JP2012150258A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suitably separate toner from a carrier of a developer in a developing unit during the deterioration of the carrier by suppressing an increase in a size of the developing unit.SOLUTION: In a printer having a developing unit 20 using a two-component developer having a carrier and toner, toner remaining on a photoreceptor drum 11 after a toner image is transferred is collected by a toner collecting mechanism 40 and conveyed to the developing unit 20. When the carrier is deteriorated, control is performed such that the toner on the photoreceptor drum 11 is collected and accumulated by the toner collecting mechanism 40 without transferring to a recording sheet. During the performance of the control, the amount of the toner adhering on the photoreceptor 11 is optically detected by a toner adhesion amount sensor 28 and the formation of an electrostatic latent image is stopped.

Description

  The present invention relates to an image forming apparatus including a developing unit that develops an electrostatic latent image formed on an image carrier such as a photoconductor using a two-component developer.

  In an electrophotographic image forming apparatus such as a printer or a copying machine, an electrostatic latent image corresponding to image data is usually formed on a photosensitive drum as an image carrier, and toner is developed by a developing device. ing. The toner image formed on the image carrier is directly or after being transferred to an intermediate transfer belt or the like, and then transferred to a recording sheet such as recording paper or an OHP sheet, and is fixed on the recording sheet by a fixing device. The

In the developing device of such an image forming apparatus, there are a case where a one-component developer composed only of toner is used and a case where a two-component developer including toner and a magnetic carrier is used.
In a developing device using a two-component developer, the developer containing toner and carrier is usually stirred and mixed to charge the toner, and the charged toner is supplied to the developing roller together with the carrier. It has become. The developer supplied to the developing roller forms a magnetic brush on the peripheral surface of the developing roller, and the toner attached to the magnetic brush adheres to the electrostatic latent image on the photosensitive drum. Develop.

  In a developing device using a two-component developer, the toner concentration of the developer in the developing device is detected by a toner concentration sensor, and when the toner concentration in the developing device decreases, the toner is supplied into the developing device by a toner replenishment mechanism. Replenishment is also done. The toner replenishing mechanism is detachable from a toner cartridge containing toner, and can replenish toner by replacing the toner cartridge.

However, when the developing device is used for a long period of time, there is a possibility that the carrier in the developer contained in the developing device deteriorates and the toner cannot be charged properly. If the toner is not properly charged, the toner conveyed on the developing roller may scatter and cause background fogging or the like to which the toner adheres in addition to the electrostatic latent image on the photoreceptor.
For this reason, for example, in Patent Document 1, when the carrier in the developing device deteriorates, the carrier in the developing device is separated and collected from the toner, and a carrier that has not deteriorated is newly supplied into the developing device. A configuration is disclosed. In Patent Document 1, a sleeve is provided in the developing device, a magnet is provided in the sleeve, a bias voltage is applied to the sleeve, the carrier and the toner are separated, and the separated toner is placed in the developing device. It is configured to carry.

Japanese Patent No. 3581720

In the configuration disclosed in Patent Document 1, it is necessary to provide a sleeve and a magnet to which a bias voltage is applied separately from the developing roller in the developing device, and the toner separated from the carrier is conveyed into the developing device. It is also necessary to have a transport route for this. For this reason, there is a possibility that the number of parts increases and the developing device becomes large.
Also, when collecting the carrier in the developing device, it is impossible to detect that all of the carrier in the developing device has been collected, so that a large amount of carrier remains in the developing device. In some cases, the collection operation is terminated. In this case, a large amount of deteriorated carrier remains in the developing device, and if the image forming operation is executed in such a state, the toner becomes poorly charged and the quality of the formed image may be lowered. There is.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an image forming apparatus capable of appropriately separating the toner and the carrier while suppressing an increase in the size of the developing device. There is to do.

  In order to achieve the above object, an image forming apparatus according to the present invention comprises a two-component development comprising an image carrier, electrostatic latent image forming means for forming an electrostatic latent image on the image carrier, a carrier and toner. A developer that supplies toner to the electrostatic latent image on the image carrier using an agent for visualization; a transfer unit that transfers the toner on the image carrier to a recording sheet; and the toner is recorded by the transfer unit. The toner remaining on the image carrier after being transferred to the sheet can be collected, and the collected toner can be switched between a state where the toner is conveyed to the developing unit and a state where the collected toner is accumulated without being conveyed to the developing unit. A toner collection mechanism, a toner adhesion amount detection means for optically detecting the amount of toner adhered on the image carrier, and the toner collection mechanism at a predetermined timing to a state where toner is accumulated, On carrier Toner accumulation control is performed in which an electrostatic latent image is formed and toner is adhered, and then accumulated in the toner recovery mechanism. During the execution of this control, the toner adhesion amount detected by the toner adhesion amount detection means is determined. And a control means for stopping the formation of the electrostatic latent image by the electrostatic latent image forming means when the value becomes a predetermined value or less.

In the image forming apparatus of the present invention, when the carrier in the developing device deteriorates, an electrostatic latent image is formed on the image carrier in the same manner as a normal image forming operation. By performing the developing operation, the toner separated from the carrier can be accumulated in the toner recovery mechanism. Therefore, it is possible to suppress an increase in the size of the developing device.
Further, when the control for accumulating the toner is executed by the toner collecting mechanism, the toner amount adhering to the image carrier is optically detected by the toner adhering amount detecting means, and thus separated from the carrier. Thus, the toner amount in the developing device can be accurately detected with high accuracy based on the toner amount adhering to the image carrier.

Further, in the control for accumulating the toner by the toner collecting mechanism, if the toner amount in the developing device becomes too small by separating the toner from the carrier, the carrier may adhere to the image carrier and damage the image carrier. is there. For this reason, it is desirable to reduce the amount of toner in the developing device as much as possible within a range where the carrier does not adhere to the image carrier.
In the present invention, since the toner amount in the developing device can be detected with high accuracy by the toner adhesion amount detecting means for optically detecting the toner amount adhered on the image carrier, there is a possibility that the carrier adheres to the image carrier. In such a range, the electrostatic latent image forming means can be stopped at an appropriate timing when the toner amount in the developing device is reduced most. As a result, it is possible to collect and accumulate more toner from the developing device while preventing the carrier from adhering to the image carrier.

