阅读说明：本技术 带电装置及图像形成装置 (Charging device and image forming apparatus ) 是由 饭田纮史 于 2020-03-13 设计创作，主要内容包括：本发明提供带电装置及图像形成装置。带电装置具备：第1带电单元,其与被带电单元接触,使所述被带电单元以第1带电电位带电；以及第2带电单元,其在比所述第1带电单元靠所述被带电单元的移动方向的上游的一侧与所述被带电单元接触,使所述被带电单元以第2带电电位带电,其中,所述第2带电电位低于所述第1带电电位。(The invention provides a charging device and an image forming apparatus. The charging device is provided with: a 1 st charging unit that is brought into contact with a charged unit and charges the charged unit at a 1 st charging potential; and a 2 nd charging unit that is brought into contact with the charged unit on an upstream side in a moving direction of the charged unit with respect to the 1 st charging unit, and charges the charged unit at a 2 nd charging potential, wherein the 2 nd charging potential is lower than the 1 st charging potential.)

1. A charging device, wherein,

the charging device is provided with:

a 1 st charging unit that is brought into contact with a charged unit and charges the charged unit at a 1 st charging potential; and

and a 2 nd charging unit that contacts the charged unit on an upstream side in a moving direction of the charged unit from the 1 st charging unit and charges the charged unit at a 2 nd charging potential, wherein the 2 nd charging potential is lower than the 1 st charging potential.

2. The charging device according to claim 1,

the apparatus includes a voltage applying unit for applying a voltage to the 1 st charging unit and the 2 nd charging unit in common.

3. The charging device according to claim 2,

the 2 nd charging unit is formed to have a higher resistance than the 1 st charging unit.

4. The charging device according to claim 2,

the contact load of the 2 nd charging unit and the charged unit is lower than the contact load of the 1 st charging unit and the charged unit.

5. The charging device according to claim 1,

the 1 st charging unit and the 2 nd charging unit each have a voltage applying unit that applies a voltage.

6. The charging device according to claim 5,

an application voltage of the voltage applying unit that applies the voltage to the 2 nd charging unit is lower than an application voltage of the voltage applying unit that applies the voltage to the 1 st charging unit.

7. An image forming apparatus in which, when a toner image is formed,

the image forming apparatus includes:

an image holding unit that holds an image; and

a charging unit for charging the image holding unit,

the charging device according to any one of claims 1 to 6 is used as the charging means.

Technical Field

The present disclosure relates to a charging device and an image forming apparatus.

Background

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).

The aforementioned Japanese patent laid-open No. 2007-33835 is configured to have: a 1 st charging member that charges a charged object at a target potential; a 2 nd charging member that preliminarily charges the charged body with a polarity opposite to a target potential; and a control unit that controls a charging potential charged with an opposite polarity by the 2 nd charging member so that a direct current flowing through the 1 st charging member becomes equal to or greater than a predetermined value when the charging member is charged by the 1 st charging member.

Disclosure of Invention

The purpose of the present disclosure is to suppress variation in the charging potential of a charged object, compared to a case where the 2 nd charging potential of a 2 nd charging unit that is in contact with a charged unit on the upstream side in the moving direction of the charged unit from the 1 st charging unit is higher than the 1 st charging potential.

According to the 1 st aspect of the present disclosure, there is provided a charging device including: a 1 st charging unit that is brought into contact with a charged unit and charges the charged unit at a 1 st charging potential; and a 2 nd charging unit that is brought into contact with the charged unit on an upstream side in a moving direction of the charged unit with respect to the 1 st charging unit, and charges the charged unit at a 2 nd charging potential, wherein the 2 nd charging potential is lower than the 1 st charging potential.

According to the 2 nd aspect of the present disclosure, the charging device includes a voltage applying unit that commonly applies a voltage to the 1 st charging unit and the 2 nd charging unit.

According to the 3 rd aspect of the present disclosure, the 2 nd charging unit is formed to have a higher resistance than the 1 st charging unit.

According to the 4 th aspect of the present disclosure, the contact load of the 2 nd charging unit with the charged unit is lower than the contact load of the 1 st charging unit with the charged unit.

According to the 5 th aspect of the present disclosure, the 1 st charging unit and the 2 nd charging unit each have a voltage applying unit that applies a voltage.

According to the 6 th aspect of the present disclosure, the voltage application unit that applies the voltage to the 2 nd charging unit has a lower application voltage than the voltage application unit that applies the voltage to the 1 st charging unit.

According to the 7 th 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.

(Effect)

According to the above-described aspect 1, the variation in the charging potential of the charging target can be suppressed compared to a case where the 2 nd charging potential of the 2 nd charging unit that is in contact with the charging target unit on the upstream side in the moving direction of the charging target unit from the 1 st charging unit is higher than the 1 st charging potential.

According to the above-described aspect 2, the number of the voltage applying units can be 1, compared to a case where the voltage applying units are independently provided in each of the 1 st charging unit and the 2 nd charging unit.

