Developing device having magnetic seals

A developing device includes a developer container containing magnetic particles; a developer holding unit formed at an opening portion of the developer container, the developer holding unit rotating while holding a developer; a magnetic field generating unit placed in the developer holding unit; a magnet placed along a circumferential direction of the developer holding unit to form a magnetic seal between the magnet and the developer holding unit; and a magnetic unit placed to be adjacent to the magnet. The developing device attains a reduction in driving torque and a high magnetic sealing performance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a developing device used in an image 
forming apparatus of an electrophotographic or electrostatic recording 
scheme to develop an electrostatic image on an image holding member. 
2. Description of the Related Art 
In an image forming apparatus of the electrophotographic scheme, an 
electrostatic image formed on the photosensitive member is developed with 
toner in the developing device. 
In such a developing device, a developer is held by the developing sleeve 
placed so as to oppose the photosensitive member in order to perform 
developing. So, toner must be prevented from leaking out via the end 
portions of the developing sleeve. 
To do this, seal members are provided for the developing sleeve end 
portions. As such a seal member for preventing leakage of toner, an 
elastic member, such as a felt or foam rubber member has widely been used. 
FIGS. 9 and 10 show a typical example of this member. FIG. 9 is a 
sectional front view showing the main part of a developing device. FIG. 10 
is a sectional side view showing the main part of the developing device. 
As shown in FIG. 9, a developing sleeve 5 incorporates a magnetic roller 6. 
The developing sleeve 5 is rotatably supported on a developer container 3 
through a sleeve bearing 12, as shown in FIG. 10. With this arrangement, 
toner supplied from the developer container 3 adheres to the surface of 
the developing sleeve 5 owing to the magnetic force of the magnetic roller 
6, and is regulated by a developing blade 7 to a predetermined thickness. 
Then, with rotation of the developing sleeve 5, the toner adheres to a 
latent image on the photosensitive drum 1 at a position opposite thereto, 
thus developing the image. Elastic seal members 8 are mounted on the two 
longitudinal end portions of the developing sleeve 5 outside the 
developing area. More specifically, the elastic seal members 8 are mounted 
on the front portion (on the opening side) of the developing sleeve 5, 
which is mounted on the developer container 3, and the rear portion (on 
the opposite side to the opening side) of the developing sleeve 5. These 
elastic seal members 8 are pressed against the outer surface of the 
developing sleeve 5 to prevent leakage of the toner. 
In this sealing method of pressing the elastic members against the 
developing sleeve, the torque required to drive the developing sleeve 
during a developing operation becomes large. 
In addition, as the number of times the elastic seal members are used 
increases, the sealing ability of each seal member deteriorates. 
The sealing method of pressing the elastic members against the developing 
sleeve is not suitable for a developing device to realize a higher 
operation speed and a longer service life. 
Under the circumstances, a technique of forming magnetic seals has been 
proposed. According to this technique, magnets are arranged at the two 
ends of a developing sleeve through a gap, and magnetic seals are formed 
by the magnetic fields generated by the magnets outside the developing 
sleeve and the magnet inside the developing sleeve. 
FIG. 11 is a sectional front view showing an example of a developing device 
using magnetic seal members. Referring to FIG. 11, magnetic seal members 
20, made of magnets, are wound around the two end portions of a developing 
sleeve 5 to oppose its outer surface through a predetermined gap g. In 
this state, the magnetic seal members 20 are mounted on a developer 
container 3, together with the developing sleeve 5. Each magnetic seal 
member 20 is magnetized to a magnetic pole pattern like the one shown in 
FIG. 12. The gap g between the outer surface of the developing sleeve 5 
and the surface of each magnetic seal member 20 is filled with a magnetic 
brush due to a triboelectric brush of the toner formed along magnetic 
lines of force 24, thereby preventing the toner from leaking out of the 
developing area. 
As a magnetic seal member, a member having the magnetization pattern shown 
in FIG. 13 or 14 may be used. 
