Image forming apparatus

Abstract

To provide an image forming apparatus capable of effectively removing deposits attached to a charged member. [Solution] The device comprises a photoreceptor having a charge injection layer containing conductive particles, a charging member in contact with the surface of the photoreceptor, a developing unit that develops an electrostatic latent image as a toner image, a transfer unit, and a control unit that controls the voltage applied to the charging member, and is capable of executing an image forming mode for transferring a toner image to a transfer member and a cleaning mode executed at a different timing from the image forming mode. In the cleaning mode, the control unit (i) periodically switches the voltage applied to the charging member within a range less than a discharge threshold relative to the surface potential of the photoreceptor before passing through the charging member, and (ii) sets the period of the voltage waveform pattern such that 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865, where Vpp_t is the amplitude of the voltage and Vpp_d is the amplitude of the surface potential of the photoreceptor after passing through the charging member.

Description

Technical Field

[0001] The present invention relates to an image forming apparatus that forms an image on a sheet.

Background Art

[0002] According to Patent Document 1, an image forming apparatus of an injection charging method is disclosed in which a charging brush is brought into contact with a photosensitive drum, and charges are injected from the charging brush into a charge injection layer of the photosensitive drum to charge the photosensitive drum. This image forming apparatus has a cleaning device that removes transfer residual toner remaining on the photosensitive drum after transfer of the toner image to a transfer material.

Prior Art Document

Patent Document

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, generally, an image forming apparatus of a cleanerless method that omits a cleaning device for removing transfer residual toner and recovers transfer residual toner in a developing unit is known. When the transfer residual toner has a normal charge with the same polarity as the voltage applied to the charging brush, the transfer residual toner can easily pass through the charging brush. However, actual transfer residual toner includes toner having a polarity opposite to the normal polarity or almost no charge. Therefore, in an image forming apparatus of an injection charging method and a cleanerless method, transfer residual toner may not be able to pass through the charging brush and may adhere to the charging brush.

[0005] Therefore, an object of the present invention is to provide an image forming apparatus capable of satisfactorily removing deposits adhering to a charging member.

Means for Solving the Problems

[0006] According to one aspect of the present invention, an image forming apparatus comprises a photoreceptor having a charge injection layer containing conductive particles and configured to be rotatable; a charging member that contacts the surface of the photoreceptor and charges the surface; a developing unit that develops an electrostatic latent image formed on the surface as a toner image; a transfer unit that transfers the toner image to a transfer member; and a control unit that controls the voltage applied to the charging member, and is capable of performing an image forming mode for transferring the toner image to the transfer member and a cleaning mode performed at a timing different from the image forming mode, wherein in the cleaning mode, the control unit (i) periodically switches the voltage applied to the charging member within a range less than a discharge threshold with respect to the surface potential of the photoreceptor before passing through the charging member, and (ii) sets the period of the waveform pattern of the voltage such that 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865, where Vpp_t is the amplitude of the voltage and Vpp_d is the amplitude of the surface potential of the photoreceptor after passing through the charging member. [Effects of the Invention]

[0007] According to the present invention, deposits adhering to the charged member can be effectively removed. [Brief explanation of the drawing]

[0008] [Figure 1] A schematic diagram showing the image forming apparatus according to this embodiment. [Figure 2] A cross-sectional view showing the layer structure of the photosensitive drum. [Figure 3] A schematic diagram showing a comb-shaped electrode. [Figure 4] A timing chart showing the state of each component in image formation mode and electrostatic brush cleaning mode. [Figure 5] (a) is a figure showing the waveform of the applied voltage for pattern 2 in Table 1, and (b) is a figure showing the drum potential at the developing nip when the applied voltage for pattern 2 in Table 1 is applied to the charging brush. [Figure 6](a) is a diagram showing the injection charging configuration as an electrical circuit diagram, (b) is a diagram showing the charging characteristics in an RC circuit, and (c) is a diagram showing the frequency characteristics of the capacitance of the injection charging configuration. [Figure 7] A block diagram showing the control block of an image forming apparatus. [Modes for carrying out the invention]

[0009] (1. Image forming apparatus configuration) Figure 1 is a schematic diagram of the image forming apparatus 100 equipped with process cartridges PY to PK according to this embodiment. In this embodiment, the image forming apparatus 100 is an electrophotographic laser beam printer compatible with Legal size paper, capable of forming images at a process speed of 210 mm / s and 600 dpi.

[0010] The image forming apparatus 100 shown in Figure 1 includes an image forming unit 40 for forming an image on a sheet K, a sheet feeding unit 50, and a fixing device 17. The sheet K includes paper such as paper and envelopes, plastic films such as overhead projector sheets (OHP), and cloth. Furthermore, the term "image forming apparatus" includes printers, copiers, facsimile machines, and multifunction devices, and refers to a device that forms an image on a sheet used as a recording medium based on image information input from an external PC or image information read from a document. In addition to the main unit with image forming function, an image forming apparatus may also include auxiliary equipment such as optional feeders, image readers, and sheet processing devices, and the entire system with such auxiliary equipment connected is also a type of image forming apparatus.

[0011] The image forming unit 40 includes four process cartridges PY, PM, PC, and PK, which each form toner images of four colors: yellow (Y), magenta (M), cyan (C), and black (K), and a laser scanner 7.

[0012] Note that the four process cartridges PY, PM, PC, and PK have the same configuration except for the color of the image they form. Therefore, only the configuration and image formation process of process cartridge PY will be explained, and the explanations of process cartridges PM, PM, and PK will be omitted.

[0013] The process cartridge PY comprises a photosensitive drum 1, a charging brush 2, a developing roller 3, and a toner container 23 that houses the developing roller 3. The photosensitive drum 1 is a cylindrical body with multiple functional layers, each 24 mm in diameter, formed on its outermost surface, which have a charge injection function. The configuration of the photosensitive drum 1 will be described in detail later.

[0014] The charging brush 2, acting as a charging component, is pressed against the photosensitive drum 1 with a predetermined pressure to form a charging nip. The charging brush 2 is configured to have a charging voltage applied by a charging power supply 61 (see Figures 6(a) and 7). The charging brush 2 and the injection charging configuration will be described in detail later.

[0015] The developing roller 3, which serves as the developing unit, is a rubber roller formed by molding conductive rubber onto a metal shaft. It is pressed against the photosensitive drum 1 with a predetermined pressure, forming a developing nip. The developing roller 3 is driven by a driving mechanism (not shown) so that it can rotate at a speed faster than the photosensitive drum. The developing roller 3 is also configured to transition between a contact state, where it is in contact with the photosensitive drum 1, and a separated state, where it is separated from the photosensitive drum 1.

[0016] The laser scanner 7 irradiates the photosensitive drum 1 with a laser based on the image signal, thereby exposing the photosensitive drum 1. In this embodiment, one laser scanner 7 is provided for each process cartridge's photosensitive drum 1, but one laser scanner may have four light-emitting units.

[0017] When a voltage of a predetermined negative polarity is applied to the charging brush 2, the photosensitive drum 1 is charged to a dark part potential of a predetermined negative polarity. In the part of the photosensitive drum 1 exposed by the laser irradiated from the laser scanner 7, the potential decreases, and an electrostatic latent image of a predetermined bright part potential is formed. Hereinafter, the part charged to the dark part potential (Vd) is referred to as the dark part Vd, and the part charged to the bright part potential (Vl) is referred to as the bright part Vl.

[0018] By applying a voltage (Vdc) of a predetermined negative polarity to the developing roller 3 and providing an appropriate potential difference between the dark part Vd and the bright part Vl, when the electrostatic latent image passes through the developing nip, the toner on the developing roller 3 transfers only to the bright part Vl, and the electrostatic latent image is visualized. The difference between the voltage (Vdc) applied to the developing roller 3 and the bright part potential (Vl) of the photosensitive drum 1 is called the developing contrast, and the amount of toner developed from the developing roller 3 to the photosensitive drum 1 can be controlled by this potential difference. Further, the difference between the bright part potential (Vd) of the photosensitive drum 1 and the voltage (Vdc) applied to the developing roller is called the back contrast, and this potential difference suppresses the occurrence of fog in which unnecessary toner is developed in the non-exposed part of the photosensitive drum 1, that is, the dark part Vd. In the present embodiment, by setting Vd = -520 [V], Vl = -120 [V], and Vdc = -320 [V], a setting in which the developing contrast and the back contrast become 200 [V] is adopted.

[0019] The toner used in the present embodiment is composed of toner particles having an average particle diameter of 6.4 [μm] and silica fine particles having an average particle diameter of 20 [nm] externally added thereto, and is charged to a negative polarity. The average particle diameter is, for example, the average particle diameter obtained from the particle volume, which can be measured by the Coulter method.

[0020] Furthermore, the image forming unit 40 is provided with an intermediate transfer belt 8 wrapped around a drive roller 9, a tension roller 10, and an opposing roller 28. Inside the intermediate transfer belt 8 are primary transfer rollers 6Y, 6M, 6C, and 6K. The primary transfer rollers 6Y, 6M, 6C, and 6K are each positioned to face the photosensitive drum 1 of each process cartridge. The intermediate transfer belt 8, as the material to be transferred, is an endless belt made of two layers of resin material, with a 2-micron thick resin surface layer coated on a 60-micron thick base layer, and is rotated in the direction of arrow Z by the drive roller 9.

[0021] The fixing device 17 includes a fixing film 18 heated by a heater (not shown) and a pressure roller 19 that presses against the fixing film 18. The sheet feeding unit 50 is located at the bottom of the image forming apparatus 100 and includes a cassette 13 that supports the sheet, a feeding roller 14 for feeding the sheet, and a pair of transport rollers 15. The feeding roller 14 and the pair of transport rollers 15 constitute the feeding and transport device 12.

[0022] Next, the image forming operation of the image forming apparatus 100 configured in this way will be described. When an image signal is input to the laser scanner 7 from a personal computer (not shown), the laser scanner 7 irradiates a laser corresponding to the image signal onto the photosensitive drum 1 of the process cartridge PY.

[0023] At this time, the surface of the photosensitive drum 1 is uniformly charged to a negative potential by the charging brush 2, and an electrostatic latent image is formed on the surface when a laser is shone from the laser scanner 7. The electrostatic latent image formed on the photosensitive drum 1 is developed by the developing roller 3, and a yellow (Y) toner image is formed on the photosensitive drum 1.

