June 27, 2014 | By David Pommerenke, Kai Parthy, Jiao Xiangyang

Two years ago, desperately trying to get a $350 Chinese 3D printer to operate the idea was born to modify the filament to print materials that can influence the electromagnetic field, in a more complex way than just ordinary plastic. It is well known that adding particles will modify the electromagnetic behavior of plastics. Here ceramic is added to increase the permittivity (changes the interaction with the electric field), ferrite and iron are added to change the interaction with the magnetic field, carbon, or metal particles, or metal fibers carbon fiber are added to create conductivity that can be used for creating electrical connections and shielding, etc. The problem is to create a filament that can be printed. To maintain printability the viscosity must be OK and about 101 parameters need to be set correctly. After a few failed tries to mix and extrude we came across 3ders article in which Kai Parthy describes his chalk filled polymers.

Quickly he was able to create a TiO2 filled polymer that increases the permittivity of the plastic (ϵr= 3.5) to a value of 7. This allows to design and print structure that have a strong interaction with the electromagnetic field. Examples are antennas, or to print waveguiding or filtering structures. As mentioned, for achieving this, polymers are filled with dielectric, magnetic or conductive fillers. Further improving the material composition Kai Parthy and the research group at the Missouri University of Science and Technology (Prof. David Pommerenke, davidjp@mst.edu) was able has to develop a printable dielectric material having a relative permittivity of up to 9 by using a 76% volume fill of TiO2 in a polymer. TiO2 is the pigment used in most white paints. The permittivity is rather constant over a wide frequency range covering > 20GHz.

Figure 1: Permittivity of different TiO2 + Polymer filaments

A difficulty that needed to be overcome was to maintain the printability at high volume fill ratios. In injection molding the high pressure allows to force materials of high viscosity into the mold. However, FDM printers operate at much lower pressures. Due to the selection of the right base polymer a filament could be created that will not break and can be printed in a standard printer.

As it is possible to locally adjust the fill ratio it is possible to locally vary the permittivity of the resulting material. A 100% fill will lead to a permittivity of 9, which can be reduced to nearly 1 by reducing the volume fill ratio. This can be realized by either designing a locally varying inner structure, or using slicer settings that change the fill ratio throughout the volume. This tapered structures allow to guide waves within the highly filled sections and allow them to transition gently to the lightly filled regions from which they can be radiated. This type of tapering was used to create a dielectric antenna. Traditionally, multiple layers of different polymers would have to be extruded to guide the waves resulting in reflections between the layers and in a complex production process. Instead a locally varying density of air filled pockets was selected to create a dielectric antenna.

Figure 2: Cross section of the dielectric antenna and its coaxial feed

Figure 3: Photo of the printed dielectric antenna placed on a printed circuit board

The antenna matched to the coax cable at a frequency of 3.3 GHz. With reduced specifications it operates from 3-7 GHz. Due to the consistency achieved in material production and printing it is possible to predict the antenna performance by simulation. The numerical simulation is performed in CST Microwave studio. An example of a predicted and a measured radiation pattern is shown below.

Figure 4: Simulated and measured radiation pattern of the FDM printed dielectric antenna at 3.3 GHz

While the prediction is not perfect it still indicates the success in designing FDM-printable materials that have defined electromagnetic properties. Further development will include the addition of magnetic materials for high frequency shielding and absorption, possibly combined with carbon to achieve conductivity.

Posted in 3D Printing Materials

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