Lecture 29 min.
Laser printer — a type of printer that can quickly produce high-quality prints of text and graphics on plain (non-special) paper. Like photocopiers, laser printers rely on the xerographic printing process, but they differ in that the image is formed by directly exposing (illuminating) the printer's photosensitive elements with a laser beam.
Prints made this way are not damaged by moisture and resist abrasion and fading. The image quality is very high.
Laser printer 1993 Apple LaserWriter Pro 630
Kyocera FS-1110 laser printer
Fuji Xerox color laser printer C1110B
While the history of inkjet printing is full of science and packed with research and discoveries, the history of the invention and development of laser printers probably has a more commercial slant and is, to a certain extent, tied more to marketing than to science.
In 1938 a law student named Chester Carlson (who, incidentally, later became a patent attorney in order to back up his talents as an inventor) obtained the first xerographic image, the successful outcome of many years of work that he had begun out of frustration with the slowness of the mimeographs of the day and the high cost of the copies they produced. The word "xerography" itself was coined from the Greek words for "dry" and "to write." The essence of the new technology was the use of static electricity to transfer dry ink (toner) onto paper.
It took another 8 years, however — and rejections from IBM and even from the U.S. Army Signal Corps — before Carlson managed in 1946 to find a company willing to manufacture the electrostatic copiers he had devised. That company was the Haloid Company, which later became the world-famous Xerox Corporation.
The first Xerox machine reached the market in 1949 under the name Model A. It was a rather bulky and complicated device. Getting a copy of a document out of it required a series of manual operations. Only ten years later was a fully automatic xerographic copier commercialized — the Xerox 914, which could turn out 7 copies per minute. This model became the prototype for all the copiers and laser printers that followed.
Xerox began work on laser printers in 1969. Success came in 1978 to company employee Gary Starkweather, who managed to add a laser beam to the technology of the existing Xerox copiers, thereby creating the first laser printer. The full-duplex Xerox 9700 could print 120 pages per minute (and it still remains the world's fastest laser printer). The machine was simply enormous, however, and its price of 350 thousand dollars (unadjusted for the exchange rate of the time) did not fit the idea of "a printer in every home" at all.
In the early 1980s, demand for devices that would surpass the existing dot matrix printers in print quality reached a critical level. In 1979 an offering came from Canon, which introduced the first desktop laser printer, the LBP-10. The following year the company privately demonstrated its new LBP-CX model to Apple, Diablo and HP in California.
At that point Canon needed strong partners to market its products in a market that was new to the company, since it held a solid position in cameras and office solutions (copiers among them) but lacked the connections required for effective sales in the data-processing equipment market. Canon first approached Diablo Systems, a division of Xerox Corporation. This was an obvious and logical step, since Diablo held most of the daisy wheel printer market and its marketing people had expressed a willingness to put the Diablo logo on other manufacturers' products as well. Xerox thus became the first company to be offered the chance to bring the CX system with the Canon controller to market.
Xerox turned the offer down, however, because together with Japan's Fuji-Xerox it was itself developing a machine that was intended to be the best desktop laser printer on the market. But although the new model 4045 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 delivered print quality that was far from the best. Former Diablo marketing staff later admitted that letting Canon's offer slip away had been a fairly large mistake, and that the HP LaserJet, which appeared somewhat later, could have been the Xerox LaserJet.
In any case, after Diablo turned down Canon's offer in Fremont, Canon's representatives drove a few miles and called on the HP offices in Palo Alto and Apple Computer in Cupertino. Hewlett-Packard was the second logical choice, since it worked closely with Diablo and had fairly broad lines of dot matrix and daisy wheel printers.
The collaboration between Canon and HP resulted in the 1984 release of the LaserJet printers, capable of printing 8 pages per minute. Their sales grew very quickly and led to Hewlett-Packard capturing almost the entire desktop laser printer market by 1985. It should be borne in mind that, as with inkjet printers, the new machines became truly affordable only after replaceable toner cartridges were developed for them (in this case the developer was Hewlett-Packard).
