Sharp AM-900 Service Manual page 61

STEP 3 (Development): Toner is attached to the latent electrostatic
images on the OPC drum to form visible
images.
: Toner (Negatively charged)
Doctor
DC-180V
Toner
supply
roller
DC
Developing roller
-280V
Fig. 14
Toner is transported to the scraper area by the toner supply roller and
the developing roller. The quantity of toner to be transported to the
doctor section is controlled by the scraper. Toner transported to the
doctor section is then passed between the developing roller and the
doctor to form a thin toner layer on the developing roller by the pres-
sure applied by the doctor.
When toner passes between the developing roller and the doctor, it is
charged negatively by friction.
When an area of OPC drum which was exposed to laser beam and
lost its charge comes in contact with the developing roller, toner moves
from the developing roller to the OPC drum surface.
The principle of toner movement from the developing roller to the OPC
drum surface is as follows.
The bias voltage of DC-280V is applied to the developing roller. Toner
is charged negatively by the difference (electrical energy) between the
bias voltage and the OPC drum surface potential and is attracted to
the OPC drum surface which is positively charged.
At that time, the potential of the area of the OPC drum which was
exposed to the laser beam and lost its charge is higher than that of the
developing roller.
Aluminum layer
(Drum base)
CTL
CGL
Exposed area
(exposed
to laser beam)
Non-exposed area
(Not exposed to
laser beam)
OPC drum
3 – 14
On the other hand, when an area of OPC drum which was not
exposed to the laser beam and did not lose its charge comes in con-
tact with the developing roller, any residual toner attached to the OPC
drum is transferred to the developing roller which is more positively
charged.
As a result, unnecessary toner on the OPC drum is collected by the
developing unit.
The operating principle for that case is contrary to that for transfer of
toner from the developing roller to the OPC drum surface. (The electric
field energy direction is contrary.)
STEP 4 (Transfer): Visible images of toner on the OPC drum are
transferred to the paper.
CTL
CGL
Aluminum layer
(Drum base)
OPC drum
Fig. 15
The high voltage of DC+3600V plus AC760V (P-P) is applied to the
transfer roller to generate electric discharge between the roller and the
OPC drum, generating positive and negative charges. Positive
charges are attracted to the OPC drum and attached to the paper
transported between the transfer roller and the OPC drum. Therefore
the paper has a strong positive charge.
Negatively charged toner on the OPC drum is attracted by the paper
which is positively charged, and the visible images of toner are trans-
ferred onto the paper.
AM-900U
Paper
Transfer roller
DC+3600V
AC760V(P-P)