Charging device and image forming device

Abstract

The present invention provides a charging device and an image forming apparatus. The charging device comprises: a first charging unit that contacts a charged unit and charges the charged unit at a first charging potential; and a second charging unit that contacts the charged unit upstream of the first charging unit in a direction of movement of the charged unit and charges the charged unit at a second charging potential, wherein the second charging potential is lower than the first charging potential.

Description

Technical Field

[0001] The present disclosure relates to a charging device and an image forming apparatus. Background Art

[0002] Conventionally, as a technique related to a charging device, for example, a technique including a plurality of charging members has been proposed (Japanese Patent Application Laid-Open No. 2007-33835).

[0003] The Japanese Patent Gazette No. 2007-33835 is constructed to include: a first charging member that charges the charged body at a target potential; a second charging member that pre-charges the charged body with a polarity opposite to the target potential; and a control unit that controls the charged potential with the opposite polarity passed through the second charging member so that when charging is passed through the first charging member, the direct current flowing in the first charging member becomes greater than a specified value. Summary of the Invention

[0004] The purpose of the present disclosure is to suppress the deviation of the charged body's charged potential compared to the case where the second charged unit in contact with the charged unit on the upstream side of the charged unit in the moving direction has a second charged potential higher than the first charged potential.

[0005] According to the first embodiment of the present disclosure, a charging device is provided, wherein the charging device comprises: a first charging unit, which contacts the charged unit so that the charged unit is charged with a first charging potential; and a second charging unit, which contacts the charged unit on the side upstream of the moving direction of the charged unit than the first charging unit so that the charged unit is charged with a second charging potential, wherein the second charging potential is lower than the first charging potential.

[0006] According to the second aspect of the present disclosure, a voltage applying unit is provided for applying a voltage commonly to the first charging unit and the second charging unit.

[0007] According to the third aspect of the present disclosure, the second charging unit is formed to have a higher resistance than the first charging unit.

[0008] According to the fourth aspect of the present disclosure, the contact load between the second charging unit and the charged unit is lower than the contact load between the first charging unit and the charged unit.

[0009] According to the fifth aspect of the present disclosure, each of the first charging unit and the second charging unit includes a voltage applying unit for applying a voltage.

[0010] According to the sixth aspect of the present disclosure, the voltage applied by the voltage applying means for applying the voltage to the second charging means is lower than the voltage applied by the voltage applying means for applying the voltage to the first charging means.

[0011] According to a seventh aspect of the present disclosure, there is provided an image forming apparatus including: an image holding unit that holds an image; and a charging unit that charges the image holding unit, wherein the charging device is used as the charging unit.

[0012] (Effect)

[0013] According to the first aspect, the variation in the charged potential of the charged body can be suppressed compared to the case where the second charged unit in contact with the charged unit upstream of the first charged unit in the moving direction has a second charged potential higher than the first charged potential.

[0014] According to the second aspect, compared to the case where the first charging unit and the second charging unit are each independently provided with a voltage applying unit, a single voltage applying unit can be provided.

[0015] According to the third aspect, compared to a case where the second charging unit is not formed to have a higher resistance than the first charging unit, the charging potential can be changed without adding any additional components.

[0016] According to the fourth aspect, compared to the case where the contact load of the second charging unit with the charged unit and the contact load of the first charging unit with the charged unit are equal, the charging potential can be changed without adding any components.

[0017] According to the fifth aspect, compared with a case where the first charging unit and the second charging unit both have a voltage applying unit for applying a voltage, setting of the applied voltage becomes easier.

[0018] According to the sixth aspect, the charging potential of the charged unit is easier to set than when the voltage applied by the voltage applying unit to the second charging unit is not lower than the voltage applied by the voltage applying unit to the first charging unit.

[0019] According to the seventh aspect, it is possible to suppress variations in the charging potential of the image holding unit, compared to a case where the charging device is not used as the charging unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram showing the configuration of an image forming apparatus to which the charging device according to the first embodiment of the present disclosure is applied.

[0021] Figure 21 is a diagram schematically illustrating the configuration of an image forming apparatus of the image forming apparatus according to Embodiment 1 of the present disclosure.

[0022] Figure 3 This is a structural diagram showing a charging device according to Embodiment 1 of the present disclosure.

[0023] Figure 4 It is a cross-sectional structural diagram showing the charging state of the charging device.

[0024] Figure 5 This is a graph showing the relationship between the applied voltage of the charging roller and the charged potential of the photosensitive drum.

[0025] Figure 6 This is a graph showing the relationship between the charging potential generated by the second charging roller and the charging potential generated by the first charging roller.

[0026] Figure 7 This is a graph showing the relationship between the charging potential generated by the second charging roller and the charging potential generated by the first charging roller in the comparative example.

