JP5375729B2 - Fixing apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device and an image forming apparatus capable of efficiently preventing generation of paper wrinkles and gloss unevenness. <P>SOLUTION: The fixing device employs a belt-nip system, in which a separation roller disposed on a downstream side of a nip and pressing a stretched fixing belt from an inner side of the belt is in a multi-step crown shape having an increased diameter from each end toward the center. More particularly, the separation roller comprises: a center portion having a shape obtained by rotating a first hyperbola around an axis, the first hyperbola having a first crown amount with the center in the axial direction as a vertex; end portions having a shape obtained by rotating a second hyperbola around the axis, the second hyperbola having a second crown amount equal to or smaller than the first crown amount, with the center in the axial direction as a vertex; and intermediate portions having a shape obtained by rotating a connecting line connecting the first hyperbola and the second hyperbola around the axis. The average slope angle of the intermediate portions is larger than the average slope angles of the center portion and the end portions. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

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

  In general, in an image forming apparatus such as an electrophotographic printer or copying machine, a toner image is developed on a photosensitive drum based on image data, and the toner image is transferred onto a sheet. In the fixing device, an image is formed on the paper by fixing the toner image by thermocompression while nipping and conveying the paper at the nip portion. As a fixing device, for example, a belt nip type fixing device is known in which a nip portion is formed by press-contacting an endless belt stretched around a plurality of support rollers to a fixing roller having a heating source therein. This belt nip type fixing device has an advantage that a wide fixing nip can be formed in a compact size.

On the other hand, in the belt nip type fixing device, the pressure distribution in the axial direction (paper width direction) of the nip portion is not uniform, and particularly when the pressure at both ends in the axial direction is small, paper wrinkles are likely to occur. The urging state (pressing load) of the pressing member is adjusted so that the pressure at both ends in the axial direction becomes relatively large. For example, in a support roller (hereinafter referred to as a separation roller) as a pressing member disposed downstream of the nip portion (sheet exit side), both ends are urged against the fixing belt with a relatively large pressing load.
However, if both ends of the separation roller are urged with a large pressing load so that both ends of the nip portion are at a predetermined pressure, the separation roller is bent and the pressure in the center in the axial direction is reduced. In this case, air bubbles are likely to be generated in the toner layer at the center in the width direction on the leading end side of the paper to be passed, and unevenness of gloss such as dripping of water due to the air bubbles occurs and the image quality deteriorates. (See FIG. 6).

Therefore, conventionally, the shape of the separation roller is a regular crown shape having a larger outer diameter at the center compared to both ends. As a result, as shown in FIG. 7, when the separation roller is urged so that the pressure at both ends of the nip portion is equal to or higher than a predetermined value, the deflection amount of the separation roller (gap with the fixing roller at the center) is small. Become. That is, the pressure at both ends of the nip portion is relatively reduced, and the pressure distribution in the axial direction of the nip portion is made uniform.
Patent Documents 1 and 2 disclose that in a roller nip type fixing device in which a pressure roller is brought into pressure contact with a fixing roller, the pressure roller has a regular crown shape and the pressure distribution in the nip portion is made uniform.

  Here, the regular crown shape is a shape obtained by rotating a quadratic curve such as a hyperbola with the center in the axial direction of the roller as the top, and this quadratic curve is a crown amount ((maximum outer diameter of roller− Minimum outer diameter) / 2). That is, when a quadratic curve with a crown amount of (ab) / 2 is rotated about the axis within the length range of the roller, a regular crown shape in which the outer diameter at the center of the roller is a and the outer diameters at both ends are b. It becomes.

  As described above, in order to prevent the occurrence of uneven gloss, it is effective to make the separation roller have a regular crown shape and make the pressure distribution in the axial direction of the nip portion uniform. In addition, as shown in FIG. 8, if the pressing load at both ends is the same, the separation roller having a large crown amount has a higher effect of making the pressure distribution in the axial direction of the nip portion uniform. Conventionally, by adjusting the pressing load and the crown amount when urging both ends of the separation roller, a pressure distribution as uniform as possible is realized while maintaining a pressure of a predetermined value or more at both ends of the nip portion.

