Electrophotography – Control of electrophotography process – Having temperature or humidity detection
Reexamination Certificate
2002-10-07
2003-12-02
Chen, Sophia S. (Department: 2852)
Electrophotography
Control of electrophotography process
Having temperature or humidity detection
C399S066000
Reexamination Certificate
active
06658220
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile machine, and the like, that forms an image utilizing an electrophotography system.
2. Related Background Art
In
FIG. 9
, a structure of a so-called tandem type color image forming apparatus is schematically shown. The tandem type color image forming apparatus has a plurality of image forming portions that form toner images of different colors, respectively, where a color image is formed by sequentially superposing the individual color images on a transferring material. In the image forming portions Pa, Pb, Pc and Pd, toner images of respective colors, i.e., magenta, cyan, yellow and black, are formed in accordance with an electrophotography system.
In the respective image forming portions Pa, Pb, Pc and Pd, upon emission of a signal for starting an image forming operation, photosensitive drums
1
a
,
1
b
,
1
c
and
1
d
, which are driven to rotate at a specific process speed, are charged by charging rollers
2
a
,
2
b
,
2
c
and
2
d
, respectively. On the other hand, exposing apparatuses
6
a
,
6
b
,
6
c
and
6
d
convert input image signals that have been separated for respective colors into light signals. The photosensitive drums
1
a
,
1
b
,
1
c
and
1
d
that have been charged as above are exposed to laser beams modulated as light signals so as to be scanned, respectively, whereby electrostatic latent images are formed on the photosensitive drums
1
a
,
1
b
,
1
c
and
1
d.
The electrostatic latent image formed on the photosensitive drum
1
a
is first visualized (developed) as a toner image by applying yellow toner onto the electrostatic latent image using a developing apparatus
3
a.
In synchronization with this visualization (development), a transferring material P, such as a paper sheet, which is electrostatically absorbed to a surface of a transferring material conveying belt
9
, and which is moved by a driving roller
10
a
, is conveyed to a transferring portion located between the photosensitive drum
1
a
and a transferring blade
4
a
of the image forming portion Pa, whereby the yellow toner image on the photosensitive drum
1
a
is transferred onto the transferring material P with the aid of the transferring blade
4
a
, to which a transferring bias is applied. The transferring material conveying belt
9
is looped around the driving roller
10
a
and a driven roller
10
b.
Similarly, in the image forming portion Pb, a magenta toner image formed on the photosensitive drum
1
b
is transferred with the aid of a transferring blade
4
b
, to which a transferring bias is applied, onto the transferring material P in such a way that the magenta toner image is superposed on the yellow toner image on the transferring material P. Subsequently, cyan and black toner images formed on the photosensitive drums
1
c
and
1
d
are sequentially transferred with the aid of transferring blades
4
c
and
4
d
, to which a transferring bias is applied, onto the transferring material P in such a way that the cyan and black toner images are superposed on the yellow and magenta toner images that have been superposed on the transferring material P. Thus, a full color toner image is formed on the transferring material P.
The transferring material P, on which the full color toner image has been transferred, is conveyed to a fixing apparatus
13
. The fixing apparatus
13
applies heat and pressure to the transferring material at a fixing nip between a fixing roller
13
a
and a pressurizing roller
13
b
, so that the full color toner image is fixed on the transferring material P. After the fixing operation, the transferring material P is discharged from the apparatus. In the above transferring process, residual toner remaining on the photosensitive drums
1
a
,
1
b
,
1
c
and
1
d
is removed and collected by cleaning apparatuses
5
a
,
5
b
,
5
c
and
5
d.
Recently, a contact transferring member, such as a transferring blade or a transferring roller, has become widely used as transferring means of a contact transfer scheme, as is the case in the above-described example of the image forming apparatus. The contact transferring member is adopted in view of its advantageous stability in transferring images. A contact transfer system that uses a contact transferring member such as a transferring blade or a transferring roller has advantages in that a transferring voltage is reduced, as compared to a conventional corona transfer system, and the amount of ozone generated during the process is small.
As means for applying transferring bias to such a contact transferring member, a constant current system that applies a constant current is sometimes used, in order to cope with variations, such as resistance variations due to environmental variations, in the electrical resistance of the contact transferring member.
The constant current application system as described above suffers from the following problems.
In a portion of a photosensitive drum extending along the longitudinal direction of the drum that is located at a transferring portion at which a contact transferring member (i.e., transferring blade or transferring roller, etc.) abuts the photosensitive drum, a relatively large transferring current flows in a portion (or area) in which a toner image is absent; it is more difficult for current to flow in a portion in which a toner image is present due to the toner's resistance. Therefore, when an image in which an image ratio (or percentage of image) along the longitudinal direction of the photosensitive drum is high is to be formed, a high quality image that does not involve transferring errors would be obtained with a small transferring current; on the otherhand, when an image in which an image ratio along the longitudinal direction of the photosensitive drum is low is to be formed, a relatively large transferring current is required in order to obtain a high quality image.
In such an image forming apparatus, when the transferring current is set to a value optimum for images in which the image ratio along the longitudinal direction of the photosensitive drum is high, transferring errors would occur upon forming images in which the image ratio along the longitudinal direction of the photosensitive drum is low, due to a shortage of the transferring current. On the other hand, when the transferring current is set to a value optimum for images in which the image ratio along the longitudinal direction of the photosensitive drum is low, transferring errors such as inversion in the polarity of toner would occur upon forming images in which the image ratio along the longitudinal direction of the photosensitive drum is high due to an excessive current flow in the toner image.
In view of the above situations, a method has been conceived that controls the transferring current in accordance with the image ratio so as to avoid variations in transferring performance due to variations in the image ratio, as described in, for example Japanese Patent Application Laid-Open No. 08-083006.
However, in an image forming apparatus like the apparatus shown in
FIG. 9
, which forms a color image by superposing toner images of different colors, a preceding color toner image(s) already is present on the transferring material when a subsequent color image is transferred onto the transferring material, so transferring errors cannot be fully avoided by taking into account only the image ratio of a monochromatic (or single-color) toner images, like in the above-mentioned Japanese Patent Application Laid-Open No. 08-083006, and transferring errors can still occur.
In the following, the way in which such transferring errors occurs will be described, referring to
FIGS. 10A and 10B
.
As shown in
FIG. 10A
, in the image forming portion Pa a first color toner image (i.e., yellow toner image) denoted by t is formed on the photosensitive drum
1
a
at about the center with respect to the longitudinal direction of the drum
1
a
. Then, a
Bessho Yuji
Takayanagi Hiroki
Canon Kabushiki Kaisha
Chen Sophia S.
Fitzpatrick ,Cella, Harper & Scinto
Gleitz Ryan
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