Air Entrapment and Drop Formation in Piezo Inkjet Printing

Piezo drop-on-demand (DOD) inkjet printers are used in an increasing number of applications for their reliable deposition of droplets onto a substrate. Droplets of a few picoliters are ejected from an ink jet nozzle at a frequency of up to 50 kHz. However, entrapment of an air microbubble into the ink channel can severely impede the productivity and reliability of the printing system. The air bubble disturbs the channel acoustics resulting in disrupted drop formation and failure of the ink channel. In this research we study air entrapment in a normal inkjet printhead and a Micro-Electro-Mechanical Systems (MEMS) based inkjet printhead. By using the actuating piezo transducer in receive the acoustics inside the channel could be monitored, clearly identifying the presence of an air microbubble inside the channel during a channel failure. A model was developed to calculate the two-way coupling between the channel acoustics and the disturbing bubble. The model was validated by simultaneous acoustical and infrared detection of the bubble. Using a glass inset in the normal printhead and a infrared visualization technique for the MEMS printhead allowed for an accurate depiction of the bubble size and its translational dynamics inside the print head. The model proves to be a very valuable tool in calculating the presence, the size and the position of entrapped air bubbles inside an operating ink printhead, purely from its acoustic response. We also studied the details of droplet formation with the Brandaris ultrahigh-speed imaging facility at frame rates of up to 10 million frames per second and using an even faster stroboscopic technique of order 5 nanoseconds. The highly reproducible recordings were analyzed to give the local flow rate and velocity distribution within the droplet. The results are compared to a 1-dimensional model based on the lubrication approximation. In the model the equations are solved on an Eulerian grid with a second order accurate scheme. The pinch-off singularity is prevented by regularization of the surface tension at small radius and overall we find excellent agreement with the experimental observations