At the same time, the questions of making new cartridges cheaper and of recycling used ones, whose numbers were beginning to hint at environmental problems, gave rise to an entire recycling industry, whose birth can be dated to 1986.
There are three ways of transferring toner:
In a two-component system the developer stays on the magnetic roller of the developing unit and continues to serve on (the toner, naturally, is consumed). In the technical specifications of many machines the manufacturers state that the developer needs no replenishment at all, but in practice its working characteristics deteriorate over time, which affects copy quality.
Toner — the powder used to lay down the image.
Carrier (Eng. Carrier) — a ferromagnetic powder (structurally, fine particles) used in two-component machines to hold toner on the surface of the magnetic roller by electrostatic forces (when mixed with toner it charges the toner with a positive static potential by mutual friction), and from there, under the action of the discharge at the corotron, onto the surface of the photoconductor drum; the developer itself, by virtue of its magnetic properties, stays on the magnetic roller and is hardly consumed at all (though it loses its properties over time and also has to be replaced).[3]
Developer (Eng. Developer) (occasionally called starter) — a mixture of materials fed to the photoconductor drum. In two-component machines it is a mixture of toner and carrier, while in single-component machines it is toner alone. The term is analogous to the term developer used in photography, but in Russian-language literature it is usually left untranslated.
The laser printing process consists of five sequential steps:
The laser printing process
Charging the photoconductor drum — applying a uniform electric charge to the surface of the rotating photoconductor drum (1). The most commonly used photoconductor drum material — organic photoconductor — requires the use of a negative charge, but there are materials (silicon, for example) that allow a positive charge to be used.
Initially, charging was carried out with a corotron (Eng. scorotron) — a taut wire to which a voltage is applied relative to the photoconductor drum. A metal grid is usually placed between the wire and the photoconductor drum to even out the electric field.
Later, charging with a charge roller (Eng. Charge Roller) (2) came into use. Such a system made it possible to lower the voltage and reduce the problem of ozone generation in the corona discharge (the conversion of O2 molecules into O3 under the action of the high voltage), but it brings with it the problem of direct mechanical contact and wear of the parts, as well as cleaning off contamination.
Laser scanning (exposure) — the process in which the negatively charged surface of the photoconductor drum passes under the laser beam. The laser beam (3) is deflected by a rotating mirror (4) and, passing through a distributing lens (5), is focused on the photoconductor drum (1). The laser is activated only at those places where toner is subsequently to land from the magnetic roller (7). Under the action of the laser, the areas of the photosensitive surface of the photoconductor drum that have been exposed by the laser become electrically conductive, and part of the charge in those areas "drains" to the metal base of the drum. An electrostatic image of the future print is thereby created on the surface of the photoconductor drum, in the form of a "pattern" of areas with a less negative charge than the general background.
When toner is fed in, the negatively charged roller imparts a negative charge to the toner and delivers it to the developing roller. The toner in the hopper is attracted to the surface of the magnetic roller by the magnet from which the roller's core is made. As the magnetic roller rotates, the toner on its surface passes through the narrow gap formed between the doctor blade and the magnetic roller. After that the toner comes into contact with the photoconductor drum and is attracted to it in those places where the negative charge has been retained. There is a very widespread misconception that the toner is attracted precisely to the places on the photoconductor drum where charge is absent. Toner, however, being a dielectric, is electrified into dipole-charged particles, which are attracted to any charged surface regardless of the sign of that surface's charge. Toner will be able to hold onto the photoconductor drum in places with no charge if the toner particles are charged (and it does not matter with what sign). At the same time, the opposite misconception is also widespread — that the toner is attracted precisely to the negatively charged places. Most brands of toner produced for consumer laser printers are labeled as negatively chargeable, which means that negatively charged toner particles cannot be attracted to like-charged areas of the photoconductor drum. Therefore, in reality, 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 magnitude does not prevent a dipole moment from forming in the surface layer of the photoconductor drum under the influence of the electrostatic field of the negatively charged toner particles.
In this way the electrostatic (invisible) image is converted into a visible one (it is developed). The toner attracted to the photoconductor drum travels onward on its surface until it comes into contact with the paper.
