JP2012173405A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that suppresses the occurrence of unstable discharge in an image formation area.SOLUTION: An image forming apparatus 10 includes a photoreceptor drum 62, and a charging part 66 that applies a DC voltage between the photoreceptor drum 62 and the charging part 66 and discharges a current so as to charge the photoreceptor drum 62. The photoreceptor drum 62 has an image formation area P in which an image is formed and a non-image formation area Q in which an image is not formed; the discharge current density of the non-image formation area Q by the charging part 66 is larger than the discharge current density of the image formation area P.

Description

  The present invention relates to an image forming apparatus.

  In Patent Document 1, for a charging member that applies a voltage to charge or charge a charged body in contact with or close to the charged body, the electric resistance value of the end side area portion of the effective charging width area is set to Disclosed is a charging member that gradually increases from the electrical resistance value toward the end of the effective charging width region.

  Patent Document 2 discloses an electrophotographic photosensitive member having a photosensitive layer on a conductive support, wherein the electrophotographic photosensitive member is charged by applying a voltage to a charging member disposed in contact with the electrophotographic photosensitive member, Disclosed is an electrophotographic photosensitive member in which the electrophotographic photosensitive member at the end of the region where the electrophotographic photosensitive member and the charging member can come into contact has high impedance.

Japanese Patent Application Laid-Open No. 07-261507 JP 07-295340 A

  An object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of unstable discharge in an image forming area.

  According to a first aspect of the present invention, there is provided an image holding body, and a charging member that charges the image holding body by applying a direct current voltage between the image holding body and discharging the image holding body. The holding body has an image forming area where an image is formed and a non-image forming area where an image is not formed, and the discharge current density of the non-image forming area by the charging member is larger than the discharge current density of the image forming area. An image forming apparatus characterized by the above.

  According to a second aspect of the present invention, the charging member is formed in a cylindrical shape, and has a surface layer that is deformed when pressed, the image carrier is formed in a cylindrical shape, and both sides in the axial direction are non-image forming regions. The non-image forming area is an image forming area. The charging member and the image holding member are in contact with each other so that the axial directions thereof are parallel to each other, and both sides of the charging member in the axial direction are held on the image. The image forming apparatus according to claim 1, further comprising a pressing unit that presses the body.

  The present invention according to claim 3 is the image forming apparatus according to claim 1 or 2, wherein the film thickness of the non-image forming area of the image carrier is smaller than the film thickness of the image forming area.

  The present invention according to claim 4 is the image forming apparatus according to claim 1 or 2, wherein the resistance of the non-image forming area of the image carrier is smaller than the resistance of the image forming area.

  The present invention according to claim 5 is the image forming apparatus according to claim 2, wherein a resistance of a portion of the charging member facing the non-image forming region is smaller than a resistance of a portion facing the image forming region.

  According to the sixth aspect of the present invention, the surface roughness Ra defined in JIS B 0601 of the portion facing the non-image forming region of the charging member is smaller than the surface roughness Ra of the portion facing the image forming region. The image forming apparatus according to claim 2.

  According to a seventh aspect of the present invention, there is provided the image forming apparatus according to any one of the first to sixth aspects, further comprising a neutralizing unit that neutralizes a non-image forming area.

  According to the first aspect of the present invention, it is possible to suppress the occurrence of unstable discharge in the image forming area.

  According to the second aspect of the present invention, in addition to the effect of the first aspect of the present invention, the charging member and the image holding member are placed before the image forming region as compared with the case without the present configuration. It is possible to easily approach the non-image forming area.

  According to the third aspect of the present invention, in addition to the effect of the first aspect of the present invention according to the first or second aspect, the unstable discharge in the image forming region can be more effectively compared to the case without the present configuration. Occurrence can be suppressed.

  According to the present invention of claim 4, in addition to the effect of the present invention of claim 1 or 2, the discharge of unstable discharge in the image forming region is more effectively compared with the case without this configuration. Occurrence can be suppressed.

  According to the present invention of claim 5, in addition to the effect of the present invention of claim 2, the generation of unstable discharge in the image forming region is more effectively performed as compared to the case without this configuration. Can be suppressed.

  According to the sixth aspect of the present invention, in addition to the effect of the present invention according to the second aspect, in comparison with the case where the present configuration is not provided, generation of unstable discharge in the image forming region is more effectively performed. Can be suppressed.

  According to the seventh aspect of the present invention, in addition to the effect of the present invention according to any one of the first to sixth aspects, the image forming region is more effectively unstable than the case without the present configuration. The occurrence of discharge can be suppressed.

1 is a cross-sectional view from the side of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic view of an image forming unit and its peripheral structure according to an embodiment of the present invention. It is an example of the schematic diagram of the cross section of the photoreceptor drum which concerns on embodiment of this invention. It is a perspective view of the charging part which concerns on embodiment of this invention. FIG. 6 is an explanatory diagram illustrating characteristics of discharge between the photosensitive drum and the charging unit. FIG. 3 is a schematic view of the periphery of a contact portion between a photosensitive drum and a charging unit. It is the schematic which shows the positional relationship with respect to the axial direction of the charging part which contacts a photoconductor drum. It is the schematic of the contact part periphery of the photoreceptor drum and charging part which concerns on 2nd embodiment.

