How a color laser printer works. Laser and inkjet printer: the principle of printing. The laser scanning unit consists

The principle of operation of all laser printers is quite similar to the operation of copiers. Initially, a magnetized area is created on the paper, to which the toner (printing powder) is then attracted. Then, a sheet of paper enters the so-called furnace, where the powder is melted.

The principle of operation of a laser printer

The principle of operation of all laser printers is quite similar to the operation of copiers. Initially, a magnetized area is created on the paper, to which the toner (printing powder) is then attracted. Then, a sheet of paper enters the so-called furnace, where the powder is melted. At the end of the process, the powder cools and solidifies. As a matter of fact, this is how the finished image on paper is obtained.

Despite the relatively high cost, in comparison with inkjet ones, even representatives of the entry-level price level will make it possible to receive even black and white images, but they will be of obvious high quality. At the same time, the print speed is also not comparable. As for maintenance, it is quite simple and unpretentious, in particular, laser printer cartridges are refilled quickly and, most importantly, inexpensively.

The main advantages of laser printers

To date, laser printers are the most popular and sought-after office equipment, this was facilitated by a number of reasons:

1. high print quality, incomparable with inkjet counterparts;
2. reliability and long service life;
3. economy of resources:

• refilling a laser printer is several times less frequent than refilling/replacing cartridges in an inkjet printer;
• if not used for a long time, the toner for laser printers does not dry out and does not become unusable;

1. affordable pricing policy (despite the fact that laser printers are somewhat more expensive than inkjet printers, their quality of work and long service life will more than pay off all costs);
2. high speed printing;
3. relatively large print volumes;
4. resistance of printed copies to water and sunlight;
5. low noise level during operation;
6. low cost of printing (about 5 kopecks per 1 sheet);
7. environmental friendliness and safety for the environment and the human body.

Specifications or how to choose a laser printer?

When deciding to purchase a laser printer, most users often make the wrong choice due to not knowing the technical characteristics.

Due to the fact that a laser printer is able to completely form the image to be printed on the sole drum, it is extremely important to have a large amount of memory and a digital processor with a high frequency. So, for a laser printer with black and white printing, the optimal memory size can be considered 4-8 MB, and for a color one - from 32 MB. In modern printers, the amount of memory can be increased with additional modules.

As for the optimal processor frequency, it varies from 25 to 150 MHz. In turn, the allowable print resolution is from 600 to 1200 dpi.

Resources of a laser printer allow you to print about 8-12 thousand copies in one calendar month. Also, when choosing a model, you should pay attention to the resource of the cartridge, which implies the number of copies that can be printed without refueling.

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V article considered principle actions and device contemporary laser printers. She opens series articles, dedicated principles and problems laser fees.

The image obtained with the help of modern laser printers (as well as matrix and inkjet) consists of dots (dots). The smaller these dots and the more frequently they are located, the higher the image quality. The maximum number of dots that the printer can separately print on a segment of 1 inch (25.4 mm) is called resolution and is characterized in dots per inch, while the resolution can be 1200 dpi or more. The quality of the text printed on a laser printer with a resolution of 300 dpi is approximately the same as a typographic one. However, if the page contains graphics containing grayscale, then a resolution of at least 600 dpi is required to obtain a high-quality graphic image. With a printer resolution of 1200 dpi, the print is almost photographic quality. If you need to print a large number of documents (for example, more than 40 sheets per day), a laser printer seems to be the only reasonable choice, since 600 dpi resolution and a print speed of 8 ... 1 2 pages per minute are standard parameters for modern personal laser printers.

OPERATING PRINCIPLE OF A LASER PRINTER

The first laser printer was introduced by Hewlett Packard. It used the electrographic principle of creating images - the same as in copiers. The difference was in the method of exposure: in copiers it occurs with the help of a lamp, and in laser printers, the light of the lamp has replaced the laser beam.

The heart of a laser printer is a photoconductive cylinder (Organic Photo Conductor), which is often called a print drum or simply a drum. It is used to transfer the image to paper. The photoconductor is a metal cylinder covered with a thin film of a photosensitive semiconductor. The surface of such a cylinder can be provided with a positive or negative charge, which is maintained as long as the drum is not illuminated. If any part of the drum is exposed, the coating becomes conductive and the charge flows from the illuminated area, forming an uncharged zone. This is a key point in understanding how a laser printer works.

