JP6620545B2 - Developing device and image forming apparatus - Google Patents

Description

  The present invention relates to a developing device and an image forming apparatus including the developing device.

  Conventionally, an electrostatic latent image formed on a photosensitive drum is developed with toner to form a toner image, the formed toner image is transferred to a sheet of paper, and the transferred toner image is heated and fixed. There is known an electrophotographic image forming apparatus for forming an image thereon.

  As a developer for developing the electrostatic latent image formed on the photosensitive drum, a one-component developer composed only of toner or a two-component developer composed of toner and carrier is generally used. Two-component developers are often used in high-speed image forming apparatuses and color image forming apparatuses because they are excellent in charging stability and adaptability to high-speed machines.

As a technique for maintaining the state of the carrier in the image forming apparatus using the above two-component developer, a dedicated carrier having a blade that comes into contact with the surface of the photosensitive drum after the deteriorated carrier is selectively discharged onto the photosensitive drum. A technique for recovering deteriorated carriers by a cleaning mechanism is disclosed (for example, see Patent Document 1).
Further, a technique is disclosed in which after the deteriorated carrier is selectively discharged onto the photosensitive drum, the deteriorated carrier is recovered by a cleaning mechanism having a blade that contacts the surface of the intermediate transfer belt via the intermediate transfer belt. (For example, refer to Patent Document 2).
Moreover, the technique of collect | recovering carriers with a dedicated mag roller is disclosed (for example, refer patent document 3).

JP 61-204647 A JP2012-113109A JP 2008-310079 A

However, since the techniques described in Patent Document 1 and Patent Document 2 are configured to apply pressure to the carrier that has entered the cleaning mechanism with the blade, the blade is worn and sufficient cleaning processing can be performed. Therefore, there is a problem that an image defect occurs. In particular, since the technique described in Patent Document 1 is configured to abut the blade on the surface of the photosensitive drum, the surface of the photosensitive drum may be damaged, and further image defects may occur.
Further, the technique described in Patent Document 3 has a configuration in which the carrier state cannot be efficiently maintained because the carrier is collected into the developing device without selecting the deteriorated carrier. Moreover, since it is the structure which newly provides the exclusive roller for carrier collection | recovery, there exists a subject that cost increases.
In addition, the techniques described in Patent Documents 1 to 3 do not have a function of selecting and reusing a deteriorated carrier whose performance can be recovered, so that the amount of replenishment of the carrier increases and the cost increases. There are challenges.

  SUMMARY An advantage of some aspects of the invention is that it provides a developing device capable of suppressing the occurrence of an image defect and suppressing an increase in cost, and an image forming apparatus including the developing device.

The invention described in claim 1 has been made to achieve the above object,
In a developing device that includes a plurality of developing rollers at a position facing the image carrier, and forms a toner image by supplying a developer composed of toner and a carrier to the electrostatic latent image formed on the image carrier.
The plurality of developing rollers are
A first developing roller;
A second developing roller, which is disposed downstream of the first developing roller in the rotation direction of the image carrier, and has a staying portion for retaining the carrier in a part of the circumferential direction;
With
A partition member movable to a position that shields between the first developing roller and the second developing roller;
A carrier recovery part that is disposed below the stay part and recovers the carrier dropped from the stay part;
A rotation control unit for controlling rotation of the first developing roller and the second developing roller;
With
The rotation control unit
At a predetermined timing when image formation is stopped, the partition member is moved to a position that shields between the first development roller and the second development roller, and the first development roller is moved in the same direction as that during image formation. The carrier is rotated to discharge the carrier, the second developing roller is rotated in the opposite direction to the image formation to collect the carrier discharged to the image carrier, and the recovered carrier is The carrier dropped from the staying part that stays is collected by the carrier collecting part.

The invention according to claim 2 is the developing device according to claim 1,
When the rotation controller rotates the first developing roller to discharge the carrier to the image carrier, the difference between the potential of the image carrier and the potential of the first developing roller is larger than that during image formation. A potential setting unit for setting the potential of the first developing roller is provided.

According to a third aspect of the present invention, in the developing device according to the first or second aspect,
The staying portion has a region in which the amount of change in magnetic flux density in a predetermined range of magnetic flux density is 0.6 mT / ° or more.
Here, the predetermined range is a range where the magnetic flux density is reduced to near 0, and is a range where the developer is peeled from the developing roller.

The invention according to claim 4 is the developing device according to any one of claims 1 to 3,
The rotation control unit rotates the second developing roller at a predetermined timing in a direction opposite to that at the time of image formation for a predetermined time, and then rotates at least one turn in the same direction as at the time of image formation. .

The invention according to claim 5 is the developing device according to any one of claims 1 to 4,
A developer supply unit for supplying the developer to the first developing roller;
A carrier replenishment unit for replenishing the carrier to the developer supply unit;
A carrier amount calculation unit for calculating the carrier amount recovered by the carrier recovery unit;
A carrier amount control unit that replenishes the carrier by the carrier supplement unit based on the carrier amount calculated by the carrier amount calculation unit;
It is characterized by providing.

