Electrophotographic printing is accomplished through a "process" rather than a "print head." The collection of components that performs the EP printing process is called an Image Formation System (or IFS).
An IFS is made up of eight distinctive areas: a photosensitive drum, cleaning blade, erasure lamp, primary corona, writing mechanism, toner, transfer corona, and fusing rollers. Each of these parts, as shown in 4-2, play an important role in the proper operation of an IFS. Trouble in any of these areas will adversely effect the printed output. A photosensitive drum is generally considered to be the heart of any IFS. An extruded aluminum cylinder is coated with a nontoxic organic compound that exhibits photoconductive properties. That is, the coating will conduct electricity when exposed to light. You might see this referred to as organic photoconductive chemicals (or OPC). The photosensitive compound gives the EP drum a bright green appearance. The aluminum base cylinder is connected to ground of the high-voltage power supply; this is an important point, serious printing problems can occur if the ground becomes loose. It is the drum that actually receives an image from a writing mechanism, develops the image with toner, then transfers the developed image to paper. Although you might think that this constitutes a print head because it delivers an image to paper, the image is not yet permanent; other operations must be performed by the IFS. Complete image development is a six-step process that involves all eight IFS components: cleaning, charging, writing, developing, transfer, and fusing. To really understand the IFS, you should know each of these steps in detail. The following sections of this chapter show you how each part of the IFS works together.
Cleaning
Before a new printing cycle can begin, the photosensitive drum must be physically cleaned and electrically erased (typically referred to as conditioning). Cleaning might sound like a rather unimportant step, but not even the best drum will transfer every microscopic granule of toner to a page every time. A rubber cleaning blade is applied across the entire length of the drum to gently scrape away any residual toner that might remain from a previous image (figure 4-3). If residual toner were not cleaned, it could adhere to subsequent pages and appear as random black speckles. Toner that is removed from the drum is deposited into a debris cavity. Keep in mind that cleaning must be accomplished without scratching or nicking the drum. Any damage to the photosensitive surface would become a permanent mark that appears on every subsequent page. Some EP printer designs actually return scrap toner back to the supply for reuse. This kind of recycling technique can substantially extend the life of your electrophotographic cartridge, and eliminate the need for a large debris cavity.
Charging
A neutral drum surface is no longer receptive to light from the writing mechanism. New images cannot be written until the drum is charged again. In order to condition the drum, a uniform electrical charge must be applied evenly across its entire surface. Surface charging is accomplished by applying a tremendous negative voltage (often about -6,000V) to a solid wire called a primary corona located close to the drum. Since the drum and high-voltage power supply share the same ground, a powerful electrical field is established between the corona wire and drum
Writing
In order to form a latent image on a drum surface, the uniform charge that has conditioned the drum must now be discharged in the precise areas where images are to be produced. Images are written to the drum using light as shown in 4-6. Any points on the drum exposed to light will discharge to a very low level (about -1 OOV), while any areas left unexposed retain their conditioning charge. The device that produces and directs light to the drum surface is called a writing mechanism.
Developing
Images written to the drum are initially invisible, merely an array of electrostatic charges. The latent image must be "developed" into a visible one before it can be transferred to paper. Toner is used for this purpose. Toner itself is an extremely fine powder of plastic resin and organic compounds bonded to iron particles. Individual granules can be seen under extreme magnification of a microscope
Transfer and Discharge
At this point, the developed toner image on the drum must be transferred onto paper. Because toner is now strongly attracted to the drum, it must be pried away by applying an even larger attractive charge to the page. A transfer corona wire charges the page as shown in figure 4-8. The theory behind the operation of a transfer corona is exactly the same as that for a primary corona, except that the potential is now positive. This places a powerful positive charge onto paper, which attracts the negatively charged toner particles. Remember that transfer is not a perfect process. Not all toner is transferred to paper, which is why a cleaning process is needed. Caution is needed here. Because the negatively charged drum and positively charged paper tend to attract each other, it is possible that paper could wrap around the drum. Even though the small-diameter drum and natural stiffness of paper tend to prevent wrapping, a static charge eliminator (or static eliminator comb) is included to counteract positive charges and remove the attractive force between paper and drum as in figure 4-9. Paper now has no net charge. Another good reason for discharging the paper is to prevent charge irregularities from shifting the toner now applied on paper before fusing can take place. Also, discharging the paper will keep subsequent sheets from repelling each other when the sheets are ejected. The drum can be cleaned and prepared for a new image.
