A 'very informative' and useful series of articles. Reprinted with authorization from Image Specialists.

Thermo-Technology

Most inkjets use thermal technology, whereby heat is used to fire ink onto the paper. There are three main stages in this process. The squirt is initiated by heating the ink to create a bubble until the pressure forces it to burst and hit the paper. The bubble then collapses as the element cools, and the resulting vacuum draws ink from the reservoir to replace the ink that was ejected. Canon and Hewlett-Packard favor this method.

Tiny heating elements are used to eject ink droplets from the print head's nozzles. Most thermal inkjets have print heads containing a total of between 300 and 600 nozzles, each about the diameter of a human hair (approx. 70 microns). These deliver drop volumes of around 8 to 10 picolitres (a picolitre is a million millionth of a liter), and dot sizes of between 50 and 60 microns in diameter. By comparison, the smallest dot size visible to the naked eye is around 30 microns. Dye - based cyan, magenta and yellow inks are normally delivered via a combined three-color (cyan, magenta and yellow) print head. Several small color ink drops - typically between four and eight - are typically combined to deliver a variable dot size. Black ink, which is generally based on bigger pigment molecules, is delivered from a separate print head in larger drop volumes of around 35pl.

Nozzle density, corresponding to the printer's native resolution, varies between 300 and 600 dpi, while enhanced resolutions of 1200 dpi are increasingly becoming available. Print speed is chiefly a function of the frequency with which the nozzles can be made to fire ink drops and the width of the swath printed by the print head. This is usually around 12MHz and half an inch respectively, giving print speeds of between 4 to 8 ppm for monochrome text and 2 to 4 ppm for color text and graphics.

Thermal technology, meanwhile, imposes the limitation that whatever type of ink is used; it must be heat-resistant because the firing process is heat-based. Using heat in thermal printers conversely also creates a need for a cooling, which adds a to the overall length of printing time.

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