U.S. patent application number 10/218054 was filed with the patent office on 2003-03-20 for magnetic toner, and developing apparatus and image forming apparatus using it.
Invention is credited to Kikushima, Seiji, Nagai, Takashi, Sugimoto, Hiroko, Takatsuna, Toru.
Application Number | 20030054275 10/218054 |
Document ID | / |
Family ID | 19105390 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030054275 |
Kind Code |
A1 |
Sugimoto, Hiroko ; et
al. |
March 20, 2003 |
Magnetic toner, and developing apparatus and image forming
apparatus using it
Abstract
A magnetic toner contains, as a magnetic powder, magnetic iron
oxide having an octahedral particle shape and an average particle
diameter of 0.21 .mu.m or larger and containing 18% or more by
weight of FeO. The magnetic toner is so prepared as to have a bulk
density A (g/cm.sup.3) and a magnetic powder content B (% by
weight) that fulfill the formulae
0.8.ltoreq.A/B.times.100.ltoreq.1.6 and 35.ltoreq.B.ltoreq.50. This
alleviates the reddishness of images produced with the magnetic
toner, and alleviates the disturbance of a thin toner layer on a
developing sleeve and thus the fogging resulting therefrom.
Inventors: |
Sugimoto, Hiroko; (Osaka,
JP) ; Nagai, Takashi; (Osaka, JP) ; Takatsuna,
Toru; (Osaka, JP) ; Kikushima, Seiji; (Osaka,
JP) |
Correspondence
Address: |
SMITH, GAMBRELL & RUSSELL, LLP
1850 M STREET, N.W., SUITE 800
WASHINGTON
DC
20036
US
|
Family ID: |
19105390 |
Appl. No.: |
10/218054 |
Filed: |
August 15, 2002 |
Current U.S.
Class: |
430/106.1 ;
399/274; 399/276; 430/111.4 |
Current CPC
Class: |
G03G 9/081 20130101;
G03G 9/0833 20130101; G03G 9/0836 20130101; G03G 15/09 20130101;
G03G 9/0835 20130101; G03G 9/0837 20130101; G03G 9/0819 20130101;
G03G 2215/0602 20130101; G03G 9/0821 20130101; G03G 9/0838
20130101; G03G 9/0817 20130101; B82Y 15/00 20130101 |
Class at
Publication: |
430/106.1 ;
430/111.4; 399/276; 399/274 |
International
Class: |
G03G 009/083 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 2001 |
JP |
2001-281592 |
Claims
What is claimed is:
1. A magnetic toner containing a binder resin and a magnetic
powder, wherein the magnetic powder is magnetic iron oxide having
an octahedral particle shape and an average particle diameter of
0.21 .mu.m or larger and containing 18% or more by weight of FeO,
and the magnetic toner has a bulk density A (g/cm.sup.3) and a
magnetic powder content B (% by weight) that fulfill the following
formulae (1) and (2):0.8.ltoreq.A/B.times.100.- ltoreq.1.6
(1)35.ltoreq.B.ltoreq.50 (2)
2. A magnetic toner as claimed in claim 1, wherein the magnetic
powder has an average particle diameter of 0.30 .mu.m or
smaller.
3. A developing apparatus comprising a developer supporting member
for supporting a developer and for transporting the developer to a
developer section and a developing blade arranged at a
predetermined distance from the developer supporting member so as
to limit an amount of the developer transported to the developing
section, wherein the developing blade is fitted with a magnet
having a magnetic force of 400 to 800 gausses, and the developer is
a magnetic toner as claimed in claim 1.
4. A developing apparatus as claimed in claim 3, wherein the
distance between the developer supporting member and the developing
blade is in a range of from 0.2 to 0.4 mm.
5. A developing apparatus as claimed in claim 3, wherein the
developer supporting member has a surface roughness in a range of
from 3.0 to 5.5 .mu.m.
6. A developing apparatus as claimed in claim 3, wherein the
developer supporting member is formed out of stainless steel.
7. An image forming apparatus comprising an image supporting
member, charging means for charging a surface of the image
supporting member with electric charge, exposing means for
irradiating the charged surface of the image supporting member with
light to form an electrostatic latent image, developing means for
developing the electrostatic latent image by feeding a developer
thereto, and transferring means for transferring the developer on
the image supporting member developed by the developing means to a
transferred-image member, wherein the developing means is realized
with a developing apparatus as claimed in one of claims 3 to 6.
