JP2016048356A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a tandem type image forming apparatus that uses a corona charging system and can prevent adhesion of a carrier in YMC stations during a monochrome image forming mode.SOLUTION: An image forming apparatus has: a rotatable image carrier 1; a corona charger 2 including a discharge electrode 304 that is supplied with current to perform discharge and a grid electrode 306, and charges the surface of the image carrier 1; and toner image forming means for forming a toner image on the surface of the image carrier 1 charged by the corona charger 2. The image forming apparatus decreases a current value to be applied to corona lines (discharge electrodes) 304 in YMC stations during a monochrome image forming mode compared with that during full-color image formation and outputs a grid voltage in accordance with a setting value to prevent adhesion of a carrier.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an image forming apparatus such as an electrophotographic system that forms a toner image on a recording medium.

  Conventionally, in the electrophotographic system, a toner image is formed as a visible image by developing a colored toner on an electrostatic latent image created by a charging and exposure process on the surface of a drum-type photoreceptor as an image carrier. To do. Here, the toner is a non-magnetic or magnetic microscopic fine particle powder that adheres to the latent image and develops the latent image. In addition to chromatic ones, there are white and transparent ones. The toner image is directly transferred from the photosensitive member to a recording material such as transfer paper as a recording medium, or transferred to the recording material via an intermediate transfer member. An unfixed toner image on the recording material is fixed with a heat roll or the like to obtain an image formed product.

  Since untransferred toner, external additives, and discharge products remain on the surface of the photoreceptor after the transfer process, it is necessary to remove them by a cleaning unit prior to the next image forming process.

  As the cleaning means for removing the transfer residual toner and the like, there are various methods such as a method using a fur brush, a magnetic brush and the like, and a method using an elastic cleaning blade. A means for scraping off the toner by rubbing the photosensitive member with a cleaning blade is generally used because it is simple and inexpensive.

  With the recent increase in speed and image quality of image forming apparatuses, the toner to be used has become nearly spherical, and it has become difficult to ensure cleanability with only a cleaning blade. Therefore, there is a cleaning assisting unit that assists in removing the transfer residual toner. For example, a fur brush that contacts the photosensitive member is disposed in front of the cleaning blade, and transfer residual toner before reaching the cleaning blade is removed by the fur brush. Since pre-cleaning can be performed with a fur brush, there is a technique for reducing the load of the cleaning blade and improving the cleaning performance.

  In order to extend the life of the photoreceptor along with changes in the cleaning technology, there are thermosetting photoreceptors in which the surface of the photoreceptor is hard to be scraped by irradiating the surface with an electron beam. If the surface of the photoconductor is difficult to scrape, damage such as chattering of the cleaning blade, reversal of the blade, chipping of the cleaning edge, and wear is likely to occur. Therefore, the role of a cleaning auxiliary member such as a fur brush that improves the cleaning property has become important.

  A tandem type full-color image forming apparatus in which a plurality of image forming units are arranged is known. A tandem type image forming apparatus transfers a toner image on a photosensitive member to an intermediate transfer member, and forms a full color image by superimposing a plurality of color toner images on the intermediate transfer member.

  For example, when forming a full color image in an image forming apparatus having an image forming unit that forms yellow (Y), magenta (M), cyan (C), and black (K) toner images, The color image forming unit is used. When forming a monochrome image, a K-color image forming unit is mainly used. At this time, as disclosed in Patent Document 1, an image forming unit for colors other than K has a technique for separating the photosensitive member and the intermediate transfer member. By thus separating them and stopping the photosensitive member, it is possible to extend the life of parts such as the photosensitive member and the developing means in the image forming unit of a color other than K color.

  However, an image forming apparatus having a large image forming unit is feared to be further increased in size and complexity, and does not have a separation mechanism between the photosensitive member and the intermediate transfer member in order to increase the positioning accuracy of the image forming unit. Further, even in a low-cost image forming apparatus, there is a concern about cost increase, and there is no separation mechanism between the photosensitive member and the intermediate transfer member.

  In an image forming apparatus that does not have a separation mechanism, a monochrome image is also formed while the photoconductor and the intermediate transfer body are in contact with each other. In other words, the photosensitive member of the image forming unit of a color other than K is rotated during monochrome image formation.

JP 2009-217061 A

  As described above, in an image forming apparatus that does not have a separation mechanism between the photosensitive member and the intermediate transfer member, it is necessary to drive the photosensitive member of an image forming unit of a color other than K even when forming a monochrome image. For this reason, in the image forming unit of a color other than K, there is a problem that the photosensitive member is charged to the opposite polarity side to that at the time of image formation by rubbing with the intermediate transfer member or the fur brush in contact with the photosensitive member. The charge polarity can be adjusted according to the charge series of the member in contact with the photoconductor, but considering the functions such as cleaning, transferability and transportability with the media, the material of the charge series with the opposite polarity is selected. There are times when it must be done.

  If the photosensitive member is charged in reverse polarity, it becomes difficult for the charging means to uniformly charge, and image unevenness occurs. For this reason, it is necessary to prevent the charging of the photosensitive member from being reverse polarity as much as possible. In order to prevent charging to the opposite polarity, there is a means for charging to the same polarity as in normal image formation by a charging means. If the negative polarity is charged to about -700 to -1000 V during normal image formation, it may be the same as the normal time, or it may be about -300 V with a weak value (hereinafter referred to as weak charge). Absent. What is necessary is just to carry out negative charge in the range which does not become positive charge of reverse polarity.

  The charging means includes a contact type in which an elastic rubber roller is brought into contact and a corona charging type having a discharge electrode and a grid electrode. In a corona charger provided with a grid, the charging potential of the drum is generally set to a desired value by controlling the grid voltage. The grid voltage adjustment circuit for adjusting the grid voltage is configured to be adjustable within a range used in normal image formation. For this reason, when the grid voltage can be controlled only by this circuit even at the time of weak charging outside the range used in the normal image formation as described above, there is a problem that the circuit scale becomes large.

  The present invention has been proposed in view of the above prior art. The purpose is to adjust the grid voltage to a desired value while suppressing the increase in the scale of the grid voltage adjustment circuit in the case where the image carrier is weakly charged when the image carrier is idly rotated. It is an object of the present invention to provide an image forming apparatus capable of performing the above.