According to the above-mentioned aspect 3, the charging potential can be changed without adding a member, as compared with a case where the 2 nd charging unit is not formed to have a higher resistance than the 1 st charging unit.

According to the above-described aspect 4, the charging potential can be changed without adding any additional member, as compared with a case where the contact load of the 2 nd charging unit with respect to the charged unit is equal to the contact load of the 1 st charging unit with respect to the charged unit.

According to the above-described aspect 5, the setting of the applied voltage is easier than in the case where the 1 st charging unit and the 2 nd charging unit have a voltage applying unit that applies a voltage in common.

According to the 6 th aspect, setting of the charging potential of the charged unit becomes easier than in the case where the applied voltage of the voltage applying unit that applies the voltage to the 2 nd charging unit is not lower than the applied voltage of the voltage applying unit that applies the voltage to the 1 st charging unit.

According to the 7 th aspect, it is possible to suppress variation in the charging potential of the image holding unit, compared to a case where the charging device is not used as the charging unit.

Drawings

Fig. 1 is a schematic configuration diagram illustrating an image forming apparatus to which a charging device according to embodiment 1 of the present disclosure is applied.

Fig. 2 is a schematic configuration diagram illustrating an image forming apparatus of an image forming apparatus according to embodiment 1 of the present disclosure.

Fig. 3 is a configuration diagram showing a charging device according to embodiment 1 of the present disclosure.

Fig. 4 is a cross-sectional configuration diagram showing a charging state of the charging device.

Fig. 5 is a graph showing a relationship between an applied voltage of the charging roller and a charging potential of the photosensitive drum.

Fig. 6 is a graph showing a relationship between the charging potential generated based on the 2 nd charging roller and the charging potential generated based on the 1 st charging roller.

Fig. 7 is a graph showing a relationship between the charging potential based on the 2 nd charging roller and the charging potential based on the 1 st charging roller in the comparative example.

Fig. 8 is a graph showing a relationship between the position in the axial direction of the charging roller and the charging characteristic in the comparative example.

Fig. 9 is a cross-sectional configuration diagram illustrating a charging device according to embodiment 2 of the present disclosure.

Fig. 10 is a configuration diagram showing a main part of an image forming apparatus to which a charging device according to embodiment 3 of the present disclosure is applied.

Fig. 11 is a graph showing a relationship between an applied voltage of the charging roller and a charging potential of the photosensitive drum.

Fig. 12 is a configuration diagram illustrating a modification of the charging device according to embodiment 3 of the present disclosure.

Detailed Description

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[ embodiment 1]

Fig. 1 and 2 show an image forming apparatus to which a charging apparatus according to embodiment 1 is applied. Fig. 1 shows an overall outline of the image forming apparatus, and fig. 2 shows a main part (an image forming apparatus and the like) of the image forming apparatus in an enlarged manner.

< integral Structure of image Forming apparatus >

The image forming apparatus 1 according to embodiment 1 is configured as a monochrome printer, for example. As shown in fig. 1, the image forming apparatus 1 includes: an image forming apparatus 10 as an example of an image forming unit that forms a toner image developed with toner constituting a developer; a paper feeding device 20 as an example of a recording medium to be fed to a transfer position of the image forming apparatus 10, which accommodates and feeds the recording paper 5; a conveying device 30 that conveys the recording paper 5 fed from the paper feeding device 20 along a conveying path; and a fixing device 40 or the like that fixes the toner image on the recording paper 5 transferred by the image forming device 10.

The image forming apparatus 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: the charging device 12 of the present embodiment; which charges the circumferential surface (image holding surface) of the photosensitive drum 11 on which an image can be formed at a desired potential; an exposure device 13 that irradiates the charged circumferential surface of the photosensitive drum 11 with light based on image information (signal) to form an electrostatic latent image having a potential difference; a developing device 14 for developing the electrostatic latent image with a toner of a black developer to form a toner image; a transfer device 15 that transfers the toner image to the recording paper 5; a charge removing device 17 that removes charges from the image holding surface of the photoreceptor drum 11 after transfer; a drum cleaning device 16; this removes and cleans off adhering matter such as toner remaining and adhering on the image holding surface of the photoreceptor drum 11 after the charge is removed.

The photosensitive drum 11 is configured by forming an image holding surface having a photoconductive layer (photosensitive layer) made of a photosensitive material on the circumferential surface of a cylindrical or columnar base material subjected to grounding treatment. The photosensitive drum 11 is supported to rotate in the direction indicated by the arrow a by power transmission from a drive device not shown. In embodiment 1, the rotation speed (peripheral speed) of the photosensitive drum 11 is set to a relatively high speed of about 400 mm/sec.

