JP6632291B2 - Image heating device - Google Patents

Description

  The present invention relates to an image heating apparatus used in an image forming apparatus such as a copying machine, a printer, and a facsimile that uses a recording method such as an electrophotographic method or an electrostatic recording method.

  2. Description of the Related Art As a fixing device for a paper as a recording material on which an unfixed image is formed and carried, which is used in an electrophotographic image forming apparatus, a contact heating type fixing system having good thermal efficiency and the like is widely known. Recently, a fixing method characterized by power saving and a short wait time (quick start) has been proposed. For example, in a configuration in which a heater included in a thin fixing film is pressed against a fixing roller via the fixing film, paper passes between the heated fixing film and the fixing roller, so that a toner image on the paper is formed. There is a film system in which fixing is performed (Patent Document 1).

  This film-type fixing device enables power saving and quick start because the heat capacity of the fixing film is small. In addition, there has been proposed an external heating type fixing device in which a heating element is disposed on the outer surface of the fixing roller and only the outer surface of the fixing roller is heated to enable power saving and quick start.

  A sponge rubber roller made of sponge rubber in which an elastic layer contains a large number of fine air bubbles is often used as a fixing roller used in a fixing device that enables power saving and quick start. The sponge rubber includes, for example, a foam obtained by dispersing a gas in silicone rubber, and a foam containing a resin microballoon. Such a sponge rubber roller has a low thermal conductivity and a high heat insulating effect due to the presence of the contained bubbles, so that heat does not escape to the core metal and the surface temperature can be kept high.

  The sponge rubber includes a closed cell sponge rubber in which bubbles inside exist independently and an open-cell sponge rubber in which bubbles are adjacent to each other. Regardless of the sponge rubber, due to the presence of air bubbles, the thermal conductivity is smaller than that of the solid rubber, and the heat insulating effect is high. However, in the case of a closed-cell sponge roller using closed-cell sponge rubber, if the pressing force is applied for a long period of time, the gas in the bubbles gradually escapes, and only the part where the pressing force is applied collapses. State.

  At the start of paper feeding, even if the rotation of the fixing roller resumes, it cannot return to the original state immediately, and it continues to rotate in a distorted shape, causing fixing failure and paper conveyance failure at that part. I will. Therefore, an open-cell sponge roller using open-cell sponge rubber that allows gas in the air to enter and exit freely and has high durability against repeated compression is generally used.

  The fixing device of the film heating system using the open-cell sponge roller as described above has a problem that paper wrinkles (hereinafter, referred to as paper wrinkles) are generated. Paper wrinkling is considered to occur when the paper transport speed is faster at the center portion than at the left and right end portions in the rotation axis direction (longitudinal direction) of the fixing roller. Conventionally, in order to solve this problem, the outer diameter of the fixing film has been processed to be larger at the left and right ends than at the center in the rotation axis direction (longitudinal direction) (hereinafter referred to as an inverted crown shape). Thus, the paper conveyance speed at the left and right end portions becomes higher than the paper conveyance speed at the central portion, thereby preventing paper wrinkles (Patent Document 2).

JP-A-4-204980 JP 2005-284089 A

  However, in recent years, as awareness of the environment has increased, it has been required to respond to paper as a thinner recording material in order to save resources. Since thin paper has low stiffness and is easily distorted and easily broken, paper wrinkles are easily generated. To prevent wrinkling of thin paper, the crown amount of the fixing film must be further increased.However, if the fixing film has an excessively large crown amount, the running of the fixing film becomes unstable and the fixing film becomes unstable. The film may be distorted.

  If the fixing film is distorted, the heat transfer may not be uniform in the direction of the rotation axis, or the image may be disturbed by being pressed against the paper while the fixing film is distorted (hereinafter, referred to as image shift). ) Occurs.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image heating apparatus capable of suppressing paper wrinkles without causing image displacement even with a thin recording material.

To solve the above object, an image heating apparatus according to the present invention, the roller having a core metal serving as a rotating shaft, provided on the outer periphery of the metal core, and the elastic layer containing therein the bubbles continuous with each other, the And, a cylindrical film whose outer surface is in contact with the outer peripheral surface of the roller, and a pressing member that is in contact with the inner surface of the film and forms a nip portion with the roller through the film , An image heating apparatus for heating an image on a recording material while nipping and conveying the recording material at the nip portion , wherein a pressure for forming the nip portion is applied between the roller and the pressing member. Thereby, the elastic layer in the central region in the axial direction of the rotation axis of the roller is crushed more than the elastic layer in the end region, and the outer peripheral length of the roller in the central region is smaller than that of the roller in the end region. Perimeter length Also it is shorter, and the nip portion of the serial Rokuzai the width of the conveying direction Nc corresponding to the central area, the front Symbol width of the nip portion corresponding to the end regions when the Ns,
Nc> Ns
Characterized in that it meets the following condition.

  According to the present invention, paper wrinkles can be suppressed even with a thin recording material without causing image displacement.

