Composite materials: tension until the part is done
The automated processing of composite materials has its perils. For example, it is very important to maintain the correct tension in the fiber tape while it is being laid over the mold. This is necessary in order to allow the machine itself to adapt to even highly demanding compaction conditions and extremely complex surface structures. But such capabilities pose a challenge in terms of controlling the system
Carbon-fiber reinforced plastics (CFRPs) are the materials that will be used to construct the aircraft of the future. But composite materials are currently more costly than the traditionally used aluminum alloys. They frequently allow productivity advances in the structure design, however, as they usually entail fewer production steps are needed.
Moreover, the weight reduction achieved is normally in the range of 20 percent, which results in considerably lower fuel consumption. The reduction in maintenance work is an additional advantage, which also allows further savings.
In order to automate the processing of composite materials, Spanish manufacturer MTorres uses so-called tape-laying machines. They are based on the concept of a movable portal frame. This allows very easy access, for example, when handling the parts and molds. Reloading the composite tape spools is also considerably simplified. The standard machine design lays composite tapes (carbon fibers incorporated into epoxy resin) in widths of 150 mm or 300 mm. It is possible, however, to adapt all machine dimensions to the user’s requirements, so that even composite material wing structures more than 30 meters long can be produced.
In order to allow very complex production processes and fulfill the airplane manufacturers’ requirements, the machine must have a powerful controller – and this was found in the Fanuc CNC Series 300i. When the tape is laid, the strips are heated to a constant temperature before being cut and laid.
For surfaces with complex contours, not only is it necessary to build up each structure from a number of layers, the alignment of each and every layer must also exactly match the design specifications. For parts, such as mounting holes, that will be subjected to heavy loads through their lifecycle, additional reinforcement layers at many different alignments are needed. The tape is normally laid into a contoured mold (layering according to the net form) or on a flat vacuum table (layering of flat forms that are later bent in a hot-forming process). The complex final product is then placed into an autoclave, where it is thermoset under the required temperature and pressure conditions.
The machine (type classification TorresLayup) comprises eleven programmable axes that are driven by Fanuc servomotors from the Alpha is and Beta is series. The X and Y axes are advanced by a pair of Alpha is motors on each side. These motors guarantee the required speed and precision. In addition, it is possible to achieve exact control of the backlash and high machine productivity at rotational speeds of up to 60 m/min on the main machine axis. A further drive system provides head rotation (C axis) and another motor allows angular alignment of the head (A axis), so that it precisely follows the pitches of complex mold contours. Two driven ultrasonic knives on four of the controlled axes help the machine also execute complex tape cuts. The compact dimensions of the Beta is series servomotors have proven to be particularly advantageous for these axes. Fanuc CNC controllers were employed in the first MTorres tape laying machines. The more complex the design of the machines became, the more important it became for the Spanish firm to work with the CNC manufacturer.
José Angel Fernández, head of Automation Engineering at MTorres, explains one of the elementary design features that could be solved with Fanuc’s help. “It is very important to maintain the correct tension in the tape when it is being laid over the mold, so that the machine itself can adapt to even particularly demanding compaction conditions and extremely complex surface structures. To achieve this, the torque of the Fanuc servomotor that helps feed the tape from the spool is carefully controlled and monitored by a Series 300i CNC controller. This special CNC functionality was developed by Fanuc in successful cooperation with us.”
The operator´s job is very simple
MTorres worked with the developers of modern CAD software for the aviation sector to develop its own software that takes the special drive requirements of the machines into consideration. Consequently, users have access to a fully integrated software package when designing new parts or their structures. Users can exactly adapt their requirements for tape laying, or select standard steps for the software in order to optimize the structure. The software allows programmers to simulate, analyze and modify all layers before the part program is actually created with the post-processor. This part program can be downloaded directly to the Fanuc Series 300i CNC controller via an Ethernet interface.
“Although the band layering process itself is very complex,” José Angel points out, “the machine operator’s job is nevertheless very simple. By designing a special human-machine interface on a Windows basis, we have ensured simplified operation.” The operator first instructs the machine to find three fixed points marked on the mold, which is attached to the machine foundation. The axes in the control programs are then automatically coordinated to these positions, and the relevant part program is started. Some very complex parts call for up to 180 tape layers. If a new 500-m tape spool is needed during a work process, the head is automatically parked in an easily accessible position. It takes the operator only two minutes to bring the magazine into position, insert a new 120-kg spool and restart the machine.
The CNC controller is permanently installed next to the tape laying machine, while the amplifiers and motors are located in a movable frame. The frame can move as much as 30 or 40 meters away from the CNC controller. The Fanuc 300i CNC controller outputs data on a fiber optic cable that is up to 100 meters long. This greatly simplifies the high voltage cabling, which in turn results in reduced costs for the components and shortened machine design times, as well as greatly reduced noise pollution and much less risk of damage when the frame is moved.
A tape defect-laser detection system with which the machine can sort out defective sections of the tape as it unrolls is a further polished auxiliary controller. Distance sensors are mounted around the moving head and the moving frame, ensuring a higher safety level for the operator. To save time and avoid potential damage during the handling and movement between machines, the Torreslayup machine is equipped with a concluding ultrasonic cutting system for cutting the uncured composite parts immediately after the layers have been laid.
German Summary
Die automatisierte Verarbeitung von Verbundwerkstoffen hat so ihre Tücken. So ist es beispielsweise sehr wichtig, die richtige Spannung im Faserband zu halten, während es über die Form gelegt wird. Nur so kann sich die Maschine selbst auch an besonders anspruchsvolle Kompressionsbedingungen und extrem komplexe Oberflächenstrukturen anpassen. Das jedoch ist eine Herausforderung an die Steuerung des Systems. Der deutschsprachige Beitrag ist nachzulesen auf www.aerotec-online.com
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