JP2022190369A - Fixing device, and image forming apparatus including the same - Google Patents

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

The present invention relates to a fixing device and an image forming apparatus having the fixing device.

2. Description of the Related Art 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 to fix a toner image on a sheet in an image forming apparatus. The first fixing member and the second fixing member are pressed against each other to form a fixing nip portion. A sheet such as a printing paper is passed through the fixing nip while 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 contacts the surface of the sheet on which the toner image is formed. In this manner, heat and pressure are applied to the toner image formed on the sheet to melt and fix the toner image on the sheet.

In such a fixing device, when the sheet passes through the fixing nip portion, part of the toner from the unfixed toner image electrostatically adheres to the outer peripheral surface of the first fixing member, and is retransferred to the subsequent sheet. image defects, a so-called electrostatic offset phenomenon, may occur. In order to suppress the occurrence of the electrostatic offset phenomenon, Japanese Patent Application Laid-Open No. 2002-100001 discloses a fixing device having a charge applying section.

The charge imparting portion described in Patent Document 1 includes the outer peripheral surface of the first fixing member and the surface of the sheet (the surface on the side that contacts the outer peripheral surface of the first fixing member, which is not yet in contact with the first fixing member). (the part where there is no charge) and charge. The charge applying section has a plurality of electrodes, and applies a high voltage between the electrodes to generate corona discharge and generate positive ions. As the positive ions move to the outer peripheral surfaces of the sheet and the first fixing member, the sheet and the outer peripheral surface of the first fixing member are charged with the same polarity (positive). This makes it difficult for the unfixed toner image on the sheet to adhere to the outer peripheral surface of the first fixing member.

JP 2017-194499 A

By the way, the outer peripheral surface of the first fixing member as described above tends to deteriorate with time, and the surface roughness tends to become rough, so that it is gradually easily charged. Therefore, if the fixing operation is continued without changing the voltage applied to the charge applying section from the value at the time of the initial startup, the amount of positive charge on the outer peripheral surface of the first fixing member gradually increases. As a result, the charge on the sheet and the charge on the outer peripheral surface of the first fixing member repel each other, and an image defect caused by scattering of the unfixed toner image on the sheet, that is, a so-called electrostatic scattering phenomenon, may occur.

In addition, when the outer peripheral surface of the first fixing member is charged for a long time or a relatively high voltage is applied, the outer peripheral surface of the first fixing member and the electrode of the charge applying portion are likely to deteriorate, and the fixing device is deteriorated. Product life may be shortened.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a fixing device excellent in durability while suppressing the occurrence of electrostatic offset and electrostatic scattering, and an image forming apparatus having the same.

A first configuration of the present invention to achieve the above object 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 the toner image on the sheet conveyed along the conveying path. The second fixing member forms a nip portion through which the sheet passes with the first fixing member. The charge imparting section generates an applied current for imparting charge, and charges the toner image on the sheet and the portion of the outer peripheral surface of the outer peripheral surface of the first fixing member on the upstream side in the moving direction of the nip portion. Grantable. The control section controls the charge applying section. The control unit 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 until a predetermined period elapses from the time of initial startup, The standby mode is executed during the second period from the time when the first period elapses until the predetermined period elapses.

According to the first configuration of the present invention, the controller executes the application mode during the first period in which the outer peripheral surface of the first fixing member is relatively difficult to be charged, so that the occurrence of the electrostatic offset phenomenon can be suppressed. can. Moreover, since the standby mode is executed in the second period when the predetermined period has passed since the initial start-up and the outer peripheral surface of the first fixing member is relatively easily charged, electrostatic scattering can be suppressed. Further, since no applied current is generated during the second period, deterioration of the outer peripheral surface of the first fixing member and the charge applying portion is less likely to occur. Therefore, it is possible to provide a fixing device that is resistant to deterioration while suppressing the occurrence of image defects.

Schematic cross-sectional view showing the internal structure of the image forming apparatus 100 according to the embodiment of the present invention. Side cross-sectional view of fixing device 13 mounted in image forming apparatus 100 FIG. 2 is a block diagram showing an example of a control path of the image forming apparatus 100; 3 is a flow chart showing an example of a control flow of the fixing device 13; Flowchart showing an example of the control flow of the fixing device 13 when executing the first application mode 3 is a flow chart showing an example of the control flow of the fixing device 13 during execution of the standby mode; Flowchart showing an example of the control flow of the fixing device 13 during execution of the second application mode

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 invention. Four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the transport direction (the right side in FIG. 1) in the main body of the image forming apparatus 100 (here, a color printer). These image forming units Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow and black). and black images are sequentially formed.

