Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent disturbance of the ends of bristles of a discharge device and maintain a stable discharge effect over a long period.SOLUTION: An image forming apparatus according to the present invention has an intermediate transfer body 6, primary transfer means 5, secondary transfer means 9, intermediate transfer body cleaning means 12, and a discharge device 27. In a high humidity environment, the image forming apparatus changes current or voltage to be supplied to the cleaning means according to an image ratio, and in a low humidity environment, it maintains the current or voltage to be supplied to the cleaning means constant irrespective of the image ratio.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that forms a toner image on a recording material. [Background technology]

[0002] Intermediate transfer belts with an elastic layer are used in the commercial printing market. The elastic layer allows for excellent transfer of embossed paper, which has an uneven surface.

[0003] However, an intermediate transfer belt with many electrically conductive portions has a problem of increased resistance, which is particularly noticeable when an ion conductive system is used to adjust the resistance of the elastic layer.

[0004] When an ionically conductive belt is used, an electric field is generated within the belt layer when a current flows. The electric field generated within the belt layer exerts a force on the ionically conductive agent, which provides ion conductivity, while the negatively charged ionically conductive agent is subjected to a force in the opposite direction of the electric field, causing it to move. For example, when a positive current is supplied to the primary transfer device, the ionically conductive agent migrates toward the inner side of the intermediate transfer belt. In contrast, the secondary transfer roller supplies a positive current from the outside of the intermediate transfer belt to transfer toner to paper. This generates an electric field in the opposite direction to the primary transfer, causing the ionically conductive agent in the intermediate transfer belt to migrate in the opposite direction to that during the primary transfer. The high-voltage application device that supplies current to the intermediate transfer belt significantly disrupts the balance between the outward and inward charge amounts, causing an imbalance in the ions within the intermediate transfer belt and increasing the resistance of the intermediate transfer belt. It has been found that as the resistance of the intermediate transfer belt increases over time and the required transfer voltage increases, image defects due to discharge at the primary and secondary transfer sections become more likely to occur.

[0005] To prevent the resistance of the intermediate transfer belt from increasing and extend its service life, a discharge device is required to suppress the resistance increase. For example, one discharge device has a configuration in which a voltage application unit, such as an electrostatic fur brush, is brought into contact with the intermediate transfer body (see Patent Document 1). According to this configuration, the discharge device supplies a current to the intermediate transfer belt that causes the total current balance of the other voltage application units of the intermediate transfer body to be approximately zero during image formation (see Patent Document 1). In a tandem full-color image forming apparatus, a positive bias voltage, opposite in polarity to the toner, is applied to the primary transfer unit at four locations, resulting in a large positive current flowing from the inner surface to the outer surface of the intermediate transfer belt. For this reason, it is desirable for the discharge device to apply a positive bias voltage from the outer surface to the inner surface. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-12861 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when using an electrostatic fur brush as a discharge device to clean the intermediate transfer belt, the brush bristles can easily collapse. This occurs because the brush is pressed against the intermediate transfer belt, causing the bristles to collapse over time. Comparing a toner-collecting cleaning brush with a discharge device located downstream, the bristles of the discharge brush collapse faster. The toner-collecting brush traps toner between the bristles, preventing bristles from collapsing. A non-toner-collecting brush, such as that described in Patent Document 1, initially has a larger diameter and moves faster than the belt, but as the diameter decreases, its circumferential speed slows. This causes bristles that have collapsed to stand up, resulting in random bristles. This random bristles cause unstable power supply. This instability reduces current flow, forcing the power supply to increase its voltage to the maximum capacity. After this, the required current cannot be supplied and replacement is required.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming apparatus that can suppress, with a simple configuration, the falling of the bristles at the ends of the fur brush of the discharge device. [Means for solving the problem]

