Laser printer
The laser printer is a printer for the production of prints on paper or foils in the electrophotography process . Laser printers are among the page printers because the entire page is exposed and printed in one pass. In common parlance, LED printers and LED plotters are usually referred to as laser printers because the functional principle is largely identical with the electrophotographic printing process, apart from the exposure, as used in copiers .
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history
The physicist Chester F. Carlson (1906–1968) applied for a patent for electrophotography in 1937. This invention was a cornerstone of today's laser printer using electrical charge . Carlson was working in the patent department of an electrical company at the time, manually making copies of patent applications. Up until now there was no machine reproduction process. The tinkerer and hobbyist went to work and on October 22, 1938, with the help of the physicist Otto Kornei, he managed to make the first photocopy with bear moss spores on a glass plate. Today's laser printer is based on the same principle.
Carlson initially struggled to market his invention. Companies such as IBM or General Electric were skeptical and disinterested. In 1944, however, he was able to sell his idea at the Batelle Memorial Institute in Ohio . He was given $ 3,000 to turn his idea into a reality. In 1950 the first dry copier from Haloid finally came onto the market (model A). When business began to run, the product and company name were changed for strategic marketing reasons. This is how electrophotography became xerography. The company name was changed to Haloid Xerox Inc., from 1961 it was only called Xerox.
In 1953, Bob Gundlach, the inventor of the first Xerox copier, produced the first full-color prints in his laboratory. In 1959, Xerox applied for the first color patent. With the Xerox 914 model, 6 copies per minute were already possible. You couldn't buy this device, you could only rent it. So the company Xerox Corp. Secure a good source of income over a long period of time.
In 1970, Xerox opened the Palo Alto Research Center (PARC). The engineer Gary Starkweather employed there developed the first laser printer. He succeeded in modulating laser beams and thus producing a print image. The first laser printer used ROS technology (raster output scanner) and was able to print 500 dots per inch ( dpi ). Thanks to the cooperation of outstanding technicians, a small PC ( Xerox Alto ) was produced in 1973 , which was equipped with a mouse , a graphic interface and the first commercially usable laser printer (EARS, Ethernet-Alto research character generator scanning laser output terminal) . With EARS it was possible to print with a resolution of 384 dpi. That same year, Xerox introduced the Xerox 6500 color copier.
In the 1960s and 1970s, pin and type wheel printers handled the majority of the print jobs. A high quality laser printer cost between $ 100,000 and $ 350,000. In May 1984 the first HP LaserJet hit the market. It only cost 3495 US dollars and met all the requirements for speed, flexibility and print quality.
The main problem in designing laser printers was modulating the laser beam . At the time of the first laser printers, there were only gas lasers that could not be switched on and off at the required speed. Special optics that were moved mechanically or piezo crystals that deflect the light by means of an applied voltage were the only way to accomplish this task. In the later common diode lasers , however, the luminous flux could be modulated very quickly.
In 1988 VEB Büromaschinenwerk "Ernst Thälmann" Sömmerda (in the Robotron combine ) developed the GDR's first laser printer , the EC7230 / LD20. Presumably only 7 pieces were built.
Printing principle
(1) laser diode , (2) collimator and focusing lenses and behind that the mirror is used for position detection by the photosensor and for line synchronization, (3) glass body for linearity correction, (4) polygon mirror with motor, (5) motor driver / control IC
The laser printer is based on the principle of electrophotography (Xerox process). The centerpiece is an imaging drum or endless belt coated with a photoconductor .
Conditioning the photoconductor
The coating of the image drum is first electrostatically charged; either by means of a charging corona (a thin wire attached to the drum that is placed under high voltage and creates a corona discharge ) or by means of charging rollers . The latter has the advantage over the charging corona that hardly any ozone is produced because the high-voltage roller is in direct contact with the imaging drum and therefore there is no ionization of the ambient air.
exposure
The charge on the photoconductor is now erased by exposure to the areas where toner is to be applied to the drum later. The more light falls on areas of the photoconductor-coated drum, the more it is discharged, since the photoconductor becomes electrically conductive when exposed to light. For exposure, a laser beam is directed line by line onto the drum via a rotating mirror (laser scanner) and is switched on and off like a grid. The image to be printed is latently projected onto the photoconductor in the form of remaining and discharged electrostatic charge.