Photoreceptor drums 1a, 1b, 1c and 1d carrying visible images (toner images) of respective colors are disposed in these image forming portions Pa to Pd. Further, an intermediate transfer belt 8 that rotates clockwise in FIG. 1 is provided adjacent to each of the image forming portions Pa to Pd. The toner images formed on these photoreceptor drums 1a to 1d are sequentially primary-transferred and superimposed on an intermediate transfer belt 8 that moves in contact with each of the photoreceptor drums 1a to 1d. After that, the toner image primarily transferred onto the intermediate transfer belt 8 is secondarily transferred onto a sheet S (sheet) as an example of a recording medium by a secondary transfer roller 9 . Further, the sheet S on which the toner image has been secondarily transferred is ejected from the main body of the image forming apparatus 100 after the toner image is fixed by the fixing device 13 . An image forming process for each of the photosensitive drums 1a to 1d is performed while the photosensitive drums 1a to 1d are rotated counterclockwise in FIG. 1 by the main motor 40 (see FIG. 3).

The paper S onto which the toner image is to be secondarily transferred is accommodated in a paper cassette 16 arranged at the bottom of the main body of the image forming apparatus 100, and is transferred to the secondary transfer roller 9 via a paper feed roller 12a and a pair of registration rollers 12b. and the driving roller 11 of the intermediate transfer belt 8 . A dielectric resin sheet 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 .

Next, the image forming units Pa to Pd will be described. Charging devices 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d and image information on the respective photosensitive drums 1a to 1d are provided around and below the rotatably arranged photosensitive drums 1a to 1d. , developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and removing developer (toner) remaining on the photosensitive drums 1a to 1d. Cleaning devices 7a, 7b, 7c and 7d are provided for cleaning.

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

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

The intermediate transfer belt 8 is stretched between a driven roller 10 on the upstream side and a drive roller 11 on the downstream side. When the rotation in the circumferential direction is started, the sheet S is transported from the pair of registration rollers 12b to the nip portion (secondary transfer nip portion) between the drive roller 11 and the secondary transfer roller 9 provided adjacent thereto at a predetermined timing. Then, the full-color image on the intermediate transfer belt 8 is secondarily transferred onto the paper S. The sheet S on which the toner image has been secondarily transferred is conveyed to the fixing device 13 .

The sheet S conveyed to the fixing device 13 is heated and pressed by a fixing belt 21 (first fixing member) and a pressure roller 22 (second fixing member) to fix the toner image on the surface of the sheet S (see FIG. 2), a desired full-color image is formed. The sheet S on which the full-color image is formed is divided in the conveying direction by the branching unit 14 branching in a plurality of directions, and is sent to the double-sided conveying path 18 as it is (or after the images are formed on both sides thereof), and is sent to the pair of discharge rollers. 15 to the ejection tray 17 .

FIG. 2 is a side sectional view of the fixing device 13 mounted in the image forming apparatus 100. As shown in FIG. Note that the upper side of FIG. 2 is the downstream side of the fixing device 13 in the paper insertion direction (sheet conveying direction), and the lower side is the upstream side of the fixing device 13 in the paper insertion direction. 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, as shown in FIG. The fixing device 13 also includes a control section 90 . The control unit 90 may be provided at an arbitrary location within the image forming apparatus 100 or may be provided within the fixing device 13 .

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

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 which is a base layer. The heat generating layer is composed of, for example, 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 mixed with metal powder such as copper, silver or aluminum. The elastic layer is made of silicon rubber or the like with a thickness of 100 μm to 500 μm, for example. 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.

The pressure roller 22 is rotatably supported by the housing of the fixing device 13 about a horizontal axis. The pressure roller 22 has a cylindrical shape and has substantially the same length in the axial direction (length in the width direction of the paper 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). By pressing the nip forming member 24 through the fixing belt 21, the outer peripheral surface of the pressure roller 22 is brought into pressure contact with the outer peripheral surface of the fixing belt 21 to form a fixing nip portion N (nip portion).

