JP2023146120A - Fixing device and image forming apparatus including the same - Google Patents

Abstract

To provide a fixing device that includes separation pawls, and that can prevent an image defect while preventing an increase in manufacturing cost.SOLUTION: A fixing device comprises a rotating body to be heated, a pressure rotating body, a heating unit, separation pawls, and an electric charge application unit. The pressure rotating body forms a fixing nip part. The separation pawls each have a contact part in contact with an outer peripheral surface of the rotating body to be heated. The electric charge application unit has a plurality of electrifying parts that project toward the outer peripheral surface and are arranged side by side parallel to the direction of the axis of rotation of the rotating body to be heated. The separation pawls each have an electric charge shield part that is arranged between leading ends of the electrifying parts and the outer peripheral surface, and blocks electric charges emitted from the electrifying parts to the outer peripheral surface.SELECTED DRAWING: Figure 3

Description

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

In an image forming apparatus, in order to fix a toner image on a sheet (recording medium such as printing paper, envelope, OHP sheet, etc.), a fixing roller or fixing belt (rotating body to be heated) and a pressure roller (rotating pressure body) are used. Fixing devices that press into contact to form a fixing nip are widely used. In this fixing device, a sheet is passed through a fixing nip portion, and heat and pressure are applied to the toner image to melt and fix the toner image onto the sheet.

Some fixing devices as described above include a separating claw and a charge applying section. The separation claw is interposed between the sheet that has passed through the fixing nip and the fixing roller. The separation claw has its tip slidably in contact with the fixing roller, and separates the sheet that has passed through the fixing nip from the fixing roller.

When the sheet passes through the fixing nip, toner components on the sheet adhere to the fixing roller, and the surface resistance of the outer circumferential surface (hereinafter simply referred to as "outer circumferential surface") of the fixing roller increases, causing the outer circumferential surface to become a negative electrode. may be charged to the opposite polarity (opposite to that of the toner). In this case, an unfixed toner image is transferred from the sheet to the fixing roller, and there is a risk that an image defect (so-called electrostatic offset phenomenon) may occur where the toner image is retransferred to a subsequent sheet. Therefore, a charge of positive polarity (same polarity as the toner) is applied to the outer circumferential surface by the charge applying section. Then, the charged polarity of the outer circumferential surface is canceled out, the outer circumferential surface is neutralized, and it is possible to suppress the electrostatic offset phenomenon from occurring.

Incidentally, even if toner components adhere to the portions of the outer peripheral surface that come into contact with the separation claws, they are scraped off by the separation claws. Therefore, the scraped portion has a smaller amount of negative charge as described above than other portions (portions to which the toner component remains attached). If a positive charge is applied to this scraped portion from the charge applying section as described above, this portion will instead be significantly charged to a positive polarity. Therefore, electrostatic scattering occurs in which the toner on the sheet scatters, and negatively charged paper powder is attracted and adheres to the fixing roller, which may lead to image defects.

To address this problem, there are fixing devices that employ a configuration for suppressing the above-described image defects (Patent Document 1 and Patent Document 2). The fixing device of Patent Document 1 includes a first charge applying section that applies a charge to a portion of the outer circumferential surface that contacts the separation claw, and a second charge applying portion that applies a charge to a portion of the outer circumferential surface that does not come into contact with the separation claw. We are prepared. By separately adjusting the applied current of the first charge applying section and the applied current of the second charge applying section, the potential of the outer circumferential surface is made uniform and the above-mentioned image defects are suppressed.

Further, the charge applying section according to the fixing device of Patent Document 2 has a plurality of separation needles arranged in the direction of the rotation axis of the fixing roller. The separation needle is arranged so as to avoid a position overlapping the separation claw in the direction of the rotation axis. The separation needle protrudes toward the outer peripheral surface. The charge applying section applies a charge to the outer circumferential surface of the separation needle from the tip thereof. As described above, since the separation needle is not arranged at a position overlapping the separation claw in the direction of the rotation axis, no electric charge is applied to the portion of the outer peripheral surface where the separation needle contacts the separation claw. This suppresses excessive positive charging of the portions of the outer peripheral surface that come into contact with the separation claws, thereby making it possible to suppress image defects.

