JP2016224310A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent image deterioration caused by fixation and to secure separability in addition to stable fixability.SOLUTION: A fixing device includes a rotatable first fixing member 21 which has flexibility, a rotatable second fixing member 22 which has an elastic layer 22b and forms a fixing nip N between the first fixing member 21 and the second fixing member 22, a nip forming member 24 which faces the second fixing member 22 through the first fixing member 21 and forms the fixing nip N, and a heating source which heats the first fixing member 21. The nip forming member 24 includes a first flat surface A, a second flat surface C which is arranged on the downstream side of the first flat surface A in the conveyance direction of a recording medium and on an approach side to the second fixing member 22, a connection surface B which connects both flat surfaces A and C, and a separation surface D which projects to the side of the second fixing member 22 from the second flat surface C on the downstream side of the second flat surface C in the conveyance direction of the recording medium. The fixing nip N is formed by the first flat surface A, the second flat surface C, and the connection surface B.SELECTED DRAWING: Figure 4

Description

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

  As a fixing device mounted on an image forming apparatus such as an electrophotographic copying machine or a printer, a belt fixing device using an endless fixing belt instead of a heating roller has been put into practical use. In this belt fixing device, the fixing belt is heated by a heating source, and a nip forming member disposed inside the fixing belt is pressed against the fixing belt to form a fixing nip between the fixing belt and the pressure roller. In addition, since the fixing belt is formed of a film material having a small heat capacity, it has a feature of excellent energy saving and quick start.

  As an example of this belt fixing device, Patent Document 1 (Japanese Patent No. 4725018) discloses that the upstream portion of the contact surface of the nip forming member with the fixing belt in the moving direction of the fixing belt is the same as the peripheral surface of the pressure roll. A fixing device is disclosed in which a curvature in the direction is set, and a downstream portion in the moving direction of the fixing belt is in a direction opposite to the circumferential surface of the pressure roll and larger in curvature than the upstream portion.

  Further, in Patent Document 2 (Japanese Patent Laid-Open No. 2014-134701), the nip forming member has a curved convex shape in the upstream region in the rotation direction of the heating belt and has a curved convex shape in the downstream region (curved concave shape). A fixing device formed into a shape, a flat shape, or the like is disclosed.

  However, as in Patent Document 1, if the nip forming member is formed with a curvature in the same direction as the peripheral surface of the pressure roller on the inlet side of the fixing nip, the surface of the fixing belt in contact with the sheet is likely to be wrinkled. In particular, the toner on the entrance side of the fixing nip is not sufficiently melted, and the adhesion of the toner to the paper is insufficient, so wrinkles generated on the belt surface cause the toner to be peeled off or lost, thereby degrading the image quality. Let

  On the other hand, as in Patent Document 2, if the upstream region of the nip forming member has a curved convex shape as opposed to Patent Document 1, the linear velocity difference between the pressure roller and the fixing belt increases. Further, the nip forming member deeply penetrates the pressure roller on the upstream side of the fixing nip, and the contact surface pressure in this region is increased. For this reason, a large displacement force acts on the toner in a region where the toner is not sufficiently melted, and this point causes deterioration of the image as in Patent Document 1.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to prevent image deterioration associated with fixing and to ensure separation in addition to stable fixing.

  In order to solve the above-described problems, the present invention provides a rotating first fixing member having flexibility and being rotatable, an elastic layer, and forming a fixing nip between the first fixing member and the first fixing member. A possible second fixing member, a nip forming member that faces the second fixing member via the first fixing member and forms the fixing nip, and a heating source that heats the first fixing member A fixing device for fixing an unfixed image on a recording medium supplied to the fixing nip, wherein the nip forming member includes a first flat surface and a recording medium conveyance direction relative to the first flat surface. A second flat surface disposed on the downstream side closer to the second fixing member, a connection surface connecting the two flat surfaces, and a downstream side of the second flat surface in the recording medium conveyance direction. A separation surface that protrudes toward the second fixing member from the second flat surface. It is characterized in that to form the fixing nip between the first planar surface and the second planar surface at the connection surface.

  According to the present invention, the entrance portion side of the fixing nip is a flat portion shaped like the first flat surface of the nip forming member. Therefore, wrinkles are unlikely to occur on the surface of the first fixing member. Further, in this flat portion, the elastic layer of the second fixing member is not so compressed, so that the linear velocity difference between the first fixing member and the second fixing member is also reduced. In addition, since the flat portion is a low surface pressure region, the displacement force acting on the unfixed developer is reduced. For the reasons described above, the displacement force acting on the developer can be reduced on the entrance side of the fixing nip where the developer is insufficiently melted, and image quality deterioration due to this displacement can be prevented. On the other hand, since the contact surface of the nip forming member compresses the elastic layer further than the first flat surface, the surface pressure can be higher on the downstream side in the recording medium conveyance direction than the first flat surface of the fixing nip. it can. Accordingly, it is possible to reliably fix the developer to the recording medium by applying sufficient heat and pressure to the developer. From the above, it is possible to achieve both prevention of image deterioration due to fixing and securing of stable fixing properties.

  In addition, according to the present invention, the nip forming member has a separation surface that protrudes further to the second fixing member side than the second flat surface on the downstream side in the recording medium conveyance direction from the second flat surface. Thus, the recording medium discharged from the fixing nip can be satisfactorily separated from the first fixing member at this separation surface. Specifically, since there is a separation surface, the trajectory of the first fixing member transitions so as to bend to the opposite side after being inclined to the second fixing member side after passing through the exit portion of the fixing nip. After the medium is discharged from the fixing nip to the second fixing member side, the medium is separated from the first fixing member at the bent portion of the first fixing member. Thereby, it is possible to reliably prevent the recording medium from being wound around the first fixing member.

