Sharp Z-21 Service Manual page 12

Step 2. Charge
A uniform negative charge is applied over the OPC drum surface by
the main corona unit. (Controlled by the grid bias.)
Step 3. Exposure (copy lamp and SELFOC lens)
The image of the original is projected onto the OPC drum through the
SELFOC lens by means of the light from the copy lamp. Where light
strikes the drum light areas of the original, the electrical resistance in
the OPC layer decreases and the charge dissipates. This forms the
electrostatic latent image on the drum surface.
Exposure
(Copy lamp)
Drum surface charge
after exposure
Dark Halftone
Black area Gray area
of original of original
Screen grid
SELFOC lens
OPC layer
Pigment layer
Aluminum
layer
OPC layer
Pigment layer
Aluminum
layer
Light
White area
of original
Step 4. Developing
Toner is attracted from the MG roller to the latent image on the drum
surface forming a visible image. With this two component magnetic
brush type developing method, the toner is charged positive because
of friction with the carrier and is, therefore, attracted to the deposits
the negatively charged areas (imaged areas) on the drum surface.
In this process, a bias potential is applied to the MG roller (magnet
roller) to prevent the toner from being attracted to the residual poten-
tial (approx. –80 to –100V) in the non-imaged areas after exposure.
Step 5. Transfer
The visible image (toner image) on the drum is transferred onto the
copy paper.
A high potential is applied to the back of the copy paper by negative
corona discharge, making the attraction between the drum and toner
less than the attraction between the copy paper and toner. As a
result, toner adheres to the copy paper.
The resulting transfer efficiency is 70 to 80% therefore 20 to 30% of
the toner remains on the drum after transfer, residual toner.
– 10 –
: Carrier
: Toner
(N) (S) Permanent magnet
(3-pole fixed type)
S
N MG roller
S