Laser printing

The laser printer is a printer for the production of printing on paper or transparencies in the electrophotography process. Laser printers are one of the page printers, as the exposure and pressure of the whole page is given in a single pass. In common parlance LED printers and LED plotters are usually referred to as laser printers, because the principle of operation is very similar.

• 5.1 toner particles
• 5.2 ozone
• 5.3 Fine dust filter
• 5.4 Ultrafine particles ( UFP )

History

The physicist Chester F. Carlson (1906-1968) reported in 1937 on electrophotography patent. This invention was a cornerstone of today's laser printer utilizing electric charge. Carlson worked at that time in the patent department of an electrical company and made manually copies of patent applications. There was so far no machine copying process. The tinkerers went to work and succeeded on October 22, 1938 Help the physicist Otto Korney the first photocopy with Lycopodium on a glass plate to customize. Today's laser printer is based on the same principle.

Carlson initially had to market his invention difficulties. Companies such as IBM or General Electric were skeptical and uninterested. In 1944, however, he could sell his idea at the Battelle Memorial Institute in Ohio. Him were made available to implement his idea into action 3000 U.S. dollars. 1950 finally came the first dry copier of the Haloid Company on the market ( Model A). As the business began to run, was changed from a marketing strategic reasons the product and company name. So was the xerography of electrophotography. The company name was changed to Haloid Xerox Inc., from 1961, she was only Xerox.

1953 produced Bob Gundlach, the inventor of the first Xerox copier, the first full color prints in his laboratory. 1959 Xerox announced to the first colored patent. With the Xerox model 914 6 copies per minute were already possible. You could not buy, but rent this unit. How could the company Xerox Corp.. secure for a long time a good source of income.

In 1970, the Xerox Palo Alto Research Center ( PARC ) opened. The employees there engineer Gary Starkweather developed the first laser printing device. He succeeded to modulate laser beams and thereby produce a printed image. The first laser printer worked after the ROS technique ( raster output scanner ) and was 500 dots per inch ( dpi) printing. Excellent technicians managed by the co- production of a small PC 1973 ( Xerox Alto) equipped with a mouse, a graphical user interface and the first commercially useful laser printer (EARS, Ethernet Alto Research character generator Scanning Laser Output Terminal ). EARS it was possible to print at a resolution of 384 dpi. That same year, Xerox introduced the Xerox 6500 color copier.

In the 1960s and 1970s, needle and Typenraddrucker completed the majority of print jobs. A high-quality laser printers cost 100000-350000 U.S. dollars. In May 1984, the first HP LaserJet came on the market. It cost only 3495 U.S. dollars and fulfilled all requirements asked, in terms of speed, flexibility and print quality.

The main problem in the development of laser printers has been to modulate the laser beam. The time of the first laser printer, there were only gas laser, which could not be at the required speed and off. Special optics were moved mechanically, or piezoelectric crystals, which distracted the light by means of an applied voltage, were the only way to accomplish this task. In the later conventional diode lasers, however, the light output could be modulated very quickly.

Pressure principle

The laser printer is the principle of electrophotography based ( Xerox process ). The centerpiece is a coated with a photoconductor drum or endless belt.

Conditioning of the photoconductor

The coating of the image drum is first electrostatically charged negatively; either by means of a charging corona (a thin, close to the drum mounted wire which is put under high voltage and the corona discharge is generated ), or by charging rollers. The latter has the advantage that hardly any ozone is produced, because the high voltage and roll is in direct contact with the image drum and therefore there is no ionization of the surrounding air in relation to the charge corona.

Exposure

The charge on the photoconductor is then removed by exposure to the positions at which the toner is to be applied later to the drum: in the exposed areas it is conductive and thereby loses its charge. For exposing a laser beam on a rotating mirror (laser scanner) is deflected line-wise to the drum, while a grid pattern on and off.

Shades of gray are in simple laser achieved by halftoning. A similar procedure is other colors in simple color laser printers. Higher-quality laser printers achieve higher resolutions in full color system. Among them, the charge on the drum is not reduced to zero in a step, but they may be decreased in increments (up to 256). Is achieved either by varying residence time of the attenuation of the laser at a location or by multiple exposure of the corresponding point.

Development

The photoconductor continues to rotate, and is accommodated in the developing unit in the immediate vicinity of the toner. The toner is negatively charged by the resin component and adheres only to the (neutralized ) Set the image drum on which the laser beam is impinged.

Toner transfer

The photoconductor moves on and brings the toner in contact either directly with the medium to be printed (paper) or first with a transfer roller or a transfer belt.

