Image-forming device

Abstract

To solve the problem that a filler or paper powder originating from a transfer material not cleaned by an intermediate transfer body cleaning device is transferred onto an image carrier via a primary transfer section, and an image defect caused by adhesion of the filler to the image carrier and toner slipping caused by jamming of the paper powder in a cleaning blade may occur in a tandem type image-forming device for cleaning an intermediate transfer body electrostatically, and the service life of an image carrier positioned upstream may become extremely short since the amount of transfer is larger in an upstream image-forming station.SOLUTION: A primary transfer current at the time of a non-image formation operation is controlled to be smaller in an upstream image-forming station.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a tandem type image forming apparatus using an intermediate transfer member. [Background technology]

[0002] There are various types of image forming apparatuses that employ electrophotographic systems. For example, there is a multiple development type in which multiple toner images formed in sequence on a specific image carrier are superimposed, and then transferred to a transfer material at once to form an image. There are also multiple transfer types in which a cycle of development and transfer is repeated, and intermediate transfer types in which toner images of each color are superimposed in sequence on an intermediate transfer body and then transferred to a transfer material at once. Among these, the intermediate transfer type has the advantages of being compact, having no risk of color mixing, and having few restrictions on the type of transfer material to which the final visible image is transferred, and is therefore particularly suitable for use in color image forming apparatuses.

[0003] Among the image forming apparatuses described above, an intermediate transfer belt having an elastic layer may be used as the intermediate transfer body. In this case, an electrostatic cleaning method for electrostatically cleaning the intermediate transfer belt is often adopted as the cleaning device. In general, the electrostatic cleaning method has a lower cleaning performance than a blade-type cleaning device that cleans with a blade. This is because the charge amount of the secondary transfer residual toner, the filler derived from the transfer material, and the paper dust is difficult to collect even if a bias voltage of either polarity is applied when the charge amount is near zero. As a result, some of the secondary transfer residual toner, filler, and paper dust are not removed from the intermediate transfer belt and continue to adhere to it and are carried around. Here, a large pressure force is applied to the primary transfer portion to form a transfer portion between the primary transfer roller and the photosensitive drum. Also, a large pressure force is applied to the secondary transfer portion between the secondary transfer roller and the secondary transfer counter roller. Also, a transfer voltage is applied to each transfer portion to form an electric field. Therefore, when the secondary transfer residual toner, filler, and paper dust pass through the primary transfer portion or the secondary transfer portion, some of them are transferred to the photosensitive drum or the secondary transfer roller. If the filler is transferred to the photosensitive drum, it will adhere to the photosensitive drum, causing image defects. In addition, paper dust will get caught in the photosensitive drum cleaning blade, creating a gap that allows the toner to slip through. In particular, in the case of a tandem-type image forming apparatus, the transfer rates at each image forming station are almost the same under the same image forming conditions, so the amount of transfer of filler and paper dust increases the more upstream the image forming station is. This causes a problem that the amount of transfer increases the more upstream the image forming station is, and the lifespan of the image forming station becomes shorter the more upstream it is.

[0004] Patent Document 1 discloses a configuration for improving cleaning performance by refreshing toner and paper dust accumulated on the brush member of an intermediate transfer belt cleaning device. This involves forming a toner image for refreshing the brush member, cleaning it with the intermediate transfer belt cleaning device, and then spitting out the paper dust attached to the brush member together with the refreshing toner. Then, when the paper dust spit out together with the toner on the intermediate transfer belt passes through the primary transfer section of each image forming station, an electric field is formed in the primary transfer section, and the paper dust is collected in each image forming station. At this time, a configuration is disclosed in which the direction of the electric field formed in the primary transfer section is set to the same direction as during image formation operation or the opposite direction, and the amount of collected dust is shared among multiple image forming stations by combining these methods.

[0005] However, in the case of the configuration of Patent Document 1, although the paper dust expelled during the refresh operation can be dispersed and collected in each image forming section according to the polarity, there was a problem that in image forming sections to which the same polarity is applied, the more upstream the image forming section is, the more paper dust is likely to accumulate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2017-107090 A Summary of the Invention [Problem to be solved by the invention]

[0007] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS An object of the present invention is to prevent paper dust adhering to an intermediate transfer body from being transferred in large quantities to an upstream image forming section in a tandem type image forming apparatus. [Means for solving the problem]

[0008] The configuration of the present invention for achieving the above object includes a first image carrier carrying a toner image, a second image carrier carrying a toner image, an intermediate transfer body onto which the toner images carried by the first image carrier and the second image carrier are primarily transferred, a cleaning unit that electrostatically cleans toner remaining on the intermediate transfer body after the toner image primarily transferred onto the intermediate transfer body is secondarily transferred onto a recording material, a first transfer member that primarily transfers the toner carried on the first image carrier to the intermediate transfer body in a first transfer section, a second transfer member that primarily transfers the toner carried on the second image carrier to the intermediate transfer body in a second transfer section, and a transfer mechanism for transferring the toner carried on the first transfer member and the intermediate transfer body to each other. and a control unit that controls the power supply unit, the first image carrier being disposed upstream of the second image carrier in the direction of movement of the intermediate transfer body, and the control unit being capable of executing a mode in which, during non-image formation, the power supply unit supplies a first current to the first transfer member, and the control unit is capable of executing a mode in which the power supply unit controls the power supply unit to supply a second current to the second transfer member when an area of ​​the intermediate transfer body that has passed the first transfer section when the first current is supplied passes the second transfer section, the first current being smaller than the second current. Effect of the Invention

[0009] According to the present invention, in a tandem type image forming apparatus, it is possible to prevent paper dust adhering to an intermediate transfer body from being transferred in large quantities to an upstream image forming section. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating a schematic cross-sectional configuration of an image forming apparatus according to a first embodiment. [Diagram 2] FIG. 11 is a diagram showing the intensities of metal elements contained in each recovered toner. [Diagram 3] 13 is a diagram showing the intensity of metal elements contained in recovered toner when the primary transfer current is changed. FIG. [Figure 4]FIG. 13 is a diagram showing the set values ​​of primary transfer currents of the image forming stations during image forming operations in the first and second embodiments. [Diagram 5] FIG. 13 is a diagram showing the set values ​​of primary transfer currents of the image forming stations during non-image forming operations in the first and second embodiments. [Figure 6] FIG. 1 is a block diagram showing the configuration of the first and second embodiments. [Figure 7] FIG. 1 is a flowchart according to the first embodiment. [Figure 8] FIG. 11 is a flowchart in the second embodiment. [Figure 9] FIG. 11 is a diagram illustrating a schematic cross-sectional configuration of an image forming apparatus according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiment of the present invention will be described in detail with reference to the drawings. EXAMPLES

[0012] FIG. 1 is a simple illustration of an example of a schematic cross-sectional view of an image forming apparatus according to the present embodiment. This is merely an example, and is not intended to be limiting. Furthermore, unless otherwise specified, the dimensions, materials, shapes, and relative positions of the components described in the following embodiments are not intended to limit the scope of the present invention. Furthermore, the materials, shapes, and the like of the members once described in the following explanations are the same as those described initially, unless otherwise specified. Furthermore, the same reference numerals are attached to the components in each drawing to have the same configuration or function, and duplicate explanations of these components are omitted as appropriate.

[0013] The image forming apparatus shown in Fig. 1 is a four-color full-color electrophotographic image forming apparatus, and Fig. 1 is a vertical sectional view showing a schematic configuration thereof. The overall configuration and operation of image formation will be described with reference to Fig. 1.

