Comprehensive analysis of the structure and working principle of digital laser printers
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Basic structure
The laser printer is composed of a laser, an acousto-optic modulator, a high-frequency drive, a scanner, a synchronizer, and a light deflector. The function of the laser printer is to modulate the binary dot matrix information sent from the interface circuit on the laser beam, and then scan to the photosensitive light. Physically. The photoreceptor and the camera mechanism form an electrophotographic transfer system, and the image image projected onto the photosensitive drum is transferred onto the printing paper, and the principle is the same as that of the copying machine. Laser printers are non-impact output devices that combine laser scanning technology with electronic imaging technology. Its model is different, the printing function is also different, but the working principle is basically the same, it has to go through: charging, exposure, development, transfer, power-off, cleaning, fixing seven processes, five of which are carried out around the photosensitive drum . When the text or image to be printed is input to a computer, it is preprocessed by computer software. Then the printer driver converts the print command (printer language) that the printer can recognize to the high frequency drive circuit to control the opening and closing of the laser emitter to form a lattice laser beam, and then scans the mirror to the electronic display system. The photosensitive drum is subjected to axial scanning exposure, and the longitudinal scanning is realized by the rotation of the photosensitive drum itself.
The photosensitive drum is a photosensitive device that is light-conducting. The photoconductive coating of the surface is filled with a uniform charge by the charging roller before the scanning exposure. When the laser beam is scanned in a dot matrix onto the photosensitive drum, the scanned spot is turned on by exposure, and the charge is rapidly released from the conductive substrate to the ground. The spot that is not exposed still maintains the original charge, so that a potential difference latent image (electrostatic latent image) is formed on the surface of the photosensitive drum, when the photosensitive drum with the electrostatic latent image is rotated to the position where the toner magnetic roller is loaded. The oppositely charged toner is adsorbed onto 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 sheet of printing paper is simultaneously sent to the middle of the photosensitive drum and the image transfer device, at which time the image transfer device applies a strong voltage on the back side of the printing paper. The toner image on the photosensitive drum is attracted to the printing paper, and the printing paper carrying the toner image is sent to a high-temperature fixing device for heating and pressure hot melting, the toner is melted, immersed in the printing paper, and finally output. Is the printed text or image.
The composition, design structure, control method and components of the control devices and components required for the working process of the laser printer will vary depending on the brand and model, such as:
1The polarity of charging the drum is different.
2 The drum is charged with different parts. Some models use a wire discharge method to charge the drum, and some models use a charging roller (FCR) to charge the drum.
3 The parts used in high-pressure transfer are different.
4 The drum exposure is in different forms. Some models use a scanning mirror to directly scan the drum, and some models use the scanned reflected laser beam to expose the drum.
But they work basically the same way. The laser beam emitted by the laser is incident on the acousto-optic deflection modulator through the mirror, and at the same time, the binary graphic lattice information sent by the computer is sent from the interface to the font generator to form a binary pulse of the desired glyph. Information, the signal generated by the synchronizer controls 9 high frequency oscillators, and then added to the acousto-optic modulator via a frequency synthesizer and a power amplifier to modulate the laser beam incident by the mirror. The modulated beam is incident on the polygon mirror, and then the beam is focused by a wide-angle focusing mirror and then incident on the surface of the photoconductor drum (toner), so that the angular velocity scanning becomes a linear velocity scan, and the entire scanning process is completed.
The surface of the toner cartridge is first charged by the charging pole to obtain a certain potential. Then, after exposure of the laser beam carrying the image information, an electrostatic latent image is formed on the surface of the toner cartridge, and the latent image is converted into a magnetic image developed by the developing device. The visible toner image is transferred to the plain paper under the electric field of the transfer electrode when passing through the transfer zone, and finally fixed by the preheating plate and the high temperature hot roll, that is, melted on the paper. Text and images. Before printing the graphic information, the cleaning roller removes the untransferred toner, and the cleaning lamp removes the residual electric charge on the drum, and then thoroughly cleans the cleaning paper system to enter a new working cycle.
history
Laser printers were invented by Xerox in the late 1960s using electro-photo-graphy. The technology uses a laser beam to scan a light drum, and by controlling the opening and closing of the laser beam, the sensing light is sucked and not sucked, and the light drum transfers the adsorbed toner to the paper to form a printing result. The entire printing process of a laser printer can be divided into a controller processing stage, an ink shadow, and a transfer stage.
Laser printers have significant advantages over dot matrix printers and inkjet printers. (1) High density. Laser printers have print resolutions as low as 300dpi and high resolutions of 400dpi, 600dpi, 800dpi, 1200dpi, and 2400dpi and 4800dpi. (2) High speed. Laser printers print at a minimum of 4ppm, typically 12ppm and 16ppm. Some laser printers can print at speeds above 24ppm. (3) Low noise. Generally below 53dB, it is ideal for use in quiet office space. (4) Strong processing ability. The controller of the laser printer has a CPU, has memory, and the controller is equivalent to the motherboard of the computer, so it can perform complicated word processing, image processing, and graphic processing, which cannot be done by the dot matrix printer and the inkjet printer. The difference between a page printer and a line printer.
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, images are produced before the printing action takes place. First, the computer converts the content that needs to be printed into a code that the computer can understand, and then transmits the code to the printer. At this point, the printer language then deciphers the code into a dot matrix pattern - this deciphering process is quite important. The dot matrix pattern produced by the excellent printer language is exactly the same as the pattern on the display screen. This consistency is what the laser printer has been pursuing for "What You See Is What You Get". The deciphered dot pattern is sent to the laser generator, and the laser generator quickly reacts to the opening and closing according to the content of the pattern, projecting the laser beam onto a charged rotating drum, and the surface of the drum is irradiated by the laser. The local charge is released, and the places where the laser is not illuminated still carry a charge.