Japanese Patent Application No. 2008-175911 Japanese Patent Application No. 2005-284089

However, in recent years, speeding up of image formation has been demanded, and it is difficult to achieve a desired pressure distribution in the nip portion by the method of adjusting the pressing load and the crown amount when urging both ends of the separation roller. It has become. That is, if the pressing load at both ends of the separation roller is increased from the viewpoint of preventing the occurrence of paper wrinkles in order to cope with the higher speed of image formation, the amount of deflection of the separation roller increases and the pressure at the center decreases, resulting in uneven gloss. Will occur. When the crown amount of the separation roller is increased to eliminate this uneven glossiness, the pressure distribution can be made uniform, but the pressure at both ends is reduced, so that paper wrinkles are likely to occur.
In particular, the separation roller has a small diameter so that the passed paper can be easily peeled off from the fixing roller. Therefore, the amount of bending when both ends are urged by a predetermined pressing load is smaller than that of the pressure roller. large. For this reason, it is not easy to balance the pressing load at both ends of the separation roller and the crown amount, and it is extremely difficult to achieve both prevention of uneven gloss and prevention of paper wrinkles.

  SUMMARY An advantage of some aspects of the invention is that it provides a fixing device and an image forming apparatus that can efficiently prevent generation of paper wrinkles and uneven gloss.

The invention according to claim 1 includes a fixing roller heated to a predetermined temperature by a heating source and a fixing belt pressed against the fixing roller, and the fixing belt is pressed against the fixing roller. A belt nip type fixing device that fixes the toner image onto the paper by thermocompression while nipping and conveying the paper on which the toner image is formed at the nip portion,
A separation roller that is arranged on the downstream side of the nip portion and presses the tensioned fixing belt from the inside,
It has a crown shape that increases in diameter from both ends toward the center,
A central portion having a shape obtained by rotating a first hyperbola having a first crown amount centered in the axial direction about the axis;
Both ends having a shape in which the second hyperbola of the second crown amount equal to or less than the first crown amount is rotated about the axis with the center in the axial direction as a vertex;
An intermediate part having a shape obtained by rotating a connecting line connecting the first hyperbola and the second hyperbola around an axis;
The average slope of the intermediate portion is larger than the average slope of the central portion and the both end portions.

  According to a second aspect of the present invention, in the fixing device according to the first aspect, a minimum outer diameter of a rotating body obtained by rotating the first hyperbola within the length range of the separation roller is such that the second hyperbola is separated from the second hyperbola. It is characterized by being larger than the minimum outer diameter of the rotating body rotated within the roller length range.

  The invention according to claim 3 is the fixing device according to claim 2, wherein the first crown amount is the same as the second crown amount.

According to a fourth aspect of the present invention, in the fixing device according to the first aspect, a minimum outer diameter of a rotating body obtained by rotating the first hyperbola within a length range of the separation roller is such that the second hyperbola is separated from the second hyperbola. It is the same as the minimum outer diameter of the rotating body rotated in the roller length range,
The first crown amount is larger than the second crown amount.

  According to a fifth aspect of the present invention, there is provided an image forming apparatus comprising the fixing device according to any one of the first to fourth aspects, wherein a toner image formed on a sheet is fixed by the fixing device. It is.

  According to the present invention, in the fixing device or the image forming apparatus, the pressure distribution in the axial direction of the nip portion can be made uniform while maintaining a pressure higher than a predetermined value at both ends of the nip portion. Can be efficiently prevented.

1 is a diagram illustrating an internal configuration of an image forming apparatus according to an embodiment. FIG. 2 is a diagram illustrating a detailed configuration of a fixing device. It is a figure which shows the crown shape of a separation roller. It is a figure which shows the external shape profile of the separation roller which concerns on Example 1. FIG. FIG. 6 is a diagram illustrating an outer profile of a separation roller according to a second embodiment. It is a figure which shows about the gloss unevenness which generate | occur | produces on a paper. FIG. 4 is a diagram illustrating a pressure contact state of a separation roller with respect to a fixing roller. It is a figure which shows the pressure distribution of the axial direction in a nip part.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In the present embodiment, an example in which the image forming apparatus according to the present invention is applied to a digital multi-function peripheral (MFP) having functions such as copying and printers will be described. The image forming apparatus according to the present invention is not limited to a digital multi-function peripheral, and may be a facsimile machine, a copying machine, or a single printer as long as the image forming apparatus forms an image on a sheet.

