Laser printer

In 1938, Chester Carlson , a law student, received the first xerographic image, the technology of which was to use static electricity when transferring toner (dry ink) to paper, which was the result of many years of work in order to switch from using existing mimeographs and getting rid of the high cost of the prints. However, only eight years later, having received a waiver from IBM and the US communications forces , in 1946, Carlson managed to find a company that agreed to produce the electrostatic copiers he invented. This company was Haloid Company, which was later renamed Xerox Corporation.

The first Xerox device entered the market in 1949 under the name Model A. This bulky and complex device required a number of manual operations to make a copy of the document. And only ten years later, a fully automatic xerograph, the Xerox 914, was able to issue 7 copies per minute. This model became the prototype of all copiers and laser printers that appeared later.

Xerox began working on laser printers in 1969. Success was achieved in 1978 by , an employee of the company, who was able to add a laser beam to the technology of existing Xerox copiers, thus creating the first laser printer. The full-duplex Xerox 9700 could print 120 pages per minute (by the way, it still remains the fastest laser printer in the world). However, the size of the device was simply enormous, and the price of 350 thousand dollars (without adjusting for the course of that time) did not fit the idea of ​​a printer in every home.

In the early 1980s, demand for devices that outperformed existing dot matrix printers in print quality reached a critical point. In 1979, the offer came from Canon , which introduced the first LBP-10 desktop laser printer. The following year, the company privately demonstrated the new LBP-CX model to California-based Apple , and HP .

At that time, Canon needed strong partners in marketing its products in a new market for the company, since the company had a strong position in the field of cameras and office solutions (the same copiers), but did not have the connections necessary for effective sales of data processing devices in the market. Canon first turned to Diablo Systems, a division of Xerox Corporation. This was obvious and logical, since Diablo owned most of the market for printers, and its marketers expressed a desire to put the Diablo logo on products from other manufacturers. Thus, Xerox was the first company to offer a CX system with a Canon controller.

However, Xerox rejected this offer because, together with the Japanese Fuji-Xerox, it was engaged in the development of the device, which was planned to be the best desktop laser printer on the market. But, although the new 4045 model combined a copier and a laser printer, it weighed about 50 kilograms, cost twice as much as the CX, had no replaceable toner cartridge, and did not provide the best print quality. Subsequently, former Diablo marketers admitted that it was a pretty big mistake to miss Canon’s offer, and the HP LaserJet printer that came out a little later might be Xerox LaserJet.

In any case, after Diablo rejected Canon’s offer in Fremont, representatives of the latter, after driving a few miles, visited the HP offices in Palo Alto and Apple Computer in Cupertino. Hewlett-Packard was the second logical choice because it worked closely with Diablo and had a fairly wide line of dot matrix and printers.

The collaboration between Canon and HP resulted in the release in 1984 of HP LaserJet printers capable of printing 8 pages per minute. Their sales grew quite rapidly and led to the fact that by 1985 Hewlett-Packard had taken possession of almost the entire market for desktop laser printers. It should be noted that, as in the case of inkjet printers, new devices became truly available only after the development of replaceable toner cartridges for them (in this case, the developer was Hewlett-Packard).

At the same time, questions of cheapening new and recycling of used cartridges, the number of which began to hint at environmental problems, gave rise to a whole branch of the processing industry, the date of birth of which can be considered 1986.

There are three ways to transfer toners:

• two-component (a system with two development components - with a separate toner and a developer ) - coloring particles intended for transfer to the photoconductor can not be held independently on the magnetic shaft of the development unit, but stick to the particles of a special magnetic carrier powder (developer), which when mixed charge due to mutual friction.
• two-component, where the toner and developer are already mixed in advance in the factory cartridge.
• single-component (for example, in modern Canon and HP printers) - only a toner without any impurities, the coloring particles of which themselves have magnetic properties. Xerox / Samsung / Brother printers use non-magnetic toner with an electrostatic toner system.

