Title: Image forming method and apparatus
Abstract: A method for developing an electrostatic latent image, including forming a magnet brush of a developer including a toner and a carrier on a developing sleeve including a main magnet and auxiliary magnets; and developing the electrostatic latent image with the magnet brush to form a toner image at a rubbing region, wherein the magnetic flux density in a normal line direction, half width, and attenuation ratio of the main magnet and the angle between the main magnet and auxiliary magnets are specified, and the magnetic sleeve has specific grooves thereon, and wherein the toner has a volume average particle diameter of from 4.0 to 7.0 μm, and includes fine particles having a circle equivalent diameter not greater than 2 μm in an amount not greater than 20% by number.
Patent Number: 6,947,692 Issued on 09/20/2005 to Kondo, et al.
| Inventors:
|
Kondo; Maiko (Numazu, JP);
Sasaki; Fumihiro (Fuji, JP);
Nanya; Toshiki (Mishima, JP);
Yagi; Shinichiro (Numazu, JP);
Tomita; Masami (Numazu, JP);
Emoto; Shigeru (Numazu, JP);
Shimota; Naohito (Suntoh-gun, JP);
Higuchi; Hiroto (Numazu, JP);
Ichikawa; Tomoyuki (Numazu, JP)
|
| Assignee:
|
Ricoh Company Limited (Tokyo, JP)
|
| Appl. No.:
|
666254 |
| Filed:
|
September 22, 2003 |
Foreign Application Priority Data
| Sep 20, 2002[JP] | 2002-275550 |
| Current U.S. Class: |
399/277; 399/276 |
| Intern'l Class: |
G03G 015/09 |
| Field of Search: |
399/111,252,265,267,276,277
|
References Cited [Referenced By]
U.S. Patent Documents
| 6593048 | Jul., 2003 | Sasaki et al.
| |
| 6597883 | Jul., 2003 | Muramatsu et al.
| |
| 6653037 | Nov., 2003 | Sawada et al.
| |
| 6660443 | Dec., 2003 | Sugiyama et al.
| |
| 6667141 | Dec., 2003 | Iwamoto et al.
| |
| 6778805 | Aug., 2004 | Kai et al.
| |
| 6792234 | Sep., 2004 | Ikeguchi et al.
| |
| Foreign Patent Documents |
| 2001-10336 | Jan., 2000 | JP.
| |
| 2000/-305360 | Nov., 2000 | JP.
| |
| 2000/-347506 | Dec., 2000 | JP.
| |
Primary Examiner: Tran; Hoan
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
1. An image forming apparatus comprising:
an image bearing member configured to bear an electrostatic latent image thereon;
a developing sleeve comprising:
a nonmagnetic sleeve having grooves with a depth of from 0.1 to 0.2 mm on an
outer surface thereof in a longitudinal direction thereof at an interval of from
0.4 to 0.6 mm; and
a magnet roller fixedly set in the nonmagnetic sleeve,
wherein the developing sleeve magnetically bears thereon a magnetic two component
developer comprising a toner and a carrier while rotating to form a magnet brush
thereon,
wherein the developing sleeve rubs the image bearing member with the magnet brush
to visualize the electrostatic latent image at a rubbing region,
wherein the magnet roller comprises a main magnet pole, which faces the latent
image bearing member and which comprises a main magnet and auxiliary magnets adjacent
to the main magnet,
wherein the main magnet has a magnetic flux density in a normal line direction
of from 100 to 200 mT at the rubbing region, an attenuation ratio of the magnetic
flux not less than 40% and a half width not greater than 25°, and each of
the auxiliary magnets has an attenuation ratio of a magnetic flux density in a
normal line direction not less than 40%, and is arranged at an angle not greater
than 35° from the main magnet,
and wherein the toner has a volume average particle diameter of from 4.0 to 7.0
μm, and includes fine particles having a circle equivalent diameter not greater
than 2 μm in an amount not greater than 20% by number.
2. The image forming apparatus according to claim 1, wherein the toner comprises
at least a wax and a binder resin, and wherein when a cross section of particles
of the toner was observed with a transmission electron microscope, a surface portion
of the particles of the toner, which surface portion has a depth of from 0 to 1
μm, has a wax area of from 5 to 30%.
3. The image forming apparatus according to claim 2, wherein the wax exists in
an outer portion of the particles of the toner, which outer portion has a depth
of from 0 to half a radius of the particles, in an amount not less than 65% by
number of the wax dispersed in the entire toner.
4. The image forming apparatus according to claim 3, wherein the wax dispersed
in the toner does not appear on a surface of the toner.
5. The image forming apparatus according to claim 2, wherein particles of the
wax having a dispersion diameter of from 0.5 to 3 μm are present in the particles
of the toner in an amount not less than 70% by number based on total wax particles
in the particles of the toner.
6. The image forming apparatus according to claim 2, wherein the wax is selected
from carnauba waxes subjected to a treatment of removing a free aliphatic fatty
acid, rice waxes, montan waxes and combinations thereof.
7. A method for developing an electrostatic latent image, comprising:
forming a magnet brush of a magnetic developer comprising a toner and a carrier
on a developing sleeve comprising a nonmagnetic sleeve and a magnet roller located
in the nonmagnetic sleeve; and
rubbing a surface of an image bearing member bearing the electrostatic latent
image thereon with the magnet brush to from a toner image on the image bearing
member,
wherein the magnet roller comprises a main magnet pole, which faces the latent
image bearing member and which comprises a main magnet and auxiliary magnets adjacent
to the main magnet,
wherein the main magnet has a magnetic flux density in a normal line direction
of from 100 to 200 mT at the rubbing region, an attenuation ratio of the magnetic
flux not less than 40% and a half width not greater than 25°, and each of
the auxiliary magnets has an attenuation ratio of a magnetic flux density in a
normal line direction not less than 40%, and is arranged at an angle not greater
than 35° from the main magnet, wherein the nonmagnetic sleeve has grooves
with a depth of from 0.1 to 0.2 mm on an outer surface thereof in a longitudinal
direction thereof at an interval of from 0.4 to 0.6 mm,
and wherein the toner has a volume average particle diameter of from 4.0 to 7.0
μm, and includes fine particles having a circle equivalent diameter not greater
than 2 μm in an amount not greater than 20% by number.