With simple laser printers, shades of gray are achieved by halftoning . The same applies to other colors in simple color laser printers. Higher quality laser printers achieve higher resolutions in the full color system . With them, the charge on the drum is not reduced to zero in one step, but rather weakened in up to 256 steps.
A black pen exposes the areas of the image to be blackened with toner, a white pen exposes the areas of the image to be left white. With the black pen, narrow lines are reproduced more clearly, while with the white pen they tend to tear. White writers can depict black areas better. The difference is not just in the way the exposure is controlled; Modified high voltages for photoconductors and toner are further requirements. The principle of the black writer was used in copiers that copy microfilm recordings back onto paper .
development
The photoconductor continues to rotate and is in the developing unit in the immediate vicinity of the toner accommodated. The remaining static charge draws the toner onto the photoconductor. In the developer unit, the toner is brought to a potential that determines the contrast of the image and prevents too much or too little toner from being transferred. The toner is electrically insulating due to the synthetic resin content.
Toner transfer
The photoconductor moves on and brings the toner into contact either directly with the medium (paper) to be printed or first with a transfer roller or a transfer belt.
With multi-color printing, older laser printers apply all the toner colors to the transfer medium one after the other. In order to stay in register , i.e. not to offset the individual color separations from one another, the positioning must be particularly precise. The transfer to the paper is then carried out in one step for all four colors. As an alternative to the transfer medium, electrostatically pre-charged paper transport belts are used, on which the paper can be positioned as if it were glued. In newer devices, the color laser printer consists of four individual printing units (one unit for each color), and the paper passes through all four printing units one after the other. An even more precise positioning of the paper is necessary here. This printing process has the advantage that you can work continuously. While old devices with transfer ribbon could only ever coat the ribbon with one color (the four toner cartridges are located in a revolver drum and are used one after the other, so that the transfer ribbon needs four revolutions to complete one side), work modern printer with four individual color units, from which the toner is transferred to an intermediate transfer belt (ITB ). Due to the four separate printing units, the following page can already be exposed in the first printing unit, while the last printing unit is still busy exposing the previous page. This enables you to achieve the same page output (pages per minute) in color printing as in monochrome printing. For old devices with a revolver drum, the page yield for color printing is reduced to 25% compared to monochrome printing.
The toner is then made to jump from the intermediate belt onto the paper by an electrostatic charge. For this purpose, a strong electrical charge is applied to the back of the paper by means of a transfer roller, which is opposite to that of the toner.
Fixation
The paper moves on to the fusing unit , which essentially consists of two rollers with a special coating (usually Teflon or silicone rubber ). At least one of the rollers is hollow and has a heating rod inside that heats the roller to around 180 ° C (± 10 ° C, depending on the medium used). As it passes through the sheet, the toner melts and sticks to the paper. On the one hand, the coating ensures that as little toner as possible adheres to the heating rollers, and on the other hand, a corresponding slight electrostatic charge of the rollers, which repels or attracts the toner (upper roller) or attracts (lower roller, beyond the paper). The toner remaining on the heating rollers is removed by cleaning rollers or a cleaning fleece in higher quality devices. In earlier systems, a wet fixation was used, in which the heating is supplied with silicone oil via exchangeable cartridges, which wets the cleaning rollers, which also have to be replaced by the user. Depending on the structural complexity of the fixing units, the service life is between 40,000 and 400,000 printed pages.
To save energy, only one roller is used in modern fixtures, which is heated by induction heating. On the opposite side, a non-stick coated tape is used that rotates around a cylinder and does not have to be heated. With this construction, the fixation can be warmed up to the necessary fixation temperature in a shorter time and can be lowered again more quickly after use until the next page is to be printed.
There are also more complex fixing processes that work at a lower fixing temperature (only approx. 70 ° C), but build up a higher pressure. The method is structurally more complex and so expensive that it is not suitable for home printers. But it has the advantage that more heat-sensitive media can be printed on. Furthermore, at lower temperatures there is less risk of harmful emissions escaping from the plastic resins of the toner or the paper itself.
