Title: Charging member and image forming apparatus provided with the same
Abstract: A charging member for charging a member to be charged and an image forming apparatus provided with the same have electrically conductive particles and an electrically conductive particle bearing member having elasticity and bearing the electrically conductive particles thereon, and the degree of cohesion of the electrically conductive particles is 0.1 to 85%. Thereby, the non-uniformity of a halftone image peculiar to particle charging can be improved.
Patent Number: 6,847,796 Issued on 01/25/2005 to Chigono, et al.
| Inventors:
|
Chigono; Yasunori (Shizuoka, JP);
Okuda; Koichi (Tokyo, JP);
Shimizu; Yasushi (Shizuoka, JP);
Yoshida; Masahiro (Tokyo, JP)
|
| Assignee:
|
Canon Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
413380 |
| Filed:
|
April 15, 2003 |
Foreign Application Priority Data
| Apr 17, 2002[JP] | 2002-114427 |
| Current U.S. Class: |
399/174; 361/225; 399/149; 399/176 |
| Intern'l Class: |
G03G 015/02 |
| Field of Search: |
399/174,175,176,168,149,150
430/902,108.1,108.24
361/221,225
|
References Cited [Referenced By]
U.S. Patent Documents
| 5380614 | Jan., 1995 | Totsuka et al. | 430/108.
|
| 5502548 | Mar., 1996 | Suzuki et al. | 399/110.
|
| 6038414 | Mar., 2000 | Inami et al. | 399/104.
|
| 6061539 | May., 2000 | Ishiyama et al. | 399/175.
|
| 6212346 | Apr., 2001 | Hirabayashi et al. | 399/176.
|
| 6226480 | May., 2001 | Hirabayashi et al. | 399/149.
|
| 6291123 | Sep., 2001 | Ohno et al. | 430/111.
|
| 6298205 | Oct., 2001 | Chigono et al. | 399/174.
|
| 6301455 | Oct., 2001 | Ishiyama et al. | 399/102.
|
| 6389254 | May., 2002 | Ishiyama et al. | 399/176.
|
| 6400919 | Jun., 2002 | Inoue et al. | 399/176.
|
| 6549742 | Apr., 2003 | Chigono et al. | 399/174.
|
| 6615010 | Sep., 2003 | Hirabayashi et al. | 399/174.
|
| 2002/0181970 | Dec., 2002 | Shimizu et al. | 399/149.
|
| Foreign Patent Documents |
| 0 864 936 | Sep., 1998 | EP.
| |
| 06-003851 | Jan., 1994 | JP.
| |
| 2002-132017 | May., 2002 | JP.
| |
Primary Examiner: Chen; Sophia S.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper & Scinto
Claims
What is claimed is:
1. A charging member for charging a member to be charged, comprising:
electrically conductive particles with a degree of cohesion, which ranges
from 0.1 to 85%; and
an electrically conductive particle bearing member having elasticity and
bearing said electrically conductive particles thereon,
wherein the degree of cohesion of said electrically conductive particles is
measured by vibrating a set screen combining meshes of 250 .mu.m openings,
150 .mu.m openings, and 75 .mu.m openings, respectively, stacked in order
from a top of the set screen.
2. A charging member according to claim 1, wherein a value obtained by
dividing an amount of said electrically conductive particle borne by said
electrically conductive particle bearing member by a surface roughness Ra
(.mu.m) of said electrically conductive particle bearing member is 0.005
to 1 mg/cm.sup.2 /.mu.m.
3. A charging member according to claim 1, wherein an average particle
diameter of said electrically conductive particles is 0.1 to 5 .mu.m.
4. A charging member according to claim 1, wherein the degree of cohesion
of said electrically conductive particles is 0.5 to 60%.
5. A charging member according to claim 1, wherein surfaces of said
electrically conductive particles are subjected to hydrophobic treatment.
6. A charging member according to claim 1, wherein said electrically
conductive particles are surface-treated by the use of a lubricant.
7. A charging member according to claim 1, wherein surfaces of said
electrically conductive particles are subjected to hydrophobic treatment,
and thereafter said electrically conductive particles are surface-treated
by the use of a lubricant.
8. A charging member according to claim 1, wherein a resistance of said
electrically conductive particles is 10.sup.-1 to 10.sup.12
.OMEGA..multidot.cm.
9. A charging member according to claim 1, which brings said electrically
conductive particles into contact with said member to be charged to
thereby charge said member to be charged.
10. A charging member according to claim 1, wherein said electrically
conductive particle bearing member is provided with a foamed layer on the
surface thereof.
11. An image forming apparatus comprising:
a member to be charged capable of bearing an image thereon; and
a charging member for bringing electrically conductive particles with a
degree of cohesion, which ranges from 0.1 to 85% into contact with said
member to be charged to thereby charge said member to be charged, said
charging member including the electrically conductive particles and an
electrically conductive particle bearing member having elasticity and
bearing said electrically conductive particles thereon,
wherein the degree of cohesion of said electrically conductive particles is
measured by vibrating a set screen combining meshes of 250 .mu.m openings,
150 .mu.m openings, and 75 .mu.m openings, respectively, stacked in order
from a top of the set screen.
12. An image forming apparatus according to claim 11, wherein a value
obtained by dividing an amount of said electrically conductive particles
borne by said electrically conductive particle bearing member by a surface
roughness Ra (.mu.m) of said electrically conductive particle bearing
member is 0.005 to 1 mg/cm.sup.2 /.mu.m.
