Electrophotography – Image formation – Fixing
Reexamination Certificate
2002-08-08
2004-09-14
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Fixing
C399S326000
Reexamination Certificate
active
06792236
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus, such as a printer, a facsimile machine or a copy machine, which is capable of forming a toner image on a recording medium, such as a paper sheet, by the electrophotography technique. More particularly, the invention relates to a fixing device for the image forming apparatus.
Generally, the image forming apparatus for forming a toner image on a recording medium by the electrophotography technique includes a photosensitive member to be driven to rotate, an exposure mechanism for forming an electrostatic latent image on the surface of the photosensitive member, a developing mechanism for developing the latent image into a toner image, a transfer mechanism for transferring the toner image onto a recording medium, and a fixing device for thermally fixing the toner image on the recording medium while allowing the recording medium having the toner image transferred by the transfer mechanism to pass therethrough.
FIG. 27
shows a fixing device of the belt fixing type (JP-A-8-334997).
In the fixing device, a fixing belt
1
extends around a fixing roller
2
driven to rotate and a heating roller
3
. An pressure roller
4
is pressed against the fixing roller
2
with the fixing belt
1
being interposed therebetween. A recording medium S on which a toner image T is formed is moved to pass through a pressure contact portion (fixing nip) N therebetween in the direction of an arrow in the figure, whereby the toner image T is fused and permanently affixed onto the recording medium S.
In the fixing device thus constructed, if a peripheral speed difference is present between two rotary members, a toner image on the recording medium passing through the press contact portion between the rotary members is blurred to disturb the image. For this reason, usually, such a method that the rotary members are both driven to rotate is not employed, and one of the rotary members is driven to rotate, while the other rotary member is rotated as a follower. That is, the fixing roller
2
is driven to rotate, whereas the fixing belt
1
follows the fixing roller
2
and the pressure roller
4
follows the fixing belt
1
. In case of another type of a fixing device in which a fixing belt is not used, one of a fixing roller and a pressure roller which cooperatively form a nip is driven to rotate, whereas the other of the fixing roller and the pressure roller follows the one.
The heating roller
3
includes guide rings
3
a
as restricting portions which come in contact with the side ends
1
b
of the fixing belt
1
to restrict lateral offset of the fixing belt
1
.
To prevent such a phenomenon that toner is transferred from the recording medium onto the surface of the fixing belt
1
(called offset phenomenon), the fixing device includes an oil application roller
5
for applying release oil, such as silicone oil, as release agent onto the surface of the fixing belt
1
.
A length (as viewed in the belt width) of the oil application roller
5
as a oil application mechanism is longer than the width of the fixing belt
1
, so that the oil is applied to the belt
1
over its entire width.
The oil application roller
5
is pressed against the fixing belt
1
at a position other than positions where the fixing belt
1
is wound on the rollers
2
and
3
. Therefore, it also functions as a tension applying mechanism for applying a tension to the fixing belt.
In the fixing device, an oil application width by the oil application roller
5
of the release agent application mechanism is longer than the width of the fixing belt
1
. Therefore, the oil that has been once applied to the top surface of the fixing belt
1
is easy to flow to the back surface
1
a
of the fixing belt
1
.
Generally, the oil application roller
5
includes a shaft and an oil holding layer, made of, for example, felt, provided around the shaft. The oil application roller
5
is pressed against a rotary member (here, fixing belt or endless belt
1
), so that oil is squeezed out of the oil holding layer and it is applied to the rotary member. Because of the structure, a pressing force to the rotary member by the oil application roller
5
at the ends of the roller is larger than that at the central portion. Therefore, the end portions of the roller are more greatly compressed than the central portion thereof, and hence the oil application amount at the end portions of the roller is larger than that at the central portion thereof.
The fixing device of the image forming apparatus is designed such that the width (length in the axial direction) of the rotary member is longer than the width (maximum passing width) of a recording medium of which the passing width is the largest of those media that may be supplied for the image formation, although such a design is a nature choice when considering its function.
For this reason, both ends of the rotary member (including the fixing belt
1
), which are not in contact with the recording medium having the maximum passing width, are coated with a relatively large amount of oil, which, however, is not absorbed by or transferred to the recording medium. As a result, a relatively large amount of oil is accumulated there. Where the amount of the accumulated oil is excessive, it creeps to the central portion of the rotary member. Because of the creeping oil, a slip will occur between the driving rotary member and the follower rotary member or between the rotary member and the recording medium. This possibly results in that the toner image on the recording medium is blurred to disturb the image.
In particular, in case where the recording medium is a sheet which does not absorb oil, such as a synthetic resin sheet, the slip is likely to occur.
Further, as described above, a relatively large amount of oil applied to both ends of the surface of the fixing belt
1
is easy to flow to the back side
1
a
of the fixing belt
1
.
The oil that has flowed to the back side
1
a
of the fixing belt
1
gradually moves to between the fixing belt
1
and the fixing roller
2
as its drive roller. When the amount of the oil moving thereto exceeds a predetermined level, a slip occurs between the fixing roller
2
and the fixing belt
1
, and as a result, the fixing operation will be instable.
Accordingly, an object of the present invention is to solve the above problems and to provide a fixing device which eliminates the above-mentioned slip, thereby providing a stable fixing operation.
In the publication, JP-A-8-334997, there is no description about the heating width (heating width as viewed in the width direction of the fixing belt
1
) by a heater contained in the heating roller
3
.
Oil impregnated in the oil holding layer stays within the layer at normal temperature since its viscosity is high, and never leaks out of the layer. In a fixing operation, the belt is heated, and the viscosity of the oil decreases while it is thermally expanded, so that it leaks out of the layer and flows to the belt.
Accordingly, if in the fixing device of
FIG. 27
, the heating width by the heater of a heating mechanism is longer than the oil application width by the oil application roller
5
of the oil application mechanism (equal to the width of the fixing belt
1
in FIG.
27
), the oil application roller
5
is heated over its entire oil application width (the full width of the fixing belt
1
in FIG.
27
), the oil is applied to the fixing belt
1
over its entire width.
If the oil is applied to the fixing belt
1
over the entire oil application width, the oil applied to the end portions of the surface of the fixing belt
1
will flow to the back side
1
a
of the fixing belt
1
.
The oil flowing to the back side
1
a
of the fixing belt
1
gradually moves to between the fixing belt
1
and the fixing roller
2
as its drive roller. When the amount of the oil exceeds a predetermined level, a slip will occur between the fixing roller
2
and the fixing belt
1
because of presence of the oil. As a result, there is a possibility that a stable fixing
Fujisawa Kazutoshi
Ishiwatari Tahei
Okumura Naoyuki
Tanaka Hiroshi
Yoda Kaneo
Grainger Quana M.
Seiko Epson Corporation
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