Electrophotography – Image formation – Development
Reexamination Certificate
2002-04-25
2004-03-23
Royer, William J. (Department: 2852)
Electrophotography
Image formation
Development
Reexamination Certificate
active
06711371
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a developing device, and more particularly to a developing device in an electrophotographic recording apparatus.
RELATED ART
Generally, in an electrophotographic recording apparatus, a photosensitive body, the surface of which is uniformly charged, is irradiated with light to form an electrostatic latent image, and toner is deposited on the electrostatic latent image portion by an electrostatic force to produce a visible image, which is transferred to a printing medium, and fixed by heating the toner on the printing medium.
FIG. 11
is a block diagram of a developing device of reversal development type in a conventional electrophotographic recording apparatus. The device in
FIG. 11
comprises a photosensitive drum
11
, a charging roller
12
, a print head
13
, a developing device
14
, and a transfer roller
15
.
The photosensitive drum
11
is a cylindrical photosensitive body, which is a working part to have an electrostatic latent image formed by the charging roller
12
and the print head
13
, have toner from the developing device
14
deposited on the latent image, and get the toner transferred to a printing medium
16
. The charging roller
12
is a cylindrical roller set face to face with the photosensitive drum
11
to uniformly electrify the surface of the photosensitive drum
11
. The print head
13
is arranged downstream of the charging roller
12
in the rotating direction of the photosensitive drum
11
to form an electrostatic latent image by irradiating the photosensitive drum
11
with light for exposure.
The developing device
14
to develop the electrostatic image to produce a toner image is arranged downstream of the print head
13
in the rotating direction of the photosensitive drum
11
, and comprises a developing roller
21
, sponge roller
22
, toner
23
, an agitator rod
24
, a developer scraping blade
25
, and a case
26
. The case
26
contains and holds the toner
23
.
The developing roller
21
is a cylindrical roller, which rotates in the arrow direction
21
a
, is arranged face to face with the photosensitive drum
11
to deposit toner on the electrostatic latent image formed on the photosensitive drum
11
. The sponge roller
22
is in contact with the developing roller
21
and rotates in the same direction (in the arrow direction
22
a
) as the developing roller
21
. After the toner is deposited on the photosensitive drum
11
, the sponge roller
22
removes surplus toner from the developing roller
21
, and, on the other hand, supplies toner afresh to the developing roller
21
. The sponge roller
22
has at its center a conductive rod
22
b
to which a voltage Vs is applied.
The toner
23
is developing powder which is deposited on the electrostatic latent image on the photosensitive drum
11
to develop the image. The agitator rod
24
agitates the toner
23
in the case
26
to homogenize it. The developer scraping blade
25
thins down the toner
23
deposited on the surface of the developing roller
21
to a thin film of a constant thickness.
The transfer roller
15
rotates in the direction of the arrow and is arranged downstream of the developing device
14
in the rotating direction of the photosensitive drum
11
and is pressed to the photosensitive drum
11
. The transfer roller
15
has a toner image transferred to the printing medium
16
fed between the photosensitive drum
11
and the transfer roller
15
.
In the electrophotographic recording apparatus configured as described, when a motor, not shown, rotates to feed the printing medium
16
in the direction of the arrow
16
a
, the developing roller
21
and the sponge roller
22
in the developing device
14
rotate in the directions of arrows
21
a
and
22
a
respectively, and the toner
23
in the developing device
14
is agitated by the agitator rod
24
which rotates in the direction of the arrow
24
a
. At this time, negative voltages are applied to the developing roller
21
and the sponge roller
22
simultaneously, but in such a way that the absolute value of the applied negative voltage of the developing roller
21
is smaller. In the case of reversal development, negatively-charged toner
23
that was agitated is deposited on the surface of the sponge roller
22
, and the deposited toner
23
is supplied to the surface of the developing roller
21
through contact of the sponge roller
22
with the surface of the developing roller
21
having a negative voltage the absolute value of which is smaller than that of the sponge roller
22
. The toner deposited on the surface of the developing roller
21
is made thinner by the developer scraping blade
25
to a thin toner layer.
On the other hand, simultaneously with the rotation of the above-mentioned motor, not shown, a negative voltage is applied to the charging roller
12
, and the photosensitive drum
11
is charged uniformly while rotating in the direction of the arrow
11
a
. After this, when the photosensitive drum
11
is irradiated with light from the print head
13
, the regions exposed to light decrease in the level of charge to 0[V], so that an electrostatic latent image is formed on the surface of the photosensitive drum
11
. The toner
23
deposited on the developing roller
21
is negatively-charged, and when the developing roller
21
contacts the electrostatic latent image on the photosensitive drum
11
or reaches a maximum proximate point, the toner
23
is deposited on the exposed regions. Meanwhile, because the regions not exposed to light are more negatively charged than the negatively-charged toner
23
on the developing roller
21
, the toner
23
is not deposited on the unexposed regions. Subsequently, the toner image is transferred from the photosensitive drum
11
to a printing medium
16
, and the toner image is fixed to the printing medium
16
by a fixing unit, not shown.
However, the conventional developing device has problems as follows.
FIG. 12
is a diagram for explaining the conventional problem.
The residues of toner (residual toner)
101
on the developing roller
21
, which was not used by the photosensitive drum
11
and returned to the developing device
14
, are scraped off on the downstream side of the contact area by the sponge roller
22
. However, in a continuous printing process, for example, the residual toner
101
gets into a sponge portion
22
c
(indicated as toner
102
) covering the periphery of the sponge roller
22
, and solidifies to give rise to clogging of the sponge, thus deteriorating the toner removing performance of the sponge roller
22
. Under this condition, the residual toner
101
is not detached completely on the downstream side of the contact area, and stays deposited on the developing roller
21
as indicated by the residual toner
101
a
. Consequently, there arises a difference in toner thickness on the surface of the developing roller
21
or a difference in potential of the toner between its first rotation and the second rotation when the residual toner
101
a
remains deposited on the developing roller
21
, which is responsible for irregular print density.
In solid print, the image, which was used for printing in the first rotation, remains as a thin residual image in the second rotation. In other words, toner is deposited on the previous toner image which was not consumed on the photosensitive drum
11
and remains on the developing roller
21
, with the result that a reverse image of the image in the first rotation is printed.
Further, irregular print density occurs for the reason as follows. The residual toner, which was once detached by scraping off from the developing roller
21
, still stays deposited on the sponge roller
22
. As the sponge roller
22
rotates, the residual toner is supplied together with other toner to the surface of the developing roller
21
, making the toner layer not uniform and giving rise to irregular print density.
Irregular print density occurs for another reason. Immediately before starting printing, the image forming process starts, and
Koyama Tetsu
Kuroda Teruaki
Okiyama Yoshitatsu
Oki Data Corporation
Rabin & Berdo PC
Royer William J.
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