[0009] The image forming apparatus of the present invention includes an image carrier that carries a toner image, an intermediate transfer belt to which the toner image is primarily transferred from the image carrier at a primary transfer section and which secondarily transfers the toner image to a recording material at a secondary transfer section, a first cleaning brush provided in contact with the intermediate transfer belt at a first cleaning section and which supplies a first cleaning current to the intermediate transfer belt to recover toner charged to a normal polarity on the intermediate transfer belt, a second cleaning brush provided in contact with the intermediate transfer belt at a second cleaning section and which supplies a second cleaning current to the intermediate transfer belt to recover toner charged to a polarity opposite to the normal polarity on the intermediate transfer belt, and a second cleaning brush provided in contact with the intermediate transfer belt at a second cleaning section and which supplies a second cleaning current to the intermediate transfer belt to recover toner charged to a polarity opposite to the normal polarity on the intermediate transfer belt. In an image forming apparatus comprising: a discharge brush that is in contact with the intermediate transfer belt at a discharge section that is located upstream of the primary transfer section and downstream of the primary transfer section, and that supplies a discharge current to the intermediate transfer belt; the first cleaning brush, the second cleaning brush; a power supply section that supplies a current to the discharge brush; and a control section that controls the power supply section, the control section is configured to change the current or voltage supplied to the first cleaning brush according to an image ratio when the environmental humidity is a first predetermined humidity, and to control the power supply section so that the current or voltage supplied to the first cleaning brush is constant regardless of the image ratio when the environmental humidity is a second predetermined humidity that is lower than the first predetermined humidity. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an image forming apparatus that can suppress, with a simple configuration, the falling of the bristles of the fur brush of the discharge device. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Figure 2] FIG. 2 is a diagram illustrating a cleaning device. [Figure 3]FIG. [Figure 4] This is an example of a value used when calculating the discharge current. [Figure 5] 10A and 10B are diagrams illustrating a case where the peripheral speed of the brush is higher than the peripheral speed of the metal roller. [Figure 6] 10A and 10B are diagrams illustrating a state in which the peripheral speed of the brush is lower than the peripheral speed of the metal roller. [Figure 7] 10 is a table showing current values ​​according to image ratios. [Figure 8] FIG. 10 is a diagram showing the relationship between the amount of remaining toner after the downstream brush and the cleaning current. [Figure 9] FIG. 2 is a control block diagram according to the present embodiment. [Figure 10] 1 is a flowchart of the present embodiment. [Figure 11] 10 is a table showing the results of changes in brushes in a paper feed test of this embodiment. [Figure 12] FIG. 2 is a diagram illustrating an image in the first embodiment. [Figure 13] 10 is a table showing current values ​​according to image ratios in the second embodiment. [Figure 14] FIG. 10 is a diagram illustrating an image in the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0013] <Image forming device> Figure 1 shows a schematic configuration of one embodiment of an image forming apparatus according to the present invention. As shown in Figure 1, reference numerals 1Y, 1M, 1C, and 1K denote photosensitive drums (latent image carriers) that rotate in the direction of arrow A, and their surfaces are uniformly charged by charging devices 2Y, 2M, 2C, and 2K. Reference numerals 3Y, 3M, 3C, and 3K denote exposure devices that expose based on image information. Electrostatic latent images corresponding to the image information are formed on the photosensitive drums 1Y, 1M, 1C, and 1K by a well-known electrophotographic process.

[0014] Developing devices 4Y, 4M, 4C, and 4K contain the chromatic toners yellow (Y), magenta (M), cyan (C), and black (K), respectively. The aforementioned electrostatic latent images are developed by these developing devices 4Y, 4M, 4C, and 4K, and toner images are formed on the surfaces of the photosensitive drums 1Y, 1M, 1C, and 1k. A reversal development method is used, in which toner is deposited on the exposed areas of the electrostatic latent image for development.

[0015] Further, reference numeral 6 denotes an intermediate transfer belt arranged to abut against the surface of the photosensitive drum 1, and is stretched around a plurality of tension rollers 20, 21, 22, 23, 25, and 26, rotating at 150 to 470 mm / sec in the direction of arrow G. In this embodiment, tension roller 20 is a tension roller that controls the tension of intermediate transfer belt 6 to a constant level, tension roller 22 is a drive roller for intermediate transfer belt 6, and tension roller 21 is an opposing roller for secondary transfer. Reference numeral 9 denotes a secondary transfer roller that sandwiches and transports the transfer material with the intermediate transfer belt 6.

