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The wet winding technique was significantly improved by the introduction of a ring winding head with radially movable arms and integrated modular siphon impregnation units. This sustainable concept makes it possible to mass-produce fibre-reinforced high-pressure vessels more efficiently, and thus more economically.
RALF SCHLEDJEWSKI GROUP LEADER, MANUFACTURING SCIENCE INSTITUT FÜR VERBUNDWERKSTOFFE GMBH KAISERSLAUTERN, GERMANY
(Published on July-August 2007 - JEC Magazine #34)
Pressure vessel systems are gaining more and more importance, especially in automotive applications. Today, mainly fossil fuels are used, but due to the predictable shortage of these energy sources, hydrogen is gaining more and more importance. Although the energy content of hydrogen is clearly higher than that of gasoline, new storage systems need to be developed because of the different densities. In principle, hydrogen can be stored in gaseous or liquid form, or compacted as a solid material. However, there are still many challenges to overcome for all three possibilities.
The storage capacity of pressure vessels is a major challenge. For hydrogen, a sufficient storage density can be achieved only by applying a very high operating pressure of up to 70 MPa. At the same time, a manufacturing technology ready for mass production of typical automotive quantities exceeding 100,000 tank systems per annum has to be made available. To fulfil the requirements of such applications, we need to give pressure vessels specific properties, notably low weight and the potential for cost-effective mass production. While wet winding is an established manufacturing technology in the market of fibrereinforced pressure vessels, it is limited with regard to high production rates.
The big challenge for new and advanced manufacturing technologies is to achieve cost-effective mass production. Within the scope of the StorHy project (Hydrogen Storage Systems for Automotive Application), a ring winding head was developed as an extension for a conventional filament winding machine.
When winding pressure tanks, it is often necessary to reduce cycle times. This can be achieved through simultaneous feed of impregnated fibre material at several locations around the perimeter. If the free filament length (the distance between feed eye and mandrel) is to be kept constant while winding a pressure vessel, the feed eyes need to be radially movable. To give a further advantage, the axial movement of the head during winding must be reduced, so that the installation length of the winding machine can be reduced. Previously-manufactured ring winding heads did not feature the radial movement option. The optimized utilization of this new option requires not only plantspecific implementation, but also customized path generation. The Institut fuer Verbundwerkstoffe GmbH has developed such an expanded system technology for wet winding of hydrogen pressure tanks. The prototype installed in the winding facility is equipped with eight feed eyes distributed around the perimeter (Figure 1). Each arm can simultaneously impregnate and lay down four carbon-fibre rovings. The system allows three times as much material throughput as comparable conventional winding systems, at the same processing velocity. For the path generation, the CADWIND software from Material S.A. has been upgraded. It is therefore possible to automatically generate winding paths that meet the specifications of the multi-feed-eye ring winding head.
Winding system with several feed eyes
Figure 2 shows the system during winding. The actuation of the ring winding head is directly connected to the CNC control of the winding machine (EHA Spezialmaschinenbau GmbH) via INTERBUS, from where it receives its position data. Programming, referencing, and visualization take place on a Windows-based industrial PC. The modified software program CADWIND is used to generate the paths according to the required laminate build-up.
The exact lay-down of the rovings, especially in the area of the dome section, is carried out via rotatable feed eyes. The special construction keeps the roving width constant in the cylindrical area of the tank and during winding of the polar caps, smoothes the rovings, and wipes off excess resin.
Modular siphon impregnation unit
The impregnation and supply of the many rovings distributed at the perimeter of the mandrel poses another challenge. Because commercial impregnation units are suitable only to a limited extent for this process and because the process also usually requires batch production, a new impregnation method was developed. Here, the rovings are pulled through a fully covered siphon-like impregnation unit and are fully saturated by an exactly dosed amount of resin (Figure 3). In addition, there is no contamination by dripping resin and pollution of the air by styrene releases (UP resins), for example. The rovings are led through the flexible arms of the winding head directly to the container surface and placed accurately by rotatable feed eyes. The suitability of this impregnation method for continuous mass production was demonstrated by an endurance test. The impregnation units worked failure-free for one entire eight-hour shift without notable wear at the roving feeding hoses and without fibre rupture. Replacement of the fibre bobbins during the ongoing winding process was possible.
Several pressure vessels were wound successfully, using ring winding heads with siphon impregnation units (Figure 4). The cross section of the wound laminate was examined to characterize the part quality. For example, the laminate void content is similar to that achieved with the standard wet winding technique. Besides the impregnation technique, other factors that significantly affect the resulting void content are the feed eye itself and whether or not a doctor blade is used (Figure 5). Wound vessels with a laminate structure designed for a 200- bar in-service pressure were tested and a burst pressure of more than 750 bar was achieved.
The development of the ring winding head with modular siphon impregnation units is a major advancement in wet winding technology, which is an established method for the production of rotation-symmetrical components. The essential benefit of this method is the considerably increased lay-down rate. Constraints with the conventional winding method are the limitations on the haul-off speed (conventional impregnation unit) and the free choice of winding angles. For the same laminate quality, the large number of rovings handled by the ring winding head is able to significantly reduce the time required for full saturation of the mandrel.
Other positive effects are achieved due to the changed winding patterns, with significantly fewer crossing points of the rovings. The better utilization of the fibre performance based on reduced undulations further reduces the weight. The radial movement of the feed eyes during the winding process also helps to significantly reduce the length of the gantry system. This leads not only to reduced cycle times, but also to reduced installation space in the winding facility, which can then be made smaller.