Fixing device and image forming apparatus equipped with same

Abstract

To provide a fixing device that prevents the occurrence of electrostatic offset and electrostatic scattering and is excellent in durability, and an image forming apparatus including the same.SOLUTION: A fixing device comprises a first fixing member, a second fixing member, an electric charge application unit, and a control unit. The first fixing member is in contact with a toner image on a sheet. The second fixing member forms a nip part with the first fixing member. The electric charge application unit can apply an electric charge between the toner image on the sheet and a part on an outer peripheral surface of the first fixing member on the upstream side of the nip part in the direction of movement of the outer peripheral surface. The control unit controls the electric charge application unit. The control unit can execute an application mode for generating an application current and a standby mode for not generating the application current, executes the application mode in a first period from the initial start-up until when a predetermined period elapses, and executes the standby mode in a second period from the time when the first period has elapsed until when the predetermined period elapses.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a fixing device and an image forming apparatus including the same. [Background technology]

[0002] In order to fix a toner image on a sheet of paper in an image forming apparatus, a fixing device including a first fixing member such as a fixing roller or a fixing belt, and a second fixing member such as a pressure roller is widely used. The first fixing member and the second fixing member are pressed together to form a fixing nip portion. With the outer circumferential surface of the first fixing member heated to a predetermined temperature, a sheet such as a print sheet is passed through the fixing nip portion. At this time, the outer circumferential surface of the first fixing member abuts against the surface of the sheet on which the toner image is formed. In this way, heat and pressure are applied to the toner image formed on the sheet, and the toner image is melted and fixed to the sheet.

[0003] In such a fixing device, when a sheet passes through the fixing nip, a portion of the toner from the unfixed toner image electrostatically adheres to the outer peripheral surface of the first fixing member, and there is a risk of an image defect in which the toner is retransferred to the succeeding sheet, that is, a so-called electrostatic offset phenomenon. In order to suppress the occurrence of this electrostatic offset phenomenon, Patent Document 1 discloses a fixing device equipped with a charge applying section.

[0004] The charge applying section described in Patent Document 1 applies a charge to the outer peripheral surface of the first fixing member and to the surface of the sheet (the surface that contacts the outer peripheral surface of the first fixing member and that has not yet contacted the first fixing material). The charge applying section has multiple electrodes, and applies a high voltage between the electrodes to generate a corona discharge and generate positive ions. These positive ions move to the sheet and the outer peripheral surface of the first fixing member, causing the sheet and the outer peripheral surface of the first fixing member to be charged with the same polarity (positive). This makes it difficult for an unfixed toner image on the sheet to adhere to the outer peripheral surface of the first fixing member. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-194499 A Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, the outer peripheral surface of the first fixing member as described above tends to deteriorate over time and become rougher, and gradually becomes more susceptible to charging. For this reason, if the fixing operation is continued without changing the voltage applied to the charge applying unit from the value at the time of initial startup, the amount of positive charge on the outer peripheral surface of the first fixing member gradually increases. This may cause the charge on the sheet and the charge on the outer peripheral surface of the first fixing member to repel each other, resulting in image defects caused by the scattering of unfixed toner images on the sheet, known as electrostatic scattering.

[0007] Furthermore, if an electric charge is applied to the outer surface of the first fixing member for a long period of time or if a relatively high voltage is applied, the outer surface of the first fixing member and the electrodes of the charge application section are likely to deteriorate, which may shorten the product life of the fixing device.

[0008] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS In view of the above, an object of the present invention is to provide a fixing device which is excellent in durability while suppressing the occurrence of electrostatic offset and electrostatic scattering, and an image forming apparatus including the fixing device. [Means for solving the problem]

[0009] In order to achieve the above object, a first configuration of the present invention is a fixing device including a first fixing member, a second fixing member, a charge applying section, and a control section. The first fixing member contacts a toner image on a sheet conveyed along a conveying path. The second fixing member forms a nip section through which the sheet passes between the first fixing member and the second fixing member. The charge applying section generates an applied current for applying a charge, and is capable of applying a charge to a portion of the outer peripheral surface of the first fixing member that is upstream of the nip section in the moving direction of the outer peripheral surface, and to the toner image on the sheet. The control section controls the charge applying section. The control section is capable of executing an application mode in which an applied current is generated, and a standby mode in which an applied current is not generated, and executes the application mode during a first period from the initial startup until a predetermined period has elapsed, and executes the standby mode during a second period from the first period until the predetermined period has elapsed. Effect of the Invention

