C-RTM Technology, The Hope For Future Low-cost Aviation Structures

 

Composites have established themselves in commercial airliner airframe construction, making them strong, lightweight structures that are lower maintenance than metals. Current forecasts for both next-generation wide- and narrow-body aircraft indicate that composite airframes will continue to grow, provided that materials and processes meet challenging targets for low cost and high productivity.

 

While autoclave-cured epoxy prepregs have dominated carbon fiber reinforced polymer (CFRP) airframes to date, developers of next-generation aircraft are actively exploring out-of-autoclave (OOA) technologies with integrated automation and in-line inspection as a key enabler for future Industry 4.0 production.

 

Equipment and automation supplier Modulo Engineering and its Composites Alliance Company (CAC) have collaborated with advanced materials supplier Hexcel’s UK and French operations to demonstrate OOA automated production of CFRP wing ribs and stringers.

 

The two parts were made using Hexcel’s automated preforming and compression resin transfer molding (C-RTM) process for dry carbon fiber reinforcements, which are rapidly infiltrated with Hexcel’s HexFlow RTM6 liquid epoxy resin to produce parts with 60% fiber content and less than 1% porosity in a 2.5 to 3.0 hour process cycle, which can be scaled up to produce multiple parts for high production rates.

 

1. Development of Dry Reinforcement Materials

 

Liquid composite molding (LCM) provides a solution to the three challenges of reducing costs, improving productivity, and providing primary load-bearing structural performance. Hexcel has developed HiMax non-crimp fabric (NCF) and HiTape unidirectional (UD) carbon fiber materials to address the need for basic structural performance when using the LCM process. HiMax enables large, flat structures (such as wing skins) to be laid up quickly, while HiTape enables tailored layups for large, complex structures with minimal waste. The application of HiMax can achieve the same performance as the latest generation of unidirectional prepreg.

 

Hexcel is committed to reducing volume and improving drape through HiMaxNCF, which includes a knitted yarn with a linear density of 20 dtex. The HiMax material is manufactured in the UK plant of Formax, which was acquired by Hexcel in 2016. The plant produces lightweight, towed multi-axis equipment and has a long history of providing solutions for demanding applications such as racing boats, supercars and Formula One cars.

 

NCF materials have been used in a wing demonstrator project completed by Airbus Defence and Space with Spanish company Danobat, using the latter’s Automated Dry Material Placement (ADMP) technology, an NCF automated fiber placement (AFP) process, and in zero-defect CFRP structures in the Airbus-sponsored ZAero project. Since 2016, Hexcel has been working on HiMax solutions for aerospace applications with major OEMs.

 

With HiMax and HiTape, Hexcel integrates low-area-weight thermoplastic filament yarn layers that act as a binder, eliminating the need for powder binders traditionally used for dry material preforming and liquid forming. For HiMax, the yarns are inserted between NCF plies; for HiTape, the yarns are applied to both sides of a carbon fiber unidirectional tape.

 

As a result, there is no need to use a powder binder to hold the unidirectional plies in place. HiTape is calibrated, not twistless, so it does not produce fuzz and has less width variation, which improves the AFP process. Thermoplastic tulle also adds toughness to the final laminate, and Hexcel has demonstrated that high material deposition rates can be achieved using next-generation AFP machines.

 

In a May 2015 SAMPE paper, Hexcel described a single-curvature preform made using a 0.25-inch-wide HiTape and a laser-equipped Coriolis Composites AFP robot at a layup rate of 1 meter per second. It has also demonstrated deposition rates of up to 150 kilograms per hour for a full-scale wing spar structure, working with an Electro-Shock AFP machine.

 

2. Let C-RTM Enter Aerospace

 

C-RTM was originally introduced with high-pressure RTM (HP-RTM) for automotive composites. Like conventional RTM, C-RTM requires placing a dry fiber preform into a matching metal mold, injecting liquid resin into the preform, and then applying heat and pressure using a press.