The charging device 12 is constituted by a contact type charging roller disposed in contact with the photosensitive drum 11. The charging device 12 includes a 1 st charging roller 121 as an example of a 1 st charging unit, and a 2 nd charging roller 122 as an example of a 2 nd charging unit disposed on an upstream side of the 1 st charging roller 121 in the rotation direction a of the photosensitive drum 11. The charging device 12 is supplied with a charging voltage. As the charging voltage, when the developing device 14 is a device that performs reversal development, a voltage or a current having the same polarity as the charging polarity of the toner supplied from the developing device 14 is supplied. The charging device 12 will be described in detail later.

The exposure device 13 is constituted by an LED print head that forms an electrostatic latent image by irradiating the photosensitive drum 11 with light corresponding to image information using an LED (light Emitting diode) as a plurality of light Emitting elements arranged along the axial direction of the photosensitive drum 11. As the exposure device 13, a device that performs deflection scanning of laser light configured according to image information along the axial direction of the photosensitive drum 11 may be used.

As shown in fig. 2, the developing device 14 is configured by arranging the following members and the like inside a housing 140 in which an opening 140a and a developer accommodating chamber are formed: a developing roller 141 as an example of a developer holding unit that holds the developer 4 and conveys the developer 4 to a developing area facing the photosensitive drum 11; 2 agitating and conveying members 142, 143 such as propellers and the like that convey the developer 4 while agitating the developer 4 so that the developer 4 passes through the developing roller 141; a layer thickness regulating member (not shown) for regulating the amount (layer thickness) of the developer held by the developing roller 141; and a supply roller 144 that supplies the developer collected from the outer peripheral surface of the developing roller 141 to the stirring and conveying member 143. In the developing device 14, a developing bias voltage is supplied from a power supply device, not shown, between the developing roller 141 and the photosensitive drum 11. The developing roller 141, the stirring and conveying members 142, 143, and the supply roller 144 are rotated in a desired direction by power transmission from a driving device not shown. Further, as the developer, a two-component developer including a nonmagnetic toner and a magnetic carrier is used.

As shown in fig. 1, the transfer device 15 is a contact type transfer device including a transfer roller 151 that rotates in contact with the periphery of the photosensitive drum 11 via the recording paper 5 at the time of 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 driving roller 153 and a driven roller 154. A direct current voltage having a polarity opposite to the charging polarity of the toner is supplied as a transfer voltage from a power supply device not shown.

The static elimination device 17 is composed of a corona or the like, and applies a charge having a polarity opposite to the charging polarity of the charging device 12 to the image holding surface of the photoreceptor drum 11 after transfer to remove the static.

As shown in fig. 2, the drum cleaning device 16 is constituted by the following members and the like: a container-like body 160 having a part opened; a cleaning plate 161 configured to be brought into contact with the circumferential surface of the photoreceptor drum 11 after transfer at a required pressure, and to remove and clean adhering matter such as residual toner; a cleaning brush 162 which is also disposed so as to contact the circumferential surface of the photosensitive drum 11 with a required pressure and which removes and cleans off deposits such as residual toner; and a feeding member 163 such as a screw for conveying the deposits such as the toner removed by the cleaning plate 161 and the cleaning brush 162, collecting them, and feeding them to a collection container, not shown. As the cleaning plate 161, a plate-shaped member (for example, a blade) made of a material such as rubber is used.

As shown in fig. 1, the fixing device 40 is configured by arranging the following members and the like inside a device case 43 in which an inlet and an outlet for the recording paper 5 are formed: a heating roller 41 as an example of a rotating body for heating (fixing unit) that rotates in the direction indicated by the arrow and is heated by the heating unit so that the surface temperature is maintained at a predetermined temperature; and a pressure roller 42 as an example of a rotating member for pressing, which rotates in contact with the heating roller 41 with a predetermined pressure in a state substantially along the axial direction of the heating roller. In the fixing device 40, a contact portion where the hot roller 41 and the pressure roller 42 contact each other serves as a fixing nip portion for performing a required fixing process (heating and pressing).

The paper feeding device 20 is disposed below the device main body 1 a. The sheet feeding device 20 mainly includes: a plurality of (or a single) sheet containers 22 that store recording sheets 5 of a desired size, type, and the like in a state of being loaded on the loading plate 21; and a feeding device 23 that feeds out the recording paper 5 one by one from the paper storage 22. The paper feeding device 20 is detachable from the apparatus main body 1a of the image forming apparatus 1 by gripping a gripping portion, not shown, provided in the paper accommodating body 22 with a hand and drawing out the gripping portion.

Examples of the recording paper 5 include plain paper, thin paper such as tracing paper, and OHP sheets used in copying machines and printers of an electrophotographic system. In order to further improve the smoothness of the image surface after fixing, the surface of the recording paper 5 is preferably as smooth as possible, and for example, a relatively heavy so-called thick paper such as coated paper obtained by coating the surface of plain paper with a resin or the like, art paper for printing, or the like can be suitably used.