[0010] According to the first configuration of the present invention, the control unit executes the application mode in the first period when the outer peripheral surface of the first fixing member is relatively difficult to charge, thereby suppressing the occurrence of the electrostatic offset phenomenon. In addition, the control unit executes the standby mode in the second period when a predetermined period has passed since the initial startup and the outer peripheral surface of the first fixing member is relatively easy to charge, thereby suppressing electrostatic scattering. In addition, since no applied current is generated during the second period, the outer peripheral surface of the first fixing member and the charge application unit are less likely to deteriorate. Therefore, it is possible to provide a fixing device that is less likely to deteriorate while suppressing the occurrence of image defects. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an internal structure of an image forming apparatus 100 according to an embodiment of the present invention. [Diagram 2] A side cross-sectional view of a fixing device 13 mounted in an image forming apparatus 100. [Diagram 3] A block diagram showing an example of a control path of the image forming apparatus 100. [Figure 4] 1 is a flowchart showing an example of a control flow of the fixing device 13. [Diagram 5]1 is a flowchart showing an example of a control flow of the fixing device 13 when the first application mode is executed. [Figure 6] A flowchart showing an example of a control flow of the fixing device 13 during standby mode execution. [Figure 7] 1 is a flowchart showing an example of a control flow of the fixing device 13 during execution of the second voltage application mode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic cross-sectional view showing the internal structure of an image forming apparatus 100 according to an embodiment of the present invention. Inside the main body of the image forming apparatus 100 (here, a color printer), four image forming sections Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (the right side in Fig. 1). These image forming sections Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and sequentially form images of cyan, magenta, yellow, and black through the respective processes of charging, exposure, development, and transfer.

[0013] These image forming units Pa to Pd are provided with photoconductor drums 1a, 1b, 1c, and 1d carrying visible images (toner images) of each color. Furthermore, an intermediate transfer belt 8 rotating in a clockwise direction in FIG. 1 is provided adjacent to each image forming unit Pa to Pd. The toner images formed on these photoconductor drums 1a to 1d are sequentially primarily transferred and superimposed on the intermediate transfer belt 8 moving while in contact with each of the photoconductor drums 1a to 1d. Thereafter, the toner images primarily transferred onto the intermediate transfer belt 8 are secondarily transferred onto a sheet of paper S (an example of a recording medium) by a secondary transfer roller 9. Furthermore, the sheet of paper S onto which the toner image has been secondarily transferred is discharged from the main body of the image forming apparatus 100 after the toner image is fixed by a fixing device 13. An image forming process for each of the photoconductor drums 1a to 1d is performed while the photoconductor drums 1a to 1d are rotated counterclockwise in FIG. 1 by a main motor 40 (see FIG. 3).

[0014] The paper S onto which the toner image is secondarily transferred is stored in a paper cassette 16 arranged at the bottom of the main body of the image forming apparatus 100, and is transported to a nip between a secondary transfer roller 9 and a drive roller 11 of the intermediate transfer belt 8 via a paper feed roller 12a and a pair of registration rollers 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is mainly used. In addition, a blade-shaped belt cleaner 19 is arranged downstream of the secondary transfer roller 9 to remove toner and the like remaining on the surface of the intermediate transfer belt 8.

[0015] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photoconductor drums 1a to 1d, there are provided charging devices 2a, 2b, 2c, and 2d that charge the photoconductor drums 1a to 1d, an exposure device 5 that exposes image information onto each of the photoconductor drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d that form toner images on the photoconductor drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d that remove developer (toner) and the like remaining on the photoconductor drums 1a to 1d.

[0016] When image data is input from a host device such as a personal computer, first, the surfaces of the photoconductor drums 1a-1d are uniformly charged by the charging devices 2a-2d. Next, the exposure device 5 irradiates light according to the image data, and electrostatic latent images corresponding to the image data are formed on the photoconductor drums 1a-1d. The developing devices 3a-3d are filled with a predetermined amount of two-component developer containing toner of each color of cyan, magenta, yellow, and black. When the ratio of toner in the two-component developer filled in each developing device 3a-3d falls below a specified value due to the formation of a toner image described later, toner is replenished from the toner containers 4a-4d to each developing device 3a-3d. The toner in the developer is supplied onto the photoconductor drums 1a-1d by the developing devices 3a-3d and electrostatically adheres to them. As a result, a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5 is formed.

