Electrophotography – Image formation – Transfer
Reexamination Certificate
2002-01-10
2002-07-09
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S227000
Reexamination Certificate
active
06418296
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a color image forming apparatus used in color printers, color copying machines or color facsimiles. More specifically, the present invention relates to a color image forming apparatus that forms a color toner image by overlapping several toner images of various colors on an intermediate transfer device in a primary transfer from several photosensitive drums, and transcribing the color toner image in a secondary transfer to a transfer material (recording paper).
BACKGROUND OF THE INVENTION
FIG. 11
shows the internal structure of a prior art example of a color image forming apparatus, as disclosed in Publication of Unexamined Patent Application (Tokkai) No. Hei 7-36246.
The printer comprises an intermediate transfer belt unit
101
including an intermediate transfer belt
102
, a primary transfer roller
103
, a secondary transfer roller
104
, a cleaner roller
105
, and a waste toner reservoir
106
. Composition or superposition of color toner images is performed on the transfer belt
102
. A group of image forming units
108
is made up of four image forming units
107
Bk,
107
Y,
107
M and
107
C, each unit being of sector shape in cross section. As can be seen in
FIG. 11
, the image forming units are arranged circularly in the middle of the printer.
When an image forming unit
107
Bk,
107
Y,
107
M or
107
C is set properly in the printer, mechanical and electrical connection systems are established between one of the image forming units
107
Bk,
107
Y,
107
M and
107
C and the machine body side via mutual coupling members. The image forming units
107
Bk,
107
Y,
107
M and
107
C are supported by a supporter, which is rotationally driven by a motor via a cylindrical shaft
109
. Each image forming unit
107
Bk,
107
Y,
107
M, and
107
C is successively moved by rotation to an image forming position
110
. The image forming position
110
is the position where a photosensitive drum
118
of the image forming unit faces the intermediate transfer belt
102
on the primary transfer roller
103
, and is also the exposure position for exposure by a laser beam
111
.
A laser exposing device
112
is provided in the lower part of the printer. The laser signal beam
111
from the laser exposing device
112
passes through an opening
113
between the image forming units
107
M and
107
C, and through an opening provided in the cylindrical shaft
109
, and enters a mirror
114
. This mirror
114
is positioned inside the shaft
109
and fixed directly to the machine body. The reflected laser beam
111
enters the image forming unit
107
Bk located at the image forming position
110
through an opening
115
, and passes through the space between a developing device
116
and a cleaner
117
of the image forming unit
107
Bk, and enters an exposure portion of the photosensitive drum
118
. The laser signal beam
111
is scanned by the exposing device in the direction of the axis of the photosensitive drum
118
. The toner image, which is formed on the photosensitive drum
118
by exposure with the laser signal beam
111
and subsequent development with the developing device
116
, is transferred to the intermediate transfer belt
102
.
Then, the group of image forming units
108
rotates by 90 degrees, so that the yellow image forming unit
107
Y moves to the image forming position
110
to replace the black image forming unit
107
Bk. An operation similar to the operation explained above for the black image is performed to form a yellow image overlaying the black image formed on the intermediate transfer belt
102
. Subsequently, the magenta and cyan image forming units
107
M,
107
C are moved to the image forming position
110
, and similar operations as explained above are repeated to compose a full color image on the intermediate transfer belt
102
. This full color image is further transferred from the intermediate transfer belt
102
onto a recording paper using a secondary transfer roller
119
, and the image on the paper is fixed by a fixing device
120
.
In the color image forming apparatus as explained above, precise registration of the toner images of all four colors is very important for obtaining a high quality full color image. However, an image forming unit system of the prior art as explained above, which forms a color image by overlaying successively four toner images of four photosensitive drums at one image forming position onto an intermediate transfer device (belt) and forms a color image on the recording paper by a secondary transfer from the intermediate transfer device, has the following disadvantage: After the start-up (power on), variations of the time until a rotation of the intermediate transfer device is in a stable condition (start-up time) and loss of the driving system can occur easily. Therefore, the rotational position of the intermediate transfer device at a predetermined time after the start-up is not necessarily a predetermined position. Consequently, it is necessary to ensure that sufficient time has passed so that the rotation of the intermediate transfer device is in a stable condition in order for a high quality image to be provided.
On the other hand, there is the strong desire to accelerate the recording operation. The time spent until the four image forming units comprising a photosensitive drum have been switched, and the rotation of the intermediate transfer device has been stabilized, is reflected in the time that the color image forming device needs for forming an image.
Moreover, the circumference of the intermediate transfer belt has an influence on the size of the entire device. Therefore, to make the circumference of the intermediate transfer belt as small as possible is desirable in order to miniaturize the device.
A main object of the present invention is to solve the problems mentioned above by providing a color image forming apparatus combining improvement of the image quality and the image forming speed with miniaturization of the entire device.
SUMMARY OF THE INVENTION
A color image forming apparatus according to the present invention comprises: a plurality of image forming units corresponding to various colors, the image forming units including a photosensitive drum, a charging device and a developing device; a unit retaining member for retaining the image forming units and moving the image forming units between an image forming position and a waiting position; an exposing device for exposing the photosensitive drum of the image forming units when located at the image forming position; an intermediate transfer belt for successive transfer and superposition of toner images of various colors from the photosensitive drum of the image forming units located at the image forming position so as to form a color toner image; means for driving the photosensitive drum and the intermediate transfer belt; a detector for detecting a reference position of the intermediate transfer belt when the intermediate transfer belt is driven, and outputting a reference position detection signal; a secondary transfer device for transferring the color toner image on the intermediate transfer belt onto paper; and a controller for controlling the operation of the above structural elements. The controller determines an operation start time of the charging device and the developing device using a rotation command signal to the driving means as a reference, and an operation start time of the exposing device, the intermediate transfer belt and the secondary transfer device using the reference position detection signal as a reference.
Usually, the start and the stop of the photosensitive drum and the intermediate transfer belt is performed for each color. In that case however, variations of the time until a rotation of the intermediate transfer device is in a stable condition (start-up time) and loss of the driving system can occur easily. In the configuration according to the present invention however, the detector detects a reference position of the intermediate transfer belt after the driving
Aizawa Masahiro
Asakura Kenji
Katakabe Noboru
Yoshikawa Masanori
Chen Sophia S.
Merchant & Gould P,C,
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