Full discharge
As the drum continues to rotate , the remaining toner is wiped off the drum using wipers (so-called wiper blades) and a cleaning brush. To support the mechanical cleaning, the image drum is exposed to light by means of an LED strip in order to divert the remaining charge and to suppress the shadows of the previous print. In the case of more complex designs, this waste toner is collected in a waste toner container, which can be exchanged for a new, empty container (the full waste toner container can be disposed of by the manufacturer). In the case of smaller devices with integrated toner drum cartridges, the residual toner is placed in a small chamber, the contents of which are disposed of with the used cartridge. The boundary between small devices without waste toner containers and high volume printers with waste toner disposal is shifting both due to the general increase in printing performance and technical developments. With many black-and-white laser printers and multifunctional systems, the scraped off residual toner is fed back into the developer unit and mixed with new toner so that waste toner disposal is not necessary. This recycling of toner is not possible with color systems because after the development process the color components can no longer be separated into individual colors.
Software driver
The printing mechanism of a laser printer requires special raster data. These are generated by the Raster Image Processor (RIP). There are three approaches to this.
- In particular, inexpensive printers use the operating system of the controlling computer to generate the raster image. Such devices are known as GDI printers and, due to the lack of an image processor, are considerably cheaper than devices with a built-in RIP. The disadvantage of the GDI method is that the printers only work with a special device driver that is supplied by the printer manufacturer. Many of these printers therefore only work with popular or older operating systems because the manufacturer does not provide drivers for niche solutions or discontinue support for the product.
- In the case of mid-range devices, the “ripping” takes place in the printer itself - no raster data is sent to it, but documents in page description languages such as PCL or PostScript. The printer does the processing itself.
- Many large devices use a separate RIP server that is separate from the printer. It is connected directly to the imagesetter unit with a high-speed connection and has a correspondingly large amount of memory. The RIP of a single A3 page with CMYK requires around 125 MB of memory.
Quality, advantages and disadvantages compared to other printing systems
The scope of performance of laser printers ranges from four to around 500 A4 pages per minute for cut paper and up to around 1600 pages per minute for continuous paper .
Laser printers are unmatched in terms of quality for pure text printing (with color laser printers also colored text) and for business graphics. Only a few inkjet printers achieve a similar sharpness of edges and a comparable depth of blackness on special paper. The main advantages are the insensitivity to different types of paper, furthermore, due to the principle, there is less bleeding of fine contours, as is often observed with inkjet printers. The printouts are also more resistant to solar radiation and contact with water, which can only be achieved with inkjet printers using special ink. Laser printing costs are significantly lower and the life expectancy of the devices is higher than that of inkjet printers. Some laser printers are able to change the raster resolution (e.g. from 300 dpi to 400 dpi) and can thus print print data streams that have been optimized for different print resolutions without any loss of quality.
- Other advantages of the laser printer are
- Resistance of the printouts: The printing ink is not only resistant to UV radiation, it also withstands moisture very well.
- Low maintenance: A laser printer can bridge longer downtimes without the need for maintenance (for comparison: the nozzles of inkjet printers dry out, the ink ribbons of dot matrix printers), although even toner cannot be stored indefinitely, as it attracts humidity as it becomes more fine-grained also ages while standing. Persistent heat can also be problematic.
Because of the high quality of the print results from color laser printers, measures to avoid and track forgery of documents were considered early on. Many current devices put an invisible watermark, the machine identification code , on every printout in order to enable the printouts to be traced back to a specific laser printer.
The composition of the toner has a lot and manufacturer-specific magnetic fingerprint. Removed developer from two-component toner also provides information about a device.
- disadvantage
- can be observed in the print quality in relation to photos. Commercially available laser printers are inferior to a good inkjet printer on special paper in terms of color space and contrast levels. Most of the time, the images lack depth and color fastness, and a slight raster may also be visible on inexpensive devices. Although the results of a laser printer are smudge-proof, they are not necessarily resistant to peeling (e.g. at kinks). This applies in particular to full color systems, where the often plastic application of paint can also be seen as a disadvantage. In principle, today's laser printers are not the first choice for printing photos.
Printing cannot be interrupted with a laser printer. A page must be printed completely in one pass, as stopping the paper transport would, among other things, burn the paper in the fuser unit. For this reason, the entire page must already find space in the printer's memory for printing, which means that there is a direct relationship between the RAM installed in the printer and the maximum print resolution dpi . Modern raster image processors can still rasterize text at the time of printing, but in the case of images that are already available as bitmaps, the memory size of modern printers determines the maximum resolution for full-page image printouts. In contrast to this, inkjet printers get by with significantly less built-in memory, as they can easily wait for the data for the next line after each printed line. Since the required main memory was relatively expensive for a long time, this was reflected in the costs.
