Fixing device, and image forming apparatus including the 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 that is conveyed along a conveyance path. The second fixing member forms, with the first fixing member, a nip part through which the sheet passes. The electric charge application unit can generate an application current for applying an electric charge and apply the 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 fixing device has a surface roughness detection unit that detects the surface roughness of the outer peripheral surface of the first fixing member. The control unit decreases the application current as the surface roughness detected by the surface roughness detection unit increases.SELECTED DRAWING: Figure 4

Description

Technical Field

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

Background Art

[0002] In an image forming apparatus, in order to fix a toner image onto a sheet, 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 in pressure contact with each other to form a fixing nip portion. A sheet such as printing paper is passed through the fixing nip portion in a state where the outer peripheral surface of the first fixing member is heated to a predetermined temperature. At this time, the outer peripheral surface of the first fixing member abuts on the surface of the sheet where 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] When such a sheet passes through the fixing nip portion, a part of the toner from the unfixed toner image may be electrostatically attached to the outer peripheral surface of the first fixing member, and an image defect called electrostatic offset phenomenon, which is retransferred to a subsequent sheet, may occur. In order to suppress the occurrence of this electrostatic offset phenomenon, Patent Document 1 discloses a fixing device including a charge applying unit.

[0004] The charge applying unit described in Patent Document 1 applies charges to the outer peripheral surface of the first fixing member and the surface of the sheet (the surface on the side in contact with the outer peripheral surface of the first fixing member and not yet in contact with the first fixing member). The charge applying unit has a plurality of electrodes, and a high voltage is applied between these electrodes to generate a corona discharge (applied current) and generate positive ions. By moving these positive ions to the sheet and the outer peripheral surface of the first fixing member, the sheet and the outer peripheral surface of the first fixing member are charged with the same polarity (positive) charges. As a result, the unfixed toner image on the sheet is less likely to adhere to the outer peripheral surface of the first fixing member.

Prior Art Documents

Patent Documents

[0005] Patent Document 1 Japanese Patent Application Laid-Open No. 2017-194499 Summary of the Invention Problems to be Solved by the Invention

[0006] By the way, the outer peripheral surface of the first fixing member as described above deteriorates over time, the surface roughness tends to become rough, and it gradually becomes easier to be charged. For this reason, if the fixing operation is continued without changing the applied voltage to the charge applying portion from the value at the first startup, the amount of positive charge on the outer peripheral surface of the first fixing member gradually increases. Then, the charge on the sheet and the charge on the outer peripheral surface of the first fixing member repel each other, and there is a risk of occurrence of an image defect in which the unfixed toner image on the sheet scatters, that is, a so-called electrostatic scattering phenomenon.

[0007] In addition, if an electric charge is applied to the outer peripheral surface of the first fixing member for a long time or a relatively high voltage is applied, the outer peripheral surface of the first fixing member and the electrodes of the charge applying portion are likely to deteriorate, and the product life of the fixing device may be shortened.

[0008] Therefore, an object of the present invention is to provide a fixing device excellent in durability while suppressing the occurrence of electrostatic offset and electrostatic scattering, and an image forming apparatus including the same. Means for Solving the Problems

[0009] 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 unit, and a control unit. The first fixing member abuts against a toner image on a sheet conveyed along a conveyance path. The second fixing member forms a nip portion through which the sheet passes between the first fixing member. The charge applying unit can generate an applied current for applying a charge and apply the charge to an upstream portion in the moving direction of the outer peripheral surface of the first fixing member with respect to the nip portion on the outer peripheral surface and the toner image on the sheet. The control unit controls the charge applying unit. The fixing device has a surface roughness detection unit that detects the surface roughness of the outer peripheral surface of the first fixing member. The control unit decreases the applied current as the surface roughness detected by the surface roughness detection unit becomes rougher.