Japanese Patent Application Publication No. 2008-65235 Japanese Patent Application Publication No. 2004-354778

However, in the fixing roller of Patent Document 1, it is necessary to separately control the first charge applying section and the second charge applying section, which may complicate the configuration and current control and increase manufacturing costs. Further, the above-mentioned separation claw is supported with enough space so that it can be moved slightly in the direction of the rotation axis. For this reason, in the fixing roller of Patent Document 2, there is a risk that the separation claw may shift to a position where it overlaps with the separation needle in the direction of the rotation axis. Then, the separation needle faces a portion of the outer circumferential surface that does not come into contact with the separation claw, and there is a risk that an electric charge may be unintentionally applied to this portion. On the other hand, there is a risk that the location where the separation needle is not placed and the location on the outer circumferential surface that is not in contact with the separation claw overlap in the direction of the rotation axis, and no charge will be applied to the location where the separation claw is not in contact. . In these cases, the charged state of the outer circumferential surface of the fixing roller will not be uniform, and image defects such as the above-mentioned electrostatic offset and electrostatic scattering may occur.

SUMMARY OF THE INVENTION Accordingly, the present invention provides a fixing device including a separation claw that can suppress image defects while suppressing an increase in manufacturing costs, and an image forming apparatus equipped with the fixing device.

In order to achieve the above object, a first configuration of the present invention is a fixing device including a heated rotating body, a pressurizing rotating body, a heating section, a separating claw, and a charge applying section. The pressure rotating body is pressed against the heated rotating body to form a fixing nip. The heating section heats the rotating body to be heated. The separation claw has a contact portion that contacts the outer circumferential surface of the heated rotating body on the downstream side of the fixing nip with respect to the moving direction of the outer circumferential surface of the heated rotating body, and separates the sheet that has passed through the fixing nip from the heated rotating body. do. The charge applying section is arranged to face the outer circumferential surface at a predetermined interval, and applies charges to the outer circumferential surface that have a polarity opposite to that of the outer circumferential surface. The fixing device melts and fixes the unfixed toner image on the sheet by heating and pressurizing the sheet passing through the fixing nip. The charge applying section includes a plurality of charging sections that protrude toward the outer circumferential surface and are arranged in parallel to the rotation axis direction of the rotating body to be heated. The separation claw has a charge shielding part that is disposed between the tip of the charging part and the outer peripheral surface and shields the charge released from the charging part to the outer peripheral surface.

According to the first configuration of the present invention, the electric charge discharged from the charging part located at a position overlapping the separation claw in the direction of the rotation axis is shielded by the charge shielding part. Therefore, the portions of the outer circumferential surface of the rotating body to be heated that overlap with the separation claws are difficult to be charged, and the portions that do not overlap with the separation claws are charged with opposite polarity and are neutralized. Further, by employing a relatively simple configuration including the separating claw and the charge applying section, it is possible to suppress the complexity of the configuration and current control. Therefore, it is possible to provide a fixing device that can suppress image defects while suppressing increases in manufacturing costs.

A schematic sectional view showing the internal structure of an image forming apparatus 100 in which a fixing device 13 according to an embodiment of the present invention is mounted. A side view showing an enlarged view of the periphery of the fixing device 13 according to an embodiment of the present invention A plan view of the outer circumferential surface 38 of the heating roller 20 viewed from above in the radial direction. A plan view of the fixing device 13 when the fixing device 13 is viewed from above along the protruding direction of the charging section 42. A block diagram showing an example of a control path of the image forming apparatus 100 A graph showing the relationship between the surface potential of the outer circumferential surface 38 and the current flowing through the charging section 42 when the cumulative printed value increases.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing the internal structure of an image forming apparatus 100 in which a fixing device 13 according to an embodiment of the present invention is mounted. The image forming apparatus 100 according to this embodiment is a 4-tandem color copying machine. First, the image forming apparatus 100 will be described with reference to FIG.

As shown in FIG. 1, inside the main body of the image forming apparatus 100, four image forming sections Pa, Pb, Pc, and Pd are provided in order from one side in the horizontal direction (left side in FIG. 1). These image forming sections Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and are formed by magenta, cyan, and yellow through each charging, exposure, development, and transfer process. and black images are sequentially formed.

