Apparatus for collecting developer carrier in a electrophotographic machine

In an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer, for performing development by using a two-component developer, a carrier collecting device for collecting a carrier deposited on a photosensitive member in development is provided. The carrier collecting device includes a fixed magnet disposed in such a manner that an intermediate position between an S pole and an N pole of the fixed magnet is closest to the photosensitive member. Accordingly, the carrier can be effectively collected from the photosensitive member, and the carrier captured to the fixed magnet can be prevented from rubbing against the photosensitive member.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a carrier collecting device for collecting 
a carrier deposited on a photosensitive member in development for use in 
an image forming apparatus such as an electrophotographic copying machine 
or an electrophotographic printer for performing development by using a 
two-component developer. 
2. Description of the Related Art 
In an image forming apparatus such as an electrophotographic copying 
machine or an electrophotographic printer, a two-component developer 
having a toner and a carrier is conventionally widely used in performing 
development. A developing unit using such a two-component developer is so 
designed as to deposit only the toner onto a photosensitive member and 
recover the carrier not deposited onto the photosensitive member. However, 
the carrier is not completely recovered but some of the carrier is 
deposited onto the photosensitive member. If the carrier is deposited onto 
the photosensitive member, the toner present in the periphery of the 
carrier on the photosensitive member cannot enough come into contact with 
a transfer paper, causing the generation of white spots or the like. 
To prevent this defect, there has conventionally been proposed a device for 
collecting the carrier deposited onto the photosensitive member. For 
example, there is proposed in Japanese Patent Laid-open No. Sho 62-262074 
a device for collecting the carrier by forming an electric field between a 
collecting member and a photosensitive member. Further, there is proposed 
in Japanese Utility Model Publication No. Sho 53-32599 a device for 
collecting the carrier by a magnetic attraction force. 
In recent years, a copying machine or a printer using electrophotography 
has a tendency that a gap between a photosensitive member and a developing 
roller is reduced to perform development with a strong development field 
in order to prevent a line of electric force from turning about from an 
edge portion of an image formed on the photosensitive member to a nonimage 
portion, thereby suppressing an edge effect, from the viewpoint of 
improvement in image quality such as reproducibility of a solid portion 
having a uniform density or reproducibility of a fine line. Further, while 
a magnetic brush composed of carrier and toner is formed in development, 
it is necessary to reduce a scavenging force (a force of scavenging the 
photosensitive member) by the magnetic brush from the viewpoint of 
eliminating scavenging lines generated at the solid portion by the 
magnetic brush to perform uniform development. To meet this requirement, 
there is a tendency to use a spherical carrier having a small particle 
size of 50 .mu.m, for example, as compared with a conventional carrier 
having a particle size of 100 .mu.m. 
The combination of such a strong development field and a small-sized 
carrier can provide a high-quality image. However, a magnetic deposition 
force of the small-sized carrier is extremely smaller than that of the 
conventional carrier having a particle size of about 100 .mu.m, and the 
strong development field has an adverse effect to cause a problem that the 
small-sized carrier is easily deposited to a nonimage portion of the 
photosensitive member by an electric field. As mentioned above, if the 
carrier is deposited onto the photosensitive member, the toner present in 
the periphery of the carrier on the photosensitive member cannot enough 
come into contact with a paper in a transfer step, causing transfer defect 
to remarkably reduce the image quality. 
As mentioned above, various devices for collecting the carrier deposited 
onto the photosensitive member have conventionally been proposed. To 
collect the small-sized carrier deposited onto the photosensitive member, 
it is necessary to apply a magnetic field or an electric field stronger 
than that in the case of collecting the conventional large-sized carrier. 
The magnetic field or the electric field can be enhanced by locating the 
collecting device near the photosensitive member. However, mere location 
of the collecting device near the photosensitive member causes a problem 
that the carrier collected by the collecting device may rub against an 
image portion of the photosensitive member during feeding the carrier to 
generate while lines. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a carrier 
collecting device which can effectively collect a carrier from a 
photosensitive member and prevent the carrier collected from rubbing 
against the photosensitive member. 
According to a first aspect of the present invention, there is provided in 
an image forming apparatus for forming a latent image on a photosensitive 
member, developing the latent image by using a two-component developer 
having a carrier and a toner, and transferring a developed image; a 
carrier collecting device for collecting the carrier deposited on the 
photosensitive member in development of the latent image, the carrier 
collecting device comprising a fixed magnet having two poles different in 
polarity, the fixed magnet being disposed so that a peak position of a 
tangential magnetic force between the two poles is closer to the 
photosensitive member than any one of peak positions of normal magnetic 
forces over the two poles. 
