Image forming device

An image forming device includes: a developer holder that is rotatably arranged opposing a latent image holder holding an electrostatic latent image, holds developer including carrier and toner charged to a predetermined regular polarity, has a potential of the same polarity as the regular polarity applied thereto, and develops the electrostatic latent image using the toner; an opposite electrode that is arranged opposing the developer holder with a predetermined gap therebetween; and a controller that changes at least one of a potential applied to the opposite electrode and a rotation direction of the opposite electrode in accordance with an area coverage of the electrostatic latent image of the latent image holder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2019-066711 filed Mar. 29, 2019.

BACKGROUND

(i) Technical Field

The present disclosure relates to an image forming device.

(ii) Related Art

There is a known image forming device that has: an image carrier that has a photosensitive layer; a charging unit that abuts and charges the image carrier to a predetermined potential; an exposing unit that forms an electrostatic latent image on a charged surface of the image carrier; a developing unit that visualizes the electrostatic latent image as a toner image and is provided with a developer carrier arranged so as to oppose the image carrier and a supply unit that supplies developer to the developer carrier; and a transfer unit that abuts the image carrier and transfers the formed toner image to a transferred material. The image forming device includes a reversed polarity toner recovery process in which, after image forming has been completed, a voltage applied to the transfer unit is made to have the reverse polarity compared to when image forming is carried out, a voltage applied to the charging unit is made to have the same polarity and to be less than the absolute value compared to when image forming is carried out, a voltage that is less than or equal to a surface potential of the image carrier is applied to the developer carrier of the developing unit, and a voltage with which a potential difference with the voltage applied to the developer carrier decreases compared to during printing is applied to the supply unit (Japanese Unexamined Patent Application Publication No. 2004-191766).

There is also a known developing device that is provided with: a developer carrier that faces an image carrier on which an electrostatic latent image is carried, can be rotated, and carries developer on a surface thereof; a charging member that comes into contact with or close to the developer on the developer carrier and regulates charging of the developer; and a bias power source that applies a predetermined bias to the charging member. The bias power source is provided with: a charging bias applying unit that applies a charging bias to between the charging member and the developer carrier and charges the developer on the developer carrier when image forming is carried out; a discharging bias applying unit that applies a discharging bias to between the charging member and the developer carrier and discharges the developer that has adhered to the charging member when image forming is not carried out; and a bias switching unit that switches a connection between the charging bias applying unit and the discharging bias applying unit (Japanese Unexamined Patent Application Publication No. 2007-86361).

SUMMARY

Aspects of non-limiting embodiments of the present disclosure relate to suppressing image quality defects caused by toner deterioration and toner charge fluctuation during continuous running.

According to an aspect of the present disclosure, there is provided an image forming device provided with: a developer holder that is rotatably arranged opposing a latent image holder holding an electrostatic latent image, holds developer including carrier and toner charged to a predetermined regular polarity, has a potential of the same polarity as the regular polarity applied thereto, and develops the electrostatic latent image using the toner; an opposite electrode that is arranged opposing the developer holder with a predetermined gap therebetween; and a controller that changes at least one of a potential applied to the opposite electrode and a rotation direction of the opposite electrode in accordance with an area coverage of the electrostatic latent image of the latent image holder.

DETAILED DESCRIPTION

Next, with reference to the drawings, exemplary embodiments and specific examples will be given hereinafter for the present disclosure to be described in greater detail; however, the present disclosure is not restricted to these exemplary embodiments and specific examples.

Furthermore, in the description using the drawings hereinafter, please be aware that the drawings are schematic and the ratios of the dimensions and so forth are different from those in reality, and members other than those required for the description are not depicted as appropriate to aid understanding.

First Exemplary Embodiment

(1) Overall Configuration and Operation of Image Forming Device

(1.1) Overall Configuration of Image Forming Device

FIG. 1is a cross-sectional schematic view depicting an example of a schematic configuration of an image forming device1according to the present exemplary embodiment.