FIG. 1 is a schematic sectional view of a fixing device as an image heating device according to a first embodiment of the present invention. (A) is a schematic diagram of the fixing device as viewed from the fixing film side when the paper conveyance speed is vc> vs, and is a diagram showing a state in which the paper is wavy in front of the fixing nip portion, and (b) is a diagram illustrating the fixing. FIG. 5 is a diagram illustrating a paper conveyance speed distribution in a rotation axis direction of a fixing roller in a nip portion. It is a figure which shows the state in which the waving occurred in the thin paper. 7A and 7B show the behavior of the paper before and after the occurrence of waving, FIG. 7A is a diagram showing the state of the paper at the time of waving, FIG. 7B is a diagram showing a slip mode, and FIG. 7C is a diagram showing a buckling mode. (A) is a schematic diagram of the fixing device viewed from the fixing film side when the paper conveyance speed is vs> vc, and shows a paper behavior before a fixing nip portion, and (b) is a diagram illustrating rotation of a fixing roller in the fixing nip portion. FIG. 4 is a diagram illustrating a paper conveyance speed distribution in an axial direction. (A) is a schematic diagram of the fixing device viewed from the upstream side in the paper conveyance direction when the fixing film has an inverted crown shape, and (b) is a diagram showing a paper conveyance speed distribution in a fixing roller rotation axis direction of a fixing nip portion. is there. 7A and 7B are diagrams illustrating a deformation behavior of the fixing roller at the time of pressing, in which FIG. 7A illustrates a case of a solid roller, and FIG. 8B illustrates a case of an open-cell sponge roller. FIG. 4 is a schematic diagram of an apparatus for measuring a relationship between a fixing nip width and a paper conveyance speed. FIG. 9 is a diagram illustrating a result of measuring a relationship between a fixing nip width and a paper conveyance speed. FIG. 6 is a diagram illustrating a relationship between a fixing nip width difference ΔN and a paper conveyance speed difference Δv. FIG. 2A is a schematic diagram of the fixing device according to the first embodiment viewed from the upstream side in the paper conveyance direction, and FIG. 2B is a diagram illustrating a shape of a fixing nip portion. FIG. 6 is a schematic diagram of a fixing device according to a second embodiment. FIG. 7A is a schematic diagram of a fixing device according to a second embodiment as viewed from the upstream side in the paper conveyance direction, and FIG. 8B is a diagram illustrating a shape of a fixing nip portion. FIG. 9 is a schematic diagram of a fixing device according to a third embodiment. FIG. 7A is a schematic diagram of a fixing device according to a third embodiment as viewed from the upstream side in the paper conveyance direction, and FIG. 8B is a diagram illustrating a shape of a fixing nip portion. FIG. 9 is a schematic diagram of a fixing device according to another embodiment. FIG. 9 is a schematic diagram of a fixing device according to another embodiment. 1 is a schematic sectional view of an image forming apparatus equipped with a fixing device as an image heating device according to an embodiment of the present disclosure.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<< 1st Embodiment >>
(Image forming device)
FIG. 18 is a schematic configuration diagram of an image forming apparatus equipped with the image heating device according to the embodiment of the present invention. The present image forming apparatus is a laser beam printer using a transfer type electrophotographic process. Reference numeral 1 denotes an electrophotographic photosensitive drum as an image carrier, which is rotationally driven in a clockwise direction indicated by an arrow at a surface moving speed (process speed) of 160 mm / s. Reference numeral 2 denotes charging means such as a contact charging roller, which uniformly charges (primarily charges) the surface of the photosensitive drum 1 to a predetermined polarity and potential.

  Reference numeral 3 denotes a laser beam scanner as image exposure means, which is a laser beam whose on / off is modulated in accordance with a time-series electric digital pixel signal of target image information input from an external device such as an image scanner or computer (not shown). L is output. Then, the charged surface of the photosensitive drum 1 is scanned and exposed (irradiated). By this scanning exposure, the charge of the exposed light portion on the surface of the photoconductor drum 1 is eliminated, and an electrostatic latent image corresponding to the target image information is formed on the surface of the photoconductor drum 1.

  Reference numeral 4 denotes a developing device, which is supplied with toner as a developing material from the developing sleeve 4a to the surface of the photosensitive drum 1 and sequentially develops a portion of the surface of the photosensitive drum 1 where an electrostatic latent image is formed. In the case of a laser beam printer, a reversal development method is generally used in which toner is adhered to the exposed portion of an electrostatic latent image and developed.

  Reference numeral 5 denotes a paper feed cassette in which paper (also referred to as a sheet) P serving as a recording material is loaded and stored. The paper feed roller 6 is driven based on the paper feed start signal, and the paper P in the paper feed cassette 5 is separated and fed one by one. The fed paper P passes through a registration roller 7 and a sheet path 8a, and is introduced into a transfer portion T, which is a nip portion where the photosensitive drum 1 comes into contact with a transfer roller 9 as a contact / rotation type transfer member. Is done. At this time, the conveyance of the paper P is controlled by the registration roller 7 so that the leading end of the toner image on the photosensitive drum 1 reaches the transfer site T, and the leading end of the paper P reaches the transfer site T. Is done.

  The paper P introduced into the transfer site T is nipped and conveyed at the transfer site T. During this time, a transfer voltage (transfer bias) controlled in a predetermined manner from a transfer bias application power source (not shown) is applied to the transfer roller 9. When a transfer bias having a polarity opposite to that of the toner is applied to the transfer roller 9, the toner image on the surface of the photosensitive drum 1 is electrostatically transferred to the surface of the paper P at the transfer portion T. The paper P on which the toner image has been transferred at the transfer portion T is separated from the surface of the photosensitive drum 1, conveyed to the fixing device 11 as an image heating device through the sheet path 8 b, and heat-presses and fixes the toner image. Receive processing.

  On the other hand, the surface of the photosensitive drum 1 after the separation of the paper (after the transfer of the toner image to the paper P) is cleaned by the removal of the untransferred toner and paper dust by the cleaning device 10, and the next charging, exposure, development, and transfer are performed. Prepare for the process.

  The paper P that has passed through the fixing device 11 is conveyed toward a paper discharge port 13 by a paper discharge roller 15, and is discharged from the paper discharge port 13 onto a paper discharge tray 14. The process speed of the image forming apparatus according to the present embodiment is 160 mm / s, and 30 sheets of image forming processing are performed per minute when the A4 paper is passed in the vertical direction.

(Image heating device)
FIG. 1 is a schematic configuration diagram of a fixing device as an image heating device according to an embodiment of the present invention. In the present embodiment, the elastic layer of the fixing roller 24 is an open-cell sponge rubber layer, and the width of the fixing nip is more centered than the left and right ends in the rotation axis direction of the fixing roller 24 (longitudinal direction intersecting the recording material conveyance direction). The configuration is large in the section.

  A thin, cylindrical (endless belt) rotatable fixing film 22 containing a heater 23 as a heating element and a fixing roller (rotating roller) 24 as an opposing element form a fixing nip (nip) Nip. It is pressed like so. Then, the paper P as a recording material is nipped and conveyed between the heated fixing film 22 and the fixing roller 24, so that the toner image on the paper is fixed.

