JP5719964B2 - Fixing apparatus and image forming apparatus having the same - Google Patents

Description

  In the present invention, a fixing belt is pressed against a pressure roller by a pressing member, a recording medium carrying an unfixed toner image is inserted into a fixing nip formed by the fixing belt and the pressure roller, and heated and pressed. The present invention relates to a fixing device for fixing to a recording medium, and an image forming apparatus using an electrophotographic system having the same.

Conventionally, in an image forming apparatus, a toner image formed on an image carrier such as a photosensitive drum is transferred to a recording medium, and the recording medium carrying the toner image is conveyed toward a fixing device. The toner image on the recording medium is fixed on the recording medium by applying heat and pressure. In the fixing device, toner is applied to the recording medium by inserting a recording medium carrying an unfixed toner image through a fixing nip formed by a heated endless fixing belt and a pressure roller pressed against the fixing belt. There is a belt fixing method for fixing.

  In this belt fixing method, a pressing member is disposed inside the fixing belt, a pressure roller is pressed against the pressing member from the outside of the fixing belt, and the fixing belt is rotated by sliding the inner surface of the fixing belt against the pressing member. I am letting. And, by making the shape of the sliding surface of the pressing member flat or arcuate, a wide nip width is obtained to improve the fixing performance, and the separation property of the paper is improved.

  However, when the sliding surface of the pressing member is formed in an arc shape, when an unfixed toner image on a recording medium such as an envelope is fixed, the conveyance speed between the front surface and the back surface of the envelope is different in the fixing nip portion. There was a problem that wrinkles were generated on the front and back surfaces, and the flaps of the envelope were shifted from a predetermined position.

  Thus, related techniques for preventing the generation of wrinkles on envelopes during fixing processing have been known. For example, the fixing device disclosed in Patent Document 1 can be switched between a pressing member having a soft pad made of an elastic soft material and a hard pad made of a hard material, and a standard mode and an envelope mode for fixing plain paper such as A4 paper. Switching mechanism. In the standard mode, the fixing nip portion is formed by the soft pad and the hard pad contacting the fixing belt. On the other hand, in the envelope mode, the hard pad is separated by the switching mechanism, and only the soft pad comes into contact with the fixing belt to form a fixing nip portion. As a result, the nip pressure is reduced, and the wrinkle and flap deviation of the envelope are reduced. ing.

JP 2008-233133 A (paragraphs [0043] to [0045], [0054] to [0058], FIGS. 2 and 3)

  However, in the fixing device described in Patent Document 1, in the standard mode, the nip pressure becomes unstable at the boundary between the soft pad and the hard pad, and in the envelope mode, the nip pressure becomes unstable in the soft pad made of a soft material. In this mode, there is a problem that it is difficult to obtain good fixing performance.

  The present invention has been made in order to solve the above-described problems, and a fixing device capable of reducing envelope wrinkles and envelope flap displacement of recording media such as envelopes and obtaining good fixing performance, and An object is to provide an image forming apparatus provided.

  In order to achieve the above object, the first invention provides a rotatable endless fixing belt heated by a heating means, a pressure roller disposed opposite to the outer peripheral surface of the fixing belt, and the fixing belt. And a pressing member that presses the fixing belt in contact with the pressure roller, and the recording medium is passed through a fixing nip formed by the fixing belt and the pressure roller and is carried on the recording medium. In the fixing device for fixing an unfixed toner image, a flat portion substantially parallel to the conveyance direction of the recording medium is provided on a surface of the pressing member facing the fixing belt, and a downstream side of the flat portion in the conveyance direction of the recording medium. An arcuate portion that curves toward the pressure roller, and a sliding surface on which an inner peripheral surface of the fixing belt slides is formed, and the pressure roller passes through the fixing belt. Flat part and A first mode in which the fixing nip portion is formed by pressure contact with the arcuate portion, and a second mode in which the pressure roller is in pressure contact with only the flat portion via the fixing belt. A switching mechanism for switching between modes is provided.

  According to a second aspect of the present invention, in the above fixing device, the switching mechanism reciprocates the pressure roller in a direction substantially parallel to the flat portion, so that the first mode and the second mode are changed. It is characterized by switching.