The image forming device1is configured including an image forming unit10, a paper supply device20mounted at one end of the image processing unit10, a paper recovery unit30mounted at the other end of the image processing unit10and in which printed paper is recovered; an operation display unit40, and an image processing unit50that generates image information from printing information transmitted from a higher-level device.

The image forming unit10is configured including a system control device11, exposure devices12, photoconductor units13, developing devices14, a transfer device15, a paper conveying device16, a fixing device17, a driving device18(not depicted; seeFIG. 9), and a power source device19(not depicted; seeFIG. 9), and forms image information received from the image processing unit50as a toner image on continuous paper S that is fed from the paper supply device20.

The paper supply device20has a paper feeding member20athat is rotatably supported and has the continuous paper S wound therearound in the form of a roll, and is configured so as to supply the continuous paper S to the image forming unit10while tensioning the continuous paper S.

The paper recovery unit30recovers the continuous paper S on which image forming has been carried out by the image forming unit10, by winding in the continuous paper S using a rotationally driven winding roll30a.

The operation display unit40is used for inputting various types of settings and instructions and displaying information. In other words, the operation display unit40corresponds to a user interface so to speak, and, to be specific, is configured by combining a liquid crystal display panel, various types of operation buttons, a touch panel, or the like.

(1.2) Configuration and Operation of Image Forming Unit

In the image forming device1having this kind of configuration, continuous paper S extending from the paper feeding member20aof the paper supply device20is conveyed to the image forming unit10in accordance with an image forming timing.

The photoconductor units13are respectively disposed in parallel below the exposure devices12and provided with photoconductor drums31serving as rotationally driven latent image holders. A charger32, an exposure device12, a developing device14, a first transfer roller52, and a cleaning device34are arranged around each photoconductor drum31in the rotation direction thereof.

In the developing devices14, developing rollers42serving as developer holders are arranged opposing the photoconductor drums31. The developing devices14are configured in substantially the same manner except for the developer, and form toner images of yellow (Y), magenta (M), cyan (C), and black (B) on the photoconductor drums31using the respective developing rollers42.

Exchangeable toner cartridges TC that house developer and developer supply devices43that supply developer from the respective toner cartridges TC to the developing devices14are arranged above the developing devices14.

The surface of the photoconductor drums31, which rotate, are charged by the chargers32, and electrostatic latent images are formed by latent image-forming light emitted from the exposure devices12. The electrostatic latent images formed on the photoconductor drums31are developed as toner images by the developing rollers42.

The transfer device15is configured from: an intermediate transfer belt51serving as an example of an image holder with which multiple transfer of the toner images formed by the photoconductor drums31of the photoconductor units13is carried out; the first transfer rollers52that sequentially transfer the toner images formed by the photoconductor units13to the intermediate transfer belt51(first transfer); and a second transfer belt53serving as an example of a transfer member that carries out batch transfer of the toner images superposed and transferred on the intermediate transfer belt51to paper, which is a recording medium (second transfer).

The second transfer belt53is stretched by a second transfer roller54and a separation roller55and is held between a backup roller65and the second transfer roller54to form a second transfer part TR, the backup roller65being arranged at the rear surface side of the intermediate transfer belt51.

The toner images formed on the photoconductor drums31of the photoconductor units13are sequentially electrostatically transferred (first transfer) onto the intermediate transfer belt51by the first transfer rollers52to which a predetermined transfer voltage is applied from a power source device or the like (not depicted) controlled by the system control device11, and superposed toner images in which the toner images are superposed are formed.

The superposed toner images on the intermediate transfer belt51are conveyed to the region in which the second transfer belt53is arranged (second transfer part TR) due to the movement of the intermediate transfer belt51. The continuous paper S is supplied to the second transfer part TR from the paper supply device20in accordance with the timing at which the superposed toner images are conveyed to the second transfer part TR. Then, the transfer voltage is applied to the backup roller65that opposes the second transfer roller54with the second transfer belt53interposed, and the superposed toner images on the intermediate transfer belt51are batch-transferred onto the continuous paper S.