The pressure roller 22 is connected to a fixing drive motor 45 (see FIG. 3) and rotates counterclockwise in FIG. The pressure roller 22 comes into contact with the outer peripheral surface of the fixing belt 21 and imparts clockwise rotational driving force to the fixing belt 21 .

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 cored bar 22a is made of metal such as aluminum having a diameter of about 20 mm, for example. The elastic layer 22b is made of silicon rubber or the like having a thickness of about 8 mm, for example. The release layer is made of a fluorine-based resin such as PFA with a thickness of, for example, about 10 μm to 50 μm.

The heating unit 23 is an induction heating (IH) type heater that heats the heat generating layer of the fixing belt 21 by induction heating. The heating unit 23 is arranged opposite to 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 arranged with respect to the fixing belt 21 . The heating portion 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).

The heating section 23 heats the fixing belt 21 . It includes an exciting coil 23a, a holding member, a core, and the like (not shown). The exciting coil 23a and the core are held at predetermined positions by a holding member. The exciting coil 23 a is composed of a litz wire in which a plurality of conductive wires are bundled, and is wound so as to extend along the axial direction of the fixing belt 21 . The excitation coil 23 a 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 .

The nip forming member 24 is arranged inside the fixing belt 21 so as to face 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 .

The nip forming member 24 has a substantially rectangular parallelepiped shape extending 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, for example, metal such as aluminum or heat-resistant resin such as liquid crystal polymer.

The surface roughness of the outer peripheral surface of the fixing belt 21 becomes rougher as the accumulated number of sheets n (total number of sheets S passed through the fixing nip portion N) increases. Therefore, the outer peripheral surface of the fixing belt 21 is more likely to be charged as the cumulative number of sheets n increases.

The belt guide 25 is arranged inside the fixing belt 21 so as to face the heating section 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 made 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 made of an elastic magnetic metal such as SUS430 with a thickness of 0.1 mm to 0.5 mm.

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 substantially at the radial center 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 .

A fixing entry guide 27 is arranged on the upstream side (lower side in FIG. 2) of the fixing nip portion N with respect to the paper insertion direction. The fixing entrance guide 27 guides the sheet S, which has passed through the secondary transfer nip portion (see FIG. 1), to the fixing nip portion N. As shown in FIG.

A separation claw 29 is arranged downstream of the fixing nip portion N (upper side in FIG. 2) with respect to the paper insertion direction. The separation claw 29 separates the sheet S after the fixing process from the surface of the fixing belt 21 . Separation claw 29 is arranged at a predetermined angle so that the tip is directed upstream (in the counter direction) with respect to the rotation direction of fixing belt 21 and the tip is close to the outer peripheral surface of fixing belt 21 .

The charge imparting section 31 is arranged upstream of the fixing belt 21 and the pressure roller 22 with respect to the paper insertion direction. The charge imparting unit 31 imparts a positive charge to the outer peripheral surface of the fixing belt 21 and the surface of the sheet S on which the toner image is formed.

The charge applying section 31 includes counter electrodes 32 a and 32 b , a discharge electrode 33 , and an electrode holding section 34 . The counter electrodes 32a and 32b are electrodes facing each other with the discharge electrode 33 interposed therebetween. The electrode holding portion 34 is supported by the housing portion of the fixing device 13 and holds the counter electrodes 32 a and 32 b and the discharge electrode 33 .

One ends of the counter electrodes 32a and 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 plate-like electrode made of stainless steel and having 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.

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

When corona discharge occurs, positive ions A are generated around the tip of the discharge electrode 33 . Some of the cations A leave the tip portion of the discharge electrode 33 and some of them reach the outer peripheral surface of the fixing belt 21 . As a result, the outer peripheral surface of the fixing belt 21 is positively charged, and the fixing belt 21 is positively charged. Although the amount of charge on the outer peripheral surface of the fixing belt 21 decreases with time, it remains constant because positive charges continue to be applied while corona discharge continues to occur.

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 forms a toner image on the sheet S before passing through the fixing nip portion N. It reaches the surface (the surface on the fixing belt 21 side). Therefore, the surface of the sheet S on which the toner image is formed before passing through the fixing nip portion N is given a positive charge and is positively charged.