  A typical configuration of an image forming apparatus according to the present invention for achieving the above object includes a rotatable image carrier, a discharge electrode that discharges when supplied with a current, and a grid electrode, A corona charger for charging the surface of the image carrier, a toner image forming means for forming a toner image on the surface of the image carrier charged by the corona charger, and a first for supplying a current to the discharge electrode. When the grid voltage is applied to the grid electrode and the first current is supplied from the first power source to the discharge electrode during image formation, the voltage value of the grid voltage is an absolute value thereof. When the image carrier is rotated without forming a toner image with respect to the image carrier, the first power source is adjustable so as to be within a predetermined range. Said first from a power source A second current having a current value smaller than the current value of the current is supplied, and the voltage value of the grid voltage applied from the second power source is set to a value whose absolute value is smaller than the lower limit value in the predetermined range. And a control unit that controls to execute a mode for charging the image carrier.

  In order to achieve the above object, another typical configuration of the image forming apparatus according to the present invention includes a rotatable first image carrier, a discharge electrode that discharges when supplied with an electric current, and the like. A corona charger having a grid electrode and charging the surface of the first image carrier, and forming a first toner image on the surface of the first image carrier charged by the corona charger. A first image forming section having a first toner image forming means; a second image carrier; and a second toner image for forming a second toner image on the surface of the second image carrier. A second image forming unit having a forming unit; a first power source for supplying a current to the discharge electrode; and a grid voltage applied to the grid electrode to form the first toner image. A first current is supplied from the first power source to the discharge electrode. A second power supply capable of adjusting the voltage value of the grid voltage so that the absolute value thereof is within a predetermined range, and the first image carrier by the first image forming unit. A transfer device for transferring the first toner image formed on the surface of the first toner image and the second toner image formed on the surface of the second image carrier by the second image forming unit to an intermediate transfer medium; In the state where the first image carrier is rotated, the second toner image is formed in the second image forming unit without forming the first toner image in the first image forming unit. The grid that is supplied from the second power source while supplying the second current having a current value smaller than the current value of the first current from the first power source when executing the image forming mode for performing The absolute value of the voltage value is within the predetermined range. And having kicking and a control unit for controlling to execute a mode for charging the first image carrier to a value smaller than the lower limit value.

  According to the present invention, when an image is not formed but is weakly charged when the image carrier is idly rotated, the grid voltage is set to a desired value while suppressing an increase in the scale of the second power source (grid voltage adjustment circuit). An image forming apparatus that can be adjusted to a value can be provided.

Schematic configuration diagram of an image forming apparatus in an embodiment 1 is a partially enlarged view of the image forming apparatus of FIG. Block diagram of control system Schematic configuration diagram of corona charger and its high voltage circuit in the embodiment Graph of output relationship between corona wire and grid electrode in Example Control chart of image forming unit for each color in full color image forming mode Control chart of image forming unit of color other than K color in monochrome image forming mode Schematic configuration diagram of an image forming apparatus having another configuration

<Example 1>
The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

(Schematic configuration of image forming apparatus)
FIG. 1 is a schematic configuration diagram of an image forming apparatus in the present embodiment. FIG. 2 is a partially enlarged view thereof. FIG. 3 is a block diagram of the control system. This image forming apparatus is an electrophotographic system, a laser beam scanning exposure system, an intermediate transfer belt system, and a tandem type full-color image forming apparatus. In FIG. 1, a plurality of image forming units (image forming stations, image forming units) U are arranged in order from the left side to the right side, and in this embodiment, four image forming units UY, UM, UC, UK are provided. ing.

  Each image forming unit U has the same image forming process mechanism except that the color of the toner image to be formed is different from yellow (Y), magenta (M), cyan (C), and black (K). An electrophotographic image forming process mechanism. FIG. 2 is an enlarged schematic view of one image forming unit U in FIG.

  Each image forming unit U includes a drum-type electrophotographic photosensitive member (hereinafter referred to as a drum) 1 as a rotatable image carrier. The drum 1 is driven to rotate in the direction of arrow A at a predetermined peripheral speed. Further, charging means 2, exposure means 3, developing means 4, primary transfer means 5, and cleaning means as image forming process means acting on the drums 1 sequentially arranged around the drum along the rotation direction of the drum 1. 30.

  The image forming unit UY contains Y color toner in the developing unit 4 and forms a Y color toner image on the drum surface (hereinafter referred to as a Y color image forming unit). The image forming unit UM stores M color toner in the developing unit 4 and forms an M color toner image on the drum surface (hereinafter referred to as an M color image forming unit). The image forming unit UC contains C toner in the developing unit 4 and forms a C toner image on the drum surface (hereinafter referred to as C color image forming unit). In the image forming unit UK, K developing toner is accommodated in the developing unit 4, and a K toner image is formed on the drum surface (hereinafter referred to as K image forming unit).

  In this embodiment, the exposure means 3 and the developing means 4 in each of the image forming units U are toner image forming means for forming a toner image on the surface of the drum 1 charged by the charging means 2. Alternatively, the charging unit 2, the exposing unit 3, and the developing unit 4 are toner image forming unit that forms a toner image on the surface of the drum 1.

  An intermediate transfer belt unit 8 is disposed below the image forming units UY, UM, UC, and UK. This unit 8 includes an endless intermediate transfer belt (intermediate transfer medium: transfer target) 9 having flexibility, and a belt tension member in which the intermediate transfer belt (hereinafter referred to as a belt) 9 is suspended. It has a secondary transfer counter roller 10, a tension roller 11, and an auxiliary roller 12 as a frame member.

  The secondary transfer counter roller 10 is disposed on the Y color image forming unit UY side. The tension roller 11 is disposed on the K color image forming unit UK side. The auxiliary roller 12 is disposed below the rollers 10 and 11 between the rollers 10 and 11. The belt 9 is rotationally driven in the arrow B direction at a predetermined peripheral speed.

  A primary transfer roller 5 serving as a primary transfer unit (transfer device) in each color image forming unit U is disposed inside the belt 9, and the corresponding image forming unit via a belt portion between the rollers 10 and 11, respectively. It contacts the lower surface of the U drum 1. In each color image forming unit U, the contact portion between the drum 1 and the belt 9 is a primary transfer unit (transfer unit) N1. A secondary transfer roller 13 as a secondary transfer means (transfer device) is in contact with the secondary transfer counter roller 10 via a belt 9. A contact portion between the secondary transfer roller 13 and the belt 9 is a secondary transfer portion (transfer portion) N2.