Effects of the Invention

[0010] According to the first configuration of the present invention, as the surface roughness of the outer peripheral surface of the first fixing member becomes rougher, that is, as the elapsed time from the initial startup becomes longer, the applied current flowing through the charge applying unit becomes smaller. For this reason, the burden on the outer peripheral surface of the first fixing member and the charge applying unit becomes smaller, and the durability is improved. Further, the outer peripheral surface of the first fixing member becomes more easily charged as the surface roughness becomes rougher. For this reason, even when the surface roughness of the outer peripheral surface becomes relatively rough and the applied current becomes smaller, the electrostatic offset phenomenon can be suppressed. 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

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0012] Hereinafter, embodiments 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 image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the conveyance direction (the right side in FIG. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, yellow, and black images are sequentially formed through the steps of charging, exposure, development, and transfer, respectively.

[0013] Photoconductor drums 1a, 1b, 1c, and 1d for carrying visible images (toner images) of respective colors are arranged in these image forming units Pa to Pd. Further, an intermediate transfer belt 8 that rotates in the clockwise direction in FIG. 1 is provided adjacent to each of the image forming units Pa to Pd. The toner images formed on these photoconductor drums 1a to 1d are sequentially primary-transferred and superimposed onto the intermediate transfer belt 8 that moves while being in contact with each of the photoconductor drums 1a to 1d. Thereafter, the toner image primary-transferred onto the intermediate transfer belt 8 is secondary-transferred onto a sheet S (sheet), which is an example of a recording medium, by a secondary transfer roller 9. Further, the sheet S onto which the toner image has been secondary-transferred is discharged from the main body of the image forming apparatus 100 after the toner image is fixed by a fixing device 13. While rotating the photoconductor drums 1a to 1d in the counterclockwise direction in FIG. 1 by a main motor 40 (see FIG. 3), an image forming process for each of the photoconductor drums 1a to 1d is executed.

[0014] The paper S onto which the toner image is secondarily transferred is housed in a paper cassette 16 arranged at the lower part of the main body of the image forming apparatus 100, and is conveyed to the nip portion between the secondary transfer roller 9 and the driving roller 11 of the intermediate transfer belt 8 via a paper feed roller 12a and a resist roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is mainly used. Further, a blade-shaped belt cleaner 19 for removing toner and the like remaining on the surface of the intermediate transfer belt 8 is arranged on the downstream side of the secondary transfer roller 9.

[0015] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photosensitive drums 1a to 1d, there are charging devices 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d, an exposure device 5 for exposing the photosensitive drums 1a to 1d with image information, developing devices 3a, 3b, 3c, and 3d for forming toner images on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d for removing the developer (toner) and the like remaining on the photosensitive drums 1a to 1d.

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

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

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

[0019] The sheet S conveyed to the fixing device 13 is heated and pressed by the fixing belt 21 (first fixing member) and the pressure roller 22 (second fixing member) so that the toner image is fixed on the surface of the sheet S (see FIG. 2), and a predetermined full-color image is formed. The sheet S on which the full-color image has been formed has its conveyance direction sorted by the branching portion 14 that branches in a plurality of directions, and is then discharged to the discharge tray 17 by the discharge roller pair 15 (either as it is or after being sent to the duplex conveyance path 18 to form images on both sides).

[0020] FIG. 2 is a side cross-sectional view of the fixing device 13 mounted on the image forming apparatus 100. Note that the upper side of FIG. 2 is the downstream side in the paper insertion direction (sheet conveyance direction) with respect to the fixing device 13, and the lower side is the upstream side in the paper insertion direction with respect 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 unit 23, a nip forming member 24, a belt guide 25, a frame member 26, and a charge applying unit 31. The fixing device 13 also includes a control unit 90. The control unit 90 may be provided at any location within the image forming apparatus 100 or may be provided within the fixing device 13.