At the point where the photoconductor drum meets the paper there is another roller underneath the paper, called the transfer roller. A positive charge is applied to it, which it passes on to the paper it touches. Toner particles that come into contact with the positively charged paper are transferred onto it and held on its surface by electrostatic force.
If you were to look at the paper at this moment, a completely finished image would already be formed on it – one that is easily destroyed by running a finger across it, because the image consists of toner powder attracted to the paper and held there by nothing but electrostatics. To obtain the final print, the image has to be fused.
The paper with the "sprinkled on" toner image moves on to the fusing unit (the fuser). The image is fused by heat and pressure. The fuser consists of two rollers:
The temperature of the heat roller is monitored by a temperature sensor (a thermistor). The fuser is essentially two rollers in contact with each other, with the paper passing between them. As the paper is heated (180–220 °C), the toner attracted to it melts and, in liquid form, is pressed into the texture of the paper. Once out of the fuser, the toner solidifies quickly, creating a permanent image that resists external influences. To keep the paper carrying the toner from sticking to the heat roller, paper separators are fitted on it.
The heat roller, however, is not the only way a heater can be implemented. An alternative is a different fuser design that uses a fuser film: a special flexible material in the form of a sleeve that completely wraps around a supporting structure holding a thin, long ceramic strip, which is in fact the heating element; besides the heater conductors, the ceramic strip itself also contains a built-in low-voltage temperature sensor on its other side. Incorrect installation of the ceramic strip by poorly qualified service-center technicians causes the temperature sensor to burn out rapidly and irreversibly.
This is true, for example, of the HP LaserJet 1100/1100A, 1200 and similar laser printers, which were excellent in their day. In later printer models (HP LJ 1010, 1018, 1020 and so on) the temperature sensor was moved out of the ceramic strip.
With this fuser-film design, a special high-temperature silicone grease must be used, because considerable sliding-friction forces arise against the ceramic as the fuser film rotates while a sheet is fed through the fusing unit.
The fuser film rests and rotates mainly on its outer edges, on plastic support posts at the sides.
It should be noted that there are the following drawbacks common to all types of fuser film. These are their tendency to tear on staples left in the paper, to burn through because of caked toner sticking to excess fuser grease inside the frame of the fusing unit beneath the film, and other harmful effects caused by careless users and service technicians.
The principle of multicolor laser printing is as follows. At the initial stage of the printing process the rendering engine takes the digital document and processes it one or more times, producing a page-by-page raster image of it separated into color components corresponding to the colors of the toners used. At the second stage a laser or an array of LEDs forms a charge distribution on the surface of the rotating photosensitive drum that matches the image being produced. The charged fine particles of toner, which consists of coloring pigment, resins and polymers, are attracted to the discharged areas of the drum surface.
Next the paper comes into contact with the drum and the toner is transferred onto it. Most color laser printers use four separate passes, one for each of the different colors. The paper then passes through the "fuser", which melts the resins and polymers in the toner and fixes it to the paper, creating the final image.
Lasers can be focused precisely, producing very fine beams that discharge exactly the required areas of the photosensitive drum. Thanks to this, modern laser printers, both color and black-and-white, have high resolution.
Advantages
As a rule, the resolution for black-and-white printing ranges from 600x600 to 1200x1200 dots per inch, while for color printing it reaches 9600x1200. In practice, color and black-and-white laser printers work the same way. The difference is that color printing uses four kinds of coloring toner. Each color makes its own contribution to the final image applied to the sheet of paper. Compared with inkjet printers, laser printers have quite a few advantages:
Disadvantages
Hidden markings (see Printer steganography, yellow dots) – many models of color printer add a hidden image to the printout indicating the date and time of printing as well as the serial number of the device, which is presumably done in order to prevent color copies of banknotes and other documents and securities from being printed[4].
The image produced by modern laser printers (as well as by dot-matrix and inkjet printers) consists of dots. The smaller these dots are and the more closely they are spaced, the higher the image quality. The maximum number of dots a printer can print separately over a length of 1 inch (25.4 mm) is called the resolution and is expressed in dots per inch (dpi – dot per inch). A printer is considered fairly good if its resolution is 300 dpi (the notation 300 x 300 dpi is sometimes used, meaning 300 dpi horizontally and 300 dpi vertically).
