JP2017227717A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suitably realize separability for a fixing belt of a sheet after fixing, while suppressing an increase of a print cost even when the number of integrated outputs of the sheet is increased.SOLUTION: A fixing device 40 of an image forming device 1 includes: an information detection part 60 for detecting information relating to a surface state of a first pressure roller 41; and a first drive motor 47 and a second drive motor 48 individually provided for the first pressure roller 41 and a second pressure roller 42. The image forming device 1 includes: a normal model set up at the normal fixing time and a belt bounce avoidance mode selected based on the information relating to the surface state of the first pressure roller 41 detected by the information detection part 60. The image forming device 1, upon selection of the belt bounce avoidance mode, executes a speed change which makes the surface speed of the fixing belt 44 in a fixing nip part N increase more than the surface speed of the second pressure roller 42.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus including a fixing device.

  In an image forming apparatus such as a copying machine, a printer, or a facsimile, a thermal fixing method is widely adopted as a method for fixing an unfixed toner image transferred onto a sheet such as a sheet onto the sheet. A fixing device is known that includes a fixing belt that is formed endlessly, is wound around a first pressure roller and a tension roller, and is sandwiched between the first pressure roller and the second pressure roller. ing.

  Nowadays, there is an increasing need for image formation on a relatively thin sheet. However, in an electrophotographic image forming apparatus, it is difficult to obtain a suitable fixing property for a thin sheet. The reason is that a thin sheet has low separability from the fixing belt on the downstream side in the sheet conveying direction of the fixing nip portion after fixing.

  In order to improve the separation of the sheet from the fixing belt, the fixing belt winding area (hereinafter simply referred to as “belt winding area”) around the first pressure roller on the downstream side of the fixing nip portion in the sheet conveying direction may be referred to. ) Is increased, and the fixing belt tension is lowered. However, the configuration in which the winding area of the fixing belt is increased, and the configuration in which the tension of the fixing belt is decreased tend to cause the fixing belt to be loosened on the downstream side of the fixing nip portion in the sheet conveying direction. A phenomenon in which the tension of the fixing belt returns to its original value (hereinafter, this phenomenon is referred to as “belt jumping”) is likely to occur so that the fixing belt jumps suddenly after the slack occurs. As a result, there is a problem that noise tends to occur in an image formed on a sheet.

  Therefore, the tension of the fixing belt that does not cause belt jumping is designed in advance according to the belt winding area and the surface friction coefficient of the first pressure roller. However, when the integrated output number of sheets increases, the surface coating of the first pressure roller may peel off. As a result, the surface friction coefficient of the first pressure roller increases, and belt jumping easily occurs. Therefore, when the cumulative number of output sheets increases and the surface coating of the first pressure roller is expected to be peeled off, it has been considered to replace the first pressure roller before the belt jump occurs. However, there was concern about an increase in printing cost.

  Patent Documents 1 and 2 disclose conventional techniques related to a fixing device that considers the possibility of application in order to solve the above problems.

  The fixing device described in Patent Document 1 includes a fixing belt wound around a plurality of rollers including a fixing roller, and a pressure roller that presses and contacts the fixing roller via the fixing belt. This fixing device executes control to make the surface speed of the pressure roller slower than the surface speed of the fixing belt until the leading end (downstream end in the conveying direction) of the recording medium (sheet) is separated from the fixing belt. That is, since the surface speed of the fixing belt is higher than the surface speed of the pressure roller, it can be expected that the occurrence of belt jump is suppressed.

  The heating device described in Patent Document 2 includes a driving roller, a tension roller, and a fixing film (endless belt) wound around a low heat capacity heating body, and a pressure roller that presses and contacts the low heat capacity heating body through the fixing film. And tension roller urging means. When this heating device detects a shift in the rotation axis direction of the fixing film, the tension roller is moved by an urging means to change the tension of the fixing film and to control the fixing film to return to a predetermined position.

JP 2012-47905 A JP-A-3-260682

  However, in the fixing device described in Patent Document 1, the surface speed of the fixing belt is set higher than the surface speed of the pressure roller in response to an increase in the surface friction coefficient of the fixing roller based on an increase in the cumulative number of output sheets of recording media (sheets). Is not increasing. Further, this fixing device controls the surface speed of the pressure roller to be equal to the surface speed of the fixing belt after the front end (downstream end in the transport direction) of the recording medium is separated from the fixing belt. For these reasons, it has been a problem that there is a possibility that the occurrence of belt jumping of the fixing belt cannot be suppressed. Further, if the surface speed of the fixing belt is increased from the beginning of the operation of the fixing device, the surface speed of the fixing roller is promoted, and the life of the fixing roller may be shortened.

  The heating device described in Patent Document 2 is also fixed in correspondence with the increase in the surface friction coefficient of the low heat capacity heating body based on the increase in the cumulative number of output sheets of recording materials (sheets), similarly to the fixing device in Patent Document 1. It does not change the tension of the film. Therefore, there is a problem that there is a possibility that the occurrence of belt jump of the fixing film may not be suppressed.