  In such a fixing method, since the fixing film 22 is thin and has a small heat capacity, power saving and quick start are enabled. Further, the fixing roller 24 in the present embodiment is an open-cell sponge roller using an open-cell sponge rubber having a low heat capacity and a low thermal conductivity for an elastic layer on the outer periphery of a core metal serving as a rotating shaft. Such a fixing roller 24 has a configuration in which heat is less likely to escape to the core metal and the surface temperature is easily maintained at a high temperature, so that further power saving and quick start can be achieved.

  In FIG. 1, the film unit 500 includes a fixing film 22, a heater 23, a heater holder 21 holding the heater 23, and an iron stay 25 supporting the heater holder 21. The heater holder 21 functions as a first pressing member that guides the fixing film 22 and presses the nip portion Nip. The heater holder 21 can be made of a high heat resistant resin such as polyimide, polyamide imide, PEEK, PPS, liquid crystal polymer, or a composite material of these resins with ceramics, metal, glass, and the like. In the present embodiment, PPS is used.

  The stay 25 is configured to be pressed in the direction of arrow W in the drawing by a spring (not shown) from the left and right ends of the fixing film 22 in the rotation axis direction (longitudinal direction). Thus, the heater 23 and the heater holder 21 holding the heater 23 are pressed by the open-cell sponge roller 24 via the fixing film 22 to form the fixing nip Nip.

In this embodiment, the total pressing force in the direction of arrow W in the drawing is 15 kgf, and the width of the fixing nip Nip is 6 to 8 mm. The width of the elastic layer of the fixing roller 24 as the open-cell sponge roller in the rotation axis direction is 225 mm, and the pressing force per unit area in the fixing nip is 0.82 × 10 ⁵ Pa to 1.1 × 10 ^5. Pa.

  The fixing roller 24 as an open-cell sponge roller is driven by a drive source (not shown) as a driving rotating body, and the surface moving speed of the open-cell sponge roller is set to the same process speed as the electrophotographic photosensitive drum, ie, 160 mm / s. 1 has been rotated in the direction of the arrow R1. Then, the fixing film 22 receives power from the fixing roller 24 at the fixing nip Nip, and is driven to rotate in a direction indicated by an arrow R2 in FIG. As described above, the fixing film 22 is heated by the heater 23 while rotating, and the heat is gradually transmitted to the fixing roller 24 and is stored in the fixing roller 24.

  The temperature of the heater 23 is controlled at a regulated temperature of 180 ° C. by a temperature detecting element 27 disposed on the back of the heater 23. The paper P as a recording material on which the unfixed toner image is recorded is conveyed from A1 to A2 in the drawing at the fixing nip portion Nip, and is supplied by the heated fixing film 22 and the open-cell sponge roller 24 storing heat during that time. The toner image is fixed on the paper P by the applied heat and the pressing force.

(Fixing film)
The fixing film 22 of the present embodiment has a cylindrical shape that is not deformed and has an outer diameter of 18 mm. The film has a multilayer structure in the thickness direction. The layer structure includes a base layer 22a for maintaining the strength of the film and a release layer 22b for reducing the adhesion of dirt to the surface. The material of the base layer 22a needs to have heat resistance to receive the heat of the heater 23, and also needs strength to slide with the heater 23. A metal such as stainless steel (SUS) or nickel, or a high heat-resistant resin such as polyimide, polyamide imide, or PEEK may be used.

  Metals have strength compared to resins and can be made thinner, and have high thermal conductivity, so that the heat of the heater 23 can be easily transmitted to the surface of the fixing film 22. On the other hand, resin has the advantage that it has a small heat capacity and is easy to warm because it has a lower specific gravity than metal.

  In the present embodiment, a polyimide resin is used as the material of the base layer 22a of the fixing film 22, and a carbon-based filler is added to improve the thermal conductivity and strength. As the thickness of the base layer 22a is smaller, the heat of the heater 23 is more easily transmitted to the surface of the fixing roller 24, but the strength is reduced. Therefore, the thickness is preferably about 15 μm to 100 μm, and in this embodiment, it is 60 μm.

  The release layer 22b of the fixing film 22 is preferably made of a fluororesin such as perfluoroalkoxy resin (PFA), polytetrafluoroethylene resin (PTFA), and tetrafluoroethylene hexafluoropolypropylene resin (FEP). In the present embodiment, among these fluororesins, PFA having excellent release properties and heat resistance is used.

  The release layer 22b may be coated with a tube, or may be coated with a paint on the surface. In the present embodiment, the release layer 22b is molded by a coat excellent in thin-wall molding. As the release layer 22b is thinner, the heat of the heater 23 is more easily transmitted to the surface of the fixing film 22, but if it is too thin, the durability is poor. In this embodiment, the thickness is 10 μm. Further, the shape of the fixing film of the present embodiment was a straight shape in which traveling was stable.

(Fusing roller)
The fixing roller 24 of the present embodiment is an open-cell sponge roller that enables quick start with low heat conductivity and low heat capacity. In the fixing roller 24, an elastic layer is formed on the outer periphery of an iron core 26, and the elastic layer includes a layer containing continuous bubbles (a large number of continuous bubbles). In this embodiment, the elastic layer has a continuous foam sponge rubber layer 24a, and further has a release layer 24b on its outer periphery.

  The open-cell sponge rubber used in the present embodiment is an open-cell sponge rubber in which a resin balloon and an open-cell agent are included in solid rubber, and the resin balloons are connected by vaporizing the open-cell agent. The thermal conductivity and heat capacity of the obtained open-cell sponge rubber become smaller as the number of resin balloons contained therein becomes smaller and the specific gravity becomes smaller. However, if it is too small, the strength is reduced and the durability is reduced.

  Therefore, it is necessary to use a material having an appropriate specific gravity, and it is preferable to set the range of 0.40 to 0.72. At this time, the thermal conductivity of the open-cell sponge rubber is 0.11 to 0.16 W / m · K. In the present embodiment, an open-cell sponge rubber having a specific gravity of 0.55 and a thermal conductivity of 0.13 W / m · K is used.

