Citation: Dietemann, B.; Wahl, L.;
Travitzky, N.; Kruggel-Emden, H.;
Kraft, T.; Bierwisch, C. Reorientation
of Suspended Ceramic Particles in
Robocasted Green Filaments during
Drying. Materials 2022, 15, 2100.
https://doi.org/10.3390/
ma15062100
Academic Editors: Ludwig Cardon
and Clemens Holzer
Received: 31 January 2022
Accepted: 9 March 2022
Published: 12 March 2022
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Article
Reorientation of Suspended Ceramic Particles in Robocasted
Green Filaments during Drying
Bastien Dietemann
1,
* , Larissa Wahl
2
, Nahum Travitzky
2
, Harald Kruggel-Emden
3
, Torsten Kraft
1
and Claas Bierwisch
1
1
Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstraße 11, 79108 Freiburg, Germany;
torsten.kraft@iwm.fraunhofer.de (T.K.); claas.bierwisch@iwm.fraunhofer.de (C.B.)
2
Institute of Glass and Ceramics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Martensstraße 5,
91058 Erlangen, Germany; larissa.wahl@fau.de (L.W.); nahum.travitzky@fau.de (N.T.)
3
Mechanical Process Engineering and Solids Processing, TU Berlin, Ernst-Reuter-Platz 1,
10587 Berlin, Germany; kruggel-emden@tu-berlin.de
* Correspondence: bastien.dietemann@iwm.fraunhofer.de
Abstract:
This work considers the fabrication of ceramic parts with the help of an additive manu-
facturing process, robocasting, in which a paste with suspended particles is robotically extruded.
Within the final part, the material properties depend on the orientation of the particles. A prediction
of the particle orientation is challenging as the part usually undergoes multiple processing steps with
varying contributions to the orientation. As the main contribution to the final particle orientation
arises from the extrusion process, many corresponding prediction models have been suggested.
Robocasting involves, however, further processing steps that are less studied as they have a smaller
influence on the orientation. One of the processing steps is drying by natural convection, which
follows directly after the extrusion process. A quantification of the reorientation that occurs during
drying is mostly unknown and usually neglected in the models. Therefore, we studied the amount
of reorientation of suspended particles in robocasted green filaments during drying in detail. For our
study, we applied the discrete element method, as it meets various requirements: The exact particle
geometry can be resolved precisely; particle–particle interactions can be described; the paste composi-
tion is reproduced exactly; the initial particle orientation can be set in accordance with the prediction
from the analytical models for the extrusion part; macroscopic force laws exist to represent capillary
forces due to the remaining fluid phase that remains during drying. From our study, we concluded
that the magnitude of particle reorientation during drying is small compared to the orientation occur-
ring during the extrusion process itself. Consequently, reorientation during drying might further be
neglected within analytical orientation prediction models.
Keywords:
evaporation; orientation prediction; particle reorientation; additive manufacturing;
material extrusion
1. Introduction
Robocasting, a material extrusion based additive manufacturing (EAM) technology
technology, produces continuous rod-like filaments from a paste consisting of suspended
microsized ceramic particles. After extrusion, robocasted filaments are subjected to at
least two further processing steps, which are (i) drying by natural convection at room
temperature followed by (ii) sintering. Of interest is the particle orientation within the fi-
nally processed filaments as it affects the material properties of the printed objects [
1
,
2
].
Therefore, a prediction of the particle orientation after an extrusion process is the topic
of ongoing research. However, while there exist a burgeoning number of analytical orien-
tation prediction models (OPMs) [
3
–
8
] or microscopic simulations [
9
–
11
] to predict such
a particle orientation within the still-wet filament, little is known about the magnitude
of particle reorientation in robocasted filaments during the drying step. Experimentally,
Materials 2022, 15, 2100. https://doi.org/10.3390/ma15062100 https://www.mdpi.com/journal/materials
