Printing conductive plastics

Abstract

Researchers at Loughborough University, UK, have been looking at how 3D printed conductive plastic materials respond to strain, and how this should influence design of printed electronic components like sensors. 3D printing initially started as a rapid prototyping technique that was only capable of printing single material models using non-functional thermoplastics. Recently, both research and industry has focused on printing multiple materials with functional electronics. Industry has released 3D printers that purely focus on interconnect, such as Voxel8 and Nano-dimension, which are a modern take on printed circuit board manufacturing. Currently, research is focusing on the 3D printing of passive electronic components, such as sensors. This progression has mirrored the rise in 2D printed electronics where development of interconnect led to printed passive components and then to fully functional printed devices. 3D printing is a sustainable, efficient manufacturing process with little waste. The ability to fully print a multi-material, three-dimensional and functional object gives the opportunity for electronic engineers and researchers to rapidly iterate and revolutionise the design of electronic systems. Jack McGhee with the 3D printer used to print conductive plastics at Loughborough University Printed electronic sensor made of conductive plastic To date, 3D printable conductive plastics tend to be the commonly-used thermoplastics polylactic acid and acrylonitrile butadiene styrene filaments, which have been mixed with conductive carbon. The electronic and mechanical characteristics of these materials are important for 3D printing and additive manufacturing to initiate a new industrial revolution. Material extrusion-based 3D printing uses layer-by-layer deposition. The effect of this manufacturing method on the electrical properties of the plastic to create reproducible applications needs to be better understood. In this Issue of Electronics Letters, Jack McGhee and colleagues at Loughborough take three sets of 3D printable conductive plastics and measure changes in resistance of the material in response to an applied strain. This was repeated for all three plastics in order to quantify the effect of the three-dimensionality of the material with different layer thicknesses. Altering the layer thickness showed consistent, repeatable and reproducible strain-sensing measurements until a specific material thickness was reached. At a certain thickness, the mechanical strength of the material increases the resistance upon bending. However, the manufacturing process creates a solid structure from tightly packed threads where upon bending threads move closer together to increase conductivity. This combination of factors creates erratic behaviour in the material. Analysis of this enabled the authors to create guidance for the design and fabrication of functional, reliable and repeatable 3D printed electronic sensors. A force sensing resistor platform that outputs the measurements to an LCD display was printed in this Letter to demonstrate an application for the work. For the additive manufacturing of reproducible devices and applications, how the manufacturing process affects the materials and the material properties needs to be understood. This Letter provides insight into how the manufacturing method, material extrusion 3D printing, and material structure fabricated from the technique, affects the bulk properties of a printed functional device. The results show how the layer-by-layer nature of the printing process can affect the properties of strain sensing devices made using conductive plastics. In the short term, the characterisations reported in this Letter can inform other researchers working on 3D printed electronics. Also, any person with a 3D printer can follow the guidance for the design and fabrication for 3D-printed strain sensors, which can be used with low-cost, low-profile and easily accessible parts, rather than with bulky signal amplification equipment. In the longer term, if 3D functional devices are to be mass produced, the manufacturers need to know a set of design rules to follow. This paper contributes to a larger set of knowledge that manufacturers will need to know to print devices. In general, McGhee and colleagues work with 2D printed electronics and ink formulations with new materials. Since this work, the authors have mainly focused on formulating conductive inks from transparent conducting oxides. “This paper was largely an exploratory study into the potential of 3D printed electronics and we are planning on using the insight to start introducing inks for 2D printed electronics into 3D printed objects” explains McGhee. Over the next 10 years, McGhee believes it would be interesting to see a conductive plastic made with metallic conductive filler, such as silver rather than graphite. McGhee says “seeing how the properties of this material compares to printed silver ink interconnect could enable new avenues of 3D printed electronics to be opened.” The general direction of 3D printed electronics currently suggests functional materials will be printed as inks and pastes in combination with non-functional thermoplastics.