JP2021081688A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that can discharge a reverse-polarity toner from a developing device to a photoconductor drum, while preventing the occurrence of carrier adhesion.SOLUTION: A CPU determines the average image ratio on immediate k recording materials (S4) from the average image ratio until "N-1"-th recording materials stored in a memory (S1) and the image ratio on an N-th recording material (S3). When the average image ratio is equal to or less than a threshold (YES in S5), the CPU performs Vback control (S6). In the Vback control, the CPU increases a "Vback value" compared with that during image formation without inverting the polarity of a voltage applied to a developing sleeve. By doing so, the CPU can efficiently discharge a reverse-polarity toner from a developer container to a photoconductor drum, while preventing the occurrence of carrier adhesion.SELECTED DRAWING: Figure 5

Description

The present invention relates to an image forming apparatus such as a printer, a copier, a facsimile or a multifunction device using electrophotographic technology.

In an image forming apparatus that forms an image with toner such as an electrophotographic method, a two-component developer (hereinafter, simply referred to as a developer) in which a non-magnetic toner and a magnetic carrier are mixed has been widely used. In this image forming apparatus, a toner image is developed by supplying toner to a photosensitive drum from a developing sleeve that rotates while carrying a developer. That is, the surface of the photosensitive drum is charged to a predetermined surface potential, and the charged surface is further exposed to form an electrostatic latent image. Then, when a developing voltage having the same polarity as the charging polarity of the photosensitive drum is applied to the developing sleeve, the toner charged with the same polarity as the charging polarity of the photosensitive drum adheres to the electrostatic latent image, and the toner image is developed (so-called). , Reverse development method).

By the way, as the above-mentioned developing process is repeated for a long period of time, a so-called fog phenomenon in which a large number of small black spots appear on a white background of a recording material is likely to occur. This is because the deterioration of the toner increases the amount of uncharged toner and low-charged toner in the developing agent. In view of this point, conventionally, it has been proposed to make the absolute value (referred to as Vback value) of the potential difference between the surface potential of the photosensitive drum and the potential of the developing sleeve larger than that at the time of image formation during non-image formation (patented). Document 1). In this case, the uncharged toner and the low-charged toner contained in the developing agent are discharged to the photosensitive drum, and then removed from the photosensitive drum by the cleaning means.

Further, in the developer that has been repeatedly used for a long period of time, in addition to the uncharged toner and the low-charged toner, there is a reverse-polarity toner whose polarity is reversed, and this reverse-polarity toner also causes the above-mentioned fog phenomenon. Become. Therefore, in the apparatus described in Patent Document 1, at the time of non-image formation, a voltage having a polarity opposite to that at the time of image formation is applied to the developing sleeve to discharge the reverse polarity toner to the photosensitive drum.

Japanese Unexamined Patent Publication No. 2005-115003

As described above, when a reverse polarity voltage is applied to the developing sleeve in order to discharge the reverse polarity toner to the photosensitive drum, the carriers in the developing agent supported on the developing sleeve adhere to the photosensitive drum. Can occur. However, if carrier adhesion occurs, there is a risk of damaging the surface of the photosensitive drum. Therefore, conventionally, it has been desired to discharge the reverse polarity toner from the developing device to the photosensitive drum while suppressing the occurrence of carrier adhesion, but such a thing has not been proposed yet.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of discharging reverse polarity toner from a developing apparatus to a photosensitive drum while suppressing the occurrence of carrier adhesion.

The image forming apparatus according to the present invention forms an electrostatic latent image by exposing a rotating image carrier, a charging means for charging the surface of the image carrier to a surface voltage, and the charged image carrier. An exposure means, a developing container containing a developer containing a non-magnetic toner and a magnetic carrier, a transport screw capable of stirring and transporting the developer in the developing container to charge the toner to a predetermined polarity, and a developer. A developing apparatus having a developer carrier that supports and rotates, and developing an electrostatic latent image formed on the image carrier by applying a voltage to the developer carrier, and the developing apparatus. A voltage applying means for applying a voltage to the agent carrier and a transfer nip portion are formed in contact with the image carrier, and a toner image formed on the image carrier is formed on a recording material by applying a voltage. When the transfer means to be transferred and the average image ratio per predetermined number of recording materials at the end of the image forming job are equal to or less than the threshold value, the polarity of the surface potential of the image carrier and the polarity of the potential of the developer carrier The absolute value of the potential difference between the surface potential of the image carrier and the potential of the developer carrier is made larger than the absolute value of the potential difference at the time of image formation, and the image carrier is transferred from the developing container to the image carrier. It is characterized by comprising a control means for executing an ejection mode for forcibly ejecting a toner having a polarity opposite to that of the predetermined polarity.

The image forming apparatus according to the present invention forms an electrostatic latent image by exposing a rotating image carrier, a charging means for charging the surface of the image carrier to a surface voltage, and the charged image carrier. An exposure means, a developing container containing a developer containing a non-magnetic toner and a magnetic carrier, a transport screw capable of stirring and transporting the developer in the developing container to charge the toner to a predetermined polarity, and a developer. A developing apparatus having a developer carrier that supports and rotates, and developing an electrostatic latent image formed on the image carrier by applying a voltage to the developer carrier, and the developing apparatus. A voltage applying means for applying a voltage to the agent carrier and a transfer nip portion are formed in contact with the image carrier, and a toner image formed on the image carrier is formed on a recording material by applying a voltage. The transfer means to be transferred and the period from when the preceding recording material passes through the transfer nip portion to when the subsequent recording material reaches the transfer nip portion during the image forming job, per predetermined number of recording materials. When the average image ratio is equal to or less than the threshold value, the surface potential of the image carrier and the potential of the developer carrier are maintained while maintaining the polarity of the surface potential of the image carrier and the polarity of the potential of the developer carrier. An discharge mode is executed in which the absolute value of the potential difference between the two is made larger than the absolute value of the potential difference at the time of image formation, and the toner having the opposite polarity to the predetermined voltage is forcibly discharged from the developing container toward the image carrier. It is characterized in that it is provided with a control means for processing.

According to the present invention, the reverse polarity toner can be efficiently discharged from the developing device to the photosensitive drum while suppressing the occurrence of carrier adhesion.