Preferably, the transfer unit includes a transfer member to which a voltage is applied, and a moving unit that moves the transfer member to a state of being pressed against the image carrier and a state of being separated from the image carrier. The control means controls the moving means so as to separate the transfer member from the image carrier during execution of the toner accumulation control.
Preferably, the toner collecting mechanism includes a shutter that opens and closes a toner supply port through which the collected toner passes and flows into the developing unit, and the control unit supplies the toner when the toner accumulation control is executed. The shutter is controlled so that the mouth is closed.

The developing device includes a toner concentration sensor that detects a toner concentration in the developing device based on a change in magnetic permeability, and a toner replenishing mechanism that replenishes toner to the developing device, and the control unit includes The toner supply by the toner supply mechanism is controlled based on the toner concentration detected by the toner concentration sensor.
Preferably, the developing device includes a developing roller that supplies toner to the image carrier, and a developing bias voltage applying unit that applies a developing bias voltage to the developing roller, and the control unit includes the toner accumulation unit. The developing bias voltage applying unit is controlled so that the developing bias voltage applied to the developing roller is increased when the control is performed so that the amount of toner attached to the image carrier is larger than that during the image forming operation. And

Preferably, the control means controls the electrostatic latent image forming means so that the toner adheres to the entire image forming area of the image carrier when the toner accumulation control is executed.
Preferably, the developing device is replaceable.

1 is a schematic diagram schematically illustrating a configuration of a printer that is an example of an image forming apparatus according to an embodiment of the present invention. FIG. 4 is a schematic diagram for explaining a configuration of a toner adhesion amount sensor provided in the printer of the present embodiment. 6 is a graph showing a relationship between an output voltage of a toner adhesion amount sensor used for toner accumulation control executed in the printer of the present embodiment and a toner concentration (% by weight) in a developing device. 4 is a graph showing a relationship between an output voltage of a magnetic permeability detection type toner density sensor provided in a developing device of the printer of the present embodiment and a toner concentration in the developing device. 6 is a flowchart illustrating a procedure of toner accumulation control executed by the control unit of the printer according to the embodiment.

<Configuration of image forming apparatus>
FIG. 1 is a schematic diagram schematically illustrating a configuration of a printer that is an example of an image forming apparatus according to an embodiment of the present invention. This printer forms a monochrome toner image corresponding to image data transmitted from a LAN or the like on a recording sheet such as a recording sheet or an OHP sheet using a two-component developer having a carrier and black toner. It is configured to do.

  The printer shown in FIG. 1 includes a photosensitive drum 11 as an example of an image carrier that is rotationally driven in a direction indicated by an arrow A. An electrophotographic system (electrostatic transfer system) is provided around the photosensitive drum 11. ), A charger 12 for forming a toner image on the recording sheet S, an exposure device 13, a developing device 20, and a transfer roller 15 are provided in that order along the rotation direction of the photosensitive drum 11. It has been.

  The photosensitive drum 11 is arranged on the right side (right side in FIG. 1) when viewed from the front side of the printer. The surface of the photosensitive drum 11 is uniformly charged by the charger 12 disposed on the side of the photosensitive drum 11 (left side of the photosensitive drum 11), and then the photosensitive drum 11 is in contact with the charger 12. The exposure is performed by the laser light L emitted from the exposure device 13 located on the downstream side in the rotation direction, whereby an electrostatic latent image corresponding to the image data is formed on the surface of the photosensitive drum 11. Is done.

The developing unit 20 disposed below the exposure unit 13 uses an electrostatic latent image formed on the surface of the photosensitive drum 11 by using, for example, a two-component developer having a black toner and a magnetic carrier. Develop toner. As a result, black toner adheres to the electrostatic latent image on the surface of the photosensitive drum 11 to form a toner image, which is visualized.
The developing device 20 can be replaced with the main body of the image forming apparatus. For example, when the total number of prints (the number of image forming operations) using the developing device 20 reaches a predetermined number set in advance, Assuming that the carrier in the developing device 20 has deteriorated, replacement with a new developing device is instructed.

  The developing device 20 can be detached from the main body of the image forming apparatus, for example, by sliding it to the front side of the main body of the image forming apparatus (the front side of the sheet of FIG. 1). The new developing device is attached to the image forming apparatus by sliding from the front side of the main body of the image forming apparatus toward the inner back side in a space formed by removing the developing device 20. Details of the configuration of the developing device 20 will be described later.

  A transfer roller 15 is provided on the downstream side in the rotation direction of the photosensitive drum 11 with respect to the developing device 20 (on the right side of the photosensitive drum 11) so as to face the photosensitive drum 11. The transfer roller 15 is configured to be movable between a state in which it is pressed against the photoconductive drum 11 and a state in which the transfer roller 15 is separated from the photoconductive drum 11 by a transfer roller contact / separation mechanism 15 a having a motor, for example. The transfer roller 15 is pressed against the photosensitive drum 11 to form a transfer nip portion with the photosensitive drum 11 and rotates in the direction indicated by the arrow C following the rotation of the photosensitive drum 11. .

A predetermined transfer bias voltage is applied to the transfer roller 15 by a transfer bias voltage application mechanism (not shown).
Below the developing unit 20, there is provided a recording sheet cassette 16 that can store recording sheets S such as recording paper and OHP sheets. The recording sheet S stored in the recording sheet cassette 16 is fed by a paper feed roller 17. The recording sheet cassette 16 is fed out one by one. The recording sheet S fed out from the recording sheet cassette 16 is conveyed to the transfer nip portion by a timing roller pair 18 at a predetermined timing.

The toner image formed on the photosensitive drum 11 is subjected to a transfer electric field generated by a transfer bias voltage applied to the transfer roller 15 while the recording sheet S conveyed to the transfer nip passes through the transfer nip. Thus, the image is transferred onto the recording sheet S. The recording sheet S to which the toner image has been transferred is conveyed to a fixing device 30 provided above the transfer roller 15.
The fixing device 30 includes a pressure roller 32 as a pressure member, and a cylindrical fixing roller 31 that rotates while being in pressure contact with the pressure roller 32, and the recording sheet S is in the pressure contact portion between the two. A fixing nip that passes therethrough is formed. A heater 33 is disposed at the axial center of the fixing roller 31, and the outer peripheral surface of the fixing roller 31 is heated by the heater 33.