1 is a schematic view of an image forming apparatus according to an embodiment of the present invention. 1 is a cross-sectional view of a fixing device according to an embodiment of the present invention. FIG. 2 is a perspective view illustrating a schematic configuration of a main part of the fixing device. FIG. 2 is an enlarged view illustrating a fixing nip of the fixing device. FIG. 5 is an enlarged view showing a fixing nip of a fixing device according to another embodiment of the present invention. It is a figure which shows the state which moved the pressure roller to the direction approaching a nip formation member. FIG. 7 is a diagram showing a comparison of the fixing belt trajectory on the separation surface between a state before the pressure roller is moved in a direction approaching the nip forming member side and a state after the pressure roller is moved. FIG. 6 is a diagram illustrating a state in which a pressure roller is moved to the upstream side in the paper conveyance direction. FIG. 10 is a diagram illustrating another example of a state in which the pressure roller is moved upstream in the paper conveyance direction.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In each of the drawings for explaining the embodiments of the present invention, constituent elements such as members and components having the same function or shape are described once by giving the same reference numerals as much as possible. Then, the explanation is omitted.

First, an overall configuration and operation of an image forming apparatus according to an embodiment of the present invention will be described with reference to FIG.
An image forming apparatus 1 shown in FIG. 1 is a color laser printer, and four image forming units 4Y, 4M, 4C, and 4K are provided in the center of the apparatus main body. Each of the image forming units 4Y, 4M, 4C, and 4K contains developers of different colors of yellow (Y), magenta (M), cyan (C), and black (K) corresponding to the color separation components of the color image. The configuration is the same except that.

  Specifically, each of the image forming units 4Y, 4M, 4C, and 4K has a drum-shaped photoconductor 5 as a latent image carrier, a charging device 6 that charges the surface of the photoconductor 5, and a surface of the photoconductor 5. A developing device 7 for supplying toner and a cleaning device 8 for cleaning the surface of the photoreceptor 5 are provided. In FIG. 1, only the photoconductor 5, the charging device 6, the developing device 7, and the cleaning device 8 included in the black image forming unit 4 </ b> K are denoted by reference numerals. In the other image forming units 4 </ b> Y, 4 </ b> M, and 4 </ b> C, The reference numerals are omitted.

  Under the image forming units 4Y, 4M, 4C, and 4K, an exposure device 9 that exposes the surface of the photoreceptor 5 is disposed. The exposure device 9 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and irradiates the surface of each photoconductor 5 with laser light based on image data.

  A transfer device 3 is disposed above the image forming units 4Y, 4M, 4C, and 4K. The transfer device 3 includes an intermediate transfer belt 30 as an intermediate transfer member, four primary transfer rollers 31 as primary transfer means, a secondary transfer roller 36 as secondary transfer means, a secondary transfer backup roller 32, A cleaning backup roller 33, a tension roller 34, and a belt cleaning device 35 are provided.

  The intermediate transfer belt 30 is an endless belt and is stretched by a secondary transfer backup roller 32, a cleaning backup roller 33, and a tension roller 34. Here, by rotating the secondary transfer backup roller 32, the intermediate transfer belt 30 runs (rotates) in the direction indicated by the arrow in the figure.

  Each of the four primary transfer rollers 31 sandwiches the intermediate transfer belt 30 with each photoconductor 5 to form a primary transfer nip. Further, a power source (not shown) is connected to each primary transfer roller 31 so that a predetermined DC voltage (DC) and / or AC voltage (AC) is applied to each primary transfer roller 31.

  The secondary transfer roller 36 sandwiches the intermediate transfer belt 30 with the secondary transfer backup roller 32 to form a secondary transfer nip. Similarly to the primary transfer roller 31, a power source (not shown) is also connected to the secondary transfer roller 36, and a predetermined DC voltage (DC) and / or AC voltage (AC) is applied to the secondary transfer roller 36. It has come to be.

  The belt cleaning device 35 includes a cleaning brush and a cleaning blade disposed so as to contact the intermediate transfer belt 30. The waste toner collected by the belt cleaning device 35 is accommodated in a waste toner container via a waste toner transfer hose.

  A bottle container 2 is provided at the upper part of the main body of the image forming apparatus, and four toner bottles 2Y, 2M, 2C, and 2K that store replenishing toner are detachably attached to the bottle container 2. ing. Toner is supplied from each toner bottle 2Y, 2M, 2C, 2K to each developing device 7 via a supply path provided between each toner bottle 2Y, 2M, 2C, 2K and each developing device 7.

  On the other hand, at the lower part of the main body of the image forming apparatus, a paper feed tray 10 that stores paper P as a recording medium, a paper feed roller 11 that carries out the paper P from the paper feed tray 10, and the like are provided. In addition to plain paper, the recording medium includes cardboard, postcard, envelope, thin paper, coated paper (coated paper, art paper, etc.), tracing paper, OHP sheet, and the like. Although not shown, a manual paper feed mechanism may be provided.

  In the image forming apparatus main body, a conveyance path R is disposed for discharging the paper P from the paper feed tray 10 through the secondary transfer nip and out of the apparatus. In the transport path R, a pair of registration rollers 12 serving as timing rollers for transporting the paper P to the secondary transfer nip at a transport timing is disposed upstream of the position of the secondary transfer roller 36 in the paper transport direction. ing.

  Further, a fixing device 20 for fixing the unfixed image transferred onto the paper P is disposed downstream of the position of the secondary transfer roller 36 in the paper transport direction. Further, a pair of paper discharge rollers 13 for discharging the paper to the outside of the apparatus is provided downstream of the fixing device 20 in the paper conveyance direction of the conveyance path R. A discharge tray 14 for stocking sheets discharged outside the apparatus is provided on the upper surface of the apparatus main body.