  In the full-color image forming mode, a belt 9 in which toner images of each color Y, M, C, and K formed on the surface of the drum 1 of the image forming unit U for each color rotate in the primary transfer unit N1 in each image forming unit U. The images are sequentially superimposed and transferred on the surface. In the transfer of the toner image, a predetermined primary transfer bias having a polarity opposite to the charging polarity of the toner is applied to the primary transfer roller 5 in the image forming unit U of each color from the high voltage power source E5 controlled by the control unit 100. Done in Thus, a four color superimposed toner image of Y color + M color + C color + K color is formed on the surface of the belt 9.

  In each color image forming unit U, the residual toner on the drum surface after the primary transfer of the toner image to the belt 9 is removed from the drum surface by the cleaning means 30, and the cleaned drum surface is repeatedly used for image formation.

  In the monochrome image forming mode, only the K-color image forming unit UK performs an image forming operation to form a K-color toner image on the surface of the drum 1, and the K-color toner image is formed on the belt 9 at the primary transfer unit N1. Primary transfer to the surface. In the image forming units UY, UM, and UC of colors other than K, the drum 1 is rotationally driven (idle rotation), but the image forming process is not executed.

  As described above, the four-color superimposed toner image formed on the surface of the belt 9 by the full-color image forming mode or the K-color toner image formed on the surface of the belt 9 by the monochrome image forming mode is secondary by the subsequent rotation of the belt 9. It is conveyed to the transfer unit N2.

  On the other hand, the recording material 14 is separated and fed by the operation of the feeding member 16 from the recording material accommodating portion 15 on which the sheet-like recording material (medium: transfer target) 14 is stacked. The recording material 14 is introduced into the secondary transfer portion N2 by the registration roller pair 17 at a predetermined control timing, and K color which is a four-color superimposed toner image or monochrome image on the belt 9 side with respect to the surface of the recording material 14. The toner images are secondarily transferred sequentially. The transfer of the toner image is performed by applying a predetermined primary transfer bias having a polarity opposite to the charging polarity of the toner to the secondary transfer roller 13 from the high voltage power source E13 controlled by the control unit 100.

  Then, the recording material 14 that has received the secondary transfer of the toner image is introduced into the fixing means 18 and undergoes a fixing process of the toner image, and is printed out as a full-color image formation product or a monochrome image formation product. Further, the residual toner on the belt surface after the secondary transfer of the toner image to the recording material 14 is removed from the belt surface by the belt cleaning means 19, and the cleaned belt surface is repeatedly used for image formation.

(drum)
In this embodiment, the drum 1 as an image carrier is a negatively charged organic photoconductor (OPC) having an axial length of 360 mm and an outer diameter of 84 mm. In the drum 1, a photosensitive layer including a photoconductive layer mainly composed of an organic photoconductor is formed on a drum-type conductive substrate. OPC is generally configured by laminating a charge generation layer, a charge transport layer, and a surface protective layer made of an organic material on a metal substrate as a conductive substrate. In this example, each layer was formed using the materials described in JP-A-2005-43806.

  In this embodiment, the drum 1 in the image forming unit U for each color is driven in the direction of arrow A at a process speed (circumferential speed) of typically 300 mm / s by a driving device (main motor) M1 controlled by a control unit (CPU) 100. Is driven to rotate.

  Referring to FIG. 3, a control unit 100 is a control circuit unit having, for example, a microcomputer as a main control circuit, and exchanges various electrical information signals with a host device 200 such as an image reader, a host computer, a network, and a facsimile. do. Also, processing of electrical information signals input from various process devices and sensors of the image forming mechanism section, processing of command signals to various process devices, predetermined initial situation control, predetermined image forming sequence control, etc. Control. Predetermined control is executed according to a control program and a reference table stored in the memory unit 101.

(Charging means)
The charging unit 2 is a unit that uniformly charges the surface (outer peripheral surface) of the drum 1 to a predetermined polarity and potential in the charging unit F. In this embodiment, the charging means 2 is a corona charger that is disposed in a non-contact manner so as to face the drum 1. FIG. 4 is a schematic cross-sectional view of the corona charger 2 and a configuration diagram of a power source for the charger. The corona charger 2 is a scorotron including a discharge electrode (discharge wire: hereinafter referred to as a corona wire) 304 that discharges when supplied with an electric current, a conductive shield (case) 305, and a grid electrode 306. A type of discharge device.

  The conductive shield 305 is a channel material obtained by forming a metal plate into a U-shaped cross section, and has a length corresponding to a predetermined charging area width of the drum 1. Terminal members (not shown) made of an insulating material are attached to both longitudinal ends of the channel material, and the corona wire 304 is located at a substantially central portion in the groove of the channel material, and both end portions are on the side thereof. The terminal member is locked and stretched. A grid electrode 306 is disposed along the length of the opening of the channel material.

  The corona charger 2 configured as described above is arranged in parallel with the generatrix direction of the drum 1 with the grid electrode 306 facing the drum 1, and the corona wire 304 is separated from the surface of the drum 1 by a predetermined distance. It is positioned and arranged with respect to the drum 1.

  A primary charging high-voltage power source (first power source for supplying a current to the corona wire) 302 that applies a voltage to the corona wire 304 of the corona charger 2 and a grid high-voltage circuit (grid) that applies a voltage to the grid electrode 306. Voltage adjustment circuit: second power source) 307.

  The primary charging high voltage power supply 302 and the grid high voltage circuit 307 are respectively controlled by the control unit 100. In this embodiment, the control unit 100 controls the power supplied to the corona charger 2 so that the charging potential (dark portion potential) of the drum 1 at the time of image formation becomes −700V. More specifically, the control unit 100 performs constant current control on the value of the current flowing through the corona wire 304 at −1000 μA (the current value of the first current), and the DC applied voltage to the grid electrode 306 is controlled to control the charging. The potential is adjusted. Details of the primary charging high-voltage power supply 302 and the grid high-voltage circuit 307 of the corona charger 2 will be described later.

  Further, the high voltage value applied during image formation is not limited to this value, and is appropriately set to a potential suitable for good image formation in accordance with the environment and the usage durability of the drum 1 and the charging means 2. Is done.

(Exposure means)
In this embodiment, the exposure means 3 is a laser scanner unit, and includes a semiconductor laser that performs image exposure in the exposure unit G on the drum 1 whose surface is uniformly charged by the corona charger 2 based on image information. ing. The exposure potential (light portion potential) by the laser beam is −200 V in this embodiment. An electrostatic latent image corresponding to the image exposure pattern is formed on the drum surface by the electrostatic contrast between the charging potential of the drum 1 of −700 V and the exposure potential of −200 V.