The fixing roller 31 and the pressure roller 32 that are in pressure contact with each other, for example, so that the fixing roller 31 rotates following the rotation of the pressure roller 32 when the pressure roller 32 is rotated by a motor (not shown). It has become.
While the recording sheet S conveyed to the fixing nip portion passes through the fixing nip portion, the unfixed toner image on the recording sheet S is heated and pressed by the fixing roller 31 and the pressure roller 32. As a result, the image is fixed on the recording sheet S. The recording sheet S that has passed through the fixing nip is peeled off from the fixing roller 31 and discharged onto the paper discharge tray 14 by the paper discharge roller pair 19.

  Note that a toner collection mechanism that collects residual toner remaining on the photosensitive drum 11 after the toner image is transferred onto the recording sheet S and conveys it to the developing unit 20 above the photosensitive drum 11 and the developing unit 20. 40 is provided. The toner collection mechanism 40 includes a toner collection unit 41 that collects toner from the surface of the photosensitive drum 11 and a toner conveyance unit 43 that conveys the collected toner to the developing device 20. The toner conveying unit 43 is configured to be able to switch to a state in which the toner collected by the toner collecting unit 41 is accumulated without being conveyed to the developing device 20.

  The toner conveying unit 43 of the toner collecting mechanism 40 is in a state of conveying the toner collected by the toner collecting unit 41 to the developing device 20 when the printing operation is executed. On the other hand, if it is determined that the carrier in the developing device 20 is in a deteriorated state, the toner transport unit 43 is switched to a state in which toner is accumulated, and the toner in the developing device 20 is transferred to the toner transport unit. The toner accumulation control accumulated in 43 is executed. When the toner accumulation control is completed, the replacement of the developing device 20 is instructed. Details of the configuration of the toner recovery mechanism 40 will be described later.

When the toner accumulation control is executed at a position downstream of the developing device 20 in the rotation direction of the photosensitive drum 11 and upstream of the transfer roller 15 in the rotation direction of the photosensitive drum 11, the photosensitive member is controlled. A toner adhesion amount sensor 28 that detects the amount of toner adhered to the surface of the drum 11 is provided.
In the toner accumulation control, the photosensitive drum 11, the charger 12, the exposure device 13, and the developing device 20 are driven, whereby a predetermined high density in which toner uniformly adheres to the entire image forming area on the photosensitive drum 11. The “solid” image is formed in a predetermined pattern. The “solid” image formed on the surface of the photosensitive drum 11 is transferred to the toner recovery mechanism 40 without being transferred to the recording sheet S, and is “solid” by the toner recovery unit 41 of the toner recovery mechanism 40. Is collected. The toner collected by the toner collection unit 41 is accumulated in the toner conveyance unit 43 without being conveyed to the developing device 20.

  In this manner, the toner in the developing device 20 is accumulated in the toner conveying section 43, so that the toner amount in the developing device 20 decreases sequentially. The toner adhesion amount sensor 28 detects the toner adhesion amount of the “solid” image formed on the photosensitive drum 11 while such toner accumulation control is being executed, and based on the detection result, The driving of the photosensitive drum 11, the charger 12, the exposure device 13, and the developing device 20 is stopped, and the formation of the “solid” image is completed. Thereafter, replacement of the developing device 20 is instructed.

<Developer configuration>
The developing device 20 has a housing 21 in which a two-component developer having a carrier and toner is accommodated. The housing 21 extends long along the axial direction of the photosensitive drum 11, and a developing roller 24 disposed in parallel to face the photosensitive drum 11 is provided in the housing 21. The developing roller 24 is opposed to almost the entire area of the photosensitive drum 11 in the axial direction.

  Provided inside the housing 21 are a supply screw 23 for supplying the two-component developer to the developing roller 24 and a stirring screw 22 for supplying the developer to the supply screw 23. The supply screw 23 is disposed in parallel with the developing roller 24 at a position facing the photosensitive drum 11 with the developing roller 24 interposed therebetween. The stirring screw 22 is disposed in parallel with the supply screw 23 at a position facing the developing roller 24 with the supply screw 23 interposed therebetween.

The interior of the housing 21 is divided by a partition wall 21c into a stirring chamber 21a in which a stirring screw 22 is disposed and a supply chamber 21b in which a supply screw 23 is disposed. In the developing roller 24 disposed in the supply chamber 21b, a portion close to the photosensitive drum 11 is opposed to the surface of the photosensitive drum 11 along the axial direction.
The stirring screw 22 is rotated in a predetermined direction by a driving unit (not shown), and conveys the developer (carrier and toner) in the stirring chamber 21a along the axial direction. The developer conveyed by the agitating screw 22 is charged with the toner and the carrier being agitated and mixed during the conveyance.

An opening (not shown) is formed in the partition wall 21c so as to face the downstream side of the developer in the stirring screw 22 in the conveyance direction of the developer, and the developer conveyed by the stirring screw 22 through this opening. It is supplied from the stirring chamber 21a to the supply chamber 21b.
The supply screw 23 is configured to convey the developer in a direction opposite to the conveyance direction by the stirring screw 22. The developer conveyed by the supply screw 23 is supplied to the developing roller 24 during the conveyance.

  The developing roller 24 is provided in the developing sleeve 24a and a cylindrical developing sleeve 24a arranged in parallel to the photosensitive drum 11 in the same manner as the developing roller used in a developing device using a conventional two-component developer. Magnet roller 24b. The magnet roller 24b is fixed to the housing 21, and a plurality of magnetic poles are formed at predetermined positions along the circumferential direction.

  The developing sleeve 24a is rotatably supported by the housing 21, and rotates around the magnet roller 24b in the direction indicated by the arrow B. A predetermined developing bias voltage is applied to the developing sleeve 24a by the developing bias voltage applying means 25. The developing bias voltage applying means 25 is controlled by the control unit 45 and develops so that the charged toner adheres to the electrostatic latent image on the photosensitive drum 11 with respect to the developing sleeve 24a. Apply a bias voltage.

  The amount of developer supplied to the developing roller 24 is regulated by the regulating member 26 to the photosensitive drum 11. The developer conveyed on the peripheral surface of the developing sleeve 24a of the developing roller 24 without being regulated by the regulating member 26 is in a state where toner is attached to the magnetic brush formed by the carrier, and the developer sleeve 24a The toner adheres to the electrostatic latent image on the photosensitive drum 11 by the applied developing bias voltage.