The control unit 90 executes the application mode (first application mode) during a first period after a predetermined period of time has passed, and waits to stop the applied current Ci during a second period from the time when the first period has passed until the predetermined period of time has passed. run mode. Also, after the second period has elapsed, the control unit 90 executes the application mode (second application mode) again. During the first period, the control unit 90 controls the voltage to be applied to the charge applying unit 31 so that the applied current Ci becomes smaller as the elapsed time from the initial activation of the fixing device 13 becomes longer. Also, the control unit 90 controls the voltage applied to the charge applying unit 31 so that the applied current Ci in the second application mode is smaller than the applied current Ci in the first application mode. The details of the control of the applied current Ci by the controller 90 will be described later.

Next, control paths of the image forming apparatus 100 and the fixing device 13 will be described with reference to FIG. Since various parts of the apparatus are controlled when the image forming apparatus 100 is used, a part of the control path necessary for carrying out the present invention will be mainly described here. Further, the description of the already described parts will be omitted.

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

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 section 70 is converted into a digital signal and sent to the temporary storage section 94 .

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

The control unit 90 includes 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), a plurality of (here, two) I A /F (interface) 96 is provided at least. CPU91 functions as a central processing unit. The ROM 92 is a read-only storage unit. The RAM 93 is a readable/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 in the image forming apparatus 100 and receives input signals from the operation unit 80 .

The ROM 92 stores a program for controlling the image forming apparatus 100 and data such as numerical values necessary for control that are not changed while the image forming apparatus 100 is in use. The RAM 93 stores necessary data generated during control of the image forming apparatus 100, data temporarily necessary for controlling the image forming apparatus 100, and the like.

For example, the ROM 92 stores the length of the period for the first period from the time of initial activation until the predetermined period has passed. Specifically, the first period is stored as a period (n≦n1 period) in which the cumulative number of sheets n passed since the initial startup is equal to or less than a predetermined first number n1. Also, the length of the second period from the time when the first period elapses until the predetermined period elapses is stored. Specifically, the second period is stored as a period in which the cumulative number of sheets n passed is greater than the first number n1 and equal to or less than a predetermined second number n2 (period where n1<n≦n2). Also, the control unit 90 controls the voltage at a predetermined timing during the first period. This timing is stored as the timing when the cumulative number n of sheets passed is equal to or greater than the third number n3.

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

In addition, the ROM 92 stores different values for the voltage to be applied to the charge imparting section 31 according to the elapsed time from the initial start-up.

The temporary storage unit 94 temporarily stores the image signal input from the image input unit 70 and converted into a digital signal. A counter 95 (counting section) accumulates and counts the number of sheets S inserted into the fixing nip section N (accumulated number of sheets passed) n.

The power supply unit 52 is connected to a commercial power source (not shown) via the power supply section 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 applying unit 31 ) in the image forming apparatus 100 .

Next, an example of control of the applied current Ci flowing through the charge applying section 31 of the fixing device 13 of this embodiment will be described with reference to flowcharts shown in FIGS. 4 to 7. FIG. FIG. 4 is a flowchart showing an example of the control flow of the fixing device 13. As shown in FIG. FIG. 5 is a flow chart showing an example of the control flow of the fixing device 13 when executing the first application mode. FIG. 6 is a flow chart showing an example of the control flow of the fixing device 13 during execution of the standby mode. FIG. 7 is a flow chart showing an example of the control flow of the fixing device 13 during execution of the second application mode.

As shown in FIG. 4, the control unit 90 determines whether or not an image forming command has been input from a host device such as a personal computer (step S1). If the image forming command is not input (No in step S1), the standby state continues until the image forming command is input. When an image forming command is input (Yes in step S1), it is determined whether or not it is the first period (n≦n1) (step S2). If it is the first period (n≦n1) (Yes in step S2), the controller 90 executes the first application mode (step S3). The details of the control during execution of the first application mode will be described later.

If it is not the first period (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 and the second period (n>n2 (No in step S4)), the second application mode is executed (step S6). Details of the control during execution of the standby mode and the second application mode will be described later.