  Next, a basic operation of the printer according to the present embodiment will be described with reference to FIG. When the image forming operation is started, the respective photoconductors 5 in the respective image forming units 4Y, 4M, 4C, and 4K are rotationally driven clockwise in the drawing, and the surface of each photoconductor 5 is set to a predetermined polarity by the charging device 6. Uniformly charged. The surface of each charged photoconductor 5 is irradiated with laser light from the exposure device 9 to form an electrostatic latent image on the surface of each photoconductor 5. At this time, the image information to be exposed on each photoconductor 5 is single-color image information obtained by separating a desired full-color image into color information of yellow, magenta, cyan, and black. In this way, toner is supplied to each electrostatic latent image formed on each photoconductor 5 by each developing device 7, whereby the electrostatic latent image is visualized (visualized) as a toner image. .

  When the image forming operation is started, the secondary transfer backup roller 32 is driven to rotate counterclockwise in the figure, and the intermediate transfer belt 30 is caused to run in the direction indicated by the arrow in the figure. Further, by applying a constant voltage having a polarity opposite to the toner charging polarity or a voltage controlled by a constant current to each primary transfer roller 31, the primary transfer nip between each primary transfer roller 31 and each photoreceptor 5 is applied. A transfer electric field is formed.

  Thereafter, when each color toner image on the photoconductor 5 reaches the primary transfer nip as each photoconductor 5 rotates, the toner image on each photoconductor 5 is generated by the transfer electric field formed in the primary transfer nip. Are sequentially superimposed and transferred onto the intermediate transfer belt 30. Thus, a full color toner image is carried on the surface of the intermediate transfer belt 30. Further, the toner on each photoconductor 5 that could not be transferred to the intermediate transfer belt 30 is removed by the cleaning device 8. Then, the surface of each photoconductor 5 is neutralized by a neutralizing device (not shown), and the surface potential is initialized.

  In the lower part of the printer, the paper feed roller 11 starts to rotate, and the paper P is sent out from the paper feed tray 10 to the transport path R. The paper P sent to the transport path R is temporarily stopped by the registration rollers 12.

  Thereafter, rotation of the registration roller 12 is started at a predetermined timing, and the paper P is conveyed to the secondary transfer nip in accordance with the timing at which the toner image on the intermediate transfer belt 30 reaches the secondary transfer nip. At this time, a transfer voltage having a polarity opposite to the toner charging polarity of the toner image on the intermediate transfer belt 30 is applied to the secondary transfer roller 36, thereby forming a transfer electric field in the secondary transfer nip. . Then, the toner images on the intermediate transfer belt 30 are collectively transferred onto the paper P by this transfer electric field. At this time, the residual toner on the intermediate transfer belt 30 that could not be transferred onto the paper P is removed by the belt cleaning device 35 and conveyed to a waste toner container.

  Thereafter, the paper P is conveyed to the fixing device 20, and the toner image on the paper P is fixed to the paper P by the fixing device 20. Then, the paper P is discharged out of the apparatus by the paper discharge roller 13 and stocked on the paper discharge tray 14.

  The above description is an image forming operation when a full-color image is formed on a sheet. A single color image can be formed using any one of the four image forming units 4Y, 4M, 4C, and 4K. Two or three image forming units can be used to form a two-color or three-color image.

FIG. 2 is a cross-sectional view of the fixing device of this embodiment.
Hereinafter, the configuration of the fixing device 20 will be described with reference to FIG.
As shown in FIG. 2, the fixing device 20 includes a fixing belt 21 as a first fixing member, a pressure roller 22 as a second fixing member in contact with the outer peripheral surface of the fixing belt 21, and the fixing belt 21. A halogen heater 23 as a heating source for heating, a nip forming member 24 for forming the fixing nip N, a stay 25 as a supporting member for supporting the nip forming member 24, and heat from the halogen heater 23 are reflected to the fixing belt 21. A reflecting member 26 that shields heat from the halogen heater 23, and a temperature sensor 28 that serves as a temperature detecting means that detects the temperature of the fixing belt 21.

  The fixing belt 21 is formed of a film-like endless belt having flexibility. Specifically, the fixing belt 21 includes a base material on the inner peripheral side formed of a metal material such as nickel or SUS or a resin material such as polyimide (PI), and a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA). Or it is comprised by the release layer of the outer peripheral side formed with polytetrafluoroethylene (PTFE) etc. An elastic layer formed of a rubber material such as silicone rubber, foamable silicone rubber, or fluorine rubber may be interposed between the base material and the release layer.

  When there is no elastic layer, the heat capacity is reduced and improvement in fixing property can be achieved.However, when the unfixed toner is crushed and fixed, minute irregularities on the belt surface are transferred to the image, causing uneven glossiness on the solid portion of the image. It can happen. In order to prevent this, it is desirable to provide an elastic layer having a thickness of 100 μm or more. By providing an elastic layer having a thickness of 100 μm or more, minute unevenness can be absorbed by elastic deformation of the elastic layer, so that occurrence of uneven gloss can be avoided.

  In this embodiment, in order to reduce the heat capacity of the fixing belt 21, the fixing belt 21 is thin and has a small diameter. Specifically, the thicknesses of the base material, the elastic layer, and the release layer constituting the fixing belt 21 are set in a range of 20 to 50 μm, 100 to 300 μm, and 10 to 50 μm, and the overall thickness is set. It is set to 1 mm or less. The diameter of the fixing belt 21 is set to 20 to 40 mm. In order to further reduce the heat capacity, the thickness of the entire fixing belt 21 is desirably 0.2 mm or less, and more desirably 0.16 mm or less. The diameter of the fixing belt 21 is desirably 30 mm or less.