 

However, in C-RTM, the mold is only partially closed, leaving a gap between the dry preform and the upper surface of the mold. A vacuum is then drawn, a precise dose of mixed resin is injected, and the press closes the gap in the mold, forcing the liquid resin down through the entire part in the Z direction into the preform. This process is much faster than conventional RTM, which injects resin in the plane of the part.

 

As a partner in the Clean Sky 2 “Optimized Composite Structures for Small Aircraft” (OPTICOMS) project, Technical Tool Engineering saw an opportunity to apply C-RTM to aerospace. In the work package “More Affordable Composite Structures”, OPTICOMS aims to reduce the production cost of small aircraft, such as regional jets, through integrated structures and automated manufacturing, exploring prepreg and liquid resin methods.

 

OPTICOMS has designed a composite wing demonstrator, including an upper wing skin with three spars, produced in one go as a monolithic structure. The full-scale wing is part of the Airframe Innovative Technology Demonstrator (ITD), which is used to evaluate and mature technologies to enable the next generation of aircraft to reach Technology Readiness Level (TRL) 6 starting in 2025.

 

 

C-RTM is well known in the industry, but like HP-RTM, still requires large compression molders. Development work at Technical Tool Engineering is to adapt C-RTM for rapid injection at lower pressures, making aerospace parts cheaper to produce and meeting stringent requirements for high fiber volume content, fiber alignment and low porosity.

 

In the OPTICOMS project, injection time for a 0.7-meter-long, 0.2-meter-wide rib was reduced from 40 minutes to 5 minutes; in a separate trial project for an I-beam 900 mm long and 150 mm high, injection time was reduced from 1 hour to less than 5 minutes. The reduction in injection time in C-RTM will be even more significant for large parts such as complete wing skins or helicopter rotor blades. C-RTM also enables the injection of high-viscosity resins and the use of low-pressure injection systems and low-tonnage compression molders, which further reduces costs.

 

The mold pressure in the C-RTM process is only 6 bar, which is much lower than the pressure used in HP-RTM. The process can achieve aerospace-quality composites while also being suitable for large, thin-walled parts and smaller complex-shaped parts.

 

3. Automation of Preforming

 

While faster, lower-cost resin injection and OOA molding are key parts of C-RTM for more affordable aerospace composite production, the entire process chain requires multiple steps for cutting and laying materials and preforming. The OPTICOMS project is also about automation, with Technology Tool Engineering providing not only a C-RTM injection system, but also a pick-and-place robot and a hot diaphragm forming (HDF) machine, all integrated into a fully automated production cell.

 

Shortly after the OPTICOMS project was launched in 2016, CAC won the Equipment and Tool Innovation Award at the 2016 CAMX North American Composites Show for its automated 3D preforming cell at the Composites Manufacturing Excellence (ACE) Awards. The cell is capable of producing 3D preforms from dry fiber or prepreg, using vacuum suction cups to pick up the cut plies, place them and fold them onto a heated preforming tool.

Technical Tool Engineering and CAC have developed software that can shape cut plies of 2D materials into complex and developable surfaces. This unit was further developed in the OPTICOMS and I-Beam pilot projects.

 

The C-RTM process has automated the classic manual stacking of plies using robotic pick and place. The robot picks up the ply from an automated cutter and transports it to a heated preforming mold located on a compacting table. A release ply, a breather layer and an articulated reusable vacuum membrane are then applied, which is then evacuated and heated to preform, with hot diaphragm molding removing air from the textile layup while a thermoplastic veil is melted to form a compacted preform.

 

The Robot Places The Layup Onto The Preform Mold OnThe Compacting Table

 

How many layers can be compacted at a time depends on the material and shape of the part. For parts with low curvature, such as wing skins, compaction can be done every 50 layers. However, the ribs for the OPTICOMS project have 90-degree angles, and the I-beams tested have a T-shape, so care must be taken not to form wrinkles in the layup during preforming.

 

Such complex shapes may require compaction every 5 to 8 layers as part of this high-volume production process: stack the layers, compact them for 2 minutes with the HDF, reopen and stack them again, and then repeat the compaction cycle, with a final cooling of the preform on the tool before transporting to the RTM tool.