Next, an example of control during execution of the first application mode will be described with reference to FIG. If the first application mode is to be executed (Yes in step S2), first, it is determined whether or not the cumulative number of sheets n passed is equal to or less than the third number n3 (n≦n3) (step S7). When the cumulative number of sheets n passed is less than or equal to the third number n3 (Yes in step S7), 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 imparting section 31 becomes 7 μA. , the first voltage is applied to the charge applying unit 31 (step S8).

When the cumulative number of sheets n passed is larger than the third number n3 (No in step S7), 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 imparting section 31 is 5 μA. A second voltage is applied to the charge applying unit 31 (step S8).

After that, it is determined whether or not the print job has ended (step S10). If the print job has not ended (No in step S10), the applied current Ci is continuously generated. When it is determined that the print 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), and the charge imparting section 31 (step S11), and the control of the applied current Ci ends. Note that when one print job includes printing a plurality of sheets, even if the accumulated number of printed sheets n exceeds the third number of sheets n3 at the end of printing for one sheet, the current applied during the continuation of the print job Do not change Ci.

Next, an example of control when the standby mode is executed will be described with reference to FIG. When the standby mode is to be executed (Yes in step S4), a control signal is transmitted to the power supply unit 52 so that the applied current Ci becomes 0 μA (Ci=0 μA), and a state in which no voltage is applied to the charge applying section 31 is set. (Step S12), and continue to do so until the print job ends (No in Step S13). When the print job ends (Yes in step S13), the control of the applied current Ci ends.

Next, an example of control when executing the second application mode will be described with reference to FIG. When the standby second application mode is to be executed (No in step S4), a control signal is transmitted to the power supply unit 52 so that the applied current Ci becomes 2 μA (Ci=2 μA), and the third voltage is applied to the charge imparting section 31. is applied.

Thereafter, it is determined whether or not the print job has ended (step 15). If the print job has not ended (No in step S15), the voltage described above continues to be applied to continuously generate the applied current Ci. If it is determined that the print job has ended (Yes in step S15), the voltage application to the charge applying unit 31 is stopped so that the applied current Ci becomes 0 μA (Ci=0 μA) (step S16). , terminates the control of the applied current Ci.

As described above, the outer peripheral surface of the fixing belt 21 is more likely to be charged as the cumulative number of sheets n increases. In other words, charging is relatively difficult during the first period, which is a predetermined period from the time of the initial start-up, and the electrostatic offset phenomenon is likely to occur if no countermeasures are taken. Therefore, the control unit 90 of the fixing device 13 according to the above-described embodiment executes the application mode during the first period to apply positive charges to the outer peripheral surface of the fixing belt 21 and the surface of the sheet S. FIG. Then, during the first period, the sheet S and the outer peripheral surface of the fixing belt 21 are charged with the same polarity (positive polarity). Therefore, it becomes difficult for a part of the toner from the unfixed toner image on the paper S to adhere to the fixing belt 21, and the occurrence of the electrostatic offset phenomenon can be suppressed.

Further, the outer peripheral surface of the fixing belt 21 during the second period is more easily electrified than the outer peripheral surface during the first period. If a relatively high voltage is applied to the charge imparting section 31 during the second period, the amount of positive charge on the outer peripheral surface of the fixing belt 21 increases. Then, electrostatic scattering may occur on the surface of the sheet S to which charges of the same polarity (positive polarity) are applied. Therefore, the control unit 90 of the fixing device 13 according to the above embodiment executes the standby mode during the second period. Therefore, during the second period, the amount of positive charge on the outer peripheral surface of the fixing belt 21 does not increase. Therefore, it is possible to suppress the occurrence of electrostatic scattering.

Further, as described above, the outer peripheral surface of the fixing belt 21 is easily charged during the second period. For this reason, the positive charge that is charged on the outer peripheral surface of the fixing belt 21 during the first period is likely to be charged on the outer peripheral surface as it is. Therefore, even if the standby mode is executed during the second period, the toner on the sheet 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 in the second period.

Further, since the applied current Ci is not generated during the second period, 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 the fixing device 13 that is resistant to deterioration while suppressing the occurrence of image defects.

Further, as described above, even during execution of the application mode, the voltage applied to the charge applying unit 31 is decreased as the elapsed time from the initial start-up increases. Specifically, the voltage applied to the charge imparting unit 31 is changed so that the current generated in the charge imparting unit 31 decreases from 7 μA to 5 μA at the timing when the cumulative number n of sheets passed exceeds the third number n3 during the first period. making it smaller. By doing so, 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.