For example, if there is a dot in the third row and third column in the bitmap generated by the printer language, the rest is blank, so the laser generator emits only a laser at this position. On the induction drum, the laser generators remain in the remaining positions. At this time, the surface of the induction drum has only the point at the third row and the third column without charge, and the rest remains charged. At this time, the laser printer has two treatment methods: 1. Only color this point. The rest is not colored (black dots are produced on white); 2. Only the rest is colored, and the dots are not colored (black dots are produced). The first method of processing is called "write black" and the second type of processing is called "write white". Obviously, if in this example we want to print a black dot in a white way, then we must let the laser shine through all the positions except the point on the surface of the drum (note that the laser is only discharging, not charging In other words, it is guaranteed that only this point is charged, and the rest of the charge is released by the laser. In this case, since the laser beam must illuminate all areas except this point, it takes a considerable time for the printer to print out this small black dot. In most environments, the printer actually needs to print at least one-third of the entire page, so most laser printers today use black-and-white printing to reduce laser scanning time.
Work process
Of course, the entire printing process of a laser printer does not only include a laser generator and an induction drum, but many other components are also involved in the printing process.
(1) Printer controller: The printer controller is responsible for receiving print data from the host and converting the data into an image. The printer controller needs to process a lot of programs, including communicating with the host, interpreting the host's print commands, formatting the print content (ie preparing to create images, including setting the paper size, side pages, selecting fonts, etc.), rasterizing (creating bitmap images) ), 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 set of electronic and mechanical systems that can print the dot pattern generated by the printer controller. The printing device has its own processor for controlling the engine and circuitry. In general, a printing apparatus is composed of a laser scanning device, an induction drum, a toner cartridge, a developing device, an electrostatic roller, a bonding device, a paper conveying device, a cleaning blade, a paper feeder, and a paper discharge tray.
Laser scanning devices, sometimes referred to as "grating output scanning devices (ROS)", include a laser generator, a rotating mirror, and a lens. The laser generator projects the laser onto all points on the surface of the drum that need to be printed, and remains off where it is not needed to print (the opposite is true for white-printing printers). The laser generator itself is fixed, and the laser beam passes through a rotating mirror to achieve lateral movement of the laser on the surface of the sensing drum. The laser generator and the rotating mirror must be designed to be extremely precise to ensure that they work in sync 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 the “receiver” or directly called the “drum”. The induction drum is usually cylindrical and the surface is extremely smooth. Its surface can be electrostatically charged, and this static electricity is released when it encounters strong light. Before the laser is touched, 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 released, so that an uncharged surface is generated on the surface of the drum. point. The drum rotates at a relatively slow but absolutely constant speed, enabling the laser to form a continuous, void-free longitudinal projection on the surface of the drum. Thus, the lateral movement of the rotating mirror and the longitudinal movement of the sensing drum cause the laser to "write" an invisible, non-static image on the surface of the drum.
The toner cartridge is a device for holding toner. Some printers have cartridges that are attached to the induction drum and are called "printing assemblies." Toner is produced from many special synthetic plastic charcoal and iron oxide. The toner material is mixed, melted, resolidified, and then pulverized into extremely small particles of uniform size. The finer and more uniform the toner, the more detailed the resulting image. Among all types of toner, HP's Microfine toner particles are 20% to 50% smaller than 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 adhere to the surface of the roller, like an extremely fine "brush." The roller is closely abutted with the induction drum and the toner cartridge respectively. When the roller rolls, the small particles on the surface of the roller are first "brushed" from the toner cartridge to a uniform layer of toner, and then the toner passes through the induction drum. It is then adsorbed to the surface of the induction drum. The developing device for writing a black printer has a function of charging the toner, because if the toner is to be adsorbed only by the portion of the surface of the drum that is not electrostatically charged (i.e., the spot scanned by the laser), the carbon must be made. The powder carries a charge (for a white printer, the process is exactly the opposite). At this time, the surface of the drum is adsorbed with toner, which forms an extremely clear image. The next step is to transfer the image onto the paper.
Paper Conveyor The paper conveying device is the most important mechanical device for laser printers. This device transfers paper through two motor-driven rollers. The paper starts with the feeder, passes through the induction drum, the heating roller, and so on, and is finally sent out of the printer. Rolling equipment in laser printers, such as drums, magnetic rollers and paper feed rollers, must be synchronized, and their speed must be consistent to ensure accurate printouts. Generally, these rollers are centered on the paper feeding device and are rotated at the same speed by meshing gears.
When the bonding device passes through the induction drum through the conveying device, the carbon powder adhered to the surface of the drum is adsorbed to the surface of the paper. At this time, although the surface of the paper is formed by the toner, the adsorption of the toner on the paper is Not very strong, a little stronger wind can blow these toner off the surface of the paper. In order for the toner to adhere permanently to the surface of the paper, the toner must be bonded. We know that the raw material of the toner is synthetic plastic char, which can be melted at high temperatures. The melted toner re-solidifies and can be permanently adhered to the surface of the paper. Inside the laser printer, there are two very hot rollers that are close together, and their function is to heat the paper passing between them. The toner is melted to adhere to the surface of the paper. The heated paper is finally output to the printer's paper output tray, at which point the entire printing process ends.