[0027] Figure 8 Graph showing the relationship between the axial position of the charging roller and the charging characteristics in a comparative example.

[0028] Figure 9 It is a cross-sectional structural diagram showing a charging device according to a second embodiment of the present disclosure.

[0029] Figure 10 1 is a structural diagram showing a main part of an image forming apparatus to which a charging device according to a third embodiment of the present disclosure is applied.

[0030] Figure 11 This is a graph showing the relationship between the applied voltage of the charging roller and the charged potential of the photosensitive drum.

[0031] Figure 12 This is a structural diagram showing a modified example of the charging device according to the third embodiment of the present disclosure. DETAILED DESCRIPTION

[0032] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.

[0033] [Implementation Method 1]

[0034] Figure 1 and Figure 2 An image forming apparatus to which the charging device of the first embodiment is applied is shown. Figure 1 The overall outline of the image forming apparatus is shown. Figure 2 The main parts (imaging device, etc.) of the image forming apparatus are shown in an enlarged manner.

[0035] <Overall Structure of Image Forming Apparatus>

[0036] The image forming apparatus 1 of the first embodiment is configured as, for example, a monochrome printer. Figure 1 As shown, the image forming apparatus 1 includes: an image forming device 10 as an example of an image forming unit, which forms a colorant image developed using a colorant constituting a developer; a paper feeding device 20 as an example of a recording medium to be supplied to a transfer position of the image forming device 10, which accommodates and supplies recording paper 5; a conveying device 30, which conveys the recording paper 5 supplied from the paper feeding device 20 along a conveying path; and a fixing device 40, etc., which fixes the colorant image on the recording paper 5 transferred by the image forming device 10.

[0037] The imaging device 10 includes a rotating photosensitive drum 11 as an example of an image holding unit (charged unit). The following devices are mainly arranged around the photosensitive drum 11. The main devices are the following devices, etc.: the charging device 12 of this embodiment; it charges the peripheral surface (image holding surface) of the photosensitive drum 11 that can form an image with a required potential; the exposure device 13, which irradiates the charged peripheral surface of the photosensitive drum 11 with light based on image information (signal) to form an electrostatic latent image with a potential difference; the developing device 14, which uses a black developer colorant to develop the electrostatic latent image into a colorant image; the transfer device 15, which transfers the colorant image to the recording paper 5; the de-staticizing device 17, which de-staticizes the image holding surface of the photosensitive drum 11 after transfer; the drum cleaning device 16; it removes and cleans the toner and other attachments remaining and adhering to the image holding surface of the photosensitive drum 11 after de-staticization.

[0038] The photosensitive drum 11 is constructed by forming an image-retaining surface having a photoconductive layer (photosensitive layer) made of a photosensitive material on the circumferential surface of a cylindrical or columnar substrate that has been subjected to grounding. The photosensitive drum 11 is supported for rotation in the direction indicated by arrow A by power transmitted from a driving device (not shown). In this first embodiment, the rotational speed (circumferential speed) of the photosensitive drum 11 is set to a relatively high speed of approximately 400 mm / s.

[0039] The charging device 12 is composed of a contact-type charging roller arranged in contact with the photosensitive drum 11. The charging device 12 includes a first charging roller 121 as an example of a first charging unit, and a second charging roller 122 as an example of a second charging unit arranged on the upstream side of the first charging roller 121 along the rotation direction A of the photosensitive drum 11. A charging voltage is supplied to the charging device 12. When the developing device 14 is a device that performs reverse development, a voltage or current having the same polarity as the charging polarity of the toner supplied from the developing device 14 is supplied as the charging voltage. The charging device 12 will be described in detail later.

[0040] The exposure device 13 is composed of an LED print head. This LED print head uses a plurality of LEDs (Light Emitting Diodes) arranged along the axial direction of the photosensitive drum 11 to irradiate the photosensitive drum 11 with light corresponding to image information, thereby forming an electrostatic latent image. Alternatively, a device that deflects and scans laser light generated according to image information along the axial direction of the photosensitive drum 11 may be used as the exposure device 13.

[0041] like Figure 2 As shown, the developing device 14 is constructed within a housing 140 having an opening 140a and a developer storage chamber. The following components are arranged: a developing roller 141, an example of a developer holding unit, which holds the developer 4 and conveys it to a development area facing the photosensitive drum 11; two agitating and conveying members 142 and 143, such as screws, which convey the developer 4 while stirring it as it passes through the developing roller 141; a layer thickness regulating member (not shown) that regulates the amount (layer thickness) of the developer held by the developing roller 141; and a supply roller 144 that supplies the developer recovered from the outer circumference of the developing roller 141 to the stirring and conveying member 143. In this developing device 14, a development bias voltage is supplied between the developing roller 141 and the photosensitive drum 11 from a power supply (not shown). Furthermore, the developing roller 141, the stirring and conveying members 142 and 143, and the supply roller 144 are driven by power transmitted from a driving device (not shown) to rotate in a desired direction. Also, as the developer, a two-component developer including a non-magnetic toner and a magnetic carrier is used.

