Electrophotography – Image formation – Development
Reexamination Certificate
2000-09-07
2002-05-28
Grimley, Arthur T. (Department: 2852)
Electrophotography
Image formation
Development
Reexamination Certificate
active
06397032
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrophotographic image forming apparatuses used in printers, copying machines, and facsimile machines, and more particularly, to an image forming apparatus including a nonmagnetic single component developing device.
2. Description of the Related Art
As an image forming apparatus using a single component developer, a so-called “contact developing device” in which an image bearing member (also referred to as a photosensitive drum) is brought into contact with a developer bearing member (also referred to as a development roller) has been disclosed, for example, in Japanese Patent Publication No. 2-26224 and Japanese Patent Laid-Open No. 3-261978.
FIG. 5
is a schematic diagram showing an example of a conventional image forming apparatus in which images are formed by a contact developing device using a nonmagnetic single component developer (hereinafter, a developer is also referred to as toner). As shown in
FIG. 5
, in such an image forming apparatus, there are disposed in the periphery of a photosensitive drum
100
, which is an image bearing member rotating in the X direction in the drawing, a charging roller
101
as a primary charging device, an exposure unit
102
as an electrostatic latent image forming device, a developer unit
103
as a developing device, a transfer roller
104
as a transfer device, and a cleaning device
105
.
The developer unit
103
includes a development roller
107
which performs development while rotating in the Y direction in the drawing, a feed roller
108
as a toner-feeding means which feeds toner T′ for nonmagnetic single component development to the development roller
107
while rotating in the Z direction in the drawing, a development blade
110
which is a toner-regulating means for regulating the amount of the toner T′ coated on the development roller
107
as well as the amount of charge, a stirring member
109
for feeding the toner T′ to the feed roller
108
and stirring the toner T′, etc.
In the contact developing device using the rigid photosensitive drum
100
which is brought into contact with the development roller
107
so as to perform development, the development roller
107
is preferably provided with an elastic body.
Conventionally, as the development roller
107
provided with the elastic body, an elastic development roller with a so-called “single solid layer” is used in which a silicone rubber or a nitrile-butadiene rubber (NBR) is formed on a metallic core bar. As the development blade
110
, a development blade is generally used formed of a thin plate composed of a stainless steel or the like to which a rubber member composed of a polyurethane rubber or the like is attached at the section abutting on the development roller
107
.
Next, the image formation operation of the image forming apparatus will be described.
In response to a print signal from the outside, the photosensitive drum
100
starts to rotate in the X direction. First, the photosensitive drum
100
is uniformly charged by the charging roller
101
. Next, an electrostatic latent image is formed on the photosensitive drum
100
due to exposure by the exposure unit
102
, and the electrostatic latent image reaches the section which is in contact with the developer unit
103
when the photosensitive drum
100
is rotated.
In connection with the above operation, the developer unit
103
performs an operation described below.
The toner T′ stirred by the stirring member
109
is fed onto the development roller
107
due to sliding friction between the development roller
107
rotating in the Y direction and the feed roller
108
rotating in the Z direction. A predetermined amount of charge is applied to the toner T′ on the development roller
107
and the amount of the toner T′ is regulated by the development blade
110
, and thus the toner is born on the development roller
107
.
When the toner borne on the development roller
107
reaches the section which is in contact with the photosensitive drum
100
, i.e., a development section, a developing bias is applied to the development roller
107
by a power source (not shown in the drawing), and thus the electrostatic latent image formed on the photosensitive drum
100
is developed by the toner T′ borne on the development roller
107
so as to become visible. The toner which is not used for development and which remains on the surface of the development roller
107
is recovered by the developer unit
103
via the feed roller
108
.
The toner on the photosensitive drum
100
reaches the section opposite to the transfer roller
104
due to the rotation of the photosensitive drum
100
, and is transferred to a sheet of transfer paper P′. The toner T′ on the transfer paper P′ is subjected to thermofusion fixing by the fixing device
106
, and thus a permanent image is produced.
The remaining toner T′ which is not transferred to the transfer paper P′ is recovered by the cleaning device
105
.
By repeating the operation described above, the image formation is repeated.
However, there are problems as described below depending on the characteristics of the development roller.
(1) Development Roller Having Electron Conduction System
When a silicone rubber is used as the development roller, a predetermined resistance of the development roller is obtained by dispersing carbon particles or metal particles into the silicone rubber. Such a conduction mode in which a predetermined resistance is obtained by dispersed particles is generally referred to as an electron conduction system. However, when a development roller having the electron conduction system with a low resistance of approximately 1×10
4
&OHgr; is used, development characteristics become binary and it is not possible to obtain desired tone characteristics. Although the binary development characteristics are advantageous for text images (i.e., linear images), they are disadvantageous for photographic images (i.e., picture images) because it is impossible to reproduce images with highlights.
As described above, in the development roller having the electron conduction system, it is difficult to obtain satisfactory tone reproduction in output images.
On the other hand, in a development roller with a high resistance of 1×10
6
&OHgr; or more, tone characteristics with gentle gradation can be obtained. However, if the resistance is increased in the development roller having the electron conduction system, for reasons which are not yet clear, when the longitudinal image width is changed, the density of the developed image varies even for the same latent image condition, which is disadvantageous. Such a phenomenon is noticeable, particularly in an image with highlights having a low density. Although a desired density can be obtained when the image width is narrow, it is not possible to obtain the desired density when the image width is wide. That is, when an image with highlights is output in the full longitudinal width, it is not possible to obtain a desired density.
Additionally, with respect to the electron conduction system, when an applied voltage is low, the resistance of the elastic body is increased, and in an extreme state, the resistance may vary by 3 orders of magnitude or more, resulting in difficulty in forming a desired image.
(2) Development Roller Having Ionic Conduction System
In a development roller, by adding an ion-conductive agent to an NBR, a polyurethane rubber, or the like so that the material itself is ionized to form a conductive path, a predetermined resistance of the development roller is obtained. Such a conduction system based on the movement of ions is generally referred to as an ionic conduction system. When a development roller having the ionic conduction system is used, in a manner differing from that of the development roller having the electron conduction system, a substantially constant resistance is obtained in response to an applied voltage. Therefore
Ohzeki Yukihiro
Sakaizawa Katsuhiro
Sekiguchi Manami
Grimley Arthur T.
Ngo Hoang
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