Nozzle Wetting and Instabilities During Droplet Formation of Molten Nylon Materials in an Inkjet Printhead

Inkjet technology can offer exciting benefits in new additive manufacturing processes, including deposition of multiple materials. This in turn requires high reliability in terms of droplet formation consistency and placement accuracy. This paper presents research into such requirements during a research where drop-on-drop deposition of two reactive molten mixtures of caprolactam is used to print nylon 6. Using an inkjet system based on a graphics industry printhead, reliable jetting of the nylon materials through an array of nozzles has been established and as the subject of this paper, droplet formation instabilities (abnormalities in consistent formation of droplets train) initiated by the process conditions were investigated using high speed imaging. With image analysis, nozzle wetting around the actuating nozzle and the droplets train trajectory error were studied. High speed imaging revealed that the nozzle wetting decreased with increasing jetting frequency. Asymmetric development of the wetting area was observed in two situations: (1) when contamination existed near to the actuating nozzle and (2) when air motion changed the trajectory of low kinetic energy droplets formed by low jetting voltages. In both, separation of the droplet tail was observed to occur toward the asymmetric side of the wetting area. The droplet instability behavior initiated by asymmetric nozzle wetting and air motion were understood. For a reliable droplet placement, use of a uniformly wet nozzle plate and higher jetting voltages were recommended to avoid trajectory errors and jet failures as seen in this paper.