How Does a Printer Work ~ The printing process ~ Types of Printers

Although many different manufacturers make printers, you’re likely to encounter only three real categories of printers (so far). The three types of printers discussed in this article are:

Laser Printers

Inkjet Printers

Dot-matrix Printers

The exam touches base on each of these, but the wise technician should focus on laser printers, not only for the exam’s sake, but because these printers are the most common type in today’s computing environment.

Laser printers

A laser printer was, at one time, also called a page printer because the computer’s operating system instructs it to print an entire page at a time, rather than a dot at a time. It’s the laser, the printer’s process, and the code sent to the printer that makes this process not only possible, but fast.

Typically more expensive than its counterparts, the laser printer is also considered more reliable and its output of better quality. As a high-end, non-impact printer, the laser printer uses a combination of processes to print images: electricity, chemistry, magnetism, optics, heat, and friction. Each component of a laser printer has its own effect on the trip that a sheet of paper takes through the printer.

The paper feeder mechanism

As discussed earlier, laser printers use a friction feeder to feed paper into the print device. Most laser printers use a paper tray to hold the paper before it is printed on. Some printers, however, use gravity as a way to hold and begin the print process. These printers hold the paper vertical in a chute above the print components. Regardless of the feed device, each printer requires some mechanism to feed the paper into the printer.

The paper transport path

Paper moves through the printer by making contact with several sets of rollers that work much like the paper feeder. Registration rollers are synchronized with the print process so the paper doesn’t lag or enter the printer too soon. Fuser rollers help fix the image to the paper in the final steps of the print process. Exit rollers (the final set) eject the finished, printed page.

The toner cartridge The toner cartridge contains three indispensable elements of the printing process:

Toner. This mixture of carbon, polyester, and iron particles does the job of ink. Carbon (the black, chalky substance) provides the outline of the image. Polyester helps the toner flow from the cartridge, through the print process, and onto the paper. Iron particles make the toner responsive to electrical charges and help it melt into the paper at the end of the print process.

Print drum. A main component in the laser print process, this photosensitive cylinder holds an electromagnetic charge when not exposed to light—but loses its charge when exposed. direct light, as light causes the drum to lose its charge.

Cleaning blade. This blade removes excess toner from the print drum as the drum rotates. The actual laser in the laser printer is contained in a special assembly that allows the laser to operate only when the printer is closed (direct laser light can damage your eyes). The laser shines on a mirror above the photosensitive drum; the reflected laser light shines on the turning drum. Because the drum cannot hold a charge when exposed to light, the laser and the drum work together to attract the charged toner to the drum—but only enough to make images on the paper. Toner will only be attracted to the areas on the print drum where the laser has shone.

Power supply

A laser printer requires high-voltage electricity to charge the corona wire and the transfer corona wire. The power supply charges these two components by converting standard AC current to a higher-voltage DC current, as required by each printer type. In addition, the printer requires DC power for the printer’s cooling system, logic board, and the motors that turn the wheels to guide the paper through the device.

Transfer corona wire

The transfer corona wire gives the paper a positive electrical charge to attract the toner onto the paper (discussed in more detail in a moment). The transfer corona also contains a static-eliminator to remove the charge after the toner has passed onto the paper.

Fuser

This ominous-sounding device is responsible for fusing (melting) the toner into the paper. The fuser contains a halogen lamp to heat a Teflon-coated roller. Opposite the Teflon-coated roller is a thick rubber roller. As a sheet of paper passes between these rollers, the iron particles and toner are melted into the paper. This is why a laser-printed image is always warm when the paper is ejected from the printer.

The laser printing process

The components of a laser printer are the starting point; how they work together is the printing process. Knowing the process is the basis for effective printer support. Laser printers are far more complex than inkjets or dot matrix printers, so a more in-depth discussion is warranted.

Charging the drum

When the printer receives a command from the computer’s operating system to begin the print process, the photosensitive drum is negatively charged (–600Vdc). This charging is why the printer requires a high-voltage power supply.

Exposing the drum

After the drum is charged, the laser flashes like a strobe light, the beam reflecting from the mirror to the negatively charged drum. In the areas where the light touches the drum, the charge changes from –600Vdc to approximately –100Vdc. As the drum spins, the laser creates the outline of the image on the drum by building a pattern of strongly charged and less-than-strongly-charged areas of negative voltage.

Developing the image

After the image is electromagnetically “written” to the drum, the toner must be applied to the print drum so the image can be transferred to paper. Adjacent to the print drum and the reservoir of toner is a smaller drum called the developer. Charged with –600Vdc by the high-voltage power supply, the developer attracts toner powder to itself as it rotates between the toner and the print drum. The print drum and the developer drum are both charged with –600Vdc, except for the areas of the print drum previously exposed to laser light. Those areas have a weaker negative charge, which attracts the toner. When the print drum has toner on only the areas of slight negative charge, then the image is ready for transfer to paper.