13. An image forming apparatus according to claim 11, wherein an average
particle diameter of said electrically conductive particles is 0.1 to 5
.mu.m.
14. An image forming apparatus according to claim 11, wherein the degree of
cohesion of said electrically conductive particles is 0.5 to 60%.
15. An image forming apparatus according claim 11, wherein surfaces of said
electrically conductive particles are subjected to hydrophobic treatment.
16. An image forming apparatus according to claim 11, wherein said
electrically conductive particles are surface-treated by the use of a
lubricant.
17. An image forming apparatus according to claim 11, wherein surfaces of
said electrically conductive particles are subjected to hydrophobic
treatment, and thereafter said electrically conductive particles are
surface-treated by the use of a lubricant.
18. An image forming apparatus according to claim 11, wherein a resistance
of said electrically conductive particles is 10.sup.-1 to 10.sup.12
.OMEGA..multidot.cm.
19. An image forming apparatus according to claim 11, wherein said
electrically conductive particle bearing member is provided with a foamed
layer on the surface thereof.
20. An image forming apparatus according to claim 11, wherein said
electrically conductive particle bearing member is moved with a peripheral
speed difference relative to said member to be charged in a contact
portion between said electrically conductive particle bearing member and
said member to be charged.
21. An image forming apparatus according to claim 20, wherein said
electrically conductive particle bearing member is rotated in a direction
opposite to a direction of movement of said member to be charged in a
contact portion between said electrically conductive particle bearing
member and said member to be charged.
22. An image forming apparatus according to claim 11, further comprising
developing means for developing an electrostatic image formed on said
member to be charged by a developer, said developing means being capable
of collecting a residual developer on said member to be charged.
23. An image forming apparatus according to claim 22, wherein said
developing means is capable of supplying said electrically conductive
particles to said member to be charged, and said electrically conductive
particles supplied to said member to be charged are supplied to said
electrically conductive particle bearing member.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a charging member for charging a member to be
charged. The invention also relates to an image forming apparatus such as
a copying machine or a printer in which a charging member is brought into
contact with a member to be charged to thereby charge the surface of the
member to be charged.
2. Related Background Art
Heretofore, a contact charging apparatus has been one in which an
electrically conductive charging member (a contact charging member or a
contact charger) of a roller type (charging roller), a fur brush type, a
magnetic brush type, a blade type or the like is brought into contact with
a member to be charged such as an image bearing member, and a
predetermined charging bias is applied to this contact charging member to
thereby charge the surface of the member to be charged to a predetermined
polarity and potential.
These charging apparatuses are expressed together as a contact charging
apparatus, but the individual charging apparatuses differ greatly from one
another in the viewpoint of the charging mechanism (the mechanism of
charging and the principle of charging) thereof. In the charging mechanism
of contact charging, there exist I. a discharging type charging mechanism
and II. a direct injecting type charging mechanism. The feature of the
charging apparatus is determined depending on by which charging mechanism
the charging apparatus is. The principles and features of the discharging
type charging mechanism and the direct injecting type mechanism will
hereinafter be described.
I. Discharging Type Charging Mechanism
This is a mechanism in which the surface of a member to be charged is
charged by a discharge product by a discharging phenomenon occurring in
the gap between a contact charging member and the member to be charged.
A discharging type charging system has a constant discharging threshold
value in the contact charging member and the member to be charged and
therefore, as shown by A (a conventional type roller charging apparatus)
in FIG. 5 of the accompanying drawings, it is necessary to apply a voltage
greater than the potential of the member to be charged to the contact
charging member. Also, as compared with a corona charger, a discharge
product is created in principle though marked small in the amount created.
A roller charging process (roller charging apparatus) using an electrically
conductive roller (charging roller) as the contact charging member by
discharge is preferable in respect of the stability of discharge, and is
widely used.
This charging roller for discharge is made by forming a rubber material or
a foamed material of electrical conductivity or medium resistance into a
roller shape as a base layer, and covering the surface thereof with a high
resistance layer. In this construction, discharging phenomenon occurs in a
gap with several of tens .mu.m a little distance from the portion of
contact between the roller and the member to be charged. Accordingly, in
order to stabilize the discharging phenomenon, the surface layer of the
roller is flat and the average roughness Ra of the surface is sub-.mu.m or
less, and the surface has high roller hardness.
Also, the roller charging by discharging is high in applied voltage and if
there is a pinhole (the exposure of a substrate by the injury of the film
of the member to be charged), a voltage drop will spread to even the
periphery thereof and faulty charging will occur. Accordingly, the surface
resistance of the surface layer is made equal to or greater than
10.sup.11.OMEGA. to thereby prevent the voltage drop.
II. Direct Injecting Type Charging Mechanism
Direct injecting type charging is a charging mechanism in which the
exchange of charges is directly done by the contact at a molecular level
between the contact charging member and the member to be charged to
thereby charge (electrify) the surface of the member to be charged. It is
referred to also as direct type charging or injecting type charging.
In this charging mechanism, the potential difference between the contact
charging member and the member to be charged is of the order of several V
to several tens of V. The charging characteristic thereof is shown by B
(magnetic brush charging apparatus) in FIG. 5. The charging potential is
equal to an applied voltage, and there is no applied voltage difference
causing discharge. Also, the voltage necessary for charging is suppressed
to a low level.
As described above, this direct type charging system as a charging
mechanism does not result in the production of ions and therefore does not
cause any evil by a discharge product. That is, it is a charging process
excellent in terms of the safety of environment, the deterioration of the
member and low electric power.