  An opening (not shown) is formed in the partition wall 21c so as to face the downstream side of the developer in the supply screw 23 in the conveyance direction of the developer, and developer (toner and carrier) that is not supplied to the developing roller 24 is formed in the opening. It is supplied from the supply chamber 21b to the stirring chamber 21a through the section. Therefore, inside the housing 21, the carrier is normally circulated by the stirring screw 22 and the supply screw 23 without being supplied to the photosensitive drum 11.

  The housing 21 is provided with a toner concentration sensor 27 that detects the toner concentration of the developer in the stirring chamber 21a. The toner concentration sensor 27 is attached to the outer peripheral surface corresponding to the bottom surface of the stirring chamber 21a. As the toner concentration sensor 27, for example, a magnetic permeability detection type sensor using the inductance of the coil is used. By using the inductance of the coil, the developer in a fixed volume near the sensor in the stirring chamber 21a is used. An apparent change in magnetic permeability is detected, and a toner concentration is detected based on the detected change in magnetic permeability.

  The output of the toner density sensor 27 is given to the control unit 45. Based on the toner concentration in the housing 21 detected by the toner concentration sensor 27, the control unit 45 drives the toner supply mechanism 44 provided in the toner recovery mechanism 40 to supply toner into the developing device 20. Execute control. Details of the toner supply mechanism 44 and toner supply control will be described later.

<Configuration of toner recovery mechanism>
As shown in FIG. 1, the toner collection mechanism 40 includes a toner conveyance unit 43 arranged horizontally on the side of the photosensitive drum 11 (left side in FIG. 1), and the toner collection unit 43 on the toner conveyance unit 43. 41 is arranged in a horizontal state.
The toner recovery unit 41 includes a housing 41 a that stores toner recovered from the photosensitive drum 11. The housing 41a includes a first linear portion 41x disposed horizontally along the axial direction of the photosensitive drum 11, and an end of the first linear portion 41x on the inner back side of the image forming apparatus above the developing device 20. And a second linear portion 41y extending horizontally along a direction orthogonal to the axial direction of the photosensitive drum 11 so as to pass through the photosensitive drum 11.

An opening 41b that faces the surface of the photosensitive drum 11 is provided in the first linear portion 41x of the housing 41a along the axial direction of the photosensitive drum 11. The opening 41b faces the surface of the photosensitive drum 11 over almost the entire length in the axial direction.
A strip-like cleaning blade 41c that scrapes off the toner on the surface of the photosensitive drum 11 is provided along the side edge of the opening 41b on the downstream side in the rotation direction of the photosensitive drum 11 in the first linear portion 41x. ing. The cleaning blade 41 c is arranged so that one side edge in the width direction orthogonal to the longitudinal direction is in sliding contact with the entire length of the surface of the photosensitive drum 11 in the axial direction.

Inside the first linear portion 41x, a toner feeding member 41d disposed along the axial direction of the photosensitive drum 11 is provided. The toner feeding member 41d is constituted by a conveying screw that conveys the toner scraped off from the surface of the photosensitive drum 11 by the cleaning blade 41c toward the inner side of the image forming apparatus.
A toner transfer plate 41e that feeds the conveyed toner into the second linear portion 41y is provided at the inner back end of the toner feeding member 41d. The toner transfer plate 41e is rotated integrally with the toner feed member 41d, and the toner transported by the toner feed member 41d is accommodated in the housing by the toner transfer plate 41e provided on the toner feed member 41d. It is fed into the second straight portion 41y of 41a.

  In the second straight portion 41y of the housing 41a, a toner recovery screw 41f that transports the toner fed by the toner transfer plate 41e along the second straight portion 41y is provided. The toner recovery screw 41f is rotated in a predetermined direction so as to convey the toner in the second linear portion 41y in a direction away from the first linear portion 41x.

  The second linear portion 41y of the housing 41a extends beyond the developing device 20 in a direction away from the photosensitive drum 11 (left side in FIG. 1), and a toner flow is formed on the lower surface of the end portion on the extending side. An outlet 41g is provided. The toner outlet 41g faces the downstream end of the toner collection screw 41f in the toner conveyance direction, and the toner conveyed by the toner collection screw 41f flows downward through the toner outlet 41g.

  The toner conveying portion 43 of the toner collecting mechanism 40 is provided below the second linear portion 41y, and has a housing 43a that accommodates toner that flows in through the toner outlet 41g. The housing 43a is configured in a straight line along the second linear portion 41y, and includes a first end 43x disposed on a side portion (right side in FIG. 1) close to the photosensitive drum 11, and the photosensitive drum 11. And a second end 43y disposed on the opposite side (left side in FIG. 1). The second end portion 43y is located in a direction away from the photosensitive drum 11 (on the left side in FIG. 1) relative to the second linear portion 41y of the housing 41a in the toner recovery portion 41.

  Inside the housing 43a, a toner supply screw 43b disposed along the longitudinal direction from the second end 43y to the first end 43x is provided. The toner supply screw 43b is rotated in a predetermined direction by a motor 43c provided outside the second end 43y of the housing 43a, and the toner supply screw 43b is rotated in a predetermined direction, whereby the housing The toner in 43a is conveyed to the first end 43x.

  The rotational force of the motor 43c is also transmitted to the toner feeding member 41d and the toner collecting screw 41f provided in the toner collecting unit 41, and the toner feeding member 41d and the toner collecting screw are driven by driving the motor 43c. 41f is rotated in synchronization with the toner supply screw 43b. The motor 43c is controlled by the control unit 45.

  The first end 43x of the housing 43a is located on the stirring chamber 21a in the developing device 20. The toner outlet 41g of the second linear portion 41y is located in the middle of the first end portion 43x and the second end portion 43y in the housing 43a of the toner conveying portion 43 (the middle in the left-right direction in FIG. 1). , Communicated with the inside of the housing 43a. Therefore, when the toner conveyed to the toner outlet 41g by the toner collecting screw 41f of the toner collecting unit 41 is supplied into the housing 43a of the toner conveying unit 43, the toner is brought close to the photosensitive drum 11 by the toner replenishing screw 43b. It is conveyed to the first end 43x.

  A toner replenishing port 43d communicating with the stirring chamber 21a is provided on the lower surface of the first end 43x of the housing 43a located on the stirring chamber 21a in the developing device 20. A shutter 43e that opens and closes the toner supply port 43d is provided on the lower surface of the first end 43x. The shutter 43e is slid in the horizontal direction by, for example, a shutter opening / closing mechanism 43f having a motor or the like, and the toner supply port 43d is closed by being slid downstream in the toner transport direction by the toner supply screw 43b. The toner supply port 43d is opened by sliding in the opposite direction.

