JEC Group have brought together the international community of composites leaders and executives in our Composites Circle as an unique networking opportunity to meet with both peers and future partners.
Pressure tank systems are gaining in importance, especially in the automotive industry with cars powered by natural gas or hydrogen. As a result, demand is growing for efficient mass-production of lightweight, high-quality tanks. Thermoset winding is an established technology in the area of fibre-reinforced pressure tanks. In the short or medium term, 100,000 tank systems per annum will be required just to meet the automotive needs generated by hydrogen technology. As part of the European StorHy project, Prf. Alois K. Schlarb and Dr Ralf Schledjewski from the IVW (Institut für Verbundwerkstoffe, or Composite Materials Institute) in Kaiserslautern, Germany, worked with Mr Axel Seifert from Material S.A (Zaventem, Belgium) to find a way to produce pressure vessels at a reasonable cost.
(Published on September 2007 – JEC Magazine #35)
Composites are essential
High-pressure hydrogen vessels have to withstand an operating pressure of 700 bar, and the minimum burst pressure for them is 2,100 bar. In order to obtain lightweight tanks for such pressures, carbon fibres are used. A cylindrical shape is obviously needed for this application, and this type of composite structure can be obtained in a single shot using wet filament winding. The process needs to be made more cost-effective to reach the large-scale automotive market and allow the development of hydrogen technology in this segment.
It took the IVW several years to develop and build the prototype for a winding system equipped with eight feed eyes (Fig. 1) distributed around the perimeter of a ring (Fig. 2). Each arm can simultaneously impregnate and lay down four 12k carbon-fibre rovings accurately, even in the dome sections. The system allows three times as much material throughput as comparable conventional winding systems, at the same processing velocity. These rotatable feed eyes give more freedom in the choice of winding angles. The multi-feed-eye ring also allows winding more complex geometries and requires a significantly shorter gantry system, due to the radial movement of the feed eyes. The resulting change in the winding patterns with significantly fewer crossing points of the rovings leads to better utilisation of the fibre performance based on reduced waves, which increases the lightweight quality.
The Institut für Verbundwerkstoffe and Material S.A. worked together to upgrade Material S.A.’s CADWIND software for path generation, adding special capabilities to the rotatable feed-eye ring to allow winding paths to be generated automatically (Fig. 3). A quick and focussed optimization of the winding pattern is performed to optimize the final structure.
New fibre impregnation process
This new winding technology has many advantages. Commercial impregnation units are not suitable for it, however (due to the required directional changes of the rotatable feed eyes), and usually require batch production. IVW had to develop a new method. During their process, the rovings are pulled through a siphon-like impregnation unit (Fig. 4 on the right) and fully saturated by an exactly dosed amount of resin. They are led through the flexible arms of the winding head directly to the container surface, and placed accurately by rotatable feed eyes.
The siphon impregnation system is almost a closed one, reducing the potential for resin leakage and air pollution from styrene release (unsaturated polyester resins). The hose connections simply need to be removed and disposed of at the end of the work cycle. Resin consumption is reduced, and the process can be a continuous one.
The Institut für Verbundwerkstoffe and Material S.A. have developed a process offering the possibility to mass-produce fibre-reinforced high-pressure vessels for hydrogen storage tanks for future cars more efficiently, and thus more economically.
The technique’s lay-down rate is three times faster than a conventional winding process, achieving significantly reduced cycle times. The technique also allows more complex geometries. The winding unit can be made smaller, or the ring winding head can be easily integrated into existing winding units where necessary. This technology could benefit the development of hydrogen-powered cars in the automotive sector.
An eight-hour long endurance test has already been performed without any problem. Further development is ongoing to increase the number of rovings per feed eye and to allow processing of 24 k rovings.
Mr Axel Seifert
“We already knew each other. Filament winding is a small family. The specific request of the IVW aimed at increasing productivity in the filament winding process. With the benefit of our know-how, we upgraded our CADWIND software to drive the machine equipped with rotatable feed eyes. The key point in this development was the contact among people.”
Dr Ralf Schledjewski
IVW designed and constructed the ring winding head based on the requirements of different pressure vessel manufacturers involved in the StorHy project. The leading software tool for programming filament winding machines, Material S.A.’s CADWIND, was modified by Material to reflect the requirements arising from the additional z-axis used for the ring winding head. Material’s new programming tool enables IVW to run the ring winding head.