Research Summary Report of A01

Meier, Niklas; Researcher, niklas.meier@tu-braunschweig.de

Zetzener, Harald; Leading researcher, h.zetzener@tu-braunschweig.de

Kwade, Arno; Project Leader, a.kwade@tu-braunschweig.de

all: TU Braunschweig, Institute for particle technology

The fundamental goal of project A01 is to understand material process interactions in particle-bed 3D printing by Selective Cement Activation (SCA). In SCA, a particle-bed consisting of fine aggregates and cement is applied layerwise. Inbetween the layerwise application, a liquid is applied selectively on the upper layer of the particle-bed. Thereby, a the cement hydration reaction is induced locally and the particle-bed hardens at the desired places. In the second funding period of this project, there is a focus on sustainability, process enhancement and material models.

Summary

At the Institute for particle technology the main focus is on the material and thus the particles. Regarding sustainability, there are three topics to be investigated in the second funding period: 1 A reduction of cement clinker content by optimising the particle size distribution. 2 The substitution of fresh aggregate by recycled aggregate. 3 The reuse of unbound material. The latter is especially important for all particle- or powder-bed based 3D printing processes, as depending on the geometry of the printed part, a lot of material might not be bound in the printed part. This material has to be reused in subsequent prints, to achieve an economically and ecologically efficient process. The reuse of the material in SCA is especially challenging, due to multiple reasons: The unbound cement in the particle-bed can react with water and thereby degrade. The water can be provided for example by moisture in the surrounding air, or water from the printing process, which can spread into regions of the particle-bed, which are not meant to be hydrated. Furthermore, the cement particles are partially smaller than 10 µm and can easily stay in the surrounding air in form of dust. Therefore, especially the fines content of the material might be lost after multiple printing processes. Lastly, there is a risk of segregation, as the material has a wide particle size distribution, ranging from below 1 µm up to 1000 µm. Therefore, for example percolation can occur, where small particles sink to the bottom of a container, while larger particles rise to the top, e.g. under vibration. This is shown exemplarily in the Figures 1 and 2: In Figure 1 there is a container with sand on the bottom and cement on the top. After vibration was applied for a while, the cement sank to the bottom (Figure 2), due to percolation.

Current state of research

To investigate the effect of the material reuse, experiments where carried out, in which material was passed through the printing process multiple times. Three different material mixtures have been testet, two of them with modified particle surfaces, which improves the flow behavior and thus, leading to higher packing densities. At different points in the test, samples of the unbound material where taken, as well as test specimen have been printed. The characterization of the unbound material showed, that there is a reduced fines content after multiple passes through the printer. Especially, one mixture with a specific particle surface modification showed high material losses. This is probably due to the reduced interparticulate forces, leading to the particles being easier dispersed in the air. Or in other terms: A lot more dust was generated by this material with modified particle surfaces. This was clearly observable during printing as a higher dust pollution. Although this specific material showed excellent packing behavior and nice mechanical properties, it is therefore currently not considered suitable for printing.