Residual toner on the surfaces of the photoconductor drums31is removed by the cleaning devices34and recovered to a waste toner housing unit (not depicted). The surfaces of the photoconductor drums31are recharged by the chargers32.

The fixing device17has an endless fixing belt17athat rotates in one direction and a pressure roller17bthat comes into contact with the peripheral surface of the fixing belt17aand rotates in one direction, and a nip part (fixing region) is formed by a pressure contact region between the fixing belt17aand the pressure roller17b.

The continuous paper S on which the toner images are transferred in the transfer device15is conveyed to the fixing device17via the paper conveying device16with the toner images in a non-fixed state. The toner images are fixed to the continuous paper S conveyed to the fixing device17, by the pair of the fixing belt17aand the pressure roller17bdue to a heating and pressure-attaching action.

The continuous paper S for which fixing has been completed is fed to the paper recovery unit30. The continuous paper S fed to the paper recovery unit30is wound in by the winding roll30awhile being tensioned.

(2) Configuration of Main Parts

FIG. 2is a cross-sectional schematic view depicting the photoconductor unit13and the developing device14, andFIG. 3is a partial cross-sectional view of the developing device14depicting the developing roller42and an opposite electrode46.

Hereinafter, a configuration and operation of the developing device14will be described with reference to the drawings.

(2.1) Overall Configuration of Developing Device

The developing device14is provided with: a developing housing41that houses developer; the developing roller42arranged opposing the photoconductor drum31; a first stirring auger44A that conveys toner supplied from the developer supply device43, while stirring the toner so to be mixed with the developer; a supply auger44B that supplies the developer to the developing roller42; a second stirring auger44C that stirs the developer that has separated from the developing roller42; a layer regulating member45that trims the developer on the developing roller42to a predetermined developer layer thickness; and the opposite electrode46that adsorbs and recovers a toner cloud generated at a site where the developing roller42and the photoconductor drum31oppose each other, and also adsorbs and recovers toner by way of the developing action from the developer held on the outer periphery of the developing roller42.

(2.2) Developing Roller

The developing roller42is arranged opposing the outer peripheral surface of the photoconductor drum31through an opening41aformed in the developing housing41, as depicted inFIG. 2. Furthermore, the developing roller42is provided with a cylindrical developing sleeve42A that is rotatably supported with respect to the developing housing41, and a magnet42B that is a columnar magnet member provided in the space inside the developing sleeve42A and fixed to the developing housing41.

The developing sleeve42A is configured in such a way that the developer is held on the outer peripheral surface thereof due to the magnetic force of the magnet42B, and the developer is conveyed and supplied to an electrostatic latent image on the photoconductor drum31due to rotation of the developing sleeve42A.

In the magnet42B, as depicted inFIG. 3, magnetic poles are formed in the order of N3, S2, N2, S1, and N1in the rotation direction of the developing sleeve42A, and the developer that is drawn up by the N3magnetic pole is held by the S2magnetic pole up to the layer regulating member45and is trimmed by the layer regulating member45. The N2magnetic pole holds the developer regulated by the layer regulating member45.

The S1magnetic pole is arranged opposing the photoconductor drum31, and holds the developer that has been conveyed from the N2magnetic pole due to rotation of the developing sleeve42A. A developing bias voltage is applied from a developing power source91for the image forming device1main body and an electric field is formed between the developing sleeve42A and the photoconductor drum31, toner within the developer moves toward the photoconductor drum31, and carrier within the developer is held by the S1magnetic pole and adhesion to the photoconductor drum31is suppressed.

