Inkjet Printer

Inkjet printing,  is a non-impact method. Ink is emitted from nozzles as they pass over a page, and the operation of an inkjet printer is easy to visualise: liquid ink from a tank for each of the three primary colours and black is squirted at the paper to build up an image. A print head scans the page in horizontal strips, using a motor assembly to move it from left to right and back, as another motor assembly rolls the paper in vertical steps. A strip of the image is printed, then the paper moves on, ready for the next strip. To speed things up, the print head doesn’t print just a single row of pixels in each pass, but a vertical row of pixels at a time.

Most inkjets use thermal technology, whereby heat is used to fire ink onto the paper. There are three main stages with this method. 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. This is the method favoured by Canon and Hewlett-Packard.

Thermal Inkjet Technology

Thermal technology imposes certain limitations on the printing process in that whatever type of ink is used, it must be resistant to heat because the firing process is heat-based. The use of heat in thermal printers creates a need for a cooling process as well, which levies a small time overhead on the printing process. Tiny heating elements are used to eject ink droplets from the print-head’s nozzles. Today’s thermal inkjets have print heads containing between 300 and 600 nozzles in total, each about the diameter of a human hair (approx. 70 microns). These deliver drop volumes of around 8 – 10 picolitres (a picolitre is a million millionth of a litre), 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 CMY print-head. Several small colour ink drops – typically between four and eight – can be combined to deliver a variable dot size, a bigger palette of non-halftoned colours and smoother halftones. 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 600dpi, with enhanced resolutions of 1200dpi increasingly 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. Typically this is around 12MHz and half an inch respectively, giving print speeds of between 4 to 8ppm (pages per minute) for monochrome text and 2 to 4ppm for colour text and graphics.

Epson’s proprietary inkjet technology uses a piezo crystal at the back of the ink reservoir. This is rather like a loudspeaker cone – it flexes when an electric current flows through it. So, whenever a dot is required, a current is applied to the piezo disk, the element flexes and in so doing forces a drop of ink out of the nozzle.

Piezoelectric Inkjet Technology

There are several advantages to the piezo method. The process allows more control over the shape and size of ink droplet release. The tiny fluctuations in the crystal allow for smaller droplet sizes and hence higher nozzle density. Also, unlike with thermal technology, the ink does not have to be heated and cooled between each cycle. This saves time, and the ink itself is tailored more for its absorption properties than its ability to withstand high temperatures. This allows more freedom for developing new chemical properties in inks.

Epson’s mainstream inkjets have black print-heads with 128 nozzles and colour (CMY) print-heads with 192 nozzles (64 for each colour), addressing a native resolution of 720 by 720dpi. Because the piezo process can deliver small and perfectly formed dots with high accuracy, Epson is able to offer an enhanced resolution of 1440 by 720dpi – although this is achieved by the print-head making two passes, with a consequent reduction in print speed. The tailored inks Epson has developed for use with its piezo technology are solvent-based and extremely quick-drying. They penetrate the paper and maintain their shape rather than spreading out on the surface and causing dots to interact with one another. The result is extremely good print quality, especially on coated or glossy paper.

Piezoelectric Print Head

Read page 77 of the textbook “Exploring Computer Hardware”.