[0042] like Figure 1 As shown, the transfer device 15 is a contact-type transfer device comprising a transfer roller 151 that rotates in contact with the periphery of the photosensitive drum 11 via the recording paper 5 during image formation and supplies a transfer voltage, and a transfer belt 152 that conveys the recording paper 5. The transfer belt 152 is stretched between a drive roller 153 and a driven roller 154. A DC voltage having a polarity opposite to the charged polarity of the toner is supplied as the transfer voltage from a power supply device (not shown).

[0043] The static eliminating device 17 is constituted by a corona discharger or the like, and this corona discharger applies a charge having a polarity opposite to the charging polarity of the charging device 12 to the image holding surface of the photosensitive drum 11 after transfer, thereby performing static elimination.

[0044] like Figure 2As shown, the drum cleaning device 16 is composed of the following components: a container-shaped body 160 with a portion open; a cleaning plate 161, which is configured to contact the circumference of the photosensitive drum 11 after transfer with a required pressure to remove and clean residual toner and other attached materials; a cleaning brush 162, which is also configured to contact the circumference of the photosensitive drum 11 with a required pressure to remove and clean residual toner and other attached materials; and a delivery member 163, such as an auger, which conveys the toner and other attached materials removed by the cleaning plate 161 and the cleaning brush 162 so that they are recovered and sent to a recovery container (not shown). As the cleaning plate 161, a plate-shaped member (e.g., a scraper) made of a material such as rubber is used.

[0045] like Figure 1 As shown, the fixing device 40 is constructed by arranging the following components within a device housing 43, which is formed with an inlet and an outlet for recording paper 5: a heating roller 41, an example of a heating rotating body (fixing unit), which rotates in the direction indicated by the arrow and is heated by the heating unit to maintain a predetermined surface temperature; and a pressure roller 42, an example of a pressing rotating body, which rotates substantially along the axial direction of the heating roller 41 while in contact with the pressure roller 42 at a predetermined pressure. In this fixing device 40, the contact area between the heating roller 41 and the pressure roller 42 forms a fixing nip where the necessary fixing process (heating and pressing) is performed.

[0046] The paper feed device 20 is located below the main body 1a. It primarily comprises a plurality of (or a single) paper containers 22, which hold recording paper 5 of a desired size and type, placed on a loading plate 21; and a feed device 23, which feeds recording paper 5 one sheet at a time from the paper containers 22. The paper feed device 20 can be attached to and detached from the main body 1a of the image forming apparatus 1 by manually grasping and pulling out a grip (not shown) provided on the paper container 22.

[0047] Examples of the recording paper 5 include plain paper used in electrophotographic copiers and printers, thin paper such as tracing paper, and OHP sheets. To further improve the smoothness of the image surface after fixing, the surface of the recording paper 5 is preferably as smooth as possible. For example, coated paper obtained by coating the surface of plain paper with a resin or the like, or relatively heavy paper such as art paper for printing can also be used.

[0048] like Figure 1As shown, between the paper feed device 20 and the transfer device 15, a paper feed conveying path 33, comprised of multiple (or a single) paper conveying roller pairs 31a to 31f or a conveying guide 32, is provided. The conveying path is configured to curve upward vertically along the apparatus main body 1a and toward the interior of the apparatus main body 1a. The paper conveying roller pair 31f, positioned just before the transfer position in the paper feed conveying path 33, serves as a roller (registration roller) for adjusting the conveyance timing of the recording paper 5. Furthermore, the transferred recording paper 5 is conveyed from the transfer device 15 to the fixing device 40 via the transfer belt 152 of the transfer device 15.

[0049] A discharge path 37 is provided above the discharge port of the fixing device 40. The recording paper is conveyed to a paper discharge section 36 via a plurality of conveying roller pairs 34a to 34d and a conveying guide 35. The paper discharge section 36 is provided on the upper end surface of the apparatus body 1a.

[0050] Further, a double-sided conveyance path 39 is provided below the discharge port of the fixing device 40 , for conveying the recording paper 5 toward the paper feed conveyance path 33 via a plurality of conveyance roller pairs 38 a to 38 e and a conveyance guide 38 f .

[0051] The recording paper 5 discharged from the fixing device 40 is temporarily conveyed to the discharge conveying path 37 by a switching gate (not shown), and then reversed by rotating the conveying roller pairs 34 a and 34 b in opposite directions and conveyed to the double-sided conveying path 39 .