The N1magnetic pole is arranged opposing the opposite electrode46(described later) below the developing roller42, and causes developer that has been conveyed from the S1magnetic pole side due to rotation of the developing sleeve42A to separate from the developing sleeve42A. Specifically, due to the repulsion force of magnetic fields directed toward each of the N1and N3magnetic pole which are like poles, the magnetic force of the developing sleeve42A becomes approximately 0, and the developer from which toner has been consumed due to developing separates from the developing sleeve42A.

The opposite electrode46is arranged below the developing roller42in the opening41aopposing the photoconductor drum31in the developing housing41. The opposite electrode46is configured of a nonmagnetic SUS, for example, and is arranged in a position opposing the N1magnetic pole of the developing roller42with there being a predetermined gap G with the outer peripheral surface of the developing roller42. In the present exemplary embodiment, the gap G between the outer peripheral surface of the developing roller42and the outer peripheral surface of the opposite electrode46is narrower than the gap between the photoconductor drum31and the developing roller42, and, specifically, is 0.4 mm or less.

Furthermore, the opposite electrode46is rotated in the same direction (the direction of arrow A1) as the rotation direction (the direction of arrow A) of the developing roller42by the driving device18, which is not depicted. A bias power source92that applies a bias voltage so as to impart a predetermined surface potential is connected to the opposite electrode46. In the present exemplary embodiment, the bias power source92is provided independently from the developing power source91.

Then, the opposite electrode46has a predetermined bias voltage applied thereto by the bias power source92, adsorbs and recovers a toner cloud generated further downstream in the rotation direction of the developing roller42than the site where the developing roller42and the photoconductor drum31oppose each other, and also adsorbs and recovers toner by way of the developing action from the developer held on the outer periphery of the developing roller42(developing sleeve42A).

A first scraper47A is arranged with the tip end thereof abutting the outer peripheral surface of the region of the opposite electrode46at the opposite side to the region that opposes the developing roller42. The first scraper47A and a second scraper47B are arranged with the tip ends thereof abutting. The first scraper47A scrapes off cloud toner that has been adsorbed to the opposite electrode46, and scrapes off toner that has been adsorbed by way of the developing action from the developer on the developing roller42.

(2.4) Developing Operation

In the developing device14, developer is stirred and conveyed within the developing housing41due to the first stirring auger44A, the supply auger44B, and the second stirring auger44C rotating, and the toner and carrier that constitute the developer rub together such that the toner is charged to a negative polarity and the carrier is charged to a positive polarity. Then, when the developer that has been stirred and conveyed reaches a section opposing the developing roller42, due to a magnetic force that acts between the N3magnetic pole, which is a drawing-up pole, and the carrier included in the developer, some of the carrier moves toward the developing roller42, and a developer layer produced by the developer is formed on the outer peripheral surface of the developing sleeve42A.

The developer layer formed on the developing sleeve42A is conveyed due to rotation of the developing sleeve42A, and is held by the N2magnetic pole and carried to the opening41ain the developing housing41opposing the photoconductor drum31, while the thickness of the developer layer is regulated using a magnetic field that is generated between the developing roller42and the layer regulating member45by the S2magnetic pole constituting a layer regulating pole, when passing through the section opposing the layer regulating member45. It should be noted that when passing through the section opposing the layer regulating member45, pressure caused by packing is applied to the developer on the developing sleeve42A, and the toner that has passed through the opposing section is additionally charged due to friction with the carrier.

A predetermined developing bias voltage is applied as a developing bias from the developing power source91such that a developing electric field acts on the developing sleeve42A, in a developing region opposing the photoconductor drum31, on the developer that has been carried to the opening41ain the developing housing41, in a state in which a strong holding force of the S1magnetic pole constituting the main developing pole is acting.

Thus, in the developing region, toner is electrostatically transferred to the electrostatic latent image on the photoconductor drum31from the developer layer on the developing sleeve42A, and the electrostatic latent image is visualized as a toner image.