According to a second aspect of the present invention, there is provided in 
an image forming apparatus for forming a latent image on a photosensitive 
member, developing the latent image by using a two-component developer 
having a carrier and a toner, and transferring a developed image; a 
carrier collecting device for collecting the carrier deposited on the 
photosensitive member in development of the latent image, the carrier 
collecting device comprising a fixed magnet having two poles different in 
polarity, the fixed magnet forming between the two poles a region having a 
tangential magnetic force greater in value than any one of maximal values 
of normal magnetic forces over the two poles, the fixed magnet being 
disposed so that any position in the region is closest to the 
photosensitive member. 
Preferably, in the carrier collecting device according to the first aspect 
or the second aspect of the present invention, the fixed magnet comprises 
a columnar fixed magnet having a plurality of poles arranged in a 
circumferential direction of the magnet, the columnar fixed magnet 
extending along the photosensitive member; the carrier collecting device 
further comprising a cylindrical capture sleeve located close to the 
photosensitive member for capturing the carrier deposited on the 
photosensitive member, the capture sleeve surrounding the fixed magnet and 
being adapted to rotate along a circumference of the fixed magnet; a 
scraper for scraping off the carrier captured by the capture sleeve from 
the capture sleeve, the scraper being located downstream of a position of 
the capture sleeve closest to the photosensitive member in respect of a 
direction of rotation of the capture sleeve, the scraper being kept in 
contact with or close to the capture sleeve; and a storing case for 
storing the carrier scraped off by the scraper. 
The carrier captured by the magnetic force of the fixed magnet forms a 
carrier chain along a line of magnetic force. Further, a carrier cluster 
(magnetic brush) is formed over an S pole or an N pole. 
Conventionally, the S pole or the N pole of the fixed magnet is usually 
located closest to the photosensitive member. To the contrary, the carrier 
collecting device according to the first aspect or the second aspect of 
the present invention is characterized in that a substantially 
intermediate position between the S pole and the N pole of the fixed 
magnet is closest to the photosensitive member. 
In the carrier collecting device according to the first aspect of the 
present invention, the substantially intermediate position between the S 
pole and the N pole closest to the photosensitive member is defined as a 
geometrical fixed position of the fixed magnet determined from peak 
positions of normal and tangential magnetic forces of the fixed magnet. In 
the carrier collecting device according to the second aspect of the 
present invention, the substantially intermediate position between the S 
pole and the N pole closest to the photosensitive member is defined as a 
geometrical fixed position of the fixed magnet determined by using a 
region present between the S pole and the N pole, the region having a 
tangential magnetic force greater in value than any one of maximal values 
of normal magnetic forces over the S pole and the N pole. 
Thus, in the carrier collecting device according to the first aspect or the 
second aspect of the present invention, the substantially intermediate 
position between the S pole and the N pole of the fixed magnet is closest 
to the photosensitive member. Therefore, the carrier chain is formed in 
the tangential direction with respect to the photosensitive member, and 
the magnetic brush is formed at a position remote from the photosensitive 
member, so that the carrier captured hardly comes into contact with the 
photosensitive member. Accordingly, a high-quality image can be obtained 
as compared with the related art. Otherwise, if the possibility of contact 
is allowed to the same degree as that in the related art, the fixed magnet 
may be disposed closer to the photosensitive member by the corresponding 
distance, thereby generating a stronger magnetic field to increase a 
carrier capturing force. 
Further, since the portion of the fixed magnet closest to the 
photosensitive member is positioned substantially intermediately between 
the S pole and the N pole, a carrier feeding force at this closest portion 
is stronger than that at the other portion over the S pole or the N pole. 
Accordingly, the carrier captured can be moved away from the 
photosensitive member more easily and quickly. 
Further, the stronger the magnetic force over the S pole or the N pole of 
the fixed magnet, the more advantageous. Therefore, the S pole or the N 
pole is magnetized so as to generate a maximum magnetic force determined 
from the material, size, etc. of the fixed magnet, and it is usually 
difficult to obtain a magnetic force greater than the maximum magnetic 
force. In the present invention, however, even when the normal magnetic 
force over the S pole or the N pole is the same as that in the related 
art, the tangential magnetic force at the intermediate portion between the 
S pole and the N pole, i.e., at the portion closest to the photosensitive 
member, can be made stronger than the normal magnetic force over the S 
pole or the N pole by reducing the distance between the S pole and the N 
pole, because the substantially intermediate portion between the S pole 
and the N pole is closest to the photosensitive member. Accordingly, even 
when the distance between the fixed magnet and the photosensitive member 
is the same as that in the related art, the carrier deposited on the 
photosensitive member can be captured by a stronger force by adopting such 
a fixed magnet having the reduced distance between the S pole and the N 
pole. 