The multi-color printing in succession all the toner colors are applied to the transfer medium in older laser printers. In order to remain in register, so do not enable the individual color separations against each other, must be particularly precise positioned here. The transfer to the paper is then carried out in one step for all four colors. Alternatively, the transfer medium also electrostatically precharged paper conveyor belts (Electrostatic Transport Belt) are used, where the paper can be positioned as though glued. For newer devices, the color laser printer consists of four individual printing units ( for each color one plant), and the paper goes through all four printing units one by one. Here is an even more accurate positioning of the paper is necessary. However, this printing method offers the advantage that work can be continued. While the old devices with transfer tape, the tape was always coated with only one color ( the four toner cartridges are in a revolver drum and are successively brought into use, so that the transfer belt needs four rounds to complete one page), work modern printer with four color units each of which the toner on an intermediate belt (ITB - Intermediate transfer Belt ) are transmitted. By the four separate printing units in the first printing unit can already be exposed to the subsequent page, while the last printing unit is not busy to expose the previous page. Thus they achieve the same page output ( printed pages per minute) as in monochrome printing in color printing. For old devices with revolver drum page yield decreased color printing at 25 % compared to monochrome printing.

The toner is then caused to skip by the intermediate belt on the paper by means of a transfer roller, a large electric charge is applied on the back of the paper, which is opposite to the charge of the toner.

Fixation

The paper moves on to the fixing unit and it consists essentially of two rollers, which have a special coating (usually Teflon or silicone rubber). At least one of the rollers is hollow and has a heating element inside, the (, ± 10 ° C depending on the medium used ) heats the roller to approximately 180 ° C. While passing through the sheet, the toner melts and is glued to the paper. That as little toner sticks to the fuser rollers, ensures on the one hand, the coating, on the other hand, turn a corresponding slight electrostatic charge of the rollers, the toner repel (upper roller) or tighten ( lower roller, beyond the paper). The remaining still on the heating rolls toner is removed by cleaning rollers or a cleaning pad for higher value devices. In previous systems, wet fixation was used, in which the heating is supplied by replaceable cartridges with silicone oil, which is also the wetted by the user to be exchanged cleaning rollers. Depending on the structural complexity of the fuser are the times in the range 40000-400000 printed pages.

To save energy, only a roller is used in modern fixations, which is heated by an induction heater. On the opposite side a non-stick - coated tape is used which rotates about a cylinder and does not have to be heated. By this construction, the fixation can be heated in a shorter time to the required fixing, and can be lowered more quickly until the next page to be printed after use.

There are also more elaborate fixing processes which operate at a lower fusing temperature (only about 70 ° C), but to build up a higher pressure. The process is complex design and so expensive that it is not suitable for home users printer. It has the advantage that wärmeempfindlichere media can be printed on. Furthermore, at lower temperatures there is less risk that harmful emissions from the plastic resins of the toner or paper to escape itself.

Fully discharge

During further rotation of the drum, the residual toner remaining on the drum by means of scrapers, so-called Scope of supply stripped. More complicated constructions of this waste toner is collected in a waste toner container, which can be replaced with a new, empty container ( the full waste toner container can be disposed of through the manufacturer). For smaller devices with integrated toner drum cartridges, the residual toner is slipped into a small chamber, the contents of which is disposed of with the spent cartridge. The boundary between small devices without residual toner collection unit and high volume printer with toner waste disposal shifts both by the general increase in printing services as well as by the technical development.

Software drivers

The printing unit of a laser printer requires special raster data. These are generated by the raster image processor (RIP). There are three approaches.

• Before Inexpensive printers use the operating system of the controlling computer for generating the raster image. Such devices are called GDI printers and are significantly cheaper than devices with a built- RIP due to the missing picture processor. The disadvantage of the GDI method is that the printer will only work with a specific device driver, which is supplied by the printer manufacturer. Many of these printers therefore only work with popular or older operating systems, because the manufacturer of niche solutions does not provide drivers or sets the support for the product. For this reason you should make sure that they have mastered PostScript or PCL for high-quality printers. In these devices, which are already available in the lowest price segment today, it is extremely likely that at an operating system change continue to be drivers. In GDI printers which is not often the case, so you may be forced out one of fully functional printer due to lack of driver support.

• For devices of the middle class, the " ribs " is done in the printer itself - in him is no raster data, but documents are sent in page description languages ​​such as PCL or PostScript. The preparation is performed by the printer itself

• In many large-scale equipment independent and separate from the printer RIP server is used. It is connected with a high-speed connection directly to the platesetter and has sufficient memory. The RIP a single A3 page in CMYK requires approximately 125 MB of memory.

Quality, advantages and disadvantages compared to other printing systems

The scope of laser printers ranging in cut paper from four up to 500 A4 pages per minute and at continuous paper to about 1600 pages per minute.

Laser printers are in high quality with pure text expression (for color laser printers and color text ) as well as in business graphics unmatched. Only some inkjet printers achieve a similar edge sharpness and on special paper a similar blackness depth. The main advantages are the immunity to various types of paper, there are also inherently less bleeding of fine contours, as is often observed in ink jet printers. The prints are resistant to sunlight and water contact, which can only be achieved using a special ink with inkjet printers. The laser printing costs are significantly lower, and life expectancy of the equipment is higher than inkjet printers. Some laser printers are able to change the screen resolution (eg 300 dpi to 400 dpi) and can therefore print data streams that have been optimized for different print resolutions, print, without sacrificing quality.