  According to a third aspect of the present invention, in the fixing device, the switching mechanism holds the pressure roller rotatably and holds the holding member movably in a direction substantially parallel to the flat portion. A first urging member that urges the member downstream in the conveyance direction of the recording medium; and an operation member that moves the holding member upstream in the conveyance direction of the recording medium against the urging force of the first urging member. It is characterized by having.

  According to a fourth aspect of the present invention, in the fixing device, the switching mechanism causes the fixing nip pressure in the second mode to be smaller than that in the first mode when switching from the first mode to the second mode. It is characterized by switching to.

  According to a fifth aspect of the present invention, in the fixing device, the switching mechanism is located at a position upstream of the fixing nip portion in the recording medium conveyance direction and with respect to the rotation center of the pressure roller. A holding member that rotatably holds the pressure roller with a position offset toward the side as a rotation fulcrum, and a second biasing member that biases the holding member in a direction in which the pressure roller presses against the pressing member, And an operating member that moves the pressure roller to the upstream side in the conveyance direction of the recording medium by rotating the holding member in a direction in which the pressure roller is separated from the pressing member.

  A sixth aspect of the invention is an image forming apparatus including the fixing device having the above-described configuration.

  According to the first invention, when fixing plain paper such as A4 paper, the fixing nip portion is formed in a flat shape and an arc shape by switching to the first mode by the switching mechanism. The unfixed toner image is stably and satisfactorily fixed. When fixing a recording medium such as an envelope, the switching mechanism switches to the second mode, so that the fixing nip portion is formed in a flat shape. The received toner image is fixed satisfactorily.

1 is a diagram illustrating a schematic configuration of an image forming apparatus including a fixing device according to a first embodiment of the present invention. 1 is a side sectional view showing a fixing device according to a first embodiment. Side surface sectional drawing which shows the press member of the fixing device which concerns on 1st Embodiment. 1 is a side view schematically showing a switching mechanism in a first mode of a fixing device according to a first embodiment. FIG. 3 is a side view schematically showing a switching mechanism in a second mode of the fixing device according to the first embodiment. FIG. 9 is a side view schematically showing a switching mechanism in the first mode of the fixing device according to the second embodiment. FIG. 9 is a side view schematically showing a switching mechanism in the second mode of the fixing device according to the second embodiment.

  Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments. Further, the use of the invention and the terms shown here are not limited thereto.

(First embodiment)
FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus including a fixing device according to an embodiment of the present invention. The image forming apparatus 1 is provided with a sheet feeding unit 2 disposed in a lower portion thereof, a sheet conveying unit 3 disposed on a side of the sheet feeding unit 2, and an upper side of the sheet conveying unit 3. The image forming unit 4 includes a fixing device 5 disposed on the discharge side of the image forming unit 4, and an image reading unit 6 disposed above the image forming unit 4 and the fixing device 5.

  The paper feed unit 2 includes a plurality of paper feed cassettes 7 that store paper 9 that is a recording medium, and a manual feed tray 22 that supplies manual paper. By rotation of the paper feed roller 8, the paper 9 is sent out one by one from the paper feed cassette 7 selected among the plurality of paper feed cassettes 7 to the paper transport unit 3. On the manual feed tray 22, paper having a size different from that of the paper 9 stored in the paper feed cassette 7 or a recording medium such as an envelope or an OHP sheet is placed, and the recording medium is sent to the paper transport unit 3.

  The paper 9 sent to the paper transport unit 3 is transported toward the image forming unit 4 via the paper transport path 10. The image forming unit 4 forms a toner image on a sheet 9 by an electrophotographic process. The image forming unit 4 is a photoconductor 11 supported so as to be rotatable in the direction of an arrow in FIG. A charging unit 12, an exposure unit 13, a developing unit 14, a transfer unit 15, a cleaning unit 16, and a charge eliminating unit 17 are provided.

  The charging unit 12 includes a charging wire to which a high voltage is applied. When a predetermined potential is applied to the surface of the photoreceptor 11 by corona discharge from the charging wire, the surface of the photoreceptor 11 is uniformly charged. When light based on the image data of the original read by the image reading unit 6 is irradiated onto the surface of the photoconductor 11 by the exposure unit 13, the potential on the surface of the photoconductor 11 is selectively attenuated, and the surface of the photoconductor 11 An electrostatic latent image is formed.

  Next, the developing unit 14 develops the electrostatic latent image on the surface of the photoconductor 11, and a toner image is formed on the surface of the photoconductor 11. This toner image is transferred by the transfer unit 15 to the sheet 9 supplied between the photoconductor 11 and the transfer unit 15.