Transferring the image

Now the registration rollers feed the paper into the printer and over the transfer corona wire, which gives the paper a strong positive charge (+600Vdc). As the paper moves beneath the print drum, the weakly charged negative toner particles are strongly attracted to the positively charged paper. As always, opposites attract; the image, outlined in toner, moves onto the paper. The paper continues to move through the assembly, passing over a static eliminator strip that removes all electromagnetic charge from the paper. (This step is crucial; a positively charged piece of paper would be attracted to the still-negative print drum, stick there, and cause a major paper jam.)

Fusing the image

As the paper leaves the print drum, all that holds the toner on the paper is a slight positive charge and a bit of gravity. The fuser assembly finishes the printing by melting the toner into the paper as the sheet moves between a heated, Tefloncoated roller and a rubber roller. The paper is then ejected from the print device.

Cleaning up the mess

As with any good party, you have to clean up a mess when you’re done. In this case, the actual mess is tiny—leftover toner in the cartridge. The cleaning blade in the cartridge scrapes the toner off the print drum in preparation for the next print job. Any unused toner is caught and held in a tray beneath the cleaning blade. (Don’t worry—the tray won’t fill up. You’ll run out of toner first. Just be careful not to breathe the old toner when you change cartridges.)

Inkjet printers

An inkjet printer is a fine, acceptable printer for environments such as home, home office, or road trips (where large print runs probably don’t take place). Though not as complex as a laser printer—nor as expensive—inkjets have evolved into highquality short-run printers.

The early inkjet printers were clunky, unreliable, and messy. Their cartridges had an ink reservoir, a pump that forced the ink into a nozzle, and a reputation for leaking ink, ruining work, and staining hands. Today’s inkjets are much more civilized. The ink cartridge contains all the working elements needed to get an image from the computer onto a sheet of paper: compartments of ink sealed with a metal plate, thin tubes from the ink source to each well, and the jet at the bottom of each compartment (a tiny pinhole that sends ink onto the page).

When the print device receives the command from the computer’s operating system to print an image, the printer starts the following process: An electrical current warms the heating element, which is submerged in the ink source. As the element heats, the ink vaporizes, creating pressure in the compartments. The pressure forces the expanding ink out of the microscopic pinhole, one tiny bubble at a time. The cartridge doesn’t spray ink like a squirt gun; the pressure in the cartridge is controlled by the degree of heat from the heating element. Only one drop at a time can escape from the ink cartridge—but it happens fast enough to create entire images in less than a minute.

Eventually the ink cartridge runs out of ink and should be replaced. Although kits are available for refilling your own ink cartridges, they can enlarge the pinhole, may contain a different formula of ink, and are generally messy and inefficient. Inkjet cartridges can also dry out if they are not used often. Most inkjet printers park the cartridge inside the printer when not in use. Other models have a separate storage unit that allows the cartridge to be removed from the print device.

Inkjet printers (especially color models) have grown in popularity with the advent of affordable digital cameras. Color inkjet printers often require two cartridges: one for black ink and one for the colors (whether red, green, and blue or cyan, yellow, and magenta). Cartridges that bundle the black ink with the other colors are called CYMK (for cyan, magenta, yellow, and black). Note that a “K,” instead of a “B,” designates “black.” The printing industry has done this for years so as not to confuse “black” with “blue.”

When replacing an ink cartridge (whether multicolor or single-color), always have an extra cartridge on hand. If you don’t have an extra, put the old cartridge in a small plastic bag and take it with you when you purchase a new cartridge. This eliminates the guesswork when you’re staring at 487 different types of printer cartridges.

Dot-matrix printers

Dot-matrix printers are impact printers; they strike an inked ribbon to put characters on paper. A dot-matrix printer triggers rows of pins that strike the ribbon in patterns, leaving closely grouped dots that make up numbers, characters, or even images.

The pins are actually solenoids—metal stubs wrapped with a short coil of wire, held in place with a spring and a small magnet. When a particular solenoid is needed to strike, an electrical current is sent to the coil of wires around the solenoid. This creates a miniature electromagnet, which causes the pin to repel against the magnet holding it in place – striking the ribbon and transferring a dot onto the paper in the process.

The print head, which houses the solenoids, moves across the paper, printing one line of dots—not characters or numbers—at a time. The rapidly striking pins make a whining screech. If you’ve never experienced the ear-splitting effect of a dotmatrix printer, you’re lucky. Some models had plastic shields to suppress the noise—somewhat.

Early dot-matrix printers used only 9 pins to print; these draft-quality printers gave way to printers boasting 17 (or even 24) pins. They produced sketchy, crude-looking pages (compared to the output of today’s printers), but they were faster than most typists.

By Barry Geen