  The present invention has been made in view of the above points, and even when the cumulative number of output sheets increases, the separation of the sheet after fixing from the fixing belt is suppressed while suppressing an increase in printing cost. An object is to provide an image forming apparatus that can be realized.

  In order to solve the above problems, the present invention has an endless fixing belt that is wound around a first pressure roller and a tension roller and is sandwiched between the first pressure roller and the second pressure roller. In the image forming apparatus for fixing the unfixed toner image transferred to the sheet at the fixing nip portion where the first pressure roller and the second pressure roller are in contact with each other with the fixing belt interposed therebetween, A detection unit that detects information relating to the surface state of one pressure roller, and two drive sources provided separately for the first pressure roller and the second pressure roller or the tension of the fixing belt are changed. At least one of the tension changing sections for performing the fixing, and as the operation mode at the time of fixing, the first pressure roller and the second pressure roller are set to predetermined surface speeds at the time of normal fixing and the predetermined surface speed is set. A normal mode in which the belt is set to a predetermined tension; and a belt bounce avoiding mode selected based on information relating to the surface state of the first pressure roller detected by the detecting unit; Is selected, a speed change for increasing the surface speed of the fixing belt in the fixing nip portion to be higher than the surface speed of the second pressure roller, or increasing the tension of the fixing belt as compared with that in the normal mode. It is characterized in that at least one of the tension changes is executed.

  According to this configuration, the occurrence of belt bounce on the downstream side of the fixing nip portion in the sheet conveying direction is suppressed by changing the speed of the fixing belt or changing the tension of the fixing belt. Therefore, even when the cumulative number of output sheets increases, the first pressure roller need not be replaced. At this time, the tension (exit curvature) on the fixing nip outlet side becomes the same as that before the surface coating of the first pressure roller is peeled off, and separation can be maintained while avoiding belt jump.

  In the image forming apparatus having the above-described configuration, the detection unit detects an integrated output number of sheets as information relating to a surface state of the first pressure roller.

  Further, in the image forming apparatus having the above-described configuration, the detection unit detects light reflectance of the surface of the first pressure roller as information relating to the surface state of the first pressure roller.

  Further, in the image forming apparatus having the above-described configuration, the detection unit detects a displacement of the slack of the fixing belt around the fixing nip as information relating to the surface state of the first pressure roller. .

  The image forming apparatus having the above-described configuration further includes an operation unit capable of setting either automatic selection of the belt jump avoidance mode or manual selection by the user of the belt jump avoidance mode.

  In the image forming apparatus configured as described above, the speed increase in the speed change or the tension increase in the tension change is executed stepwise or continuously in the belt jump avoiding mode.

  According to the configuration of the present invention, even when the cumulative number of output sheets increases, the first pressure roller does not need to be replaced, and the occurrence of belt jumping on the downstream side in the sheet conveyance direction of the fixing nip portion is prevented. It is possible to suppress. Therefore, it is possible to provide an image forming apparatus capable of suitably realizing separation of the sheet after fixing from the fixing belt while suppressing an increase in printing cost.

1 is a partial vertical sectional front view of an image forming apparatus according to a first embodiment of the present invention. 1 is a schematic front view of a fixing device of an image forming apparatus according to a first embodiment of the present invention. 1 is a block diagram illustrating a configuration related to a fixing device of an image forming apparatus according to a first embodiment of the present invention. It is a graph which shows the relationship between the durable printed sheet number and the surface friction coefficient of a 1st pressure roller. 6 is a graph showing the relationship between the surface speed of the fixing belt and the belt jump generation cycle in the fixing device of the image forming apparatus according to the first embodiment of the present invention. FIG. 6 is a schematic front view of a fixing device of an image forming apparatus according to a second embodiment of the present invention. FIG. 5 is a schematic front view of a fixing device of an image forming apparatus according to a second embodiment of the present invention, showing a state in which the tension of the fixing belt is changed. FIG. 6 is a schematic front view of a fixing device of an image forming apparatus according to a third embodiment of the present invention. FIG. 7 is a schematic front view of a fixing device of an image forming apparatus according to a third embodiment of the present invention, showing a state in which the tension of the fixing belt is changed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following contents.

<First Embodiment>
First, an image output operation of the image forming apparatus according to the first embodiment of the present invention will be described while explaining the outline of the structure with reference to FIG. FIG. 1 is an example of a partial vertical sectional front view of an image forming apparatus. In the drawing, a two-dot chain line with an arrow indicates a sheet conveyance path and a conveyance direction.

  As shown in FIG. 1, the image forming apparatus 1 is a so-called tandem color copying machine, and includes an image reading unit 2 that reads an image of a document, a printing unit 3 that prints the read image on a sheet such as paper, and printing. An operation unit 4 for inputting conditions and displaying an operation status and a main control unit 5 are provided.

