Sharp SF-1120 - Manual (Page 39)

(3) Details of image forming process
Step 1 (Main Charging)
By negative discharging of the main charger, uniform negative char-
ges are applied to the OPC drum surface.
The OPC drum surface potential is controlled by the screen grid
voltage to maintain the grid voltage at a constant level.
• When the drum surface potential is lower than the grid voltage,
electric charges generated by discharging of the charger go
through the screen grid to charge the drum surface potential until it
becomes equal to the grid voltage.
• When the drum surface potential virtually reaches the grid potential
level, electric charges generated by discharging of the charger
flows through the electrode of the screen grid to the high voltage
unit grid voltage output circuit, thus always maintaining the drum
surface potential at a level virtually equal to the grid voltage.
• The main corona unit employs the scorotron system to charge the
photoconductor surface to a certain level uniformly.
In addition, the conventional corona wire is replaced with the
corona charging mechanism by saw-teeth plate (stainless steel
plate of 0.1 mm thick). In corona discharge, oxygen molecules in
the air are ionized to generate ozone (O
3 ). The mechanism restrict
the generation of ozone.
Step 2 (Exposure)
Light from the copy lamp is radiated on the document, and the optical
image of the document is reflected by the mirrors and projected
through the lens to the OPC drum.
The lighter portion of the document reflects more light (high intensity)
to the OPC drum, and the darker portion of the document reflect less
light (low intensity) to the OPC drum. Positive or negative charges are
generated in the CGL of the OPC drum where lights are radiated.
Negative charges generated in the CGL move towards the positive
charges in the aluminum layer generated in step 3. While the positive
charges in the CGL move towards the negative charges on the CPU
drum surface generated in step 3. Therefore, positive charges and
negative charges are neutralized in the aluminum layer and the OPC
drum surface at the light radiating position, decreasing the OPC drum
surface potential. The CGL electric charge generating amount in-
creases in proportion to the document density, that is, reflected light
intensity (the OPC drum surface intensity). Therefore, electric char-
ges are generated less in the CGL layer corresponding to the lighter
density of document (higher intensity of the OPC drum surface), and
a greater quantity of the negative charges on the OPC drum surface
is neutralized, decreasing the OPC drum surface potential more. On the contrary, electric charges are generated more in the CGL
layer corresponding to the darker density of document (lower intensity
of the OPC drum surface), and less quantity of the negative charges
on the CPU drum surface is neutralized, decreasing the OPC drum
surface less. Therefore, the OPC drum surface potential correspond-
ing to the lighter portion of the document is lower, and that cor-
responding to the darker portion of the document is higher. Latent
static-electricity images are formed in the above manner.
CGL
CTL
OPC drum
Main corona unit Blank lamp
Cleaning blade
Discharge lamp
Seperation pawl
Seperation corona unit
Transfer corona unit
Resist roller
MG roller
Developer unit
Main corona unit
Screen grid
High voltage unit
Aluminum
layer
5 – 10