8. An image forming apparatus as claimed in claim 7, wherein a
distance between the image supporting member and the developer
supporting member is greater than a thickness of a thin layer of
the magnetic toner formed on the developer supporting member, and
development of the electrostatic latent image is achieved by
feeding the developer thereto with an alternating-current bias
voltage applied between the image supporting member and the
developer supporting member.
9. An image forming apparatus as claimed in claim 7, wherein, in
the developing means, development of the electrostatic latent image
is achieved by letting the magnetic toner charged with electric
charge of a same polarity as an unexposed portion of the image
supporting member attach to an exposed portion thereof.
10. An image forming apparatus as claimed in claim 7, wherein the
image supporting member is an amorphous silicon photoconductor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a magnetic toner, and to a
developing apparatus and an image forming apparatus using such a
magnetic toner.
[0003] 2. Description of the Prior Art
[0004] Toners for use in the development of electrostatic latent
images are roughly grouped into non-magnetic and magnetic toners.
In image development, non-magnetic toners are usually used in the
form of two-component developers, i.e., mixed with a carrier.
Two-component developers maintain the electric charge of the toner
stably, and thus offer satisfactory images for an extended period.
However, they are susceptible to variations in the mixing ratio
between the toner and the carrier and to deterioration of the
carrier.
[0005] On the other hand, magnetic toners are usually used singly,
i.e., in the form of one-component developers. One-component
developers, containing no carrier, permit size and weight reduction
of developing apparatus, and do not require maintenance such as
replacement of the carrier. For these reasons, in recent years,
magnetic toners have come to be used not only in low-speed,
small-size copiers and printers but also medium- and high-speed
copiers and printers.
[0006] Magnetic toners usually do not contain colorants, and their
colors originate from magnetic powders. However, in general,
magnetic powders are not purely black but reddish, making images
developed with a magnetic toner appear reddish. That is, magnetic
toners do not always offer a satisfactory tone of color.
[0007] The easiest way to alleviate this reddishness of images
formed with a magnetic toner is to reduce the magnetic powder
content of the toner. However, reducing the magnetic powder content
of a toner results in reducing the coercivity of the toner, and
thus leads to disturbance of a thin toner layer on a developer
supporting member (hereinafter referred to as a "developing sleeve"
also) and to increased toner adhesion in a non-image area
(fogging).
SUMMARY OF THE INVENTION
[0008] An object of the present invention is to provide a magnetic
toner that produces less reddish images with less disturbance of a
thin toner layer on a developing sleeve and thus with reduced
fogging resulting therefrom.
[0009] Another object of the present invention is to provide a
developing apparatus and an image forming apparatus that produce
less reddish images with reduced fogging.
[0010] To achieve the above objects, according to one aspect of the
present invention, a magnetic toner contains, as a magnetic powder,
magnetic iron oxide having an octahedral particle shape and an
average particle diameter of 0.21 .mu.m or larger and containing
18% or more by weight of FeO. Moreover, the magnetic toner is so
prepared as to have a bulk density A (g/cm.sup.3) and a magnetic
powder content B (% by weight) that fulfill formulae (1) and (2)
below. Prepared in this way, the magnetic toner produces less
reddish images with less disturbance of a thin toner layer on a
developing sleeve and thus with less fogging resulting
therefrom.
0.8.ltoreq.A/B.times.100.ltoreq.1.6 (1)
35.ltoreq.B.ltoreq.50 (2)
[0011] To achieve the above objects, according to another aspect of
the present invention, in a developing apparatus and an image
forming apparatus, a developing blade is provided that is fitted
with a magnet having a magnetic force of 400 to 800 gausses, and a
magnetic toner as described above is used as a developer. This
makes it possible to produce less reddish images with less
fogging.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] This and other objects and features of the present invention
will become clear from the following description, taken in
conjunction with the preferred embodiments with reference to the
accompanying drawings in which:
[0013] FIG. 1 is a sectional view showing an example of a
developing apparatus according to the invention; and
[0014] FIG. 2 is a sectional view showing an example of an image
forming apparatus according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] As a result of an intensive study made in search of a way to
alleviate the reddishness of images produced with a magnetic toner
without disturbing a thin toner layer on a developing sleeve, the
inventors of the present invention have found out that the aim is
achieved by using a particular, less reddish, type of magnetic
powder and by preparing a magnetic toner so that its magnetic force
and flowability fulfill a particular relationship, which findings
have led to the present invention.
