Complex Permittivity and Anisotropy Measurement of 3D-Printed PLA at Microwaves and Millimeter-waves

The use of 3D-printing based on extrusion depositing technologies for electromagnetic applications (such as dielectric antenna design or human mimicking tissues) is increasing as a low cost alternative to other small-scale fabrication technologies. As a result, it is of utmost scientific interest to know the exact dielectric properties of commercially available printable materials, such as polylactide acid (PLA). The extrusion depositing process may originate an anisotropic behavior of the material permittivity especially at mm-waves. We investigate this plastic's complex permittivity across the microwave spectrum from 0.5 GHz to 20 GHz and at 40 GHz and 60 GHz. We propose a new indirect method for the complex permittivity determination at microwave frequencies based on a microstrip line with stubs and a superstrate. At 40 GHz and 60 GHz, the printed PLA samples are characterized using waveguide and open resonator methods. Results show that throughout the tested frequency range the dielectric constant along the normal direction to the printing surface is around 2.75 +/- 0.05 and tan delta = (1.1 +/- 0.2) x10(-2). However, at least for the tested mm-wave bands, the dielectric constant along the parallel direction to the printing surface is around 2.96, corresponding to nearly 7% anisotropy.