JP4947066B2 - Fixing apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to a fixing device that thermally fixes an unfixed image formed on a recording sheet to a recording sheet, and an image forming apparatus including the fixing device.

  In an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a complex machine of these, usually, a toner image corresponding to image data is formed and transferred to a recording sheet, and the transferred toner image is fixed on the recording sheet by a fixing device. It has a configuration to let you. As a fixing device, a belt fixing method in which a fixing nip is formed using a pair of belts has been put into practical use. Since the belt fixing method has a small heat capacity, warm-up can be performed in a short time, and power saving can be achieved.

  However, the belt fixing method has a problem that it is not easy to obtain a sufficient nip pressure in the fixing nip because the fixing nip is formed by pressing the pair of belts together. Patent Document 1 discloses an endless pressure belt that has a thin cylindrical shape in a state in which no external force is applied to a heating belt that is wound around a belt support member including a planar heating element and a support roller. A fixing device is disclosed in which is pressed by pressing means such as a compression spring. Thus, the nip pressure can be increased by pressing the heating belt, to which the pressure belt is pressed by the pressing means, against the belt support member that is fixedly disposed.

JP 2006-58418 A

  In the fixing device disclosed in Patent Document 1, since the heating belt circulates while being in sliding contact with the belt support member that is fixedly arranged, the heating belt smoothly circulates by frictional force with the belt support member. Therefore, there is a possibility that the recording sheet cannot smoothly pass through the fixing nip. In addition, a large torque is required to smoothly move the heating belt that is in sliding contact with the belt support member. This may increase power consumption and reduce energy efficiency. Furthermore, since the heating belt is worn by sliding contact with the belt support member, the heating belt may not be used stably over a long period of time.

  The present invention has been made in view of the above-described conventional problems, and the object thereof is to suppress the belt wear without requiring a large torque in the belt fixing system using a pair of mutually opposed belts. An object of the present invention is to provide a fixing device capable of stably ensuring a high nip pressure in the entire nip portion formed between belts, and an image forming apparatus including the same.

  In order to achieve the above object, a fixing device according to the present invention includes a first belt wound around a pair of first rollers so as to be able to move around and a second belt wound around the pair of second rollers so as to be turned around. A fixing device that heat-fixes an unfixed image formed on a recording sheet by heating and pressurizing while the recording sheet passes through a fixing nip formed by pressure contact with the belt, When the first belt is separated from the second belt, each belt portion facing each other between the pair of first rollers has rigidity capable of running in a curved state protruding outward, In the second belt, one traveling portion between the pair of second rollers is in pressure contact with one traveling portion protruding outside the first belt, whereby one of the first belts First belt deforming means for deforming the first belt so that the nip pressure is increased in a traveling portion of the first belt pressed into contact with the second belt; It is provided.

  An image forming apparatus according to the present invention is an image forming apparatus that thermally fixes an unfixed image formed on a recording sheet by a fixing unit, and includes the fixing device as the fixing unit. .

  In the fixing device of the present invention, the belt portion of the second belt pressed against the belt portion of the first belt travels in a concavely curved state due to the rigidity of the first belt, and the first belt and the second belt Since the curved belt portions are pressed against each other to form a fixing nip, a special member such as a pressure pad for forming the fixing nip becomes unnecessary, and the fixing nip is high. The nip pressure can be secured stably over a long period of time. In addition, the first belt is deformed by the first belt deforming means, whereby the nip pressure can be increased in the entire area of the pressure contact portion between the first belt and the second belt. Thereby, gloss can be imparted to the toner image fixed on the recording sheet, and the image quality of the toner image can be improved. In addition, since it is not necessary to provide a member that is in sliding contact with the first belt, a large torque is not required to rotate the first belt, and it is possible to suppress wear of the first belt. Therefore, the fixing device can be used stably over a long period of time.

Preferably, the first belt deforming means is a pressing roller that presses the other traveling portion of the first belt located on the far side with respect to the second belt from the outside.
Preferably, the first belt deforming means has a first downstream side disposed on the downstream side with a peripheral speed of a first upstream roller disposed on the upstream side in the recording sheet passing direction of the pair of first rollers. It is a roller peripheral speed changing means for making it faster than the peripheral speed of the roller.

  Preferably, each of the pair of first rollers and each of the pair of second rollers are pressed against each other via the first belt and the second belt, so that An upstream nip portion and a downstream nip portion are formed in which the first belt and the second belt are pressed against each other, and the nip pressure of the upstream nip portion is lower than the nip pressure of the downstream nip portion. It is characterized by being.