The schematic diagram which shows the structure of the image forming apparatus of this embodiment. The schematic diagram for demonstrating the image forming part. The schematic diagram for demonstrating the transport screw. A control block diagram for explaining a control unit. The flowchart which shows the reverse polarity toner discharge processing. A timing chart showing a potential change during Vback control. The graph which shows the relationship between the number of carriers adhering on a photosensitive drum, and the Vback value in the case of different average image ratios. It is a figure which shows the charge amount distribution of toner, (a) is this embodiment, (b) is comparative example 1, (c) is comparative example 2. The graph which shows the relationship between the number of carriers adhering on a photosensitive drum, and the Vback value in the case of different humidity.

[Image forming device]
First, the configuration of the image forming apparatus of this embodiment will be described with reference to FIGS. 1 to 3. The image forming apparatus 100 shown in FIG. 1 is a color laser printer using a transfer type electrophotographic process, a contact charging method, and a reverse development method. The image forming apparatus 100 forms a color image on the recording material S and outputs it according to the image information from an external device such as a personal computer or an image reading apparatus that is communicably connected to the apparatus main body. Examples of the recording material S include various types of sheet materials such as plain paper, thick paper, rough paper, uneven paper, coated paper, glossy paper, paper such as photographic paper, plastic film, and cloth.

As shown in FIG. 1, the image forming apparatus 100 arranges a plurality of image forming portions PY, PM, PC, and PBk side by side along the moving direction of the intermediate transfer belt 91, that is, so-called tandem type intermediate transfer method image forming. It is a device. The image forming apparatus 100 has image forming units PY, PM, PC, and PBk for forming images of each color of yellow (Y), magenta (M), cyan (C), and black (Bk), respectively. Explaining the overall operation when forming a four-color color image, a color-separated image signal is generated according to a signal from an external device communicably connected to the image forming apparatus 100. In response to this image signal, toner images of each color are formed in the image forming units PY, PM, PC, and PBk.

The image forming portions PY, PM, PC, and PBk are photosensitive drums 1Y, 1M, 1C, and 1Bk, which are rotatable drum-type electrophotographic photosensitive members (photoreceptors) having a photosensitive layer of an organic substance on a conductive support. Has. The surface of the photosensitive drums 1Y, 1M, 1C, and 1Bk as the image carrier is charged to the surface potential by applying a predetermined charging voltage to the charging rollers 2Y, 2M, 2C, and 2Bk. By scanning and exposing the uniformly charged surface with an exposure device 3Y, 3M, 3C, 3Bk (laser beam scanner or the like) as an exposure means, the uniformly charged surface is placed on the photosensitive drums 1Y, 1M, 1C, and 1Bk (on the image carrier). Form an electrostatic latent image. Then, the toner image as a developer is supplied to the electrostatic latent image by the developing devices 4Y, 4M, 4C, and 4Bk as the developing means, so that the toner image is formed on the photosensitive drums 1Y, 1M, 1C, and 1Bk.

The toner images of each color formed on the photosensitive drums 1Y, 1M, 1C, and 1Bk are sequentially superposed on the endless belt-shaped intermediate transfer belt 91 in each primary transfer unit d (see FIG. 2) and primary transfer is performed. .. Primary transfer rollers 92Y, 92M, 92C, and 92Bk are provided on the inner peripheral surface side of the intermediate transfer belt 91 as the image carrier so as to face the photosensitive drums 1Y, 1M, 1C, and 1Bk. By the action of the primary transfer rollers 92Y, 92M, 92C and 92Bk, the primary transfer of the toner image from the photosensitive drums 1Y, 1M, 1C and 1Bk to the intermediate transfer belt 91 is performed. Then, the color toner image formed on the intermediate transfer belt 91 has been conveyed to the secondary transfer portion N (transfer nip portion) where the secondary transfer roller 10 as the transfer means and the intermediate transfer belt 91 face each other. The secondary transfer is collectively performed on the recording material S. Next, the recording material S to which the toner image is transferred is conveyed to a fixing device 13 as a fixing means, where the toner image is fixed, and then discharged to the outside of the machine. In the present embodiment, the fixing device 13 applies heat and pressure to the recording material S by a pair of fixing rollers to melt and fix the toner image on the recording material S.

[Image forming part]
Next, the image forming unit PY, PM, PC, and PBk will be described in more detail. The four image forming units PY, PM, PC, and PBk are configured in substantially the same manner except that the toner colors used in the developing devices 4Y, 4M, 4C, and 4Bk are different from those of yellow, magenta, cyan, and black. Therefore, hereinafter, when no particular distinction is required, the subscripts Y, M, C, and Bk of the symbols indicating that they are elements of each image forming unit will be omitted. As shown in FIG. 2, in the image forming unit P, a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 92, and a cleaning device 7 are arranged so as to surround the photosensitive drum 1.

In the case of the present embodiment, the photosensitive drum 1 is an organic photoconductor (OPC) drum, and its outer diameter is, for example, 30 mm. Further, the photosensitive drum 1 is rotationally driven in the direction of arrow R1 (counterclockwise) shown in the figure at a process speed (peripheral speed) of, for example, 240 mm / sec, about the central support axis.

The charging roller 2 is a charging member formed in a roller shape. By applying a charging voltage under predetermined conditions to the charging roller 2 as a charging means, the surface of the photosensitive drum 1 is uniformly charged to a negative electrode surface potential (also referred to as a charging potential or a dark area potential Vd). A DC voltage is applied to the charging roller 2 from the charging power supply V1 via a core metal. As a result, the surface of the rotating photosensitive drum 1 is charged to a predetermined surface potential. The contact portion between the charging roller 2 and the photosensitive drum 1 is the charging position a. In the present embodiment, the voltage (charging voltage) applied to the charging roller 2 at the time of image formation is a DC voltage of “−1000V”. By applying such a direct current charging voltage, the surface of the photosensitive drum 1 is uniformly charged to a surface potential of "-750V".

The photosensitive drum 1 is uniformly charged to a predetermined polarity and potential by the charging roller 2, and then receives an image exposure (laser light L) by the exposure apparatus 3. In the present embodiment, the exposure apparatus 3 is a color separation / imaging exposure optical system for a color original image, and a scanning exposure system by laser scanning that outputs a laser beam modulated in response to a time-series electric digital pixel signal of image information. And so on. The exposure apparatus 3 forms electrostatic latent images of color components corresponding to the image forming portions PY, PM, PC, and PBk of the target color image on the photosensitive drums 1Y, 1M, 1C, and 1Bk (FIG. 1). The exposure apparatus 3 is provided on the downstream side in the rotation direction of the photosensitive drum 1 with respect to the charging roller 2.

