Charge conditioning device and image forming apparatus

Abstract

To provide a configuration that can stably adjust electrical charges on a sheet regardless of the width of the sheet and a change in the resistance of an electrical charge adjustment roller 910 of an electrical charge adjustment unit.SOLUTION: An electrical charge adjustment device 9 adjusts electrical charges on a sheet to which a toner image is fixed by a fixing device 7. A control unit applies predetermined voltage to the electrical charge adjustment device 9 from an electrical charge adjustment high voltage power supply 90, with the sheet not conveyed to an electrical charge adjustment nip part N formed by an electrical charge adjustment roller 910 and a counter roller 92. At that time, the control unit executes a mode for determining the relationship between current and voltage from the predetermined voltage and a current value detected by a current detection circuit 93. The control unit determines the voltage applied to the electrical charge adjustment device 9 from the electrical charge adjustment high voltage power supply 90 when the sheet passes through the electrical charge adjustment nip part N based on the relationship determined in the mode and the length of the sheet in the sheet width direction acquired by an operation unit.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copier, printer, facsimile, or a multifunction machine having multiple functions of these. [Background technology]

[0002] In an image forming apparatus, a toner image formed in an image forming unit is transferred to a sheet in a transfer unit, and the toner image is fixed to the sheet in a fixing unit. After that, the sheet is stacked on an output tray or the like. At this time, the sheets may stick together due to electrostatic force between them. For this reason, a configuration has been proposed that includes a charge adjustment unit that applies a voltage to the sheet on which the toner image has been fixed by the fixing unit to adjust the charge on the sheet (Patent Document 1).

[0003] In Patent Document 1, the charge adjustment unit includes a pair of conductive rubber rollers arranged opposite each other and a power source that applies a voltage to the conductive rubber rollers, and is configured to apply a voltage to a sheet passing through the nip portion of the pair of conductive rubber rollers. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-122156 Summary of the Invention [Problem to be solved by the invention]

[0005] However, with the configuration described in Patent Document 1, it is difficult to control the charge applied to the sheet when the width of the sheet or the electrical resistance of the conductive rubber roller (the rotating body of the charge adjustment unit) changes. This is because when a constant current is supplied from a power source to the roller, the current flowing into the sheet and the current flowing out of the sheet fluctuates depending on the width of the sheet and the electrical resistance of the roller. If the current flowing through the sheets is not appropriate, it is not possible to prevent sheets from sticking to each other.

[0006] For example, if the resistance of the sheet is higher than expected due to factors such as the moisture content of the surrounding environment, current may flow only to areas of the nip where there is no sheet, rather than areas where there is a sheet, resulting in an insufficient charge to prevent sticking. Also, if the resistance of the conductive rubber roller is higher than expected due to factors such as deterioration of the electrical conductivity of the material, the current in areas where there is no sheet may decrease, causing excessive charge to be supplied to areas where there is a sheet, which may result in sticking.

[0007] SUMMARY OF THE INVENTION An object of the present invention is to provide a configuration that can stably adjust the charge of a sheet regardless of the width of the sheet or changes in the resistance of the rotating body of the charge adjustment unit. [Means for solving the problem]

[0008] The image forming apparatus of the present invention comprises a transfer unit that transfers a toner image onto a sheet, a fixing unit that heats and pressurizes the sheet onto which the toner image has been transferred in the transfer unit to fix the toner image to the sheet, a charge adjustment unit that has a pair of rotating bodies that sandwich and transport the sheet onto which the toner image has been fixed in the fixing unit and adjusts the charge on the sheet by applying a voltage, a power supply that applies a voltage to the charge adjustment unit, a current detection unit that detects the current flowing through the charge adjustment unit, an information acquisition unit that can acquire information on the length of the sheet in the sheet width direction perpendicular to the sheet transport direction, and a control unit that controls the power supply, wherein the power supply is controlled at a constant voltage, and the control unit applies a predetermined voltage from the power supply to the charge adjustment unit when the sheet is not transported to a nip portion formed by the pair of rotating bodies, and at that time executes a mode that determines the relationship between current and voltage from the current value detected by the current detection unit and the predetermined voltage, and determines the voltage to be applied from the power supply to the charge adjustment unit when the sheet passes through the nip portion based on the relationship determined in the mode and the length of the sheet in the sheet width direction acquired by the information acquisition unit.

[0009] The image forming apparatus of the present invention comprises a transfer unit that transfers a toner image onto a sheet, a fixing unit that heats and pressurizes the sheet onto which the toner image has been transferred in the transfer unit to fix the toner image to the sheet, a charge adjustment unit that has a pair of rotating bodies that sandwich and transport the sheet onto which the toner image has been fixed in the fixing unit and adjusts the charge on the sheet by applying a voltage, a power supply that supplies current to the charge adjustment unit, a voltage detection unit that detects the voltage applied to the charge adjustment unit, an information acquisition unit that can acquire information on the length of the sheet in the sheet width direction perpendicular to the sheet transport direction, and a control unit that controls the power supply, wherein the power supply is controlled with a constant current, and the control unit supplies a predetermined current from the power supply to the charge adjustment unit when the sheet is not transported into a nip portion formed by the pair of rotating bodies, and at that time executes a mode that determines the relationship between current and voltage from the voltage value detected by the voltage detection unit and the predetermined current, and determines the voltage to be applied to the charge adjustment unit from the power supply when the sheet passes through the nip portion based on the relationship determined in the mode and the length of the sheet in the sheet width direction acquired by the information acquisition unit. [Effects of the Invention]

[0010] According to the present invention, the charge of the sheet can be stably adjusted regardless of the width of the sheet or the change in the resistance of the rotor of the charge adjusting section. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to a first embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing the schematic configuration of an image forming unit according to the first embodiment. [Figure 3] 1 is a cross-sectional view showing a schematic configuration of a charge adjustment device according to a first embodiment. [Figure 4] FIG. 2 is a block diagram relating to voltage control of the charge adjustment device according to the first embodiment. [Figure 5] 4 is a flowchart of voltage control of the charge adjustment device according to the first embodiment. [Figure 6]4 is a graph showing a current load in voltage control of the charge adjustment device according to the first embodiment. [Figure 7] 4 is a graph showing a table of voltages applied to the charge adjustment device according to the first embodiment. [Figure 8] FIG. 2 is a schematic diagram illustrating a current flowing in the charge adjustment device according to the first embodiment. [Figure 9] 10 is a flowchart of voltage control of a charge adjustment device according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] First Embodiment The first embodiment will be described with reference to Figures 1 to 8. First, the schematic configuration of an image forming apparatus according to this embodiment will be described with reference to Figures 1 and 2.