[0016] Specifically, one of the main features of a magnetic toner
according to the invention is that it contains, as a magnetic
powder, magnetic iron oxide having an octahedral particle shape and
an average particle diameter of 0.21 .mu.m or larger and containing
18% or more by weight of FeO.
[0017] First, the magnetic powder used in the invention will be
described. The tone of color of a magnetic powder depends greatly
on its particle diameter; specifically, a magnetic powder having an
average particle diameter smaller than 0.21 .mu.m appears
distinctly reddish. Accordingly, in the invention, a magnetic
powder having an average particle diameter of 0.21 .mu.m or larger
is used. There is no particular upper limit to the average particle
diameter of the magnetic powder. However, an average particle
diameter larger than 0.30 .mu.m may result in too low coercivity,
leading to increased fogging, and accordingly the average particle
diameter of the magnetic powder is preferably 0.30 .mu.m or
smaller.
[0018] The tone of color of a magnetic powder depends also on its
FeO content. A magnetic powder containing less than 18% by weight
of FeO appears distinctly reddish. Accordingly, in the invention, a
magnetic powder containing 18% or more by weight of FeO is used. A
further preferred FeO content is 20% or more by weight.
[0019] The particle shape of the magnetic iron oxide affects not
only the tone of color of the magnetic toner but also how easily
the magnetic iron oxide unwantedly separates from toner particles.
There are other types of magnetic iron oxide having different
particle shapes such as spherical and cubic. The study by the
inventors of the present invention, however, has shown that
magnetic iron oxide with an octahedral particle shape surpasses
other types in both blackness and unlikeliness to separate from
toner particles. Accordingly, in the invention, magnetic iron oxide
with an octahedral particle shape is used. The magnetic powder may
be subjected to surface treatment using a silane coupler, a
titanium-based coupler, or the like.
[0020] Another main feature of a magnetic toner according to the
invention is that its bulk density and magnetic powder content
fulfill formulae (1) and (2) noted earlier. Specifically, the
magnetic toner has a lower magnetic powder content B than
conventionally usual to alleviate its reddishness, and in addition,
to prevent the resulting disturbance of a thin toner layer on a
developing sleeve, is so prepared as to have a controlled
flowability. Here, the bulk density of the toner is used as an
index of its flowability. The higher the flowability of a toner,
the higher its bulk density; the lower the flowability of a toner,
the lower its bulk density.
[0021] As will be understood from formula (2), a preferred range of
the magnetic powder content B is from 35 to 50% by weight. A
magnetic powder content higher than 50% by weight results in
distinct reddishness of the produced images and also in inadequate
initial image density in a high-temperature, high-humidity
environment. This is because, in a high-humidity environment,
whereas the toner is charged with less electric charge, the high
magnetic powder content increases the magnetic attraction by which
the toner is attracted toward the developing sleeve. On the other
hand, a magnetic powder content lower than 35% by weight results in
fogging. This is because the low magnetic powder content of the
toner lowers its coercivity and thus permits the toner to detach
easily from the developing sleeve. A further preferred lower limit
of the magnetic powder content is 40% by weight, and a further
preferred upper limit thereof is 45% by weight.
[0022] It is also necessary that the bulk density A and the
magnetic powder content B of the toner fulfill formula (1) noted
earlier. Through a series of tests performed with different
magnetic powder contents lower than conventionally usual, the
inventors of the present invention found out that disturbance of a
thin toner layer on a developing sleeve was particularly likely in
a low-temperature, low-humidity environment. An extensive study in
search of a way to prevent such disturbance of a thin toner layer
on a developing sleeve led us to find out that the aim was achieved
by increasing the magnetic powder content to such a degree as not
to make the toner distinctly reddish and simultaneously making the
bulk density of the toner, i.e., the index of its flowability,
lower than usual. Then, under such conditions, we investigated the
relationship between the magnetic powder content and the bulk
density, and derived therefrom the inequality of formula (1).
Falling out of the range defined by formula (1) beyond its lower
limit results in fogging all over the produced image. On the other
hand, falling out of the range defined by formula (1) beyond its
upper limit results in disturbance of a thin toner layer on a
developing sleeve, and thus in partial fogging in the produced
image. To maintain adequate image density for an extended period, a
further preferred lower limit of formula (1) is 1.0, and a further
preferred upper limit thereof is 1.4.
