also reduces the pressure in the mold.
At the same time, the pressing tonnage depends on the part size. The effect of manufacturing parts with the size of 2m is very good (the pressure is about 1500 tons), but the part with the size of 4-6m needs to calculate the commercial feasibility, because the investment of the molding machine is very high. Liu Yawei introduced CTC's 1.5mr0.5m CFRP rib in the JEC exhibition in 2018 in his last "c-rtm process, the star of hope of future aviation structure" (the original link is Liu Yawei's | c-rtm process, the hope of future aviation low-cost structure), which represents that the process can manufacture any possible Aerospace main structure. At the end of 2019, CTC obtained a higher tonnage molding machine.
Main structure of wing rib shown by CTC
5、 2K resin and sensor based quality assurance in the past, all RTM aircraft structures produced by Airbus helicopter only use the 1K system mixed by the supplier, and then the supplier shall certify the mixed resin, and bear the responsibility to ensure that the resin meets the qualification requirements. Now they are considering 2K system, but the challenge is to ensure the mixing quality. The premixed 1K system needs to be refrigerated, transported and stored at low temperature to prevent reaction before parts are manufactured. Therefore, the advantages of 2K system include eliminating the cost of transportation and storage and higher reaction activity, so that the forming can be carried out faster. However, one problem is that quality assurance (QA) will be transferred from the resin supplier to the part manufacturer, and this quality inspection requires testing the mix ratio and the quality of the mixed resin. Both are required by Airbus to identify new materials, so this means that the two components are measured before mixing and the resin is measured after mixing. At present, only sensors are installed before the mixing head, which can show how much resin and hardener are measured, but this is not enough to complete the qualification.
At the same time, ALPEX is working to develop an online quality system to verify the 100% correct mix of resins required by Airbus. ALPEX and Netzsch are working together on the in mold sensor application. Netzsch and Qishile have established a partnership to make this new technology come out, which can monitor temperature, pressure and resin curing through a sensor (in the mold).
ALPEX wants to use in mold sensors, while the hybrid / injection equipment supplier says the sensors should be in their system, which is right, but the latter also needs to be improved. In the automobile hp-rtm, the resin mixing ratio is measured in the whole forming process, so this is an average value. However, the aviation industry needs to make this measurement at each discrete point in the process so that it can know that the mixing ratio is always correct, even in the last half second of the injection process. Therefore, a better time resolution of the measurement system is needed, and sensors may be needed before, inside and in the mold of the mixing head. Airbus helicopters are developing QA at the same time as the evaluation of 2K resin in aerospace applications, and working with machine tool suppliers to ensure that