Image forming apparatus

Abstract

This suppresses image defects caused by toner being reverse-transferred to the photoreceptor. [Solution] The image forming apparatus comprises a photoreceptor, a charging member, a developing unit, a transfer unit, a recovery member disposed downstream of the transfer unit and upstream of the charging member, which electrostatically recovers the reverse transfer toner having a first color transferred from the transfer member to the surface of the photoreceptor by the transfer unit, and a control unit that controls the voltage applied to the charging member and the recovery member, respectively. When the absolute value of the potential of the recovery member during image formation is V1 and the absolute value of the surface potential of the photoreceptor before passing through the recovery member is V2, the control unit controls V2

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, a tandem type image forming apparatus is proposed in which four photosensitive drums each carrying a toner image of a different color are provided with respect to the moving direction of an intermediate transfer belt. In this image forming apparatus, a cleaning device for removing the residual transfer toner remaining on the photosensitive drum after the transfer of the toner image is omitted, and a cleanerless method for recovering the residual transfer toner in the developing unit is adopted. In such a configuration, when the toner image transferred from the upstream photosensitive drum to the intermediate transfer belt passes through the position where the downstream photosensitive drum and the intermediate transfer belt contact, a phenomenon called reverse transfer in which the toner image moves to the downstream photosensitive drum may occur. Reverse transfer is caused by the discharge occurring between the photosensitive drum and the intermediate transfer belt, resulting in the inversion of the polarity of the toner of the toner image to be reversely transferred.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In an image forming apparatus adopting the cleanerless method as in Patent Document 1 above, the toner reversely transferred to the photosensitive drum often adheres to the charging roller which is a charging member. When a large amount of toner adheres to the surface of the charging roller, the charging performance may change and deviate from the desired drum potential.

[0005] Therefore, an object of the present invention is to provide an image forming apparatus capable of suppressing image defects caused by toner reversely transferred to the photoreceptor.

Means for Solving the Problem

[0006] According to one aspect of the present invention, an image forming apparatus transfers a second toner image having a second color different from the first color to a transfer member so as to overlap the first toner image having the first color. In the image forming apparatus, a photosensitive member configured to be rotatable in a predetermined rotation direction, a charging member that contacts the surface of the photosensitive member to charge the surface, a developing unit that develops an electrostatic latent image formed on the surface as the second toner image, a transfer unit that transfers the second toner image to the transfer member, and a recovery member disposed downstream of the transfer unit and upstream of the charging member in the rotation direction, the recovery member electrostatically recovers the reverse transfer toner having the first color transferred from the transfer member to the surface of the photosensitive member by the transfer unit, and a control unit that controls voltages applied to the charging member and the recovery member, respectively. When the absolute value of the potential of the recovery member during image formation is V1 and the absolute value of the surface potential of the photosensitive member before passing through the recovery member is V2, the control unit controls the voltages applied to the charging member and the recovery member, respectively, so as to satisfy V2 < V1 and (V1 - V2) is less than the discharge threshold value.

Advantages of the Invention

[0007] According to the present invention, image defects can be suppressed.

Brief Description of the Drawings

[0008] [Figure 1] An overall schematic view showing an image forming apparatus according to the present embodiment. [Figure 2] A cross-sectional view showing the layer structure of a photosensitive drum. [Figure 3] A schematic view showing a comb-shaped electrode. [Figure 4] A view showing the peripheral configuration of the photosensitive drum 1 in Example 3. [Figure 5] A block diagram showing a control block of the image forming apparatus.

Embodiments 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 a laser beam printer using an electrophotographic process method that is compatible with legal-sized paper and capable of forming images at a process speed of 310 mm / s, a letter-size throughput of 60 ppm, 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 includes a photosensitive drum 1 as a photoreceptor, a temporary recovery brush 2, a developing roller 3, a toner container 23 that houses the developing roller 3, a charging roller 4, and a pre-exposure device 5. The photosensitive drum 1 is a cylindrical body with multiple functional layers formed thereon, each with a diameter of 24 mm. The photosensitive drum 1 is configured to rotate in a predetermined rotational direction, indicated by the arrow in Figure 1. The configuration of the photosensitive drum 1 will be described in detail later.

[0014] The temporary recovery brush 2 is pressed against the photosensitive drum 1 with a predetermined pressure to form a brush nip. The temporary recovery brush 2 is a bar brush made of conductive nylon fibers processed into a pile fabric with a width of 5 mm, which is attached to a plated steel sheet. In other words, the temporary recovery 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. A voltage of opposite polarity is applied to the temporary recovery brush 2 by a voltage application unit (not shown). The configuration of the temporary recovery brush 2 and the applied voltage will be described in detail later.

[0015] The charging roller 4, acting as a charging component, is positioned downstream of the temporary recovery brush 2 in the rotational direction of the photosensitive drum 1. The charging roller 4 is a rubber roller with a diameter of 8 mm, formed by molding conductive rubber onto a metal shaft. The charging roller 4 is pressed against the photosensitive drum 1 with a predetermined pressure, forming a charging nip. The charging roller 4 is also configured to rotate in accordance with the photosensitive drum 1. The charging roller 4 is configured to have a charging voltage applied by a charging voltage pressurizing unit (not shown). In this embodiment, the charging roller 4 is subjected to a charging voltage of -1070 V during image formation. The configuration of the charging roller 4 will be described in detail later.

[0016] The developing roller 3 as the developing unit is a rubber roller formed by molding conductive rubber on a metal shaft, and is pressed against the photosensitive drum 1 with a predetermined pressing force to form a developing nip. The developing roller 3 is driven by a driving means (not shown) so as to be rotatable at a speed higher than that of the photosensitive drum. Further, the developing roller 3 is configured to be able to transition between a contacting state in which it contacts the photosensitive drum 1 and a separating state in which it is separated from the photosensitive drum 1. A negative voltage is applied to the developing roller 3 by a voltage applying unit (not shown), and in this embodiment, a developing voltage of -320 [V] is applied during image formation.

[0017] The laser scanner 7 irradiates the photosensitive drum 1 with a laser based on an image signal to expose the photosensitive drum 1. In this embodiment, the laser scanner 7 is provided one by one for each photosensitive drum 1 of each process cartridge, but one laser scanner may have four light emitting portions.

[0018] The photosensitive drum 1 is charged to a predetermined dark part potential of a predetermined negative polarity by applying a voltage of a predetermined negative polarity to the charging roller 4. In the part of the photosensitive drum 1 exposed by the laser irradiated from the laser scanner 7, the potential drops, and an electrostatic latent image of a predetermined light 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 light part potential (Vl) is referred to as the light part Vl.

[0019] A voltage (Vdc) with a predetermined negative polarity is applied to the developing roller 3, and an appropriate potential difference is provided 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, thereby visualizing the electrostatic latent image. 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. Also, the difference between the dark 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 is adopted in which the developing contrast and the back contrast become 200 [V].

[0020] Note that the toner used in the present embodiment is composed of toner particles with an average particle size of 6.4 [μm] to which silica fine particles with an average particle size of 20 [nm] are externally added, and is charged with a negative polarity. The average particle size is, for example, the average particle diameter obtained from the particle volume, which can be measured by the Coulter method.