8. The image forming method according to claim 7, wherein the toner comprises
at least a wax and a binder resin, and wherein when a cross section of particles
of the toner was observed with a transmission electron microscope, a surface portion
of the particles of the toner having a depth of from 0 to 1 μm has a wax
area of from 5 to 30%.
9. The image forming method according to claim 8, wherein the wax exists in an
outer portion of the particles of the toner, which outer portion has a depth of
from 0 to half a radius of the particles, in an amount not less than 65% by number
of the wax dispersed in the entire toner.
10. The image forming method according to claim 9, wherein the wax dispersed
in the toner does not appear on a surface of the toner.
11. The image forming method according to claim 8, wherein particles of the wax
having a dispersion diameter of from 0.5 to 3 μm are present in the particles
of the toner in an amount not less than 70% by number based on total wax particles
in the toner.
12. The image forming method according to claim 8, wherein the wax is selected
from carnauba waxes subjected to a treatment of removing a free aliphatic fatty
acid, rice waxes, montan waxes and combinations thereof.
13. A process cartridge for an image forming apparatus, comprising:
an image bearing member configured to bear an electrostatic latent image thereon;
and
a developing device configured to develop the electrostatic latent image with
a developer comprising a toner to form a toner image on the image bearing member,
wherein the developing device comprises:
a developing sleeve comprising:
a nonmagnetic sleeve having grooves with a depth of from 0.1 to 0.2 mm on an
outer surface thereof in a longitudinal direction thereof at an interval of from
0.4 to 0.6 mm; and
a magnet roller fixedly set in the nonmagnetic sleeve,
wherein the developing sleeve magnetically bears thereon a magnetic two component
developer comprising a toner and a carrier while rotating to form a magnet brush
thereon,
wherein the developing sleeve rubs the image bearing member with the magnet brush
to visualize the electrostatic latent image at a rubbing region,
wherein the magnet roller comprises a main magnet pole, which faces the latent
image bearing member and which comprises a main magnet and auxiliary magnets adjacent
to the main magnet,
wherein the main magnet has a magnetic flux density in a normal line direction
of from 100 to 200 mT at the rubbing region, an attenuation ratio of the magnetic
flux not less than 40% and a half width not greater than 25°, and each of
the auxiliary magnets has an attenuation ratio of a magnetic flux density in a
normal line direction not less than 40%, and is arranged at an angle not greater
than 35° from the main magnet,
and wherein the toner has a volume average particle diameter of from 4.0 to 7.0
μm, and includes fine particles having a circle equivalent diameter not greater
than 2 μm in an amount not greater than 20% by number.
14. The process cartridge according to claim 13, wherein the toner comprises
at least a wax and a binder resin, and wherein when a cross section of particles
of the toner was observed with a transmission electron microscope, a surface portion
of the particles of the toner, which surface portion has a depth of from 0 to 1
μm, has a wax area of from 5 to 30%.
15. The process cartridge according to claim 14, wherein the wax exists in an
outer portion of the particles of the toner, which outer portion has a depth of
from 0 to half a radius of the particles, in an amount not less than 65% by number
of the wax dispersed in the entire toner.
16. The process cartridge according to claim 15, wherein the wax dispersed in
the toner does not appear on a surface of the toner.
17. The process cartridge according to claim 14, wherein particles of the wax
having a dispersion diameter of from 0.5 to 3 μm are present in the particles
of the toner in an amount not less than 70% by number based on total wax particles
in the particles of the toner.
18. The process cartridge according to claim 14, wherein the wax is selected
from carnauba waxes subjected to a treatment of removing a free aliphatic fatty
acid, rice waxes, montan waxes and combinations thereof.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming method and an image forming
apparatus. Specifically, the present invention relates to a method for developing
an electrostatic latent image with a developer, and to a developing apparatus including
magnetic poles for forming a magnetic brush of a developer in a developing region
on a surface of a developer bearing member. In addition, the present invention
also relates to a process cartridge which produces images using a toner, and a
method for fixing a toner image.
2. Discussion of the Background
Copiers, printers, facsimile apparatus and similar electrophotographic or
electrostatic image forming apparatus generally include a latent image bearing
member such as photoconductive drums or photoconductive belts. A latent image is
formed on the image bearing member in accordance with image data. It is popular
to use a magnet brush developing method using a two-component developer made of
a toner and a carrier from the view point of image transferability, halftone reproducibility
and temperature/humidity stability of developing characteristics. In such a developing
method, the two-component developer forms brush chains on a developer bearing member
and is fed to a developing region where the developer bearing member faces the
image bearing member. At the developing region, the toner in the developer adheres
to an electrostatic latent image portion formed on the latent image bearing member.
The above-mentioned developer bearing member usually includes a cylindrical sleeve
and a magnet roller located in the sleeve for forming a magnetic field by which
a developer forms a magnet brush on the surface of the sleeve.
By rotating at least one of the above-mentioned sleeve and magnet roller, erected
chains of the developer are moved on the surface of the sleeve. The developer conveyed
to the developing region is erected along lines of the magnetic force caused by
a main development magnetic pole. The brush chains contact the surface of the latent
image bearing member while yielding, and the brush chains rub the latent image
because of moving at a linear velocity different from that of the latent image
bearing member. At this time, the developer provides the toner for the latent image,
resulting in development of the latent image.
Published unexamined Japanese Patent Applications Nos. 2000-305360 and
2000-347506 have proposed image forming technologies to improve image quality of
both a high density image portion and a low density image portion at the same time.
