Thanks to RTM & SQRTM aircraft wings exit autoclave, what is next?

Back in 1989, Radius Engineering published: “a versatile Resin Transfer Moulding (RTM) process for producing high quality aerospace composite laminates has been demonstrated. Low-void-content components have been fabricated with a standard high temperature prepregging epoxy resin” [1]. The study introduced that a piston connected to a closed mould can do what an autoclave will never do, to give independent control to the 6 key process parameters that drive composite material quality: time, temperature, vacuum, ply thickness, fibre bed pressure, resin pressure [2]. This paper is a masterpiece because advanced process and flow simulation, validated with 100 flow sensors on a mould, were developed to fix the root cause of porosity, not to develop a flow simulation software.

A vacuum tight process was defined, associated moulds and equipment were specified, and since, hundreds of moulds were successfully filled at first part without any flow simulation. The specified equipment and moulds were for aerospace market qualification, with its own quality requirements, and Radius Engineering should have called the process “Qualified Resin Transfer Moulding”, QRTM. The autoclave qualification is reproduced out-of-autoclave (OOA). A lot of OOA processes were developed with tens of acronyms, but most lost focus on equivalence to autoclave and aerospace quality. Many do not control key process parameters well, making process qualification difficult.

Finding a way out of autoclave

Autoclave is in fact an absurd challenge: to heat and to conform to shape a liquid plastic with a pressurised gas. A piston for filling a mould cavity with liquid resin, and controlling pressure is more logical and industrial. But RTM faced the limitation of prepreg resins that, along years, became more viscous with addition of thermoplastic for impact resistance. The logic at Radius Engineering was to use the pre-impregnated laminate as preform in the RTM mould, injecting with a piston a small quantity of the prepreg resin around the laminate to directly control resin pressure within the laminate. This process, equivalent to autoclave, was called “Same Qualified Resin Transfer Moulding”, SQRTM. Today, SQRTM is qualified by all major OEMs and the exit way out of autoclave is clearly indicated. A QRTM process could be associated to SQRTM for a clear distinction from other infusion processes.

Aeronautics industry is facing the opposite risk. The risk with composites is that the material and parts are produced at the same time: with composites, part producers become material fabricants with the risk to produce zero quality material. This is why curing pre-impregnated laminate was a cultural revolution for the aeronautics metallic industry. With RTM, the part producer takes further responsibility of impregnating the fibres. New risks emerge with handling and degassing of a self-reactive product, the resin. Equipment designed by Radius Engineering addressed these risks properly, but poor practice remained in place, like insufficient hot resin degassing, and unsafe equipment such as the pressure pot continued to be used. For intensive production, like engine platforms or fan blades, the right equipment quickly became dominant.

Maturing further RTM and SQRTM

A close partnership was established with Radius Engineering to reproduce their business model in Europe, to develop processes for delivering optimal equipment. Driven by engineers coming from tier ones, the focus of Coexpair is to bring risk level of RTM and SQRTM as acceptable to aerostructure in balance of proven cost savings figures. Coexpair R&D for SQRTM process is scienced based and advanced sensors for live monitoring of process are used. Optical fibres with 40 Bragg’s sensors measure temperatures and detect fibres movement. Thin pressure pads measure pressure within mould at 256 locations. For each resin system, a full understanding of SQRTM resin flow, viscosity variation and resin retraction at gel point is developed. Visco-elastic simulation is used to optimise moulds. Process parameters databases with associated analysis software are developed.

Coexpair coordinates R&D building blocks, from coupon to demonstrator. IMS&CPS project, FP7 funded, was a first illustration with coupon tests demonstrating SQRTM equivalence to autoclave for HexplyTM M21 and fabrication of 2 demonstrators (typical A350 nose landing gear door)[5]. IMS&CPS demonstrators did not present porosity and were close tolerances. The potential to mould net-edge stiffeners was discovered and became a standard for SQRTM, as example on E2 flaps from Sonaca. After this, Airbus proposed Coexpair to re-engineer the actual A320 nose landing gear door for SQRTM, removing the sandwich. In 3 years, within Aflonext project, Coexpair reached the target of flight test qualification level for SQRTM. Safran Aerostructure provided key engineering and manufacturing resources.

