In most on the market available type-IV pressure vessels, a rotationally molded polymer-based liner forms the hydrogen barrier and serves as a winding mandrel for the load-bearing fiber composite. However, due to the high tooling costs for rotational molding, small series of liners for prototypes or several different liner geometries for efficient use of installation space are hardly feasible. This can lead to lower volumetric energy densities of demonstrators and prototypes, resulting in poorer performance indicators in the competitive field of energy storage and therefore slowing down the transition towards emission-free energy storage.
The boost in 3D printing technology in recent years has led to the development of efficient large-format printing. Due to the tool-free manufacturing method and great flexibility, 3D-printed liners are suitable for small series and prototypes from a structural point of view. However, the requirements for the liner are more challenging, as it must guarantee gas-tightness and a good barrier property against hydrogen diffusion. These properties are greatly influenced by the structure of the polymer chains. Especially in 3D printing, the manufacturing process significantly affects this microstructure due to the deposition of individual layers. Therefore, investigations into the influence of the manufacturing process of polymers on gas permeability were conducted based on two polyamides (PA6 and PA11). Both unreinforced and short-fiber reinforced compounds were used for manufacturing permeability specimens in an injection-molding process as well as in a 3D-printing process.
The experiments confirmed that PA6 shows better barrier properties than PA11 and that the addition of short fibers improves the barrier properties as well. A comparison of the manufacturing methods showed that the barrier properties of 3D-printed samples are comparable to and, in some cases, even slightly better than those of samples manufactured through injection molding. However, 3D-printed samples show a slightly higher standard deviation, which is likely due to the multitude of influencing factors and the more sensitive manufacturing process. Nevertheless, it was demonstrated that the manufacturing of liners with sufficient barrier properties using 3D printing processes is feasible.