Preferably, the unfixed image on the recording sheet is heated by a heating unit provided between the pair of first rollers inside the circumferential movement area of the first belt.
Preferably, the heating means is a heater lamp or a heating wire heater.

  Preferably, the unfixed image on the recording sheet is heated by an electromagnetic induction heating type heating unit.

1 is a schematic diagram illustrating an overall schematic configuration of a printer in which a fixing device according to an embodiment of the present invention is mounted as a fixing unit. 2 is a schematic diagram for explaining a configuration of a fixing device according to an embodiment of the present invention provided as a fixing unit in the printer. FIG. In order to explain the configuration of the fixing device, the second belt and the pressure roller are separated from the first belt. FIG. 4 is a schematic diagram for explaining that an intermediate nip portion between a first belt and a second belt in the fixing device becomes high pressure. It is a graph which shows the pressure distribution of the nip part between the 1st belt and the 2nd belt obtained by the experimental result on the predetermined conditions in the fixing device. It is a table | surface which shows the conditions of the said experiment. FIG. 6 is a schematic diagram for explaining a configuration of a fixing device according to another embodiment of the present invention. It is a graph which shows the pressure distribution of the nip part between the 1st belt and the 2nd belt obtained by the experimental result on the predetermined conditions in the fixing device. It is a schematic diagram which shows the other example of the fixing device of this invention.

  FIG. 1 is a schematic diagram showing a configuration of a tandem color digital printer (hereinafter simply referred to as “printer”) using a fixing device according to an embodiment of the present invention as a fixing unit. As shown in FIG. 1, the printer 1 according to the present embodiment includes an image processing unit 30 that forms a toner image, a sheet conveying unit 40 that conveys a recording sheet S to which the toner image is transferred, and a recording unit. And a fixing device 50 constituting a fixing unit for fixing the toner image transferred onto the sheet S to the recording sheet S. When the printer 1 receives a print execution instruction (print job) from a terminal device connected to a network (for example, an in-house LAN), the printer 1 forms a toner image on the recording sheet S according to the instruction.

  The image processing unit 30 is arranged in order from the upstream side along the circumferential movement direction of the intermediate transfer belt 36 below the intermediate transfer belt 36, and the intermediate transfer belt 36 that rotates in the direction indicated by the arrow A. The image forming units 39Y, 39M, 39C, and 39K, and the exposure device 21 that irradiates the image forming units 39Y, 39M, 39C, and 39K with the laser beam L, respectively. The intermediate transfer belt 36 is wound around a driven roller 37 disposed close to the image forming unit 39Y and a driving roller 38 disposed close to the image forming unit 39K. The driving roller 38 is not shown. By being rotationally driven by the motor, it moves around in the direction indicated by arrow A. The intermediate transfer belt 36 is semiconductive.

  The image forming unit 39Y disposed on the upstream side in the moving direction of the intermediate transfer belt 36 includes a photoconductive drum 31Y, and yellow (on the photoconductive drum 31Y through known charging, exposure, and development steps in order. A toner image of color Y) is formed. For this purpose, the image forming unit 39Y includes a roller charger 32Y that charges the outer peripheral surface of the photosensitive drum 31Y, and a laser beam L emitted from the exposure device 21 to the outer peripheral surface of the charged photosensitive drum 31Y. A developing device 33Y for developing the electrostatic latent image formed by the Y toner, a primary transfer roller 34Y for transferring the toner image formed on the outer peripheral surface of the photosensitive drum 31Y to the intermediate transfer belt 36, and a toner. A cleaner 35Y for cleaning the surface of the photosensitive drum 31Y after the image is transferred is provided in order along the rotation direction of the photosensitive drum 31Y.

  The other image forming units 39M, 39C, and 39K have the same configuration as the image forming unit 39Y, and are formed by a charger that charges the surface of the photosensitive drum and the laser light L emitted from the exposure device 21. A developing unit that develops the electrostatic latent image with toner, a primary transfer roller that transfers the toner image to the intermediate transfer belt 36, and a cleaner that cleans the surface of the photosensitive drum are provided, and magenta (M), cyan ( C) and black (K) toner images are formed with the respective toners and transferred to the intermediate transfer belt 36.

