From the Wikipedia article “Laser printer.”
A laser printer is a common type of computer printer that rapidly produces high quality text and graphics on plain paper. As with digital photocopiers and multifunction printers (MFPs), laser printers employ a xerographic printing process but differ from analog photocopiers in that the image is produced by the direct scanning of a laser beam across the printer’s photoreceptor.
- 1 Overview
- 3 How it works
- 6 Laser printer maintenance
A laser beam projects an image of the page to be printed onto an electrically charged rotating drum coated with selenium or, more common in modern printers, organic photoconductors. Photoconductivity removes charge from the areas exposed to light. Dry ink (toner) particles are then electrostatically picked up by the drum’s charged areas. The drum then prints the image onto paper by direct contact and heat, which fuses the ink to the paper.
Unlike impact printers, laser printer speed can vary widely, and depends on many factors, including the graphic intensity of the job being processed. The fastest models can print over 200 monochrome pages per minute (12,000 pages per hour). The fastest colour laser printers can print over 100 pages per minute (6000 pages per hour). Very high-speed laser printers are used for mass mailings of personalized documents, such as credit card or utility bills, and are competing with lithography in some commercial applications.
The cost of this technology depends on a combination of factors, including the cost of paper, toner, and infrequent drum replacement, as well as the replacement of other consumables such as the fuser assembly and transfer assembly. Often printers with soft plastic drums can have a very high cost of ownership that does not become apparent until the drum requires replacement.
A duplexing printer (one that prints on both sides of the paper) can halve paper costs and reduce filing volumes. Formerly only available on high-end printers, duplexers are now common on mid-range office printers, though not all printers can accommodate a duplexing unit. Duplexing can also give a slower page-printing speed, because of the longer paper path.
In comparison with the laser printer, most inkjet printers and dot-matrix printers simply take an incoming stream of data and directly imprint it in a slow lurching process that may include pauses as the printer waits for more data. A laser printer is unable to work this way because such a large amount of data needs to output to the printing device in a rapid, continuous process. The printer cannot stop the mechanism precisely enough to wait until more data arrives, without creating a visible gap or misalignment of the dots on the printed page.
Instead the image data is built up and stored in a large bank of memory capable of representing every dot on the page. The requirement to store all dots in memory before printing has traditionally limited laser printers to small fixed paper sizes such as letter or A4. Most laser printers are unable to print continuous banners spanning a sheet of paper two meters long, because there is not enough memory available in the printer to store such a large image before printing begins.
How it works
There are typically seven steps involved in the laser printing process:
Raster image processing
Each horizontal strip of dots across the page is known as a raster line or scan line. 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), HP Printer Command Language (PCL), or Microsoft XML Page Specification (XPS), 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.
The first Hewlett Packard LaserJet only had 128 kilobytes of memory. It typically was used to print text only, and did not operate like modern graphical printers. The page character information was stored in only a few kilobytes, and during printing the actual dot patterns for each raster scan line was looked up in font bitmap tables stored in Read Only Memory (ROM). Additional fonts were stored on ROM cartridges, plugged into expansion slots.
For fully graphical output using a page description language, a minimum of 1 megabyte of memory is needed to store an entire monochrome letter/A4 sized page of dots at 300 dpi. At 300 dpi, there are 90,000 dots per square inch (300 dots per linear inch). A typical 8.5 x 11 sheet of paper has 0.25 inch margins, reducing the printable area to 8.0 x 10.5 inches, or 84 square inches. 84 sq/in x 90,000 dots per sq/in = 7,560,000 dots. Meanwhile 1 megabyte = 1,048,576 bytes, or 8,388,608 bits, which is just large enough to hold the entire page at 300 dpi, leaving about 100 kilobytes to spare for use by the raster image processor.
In a colour printer, each of the four CYMK toner layers is stored as a separate bitmap, and all four layers are typically preprocessed before printing begins, so a minimum of 4 megabytes is needed for a full-colour letter-size page at 300 dpi.
Memory requirements increase with the square of the dpi, so 600 dpi requires a minimum of 4 megabytes for monochrome, and 16 megabytes for colour at 600 dpi. Some printers are capable of variable size dots and interstitial dots; these additional functions may require many times more memory over the minimums described here.
Printers capable of tabloid and larger size may include memory expansion slots. If insufficient memory is available, some features may be disabled, such as being able to print in colour at letter size but only capable of monochrome at tabloid size. Purchasing additional memory may permit printing in colour at the larger size.
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In older printers, a corona wire positioned parallel to the drum, or in more recent printers, 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.
An AC bias is applied to the primary charge roller to remove any residual charges left by previous images. The roller will also apply a DC bias on the drum surface to ensure a uniform negative potential. The desired print density is modulated by this DC bias.
