Electrophotography – Image formation – Development
Reexamination Certificate
2001-09-13
2002-10-29
Lee, Susan S. Y. (Department: 2852)
Electrophotography
Image formation
Development
C399S267000, C399S277000
Reexamination Certificate
active
06473586
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a developing unit for use in an image forming apparatus such as a copying machine, a printer, and the like, and more particularly to improvements of a developing unit using a two-component developer having a magnetic carrier and a toner and an image forming apparatus using the same.
2. Related Art
Generally, development systems used in the electrophotographic process are largely classified into a one-component development system which uses as a developer only a toner constituted by pigment particles and a two-component development system using as the developer one in which magnetic substance particles and a toner constituted by pigmented particles are mixed and agitated.
The one-component development system includes magnetic one-component development using a magnetic toner and a nonmagnetic one-component development using a nonmagnetic toner, and since the magnetic toner does not allow light to transmit therethrough depending on the magnetic powder, the magnetic toner is mainly used as a black toner. Meanwhile, the nonmagnetic toner is suitable for color, but the nonmagnetic toner is currently used principally in low-speed machines in view of the fact that its maintainability is short.
In contrast, since the two-component development system handles the toner with the medium of magnetic substance particles, the two-component development system is able to make use of the magnetically attracting force, handling is facilitated concerning the nonmagnetic toner as well, and it excels particularly in the maintainability. Hence the two-component development system is widely used principally in full-color machines and high-speed machines.
As a conventional developing unit using the two-component development system, one shown in
FIG. 33
, for example, is generally used.
In the drawing, a developing unit
210
has a development housing
211
with an opening
212
for development formed therein in such a manner as to oppose a latent image carrier
200
such as a photoconductor drum, and development roll
213
serving as a developer carrier is disposed in face-to-face relation to the opening
212
for development, a two-component developer G having a magnetic carrier and a toner is carried on the surface of the development roll
213
, and after the thickness of the developer layer is restricted to a predetermined layer thickness by a trimming member
220
, an electrostatic latent image on the latent image carrier
200
is converted into a visible image in a development region opposing the latent image carrier
200
of the development roll
213
.
Here, the development roll
213
has a development sleeve
214
disposed rotatably so as to rotate, for example, from the upper side toward the lower side in a development region, as well as a magnet member
215
which is fixedly provided inside the development sleeve
214
and in the periphery of which a plurality of magnetic poles
216
are arranged. As the layout of the magnetic poles
216
of the magnet member
215
, a development magnetic pole
216
is disposed at a position corresponding to the development region, fixed magnetic poles
216
b
to
216
e
are disposed in the order of an S pole, an N pole, an N pole, and an S pole on the downstream side, in the rotating direction of the development sleeve
214
, of the development magnetic pole
215
a
(e.g., N pole). A portion in which the magnetically attracting force is weak or nil is secured midway between the fixed magnetic poles
216
c
and
216
d
of the same polarity, and the fixed magnetic pole
216
d
is disposed before the trimming member
220
, thereby allowing the fixed magnetic pole
216
d
to function as a pickup magnetic pole for capturing the developer.
Further, in this example, a transport paddle
221
which rotates in a direction opposite to the rotating direction of the development roll
213
is disposed at a position adjacent to the development roll
213
inside the development housing
211
, e.g., at an opposing portion. Furthermore, a pair of screw augers
222
and
223
for agitating and mixing the developer and circulatingly transporting the developer are disposed on the inner side behind it.
In such a developing unit, the magnetically attracting force acts inside the development roll
213
by the distribution of the magnetic force of the magnet member
215
inside it. The developer G agitated and mixed by the screw augers
222
and
223
is carried and transported on the surface of the development roll
213
by the magnetically attracting force, and after it is restricted to a predetermined layer thickness by the trimming member
220
, the developer G reaches the development region, and develops an electrostatic latent image on the latent image carrier
200
.
Then, the developer G which lost part of the toner upon completion of development is transported to the downstream side of the development roll
213
. However, since a portion in which the magnetically attracting force is weak or nil is formed midway between the fixed magnetic poles
216
c
and
216
d
of the same polarity, the transported developer G drops from the surface of the development roll
213
due to the action of the gravity.
Subsequently, the dropped developer G is transported to the screw auger
222
by the transport paddle
221
, is further transported in the direction of the axis of rotation of the screw auger
222
and is delivered to the screw auger
223
at an unillustrated delivery section. Even if, for instance, a fresh toner is added there, the added developer G, while being agitated by the screw auger
223
, is sent in a direction opposite to the screw auger
222
, is delivered again to the screw auger
222
at an opposite delivery section and is transported in the axial direction. At the same time, this developer G is gradually supplied to the vicinity of the development roll
213
by the transport paddle
221
.
In this state, the supplied developer adheres to the surface of the development roll
213
by the magnetically attracting force, and the processes in which the developer is subjected to transport, layer formation, and development are repeated again.
Incidentally, in the two-component development system, the attraction between the toner and the carrier making up the developer makes use of the fact that they electrostatically attract each other as a result of the fact that the surfaces of the toner and the carrier respectively charged to mutually opposite polarities by the frictional charging at the time when the toner and the carrier are agitated and mixed. For this reason, it becomes necessary to agitate and mix the developer replenished with the toner, as described above, while the developer is being transported.
In addition, in the development region, it is necessary to control the rate of developer transported within a fixed range so that the developer passes that region with a sufficient amount and does not cause clogging or the like in making contribution to development. For this reason, a method is adopted in which the trimming member is provided to forming the thickness of the developer layer to a predetermined thickness.
Further, in the development region, since the toner in the developer is developed by an electric field formed between the latent image carrier and the developer on the development roll, a development bias voltage is generally applied to the development roll.
The quantity of toner developed at that time is most important among various characteristics that concern the image quality, since it determines the image density. Since the quantity of development toner is determined by neutralizing the electric field in the aforementioned development region by the charge of the toner, it is very important to maintain the charge of the toner to a fixed level.
In the two-component development system, as shown in
FIG. 34
, the density of the toner in the developer and the amount of toner charge are in an inversely proportional relationship, so that the amount of toner charge can be set to a s
Noda Akihiko
Oba Shota
Yamaura Masaaki
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