Laser printing
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A laser printer is a common computer peripheral 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.
Overview
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 allows charge to leak away from the areas exposed to light. Dry ink (toner) particles are then electrostatically picked up by the drum's charged areas, which have not been exposed to light. 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 color 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.[1]
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.
History
The laser printer was invented at Xerox in 1969 by researcher Gary Starkweather, who had an improved printer working by 1971[2] and incorporated into a fully functional networked printer system by about a year later.[3] The prototype was built by modifying an existing xerographic copier. Starkweather disabled the imaging system and created a spinning drum with 8 mirrored sides, with a laser focused on the drum. Light from the laser would bounce off the spinning drum, sweeping across the page as it traveled through the copier. The hardware was completed in just a week or two, but the computer interface and software took almost 3 months to complete.[citation needed]
The first commercial implementation of a laser printer was the IBM 3800 in 1976, used for high-volume printing of documents such as invoices and mailing labels. It is often cited as "taking up a whole room," implying that it was a primitive version of the later familiar device used with a personal computer. While large, it was designed for an entirely different purpose. Many 3800s are still in use.[citation needed]
The first laser printer designed for use in an office setting was released with the Xerox Star 8010 in 1981. Although it was innovative, the Star was an expensive (17 000 USD) system that was purchased by only a relatively small number of businesses and institutions. After personal computers became more widespread, the first laser printer intended for a mass market was the HP LaserJet 8 ppm, released in 1984, using a Canon engine controlled by HP software. The HP LaserJet printer was quickly followed by laser printers from Brother Industries, IBM, Apple and others. First-generation machines had large photosensitive drums, of circumference greater than the paper length. Once faster-recovery coatings were developed, the drums could touch the paper multiple times in a pass, and could therefore be smaller in diameter.
Laser printers brought fast, high quality text printing with multiple fonts on a page to the business and consumer markets. No other commonly available printer could offer this combination of features.
As with most electronic devices, the cost of laser printers has fallen markedly over the years. In 1984, the HP LaserJet sold for 3500 USD,[4] had trouble with even small, low resolution graphics, and weighed 32 kg (71lb). Low end monochrome laser printers often sell for less than 75 USD as of 2008. These printers tend to lack onboard processing and rely on the host computer to generate a raster image (see Winprinter), but still will outperform the LaserJet Classic in nearly all situations.
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, BR-Script), 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.
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 × 11 sheet of paper has 0.25-inch (6.4 mm) margins, reducing the printable area to 8.0 × 10.5 inches (270 mm), or 84 square inches. 84 sq/in × 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 color 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-color 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 color at 600 dpi. Printers capable of tabloid and larger size may include memory expansion slots.
Charging
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 photo conductor 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.
Numerous patents[specify] 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 version[specify] uses 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
Exposing
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 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. 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.
Some non-laser printers expose by an array of light emitting diodes spanning the width of the page, rather than by a laser ("exposing" is also known as "writing" in some documentation).
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 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.
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.
Fusing
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.
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. 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
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.
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.
Color laser printers
Color laser printers use colored toner (dry ink), typically cyan, magenta, yellow, and black (CMYK).
While monochrome printers only use one laser scanner assembly, color printers often have two or more scanner assemblies.
Color printing adds complexity to the printing process because very slight misalignments known as registration errors can occur between printing each color, causing unintended color fringing, blurring, or light/dark streaking along the edges of colored regions. To permit a high registration accuracy, some color laser printers use a large rotating belt called a "transfer belt". The transfer belt passes in front of all the toner cartridges and each of the toner layers are precisely applied to the belt. The combined layers are then applied to the paper in a uniform single step.
Color printers usually have a higher cost per page production cost than monochrome printers.
DPI Resolution
- 1200 DPI printers were commonly available during 2008.
- 2400 DPI electrophotographic printing plate makers, essentially laser printers that print on plastic sheets, are also available.
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.