[0014] The image forming apparatus shown in Fig. 1 has four (four color) image forming stations (image forming units), namely image forming stations Y, M, C, and K, which form yellow, magenta, cyan, and black toner images, respectively. The toner images of each color formed at these image forming stations Y, M, C, and K are sequentially primarily transferred onto an intermediate transfer body and superimposed. After that, they are all secondarily transferred collectively onto a transfer material (recording material) P such as paper. The four color toner images thus secondarily transferred are then fixed to obtain a four-color full-color image.

[0015] The image forming stations Y, M, C, and K are provided with drum-shaped photoconductors (hereinafter, photoconductor drums) 11Y, 11M, 11C, and 11K as image carriers (photoconductors, photoconductor drums), respectively. Each of the photoconductor drums 11Y, 11M, 11C, and 11K in this embodiment is configured by applying an OPC (organic photoconductor) as a photosensitive layer to the outer peripheral surface of an aluminum cylinder having an outer diameter of 30 mm, and using a curable resin as the resin for the charge transport layer.

[0016] The photosensitive drums 11Y, 11M, 11C, and 11K are in contact with charging rollers 12Y, 12M, 12C, and 12K, each having an outer diameter of 14 mm, which are pressed against the surface of the photosensitive drums 11Y, 11M, 11C, and 11K. A charging bias, which is a DC voltage superimposed on an AC voltage, is applied to the charging rollers 12Y, 12M, 12C, and 12K from a primary charging bias power supply (not shown). In this way, the photosensitive drums 11Y, 11M, 11C, and 11K are uniformly charged. At this time, by using a charging bias in which a DC voltage is superimposed on an AC voltage, charging can be performed efficiently.

[0017] These charging rollers 12Y, 12M, 12C, and 12K have an elastic layer 3 mm thick on a conductive support (a round bar made of a metal material such as iron, copper, stainless steel, aluminum, or nickel). The elastic layer is appropriately doped with conductive agents having an electronic conduction mechanism, such as carbon black, graphite, and conductive metal oxides, and conductive agents having an ion conduction mechanism, such as alkali metal salts and quaternary ammonium salts. This allows the resistance value to be increased to 10 10The electrical conductivity is adjusted to less than Ω. Specific elastic materials for the elastic layer include, for example, natural rubber, ethylene propylene rubber (EPDM), styrene butadiene rubber (SBR), silicone rubber, urethane rubber, epichlorohydrin rubber, etc. In addition, synthetic rubbers such as isoprene rubber (IR), butadiene rubber (BR), nitrile butadiene rubber (NBR), and chloroprene rubber (CR), as well as polyamide resin, polyurethane resin, and silicone resin, etc. can be used.

[0018] Then, the photosensitive drums 11Y, 11M, 11C, and 11K are uniformly charged by the charging rollers 12Y, 12M, 12C, and 12K, and then electrostatic latent images corresponding to the respective colors are formed on the photosensitive drums 11Y, 11M, 11C, and 11K by the exposure devices 13Y, 13M, 13C, and 13K. The exposure devices 13Y, 13M, 13C, and 13K irradiate the photosensitive drums 11Y, 11M, 11C, and 11K with laser light in accordance with image information to form electrostatic latent images. In this embodiment, the laser light having a wavelength of 780 nm is irradiated.

[0019] The electrostatic latent images formed on the photosensitive drums 11Y, 11M, 11C, and 11K are developed by developing devices 14Y, 14M, 14C, and 14K that contain yellow, magenta, cyan, and black toner, respectively. The developing devices 14Y, 14M, 14C, and 14K develop the electrostatic latent images formed on the photosensitive drums 11Y, 11M, 11C, and 11K into toner images by applying toner of each color.

[0020] As the developer, a two-component developer in which a magnetic carrier is mixed with toner particles is used. The developing device 14 conveys the two-component developer by magnetic force and develops it in contact with the photosensitive drum. There is no particular restriction on the shape and particle size of the toner particles, as long as they are accurately placed on the electrostatic latent images on the photosensitive drums 11Y, 11M, 11C, and 11K. However, from the viewpoint of obtaining good image quality, it is preferable to use a spherical toner. From the viewpoint of durability when added to the toner, it is preferable that the particle size of the external additive added to the toner is 1 / 10 or less of the weight average particle size of the toner particles. The particle size of this external additive means the average particle size obtained by observing the surface of the toner particles with a microscope. The external additive in this embodiment is used in an amount of 0.01 to 15 parts by weight, preferably 0.05 to 12 parts by weight, per 100 parts by weight of the toner.

[0021] As the external additive, metal oxides such as aluminum oxide, titanium oxide, strontium titanate, cerium oxide, magnesium oxide, chromium oxide, tin oxide, zinc oxide, etc., nitrides such as silicon nitride, and carbides such as silicon carbide can be used. Metal salts such as calcium sulfate, barium sulfate, calcium carbonate, etc., fatty acid metal salts such as zinc stearate, calcium stearate, etc., carbon black, silica, etc. can also be used. These external additives may be used alone or in combination. Those that have been subjected to hydrophobic treatment are preferred.

[0022] The charge polarity of the toner composed of the above components (charge polarity of normal toner) can be either negative or positive, but in this embodiment, a toner with a negative charge polarity was used. Also, the two-component developer in this embodiment has an average charge amount (charge amount per unit weight; hereinafter Q / M) charged by friction with the carrier of -1.0×10 -2 C / kg-6.0×10 -2 C / kg was used.

[0023] An intermediate transfer belt 16 is disposed below the four image forming stations Y, M, C, and K. Primary transfer rollers 15Y, 15M, 15C, and 15K are disposed in contact with the inner side of the intermediate transfer belt 16 at positions corresponding to the photosensitive drums 11Y, 11M, 11C, and 11K as transfer members. The outer diameter of the primary transfer rollers 15Y, 15M, 15C, and 15K in this embodiment is 18 mm. The intermediate transfer belt 16 is pressed against the surfaces of the photosensitive drums 11Y, 11M, 11C, and 11K by the primary transfer rollers 15Y, 15M, 15C, and 15K. As a result, a primary transfer portion (primary transfer nip portion) N1 is formed between the photosensitive drums 11Y, 11M, 11C, and 11K and the intermediate transfer belt 16. Then, a positive polarity primary transfer bias is applied from a primary transfer bias power source (not shown) while being driven to rotate by the intermediate transfer belt 16. For this reason, the toner images on the photosensitive drums 11Y, 11M, 11C, and 11K are primarily transferred onto the intermediate transfer belt 16 via the primary transfer portion N1. The primary transfer rollers 15Y, 15M, 15C, and 15K are made of EPDM, urethane rubber, CR, NBR, or the like, with a resistance adjuster such as carbon added thereto, and the resistance value is set to 10 7 It is common to use materials adjusted to below Ω.

[0024] The intermediate transfer belt 16 used in this embodiment is a base body molded to a thickness of 100 μm made of polyimide resin, an elastic layer made of urethane rubber to a thickness of 300 μm on the upper surface thereof, and a surface layer made of fluororesin to a thickness of 5 μm. Although polyimide resin is used as the base body, polystyrene or the like can be used. As for the thickness, any material within the range of 50 μm to 500 μm can be used in the same manner. If the thickness is less than 50 μm, sufficient durability cannot be obtained due to wear, and if the thickness exceeds 500 μm, the intermediate transfer belt 16 becomes difficult to bend appropriately on the support shaft, and the running becomes unstable. In addition, although urethane rubber is used as the elastic layer, elastic materials such as natural rubber and isoprene rubber can be used. Furthermore, if necessary, the volume resistivity of the intermediate transfer belt 16 can be appropriately adjusted by using conductive fillers such as carbon, ZnO, SnO2, and TiO2. If the thickness is less than 20 μm, the elastic layer cannot contract sufficiently and cannot fully absorb the fluctuations in the contact pressure of the photosensitive drums 11Y, 11M, 11C, and 11K against the intermediate transfer belt 16. Also, if the thickness exceeds 1000 μm, the intermediate transfer belt 16 cannot bend properly on the support shaft regardless of the hardness, and the running becomes unstable.