  In this embodiment, the diameter of the pressure roller 22 is set to 20 to 40 mm, and the diameter of the fixing belt 21 and the diameter of the pressure roller 22 are configured to be equal. However, the present invention is not limited to this configuration. For example, the fixing belt 21 may be configured such that the diameter of the fixing belt 21 is smaller than the diameter of the pressure roller 22.

  As shown in FIG. 3, belt holding members 40 are inserted on the inner circumferences of both ends of the fixing belt 21. The fixing belt 21 is rotatably supported by belt holding members 40 at both ends, and basically there is no member that supports the fixing belt 21 other than the belt holding member 40. That is, the fixing belt 21 is in a non-tensioned state that is not stretched over a roller or the like. The belt holding member 40, together with the halogen heater 23 and the stay 25, is fixed to a pair of side plates provided on both sides in the axial direction of the fixing device 20.

  As shown in FIG. 2, the pressure roller 22 includes a cored bar 22a, an elastic layer 22b made of a rubber material such as foamable silicone rubber, silicone rubber, or fluorine rubber provided on the surface of the cored bar 22a, and an elastic layer. And a release layer 22c made of PFA or PTFE provided on the surface of the layer 22b. The pressure roller 22 is pressed toward the fixing belt 21 by the switching mechanism, and is in contact with the nip forming member 24 via the fixing belt 21. At the location where the pressure roller 22 and the fixing belt 21 are in pressure contact, the elastic layer 22b of the pressure roller 22 is crushed to form a fixing nip N having a predetermined width.

  The pressure roller 22 is configured to be rotationally driven by a driving source such as a fixing motor (not shown) provided in the apparatus main body. When the pressure roller 22 rotates, the driving force is transmitted to the fixing belt 21 at the fixing nip N, and the fixing belt 21 is driven to rotate.

  In the present embodiment, the pressure roller 22 is a solid roller, but may be a hollow roller. In that case, a heating source such as a halogen heater may be disposed inside the pressure roller 22. The elastic layer 22b may be solid rubber, but if there is no heat source inside the pressure roller 22, sponge rubber may be used. Sponge rubber is more preferable because heat insulation is enhanced and heat of the fixing belt 21 is less likely to be taken away.

  The halogen heater 23 is disposed on the inner peripheral side of the fixing belt 21 and on the upstream side in the sheet conveying direction of the fixing nip N so that a portion other than the fixing nip N of the fixing belt 21 can be heated. The halogen heater 23 is configured to generate heat by being output controlled by a power supply unit provided in the apparatus main body, and the output control is performed based on the detection result of the surface temperature of the fixing belt 21 by the temperature sensor 28. . By such output control of the heater 23, the temperature of the fixing belt 21 (fixing temperature) can be set to a desired temperature. A temperature sensor (not shown) for detecting the temperature of the pressure roller 22 is provided in place of the temperature sensor for detecting the temperature of the fixing belt 21, and the temperature of the fixing belt 21 is predicted based on the temperature detected by the temperature sensor. You may make it do.

  In this embodiment, two halogen heaters 23 are provided, but the number of halogen heaters 23 may be one or three or more according to the size of the paper used in the printer. In addition to the halogen heater, an IH, a resistance heating element, a carbon heater, or the like can be used as a heating source for heating the fixing belt 21.

  The reflecting member 26 is fixedly supported by the stay 25 so as to face the halogen heater 23. By reflecting the radiant heat from the halogen heater 23 to the fixing belt 21 by the reflecting member 26, heat is prevented from being transmitted to the stay 25 and the like, so that the fixing belt 21 can be efficiently heated to save energy. I am trying. As the material of the reflecting member 26, aluminum, stainless steel or the like is used. In particular, in the case of using a material obtained by vapor-depositing silver having a low emissivity (high reflectivity) on an aluminum base material, the heating efficiency of the fixing belt 21 can be improved.

  The shielding member 27 is configured by forming a metal plate having a thickness of 0.1 mm to 1.0 mm into an arc-shaped cross-sectional shape along the inner peripheral surface of the fixing belt 21. The shielding member 27 is movable between the fixing belt 21 and the halogen heater 23 in the circumferential direction. In the present embodiment, as shown in FIG. 2, the circumferential region of the fixing belt 21 is opposed to the halogen heater 23 and is directly heated by the halogen heater 23. Other members (reflecting member 26, stay 25, nip forming member 24, etc.) fixed to the side plate or the like are interposed, and a non-heated region β that is not directly heated by the halogen heater 23 is formed. When there is no need for heat shielding, as shown in FIG. 2, the shielding member 27 is moved to the non-heated region β side, and when it is necessary to heat shield, the shielding member 27 is moved to the heated region α side. It is possible.

  By rotating the shielding member 27 in this manner, the area of the heated region α of the fixing belt 21 is changed, and the amount of radiant heat applied to the fixing belt 21 from the halogen heater 23 is adjusted. Thereby, an excessive temperature rise in the non-sheet passing region of the fixing belt 21 during continuous sheet feeding can be suppressed, and deterioration and damage of the fixing belt 21 due to heat can be prevented. Since the shielding member 27 requires heat resistance, it is preferable to use a metal material such as aluminum, iron, stainless steel, or ceramic as the material.