[0017] Then, an electric field is applied between the primary transfer rollers 6a-6d and the photoconductor drums 1a-1d by the primary transfer rollers 6a-6d at a predetermined transfer voltage, and the cyan, magenta, yellow and black toner images on the photoconductor drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed with a predetermined positional relationship for forming a predetermined full-color image. After that, in preparation for the subsequent formation of a new electrostatic latent image, the toner and the like remaining on the surfaces of the photoconductor drums 1a-1d after the primary transfer are removed by the cleaning devices 7a-7d.

[0018] The intermediate transfer belt 8 is stretched over a driven roller 10 on the upstream side and a driving roller 11 on the downstream side, and when the intermediate transfer belt 8 starts to rotate clockwise with the rotation of the driving roller 11 by a belt driving motor (not shown), the paper S is transported at a predetermined timing from the registration roller pair 12b to a nip portion (secondary transfer nip portion) between the driving roller 11 and a secondary transfer roller 9 provided adjacent thereto, and the full-color image on the intermediate transfer belt 8 is secondarily transferred onto the paper S. The paper S to which the toner image has been secondarily transferred is transported to a fixing device 13.

[0019] The paper S conveyed to the fixing device 13 is heated and pressurized by a fixing belt 21 (first fixing member) and a pressure roller 22 (second fixing member) to fix the toner image onto the surface of the paper S (see FIG. 2), and a predetermined full-color image is formed. The paper S on which the full-color image has been formed is then directed to a different conveying direction by a branching section 14 that branches into multiple directions, and is discharged directly onto a discharge tray 17 by a pair of discharge rollers 15 (or after being sent to a double-sided conveying path 18 and having images formed on both sides).

[0020] 2 is a side cross-sectional view of the fixing device 13 mounted on the image forming apparatus 100. The upper side in FIG. 2 is the downstream side in the paper insertion direction (sheet conveying direction) relative to the fixing device 13, and the lower side is the upstream side in the paper insertion direction relative to the fixing device 13. As shown in FIG. 2, the fixing device 13 includes a fixing belt 21, a pressure roller 22, a heating section 23, a nip forming member 24, a belt guide 25, a frame member 26, and a charge applying section 31. The fixing device 13 also includes a control section 90. The control section 90 may be provided at any position in the image forming apparatus 100, or may be provided within the fixing device 13.

[0021] Fixing belt 21 is supported rotatably about a horizontal axis by a housing (not shown) of fixing device 13. Fixing belt 21 is endless and configured in a cylindrical shape with an outer diameter of 20 mm to 50 mm, for example, and has approximately the same axial length (width direction length of paper S) as pressure roller 22. Fixing belt 21 rotates in the clockwise direction in FIG. 2 along the insertion direction of paper S.

[0022] The fixing belt 21 has a laminated structure in which an elastic layer and a release layer are laminated on the outer periphery of a heat generating layer, which is a base layer. The heat generating layer is made of a metal film such as nickel having a thickness of 30 μm to 50 μm, or a polyimide film having a thickness of 50 μm to 100 μm containing metal powder such as copper, silver, or aluminum. The elastic layer is made of silicone rubber having a thickness of 100 μm to 500 μm. The release layer is made of a fluorine-based resin such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) having a thickness of 30 μm to 50 μm.

[0023] Pressure roller 22 is supported on the housing of fixing device 13 so as to be rotatable about a horizontal axis. Pressure roller 22 is cylindrical, and has approximately the same axial length (widthwise length of paper S) as fixing belt 21. A predetermined pressure is applied to pressure roller 22 by pressure mechanism 30 (see FIG. 3) toward fixing belt 21. The outer circumferential surface of pressure roller 22 presses nip forming member 24 through fixing belt 21, thereby being pressed against the outer circumferential surface of fixing belt 21 to form fixing nip portion N (nip portion).

[0024] The pressure roller 22 is connected to a fixing drive motor 45 (see FIG. 3) and rotates in a counterclockwise direction in FIG. 2. The pressure roller 22 contacts the outer peripheral surface of the fixing belt 21 and applies a rotational drive force to the fixing belt 21 in the clockwise direction.

[0025] The pressure roller 22 has a laminated structure in which an elastic layer 22b is laminated on the outer periphery of a core metal 22a, and a release layer (not shown) is laminated on the surface of the elastic layer 22b. The core metal 22a is made of a metal such as aluminum with a diameter of about 20 mm. The elastic layer 22b is made of silicone rubber with a thickness of about 8 mm. The release layer is made of a fluorine-based resin such as PFA with a thickness of about 10 μm to 50 μm.