  As the charger provided in each of the image forming units 39Y, 39M, 39C, and 39K, a roller-shaped charging member is used. However, the present invention is not limited to this configuration, and a corona discharge charging charger or blade-shaped charging member is used. A member, a brush-shaped charging member, or the like can also be used. The exposure device 21 is provided with light emitting elements such as laser diodes driven by drive signals corresponding to image data of each color of Y, M, C, and K. From each light emitting element, each image forming unit 39Y, Laser light L is irradiated to the photosensitive drums (31Y, etc.) of 39M, 39C, and 39K, respectively.

  In the printer 1 of the present embodiment, a monochrome mode in which a monochrome image is formed using one color (for example, K color) toner, and a color image is formed using four color toners of Y, M, C, and K. It is possible to switch between modes. In the case of an image forming operation in the color mode, in each of the image forming units 39Y, 39M, 39C, and 39K, Y and Y formed on the outer peripheral surface of a photosensitive drum (31Y or the like) that is rotationally driven at a predetermined peripheral speed. The toner images of M, C, and K are transferred onto the same area of the intermediate transfer belt 36 by the primary transfer rollers (34Y and the like) of the image forming units 39Y, 39M, 39C, and 39K, respectively. In the case of an image forming operation in the monochrome mode, a toner image formed by one selected image forming unit is transferred onto a predetermined area on the intermediate transfer belt 36.

  The toner remaining on the outer peripheral surface of the photosensitive drum (31Y, etc.) without being transferred onto the intermediate transfer belt 36 is brought into contact with the cleaner (35Y, etc.) by the rotation of the photosensitive drum (31Y, etc.). When the toner reaches the part, it is removed from the outer peripheral surface of the photosensitive drum by a cleaner (35Y or the like). As the cleaner (35Y or the like), a plate-like blade, a fixed brush, a rotating brush, a roller, or a combination of these members can be used. Further, without providing such a cleaner, it is possible to employ a cleanerless system in which the toner remaining on the outer peripheral surface of the photosensitive drum (31Y, etc.) is collected by a developing device (33Y, etc.).

  Above the intermediate transfer belt 36, hoppers 22Y and 22M containing Y, M, C, and K color toners supplied to the developing devices (33Y and the like) of the image forming units 39Y, 39M, 39C, and 39K. , 22C, and 22K are provided, and the toners of the respective colors accommodated in the respective hoppers 22Y, 22M, 22C, and 22K are replenished to the developing devices (33Y and the like) of the respective image forming units 39Y, 39M, 39C, and 39K. .

  A sheet conveying unit 40 that conveys the recording sheet S includes a sheet feeding cassette 41 that is detachably provided on the lower side of the exposure device 21 and a recording sheet S that is fed from the sheet feeding cassette 41 to the side of the intermediate transfer belt 36. And a sheet conveyance path 42 that conveys the toner toward the fixing device 50 thereabove. In the paper feed cassette 41, recording sheets and the like are stored as recording sheets S in a stacked state, and the uppermost recording sheet S is fed into the sheet conveyance path 42 by the rotation of the paper feed roller 43.

  In the sheet conveyance path 42, a secondary transfer roller 45 is provided so as to face the drive roller 38 that rotates the intermediate transfer belt 36 around the intermediate transfer belt 36, and the intermediate transfer along the peripheral surface of the drive roller 38. A transfer nip is formed between the circumferential movement area of the belt 36 and the secondary transfer roller 45, and this transfer nip is a secondary transfer unit 46 that transfers the toner image onto the recording sheet S. The toner image transferred onto the intermediate transfer belt 36 is secondarily transferred by the secondary transfer roller 45 onto the recording sheet S that is conveyed upward in the sheet conveyance path 42.

  A registration roller pair 44 is provided below the secondary transfer unit 46 in the sheet transport path 42 to transport the recording sheet S fed from the paper feed cassette 41 to the secondary transfer unit 46 at a predetermined timing. The registration roller pair 44 controls the timing at which the recording sheet S is conveyed to the secondary transfer unit 46 so as to coincide with the timing at which the toner image transferred onto the intermediate transfer belt 36 is conveyed to the secondary transfer unit 46. To do. The recording sheet S conveyed to the secondary transfer unit 46 at a predetermined timing by the registration roller pair 44 is transferred with the toner image formed on the intermediate transfer belt 36 in the secondary transfer unit 46, and the secondary transfer unit 46. Is conveyed to the fixing device 50 disposed above. Note that the intermediate transfer belt 36 after the secondary transfer is cleaned by the cleaning blade 23 disposed opposite to the driven roller 37 via the intermediate transfer belt 36.