As described above, the carrier collecting device of the present invention 
is characterized in that the substantially intermediate position between 
the S pole and the N pole of the fixed magnet is closest to the 
photosensitive member. Accordingly, the carrier captured forms a carrier 
chain in the tangential direction in the vicinity of the photosensitive 
member, thus reducing the possibility of the carrier rubbing against the 
photosensitive member. Furthermore, since the portion closest to the 
photosensitive member has a great carrier feeding force, the carrier 
captured can be fed to the position of the scraper easily and quickly by 
the rotation of the capture sleeve. 
Accordingly, the image forming apparatus employing the carrier collecting 
device of the present invention can provide a high-quality image with 
white spots and white lines reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, reference numeral 10 denotes a photosensitive drum 
rotatably provided in an image forming apparatus. 
A developing device 20 is located close to the photosensitive drum 10. The 
developing device 20 is provided with a developing roller 23 composed of a 
columnar fixed magnet 21 extending in a direction perpendicular to the 
plane of the sheet of FIG. 1 and a cylindrical sleeve 22 surrounding the 
fixed magnet 21. A two-component developer (not shown) composed of carrier 
and toner is stored in a developer tank 24. The developer is agitated by 
an upper auger 25, a lower auger 26, and a counter auger 27. The toner 
attached to the carrier by the agitation is attracted together to the 
fixed magnet 21 and is deposited to the sleeve 22. The sleeve 22 is 
rotated in a direction of arrow B. The developer deposited to the sleeve 
22 is restricted in its layer thickness by a layer thickness restricting 
member 28 during the rotation of the sleeve 22. Thereafter, the developer 
on the sleeve 22 is fed to a position close to the photosensitive drum 10 
by the rotation of the sleeve 22. On the other hand, the photosensitive 
drum 10 is rotated in a direction of arrow A. A latent image formed on the 
photosensitive drum 10 at its upstream portion is developed by the toner 
of the developer on the sleeve 22. The toner splashed in the development 
is recovered through ducts 29 and 30 kept under vacuum. The carrier after 
the development is kept deposited on the sleeve 22 and is returned to the 
developer tank 24. In the development, some of the carrier is deposited to 
the photosensitive drum 10. To collect this carrier from the 
photosensitive drum 10, a carrier collecting device 40 is located on the 
downstream side of the developing device 20. 
The carrier collecting device 40 is provided with a capture roller 43 
composed of a columnar fixed magnet 41 and a capture sleeve 42 surrounding 
the fixed magnet 41 and adapted to rotate in a direction of arrow C. The 
fixed magnet 41 is disposed so that an intermediate position between an 
upstream S pole and an N pole is closest to the photosensitive drum 10. 
The carrier deposited on the photosensitive drum 10 and fed to a position 
opposed to the fixed magnet 41 is attracted by the fixed magnet 41 and is 
deposited to the capture sleeve 42. Then, the carrier deposited to the 
capture sleeve 42 is fed in the direction C. A scraper 44 for scraping off 
the carrier fed by the capture sleeve 42 is located at a position just 
over a downstream S pole of the fixed magnet 41 with respect to the 
direction of rotation (the direction C) of the capture sleeve 42 in such a 
manner as to be in contact with or close to the capture sleeve 42. The 
carrier scraped off by the scraper 44 is stored into a storing case 45. 
FIG. 2A schematically shows the arrangement of the fixed magnet 41 provided 
in the carrier collecting device 40 in this preferred embodiment, whereas 
FIG. 2B schematically shows the arrangement of a fixed magnet 41' in the 
related art for the purpose of comparison with the arrangement shown in 
FIG. 2A. 
As shown in FIG. 2A, the fixed magnet 41 in this preferred embodiment is 
fixedly disposed so that the intermediate position between the upstream S 
pole and the N pole is closest to the photosensitive drum 10. In contrast, 
the related art shown in FIG. 2B uses a strong normal magnetic force in 
the vicinity of a pole (an N pole in this case) of the fixed magnet 41', 
and accordingly the pole is closest to a photosensitive drum 10'. 
FIG. 3 schematically shows such a manner that the carrier at 1 deposited on 
the photosensitive drum 10 is captured by the capture roller 43. 
As shown in FIG. 3, the carrier 1 deposited on the photosensitive drum 10 
is magnetically attracted by the fixed magnet 41 constituting the capture 
roller 43. The carrier 1 attracted by the fixed magnet 41 forms a carrier 
chain such that many carrier particles are chained along a line of 
magnetic force. As the line of magnetic force extends tangentially between 
each S pole and the N pole, the carrier chain is also formed in such a 
direction as to be closely deposited on the capture sleeve 42. 