  The shutter opening / closing mechanism 43f is controlled by the control unit 45. The controller 45 slides the shutter 43e so that the toner replenishing port 43d is opened when the developing device 20 performs a printing operation based on the image data, and replenishes the toner when executing the toner accumulation control. The shutter 43e is slid so that the opening 43d is closed.

  When the shutter 43e opens the toner supply port 43d, the toner conveyed by the toner supply screw 43b flows into the stirring chamber 21a of the developing device 20 through the toner supply port 43d. On the other hand, when the shutter 43e closes the toner replenishing port 43d, the toner conveyed by the toner replenishing screw 43b does not flow into the stirring chamber 21a of the developing device 20 and flows inside the housing 43a. Accumulated in.

A toner replenishing mechanism 44 is provided on the second end 43 y of the housing 43 a in the toner transport unit 43. The toner supply mechanism 44 includes, for example, a detachable toner bottle, and the toner contained in the toner bottle is moved into the housing 43 a of the toner transport unit 43 by rotating the toner bottle. Supplied.
The toner replenishing mechanism 44 is controlled by a control unit 45, and the control unit 45 provides a toner density sensor provided in the developing device 20 when the developing device 20 performs a printing operation based on image data. The toner replenishing mechanism 44 is controlled based on the toner density in the developing device 20 detected by 27.

  The toner supplied from the toner supply mechanism 44 into the second end 43y of the housing 43a is conveyed to the first end 43x of the housing 43a by the toner supply screw 43b. In this case, since the developing device 20 is executing a printing operation based on the image data, the toner supply port 43d provided at the first end 43x is open. Accordingly, the toner conveyed to the first end portion 43x is supplied into the stirring chamber 21a of the developing device 20 through the toner supply port 43d.

<Configuration of toner adhesion amount sensor>
FIG. 2 is a schematic diagram for explaining the configuration of the toner adhesion amount sensor 28 disposed to face the photosensitive drum 11. In FIG. 2, the state shown in FIG. 1 is shown upside down. As the toner adhesion amount sensor 28, for example, a reflective optical sensor having a light emitting element 28a and a light receiving element 28b is used. The light emitting element 28a is arranged so that the light emitted from the light emitting element 28a is irradiated to the central portion in the axial direction on the surface of the photosensitive drum 11, and the light receiving element 28b is emitted from the light emitting element 28a. It arrange | positions so that the light reflected by the surface of the photosensitive drum 11 may be received.

  The toner adhesion amount sensor 28 using such a reflection type optical sensor emits light to a “solid” image as a specific pattern formed on the surface of the photosensitive drum 11 by the light emitting element 28a when the toner accumulation control is executed. , And the amount of reflected light reflected from the “solid” image is received by the light receiving element 28b. The light receiving element 28 b outputs a voltage corresponding to the received light amount to the control unit 45.

The surface of the “solid” image formed on the photoconductive drum 11 has unevenness due to toner particles being stacked in layers, and the unevenness of the surface of the “solid” image depends on the amount of toner attached. By changing, the amount of reflected light changes.
That is, when the amount of toner adhering to the surface of the photosensitive drum 11 is small, the toner constituting the “solid” image is relatively thinly laminated, and the surface of the “solid” image is compared with the toner. Small irregularities (steps) are formed. Thereby, a large amount of light is specularly reflected (indicated by an arrow M in FIG. 2), and the amount of light (indicated by an arrow R in FIG. 2) that is irregularly reflected on the surface of the “solid” image is reduced.

  On the other hand, when the amount of toner of the “solid” image formed on the surface of the photosensitive drum 11 is large, the toner constituting the “solid” image is in a relatively thick laminated state. For this reason, a relatively large unevenness (step) is formed on the surface of the “solid” image by the toner. This increases the amount of light that is irregularly reflected on the surface of the “solid” image, while reducing the amount of light that is specularly reflected.

  In the toner accumulation control, the toner adhesion amount of the “solid” image formed on the surface of the photosensitive drum 11 corresponds to the toner concentration in the developing device 20, and as the toner concentration in the developing device 20 decreases, The amount of toner that forms a “solid” image on the surface of the photosensitive drum 11 is reduced. Therefore, based on the relationship between the density of the “solid” image on the photosensitive drum 11 and the toner density in the developing device 20, development is performed based on the toner adhesion amount of the “solid” image detected by the toner adhesion amount sensor 28. The toner concentration in the container 20 can be detected.

  FIG. 3 shows the relationship between the toner concentration (% by weight) in the developing device 20 and the output voltage of the toner adhesion amount sensor 28 when a “solid” image is formed on the surface of the photosensitive drum 11 in the toner accumulation control. It is a graph. As shown in FIG. 3, the toner density in the developing device 20 is in a range of about 1% to 4% (a broken line in FIG. 3), which is lower than an appropriate range (about 4% to 6%) when the printing operation is executed. The output voltage of the toner adhesion amount sensor 28 decreases linearly so as to be almost inversely proportional to the toner density. Therefore, the toner density in the developing device 20 can be accurately detected based on the output voltage of the toner adhesion amount sensor 28 within a low density range of about 2%.

  In the toner accumulation control, the “solid” image formed on the surface of the photosensitive drum 11 is conveyed to the toner collecting mechanism 40 without being transferred to the recording sheet S by the toner in the developing device 20, and the toner is collected. After being collected from the surface of the photosensitive drum 11 by the unit 41, it is accumulated in the toner transport unit 43. Accordingly, as the toner accumulation control execution time becomes longer, the amount of toner remaining in the developing device 20 decreases sequentially, and the toner density in the developing device 20 decreases sequentially.

When the toner density in the developing device 20 decreases, the amount of toner necessary for supplying the electrostatic latent image to the electrostatic latent image does not change even if the potential of the electrostatic latent image of the “solid” image does not change from the initial state. It decreases in the developing device 20. Therefore, the toner amount per unit area of the “solid” image is also reduced.
When the toner adhesion amount of the “solid” image on the photosensitive drum 11 detected by the toner adhesion amount sensor 28 is reduced to a predetermined value (for example, 2%) or less, the control unit 45 causes the toner in the developing device 20 to be Assuming that the density has dropped below a predetermined value, the driving of the photosensitive drum 11, the charger 12, the exposure device 13, and the developing device 20 is stopped.