In a two-component system, the developer remains on the magnetic shaft of the development unit and continues to serve further (toner, of course, is consumed). In the technical descriptions of many devices, manufacturers claim that the developer does not require replenishment at all, however, in practice, its performance deteriorates over time, which affects the quality of the copies.

Printing is carried out with a toner, which is a finely divided magnetic polymer that melts at a temperature of 200 degrees. The toner is sprinkled into the toner cartridge, and, thanks to the magnetic shaft located at the bottom of the cartridge, is evenly distributed over it.

To construct an image in laser printers, a photographic method is used: the laser beam (or LED beam) enters the photocell, which is pre-charged without access to light by a charge corotron. The charge Korotron is located above the photovoltaic and is made in the form of a wire stretched parallel to the photovoltaic from a high-resistance material ( manganin , constantan ) or in the form of a rubber roller (contact charge) that is in contact with the photovoltaic. A constant high voltage is applied to the charge corotron, which electrifies the surface of the photovolta due to impact ionization of air resulting from a high voltage corona discharge .

The printing mechanism

• Drum unit (drum unit) - the main unit of each copy machine . Used to transfer the image to paper. It includes a drum unit, a doctor blade, a waste toner bin.
  • Drum (Photoval, photoreceptor) - an aluminum cylinder coated with a photosensitive material that can change its electrical resistance under lighting. In some systems, instead of a photo cylinder, a photo belt was used - an elastic looped strip with a photo layer.
  • Magnetic shaft — the shaft in the cartridge used to transfer toner from the hopper to the drum unit (or the development roller in Xerox / Samsung devices that use non-magnetic toner.)
  • Squeegee knife (cleaning blade)
  • Mining hopper
• Laser unit (laser beam unit) (or LED bar , in LED printers )
• Korotron (coronator, charge roller, Corona Wire)
• Transfer belt (transfer belt) - a tape in color laser printers, which is applied to the intermediate image from the drums of 4 color cartridges, which is then transferred to the final carrier - paper ^[1] .
• A developing unit is used to transfer toner to an electrostatic image formed on the surface of a photoconductive drum ^[2] .

Consumables

Toner is a powder for applying an image.

Carrier is a ferromagnetic powder (it is composed of small particles) used in two-component machines to hold the toner on the surface of the magnetic shaft due to electrostatic forces (when mixed with the toner it charges it with positive static potential during mutual friction), and from there , under the influence of a discharge on a corotron, on the surface of the photodrum; and the developer himself, due to his magnetic properties, remains on the magnetic shaft and is almost not consumed (however, it loses its properties over time and also requires replacement) ^[3] .

Developer (Eng. Developer) (sometimes called a starter) - a mixture of materials supplied to the drum. In two-component machines, this is a mixture of toner and carrier, and in single-component machines, it is only toner. The term is similar to the term developer used in photography, but is usually not translated into Russian-language literature.

The laser printing process consists of five successive steps:

Photo Charging

Photocell charging - applying a uniform electric charge to the surface of a rotating photodrum (1). The most commonly used drum material — the photoorganic — requires the use of a negative charge, but there are materials (such as silicon ) that allow the use of a positive charge.

Initially, charging was carried out using a scorotron ( English scorotron ) - a stretched wire that is supplied with voltage relative to the drum. A metal mesh is usually placed between the wire and the drum, which serves to level the electric field.

Later they began to use charging using a charging roller ( Eng. Charge Roller ) (2). Such a system made it possible to reduce the stress and reduce the problem of ozone emission in the corona discharge (the conversion of O ₂ molecules to O ₃ under the action of high voltage), however, it entails the problem of direct mechanical contact and wear of parts, as well as cleaning from contaminants.

Laser Scanning

Laser scanning (exposure) is the process of passing a negatively charged surface of a photocell under a laser beam. The laser beam (3) is deflected by a rotating mirror (4) and, passing through the distribution lens (5), focuses on the photo camera (1). The laser is activated only in those places where toner from the magnetic shaft (7) will subsequently have to get. Under the action of the laser, the sections of the photosensitive surface of the photovoltaic that were illuminated by the laser become electrically conductive, and part of the charge in these areas "flows" onto the metal base of the photovoltaic. Thus, an electrostatic image of the future print in the form of a “pattern” of areas with a less negative charge than the general background is created on the surface of the photo-camera.