  It should be noted that a potential measuring means 102 capable of measuring the potential of the drum 1 after exposure is arranged so that it can be confirmed whether the charging potential and the exposure potential of the drum 1 are actually predetermined potentials.

  In this embodiment, an example in which a semiconductor laser is used as the exposure unit 3 will be described. However, another unit such as an LED may be used as the exposure unit 3.

(Development means)
The developing means 4 will be described with reference to FIG. The developing means 4 in this embodiment is a developing device using a two-component developer that is a mixture of a non-magnetic toner and a magnetic carrier. The developing device 4 includes a developing container 4a containing a two-component developer, and a developing sleeve 4b as a developer carrying member rotatably provided at an opening of the developing container 4a.

  In the present embodiment, the axial length of the developing sleeve 4b is 325 mm. The developing sleeve 4b has a magnet roller (magnet) 4c fixedly arranged in a non-rotating manner therein. The developing sleeve 4b is rotationally driven in the direction of arrow C at a predetermined peripheral speed around the outside of the magnet roller 4c.

  The two-component developer in the developing container 4a is agitated and conveyed by the conveying member 4d and supplied to the developing sleeve 4b. The toner is configured to be triboelectrically charged to a negative polarity by rubbing against the magnetic carrier. In this embodiment, a toner having an average particle diameter of about 6 μm obtained by pulverizing and classifying a kneaded pigment in a resin binder mainly composed of polyester is used. The average charge amount of the toner attached to the exposure potential of the drum 1 is about −30 μC / g.

  The two-component developer supplied to the developing sleeve 4b is carried as a magnetic adsorption layer on the surface of the developing sleeve 4b by the magnetic force of the magnet roller 4c, and is conveyed along with the rotation of the developing sleeve 4b. Then, the layer pressure is regulated to a predetermined value by the developing blade 4e when it goes out of the developing container 4a, and it is conveyed to the developing part D which is a predetermined gap part between the developing sleeve 4b and the drum 1 by the subsequent rotation of the developing sleeve 4b. Applied to the drum surface.

  In the present embodiment, a developing bias in which a DC voltage (−550 V) and an AC voltage (Vpp is 1600 V) are superimposed is applied to the developing sleeve 4 a from a high voltage power source E4 controlled by the control unit 100. With this developing bias, the toner in the developer carried on the developing sleeve 4a side adheres to the electrostatic latent image on the drum 1 side in the developing portion D, thereby developing the latent image (the toner adheres to the exposure potential portion of the drum 1). Is performed). In the present embodiment, the developing sleeve 4a has a diameter of 20 mm and is rotated at a speed of 160% with respect to the rotational speed of the drum 1.

  The developer used for developing the latent image in the developing section D is transported back into the developing container 4a by the subsequent rotation of the developing sleeve 4a, peeled off from the surface of the developing sleeve 4a, and new developer is applied to the surface of the developing sleeve 4a. Supplied.

(Cleaning means)
The cleaning means 30 will be described with reference to FIG. The cleaning unit 30 is disposed on the downstream side of the primary transfer unit N1 and the upstream side of the charging unit F in the rotation direction of the drum 1, and is a cleaning unit that cleans the surface of the drum 1. The cleaning means 30 in the present embodiment includes a first cleaning member 6, a second cleaning member 7, and a housing 20 that encloses these cleaning members 6 and 7.

  In the present embodiment, the first cleaning member 6 is a fur brush roller (fur brush) 6 as a rotating member that is disposed so as to contact the drum 1 and is rotationally driven to rub the surface of the drum 1. The second cleaning member 7 is a cleaning blade made of urethane rubber having a length of 340 mm that is in contact with the drum surface downstream of the fur brush roller 6 in the rotation direction of the drum 1. The cleaning blade 7 is disposed in a fixed manner by contacting the surface of the drum 1 with a predetermined contact pressure with the tip edge portion as a counter with respect to the drum rotation direction.

  Residue such as transfer residual toner remaining on the surface of the drum 1 after the toner image is transferred to the belt 9 in the primary transfer portion N1 is disturbed by the fur brush 6 that is rotationally driven in the cleaning means 30. As a result, after the adhesive force with the drum 1 is weakened, it is removed from the surface of the drum 1 by the cleaning blade 7.

  Residues such as transfer residual toner removed from the drum surface by the cleaning blade 7 are temporarily held by the fur brush 6 positioned below the blade. Then, the fur brush 6 is carried to a contact position with the scraper 60 that is in contact with the peripheral surface of the fur brush 6 by the rotation of the fur brush 6. Residue such as transfer residual toner jumps out of the fur brush due to the repulsive force of the fiber of the fur brush 6 elastically deformed by the contact between the scraper 60 and the fur brush 6, and a conveying spiral (rotating spiral shaft) on the bottom surface of the housing 20. Drops near 61.

  Residue such as transfer residual toner that has dropped near the conveyance spiral 61 is conveyed in the axial direction of the drum 1 by the conveyance spiral 61 extending in the rotation axis direction of the drum 1 and further passes through a collected toner conveyance path (not shown). Then, it is recovered in a toner recovery container (not shown).

  A fur brush (rotating member) 6 as a first cleaning member functions as a cleaning unit for scraping off toner from the surface of the drum 1 together with the cleaning blade 7. In addition, it functions as drum polishing means (image carrier polishing means) for polishing the surface of the drum 1.

The fur brush roller 6 is obtained by planting fibers 6b on a rotating shaft 6a. It is manufactured by winding a cloth material in which fibers 6b are planted around a metal rotating shaft 6a having a diameter of 12 mm. The fur brush fiber 6b is a fiber obtained by bundling 6 denier single fibers made of acrylic on a base material at a flocking density of 50 kF / inch ² (flocking density per unit area). The fiber 6b has a length of 4.5 mm and is arranged so as to enter the drum 1 by about 1.0 mm.

  In general, acrylic (particularly fibrous) is a material that tends to be negative in the triboelectric charging series. Other fiber materials may be resin fibers such as polyester, rayon, and PET.

  As the material of the fur brush fiber 6b, a charge series material (such as acrylic or polyester) that is easily charged to a polarity opposite to the charging potential of the toner and the drum 1 may be used. The average charge amount of the transfer residual toner may be on the positive side. This is an influence when passing through the transfer means 5, and the average charge amount of the toner that has not been transferred is greatly reduced, and the polarity may be reversed from negative to positive. For this reason, the fur brush fiber 6b having a charge series that tends to be negative tends to collect positively-polarized toner and the like. This has the effect of facilitating the removal of transfer residual toner from the drum 1 and assisting in cleaning.

