Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-02-08
2002-01-01
Pendegrass, Joan (Department: 2852)
Electrophotography
Image formation
Transfer
C347S116000, C347S234000
Reexamination Certificate
active
06336024
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Filed of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus capable of reducing, without using a special mechanism, a color misregistration caused by a variation in the load of a belt-shaped intermediate transfer medium or a belt-shaped photoconductor.
2. Description of the Related Art
A color image forming apparatus which uses a belt-shaped intermediate transfer medium or a belt-shaped photoconductor as an image carrier has heretofore been known as a xerographic printer. In a color image forming apparatus which uses a belt-shaped intermediate transfer medium, a latent image of a first color is formed by scanning the surface of a photoconductor with a laser beam, and the latent image is developed to form toner image of the first color and this toner image is transferred to the intermediate transfer medium. Then, a latent image of a second color is formed on the surface of the photoconductor and a toner image of the second color is formed, and this toner image is superimposed on and transferred to the toner image of the first color on the intermediate transfer medium. In this manner, toner images of individual colors Y (yellow), M (magenta), C (cyan) and K (black) are formed on the surface of the intermediate transfer medium in a superimposed state, and the color toner images formed on the intermediate transfer medium are collectively transferred to a recording sheet which is an image carrier.
In a color image forming apparatus which uses a belt-shaped photoconductor, a latent image of a first color is formed by scanning the surface of a photoconductor with a laser beam, and the latent image is developed to form a toner image of the first color. Then, a latent image of a second color is formed on the surface of the photoconductor and a toner image of the second color is formed. In this manner, toner images of plural colors are formed on the surface of the photoconductor in a superimposed state, and the color toner images formed on the photoconductor are collectively transferred to a recording sheet which is an image carrier.
In such a color image forming apparatus, a variation in the speed of the intermediate transfer medium or the photoconductor which is a belt-shaped image carrier causes a variation in the print position of each of the color toner images, resulting in color misregistration or non-uniform density. In general, a variation in the speed of the belt is a periodic variation due to the eccentricity of a belt driving roll, but a variation in speed due to a variation in load acting on the belt is also a large problem.
FIG. 13
is a graph showing the relationship between a load acting on the belt-shaped image carrier and the average speed of the belt-shaped image carrier. When the load acting on the belt is small, a small slip or elongation occurs in the belt, and the average speed of the belt linearly decreases with an increase in the load. On the other hand, if the load acting on the belt is a predetermined or more, a slip occurs and it becomes impossible to transport the belt, so that the speed of the belt linearly decreases. In a region in which the average speed linearly varies, if the load increases, the average speed of the belt decreases from V to V′ as shown in FIG.
14
A. In the case of printing at the average speed V′, since the average speed V′ is lower than the average speed V, color misregistrations accumulate while the load is increasing, and even if the leading edges of transfer positions of toner images coincide with one another in the sub-scan direction, a color misregistration amount R occurs as shown in FIG.
14
B.
On the other hand, a decrease in speed due to the flexure of a driving system such as a driving gear which drives a belt driving roll is different from an average decrease in speed due to a slip or an elongation of the belt, and as shown in
FIG. 15A
, when a load is applied, the speed instantaneously decreases and is immediately restored. Accordingly, as shown in
FIG. 15B
, only when a load is applied, a misregistration of a print position instantaneously occurs, and after that no misregistration occurs so that, the misregistration quantities of print positions do not accumulate.
In other words, when a transfer roll or a cleaner to be used during transfer to the belt-shaped intermediate transfer medium makes contact with the belt-shaped intermediate transfer medium, a difference occurs between the speeds of the belt-shaped intermediate transfer medium before and after the transfer roll or the cleaner makes contact with the same, and the difference in speed causes small variations in print position, thus causing a color misregistration of a print position.
The problem of the transfer misregistration also occurs in the case of a drum-shaped photoconductor or intermediate transfer medium.
A general method for ameliorating such small variations in print position is to detect a variation in the speed or the position of a belt-shaped image carrier and correct a print position. Japanese Patent Laid-Open No. 234064/1992 discloses the art of securing an encoder to a roll shaft to be driven by a belt-shaped image carrier and detecting the speed of the belt-shaped image carrier from an angular velocity obtained from the encoder. Japanese Patent Laid-Open No. 175687/1997 describes the art of printing a mark on a belt-shaped image carrier in advance and detecting the mark through a sensor to detect and control the speed of the belt-shaped imager carrier.
However, either of these arts is suited to the art of controlling the speed of a belt-shaped image carrier with high precision, but needs devices such as a detecting device which detects the speed of the image carrier, a computing device which computes a correction amount relative to the detected speed, and a control device which controls a belt driving speed on the basis of the computed result. Either of the arts has, therefore, the problem that the entire size of the image forming apparatus becomes so large as to be disadvantageous in terms of cost and space.
Japanese Patent Laid-Open No. 80853/1997 describes a color image forming apparatus in which a photoconductor and the intermediate transfer medium are driven independently of each other. The color image forming apparatus utilizes an art for correcting a color misregistration due to the fact that a latent image writing part and an intermediate transfer medium are asynchronous with each other. In the art, the phase difference between the rotational period of the latent image writing part and the rotational period of the intermediate transfer medium is detected on the basis of a reference position signal relative to the intermediate transfer medium, and if a color misregistration occurs when a toner image formed on the photoconductor is superimposed on a toner image previously transferred to the intermediate transfer medium, the rotational speed of the intermediate transfer medium is increased or decreased while a toner image is not being transferred to the intermediate transfer medium, thereby correcting the phase difference. Japanese Patent Laid-Open No. 80853/1997 also describes the art of, when a latent image is not being written to the photoconductor, increasing or decreasing the rotational period of the intermediate transfer medium and correcting the phase difference in order to correct a color misregistration occurring in case that the photoconductor and the intermediate transfer medium are driven by identical or different driving devices.
However, the art of Japanese Patent Laid-Open No. 80853/1997 has the problem that since the phase difference is detected on the basis of the reference position signal, a color misregistration or a density deviation cannot be corrected if a variation in load occurs in the intermediate transfer medium or the photoconductor after the reference position signal has been outputted.
SUMMARY OF THE INVENTION
The invention has been made to solve the above-described problems, and provide
Kanaya Shinichi
Murakami Jun-ichi
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
Pendegrass Joan
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