[0026] Heating unit 23 is an induction heating (IH) type heater that generates heat by induction heating the heat generating layer of fixing belt 21. Heating unit 23 is disposed in an area of ​​fixing belt 21 opposite the side where pressure roller 22 is disposed, facing the outer circumferential surface of fixing belt 21 with a predetermined gap therebetween. Heating unit 23 extends slightly longer than fixing belt 21 along the axial direction of fixing belt 21 (the width direction of paper S, the direction perpendicular to the paper surface of FIG. 2). The temperature of fixing belt 21 is detected by a thermistor 47 (see FIG. 3).

[0027] The heating unit 23 heats the fixing belt 21. It includes an exciting coil 23a, a holding member, a core, etc. (not shown). The exciting coil 23a and the core are held in a predetermined position by the holding member. The exciting coil 23a is made of a Litz wire made of a bundle of multiple conductive wires, and is wound so as to extend along the axial direction of the fixing belt 21. The exciting coil 23a is configured in an arc shape along the outer circumferential surface of the fixing belt 21 in the circumferential direction of the fixing belt 21.

[0028] The nip forming member 24 is disposed on the inside of the fixing belt 21, facing the pressure roller 22 across the fixing belt 21. The nip forming member 24 contacts the inner circumferential surface of the fixing belt 21, and forms a fixing nip portion N between the fixing belt 21 and the pressure roller 22.

[0029] The nip forming member 24 has a generally rectangular parallelepiped shape that extends along the axial direction of the fixing belt 21 with substantially the same length as the fixing belt 21. The nip forming member 24 has a base material made of a metal such as aluminum or a heat-resistant resin such as a liquid crystal polymer.

[0030] The surface roughness of the outer peripheral surface of the fixing belt 21 becomes rougher as the cumulative number of sheets n (the cumulative number of sheets S inserted into the fixing nip N) increases. Therefore, the outer peripheral surface of the fixing belt 21 becomes more easily charged as the cumulative number of sheets n increases.

[0031] The belt guide 25 is disposed on the inside of the fixing belt 21, facing the heating unit 23 across the fixing belt 21. The belt guide 25 contacts the inner peripheral surface of the fixing belt 21 except for the fixing nip portion N, and supports the fixing belt 21 from the inside. The belt guide 25 is made of a metal plate that extends along the axial direction of the fixing belt 21 and has substantially the same length as the fixing belt 21. The belt guide 25 is made of an elastic magnetic metal such as SUS430 having a thickness of 0.1 mm to 0.5 mm.

[0032] The frame member 26 is supported by a housing portion of the fixing device 13, and holds the nip forming member 24 and the belt guide 25. The frame member 26 is disposed at approximately the center in the radial direction of the fixing belt 21, between the belt guide 25 and the nip forming member 24. The frame member 26 extends slightly longer than the fixing belt 21 along the axial direction of the fixing belt 21.

[0033] A fixing entrance guide 27 is disposed upstream of the fixing nip N in the paper insertion direction (lower side in FIG. 2). The fixing entrance guide 27 guides the paper S that has passed through the secondary transfer nip (see FIG. 1) to the fixing nip N.

[0034] A separation claw 29 is disposed downstream of the fixing nip N in the paper insertion direction (upper side in FIG. 2). The separation claw 29 separates the paper S after fixing processing from the surface of the fixing belt 21. The separation claw 29 is disposed at a predetermined angle such that its tip faces upstream with respect to the rotation direction of the fixing belt 21 (counter direction) and its tip approaches the outer circumferential surface of the fixing belt 21.

[0035] The charge applying unit 31 is disposed upstream of the fixing belt 21 and the pressure roller 22 in the paper insertion direction. The charge applying unit 31 applies a positive charge to the outer peripheral surface of the fixing belt 21 and to the surface of the paper S on which the toner image is formed.

[0036] The charge applying section 31 includes counter electrodes 32a and 32b, a discharge electrode 33, and an electrode holding section 34. The counter electrodes 32a and 32b are electrodes that face each other with the discharge electrode 33 sandwiched therebetween. The electrode holding section 34 is supported by a housing section of the fixing device 13, and holds the counter electrodes 32a and 32b and the discharge electrode 33.

[0037] One end of each of the counter electrodes 32a, 32b and the discharge electrode 33 is fixed to the electrode holding portion 34, and the other end faces the outer circumferential surface of the fixing belt 21 with a predetermined gap therebetween. The discharge electrode 33 is a flat electrode made of stainless steel and has a relatively thin plate thickness (for example, about 0.1 mm). The end of the discharge electrode 33 on the fixing belt 21 side is formed in a sawtooth shape.

