Droplet Formation in Continuous Inkjet (CIJ) and Drop-on-Demand (DOD)
Concepts of Continuous Inkjet (CIJ) and Drop-on-Demand (DOD)
In both Continuous Inkjet (CIJ) and Drop-on-Demand (DOD) methods, liquid ink flows through a small orifice, commonly referred to as a nozzle.
The fundamental difference between the two methods lies in the nature of the flow through the nozzle. In CIJ, as the name implies, the flow is continuous, whereas in DOD it is intermittent and pulsed.
A CIJ system generates a continuous stream of droplets, from which selected droplets are directed to print onto the substrate as needed. In contrast, in DOD printing, ink is ejected through the nozzle to form short liquid jets that condense into droplets only when required.
A continuous liquid jet emerging from a nozzle is inherently unstable and will eventually break up into a stream of droplets under the influence of surface tension.
Continuous Inkjet (CIJ)
Liquid Formation Process in CIJ
In the liquid jet illustrated above, vibrations applied to the fluid upstream of the nozzle stimulate controlled jet breakup. Even without stimulation, an unstimulated jet will eventually break up in a similar manner after traveling a longer distance.
In modern CIJ printing, droplets intended for deposition on the substrate are typically controlled electrostatically. As each droplet separates from the end of the continuous jet, an electric charge is induced on it. When the charged droplet subsequently passes through a fixed electric field, it is deflected by a precise amount so that it lands at the correct position on the substrate. In contrast, uncharged droplets are directed into a gutter, allowing excess ink to be recovered and recirculated back to the nozzle.
In this way, a stream of droplets from a single nozzle, combined with substrate motion, can be used to print large areas of text or images.
CIJ is also employed in multi-nozzle configurations, where each jet addresses a specific pixel position within a printed swath. As described above, droplets from a single jet are either deflected into a gutter or directed onto the substrate via an electric field or other mechanisms, thereby forming the printed pattern.
Drop-on-Demand (DOD)
Liquid Formation Process in DOD
The figure above shows jets ejected from three nozzles in DOD mode and traveling to the right. Behind the primary droplet at the head of each jet is a long liquid ligament, which eventually detaches from the nozzle (at the left edge of the image) and is pulled toward the leading droplet while thinning. In this process, a series of smaller “satellite” droplets may form.
The final jet structure may consist of a single spherical droplet (the ideal case), or more commonly, a primary droplet followed by one or more smaller satellite droplets.
In DOD printing, droplets are not deflected. Instead, each printhead contains a large array of nozzles (ranging from hundreds to thousands). The position where each droplet lands on the substrate is controlled by the relative motion between the droplet and the substrate, the timing of droplet ejection, and the selection of the appropriate nozzle within the array.
Most modern DOD printers use one of two methods to generate the pressure pulse required for droplet ejection. Many printheads rely on the deformation of piezoelectric ceramic elements, while in other printheads (thermal inkjet, sometimes referred to as bubble jet), the pressure pulse is generated by the expansion of a small vapor bubble formed when a tiny resistive heater rapidly heats the liquid.
Each actuation method has its advantages and disadvantages. Compared with thermal inkjet printheads, piezoelectric printheads can handle a much wider range of liquids (thermal inkjet is limited to inks that can be vaporized satisfactorily). However, thermal inkjet printheads are generally simpler in structure and less expensive to manufacture.
Typical Droplet Size and Velocity
The droplet diameters used in inkjet printing typically range from 10 to 100 μm, corresponding to droplet volumes of approximately 0.5 to 500 pL. At the point of impact on the substrate, droplet velocities are typically 5–8 m/s for DOD printing and 10–30 m/s for CIJ printing.