  The sheet 9 on which the toner image has been transferred is conveyed toward the fixing device 5 disposed downstream of the image forming unit 4 in the sheet conveying direction. In the fixing device 5, the paper 9 is heated and pressed, and the toner image is melted and fixed on the paper 9. Next, the sheet 9 on which the toner image is fixed is discharged onto the discharge tray 21 by the discharge roller pair 20.

  After the transfer of the toner image onto the paper 9 by the transfer unit 15, the toner remaining on the surface of the photoconductor 11 is removed by the cleaning unit 16, and the residual charge on the surface of the photoconductor 11 is removed by the charge eliminating unit 17. Then, the photosensitive member 11 is charged again by the charging unit 12, and image formation is performed in the same manner.

  The fixing device 5 is configured as shown in FIG. FIG. 2 is a side sectional view schematically showing the fixing device.

  The fixing device 5 is a fixing method using electromagnetic induction heating, and includes a fixing belt 26, a pressure roller 19, an induction heating unit 30 that heats the fixing belt 26, a thermistor 25 that is a temperature detection unit, and a belt. And a support member 55.

  The fixing belt 26 is an endless heat-resistant belt, and in order from the inner peripheral side, for example, an induction heating layer 26a made of electroformed nickel having a thickness of 30 to 50 μm, and an elastic material made of silicone rubber having a thickness of 100 to 500 μm, for example. The layer 26b and a release layer 26c made of, for example, a fluororesin having a thickness of 30 to 50 μm and improving the releasability when the unfixed toner image is melted and fixed at the fixing nip portion N are laminated. . The induction heating layer 26a may be formed by mixing metal powder such as copper, silver, or aluminum into polyimide resin having a thickness of 50 to 100 μm, for example.

  The belt support member 55 includes a guide portion 59 and a pad holding portion 56, and is made of a metal such as aluminum, a heat resistant resin, or the like. The guide portion 59 has an arc shape in a cross-sectional view, and holds the fixing belt 26 at a predetermined interval with respect to the induction heating portion 30. The pad holding part 56 holds a pressing pad 60 that is a pressing member. The pressing pad 60 is disposed on the inner peripheral surface of the fixing belt 26 and faces the pressure roller 19 through the fixing belt 26. The pressing pad 60 presses the fixing belt 26 toward the pressure roller 19 side. The pad holding portion 56 may be provided separately from the guide portion 59 and supported by the apparatus main body.

  The pressure roller 19 includes a cylindrical cored bar 19a such as stainless steel, an elastic layer 19b made of, for example, silicone rubber formed on the cored bar 19a, and a mold release made of a fluororesin or the like covering the surface of the elastic layer 19b. A layer 19c. The pressure roller 19 is rotated by a drive source such as a motor (not shown), and the fixing belt 26 is driven to rotate by the rotation of the pressure roller 19. A fixing nip portion N is formed at a portion where the pressure roller 19 and the fixing belt 26 are in pressure contact. The fixing nip portion N heats and presses an unfixed toner image on the paper 9 to be conveyed, and the toner image on the paper 9. To fix.

  The induction heating unit 30 includes a coil 37, a bobbin 38, and a magnetic core 39, and heats the fixing belt 26 by electromagnetic induction. The induction heating unit 30 extends in the width direction of the fixing belt 26 (the front and back surfaces of the paper surface in FIG. 2) and is disposed to face the fixing belt 26 so as to surround substantially half of the outer periphery of the fixing belt 26.

  The exciting coil 37 is attached to the bobbin 38 by winding a litz wire in a loop shape a plurality of times along the width direction of the fixing belt 26 (front and back direction in FIG. 2). The exciting coil 37 is connected to a power source (not shown) and generates an alternating magnetic flux by a high frequency current supplied from the power source. The magnetic flux from the exciting coil 37 passes through the magnetic core 39, is guided in a direction parallel to the paper surface of FIG. 2, and passes along the induction heating layer 26 a of the fixing belt 26. An eddy current is generated in the induction heat generation layer 26a due to a change in the alternating strength of the magnetic flux passing through the induction heat generation layer 26a. When an eddy current flows through the induction heating layer 26a, Joule heat is generated by the electrical resistance of the induction heating layer 26a, and the fixing belt 26 generates heat.