As shown in fig. 1, between the paper feed device 20 and the transfer device 15, a paper feed conveyance path 33, which is formed of a plurality of (or single) paper conveyance roller pairs 31a to 31f or a conveyance guide 32 that conveys the recording paper 5 fed out from the paper feed device 20 to a transfer position, is provided in a shape curved toward the inside of the device main body 1a from the midway toward the upper side along the vertical direction of the device main body 1 a. The paper transport roller pair 31f arranged in the paper feed transport path 33 at a position immediately before the transfer position is configured as a roller (registration roller) that adjusts the transport timing of the recording paper 5, for example. Further, from the transfer device 15 to the fixing device 40, the recording paper 5 after transfer is conveyed to the fixing device 40 by the transfer belt 152 of the transfer device 15.

Further, above the discharge port of the fixing device 40, a discharge transport path 37 is provided for transporting the recording paper to the paper discharge portion 36 via the plurality of sets of transport roller pairs 34a to 34d and the transport guide 35 and discharging the recording paper. The paper ejection section 36 is provided on the upper end surface of the apparatus main body 1 a.

Further, below the discharge port of the fixing device 40, a duplex conveyance path 39 is provided that conveys the recording paper 5 to the paper feed conveyance path 33 via a plurality of sets of conveyance roller pairs 38a to 38e and a conveyance guide 38 f.

The recording paper 5 discharged from the fixing device 40 is once conveyed to the discharge conveyance path 37 by a switching gate not shown, and then the conveyance roller pairs 34a and 34b are rotated in the reverse direction, thereby reversing the front and back and conveying the recording paper to the duplex conveyance path 39.

Note that reference numeral 200 in fig. 1 denotes a control device that collectively controls the operation of the image forming apparatus 1. The control device 200 includes a CPU (central Processing unit), a ROM (read Only memory), a ram (random Access memory), a bus connecting the CPU and the ROM, a communication interface, and the like, which are not shown. Reference numerals 201 and 202 denote an image reading apparatus and a document conveying apparatus, respectively, which are disposed above the apparatus main body 1a of the image forming apparatus 1.

< basic operation of image Forming apparatus >

The following describes a basic image forming operation performed by the image forming apparatus 1.

The image forming apparatus 1 is controlled by the control device 200, and when command information for requesting a monochromatic image forming operation (printing) is received from an operation panel, a user interface, a printer driver, and the like, not shown, mounted on the apparatus main body 1a, the image forming apparatus 10, the paper feeding device 20, the conveying device 30, the fixing device 40, and the like are started up.

Then, in the image forming apparatus 10, as shown in fig. 1, first, the photosensitive drum 11 is rotated in the direction indicated by the arrow a, and the charging device 12 charges the surface of the photosensitive drum 11 with a desired polarity (negative polarity in embodiment 1) and potential. Next, the exposure device 13 irradiates the charged surface of the photosensitive drum 11 with light based on information of an image inputted to the image forming apparatus 1, and forms an electrostatic latent image having a desired potential difference on the surface.

Next, the developing device 14 supplies black toner charged with a required polarity (negative polarity) from the developing roller 141 to the electrostatic latent image formed on the photosensitive drum 11, and electrostatically adheres the toner to develop the electrostatic latent image. By this development, the electrostatic latent image formed on the photosensitive drum 11 is visualized as a toner image developed with black toner.

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

In the image forming apparatus 10 after the end of the transfer, after the residual charge on the surface of the photosensitive drum 11 is removed by the charge removal device 17, the drum cleaning device 16 removes the adhering matter by scraping and cleans the surface of the photosensitive drum 11. Thereby, the imaging apparatus 10 is in a state in which the next imaging operation can be performed.

The paper feed device 20 feeds out a desired recording paper 5 to the paper feed conveyance path 33 in cooperation with the image forming operation. In the paper feed conveyance path 33, a paper conveyance roller pair 31f as registration rollers feeds out the recording paper 5 to the transfer position in accordance with the transfer timing.

Next, the recording paper 5 to which the toner image is transferred is conveyed to the fixing device 40 by the transfer belt 152. In the fixing device 40, the transferred recording paper 5 is introduced into and passed through a fixing nip between the rotating heating roller 41 and the pressure roller 42, and thereby a desired fixing process (heating and pressing) is performed to fix the unfixed toner image to the recording paper 5. When the recording paper 5 after the fixing is completed performs an image forming operation for forming an image only on one side thereof, the recording paper is discharged by the discharge roller pair 34d along the discharge conveyance path 37 to the paper discharge portion 36 provided at the upper end portion of the apparatus main body 1 a.

When images are formed on both sides of the recording paper 5, the recording paper 5 on one side of which an image is formed is conveyed to the conveying roller pairs 34a and 34b by a switching gate not shown, and the recording paper 5 is temporarily conveyed in the discharge direction by the conveying roller pairs 34a and 34 b. Then, the rotation direction of the transport roller pairs 34a and 34b is reversed with the transport roller pairs 34a and 34b sandwiching the rear end of the recording paper 5, and after the front and back of the recording paper 5 are reversed, the transport roller pairs 38a to 38e transport the recording paper to the transfer device 15 again through the duplex transport path 39, and the toner image is transferred to the back surface of the recording paper 5. The recording paper 5 having the toner image transferred on the back surface thereof is conveyed to the fixing device 40 via the transfer belt 152 of the transfer device 15, subjected to fixing treatment (heating and pressing) by the fixing device 40, and discharged to the paper discharge portion 36 by the conveying roller pairs 34a to 34 d.