[First embodiment]
Embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view from the side of an image forming apparatus 10 as an embodiment of the present invention.

  The image forming apparatus 10 includes an image forming apparatus main body 12, and the upper part of the image forming apparatus main body 12 is used as a discharge unit 14 from which a recording medium on which an image is formed is discharged.

  The image forming apparatus main body 12 is provided with a mounting opening / closing section (not shown) and a paper feeding opening / closing section 24 that can be freely opened and closed with respect to the image forming apparatus main body 12.

  The mounting opening / closing portion is used when mounting the storage containers 30Y, 30M, 30C, and 30K used as the image forming agent storage container in the image forming apparatus main body 12, or when mounting the storage containers 30Y, 30M, 30C, and 30K to the image forming apparatus. Opened when removed from the body 12. The mounting opening / closing unit is closed when image formation is performed.

  The paper feed opening / closing unit 24 is opened when the recording medium is supplied from the front side of the image forming apparatus main body 12.

The storage containers 30Y, 30M, 30C, and 30K store yellow (Y), magenta (M), cyan (C), and black (K) toners used as image forming agents, respectively.
The storage containers 30Y, 30M, and 30C are similarly configured in shape and size, and store toner of substantially the same volume.
The storage container 30K is configured to be vertically longer than the storage containers 30Y, 30M, and 30C, has a larger volume than the storage containers 30Y, 30M, and 30C, and has a larger volume of toner that is stored than the storage containers 30Y, 30M, and 30C. It is supposed to be.

  Although the storage containers 30Y, 30M, and 30C and the storage container 30K have different volumes of toner that can be stored, they have the same members and functions.

In the image forming apparatus main body 12, an image forming unit 40, a recording medium supply device 42 that supplies a recording medium to the image forming unit 40, and a conveyance path 44 that is used for conveying the recording medium are provided.
The image forming unit 40, the recording medium supply device 42, and the conveyance path 44 constitute an image forming unit that forms an image on a recording medium.

The image forming unit 40 includes, for example, four image forming units 52Y, 52M, 52C, and 52K, a latent image forming device 54, and a transfer device 56. The image forming units 52Y, 52M, 52C, and 52K form developer images using Y, M, C, and K toners, respectively.
The image forming units 52Y, 52M, 52C, and 52K have the same configuration although the corresponding colors are different. Therefore, hereinafter, Y, M, C, and K corresponding to each color are omitted, and the “image forming unit 52” will be described. And may be generically named. The same applies to other configurations corresponding to the respective colors (such as the container 30 and the photosensitive drum 62).

  Each of the image forming units 52 includes a photosensitive drum 62 used as an image holding member, a cleaning device 64 that cleans the surface of the photosensitive drum 62, a charging unit 66 that charges the photosensitive drum 62, and a latent image forming device. And a developing device 68 that develops the electrostatic latent image formed on the surface of the photosensitive drum 62 by toner with toner to form a toner image.

  Each developing device 68 is supplied with the corresponding color toner from the container 30.

  The transfer device 56 includes a belt-shaped intermediate transfer body 72 used as a transfer medium, primary transfer rolls 74Y, 74M, 74C, and 74K used as a primary transfer device, and secondary transfer rolls used as a secondary transfer device. 76 and a cleaning device 78 for cleaning the surface of the intermediate transfer member 72.

  The intermediate transfer member 72 is transferred such that the toner images formed on the respective photosensitive drums 62 are superimposed. The intermediate transfer body 72 is rotatably supported by, for example, four support rolls 82a, 82b, 82c, and 82d used as support members.

  The primary transfer rolls 74Y, 74M, 74C, and 74K transfer toner images corresponding to the respective colors formed on the photosensitive drums 62Y, 62M, 62C, and 62K to the intermediate transfer body 72, respectively.

  The secondary transfer roll 76 transfers each color toner image transferred to the intermediate transfer body 72 to a recording medium.

  The recording medium supply device 42 includes, for example, a recording medium storage container 92 that stores recording media in a stacked state, an extraction roll 94 that extracts the uppermost recording medium stored in the recording medium storage container 92, and the extraction roll A conveyance roll 96 that conveys the recording medium extracted in 94 toward the image forming unit 40; and a separation roll 98 that contacts the conveyance roll 96 and separates the recording medium from the conveyance roll 96.

  The recording medium storage container 92 can be pulled out, for example, to the front side (left side in FIG. 1) of the image forming apparatus main body 12, and the recording medium is replenished while being pulled out from the image forming apparatus main body 12.

  The conveyance path 44 includes a main conveyance path 100, a reverse conveyance path 102, and an auxiliary conveyance path 104.

  The main conveyance path 100 is a conveyance path that conveys the recording medium supplied from the recording medium supply device 42 toward the discharge unit 14. In the main conveyance path 100, a registration roll 112, a secondary transfer roll 76, a fixing device 114, and a discharge roll 116 are arranged in order from the upstream side in the conveyance direction of the recording medium.

  The registration roll 112 starts to rotate at a predetermined timing from the stopped state, and the recording medium is transferred to the intermediate transfer body 72 and the secondary transfer roll 76 so as to coincide with the timing at which the toner image is transferred to the intermediate transfer body 72. To the contact portion (nip portion).

