Mechanisms of Droplet Formation and Deposition in Drop-On-Demand Needle-Valve Inkjets for Precision 3D Microprinting

Understanding the droplet formation and deposition process in 3D printing is essential for the optimization of printhead design and jetting parameters to print a desired object. A seminal computational study is presented to comprehensively investigate the entire process from liquid flow inside the nozzle chamber to droplet formation and deposition in needle-valve inkjet printing of non-Newtonian inks. It is found that the competing effect between flow cohesion and surrounding air disruption causes an oscillated instability on the inkjet surface. Various defects in droplet formation have been identified, which include excessive liquid suction into the nozzle tube due to the relative vacuum created by the valve needle motion, liquid accumulation on the nozzle mouth due to the insufficiency of liquid momentum to overcome the surface tension cohesion, and satellite droplet formation as a result of the overflow of liquid ink through the nozzle at a high droplet velocity. The effect of jetting parameters and nozzle design on the droplet volume, velocity and deposition process are amply discussed. It is shown that this study can allow the volume and velocity of the generated droplets to be controlled reasonably precisely to as small as 1.75 nL and 1 m/s respectively to avoid droplet defects and liquid splashing during printing, so can the droplet diameter at the equilibrium stage of deposition be controlled to achieve printed objects of desired geometry and accuracy. The model verification shows a good agreement between the model predictions and the corresponding experimental data. This study has provided a deeper understanding of the complex flow dynamics involved in the needle-valve inkjet, which forms a fundamental basis for the optimization of printhead design and jetting parameters to improve the resolution and quality of printed micro-objects.

Automated summary

Understanding the process of droplet formation and deposition in 3D printing is crucial for optimizing printhead design and jetting parameters. A computational study on needle-valve inkjet printing of non-Newtonian inks is presented, exploring the entire process from liquid flow inside the nozzle chamber to droplet formation and deposition. Various defects in droplet formation have been identified, and the effect of jetting parameters and nozzle design on droplet characteristics and deposition process is discussed. The study shows that precise control of droplet volume, velocity, and diameter can be achieved, allowing for improved resolution and quality in printed micro-objects.