  As another effect, there is an effect of neutralizing the potential charged to the negative of the drum 1 to the zero potential by the contact of the fur brush fiber 6b. From the above effects, acrylic or polyester was selected as the material of the fur brush fiber 6b.

  Further, when attached to the image forming apparatus, the fur brush fibers 6b are made of conductive material, and the metal rotating shaft 6a of the fur brush roller 6 is attached so as to be grounded. As indicated by arrows A and H, the drum 1 and the fur brush roller 6 are set to rotate in the same direction at the contact nip portion H of both 1 and 6. The rotation of the fur brush roller 6 can be set to an arbitrary rotation speed by the control device 62 controlled by the control unit. In the present embodiment, the fur brush roller 6 rotates at a peripheral speed of 140% of the rotational speed of the drum 1.

  In the present embodiment, the rotating member as the first cleaning member is the fur brush roller 6 that can be arbitrarily rotated as described above. However, the rotating member is not limited to the fur brush roller but may be a roll member having another structure that can rotate. Good.

(Intermediate transfer member)
In this embodiment, an endless flexible belt member is used as the intermediate transfer member (transfer target member) 9. As the belt 9, an appropriate amount of carbon black as an antistatic agent is contained in a resin such as polyimide and polycarbonate, or various rubbers, the volume resistivity is 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm, and the thickness is 0.1. A thickness of 07 to 0.3 mm is used.

Further, as a belt 9, a rubber layer in which an appropriate amount of carbon black as an antistatic agent is contained in various rubbers on a base material having a thickness of 0.07 to 0.2 mm made of a resin such as polyimide or polycarbonate. A thickness of 0.05 to 0.5 mm is formed. Furthermore, as a surface layer, a belt having a three-layer structure in which a material containing a fluororesin or the like is coated with a thickness of 5 to 30 μm can be used. At that time, the volume resistivity is preferably 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm.

  The belt 9 has a circumferential speed difference with respect to the drum 1. In this embodiment, the belt 9 is driven at a speed of 102% with respect to the drum 1. As described above, the belt 9 is in contact with the drum 1 with a predetermined pressure by the primary transfer roller 5, and contact charging is generated by rubbing against the drum 1.

(Image forming operation)
Next, image forming operations in the full color image forming mode and the monochrome image forming mode will be described.

1) Full Color Image Forming Mode FIG. 6 is a control chart of the image forming units UY, MU, UC, and UK for each color in the full color image forming mode. In each color image forming unit UY, MU, UC, UK, each rotating drum 1 is charged to a charging potential of −700 V by the corona charger 2, and a latent image is formed by the exposure unit 3. As described above, the corona charger 2 applied −1000 μA (current value of the first current) to the corona wire 304, and −770 V was applied to the grid electrode 306 so that the charging potential was −700 V.

  The latent image formed by the exposure unit 3 is developed with a DC voltage of −550 V and an AC voltage of 1600 V applied to the developing sleeve 4 b of the developing unit 4 to form a toner image on the drum 1. The toner images on the drums 1 of the image forming units UY, MU, UC, and UK for each color are sequentially superimposed and transferred onto the belt 9 by the primary transfer unit 5.

  The superimposed transfer toner image is secondarily transferred onto the recording material 14 at once in the secondary transfer portion N2, fixed on the recording material 14 by the fixing means 11, and the full-color image formed product is discharged. Untransferred toner on the drum 1 that has not been transferred to the belt 9 by the primary transfer means 5 is cleaned by the rotating fur brush roller 6 and the cleaning blade 7.

2) Monochrome Image Forming Mode FIG. 7 is a control chart of the image forming units UY, MU, UC, and UK for each color in the monochrome image forming mode. In the monochrome image forming mode, the K color image forming unit UK that forms the K color toner image performs the K color toner image forming operation by the same operation as in the full color image forming mode (FIG. 6). Since the drum 1 in the image forming units UY, UM, and UC of colors other than the K color is in contact with the belt 9, the drum 1 starts to be rotated simultaneously with the rotation driving of the belt 9.

  Further, in the image forming units UY, UM, and UC of colors other than K, normal charging (charging at the time of image formation) with respect to the drum 1 is not performed, and the charging potential is −. Weakly charge to 270V.

  In this embodiment, the weak charge is set by the control unit 100 so that the current value supplied to the corona wire 304 of each corona charger 2 in the image forming units UY, UM, and UC is set to −500 μA. This current value is a second current having a current value smaller than the current value of the first current: −1000 μA. And it is performed by setting the grid voltage applied to the grid electrode 306 to -300V.

  In addition, the control unit 100 performs the above-described weak charging on the drum 1 in the image forming units UY, UM, and UC of colors other than K, and at the same time, the developing sleeve 4b of the developing device 4 also includes the drum 1 and a predetermined amount A DC voltage of −200 V that forms a potential difference is applied. This prevents unnecessary toner and carrier from adhering to the drum 1 with respect to the drum 1 in the image forming units UY, UM, and UC. Since development is not performed, the development sleeve 4b is held in a rotation stopped state in order to prevent deterioration of the developer.

  Due to the above-described weak charging of the drums 1 in the image forming units UY, UM, and UC of colors other than K, the rotating drum 1 in these image forming units is positively charged by rubbing with the fur brush roller 6 or the belt 9. To form a monochrome image. After the primary transfer, the same operation as full color image formation is performed. Incidentally, -100V (voltage having the same polarity as the charging polarity of the toner) is applied to each primary transfer roller 5 in the image forming units UY, UM, and UC to prevent contact charging at the primary transfer unit N1.

  When the drum 1 is weakly charged in the image forming units UY, UM, and UC, the supply current value to the corona wire 304 of the corona charger 2 is changed to −500 μA instead of −1000 μA, which is the same as charging during normal image formation. . As a result, the grid voltage can be set and output to -300V as intended. If the grid voltage is set to −300V and the output is not output as set to −440V, there is a concern about carrier adhesion to the drum 1 and toner fogging.

  In particular, when carrier adhesion occurs, the carrier adhered on the drum 1 is transported to the cleaning blade 7 and is sandwiched between the contact portions of the cleaning blade 7, scratching the surface of the drum 1, or missing the contact portion of the cleaning blade 7. Cause damage. Therefore, it is necessary to output the grid voltage reliably as set. Therefore, the supply current value for the corona wire 304 is set to -500 μA.