The invention described in claim 6
In the image forming apparatus,
The developing device according to claim 1, wherein a toner image is formed by supplying a developer including toner and a carrier to an image carrier and an electrostatic latent image formed on the image carrier. And an image forming unit that forms a toner image formed by the developing device on a sheet.

  According to the present invention, it is possible to suppress the occurrence of image defects and to suppress an increase in cost.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an exemplary embodiment. FIG. 3 is a functional block diagram illustrating a control structure of the image forming apparatus according to the present embodiment. It is a side view which shows schematic structure of a developing device. FIG. 10 is a diagram illustrating an image forming operation performed by the developing device. It is a figure which shows the collection | recovery operation | movement of the carrier which a developing device performs. It is a figure which shows an example of a mode that the deterioration carrier which can recover performance is sorted in the retention area | region of a 2nd developing roller. It is a figure which shows the correspondence of the position (angle) of a 2nd developing roller, and magnetic flux density. It is a figure which shows an example of the variation | change_quantity of the magnetic flux density in a developing roller. It is a figure which shows an example of the correspondence of magnetic flux density variation | change_quantity and the presence or absence of the stay of the developer in a peeling part. 5 is a flowchart showing an operation of the image forming apparatus according to the present embodiment. FIG. 6 is a diagram illustrating an example of potentials of a photosensitive drum, a first developing roller, and a second developing roller during image formation and collection operation. FIG. 10 is a diagram illustrating an example of a table indicating a correspondence relationship between a developer usage history and a time for performing a carrier recovery operation. It is a figure which shows an example of the table which shows the correspondence of the usage history of a developer, and the amount of waste carriers. It is a figure which shows an example of the verification result in an Example and a comparative example.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  An image forming apparatus 100 according to the present embodiment is an image forming apparatus using an electrophotographic process technology. As illustrated in FIGS. 1 and 2, an automatic document feeder 1, an image reading unit 2, an image forming unit 3, The image forming apparatus includes a fixing unit T, a sheet feeding unit 5, a storage unit 6, an operation display unit 7, and a control unit 10.

The automatic document feeder 1 feeds the documents one by one and conveys them to the image reading position, and discharges the document after the image reading to a predetermined place (document delivery table 109).
The automatic document feeder 1 includes a document table 101 on which documents are placed, a document separating unit 103 that separates the placed documents one by one, and a document transport unit 105 that transports the separated documents toward the image reading position. In order to reverse the front and back of the original in the original discharge means 107 for discharging the conveyed original, the original discharge base 109 on which the discharged original is placed, and the double-sided copy mode for reading images on both sides of the original A document reversing unit 111 including a pair of rollers to be used is provided.
The document reading position is provided below the document conveying unit 105, and an image of the document is read through the slit 201 constituting the image reading unit 2, and the read document is discharged by the document discharge unit 107. It is discharged to the table 109.

The image reading unit 2 reflects light from the first mirror unit 205 and the first mirror 215, which are formed by integrating a slit 201, a lamp 213 that irradiates a document, and a first mirror 215 that reflects reflected light from the document. A second mirror unit 207 in which the second mirror 217 and the third mirror 219 are integrated in a substantially V shape, an imaging lens 209 that forms an image of reflected light from the third mirror 219 on the image sensor, and an imaging lens A line-shaped image sensor (hereinafter referred to as a CCD) 211 that obtains image information by photoelectrically converting the light image formed by 209 is configured.
The image signals read by the image reading unit 2 are sequentially taken out from a memory (not shown) and input as electric signals to the respective exposure optical systems as exposure means.

The image forming unit 3 includes a photosensitive drum (image carrier) 4 in which a photosensitive layer made of a resin such as polycarbonate containing an organic photoconductor is provided on a drum-shaped metal substrate, and a charging means having a scorotron type. A charger 320, an exposure optical system 330 that is an exposure unit (exposure means) constituted by a laser optical system, a developing device 8 that visualizes an electrostatic latent image, and a part of the peripheral surface of the photosensitive drum 4. The image forming apparatus includes a transfer unit 340, a cleaning device 350 including a blade (cleaning unit) for cleaning the surface of the photosensitive drum 4 after transfer, and the like.
The developing device 8 contains a two-component developer including a magnetic carrier (hereinafter referred to as carrier) and a non-magnetic toner (hereinafter referred to as toner). The developing device 8 includes a plurality of (two in the present embodiment) developer that is fixed in position and includes a magnet in which a plurality of magnetic poles are arranged in the circumferential direction and has a non-magnetic rotatable cylindrical shape. It has a carrier (developing roller) and the like.