Thereafter, the developer layer on the developing sleeve42A that has passed through the developing region returns to inside the developing housing41due to rotation of the developing sleeve42A, separates from the developing roller42and drops inside the developing housing41due to a repelling magnetic field formed by the N1magnetic pole constituting a separation pole, is once again stirred and conveyed by the first stirring auger44A, the supply auger44B, and the second stirring auger44C, and waits for the next developing operation.

Here, as depicted inFIG. 1, in a case where image forming is carried out by the image forming unit10with the continuous paper S being supplied from the paper supply device20, the image forming operation becomes continuous running so to speak, and there is a risk of an image quality defect occurring due to limited expelling (replacing) of the toner within the developer obtained after developing that has passed through the developing region. In particular, in a case where the area coverage at which image forming is carried out is low, the consumption of toner in the developer is low and toner is liable to remain in the developer. As a result, there is an excessive increase in the embedding of an external additive in the toner surface and the toner charge amount, and there is a risk of there being a deterioration in granularity, which is referred to as color noise (CN) and quantifies density unevenness at a pitch of 1 mm or less.

FIG. 4depicts an example of a relationship between area coverage AC, color noise CN, and an external additive embedding grade, for the case where the continuous paper S is supplied and continuous running is carried out. Here, color noise CN quantifies granularity which is a sensory evaluation value, and signifies that image quality improves as the color noise CN decreases. Furthermore, the external additive embedding grade signifies that, as the external additive embedding grade decreases, the embedding of an external additive into toner decreases and the toner charge properties and fluidity improve.

As depicted inFIG. 4, in a case where the area coverage AC is low, the embedding grade of an external additive is high (in a case where the area coverage AC is 1%, the external additive embedding grade is 4.5) and the color noise exceeds 5. It is apparent that the color noise CN decreases if there is an increase in the area coverage AC when continuous running is carried out, and if the area coverage AC is 4% or higher, the color noise CN becomes 4.5 or less which is unremarkable in sensory terms as color noise CN.

Here, in the present exemplary embodiment, in a case where the area coverage AC is 4%, the amount of toner consumed by developing is 0.01 g/m2, and the toner included in the developer obtained after developing is consumed (expelled) at an amount of 0.01 g/m2or more. It is therefore surmised that it may be possible to suppress deterioration in the color noise CN with the embedding of an external additive being suppressed even when continuous running is carried out.

(2.5) Controlling Change in Rotation Direction and Bias Voltage of Opposite Electrode

In the developing device14in the present exemplary embodiment, deterioration in the color noise CN is suppressed with expelling of the toner within the developer obtained after developing being promoted by changing at least one of the bias voltage applied to the opposite electrode46and the rotation direction of the opposite electrode46in accordance with the area coverage AC of an electrostatic latent image formed on the photoconductor drum31.

(2.5.1) Adsorption of Cloud Toner

FIG. 5depicts a rotation direction and an applied bias voltage of the opposite electrode46when cloud toner is adsorbed.

In the present exemplary embodiment, as depicted inFIG. 5, a potential Vs that has the same polarity (minus) as the toner and the same magnitude as the developing potential of the developing roller42(developing sleeve42A) is applied to the opposite electrode46, and the opposite electrode46is rotated in the same direction as the developing roller42(arrow A1inFIG. 5; the outer peripheral surfaces where the developing roller42and the opposite electrode46face each other are rotated in opposite directions to each other).

Thus, the opposite electrode46has the same potential as the developing roller42and the toner within the developer obtained after developing is not expelled; however, a toner cloud that is generated further downstream in the rotation direction of the developing roller42than the site where the developing roller42and the photoconductor drum31oppose each other is adsorbed and recovered (see the arrow inFIG. 5).

The adsorbed cloud toner is scraped off by the first scraper47A and is sucked by an undepicted suction device and recovered into a waste toner recovery container (not depicted) via a duct41bserving as an example of an air passage provided below the developing housing41. Thus, it may be possible to suppress leakage of the cloud toner.