Laser printers are less demanding about paper than, for example, inkjets, and the cost of printing one page of a text document is several times lower. At the same time, inexpensive models of laser and LED monochrome printers are already able to compete on price with high-quality color inkjet printers.
Most of the laser printers on the market are intended for black-and-white printing; color laser printers are quite expensive and are aimed at corporate users.
Laser printers print on any heavy paper (from 60 g/m²) at a speed of 6 to ... (this figure keeps growing) sheets per minute (ppm – page per minutes), and the resolution can be 1200 dpi or more. The quality of text printed on a laser printer at 300 dpi is roughly equivalent to typeset quality. However, if the page contains images with shades of gray, a resolution of at least 600 dpi will be needed to obtain a high-quality graphic image. At a printer resolution of 1200 dpi the print is of almost photographic quality. If you need to print a large number of documents (more than 40 sheets a day, for example), a laser printer appears to be the only sensible choice, since for modern personal laser printers a resolution of 600 dpi and a printing speed of 8 to 12 pages per minute are standard specifications.
The laser printer was first introduced by Hewlett Packard. It used the electrographic principle of image creation – the same one as in photocopiers. The difference lay in the method of exposure: in photocopiers it is done with a lamp, while in laser printers the light of the lamp was replaced by a laser beam (Fig. 1).
Fig. 1. Laser printer design
The heart of a laser printer is the photoconducting cylinder (Organic Photo Conductor), often called the printing photoconductor drum or simply the drum. It is what transfers the image onto the paper. The photoconductor drum is a metal cylinder coated with a thin film of photosensitive semiconductor. The surface of such a cylinder can be given a positive or negative charge, which is retained as long as the drum is not illuminated. If some part of the drum is exposed, the coating becomes conductive and the charge drains away from the illuminated area, forming an uncharged zone. This is the key point in understanding how a laser printer works.
Another crucial part of the printer is the laser together with the opto-mechanical system of mirrors and lenses that moves the laser beam across the surface of the drum. A compact laser generates a very fine beam of light. Reflected off rotating mirrors (usually four-sided or six-sided), this beam illuminates the surface of the photoconductor drum, removing its charge at the point of exposure.
To produce a dot image, the laser is switched on and off by a controlling microcontroller. The rotating mirror sweeps the beam out into a line of the latent image on the surface of the photoconductor drum.
Once a line has been formed, a special stepper motor turns the drum to form the next one. This shift corresponds to the printer's vertical resolution and is usually 1/300 or 1/600 inch. The process by which the latent image is formed on the drum resembles the way a raster is formed on a television monitor screen.
Two main methods are used for the preliminary (primary) charging of the photoconductor cylinder surface:
• by means of a thin wire or grid called a "corona wire". The high voltage applied to the wire causes a glowing ionized region to appear around it, which is called the corona and gives the drum the necessary static charge;
• by means of a primary charge roller (PCR).
So, an invisible image in the form of statically discharged dots has been formed on the drum. What happens next?
Before describing how the image is transferred to paper and fused there, let us look at the design of the cartridge for the Hewlett Packard Laser Jet 5L printer. Two main compartments can be identified in this typical cartridge:
the waste toner compartment and the toner compartment.
The main components of the waste toner compartment (Fig. 2):
1 – Photosensitive drum (Organic Photo Conductor (OPC) Drum). This is an aluminum cylinder coated with an organic photosensitive, photoconductive material (usually zinc oxide) that is able to hold the image written by the laser beam;
2 – Primary charge roller (PCR). Applies a uniform negative charge to the drum. It consists of a conductive rubber or foam layer on a metal shaft;
3 – "Wiper", doctor blade, cleaning blade (Wiper Blade, Cleaning Blade). Cleans the drum of the toner residue that was not transferred to the paper. It is built as a metal frame (stamping) with a polyurethane blade at its end;
4 – Recovery blade. Covers the gap between the drum and the waste toner hopper. The recovery blade lets the toner remaining on the drum pass into the hopper and keeps it from spilling back out (from the hopper onto the paper).
