Abstract
As new printing approaches emerge, in situ diagnostics to monitor the print quality in real-time become essential for long-term monitoring and feedback control. In this article, we present a millimeter-wave electromagnetic monitoring approach for liquid metal droplet-on-demand printing to support the high-speed and real-time evaluation of droplet ejection. An open-ended rectangular waveguide is placed perpendicular to a jetted droplet stream and operated at a continuous-wave frequency of 40 GHz. Liquid metal droplets with diameters as low as 1.2 mm are characterized, and droplet jetting events on the order of 500 μm are detected at ejection rates up to 80 Hz. The measured results demonstrate that trends at the macro-level (large-scale print variation and anomalies at the nozzle tip) as well as micro-level (droplet size, position, and dynamics) can be detected using this technique.
Highlights
Advanced material and manufacturing techniques have enabled the additive manufacturing of a broad range of materials
As new printing approaches emerge, in situ diagnostics to monitor the print quality in real-time become essential for long-term monitoring and feedback control
Because the quality of the jetted liquid metal droplets is influenced by a multitude of factors, in situ diagnostics are critical to monitor the droplet print quality over the entire duration of the build process.[3–5]
Summary
Advanced material and manufacturing techniques have enabled the additive manufacturing of a broad range of materials. We present a millimeter-wave electromagnetic monitoring approach for droplet detection and characterization during LMJ-DoD printing. In this approach, an open-ended waveguide is placed perpendicular to the droplet stream such that jetted droplets are within the electromagnetic near-field of the waveguide aperture. Operation at millimeter-wave frequencies can yield a major reduction in the required data volume when compared with high-speed video, if the required droplet information can be extracted from the reflected millimeter-wave signal amplitude and phase alone (as opposed to n  n pixels) This reduction was demonstrated in our previous work in which an open-ended T-junction was used to monitor impedance changes as droplets passed through the lateral arms of the T-junction.[8,9]. For three-dimensional additive manufacturing, the crucible is controlled with a motorized Z stage, and the build plate is controlled with an X–Y stage
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