[0021] Also, the image forming unit 40 is provided with an intermediate transfer belt 8 wound around a driving roller 9, a tension roller 10, and a counter roller 28. Inside the intermediate transfer belt 8, primary transfer rollers 6Y, 6M, 6C, and 6K are provided. The primary transfer rollers 6Y, 6M, 6C, and 6K as transfer units are arranged so as to face the photosensitive drums 1 of the respective process cartridges. The intermediate transfer belt 8 as the transfer member is an endless belt made of a two-layer resin material in which a resin surface layer with a thickness of 2 [μm] is coated on a base layer with a thickness of 60 [μm], and is rotated in the arrow Z direction by the driving roller 9. A positive transfer voltage is applied to the primary transfer rollers 6Y, 6M, 6C, and 6K by a voltage application unit (not shown). In the present embodiment, a transfer voltage of +300 [V] is applied during image formation.

[0022] 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.

[0023] 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.

[0024] At this time, the surface of the photosensitive drum 1 is uniformly charged to a negative potential by the temporary recovery brush 2 and the charging roller 4, 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.

[0025] 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.

[0026] After the toner image is first transferred from the photosensitive drum 1 to the intermediate transfer belt 8, the surface potential of the photosensitive drum 1 is discharged to approximately 0[V] by the pre-exposure device 5. This is done to equalize the potential unevenness on the surface of the photosensitive drum 1, which is caused by exposure by the laser scanner 7 and discharge by the primary transfer roller, and to obtain a uniform dark area potential (Vd).

[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 is mostly negative polarity toner, which is the normal charging 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 it passes through the brush nip and the charging nip. This is because the surface potentials of the primary recovery brush 2 and the charging roller 4 have a negatively higher potential than the surface potential of the photosensitive drum 1 (hereinafter referred to as the drum surface potential). At this time, the drum surface potential is approximately equal to the light area potential (Vl). When passing through the charging nip, the drum potential is charged again to the dark area potential (Vd), and 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, in full-color image forming apparatuses, it is known that reverse transfer can occur in which toner transferred to the intermediate transfer belt upstream of the multiple transfer processes is transferred to the photosensitive drum in the downstream transfer process. More specifically, reverse transfer of toner is a phenomenon in which the polarity of the upstream color toner on the intermediate transfer belt is reversed by an electrical discharge that occurs between the photosensitive drum and the intermediate transfer belt, and the toner with reversed polarity is electrostatically reverse-transferred to the photosensitive drum downstream. Hereinafter, the reverse-transferred toner will be referred to as reverse-transferred toner. In this embodiment, reverse-transferred toner is a toner with positive polarity, opposite to the normal polarity of negative polarity.

[0030] In this embodiment, for example, some of the yellow (Y) and magenta (M) toner on the intermediate transfer belt 8 may be reverse-transferred to the photosensitive drums of the process cartridges PC and PK as they pass through the primary transfer section between the primary transfer rollers 6C and 6K and the photosensitive drum 1. Most of the reverse-transferred toner becomes positive-polarity toner due to the discharge in the primary transfer section as described above. Therefore, if the surface potential of the temporary recovery brush 2 and the charging roller 4 is negatively higher than the drum potential at the reverse transfer generation section, the reverse-transferred toner will transfer to and adhere to the temporary recovery brush 2 and the charging roller 4. On the other hand, if the surface potential of the temporary recovery brush 2 and the charging roller 4 is negatively lower than the drum potential at the reverse transfer generation section, the reverse-transferred toner will remain on the surface of the photosensitive drum 1 without transferring to the temporary recovery brush 2 and the charging roller 4.

[0031] In particular, in this embodiment, the potential of the charging roller 4 is -1070[V], which is always higher than the drum potential. Therefore, when reverse transfer toner passes through the temporary recovery brush 2, the reverse transfer toner basically adheres to the charging roller 4. The relationship between the drum potential and the potential of the temporary recovery brush 2 in the reverse transfer generation section will be described later.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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 5 is a block diagram showing the control block of the image forming apparatus 100. As shown in Figure 5, 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.

[0036] 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 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 roller 4, the developing roller 3, and each transfer roller, respectively, by controlling the charging power supply 61, the developing power supply 62, and the transfer power supply 63. The CPU 26 may also control the voltage applied to the temporary recovery brush 2 by controlling the charging power supply 61, or it may control the voltage applied to the temporary recovery brush 2 by a power supply separate from the charging power supply 61. 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, respectively, and a secondary transfer power supply that controls the voltage applied to the secondary transfer roller 11. Furthermore, a separate CPU may be provided for controlling the various motors mentioned above, and a separate CPU for controlling the various power supplies mentioned above.

[0037] The following describes the charging configurations of Examples 1 to 3, followed by a description of potential control suitable for the configuration of each example.

[0038] <Example 1> (2. Method for manufacturing a photosensitive drum) (2-1.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.

[0039] (2-2. 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.

[0040] (2-3. 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].

[0041] [ka]

[0042] (2-4. 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].

[0043] 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]

[0044] (2-5. Outermost layer) The outermost layer 105 is provided on the charge transport layer 104c of the photosensitive layer 104 and is manufactured as follows. Specifically, a coating solution prepared by mixing a siloxane-modified acrylic compound (product name: Cymac US270, manufactured by Toagosei Co., Ltd.) as a binder resin with a mixed solvent of 1-propanol and cyclohexane is applied to the charge transport layer by immersion to form a coating film, which is then dried at 40°C for 5 minutes. After that, the coating film is irradiated with an electron beam under a nitrogen atmosphere, and then the temperature of the coating film is raised to 100°C under a nitrogen atmosphere. Next, the coating film is allowed to cool naturally in air until the temperature reaches 25°C, and then heat treatment is performed for 20 minutes under conditions that bring the temperature of the coating film to 100°C to form the outermost layer 105 with a thickness of 2 [μm]. In Example 1, the coating solution was mixed in the following proportions. • Siloxane-modified acrylic compound 0.1 part 1-Propanol 70 units • Cyclohexane 30 parts

[0045] (3. Configuration of the temporary retrieval brush) In Example 1, the temporary recovery brush 2 is located downstream of the primary transfer section and upstream of the charged nip in the rotational direction of the photosensitive drum 1. The temporary recovery 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 temporary recovery brush 2 has a support member made of sheet metal and a brush portion attached to the support member and made of conductive fibers that rubs against the photosensitive drum 1.

[0046] The conductive nylon fibers have a fineness of 2 denier and a flocking density of 240 fibers / mm². 2 The temporary recovery brush 2 has a pile length of 6 mm and is in contact with the photosensitive drum 1 such that the amount of penetration from the tip of the bristles is 0.5 mm. Because there is variation in the height of the bristles of the temporary recovery brush 2, if the amount of penetration into the photosensitive drum 1 is too small, the contact state with the photosensitive drum 1 becomes unstable. In this case, there is a risk that the toner recovery performance of the temporary recovery brush 2 on the photosensitive drum 1 will be insufficient.

[0047] On the other hand, if the amount of temporary recovery brush 2 that penetrates the photosensitive drum 1 is too large, the contact pressure between the temporary recovery brush 2 and the photosensitive drum 1 will increase, which may cause scratches on the photosensitive drum 1 caused by the temporary recovery brush 2. Also, if the amount of penetration by the temporary recovery brush 2 is too large, the temporary recovery brush 2 may block the residual transfer toner remaining on the surface of the photosensitive drum 1, and there is a risk that the residual transfer toner will fall into the image forming apparatus 100 and contaminate the inside of the machine. Therefore, the design values ​​of the temporary recovery brush 2, such as the bristle density, fineness, pile length, and penetration amount, need to be set in a balanced manner from the above perspectives.