  The image reading unit 2 is a known unit that reads an image of a document placed on the upper surface of a platen glass (not shown) by moving a scanner (not shown). An image of a document is separated into three colors of red (R), green (G), and blue (B), and is converted into an electrical signal by a CCD (Charge Coupled Device) image sensor (not shown). Accordingly, the image reading unit 2 obtains image data for each color of red (R), green (G), and blue (B).

  The image data for each color obtained by the image reading unit 2 is subjected to various processes in the main control unit 5, and image data for each reproduction color of yellow (Y), magenta (M), cyan (C), and black (K). And stored in the storage unit 5b (see FIG. 3) of the main control unit 5. The image data for each reproduction color stored in the storage unit 5b is subjected to a process for correcting misregistration, and then scanned in synchronization with the sheet conveyance in order to perform optical scanning on the photosensitive drum 21 as an image carrier. Read line by line.

  The printing unit 3 forms an image by electrophotography and transfers the image to a sheet. The printing unit 3 includes an intermediate transfer belt 11 in which an intermediate transfer member is formed as an endless belt. The intermediate transfer belt 11 is wound around a driving roller 12 and driven rollers 13 and 14. The intermediate transfer belt 11 is rotated by the driving roller 12 counterclockwise in FIG.

  The driving roller 12 presses and contacts the secondary transfer roller 15 facing the intermediate transfer belt 11. At the position of the driven roller 14, an intermediate transfer cleaning unit 16 provided so as to face the driven roller 14 with the intermediate transfer belt 11 interposed therebetween contacts the outer peripheral surface of the intermediate transfer belt 11. After the toner image formed on the outer peripheral surface of the intermediate transfer belt 11 has been transferred to the sheet, the intermediate transfer cleaning unit 16 removes and removes deposits such as toner remaining on the outer peripheral surface of the intermediate transfer belt 11.

  Below the intermediate transfer belt 11, image forming units 20Y, 20M, 20C, and 20K corresponding to reproduction colors of yellow (Y), magenta (M), cyan (C), and black (K) are provided. In this description, the description of “Y”, “M”, “C”, and “K” identification symbols is omitted unless it is particularly limited, and is generically called “image forming unit 20”, for example. There are things to do. The four image forming units 20 are arranged in a line along the rotation direction of the intermediate transfer belt 11 from the upstream side to the downstream side in the rotation direction. The four image forming units 20 have the same configuration, and a charging unit, an exposure unit, a developing unit, a drum cleaning unit, and a primary transfer roller are provided around the photosensitive drum 21 that rotates clockwise in FIG. Prepare.

  Above the intermediate transfer belt 11, toner bottles 31 and toner hoppers 32 corresponding to the four image forming units 20 of the respective reproduction colors are provided. The developing unit and the toner hopper 32 are provided with a toner remaining amount detection unit (not shown) that detects the amount of toner inside each. Further, a toner replenishing device (not shown) is provided between the developing unit and the toner hopper 32 and between the toner hopper 32 and the toner bottle 31. When a decrease in the toner amount inside the developing unit is detected by the remaining amount detecting unit, the replenishing device is driven to replenish toner from the toner hopper 32 to the developing unit. Further, when the remaining amount detection unit detects a decrease in the toner amount inside the toner hopper 32, the replenishing device is driven to replenish the toner from the toner bottle 31 to the toner hopper 32. The toner bottle 31 is detachably attached to the apparatus main body, and can be replaced with a new one as appropriate.

  Below the image forming unit 20, a scanning optical device 23 as an exposure unit is arranged. One scanning optical device 23 corresponds to four image forming units 20, and has four light sources such as four semiconductor lasers (not shown) individually corresponding to the four photosensitive drums 21. The scanning optical device 23 modulates the four semiconductor lasers according to the image gradation data of each reproduction color, and individually emits laser light corresponding to each reproduction color to the four photosensitive drums 21.

  Below the scanning optical device 23, there is provided a sheet supply device 91 for stacking and storing a plurality of sheets S such as paper. The sheets S accommodated in the sheet supply apparatus 91 are sent out one by one from the uppermost layer to the sheet conveyance path Q. The sheet S sent out from the sheet stacking device 91 to the sheet conveying path Q reaches the position of the registration roller pair 94. Then, the registration roller pair 94 corrects the skew feeding of the sheet S (skew correction) and synchronizes with the rotation of the intermediate transfer belt 11, and the contact portion (secondary transfer) between the intermediate transfer belt 11 and the secondary transfer roller 15. The sheet S is sent out toward the nip portion.