  The fixing device 114 fixes the toner image on the recording medium onto which the toner image has been transferred by the transfer device 56.

  The discharge roll 116 discharges the recording medium on which the toner image is fixed by the fixing device 114 to the discharge unit 14. Further, when forming images on both sides of the recording medium, the discharge roll 116 rotates in a direction opposite to that when the recording medium is discharged to the discharge unit 14, and removes the recording medium on which the image is formed on one side. It conveys to the reverse conveyance path 102 from the end side.

  The reverse conveyance path 102 is a conveyance path that reverses the recording medium on which an image is formed on one surface and conveys the recording medium toward the upstream side of the registration roll 112 again. In the reverse conveyance path 102, for example, two reverse conveyance rolls 118a and 118b are arranged.

  The auxiliary conveyance path 104 is used when a recording medium is supplied from the front side of the image forming apparatus main body 12 in a state where the paper feeding opening / closing section 24 is opened with respect to the image forming apparatus main body 12. In the auxiliary conveyance path 104, an auxiliary conveyance roll 120 that conveys the recording medium toward the registration roll 112 and a separation roll 122 that contacts the auxiliary conveyance roll 120 and separates the recording medium are arranged.

In the image forming apparatus main body 12, a film thickness measuring unit 130 that measures the film thickness (film thickness reduction) of the photosensitive drum 62 is provided.
Regarding the measurement of the film thickness of the photosensitive drum 62, the film thickness measuring unit 130 measures, for example, the number of printed recording media (hereinafter referred to as “number of printed sheets”), the rotation number of the photosensitive drum 62, or You may make it perform the measurement of the usage-amount of a toner, or what combined these.

Further, a voltage setting unit 140 that sets a voltage to be applied to the charging unit 66 is provided in the image forming apparatus main body 12. The voltage setting unit 140 is notified of the measurement result measured by the film thickness measurement unit 130.
The voltage setting unit 140 sets the voltage to be applied based on the measurement result of the film thickness measurement unit 130.

Next, details of the image forming unit 52 will be described.
FIG. 2 shows a schematic diagram of the image forming unit 52 and its peripheral structure.

  Around the photosensitive drum 62, a cleaning device 64, a charging unit 66, a developing device 68, and a primary transfer roll 74 are disposed via an intermediate transfer member 72.

  The cleaning device 64 includes a cleaning blade 200 that removes toner, paper dust, and the like remaining on the surface of the photosensitive drum 62, a collection container 202 that collects toner and the like removed by the cleaning blade 200, and a collection container 202 that collects the toner and the like. And a leakage prevention member 204 for preventing leakage of toner and the like.

The charging unit 66 is provided along the rotation direction of the photosensitive drum 62 (hereinafter, simply referred to as “rotation direction”).
A side closer to the developing device 68 with respect to the rotation direction is defined as a downstream side in the rotation direction.

The charging unit 66 is formed in a roll shape, and is configured as a charging roll that is charged (contact charging) in contact with the photosensitive drum 62.
It is assumed that the contact charging includes charging the charging unit 66 and the photosensitive drum 62 as close as possible to cause discharge.

  The charging unit 66 is provided with a cleaning member 226 that cleans the surface of the charging unit 66. The cleaning member 226 is rotated by the rotation of the charging unit 66.

The charging unit 66 is provided with an applying unit 210 that applies a voltage to the charging unit 66.
In the present embodiment, the application unit 210 applies a DC voltage to the charging unit 66 (DC charging method).
Note that the application unit 210 may apply a method in which an AC voltage is superimposed on a DC voltage (AC + DC charging method).

As described above, the voltage setting unit 140 is notified of the measurement result from the film thickness measurement unit 130.
As described above, the voltage setting unit 140 is configured to set the voltage applied to the applying unit 210 based on the measurement result of the film thickness measuring unit 130.

Next, details of the photosensitive drum 62 will be described.
FIG. 3 shows an example of a schematic diagram of a cross section of the photosensitive drum 62.

  In the photosensitive drum 62, an undercoat layer 172 is provided on a conductive substrate 170, and a charge generation layer 174, a charge transport layer 176, and a protective layer 178 are formed on the undercoat layer 172. Is provided.

Examples of the material of the conductive substrate 170 include, for example, a metal plate, a metal drum, and a metal plate formed using a metal or an alloy such as aluminum, copper, zinc, stainless steel, chromium, nickel, molybdenum, vanadium, indium, gold, and platinum. In addition, a metal belt, or a paper, a plastic film, a belt, or the like on which a conductive compound such as a conductive polymer or indium oxide, a metal or an alloy such as aluminum, palladium, or gold is applied, vapor-deposited, or laminated is used.
Here, the conductivity means that the volume resistivity is less than 10 ¹³ Ωcm.

When the photosensitive drum 62 is used in a laser printer, the surface of the conductive substrate 170 has a centerline average roughness (Ra ₇₅ ) of 0.04 in order to prevent interference fringes generated when laser light is irradiated. Not less than μm and not more than 0.5 μm.
When the center line average roughness (Ra ₇₅ ) is less than 0.04 μm, the interference prevention effect tends to be insufficient, and when it exceeds 0.5 μm, the image quality tends to be coarse.