Image forming sections Pa to Pd are provided with photoreceptor drums 1a to 1d, respectively. Charging devices 2a to 2d, an exposure device 5, developing devices 3a to 3d, and cleaning devices 7a to 7d are provided around and below the photoreceptor drums 1a to 1d. The charging devices 2a to 2d charge the photoreceptor drums 1a to 1d. The exposure device 5 exposes the photosensitive drums 1a to 1d based on the image data. The developing devices 3a to 3d develop the electrostatic latent images formed on the photoreceptor drums 1a to 1d with toner. The cleaning devices 7a to 7d collect and remove the developer (toner) remaining on the photoreceptor drums 1a to 1d after the toner images are transferred.

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 photoreceptor drums 1a to 1d. Next, the exposure device 5 irradiates light according to the image data to form electrostatic latent images on each of the photosensitive drums 1a to 1d according to the image data. 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. The toner in this developer is supplied onto the photoreceptor drums 1a to 1d by the developing devices 3a to 3d and electrostatically adheres thereto. As a result, toner images (visible images) corresponding to the electrostatic latent images are formed on the photoreceptor drums 1a to 1d.

Toner containers 4a to 4d are provided above the developing devices 3a to 3d. When the ratio of toner in the two-component developer filled in each developing device 3a to 3d falls below a specified value, toner is replenished from the toner container 4a to 4d to each developing device 3a to 3d.

A driven roller 10, a driving roller 11, and an intermediate transfer belt 8 are provided adjacent to the image forming sections Pa to Pd. The driven roller 10 and the driving roller 11 are rotatably supported. The driven roller 10 and the driving roller 11 are arranged parallel to the horizontal direction. The drive roller 11 is connected to a drive source (not shown) such as a motor, and is rotated by the driving force of this drive source.

The intermediate transfer belt 8 is formed of a sheet made of dielectric resin. It is preferable to use a seamless belt as the intermediate transfer belt 8. The intermediate transfer belt 8 is stretched around a driven roller 10 and a driving roller 11 . The inner circumferential surface of the intermediate transfer belt 8 and the outer circumferential surface of the drive roller 11 are in contact with each other so that the intermediate transfer belt 8 rotates as a result of the drive roller 11 . The outer peripheral surface of the driven roller 10 and the inner peripheral surface of the intermediate transfer belt 8 are in contact with each other so that the driven roller 10 rotates as a result of the intermediate transfer belt 8 .

The intermediate transfer belt 8 is in contact with the photosensitive drums 1a to 1d. Primary transfer rollers 6a to 6d are arranged to face the photosensitive drums 1a to 1d, respectively, with the intermediate transfer belt 8 in between. The primary transfer rollers 6a to 6d are driven to rotate at the same linear speed as the photosensitive drums 1a to 1d and the intermediate transfer belt 8 by a primary transfer drive motor (not shown).

A secondary transfer roller 9 is arranged at a position facing the drive roller 11 with the intermediate transfer belt 8 in between. The secondary transfer roller 9 is in pressure contact with the intermediate transfer belt 8 to form a secondary transfer nip portion N1. A cleaning blade 19 is disposed downstream of the secondary transfer nip N1 with respect to the rotation direction of the intermediate transfer belt 8 (counterclockwise as shown in FIG. 1). The cleaning blade 19 is in contact with the outer peripheral surface of the intermediate transfer belt 8 and removes toner and the like remaining on the surface of the intermediate transfer belt 8.

When the drive roller 11 rotates, the intermediate transfer belt 8 rotates while contacting the outer peripheral surfaces of the photoreceptor drums 1a to 1d. At this time, the primary transfer rollers 6a to 6d apply a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d. Then, the toner images on the photoreceptor drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8.

Then, the sheet S is conveyed to the secondary transfer nip portion N1 at a predetermined timing. In the secondary transfer nip portion N1, the toner image that has been primarily transferred to the intermediate transfer belt 8 is secondarily transferred to the surface of the sheet S. Thereafter, the toner remaining on the surfaces of the photoreceptor drums 1a to 1d is removed by cleaning devices 7a to 7d, and a new electrostatic latent image is subsequently formed.

The sheet S onto which the toner image has been transferred is conveyed to the fixing device 13. The fixing device 13 fixes the toner image on the conveyed sheet S onto the sheet S. Details of the fixing device 13 will be described later.