It is disclosed therein a developing apparatus which visualizes an electrostatic
latent image on an image bearing member and which includes a developing sleeve
including a nonmagnetic sleeve, and a magnet roller fixedly set within the nonmagnetic
sleeve and including plural magnets arranged at a regular angle, wherein the developing
sleeve magnetically bears a magnetic two-component developer including a toner
and a carrier to form a magnet brush thereon, and wherein the developing sleeve
rubs the image bearing member with the magnet brush to visualize the electrostatic
latent image at a rubbing region. In this developing apparatus, the attenuation
ratio of a magnetic flux density at the rubbing region in a normal line direction
is specified. In addition, the attenuation ratio of magnetic flux densities of
a main magnet and a magnet adjacent thereto at the rubbing region in a normal line
direction, or an angle between the main magnet and the magnet adjacent to the main
magnet at the rubbing region are specified.
However, in such a high-efficiency developing method in which a magnetic
force of a main development magnetic pole is high, and a developer having a short
length of magnet brush rubs a surface of a photoreceptor at a rotating speed of
from 1.1 to 3.0 times that of the photoreceptor, the toner is insufficiently supplied
i.e., the resultant images have a low image density or the resultant images are
unclear when the rotating speed is less than 1.5 times. Therefore the rotating
speed ratio is preferably not less than 1.5 times. In this case, a rear-end omission
problem in that the rear end of a solid image is omitted occurs. Such a problem
tends to be caused under a condition in which the rotating speed ratio is greater
than 1.0. This problem is seriously caused as the rotating speed of the magnetic
brush increases.
In order to prevent the rear-end omission problem of toner in such a developing
process, i.e., in order to obtain satisfactory image density and image qualities,
it is necessary to improve developing ability by another method.
Currently, a toner having a smaller particle size is desired to produce
high quality images.
When the particle size of a toner is miniaturized, the content of fine particles
in the toner increases. It is confirmed by experiment that a toner having a small
particle diameter remarkably contaminates a developing sleeve. The mechanism of
this phenomenon is as follows. Toner particles present on a portion of a developing
sleeve corresponding to a non-image portion of a photoreceptor is pushed toward
the developing sleeve by an electric field. Normally, the toner particles are quickly
re-adhered to a surface of the carrier due to electrostatic attraction. However,
since fine toner particles have extremely bad fluidity (characteristic specific
to a fine powder), the fine toner particles adhered to the, developing sleeve are
hardly re-adhered to the carrier surface. Namely, adhesion strength of find toner
particles against the developing sleeve is extremely strong. Furthermore, when
the fine toner particles adhered to the developing sleeve are rubbed with the carrier
many times, fusion-bonding of the toner to the sleeve occurs (hereinafter this
phenomenon is referred to as on-sleeve toner fixation). When this on-sleeve toner
fixation occurs, the image density decreases with time. In particular, when a solid
image is printed continuously on four sheets of paper after a 100,000-copy running
test, it is found that the image density of the solid image gradually decreases
from the first sheet to the fourth sheet. Namely, since an electrically insulating
layer constituted of a toner ingredient is formed on a surface of an electroconductive
sleeve, the effective bias of the developing bias applied lowers, and thereby the
developing ability of the developing sleeve is deteriorated.
Published unexamined Japanese Patent Application No. 2000-10336 proposes
that a developing sleeve is subjected to a blast treatment with spherical particles
to form smooth unevenness portion thereon in order to prevent adhesion of a toner
to a developing sleeve. The adhesion of a toner to a sleeve can be prevented to
some extent by this method, but the ability of the sleeve to feed a developer is
not enough for current fast printing machines, and high quality images cannot be
obtained easily.
On the other hand, a wax is conventionally included in a toner in order to impart
a releasing property to the toner at fixation.
Since waxes have a smaller molecular weight and is softer than a binder resin,
so-called a filming phenomenon in that the waxes adhere to a carrier and a photoreceptor
and thereby a wax film is formed thereon tends to occur. When the filming phenomenon
occurs on the carrier (i.e., a spent carrier problem), the toner cannot be friction-charged
with such a carrier. As a result, defective charging occurs and the resultant images
have background fouling. In addition, a white stripe abnormal image appears on
a halftone image when a wax film is formed on a photoreceptor. In addition, waxes
tend to cause the on-sleeve toner fixation. These phenomena turn worse, i.e., it
is difficult to maintain the initial image qualities, when copying processes are repeated.
In addition, it is known that temperature increases in a developing apparatus
with repetition of copying processes, resulting in increase of the atmospheric
temperature at a nip region. The heat is easy to stay in the above-mentioned developer,
which has a high density of brush chains, i.e., the heat tends to hardly leak from
the developing region. As a result, the wax in the toner easily bleeds out, resulting
in occurrence of fixation of the wax on the sleeve, and filming of the wax on the
photoreceptor and the carrier used.
Because of these reasons, a need exists for an image forming method by which
high quality images are stably produced for a long period of time.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an image
forming method and apparatus by which high quality images are produced for a long
period of time without causing the rear-end omission problem.
To achieve such a object, the present invention provides an image forming apparatus
including an image bearing member bearing an electrostatic latent image thereon;
a developing sleeve including a nonmagnetic sleeve; and a magnet roller fixedly
arranged within the nonmagnetic sleeve and including plural magnets.
The developing sleeve magnetically bears a magnetic two-component developer including
a toner and a carrier to form a magnet brush thereon. The developing sleeve rubs
the image bearing member with the magnet brush to visualize the electrostatic latent
image at a rubbing region. The magnet roller has a main magnet pole including a
main magnet and auxiliary magnets adjacent to the main magnet, which are positioned
so as to face the latent image bearing member.
The main magnet has a magnetic flux density of from 100 to 200 mT at the rubbing
region in a normal line direction, and a half width of the magnetic flux density
not greater than 25°. The auxiliary magnet has an attenuation ratio of a magnetic
flux density in a normal line direction not less than 40%, while the magnets are
arranged at an angle not greater than 35°. The nonmagnetic sleeve has grooves
on an outer surface thereof which is formed in a longitudinal direction thereof
at an interval of from 0.4 to 0.6 mm with a depth of from 0.1 to 0.2 mm.
