JP4962025B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus and a method for supplying lubricating toner to a cleaning elastic plate member.

JP 63-202782 A Japanese Patent Laid-Open No. 03-007977 JP 2004-191737 A

  Conventionally, as an electrophotographic image forming apparatus, for example, an electrostatic latent image formed by charging and exposing the surface of a photoreceptor is developed to form a toner image, the toner image is primarily transferred to an intermediate transfer body, and then recorded. It is known that a secondary transfer is performed on a recording medium such as paper, and this is heated and pressed to fix a toner image to form a desired image. In general, in such an electrophotographic image forming apparatus, for example, a plate-like elastic member (so-called cleaning blade) is used as a means for cleaning (cleaning and removing) transfer residual toner remaining on the surface of the photoreceptor after transfer. (For example, refer to Patent Documents 1 to 3).

  Here, Patent Document 1 discloses an image forming apparatus configured to form a toner image on a photoreceptor and supply toner and an external additive to a cleaning blade when the apparatus is installed or replaced.

  Patent Document 2 discloses an image forming apparatus configured to form a toner image on a photoreceptor and to supply toner and external additives to a cleaning blade every time an image is formed or intermittently.

  Further, in Patent Document 3, a toner image is formed on the photoconductor when the photoconductor is not used or when a certain running cycle is reached, and toner and external additives are supplied to the cleaning blade. A configured image forming apparatus is disclosed.

  The object of the present invention is to suppress the generation of discharge products and to reduce the stress caused by the electric field on the electrostatic latent image holding member, compared to the case of supplying a toner or the like by applying a voltage. Another object of the present invention is to provide a method for supplying lubricating toner to an image forming apparatus and a cleaning elastic plate member.

  In order to achieve the above object, an image forming apparatus according to claim 1, wherein an electrostatic latent image is held on a surface of the image forming apparatus and the electrostatic latent image holding body formed to be rotatable, and a charging voltage is applied. A charging device for charging the surface of the electrostatic latent image holding member to a predetermined potential, a developing device for developing the electrostatic latent image formed on the electrostatic latent image holding member with toner, and the electrostatic device And a cleaning device having a cleaning plate-like elastic member that contacts the surface of the latent image holding member and cleans the transfer residual toner, and the electrostatic device without applying a charging voltage to the charging device during non-image formation. The latent image holding member is rotated to supply toner and external additives to the cleaning elastic plate member.

  The image forming apparatus according to claim 2, in the configuration according to claim 1, when supplying toner and an external additive to the cleaning plate-like elastic member, a developing voltage is not applied to the developing device. It is characterized by.

  According to a third aspect of the present invention, in the configuration according to the first or second aspect, when the toner and the external additive are supplied to the cleaning elastic plate member, the developing roller of the developing device is rotated. It is characterized by making it.

  According to a fourth aspect of the present invention, there is provided the image forming apparatus according to any one of the first to third aspects, wherein the toner and the external additive are supplied to the cleaning plate-like elastic member. The supply area of the toner and the external additive on the electrostatic latent image holding member is changed according to the toner density.

  According to a fifth aspect of the present invention, in the configuration according to the fourth aspect, when the toner density in the developing device decreases below a predetermined density, the rotational speed or rotation of the electrostatic latent image holding member. It is characterized by increasing time.

  The method for supplying lubricating toner to the cleaning plate-like elastic member according to claim 6 applies an electrostatic latent image holding member formed on a surface of the electrostatic latent image holding member so as to be rotatable, and a charging voltage. Accordingly, a charging device that charges the surface of the electrostatic latent image holding member to a predetermined potential, a developing device that develops the electrostatic latent image formed on the electrostatic latent image holding member with toner, and the static device A cleaning device having a cleaning plate-like elastic member that contacts the surface of the electrostatic latent image holding member and cleans transfer residual toner, and the static electricity is applied to the charging device without applying a charging voltage during non-image formation. The electrostatic latent image holding member is rotated, the developing roller is rotated without applying a developing voltage to the developing device, and the toner and the external additive are supplied to the cleaning plate-like elastic member. .

  According to the first aspect of the present invention, the generation of the discharge product is suppressed and the stress due to the electric field on the electrostatic latent image holding member is reduced as compared with the case where the toner is supplied by applying the charging voltage. be able to.