Description will now be made of a charging apparatus by the direct
injecting type charging mechanism.
In the direct type charging mechanism, an important factor which determines
charging performance is the contacting property between the contact
charging member and the member to be charged. The contacting property
herein referred to means the performance of the contact type charging
member being capable of microscopically contact with how much of the
surface of the member to be charged while the latter passes through the
charging apparatus.
As a form of the contact charging member used in the direct injecting type
charging apparatus, an attempt by a charging roller for discharging or the
like has been made, but direct injecting type charging has been impossible
by the charging roller for discharging. This is because in the
high-hardness and smooth surface structure as previously described, the
contact charging member appears to be in close contact with the member to
be charged, but is scarcely in contact with the latter in the sense of a
microscopic contacting property at a molecular level necessary for charge
injection.
As a direct injecting type charging process proposed at present, there is
particle charging using a magnetic brush.
Thinking of improvements in particle charging and contact density, a
charging process (particle charging) using electrically conductive
particles is advantageous. The electrically conductive particles used at
this time are referred to as the "charging particles". As examples of an
apparatus of a charging type using the charging particles, there have been
proposed A. a magnetic brush charging apparatus using a magnetic brush
charging member having magnetically restrained electrically conductive
magnetic particles as the charging particles as a brush by a magnet, and
B. a charging apparatus using a charging member having a thin electrically
conductive particle layer formed on an elastic roller.
A. Magnetic Brush Charging Apparatus
FIG. 6 of the accompanying drawings is a model view schematically showing
the construction of an example of the magnetic brush charging apparatus
100. The reference numeral 120 designates a magnetic brush charging member
comprising a fixedly supported magnet roll 122, including magnetic poles
N1, N2, S1, and S2, a nonmagnetic and electrically conductive charging
sleeve 121 rotatably fitted around and concentrically with the magnet roll
122, and a magnetic brush layer (magnetic brush portion) 124 of
electrically conductive magnetic particles C formed while being attracted
to and held on the outer peripheral surface of the charging sleeve 121 by
the magnetic force of the magnetic roll 122 in the charging sleeve. The
reference numeral 123 denotes a casing to which the magnetic brush
charging member 120 is assembled and in which a suitable amount of
electrically conductive magnetic particles C is contained and stored. The
reference numeral 125 designates a magnetic brush layer thickness
regulating blade provided in the casing 123.
As the electrically conductive magnetic particles C which are charging
particles causing the magnetic brush layer 124 to be constituted, use is
made of magnetic metal particles such as ferrite or magnetite or these
magnetic particles bound by resin. The resistance value thereof is
1.times.10.sup.6 to 10.sup.9 .OMEGA.cm. The particle diameter thereof is
10 to 50 .mu.m.
The charging sleeve 121 is rotatively driven in the same clockwise
direction of the arrow as e.g. a photosensitive drum 1 as a member to be
charged. The magnetic brush layer 124 is rotatively conveyed in a
clockwise direction with the charging sleeve 121, and is regulated to a
predetermined layer thickness by the blade 125, and the
layer-thickness-regulated magnetic brush layer 124 contacts with the
photosensitive drum 1 and rubs against the surface of the photosensitive
drum 1 in a charging contact portion n. The magnetic brush layer 124
having passed through the charging contact portion n is return-conveyed to
an electrically conductive magnetic particle reservoir portion in the
casing 123 by the continued rotation of the charging sleeve 121, and is
circularly conveyed and used.
A predetermined charging bias is applied from a charging bias applying
voltage source V1 to the charging sleeve 121, and the surface of the
photosensitive drum 1 is uniformly charged to a predetermined polarity and
potential in the charging contact portion n by a direct injecting type
charging mechanism with the aid of the rubbing by the magnetic brush layer
124 and the applied charging bias.
B. Charging Apparatus by Thin Layer Electrically Conductive Particles
FIG. 7 of the accompanying drawings is a model view schematically showing
the construction of an example of a charging apparatus 20 by thin layer
electrically conductive particles. This charging apparatus 20 has a
charging roller 2 as a contact charging member, a charging bias applying
voltage source S1 for the charging roller, and a charging particle
supplying device 3.
The charging roller 2 comprises a mandrel 2a and an elastic
medium-resistance layer 2b of rubber or a foamed material as a charging
particle bearing member formed into a roller shape concentrically and
integrally with the outer periphery of the mandrel 2a, and further has a
thin layer of charging particles (electrically conductive particles) m
borne on the outer peripheral surface of the elastic medium-resistance 2b.
This charging roller 2 is pressed into contact with the photosensitive drum
1 as the member to be charged with a predetermined amount of entry to
thereby form a charging contact portion n of a predetermined width. The
charging particles m borne on the charging roller 2 contact with the
photosensitive drum 1 in the charging contact portion n.
The charging roller 2 is rotatively driven in the same clockwise direction
of the arrow as the photosensitive drum 1, and is rotated in a direction
opposite to the direction (counter-clockwise) of rotation of the
photosensitive drum 1 in the charging contact portion n, whereby it
contacts with the surface of the photosensitive drum 1 with a speed
difference with the charging particles m interposed therebetween.