(2.5.2) Expelling Toner from Developer Obtained after Developing

FIG. 6depicts a rotation direction and an applied bias voltage of the opposite electrode46when the toner within developer is expelled.

In the present exemplary embodiment, as depicted inFIG. 6, a potential Vs that has the same polarity (minus) as the toner and a larger absolute value than the developing potential of the developing roller42(developing sleeve42A) is applied to the opposite electrode46, and the opposite electrode46is rotated in the same direction as the developing roller42. Specifically, the developing voltage of the developing roller42is −150 v to −450 v, and therefore a bias voltage of 0 v to −100 v is applied. Here, 0 v is the ground state.

Thus, the opposite electrode46has a potential that is 50 v to 450 v higher than the developing roller42, and the toner within the developer obtained after developing flies (develops) toward the opposite electrode46and is adsorbed onto the outer peripheral surface of the opposite electrode46(see the arrows inFIG. 6).

The adsorbed toner is scraped off by the first scraper47A and is sucked by the suction device and recovered into the waste toner recovery container via the duct41bprovided below the developing housing41. Thus, the toner within the developer obtained after developing is expelled, new developer is supplied from the supply auger44B to the developing roller42, and it may be possible to suppress image quality defects caused by toner deterioration and toner charge fluctuation even when images having a low area coverage AC are continuously formed.

FIG. 7depicts a relationship between the relative positions of the opposite electrode46and the N1magnetic pole constituting the separation pole of the developing roller42, and the amount of toner that flies toward and is consumed by the opposite electrode46. As depicted inFIG. 7, the amount of consumed toner is the highest at a position (0°) where the opposite electrode46opposes the N1magnetic pole, and the amount of consumed toner decreases the more the N1magnetic pole and the opposite electrode46deviate from facing each other. In the present exemplary embodiment, the opposite electrode46is arranged in a position opposing the N1magnetic pole of the developing roller42, and a decline in the amount of toner adsorbed by the opposite electrode46is limited.

FIG. 8depicts a relationship between the gap G between the opposite electrode46and the developing roller42and the amount of toner that flies toward and is consumed by the opposite electrode46. As depicted inFIG. 8, the amount of consumed toner increases as the gap G between the opposite electrode46and the developing roller42becomes smaller. In the present exemplary embodiment, the gap G between the outer peripheral surface of the opposite electrode46and the outer peripheral surface of the developing roller42is set to 0.4 mm or less, and the amount of consumed toner becomes 0.01 g/m2or more which is equivalent to an area coverage of 4% with which a deterioration in the color noise CN is difficult to be perceived in sensory terms.

(2.5.3) Operation of Developing Device

FIG. 9is a functional block diagram depicting a functional configuration of the image forming unit10, andFIG. 10is a flowchart depicting an operation flow of the developing device14during continuous running.

As depicted inFIG. 10as a flowchart, in the developing device14which is a main part of the image forming unit10in the present exemplary embodiment, printing control is carried out with predetermined developing conditions (parameters) being set by the system control device11which serves as an example of a controller.

The system control device11functions as a controller that changes at least one of the bias voltage applied to the opposite electrode46and the rotation direction of the opposite electrode46in accordance with the area coverage AC of an electrostatic latent image formed on the photoconductor drum31.

The system control device11, upon receiving a print job (S101), measures the rotational drive time T1of the developing device14(S102), and also measures the total image pixels P for the print job using the image processing unit50(S103). Then, an area coverage calculation unit110calculates the area coverage AC for the print job on the basis of the rotational drive time T1of the developing device14in the print job and the total image pixels P for the print job (S104), and determines whether or not the area coverage AC is higher than a predetermined threshold value ACth (S105). In the present exemplary embodiment, the threshold value ACth is set to 4%.

