JP2022190370A - 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 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

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 (applied current) 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 fixing device has a surface roughness detection section that detects the surface roughness of the outer peripheral surface of the first fixing member. The controller decreases the applied current as the surface roughness detected by the surface roughness detector increases.

According to the first configuration of the present invention, as the surface roughness of the outer peripheral surface of the first fixing member increases, that is, as the elapsed time from the initial start-up increases, the applied current flowing through the charge applying section decreases. Become. As a result, the burden on the outer peripheral surface of the first fixing member and the charge-applying portion is reduced, and the durability is improved. In addition, the outer peripheral surface of the first fixing member is more likely to be charged as the surface roughness increases. Therefore, even if the surface roughness of the outer peripheral surface becomes relatively rough and the applied current becomes small, the electrostatic offset phenomenon can be suppressed. 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; FIG. 3 is a block diagram showing a modified example of the control section 90 related to the fixing device 13 of the present invention;

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 with a relatively thin plate thickness (for example, about 0.1 mm). The end portion 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 adjusts the magnitude of the voltage applied to the charge applying unit 31 so that the applied current Ci becomes smaller as the surface roughness of the outer peripheral surface of the fixing belt 21 becomes rougher than when the fixing device 13 is first started. to control. Specifically, the control unit 90 controls a first period in which the surface roughness of the outer peripheral surface of the fixing belt 21 is roughened from the state at the time of the initial startup to a predetermined state, and a period from the state after the first period to the predetermined state. The applied current Ci is reduced for each period of a second period in which the state becomes rougher and a third period in which the state after the second period has passed becomes rougher.

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 or more and less than 4/7 of 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. 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, and a surface roughness calculator 35 (surface roughness detector). , at least a plurality of (here, two) I/Fs (interfaces) 96 . 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 a first number n1 and a second number n2, which are the predetermined accumulated number n of sheets. The first number n1 is the cumulative number n of sheets passed after the first period. The second number n2 is the accumulated number n of sheets passed after the second period has elapsed. In other words, the above-described first period is a period (n≦n1 period) in which the cumulative number of sheets n passed is equal to or less than the first number n1. The second period described above is 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 (a period in which n1<n≦n2). The above-described third period is a period (n2≦n) in which the cumulative number of sheets passed n is greater than that in the second period.

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. The counter 95 includes a sheet count unit 97 that accumulates and counts the number of sheets S (cumulative number of sheets passed) n passed through the fixing nip portion N, and a time count unit 98 that counts the elapsed time from the time of the initial activation. and have This elapsed period can be the sum of the times during which the rotational driving force is applied to the fixing belt 21 . The counted numbers of the passed paper counting section 97 and the time counting section 98 are stored in the temporary storage section 94 .

The surface roughness calculation section 35 includes a counter 95, a ROM 92, a RAM 93, and a temporary storage section 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 number n of sheets counted by the sheet count unit 97 and stored in the temporary storage unit 94, the first number n1 and the second number n2 stored in the ROM, the first It is calculated which of the first period, the second period, and the third period has elapsed since the start. As described above, the surface roughness of the outer peripheral surface of the fixing belt 21 becomes rougher in the first period, the second period, and the third period. to control the applied current Ci.

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.

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 control unit 90 transmits a control signal to the power supply unit 52 so that the applied current Ci of corona discharge generated in the charge imparting unit 31 is 7 μA. Then, the first voltage is applied to the charge applying section 31 (step S3).

Next, it is determined whether or not the print job has ended (step S4). If the print job has not ended (No in step S4), the applied current Ci continues to be generated. Note that if one print job includes the printing of multiple sheets, even if the cumulative number of sheets passed n exceeds the first number n1 at the end of printing for one sheet, this is applied while the print job continues. Do not change the current Ci. When it is determined that the print job has ended (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 the charge imparting section 31 is stopped (step S5), and the control of the applied current Ci ends.

If it is determined in step S2 that 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 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 applied current Ci of the corona discharge generated in the charge imparting section 31 becomes 5 μA, and the charge imparting section 31 receives the second period. A voltage is applied (step S7).

Next, it is determined whether or not the print job has ended (step S8). If the print job has not ended (No in step S8), the applied current Ci is continued. Note that if one print job includes the printing of multiple sheets, even if the cumulative number of sheets n passed exceeds the second number n2 at the end of printing for one sheet, the Do not change the current Ci. If it is determined that the print 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), and the charge imparting section 31 is stopped (step S5), and the control of the applied current Ci ends.