[0024] Similarly, laser light from the laser scanner is shone onto each photosensitive drum of process cartridges PM, PC, and PK, forming magenta (M), cyan (C), and black (K) toner images on each drum. The toner images of each color formed on each drum are transferred to the intermediate transfer belt 8 by primary transfer rollers 6Y, 6M, 6C, and 6K to which a positive voltage is applied, and then transported by the intermediate transfer belt 8 to the secondary transfer roller 11. The image formation process for each color is performed at a timing that overlaps with the upstream toner image that has been primary transferred onto the intermediate transfer belt 8.

[0025] In parallel with this image formation process, the sheet K housed in the cassette 13 of the sheet feeding unit 50 is transported to the registration roller pair 16 by the feeding and transport device 12. The sheet K's skew is corrected when its leading edge strikes the nip of the registration roller pair 16, which is in a stationary state. The sheet K, whose skew has been corrected by the registration roller pair 16, is transported at a predetermined transport timing. Then, the full-color toner image on the intermediate transfer belt 8 is transferred to the sheet K by a positive polarity secondary transfer bias applied to the secondary transfer roller 11, which acts as a secondary transfer unit.

[0026] The sheet K onto which the toner image has been transferred is subjected to predetermined heat and pressure by the fixing film 18 and pressure roller 19 of the fixing device 17, causing the toner to melt and solidify (fix) to the sheet. The sheet K that has passed through the fixing device 17 is discharged into the discharge tray 29 by the discharge roller pair 20.

[0027] The image forming apparatus 100 of this embodiment employs a cleanerless system using a development-simultaneous cleaning method. That is, after the toner image is first transferred from the photosensitive drum 1 to the intermediate transfer belt 8, the toner remaining on the surface of the photosensitive drum 1, which is the primary transfer residue toner, is electrostatically collected at the development nip. The primary transfer residue toner includes toner with negative polarity, which is the normal charging polarity, and toner with positive polarity, which is the opposite polarity.

[0028] The negatively polarized primary transfer residue toner on the surface of the photosensitive drum 1 remains on the surface of the photosensitive drum 1 when passing through the charged nip. This is because the surface potential of the charged brush 2 has a negatively higher potential than the surface potential of the photosensitive drum 1 (hereinafter referred to as the drum surface potential). Subsequently, the primary transfer residue toner is transferred from the photosensitive drum 1 to the developing roller 3 and recovered at the developing nip. This is because the potential of the developing roller 3, to which a negatively polarized voltage (Vdc) is applied, is lower than the dark area potential (Vd) of the photosensitive drum 1.

[0029] On the other hand, positively polarized primary transfer residue toner is transferred to the charged brush 2, which has a negative potential higher than the drum surface potential, as it passes through the charged nip, and adheres to the charged brush 2. If toner continues to accumulate on the charged brush 2, it may hinder the ability of the charged brush 2 to inject charge into the surface of the photosensitive drum 1. For this reason, in the image forming apparatus 100 with this configuration, a charged brush cleaning mode is performed after the image forming operation on the sheet K to electrostatically remove toner and other deposits from the charged brush 2. The details of the charged brush cleaning mode will be described later.

[0030] Furthermore, after the toner image is secondarily transferred from the intermediate transfer belt 8 to the sheet K, the toner remaining on the surface of the intermediate transfer belt 8, known as secondary transfer residue toner, is mechanically scraped off by the cleaning blade 21, which acts as a cleaning component. The secondary transfer residue scraped off by the cleaning blade 21 is then collected in the waste toner collection container 22.

[0031] Furthermore, the image forming apparatus 100 has a control board 25 on which electrical circuits for controlling the image forming apparatus 100 are mounted. Figure 7 is a block diagram showing the control block of the image forming apparatus 100. As shown in Figure 7, the control board 25 is equipped with a CPU (Central Processing Unit) 26 as a control unit, a ROM (Read Only Memory) 71, and a RAM (Random Access Memory) 72. Various programs are stored in the ROM 71, and the CPU 26 reads and executes the programs stored in the ROM 71. The RAM 73 is used as a workspace for the CPU 26.

[0032] The CPU 26 is responsible for controlling the intermediate transfer belt drive motor 73, the transport drive motor 74, and the drum motor 75, as well as controlling various image signals related to image formation. The intermediate transfer belt drive motor 73 is a motor for driving the intermediate transfer belt 8. The transport drive motor 74 is a motor for driving the feed transport device 12, the registration roller pair 16, and the fixing device 17. The drum motor 75 is the drive source for the process cartridge. The CPU 26 controls the voltage applied to the charging brush 2, the developing roller 3, and each transfer roller, etc., by controlling the charging power supply 61, the developing power supply 62, and the transfer power supply 63. The transfer power supply 63 may be divided into multiple primary transfer power supplies that control the voltage applied to the primary transfer rollers 6Y, 6M, 6C, and 6K, and a secondary transfer power supply that controls the voltage applied to the secondary transfer roller 11. In addition, a separate CPU may be provided for controlling the various motors and a separate CPU for controlling the various power supplies.

[0033] (2. Photosensitive drum) Next, the layer structure of the photosensitive drum 1 will be described with reference to Figure 2. Figure 2 is a cross-sectional view showing the layer structure of the photosensitive drum 1. The photosensitive drum 1 has a layer structure from bottom to top consisting of a support 101, a conductive layer 102, an undercoat layer 103, a photosensitive layer 104, and a charge injection layer 105. The photosensitive layer 104 has a charge generation layer 104p and a charge transport layer 104c, and the charge injection layer 105 contains conductive particles 105p. The photosensitive drum 1, as a photoreceptor, is formed by providing multiple layers on a cylindrical support 101, but a belt-shaped or sheet-shaped photosensitive member may be used instead of the photosensitive drum 1.

[0034] One method for manufacturing the photosensitive drum 1 is to prepare coating solutions for each layer, apply them in the desired order, and then dry them. Methods for applying the coating solutions include immersion coating, spray coating, inkjet coating, roll coating, die coating, blade coating, curtain coating, wire bar coating, and ring coating. Among these, immersion coating is preferred from the viewpoint of efficiency and productivity. The layers of the photosensitive drum 1 will now be described.

[0035] (2-1.Support) The support 101 of the photosensitive drum 1 is preferably a conductive support. The shape of the support 101 can be cylindrical, belt-shaped, or sheet-shaped, but cylindrical is preferred. The surface of the support 101 may also be subjected to electrochemical treatments such as anodizing, blasting, or cutting. The material of the support 101 is preferably metal, resin, or glass. Examples of metals include aluminum, iron, nickel, copper, gold, stainless steel, and alloys thereof. Among these, the support 101 is preferably made of aluminum. Furthermore, resins and glass may be imparted with conductivity by mixing or coating them with a conductive material.

[0036] (2-2. Conductive layer) In the photosensitive drum 1, a conductive layer 102 can be provided on the support 101. By providing the conductive layer 102, scratches and irregularities on the surface of the support 101 can be concealed, and the reflection of light on the surface of the support 101 can be controlled. Preferably, the conductive layer 102 contains conductive particles and a resin. Examples of materials for the conductive particles include metal oxides, metals, and carbon black.

[0037] Examples of metal oxides include zinc oxide, aluminum oxide, indium oxide, silicon oxide, zirconium oxide, tin oxide, titanium oxide, magnesium oxide, antimony oxide, and bismuth oxide. Examples of metals include aluminum, nickel, iron, nichrome, copper, zinc, and silver.

[0038] The average thickness of the conductive layer 102 is preferably 1 μm or more and 50 μm or less, and particularly preferably 3 μm or more and 40 μm or less. The conductive layer 102 can be formed by preparing a coating solution for the conductive layer containing the above-mentioned materials and solvents, forming this coating film, and drying it. Examples of solvents used in the coating solution include alcohol-based solvents, sulfoxide-based solvents, ketone-based solvents, ether-based solvents, ester-based solvents, and aromatic hydrocarbon-based solvents. Methods for dispersing conductive particles in the coating solution for the conductive layer include using a paint shaker, sand mill, ball mill, or liquid impaction type high-speed disperser.

[0039] (2-3. Lower layer) The photosensitive drum 1 may have an undercoat layer 103 provided on the support 101 or the conductive layer 102. The average thickness of the undercoat layer 103 is preferably 0.1 [μm] or more and 50 [μm] or less, more preferably 0.2 [μm] or more and 40 [μm] or less, and particularly preferably 0.3 [μm] or more and 30 [μm] or less.

[0040] Examples of resins for the undercoat layer 103 include polyacrylic acid resin, polyvinyl alcohol resin, polyvinyl acetal resin, polyethylene oxide resin, polypropylene oxide resin, ethylcellulose resin, methylcellulose resin, polyamide resin, polyamic acid resin, polyurethane resin, polyimide resin, polyamide-imide resin, polyvinylphenol resin, melamine resin, phenol resin, epoxy resin, and alkyd resin. In addition, the undercoat layer 103 may contain inorganic compounds or organic compounds other than resin.

[0041] Examples of inorganic compounds include gold, silver, aluminum, zinc oxide, lead white, aluminum oxide, indium oxide, silicon oxide, zirconium oxide, tin oxide, titanium oxide, magnesium oxide, antimony oxide, bismuth oxide, indium oxide, tin oxide, zirconium oxide, barium sulfate, and strontium titanate.

[0042] Examples of organic compounds include electron transport compounds and conductive polymers. Examples of conductive polymers include polythiophene, polyaniline, polyacetylene, polyphenylene, and polyethylenedioxythiophene. Examples of electron transport materials include quinone compounds, imide compounds, benzimidazole compounds, cyclopentadienylidene compounds, fluorenone compounds, xanthone compounds, benzophenone compounds, cyanovinyl compounds, aryl halides, silole compounds, and boron-containing compounds.

[0043] The undercoat layer 103 is obtained by preparing an undercoat coating solution containing the above-mentioned materials, applying it to the support 101 or the conductive layer 102, and then drying or curing the coating film. Examples of solvents used when preparing the coating solution include alcohol-based solvents, ketone-based solvents, ether-based solvents, ester-based solvents, or aromatic hydrocarbon-based solvents. Dispersion methods for dispersing particles in the coating solution include using a paint shaker, sand mill, ball mill, or liquid impaction type high-speed disperser.