Hereinafter, a series of image forming operations performed by the image forming unit 3 will be described.
With the start of the image forming process, the surface of the photosensitive drum 4 that rotates clockwise is charged to a predetermined polarity by the charger 320. Next, exposure by an image signal by the exposure optical system 330 is performed, and an electrostatic latent image is formed on the photosensitive drum 4. The electrostatic latent image is reversely developed by contact or non-contact development processing with a developer in the developing device 8 and converted into a toner image which is a visible image, and then a transfer medium (hereinafter referred to as copy paper) P by the action of the transfer unit 340. Transcribed above.
On the other hand, the surface of the photosensitive drum 4 after the transfer process is completed is cleaned by the cleaning unit 350, and preparations for new image formation are made.
The copy paper P onto which the toner image has been transferred is separated from the photosensitive drum 4 while being subjected to, for example, an action of a separation claw (not shown), and is conveyed toward the fixing unit T.

The fixing unit T includes a fixing roller T1 having a built-in fixing heater and a pressure roller T2 that rotates while being pressed against the fixing roller T1. The toner forming the image is melted by heating and pressurizing with both rollers and fixed on the copy paper P.
After completion of the fixing process by the fixing unit T, the copy paper P is conveyed by a paper discharge roller 603 provided downstream thereof, and is discharged onto a paper discharge tray 604 provided outside the apparatus main body.

  The paper feed unit 5 includes a plurality of paper feed trays 51 and 52, and each of the paper feed trays 51 and 52 stores a plurality of different types of paper. The paper feeding unit 5 feeds paper stored in a predetermined conveyance path to the image forming unit 3.

  The storage unit 6 includes, for example, an HDD (Hard Disk Drive), a semiconductor memory, and the like, and stores data such as program data and various setting data in a readable / writable manner from the control unit 10. In addition, the storage unit 60 is a table (see FIG. 12) showing a correspondence relationship between the developer usage history and the carrier collection time, or a table showing a correspondence relationship between the developer usage history and the amount of discarded carriers. (See FIG. 13) and the like are stored.

The operation display unit 7 is configured by, for example, a liquid crystal display (LCD) with a touch panel, and functions as the display unit 71 and the operation unit 72.
The display unit 71 displays various operation screens, operation states of functions, and the like according to a display control signal input from the control unit 10. In addition, a touch operation by the user is received and an operation signal is output to the control unit 10.
The operation unit 72 includes various operation keys such as a numeric keypad and a start key, receives various input operations by the user, and outputs an operation signal to the control unit 10. The user can operate the operation display unit 70 to perform settings relating to image formation such as image quality setting, magnification setting, application setting, output setting, and paper setting, a paper conveyance instruction, and a device stop operation.

  The control unit 10 is configured to include a CPU, a RAM, a ROM, and the like. The CPU expands various programs stored in the ROM to the RAM, and cooperates with the expanded various programs to perform the automatic document conveyance unit 1, The operation of each part of the image forming apparatus 100 such as the image reading unit 2, the image forming unit 3, the fixing unit T, the paper feeding unit 5, the storage unit 6, and the operation display unit 7 is comprehensively controlled (see FIG. 2). For example, the control unit 10 inputs an electric signal from the image reading unit 2 to perform various image processing, and outputs image data generated by the image processing to the image forming unit 3. The control unit 10 controls the operation of the image forming unit 3 to form an image on a sheet.

  Next, a more detailed configuration of the developing device 8 according to the present embodiment will be described with reference to FIGS. 4 to 6, a is a rotation direction of the photosensitive drum 4, b is a rotation direction of the first developing roller 810, c1 and c2 are rotation directions of the second developing roller 820, and D1 is a developing roller 810. D2 is a flow of developer through the second developing roller 820, D3 is a flow of deteriorated developer (deteriorated carrier) through the photosensitive drum 4 and the second developing roller 820, and M1 is a first development. A magnetic force distribution in the vicinity of the roller 810, M2 indicates a magnetic force distribution in the vicinity of the second developing roller 820.

  The developing device 8 is disposed on the upstream side (left side in the figure) facing the photosensitive drum 4 and in the moving direction (rotating direction) of the photosensitive drum 4, and includes a first developing roller 810 that conveys the developer, and the photosensitive drum. 4, a second developing roller 820 that is disposed on the downstream side (right side in the drawing) of the photosensitive drum 4 in the moving direction (rotating direction) of the first developing roller 810, and conveys the developer, and the first developing roller 810 and the 2nd developing roller 820, respectively, The 1st screw 830 and the 2nd screw 840 which are each arrange | positioned are comprised. Further, the bottom surface side of the developing device 8 is divided into a first developer collecting chamber 850 for collecting the developer dropped from the first developing roller 810 and a developer for collecting the developer dropped from the second developing roller 820 by the partition 870. 2 developer recovery chamber 860. The first developer recovery chamber 850 contains a two-component developer mainly composed of toner and carrier.

The developer stored in the first developer recovery chamber 850 is supplied to the first developing roller 810 by the first screw 830. That is, the first developer recovery chamber 850 and the first screw 830 function as a developer supply unit of the present invention.
Further, the developing device 8 is provided with a carrier replenishing unit 80 (see FIG. 2) that replenishes the developer supplying unit (first developer collecting chamber 850).

