Fixing device and image forming apparatus including the same

Abstract

To provide a fixing device of a belt fixing system that can maintain the constant contact pressure force between a fixing belt and a pressure roller at a fixing nip part, and an image forming apparatus including the same.SOLUTION: A fixing device comprises a fixing belt, a nip forming member, a pressure roller, a housing, a planar heater, a pair of belt holders, and a pressure mechanism. The pressure mechanism has a pair of pressure springs that urge the pair of belt holders in a direction approaching the pressure roller, and a pair of pressure plates that are arranged opposite to the pair of belt holders and swing in a direction approaching or moving away from the belt holders, and can adjust an urging force of the pressure springs by swinging the pressure plates. The pressure spring is a compression coil spring. The diameter of the pressure spring is larger than the length in a width direction of the planar heater that is the same direction as the direction of rotation of the fixing belt. The central axis of the pressure spring is arranged to overlap a straight line that passes through the center of rotation of the fixing belt and the center of rotation of the pressure roller.SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present invention relates to a fixing device used in an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine thereof, and an image forming apparatus including the same.

Background Art

[0002] In an image forming apparatus using an electrophotographic method, in order to fix a toner image on a sheet, a fixing device including a fixing member formed by pressing a fixing roller or a fixing belt (a rotating member to be heated) and a pressure roller (a rotating member for applying pressure) is widely used. In this fixing device, heat and pressure are applied to the toner image through a sheet in a fixing nip portion formed by a fixing roller or a fixing belt and a pressure roller, and the toner image is melted and fixed on the sheet.

[0003] In the fixing device as described above, it is necessary to press the rotating member to be heated and the rotating member for applying pressure against each other with a predetermined pressure. For example, Patent Document 1 discloses a fixing device in which a bearing for a pressure roller advances by the spring force of a pressure spring, and the pressure roller is pressed against a fixing roller. In the configuration of Patent Document 1, the pressing position of the pressure spring is arranged on a straight line passing through the rotation centers of the fixing roller and the pressure roller.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a belt fixing type fixing device that uses a fixing belt as a heated rotating body, the pressing position is often not arranged on the straight line connecting the rotation center of the pressure roller and the rotation center of the fixing belt. This is because, in order to suppress the load (spring constant) of the pressure spring, the pressing position is set at a position farther from the fixing nip portion than from the fulcrum of the pressing member (pressure plate). That is, the load of the pressure spring is reduced by using the principle of a lever.

[0006] However, when the pressing position is not arranged on the straight line passing through the rotation center of the pressure roller and the rotation center of the fixing belt, the pressing direction becomes the circumferential direction centered on the fulcrum of the pressing member. Also, a positional tolerance of the pressing position occurs. Therefore, variations are likely to occur in the pressing force in the originally required pressing direction (the straight line passing through the rotation center of the pressure roller and the rotation center of the fixing belt). Furthermore, there was a possibility that the nip forming member for forming the fixing nip portion would incline.

[0007] In view of the above problems, an object of the present invention is to provide a fixing device capable of maintaining a constant pressure contact force between a fixing belt and a pressure roller in a fixing nip portion with a simple configuration in a belt fixing type, and an image forming apparatus including the same.

Means for Solving the Problems

[0008] To achieve the above object, a first configuration of the present invention includes an endless fixing belt, a nip forming member, a pressure roller, a housing, a planar heater, a pair of belt holders, and a pressure mechanism. The fixing device melts and fixes an unfixed toner image on a sheet by heating and pressing the sheet that passes through the fixing nip portion with the fixing belt and the pressure roller rotated. The nip forming member is disposed opposite to the inner peripheral surface of the fixing belt. The pressure roller is pressed against the nip forming member to form a fixing nip portion between the pressure roller and the fixing belt. The housing houses the fixing belt, the nip forming member, and the pressure roller. The planar heater is disposed on the opposing surface of the nip forming member facing the fixing belt to heat the fixing belt. The pair of belt holders hold both ends of the fixing belt and the nip forming member. The pressure mechanism adjusts the pressure contact force between the fixing belt and the pressure roller. The pressure mechanism includes a pair of pressure springs that bias each of the pair of belt holders in a direction approaching the pressure roller, and a pair of pressure plates that are disposed opposite to each of the pair of belt holders and swing in a direction approaching or separating from the belt holders. By swinging the pressure plates, the biasing force of the pressure springs can be adjusted. The pressure spring is a compression coil spring, and the diameter of the pressure spring is larger than the length in the width direction of the planar heater in the same direction as the rotation direction of the fixing belt. The central axis of the pressure spring is arranged to overlap with a straight line passing through the rotation center of the fixing belt and the rotation center of the pressure roller.