[0048] In this embodiment, a temporary recovery brush 2 with a resistance of 1 × 10^5 [Ω] was used. The resistance of the temporary recovery brush 2 was calculated from the current flowing when the temporary recovery 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 temporary recovery brush 2 can be controlled by changing the material of the conductive fibers of the temporary recovery brush 2, for example.

[0049] (4. Configuration of the electrostatic roller) Furthermore, the charging roller 4 according to Example 1 employs a so-called contact charging method, generating a spark discharge by creating a potential difference exceeding the discharge threshold with respect to the photosensitive drum 1 at the charging nip, thereby charging the surface of the photosensitive drum 1. In order to suppress potential unevenness on the surface of the photosensitive drum 1, it is desirable that the charging roller 4 be configured to obtain stable spark discharge performance. Therefore, in Example 1, a single-layer charging roller 4 made of conductive hydrin rubber obtained by extrusion processing was adopted. The resistance value of the charging roller 4 is approximately 1 × 10^6 [Ω]. The resistance value of the charging roller 4 was calculated from the current value that flows when the charging roller 4 is brought into contact with a metal cylinder and a voltage of -100 [V] is applied, similar to the temporary recovery brush 2.

[0050] <Example 2> Next, the configuration of the photosensitive drum 1 according to Example 2 will be described. The photosensitive drum 1 according to Example 2 has a support 101, a conductive layer 102, an undercoat layer 103, and a photosensitive layer 104, similar to Example 1. On the other hand, the photosensitive drum 1 according to Example 2 differs from Example 1 only in the configuration of the outermost layer 105.

[0051] (5. Outermost layer) The outermost layer 105, which serves as the charge implantation layer in Example 2, is provided on the charge transport layer 104c of the photosensitive layer 104 and is manufactured as follows. Specifically, a coating solution 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 is immersed and applied onto the charge transport layer 104c to form a coating film, which is then dried at 50°C for 6 minutes. After that, the coating film is irradiated with an electron beam under a nitrogen atmosphere, and then the temperature of the coating film is raised to 117°C under a nitrogen atmosphere. Next, the coating film is allowed to cool naturally in air until its temperature reaches 25°C, and then a heat treatment is performed for 1 hour under conditions that bring the temperature of the coating film to 120°C to form a charge implantation layer with a thickness of 2 [μm] as the outermost layer 105. [ka]

[0052] In Example 2, the coating solution was mixed in the following proportions and applied to the charge transport layer 104c by immersion. 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

[0053] The volume resistivity of the outermost layer 105 can be measured using a pA (picoampere) meter. The method for measuring the volume resistivity of the outermost 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 outermost layer 105. A comb-shaped electrode 105a as shown in Figure 3, with an electrode-to-electrode distance (D) = 180 [μm] and a length (L) = 59 [mm], is fabricated on a PET film by vapor deposition, and the outermost layer 105 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) is measured when a DC voltage (V) of 1000 [V] is applied between the comb-shaped electrodes 105a, and the surface resistivity ρs of the outermost layer (charge injection layer) 105 is calculated from DC voltage (V) / DC voltage (I). Using the obtained surface resistivity ρs and the film thickness t [cm] of the outermost layer 105, the volume resistivity ρv [Ω·cm] can be obtained by the following formula (1). ρv = ρs × t ···(1) (ρv: volume resistivity, ρs: surface resistivity, t: thickness of the outermost layer)

[0054] (6. Configuration of the temporary retrieval brush) It is known that in the injection charging method, the electrostatic charge of the photosensitive drum is better when the resistance of the charging member and the outermost layer of the photosensitive drum is low, and the contact area of ​​the charging member is large. When a conductive bar brush is used as the temporary recovery brush 2, it has been confirmed that the injection charging performance decreases when the resistance value exceeds 1.0 × 10^9 [Ω]. Also, while the contact area increases and the injection charging performance improves as the penetration depth of the temporary recovery brush 2 into the photosensitive drum 1 increases, there is a risk of scratching the photosensitive drum 1 and mechanically blocking the transfer residue toner. For this reason, it has been confirmed that the penetration depth of the temporary recovery brush 2 into the photosensitive drum 1 is preferably around 0.5 to 1.0 [mm]. In Example 2, as in Example 1, a temporary recovery brush 2 with a resistance value of 1 × 10^5 [Ω] was used. The temporary recovery brush 2 also had a bristle density of 240 [hairs / mm]. 2Furthermore, the amount of material penetrating the photosensitive drum 1 is 0.5 mm, which ensures a sufficient contact area with the photosensitive drum 1 and provides good injection charging performance.

[0055] (7. Configuration of the electrostatic roller) The charging roller 4 neutralizes the potential unevenness of the photosensitive drum 1 caused by the temporary recovery brush 2 and secondary charges the photosensitive drum 1 by spark discharge in order to raise the drum potential to the desired dark area potential (Vd). In other words, the charging configuration in Example 2 combines direct charge injection from the temporary recovery brush 2 and spark discharge by the charging roller 4 to charge the outermost layer (charge injection layer) 105 of the photosensitive drum 1. It is desirable that the charging roller 4 has a low injection charging performance and a configuration that can obtain stable spark discharge performance.

[0056] To suppress electrocharging from the electrocharging roller 4, a configuration in which the surface of the electrocharging roller 4 has high resistance and a small contact area with the photosensitive drum 1 is effective. Therefore, in this embodiment, an electrocharging roller 4 with a two-layer structure having a high-resistance surface layer was adopted, in which a coating liquid, which is a mixture of urethane-based resin material and particles with a particle size of approximately 20 [μm], was spray-coated onto the surface of conductive hydrin rubber obtained by extrusion. The volume resistivity of the high-resistance surface layer is approximately 1 × 10^14 [Ω·cm]. In addition, the arithmetic mean roughness Ra, which is a numerical representation of the surface roughness of the high-resistance surface layer, is approximately 2.0 [μm]. From the viewpoint of suppressing electrocharging, it is desirable that the volume resistivity of the high-resistance surface layer be 1 × 10^12 [Ω·cm] or more, and that the arithmetic mean roughness Ra of the high-resistance surface layer be 0.5 to 3.0 [μm].

[0057] <Example 3> (8. Pre-exposure equipment) Example 3, in addition to the charging configuration of Example 1, includes a pre-exposure device 5, as shown in Figure 4. Figure 4 shows the peripheral configuration of the photosensitive drum 1 in Example 3. The pre-exposure device 5 is positioned downstream of the primary transfer section between the photosensitive drum 1 and the primary transfer roller 6, and upstream of the temporary recovery brush 2, in the rotational direction of the photosensitive drum 1. The pre-exposure device 5 discharges both the dark area Vd and the bright area Vl of the photosensitive drum 1 to 0[V]. Therefore, in Example 3, the drum potential before passing through the temporary recovery brush 2 (hereinafter referred to as the pre-brush potential) becomes 0[V] regardless of the image pattern.

[0058] <Comparative Example> Furthermore, a comparative example was provided in which the temporary recovery brush 2 was not provided, compared to the configuration of Example 1.