[0052] also, Figure 1 Reference numeral 200 in the figure denotes a control device that centrally controls the operation of image forming apparatus 1. Control device 200 includes a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), or a bus or communication interface connecting these CPU and ROM (not shown). Reference numerals 201 and 202 denote an image reading device and a document feeder, respectively, disposed on the upper portion of the main body 1a of image forming apparatus 1.

[0053] <Basic Operation of Image Forming Device>

[0054] Hereinafter, a basic image forming operation performed by the image forming apparatus 1 will be described.

[0055] The image forming device 1 is controlled by a control device 200. When an instruction information requesting a monochrome image forming action (printing) is received from an unillustrated operation panel or an unillustrated user interface, printer driver, etc. installed on the device body 1a, the imaging device 10, paper feeding device 20, conveying device 30 and fixing device 40, etc. are started.

[0056] Then, in the imaging device 10, as shown in FIG. Figure 1 As shown, first, the photosensitive drum 11 rotates in the direction indicated by arrow A, and the charging device 12 charges the surface of the photosensitive drum 11 with a desired polarity (negative polarity in the first embodiment) and potential. Next, the exposure device 13 irradiates the charged surface of the photosensitive drum 11 with light based on image information input to the image forming apparatus 1, thereby forming an electrostatic latent image having a desired potential difference on the surface.

[0057] Next, the developing device 14 supplies black toner charged with a desired polarity (negative polarity) from the developing roller 141 to the electrostatic latent image formed on the photosensitive drum 11, causing it to electrostatically adhere and thereby develop the image. Through this development, the electrostatic latent image formed on the photosensitive drum 11 is visualized as a toner image developed with the black toner.

[0058] Next, when the toner image formed on the photosensitive drum 11 is transported to the transfer position, the transfer device 15 transfers the toner image to the recording paper 5 .

[0059] After transfer is completed, the static eliminator 17 removes residual charge from the surface of the photosensitive drum 11. The drum cleaner 16 then scrapes away any deposits and cleans the surface of the photosensitive drum 11. This allows the image forming apparatus 10 to be ready for the next image forming operation.

[0060] The paper feed device 20 feeds the required recording paper 5 to the paper feed path 33 in conjunction with the image forming operation. In the paper feed path 33, a paper feed roller pair 31f serving as registration rollers feeds the recording paper 5 to the transfer position in conjunction with the transfer timing.

[0061] Next, the recording paper 5 to which the toner image has been transferred is conveyed by the transfer belt 152 to the fixing device 40. In the fixing device 40, the transferred recording paper 5 is guided into the fixing nip between the rotating heating roller 41 and the pressure roller 42 and passed through. The necessary fixing process (heating and pressure) is thereby applied, and the unfixed toner image is fixed to the recording paper 5. When the recording paper 5 after fixing is performed, the recording paper 5 is discharged along the discharge conveying path 37 by the discharge roller pair 34d to the paper discharge portion 36 provided at the upper end of the apparatus main body 1a during the image forming operation in which an image is formed on only one side of the recording paper 5.

[0062] When an image is formed on both sides of the recording paper 5, the recording paper 5 with an image formed on one side is fed to the transport roller pair 34a, 34b via a switching gate (not shown). The transport roller pair 34a, 34b then temporarily transports the recording paper 5 in the discharge direction. Then, while the rear end of the recording paper 5 is held between the transport roller pair 34a, 34b, the rotation direction of the transport roller pair 34a, 34b is reversed. After the recording paper 5 is reversed, the transport roller pair 34a, 34b is fed again to the transfer device 15 via the double-sided transport path 39. A toner image is transferred to the back side of the recording paper 5. The recording paper 5 with the toner image transferred to the back side is then fed to the fixing device 40 via the transfer belt 152 of the transfer device 15. The fixing device 40 performs a fixing process (heating and pressurization) on the recording paper 5 before the recording paper 5 is discharged to the paper discharge section 36 via the transport roller pairs 34a, 34d.

[0063] Through the above-described operation, the recording paper 5 having a monochrome image formed on one or both sides is output.

[0064] <Structure of the live device>

[0065] Figure 2 1 is a diagram showing the configuration of the charging device according to the first embodiment.

[0066] like Figure 2 As shown, the charging device 12 of embodiment 1 includes: a first charging roller 121, which is arranged in contact with the photosensitive drum 11 as an example of a charged unit, so that the photosensitive drum 11 is charged at a first charging potential determined by an applied voltage and a discharge start voltage; and a second charging roller 122, which is arranged in contact with the photosensitive drum 11 on the upstream side along the rotation direction A of the photosensitive drum 11 than the first charging roller 121, so that the photosensitive drum 11 is charged at a second charging potential determined by an applied voltage and a discharge start voltage, wherein the second charging potential is lower than the first charging potential.