[0023] The binder resin used in a toner according to the invention
may be of any type, examples including styrene-acrylic resin and
polyester resin. Needless to say, as required, these types of resin
may be used in combination with another type of resin.
[0024] Examples of the monomers that are used as the base of the
styrene-acrylic resin include: derivatives of styrene such as
styrene, .alpha.-methylstyrene, p-methylstyrene, p-t-butylstyrene,
p-chlorstyrene, and hydroxystyrene; and esters of (meth)acrylic
acid such as methacrylic acid, methyl (meth)acrylate, ethyl
(meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate,
glycidyl (meth)acrylate, methoxyethyl (meth)acrylate, propoxyethyl
(meth)acrylate, methoxydiethylene glycol (meth)acrylate,
ethoxydiethylene glycol (meth)acrylate, benzil (meth)acrylate,
cyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate,
(meth)acrylonitrile, (meth)acrylamide, N-methylol (meth)acrylamide,
ethylene glycol di(meth)acrylate, 1,3-butylene glycol
di(meth)acrylate, 1,4-butanediol di(meth)acrylate, and trimethylol
ethane tri(meth)acrylate.
[0025] A mixture of some of these monomers is made into the binder
resin used in the invention by polymerizing the mixture by an
appropriate process such as solution polymerization, block
polymerization, emulsion polymerization, or suspension
polymerization. In the polymerization process here, any
conventionally known polymerization initiator can be used, examples
including: acetyl peroxide, decanoyl peroxide, lauroyl peroxide,
benzoyl peroxide, azobisisobutyronitrile,
2,2'-azobis-2,4-dimethylvaleronitrile, and
2,2'-azobis-4-methoxy-2,4-dime- thylvaleronitrile. Preferably 0.1
to 15% by weight of one of these polymerization initiators is added
to the total weight of the monomers.
[0026] The polyester resin is produced mainly through condensation
polymerization of a polycarboxylic acid and a polyhydric alcohol.
Examples of the polycarboxylic acid include: aromatic
polycarboxylic acids such as phthalic acid, isophthalic acid,
terephthalic acid, succinic acid, 1,2,4-benzenetricarboxylic acid,
2,5,7-naphthalenetricarbo- xylic acid,
1,2,4-naphthalenetricarboxylic acid, and pyromellitic acid;
aliphatic dicarboxylic acids such as maleic acid, fumaric acid,
succinic acid, adipic acid, sebacic acid, malonic acid, azelaic
acid, mesaconic acid, citraconic acid, and glutaconic acid;
alicyclic dicarboxylic acids such as cyclohexane dicarboxylic acid
and cyclohexenedicarboxylic acid; and anhydrides and lower alkyl
esters of these carboxylic acids. These are used singly or as a
mixture of two or more of them.
[0027] Here, the content of components with three or more carboxyl
or hydroxy groups depends on the degree of cross-linking, and
therefore the desired degree of cross-linking can be achieved by
controlling the amount of such components added. In general, a
preferred content of components with three or more carboxyl or
hydroxy groups is 15 mol % or lower.
[0028] On the other hand, examples of the polyhydric alcohol used
in the polyester resin include: alkylene glycols such as ethylene
glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol,
1,4-butenediol, neopentyl glycol, 1,5-pentane glycol, and
1,6-hexane glycol; alkylene ether glycols such as diethylene
glycol, triethylene glycol, dipropylene glycol, polyethylene
glycol, polypropylene glycol, and polytetramethylene glycol;
alicyclic polyhydric alcohols such as 1,4-cyclohexame dimethanol
and hydrogenated bisphenol A; and bisphenols such as bisphenol A,
bisphenol F, and bisphenol S, and alkylene oxides of such
bisphenols. These are used singly or as a mixture of two or more of
them.
[0029] As required, monocarboxylic acids and monohydric alcohols
may be used for the purpose of adjusting the molecular weight and
controlling the reaction. Examples of monocarboxylic acids include
benzoic acid, p-hydroxybenzoic acid, toluenecarboxylic acid,
salicylic acid, acetic acid, propionic acid, and stearic acid.
Examples of monohydric alcohols include benzil alcohol,
toluene-4-methanol, and cyclohexane methanol.