[0059] (9. Potential before brushing) Next, the pre-brush potential during image formation will be explained using Table 1 below. Table 1 shows the pre-brush potential measured in a cyan (C) process cartridge PC while changing the printed images of cyan and its upstream yellow and magenta. The pre-brush potential was measured by modifying the cyan process cartridge PC of the image forming apparatus 100 and placing a potential measurement probe (model 3800-S2: manufactured by Trek Japan Co., Ltd.) between the primary transfer unit and the temporary recovery brush 2 at a distance of 1.0 [mm] from the drum surface and in the center in the width direction. In the configuration of Example 3, the above potential measurement probe was placed between the pre-exposure device 5 and the temporary recovery brush 2 to measure the pre-brush potential. The placed potential measurement probe was connected to a surface potential meter (model 370: manufactured by Trek Japan Co., Ltd.) to measure the surface potential of the photosensitive drum 1. [Table 1]

[0060] As shown in Table 1, in Examples 1 and 2 and the Comparative Example, the pre-brush potential differed depending on the printed image of the cyan toner color of the process cartridge PC being measured, as well as the upstream yellow and magenta toners. In Example 3, due to the static elimination effect of the pre-exposure device 5, the pre-brush potential (hereinafter simply referred to as pre-brush potential) of the photosensitive drum 1 of the process cartridge PC was 0 [V] for all printed images (image patterns). The differences in pre-brush potential in Examples 1 and 2 and the Comparative Example are described below.

[0061] When the yellow and magenta print images were entirely white, i.e., with a print density of 0%, and the cyan print image was entirely black, i.e., with a print density of 100%, the pre-brush potential was -120[V] in all of Examples 1, 2, and the Comparative Example, and was equal to the bright area potential (Vl). A print density of 0% means that no image is formed at all in the image-forming area of ​​the sheet K or photosensitive drum 1, and a print density of 100% means that a solid black image is formed, i.e., filled in, across the entire image-forming area of ​​the sheet K or photosensitive drum 1.

[0062] On the other hand, when the printed images for yellow, magenta, and cyan were entirely white, the pre-brush potential was -400[V] in all of Examples 1 and 2 and the Comparative Example, which was lower than the dark area potential (Vd) = -520[V]. In this description of the embodiment, the high and low potentials are described as high and low when compared in terms of absolute value of the potential. This is thought to be because, when the photosensitive drum 1 passed through the primary transfer section, the potential difference between the dark area potential (Vd) and the surface of the intermediate transfer belt 8 was above the discharge threshold, and the drum potential after primary transfer decreased due to discharge.

[0063] It has been confirmed that discharge begins in the primary transfer section when the potential difference between the surface of the intermediate transfer belt 8 and the photosensitive drum 1C (see Figure 1) that forms the cyan toner image is 550[V] or more, and the potential difference between the voltage applied to the primary transfer roller 6C and the photosensitive drum 1C is 640[V] or more. In this implementation, the dark area potential (Vd) is -520[V], the transfer voltage of the primary transfer roller 6C is +300[V], and a potential difference of 820[V] is formed in the primary transfer section. Therefore, when the printed images of yellow, magenta, and cyan are all white, the potential difference in the primary transfer section is above the discharge threshold, and it is considered that discharge occurs near the primary transfer section. On the other hand, since the bright area potential (Vl) is -120[V], when the yellow and magenta printed images are entirely white and the cyan printed image is entirely black, the potential difference in the primary transfer area is 420[V], which is below the discharge threshold. Therefore, it is thought that the drum potential after primary transfer did not change from the bright area potential (Vl).

[0064] Next, when the cyan print image was entirely white, and the yellow and magenta print images were not entirely white, the pre-brush potential was higher than -400[V] in all cases of Examples 1 and 2 and the Comparative Example. Furthermore, the pre-brush potential tended to be higher as the print density of yellow and magenta increased. More specifically, the pre-brush potential increased in the following order: a full-screen halftone image of magenta with 25% print density (M Halftone 25%), a full-screen halftone image of magenta with 50% print density (M Halftone 50%), a full-screen solid magenta image with 100% print density (M Full Black 100%), and an image created by superimposing full-screen solid magenta and yellow images with 100% print density (YM Full Black 200%). In the YM Full Black 200% print image, the pre-brush potential was -510[V], which was a high potential close to the dark area potential (Vd). This is thought to be because the accumulation of negatively charged yellow and magenta toners on the surface of the intermediate transfer belt 8 effectively increases the surface potential of the intermediate transfer belt 8 to the negative side, reducing the potential difference between the dark area potential (Vd) and the surface of the intermediate transfer belt 8. In particular, when both yellow and magenta print entirely black, the potential difference is thought to have decreased to a level where almost no discharge occurs.

[0065] As shown above, the pre-brush potential changes not only with the printed image of the color in which the image is formed (cyan in the above example), but also with the printed image of its upstream color (yellow and magenta in the above example). In particular, printed images with a high pre-brush potential are those where the amount of toner of the upstream color is large and the color in which the image is formed is an all-white image. Therefore, in order to effectively recover the reverse-transfer toner of the upstream color with the temporary recovery brush 2 for various printed images, it is necessary to appropriately select the potential of the temporary recovery brush 2.

[0066] The following describes the results of testing the charging ghost and the recovery performance of reverse transfer toner by the temporary recovery brush 2 when the potential of the temporary recovery brush 2 was set to -100[V] to -1200[V], for the configurations of Examples 1 to 3 and the comparative example. Then, the appropriate potential of the temporary recovery brush 2 will be discussed. Note that charging ghost is an image defect caused by reverse transfer toner adhering to the charged component (charged roller 4). When a lot of toner adheres to the surface of the charged component (charged roller 4), the charging performance changes, deviating from the desired drum potential, and the image density may become lighter or darker than the original. In particular, if reverse transfer toner adheres in a concentrated area, the density will change only at the area where it adheres.

[0067] (10-1. Evaluation Methods) The effectiveness of this configuration was confirmed by checking the amount of toner accumulated in the temporary recovery brush 2 and the charging roller 4 after continuous printing, and by checking for the presence or absence of static ghosting. The evaluation conditions were as follows: under a temperature of 23°C and a humidity of 50%, 59 pages of images forming three vertical bands with a width of 40 mm and a length of 200 mm were continuously printed, followed by the printing of one page of a full-surface halftone image of cyan with a print density of 25% (C25% halftone). The three vertical bands were images created by superimposing full-surface fill images of magenta and yellow with a print density of 100% (YM full black), a full-surface fill image of magenta with a print density of 100% (M full black), and a full-surface halftone image of magenta with a print density of 50% (M50% halftone), making them images prone to reverse transfer.

[0068] Furthermore, A4-sized GF-C081 (manufactured by Canon) was used as the recording material. For the configurations of Examples 1 to 3 and the comparative example, the potential of the temporary recovery brush 2 was changed in 100V steps within the range of -100[V] to -1200[V], and evaluations were performed with 12 potential settings for each configuration. The surfaces of the temporary recovery brush 2 and the charging roller 4 were cleaned after each evaluation setting.

[0069] The detection of charged ghosting involved examining the density difference of the halftone image in each vertical band in the full-screen cyan halftone image with a print density of 25% on page 60. Configurations with a slight density difference that were acceptable were marked with ○, and configurations with a large density difference that were unacceptable were marked with ×. In Table 2, the three vertical bands are shown as the YM full black area, the M full black area, and the M 50% halftone area, and the pre-brush potential corresponding to each vertical band after primary transfer is also shown in Table 2.