[0038] The discharge electrode 33 is connected to a power supply unit 52 (see FIG. 3) provided in the image forming apparatus 100. When executing the application mode, the control unit 90 applies a high voltage to the discharge electrode 33. Then, a corona discharge is generated between the sawtooth tip portion formed on the discharge electrode 33 and the counter electrodes 32a and 32b, and an applied current Ci flows.

[0039] When corona discharge occurs, positive ions A are generated around the tip of the discharge electrode 33. Some of these positive ions A leave the tip of the discharge electrode 33, and some of them reach the outer peripheral surface of the fixing belt 21. As a result, a positive charge is imparted to the outer peripheral surface of the fixing belt 21, and the fixing belt 21 becomes positively charged. The amount of charge on the outer peripheral surface of the fixing belt 21 decreases over time, but remains constant as long as corona discharge continues to occur because positive charges continue to be imparted.

[0040] Another part of the positive ions A generated by the corona discharge passes through the gap between the fixing belt 21 and the counter electrodes 32a, 32b and reaches the surface (the surface on the fixing belt 21 side) of the paper S on which the toner image is formed before passing through the fixing nip N. Therefore, the surface of the paper S on which the toner image is formed before passing through the fixing nip N is given a positive charge and becomes positively charged.

[0041] The control unit 90 executes an application mode (first application mode) during a first period in which a predetermined period has elapsed, and executes a standby mode in which the application current Ci is stopped during a second period from when the first period has elapsed until when the predetermined period has elapsed. The control unit 90 also executes an application mode (second application mode) again after the second period has elapsed. During the first period, the control unit 90 controls the voltage applied to the charge applying unit 31 so that the application current Ci decreases as the elapsed period from the initial startup of the fixing device 13 increases. The control unit 90 also controls the voltage applied to the charge applying unit 31 so that the application current Ci in the second application mode is smaller than the application current Ci in the first application mode. Details of the control of the application current Ci by the control unit 90 will be described later.

[0042] Next, the control paths of the image forming apparatus 100 and the fixing device 13 will be described with reference to Fig. 3. Since various controls are performed on each part of the image forming apparatus 100 when it is used, the following description will focus on the parts of the control paths that are necessary for implementing the present invention. Also, the description of the parts that have already been described will be omitted.

[0043] 3 is a block diagram showing an example of a control path of the image forming apparatus 100. As shown in FIG. 3, the control path of the image forming apparatus 100 includes an image input section 70, an operation section 80, a control section 90, image forming sections Pa to Pd, a power supply section 51, a power supply unit 52, and a fixing device 13.

[0044] The image input unit 70 is a receiving unit that receives image data transmitted from a personal computer or the like to the image forming apparatus 100. The image signal input from the image input unit 70 is converted into a digital signal and then sent to a temporary storage unit 94.

[0045] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 that indicate various states, and is configured to indicate the state of the image forming apparatus 100, the image formation status, and the number of copies to be printed. In addition, the type and size of the paper S can be input by specifying the paper cassette 16 or manual paper feed tray (not shown) from which the paper S is fed from the operation unit 80. Various settings of the image forming apparatus 100 are made from the printer driver of the personal computer.

[0046] The control unit 90 includes at least a CPU (Central Processing Unit) 91, a ROM (Read Only Memory) 92, a RAM (Random Access Memory) 93, a temporary storage unit 94, a counter 95 (counting unit), and multiple (here, two) I / Fs (interfaces) 96. The CPU 91 functions as a central processing unit. The ROM 92 is a read-only storage unit. The RAM 93 is a readable and writable storage unit. The temporary storage unit 94 temporarily stores image data, etc. The I / F 96 transmits control signals to each device in the image forming apparatus 100 and receives input signals from the operation unit 80.

[0047] The ROM 92 stores data such as the control program of the image forming apparatus 100 and numerical values necessary for control that are not to be changed during the use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100 and data temporarily required for the control of the image forming apparatus 100.

