The term inkjet printeris very descriptive of the process at work -- these printers put an image on paper using tiny jets of ink. The term laser printer, on the other hand, is a bit more mysterious -- how can a laserJet Printers, a highly focused beam of light, write letters and draw pictures on paper? Show The Basics: Static Electricity The path of a piece of paper through a laser printer The basic components of a laser printer The Basics: Drum Initially, the drum is given a total positive chargeby the charge corona wire, a wire with an electrical current running through it. (Some printers use a charged rollerinstead of a corona wire, but the principle is the same.) As the drum revolves, the printer shines a tiny laser beam across the surface to discharge certain points. In this way, the laser "draws" the letters and images to be printed as a pattern of electrical charges -- an electrostatic image. The system can also work with the charges reversed -- that is, a positive electrostatic image on a negative background. The laser “writes on a photoconductive revolving drum. After the pattern is set, the printer coats the drum with positively charged toner -- a fine, black powder. Since it has a positive charge, the toner clings to the negative discharged areas of the drum, but not to the positively charged "background." This is something like writing on a soda can with glue and then rolling it over some flour: The flour only sticks to the glue-coated part of the can, so you end up with a message written in powder. With the powder pattern affixed, the drum rolls over a sheet of paper, which is moving along a belt below. Before the paper rolls under the drum, it is given a negative charge by the transfer corona wire(charged roller). This charge is stronger than the negative charge of the electrostatic image, so the paper can pull the toner powder away. Since it is moving at the same speed as the drum, the paper picks up the image pattern exactly. To keep the paper from clinging to the drum, the corona wire discharges it immediately after picking up the toner. The basic components of a laser printer Fuser The Printer Controller: In an office, a laser printer will probably be connected to several separate host computers, so multiple users can print
documents from their machine. The controller handles each one separately, but may be carrying on many "conversations" concurrently. This ability to handle several jobs at once is one of the reasons why laser printers are so popular. These days, you might have hundreds of different fonts to choose from, and you wouldn't think twice about printing a complex graphic. To handle all of this diverse information, the printer needs to speak a more advanced language. The primary printer languages these days are Hewlett Packard's Printer Command Language(PCL) and Adobe's Postscript. Both of these languages describe the page in vectorform -- that is, as mathematical values of geometric shapes, rather than as a series of dots (a bitmapimage). The printer itself takes the vector images and converts them into a bitmap page. With this system, the printer can receive elaborate, complex pages, featuring any sort of font or image. Also, since the printer creates the bitmap image itself, it can use its maximum printer resolution. Some printers use a graphical device interface(GDI) format instead of a standard PCL. In this system, the host computer creates the dot array itself, so the controller doesn't have to process anything -- it just sends the dot instructions on to the laser. But in most laser printers, the controller must organize all of the data it receives from the host computer. This includes all of the commands that tell the printer what to do -- what paper to use, how to format the page, how to handle the font, etc. For the controller to work with this data, it has to get it in the right order. Once the data is structured, the controller begins putting the page together. It sets the text margins, arranges the words and places any graphics. When the page is arranged, the raster image processor(RIP) takes the page data, either as a whole or piece by piece, and breaks it down into an array of tiny dots. As we'll see in the next section, the printer needs the page in this form so the laser can write it out on the photoreceptor drum. In most laser printers, the controller saves all print-job data in it’s own memory. This lets the controller put different printing jobs into a queueso it can work through them one at a time. It also saves time when printing multiple copies of a document, since the host computer only has to send the data once. The Laser Assembly • A laser The laser receives the page data -- the tiny dots that make up the text and images -- one horizontal line at a time. As the beam moves across the drum, the laser emits a pulse of light for every dot to be printed, and no pulse for every dot of empty space. The laser doesn't actually move the beam itself. It bounces the beam off a movable mirrorinstead. As the mirror moves, it shines the beam through a series of lenses. This system compensates for the
image distortion caused by the varying distance between the mirror and points along the drum. Some laser printers use a strip of light emitting diodes (LED) to write the page image, instead of a single laser. Each dot position has its own dedicated light, which means the printer has one set print resolution. These systems cost less to manufacture than true laser assemblies, but they produce inferior results. Typically, you'll only find them in less expensive printers. Toner Basics So what is toner? The short answer is: It's an electrically charged powder with two main ingredients: pigmentand plastic. The role of the pigment is fairly obvious -- it provides the coloring (black, in a monochrome printer) that fills in the text and images. This pigment is blended into plastic particles, so the toner will melt when it passes through the heat of the fuser. This quality gives toner a number of advantages over liquid ink. Chiefly, it firmly binds to the fibers in almost any type of paper, which means the text won't smudge or bleed easily. Applying Toner Because they are negatively charged, the developer beads collect the positive toner particles as they pass through. The roller then brushes the beads past the drum assembly. The electrostatic image has a stronger negative charge than the developer beads, so the drum pulls the toner particles
away. The drum then moves over the paper, which has an even stronger charge and so grabs the toner. After collecting the toner, the paper is immediately discharged by the negative electrons, which, are prepared by corona wire. At this point, the only thing keeping the toner on the page is gravity if you were to blow on the page, you would completely lose the image. The page must pass through the fuser to affix the toner. Internal quartz tube lamps heat the fuser rollers, so the plastic in the toner melts as it passes through. But what keeps the toner from collecting on the fuser rolls, rather than sticking to the page? To keep this from happening, the fuser rolls must be coated with Teflon, the same non-stick material that keeps your breakfast from sticking to the bottom of the frying pan. Color Printers Inside a
color laser printer Which component in a laser printer charges the paper?Which component in a laser printer charges the paper to attract toner? The transfer roller charges the paper to attract the toner. The primary corona prepares the photosensitive drum for writing by causing it to receive a negative electrostatic charge.
Which part of the laser printer applies a positive charge?The basic components of a laser printer
Initially, the drum is given a total positive charge by the charge corona wire, a wire with an electrical current running through it. (Some printers use a charged roller instead of a corona wire, but the principle is the same.)
Which component inside the printer applies a charge to the paper?A transfer roller (or a transfer corona) applies a charge to the paper as it passes. The charged paper attracts the toner and the toner is transferred from the imaging drum to the paper.
Which laserjet printer part puts a positive charge on the paper in order to pull toner from the drum?The secondary corona wire, or transfer roller, applies a positive charge onto the paper. The agitator unit inside the toner cartridge hopper spins, and the toner begins to heat up. The toner adder spins, pulling toner in, gathering toner dust on its surface.
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