Image formation system
Laser printing requires the interaction of several different technologies including electronics,
optics, and electrophotographics to provide a printed page. Each process functions
independently and must be coordinated with the other printer processes. Image formation
consists of seven processes:
Electrostatic latent image formation stage
Paper direction
Direction of drum rotation
2. Scanning exposure
3. Developing
4. Transfer5. Separation
6. Fusing
7. Drum cleaning
Paper delivery
Pick-up
1. Primary charging
Fusing stage
Transfer stage
Developing stage
Drum cleaning
stage
Figure 4-5. Image formation block diagram
Print cartridge
As the focal point of the image formation system, the print cartridge contains toner and
houses the cleaning, conditioning, and developing stages of the process. The print cartridge
contains the photosensitive drum, primary charging roller, developing station, toner cavity,
and cleaning station. By including these components (which wear, degrade, or are
consumed) in the replaceable print cartridge, the need for many service calls is eliminated.
The special photosensitive properties of the drum form an image on the drum surface and
then transfer the image to media.
CAUTION
The print cartridge does not include a light-blocking shutter. Do not expose the drum to light,
which can permanently damage the drum. Protect the print cartridge whenever you remove it
by covering it with paper.
The seven image-formation processes
1. Conditioning stage (primary charging)—This process applies a uniform negative
charge to the surface of the drum with the primary charging roller, which is located in the
print cartridge. The primary charging roller is coated with conductive rubber. An AC bias
is applied to the roller to erase any residual charges from any previous image. In
addition, the primary charging roller applies a negative DC bias to create a uniform
negative potential on the drum surface. The print density setting modifies the DC voltage.
2. Writing stage (scanning exposure)—During this process, a modulated laser diode
projects the beam onto a rotating scanning mirror. As the mirror rotates, the beam
reflects off the mirror, first through a set of focusing lenses, then off a mirror, and finally
through a slot in the top of the print cartridge, and onto the photosensitive drum. The
beam sweeps the drum from left to right, discharging the negative potential wherever the
beam strikes the surface. This creates a latent electrostatic image, which later is
developed into a visible image. Because the beam sweeps the entire length of the drum
while the drum rotates, the entire surface area of the drum can be covered. At the end of
each sweep, the beam strikes the beam-detect lens, generating the beam-detect signal
(BD signal). The BD signal is sent to the ECU where it is converted to an electrical signal
used to synchronize the output of the next scan line of data.
ENWW Printer functions 43
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