Advantages of the Invention

[0009] According to the first configuration of the present invention, by making the diameter of the pressure spring larger than the length in the width direction of the planar heater, the pressure contact force can be uniformly applied in the width direction of the planar heater, and the inclination of the planar heater with respect to the fixing nip portion can be suppressed. Further, by arranging the central axis of the pressure spring to overlap with a straight line passing through the rotation center of the fixing belt and the rotation center of the pressure roller, variations in the pressure applied by the pressure mechanism and the inclination of the nip forming member can be suppressed.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing the internal structure of an image forming apparatus 100 according to an embodiment of the present invention. Inside the image forming apparatus 100 (here a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the conveyance direction (left side in FIG. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and form yellow, cyan, magenta, and black images sequentially through the steps of charging, exposure, development, and transfer, respectively.

[0012] In these image forming units Pa to Pd, there are disposed photosensitive drums (image carriers) 1a, 1b, 1c, and 1d that carry visible images (toner images) of respective colors. Further, an intermediate transfer belt (intermediate transfer member) 8 that rotates in the counterclockwise direction in FIG. 1 by a belt drive motor (not shown) is provided adjacent to each of the image forming units Pa to Pd. The toner images formed on these photosensitive drums 1a to 1d are sequentially primary transferred and superimposed onto the intermediate transfer belt 8 that moves while contacting each of the photosensitive drums 1a to 1d. Thereafter, the toner image primary transferred onto the intermediate transfer belt 8 is secondary transferred onto transfer paper P as an example of a recording medium by a secondary transfer roller 9. Further, after the toner image is fixed in a fixing device 14, the transfer paper P onto which the toner image has been secondary transferred is discharged from the main body of the image forming apparatus 100. While rotating the photosensitive drums 1a to 1d in the clockwise direction in FIG. 1, an image forming process for each of the photosensitive drums 1a to 1d is executed.

[0013] The transfer paper P onto which the toner image is secondary transferred is housed in a paper cassette 16 disposed at the lower part of the main body of the image forming apparatus 100, and is conveyed along a paper conveyance path 19 to a nip portion between the secondary transfer roller 9 and a drive roller 11 of the intermediate transfer belt 8 via a paper feed roller 12 and a registration roller pair 13. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a seamless belt having no seam is mainly used. Also, a blade-shaped belt cleaner 25 for removing toner and the like remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

[0014] Next, the image forming units Pa to Pd will be described. Around and below the photosensitive drums 1a to 1d rotatably disposed, there are provided charging devices 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d, an exposure device 5 for exposing each of the photosensitive drums 1a to 1d with image information, developing devices 3a, 3b, 3c, and 3d for forming a toner image on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d for removing developer (toner) and the like remaining on the photosensitive drums 1a to 1d.

[0015] When image data is input from a host device such as a personal computer, first, the charging devices 2a to 2d uniformly charge the surfaces of the photosensitive drums 1a to 1d. Next, the exposure device 5 irradiates light according to the image data, and an electrostatic latent image corresponding to the image data is formed on each of the photosensitive drums 1a to 1d. The developing devices 3a to 3d are each filled with a predetermined amount of a two-component developer containing yellow, cyan, magenta, and black toner. When the ratio of the toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a specified value due to the formation of a toner image described later, toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in this developer is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and electrostatically adheres thereto. Thereby, a toner image corresponding to the electrostatic latent image formed by the exposure from the exposure device 5 is formed.

[0016] Then, a predetermined transfer voltage is applied between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d by the primary transfer rollers 6a to 6d, and the yellow, cyan, magenta, and black toner images on the photosensitive drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. These images are formed with a predetermined positional relationship in advance. Thereafter, in preparation for the formation of a new electrostatic latent image that is subsequently performed, toner and the like remaining on the surfaces of the photosensitive drums 1a to 1d after primary transfer are removed by the cleaning devices 7a to 7d.

[0017] The intermediate transfer belt 8 is stretched between the upstream driven roller 10 and the downstream driving roller 11. When the intermediate transfer belt 8 starts to rotate counterclockwise as the driving roller 11 rotates by a belt driving motor (not shown), the transfer paper P is conveyed from the registration roller pair 13 to the secondary transfer nip portion N1 (see FIG. 2) between the driving roller 11 and the secondary transfer roller 9 provided adjacent thereto at a predetermined timing. Then, the toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P passing through the secondary transfer nip portion N1.

