Electrophotography – Image formation – Charging
Reexamination Certificate
2001-05-23
2002-12-17
Tran, Hoan (Department: 2852)
Electrophotography
Image formation
Charging
C399S313000
Reexamination Certificate
active
06496673
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic process or an electrostatic recording process such as a copying apparatus or a printer.
2. Related Background Art
In the image forming apparatus employing the electrophotographic process, the photosensitive member is generally charged by corona discharge.
In recent years, instead of such discharge method, there is being conceived an injection charging process practically involving no discharge.
In such injection charging process, it is required to achieve contact with the photosensitive member in a high density, but it is practically very difficult to maintain a roller or a blade in contact, without leaving a small gap, with the rotating photosensitive member.
It is therefore conceived to achieve injection charging by contacting a fine particle layer with the photosensitive member.
The Japanese Patent Application Laid-open No. 5-127490 proposes a charging apparatus of a magnetic brush type.
In the contact charging apparatus of magnetic brush type, conductive magnetic particles are held as a magnetic brush either directly on a magnet or on a sleeve incorporating a magnet, and the charging is initiated by contacting such magnetic brush in a stopped or rotating state with an image bearing body to be charged (hereinafter called photosensitive drum) and applying a voltage thereto. As the magnetic particles constituting the magnetic brush, the aforementioned Japanese Patent Application Laid-open No. 05-127490 describes that there are preferred magnetic particles with an average particle size not less than 30 &mgr;m and not more than 70 &mgr;m, free from protruding portions such as needle-shaped portions or edges, and made into a spherical shape in such a manner that the ratio of the longer axis and the shorter axis does not exceed 3 times.
However, in the image forming apparatus employing the contact charging apparatus of magnetic brush type as disclosed in the aforementioned Japanese Patent Application Laid-open No. 05-127490 as the charging means for the photosensitive drum and also employing the contact transfer charging process as the transfer means for a developer image from the photosensitive drum to a recording medium, there has been encountered a drawback of image defects such as image fog or ghost image, resulting presumably from defective charging.
Such situation occurs in case of paper-passing durability test utilizing, as the conductive magnetic particles for constituting the magnetic brush (hereinafter called carrier), “spherical carrier” particles formed by a polymerization process and having an average particle size of 50 &mgr;m, a saturation magnetization of 200 emu/cm
3
and a resistivity of 5×10
6
&OHgr;cm.
However, satisfactory image formation, without the aforementioned image defects resulting from the defective charging, can be achieved with “crushed carrier” formed by a crushing process and having an average particle size of 25 &mgr;m, a saturation magnetization of 200 emu/cm
3
and a resistivity of 5×10
6
&OHgr;cm instead of the spherical carrier.
This is presumably because the finely powdered carrier particles formed at the crushing operation increase the contact area between the photosensitive drum and the carrier thereby increasing the charging ability to the photosensitive drum.
However, if the crushed carrier is used, the carrier may leak for example by adhesion of the fine carrier particles to the photosensitive drum as described in the aforementioned Japanese Patent Application Laid-open No. 05-127490 and such leaking carrier may be subjected to the pressure of the contact transfer charging apparatus in the transfer portion to cause damage to the photosensitive drum, thereby significantly deteriorating the service life thereof.
Also, the image forming apparatuses have been made more compact in recent years, but such compactization of the entire image forming apparatus is inevitably limited by the compactization of the individual means or device of the image forming process steps such as charging, exposure, development, transfer, fixing and cleaning.
Also the toner remaining on the image bearing body after the image transfer (residual developer) is collected as waste toner by cleaning means (cleaner), but such waste toner is preferably absent in consideration of the environmental protection.
For this reason, there is already commercialized the image forming apparatus of a “cleanerless process” (cleanerless system, toner recycling system) in which the cleaner is dispensed with and the residual toner remaining on the image bearing body after the transfer step is removed from the image bearing body by “cleaning simultaneous with development” of the development means (developing device, developing unit) and is collected and reused in the development means.
In such cleaning simultaneous with development, the toner remaining in a certain amount on the image bearing body after the image transfer is collected, at the development in one of the subsequent process cycles, by a fog-eliminating bias (a fog-eliminating potential difference Vback between a DC voltage applied to the development means and the surface potential of the image bearing body).
In such process, since the residual toner after the development is collected in the development means and is used again in one of the subsequent process cycles, it is rendered possible to eliminate the waste toner and to reduce the amount of cumbersome maintenance work. Also, the absence of the cleaner provides a large advantage in space, thereby enabling significant compactization of the image forming apparatus.
However, the use of such cleanerless system in combination with the magnetic brush charging process results in drawbacks of a pass-through ghost and a hue variation in case of a color image.
FIG. 26
shows an example of image defect by a pass-through ghost.
FIG. 26
shows an A4-sized sheet on which a character “A” is formed in a position (
1
) (real image or main image). The character “A” in the above-mentioned position (
1
) is not completely transferred but partially remains on the photosenstive drum as residual toner, which should be collected in the developing portion but is not completely collected and is transferred again onto the recording material to form an image in a position (
2
), called pass-through ghost, that should not be present ideally. This explanation of the phenomenon is also supported by a fact that the distance a shown in
FIG. 26
coincides with the peripheral length of the photosensitive drum used.
It will be understood that the aforementioned pass-through ghost phenomenon breaks the basic concept of the cleanerless system and results in the destruction of the system itself. In the following there will be explained why the cleanerless system is broken by the pass-through ghost phenomenon.
The investigation of the present inventors has revealed that, in the cleanerless system, the collection of the transfer residual toner in the developing portion is limited and that the efficiency of collection and the mass of the transfer residual toner per unit area are mutually correlated.
More specifically, in a chart shown in
FIG. 27
, the ordinate indicates the level of the image defect (pass-through ghost) as shown in
FIG. 26
, evaluated in five levels, while the abscissa indicates the amount of the transfer residual toner per unit area. The amount of the transfer residual toner is adjusted by regulating the transfer current, thereby varying the transfer efficiency. In
FIG. 27
, marks ◯ indicate the evaluated levels of the images in the position (
1
) in
FIG. 26
while the marks x indicate the evaluated levels of the image in the position (
2
) (namely the levels of generation of the pass-through ghost images).
The marks x in
FIG. 27
indicate that the pass-through ghost starts to be generated about where the amount of the transfer residual toner exceeds 0.06 mg/cm
2
, and becomes more severe as the amount of the transfer resi
LandOfFree
Image forming apparatus capable of preventing damage to a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Image forming apparatus capable of preventing damage to a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Image forming apparatus capable of preventing damage to a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2932713