(6b) Influence of Interfaces in Electrical Properties of 3D Printed Structures

Additive Manufacturing technology has advanced to being a major player in the field of manufacturing in the last two decades revolutionizing the manufacturing of mechanical parts for aerospace and automotive applications, prosthetics, and dentistry. The application of additive manufacturing in the production of sensors and actuators can alleviate the delivery and storage related constraints for remote locations such as offshore platforms and space stations. Material extrusion-based additive manufacturing (MEAM) is a uniquely suited additive manufacturing technique for cost-effective printing of sensors and actuators. In MEAM, a 3D structure is built in a layer-by-layer manner as instructed by a g-code, by extruding a thermoplastic material through a nozzle heated above the melting point of the plastic. Several parameters including extrusion temperature, print speed, bed temperature, part orientation, layer height, raster angle, and infill can be varied in MEAM. Contemporary simulation and experimental studies, both show that the print parameters play an important role in determining the strength of interlayer bonding and the resulting anisotropy in mechanical strength of printed parts. To develop MEAM into an industrially feasible technique for 3D printing sensor and actuator components, similar fundamental studies are necessary to predict electrical part performance based on print parameters.

In this study, we conduct work using a commercial conductive composite of polylactic acid and carbon black. We adopt a hollow box structure specimen to examine the effect of extrusion temperature and print speed on the electrical characteristics of the bond interfaces, specifically those between two subsequent layers. The walls of the hollow box compose of single extruded fibers stacked on top of each other. The electrical characteristics are measured using small signal impedance spectroscopy to avoid Joule heating effects. The impact of varying the extrusion temperature and print speed on electrical impedance was examined in two directions, across the interlayer bond interfaces (Z-direction), and along the interlayer bond interfaces (F-direction). A variation of impedance in F-direction with extrusion temperature was observed and not print speed, indicating that the material resistivity varied with extrusion temperature. Thus, the Z/F ratio was used to quantify anisotropy in electrical impedance as the extrusion temperature and print speed were varied. It was found that the Z/F ratio remains relatively constant with variation in extrusion temperature and print speed. Variation in sample cross-section in Z- and F- direction were found to result in a Z/F ratio of 1 in COMSOL Multiphysics simulations. This showed that the bond interfaces were primarily responsible for the observed Z/F of 2.15 ± 0.23, that is, the bond interfaces contribute nearly two-third of the impedance when measuring across the fibers.

To understand how the impedance scales with the number of interfaces, impedance was measured across different number of bond interfaces in samples printed with extreme parameters - high/low print speed and high/low extrusion temperature. Impedance was found to scale linearly with the number of interfaces. This implied that tests across single bond interfaces could be used to predict results over multiple interfaces; thus, allowing to study effects of multiple parameters in a relatively short time. The scaling law was found to change when using different extrusion temperature; high extrusion temperature has a lower impedance per interface.

Thus by investigating the electrical behavior of bond interfaces, we provide a framework to model and predict anisotropy in electrical impedance and predict electrical impedance based on print parameters.