Numerous patents describe the photosensitive drum coating as a silicon sandwich with a photocharging layer, a charge leakage barrier layer, as well as a surface layer. One versionuses amorphous silicon containing hydrogen as the light receiving layer, Boron nitride as a charge leakage barrier layer, as well as a surface layer of doped silicon, notably silicon with oxygen or nitrogen which at sufficient concentration resembles machining silicon nitride; the effect is that of a light chargeable diode with minimal leakage and a resistance to scuffing.
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 light emitting diodes spanning the width of the page, but these devices are not “Laser Printers”.) Lasers are used because they generate a narrow beam over great distances. The laser beam neutralizes (or reverses) the charge on the black parts of the image, leaving a static electric negative image on the photoreceptor surface to lift the toner particles.
A beam detect (BD) sensor is used to synchronize the laser sweeping process at the end of each sweep cycle. 
The surface with the latent image is exposed to toner, fine particles of dry plastic powder mixed with carbon black or colouring agents. The charged toner particles are given a negative charge, and are electrostatically attracted to the photoreceptor’s latent image, the areas touched by the laser. Because like charges repel, the negatively charged toner will not touch the drum where the negative charge remains.
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.
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.
The paper passes through rollers in the fuser assembly where heat (up to 200 Celsius) and pressure bond the plastic powder to the paper.
One roller is usually a hollow tube (heat roller) and the other is a rubber backing roller (pressure roller). A radiant heat lamp is suspended in the center of the hollow tube, and its infrared energy uniformly heats the roller from the inside. For proper bonding of the toner, the fuser roller must be uniformly hot.
The fuser accounts for up to 90% of a printer’s power usage. The heat from the fuser assembly can damage other parts of the printer, so it is often ventilated by fans to move the heat away from the interior. The primary power saving feature of most copiers and laser printers is to turn off the fuser and let it cool. 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 fuser roller, so there is less mass to be heated and the fuser can more quickly reach operating temperature. This both speeds printing from an idle state and permits the fuser to turn off more frequently to conserve power.
If paper moves through the fuser more slowly, there is more roller contact time for the toner to melt, and the fuser can operate at a lower temperature. Smaller, inexpensive laser printers typically print slowly, due to this energy-saving design, compared to large high speed printers where paper moves more rapidly through a high-temperature fuser with a very short contact time.
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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. There are some exceptions however, most notably some Brother and Toshiba laser printers, which use a patented method to clean and recycle the waste toner.
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, called “Economode” by Hewlett-Packard, which uses about half as much toner but produces a lighter draft-quality output.
Laser printer maintenance
Most consumer and small business laser printers use a toner cartridge that combines the photoreceptor (sometimes called “photo conductor unit” or “imaging drum”) with the toner supply bin, the waste toner hopper, and various wiper blades. When the toner supply is consumed, replacing the toner cartridge automatically replaces the imaging drum, waste toner hopper, and wiper blades.
Some laser printers maintain a page count of the number of pages printed since last maintenance. On these models, a reminder message will appear informing the user it is nearing time to replace standard maintenance parts. On other models, no page count is kept or no reminder is displayed, so the user must keep track of pages printed manually or watch for warning signs like paper feed problems and print defects.
Manufacturers usually provide life expectancy charts for common printer parts and consumables. Manufacturers rate life expectancy for their printer parts in terms of “expected page-production life” rather than in units of time.
Consumables and maintenance parts for Business-class printers will generally be rated for a higher page-production expectancy than parts for personal printers. In particular, toner cartridges and fusers usually have a higher page production expectancy in business-class printers than personal-class printers. Colour laser printers can require more maintenance and parts replacement than monochrome laser printers since they contain more imaging components.
For rollers and assemblies involved in the paper pickup path and paper feed path, typical maintenance is to vacuum toner and dust from the mechanisms, and replace, clean, or restore the rubber paper-handling rollers. Most pickup, feed, and separation rollers have a rubber coating which eventually suffers wear and becomes covered with slippery paper dust. In cases where replacement rollers are discontinued or unavailable, rubber rollers can be cleaned safely with a damp lint-free rag. Commercial chemical solutions are also available which may help temporarily restore the traction of the rubber.
The fusing assembly (also called a “fuser”) is generally considered a replaceable consumable part on laser printers. The fusing assembly is responsible for melting and bonding the toner to the paper. There are many possible defects for fusing assemblies: defects include worn plastic drive gears, electronic failure of heating components, torn fixing film sleeves, worn pressure rollers, toner buildup on heating rollers and pressure rollers, worn or scratched pressure rollers, and damaged paper sensors.
Some manufacturers offer preventative maintenance kits specific to each printer model; such kits generally include a fuser and may also include pickup rollers, feed rollers, transfer rollers, charge rollers, and separation pads.
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