If it is determined as a result of the determination that the area coverage AC for the print job is higher than the threshold value ACth (S105: yes), a potential Vs that has the same polarity (minus) as the toner and the same magnitude as the developing roller42(developing sleeve42A) is applied to the opposite electrode46(S106), and the opposite electrode46is rotated in the same direction as the developing roller42(S107; seeFIG. 5). Thus, a toner cloud that is generated further downstream in the rotation direction of the developing roller42than the site where the developing roller42and the photoconductor drum31oppose each other is adsorbed and recovered, and it may be possible to suppress leakage of the cloud toner.

If it is determined in step S105that the area coverage AC for the print job is lower than the threshold value ACth (S105: no), a potential Vs that has the same polarity (minus) as the toner and a larger absolute value than the developing potential of the developing roller42(developing sleeve42A) is applied to the opposite electrode46(S108), and the opposite electrode46is rotated in the same direction as the developing roller42(S109; seeFIG. 6).

The system control device11measures the rotation time T2of the opposite electrode46(S110), and if the rotation time T2of the opposite electrode46has elapsed a predetermined time (Nsec) (S111: yes), the opposite electrode46is switched to a potential Vs having the same magnitude as the developing roller42(developing sleeve42A). Thus, the toner within the developer obtained after developing is expelled, and it may be possible to suppress image quality defects caused by toner deterioration and toner charge fluctuation even when images having a low area coverage AC are continuously formed.

Second Exemplary Embodiment

FIG. 11is a partial cross-sectional view depicting the developing roller42and the opposite electrode46of the developing device14in which a second exemplary embodiment is applied. It should be noted that configurations that are the same as in the first exemplary embodiment are denoted by the same reference characters, and detailed descriptions thereof are omitted here.

(1) Configuration of Developing Device14

Below the developing roller42in the opening opposing the photoconductor drum31in the developing housing41, the opposite electrode46is arranged in a position opposing the N1magnetic pole of the developing roller42with there being the predetermined gap G with the outer peripheral surface of the developing roller42. Similar to the first exemplary embodiment, the gap G between the outer peripheral surface of the developing roller42and the outer peripheral surface of the opposite electrode46is 0.4 mm or less, which is narrower than the gap between the photoconductor drum31and the developing roller42.

Furthermore, the opposite electrode46is rotated in the same direction (the direction of arrow A1) or the opposite direction (the direction of arrow A2) with respect to the rotation direction (the direction of arrow A) of the developing roller42(developing sleeve42A). The bias power source92that applies a bias voltage so as to impart a predetermined surface potential is connected to the opposite electrode46. In the present exemplary embodiment, the bias power source92is provided independently from the developing power source91.

Then, the opposite electrode46has a predetermined bias voltage applied thereto by the bias power source92, adsorbs and recovers a toner cloud generated further downstream in the rotation direction of the developing roller42than the site where the developing roller42and the photoconductor drum31oppose each other, and also adsorbs and recovers toner by way of the developing action from the developer held on the outer periphery of the developing roller42(developing sleeve42A).

The first scraper47A is arranged with the tip end thereof abutting the outer peripheral surface of the region of the opposite electrode46at the opposite side to the region that opposes the developing roller42. Furthermore, the second scraper47B is arranged with a tip end thereof abutting the outer peripheral surface of the region of the opposite electrode46that opposes the developing roller42.

The first scraper47A scrapes off toner that has adsorbed to the opposite electrode46by way of the developing action from the developer on the developing roller42. The second scraper47B scrapes off cloud toner that has adsorbed to the opposite electrode46.

(2) Action of Opposite Electrode

(2.1) Adsorption of Cloud Toner

FIG. 12depicts a rotation direction and an applied bias voltage of the opposite electrode46when cloud toner is adsorbed.