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

[0022] The fixing belt 21 has a laminated structure in which an elastic layer and a release layer are laminated on the outer peripheral side of a heat generating layer that is a base material layer. The heat generating layer is configured, for example, by a metal film such as nickel with a thickness of 30 μm to 50 μm, or a polyimide film with a thickness of 50 μm to 100 μm mixed with metal powders such as copper, silver, and aluminum. The elastic layer is configured, for example, by a silicon rubber or the like with a thickness of 100 μm to 500 μm. The release layer is configured, for example, by a fluororesin such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) with a thickness of 30 μm to 50 μm.

[0023] The pressure roller 22 is rotatably supported about a horizontal axis by a housing of the fixing device 13. The pressure roller 22 has a cylindrical shape and has substantially the same axial length (width direction length of the sheet S) as the fixing belt 21. A predetermined pressure is applied to the pressure roller 22 toward the fixing belt 21 by a pressure mechanism 30 (see FIG. 3). The outer peripheral surface of the pressure roller 22 is pressed against the nip forming member 24 via the fixing belt 21, and is brought into pressure contact with the outer peripheral surface of the fixing belt 21 to form a fixing nip portion N (nip portion).

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

[0025] The pressure roller 22 has a laminated structure in which an elastic layer 22b is laminated on the outer peripheral side 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 having a diameter of about 20 mm. The elastic layer 22b is made of, for example, silicon rubber having a thickness of about 8 mm. The release layer is made of a fluororesin such as PFA having a thickness of about 10 μm to 50 μm.

[0026] The heating unit 23 is an induction heating (IH) type heater that generates heat in the heat generating layer of the fixing belt 21 by induction heating. The heating unit 23 is disposed to face the outer peripheral surface of the fixing belt 21 with a predetermined gap in a region opposite to the side where the pressure roller 22 is disposed with respect to the fixing belt 21. The heating unit 23 extends slightly longer than the fixing belt 21 along the axial direction of the fixing belt 21 (the width direction of the sheet S, the direction perpendicular to the paper surface of FIG. 2). The temperature of the 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 and a holding member, a core, etc. (not shown). The exciting coil 23a and the core are held at predetermined positions by the holding member. The exciting coil 23a is composed of a litz wire formed by bundling a plurality of conducting wires and is wound so as to extend along the axial direction of the fixing belt 21. The exciting coil 23a is formed in an arc shape along the outer peripheral surface of the fixing belt 21 in the circumferential direction of the fixing belt 21.

[0028] The nip forming member 24 is disposed inside the fixing belt 21 opposite to the pressure roller 22 with the fixing belt 21 interposed therebetween. The nip forming member 24 contacts the inner peripheral 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 substantially rectangular parallelepiped shape extending along the axial direction of the fixing belt 21 and having 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 passed n (the cumulative number of sheets of the paper S inserted through the fixing nip portion N) increases. For this reason, the outer peripheral surface of the fixing belt 21 becomes more likely to be charged as the cumulative number of sheets passed n increases.

[0031] The belt guide 25 is disposed inside the fixing belt 21 facing the heating unit 23 with the fixing belt 21 interposed therebetween. The belt guide 25 contacts the inner peripheral surface of the fixing belt 21 other than the fixing nip portion N and supports the fixing belt 21 from the inside. The belt guide 25 is formed of a sheet metal extending along the axial direction of the fixing belt 21 and having substantially the same length as the fixing belt 21. The belt guide 25 is formed of a magnetic metal having elasticity such as SUS430 with a thickness of, for example, 0.1 mm to 0.5 mm.

[0032] The frame member 26 is supported by the 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 a substantially central portion in the radial direction of the fixing belt 21 and 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] On the upstream side (the lower side in FIG. 2) of the fixing nip portion N with respect to the paper insertion direction, a fixing entry guide 27 is disposed. The fixing entry guide 27 guides the paper S that has passed through the secondary transfer nip portion (see FIG. 1) to the fixing nip portion N.