  In the image forming unit 20, an electrostatic latent image is formed on the surface of the photosensitive drum 21 by the laser light emitted from the scanning optical device 23, and the electrostatic latent image is visualized as a toner image by the developing unit. The toner image formed on the surface of the photosensitive drum 21 is primarily transferred to the outer peripheral surface of the intermediate transfer belt 11 at a position where the photosensitive drum 21 faces the primary transfer roller with the intermediate transfer belt 11 interposed therebetween. The toner images of the image forming units 20 are sequentially transferred to the intermediate transfer belt 11 at a predetermined timing along with the rotation of the intermediate transfer belt 11, so that yellow, magenta, cyan, black on the outer peripheral surface of the intermediate transfer belt 11. A color toner image is formed by superimposing the four color toner images.

  The color toner image primarily transferred to the outer peripheral surface of the intermediate transfer belt 11 is formed by contacting the intermediate transfer belt 11 and the secondary transfer roller 15 with the sheet S sent in synchronization by the registration roller pair 94. The secondary transfer nip is transferred.

  A fixing device 40 is provided above the secondary transfer nip portion. The sheet S on which the unfixed toner image is transferred at the secondary transfer nip portion is sent to the fixing device 40, where the toner image is heated and pressurized and fixed on the sheet S. The sheet S that has passed through the fixing device 40 is discharged to a sheet discharge unit 96 provided above the intermediate transfer belt 11.

  The operation unit 4 is provided on the front side of the image reading unit 2. The operation unit 4 inputs settings such as the printing conditions such as the type and size of the sheet S used for printing by the user, enlargement / reduction, and presence / absence of double-sided printing, and settings such as a fax number and a sender name in facsimile transmission. Accept. In addition, the operation unit 4 also serves as a notification unit for notifying the user of the status, cautions, error message, and the like of the device on the display unit 4w.

  In addition, the image forming apparatus 1 is provided with a main control unit 5 composed of a CPU 5a, an image processing unit, and other electronic parts (not shown) for overall operation control (see FIG. 3). The main control unit 5 uses a central processing unit CPU 5a and an image processing unit, and controls components such as the image reading unit 2 and the printing unit 3 based on programs and data stored and input in the storage unit 5b. A series of image forming operations and printing operations are realized.

  Next, the configuration of the fixing device 40 of the image forming apparatus 1 will be described with reference to FIGS. FIG. 2 is a schematic front view of the fixing device 40. FIG. 3 is a block diagram illustrating a configuration related to the fixing device 40 of the image forming apparatus 1. FIG. 4 is a graph showing the relationship between the number of durable prints and the surface friction coefficient of the first pressure roller. In FIG. 2, the configuration of the fixing device 40 is shown with the sheet conveyance path Q substantially horizontal. In FIG. 2, the left side is the upstream side in the sheet conveyance direction with respect to the fixing device 40, and the right side is the downstream side in the sheet conveyance direction with respect to the fixing device 40.

  As shown in FIGS. 2 and 3, the fixing device 40 includes a first pressure roller 41, a second pressure roller 42, a tension roller 43, a fixing belt 44, a first heating unit 45, a second heating unit 46, and a first heating unit. A drive motor 47, a second drive motor 48, an urging unit 50, and an information detection unit 60 are provided.

  The first pressure roller 41 and the second pressure roller 42 have a cylindrical shape, and are arranged side by side so that their peripheral surfaces face each other with the paper transport path Q interposed therebetween. The first pressure roller 41 and the second pressure roller 42 have their rotation axes extending along the sheet width direction, that is, the front-rear direction of the image forming apparatus 1. The first pressure roller 41 and the second pressure roller 42 have a length corresponding to the entire area of the paper conveyance path Q in the paper width direction. The first pressure roller 41 is rotatably supported at its rotating shaft (not shown) by a bearing provided in the casing of the fixing device 40. The second pressure roller 42 is rotatably supported by a bearing provided on a housing portion of the fixing device 40 with a rotation shaft (not shown).

  The first pressure roller 41 and the second pressure roller 42 have a laminated structure in which, for example, a release layer (for example, fluorine coating), a heat generation layer, an elastic layer, and the like are provided sequentially from the outer peripheral surface side toward the radial center. . The first pressure roller 41 and the second pressure roller 42 have a diameter of about 40 mm to 60 mm, for example, and each elastic layer has a thickness of about several mm, for example. The sheet on which the surface of the first pressure roller 41 is heated by the action of the first heating unit 45 and the surface of the second pressure roller 42 is heated by the action of the second heating part 46 and the unfixed toner image is transferred. S is heated to fix the toner to the sheet S. The second pressure roller 42 accommodates the second heating unit 46 therein.

  The second pressure roller 42 is given a predetermined pressure by a pressure mechanism using a pressure spring or the like (not shown), and its peripheral surface presses and contacts the peripheral surface of the first pressure roller 41 to fix the nip portion. N is formed. In the fixing nip portion N, for example, the first pressure roller 41 is depressed inward in the radial direction. The fixing device 40 fixes the unfixed toner image transferred onto the sheet S at the fixing nip N where the first pressure roller 41 and the second pressure roller 42 are in contact with each other with the fixing belt 44 interposed therebetween.