[0044] (2-4. Photosensitive layer) The photosensitive layer 104 comprises a charge generation layer 104p and a charge transport layer 104c. The charge generation layer 104p preferably contains a charge generation substance and a resin. Examples of charge generation substances include azo pigments, perylene pigments, polycyclic quinone pigments, indigo pigments, and phthalocyanine pigments. Among these, azo pigments and phthalocyanine pigments are preferred. Among phthalocyanine pigments, oxytitanium phthalocyanine pigments, chlorogallium phthalocyanine pigments, and hydroxygallium phthalocyanine pigments are preferred. The content of the charge generation substance in the charge generation layer 104p is preferably 40% to 85% by mass, and more preferably 60% to 80% by mass, based on the total mass of the charge generation layer 104p.

[0045] Examples of resins include polyester resin, polycarbonate resin, polyvinyl acetal resin, polyvinyl butyral resin, acrylic resin, silicone resin, epoxy resin, melamine resin, polyurethane resin, phenolic resin, polyvinyl alcohol resin, cellulose resin, polystyrene resin, polyvinyl acetate resin, and polyvinyl chloride resin. Among these, polyvinyl butyral resin is more preferred.

[0046] Furthermore, the charge generation layer 104p may further contain additives such as antioxidants and ultraviolet absorbers. Specifically, examples include hindered phenol compounds, hindered amine compounds, sulfur compounds, phosphorus compounds, benzophenone compounds, and the like.

[0047] The charge generation layer 104p can be formed by preparing a coating solution for the charge generation layer containing the above-mentioned materials and solvents, forming this coating film on the undercoat layer 103, and drying it. Examples of solvents used in the coating solution include alcohol-based solvents, sulfoxide-based solvents, ketone-based solvents, ether-based solvents, ester-based solvents, and aromatic hydrocarbon-based solvents. The film thickness of the charge generation layer 104p is preferably 0.1 [μm] to 1.5 [μm], and more preferably 0.15 [μm] to 1.0 [μm].

[0048] The charge transport layer 104c preferably contains a charge transport material and a resin. Examples of charge transport materials include polycyclic aromatic compounds, heterocyclic compounds, hydrazone compounds, styryl compounds, enamine compounds, benzidine compounds, triarylamine compounds, and resins having groups derived from these materials. Among these, triarylamine compounds and benzidine compounds are preferred.

[0049] The content of the charge transport material in the charge transport layer 104c is preferably 25% by mass or more and 70% by mass or less, and more preferably 30% by mass or more and 55% by mass or less, based on the total mass of the charge transport layer 104c.

[0050] Examples of resins include polyester resin, polycarbonate resin, acrylic resin, and polystyrene resin. Among these, polycarbonate resin and polyester resin are preferred. Polyarylate resin is particularly preferred among polyester resins. The content ratio (mass ratio) of charge transport material to resin is preferably 4:10 to 20:10, and more preferably 5:10 to 12:10.

[0051] Furthermore, the charge transport layer 104c may contain additives such as antioxidants, ultraviolet absorbers, plasticizers, leveling agents, lubrication agents, and wear resistance improvers. Specifically, examples include hindered phenol compounds, hindered amine compounds, sulfur compounds, phosphorus compounds, benzophenone compounds, siloxane-modified resins, silicone oils, fluororesin particles, polystyrene resin particles, polyethylene resin particles, silica particles, alumina particles, and boron nitride particles.

[0052] The charge transport layer 104c can be formed by preparing a coating solution for the charge transport layer containing the above-mentioned materials and solvents, forming this coating film on the charge generating layer 104p, and drying it. Examples of solvents used in the coating solution include alcohol-based solvents, ketone-based solvents, ether-based solvents, ester-based solvents, and aromatic hydrocarbon-based solvents. Among these solvents, ether-based solvents or aromatic hydrocarbon-based solvents are preferred.

[0053] The thickness of the charge transport layer 104c is preferably 3 [μm] or more and 50 [μm] or less, more preferably 5 [μm] or more and 40 [μm] or less, and particularly preferably 10 [μm] or more and 30 [μm] or less.

[0054] (2-5. Charge injection layer) The photosensitive drum 1 used in this embodiment is characterized by having a charge implantation layer 105 containing conductive particles 105p on its outermost surface. That is, the charge implantation layer 105 includes the surface of the photosensitive drum 1. The charge implantation layer 105 may contain polymers and resins of compounds having polymerizable functional groups. Examples of polymerizable functional groups include isocyanate groups, blocked isocyanate groups, methylol groups, alkylated methylol groups, epoxy groups, metal alkoxide groups, hydroxyl groups, amino groups, carboxyl groups, thiol groups, carboxylic acid anhydride groups, carbon-carbon double bond groups, alkoxysilyl groups, and silanol groups. Monomers having charge transport ability may be used as compounds having polymerizable functional groups. Examples of resins include polyester resins, acrylic resins, phenoxy resins, polycarbonate resins, polystyrene resins, phenolic resins, melamine resins, and epoxy resins. Among these, acrylic resins are preferred.

[0055] The conductive particles 105p contained in the charge injection layer 105 include metal oxide particles such as titanium oxide, zinc oxide, tin oxide, and indium oxide. When metal oxides are used as conductive particles 105p, the metal oxides may also contain elements such as niobium, phosphorus, and aluminum, or their oxides. The conductive particles 105p may also have a laminated structure having core material particles and a coating layer covering the particles. Examples of core material particles include titanium oxide, barium sulfate, and zinc oxide. Examples of coating layers include metal oxides such as titanium oxide and tin oxide.

[0056] The conductive particles 105p are preferably titanium oxide particles containing niobium. In other words, the conductive particles 105p are preferably metal oxides containing titanium atoms and niobium atoms. As niobium-containing titanium oxide particles, various shapes can be used, such as spherical, polyhedral, ellipsoidal, flaky, and needle-shaped. Among these, spherical, polyhedral, and ellipsoidal shapes are preferred from the viewpoint of reducing image defects such as black spots. In this embodiment, it is even more preferable that the niobium-containing titanium oxide particles are spherical or polyhedral in shape close to a sphere. The niobium-containing titanium oxide particles are preferably anatase-type or rutile-type titanium oxide particles, and it is even more preferable that they are anatase-type titanium oxide particles. Using anatase-type titanium oxide results in good implantation charging properties.

[0057] In this embodiment, it is particularly preferable that the conductive particles 105p are particles having anatase-type titanium oxide particles as a core material and titanium oxide that coats the surface of the core material and contains niobium. When the conductive particles 105p have niobium-containing titanium oxide particles, the niobium content in the conductive particles 105p is preferably 0.5% by mass or more and 15.0% by mass or less, and more preferably 2.6% by mass or more and 10.0% by mass or less. If the niobium content in the conductive particles 105p is 0.5% by mass or more, a high effect of suppressing image flow can be obtained, and if it is 15.0% by mass or less, the volume resistivity of the charge injection layer 105 does not become too large.

[0058] When using a metal oxide as the conductive particle 105p, it is preferable to treat the surface of the metal oxide particles with a silane coupling agent or the like from the viewpoint of dispersibility and liquid stability. Furthermore, when using a metal oxide as the conductive particle 105p, its volume average particle size is preferably 1 nm or more and 500 nm or less, more preferably 3 nm or more and 300 nm or less, and even more preferably 5 nm or more and 100 nm or less.

[0059] The charge injection layer 105 may contain additives such as antioxidants, ultraviolet absorbers, plasticizers, leveling agents, lubricity enhancers, and wear resistance improvers. Specific examples of additives include hindered phenol compounds, hindered amine compounds, sulfur compounds, phosphorus compounds, benzophenone compounds, siloxane-modified resins, silicone oils, fluororesin particles, polystyrene resin particles, polyethylene resin particles, silica particles, alumina particles, and boron nitride particles.

[0060] The charge implantation layer 105 can be formed by preparing a coating solution for the charge implantation layer containing the above-mentioned materials and solvents, forming this coating film on the photosensitive layer 104, and drying and / or curing it. Examples of solvents used in the coating solution include alcohol-based solvents, ketone-based solvents, ether-based solvents, sulfoxide-based solvents, ester-based solvents, and aromatic hydrocarbon-based solvents. The film thickness of the charge implantation layer 105 is preferably 0.2 [μm] to 5 [μm], and more preferably 0.5 [μm] to 3 [μm].

[0061] (2-6. Method for measuring the volume resistivity of the charge injection layer) The volume resistivity of the charge injection layer 105 can be measured using a pA (picoampere) meter. The method for measuring the volume resistivity of the charge injection layer 105 will be explained with reference to Figure 3. Figure 3 is a schematic diagram showing a comb-shaped electrode 105a that can be used when measuring the volume resistivity of the charge injection layer 105. A comb-shaped electrode 105a as shown in Figure 3, with an inter-electrode distance (D) = 180 [μm] and a length (L) = 59 [mm], is fabricated on a PET film by vapor deposition, and a charge injection layer 105 with a thickness (T1) of 2 [μm] is placed on top of it. The comb-shaped electrode 105a consists of two comb-shaped electrodes that are insulated from each other. Under conditions of 23°C / 50%RH, the DC voltage (I) when a DC voltage (V) of 100 [V] is applied between the comb-shaped electrodes 105a is measured, and the volume resistivity ρv (Ω·cm) is obtained by the following formula (1). Volume resistivity ρv(Ω cm)=V(V)×T1(cm)×L(cm) / {I(A)×D(cm)}···(1)

[0062] When measuring the volume resistivity of the charge injection layer 105, which exists as a surface layer of the photosensitive drum 1 rather than the charge injection layer 105 alone, the surface resistivity of the charge injection layer 105 is measured, and the obtained value is converted to volume resistivity. A comb-shaped electrode 105a with an electrode distance (D) = 180 [μm] and a length (L) = 59 [mm] as shown in Figure 3 is fabricated on the surface of the charge injection layer 105 of the photosensitive drum 1 by gold deposition. The DC voltage (I) is measured when a DC voltage (V) of 1000 [V] is applied between the comb-shaped electrodes 105a in an environment of 23°C / 50%RH, and the surface resistivity ρs of the charge injection layer is calculated from DC voltage (V) / DC voltage (I). Using the obtained surface resistivity ρs and the film thickness t (cm) of the charge injection layer 105, the volume resistivity ρv (Ω·cm) can be obtained by the following equation (2). ρv = ρs × t ···(2) (ρv: volume resistivity, ρs: surface resistivity, t: thickness of the charge injection layer)

[0063] In this measurement, since a minute amount of current is to be measured, it is preferable to use a resistance measuring device capable of measuring minute currents. For example, the Hewlett-Packard picoammeter 4140B is one such device. The comb-type electrodes used and the applied voltage should be selected to obtain an appropriate signal-to-noise ratio based on the material and resistance value of the charge injection layer.