  The thermistor 25 is disposed on the surface of the fixing belt 26 so as to face the center and both ends in the width direction, and detects the temperature of each region. Based on the temperature detected by the thermistor 25, the current supplied to the exciting coil 37 of the induction heating unit 30 is controlled.

  When the fixing belt 26 is heated by the induction heating unit 30 serving as a heating unit and reaches a fixing temperature, the sheet 9 sandwiched by the fixing nip N is heated and is pressed by the pressure roller 19. The powdered toner on the paper 9 is melted and fixed. The sheet 9 after the fixing process is conveyed in close contact with the surface of the fixing belt 26, but is separated from the surface of the fixing belt 26 by a separation member (not shown) and conveyed downstream of the fixing device 5.

  A detailed configuration around the pressure pad 60 and the fixing nip N is shown in FIG. FIG. 3 is a side sectional view of the periphery of the pressing pad 60 and the fixing nip N. 3 is a view of the fixing nip portion N of FIG. 2 as viewed from the back side of the paper surface, and therefore the sheet 9 is conveyed from the arrow A to the arrow B direction with respect to the fixing nip portion N.

  The pressing pad 60 is made of a heat resistant resin such as a liquid crystal polymer resin or an elastic material such as silicone rubber, and an elastomer may be disposed on the sliding surface 60 a facing the fixing belt 26. In addition, a fluororesin-based sliding member (not shown) such as a PTFE sheet may be interposed on the sliding surface 60a in order to reduce the sliding load on the contact surface with the fixing belt 26. When the pressing pad 60 is formed of a liquid crystal polymer resin, the deformation of the pressing pad 60 due to heating and pressing can be suppressed and the shape of the fixing nip portion N can be maintained constant over a long period of time.

  The sliding surface 60a has a flat portion 61 substantially parallel to the paper conveyance direction (in the direction of arrow A in the figure) on the upstream side of the fixing nip portion N, and the pressure roller 19 side on the downstream side of the flat portion 61 in the paper conveyance direction. And an arcuate portion 63 that curves.

  The arc-shaped part 63 has a configuration having a radius of curvature larger than the radius of curvature of the pressure roller 19. With this configuration, the pressure contact force of the fixing nip portion N can be reduced from the upstream side to the downstream side in the sheet conveyance direction, and accordingly, the deformation of the pressure pad 60 due to the heat of the fixing belt 26 and the pressure of the pressure roller 19. And the separation of the sheet 9 at the downstream end of the fixing nip N can be maintained for a long period of time.

  In the present embodiment, the pressure roller 19 abuts against the flat portion 61 and the arc-shaped portion 63 via the fixing belt 26 to form the fixing nip portion N, and the pressure roller 19 is the fixing belt. The second mode in which the fixing nip portion N is formed can be switched by contacting only the flat portion 61 via the H.26.

  When fixing plain paper such as A4 paper, the pressure roller 19 is pressed against the flat portion 61 and the arc-shaped portion 63 through the fixing belt 26 with a predetermined pressure. With this configuration, since the fixing nip portion N in which the flat portion 61 and the arc-shaped portion 63 continue in this order from the upstream side with respect to the paper conveyance direction is formed, the sheet 9 that has entered the fixing nip portion N is first flat. After passing through 61, it passes through the arc-shaped portion 63. Accordingly, the sheet 9 can be stably inserted from the flat portion 61 into the fixing nip portion N. Further, since the toner on the sheet 9 is melted and fixed to some extent when passing through the arc-shaped portion 63, the fixing image is less likely to be disturbed when passing through the arc-shaped portion 63 where the nip pressure tends to vary. The performance can be stabilized.

  On the other hand, when a fixing process is performed on a recording medium such as an envelope, the pressure roller 19 is configured to be pressed only to the flat portion 61 with a predetermined pressure via the fixing belt 26. With this configuration, the fixing nip N is formed in a flat shape with a predetermined nip pressure, and the contact width (fixing nip width) between the pressing pad 60 and the pressure roller 19 is relatively short. When the recording medium passes through the fixing nip portion N, the conveyance speed does not change between the front surface and the back surface of the envelope, and the nip pressure applied to the envelope is generally reduced, so that wrinkles and flap deviation of the envelope can be reduced. .