By the above operation, the recording paper 5 on which the monochrome image is formed on one side or both sides is output.

< Structure of charged device >

Fig. 2 is a configuration diagram showing the charging device according to embodiment 1.

As shown in fig. 2, the charging device 12 according to embodiment 1 includes: a 1 st charging roller 121 disposed in contact with the photosensitive drum 11 as an example of a unit to be charged, and configured to charge the photosensitive drum 11 at a 1 st charging potential determined by an applied voltage and a discharge start voltage; and a 2 nd charging roller 122 disposed in contact with the photosensitive drum 11 on an upstream side in the rotational direction a of the photosensitive drum 11 from the 1 st charging roller 121 so as to charge the photosensitive drum 11 at a 2 nd charging potential determined by the applied voltage and the discharge start voltage, wherein the 2 nd charging potential is lower than the 1 st charging potential.

The 1 st charging roller 121 and the 2 nd charging roller 122 are basically configured similarly. As shown in fig. 3, the 1 st charging roller 121 and the 2 nd charging roller 122 are formed in a cylindrical shape, and include columnar mandrels 123 and 124 made of metal such as stainless steel or iron, semiconductive elastomer layers 125 and 126 covering the outer peripheries of the mandrels 123 and 124 with a desired thickness and having conductivity, and surface layers 127 and 128 covering the surfaces of the elastomer layers 125 and 126 with a small thickness. The mandrels 123, 124 also serve as rotating shafts provided to protrude from both end portions of the 1 st charging roller 121 and the 2 nd charging roller 122 in the axial direction. The 1 st charging roller 121 and the 2 nd charging roller 122 are pressed against the outer peripheral surface of the photosensitive drum 11 via the mandrels 123 and 124 by biasing means such as coil springs, not shown, and are rotated in accordance with the rotation of the photosensitive drum 11 in a state in which the elastic layers 125 and 126 and the surface layers 127 and 128 are elastically deformed so as to have a required nip width.

As shown in fig. 2, the mandrels 123 and 124 of the 1 st charging roller 121 and the 2 nd charging roller 122 are connected to a high-voltage power supply device 129 (an example of a voltage applying means) via a bearing member (not shown). The control device 200 controls the value of the high voltage applied to the charging rollers 121 and 122 by the high-voltage power supply device 129 and the application timing of the voltage. The high-voltage power supply 129 supplies a predetermined dc high voltage or high current having a negative polarity, which is the same polarity as the charge polarity of the toner supplied from the developing device 14.

The elastic body layers 125 and 126 of the 1 st charging roller 121 and the 2 nd charging roller 122 are formed of, for example, a porous foam having cavities and irregularities in the inner and surface thereof. The elastomer layers 125 and 126 are configured such that a resistance adjusting agent such as carbon black or an ionic conductive agent is dispersed in a material having a desired resistance value, such as a foamable resin such as polyurethane, polyethylene, polyamide, olefin, melamine, or polypropyleneA material, or a foamable 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, or the like. The volume resistivity of the elastomer layers 125 and 126 is set to 10, for example⁴～10⁸Range of Ω cm. The elastomer layers 125 and 126 may be an unfoamed solid rubber.

The surface layers 127 and 128 of the 1 st charging roller 121 and the 2 nd charging roller 122 are provided by coating the outer peripheral surfaces of the elastomer layers 125 and 126 with a particulate filler dispersed therein, for example. The surface layers 127 and 128 of the charging rollers 121 and 122 may be formed by, for example, dispersing a conductive material in a tubular body such as Polytetrafluoroethylene (PTFE) or Perfluoroalkoxyalkane (PFA), and covering the outer peripheral surfaces of the elastic layers 125 and 126 with the conductive material.

The 1 st charging roller 121 and the 2 nd charging roller 122 may not have the surface layers 127 and 128 other than the elastic layers 125 and 126.

However, in the charging device 12 of embodiment 1, the charging potential (2 nd charging potential) of the photosensitive drum 11 by the 2 nd charging roller 122 determined by the applied voltage and the discharge start voltage is configured to be lower than the charging potential (1 st charging potential) of the photosensitive drum 11 by the 1 st charging roller 121.

In other words, in the charging device 12 according to embodiment 1, the 1 st charging roller 121 located on the downstream side in the rotational direction of the photosensitive drum 11 is configured such that the charging potential of the photosensitive drum 11 determined by the applied voltage and the discharge start voltage becomes higher than the 2 nd charging roller 122 located on the upstream side in the rotational direction of the photosensitive drum 11.