Another important part of the printer is the laser and the optical-mechanical system of mirrors and lenses that moves the laser beam along the surface of the drum. The small-sized laser generates a very thin light beam. Reflected from rotating mirrors (usually tetrahedral or hexagonal in shape), this beam illuminates the surface of the photoconductor, removing its charge at the exposure point.

To obtain a dot image, the laser is turned on and off using a control microcontroller. The rotating mirror unfolds the beam as a line of latent image on the surface of the photoconductor.

After the formation of the line, a special stepper motor turns the drum to form the next one. This offset corresponds to the vertical resolution of the printer and is usually 1/300 or 1/600 of an inch. The process of formation of the latent image on the drum resembles the formation of a raster on the screen of a television monitor.

Two main methods of preliminary (primary) charge of the photocylinder surface are used:

Ø using a thin wire or mesh called "corona wire". A high voltage applied to the wire causes a glowing ionized area around it, called the corona, and gives the drum the necessary static charge;

Ø using a pre-charged rubber roller (PCR).

So, an invisible image is formed on the drum in the form of statically discharged dots. What's next?

DEVICECARTRIDGE

Before we talk about the process of transferring and fixing an image on paper, let's consider the design of a cartridge for a Hewlett Packard Laser Jet 5L printer. There are two main compartments in this typical cartridge: the waste toner compartment and the toner compartment.

The main structural elements of the waste toner compartment:

1 - photoconductor(Organic Photo Conductor (OPC) Drum). It is an aluminum cylinder coated with an organic light-sensitive and photoconductive material (usually zinc oxide), which is able to preserve the image applied by the laser beam;

2 - Shaft primary charge(Primary Charge Roller (PCR)). Provides uniform negative drum charge. It is made of a conductive rubber or foam base applied to a metal shaft;

3 - « Viper» , squeegee, cleaning blade(Wiper Blade, Cleaning Blade). Cleans the drum of toner residue that has not been transferred to the paper. Structurally made in the form of a metal frame (stamping) with a polyurethane plate (blade) at the end;

4 - Blade cleaning (Recovery Blade). Covers the area between the drum and the waste toner box. The Recovery Blade lets the toner left on the drum into the hopper and prevents it from spilling out in the opposite direction (from the hopper to the paper).

The main structural elements of the toner compartment:

1 - Magnetic shaft(Magnetic Developer Roller, Mag Roller, Developer Roller). It is a metal tube with a non-moving magnetic core inside. Toner is attracted to the magnetic shaft, which, before being fed to the drum, acquires a negative charge under the action of a constant or alternating voltage;

2 - « Doctor» (Doctor Blade, Metering Blade). Provides even distribution of a thin layer of toner on the magnetic roller. Structurally made in the form of a metal frame (stamping) with a flexible plate (blade) at the end;

3 - Sealing blade magnetic shaft(Mag Roller Sealing Blade). A thin plate similar in function to the Recovery Blade. Covers the area between the magnetic roller and the toner supply compartment. Mag Roller Sealing Blade allows toner remaining on the magnetic roller to enter the compartment, preventing toner from leaking in the opposite direction;

4 - Bunker for toner (toner Reservoir). Inside it is the "working" toner, which will be transferred to the paper during the printing process. Besides, the activator of a toner (Toner Agitator Bar) - the wire frame intended for hashing of a toner is built in the bunker;

5 - Seal, check (Seal). In a new (or regenerated) cartridge, the toner hopper is sealed with a special seal that prevents toner from spilling during transportation of the cartridge. This seal is removed prior to use.

PRINCIPLE OF LASER PRINTING

The figure shows the cartridge in section. When the printer is turned on, all components of the cartridge begin to move: the cartridge is being prepared for printing. This process is similar to the printing process, but the laser beam does not turn on. Then the movement of the cartridge components stops - the printer enters the ready-to-print state.

After sending a document for printing, the following processes take place in the laser printer cartridge:

Charger drum. The primary charge roller (PCR) evenly transfers negative charge to the surface of the rotating drum.

Exposure. The negatively charged surface of the drum is only exposed to the laser beam where the toner will be applied. Under the action of light, the photosensitive surface of the drum partially loses its negative charge. Thus, the laser exposes the latent image to the drum in the form of dots with a weakened negative charge.