Accordingly, the possibility that the carrier 1 captured by the capture 
sleeve 42 may come into contact with the photosensitive drum 10 can be 
reduced. Further, the carrier is fed by the rotation of the capture sleeve 
42 more easily between each S pole and the N pole than over each pole. 
Accordingly, the carrier captured by the capture sleeve 42 fast moves away 
from the photosensitive drum 10 in this preferred embodiment employing the 
arrangement of the fixed magnet 41 such that the intermediate position 
between the upstream S pole and the N pole is closest to the 
photosensitive drum 10. The line of magnetic force over each pole of the 
fixed magnet 41 extends normally, so that a carrier cluster (magnetic 
brush) is formed over each pole. However, since each pole of the fixed 
magnet 41 is positioned remote from the photosensitive drum 10, the 
possibility that the magnetic brush may rub against the photosensitive 
drum 10 can also be reduced. 
In this manner, the generation of white lines due to rubbing of the carrier 
against an image region on the photosensitive drum 10 can be avoided and 
the carrier can be effectively collected according to this preferred 
embodiment. 
FIG. 4 shows measured values of magnetic forces of the fixed magnet, 
wherein the distribution of normal magnetic force is shown by a solid 
line, and the distribution of tangential magnetic force is shown by a 
broken line. 
As shown in FIG. 4, the normal magnetic force is strong over each pole, and 
the tangential magnetic force is strong between poles. Further, the 
distance between the upstream S pole (S1 pole) and the N pole is smaller 
than the distance between the N pole and the downstream S pole (S2 pole). 
In the fixed magnet with the smaller distance between the upstream S pole 
and the N pole, there is formed therebetween a region D having a magnetic 
force (tangential magnetic force) stronger than the magnetic force over 
each pole (normal magnetic force). Accordingly, by disposing the fixed 
magnet in such a manner that any position in the region D (typically, a 
peak position of the tangential magnetic force) is closest to the 
photosensitive drum 10, the carrier deposited on the photosensitive drum 
10 can be more effectively captured by a magnetic force stronger than that 
in the case where a pole position is closest to the photosensitive drum 
10, in addition to the above-mentioned advantages that the carrier chain 
is tangentially formed between poles to thereby reduce the possibility of 
the carrier rubbing against the photosensitive drum 10 and that the 
feedability of the carrier is high between poles. 
FIG. 5 shows test data in which the number of white spots is compared 
between the preferred embodiment and the related art. In this test, a 
magnet having the magnetic force distribution shown in FIG. 4 was used, 
and the magnet was fixed in such a manner that the intermediate position 
between the S1 pole and the N pole was closest to the photosensitive drum 
as shown in FIG. 2A and that the N pole was closest to the photosensitive 
drum as shown in FIG. 2B. In FIG. 5, the preferred embodiment (interpole 
type) is shown by a broken line, and the related art (over-pole type) is 
shown by a solid line. 
The distance between the photosensitive drum 10 and the capture sleeve 42 
(see FIG. 1) was 0.48 mm at one axial end, 0.49 mm at the axial center, 
and 0.48 mm at the other axial end in both the interpole type (the 
preferred embodiment) and the over-pole type (the related art) as the 
results of measurement. In a development step for magenta with use of a 
color printer, a potential difference .vertline.V.sub.bkg -V.sub.bias 
.vertline. between a charge potential V.sub.bkg as a background of the 
photosensitive drum 10 and a bias potential V.sub.bias applied to the 
developing roller 23 was changed. After the development of magenta, a 
half-tone image having a cyan image density of 20% was developed to obtain 
a visible image. In this visible image, the number of white spots per B4 
size was measured. 
As shown in FIG. 5, the result of measurement showed that the number of 
white spots in the interpole type (the preferred embodiment) wherein the 
intermediate position between the S1 pole and the N pole was closest to 
the photosensitive drum was reduced to substantially half the number of 
white spots in the over-pole type (the related art) wherein the N pole was 
closest to the photosensitive drum. 
In addition to the evaluation shown in FIG. 5, the frequency of occurrence 
of white lines was also evaluated. As the result, it was confirmed that 
the frequency of occurrence of white lines in the preferred embodiment was 
greatly reduced as compared with that in the related art. 
While the invention has been described with reference to a specific 
embodiment, the description is illustrative and is not to be construed as 
limiting the scope of the invention. Various modifications and changes may 
occur to those skilled in the art without departing from the spirit and 
scope of the invention as defined by the appended claims.