The toner has a volume average particle diameter of from 4.0 to 7.0 μm,
and includes fine powders having a circle equivalent diameter not greater than
2 μm in an amount not greater than 20% by number.
The toner preferably includes at least a wax and a binder resin. When a cross
section of the toner is observed with a transmission electron microscope, a surface
portion of the toner, which portion has a depth of from 0 to 1 μm has a wax
area of from 5 to 30%.
The wax preferably exists in the outer portion of toner particles, which outer
portion is defined as an outer portion of toner particles having a depth from 0
to half the radius of the toner particles, in an amount not less than 65% by number
of the wax dispersed in the entire toner particles.
It is preferable that the wax dispersed in the toner does not appear on a surface
of the toner.
It is preferable that particles of the wax having a dispersion diameter of from
0.5 to 3 μm are dispersed in the toner in an amount not less than 70% by
number based on total particles in the toner.
The wax is preferably selected from carnauba waxes subjected to a treatment of
removing a free aliphatic fatty acid, rice waxes, montan waxes and combinations thereof.
As another aspect of the present invention, a method for developing an electrostatic
latent image is provided, which includes forming a magnet brush of a magnetic developer
including a toner and a carrier on the developing sleeve mentioned above and rubbing
a surface of an image bearing member bearing the electrostatic latent image thereon
with a magnet brush to form a toner image on the image bearing member.
As yet another aspect of the present invention, a process cartridge for an image
forming apparatus is provided which includes:
at least an image bearing member configured to bear an electrostatic latent image
thereon; and
a developing device configured to develop the electrostatic latent image using
a developer including the toner mentioned above and the developing sleeve mentioned
above to form a toner image on the image bearing member.
The process cartridge may include a charger configured to charge the image bearing
member; a cleaner configured to clean a surface of the image bearing member; and
other devices for use in the image forming apparatus of the present invention.
These and other objects, features and advantages of the present invention will
become apparent upon consideration of the following description of the preferred
embodiments of the present invention taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Various other objects, features and attendant advantages of the present invention
will be more fully appreciated as the same becomes better understood from the detailed
description when considered in connection with the accompanying drawings in which
like reference characters designate like corresponding parts throughout and wherein:
FIG. 1 is a schematic diagram illustrating an example of the image forming apparatus
of the present invention;
FIG. 2 is a schematic diagram for explaining the magnetic flux density of a
developing sleeve for use in the image forming apparatus of the present invention;
FIG. 3 is a schematic diagram for explaining the constitution of a developing
sleeve for use in the image forming apparatus of the present invention;
FIG. 4 is a schematic diagram illustrating the constitution and the magnetic
flux density of a conventional developing sleeve;
FIG. 5 is a schematic view illustrating the cross section of an embodiment of
the process cartridge of the present invention; and
FIGS. 6A-6D are schematic views of the developing portion of a magnet brush
developing device and views for explaining the mechanism of formation of white
spots at a rear end of an image.
DETAILED DESCRIPTION OF THE INVENTION
The image forming method and apparatus of the present invention will be explained
in detail referring to drawings.
As illustrated in FIG. 1, the image forming apparatus includes a photoreceptor
A serving as an electrostatic latent image bearing member, a charger
2 configured
to charge a surface of the photoreceptor A, a laser beam
3 configured to
form a latent image on the uniformly charged surface of the photoreceptor A, a
developing device
4 configured to form a toner image by developing the latent
image on the photoreceptor with a developer including a toner, a transferer
5
configured to transfer the toner image formed on the photoreceptor to a recording
paper and a cleaner configured to remove residual toner particles on the photoreceptor.
In such a constitution, the photoreceptor
1, the surface of which is uniformly
charged by the charger
2, forms an electrostatic latent image by being exposed
to the laser beam
3. The latent image is developed with the developing apparatus
4, and thereby a toner image is formed on the photoreceptor
1. The
toner image is transferred from the surface of the photoreceptor
1 to the
recording paper, which is fed from a sheet feeding tray (not shown), by the transferer
5 including a transfer belt or the like. The recording paper electrostatically
adhered to the photoreceptor during the image transfer process is separated from
the photoreceptor by a separation pick. Then the unfixed toner image on the recording
paper is fixed to the recording paper by a fixer (not shown). On the other hand,
the residual toner on the photoreceptor
1 is removed therefrom by a cleaner
6 and the toner is collected. Thus the photoreceptor
1 is initialized
to be used for the next image forming process.
In the developing device
4, a developing roller
46 serving as a
developer bearing member is provided close to the photoreceptor
1 and a
developing region is formed at a position at which the developing roller
46
and the photoreceptor A are opposed to each other. The developing roller
46
includes a cylindrical sleeve
46s which is formed of a nonmagnetic
material such as aluminum, brass, stainless and electroconductive resins and which
is rotated in a counterclockwise direction by a rotating drive system (not shown).
In this example, the inner diameter of the drum of the photoreceptor 1 is 100 mm
and the linear velocity of the drum is set to 330 mm/second. The inner diameter
of the cylindrical sleeve
46s is 25 mm and the linear velocity of
the sleeve is set to 660 mm/second. Therefore, the ratio of the linear velocity
of the photoreceptor drum to the linear velocity of the cylindrical sleeve is 2.0.
In addition, the developing gap, i.e., a gap between the photoreceptor
1
and the developing sleeve
46s, is set to 0.5 mm.
Normally, the surface of the developing sleeve
46s is subjected
to a surface roughening treatment so as to have grooves having a width of 0.2 mm
in the longitudinal direction thereof at an interval of from 0.7 mm to 1.0 mm.
In the present invention, the interval of the grooves is set to be from 0.4 mm
to 0.6 mm to increase the surface area of the sleeve, resulting in increase of
density of the brush chain.
A doctor blade
47 is positioned on an upstream side of the developing
region
relative to the developer feeding direction (i.e., a counterclockwise direction
in FIG.