[0025] There is no particular restriction on the surface layer, but it is required to improve the secondary transfer property by reducing the adhesion of the toner to the surface of the intermediate transfer belt 16. Although a fluororesin is used in this embodiment, polyurethane or the like can also be used.

[0026] In addition, a conductive agent for adjusting resistance is added to the base and the elastic layer. There is no particular limitation on the conductive agent for adjusting resistance, but it may be, for example, carbon black, graphite, etc. Note that the conductive agent for adjusting resistance is not limited to the above conductive agents.

[0027] The intermediate transfer belt 16 used in this embodiment has a volume resistivity of 1.0×10 10 [Ω cm], surface resistance is 6.0×10 11In the present embodiment, an intermediate transfer belt having a multi-layer structure with an elastic layer is used, but an intermediate transfer belt having a single layer structure made of a fluorine-containing resin such as a polyethylene-tetrafluoroethylene copolymer may also be used.

[0028] The intermediate transfer belt 16 is stretched over an intermediate transfer belt driving roller 21, an intermediate transfer belt tension roller 22, and a secondary transfer opposing roller 23. The intermediate transfer belt driving roller 21 is rotated in the direction of the arrow (clockwise) by a driving force transmitted from a driving motor (not shown). The intermediate transfer belt 16 is driven by a driving system separate from the photosensitive drums 11Y, 11M, 11C, and 11K, so that the speeds of the photosensitive drums 11Y, 11M, 11C, and 11K and the speed of the intermediate transfer belt 16 can be controlled independently. In this embodiment, the photosensitive drums 11Y, 11M, 11C, and 11K and the intermediate transfer belt 16 are driven so that the speed difference between the surfaces of the photosensitive drums 11Y, 11M, 11C, and 11K and the surface of the intermediate transfer belt 16 is within a range of 1 to 5%. The intermediate transfer belt driving roller 21 has a conductive rubber layer on a conductive support, and is driven by a conductive rubber layer. 2 ~10 6 The intermediate transfer belt tension roller 22 is formed by covering an insulating roller with a semiconductive thin film having a thickness of 10 μm to 200 μm, and the surface resistivity of the roller is 10 7 ~10 11 It is adjusted to Ω / □ (□: unit area).

[0029] At a portion of the intermediate transfer belt 16 facing the secondary transfer opposing roller 23, a secondary transfer roller 24 having an outer diameter of 24 mm is disposed facing the secondary transfer opposing roller 23 as a backup roller. The secondary transfer opposing roller 23 and the secondary transfer roller 24 form a second transfer means for transferring the toner image 20 primarily transferred onto the intermediate transfer belt 16 to the transfer material. A secondary transfer bias voltage of a polarity opposite to the charge polarity of the toner is applied to the secondary transfer roller 24 from a secondary transfer bias power source (not shown). In this way, the toner images 20 on the intermediate transfer belt 16 are secondarily transferred collectively onto the surface of the transfer material P via the secondary transfer portion (secondary transfer nip portion) N2 on the secondary transfer opposing roller 23.

[0030] A positive primary transfer bias is applied to the primary transfer rollers 15Y, 15M, 15C, and 15K. In this manner, the toner images of each color formed on each of the above-mentioned photosensitive drums 11Y, 11M, 11C, and 11K are sequentially primarily transferred to the intermediate transfer belt 16 at the respective primary transfer portions N1. As a result, the four color toner images are superimposed on the intermediate transfer belt 16, and a toner image 20 after primary transfer is formed on the intermediate transfer belt 16. After the toner image transfer, the primary transfer residual toner remaining on the surface of each of the photosensitive drums 11Y, 11M, 11C, and 11K is removed by the photosensitive drum cleaning blades 17Y, 17M, 17C, and 17K. The photosensitive drum cleaning blades 17Y, 17M, 17C, and 17K are supported by metal plates (not shown). The primary transfer residual toner removed from each of the photosensitive drums 11Y, 11M, 11C, and 11K by the cleaning blade is sent to a primary transfer residual toner collection container by a primary transfer residual toner conveying screw. In this embodiment, the cleaning blade is made of polyurethane rubber, which has high hardness and elasticity, and is excellent in abrasion resistance, mechanical strength, oil resistance, ozone resistance, and the like, and does not damage the photosensitive drum. Of course, the cleaning blade may be made of other materials as long as they have appropriate elasticity and hardness. Examples of the material include styrene-butadiene copolymer, chloroprene, butadiene rubber, ethylene-propylene-diene rubber, and chlorosulfonated polyethylene rubber. Other examples include elastomers such as fluororubber, silicone rubber, acrylic rubber, and nitrile rubber.

[0031] The four-color toner image 20 superimposed on the intermediate transfer belt 16 is transported to the secondary transfer section N2 as the intermediate transfer belt 16 rotates in the direction of the arrow (clockwise). Meanwhile, a transfer material P (e.g., paper, transparent film) stored in a paper feed cassette (not shown) is fed by a paper feed roller and transported to a registration roller by a transport roller. This transfer material P is supplied to the secondary transfer section N2 by the registration roller so as to timely align the four-color toner image 20 on the intermediate transfer belt 16. Then, the toner image is transferred to the transfer material P at the position of the secondary transfer section N2, and the toner is fused by applying heat and pressure in a fixing device, and the toner image is fixed on the transfer material P, which is then discharged outside the device main body.

[0032] Furthermore, residual toner that has not been secondarily transferred to the transfer material P at the secondary transfer portion N2 is sent to an intermediate transfer belt cleaning device for the intermediate transfer belt 16 and removed. The intermediate transfer belt cleaning device is made up of an intermediate transfer belt cleaning brush 25, a bias roller 26, and a bias roller cleaning blade 27, which are all arranged in the same housing. Similarly, the intermediate transfer belt cleaning device has an intermediate transfer belt cleaning brush 28, a bias roller 29, and a bias roller cleaning blade 30.

[0033] The intermediate transfer belt cleaning brush 28 is disposed downstream of the intermediate transfer belt cleaning brush 25 in the moving direction of the intermediate transfer belt 16 .

[0034] The secondary transfer residual toner removed from the intermediate transfer belt 16 by the intermediate transfer belt cleaning brushes 25, 28 is electrostatically transferred to the bias rollers 26, 29. The secondary transfer residual toner transferred to the bias rollers 26, 29 is then cleaned by the bias roller cleaning blades 27, 30. The collected toner cleaned by the bias roller cleaning blades 27, 30 is stored in a secondary transfer residual toner container (not shown).

[0035] In this embodiment, the intermediate transfer belt cleaning brush 25 as the first brush member is a woven roll brush wound in a spiral shape around a rotatable conductive support. Specifically, a conductive back coat is applied to the back side of the tape base fabric, which is made of a flat base fabric consisting of warp and weft threads and has fibers (pile yarns) forming the brush woven into a U-shape by pile weaving. The intermediate transfer belt cleaning brush 25 is brought into contact with the surface of the intermediate transfer belt 16 so as to rotate in the same direction (clockwise) as the surface of the intermediate transfer belt 16.