(Grid high voltage circuit, primary charging high voltage power supply)
Hereinafter, a grid high voltage circuit in which the grid electrode 306 is output as set when the output value of the corona wire 304 is changed will be described. In FIG. 4, 307 is a block diagram of a grid high voltage circuit. As described above, the corona charger (scorotron charger) 2 for charging the surface of the drum 1 is provided on the outer periphery of the drum 1. The corona charger 2 includes a conductive shield 305 that is a channel material formed by molding a metal plate into a U-shaped cross-sectional shape. A predetermined high voltage is applied.

  The surface potential of the drum 1 is controlled by applying a negative grid voltage Vg_out to the grid electrode 306 from a grid high voltage circuit (grid voltage adjustment circuit) 307 described in detail later. The grid electrode 306 is at the same potential as the conductive shield 305.

  Next, the grid high voltage circuit 307 will be described. The collector of the transistor Tr1, the resistor R1, and the voltage detection circuit 308 are connected to the grid electrode 306 of the corona charger 2 via the resistor Rc. The voltage detection circuit 308 generates a detection signal Vsns obtained by converting the level of the grid voltage Vg_out.

  The detection signal Vsns is input to the non-inverting input terminal of the operational amplifier OP1. A control signal Vcont output from the control unit 100 and corresponding to the target value of the grid voltage Vg_out is input to the inverting input terminal of the operational amplifier OP1. The inverting input terminal and output terminal of the operational amplifier OP1 are connected via a capacitor C1 for negative feedback.

  The operational amplifier OP1 is connected to + 24V for a single power supply operation. A resistor R2 is connected between the output terminal of the operational amplifier OP1 and the base of the transistor Tr1, and a resistor R3 is connected between the base and emitter of the transistor Tr1. The emitter of the transistor Tr1 is connected to + 24V.

  Next, the operation of the grid high voltage circuit 307 will be described. The grid voltage Vg_out is generated when a high voltage is applied from the primary charging high-voltage power supply 302 to the corona wire 304 of the corona charger 2. The primary charging high-voltage power supply 302 is controlled by the control unit 100 so that the current value Ichg flowing through the corona wire 304 becomes a desired value.

  When the primary charging high-voltage power supply 302 outputs a high voltage, a high voltage is applied to the corona wire 304, and a potential difference between the corona wire 304 and the grid electrode 306 increases, and a corona discharge is generated between the corona wire 304 and the grid electrode 306. Therefore, current Ichg flows from grid electrode 306 to corona wire 304.

  The current Ichg is a total value of the current Id flowing from the drum 1 to the grid electrode 306, the collector current Ic of the transistor Tr1, the current Ir flowing through the resistor R1, and the current Is flowing from the voltage detection circuit 308. Here, the values of the current Id and the current Is are small, and the total value of the current Ir and the current Ic occupies a large ratio with respect to the current Ichg. That is, Ichg≈Ir + Ic. On the other hand, since the base potential of the transistor Tr1 is + 24V−Vbe, Vg_out≈R1 × Ir.

  The grid voltage Vg_out generated when the primary charging high voltage power supply 302 outputs a high voltage and the current Ichg flows through the corona wire 304 is controlled so that the output voltage value becomes a desired value. That is, the voltage of the output terminal of the operational amplifier OP1 is adjusted so that the voltage detection signal Vsns generated by the voltage detection circuit 308 is equal to the control signal Vcont output from the CPU 9. Thus, the value of the current Ic flowing through the series circuit of the transistor Tr1 and the resistor Rc and the value of the current Ir flowing through the resistor R1 are controlled so that the grid voltage Vg_out becomes a desired voltage value.

  When the grid voltage Vg_out is smaller than the target value (the difference from the ground potential is large), Vsns <Vcont, and the OP amplifier OP1 performs an operation of decreasing the voltage of the output terminal. When the output voltage of the OP amplifier OP1 is lowered, the current flowing through the resistor R2 increases and the base current of the transistor Tr1 increases, so that Ic increases. In this case, the current Ir flowing through the resistor R1 is decreased, and the grid voltage Vg_out is controlled so as to shift in a direction in which the difference from the ground potential is small and converge to the target value.

  When the grid voltage Vg_out is larger than the target value (the difference from the ground potential is small), Vsns> Vcont, and the OP amplifier OP1 performs an operation of increasing the voltage of the output terminal. Since the base potential of the transistor Tr1 is substantially constant during the output of the grid voltage Vg_out, when the output voltage of the OP amplifier OP1 is increased, the current flowing through the resistor R2 decreases and the base current of the transistor Tr1 decreases, so that Ic decreases. In this case, the current Ir flowing through the resistor R1 is increased, and the grid voltage Vg_out is controlled to shift toward a large difference from the ground potential and converge to the target value.

  As described above, the grid high voltage circuit 307 performs constant voltage control so that the grid voltage Vg_out is maintained at a desired voltage value.

  Next, a method for determining the constants of the resistors R1 and Rc will be described. In the image forming apparatus according to this embodiment, when a current of −1000 uA is supplied as the current Ichg during the image forming operation, the grid voltage Vg_out is controlled within a range of −500 to −1000V.

  The grid voltage Vg_out becomes the maximum value when the output voltage of the OP amplifier OP1 rises due to the operation of the OP amplifier OP1 and the transistor Tr1 is turned off, and Ichg≈Ir and Vg_out≈Ir × R1. Since the maximum control voltage of the grid voltage Vg_out is −1000V, Ir × R1≈1000V. Since the current Ir is 1000 uA, the resistor R1 needs to be 1 MΩ or more. Here, 1.2 MΩ is selected as the resistor R1 in consideration of the margin.

  On the other hand, when the minimum value of the grid voltage Vg_out is −500 V, the output voltage of the OP amplifier OP1 decreases and the transistor Tr1 is turned on, and Ichg≈Ir + Ic. Assuming that a sufficient base current is applied and the Vce of the transistor Tr1 is approximately 0 V, the current Ichg (1000 uA) flows through the parallel resistance of the resistor R1 (1.2 MΩ) and the resistor Rc.

  When 680 kΩ is selected as the resistance Rc, R1 // Rc is 434 kΩ, and −434 V is generated with respect to a current of 1000 uA, and −500 V can be obtained as the grid voltage Vg_out by controlling the base current of the transistor Tr1.