  For each of the first pressure roller 41 and the second pressure roller 42, a first drive motor 47 and a second drive motor 48, which are two drive sources, are provided. For example, a stepping motor is used as the first drive motor 47, and an AC servo motor is used as the second drive motor 48, for example. The first pressure roller 41 receives power from the first drive motor 47 and rotates counterclockwise in FIG. The second pressure roller 42 receives power from the second drive motor 48 and rotates clockwise in FIG.

  The tension roller 43 has a cylindrical shape, and accommodates a first heating unit 45 for heating the first pressure roller 41 therein. The tension roller 43 is disposed so that the peripheral surface thereof is spaced apart from the peripheral surface of the first pressure roller 41. The tension roller 43 has a rotation axis parallel to the rotation axis of the first pressure roller 41 and extends in the sheet width direction, that is, the front-rear direction of the image forming apparatus 1. The tension roller 43 has a length corresponding to the entire area of the sheet conveyance path Q in the sheet width direction. The rotation shaft 43a of the tension roller 43 is rotatably supported by a later-described bearing 51 of the urging unit 50. The tension roller 43 is supported so as to be displaceable in a direction approaching and separating from the first pressure roller 41.

  The fixing belt 44 is formed in an endless shape, and the length in the sheet width direction is the same as that of the first pressure roller 41 and the tension roller 43. The fixing belt 44 has a laminated structure in which an elastic layer, a release layer, and the like are provided on a base material layer made of, for example, a polyimide film. The fixing belt 44 has a diameter of about 100 mm, for example. The fixing belt 44 is wound around the first pressure roller 41 and the tension roller 43, and is sandwiched between the first pressure roller 41 and the second pressure roller 42 in the fixing nip portion N. A predetermined tension is applied to the fixing belt 44 by the urging unit 50.

  The urging unit 50 includes a bearing 51, an urging spring 52, and a support member 53. The bearing 51 rotatably supports the rotating shaft 43a of the tension roller 43. The urging spring 52 is constituted by, for example, a tension spring, and one end is connected to the bearing 51 and the other end is connected to the support member 53, and is interposed between the bearing 51 and the support member 53. The support member 53 is fixed to the casing of the fixing device 40 so that it cannot be displaced. The biasing spring 52 causes a biasing force to act on the tension roller 43 in a direction in which the tension roller 43 is separated from the first pressure roller 41 via the bearing 51, thereby generating a predetermined tension on the fixing belt 44.

  The information detection unit 60 is disposed in the vicinity of the first pressure roller 41, for example. The information detection unit 60 detects information related to the surface state of the first pressure roller 41.

  The information detection unit 60 detects, for example, the accumulated output number of sheets S as information relating to the surface state of the first pressure roller 41. In this case, the information detection unit 60 includes, for example, a sheet detection sensor, detects the sheet S passing through the fixing nip N, and counts the number of sheets. Note that an information detection unit 60 may be provided in the main control unit 5 to detect the cumulative output number of sheets S as digital information.

  Alternatively, the information detection unit 60 detects, for example, the light reflectance of the surface of the first pressure roller 41 as information relating to the surface state of the first pressure roller 41. In this case, the information detection unit 60 includes, for example, a non-contact type color discrimination sensor, and detects a change in glossiness of the surface of the first pressure roller 41 as a light reflectance.

  Alternatively, the information detection unit 60 detects, for example, the displacement of the slack of the fixing belt 44 around the fixing nip N as information relating to the surface state of the first pressure roller 41. In this case, the information detection unit 60 includes, for example, a non-contact type displacement sensor, and detects the loose displacement of the fixing belt 44.

  The image forming apparatus 1 has a normal mode and a belt jump avoidance mode as operation modes during fixing.

  The normal mode is an operation mode used during normal fixing. In the normal mode, the first pressure roller 41 and the second pressure roller 42 are set to a predetermined surface speed, and the fixing belt 44 is set to a predetermined tension.

  On the other hand, the belt bounce avoiding mode is automatically selected based on information relating to the surface state of the first pressure roller 41 detected by the information detection unit 60.

  For example, the belt jump avoidance mode is automatically selected based on the cumulative output number of sheets S detected by the information detection unit 60. FIG. 4 shows the relationship between the number of durable prints and the surface friction coefficient of the first pressure roller 41. The vertical axis represents the surface friction coefficient of the first pressure roller 41, and the horizontal axis represents the number of durable prints. Represent. Then, “100%” of the durable printed sheet number in FIG. 4 means a standard durable printed sheet number in the image forming apparatus 1 preset in the design stage.

  4 that the surface friction coefficient of the first pressure roller 41 increases when the number of durable printed sheets exceeds 100%. This is considered to be caused by peeling of the surface coating (release layer or the like) of the first pressure roller 41. As a result, there is a possibility that belt jumping is likely to occur. For example, when the number of durable printed sheets exceeds 100%, the belt jump avoiding mode is selected.