Thereafter, the sheet S is discharged via a pair of discharge rollers 15 to a discharge tray 17 formed on the top surface of the main body of the image forming apparatus 100. When double-sided printing is performed on the sheet S, a branching section 14 provided on the downstream side of the fixing device 13 distributes the sheet S that has passed through the fixing device 13 to a reversing conveyance path 18 . The reversing conveyance path 18 conveys the sheet S again to the secondary transfer nip while inverting the front and back sides. The toner image is secondarily transferred to the back surface of the re-conveyed sheet S, and the toner image is fixed by the fixing device 13. Then, the sheet S is discharged onto the discharge tray 17 by the discharge roller pair 15.

Next, the fixing device 13 will be explained. FIG. 2 is an enlarged side view showing the vicinity of the fixing device 13 according to the embodiment of the present invention. As shown in FIG. 2, the fixing device 13 employs a roller fixing method. The fixing device 13 includes a heating roller 20 (rotating body to be heated), a heating section 23, a pressure roller 21 (rotating pressure body), a heating side drive source 24 (drive source), a separation claw 22, and a charger. The application section 31 is provided.

The heating roller 20 includes a core metal 26 and a release layer 27. The core metal 26 is a hollow cylindrical rod-shaped body made of a metal such as aluminum or iron that has excellent thermal conductivity. The mold release layer 27 is made of a material with excellent mold release properties, such as fluororesin. The mold release layer 27 covers the core bar 26 .

The heating section 23 is provided inside the core metal 26. The heating section 23 is a heater such as a halogen lamp or a xenon lamp. The heating unit 23 heats the mold release layer 27 via the core metal 26 and adjusts the temperature of the surface of the mold release layer 27 to a predetermined temperature. The temperature of the heating roller 20 is detected by a thermistor 47 (see FIG. 5) and controlled by a control section 90, which will be described later.

Pressure roller 21 is provided at a position facing heating roller 20 . The pressure roller 21 includes a base material 32, an elastic layer 33, and a release layer 34. The base material 32 is a metal core made of aluminum. The base material 32 is rotatably supported by the housing of the fixing device 13.

The elastic layer 33 is a layer formed on the base material 32 and made of silicone rubber. The elastic layer 33 provides elasticity to the fixing nip portion N2. The release layer 34 is a layer made of fluororesin and formed into a tube shape so as to cover the surface of the elastic layer 33. The release layer 27 and the release layer 34 make it easy to separate the sheet S from the heating roller 20 and the pressure roller 21 when the unfixed toner image is melted and fixed in the fixing nip N2.

A fixing nip portion N2 is formed at the contact portion between the pressure roller 21 and the heating roller 20. The pressure roller 21 is urged toward the heating roller 20 by a pressure mechanism 30 (see FIG. 5), and the urging force of the pressure mechanism 30 applies a predetermined nip pressure to the fixing nip portion N2.

The heating side drive source 24 is a motor that outputs rotational driving force. The heating side drive source 24 is connected to the heating roller 20 via a first drive transmission mechanism 35 composed of a plurality of gears and the like. The heating side drive source 24 outputs rotational driving force to the heating roller 20 via the first drive transmission mechanism 35 . The heating roller 20 receives the rotational driving force of the heating side drive source 24 and rotates in the circumferential direction of a rotating shaft 37 located at the center of the core bar 26 (the radial center of the heating roller 20). Hereinafter, the direction along the rotation axis 37 will be referred to as the "rotation axis direction." Further, the direction perpendicular to the rotating shaft 37 is referred to as the "radial direction."

The pressure roller 21 receives the rotational driving force of the heating roller 20 and rotates as a result. Pressure roller 21 rotates in the opposite direction to heating roller 20 . As a result, in the fixing nip portion N2, the outer circumferential surface 38 of the heating roller 20 and the outer circumferential surface of the pressure roller 21 move in the same direction. The rotation direction of the heating roller 20 and the pressure roller 21 is the conveyance direction of the sheet S.

As described above, the sheet S on which the toner image has been secondarily transferred in the secondary transfer nip portion N1 is conveyed to the fixing nip portion N2. Then, the sheet S is held between the heating roller 20 and the pressure roller 21 at the fixing nip N2, and is conveyed to the downstream side of the fixing nip N2 while being heated and pressurized. As a result, the powdered toner on the sheet S is melted and fixed on the sheet S under pressure.