[0016] Reference numerals 5y, 5m, 5c, and 5k denote primary transfer rollers, which are disposed opposite the photosensitive drums 1Y, 1M, 1C, and 1K across the intermediate transfer belt 6. A constant voltage-controlled transfer bias of opposite polarity to the toner image is applied to the primary transfer rollers in synchronization with the transport of each color toner image to the primary transfer nip (primary transfer portion). In this way, the primary transfer rollers primarily transfer the toner images on the photosensitive drums 1Y, 1M, 1C, and 1K onto the intermediate transfer belt 6. The primary transfer rollers 5 are made of an elastic layer of ion-conductive foam rubber and a core metal, and have an outer diameter of 15 to 20 mm and a resistance of 1E+5 to 1E+8 Ω when measured at N / N (23°C, 50% RH) and 2 kV.

[0017] The secondary transfer roller 9 is made of an elastic layer of ion-conductive foam rubber and a core metal, and has an outer diameter of 20 to 25 mm, a resistance value measured as N / N (23°C, 50% RH), and a resistance of 1E+5 to 1E+8 Ω when 2 kV is applied.

[0018] Furthermore, the tension roller 21 is made of an elastic layer of electronically conductive rubber and a core metal, and has an outer diameter of 20 to 22 mm, a resistance value measured as N / N (23°C, 50% RH), and a transfer roller of 1E+5 to 1E+8 Ω when 50 V is applied.

[0019] Reference numeral 8 denotes a registration roller, which supplies a transfer material 7 to the secondary transfer nip (secondary transfer portion) in synchronization with the toner image on the intermediate transfer belt 6 being conveyed to the secondary transfer portion.

[0020] At this time, a constant voltage controlled transfer bias of opposite polarity to the toner image is applied to the secondary transfer roller 9. For example, a transfer voltage of +1 to +7 kV is applied and a current of +40 to +120 μA is passed, and the toner image on the intermediate transfer belt 6 is transferred to the transfer material 7.

[0021] The transfer material 7 is then transported by a pre-fixing transport device 41 into the fixing device 30, where it undergoes a heat and pressure fixing process for the toner image. The pre-fixing transport device 41 has a rotating belt in the center, 100 to 110 mm wide and 1 to 3 mm thick, made of a rubber material such as EPDM, on which the transfer material 7 is placed and transported. The belt has holes with a diameter of 3 to 7 mm, and suction is applied from the inside to increase the holding force of the transfer material 7 and stabilize the transport performance.

[0022] Reference numeral 12 denotes a belt cleaning device that electrostatically collects and cleans the secondary transfer residual toner on the intermediate transfer belt 6. The cleaned intermediate transfer belt 6 is reused in the image formation and imaging process.

[0023] <Intermediate transfer body> In this embodiment, the intermediate transfer belt 6 is composed of a base layer (backside layer), an elastic layer (intermediate layer), and a surface layer. The base layer is made of resin such as polyimide or polycarbonate, or various rubbers, etc., containing an appropriate amount of carbon black as an antistatic agent, and has a thickness of 0.05 to 0.15 mm. The elastic layer is made of various rubbers, such as CR rubber, urethane rubber, or silicone rubber, containing an appropriate amount of ion conductive agent, and has a thickness of 0.1 to 0.500 mm. The surface layer is made of resin such as urethane resin or fluororesin, and has a thickness of 0.0002 to 0.020 mm.

[0024] The intermediate transfer belt 6 has a volume resistivity of 5E+8 to 1E+14 Ω / cm (23°C, 50% RH), an MD1 hardness of 60 to 85° (23°C, 50% RH), and a static friction coefficient of 0.15 to 0.6 (23°C, 50% RH, HEIDON type 94i).

[0025] <Electrostatic cleaning device> As shown in Figure 2, electrostatic brush cleaning device 12, which is a belt cleaning device, has device housing 121 arranged near intermediate transfer belt 6. Inside this housing, electrostatic fur brushes 122 and 123 are provided as cleaning brushes. Also provided are aluminum metal rollers 124 and 125 and cleaning blades 126 and 127. The fur brushes are made of carbon-dispersed nylon fibers, acrylic fibers, or polyester fibers with a thread resistance of 3E+5 to 1E+13 (Ω / cm) and a fiber thickness of 2 to 15 denier, planted on the metal roller at a planting density of 50,000 to 500,000 fibers per inch^2.