  The nip forming member 24 is disposed inside the fixing belt 21 and at a position facing the pressure roller 22 via the fixing belt 21. The nip forming member 24 includes a base 241 and a thin plate-like nip forming portion 242 fixed to the fixing belt 21 side of the base 241. The base 241 and the nip forming portion 242 are both heat-resistant resin materials such as polyethersulfone (PES), polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyethernitrile (PEN), and polyamideimide (PAI). , Polyether ether ketone (PEEK) and the like. In addition, the base 241 and the nip forming portion 242 can be formed of metal or ceramic. The base 241 and the nip forming portion 242 can be formed of the same kind of material as well as different materials. In the latter case, the base 241 and the nip forming part 242 can be integrated into one component.

  The base 241 of the nip forming member 24 is supported by the stay 25. Thereby, it is possible to prevent the nip forming portion 242 from being bent due to the pressure applied by the pressure roller 22, and to form a uniform nip width in the entire axial direction of the region where the fixing belt 21 and the pressure roller 22 face each other. . The stay 25 is made of a metal material such as a steel material such as stainless steel in order to satisfy the bending prevention function of the nip forming member 24. However, the stay 25 is made of a resin material if there is a sufficient effect for preventing the bending. You can also

  A low friction sheet 243 is attached to the surface of the nip forming portion 242 that slides with the fixing belt 21. When the fixing belt 21 rotates, the inner surface of the fixing belt 21 slides with respect to the low friction sheet 243, so that the frictional resistance acting on the fixing belt 21 can be reduced.

FIG. 4 shows the fixing nip N in an enlarged manner.
In FIG. 4, in order to clearly represent the shape of the fixing nip N, the unevenness or step of the nip forming member 24 is exaggerated. Therefore, the shapes of the nip forming member 24 and the fixing nip N are different from actual shapes.

  As shown in FIG. 4, a surface 242a (front surface) of the nip forming portion 242 facing the pressure roller 22 has a width dimension larger than the fixing nip N in the sheet conveyance direction. A first flat surface A, a connection surface B, a second flat surface C, and a separation surface D are formed on the front surface 242a from the entrance side (lower side of the drawing) of the fixing nip N. In FIG. 2, the separation surface D is omitted in the cross-sectional shape of the nip forming member 24.

Two of the flat surfaces A and C are formed so as to be parallel to each other. One flat surface A (first flat surface) is disposed on the inlet side of the fixing nip N, and the other flat surface C (second flat surface) is more at the outlet of the fixing nip N than the first flat surface A. Is disposed on the side (downstream in the paper conveyance direction). Further, the second flat surface C is located closer to the pressure roller 22 than the first flat surface A (right side of the drawing). The first and second flat surfaces A and C are preferably formed so as to be parallel to the entry direction of the paper P with respect to the inlet portion N1 of the fixing nip N. Specifically, in the present embodiment, an imaginary line m (see FIG. 4) connecting the rotation center O ₁ of the fixing belt 21 (center of the belt holding member 40 shown in FIG. 3) and the rotation center O ₂ of the pressure roller 22. The first flat surface A and the second flat surface C are arranged along a direction substantially perpendicular to the first flat surface A.

  The first flat surface A and the second flat surface C are smoothly connected via the connection surface B. The shape of the connection surface B is arbitrary, and can be formed as a flat surface, a curved surface, or a composite surface combining these. FIG. 4 illustrates a case where the connection surface B is formed by a composite curved surface including a first curved surface portion B1 on the inlet side of the fixing nip N and a second curved surface portion B2 on the outlet side of the fixing nip N.

  In this embodiment, the first curved surface portion B1 and the second curved surface portion B2 have opposite directions of curvature. Specifically, the first curved surface portion B1 is a cylindrical surface that is concave when viewed from the pressure roller 22 side, and the second curved surface portion B2 has a single curvature radius R2. The cylindrical surface is convex when viewed from the pressure roller 22 side.

  The separation surface D is disposed downstream of the second flat surface C in the paper transport direction. The separation surface D includes a first separation surface D1 and a second separation surface D2 that are curved surfaces having curvatures opposite to each other. The first separation surface D1 is continuously provided downstream from the downstream end portion of the second flat surface C in the paper conveyance direction, and the second separation surface D2 is downstream of the first separation surface D1 in the paper conveyance direction. It is provided continuously from the end to the downstream side. The first separation surface D1 and the second flat surface C are connected so as to have a common tangent at the boundary.

  The first separation surface D1 is formed in a cylindrical surface shape having a convex side opposite to the pressure roller 22 side. On the other hand, the 2nd separation surface D2 is formed in the shape of a cylindrical surface which makes the pressure roller 22 side convex, contrary to the 1st separation surface D1. Further, the second separation surface D2 is configured with a curvature larger than that of the first separation surface D1 (a curvature radius R4 smaller than the curvature radius R3 of the first separation surface D1). Here, the first curved surface portion B1, the second curved surface portion B2, the first separation surface D1, and the second separation surface D2 are described as having a single radius of curvature R1, R2, R3, and R4, respectively. However, each of these surfaces B1, B2, D1, and D2 may be composed of a plurality of curved surfaces having different curvatures.

  When the pressure roller 22 is pressed against the nip forming portion 242 described above via the fixing belt 21, the elastic layer 22 b of the pressure roller 22 is elastically deformed, and the fixing is performed between the fixing belt 21 and the pressure roller 22. A nip N is formed. At this time, the fixing belt 21 is deformed following the shapes of the first flat surface A, the connection surface B, and the second flat surface C of the nip forming portion 242.

  Of the fixing nip N, the portion formed by the first flat surface A and the second flat surface C of the nip forming portion 242 has a flat shape corresponding to the shapes of both flat surfaces A and C. Further, the portion formed on the connection surface B of the nip forming portion 242 has a shape corresponding to the shape of the connection surface B (curved surface shape in the present embodiment). In the following description, in the fixing nip N, a region formed by the first flat surface A of the nip forming portion 242 is referred to as a first flat portion Na, and a region formed by the connection surface B is referred to as a connection portion Nb. A region formed by the second flat surface C is referred to as a second flat portion Nc.