[0018] The transfer paper P onto which the toner image has been secondarily transferred is conveyed to the fixing device 14. The fixing device 14 has a fixing belt 14a and a pressure roller 14b (both are shown in FIG. 2). The fixing belt 14a is heated by a planar heater 43 (shown in FIG. 8). The pressure roller 14b is pressed against the fixing belt 14a to form a fixing nip portion N2 (shown in FIG. 5), and applies a rotational driving force to the fixing belt 14a.

[0019] The transfer paper P conveyed to the fixing device 14 is heated and pressed by the fixing belt 14a and the pressure roller 14b, so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full-color image is formed. After the transfer paper P on which the full-color image has been formed passes through the fixing discharge roller pair 24 (shown in FIG. 2), the conveyance direction is distributed by a branch portion 15 branched in a plurality of directions, and then (or after being sent to the duplex conveyance path 20 and having images formed on both sides), it is discharged to the discharge tray 18 by the discharge roller pair 17.

[0020] FIG. 2 is a partial cross-sectional view around the paper conveyance path 19 and the duplex conveyance path 20 in the image forming apparatus 100 of the present embodiment. The opening / closing cover 21 constitutes the side surface 102 of the image forming apparatus 100 and is rotatably supported by a cover support shaft 21a provided below the main body of the image forming apparatus 100. The inner surface of the opening / closing cover 21 constitutes one (outer) conveyance surface of the duplex conveyance path 20.

[0021] A handle portion 22 is provided on the side surface of the opening / closing cover 21. One end of the handle portion 22 engages with an engagement pin (not shown) provided on the front frame and the rear frame of the main body of the image forming apparatus 100 to hold the opening / closing cover 21 in a closed state. When opening the opening / closing cover 21, the handle portion 22 is rotated to release the engagement with the engagement pin.

[0022] Inside the opening / closing cover 21, a conveyance unit 23 is arranged. The conveyance unit 23 is rotatably supported by the main body of the image forming apparatus 100 around a unit support shaft 23a, and forms a part of the conveyance surfaces of the duplex conveyance path 20 and the paper conveyance path 19. The duplex conveyance path 20 extends vertically along the side surface 102 of the image forming apparatus 100 between the inner surface of the opening / closing cover 21 and the outer surface of the conveyance unit 23, and is curved in a substantially C shape to merge with the paper conveyance path 19. On the inner surface of the conveyance unit 23, in order from the upstream side (the lower side in FIG. 2) in the conveyance direction of the transfer paper P, a roller 13b on one side constituting the resist roller pair 13 and a secondary transfer roller 9 are attached.

[0023] By rotating only the opening / closing cover 21 in the opening direction with respect to the image forming apparatus 100 to an open state, the duplex conveyance path 20 is widely exposed. Also, by rotating the opening / closing cover 21 together with the conveyance unit 23 in the opening direction, the conveyance unit 23 is separated from the main body side of the image forming apparatus 100 and the paper conveyance path 19 is widely exposed. On the other hand, by rotating the opening / closing cover 21 together with the conveyance unit 23 in the closing direction to a closed state, the conveyance unit 23 abuts on the main body side of the image forming apparatus 100, and the secondary transfer roller 9 is pressed against the driving roller 11 via the intermediate transfer belt 8, and a secondary transfer nip portion N1 is formed.

[0024] Next, the configuration of the fixing device 14 will be described. FIG. 3 is a perspective view of the fixing device 14 as viewed from the downstream side (the left side in FIG. 2) in the discharge direction of the transfer paper P from the image forming apparatus 100. FIG. 4 is a side cross-sectional view of the fixing device 14 cut at the central portion in the longitudinal direction.

[0025] The fixing device 14 has a housing 30, side covers 31a, 31b, and a stay 31c. Inside the housing 30, a fixing belt 14a and a pressure roller 14b are accommodated. Above the housing 30, a roller 24a on one side (the left side in FIG. 2) constituting the fixing discharge roller pair 24 is supported.

[0026] The side covers 31a and 31b are fixed to the side plates 30a and 30b disposed at both longitudinal ends of the housing 30, respectively. The stay 31c is made of metal and is a plate-like member fixed along the longitudinal direction of the housing 30. At both longitudinal ends of the stay 31c, a pair of spring receiving portions 40 for supporting one end side (lower end portion) of the pressure spring 35 (see FIG. 5) of the pressure mechanism 32 are formed.

