Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-06-02
2001-10-02
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S308000
Reexamination Certificate
active
06298212
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as a electrophotographic copier or laser printer, and relates more specifically to an improvement of an image forming apparatus for transferring to paper or other recording medium a toner image formed on an image carrier such as a photoconductive drum by way of an intervening intermediate transfer belt.
2. Description of Related Art
An image forming apparatus of the type related to the present invention typically has developing units for black (Bk), yellow (Y), magenta (M), and cyan (C) color components disposed around an image carrier such as a photoconductive drum, and a transfer drum disposed facing the image carrier. A recording medium such as paper is affixed to the transfer drum so that with each rotation of the image carrier a toner image of each color formed on the image carrier is transferred in sequence to the paper.
A problem with this type of image forming apparatus, however, is that because unfixed toner images of each color are directly transferred in layers to the paper, numerous factors, including the thickness and surface characteristics of the paper, and the properties of the system transporting the paper to the image carrier, easily affect the quality of the resulting color image formed on the paper.
An image forming apparatus using an intermediate transfer unit to resolve such problems is taught in, for example, Japanese Examined Patent Application Publication (kokoku) S49-209, and Japanese Unexamined Patent Application Publications (kokai) S62-206567 and H2-213879.
As taught in these applications, developing units for black (Bk) yellow (Y), magenta (M), and cyan (C) color components are disposed around an image carrier such as a photoconductive drum, and an intermediate transfer unit, typically having a belt shape and called an intermediate transfer belt, is disposed facing the image carrier. An unfixed toner of each color component formed on the image carrier at each rotation of the image carrier is first sequentially transferred to the intermediate transfer belt, producing a composite first bias transfer image of superimposed toner images on the intermediate transfer belt. This composite toner image is then transferred in a second bias transfer step to the paper recording medium to form the desired image on the paper.
Advantages of this type of image forming apparatus are that because a composite toner image of multiple superimposed toner images already transferred to the intermediate transfer belt is transferred to the recording medium in a single step, the above-noted unstable factors can be eliminated, and image alignment and color offset problems occurring when transferring multiple toner image layers can be effectively prevented.
A problem that is found with this type of image forming apparatus, however, is scattering of the toner image on the intermediate transfer belt after the first bias transfer step. This results in line image parts of the toner image blurring.
It should be noted that this technical problem occurring in the above-described type of image forming apparatus also occurs in so-called tandem image forming apparatuses as taught, for example, in Japanese Unexamined Patent Application Publication (kokai) H10-260593. This tandem image forming apparatus has plural parallel image formation units with an intermediate transfer belt moving circularly along the direction in which these image formation units are arranged. Images in each of the color components (typically black, cyan, magenta, and yellow) formed on each of the image formation units are sequentially transferred to the intermediate transfer belt (first bias transfer step), and the composite color image now on the intermediate transfer belt is then transferred to the paper recording medium (second bias transfer step, or batch transfer) to form a desired image on the paper.
SUMMARY OF THE INVENTION
With consideration for the above noted problems, the present invention provides an image forming apparatus capable of effectively preventing scattering of a toner image transferred to the intermediate transfer belt.
To overcome the above problems, an image forming apparatus according to a first aspect of the present invention has, as shown in
FIG. 1
, an image carrier
1
on which a toner image is formed according to image data and held; an intermediate transfer belt
3
disposed facing image carrier
1
and supported on so as to move circularly about plural rollers
2
with tension applied; a first bias transfer unit
4
for sequentially transferring a toner image T on image carrier
1
to intermediate transfer belt
3
; and a second bias transfer unit
6
for batch transferring the toner image T on intermediate transfer belt
3
to a recording medium
5
; and is characterized by a potential holding unit
8
disposed to a first contact member
7
first contacting the intermediate transfer belt
3
after it passes the first bias transfer unit
4
, and holding the surface potential of the first contact member
7
at or above the charge potential of the back of intermediate transfer belt
3
.
This first aspect of the invention shall not be limited to a configuration in which one image carrier
1
rotates plural times to accomplish the first bias transfer of a toner image T to the intermediate transfer belt
3
, and includes configurations having plural image carriers
1
. For example, four image carriers could be parallel disposed with each used for the first transfer of a toner image T formed thereon to the intermediate transfer belt
3
.
The image carrier
1
can also be a drum or a belt, and toner image T formation can be accomplished using an electrophotographic, electrostatic recording, or other technique as appropriate.
Furthermore, the intermediate transfer belt
3
can be variously comprised insofar as it supported in tension on plural rollers
2
, and has resistivity sufficient to electrostatically hold the toner image T.
Furthermore, insofar as the first bias transfer unit
4
is capable of forming a transfer field and transferring a toner image T formed on the image carrier
1
to the intermediate transfer belt
3
, it can be, for example, a contact transfer type using a transfer roller or similar unit, or a non-contact transfer type using a corotron or similar unit.
Furthermore, the first contact member
7
is the first member that the intermediate transfer belt
3
contacts after it passes the first bias transfer unit
4
, and is conceivably any of various unit other than a roller
2
for holding the intermediate transfer belt
3
in tension. It should be noted that in the configuration shown in
FIG. 1
, one of the plural rollers
2
is the first contact member
7
.
Furthermore, the potential holding unit
8
can be comprised in many ways insofar it is disposed to the first contact member
7
and has the ability to hold the surface potential of first contact member
7
at or above the charge potential of the back of the intermediate transfer belt
3
.
A specific example of potential holding unit
8
is to dispose the first contact member
7
ungrounded.
A further example of potential holding unit
8
is to use a resistance grounding unit for grounding the first contact member
7
through a high resistance resistor.
The resistance of this high resistance resistor must be sufficient to hold the surface potential of the first contact member
7
at or above the charge potential of the back of intermediate transfer belt
3
.
While this high resistance resistor can be disposed in various ways, from the perspective of the ease with which it can be achieved and the compactness of the image forming apparatus it is preferably a high resistance coating formed on the surface of first contact member
7
. Furthermore, this coating is preferably made from a PET (polyethylene terephthalate) resin or PFA (perfluoro alkoxy) resin. Furthermore, the potential holding unit
8
can be a biasing unit for applying a dc bias of the same polarity as the surface potential of the inter
Enomoto Yoshihiro
Hayashi Yukio
Kono Noriyoshi
Takahashi Nobukazu
Fuji 'Xerox Co., Ltd.
Grainger Quana M.
Morgan & Lewis & Bockius, LLP
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