The toner adhesion amount sensor 28 is used when toner accumulation control is executed, and is not used when a print operation based on image data is executed.
<Control during image forming operation by control unit>
The control unit 45 adjusts the transfer bias voltage applied to the driving means for rotating the photosensitive drum 11, the charger 12, the exposure device 13, and the transfer roller 15, and the stirring screw 22 in the developing device 20, as in the past. Then, a printing operation for forming a toner image based on the image data is executed by controlling the supply screw 23, driving means for rotating the developing sleeve 24a of the developing roller 24, and the like.

  In this case, the control unit 45 controls the transfer roller contact / separation mechanism 15 a so that the transfer roller 15 is pressed against the photosensitive drum 11. Further, the control unit 45 controls the developing bias voltage applying unit 25 so that a predetermined developing bias voltage is applied to the developing sleeve 24 a of the developing roller 24 in the developing device 20. In the present embodiment, the toner is charged to a negative potential during the printing operation, and a voltage of −500 V is applied to the developing sleeve 24a as the developing bias voltage.

  The control unit 45 further drives each of the stirring screw 22 of the developing device 20, the supply screw 23, and the developing sleeve 24 a of the developing roller 24 to rotate, and controls the motor 43 c to control the toner recovery mechanism 40. The toner feeding member 41d and the toner collecting screw 41f of the toner collecting unit 41 and the toner replenishing screw 43b of the toner conveying unit 43 are rotated. Further, the shutter opening / closing mechanism 43f is controlled so that the shutter 43e provided in the toner conveying portion 43 opens the toner supply port 43d.

In such a state, a printing operation based on the image data is executed. In this case, the rotated photosensitive drum 11 is uniformly charged by the charger 12 and then exposed by the exposure device 13, whereby an electrostatic latent image corresponding to image data is formed on the surface of the photosensitive drum 11. It is formed.
In the developing device 20, when the developer is conveyed by the developing sleeve 24 a of the developing roller 24, the developing bias voltage is applied to the developing sleeve 24 a by the developing bias voltage applying unit 25, so Toner adheres to the electrostatic latent image formed on the surface. Thereby, the electrostatic latent image is developed (visualized) with the toner.

The toner image formed on the surface of the photosensitive drum 11 is transferred to the recording sheet S conveyed from the recording sheet cassette 16 at the transfer nip portion where the transfer roller 15 is pressed against the photosensitive drum 11. The recording sheet S to which the toner image is transferred is conveyed to the fixing device 30, and the toner image is fixed to the recording sheet S in the fixing device 30.
When the toner image is transferred to the recording sheet S, the toner remaining on the surface of the photosensitive drum 11 is scraped off by the cleaning blade 41 c provided in the toner recovery unit 41 of the toner recovery mechanism 40. . The toner scraped off is transported along the axial direction of the photosensitive drum 11 through the first linear portion 41x of the housing 41a by the toner feeding member 41d.

  The toner conveyed in the first linear portion 41x is fed into the second linear portion 41y from the first linear portion 41y by the toner transfer plate 41e of the toner feeding member 41d, and the second linear portion by the toner recovery screw 41f. It is conveyed in 41y. The toner conveyed in the second linear portion 41y passes through the toner outlet 41g, is supplied into the housing 43a of the toner conveying portion 43, and is conveyed to the toner supply port 43d by the toner supply screw 43b.

In this case, since the toner replenishing port 43d is opened without being closed by the shutter 43e, the toner conveyed to the toner replenishing port 43d passes through the toner replenishing port 43d and is stirred in the stirring chamber 21a of the developing device 20. Supplied in.
Further, the toner concentration in the stirring chamber 21 a of the developing device 20 is detected by the toner concentration sensor 27, and the control unit 45 determines that the toner concentration in the developing device 20 detected by the toner concentration sensor 27 is the photosensitive drum 11. When the upper electrostatic latent image falls below a predetermined range (for example, 6%) required for developing the toner, the toner bottle in the toner replenishing mechanism 44 reaches the density within the predetermined range for a predetermined time. Rotate over time. As a result, the toner in the toner bottle is supplied into the housing 43 a of the toner transport unit 43.

  The toner supplied into the housing 43a is conveyed through the housing 43a to the first end 43x by the toner supply screw 43b, and is supplied into the stirring chamber 21a of the developing device 20 through the toner supply port 43d. . As a result, the toner density of the developer in the developing device 20 becomes high, and the toner density becomes an appropriate toner density required for developing the electrostatic latent image on the photosensitive drum 11 with toner.

  FIG. 4 is a graph showing the relationship between the toner density and the output voltage in the toner density sensor 27 of the magnetic permeability detection type. As shown in FIG. 4, in the magnetic permeability detection type toner density sensor 27, when the toner density is increased, the apparent magnetic permeability in the developer is lowered and the output voltage value is lowered. On the contrary, when the toner concentration is lowered, the apparent magnetic permeability in the developer is increased and the output voltage value is increased. The toner density sensor 27 is a straight line that is almost inversely proportional to the toner density in the toner density range (about 3 to 6%) required in the printing operation (image forming operation) (the range indicated by R1 in FIG. 4). As a result, the output voltage value decreases.

As described above, the toner density sensor 27 can detect the toner density in the developing device 20 with high accuracy in the range of 3 to 6% which is an appropriate toner density during the printing operation. Therefore, the toner concentration in the developing device 20 can be reliably maintained within an appropriate range of 3 to 6%.
The magnetic permeability detection type toner density sensor 27 responds to changes in toner density in a range (range indicated by R2 in FIG. 4) other than the appropriate toner density range (3 to 6%) during the printing operation. The change in the output voltage becomes small and tends to deviate from the proportional relationship with the toner density. Therefore, it is not suitable for toner accumulation control in which a toner density of about 2% is a detection target.

<Toner accumulation control by control unit>
The control unit 45 is provided with a counter for accumulating the number of image forming operations (the number of prints) every time a print operation is executed, and a predetermined number of prints accumulated by the counter is set in advance. When the value reaches, it is assumed that the carrier in the developing device 20 has deteriorated. For example, the control unit 45 displays an instruction to replace the developing device 20 on an operation panel (not shown). The counter is reset when the developing device 20 is replaced.