Wiper blades can fail even in new, unused toners that have been stored for a few years; a failed wiper blade will manifest itself as ghosting and/or full page shading.
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.
Some color laser printers, notably some Lexmark models[citation needed] run "calibration" cycles even when no printing has occurred for weeks. These are widely reported[by whom?] to waste a significant amount of toner from each reservoir, in addition to consuming electricity. This has a significant impact on printing economy, especially in low-volume applications. On some models these calibration cycles can be disabled via a menu choice, for others the printer must be unplugged to avoid this waste. Printers that have this issue have a replaceable "waste toner bin", which is another periodic operating expense.
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. Color 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 and third-party parts suppliers 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. These same sources may offer a trade-in credit for the return of failed fuser assemblies, which they later rebuild for resale.
Steganographic anti-counterfeiting ("secret") marks
Many modern color laser printers mark printouts by a nearly invisible dot raster, for the purpose of identification.
The dots are yellow and about 0.1 mm in size, with a raster of about 1 mm. This is purportedly the result of a deal between the U.S. government and printer manufacturers to help track counterfeiters.[5]
The dots encode data such as printing date, time, and printer serial number in binary-coded decimal on every sheet of paper printed, which allows pieces of paper to be traced by the manufacturer to identify the place of purchase, and sometimes the buyer.
Digital rights advocacy groups such as the Electronic Frontier Foundation are concerned about this erosion of the privacy and anonymity of those who print.[6]
Air transport ban
After the October 2010 cargo planes bomb plot, in which shipments of laser printers with explosive-filled toner cartridges were discovered on separate cargo airplanes, the US prohibited pass-through passengers from carrying certain printer cartridges on flights.[7] The US Transportation Security Administration said it would ban toner and ink cartridges weighing over 453 grams (16oz) from all passenger flights.[8][9] U.S. Homeland Security Secretary Janet Napolitano said the ban would apply to both carry-on bags and checked bags on domestic and international flights in-bound to the U.S.[9] PC Magazine opined that the ban would not affect average travelers, whose ink cartridges are generally lighter, but would affect the importing of laser printer supplies, as many laser toner cartridges weigh well in excess of a pound.[9]
Safety hazards, health risks, and precautions
Shock hazards
Although modern printers include many safety interlocks and protection circuits, it is possible for a high voltage or a residual voltage to be present on the various rollers, wires, and metal contacts inside a laser printer. Care should be taken to avoid unnecessary contact with these parts to reduce the potential for painful electrical shock.
Toner clean-up
Toner particles are designed to have electrostatic properties and can develop static-electric charges when they rub against other particles, objects, or the interiors of transport systems and vacuum hoses. Because of this and its small particle size, toner should not be vacuumed with a conventional home vacuum cleaner. Static discharge from charged toner particles can ignite dust in the vacuum cleaner bag or create a small explosion if sufficient toner is airborne. This may damage the vacuum cleaner or start a fire. In addition, toner particles are so fine that they are poorly filtered by conventional household vacuum cleaner filter bags and blow through the motor or back into the room.
Toner particles melt (or fuse) when warmed. Small toner spills can be wiped up with a cold, damp cloth.
If toner spills into the laser printer, a special type of vacuum cleaner with an electrically conductive hose and a high efficiency (HEPA) filter may be needed for effective cleaning. These are called ESD-safe (Electrostatic Discharge-safe) or toner vacuums. Similar HEPA-filter equipped vacuums should be used for clean-up of larger toner spills.
Toner is easily cleaned from most water-washable clothing. As toner is a wax or plastic powder with a low melting temperature, it must be kept cold during the cleaning process. Washing a toner stained garment in cold water is often successful. Even warm water is likely to result in permanent staining. The washing machine should be filled with cold water before adding the garment. Washing through two cycles improves the chances of success. The first may use hand wash dish detergent, with the second cycle using regular laundry detergent. Residual toner floating in the rinse water of the first cycle will remain in the garment and may cause a permanent graying. A clothes dryer or iron should not be used until it is certain that all the toner has been removed.