  The smaller the outer diameter of the fixing roller 24 is, the smaller the heat capacity is. However, if the outer diameter is too small, the width of the fixing nip Nip becomes narrow, and the heat transfer to the paper becomes small, and the fixing property is deteriorated. A diameter is required. In the present embodiment, the outer diameter is 18 mm. Regarding the thickness of the open-cell sponge rubber layer 24a, if it is too thin, heat escapes to the iron core 26, so that an appropriate thickness is required. In the present embodiment, the thickness of the open-cell sponge rubber layer 24a is 3.5 mm. Accordingly, the diameter of the cored bar 26 is 11 mm.

  If the hardness of the open-cell sponge roller is too high, the width of the fixing nip Nip becomes narrow, and the heat transfer to the paper becomes small, thereby deteriorating the fixing property. Will fall. Therefore, the hardness is preferably in the range of 30 ° to 60 ° in Asker C hardness (500 g load). In the present embodiment, the one at 50 ° is used.

  The release layer 24b of the toner was made of a perfluoroalkoxy resin (PFA) similarly to the release layer 22b of the fixing film 22. The release layer 24b may have a surface coated with a paint or a tube. In the present embodiment, the release layer 22b is formed from a tube having excellent durability. As a material of the release layer 24b, in addition to PFA, a fluororesin such as PTFE and FEP, a fluororubber or a silicone rubber having good release properties may be used.

(heater)
The heater 23 is formed by applying a current-generating resistance layer of Ag / Pd (silver palladium) to a surface of an alumina substrate having a width of 5 mm and a thickness of 1 mm by screen printing to a thickness of 10 μm. The one covered with the thickness was used. On the back surface of the heater 23, a temperature detecting element 27 for detecting the temperature of the ceramic substrate which has been heated according to the heat generated by the energized heat generating resistance layer is arranged.

  The temperature of the heater 23 is adjusted by appropriately controlling the current flowing from the electrode (not shown) at the longitudinal end to the current-carrying resistance layer in accordance with the signal from the temperature detecting element 27. Then, the heat of the heater 23 is transmitted from the inner surface of the fixing film 22 to the surface thereof, and heats the surface of the fixing roller 24 as an open-cell sponge roller via the fixing nip Nip. When the paper P to which the unfixed toner image is transferred as described above is conveyed to the fixing nip N, the heat of the fixing film 22 and the open-cell sponge roller 24 is transmitted to the unfixed toner image and the paper P, and the toner is transferred to the paper P. The image is fixed.

(Mechanism of paper wrinkling)
In the fixing device configured to pinch and convey paper as described above, paper wrinkling may occur. Hereinafter, the mechanism by which paper wrinkles occur will be described. Paper wrinkling is frequently seen when the paper transport speed vc at the center in the rotation axis direction (longitudinal direction) of the fixing roller is faster than the paper transport speed vs at the left and right ends. When the paper conveyance speed is vc> vs, the paper behaves roughly as follows.

<Paper Behavior when Paper Conveyance Speed is vc>vs>
・ The paper is wavy near the entrance of the fixing nip.
・ The tip of the waving bites into the fixing nip.
・ Ripples continue to bite into the fixing nip.

  First, the paper behavior (A) will be described with reference to FIG. FIG. 2A is a view of the fixing device shown in FIG. 1 as viewed from the direction of the arrow W. The paper P is conveyed in the direction from A1 to A2 in the drawing, and has a width N indicated by a thick line in the drawing. This shows a state in which waving occurs before the nip portion Nip. FIG. 2B shows the paper conveyance speed distribution in the rotation axis direction (longitudinal direction) of the fixing roller 24 as the open-cell sponge roller at that time.

  When the paper transport speed vc at the central portion is faster than the paper transport speed vs at the left and right ends, the central portion and the left and right edges of the paper assuming that the paper P has been transported horizontally to an arbitrary point Nn within the fixing nip width. Changes from the straight line B1-OB2 connecting the positions of. That is, the rotation about the point O by the angle θn1 occurs, and the positional relationship between the central portion and the left and right end portions of the paper becomes as B1′-O-B2 ′. At this time, in a region before the fixing nip portion Nip, a force is applied to the paper in the direction of drawing toward the center as shown by arrows D1 and D2 in the figure, and the center portion has a waving height h and a waving angle α. You will just be lifted.

  The occurrence of the waving also depends on the stiffness of the paper. When the basis weight of the paper is large and the stiffness of the paper is sufficiently strong, waviness is unlikely to occur, but when the basis weight of the paper is small and the stiffness of the paper is thin, the undulation height h and the undulation angle α are both small. Large undulations are likely to occur.

  In the case of thinner paper, a plurality of undulations may occur in the rotational axis direction (longitudinal direction) of the open-cell sponge roller as shown in FIG. Even if there is a slight bias in the distribution of the pressing force in the fixing nip, such as unevenness in the hardness distribution and outer diameter of the fixing roller and fixing film, and the grain of the paper, thin paper with weak stiffness will be affected and undulation will occur. This is because Such waving has a small height h but a large angle α.

  Next, the paper behaviors (a) to (c) will be described with reference to FIG. FIG. 4 shows a state near the fixing nip portion Nip when the fixing device is viewed from A1 in FIG. FIG. 4A shows a state at the time when the tip of the corrugation of the paper P is caught between the fixing film 22 and the fixing roller 24 pressed by the force of W in the figure (paper behavior (a)). It is.

  FIGS. 4B and 4C show that the paper P is further conveyed from the state of FIG. 4A, and that the undulation of the paper P is between the fixing film 22 and the fixing roller 24 pressed against each other by the pressing force W. The two paper deformation modes that can be considered when the paper is gradually bitten (paper behavior (C)) are shown. FIGS. 4B and 4C show a sliding mode and a buckling mode, respectively.

  First, in the sliding mode of FIG. 4B, the paper has a strong stiffness, and the forces F1 and F2 for keeping the paper flat are strong. This is the case where slippage occurs so that P spreads right and left. At this time, paper wrinkles do not occur.