As a method for adjusting the surface roughness, liquid honing is performed by suspending an abrasive in water and spraying it on the support, centerless grinding in which the support is pressed against a rotating grindstone, and grinding is continuously performed. An oxidation treatment or the like is used.
Further, a method is used in which conductive or semiconductive powder is dispersed in a resin, a layer is formed on the surface of the support, and the surface is roughened by particles dispersed in the layer.

The undercoat layer 172 is a layer that imparts leak resistance and carrier blocking properties.
For example, the undercoat layer 172 is configured to contain inorganic particles in the binder resin.

  Examples of the binder resin used for the undercoat layer 172 include acetal resins such as polyvinyl butyral, polyvinyl alcohol resin, casein, polyamide resin, cellulose resin, gelatin, polyurethane resin, polyester resin, methacrylic resin, acrylic resin, Polymer resin compounds such as vinyl chloride resin, polyvinyl acetate resin, vinyl chloride-vinyl acetate-maleic anhydride resin, silicone resin, silicone-alkyd resin, phenol resin, phenol-formaldehyde resin, melamine resin, urethane resin, or charge Examples thereof include a charge transporting resin having a transporting group and a conductive resin such as polyaniline.

Various additives may be used in the undercoat layer 172 to improve electrical characteristics, environmental stability, and image quality.
As the additive, polycyclic condensation type, azo type electron transporting pigments, zirconium chelate compounds, titanium chelate compounds, aluminum chelate compounds, titanium alkoxide compounds, organic titanium compounds, silane coupling agents and the like are used.

As the inorganic particles, for example, powder resistance (volume resistivity) of 10 ² Ωcm or more and 10 ¹¹ Ωcm or less is used.
If the volume resistivity is less than 10 ² Ωcm, sufficient leakage resistance cannot be obtained, and if it is higher than 10 ¹¹ Ωcm, a residual potential may be increased.
As the inorganic particles, for example, inorganic particles (conductive metal oxide) such as tin oxide, titanium oxide, zinc oxide, and zirconium oxide are used.

  In addition, the inorganic particles may be subjected to a surface treatment, or may be used as a mixture of two or more kinds such as those having different surface treatments or particles having different particle diameters. The volume average particle diameter of the inorganic particles is, for example, in the range of 50 nm to 2000 nm.

As the inorganic particles, for example, those having a specific surface area of 10 m ² / g or more by BET method are used. If the specific surface area value is less than 10 m ² / g, the chargeability tends to be lowered, and it tends to be difficult to obtain good electrophotographic characteristics.

  For example, the undercoat layer 172 has a Vickers hardness of 35 or more.

The undercoat layer 172 is, for example, 15 μm or more and 50 μm or less.
When the thickness of the undercoat layer 172 is less than 15 μm, sufficient leakage resistance cannot be obtained, and when it is 50 μm or more, residual potential tends to remain when used for a long period of time, resulting in an image density abnormality. There is a case.

  The charge generation layer 174 is a layer containing a charge generation material and a binder resin.

Examples of the charge generating material include azo pigments such as bisazo and trisazo, fused aromatic pigments such as dibromoanthanthrone, perylene pigments, pyrrolopyrrole pigments, phthalocyanine pigments, zinc oxide, and trigonal selenium.
As the charge generation material, for example, an inorganic pigment is desirable when the exposure wavelength of the light source is 380 nm or more and 500 nm or less, and a metal or metal-free phthalocyanine pigment is used when the exposure wavelength is 700 nm or more and 800 nm or less.

Examples of the binder resin used for the charge generation layer 174 include an insulating resin or an organic photoconductive polymer such as poly-N-vinylcarbazole, polyvinyl anthracene, polyvinyl pyrene, and polysilane.
Specifically, the binder resin includes polyvinyl butyral resin, polyarylate resin (polycondensate of bisphenols and aromatic divalent carboxylic acid, etc.), polycarbonate resin, polyester resin, phenoxy resin, vinyl chloride-vinyl acetate. A polymer, polyamide resin, acrylic resin, polyacrylamide resin, polyvinyl pyridine resin, cellulose resin, urethane resin, epoxy resin, casein, polyvinyl alcohol resin, polyvinyl pyrrolidone resin, and the like are used.
These binder resins are used singly or in combination of two or more. The mixing ratio of the charge generation material and the binder resin is, for example, in the range of 10: 1 to 1:10 by mass ratio.
Here, “insulating” means that the volume resistivity is 10 ¹³ Ωcm or more.

  The film thickness of the charge generation layer 174 is, for example, not less than 0.1 μm and not more than 5.0 μm.

  The charge generation layer 174 is formed using a coating liquid in which the charge generation material and the binder resin are dispersed in a solvent.

  Solvents used for dispersion include methanol, ethanol, n-propanol, n-butanol, benzyl alcohol, methyl cellosolve, ethyl cellosolve, acetone, methyl ethyl ketone, cyclohexanone, methyl acetate, n-butyl acetate, dioxane, tetrahydrofuran, and methylene chloride. , Chloroform, chlorobenzene, toluene and the like, and these are used singly or in combination of two or more.

