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Laser Printers Laser printers and inkjet printers are referred to as page printers because they receive their print job instructions one page at a time rather than receiving instructions one line at a time. There are two major types of page printers that use the electrophotographic (EP) imaging proc...

Laser Printers Laser printers and inkjet printers are referred to as page printers because they receive their print job instructions one page at a time rather than receiving instructions one line at a time. There are two major types of page printers that use the electrophotographic (EP) imaging process. The first uses a laser to scan the image onto a photosensitive drum, and the second uses an array of light-emitting diodes (LEDs) to create the image on the drum. Even though they write the image in different ways, both types still follow the laser printer imaging process. Since the A+ exam focuses on the laser printer imaging process and not on differences between laser and LED, we’ll focus on the same here. Xerox, Hewlett-Packard, and Canon were pioneers in developing the laser printer technology we use today. Scientists at Xerox developed the electrophotographic (EP) imaging process in 1971. HP introduced the first successful desktop laser printer in 1984, using Canon hardware that used the EP process. This technology uses a combination of static electric charges, laser light, and a black powdery ink-like substance called toner. Printers that use this technology are called EP process laser printers, or just laser printers. Every laser printer technology has its foundations in the EP printer imaging process. Let’s discuss the basic components of the EP laser printer and how they operate so that you can understand the way an EP laser printer works. Basic Components Most printers that use the EP imaging process contain nine standard assemblies: the toner cartridge, laser scanner, high-voltage power supply, DC power supply, paper transport assembly (including paper-pickup rollers and paper-registration rollers), transfer corona, fusing assembly, printer controller circuitry, and ozone filter. Let’s discuss each of the components individually, along with a duplexing assembly, before we examine how they all work together to make the printer function. The Toner Cartridge The EP toner cartridge (see Figure 5.10), as its name suggests, holds the toner. Toner is a black carbon substance mixed with polyester resins to make it flow better and iron oxide particles to make it sensitive to electrical charges. These two components make the toner capable of being attracted to the photosensitive drum and of melting into the paper. In addition to these components, toner contains a medium called the developer (also called the carrier), which carries the toner until it is used by the EP process. 250 Figure 5.10 An EP toner cartridge The toner cartridge also contains the EP print drum. This drum is coated with a photosensitive material that can hold a static charge when not exposed to light but cannot hold a charge when it is exposed to light—a curious phenomenon and one that EP printers exploit for the purpose of making images. Finally, the drum assembly contains a cleaning blade that continuously scrapes the used toner off the photosensitive drum to keep it clean.  Exposing a photosensitive drum to dust or light can damage it, but touching it will most likely render the drum inoperable! It’s best to just not mess around with them.  In most laser printers, toner cartridge means an EP toner cartridge that contains toner and a photosensitive drum in one plastic case. In some laser printers, however, the toner and photosensitive drum can be replaced separately instead of as a single unit. If you ask for a toner cartridge for one of these printers, all you will receive is a cylinder full of toner. Consult the printer’s manual to find out which kind of toner cartridge your laser printer uses. 251  Never ship a printer anywhere with a toner cartridge installed! The jostling that happens during shipping could cause toner to spill out of the cartridge and all over the inside of the printer. This will be a huge mess to clean up! If the printer is a laser printer, remove the toner cartridge first. You can put it in a sealed, airtight bag to ship, if needed. If it’s an LED page printer, there is a method to remove the photosensitive drum and toner cartridge. Check your manual for details. The Laser Scanning Assembly As we mentioned earlier, the EP photosensitive drum can hold a charge if it’s not exposed to light. It is dark inside an EP printer, except when the laser scanning assembly shines on particular areas of the photosensitive drum. When it does that, the drum discharges, but only in the area that has been exposed. As the drum rotates, the laser scanning assembly scans the laser across the photosensitive drum, exposing the image onto it. Figure 5.11 shows the laser scanning assembly. Figure 5.11 The EP laser scanning assembly (side view and simplified top view)  Laser light is damaging to human eyes. Therefore, the laser is kept in an enclosure and will operate only when the laser printer’s cover is closed. 252 High-Voltage Power Supply The EP process requires high-voltage electricity. The high-voltage power supply (HVPS) provides the high voltages used during the EP process. This component converts AC current from a standard wall outlet (120V and 60 Hz) into higher voltages that the printer can use. This high voltage is used to energize both the charging corona and the transfer corona.  