Toner Overlay

When a toner is supplied, a negatively charged roller gives the toner a negative charge and feeds it to the development roller. The toner in the hopper is attracted to the surface of the magnetic shaft by the magnet from which the shaft core is made. During rotation of the magnetic shaft, the toner located on its surface passes through a narrow slot formed between the metering blade and the magnetic shaft. After this, the toner comes into contact with the photocell and is attracted to it in those places where a negative charge has been preserved. It is a very common misconception that toner is attracted precisely to places where there is no charge on the photocell. However, the toner, being a dielectric, is electrified into dipole-charged particles, which are attracted to any charged surfaces, regardless of the sign of the charge of the latter. The toner will be able to stay on the photocell in places without a charge if the toner particles are charged (and it does not matter with which sign). At the same time, the reverse misconception is widespread that toner is attracted precisely to places charged negatively. Most brands of manufactured toners for household laser printers are marked as negatively charged, which indicates the impossibility of attracting negatively charged particles of toner to the same charged areas of the drum. Therefore, in fact, a negatively charged toner cannot be attracted to all charged areas, regardless of the sign of the charge, but only to those that have no charge, or at least to those whose charge does not interfere with the formation in the surface layer the drum of the dipole moment under the influence of the electrostatic field of negatively charged toner particles.

Thus, the electrostatic (invisible) image is converted into visible (manifested). The toner attracted to the photo camera moves on it until it comes in contact with the paper.

Toner Transfer

In the place where the photo-paper contacts the paper, there is another roller under the paper called the transfer roller. A positive charge is applied to it, which it communicates to the paper with which it is in contact. Particles of toner, coming into contact with positively charged paper, are transferred to it and held on the surface due to electrostatics.

If at this moment you look at the paper, a completely finished image will be formed on it, which can be easily destroyed by swiping it with your finger, because the image consists of toner powder attracted to the paper, which cannot be held on paper other than electrostatics. To obtain the final print, the image must be fixed.

Fixing Toner

The paper with the “poured” toner image moves further to the fusing unit (the stove). The image is fixed due to heating and pressure. The stove consists of two shafts:

• the top, inside of which there is a heating element (usually a halogen lamp ), called a thermal bulb;
• lower (pressure roller), which presses the paper to the upper due to the retaining spring.

The temperature was monitored by a thermal sensor ( thermistor ). The stove consists of two contacting shafts between which paper passes. When the paper is heated (180–220 ° C), the toner attracted to it melts and is squeezed into the paper texture in liquid form. Leaving the stove, the toner quickly hardens, which creates a constant image that is resistant to external influences. So that the paper on which the toner is applied does not adhere to the thermal, paper separators (fangs) are made on it.

However, the term is not the only implementation of the heater. An alternative is another device of the stove in which thermal film is used: that is, a special flexible material in the form of a tube that completely wraps the supporting structure with a thin and long ceramic plate, which is just a heating element containing in the structure of the ceramic plate, in addition to the heater conductors, built-in low voltage temperature sensor on the other side of the plate. Erroneous installation of a ceramic plate by unskilled service center employees leads to a rapid and irrevocable burnout of the temperature sensor.

For example, this is true for HP LaserJet 1100 / 1100A, 1200, and more. In subsequent models of printers (HP LaserJet 1010,1018,1020, etc.), the temperature sensor was removed from the structure of the ceramic plate.

With this embodiment of the oven with thermal film, it is necessary to use a special high-temperature silicone grease due to the presence of significant forces during friction-sliding on ceramics during rotation of the thermal film when the sheet is run through the thermal block.

Thermofilm mainly relies on its extreme sides and rotates on the side plastic support posts.