[0013] [Image forming device] 1, the image forming apparatus 100 of this embodiment is a laser beam printer that forms a full-color image on a sheet P (paper, an OHP sheet, cloth, etc.) as a recording material using an electrophotographic method. The image forming apparatus 100 is an intermediate transfer tandem type in which image forming units Pa, Pb, Pc, and Pd, which are means for forming yellow, magenta, cyan, and black toner images, are arranged along an intermediate transfer belt 51.

[0014] The image forming stations Pa, Pb, Pc, and Pd each include photosensitive drums 1a, 1b, 1c, and 1d, which serve as image carriers and photosensitive bodies that carry electrostatic latent images. At the image forming station Pa, a yellow toner image is formed on the photosensitive drum 1a and is then primarily transferred to an intermediate transfer belt 51, which serves as an intermediate transfer body. At the image forming station Pb, a magenta toner image is formed on the photosensitive drum 1b and is then primarily transferred and superimposed on the yellow toner image on the intermediate transfer belt 51. At the image forming stations Pc and Pd, a cyan toner image and a black toner image are formed on the photosensitive drums 1c and 1d, respectively, and are similarly primarily transferred in sequence and superimposed on the toner image on the intermediate transfer belt 51. In this embodiment, the photosensitive drums and the intermediate transfer belt serve as image carriers that carry toner images.

[0015] The four-color toner images that have been primarily transferred onto the intermediate transfer belt 51 are secondarily transferred all at once onto the sheet P that has been fed to the secondary transfer section N2 formed by the intermediate transfer belt 51 and the secondary transfer roller 56. The sheet P onto which the toner images have been secondarily transferred at the secondary transfer section N2 is heated and pressurized by the fixing device 7 as a fixing section, and the toner image is fixed to the surface thereof. Then, the sheet P is discharged to the outside and stacked on the discharge tray 86.

[0016] In the feeding device 8, the sheet P is pulled out from the cassette 81 by the pickup roller 82, separated one by one by the separation device 83, and sent to the registration rollers 84. The registration rollers 84 receive the sheet P in a stopped state and make it wait, and then send the sheet P to the secondary transfer section N2 in synchronization with the toner image on the intermediate transfer belt 51.

[0017] In the intermediate transfer unit 5, an intermediate transfer belt 51, which is an example of an image carrier, is wound around a drive roller 52, support rollers 58 and 59, a tension roller 53, and an opposing roller 54, and rotates the belt in the direction of arrow R2. The opposing roller 54 is disposed in a position facing a secondary transfer roller 56 across the intermediate transfer belt 51. The outer peripheral surface of the intermediate transfer belt 51 wound around the opposing roller 54 and the secondary transfer roller 56 form a secondary transfer portion N2 that nips a sheet.

[0018] The secondary transfer roller 56 is made of an elastic layer of ion-conductive foam rubber (NBR rubber) and a core metal, and has an outer diameter of 24 mm, a roller surface roughness Rz of 6.0 μm or more and 12.0 μm or less, a resistance value of 1E+5 to 1E+7 Ω when 2 kV is applied (measured in an N / N (23°C, 50% RH) environment), and an Asker-C hardness of the elastic layer of approximately 30 or more and 40 or less.

[0019] A variable supply bias secondary transfer high-voltage power supply D2 is connected to the secondary transfer roller 56, and a secondary transfer bias is applied to the secondary transfer roller 56. When performing secondary transfer, a high-voltage positive (positive polarity) transfer voltage (secondary transfer bias) is applied to the secondary transfer roller 56, thereby electrostatically attracting the negatively charged toner image to the sheet. As a result, the toner image carried on the intermediate transfer belt 51 is secondarily transferred to the sheet P passing through the secondary transfer section N2.

[0020] The fixing device 7 forms a heating nip by pressing a pressure roller 73 against a fixing roller 72 disposed around a lamp heater 71. Then, in the heating nip, the sheet P onto which the toner image has been transferred at the secondary transfer portion N2 is heated and pressurized to fix the toner image to the sheet P. After the fixing process, the sheet P is discharged outside the apparatus by discharge rollers 85, which serve as a discharge portion, and is stacked on a discharge tray 86 or the like.

[0021] The belt cleaning device 57 rubs a cleaning blade against the intermediate transfer belt 51 to remove residual toner, paper dust, etc. remaining on the surface of the intermediate transfer belt 51 after the sheet P has passed through the secondary transfer section N2 and been separated.

[0022] The image forming units Pa, Pb, Pc, and Pd are configured almost identically, except that the colors of toner used in the developing devices 4a, 4b, 4c, and 4d attached to the photosensitive drums 1a, 1b, 1c, and 1d, respectively, are different: yellow, magenta, cyan, and black. The image forming unit Pa will be described below with reference to Figure 2, and the other image forming units Pb, Pc, and Pd will be described by replacing the "a" at the end of the reference numerals with "b," "c," and "d."

[0023] 2, the image forming unit Pa has a photosensitive drum 1a around which a charging roller 2a, an exposure device 3a, a developing device 4a, a primary transfer roller 55a, and a cleaning device 6a are arranged. The photosensitive drum 1a is an aluminum cylinder having an organic photoconductor layer (OPC) with a negative charging polarity formed on the outer peripheral surface thereof, and rotates in the direction of arrow R1 at a process speed of 100 mm / sec or more and 700 mm / sec or less (e.g., 240 mm / sec).