The first developing roller 810 and the second developing roller 820 are cylindrical members made of a nonmagnetic metal material such as aluminum or stainless steel, and incorporate magnets in a fixed position.
The rotation of the first developing roller 810 and the second developing roller 820 is controlled by the control unit 10. That is, the control unit 10 functions as a rotation control unit of the present invention.

Here, an image forming operation performed by the developing device 8 will be described with reference to FIG. During image formation, the first developing roller 810 rotates counterclockwise, while the second developing roller 820 rotates clockwise.
At the time of image formation, first, a developer is supplied from the first screw 830 to the first developing roller 810, and is regulated by a doctor blade 880 that regulates the thickness of the developer so that a uniform thickness is formed on the first developing roller 810. A developer layer is formed.
Next, the developer on the first developing roller 810 is conveyed as it is to the developing region by the action of a magnetic field before development is performed in the developing region where the photosensitive drum 4 and the first developing roller 810 face each other. And the one that is delivered to the opposing second developing roller 820. The developer conveyed to the developing area as it is is developed in the developing area, and then separated from the first developing roller 810 and falls into the first developer collecting chamber 850. On the other hand, the developer delivered to the second developing roller 820 is transported by the second developing roller 820, and development is performed in the developing region where the photosensitive drum 4 and the second developing roller 820 face each other.
Next, the developer that has passed through the developing region is separated from the second developing roller 820 by the action of the magnetic field and falls into the second developer collecting chamber 860. The developer in the second developer recovery chamber 860 flows back to the first developer recovery chamber 850 through an opening (not shown) provided in the partition 870.

Next, the carrier collecting operation performed by the developing device 8 will be described with reference to FIG. Note that, during the collecting operation, the first developing roller 810 and the second developing roller 820 both rotate counterclockwise. That is, the rotation direction of the second developing roller 820 is different between the image forming time and the collecting operation time.
During the collecting operation, first, a developer is supplied from the first screw 830 to the first developing roller 810 and is regulated by the doctor blade 880 to form a developer layer having a uniform thickness on the first developing roller 810.

Next, the developer on the first developing roller 810 is transferred to the opposing second developing roller 820 by the partition member 890 that has moved to a position that shields between the first developing roller 810 and the second developing roller 820. The photosensitive drum 4 and the first developing roller 810 are conveyed to a developing area facing each other, and only the deteriorated carrier is discharged to the photosensitive drum 4 by the action of a magnetic field. The partition member 890 may be any member that can shield between the first developing roller 810 and the second developing roller 820. For example, a partial cylinder that rotates between the first developing roller 810 and the second developing roller 820 in the outer peripheral direction. Or blinds.
In general, a magnetic force attracting the carrier on the first developing roller 810 from the first developing roller 810 to the first developing roller 810 side acts in the developing region, and the photosensitive drum 4 and the first developing roller 810 The Coulomb force generated in the direction of attracting to the photosensitive drum 4 side works due to the potential difference. During image formation, the potential difference between the photosensitive drum 4 and the first developing roller 810 is controlled so that the magnetic force is stronger than the Coulomb force so that the carrier is not attracted to the photosensitive drum 4 side. On the other hand, during the collecting operation, a normal carrier that has not deteriorated is left on the first developing roller 810, while the deteriorated carrier (for example, the carrier with reduced magnetic force or the surface is contaminated to increase resistance (the Coulomb force is increased). In order to attract only the carrier) to the photosensitive drum 4 side, the potential difference between the photosensitive drum 4 and the first developing roller 810 is controlled so that the Coulomb force is stronger than during image formation.

The normal undegraded carrier is developed in the development area, and then separated from the first development roller 810 and falls into the first developer recovery chamber 850.
On the other hand, the deteriorated carrier attracted to the photoconductive drum 4 is conveyed by the photoconductive drum 4 and collected by the second developing roller 820 rotating in the opposite direction to that during image formation. Next, the deteriorated carrier recovered by the second developing roller 820 is conveyed to a staying region (stagnation part) E1 formed in a part of the circumferential direction of the second developing roller 820, and the deteriorated carrier whose performance can be recovered. A sorting process is performed (see FIG. 6 and the like).
Here, the deteriorated carrier whose performance can be recovered is a carrier (k1 in the figure) whose surface is contaminated and has increased resistance, and the unrecoverable deteriorated carrier has a reduced magnetic force. It is a carrier (k2 in the figure). Further, the staying region E1 is a region in which the change in magnetic flux density is steep to improve the staying property of carriers (see FIG. 7). Note that the developer separation region E2 (see FIG. 4) during image formation is a region in which the change in magnetic flux density is slow (see FIG. 7), and the change of the developer is performed by smoothing the change in magnetic flux density. Property (peelability) is improved.

FIG. 8 shows an example of the amount of change in magnetic flux density in the developing roller.
The developer is peeled from the developing roller (the first developing roller 810 and the second developing roller 820) in a predetermined range (hereinafter referred to as a peeling portion) in which the magnetic flux density decreases to near zero. In this embodiment, as shown in FIG. 8, the peeled portion H1 is defined as a magnetic flux density around 10 mT (for example, 7 to 13 mT), and the magnetic flux density change amount is inclined around the magnetic flux density of 10 mT (magnetic flux density change width / developing roller). Position (angle)).