  The fixing device 50 includes a first belt 51 and a second belt 52 that form a fixing nip by being pressed against each other, a pressing roller 59 that presses an outer peripheral surface of the first belt 51, and a circular movement of the first belt 51. The toner image secondarily transferred to the recording sheet S in the lower secondary transfer unit 46 is conveyed to the upper side of the toner image 57, and the recording sheet S is conveyed upward. The recording sheet S is heat-fixed by being heated and pressed while passing through the fixing nips of the first belt 51 and the second belt 52 that are pressed against each other. The recording sheet S that has passed through the fixing device 50 is discharged by a paper discharge roller 61 onto a paper discharge tray 62 provided above the hoppers 22Y, 22M, 22C, and 22K.

  FIG. 2 is a schematic diagram for explaining the configuration of the fixing device 50, and FIG. 3 is a schematic diagram showing the second belt 52 and the pressing roller 59 separated from the first belt 51. The first belt 51 is an endless belt having a predetermined rigidity that maintains a cylindrical shape with a circular cross section in a natural state, and is disposed on the upstream side (lower side) in the conveyance direction of the recording sheet S as shown in FIG. The first upstream roller 53 and a first downstream roller 54 disposed on the downstream side (upper side) in the conveyance direction of the recording sheet S in a state parallel to the first upstream roller 53 have a predetermined tensile force ( By applying the belt tension), the belt is rigid so that it can move around while maintaining a substantially elliptical shape. The first upstream roller 53 and the first downstream roller 54 have the same radius r, and are aligned in the Y-axis direction (vertical direction) so that they have the same X-axis coordinate position (horizontal position). In this state, they are arranged with a predetermined distance D between the axes.

  The first belt 51 wound around the first upstream roller 53 and the first downstream roller 54 with tension due to the rigidity of the belt itself is separated from each of the second belt 52 and the pressing roller 59 in the first state. The mutually opposite belt running portions between the upstream roller 53 and the first downstream roller 54 are forced to return to the cylindrical state before the belt tension is applied (restoring force), so that the first upstream roller 53 And a curved state projecting outward from the common tangent L1 of the first downstream roller 54 by the same projection amount (deflection amount) δ. The first belt 51 is disposed so as to face the surface of the recording sheet S on which the toner image is transferred. For example, the first downstream roller 54 is driven to rotate by a drive motor, so that the arrow in FIG. Move in the direction indicated by M.

  A cylindrical heater lamp 57 formed in a straight line is provided at the intermediate position between the first upstream roller 53 and the first downstream roller 54 inside the circumferential movement area of the first belt 51, and the first upstream roller 53 and the first downstream roller 54 are arranged in parallel. As the heater lamp 57, a halogen heater, a carbon heater, a xenon lamp heater, or the like can be used. Further, not only the heater lamp 57 but also a heating wire heater formed in a linear shape can be used.

  The second belt 52 is an endless belt that is lower in rigidity than the first belt 51 and has rigidity (flexibility) that is bent when the first belt 51 is pressed, and is upstream of the recording sheet S in the conveyance direction ( A second upstream roller 55 disposed on the lower side and a second downstream roller 56 disposed on the downstream side (upper side) in the conveyance direction of the recording sheet S in a state parallel to the second upstream roller 55. For example, when the belt is applied with a predetermined tension by the second upstream roller 55 and is separated from the first belt 51, the second upstream roller 55 and the second downstream roller are wound. The belt running portions facing each other 56 are rotated around in parallel with each other. The second upstream roller 55 and the second downstream roller 56 also have radii r equal to the radii of the first upstream roller 53 and the first downstream roller 54, respectively, so that they have the same X coordinate position. In the state where they are aligned in the axial direction, the first upstream roller 53 and the first downstream roller 54 are disposed with the same inter-axis distance D as the inter-axis distance D. The second upstream roller 55 is moved by a tension spring (not shown) so that a belt tension (tensile force) F along the conveyance direction of the recording sheet S is applied to the second belt 52. It is biased downward.

