At present, when original equipment manufacturers and suppliers are exploring the materials and processes of next-generation aircraft, two different methods are coming into view. The first is liquid molding of dry fiber preforms, for example, the high-speed resin of Spirit AeroSystem in Scotland In the resin transfer molding (RTM) production line and in the Wing of Tomorrow (WOT) project led by Airbus, the project uses automatic placement of noncrimp fabrics (NCF) and liquid resin Pouring and then autoclave (out of autoclave, OOA) curing in an oven.
The second is the use of thermoplastic composites (TPC). For example, in the Clean Sky 2 project Multifunctional Fuselage Demonstrator (MFFD), automatic fiber placement is used (the lower part is reinforced on site and the upper part is Partially cured by autoclave) to manufacture hardened skins, which are then assembled by welding. These procedures use automation to increase productivity and quality while reducing costs. But which of these two technologies is better?
The German Aerospace Center (DLR) Institute of Structure and Design has a Light Production Technology Center (ZLP) in Augsburg. Among many composite manufacturing projects, PROTEC NSR and Fast Lane RPB provide a unique opportunity to compare the liquid-molded thermoset rear pressure bulkhead (RPB) of the dual-channel Airbus A350 respectively (Figure 1) And single-channel Airbus A320 thermoplastic RPB. Both projects worked with Premium Aerotec Group, a Tier 1 supplier of these structures, and demonstrated automation while evaluating cycle times and costs.
Liquid molded thermoset separator
Preforms and fixtures: The process steps for manufacturing vacuum bagged dry preforms are shown in Figure 2 below. RTM6 epoxy resin is injected into it through the use of Airbus's patented technology-vacuum assisted process (VAP), which uses a semi-permeable membrane to reduce porosity. The preform laminate consists of two groups of 16 layers, with a maximum length of 5 meters, using 5 strands of satin carbon fiber fabric with a total width of 1.27 meters. Between these two devices are embedded 25 reinforcements of complex shapes, the size of which reaches 1.5×2.5 meters, and eight stringers are placed on them.
In this process, developing tools and fixtures that can achieve the necessary precision in covering and handling is one of the biggest challenges. Part of the reason is that the complex layer mixing requires three different overhanging mechanisms:
First, apply the full-width material that must be adapted to the shape of the mold directly from the roller.
The second is the drape of the large-area structural ply, in which two robots cooperate, one on each edge of the fabric, to pick and create the target geometry of the mold to ensure that there is no wrinkle before fitting.
Third, it is suitable for small special-shaped pavement of 1.5 to 2.5 meters.
For collaborative robots, experts have developed an end effector, which contains six modules connected by a ball joint, which allows the end effector to deform in a snake-like manner to conform to the target geometry of the mold. The integrated heating device activates the adhesive during the fabric transportation and forming process, and maintains its 3D shape and position once placed.
For small-shaped layers up to 1.5 x 2.5 meters, a second gripper was developed. This gripper uses 127 modules equipped with vacuum suction devices for picking. This jig picks up the material in the 2D state and then bends it to the target geometry, but it must decide which of the 127 modules to be fixed and the module to slide to convert the 2D layer to the 3D shape. Therefore, it is very similar to the drape of the hand.
The end effector has a gripper that uses 127 modules equipped with vacuum suction to pick up the cut fabric layer, then transform it into a 3D shape, and then heat it before putting it into the mold
Online inspection: The optical sensor in the modular gripper monitors the overhang process. After placing the laminate, the end effector will check the quality with the Leica T-SCAN and the fiber angle measurement system based on the PrimeBaseTM camera. During the test, the fiber angle was first measured and compared with the CAD file, and then the edge of each piece was measured, and its position was checked against the CAD file.
Stringer and vacuum bag: After completing the prefab, connect eight stiffeners (stringers) to the top. To this end, a multi-motion gripper was developed and applied to vacuum bag auxiliary materials. The gripper consists of three independent 6-DOF small robots and a rigid arm, all of which are installed on the center flange of an industrial 6-DOF robot.