FIG. 9 shows an example of a correspondence relationship between the change amount of the magnetic flux density and the presence or absence of the developer in the peeled portion.
In the example shown in FIG. 9, when the amount of change in magnetic flux density is less than 0.6 mT / ° (0.3 mT / °, 0.4 mT / °, 0.5 mT / °), the developer stays in the peeled portion. It is shown that it is not possible. Further, it is shown that when the magnetic flux density change amount is 0.6 mT / °, the developer stays slightly. In addition, it is indicated that the developer stays when the magnetic flux density change amount exceeds 0.6 mT / ° (0.7 mT / °, 0.8 mT / °, 0.9 mT / °).
That is, when the change in magnetic flux density at the peeling portion is slow (when the change in magnetic flux density is less than 0.6 mT / °), the developer peeling efficiency is good and the developer does not remain on the developing roller. On the other hand, when the change in magnetic flux density at the peeled portion is steep (when the change in magnetic flux density is 0.6 mT / ° or more), the developer peeling efficiency is poor and the developer stays in the peeled portion.

In this embodiment, in the separation area of the first development roller 810 and the separation area E2 of the second development roller 820, the developer is efficiently peeled by setting the magnetic flux density change amount to less than 0.6 mT / °. I am letting.
In the present embodiment, the developer is intentionally retained in the retention region E1 of the second developing roller 820 by setting the magnetic flux density change amount to 0.6 mT / ° or more. As a result, a process of selecting a deteriorated carrier whose performance can be recovered (a carrier whose surface is contaminated and increased in resistance) and a non-recoverable deteriorated carrier (a carrier whose magnetic force has decreased) is performed.

  As a result of the sorting process, the deteriorated carrier whose performance can be recovered stays in the staying region E1, and is rubbed against the second developing roller 820 to remove the dirt on the surface layer, thereby recovering the performance. On the other hand, the deteriorated carrier whose performance cannot be recovered is ejected from the staying region E1 due to its low magnetic force, and dropped into the deteriorated carrier recovery chamber (carrier recovery unit) 900 disposed below the staying region E1 and recovered ( (See FIG. 6). The deteriorated carrier recovered in the deteriorated carrier recovery chamber 900 is discharged to a carrier recovery box (not shown) by the discharge screw 910 and discarded.

Next, the operation of the image forming apparatus 100 according to the present embodiment will be described with reference to FIG.
First, the control unit 10 stops the image forming operation by the image forming unit 3 (step S101). The processing in step S101 is performed when a job related to image formation is completed or the development rollers (the first development roller 810 and the second development roller 820) are driven for a predetermined time. The period of the predetermined time is not particularly limited, but is, for example, 10 hours.

  Next, the control unit 10 inserts a partition 870 between the first developing roller 810 and the second developing roller 820 (step S102). Thereby, it is possible to prevent the developer on the first developing roller 810 from being directly transferred between the first developing roller 810 and the second developing roller 820.

Next, the control unit 10 rotates the second developing roller 820 arranged on the downstream side in the moving direction (rotating direction) of the photosensitive drum 4 in the same direction (clockwise) as in image formation (step S103). Thereby, the developer on the second developing roller 820 can be peeled off once.
Next, the control unit 10 stops the second developing roller 820 (step S104).

  Next, the control unit 10 rotates the photosensitive drum 4, the first developing roller 810 and the second developing roller 820 arranged on the upstream side in the moving direction (rotating direction) of the photosensitive drum 4 (step S105). Thereby, the carrier collecting operation is started. In the process of step S105, the photosensitive drum 4 and the first developing roller 810 rotate in the same direction as during image formation. On the other hand, the second developing roller 820 rotates in the opposite direction (counterclockwise) to that during image formation.

  In the process of step S105, the control unit 10 enables the recovery operation of the deteriorated carrier by controlling the potential difference between the photosensitive drum 4 and each of the first developing roller 810 and the second developing roller 820. Specifically, the control unit 10 sets the potential Vdc of the first developing roller 810 so that the difference between the potential V0 of the photosensitive drum 4 and the potential Vdc of the first developing roller 810 is larger than that during image formation. . That is, the control unit 10 functions as a potential setting unit of the present invention. As a result, the Coulomb force acts on the deteriorated carrier on the first developing roller 810 more strongly than when forming an image. be able to. Further, the control unit 10 sets the potential Vdc of the second developing roller 820 so that the difference between the potential V0 of the photosensitive drum 4 and the potential Vdc of the second developing roller 820 is smaller than that during image formation. As a result, the Coulomb force acts on the deteriorated carrier on the photoconductive drum 4 more weakly than during image formation, so that the deteriorated carrier is attracted to the second developing roller 820 side by the magnetic force and collected.

