Fully automated RTM production line
This paper gives an overview of an automated RTM production line concept, using the example of attach-fitting manufacturing for the Airbus single-aisle airplane family. The process was developed at the Composite Technology Centre GmbH, Stade, a member of the CFK-Valley Stade e.V.
(Published on June 2008 – JEC Magazine #41)
PROF. DR.-ING. AXEL HERRMANN, CEO
CLAUS FASTERT, PROJECT ENGINEER CTC GMBH
At present, the production of fibre-reinforced plastic components in aircraft construction uses manual manufacturing steps almost exclusively. Only within a few fields of application such as assembling or tape-laying, for example, have automated systems been generally accepted so far.
Because this is inconsistent with the fast-growing use of fibre-reinforced plastics and the constantly increasing manufacturing rates in the civil aircraft industry, a publiclyfunded project was started at the CTC GmbH, Stade in 2003 to find ways to simulate processes that might be used for fully automated CFRP production and convert them into equipment.
Automation is only reliable for parts produced in large quantities, so an RTM-moulded attach fitting for the A320 vertical tail plane was chosen from the range of parts at the
Airbus Stade plant for the simulation. This component is produced for each single-aisle vertical tail plane in twelve different versions. The vertical tail for the A318, A319, A320 or A321 consists of two large shell segments that are joined to the centre box. Each shell has six attach fittings, which are bonded to the shell skin in three pairs. These fittings serve to connect the vertical tail to the fuselage (figure 1). The aircraft production rate is 40 per month. About 5,800 components are produced per year.
|CTC GmbH is the CFRP competence centre for Airbus Germany. It was founded in 2000 in the CFK Valley Stade e.V., close to the Airbus Stade plant. Its main skills cover developing advanced preforming technologies; developing, testing, and enhancing injection and infusion technologies; process automation; and feasibility studies for serial production.|
The serial production process
These components are currently manufactured in several individual steps, which are described in detail below. The process starts on an automatic cutting table, where the individual blanks are cut. The plies are collected manually and placed in a temporary storage place in reverse order for later placement in the RTM tool, also manually. The lid is screwed onto the tool, the injection hoses connected, and the tool transferred to the oven where injection and cure take place. Following cure, the tool is transferred to a circulating-air cooling track. At the end of the cooling phase, the tool is delivered to a workplace and opened with a crane. The cured component is then released, and the mould can be cleaned and coated with a release agent. At this point, the cycle starts again.the solution.
The automated process
A fully automated system was developed for the described process, based on the example of one of the twelve attach fittings (figure 1). Nearly 50 individual plies were used to make the selected demonstration component. First, the entire process was planned and simulated, along with several variants, using DELMIA process-engineering software. The simulation showed that splitting the process up into a preforming step and a curing step would make sense, as it would shorten the RTM tool holding time and allow parallel processes. After the simulation, the most promising process variant was converted into a complete pilot plant to validate the simulation data and to improve the digital model. Several new technologies had to be developed to implement this system. The most important achievements are described below. As in serial production, the preforming process starts on an automated cutting table. A machine with material feeder is installed at CTC to produce the blanks. The cutter has a robot equipped with an adjustable vacuum effector (figure 2) for picking up the blanks automatically. Individual blanks are distributed and stacked so as to optimize material use and minimize waste. The handling device makes it possible to decollate every single blank without impairing the flanking blanks and without changing the effector. At the same time, the single layer is draped at the grab of a blank by the effector’s 3D-contoured effective area. The picked-up blanks are placed in a microwave-transparent preforming tool in the correct order. Preform consolidation takes place under vacuum in a 12 kW microwave oven, so that processing time is significantly shortened.
The finished preform is taken out of the preform mould by a second robot with a vacuum-assisted effector and placed in
the opened curing mould. The curing mould is transferred to the lid station by a roller conveyor and fitted with the RTM tool lid in a fully automatic process (figure 3). The closed mould is moved into a heating press and fitted with hoses for resin and vacuum connection by a docking station especially developed for this purpose. The advantage of this automatic docking station is that no manual interference is necessary and that the volume of throw-away parts is remarkably reduced (figure 4). With the system developed, only two copper tubes about 15 cm long are wasted per injection, while with the serial process, two resin-filled hoses at least 2 m long, including screwed fittings, have to be thrown away. After closing the press and connecting the resin injection lines, injection begins, using a fullyautomated injection system with resin trap.
This injection system is further characterized by the preliminary heating of resin by microwave and by the control of injection duration by determining the resin mass that flows through the preform. Injection pressures and temperatures are permanently monitored and logged. After curing, the injection lines are disconnected automatically. The thermal characteristics of the single-component RTM 6 resin used make an absolutely clean and drip-free separation possible. The tool is transferred to the lid opening station and opened up again by electrically-driven mould opening cotter pins. Applying the required force for separating the lid from the mould is the main challenge at this stage. The lid is taken off over elaborated kinematics and a third robot releases the finished part from the mould with a suction gripper. Then, the robot changes its effector to clean the mould. This cleaning effector works with a combination of air jet and vacuum cleaner. The curing mould is then available for a further process.
In summary, the following developments were accomplished:
- Vacuum effector for sorting and stacking the individual blanks
- Microwave-assisted preforming process
- Automated lid station for opening and closing the curing mould
- Automatic docking unit for resin and vacuum supply
- Fully automated injection system resin trap
- Automated cleaning method for RTM moulds (figure 5).
The individual processing steps are being implemented into serial production at the Airbus Stade plant in 2008 to optimize the whole procedure. The goals are to cut the production time for one attach fitting by about half, increase the RTM production capacities, and benefit from state-of-the-art RTM technology. Moreover, substantial progress is made towards process automation.