Anything with the words high voltage in it should make you pause before opening a device and getting your hands into it. The HVPS can hurt or kill you if you’re working inside a laser printer and don’t know what you’re doing. DC Power Supply The high voltages used in the EP process can’t power the other components in the printer (the logic circuitry and motors). These components require low voltages, between +5VDC and +24VDC. The DC power supply (DCPS) converts house current into three voltages: +5VDC and –5VDC for the logic circuitry and +24VDC for the paper transport motors. This component also runs the fan that cools the internal components of the printer. Paper Transport Assembly The paper transport assembly is responsible for moving the paper through the printer. It consists of a motor and several rubberized rollers that each performs a different function. The first type of roller found in most laser printers is the feed roller, or paper pickup roller (see Figure 5.12). This D-shaped roller, when activated, rotates against the paper and pushes one sheet into the printer. This roller works in conjunction with a special rubber separator pad to prevent more than one sheet from being fed into the printer at a time. Figure 5.12 Paper transport rollers Another type of roller that is used in the printer is the registration roller (also shown in Figure 5.12). There are actually two registration rollers, which work together. These rollers synchronize the paper movement with the image-formation process in the EP cartridge. The rollers don’t feed the paper past the EP cartridge until the cartridge is ready for it. 253 Both of these rollers are operated with a special electric motor known as an electronic stepper motor. This type of motor can accurately move in very small increments. It powers all the paper transport rollers as well as the fuser rollers. The Transfer Corona Assembly When the laser writes (exposes) the images on the photosensitive drum, the toner then sticks to the exposed areas. (We’ll cover this in the “Electrophotographic Imaging Process” section.) How does the toner get from the photosensitive drum onto the paper? The transfer corona assembly (see Figure 5.13) is given a high-voltage charge, which is transferred to the paper, which, in turn, pulls the toner from the photosensitive drum. Figure 5.13 The transfer corona assembly Included in the transfer corona assembly is a static-charge eliminator strip that drains away the charge imparted to the paper by the corona. If you didn’t drain away the charge, the paper would stick to the EP cartridge and jam the printer. There are two types of transfer corona assemblies: those that contain a transfer corona wire and those that contain a transfer corona roller. The transfer corona wire is a small-diameter wire that is charged by the HVPS. The wire is located in a special notch in the floor of the laser printer (under the EP print cartridge). The transfer corona roller performs the same function as the transfer corona wire, but it’s a roller rather than a wire. Because the transfer corona roller is directly in contact with the paper, it supports higher speeds. For this reason, the transfer corona wire is used infrequently in laser printers today. Fusing Assembly The toner in the EP toner cartridge will stick to just about anything, including paper. This is true because the toner has a negative static charge and most objects have a net positive charge. However, these toner particles can be removed by brushing any object across the page. This could be a problem if you want the images and letters to stay on the paper permanently. To solve this problem, EP laser printers incorporate a device known as a fuser (see Figure 5.14), which uses two rollers that apply pressure and heat to fuse the plastic toner particles to the paper. You may have noticed that pages from either a laser printer or a copier (which uses a similar device) come out warm. This is because of the fuser. Figure 5.14 The fuser 254 The fuser is made up of three main parts: a halogen heating lamp, a Teflon-coated aluminum-fusing roller, and a rubberized pressure roller. The fuser uses the halogen lamp to heat the fusing roller to between 329° F (165° C) and 392° F (200° C). As the paper passes between the two rollers, the pressure roller pushes the paper against the fusing roller, which melts the toner into the paper.  The fuser can cause severe burns! Be careful when working with it. Printer Controller Circuitry Another component in the laser printer that we need to discuss is the printer controller assembly. This large circuit board converts signals from the computer into signals for the various assemblies in the laser printer using a process known as rasterizing. This circuit board is usually mounted under the printer. The board has connectors for each type of interface and cables to each assembly. When a computer prints to a laser printer, it sends a signal through a cable to the printer controller assembly. The controller assembly formats the information into a page’s worth of line-by-line commands for the laser scanner. The controller sends commands to each of the components, telling them to wake up and begin the EP imaging process. Ozone Filter Your laser printer uses various high-voltage biases inside the case. As anyone who has been outside during a lightning storm can tell you, high voltages create ozone. Ozone is a chemically reactive gas that is created by the high-voltage coronas (charging and transfer) inside the printer. Because ozone is chemically reactive and can severely reduce the life of laser printer components, many older laser printers contain a filter to remove ozone gas from inside the printer as it is produced. This filter must be removed and cleaned with compressed air periodically. (Cleaning it whenever the toner cartridge is replaced is usually sufficient.) Most newer laser printers don’t have ozone filters. This is because these printers don’t use transfer corona wires but instead use transfer corona rollers, which dramatically reduce ozone emissions. 