1). The doctor blade
47 controls the height of the brush
chains, i.e., the amount of the developer on the developing sleeve. In this example,
the gap between the doctor blade
47 and the developing sleeve
46s
is set to 0.48 mm. Further, a screw
45 is provided at a location opposite
to the photoreceptor
1 relative to the developing roller
46 to transport
the developer in the developing casing
40 to the developing roller
46
while agitating the developer.
Then the configuration of the magnet roller in the developing roller
46
will be explained. The magnet roller forming a magnetic field is fixedly arranged
in the sleeve such that the developer rises on the developing sleeve
46s
in the form of chains. The carrier in the developer is raised on the developing
sleeve
46s along the magnetic lines in normal direction in the form
of chains. Charged toner particles are adhered to the carrier chains, thereby forming
a magnetic brush. The developing sleeve
46s conveys the magnetic
brush counterclockwise, i.e., in the rotation direction of the sleeve
46s.
FIG. 4 is a schematic view illustrating a conventional developing sleeve including
only one magnet as a main magnet pole.
In FIG. 4, a main development magnetic pole P
1 is a north pole which forms
a magnetic brush for developing an electrostatic latent image. The developing roller
further includes magnets P
2, P
3, P
4, P
5 and P
6.
In contrast, as illustrated in FIG. 2, the magnet roller of the present invention
has a plurality of magnets, P
1a, P
1b and P
1c,
as the main development magnetic pole. The magnets P
1a, P
1b
and P
1c are positioned so as to face the latent image bearing
member, i.e., the magnets are located in a region in which the latent image bearing
member is rubbed with the magnet brush.
As illustrated in FIG. 2, the main development magnetic pole P
1 includes
three magnets, P
1a, P
1b and P
1c each
of which has a small cross-section area, are arranged in this order in the developer
feeding direction. The magnet P
1b is the main. magnet and the magnets
P
1a and P
1c are auxiliary magnets. These magnets are
formed of a rare earth metal alloy.
Then the magnetic properties of the developing roller will be explained in detail.
The magnetic flux density at the surface of the developing sleeve in the normal
direction is shown in dashed lines in FIGS. 2 and 3. A gauss meter HGM-8300 and
an axial probe TYPE A1 both manufactured by ADS Co., Ltd. are used for measuring
the magnetic flux densities in the normal direction and the magnetic flux densities
are recorded in a circle chart.
The attenuation rate is defined as a ratio of a peak value of the magnetic flux
density in the normal line direction at a point distanced from the surface of the
developing sleeve by 1 mm to the peak value of the magnetic flux density in the
normal direction at the surface of the developing sleeve (in units of %) The magnetic
flux density at the point distanced from the surface of the developing sleeve by
1 mm is indicated in dotted lines in FIGS. 2 and 4.
Then the half value central angle will be explained referring to FIG.
3.
The half value central angle of the magnet P
1a is defined as an angle
formed by a line L
1 (i.e., the maximum magnetic force line) and a line L
2
passing through a point having a half magnetic force of the maximum magnetic force.
If the maximum magnetic force of the magnet is 120 mT, the half value is 60 mT.
In this example, the main magnet P
1b, a magnet P
4 for drawing
the developer onto the developing sleeve
46, a magnet P
6 feeding
the drawn developer to a developing region and magnets P
2 and P
3
feeding the developer in a region after the developing region form N poles. The
auxiliary magnets P
1a and P
1c and a magnet P
5
feeding the drawn developer form S poles. A magnet having a normal direction magnetic
force not less than 120 mT at the surface of the developing roller is used as a
main magnet P
1b. When both the main magnet P
1b and
the auxiliary magnet P
1c positioned on a downstream side of the main
magnet P
1b have a magnetic force, for example, not less than 100
mT, problems such as adhesion of carrier particles on a photoreceptor
1
are not caused. When the magnets have a magnetic force not greater than 100 mT,
the carrier adhesion problem is caused. The tangential magnetic force mainly influences
on the carrier adhesion problem. In order to increase the tangential magnetic force,
the magnetic force of P
1b and P
1c has to be increased.
Occurrence of the carrier adhesion problem can be prevented by sufficiently increasing
the magnetic force of either the main magnet or the auxiliary magnets. In this
example, the width of the magnets P
1a, P
1b and P
1c
is 2 mm. In addition, the half value central angle is 16° in this case.
When the half value central angle of the main magnet is greater than 25°,
an abnormal image tends to be produced. For comparison, magnetic forces of a conventional
magnetic roller are illustrated in FIG.
4.
The half value central angles of the auxiliary magnets P
1a and
P
1c are preferably not greater than 35°. In addition, as illustrated
in FIG. 3, the angle formed by the auxiliary magnet P
1a or P
1c
and the main magnet P
1b is preferably not greater than 30°.
In the above-mentioned example, the angle is set to 25° such that the half
value central angle of the main magnet is 16°. Further, the angle between
the transition point, where polarity changes from the N pole to the S pole or vice
versa, of the auxiliary magnetic pole P
1a and the magnetic pole P
6
and the transition point of the auxiliary magnetic pole P
1c and the
magnetic pole P
2 is set to not greater than 120°.
The magnetic flux density of the main magnetic pole P
1b in a normal
line direction is 120 mT at the sleeve surface and is 55.8 mT at a point distanced
from the sleeve surface by 1 mm. Namely, the variation of the magnetic flux density
thereof in a normal line direction is 64.2 mT, and the attenuation ratio thereof
is 53.5% (i.e., (64.2/120)×100). The magnetic flux density of the auxiliary
magnetic pole P
1a located on an upstream side from the main magnetic
pole P
1b in a normal line direction is 100 mT at the sleeve surface
and is 53.3 mT at a point distanced from the sleeve surface by 1 mm. The variation
of the magnetic flux density thereof in a normal line direction is 46.7 mT, and
the attenuation ratio thereof is 46.7%. The auxiliary magnetic pole P
1c
located on an downstream side from the magnetic flux density of the main magnetic
pole P
1b in a normal line direction is 120 mT at the sleeve surface
and is 67.4 mT at a point distanced from the sleeve surface by 1 mm. The variation
of the magnetic flux density thereof in a normal line direction is 52.6 mT, and
the attenuation ratio thereof is 43.8%.