  The second upstream roller 55 is arranged side by side in the X-axis direction (horizontal direction) so as to have the same Y coordinate position as the first upstream roller 53, and a predetermined roller load P1 is applied to the first upstream roller 53. Therefore, it is pressed through the second belt 52 and the first belt 51. By pressing the second upstream roller 55 against the first upstream roller 53, an upstream nip portion along the X-axis direction is formed between the first belt 51 and the second belt 52. The second downstream roller 56 is also arranged side by side in the X-axis direction (horizontal direction) so that it has the same Y coordinate position as the first downstream roller 54, and a predetermined roller load P2 is applied to the first downstream roller 54. Thus, it is pressed through the second belt 52 and the first belt 51. By pressing the second downstream roller 56 against the first downstream roller 54, a downstream nip portion having a predetermined nip pressure along the X-axis direction is formed between the first belt 51 and the second belt 52. Is done.

  As shown in FIG. 2, the first belt 51 and the second belt 52 configured as described above are configured such that the belt portion of the first belt 51 that travels in a curved state protruding outwardly faces the belt portion. The belt portion of the second belt 52, which is pressed against the belt portion 52 and has a rigidity lower than that of the first belt 51, is curved in a concave shape along the belt portion of the first belt 51. When the first belt 51 is rotated by the rotational driving of the side roller 54, the second belt 52 follows the first belt 51 by being in pressure contact with the first belt 51. 2 in the direction indicated by the arrow N.

  In this case, since the second belt 52 is lower in rigidity and flexible than the first belt 51, the curved belt portion that protrudes outside the first belt 51 is the first belt 51 as shown in FIG. When the belt portion of the second belt 52 is pressed with the pressing load W, the belt running portion of the second belt 52 follows the curved shape of the first belt 51 by the restoring force (reaction force) W of the second belt 52 itself. The curved belt traveling portion of the second belt 52 presses against the belt traveling portion of the first belt 51. Such a state is maintained even when the first belt 51 and the second belt move around. As a result, an intermediate nip portion in which the outer peripheral surfaces of the first belt 51 and the second belt 52 are in pressure contact with each other is formed in the entire region from the upstream nip portion to the downstream nip portion.

  In this state, as shown in FIG. 2, the pressing roller 59 is pressed against the traveling portion of the first belt 51 on the far side with respect to the second belt 52, and the pressed belt traveling portion is recessed in a concave shape. It is pressed to be in a state. As a result, the first belt 51 having rigidity is deformed so that the belt traveling portion forming the intermediate nip portion pressed against the second belt further protrudes outward, and the second belt in the intermediate nip portion is deformed. The belt running portion of the belt 52 is strongly pressed. As a result, in the intermediate nip portion, the belt running portion of the second belt 52 is curved in a larger concave shape than the case where the pressing roller 59 is not pressed, and the bending stress causes the entire region of the intermediate nip portion to The first belt 51 is strongly pressed against the outer peripheral surface. Accordingly, a high nip pressure can be obtained in the entire intermediate nip portion.

  The first belt 51 is, for example, a base having a predetermined thickness made of a material such as Ni (nickel), iron, SUS (stainless steel), W (tungsten), PI (polyimide) having a large elastic modulus (Young's modulus). The cross section is endlessly formed into a circular cylinder having a predetermined outer diameter. The thickness of the base material is, for example, 50 to 200 μm. On the surface (outer peripheral surface) of the base material, an elastic layer such as a fluorine-based resin such as PFA, PTFA, and ETFA, a silicon-based rubber, and the elastic layer A release layer composed of a PFA tube, a PFA coat, or the like is provided on the surface. The thickness of the elastic layer is, for example, 200 μm, and the thickness of the release layer is, for example, 30 μm.

  The second belt 52 has a predetermined rigidity lower than that of the first belt 51 so that the belt portion to which the first belt 51 is pressed is curved in a concave shape, and has heat resistance such as polyimide resin, for example. It has a laminated structure of a heat-resistant layer and a release layer laminated on the surface (outer peripheral surface) of this heat-resistant layer. The release layer is made of a material excellent in releasability with the toner, and for example, a fluororesin is used. The thickness of the heat-resistant layer is, for example, 30 to 100 μm, and the thickness of the release layer is, for example, about 30 μm. The second belt 52 is not limited to such a two-layer structure, and one or more layers may be provided between the heat-resistant layer and the release layer.