[0067] The first charging roller 121 and the second charging roller 122 are basically constructed in the same manner. Figure 3As shown, the first charging roller 121 and the second charging roller 122 are formed into a cylindrical shape, and each of them includes cylindrical core rods 123 and 124 made of a metal such as stainless steel or iron, semi-conductive elastic layers 125 and 126 that are provided with conductivity and cover the outer circumference of the core rods 123 and 124 to a desired thickness, and surface layers 127 and 128 that are thinly coated on the surfaces of the elastic layers 125 and 126. The core rods 123 and 124 also serve as rotating shafts that protrude from both ends of the first charging roller 121 and the second charging roller 122 in the axial direction. In addition, the first charged roller 121 and the second charged roller 122 are pressed against the outer peripheral surface of the photosensitive drum 11 via core rods 123 and 124 by a force-applying unit such as a coil spring (not shown), and are driven to rotate along with the rotation of the photosensitive drum 11 while being elastically deformed so that the elastic layer 125, 126 and the surface layer 127, 128 have the required clamping width.

[0068] like Figure 2 As shown, the core rods 123 and 124 of the first and second charging rollers 121 and 122 are connected to a high-voltage power supply 129 (an example of a voltage application unit) via bearing members (not shown). The control device 200 controls the value and timing of the high voltage applied to the charging rollers 121 and 122 by the high-voltage power supply 129. The high-voltage power supply 129 supplies a predetermined negative-polarity DC high voltage or high current, which is the same polarity as the charged toner supplied from the developing device 14.

[0069] The elastomer layers 125 and 126 of the first charging roller 121 and the second charging roller 122 are composed of, for example, a porous foam having cavities and concave-convex portions inside and on the surface. The elastomer layers 125 and 126 are constructed to have the required resistance value by dispersing a resistance adjuster such as carbon black or an ionic conductive agent in a foaming resin material such as polyurethane, polyethylene, polyamide, olefin, melamine, polypropylene, or a foaming rubber material such as EPDM (ethylene-propylene-diene copolymer rubber), NBR (acrylonitrile-butadiene copolymer rubber), styrene-butadiene rubber, chloroprene rubber, silicone rubber, nitrile rubber, natural rubber, etc. The volume resistivity of the elastomer layers 125 and 126 is, for example, set to 10 4 ~10 8 In addition, the elastic layers 125 and 126 may be solid rubber without foaming.

[0070] The surface layers 127 and 128 of the first and second charging rollers 121 and 122 are formed, for example, by coating the outer circumferences of the elastomer layers 125 and 126 with a granular filler. Furthermore, the surface layers 127 and 128 of the charging rollers 121 and 122 may be formed, for example, by coating the outer circumferences of the elastomer layers 125 and 126 with a conductive material dispersed in a tubular body such as polytetrafluoroethylene (PTFE) or perfluoroalkoxyalkane (PFA).

[0071] In addition, the first charging roller 121 and the second charging roller 122 may not have the surface layers 127 and 128 other than the elastic layers 125 and 126 .

[0072] However, in the charging device 12 of this embodiment 1, the charging potential (second charging potential) of the photosensitive drum 11 generated by the second charging roller 122 determined by the applied voltage and the discharge start voltage is lower than the charging potential (first charging potential) of the photosensitive drum 11 generated by the first charging roller 121.

[0073] In other words, in the charging device 12 of this embodiment 1, the first charging roller 121 located on the downstream side along the rotation direction of the photosensitive drum 11 is configured so that the charging potential of the photosensitive drum 11 determined by the applied voltage and the discharge start voltage becomes higher than the second charging roller 122 located on the upstream side along the rotation direction of the photosensitive drum 11.

[0074] like Figure 4 As shown, the first and second charging rollers 121 and 122 are pressed against the outer peripheral surface of the photosensitive drum 11 via core rods 123 and 124 by a biasing means such as a coil spring (not shown), thereby elastically deforming the elastomer layers 125 and 126 and the surface layers 127 and 128 so as to form a desired clamping width. Then, a predetermined negative-polarity DC high voltage is applied to the core rods 123 and 124 by a high-voltage power supply device 129. This generates a micro-gap discharge in the discharge area D formed between the first and second charging rollers 121 and 122 and the outer peripheral surface of the photosensitive drum 11. The charged particles generated by this micro-gap discharge charge the outer peripheral surface of the photosensitive drum 11 to the desired negative-polarity charge potential. Figure 4 In FIG. 1 , reference numeral 111 denotes a conductive substrate of the photosensitive drum 11, and reference numeral 112 denotes a photosensitive layer of the photosensitive drum 11. The conductive substrate 111 of the photosensitive drum 11 is grounded.