[0064] (3. Injection and charging configuration) The charging configuration in this embodiment is characterized by directly injecting charge into the charge injection layer 105 of the photosensitive drum 1 from the charging brush 2 to charge it. The charging brush 2 is a bar brush made by attaching a 5 mm wide fabric, which is made of conductive nylon fibers processed into a pile fabric using carbon as a conductive agent, to a 1 mm thick plated steel sheet. In other words, the charging brush 2 has a support member made of sheet metal and a brush portion made of conductive fibers attached to the support member that rubs against the photosensitive drum 1.

[0065] The conductive nylon fibers have a fineness of 2 denier and a flocking density of 240 fibers / mm². 2The pile length is 6 mm, and the brush is in contact with the photosensitive drum 1 such that the amount of penetration from the tip of the brush is 0.5 mm. The direct electrostatic charge injection by the electrostatic brush 2 improves as the contact area between the electrostatic brush 2 and the photosensitive drum 1 increases. Therefore, the greater the amount of penetration of the electrostatic brush 2 into the photosensitive drum 1, the greater the contact area and the better the electrostatic charge injection.

[0066] On the other hand, if the amount of charging brush 2 that penetrates the photosensitive drum 1 exceeds a predetermined amount, the contact pressure between the charging brush 2 and the photosensitive drum 1 increases, which may cause scratches on the photosensitive drum 1 caused by the charging brush 2. Also, in the cleanerless image forming apparatus 100, if the amount of charging brush 2 that penetrates is too high, the charging brush 2 may block the residual toner remaining on the surface of the photosensitive drum 1, which may reduce the charging ability of the charging brush 2. Therefore, the design values ​​of the charging brush 2, such as the bristle density, fineness, pile length, and penetration amount, need to be set in a way that balances the above considerations with the charging ability.

[0067] In this embodiment, a charging brush 2 with a resistance of 1 × 10^5 [Ω] was used. The resistance of the charging brush 2 was calculated from the current flowing when the charging brush 2 was brought into contact with a metal cylinder of the same diameter as the photosensitive drum 1 under the same conditions and a voltage of -100 [V] was applied. The resistance of the charging brush 2 can be controlled by changing the material of the conductive fibers of the charging brush 2, but a lower resistance of the charging brush 2 improves the charging ability of the charging brush 2.

[0068] Incidentally, in a cleanerless image forming apparatus that uses an injection charging method and a development-simultaneous cleaning method, in which an injection brush is brought into contact with the photosensitive drum to inject an electric charge into the photosensitive drum, as in this embodiment, as described above, residual toner from the transfer may not be able to pass through the charging brush and may adhere to the charging brush. Also, in the case of a direct transfer method in which the photosensitive drum and the sheet are in contact, substances such as fillers and fibers contained in the sheet are transferred to the photosensitive drum at the transfer section, and these substances may not be able to pass through the charging brush and may adhere to the charging brush.

[0069] Possible cleaning operations to remove deposits from charged components include the following: For example, applying a voltage of the opposite polarity to the charged component compared to the image formation process can transfer the reverse-polarity deposits from the charged component to the photosensitive drum. Alternatively, a configuration can be considered in which the voltage applied to the charged component is weakened (turned off) after one rotation of the photosensitive drum's surface, which has been charged with the normal polarity, thereby reversing the contrast.

[0070] Furthermore, in cleanerless image forming apparatuses using injection charging (adhesion charging), a known problem is latent image flow, where discharge products adhering to the surface of the photosensitive drum reduce the resistance of the drum surface, making it impossible to maintain the latent image. In injection charging, when a voltage exceeding the discharge threshold is applied to the charged member, a spark discharge occurs between the charged member and the photosensitive drum. This allows the drum surface to be controlled to a desired potential. On the other hand, when a spark discharge occurs between the charged member and the photosensitive drum, discharge products are generated on the drum surface. In image forming apparatuses equipped with a cleaning device, the cleaning blade and the like mechanically clean the surface of the photosensitive drum, making it difficult for discharge products to accumulate on the drum surface. On the other hand, in cleanerless image forming apparatuses, discharge products continue to accumulate on the drum surface, making latent image flow a likely problem.

[0071] Furthermore, in the injection charging method, when a voltage of reverse polarity is applied to the charged member, or when the applied voltage to the charged member is weakened, the drum potential fluctuates rapidly in accordance with the applied voltage. In other words, the higher the injection performance, the higher the potential tracking ability between the charged member and the photosensitive drum, making it difficult to secure the potential contrast necessary for removing deposits from the charged member.

[0072] (4. Charging brush cleaning mode) As described above, in the image forming apparatus 100 according to this embodiment, a charging brush cleaning mode is performed after the image forming operation on the sheet K in order to electrostatically remove deposits such as toner that have adhered to the charging brush 2. The details of the image forming mode and charging brush cleaning mode that the CPU 26 of the image forming apparatus 100 can execute will be described below with reference to Figure 4.

[0073] Figure 4 is a timing chart showing the rotation state of the photosensitive drum 1 and intermediate transfer belt 8, the contact state of the developing roller 3, and the applied voltage state to the charging brush 2, developing roller 3, and primary transfer roller 6 in the image forming mode and the charging brush cleaning mode when the image forming apparatus 100 receives a print signal. When the image forming apparatus 100 receives a print signal from an external computer or the like, the CPU 26 executes the image forming mode.

[0074] In image formation mode, the photosensitive drum 1 and the intermediate transfer belt 8 begin to rotate, respectively, due to a drum motor and an intermediate transfer belt drive motor (not shown). Subsequently, a voltage of -700[V] is applied to the charging brush 2 by a charging power supply 61 (see Figures 6(a) and 7) so that the dark area potential (Vd) of the photosensitive drum 1 in image formation mode becomes -520[V].

[0075] Next, the developing roller 3 transitions from a separated state to a contact state, and simultaneously, a voltage of -350[V] is applied to the developing roller 3. Then, a voltage of +800[V] is applied to the primary transfer roller 6. After the printing operation on sheet K is completed, the developing roller 3 transitions from a contact state to a separated state, and simultaneously, the applied voltage to the developing roller 3 is turned OFF. Furthermore, a voltage of -800[V] is applied to the primary transfer rollers 6Y~6K, and the image formation mode ends. Then, the CPU 26 starts the charged brush cleaning mode as a cleaning mode following the image formation mode.

[0076] In the charging brush cleaning mode of each embodiment described later, the CPU 26 periodically switches the applied voltage (voltage) to the charging brush 2 within a range that is below the discharge threshold relative to the surface potential of the photosensitive drum 1 before it passes through the charging brush 2. The control of the applied voltage to the charging brush 2 in the charging brush cleaning mode and its effects will be described in detail later.

[0077] When the voltage waveform of the charging brush cleaning mode is applied to the charging brush 2, accumulated toner and other deposits (mainly positively charged) are removed from the charging brush 2 and pass through the nip between the developing roller 3 and the photosensitive drum 1, which are separated. Then, as the deposits pass through the primary transfer nip formed between the photosensitive drum 1 and the primary transfer roller, they are transferred to the intermediate transfer belt 8, which has a potential negatively higher than the surface potential of the photosensitive drum 1, and are subsequently collected by the cleaning blade 21. After performing the charging brush cleaning mode for a predetermined time, the CPU 26 turns OFF the voltage applied to the charging brush 2 and the voltage applied to the primary transfer roller, stops the driving of the photosensitive drum 1 and the intermediate transfer belt 8, and ends the operation.

[0078] <Examples> The following describes an example of this embodiment.

[0079] (5. Method for manufacturing a photosensitive drum) (5-1.Support) As shown in Figure 2, the support 101 of the photosensitive drum 1 is made up of an aluminum cylinder with a diameter of 24 mm and a length of 257.5 mm.

[0080] (5-2. Conductive layer) The conductive layer 102 of the photosensitive drum 1 is manufactured as follows: A dispersion consisting of titanium oxide (TiO2) particles coated with oxygen-deficient tin oxide (SnO2) as metal oxide particles, a phenolic resin as a binder, 1-methoxy-2-propanol as a solvent, silicone resin particles as a surface roughening agent, and silicone oil as a leveling agent is immersed and coated onto the support 101, and heated at 140°C for 1 hour to form a conductive layer 102 with a thickness of 30 [μm] on the support 101.

[0081] (5-3. Lower layer) An electron transport material represented by the following formula (E-1), a blocked isocyanate, a styrene-acrylic resin, and a silica slurry were dissolved in a mixed solvent of 1-butanol and acetone, and this mixture was immersed and coated onto the conductive layer 102. This mixture was then heated at 170°C for 30 minutes to form an undercoat layer 103 with a thickness of 0.7 [μm].

[0082] [ka]

[0083] (5-4. Photosensitive layer) As described above, the photosensitive layer 104 has a charge generation layer 104p and a charge transport layer 104c. A coating solution prepared by diluting crystalline hydroxygallium phthalocyanine and polyvinyl butyral resin with cyclohexanone and ethyl acetate was immersed and coated onto the undercoat layer 103, and dried at 95°C for 10 minutes to form a charge generation layer 104p with a film thickness of 0.20 [μm].