[0070] The recovery performance of reverse-transferred toner by the temporary recovery brush 2 was evaluated by disassembling the cyan process cartridge PC after 60 pages of continuous printing and checking the visual appearance of the reverse-transferred toner recovery state by the temporary recovery brush 2 and the state of reverse-transferred toner adhesion to the charging roller 4. For the reverse-transferred toner recovery state by the temporary recovery brush 2 (brush recovery state), a configuration with a large amount of reverse-transferred toner recovered was marked with ○, and a configuration with almost no reverse-transferred toner recovered was marked with ×. For the state of reverse-transferred toner adhesion to the charging roller 4 (charging roller adhesion state), a state with almost no reverse-transferred toner adhesion was marked with ○, and a state with a large amount of reverse-transferred toner adhesion was marked with ×.

[0071] (10-2. Evaluation Results) Table 2 summarizes the evaluation results of charged ghosts in the combined evaluation of the configurations of Examples 1-3 and the comparative example, and the potential of the temporary recovery brush 2. Table 3 summarizes the evaluation results of the recovery state of reverse transfer toner by the temporary recovery brush 2 and the adhesion state of reverse transfer toner to the charged roller 4. [Table 2] [Table 3]

[0072] (10-3. Evaluation results of Example 1) As shown in Table 2, the configuration of Example 1 confirmed that static ghosting could be suppressed in any printed image by setting the potential of the temporary recovery brush 2 (hereinafter sometimes simply referred to as the temporary recovery brush potential) to -600[V] or higher. Furthermore, while a potential of -600[V] was required for the temporary recovery brush 2 in the YM all-black area, static ghosting disappeared in the M all-black area and the M 50% halftone area at a potential of -500[V]. Thus, it was confirmed that the appropriate potential of the temporary recovery brush 2 to suppress static ghosting differed depending on the printed image of each color.

[0073] More specifically, in the YM all-black area where the pre-brush potential is -510[V], it is preferable that the potential of the temporary recovery brush 2 is -600[V] or higher. In the M all-black area where the pre-brush potential is -460[V], it is preferable that the potential of the temporary recovery brush 2 is -500[V] or higher. In the M 50% halftone area where the pre-brush potential is -440[V], it is preferable that the potential of the temporary recovery brush 2 is -500[V] or higher. As mentioned above, the high and low potentials are described in terms of the absolute value of the potentials. That is, below, when the magnitude relationship of the potentials is expressed with an inequality sign, it is shown as an inequality that holds true for the absolute values ​​of those potentials. In all printed images, good static ghosting is achieved in configurations that satisfy the relationship pre-brush potential < temporary recovery brush potential, and it was confirmed that the occurrence of static ghosting can be suppressed by setting pre-brush potential < temporary recovery brush potential.

[0074] As shown in Table 3, in the configuration of Example 1, it was confirmed that the reverse transfer toner could be recovered well by the temporary recovery brush 2 at a temporary recovery brush potential of -600[V] to -1000[V] in the YM all-black area and -500[V] to -900[V] in the M 50% halftone area. Furthermore, the adhesion of reverse transfer toner to the charging roller 4 was suppressed at a temporary recovery brush potential of -600[V] or higher in the YM all-black area and -500[V] or higher in the M 50% halftone area.

[0075] The behavior of the temporary recovery brush 2 when its potential was increased differed from the adhesion state of the charged roller 4 and the recovery state of the temporary recovery brush 2. In the recovery state of the temporary recovery brush 2, the recovery efficiency of the reverse transfer toner tended to decrease when the potential of the temporary recovery brush 2 was too high. This is thought to be because the potential of the temporary recovery brush exceeded the discharge threshold relative to the potential before the brush, causing discharge to occur on the upstream side of the temporary recovery brush 2, resulting in a negative polarity reversal of the reverse transfer toner. Furthermore, it was confirmed that, similar to the primary transfer section, discharge begins in the brush nip between the photosensitive drum 1 and the temporary recovery brush 2 when the potential difference between the temporary recovery brush 2 and the photosensitive drum 1 is 550[V] or higher.

[0076] Therefore, in a configuration where the temporary recovery brush potential is 550[V] or more higher than the pre-brush potential, the polarity of the reverse transfer toner is reversed to negative due to discharge, and it is thought that the negative polarity reverse transfer toner cannot be recovered by the temporary recovery brush 2, which has a high negative potential relative to the photosensitive drum 1. On the other hand, the reverse transfer toner, which has become negative polarity, passes through the charging roller 4, which has a high negative potential, and is then recovered by the developing roller 3. For this reason, even if the temporary recovery brush potential is set to an excessively high potential, the adhesion of the reverse transfer toner to the charging roller 4 remains good, and it is thought that the occurrence of charged ghosting is suppressed. On the other hand, since the reverse transfer toner recovered by the developing roller 3 is the toner of the upstream color, there is a risk that color changes due to color mixing will become a problem when the electrostatic latent image is developed again by the process cartridge PC. For this reason, it is not desirable to recover the reverse transfer toner with the developing roller 3 (developing section).

[0077] (10-4. Evaluation results of Example 2) As shown in Table 2, it was confirmed that, in the configuration of Example 2, similar to Example 1, charging ghosting can be suppressed by setting the temporary recovery brush potential to -600[V] or higher in the YM all-black area and -500[V] or higher in the M all-black area and M 50% halftone area. In other words, it was confirmed that, in the configuration of Example 2, similar to Example 1, the occurrence of charging ghosting can be suppressed by setting the pre-brush potential < temporary recovery brush potential.

[0078] As shown in Table 3, in the configuration of Example 2, it was confirmed that the reverse transfer toner could be recovered well by the temporary recovery brush 2 at a temporary recovery brush potential of -600[V] to -900[V] in the YM all-black area and -500[V] to -800[V] in the M 50% halftone area. Furthermore, the adhesion of reverse transfer toner to the charging roller 4 was suppressed at a temporary recovery brush potential of -600[V] or higher in the YM all-black area and -500[V] or higher in the M 50% halftone area.

[0079] Unlike Example 1, Example 2 showed a tendency for reduced recovery of reverse transfer toner by the temporary recovery brush 2 even at lower temporary recovery brush potentials when the temporary recovery brush potential was excessively high. This is thought to be because, in Example 2, the temporary recovery brush 2 injected charge into the outermost layer 105 of the photosensitive drum 1, resulting in charge injection into the reverse transfer toner as well. When the temporary recovery brush potential increases, the amount of charge injected into the reverse transfer toner also increases, and it is thought that the polarity of the reverse transfer toner reversed to negative even when the potential difference between the pre-brush potential and the temporary recovery brush potential was lower than the discharge threshold. Similar to Example 1, the reverse transfer toner whose polarity has turned negative cannot be recovered by the temporary recovery brush 2, and after passing through the charging roller 4, it is recovered by the developing roller 3. Therefore, even when the temporary recovery brush potential is set to an excessively high potential, the adhesion of the reverse transfer toner to the charging roller 4 remains good, and it is thought that the occurrence of charged ghosting is suppressed.

[0080] (10-5. Evaluation results of Example 3) As shown in Table 2, it was confirmed that in the configuration of Example 3, static ghosting can be suppressed in all printed images by setting the temporary recovery brush potential to -100[V] or higher. In the configuration of Example 3, the pre-brush potential becomes 0V due to the static elimination effect of the pre-exposure device 5, so it is thought that static ghosting could be suppressed at a low potential in all printed images. In other words, it was confirmed that, in the configuration of Example 3 as in Examples 1 and 2, the occurrence of static ghosting can be suppressed by setting the pre-brush potential < temporary recovery brush potential.