When it is determined in step S6 that it is not the second period (n>n2 (No in step S6)), the power supply unit 52 is controlled so that the applied current Ci of the corona discharge generated in the charge imparting section 31 is 2 μA. A signal is transmitted to apply the third voltage to the charge applying unit 31 (step S9).

Next, 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. Note that if one print job includes the printing of multiple sheets, even if the cumulative number of sheets passed n exceeds the third number n3 at the end of printing for one sheet, this is applied while the print job continues. Do not change the current Ci. 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 is stopped (step S5), and the control of the applied current Ci ends.

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 applies a predetermined first voltage to the charge applying unit 31 during the first period to apply a positive charge to the outer peripheral surface of the fixing belt 21 and the surface of the sheet S. Give. 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 (first voltage) is applied to the charge applying unit 31 during the second period, the amount of positive charge on the outer peripheral surface of the fixing belt 21 is higher than the amount of charge during the first period. growing. 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 section 90 of the fixing device 13 according to the above embodiment applies a second voltage, which is lower than the first voltage, to the charge applying section 31 . Therefore, during the second period, an increase in the amount of positive charge 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.

Similarly, the outer peripheral surface of the fixing belt 21 during the third period is more easily electrified than the outer peripheral surface during the second period. Therefore, by applying the third voltage, which is lower than the second voltage, to the charge applying section 31 during the third period, it is possible to suppress the occurrence of electrostatic scattering while suppressing the electrostatic offset phenomenon.

In addition, as the surface roughness of the outer peripheral surface of the fixing belt 21 becomes rougher, that is, as the elapsed time from the initial startup becomes longer, the applied current Ci becomes smaller. Deterioration of the outer peripheral surface of the 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, 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, 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, based on the surface roughness detected by the surface state detection mechanism 36, the control unit 90 determines the elapsed period (first period, second period, third period) from the time of the initial activation, and the charge applying unit 31 Determine the voltage to be applied to

Further, as another example of the above example, a configuration may be employed in which the ROM 92 stores the cumulative driving period from the time of initial activation as the first period and the cumulative driving period after the first period has elapsed as the second period. can also In this case, the control unit 90 determines the period (first period, second period, third period) based on the cumulative driving period counted by the time counting unit 98 and stored in the temporary storage unit 94 .

Further, as still 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 a 1st 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 35 surface roughness calculator (surface roughness detector)
36 surface condition detection mechanism (surface roughness detection unit)
90 control unit 97 sheet count unit 98 time count unit 100 image forming apparatus A cation (electric charge)
N Fixing nip portion (nip portion)
Pa to Pd Image forming unit S Paper (sheet)
n Cumulative number of sheets passed

Claims (9)

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
a surface roughness detection unit that detects the surface roughness of the outer peripheral surface of the first fixing member;
The fixing device, wherein the control section reduces the applied current as the surface roughness detected by the surface roughness detection section increases. The control unit controls a first period in which the surface roughness is roughened from the state at the time of initial startup to a predetermined state, a second period in which the surface roughness is further roughened from the state at the time of the first period to a predetermined state, and the second period. 2. The fixing device according to claim 1, wherein the applied current is reduced stepwise for each period of a third period in which the state after the lapse of two periods becomes rougher. When the applied currents after the lapse of the first period, the lapse of the second period, and the lapse of the third period are defined as a first applied current, a second applied current, and a third applied current, respectively, the 2 The 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 or more and less than 4/7 times the first applied current 3. The fixing device according to claim 2, wherein: a passing sheet counting unit that counts the cumulative number of sheets passing through the nip portion since the initial start-up, and
4. The fixing device according to claim 2, wherein the surface roughness detection section calculates the surface roughness based on the cumulative number of passed sheets. 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 claim 4, wherein the fixing device is twice or less. Having a time counting unit that measures the cumulative driving period from the time of initial startup,
4. The fixing device according to claim 1, wherein the surface roughness detection section calculates the surface roughness according to the cumulative driving period. a surface state detection mechanism capable of detecting the surface state of the outer peripheral surface of the fixing member;
4. The fixing device according to claim 1, wherein the surface roughness detection section calculates the surface roughness based on the detection result of the surface state detection mechanism. The charge applying unit 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 electric charge. The fixing device according to any one of claims 1 to 7, wherein the fixing device generates an electric current. 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 8, which fixes the toner image on the sheet onto the sheet;
An image forming apparatus comprising:

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