Vacuum packaging auxiliary materials-peeling layer, perforated release film and resin flow medium (tool side auxiliary material)-pre-cut and pre-connected, designed for placement. They don't need to be stacked, just need to be placed. The prefabricated semi-permeable membrane is applied semi-automatically through an end effector with an "umbrella" mechanism, while the placement of the adhesive tape and external vacuum bag is still manual, but can also be automated.
Cycle time and cost: The biggest challenge is to build a modular manufacturing execution system (MES) based on artificial intelligence (AI) to monitor the process chain. In the whole process, a data management system must be established to integrate completely different processes, and then use MES to command through a data exchange port. PROTEC NSR technology was verified by manufacturing a full-scale demonstrator in January 2019, and reached the maturity level of TRL 5-6 by mid-2019. Compared with PAG's current state-of-the-art technology, this automated process chain has shortened the cycle time of rolling fabric application by 58%, and the selection and placement of cutting layers has been reduced by 50%. The manufacturing costs of these businesses have been reduced by 11.5% and 31% respectively.
Thermoplastic RPB
The project started in 2018 by PAG and the Institute of Institut für Verbundwerkstoffe to demonstrate the possibility of thermoplastic composites in large parts and main structures. RPB is not a real main component because its mechanical requirements do not have wings or The fuselage is so high, but it shows that it is possible for a large, flat, slightly curved structure. In just four months, an A320 RPB demonstrator was developed and used as an example of how to convert existing aluminum structures into thermoplastic composites.
The demonstration uses Cetex carbon fiber fabric/polyphenylene sulfide (PPS) organic board (Toray Advanced Composites) and resistance welding. The resistance element between the two welded surfaces generates heat and remains in the welded structure. GKN Fokker has been using this technology for decades to produce aircraft landing gear doors and fixed leading edges. For this A320 RPB, the ZLP team used CF resistor elements instead of the traditional stainless steel mesh.
As the price of the thermoplastic composite RPB is the same or cheaper, the material is much more expensive. Therefore, reducing production costs through automation is the key, and using eight identical petal parts is also the key. Thermoforming as a separate part will require a very large pressure, which will be too expensive. The degree of automation of thermoplastic compression molding is higher than that of thermoset composites. Matching metal molds are mainly used, but constant temperature is the main problem, but this also makes the automatic stamping cycle very fast.
Integrated quality inspection: Use a standard test bench to make multiple welding samples, measure current, voltage and temperature, and then use ultrasonic testing (UT) to inspect them, and get the correlation between process parameters and good consolidation Sex. A process simulation was also established to compare the data obtained during the welding process with the established initial baseline.
Automation and cycle time: The whole process is very fast, reaching the scale of auto parts. TRL3 was achieved in the 2019 evaluation and has matured to TRL4. TRL6 will be achieved by the end of 2021. Before the pandemic, PAG had stated that it would put thermoplastic RPB into production by 2021. Although the future is still unclear, thermoplastic RPB is still regarded as the "future body", with its weight reduced from 41 kg to 35 kg, processing and assembly time reduced by 75%, and overall component costs reduced by more than 10%.
Comparison of TS and TP composite materials
The processing speed of thermoplastics is so fast that it can be cheaper than aluminum, and can even reach a production rate of 100 aircraft per month. RPB is suitable for both thermoplastic welding and automation. In contrast, dry fiber and liquid molded RPB are more expensive to automate.
However, this kind of automation can only achieve real benefits through the automation of some sub-processes. For example, through automated auxiliary equipment, vacuum bagging can be completed in about an hour, which is about 10 times faster than the manual process. However, cost is the main obstacle to implementing such improvements. For a part such as RPB, the cost of digital tools, robots and development is too high. However, if a modular approach can be developed and the system used for many parts, then the cost may be controlled so that it can be affordable with reduced time and labor.
Although only thermosetting and thermoplastic composite material molding processes have been compared so far, the structure of the two composite materials has not been compared in detail. Therefore, the future goal is to prove that the thermoplastic welding line also has the required performance of aircraft wings and fuselage, and This performance must also be demonstrated for large integrated liquid-forming wings. This is exactly what the MFFD and WOT programs have to accomplish.