Application toner. At this stage, the latent image on the drum is converted by toner into a visible image that will be transferred to paper. The toner located near the magnetic roller is attracted to its surface under the influence of the field of a permanent magnet, from which the core of the roller is made. When the magnetic shaft rotates, the toner passes through a narrow slot formed by the "doctor" and the shaft. As a result, it acquires a negative charge and sticks to those parts of the drum that have been exposed. "Doctor" ensures uniform application of toner on the magnetic roller.

Transfer toner on the paper. Continuing to rotate, the drum with the developed image comes into contact with the paper. On the reverse 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 produces an image "poured" with toner.

Anchoring Images. A sheet of paper with an unfixed image moves to the fixing mechanism, which consists of two contiguous shafts, between which the paper is pulled. The lower shaft (Lower Pressure Roller) presses it against the upper shaft (Upper Fuser Roller). The top roller is heated, and upon contact with it, the toner particles are melted and fixed on the paper.

cleaning drum. Some of the toner does not transfer to the paper and remains on the drum, so it needs to be cleaned. This function is performed by the viper. Any toner remaining on the drum is wiped off by the wiper into the waste toner box. At the same time, the Recovery Blade closes the area between the drum and the hopper, preventing toner from spilling onto the paper.

"Erasing" Images. At this stage, the latent image applied by the laser beam is "erased" from the surface of the drum. With the help of the primary charge roller, the surface of the photoconductor is evenly “covered” with a negative charge, which is restored in those places where it was partially removed by the action of light.

Includes seven sequential operations to create a given image on a sheet of paper. This is a very interesting and technological process, which can be divided into two main stages: drawing an image and fixing it. The first stage is associated with the operation of the cartridge, the second takes place in the fusing unit (oven). As a result, in a matter of seconds on a white sheet of paper, we get the image we are interested in.

So, what happens in such a short period of time in the printer? Let's figure this out.

Charge

Recall that the toner is a finely dispersed substance (5-30 microns), and its particles very easily accept any electric charge.

In a cartridge, the charge roller ensures that the negative charge is evenly transferred to the photoconductor. This happens when the charge roller is pressed against the photoconductor, and rotating in one direction (while imparting a negative static charge to the photoconductor evenly), causes it to rotate in the other direction.

Thus, the surface of the photoconductor has a negative charge evenly distributed over the area.

Exposure

In the next process, the future image is exposed on the photoconductor.

This is due to the laser. When the laser beam hits the surface of the photoconductor, it removes a negative charge in this place (the point becomes neutrally charged). Thus, the laser beam forms the future image according to the given coordinates in the program. Exclusively in those places where it is necessary.

So we get the exposed part of the image in the form of negatively charged dots on the surface of the photoconductor.

development

Next, toner is applied to the exposed image on the surface of the photoconductor in an even thin layer using the developing roller. The toner particles take on a negative charge and form a future image on the surface of the drum.

Transfer

The next step is to transfer the negatively charged toner image from the drum unit onto a blank sheet of paper.

This is caused by the contact of the image transfer roller with the sheet of paper (the sheet passes between the transfer roller and the drum unit). The transfer roller has a high positive potential so that all negatively charged toner particles (in the form of a formed image) are transferred to the paper sheet.

Anchoring

The next step in laser printing is to fix the image from the toner onto a sheet of paper in the fusing unit (in the oven).

At its core, this is the process of "baking" on paper. The sheet with toner, passing between the thermal shaft and the pressure roller, is subjected to thermo-baric (temperature and pressure) treatment, as a result of which the toner is fixed on the sheet and becomes resistant to external mechanical influences.

In our picture you can see a thermal shaft and a pressure roller. The thermal roll is used in a number of laser-type printing devices. A halogen lamp is used inside the thermal shaft, which provides heating (heating element).

There are other models of laser-type printing devices, where a thermal film is used instead of a thermal shaft (as a heating element). The difference between them is that the halogen heater requires more time to operate. It is worth noting the fact that devices with thermal film are very susceptible to mechanical influences of foreign objects (paper clips, staples from a stapler) on a sheet of paper. This is fraught with failure of the thermal film itself. She is very sensitive to damage.

cleaning

Since this whole process leaves a small amount of toner on the surface of the drum unit, a squeegee (cleaning blade) is installed in the cartridge to clean the remaining microparticles of toner from the drum shaft.

Scrolling, the shaft is cleaned. The residual powder goes into the waste toner box.

