Thermal and Piezo Inkjet DOD

Written October 16, 2019

Drop on demand (DOD) is a broad classification of inkjet printing technology where drops are ejected from the print head only when required. The drops are formed by the creation of a pressure pulse within the print head. The method used to generate this pressure pulse creates the primary sub-categories within DOD, namely thermal and piezoelectric (piezo). Disposable: Inkjet head is bonded onto the ink cartridge (image on left). Thermal inkjet, HP, Lexmark, Kodak, Canon, others. There are also versions of thermal heads where the head component is separated from the ink cartridge proper.

Thermal inkjet heads all have a finite life span due to the construction of the head proper. Current versions of thermal inkjet heads have very good longevity measured in volume of ink (ml) pumped before failure. Non-disposable: inkjet head and ink supply cartridges are separated. Piezo drop on demand, and continuous inkjet (image on right) heads are used in most industrial printing equipment, large format, wide format, and inkjet web presses.

Head Manufacturers

Aprion, Canon, FujiFilm (Dimatix/Spectra), Epson, Konica Minolta, Hitachi, HP, Lexmark, Jetrion, Ricoh, Samsung, Benq, Brother, Lexmark, Pixdro, Seiko, Trident, Videojet, Olympus, Toshiba, Kodak, Versamark, Xaar, and more.

Head Cleaning

The primary cause of inkjet printing problems is due to ink drying on the printhead's nozzles, causing the pigments and dyes to dry out and form a solid block of hardened mass that plugs the microscopic ink passageways. Most printers attempt to prevent this drying from occurring by covering the printhead nozzles with a rubber cap when the printer is not in use. Abrupt power losses or unplugging the printer before it has capped the printhead, can cause the printhead to be left in an uncapped state. Even when the head is capped, this seal is not perfect, and over a period of several weeks the moisture (or other solvent) can still seep out, causing the ink to dry and harden.

Once ink begins to collect and harden, the drop volume can be affected, drop trajectory can change, or the nozzle can completely fail to jet ink. To combat this drying, nearly all inkjet printers include a mechanism to reapply moisture to the printhead. Typically, there is no separate supply of pure ink-free solvent available to do this job, and so instead the ink itself is used to remoisten the printhead. The printer attempts to fire all nozzles at once, and as the ink sprays out, some of it wicks across the printhead to the dry channels and partially softens the hardened ink. After spraying, a rubber wiper blade is swept across the printhead to spread the moisture evenly across the printhead, and the jets are again all fired to dislodge any ink clumps blocking the channels.