The patent and literature research was updated in order to incorporate the latest scientific findings into the project work. With the help of the PA, a catalogue of requirements was also drawn up in which all the necessary/required properties of the acoustic absorbers to be manufactured were defined.

A simulation approach was chosen that makes it possible to simulate the sound absorption capacity of absorbers made from different renewable raw materials with any internal structures. A special material model was integrated into an optimised finite element model to map the absorption properties of the composite material. With this material model, the structural material can be modelled as an equivalent acoustic material (degree of freedom sound pressure). This simulation basis was used to specifically design internal absorber geometries in order to optimise the behaviour of the absorber selectively for different frequency ranges. The model was continuously optimised and validated by measuring the sound absorption coefficient of samples produced using binder jetting during the course of the project.

A selection of potential starting materials first had to be made and analysed. In the course of the project, four materials were identified that are suitable for the binder jetting process and are also likely to be available as a residual material in the foreseeable future: Miscanthus, beech wood, birch wood and chaff straw.

These four materials are available as powdery residues from industry. Various processing techniques (sieving, mixing) were used for these selected materials. The use of the individual techniques depends on the initial properties (especially the flowability and particle size) of the materials used. Only materials that do not require energy-intensive additional comminution were explicitly selected. The increase in fines content required for processability with the available binder jetting system was achieved by adding Miscanthus dust or, in the case of chaff straw, chaff straw dust. Depending on the material, a processable powder could be generated with a dust content of 10 % to 50 %.

The powders were analysed both as pure material and in the various processed mixing ratios in terms of particle size distribution, particle shape, bulk and tap density and the resulting Hausner number as a key figure for flowability.

A variety of gelatine was tested as a binder, which was to be used as a liquid binder in the process. Several gelatine solutions were developed that would be compatible with the print head available in the BJT system in terms of the liquid properties achieved. However, in a model run with a comparable single nozzle for droplet generation, it was found that these solutions would lead to blockages in the fine liquid channels of print heads despite the heated system.

For this reason, an alternative binder system was developed. A gelatine hydrolysate, which is also soluble in cold water, is added to the residues as a powdered solid binder. This can be dissolved by a pressurised liquid containing water and ensure that the component solidifies during the subsequent drying process.

Printable parameters were determined for the selected materials. In addition to the amount of binder discharged and the layer thickness, the bulk properties of the powders in particular were adapted to the machine. This was achieved by optimising the ratio of the finest particles (dust) to the base powder. It was found that the various defects during coating application, such as layer shift or cracks, can be reduced in this way.

First of all, various methods for depositing protective layers from organosilicon precursors were investigated. However, this could not be carried out without damaging the surface due to the process. Alternatively, a coating of the test specimens by spraying or dipping in gelatine hydrolysate solution was investigated. This method was initially not to be used, as there were fears that the porous structure of the component surface would become clogged. However, this was not observed, meaning that the coated samples actually exhibited better sound absorption properties than uncoated samples.

An additional method was also developed to improve the component properties. The storage of finished components in a saturated vapour atmosphere with subsequent drying. This is beneficial for the mechanical and acoustic properties of the components.

Test specimens were manufactured from the selected and optimised materials to measure the sound absorption coefficient. In addition, resonator structures designed using simulation were printed and analysed in order to optimise and validate the simulation models. It was found that, particularly in the low frequency range below 800 Hz, the sound absorption properties are better than those of a commonly used commercial material, assuming the same material thickness. The sound absorption properties can then be further improved by introducing a resonator structure.

The mechanical characterisation of the components was carried out using bending and abrasion tests. Analyses of fire behaviour and microbial resistance were also carried out. [M1]

[M1]Publication of results would probably have to be agreed with Filk and IWU. The final report is still final and public, if so, it could be used to include strength diagrams etc. with source.