[0048] For example, the ROM 92 stores the length of the first period from the initial startup until a predetermined period elapses. Specifically, the first period is stored as a period (a period where n ≤ n1) in which the cumulative number of sheets passed n from the initial startup is equal to or less than a predetermined first number of sheets n1. Also, for the second period from when the first period elapses until a predetermined period elapses, the length of that period is stored. Specifically, the second period is stored as a period (a period where n1 < n ≤ n2) in which the cumulative number of sheets passed n is more than the first number of sheets n1 and equal to or less than a predetermined second number of sheets n2. Further, the control unit 90 controls the voltage at a predetermined timing during the first period. Regarding this timing, it is stored as the timing when the cumulative number of sheets passed n becomes equal to or more than a third number of sheets n3.

[0049] The second number of sheets n2 is 14 times or more and 18 times or less the first number of sheets. The third number of sheets n3 is 1 / 2 times or more and 2 / 3 times or less the first number of sheets. As specific numerical values for each, for example, the first number of sheets n1 can be set to 500 sheets, the second number of sheets n2 to 8000 sheets, and the third number of sheets n3 to 500 sheets.

[0050] Also, the ROM 92 stores different values for the voltage applied to the charge applying unit 31 according to the elapsed period from the initial startup.

[0051] The temporary storage unit 94 temporarily stores the image signal input from the image input unit 70 and converted into a digital signal. The counter 95 (counting unit) accumulates and counts the number of sheets (cumulative number of sheets passed) n of the paper S inserted through the fixing nip portion N.

[0052] Power supply unit 52 is connected to a commercial power source (not shown) via power supply unit 51. Power supply unit 52 distributes the power supplied from power supply unit 51 in response to an output signal from control unit 90 to each device (including charge application unit 31) in image forming apparatus 100.

[0053] Next, an example of control of the applied current Ci flowing through the charge applying unit 31 of the fixing device 13 of this embodiment will be described with reference to the flowcharts shown in Figs. 4 to 7. Fig. 4 is a flowchart showing an example of a control flow of the fixing device 13. Fig. 5 is a flowchart showing an example of a control flow of the fixing device 13 when the first application mode is being executed. Fig. 6 is a flowchart showing an example of a control flow of the fixing device 13 when the standby mode is being executed. Fig. 7 is a flowchart showing an example of a control flow of the fixing device 13 when the second application mode is being executed.

[0054] As shown in Fig. 4, the control unit 90 judges whether or not an image formation command has been input from a higher-level device such as a personal computer (step S1). If an image formation command has not been input (No in step S1), the control unit 90 continues to wait until an image formation command is input. If an image formation command has been input (Yes in step S1), the control unit 90 judges whether or not the above-mentioned first period (n <= n1) is in progress (step S2). If the first period (n <= n1) is in progress (Yes in step S2), the control unit 90 executes the first application mode (step S3). Details of the control during execution of the first application mode will be described later.

[0055] If it is not the first period (if n>n1 (No in step S2)), then it is determined whether it is the second period (n≦n2) (step S4). If it is the second period (Yes in step S4), the standby mode is executed (step S5). If it is not the first period or the second period (if n>n2 (No in step S4)), the second application mode is executed (step S6). Details of the control during the standby mode and the second application mode will be described later.

[0056] Next, a control example when the first application mode is executed will be described with reference to Fig. 5. When the first application mode is executed (Yes in step S2), first, it is determined whether the cumulative number of passed sheets n is equal to or less than the third number n3 (n≦n3) (step S7). If the cumulative number of passed sheets n is equal to or less than the third number n3 (Yes in step S7), a control signal is sent to the power supply unit 52 to apply a first voltage to the charge applying unit 31 so that the applied current Ci of the corona discharge generated in the charge applying unit 31 becomes 7 μA (step S8).

[0057] If the cumulative number of sheets n passed is greater than the third number n3 (No in step S7), a control signal is sent to the power supply unit 52 to apply a second voltage to the charge applying section 31 so that the applied current Ci of the corona discharge generated in the charge applying section 31 is 5 μA (step S8).

[0058] Then, it is determined whether the print job has ended (step S10). If the print job has not ended (No in step S10), the applied current Ci is continued to be generated. If it is determined that the print job has ended (Yes in step S10), a control signal is sent to the power supply unit 52 so that the applied current Ci becomes 0 μA (Ci=0 μA), and the application of voltage to the charge applying unit 31 is stopped (step S11), and the control of the applied current Ci is terminated. Note that if one print job includes printing of multiple sheets, the applied current Ci is not changed while the print job is continuing, even if the cumulative number of printed sheets n exceeds the third number of sheets n3 at the end of printing one sheet.