[0024] The charging roller 2a, which is a charging member, is formed by covering the surface of a metallic central shaft with a resistive elastic layer, and is pressed against the photosensitive drum 1a and rotates in response. A power source D3 applies a DC voltage superimposed on an AC voltage to the charging roller 2a, thereby charging the surface of the photosensitive drum 1a to a uniform negative potential.

[0025] The exposure device 3a scans a laser beam, which is ON-OFF modulated based on scanning line image data obtained by developing a yellow separation color image, with a rotating mirror, to write an electrostatic image onto the surface of the charged photosensitive drum 1a.

[0026] The developing device 4a stirs a two-component developer, which is a mixture of non-magnetic toner and magnetic carrier, to charge the non-magnetic toner negatively and the magnetic carrier positively. The charged two-component developer is carried in a brush-like state on a developing sleeve 41a, which rotates around a fixed magnetic pole 42a in the counter-rotating direction to the photosensitive drum 1a, and rubs against the photosensitive drum 1a. A power source D4 applies a developing voltage, which is a negative DC voltage superimposed on an AC voltage, to the developing sleeve 41a, moving the toner to the exposed area of ​​the photosensitive drum 1a, which is relatively more positively polarized than the developing sleeve 41a, thereby reverse-developing the electrostatic image.

[0027] The primary transfer roller 55a, which is a primary transfer member, is pressed against the photosensitive drum 1a side so as to sandwich the intermediate transfer belt 51, forming a primary transfer portion N1a between the photosensitive drum 1a and the intermediate transfer belt 51. The power source D1a is a transfer output unit that applies a voltage to the primary transfer roller 55a, and applies a positive DC voltage of 500 V or more and 7000 V or less (for example, +900 V) to the primary transfer roller 55a as a primary transfer bias. As a result, the negatively charged toner image carried on the photosensitive drum 1a is primarily transferred to the intermediate transfer belt 51, which passes through the primary transfer portion N1a.

[0028] The primary transfer roller 55a has a resistance of 1×10 when 2000 V is applied. 2 ~10 8 A semiconductive roller with a resistance of 1×10 Ω was used. Specifically, an ion-conductive sponge roller with an outer diameter of φ16 mm and a core diameter of φ8 mm, formed by a blend of nitrile rubber and ethylene-epichlorohydrin copolymer, was used. The resistance value of the primary transfer roller 55a was 1×10 Ω when an applied voltage of 2 kV was applied in an environment of a temperature of 23°C and a humidity of 50% RH. 6 ~10 8 It is about Ω.

[0029] The cleaning device 6a slides a cleaning blade over the photosensitive drum 1a to remove the untransferred toner adhering to the surface of the photosensitive drum 1a that has passed through the primary transfer portion N1a.

[0030] In recent years, the variety of sheets available has expanded, resulting in a wide range of sheet thicknesses and electrical resistivities, leading to the adoption of the intermediate transfer method. To prevent variations in the amount of charge supplied to the toner image due to differences in the image ratio in the main scanning direction, sheet width, and other factors, constant voltage control is employed in the transfer unit (secondary transfer unit in the above example) that transfers the toner image to the sheet. Furthermore, changes in the ambient environment, such as temperature and humidity, or the accumulated film thickness of the surface layer of the photosensitive drum, can change the electrical resistance of the intermediate transfer belt and transfer roller, or the thickness of the surface layer of the photosensitive drum. To accommodate these changes, Active Transfer Voltage Control (ATVC) is implemented to determine the control value for constant voltage control prior to image formation in order to optimize the voltage applied to the transfer roller during image formation.

[0031] ATVC control is a control in which, when there is no sheet at secondary transfer portion N2, a plurality of different test voltages are applied to secondary transfer roller 56, the current detection sensor is used to detect the current at each transfer voltage, the relationship between the transfer voltage and the current is found, and the transfer voltage (secondary transfer bias) to be applied to secondary transfer portion N2 is set based on this. The entire control of image forming apparatus 100, including this ATVC control, is performed by control unit 110 (FIG. 1).

[0032] 4, which will be described later, the control unit (control circuit) 110 has a CPU (Central Processing Unit) 111, a RAM (Random Access Memory) 112, and a ROM (Read Only Memory) 113. The CPU 111 controls each unit while reading out a program corresponding to a control procedure stored in the ROM 113. The RAM 112 also stores working data and input data, and the CPU 111 performs control by referring to the data stored in the RAM based on the aforementioned programs, etc.

[0033] [Charge adjustment device] In this embodiment, in order to prevent sheets stacked on the discharge tray 86 from sticking together due to electrostatic force, a charge adjustment unit 9 is disposed downstream of the fixing device 7 in the sheet conveyance direction and upstream of the discharge roller 85 (FIG. 1), as shown in FIG. 3. The charge adjustment unit 9 adjusts the charge on the sheet on which the toner image has been fixed by the fixing device 7 by applying a voltage.

[0034] In Figure 3, the dashed line indicates the transport path of the sheet P, and the charge adjustment device 9 is located downstream of the fixing device 7. The charge adjustment device 9 has a charge adjustment roller 910 and an opposing roller 92 as a pair of rotating bodies that sandwich and transport the sheet. The charge adjustment roller 910 is connected to a charge adjustment high-voltage power supply 90 as a power source, and is in pressure contact with the opposing roller 92 to form a charge adjustment nip portion N. The opposing roller 92 is grounded. When the sheet P passes through the charge adjustment nip portion N formed by the opposing roller 92 and the charge adjustment roller 910, charge adjustment after passing through the charge adjustment nip portion N is performed by a neutralization current controlled by the charge adjustment high-voltage power supply 90.

