Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-10-15
2003-06-10
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S329000, C430S111400, C430S124300, C430S126200
Reexamination Certificate
active
06577837
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an image forming method in which an electrostatic latent image is developed and transferred onto an intermediate transfer member and the image is transferred and fixed onto a recording medium through heating in an electrophotographic method or an electrostatic recording method, and an image forming apparatus used in this method.
DESCRIPTION OF THE RELATED ART
In an image forming apparatus of an ordinary electrophotographic system, for example, an image forming apparatus in which an electrostatic latent image is formed on an image bearing member and developed with a dry toner to form a toner image and the toner image is then electrostatically transferred and fixed onto a recording medium to obtain the image, there arise often problems that non-uniform density occurs in the image or a toner powder is scattered to impair the resolution of the image or the dot reproducibility. The non-uniform density or the toner scattering mainly occurs while the toner image on the image bearing member is electrostatically transferred onto the recording medium.
In the electrostatic transfer method, a toner transfer efficiency is increased in proportion to an intensity of an electric field to be applied to the toner layer. However, when the intensity of the electric field becomes higher than a certain degree, so-called Paschen discharge occurs to decrease the transfer efficiency. That is, the transfer efficiency shows a peak value in a certain intensity of the electric field. Generally, the peak value of the transfer efficiency, in many cases, does not reach 100%, and remains approximately 95% at the highest.
The transfer efficiency of the toner layer thus depends on the intensity of the electric field. Accordingly, as the intensity of the electric field is changed owing to a non-uniform thickness of the toner layer, or an uneven surface or non-uniform electric properties of a recording medium such as paper, the transfer efficiency is also changed. When the toner image formed on the recording medium is monochromic and a layer is thin, the intensity of the electric field is mainly changed owing to the uneven surface or the non-uniform electric properties of the recording medium, with the result that the image mottle occurs. When monochromic toner images formed independently on the image bearing member are overlaid and transferred onto the recording medium to form a color image, the image mottle also occurs owing to the uneven surface or the non-uniform electric properties of the recording medium. In the electrostatic transfer method, a difference in transfer efficiency by the change in a thickness of a layer between a portion of a high layer thickness formed by overlaying and transferring plural toner images and a portion of a low layer thickness formed by transferring a monochromic toner image is small, but the transfer efficiency is greatly changed and the image mottle tends to occur owing to the uneven surface or the non-uniform electric properties of the recording medium.
Meanwhile, in a so-called color image forming apparatus of an intermediate transfer system in which plural toner images formed independently on an image bearing member are electrostatically transferred primarily onto an intermediate transfer member having a less uneven surface with less non-uniform electric properties to be overlaid thereon in order and the multicolor toner image formed on the intermediate transfer member is secondarily transferred onto a recording medium, the transfer efficiency is less changed, so that an image with less image mottle can be obtained.
For transferring a toner image onto a recording medium such as paper electrostatically uniformly, it is required to apply a fixed electric field. With respect to a multicolor toner image formed on an intermediate transfer member, there is an area in which a toner image with plural layers, such as three or more layers is formed, while there is an area in which no toner image layer is formed. Accordingly, it is difficult to apply a fixed electric field to the toner image of which the layer thickness varies greatly, and the intensity of the electric field tends to be non-uniform. Consequently, in the secondary transfer by the electrostatic transfer method, not all of the multicolor toner images formed on the intermediate transfer member are transferred onto the recording medium, and a part thereof remain on the intermediate transfer member. The amount of the toner remaining on the intermediate transfer member varies depending on the thickness of the toner layer on the intermediate transfer member. As a result, the color balance of the color images obtained on the recording medium is lost, and desired color images are hardly obtained. Besides, due to the uneven surface of the recording medium, the recording medium and the intermediate transfer member are not completely adhered. The transfer electric field becomes non-uniform owing to a non-uniform gap generated therebetween, or the transfer efficiency is decreased with a Coulomb repulsion force of toners to decrease the image quality.
In order to solve these problems, Japanese Patent Publication No. 41679/1971 discloses an image forming method which has steps of adhesively transferring a toner image formed on an image bearing member onto a surface of an elastic intermediate transfer member, then heating a recording medium fed between the intermediate transfer member and a heating roller using the heating roller, and fusing the toner image on the intermediate transfer member to thermally transfer the toner image on the transfer member onto the recording medium. Further, Japanese Patent Publication Nos. 1024/1989 and 1027/1989 disclose a method in which an endless belt-like intermediate transfer member and a recording medium superposed with a toner image transferred onto the intermediate transfer member therebetween are urged with a heating roll and a pressure roll to transfer and fix the toner image on the intermediate transfer member onto a recording medium. Still further, Japanese Patent Publication Nos. 20632/1982, 36341/1983 and 1023/1989 disclose a method which has steps of heating a toner image transferred onto an endless belt-like intermediate transfer member to a temperature above a melting point of a toner, and then urging the intermediate transfer member against a recording medium to transfer and fix the toner image on the intermediate transfer member onto the recording medium, wherein after the intermediate transfer member is urged against the recording medium, the intermediate transfer member and the recording medium are circulated and moved while being contacted with each other for a long period of time, and heat transfer from the intermediate transfer member to the recording medium is satisfactorily conducted in this contact state to surely transfer and fix the toner image on the intermediate transfer medium onto the recording medium.
In these non-electrostatic transfer methods, the troubles caused by the non-uniformity of the electric field which are found in the foregoing electrostatic transfer method do not occur, so that a high-quality image with a good color balance can be obtained in a color image with a high transfer efficiency of a toner image and a high sharpness. However, in the methods disclosed in Japanese Patent Publication Nos. 41679/1989, 1024/1989 and 1027/1989, there are problems that since a pressure roll mounted on the reverse side of the recording medium is not provided with a heating unit, the recording medium takes out a large amount of heat so that the toner of the toner image in contact with the recording medium is hardly fused on the recording medium and insufficient fixing tends to occur in the image formation at a high speed in particular.
Furthermore, in the methods disclosed in Japanese Patent Publication Nos. 20632/1982, 36341/1983 and 1023/1989, there are problems that while the intermediate transfer member and the recording medium are moved in contact with each other for a long period of time, they come sometimes ou
Iida Yoshifumi
Ishizuka Daisuke
Nakamura Masaki
Chen Sophia S.
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
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