FIG. 3 is a plan view of the outer circumferential surface 38 of the heating roller 20 viewed from above in the radial direction. FIG. 4 is a plan view of the fixing device 13 when the fixing device 13 is viewed from above along the protruding direction of the charging section 42. As shown in FIG. The heating roller 20 rotates in the direction of arrow A in FIGS. 3 and 4. In other words, the direction of arrow A in FIGS. 3 and 4 is the conveyance direction of the sheet S.

The separation claws 22 are formed by coating the surface of a highly insulating heat-resistant resin (eg, polyimide, etc.) with a fluororesin. As shown in FIGS. 2 to 4, the separation claw 22 is disposed on the downstream side of the fixing nip N2 with respect to the conveyance direction of the sheet S. A plurality of separation claws 22 are arranged linearly along the rotation axis direction.

The separation claw 22 is in contact with the outer peripheral surface 38 of the heating roller 20. A portion of the separating claw 22 that contacts the outer circumferential surface 38 is referred to as a contact portion 39 . Even if the sheet S that has passed through the fixing nip portion N2 is stuck to the outer circumferential surface 38 due to the melted toner, it is peeled off from the outer circumferential surface 38 and separated by contacting the contact portion 39.

The separation claw 22 includes a charge shielding portion 40 . The width of the charge shielding portion 40 is approximately equal to the width of the contact portion 39 in the direction of the rotation axis. More specifically, the width of the charge shielding part 40 is 9/10 times or more and 11/10 times or less the width of the contact part 39. As described above, the separation claws 22 are made of a highly insulating material, and the charges that reach the separation claws 22 are blocked by the charge shielding portion 40 .

The charge applying section 31 is arranged to face the outer circumferential surface with a predetermined distance therebetween. The charge applying unit 31 applies charges to the outer circumferential surface 38 that have a polarity opposite to the charging polarity (here, negative polarity) of the outer circumferential surface 38 (here, positive polarity). Hereinafter, the charge applying section 31 will be explained in more detail.

As shown in FIGS. 2 to 4, the charge applying section 31 includes a base section 41 and a plurality of charging sections 42. As shown in FIGS. The base portion 41 is a member extending parallel to the rotation axis direction. The base portion 41 is fixed to a part of the housing, a frame, or the like of the image forming apparatus 100. The charging section 42 is a needle-shaped member made of a conductive material (such as a metal material or a resin material with a low electrical resistance value). The charging portion 42 protrudes from the base portion 41 toward the outer circumferential surface 38 .

A plurality of charging units 42 are arranged at equal intervals along the rotation axis direction so as to overlap at least the entire paper passing area A1 of the outer circumferential surface 38 with respect to the rotation axis direction. The paper passing area A1 is an area of the outer circumferential surface 38 that is contacted by the sheet S passing through the fixing nip N2. The charging unit 42 is also arranged on both sides of the paper passing area A1 in the direction of the rotation axis.

The charge applying section 31 is connected to a power supply unit 52 (see FIG. 5). The power supply unit 52 is connected to the power supply section 51 and distributes the power supplied from the power supply section 51 to various devices in the image forming apparatus 100 .

When power is supplied from the power supply unit 52 to the charge applying section 31, a current flows through the charging section 42, and a predetermined amount of charge is released from the tip of the charging section 42. The charge moves from the charging section 42 to the outer circumferential surface along a straight line L1 that connects the tip of the charging section 42 and the outer circumferential surface 38 at the shortest distance. The surface potential of the outer circumferential surface 38 changes due to the movement of this charge. Hereinafter, this direction of charge movement will be referred to as the "discharge direction." A control section 90 is connected to the power supply unit 52, and controls the current by the control section 90 to adjust the amount of charge applied to the outer circumferential surface 38. Details regarding the control unit 90 will be described later.

Hereinafter, among the plurality of charging parts 42, the one located at a position overlapping the charge shielding part 40 in the direction of the rotation axis will be referred to as a first charging part 42a, and the other ones will be referred to as a second charging part 42b. When the first charging section 42a and the second charging section 42b are not particularly distinguished, they are simply referred to as the charging section 42. The charge shielding section 40 faces the first charging section 42a along the discharge direction. The charge shielding section 40 is located downstream of the first charging section 42a with respect to the discharge direction.

