Electrophotography – Image formation – Development
Reexamination Certificate
2001-02-22
2002-10-08
Pendegrass, Joan (Department: 2852)
Electrophotography
Image formation
Development
C399S128000, C399S325000
Reexamination Certificate
active
06463240
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a potential controlling method and potential controller of an image forming apparatus, such as a copying machine, a printer, or a facsimile, using an electrophotographic system, and particularly to a potential controlling method and potential controller of an image forming apparatus, which can certainly prevent a picture quality defect or the like caused by adhesion of toner or carrier to the surface of an image bearing body at the time of rising or falling of a charged potential of the image bearing body and a developing bias voltage.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a copying machine, a printer, or a facsimile, using the electrophotographic system is constructed such that after the surface of a photoreceptor drum is charged to a predetermined potential, the image exposure of the surface of the photoreceptor drum is carried out to form an electrostatic latent image, and the electrostatic latent image is developed by a developing unit, so that a desired image is formed.
In the image forming apparatus like this, at the time of start of charging of the photoreceptor drum, as shown in
FIG. 9A
, when a non-charged region of the surface of the photoreceptor drum which is not charged is moved to a development position facing a developing roll of the developing unit as the photoreceptor drum is rotated, and when a developing bias voltage Vdc is applied to the developing roll, an electric field in the direction to move toner from the developing roll to the surface of the photoreceptor drum is formed between the photoreceptor drum and the developing roll by a potential difference Vimg between the developing bias voltage Vdc and the potential of the non-charged region of the photoreceptor drum, that is, 0 V. Thus, the toner or the toner and carrier in a developer adhere to the surface of the photoreceptor drum.
Besides, in the above image forming apparatus, at the time of start of charging of the photoreceptor drum, as shown in
FIG. 9C
, when a charged region of the surface of the photoreceptor drum is moved to the development position facing the developing roll of the developing unit as the photoreceptor drum is rotated, and when the developing bias voltage Vdc of the developing roll remains OFF, an electric field in the direction to move the carrier charged with reverse polarity of the toner from the developing roll to the surface of the photoreceptor drum is formed between the photoreceptor drum and the developing roll by a potential difference between a charged potential Vh of the photoreceptor drum and 0 V of an OFF potential of the developing bias voltage. Thus, the carrier immediately adheres to the charged region of the surface of the photoreceptor drum by the above electric field.
The adhesion phenomenon of the toner and the carrier occurs not only at the time of start of charging of the photoreceptor drum as the time of start of image formation, but also at the time of stop of charging of the photoreceptor drum as the time of end of image formation with the same reason as the above.
Like this, when the toner and the carrier adhere to the surface of the photoreceptor drum, in the image forming apparatus using an intermediate transfer belt, there have been problems that the photoreceptor drum, the intermediate transfer belt or the like is fouled or damaged, and in the region where the carrier adheres to the surface of the photoreceptor drum, transfer of a toner image is not excellently carried out, so that a defect in picture quality, such as a white point or white streak on a printed sheet, is caused.
As a technique which can solve the problems caused from the adhesion of the toner and the carrier to the surface of the photoreceptor drum at the time of start or stop of image formation, there has been already proposed a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 9-329946 or No. Hei. 7-253693.
An image forming apparatus of the foregoing Japanese Patent Unexamined Publication No. Hei. 9-329946 includes a charging part including a charging member for uniformly charging the surface of an image bearing body and a charging power source for applying a voltage to the charging member, an exposure part for attenuating the potential of the surface of the image bearing body charged by the charging part with image exposure to form an electrostatic latent image on the image bearing body, and a developing part including a developer bearing body for bearing a two-component developer containing a toner charged in the same polarity as the charging polarity of the charging part and a carrier charged in the reverse polarity to the charging polarity, and a developing bias power source for applying a developing bias voltage of the same polarity as the charging polarity to the developer bearing body, and the image forming apparatus further includes a control part for controlling the turning ON/OFF of the charging power source and the developing bias voltage power source so that application of the developing bias voltage to the developer bearing body starts when the end portion of charging start of the surface of the image bearing body faces the developer bearing body, and the application of the developing bias voltage is stopped when the end portion of charging stop of the surface of the image bearing body faces the developer bearing body, and the ON/OFF timing of the charging power source and the developing bias power source is set so as to make the value of the potential of the surface of the image bearing body approach the developing bias voltage under the condition that the absolute value of the potential of the surface of the image bearing body, which passes through the facing position to the developer bearing body, becomes equal to or less than the absolute value of the developing bias voltage when the developing bias voltage is transiently changed by ON/OFF of the developing bias power source.
A potential control method in an image forming apparatus of Japanese Patent Unexamined Publication No. Hei. 7-253693 uses a two-component developer made of a toner and a carrier, and in an image forming apparatus in which the toner is made to adhere to a photoreceptor by applying a developing bias voltage to a developing sleeve, both the surface potential of the photoreceptor and the developing bias are controlled in stages at the time of start and stop of the rotation of the photoreceptor.
However, the foregoing prior art has problems as follows. That is, in the case of the foregoing image forming apparatus of Japanese Patent Unexamined Publication No. Hei. 9-329946, the ON/OFF timing of the charging power source and the developing bias power source is set so as to make the value of the potential of the surface of the image bearing body approach the developing bias voltage under the condition that the absolute value of the potential of the surface of the image bearing body, which passes through the facing position to the developer bearing body, becomes equal to or less than the absolute value of the developing bias voltage when the developing bias voltage is transiently changed by ON/OFF of the developing bias power source. Thus, as shown in
FIG. 9B
, if the timing of the charged potential of the image bearing body is coincident with that of the developing bias voltage at the developing position, any problem does not occur. However, the charging power source and the developing bias power source have difference and fluctuation in rising/falling speed, and the image bearing body also has rotation variation or the like. Thus, there have been problems that it is difficult to make the timing of the charged potential of the image bearing body coincident with that of the developing bias voltage at the developing position, and it is difficult to certainly prevent the toner and carrier from adhering to the surface of the image bearing body at the time of start and stop of the image formation. The problems become more remarkable when the rotatio
Hama Jun-ichi
Onuki Tomio
Fuji 'Xerox Co., Ltd.
Morgan & Lewis & Bockius, LLP
Pendegrass Joan
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