[0084] A coating solution was prepared by dissolving a charge transport material represented by formula (C-1), a charge transport material represented by formula (C-2), a charge transport material represented by formula (C-3), polycarbonate, and a polycarbonate resin having copolymer units of formulas (C-4) and (C-5) in a mixed solvent of orthoxylene, methyl benzoate, and dimethoxymethane. This coating solution was then applied to the charge generating layer 104p by immersion to form a coating film, and the coating film was dried at 120°C for 30 minutes to form a charge transport layer 104c with a thickness of 12 [μm]. [ka] [ka] [ka] [ka] [ka]

[0085] (5-5. Charge injection layer) A coating solution was prepared by mixing a compound represented by the following structural formula (O-1) as a binder resin and niobium-containing titanium oxide particles as conductive particles in a mixed solvent of 1-propanol and cyclohexane. This solution was then immersed and coated onto the charge transport layer 104c to form a coating film, which was dried at 50°C for 6 minutes. Subsequently, the coating film was irradiated with an electron beam under a nitrogen atmosphere, and then the temperature of the coating film was raised to 117°C under a nitrogen atmosphere. Next, the coating film was allowed to cool naturally in air until its temperature reached 25°C, and then heat-treated for 1 hour under conditions that brought the temperature of the coating film to 120°C to form a charge implantation layer 105 with a thickness of 2 [μm]. [ka]

[0086] In this embodiment, the coating solution was mixed in the following proportions to be immersed and applied onto the charge transport layer 104c, and the volume resistivity of the coating film formed by the coating solution was approximately 1.0 × 10^13 [Ω·cm]. Compounds represented by structural formula (O-1) (79 parts) 76 parts of niobium-containing titanium dioxide particles 1-Propanol 100 units 100 copies of cyclohexane

[0087] (6. Voltage control waveform during electrostatic brush cleaning mode) In the charging brush cleaning mode, the CPU 26 periodically switches the voltage applied to the charging brush 2 within a range that keeps the surface potential of the photosensitive drum 1 below the discharge threshold before passing through the charging brush 2. Table 1 shows the voltage patterns applied to the charging brush 2 in the charging brush cleaning mode. [Table 1]

[0088] The applied voltage pattern will be explained with reference to Figure 5(a). Figure 5(a) shows the waveform of the applied voltage shown in pattern 2 at the timing of transition from image formation mode to charging brush cleaning mode. As shown in Figure 5(a), the value Vmax is the maximum value of the negative polarity voltage of the applied waveform pattern, and the value Vmin is the minimum value of the negative polarity voltage or the maximum value of the positive voltage of the applied waveform pattern. In other words, the values ​​Vmax and Vmin are the upper and lower limits, respectively, of the amplitude of the applied voltage applied to the charging brush 2. Note that the values ​​Vmax and Vmin are not the overshoot and undershoot values ​​within the applied voltage waveform, respectively, but the target voltage after the applied voltage waveform has stabilized. The amplitude Vpp_t is the absolute value of the difference between Vmax and Vmin, i.e., the amplitude of the applied voltage. The period is one period of the waveform, and in the case of a square wave, it is calculated as (application time of value Vmax) + (application time of value Vmin). In the embodiments described later, the time for applying the voltage pattern to the charging brush 2 in the charging brush cleaning mode was standardized to 2 seconds. Therefore, the control was set so that the shorter the period of the voltage pattern, the greater the number of waves in the applied voltage applied to the charging brush 2. The duty cycle (Duty) indicates what proportion of the overall period the voltage applied to the charging brush 2 is at a high voltage. For a square wave, the duty cycle (Duty) is Vmax application time / (Vmax application time + Vmin application time), and a larger value indicates a longer time for which Vmax is applied. For sine waves and triangular waves, since they are symmetrical waveforms, the duty cycle (Duty) was set to 0.5.

[0089] It has been confirmed that discharge current begins to flow in the charged nip and primary transfer nip when the potential difference is 550[V] or higher. The dark area potential (Vd), which is the drum potential of the photosensitive drum 1 when the charged brush cleaning mode is performed, is -520[V]. Even when the photosensitive drum 1 passes through the primary transfer nip formed with the primary transfer roller to which a voltage of -800[V] is applied, the potential difference is 280[V], which is below the discharge threshold. Therefore, the drum potential hardly changes, and the drum potential after passing through the charged brush 2 is also approximately equal to Vd = -520[V].

[0090] In patterns 1-14, 16, and 17 of Table 1, the potential difference between the value Vmax and the value Vmin and the photosensitive drum 1 before passing through the charging brush 2 is set to be less than the discharge threshold (550[V]). Pattern 15 is the only example where a pattern exceeding the discharge threshold is shown.

[0091] (7. Measurement of drum potential amplitude Vpp_d after passing through the charged brush) As mentioned above, in the injection charging method, even if a periodically changing voltage waveform is applied to the charging brush 2, the potential (Vd) of the photosensitive drum 1 quickly follows, making it difficult to secure the potential contrast, which is the potential difference between the charging brush 2 and the photosensitive drum 1 necessary for removing deposits.

[0092] In order to understand the tracking ability and potential contrast of the surface potential of the photosensitive drum 1 (hereinafter also referred to as the drum potential) with respect to the voltage applied to the charging brush 2 in each embodiment described later, the drum potential after passing through the charging brush 2 was measured. The drum potential was measured by modifying the black process cartridge PK of the image forming apparatus 100 and placing a potential measuring probe (model 3800-S2: manufactured by Trek Japan Co., Ltd.) at a distance of 0.5 [mm] from the drum surface and in the center in the width direction at the position of the developing roller 3. The placed potential measuring probe was connected to a surface potential meter (model 370: manufactured by Trek Japan Co., Ltd.) to enable measurement of the surface potential of the photosensitive drum 1.

[0093] The behavior of the drum potential will be explained with reference to Figure 5(b). Figure 5(b) shows the drum potential at the developing nip when the applied voltage of Pattern 2 in Table 1 is applied to the charging brush 2 at the timing of transitioning from image formation mode to cleaning mode. Figures 5(a) and 5(b) are shown aligned on the time axis. It should be noted that the drum potential shown in Figure 5(b) is measured at the developing nip, so the potential change is measured with a time delay corresponding to the distance from the charging brush 2 to the developing roller 3 along the circumference of the photosensitive drum 1. The drum potential is Vd = approximately 520 [V] until the applied voltage to the charging brush 2 is periodically switched. After that, when the applied voltage to the charging brush 2 is periodically switched, the drum potential changes with a certain amplitude. The amplitude of the drum potential at this time is defined as the drum potential amplitude Vpp_d, and the drum potential amplitude Vpp_d was measured as an indicator of the effect of each embodiment described later. In other words, the drum potential amplitude Vpp_d represents the amplitude of the drum potential in the portion of the surface of the photosensitive drum 1 that has passed through the charged nip. Note that this measurement requires a surface potential meter with a fast response speed and high spatial resolution to measure the high-speed change in drum potential.

[0094] (8. Evaluation Methods) The effects of each embodiment described later were verified under the following conditions. In the image forming apparatus 100 shown in Figure 1, the paper flow durability was evaluated while applying voltages according to patterns 1 to 17 shown in Table 1 to the charged brush 2 in the charged brush cleaning mode, and the appearance of the charged brush 2, the injection charging performance, and the latent image flow were evaluated.

[0095] The paper feed durability evaluation was conducted under conditions of 23°C and 50% humidity. Each color process cartridge (PY-PK) was used to feed 2,000 sheets of text images with a 1% print density per day, up to a maximum of 10,000 sheets. For the paper feed durability test, A4-sized GF-C081 (Canon) paper was used as the recording material. Each printing job printed two pages, and the electrostatic brush cleaning mode was performed every two pages.

[0096] The appearance of the charging brush 2 was checked by removing the photosensitive drum 1 from the process cartridge PY~PK after paper feeding durability testing, and observing the amount of toner accumulated on the charging brush 2 under magnification. A large amount of toner or other deposits were attached to the bristles of the charging brush 2, which was marked as ×, and a small amount of toner or other deposits, where the bristles of the charging brush 2 were visible, was marked as ○.

[0097] The injection charge retention rate was determined by measuring the charging current flowing through one rotation of the photosensitive drum at the start of the image formation operation. A score of ○ was given if the charging current after durability was maintained at 80% or more of the charging current before durability (change of less than 20%), and a score of × was given if the retention rate was less than 80% (change of 20% or more).

[0098] Latent image flow was tested using a post-durability process cartridge under high temperature and high humidity conditions of 32.5°C and 80% humidity, by printing 10% halftones. A score of ○ indicated that a proper image could be formed, while a score of × indicated that it could not.

[0099] (9. Evaluation Results) The evaluation results for patterns 1 to 17 in Table 1 are shown in Table 2 below. As shown in Table 2, good evaluation results were obtained for patterns 1 to 11, 16, and 17, while patterns 12 to 15 showed insufficient performance in removing deposits or in preventing latent flow. In other words, patterns 1 to 11, 16, and 17 can be said to be examples 1 to 13, respectively, and patterns 12 to 15 can be said to be comparative examples 1 to 4. [Table 2]

[0100] (10. Effect of cleaning the applied voltage period and amplitude within an appropriate range) Examples 1, 2, 3, 4, 5, 12, and 13, and Comparative Examples 1, 2, and 3 all had the same waveform, values ​​Vmax and Vmin, amplitude Vpp_t, and duty cycle applied to the charging brush 2, differing only in their period. While good results were obtained in Examples 1, 2, 3, 4, 5, 12, and 13, a decrease in injection charging performance (injection current maintenance rate) due to paper handling durability was observed in Comparative Examples 1, 2, and 3.

[0101] In Comparative Examples 1 and 2, the value of (Vpp_d / Vpp_t), i.e., the change in drum potential amplitude Vpp_d relative to amplitude Vpp_t, is large. As a result, the drum potential follows the voltage applied to the charging brush 2, and the potential contrast between the charging brush 2 and the photosensitive drum 1, which is necessary for removing deposits attached to the charging brush 2, is small. Consequently, it is difficult to remove deposits from the charging brush 2, deposits accumulate on the charging brush 2 throughout the paper feeding period, and it is thought that the injection charging ability (injection current maintenance rate) that charges the photosensitive drum 1 by the charging brush 2 decreases.

[0102] In Examples 1, 2, 3, 4, 5, 12, 13, and Comparative Example 3, the shorter the period of the applied voltage waveform pattern, the smaller the drum potential amplitude Vpp_d became. This indicates that waveforms that switch the applied voltage at short intervals were able to secure a larger potential contrast between the charging brush 2 and the photosensitive drum 1. On the other hand, in Comparative Example 3, despite having the smallest drum potential amplitude Vpp_d and the largest potential contrast, deposits accumulated on the charging brush 2 throughout the paper feeding process, resulting in a decrease in injection charging performance.

[0103] The difference in deposit removal performance due to the waveform pattern of the voltage applied to the charged brush 2 in these charged brush cleaning modes is thought to be due to the electrical circuit characteristics of the injection charging configuration. The injection charging configuration will be explained with reference to Figures 6(a) to 6(c). Figure 6(a) is a diagram showing the injection charging configuration as an electrical circuit, Figure 6(b) is a diagram showing the charging characteristics in a resistor-capacitor series circuit (RC circuit), and Figure 6(c) is a diagram showing the frequency characteristics of the capacitance of the injection charging configuration.