  In the fixing device 50 having such a configuration, when the recording sheet S on which the toner image is transferred by the secondary transfer unit 46 is conveyed, the recording sheet S is divided into the first upstream roller 53 and the second upstream roller 55. And enters the upstream nip portion between the first belt 51 and the second belt 52, which has reached a predetermined nip pressure P1, and is pressed by the pressing roller 59, so that it is in a curved state that largely protrudes outward. After passing through the intermediate nip portion in a state of being strongly pressed between the belt running portion of the first belt 51 and the belt running portion of the second belt 52 curved in a concave shape, the first downstream roller 54 and the second belt The paper is discharged through the downstream nip portion between the first belt 51 and the second belt 52, which has reached a predetermined nip pressure P2 by the downstream roller 56. The toner image on the recording sheet S is heated and melted by the heater lamp 57 while being passed through the upstream nip portion, the intermediate nip portion, and the downstream nip portion, and is pressed onto the recording sheet S. Heat fixing.

  In this case, in the intermediate nip portion, the first belt 51 is pressed by the pressing roller 59, so that the first belt 51 and the second belt 52 are pressed against each other with a strong pressure over the entire area. Since the high nip pressure is ensured, the toner image on the recording sheet S is reliably pressed and thermally fixed in the entire area of the intermediate nip portion of the recording sheet S. The toner image on the recording sheet S is melted while passing through the intermediate nip portion, and is pressed against the recording sheet S with a strong pressure over the entire area of the intermediate nip portion. As a result, the toner image is smoothed over the entire image without causing image misalignment. Further, since a high nip pressure is obtained in the entire intermediate nip portion, vibration of the recording sheet S can be reliably suppressed, and wrinkles are generated in the recording sheet S when passing through the downstream nip portion. Is also reliably suppressed.

  The nip pressure P1 at the upstream nip portion and the nip pressure P2 at the downstream nip portion may be equal to each other, but the nip pressure P2 at the downstream nip portion is greater than the nip pressure P1 at the upstream nip portion. It is preferable to increase the value. Thus, by making the nip pressure of the downstream nip portion larger than the nip pressure of the upstream nip portion, the toner heated and melted in the entire intermediate nip portion is pressurized by the high nip pressure in the downstream nip portion. Thus, the toner image is further flattened. As a result, the toner image is further glossed and the image quality is further improved.

  The nip pressure of the fixing nip in the fixing device 50 of this embodiment shown in FIG. 2 was measured under the following conditions. The first belt 51 is formed by using a cylindrical nickel (Ni) belt having a thickness of 120 μm, a width of 350 mm, and a diameter of 34 mm as a base material, and an elastic layer and a release layer are laminated on the surface. By being wound around the first upstream roller 53 and the first downstream roller 54 which are arranged in a fixed state so as to be 8 mm, the circular movement is performed while maintaining a predetermined elliptical shape. As the second belt 52, a belt having a heat-resistant layer made of polyimide resin is used similarly to the belt generally used in the conventional fixing device, and the second upstream roller 55 and the second downstream side are used. A tension is applied to the second upstream roller 55 so as to be wound around the roller 56 and to have a belt tension of 20N. The roller load (nip pressure P1 of the upstream nip portion) between the first upstream roller 53 and the second upstream roller 55 is 50 N, and the roller load (downstream side) between the first downstream roller 54 and the second downstream roller 56 The nip pressure P2) of the nip portion was set to 300N, the pressing roller load by the pressing roller 59 was set to 150N, and the first downstream roller 54 was driven at a constant rotational speed to rotate the first belt 51. In this case, the first upstream roller 53 is rotated by the circular movement of the first belt 51, so that the rotational speed of the first upstream roller 53 becomes equal to the rotational speed of the first downstream roller 54. ing. Further, since the first upstream roller 53 and the first downstream roller 54 have the same radius r, their peripheral speeds are equal, and the first downstream roller 54 and the first downstream roller 54 have the same peripheral speed. The ratio of the peripheral speed of one upstream roller 53 is 100%. The nip pressure (pressure per unit area) in the entire fixing nip was measured with a pressure sensor (trade name “Tactile Sensor” manufactured by Nitta Corporation). As shown by the solid line in FIG. A high nip pressure was obtained. In this case, the minimum nip pressure (pressure per unit area) at the intermediate nip portion was as high as 9 kPa. When the toner image is fixed on the recording sheet S by the fixing device 50 under such conditions, the toner image fixed on the recording sheet S does not cause image shift and has high image quality with gloss. It was.

