Electrophotography – Image formation – Transfer
Reexamination Certificate
1999-09-02
2001-05-22
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S299000
Reexamination Certificate
active
06236828
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus, for example, a copying machine, a printer, a facsimile machine, or the like. In particular, it relates to an image forming apparatus which forms an image on a piece of transfer medium borne on a belt for conveying the transfer medium.
Generally, an image forming apparatus which employs an electrophotographic process comprises an image forming stations, a conveying means, and a fixing means. In the image forming station, a latent image is formed with the use of light, magnetism, and electrical charge, and the latent image is developed into a visible image. The conveying means conveys the transfer medium to the image forming station so that the visible image formed in the image forming station is transferred onto the transfer medium. The fixing means fixes the visible image onto the transfer medium after the image is transferred to the transfer medium.
The image forming station comprises a medium, for example, an electrophotographic photosensitive member, on which an image is formed. Such a medium varies in characteristics, for example, shape. The image forming station comprises a means for forming a latent image, and a means for developing a latent image. These means also vary in characteristics so that they match with an image forming means.
A substantial number of image forming apparatuses, in particular, full-color image forming apparatuses, employ an electrostatic conveying means, that is, a conveying means which uses electrostatic force to hold a transfer medium on its surface while conveying the transfer medium from the transfer means to the fixing means. This is due to the fact that an electrostatic conveying means is superior to other conveying means in many respects. A color image forming apparatus is an apparatus which forms a full-color image by laying in layers multiple images on a transfer medium with the use of multiple image forming stations.
One of the well known patent documents which disclose image forming apparatuses which employ the above described system or the like is Japanese Patent Laid-Open Application No. 13,976/1990.
FIG. 7
depicts, in general structure, one of the image forming apparatuses disclosed in that document.
The image forming apparatus in
FIG. 7
comprises three image forming stations: I, II, and III, a conveying means
139
, and a fixing means
56
. The conveying means
139
has a conveyer belt
126
for conveying transfer medium, and extends below the three image forming stations I, II, and III. The fixing means is equipped with thermal rollers
56
a
and
57
b
for fixing the images on the transfer medium to the transfer medium, and is disposed at the exit portion of the conveying means
139
. The image forming stations I, II, and III are equipped with photosensitive drums
111
,
112
, and
113
, charging devices
114
,
115
, and
116
, developing devices
117
,
118
, and
119
, charging devices
120
,
121
, and
122
for image transfer, and cleaners
123
,
124
, and
125
, correspondingly.
The conveyer belt
126
is formed of resin. In an image forming operation, its surface is electrically charged by a charging device
133
for adhesion, to adhere a transfer medium to the surface of the conveyer belt
126
, and charge it so that the transfer medium is reliably conveyed.
The conveyer belt
126
is supported by a driver roller
131
and a follower roller
134
, and is stretched between them with the application of a predetermined amount of tension. As the driver roller
131
is rotationally driven, the transfer belt
126
moves at a predetermined velocity.
After being released into the apparatus by a registration roller
49
, a transfer medium is pinched by the follower roller
134
of the transfer medium conveying means
139
, and a pressing roller
52
pressed upon the follower roller
134
through the conveyer belt
126
, and while the transfer medium
46
is passed between the two rollers, it is pressed upon the electrically charged conveyer belt
126
, so that the transfer medium
46
is electrostatically adhered to the conveyer belt
126
in a desirable manner, that is, without becoming wavy.
The rotational velocity of the registration roller
49
is set to be slightly higher than the velocity, or conveying velocity, of the conveyer belt
126
, to cause the transfer medium
46
to elastically bend slightly so that the transfer medium
46
is not affected by the rotational velocity of the registration roller
49
.
Under a high humidity-high temperature environment, the conveyer belt
126
is not sufficiently charged, which sometimes allows portions or the entirety of the transfer medium to behave as if “floating” on the surface of the transfer medium. If the floating of the transfer medium occurs, transfer failures may occur. For example, a portion or portions of an image may become misaligned, or the entirety of an image may become misaligned relative to the transfer medium. Further, a portion or portions of an image may drop out. In order to prevent such problems, an auxiliary roller
200
and an idler roller
203
are provided, which are enabled to freely rotate while pinching the conveyer belt
126
.
With the provision of the auxiliary roller
200
described above, the transfer medium is prevented from floating from the conveyer belt
126
when an image formed in the image forming station is transferred onto the transfer medium which is being moved forward by the conveyer belt
126
. In other words, it is assured that the transfer medium remains adhered to the conveyer belt
126
. Therefore, an excellent image, that is, an image which does not suffer from the above described partial misalignment, registration failure, or partial image drop-out, can be formed.
On the other hand, if the conveyer belt
126
meanders or deviates while running, an image transferred onto the transfer medium in one of the image forming stations may not align with an image from another image forming station (hereinafter, “registration misalignment”). In order to prevent this problem, an image forming apparatus is usually provided with one of several means for correcting the meandering or deviating. For example, an image forming apparatus may be provided with guiding ribs, which may be arranged along one edge, or both edges, of the conveyer belt
126
, along the entire length of the belt, or some of the aforementioned rollers which support and/or stretch the conveyer roller
126
may be provided with guiding grooves. Further, the rollers may be provided with “shoulder ribs” for guiding the conveyer belt
126
.
However, in the case of the above described conventional means for preventing the meandering or deviating of the conveyer belt, the driver roller
131
is positioned at the most downstream end, in terms of the transfer medium movement, of the range in which the conveyer belt conveys the transfer medium. Therefore, there are the following problems.
That is, one of the methods for making it difficult for the conveyer belt
126
to slip on the driver roller
131
is to increase the diameter of the driver roller
131
(standard method is to increase it to 20 mm or larger). However, if increasing the driver roller diameter becomes the priority, a problem occurs. More specifically, the larger the diameter of the driver roller
131
, the smaller the curvature of the conveyer belt
126
at the predetermined point at which the transfer medium
46
becomes separated from the conveyer belt
126
to be delivered to a fixing means
56
, and the smaller the curvature of the conveyer belt
126
at the separation point, the more difficult it is for the transfer medium
46
to be separated from the conveyer belt
126
by the curvature of the conveyer belt
126
and the resiliency of the transfer medium
46
. Thus, if the transfer medium
46
is weak in resiliency, or temperature and/or humidity are very high, the transfer medium
46
is liable to fail to separate from the conveyer belt
126
, and jam the apparatus.
On the contrary, if the driver roller diamet
Munenaka Katsumi
Oyama Kiyoshi
Canon Kabushiki Kaisha
Chen Sophia S.
Fitzpatrick ,Cella, Harper & Scinto
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