[0026] The electrostatic fur brushes 122 and 123 are provided so as to contact the intermediate transfer belt 6 at the first and second cleaning sections, respectively. That is, the electrostatic fur brushes 122 and 123 are disposed in sliding contact with the intermediate transfer belt 6, maintaining a penetration depth of approximately 1.0 to 2.0 mm. The electrostatic fur brushes 122 and 123 are configured to rotate in the direction of the arrows by a drive motor (not shown) at a speed that is 20 to 80% of the conveyance speed of the intermediate transfer belt 6. The metal rollers 124 and 125 are disposed so as to maintain a penetration depth of 1.5 to 2.5 mm relative to the electrostatic fur brushes 122 and 123, and are disposed so as to rotate in the direction of the arrows at the same speed as the electrostatic fur brushes 122 and 123. The cleaning blades 126 and 127 that contact the metal rollers 124 and 125 are made of plate-shaped rubber such as urethane. The cleaning blades 126 and 127 have a thickness of 1.6 to 2.2 mm and an IRHD hardness of 70 to 78° (23° C., 50% RH), and are positioned so as to maintain a penetration depth of 0.5 to 2.0 mm into the metal roller.

[0027] A negative DC voltage of -60 to -73 uA, which is constant current controlled, is applied to the metal roller 124 of the cleaning member, which is located upstream in the rotation direction of the intermediate transfer belt 6, by a DC power supply serving as a power source. (In this embodiment, the DC voltage is -60 to -73 uA, but this is not limited to this.) In other words, when a DC voltage is applied from the DC power supply, the potential of the electrostatic fur brush 122 becomes negative. In other words, the electrostatic fur brush 122 is capable of collecting toner charged to the polarity opposite to the normal polarity (+).

[0028] Meanwhile, a DC voltage of +20 to +73 μA, controlled as a constant current, is applied to the metal roller 125, a cleaning member located downstream of the intermediate transfer belt 6 in the direction of rotation, from a DC power supply. (In this embodiment, the DC voltage is +20 to +73 μA, but this is not limited to this.) That is, when a DC voltage is applied from the DC power supply, the potential of the electrostatic fur brush 123 becomes positive. Therefore, the electrostatic fur brush 123 can collect toner charged with the same polarity (negative) as the normal polarity. A cleaning electric field suitable for the cleaning toner is formed between the fur brush and the fur brush, and residual toner on the intermediate transfer belt 6 is attracted to and removed by the fur brushes 122 and 123. The attracted and removed toner is further transferred from the fur brushes 122 and 123 to the metal rollers 124 and 125 by the electric field, and then scraped off by cleaning blades 126 and 127. Note that, although the tension roller 22 serves as the opposing roller in this embodiment, separate opposing rollers may be provided for the electrostatic fur brushes 122 and 123. In this embodiment, the metal roller of the cleaning member is not energized, but if the opposing rollers are independent upstream and downstream, cleaning can be performed in the same way by applying a voltage to the opposing roller and grounding the metal roller of the cleaning member. In this way, the toner remaining on the intermediate transfer belt after the secondary transfer is electrostatically cleaned by the electrostatic fur brushes 122 and 123.

[0029] <Discharge device> As shown in FIG. 1, the discharge device 27 is disposed downstream of the secondary transfer roller 9 in the rotation direction of the intermediate transfer body 6.

[0030] As shown in Figure 3, the discharge device 27 comprises a device housing 275 disposed near the intermediate transfer belt 6, and an electrostatic fur brush 271, an aluminum metal roller 272, and a cleaning blade 273 provided inside the device housing. The electrostatic fur brush 271 serving as a discharge brush has a thread resistance of 3E+5 to 1E+13 (Ω / cm) and a fiber thickness of 2 to 15 denier. The electrostatic fur brush 271 is made of carbon-dispersed nylon fiber, acrylic fiber, or polyester fiber, and is planted on the metal roller at a density of 50,000 to 500,000 fibers per inch^2.