[0104] As shown in Figure 6(a), the injection charging configuration can be simplified to be a circuit in which the charging power supply 61, the charging brush 2, and the photosensitive drum 1 are connected in series as a resistor and a capacitor. That is, as shown in the lower circuit diagram of Figure 6(a), the photosensitive drum 1 can be considered as a capacitor 1a, and the charging brush 2 can be considered as a contact resistor 2a and the resistance 2b of the charging brush 2 itself.

[0105] Generally, in a series resistor-capacitor circuit, when a voltage is applied, the potential of the electrodes of capacitor 1a stores charge with a certain time characteristic, as shown in Figure 6(b). The time constant τ = RC (resistance × capacitance) is a well-known indicator of this characteristic. Figure 6(b) shows the time characteristic of the potential Vout of the capacitor electrodes under the conditions of applied voltage Vin = 1 [V] and RC = 1. The potential Vout of the capacitor electrodes in an RC circuit is expressed by the following equation (3). Vout=Vin(1-e^(-t / RC))···(3)

[0106] When the time t for which a voltage is applied to capacitor 1a satisfies the relationship t=τ=RC=1, the potential Vout of the capacitor electrode becomes 0.632[V], reaching 63.2% of the input voltage. Similarly, the potential Vout of the capacitor electrode becomes 86.5% of the input voltage at t=2τ, 95.0% at t=3τ, 98.2% at t=4τ, and 99.3% at t=5τ. The potential Vout of the capacitor electrode becomes 90% or more of the input voltage when the time is approximately 2.3τ or greater.

[0107] In the injection charging circuits of each embodiment, the circuit-specific time constant τ is thought to be maintained by the R component (resistive component) consisting of the resistance 2b and contact resistance 2a of the charging brush 2, and the C component (capacitive component) consisting of the capacitance of the surface of the photosensitive drum 1 (capacitor 1a). Furthermore, in Table 2, the amplitude Vpp_t is considered to correspond to the applied voltage Vin, and the drum potential amplitude Vpp_d is considered to correspond to the potential Vout of the capacitor electrode.

[0108] In Examples 1, 2, 3, 4, 5, 12, and 13, where good adhesion removal performance was obtained for the charged brush 2, the maximum Vpp_d / Vpp_t (corresponding to Vout / Vin) was 0.865, suggesting that charging and decharging were repeatedly performed on the surface of the photosensitive drum 1 in a time range shorter than the time constant τ. On the other hand, in Comparative Examples 1 and 2, the Vpp_d / Vpp_t (corresponding to Vout / Vin) were 0.906 and 0.971, respectively, suggesting that charging and decharging were repeatedly performed on the surface of the photosensitive drum 1 in a time range longer than twice the time constant τ (2τ). In other words, in a configuration that implements a charged brush cleaning mode in which the applied voltage is periodically switched by injection charging, as in each example, it is desirable to switch the applied voltage at a short period of twice the time constant τ or less (2τ or less), and it is desirable to switch the applied voltage applied to the charged brush 2 so as to satisfy the relationship Vpp_d / Vpp_t ≤ 0.865.

[0109] On the other hand, in Comparative Example 3, Vpp_d / Vpp_t = 0.034, despite being the smallest, deposits accumulated on the charging brush 2 throughout the paper-passing endurance test, and a decrease in injection charging performance due to paper-passing endurance was observed. This is presumed to be because the switching cycle of the applied voltage is too fast, preventing the charging circuit from following the changes in the applied voltage, and consequently, the deposits also cannot follow the changes in the applied voltage, making deposit removal difficult.

[0110] Figure 6(c) shows the frequency characteristics of the capacitance of the charging circuit in each embodiment. Capacitance was measured using an LCR meter (4284A: Hewlett-Packard) with the photosensitive drum 1 stationary and not rotating, at an applied voltage of 10V in the frequency range of 20 to 10 [MHz]. As shown in Figure 6(c), a tendency for the capacitance to decrease was observed from around 1.0 × 10^4 [Hz] (10 [kHz], 0.1 [ms] period), and at a period of 0.01 [ms] (100 [kHz]) in Comparative Example 3, the capacitance was approximately 40% of that in the low-frequency range. In other words, it is considered that the charging circuit is in a region where it cannot respond to the applied voltage at the applied voltage switching period of Comparative Example 3.

[0111] Similarly, deposits such as toner attached to the charged brush 2 could not respond to the switching of the applied voltage, and in the configuration of Comparative Example 3, it is thought that the deposits on the charged brush 2 could not be transferred to the surface of the photosensitive drum 1. Therefore, in the charged brush cleaning mode, it is desirable to switch the applied voltage at a longer period than in Comparative Example 3, where Vpp_d / Vpp_t = 0.034. Specifically, from the results of Example 12, it is desirable to periodically switch the applied voltage so as to satisfy the relationship 0.05 ≤ Vpp_d / Vpp_t. As shown above, it was confirmed that good deposit removal performance on the charged brush 2 can be ensured by periodically switching the applied voltage so as to satisfy 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865.

[0112] (11. Influence of the Vpp_t range) In Examples 6 and 7, the charging brush cleaning mode was performed using the same square wave waveform, value Vmax, duty cycle, and period as in Example 2, but with different absolute values ​​and polarities of the value Vmin and amplitude Vpp_t. In Example 6, the value Vmin has the same polarity as the voltage applied to the charging brush 2 when the image formation mode is performed, while in Example 7, the value Vmin has the opposite polarity to the voltage applied to the charging brush 2 when the image formation mode is performed.

[0113] The Vpp_d / Vpp_t values ​​for Examples 6 and 7 were 0.408 and 0.458, respectively, which are approximately equal to the Vpp_d / Vpp_t value of 0.404 for Example 2. Similarly, the evaluation results were also good throughout the paper-passing endurance test. In other words, even in configurations where the absolute value and polarity of the value Vmin and the amplitude Vpp_t differed, a charged brush 2 with good deposit removal performance was obtained by periodically switching the applied voltage to satisfy 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865, similar to Examples 1 to 5.

[0114] Furthermore, when the dark area potential (Vd) of the photosensitive drum 1 is higher than -550[V], it is desirable to set the value Vmin as low as possible without exceeding the discharge threshold. On the other hand, as in each embodiment, when the dark area potential (Vd) is lower than -550[V], it is desirable to select 0[V] or a potential of the opposite polarity. When performing charged brush cleaning with an applied voltage having a square wave waveform, the configuration in Example 2, where the applied voltage to the charged brush 2 is turned OFF and the value Vmin is set to 0[V], results in a faster fall rate of the applied voltage and the waveform of the applied voltage approaches an ideal square wave, compared to controlling the waveform to aim for a predetermined voltage value (-100[V]) as in Example 6. As a result, it is easier to secure the potential contrast between the charged brush 2 and the photosensitive drum 1, and the adhesion removal performance tends to be good. Also, when a voltage of the opposite polarity is applied as in Example 7, adhesions with the opposite polarity charge are more easily removed, resulting in a good removal effect, but it is necessary to add a power supply capable of outputting the opposite polarity. Therefore, there are concerns that this could lead to increased manufacturing costs and larger size of the image forming apparatus 100. Thus, it is desirable to select the value Vmin considering the balance between the adhering material removal performance, the dark area potential (Vd) of the photosensitive drum 1, the manufacturing cost of the image forming apparatus 100, and its size.

[0115] (12. The effect of duty cycle) In Examples 8 and 9, the charging brush cleaning mode was performed with the same values ​​Vmax, Vmin, amplitude Vpp_t, and period as in Example 2, but with only the waveform duty cycle differing. The Vpp_d / Vpp_t values ​​in Examples 8 and 9 were 0.416 and 0.393, respectively, which are approximately equal to the Vpp_d / Vpp_t value of 0.404 in Example 2. Similarly, good results were obtained throughout the paper-passing durability test. In other words, even with configurations where the duty cycle of the applied voltage waveform differed, a charging brush 2 with good deposit removal performance was obtained by periodically switching the applied voltage to satisfy 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865, similar to Examples 1 to 7.

[0116] (13. Waveform Influence) In Examples 10 and 11, the charging brush cleaning mode was performed using the same values ​​Vmax, Vmin, amplitude Vpp_t, duty cycle, and period as in Example 2, but with only the shape of the applied voltage waveform differing. The Vpp_d / Vpp_t values ​​for Examples 10 and 11 were 0.601 and 0.640, respectively, which were slightly larger than the 0.404 in Example 2. This is thought to be because the square wave applies values ​​Vmax and Vmin instantaneously, causing a time delay in the potential of the photosensitive drum 1 to follow, making it easier to secure the potential contrast necessary for removing deposits. In contrast, the sine wave and triangular wave change the applied voltage with a slope, making it easier for the potential of the photosensitive drum 1 to follow in time, thus making it more difficult to secure potential contrast. On the other hand, good evaluation results were obtained throughout the durability test in Examples 10 and 11 as well. In other words, even in configurations with different waveform shapes, a charged brush 2 with good deposit removal performance was obtained by periodically switching the applied voltage so as to satisfy 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865, similar to Examples 1 to 9.

[0117] (14. The effect of performing cleaning below the discharge threshold) As shown in Comparative Example 4, latent image flow occurred only in the configuration with an amplitude Vpp_t of 1800[V]. This is thought to be because, while the drum potential before passing through the charged brush 2 was Vd = -520[V], cleaning was performed with a potential difference exceeding the discharge threshold, resulting in the accumulation of discharge products on the surface of the photosensitive drum 1. Therefore, it was confirmed that in order to maintain the advantage of latent image flow in injection charging while demonstrating the deposit removal performance of the charged brush 2, it is desirable to perform the cleaning operation with an applied voltage pattern below the discharge threshold.

[0118] (15. Summary) As described above, each embodiment shown in Tables 1 and 2 uses a cleanerless image forming apparatus 100 employing an injection charging method and a development-simultaneous cleaning method. In each embodiment shown in Tables 1 and 2, in the charging brush cleaning mode, the applied voltage applied to the charging brush 2 was periodically switched within a range below the discharge threshold relative to the surface potential of the photosensitive drum 1 before passing through the charging brush 2. In addition, in each embodiment shown in Tables 1 and 2, in the charging brush cleaning mode, when the amplitude of the applied voltage is the amplitude Vpp_t and the amplitude of the surface potential of the photosensitive drum 1 after passing through the charging brush 2 is the drum potential amplitude Vpp_d, the period of the waveform pattern of the applied voltage was set to satisfy 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865.

