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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.
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