[0031] The electrostatic fur brush 271 is disposed so as to contact the intermediate transfer belt 6 at the discharge portion. That is, the electrostatic fur brush 271 is disposed in sliding contact with the intermediate transfer belt 6, maintaining a penetration depth of approximately 1.0 to 2.0 mm. The electrostatic fur brush 271 is configured to rotate in the direction of the arrow by a drive motor (not shown) at a speed of 20 to 80% of the conveyance speed of the intermediate transfer belt 6. The metal roller 272 is disposed so as to maintain a penetration depth of 1.5 to 2.5 mm relative to the electrostatic fur brush 271 and rotate in the direction of the arrow at the same speed as the electrostatic fur brush 271. The cleaning blade 273 that contacts the metal roller 272 is made of a plate-shaped rubber such as urethane, has a thickness of 1.6 to 2.2 mm, and an IRHD hardness of 70 to 78° (23°C, 50% RH). The cleaning blade 273 is disposed so as to maintain a penetration depth of 0.5 to 2.0 mm relative to the metal roller.

[0032] Next, a formula for calculating the current to be supplied to the discharge member will be described.

[0033] Current is supplied to the intermediate transfer belt 6 by the primary transfer roller 5, the secondary transfer roller 9, and the electrostatic cleaning device 12. In this embodiment, as shown in Equation 1, the current Idis supplied to the discharge member 271 during image formation is set as follows: That is, the current density is calculated by subtracting the current densities obtained by dividing the currents supplied to the primary transfer roller 5, the secondary transfer roller 9, and the first and second electrostatic cleaning devices 12 by their respective longitudinal widths. In this embodiment, the direction of the current flowing from the inner surface to the outer surface of the intermediate transfer belt is defined as positive. That is, the subtracted current density is ((It1y + It1m + It1c + It1k) / Rt1-It2 / Rt2+Icl1 / Rcl1-Icl2 / Rcl2). Then, this subtracted current density ((It1y+It1m+It1c+It1k) / Rt1-It2 / Rt2+Icl1 / Rcl1-Icl2 / Rcl2) is multiplied by the longitudinal width Rdis of the discharge member 271. Then, this value is added with the discharge correction current Idis_offset. In this way, the current Idis supplied to the discharge member 271 can be obtained. Idis={(It1y+It1m+It1c+It1k) / Rt1 -It2 / Rt2 +Icl1 / Rcl1 -Icl2 / Rcl2}×Rdis +Idis_offset equation (1) Rt1: Primary transfer roller length Ity,m,c,k: Primary transfer current y,m,c,k Rt2: Secondary transfer roller length It2: Secondary transfer current Rcl1: longitudinal width of first electrostatic cleaning device Icl1: first electrostatic cleaning current Rcl2: longitudinal width of second electrostatic cleaning device Icl2: second electrostatic cleaning current Rdis: Discharge member length Idis: Discharge current Idis_offset: Discharge compensation current Figure 4 shows the initial values ​​at power-on in this embodiment, but these values ​​are not limited to these as long as the difference is calculated. The discharge correction current was set to 11 μA, but this was set at 5% of approximately 220 μA because the power supply output discharge current can fluctuate within a range of ±5%. Note that fluctuation means that even if 220 μA is set based on the power supply's accuracy, the output may be 209 μA or 231 μA.

[0034] Next, we will explain why the bristles of the brush in the discharge device become distorted. Figure 5 shows a brush in its initial state, with the bristles not falling down much. The bristles of the brush are located at the position indicated by the solid line in Figure 5, and the outer diameter of the brush remains the same. Because the brush 271 strikes the intermediate transfer belt 26 in the counter direction, the bristles begin to flap to the left. They also strike the metal roller 272 in the with direction. Initially, the bristles of the brush are located at the position indicated by the solid line in Figure 5, and the peripheral speed Vbr of the brush is greater than the peripheral speed Vr of the metal roller. At this time, the bristles begin to flap to the left.