The thermal load on the print media in the fuser requires special print media (such as foils, self-adhesive labels, window envelopes) to be heat resistant up to 200 ° C.
The fundamentally not completely straight paper path and the toner transfer through electrostatic charge make the processing of thick paper difficult, which is why at least the processing speed must be reduced significantly. Processing of electrically (partially) conductive media (such as ESD cardboard) or completely rigid templates (such as for CD blanks) is therefore not possible.
In addition to photo printouts, laser printers are also unsuitable for reference printouts ( proof ) , as the color quality does not remain constant due to changes in temperature and humidity. In addition, laser printers often work with non-linear intensity curves, so that dark colors taper and light ones are outshone.
Health hazard
Laser printers (like copiers) have long been suspected of being a health hazard. However, various studies have not yet been able to clarify this specifically. The Federal Institute for Risk Assessment carried out a pilot study. According to initial results, it was recommended to use devices that comply with the standard of the Blue Angel eco- label . The Federal Institute for Occupational Safety and Health published the results of a detailed study in mid-2015, which came to the conclusion that there is no acute threat from fine dust pollution from laser printers, as the general dust limit value is undercut by a factor of 100 in its measurements. However, the BAuA does not rule out possible allergic reactions and nevertheless recommends that laser printers are best operated in large, well-ventilated rooms.
Toner particles
Like photocopiers, laser printers work with dry toners , which contain carbon black as a black color pigment and heavy metals such as lead and cadmium in the case of non- RoHS- compliant grades , which can be harmful.
If toner escapes, for example due to improper handling or damage to the device, the substances it contains can affect the mucous membranes, especially those of the respiratory tract, or the skin. In principle, laser printers are never completely “clean”: Part of the toner is blown out of the printer by the ventilation system, which is essential for cooling the internal components. However, modern devices have a filter that filters the particles almost completely. The use of additional filters, which can be purchased in stores, is not recommended, as the fans otherwise run at excessive speed due to the air jam and cause even greater pollution. They are microscopic particles. Service technicians and employees in the refill and recycling area are naturally more exposed to the pollutants. It is recommended to use protective devices in the form of disposable gloves and respiratory protection.
ozone
Some laser printers also release ozone for technical reasons . The application of electrical charges to the imaging drum takes place with high to very high electrical field strengths through fine platinum wires, the so-called corona wires, or tapered metal combs, which are placed under high voltage. In the area of high field strengths, the ambient air is ionized , whereby the atmospheric oxygen (O 2 ) is split up and converted into ozone (O 3 ). Devices with high ozone emissions should therefore be located in well-ventilated rooms. Exchangeable ozone filters do not have a noticeable air cleaning effect at reasonable, practicable change intervals of several months.
Since the mid-1990s, devices with low printing speeds have been using jagged metal strips or contact rollers instead of corona wires to transfer charges. The voltages required can be reduced to such an extent that there is no significant air ionization and therefore no ozone emissions that are subject to declaration. These devices are declared "ozone-free" by the manufacturers.
Fine dust filter
The fine dust filters for laser printers have been developed to reduce possible pollution of fine dust concentration and toner particles in the air. The odor pollution of the environment is also improved by these filters.
Many printers bring a fine dust filter with them from the factory. Some have to be changed separately, others are contained in the toner cartridge and are replaced with it. Depending on the printing performance, a shelf life of six to twelve months should be assumed and the fine dust filter should be replaced after a year at the latest.
Filters can be retrofitted to reduce emissions from laser printers, for example with adhesive strips on the printer's ventilation opening. However, paper output trays cannot be fitted with filters and are therefore a significant source of particulate emissions. The Federal Environment Agency has investigated whether retrofit filters are suitable for separating ultra-fine particles and has determined that filters are not recommended without reservation due to their partially reduced effect. In addition, recommendations as to which filter type fits which printer are sometimes inadequate.
Ultra fine particles (UFP)
During operation, especially at the beginning of a printing process, laser printers emit ultrafine particles (UFP). At the beginning of 2013, however, it was not clear how much this ultrafine dust actually poses a health risk. The criteria for the Blue Angel, which will be binding from 2014 onwards, nevertheless provide for a limit value for UFP based on the RAL-UZ 171 award basis, which, however, is viewed by critics as far too high.