255 Duplexing Assembly Any laser printer worth its money today can print on both sides of the paper (as can some nicer models of inkjet printers, mentioned earlier). This is accomplished through the use of a duplexing assembly. Usually located inside or on the back of the printer, the assembly is responsible for taking the paper, turning it over, and feeding back into the printer so the second side can be printed. Electrophotographic Imaging Process The electrophotographic (EP) imaging process is the process by which an EP laser printer forms images on paper. It consists of seven major steps, each designed for a specific goal. Although many different manufacturers call these steps different things or place them in a different order, the basic process is still the same. Here are the steps in the order you will see them on the exam: Processing Charging Exposing Developing Transferring Fusing Cleaning Before any of these steps can begin, however, the controller must sense that the printer is ready to start printing (toner cartridge installed, fuser warmed to temperature, and all covers in place). Printing cannot take place until the printer is in its ready state, usually indicated by an illuminated Ready LED light or a display that says something like 00 READY (on HP printers). The computer sends the print job to the printer, which begins processing the data as the first step to creating output. Step 1: Processing The processing step comprises two parts: receiving the image and creating the image. The computer sends the print job to the printer, which receives it via its print interface (USB, wireless, etc.). Then, the printer needs to create the print job in such a way that it can accurately produce the output. If you think back to our discussion of dot-matrix printing earlier in this chapter, you might recall that dot-matrix printers produce images by creating one strip of dots at a time across the page. Laser printers use the same concept of rendering one horizontal strip at a time to create the image. Each strip across the page is called a scan line or a raster line. A component of the laser printer called the Raster Image Processor (RIP) manages raster creation. Its responsibility is to generate an image of the final page in memory. How the raster gets created depends on the page-description language that your system is using, such as PostScript (PS) or Printer Control Language (PCL). Ultimately, this collection of lines is what gets written to the photosensitive drum and onto the paper. 256 Step 2: Charging The next step in the EP process is charging (see Figure 5.15). In this step, a special wire or roller (called a charging corona) within the EP toner cartridge (above the photo sensitive drum) gets high voltage from the HVPS. It uses this high voltage to apply a strong, uniform negative charge (around –600VDC) to the surface of the photosensitive drum. Figure 5.15 The charging step of the EP process Step 3: Exposing Next is exposing the drum to the image, often referred to as the exposing step. In this step, the laser is turned on and scans the drum from side to side, flashing on and off according to the bits of information that the printer controller sends it as it communicates the individual bits of the image. Wherever the laser beam touches, the photosensitive drum’s charge is severely reduced from –600VDC to a slight negative charge (around –100VDC). As the drum rotates, a pattern of exposed areas is formed, representing the image to be printed. Figure 5.16 shows this process. Figure 5.16 The exposing step of the EP process At this point, the controller sends a signal to the pickup roller to feed a piece of paper into the printer, where it stops at the registration rollers. 257 Step 4: Developing Now that the surface of the drum holds an electrical representation of the image being printed, its discrete electrical charges need to be converted into something that can be transferred to a piece of paper. The EP process step that accomplishes this is developing (see Figure 5.17). In this step, toner is transferred to the areas that were exposed in the exposing step. Figure 5.17 The developing step of the EP process A metallic roller called the developing roller inside an EP cartridge acquires a –600VDC charge (called a bias voltage) from the HVPS. The toner sticks to this roller because there is a magnet located inside the roller and because of the electrostatic charges between the toner and the developing roller. While the developing roller rotates toward the photosensitive drum, the toner acquires the charge of the roller (–600VDC). When the toner comes between the developing roller and the photosensitive drum, the toner is attracted to the areas that have been exposed by the laser (because these areas have a lesser charge, –100VDC). The toner also is repelled from the unexposed areas (because they are at the same –600VDC charge and like charges repel). This toner transfer creates a fog of toner between the EP drum and the developing roller. The photosensitive drum now has toner stuck to it where the laser has written. The photosensitive drum continues to rotate until the developed image is ready to be transferred to paper in the next step. Step 5: Transferring At this point in the EP process, the developed image is rotating into position. The controller notifies the registration rollers that the paper should be fed through. The registration rollers move the paper underneath the photosensitive drum, and the process of transferring the image can begin; this is the transferring step. The controller sends a signal to the charging corona wire or roller (depending on which one the printer has) and tells it to turn on. The corona wire/roller then acquires a strong positive charge (+600VDC) and applies that charge to the paper. Thus charged, the paper pulls the toner from the photosensitive drum at the line of contact between the roller and the paper because the paper and toner have opposite charges. Once the registration rollers move the paper past the corona wire, the static-eliminator strip removes all charge from258that line of the paper. Figure 5.18 details this step. If the strip didn’t bleed this charge away, the paper would attract itself to the toner cartridge and cause a paper jam. Figure 5.18 The transferring step of the EP process The toner is now held in place by weak electrostatic charges and gravity. It will not