Serving customer by developing and industrialising SQRTM solutions

It started in 2008 with SABCA and a first combination of ATL and SQRTM. Applied to a highly integrated part, it was finalist of JEC Awards [4]. Sonaca introduced SQRTM in serial production with the Embraer E2 flaps. At JEC 2019, Stelia, now Airbus Atlantic, presented a section of A220 Vertical Tail Plane made of Hexply M21E and cured by SQRTM. Integration features net-edge mouse holes and stiffeners. The 2020 crisis did not stop SQRTM progression. At Paris Airshow 2023, Turkish Aerospace presented a multicell section of aileron made of Hexply M21E. The first combination of movable optimal architecture and unidirectional tape.

In composites, process and equipment is a single joint solution. Our RTM / SQRTM workcell is not a press but a specific piece of equipment that controls the shape of the moulds from the injection stage to the point that the composite gels and reaches its final dimensions. Among the 6 key process parameters, the workcell controls the fibre bed pressure (the pressure on the solid part of laminate, the carbon fibres). Mould kinematics is key. This pressure prevents fibres distortion and is independent of the resin pressure controlled by the piston (the pressure on the liquid part of laminate, the resin). This independence in pressure control is fundamental to understanding why RTM or SQRTM work better with a high volume fraction of fibres (60% or more). Our workcell solution is elegant and Coexpair has scaled it to the largest RTM / SQRTM equipment ever done for aerospace (36 m, 4,000 tons clamping pressure, 1,000 Amps). To succeed, advanced multi-physics finite element models were developed at Coexpair.

A cost advantage and performant TCO

Between autoclave and RTM or SQRTM, the cost of investment in tools shall not be limited to a mould comparison. Autoclave’s cascade of tools also includes preforming, trimming and assembly jigs that can be strongly simplified or eliminated thanks to RTM or SQRTM. This brings a cost advantage of 20% to 30% in favor of RTM or SQRTM.

Total cost of ownership of moulds includes capital to invest, maintenance cost, and replacement cost. It also involves a rejection rate that could vary drastically with mould quality. Program risks, lead time and rampups shall be addressed correctly. Coexpair built a cost model that includes mould material options and Coexpair aluminium moulds were selected by Spirit AeroSystems for A320 RTM spoilers. Automation would have been problematic with steel moulds. Invar was not an option.

Thermal expansion mismatch of mould material versus carbon fibre part cured at 180°C shall not be the driver of mould material selection. Heat conductivity, accuracy, lead time, cost, repairability, and weight are more important factors. Coexpair published [6] that an 8-meter SQRTM aluminium mould featuring pad-ups and drop-offs, representative of a flap skin, can cure unidirectional laminate with perfect quality. The mould expands by 3 cm when part gelled at 180°C, and the laminate conforms to this expansion according to predictions done at mould design stage. The aluminium moulds are coated with 50 μm of hard ceramic, comparable to steel for hardness and wear. The optimised treatment is proprietary, and supplier shops are qualified according to Coexpair EN9100 quality system. Test coupons follow each surface treatment. For mould usage, optimal release agent was selected, and repair solutions were developed. R&D continue on anti-adhesive coating. Moulding of large RTM and SQRTM parts is a long story. End of the nineties, Radius Engineering studied RTM wing panel (Airbus Tango Project) and I lead, for Sonaca, a study on large RTM fuselage panel (ESA reusable launcher). In 2014, Dr Herrmann presented at CTC a white paper about SQRTM manufacturing for spars and skins of a wing, Aircraft A320 size. In 2017, for high production rate, Coexpair proposed to Airbus combinations of Iso-Thermal RTM process and original equipment.