[0035] Figure 6 shows the brush after passing about 500,000 sheets and the bristles are tilted. The brush bristles are positioned as shown by the solid lines in Figure 6, and the brush outer diameter is small. Because the brush 271 hits the intermediate transfer belt 26 in the counter-direction, the bristles are bent to the left. They also hit the metal roller 272 in the width direction. The brush peripheral speed Vbr is smaller than the metal roller peripheral speed Vr. At this time, the brush bristles were bent to the left, but now the tips are raised, and the bristles are disordered after passing through the nip with the metal roller 272. This again hits the intermediate transfer belt in the counter-direction, further deforming the bristles. [Example]

[0036] To prevent deformation of the bristles, it is effective to also supply toner to the brush of the discharge device. In this embodiment, the cleaning current of the cleaning member 124 located upstream and the cleaning member 125 located downstream in the rotation direction of the intermediate transfer belt 6 is reduced so that the toner is not completely collected and is also sent to the brush of the discharge device. Experiments have confirmed that the bristles are less likely to collapse if the amount of toner going to the brush of the discharge device is set to 5 to 40% of the total. Essentially, it is sufficient to reduce the current of the cleaning member 125 located downstream, which has the same polarity as the discharge device (+).

[0037] The current value should be adjusted depending on the image ratio, as shown in Figure 7. The image ratio is the proportion of YMCK toner on the paper. For example, an all-blue image would have 100% M and 100% C, resulting in an image ratio of 200%. The smaller the image ratio, the smaller the current value. This is to prevent the small amount of toner from being completely removed before it reaches the discharge brush. When the image ratio is high, even if a certain amount is removed by the downstream brush, there is still some left to go to the discharge brush, so the current value is increased. Figure 8 shows the relationship between the amount of remaining toner after the downstream brush and the cleaning current. It can be seen that, to ensure the same required amount of toner, the current must be reduced when the image ratio is low. It can also be seen that the current can be increased when the image ratio is high. While it would be possible to change only the downstream current, the discharge current derived from equation (1) would increase if only the downstream current were reduced. To minimize this, the upstream current is also adjusted according to the image ratio.

[0038] <Image formation operation> 9 is a schematic block diagram showing the control mode of the main parts of the image forming apparatus of this embodiment. The control unit 50 as a control means is configured to have a CPU 51 as an arithmetic control means which is a central element for performing arithmetic processing, and memories (storage media) such as RAM 52 and ROM 53 as storage means. The RAM 52, which is a rewritable memory, stores information input to the control unit 50, detected information, arithmetic results, etc., while the ROM 53 stores control programs, pre-determined data tables, etc. The CPU 51 and memories such as RAM 52 and ROM 53 can transfer and read data to and from each other.

[0039] The control unit 50 is connected to the operation unit and image reading unit of the image forming apparatus, and external devices such as a personal computer. The control unit 50 comprehensively controls each unit of the image forming apparatus to perform image formation operations based on instructions from the operation unit of the image forming apparatus, image data from the image reading unit, or image formation signals (image data, control commands) from external devices. In this embodiment, the control unit 50 is connected to a primary transfer power supply E1, a secondary transfer power supply E2, first and second cleaning power supplies 25A and 25B, and a discharge power supply E35. The control unit 50 is also connected to a sheet number counter 70 as a counting unit (counting means) that counts the number of recording materials P on which images have been formed and output from the image forming apparatus. The image forming apparatus executes an image formation operation (print job), which is initiated by a start command and is a series of operations for forming and outputting images on one or more recording materials P.

[0040] Next, FIG. 10 shows a flowchart of this embodiment.

[0041] As shown in FIG. 10, the control unit 50 starts image formation (S1) and stores the detection results of the primary transfer current during image formation and the secondary transfer current during paper feed in the RAM 52 (S2). Next, it determines whether it is time to change the discharge current (S3). If it is time to change the discharge current (Yes in S3), it acquires the image ratio of the image to be formed. If it is determined in S3 that it is not time to change the discharge current (No in S3), it proceeds to S7. Then, it determines the current to be supplied to the electrostatic fur brushes 122 and 123 based on the acquired image ratio and the upstream current value and downstream brush current value stored in the ROM 52 (S4). After this, it calculates the target current to be supplied to the discharge member 271 using the above-mentioned calculation method (S5), and changes the supplied current along with the upstream current value and downstream brush current value (S6). Next, it determines whether image formation is to be completed. If image formation is not completed (No in S7), it returns to image formation. If image formation is completed (Yes in S7), it ends.