  Here, the role of the resistor Rc is considered. When the current Ichg≈−1000 uA and the grid voltage Vg_out≈−500 V, the current Ic is 580 uA. When the resistance Rc = 0Ω, since the current Ic of the transistor Tr1 is 580 uA and Vce (= Vg_out) is 500 V, the loss of the transistor Tr1 is 0.29 W. To allow this loss, the transistor Tr1 becomes large and expensive.

  When the resistance Rc = 680 kΩ, when the grid voltage Vg_out = −500V, Vce of the transistor Tr1 is 100V, and the loss of the transistor Tr1 can be suppressed to 0.06W. Although the resistor Rc has a role of taking heat loss instead of the transistor Tr1, an inexpensive resistor or the like having a rated power of about 1 W can be used.

  The grid high voltage circuit 307 determines the control range of the grid voltage Vg_out corresponding to the current Ichg as shown in FIG. 5 by the constants of the resistance R1 and the resistance Rc determined in this way. That is, when the current Ichg is −1000 uA, the grid voltage Vg_out can be controlled from −1000 V (point A) to −500 V (point B). When the current Ichg is lowered to −500 uA, the grid voltage Vg_out can be controlled in the range of −550 V (point C) to −250 V.

  As described above, by setting the set value of the corona wire 304 to −500 μA, the output value of the grid electrode 306 can be controlled according to the set value. Further, by reducing the current applied to the corona wire 304, the amount of discharge products is also reduced, so that the image flow can be improved. As a result, unnecessary toner adhesion and carrier adhesion to the surface of the drum 1 can be prevented, and a good image can be formed over a long period of time.

  The first characteristic configuration of the image forming apparatus of the present embodiment that exhibits the above-described effects is summarized as follows.

  It has a drum 1 as a rotatable image carrier. A corona charger 304 that includes a corona wire 304 as a discharge electrode that discharges when supplied with a current and a grid electrode 306 and charges the surface of the drum 1 is provided. Toner image forming means 3 and 4 for forming a toner image on the surface of the drum 1 charged by the corona charger 2 are provided. A primary charging high-voltage power supply 302 is provided as a first power supply for supplying a current to the corona wire 304. A grid high voltage circuit 307 is provided as a second power source for applying the grid voltage Vg_out to the grid electrode 306.

  When the first current is supplied from the primary charging high voltage power supply 302 to the corona wire 304 at the time of image formation, the grid high voltage circuit 307 sets the voltage value of the grid voltage Vg_out so that its absolute value is within a predetermined range. Can be adjusted.

  And it has the control part 100 which controls to perform the following charging mode. That is, when the drum 1 is rotated without forming a toner image on the drum 1, a second current having a current value smaller than the current value of the first current is supplied from the primary charging high-voltage power supply 302. . At the same time, the voltage value of the grid voltage Vg_out applied from the grid high voltage circuit 307 is controlled so that the absolute value thereof is smaller than the lower limit value in the predetermined range and the drum 1 is charged.

  The second characteristic configuration of the image forming apparatus of the present embodiment that exhibits the above-described effects is summarized as follows.

  A first image forming unit UY, UM, UC having the following configuration is included. That is, it has a drum 1 as a rotatable first image carrier. The apparatus includes a corona charger 2 that includes a corona wire 304 serving as a discharge electrode that discharges when supplied with a current and a grid electrode 306, and charges the surface of the first drum 1. First toner image forming means 3 and 4 for forming a first toner image on the surface of the first drum 1 charged by the corona charger 2 are provided.

  The second image forming unit includes a drum 1 as a second image carrier and second toner image forming units 2, 3, and 4 for forming a second toner image on the surface of the second drum 1. Part UK.

  In addition, a primary charging high-voltage power supply 302 is provided as a first power supply for supplying a current to the corona wire 304. A grid high voltage circuit 307 is provided as a second power source for applying the grid voltage Vg_out to the grid electrode 306.

  The grid high voltage circuit 307 can be adjusted as follows. That is, when the first current is supplied from the primary charging high-voltage power supply 302 to the corona wire 304 at the time of image formation for forming the first toner image, the absolute value of the voltage value of the grid voltage Vg_out is within a predetermined range. It can be adjusted to be within the range.

  The first toner image formed on the surface of the first drum 1 by the first image forming units UY, UM, UC and the second toner image formed on the surface of the second drum 1 by the second image forming unit UK. The toner image is transferred to a belt 19 as an intermediate transfer medium.

  In the state where the first drum 1 is rotated, the second toner image is formed in the second image forming unit UK without forming the first toner image in the first image forming units UY, UM, UC. When executing the image forming mode to be performed, the control unit 100 performs the following control. That is, a second current having a current value smaller than the current value of the first current is supplied from the primary charging high-voltage power supply 302. At the same time, the voltage value of the grid voltage Vg_out applied from the grid high voltage circuit 307 is controlled so that the absolute value thereof is smaller than the lower limit value in the predetermined range and the first drum 1 is charged. .

<Example 2>
Since the configuration of the second embodiment is the same as that of the first embodiment, description thereof is omitted. In the second embodiment, an example in which the set values for the corona wire 304 and the grid electrode 306 are changed according to the driving conditions of the developing sleeve 4b of each image forming unit U during image formation will be described.

  Let S1 be the rotational speed of the developing sleeve 4b of each color image forming portion UY, UM, UC, UK in the full-color image forming mode. Let S2 be the rotation speed of the developing sleeve 4b of the image forming portions UY, UM, UC of colors other than K in the monochrome image forming mode. As in the first embodiment, in the monochrome image forming mode, since the developing sleeves 4b of the image forming portions UY, UM, UC other than K are stopped, S2 = 0. Therefore, the relationship is S1> S2.

  Therefore, the rotation speeds S1 and S2 of the developing sleeve 4b in the full-color image formation mode and the monochrome image formation mode are compared, and the set values for the corona wire 304 and the grid electrode 306 when S1> S2. May be changed.

  Further, from the viewpoint of image flow, the developing sleeve 4b is rotated while the drum 1 is rotated as the D2 / D1 of the developing distance 4 of the developing sleeve 4b with respect to the traveling distance D1 of the drum 1 per unit time becomes smaller. This indicates that the motor is not rotating. When the developing sleeve 4 b rotates, a slight amount of fog toner adheres onto the drum 1 and is supplied to the cleaner blade 7. However, when the developing sleeve 4b is stopped, even the fog toner is not supplied to the cleaner blade 7.