The relative speed difference of the charging roller 2 relative to the
photosensitive drum 1 can be provided by rotatively driving the
photosensitive drum in a direction counter to the direction of rotation of
the charging roller 2 (a direction of rotation forward to the rotation of
the photosensitive drum 1) at a different peripheral speed. The charging
property of direct injecting type charging, however, depends on the ratio
between the peripheral speed of the photosensitive drum 1 and the
peripheral speed of the charging roller 2 and therefore, it is more
advantageous in respect of the number of revolutions to rotatively drive
the charging roller 2 in the same direction as the photosensitive drum 1,
and it is also preferable in respect of the retain ability of the
particles to adopt this construction.
During the image recording by an image recording apparatus, a predetermined
charging bias is applied from a charging bias applying voltage source S1
to the mandrel 2a of the charging roller 2.
Thereby, the peripheral surface of the photosensitive drum 1 is uniformly
contact-charged to a predetermined polarity and potential by a direct
injecting type charging process.
The charging particles m applied to the outer peripheral surface of the
charging roller 2 adhere to and are taken away by the surface of the
photosensitive drum 1 with the charging of the photosensitive drum 1 by
the charging roller 2. Accordingly, in order to make up for it, a charging
particle supplying device 3 for the charging roller 2 is required. The
application of the charging particles m to the charging roller 2 by the
charging particle supplying device 3 is effected by agitating the charging
particles m stored in the housing container 3a of the charging particle
supplying device 3 by an agitating vane 3b and supplying them to the outer
peripheral surface of the charging roller 2. Any charging particles m
which become excessive in conformity with a target amount of application
are scraped off by a fur brush 3c to thereby effect the application of a
proper amount of charging particles. The control of the amount of
application of the charging particles is adjustable at any time by the
control of the number of revolutions of the fur brush 3c.
C. Aptitude of Particle Charging to a Cleanerless System
Particle charging is suitable for the toner recycle system of an image
forming apparatus. That is, a toner recycle process is an excellent
construction in a transfer type image recording apparatus wherein waste
toner (untransferred toner) is used again for image forming to thereby
effectively make the most of the toner and eliminate a space for a cleaner
container and realize the downsizing of the image recording apparatus.
The untransferred toner is once introduced into a contact charging member
and is made ready for reuse (the original amount of charge of the toner)
and is returned to a developing apparatus through an image bearing member
and is used again for developing, or if unnecessary, is collected,
whereby, toner recycle becomes possible. A charging apparatus used here is
required to charge the image bearing member and in addition, to collect
the untransferred toner and recharge the toner.
From the viewpoint as described above, an attempt is made to think of the
aptitude of particle charging to toner recycle. A magnetic brush has the
features that itself is comprised of particles and can move with a degree
of freedom, and is great in contact area. Accordingly, in the magnetic
brush, it becomes possible to advantageously realize such functions
requisite in toner recycle as collecting the untransferred toner from on
the image bearing member, and further making the charges of the introduced
toner proper.
In the conventional charging technique as described above, however, it has
become apparent that the image recording apparatus causes the following
deterioration of the quality of image. Firstly, the problem of the
uniformity of a halftone image. When a uniform image of a medium density
area has been outputted, the has been a black streaked faulty image like a
trace swept by a broom, and also in a halftone image, there has occurred a
white spot-like faulty image of the order of 0.1 to 0.5 mm. Further, there
has occurred a faulty image having its ground slightly developed, i.e.,
fog. Observing the state of the fog well, it has been characteristic that
the fog toner is distributed with a certain unit. Particularly these are
remarkable in the lowering of performance under a high-temperature
high-humidity environment. Also, they have been remarkable in a printing
test after the image recording apparatus has been left as it is for a long
period.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a charging member and
an image forming apparatus which improve the non-uniformity of a halftone
image peculiar to particle charging.
It is another object of the present invention to provide a charging member
and an image forming apparatus which improve fog peculiar to particle
charging.
It is another object of the present invention to provide a charging member
and an image forming apparatus which reduce the cohesiveness of
electrically conductive particles.
It is another object of the present invention to provide an image forming
apparatus which is suited for collecting a developer by a developing
device by preventing the cohesion of electrically conductive particles
coming off a charging member.
Further objects and features of the present invention will become more
fully apparent from the following detailed description when read with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an image recording apparatus according to
Embodiment 1.
FIG. 2 is a model view showing the layer construction of a photosensitive
drum.
FIG. 3A is an illustration of a method of measuring the resistance value of
a charging roller.
FIG. 3B is an illustration of the method of measuring the resistance value
of the charging roller.
FIG. 4 is a schematic view of an image recording apparatus according to
Embodiment 2.
FIG. 5 is a charging characteristic graph of a conventional type roller
charging apparatus and a magnetic brush charging apparatus.
FIG. 6 is a schematic view of an example of the magnetic brush charging
apparatus.
FIG. 7 is a schematic view of an example of a charging apparatus by
thin-layer electrically conductive particles.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
<Embodiment 1>
FIG. 1 schematically shows the construction of a charging member according
to the present invention or an image recording apparatus using the
charging member. This image recording apparatus is a laser printer of a
direct injecting type charging type utilizing a transfer type
electrophotographic process.
(1) General Schematic Construction of the Image Recording Apparatus
The reference numeral 1 designates an image bearing member as a member to
be charged, and in the present embodiment, it is a rotary drum-shaped
negative polarity OPC photosensitive member (a negative photosensitive
member, and hereinafter referred to as the photosensitive drum) having a
diameter of 24 mm. This photosensitive drum is rotatively driven at a
constant speed of a peripheral speed 47 mm/sec. (=process speed PS, i.e.,
a printing speed) in the clockwise direction of arrow. This photosensitive
drum 1 will be described later in greater detail.