With the use of this technical means, since the developing sleeve 5 and the 
magnetic seal members 20 can be arranged in a non-contact state, the 
rotational torque of the developing sleeve 5 is considerably small. For 
this reason, a compact, inexpensive driving motor can be used. In 
addition, since variations in rotational torque are small, the developing 
sleeve 5 and the photosensitive drum 1 do not easily undergo rotation 
variations. This technique is therefore suitably used to increase the 
operation speed of the device. 
Furthermore, since the magnetic seal members 20 are free from wear and the 
like, they can be used semipermanently and recycled. 
In this magnetic sealing method, the sealing performance is influenced by 
the magnetic flux density. 
Although the sealing effect can be enhanced by using magnets having strong 
magnetic forces, the device increases in size, and the magnetic forces may 
inflict adverse effects on the developing area. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a developing device 
which attains a reduction in driving torque for a developer holding 
member. 
It is another object of the present invention to provide a developing 
device which can obtain a high magnetic sealing performance. 
It is still another object of the present invention to provide a developing 
device comprising: 
a developer container containing magnetic particles; 
a developer holding member formed at an opening portion of the developer 
container, the developer holding member rotating while holding a 
developer; 
a magnetic field generating member placed in the developer holding member; 
a magnet placed along a circumferential direction of the developer holding 
member to form a magnetic seal in cooperation with the magnetic field 
generating member; and 
a magnetic member placed so as to be adjacent to the magnet. 
The above and other objects, features, and advantages of the present 
invention will be apparent from the following detailed description in 
conjunction with the accompanying drawings and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiments of the present invention will be described below with 
reference to the accompanying drawings. 
&lt;First Embodiment&gt; 
FIG. 1 is a sectional view showing a process cartridge which uses a 
developing device of an embodiment of the present invention and which is 
detachably mounted in an image forming apparatus body. 
This process cartridge is a unit including at least a photosensitive member 
as an image holding member for holding an electrostatic image and a 
developing device for developing the electrostatic image on the 
photosensitive member. 
As shown in FIG. 1, in the process cartridge according to the configuration 
of this embodiment, a developing means 4 including a charging means 2, a 
developer container 3, a developing sleeve 5 as a developer holding 
member, a developing blade 7, and a cleaning means 11 are arranged around 
a photosensitive drum 1. These components are covered with a housing 
constituted by frames 15, 16, 17, and 18, and are integrated into a 
cartridge. This cartridge is detachably mounted in an image forming 
apparatus body (not shown). 
The developing sleeve 5 incorporates a magnetic roller 6, and is rotatably 
mounted on the developer container 3 through a sleeve bearing (not shown). 
Monocomponent magnetic toner as a developer supplied from the developer 
container 3 adheres to the surface of the developing sleeve 5 owing to the 
magnetic force of the magnetic roller 6, and is regulated by the 
developing blade 7 to a predetermined thickness. The resultant toner is 
brought to a position where it opposes a latent image on the 
photosensitive drum 1 upon pivotal movement of the developing sleeve 5. At 
this position, the toner adheres to the latent image to develop it. 
The developing means 4 has magnetic seal members 21 arranged along the 
outer surfaces of the two end portions of the developing sleeve 5. Each 
magnetic seal member 21 is mounted on the developer container 3 with a gap 
g being ensured between the magnetic seal member 21 and the outer surface 
of the developing sleeve 5. As shown in FIG. 2, the magnetic seal member 
21 is formed by joining a magnetic plate (magnetic member) 23 to a side 
surface of a magnet 22 in its widthwise direction which coincides with the 
longitudinal direction of the developing sleeve 5. 
Each magnetic seal member 21 in this embodiment will be described in 
detail. 
The magnetic seal member 21 is constituted by the following two constituent 
elements: the magnet 22 which is an injection molded member which is 3 mm 
wide and made of a nylon binder containing an Nd-Fe-B magnetic powder; and 
the magnetic plate 23 which is a 1-mm thick iron member. 