stay there, however, unless it is made permanent, which is the reason for the fusing step. Step 6: Fusing The penultimate step before the printer produces the finished product is called fusing. Here the toner image is made permanent. The registration rollers push the paper toward the fuser rollers. Once the fuser grabs the paper, the registration rollers push for only a short time longer. The fuser is now in control of moving the paper. As the paper passes through the fuser, the 350° F fuser roller melts the polyester resin of the toner, and the rubberized pressure roller presses it permanently into the paper (see Figure 5.19). The paper continues through the fuser and eventually exits the printer. Figure 5.19 The fusing step of the EP process Once the paper completely exits the fuser, it trips a sensor that tells the printer to finish the EP process with the cleaning step. Step 7: Cleaning In the last part of the laser imaging process, a rubber blade inside the EP cartridge scrapes any toner left on the drum into a used toner receptacle inside the EP cartridge, and a fluorescent lamp discharges any remaining charge on the photosensitive drum. (Remember that the drum, being photosensitive, loses its charge when exposed to light.) This step is called cleaning (see Figure 5.20). Figure 5.20 The cleaning step of the EP process 259 The EP cartridge is constantly cleaning the drum. It may take more than one rotation of the photosensitive drum to make an image on the paper. The cleaning step keeps the drum fresh for each use. If you didn’t clean the drum, you would see ghosts of previous pages printed along with your image.  The amount of toner removed in the cleaning process is quite small, and the cartridge will run out of toner before the used toner receptacle fills up. The toner that’s in the receptacle is useless because the imaging process has already chemically altered it. In addition, it’s considered a hazardous substance. Recycle the print cartridge and don’t pay attention to the leftover toner. At this point, the printer can print another page, and the EP process can begin again. Summary of the EP Imaging Process Figure 5.21 provides a diagram of all the parts involved in the EP printing process. Here’s a summary of the process, which you should commit to memory: The printer receives and processes the image and stores a page in memory. The printer places a uniform –600VDC charge on the photosensitive drum by means of a charging corona. The laser “paints” an image onto the photosensitive drum, discharging the image areas to a much lower voltage (–100VDC). The developing roller in the toner cartridge has charged (–600VDC) toner stuck to it. As it rolls the toner toward the photosensitive drum, the toner is attracted to (and sticks to) the areas of the photosensitive drum that the laser has discharged. The image is then transferred from the drum to the paper at its line of contact by means of the transfer corona wire (or corona roller) with a +600VDC charge. The static-eliminator strip removes the high, positive charge from the paper, and the paper, now holding the image, moves on. The paper then enters the fuser, where a fuser roller and the pressure roller make the image permanent. The paper exits the printer. The printer uses a rubber scraper to clean the photosensitive drum. At that point, it is ready to print the next page or it returns to the ready state. Figure 5.21 The EP imaging process 260  There have been a lot of concepts covered in the laser printing section. For the A+ certification exam, be sure you are familiar with laser printer parts, including the imaging drum, fuser assembly, transfer belt, pickup rollers, separator pads, and duplexing assembly. Also know the steps of the imaging process in order. They are processing, charging, exposing, developing, transferring, fusing, and cleaning. Thermal Printers The types of printers that you have learned about so far in this chapter account for 90 percent of all paper printers that are used with home or office computers and that you will see as a repair technician. The remaining 10 percent consist of other types of printers that primarily differ by the method they use to put colored material on the paper to represent what is being printed. Examples of these include solid ink, dye sublimation, and thermal printers. Keep in mind that, for the most part, these printers operate like other paper printers in many ways: They all have a paper feed mechanism (sheet-fed or roll); they all require consumables such as ink or toner and paper; they all use the same interfaces, for the most part, as other types of printers; and they are usually about the same size. Thermal printing technology is used in many point-of-sale (POS) terminals and older fax machines. (Newer fax machines usually use inkjet or laser technology.) They print on special thermal paper, which is a kind of waxy paper that comes on a roll; the paper turns black when heat passes over it. Thermal printers work by using a print head that is the width of the paper. When it needs to print, a heating element heats certain spots on the print head. The paper below the heated print head turns black in those spots. As the paper moves through the printer, the pattern of blackened spots forms an image on the page of what is being printed. Another type of thermal printer uses a heat-sensitive ribbon instead of heat-sensitive paper. A thermal print head melts wax-based ink from the ribbon onto the paper. These are called thermal transfer printers or thermal wax-transfer printers. 261 Thermal direct printers typically have long lives because they have few moving parts. The only unique part that you might not be as familiar with is the paper feed assembly, which often needs to accommodate a roll of paper instead of sheets. The paper is somewhat expensive, doesn’t last long (especially if it is left in a very warm place, like a closed car in summer), and produces poorer-quality images than the paper used by most of the other printing technologies.