In this example, among the magnet brushes formed by the developer along the lines
of magnetic forces of the magnet roller, only the brush formed on the main magnet
P
1b is brought into contact with a photoreceptor and an electrostatic
latent image on the photoreceptor is developed with the brush. When the magnet
brush is observed while not being brought into contact with the photoreceptor,
it is found that the magnet brush has a length of about 1 mm, and is shorter than
the magnet brushes formed by conventional magnet rollers. Namely, the magnetic
brush is thicker than the conventional magnetic brushes.
When the gap between the developer controlling member and the developing sleeve
is the same as that of conventional developing devices, the amount of the developer
passing the gap is the same. Therefore, it is confirmed that the magnetic brush
in the developing region in the present invention is shorter and thicker than that
of the conventional developing devices. The reason therefor is as follows. Since
the magnetic flux density in a normal line direction at a point distanced from
the developing sleeve by 1 mm is greatly decreased in the present invention, a
brush chain cannot be formed at a point distanced from the developing sleeve and
therefore the magnet brush is short, i.e., a thick magnet brush is formed on the
surface of the developing sleeve.
In a case of the conventional magnet roller illustrated in FIG. 4, the magnetic
flux density in a normal line direction at the surface of the sleeve is 90 mT,
the magnetic flux density in a normal line direction at a point distanced from
the surface of the sleeve by 1 mm is 63.9 mT, the variation of the magnetic flux
density in a normal line direction is 26.1 mT, and the attenuation ratio is 29%
which is much smaller than that in the developing roller in the present invention.
In addition, it is possible to control the attenuation ratio so as to be not
less
than 40% or to control the half value central angle so as to be less than 25°
by setting magnets such that the angle therebetween is not greater than 35°.
In addition, it is preferable that the magnetic flux density in a normal line
direction of the main magnetic pole P
1b is 120 mT at the surface
of the developing sleeve (i.e., within a range of from 100 to 200 mT).
When the attenuation ratio is less than 40%, the magnet brush tends to be long.
Since the developing gap is narrow in the present embodiment, the magnet brush
contacts a surface of the photoconductive drum, which surface has not reached to
a developing nip region, and thereby appropriate development cannot be performed.
In order to increase the attenuation ratio, methods such as selecting proper
magnet
materials for the development magnetic pole and strongly turning the line of magnetic
force generating from the development magnetic pole inside. Specific examples of
the latter method include a method in which the development magnetic pole is constituted
of a main magnetic pole erecting the magnet brush and auxiliary magnets which have
an opposite pole and are positioned on upstream and downstream sides of the main
magnet relative to the rotating direction of the developer bearing member. In addition,
there is another method in which by providing a magnetic pole, such as a transfer
magnetic pole, other than the development magnetic pole, the line of magnetic force
generating from the development magnetic pole is turned inside, resulting in narrowing
of the half width of the development magnetic pole. The half width is preferably
set to not greater than 22° and more preferably not greater than 18°.
It is experimentally confirmed that the attenuation ratio increases when the half
width of the development magnetic pole is narrowed. When the half width is not
greater than 25°, the magnetic flux density in a radial direction is decreased,
resulting that the magnet brush hardly has a high density.
In addition, the line of magnetic force of the main magnetic pole (P
1b)
can be turned inside by providing auxiliary magnetic poles (P
1a and
P
1c). In this case, the magnet brush is formed uniformly without
changing the length in the longitudinal direction in the developing region and
thereby the rear-end omission problem in that white spots are formed at a rear
end in the longitudinal direction of an image can be avoided.
When the above conditions are fulfilled in the method in which a magnet brush
formed by the main magnet is brought into contact with a photoreceptor to develop
a latent image, and the developing nip is set to be not less than the particle
diameter of a developer and not greater than 2 mm, a problem in that a white spots
(omissions) are formed at an end portion of images can be avoided, and small images
such as horizontal thin lines and 1-dot images can be well produced.
FIG. 5 is a schematic view illustrating the cross section of an embodiment of
the process cartridge of the present invention. Numeral
21 denotes a process
cartridge. The process cartridge
21 includes a photoreceptor
22 serving
as an image bearing member bearing an electrostatic latent image thereon, a charger
23 which charges the photoreceptor
22, a developing roller
24
serving as a member of a developing device which develops the electrostatic latent
image on the photoreceptor
22 with the developer of the present invention
to form a toner image on the photoreceptor
22, and a cleaning blade
25
which serves as a cleaner and which removes toner particles remaining on the surface
of the photoreceptor
22 after the toner image on the photoreceptor
22
is transferred onto a receiving material (not shown).
The process cartridge is not limited to the process cartridge
21 illustrated
in FIG.
3. Any process cartridges including at least an image bearing member
and a developing device including the toner of the present invention can be used
as the process cartridge of the present invention.
The process cartridge of the present invention is detachably set in an image
forming apparatus. In the image forming apparatus in which the process cartridge
is set, the photoreceptor
22 is rotated at a predetermined rotation speed.
The photoreceptor
22 is charged with the charger
23 and thereby the
photoreceptor
22 is uniformly charged positively or negatively. Then an
image irradiating device (not shown) irradiates the charged surface of the photoreceptor
22 with light using a method such as slit irradiation methods and laser
beam irradiation methods, resulting in formation of electrostatic latent image
on the photoreceptor
22.
The thus prepared electrostatic latent image is developed by the developing roller
24 bearing the developer of the present invention thereon, resulting in
formation of a toner image on the photoreceptor
22. The toner image is then
transferred onto a receiving material (not shown) which is timely fed by a feeding
device (not shown) to a transfer position between the photoreceptor
22 and
a transfer device (not shown).
The toner image formed on the receiving material is then separated from the photoreceptor
22 and fixed by a heat/pressure fixing device (not shown) including a fixing
roller. The fixed image is discharged from the image forming apparatus. Thus, a
hard copy is produced.