[0036] The brush fibers are preferably conductive and flexible in order to suppress mechanical stress on the surface of the intermediate transfer belt 16. There are no particular limitations on the flexible brush fiber material, and it can be appropriately selected according to the purpose. Examples of the flexible brush fiber material include polyolefin resins (e.g., polyethylene and polypropylene); polyvinyl resins or polyvinylidene resins (e.g., polystyrene, acrylic resin, polyacrylonitrile, and polyvinyl acetate). Examples of the flexible brush fiber material include polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride, polyvinyl carbazole, polyvinyl ether, and polyvinyl ketone; vinyl chloride-vinyl acetate copolymers; and styrene-acrylic acid copolymers. Examples of the flexible brush fiber material include styrene-butadiene resins; fluororesins (e.g., polytetrafluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, and polychlorotrifluoroethylene); polyesters; nylons; acrylics; rayon; and polyurethanes. Examples of the flexible brush fiber material include polycarbonates; phenolic resins; and amino resins (e.g., urea-formaldehyde resins, melamine resins, benzoguanamine resins, urea resins, and polyamide resins). A conductive agent for adjusting resistance is dispersed in the brush fibers to give them electrical conductivity. The conductive agent for adjusting resistance is not particularly limited, and examples thereof include carbon black, graphite, metal powder such as aluminum or nickel, tin oxide, titanium oxide, antimony oxide, indium oxide, and potassium titanate. Conductive metal oxides such as antimony oxide-tin oxide composite oxide (ATO) and indium oxide-tin oxide composite oxide (ITO) may also be conductive metal oxides coated with insulating fine particles such as barium sulfate, magnesium silicate, and calcium carbonate. The conductive agent for adjusting resistance is not limited to the above conductive agents.

[0037] The thickness of the brush fibers is preferably in the range of 5 μm to 50 μm. If the thickness of the brush fibers is smaller than 5 μm, the cleaning performance is maintained initially, but the secondary transfer residual toner is easily accumulated between the brush fibers. As a result, the cleaning performance is significantly reduced when the intermediate transfer belt cleaning brush 25 accumulates a certain amount of secondary transfer residual toner. In addition, if the thickness of the brush fibers is larger than 50 μm, the number of brush fibers that can exist per unit area is reduced. Therefore, the cleaning unevenness between the places where the brush fibers contact the intermediate transfer belt 16 and the places where they do not contact becomes noticeable, and the cleaning performance becomes unstable. Furthermore, since the rigidity of the brush fibers becomes high, the brush fibers damage the surface of the intermediate transfer belt 16 when the intermediate transfer belt cleaning brush 25 rotates, which is not preferable.

[0038] The length of the brush fibers is not particularly limited, but is preferably 1 to 15 mm. If the length of the brush fibers is 1 mm or less, the conductive support of the intermediate transfer belt cleaning brush 25 and the intermediate transfer belt 16 are arranged very close to each other, which is undesirable because the conductive support comes into contact with the intermediate transfer belt 16 and is easily scratched. If the length is 15 mm or more, the force with which the tips of the brush fibers contact the intermediate transfer belt 16 becomes weak, which reduces the cleaning ability of the intermediate transfer belt 16 and makes the brush fibers more likely to come off, which is undesirable.

[0039] The brush fiber density is preferably 10,000 to 1,000,000 fibers per square inch. A brush fiber density of 10,000 fibers per square inch or less is not preferred because uneven cleaning occurs between the areas where the brush fibers contact the intermediate transfer belt 16 and the areas where they do not contact the intermediate transfer belt 16. In addition, in order to increase the brush fiber density to 1,000,000 fibers per square inch or more, the diameter of the brush fibers must be made very small, which is not preferred.

[0040] In this embodiment, the conductive support of the intermediate transfer belt cleaning brush 25 has an outer diameter of 8 mm, and the intermediate transfer belt cleaning brush 25 uses nylon fibers with carbon dispersed therein, with a thickness of 20 μm and a pile length of 5 mm. The brush density is approximately 5.0×10 per square inch. 4 It is a book.

[0041] The bias roller 26 arranged in contact with the intermediate transfer belt cleaning brush 25 is an aluminum metal roller with an outer diameter of 15 mm and a hard alumite surface. The bias roller cleaning blade 27 arranged in contact with the bias roller 26 is the same as the photosensitive drum cleaning blades 17Y, 17M, 17C, and 17K described above.

[0042] The intermediate transfer belt cleaning brush 28 as the second brush member has the same configuration as the intermediate transfer belt cleaning brush 25. The bias roller 29 and the bias roller cleaning blade 30 have the same configurations as the bias roller 26 and the bias roller cleaning blade 27.

[0043] The intermediate transfer belt cleaning brushes 25 and 28 are disposed in sliding contact with the intermediate transfer belt 16 with an intrusion of about 1.0 mm. The intermediate transfer belt cleaning brushes 25 and 28 rotate in the same direction (clockwise) as the intermediate transfer belt 16 via a gear connected to the intermediate transfer belt drive roller 21, and the peripheral speed ratio is set to 30% in the reverse direction. The bias rollers 26 and 29 are disposed in a intrusion of about 1.0 mm with respect to the intermediate transfer belt cleaning brushes 25 and 28, and rotate in the reverse direction (counterclockwise) at a uniform speed via a gear connected to the intermediate transfer belt cleaning brushes 25 and 28. The bias roller cleaning blades 27 and 30 that come into contact with the bias rollers 26 and 29 are disposed so that their intrusion into the bias rollers 26 and 29 is 1.0 mm.

[0044] A bias power supply as a voltage application means (not shown) is connected to the bias rollers 26 and 29 of the intermediate transfer belt cleaning brushes 25 and 28, and a DC bias voltage (cleaning voltage) is applied to the bias power supply. This bias power supply is designed to output a negative bias voltage, which is the same polarity as the toner, to the intermediate transfer belt cleaning brush 25, and a positive bias voltage, which is the opposite polarity, to the intermediate transfer belt cleaning brush 28.

[0045] Also, 18Y, 18M, 18C, and 18K in FIG. 1 are toner supply containers that store toners to be supplied to the developing devices 14Y, 14M, 14C, and 14K of each color. 19 is a detection sensor for detecting the density of a toner image 20 transferred onto the intermediate transfer belt 16 in sequence from the photosensitive drums 11Y, 11M, 11C, and 11K. To control the density, a toner image 20 is formed on each of the photosensitive drums 11Y, 11M, 11C, and 11K or the intermediate transfer belt 16. Then, the toner image 20 is irradiated with light to detect the intensities of the specular reflection light component and the scattered light component, and the density is controlled based on the relationship between the amount of light and the density. In this embodiment, a method is adopted in which the density is controlled by detecting the toner image 20 of each color on the intermediate transfer belt 16 by the amount of light of the scattered light component in the case of yellow, magenta, and cyan, and the amount of light of the specular reflection light component in the case of black.