  Alternatively, for example, the belt jump avoidance mode is automatically selected based on the light reflectance of the surface of the first pressure roller 41 detected by the information detection unit 60. When the surface coating of the first pressure roller 41 is peeled as described above, the glossiness of the surface of the first pressure roller 41 is increased. Therefore, when the information detection unit 60 detects a change in glossiness of the surface of the first pressure roller 41, the belt jump avoidance mode is selected.

  Alternatively, for example, the belt jump avoidance mode is automatically selected based on the slack displacement of the fixing belt 44 detected by the information detection unit 60. As described above, when the surface friction coefficient of the first pressure roller 41 increases, the fixing belt 44 is likely to be loosened near the downstream side of the fixing nip portion N in the sheet conveying direction. Therefore, when the information detecting unit 60 detects the displacement of the slack of the fixing belt 44, the belt jump avoiding mode is selected. For example, when the displacement of the slack of the fixing belt 44 reaches 500 μm in the normal direction of the surface of the fixing belt 44, the belt jump avoiding mode is selected.

  In the belt jump avoiding mode, either the automatic selection or the manual selection by the user can be set using the operation unit 4. For example, the user can make a judgment by looking at the state of the image, text, pattern, etc. printed on the sheet S, and manually select the belt jump avoidance mode.

  Next, the operation of the belt jump avoidance mode of the fixing device 40 will be described with reference to FIG. FIG. 5 is a graph showing the relationship between the surface speed of the fixing belt 44 and the belt jump generation cycle in the fixing device 40.

  Here, the surface speed of the fixing belt 44 and the surface speed of the second pressure roller 42 at each separation before the first pressure roller 41 and the second pressure roller 42 contact each other with the fixing belt 44 interposed therebetween. Is, for example, about 290 mm / s to 300 mm / s, 320 mm / s. In the configuration of the present embodiment, the surface speed of the fixing belt 44 is substantially equal to the surface speed of the second pressure roller 42 when the number of pulses of the first drive motor 47 that is a stepping motor is about 24100.

  Then, when the belt jump avoidance mode is selected, the image forming apparatus 1 executes a speed change that increases the surface speed of the fixing belt 44 at the fixing nip portion N more than the surface speed of the second pressure roller 42. At this time, in order to increase the surface speed of the fixing belt 44, the number of pulses of the first drive motor 47 which is a stepping motor is increased. Specifically, the image forming apparatus 1 performs control to transmit a command for increasing the number of pulses of the first drive motor 47 from about 24100 to the first drive motor 47 when the belt jump avoidance mode is selected. Run.

  As a result, according to FIG. 5, it can be seen that when the surface speed of the fixing belt 44 is increased more than the surface speed of the second pressure roller 42, the occurrence of belt jumping can be suppressed. On the other hand, according to FIG. 5, when the surface speed of the fixing belt 44 is slower than the surface speed of the second pressure roller 42 in a state where the surface of the first pressure roller 41 requires a fixing operation in the belt jump avoiding mode. That is, when the number of pulses of the first drive motor 47 is lower than about 24100, it can be seen that belt jumping is likely to occur.

  As in the above-described embodiment, the fixing device 40 has an information detection unit 60 that detects information related to the surface state of the first pressure roller 41, and the first pressure roller 41 and the second pressure roller 42. A first drive motor 47 and a second drive motor 48 provided separately are provided. In the image forming apparatus 1, as an operation mode at the time of fixing, the first pressure roller 41 and the second pressure roller 42 are set to a predetermined surface speed and the fixing belt 44 is set to a predetermined tension during normal fixing. It has a normal mode and a belt bounce avoiding mode selected based on information related to the surface state of the first pressure roller 41 detected by the information detection unit 60. Further, when the belt jump avoiding mode is selected, the image forming apparatus 1 executes a speed change that increases the surface speed of the fixing belt 44 at the fixing nip portion N more than the surface speed of the second pressure roller 42.

  According to this configuration, by executing the speed change of the fixing belt 44, it is possible to suppress the occurrence of the belt bounce on the downstream side of the fixing nip portion N in the sheet conveying direction. Therefore, even when the cumulative output number of sheets S increases, the first pressure roller 41 does not need to be replaced. At this time, the tension (exit curvature) on the fixing nip outlet side becomes the same as that before the surface coating of the first pressure roller is peeled off, and separation can be maintained while avoiding belt jump. Therefore, it is possible to suitably realize the separation of the sheet S after fixing from the fixing belt 44 while suppressing an increase in printing cost.

  Further, since the information detection unit 60 detects the cumulative output number of sheets S as information relating to the surface state of the first pressure roller 41, an increase in the surface friction coefficient of the first pressure roller 41 that causes belt jumping. Can be predicted based on the cumulative number of sheets S output. Therefore, the occurrence of belt jump can be effectively suppressed.