  According to the second aspect of the present invention, it is possible to further reduce the stress due to the electric field applied to the electrostatic latent image holding member as compared with the case where the toner is supplied by applying the developing voltage.

  According to the third aspect of the present invention, the toner and the external additive can be supplied to a wider surface area of the electrostatic latent image holding member as compared with the configuration in which the developing roll is stopped.

  According to the fourth aspect of the present invention, a constant amount of lubricating toner can always be stably supplied to the cleaning plate-like elastic member.

  According to the fifth aspect of the present invention, it is possible to easily realize an image forming apparatus capable of changing the supply area of the toner and the external additive on the electrostatic latent image holding member according to the toner density. .

  According to the sixth aspect of the present invention, voltage application to the electrostatic latent image holding member and generation of discharge products are suppressed to the minimum necessary, and stress on the electrostatic latent image holding member is reduced and cleaning is performed. The toner and the external additive can be stably supplied to the plate-like elastic member, and the increase in the driving torque of the electrostatic latent image holding member with time can be stably suppressed.

  Embodiments according to the present invention will be described below with reference to the drawings.

  First, a schematic configuration of an image forming apparatus according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is an overall configuration diagram of a tandem type color image forming apparatus according to the present invention.

  In FIG. 1, an image forming apparatus and an original reading apparatus 10 that reads an original 12 on a platen 11 and four color components (for example, yellow, magenta, cyan, and black) images (toner images) by, for example, electrophotography. An image forming unit 20 to be formed (in this embodiment, a yellow image forming unit: 20a, a magenta image forming unit: 20b, a cyan image forming unit: 20c, a black image forming unit: 20d) and each image forming unit 20 An intermediate transfer belt 30 that sequentially transfers (primary transfer) the formed color component images, and a secondary transfer device 50 that collectively transfers (secondary transfer) the superimposed images transferred on the intermediate transfer belt 30 to the recording paper 100. A fixing device 70 for fixing the batch-transferred image on the recording paper 100, and the recording paper 10 at the batch transfer portion. It is obtained by a paper transportation system 80 for supplying a. Note that the reference CR is an apparatus controller that includes a CPU, a ROM, a RAM, and the like, and controls various components in the image forming apparatus and various processes such as a lubricating toner supply mode to be described later.

  In the present embodiment, the document reading apparatus 10 includes, for example, a light source 13 that is disposed below the platen 11 and irradiates light on the surface of the document 12, and a CCD that converts reflected light from the surface of the document 12 into an electrical signal. The photoelectric conversion element 14, an appropriate number of mirrors 15 that form a path for guiding the reflected light from the surface of the document 12 to the photoelectric conversion element 14, and the reflected light from the surface of the document 12 are coupled on the imaging surface of the photoelectric conversion element 14. And an imaging lens 16 for imaging.

  Each of the image forming units 20a to 20d includes photoconductors 21a to 21d that are examples of electrostatic latent image holders, and the photoconductors 21a to 21d are charged around the photoconductors 21a to 21d. Electrostatic latent images (mainly based on image data read by the photoelectric conversion element 14 or image data taken from another recording medium or the like on the charging devices 22a to 22d and the charged photoconductors 21a to 21d. Developing devices 24a to 24d and photosensitive members 21a to 21d in which electrostatic latent images written on the photosensitive members 21a to 21d are developed with respective color component toners. Residual toner on the primary transfer devices 25a to 25d and the photoconductors 21a to 21d, such as a transfer roll, on which the toner image on the upper surface is transferred to the intermediate transfer belt 30 is cleaned with a cleaning plate-like elastic member. Drum cleaning device 26a~26d removed by ring blade 260 is disposed, respectively.

  Here, the image forming units 20a to 20d have the same configuration, and these components (for example, the photoconductors 21a to 21d) have the same structure. For example, the photoconductor 21) is used.

  The photosensitive member 21 in the present embodiment is a small-diameter drum-like member having an outer diameter of φ30 mm, and has, for example, an organic photosensitive material, an amorphous selenium photosensitive material, an amorphous material on the surface of a metal drum that rotates in the arrow direction. A photosensitive layer made of a crosslinked resin having a charge transporting property such as a silicon-based photosensitive material is formed as a film. The charging roll 22, which is a contact type charging device that comes into contact with the surface of the photoreceptor 21, causes the photosensitive layer of the photoreceptor 21 to have a predetermined potential by a so-called charging bias in which an AC voltage (current) is superimposed on a DC voltage (current). It is comprised so that it may be electrically charged. The charging roll 22 comes into contact with the photoconductor 21 with a constant load and a biting amount, and is rotated at the same speed as the photoconductor 21 following the rotation of the photoconductor 21 by the frictional force of the contact portion.