As shown in fig. 4, the 1 st charging roller 121 and the 2 nd charging roller 122 are pressed against the outer peripheral surface of the photosensitive drum 11 via the mandrels 123 and 124 by biasing means such as coil springs, not shown, and thereby are elastically deformed so that the elastic layers 125 and 126 and the surface layers 127 and 128 have a required nip width. Then, the 1 st charging roller 121 and the 2 nd charging roller 122 apply a predetermined high voltage of a negative dc voltage to the mandrels 123 and 124 by the high-voltage power supply device 129, thereby generating a fine gap discharge in the discharge region D formed between the 1 st charging roller 121 and the 2 nd charging roller 122 and the outer peripheral surface of the photosensitive drum 11, and charging the outer peripheral surface of the photosensitive drum 11 with a desired negative charging potential by the charged particles generated by the fine gap discharge. In fig. 4, reference numeral 111 denotes a conductive base of the photosensitive drum 11, and reference numeral 112 denotes a photosensitive layer of the photosensitive drum 11. The conductive base 111 of the photosensitive drum 11 is grounded.

The 1 st charging roller 121 and the 2 nd charging roller 122 have the following relationship between the dc high voltage applied to the cored bars 123 and 124 of the 1 st charging roller 121 and the 2 nd charging roller 122 and the charging potential of the outer peripheral surface of the photosensitive drum 11.

The charging potential of the photosensitive drum 11 generated by the 1 st charging roller 121 and the 2 nd charging roller 122 is determined by the application voltage and the discharge start voltage. It is known that the discharge start voltage follows paschen's law. Now, the charging potential of the photosensitive drum 11 by the 1 st charging roller 121 is Vh1, the charging potential of the photosensitive drum 11 by the 2 nd charging roller 122 is Vh2, the application voltage of the 1 st charging roller 121 is Vdc1, the application voltage of the 2 nd charging roller 122 is Vdc2, the discharge start voltage of the 1 st charging roller 121 is V α 1, and the discharge start voltage of the 2 nd charging roller 122 is V α 2.

As shown in fig. 5, the 1 st charging potential Vh1 and the 2 nd charging potential Vh2 of the photosensitive drum 11 generated by the 1 st charging roller 121 and the 2 nd charging roller 122 are values obtained by subtracting the discharge start voltages V α 1 and V α 2 from the applied voltages Vdc1 and Vdc2, and can be expressed by the following formula. In addition, the gradient of the straight line is 1.

Vh1＝Vdc1-Vα1

Vh2＝Vdc2-Vα2

In embodiment 1, the volume resistivity of the elastomer layer 126 in the 2 nd charging roller 122 is set to be larger than the volume resistivity of the elastomer layer 125 in the 1 st charging roller 121. That is, the amount of the resistance adjusting agent such as carbon black or an ionic conductive agent dispersed in the elastomer layer 126 of the 2 nd charging roller 122 is set to be smaller than that of the resistance adjusting agent in the elastomer layer 125 of the 1 st charging roller 121, and the volume resistivity is set to be larger than that of the elastomer layer 125 of the 1 st charging roller 121. Therefore, even when the applied voltage Vdc2 (Vdc 1) equal to that of the 1 st charging roller 121 is applied to the 2 nd charging roller 122, the discharge start voltage V α 2 at which the discharge is started to start the rise of the 2 nd charging potential Vh2 of the photosensitive drum 11 becomes a value higher than the discharge start voltage V α 1 of the 1 st charging roller 121 (V α 2 > V α 1).

As a result, in the charging device 12 of embodiment 1, when the same high-voltage power supply device 129 applies an equal application voltage (Vdc1 ═ Vdc2) to the 1 st charging roller 121 and the 2 nd charging roller 122, the 2 nd charging potential Vh2 of the photosensitive drum 11 by the 2 nd charging roller 122 determined by the application voltage and the discharge start voltage becomes a value lower than the 1 st charging potential Vh1 by the 1 st charging roller 121 (Vh1 > Vh 2). That is, the 2 nd charging potential Vh2 generated by the 2 nd charging roller 122 determined by the application voltage and the discharge start voltage is set to a value lower than the 1 st charging potential Vh1 that is the target charging potential V β of the photosensitive drum 11 achieved by the charging device 12.

< action of charged device >

In the image forming apparatus to which the charging device of embodiment 1 is applied, as shown in fig. 2, at the start of an image forming operation, the outer peripheral surface of the photosensitive drum 11 is charged at a predetermined charging potential by the charging device 12.

In the charging device 12, as shown in fig. 2, first, the outer peripheral surface of the photosensitive drum 11 is charged by the 2 nd charging roller 122. At this time, the applied voltage Vdc2 is applied to the 2 nd charging roller 122 by the high-voltage power supply device 129. As shown in fig. 5, the 2 nd charging roller 122 charges the outer peripheral surface of the photosensitive drum 11 so as to have a 2 nd charging potential Vh2 determined by the applied voltage Vdc2 and the discharge start voltage V α 2.

