Device and principle of operation of a laser printer

Lots of people have used laser printers, some have them at home, but does everyone know how a laser printer works? The reader will find the answer to this question in this article.

A laser printer is a peripheral device that prints text and graphics quickly and efficiently on ordinary office and specialty paper. The main advantages of these printers, such as low cost of printing, high speed of operation, high resource and resolution, resistance to moisture and fading, have made them the most commonly used not only among office workers, but also among ordinary users.

Creation and development of laser printers

The first image using dry ink and static electricity was obtained by Chester Carlson back in 1938. And only 8 years later, he was able to find a manufacturer of the devices he invented. It was the company we all know today as Xerox. And in the same year, 1946, the first copier entered the market. It was a huge and complex machine requiring a number of manual operations. It was only in the mid-1950s that the first fully automated mechanism was created, which was the prototype of the modern laser printer.

In late 1969, Xerox began work on the development of laser printers, adding a laser beam to existing designs at the time. But it was worth a third of a million dollars by those standards and had a huge size, which did not allow using such a device even in small enterprises, let alone in everyday life.

the first personal laser printer from HP

The collaboration between the current giants of the printing industry Canon and HP is the launch of a series of LaserJet printers that can print up to 8 pages of text per minute. Such devices became more affordable with the introduction of the first replaceable cartridge for a laser printer.

Principle of operation

The basis for image formation is the dye contained in the toner. Under the influence of static electricity, it sticks and is literally imprinted on the paper. But how does this happen?

Any laser printer consists of three main functional blocks: a printed circuit board, an image transfer unit (cartridge) and a printing unit. The paper feed unit feeds the printing paper. They are designed in two ways – paper feed from the lower tray and paper feed from the upper tray.

Its structure is quite simple:

  • roller – needed to grab paper;
  • block for gripping and feeding one sheet;
  • a roller that transfers static electricity to the paper.
  • The laser printer cartridge consists of two parts, the toner and the drum or photo cylinder.

Toner

Toner consists of microscopic polymer particles that are coated with a dye, with the inclusion of magnenite and a charge regulator. Each firm produces powder with unique characteristics for its own printers and multifunctional devices. All powders differ in magnetism, density, dispersity, grain size and other physical characteristics. Therefore, do not refill the cartridges with random toner. The advantages of toner over ink are the clarity of the printed image and moisture resistance, which is ensured by imprinting the powder into the paper. Among the disadvantages are shallow color depth, saturation in color printing and negative effects on the human body when interacting with toner, for example, while charging a cartridge.

The structure and stages of printing images

The imaging drum is made in the form of a longitudinal aluminum shaft with a thin layer of material applied to it, sensitive to light rays with certain parameters. The cylinder is covered with a protective layer. In addition to aluminum, drums are made from inorganic photosensitive substances. The main property of a drum is a change in conductivity (charge) under the influence of a laser beam. This means that if the cylinder is charged, it will store it for a significant period of time. But if you illuminate any area of ​​the shaft with light, they immediately lose their charge and become neutrally charged due to an increase in conductivity (that is, a decrease in electrical resistance) in these zones. The charge drains from the surface through the inner conductive layer.

When a document arrives for printing, the printed circuit board processes it and sends the appropriate light pulses to the image transfer unit, where the digital picture is converted into an image on paper. The drum is rotated by the shaft and receives a primary negative or positive charge from the nearby roller. Its value is determined by the print settings, which are reported by the printed circuit board.

After charging the cylinder, a horizontal laser beam scans it with tremendous frequency. The exposed areas of the photocylinder, as mentioned above, become uncharged. These uncharged zones form the desired image on the drum in a mirror image. Further, in order for the image to appear on the paper, the uncharged areas must be filled with toner. The laser scanning unit consists of a mirror, a semiconductor laser, several shaping lenses and one focusing lens.

The drum contacts a roller made primarily of magnesium and delivers toner to the photocylinder from the container. The roller, in which the permanent magnet is located, is made in the form of a hollow cylinder with a conductive layer. Under the influence of the magnetic field, the toner from the hopper is attracted to the roller by the force of the magnetized core.

Under the influence of electrostatic voltage, the toner from the roller will be transferred to the image formed by the laser beam on the surface of the drum, rotating close to the roller. The toner has nowhere to go, because his negatively charged particles are attracted to the positively charged areas of the photocylinder, on which the desired image is formed. The negative drum charge pushes waste toner back into the laser-scanned areas.

Let’s note one nuance. There are two types of imaging. The most common is to use positively charged toner. This powder remains on the neutral charged areas of the photocylinder. That is, the laser illuminates the areas where our future image will be. The drum is negatively charged. The second mechanism is less common and uses negatively charged toner. The laser beam “discharges” the areas of the positively charged photocylinder, on which the image should not be. It is worth remembering when choosing a laser printer, because in the first case there will be a more accurate transfer of parts, and in the second – a more uniform and dense filling. The first printers were perfect for printing text documents, which is why they became widespread.

Before touching the cylinder, the paper is charged with static electricity by the charge transfer roller. By the influence of which toner is attracted to the paper when it comes into contact with the drum. The static charge eliminator is immediately removed from the paper. This eliminates the attraction of the sheet to the photocylinder. During the passage of the paper through the laser scanning unit, a formed image becomes visible on the sheet, which is easily destroyed by the slightest touch. For its durability, it is necessary to fix by melting the additives included in the toner. This process takes place in the imaging unit – this is the third key unit of the laser printer. It is also called the “stove”. In short, the substances that make up the toner melt. After they are pressed and solidified, these polymers seem to cover the ink, protecting it from external influences. The reader will now understand why the printed sheets coming out of the printer are so warm.

By design, the so-called “stove” consists of two shafts, one of which contains a heating element. The second, often the bottom one, is needed to press the molten polymer into the paper. Heating elements are made in the form of thermistors made in the form of thermal films. When voltage is applied to them, these elements are heated to high temperatures (about 200 ° C) in a split second. A pressure roller presses the sheet against the heater, during which liquid microscopic particles of toner are pressed into the texture of the paper. There are spacers at the exit from the fixing unit so that the paper does not stick to the thermal film.

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