  The charging roll 22 according to the present embodiment is a surface of a roll-shaped member configured by covering a surface of a metal core made of metal such as stainless steel with a conductive elastic layer made of conductive synthetic resin or synthetic rubber. Further, a resistance control layer whose resistance value is adjusted to a predetermined value is laminated, and a release layer is further formed on the surface of the resistance control layer as necessary. Then, for example, by applying a DC voltage on which an AC voltage is superimposed to the metal core, a gap discharge is generated in a minute gap between the charging roll 22 and the photoconductor 21, and the surface of the photoconductor 21 is generated by the discharge. Is to be charged.

  Further, the developing device 24 includes a developing roll 24R to which a predetermined voltage or the like (so-called developing bias) is applied, and a developing container that contains a developer containing polymerized toner that is negatively charged, for example. The developer is agitated by a developer agitating member (not shown) that is rotatably disposed on the photosensitive drum 21 and conveyed to the developing roll 24R, and the photosensitive member 21 and the developing roll 24R are opposed to each other on the photosensitive member 21 in the developing region. The electrostatic latent image is developed (visualized) into a toner image. In the present embodiment, the developing device 24 further includes a density sensor (not shown) that detects the toner density of the developer stored therein, and a toner cartridge (not shown) according to consumption of the developer based on image formation or the like. From now on, a new developer is appropriately supplied.

  On the downstream side of the developing area along the peripheral surface of the photoconductor 21, a primary transfer area is formed in contact with the photoconductor 21 via the intermediate transfer belt 30 so as to face the photoconductor 21 and form a primary transfer area. A transfer roll 25 is provided.

  Further, on the downstream side of the primary transfer area (in this example, substantially on the left side of the photosensitive member 21), the transfer residual toner that has not been transferred to the intermediate transfer belt 30 in the transfer region and has adhered to the photosensitive member 21 is transferred. A drum cleaning device 26 for cleaning is provided. The drum cleaning device 26 is disposed in a housing having a substantially U-shaped cross section disposed so as to extend in the axial direction of the photoconductor 21, and the tip of the drum cleaning device 26 is in contact with the surface of the photoconductor 21 to form a shaft. A cleaning blade 260 is provided so as to extend in the direction.

  The cleaning blade 260 in the present embodiment is attached to the tip of a support member such as a sheet metal and has a flat blade portion formed of an elastic member such as rubber, and this blade portion is the rotation direction of the photosensitive member 21 ( In this example, it is arranged so as to be in contact with the surface of the photosensitive member 21 in the clockwise direction.

  The intermediate transfer belt 30 is an intermediate transfer body to which toner images of respective colors sequentially formed by the image forming units 20 a to 20 d are transferred in a state of being superimposed on each other. It is inserted between each of the primary transfer rolls 25 arranged to face the photoconductor 21, is spanned by a plurality of support rolls 31 to 34, etc., and can be circulated and moved in the direction of the arrow. In this embodiment, the support roll 31 is used as a drive roll driven by a belt drive motor (not shown), the support roll 32 is used as a driven roll, and the support roll 33 is used as the intermediate transfer belt 30. Used as a correction roll for controlling meandering in a direction substantially perpendicular to the moving direction (steering roll: provided so as to be tiltable with one end in the axial direction as a fulcrum), and further, a support roll 34 constitutes a secondary transfer device 50. It is used as a backup roll for the transfer roll 51.

  Further, a belt cleaner 35 for removing residual toner on the intermediate transfer belt 30 after the secondary transfer is provided on the surface side of the intermediate transfer belt 30 facing the support roll 31 so as to be retractable with respect to the intermediate transfer belt 30. In addition, a conveyance belt 53 and a fixing device 70 are disposed on the downstream side of the secondary transfer device 50 in the sheet conveyance direction.

  The intermediate transfer belt 30 in the present embodiment is formed, for example, by forming a flexible synthetic resin film such as polyimide in a strip shape, and connecting both ends of the synthetic resin film formed in the strip shape by means such as welding. By doing so, an endless belt can be used.