The surface of the photoreceptor
22 is cleaned by the cleaning blade
25
to remove toner remaining on the photoreceptor
22, followed by discharging,
to be ready for the next image forming operation.
FIG. 6A illustrates the developing portion of a magnet brush developing device
using a negative-positive developing method and a two-component developer. A developing
roller
46 serving as a developer bearing member is illustrated in a right
side of FIG. 6A and a photoreceptor
1 is illustrated in a left side of FIG.
6A. The developing roller
46 includes the developing sleeve
46s
rotating in a direction (D) and development magnets poles fixed therein.
The two-component developer including a non magnetic toner and a magnetic carrier
is transferred to a portion of the developing roller facing the photoreceptor
1
by a rotation of the developing sleeve
46s. In the portion facing
the photoreceptor
1, the carrier of the two-component developer is erected
by the magnetic force of a development magnetic pole, resulting in formation of
a magnet brush.
In FIG. 6A, a small circle represents the toner particles and a large circle
represents
the carrier particles. For explanation convenience, only one pile of the magnet
brush in the developing portion is illustrated in a full line while other magnet
brushes are illustrated in dot lines and the toner particles therein are not illustrated.
On the other hand, the photoreceptor
1 bears an electrostatic latent image
on the surface thereof and rotates in a direction (C). In FIG. 6A, a non-image
portion of the electrostatic latent image illustrated as (A) is negatively charged.
In the portion where the photoreceptor
1 faces the developing roller
46,
the latent image on the surface of the photoreceptor is rubbed with the magnet
brush and toner particles adhere to an image portion due to the development electric
field. As a result, on a downstream side from the developing portion, a toner image
is formed in the image portion of the latent image on the surface of the photoreceptor
1 as illustrated as (B). In this case, the portion of the photoreceptor
where the magnet brush rubs the surface of the portion is referred to as a nip
portion. In addition, a proper image density cannot be obtained when only a point
of the developer bearing member rubs a point of the photoreceptor, and therefore
the photoreceptor and the developing sleeve rotate at a different speed such that
plural points of the developer bearing member rub a point of the photoreceptor.
Namely, the developing sleeve rotates faster than the photoreceptor.
FIGS. 6B,
6C and
6D are views for explaining the mechanism of
formation of white spots at a rear end of an image referring to this example. All
of FIGS. 6B,
6C and
6D are enlarged views of the portion where the
photoreceptor
1 and the developing sleeve faces each other in FIG.
6A.
The edge of the magnet brush illustrated on a right side of each of FIGS. 6B,
6C
and
6D approaches the photoreceptor illustrated on a left side of the figures.
FIGS. 6B,
6C and
6D illustrate chronologically the rotation of the
magnet brush in this order. Referring to FIGS. 6B,
6C and
6D, at
the portion where the photoreceptor faces the developing roller, a border between
a non-image portion and a black solid image is to be developed (namely "white spots"
are to be formed at a rear portion of an image), and a toner image just developed
is located on a downstream side from the portion in the rotating direction (C).
One of the magnet brush (M) approaches the photoreceptor in this state. Actually,
the photoreceptor rotates in a counterclockwise direction (C), but as mentioned
above, the developing sleeve is rotating faster than the photoreceptor, the magnet
brush overtakes the photoreceptor. Therefore, in FIGS. 6B,
6C and
6D,
the photoreceptor is illustrated as being stopped.
In FIG. 6A, the magnet brush approaching the photoreceptor passes through a non-image
portion (N) before arriving at an edge (E) of the image portion to be developed.
At this time, the non-image portion (N) and the toner particles repulse due to
a repulsion force (R) therebetween, the toner particles are gradually removed from
the photoreceptor, resulting in transfer of the toner particles toward the sleeve.
Hereinafter this phenomenon is referred to as "toner drift". As a result of the
toner drift, as illustrated in FIG. 6C, when the magnet brush reaches the edge
(E) of the toner image, the surface of the positively charged carrier particles
is exposed. Therefore, there is no toner particle on the magnet brush (M) for developing
the edge (E) of the latent image and the edge (E) is not developed. Further, referring
to FIG. 6D, when the magnet brush reaches a position P, the toner particles once
adhered to the photoreceptor are transferred to the photoreceptor, if the adhesive
force between the toner and the photoreceptor is low. As a result, there is a case
when a development is not performed at a border between an image portion and a
non-image portion, resulting in occurrence of the rear-end omission problem.
Then the toner for use in the present invention will be explained.
The present inventors discover that when a fine particle toner having a particle
diameter of from 4.0 to 7.0 μm is used as the toner for the above-mentioned
developing apparatus, fine components in the toner, especially fine particles not
greater than 2 μm, mainly cause the on-sleeve toner fixation problem. Since
adhesion between the toner and the sleeve is considered to be higher in a fine
particle side of a toner than in a large particle side thereof, the on-sleeve toner
fixation tends to proceed when the amount of fine particles included in the toner increases.
Namely, it is found that when the amount of fine particles having a particle
size (i.e., a circle-equivalent particle diameter) not greater than 2 μm
is not greater than 20% by number when measured by a flow particle image analyzer,
the toner can produce images having high image qualities for a long period of time
without causing the rear-end omission problem.
Conventionally, COULTER COUNTER, or the like instruments are used
for measuring a particle diameter of a toner. Since this measuring method measures
a particle diameter utilizing changes of resistance when the toner passes a fine
pore, measurements of particle size not greater than 2 μm are greatly influenced
by a noise, i.e., measurements are impossible due to lack of measuring accuracy.
In contrast, the flow particle image analyzer measuring a particle size while performing
an image analysis can measure the particle diameter of fine particles having a
circle-equivalent particle diameter not greater than 2 μm. By using the flow
particle image analyzer, it can be found that a toner including toner particles
having a circle-equivalent particle diameter not greater than 2 μm in an
amount not greater than 20% by number does not cause the on-sleeve toner fixation
problem even when being repeatedly used for a long period of time.
The circle-equivalent particle diameter means the diameter of a circle having
the same area as the projected area of a toner particle, and can be determined
using a flow-type particle analyzer manufactured by SYSMEX. The method for determining
the circle-equivalent particle diameter of a toner is as follows.