FIG. 1 is a diagram illustrating an internal configuration of the image forming apparatus according to the present embodiment.
The image forming apparatus 1 reads a color image on a sheet based on image data obtained by reading a color image formed on a document or image data input from an external information device (for example, a personal computer) via a network. An image is formed by superimposing colors. In the image forming apparatus 1, the photosensitive drums 23 (23Y, 23M, 23C, and 23K) corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K) are connected and once. The tandem method is employed in which the toner images of the respective colors are sequentially transferred by the above procedure to form a color image on the paper.

As shown in FIG. 1, the image forming apparatus 1 includes an image reading unit 10, an image forming unit 20, a transport unit 30, a fixing device 60, and the like.
The image reading unit 10 includes an automatic document feeder 11 called an ADF (Auto Document Feeder), a document image scanning device 12, and the like. The automatic document feeder 11 transports the document d placed on the document tray by the transport mechanism and sends it out to the document image scanning device 12. The document image scanning device 12 optically scans the conveyed document d, and performs photoelectric conversion by a CCD (Charge Coupled Device) to read a document image. Here, the image includes text data such as characters and symbols in addition to image data such as graphics and photographs.

An image (analog image signal) read by the image reading unit 10 is output to the control unit 50 described later, and after various image processing such as A / D conversion processing and shading correction processing is performed, yellow (Y), Color separation into magenta (M), cyan (C), and black (K) colors is performed and output to the image forming unit 20 as output image data.
The automatic document feeder 11 can continuously read images (including both sides) of a large number of documents d placed on a document tray at a time. The read document image data is stored in an image memory (not shown) in the image forming unit 20 and sequentially read out as output image data.

  The image forming unit 20 includes an exposure device 21 (21Y, 21M, 21C, 21K), a developing device 22 (22Y, 22M, 22C, 22K), and a photosensitive drum, which are provided for each of Y, M, C, and K colors. 23 (23Y, 23M, 23C, 23K), charging device 24 (24Y, 24M, 24C, 24K), cleaning device 25 (25Y, 25M, 25C, 25K), intermediate transfer unit 26, cleaning device 27, and the like. ing.

  In the image forming unit 20, the charging device 24 charges the photosensitive drum 23, and the exposure device 21 irradiates the charged photosensitive drum 23 with light according to the image data of each color to form an electrostatic latent image. The developing device 22 develops the electrostatic latent image by attaching each color toner to the surface of the photosensitive drum 23 on which the electrostatic latent image is formed.

The intermediate transfer unit 26 includes an endless belt (hereinafter referred to as an intermediate transfer belt) 26a that is stretched around a support roller 26b. When the intermediate transfer belt 26a is pressed by the primary transfer roller 101 (101Y, 101M, 101C, 101K) to the photosensitive drum 23 to which the toner is attached, the respective color toner images are sequentially superimposed on the intermediate transfer belt 26a. Transcribed. When the primary transfer intermediate transfer belt 28a is pressed against the sheet by the secondary transfer roller 102, the toner image is secondarily transferred to the sheet.
The cleaning device 25 removes the toner remaining on the surface of the photosensitive drum 23 after the primary transfer. The cleaning device 27 removes the toner remaining on the intermediate transfer belt 26a after the secondary transfer.

  The fixing device 60 is a belt nip type fixing device, and includes a fixing roller 61 and a pressure unit 62 having a fixing belt 625 pressed against the fixing roller 61. The toner image is fixed to the paper by thermocompression while the paper on which the toner image is formed is nipped and conveyed at the nip formed by the fixing belt 625 being pressed against the fixing roller 61. The detailed configuration of the fixing device 60 will be described later.