  Next, a basic image formation process during normal image formation (image formation mode) of the image forming apparatus according to the present embodiment will be described.

  First, the surface of the photoconductor 21 (photosensitive layer) is uniformly charged by applying a predetermined charging bias in which an AC voltage (current) is superimposed on a predetermined DC voltage (current) to the charging roll 22. Is done. Next, for example, when the image data of each color component image (black, yellow, magenta, cyan) read by the document reading device 10 is sent to the exposure device 23 of each image forming unit 20 (20a to 20d), An electrostatic latent image for each color component is formed on the photoreceptor 21 of each image forming unit 20. Thereafter, the electrostatic latent image is visualized by applying a predetermined developing bias between the developing roller 24R of the developing device 24 in which the corresponding toner of each color is accommodated and the photosensitive member 21, and each color is not fixed. It is formed as a toner image.

  The unfixed toner image of each color component is opposite to the charged polarity of the toner with respect to the base material of the intermediate transfer belt 30 by the primary transfer roll 25 at the primary transfer portion where each photoconductor 21 and the intermediate transfer belt 30 are in pressure contact. By applying a voltage of polarity, primary transfer is performed by sequentially superimposing on the surface of the intermediate transfer belt 30 by pressure contact force and electrostatic attraction force. The unfixed toner image primarily transferred to the intermediate transfer belt 30 in this way is conveyed to a secondary transfer portion facing the conveyance path of the recording paper 100 as a recording material as the intermediate transfer belt 30 rotates. The Each color toner remaining on the photoreceptor 21 after the primary transfer is scraped off by the elastic cleaning blade 260 of the drum cleaning device 26 to prepare for the next image forming cycle.

  At the secondary transfer site, the secondary transfer roll 51 is pressed by the backup roll 34 disposed in the space in the intermediate transfer belt 30 via the intermediate transfer belt 30 and fed from the paper feed tray 82 (paper transport system 80). The recording paper 100 unloaded at a predetermined timing by 81 (paper transport system 80) is inserted between the secondary transfer roll 51 and the intermediate transfer belt 30 through a predetermined number of transport rollers 83 (paper transport system 80). .

  The unfixed toner images held on the intermediate transfer belt 30 are collectively transferred to the recording paper 100 at the secondary transfer site by a transfer electric field formed between the backup roll 34 and the secondary transfer roll 51. Electrotransfer is performed.

  Thereafter, the recording paper 100 onto which the unfixed toner image has been transferred is sent to the fixing device 70 via the conveyance belt 53, and the fixing device 70 fixes the unfixed toner image on the recording paper 100 with heat and pressure. The The residual toner is removed by the belt cleaner 35 from the intermediate transfer belt 30 after the transfer of the unfixed toner image to the recording paper 100 is completed.

  By the way, as schematically shown in FIG. 2, the toner and the external additive in the developer generally enter the minute gap (nip) between the cleaning blade 260 and the surface of the photosensitive member 21 to play the role of a lubricant. It will be. Specifically, a toner image is formed on the photoconductor 21, and toner is supplied to the end of the cleaning blade 260 of the drum cleaning device 26 (the end that contacts the photoconductor 21), whereby the photoconductor 21. The driving torque can be reduced.

  On the other hand, as described above, when a contact-type charging roll is used as the charging device 22 for the photoreceptor 21, a discharge product is generated due to discharge in a minute gap between the photoreceptor 21 and the charging roll 22. As shown in FIG. 3, this discharge product increases the friction coefficient between the cleaning blade 260 of the drum cleaning device 26 and the photosensitive member 21, and increases the driving torque of the photosensitive member 21 over time. End up.

  Therefore, in the image forming apparatus according to the present embodiment, a lubricating toner supply mode for supplying toner and external additives to the cleaning blade 260 of the drum cleaning device 26 during non-image formation is provided, and lubrication is performed in the following procedure. By supplying the toner, the generation of discharge products is suppressed as much as possible, and the increase in driving torque of the photoconductor 21 over time is suppressed. The outline of the procedure for supplying the lubricating toner in the lubricating toner supply mode according to the present embodiment will be described below.

  First, the charging bias of the charging device 22 and the developing bias of the developing device 24 are turned off in a standby state when the apparatus is started up or during non-image forming such as between a series of image formations.