  The configuration of the switching mechanism is shown in FIGS. FIG. 4 is a side view schematically showing the configuration of the switching mechanism in the first mode, and FIG. 5 is a side view schematically showing the configuration of the switching mechanism in the second mode. A pair of holding plates 71 and a pair of side plates 65 are disposed on the front and back sides of the fixing belt 26 in FIGS. 4 and 5. In FIGS. In order to facilitate understanding, the side plate 65 on the front side of the drawing is omitted.

  As shown in FIG. 4, the switching mechanism 70 includes a holding plate 71 that is a holding member, a cam plate 73 that is an operating member, and a spring member 72 that is a first biasing member.

  The holding plate 71 is disposed on both ends of the support shaft of the pressure roller 19 and is formed of a substantially rectangular flat plate. The upper edge of the pair of holding plates 71 is provided with a pressure roller holding portion 71a formed by a U-shaped long hole, and the pressure roller holding portion 71a holds the support shaft of the pressure roller 19 rotatably. To do. In addition, a guide hole 71b made of a long hole extending in the paper transport direction A is formed on the left edge of the holding plate 71 (upstream side in the paper transport direction A). A guide pin 67 is fixed to a side plate 65 constituting a part of the apparatus main body of the fixing device 5, and the guide pin 67 is engaged with the guide hole 71b. Accordingly, the holding plate 71 is held so as to be movable along the guide hole 71b in the paper transport direction A, that is, in a direction substantially parallel to the flat portion 61 (see FIG. 3). A guide hole 71b with which the guide pin 67 engages is further formed on the right edge of the holding plate 71 (on the downstream side in the paper conveyance direction A) so that the holding plate 71 can move more smoothly in the paper conveyance direction A. You may comprise as follows. Further, the pressure roller holding portion 71a may be a circular hole that rotatably holds the support shaft of the pressure roller 19.

  A pressure spring 66 made of a tension coil spring or the like is hung between the right edge portion (downstream in the sheet conveying direction A) of the pair of holding plates 71 and the side plate 65, and the holding plate 71 is attached with the pressure spring 66. It is urged to the upper side of FIG. 4 by the force. When the holding plate 71 is urged upward by the pressure spring 66 in FIG. 4, the pressure roller 19 is pressed against the pressure pad 60 via the fixing belt 26, and the pressure roller 19 and the fixing belt 26 are fixed. A nip portion N1 is formed. The fixing nip portion N1 is formed when the pressure roller 19 contacts the flat portion 61 and the arc-shaped portion 63 (see FIG. 3) of the sliding surface 60a via the fixing belt 26. When the fixing nip portion N1 is formed, it is suitable for fixing ordinary paper such as A4 paper.

  A spring member 72 that urges the holding plate 71 toward the downstream side in the paper transport direction A is disposed at the left edge of the pair of holding plates 71. A cam plate 73 is disposed in the vicinity of the right edge of the pair of holding plates 71. The cam plate 73 is attached to an apparatus main body (not shown) so as to be rotatable around a rotation shaft 73a. The cam plate 73 has a circular outer edge portion, and the outer edge portion is in contact with a cam contact surface 71 c formed on the right edge portion of the holding plate 71. A rotating shaft 73a is provided at a position eccentric with respect to the circular center of the cam plate 73. When the rotating shaft 73a is rotated around the rotating shaft 73a by a drive source such as a motor (not shown), the cam plate 73 is eccentric. Rotate while. When the cam plate 73 is rotated about the rotation shaft 73 a by 180 degrees, the holding plate 71 resists the urging force of the spring member 72 and engages the guide hole 71 b and the guide pin 67 to press the pressure roller 19. 4 is moved in parallel to the left side of FIG. 4 (upstream side in the sheet conveying direction A) to enter the second mode (state shown in FIG. 5).

  In the second mode shown in FIG. 5, since the pressure roller 19 is in the state of moving upstream in the paper transport direction A (in a direction substantially parallel to the flat portion 61 of the sliding surface 60a), the fixing nip portion N2 is The pressure roller 19 is formed by contacting only the flat portion 61 (see FIG. 3) of the sliding surface 60a via the fixing belt 26. Since the width of the fixing nip portion N2 is shorter than the width of the fixing nip portion N1 in the first mode and the fixing nip portion N2 is formed in a flat shape, it is suitable for fixing a recording medium such as an envelope. Become.

