A printer is the computer component that lets you create copies of information stored in the computer on paper. The printed material is often called hard copy, to differentiate it from the data stored on a disk, or held in the computer’s memory.
A basic classification of printers is the division into impact printers and non-impact printers. The impact printer prints by mechanical impacts on the paper using needles or type wheels similar to conventional type writers. For example, the dot–matrix printer creates text and images on the page by hammering several small pins against the paper with an inked ribbon between the pins and the paper. By pressing the inked ribbon against the paper, each pin leaves a dot on the paper. The more pins used, the higher the resolution of the printed image – 9–pin and 24–pin was common options. Dot–matrix printers are practically out of use because of development in inkjet and laser printing technology. Nowadays these two methods are being low priced.
Inkjet printers do not use any mechanical impact and that’s why they are example of non-impact printers. The non–impact printer prints without any mechanical impact. Laser printer uses electrostatic forces to apply black or colored toner particles on the paper. Thermo printers are non–impact printers, too. Non–impact printer is far more quite at work than impact printers.
Inkjet printers use black or colored inks which are placed on the paper in fine droplets. The ink is stored in an ink cartridge. The printer head is located the bottom side of the ink cartridge. The printer head consists of many fine jets arranged in a matrix. Ink is fed from the cartridge reservoir through fine channels to each jet. Just before the opening of each jet is a device which forces the ink
out of the jet opening and so squirts a drop of ink onto the paper. Where the printer head works on the inkjet principle, a piezoelectric crystal is located just before the jet opening around the ink channel. This Piezo–electrical crystal can be caused to produce a single vibration by subjecting it to an electrical pulse. If the vibration causes the crystal to contract, the crystal squeezes the channel together enough to force ink through the jet opening and a drop of ink is squirted onto the paper. Where the printer head works on the bubble jet principle, a heating element is located just before the jet opening. This heating element can heat the ink so strongly that a steam bubble is formed. As this steam bubble expands, the ink is forced through the jet opening until a drop of ink is released and is sprayed onto the paper. One or more ink cartridges are mounted on a carriage. As the carriage moves at right angles across the paper, the ink cartridges apply ink in lines to the paper via the printer heads. The paper, in
turn, is drawn in by one step once a line has been printed. Standard inkjet printers achieve resolutions of 600 by 300 dpi. The user can choose between different print qualities. At a low print quality, inkjet printers can print five or more pages per minute.
Laser and LED printers
Laser and LED printers use black or colored toner. Toner is fine ink dust which is applied to the paper by electrostatic attraction and then fixed onto the paper by heat and pressure. The first step involves transferring the digital image information onto a photoelectric drum (known as a photoconductive drum) using a laser or a row of LEDs (Light Emitting Diodes). In the case of a Laser printer, a laser beam is controlled by the data received by the printer. This laser beam is directed onto the photoconductive drum via mirrors and lenses. As the drum turns, the laser strikes points in a line at right angles across the surface of the
drum. In the case of a LED printer, a row of LEDs is mounted along the PC drum controlled by the digital data the LEDs light up. The surface of the drum is electrically charged, e.g. with a negative charge. The negative charge is eliminated at the points on the surface of the drum which have been struck by the laser beam or lit up by the LEDs respectively. The laser beam or the light of the LEDs thus mark out the areas of the drum where toner is to be applied in the next step. As the drum rotates, the area of the drum surface that has already
been marked out by the laser or the LEDs is passed by the developing unit. The toner is negatively-charged and, due to the electrostatic force of attraction, adheres to all the points on the drum surface where the laser beam has generated a positive charge. The next step involves transferring the toner from the drum to the paper. The paper is given a positive charge by an electrical field before it is fed past the drum. The positive charge on the paper is greater than
the positive charge at the points on the drum to which toner is adhering. When the drum carrying the toner is rolled over the paper, the paper, which has the greater charge, exerts a stronger force of attraction on the toner than the drum does. The toner is released rom the drum and adheres to the paper. The drum is then cleaned of residual toner and the original negative electrical charge is restored. Once the toner has been transferred to the paper by electrostatic
attraction, the toner must be fixed on the paper. This is achieved by pressure and heat, for example. The heat melts the wax in the toner and the two rollers press the resin/toner mixture onto the paper. The toner is bonded to the paper by the resin and pressure. The exposure of the drum by Laser or LEDs, the transfer of toner onto the drum and finally the transfer of the toner from the drum onto the paper form a single process in which ink can be transferred to the paper. A monochrome printer performs this process once with black toner. A full color printer performs this process four times with toner in one of the ink colors (yellow, magenta, cyan or black) being applied to the paper at each pass.
Thermo printers use heat and special printer paper or transfer film to bring color on a copy. The major methods of thermo printers are • the autochrom method,
the thermo transfer method and
the thermo sublimination.
Special printer paper is used for the autochrome method. This paper is already impregnated with the inks. The printer paper comprises a carrier layer formed from three ink layers containing yellow, magenta and cyan emulsions, and a final heat-resistant protective layer. The external application of heat causes the ink emulsions in the various layers to reveal their color. With autochrome printers, the autochrome paper is conveyed past a heating element in three passes. The yellow layer is heated in the first pass, the magenta layer in the second pass and the cyan layer in the final pass. The yellow layer reacts at low heat, the magenta layer at moderate heat and the cyan layer at high heat. The quantity of colour “activated” can be regulated by finely graduating the amount of heat applied to an ink layer, thus allowing half-tones to be produced. The resolution that can be achieved using the autochrome method is determined by the design of the heating element. Standard autochrome printers for the private sector can achieve a resolution of just above 300 dpi and use A6 format paper. With thermotransfer methods, the ink is transferred from a carrier film onto the paper. The carrier film is coated in consecutive sections with yellow, magenta and cyan wax. The paper is conveyed past one section of the carrier film at a time in three consecutive passes. A thermo printer head is positioned at right angles across the back of the carrier film. Individual heating elements melt the colored wax and thus transfer the ink from the carrier film onto the paper in the
form of screen dots. Thermotransfer printers achieve a resolution of 600 dpi for color printing. The thermosublimation process basically works in the same way as the thermotransfer process. There is colored ink on the carrier foil instead of coloured wax. The coloured ink is heated so strongly by the heating elements that the ink evaporates into the paper. Thermosublimation printers reach a resolution of up to 600 dpi and, depending on their design, can print up to A3 sheet size.