In this embodiment, a laser beam scanner using a semiconductor laser is used as the exposure apparatus 3. The laser beam scanner outputs the modulated laser light corresponding to the image signal sent from the external device to the image forming apparatus 100 side, and laser scans and exposes the surface of the charged photosensitive drum 1. By this laser scanning exposure, the potential of the portion irradiated with the laser beam L on the photosensitive drum 1 is lowered, so that an electrostatic latent image corresponding to the image information scanned and exposed is formed on the rotating photosensitive drum 1. To. In this embodiment, the exposed potential is "-390V". The irradiation position of the image exposure (laser light L) on the photosensitive drum 1 is the exposure position b.

The electrostatic latent image formed on the photosensitive drum 1 is developed with toner in the developing device 4. In this embodiment, the developing device 4 is a two-component contact developing device. The developing apparatus 4 includes a developing container 40, a developing sleeve 41 as a developing agent carrier having a magnet roller fixedly arranged inside, a regulating blade 42, and a transport screw 43 arranged in the developing container 40 (inside the developing container). , 44 and the like. The developing container 40 is divided into a developing chamber 462 and a stirring chamber 461 by a partition wall 49 extending in the vertical direction (see FIG. 3), the developing chamber 462 has a first transfer screw 43, and the stirring chamber 461 has a second. Conveying screws 44 are arranged respectively. The first transfer screw 43 can agitate and transfer the developer in the developing chamber 462 and supply the developer to the developing sleeve 41. The second transfer screw 44 agitates and conveys the developer in the stirring chamber 461. Then, when the replenisher is replenished from the toner replenishment unit 5 as described later, the second transport screw 44 mixes the replenisher with the developer, and the toner concentration in the developer (weight of the toner in the developer). Shows the ratio, also referred to as T / D) to homogenize. In this embodiment, the toner concentration in the developer in the initial state, which has never been used for image formation, is "10%".

The developing container 40 mainly contains a two-component developer which is a mixture of a non-magnetic toner and a magnetic carrier. Non-magnetic toner is a low-temperature fixing toner with a low melting point (softening point) in which a colorant, wax component, etc. are encapsulated in a resin such as polyester or styrene and pulverized or polymerized to form a powder (for example, a melting point of 80 ° C. to 130 ° C. ℃). The magnetic carrier is obtained by applying a resin coating to the surface layer of a core made of resin particles obtained by kneading ferrite particles or magnetic powder. In this embodiment, for example, a non-magnetic toner having a negative charge characteristic with an average particle diameter of 5.5 ^{μm and a magnetic carrier having a positive charge characteristic having a saturation magnetization of 205 emu / cm 3} and an average particle diameter of 35 μm are used.

The developing sleeve 41 is provided with a magnetic field generating means (not shown) in a non-rotating manner, and a part of the outer peripheral surface that is rotatable with respect to the magnetic field generating means is exposed to the outside of the developing container 40. It is arranged in the developing container 40. Then, the developer that is agitated and conveyed by the transfer screws 43 and 44 is supported and rotated. The transport screws 43 and 44 can charge the toner to the negative electrode property and the carrier to the positive electrode property by stirring and transporting the developer.

The regulating blade 42 faces the developing sleeve 41 with a predetermined gap, and forms a developer thin layer on the developing sleeve 41 as the developing sleeve 41 rotates in the R2 direction (clockwise direction) shown in the drawing. .. In the case of the present embodiment, the developing sleeve 41 is arranged so as to be close to and opposed to the photosensitive drum 1 so that the closest distance (SD gap) to the photosensitive drum 1 is maintained at, for example, 310 μm. The position where the photosensitive drum 1 and the developing sleeve 41 face each other is the developing position c. The developing sleeve 41 is rotationally driven by a motor 60 as a driving means so that its surface moves in the same direction as the traveling direction (R1 direction in the drawing) of the surface of the photosensitive drum 1 at the developing position c.

The developer thin layer on the developing sleeve 41 comes into contact with the surface of the photosensitive drum 1 at the developing position c and appropriately rubs the photosensitive drum 1. A voltage is applied to the developing sleeve 41 from the developing power supply V2 as a voltage applying means. The voltage applied to the developing sleeve 41 is a voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). That is, the developing power supply V2 has a DC power supply and an AC power supply. For example, the developing power supply V2 applies a vibration voltage obtained by superimposing a DC voltage of "-650V" and an AC voltage having a frequency of "11kHz" and an inter-peak voltage of "1800V" to the developing sleeve 41 as a developing voltage at the time of image formation. To do. Therefore, when the surface potential of the photosensitive drum 1 of each color is charged to "-750V", the "Vback value" at the time of image formation is "100V (750-650)". The "Vback value" at the time of image formation creates an electric field that pulls the toner back from the photosensitive drum 1 to the developing sleeve 41. This is to suppress the fog phenomenon in which the toner adheres to the non-image portion of the photosensitive drum 1.

The toner in the developer conveyed to the developing position c by the rotating developing sleeve 41 selectively adheres to the electrostatic latent image formed on the photosensitive drum 1 by the electric field due to the developing voltage. As a result, the electrostatic latent image on the photosensitive drum 1 is developed as a toner image. In the present embodiment, toner adheres to the exposed bright portion on the photosensitive drum 1, and the electrostatic latent image is inverted and developed.

The developer thin layer on the developing sleeve 41 that has passed through the developing position c is continuously returned to the developing container 40 as the developing sleeve 41 rotates. Conveying screws 43 and 44 are provided in the developing container 40. The transport screws 43 and 44 rotate in synchronization with the rotation of the developing sleeve 41, and have a function of stirring and mixing the toner with the carrier to give a predetermined charge to the toner. Further, the transport screws 43 and 44 transport the developer in opposite directions in the longitudinal direction, and supply the developer to the developing sleeve 41.

An inductance sensor 45 is provided on the upstream side wall surface of the transport screw 44 of the developing apparatus 4 as a density detecting means for detecting the toner concentration in the developing agent based on the change in the magnetic permeability of the developing agent. The inductance sensor 45 uses the inductance to output an output voltage according to the magnetic permeability in the vicinity of the detection surface. When the T / D is small, the proportion of carriers contained in the developer in the unit volume near the detection surface is large, so that the apparent magnetic permeability in the developer is high and the output value is high. On the contrary, when the T / D is large, the proportion of carriers contained in the developer in the unit volume near the detection surface of the inductance sensor 45 becomes small, so that the apparent magnetic permeability in the developer becomes low and the output value becomes low. ..