[0075] The first and second charging rollers 121 and 122 have the following relationship between the DC high voltage applied to the cored rods 123 and 124 of the first and second charging rollers 121 and 122 and the charged potential of the outer peripheral surface of the photosensitive drum 11 .

[0076] The charge potential of the photosensitive drum 11 generated by the first charging roller 121 and the second charging roller 122 is determined by the applied voltage and the discharge start voltage. It is known that the discharge start voltage follows Paschen's law. Now, let the charge potential of the photosensitive drum 11 generated by the first charging roller 121 be Vh1, the charge potential of the photosensitive drum 11 generated by the second charging roller 122 be Vh2, the applied voltage of the first charging roller 121 be Vdc1, the applied voltage of the second charging roller 122 be Vdc2, the discharge start voltage of the first charging roller 121 be Vα1, and the discharge start voltage of the second charging roller 122 be Vα2.

[0077] In this way, Figure 5 As shown, the first and second charging potentials Vh1 and Vh2 of the photosensitive drum 11 generated by the first and second charging rollers 121 and 122 are obtained by subtracting the discharge start voltages Vα1 and Vα2 from the applied voltages Vdc1 and Vdc2 and can be expressed as follows.

[0078] Vh1=Vdc1-Vα1

[0079] Vh2=Vdc2-Vα2

[0080] In this first embodiment, the volume resistivity of the elastomer layer 126 in the second charging roller 122 is set to be greater than the volume resistivity of the elastomer layer 125 in the first charging roller 121. Specifically, the amount of resistance adjusting agent, such as carbon black or an ionic conductive agent, dispersed in the elastomer layer 126 of the second charging roller 122 is set to be less than the amount of resistance adjusting agent in the elastomer layer 125 of the first charging roller 121, and the volume resistivity is set to be greater than the volume resistivity of the elastomer layer 125 of the first charging roller 121. Therefore, even when the second charging roller 122 is applied with an applied voltage Vdc2 (=Vdc1) equal to that of the first charging roller 121, the discharge start voltage Vα2 at which discharge begins and the second charged potential Vh2 of the photosensitive drum 11 begins to rise is higher than the discharge start voltage Vα1 of the first charging roller 121 (Vα2 > Vα1).

[0081] As a result, in the charging device 12 of this first embodiment, when an equal applied voltage (Vdc1=Vdc2) is applied to the first charging roller 121 and the second charging roller 122 from the same high-voltage power supply device 129, the second charged potential Vh2 of the photosensitive drum 11 generated by the second charging roller 122, which is determined by the applied voltage and the discharge start voltage, becomes a value lower than the first charged potential Vh1 generated by the first charging roller 121 (Vh1>Vh2). In other words, the second charged potential Vh2 generated by the second charging roller 122, which is determined by the applied voltage and the discharge start voltage, is set to a value lower than the target charged potential Vβ of the photosensitive drum 11 achieved by the charging device 12, that is, the first charged potential Vh1.

[0082] <Operation of live equipment>

[0083] In an image forming apparatus to which the charging device of the first embodiment is applied, as shown in FIG. Figure 2 As shown, when the image forming operation starts, the outer peripheral surface of the photosensitive drum 11 is charged to a predetermined charging potential by the charging device 12 .

[0084] In the charging device 12, as Figure 2 As shown in FIG. 1 , first, the outer peripheral surface of the photosensitive drum 11 is charged by the second charging roller 122. At this time, a voltage Vdc2 is applied to the second charging roller 122 by the high voltage power supply device 129. Figure 5 As shown, the second charging roller 122 charges the outer peripheral surface of the photosensitive drum 11 to a second charging potential Vh2 determined by the applied voltage Vdc2 and the discharge start voltage Vα2.

[0085] Then, in the charging device 12, the outer peripheral surface of the photosensitive drum 11 is charged by the first charging roller 121. At this time, the same applied voltage Vdc1 as that of the second charging roller 122 is applied to the first charging roller 121 by the high voltage power supply device 129. Figure 5 As shown, the first charging roller 121 charges the outer peripheral surface of the photosensitive drum 11 to a first charging potential Vh1 determined by the applied voltage Vdc1 and the discharge start voltage Vα1 .

[0086] Therefore, in the charging device 12 of the above-mentioned embodiment 1, if Figure 6 As shown, the second charging potential Vh2 of the second charging roller 122 located upstream in the rotational direction of the photosensitive drum 11 is set to a value lower than the target charging potential Vβ of the photosensitive drum 11 achieved by the charging device 12 .

