Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-07-09
2002-09-10
Lee, Susan S. Y. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S308000, C399S310000
Reexamination Certificate
active
06449453
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a transfer device for transferring a visible image such as a toner image from a visible image carrying belt such as a photosensitive member or an intermediate transfer belt onto a transfer member such as a transfer sheet, and to an image forming apparatus such as a facsimile, a printer or a copying machine provided with the transfer device.
BACKGROUND OF THE INVENTION
FIG. 4
illustrates a conventionally known transfer device of this type. In
FIG. 4
, a transfer device
100
comprises an intermediate transfer belt
101
serving as a visible image carrying belt, a secondary transfer bias roller
102
serving as a transfer current applying member, a secondary transfer bias power source, not illustrated, for applying a secondary transfer bias to the secondary transfer bias roller
102
, a secondary transfer facing roller
103
, a drive roller
104
, a tension roller
105
, a primary transfer bias roller
106
and a primary transfer grounding roller
107
.
The intermediate transfer belt
101
is driven to be rotated in a direction indicated by an arrow B via the drive roller
104
while being stretched across the secondary transfer facing roller
103
, the drive roller
104
, the tension roller
105
, the primary transfer bias roller
106
and the primary transfer grounding roller
107
. A portion of the intermediate transfer belt
101
, positioned between the primary transfer bias roller
106
and the primary transfer grounding roller
107
is urged toward a photosensitive drum
1
in an image forming apparatus via the rollers
106
and
107
, to be positively brought into contact with the photosensitive drum
1
, and thus, it forms a primary transfer position.
At the primary transfer position, a primary transfer current is applied to the intermediate transfer belt
101
from the primary transfer bias roller
106
, thereby forming a primary transfer electric field therebetween. Most of the primary transfer current applied to the intermediate transfer belt
101
is introduced to a ground via the primary transfer grounding roller
107
.
The intermediate transfer belt
101
is sandwiched between the secondary transfer bias roller
102
and the secondary transfer facing roller
103
, thereby forming a secondary transfer nip as a transfer position. At the secondary transfer nip, a secondary transfer electric field is formed by a secondary transfer bias which has a polarity opposite to that of a toner and is applied from the secondary transfer bias roller
102
to the intermediate transfer belt
101
.
The drive roller
104
and the tension roller
105
are brought into contact with the reverse of the intermediate transfer belt
101
downstream and upstream of the secondary transfer nip (hereinafter referred simply to as a nip downstream side and a nip upstream side, respectively), respectively, and thus, introduce a residual electric charge on the intermediate transfer belt
101
to grounds.
A toner image formed on the photosensitive drum
1
is primarily transferred onto the intermediate transfer belt
101
by the effect of the primary transfer electric field or the like when the intermediate transfer belt
101
passes through the primary transfer position as it is driven to be rotated. The intermediate transfer belt
101
having the primarily transferred toner image formed thereon intrudes in the secondary transfer nip as the intermediate transfer belt
101
is transported.
In the meantime, a sheet feeder, not shown, in the image forming apparatus feeds a transfer sheet
10
toward the secondary transfer nip at such a timing that the toner image is superimposed on the transfer sheet
10
. The toner image superimposed on the transfer sheet
10
at the secondary transfer nip is secondarily transferred from the intermediate transfer belt
101
onto the transfer sheet
10
by the effect of a nip inner pressure, the secondary transfer electric field or the like.
In the transfer device
100
such configured as described above, in the case where the secondary transfer bias power source is of a type for applying the secondary transfer bias of a predetermined voltage to the secondary transfer bias roller
102
, the secondary transfer current flowing from the secondary transfer bias roller
102
to the intermediate transfer belt
101
is undesirably varied when the electric resistance of the intermediate transfer belt
101
is varied according to environmental fluctuations. In this case, if the secondary transfer current is varied, secondary transfer performance becomes unstable, so that a secondary transfer image of a stable quality cannot be obtained.
Therefore, there are generally used secondary transfer bias power sources of a type for making constant the secondary transfer current from the secondary transfer bias roller
102
to the intermediate transfer belt
101
by a constant current control or the like. Such a secondary transfer bias power source can apply a constant secondary transfer current to the intermediate transfer belt
101
irrespective of the fluctuations in electric resistance of the intermediate transfer belt
101
, so as to stabilize the secondary transfer performance.
However, in the transfer device
100
illustrated in
FIG. 4
, even if the secondary transfer performance is stabilized by applying the constant secondary transfer current to the intermediate transfer belt
101
, the edge of the secondary transfer image secondarily transferred onto the transfer sheet
10
may often become unclear depending upon the environment of temperature or humidity.
The present inventor has earnestly studied on causes which make the edge of the secondary transfer image unclear. As a result, he or she has found a phenomenon as follows: the secondary transfer current flowing from the secondary transfer bias roller
102
to the intermediate transfer belt
101
is divided into a downstream current A
1
which is transmitted downstream of the nip inside the belt so as to flow in the drive roller
104
, a nip current A
2
which is transmitted in a belt thickness direction inside the nip so as to flow in the secondary transfer facing roller
103
, and an upstream current A
3
which is transmitted upstream of the nip inside the belt so as to flow in the tension roller
105
, as shown in FIG.
5
. It has been found that among these currents, the upstream current A
3
disperses some of innumerous toners forming the toner image on the intermediate transfer belt
101
before intruding into the secondary transfer nip from an image portion to the surroundings of a non-image portion, thereby generating toner dispersion.
A downstream resistance R
1
, a nip resistance R
2
and an upstream resistance R
3
act on the downstream current A
1
, the nip current A
2
and the upstream current A
3
, respectively. As illustrated in
FIG. 6
, the downstream resistance R
1
is equivalent to the sum of an electric resistance over a length L
1
from the secondary transfer nip of the intermediate transfer belt
101
to a contact position with the drive roller
104
and an electric resistance of the drive roller
104
; the nip resistance R
2
is equivalent to the sum of an electric resistance of the intermediate transfer belt
101
in a thickness direction and an electric resistance of the secondary transfer facing roller
103
; and the upstream resistance R
3
is equivalent to the sum of an electric resistance over a length L
2
from the secondary transfer nip of the intermediate transfer belt
101
to a contact position with the tension roller
105
and an electric resistance of the tension roller
105
.
The reason why unclearness of the edge is found or not depending upon the environment is as follows. Namely, variations in electric resistance according to the environmental fluctuations are generated in not only the intermediate transfer belt but also the secondary transfer facing roller
103
, the drive roller
104
or the tension roller
105
. If at least one of these rollers is varied in electric resistance at a varying rate different from those of the oth
Lee Susan S. Y.
Ricoh & Company, Ltd.
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