The reference numeral 20 denotes a charging apparatus which uniformly
charges the peripheral surface of the photosensitive drum 1 being rotated
to a predetermined polarity and potential. This charging apparatus 20 is
similar to the aforedescribed charging apparatus of FIG. 7 by the thin
layer electrically conductive particles, and has a charging roller 2 as a
contact charging member, a charging bias applying voltage source S1 for
the charging roller, and a charging particle supplying device 3 for the
charging roller.
In the present embodiment, the peripheral surface of the photosensitive
drum 1 is uniformly contact-charged to a predetermined polarity and
potential by this charging apparatus 20 in a direct injecting type
charging process. In the present embodiment, a charging bias of -600V has
been applied from the charging bias applying voltage source V1 to the
mandrel 2a of the charging roller 2 to thereby obtain substantially the
same charging potential as the applied charging bias on the surface of the
photosensitive drum 1. The charging apparatus 2 and the direct injecting
type charging will be described later in greater detail.
The reference numeral 4 designates a laser beam scanner (exposure
apparatus) including a laser diode, a polygon mirror, etc. This laser beam
scanner 4 outputs a laser beam intensity-modulated correspondingly to the
time-serial electrical digital pixel signal of desired image information,
and effects the scanning exposure L of the uniformly charged surface of
the rotary photosensitive drum 1 by the laser beam.
By this scanning exposure L, an electrostatic latent image corresponding to
the desired image information is formed on the surface of the rotary
photosensitive drum 1.
The reference numeral 60 denotes a developing apparatus (developing
device). The developing apparatus 60 in the present embodiment retains a
magnetic toner (negative toner) t and coats a developing sleeve 60a with a
constant amount of toner t. The toner t carries constant frictional
electrification by the rubbing thereof against the developing sleeve 60a,
and reversal-develops and visualizes the electrostatic latent image on the
photosensitive drum 1 in a developing area a by a developing bias applied
to between the developing sleeve 60a and the photosensitive drum 1 by a
developing bias applying voltage source V2. The developing apparatus 60
will be described later in greater detail.
The reference numeral 6 designates a transfer roller of medium resistance
as contact transferring means, and it is brought into predetermined
pressure contact with the photosensitive drum 1 to thereby form a
transferring nip portion b, a transfer material P as a recording medium is
fed from a feed portion, not shown, to this transferring nip portion b at
predetermined timing and a predetermined transfer bias voltage is applied
from a transfer bias applying voltage source V3 to the transfer roller 6,
whereby the toner image on the photosensitive drum 1 is sequentially
transferred to the surface of the transfer material P fed to the
transferring nip portion b.
The transfer roller 6 used in the present embodiment is one of a roller
resistance value 5.times.10.sup.8.OMEGA. comprising a mandrel 6a and a
medium-resistance foamed layer 6b formed thereon, and transfer has been
effected with a voltage of +2.0 kV applied to the mandrel 6a. The transfer
material P introduced into the transferring nip portion b is nipped and
transported by this transferring nip portion b, and the toner image formed
and borne on the surface of the rotary photosensitive drum 1 is
sequentially transferred to the surface side of the transfer roller by an
electrostatic force and a pressure force.
The reference numeral 7 denotes a fixing device of a heat fixing type or
the like. The transfer material P fed to the transferring nip portion b
and having received the transfer of the toner image from the
photosensitive drum 1 is separated from the surface of the rotary
photosensitive drum 1 and is introduced into this fixing device 7, and is
subjected to the fixing of the toner image and is delivered out of the
image recording apparatus as an image-formed article (printed copy).
Then, the photosensitive drum 1 is again charged by the charging apparatus
20 and is repetitively used for image forming.
The reference numeral 8 designates a photosensitive drum cleaning apparatus
for scraping off any untransferred toner residual on the photosensitive
drum 1 by a cleaning blade 8a and collecting it in a waste toner container
8b.
Then, the photosensitive drum 1 is again charged by the charging apparatus
20 and is repetitively used for image forming.
(2) Photosensitive Drum 1
FIG. 2 is a model view showing the layer construction of the photosensitive
drum (electrophotographic photosensitive member) 1 used in the present
embodiment. This photosensitive drum 1 is improved in charging performance
by applying a charge injection layer 16 to an ordinary organic
photosensitive drum comprising an aluminum drum base (Al drum base) 11
coated with an underlying layer 12, a positive charge injection preventing
layer 13, a charge generating layer 14 and a charge transporting layer 15
in the named order.
The charge injection layer 16 comprises SnO.sub.2 ultra-fine particles 16a
(having a diameter of about 0.03 .mu.m) as electrically conductive
particles (electrically conductive filter), a polymerization initiator,
etc. mixed with and dispersed in photo-curing type acrylic resin as a
binder, and formed into film by a photo-curing method after coating.
Also, in addition, by causing a lubricant such as tetrafluoroethylene resin
to be contained in it, there is the effect of suppressing the surface
energy of the surface of the photosensitive drum to thereby generally
suppress the adherence of the charging particles m. The surface energy,
when expressed in terms of the contact angle of water, may probably be 85
degrees or greater, and more preferably be 90 degrees or greater.