[0087] Therefore, in the charging device 12 of this embodiment 1, the surface of the photosensitive drum 11 is charged with the second charging potential Vh2 by the second charging roller 122 located on the upstream side along the rotation direction of the photosensitive drum 11, and then, the surface of the photosensitive drum 11 is charged with the first charging potential Vh1 equal to the target charging potential Vβ by the first charging roller 121 located on the downstream side along the rotation direction of the photosensitive drum 11, thereby generating a small gap discharge between the first charging roller 121 and the surface of the photosensitive drum 11 and charging with the first charging potential Vh1.

[0088] At this time, the surface potential of the photosensitive drum 11 becomes a second charging potential Vh2 lower than the target charging potential Vβ, thereby suppressing the so-called charge injection in which charges generated when charging by the first charging roller 121 directly move from the first charging roller 121 to the surface of the photosensitive drum 11.

[0089] As described above, in the charging device 12 of the first embodiment, the variation in the charging potential of the photosensitive drum 11 is suppressed compared to a case where the relationship between the charging potential determined by the applied voltage and the discharge start voltage is not considered in the first charging roller 121 and the second charging roller 122 .

[0090] Furthermore, the charging device 12 includes a plurality of charging units, including a first charging roller 121 and a second charging roller 122. This improves the charging capability compared to a case where a single charging unit is used to charge the photosensitive drum 11. Therefore, even when the rotational speed of the photosensitive drum 11 is significantly increased to 400 mm / s and the number of recording paper sheets 5 that can be imaged per unit time is increased, the outer peripheral surface of the photosensitive drum 11 can be uniformly charged at the desired potential, thereby suppressing the occurrence of charged ghosts.

[0091] Comparative Example

[0092] On the other hand, as a comparative example, the inventors conducted the following comparative test: Identical charging rollers with equal applied voltages and discharge start voltages were used as the first charging roller 121 and the second charging roller 122, and the surface of the photosensitive drum 11 was charged to a target charge potential Vβ by the first charging roller 121 and the second charging roller 122. In this case, three types of charging rollers I to III with the same structure but from different batches were used as the first charging roller 121.

[0093] Figure 7 1 is a graph showing the results of the comparative example. In this graph, the horizontal axis represents the charged potential of the photosensitive drum 11 by the second charging roller 122 , and the vertical axis represents the charged potential of the photosensitive drum 11 by the first charging roller 121 .

[0094] according to Figure 7It can be seen that when the same charging rollers with equal applied voltage and discharge start voltage are used as the first charging roller 121 and the second charging roller 122, and the surface of the photosensitive drum 11 is charged with a target charging potential by the first charging roller 121 and the second charging roller 122, it can be seen that the charging potential of the photosensitive drum 11 after being charged by the first charging roller 121 deviates greatly depending on the characteristics I to III (different batches) of the first charging roller 121.

[0095] According to the inventor's investigation, the reasons are as follows: Figure 8 As shown, when the same charging rollers with the same applied voltage and discharge start voltage are used as the first charging roller 121 and the second charging roller 122, and the surface of the photosensitive drum 11 is charged with the target charging potential by the first charging roller 121 and the second charging roller 122, a charge injection phenomenon occurs at the axial end of the first charging roller 121, and the deviation of the axial charging potential generated by the second charging roller 122 before is enlarged.

[0096] To further explain, when the same charging rollers having the same applied voltage and discharge start voltage are used as the first charging roller 121 and the second charging roller 122, and the surface of the photosensitive drum 11 is charged to the target charging potential by the first charging roller 121 and the second charging roller 122, as shown in FIG. Figure 8 As shown, although a discharge phenomenon occurs in the central portion of the first charging roller 121 and the charged potential of the photosensitive drum 11 increases due to the discharge phenomenon, the charged potential of the photosensitive drum 11 has been increased to a high negative polarity potential as the target potential by the second charging roller 122 at both end portions along the axial direction of the first charging roller 121 where the clamping pressure tends to increase compared to the central portion. Therefore, it is considered that the charge injection phenomenon caused by the first charging roller 121 is triggered.

[0097] [Implementation Method 2]

[0098] Figure 9 A charging device according to Embodiment 2 is shown. In the charging device 12 according to Embodiment 2, a contact load of the second charging unit with the charged object is set to be lower than a contact load of the first charging unit.

[0099] That is, in the charging device 12 of the second embodiment, as Figure 9 As shown, the contact load of the second charging roller 122 with the photosensitive drum 11 is set to be lower than the contact load of the first charging roller 121. Specifically, the spring constant of the urging means such as a coil spring (not shown) that presses the second charging roller 122 against the outer peripheral surface of the photosensitive drum 11 is set to be smaller, or the deformation amount of the urging means such as the coil spring (not shown) is configured to be smaller.