  In this state, the photosensitive member 21 is rotated, and the developing roll 24R of the developing device 24 is rotated at a predetermined rotational speed, so that the toner and the external additive on the developing roll 24R can only be electrostatically attracted and adhered. The toner and the external additive are deposited on the photoreceptor 21.

  In the lubricating toner supply mode according to such a simple procedure, the surface potential of the photosensitive member 21 becomes 0 V by not applying the charging bias, and further, the developing bias is set to 0 V, and the photosensitive member 21 and the developing roll 24R are rotated to develop. Since an electric field is not generated, normal image formation is not performed, but the toner adheres to the photoconductor 21 due to electrostatic attraction force due to charging of the toner itself or an adhesion force with the photoconductor 21. Thus, since no unnecessary electric field is applied to the photoconductor 21 during non-image formation, the generation of discharge products is reliably suppressed, and stress due to the electric field on the photoconductor 21 is reduced. It is possible to contribute to the extension of the service life, and the toner and the external additive adhering to the photoconductor 21 can be reliably supplied to the cleaning blade as a lubricant, and the driving torque of the photoconductor 21 can be reduced. The rise can be effectively reduced.

  Here, the relationship between the toner concentration in the developing device 24 and the amount of toner adhering to the photosensitive member 21 is shown in FIG. As can be understood from FIG. 4, the amount of toner deposited on the photoreceptor 21 decreases as the toner concentration in the developing device 24 decreases.

  Therefore, in the lubricating toner supply mode according to the present embodiment, the toner concentration in the developer is further detected by the density sensor provided in the developing device 24, and the toner is adhered onto the photoreceptor 21 based on the detection result. The toner supply region to be changed is changed so that a constant amount of toner is always supplied to the cleaning blade 260.

  Specifically, when the toner density in the developing device 24 decreases, the rotational speed and the rotation time of the photosensitive member 21 and the developing roll 24R in the lubricating toner supply mode are increased, so that the circumferential surface of the photosensitive member 21 is increased. The width of the lubricating toner to be adhered (the width of the toner image along the circumferential direction of the photoreceptor 21) is increased.

  In the present embodiment, the photosensitive member 21 and the developing roller 24R are rotated in the lubricating toner supply mode. However, the developing roller 24R is stopped and only the photosensitive member 21 is rotated. Also good. However, in order to adhere the lubricating toner over a wider peripheral surface of the photosensitive member 21, the photosensitive member 21 and the developing roll 24R can be rotated together in the lubricating toner supply mode as in the present embodiment. preferable.

  Further, in the present embodiment, from the viewpoint of reducing stress due to an unnecessary electric field on the photoconductor 21, the development bias is not applied in addition to the charging bias. From the viewpoint of adhering the toner to the photoreceptor 21, a predetermined developing bias may be applied.

  The results of comparing and verifying the transition of the driving torque of the photosensitive member 21 with the image forming apparatus having the conventional configuration in the image forming apparatus according to the present invention configured as described above will be described below as an example.

As an image forming apparatus used for verification, an image forming apparatus having a tandem configuration having four image forming units 20a to 20d as shown in FIG. 1 is used. The main specifications of the image forming apparatus used for the verification are shown below.
・ Process speed: 165mm / s
-Photoconductor 21: φ30 mm
-Charging roll 22: A two-layer structure having an elastic layer and a surface layer with a diameter of 12 mm.
Developer: Polymerized toner having an average particle size of 5.8 μm to which Si-based and F-based external additives are added, and the toner concentration is set to 8.5%.
Cleaning blade 260: A stainless steel plate having a thickness of 80 μm and a free length of 8.6 mm was bonded with a urethane rubber having a thickness of 1.2 mm and a free length of 5 mm and fixed to a sheet metal having a thickness of 1.8 mm. The cleaning blade 260 is pressed against the photosensitive member 21 in a doctor direction (a direction opposite to the rotation direction of the photosensitive member 21), a pressing force of 3.8 gf / mm, and an attachment angle (a tangent line between the cleaning blade 260 and the photosensitive member 21). Angle) was set to be 12 °.