• Resistance of prints: The printer ink is not only resistant to UV radiation, it also keeps the humidity was very good. Who is traveling a lot and prints for outdoor use, should use a laser printer.
• Low maintenance: A laser printer can bridge longer service life without any maintenance would be necessary (for comparison: in inkjet printers dry the nozzles in dot matrix printers the color bands from ), but also toner is not stored indefinitely, as it attracts humidity with increasing fineness of grain and thus also ages in the state. Likewise, permanent heat is problematic under certain circumstances.

Because of the high quality of printouts of color laser printers was thought early on measures for the prevention and prosecution of falsifying documents. So many current devices bring an invisible watermark, the machine identification code to each expression to make the tracking of printing on a particular laser printer.

The composition of the toner has a lot and manufacturer-specific magnetic fingerprint. Also confiscated developers in two-component toner gives conclusions on a device.

The printing operation can not be interrupted in a laser printer. One side must be completely printed in one pass, since the stopping of the paper transportation would burn among other things, the paper in the fuser. Therefore, the whole page for the print already in the printer's memory, must be found, so there is a direct relationship between the built- in printer RAM and the maximum print resolution dpi. Although modern Raster Image Processors can not rasterize text at print time, when existing as bitmap images are also in modern printers, the memory size is the maximum resolution for full-page frame print from. In contrast, inkjet printers come with much less built-in memory, since it after each printed line can wait for the data for the next line without any problems. Since the required memory for a long time was relatively expensive, it struck this down on costs.

The thermal load of the print media in the fixation requires for special print media (such as foils, self adhesive labels, window envelopes ) Heat resistance up to 200 ° C.

The theoretically not completely straight paper path and toner transfer by electrostatic charge reversal complicates the processing of high strength paper, which is why at least the processing speed must be greatly reduced. Processing of electric ( partial) conductive media (such as ESD cardboard ) or completely rigid templates ( such as for CD-R ) is therefore not possible.

Of the photo print are laser printers also unsuitable for reference prints (proof ), as the color quality is not constant due to temperature and humidity changes. In addition, laser printers often work with non-linear intensity curves, so run dark shades and are outshone bright.

Health hazard

For a long laser printers are available (as well as copier ) suspected to endanger health. Various studies have so far not clarify the concrete. The Federal Institute for Risk Assessment carried out a pilot study. According to initial results, it was recommended to use equipment which adhere to the standard of the eco-label Blue Angel.

Toner particles

Laser printers work like photocopiers with dry toners containing a black pigment carbon black and not RoHS compliant varieties also heavy metals such as lead and cadmium, so sometimes may be harmful.

Kick out of toner, such as improper handling or damage to the equipment, the materials contained on the mucous membranes, especially those of the respiratory tract can, or act on the skin. Laser printers are in principle never completely " clean ": A portion of the toner is blown through the ventilation system, which is essential to cool the internal components of the printer. However, modern devices have a filter which almost completely filtered by the particles. The use of additional filters, which can be purchased commercially, is not recommended because the fan due to the air traffic congestion otherwise run with excessive speed and provide an even greater pollution. These are microscopic particles. Service technicians and workers in the field Refill and recycling are naturally exposed to the pollutants to an increased extent. It is recommended to use protective devices in the form of disposable gloves and respiratory protection.

Ozone

Furthermore, for technical reasons released ozone in some laser printers. The application of electric charges on the image drum is done with high to very high electric field strengths enforced under high voltage fine platinum wires, the so-called corona wires or pointed metal combs. In the high field strengths, the surrounding air is ionized, with the air oxygen split (O2 ) and ozone (O3 ) is converted. Devices with high ozone output should be in well-ventilated areas. Replaceable ozone filters have at reasonable practicable replacement intervals of several months over no appreciable air cleaning effect.

For devices with low speeds, the charges since the mid- 1990s rather than with corona wires with the help of serrated metal strip or Contact scooters transferred. The voltages required can be reduced to the extent that no appreciable air ionization and therefore no be declared ozone emissions occur. These devices are declared by the manufacturers as " ozone- free".

Fine dust filter

The fine dust filters for laser printers were developed to reduce the potential burden of particulate matter concentration and toner particles in the air. The odor stress on the environment is also improved by this filter.

Many printers bring a fine dust filter with from the factory. Some need an extra change you, others are contained in the toner cartridge and replaced with this one. Depending on the printing performance should assume a shelf life of 6 to 12 months and the fine dust filter to be replaced after one year.

Ultrafine particles ( UFP )

In operation, particularly at the start of printing, laser printers come ultrafine particles ( UFP ) from. However, it was not released in early 2013, from which amount of these ultrafine particles at all constitutes a health hazard. See the 2014 compulsory criteria for the blue angel with the Basic Criteria RAL -UZ 171 before still a limit on UFP.