Regarding the circulation direction of the developer in the developing container 40 (see the arrow in FIG. 3), a replenisher supply port 47 is provided on the downstream side of the inductance sensor 45 (see FIG. 2). After performing the developing operation, the developer is carried to the detection surface of the inductance sensor 45, where the toner concentration is detected. Depending on the detection result, the replenisher is replenished from the toner replenishment unit 5 through the replenisher replenishment port 47 of the developing device 4 by the rotation of the replenishment screw 51 of the toner replenishment unit 5 connected to the developing device 4. By replenishing the replenishing agent, the toner concentration in the developing agent in the developing container 40 is kept constant. In this embodiment, a mixture of toner and carrier at a weight ratio of 9: 1 was used as a replenisher.

The replenished replenisher is conveyed in the developing container 40 while being stirred by the conveying screws 43 and 44, whereby an appropriate charge is applied to the toner. Then, when the developer is carried in the vicinity of the developing sleeve 41 by the transport screw 43, the developer is formed into a thin layer on the developing sleeve 41 and used for development.

The toner (transfer residual toner) remaining on the photosensitive drum 1 without being transferred to the intermediate transfer belt 91 by the primary transfer unit d is removed from the photosensitive drum 1 by the cleaning device 7. The cleaning device 7 removes the transfer residual toner on the photosensitive drum 1 by a cleaning blade 7a provided in contact with the photosensitive drum 1.

[Control unit]
As shown in FIG. 1, the image forming apparatus 100 has a control unit 101 as a control means for controlling the image forming apparatus 100. The control unit 101 will be described with reference to FIG. In addition to the illustration, various devices such as a motor and a power supply for operating the image forming apparatus 100 may be connected to the control unit 101, but the illustration and description are omitted here because it is not the main purpose of the invention.

The control unit 101 has a CPU (Central Processing Unit) 102 and a memory 103 such as a ROM, RAM, or hard disk device, and controls various motors and power supplies based on image information and inputs of various sensors. The memory 103 stores, for example, an image forming job, various programs such as "reverse polarity toner discharge processing" (see FIG. 5) described later, and various data such as an average image ratio and "Vback value" described later. The CPU 102 can execute various programs stored in the memory 103, and can execute various programs to operate the image forming apparatus 100. For example, the CPU 102 can perform various controls such as controlling the rotation start / stop of the photosensitive drum 1, the developing sleeve 41 or the intermediate transfer belt 91, starting / stopping the voltage application of the charging power supply V1 and the developing power supply V2, and adjusting the voltage value. .. The CPU 102 can adjust the surface potential of the photosensitive drum 1Y by controlling the charging power supply V1, and the CPU 102 can adjust the potential of the developing sleeve 41 by controlling the developing power supply V2. That is, the CPU 102 can change the above-mentioned "Vback value" by controlling the charging power supply V1 and the developing power supply V2. The memory 103 can also temporarily store the calculation processing results and the like associated with the execution of various programs.

Here, the image forming job is a series of periods from the start of image formation to the completion of the image forming operation based on the print signal for forming an image on the recording material S. Specifically, it refers to the period from the front rotation (preparatory operation before image formation) after receiving the print signal (job input) to the rear rotation (operation after image formation), the image formation period, and the paper. It is a period including the interval. In the present specification, the term “paper-to-paper” means that the preceding recording material S passes through the secondary transfer unit N (see FIG. 1) during the image forming job, and then the subsequent recording material S reaches the secondary transfer unit T2. Is the period until. In other words, the space between papers is a period during which the region corresponding between the recording material S and the recording material S passes through the secondary transfer unit N during the image forming job.

Further, in the present embodiment, a predetermined patch image is formed on the intermediate transfer belt 91 at a predetermined interval (for example, for each predetermined number of images formed) by a command of the control unit 101. Then, by detecting the amount of toner on the patch image, the amount of charge of the toner in the developing device 4 is detected. That is, as shown in FIG. 1, a toner loading amount detection sensor 97 for detecting the toner loading amount is arranged downstream of the image forming portion PBk of the intermediate transfer belt 91. The toner loading amount detection sensor 97 is an optical reflection type sensor. Then, the control unit 101 transfers the intermediate transfer from the difference between the reflected light amount of the intermediate transfer belt 91 when the patch image is not formed and the reflected light amount when the patch image is formed, which is detected by the toner loading amount detection sensor 97. The amount of toner loaded on the belt 91 is calculated.

The control unit 101 obtains the amount of toner charged in the developing apparatus 4 from the amount of toner loaded. Here, the control unit 101 determines that the toner charge amount is too low when the amount of toner on the intermediate transfer belt 91 is large, and conversely when the amount of toner on the intermediate transfer belt 91 is small. It is determined that the toner charge amount is too high. Then, the control unit 101 sets the timing and frequency of toner replenishment so that the toner concentration detected by the inductance sensor 45 becomes a desired concentration (target concentration) based on the detection result of the toner loading amount detection sensor 97. adjust. Specifically, when the amount of toner charged in the developing device 4 is too high, the developing contrast (also referred to as bright area potential), which is the difference between the developing voltage and the exposed area potential (also referred to as bright area potential) on the surface of the photosensitive drum exposed by the developing device. Since the amount of toner developed by Vcont) is reduced, the target concentration is increased. On the contrary, when the amount of toner charged in the developing apparatus 4 is too low, the amount of toner developed at the same development contrast (Vcont) increases, so that the target density is lowered.

Then, in the case of the present embodiment, the CPU 102 has a "Vback value" (potential difference) which is changed in detail in the "reverse polarity toner discharge process" (see FIG. 5) described later according to the toner concentration detected by the inductance sensor 45. The size of (absolute value) can be set. That is, when the CPU 102 executes the "reverse polarity toner discharge process", the "Vback value" is set as the first potential difference when the toner concentration is the first concentration, and the toner concentration is a second concentration higher than the first concentration. In addition, the "Vback value" is set to the second potential difference larger than the first potential difference.