  Moreover, since the information detection part 60 detects the light reflectivity of the surface of the 1st pressurization roller 41 as information regarding the surface state of the 1st pressurization roller 41, the 1st pressurization roller 41 which causes a belt jump. It is possible to predict the increase in the surface friction coefficient based on the light reflectance of the surface of the first pressure roller 41. Therefore, the occurrence of belt jump can be effectively suppressed.

  Further, since the information detector 60 detects the slack displacement of the fixing belt 44 around the fixing nip portion N as information relating to the surface state of the first pressure roller 41, the first pressure that causes the belt to jump. An increase in the surface friction coefficient of the roller 41 can be predicted based on the slack displacement of the fixing belt 44. Therefore, the occurrence of belt jump can be effectively suppressed.

  In addition, since the operation unit 4 capable of either automatic selection of the belt bounce avoidance mode or manual selection by the user of the belt bounce avoidance mode is provided, selection and deselection of the belt bounce avoidance mode are appropriately performed as necessary. It is possible to select arbitrarily.

Second Embodiment
Next, an image forming apparatus according to a second embodiment of the present invention will be described with reference to FIGS. FIG. 6 is a schematic front view of the fixing device of the image forming apparatus. FIG. 7 is a schematic front view of the fixing device, and shows a state in which the tension of the fixing belt is changed. Since the basic configuration of this embodiment is the same as that of the first embodiment described above, the same components as those in the first embodiment are assigned the same names and the same reference numerals as before, and the details thereof are the same. The description may be omitted.

  As shown in FIG. 6, the fixing device 40 of the image forming apparatus 1 according to the second embodiment includes a cam member 54 whose urging unit 50 is a tension changing unit.

  The cam member 54 is disposed adjacent to the support member 53 between the support member 53 and the tension roller 43. For example, the cam member 54 has an elliptical shape when viewed from the sheet width direction, and can rotate around an axis extending in parallel with the sheet width direction. When the cam member 54 is rotated, the support member 53 adjacent to the cam member 54 is displaced along the outer shape of the cam member 54. That is, the support member 53 is guided by a guide portion (not shown) and is displaced in a direction in which the support member 53 approaches and separates from the tension roller 43.

  When the support member 53 is displaced in the direction away from the tension roller 43, the biasing spring 52 extends, and the biasing force by the biasing spring 52 is increased in the direction separating the tension roller 43 from the first pressure roller 41. To increase. As a result, the tension of the fixing belt 44 also increases.

  Then, when the belt jump avoidance mode is selected, the image forming apparatus 1 executes a tension change that increases the tension of the fixing belt 44 as compared with the normal mode. At this time, the cam member 54 is rotated to increase the tension of the fixing belt 44.

  According to the configuration of the second embodiment, by executing the tension change of the fixing belt 44, it is possible to suppress the occurrence of the belt jump on the downstream side of the fixing nip portion N in the sheet conveying direction. Therefore, even when the accumulated output number of sheets S increases, the first pressure roller 41 does not have to be replaced, and further, the fixing nip portion N after the fixing of the relatively thin sheet S is downstream in the sheet conveying direction. It is possible to maintain the separability from the fixing belt 44 while avoiding belt jumping.

<Third Embodiment>
Next, an image forming apparatus according to a third embodiment of the present invention will be described with reference to FIGS. FIG. 8 is a schematic front view of the fixing device of the image forming apparatus. FIG. 7 is a schematic front view of the fixing device, and shows a state in which the tension of the fixing belt is changed. Since the basic configuration of this embodiment is the same as that of the first embodiment described above, the same components as those in the first embodiment are assigned the same names and the same reference numerals as before, and the details thereof are the same. The description may be omitted.

  As shown in FIG. 6, the fixing device 40 of the image forming apparatus 1 according to the third embodiment includes a tension changing roller 55 in which the urging unit 50 is a tension changing unit.

  The tension change roller 55 is disposed adjacent to the inside of the fixing belt 44 between the support member 53 and the tension roller 43. The tension changing roller 55 can rotate around an axis extending in parallel with the sheet width direction. The tension changing roller 55 is displaced in the direction from the inner side to the outer side of the fixing belt 44 by using a cam mechanism or the like (not shown).

  When the tension change roller 55 is displaced in the direction from the inner side to the outer side of the fixing belt 44, the outer shape of the fixing belt 44 is pushed outward and the fixing belt 44 is extended. As a result, the tension of the fixing belt 44 increases.

  The tension changing roller 55 may be disposed adjacent to the outside of the fixing belt 44 in advance and displaced in the direction from the outside to the inside of the fixing belt 44.

  Then, when the belt jump avoidance mode is selected, the image forming apparatus 1 executes a tension change that increases the tension of the fixing belt 44 as compared with the normal mode. At this time, the tension changing roller 55 is displaced in order to increase the tension of the fixing belt 44.