The main components of the toner compartment (see Fig. 3):
1 – Magnetic roller (Magnetic Developer Roller, Mag Roller, Developer Roller). This is a metal tube containing a stationary magnetic core. Toner is attracted to the magnetic roller and, before being fed to the drum, acquires a negative charge under a DC or AC voltage;
2 – "Doctor" (Doctor Blade, Metering Blade). Spreads a thin, even layer of toner over the magnetic roller. It is built as a metal frame (stamping) with a flexible blade at its end;
3 – Magnetic roller sealing blade (Mag Roller Sealing Blade). A thin blade with the same function as the recovery blade. It covers the gap between the magnetic roller and the toner supply compartment. The mag roller sealing blade lets the toner remaining on the magnetic roller pass into the compartment, preventing toner from leaking back out;
4 – Toner reservoir. It holds the "working" toner that will be transferred to the paper during printing. The reservoir also contains a toner agitator bar – a wire frame that stirs the toner;
5 – Seal. In a new (or remanufactured) cartridge the toner reservoir is closed with a special seal that prevents toner from spilling while the cartridge is being transported. This seal is removed before the cartridge is put into service.
Fig. 4 shows a cutaway view of the cartridge. When the printer is switched on, all of the cartridge components start to move: the cartridge is being prepared for printing. This process is the same as the printing process, except that the laser beam is not turned on. The cartridge components then stop moving and the printer enters its Ready state.
Fig. 4. Cutaway view of the cartridge
Once a document has been sent to print, the following processes take place inside the laser printer cartridge:
Charging the drum (Fig. 5). The primary charge roller (PCR) applies a uniform negative charge over the surface of the rotating drum.
Fig. 5. Charging the drum
Exposure (Fig. 6). The negatively charged surface of the drum is exposed by the laser beam only at those places where toner is to be applied. Under the light, the photosensitive surface of the drum partially loses its negative charge. The laser thus writes a latent image onto the drum as dots with a weakened negative charge.
Fig. 6. Exposure
Applying the toner (Fig. 7). At this stage the latent image on the drum is turned by the toner into a visible image that will be transferred to the paper. The toner near the magnetic roller is attracted to its surface by the field of the permanent magnet that forms the core of the roller. As the magnetic roller turns, the toner passes through the narrow gap formed by the "doctor" and the roller. As a result it acquires a negative charge and sticks to those areas of the drum that were exposed. The "doctor" ensures that the toner is applied evenly to the magnetic roller.
Fig. 7. Applying the toner
Transferring the toner to the paper (Fig. 8). Continuing to rotate, the drum with the developed image comes into contact with the paper. On the other side, the paper is pressed against the transfer roller, which carries a positive charge. As a result the negatively charged toner particles are attracted to the paper, which receives an image "dusted" onto it in toner.
Fig. 8. Transferring the toner to the paper
Fusing the image (Fig. 9). The sheet of paper with the unfused image moves to the fusing mechanism, which consists of two rollers in contact with each other, with the paper drawn between them. The lower roller (lower pressure roller) presses it against the upper roller (upper fuser roller). The upper roller is heated, and on contact with it the toner particles melt and are fused to the paper.
Fig. 9. Fusing the image
Cleaning the drum (Fig. 10). Some of the toner is not transferred to the paper and remains on the drum, so the drum has to be cleaned. This is the job of the "wiper". All of the toner left on the drum is scraped by the wiper into the waste toner hopper. At the same time the recovery blade closes off the gap between the drum and the hopper, keeping the toner from spilling onto the paper.
Fig. 10. Cleaning the drum
"Erasing" the image (Fig. 11). At this stage the latent image written by the laser beam is "erased" from the surface of the drum. With the help of the primary charge roller the surface of the photosensitive drum is uniformly "covered" with a negative charge, which is restored in those places where it had been partially removed by the light.
Fig. 11. "Erasing" the image
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