  As a method for dispersing the charge generating material and the binder resin in the solvent, a ball mill dispersion method, an attritor dispersion method, a sand mill dispersion method, or the like is used. By these dispersion methods, changes in the crystal form of the charge generation material due to dispersion are prevented.

  Further, when the charge generation layer 174 is formed, a blade coating method, a Mayer bar coating method, a spray coating method, a dip coating method, a bead coating method, an air knife coating method, a curtain coating method, or the like is used.

  The charge transport layer 176 is formed containing a charge transport material and a binder resin, or containing a polymer charge transport material.

Charge transport materials include quinone compounds such as p-benzoquinone, chloranil, bromanyl, anthraquinone, tetracyanoquinodimethane compounds, fluorenone compounds such as 2,4,7-trinitrofluorenone, xanthone compounds, benzophenone compounds , Electron transport compounds such as cyanovinyl compounds and ethylene compounds, triarylamine compounds, benzidine compounds, arylalkane compounds, aryl-substituted ethylene compounds, stilbene compounds, anthracene compounds, hydrazone compounds, etc. A hole transporting compound is used.
These charge transport materials may be used singly or in combination of two or more, but are not limited thereto.

The binder resin used for the charge transport layer 176 includes polycarbonate resin, polyester resin, polyarylate resin, methacrylic resin, acrylic resin, polyvinyl chloride resin, polyvinylidene chloride resin, polystyrene resin, polyvinyl acetate resin, styrene-butadiene. Copolymer, vinylidene chloride-acrylonitrile copolymer, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-maleic anhydride copolymer, silicone resin, silicone alkyd resin, phenol-formaldehyde resin, styrene-alkyd resin , Poly-N-vinylcarbazole, polysilane and the like.
These binder resins are used singly or in combination of two or more.
The blending ratio of the charge transport material and the binder resin is, for example, 10: 1 to 1: 5 by mass ratio.

  As the polymer charge transport material, poly-N-vinylcarbazole, polysilane, or the like is used. The polymer charge transport material may be formed alone or mixed with a binder resin to form a film.

  The film thickness of the charge transport layer 176 is, for example, 5 μm or more and 50 μm or less.

The charge transport layer 176 is formed using a charge transport layer forming coating solution containing the above-described constituent materials.
Solvents used in the coating solution for forming the charge transport layer include aromatic hydrocarbons such as benzene, toluene, xylene and chlorobenzene, ketones such as acetone and 2-butanone, and halogenation such as methylene chloride, chloroform and ethylene chloride. Organic solvents such as aliphatic hydrocarbons, cyclic ethers such as tetrahydrofuran and ethyl ether, or straight-chain ethers may be used alone or in admixture of two or more.

  Examples of the coating method for applying the charge transport layer forming coating solution on the charge generation layer 174 include a blade coating method, a Mayer bar coating method, a spray coating method, a dip coating method, a bead coating method, an air knife coating method, A curtain coating method or the like is used.

The protective layer 178 is the outermost surface layer of the photosensitive drum 62, and is a layer provided to give resistance to abrasion, scratches, etc. on the outermost surface and to increase toner transfer efficiency.
As the protective layer 178, conductive particles dispersed in a binder resin, lubricant particles such as fluororesin and acrylic resin dispersed in a normal charge transport layer material, a hard coating agent such as silicone or acrylic, or In view of strength, electrical characteristics, image quality maintenance, etc., those having a crosslinked structure, those containing a charge transporting material that is easily oxidized, and the like are used.
As what forms a crosslinked structure, a phenol resin, a urethane resin, a siloxane resin etc. are used, for example.

  Thus, the photosensitive drum 62 has a structure in which a chargeable film is formed on the surface.

The photosensitive drum 62 is not limited to the above configuration, and an undercoat layer 172 is provided on the conductive substrate 170, and a charge transport layer 176, a charge generation layer 174, and a protective layer 178 are sequentially formed on the undercoat layer 172. An exposed photosensitive layer may be provided.
Further, the charge generation material and the charge transport material may be formed as the same layer (single layer type photosensitive layer).
Note that the undercoat layer 172 may not be provided.

Next, details of the charging unit 66 will be described.
FIG. 4 is a perspective view of the charging unit 66.

The charging unit 66 includes a cored bar (shaft) 230, an elastic layer 232 disposed on the outer peripheral surface of the cored bar 230, and a surface layer 234 disposed on the outer peripheral surface of the elastic layer 232. The
The elastic layer 232 and the surface layer 234 of the charging unit 66 are deformed by being pressed.
The charging unit 66 is not limited to the above configuration, and an adhesive layer (primer layer) is provided between the metal core 230 and the elastic layer 232, and a resistance adjusting layer or a prevention layer is provided between the elastic layer 232 and the surface layer 234. A protective layer (coating layer) may be provided outside the surface layer 234.

The core metal 230 is formed in a rod shape by a conductive member. The core metal 230 may be a hollow (tubular) member or a non-hollow member.
As a material of the core metal 230, for example, a metal such as iron, copper, brass, stainless steel, aluminum, nickel, or a member (for example, resin or ceramic) whose outer peripheral surface is plated is dispersed. A member or the like is used.

  The elastic layer 232 is composed of, for example, an elastic material and a conductive agent. Further, the elastic layer 232 contains an additive as necessary.