FIG. 11 shows the potentials of the photosensitive drum 4, the first developing roller 810, and the second developing roller 820 at the normal time (at the time of image formation) and the collecting operation in the present embodiment. In the example shown in FIG. 11, the potential V0 of the photosensitive drum 4 is set to −700 V, the potential Vdc of the first developing roller 810, and the potential Vdc of the second developing roller 820 are set to −500 V during normal times. Accordingly, the potential difference between the photosensitive drum 4 and each of the first developing roller 810 and the second developing roller 820 during image formation is 200V.
On the other hand, during the collecting operation, the potential V0 of the photosensitive drum 4 is set to −700V, the potential Vdc of the first developing roller 810 is set to −300V, and the potential Vdc of the second developing roller 820 is set to −650V. Therefore, the potential difference between the photosensitive drum 4 and the first developing roller 810 and the second developing roller 820 at the time of image formation is 400V and 50V, respectively. That is, in this embodiment, the potential difference between the photosensitive drum 4 and the first developing roller 810 is set to be larger than that during image formation, and the potential difference between the photosensitive drum 4 and the second developing roller 820 is set during image formation. It is set to be smaller.
The control unit 10 realizes the discharge operation of the deteriorated carrier from the first developing roller 810 and the recovery operation of the deteriorated carrier at the second developing roller 820 by the above-described potential control.

Next, the control unit 10 determines whether or not a predetermined time has elapsed (step S106). The predetermined time is a time during which the retention amount of the deteriorated carrier in the stay region E1 of the second developing roller 820 increases so much that the deteriorated carrier whose performance can be recovered may fall into the deteriorated carrier recovery chamber 900. For example, 30 seconds.
When it is determined that the predetermined time has elapsed (step S106: YES), the controller 10 proceeds to the next step S107.
On the other hand, when it is determined that the predetermined time has not elapsed (step S106: NO), the control unit 10 repeats the process of step S106 until the predetermined time has elapsed.

  Next, the control unit 10 stops the photosensitive drum 4, the first developing roller 810, and the second developing roller 820 (step S107). By stopping the photosensitive drum 4, the first developing roller 810, and the second developing roller 820 at the timing when a predetermined time has elapsed, it is possible to prevent the degradation carrier that can recover its performance from being discarded.

Next, the control unit 10 rotates the second developing roller 820 in the same direction (clockwise) as during image formation (step S108). The operation of rotating the second developing roller 820 is preferably performed at least once. Thereby, the deteriorated carrier that has stayed in the stay region E1 of the second developing roller 820 and recovered its performance can be recovered in the second developer recovery chamber 860.
Next, the control unit 10 determines whether or not it is necessary to continue the carrier collecting operation (step S109). Specifically, the control unit 10 needs to continue the carrier collecting operation with reference to a table (see FIG. 12) showing a correspondence relationship between the developer usage history and the time for performing the carrier collecting operation. It is determined whether or not. The duration of the carrier recovery operation is preferably set to increase as the developer usage history increases. This is because the proportion of deteriorated carriers in the developer increases as the usage history of the developer increases.
When it is determined that the carrier collection operation needs to be continued (step S109: YES), the control unit 10 proceeds to step S105 and continues the carrier collection operation.
On the other hand, when it determines with the control part 10 not having to continue the collection | recovery operation | movement of a carrier (step S109: NO), it transfers to the following step S110.

Next, the control unit 10 stops the second developing roller 820 (step S110).
Next, the control unit 10 controls the carrier replenishing unit 80 to replenish the first developer collecting chamber 850 of the developing device 8 with the same amount of carriers as collected and discarded in the deteriorated carrier collecting chamber 900 ( Step S111). Specifically, the control unit 10 calculates the waste carrier amount with reference to a table (see FIG. 13) showing the correspondence between the developer usage history and the waste carrier amount, and the same amount as the calculated waste carrier amount. Is replenished to the first developer recovery chamber 850. That is, the control unit 10 functions as a carrier amount calculation unit and a carrier amount control unit of the present invention.

Hereinafter, comparison verification between an example in the image forming apparatus 100 according to the present embodiment and a comparative example in the image forming apparatus described in Patent Document 2 (Japanese Patent Laid-Open No. 2012-113109) cited in the prior art document will be shown.
In both the examples and comparative examples, a developer having a usage history of 100 [hour] was used, and the time for performing the carrier recovery operation was 90 [sec].

In FIG. 14, the verification result in an Example and a comparative example is shown.
Referring to N1 and N4 in the figure, the number of spots on the image due to carrier adhesion disappeared by the discharging operation in both the example and the comparative example (5 → 0), and there was no difference between the two.
Further, referring to N2 and N5 in the figure, the toner charge amount was improved (increased) by the discharging operation in both the example and the comparative example (30 → 35), and there was no difference between the two.
On the other hand, referring to N3 in the figure, the developer discharge amount was 10 g in the example and 30 g in the comparative example, and the amount in the example was smaller. This is due to the fact that only carriers with reduced magnetic force are discharged. That is, it was found that the example maintained the state of the developer in the same manner as the comparative example while suppressing the discharge amount of the carrier.
Further, referring to N6 in the figure, the image defect did not occur after the discharge operation in the embodiment, whereas the image defect occurred after the discharge operation in the comparative example. Specifically, in the comparative example, the blade was worn by the carrier that entered the cleaning mechanism of the intermediate transfer belt, and the cleaning failure of the intermediate transfer belt occurred, and image streaks due to the cleaning failure occurred. Further, the carrier sandwiched between the blades and the intermediate transfer belt rubbed to cause streak-like scratches on the intermediate transfer belt, and image streaks due to the belt scratches were generated.
From the above verification results, it was found that the example obtained better results than the comparative example.