[0027] On the side cover 31a side, a drive input gear 60 and a pressure release gear 61 are arranged. The drive input gear 60 meshes with a roller drive gear (not shown) fixed to the rotation shaft 141 of the pressure roller 14b. When a rotational driving force is transmitted from a fixing drive motor (not shown) to the drive input gear 60 via a drive output gear (not shown) on the main body side of the image forming apparatus 100, the pressure roller 14b rotates at a predetermined speed. Thereby, the fixing belt 14a pressed against the pressure roller 14b also rotates following the pressure roller 14b.

[0028] Inside the fixing belt 14a, a nip forming member 41 and a belt guide 42 are arranged. The nip forming member 41 forms a fixing nip portion N2 (see FIG. 5) through which the transfer paper P is inserted by contacting the pressure roller 14b via the fixing belt 14a. The nip forming member 41 is made of a heat-resistant resin such as liquid crystal polymer or an elastic material such as silicone rubber, and an elastomer may be arranged on the opposing surface to the fixing belt 14a to enhance slidability.

[0029] The belt guide 42 has an arcuate shape in cross-section and contacts the inner peripheral surface of the fixing belt 14a except for the surface facing the nip forming member 41. The belt guide 42 applies a predetermined tension to the fixing belt 14a and holds the fixing belt 14a in an arcuate shape from the inside. The belt guide 42 is composed of a metal plate extending along the axial direction of the fixing belt 14a and having substantially the same length as the fixing belt 14a.

[0030] FIG. 5 is a side cross-sectional view of the fixing device 14 cut at the longitudinal end (the right end in FIG. 3). Pressing mechanisms 32 for adjusting the pressing force between the fixing belt 14a and the pressure roller 14b are arranged at both longitudinal ends of the fixing device 14. The pressing mechanisms 32 are provided in a pair at both axial ends of the fixing belt 14a and the pressure roller 14b. The pair of pressing mechanisms 32 each have a pressing plate 33, a pressing spring 35, and an eccentric cam 37.

[0031] The pressing plate 33 is disposed opposite to a pair of belt holders 143 that support both longitudinal ends of the fixing belt 14a, the nip forming member 41, and the belt guide 42. The pressing plate 33 has a fulcrum portion 33a supported by the housing 30 of the fixing device 13 and is swingable in a direction approaching or separating from the belt holder 143.

[0032] The pressing spring 35 is a compression coil spring and biases the belt holder 143 in a direction approaching the pressure roller 14b. More specifically, one end of the pressing spring 35 is supported by a spring receiving portion 40 formed on the stay 31c. The other end of the pressing spring 35 is externally inserted into a second positioning boss 143a (see FIG. 9) of the belt holder 143 that protrudes through a through hole 33b formed in the pressing plate 33 and is in contact with the pressing plate 33. The inner diameter of the through hole 33b is smaller than the outer diameter of the pressing spring 35, and the pressing spring 35 contracts between the pressing plate 33 and the spring receiving portion 40 due to the swing of the pressing plate 33.

[0033] The eccentric cam 37 is disposed on the same side as the belt holder 143 with respect to the pressing plate 33 (the right side in FIG. 5). The eccentric cam 37 is integrally formed with a pressure release gear 61 (see FIG. 3) that inputs a driving force to the pressing mechanism 32. When the eccentric cam 37 rotates together with the pressure release gear 61, the outer diameter of the eccentric cam 37 that contacts the pressing plate 33 changes.

[0034] Figures 6 and 7 are side views showing the pressurized state and the pressure release state by the pressurizing mechanism 32, respectively. As shown in Figure 6, when the small-diameter portion of the eccentric cam 37 faces the pressure plate 33, a constant pressing force acts on the belt holder 143 by the biasing force of the pressure spring 35. As a result, the pressure roller 14b is pressed against the nip forming member 41 to form a fixing nip portion N2 (see Figure 5) between the fixing belt 14a. In the state of Figure 6, the light-shielding portion 33c formed at the swing end of the pressure plate 33 has retreated from the detection portion of the PI sensor 70, and the PI sensor 70 detects that it is in the pressurized state.