Examples of elastic materials include isoprene rubber, chloroprene rubber, epichlorohydrin rubber, butyl rubber, polyurethane, silicone rubber, fluorine rubber, styrene-butadiene rubber, butadiene rubber, nitrile rubber, ethylene propylene rubber, epichlorohydrin-ethylene oxide copolymer. Examples thereof include rubber, epichlorohydrin-ethylene oxide-allyl glycidyl ether copolymer rubber, ethylene-propylene-diene terpolymer rubber (EPDM), acrylonitrile-butadiene copolymer rubber (NBR), natural rubber, and blended rubbers thereof.
The elastic material may be foamed or non-foamed.

As the conductive agent, an electronic conductive agent or an ionic conductive agent is used.
Examples of the electronic conductive agent include carbon black such as ketjen black and acetylene black, pyrolytic carbon, graphite, or conductive metals or alloys such as aluminum, copper, nickel, and stainless steel, or tin oxide and indium oxide. In addition, conductive metal oxides such as titanium oxide, tin oxide-antimony oxide solid solution, tin oxide-indium oxide solid solution, or the like, which are obtained by conducting the surface of an insulating substance, are used.
Examples of the ionic conductive agent include perchloric oxygen salts or chlorates such as tetraethylammonium and lauryltrimethylammonium, or perchloric oxygen salts or chlorates of alkali metals such as lithium and magnesium, or alkaline earth. These include perchloric oxygen salts or chlorates of similar metals.
A electrically conductive agent may be used independently and may be used in combination of 2 or more type.

  Examples of additives that can be used include softeners, plasticizers, curing agents, vulcanizing agents, vulcanization accelerators, antioxidants, surfactants, coupling agents, fillers (silica, calcium carbonate), and the like.

The elastic layer 232 has a thickness of, for example, about 1 mm to 10 mm and a volume resistivity of about 10 ³ Ωcm to 10 ¹⁴ Ωcm.

  The surface layer 234 is made of, for example, a resin. The surface layer 234 contains roughness-providing particles, a conductive agent, and an additive that impart a predetermined surface roughness, if necessary.

  Resins include acrylic resin, cellulose resin, polyamide resin, copolymer nylon, polyurethane resin, polycarbonate resin, polyester resin, polyethylene resin, polyvinyl resin, polyarylate resin, styrene butadiene resin, melamine resin, epoxy resin, urethane resin, silicone A resin, a fluororesin (for example, a tetrafluoroethylene perfluoroalkyl vinyl ether copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, a polyvinylidene fluoride, or the like), a urea resin, or the like is used.

The copolymer nylon includes nylon 610, nylon 11, nylon 12, and one or more of these as polymerized units. Further, this copolymer may contain nylon 6 or nylon 66 as a polymerization unit.
Further, an elastic material used for the elastic layer 232 may be applied as the resin.

As the roughness-imparting particles, conductive particles or non-conductive particles are used.
Here, “conductive” indicates that the volume resistivity is less than 10 ¹³ Ωcm, and “non-conductive” indicates that the volume resistivity is 10 ¹³ Ωcm or more.

As the conductive particles, for example, a conductive agent used for the elastic layer 232 is applied.
Non-conductive particles include resin particles (polyimide resin particles, methacrylic resin particles, polystyrene resin particles, fluorine resin particles, silicone resin particles, etc.), inorganic particles (clay particles, kaolin particles, talc particles, silica particles, alumina particles, etc.) ), And ceramic particles.

  The roughness imparting particles may be composed of the same kind as that used for the resin. In such a configuration, the compatibility and adhesion between the roughness-imparting particles and the resin are improved as compared with the case without this configuration.

  As the conductive agent and additive used for the surface layer 234, for example, the conductive agent and additive used for the elastic layer 232 described above are applied.

Next, the configuration of the photosensitive drum 62 and the charging unit 66 will be described.
FIG. 5 is an explanatory diagram illustrating characteristics of discharge between the photosensitive drum 62 and the charging unit 66.
FIG. 6 is a schematic view of the periphery of the contact portion between the photosensitive drum 62 and the charging unit 66.

  First, the discharge characteristics will be described using the photosensitive drum 62 and the charging unit 66.

In discharging, as the voltage for charging the photosensitive drum 62 (hereinafter referred to as “charging voltage”) increases, the distance between the photosensitive drum 62 and the charging portion 66 (hereinafter referred to as “gap length”) increases. Occurs in places.
Here, the “charging voltage” indicates a voltage required to charge the photosensitive drum 62.
For example, when the value set for the photosensitive drum 62 is “−600 V”, when the photosensitive drum 62 is changed from “0 V” to “−600 V”, the charging voltage becomes “−600 V”. When the photosensitive drum 62 is changed from “−200 V” to “−600 V”, the charging voltage becomes “−400 V”.

  Discharge tends to become unstable as the gap length increases. For this reason, a discharge having a long gap length causes a defect in image formation (image defect: for example, a horizontal stripe-like defect).

Further, the previously generated discharge (first discharge) charges the member to be charged (photosensitive drum 62) within a predetermined range. For this reason, the discharge (second discharge) that occurs later within the predetermined range in the first discharge has a smaller charging voltage and a shorter gap length than the first discharge.
Similarly, the discharge (third discharge) within the range in which the second discharge is charged and generated after the second discharge further reduces the charging voltage and the gap length.

