Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-02-26
2003-02-25
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S312000
Reexamination Certificate
active
06526249
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color image forming apparatus for forming a color image on a sheet by using toners, and a color image forming method there of, and more particularly to a color image forming apparatus which has a tandem type engine arranged a plurality of toner image forming units, and a color image forming method thereof.
2. Description of the Related Art
A tandem type color image forming apparatus as a color image forming apparatus, which has a plurality of toner image forming units parallel-disposed in a carrying path thereof, continuously forms the toner images with different colors on a sheet to enable a high-speed printing.
FIG. 7
shows a conventional tandem type color electro-photographic apparatus.
In
FIG. 7
, reference numbers
10
,
20
,
30
and
40
indicate the OPC (Organic Photoconductor) drums of the Yellow-color, Magenda-color, Cyan-color, and Black-color toner process units, respectively. The electrostatic latent images are formed on the OPC drums
10
,
20
,
30
and
40
and developed with Yellow-color, Magenda-color, Cyan-color, and Black-color toners by the unshown developing members in the Yellow-color, Magenda-color, Cyan-color, and Black-color toner process units.
The developed toners are transferred onto a sheet
100
by the strength of an electric field, which has been generated between the OPC drums
10
to
40
and the sheet
100
by a voltage applied from transfer members
80
to
84
such as transfer rollers, etc. The sheet
100
is electrically charged by a sheet adsorption roller
60
, and then it is adsorbed onto a dielectric belt
50
.
The sheet
100
is carried to the transfer positions of the OPC drums
10
,
20
,
30
, and
40
by the movement of the dielectric belt
50
, and all of the four colors are transferred onto the sheet
100
. Then, the sheet
100
is taken off the dielectric belt
50
, and the toner images on the sheet
100
are fixed by an unshown fixing member. Even when the four colors are transferred at different positions, the dielectric belt
50
adsorbs the charged sheet
100
, so that a high-quality color image can be formed without a position deviation of each color on the sheet
100
.
As disclosed in U.S. Pat. No. 5,907,758 (Japanese Unexamined Published Patent 10-198120), and U.S. Pat. No. 6,021,286 (Japanese Unexamined Published Patent 11-161035), etc., in a conventional tandem type color electro-photographic process, the dielectric belt
50
is charged to a high electric potential such as approximately 1000V by a charging device
70
. The reason why the dielectric belt
50
is charged to the high electric potential is explained. An electric potential difference between the sheet
100
and the OPC drums
10
to
40
can be increased for the charged electricity of the dielectric belt
50
, even when the transfer voltage applied to the four color toner transfer members
80
,
81
,
82
, and
83
is lowered. The strength of the electric field generated between the sheet
100
and the OPC drum is caused by increasing the potential difference between the sheet
100
and the OPC drum to a degree that no electric discharge occurs, so that the transfer efficiency can be improved.
The above process is explained below, taking an example. The conductive brush
70
charges the dielectric belt
50
to 1000V. At the same time, the sheet adsorption roller
60
charges the sheet
100
to adsorb the sheet
100
onto the dielectric belt
50
. At this time, the sheet
100
must be charged so that the potential difference between the front and back faces of the sheet
100
can be set to 2000V.
Then, the Yellow-color toners contained in the OPC drum
10
are transferred onto the sheet
100
in the Yellow-color toner transfer position. A voltage of −100V (direct current) is applied to a transfer roller
80
. At this time, electric charges on the sheet move to a photosensitive body
10
. Therefore, the potential difference between the front and back faces of the dielectric belt
50
is lowered from 1000V to 400V.
Then, the Magenda-color toners contained in the OPC drum
20
are transferred onto the sheet
100
in the Magenda-color toner transfer position. A voltage of 500V (direct current) is applied to a transfer roller
82
. At this time, the electric charges move to a photosensitive body
20
. Therefore, the potential difference between the front and back faces of the dielectric belt
50
is lowered from 400V to 200V.
Then, the Cyan-color toners contained in the OPC drum
30
are transferred onto the sheet
100
in the Cyan-color toner transfer position. A voltage of 700V (direct current) is applied to a transfer roller
83
. At this time, the electric charges move to a photosensitive body
30
. Therefore, the potential difference between the front and back faces of the dielectric belt
50
is lowered from 200V to 0V.
Finally, the Black-color toners contained in the OPC drum
40
are transferred onto the sheet
100
in the Black-color toner transfer position. A voltage of 900V (direct current) is applied to a transfer roller
84
.
In the above sequential transfer process, the potential difference between the surfaces of the sheet
100
and the photosensitive bodies
10
to
40
is always maintained at 1200V to obtain an even transfer efficiency.
However, viewing from the characteristics of the dielectric belt, it is necessary to keep the charge carrying function to lower the transfer voltage until at least the four-color transfer process is completed, so the resistance value of the dielectric belt must be high and constant. Therefore, the dielectric belt needs to be selected in a limited and permissible range, so that there is a problem that it is difficult to lower the apparatus cost.
It is known that when the running (printing) operation is executed to some degree, the surface-resistance on the dielectric belt as well as the electric charge carrying ability of the dielectric belt are lowered by the adsorption of impurities such as toners, etc. For example,
FIG. 8
shows the result of measuring the electric potential fluctuations on the dielectric belt surface for the time (seconds) when a new dielectric belt (New Belt) before running and an old dielectric belt (Old Belt) after running during a specific time are charged to about 900V.
It is judged from this result that the electric charge carrying ability of the dielectric belt has been lowering. When the dielectric belt with the material characteristics in
FIG. 8
is mounted onto the apparatus, the electric potential of the dielectric belt located in the toner transfer position is set to approximately 900V before running, but is lowered to approximately 500V after running, supposing that the electric potential of the dielectric belt is set to approximately 900V and it takes two seconds for the dielectric belt to be carried from the charging roller
70
to the transfer position. When the electric potential of the dielectric belt is lowered, the effective electric potential difference between the sheet
100
and the OPC drums is also lowered, so there is a problem that the transfer efficiency is lowered, depending on the apparatus running time (operation time).
Additionally, the tandem type color electro-photographic process contains many components, viewing from its characteristics that four image forming process units are parallel-disposed therein. For example, a general tandem type color electro-photographic process contains four sets of photosensitive bodies, photosensitive body chargers (containing the power source), exposure units, developing units, photosensitive body cleaning blades, transfer units (containing the power source), etc., respectively. Therefore, there is a problem that the tandem type color electro-photographic process is produced at higher costs than other color electrophotographic processes.
To reduce the number of components, it is considered as an example that a transfer power source should be used commonly. However, as explained above, when the electric potential of the dielectric
Armstrong Westerman & Hattori, LLP
Fujitsu Limited
Grainger Quana M.
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