[0035] When the eccentric cam 37 rotates by a predetermined amount from the state of Figure 6 and the large-diameter portion of the eccentric cam 37 contacts the pressure plate 33 as shown in Figure 7, the pressure plate 33 is pressed in a direction away from the belt holder 143 against the biasing force of the pressure spring 35. As a result, the pressure spring 35 is compressed between the pressure plate 33 and the spring receiving portion 40, and the pressure acting on the belt holder 143 from the pressure plate 33 is weakened. In the state of Figure 7, the light-shielding portion 33c shields the detection portion of the PI sensor 70, and the PI sensor 70 detects that it is in the pressure release state.

[0036] Figure 8 is an enlarged partial view of the periphery of the belt holder 143 and the pressure spring 35 in Figure 5. As shown in Figure 8, the pressure spring 35 is arranged on a straight line L passing through the rotation center O1 of the fixing belt 14a and the rotation center O2 of the pressure roller 14b. More specifically, the center line in the axial direction (contraction direction) of the pressure spring 35, which is a compression coil spring, is arranged so as to overlap with the straight line L.

[0037] The required pressing direction between the fixing belt 14a and the pressure roller 14b is a direction perpendicular to the fixing nip portion N2 between the fixing belt 14a and the pressure roller 14b, and is the direction of the straight line L connecting the rotation center of the fixing belt 14a and the rotation center of the pressure roller 14b.

[0038] By arranging the compression spring 35 as described above, it is possible to apply the necessary load (compression contact force) in the necessary compression direction (the direction of the straight line L). As a result, it is possible to suppress variations in the pressing force by the pressing mechanism 32 and the inclination of the nip forming member 41.

[0039] Further, a planar heater 43 is disposed on the surface of the nip forming member 41 that faces the pressure roller 14b. The planar heater 43 extends with a predetermined width R2 over substantially the entire region in the rotational axis direction (width direction) of the fixing belt 14a. The planar heater 43 is a heating member that heats the fixing belt 14a to the fixing temperature.

[0040] In the present embodiment, the diameter R1 of the compression spring 35 is made larger than the width R2 of the planar heater 43. Thereby, the compression contact force can be uniformly applied in the width direction of the planar heater 43, and the inclination of the planar heater 43 with respect to the fixing nip portion can be suppressed.

[0041] FIG. 9 is a perspective view of the configuration around the pressing mechanism 32 as viewed from the side of the stay 31c. Note that FIG. 9 shows the pressing mechanism 32 on one end side (the right side in FIG. 3) of the fixing device 15, but the pressing mechanism 32 on the other end side (the left side in FIG. 3) of the fixing device 14 has the same configuration except for being symmetric about the left and right.

[0042] As shown in FIG. 9, a first positioning boss 40a for positioning one end portion of the compression spring 35 (see FIG. 8) is formed on the spring receiving portion 40. A second positioning boss 143a for positioning the other end portion of the compression spring 35 is formed on the belt holder 143.

[0043] By inserting the first positioning boss 40a and the second positioning boss 143a into both end portions of the compression spring 35, respectively, the center line in the axial direction of the compression spring 35 is arranged to overlap with the straight line L as shown in FIG. 8.

[0044] Below the spring receiving portion 40 of the stay 31c, slits 80a and 80b are formed. Hook portions 81 of the housing 30 and unit positioning protrusions 82 are engaged with the slits 80a and 80b, respectively. By engaging the hook portion 81 with the slit 80a, the lower end portion of the stay 31c and the housing 30 are connected. When the fixing device 14 is inserted into the main body of the image forming apparatus 100, the unit positioning protrusion 82 engages with a positioning hole (not shown) on the main body side of the image forming apparatus 100 to position the fixing device 14 with respect to the main body of the image forming apparatus 100.

[0045] When attaching the stay 31c to the housing 30, first, the first positioning boss 40a is inserted into one end portion of the compression spring 35. In this state, the slits 80a and 80b are engaged with the hook portion 81 and the unit positioning protrusion 82 of the housing 30. Then, while compressing the compression spring 35, the stay 31c is swung in a direction approaching the housing 30 with the lower end portion of the stay 31c as a fulcrum.

[0046] After that, with the upper portion of the stay 31c in contact with the housing 30, a screw 83 is fastened to a screw hole (not shown) formed in the stay 31c. By performing the above procedure at both ends in the longitudinal direction of the stay 31c, the attachment of the stay 31c is completed.

[0047] According to the present embodiment, by providing the spring receiving portion 40 on the stay 31c of the fixing device 14, the number of parts and the number of assembly steps can be reduced compared to a configuration in which the spring receiving portion 40 is a separate member, and the cost of the fixing device 14 can be reduced.