Specifically, as shown in FIG. 5, it is assumed that discharge occurs in the order of positions A, B, and C along the rotation axis direction of the charging unit 66. In this case, due to the discharge at the position A, the photosensitive drums 62 corresponding to the surrounding positions B and C are charged (in this example, the photosensitive drum 62 is charged to a negative potential).
For this reason, the charging voltage at positions B and C is smaller than the charging voltage at position A.

Further, the subsequent discharge at the position B charges the portion of the photosensitive drum 62 corresponding to the position C which is the periphery thereof.
For this reason, the charging voltage at the position C is smaller than the charging voltage at the position B.

Accordingly, the charging voltage decreases in the order of positions A, B, and C.
That is, the discharge gap length at the position B is shorter than the position A, and the discharge at the position B is more stable than the position A.
Furthermore, the gap length of the discharge at the position C is shorter than the positions A and B, and the discharge at the position C is more stable than the positions A and B.

The larger the current density of the generated discharge (discharge current density), the smaller the surrounding charging voltage.
Here, the discharge current density indicates a value obtained by dividing a current value flowing between electrodes (between the photosensitive drum 62 and the charging unit 66) by discharge by an area in a direction perpendicular to the current flow in the discharge space.

  Discharge is more likely to occur earlier at locations where the discharge current density is higher (eg, locations where resistance is lower).

In addition, the discharge is likely to occur first at a location where the photosensitive drum 62 and the charging unit 66 come into contact with each other first (approaching a distance that causes discharge) as the photosensitive drum 62 and the charging unit 66 rotate.
For example, the photosensitive drum 62 and the charging unit 66 may be distorted or have a variation in surface shape due to manufacturing accuracy, and due to these, a difference occurs in the order of the discharged parts. There is a case where there is a first difference (there is a slight difference that the generation of the discharge is preceded by the time of contact with the rotation axis direction).

As shown in FIG. 6, the photosensitive drum 62 and the charging unit 66 have an image forming area P in which an image is formed so that the axial direction is parallel, and the image forming area P with respect to the rotational axis direction. It is in contact (or close) with the non-image forming area Q on the outside (on the end side with respect to the rotation axis direction) where no image is formed.
The image forming area P indicates an area that affects the image printed on the recording medium, and the non-image forming area Q indicates an area that does not affect the image printed on the recording medium.

The discharge current density in the non-image forming area Q is configured to be larger than the discharge current density in the image forming area P.
Specifically, for example, the resistance of the charging unit 66 is configured such that the portion q1 corresponding to the non-image forming region Q is smaller than the portion p1 corresponding to the image forming region P.
The resistance is indicated by surface resistivity (Ω / □), volume resistivity (Ωcm), or the like.

As described above, the discharge having a large discharge current density occurs in the non-image forming area Q, so that the charging voltage in the image forming area P becomes smaller than that in the case where the present configuration is not provided.
In the present embodiment, at the end that is the non-image forming region Q, a discharge having a large discharge current density occurs before the image forming region P located in the center. Therefore, the charging voltage in the image forming region P is smaller than in the case where the present configuration is not provided.

Not limited to the above configuration, the surface roughness of the charging unit 66 may be configured such that the portion q1 corresponding to the non-image forming region Q is smaller than the portion p1 corresponding to the image forming region P.
The surface roughness is indicated by arithmetic average roughness (Ra), ten-point average roughness (Rzjis), maximum height roughness (Rz), etc. defined by JIS B 0601-2001.

  Alternatively, the resistance of the photosensitive drum 62 may be configured such that the portion q2 corresponding to the non-image forming region Q is smaller than the portion p2 corresponding to the image forming region P.

  Further, the film thickness of the photosensitive drum 62 may be configured such that the portion q2 corresponding to the non-image forming region Q is smaller than the portion p2 corresponding to the image forming region P.

  The capacitance of the portion q2 corresponding to the non-image forming region Q may be configured to be larger than the capacitance of the portion p2 corresponding to the image forming region P.

In addition, a pressing unit that presses the charging unit 66 and the photosensitive drum 62 so that the non-image forming area Q approaches the image forming area P is provided.
For example, the pressing unit presses the charging unit 66 toward the photosensitive drum 62 such that the non-image forming area Q has a higher pressing force than the image forming area P.

In this embodiment, the pressing means is an urging means 250 made of an elastic member such as a spring, and the charging unit 66 is urged toward the photosensitive drum 62 by the urging means 250.
The urging means 250 is provided so as to urge both end portions of the charging portion 66 with respect to the rotation axis direction toward the photosensitive drum 62 in the non-image forming region Q.

The positional relationship in the axial direction of the charging unit 66 that contacts the photosensitive drum 62 will be described.
FIG. 7 is a schematic view of the contact portion between the photosensitive drum 62 and the charging portion 66 as viewed from the end of the charging portion 66 in the axial direction.

When the charging unit 66 is biased (pressed) by the biasing means 250, the elastic layer 232 and the surface layer 234 are deformed. For this reason, the shape of the contact portion of the charging unit 66 with the photosensitive drum 62 differs depending on the position in the axial direction.
The charging unit 66 has a shape that is greatly distorted toward the axial end portion urged by the urging means 250.

