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Copiers and laser printers have a lot in common.
The major difference is in how the image is formed on a photosensitive
drum:
- A copier uses a bright light and lens to focus an image of the
original (actually, a strip at a time which is scanned in most
modern low to medium performance copiers) onto the drum. Adjusting
the lens-to-original and lens-to-drum distance is used to vary
the reduction or magnification.
- A laser printer uses a low power sharply focused laser beam
to scan one line at a time on the drum. Modern laser printers
use infra-red solid state laserdiodes similar to those used in
CD players and optical disk drives while older ones used helium
neon lasers.
- The digital image is generated from a bit map stored in the
printer's memory and modulates the laser beam. Scanning is mechanical
- a high speed motor spins a multifaceted deflection mirror to
get the X-axis and the paper moves to get the Y axis.
LED printers use a large array of LEDs as the image
source but are otherwise similar to laser printers. Plain paper
fax machines use similar techniques in their printing mechanism.
Beyond this, copiers and laser printers are nearly
identical (at least in principle) except that copiers use a positive
process (dark areas in the original result in marks on the paper)
and laser printers commonly use a negative process (a spot of light
results in a dark mark on the paper).
The most sophisticated machines are now actually
scanner-laser printer combinations with buffer memory so that multiple
copies can be made without rescanning the original, sorting and
collating is more flexible, scaling and rotation can be done digitally,
and other features not possible with simple copiers.
(Portions from: Copenhagen Cowboy (cowboy@fastlane.net).)
The photosensitive drum is the heart of the laser
printer or copier. In larger machines, it may be a separately replaceable
unit. In most laser printers and smaller copiers, it is part of
the 'toner cartridge' and is a throw-away (or may be recycled).
The drum is coated with a photosensitive material
which has an extremely high resistance when in darkness. It's resistance
drops to a low value when illuminated.
All of the following takes place as a continuous
process as the drum rotates. Note that the actual photosensitive
drum in most copiers and laser printers has a circumference that
is much smaller than the length of the printed page. Therefore,
only a portion fits at any given time and the charging, exposure,
transfer to the paper, cleaning, and erasing is a continuous process:
- The drum's surface is charged to a high positive voltage (typically
5 to 6 kV) by a set of charging corona wires in close proximity
to the drum.
- The exposure process differs for copiers and laser printers:
- For copiers, a swath of the original is focused onto the drum.
As the drum turns, a quartz lamp and strip mirror moves along
the original and second strip turning mirror moves at half this
speed. The result is that the entire original's image is kind
of 'peeled' onto the rotating drum. (Look through the glass platform
that supports the original of a copier as it is copying and you
will see what I mean.)
- For laser printers, the negative image of the page stored in
the printer's buffer memory (the laser is turned on where the
print is to be black) is read out and scanned onto the drum one
line (i.e., 1/300th or 1/600th of an inch) at a time.
- Where the light hits the drum's surface, its resistance drops
dramatically and the charge in these areas is dissipated. At this
point, a swath of the image of your ultimate copied or printed
page resides as areas of electrostatic charge on the drum. This
is a 'latent' image and must be 'developed'.
- As the drum continues to turn, the latent image rotates past
the 'developer unit' which contains a mixture of developer and
toner. For the most part, developer is not really used up during
the printing process but some is lost and may need to be replenished
from time-to-time (depends on design).
- Developer is a material which includes powdered iron or other
powder which is attracted by a magnet.
- Toner is the actual 'ink' and consists of very finely powdered
thermo plastic particles. These are 'fixed' in the fuser by literally
melting the image onto the paper.
Depending on design, the developer material may
be separate or actually combined with the toner.
A magnet in the developer unit which is as long
as the page is wide causes the developer along with trapped toner
to stand out following its lines of force off of its long N-S pole
pieces. This forms a kind of brush of toner and developer material
which is in contact with the drum as it rotates with its latent
image. Normally, the developer material brush is C-shaped, and toner
particles are carried in the C-shape (the back of the 'C' is against
the drum).
Here is where the developing processes of copiers
and laser printers differ:
- For copiers, the relative charges of the drum and toner are
set up so that toner is drawn to the unexposed (dark parts of
the original) portions of the drum resulting in a positive image
on the paper.
- For laser printers, the relative charges of the drum and toner
are set up so that toner is drawn to the exposed (where the laser
beam was turned on) portions of the drum resulting in a negative
image on the paper.
- The drum continues to rotate around and comes in contact with
the paper.
- Below the paper is another corona, the 'transfer corona'. Another
high voltage is applied to the back of the paper (once again,
around 7 or 8 kV DC) to draw the toner from the drum to the paper.
(Remember, all this is going on in a continual cycle and it is
all in motion).
- Depending on the manufacturer of the machine, you may or may
not have a third corona, the 'separation corona'. This is needed
to separate the paper from the drum, but not disturb the toner
on the paper (the separation corona is usually 4 or 5 kV AC (if
it was DC, you would separate the paper, but have *very* smeared
toner all over the page as to make it unreadable). The separation
corona usually has guides over it to keep the paper from 'dipping'
down too far into the corona shell.
- Paper is then transported to the fuser which 'fixes' the toner
to the paper via heat (to soften the toner particles) and pressure
(to embed them in the paper fiber). There are parts in the fuser
which also keep the paper from sticking to the hot rollers. A
thermostatically controlled quartz tube lamp provides the heat
inside the anti-stick (Teflon coated) fuser roller.
- Finally, your copy or printed page is ready!
- However, we are not done as there is still some toner on the
drum - it is not possible to get it all off electrically) so there
is usually a rubber or plastic blade which rubs in direct contact
with the drum. This 'drum blade' scrapes the toner off the drum,
and the 'recovery blade' catches it to keep it from falling back
into the machine. A 'used toner auger' transports the used toner
(which is now changed both physically and electrically and is
also contaminated with paper dust (don't reuse your used toner)
because it can eventually damage the developer unit, cleaning
blades, fuser sections and other parts of the mechanism.
- Now that all the toner has been scraped off the drum, there
is still some residual charge on the drum from the previous exposure
process. You can't scrape the static charge off the drum, so the
cleaned drum is now fully exposed to a bright light to discharge
the drum surface and prepare it again for a new charge, which
comes right after the discharge lamps.
That is the basic process. Many variations are
possible and depending upon the machine and manufacturer, some of
this may be a little different. Where a (disposable) toner cartridge
is used, many of these components are replaced with the cartridge
- typically the drum, toner itself and developer (usually combined
into a single powder), developer magnet (really neat!), cleaning
blades, some of the corona wires.
Back to Printer and Photocopier Technology
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