The transport unit 30 includes a paper feeding device 31, a transport device 32, a paper discharge device 33, and the like. The paper feeding device 31 includes three paper feeding tray units 31a to 31c. In these paper feeding tray units 31a to 31c, standard papers and special papers identified based on the amount and size of paper are set in advance. Each type is housed. The sheets stored in the sheet feed tray units 31 a to 31 c are sent one by one from the top and conveyed to the image forming unit 20 by a conveying device 32 having a plurality of conveying rollers such as the registration rollers 103. At this time, the registration unit provided with the registration roller 103 corrects the inclination of the fed paper and adjusts the conveyance timing.
Then, in the image forming unit 20, the toner image on the intermediate transfer belt 26 a is secondarily transferred onto one side of the sheet at a time and subjected to a fixing process by the fixing device 60. The sheet on which the image is formed is discharged to a discharge tray 33a outside the apparatus by a discharge device 33 provided with a discharge roller 104.

FIG. 2 is a diagram illustrating a detailed configuration of the fixing device 60.
As shown in FIG. 2, the fixing device 60 is a belt nip type fixing device and includes a fixing roller 61, a pressure unit 62, an external heating unit 63, a cleaning unit 64, a refresh roller 65, and the like.
In the fixing roller 61, for example, a 1.5 mm-thick elastic layer 612 made of silicone rubber is formed on a hollow Al core bar 611 having an outer diameter φ80, and a release layer 613 made of a 50 μm-thick PFA tube is formed thereon. And a halogen heater 614 as a heating source is incorporated. Note that the release layer 613 may be formed by coating a release material on the elastic layer 612.

The pressure unit 62 presses the fixing belt 625 as a pressure member stretched around the separation roller 621, the inlet roller 622, the steering roller 623, and the support roller 624, and the fixing belt 625 from the inside toward the fixing roller 61. A fixed pressing member 627.
The fixing belt 625 is an endless belt having a configuration in which a 180 μm-thick elastic layer made of silicone rubber is formed on a polyimide base material having a thickness of 70 μm, and a release layer made of a 50 μm-thick PFA tube is formed thereon. is there.

The separation roller 621 is a solid SUS roller having a minimum outer diameter φ20 and a width of 380 mm, for example, and presses the fixing belt 625 toward the fixing roller 61 at the exit of the nip portion. The separation roller 621 is formed in a regular crown shape in which the diameter increases from both ends in the axial direction toward the central portion in order to maintain an appropriate pressure distribution in the axial direction when the fixing belt 625 is pressed.
In this embodiment, by improving the crown shape of the separation roller 621, the generation of paper wrinkles and the occurrence of uneven gloss are efficiently prevented.

The fixed pressing member 627 has a configuration in which a curved elastic layer close to the curvature of the fixing roller 61 is formed on a metal stay 626 such as SUS or Al, and between the separation roller 621 and the inlet roller 622. It is arranged. For example, the upstream side (paper inlet side) of the elastic layer of the fixed pressing member 627 is made of silicone sponge, and the downstream side is made of silicone rubber having a hardness of JIS-A 10 °.
In order to reduce the sliding resistance between the fixed pressing member 627 and the fixing belt 625, the surface of the fixed pressing member 627 is covered with, for example, a 70 μm thick sliding sheet 628 made of a heat resistant resin. The sliding sheet 628 is attached to the stay 626 along the sliding surface of the fixed pressing member 627.
When the fixing belt 625 is pressed against the fixing roller 61 by the separation roller 621 and the fixed pressing member 627, a wide nip portion (for example, nip width: 20 mm) is formed.

An inlet roller 622 disposed on the paper entry side of the pressure unit 62 incorporates a heater (not shown) and heats the fixing belt 625. The inlet roller 622 is in contact with a lubricant supply member 629 so that the lubricant is supplied to the inner peripheral surface of the fixing belt 625 via the inlet roller 622.
The lubricant supply member 629 is attached to the stay 626 and has a configuration in which an oil regulating film made of a porous PTFE is formed in a bag shape on an oil impregnated member made of felt such as aramid fiber. The lubricant supply member 629 applies the lubricant supplied from the oil impregnated member to the inlet roller 622 via the oil regulating film. As the lubricant, silicon oil such as dimethyl silicone oil having a viscosity of 100 to 1000 cs or methylphenyl silicone oil having a viscosity of 100 to 1000 cs is used.
The steering roller 623 disposed on the upstream side in the belt conveyance direction of the inlet roller 622 corrects the bias (meandering) of the fixing belt 625 by, for example, moving one end up and down and swinging the rotation shaft.

