Graduate Students Research 3D Printing with Droplet Deposition


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In ‘Computational Modeling of Droplet Deposition and Coalescence for Dropwise Additive Manufacturing,’ master’s student Priyanshu Vishnoi submits a thesis to the Graduate School of the University at Buffalo (School of Chemical and Biological Engineering) centered around the study of 3D printing with metal.

Using alloy Aluminum 6061 (a material often used for testing), Vishnoi observed not only the benefits of additive manufacturing over subtractive, but also analyzed the use of a new 3D printer being developed by Vader Systems while examining the use of droplet deposition. The researcher and his team used a variety of mathematical programs to understand more about droplet-air and droplet-substrate interactions and studied the structures that were actually 3D printed to find out more about how the parameters they set affected fabrication.

Vishnoi points out that there was positive ‘agreement’ between computational models and the data they extracted. His study goes into some detail regarding subtractive processes but points out that 3D printing can supersede such traditional manufacturing as it grants the user the ability to create metal prototypes that would have been too challenging or expensive otherwise. He also points out added benefits to 3D printing such as speed and added efficiency; however, Vishnoi and his team are certainly not in the dark about drawbacks either for industrial manufacturing such as overall startup costs to put AM processes in place and then control them. Metal of course is not the only material being widely used in additive manufacturing either—and the list of substances available continues to grow, from ceramics to concrete to the more ubiquitous polymers which continue to reign.

For AM of metals using droplet ejection techniques, Vishnoi points out that operational obstacles continue to be a problem in terms of thermal management, droplet ejection, and droplet patterning with deposition, coalescence and solidification. The current research focuses on drop-on demand printing with metal, with the droplets being extruded at a consistent interval.

“Drop-on-demand 3D printers are commonly made up of a small-sized orifice, a reservoir and a printhead that generates a pressure pulse so as to create a discontinuity in the ejected fluid stream,” states Vishnoi in his paper.

“In order to build accurate droplet patterning, a substrate is set up beneath the printhead. The substrate moves at a pre-programmed velocity, which must be matched to the frequency of the droplet ejection. Droplet ejection frequency varies according to the shape of the structure to be printed.”

Vishnoi and his team focused solely on magnetohydropdynamic (MHD) droplet ejection. The goal was to be able to manufacture more complex structures using the Magnetojet process, along with detailing their computational model.

Conceptual schematic of MHD jetting process

MHD is hardly a new concept, in use since 1907, with potential for modern applications in the following fields:

  • Geophysics
  • Astrophysics
  • Sensors
  • Magnetic drug targeting
  • Power generation

MHD is still used widely for casting processes, but here the researchers attempt to convert it to a new method for 3D printing.

 “The MagnetojetTM printing process has been used to create aluminum parts with a repetition rate up to 1000 droplets/sec, with a droplet placement resolution of 500 µm. It has achieved a mass deposition rate of up to 1 lb per hour based on a single orifice that generates droplets with a 500 µm diameter,” states Vishnoi in his paper. “In addition, it is a relatively low-cost process that can print parts with improved mechanical properties owing to the presence of a unique metal grain structure.”

Schematic design of the MHD-based DOD liquid metal printing system

The team used Flow-3D software for their computational analysis, studying thermo-fluidic dynamics. They found that droplet solidification was affected by the following:

  • Ejection frequency
  • Temperature
  • Velocity
  • Size
  • Spacing
  • Substrate factors

They also used wall and pressure boundaries to apply no-slip conditions as well as static and stagnation pressure. Testing was performed on a simple model with ten droplets being 3D printed first—and more complex geometries fabricated later in testing.

The university researchers discovered that shape of the product had a lot to do with their success in 3D printing. Droplet overlap was an important parameter in this 3D printing research, along with overlap fraction, and droplet ejection.

“The process is extremely cost effective since it uses metal wire feedstock, thus eliminating the need of specially prepared powder. Still, challenges remain in realizing the optimum operating parameters of operation, and improvement of overall process performance. Isolating the critical droplet deposition parameters will allow the process to build a broader range of metallic structures, such as inclined pillars, horizontal overhangs etc. with high mechanical strength and minimum material wastage,” stated the researchers in conclusion. “To address this, we have presented a computational modeling approach, focusing on the droplet deposition model. This model can be used for a rational study and design of MagnetojetTM process, as well as similar drop-on-demand processes and will be improved in the 45 future to include additional physics, in order to faithfully model the process in its entirety.”

For the future, the research team looks forward to designs for similar 3D printed models using a variety of different types of versatile metals and alloys like iron, titanium, and nickel—all of which can be printed at a high temperature.

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[Source / Images: Computational Modeling of Droplet Deposition and Coalescence for Dropwise Additive Manufacturing]

The effect of droplet overlap fraction on the printed structure: (a) 10 droplets at 100 Hz, 0.5 O.F. (b) 10 droplets at 100 Hz, 0.80 O.F. (c) 10 droplets at 100 Hz, 0.90 O.F. (d) 10 droplets at 100 Hz, 1.0 O.F.

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