  On the other hand, the buckling mode (c) is a mode in which buckling Z occurs on the paper P due to the pressing force W. The buckling Z is more likely to occur due to the pressing force W as the stiffness of the paper is weaker and the force for keeping the paper flat is weaker or the generated waving height h or the waving angle α is larger. At this time, the paper P is folded and paper wrinkles occur.

  In summary, paper wrinkles are generated by the following mechanism. First, when the paper conveyance speed at the center in the rotation axis direction of the fixing roller is faster than the paper conveyance speed at the right and left ends, a force acts on the paper in a region before the fixing nip toward the center. I do. At this time, the thinner the paper basis weight and the lower the stiffness, the easier the waving is generated. Further, when the paper is further conveyed and the leading edge of the waving is caught between the fixing roller and the fixing film, which are open-cell sponge rollers, when the paper has a strong stiffness, a slip mode is set and no paper wrinkles are generated. If the paper is weak, the buckling mode is likely to occur and paper wrinkles will occur.

(Paper wrinkle prevention mechanism)
In order to prevent the occurrence of paper wrinkles, the paper transport speed vs at the left and right ends in the rotation axis direction (longitudinal direction) of the fixing roller is higher than the paper transport speed vc at the center, contrary to the above configuration. It is good to FIG. 5 is a diagram showing the state of paper conveyance in FIG. 5A and FIG. 5B showing the paper conveyance speed distribution, as in FIG. 2, and shows a case where the paper conveyance speed distribution is vs> vc. ing.

  When the paper transport speed vs at the left and right end portions is faster than the paper transport speed vc at the central portion, the central portion and the left and right edge portions of the paper assuming that the paper P has been transported horizontally to an arbitrary point Nn within the fixing nip width. Are rotated about the point O by an angle θn2 from a straight line B1-OB2 connecting the positions of. The positional relationship between the center and the left and right ends of the paper is as shown by B1 "-OB2". At this time, in the area before the fixing nip portion, as shown by arrows E1 and E2 in the drawing, a force is applied to the paper in the direction of pulling the paper toward the left and right end portions, so that the wavy which originally causes the paper wrinkles. Is suppressed.

  However, in the case of a thin paper having a small basis weight and a low stiffness, as described above, the undulation having a large undulation height h and a large undulation angle α is likely to occur. Therefore, it is necessary to further increase the force in the direction of being pulled toward the left and right end portions. For this purpose, the paper conveyance speed is set to vs> vc, and the difference may be increased. In other words, by increasing the difference by setting the paper conveyance speed to vs> vc, it is possible to eliminate waving before the fixing nip even with thin paper, and to prevent paper wrinkles.

(Comparative Example 1 is a method in which the fixing film has an inverted crown shape so that vs> vc).
As described above, in order to prevent paper wrinkles on paper having a small basis weight and low stiffness, the paper transport speed must be set to vs> vc, and the difference Δv must be increased. Hereinafter, the paper transport speed difference Δv is determined by using the paper transport speed vs at the left and right ends in the same direction as the paper transport speed vc at the center in the rotation axis direction (longitudinal direction) of the fixing roller 24 as the open-cell sponge roller. It shall be given as follows.
Δv = vs−vc (Equation 1)
Conventionally, as a means for satisfying vs> vc, a method is adopted in which the fixing film 22 has a shape in which the outer diameter at the left and right ends is larger than the outer diameter at the center in the rotation axis direction (longitudinal direction), that is, an inverted crown shape. It had been. FIG. 6 shows a schematic diagram of the configuration of the fixing device 11 of the proportional example 1 as viewed from the direction A1 in FIG.

  The reverse crown-shaped fixing film 22 rotates at a lower surface moving speed vFc at the central portion having a smaller outer peripheral length than at the left and right end portions having a larger outer peripheral length. For this reason, it is considered that the central portion of the fixing film 22 is rotating while applying a brake to the fixing roller 24, which is an open-cell sponge roller, in a shaded area J in FIG.

  When the paper is passed here, the paper transport speed vc at the central portion where the brake is applied becomes slower, so that the paper transport speed vs at the left and right ends becomes faster. In order to increase Δv, the difference between the outer diameter of the central portion and the outer diameter of the left and right end portions in the rotation axis direction (longitudinal direction), that is, the amount of inverted crown may be increased.

  Here, Table 1 shows the results of examining the occurrence of the paper conveyance speed difference Δv and the effect of preventing paper wrinkles due to the inverted crown shape of the fixing film. The paper transport speed difference Δv is obtained as follows. That is, Extra80g paper P cut into a strip having a width of 30 mm is passed through the center of the fixing roller rotating shaft direction and the left and right ends of the image forming apparatus. Then, the transport speed of the paper P at the time when the paper P reached the discharge tray was measured and calculated by a digital laser Doppler speedometer (Canon Inc.).

  The fixing roller used for the measurement is the same as the open-cell sponge roller of the present embodiment, and has an outer diameter of 18 mm, an Asker C hardness of 50 °, and a reverse crown amount in the rotation axis direction of 0 μm (straight shape). The pressing force W was 15 kgf, and the process speed was 160 mm / s. In Table 1, the measured paper transport speed vs. the right and left end portions, the central portion paper transport speed vc, and the paper transport speed difference Δv are shown.

  The effect of preventing paper wrinkles was confirmed by examining the presence or absence of paper wrinkles with respect to the basis weight of paper, since paper wrinkles tend to occur as the weight of the paper is smaller and the stiffness is weaker. For the fixing film, the outer diameter of the central portion was unified to 18 mm, and the cases where the reverse crown amount was 0 μm (straight shape), 25 μm, and 80 μm were examined. During continuous paper passing of 10 sheets, x indicates that paper wrinkles occurred, and o indicates that no wrinkles occurred. Further, the image displacement was also evaluated. Image misalignment is an image defect in which unfixed toner on paper rubs against a film in front of a fixing nip to disturb an image.

  As described above, if the fixing film has an excessively large crown amount, an image shift occurs. X indicates that an image shift has occurred, and × indicates that no image shift has occurred.