[0035] The charge adjustment roller 910 has a metal core (rotating shaft) 910a as a conductive shaft portion, and an elastic layer 910b as an outer peripheral portion containing an ion-conductive material formed on the outer periphery of the core 910a. The opposing roller 92 also has a metal core (rotating shaft) 92a as a conductive shaft portion, and an elastic layer 92b as an outer peripheral portion containing an ion-conductive material formed on the outer periphery of the core 92a.

[0036] The charge adjustment roller 910 and the counter roller 92 are semiconductive rollers, and the elastic layers 910b and 92b are made of an ion-conductive material formed by blending nitrile rubber and ethylene-epichlorohydrin copolymer. The charge adjustment roller 910 and the counter roller 92 are semiconductive rollers with an outer diameter of 20 mm and a core diameter of 16 mm, but the materials for the charge adjustment roller 910 and the counter roller 92 are not limited to these.

[0037] The opposing roller 92 is disposed so as to sandwich the sheet between itself and the charge adjustment roller 910. Specifically, both ends of the core of either the core 910a of the charge adjustment roller 910 or the core 92a of the opposing roller 92 are urged toward the other roller by a spring member, whereby the elastic layers 910b and 92b are pressed against each other to form a nip N. Therefore, the sheet that has passed through the fixing device 7 passes through the nip N formed between the charge adjustment roller 910 and the opposing roller 92.

[0038] [Voltage control of charge regulator] Next, the voltage control of the charge adjustment device 9 in this embodiment will be described with reference to Figures 4 and 5. As shown in Figure 4, the image forming apparatus 100 of this embodiment has an environment sensor 75, an operation unit 76, a charge adjustment high-voltage power supply 90, a current detection circuit 93, and a voltage detection circuit 94, which are connected to the control unit 110.

[0039] An environment sensor 75 serving as an environment detection unit detects the temperature and humidity inside the main body (inside the device) of the image forming apparatus 100. An operation unit 76 serving as an information acquisition unit is provided with, for example, a touch panel capable of displaying and inputting information, allowing the user to input various information and set various conditions. It is also capable of displaying the status of the image forming apparatus 100. In this embodiment, by inputting sheet information using the operation unit 76 by the user, it is possible to acquire information, for example, about the length of the sheet in the sheet width direction perpendicular to the sheet conveyance direction (sheet width).

[0040] As described above, the charge adjustment high-voltage power supply 90 is connected to the charge adjustment device 9 and applies a voltage to the charge adjustment device 9. Specifically, the charge adjustment high-voltage power supply 90 is connected to the core 910a of the charge adjustment roller 910 and applies a voltage to the core 910a. In this embodiment, the charge adjustment device 9 is controlled at a constant voltage. A current detection circuit 93 serving as a current detection unit detects the current flowing through the charge adjustment device 9. A voltage detection circuit 94 serving as a voltage detection unit detects the voltage applied to the charge adjustment device 9.

[0041] In this embodiment, the upper and lower limits of the static elimination current when the sheet P passes through the charge adjustment nip N are determined based on information such as the environmental sensor 75, conditions specified by the operation unit 76, and the electrical resistance of the charge adjustment nip N detected before the sheet P passes through the charge adjustment nip N. Then, when the sheet P is transported through the charge adjustment nip N, the control unit 110 controls the high voltage output at a constant voltage from the charge adjustment high-voltage power supply 90 while detecting the static elimination current flowing through the charge adjustment nip N so that the static elimination current falls within the range of the upper and lower limits. To perform this control, the high-voltage board of the charge adjustment high-voltage power supply 90 includes a current detection circuit 93 for detecting the static elimination current and a voltage detection circuit 94 for detecting the high voltage being output.

[0042] Specifically, the control unit 110 can execute a mode (first mode) in which a predetermined voltage is applied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 while no sheet is being conveyed to the charge adjustment nip N, and the relationship between the current and voltage is determined from the current value detected by the current detection circuit 93 and the predetermined voltage. In this mode, the control unit 110 applies a plurality of different voltages as the predetermined voltages, and determines the relationship between the current value and the voltage value from the plurality of current values ​​detected by the current detection circuit 93 and the plurality of voltage values ​​applied when each voltage is applied.

[0043] Then, the control unit 110 determines the voltage to be applied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 when the sheet passes through the charge adjustment nip portion N based on the relationship determined in the mode and the length of the sheet in the sheet width direction (sheet width) obtained by the operation unit 76.

[0044] In particular, in this embodiment, the control unit 110 determines a target current value to be passed through the charge adjustment device 9 and upper and lower limit values ​​of a current that can be passed through an area where a sheet is sandwiched in the charge adjustment nip N, based on information detected by the environmental sensor 75. Next, the control unit 110 determines a first voltage at which the target current value flows through an area where no sheet is present in the charge adjustment nip N, based on the relationship obtained in the above-described mode, and also determines a second voltage related to the electrical resistance of the sheet, based on information detected by the environmental sensor 75. Next, when the sheet is transported to the charge adjustment nip N, a third voltage obtained by adding the first and second voltages described above is applied to the charge adjustment device 9, and the current value at that time is detected by the current detection circuit 93.

[0045] Furthermore, the control unit 110 calculates the sheet presence region current value that flows in the region where the sheet is present in the charge adjustment nip N based on the current value when the above-mentioned third voltage is applied, the relationship determined in the above-mentioned mode, and the sheet width acquired by the operation unit 76. Then, the control unit 110 determines the voltage to be applied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 when the sheet passes through the charge adjustment nip N so that the sheet presence region current value falls within the range between the above-mentioned upper and lower limit values.

[0046] An example of the above-mentioned control flow will be described below using the flowchart in Fig. 5. First, when job information is transmitted from the operation unit 76, the control unit 110 starts the image forming process by the image forming apparatus 100 (S101). The job information specified by the operation unit 76 includes image information specified by the user, the size (width, length) of the sheet P on which the image is to be printed, information related to the thickness of the sheet P (thickness or basis weight), and information related to the surface properties of the sheet P, such as whether the sheet P is coated paper or not, and the CPU 111 of the control unit 110 writes this information to the RAM 112 (S102).