Further, in the present embodiment, as shown in FIG. 1, a temperature / humidity sensor 50 as a humidity detecting means is arranged in the main body of the image forming apparatus 100. Then, the CPU 102 makes it possible to set the magnitude of the "Vback value" that is changed in the "reverse polarity toner discharge process" (see FIG. 5) described later, according to the humidity detected by the temperature / humidity sensor 50. That is, when the "reverse polarity toner discharge process" is executed, the CPU 102 sets the "Vback value" as the first potential difference when the humidity is the first humidity, and when the humidity is the second humidity lower than the first humidity, " Let the "Vback value" be a second potential difference larger than the first potential difference.

Further, the CPU 102 makes it possible to change the execution frequency of the "reverse polarity toner discharge process" (see FIG. 5), which will be described later, according to the humidity detected by the temperature / humidity sensor 50. That is, the CPU 102 sets the "threshold value" (see S5 and S10 in FIG. 5) as the first threshold value when the humidity is the first humidity, and sets the "threshold value" when the humidity is the second humidity lower than the first humidity. The second threshold is smaller than the first threshold. In this way, in the "reverse polarity toner discharge process" described in detail later, the execution frequency of the Vback control is changed by changing the "threshold value" (see S5 and S10 in FIG. 4) to be compared with the average image ratio as the execution condition of the Vback control. I am trying to be able to change. In the case of the present embodiment, by increasing the above "threshold value" (for example, changing from 3% to 8%) in the low humidity environment as compared with the high humidity environment, the execution frequency of the Vback control is increased. Good.

By the way, as already described, in the developer that has been repeatedly used for a long period of time, in addition to the uncharged toner and the low-charged toner, there is a reverse polarity toner whose polarity is reversed. In particular, when the image formation with a small image ratio is continued, the toner consumption is low and it is difficult to replace the toner by replenishment as compared with the case where the image formation with a large image ratio is performed. The amount of toner tends to increase. Then, as the deterioration of the toner progresses, the charge of the toner is attenuated, but the polarity of some toners is reversed. Further, when the toner is newly replenished in a state where the amount of deteriorated toner is increased, the polarity is further reversed because the triboelectric charge between the replenished toner and the deteriorated toner promotes the attenuation of the charge of the deteriorated toner. The amount of reverse polarity toner is increased. When the amount of reverse-polarity toner increases, the fog phenomenon is likely to occur during image formation.

Therefore, conventionally, at the time of non-image formation, the reverse polarity toner is discharged to the photosensitive drum by applying a voltage having the opposite polarity to that at the time of image formation to the developing sleeve. However, in such a case, so-called carrier adhesion may occur in which the magnetic carrier adheres to the photosensitive drum 1. If carrier adhesion occurs, the surface of the photosensitive drum may be damaged. Further, if carrier adhesion occurs continuously, the amount of the developer in the developing apparatus 4 decreases as the carriers are discharged, so that screw marks mainly derived from the shape of the transport screw 43 appear as uneven image defects. May occur. Therefore, in the present embodiment, the reverse polarity toner in the developing container 40 can be forcibly discharged to the photosensitive drum 1 without causing carrier adhesion as much as possible. This will be described below.

[Reverse polarity toner discharge processing]
The “reverse polarity toner discharge treatment” (discharge mode) of the present embodiment will be described with reference to FIGS. 5 and 6 with reference to FIG. The “reverse-polarity toner discharge process” shown here is started by the CPU 102 when the execution of the image forming job is started.

As shown in FIG. 5, the CPU 102 reads out the average image ratio up to the "N-1" image stored in the memory 103 (S1). The CPU 102 acquires a video count value “Vc (N)” related to an image formed on the Nth recording material S (S2). Then, the CPU 102 obtains the image ratio “Duty (N)” in the Nth recording material S based on the video count value by Equation 1 (S3). Here, the video count value is an integrated value when the levels (0 to 255 levels) of the input image information for each pixel are integrated for one surface of the output image. In the present embodiment, the video count value of the output image formed for each recording material S is acquired, and the image ratio is calculated using this.
Duty (N) = Vc (N) / 1023 ... (Equation 1)

The CPU 102 obtains the average image ratio "Ave_Duty (N)" per the most recent predetermined number of images including the Nth image (for example, k = 1000 images) according to the following formula 2 (S4).
Ave_Duty (N) = Ave_Duty (N-1) x (k-1) / k
+ Duty (N) / k ... (Equation 2)

The CPU 102 determines whether or not the obtained average image ratio is equal to or less than a threshold value (for example, 3%) (S5). When the average image ratio is larger than the threshold value (NO in S5), the CPU 102 jumps to the process in step S7 and starts forming the Nth image without performing the Vback control described later (S7). After that, the CPU 102 proceeds to the process of step S8.

On the other hand, when the obtained average image ratio is equal to or less than the threshold value (YES in S5), the CPU 102 performs Vback control between the papers (S6), and then starts forming the Nth image (S7). The V-back control is a control in which the "V-back value" is made larger than that at the time of image formation in order to discharge the reverse-polarity toner, which has increased with the deterioration of the toner, onto the photosensitive drum 1. Vback control will be described later (see FIG. 6).

Then, the CPU 102 determines whether or not to end the image forming job being executed (S8). When the image forming job is not completed (NO in S8), the CPU 102 sets "N = N + 1" (S9) to form an image on the next recording material S, and obtains the Nth video count value. The process returns to the process of step S2, and the process of steps S2 to S8 described above is repeated. When the image formation job is terminated (YES in S8), the CPU 102 determines whether or not the average image ratio is equal to or less than the threshold value (S10). When the average image ratio is larger than the threshold value (NO in S10), the CPU 102 ends the "reverse polarity toner discharge process". On the other hand, when the average image ratio is equal to or less than the threshold value (YES in S10), the CPU 102 performs Vback control during the rear rotation (S11), and then ends the "reverse polarity toner discharge process".

The above Vback control will be described. FIG. 6 shows the surface potential of the photosensitive drum 1 and the surface potential of the developing sleeve 41 during image formation and Vback control. In the present embodiment, when the "Vback value" at the time of image formation is "100V", the "Vback value" at the time of Vback control is set to "180V" (+ 80V from the time of image formation). Here, in order to set the "Vback value" to "180V", the voltage applied to the developing sleeve 41 by controlling the developing power supply V2 is changed from "-650V" at the time of image formation to "-570V". In the present embodiment, the polarity of the surface potential of the photosensitive drum 1 and the polarity of the potential of the developing sleeve 41 are maintained from the time of image formation, and the "Vback value" is changed.

