Editor's note: This article explains in more detail the basic working principle of laser printers on the market. Because many digital printing equipment also uses this technology, so I hope this article can help friends who use digital equipment.
basic structure
The laser printer is composed of laser, acousto-optic modulator, high frequency drive, scanner, synchronizer and optical deflector, etc. Its function is to modulate the binary dot matrix information sent by the interface circuit on the laser beam, and then scan to the photosensitive Physically. The photoreceptor and the photographic mechanism form an electrophotographic transfer system, which transfers the image of the image shot onto the photoreceptor drum to the printing paper. The principle is the same as that of the copier. A laser printer is a non-impact output device that combines laser scanning technology and electronic imaging technology. Different models have different printing functions, but the working principle is basically the same, all have to go through: charging, exposure, development, transfer, electricity elimination, cleaning, fixing seven processes, of which five processes are carried out around the photosensitive drum . When the text or image to be printed is input to the computer, it is preprocessed by computer software. Then the printer driver converts it into a print command (printer language) that can be recognized by the printer and sends it to the high-frequency drive circuit to control the laser transmitter on and off to form a dot-matrix laser beam. The photosensitive drum in is subjected to axial scanning exposure, and longitudinal scanning is achieved by the rotation of the photosensitive drum itself.
The photosensitive drum is a photosensitive device and has the characteristics of being conducted by light. The photoconductive coating on the surface is uniformly charged by the charging roller before scanning exposure. When the laser beam is scanned onto the photosensitive drum in the form of a dot matrix, the scanned dot is turned on by exposure, and the charge is quickly released by the conductive base pair. The unexposed points still maintain the original charge, so that a latent image (electrostatic latent image) of potential difference is formed on the surface of the photosensitive drum. When the photosensitive drum with the electrostatic latent image rotates to the position carrying the toner magnetic roller The toner with the opposite charge is attracted to the surface of the photosensitive drum to form a toner image.
When the photosensitive drum carrying the toner image continues to rotate and reaches the image transfer device, a piece of printing paper is also sent between the photosensitive drum and the image transfer device. At this time, the image transfer device applies a strong voltage on the back of the printing paper. The toner image on the photosensitive drum is attracted to the printing paper, and then the printing paper loaded with the toner image is fed into a high-temperature fixing device to be heated, pressurized and melted. After the toner is melted, it is immersed in the printing paper and finally output Is the printed text or image.
The composition, design structure, control method and adopted components of the control device and components required for the working process of the laser printer will vary depending on the brand and model, such as:
â‘ The polarity of the photosensitive drum is different.
â‘¡The parts used for charging the photosensitive drum are different. Some models use the wire discharge method to charge the photosensitive drum, and some models use a charging roller (FCR) to charge the photosensitive drum.
â‘¢The parts used for high-pressure transfer are different.
â‘£The exposure mode of the photosensitive drum is different. Some models use a scanning mirror to directly expose the photosensitive drum, and some models use the reflected laser beam after scanning to expose the photosensitive drum.
But they work basically the same. The laser beam emitted by the laser enters the acousto-optic deflection modulator through the mirror, and at the same time, the binary image and lattice information sent by the computer is sent from the interface to the font generator to form the binary pulse of the desired font For information, nine high-frequency oscillators are controlled by the signal generated by the synchronizer, and then added to the acousto-optic modulator through a frequency synthesizer and power amplifier to modulate the laser beam incident from the mirror. The modulated light beam enters the multi-faceted rotating mirror, and then the beam is focused by the wide-angle focusing lens and then hits the surface of the photoconductor drum (cartridge), so that the angular velocity scan becomes a linear velocity scan to complete the entire scanning process.
The surface of the drum is charged by the charging electrode to obtain a certain potential, and then exposed by a laser beam carrying image information, an electrostatic latent image is formed on the surface of the drum, which is developed by a magnetic brush developer, and the latent image is transformed into The visible toner image, when passing through the transfer area, is transferred to the plain paper under the action of the electric field of the transfer electrode, and finally fixed by the preheating plate and high temperature hot roller, that is, fused on the paper Text and images. Before printing the graphic information, the cleaning roller removes the untransferred toner, the discharge lamp removes the residual charge on the drum, and then cleans it thoroughly through the cleaning paper system, then you can enter a new round of work cycle.
