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How Laser Printers Create Sharp Images Using Static Electricity

written by: John Garger•edited by: J. F. Amprimoz•updated: 8/25/2010

An ink jet printer works by squirting ink onto a page. Laser printers, however, are a bit more complicated, relying on some familiar properties of matter and energy learned in grammar school.

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    During the 1980s, laser printers were expensive printing machines found only in the large organizations who could afford their price tag. New processes utilized throughout the 1990s made laser printers much more economical, leading to prices feasible for the home user. Unlike the simple concept of printing with ink, laser printers are largely an enigma. What does a laser have to do with printing anyway?

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    How a Laser Printer Works

    Laser printer technology starts with a concept familiar to younger siblings the world over. The static electricity used to annoy your little sister when you would walk with socks over a carpet and shock her is the same scientific principle used to print with lasers.

    Static electricity forms when an electrical charge builds up on an insulated object. Recall that objects that are either both positively or negatively charged repel one another, but objects with opposite charges are attracted to one another. The most important part of the laser printer is the drum; a photoreceptor made out of a photoconductive material. This simply means that the drum is capable of holding either a positive or negative charge and can hold both simultaneously on different areas of its surface, even areas right next to one another.

    Initially, a wire with electricity running through it, called a corona wire, gives the entire drum a positive charge. At this point, the printer looks at the data it has received from the computer about what the final image should look like.

    The drum rotates and the printer uses a small laser to draw on the drum the image that is to be printed. The laser draws the image by changing the charge on parts of the drum from a positive to a negative charge. The result is an electrostatic image on the drum.

    When this is complete, positively charged toner coats the negatively charged parts of the drum that were drawn on by the laser. The toner, since it is positively charged, only sticks to the negatively-charged areas of the drum. At this point, a mechanism rolls the drum over a piece of paper which itself has been negatively charged so the toner will temporarily stick. In fact, the negative charge of the paper is stronger than that of the drum holding the toner so that the toner will release from the drum and stick to the paper.

    The final process differs slightly from printer to printer but the basic idea is that the toner needs to be heated so it forms a strong bond with the paper. This is usually done with a pair of rollers called a fuser. The piece of paper passes between the heated rollers of the fuser and leaves the printer ready to be picked up. Since the process does not use inks, there is no need to wait for the printing to dry; it is a completely dry process. After the printing process, the drum passes over a discharge lamp which erases the text and/or images that have been electrostatically imprinted on it and the whole process is ready to begin again.

    Of course, coordinating this process to work in only a few seconds takes some feats of mechanical and programming magic. Just imagine how fast a laser printer is able to print multiple pages in a row and you can see how the drum, especially, gets quite a workout. As toner needs to be refilled, the drum of a laser printer does not last forever and needs to be replaced. In most lasers printers, a drum will not last far beyond about 12,000 to 15,000 pages, about 24 to 30 reams of paper. Eventually the photoconductive material found in the drum breaks down and will not properly hold the correct charge. This is why when the drum begins to deteriorate, the printed pages can be too light or have a grey haze to them.