Manipulating Drop Formation in Piezo Acoustic Inkjet

Inkjet developments move towards higher productivity and quality, requiring adjustable small droplet sizes fired at high repetition rates. Normally, maximum jetting efficiency is achieved by tuning the slopes of the driving waveform to the travel times of acoustic waves inside the channel. Important parameters are channel length, compliance of channel cross-section (through its impact on the effective speed of sound in the ink) and the inlet geometry (through its impact on the phase shift of reflected acoustic waves). In addition, nozzle size and drop speed are the main factors determining the default drop size. The shape of the nozzle is important for the acoustic impedance and thus for efficiency and damping (enabling high drop-on-demand frequencies). In this paper eight drop-size-modulation (DSM) techniques are identified. First, DSM can be achieved by modifying the pulse width and changing the fill-before-fire level. Next, droplet size can be enlarged by using acoustic resonances, either by pre-actuating the meniscus movement or by firing bursts of multiple drops in phase with the channel acoustics. Droplet size can be reduced by using break pulses, especially when combined with a satellite drop formation mechanism. Finally meniscus resonances and drop formation resonances can be used. It is shown that using an appropriate combination of these modulation mechanisms droplet sizes of 7 to 65 pl can be fired at 20 kHz with one printhead geometry. Introduction Inkjet is an important technology in document printing and many new industrial applications. Océ applies inkjet in its wide format color printing systems. New inkjet developments move towards higher productivity and quality. Understanding the drop formation process and all relevant phenomena [1] is the starting point to control the drop formation [2], reduce cross-talk effects and to achieve a maximum reliability, necessary for all productive applications. This paper focuses on the mechanisms involved in firing multiple drop sizes. Operating Principle A long ink channel with a nozzle at the right and a large reservoir at the left is the basic geometry of the inkjet device as shown in figure 1. A piezo actuator element drives each channel. To fire a droplet, an electric voltage is applied and the channel cross-section will be deformed by the inverse piezo-electric effect. This results in pressure waves inside the channel. The pressure waves propagate in both directions and will be reflected at changes in characteristic impedances (variations in cross-section and compliance of the channel structure). In the simplified diagram in figure 1 a first slope of the driving waveform enlarges the channel cross-section and the resulting negative pressure wave will be reflected at the reservoir at the left. The reservoir acts as an open end and the acoustic wave returns as a positive pressure wave. Figure 1. Actuation and resulting channel acoustics The second slope of the driving waveform will reduce the channel cross-section to its original size and will amplify the positive pressure wave when tuned to the travel time of this acoustic wave. The resulting pressure just before the nozzle is shown in figure 2.