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history
The laser printer was invented by Xerox in the late 1960s and used electro-photo-graphy technology. This technology uses a laser beam to scan the optical drum, and controls the on and off of the laser beam to make the sensor drum absorb and not absorb toner. The drum then transfers the absorbed toner to the paper to form a print result. The entire printing process of the laser printer can be divided into controller processing stage, ink shadow and transfer stage.
Compared with needle printers and inkjet printers, laser printers have very obvious advantages. (1) High density. Laser printers have a minimum print resolution of 300dpi, as well as high resolutions of 400dpi, 600dpi, 800dpi, 1200dpi, 2400dpi and 4800dpi. (2) High speed. The printing speed of laser printers is at least 4ppm, generally 12ppm, 16ppm, some laser printers can print speeds above 24ppm. (3) Low noise. Generally below 53dB, it is very suitable for use in a quiet office. (4) Strong processing ability. The controller of the laser printer has a CPU and memory, and the controller is equivalent to the main board of the computer, so it can perform complex word processing, image processing, and graphics processing, which cannot be completed by the needle printer and the inkjet printer. The difference between page printers and line printers.
How does a laser printer print beautiful text and graphics? In fact, the printing process of laser printers and photocopiers is basically the same. The difference is that for laser printers, the image is generated before the printing action occurs. First, the computer converts the content to be printed into codes that the computer can understand, and then transmits these codes to the printer. At this time, the printer language decodes these codes into dot-matrix patterns-this deciphering process is very important. The dot pattern produced by the excellent printer language is exactly the same as the pattern on the monitor screen. This consistency is the “What You See Is What You Get†that laser printers have been pursuing. The decoded dot pattern is sent to the laser generator, and the laser generator quickly responds to the on and off according to the content of the pattern, and projects the laser beam onto a charged rotating drum. The surface of the drum is usually irradiated by the laser. Local charges are discharged, and those areas that are not illuminated by the laser are still charged.
For example, if there is a dot in the third row and third column of the bitmap generated by the printer language, and the rest are blank, then the laser generator will only emit a beam of laser light at this position to illuminate On the induction drum, the laser generators in the remaining positions are kept off. At this time, only the point on the third row and third column of the surface of the induction drum is not charged, and the rest is still charged. At this time, the laser printer has two processing methods: 1. Only color this point, The rest of the part is not colored (black dots on a white background); 2. Only the remaining part is colored, and the point is not colored (white dots on a black background). The first processing method is called "writing black", and the second processing method is called "writing white". Obviously, if in this example we want to print a black dot by writing it in white, then we must let the laser illuminate all positions on the surface of the sensor drum except this point (note that the role of the laser is only discharge, not charge ), In other words, to ensure that only this point is charged, the rest of the charge is discharged by the laser. In this case, since the laser beam must illuminate all areas except this point, it takes quite some time for the printer to print this small black dot. In most environments, the actual part of the printer that needs to be printed only occupies at most one-third of the entire page, so most laser printers today use black writing to print, which can shorten the time of laser scanning.
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work process
Of course, the entire printing process of the laser printer does not only include the laser generator and the induction drum, but many other parts also participate in the printing job.
(1) Printer controller: The printer controller is responsible for receiving the print data from the host computer and converting these data into images. The printer controller needs to handle many programs, including communicating with the host, interpreting the host ’s print commands, formatting the print content (that is, preparing to create an image, including setting the paper size, margins, selecting fonts, etc.), rasterizing (creating a dot matrix image ), And finally send the image to the print engine. Different printer languages ​​issue different commands to the controller, and different manufacturers use different methods to design their respective printer controllers.