As described above, the developing device 8 of the image forming apparatus 100 according to the present embodiment is disposed on the downstream side of the first developing roller 810 and the first developing roller 810 in the rotation direction of the image carrier (photosensitive drum 4). And a second developing roller 820 having a staying part (staying area E1) for retaining the carrier in a part of the circumferential direction. Also, a carrier recovery unit (deteriorated carrier recovery chamber 900) that is disposed below the retention unit and collects the carrier that has dropped from the retention unit, and a rotation control unit that controls the rotation of the first development roller 810 and the second development roller 820. (Control unit 10). In addition, the rotation control unit rotates the first developing roller 810 in the same direction as that during image formation at a predetermined timing while image formation is stopped to discharge the carrier to the image carrier, and causes the second developing roller 820 to form an image. The carrier that has been rotated in the opposite direction to the time and is discharged to the image carrier is collected, and the carrier that has dropped from the staying part where the collected carrier stays is collected by the carrier collecting part.
Therefore, according to the developing device 8 according to the present embodiment, since the carrier is not collected using a blade, the image carrier (such as the photosensitive drum 4 or the intermediate transfer belt) is not damaged, and the image carrier is not damaged. It is possible to suppress the occurrence of image defects due to scratches. Moreover, since it is not necessary to provide a dedicated roller for collecting the carrier, an increase in cost can be suppressed. Further, a staying portion can be formed on the second developing roller 820 to select a deteriorated carrier that can recover its performance and a non-recoverable deteriorated carrier, and the recoverable deteriorated carrier can be recovered by rubbing in the staying portion. Therefore, it is possible to reduce the carrier discharge amount and suppress the cost increase.

Further, when the developing device 8 according to the present embodiment rotates the first developing roller 810 by the rotation control unit to discharge the carrier to the image carrier, the potential of the image carrier and the potential of the first developing roller 810 are calculated. A potential setting unit (control unit 10) is provided for setting the potential of the first developing roller 810 so that the difference is larger than that during image formation.
Therefore, according to the developing device 8 according to the present embodiment, the Coulomb force acts more strongly on the deteriorated carrier on the first developing roller 810 than at the time of image formation. Therefore, the image carrier can efficiently collect the deteriorated carrier. It can be carried out.

Further, according to the developing device 8 according to the present embodiment, the staying portion has a region where the amount of change in magnetic flux density in the predetermined range of the magnetic flux density is 0.6 mT / ° or more.
Therefore, according to the developing device 8 according to the present embodiment, since the developer can be intentionally retained in the retention portion, the performance of the recoverable deteriorated carrier is discharged while discharging the deteriorated carrier whose performance cannot be recovered. Can be recovered by rubbing with the image carrier.

Further, according to the developing device 8 according to the present embodiment, the rotation control unit rotates the second developing roller 820 in a direction opposite to that at the time of image formation for a predetermined time at a predetermined timing, and then the same direction as at the time of image formation. To at least one turn.
Therefore, according to the developing device 8 according to the present embodiment, the retention amount of the deteriorated carrier in the retention portion can be prevented from being excessively discharged and discharged to the deteriorated carrier whose performance can be recovered. The amount of discharge can be further reduced and the cost increase can be suppressed.

Further, the developing device 8 according to the present embodiment has a developer supply unit (first developer recovery chamber 850, first screw 830) for supplying the developer to the first developing roller 810, and a carrier for the developer supply unit. Based on the carrier amount calculated by the carrier replenishing unit 80, the carrier amount calculating unit (control unit 10) for calculating the carrier amount collected by the carrier collecting unit, and the carrier amount calculated by the carrier amount calculating unit, A carrier amount control unit (control unit 10) for replenishing the carrier.
Therefore, according to the developing device 8 according to the present embodiment, since an appropriate amount of carrier can be replenished, the amount of carrier in the developing device 8 can be appropriately controlled.

  As mentioned above, although concretely demonstrated based on embodiment which concerns on this invention, this invention is not limited to the said embodiment, It can change in the range which does not deviate from the summary.

  For example, in the above embodiment, the potential of the first developing roller 810 is set so that the difference between the potential of the photosensitive drum 4 and the potential of the first developing roller 810 is larger than that during image formation. It is not limited to this. That is, it is not always necessary to set the difference between the potential of the photosensitive drum 4 and the potential of the first developing roller 810 to be larger than that at the time of image formation. Generation | occurrence | production can be suppressed and a cost increase can be suppressed.