  In the fixing nip N, the inlet portion N1 is composed of the first flat portion Na (the first flat surface A of the nip forming portion 242 forms the inlet portion N1), and the outlet portion N2 is the second flat portion Nc. (The second flat surface C of the nip forming part 242 forms the outlet part N2).

  The sheet P reaches the entrance N1 of the fixing nip N with the toner T directed toward the fixing belt 21 and enters the first flat portion Na. In the first flat portion Na, the compressibility Xa of the elastic layer 22b of the pressure roller 22 increases toward the downstream side, and the surface pressure acting on the paper P gradually increases accordingly. Next, the paper P enters the connection portion Nb. At the connection portion Nb, the compression rate Xb of the elastic layer 22b increases rapidly to take a maximum value, and the surface pressure acting on the paper P becomes maximum. Thereafter, while the sheet P moves through the connecting portion Nb and the second flat portion Nc, the compression rate Xc of the elastic layer 22b decreases toward the downstream side, so that the surface pressure acting on the sheet P gradually decreases. Accordingly, the toner fixing process (melting / fluidizing / merging / penetrating / solidifying) proceeds mainly in a region from the contact portion Nb to the second flat portion Nc.

As described above, in the present invention, the high surface pressure region in the fixing nip N is provided on the outlet side of the fixing nip N. In order to realize this surface pressure distribution, a portion of the fixing nip N in which the compression rate of the elastic layer 22b is maximized (in the present embodiment, the connecting portion Nb) is closer to the fixing nip than the rotation center O ₂ of the pressure roller 22. N exit side, that is, on the exit side of the fixing nip N from the intersection when the perpendicular is drawn from the rotation center O ₂ of the pressure roller 22 to the first flat surface A (or an extension thereof) of the nip forming portion 242. Provide. In the present embodiment, as shown in FIG. 4, the rotational center O ₁ and in the vicinity of the imaginary line m connecting the rotational center O ₂ of the pressure roller 22 the boundary portion of the connecting portion Nb with the first flat portion Na of the fixing belt 21 Therefore, the maximum compression portion of the elastic layer 22b can be reliably arranged under the above conditions.

  Further, in the first flat portion Na of the fixing nip N, the surface of the fixing belt 21 is less likely to be wrinkled as compared to the case where the first flat portion Na is formed in a concave curved surface shape when viewed from the pressure roller 22 side. Further, in the first flat portion Na, since the compression amount Xa of the elastic layer 22b of the pressure roller 22 is small, the linear velocity difference between the fixing belt 21 and the pressure roller 22 is also small. Further, even when considering the displacement force acting on the unfixed toner, the influence of the displacement force is small because the first flat portion Na is in the low surface pressure region. For the above reasons, it is possible to suppress the deviation of the unfixed toner on the upstream side of the fixing nip N and prevent the image quality from being deteriorated due to the fixing.

  On the other hand, since the second flat surface C of the nip forming member 24 is disposed closer to the pressure roller 22 than the first flat surface A, the second flat surface C is located downstream of the first flat portion Na. A compression amount larger than the compression amount Xa of the elastic layer 22b in the first flat portion Na can be obtained at least at the connection portion Nb (or both the connection portion Nb and the second flat portion Nc). As a result, the exit side of the fixing nip N can be set to a high surface pressure region, and sufficient heat and pressure can be applied to the toner to reliably fix the unfixed image. Further, by locally increasing the contact surface pressure of the fixing nip as described above, it is possible to save energy by suppressing the heating amount by the heating source 23.

  As described above, according to the present invention, in the fixing nip, in the region where the toner T is not sufficiently heated and the unfixed image is likely to be displaced, the fixing nip N bites into the elastic layer 22b of the pressure roller 22. This is a flat surface having a small surface to suppress the deviation of the unfixed image. On the other hand, when the toner T is sufficiently heated, the nip forming member 24 is deeply cut into the elastic layer 22b to surely fix the toner. is there. As a result, it is possible to achieve both prevention of image deterioration due to fixing and securing of stable fixing properties.

  By the way, in the existing fixing device in which the rollers are pressed against each other to form the fixing nip N, the fixing nip is a concave curved surface or a convex curved surface. Often wrinkles. On the other hand, in the fixing device of the present invention, since the inlet side and the outlet side of the fixing nip N are formed by the flat portions Na and Nc that are parallel to each other, they reach from the first flat portion Na to the connecting portion Nb. The bending direction of the envelope up to and the bending direction of the envelope from the connecting portion Nb to the second flat portion Nc are reversed, and the bending in both directions is offset. Therefore, the generation of envelope wrinkles can be prevented.

  Further, in the fixing device of the present invention, since the nip forming member 24 has the separation surface D, the sheet P discharged from the fixing nip N is separated well from the fixing belt 21 at the position of the separation surface D. It is done. Specifically, the presence of the separation surface D causes the trajectory of the fixing belt 21 to pass through the exit portion N2 of the fixing nip N and then incline to the pressure roller 22 side and then bend to the opposite side with a large curvature. . As a result, the sheet P that has reached the outlet portion N2 of the fixing nip N is discharged in a direction F inclined toward the pressure roller 22 with respect to the extending direction of the second flat portion Nc, and then the fixing belt 21 is enlarged. The belt is separated from the fixing belt 21 at the bending portion.