Ozone hazards
As a natural part of the printing process, the high voltages inside the printer can produce a corona discharge that generates a small amount of ionized oxygen and nitrogen, forming ozone and nitrogen oxides. In larger commercial printers and copiers, a carbon filter in the air exhaust stream breaks down[citation needed] these oxides to prevent pollution of the office environment.
However, some ozone escapes the filtering process in commercial printers, and ozone filters are not used in many smaller consumer printers. When a laser printer or copier is operated for a long period of time in a small, poorly ventilated space, these gases can build up to levels at which the odor of ozone or irritation may be noticed. A potential for creating a health hazard is theoretically possible in extreme cases.[10]
Respiratory health risks
According to a recent study conducted in Queensland, Australia, some printers emit sub-micrometre particles which some suspect may be associated with respiratory diseases.[11] Of 63 printers evaluated in the Queensland University of Technology study, 17 of the strongest emitters were made by Hewlett-Packard and one by Toshiba. The machine population studied, however, was only those machines already in place in the building and was thus biased toward specific manufacturers. The authors noted that particle emissions varied substantially even among the same model of machine. According to Professor Morawska of Queensland University, one printer emitted as many particles as a burning cigarette.[12]
- "The health effects from inhaling ultrafine particles depend on particle composition, but the results can range from respiratory irritation to more severe illness such as cardiovascular problems or cancer." (Queensland University of Technology).[13]
A 2006 study in Japan found that laser printers increase concentrations of styrene, xylenes, and ozone, and that ink-jet printers emitted pentanol.[14]
Muhle et al. (1991) reported that the responses to chronically inhaled copying toner, a plastic dust pigmented with carbon black, titanium dioxide and silica were also similar qualitatively to titanium dioxide and diesel exhaust.[15]
See also
References
- ^ "Facts about laser printing(access date 2010-09-01)". Papergear.com. 2010-09-01. Retrieved 2010-11-17.
- ^ Edwin D. Reilly (2003). Milestones in Computer Science and Information Technology. Greenwood Press. ISBN 1-57356-521-0.
- ^ Roy A. Allan (2001). A History of the Personal Computer: The People and the Technology. Allan Publishing. ISBN 0-9689108-0-7.
- ^ "HP Virtual Museum: Hewlett-Packard LaserJet printer, 1984". Hp.com. Retrieved 2010-11-17.
- ^ "Electronic Frontier Foundation- privacy on printers". Eff.org. Retrieved 2010-11-17.
- ^ "Electronic Frontier Foundation Threat to privacy". Eff.org. 2008-02-13. Retrieved 2010-11-17.
- ^ Matt Apuzzo and Eileen Sullivan (November 3, 2010). "Officials suspect Sept. dry run for bomb plot". The Washington Post. Retrieved November 2, 2010.
- ^ "UK: Plane Bombs Explosions Were Possible Over U.S". Fox News. Retrieved 2010-11-17.
- ^ a b c Hoffman, Tony. "U.S. Bans Large Printer Ink, Toner Cartridges on Inbound Flights". PC Mag. Retrieved 2010-11-17.
- ^ "Photocopiers and Laser Printers Health Hazards".
- ^ "Particle Emission Characteristics of Office Printers" (PDF). The San Francisco Chronicle.
- ^ "Particle Emission Characteristics of Office Printers". The Sydney Morning Herald. 2007-08-01.
- ^ "Study reveals the dangers of printer pollution".
- ^ "Are Laser Printers Hazardous to Your Health? – Yahoo! News".
- ^ "11.6 METALS" (PDF). 070821 epa.gov
External links
- Howstuffworks "How Laser Printers Work"
- " Daniel Singermann`s Forum, author of book "repair of printers ... the definitive guide professional"
- Is Your Printer Spying On You? (by EFF)
- Detailed description, modelling and simulation of the electrophotographic print process (technical; 7.2 MB)
- How a Laser Printer Works Video