  According to the configuration of the third embodiment, by changing the tension of the fixing belt 44, it is possible to suppress the occurrence of belt jumping on the downstream side of the fixing nip portion N in the sheet conveyance direction. Therefore, even when the accumulated output number of sheets S increases, the first pressure roller 41 does not have to be replaced, and further, the fixing nip portion N after the fixing of the relatively thin sheet S is downstream in the sheet conveying direction. It is possible to maintain the separability from the fixing belt 44 while avoiding belt jumping.

<Fourth embodiment>
Next, an image forming apparatus according to a fourth embodiment of the present invention will be described. Since the basic configuration of this embodiment is the same as that of the first embodiment described above, the same components as those in the first embodiment are assigned the same names and the same reference numerals as before, and the details thereof are the same. The description may be omitted.

  In the belt bounce avoiding mode, the image forming apparatus 1 according to the fourth embodiment increases the speed when changing the speed of the fixing belt 44 or the tension when changing the tension of the fixing belt 44 as the number of durable prints increases as shown in Table 1. Run it.

  Table 1 shows the difference in the surface speed of the fixing belt 44 with respect to the surface speed of the second pressure roller 42 when the speed of the fixing belt 44 is changed when the number of durable printed sheets increases to 100%, 175%, and 250%, for example. For example, it means increasing stepwise to 0.2%, 0.4%, and 0.6%.

  Table 1 also shows that when the number of durable printed sheets increases to, for example, 100%, 175%, and 250%, the tension of the fixing belt 44 is changed by, for example, twice or four times that in the normal mode when the tension of the fixing belt 44 is changed. , Means to increase stepwise by 6 times.

  According to the configuration of the fourth embodiment, the speed of the fixing belt 44 is changed stepwise in response to an increase in the surface friction coefficient of the first pressure roller 41 as the cumulative number of sheets S output increases. A tension change can be performed. As a result, it is possible to effectively suppress the occurrence of belt bounce on the downstream side of the fixing nip N in the sheet conveying direction.

  Note that when the number of durable printed sheets increases from 100% to 250%, for example, the speed change of the fixing belt 44 or the tension change of the fixing belt 44 may be continuously increased.

  Although the embodiments of the present invention have been described above, the scope of the present invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the invention.

  For example, the image forming apparatus 1 includes a first drive motor 47 and a second drive motor 48 that are two drive sources individually provided for the first pressure roller 41 and the second pressure roller 42, or a fixing belt. It is sufficient that at least one of the cam member 54 or the tension changing roller 55 which is a tension changing portion for changing the tension of 44 is provided, but both the driving source and the tension changing portion may be provided.

  The present invention can be used in an image forming apparatus including a fixing device.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 40 Fixing apparatus 41 1st pressure roller 42 2nd pressure roller 43 Tension roller 44 Fixing belt 47 1st drive motor (drive source)
48 Second drive motor (drive source)
50 Biasing part 54 Cam member (Tension changing part)
55 Tension change roller (Tension change part)
60 Information detector (detector)
N Fixing nip S Sheet

Claims (6)

An endless fixing belt wound around a first pressure roller and a tension roller and sandwiched between the first pressure roller and the second pressure roller, and the first pressure roller is sandwiched between the first pressure roller and the tension roller. In an image forming apparatus for fixing an unfixed toner image transferred onto a sheet at a fixing nip where the pressure roller and the second pressure roller are in contact with each other,
A detection unit for detecting information relating to a surface state of the first pressure roller;
At least one of two drive sources provided separately for the first pressure roller and the second pressure roller or a tension changing unit for changing the tension of the fixing belt;
With
As an operation mode at the time of fixing, a normal mode in which the first pressure roller and the second pressure roller are set to a predetermined surface speed and the fixing belt is set to a predetermined tension during normal fixing; A belt bounce avoiding mode selected based on information relating to the surface state of the first pressure roller detected by the detector;
When the belt bounce avoiding mode is selected, a speed change that increases the surface speed of the fixing belt at the fixing nip portion higher than the surface speed of the second pressure roller, or the tension of the fixing belt is set to the normal mode. An image forming apparatus that executes at least one of tension changes that increase more than the time.   The image forming apparatus according to claim 1, wherein the detection unit detects an accumulated output number of sheets as information related to a surface state of the first pressure roller.   The image forming apparatus according to claim 1, wherein the detection unit detects a light reflectance of a surface of the first pressure roller as information relating to a surface state of the first pressure roller.   2. The image forming apparatus according to claim 1, wherein the detection unit detects a slack displacement of the fixing belt in the vicinity of the fixing nip as information relating to a surface state of the first pressure roller. .   5. The operation unit according to claim 1, further comprising an operation unit that can be set to either automatic selection of the belt bounce avoidance mode or manual selection by a user of the belt bounce avoidance mode. Image forming apparatus.   6. The image forming apparatus according to claim 1, wherein in the belt jump avoiding mode, speed increase in the speed change or tension increase in the tension change is executed stepwise or continuously. .