Fundamentals of Inkjet Technology – Droplet Formation

I. History of Inkjet Printing Development

 The principle of inkjet printing was first commercially developed in the 1970s and 1980s, initially applied to product date marking, coding, and mailing addresses. The technology used for these applications has now matured completely, requiring high operational speed while tolerating relatively low resolution in the printed text. These "Continuous Inkjet" (CIJ) printers are now widely used as standard equipment in factories globally. The following developments, starting in the 1990s, involved "Drop-On-Demand" (DOD) printing, which achieves higher resolution than the early coders and provides affordable digital reproduction of text and images in home and small office environments. Recently, inkjet printing has rapidly advanced in commercial applications and other areas, mainly using DOD technology.

II. Droplet Formation: Continuous Inkjet vs. Drop-On-Demand

In both CIJ and DOD methods, liquid ink flows through a small opening, typically called a nozzle. The essential difference between the two methods lies in the nature of the flow through the nozzle. In CIJ, as the name suggests, the flow is continuous, whereas in DOD, the flow is pulsed.
 CIJ systems generate a continuous flow of droplets, selecting those that need to be printed onto the substrate, while in DOD printing, droplets are ejected only when needed. These ink droplets are created by generating a pressure pulse inside the ink chamber. Subcategories of DOD printing are defined by the various methods used to create the pressure pulse, with three primary types being thermal inkjet, piezoelectric, and electrostatic.

a. Continuous Jet Decomposition Forming Droplets
b. Droplet and Jet Formation in DOD Mode from Three Nozzles, Moving Right

In 1833, Savart conducted the first experiments on this process, and in 1879, Rayleigh performed quantitative analysis, showing that the fastest-growing disturbances in the jet (and therefore the resulting distance between the droplet centers) were approximately 4.5 times the diameter of the jet.
This phenomenon, commonly referred to as Rayleigh breakup, was first applied by Sweet at Stanford University in the early 1960s as the basis for CIJ printers (used for recording oscilloscope traces).

Sweet's design introduced the key concept of stimulating jet breakup by modulating the jet at an appropriate frequency and using electrostatic forces to deflect the droplets. In the flow shown in Figure a, the breakup is stimulated by applying vibrations to the upstream fluid of the nozzle, though even the unperturbed jet will break up in a similar manner after traveling a longer distance.
In modern CIJ printing, droplets that need to be printed onto the substrate are typically guided using electrostatic forces. When each droplet detaches from the end of the continuous flow, it acquires a charge so that, when it passes through a fixed electric field, it is correctly deflected to the right location on the substrate. In contrast, uncharged droplets are directed to a catchment where excess ink can be recovered and recycled back to the nozzle. In this way, the droplets from a single nozzle are combined with a moving substrate, used to print a line of text or an image. CIJ is also used in multi-nozzle configurations, where each jet handles a pixel location on the print strip. The printed pattern is created by deflecting droplets from a single jet through an electric field (as described above) or by other means to either a catchment or to the substrate.

Figure b shows the jet formation from three nozzles in DOD mode, moving right. The primary droplet forms at the head of the jet, followed by a long liquid filament that eventually detaches from the nozzle (at the left edge of the image), being pulled toward the jet head, thinning, and forming a series of "satellite" droplets. The final form of the jet could be a single spherical droplet (ideal), or more commonly, a primary droplet followed by one or more smaller "satellite" droplets.

In DOD printing, droplets are not "guided"; many nozzles (ranging from hundreds to thousands) are arranged in an array in each printhead. The position where each droplet falls on the substrate is controlled by the relative motion between the droplet and the substrate, the timing of droplet ejection, and selecting the appropriate nozzle from the array.
Most current DOD printers use one of two methods to generate the pressure pulses required to eject ink droplets. Many printheads use the deformation of piezoelectric ceramic elements to achieve this, while in other types (thermal inkjet, sometimes referred to as bubble jet), the pressure pulse for ejecting the droplet is generated by the expansion of a small vapor bubble formed by the action of a small electric heating element on the liquid.

Both driving types have their advantages and disadvantages. Piezoelectric printheads can handle a broader range of fluids than thermal printheads (which are limited to inks that can be satisfactorily vaporized), while the latter are simpler and cheaper to manufacture. Piezoelectric DOD printheads were first designed in the 1970s, with thermal DOD developed a bit later.
The droplet diameters used in inkjet printing typically range from 10 to 100 μm, corresponding to droplet volumes of 0.5 to 500 pl.
The speed of droplets impacting the substrate is typically 5–8 m/s for DOD printing and 10–30 m/s for CIJ.