[0034] On the downstream side of the fixing nip portion N (the upper side in FIG. 2) with respect to the paper insertion direction, a separation claw 29 is arranged. The separation claw 29 separates the paper S after the fixing process from the surface of the fixing belt 21. The separation claw 29 is arranged at a predetermined angle with its tip facing upstream (counter direction) with respect to the rotation direction of the fixing belt 21 and close to the outer peripheral surface of the fixing belt 21.

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

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

[0037] One ends of the opposed electrodes 32a, 32b and the discharge electrode 33 are fixed to the electrode holding portion 34, and the other ends face the outer peripheral surface of the fixing belt 21 with a predetermined gap therebetween. The discharge electrode 33 is a flat plate-shaped electrode made of stainless steel with a relatively thin plate thickness (for example, about 0.1 mm). The end portion of the discharge electrode 33 on the side of the fixing belt 21 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 the control unit 90 executes the application mode, a high voltage is applied to the discharge electrode 33. Then, corona discharge occurs between the serrated tip portion formed on the discharge electrode 33 and the opposed electrodes 32a, 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. A part of these positive ions A separates from the tip portion of the discharge electrode 33, and a part of them reaches 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 is positively charged. Although the amount of charge on the outer peripheral surface of the fixing belt 21 decreases over time, it becomes constant because positive charges are continuously imparted while corona discharge continues to occur.

[0040] Also, 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 and 32b and reaches the surface of the sheet S (the surface on the fixing belt 21 side) where the toner image is formed before passing through the fixing nip portion N. For this reason, the surface of the sheet S where the toner image is formed before passing through the fixing nip portion N is imparted with a positive charge and is positively charged.

[0041] The control unit 90 controls the magnitude of the voltage applied to the charge applying unit 31 so that the applied current Ci decreases as the surface roughness of the outer peripheral surface of the fixing belt 21 becomes coarser from the state at the first startup of the fixing device 13. Specifically, the control unit 90 includes a first period in which the surface roughness of the outer peripheral surface of the fixing belt 21 becomes coarser from the state at the first startup to a predetermined state, a second period in which it becomes even coarser from the state when the first period has elapsed to a predetermined state, and a third period in which it becomes even coarser from the state when the second period has elapsed, and decreases the applied current Ci for each period.

[0042] The second applied current Ci flowing in the second period is preferably 4 / 7 times or more and less than 6 / 7 times the first applied current Ci flowing in the first period. Also, the third applied current Ci flowing in the third period is preferably 1 / 7 times or more and less than 4 / 7 times the first applied current Ci. The control unit 90 controls the applied current Ci by changing the voltage applied to the charge applying unit 31 to the first voltage, the second voltage, or the third voltage. Details of the control of the applied current Ci by the control unit 90 will be described later.

[0043] Next, the control path of the image forming apparatus 100 and the fixing device 13 will be described with reference to FIG. 3. Since various controls of each part of the apparatus are performed when using the image forming apparatus 100, here, among the control paths, the parts necessary for the implementation of the present invention will be mainly described. Also, the description of the parts already described will be omitted.

[0044] FIG. 3 is a block diagram showing an example of the 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 unit 70, an operation unit 80, a control unit 90, image forming units Pa to Pd, a power supply unit 51, a power supply unit 52, and a fixing device 13.

[0045] 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 the temporary storage unit 94.

[0046] The operation unit 80 is provided with a liquid crystal display unit 81 and LEDs 82 indicating various states, and is configured to indicate the state of the image forming apparatus 100, display the image forming status and the number of printed sheets. Also, by designating a paper cassette 16 from which the paper S is fed or a manual feed tray (not shown) from the operation unit 80, the type and size of the paper S can be input. Various settings of the image forming apparatus 100 are made from the printer driver of the personal computer.

[0047] 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, a surface roughness calculation unit 35 (surface roughness detection unit), and a plurality (here, two) of 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 and the like. The I / F 96 transmits control signals to each device within the image forming apparatus 100 and receives input signals from the operation unit 80.