[0042] Figure 11 shows the results of the brush changes when a paper feed test was actually conducted. The image is an A4 size image with a combination of yellow, magenta, cyan, and black, as shown in Figure 12. (a): 40% images of yellow, magenta, cyan, and black are arranged side by side. (b): 70% images of yellow, magenta, cyan, and black are arranged side by side. (c): 120% red and 120% blue images are lined up. 120% red is a secondary color of 60% yellow and 60% magenta, and 120% blue is a secondary color of 60% magenta and 60% cyan. (d): 200% blue image. 200% blue is a secondary color of 100% magenta and 100% cyan. In an environment of 23°C and 50%, images were switched every 100,000 sheets in the order of (a), (b), (c), and (d). Comparison was made between this embodiment and a conventional example, where the upstream brush was at -73 μA and the downstream brush was at +73 μA, and all toner was collected. The shape of the discharge brush was visually inspected every 400,000 sheets. In Figure 11, an X indicates that the brush tip was distorted, and an O indicates that it was not distorted. As shown in Figure 11, the bristles of both brushes remained undisturbed up to 400,000 sheets. In the conventional example, the bristles collapsed, resulting in a narrower outer diameter. However, in this embodiment, the bristles collapsed less, and the outer diameter was the same for the upstream and downstream brushes. This suggests that the toner supply traps the toner between the bristles, preventing them from collapsing. At 800,000 sheets, the bristles of the conventional example were distorted. A high voltage was output from the power supply to ensure the required current. In this example, the bristles of both the upstream and downstream brushes all flapped in the same direction, showing no disturbance. The output from the power supply was also kept low. In the conventional example, the necessary current could not be secured at 1000 sheets, and paper feed was stopped. In this example, the bristle tips remained intact even after 2000 sheets, and the output from the power supply was kept low, achieving high durability.

[0043] In this embodiment, a constant current power supply is used to output the brush current, but a constant voltage power supply may also be used. When using a constant voltage power supply, the current and voltage values ​​are measured periodically and the voltage value is fixed so that the target current is achieved. [Example]

[0044] Example 1 shows results in a normal humidity environment of 23°C and 50% humidity. Our experiments have shown that the tendency for brush bristles to collapse depends on the environmental humidity. In a high-humidity environment (first predetermined humidity), the brush bristles collapse just as easily as in a normal humidity environment. However, in a low-humidity environment (second predetermined humidity), the bristles are less likely to become distorted, even without toner. This is thought to be because the bristles retain their elasticity in low humidity, preventing them from collapsing even when pressed against the intermediate transfer belt or metal roller. Ideally, the discharge brush is used purely for applying current and should not contain toner. At an environmental humidity of less than 10%, the bristles collapse at the same rate as the upstream and downstream brushes, even without toner, so the downstream brush can remove all the toner. Therefore, at the humidity level when switching from low humidity to normal humidity, which is set to 73 μA regardless of the image ratio, the bristles collapse more easily than in low humidity. Therefore, we set the current to 60 μA regardless of the image ratio so that a small amount of toner reaches the discharge brush. Figure 13 shows examples of settings for each environmental humidity. 13 and information from an environmental sensor (not shown), the CPU 51 controls the cleaning current supplied to the electrostatic fur brushes 122, 123 based on the image ratio in the case of a high-humidity environment (first predetermined humidity). On the other hand, in the case of a low-humidity environment (second predetermined humidity), the CPU 51 controls the cleaning current supplied to the electrostatic fur brushes 122, 123 to be constant current, regardless of the image ratio. Note that, although constant current control has been described as an example in this embodiment, constant voltage control may also be used.