[0100] Thus, the contact load of the second charging roller 122 with the photosensitive drum 11 is set to be low, thereby increasing the distance between the electrodes of the second charging roller 122 and the photosensitive drum 11. Figure 5 In the case shown, it can be understood from Paschen's law that the discharge start voltage Vα2 is higher than the discharge start voltage of the first charging roller 121 .

[0101] Therefore, the second charging roller 122 is configured so that the charged potential of the photosensitive drum 11 determined by the applied voltage and the discharge start voltage is lower than the charged potential of the photosensitive drum 11 by the first charging roller 121 .

[0102] In the charging device 12 of the second embodiment, the same charging rollers can be used as the first charging roller 121 and the second charging roller 122 , thereby avoiding an increase in cost.

[0103] The other structures and functions are the same as those in the above embodiment, so their description is omitted.

[0104] [Implementation Method 3]

[0105] Figure 10 A charging device according to Embodiment 3 is shown. In the charging device 12 according to Embodiment 3, the first charging roller 121 and the second charging roller 122 are configured to have voltage applying means independently of each other.

[0106] Furthermore, in this embodiment 3, the applied voltage of the second voltage applying means for applying a voltage to the second charging roller 122 is set to be lower than the applied voltage of the first voltage applying means for applying a voltage to the first charging roller. In addition, in this embodiment 3, the same charging roller is used as the first charging roller 121 and the second charging roller 122.

[0107] That is, Figure 10 As shown, the charging device 12 of this third embodiment includes a first high-voltage power supply device 129a as a first voltage applying unit for applying a voltage to a first charging roller 121, and a second high-voltage power supply device 129b as a second voltage applying unit for applying a voltage to a second charging roller 122. The voltage applied by the second high-voltage power supply device 129b is set to be lower than the voltage applied by the first high-voltage power supply device 129a (Vdc1>Vdc2).

[0108] In the charging device 12 of the third embodiment, the applied voltages of the first high-voltage power supply device 129a and the second high-voltage power supply device 129b are set as follows:

[0109] Vh1=Vdc1-Vα1

[0110] Vh2=Vdc2-Vα2

[0111] Here, the relationship of Vdc1>Vdc2 is satisfied.

[0112] Therefore, in the charging device 12 of the third embodiment, as shown in FIG. Figure 11 As shown, the charging potential Vh2 of the photosensitive drum 11 by the second charging roller 122 is lower than the charging potential Vh1 of the photosensitive drum 11 by the first charging roller 121 .

[0113] In the charging device 12 of this embodiment 3, the same charging roller can be used as the first charging roller 121 and the second charging roller 122 , which can reduce costs compared to using different charging rollers as the first charging roller 121 and the second charging roller 122 .

[0114] In the third embodiment, the first high-voltage power supply device 129a and the second high-voltage power supply device 129b are independently provided. Figure 12 As shown, the first high-voltage power supply device 129 a and the second high-voltage power supply device 129 b may be shared as the high-voltage power supply device 129 , and a high voltage may be applied from the shared high-voltage power supply device 129 via the resistor R to the second charging roller 122 .

[0115] With this configuration, while the high-voltage power supply device 129 is made common, the voltage applied to the second charging roller 122 can be set to a lower value than the voltage applied to the first charging roller 121 .

[0116] The other structures and functions are the same as those in the above embodiment, so their description is omitted.

[0117] In addition, in the embodiment, a monochrome image forming device is described as the image forming device, but the same can of course be applied to a full-color image forming device having imaging devices 10 (Y, M, C, K) of yellow (Y), magenta (M), blue (C) and black (K).

[0118] Furthermore, in the above embodiment, the case of two charging members consisting of a first charging rod and a second charging roller is described, but it is also possible to configure a configuration with three or more charging members. In this case, the charging potential of the charged unit arranged on the upstream side along the moving direction of the charged unit is set lower than that of the charged unit arranged on the downstream side.

Claims

1. A charging device, wherein: The charging device has: a first charging unit that contacts the charged unit and charges the charged unit at a first charging potential; a second charging unit that contacts the charged unit on a side upstream of the first charging unit in the moving direction of the charged unit and charges the charged unit in such a manner that a second charging potential has an absolute value greater than that of the first charging potential at the same polarity as the first charging potential; as well as A voltage applying unit applies a voltage to the first charging unit and the second charging unit in common.

2. The charging device according to claim 1, wherein The second charging unit is formed to have a higher resistance than the first charging unit.

3. The charging device according to claim 1, wherein: A contact load between the second charging unit and the charged unit is lower than a contact load between the first charging unit and the charged unit.

4. An image forming apparatus, wherein: The image forming apparatus includes: an image holding unit for holding an image; and a charging unit for charging the image holding unit, As the charging unit, the charging device according to any one of claims 1 to 3 is used.

Images