[0047] The control unit 110 reads environmental information from the environmental sensor 75, which reads environmental information such as the temperature and humidity inside the machine (S103). The ROM 113 in the control unit 110 has a correlation between the environmental information and the target current Itarget for the static elimination current, and based on the environmental information, the CPU 111 in the control unit 110 calculates the target current Itarget corresponding to the environment when charge adjustment is performed by the charge adjustment device 9, and writes this to the RAM 112 (S104). The target current is changed depending on the environmental information because the amount of charge required to cancel the amount of charge carried by the sheet after passing through the fixing device 7 changes depending on the environment.

[0048] Furthermore, when the sheet P passes through the charge adjustment nip N, the range of current that can be allowed to flow in an area where there is a sheet that can prevent sheets from sticking together (hereinafter, for convenience, also referred to as the sheet presence area or the paper passing area) is determined by referring to a table such as that shown in Fig. 6, which is created based on the results of a preliminary study and stored in ROM 113, in accordance with the ambient environment information acquired in S103 (S105). The range of current that can be allowed to flow in the paper passing area is a value between an upper limit current (upper limit value) Imax and a lower limit current (lower limit value) Imin.

[0049] Furthermore, since the current range that can prevent sheets from sticking together varies not only depending on the surrounding environmental information but also on the thickness and surface properties of the sheet P, it is desirable that the table of the current range that can be passed through the paper passing section also changes depending on information related to the thickness of the sheet P (basis weight) and information related to the surface properties of the sheet P.

[0050] Furthermore, before the sheet P on which the toner image has been fixed reaches the charge adjustment nip N, the control unit 110 supplies a predetermined charge removal voltage (predetermined voltage) from the charge adjustment high-voltage power supply 90 while the opposing roller 92 and the charge adjustment roller 910 are in contact with each other, and reads the electrical resistance of the charge adjustment nip N, or in other words, the voltage-current relationship (S106). That is, the above-mentioned mode (first mode) is executed.

[0051] The electrical resistance of the charge adjustment nip portion N may not be linearly proportional to the current but may be expressed by a polynomial of quadratic or higher order. Therefore, in this embodiment, a predetermined static elimination voltage is supplied for detecting the electrical resistance at three or more stages so that the voltage-current relationship before the sheet P reaches the charge adjustment nip portion N can be expressed by a polynomial, and the current flowing at that time is detected. In this embodiment, the coefficients a, b, and c of the following quadratic expression are calculated to represent the relationship between current and voltage from an approximation of the relationship between current I and voltage V at three points: I=aV 2 +bV+c

[0052] Based on the target current Itarget written to RAM 112 in S104 and the voltage-current relationship calculated in S106, CPU 111 calculates a neutralization voltage value (first voltage) Vb to be output from charge adjustment high-voltage power supply 90 in order to pass the target current Itarget when sheet P is not being transported to charge adjustment nip portion N (hereinafter, for convenience, also referred to as non-paper passing).

[0053] 7, the ROM 113 stores a table for calculating the neutralization voltage Vp to be output with the addition of the electrical resistance of the sheet P from user information such as the basis weight of the sheet P and the amount of moisture contained in the atmosphere inside the main body calculated from the ambient environment information. Then, the CPU 111 calculates the neutralization voltage (second voltage) Vp to be output with the addition of the electrical resistance of the sheet P from the job information acquired in S102 and the environmental information acquired in S103. Then, the CPU 111 writes the initial value of the neutralization voltage (high neutralization voltage, third voltage) Vj to be applied when the sheet P reaches the charge adjustment nip portion N as Vb+Vp, which is the sum of Vb and Vp, into the RAM 112, and prepares for the timing when the sheet P reaches the charge adjustment nip portion N (S107).

[0054] The table for calculating Vp as shown in Fig. 7 was determined through prior studies. The neutralization voltage Vp for the electrical resistance component of the sheet P varies depending on the surface properties of the sheet P as well as on information related to the thickness of the sheet P (basis weight). Therefore, it is desirable that the table also varies depending on information related to the surface properties of the sheet P. Furthermore, in this embodiment, the information related to the thickness of the sheet P and the information related to the surface properties of the sheet P are included in the job information acquired in S102, but it is also possible to provide separate measuring means for detecting the thickness of the sheet P and the surface properties of the sheet P, and to calculate Vp based on the detection results obtained from these measuring means.

[0055] The neutralization current Ij is detected (S108) and the sheet passing section current Ip is calculated (S109) from the time when the leading edge of the sheet P in the transport direction reaches the charge adjustment nip portion N and has advanced sufficiently until the trailing edge of the sheet P in the transport direction exits the charge adjustment nip portion N. In this embodiment, the position where the leading edge of the sheet P has advanced sufficiently is defined as the position where the leading edge of the sheet P has advanced 10 mm from the charge adjustment nip portion N. Also, in this embodiment, the position where the trailing edge of the sheet P in the transport direction exits the charge adjustment nip portion N is defined as the position 10 mm before the trailing edge of the sheet P exits the charge adjustment nip portion N. The neutralization current Ij is detected at a predetermined timing, and in this embodiment, detection is performed every 8 msec.

[0056] Fig. 8 is a schematic diagram of the charge adjustment device 9 viewed from the sheet transport direction. As shown in Fig. 8, the neutralization current Ij detected by the current detection circuit 93 is divided into a current flowing in a paper-passing portion and a current flowing in a non-paper-passing portion, and is expressed by the following equation, where Lr is the length (width) of the charge adjustment roller 910 in a direction approximately perpendicular to the transport direction of the sheet P, and Lp is the length (sheet width) of the sheet P in a direction approximately perpendicular to the transport direction of the sheet P. Ij=Lp / Lr×Ip+(Lr-Lp) / Lr×Inp

[0057] Here, the paper-passing portion current Ip is a current that flows in the portion where the sheet P is present within the entire area of ​​the charge adjustment nip portion N in the direction (longitudinal direction) that is approximately perpendicular to the conveying direction of the sheet P. Inp is a current (non-paper-passing portion current) that flows in the portion where the sheet P is not present within the entire area of ​​the charge adjustment nip portion N in the longitudinal direction.