  The external heating unit 63 is configured by an endless belt (hereinafter referred to as a heating belt) 633 stretched between a heating roller 631 having a built-in heat source and a support roller 632, and the heating belt 633 extends along the surface of the fixing roller 61. By the pressure contact, the fixing roller 61 is supplementarily heated to stabilize the temperature. The heating roller 631 has a configuration in which, for example, the surface of an aluminum core is covered with a PFA tube (or coating layer).

  The cleaning unit 64 is a cleaning web type cleaning device. The cleaning unit 64 is a roll of a cleaning web 644 made of paper or cloth, and the cleaning web 644 fed out from the roll 641 is heated to a heating roller 631. A pressure roller 643 that is in pressure contact therewith, and a winding unit 642 that winds up the cleaning web 644 after cleaning. The cleaning web 644 delivered from the original winding portion 641 is pressed against the heating belt 633 (heating roller 631) by the pressing roller 643, and the residual toner attached to the heating belt 633 is removed while being wound up. That is, the toner adhering to the fixing roller 61 is transferred to the heating belt 633 and removed by the cleaning web 644.

  The refresh roller 65 has a predetermined surface roughness, and recovers the surface property of the fixing roller 61 by rubbing the surface of the fixing roller 61. The refresh roller 65 is, for example, a solid SUS roller having an outer diameter of φ80, and the surface is blasted with glass beads.

FIG. 3 is a view showing the crown shape of the separation roller 621. In FIG. 3, the outer diameter difference in the axial direction is greatly shown in order to remarkably represent the characteristic shape of the separation roller 621, but actually the minimum outer diameter is φ20 mm, whereas the center and both ends And the outer diameter difference is less than 1 mm.
As shown in FIG. 3, the separation roller 621 has a crown shape that increases in diameter from both ends L and R toward the center C, and has a central portion P1 having an average inclination θ ₁ and a width of 160 mm and a moderate inclination P1 and an average inclination. the intermediate portion of the steep width 30mm in degrees theta ₃ P3, partitioned by average inclination theta ₂ at both end portions P2 gentle slope of width 80 mm.
Here, the average inclination θ is defined by tan θ = outer diameter difference in each region / axial width (axial width / 2 for the central portion P1), and has a curved shape that defines the outer shape. Unaffected.

The center portion P1 has a shape obtained by rotating the first hyperbola L1 having the center C in the axial direction as a vertex about the axis. Both end portions P2 have a shape obtained by rotating the second hyperbola L2 having the center in the axial direction as a vertex about the axis. Further, the crown amount (first crown amount) of the first hyperbola L1 forming the central portion P1 is equal to or greater than the crown amount (second crown amount) of the second hyperbola L2 forming both ends P2.
The first hyperbola L1 and the second hyperbola L2 have a tangential slope that increases from the axial center C toward both ends L and R. Therefore, regardless of the length of the central part P1 and both ends P2, the average slope of the central part P1 The degree θ ₁ is smaller than the average inclination θ ₂ of both end portions P2.

Here, the crown amount of the first hyperbola L1 and the second hyperbola L2 is the rotation amount of the rotating body formed when the first hyperbola L1 or the second hyperbola L2 is rotated about the axis within the length range of the separation roller 621. Maximum outer diameter−minimum outer diameter) / 2.
In the separation roller 621, when the crown amount of the first hyperbola L1 is, for example, 0.8 mm, the rotating body when the hyperbola is rotated about the axis has a maximum outer diameter of 20.8 mm at the center C and a minimum outer diameter. Is 20 mm at both ends L and R, and a part of the rotating body is the central portion P1.

The intermediate part P3 has a shape obtained by rotating a connecting line L3 connecting the first hyperbola L1 and the second hyperbola L2 about the axis. R processing is applied to the boundary between the intermediate portion P3 and both end portions P2 or the central portion P1, and the intermediate portion P3 and both end portions P2 or the central portion P1 are smoothly connected.
Also, average inclination theta ₃ of the intermediate portion P3 is larger than the average inclination theta ₂ of the average inclination theta ₁ and both ends P2 of the central portion P1. That is, the separation roller 621 has a two-stage crown shape in which a steeply inclined intermediate portion P3 is formed between both end portions P2 and the central portion P1 which are gently inclined.
Although the separation roller 621 has a complicated shape as compared with the conventional separation roller, the desired shape can be realized relatively easily by lathe processing or the like.