[0048] The ROM 92 stores data such as a control program for the image forming apparatus 100 and numerical values necessary for control that should not 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 that is temporarily required for the control of the image forming apparatus 100.

[0049] For example, the ROM 92 stores a first sheet count n1 and a second sheet count n2, which are a predetermined cumulative paper feed count n. The first sheet count n1 is the cumulative paper feed count n when the first period has elapsed. The second sheet count n2 is the cumulative paper feed count n when the second period has elapsed. In other words, the above-mentioned first period is a period during which the cumulative paper feed count n is less than or equal to the first sheet count n1 (the period n ≦ n1). The above-mentioned second period is a period during which the cumulative paper feed count n is more than the first sheet count n1 and less than or equal to a predetermined second sheet count n2 (the period n1 < n ≦ n2). The above-mentioned third period is a period during which the cumulative paper feed count n is greater than the second period (the period n2 ≦ n).

[0050] The second sheet count n2 is 14 times or more and 18 times or less the first sheet count. The third sheet count n3 is 1 / 2 times or more and 2 / 3 times or less the first sheet count. As specific numerical values for each, for example, the first sheet count n1 can be set to 500 sheets, the second sheet count n2 to 8000 sheets, and the third sheet count n3 to 500 sheets.

[0051] Further, in the ROM 92, different values are stored for the voltage applied to the charge application unit 31 according to the elapsed time since the first startup.

[0052] 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 includes a paper passage counting unit 97 that accumulatively counts the number of sheets of paper S (cumulative paper passage number) n inserted into the fixing nip portion N, and a time counting unit 98 that counts the elapsed time since the first startup. This elapsed time can be the cumulative time when the rotational driving force is applied to the fixing belt 21. The counted values of the paper passage counting unit 97 and the time counting unit 98 are stored in the temporary storage unit 94.

[0053] The surface roughness calculation unit 35 is configured to include the counter 95, the ROM 92, the RAM 93, and the temporary storage unit 94, and calculates the surface roughness of the outer peripheral surface of the fixing belt 21 from the values stored therein. Specifically, based on the cumulative paper passage number n counted by the paper passage counting unit 97 and stored in the temporary storage unit 94, the first sheet number n1 and the second sheet number n2 stored in the ROM, it is calculated which of the first period, the second period, and the third period the elapsed time since the first startup is. As described above, since the surface roughness of the outer peripheral surface of the fixing belt 21 gradually becomes rougher in the first period, the second period, and the third period, the control unit 90 controls the applied current Ci according to the period calculated by the surface roughness calculation unit 35.

[0054] The power supply unit 52 is connected to a commercial power supply (not shown) via the power supply unit 51. The power supply unit 52 distributes the power supplied from the power supply unit 51 according to the output signal from the control unit 90 to each device (including the charge application unit 31) in the image forming apparatus 100.

[0055] Next, a control example of the applied current Ci flowing through the charge application unit 31 of the fixing device 13 according to the present embodiment will be described using the flowcharts shown in FIGS. 4 to 7. FIG. 4 is a flowchart showing an example of the control flow of the fixing device 13.

[0056] As shown in FIG. 4, the control unit 90 determines whether an image formation command has been input from a host device such as a personal computer (step S1). If the image formation command is not input (No in step S1), the standby state is continued until the image formation command is input as it is.

[0057] When the image formation command is input (Yes in step S1), it is determined whether it is the above-described first period (n ≤ n1) (step S2). If it is the first period (n ≤ n1) (Yes in step S2), the control unit 90 transmits a control signal to the power supply unit 52 so that the corona discharge application current Ci generated in the charge application unit 31 becomes 7 μA, and applies a first voltage to the charge application unit 31 (step S3).