Thereafter, in the charging device 12, the outer peripheral surface of the photosensitive drum 11 is charged by the 1 st charging roller 121. At this time, an applied voltage Vdc1 equal to that of the 2 nd charging roller 122 is applied to the 1 st charging roller 121 by the high-voltage power supply device 129. As shown in fig. 5, the 1 st charging roller 121 charges the outer peripheral surface of the photosensitive drum 11 to a 1 st charging potential Vh1 determined by the applied voltage Vdc1 and the discharge start voltage V α 1.

Therefore, in the charging device 12 of embodiment 1 described above, as shown in fig. 6, the 2 nd charging potential Vh2, which is the charging potential of the 2 nd charging roller 122 located on the upstream side in the rotation direction of the photosensitive drum 11, is set to a value lower than the target charging potential V β of the photosensitive drum 11 achieved by the charging device 12.

Therefore, in the charging device 12 according to embodiment 1, the surface of the photosensitive drum 11 is charged at the 2 nd charging potential Vh2 by the 2 nd charging roller 122 located on the upstream side in the rotational direction of the photosensitive drum 11, and thereafter, the surface of the photosensitive drum 11 is charged at the 1 st charging potential Vh1 equal to the target charging potential V β by the 1 st charging roller 121 located on the downstream side in the rotational direction of the photosensitive drum 11, whereby a minute gap discharge can be generated between the 1 st charging roller 121 and the surface of the photosensitive drum 11 and the surface can be charged at the 1 st charging potential Vh 1.

At this time, since the surface potential of the photosensitive drum 11 becomes the 2 nd charging potential Vh2 lower than the target charging potential V β, so-called charge injection in which charges directly move from the 1 st charging roller 121 to the surface of the photosensitive drum 11 when charged by the 1 st charging roller 121 can be suppressed.

As described above, in the charging device 12 according to embodiment 1, the variation in the charging potential of the photosensitive drum 11 is suppressed compared to the case where the relationship of the charging potential determined by the applied voltage and the discharge start voltage is not considered in the 1 st charging roller 121 and the 2 nd charging roller 122.

Further, since the charging device 12 includes a plurality of charging units including the 1 st charging roller 121 and the 2 nd charging roller 122, the charging capability can be improved as compared with a case where the photosensitive drum 11 is charged by a single charging unit. Therefore, even when the rotation speed of the photosensitive drum 11 is set to 400 mm/sec at a significantly high speed and the number of recording sheets 5 on which images can be formed per unit time is increased, the outer peripheral surface of the photosensitive drum 11 can be uniformly charged at a desired potential, and the occurrence of a charged ghost can be suppressed.

Comparative example

On the other hand, as comparative examples, the present inventors conducted the following comparative tests: the same charging roller having the same applied voltage as the discharge start voltage is used as the 1 st charging roller 121 and the 2 nd charging roller 122, and the surface of the photosensitive drum 11 is charged at the target charging potential V β by the 1 st charging roller 121 and the 2 nd charging roller 122. At this time, 3 kinds of charging rollers I to III having the same configuration and different in batches were used as the 1 st charging roller 121.

Fig. 7 is a graph showing the results of the comparative example. In the graph, the horizontal axis represents the charging potential of the photosensitive drum 11 by the 2 nd charging roller 122, and the vertical axis represents the charging potential of the photosensitive drum 11 by the 1 st charging roller 121.

As can be seen from fig. 7, when the same charging roller having the same applied voltage and the same discharge start voltage is used as the 1 st charging roller 121 and the 2 nd charging roller 122, and the surface of the photosensitive drum 11 is charged by the 1 st charging roller 121 and the 2 nd charging roller 122 at the target charging potential, it is known that the charging potential of the photosensitive drum 11 charged by the 1 st charging roller 121 greatly varies depending on the characteristics I to III (lot-to-lot) of the 1 st charging roller 121.

According to the examination by the present inventors, the reason is considered as follows: as shown in fig. 8, when the same charging roller having the same applied voltage and the same discharge start voltage is used as the 1 st charging roller 121 and the 2 nd charging roller 122 and the surface of the photosensitive drum 11 is charged at the target charging potential by the 1 st charging roller 121 and the 2 nd charging roller 122, an injection phenomenon of charges occurs at an end portion of the 1 st charging roller 121 in the axial direction, and a variation in the charging potential in the axial direction due to the 2 nd charging roller 122 before that is expanded.

To explain further, when the same charging roller having the same applied voltage and the same discharge start voltage is used as the 1 st charging roller 121 and the 2 nd charging roller 122, and the surface of the photosensitive drum 11 is charged at the target charging potential by the 1 st charging roller 121 and the 2 nd charging roller 122, as shown in fig. 8, a discharge phenomenon occurs in the center portion of the 1 st charging roller 121, and the charging potential of the photosensitive drum 11 is raised by the discharge phenomenon, but the charging potential of the photosensitive drum 11 is already raised to a potential having a negative polarity as the target potential by the 2 nd charging roller 122 at both end portions of the 1 st charging roller 121 in the axial direction, at which the nip pressure tends to rise compared with the center portion, and therefore, it is considered that the charge injection phenomenon by the 1 st charging roller 121 is caused.

