Specifications
AL-1551 COPY PROCESS 6-4
Step-7: Optical discharge (Semiconductor laser)
Before the drum rotation is stopped, the semiconductor laser is radiated
onto the drum to reduce the electrical resistance in the OPC layer and
elimate residual charge, providing a uniform state to the drum surface for
the next page to be printed.
When the electrical resistance is reduced, positive charges on the
aluminum layer are moved and neutralized with negative charges on the
OPC layer.
Charge by the Scorotron charger
Function
The Scorotron charger functions to maintain uniform surface potential on
the drum at all times, It control the surface potential regardless of the
charge characteristics of the photoconductor.
Basic function
A screen grid is placed between the saw tooth and the photoconductor. A
stable voltage is added to the screen grid to maintain the corona current
on the photoconductor.
As the photoconductor is charged by the saw tooth from the main corona
unit, the surface potential increases. This increases the current flowing
through the screen grid. When the photoconductor potential nears the
grid potential, the current turns to flow to the grid so that the
photoconductor potential can be maintained at a stable level.
Process controlling
Function
The print pattern signal is converted into an invisible image by the
semiconductor laser using negative to positive (reversible) developing
method. Therefore, if the developing bias is added before the drum is
charged, toner is attracted onto the drum. If the developing bias is not
added when the drum is charged, the carrier is attracted to the drum
because of the strong electrostatic force of the drum.
To avoid this, the process is controlled by adjusting the drum potential
and the grid potential of the Scorotron charger.
Basic function
Voltage added to the screen grid can be selected, high and low. To make
it easily understood, the figure below shows voltage transition at the
developer unit.
Start
1) Because the grid potential is at a low level, the drum potential is at
about -400V. (Carrier may not be attracted though the carrier is
pulled towards the drum by the electrostatic force of -400V.
2) Developing bias (-400V) is applied when the photoconductor
potential is switched from LOW to HIGH.
3) Once developing bias (-400V) is applied and the photo conductor
potential rises to HIGH, toner will not be attracted to the drum.
Stop
The reverse sequence takes place.
Retaining developing bias at an abnormal occurrence
Function
The developing bias will be lost if the power supply was removed during
print process. In this event, the drum potential slightly abates and the
carrier makes deposits on the drum because of strong static power. To
prevent this, the machine incorporates a function to retain the developing
bias for a certain period and decrease the voltage gradually against
possible power loss.
Basic function
Normally, the developing bias voltage is retained for a certain time before
the drum comes to a complete stop if the machine should stop before
completing the normal print cycle. The developing bias can be added
before resuming the operation after an abnormal interruption. Therfore,
carrier will not make a deposit on the drum surface.
Semiconductor laser
0
START STOP
Print potentioal
Toner attract
potential
2)
3)
1)
Low
4)
High
Drum potential
Developing bias
Time