The charge shielding section 40 shields the charges emitted from the first charging section 42a. As described above, the charge shielding section 40 is located at a position overlapping the contact section 39 in the direction of the rotation axis. Therefore, the portion of the outer circumferential surface 38 that is in contact with the contact portion 39 is less likely to be charged.

Next, control paths for the image forming apparatus 100 and the fixing device 13 will be explained using FIG. 5. Note that when using the image forming apparatus 100, various controls are performed on each part of the apparatus, so here, the explanation will focus on the parts of the control path that are necessary for implementing the present invention. Furthermore, descriptions of parts that have already been described will be omitted. 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.

FIG. 5 is a block diagram showing an example of a control path of the image forming apparatus 100. As shown in FIG. 5, the control path of the image forming apparatus 100 includes an image input section 70, an operation section 80, a control section 90, image forming sections Pa to Pd, a power supply section 51, a power supply unit 52, and a fixing device 13. It is made up of.

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

The operation unit 80 is provided with a liquid crystal display unit 81 and an LED 82 that indicates various statuses, and is configured to indicate the status of the image forming apparatus 100, the image forming status, and the number of copies to be printed. Furthermore, the type and size of the sheet S can be input by specifying the sheet cassette 16 or manual paper feed tray (not shown) from which the sheet S is fed from the operation unit 80. Various settings of the image forming apparatus 100 are performed from a printer driver of a 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 counting unit 95, and a plurality of (here, two) I/Fs ( interface) 96. The CPU 91 functions as a central processing unit. The ROM 92 is a read-only storage section. 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 in the image forming apparatus 100 and receives input signals from the operation unit 80.

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

Further, the ROM 92 stores different values for the voltage applied to the charge applying section 31, that is, the current generated in the charging section 42, depending on the elapsed period from the initial startup. Specifically, as the cumulative print value (described later) increases, the current generated in the charging section 42 increases.

The temporary storage section 94 temporarily stores an image signal input from the image input section 70 and converted into a digital signal. The counting section 95 cumulatively counts the number of sheets S inserted into the fixing nip section N2 (cumulative number of sheets passed). Further, the counting section 95 counts the cumulative print value, which is the cumulative value of the print values of the sheets S passed through the fixing nip section N2.

Counting of the cumulative print value starts from the first startup, and each time the sheet S passes through the fixing nip portion N2, the printing rate of the passed sheet S is accumulated and added. For example, a sheet S on which an image with a printing rate of 5% is formed passes through the fixing nip portion N2, followed by a sheet S on which an image with a printing rate of 10% is formed, and then an image with a printing rate of 15% is formed. When the printed sheet S passes through, the respective printing rates (here, 0.05, 0.1, and 0.15) are added, and the cumulative printing value becomes 0.3. When images are formed on both sides of the sheet S, only the printing rate of the side that is in pressure contact with the outer circumferential surface 38 is added.

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 power supplied from the power supply section 51 to each device (including the charge application section 31) in the image forming apparatus 100 in response to an output signal from the control section 90.

FIG. 6 is a graph showing the relationship between the surface potential of the outer circumferential surface 38 and the current flowing through the charging section 42 when the cumulative printed value increases. In FIG. 6, the horizontal axis is the cumulative printed value (dimensionless number), the first vertical axis (left vertical axis) is voltage [kV], and the second vertical axis (right vertical axis) is current [μA].

In FIG. 6, the current flowing through the charging section 42 is C1. Further, the surface potential of a portion of the outer circumferential surface 38 that is not in contact with the contact portion 39 (area A3 shown in FIG. 4) is set as C2. Further, if no charge is applied by the charge applying unit 31, the surface potential of a portion of the outer circumferential surface 38 that is not in contact with the contact portion 39 (area A2 shown in FIG. 4) is set as C3. In addition, similar to the conventional fixing device 13, the surface potential when a charge is directly applied to the downstream portion in the rotational direction of the heating roller 20 (area A3 shown in FIG. 4) at the location where the outer circumferential surface 38 contacts the contact portion 39. is defined as C4.