[0046] In this manner, in this embodiment, an electrostatic cleaning method is adopted as a cleaning device for the intermediate transfer belt 16, which performs electrostatic cleaning by applying a bias voltage to a roll-shaped brush. In such an image forming apparatus, the cleaning performance generally tends to be inferior to that of a blade cleaning method in which cleaning is performed by a blade such as the photosensitive drum cleaning blades 17Y, 17M, 17C, and 17K. The reason is that it is difficult to collect the toner with either polarity when the charge amount is near zero. In addition, the adhesion force with the intermediate transfer belt 16 is large, and the electric field due to the bias voltage applied to the intermediate transfer belt cleaning brushes 25 and 28 often cannot collect the toner completely. Furthermore, since the mechanical scraping force of the intermediate transfer belt cleaning brush is smaller than that of the cleaning blade, the electrostatic cleaning method generally has inferior cleaning performance in many cases. If the intermediate transfer belt cleaning device cannot sufficiently clean the residual toner, filler, paper powder, etc. attached to the intermediate transfer belt 16, some of them will be carried around while adhering to the surface of the intermediate transfer belt 16. This causes problems such as a decrease in gloss. On the other hand, in the primary transfer section and the secondary transfer section, an electric field is formed according to the bias voltage applied to the primary transfer rollers 15Y, 15M, 15C, and 15K and the secondary transfer roller 24. Therefore, due to these electrostatic forces, some of the transfer residual toner, filler, paper powder, etc. are transferred to the photosensitive drums 11Y, 11M, 11C, and 11K and the secondary transfer roller 24. Furthermore, compared with the pressure force of the intermediate transfer belt cleaning brushes 25 and 28 on the intermediate transfer belt 16, the pressure force of the primary transfer rollers 15Y, 15M, 15C, and 15K on the photosensitive drums 11Y, 11M, 11C, and 11K is larger. Therefore, when the adhesion force on the photosensitive drums 11Y, 11M, 11C, and 11K becomes larger than the adhesion force on the intermediate transfer belt 16, the adhesion on the intermediate transfer belt 16 may be transferred to the photosensitive drums 11Y, 11M, 11C, and 11K. Similarly, the pressure applied by the secondary transfer roller 24 to the secondary transfer opposing roller 23 is significantly larger than the pressure applied by the intermediate transfer belt cleaning brushes 25 and 28 .Therefore, when the adhesion force to the secondary transfer roller 24 becomes larger than the adhesion force to the intermediate transfer belt 16, the deposit on the intermediate transfer belt 16 may be transferred to the secondary transfer roller 24. When the secondary transfer residual toner, the filler derived from the transfer material P, and the paper powder adhere to the photosensitive drums 11Y, 11M, 11C, and 11K, there is no big problem with the secondary transfer residual toner. That is, it is easy to clean by the photosensitive drum cleaning blades 17Y, 17M, 17C, and 17K in the same way as the normal primary transfer residual toner. However, when the filler or the paper powder is transferred to the photosensitive drums 11Y, 11M, 11C, and 11K, various problems occur. As an example, when a filler such as talc adheres to the photosensitive drum at the primary transfer section or the cleaning blade, the surface becomes highly resistive, and the charging ability is insufficient under normal image forming conditions. For this reason, a density step occurs between the part on the image where talc is attached and the part on the image where it is not attached. As for paper dust, when fibrous paper dust is transferred to the photosensitive drum via the primary transfer section, it may not be cleaned and may get caught by the cleaning blade. In this case, a gap is generated between the photosensitive drum and the cleaning blade, causing the problem that the primary transfer residual toner may slip through the gap. On the other hand, when the dust is transferred to the secondary transfer roller 24, the electrical resistance of the secondary transfer roller 24 may increase. In that case, the high voltage capacity of the secondary transfer bias power supply that applies the secondary transfer bias voltage (not shown) may become insufficient, causing image defects such as secondary transfer defects.

[0047] When the image forming conditions in each image forming station are the same, the transfer rate of the filler and paper powder to the photosensitive drum is considered to be the same. Therefore, the more the amount of the filler that reaches the primary transfer portion N1 in each image forming station, the more the transfer amount itself becomes. Therefore, the amount of the filler and paper powder transferred to the photosensitive drum is the largest in the photosensitive drum 11Y. Figure 2 shows the metal element strength contained in the recovered toner recovered by each cleaning device when 10,000 sheets of transfer material using talc and calcium bicarbonate as the main filler were passed through each. In the experiment of Figure 2, the voltage applied to the primary transfer roller was set so that the primary transfer current flowed at 45 μA during both image formation and non-image formation (pre-rotation, post-rotation, and between sheets).

[0048] The recovered toner shown in FIG. 2 is the toner cleaned by the intermediate transfer belt cleaning device, the photosensitive drum unit Y, the photosensitive drum unit M, and the photosensitive drum unit K. The CLN recovered toner in FIG. 2 is the toner recovered by the intermediate transfer belt cleaning device. Similarly, the 1st recovered toner, the 2nd recovered toner, and the 4th recovered toner are the toners recovered by the photosensitive drum unit Y, the photosensitive drum unit M, and the photosensitive drum unit K, respectively. The method for measuring the strength of the metal elements contained in the recovered toner is as follows. That is, about 1 g of each of the recovered toners was placed in a dedicated aluminum ring for pressing with a diameter of 10 mm, flattened, and a measurement sample was prepared using a tablet molding compression machine "BRE-32" (manufactured by Maekawa Test Machinery Manufacturing Co., Ltd.). The preparation conditions were pressurization at 20 MPa for 60 seconds, and the pellets molded to a thickness of about 2 mm were measured with fluorescent X-rays to confirm the element strength. The fluorescent X-rays were measured using a wavelength dispersive fluorescent X-ray analyzer Axios Advanced manufactured by PANalytical. Talc contains magnesium element as its composition, so it is detected as Mg element intensity, and calcium bicarbonate contains calcium element as its composition, so it is detected as Ca element intensity. From the results shown in Figure 2, it can be confirmed that talc and calcium bicarbonate are indeed detected in the recovered toner other than that from the intermediate transfer belt cleaning device. It can also be confirmed that the detected amounts of these substances increase the further upstream the photosensitive drum unit is located in the movement direction of the intermediate transfer belt.

[0049] Therefore, in order to control the amount of the filler or paper dust transferred to each of the photosensitive drums 11Y, 11M, 11C, and 11K, it is considered that the transfer rate of the filler or paper dust may be controlled. One of the means for controlling the transfer rate is electrostatic force at the primary transfer portion. That is, the amount of the filler or paper dust transferred to the photosensitive drums 11Y, 11M, 11C, and 11K can be adjusted by controlling the electric field from the intermediate transfer body to each of the photosensitive drums. In this embodiment, a positive secondary transfer bias voltage is applied at the secondary transfer portion N2, and bias voltages of substantially equal negative and positive polarities are applied from the intermediate transfer belt cleaning brushes 25 and 28, respectively. Therefore, it is considered that the filler or paper dust derived from the transfer material P is positively charged. Therefore, during non-image formation, the primary transfer bias voltage is controlled so that the repulsive force between the intermediate transfer belt 16 and the filler or paper dust at the primary transfer portion N1 becomes smaller as the image forming unit is more upstream. In this way, the amount of transfer to the photosensitive drums 11Y, 11M, 11C, and 11K can be controlled. As in the case described in FIG. 2, 10,000 sheets of transfer material using calcium bicarbonate as the main filler were passed through each of the transfers. The primary transfer current was set to 60 μA and 45 μA. The Ca element intensity of the toner collected by the photosensitive drum unit was measured by fluorescent X-rays at each setting, and the results are shown in FIG. 3. It can be seen that the amount of transfer to the photosensitive drum is reduced when the primary transfer current is 45 μA because the repulsive force with the positive polarity filler and paper powder is smaller. As described above, the amount of the filler and paper powder that reaches the photosensitive drum unit located upstream is greater. For this reason, in this embodiment, in order to make these uniform, the transfer rate of the filler and paper powder is reduced in the image forming station located upstream. Specifically, the image forming station located upstream is controlled to perform an operation to reduce the primary transfer current. However, if the primary transfer current is controlled to be smaller than the appropriate primary transfer current during the image forming operation, the transfer efficiency of the toner image is reduced and image defects are generated. For this reason, the operation of reducing the primary transfer current for the image forming station located further upstream is performed during non-image formation, which includes the preparatory sequence (hereinafter referred to as the pre-rotation sequence) that starts after the image formation signal is input.It also includes the period from the primary transfer of the toner image formed in the previous image forming operation to the formation of the toner image formed in the next image forming operation (hereinafter, the sheet interval sequence) when image formation is continued continuously. It also includes the period during the fall sequence after all image formation is completed (hereinafter, the post-rotation sequence). At this time, the setting value of the primary transfer current in each image forming station controlled during the image forming operation is shown in FIG. 4, and the setting value of the primary transfer current controlled during the non-image forming operation is shown in FIG. 5. In this embodiment, the same photosensitive drum units Y, M, C, and K and the primary transfer rollers 15Y, 15M, 15C, and 15K are used, and various bias voltages applied during image formation are also equivalent. Note that the primary transfer current is set as shown in FIG. 4 so that the primary transfer efficiency of the toner image is close to the extreme value during the image forming operation, but it is not limited to these values, including the settings during the non-image forming operation, and can be changed appropriately according to the design of the image forming system.