[0058] Next, it is determined whether the printing job has ended (step S4). If the printing job has not ended (No in step S4), the application current Ci is continuously generated. When a plurality of prints are included in one printing job, even if the cumulative paper feed number n exceeds the first number n1 at the end of printing for one sheet, the application current Ci is not changed during the continuation of the printing job. When it is determined that the printing job has ended (Yes in step S4), a control signal is transmitted to the power supply unit 52 so that the application current Ci becomes 0 μA (Ci = 0 μA), and the application of the voltage to the charge application unit 31 is stopped (step S5), and the control of the application current Ci is ended.

[0059] If it is determined in step S2 that it is not the first period (when n > n1 (No in step S2)), next, it is determined whether it is the second period (n ≤ n2) (step S6). If it is the second period (Yes in step S6), a control signal is transmitted to the power supply unit 52 so that the corona discharge application current Ci generated in the charge application unit 31 becomes 5 μA, and a second voltage is applied to the charge application unit 31 (step S7).

[0060] Next, it is determined whether the printing job has ended (step S8). If the printing job has not ended (No in step S8), the applied current Ci is continuously generated. Note that even if the cumulative number of sheets passed n exceeds the second number of sheets n2 at the end of printing for one sheet, the applied current Ci is not changed during the continuation of the printing job. When it is determined that the printing job has ended (Yes in step S8), a control signal is transmitted to the power supply unit 52 so that the applied current Ci becomes 0 μA (Ci = 0 μA), the application of voltage to the charge application unit 31 is stopped (step S5), and the control of the applied current Ci is terminated.

[0061] When it is determined in step S6 that it is not the second period (when n > n2 (No in step S6)), a control signal is transmitted to the power supply unit 52 so that the applied current Ci of the corona discharge generated in the charge application unit 31 becomes 2 μA, and a third voltage is applied to the charge application unit 31 (step S9).

[0062] Next, it is determined whether the printing job has ended (step S10). If the printing job has not ended (No in step S10), the applied current Ci is continuously generated. Note that even if the cumulative number of sheets passed n exceeds the third number of sheets n3 at the end of printing for one sheet, the applied current Ci is not changed during the continuation of the printing job. When it is determined that the printing job has ended (Yes in step S10), a control signal is transmitted to the power supply unit 52 so that the applied current Ci becomes 0 μA (Ci = 0 μA), the application of voltage to the charge application unit 31 is stopped (step S5), and the control of the applied current Ci is terminated.

[0063] As described above, the outer peripheral surface of the fixing belt 21 becomes more likely to be 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, it is relatively difficult to be charged, and if no countermeasures 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 applies a predetermined first voltage to the charge applying unit 31 during the first period, and imparts positive charges to the outer peripheral surface of the fixing belt 21 and the surface of the sheet S. Then, during the first period, the sheet S and the outer peripheral surface of the fixing belt 21 will be charged with the same-polarity (positive-polarity) charges. For this reason, it becomes difficult for a part of the toner to adhere to the fixing belt 21 from the unfixed toner image on the sheet S, and the occurrence of the electrostatic offset phenomenon can be suppressed.

[0064] Also, the outer peripheral surface of the fixing belt 21 during the second period is more likely to be charged than the outer peripheral surface during the first period. If a relatively high voltage (first voltage) is applied to the charge applying unit 31 during the second period, the amount of positive charges on the outer peripheral surface of the fixing belt 21 will become larger than the amount of charges during the first period. Then, there is a possibility that electrostatic scattering may occur on the surface of the sheet S to which the same-polarity (positive-polarity) charges are applied. Therefore, the control unit 90 of the fixing device 13 according to the above embodiment applies a second voltage smaller than the first voltage to the charge applying unit 31. For this reason, during the second period, an increase in the amount of positive charges on the outer peripheral surface of the fixing belt 21 is suppressed, and the occurrence of electrostatic scattering can be suppressed while suppressing the electrostatic offset phenomenon.

[0065] Similarly, the outer peripheral surface of the fixing belt 21 during the third period is more likely to be charged than the outer peripheral surface during the second period. For this reason, during the third period, by applying a third voltage smaller than the second voltage to the charge applying unit 31, the electrostatic offset phenomenon can be suppressed while suppressing the occurrence of electrostatic scattering.

