XEROGRAPHY step by stepStep 1. Charging An electrostatic charge of í600 volts is uniformly distributed over the surface of the drum by a coronadischarge from a Corona unit (Corotron), with output limited by a control grid or screen. The complete unitis correctly called a Screened Corotron or Scorotron for short. This effect can also be achieved with theuse of a contact roller with a charge applied to it. Essentially, a corona discharge is generated by anarrow wire 1/4 to 1/2 inch apart from the photoconductor. A negative charge is placed on the wire, whichwill ionize the space between the wire and conductor, so electrons will be repelled and pushed away ontothe conductor. The conductor is set on top of a conducting surface, kept at ground potential.The polarityis chosen to suit the Positive or Negative process. Positive process is used for producing black on whiteanalogue copies. Negative process is used for producing black on white from negative originals (mainlymicrofilm) and all digital printing and copying. This is to economize on the use of laser light by theBlackwriting or Write to Black exposure method. Step 2. Exposure The document or microform to be copied is illuminated by flash lamps on the platen and either passedover a lens or is scanned by a moving light and lens, such that its image is projected onto andsynchronized with the moving drum surface. Where there is text or image on the document, thecorresponding area of the drum will remain unlit. Where there is no image the drum will be illuminatedand the charge will be dissipated. The charge that remains on the drum after this exposure is a latentimage and is a negative of the original document.Alternatively, the image may be Flash Exposed, using aXenon strobe, onto the surface of the moving drum or belt, fast enough to render a perfect latent image.Whether in a scanning or a stationary optical system, combinations of lenses and mirrors are used toproject the original image on the platen (scanning surface) onto the photoconductor. Additional lenses,with different focal lengths or zooming lenses are utilized to enlarge or reduce the image. The scanningsystem, though, must change its scanner speed to adapt to elements or reductions. A drum is inferior to a belt in the sense that although it is simpler than a belt, it must be buffered graduallyin parts rolling on the drum. As a result, the belt is more efficient to use one exposure to make a directpassage.In a laser or LED printer, modulated light is projected onto the drum surface to create the latent image.The modulated light is used only to create the positive image, hence the term blackwriting. Step 3. Development Youre reading a free preview. Pages 2 to 3 are not shown in this preview. Read the full version In high-volume copiers, the drum is presented with a slowly turbulent mixture of toner particles and larger,iron, reusable carrier particles. The carrier particles have a coating which, during agitation, generatesa tribo-electric charge (a form of static electricity), which attracts a coating of toner particles. In addition,the mix is manipulated with a magnetic roller to present to the surface of the drum or belt a brush of toner.By contact with the carrier each neutral toner particle has an electric charge of polarity opposite to thecharge of the latent image on the drum. The charge attracts toner to form a visible image on the drum. Tocontrol the amount of toner transferred, a bias voltage is applied to the developer roller to counteract theattraction between toner and latent image.Where a negative image is required, as when printing from a microform negative, then the toner has thesame polarity as the corona in step 1. Electrostatic lines of force drive the toner particles away from thelatent image towards the uncharged area, which is the area exposed from the negative.Early color copiers and printers used multiple copy cycles for each page output, using colored filters andtoners. Modern units use only a single scan to four separate, miniature process units, operatingsimultaneously, each with its own coronas, drum and developer unit. Step 4. Transfer Paper is passed between the drum and the transfer corona, which has a polarity that is the opposite of the charge on the toner. The toner image is transferred by a combination of pressure and electrostaticattraction, from the drum to the paper. On many color and high-speed machines, it is common to replacethe transfer corona with one or more charged Bias Transfer Rollers (BTRs), which apply greater pressureand produce a higher quality image. Step 5. Separation Electric charges on the paper are partially neutralized by AC from a second corona, usually constructed intandem with the transfer corona and immediately after it. As a result, the paper, complete with most (butnot all) of the toner image, is separated from the drum or belt surface. Step 6. Fixing The toner image is permanently fixed to the paper using either a heat and pressure mechanism (Hot RollFuser) or a radiant fusing technology (Oven Fuser) to melt and bond the toner particles into the medium(usually paper) being printed on. There also used to be available Offline vapor fusers. These were trayscovered in cotton gauze which was sprinkled with a volatile liquid, such as ether. When the transferredimage was brought into proximity with the vapor from the evaporating liquid the result was a perfectlyfixed copy without any of the distortion or toner migration which can occur with the other methods. Thismethod is now outlawed by the Health and Safety authorities, for obvious reasons. Step 7. Cleaning Youre Reading a Free Preview Page 2 is not shown in this preview. Download The drum, having already been partially discharged during detack, is further discharged, by light, and anyremaining toner, that did not transfer in Step 6, is removed from the drum surface by a rotating brushunder suction, or a squeegee known as the Cleaning Blade. In most cases, this waste toner is routedinto a waste toner compartment for later disposal; however, in some systems it is routed back into thedeveloper unit for reuse. This process, known as Toner Reclaim, is much more economical but canpossibly lead to a reduced overall toner efficiency through a process known as toner polluting wherebyconcentration levels of toner/developer having poor electrostatic properties are permitted to build up inthe developer unit, reducing the overall efficiency of the toner in the system.Note: Some systems have abandoned entirely the use of a separate developer (carrier). These systems,known as Mono Component, operate as above but use either a magnetic toner or fusible developer (however one wishes to view it). This results in the complete removal of the need to replace worn-outdeveloper, as the user effectively replaces it along with the toner. An alternative developing system,developed by KIP from an abandoned line of research by Xerox, completely replaces magnetic toner manipulation and the cleaning system, with a series of computer-controlled, varying biases. The toner isprinted directly onto the drum, by direct contact with a rubber developing roller which, by reversing thebias, removes all the unwanted toner and returns it to the developer unit for re-use.The development of xerography has led to new technologies that some predict will eventually eradicatetraditional offset printingmachines. These new machines that print in full CMYK color, such as Xeikon,use xerography but provide nearly the quality of traditional ink prints. Youre Reading a Free Preview Page 3 is not shown in this preview. 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Posted on: Fri, 18 Oct 2013 00:26:36 +0000
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