  When the fixing film has a straight shape with an inverted crown amount of 0 μm, the difference in paper conveyance speed Δv is almost 0 mm / s, and at this time, paper wrinkles are generated even with paper having a basis weight of 64 g. When the fixing film was formed in an inverted crown shape having an inverted crown amount of 25 μm, the paper conveyance speed difference Δv was 0.3 mm / s, and it was possible to prevent paper wrinkles of paper having a basis weight of 64 g. When the reverse crown amount of the fixing film was set to 80 μm, the difference Δv in the paper transport speed was increased to 1.3 mm / s, and paper wrinkles could be prevented up to a paper weight of 52 g. However, an image shift has occurred.

  As described above, when the inverted crown shape of the fixing film is too large, the running of the fixing film becomes unstable, and the unfixed toner on the paper pressed against the fixing film distorted due to the unstable running and the fixing roller. Is disturbed, and an image shift occurs.

(Comparative Example 2 where the fixing roller has an inverted crown shape so that vs> vc)
Next, a description will be given of a method of setting the paper conveyance speed to a relation of vs> vc by the fixing roller. The optimum means is different between the solid roller and the open-cell sponge roller. First, in the case where a solid roller is used as the fixing roller 24, the shape of the fixing roller 22 is formed into an inverted crown shape, and the paper transport speed is set to vs> vc on substantially the same principle as when the fixing film 22 is formed as an inverted crown. It is possible to increase Δv.

  Next, a case where an open-cell sponge roller is used as the fixing roller 24 will be described. In the case of an open-cell sponge roller, it is difficult to make the paper transport speed vs. vc even if the shape is an inverted crown shape like a fixing film or a solid roller. Hereinafter, the reason will be described while showing the difference between the solid roller and the open-cell sponge roller.

  There is a difference in the deformation behavior between the solid roller and the open-cell sponge roller when a pressing force is applied. FIGS. 7A and 7B show the deformation behavior of each fixing roller when the pressing force W is applied. 7A shows a case where the fixing roller is a solid roller, and FIG. 7B shows a case where the fixing roller is an open-cell sponge roller. The dotted line in the figure indicates the shape of the fixing roller in a state where the pressing force W is not applied.

  In the case of a solid roller, a pressing force W is applied, and when the rubber contracts in the direction of the pressing force W, the rubber expands in the direction of arrow 70 in the drawing. Therefore, the more the paper is crushed, the more the rubber is stretched, the longer the outer peripheral length, and the faster the paper transport speed. On the other hand, in the case of the open-cell sponge roller, when the pressing force W is applied, the air in the bubbles existing in the open-cell sponge rubber layer goes out and the bubbles are crushed, so that the outer peripheral length of the open-cell sponge roller is reduced. Make such a deformation. For this reason, the more the paper is crushed, the smaller the outer peripheral length is, and the lower the paper transport speed is.

  Due to such a difference in the deformation behavior, the solid roller and the open-cell sponge roller differ in the paper conveyance speed difference that can be caused by the inverted crown shape. As the outer diameter of the solid roller increases, the outer peripheral length related to the paper transport speed increases, and the amount of rubber increases, so the elongation and deformation in the direction of arrow 70 in the figure also increases. Become. On the other hand, even if the outer diameter of the open-cell foam sponge roller is large, if the open-cell foam sponge roller is crushed by the pressing force W, the outer peripheral length of the open-cell foam sponge roller is reduced.

  Here, the results of examining the occurrence of the paper conveyance speed difference Δv due to the fixing roller having an inverted crown shape and the effect of preventing paper wrinkles will be described. The experiment was performed in the same manner as when the occurrence of the paper conveyance speed difference Δv due to the inverted crown shape of the fixing film and the effect of preventing paper wrinkles were examined. The reverse crown amount of the fixing film was 0 μm (straight shape). The comparison was made with the reverse crown amounts of the fixing rollers being 0 μm, 150 μm, and 200 μm. The results are shown in Table 2.

  By making the solid roller have an inverted crown shape, the paper transport speed at the left and right ends was increased, and a paper transport speed difference Δv could be generated. In the case of a solid roller, by setting the reverse crown amount to 200 μm, Δv could be made 1.8 mm / s, and wrinkles of a thin paper having a basis weight of 42 g could be prevented.

  However, even if the open-cell sponge roller has an inverted crown shape with an inverted crown amount of 200 μm, the paper transport speed vs. at the left and right ends does not change so much as the solid roller, and the paper transport speed Δv cannot be increased so much. Therefore, the effect of preventing paper wrinkles was small. As described above, when the fixing roller is an open-cell sponge roller, it is difficult to make the paper conveyance speed satisfy the relationship of vs> vc by a method in which the shape of the fixing roller is an inverted crown shape like a solid roller.

(Method of setting vs> vc by providing a nip width difference in the fixing roller in the present embodiment)
As described above, when the fixing roller 24 is an open-cell sponge roller, the outer peripheral length of the open-cell sponge roller can be reduced by crushing bubbles in the elastic rubber layer by applying a pressing force W. Utilizing this property, if the central portion of the open-cell sponge roller is crushed more than the left and right end portions, the paper transport speed can be in a relationship of vs> vc.

  Here, a change in the paper conveyance speed when the open-cell sponge roller was gradually crushed was examined, and the above verification was performed. The experiment was examined using a fixing device as shown in FIG. In order to exclude the influence of the bending of the fixing roller due to the pressing force, a fixing device having a reduced length in the rotation axis direction was used. The length of the rubber layer of the open-cell foam sponge roller in the rotation axis direction and the length of the fixing film in the rotation axis direction were 40 mm. The amount of collapse of the open-cell sponge roller by the pressing force W was determined by measuring the fixing nip width N.

  It can be said that as the fixing nip width N is larger, the open-cell sponge roller is more crushed and has a smaller outer diameter. The fixing nip width N was measured by a pressure distribution measuring system (Nitta Corporation). The configuration of the fixing device used in the experiment is as follows. In FIG. 8, a fixing film 81 includes a pressing body 82 and is pressed against a fixing roller 84 by a spring 83. When the pressing force was adjusted by a spring 83 to be 2.1 to 3.9 kgf, the fixing nip width N was about 6.8 to 7.4 mm.