Further, as described above, the standby mode is executed during the second period, so the charge amount on the outer circumferential surface of the fixing belt 21 decreases with 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. In addition, the outer peripheral surface of the fixing belt 21 after the second period is more easily electrified than the outer peripheral surface during the first period and the second period. 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, image defects can be suppressed while suppressing deterioration of the charge applying section 31 and the fixing belt 21 .

Further, as described above, the counter electrodes 32a and 32b are arranged to face each other with the discharge electrode 33 interposed therebetween. This stabilizes the generated corona discharge.

In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope 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 n passed. It may be calculated by As an example, the counting unit can count and store the cumulative time (cumulative driving time) during which the rotational driving force is applied to the pressure roller 22 . In this case, the ROM 92 stores the accumulated driving time in the first period and the accumulated driving time in the second period. The control unit 90 calculates the elapsed period from the initial activation based on the cumulative driving time stored in the counting unit and the value of the cumulative driving time for each period stored in the ROM 92 .

As another example of the above example, the maintenance worker periodically measures the surface roughness of the fixing belt 21 and inputs the measured value. You may make it calculate 1 period and a 2nd period. Alternatively, the control unit 90 may directly determine the voltage to be applied to the charge applying 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 applying section 31 according to the input measured value of surface roughness.

Further, in each of the above-described embodiments, the fixing device 13 of the sliding belt type in which the endless fixing belt 21 slides against the nip forming member 24 as the heated rotating body was exemplified. The same applies to a uniaxial belt type fixing device in which the fixing belt 21 is wound around a roller, a biaxial belt type fixing device in which the fixing belt 21 is stretched between a fixing roller and a heating roller, or a fixing device equipped with a heated rotator other than the fixing belt 21. Of course, it can be applied to

The present invention includes a fixing device for inserting a recording medium into a fixing nip portion formed by a heated rotating body and a pressure member, applying heat and pressure to the toner image, and fusing and fixing the toner image on the recording medium. It can be used for an image forming apparatus. By using the present invention, it is possible to provide an image forming apparatus with a relatively long product life while suppressing the occurrence of image defects.

13 fixing device 21 fixing belt (first fixing member)
22 pressure roller (second fixing member)
31 charge applying unit 32a, 32b counter electrode 33 discharge electrode 90 control unit 95 counter (counting unit)
100 Image forming apparatus A Cation (electric charge)
Ci Applied current N Fixing nip portion (nip portion)
Pa to Pd Image forming unit S Paper (sheet)
n Cumulative number of sheets passed

Claims (7)

a first fixing member that contacts the toner image on the sheet conveyed along the conveying path;
a second fixing member forming a nip portion through which the sheet passes between itself and the first fixing member;
An applied current for imparting electric charge is generated to apply an electric charge to a portion of the outer peripheral surface of the first fixing member on the upstream side in the moving direction of the nip portion and to the toner image on the sheet. a charge imparting unit capable of imparting;
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 executing the standby mode during a second period from when the first period elapses to when a predetermined period elapses. 2. The fixing device according to claim 1, wherein, during the first period, the control section reduces the applied current as the elapsed period from the initial start-up increases. 3. The fixing device according to claim 1, wherein the controller executes the application mode again after the second period has elapsed. a counting unit that counts the cumulative number of sheets passed through the nip portion since the initial start-up;
The fixing device according to any one of claims 1 to 3, wherein the control section calculates the elapsed time from the initial startup based on the cumulative number of sheets passed. When the cumulative number of sheets passed after the first period has passed and the cumulative number of sheets passed after the second period has passed are respectively the first number and the second number, the second number is 14 times or more and 18 times the first number. 5. The fixing device according to any one of claims 1 to 4, wherein the fixing device is twice or less. The charge applying section has a discharge electrode and a counter electrode spaced 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. 6. The fixing device according to any one of claims 1 to 5, wherein the fixing device generates an image forming unit arranged upstream of the nip portion in the conveying direction of the sheet and forming the toner image on the sheet;
The fixing device according to any one of claims 1 to 6, which fixes the toner image on the sheet onto the sheet;
An image forming apparatus comprising:

Images