Drop-on-demand

A Canon inkjet with CMYKcartridges

Drop-on-demand (DOD) is divided into thermal DOD and piezoelectric DOD.

Most consumer inkjet printers, including those from Canon, Hewlett-Packard, and Lexmark, use the thermal inkjet process. The thermal inkjet principle was discovered by Canon engineer Ichiro Endo in August 1957. In the thermal inkjet process, the print cartridges contain a series of tiny chambers, each containing a heater, all of which are constructed by photolithography. To eject a droplet from each chamber, a pulse of current is passed through the heating element causing a rapid vaporization of the ink in the chamber to form a bubble, which causes a large pressure increase, propelling a droplet of ink onto the paper (hence Canon's trade name of Bubble Jet). The ink's surface tension, as well as the condensation and thus contraction of the vapor bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir. The inks used are usually water-based and use either pigments or dyes as the colorant. The inks used must have a volatile component to form the vapor bubble, otherwise droplet ejection cannot occur. As no special materials are required, the print head is generally cheaper to produce than in other inkjet technologies.

Most commercial and industrial inkjet printers and some consumer printers (those produced by Epson and Brother Industries) use apiezoelectric material in an ink-filled chamber behind each nozzle instead of a heating element. When a voltage is applied, the piezoelectric material changes shape, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. Piezoelectric (also called Piezo) inkjet allows a wider variety of inks than thermal inkjet as there is no requirement for a volatile component, and no issue with kogation (buildup of ink residue), but the print heads are more expensive to manufacture due to the use of piezoelectric material (usually PZT, lead zirconium titanate). A DOD process uses software that directs the heads to apply between zero to eight droplets of ink per dot, only where needed. Piezo inkjet technology is often used on production lines to mark products. For instance, the "use-before" date is often applied to products with this technique; in this application the head is stationary and the product moves past. Requirements of this application are high speed, a long service life, a relatively large gap between the print head and the substrate, and low operating cost.

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