[0030] It is preferable that the binder resin used have a glass
transition point in the range from 45 to 90.degree. C. With a glass
transition point below 45.degree. C., the binder resin may gather
together inside a toner cartridge or a developing apparatus. On the
other hand, with a glass transition point over 90.degree. C., the
toner may not fuse satisfactorily onto a transfer material such as
paper.
[0031] As required, a magnetic toner according to the invention may
contain a charge control agent, a mold release agent, a surface
treatment agent, or the like. The charge control agent may be of
any conventionally known type, examples including: as charge
control agents that tend to be positively charged, nigrosine dyes,
nigrosine dyes denatured with a fatty acid, nigrosine dyes
denatured with a fatty acid containing a carboxyl group, quaternary
ammonium salts, amine-based compounds, and organic metallic
compounds; and, as charge control agents that tend to be negatively
charged, metallic complexes of a hydroxycarboxylic acid, metallic
complexes of an azo compound, metal complex dyes, and salicylic
acid derivatives.
[0032] The mold release agent may be one of various types of wax or
low-molecular-weight olefin resin. Examples of wax include: esters
of a fatty acid with a polyhydric alcohol; esters of a fatty acid
with a higher alcohol; amides of an alkylenebis fatty acid; and
natural waxes. Examples of low-molecular-weight olefin resin
include: polypropylene, polyethylene, and propylene-ethylene
copolymer with a number-average molecular weight in the range from
1,000 to 10,000, in particular in the range from 2,000 to 6,000.
Among these, polypropylene is particularly suitable.
[0033] The surface treatment agent may be any substance that
improves the charge controllability and bulk density (flowability)
of the toner, examples including: inorganic fine particle powder
such as silica, alumina, titanium oxide, zinc oxide, magnesium
oxide, and calcium carbonate; organic fine particle powder such as
polymethyl methacrylate; and metallic salts of a fatty acid such as
zinc stearate. These are used singly or as a mixture of two or more
of them. Preferably 0.1 to 2.0% by weight of the surface treatment
agent is added to the toner. The surface treatment agent is mixed
with the toner, for example, in a Henschel mixer, V-blender,
tumbler mixer, or hybridizer.
[0034] A magnetic toner according to the invention can be
manufactured by a process that itself is conventionally known, such
as crushing-and-classifying, melt granulation, spray granulation,
or suspension/emulsification polymerization. Among these, from the
viewpoint of manufacturing equipment and productivity,
crushing-and-classifying is preferred. Crushing-and-classifying is
performed in the following manner. First, a toner composition
containing a binder resin and a magnetic powder, with a charge
control agent, a mold release agent, and the like added thereto as
required, is premixed in a Henschel mixer or a V-blender, and is
then melt and kneaded in a melting-kneading machine such as a
twin-screw extruder. The toner composition thus melted and kneaded
is cooled, is then subjected to coarse/fine crushing, and is then,
as required, classified to obtain toner particles having the
desired particle size distribution. As required, the surfaces of
the toner particles are treated with a surface treatment agent to
finish the toner. The magnetic powder may be contained inside the
toner particles, or may be made to adhere to the surfaces of the
toner particles. The toner according to the invention may be used
as it is as a one-component developer, or may be mixed with a
carrier so as to be used as a two-component developer.
[0035] Next, a developing apparatus according to the invention will
be described. The main features of a developing apparatus according
to the invention are that a developing blade provided with a magnet
having a magnetic force of 400 to 800 gausses is arranged at a
predetermined distance from a developing sleeve, and that a
magnetic toner prepared as described above is used as a developer.
In this structure, even with a toner having a low magnetic powder
content, it is possible to form a thin toner layer uniformly on the
developing sleeve.
[0036] FIG. 1 is a sectional view showing an example of a
developing apparatus according to the invention. This developing
apparatus 4 is provided with a developing sleeve (developer
supporting member) 41 composed of a sleeve 41a and a magnet 41b
housed inside and fixed to it, a first agitating/transporting
member 42 having a spiral shape, and a second
agitating/transporting member 43 having a spiral shape. To the
upper right of the developing sleeve 41, a developing blade 45 for
restricting the amount of toner transported to a developing section
and charging the toner with electric charge by friction is arranged
at a predetermined distance from the developing sleeve 41. On the
bottom surface of this developing blade 45, a magnet 45a having a
predetermined magnetic force is fitted. On a side wall to the right
of the second agitating/transporting member 43, a toner sensor 44
for detecting the amount of toner is arranged.