[ embodiment 2]

Fig. 9 shows a charging device according to embodiment 2. In the charging device 12 according to embodiment 2, the contact load of the 2 nd charging unit with respect to the charged object is set to be lower than the contact load of the 1 st charging unit.

That is, in the charging device 12 of embodiment 2, as shown in fig. 9, the contact load of the 2 nd charging roller 122 with the photosensitive drum 11 is set to be lower than the contact load of the 1 st charging roller 121. Specifically, the spring constant of the biasing means such as a coil spring not shown that presses the 2 nd charging roller 122 against the outer peripheral surface of the photosensitive drum 11 is set small, or the amount of deformation of the biasing means such as a coil spring not shown is set small.

In this way, the contact load of the 2 nd charging roller 122 with the photosensitive drum 11 is set low, which corresponds to an increase in the distance between the electrodes of the 2 nd charging roller 122 and the photosensitive drum 11, and as can be understood from paschen's law similarly to the case shown in fig. 5, the discharge start voltage V α 2 is higher than the discharge start voltage of the 1 st charging roller 121.

Therefore, the 2 nd charging roller 122 is configured such that the charging potential of the photosensitive drum 11 determined by the applied voltage and the discharge start voltage is lower than the charging potential of the photosensitive drum 11 generated by the 1 st charging roller 121.

In the charging device 12 according to embodiment 2, the same charging roller can be used as the 1 st charging roller 121 and the 2 nd charging roller 122, and an increase in cost can be avoided.

Other structures and operations are the same as those of the above-described embodiment, and therefore, the description thereof is omitted.

[ embodiment 3]

Fig. 10 shows a charging device according to embodiment 3. In the charging device 12 according to embodiment 3, the 1 st charging roller 121 and the 2 nd charging roller 122 are configured to have voltage applying means independently of each other.

In embodiment 3, the voltage applied by the 2 nd voltage application means for applying a voltage to the 2 nd charging roller 122 is set to be lower than the voltage applied by the 1 st voltage application means for applying a voltage to the 1 st charging roller. In embodiment 3, the same charging roller is used as the 1 st charging roller 121 and the 2 nd charging roller 122.

That is, as shown in fig. 10, the charging device 12 according to embodiment 3 includes: a 1 st high-voltage power supply device 129a as a 1 st voltage applying unit that applies a voltage to the 1 st charging roller 121, and a 2 nd high-voltage power supply device 129b as a 2 nd voltage applying unit that applies a voltage to the 2 nd charging roller 122. The applied voltage of the 2 nd high-voltage power supply device 129b is set to be lower than the applied voltage of the 1 st high-voltage power supply device 129a (Vdc1 > Vdc 2).

In the charging device 12 according to embodiment 3, the voltages applied by the 1 st high-voltage power supply device 129a and the 2 nd high-voltage power supply device 129b are set as follows:

Vh1＝Vdc1-Vα1

Vh2＝Vdc2-Vα2

wherein, the relation of Vdc1 > Vdc2 is satisfied.

Therefore, in the charging device 12 according to embodiment 3, as shown in fig. 11, the charging potential Vh2 of the photosensitive drum 11 generated by the 2 nd charging roller 122 is lower than the charging potential Vh1 of the photosensitive drum 11 generated by the 1 st charging roller 121.

In the charging device 12 according to embodiment 3, the same charging roller can be used as the 1 st charging roller 121 and the 2 nd charging roller 122, and cost reduction can be achieved compared to a case where different charging rollers are used as the 1 st charging roller 121 and the 2 nd charging roller 122.

In addition, although the above-described embodiment 3 has been described with respect to the case where the 1 st high-voltage power supply device 129a and the 2 nd high-voltage power supply device 129b are provided independently, as shown in fig. 12, the 1 st high-voltage power supply device 129a and the 2 nd high-voltage power supply device 129b 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 to the 2 nd charging roller 122 via the resistor R.

With such a configuration, the voltage applied to the 2 nd charging roller 122 can be set to a value lower than the voltage applied to the 1 st charging roller 121 in a state where the high-voltage power supply devices 129 are shared.

Other structures and operations are the same as those of the above-described embodiment, and therefore, the description thereof is omitted.

In the above-described embodiment, although the monochrome image forming apparatus is described as the image forming apparatus, it is needless to say that the present invention can be similarly applied to a full-color image forming apparatus having each of the image forming apparatuses 10(Y, M, C, K) of yellow (Y), magenta (M), blue (C), and black (K).

In the above embodiment, the case of 2 charging members including the 1 st charging roller and the 2 nd charging roller was described, but 3 or more charging members may be provided. In this case, the charging unit disposed on the upstream side in the moving direction of the charged unit has a lower charging potential than the charging unit disposed on the downstream side.