Removing charge

In the last step, the drum shaft is in contact with the charge roller. This leads to the fact that the "map" of the negative charge is again aligned on the surface of the drum (up to this point, both negatively charged places and neutrally charged ones remained on the surface - they were the projection of the image).

Thus, the charge roller again imparts a uniformly distributed negative potential to the surface of the photoconductor.

This ends the print cycle of one sheet.

Conclusion

Thus, the laser printing technology includes seven successive stages of transferring and fixing the image on paper. On modern devices, such a process of printing one image on A4 paper takes only a few seconds.

This replaces worn internal parts such as the photoconductor, charge roller, or magnetic roller. These parts are inside the cartridge and you can see them in the picture above. Due to the wear of these elements, print quality is significantly degraded.

A bit about the history of laser printing

And finally, a little about the development of laser printing technology. Surprisingly, laser printing technology appeared earlier, for example, the same dot-matrix printing technology. Chester Carlson in 1938 invented a printing method called electrography. It was used in copiers of that time (60-70s of the last century).

The very development and creation of the first laser printer is prescribed by Gary Starkweather. He was an employee of Xerox. His idea was to use copier technology to create a printer.

First appeared in 1971 first laser printer firm Xerox. It was called the Xerox 9700 Electronic Printing System. Serial production was established later - in 1977.

Laser printers have become indispensable attributes of office equipment. Such popularity is explained by the high speed and low cost of printing. To understand how this technique works, you should know the device and principle of operation of a laser printer. In fact, all the magic of the device is explained by simple design solutions.

Back in 1938, Chester Carlson patented a technology that transferred an image to paper using dry ink. The main engine of work was static electricity. Electrographic method(and it was he) became widespread in 1949, when the Xerox Corporation took it as the basis for the operation of its very first device. However, it took another decade of work to achieve logical perfection and complete automation of the process - only after that did the first Xerox appear, which became the prototype of modern laser printing devices.

First Xerox 9700 Laser Printer

The very first laser printer appeared only in 1977 (it was the Xerox 9700 model). Then printing was done at a speed of 120 pages per minute. This device was used exclusively in institutions and enterprises. But already in 1982, the first Canon desktop unit was released. Since that time, numerous brands have been involved in the development, which to this day offer more and more new options for desktop laser printing assistants. Each person who decides to use such a technique will be interested to learn more about the internal structure and principle of operation of such a unit.

What's inside

Despite the large assortment, the laser printer device of all models is similar. The work is based on photoelectric part of xerography, and the device itself is divided into the following blocks and nodes:

• laser scanning unit;
• a node that transfers the image;
• node for fixing the image.

The first block is presented lens and mirror system. It is here that a semiconductor type laser with a focusable lens is located. Next are mirrors and groups that can rotate, thereby forming an image. We pass to the node responsible for transferring the image: it contains the toner cartridge and roller carrying charge. Already in the cartridge alone, there are three main image-forming elements: a photocylinder, a pre-charge roller and a magnetic roller (working in conjunction with the drum of the device). And here the possibility of a photocylinder to change its conductivity under the action of light that has fallen on it acquires great relevance. When the photocylinder is charged, it retains it for a long time, but when illuminated, its resistance decreases, which leads to the fact that the charge begins to drain from its surface. This gives us the impression we need.

In general, there are two ways to create a picture.

Getting into the unit, immediately before the future contact with the photocylinder, the paper itself receives the corresponding charge. The transfer roller helps her with this. After the transfer, the static charge disappears with the help of a special neutralizer - this is how the paper ceases to be attracted to the photo cylinder.

How is the image captured? This is due to those additives that are in the toner. They have a specific melting point. Such a "stove" presses the molten toner powder into the paper, after which it quickly hardens and becomes durable.

Images printed on paper by a laser printer have excellent resistance to numerous external influences.

How the cartridge works

The defining link in the operation of a laser printer is the cartridge. It is a small hopper with two compartments - for working toner and for waste material. There is also a light-sensitive drum (photocylinder) and mechanical gears for turning it.

The toner itself is a fine-dispenser powder, which consists of polymer balls - they are covered with a special layer of magnetic material. If we are talking about color toner, then it also contains dyes.

It is important to know that each manufacturer produces its own original toner - all of them have their own magneticness, dispersion and other properties.

That is why in no case should you refill cartridges with random toners - this can adversely affect its performance.