[0048] Also, the distance D1 from the spring receiving portion 40 (first positioning boss 40a) to the screw fixing portion 85 is smaller than the distance D2 from the engagement position of the slit 80a and the hook portion 81 to the spring receiving portion 40 (first positioning boss 40a). Therefore, when the stay 31c is swung, the compression spring 35 can be compressed with a small force according to the lever principle. Accordingly, the attachment work of the stay 31c can be smoothly performed, and the assemblability of the fixing device 14 can be improved.

[0049] In addition, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, the slit 80a is provided below the spring receiving portion 40 of the stay 31c, and the hook portion 81 is provided on the housing 30 side. However, the hook portion 81 may be provided on the stay 31c side, and the slit 80a may be provided on the housing 30 side.

[0050] Also, in the above-described embodiment, the color printer as shown in FIG. 1 has been described as an example of the image forming apparatus 100. However, the present invention is not limited to the color printer, and can be applied to image forming apparatuses provided with a fixing device, such as a color copier, a color multifunction machine, a monochrome printer, and a monochrome copier.

Industrial Applicability

[0051] The present invention can be used for a fixing device used in an image forming apparatus such as a copier, a printer, a facsimile machine, and a combination machine thereof. By using the present invention, in a belt fixing type, it is possible to provide a fixing device capable of maintaining a constant pressure contact force between a fixing belt and a pressure roller in a fixing nip portion with a simple configuration, and an image forming apparatus including the same.

Explanation of Reference Numerals

[0052] Pa~Pd Image forming unit 14 Fixing device 14a Fixing belt 14b Pressure roller 143 Belt holder 30 Housing 30a, 30b Side plates 31a, 31b Side covers 31c Stay 32 Pressing mechanism 33 Pressing plate 35 Pressing spring 37 Eccentric cam 40 Spring receiving portion 41 Nip forming portion 42 Belt guide 43 Planar heater 80a and 80b slits (engagement parts) 81 hook part (engaged part) 82 unit positioning projection 85 screw fixing part 100 image forming apparatus P transfer paper (recording medium) N2 fixing nip part O1 rotation center (fixing belt) O2 rotation center (pressure roller)

Claims

1. An endless fixing belt, A nip forming member disposed opposite to the inner peripheral surface of the fixing belt, A pressure roller pressed against the nip forming member to form a fixing nip portion between the fixing belt, A housing for housing the fixing belt, the nip forming member, and the pressure roller, A planar heater disposed on the opposing surface of the nip forming member facing the fixing belt for heating the fixing belt, A pair of belt holders for holding both ends of the fixing belt and the nip forming member, A pressurizing mechanism for adjusting the pressure contact force between the fixing belt and the pressure roller, In a fixing device for melting and fixing an unfixed toner image on a sheet by heating and pressurizing the sheet passing through the fixing nip portion with the fixing belt and the pressure roller rotated, The pressurizing mechanism, A pair of pressurizing springs for urging each of the pair of belt holders in a direction approaching the pressure roller, A pair of pressure plates disposed opposite to each of the pair of belt holders and oscillating in a direction approaching or separating from the belt holder, And the urging force of the pressurizing spring can be adjusted by oscillating the pressure plate, The pressurizing spring is a compression coil spring, and the diameter of the pressurizing spring is larger than the length in the width direction of the planar heater in the same direction as the rotation direction of the fixing belt, A fixing device characterized in that the central axis of the pressurizing spring is arranged to overlap a straight line passing through the rotation center of the fixing belt and the rotation center of the pressure roller.

2. On the side surface of the housing, a metal stay is attached along the longitudinal direction, A pair of spring receiving portions for supporting the ends of the pair of pressurizing springs on the side opposite to the belt holder are formed at both ends of the stay in the longitudinal direction. The fixing device according to claim 1.

3. The stay, at both ends in the longitudinal direction, A pair of engaging portions formed on one side in a direction orthogonal to the longitudinal direction and engaging with an engaged portion formed on the housing, A pair of screw fixing portions formed on the other side in a direction orthogonal to the longitudinal direction and screwed to the housing, And has, The distance between the spring receiving portion and the screw fixing portion at one end in the longitudinal direction of the stay is larger than the distance between the spring receiving portion and the engaging portion. The fixing device according to claim 2.

4. An image forming unit that forms a toner image on a recording medium, The fixing device according to any one of claims 1 to 3, which heats and presses the recording medium on which the toner image has been formed by the image forming unit to fix the toner image on the recording medium, An image forming apparatus comprising the same.

Images