Filament winding is a well-established process for the production of high-end fully wrapped composite pressure vessels. This type of tanks can be designed for service pressures that exceed 700 bar and are ideal for storage of gas fuels like compressed hydrogen in automotive and lightweight applications. As the demand for composite pressure vessels increases, lower costs and better product quality become very important. Impregnation is one of the most important steps in the wet winding process. During this step the dry continuous fibers are combined with the liquid matrix in order to create a fully impregnated semi-finished product. The properties of the impregnated roving have a major effect on the laminate quality and the efficient processing of the liquid matrix has a big influence on the manufacturing costs. The present work is related to the development of a new impregnation method for the processing of carbon fiber rovings. The developed impregnation unit (siphon impregnation system) consists of a sinusoidal cavity without any moving parts. This combined with an automated resin mixing-dosing system this allows complete wet-out of the fibers, precise calibration of the resin fraction, and stable processing conditions. The paper focuses on the modeling of the impregnation process inside the siphon unit. Mathematical expressions for the fiber compaction, the gradual increase of the roving tension, the static pressure, the capillarity of the roving, and the fiber permeation are presented, discussed, and experimentally verified. These expressions were implemented in an algorithm which can model the impregnation process by taking input parameters into account like winding speed, resin dosing, viscosity, and roving tex. The model was solved and the processing parameters of winding tension, fiber volume fraction, and impregnation degree have been simulated. An experimental set-up based on a filament winding machine was used for the validation of the model. Trials with different processing parameters and long run tests have been performed. The results proved that the model can accurately simulate the impregnation process. The good impregnation degree of the wound samples confirmed the efficiency of the siphon impregnation unit.

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For automotive applications high pressure storage of compressed hydrogen (CH2) becomes more and more important especially regarding the future sustainable mobility based on renewable energy. The filament winding technology is highly industrialized today to meet the requirements for a high-quantity production of lightweight fiber reinforced pressure vessels used for hydrogen-powered cars using fuel cells or combustion engines. However, the main disadvantage of the conventional wet winding process is the low lay-down rate. A decrease of the cycle time to increase the production rate can be realized by the simultaneous feed of a large number of rovings circumferentially arranged around the mandrel. A team of engineers at the Institut fuer Verbundwerkstoffe (Institute for Composite Materials) now further developed the ring winding technology to manufacture pressure tanks with a diameter up to 500 mm (20 inch) with fully wrapped dome sections. This large ring winding head with 12 radial movable arms and multiple payout eyes allows an accordingly higher material output and reduces the cycle time significantly. The profitability analysis considering ring winding head configurations with different number of feed-eyes show the optimization potential regarding the reduction of the production costs of FRP pressure cylinders. A critical review regarding reachable process efficiency is also given in this paper. A modified “tube siphon impregnation unit” is the most important component of the ring winding head due to its compact and modular design. This clean impregnation of 48 carbon fiber rovings near the winder minimizes possible resin leakages. In comparison to a conventional resin bath the amount of hazardous waste, for example contaminated acetone, can be reduced. FRP laminates (fiber reinforced plastic) with a reduced number of crossing points and less ondulation of fiber bundles result higher load bearing capacity and increased mechanical properties. Regarding the degree of interweaving the multi feed-eye configuration of a ring winding head makes it more challenging to define an optimized winding pattern with the given winding angle. The split disk test method was used for comparative investigations regarding the influence of undulations on the material properties of filament wound laminates.

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