  As a result, the frictional force of the cleaner blade 7 against the surface of the drum 1 is reduced, and it becomes impossible to remove discharge products and the like adhering to the drum surface, and image flow is likely to occur. Using this result, even if the setting value of the corona wire 304 is changed from, for example, −600 μA to −500 μA according to the ratio D2 / D1 of the traveling distance between the drum 1 and the developing sleeve 4b per unit time. Good. This not only outputs the grid electrode 306 according to the set value, but also prevents the occurrence of image flow.

  For example, the drum 1 and the developing sleeve 4b per unit time when the speed of the developing sleeve 4b is changed during continuous solid white or when the speed of the drum 1 and the developing sleeve 4b is changed according to the type of the recording material 14 or the like. The ratio of D2 / D1 of the travel distance is detected. Depending on the detection result, the set values of the corona wire 304 and the grid electrode 306 can be changed.

  More specifically, the relationship between the image flow, the corona line 304, and the set output value of the grid electrode 306 is stored in advance as a database. Then, the setting value of the corona wire 304 and the grid electrode 306 can be changed from the ratio of D2 / D1 of the traveling distance between the drum 1 and the developing sleeve 4b per unit time to reliably prevent image flow.

  Although the embodiment in which the cleaning blade 30 is provided in the cleaning means 30 has been described, the present invention can also be applied to a cleanerless type image forming unit having no cleaning means for residual toner such as a cleaning blade.

<Other matters>
(1) In the embodiment, the belt 9 as an intermediate transfer member is formed by forming a rubber layer on a base material such as polyimide or polycarbonate. You may use the thing of the structure in which the rubber layer is not formed.

  (2) Not only when the cleaning blade 7 or the fur brush 6 is used as a cleaning means (cleaning member) as in the embodiment, but a member that rubs the surface of the image carrier with a non-rotating fixed brush or a rotatable sponge roller. May be used.

  (3) In the image forming apparatus of FIG. 1, the intermediate transfer belt unit 8 carries the recording material 14 as a transfer medium as shown in FIG. 8, and the transfer portions for the image forming portions UY, UM, UC, UK for each color. It is also possible to adopt an apparatus configuration in which the transport belt unit 40 transports sequentially to N1. Reference numeral 41 denotes an endless conveying belt that carries and conveys the recording material 14, and 42 to 44 are stretching rollers that suspend and stretch the belt 41.

  (4) The image forming principle and image forming process of the image forming unit of the image forming apparatus are not limited to the electrophotographic system using a photoconductor as the image carrier of the embodiment. An electrostatic recording method using a dielectric as an image carrier may be used.

  (5) The image carrier 1 is not limited to the drum type of the embodiment. A flexible endless belt type can also be used. The intermediate transfer body 19 and the recording material transport body 41 may be drum-type.

  (6) The developing method of the developing device 4 is not limited to the developing method using the two-component developer of the embodiment. A developing method using a so-called one-component developer composed of only one-component magnetic toner or one-component non-magnetic toner may be used.

  1. Image carrier 2. Corona charger 304. Discharge electrode 306 Grid electrode 3, 4 Toner image forming means (exposure means, developing means) 302 First power supply 307 ··· Second power source 100 ··· Control unit

Claims (6)

A rotatable image carrier;
A corona charger comprising a discharge electrode that discharges when supplied with an electric current, and a grid electrode, and charges the surface of the image carrier;
Toner image forming means for forming a toner image on the surface of the image carrier charged by the corona charger;
A first power source for supplying current to the discharge electrode;
When a grid voltage is applied to the grid electrode and a first current is supplied from the first power source to the discharge electrode during image formation, the absolute value of the voltage value of the grid voltage is within a predetermined range. A second power supply that is adjustable to be within the range of
When rotating the image carrier without forming a toner image with respect to the image carrier, a second current having a current value smaller than the current value of the first current is applied from the first power source. Control is performed so that the voltage value of the grid voltage applied from the second power supply is set to a value whose absolute value is smaller than a lower limit value in the predetermined range and the image carrier is charged. A control unit,
An image forming apparatus comprising:   A transfer unit that transfers the toner image formed on the surface of the image carrier by the toner image forming unit to a transfer target; and a downstream side of the transfer unit in the rotation direction of the image carrier and charged by the corona charger. A cleaning member that is disposed in contact with the image carrier on the upstream side of the charging unit and that cleans the surface of the image carrier by rubbing against the surface of the image carrier. The image forming apparatus according to claim 1, wherein:   The image forming apparatus according to claim 2, wherein the cleaning member is a brush in which a plurality of fibers are implanted. A rotatable first image carrier;
A corona charger comprising a discharge electrode that discharges when supplied with a current, and a grid electrode, and charges the surface of the first image carrier;
A first image forming unit comprising: a first toner image forming unit that forms a first toner image on the surface of the first image carrier charged by the corona charger;
A second image carrier;
A second image forming unit having a second toner image forming unit for forming a second toner image on the surface of the second image carrier;
A first power source for supplying current to the discharge electrode;
A voltage of the grid voltage is applied when a first current is supplied from the first power source to the discharge electrode during image formation in which a grid voltage is applied to the grid electrode to form the first toner image. A second power supply whose value is adjustable so that its absolute value is within a predetermined range;
The first toner image formed on the surface of the first image carrier by the first image forming unit and the second toner image formed on the surface of the second image carrier by the second image forming unit. A transfer device for transferring the second toner image to the intermediate transfer medium;
The second image forming unit forms the second toner image in the second image forming unit without forming the first toner image in the first image forming unit while the first image carrier is rotated. The grid voltage applied from the second power supply while supplying a second current having a current value smaller than the current value of the first current from the first power supply when executing the image forming mode to be performed. A control unit that controls the absolute value of the voltage value to be smaller than a lower limit value in the predetermined range to execute a mode for charging the first image carrier;
An image forming apparatus comprising:   The first image carrier is in contact with the first image carrier on the downstream side of the transfer portion of the toner image onto the intermediate transfer medium in the rotation direction of the first image carrier and on the upstream side of the charging portion charged by the corona charger. 5. The image forming apparatus according to claim 4, further comprising: a cleaning member that is disposed on the surface of the first image carrier to clean the surface of the first image carrier by rubbing against the surface of the first image carrier. apparatus.   The image forming apparatus according to claim 5, wherein the cleaning member is a brush in which a plurality of fibers are implanted.