Fusing
Once the toner image has reached paper, it is only held to the page by gravity and weak electrostatic attraction. Toner must be fixed permanently (or fused) to the page before it can be handled. Fusing is accomplished with a heat and pressure assembly like the one shown in 4-10. A high-intensity quartz lamp heats a nonstick roller to about 180°C. Pressure is applied with a pliable rubber roller. When a developed page is passed between these two rollers, heat from the top roller melts the toner, and pressure from the bottom roller squeezes molten toner into the paper fibers where it cools and adheres permanently. The finished page is then fed to an output tray. Note that both rollers are referred to as fusing rollers though only the top roller actually "fuses."
Writing Mechanism
As discussed earlier in this blog, the newly charged photosensitive drum contains a uniform electrostatic charge across its surface. To form a latent image, the drum must be discharged at any and all points that comprise the image. Directed light is used to discharge the drum as needed. Images are scanned onto the drum one horizontal line at a time. A single pass across the drum is called a trace or scan line. Light is directed to any points along the scan line where dots are required. When a scan line is completed, the drum increments in preparation for another scan line. It is up to the printer's control circuits to break down an image into individual scan lines, then direct the writing mechanism accordingly.
Lasers
Lasers have been around since the early 1960s, and they have developed to the point where they can be manufactured in a great variety of shapes, sizes, and power output. To understand why lasers make such a useful writing mechanism, you must understand the difference between laser light, and ordinary "white" light.
LED
Fortunately, a photosensitive drum is receptive to light from many different sources. Even light from light-emitting diodes (LEDs) can expose the drum. By fabricating a series of microscopic LEDs into a single scan line, as shown in 4-14, an individual LED can be provided for every possible dot in a scan line. For example, the ROHM JE3008SS02 is an LED print bar containing 2,560 microscopic LEDs over 8.53 inches.
What is Electrophotographic Cartridge
Electrophotographic printers mandate the use of extremely tight manufacturing tolerances to ensure precise, consistent operation. A defect of only a few thousandths of an inch could cause unacceptable image formation. Even the effects of normal mechanical wear can have an adverse effect on print quality. Many key IFS components would have to be replaced every 5,000 to 10,000 pages to maintain acceptable performance. Clearly it would be undesirable to send your printer away for a complete (and time-consuming) overhaul every 10,000 pages. In order to ease manufacturing difficulties and provide fast, affordable maintenance to every EP printer user, critical components of the IFS, as well as a supply of toner, are assembled into a replaceable electrophotographic cartridge. This is sometimes referred to as an engine. As 4-16 shows, a typical EP cartridge contains the toner roller, toner supply, debris cavity, primary corona (and primary grid), photosensitive drum, and cleaning blade assembly. All necessary electrical connectors and drive gears are included. By assembling sensitive components into a single replaceable cartridge, printer reliability is substantially improved by preventing problems before they ever become noticeable.
How to Protect the Electrophotographic Cartridge ?
As you might imagine, the precision components in an EP cartridge are sensitive and delicate. The photosensitive drum and toner supply are particularly sensitive to light and extreme environmental conditions, so it is important to follow several handling and storage guidelines. First, the photosensitive drum is coated with an organic material that is extremely sensitive to light. Although a metal shroud covers the drum when the cartridge is exposed, light might still penetrate the shroud and cause unwanted exposure (also known as jogging). Deactivating the printer for a time will often eliminate mild fogging. Do not defeat the shroud in open light unless absolutely necessary, and then only for short periods. This will certainly fog the drum. A seriously fogged cartridge might have to be placed in a dark area for several days. Also, never expose the EP drum to direct sunlight; direct sunlight can permanently damage the drum's coating.
0 Comments