  In this embodiment, since the fixing nip N has the second curved surface portion B2 formed with a convex cylindrical surface when viewed from the pressure roller 22 side, the sheet P passes through the fixing nip N. The sheet P is bent so that the front end of the sheet P faces the fixing belt 21 side. For this reason, it becomes easy to wind around the fixing belt 21 as compared with the configuration in which the fixing nip is formed only by a flat surface. In particular, when the paper P is a thin paper having a low rigidity (restoring force), the paper P is more easily wound around the fixing belt 21. However, even in such a case, the sheet P can be satisfactorily separated because the nip forming member 24 has the separation surface D as described above.

The configuration of another embodiment of the present invention will be described below.
As shown in FIG. 5, in the fixing device 20 according to another embodiment, the direction in which the pressure roller 22 approaches and separates from the nip forming member 24 (the arrow G direction in the drawing) and the sheet conveyance direction (the arrow in the drawing) H direction). Other than that, it is the same structure as the said embodiment.

  Thus, by making the pressure roller 22 movable, it becomes possible to improve the separation of the paper with respect to the fixing belt 21 and to further suppress the occurrence of wrinkles on the paper such as an envelope. . In the present embodiment, the pressure roller 22 is configured to be movable in a linear direction orthogonal or parallel to the first flat surface A and the second flat surface C. You may comprise so that it can move to the direction which inclines with respect to a curve direction.

  Specifically, when it is desired to improve the paper separation, the pressure roller 22 is moved in a direction approaching the nip forming member 24 as shown in FIG. Thereby, the elastic layer 22b of the pressure roller 22 is further crushed. Then, the range in which the fixing belt 21 is pressed against the nip forming member 24 side is expanded, and the fixing nip N is expanded. As a result, the fixing belt 21 approaches or comes into contact with the first separation surface D1 on the downstream side in the sheet conveyance direction from the exit portion N2 of the fixing nip N, and the fixing belt 21 follows the shape of the first separation surface D1. It will be greatly curved.

  As described above, the curvature of the fixing belt 21 on the downstream side in the paper conveyance direction from the outlet portion N2 is larger than that in the state illustrated in FIG. 5 because the fixing belt 21 is largely curved along the shape of the first separation surface D1. Increases and the separation of the paper increases. At this time, even if the fixing belt 21 is close to or in contact with the first separation surface D1, the first separation surface D1 has a cylindrical surface or a curved surface that is convex on the side opposite to the pressure roller 22 side. By being formed, excessive deformation of the fixing belt 21 can be suppressed. Thereby, it is possible to prevent the conveyance of the paper from being impaired. In the example shown in FIG. 6, the pressure roller 22 is not in contact with the separation surface D via the fixing belt 21, but if there is no problem in the transportability of the paper P, the paper on the separation surface D You may contact | abut to the part of the conveyance direction upstream.

FIG. 7 is an enlarged view showing the trajectory of the fixing belt 21 on the separation surface D. FIG. 7A shows the belt track in the state shown in FIG. 5, and FIG. 7B shows the belt track in the state shown in FIG.
Here, an angle θ formed by a tangent line Z at the most protruding portion of the separation surface D (the top of the second separation surface D2) and a straight line F indicating the conveyance direction of the sheet separated from the fixing belt 21 is defined as a separation angle. In other words, the separation angle θ is larger in the case shown in FIG. 7B than in the case shown in FIG. As described above, since the angle in the direction in which the sheet is separated from the fixing belt 21 is increased by increasing the separation angle θ, the sheet is easily separated.

  On the other hand, when using paper that is easily separated, the pressure roller 22 may be moved away from the nip forming member 24 from the state shown in FIG. 6 to return to the state shown in FIG. As a result, it is possible to avoid a large change in the curvature of the fixing belt 21 and maintain the sheet transportability.

  Next, when it is desired to suppress the occurrence of wrinkles on the paper to a higher degree, the pressure roller 22 is moved upstream in the paper conveyance direction as shown in FIG. Thus, by moving the pressure roller 22, the pressure roller 22 abuts only on the flat first flat surface A so that the fixing nip N is formed. As a result, compared to the state in which the pressure roller 22 is in contact with the curved connection surface B as shown in FIG. 5, the difference in linear velocity is less likely to occur on the front and back surfaces of the paper, and wrinkle generation is suppressed to a higher degree. Will be able to.

  In the example shown in FIG. 8, the entire fixing nip N is entirely formed only by the first flat surface A. However, in order to enhance the effect of suppressing the generation of wrinkles, the entire fixing nip N is not necessarily completely the first. It may not be formed only by one flat surface A. That is, it is possible to increase the wrinkle suppression effect simply by increasing the ratio at which the fixing nip N is formed by the first flat surface A. Therefore, for example, even if the pressure roller 22 is in contact with the connection surface B other than the first flat surface A or the second flat surface C as in the example shown in FIG. If the ratio formed by the flat surface A increases, the wrinkle suppressing effect can be enhanced. In order to suppress the generation of wrinkles to a higher degree, the proportion of the fixing flat nip N occupied by the first flat surface A is better. Therefore, as shown in FIG. It is most effective when it is formed only by the flat surface A.

  On the other hand, when using paper that does not easily wrinkle, the pressure roller 22 may be moved downstream from the state shown in FIG. 8 or 9 to return to the state shown in FIG. As a result, the pressure roller 22 comes into strong contact with the connection surface B, so that the contact surface pressure of the fixing nip is locally increased, and it is possible to improve image fixability and save energy.

  In the above-described example, the case where the pressure roller 22 is moved in the direction of approaching / separating from the nip forming member 24 or the sheet conveying direction has been described. It is also possible to adjust the separability and the suppression effect of wrinkles.

  The configuration of the present invention and its operation and effects will be described together as follows.

