Highly efficient pressure vessel manufacture

Roving & Tape Processing10News25

Driven by current political and social developments with regard to climate change, the need to reduce harmful pollutants - such as CO / CO2 - is increasing significantly. Hydrogen storage in conjunction with electrified vehicles can substantially contribute to reducing these greenhouse gases. Due to the significantly lower (volume-related) energy density of hydrogen compared to gasoline or diesel, the gas must be stored under high pressures of up to 700 bar. These high-pressure vessels, which are subjected to internal pressure, are a classic area of application for Fiber reinforced polymer composites (FRPC)). Weight savings of up to 70% are possible in comparison to steel vessels (the so-called type I pressure vessel).

Efficient manufacturing processes for pressure vessel systems are the essential factor for the use of natural gas or hydrogen for the drive technology of motor vehicles. Hydrogen technology in particular is becoming increasingly important. State-of-the-art today are so-called type IV pressure vessels (Figure 1), usually manufactured with filament winding technology. In order to achieve shorter cycle times, there are various approaches that were co-developed at Institut für Verbundwerkstoffe (IVW). For instance, the impregnated fiber material can be fed in at several points on the circumference to increase the throughput considerably. The ring winding technology developed at IVW (Figure 2) has already been awarded several innovation prizes and has been further optimized over the past decades.

The number of simultaneously radially fed rovings was increased from 32 to 48 and the process robustness increased as well.

Conventional resin baths have proven unsuitable for this new technology. For this reason, a new impregnation method - the so-called "siphon impregnation" - had to be developed. The fiber rovings are impregnated with resin only directly before the mandrel, which not only allows a large number of rovings to be used, but also significantly increases the process robustness.

The new concept of a “siphon impregnation unit” developed at IVW has a closed impregnation chamber, in which the required amount of resin is precisely metered and fed by a mixing unit. Since the resin/hardener mix is only formed shortly before the siphon, the pot life of the resin is no longer a critical problem. In addition, the cleaning effort is significantly reduced compared to using conventional resin baths. The siphon impregnation can, however, not only be used as a component of the new ring winding device, but also as an individual operation for fast-curing, functionalized or process-critical resins, for example. This innovative impregnation technology was also gradually developed for use in series production.

The ring winding device currently installed at IVW is equipped with up to 12 yarn depositors distributed around the circumference with integrated impregnation units. With this technology, 48 individual rovings can be processed at the same time, which is almost a five-fold increase in the lay-up rate compared to a conventional winding system with a maximum of 10 rovings.

Another approach to increase process speed in the manufacture of type IV tanks is the use of so-called TowPregs. These materials are characterized by the fact that the roving is already completely saturated with resin, but the resin is "frozen". This means that even at very high speeds there is no contamination from liquid resin (drips due to oversaturation or splashing due to centrifugal forces). As a result, the winding speed can be increased by a factor of 3 compared to liquid resin impregnation. The relatively high costs associated with the use of TowPregs are disadvantageous. On the one hand, the additional decoupled material creation process is an additional effort, on the other hand, most TowPreg materials require a cooling chain.

That is why IVW has developed a new “Direct TowPreg winding process” during the last two years in the research project “Speedpreg” - together with the partners Jakob Weiß und Söhne GmbH and the Chair of Polymer Chemistry at University of Bayreuth (Figure 3). The dry fibers are impregnated with a specially developed UV-reactive resin using a newly developed impregnation unit. The resin is "frozen" by activating its UV-reactive components. This amount of UV-reactive additive is - at the minimum - necessary to enable the processing of the impregnated rovings like TowPregs.

Due to the developments shown - paired with 30 years of experience in winding technology - IVW has a pioneering position in the field of highly efficient pressure vessel production.

The “Speedpreg - Development of a high-speed winding process” project is funded by the Federal Ministry for Economic Affairs and Energy on the basis of a decision by the German Bundestag (grant number ZF4052320AT7).

Dr.-Ing. Jens Schlimbach
Institut für Verbundwerkstoffe GmbH
Press & Joining Technologies
Erwin-Schrödinger-Straße 58
67663 Kaiserslautern
Phone: +49 631 2017-312
E-Mail: jens.schlimbach@ivw.uni-kl.de

Figure 1: TYP IV pressure vessel [Source: Toyota Motor Corporation]

Figure 2: Prototype of the ring winding technology developed at IVW [Source: IVW]

Figure 3: Schematic presentation of the developed direct Towpreg winding technique

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