Characterising the airflow instability in the print gap of inkjet printers.

Inkjet printer manufacturers face the challenge of designing systems that operate at elevated print gap heights to enable new applications for inkjet technology. At large print gap heights, the print quality tends to be compromised due to an airflow instability that misdirects the ink droplets, misplacing them on the paper. To characterise this instability, simultaneous high-velocity planar PIV measurements and printing tests were performed, together with numerical simulations using the dispersed-phase continuum (DPC) and particle-in-cell (P-in-C) droplet modelling techniques. The predicted and measured flow-fields showed a pair of counter-rotating vortices located near the injection-zone that became unstable with an increase in both print gap heights and number density of droplets. The results demonstrated that increasing droplet number density or print gap height induced a supercritical pitchfork bifurcation from a steady and uniform flow field across the domain to a standing wave regime with deformed vortex cores in the spanwise direction. With further increases in number density and print gap height, the flow field became oscillatory. These different flow dynamics misplace the droplets in specific patterns. The uniform flow field produces a defect-free print, whereas the standing wave regime creates a print with straight dark stripes. The oscillatory flow regime, on the other hand, leads to oblique dark stripes that are characteristic of the tiger-stripe printing defect.