[0058] From the above formula, the sheet passing portion current Ip is calculated by the following formula. Ip=Lr / Lp×{Ij-(Lr-Lp) / Lr×Inp} The non-sheet passing portion current Inp depending on the roller length in the longitudinal direction is calculated by the following formula using the electrical resistance of the charge adjustment nip portion N obtained in S106. Inp=aVj 2 +bVj+c

[0059] In order for the upper limit Imax and lower limit Imin of the neutralization current to function properly even when the width of the sheet P varies, the current value Ip in the paper-passing portion and the current value Inp in the non-paper-passing portion are normalized to the width Lr of the charge adjustment roller 910. The current Ip in the paper-passing portion can be calculated based on the average value of multiple current detection results during paper passage.

[0060] Then, the CPU 111 determines whether the sheet-passing portion current value (sheet-present region current value) Ip calculated in S109 is equal to or greater than the upper limit Imax and equal to or less than the lower limit Imin (S110, S111). If the sheet-passing portion current value Ip is equal to or greater than the upper limit Imax (No in S110), the CPU 111 decreases the neutralization voltage Vj by the voltage change amount ΔV per time and stores the result in the RAM 112 (S112). On the other hand, if the sheet-passing portion current value Ip is equal to or less than the lower limit Imin (No in S111), the CPU 111 increases the neutralization voltage Vj by the voltage change amount ΔV per time and stores the result in the RAM 112 (S113).

[0061] In this embodiment, the value of the voltage change width ΔV per change is set to 50 V. This paper-passing section current calculation, comparison with the upper and lower limits, and increase / decrease in voltage are repeated until the area 10 mm from the rear end of the recording material is reached (S114). As a result, even if the paper-passing section current value Ip initially deviates from the upper limit value Imax or the lower limit value Imin, the paper-passing section current value Ip gradually approaches the range of the upper limit value Imax and the lower limit value Imin, and typically finally reaches the upper limit value Imax or the lower limit value Imin.

[0062] This changed neutralization voltage is applied as the initial value when adjusting the charge on the next sheet P and onwards (S115). Until the job is completed, the flow of S108 to S115 is repeated using the initial value determined in S115 (S116). Note that if the sheet passing portion current value Ip is within a predetermined range, that is, if it is less than the upper limit value Imax (Yes in S110) and greater than the lower limit value Imin (Yes in S111), the neutralization voltage Vj is not changed.

[0063] In this embodiment, the charge of the sheet can be stably adjusted regardless of the width of the sheet P or changes in the resistance of the charge adjustment roller 910 of the charge adjustment device 9. That is, by performing the series of controls described above, the current that flows in the non-sheet-passing portion when the sheet P is transported to the charge adjustment nip N can be predicted by detecting the resistance before the sheet P reaches the charge adjustment device 9, and the current applied to the sheet can be controlled using the predicted current that flows in the non-sheet-passing portion. This makes it possible to perform charge adjustment of the sheet by the charge adjustment device 9 regardless of the resistance of the charge adjustment roller 910 and the sheet, which vary depending on various conditions, and to maintain a state in which sheets do not stick to each other.

[0064] <Second embodiment> The second embodiment will be described with reference to Fig. 9. In the first embodiment described above, the voltage control of the charge adjustment device 9 was performed at a constant voltage, but in this embodiment, it is performed at a constant current. Since the other configurations and operations are the same as those of the first embodiment described above, the same configurations are given the same reference numerals, and explanations and illustrations will be omitted or simplified. The following description will focus on the points that are different from the first embodiment.

[0065] As described above, this embodiment differs from the first embodiment only in the voltage control method of the charge adjustment device 9, and the configurations of the image forming apparatus 100 and the charge adjustment device 9 are the same. Therefore, the voltage control of the charge adjustment device 9 of this embodiment will be described below.

[0066] In this embodiment, similar to the first embodiment, the high voltage output from the charge adjustment high-voltage power supply 90 is controlled by the control unit 110 so that the neutralization current flowing through the charge adjustment nip portion N falls within predetermined upper and lower limit values. However, in this embodiment, the high voltage is output from the charge adjustment high-voltage power supply 90 as a constant current.

[0067] That is, in this embodiment, the control unit 110 can execute a mode (second mode) in which a predetermined current is supplied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 while no sheet is being conveyed to the charge adjustment nip N, and the control unit 110 determines the relationship between current and voltage from the voltage value detected by the voltage detection circuit 94 and the predetermined current.The control unit 110 then determines the voltage to be applied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 when the sheet passes through the charge adjustment nip N, based on the relationship determined in this mode and the length of the sheet in the sheet width direction (sheet width) acquired by the operation unit 76.

[0068] In particular, in this embodiment, the control unit 110 determines a target current value to be supplied to the charge adjustment device 9 based on information detected by the environment sensor 75, and upper and lower limit values ​​of the current value that may be supplied to a region of the sheet when the sheet is sandwiched in the charge adjustment nip N. Next, when the sheet is transported to the charge adjustment nip N, the charge adjustment high-voltage power supply 90 supplies the target current to the charge adjustment device 9, and the voltage value at that time is detected by the voltage detection circuit 94.

[0069] Furthermore, the control unit 110 calculates the sheet presence region current value that flows in the region where the sheet is present in the charge adjustment nip N based on the voltage value when the target current is supplied, the relationship determined in the above-described mode, and the sheet width acquired by the operation unit 76. Then, the control unit 110 determines the current to be supplied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 when the sheet passes through the charge adjustment nip N so that the sheet presence region current value falls within the range between the upper limit and the lower limit.