[0059] Next, an example of control during standby mode execution will be described with reference to Fig. 6. When standby mode is executed (Yes in step S4), a control signal is sent to power supply unit 52 so that applied current Ci becomes 0 µA (Ci = 0 µA), and a state in which no voltage is applied to charge applying unit 31 is maintained (step S12), and this state is maintained until the print job is completed (No in step S13). When the print job is completed (Yes in step S13), control of applied current Ci is terminated.

[0060] Next, a control example when the second application mode is executed will be described with reference to Fig. 7. When the second application mode is executed (No in step S4), a control signal is sent to the power supply unit 52 to apply the third voltage to the charge applying unit 31 so that the applied current Ci becomes 2 µA (Ci = 2 µA).

[0061] Then, it is determined whether the print job has ended (step S15). If the print job has not ended (No in step S15), the above-mentioned voltage continues to be applied, and the applied current Ci continues to be generated. If it is determined that the print job has ended (Yes in step S15), the application of voltage to the charge applying unit 31 is stopped (step S16) so that the applied current Ci becomes 0 μA (Ci=0 μA), and the control of the applied current Ci ends.

[0062] As described above, the outer peripheral surface of the fixing belt 21 becomes more easily charged as the cumulative number of sheets passed n increases. In other words, during the first period, which is a predetermined period from the initial startup, the outer peripheral surface is relatively difficult to charge, and if no measures are taken, the electrostatic offset phenomenon is likely to occur. Therefore, the control unit 90 of the fixing device 13 according to the above embodiment executes the application mode during the first period and applies positive charges to the outer peripheral surface of the fixing belt 21 and the surface of the paper S. Then, during the first period, the paper S and the outer peripheral surface of the fixing belt 21 are charged with charges of the same polarity (positive polarity). Therefore, a portion of the toner from the unfixed toner image on the paper S is less likely to adhere to the fixing belt 21, and the occurrence of the electrostatic offset phenomenon can be suppressed.

[0063] Also, the outer peripheral surface of the fixing belt 21 during the second period is more easily charged than the outer peripheral surface during the first period. If a relatively high voltage is applied to the charge applying section 31 during the second period, the amount of positive charge on the outer peripheral surface of the fixing belt 21 increases. This may cause electrostatic scattering on the surface of the paper S to which a charge of the same polarity (positive polarity) has been applied. Therefore, the control section 90 of the fixing device 13 according to the above embodiment executes a standby mode during the second period. Therefore, the amount of positive charge on the outer peripheral surface of the fixing belt 21 does not increase during the second period. This makes it possible to suppress the occurrence of electrostatic scattering.

[0064] As described above, the outer peripheral surface of the fixing belt 21 is easily charged during the second period. Therefore, the positive charge that was applied to the outer peripheral surface of the fixing belt 21 during the first period tends to remain on the outer peripheral surface. Therefore, even if the standby mode is executed during the second period, the toner on the paper S is less likely to adhere to the outer peripheral surface of the fixing belt 21. Therefore, the electrostatic offset phenomenon can be suppressed even during the second period.

[0065] Furthermore, since no applied current Ci is generated during the second period, it is possible to suppress deterioration of the counter electrodes 32a, 32b, the discharge electrode 33, and the outer circumferential surface of the fixing belt 21. As a result, it is possible to provide a fixing device 13 that is resistant to deterioration while suppressing the occurrence of image defects.

[0066] As described above, even during execution of the application mode, the voltage applied to the charge applying unit 31 is reduced as the time that has elapsed since the initial startup becomes longer. Specifically, the voltage applied to the charge applying unit 31 is reduced so that the current generated in the charge applying unit 31 decreases from 7 μA to 5 μA when the cumulative number of passed sheets n during the first period exceeds the third number n3. In this way, deterioration of the counter electrodes 32a and 32b, the discharge electrode 33, and the outer circumferential surface of the fixing belt 21 can be suppressed.

[0067] As described above, since the standby mode is executed during the second period, the amount of charge on the outer peripheral surface of the fixing belt 21 decreases over time. However, since the second application mode is executed after the second period has elapsed, the occurrence of the electrostatic offset phenomenon can be suppressed even after the second period has elapsed. Furthermore, the outer peripheral surface of the fixing belt 21 after the second period has elapsed is more easily charged than the outer peripheral surface during the first and second periods. Therefore, even if the applied current Ci in the second application mode is made smaller than the applied current Ci in the first application mode, the occurrence of the electrostatic offset phenomenon can be suppressed. Therefore, it is possible to suppress image defects while suppressing deterioration of the charge applying section 31 and the fixing belt 21.