[0066] Also, as the surface roughness of the outer peripheral surface of the fixing belt 21 increases, that is, as the elapsed time from the first startup becomes longer, the applied current Ci decreases, so that deterioration of the counter electrodes 32a and 32b, the discharge electrode 33, and the outer peripheral surface of the fixing belt 21 can be suppressed. As described above, it is possible to provide a fixing device 13 that is less likely to deteriorate while suppressing the occurrence of image defects.

[0067] Also, as described above, the counter electrodes 32a and 32b are arranged so as to face each other with the discharge electrode 33 interposed therebetween. Thereby, the generated corona discharge becomes stable.

[0068] In addition, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention. For example, in the above-described embodiment, the first period and the second period are defined based on the cumulative number of sheets passed n, but the elapsed period from the first startup may be calculated based on a value other than the cumulative number of sheets passed n. As an example, as shown in FIG. 5, a configuration in which the fixing device 13 includes a surface state detection mechanism 36 can be adopted. The surface state detection mechanism 36 is a mechanism including a sensor or the like capable of detecting the surface roughness of the outer peripheral surface of the fixing belt 21. In this case, the control unit 90 determines the elapsed period (first period, second period, third period) from the first startup based on the surface roughness detected by the surface state detection mechanism 36, and determines the voltage applied to the charge application unit 31.

[0069] Also, as another example of the above example, a configuration in which the cumulative drive period from the first startup is stored in the ROM 92 as the first period and the cumulative drive period from the end of the first period is stored as the second period can also be adopted. In this case, the control unit 90 determines the period (first period, second period, third period) based on the cumulative drive period counted by the time count unit 98 and stored in the temporary storage unit 94.

[0070] Also, as yet another example of the above example, a maintenance worker may periodically measure the surface roughness of the fixing belt 21 and input the measured value. Then, the control unit 90 may calculate the first period and the second period according to the measured value. Alternatively, the control unit 90 may directly determine the voltage applied to the charge application unit 31 according to the measured value without calculating the first period and the second period. In this case, the ROM 92 stores the magnitude of the voltage applied to the charge application unit 31 according to the measured value of the input surface roughness.

[0071] In each of the above embodiments, the fixing device 13 of the sliding belt type in which the endless fixing belt 21 slides with respect to the nip forming member 24 as the heated rotating body has been exemplified. However, for example, it can be similarly applied to a single-axis belt type in which the fixing belt 21 is wound around a fixing roller, a two-axis belt type fixing device in which the fixing belt 21 is stretched between a fixing roller and a heating roller, or a fixing device including a heated rotating body other than the fixing belt 21.

Industrial Applicability

[0072] The present invention can be used in an image forming apparatus including a fixing device that inserts a recording medium through a fixing nip portion formed by a heated rotating body and a pressure member, applies heat and pressure to a toner image, and melts and fixes the toner image on the recording medium. By using the present invention, it is possible to provide an image forming apparatus that suppresses the occurrence of image defects and has a relatively long product life.

Explanation of Signs

[0073] 13 Fixing device 21 Fixing belt (first fixing member) 22 Pressing roller (second fixing member) 31 Charge application unit 32a, 32b Opposing electrodes 33 Discharge electrode 35 Surface roughness calculation unit (surface roughness detection unit) 36 Surface state detection mechanism (surface roughness detection unit) 90 Control unit 97 Sheet passing count unit 98-hour counting unit 100 image forming apparatus A cation (charge) N fixing nip section (nip section) Pa~Pd image forming section S paper (sheet) n cumulative number of sheets passed