Also, from the viewpoint of charging performance, the resistance of the
surface layer of the surface becomes an important factor. In the direct
injecting type charging process, it is considered that the resistance of
the member to be charged is lowered, whereby the surface area of the
member to be charged which can be charged per injection point (contact
point) becomes wider. Accordingly, even if the charging roller is in the
same contact state, when the resistance of the surface of the member to be
charged is low, the efficient exchange of charges becomes possible. On the
other hand, the member to be charged is used as a photosensitive member,
it is necessary to retain an electrostatic latent image thereon for a
predetermined time and therefore, a range of 1.times.10.sup.9 to
1.times.10.sup.14 (.OMEGA..multidot.cm) is suitable as the volume
resistivity value of the charge injection layer 16.
Also, even in the case of a photosensitive drum not using the charge
injection layer 16, when for example, the charge transporting layer 15 is
within the above-mentioned resistance range, an equal effect is obtained.
Further, the use of an amorphous silicon photosensitive member or the like
of which the volume resistivity of the surface layer is 10.sup.13
.OMEGA..multidot.cm would also lead to the obtainment of a similar effect.
The resistance of the surface layer of the photosensitive drum 1 used in
the present embodiment was 10.sup.12 .OMEGA..multidot.cm.
(3) Charging Roller 2
The charging roller 2 in the present embodiment, as previously described,
comprises a mandrel 2a and an elastic medium-resistance layer 2b of rubber
or a foamed material as a charging particle bearing member formed into a
roller shape around this mandrel 2a so as to be concentric and integral
therewith. Charging particles (electrically conductive particles) m are
borne on the outer peripheral surface of the elastic medium-resistance
layer 2b of the charging roller 2. That is, the charging roller 2 and the
electrically conductive particles m are provided as a charging member.
The elastic medium-resistance layer 2b was prescribed by resin (e.g.
urethane), electrically conductive particles (e.g. carbon black), a
sulfidizing agent, a foaming agent, etc. and was formed into a roller
shape on the mandrel 2a. Thereafter, the surface thereof was polished.
The charging roller 2 in the present embodiment differs from a usually used
charging roller for discharging in
1) the surface structure for bearing charging particles m of high density
on the surface layer thereof and roughness characteristic, and
2) a resistance characteristic (volume resistivity and surface electrical
resistance) necessary for direct injecting type charging.
1) Surface Structure and Roughness Characteristic
Heretofore, the roller surface by discharging has been flat and sub-.mu.m
or less in terms of the average roughness Ra of the surface, and has been
high in roller hardness. In charging using discharging, a discharging
phenomenon occurs in a gap with several of tens .mu.m a little separate
from the contact portion between the roller and the member to be charged.
When unevenness is present on the surfaces of the roller and the member to
be charged, magnetic field intensity partly differs and therefore, the
discharging phenomenon becomes unstable and uneven charging occurs.
Accordingly, the conventional charging roller requires a smooth and highly
hard surface.
Then, considering why injection charging cannot be done by the charging
roller for discharging, it appears to be in close contact with the
photosensitive drum as the member to be charged in the surface structure
as previously described, but it is scarcely in contact with the
photosensitive drum in the sense of microscopic contact property at a
molecular level necessary for charge injection.
On the other hand, a certain degree of roughness is required of the
charging roller 2 in the present embodiment from the necessity of bearing
the charging particles m highly density. In terms of the average is
roughness Ra, 1 .mu.m to 500 .mu.m is preferable.
If the average roughness is smaller than 1 .mu.m, a surface area for
bearing the charging particles m is deficient, and when an insulator (e.g.
the toner) or the like adheres to the surface layer of the roller, the
periphery thereof becomes incapable of contacting with the photosensitive
drum as the member to be charged, and charging performance lowers.
Also, when particle retaining capability is considered, it is preferable to
have roughness greater than the particle diameter of the charging
particles used.
When conversely, the roughness is greater than 500 .mu.m, the unevenness of
the surface of the roller lowers the charging uniformity in the surface of
the member to be charged. In the present embodiment, Ra was 40 .mu.m.
For the measurement of the average roughness Ra, surface shape measuring
microscopes VF-7500 and VF7510 produced by Keyence Co., Inc. were used and
the measurement of the shape and Ra of the surface roller was effected in
non-contact by the use of an objective lens of 250 times to 1250 times.
2) Resistance Characteristic
The conventional type charging roller using discharge comprises a mandrel
and a base layer of low resistance formed thereon, and thereafter having
its surface covered with a high-resistance layer. Roller charging by
discharge, if an applied voltage is high and there is a pinhole (the
exposure of a substrate by the injury of film), voltage drop will reach
even the periphery thereof and faulty charging will occur. Accordingly, it
is necessary that the resistance of the charging roller be made equal to
or greater than 10.sup.11.OMEGA..
On the other hand, in the direct injecting type charging process in the
present embodiment, charging by a low voltage is possible and therefore,
the surface layer of the contact charging member need not be made high in
resistance, but the roller can be constituted by a single layer. Rather,
in direct injecting type charging, it is preferable that the surface
electrical resistance of the charging roller 2 be 10.sup.4 to
10.sup.10.OMEGA..
If the surface resistance becomes greater than 10.sup.10.OMEGA., a great
potential difference occurs on the surface of the roller and therefore a
discharge bias acts on the charging particles, and the charging particles
become liable to be discharged. Also, the uniformity in the charging
surface is lowered and the unevenness by the rubbing of the roller appears
as a steak shape in a halftone image, and a reduction in the quality of
image is seen.
On the other hand, when the surface electrical resistance is smaller than
10.sup.4.OMEGA., even in the case of injecting type charging, a peripheral
voltage drop by the pinhole of the drum will occur.