As shown in FIG. 6, the current C1 flowing through the charging section 42 increases stepwise every time the cumulative print value increases by a predetermined increase print value X1 to X6. Specifically, the current C1 is constant at 0 μA until the cumulative print value increases by a predetermined increased print value X1 from the state when the image forming apparatus 100 is first started (the cumulative print value is 0). , the surface potential C2 of the region A3 of the outer circumferential surface 38 decreases. When the cumulative print value reaches the increased print value X1, the current C1 increases to 0.13 μA, and the charge applying section 31 starts applying a charge to the outer circumferential surface 38. Therefore, the surface potential C2 of the region A3 of the outer circumferential surface 38 rises to about 0V.

Even if the cumulative print value further increases, the current C1 of the charging section 42 remains constant (0.13 μA) until the cumulative print value reaches the sum of the increased print value X1 and the increased print value X2, and the outer peripheral surface The surface potential C2 of the area A3 of 38 decreases again. When the cumulative print value reaches the total value of the increased print value X1 and the increased print value X2, the current C1 increases to 0.26 μA, and the surface potential C2 of the area A3 of the outer peripheral surface 38 rises to about 0V again. Thereafter, even if the cumulative print value further increases, charges are applied to the outer peripheral surface 38 by the charge applying unit 31 so that the surface potential C2 of the area A3 of the outer peripheral surface 38 becomes approximately 0V.

During this time, the portion downstream of the contact portion 39 in the rotational direction of the heating roller 20 (area A3 shown in FIG. 4) of the portion of the outer circumferential surface 38 that contacts the contact portion 39 is electrically blocked by the charge shielding portion 40. The surface potential of this portion is difficult to change from about 0V.

The increased print values X1 to X6 are set to increase as the cumulative print value increases. That is, the increased printing values are increased in the order of X1, X2, X3, X4, X5, and X6.

Conventionally, even if toner components adhere to a portion of the outer circumferential surface 38 that contacts the separation claws 22 (area A3 shown in FIG. 4), the separation claws 22 scrape off the toner components. Therefore, the surface potential of the scraped portion is lower than that of other portions (the portion where the toner component remains attached, area A2 shown in FIG. 4). When a positive charge is applied directly to this scraped part from the charge applying part 31, the surface potential C4 of this part becomes more positive (same polarity as the sheet) as shown in FIG. Therefore, electrostatic scattering may occur.

Furthermore, if no charge is applied by the charge applying unit 31, as the cumulative print value increases, the surface potential C3 of a portion of the outer circumferential surface 38 that does not come into contact with the separating claw 22 (area A2 shown in FIG. 4) decreases significantly. When the surface potential C3 becomes smaller than a predetermined value (the value of P1 shown in FIG. 6), there is a high possibility that an electrostatic offset phenomenon will occur.

On the other hand, in the fixing device 13 of this embodiment, as described above, the charge applying section 31 applies a charge to the outer circumferential surface 38. Furthermore, the charges emitted from the first charging section 42a are blocked by the charge shielding section 40. Therefore, the surface potential C1 of the portion of the outer circumferential surface 38 that overlaps with the separation claw 22 (area A3 shown in FIG. 4) is difficult to rise, and the surface potential C1 of the portion that does not overlap with the separation claw 22 (area A2 shown in FIG. 4) increases. do. As a result, even if toner components adhere to the outer circumferential surface 38 and the outer circumferential surface is negatively charged, it is possible to remove the static electricity so that the surface potential of the outer circumferential surface 38 approaches a uniform state, and it is possible to eliminate electrostatic offset. Image defects such as electrostatic scattering can be suppressed.

Further, unlike the conventional fixing device 13, there is no need to provide a plurality of charge applying sections 31, and static electricity can be removed simply by controlling the current of the charge applying section 31. Further, the charge is shielded by the charge shielding section 40. Therefore, image defects can be suppressed with a simple structure and control system. Therefore, it is possible to provide the fixing device 13 that can suppress image defects while suppressing an increase in manufacturing costs.

Further, the charge shielding portion 40 is formed as a part of the separating claw 22. Therefore, if the separation claw 22 is slightly displaced, the contact portion 39 and the charge shielding portion 40 will also move together. That is, the contact portion 39 and the charge applying portion 31 will not be misaligned in the direction of the rotation axis. Thereby, it is possible to suppress misalignment between the portion where the charge is shielded by the charge shielding portion 40 and the contact portion 39 .

As described above, the fixing device 13 of the above embodiment increases the current flowing through the charging unit 42 and increases the amount of charge applied to the outer circumferential surface 38 as the cumulative print value increases. As a result, even if the amount of negative charge remaining on the outer circumferential surface 38 increases due to adhesion of toner components, etc., it is possible to suitably eliminate the charge on the outer circumferential surface 38.