In this way, when the "Vback value" is made larger than that at the time of image formation, an electric field stronger than that at the time of image formation is formed between the photosensitive drum 1 and the developing sleeve 41, so that the reverse polarity toner is directed from the developing sleeve 41 to the photosensitive drum 1. Can move. At that time, since the developing sleeve 41 and the transport screws 43 and 44 are still rotating, the developing agent in the developing container 40 is supplied to the developing sleeve 41, and the reverse polarity toner contained in the developing agent is transferred to the photosensitive drum 1. Can be spit out. At this time, the voltage applied to the developing sleeve 41 has the same polarity as that at the time of image formation and is not opposite polarity, so that the carriers contained in the developing agent are not easily discharged.

In order to make the "Vback value" larger than that at the time of image formation during Vback control, the development power supply V2 is not limited to the control as described above. For example, the surface potential of the photosensitive drum 1 may be changed from "-750V" to "-830V" at the time of image formation by controlling the charging power supply V1 without controlling the developing power supply V2. Alternatively, both the developing power supply V2 and the charging power supply V1 may be controlled to make the "Vback value" larger than that at the time of image formation.

Here, the number of carriers attached to the photosensitive drum 1 immediately after outputting 10,000 images with an image ratio of 10% (see solid line) and immediately after outputting 10,000 images with an image ratio of 1% (see broken line). The relationship with the "Vback value" is shown in FIG. As can be understood from FIG. 7, after 10,000 images having an image ratio of 10% are output, the number of carriers attached increases as the "Vback value" increases. This is because the toner concentration in the developer in the developing container 40 is lowered by the control using the toner loading amount detection sensor 97.

That is, an external additive is added to the surface of the toner, and when the external additive is released and adheres to the magnetic carrier, the chargeability of the magnetic carrier is reduced, so that the charge of the toner is reduced. When an image having a high image ratio is continuously output, the frequency of passage of toner in the developing apparatus 4 increases as the consumption of toner increases and the frequency of replenishment operation increases. In that case, the external additive of the toner is often transferred to the magnetic carrier, and the charge of the toner can be greatly reduced.

When the toner concentration is controlled by using the toner loading amount detection sensor 97 described above, when the toner charge decreases, for example, the density of the patch image formed every 300 sheets becomes thicker, so that the developer in the developing container 40 has a higher density. The toner concentration is controlled to decrease. The toner density after actually outputting 10,000 images with an image ratio of 10% was about 6%, and the toner density after outputting 10,000 images with an image ratio of 1% was about 10%. When the toner concentration is low, the toner coverage of the magnetic carrier decreases, the area of contact with the photosensitive drum 1 increases in the developing area, and the reflection force acting between the magnetic carrier and the photosensitive drum 1 causes carrier adhesion. It will be easier.

In the present embodiment, the "reverse polarity toner discharge treatment" is executed when the toner is deteriorated and the carrier adhesion does not occur remarkably even if the "Vback value" is made larger than that at the time of image formation. I am trying to do it. That is, when the average image ratio of the output image is high, the toner is frequently replaced by the developing operation and the replenishing operation, so that the progress rate of toner deterioration is slow. In addition, when the "Vback value" is increased, carrier adhesion becomes remarkable. Therefore, when the average image ratio is high, it is not preferable to increase the "Vback value" as compared with the time of image formation. On the other hand, when the average image ratio of the output image is low, it is difficult for the toner to be replaced, so that the progress rate at which the toner deteriorates becomes high. Further, since the possibility of carrier adhesion is small even if the "Vback value" is slightly increased, it is effective to increase the "Vback value" from that at the time of image formation to control the ejection of the reverse polarity toner. That is, the "reverse polarity toner discharge process" is not executed when the average image ratio is high, but is executed only when the average image ratio is low. Therefore, in the present embodiment, when the average image ratio of the latest 1000 images is 3% or less, it is considered that the reverse polarity toner is increased due to the deterioration of the toner, and the "Vback value" is obtained between the papers and during the backward rotation. Is larger than when the image was formed.

The "Vback value" during Vback control is preferably "+ 20V or more and + 150V or less" based on the "Vback value" at the time of image formation, and more preferably "+ 50V or more and + 100V or less". As can be understood from FIG. 7, as the “Vback value” increases, the number of carriers attached to the photosensitive drum 1 exponentially increases without reversing the polarity of the voltage applied to the developing sleeve 41. Because it will increase.

Next, the experimental results when the above-mentioned "reverse polarity toner discharge treatment" is adopted are shown in FIGS. 8 (a) to 8 (c) together with a comparative example. FIG. 8A shows the experimental results of the present embodiment adopting the “reverse polarity toner discharge treatment”, FIG. 8B shows the experimental results of Comparative Example 1, and FIG. 8C shows the experimental results of Comparative Example 2. .. In the experiment, 10,000 images with an image ratio of 50% and 10,000 images with an image ratio of 1% were output in succession (immediately after endurance), and then images with an image ratio of 50% were further output to 10 recording materials S. After the image was formed, the supplement was replenished.

In Comparative Example 1 and Comparative Example 2, the above-mentioned "reverse polarity toner discharge treatment" was not executed. In Comparative Example 1, the "Vback value" was set to "100V", which is the same as when the image was formed, even between papers. Regardless of the deterioration state, the "Vback value" is always larger than that at the time of image formation between papers. In the experiment of the present embodiment, when 10,000 images with an image ratio of 50% were output, the execution condition was not satisfied and the "reverse polarity toner discharge process" was not executed. Then, the execution condition was satisfied at the 960th image after the image with the image ratio of 1% was started to be output, and thereafter, the "reverse polarity toner discharge process" was executed while the image with the image ratio of 1% was output.

In FIGS. 8 (a) to 8 (c), the charge amount distribution per unit mass of the toner immediately after durability is shown by a solid line, and the unit of toner after replenishment in which the toner is sufficiently agitated after replenishment of the replenisher is shown. The charge distribution per mass is shown by the broken line. Further, in each case, the ratio of the toner charged positively is shown in Table 1. The charge amount distribution of the toner was measured using a charge amount distribution measuring device (E-Spart Analyzer) manufactured by Hosokawa Micron Corporation. This is a measuring device that detects the movement of particles in an air vibration field in an electric field by the laser Doppler method and simultaneously measures the charge amount and particle diameter of each particle from the data.

