You know how if you rub a balloon against your shirt, it will stick to you? The principle at work there is static electricity-and without it, we wouldn’t have laser printers. Static electricity is an electrical charge that builds up on an object-and atoms with a positive charge will cling to those with a negative charge. When you rub the balloon against your shirt, the balloon and your shirt pick up a charge and the atoms stick together.
In a laser printer, static electricity functions as a sort of glue. The charged object in your printer is a drum assembly, composed of a rapidly revolving drum or cylinder made from highly photoconductive material. A wire or roller with an electrical current running through it gives the drum a positive charge. The drum revolves, and as it does, a laser beam is focused on its surface to discharge atoms from certain points-essentially drawing the letters and images that will be on the paper in an electrical-charge pattern, or an electrostatic image.
The printer sends a sheet of paper through the fuser, or a pair of rollers that have been heated. As the paper passes through, toner powder melts on its surface and fuses with the paper fibers. The paper itself is warm when it comes out of the printer, because of the heat of the rollers. The rollers are hot enough to burn the paper, but the sheets pass through so quickly that it doesn’t collect heat fast enough to ignite.
Once the paper has been printed, a discharge lamp shines general light on the drum surface, essentially erasing it in preparation for the next image. The charged wire-called the corona wire-then re-charges the drum for the next pass.
Some laser printers print in black and white only, while others have colour capability. Colour printers typically have blue, red and yellow ink in addition to black. They print in colour by sending the paper through the system several times, laying down a different layer of ink each time. Some printers lay different inks onto the drum, allowing all the layers to be printed on the page in one pass-while others continually recirculate the paper, allowing the drum to lay down a different layer of ink each time it passes. A very large colour printer may have several drum and toner sets, each one assigned to laying down a separate colour.
So how does a laser printer receive data about the printed images and print them on paper? The printer controller, a small computer chip, communicates with the computer you’re printing from-your PC or laptop-through a port such as a parallel or USB port. When the print job starts, the laser printer and the host computer essentially discuss how they will communicate data. In an office setting, a laser printer is often connected to a number of different computers, and the printer controller communicates with each of them separately-sometimes with more than one at the same time.
In earlier times, there were only a few fonts and formatting styles to choose from-and a sent file and some simple formatting directions were all that was needed. Today, printers have to be able to carry on a “conversation” to sort through hundreds of different font options and complex graphics. That’s why today’s printer controllers have a distinct language with which they communicate with computers. The most common ones are Hewlett Packard’s Printer Command Language (PCL) and Adobe Postscript. These languages both describe the printed image by assigning mathematical values to geometric shapes-as opposed to earlier printers, which mainly communicated in a series of dots in a bitmap image. Today’s printers can take the images sent from computers and convert them to bitmap images, allowing any kind of complex graphic or font to appear on the page.
Laser printers are complicated machines-but they typically cost less in consumables and tend to be faster and quieter than inkjet printers are. Still, they also tend to be more expensive up front-so many business and personal users buy an inkjet first. But a good laser printer is a good investment, especially if you need to print in high volume and high quality.