  In order to switch from the second mode to the first mode, when the cam plate 73 is rotated by 180 degrees around the rotation shaft 73a, the cam plate 73 rotates while being eccentric, so that the biasing force of the spring member 72 The holding plate 71 moves to the downstream side in the paper transport direction A along the guide hole 71b and is in the state shown in FIG.

  In the above-described embodiment, the spring member 72 urges the holding plate 71 downstream in the sheet conveying direction A, and the cam plate 73 is always in contact with the cam contact surface 71c at the right edge of the holding plate 71. However, the present invention is not limited to this, and the spring member 72 urges the holding plate 71 toward the upstream side in the sheet conveying direction A, and the cam contact surface 71 c is disposed on the left edge of the holding plate 71. The cam plate 73 may always be in contact with the cam contact surface 71c. Further, the spring member 72 urges the holding plate 71 in the paper transport direction A, and the holding plate 71 contacts the apparatus main body in one of the first mode and the second mode, thereby setting a predetermined position. Alternatively, the holding plate 71 may be in contact with the cam plate 73 in the other mode to hold a predetermined position. In these cases as well, the same effects as in the above embodiment can be obtained.

(Second Embodiment)
6 and 7 show the configuration of the switching mechanism 70 of the second embodiment, FIG. 6 is a schematic side view of the switching mechanism 70 in the first mode, and FIG. 7 shows the switching mechanism 70 in the second mode. FIG. In the second embodiment, the holding plate 71 is rotated to switch to the first mode and the second mode. The switching mechanism 70, which is different from the first embodiment, will be mainly described, and the same part as the first embodiment. The description of is omitted. A pair of holding plates 71 and a pair of side plates 65 are disposed on the front and back sides of the fixing belt 26 in FIGS. 6 and 7. In FIGS. In order to make it easier to understand, the side plate 65 on the front side of the paper surface is omitted, and the holding plate 71, the pressing pad 60, the pressure roller 19 and the like overlap in the drawing. The outer shape of 71 is indicated by a broken line.

  As shown in FIG. 6, the switching mechanism 70 includes a holding plate 71 that is a holding member, a cam plate 73 that is an operating member, and a pressure spring 66 that is a second urging member.

  The holding plate 71 is disposed on both ends of the support shaft of the pressure roller 19 and is formed of a substantially rectangular flat plate. The holding plate 71 is provided with an engagement hole 71d that rotatably holds the support shaft of the pressure roller 19. In addition, a rotation support hole 71e that is rotatably fitted to a support shaft 68 fixed to the side plate 65 is provided at the upper left edge of the holding plate 71. The rotation support hole 71 e is disposed at a position upstream of the fixing nip portion N in the sheet conveyance direction A and at a position offset toward the pressing pad 60 with respect to the rotation center O of the pressure roller 19. When the holding plate 71 is rotated clockwise (in the direction of arrow C) about the support shaft 68 by the arrangement of the rotation fulcrum, the pressure roller 19 is rotated clockwise (in the direction of arrow C) about the support shaft 68. Move. That is, the pressure roller 19 moves to the lower side in FIG. 6 (in the direction away from the pressing pad 60) and to the upstream side in the paper transport direction A.

  Further, a pressure spring 66 made of a tension coil spring or the like is hung between the right edge portion (downstream side in the sheet conveying direction A) of the pair of holding plates 71 and the side plate 65. It is urged | biased by the upper side of FIG. When the holding plate 71 is biased upward in FIG. 6 by the pressure spring 66, the pressure roller 19 is pressed against the pressure pad 60 via the fixing belt 26, and the pressure roller 19 and the fixing belt 26 are A fixing nip portion N1 having a predetermined nip pressure is formed. The fixing nip portion N1 is formed when the pressure roller 19 contacts the flat portion 61 and the arc-shaped portion 63 (see FIG. 3) of the sliding surface 60a via the fixing belt 26. When the fixing nip portion N1 is formed, it is suitable for fixing ordinary paper such as A4 paper.