As can be understood from FIGS. 8A to 8C and Table 1, immediately after the durability, the ratio of the toner charge amount of the embodiment and Comparative Example 2 is closer to the negative electrode side than that of Comparative Example 1. There is. This is because, in the embodiment and Comparative Example 2, there may be a timing when the “Vback value” becomes “180V” which is larger than that at the time of image formation, and at that time, the deteriorated reverse electrode toner is positively put on the photosensitive drum 1. This is because it is discharged, the components on the positive electrode side are reduced, and the distribution is closer to the negative electrode side. Further, even if the toner does not have its polarity reversed immediately after its durability, the toner charged by triboelectric charging takes away the charge after the supply, so that a toner whose polarity is reversed may be produced. As a result, after replenishment, the difference between the positive electrode toner and the negative electrode toner is more remarkable than immediately after the durability.

Next, Table 2 shows the fog concentration after replenishment and the presence or absence of screw marks on the image. To measure the fog density, "REFLECTMER MODEL TC-6DS" manufactured by Tokyo Denshoku Co., Ltd. was used, and the fog density (%) was calculated from the difference between the whiteness of the white background of the printed fixed image and the whiteness of the recording material S. Was calculated.

As shown in Table 2, in Comparative Example 1, the fog concentration after replenishment is higher than that in the embodiment and Comparative Example 2. This is because, as described above, the amount of reverse polarity toner or toner having a low charge amount has increased. Further, in Comparative Example 2, a screw mark was generated on the image. This is because the "Vback value" is always larger than that at the time of image formation, regardless of the average image ratio. That is, when an image having an image ratio of 50% is continuously output, it is controlled in the direction of reducing the coating ratio of the toner to the carrier, so that the "Vback value" is increased and a strong electric field is formed. , A large amount of carrier adhesion occurs. When the "Vback value" is changed to "180V" at the time between papers in this way, carrier adhesion occurs and the amount of the developer in the developing container 40 decreases. In that case, a sufficient amount of the developer cannot be supplied to the developing sleeve 41, so that screw marks derived from the shape of the transport screw 44 may occur in the image.

On the other hand, in the case of the present embodiment, when an image having an image ratio of 50% is output, the image is formed with a constant "Vback value". On the other hand, when the output of an image having a low image ratio continues and the deterioration of the toner progresses, the "Vback value" is made larger than that at the time of image formation, so that the reverse polarity toner is used while suppressing the occurrence of carrier adhesion. It can be spit out efficiently. Therefore, it is possible to suppress the occurrence of the fog phenomenon, and the screw mark does not occur in the image.

As described above, in the present embodiment, the reverse polarity toner can be discharged from the developing container 40 to the photosensitive drum 1 by controlling the "Vback value" between the papers or at the end of image formation (during rear rotation). There is. At that time, the "Vback value" is made larger than that at the time of image formation without reversing the polarity of the voltage applied to the developing sleeve 41. That is, while maintaining the polarity of the surface potential of the photosensitive drum 1 and the polarity of the potential of the developing sleeve 41, the absolute value of the potential difference between the surface potential of the photosensitive drum 1 and the potential of the developing sleeve 41 is set to the absolute value of the potential difference at the time of image formation. Greater than the value. As a result, the reverse polarity toner can be efficiently discharged from the developing container 40 to the photosensitive drum 1 while suppressing the occurrence of carrier adhesion. Since the carrier is attracted by the magnetic field generating means in the developing sleeve 41, the "Vback value" is made larger than that at the time of image formation while maintaining the polarity of the surface potential of the photosensitive drum 1 and the polarity of the potential of the developing sleeve 41. However, it is difficult to discharge from the developing container 40 to the photosensitive drum 1.

As described above, the timing of performing the Vback control of the present embodiment is preferably performed during paper spacing or backward rotation (see S6 and S11 in FIG. 5), and is not preferable during front rotation. The reason why the front rotation is not preferable is that when Vback control is performed during the front rotation, the negative electrode toner is separated from the magnetic carrier by a strong electric field, and more toner is attracted from the developing container 40 to the developing sleeve 41 and supported. Because it can be done. In that case, when the image is formed immediately after, the amount of toner supported on the developing sleeve 41 increases, and the image may become darker over the entire circumference of the developing sleeve 41. Therefore, it is preferable to perform Vback control during the forward rotation. Absent.

Next, in an environment where the humidity of 23 ° C. 50% RH and 23 ° C. 5% RH are different, the number of carriers attached and the "Vback value" after forming an image with an image ratio of 1% on 10,000 recording materials S The relationship between the above is shown in FIG. As can be understood from FIG. 9, it can be seen that the amount of carriers attached to the "Vback value" is smaller in the 23 ° C. 5% RH environment shown by the broken line than in the 23 ° C. 50% RH environment shown by the solid line. This is because the toner concentration in the developer in the developing container 40 is increased by the control using the toner loading amount detection sensor 97.

That is, the chargeability of the toner and the carrier is higher in a low humidity environment and lower in a high humidity environment. If the image with a low image ratio is continuously output in a low humidity environment, the charge of the toner increases. When the charge of the toner increases, for example, the density of the patch image formed every 300 sheets becomes thin, so that the toner density is controlled to increase. The toner concentration after actually forming an image with an image ratio of 1% on 10000 recording materials S in a 23 ° C. and 50% RH environment is about 10%, and an image having an image ratio of 1% in a 23 ° C. and 5% RH environment. The toner concentration after forming an image on 10000 sheets of recording material S was about 12%. When the toner concentration is high, the coverage of the toner of the magnetic carrier increases, and the area of contact with the photosensitive drum 1 in the developing region decreases. In that case, the mirroring force between the magnetic carrier and the photosensitive drum 1 becomes difficult to work, so that carrier adhesion is unlikely to occur.

Therefore, in a low humidity environment, it is preferable to make the "Vback value" at the time of Vback control larger than in a high humidity environment. Further, when triboelectric charging occurs between the toner whose charge has increased and the replenished toner in a low humidity environment, the difference in polarity between the two becomes larger than in a normal environment, so that the polarity reversal in the toner becomes remarkable. That is, since the amount of reverse polarity toner tends to increase, it is necessary to discharge the reverse polarity toner from the developing container 40 to the photosensitive drum 1 before replenishing the replenisher. Therefore, in a low humidity environment, it is preferable to increase the "Vback value" at the time of Vback control. Therefore, in the case of a low humidity environment, it is preferable to increase the "Vback value" as compared with the case of a high humidity environment. For example, the "Vback value" may be set to "180V" in a 23 ° C. 50% RH environment, and the "Vback value" may be set to "200V" in a 23 ° C. 5% RH environment.