As shown in FIGS. 12, 13, and 14, the magnet 22 is magnetized in a 
plurality of sections along the circumferential direction of the 
developing sleeve. It is preferable that the magnet 22 be alternately 
magnetized to south and north poles, as in the magnetic seal member 20 
shown in FIG. 12. Alternatively, the magnet 22 may be magnetized so as to 
have opposite polarities in the side surfaces thereof, as in the magnetic 
seal member 20 shown in FIG. 13. Alternatively, as in the magnetic seal 
member 20 shown in FIG. 14, the magnet 22 may be magnetized so as to have 
opposite polarities on the front side which opposes the developing sleeve 
5, and the rear side. 
The magnetic plate 23 is preferably made of a soft magnetic material, such 
as soft iron, silicon steel, or Permalloy, which has high magnetic 
susceptibility and small magnetic hysteresis loss. 
As a method of joining the magnet 22 and the magnetic plate 23, insert 
molding of injection molding is used. Even if, however, these members are 
joined to each other by a double-coated adhesive tape or an adsorbing 
manner using only a magnetic force, the same effects to be described later 
can be obtained. The gap between the developing sleeve 5 and the magnetic 
seal member 21 is 0.1 to 0.7 mm, and the magnetic flux density on the 
surface of the developing sleeve 5, caused by the magnetic force of the 
magnetic seal member 21 is about 1,000 to 2,000 Gs. The magnets 22 and the 
magnetic plates 23 of the magnetic seal members 21 are positioned such 
that the magnets 22 are located on the near sides of an opening portion 26 
(the middle portion of the developing sleeve 5 which is indicated by the 
dots in FIG. 2) of the developer container 3, and the magnetic plates 23 
are located on the far sides of the opening portion 26 (the two 
longitudinal end portions of the developing sleeve 5 in FIG. 2). 
As described above, since the magnet 22 is located on the near side of the 
opening portion 26 of the developer container 3, and the magnetic plate 23 
is located on the far side of the opening portion 26, magnetic lines of 
force 24 which run from the front side to the rear side of the magnetic 
seal member 21 are formed between the magnet 22 and the magnetic plate 23 
to enter the magnetic plate 23 with high permeability, as indicated by 
FIG. 3B which is an enlarged view of a portion A in FIG. 3A. Unlike the 
prior art shown in FIGS. 15 and 16, almost no magnetic lines of force run 
beyond the width of the magnetic seal member 21. 
Since the toner spreading along the magnetic lines of force 24 on the 
surface of each magnetic seal member 21 is not present outside the 
magnetic plate 23 on the magnetic plate 23 side (outside the opening 
portion 26), the toner does not come into contact with a spacer roller 25 
upon rotation of the developing sleeve 5. For this reason, each spacer 
roller 25 can be placed near the side surface of the magnetic seal member 
21. Apparently, therefore, the process cartridge can be reduced in size, 
and at the same time, the image forming apparatus body itself can be 
reduced in size. In addition, since the toner on each magnetic seal member 
21 does not spread outside the opening portion 26 of the developer 
container 3 by the magnetic plate 23, the toner can be reliably held 
within the range in which the magnetic force on the surface of the 
magnetic seal member 21 is strong. Even if a shock or the like acts on the 
process cartridge when the user attaches/detaches it to/from the image 
forming apparatus, no toner leaks. That is, good sealing characteristics 
can be obtained. 
Furthermore, since the magnetic plate 23 is joined to the side surface of 
each magnet 22, the magnetic lines of force 24 enter the magnetic plate 
23, as described above. That is, the diverging magnetic lines of force are 
converged onto the magnetic plate 23. As a result, the magnetic flux 
density on the surface of the magnet 22 increases to attain a further 
improvement in sealing characteristics. Moreover, since compact, 
inexpensive magnets with small magnetic forces can be used when there is a 
margin in terms of sealing characteristics, a reduction in cost can be 
achieved. 