  The fixing roller 84 was rotated in the direction of R1 in the figure at the same speed of 160 mm / s as the process speed of the image forming apparatus in the present embodiment. Then, Extra 80 g of paper P cut into a strip having a width of 30 mm was passed through a fixing nip portion Nip formed by the fixing roller 84 and the fixing film 81. Then, the transport speed of the paper after passing through the fixing nip was measured by a digital laser Doppler speedometer 80 (Canon Inc.). The results are shown in FIG. As shown in FIG. 9, the open-cell sponge roller was crushed by increasing the pressing force W, and the paper conveyance speed was decreased as the fixing nip width N was increased.

  As described above, when the open-cell sponge roller is used as the fixing roller, in order to prevent the paper wrinkles by setting the paper conveyance speed to the relation of vs> vc, the central portion may be crushed rather than the left and right ends. That is, the fixing nip width Nc at the central portion (first region) may be configured to be larger than the nip width Ns at the end portion (left and right end portions as the second region). That is, the condition of Nc> Ns may be satisfied. Further, in order to increase the paper transport speed difference Δv, the difference ΔN between the fixing nip width Nc at the center and the nip width Ns at the left and right ends may be increased.

(Fixing nip configuration based on heater holder shape of this embodiment)
As described above, in order to generate the paper conveyance speed difference Δv in order to prevent paper wrinkling in the case of using the open-cell sponge roller, the width (nip width) Nc of the fixing nip portion at the center is adjusted to the left and right ends. The width of the fixing nip portion (nip width) Ns may be larger than the fixing nip portion. Hereinafter, the width difference (nip width difference) ΔN of the fixing nip portion is obtained by using the fixing nip width Nc at the center portion in the rotation axis direction (longitudinal direction) of the open-cell sponge roller and the fixing nip width Ns at the left and right ends in the same direction. , Given below.
ΔN = Nc−Ns (Equation 2)
As can be seen from the value of Δv of Experiment No. 6 and the degree of paper wrinkle occurrence in Table 2, in the film-type fixing device of the present embodiment, the paper wrinkle of thin paper having a basis weight of 42 g has a Δv of 1.8 mm / s or more. Can be prevented. From FIG. 9, when the fixing nip width Ns at the left and right ends is 6.8 mm, the fixing nip width Nc at the center is changed, and when ΔN is changed, how much the paper conveyance speed difference Δv occurs is calculated. did. FIG. 10 shows a graph in which Δv is plotted against ΔN.

  In order to prevent the paper wrinkles of the thin paper having the basis weight of 42 g, the fixing nip width difference ΔN may be set to 0.4 mm or more so that the sheet conveyance speed difference Δv is 1.8 mm / s or more. That is, the nip width Nc at the center may be set to 7.2 mm or more.

  In the present embodiment, the nip width is adjusted by adjusting the shape of the heater holder 21 as shown in FIG. That is, the heater holder 21 as the first pressing member has a shape (convex shape) that is closer to the fixing roller 24 at the center (first region) in the longitudinal direction than at the end (second region). As a result, as can be understood from FIG. 9, the fixing roller 24 has a higher pressing force at the longitudinal central portion (first region) than at the end portion (second region), and the longitudinal central portion (second region). In the first region, the nip width is larger than in the end portion (the second region).

  FIG. 11B shows a nip width shape in the present embodiment. By setting H1 which is the protrusion amount of the central convex portion of the heater holder 21 in FIG. 11A to 550 μm, the fixing nip width Ns at the left and right ends is 6.8 mm, whereas the central nip width Ns is 6.8 mm. Nc is set to 7.2 mm larger than Ns.

(Paper wrinkle prevention effect in the present embodiment)
The results of evaluating the occurrence of paper wrinkles in the configuration of the present embodiment are shown. In the same manner as in the evaluation method described above, the presence or absence of paper wrinkles when 10 sheets of each basis weight were continuously passed was evaluated. The paper transport speed difference Δv was measured in the same manner. For comparison, the results of Experiment No. 2 and Experiment No. 3 which are the results when only the paper wrinkle countermeasures by making the fixing film of Table 1 have an inverted crown shape are also shown. Image deviation was also confirmed, and the results are shown in Table 3.

  In the configuration of the present embodiment, by making the fixing nip width at the central portion in the longitudinal direction 0.4 mm larger than the nip width at the left and right end portions, the paper transport speed at the central portion is 1.8 mm / paper higher than the paper transport speed at the left and right end portions. s could be slowed down. For this reason, paper wrinkles of a thin paper having a basis weight of 42 g, which could not be prevented by the conventional method for preventing paper wrinkles by using an inverted crown as a fixing film, could be prevented. Further, since the amount of reverse crown of the fixing film was 0 μm, no image shift occurred.

  As described above, according to the present embodiment, even in the case of using an open-cell sponge roller that enables power saving and quick start in a film-type fixing device, it is possible to prevent paper wrinkles without causing image shift on thinner paper than before. I was able to.

<< 2nd Embodiment >>
In a second embodiment of the present invention, a fixing device as an external heating type image heating device using a fixing roller having a heat storage rubber layer has a fixing nip width in the same direction as the left and right ends in the rotation axis direction of the fixing roller. By making the central portion larger, paper wrinkles are prevented. Hereinafter, the present embodiment will be described. The image forming apparatus for forming an unfixed toner image is the same as in the first embodiment, and the description is omitted. In the fixing device, the same members are denoted by the same reference numerals, and the description is omitted.

(Image heating device)
The fixing device as the image heating device of the present embodiment is a fixing device of an external heat fixing type, and is an example of a fixing device used for achieving quick start and power saving. By arranging the heating element on the outer surface of the fixing roller and heating only the outer surface of the fixing roller, quick start and power saving can be achieved.

  The configuration of the fixing device 200 of this embodiment will be described with reference to FIG. FIG. 12 is a cross-sectional view of the fixing device 200. The fixing device 200 includes a fixing roller 111 (rotating roller), a heating unit 112, and a pressure unit 113. On the outer peripheral surface of the fixing roller 111, a pressure unit 113, which is pressed against the fixing roller 111, forms a fixing nip Nip1.