[0037] When the toner sensor 44 detects a shortage of the toner
inside the developing apparatus 4, a fresh supply of toner T is fed
from a toner hopper (not shown) to the developing apparatus 4. The
supplied toner T is first transported, while being agitated, in the
direction from the front side to the back side of the figure by the
second agitating/transporting member 43, and is then, at the
back-side end, fed from the second agitating/transporting member 43
to the first agitating/transporting member 42. The toner T is then
transported, while being agitated, in the direction from the back
side to the front side of the figure by the first
agitating/transporting member 42, and meanwhile an appropriate
amount of it is fed to the developing sleeve 41.
[0038] The toner T fed to the developing sleeve 41 is, as the
developing sleeve 41 rotates counter-clockwise, fed to a position
facing a photoconductor 1 (developing section). Meanwhile, the
amount of toner fed to the developing section is controlled by the
developing blade 45 arranged at a predetermined distance from the
developing sleeve and magnetized by the magnet 45a, and
simultaneously a thin layer of the toner is formed; moreover, the
toner T is charged with electric charge by friction. Here, it is
important that the magnet 45a have a magnetic force in the range
from 400 to 800 gausses. A magnetic force weaker than 400 gausses
may lead to failure to form a thin toner layer uniformly on the
developing sleeve. On the other hand, a magnetic force stronger
than 800 gausses may cause too small an amount of toner to be
transported to the developing section. The distance between the
developing sleeve 41 and the developing blade 45 is determined
appropriately according to the particle diameter of the toner T and
the magnetic forces of the magnets 41b and 45a. In general, a
preferred range of this distance is from 0.2 to 0.4 mm.
[0039] To form a thin toner layer uniformly, it is preferable that
the developing sleeve 41 have a surface roughness in the range from
3.0 to 5.5 .mu.m. Giving the developing sleeve 41a surface
roughness of 3.0 .mu.m or finer may lead to lower transportability
of the toner T and to failure to form a thin toner layer uniformly.
On the other hand, giving it a surface roughness of 5.5 .mu.m or
coarser may cause the toner T to lodge in depressions in the
surface of the developing sleeve 41 and fuse onto the developing
sleeve 41. A further preferred range of the surface roughness is
from 3.5 to 4.5 .mu.m.
[0040] The surface roughness of the developing sleeve can be
controlled to within the aforementioned range, for example, by
treating it by blasting. The desired surface roughness is obtained
by appropriately selecting or adjusting the type of the blasted
material, the pressure of compressed air, the duration of blasting,
the distance from the blast nozzle to the developer supporting
member, and other conditions. Examples of the blasted material used
here include sand, glass beads, and steel balls. The developing
sleeve is made of, advisably, a non-magnetic material such as
stainless steel or aluminum alloy, of which stainless steel is
preferred for its high wear resistance and other properties.
[0041] Here, the developing sleeve 41 is so structured that the
magnet 41b housed inside it is stationary and the cylindrical
sleeve 41a is rotatable; however, it may be so structured that the
internal magnet 41b is rotatable and the sleeve 41a is stationary,
or that both the internal magnet 41b and the sleeve 41a are
rotatable (in the same direction or in the opposite
directions).
[0042] An electrostatic latent image on the photoconductor
(electrostatic latent image supporting member) may be developed by
charged area development or by reversal development. The
development may be achieved by contact development, in which the
thin toner layer makes contact with the photoconductor, or by toner
projection development (jumping development), in which they do not
make contact with each other. To alleviate fogging, it is
recommended to combine reversal development and toner projection
development. In that case, the photoconductor is charged with the
same polarity as the toner, and exposure removes the electric
charge of the portion corresponding to the latent image. Moreover,
in the developing section, an alternating voltage obtained by
superimposing an alternating-current voltage on a direct-current
voltage is applied, as a developing bias voltage, between the
developing sleeve and the photoconductor. As a result, the toner on
the developing sleeve jumps to and attaches to the discharged
electrostatic latent image on the photoconductor, making the
electrostatic latent image visible as a toner image.
[0043] The photoconductor used here may be made of any
conventionally known material, examples including an amorphous
silicon photoconductor, organic photoconductor, Se-based
photoconductor, ZnO photoconductor, and CdS-based photoconductor.