Further, as the cumulative number of sheets passed increases, the surface potential of the outer circumferential surface 38 usually increases due to the influence of friction and the like. Furthermore, by performing the fixing operation on the sheet S with a high printing rate, the surface potential of the outer circumferential surface 38 becomes larger. This tendency is remarkable when the cumulative print value is relatively low, and this tendency weakens as the cumulative print value becomes relatively high. In contrast, in the fixing device 13 of the embodiment described above, the values of the increased print values X1 to X6 are set larger as the cumulative print value becomes larger. Thereby, the timing for increasing the amount of charge applied to the outer circumferential surface 38 can be made to correspond to the above-described tendency. Therefore, the surface potential of the outer circumferential surface 38 can be maintained appropriately, and image defects can be suitably suppressed.

In addition, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention. For example, in the above embodiment, the fixing device 13 of a roller fixing type that includes the heating roller 20 as a rotating body to be heated is exemplified. can be similarly applied.

Further, the heating method is not limited to the configuration of the above embodiment in which the heating section 23 is provided inside the core bar 26, but another heating method such as an induction heating method including an excitation coil and a core may be adopted. .

The present invention can be used in a fixing device that melts and fixes the toner image on the sheet by inserting a sheet into a fixing nip formed by a heated rotating body and a pressure rotating body and applying heat and pressure to the toner image. It is. By utilizing the present invention, it is possible to provide an image forming apparatus equipped with a fixing device including a separation claw that can suppress image defects while suppressing an increase in manufacturing costs.

13 Fixing device 20 Heating roller (rotating body to be heated)
21 Pressure roller (pressure rotating body)
22 Separation claw 23 Heating section 24 Heating side drive source (drive source)
31 Charge applying section 37 Rotating shaft 38 Outer surface 39 Contact section 40 Charge shielding section 42 Charging section 42a First charging section 42b Second charging section 90 Control section 95 Counting section 100 Image forming apparatus A1 Paper passing area L1 Straight line N2 Fixing nip section Pa~Pd Image forming section S Sheet X1~X6 Increased printing value

Claims (6)

A rotating body to be heated;
a pressure rotating body that is pressed against the heated rotating body to form a fixing nip;
a heating unit that heats the rotating body to be heated;
a contact portion that contacts the outer circumferential surface of the heated rotating body on the downstream side of the fixing nip with respect to the moving direction of the outer circumferential surface of the heated rotating body, and the sheet that has passed through the fixing nip is transferred to the heated rotating body a separation claw that separates from the
a charge applying section that is arranged opposite to the outer circumferential surface at a predetermined interval and applies an electric charge to the outer circumferential surface that has a polarity opposite to the charged polarity of the outer circumferential surface;
A fixing device that melts and fixes an unfixed toner image on the sheet by heating and pressurizing the sheet passing through the fixing nip,
The charge applying section includes a plurality of charging sections that protrude toward the outer circumferential surface and are arranged in parallel to the rotation axis direction of the rotating body to be heated,
The fixing device is characterized in that the separating claw includes a charge shielding part that is disposed between a tip of the charging part and the outer circumferential surface and shields charges released from the charging part to the outer circumferential surface. The fixing device according to claim 1, wherein the plurality of charging units are arranged at equal intervals in the direction of the rotation axis so as to overlap the entire area of the sheet passing area that contacts the sheet on the outer peripheral surface. Device. The fixing device according to claim 1, wherein the width of the charge shielding portion is 9/10 times or more and 11/10 times or less the width of the contact portion with respect to the rotation axis direction. The fixing device according to claim 1 , wherein the fixing device fixes the toner image on the sheet onto the sheet; an image forming unit that forms an image;
An image forming apparatus comprising: a counting unit that counts a cumulative print value obtained by accumulating printing rate values of the sheet passing through the fixing nip portion;
a control unit that controls the charge applying unit;
Equipped with
The image forming apparatus according to claim 4, wherein the control unit increases the applied current as the cumulative print value increases. The control unit increases the applied current each time the cumulative print value increases by a predetermined increased print value, and as the cumulative print value increases, the increased print value increases. The image forming apparatus according to claim 5.

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