Self-closing HP-RTM moulds for compact hydrogen tanks

The tank modules are made of carbon-fibre reinforced plastics (CFRP) and are autonomously produced in self-closing moulds. The process does not require a press or further production equipment, such as autoclaves for curing.

To this end, cylindrical hollow bodies are overbraided with carbon fibre, and then a low-viscosity, resin-based mixture is injected under a high level of pressure in the heated mould. After curing in the mould, the modular CFRP cylinders can be removed and connected to form a tank.

BBG, a manufacturer of moulds, machinery and plants, has developed and manufactured molds for the high-pressure resin transfer molding (HP-RTM) process for many years. As early as in 2019, the company presented a mold for the production of hydrogen tanks at the JEC World Composite trade show.

Compact tanks, range similar to those of internal-combustion engines

Since tanks are made of individual CFRP cylinder modules, they can be tailored precisely to the installation space available in a vehicle. The modules can be integrated, for example, in the floor assembly to save space. Such a tank filled with 700 bar internal pressure facilitates a range that is similar to those offered by internal-combustion engines for hydrogen-powered passenger and commercial vehicles.

Self-closing HP-RTM mold suffices for series production

First the inner shells of the cylinders, which are also referred to "liners", are overbraided with carbon fiber. (Photo: BBG) Then the blanks thus prepared are inserted into the two-part mold. Up to 15 cylinders with a diameter of 50 mm and more can be produced simultaneously per operation.

A low-viscosity, resin-based mixture is injected under a high level of pressure once the heated mold has closed autonomously. This mixture penetrates the carbon-fiber braid, enclosing the individual fibers evenly and without defects in the process. After curing in the mold, the finished pressure-resistant components can be removed and assembled to form a tank.

Simple technology and low investment cost

In comparison to other processes, series production with self-closing molds offers the benefit of being based on a simple technology, which results in low investment cost. Replacing large cylindrical hydrogen storage units by more compact tanks and the reduction of production cost play an important role in the development of fuel cell vehicles. Among other initiatives, the research project Bryson, in which TU Dresden (Dresden Technical University), Hochschule München (Munich University) and Leichtbauzentrum Sachsen (Lightweight Construction Center Saxony) participate in addition to BMW, is looking for solutions to this problem.

Efficient and reliable production of hydrogen tanks

Gerhard Hörtrich, project and sales manager at BBG, refers to the company's large competence in tool and mold making: "Since the high pressure applied to hydrogen tanks requires a great deal of component stability, we attach particular importance to maximum precision in the design and production of HP-RTM molds.

For the production of the hydrogen tanks to be as efficient as possible at the same time, we have developed and integrated a reliable sealing system, among other things. This system ensures that virtually no manual reworking is required when the resin mixture is processed", says Hörtrich.

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