  In addition to the above conditions, Cin 0% (with respect to the total number of dots on the image electronic data) under the high temperature and high humidity environment (28 ° C., 85%) After printing a certain number of images with a black dot ratio of 0%, the static torque of the photosensitive member 21 was measured and evaluated with a torque gauge (manufactured by Nitto Seisakusho).

  In the lubrication toner supply mode, neither the charging bias nor the development bias is applied during non-image formation between the Cin 0% image formation and the image formation, and the photoreceptor surface potential is 0 [V] and the development bias is 0 [ V] toner (corresponding to a Cin 5% toner image) deposited on the photosensitive member 21 with a width of 20 mm and a 1 pv interval (about 1 mg / pv) to the blade edge (end of the cleaning blade 260 on the photosensitive member 21 side) Supplied.

  On the other hand, in the conventional method as a comparison object, both the charging bias and the developing bias are applied as in the normal image formation, and similarly, a toner image having a Cin of 5% and a width of 20 mm is formed at a 1 pv interval (about 1 mg / pv). Supplied to the blade edge.

  The result of comparison and verification under the above conditions is shown in FIG. As understood from FIG. 5, in the conventional method, the driving torque exceeds 4 kgf · cm immediately after the start of printing, whereas in the present invention, the driving torque is suppressed to 3 to 3.5 kgf · cm. It can be seen that sometimes the charging bias and the developing bias are not applied to effectively suppress the generation of discharge products, and the increase in the driving torque of the photoconductor 21 is suppressed by the supply of sufficient lubricating toner.

  Even in the case where the toner concentration in the developing device 24 is decreased, in the present embodiment, the toner adheres to the photoconductor 21 by changing the rotation speed or the rotation time of the photoconductor 21 in accordance with the change in the toner concentration. Since the width of the lubricating toner image to be adjusted is adjusted, a constant amount of toner and an external additive can be always supplied to the cleaning blade 260 regardless of the toner concentration, and the increase in the driving torque of the photosensitive member 21 can be stably suppressed. I was able to confirm.

  In the present embodiment, the configuration using a contact-charging charging roll as an example of the charging method is illustrated, but the present invention is not limited to such a contact-charging charging device. Of course, the present invention can also be applied to a non-contact charging device.

  The execution timing of the lubrication toner supply mode is not limited to non-image formation between a series of image formations, but also at any timing such as non-image formation in a standby state until the normal image is formed after the power is turned on. It can be applied during non-image formation.

1 is a schematic configuration diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 6 is a schematic diagram illustrating a lubricating action of toner and external additives supplied to a cleaning blade. It is a figure which shows the relationship between the electric discharge product on a photoreceptor, and a driving torque. FIG. 4 is a diagram illustrating a relationship between a toner concentration in a developer and a toner amount adhering to a photoreceptor. It is a figure which shows the result of having compared and verified the change with time of drive torque.

Explanation of symbols

  10: Document reading device, 20: Image forming unit, 20a-20d: Image forming unit, 21a-21d: Photoconductor, 22a-22d: Charging device, 23a-23d: Exposure device, 24a-24d: Developing device, 24Ra- 24Rd: development roll, 25a-25d: primary transfer device, 26a-26d: drum cleaning device, 30: intermediate transfer belt, 34: backup roll, 35: belt cleaner, 50: secondary transfer device, 51: secondary transfer roll 53: transport belt, 70: fixing device, 80: paper transport system, 81: feed roll, 82: paper feed tray, 83: transport roll, 100: recording paper, 260: cleaning blade, CR: device controller

Claims (1)

An electrostatic latent image is held on the surface of the electrostatic latent image holder, and is formed to be rotatable.
A charging device that charges the surface of the electrostatic latent image holding member to a predetermined potential by applying a charging voltage;
A developing device having a rotatable developing roll and developing the electrostatic latent image formed on the electrostatic latent image holding member with toner;
A cleaning device having a cleaning plate-like elastic member for cleaning the transfer residual toner in contact with the surface of the electrostatic latent image holding member,
In the non-image-forming, with respect to the charging device and the developing device, the charging voltage and the cleaning plate-like elastic member by rotating said electrostatic latent image holding member and the developing roller without applying a developing voltage Toner and supplying the external additive, this time, when said toner density in the developing device falls below a predetermined concentration, by increasing the rotation speed or rotation time of the electrostatic latent image holding member and the developing roller A toner and an external additive are supplied to the cleaning plate-like elastic member.

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