  A cam plate 73 is disposed in the vicinity of the upper edge of the pair of holding plates 71 and the pressure spring 66. The cam plate 73 is attached to the side plate 65 so as to be rotatable around the rotation shaft 73a. Further, the cam plate 73 includes a circular outer edge portion, and the outer edge portion is provided so as to be able to contact and separate from a cam contact surface 71 c formed on the upper edge portion of the holding plate 71. In the state of FIG. 6 (first mode), the cam plate 73 is disposed away from the cam contact surface 71c. A rotating shaft 73a is provided at a position eccentric with respect to the circular center of the cam plate 73. When the rotating shaft 73a is rotated around the rotating shaft 73a by a drive source such as a motor (not shown), the cam plate 73 is eccentric. Rotate while. When the cam plate 73 is rotated by 180 degrees around the rotation shaft 73a, the outer edge portion of the cam plate 73 comes into contact with the cam contact surface 71c of the holding plate 71 and further resists the urging force of the pressure spring 66. The holding plate 71 is moved downward. That is, the holding plate 71 rotates around the support shaft 68 in the direction of arrow C along with the pressure roller 19 to enter the second mode (the state shown in FIG. 7).

  In the second mode shown in FIG. 7, the pressure roller 19 is rotated in the direction of arrow C around the support shaft 68, so that the pressure roller 19 passes through the fixing belt 26 in the fixing nip portion N <b> 2. It is formed by contacting only the flat portion 61 (see FIG. 3) of the sliding surface 60a, and the nip pressure of the fixing nip portion N2 is smaller than the nip pressure in the first mode. Since the width of the fixing nip portion N2 is shorter than the width of the fixing nip portion N1 in the first mode, and the fixing nip portion N2 is formed flat and the fixing nip pressure is reduced, a recording medium such as an envelope is fixed. It becomes a state suitable for processing.

  To switch from the second mode to the first mode, when the cam plate 73 is rotated by 180 degrees around the rotation shaft 73a, the cam plate 73 is separated from the cam contact surface 71c of the holding plate 71, and the pressure spring Due to the urging force of 66, the holding plate 71 rotates counterclockwise about the support shaft 68 to be in the state shown in FIG.

  In the first and second embodiments, the induction heating unit 30 is used as the heating unit. However, the present invention is not limited to this, and the heating unit may be a halogen lamp or the like.

  In the present invention, a fixing belt is pressed against a pressure roller by a pressing member, and a recording medium carrying an unfixed toner image is inserted into a fixing nip formed by the fixing belt and the pressure roller, and heated and pressed. The present invention can be used for a fixing device for fixing to a recording medium, and an image forming apparatus using an electrophotographic system provided with the fixing device.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 5 Fixing apparatus 19 Pressure roller 26 Fixing belt 30 Induction heating part (heating means)
55 Belt support member 56 Pad holding portion 59 Guide portion 60 Press pad (press member)
60a Sliding surface 61 Flat portion 63 Arc-shaped portion 65 Side plate 66 Pressure spring (second biasing member)
67 Guide pin 68 Support shaft 70 Switching mechanism 71 Holding plate (holding member)
71a Pressure roller holding portion 71b Guide hole 71c Cam contact surface 71d Engagement hole 71e Rotation support hole 72 Spring member (first urging member)
73 Cam plate (actuating member)
73a Rotating shaft N, N1, N2 Fixing nip

Claims (1)

A rotatable endless fixing belt heated by a heating means;
A pressure roller disposed opposite to the outer peripheral surface of the fixing belt;
A pressing member that is inscribed in the fixing belt and presses the fixing belt against the pressure roller,
In a fixing device for fixing a non-fixed toner image carried on a recording medium by inserting a recording medium through a fixing nip formed by the fixing belt and the pressure roller,
On the surface of the pressing member that faces the fixing belt, a flat portion that is substantially parallel to the recording medium conveyance direction, and an arc shape that curves toward the pressure roller side downstream of the flat portion in the recording medium conveyance direction. And a sliding surface on which the inner peripheral surface of the fixing belt slides is formed.
A first mode in which the pressure roller is pressed against the flat portion and the arcuate portion via the fixing belt to form the fixing nip portion; and the pressure roller is disposed on the flat portion via the fixing belt. A switching mechanism for switching to the second mode for forming the fixing nip portion by being pressed against only,
The switching mechanism holds the pressure roller rotatably and holds the pressure roller so as to be movable in a direction substantially parallel to the flat portion, and urges the holding member to the downstream side in the conveyance direction of the recording medium . a first biasing member, and the actuating member for moving against said holding member to the biasing force of the first biasing member, was closed,
The holding member is formed with a long guide hole extending in the recording medium conveyance direction, a guide pin is provided in a part of the apparatus main body, and the guide pin is engaged with the guide hole. A fixing device.

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