[0119] This allows for effective removal of deposits attached to the charged brush 2 while suppressing the generation of latent image flow. By effectively removing deposits attached to the charged brush 2 using the charged brush cleaning mode, the accumulation of deposits on the charged brush 2 can be reduced, improving the appearance of the charged brush 2 and suppressing a decrease in injection charging ability.

[0120] When the charging brush cleaning mode is performed, the voltage applied to the charging brush 2 changes periodically, as shown in Figures 5(a) and 5(b), and the drum potential also changes periodically. In this way, the potential of the charging brush 2 and the drum potential change with a certain amplitude (Vpp_t, Vpp_d). This ensures the potential contrast between the charging brush 2 and the photosensitive drum 1 necessary to remove deposits from the charging brush 2, and allows for effective removal of deposits attached to the charging brush 2. Based on the applied voltage patterns in Tables 1 and 2, it was found that a range of 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865 is preferable for the Vpp_d / Vpp_t range. More specifically, the period of the applied voltage applied to the charging brush 2 is preferably 0.1 [msec] or more and 40 [msec] or less.

[0121] Furthermore, if the charged brush 2 is cleaned with a potential contrast (potential difference) exceeding the discharge threshold, latent image flow will be generated by discharge products. For this reason, it is preferable that the applied voltage to the charged brush 2 be in a range below the discharge threshold.

[0122] <Other Embodiments> In the embodiments described above, a drum cleanerless system was used that did not have a means for cleaning the primary transfer residue toner on the photosensitive drum 1. However, a means for cleaning the primary transfer residue toner may also be included. For example, in a so-called blade cleaning system in which a rubber blade is brought into contact with the photosensitive drum 1 to collect the primary transfer residue toner, the charging brush 2 may become contaminated by the adhesion of fine particles such as external additives and toner matrix resin that have slipped through the rubber blade. Similar to the cleanerless configuration, substances that adhere to the charging brush 2 tend to have a positive charge. Therefore, by periodically switching the applied voltage to satisfy 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865, a charging brush 2 with good adhesion removal performance was obtained for substances that have slipped through the rubber blade and adhered to the charging brush.

[0123] Furthermore, in the embodiments described above, the developing roller 3 was separated from the photosensitive drum 1 and the voltage applied to the developing roller 3 was turned OFF in the charging brush cleaning mode. However, the developing roller 3 may be kept in contact with the photosensitive drum 1 and the voltage may be kept applied. In this configuration as well, the deposits removed from the charging brush mainly have a positive charge, so when passing through the developing nip, they remain on the photosensitive drum surface, which has a higher negative potential. After passing through the developing nip, the deposits on the photosensitive drum 1 are transferred to the intermediate transfer belt, which has a negatively higher potential than the photosensitive drum surface potential, when passing through the primary transfer nip, and are then collected by the cleaning blade 21.

[0124] Furthermore, in the embodiments described above, a full-color image forming apparatus 100 was used, which performs image formation using four process cartridges containing four toners. However, a so-called monocolor image forming apparatus that uses only one process cartridge may also be used. In monocolor image forming apparatuses, a direct transfer method is often used, in which there is no intermediate transfer belt, and the photosensitive drum and transfer roller perform direct transfer via the recording material. In the direct transfer method, since the recording material is in direct contact with the surface of the photosensitive drum, fibers and fillers contained in the recording material tend to adhere to the surface of the photosensitive drum after transfer, and fibers and fillers derived from the recording material also tend to adhere to and accumulate on the charging brush. In the direct transfer method as well, since the paper components adhering to the charging brush tend to have a positive charge, a charging brush 2 with good adhesion removal performance can be obtained by periodically switching the applied voltage, as in the embodiments described above.

[0125] Furthermore, in the embodiments described above, a volume resistivity of 1.0 × 10^13 [Ω·cm] was used for the charge injection layer 105 of the photosensitive drum 1, but similar effects can be obtained even with a different volume resistivity configuration. When the volume resistivity is below 1.0 × 10^13 [Ω·cm], the tracking ability of the drum potential amplitude Vpp_d with respect to amplitude Vpp_t increases, and it has been confirmed that it is necessary to switch the applied voltage at a faster cycle. On the other hand, when the volume resistivity is above 1.0 × 10^13 [Ω·cm], the tracking ability of the drum potential amplitude Vpp_d with respect to amplitude Vpp_t decreases, and it has been confirmed that it is possible to switch the applied voltage at a slower cycle. In addition, from the viewpoint of charge retention, below 1.0 × 10^9 [Ω·cm], it becomes difficult to maintain the latent image, and there is a concern that the printing quality of fine lines will deteriorate. Furthermore, from the viewpoint of injection charging, if it exceeds 1.0 × 10^14 [Ω·cm], the injection charging is low, and it becomes necessary to apply a charging voltage exceeding the discharge threshold in order to obtain the desired dark area potential (Vd) of the photosensitive drum 1, thus losing the advantages of the injection charging configuration. From the viewpoints of the ability to remove deposits from the charged brush and the injection charging, it is desirable that the volume resistivity of the photosensitive drum 1 be between 1.0 × 10^9 [Ω·cm] and 1.0 × 10^14 [Ω·cm].

[0126] Furthermore, in the embodiments described above, a charging brush 2 consisting of a bar brush with a resistance of 1 × 10^5 [Ω] was used as the injection member, but similar effects can be obtained even with configurations of different resistance values. When the resistance value is below 1.0 × 10^5 [Ω], the tracking ability of the drum potential amplitude Vpp_d with respect to the amplitude Vpp_t increases, so it has been confirmed that it is necessary to switch the applied voltage at a faster cycle. On the other hand, when the resistance value is above 1.0 × 10^5 [Ω], the tracking ability of the drum potential amplitude Vpp_d with respect to the amplitude Vpp_t decreases, so it has been confirmed that it is possible to switch the applied voltage at a slower cycle. Also, from the viewpoint of injection charging performance, if it exceeds 1.0 × 10^9 [Ω], the injection charging performance is low, and it becomes necessary to apply a charging voltage exceeding the discharge threshold in order to obtain the desired dark area potential (Vd) of the photosensitive drum 1, thus losing the advantages of the injection charging configuration. From the viewpoints of the ability to remove deposits from the charging brush 2 and the injection charging performance described above, it is desirable that the resistance of the charging brush 2 be 1.0 × 10^9 [Ω] or less.

[0127] Furthermore, in the embodiments described above, a charging brush 2 consisting of a bar brush was used as the injection member (charging member), but similar effects can be obtained with other injection members. Specifically, a brush roller in which conductive nylon fibers are wound around a roller-shaped core, or a conductive rubber roller can be used as the injection member. The brush roller is a roller made of conductive fibers that rotates in accordance with the photosensitive drum 1. The rubber roller is a roller made of conductive rubber that rotates in accordance with the photosensitive drum 1. When the above rubber roller is used as the injection member, although the charge injection capability is reduced compared to the above bar brush, it has been confirmed that charge injection is possible with a resistance value of about 1 × 10^5 [Ω].

[0128] Unlike a fixed bar brush, a rotating roller-shaped brush roller requires switching the applied voltage for at least one cycle within the time it takes for the charged nip to pass through. The time it takes for the charged nip to pass through is obtained by dividing the width of the charged nip by the process speed. If the period of the applied voltage waveform is slower than the time it takes for the charged nip to pass through, a portion of the charged roller will only have a potential contrast for removing deposits when its potential is negatively high relative to the surface potential of the photosensitive drum 1, making it difficult to remove positively charged deposits accumulated on the charged roller. Therefore, it is desirable to apply a voltage waveform for at least one cycle or more while the charged nip is passing through. In each of the embodiments described above, the process speed, which is the peripheral speed of the surface of the photosensitive drum 1 and the conductive roller, is 210 [mm / s]. For example, if the width of the charged nip is 1 [mm], the time it takes for the charged nip to pass through is approximately 4.76 [ms], so it is desirable to set the period of the applied voltage to the charged roller to less than 4.76 [ms].

[0129] Furthermore, although the above-described embodiments used a configuration with one charging member (charging brush 2), similar effects can be obtained even with a configuration of multiple charging members. Specifically, in cases where the injection charging ability is low with one charging member and the desired dark area potential (Vd) cannot be obtained, or when uneven charging potential due to the charging brush 2 manifests as image defects, a configuration in which a second charging member is added downstream of the charging brush 2 in the rotational direction of the photosensitive drum 1 is envisioned. That is, in the charging brush cleaning mode, the CPU 26 periodically switches the second voltage applied to the second charging member within a range below the discharge threshold relative to the surface potential of the photosensitive drum 1 before passing through the second charging member, similar to the charging brush 2 as the first charging member. Also, in the charging brush cleaning mode, the CPU 26 sets the period of the waveform pattern of the second voltage so that 0.05 ≤ Vpp_d2 / Vpp_t2 ≤ 0.865 is satisfied, where Vpp_t2 is the amplitude of the second voltage and Vpp_d2 is the amplitude of the surface potential of the photosensitive drum 1 after passing through the second charging member. This makes it possible to obtain a second charged member that has good deposit removal performance, similar to the charged brush 2.

[0130] Furthermore, while the embodiments described above used a configuration in which the charging brush cleaning mode was performed after image formation, the charging brush cleaning mode may be performed at other times. Specifically, it can be performed when the power to the image forming apparatus is turned on, when it wakes up from sleep mode, before the image forming operation, or between images (between sheets of paper) during the image forming operation. In other words, the charging brush cleaning mode can be performed at a different time from the image forming mode.

[0131] Furthermore, in the embodiments described above, a configuration was used in which a negatively charged toner was used and a negative charging voltage was applied. However, a configuration in which a positively charged toner was used and a positive charging voltage was applied may also be used. In that case, the primary transfer residue toner mainly has a negative charge and accumulates on the charging brush to which a positive charging voltage is applied. However, by reversing the polarity of the voltage applied to each component shown in this embodiment, a charging brush 2 with good deposit removal performance can be obtained in a similar manner.

[0132] In the embodiments described above, the effect of removing deposits with reverse polarity (toner, paper components, etc.) contained in the charged brush 2 was mainly described. However, this configuration is also effective when deposits with normal polarity (toner, paper components, etc.) adhere to the charged brush 2. The applied voltage is switched so that a potential contrast between positive and negative polarity is obtained below the discharge threshold with respect to the drum potential of the photosensitive drum 1 before it passes through the charged brush 2. Therefore, when the potential of the charged brush 2 is positively higher than the drum potential, deposits with positive charge are mainly removed. On the other hand, when the potential of the charged brush 2 is negatively higher than the drum potential, deposits with negative charge are mainly removed, so negatively charged deposits with normal polarity can also be removed from the charged brush 2 in the same way.

