Electrophotography – Image formation – Transfer
Reexamination Certificate
1999-12-16
2002-02-12
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S312000
Reexamination Certificate
active
06347209
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording material by using an electrophotographic process, for example, an image forming apparatus such as a copier, a printer, or a facsimile.
2. Related Background Art
Conventionally, there have been suggested or worked various image forming apparatuses using an electrophotographic process. For example, there are an image forming apparatus in which toner images formed on a photosensitive drum are sequentially transferred and superposed on top of one another on a recording material which is conveyed with being borne by a transfer drum or by a transfer belt and an image forming apparatus in which toner images formed on a photosensitive drum are primarily transferred and sequentially superposed on top of one another on an intermediate transfer drum or on an intermediate transfer belt and then the toner images on the intermediate transfer drum or on the intermediate transfer belt are secondarily transferred to a recording material. The image forming apparatuses using these two methods will be described below.
Simply describing the image forming process of an image forming apparatus in which the former method (a transfer belt) is used, first a photosensitive drum rotates so as to be uniformly charged on its surface by a charger. Next, the photosensitive drum is irradiated with a laser beam modulated by an image signal of a first color, for example, magenta of an original so as to form an electrostatic latent image of magenta on the photosensitive drum. The electrostatic latent image is developed by a magenta developing unit so as to form a magenta toner image of the first color on the photosensitive drum.
On the other hand, the recording material in a sheet feeding cassette is conveyed to the transfer belt by a registration roller or the like. Simultaneously with this recording material conveyance, an adsorbing roller is pressed to a surface of the transfer belt so that the transfer belt is charged by an adsorbing charger from a back side of the transfer belt in order to adsorb the recording material electrostatically onto the transfer belt. This transfer belt is rotating in synchronism with the photosensitive drum and the magenta toner image formed on the photosensitive drum by the transfer charger is transferred to the recording material borne by the transfer belt. The transfer belt continues its rotation without change to prepare for a transfer of a cyan toner image of the subsequent second color.
Next, the cyan toner image of the second color is formed on the photosensitive drum and the cyan toner image is transferred and superposed on the magenta toner image on the recording material borne by the transfer belt. The same image forming process is repeated also for the third and fourth colors yellow and black to obtain a full color image in which four-colored, magenta, cyan, yellow, and black toner images are superposed on each other on the recording material.
The recording material to which the four-colored toner images have been transferred is separated from the transfer belt and conveyed to a fixing device. The fixing device heats and presses the toner image and the recording material by a fixing roller and a pressure roller to mix respective colors of the toner image and to fix it to the recording material, by which a full-color print image is formed and then the recording material is discharged to an outside of the apparatus.
Briefly describing the image forming process of an image forming apparatus using the latter method (an intermediate transfer belt), a photosensitive drum is driven to rotate at a predetermined peripheral speed so as to be uniformly charged on its surface by a charger and is exposed to a laser beam with scanning by an exposing device, by which an electrostatic latent image of a first color is formed on the photosensitive drum and then the latent image is developed by a developing device. The developing device contains four developing units for yellow toner, magenta toner, cyan toner, and black toner, respectively. The electrostatic latent image of the first color on the photosensitive drum is developed by a yellow developing unit so as to be visualized as a yellow toner image.
The formed yellow toner image is electrostatically transferred to an intermediate transfer belt in a primary transfer portion where the intermediate transfer belt is put in contact with the photosensitive drum (a primary transfer). Toner remaining on the photosensitive drum which has been completed to be primarily transferred is removed from its surface by a cleaner and then the photosensitive drum is supplied to the next color image formation.
In the same manner, the photosensitive drum is charged by the charger and is exposed to a laser beam to form a second color electrostatic latent image, and then the latent image on the photosensitive drum is developed by a magenta developing unit to form a magenta toner image on the photosensitive drum. The magenta toner image is transferred and superposed on the yellow toner image on the intermediate transfer belt.
The above process is repeated also for cyan and black and respective toner images are sequentially superposed on the intermediate transfer belt for transfers. Thereby a color image is formed with four-colored, yellow, magenta, cyan, and black toner images laminated on the intermediate transfer belt.
Afterward a secondary transfer charger which has been spaced from the intermediate transfer belt is made to abut against a surface of the intermediate transfer belt, and in a secondary transfer portion where the intermediate transfer belt is in contact with the secondary transfer charger, toner images of the four colors on the intermediate transfer belt are collectively transferred to a surface of a recording material conveyed at a predetermined timing (a secondary transfer).
The recording material to which the toner images of the four colors have been transferred is conveyed from the intermediate transfer belt to a fixing device, where it is subjected to a fixing process with a heat roller or the like so as to make a full-color permanent image and then discharged to an outside of the image forming apparatus.
In the transfer process of the image forming apparatus in the former and the primary transfer process and the secondary transfer process of the image forming apparatus in the latter, a constant current power supply is connected to the transfer charger, the primary transfer charger, and the secondary transfer charger to control the transfer current at constant current from a viewpoint of a stability of the image transfer.
In the former image forming apparatus, however, poor transferring may occur as described below.
In this image forming apparatus, an image can be formed on a recording material having a smaller size than the applicable maximum size (in a length in a direction perpendicular to a recording material conveying direction). At this point, a volume resistivity of the recording material varies within a range of approx. 2×10
7
to 10
14
&OHgr;cm according to a type of the recording material or hygroscopic conditions.
If a recording material having the applicable maximum size is passed for a transfer by using the transfer belt as shown in
FIG. 12
, assuming that Itr[&mgr;A] is transfer current from the transfer charger, &khgr;[cm] is a width of the recording material (a length in a thrust direction perpendicular to a transfer belt moving direction), and v[cm/s] is a rotating speed of the photosensitive drum, that is, a process speed, the transfer current flowing in &Dgr;t sec is Itr×&Dgr;t and a target area for the recording material is v×&Dgr;t×&khgr; in this condition as shown in
FIG. 13
, and therefore the surface charge density on the recording material is expressed as follows:
Itr×&Dgr;t/(&khgr;×&Dgr;t×v)=Itr/(&khgr;·v)[&mgr;C/cm
2
]
While the recording material has a relatively very hig
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Grainger Quana M.
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