[0050] In addition, the resistance of the primary transfer roller generally increases due to the deterioration of the electrical conductivity of the primary transfer roller and the adhesion of scattered toner by repeatedly performing image forming operations. As a result, the primary transfer current becomes smaller than a predetermined value, and the primary transfer efficiency of the toner image during image forming operations decreases, so the primary transfer current is controlled by constant current control. In this embodiment, the primary transfer current is controlled by a constant current control current even during non-image forming operations, following the image forming operation, but is not limited to this, and may be controlled by constant voltage control as necessary.

[0051] FIG. 6 is a block diagram of the image forming apparatus used in this embodiment. When image forming information is input to the image forming apparatus, the CPU 31 as a control unit starts a pre-rotation sequence. That is, the photosensitive drum driving motors Y, M, C, and K and the intermediate transfer belt driving motor are driven to drive the photosensitive drums 11Y, 11M, 11C, and 11K and the intermediate transfer belt 16. After that, the CPU 31 controls the charging bias voltage power supplies Y, M, C, and K to apply charging bias voltages to the charging rollers 12Y, 12M, 12C, and 12K. The CPU 31 also controls the developing bias voltage power supplies Y, M, C, and K to apply developing bias voltages to the developing devices 14Y, 14M, 14C, and 14K. The CPU 31 also controls the primary transfer bias voltage power supplies Y, M, C, and K to apply transfer bias voltages to the primary transfer rollers 15Y, 15M, 15C, and 15K. At this time, the CPU 31 reads the primary transfer current setting shown in FIG. 5 stored in the non-volatile memory 32, and performs constant current control of the primary transfer bias voltage applied to the primary transfer rollers 15Y, 15M, 15C, and 15K with the primary transfer current shown in FIG. 5. The CPU 31 performs constant current control with the primary transfer current shown in FIG. 4 in accordance with the timing when the image formation preparation is completed and the image forming operation is started. Then, the toner images formed on the photosensitive drums 11Y, 11M, 11C, and 11K according to the image formation information inputted respectively are primarily transferred onto the intermediate transfer belt 16 at the primary transfer portion N1. Thereafter, the toner image primarily transferred onto the intermediate transfer belt 16 is secondarily transferred onto the transfer material P at the secondary transfer portion N2. The toner remaining on the intermediate transfer belt without being completely secondary transferred is conveyed to the intermediate transfer belt cleaning device and cleaned. When the image forming operation based on the inputted image formation signal is completed, the post-rotation sequence is started. The CPU 31 controls the primary transfer bias voltages applied to the primary transfer rollers 15Y, 15M, 15C, and 15K with constant current so that they are controlled by the primary transfer currents shown in FIG. 5 stored in the non-volatile memory 32. When the post-rotation sequence is completed, the application of the primary transfer bias voltage, the developing bias voltage, and the charging bias voltage is stopped in sequence, and the drive of each of the photosensitive drum drive motors Y, M, C, and K and the intermediate transfer belt drive motor is stopped.When the image forming operation continues, such as during continuous image formation, the CPU 31 controls the primary transfer current shown in FIG. 5 during the paper interval. Then, when the next image forming operation starts, the operation of controlling the primary transfer current to the primary transfer current shown in FIG. 4 is repeated according to the length of the paper interval sequence. As described above, in this embodiment, the CPU 31 can execute the following operations. That is, during non-image formation, the CPU 31 controls the primary transfer bias voltage power supplies Y, M, C, and K as a power supply unit to supply a first current to the primary transfer roller 15Y as a first transfer member. Then, when the first current is being supplied to the primary transfer roller 15Y, the area of ​​the intermediate transfer belt 16 that has passed through the primary transfer portion N1 facing the photosensitive drum 11Y supplies a second current when passing through the primary transfer portion N1 facing the photosensitive drum 11M. Then, the CPU 31 controls the primary transfer bias voltage power supplies Y, M, C, and K so that the first current is smaller than the second current. This makes it possible to prevent paper dust and the like adhering to the intermediate transfer belt from being unevenly transferred to the upstream image forming unit during non-image formation.

[0052] Next, the image forming operation in this embodiment will be described with reference to the flow chart shown in FIG.

[0053] First, when an image formation signal is input to the image forming apparatus (S1), the CPU 31 starts driving the photosensitive drums 11Y, 11M, 11C, and 11K and the intermediate transfer belt 16 (S2), and controls the current to the primary transfer current shown in FIG. 5 during the pre-rotation sequence (S3). When the image forming operation starts (S4), the CPU 31 controls the current to the primary transfer current shown in FIG. 4 again (S5). When the image forming operation ends (S6), the CPU 31 controls the current to the primary transfer current shown in FIG. 5 (S7). On the other hand, if the image forming operation is to be continued ("Yes" in S8), the CPU 31 controls the current to the primary transfer current shown in FIG. 4 again (S5) when the image forming operation is resumed (S4). Thereafter, the operations from S4 to S8 are repeated. On the other hand, if the image forming operation is not to be continued ("No" in S8), the CPU 31 sequentially drops the various bias voltages when the post-rotation sequence ends. Thereafter, the CPU 31 stops driving the photosensitive drums 11Y, 11M, 11C, and 11K and the intermediate transfer belt 16 (S9).

[0054] Next, the effect of this embodiment was confirmed. That is, 200,000 sheets of A4 size images were output with the primary transfer current settings shown in Fig. 4 and Fig. 5 to confirm the effect. The recording materials were Kishu fine paper N manufactured by Hokuetsu Corporation, which contains talc as a filler, and multi-paper Super Economy + manufactured by Askul, which contains calcium bicarbonate, and the images were confirmed by passing the recording materials through the printer. As a result of outputting the images according to the flow chart shown in Fig. 7, both papers were passed through up to 200,000 sheets, and no image defects were generated, particularly due to the photosensitive drum unit, and stable output of good images could be maintained. EXAMPLES

[0055] In the first embodiment, the set value of the primary transfer current of each image forming station is changed as shown in FIG. 5 during non-image forming operations such as the pre-rotation sequence, the sheet interval sequence, and the post-rotation sequence. Then, the amount of the filler and the paper dust transferred to each of the photosensitive drums 11Y, 11M, 11C, and 11K is uniformized. However, the filler and the paper dust are not completely transferred to each of the photosensitive drums 11Y, 11M, 11C, and 11K. Therefore, in the long term, there are cases where image defects occur, and cases where the paper dust is caught between each of the cleaning blades 17Y, 17M, 17C, and 17K and the toner passes through. In order to deal with such a situation, in this embodiment, a predetermined toner image (a first predetermined toner image, a second predetermined toner image) is formed on each of the photosensitive drums 11Y, 11M, 11C, and 11K during non-image forming operations. Then, the control is performed so that the filler and paper dust are transferred onto the photosensitive drums 11Y, 11M, 11C, and 11K with a predetermined toner image interposed at the primary transfer portion N1. In this way, the filler and paper dust adhering to the intermediate transfer belt 16 can be supplied to the cleaning blade together with the toner image, and the filler can be prevented from coming into direct contact with the photosensitive drum and adhering thereto. In addition, cleaning the filler and paper dust together with the toner image can prevent them from being caught by the cleaning blade, further reducing the risk of image defects and toner slip-through.