  For comparison, the first belt 51 is not pressed by the pressing roller 59, and the first belt is subjected to the same conditions except that the nip pressure of the upstream nip and the nip pressure of the downstream nip are equal. When the nip pressure in the entire area of the fixing nip was measured by moving the circuit 51 around, the nip pressure in the intermediate nip portion was lower than the nip pressure in the present embodiment over the entire area as shown by the broken line in FIG. . In this case, the minimum nip pressure (pressure per unit area) at the intermediate nip portion is 2 kPa, and image displacement may occur depending on the image forming conditions. Each result is shown in the table of FIG.

As described above, in the fixing device 50 according to the present embodiment, the nip pressure of the intermediate nip in the fixing nip can be increased throughout the intermediate nip, and image misalignment can be reliably prevented from occurring in the fixing nip. In addition, a high-quality toner image having gloss can be formed.
In the above embodiment, the first downstream roller 54 is rotationally driven to make the first belt 51 go around, and the second belt 52 goes around following the round movement. Not limited to this configuration, the second downstream roller 56 is rotated by the drive motor to cause the second belt 52 to go around, and the first belt 51 goes around following the circumferential movement of the second belt 52. You may make it move. Alternatively, the first upstream roller 53 or the second upstream roller 55 is driven to rotate, the first upstream roller 53 and the first downstream roller 54, or the second upstream roller 55 and the second downstream roller 56. May be configured to rotate each of the four rollers 53 to 56, respectively.

  FIG. 7 is a schematic diagram showing a schematic configuration of a fixing device 50 according to another embodiment of the present invention. In the fixing device 50, the first upstream roller 53 and the first downstream roller 54 around which the first belt 51 is wound are rotationally driven by independent separate first motor 64 and second motor 65, respectively. The rotational speeds of the first motor 64 and the second motor 65 are controlled by the control unit 66. The controller 66 controls the circumferential speed of the first motor 64 and the second motor 65 so that the circumferential speed of the first upstream roller 53 is higher than the circumferential speed of the first downstream roller 54. It functions as a speed changing means. The other configuration is the same except that the first belt 51 is not pressed by the fixing device 50 and the pressing roller 59 shown in FIG.

  In the fixing device 50 having such a configuration, the peripheral speed of the first upstream roller 53 is higher than the peripheral speed of the first downstream roller 54, so that it passes through the intermediate nip portion of the first belt 51. The belt running portion is deformed into a large curved state such that the conveyance speed in the vicinity of the upstream nip portion is higher than the conveyance speed in the vicinity of the upstream nip portion and further protrudes outward. Thereby, the nip pressure can be increased in the entire area of the intermediate nip portion.

  In the fixing device 50 according to the second embodiment that does not use the pressing roller 59, the first upstream roller is set so that the ratio of the circumferential speed of the first upstream roller 53 to the circumferential speed of the first downstream roller 54 is 105%. Except that the rotational speed of 53 is higher than the rotational speed of the first downstream roller 54, the nip pressure in the entire fixing nip is measured by a pressure sensor (NITTA CORPORATION) as the same conditions as the fixing conditions in the first embodiment. Product, product name “Tactile Sensor”), a relatively high nip pressure was obtained over the entire intermediate nip as shown by the solid line in FIG. In this case, the minimum nip pressure (pressure per unit area) at the intermediate nip portion was 9 kPa. When the unfixed toner image on the recording sheet S was heat-fixed by the fixing device 50 under such conditions, the toner image fixed on the recording sheet S did not cause image shift and was given gloss. A high-quality toner image could be obtained. In addition, the broken line in FIG. 8 is the nip pressure in the comparative example shown by the broken line in FIG. The results are also shown in the table of FIG.

Also in the fixing device 50 in the present embodiment, the nip pressure of the intermediate nip in the fixing nip can be increased over the entire area of the intermediate nip, and it is possible to surely prevent image misalignment from occurring in the fixing nip and to have high gloss A high-quality toner image can be obtained.
The fixing device of the present invention is not limited to each of the above embodiments. For example, the heater lamp 57 is not limited to the configuration in which the heater lamp 57 is provided between the first upstream roller 53 and the first downstream roller 54, and another heating unit may be provided together with the heater lamp 57. For example, in a configuration in which the first upstream roller 53 and the second upstream roller 55 each have a hollow core bar, a heater lamp may be disposed together with the heater lamp 57 in each core bar. Or it is good also as a structure which arrange | positions a heater lamp only in any one metal core of the 1st upstream roller 53 and the 2nd upstream roller 55. FIG. Furthermore, a heater lamp may be provided between the second upstream roller 55 and the second downstream roller 56 together with the heater lamp 57. In any configuration, since the heating efficiency of the fixing nip portion can be improved, the warm-up time can be further shortened. Moreover, not only a heater lamp but a linear heating wire heater can also be used.