Further, it is preferable that volume resistivity be within the range of
10.sup.4 to 10.sup.7.OMEGA.. If the volume resistivity is smaller than
10.sup.4.OMEGA., the voltage drop of the voltage source by pinhole leak
becomes liable to occur. On the other hand, if the volume resistivity is
greater than 10.sup.7.OMEGA., an electric current necessary for charging
cannot be secured and the charging voltage will drop.
The surface electrical resistance and volume resistivity of the charging
roller 2 used in the present embodiment were 10.sup.7.OMEGA. and
10.sup.8.OMEGA., respectively.
The resistance measurement of the charging roller 2 was carried out by the
following procedure. The construction during the measurement is
schematically shown in FIGS. 3A and 3B. The roller resistance was measured
with an insulator drum 93 having an outer diameter of 24 mm being provided
with electrodes so that total pressure of 9.8 N (1 kgf) might be applied
to the mandrel 2a of the charging roller 2. As regards the electrode, a
guard electrode 91 was disposed around a main electrode 92, and
measurement was effected with wiring diagrams shown in FIGS. 3A and 3B.
The distance between the main electrode 92 and the guard electrode 91 was
adjusted to about the degree of the thickness of the elastic
medium-resistance layer 2b, and the main electrode 92 kept a sufficient
width relative to the guard electrode 91. As regards the measurement,
+100V was applied from a voltage source S4 to the main electrode 92 and
electric currents flowing through ammeters AV and AS were measured, and
the volume resistivity and surface electrical resistance were measured.
As has hitherto been described, in the charging roller in the present
embodiment,
1) A surface structure roughness characteristic in order to bear charging
particles of high density on the surface layer, and
2) A resistance characteristic (volume resistivity and surface electrical
resistance) necessary for direct charging are necessary.
3) Other Roller Characteristics
In the direct injecting type charging process, it is important for the
contact charging member to function as a flexible electrode.
In the magnetic brush, this is realized by the flexibility the magnetic
particle layer itself has.
In the charging apparatus 20 in the present embodiment, this is achieved by
adjusting the elastic characteristic of the elastic medium-resistance
layer 2b of the charging roller 2. In terms of Asker C hardness, 15
degrees to 50 degrees is a preferable range 20 to 40 degrees is more
preferable.
If the hardness is too high, a necessary amount of entry is not obtained,
and the charging contact portion n cannot be secured between the contact
charging member and the member to be charged and therefore, charging
performance is lowered. Also, the contact property of a substance at a
molecular level is not obtained and therefore, the contact with the
periphery thereof is hampered by the mixing or the like of a foreign
substance.
On the other hand, if the hardness is too low, the shape of the contact
charging member is not unstable and therefore the pressure of contact with
the member to be charged becomes uneven to thereby cause uneven charging.
Or there is caused faulty charging by the permanent deformation distortion
of the roller by being left as it is for a long period.
In the present embodiment, use was made of a charging roller 2 of 20
degrees in terms of Asker C hardness. Further, the charging roller 2 was
brought into contact with the photosensitive drum 1 with a total load of
1,000 g applied from the opposite end shafts of the roller. As the result,
the roller entered by about 0.2 to 0.3 mm from the surface of the drum,
and the width of the contact portion n between the roller and the drum was
2.7 mm.
4) Material, Structure and Dimensions of the Charging Roller
As the material of the elastic medium-resistance layer 2b of the charging
roller 2, mention may be made of EPDM, urethane, NBR, silicone rubber, or
a rubber material having carbon black for resistance adjustment or an
electrically conductive substance such as a metal oxide dispersed in IR or
the like. It is also possible to effect resistance adjustment by the use
of an ion conductive material without dispersing an electrically
conductive substance. Thereafter, the roughness adjustment of the surface
and the shaping by polishing are carried out as required. A construction
by a plurality of layers functionally separated from one another is also
possible.
However, as the form of the elastic medium-resistance layer 2b of the
charging roller 2, porous member structure is more preferable. This is
also advantageous in manufacture in that the aforedescribed surface
roughness can be obtained simultaneously with the molding of the roller.
The cell diameter of a foamed material is appropriately 1 to 500 .mu.m.
After molding by foaming, the surface of the foamed material is polished
to thereby expose the surface of the porous member, and it is possible to
make surface structure having the aforedescribed roughness.
Finally, an elastic medium-resistance layer 2b of a layer thickness 6 mm
having a porous member surface was formed on a mandrel 2a having a
diameter of 6 mm and a length of 240 mm, and a charging roller 2 having a
medium-resistance layer having a length of 220 mm was prepared.
(4) Charging Particles m
In the present embodiment, as the charging particles m, use was made of
electrically conductive zinc oxide having specific resistance of 10.sup.3
.OMEGA..multidot.cm and an average particle diameter 1.3 .mu.m. The
charging particles m are contained in the housing container 3a of the
charging particle supplying device 3.
As the material of the charging particles m, use can be made of one of
various electrically conductive particles such as electrically conductive
inorganic particles such as other metal oxides, a mixture with an organic
substance, or these materials subjected to surface treatment. Also, the
charging particles m in the present invention need not be magnetically
restrained and therefore need not have magnetism. Conversely, the charging
particles (electrically conductive particles) m in the present embodiment
is nonmagnetic and therefore, could be made small in particle diameter as
compared with magnetic electrically conductive particles. Consequently,
the electrically conductive particles can closely contact with the
photosensitive member and therefore, the injecting type charging property
can be improved.
