Color image forming apparatus

Electrophotography – Image formation – Photoconductive member

Reexamination Certificate

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Details

C399S299000, C399S301000, C399S302000

Reexamination Certificate

active

06625411

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a color image forming apparatus such as a color printer, a color copier, or a color facsimile using electrophotography for forming a color image. More particularly, the invention relates to a color image forming apparatus in which a plurality of toner images respectively formed by a plurality of single-color toner image formers are sequentially transferred on a transferring member or a recording medium held by the transferring member.
In general, an image forming apparatus and in particular, a tandem image forming apparatus using electrophotography has a photoconductor as an image supporting member having a photosensitive layer on the outer peripheral surface, a charger for uniformly charging the outer peripheral surface of the photoconductor, an exposing member for selectively exposing the outer peripheral surface charged uniformly by the charger for forming an electrostatic latent image thereon, and a developer for providing toner as a developing powder to the electrostatic latent image formed by the exposing member for providing a visible image (toner image).
Two types of tandem image forming apparatuses for forming a color image are known. In one type, a plurality of (for example, four) image supporting members are brought into contact with an intermediate transferring member such as a transferring belt or the like so that toner images on the image supporting members are sequentially transferred to the intermediate transferring member so as to superpose the toner images of a plurality of colors (for example, yellow, cyan, magenta, and black) thereon to provide a color image. In the other type, a recording medium, such as paper, is held and transported on a belt member, a drum member or the like, and toner images on a plurality of image supporting members are sequentially transferred to the recording medium so as to superpose the multiple-color toner images thereon to provide a color image.
In order to realize a good transfer state of toner images (therefore realizing a fine image) in the color image forming apparatus as described above, it is desirable that the circumferential velocity of the image supporting member and that of the transferring member be completely matched with each other. More practically, however, manufacturing errors and varying tolerances are usually introduced into the image supporting member, the transferring member, or the parts making up their driver units at the manufacturing stage. Thus, it is practically impossible to completely match the circumferential velocity of the image supporting member and that of the transferring member.
If the difference between the circumferential velocity of the image supporting member and that of the transferring member varies, for example, if the circumferential velocity of the image supporting member is higher than that of the transferring member at one point in time and the former becomes lower than the latter at another point in time, the transfer state becomes remarkably unstable and a fine image cannot be provided. Particularly, to superpose multiple-color toner images on each other for providing a color image, a color-to-color shift occurs and the image quality is remarkably degraded.
To prevent color shift from occurring, there is proposed a color image forming apparatus wherein a velocity of the transferring member Vt is faster than the circumferential velocity of each image supporting member Vd (Vt>Td) in Japanese Patent Publication No. 11-65222A.
As shown in
FIG. 5
, this color image forming apparatus comprises a conveyor belt
3
placed on a drive roller
1
and a driven roller
2
. The conveyor belt
3
is circulated by the drive roller
1
and the driven roller
2
, and holds and transports a recording medium P such as paper. Four single-color toner image formers
5
, each having an image supporting member
4
, are brought into contact with the conveyor belt
3
and sequentially transfer toner images on the plurality of image supporting members
4
to the recording medium P held on the conveyor belt
3
.
Here, the velocity of the conveyor belt
3
is faster than the circumferential velocity of each of the image supporting members
4
, that is, Vt>Vd.
According to the above configuration, each image supporting member
4
acts on the conveyor belt
3
as a brake. Thus, the tensed state of the conveyor belt
3
becomes stable between the drive roller
1
and the image supporting member
4
adjacent to the drive roller
1
, but the image supporting members
4
are set to the same circumferential velocity Vd and the tensed state of the conveyor belt
3
becomes unstable between the image supporting members
4
.
Therefore, the state of the transferring point from each image supporting member
4
to the recording medium P held on the conveyor belt
3
also becomes unstable and consequently, a good color image is not always provided.
Further, this publication does not describe any drive structures of the image supporting members and the conveyor belt as the transferring member.
Another way of trying to prevent color shift is disclosed in Japanese Patent Publication No. 4-324881A. This publication discloses an image forming apparatus wherein the velocity of an image supporting member is always higher than that of transferring member.
In
FIG. 6
, numeral
12
denotes an intermediate transferring drum, and four different-color photoconductive drums
11
(Y, M, C, and K) are brought into contact with the intermediate transferring drum
12
. A yellow toner image provided by the photoconductive drum
11
Y, a magenta toner image provided by the photoconductive drum
11
M, a cyan toner image provided by the photoconductive drum
11
C, and a black toner image provided by the photoconductive drum
11
K are sequentially transferred to the intermediate transferring drum
12
to form a full-color toner image, which is then transferred to a recording medium P.
The intermediate transferring drum
12
is driven by a driver unit
14
and the photoconductive drums
11
(Y, M, C, and K) are driven by driver units
13
(Y, M, C, and K). Each of the driver units
13
(Y, M, C, and K) is provided with a velocity converter
15
(Y, M, C, and K) composed of a motor and gears.
In the related art apparatus shown in
FIG. 6
, since the driver units of the image supporting members and the driver unit of the transferring member are driven by separate drive sources to set the velocity V (Y, M, C, K) of the image supporting member (photoconductive drum
11
) higher than the velocity V
1
of the transferring member (intermediate transferring drum
12
), the driving structure becomes very complicated and the apparatus size is also increased.
Japanese Patent No. 2686267 discloses an image forming apparatus having a mechanism for transmitting a driving force from an image supporting member to a transferring roller so that the circumferential velocity of the transferring roller becomes higher than that of the image supporting member, but this document teaches an apparatus for forming a single-color image and does not consider any color shift problem involved in forming a full-color image.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a color image forming apparatus which stabilizes the state of transferred toner images from respective image supporting members to a transferring member at a transferring point for providing a fine image with no color shift using a simple driving mechanism.
In order to achieve the above object, according to the present invention, there is provided a color image forming apparatus, comprising:
a drive roller and a driven roller;
a looped belt member, stretched and circulated by at least the drive roller and the driven roller so as to have a tensed side and a slack side; and
a plurality of image supporting members, each supporting a single color toner image thereon, and abutting onto the tensed side of the belt member to define a transferring position at which the toner image is transferred onto either the belt member

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