[0081] As shown in Table 3, the recovery of reverse transfer toner by the temporary recovery brush 2 was good in all printed images when the temporary recovery brush potential was in the range of -100[V] to -500[V], but reverse transfer toner could not be recovered at -600[V] or higher. Furthermore, the adhesion of reverse transfer toner to the charging roller 4 was good at all temporary recovery brush potentials above -100[V], indicating that the adhesion of reverse transfer toner was suppressed. The reason why the recovery of reverse transfer toner by the temporary recovery brush 2 deteriorated at -600[V] or higher is thought to be because, in the configuration of Example 3, the pre-brush potential is 0[V], so even at low potentials, the discharge threshold was exceeded. Similar to Example 1, the polarity of the reverse transfer toner reversed to negative due to discharge, and it is thought that the reverse transfer toner with negative polarity could not be recovered by the temporary recovery brush 2, but was recovered by the developing roller 3 after passing through the charging roller 4. Therefore, even when the temporary recovery brush potential is set to an excessively high potential, the adhesion of the reverse transfer toner to the charged roller 4 remains good, and it is believed that the generation of charged ghosting is suppressed.

[0082] (10-6. Evaluation results of comparative examples) As shown in Table 2, in the configuration of Comparative Example 1, static ghosting occurred in all printed images. Furthermore, as shown in Table 3, a large amount of reverse transfer toner adhered to the static roller 4 in all printed images.

[0083] (11. Summary of evaluation results) From the evaluation results of Examples 1-3 and the comparative example, it was confirmed that by providing a temporary recovery brush 2 for recovering reverse transfer toner downstream of the primary transfer section and upstream of the charging roller 4 in the rotation direction of the photosensitive drum 1, and by setting the brush pre-potential < temporary recovery brush potential during image formation, the occurrence of charged ghosting can be suppressed. In other words, by setting the brush pre-potential < temporary recovery brush potential during image formation, the reverse transfer toner can be recovered by the temporary recovery brush 2. This suppresses the adhesion of reverse transfer toner to the charging roller 4, and suppresses image defects such as charged ghosting, where the charging performance changes only in the part where the reverse transfer toner adheres to the charging roller 4, causing a change in image density.

[0084] Furthermore, while this implementation configuration demonstrated the effect on three printed images, in actual image forming apparatuses, various reverse-transferable image patterns are anticipated, and in those cases as well, it is necessary to satisfy the condition that pre-brush potential < temporary recovery brush potential. Printed images with a high pre-brush potential are, as mentioned above, those where the amount of toner for the upstream color is large and the image being formed is an all-white image, and in full-color image formation, when up to three colors of upstream color toner are formed on the intermediate transfer belt 8.

[0085] In the image forming apparatus 100 of this implementation configuration, all-black images of yellow, magenta, and cyan were formed on the intermediate transfer belt 8, and the pre-brush potential was measured in the black process cartridge PK. The pre-brush potential was approximately -520[V], which was almost equal to the dark area potential (Vd). Therefore, from the viewpoint of the recovery of reverse transfer toner, it is desirable that the potential of the temporary recovery brush 2 be higher than the dark area potential (Vd).

[0086] Furthermore, from the evaluation results of Examples 1 and 3, it was confirmed that good reverse transfer toner recovery can be obtained by setting the potential difference between the temporary recovery brush potential and the potential before the brush to below the discharge threshold. Even if the above potential difference is above the discharge threshold, the reverse transfer toner that has passed through the temporary recovery brush 2 is recovered by the developing roller 3, and charged ghosting is suppressed. However, if, for example, toners of other colors such as yellow and magenta are recovered in the cyan process cartridge PC, and the electrostatic latent image is developed again by the process cartridge PC, there is a risk that color changes due to color mixing will become a problem. For this reason, from the viewpoint of suppressing image defects, it is not desirable to recover the reverse transfer toner in the developing roller 3 (developing unit).

[0087] Therefore, from the viewpoint of suppressing discharge to reverse-transfer toner, it is desirable that the potential of the temporary recovery brush 2 be below the discharge threshold relative to the pre-brush potential in any printed image where reverse transfer may occur. In this embodiment, the change in recovery performance for three printed images was shown, but in an actual image forming apparatus, various image patterns that may be subject to reverse transfer are expected, and in that case as well, the potential difference between the pre-brush potential and the temporary recovery brush must be below the discharge threshold. A printed image with a low pre-brush potential is an all-black image, i.e., the bright area potential (Vl), but as mentioned above, discharge is unlikely to occur in the primary transfer section for the bright area potential (Vl), so reverse transfer hardly occurs. Therefore, under conditions where reverse transfer may occur, the printed image with the lowest pre-brush potential is an all-white image where the amount of toner of the upstream color is small and the color in question is not exposed, and in that case the pre-brush potential is approximately equal to the post-transfer potential of the dark area potential (Vd). Therefore, from the viewpoint of suppressing discharge to the reverse transfer toner, it is desirable that the potential of the temporary recovery brush 2 be below the discharge threshold relative to the post-transfer potential after the dark area Vd has passed through the primary transfer area.

[0088] Furthermore, when measuring the drum potential in the case of a halftone image, a potential higher than the bright area potential (Vl) and lower than the dark area potential (Vd) is measured. This potential can be the potential at which discharge occurs in the primary transfer area, so it seems desirable that the potential be below the discharge threshold relative to the post-transfer potential of the halftone image. However, the drum potential of a halftone image measured by a surface potential meter is an averaged result because the spot diameter is larger than the halftone dot diameter. In reality, the exposed area of ​​the dot is almost at the bright area potential (Vl), and the unexposed area is at the dark area potential (Vd). Therefore, discharge occurs in the dark area Vd (unexposed area) of the halftone image, and as described above, by setting the potential of the primary recovery brush 2 below the discharge threshold relative to the post-transfer potential of the dark area Vd, discharge to the reverse transfer toner can be suppressed.

[0089] From the evaluation results of Example 2, it was confirmed that in a configuration in which the temporary recovery brush 2 also functions as an injection charging member, good reverse transfer toner recovery can be obtained by setting the potential of the temporary recovery brush 2 to a potential difference less than the discharge threshold and approximately 100[V] lower than the potential before the brush. The amount of charge injected into the reverse transfer toner changes depending on the configuration of the temporary recovery brush 2 and the outermost layer 105 of the photosensitive drum 1, but it was confirmed that this tendency is more pronounced the higher the conductivity of each component.

[0090] It has been confirmed that when the resistance of the temporary recovery brush 2 exceeds 1.0 × 10^9 [Ω], the charge injection capability to the reverse transfer toner decreases, similar to the charge injection capability to the photosensitive drum 1. Furthermore, it has been confirmed that when the volume resistivity of the outermost layer 105 of the photosensitive drum 1 exceeds 1.0 × 10^14 [Ω·cm], the charge injection capability to both the photosensitive drum 1 and the reverse transfer toner decreases. On the other hand, when the volume resistivity of the outermost layer 105 is less than 1.0 × 10^9 [Ω·cm], it becomes difficult to maintain the latent image, and the print quality of fine lines tends to deteriorate. It has been found that, in combinations of the volume resistance of the photosensitive drum 1 and the lower limit of the resistance of the temporary recovery brush 2 that are usable from the standpoint of print quality, the potential difference between the pre-brush potential and the temporary recovery brush 2 at which the recovery capability of the reverse transfer toner decreases due to charge injection into the toner is 350 [V] or higher. Therefore, in a configuration where the temporary recovery brush 2 also functions as an injection charging member, as in Example 2, it is more desirable that the potential of the temporary recovery brush 2 be less than 350[V] relative to the post-transfer potential after the dark area Vd (unexposed area) has passed through the primary transfer section, from the viewpoint of suppressing discharge to the reverse transfer toner.