[0068] As described above, the counter electrodes 32a, 32b are disposed to face each other with the discharge electrode 33 therebetween, which stabilizes the generated corona discharge.

[0069] In addition, the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the present invention. For example, in the above embodiment, the first period and the second period are defined based on the cumulative number of sheets n passed through, but the elapsed period from the initial startup may be calculated based on a value other than the cumulative number of sheets n passed through. As an example, the counting unit can count and store the cumulative time (accumulated driving time) during which the rotational driving force is applied to the pressure roller 22. In this case, the ROM 92 stores the cumulative driving time in the first period and the cumulative driving time in the second period. The control unit 90 calculates the elapsed period from the initial startup based on the cumulative driving time stored in the counting unit and the value of the cumulative driving time in each period stored in the ROM 92.

[0070] As another example of the above, a maintenance worker may periodically measure the surface roughness of fixing belt 21 and input the measured value, so that control unit 90 calculates the first period and the second period according to the measured value. Alternatively, control unit 90 may not calculate the first period and the second period, but may directly determine the voltage to be applied to charge applying unit 31 according to the measured value. In this case, ROM 92 stores the magnitude of the voltage to be applied to charge applying unit 31 according to the input measured value of surface roughness.

[0071] In addition, in each of the above embodiments, a sliding belt type fixing device 13 in which an endless fixing belt 21 slides against a nip forming member 24 as a heated rotating body has been exemplified. However, the present invention can be applied in exactly the same way to a fixing device of a uniaxial belt type in which the fixing belt 21 is wound around a fixing roller, a biaxial belt type in which the fixing belt 21 is stretched between a fixing roller and a heating roller, or a fixing device equipped with a heated rotating body other than the fixing belt 21. [Industrial Applicability]

[0072] The present invention can be used in an image forming apparatus equipped with a fixing device that applies heat and pressure to a toner image by inserting the recording medium into a fixing nip formed by a heated rotating body and a pressure member to melt and fix the toner image on the recording medium. By using the present invention, it is possible to provide an image forming apparatus that has a relatively long product life while suppressing the occurrence of image defects. [Explanation of symbols]

[0073] 13 Fixing device 21 Fixing belt (first fixing member) 22 Pressure roller (second fixing member) 31 Charge applying section 32a, 32b Counter electrodes 33 Discharge electrode 90 Control section 95 Counter (Counting section) 100 Image forming device A cation (charge) Ci applied current N Fixing nip (nip) Pa~Pd Image forming section S Paper (sheet) n Cumulative number of sheets

Claims

1. a first fixing member that contacts a toner image on a sheet transported along a transport path; a second fixing member that forms a nip portion between the first fixing member and the second fixing member, through which the sheet passes; a charge applying section that generates an applied current for applying a charge, and is capable of applying a charge to a portion of the outer peripheral surface of the first fixing member that is upstream of the nip portion in the moving direction of the outer peripheral surface, and to the toner image on the sheet; A control unit that controls the charge applying unit; In a fixing device comprising: the control unit is capable of executing an application mode in which the application current is generated and a standby mode in which the application current is not generated, and executes the application mode during a first period from an initial startup until a predetermined period has elapsed, and executes the standby mode during a second period from the first period until a predetermined period has elapsed, The fixing device, wherein the control unit executes the application mode again after the second period has elapsed.

2. The fixing device according to claim 1 , wherein the control unit reduces the applied current during the first period as the elapsed time from the initial startup becomes longer.

3. a counting unit that counts the number of sheets that pass through the nip portion, the number being accumulated from the time of the initial startup; 3. The fixing device according to claim 1, wherein the control unit calculates the elapsed time from the initial startup based on the cumulative number of sheets passed.

4. 4. The fixing device according to claim 3, wherein when the cumulative number of sheets passed at the time when the first period has elapsed and the cumulative number of sheets passed at the time when the second period has elapsed are a first number and a second number, respectively, the second number is 14 times or more and 18 times or less of the first number.

5. 5. The fixing device according to claim 1, wherein the charge applying section has a discharge electrode and a counter electrode arranged at a distance from the outer peripheral surface of the first fixing member, and generates the applied current by generating a corona discharge between the discharge electrode and the counter electrode.

6. an image forming section disposed upstream of the nip section in a conveying direction of the sheet, the image forming section forming the toner image on the sheet; a fixing device according to claim 1 , which fixes the toner image on the sheet onto the sheet; An image forming apparatus comprising:

Images