(1) 1 mg to 10 mg of a sample to be measured is mixed with 50 to 100 ml of 1%
aqueous solution of sodium chloride which is prepared using a first grade sodium
chloride and which is filtered using a filer having openings of 0.45 μm,
and 0.1 ml to 0.5 ml of a dispersant (i.e., a surfactant) such as an alkylbenzene
sulfonic acid salt;
(2) the mixture is dispersed using an ultrasonic dispersing machine for about
1 minute to prepare a suspension including particles of 5,000 to 15,000 per 1 micro-liter
of the suspension;
(3) the circle-equivalent particle diameters of the particles of the sample are
determined by the particle analyzer mentioned above; and
(4) the percentage (% by number) of each of particle diameter ranges is calculated.
In this measurement, data of 0.6 μm or more in particle diameter are considered
to be effective.
The toner of the present invention includes at least a wax and a binder resin.
It is preferable that among the wax particles present in the toner, the wax particles
present in a surface portion of the toner, which surface portion is defined as
a surface portion having a depth of from 0 to 1 μm, have an area of from
5 to 30%. In particular, it is preferable that wax particles exist in the outer
portion of the toner, which is defined as a surface portion having a depth of from
0 to half the radius of a toner particle, in an amount not less than 65% by number
of the wax particles dispersed in the entire toner particle, so that a sufficient
amount of wax can be exuded from the surface of the toner particles when the toner
is fixed, resulting in impartment of good releasing property to the toner. In addition,
the amount of wax particles at the uppermost surface of the toner can be reduced,
and therefore transfer of the wax particles to a photoreceptor and a developing
sleeve can be avoided. In particular, this toner can produce good effects when
used for the developing method of the present invention in which a thick magnet
brush is formed in the nip portion where a magnet brush rubs a photoreceptor, and
thereby great heat and mechanical stresses are applied to the toner in a developing process.
When the surface portion of the toner having a depth of from 0 to 1 μm
has a wax area less than 5%, the toner has insufficient releasing property. In
addition, when the surface portion of the toner having a depth of from 0 to 1 μm
has a wax area greater than 30%, filming of the toner (wax) on the photoreceptor
and the developing sleeve may be seriously caused.
In addition, it is preferable for the toner of the present invention that the
wax dispersed in the toner has a particle diameter distribution such that particles
having a size of from 0.5 μm to 3 μm are present in an amount not less
than 70% by number, and-more preferably particles having a size of from 1 μm
to 2 μm are present in an amount not less than 70% by number. When particles
having a size less than 0.5 μm are included in a large amount, good releasing
property cannot be developed. In addition, when particles having a size greater
than 3 μm are included in a large amount, fluidity deteriorates due to cohesion
thereof. In addition, filming occurs, and color reproducibility and glossy property
seriously deteriorate when the toner is used for color toners.
In the present invention, a diameter of a wax in the maximum length direction
is defined as a wax dispersion diameter. Concretely, the wax dispersion diameter
is measured as follows. A toner is embedded in an epoxy resin to cut finely to
have a thickness of about 100 μm, followed by dyeing with ruthenium tetraoxide.
Then a cross-sectional surface of the toner is observed with a transmission electron
microscope with a 10,000 magnification power and photographed. By evaluating images
of 20 particles, the dispersion diameter of the toner is determined.
The wax area ratio of the surface portion of the toner having a depth of from
0 to 1 μm is determined as an area ratio of the wax present in the surface
portion of the toner having a depth of from 0 to 1 μm to that in the entire toner.
The wax particles existing in the outer portion of the toner means the wax particles
which exist in the outer portion of the toner when the toner particle is divided
into two portions by a curve connecting intermediate points between the center
of the toner particle and the surface of the toner (in this case, toner particles
existing at the surface of the toner are excluded). In this case, the wax particles
existing on the curve are considered to be included in the inner portion. This
outer portion is sometimes referred to as "an outer portion having a depth of from
0 to half a radius of the toner particle."
Suitable waxes for use as the wax in the toner of the present invention
include carnauba waxes subjected to a treatment of removing a free aliphatic fatty
acid, rice waxes and montan waxes. In particular, the carnauba waxes subjected
to a treatment of removing a free aliphatic fatty acid have small volatile component,
and thereby the effect of preventing occurrence of filming of the toner on a photoreceptor
and the spent carrier problem can be produced. In addition, since waxes exude from
a surface of the toner when fixing to impart a releasing property to the toner,
waxes preferably have a low acid value not greater than 5 KOH mg/g, for example,
by being subjected to a treatment of removing a free aliphatic fatty acid. Waxes
are preferably included in the toner in an amount of from 2.0 to 12 parts by weight,
and more preferably from 4.0 to 8.0 parts by weight, based on 100 parts by weight
of the toner to impart good fixability to the toner.
Then the binder resin of the toner will be explained in detail.
In the present invention, modified polyester resins are preferably used as the
binder resin of the toner of the present invention. Polyester prepolymers having
an isocyanate group can be used for the modified polyester resins. Specific examples
of the polyester prepolymers (A) having an isocyanate group include polyesters
prepared by poly condensing a polyol (1) and a polycarboxylic acid (2) and reacting
active hydrogen groups of the condensation product with a polyisocyanate (3). Specific
examples of the active hydrogen groups include hydroxyl groups (alcoholic hydroxyl
groups and phenolic hydroxyl groups), amino groups, carboxyl groups and mercapto
groups. Among these groups, alcoholic hydroxyl groups are most preferable.
Specific examples of the polyols (1) includes diols (1-1) and polyols (1-2)
having not less than 3 hydroxyl groups. It is preferable to use a diol (1-1) by
itself or a mixture of a diol (1-1) and a small quantity of a polyol (1-2). Specific
examples of the diols (1-1) include alkylene glycol (such as ethylene glycol, 1,2-propylene
glycol, 1,3-propylene glycol, 1,4-butanediol and 1