In the meantime, more and more scientific studies are proving the health risks posed by toner dust from laser printers and the nano-particles they contain; so z. B. several studies from Harvard University . The media are also reporting more and more frequently about the health dangers of laser printers and laser copiers. It also became known that in 2016 the federal government warned of the dangers of laser printers, whose “fine dust in ultra-fine particle size [...] is particularly dangerous”. This warning was withdrawn because of studies that scholars criticize as unscientific and interest-driven. Michael Braungart from the Hamburg Environmental Institute criticizes the fact that the studies were financed by institutions that have financial interests in this topic themselves, and that the particles released are, among other things, carcinogenic and mutagenic.
See also: Green IT
See also
literature
- Rainer Hoffmann: Modeling and Simulation of an Electrostatic Image Transfer . ( Memento of October 22, 2005 in the Internet Archive ) (PDF; 7.6 MB) Dissertation (English)
- Edwin D. Reilly: Milestones in Computer Science and Information Technology . Greenwood Press 2003, ISBN 1-57356-521-0 (English)
Web links
- Data track paper
- Michael Hirtner: Laser printer - function, components and maintenance. drucker-infos.de, accessed on August 13, 2010 .
- How does a laser printer work?
- Nano Control - International Foundation for Healthy Indoor Air - Representing the interests of those damaged by toner dust
Individual evidence
- ↑ robotrontechnik.de
- ↑ patent-de.com
- ↑ computerwoche.de
- ↑ Printing techniques in comparison - product finder printer . Website of the Stiftung Warentest . Retrieved April 26, 2012.
- ↑ Ralf Krauter : Tracking down document forgers - toner particles reveal which printer a sheet of paper comes from. In: dradio research from March 18, 2013.
- ↑ Print, scan, copy, fax - product finder printer . Website of the Stiftung Warentest . Retrieved April 26, 2012.
- ^ Danger in the office - report by ZDF magazine Frontal 21 (PDF; 95 kB), nuerenberg-it.de, accessed on April 9, 2012
- ↑ Evaluation of possible relationships between emissions from office machines, in particular from photocopiers and laser printers, and health impairments or health hazards in exposed office workers . (PDF; 11 MB) Final report of January 8, 2008 of the pilot study on behalf of the Federal Institute for Risk Assessment (BfR), 317 p .; Retrieved February 27, 2010.
- ↑ Toner dust and emissions from printers and copiers in the workplace . ( Memento of July 10, 2012 in the Internet Archive ) (PDF; 50 kB) Federal Institute for Occupational Safety and Health; accessed on October 7, 2015
- ^ Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA): Safe laser printers and copiers. Retrieved April 16, 2020 .
- ↑ Tim Gerber: Risk avoidance - banish health hazards from laser printers . In: c't , 10/13.
- ↑ Michael Houben: Poisonous printers: Why dirt throwers get the Blue Angel . Contribution to the ARD program plusminus on October 18, 2017.
- ↑ Sandra V. Pirela et al .: Consumer exposures to laser printer-emitted engineered nanoparticles: A case study of life-cycle implications from nano-enabled products . In: Nanotoxicology , 2015, 9 (6), pp. 760–768, PMC 4671491 (free full text)
- ↑ Sandra V. Pirela et al .: Effects of Laser Printer – Emitted Engineered Nanoparticles on Cytotoxicity, Chemokine Expression, Reactive Oxygen Species, DNA Methylation, and DNA Damage: A Comprehensive in Vitro Analysis in Human Small Airway Epithelial Cells, Macrophages, and Lymphoblasts . In: Environ Health Perspect , 2016 Feb, 124 (2), pp. 210–219, PMC 4749083 (free full text)
- ↑ Xiaoyan Lu et al .: In vivo epigenetic effects induced by engineered nanomaterials: A case study of copper oxide and laser printer-emitted engineered nanoparticles . In: Nanotoxicology , 2016 Jun, 10 (5), pp. 629–639, PMC 4958020 (free full text)
- ↑ Michael Houben: Poisonous printers: Why dirt throwers get the Blue Angel . Contribution to the ARD program plusminus on October 18, 2017.
- ↑ Danger from laser printers . Contribution to the WDR broadcast Westpol on January 28, 2018.
- ↑ Michael Houben: Poisonous printers: Why dirt throwers get the Blue Angel . Contribution to the ARD program plusminus on October 18, 2017.