Raster Image Processing
Each horizontal strip of dots across the page is known as a raster. Creating the image to be printed is done by a Raster Image Processor (RIP), typically built into the laser printer. The source material may be encoded in any number of special page description languages such as Adobe PostScript (PS) or HP Page Control Language (PCL), as well as unformatted text-only data. The RIP uses the page description language to generate a bitmap of the final page in the raster memory. Once the entire page has been rendered in raster memory, the printer is ready to begin the process of sending the rasterized stream of dots to the paper in a continuous stream.
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Charging
A corona wire (in older printers) or a primary charge roller projects an electrostatic charge onto the photoreceptor (otherwise named the photoconductor unit), a revolving photosensitive drum or belt, which is capable of holding an electrostatic charge on its surface while it is in the dark.
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Writing
The laser is aimed at a rotating polygonal mirror, which directs the laser beam through a system of lenses and mirrors onto the photoreceptor. The beam sweeps across the photoreceptor at an angle to make the sweep straight across the page; the cylinder continues to rotate during the sweep and the angle of sweep compensates for this motion. The stream of rasterized data held in memory turns the laser on and off to form the dots on the cylinder. Some printers switch an array of laser diodes spanning the width of the page, and they signal to both the photoreceptor and their Quartz-clocked host in time to marks on the underpassing cylinder. Lasers are used because they generate a narrow beam for great distances. The laser beam neutralizes (or reverses) the charge on the white parts, leaving a mirror image of static electricity on the photoreceptor surface to lift powdered ink.
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Developing
The surface with the latent image is exposed to toner, fine particles of dry plastic powder mixed with carbon black or coloring agents. The charged toner particles are given a negative charge, and are electrostatically attracted to the photoreceptor where the laser wrote the latent image. Because like charges repel, the negatively charged toner will not touch the drum where light has not removed the negative charge.
The overall darkness of the printed image is controlled by the high voltage charge applied to the supply toner. Once the charged toner has jumped the gap to the surface of the drum, the negative charge on the toner itself repels the supply toner and prevents more toner from jumping to the drum. If the voltage is low, only a thin coat of toner is needed to stop more toner from transferring. If the voltage is high, then a thin coating on the drum is too weak to stop more toner from transferring to the drum. More supply toner will continue to jump to the drum until the charges on the drum are again high enough to repel the supply toner. At the darkest settings the supply toner voltage is high enough that it will also start coating the drum where the initial unwritten drum charge is still present, and will give the entire page a dark shadow.
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Transferring
The photoreceptor is pressed or rolled over paper, transferring the image. Higher-end machines use a positively charged transfer roller on the back side of the paper to pull the toner from the photoreceptor to the paper.
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Fusing
The paper passes through a fuser assembly with rollers that provide heat and pressure (up to 200 degrees Celsius), bonding the plastic powder to the paper. In the fuser assembly one roller is usually a hollow tube and the other is a rubber backing roller. A radiant heat lamp is suspended in the center of the hollow tube, and infrared energy is projected onto the inside of the roller to uniformly heat it from the inside out. For proper bonding of the toner, the fuser roller needs to be uniformly hot.
The fuser tends to account for up to 90% of a printer's power usage. The intense heat from the fuser assembly can cause damage to the rest of the printer, so the hot fuser assembly is often surrounded by fans blowing the heat away from the rest of the equipment inside the printer. The primary power saving feature of most copiers and laser printers is to simply turn off the fuser and let it go cold. Resuming normal operation requires waiting for the fuser to return to operating temperature before printing can begin.
Some printers use a very thin flexible metal, so that the hollow roller has a low mass and can be quickly warmed to the correct temperature. This both speeds printing from a cold idle state and permits the fuser to turn off more frequently to conserve power.
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Cleaning
When the print is complete, an electrically neutral soft plastic blade cleans any excess toner from the photoreceptor and deposits it into a waste reservoir, and a discharge lamp removes the remaining charge from the photoreceptor.
Toner may occasionally be left on the photoreceptor when unexpected events such as a paper jam occur. The toner is on the photoconductor ready to apply, but the operation failed before it could be applied. The toner must be wiped off and the process restarted.
Waste toner cannot be reused for printing because it can be contaminated with dust and paper fibers. A quality printed image requires pure, clean toner. Reusing contaminated toner can result in splotchy printed areas or poor fusing of the toner into the paper.
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Multiple steps occurring at once
Once the raster image generation is complete all steps of the printing process can occur one after the other in rapid succession. This permits the use of a very small and compact unit, where the photoreceptor is charged, rotates a few degrees and is scanned, rotates a few more degrees and is developed, and so forth. The entire process can be completed before the drum completes one revolution. Different printers implement these steps in distinct ways. Some "laser" printers actually use a linear array of light-emitting diodes to "write" the light on the drum (see LED printer). The toner is based on either wax or plastic, so that when the paper passes through the fuser assembly, the particles of toner melt. The paper may or may not be oppositely charged. The fuser can be an infrared oven, a heated pressure roller, or (on some very fast, expensive printers) a xenon flash lamp. The Warm Up process that a laser printer goes through when power is initially applied to the printer consists mainly of heating the fuser element. Many printers have a toner-conservation mode or "economode", which can be substantially more economical with fuser consumption at the price of slightly lower contrast. |