It should be noted that there are the following disadvantages inherent in all types of thermal films. This is their tendency to breakthroughs from stapler paper clips, burn-through due to sticking of sintered toner on excess thermal grease inside the frame of the thermal unit under the film and the presence of other negative effects of unlucky users and service repairmen.

The principle of multicolor laser printing is as follows. At the initial stage of the printing process, the rendering engine takes a digital document and processes it one or several times, creating its page-by-bit raster image decomposed into color components corresponding to the colors of the toners used. At the second stage, the laser or array of LEDs forms a charge distribution on the surface of the rotating photosensitive drum, similar to the image obtained. Charged small particles of toner, consisting of a coloring pigment, resins and polymers, are attracted to the discharged areas of the drum surface.

Next, the toner from the drum is transferred to the transfer tape, on which a full-color image is formed, and with which the toner is transferred to the paper. Most color laser printers use four separate passes corresponding to different colors. Then the paper passes through the “stove”, which melts the resins and polymers in the toner and fixes it on the paper, creating the final image.

Lasers are able to accurately focus, resulting in very thin beams that discharge the necessary parts of the photosensitive drum. Thanks to this, modern laser printers, both color and black and white, have a high resolution.

Benefits

Typically, the resolution for black and white printing varies from 600 × 600 to 1200 × 1200 dpi, but with color printing it reaches 9600 × 9600. Color and black and white laser printers work the same in practice. The difference is that four types of ink toner are used for color printing. Any color contributes to the final image applied to a sheet of paper. Compared to inkjet printers , laser printers have many advantages:

• They have a higher speed, since the laser beam can move much faster than a print head with dozens or even hundreds of nozzles, from which microscopic droplets of ink are injected at a certain interval at the time of printing (exceptions: inkjet printers with linear stationary formatted heads).
• Laser beams are even more accurate and due to their compact focusing, they can achieve high resolution. Laser printers are more economical than inkjet printers, simply because toner cartridges last for more than a thousand pages, but ink cartridges run out faster and need to be refilled or replaced more often (this applies only to desktop, home inkjet printers)
• Laser prints are more stable, the clarity of the prints is not violated in conditions of high humidity. The toner can coalesce, which can easily be fixed by lightly shaking the cartridge, unlike inkjet printers, whose ink can dry in nozzles , which requires washing and, sometimes, replacing them.
• Color laser printers provide high speed printing, provide high-quality color and black-and-white prints, as well as an attractive cost of printing a page, taking into account consumables.

disadvantages

• When a laser printer is operating, ozone , nitrogen oxides ( NO₂ , N₂O ), acetone , paper dust, ultraviolet and infrared (thermal) radiation are emitted .
• When the paper is heated, formaldehyde and water vapor are actively evaporated.
• The presence in the design of elements with high energy consumption (high-voltage charging unit, thermal unit) leads to the fact that the peak power consumption of the laser printer is high enough, which makes it impossible to connect it to household uninterruptible power supplies of medium and low power.
• The print quality of color grayscale images (such as photographs ) is lower than with inkjet printing.
• Laser printers are 1.8 times more expensive than inkjet printers, and the cost of a set of cartridges for a laser printer is much more expensive than a kit for an inkjet (as a rule, the cost of a new printer). In medium and industrial laser printers, the comparative cost of consumables and equipment costs are approximately the same compared to medium and industrial inkjet printers, and the cost of printing is 3-10 times higher than the cost of inkjet printing with constant quality .
• Demanding on the quality of paper, they break when using crumpled paper or paper with a paper clip.
• There is no stable repeating full-color print preserving all the values ​​of the original, due to the inability to control electrostatic fields.

Hidden marks (see Yellow dots ) - many color printers print a hidden image on the print, indicating the date and time of printing, as well as the serial number of the device, which is supposedly done to stop printing color copies of banknotes and other documents and valuable papers ^[4] .

• a printer
• Wide oriented printer
• Virtual printer
• Multifunction device
• Copy machine
• Toner
• Rasterization
• Electrography

5. The principle of laser printers