(2) Printing device: The printing device is a combination of electronic and mechanical systems, which can print the dot matrix graphics generated by the printer controller. The printing device has its own processor used to control the engine and circuits. Generally speaking, the printing device consists of the following components: laser scanning device, induction drum, toner cartridge, developing device, electrostatic roller, bonding device, paper conveying device, cleaning blade, paper feeder and paper output tray.
The laser scanning device is sometimes referred to as a "raster output scanning device (ROS)" and includes a laser generator, rotating mirror and a lens. The laser generator projects the laser light on all the points on the surface of the induction drum that need to be printed, and keeps it turned off where it does not need to be printed (the writing white printer is just the opposite). The laser generator itself is fixed, and the laser beam uses a rotating mirror to achieve the lateral movement of the laser on the surface of the induction drum. The laser generator and the rotating mirror must be designed to be extremely precise in order to ensure that they work synchronously and accurately project the laser to the correct point. The longitudinal movement of the laser on the surface of the induction drum is achieved by the rotation of the induction drum itself.
The induction drum is also called "receiver", or directly called "drum". The induction drum is usually cylindrical and has a very smooth surface. Its surface can be charged by static electricity, and this static electricity will be released as soon as it encounters strong light. Before touching the laser, the surface of the drum is uniformly charged by the electrostatic roller. When the laser beam is projected to a certain point on the surface of the drum, the static electricity at this point is discharged, so that an uncharged point. The drum rotates at a relatively slow but absolutely constant speed, enabling the laser to form a continuous, gap-free longitudinal projection on the surface of the drum. In this way, the lateral movement of the rotating mirror and the longitudinal movement of the induction drum cause the laser to "write" an invisible, non-static image on the surface of the drum.
The toner cartridge is a device used to hold toner. The toner cartridge and induction drum of some printers are installed together and are called "printing components". Toner is produced from many special synthetic plastic carbon ash and iron oxide. Toner raw materials are mixed, melted, re-solidified, and then crushed into very small particles of uniform size. The finer and more uniform the toner, the finer the resulting image. Among all types of toner, HP's Microfine toner particles are 20% to 50% smaller than those of other brands, and therefore enjoy a high reputation in the industry.
The developing device is actually a roller covered with magnetic particles. These magnetic particles are attached to the surface of the roller, like an extremely fine "brush". This roller is close to the induction drum and the toner cartridge. When the roller rolls, the small particles on the surface of the roller "brush" a uniform layer of toner from the toner cartridge, and then these toners pass through the induction drum Will be attracted to the surface of the induction drum. The developing device of the writing black printer has the function of charging toner, because if you want the toner to be absorbed only by the part of the drum surface that does not carry static electricity (that is, the point scanned by the laser), the carbon The powder is charged (for a white printer, the process is completely reversed). At this time, the surface of the drum adsorbed toner, forming an extremely clear image, the next step is to transfer this image to the paper.
Paper conveying device The paper conveying device is the most important mechanical device of the laser printer. This device uses two motor-driven rollers to convey the paper. The paper starts from the feeder, passes through the induction drum, heating roller and other components, and is finally sent out of the printer. Rolling devices in laser printers, such as photosensitive drums, magnetic rollers, and paper feed rollers, must rotate synchronously, and their speeds must be consistent to ensure accurate printouts. Generally speaking, these rollers are centered on the paper feeding device, and achieve the same speed rotation through the gears meshing with each other.
When the paper passes through the conveying device and passes through the induction drum, the toner on the drum surface is adsorbed to the surface of the paper. Although the surface of the paper is formed of toner, the adsorption of the toner on the paper Not very strong, a little stronger wind can blow these toners off the surface of the paper. In order to make the toner adhere to the surface of the paper permanently, the toner must be bonded. We know that the raw material of carbon powder is synthetic plastic charcoal ash, which can be melted at high temperature. After the melted toner solidifies, it can be permanently stuck to the surface of the paper. Inside the laser printer, there are two very hot rollers that are close together. Their role is to heat the paper passing between them. The toner melts and sticks to the surface of the paper. The heated paper is finally output to the output tray of the printer, at which time the entire printing process is ended.
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