  The heating means is not limited to the heater lamp 57, the heating wire heater or the like. For example, an induction heating element is provided on the first belt 51, and the second belt 52 of the first belt 51 is provided as shown in FIG. The first belt 51 may be heated by electromagnetic induction heating type heating means in which a pair of exciting coils 65 are arranged in the circumferential movement direction so as to face the belt portion located on the far side.

  Furthermore, in the above-described embodiment, the first belt 51 is disposed to face the surface of the recording sheet S on which the toner image is fixed. However, the toner image on the recording sheet S is fixed to the second belt 52. You may make it arrange | position facing the made surface. In addition, each of the first upstream roller 53 and the first downstream roller 54 and the second upstream roller 55 and the second downstream roller 55 have the same radius. However, the radius of each roller can be arbitrarily set. Furthermore, although the nip pressure of the downstream nip portion is set higher than the nip pressure of the upstream nip portion, each nip pressure may be made equal.

  The image forming apparatus to which the fixing device according to the present invention is applied is not limited to a tandem color digital printer. For example, four developing devices are arranged around a rotation shaft, and these four developing devices are The image forming apparatus may be a so-called four-cycle image forming apparatus that forms a full-color image sequentially facing the electrostatic latent image carrier, or a monochrome image forming apparatus that includes only one developing device. Further, the fixing device of the present invention is not limited to a printer but can be applied to a copying machine, a FAX, an MFP (Multiple Function Peripheral), and the like.

  The present invention provides a fixing device that heats and pressurizes an unfixed image using a pair of belts and thermally fixes the fixing nip in which the belts are pressed against each other without providing a member that slides on the inner peripheral surface of the belt. A high nip pressure can be ensured stably over a long period in the entire region.

30 Image process unit 36 Intermediate transfer belt 39Y, 39M, 39C, 39K Image forming unit 40 Sheet conveying unit 50 Fixing device 51 First belt 52 Second belt 53 First upstream roller 54 First downstream roller 55 Second upstream side Roller 56 Second downstream roller 57 Heater lamp 59 Press roller 65 Excitation coil

Claims (8)

A fixing nip formed by press-contacting a first belt wound around a pair of first rollers so as to be able to move around and a second belt wrapped around a pair of second rollers so as to be able to move around. A fixing device that heat-fixes an unfixed image formed on the recording sheet by heating and pressing while the recording sheet passes,
When the first belt is separated from the second belt, each belt portion facing each other between the pair of first rollers has rigidity capable of running in a curved state protruding outward,
In the second belt, one running portion between the pair of second rollers is in pressure contact with one running portion protruding outside the first belt, whereby one running of the first belt is performed. It becomes in a state curved in a concave shape along the part,
A fixing device, comprising: a first belt deforming unit configured to deform the first belt so that a nip pressure increases in a traveling portion of the first belt pressed against the second belt.   2. The fixing according to claim 1, wherein the first belt deforming unit is a pressing roller that presses the other traveling portion of the first belt located on the far side with respect to the second belt from the outside. apparatus.   The first belt deforming means is configured such that the peripheral speed of the first upstream roller disposed on the downstream side is the peripheral speed of the first upstream roller disposed on the upstream side in the recording sheet passing direction of the pair of first rollers. The fixing device according to claim 1, wherein the fixing device is a roller peripheral speed changing unit that makes the speed higher than the speed. Each of the pair of first rollers and each of the pair of second rollers are pressed against each other via the first belt and the second belt, so that the first belt is applied to each pressing portion. And an upstream nip portion and a downstream nip portion where the second belt is pressed against each other are formed,
The fixing device according to claim 1, wherein a nip pressure of the upstream nip portion is lower than a nip pressure of the downstream nip portion.   5. The unfixed image on the recording sheet is heated by a heating means provided between the pair of first rollers inside the circular movement area of the first belt. The fixing device according to any one of the above.   The fixing device according to claim 5, wherein the heating unit is a heater lamp or a heating wire heater.   The fixing device according to claim 1, wherein the unfixed image on the recording sheet is heated by heating means of an electromagnetic induction heating method. An image forming apparatus for thermally fixing an unfixed image formed on a recording sheet by a fixing unit,
An image forming apparatus comprising the fixing device according to claim 1 as the fixing unit.

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