  The fixing device 20 according to the present invention has a flexible and rotatable fixing belt 21 and a rotatable pressure roller 22 having an elastic layer and forming a fixing nip N between the fixing belt 21 and the fixing belt 21. A sheet P supplied to the fixing nip N, and a nip forming member 24 that faces the pressure roller 22 via the fixing belt 21 and forms a fixing nip N, and a heating source 23 that heats the fixing belt 21. The upper unfixed image is fixed.

  In the fixing device 20, the nip forming member 24 includes a first flat surface A and a second flat surface that is disposed closer to the pressure roller 22 on the downstream side in the paper transport direction than the first flat surface A. Surface C, connecting surface B connecting both flat surfaces A and C, and separation surface projecting to the pressure roller 22 side from the second flat surface C on the downstream side of the second flat surface C in the sheet conveying direction. The fixing nip N is formed by the first flat surface A, the second flat surface C, and the connection surface B.

  Thus, by forming the fixing nip N with the first flat surface A, the second flat surface C, and the connection surface B, it is possible to achieve both prevention of image deterioration due to fixing and securing of stable fixing properties. It becomes possible.

  That is, the entrance nip N1 side of the fixing nip N is a flat portion having a shape that follows the first flat surface A of the nip forming member 24. Accordingly, the surface of the fixing belt 21 is less likely to be wrinkled. Further, in this flat portion A, the elastic layer 22b of the pressure roller 22 is not compressed so much, so that the linear velocity difference between the fixing belt 21 and the pressure roller 22 is also reduced. In addition, since the flat portion A is a low surface pressure region, the displacement force acting on the unfixed toner is reduced. For the above reasons, the displacement force acting on the toner can be reduced on the inlet portion N1 side of the fixing nip N where the toner is insufficiently melted, and deterioration of image quality due to this displacement can be prevented. On the other hand, since the contact surface B of the nip forming member 24 further compresses the elastic layer 22b more than the first flat surface A, it is higher on the downstream side in the paper conveyance direction than the first flat surface A of the fixing nip N. Pressure. Therefore, the toner can be reliably fixed to the paper P by applying sufficient heat and pressure to the toner. From the above, it is possible to achieve both prevention of image deterioration due to fixing and securing of stable fixing properties.

  Further, the nip forming member 24 has a separation surface D projecting toward the pressure roller 22 from the second flat surface C on the downstream side of the second flat surface C in the sheet conveyance direction, so that the fixing nip N The sheet P discharged from the sheet can be satisfactorily separated from the fixing belt 21 at the separation surface D. Specifically, since the separation surface D is present, the trajectory of the fixing belt 21 passes through the exit portion N2 of the fixing nip N and then changes so as to bend to the opposite side after being inclined to the pressure roller 22 side. After P is discharged from the fixing nip N to the pressure roller 22 side, it is separated from the fixing belt 21 where the fixing belt 21 bends. As a result, the paper P can be reliably prevented from being wound around the fixing belt 21.

  As mentioned above, although embodiment of this invention was described, it cannot be overemphasized that this invention can add a various change in the range which is not restricted to the above-mentioned embodiment and does not deviate from the summary of this invention. For example, the image forming apparatus equipped with the fixing device according to the present invention is not limited to a color printer as shown in FIG. 1, but may be a monochrome printer. In addition to a printer, a copying machine, a facsimile, or a complex machine of these can be used.

20 fixing device 21 fixing belt (first fixing member)
22 Pressure roller (second fixing member)
22b Elastic layer 23 Halogen heater (heating source)
24 Nipping member A First flat surface B Connection surface C Second flat surface D Separation surface D1 First separation surface D2 Second separation surface N Fixing nip P Paper (recording medium)
T Toner (Developer)

Japanese Patent No. 4725018 JP 2014-134701 A

Claims (9)

A flexible, rotatable first fixing member; an elastic layer; a rotatable second fixing member forming a fixing nip with the first fixing member; A nip forming member that faces the second fixing member through one fixing member and forms the fixing nip, and a heating source that heats the first fixing member, and is supplied to the fixing nip In a fixing device for fixing an unfixed image on a recording medium,
The nip forming member includes a first flat surface, a second flat surface disposed closer to the second fixing member on the downstream side in the recording medium conveyance direction than the first flat surface, A connecting surface that connects the two flat surfaces, and a separation surface that protrudes further toward the second fixing member than the second flat surface on the downstream side of the second flat surface in the recording medium conveyance direction,
The fixing device is characterized in that the fixing nip is formed by the first flat surface, the second flat surface, and the connection surface.   The separation surface includes a curved first separation surface that is convex on the opposite side of the second fixing member side, and a downstream side from the downstream end of the first separation surface in the recording medium conveyance direction. 2. The fixing device according to claim 1, further comprising a curved second separation surface that protrudes from the second fixing member side.   The fixing device according to claim 2, wherein the second separation surface is a curved surface having a larger curvature than the first separation surface.   4. The fixing device according to claim 1, wherein the second fixing member is configured to be movable toward and away from the nip forming member. 5.   The first fixing member is moved closer to the separation surface by moving the second fixing member in a direction approaching the nip forming member, and the first fixing member at a location corresponding to the separation surface is moved to the first fixing member. The fixing device according to claim 4, wherein the fixing member is changed so that the curvature of the fixing member is increased.   The fixing device according to claim 1, wherein the second fixing member is configured to be movable in a recording medium conveyance direction.   The fixing device according to claim 6, wherein the rate at which the fixing nip is formed by the first flat surface is increased by moving the second fixing member to the upstream side in the recording medium conveyance direction.   The fixing device according to claim 1, wherein the first flat surface and the second flat surface are parallel to each other.   An image forming apparatus comprising the fixing device according to claim 1.

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