&lt;Second Embodiment&gt; 
The second embodiment of the present invention will be described next with 
reference to FIGS. 4, 5A and 5B. 
The same reference numerals in FIGS. 4, 5A and 5B denote the same parts as 
in FIGS. 2, 3A and 3B, and a repetitive description thereof will be 
avoided; only the structure of a magnetic seal which is a characteristic 
feature of the second embodiment will be described below. 
A magnet 22 and a magnetic plate 23 constituting a magnetic seal member 21 
in this embodiment are positioned such that the magnetic plate 23 is 
located on the near side of the opening portion 26 of a developer 
container 3, and the magnet 22 is located on the far side of the opening 
portion 26, as shown in FIG. 4. 
Each magnetic seal member 21 is located on the near side of the opening 
portion 26 to attain a reduction in the size of the device. 
As described above, since the magnet 22 is located on the far side of the 
opening portion 26 of the developer container 3, and the magnetic plate 23 
is located on the near side of the opening portion 26, the magnetic seal 
member 21 delivers magnetic lines of force 24 between the magnet 22 and 
the magnetic plate 23, which enter the magnetic plate 23 with high 
permeability, as shown in FIGS. 5A and 5B. Unlike the prior art shown in 
FIGS. 15 and 16, therefore, no magnetic lines of force 24 run beyond the 
magnetic plate 23 in the width direction of the magnetic seal member 21. 
The toner spreading along the magnetic lines of force 24 on the surface of 
the magnetic seal member 21 does not therefore spread to the magnetic 
plate 23 side, i.e., the inner wall of the opening portion 26. That is, 
the toner in the developer container does not spread in the axial 
direction of a developing sleeve 5 so as not to flow over the outer 
surface of the developing sleeve 5 along the magnetic lines of forces from 
each seal member constituted by the magnet. For this reason, the toner is 
not deposited on the inner wall of the opening portion 26 of the developer 
container 3 owing to the magnetic force of each seal member. This prevents 
a decrease in density due to a deficiency of the toner supply at an end 
portion of a toner image. Each seal member constituted by the magnet can 
be located far from the opening portion 26 to prevent a decrease in 
density. Alternatively, the problem associated with an increase in the 
longitudinal size of the device, which occurs, for example, when the width 
of the opening portion 26 becomes larger than the image area, can be 
solved. 
In addition, a magnetic roller 6 is mounted in the developing sleeve 5, and 
the magnetic plates 23 are arranged to oppose the two ends of the magnetic 
roller 6. With this arrangement, at the position where the magnetic roller 
6 and the magnetic plate 23 oppose each other, the magnetic lines of force 
24 run as shown in FIG. 7 which is a sectional view taken along a line 
7--7 in FIG. 6. FIG. 8 shows the magnetic lines of force 24 along a 
cross-section taken along a line 8--8 in FIG. 6. As shown in FIGS. 7 and 
8, magnetic brushes are doubly formed in the longitudinal direction of the 
developing sleeve 5, i.e., the magnetic brush between the magnetic roller 
6 and each magnetic plate 23 and the magnetic brush generated by the 
magnet of each magnetic seal member 21, thereby improving the sealing 
characteristics. 
In addition, since the magnetic plates 23 are placed on the side surfaces 
of the magnets 22, the magnetic lines of force 24 from the magnets 22 
enter the magnetic plates 23. For this reason, the magnetic lines of force 
24 concentrate on the magnetic plate 23. Therefore, the magnetic flux 
density on the surface of each magnet 22 increases, and the magnetic force 
increases. Consequently, the sealing characteristics can be further 
improved. 
Moreover, since compact, inexpensive magnets with small magnetic forces can 
be used when there is a margin in terms of sealing characteristics, a 
reduction in cost can be achieved. 
The embodiments of the present invention have been described above, but the 
present invention is not limited these embodiments. Various changes and 
modifications of the embodiments can be made within the spirit and scope 
of the invention.