Thus, the separation roller 621 has a crown shape of an outer profile obtained by synthesizing two hyperbolas, instead of a regular crown shape obtained by rotating a single hyperbola around the axis as in the prior art.
According to the separation roller 621, since the outer diameter of the central portion P1 is large, the pressure drop at the axial center of the nip portion when both ends are urged by a large pressing load is suppressed. Further, since the outer diameters of both end portions P2 are clearly thinner than the central portion P1, the pressure at both axial ends of the nip portion is maintained.
Therefore, according to the fixing device 60 including the separation roller 621, the pressure distribution in the axial direction of the nip portion is made uniform while maintaining a pressure equal to or higher than a predetermined value at both ends of the nip portion. Generation of uneven gloss can be efficiently prevented. Further, according to the image forming apparatus 1 including the fixing device 60, high-quality image formation is realized without causing paper wrinkles.

[Example 1]
FIG. 4 is a diagram illustrating an outer profile of the separation roller 621 according to the first embodiment.
As shown in FIG. 4, in the first embodiment, the first hyperbola L1 that forms the central portion P1 and the second hyperbola L2 that forms both ends P2 are composed of two hyperbola with the same crown amount and different vertex coordinates. doing. Specifically, a hyperbola with a crown amount of 0.74 mm is defined as a second hyperbola L2, and a hyperbola obtained by adding 0.06 mm as a correction value to the second hyperbola L2 is defined as a first hyperbola L1. Further, a curve obtained by linearly complementing the second hyperbola L2 with 0 to 0.06 mm is defined as a connecting line L3.
That is, in Example 1, the minimum outer diameter (20.06 mm, shift amount: 0.06 mm) of the rotating body obtained by rotating the first hyperbola L1 within the length range of the separation roller 621 separates the second hyperbola L2. It is larger than the minimum outer diameter (20 mm) of the rotating body rotated within the length range of the roller 621.

  Using the fixing device 60 provided with the separation roller 621 of Example 1, the occurrence of paper wrinkles and gloss unevenness was observed when the toner image was thermocompression bonded to the paper. At this time, the sheet conveyance speed was 400 mm / sec, and the pressing load of the separation roller 621 was 680 N. As a result, neither paper wrinkles nor gloss unevenness occurred.

  In Example 1, the crown amount of the first hyperbola L1 (first crown amount) is set to be the same as the crown amount of the second hyperbola L2 (second crown amount). It may be larger than two crown amounts.

[Example 2]
FIG. 5 is a diagram illustrating an outer profile of the separation roller 621 according to the second embodiment.
As shown in FIG. 5, in Example 2, the first hyperbola L1 that forms the central portion P1 and the second hyperbola L2 that forms both ends P2 are composed of two hyperbolas with different crown amounts. Specifically, a hyperbola with a crown amount of 0.8 mm is defined as a first hyperbola L1, and a hyperbola with a crown amount of 0.74 mm is defined as a second hyperbola L2. The connecting line L3 smoothly connects the first hyperbola L1 and the second hyperbola L2.
That is, in Example 2, the minimum outer diameter (20 mm) of the rotating body obtained by rotating the first hyperbola L1 within the length range of the separation roller 621 is set to rotate the second hyperbola L2 within the length range of the separation roller 621. The crown amount (first crown amount) of the first hyperbola L1 is larger than the crown amount (second crown amount) of the second hyperbola L2 in the same manner as the minimum outer diameter of the rotating body.

  Using the fixing device 60 provided with the separation roller 621 of Example 2, a toner image was thermocompression bonded to the paper under the same conditions as in Example 1, and the occurrence of paper wrinkles and gloss unevenness was observed. As a result, neither paper wrinkles nor gloss unevenness occurred.