[0070] An example of the above-mentioned control flow will be described below using the flowchart in Fig. 9. First, when job information is transmitted from the operation unit 76, the control unit 110 starts the image forming process by the image forming apparatus 100 (S201) and acquires sheet information from the job information (S202). The control unit 110 reads environmental information such as the temperature and humidity inside the machine using the environmental sensor 75 (S203).

[0071] ROM 113 in control unit 110 stores the correlation between the target current Itarget for the neutralization current and environmental information. Based on the environmental information, CPU 111 in control unit 110 calculates the target current Itarget corresponding to the environment when charge adjustment is performed by charge adjustment device 9 and writes this to RAM 112 (S204). Furthermore, when sheet P passes through charge adjustment nip N, the range of current that can be applied to the sheet passing portion to prevent sticking between sheets is calculated by referring to a table such as that shown in FIG. 6, which was created based on the results of a preliminary study and stored in ROM 113, in accordance with the ambient environmental information acquired in S203 (S205). The range of current that can be applied to the sheet passing portion is a value between an upper limit current (upper limit value) Imax and a lower limit current (lower limit value) Imin. S201 to S205 are the same as S101 to S105 in FIG. 5.

[0072] Next, in this embodiment, before the sheet P on which the toner image has been fixed reaches the charge adjustment nip N, the control unit 110 supplies a predetermined charge removal current (predetermined current) from the charge adjustment high-voltage power supply 90 while the opposing roller 92 and the charge adjustment roller 910 are in contact with each other, and reads the electrical resistance of the charge adjustment nip N, or in other words, the voltage-current relationship (S206). That is, the above-mentioned mode (second mode) is executed.

[0073] In this embodiment, a predetermined neutralization current is supplied for detecting electrical resistance at multiple stages of three or more points, and the voltage applied at that time is detected so that the voltage-current relationship before the sheet P reaches the charge adjustment nip N can be expressed by a polynomial. In this embodiment, as the relationship between current and voltage, the coefficients a, b, and c of the following quadratic equation are calculated from an approximation of the relationship between the current I and voltage V at three points. I=aV 2 +bV+c

[0074] The CPU 111 writes the target current Itarget written to the RAM 112 in S204 as the initial value of the discharge current Ij to be applied when the sheet P reaches the charge adjustment nip N, and prepares for the timing when the sheet P reaches the charge adjustment nip N (S207).

[0075] The static elimination voltage Vj is detected (S208) and the sheet passing section current Ip is calculated (S209) from the time when the leading edge of the sheet P in the transport direction reaches the charge adjustment nip portion N and has advanced sufficiently until the trailing edge of the sheet P in the transport direction exits the charge adjustment nip portion N. In this embodiment, the position where the leading edge of the sheet P has advanced sufficiently is defined as the position where the leading edge of the sheet P has advanced 10 mm from the charge adjustment nip portion N. Also, in this embodiment, the position where the trailing edge of the sheet P in the transport direction exits the charge adjustment nip portion N is defined as the position 10 mm before the trailing edge of the sheet P exits the charge adjustment nip portion N. The static elimination voltage Vj is detected at a predetermined timing, and in this embodiment, detection is performed every 8 msec.

[0076] As shown in Figure 8 above, the applied neutralization current Ij is divided into a current flowing in the paper-passing portion and a current flowing in the non-paper-passing portion, and is expressed as follows, where Lr is the length (width) of the charge adjustment roller 910 in a direction approximately perpendicular to the conveying direction of the sheet P, and Lp is the length (sheet width) of the sheet P in a direction approximately perpendicular to the conveying direction of the sheet P. Ij=Lp / Lr×Ip+(Lr-Lp) / Lr×Inp

[0077] Here, the paper-passing portion current Ip is a current that flows in the portion where the sheet P is present within the entire area of ​​the charge adjustment nip portion N in the direction (longitudinal direction) that is approximately perpendicular to the conveying direction of the sheet P. Inp is a current (non-paper-passing portion current) that flows in the portion where the sheet P is not present within the entire area of ​​the charge adjustment nip portion N in the longitudinal direction.

[0078] From the above formula, the sheet passing portion current Ip is calculated by the following formula. I p =Lr / Lp×{Ij-(Lr-Lp) / Lr×Inp} The non-sheet passing portion current Inp depending on the roller length in the longitudinal direction is calculated by the following formula using the electrical resistance of the charge adjustment nip portion N obtained in S206. Inp=aVj 2 +bVj+c

[0079] In order for the upper limit Imax and lower limit Imin of the neutralization current to function properly even when the width of the sheet P varies, the current value Ip in the paper-passing portion and the current value Inp in the non-paper-passing portion are normalized to the width Lr of the charge adjustment roller 910. The current Ip in the paper-passing portion can be calculated based on the average value of multiple current detection results during paper passage.

[0080] Then, the CPU 111 determines whether the sheet-passing portion current value (sheet-present region current value) Ip calculated in S209 is equal to or greater than the upper limit Imax and equal to or less than the lower limit Imin (S210, S211). If the sheet-passing portion current value Ip is equal to or greater than the upper limit Imax (No in S210), the CPU 111 decreases the neutralization current Ij by the current change amount ΔI per time and stores the result in the RAM 112 (S212). On the other hand, if the sheet-passing portion current value Ip is equal to or less than the lower limit Imin (No in S211), the CPU 111 increases the neutralization current Ij by the voltage change amount ΔI per time and stores the result in the RAM 112 (S213).

[0081] In this embodiment, the value of the voltage change width ΔI per change is set to 2 μA. This paper-passing section current calculation, comparison with the upper and lower limits, and increase / decrease in current are repeated until the area 10 mm from the rear end of the recording material is reached (S214). As a result, even if the paper-passing section current value Ip initially deviates from the upper limit value Imax or the lower limit value Imin, the paper-passing section current value Ip gradually approaches the range of the upper limit value Imax and the lower limit value Imin, and typically finally reaches the upper limit value Imax or the lower limit value Imin.

