2008 Volume 49 Issue 2 Pages 331-338
The dynamics of droplet formation of liquid in a piezoelectric inkjet printing process with bipolar pulse and drop-on-demand (DOD) mode is investigated in this study. Two liquids with different viscosities and surface tension coefficients; alcohol and ethylene glycol, are studied. The effects of pulse voltage on the droplet formation are also examined. A piezoelectric actuated inkjet printhead with a nozzle orifice of 30 μm in diameter is employed to conduct the investigations at room temperature (25°C). The complex morphologies of the droplets during their formation, which include ejection and stretching of liquid, contraction of liquid column, pinch-off of liquid column from nozzle exit, breakup of liquid column into primary droplet and possible satellites, and combination of primary drop and satellites, are demonstrated. The droplet size is in the range of 23–37 μm. The investigations also show a workable pulse voltage range; between 28 and 40 volts, exists for the droplets to be smoothly generated and ejected for alcohol where viscosity and surface tension coefficient are smaller. The range is between 30 and 50 volts for ethylene glycol. Within this workable voltage range, one single droplet for each pulse can be achieved with lower voltage. For the intermediate voltage, two droplets are generated initially and collide into one during the flying stage. For the higher voltage, multiple droplets are formed without recombination. It is also found that the velocities of the main droplet and satellite droplet in the different voltage ranges are responsible for whether the multiple initial droplets can be recombined.