In a 23 ° C. 5% RH environment, the "Vback value" is set to "200V", 10,000 images with an image ratio of 50% and 10,000 images with an image ratio of 1% are output in succession, and then an image with an image ratio of 50% is further output. An experiment was conducted in which an image was formed on 10 recording materials S. As a result, the fog concentration was "0.4%", which was further improved as compared with the case where the "Vback value" was not increased in a low humidity environment, and no screw marks were generated.

In this way, under predetermined conditions, it is possible to optimize the "Vback value" during Vback control. The "Vback value" at the time of Vback control may be appropriately changed depending on, for example, the toner concentration, not limited to the example of the humidity environment. When the toner concentration is high, it is preferable to increase the "Vback value" as compared with the case where the toner concentration is low. For example, when the toner concentration is 6% or less, the "Vback value" may be set to "180V", and when the toner concentration is higher than 6%, the "Vback value" may be set to "200V".

1 (1Y to 1Bk) ... Image carrier (photosensitive drum), 2 (2Y to 2Bk) ... Charging means (charging roller), 3 (3Y to 3Bk) ... Exposure means (exposure device), 4 (4Y to 4Bk) ... Developing device, 10 ... Transfer means (secondary transfer roller), 40 ... Development container, 41 ... Developer carrier (development sleeve), 43 (44) ... Conveyor screw, 45 ... Concentration detection means (inductivity sensor), 50 ... Humidity detecting means (temperature / humidity sensor), 60 ... driving means (motor), 100 ... image forming apparatus, 101 ... controlling means (control unit), N ... transfer nip part (secondary transfer unit), V2 ... voltage applying means (V2 ... voltage applying means (secondary transfer unit) Development power supply)

Claims (8)

With a rotating image carrier,
A charging means for charging the surface of the image carrier to a surface potential,
An exposure means that exposes the charged image carrier to form an electrostatic latent image,
A developing container containing a developing agent containing a non-magnetic toner and a magnetic carrier, a transport screw capable of stirring and transporting the developing agent in the developing container to charge the toner to a predetermined polarity, and rotating while supporting the developing agent. A developing apparatus having a developing agent carrier to develop the electrostatic latent image formed on the image carrier by applying a voltage to the developing agent carrier, and a developing apparatus using toner.
A voltage applying means for applying a voltage to the developer carrier and
A transfer means that abuts on the image carrier to form a transfer nip and transfers a toner image formed on the image carrier to a recording material by applying a voltage.
At the end of the image forming job, when the average image ratio per predetermined number of recording materials is equal to or less than the threshold value, the polarity of the surface potential of the image carrier and the polarity of the potential of the developer carrier are maintained. The absolute value of the potential difference between the surface potential of the image carrier and the potential of the developer carrier is made larger than the absolute value of the potential difference at the time of image formation, and the predetermined polarity is obtained from the developing container toward the image carrier. A control means for executing a discharge mode for forcibly discharging a toner having a reverse polarity is provided.
An image forming apparatus characterized in that. The control means averages the number of recording materials per predetermined number during the period from when the preceding recording material passes through the transfer nip portion to when the subsequent recording material reaches the transfer nip portion during the image forming job. When the image ratio is equal to or less than the threshold value, the discharge mode is executed.
The image forming apparatus according to claim 1. With a rotating image carrier,
A charging means for charging the surface of the image carrier to a surface potential,
An exposure means that exposes the charged image carrier to form an electrostatic latent image,
A developing container containing a developing agent containing a non-magnetic toner and a magnetic carrier, a transport screw capable of stirring and transporting the developing agent in the developing container to charge the toner to a predetermined polarity, and rotating while supporting the developing agent. A developing apparatus having a developing agent carrier to develop the electrostatic latent image formed on the image carrier by applying a voltage to the developing agent carrier, and a developing apparatus using toner.
A voltage applying means for applying a voltage to the developer carrier and
A transfer means that abuts on the image carrier to form a transfer nip and transfers a toner image formed on the image carrier to a recording material by applying a voltage.
During the image forming job, the average image ratio per predetermined number of recording materials is equal to or less than the threshold value in the period from when the preceding recording material passes through the transfer nip portion to when the subsequent recording material reaches the transfer nip portion. When, the absolute value of the potential difference between the surface potential of the image carrier and the potential of the developer carrier is maintained while maintaining the polarity of the surface potential of the image carrier and the potential of the developer carrier. To be larger than the absolute value of the potential difference at the time of image formation, and to execute a discharge mode for forcibly discharging toner having the opposite polarity to the predetermined polarity from the developing container toward the image carrier. Prepare, prepare
An image forming apparatus characterized in that. At the end of the image forming job, the control means executes the discharge mode when the average image ratio per predetermined number of recording materials is equal to or less than a threshold value.
The image forming apparatus according to claim 3. Equipped with a humidity detection means to detect humidity
When the discharge mode is executed, the control means sets the absolute value of the potential difference as the first potential difference when the humidity is the first humidity, and when the humidity is the second humidity lower than the first humidity. Let the absolute value of the potential difference be a second potential difference larger than the first potential difference.
The image forming apparatus according to any one of claims 1 to 4. Equipped with a humidity detection means to detect humidity
When the discharge mode is executed, the control means sets the threshold value as the first threshold value when the humidity is the first humidity, and sets the threshold value when the humidity is a second humidity lower than the first humidity. A second threshold value larger than the first threshold value.
The image forming apparatus according to any one of claims 1 to 4. A concentration detecting means for detecting the toner concentration of the developer in the developing container is provided.
When the discharge mode is executed, the control means sets the absolute value of the potential difference as the first potential difference when the toner concentration is the first concentration, and the toner concentration is a second concentration higher than the first concentration. In some cases, the absolute value of the potential difference is set to a second potential difference larger than the first potential difference.
The image forming apparatus according to any one of claims 1 to 6, wherein the image forming apparatus is characterized in that. The control means changes the voltage applied to the voltage applying means with the same polarity as at the time of image formation to obtain an absolute value of the potential difference between the surface potential of the image carrier and the potential of the developer carrier. Make it larger than the absolute value of the potential difference at the time of image formation,
The image forming apparatus according to any one of claims 1 to 7.

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