When an instruction to replace the developing device 20 is displayed on the operation panel, the developing device 20 is replaced by, for example, a service person. In this case, when the execution of toner accumulation control is instructed by a service person prior to replacement of the developing device 20, the control unit 45 executes toner accumulation control.
FIG. 5 is a flowchart illustrating a procedure of toner accumulation control executed by the control unit 45. This flowchart is started when a replacement instruction for the developing device 20 is displayed. First, the control unit 45 confirms whether the start of toner accumulation control is instructed (see step S11 in FIG. 5, the same applies hereinafter). The instruction to start toner accumulation control is performed, for example, when a serviceman or the like operates a predetermined switch or the like provided on the operation panel. If the start of toner accumulation control is not instructed (“NO” in step S11), the process waits until instructed.

When the controller 45 receives an instruction to start toner accumulation control (“YES” in step S11), the controller 45 controls the transfer roller contact / separation mechanism 15a to separate the transfer roller 15 from the photosensitive drum 11 and the shutter 43e. Controls the shutter opening / closing mechanism 43f so as to close the toner supply port 43d (step S12).
Further, the controller 45 applies bias voltage applying means so that the developing bias voltage applied to the developing sleeve 24a of the developing device 20 becomes a voltage that increases the amount of toner adhering to the photosensitive drum 11 as compared with the printing operation. 25 is adjusted (step S13). For example, when the toner is charged to a negative polarity, the bias voltage applying unit 25 adjusts the developing bias voltage during printing operation to −500 V and to −700 V when performing toner accumulation control.

In such a state, the control unit 45 performs control so that each of the charger 12, the exposure device 13, and the developing device 20 is in a driving state. Further, the motor 43c of the toner collecting mechanism 40 is rotated so that the toner collecting mechanism 40 is in a driving state, and toner accumulation control for accumulating the toner in the developing device 20 in the toner transport unit 43 of the toner collecting mechanism 40 is started. (Step S14).
In the toner accumulation control, similarly to the normal printing operation, the surface of the photosensitive drum 11 that has been rotated is uniformly charged by the charger 12, and the entire surface of the image forming area on the surface of the photosensitive drum 11 is filled with toner. The entire surface of the image forming area is exposed by the exposure device 13 so that a “solid” image with the attached image is formed. In this way, by driving the charging device 12 and the exposure device 13 while the photosensitive drum 11 is rotated, an image electrostatic latent image corresponding to the “solid” image is formed on the photosensitive drum 11. .

Further, the operations of the paper feed roller 17 and the timing roller pair 18 are stopped so that the recording sheet S is not conveyed from the recording sheet cassette 16 to the transfer nip portion.
The developing device 20 develops the electrostatic latent image formed on the surface of the photosensitive drum 11 with toner, as in the case of performing a normal printing operation. As a result, a “solid” image in which the toner adheres over the entire surface of the image forming area on the surface of the photosensitive drum 11 is formed.

  In this case, the developing sleeve 24a is attached to the surface of the photosensitive drum 11 by applying a developing bias voltage so that the amount of toner attached to the photosensitive drum 11 is larger than that in the normal printing operation. The amount of toner is larger than that during normal printing operation. Therefore, the toner in the developing device 20 can be efficiently accumulated in the toner transport unit 43.

  The “solid” image formed on the surface of the photoconductive drum 11 is provided in the toner collecting unit 41 of the toner collecting mechanism 40 after passing through a region facing the transfer roller 15 separated from the photoconductive drum 11. It reaches the sliding contact portion with the cleaning blade 41c. In this case, the recording sheet S is not conveyed from the recording sheet cassette 16 to the transfer nip portion, and the transfer roller 15 is separated from the surface of the photosensitive drum 11. The toner constituting the “solid” image is conveyed to the sliding contact portion with the cleaning blade 41 c without adhering to the recording sheet S and the transfer roller 15.

  When the surface of the photosensitive drum 11 on which the “solid” image is formed reaches the cleaning blade 41 c, the cleaning blade 41 c scrapes off the toner constituting the “solid” image from the surface of the photosensitive drum 11. The toner scraped off from the surface of the photosensitive drum 11 is conveyed into the housing 43a of the toner conveying portion 43 by the toner feeding member 41d and the toner recovery screw 41f.

  The toner transported into the housing 43a of the toner transport section 43 is transported by a toner replenishing screw 43b to a toner replenishing port 43d located on the stirring chamber 21a in the developing device 20. However, since the toner replenishing port 43d is closed by the shutter 43e, the toner replenishing port 43d is not supplied from the housing 43a of the toner conveying portion 43 into the stirring chamber 21a but is accumulated in the housing 43a.

  While such toner accumulation control is being executed, the control unit 45 monitors the output voltage of the toner adhesion amount sensor 28 that detects the amount of toner adhered to the photosensitive drum 11 (step S15). When it is detected by the output voltage of the amount sensor 28 that the toner density in the developing device 20 has become a predetermined value or less (eg, 2% or less) (“YES” in step S15), the control unit 45 causes the photosensitive drum to 11. The driving of the charging device 12 and the exposure device 13 is stopped to stop the operation for forming the electrostatic latent image on the photosensitive drum 11, and the driving of the developing device 20 is also stopped (step S16). Then, the toner accumulation control is finished in a state where the shutter 43e closes the toner supply port 43d.

In such a state, the control unit 45 displays on the operation panel that the toner accumulation control has been completed (step S17).
When it is displayed on the operation panel that the toner accumulation control has been completed, the developing device 20 is normally replaced by a service person. In this case, only the low-concentration toner of about 2% and the deteriorated carrier remain in the developing device 20 removed from the image forming apparatus.

  When the replacement of the developing device 20 is completed and a new developing device 20 containing a carrier and toner in an undegraded state is attached to the image forming apparatus, the control unit 45 slides the shutter 43e to move the toner. The supply port 43d is opened. When the printing operation is executed in such a state, the toner accumulated in the toner conveying unit 43 is sequentially conveyed to the developing device 20 and used for developing the electrostatic latent image on the photosensitive drum 11. Further, when developing the electrostatic latent image on the photosensitive drum 11, an undegraded carrier in the developing device 20 is used, so that a high quality image can be formed.

  As described above, the driving of the photosensitive drum 11, the charger 12, the exposure device 13, and the developing device 20 when the toner accumulation control is executed is stopped based on the output of the toner adhesion amount sensor 28. As shown in FIG. 3, the toner adhesion amount sensor 28 can detect the toner concentration in the developing device 20 with high accuracy in a low range of 1 to 4%. When the density becomes low, the driving of the photosensitive drum 11, the charging device 12, the exposure device 13, and the developing device 20 can be surely stopped.