In addition, when the crown amount of the first hyperbola L1 is larger than the crown amount of the second hyperbola L2 as in the second embodiment, the first hyperbola L1 and the second hyperbola L2 are directly and smoothly connected to each other. A part may be the connecting line L3 of the intermediate part P3. Again, average inclination theta ₃ of the intermediate portion P3 is greater than the average inclination theta ₂ of the average inclination theta ₁ and both ends P2 of the central portion P1.

[Comparative Example 1]
In Comparative Example 1, the shape of the separation roller 621 was a regular crown shape in which a hyperbola with a crown amount of 0.74 mm was rotated around the axis.
Using the fixing device 60 provided with the separation roller 621 of Comparative Example 1, a toner image was thermocompression bonded to the paper under the same conditions as in Example 1, and the occurrence of paper wrinkles and gloss unevenness was observed. As a result, paper wrinkles did not occur, but gloss unevenness occurred. In the separation roller 621 according to the comparative example 1, it is considered that the pressure at the center of the nip portion is remarkably reduced (see the crown amount in FIG. 8: small).

[Comparative Example 2]
In Comparative Example 2, the shape of the separation roller 621 was a regular crown shape in which a hyperbola with a crown amount of 0.8 mm was rotated about the axis.
Using the fixing device 60 provided with the separation roller 621 of Comparative Example 2, a toner image was thermocompression bonded to the paper under the same conditions as in Example 1, and the occurrence of paper wrinkles and gloss unevenness was observed. As a result, gloss unevenness did not occur, but paper wrinkles occurred. It is considered that the separation roller 621 according to Comparative Example 2 has an excessively large outer diameter at the center portion, and the pressure at both ends of the nip portion has decreased (see Crown amount: large in FIG. 8).

As mentioned above, although the invention made by this inventor was concretely demonstrated based on embodiment, this invention is not limited to the said embodiment, It can change in the range which does not deviate from the summary.
For example, in the above-described embodiment, the shape of the separation roller 621 is a two-step crown shape. However, for example, the intermediate portion P3 includes a portion (gradually inclined portion) rotated about the hyperbola as the center. It is good also as a structure by which the steeply inclined part and the gentle inclined part were alternately formed, and it is good also as a multistage crown shape of 3 steps | paragraphs or more. Again, average inclination theta ₃ of the intermediate portion P3 is greater than the average inclination theta ₂ of the average inclination theta ₁ and both ends P2 of the central portion P1.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 60 Fixing apparatus 61 Fixing roller 62 Pressure unit 621 Separation roller P1 Center part P2 Both ends P3 Middle part L1 1st hyperbola L2 2nd hyperbola L3 Connection line

Claims (5)

A fixing roller heated to a predetermined temperature by a heating source and a fixing belt pressed against the fixing roller are provided, and a toner image is formed at a nip formed by pressing the fixing belt against the fixing roller. A belt nip type fixing device that fixes the toner image onto the paper by thermocompression while nipping and conveying the formed paper,
A separation roller that is arranged on the downstream side of the nip portion and presses the tensioned fixing belt from the inside,
It has a crown shape that increases in diameter from both ends toward the center,
A central portion having a shape obtained by rotating a first hyperbola having a first crown amount centered in the axial direction about the axis;
Both ends having a shape in which the second hyperbola of the second crown amount equal to or less than the first crown amount is rotated about the axis with the center in the axial direction as a vertex;
An intermediate part having a shape obtained by rotating a connecting line connecting the first hyperbola and the second hyperbola around an axis;
The fixing device according to claim 1, wherein an average inclination of the intermediate portion is larger than an average inclination of the central portion and the both end portions.   The minimum outer diameter of the rotating body obtained by rotating the first hyperbola within the length range of the separation roller is greater than the minimum outer diameter of the rotating body obtained by rotating the second hyperbola within the length range of the separation roller. The fixing device according to claim 1.   The fixing device according to claim 2, wherein the first crown amount is the same as the second crown amount. The minimum outer diameter of the rotating body obtained by rotating the first hyperbola within the length range of the separation roller is the same as the minimum outer diameter of the rotating body obtained by rotating the second hyperbola within the length range of the separation roller. ,
The fixing device according to claim 1, wherein the first crown amount is larger than the second crown amount.   An image forming apparatus comprising the fixing device according to claim 1, wherein a toner image formed on a sheet is fixed by the fixing device.

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