[0082] This changed neutralization current Ij is applied as the initial value when adjusting the charge for the next sheet P and onwards (S215). Until the job is completed, the flow of S208 to S215 is repeated using the initial value determined in S215 (S216). Note that if the sheet passing section current value Ip is within a predetermined range, that is, if it is less than the upper limit value Imax (Yes in S210) and greater than the lower limit value Imin (Yes in S211), the neutralization current Ij is not changed.

[0083] In this embodiment, too, stable charge adjustment of the sheet can be performed regardless of the width of the sheet P or changes in the resistance of the charge adjustment roller 910 of the charge adjustment device 9. That is, by performing the series of controls described above, the current that flows in the non-sheet-passing portion when the sheet P is transported to the charge adjustment nip N can be predicted by detecting the resistance before the sheet P reaches the charge adjustment device 9, and the current applied to the sheet can be controlled using the predicted current that flows in the non-sheet-passing portion. This makes it possible to perform charge adjustment of the sheet by the charge adjustment device 9 regardless of the resistance of the charge adjustment roller 910 and the sheet, which vary depending on various conditions, and to maintain a state in which sheets do not stick to each other. [Explanation of symbols]

[0084] 7. Fixing device (fixing section) 9...Charge adjustment device (charge adjustment section) 75 Environmental sensor (environmental detection unit) 76...Operation unit (information acquisition unit) 90... Charge Regulating High Voltage Power Supply (Power Supply) 92 Counter roller (rotating body) 93 Current detection circuit (current detection section) 94 Voltage detection circuit (voltage detection section) 100 Image forming device 110 Control unit 910 Charge adjustment roller (rotating body) N2: Secondary transfer unit (transfer unit)

Claims

1. a charge adjustment unit that is disposed downstream in a sheet conveying direction from a transfer unit that transfers a toner image onto a sheet, and adjusts a charge on the sheet by applying a voltage to a pair of rotating bodies; A power source that applies a voltage to the charge adjustment unit; a current detection unit that detects a current flowing in the charge adjustment unit; A control unit that controls the power supply, the control unit applies a predetermined voltage from the power source to the charge adjustment unit in a state where a sheet is not conveyed to the nip portion formed by the pair of rotating bodies, and applies a voltage from the power source to the charge adjustment unit when the sheet passes through the nip portion based on a detection result detected by the current detection unit at that time.

1. A charge balancing device comprising:

2. the control unit applies a plurality of voltages different from each other as the predetermined voltage in a state in which a sheet is not conveyed to the nip portion, 2. The charge balancing device of claim 1.

3. An environment detection unit that detects temperature and humidity inside the device, the control unit applies a voltage from the power source to the charge adjustment unit when the sheet passes through the nip portion based on the information detected by the environment detection unit.

3. A charge adjustment device according to claim 1 or 2.

4. An information acquisition unit capable of acquiring information on the length of a sheet in a sheet width direction perpendicular to a sheet conveying direction, the control unit applies a voltage from the power source to the charge adjustment unit when the sheet passes through the nip portion, based on the length of the sheet in the sheet width direction acquired by the information acquisition unit.

4. A charge balancing device according to claim 1, 2 or 3.

5. The control unit applies a voltage from the power source to the charge adjustment unit while a first sheet is being transported by the nip portion formed by the pair of rotating bodies, and based on the detection result detected by the current detection unit at that time, applies a voltage from the power source to the charge adjustment unit when a second sheet, which is transported next to the first sheet, is transported by the nip portion.

5. A charge balancing device according to claim 1, wherein the charge balancing device comprises a first electrode and a second electrode.

6. The control unit controls a voltage value so that a current value according to the detection result when a voltage is applied from the power source to the charge adjustment unit when the sheet passes through the nip portion falls within a predetermined range of a target current value.

6. A charge balancing device according to claim 1, wherein the charge balancing device comprises a first electrode and a second electrode.

7. A charge adjustment section is disposed downstream in a sheet conveying direction from a transfer section that transfers a toner image onto a sheet, and adjusts a charge on the sheet by applying a voltage to a pair of rotating bodies; A power source that applies a voltage to the charge adjustment unit; a voltage detection unit that detects a voltage applied to the charge adjustment unit; A control unit that controls the power supply, the control unit supplies a predetermined current from the power source to the charge adjustment unit in a state where a sheet is not conveyed to the nip portion formed by the pair of rotating bodies, and applies a voltage from the power source to the charge adjustment unit when the sheet passes through the nip portion based on a detection result detected by the voltage detection unit at that time.

1. A charge balancing device comprising:

8. The control unit supplies a plurality of currents different from each other as the predetermined current when a sheet is not conveyed to the nip portion.

8. The charge balancing device of claim 7.

9. An environment detection unit that detects temperature and humidity inside the device, the control unit applies a voltage from the power source to the charge adjustment unit when the sheet passes through the nip portion based on the information detected by the environment detection unit.

9. A charge adjustment device according to claim 7 or 8.

10. An information acquiring unit capable of acquiring information on the length of a sheet in a sheet width direction perpendicular to a sheet conveying direction, the control unit applies a voltage from the power source to the charge adjustment unit when the sheet passes through the nip portion, based on the length of the sheet in the sheet width direction acquired by the information acquisition unit.

10. A charge balancing device according to any one of claims 7 to 9.

11. The control unit applies a voltage from the power source to the charge adjustment unit while a first sheet is being transported by the nip portion formed by the pair of rotating bodies, and based on the detection result detected by the voltage detection unit at that time, applies a voltage from the power source to the charge adjustment unit when a second sheet to be transported next to the first sheet is transported by the nip portion. Charge conditioning device according to any one of claims 7 to 10.

12. The control unit controls a voltage value so that a current value according to the detection result when a voltage is applied from the power source to the charge adjustment unit when the sheet passes through the nip portion falls within a predetermined range of a target current value. Charge conditioning device according to any one of claims 7 to 11.

13. A charge adjustment device according to any one of claims 1 to 12, The transfer unit; An image forming apparatus comprising:

Images