Electrophotography – Image formation – Transfer
Reexamination Certificate
1999-09-01
2001-05-01
Beatty, Robert (Department: 2852)
Electrophotography
Image formation
Transfer
Reexamination Certificate
active
06226487
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transfer and fixing device, an image forming apparatus using the transfer and fixing device and an image forming method using the image forming apparatus which are used for a printer or a copying machine using an electrophotographic method.
2. Description of the Prior Art
Conventionally, image forming apparatuses which are capable of forming an electrostatic latent image on a photosensitive body, and developing the image using a dry toner and thereafter electrostatically transferring and fixing a toner image on a recording medium have been popularly used. In such image forming apparatuses, because of irregularities of the surface of a paper which constitutes a recording medium, the paper and the photosensitive body do not completely come into contact with each other and an uneven or irregular gap is formed between the paper and the photosensitive body and hence, a transfer electric field is disturbed or Coulomb repulsion occurs between toners thus disturbing the image.
To cope with such a problem, there have been proposed an image forming apparatus which is capable of forming a multiple toner image of the multicolor by eletrostatically overlapping and transferring plurality of toner images to an intermediate transfer body of an endless belt shape, and fusing the multiple toner image on the intermediate transfer body and thereafter simultaneously transferring and fixing the fused multiple toner image to a recording medium so as to obtain a color copy, and an image forming apparatus which is capable of fusing a multiple toner image of the multicolor which is formed on a photosensitive body having an endless belt shape and thereafter simultaneously transferring and fixing the fused multiple toner image to a recording medium so as to obtain a color copy. These color image forming apparatuses which are capable of transferring and fixing simultaneously have an advantage that the transfer of the toner image to the recording medium is performed non-electrostatically and hence, the previously mentioned degrading of image caused by the electrostatic image transfer hardly occurs.
As an improvement of this image forming apparatus, Japanese Patent laid-open No. 19642/1993, Japanese Patent laid-open No. 107950/1993 and Japanese Patent laid-open No. 249798/1993 and the like disclose a method where for further improving the transferability of a toner image formed on the surface of an intermediate transfer body having an endless belt shape or a photosensitive body to a recording medium, the intermediate transfer body or the photosensitive body carrying the toner image and a paper are heated or pressed under a condition that they are closely brought into contact with each other, and thereafter, the toner image is cooled and solidified while holding the intermediate transfer body or the photosensitive body and the recording medium in an overlapped manner, and then, the recording medium to which the toner image is transferred and fixed is peeled off from the intermediate transfer body or the photosensitive body.
In the image forming apparatus adopting the above method, when the cohesion between toners becomes greater than the adhering force between the toner and the medium, the toner image is peeled off from the intermediate transfer body or the photosensitive body and hence, the occurrence of a so-called offset which remains a part of the toner image on the intermediate transfer body or the photosensitive body can be prevented. Accordingly, it becomes possible for the image forming apparatus to require no oil and the transferring efficiency of the toner is enhanced thus achieving a favorable color balance of the image. Still furthermore, by using the intermediate transfer body or the photosensitive body having a smooth surface and solidifying the toner image by way of such a smooth surface, a high quality image which is excellent in luster and transparency of the toner and thus exhibits a high-grade feeling can be obtained.
In the image forming apparatus adopting the above method, however, a large number of wrinkles or creases are formed on the recording medium parallel to a recording medium transferring direction after heating and pressing operations by the transfer and fixing device and it gives rise to a phenomenon that the toner image is partially peeled off from the intermediate transfer body or the photosensitive body before the toner image is cooled and solidified. When this phenomenon occurs, in addition to the occurrence of wrinkles on the recording medium, the image quality is remarkably degraded.
FIG.
9
(
a
) is a plane view showing the condition where the wrinkles are generated on the recording medium after heating and pressing using a conventional transfer and fixing device and FIG.
9
(
b
) is a cross-sectional view taken along a line B—B of FIG.
9
(
a
).
As shown in FIG.
9
(
a
), a large number of wrinkles
9
may occur in parallel to the recording medium conveying direction A on the recording medium P after heating and pressing using the conventional transferring and fixing device. When the wrinkles
9
occur, as shown in FIG.
9
(
b
), the recording medium P is peeled off from the intermediate transfer body
1
before the toner image t is cooled and solidified and the toner image t is partially solidified under a condition that a part of the toner image t is not closely brought into contact with the intermediate transfer body
1
. Accordingly, the surface of the solidified toner image t is formed in an irregular condition where low luster portions C and high luster portions D are present in a mixed form thus the glossiness of the image is lowered and the image quality is drastically degraded.
The wrinkles which are parallel to the recording medium conveying direction A also occur in the conventional fixing device which adopts a roller method. In such a fixing device, as disclosed in Japanese Patent laid-open No. 146806/1996, for example, a nip width at both roller axial direction end portions of a nip region where a pair of fixing rollers press each other is set to be wider than the nip width at the roller axial central portion and hence, the occurrence of wrinkles can be restricted.
FIG. 10
is a schematic view showing the nip region defined by a pair of rollers having resilient body layers respectively.
As shown in
FIG. 10
, in the nip region N where a heating roller
2
and a pressure roller
3
which are mounted in the fixing device press each other, a resilient layer
3
a
which is formed on the surface of the pressure roller
3
is subjected to a compressive deformation. When a pair of rollers
2
,
3
are rotated in a C direction and a D direction respectively under this condition, to maintain the steady state, a resilient body having the same weight must pass through a portion A (a portion hatched in a rightward and upward direction) which is subjected to the compressive deformation and a portion B (a portion hatched in a leftward and downward direction) which is not subjected to the compressive deformation. Accordingly, the circumferential speed of the portion A where the cross-sectional area thereof is reduced due to the compressive deformation is greater than the circumferential speed of the portion B. Especially, at the portion A, as the part thereof comes closer to the surface of the layer, an amount of increase of the circumferential speed becomes greater. The amount of increase of the circumfereatial speed depends on the magnitude of the compressive deformation and becomes greater as an amount of deformation is increased.
FIG. 11
is a schematic view showing the principle for preventing wrinkles in a conventional fixing device.
As shown in
FIG. 11
, in a conventional fixing device which adopts a roller method, usually, a heating roller
2
and a pressure roller
3
make their respective roller axial end portions thereof have larger diameter than the diameter of the central portions thereof and hence, an amount of compressive deformation of the rollers at the respective r
Fukuda Yuichi
Matsuda Tsukasa
Yamada Tetsuo
Yamashita Takayuki
Beatty Robert
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
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