Fraunhofer Institute for Mechanics of Materials IWM
Fraunhofer Institute for Mechanics of Materials IWM

The correct particle orientation for fluid-based shaping

© Fraunhofer IWM

Calculation of the flow field

The rheology of particle-laden fluids is often complex due to shear-thinning, shear-thickening or thixotropic effects and the prediction of the flow field is not trivial. Typical issues in complex flows often concern the avoidance of dead spaces by means of an appropriately optimized geometry or ensuring a constant viscosity.

© Fraunhofer IWM

Precisely determining particle orientation

In particle-laden fluids with non-circular particles, these tend to have a certain alignment, especially in larger shear fields. Depending on the application, this may be desirable or undesirable. However, the particle orientation can be influenced. Questions include how strong the shear field needs to be for film casting or 3D printing processes such as fused filament fabrication in order to achieve a certain alignment of the primary particles and if other particle shapes can be used as an option. 

Examples of projects

© Fraunhofer IWM
Simulation of film casting with visualization of the streamlines. The color coding corresponds to the local shear rate. The aim of the investigation was to determine the orientation of the particles, which are still randomly oriented in the feed and align themselves during the process.

Optimizing particle orientation during film casting through process simulation


In the production of ceramic films, the local microstructure is a decisive factor for the material properties. However, the experimental determination of this parameter is cumbersome and simulations serve as an alternative for a sound basis for decision-making. Fraunhofer IWM has developed a coupled simulation model at microstructure and plant level. This model covers the entire ceramic casting process, including the calculation of the velocity field and particle orientation in the casting slurry and the resulting product. The simulations at the plant scale enable the process control for foil casting to be optimized cost-effectively and dead spaces to be avoided. At the microstructure level, the simulation allows conclusions to be drawn on how to specifically influence the particle orientation. In this way, simulation can help to reduce rejects, adjust the desired gradients in the particle distribution more precisely and improve the orientation of the particles in a targeted manner.

  • Further details on film casting can be found on our SimPARTIX homepage
  • P. Polfer, Z. Fu, T. Breinlinger, A. Roosen, T. Kraft, Influence of the doctor blade shape on tape casting - Compari-son between analytical, numerical and experimental results, J. Am. Ceram. Soc. 99 (2016) 3233-3240. Link
  • P. Polfer, T. Breinlinger, T. Kraft, Simulation of tape casting: capabilities and potentials, cfi/Ber. DKG 92, no 10-11 (2015) E181-E186 Link
  • Fu, Z.; Polfer, P.; Kraft, T.; Roosen, A.; Three-dimensional shrinkage behavior of green tapes derived from spherical-shaped powders: Experimentical studies and numerical simulations; Journal of the European Ceramic Society 35/8 (2015) 2413–2425 Link

© Fraunhofer IWM
Flow behavior of a silver paste in front of a squeegee during screen printing. The figure shows the streamlines and color-codes the shear rate of the paste. The high shear rate at the bottom of the screen leads to a reduction in paste viscosity.

Reduction of structure widths in screen printing of ceramic multilayer circuits


Ceramic multilayer circuit carriers can be found in microwave circuits, pacemakers, sensors and WLAN units, among other things. The fine conductor tracks are applied to the circuit boards by screen printing. For this purpose, a metal paste containing fine silver or other precious metal particles is printed onto a surface through a stencil according to the desired shape and then sintered together with the ceramic foils at a relatively low temperature. At Fraunhofer IWM, a simulation model was developed and used to fully describe the flow behavior of the paste in the screen printing process. It was shown that a hydrophobic coating on the underside of the screen significantly improves paste release, while a separate coating on the upper side of the screen is not necessary. This facilitated the development of suitably coated screens and adapted pastes for industrial partners. 

© Fraunhofer IWM
Visualization of the robocasting extrusion process, in which a continuous strand is extracted from a controllable nozzle. Of interest is the orientation of the ceramic particles in the strand.

Rheology of ceramic pastes during robocasting


Robocasting is an example of additive manufacturing based on material extrusion. In this specific example, a paste was considered that consists of ceramic particles of different shapes and sizes on a microscopic scale. The project focused in particular on the orientation of the particles in the printed strand. Specifically, an attempt was made to set a uniform particle orientation via the paste rheology and the nozzle geometry. For this purpose, a process sequence was established in which simulations on a microscopic scale were used to precisely resolve the particle geometry and then the results were used on a macroscopic scale. Specifically, the data was used to calibrate effective orientation models. This combination allows any particle geometry and size distribution to be considered as well as accurate particle orientations to be predicted using fast models such as the Folgar-Tucker model.

  • Dietemann, B.; Bosna, F.; Lorenz, M.; Travitzky, N.; Kruggel-Emden, H.; Kraft, T.; Bierwisch, C., Numerical study of texture in material extrusion: Orientation in a multicomponent system of spheres and ellipsoids, Journal of Non-Newtonian Fluid Mechanics 291 (2021) Art. 104532 Link
  • Dietemann, B.; Bosna, F.; Kruggel-Emden, H.; Kraft, T.; Bierwisch, C., Assessment of analytical orientation prediction models for suspensions containing fibers and spheres, Journal of Composites Science 5/4 (2021) Art. 107, 18 Seiten Link
  • 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 15/6 (2022) Art. 2100, 20 Seiten Link
  • Dietemann, B.; Bierwisch, C., Predicting particle orientation: Is an accurate flow field more important than the actual orientation model?, Journal of Non-Newtonian Fluid Mechanics 310 (2022) Art. 104927, 12 pages Link

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