[0056] First, as in the first embodiment, the primary transfer current shown in FIG. 4 is used for control during image forming operation, and the primary transfer current shown in FIG. 5 is used for control during non-image forming operation. In this embodiment, each image forming station further forms a toner image for buffering the adhesion of filler and paper dust. When the buffer toner image formed during non-image forming operation is primarily transferred, the primary transfer current shown in FIG. 5 is used. Therefore, unlike normal image forming operation, almost no toner is primarily transferred to the intermediate transfer belt 16, and the toner image remains supported on the photosensitive drums 11Y, 11M, 11C, and 11K. In this state, the filler and paper dust that have passed through the intermediate transfer belt cleaning device are transferred onto the photosensitive drums 11Y, 11M, 11C, and 11K, so that the formed toner image is interposed between the photosensitive drum and the filler or paper dust, and does not directly contact the drum. Therefore, adhesion of the filler to the photosensitive drum is suppressed, and image defects such as density step are less likely to occur compared to the first embodiment. Furthermore, because the paper dust and filler are carried on the toner image and reach the cleaning blade in a state where their adhesive force is reduced, they are easily cleaned together with the toner image. As a result, paper dust is prevented from being caught by the cleaning blade, and image defects such as toner slip-through can be more effectively prevented than in the first embodiment.

[0057] At this time, it is desirable that the length of the toner image formed on the photosensitive drum in the main scanning direction is formed so as to cover all areas that may be used for image formation. However, since the filler and paper dust are mainly transferred only to the area where the paper is passing, in this embodiment, from the viewpoint of suppressing toner consumption, the length is controlled to be automatically changed in synchronization with the size of the transfer material P being used. In this case, the setting of the paper feed tray (not shown) and the like may be detected, and the length of the toner image in the main scanning direction may be controlled based on the detection result. In addition, since paper dust is generally present in large amounts at the cut edge portion, the buffer toner image may be controlled to be formed only in the area including the edge portion. In other words, the position where the buffer toner image is formed may be controlled based on the paper size used in the previous image formation.

[0058] Furthermore, with regard to the length in the sub-scanning direction, when it is executed during the pre-rotation sequence, the toner image may be formed at any timing between the start of application of the charging bias voltage to the photosensitive drum for each image forming station and the start of the image forming operation. Therefore, it can be freely set according to the image forming system being used. During the sheet interval sequence, it may be formed at any timing between the end of the previous image forming operation and the start of the next image forming operation. Furthermore, the frequency can also be freely set. During the post-rotation sequence, the toner image may be formed at any timing between the end of the last image forming operation and the end of the post-rotation sequence. Therefore, it can be freely set according to the image forming system being used.

[0059] The density of the toner image formed during non-image forming operation should ideally be formed with a 16-bit image signal value FF since a sufficient amount must remain on the photosensitive drum after primary transfer with the primary transfer current setting in Fig. 5, but there is a possibility that part of the toner image may be primarily transferred onto the intermediate transfer belt 16, causing the secondary transfer roller 24 to become dirty with toner. The closer the current is to the primary transfer current during image forming operation, the less likely the buffer toner image is to remain on the photosensitive drum, so the image may be formed with a density according to the primary transfer current setting of each image forming station.

[0060] In this embodiment, the length of the toner image in the main scanning direction is set to 10 mm, and the position where the toner image is formed is controlled to be formed only around the edges on the front and back sides determined based on the size of the transfer material P described above in order to reduce the amount of toner consumed. The length in the sub-scanning direction is set to 50 mm during the post-rotation sequence and post-rotation sequence, and 10 mm during the paper interval sequence, and the density is set to be formed with a 16-bit image signal value of 80. Note that, in this embodiment, the conditions of the toner image formed during non-image forming operation are the same for all image forming stations, but they can be changed independently and appropriately depending on the type and size of the transfer material P, the image forming mode such as full color or monochrome image, and the like.

[0061] In addition, in this embodiment, the timing for forming a toner image during non-image forming operation is during the pre-rotation sequence, the paper interval sequence, and the post-rotation sequence, but these timings can be freely selected and combined for execution.

[0062] Next, the block diagram of this embodiment will be omitted since it is similar to that of the first embodiment, and the image forming operation will be described with reference to the flow chart shown in FIG.

[0063] First, when an image formation signal is input to the image forming apparatus (S1), the CPU 31 starts driving the photosensitive drums 11Y, 11M, 11C, and 11K and the intermediate transfer belt 16 (S2). Then, the CPU 31 controls the primary transfer current shown in FIG. 5 during the pre-rotation sequence to form a toner image for preventing the adhesion of filler and paper dust (S3). When the image forming operation starts (S4), the CPU 31 controls the primary transfer current again to that shown in FIG. 4 (S5). When the image forming operation ends (S6), the CPU 31 controls the primary transfer current to that shown in FIG. 5 (S7). On the other hand, if the image forming operation is to be continued ("Yes" in S8), the CPU 31 controls the primary transfer current again to that shown in FIG. 4 when the image forming operation is resumed (S4) to form a toner image (S5), and thereafter repeats the operations from S4 to S8. On the other hand, if the image forming operation is not to be continued ("No" in S8), the CPU 31 sequentially turns off the various bias voltages when the post-rotation sequence ends. Then, the CPU 31 stops driving the photosensitive drums 11Y, 11M, 11C, and 11K and the intermediate transfer belt 16 (S9).

[0064] In this embodiment, up to 300,000 sheets were passed through in the same manner as in Example 1 under the above-mentioned conditions, and no image defects caused by the photosensitive drum unit occurred, and stable, good image output was maintained. EXAMPLES

[0065] In the first and second embodiments, examples have been described in which rotatable cleaning brushes 25, 28 are used as the electrostatic cleaning means. In this embodiment, a pre-charging brush 251 is disposed as a pre-charging member (pre-charging means) instead of the cleaning brush 25 which is the upstream brush. Meanwhile, the downstream cleaning brush 28 has the same configuration. In this embodiment, other than the configuration of the pre-charging brush 251, the configuration is the same as in the first embodiment, and therefore a description thereof will be omitted.

[0066] In this embodiment, the pre-charging brush 251 is fixed and unrotatable upstream of the cleaning brush 28 in the movement direction of the intermediate transfer belt 16. The pre-charging brush 251 is arranged in contact with the intermediate transfer belt 16, and pre-charges the transfer residual toner to the normal charging polarity by applying a voltage of the same polarity (negative) as that of the toner.

[0067] In this embodiment, the transfer residual toner pre-charged by the pre-charging brush 251 is configured to be collected by the downstream cleaning brush 28. This makes it possible to reduce costs, since the bias roller 26 and bias roller cleaning blade 27 are not necessary, compared to the configuration using the rotatable cleaning brush 25 as in the first embodiment.