The process of making an impression

The appearance of an image or text on paper will consist of the following successive stages:

• drum charge;
• exposure;
• developing;
• transfer;
• fastening.

How does photocharge work? It is formed on the photodrum (where, as is already clear, the future image itself is born). To begin with, there is a supply of charge, which can be both negative and positive. This happens in one of the following ways.

1. used coronator, that is, a tungsten filament coated with carbon, gold and platinum inclusions. When a high voltage comes into play, a discharge is carried between this thread by a frame, which, accordingly, will create an electric field that transfers a charge to the photoconductor.
2. However, the use of filament led to problems with soiling and degradation of printed material over time. Works much better charge roller with similar features. He himself looks like a metal shaft, which is covered with conductive rubber or foam rubber. There is contact with the photocylinder - at this moment the roller transfers the charge. The voltage here is much lower, but the parts wear out much faster.

This is the work of illumination, as a result of which part of the photo cylinder becomes conductive and passes the charge through the metal base in the drum. And the exposed area becomes uncharged (or acquires a weak charge). At this stage, a still invisible image is formed.

Technically it works like this.

1. The laser beam falls on the surface of the mirror and is reflected onto the lens, which distributes it to the desired location on the drum.
2. So the system of lenses and mirrors forms a line along the photocylinder - the laser turns on and off, the charge either remains intact or is removed.
3. Line ended? The drum unit will rotate and the exposure will continue again.

development

In this process, it is important cartridge magnetic shaft, similar to a tube made of metal, inside of which there is a magnetic core. Part of the surface of the shaft is placed in the refill toner hopper. The magnet attracts the powder to the shaft, and it is carried out.

It is important to regulate the uniformity of the distribution of the powder layer - for this there is a special dosing blade. It passes only a thin layer of toner, throwing the rest back. If the blade is not installed correctly, black streaks may appear on the paper.

After that, the toner moves to the area between the magnetic roller and the photocylinder - here it will be attracted to the exposed areas, and repelled from the charged ones. So the image becomes more visible.

Transfer

In order for the image to appear already on paper, it comes into play transfer roller, in the metal core of which a positive charge is attracted - it is transferred to paper thanks to a special rubberized coating.

So, the particles break away from the drum and begin to move onto the page. But they are kept here so far only because of static stress. Figuratively speaking, the toner is simply poured where it is needed.

Dust and paper lint can get in with the toner, but they can be removed viper(with a special plate) and sent straight to the waste compartment on the hopper. After a full circle of the drum, the process is repeated.

To do this, the property of the toner to melt at high temperatures is used. Structurally, this is assisted by the following two shafts:

• at the top there is a heating element;
• at the bottom, melted toner is pressed into the paper.

Sometimes such a "stove" is thermal film- a special flexible and heat-resistant material with a heating component and a pressure roller. Its heating is controlled by a sensor. Just at the moment of passage between the film and the pressure part, the paper heats up to 200 degrees, which allows it to easily absorb the liquid toner.

Further cooling occurs naturally - laser printers usually do not require the installation of an additional cooling system. However, a special cleaner passes here again - usually its role is played by felt shaft.

Felt is usually impregnated with a special compound, which helps to lubricate the coating. Therefore, another name for such a shaft is oil.

How color laser printing works

But what about color printing? The laser device uses four of these basic colors - black, magenta, yellow and cyan. The principle of printing is the same as in the black and white case, however, the printer will first split the image into monochrome for each color. The successive transfer of each color by each cartridge begins, and as a result of the overlay, the desired result is obtained.

There are such technologies of color laser printing:

• multipass;
• single pass.

At multi-pass option an intermediate carrier comes into play - this is a shaft or tape that carries the toner. It works like this: 1 color is superimposed in 1 revolution, then another cartridge is fed to the right place, and the second one is placed on top of the first picture. Four passes are enough to form a complete picture - it will go to paper. But the device itself will work 4 times slower than its black and white counterpart.

How the printer works single pass technology? In this case, all four separately printing mechanisms have a common control - they are lined up in one line, each has its own laser unit with a portable roller. So the paper goes along the drum, sequentially collecting all four images of the cartridges. Only after this passage does the sheet go into the oven, where the picture is fixed.

The merits of laser printers have made them a favorite for document work, both in the office and at home. And information about the internal component of their work will help any user to notice shortcomings in time and contact the service department for technical support for the operation of the device.