Claims

1. A first fixing member that contacts a toner image on a sheet conveyed along a conveyance path, A second fixing member that forms a nip portion through which the sheet passes between the first fixing member, A charge applying unit that generates an applied current for applying a charge and can apply charges to an upstream portion of the outer peripheral surface of the first fixing member in the moving direction of the outer peripheral surface with respect to the nip portion and the toner image on the sheet, A control unit that controls the charge applying unit, Comprising, In the fixing device that applies a rotational driving force to the first fixing member by rotating the second fixing member while pressing the first fixing member and fixes the toner image on the sheet passing through the nip portion, The control unit, A detection unit that detects at least one value of the cumulative number of sheets passed, which is the cumulative number of sheets passed since the first start-up of the number of sheets passed through the nip portion by the sheet, the surface roughness of the outer peripheral surface of the first fixing member, and the cumulative driving period, which is the cumulative period since the first start-up of the period during which the rotational driving force is applied to the first fixing member, A calculation unit that calculates the elapsed period since the first start-up based on the value detected by the detection unit, Having, The control unit controls the charge applying unit so that the applied current decreases as the elapsed period calculated by the calculation unit increases. The fixing device is characterized by this.

2. The calculation unit calculates, based on the value, which of the first period in which the elapsed period increases from the value at the first start-up to a predetermined value, the second period in which the value further increases from the value at the end of the first period to a predetermined value, and the third period in which the value further increases from the value at the end of the second period the elapsed period is, The control unit controls the charge applying unit according to the calculation result calculated by the calculation unit so that the applied current decreases step by step in the order of the first period, the second period, and the third period. The fixing device according to claim 1 is characterized by this.

3. When the applied currents at the end of the first period, the end of the second period, and the end of the third period are the first applied current, the second applied current, and the third applied current, respectively, the second applied current is 4 / 7 times or more and less than 6 / 7 times the first applied current, and the third applied current is 1 / 7 times or more and less than 4 / 7 times the first applied current. The fixing device according to claim 2 is characterized by this.

4. The detection unit includes a paper passage counting unit that counts the cumulative number of sheets passed, and the value is the count number of the paper passage counting unit. The control unit further includes a storage unit that stores each preset number of sheets for each of the first period, the second period, and the third period. Each of the preset numbers of sheets is set to increase in the order of the first period, the second period, and the third period. The calculation unit determines whether the value has reached each of the preset numbers of sheets stored in the storage unit, and calculates in which of the first period, the second period, and the third period the elapsed period is. The fixing device according to claim 2 or 3.

5. When the preset numbers of sheets at the end of the first period and the end of the second period are the first number of sheets and the second number of sheets, respectively, the second number of sheets is 14 times or more and 18 times or less the first number of sheets. The fixing device according to claim 4, characterized in that.

6. The detection unit includes a time counting unit that counts the cumulative drive period, and the value is the count number of the time counting unit. The control unit further includes a storage unit that stores each preset period for each of the first period, the second period, and the third period. The calculation unit determines whether the value has reached each of the preset periods stored in the storage unit, and calculates in which of the first period, the second period, and the third period the elapsed period is. The fixing device according to claim 2 or 3, characterized in that.

7. The detection unit includes a surface state detection mechanism capable of detecting the surface roughness, and the value is the surface roughness detected by the surface state detection mechanism. The control unit further includes a storage unit that stores each preset roughness for each of the first period, the second period, and the third period. The calculation unit determines whether the value has reached each of the preset roughnesses stored in the storage unit, and calculates in which of the first period, the second period, and the third period the elapsed period is. The fixing device according to claim 2 or 3, characterized in that.

8. The charge applying unit includes a discharge electrode and a counter electrode arranged apart from the outer peripheral surface of the first fixing member, and generates a corona discharge between the discharge electrode and the counter electrode to generate the applied current. The fixing device according to any one of claims 1 to 7.

9. An image forming unit disposed upstream of the nip portion in the sheet conveyance direction, for forming the toner image on the sheet; A fixing device according to any one of claims 1 to 8, for fixing the toner image on the sheet onto the sheet; An image forming apparatus comprising the above.

Images