[0091] (12. Summary) As described above, in each embodiment shown in Tables 1 to 3, a temporary recovery brush 2 is provided as a recovery member, positioned downstream of the primary transfer roller (transfer section) and upstream of the charging roller 4 in the rotational direction of the photosensitive drum 1. The temporary recovery brush 2 can electrostatically recover the reverse transfer toner. In an image forming apparatus 100 that transfers a second toner image having a second color (e.g., cyan) different from the first color onto an intermediate transfer belt 8 so as to be superimposed on a first toner image having a first color (e.g., yellow or magenta), the temporary recovery brush 2 of the process cartridge (PC) that forms the toner image of the second color electrostatically recovers the reverse transfer toner having the first color.

[0092] In such an image forming apparatus 100, let the absolute value of the potential of the temporary recovery brush 2 during image formation be V1, and the absolute value of the surface potential (drum potential) of the photosensitive drum 1 before passing through the temporary recovery brush 2 be V2. At this time, the CPU 26 controls the voltages applied to the charging roller 4 and the temporary recovery brush 2 so that V2 < V1 is satisfied and (V1 - V2) is less than the discharge threshold. For example, in Examples 1 and 2, as shown in Tables 2 and 3, it is preferable to set V1 to -600 [V] to -900 [V]. Also, in Example 3, as shown in Tables 2 and 3, it is preferable to set V1 to -100 [V] to -500 [V]. Thereby, image defects such as charging ghosts and color changes due to color mixing can be suppressed.

[0093] Also, in each of the above-described embodiments, since the reverse transfer toner is recovered by the temporary recovery brush 2 during image formation, there is no need to perform other processes such as a cleaning operation after the image formation operation is completed. Therefore, an increase in the downtime of the image forming apparatus 100 can be suppressed, and the throughput can be improved.

[0094] Furthermore, let the absolute value of the drum potential immediately after passing through the charging roller 4 during image formation be V3, and the absolute value of the surface potential in the non-exposed portion (dark portion Vd) of the photosensitive drum 1 after passing through the primary transfer portion and before passing through the temporary recovery brush 2 be V4. At this time, the CPU 26 controls the voltages applied to the charging roller 4 and the temporary recovery brush 2 so that V3 < V1 is satisfied and (V1 - V4) is less than the discharge threshold. Thereby, even in a printing image pattern in which the potential before the brush is the highest, such as forming a full black image of yellow, magenta, and cyan on the intermediate transfer belt 8 and forming a full white image in the black process cartridge PK, image defects such as charging ghosts and color changes due to color mixing can be suppressed.

[0095] Furthermore, in the configuration described in Example 2, where the charge injection layer of the photosensitive drum 1, the outermost layer 105, is charged by combining direct charge injection from the temporary recovery brush 2 and spark discharge by the charging roller 4, it is preferable that (V1-V4) be less than 350[V]. This is because, as described above, by setting (V1-V4) to less than 350[V], discharge to the reverse transfer toner in the primary transfer section is suppressed, and the recovery of the reverse transfer toner by the temporary recovery brush 2 is improved.

[0096] Furthermore, V5 is the drum potential immediately after passing through the charging roller 4 during image formation, i.e., the drum potential in the unexposed area (dark area Vd), V6 is the drum potential in the exposed area (bright area Vl), and V7 is the voltage applied to the primary transfer roller. At this time, the CPU 26 in each of the above embodiments controls the voltage applied to the charging roller 4 so that the absolute value of (V5-V7) is greater than or equal to the discharge threshold, and the absolute value of (V6-V7) is greater than or equal to the discharge threshold. For example, in each of the above embodiments, V5 = -520[V], V6 = -120[V], V7 = +300[V], and the voltage applied to the charging roller 4 is -1070[V]. Also, discharge occurs when the potential difference between the voltage applied to the primary transfer roller and the photosensitive drum 1 is 640[V] or more. Due to this potential relationship, when the printed image of the color in which the image is formed (cyan in the above example) is 100% white in the primary transfer section (see Table 1), toner may be reverse-transferred to the photosensitive drum 1 due to discharge. However, as shown in Tables 2 and 3, by appropriately setting the potential of the primary recovery brush 2, image defects such as static ghosting and color changes due to color mixing can be suppressed.

[0097] <Other Embodiments> 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, the invention is not limited to this. For example, a direct transfer type image forming apparatus that directly transfers the toner images of each color onto a recording material (sheet) may be used. In the direct transfer type, discharge occurs between the surface of the recording material and the photosensitive drum 1 in the transfer section. It has also been confirmed that discharge begins when the potential difference between the surface of the recording material and the photosensitive drum 1 is 550[V] or higher, and that discharge occurs when the voltage applied to the transfer roller and the potential difference between the photosensitive drum 1 are higher than 640[V] due to the voltage distributed by the recording material. On the other hand, although the discharge position changes from the surface of the intermediate transfer belt 8 to the surface of the recording material, the post-transfer potential changes due to the discharge, and reverse transfer occurs, which is the same as in the embodiments described above. Therefore, even in a direct transfer type full-color image forming apparatus, by satisfying the relationship between the pre-brush potential and the temporary recovery brush potential of this embodiment, good recovery performance of reverse-transferred toner by the temporary recovery brush 2 and a charge ghost suppression effect can be obtained.

[0098] Furthermore, while this embodiment uses a tandem system with a photosensitive drum 1 for each toner color (yellow, magenta, cyan, and black), a rotary full-color image forming apparatus with four color developers for each photosensitive drum 1 may also be used. In the rotary system, the development of the electrostatic latent image on the photosensitive drum by the developer and the primary transfer onto the intermediate transfer belt are repeated for all four colors, and the photosensitive drum 1 passes through the primary transfer section four times before a full-color image is formed on the recording material. Although the rotary image forming apparatus has only one primary transfer section, the potential difference and discharge phenomenon in the primary transfer section are the same as in the image forming apparatus of this embodiment. That is, the toner image of the upstream color on the intermediate transfer belt can be reverse-transferred to the photosensitive drum when it passes through the primary transfer section in the downstream process. Therefore, even in a rotary full-color image forming apparatus, by satisfying the relationship between the pre-brush potential and the primary recovery brush potential of this embodiment, good recovery performance of reverse-transferred toner by the primary recovery brush 2 and a charge ghost suppression effect can be obtained.

[0099] Furthermore, although a bar brush was used as the component of the temporary recovery brush in this embodiment, similar effects can be obtained with brush components made of other conductive fibers. Specifically, instead of the temporary recovery brush 2, the reverse transfer toner can be recovered by a brush roller in which conductive nylon fibers are wound around a roller-shaped core.

[0100] Furthermore, in the embodiments described above, a configuration was used in which a toner with a negative charge was used and a negative charging voltage was applied. However, a configuration in which a toner with a positive charge was used and a positive charging voltage was applied may also be used. In that case, the reverse transfer toner mainly has a negative charge, and by applying a positive voltage to the temporary recovery brush 2 and satisfying the potential relationship of this embodiment, good recovery performance of the reverse transfer toner by the temporary recovery brush 2 and a charge ghost suppression effect can be obtained.