In the present exemplary embodiment, as depicted inFIG. 12, the opposite electrode46is rotated in the opposite direction to the developing roller42(arrow A2inFIG. 12; the outer peripheral surfaces where the developing roller42and the opposite electrode46face each other rotate in the same direction as each other), and a potential Vs that has the same polarity (minus) as the toner and the same magnitude as the developing roller42(developing sleeve42A) is applied.

Thus, the opposite electrode46has the same potential as the developing roller42and the toner within the developer obtained after developing is not expelled; however, a toner cloud that is generated further downstream in the rotation direction of the developing roller42than the site where the developing roller42and the photoconductor drum31oppose each other is adsorbed and recovered (see the arrows inFIG. 12).

The adsorbed cloud toner is scraped off by the second scraper47B and is recovered to inside the developing housing41. Thus, it may be possible to suppress leakage of the cloud toner.

(2.2) Expelling Toner from Developer Obtained After Developing

FIG. 13depicts a rotation direction and an applied bias voltage of the opposite electrode46when toner within developer is expelled.

In the present exemplary embodiment, as depicted inFIG. 13, the opposite electrode46is rotated in the same direction as the developing roller42, and a potential Vs that has the same polarity (minus) as the toner and a larger absolute value than the developing potential of the developing roller42(developing sleeve42A) is applied.

Thus, the opposite electrode46has a higher potential than the developing roller42, and the toner within the developer obtained after developing flies toward the opposite electrode46and is adsorbed onto the outer peripheral surface of the opposite electrode46(see the arrows inFIG. 13).

The adsorbed toner is scraped off by the first scraper47A and is sucked by the undepicted suction device and recovered into the waste toner recovery container (not depicted) via the duct41bprovided below the developing housing41. Thus, the toner within the developer obtained after developing is expelled, new developer is supplied from the supply auger44B to the developing roller42, and it may be possible to suppress image quality defects caused by toner deterioration and toner charge fluctuation even when images having a low area coverage AC are continuously formed.

(2.3) Operation of Developing Device

FIG. 14is a flowchart depicting an operation flow of the developing device14during continuous running.

The system control device11, upon receiving a print job (S201), measures the rotational drive time T1of the developing device14(S202), and also measures the total image pixels P for the print job using the image processing unit50(S203). Then, the area coverage calculation unit110calculates the area coverage AC for the print job on the basis of the rotational drive time T1of the developing device14in the print job and the total image pixels P for the print job (S204), and determines whether or not the area coverage AC is higher than the predetermined threshold value ACth (S205). In the present exemplary embodiment, the threshold value ACth is set to 4%.

If it is determined as a result of the determination that the area coverage AC for the print job is higher than the threshold value ACth (S205: yes), a potential Vs that has the same polarity (minus) as the toner and the same magnitude as the developing roller42(developing sleeve42A) is applied to the opposite electrode46(S206), and the opposite electrode46is rotated in the opposite direction to the developing roller42(S207; seeFIG. 12). Thus, a toner cloud that is generated further downstream in the rotation direction of the developing roller42than the site where the developing roller42and the photoconductor drum31oppose each other is adsorbed and recovered, and it may be possible to suppress leakage of the cloud toner.

If it is determined in step S205that the area coverage AC for the print job is lower than the threshold value ACth (S205: no), a potential Vs that has the same polarity (minus) as the toner and a larger absolute value than the developing potential of the developing roller42(developing sleeve42A) is applied to the opposite electrode46(S208), and the opposite electrode46is rotated in the same direction as the developing roller42(S209; seeFIG. 13).

The system control device11measures the rotation time T2of the opposite electrode46(S210), and if the rotation time T2of the opposite electrode46has elapsed a predetermined time (Nsec) (S211: yes), the opposite electrode46is switched to a potential Vs having the same magnitude as the developing roller42(developing sleeve42A). Thus, the toner within the developer obtained after developing is expelled, and it may be possible to suppress image quality defects caused by toner deterioration and toner charge fluctuation even when images having a low area coverage AC are continuously formed.