  As shown in FIG. 7, the axial end portion (non-image forming) of the portion p1 corresponding to the image forming region P rather than the curved surface L1 of the central portion in the axial direction of the charging portion 66 (portion p1 corresponding to the image forming region P). The curved surface L2 of the portion corresponding to the region Q1 corresponding to the region q1 has a shape in which the distance from the photosensitive drum 62 is smaller (the range in which the distance from the photosensitive drum 62 is small increases). ing.

Further, the curved surface L3 that is closer to the end in the axial direction than the curved surface L2 and that is the central portion in the axial direction of the portion q2 corresponding to the non-image forming region Q has a distance from the photosensitive drum 62. It has a smaller shape.
Similarly, the curved surface L4 that is closer to the end in the axial direction than the curved surface L3 and that corresponds to the non-image-forming region Q and that corresponds to the non-image forming area Q is the curved surface L4. The shape becomes smaller.

  For this reason, as shown in FIG. 7, compared with the case where this configuration is not provided, the charging unit 66 and the photosensitive drum 62 have portions q1 and q2 corresponding to the non-image forming region Q as they rotate. The configuration is such that the portions p1 and p2 corresponding to the image forming region P are likely to contact (or approach) earlier than the portions p1 and p2, and the portions p1 and p2 corresponding to the image forming region P are from the central side in the region. The end side is easier to contact (or approach) first.

  Accordingly, the discharge is generated first in the portions q1 and q2 corresponding to the non-image forming region Q, and subsequently the discharge is generated on the axial end side of the portions p1 and p2 corresponding to the image forming region P. In this way, discharges are generated in order on the inner side (axially central portion side).

  According to the order in which the axial discharge of the charging unit 66 occurs, the discharge current density and the charging voltage are the largest in the parts q1 and q2 corresponding to the non-image forming area Q, and then the part p1 corresponding to the image forming area P, It is large on the axial end side of p2, and gradually decreases toward the inner side (axial center side).

  Therefore, in the case of having this configuration, the discharge current density and the charging voltage in the image forming region P decrease from the axial end portion toward the axial central portion, that is, the discharge gap length decreases. In this case, image defects are hardly generated.

As the pressing means, a fixing member is provided for fixing the charging unit 66 in contact with (or in proximity to) the photosensitive drum 62 in a state in which a load is applied to the non-image forming region Q compared to the image forming region P. You may do it.
The pressing unit may be provided so as to press the photosensitive drum 62 toward the charging unit 66.

[Second Embodiment]
Next, a second embodiment will be described.
FIG. 8 is a schematic view of the periphery of the contact portion between the photosensitive drum 62 and the charging unit 66 according to the second embodiment.

In the second embodiment, a neutralizing device 260 such as a neutralizing lamp that neutralizes the portion q2 corresponding to the non-image forming area Q of the photosensitive drum 62 is provided.
For this reason, it is easier for current to flow in the non-image forming region Q than in the case without this configuration. That is, the discharge current density in the non-image forming area Q is configured to be larger than the discharge current density in the image forming area P.

  The configurations of the photosensitive drum 62, the charging unit 66, and the like are not limited to these embodiments, and the configurations may be combined, and can be changed as appropriate according to the purpose.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Image forming apparatus main body 14 Discharge part 24 Feeding opening / closing part 40 Image forming part 42 Recording medium supply apparatus 44 Conveyance path 52 Image forming unit 54 Latent image forming apparatus 56 Transfer apparatus 62 Photosensitive drum 66 Charging part 68 Developing device 72 Intermediate transfer member 114 Fixing device 130 Film thickness measuring unit 140 Voltage setting unit 210 Application unit 250 Energizing means

Claims (7)

An image carrier,
A charging member that charges the image carrier by applying and discharging a DC voltage between the image carrier and the image carrier;
Have
The image carrier has an image forming area where an image is formed and a non-image forming area where an image is not formed, and the discharge current density of the non-image forming area by the charging member is higher than the discharge current density of the image forming area. An image forming apparatus characterized by being large. The charging member is formed in a cylindrical shape and has a surface layer that is deformed by pressing,
The image carrier is formed in a cylindrical shape, both sides in the axial direction are non-image forming areas, and the space between the non-image forming areas is an image forming area,
The charging member and the image carrier are in contact so that the axial directions are parallel,
The image forming apparatus according to claim 1, further comprising a pressing unit that presses both axial sides of the charging member toward the image holding member.   The image forming apparatus according to claim 1, wherein a film thickness of the non-image forming area of the image carrier is smaller than a film thickness of the image forming area.   The image forming apparatus according to claim 1, wherein a resistance of the non-image forming area of the image carrier is smaller than a resistance of the image forming area.   The image forming apparatus according to claim 2, wherein a resistance of a portion of the charging member facing the non-image forming region is smaller than a resistance of a portion facing the image forming region.   The image forming apparatus according to claim 2, wherein a surface roughness Ra defined in JIS B 0601 of a portion of the charging member facing the non-image forming region is smaller than a surface roughness Ra of a portion facing the image forming region.   The image forming apparatus according to claim 1, further comprising a discharging unit that discharges the non-image forming area.

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