[0068] On the other hand, when a non-rotating, fixed pre-charging brush 251 is used in this manner, the paper dust collected by the pre-charging brush 251 continues to accumulate. For this reason, the paper dust removal capability is lower than that of a rotating cleaning brush. Therefore, the paper dust that should be collected by the pre-charging brush 251 (paper dust of positive polarity in this embodiment) is likely to be carried around on the intermediate transfer belt. That is, the paper dust (positive in this embodiment) that is charged with the polarity opposite to that applied to the pre-charging brush 251 (negative in this embodiment) is likely to be carried around on the intermediate transfer belt. And, as described above, the paper dust that is likely to be carried around on the intermediate transfer belt is likely to accumulate in the most upstream image forming station.

[0069] Therefore, in this embodiment, the primary transfer current of each image forming station during non-image formation is controlled so that the paper dust that is prone to the above-mentioned drag (paper dust that is charged with the polarity opposite to that applied to the pre-charging brush 251) is dispersed as much as possible at each image forming station.

[0070] Specifically, the more upstream the image forming station, the smaller the absolute value of the primary transfer current is made (in this embodiment, the polarity of the entrained paper dust is positive, so there is a repulsive relationship with the primary transfer bias (positive polarity in this embodiment)).

[0071] Since the following is the same as in the first and second embodiments, explanation of the block diagrams and flow charts will be omitted.

[0072] As described above, according to this embodiment, in an image forming apparatus using a fixed brush (pre-charging brush), it is possible to prevent paper dust from being unevenly transferred to the most upstream image forming station.

[0073] [others] In this embodiment, the normal charging polarity of the toner is negative, but the present invention is not limited to this. The same configuration can be applied to the case where the normal charging polarity of the toner is positive. In this case, the polarities of the primary transfer bias and the secondary transfer bias are simply in the opposite relationship to that of this embodiment, but the primary transfer current is made smaller the more upstream the image forming station is.

[0074] In addition, in this embodiment, a configuration has been described in which the primary transfer current is made smaller the more upstream the image forming station is, but a configuration in which the primary transfer current of the most upstream image forming station is set to zero and the primary transfer current is made smaller the more upstream the image forming station is, among the image forming stations other than the most upstream station. [Explanation of symbols]

[0075] 11Y, 11M, 11C, 11K Photosensitive drum 12Y, 12M, 12C, 12K Charging roller 13Y, 13M, 13C, 13K Exposure equipment 14Y, 14M, 14C, 14K developing device 15Y, 15M, 15C, 15K Primary transfer roller 16 Intermediate transfer belt 17Y, 17M, 17C, 17K Photosensitive drum cleaning blade 18Y, 18M, 18C, 18K Toner Refill Container 19 Intermediate transfer belt toner concentration detection sensor 20 Toner image after primary transfer 21 Drive roller 22 Tension roller 23 Secondary transfer opposing roller 24 Secondary transfer roller 25 Intermediate transfer belt upstream cleaning brush 26 Upstream bias roller 27 Upstream Bias Roller Cleaning Blade 28 Intermediate transfer belt downstream cleaning brush 29 Downstream bias roller 30 Downstream bias roller cleaning blade 31 CPU 32 Non-volatile memory 251 Pre-charged Brush P Transfer material

Claims

1. a first image carrier that carries a toner image; a second image carrier that carries a toner image; an intermediate transfer body onto which the toner images carried by the first image carrier and the second image carrier are primarily transferred; a cleaning means for electrostatically cleaning the toner remaining on the intermediate transfer body after the toner image, which has been primarily transferred to the intermediate transfer body, is secondarily transferred to a recording material; a first transfer member that primarily transfers the toner carried on the first image carrier to the intermediate transfer member in a first transfer portion; a second transfer member that primarily transfers the toner carried on the second image carrier to the intermediate transfer member in a second transfer portion; a power supply unit that applies a voltage to each of the first transfer member and the second transfer member; a control unit for controlling the power supply unit, the first image carrier is disposed upstream of the second image carrier in a moving direction of the intermediate transfer body, The control unit is capable of executing a mode in which, during non-image formation, the control unit controls the power supply unit to supply a first current to the first transfer member, and when the first current is supplied, a second current is supplied to the second transfer member when an area of ​​the intermediate transfer body that has passed the first transfer section passes the second transfer section, and the image forming apparatus is characterized in that the first current is smaller than the second current.

2. 2. The image forming apparatus according to claim 1, wherein the polarity of the voltage applied by the power supply unit when supplying the first current and the polarity of the voltage applied by the power supply unit when supplying the second current are the same polarity and are opposite to the charging polarity of regular toner.

3. 2. The image forming apparatus according to claim 1, further comprising a plurality of image carriers including the first image carrier and the second image carrier, and a plurality of transfer members including the first transfer member and the second transfer member that transfer the toner images carried on the plurality of image carriers to the intermediate transfer body, wherein the control unit controls the power supply unit so that a current flows through each of the plurality of transfer members when the mode is executed, and the current supplied to the plurality of transfer members in the mode is smaller for the transfer members that are disposed upstream in the direction of movement of the intermediate transfer body.

4. 2. The image forming apparatus according to claim 1, wherein the control unit is configured to supply a first predetermined toner image to the first transfer unit while the first current is supplied to the first transfer member, and controls the control unit to supply a second predetermined toner image to the second transfer unit while the second current is supplied to the second transfer member.

5. 2. The image forming apparatus according to claim 1, wherein the electrostatic cleaning means comprises a first brush member to which a first cleaning voltage is applied and which electrostatically cleans the intermediate transfer body, and a second brush which is disposed downstream of the first brush member in the movement direction of the intermediate transfer body, and which is applied with a second cleaning voltage having an opposite polarity to the first cleaning voltage and which electrostatically cleans the intermediate transfer body.

6. The image forming apparatus according to claim 1, characterized in that, when forming images continuously on a plurality of recording materials, the control unit controls the power supply unit so that the mode is executed while an area of ​​the intermediate transfer body corresponding to a space between a first recording material and a second recording material following the first recording material passes through the first transfer unit and the second transfer unit, respectively.

7. a first image carrier that carries a toner image; a second image carrier that carries a toner image; an intermediate transfer body onto which the toner images carried by the first image carrier and the second image carrier are primarily transferred; a pre-charging member for pre-charging a toner remaining on the intermediate transfer body after the toner image, which has been primarily transferred to the intermediate transfer body, is secondarily transferred to a recording material; a cleaning unit that is disposed downstream of the pre-charging member in the moving direction of the intermediate transfer body and that electrostatically cleans the toner charged by the pre-charging member; a first transfer member that primarily transfers the toner carried on the first image carrier to the intermediate transfer member in a first transfer portion; a second transfer member that primarily transfers the toner carried on the second image carrier to the intermediate transfer member in a second transfer portion; a power supply unit that applies a voltage to each of the first transfer member and the second transfer member; a control unit for controlling the power supply unit, the first image carrier is disposed upstream of the second image carrier in a moving direction of the intermediate transfer body, The control unit is capable of executing a mode in which, during non-image formation, the control unit controls the power supply unit to supply a first current to the first transfer member, and when the first current is supplied, a second current is supplied to the second transfer member when an area of ​​the intermediate transfer body that has passed the first transfer section passes the second transfer section, and the image forming apparatus is characterized in that the first current is smaller than the second current.

8. 8. The image forming apparatus according to claim 7, wherein the pre-charging member is a pre-charging brush fixed so as to come into contact with the intermediate transfer body.

9. 8. The image forming apparatus according to claim 7, wherein the first current is zero.

Images