Electrophotography – Document handling – Copy
Reexamination Certificate
1999-12-03
2002-07-02
Grainger, Quana M. (Department: 2852)
Electrophotography
Document handling
Copy
C271S188000, C399S406000
Reexamination Certificate
active
06415130
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a both-face (i.e. duplex) image forming apparatus, such as a copier, a printer or a combination of the two, in which a front surface and a back surface of the sheet are reversed, and images are formed on both surfaces of the sheet. As used herein, duplex and both-face are used interchangeably.
2. Related Background Art
A conventional example of a both-face image forming apparatus is a copier.
The image forming processing performed by a copier
17
will be described while referring to
FIG. 7. A
rotatably supported image bearing member (hereinafter referred to as a photosensitive drum)
1
is rotated in the direction indicated by an arrow, while the surface is uniformly charged by a primary charger
2
. Then, an image information exposure
3
is performed for the photosensitive drum
1
, and an electrostatic latent image is formed on its surface. Thereafter, a developing device
4
performs a visualization process for the electrostatic latent image and produces a toner image.
Synchronized with the rotation of the photosensitive drum
1
, a sheet P, which is a recording medium, is fed to the photosensitive drum
1
by registration rollers
11
, and the toner image on the photosensitive drum
1
is transferred to the sheet P by a transfer charger
5
. Then, the sheet P is separated from the photosensitive drum
1
by a separation charger
6
. Following this, the sheet P is conveyed by a conveying unit
7
to a fixing device
8
, whereat the toner image is fixed to the sheet P.
After the toner image has been transferred, the surface of the photosensitive drum
1
is cleaned by a cleaner
9
, and the potential held by the photosensitive drum
1
is eliminated by a pre-exposure lamp
10
. In this fashion, the photosensitive drum
1
is again prepared for the forming of another image.
The photosensitive drum
1
, the primary charger
2
, the image information exposure
3
, the developing device
4
, the transfer charger
5
and the separation charger
6
, together constitute a so-called Carlson process type image forming means
16
for the copier
17
, which is used to transfer a toner image from the photosensitive drum
1
to a plain paper sheet P.
For the image forming means
16
, the transfer charger
5
applies, from the side opposite to the transfer surface of the sheet P contacting the photosensitive drum
1
, an electrical field that has a polarity opposite to that of the charge polarity held by the toner, and thus induces the transfer of the toner image from the photosensitive drum
1
to the sheet P. Immediately after the transfer of the toner image to the sheet P by the transfer charger
5
, the separation charger
6
separates the sheet P from the photosensitive drum
1
. For this separation process, to eliminate the electrical charge held by the sheet P and to eliminate the attractive force existing between the sheet P and the photosensitive drum
1
, the separation charger
6
applies to the sheet P an AC discharge or a DC discharge which has the same polarity as the toner. Then, the rigidity of the sheet P or its weight is employed to separate the sheet P from the photosensitive drum
1
.
For a smooth separation, it is preferable that the sheet P be curled, for example, in an effective separation direction.
The effective separation direction is the direction in which, as is specifically shown in
FIG. 10A
, the leading end of the sheet P progresses while directed toward the separation charger
6
(away from the photosensitive drum
1
) after passing through the photosensitive drum
1
. The ineffective separation direction is the direction in which, as is specifically shown in
FIG. 10B
, reversely, the sheet P progresses while being curled around the photosensitive drum
1
and adheres to the surface of the photosensitive drum
1
.
Next, the sheet conveying operation of a conventional copier
17
will be described while referring to FIG.
8
.
Individual sheets P are shown in
FIG. 8
so that it can be easily understood in which direction a sheet is curled when it is fed along the sheet conveying path.
Sheets P, stacked in a sheet feed cassette
14
e
and in sheet feed decks
14
a
,
14
b
,
14
c
and
14
d
, are fed to the registration rollers
11
through sheet feeding rollers
13
, positioned on sheet feeding paths
50
and
51
. Before a sheet P is fed to the registration rollers
11
, in order to facilitate its separation from the photosensitive drum
1
it is curled a predetermined curling distance in the effective separation direction by an initial curling unit
12
or
20
. By the aid of the curl, the sheet P will smoothly separate from the photosensitive drum
1
. The initial curling units
12
and
20
individually comprise curling roller sets
12
a
and
12
b
, and
20
a
and
20
b
, each set of which is respectively composed of rollers having a small diameter and a large diameter.
As concerns the forming of images on both surfaces of a sheet P, a sheet P, to one surface (also called a first surface for this invention) of which an image has been fixed by the fixing device
8
, is conveyed to a switchback type surface reverse portion (surface reversing means) by a flapper
15
. The sheet P is routed by the surface reversing means so that it is reversed, and an image is formed on its other surface (also called a second surface for this invention). Thereafter, the sheet P is discharged from the copier
17
.
The sheet P can be a sheet of plain paper, a sheet of thin resin that is substituted for plain paper, a post card, a sheet of cardboard, an envelope, or a sheet of thin plastic.
When a conventional copier
17
is employed to form images on second surfaces, however, the sheets P do not separate smoothly from the photosensitive drum
1
, and there are frequent paper jams. Such jamming occurs because the sheets P are curled in the ineffective separation direction before images are formed on their second surfaces.
Assume that the measurement of the curling distance applied by the initial curing unit
12
or
20
is A (mm), and the measurement of the curling distance applied by a first R portion
18
a
and a second R portion
18
b
of the surface reverse portion
18
is B (mm).
It is expected that a sheet P will not be curled by components other than the initial curing unit
12
or
20
until it reaches the surface reverse portion
18
, and that while passing through the surface reverse portion
18
, the sheet P will be cooled and curled. However, while the sheet P will be curled by the first R portion
18
a
, it will seldom be curled by the second R portion
18
b.
The measurement of the curling distance in this invention indicates the degree to which a sheet if curled, and is a value obtained by performing a measurement such as is shown in
FIGS. 9A and 9B
. Specifically, as is shown in
FIG. 9A
, the curled sheet P is suspended, its curled edge Pa held horizontally, and the shape formed by the curled sheet is written on a horizontal plate
19
, as is shown in FIG.
9
B. Then the distance L, between a line D connecting the upstream end Pb and the downstream end Pc in the sheet conveying direction and the surface of the curled portion farthest from the line D, is measured, and the obtained value is used as the curling distance.
The curling distance for the first surface is the curling distance A (mm) in the effective separation direction. The curling applied to the first surface acts with the curling applied by the first R portion
18
a
to the second surface in the ineffective separation direction, and only the initial curling unit
20
acts in the effective separation direction. Thus, the curling distance is −A−B+A=−B (mm).
As is described above, with a conventional copier, while the first surface of a sheet can be satisfactorily separated from the photosensitive drum
1
, the direction of the curling of the first surface applied by the initial curling unit
12
or
20
is opposite to the direction of the curling applied to the second surface by
Fujiwara Motohiro
Katsumi Toru
Suzuki Shinya
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