Among these, an amorphous silicon photoconductor is preferred for
its high durability. The photoconductor may have any conventionally
known shape, examples including a drum-like, sheet-like, belt-like,
and web-like shape. Among these, a drum-like shape is
preferred.
[0044] Next, an image forming apparatus according to the invention
will be described. FIG. 2 is a sectional view showing an example of
an image forming apparatus according to the invention. The surface
of a photoconductor 1 is charged uniformly with positive electric
charge by a charging means 2. Next, an electrostatic latent image
(exposed portion) is formed on the surface of the photoconductor 1
by an exposing means 3. Then, by the use of the developing
apparatus 4 described above, the electrostatic latent image is
sprinkled with toner from the thin toner layer formed on the
developing sleeve having the magnet housed inside it, so that the
electrostatic latent image is turned into a visible image. Then,
the toner image on the photoconductor 1 is transferred to a
transferred-image member 7 by a transferring means 5. Thereafter,
the toner image on the transferred-image member 7 is subjected to
heat and pressure by an unillustrated fixing means so as to be
fused and fixed to the transferred-image member 7. On the other
hand, the toner left on the photoconductor 1, i.e. the toner that
has not been transferred, is roughly removed by a cleaning brush 61
and then completely removed by a cleaning blade 62. in a cleaning
means 6.
EXAMPLES
Practical Example 1
[0045] Toner ingredients, specifically 100 parts by weight of
styrene-acrylic resin serving as a binder resin, 5 parts by weight
of a charge control agent, and 40 parts by weight of a magnetic
powder (having an octahedral particle shape and an average particle
diameter of 0.25 .mu.m and containing 18% by weight of FeO), were
put and mixed in a Henschel mixer, were then melt-kneaded in a
twin-screw extruder, were then cooled in a drum flaker, and were
then coarsely crushed on a hammer mill. The resultant granules were
then finely crushed on a mechanical mill, and were then classified
with a pneumatic classifier to obtain toner particles with a
predetermined volume average particle diameter. These toner
particles were blended with 0.6% by weight of silica (with a
particle diameter of 0.012 .mu.m), and were mixed through intense
agitation in a Henschel mixer to obtain a magnetic toner that tends
to be charged positively.
[0046] The bulk density of this magnetic toner was measured in the
following manner, and the magnetic toner thus prepared was put in a
high-speed printer to evaluate image reddishness, background
fogging, partial fogging, initial image density, and image density
sustenance. The results are shown in FIG. 1. The development was by
non-contact development using as the photoconductor an amorphous
silicon photoconductor, using a developing bias voltage obtained by
superimposing a direct-current of 160 V on an alternating-current
voltage with a frequency of 2.5 kHz and a peak-to-peak voltage of
1.9 kV, with the surface voltage of the latent image on the
photoconductive drum set at 10 V in bright portions and 240 V in
dark portions, and with the gap between the developing sleeve and
the photosensitive drum set at 320 .mu.m.
[0047] Measurement of the Bulk Density of the Toners
[0048] 30 g of the toner was put in a container, from which the
toner was quietly poured into a funnel with a sieve. With a 30 ml
collecting container placed under the funnel, the toner on the
sieve was stirred with a brush for 90 seconds to make the toner
disperse and fall. Then, the weight of the toner collected in the
collecting container was measured, and the bulk density of the
toner was calculated according to the following formula:
Bulk Density (g/cm.sup.3)=Magnetic Toner Weight/Collecting
Container Volume
[0049] Evaluation of the Obtained Images
[0050] The magnetic toner prepared as described above was put in a
high-speed printer structured as shown in FIG. 2 and capable of
printing 50 sheets per minute, and image reddishness and initial
background and partial fogging were evaluated under
normal-temperature, normal-humidity conditions (20.degree. C., 65
RH). Image reddishness and partial fogging were inspected visually.
Partial fogging was inspected by measuring the density of a
non-image area by using a reflection density meter (the model TC-6D
manufactured by Tokyo Denshoku Co., Ltd., Japan). Background
fogging was evaluated as follows: a concentration of 0.008 or lower
was evaluated as "GOOD," and a concentration over 0.008 as "NG."