[0133] The present invention can also be realized by supplying a program that implements one or more of the functions of the above embodiments to a system or device via a network or storage medium, and by having one or more processors in the computer of that system or device read and execute the program. It can also be realized by a circuit (e.g., an ASIC) that implements one or more functions.

[0134] Summary of this disclosure This disclosure includes at least the following: (Composition 1) A photoreceptor having a charge injection layer containing conductive particles and configured to be rotatable, A charging member that contacts the surface of the photoreceptor and charges the surface, A developing unit that develops the electrostatic latent image formed on the surface as a toner image, A transfer unit that transfers the toner image to the material to be transferred, A control unit for controlling the voltage applied to the charged member, comprising: an image forming mode for transferring the toner image to the transfer member; and a cleaning mode performed at a different timing from the image forming mode, In the cleaning mode, the control unit, (i) Periodically switching the voltage applied to the charging member within a range less than the discharge threshold with respect to the surface potential of the photoreceptor before it passes through the charging member, and (ii) When the amplitude of the voltage is Vpp_t and the amplitude of the surface potential of the photoreceptor after passing through the charged member is Vpp_d, the period of the waveform pattern of the voltage is set such that 0.05 ≤ Vpp_d / Vpp_t ≤ 0.865. An image forming apparatus characterized by the following features. (Configuration 2) The aforementioned period is 0.1 [msec] or more and 40 [msec] or less. The image forming apparatus according to configuration 1, characterized in that it is a picture forming apparatus. (Composition 3) The aforementioned period is less than or equal to twice the time constant τ of the RC circuit when the photoreceptor and the charging member are considered as a capacitor and a resistor, respectively. An image forming apparatus according to configuration 1 or 2, characterized by the above. (Composition 4) The waveform pattern of the aforementioned voltage is a square wave. An image forming apparatus according to any one of configurations 1 to 3, characterized by the above. (Composition 5) The waveform pattern of the voltage is a sine wave or a triangular wave. An image forming apparatus according to any one of configurations 1 to 3, characterized by the above. (Composition 6) The lower limit of the amplitude of the voltage is 0[V]. An image forming apparatus according to any one of configurations 1 to 5, characterized by the above. (Composition 7) The lower limit of the amplitude of the voltage is of the same polarity as the voltage applied to the charging member when the image forming mode is performed. An image forming apparatus according to any one of configurations 1 to 5, characterized by the above. (Composition 8) The lower limit of the amplitude of the voltage is of opposite polarity to the voltage applied to the charged member when the image forming mode is performed. An image forming apparatus according to any one of configurations 1 to 5, characterized by the above. (Composition 9) The volume resistivity of the charge injection layer is 1.0 × 10⁹ [Ω·cm] or greater and 1.0 × 10¹⁴ [Ω·cm] or less. An image forming apparatus according to any one of configurations 1 to 8, characterized by the above. (Composition 10) The conductive particles include a metal oxide containing titanium atoms and niobium atoms. An image forming apparatus according to any one of configurations 1 to 9, characterized by the above. (Composition 11) The charging member comprises a support member and a brush portion attached to the support member, which is made of conductive fibers and rubs against the photoreceptor. An image forming apparatus according to any one of configurations 1 to 10, characterized by the above. (Composition 12) The resistance of the aforementioned charging member is 1.0 × 10^9 [Ω] or less. The image forming apparatus according to configuration 11, characterized by the features described above. (Composition 13) The charging member is made of conductive rubber and includes a roller that rotates in accordance with the photoreceptor. An image forming apparatus according to any one of configurations 1 to 10, characterized by the above. (Composition 14) The charging member is made of conductive fibers and includes a roller that rotates in accordance with the photoreceptor. An image forming apparatus according to any one of configurations 1 to 10, characterized by the above. (Composition 15) The roller, together with the photoreceptor, forms a charged nip. The period is shorter than the time obtained by dividing the width of the charged nip in the rotational direction of the photoreceptor by the peripheral speed of the roller. The image forming apparatus according to configuration 13 or 14, characterized by the above. (Composition 16) The charging member, the voltage, and the period are, respectively, the first charging member, the first voltage, and the first period. The photoreceptor further comprises a second charging member that contacts the surface of the photoreceptor and charges the surface, In the cleaning mode, the control unit, (i) Periodically switching the second voltage applied to the second charging member within a range less than the discharge threshold with respect to the surface potential of the photoreceptor before it passes through the second charging member, (ii) When the amplitude of the second voltage is Vpp_t2 and the amplitude of the surface potential of the photoreceptor after passing through the second charged member is Vpp_d2, the period of the waveform pattern of the second voltage is set such that 0.05 ≤ Vpp_d2 / Vpp_t2 ≤ 0.865. An image forming apparatus according to any one of configurations 1 to 15, characterized by the above. (Composition 17) The developing unit electrostatically recovers the toner remaining on the photoreceptor after the toner image has been transferred to the transfer target by the transfer unit. An image forming apparatus according to any one of configurations 1 to 16, characterized by the above. (Composition 18) The system further includes a secondary transfer unit that transfers the toner image transferred to the transfer target member to a sheet. An image forming apparatus according to any one of configurations 1 to 17, characterized by the above. (Composition 19) In the cleaning mode, the deposits adhering to the charged member are electrostatically moved from the charged member to the transfer target member via the photoreceptor. The system further includes a cleaning member for removing the deposits on the transfer member. The image forming apparatus according to configuration 18, characterized by the features described above. [Explanation of Symbols]

[0135] 1: Photoreceptor (photosensitive drum) / 2: Charging component (charging brush) / 3: Developing unit (developing roller) / 8: Transfer material (intermediate transfer belt) / 11: Secondary transfer unit (secondary transfer roller) / 21: Cleaning component (cleaning blade) / 26: Control unit (CPU) / 100: Image forming apparatus / 105: Charge injection layer / 105p: Conductive particles / Vmin: Lower limit (value)

Claims

1. A photoreceptor having a charge injection layer containing conductive particles and configured to be rotatable, A charging member that contacts the surface of the photoreceptor and charges the surface, A developing unit that develops the electrostatic latent image formed on the surface as a toner image, A transfer unit that transfers the toner image to the material to be transferred, A control unit for controlling the voltage applied to the charged member, comprising: an image forming mode for transferring the toner image to the transfer member; and a cleaning mode performed at a different timing from the image forming mode, In the cleaning mode, the control unit, (i) Periodically switching the voltage applied to the charging member within a range less than the discharge threshold with respect to the surface potential of the photoreceptor before it passes through the charging member, (ii) When the amplitude of the voltage is Vpp_t and the amplitude of the surface potential of the photoreceptor after passing through the charged member is Vpp_d, the period of the waveform pattern of the voltage is set such that 0.05 ≤ Vpp_d / Vpp_t ≤ 0.

865. An image forming apparatus characterized by the following features.

2. The aforementioned period is 0.1 [msec] or more and 40 [msec] or less. The image forming apparatus according to feature 1.

3. The period is less than or equal to twice the time constant τ of the RC circuit when the photoreceptor and the charging member are considered as a capacitor and a resistor, respectively. The image forming apparatus according to feature 1.

4. The waveform pattern of the aforementioned voltage is a square wave. The image forming apparatus according to feature 1.

5. The waveform pattern of the voltage is a sine wave or a triangular wave. The image forming apparatus according to feature 1.

6. The lower limit of the amplitude of the voltage is 0 [V]. The image forming apparatus according to feature 1.

7. The lower limit of the amplitude of the voltage is of the same polarity as the voltage applied to the charging member when the image forming mode is performed. The image forming apparatus according to feature 1.

8. The lower limit of the amplitude of the voltage is of opposite polarity to the voltage applied to the charged member when the image forming mode is performed. The image forming apparatus according to feature 1.

9. The volume resistivity of the charge injection layer is 1.0 × 10⁹ [Ω·cm] or greater and 1.0 × 10¹⁴ [Ω·cm] or less. The image forming apparatus according to feature 1.

10. The conductive particles include a metal oxide containing titanium atoms and niobium atoms. The image forming apparatus according to feature 1.

11. The charging member comprises a support member and a brush portion attached to the support member, which is made of conductive fibers and rubs against the photoreceptor. The image forming apparatus according to feature 1.

12. The resistance of the charging member is 1.0 × 10⁹ [Ω] or less. The image forming apparatus according to feature 11.

13. The charging member is made of conductive rubber and includes a roller that rotates in accordance with the photoreceptor. The image forming apparatus according to feature 1.

14. The charging member is made of conductive fibers and includes a roller that rotates in accordance with the photoreceptor. The image forming apparatus according to feature 1.

15. The roller, together with the photoreceptor, forms a charged nip. The period is shorter than the time obtained by dividing the width of the charged nip in the rotational direction of the photoreceptor by the peripheral speed of the roller. The image forming apparatus according to feature 13.

16. The charging member, the voltage, and the period are, respectively, the first charging member, the first voltage, and the first period. The photoreceptor further comprises a second charging member that contacts the surface of the photoreceptor and charges the surface, In the cleaning mode, the control unit, (i) Periodically switching the second voltage applied to the second charging member within a range less than the discharge threshold with respect to the surface potential of the photoreceptor before it passes through the second charging member, (ii) When the amplitude of the second voltage is Vpp_t2 and the amplitude of the surface potential of the photoreceptor after passing through the second charged member is Vpp_d2, the period of the waveform pattern of the second voltage is set such that 0.05 ≤ Vpp_d2 / Vpp_t2 ≤ 0.

865. The image forming apparatus according to any one of claims 1 to 15.

17. The developing unit electrostatically recovers the toner remaining on the photoreceptor after the toner image has been transferred to the transfer target by the transfer unit. The image forming apparatus according to any one of claims 1 to 15.

18. The system further includes a secondary transfer unit that transfers the toner image transferred to the transfer target member to a sheet. The image forming apparatus according to any one of claims 1 to 15.

19. In the cleaning mode, the deposits adhering to the charged member are electrostatically moved from the charged member to the transfer member via the photoreceptor. The system further includes a cleaning member for removing the deposits on the transfer member. The image forming apparatus according to feature 18.

Images