[0045] A paper feed test was also carried out in this example in the same way as in Example 1. The test was carried out in environments with an environmental humidity of 30% and 5%, and as shown in Figure 11, the tip of the brush did not become disordered even after 2000,000 sheets were printed, just like in Example 1, and the output from the power supply was kept low, realizing high durability.

[0046] The environmental humidity may be classified by relative humidity as in this embodiment, or by absolute humidity. [Example]

[0047] In the paper feed tests in Examples 1 and 2, the images passed through the paper were uniform in image ratio across the entire surface. However, if only a portion of the paper had a high image ratio, the overall image ratio would be deemed low. For example, as shown in Figure 14, if only 1 / 4 of the paper was 200% blue and the rest was blank, the overall image ratio would be 50%. In this case, if the current values ​​of the upstream and downstream brushes were lowered as shown in the figure, the amount of toner collected by the upstream and downstream brushes would decrease, and the toner would go exclusively to the discharge brush. Having too much toner in only one area is undesirable, as this can cause longitudinal unevenness in the discharge current on the intermediate transfer belt. For this reason, in this example, the image was divided and the image ratio was examined for each divided area. If any of the divided areas had an image ratio exceeding 150%, the current setting for the area exceeding 150% in the figure was used. In Figure 14, if the image is divided vertically and horizontally into two, resulting in a total of four areas, each area would be 200%, so the downstream brush current for this image would be set to +60 μA and the upstream brush current to -73 μA. By setting the setting for a high image ratio in this way, the amount of toner collected by the upstream and downstream brushes increases, preventing toner from going only to the discharge brush.

[0048] The number of image divisions should preferably be 2 to 6 for A4 size, and 3 to 12 for A3 size. If the number of divisions is too small, the image ratio of the divided area will be too small. On the other hand, if the number of divisions is too large, it will be erroneously detected as having a high image ratio even if the amount of toner can be sufficiently collected by the upstream brush and downstream brush. [Example]

[0049] In the first, second, and third embodiments, the waste toner is collected by the discharge brush, but it is also possible to periodically output a horizontal band image, turn off the current to the upstream and downstream brushes, and supply toner to the discharge brush. In this case, by using the image ratio for each area in the third embodiment, a band image is created in the position where the image ratio was low, and the image is sent, thereby preventing a situation where there is insufficient toner in some areas.

Claims

1. an image carrier that carries a toner image; an intermediate transfer belt that transfers a toner image from the image carrier to a primary transfer unit and transfers the toner image to a recording material to a secondary transfer unit; a first cleaning brush provided in contact with the intermediate transfer belt at a first cleaning section, the first cleaning brush supplying a first cleaning current to the intermediate transfer belt to collect toner charged to the normal polarity on the intermediate transfer belt; a second cleaning brush provided in contact with the intermediate transfer belt at a second cleaning section, and configured to supply a second cleaning current to the intermediate transfer belt and collect toner charged with a polarity opposite to the normal polarity on the intermediate transfer belt; a discharge brush that is provided in contact with the intermediate transfer belt at a discharge unit that is downstream of the first cleaning unit and the second cleaning unit and upstream of the primary transfer unit in the rotation direction of the intermediate transfer belt, and that supplies a discharge current to the intermediate transfer belt; a power supply unit that supplies current to the first cleaning brush, the second cleaning brush, and the discharge brush; a control unit that controls the power supply unit, The control unit is configured to change the current or voltage supplied to the first cleaning brush depending on the image ratio when the environmental humidity is a first predetermined humidity, and to control the power supply unit so that the current or voltage supplied to the first cleaning brush is constant regardless of the image ratio when the environmental humidity is a second predetermined humidity that is lower than the first predetermined humidity.

2. 2. The image forming apparatus according to claim 1, wherein the power supply unit is configured so that when the discharge current is supplied to the discharge brush, the polarity of the potential of the discharge brush becomes opposite to the normal polarity of the toner.

3. The image forming apparatus according to claim 2, characterized in that the control unit is configured to set the value of the first cleaning current so that, when the environmental humidity is the first predetermined humidity, a portion of the toner remaining on the intermediate transfer belt after secondary transfer passes through the first cleaning unit, and the toner that has passed through the first cleaning unit is supplied to the discharge brush.

Images