Moreover, initial image concentration (in a solid black area of
copied images) was measured under high-temperature, high-humidity
conditions (35.degree. C., 85 RH) by using a reflection density
meter (the model TC-6D manufactured by Tokyo Denshoku Co., Ltd.,
Japan). Initial image density was evaluated as follows: a
concentration of 1.2 or higher was evaluated as "GOOD," and any
lower concentration as "NG."
[0051] Image Density Sustenance
[0052] With the magnetic toner prepared as described above, bulk
printing was performed to check whether an image density of 1.3 or
higher was sustained after copying on 10,000 sheets, in which case
image density sustenance was evaluates as "FAIR," and after copying
on 20,000, in which case it was evaluated as "GOOD."
Practical Examples 2 to 7 and Comparative Examples 1 to 8
[0053] With toners prepared so as to contain different types of
magnetic powder and have different bulk densities as shown in Table
1, images produced with them were evaluated in the same manner as
with Practical Example 1.
[0054] Table 1 shows the following. With the magnetic toners of
Practical Examples 1 to 7, which are magnetic toners according to
the present invention, image reddishness was alleviated to such a
degree as to be ignorable in practical terms, and no background or
partial fogging was observed. Nor was observed inadequate initial
image density under high-temperature, high-humidity conditions.
Moreover, with any of these magnetic toners, an image density of
1.3 was sustained after bulk printing on 10,000 sheets. With the
magnetic toners of Practical Examples 1 to 4 and 6, an image
density of 1.3 was sustained after bulk printing on 20,000 sheets.
By contrast, with the magnetic toners of Comparative Examples 1 and
2, which used magnetic powders having average particle diameters as
small as 0.14 and 0.20 .mu.m respectively, image reddishness was
observed. Also with the magnetic toner of Comparative Example 3, of
which the FeO content was as low as 15% by weight, image
reddishness was observed. With the magnetic toners of Comparative
Examples 4 and 5, which did not fulfill formula (1), background and
partial fogging were observed respectively. With the magnetic toner
of Comparative Example 6, which had a low magnetic powder content,
background fogging was observed. With the magnetic toner of
Comparative Example 7, which had a high magnetic powder content,
inadequate initial image density was observed under
high-temperature, high-humidity conditions. With the magnetic toner
of Comparative Example 8, which had a low magnetic powder content
and did not fulfill formula (1), partial fogging was observed.
1 TABLE 1 Magnetic Magnetic Iron Initial Image Bulk Iron Oxide
Oxide Average FeO Density, at High Image Density Content Particle
Diameter Content Background Partial Temperature and Density
[g/cm.sup.3] [wt %] A/B .times. 100 [.mu.m] [wt %] Reddishness
Fogging Fogging Humidity Sustenance Practical 0.45 40 1.1 0.25 18
GOOD GOOD GOOD GOOD GOOD Example 1 Practical 0.45 40 1.1 0.21 18
GOOD GOOD GOOD GOOD GOOD Example 2 Practical 0.45 40 1.1 0.21 30
GOOD GOOD GOOD GOOD GOOD Example 3 Practical 0.45 40 1.1 0.30 25
GOOD GOOD GOOD GOOD GOOD Example 4 Practical 0.40 50 0.8 0.25 18
GOOD GOOD GOOD GOOD FAIR Example 5 Practical 0.45 35 1.3 0.25 18
GOOD GOOD GOOD GOOD GOOD Example 6 Practical 0.56 35 1.6 0.25 18
GOOD GOOD GOOD GOOD FAIR Example 7 Comparative 0.45 40 1.1 0.14 25
NG GOOD GOOD GOOD GOOD Example 1 Comparative 0.45 40 1.1 0.20 25 NG
GOOD GOOD GOOD GOOD Example 2 Comparative 0.45 40 1.1 0.25 15 NG
GOOD GOOD GOOD GOOD Example 3 Comparative 0.35 50 0.7 0.25 18 GOOD
NG GOOD GOOD FAIR Example 4 Comparative 0.59 35 1.7 0.25 18 GOOD
GOOD NG GOOD N/A.sup.(*) Example 5 Comparative 0.40 33 1.2 0.25 18
GOOD NG GOOD GOOD GOOD Example 6 Comparative 0.62 52 1.2 0.25 18
GOOD GOOD GOOD NC FAIR Example 7 Comparative 0.56 33 1.7 0.25 18
GOOD GOOD NC GOOD N/A.sup.(*) Example 8 .sup.(*)Bulk printing
impossible because of partial fogging.
* * * * *