As regards particle resistance, the exchange of charges through the
particles is effected and therefore 10.sup.12 .OMEGA..multidot.cm or less
is necessary as specific resistance, and preferably 10.sup.10
.OMEGA..multidot.cm or less is desirable. On the other hand, in order to
prevent a leak trace when there is a pinhole in the drum, 10.sup.-1
.OMEGA..multidot.cm or greater, preferably 10.sup.2 .OMEGA..multidot.cm or
greater is desirable.
Resistance measurement was effected by measuring and normalizing by the
tablet method. That is, about 0.5 g of charging particles m was put into a
cylinder having a bottom surface area of 2.26 cm.sup.2, and pressure of
147N (15 kgf) was applied to upper and lower electrodes and at the same
time, a voltage of 100V was applied thereto and the resistance value was
measured and thereafter, was normalized to thereby calculate the specific
resistance.
As regards the measurement of the particle diameter of the particles, D50
is calculated by a grain size distribution at a volume standard obtained
with a liquid module attached to LS-230 type laser diffraction type grain
size distribution measuring apparatus produced by COULTER Co., Inc. and
with particle diameters of 0.04 to 2000 .mu.m set as a measurement range.
The measurement is effected with about 10 mg of particles added to 10 ml
of methanol, and dispersed for 2 minutes by an ultrasonic dispersing
machine, and thereafter under a condition that the measuring time is 90
seconds and the frequency of measurement is one time.
There is a case where the charging particles m exist not only in the state
of primary particles, but also in the state of secondary particles in
which the primary particles have cohered, but if the physical properties
and function as the charging particles m can be realized as the secondary
particles, it is possible to function as the charging particles. However,
if the charging particles are formed by the secondary particles, an
improvement in charging performance is seen while, on the other hand, fog
and the lowering of the uniformity of a halftone image sometimes become
remarkable. This is because the secondary particles tend to cohere further
and this conversely causes a faulty image, and it becomes necessary to
adjust the degree of cohesion to an appropriate range. The details of this
will be described later.
It is desirable that the charging particles m, particularly when used for
the charging of the photosensitive member, be white or nearly transparent
so as not to hinder the exposure of the latent image. Further, considering
that the charging particles m are partly transferred from the
photosensitive member to a recording material, it is desirable in color
recording that the charging particles m be colorless or white. That is, it
is preferable that the charging particles m be nonmagnetic. Also in order
to prevent the scattering of light by the particles during image exposure,
it is desirable that the particle diameter of the charging particles m be
equal to or smaller than the size of a constituent pixel, and further,
equal to or smaller than the particle diameter of the toner. The lower
limit value of the particle diameter is considered to be 10 nm as being
stably obtained as particles.
That is, 0.01 to 10 .mu.m is usable as the particle diameter. 0.1 to 5.0
.mu.m is preferable. If the particle diameter is small, besides a problem
in manufacture, the deterioration of the toner when the charging particles
adhere to the toner is remarkable. If the particle diameter is large, it
becomes difficult to maintain the charging performance when any change in
environment is taken into consideration.
Further, in the present invention, from the viewpoint of the degree of
cohesion of the particles, 0.5 to 3 .mu.m is a preferable range of
particle diameter of the charging particles.
Also, it is necessary for the particles to have an appropriate specific
surface area. The specific surface area should preferably be
1.times.10.sup.5 to 100.times.10.sup.5 cm.sup.2 /cm.sup.3. More
preferably, it should be 5.times.10.sup.5 to 100.times.10.sup.5 cm.sup.2
/cm.sup.3. If the specific surface area is smaller than this range, even
if the charging particles are of the same particle diameter, the
performance as the charging particles will lower. This is expected to be
because if the specific surface area is small, the charging particles
assume relatively simple surface structure and therefore the points of
contact with the member to be charged are decreased. On the other hand, if
the specific surface area is too great, the lowering of the performance of
the toner has sometimes occurred particularly in a second embodiment.
Particles particularly great in specific surface area tend to become weak
in particle structure and incapable of maintaining a stable particle
diameter.
The charging performance can be greatly improved by an increase in specific
surface area, but particles great in specific surface area have the
tendency that the cohesion of the particles becomes great. As the result,
a faulty image which is a problem peculiar to the present invention
becomes liable to occur. Accordingly, it leads to the realizability of
charging particles of higher performance to increase the specific surface
area and also, pay attention to the degree of cohesion and carry out
various kinds of surface treatment for selecting particles or weakening
cohesion.
The measurement of the specific surface area of the particles was carried
out in the following manner.
First, in accordance with BET method, nitrogen gas is adsorbed to the
surface of a sample by the use of a specific surface area measuring
apparatus "Gemini 2375 Ver. 5.0" (produced by Shimazu Works Ltd.), and BET
specific surface area (cm.sup.2 /g) is calculated by the use of BET
multipoint method.
Next, true density (g/cm.sup.3) is found by the use of a dry type automatic
densimeter "Accupyc 1330" (produced by Shimazu Works Ltd.). At this time,
by the use of a sample container of 10 cm.sup.3, helium gas purge is
carried out ten times at maximum pressure of 19.5 psig as sample
pre-treatment. Thereafter, if as a pressure equilibrium judging value as
to whether the pressure in the container has reached equilibrium, the
deflection of the pressure in the sample chamber is equal to or smaller
than 0.0050/min as a standard, it is regarded as an equilibrium state and