  Moreover, in the said embodiment, although the magnetic flux density variation | change_quantity of the retention area | region E1 of the 2nd developing roller 820 is set to 0.6 mT / degree or more, it is not limited to this. That is, it is only necessary that the carrier stays in the stay region E1, and even when the value is set to a value lower than 0.6 mT / °, the occurrence of image defects can be suppressed and the cost increase can be suppressed. .

  In the above-described embodiment, the second developing roller 820 is rotated in a direction opposite to that at the time of image formation for a predetermined time and then rotated at least one turn in the same direction as at the time of image formation. It is not something. That is, it is not always necessary to rotate at least one turn in the same direction as when forming an image, and even when the rotation is not made more than one turn, the occurrence of image defects can be suppressed and the cost increase can be suppressed.

  In the above embodiment, the carrier is replenished based on the discarded carrier amount calculated by the carrier amount calculation unit, but the present invention is not limited to this. For example, a certain amount may be replenished regardless of the amount of discarded carriers.

  In the above embodiment, the configuration in which the developing device 8 is disposed below the photosensitive drum 4 is described as an example. However, the present invention is not limited to this. For example, the developing device 8 may be disposed on the side of the photosensitive drum 4 (on the right side in FIG. 1). However, in this case, it is necessary to dispose the deteriorated carrier recovery chamber 900 below the staying area E1 of the second developing roller 820. That is, the developing device 8 may be arranged in any direction of the photosensitive drum 4 as long as the deteriorated carrier recovery chamber 900 is arranged below the staying area E1.

  In addition, the detailed configuration of each apparatus constituting the image forming apparatus and the detailed operation of each apparatus can be changed as appropriate without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 100 Image forming apparatus 1 Automatic document conveyance part 2 Image reading part 3 Image forming part 4 Photosensitive drum (image carrier)
320 Charging Device 330 Exposure Optical System 8 Developing Device 810 First Developing Roller 820 Second Developing Roller E1 Retention Area (Retention Section)
830 1st screw (developer supply part)
840 Second screw 850 First developer recovery chamber (developer supply part)
860 Second developer recovery chamber 870 Partition 880 Doctor blade 890 Partition member 900 Deteriorated carrier recovery chamber (carrier recovery section)
910 Discharge screw 80 Carrier replenishment unit 340 Transfer means 350 Cleaning device T Fixing unit 5 Paper feed unit 6 Storage unit 7 Operation display unit 71 Display unit 72 Operation unit 10 Control unit (rotation control unit, potential setting unit, carrier amount calculation unit, Carrier amount control unit)

Claims (6)

In a developing device that includes a plurality of developing rollers at a position facing the image carrier, and forms a toner image by supplying a developer composed of toner and a carrier to the electrostatic latent image formed on the image carrier.
The plurality of developing rollers are
A first developing roller;
A second developing roller, which is disposed downstream of the first developing roller in the rotation direction of the image carrier, and has a staying portion for retaining the carrier in a part of the circumferential direction;
With
A partition member movable to a position that shields between the first developing roller and the second developing roller;
A carrier recovery part that is disposed below the stay part and recovers the carrier dropped from the stay part;
A rotation control unit for controlling rotation of the first developing roller and the second developing roller;
With
The rotation control unit
At a predetermined timing when image formation is stopped, the partition member is moved to a position that shields between the first development roller and the second development roller, and the first development roller is moved in the same direction as that during image formation. The carrier is rotated to discharge the carrier, the second developing roller is rotated in the opposite direction to the image formation to collect the carrier discharged to the image carrier, and the recovered carrier is A developing device characterized in that a carrier dropped from the staying part is collected by the carrier collecting part.   When the rotation controller rotates the first developing roller to discharge the carrier to the image carrier, the difference between the potential of the image carrier and the potential of the first developing roller is larger than that during image formation. The developing device according to claim 1, further comprising a potential setting unit configured to set a potential of the first developing roller.   The developing device according to claim 1, wherein the staying portion has a region in which a change amount of magnetic flux density in a predetermined range of magnetic flux density is 0.6 mT / ° or more.   The rotation control unit rotates the second developing roller at a predetermined timing in a direction opposite to that at the time of image formation for a predetermined time, and then rotates at least one turn in the same direction as at the time of image formation. The developing device according to claim 1. A developer supply unit for supplying the developer to the first developing roller;
A carrier replenishment unit for replenishing the carrier to the developer supply unit;
A carrier amount calculation unit for calculating the carrier amount recovered by the carrier recovery unit;
A carrier amount control unit that replenishes the carrier by the carrier supplement unit based on the carrier amount calculated by the carrier amount calculation unit;
The developing device according to claim 1, further comprising:   The developing device according to claim 1, wherein a toner image is formed by supplying a developer including toner and a carrier to an image carrier and an electrostatic latent image formed on the image carrier. And an image forming unit that forms a toner image formed by the developing device on a sheet.

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