  Thereby, there is no possibility that the toner accumulation control is continuously executed in a state where the toner density in the developing device 20 is remarkably lowered. Accordingly, the carrier is prevented from adhering to the surface of the photosensitive drum 11 by executing the developing operation of the electrostatic latent image on the photosensitive drum 11 in a state where the toner density in the developing device 20 is remarkably lowered. Can do. As a result, there is no possibility that the photoconductor drum 11 is damaged by the adhesion of the carrier, and therefore, it is possible to prevent the quality of the image formed during the printing operation from being deteriorated due to the photoconductor drum 11 being damaged.

In addition, since the toner density in the developing device 20 is lowered to a predetermined low density, it is possible to prevent the toner 20 from being replaced with a large amount of toner remaining in the developing device 20.
For example, the toner density sensor 27 provided in the developing device 20 is used to drive the photosensitive drum 11, the charger 12, the exposure device 13, and the developing device 20 without using the toner adhesion amount sensor 28. If the configuration is such that the following is stopped, the following problems may occur.

That is, due to the characteristics of the toner density sensor 27 shown in FIG. 4, when the density becomes 3% or less, the change in the apparent permeability in the developer becomes small, and the change in the output voltage value with respect to the change in the toner density. It tends to be smaller.
As a result, in a low toner density range of 3% or less, the toner density detection accuracy is lowered, so that the toner accumulation control cannot be completed at an appropriate timing, and a large amount of toner is discharged from the developing device 20. There is a risk that it will remain inside or there is a risk that the carrier in the developing device 20 will adhere to the photosensitive drum 11.

  In this embodiment, since the toner adhesion amount sensor 28 using an optical sensor is used instead of such a magnetic permeability detection type toner density sensor 27, the toner density in the developing device 20 is a predetermined low density. It can be detected with high accuracy. As a result, it is possible to prevent the carrier from adhering to the photoconductive drum 11 and scratching the photoconductive drum 11, and to appropriately separate the toner from the carrier without increasing the size of the developing device as in the prior art. can do.

<Modification>
In the above description, the image forming apparatus in which the developing device 20 is replaceable has been described. However, the present invention is not limited to such a configuration. For example, only the developer in the developing device may be replaced. In this case, when the total number of printed sheets reaches a predetermined number, the replacement of the developer is instructed, and after the toner accumulation control is completed, all the developer remaining in the developing device is collected and newly added. Then, a developer having an undegraded carrier and a toner having a predetermined density with respect to the carrier is replenished in the developing device.

  The image forming apparatus of the present invention is not limited to an image forming apparatus that forms a monochrome image, but can be applied to an image forming apparatus that forms a full-color image. When applied to a full-color image forming apparatus, for example, a toner recovery mechanism 41 and a toner adhesion sensor 28 are provided for each image forming unit that forms an image of each color of Y, M, C, and K.

  The present invention is useful as a technique for preventing damage to a photosensitive drum when recovering a deteriorated carrier in a developing device.

DESCRIPTION OF SYMBOLS 11 Photosensitive drum 12 Charging device 13 Exposure device 15 Transfer roller 15a Transfer roller contact / separation mechanism 20 Developing device 21 Housing 21a Stirring chamber 21b Supply chamber 22 Stirring screw 23 Supply screw 24 Developing roller 24a Developing sleeve 24b Magnet roller 25 Application of developing bias voltage Means 27 Toner concentration sensor 28 Toner adhesion amount sensor 28a Light emitting element 28b Light receiving element 40 Toner recovery mechanism 41 Toner recovery part 41b Opening part 41c Cleaning blade 41d Toner feeding member 41e Toner transfer plate 41f Toner recovery screw 43 Toner transport part 43a Housing 43b Toner Supply screw 43d Toner supply port 43e Shutter 43f Shutter opening / closing mechanism 44 Toner supply mechanism 45 Control unit

Claims (7)

An image carrier;
An electrostatic latent image forming means for forming an electrostatic latent image on the image carrier;
A developer for visualizing the electrostatic latent image on the image carrier by supplying toner using a two-component developer having a carrier and toner;
Transfer means for transferring the toner on the image carrier to a recording sheet;
Collects the toner remaining on the image carrier after the toner is transferred to the recording sheet by the transfer means, and stores the collected toner in a state where it is transported to the developing device and without being transported to the developing device. A toner recovery mechanism that can be switched between
A toner adhesion amount detecting means for optically detecting the amount of toner adhered on the image carrier;
At a predetermined timing, the toner collection mechanism is switched to a state in which toner is accumulated, and an electrostatic latent image is formed on the image carrier, and after the toner is attached, toner accumulation control for accumulation in the toner collection mechanism During the execution of this control, if the toner adhesion amount detected by the toner adhesion amount detection means falls below a predetermined value, the electrostatic latent image forming means forms an electrostatic latent image. Control means for stopping;
An image forming apparatus comprising: The transfer unit includes a transfer member to which a voltage is applied, and a moving unit that moves the transfer member to a state of being pressed against the image carrier and a state of being separated from the image carrier.
2. The image forming apparatus according to claim 1, wherein the control unit controls the moving unit to separate the transfer member from the image carrier during execution of the toner accumulation control. The toner collecting mechanism includes a shutter that opens and closes a toner supply port through which the collected toner passes and flows into the developing unit;
3. The image forming apparatus according to claim 1, wherein the control unit controls the shutter so that the toner supply port is closed when the toner accumulation control is executed. The developing device includes a toner concentration sensor that detects a toner concentration in the developing device based on a change in magnetic permeability;
A toner supply mechanism for supplying toner to the developing device,
The image forming apparatus according to claim 1, wherein the control unit controls toner supply by the toner supply mechanism based on a toner concentration detected by the toner concentration sensor. . The developing device includes a developing roller that supplies toner to the image carrier, and a developing bias voltage applying unit that applies a developing bias voltage to the developing roller.
The control means is configured to increase the developing bias voltage applied to the developing roller during execution of the toner accumulation control so that the amount of toner adhering to the image carrier is larger than that during the image forming operation. The image forming apparatus according to claim 1, wherein the application unit is controlled.   The control unit controls the electrostatic latent image forming unit so that the toner adheres to the entire image forming region of the image carrier when the toner accumulation control is executed. The image forming apparatus according to claim 5.   The image forming apparatus according to claim 1, wherein the developing unit is replaceable.

Images