For wing spars, Coexpair worked with Airbus and FidamC. The Walloon Region supported this research. The project was a joint innovation about process and equipment, targeting industrial solutions. Aluminium mould thickness was minimised because it follows the spar shape, including its kink. The workcell matches mould “V” shape and provides it with stiffness. As jointly published by Airbus, FidamC and Coexpair [7] only one resin inlet point is used, vacuum is close to 1 mbar, preform follows the 5 cm mould expansion and part accuracy is within a few tens of millimeters. Environmental impact vs autoclave is a significative division of energy consumption and consumable trash. AFP/SQRTM process is a straight-forward solution with this equipment, and it may be considered for running spars production. For large and complex curved RTM skin panels, Coexpair worked with Airbus and CTC. Again, the Walloon Region supported the research. To limit the mould mass to heat-up to a realistic target, 40 tons vs 300 tons for a full box mould, the solution is double thin shells suspended within a workcell (press function) through an interface that keep the shells free to expand in the 3 directions. The system limits mould distortion to a few tens of millimeters under 7 bars. Aside of the advanced RTM process proposed by Coexpair, SQRTM could be a straightforward solution with such equipment for stiffened panels.

The RTM and SQRTM solutions developed are prone to automation. Once the mould is within Coexpair’s workcell, automation controls pressure, heat and injection. Valves open and close automatically. Resin cartridges replace resin handling, degassing or even mixing. A central server, the Coexpair MaestroTM, records process data, provides intelligence, and generates reports. The basics of 4.0 industry.

A “zero touch time” composites shop

RTM and SQRTM automation interfaced with AFP/ATL equipment brings the vision of a “zero touch time” composites shop, from carbon fibre to moulded part. In 2019, Coexpair Dynamics was started to integrate tailored AFP/ATL from Trelleborg Sealing Solution (formerly Automated Dynamics). AFP/ATL equipment are running at Coexpair Dynamics, next to Coexpair workcells. It is a unique laboratory for composites automation. First AFP equipment will be delivered, like at Syensqo material application center.

In conclusion, RTM and SQRTM processes are solutions to produce 100 single aisle wings per month (including spars, skins, movables, winglets, ribs, etc.). A Coexpair publication is announced for Sampe Belfast 2024.

The remaining challenge is the fuselage, panel or barrel. The SQRTM process would be an evolution from Boeing 787 or Airbus A350 autoclave process. Qualified prepreg would be used, parts dimensional tolerances would be one order of magnitude better, process repeatability would be infinitively better. Cost of equipment should be affordable.

Most probably, the solution for composites single aisle fuselage will be an evolution of previous program achievements not a totally new one and, too risky one. For sure aerostructure industry shall exit autoclave to produce 100 single aisle A/C per month in composites. SQRTM may be the exit way out.

References:

[1] Aerospace RTM, a Multidisciplinary Approach. Dimitrije Milovich, Ron H. Nelson, Radius Eng. March 6-7, 1990 Radisson Plaza Hotel Manhattan Beach, California

[2] 20 Years of Shooting Glue into String: Evolution of RTM and SQRTM Technology and Large Composite Structures. Dimitrije Milovich, Andre Bertin, Tom Frazier. Radius Eng. Coexpair. Airbus Material Dialogue 2009

[3] Technology Readiness Level (TRL). High Lift Composite R&T Examples. DR.-Ing. York C. Roth, Airbus. 20 Jahre IVWS. 2010-16-17, Kaiserslautern, Germany.

[4] Highly Integrated Structure Manufactured in One Shot using Automated Processes with Prepreg Uniderctional Tape. Cedric de Roover, Bertrand Vaneghem, SABCA. Sampe Europe 2010.

[5] FP7 IMS&CPS project.

[6] Is There a Size Limit to Aluminum Mould used for 180°C cure of CFRP Unidirectional Laminates? An 8 meters long SQRTM experience. André Bertin, Charles Langlais & Bertrand Vaneghem. Coexpair. Sampe Setec 2016.

[7]High-rate, high-quality and low cost production solution for large composite aerostructure using Resin Transfer Moulding (RTM): Wing Spar demonstrator. Authors: Álvaro Calero, Betty Fantina, Jaime Sistach, Fernando Romero (FIDAMC), Alice Salmon, Antoine Vierset, André Bertin (Coexpair), Ricardo Pinillos (Airbus). Sampe Setec 2013.

Cover photo: Coexpair landing gear doors in SQRTM