 

The automated cell lays up at a rate of 15 seconds per layer, and the ribs, which totaled less than 20 layers on the project, were laid up in 20 minutes. The I-beams took longer to lay up – 45 minutes – because of their complex shape and laminate stacks with thicknesses ranging from 1.2 mm to 6 mm. This is much faster than manual processes and reduces the risk of errors, improving repeatability and quality while reducing costs.

 

4. Smart Control

 

Another key feature of the automated preforming cell of Tech Mold Engineering is its integrated control system, the backbone of which is called SMART CONTROL, a camera system and multi-purpose software that compares images taken during processing with the CAD database of the part, enabling preform shape recognition, fiber direction control, ply positioning, and defect and foreign object (FOD) detection.

 

Feedback from SMART CONTROL instructs the robot how to pick up and place plies in the correct order and timing, and also alerts the cell operator when errors are detected. The system can be configured so that the operator manually removes the ply and then restarts the system to replace it, or an automated solution can be created with the customer to remove defective plies and correct errors.

 

 

The optical images used to detect the edges and contours of the plies are also used to control the fiber orientation. If the part is large (e.g. 2 meters long instead of 200 mm), one camera may need to take a picture from a higher vertical position to detect the edge of the ply, and then move closer to the part to check the fiber orientation.

 

This sequence can be calibrated for each type of part. To detect wrinkles and FOD, there is a database of different defects to which data can be added, allowing the deep learning algorithm to improve over time. The idea is to create an adaptive system.

 

1. Production Cycle and Future Production

 

The preforms for the OPTICOMS ribs were made with HiMax and the I-beams with HiTape. The resin system for both was HexFlow RTM6, a one-component (1K) liquid epoxy resin from Hexcel, with a cure cycle of 90 to 120 minutes at 180°C. Curing is the longest step in the part production cycle for the OPTICOMS ribs and the test I-beams.

 

To date, this is the only epoxy resin that meets the requirements for RTM of aerospace structures. However, there is growing interest in qualifying a two-component system because it is mixed on the injection head, provides greater flexibility in cure time, and eliminates the need for refrigerated transportation and storage to prevent the pre-mixed RTM6 from reacting prematurely.

 

Airbus Helicopters has collaborated with Alpex Technologies in the SPARTA project to demonstrate the HP-RTM process using HexFlow RTM6 two-component (2K) resin. The process cures the A350 door frame in 30 minutes at 180°C, a complex-shaped load-bearing structure that is 2 meters long, 200-250 mm wide and 8-10 mm thick.

 

Airbus also demonstrated a 1.5m x 0.5m CFRP rib made using HP-RTM at its Composite Technology Center (CTC) in Germany, with a molding cycle of 20 minutes for a part with a fiber content of 60% and a porosity of less than 2%.

 

Over the past year, CTC has worked with a number of suppliers to transition hand-laid prepreg parts to HP-RTM for the A320. However, 2K resin systems must be qualified for aerostructure production, and CTC was concerned about whether it could consistently ensure the in-situ mixing quality of 2K resins. Alpex is using in-mold sensors from Netzsch in Germany and Kistler in Switzerland to help achieve this, and is developing other solutions.

 

Even without a 2K system, higher production volumes are possible. In the SPARTA project, the door frames were taken out after an initial cure of 30 minutes at 180°C and then post-cured in a press under vacuum to eliminate thermal stresses and ensure mechanical properties. Although the process requires an additional tool set, with only one molding and injection unit, the additional tooling costs are offset by lower process and molding costs, and can be quickly paid for even if the annual production volume is only 500-1000 parts.

 

The main achievement of these demonstrators is the ability to manufacture aerospace primary load-bearing structures with a short resin injection time compared to traditional aerospace RTM processes. In traditional RTM processes, infiltrating resin into such large unidirectional carbon fiber reinforced parts is an arduous and lengthy process. This intelligent automated preforming and C-RTM processing has the potential to enable more cost-effective and sustainable production of the integrated stiffened skin structures envisioned for future aircraft.