[0101] 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.

[0102] Summary of this disclosure This disclosure includes at least the following: (Composition 1) In an image forming apparatus that transfers a second toner image having a second color different from the first color onto a transfer member so as to be superimposed on a first toner image having a first color, A photoreceptor configured to be rotatable in a predetermined direction of rotation, A charging member that charges the surface of the photoreceptor by contacting it, A developing unit that develops the electrostatic latent image formed on the surface as the second toner image, A transfer unit that transfers the second toner image to the transfer member, A recovery member disposed downstream of the transfer portion and upstream of the charging member in the rotation direction, the recovery member electrostatically recovering the reverse transfer toner having the first color transferred from the transfer member to the surface of the photoreceptor by the transfer portion. A control unit that controls the voltages applied to the charging member and the recovery member, respectively. When the absolute value of the potential of the recovery member during image formation is V1 and the absolute value of the surface potential of the photoreceptor before passing through the recovery member is V2. The control unit controls the voltages applied to the charging member and the recovery member, respectively, so that V2 < V1 is satisfied and (V1 - V2) is less than the discharge threshold. An image forming apparatus characterized by this. (Configuration 2) When the absolute value of the surface potential of the photoreceptor immediately after passing through the charging member during image formation is V3 and the absolute value of the surface potential in the non-exposed portion of the photoreceptor after passing through the transfer portion and before passing through the recovery member is V4. The control unit controls the voltages applied to the charging member and the recovery member, respectively, so that V3 < V1 is satisfied and (V1 - V4) is less than the discharge threshold. The image forming apparatus according to Configuration 1, characterized by this. (Configuration 3) The photoreceptor has a charge injection layer containing conductive particles. The charging member and the recovery member charge the surface of the photoreceptor by contacting the charge injection layer. The image forming apparatus according to Configuration 2, characterized by this. (Configuration 4) The control unit controls the voltages applied to the charging member and the recovery member, respectively, so that (V1 - V4) is less than 350 [V]. The image forming apparatus according to Configuration 3, characterized by this. (Configuration 5) The volume resistivity of the charge injection layer is 1.0 × 10^9 [Ω·cm] or more and 1.0 × 10^14 [Ω·cm] or less. The image forming apparatus according to Configuration 3 or 4, characterized by this. (Composition 6) When V5 is the surface potential of the photoreceptor immediately after passing through the charged member during image formation, V6 is the surface potential of the photoreceptor in the exposed state, and V7 is the voltage applied to the transfer section, The control unit controls the voltage applied to the charged member such that the absolute values ​​of (V5-V7) are greater than or equal to the discharge threshold, and the absolute values ​​of (V6-V7) are greater than or equal to the discharge threshold. An image forming apparatus according to any one of configurations 1 to 5, characterized by the above. (Composition 7) The recovery 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 6 characterized by the above. (Composition 8) The resistance value of the recovery member is 1.0 × 10^9 [Ω] or less. The image forming apparatus according to configuration 7, characterized by the features described above. (Composition 9) 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 8, characterized by the above. (Composition 10) The charging member has a high-resistivity surface layer with a volume resistivity of 1.0 × 10^12 [Ω·cm] or more, and the surface is charged by spark discharge. The image forming apparatus according to configuration 9, characterized by the features described therein. (Composition 11) The developing unit electrostatically recovers the toner remaining on the photoreceptor after the second 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 10, characterized by the above. (Composition 12) The system further includes a secondary transfer unit that transfers the first toner image and the second toner image transferred to the transfer member onto a sheet. An image forming apparatus according to any one of configurations 1 to 11, characterized by the above. (Composition 13) The developing unit electrostatically recovers the toner remaining on the photoreceptor after the second toner image has been transferred to the transfer target by the transfer unit. The image forming apparatus according to configuration 12, characterized by the features described above. [Explanation of Symbols]

[0103] 1: Photoreceptor (photosensitive drum) / 2: Recovery component (temporary recovery brush) / 3: Developing unit (developing roller) / 4: Charging component (charging roller) / 6C: Transfer unit (primary transfer roller) / 8: Transfer target (intermediate transfer belt) / 11: Secondary transfer unit (secondary transfer roller) / 26: Control unit (CPU) / 100: Image forming apparatus / 105: Charge injection layer (outermost layer)

Claims

1. In an image forming apparatus that transfers a second toner image having a second color different from the first color onto a transfer member so as to be superimposed on a first toner image having a first color, A photoreceptor configured to be rotatable in a predetermined direction of rotation, A charging member that charges the surface of the photoreceptor by contacting it, A developing unit that develops the electrostatic latent image formed on the surface as the second toner image, A transfer unit that transfers the second toner image to the transfer member, A recovery member positioned downstream of the transfer unit and upstream of the charging member in the rotational direction, the recovery member electrostatically recovers the reverse transfer toner having the first color that has been transferred from the transfer member to the surface of the photoreceptor by the transfer unit, The system comprises a control unit that controls the voltage applied to the charging member and the recovery member, respectively. When V1 is the absolute value of the potential of the recovery member during image formation, and V2 is the absolute value of the surface potential of the photoreceptor before passing through the recovery member, The control unit controls the voltages applied to the charging member and the recovery member, respectively, so that V2 < V1 and (V1 - V2) is less than the discharge threshold. An image forming apparatus characterized by the following features.

2. When V3 is the absolute value of the surface potential of the photoreceptor immediately after passing through the charged member during image formation, and V4 is the absolute value of the surface potential of the unexposed portion of the photoreceptor after passing through the transfer section and before passing through the recovery member, The control unit controls the voltages applied to the charging member and the recovery member, respectively, so that V3 < V1 and (V1 - V4) is less than the discharge threshold. The image forming apparatus according to feature 1.

3. The photoreceptor has a charge injection layer containing conductive particles, The charging member and the recovery member charge the surface of the photoreceptor by contacting the charge injection layer. The image forming apparatus according to feature 2.

4. The control unit controls the voltages applied to the charging member and the recovery member, respectively, so that (V1-V4) is less than 350 [V]. The image forming apparatus according to feature 3.

5. 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 3.

6. When V5 is the surface potential of the photoreceptor immediately after passing through the charged member during image formation, V6 is the surface potential of the photoreceptor in the exposed state, and V7 is the voltage applied to the transfer section, The control unit controls the voltage applied to the charging member such that the absolute value of (V5-V7) is greater than or equal to the discharge threshold, and the absolute value of (V6-V7) is greater than or equal to the discharge threshold. The image forming apparatus according to feature 1.

7. The recovery 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.

8. The resistance value of the recovery member is 1.0 × 10⁹ [Ω] or less. The image forming apparatus according to feature 7.

9. 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.

10. The charging member has a high-resistivity surface layer with a volume resistivity of 1.0 × 10¹² [Ω·cm] or more, and the surface is charged by spark discharge. The image forming apparatus according to feature 9.

11. The developing unit electrostatically recovers the toner remaining on the photoreceptor after the second 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 10.

12. The system further includes a secondary transfer unit that transfers the first toner image and the second toner image transferred to the transfer member onto a sheet. The image forming apparatus according to any one of claims 1 to 10.

13. The developing unit electrostatically recovers the toner remaining on the photoreceptor after the second toner image has been transferred to the transfer target by the transfer unit. The image forming apparatus according to feature 12.

Images