Inkjet printing, like laser printing, is a non-impact method. Ink is emitted from nozzles as they pass over a variety of possible media, and the operation of an inkjet printer is easy to visualise: liquid ink in various colours being 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.
On ordinary inkjets, the print head takes about half a second to print a strip across a page. Since A4 paper is about 8.5in wide and inkjets operate at a minimum of 300dpi, this means there are at least 2,475 dots across the page. The print head has, therefore, about 1/5000th of a second to respond as to whether or not a dot needs printing. In the future, fabrication advances will allow bigger print-heads with more nozzles firing at faster frequencies, delivering native resolutions of up to 1200dpi and print speeds approaching those of current colour laser printers (3 to 4ppm in colour, 12 to 14ppm in monochrome).
There are several types of inkjet technology but the most common is "drop on demand" (DOD). This works by squirting small droplets of ink onto paper, through tiny nozzles: like turning a hosepipe on and off 5,000 times a second. The amount of ink propelled onto the page is determined by the driver software that dictates which nozzles shoot droplets, and when.
The nozzles used in inkjet printers are hair fine and on early models they became easily clogged. On modern inkjet printers this is rarely a problem, but changing cartridges can still be messy on some machines. Another problem with inkjet technology is a tendency for the ink to smudge immediately after printing, but this, too, has improved drastically during the past few years with the development of new ink compositions.