On the other hand, a heating unit 112 for heating the outer surface of the fixing roller 111 comes into contact from the opposite side of the pressure unit 113 to form a heating nip Nip2. In this embodiment, the total pressing force in the direction of arrow W in the drawing is 15 kgf, and the width of the fixing nip Nip is 6 to 8 mm. The width of the axial direction of the elastic layer of the continuous foam sponge roller is 225 mm, the contact pressure per nip unit area is ^{0.82 × 10 5 Pa~1.1 × 10 5} Pa. The fixing roller 111 is driven by a drive source (not shown), and rotates in the direction of arrow R1 in FIG. 12 at the same process speed as the electrophotographic photosensitive drum at 160 mm / s.

  The heating film 118 and the pressure film 120 receive power from the fixing roller 111 by Nip1 and Nip2, and are driven to rotate in directions R2 and R3 in FIG. 12, respectively. The heat of the heating unit 112 is transmitted to the surface of the fixing roller 111 at the heating nip Nip2, and the heat of the fixing roller 111 is transmitted to the paper P at the fixing nip Nip1. The heater 23 in the heating unit 112 is controlled at a temperature of 180 ° C. by a temperature detecting element 27 arranged on the back of the heater 23.

  The paper P on which the unfixed toner image is recorded is conveyed from A1 to A2 in the drawing at the fixing nip portion Nip1, and the toner P on the paper P is fixed by heat and the pressing force of the fixing roller 111 and the pressing unit. is there. Hereinafter, the detailed configuration will be described.

(Fusing roller)
The fixing roller 111 is an open-cell sponge roller whose elastic layer has a low heat capacity and a low heat conductivity in order to enable quick start, similarly to the fixing roller 24 described in the first embodiment. The fixing roller outer diameter and core diameter, the thermal conductivity and specific gravity of the open-cell sponge rubber layer, the release layer on the outermost surface of the fixing roller, and the like are substantially the same as those of the fixing roller 24. However, in this embodiment, the heat storage rubber layer 115a is provided between the elastic layer 115b and the release layer 24b.

  In the case of the fixing roller used in the fixing device of the external heat fixing type as in the present embodiment, if the heat capacity is small and the thermal conductivity is only low, the surface temperature of the fixing roller tends to increase, but only the outermost layer is warmed. . As a result, the amount of heat may be insufficient to fix the toner on the paper P as the recording material. For this reason, the fixing roller 111 of the present embodiment has a fixing roller configuration including a heat storage rubber layer 115a for storing heat and a heat insulating rubber layer (elastic layer) 115b for preventing heat from leaking into the fixing roller.

  Here, the heat insulating rubber layer (elastic layer) 115b is the same as the open-cell sponge rubber layer shown in the first embodiment, and has a specific gravity of 0.55 and a thermal conductivity of 0.13 W / m · K. It is foam sponge rubber. The heat storage rubber layer 115a preferably has a high heat conductivity in order to quickly store heat from the heating rotator, and has a heat conductivity of 0.3 W / m · K by adding a heat conductive filler to solid rubber. It is preferable to use the one described above. The heat capacity of the heat storage rubber layer 115a is preferably adjusted according to the thermal conductivity (specific gravity) of the rubber and its thickness, and is set to about 30 μm to 1 mm in accordance with the printing speed of the image forming apparatus. In this embodiment, the thickness is 200 μm.

(Heating unit)
The heating unit 112 heats the fixing roller 111 from the surface, and has the same configuration as the film unit 500 of the first embodiment. It comprises a heater 23 as a heating element and an endless belt-shaped heating film 118, and a heater holder 21 which guides the heating film 118 and is a holding member for the heater 23.

  The heating film 118 according to the present embodiment needs heat resistance and strength similarly to the fixing film 22 according to the first embodiment. Therefore, a metal such as stainless steel (SUS) or nickel, or a high heat-resistant resin such as polyimide, polyamide imide, or polyethyl ethyl ketone (PEEK) may be used. In the present embodiment, a polyimide film having an outer diameter of 18 mm is used.

  Further, on the surface layer of the heating film 118, similarly to the fixing film 22 of the first embodiment, there is a toner release layer 118a. The release layer 118a of the heating film 118 is preferably made of a fluororesin such as perfluoroalkoxy resin (PFA), polytetrafluoroethylene resin (PTFA), and tetrafluoroethylene hexafluoropolypropylene resin (FEP). In the present embodiment, among these fluororesins, PFA having excellent release properties and heat resistance is used.

(Pressure unit)
The pressure unit 113 nips and conveys the paper P, which is a recording medium, while pressing the paper P with the fixing roller 111. The pressing unit 113 includes an endless belt-shaped pressing film 120, a pressing film guide 121 which is a rotation holding member of the pressing film 120, a stay 25 supporting the pressing film guide 121, and a sliding member 122. It is configured. The pressure film needs to have heat resistance, strength, and releasability from the toner similarly to the fixing film 22 of the first embodiment. Therefore, the same film as the fixing film 22 of the first embodiment was used as the pressure film 120 of the present embodiment.

  Further, the sliding member 122 of the present embodiment is a member that slides on the pressure film to form the nip portion Nip1, and requires heat resistance and durability, and has good slidability. It is good. As the material of the sliding member 122, for example, a heat-resistant resin sheet such as heat-resistant felt, mica, ceramic, liquid crystal polymer, phenol resin, PPS, PEEK, polyimide, polyamide imide, or fluororesin is used. Note that a metal having high thermal conductivity for making the heat distribution in the rotation axis direction of the fixing roller 111 uniform may be used. Further, a sliding layer such as a glass coat or a fluorine resin for reducing frictional resistance may be provided on the surface.

  In the present embodiment, a 1 mm thick aluminum plate coated with PTFE was used. Further, here, the sliding member 122 and the pressure film guide 121 are treated as separate members, but they may be formed by integral molding with the same resin. It is possible.

(Conventional paper wrinkle prevention method and paper wrinkle prevention effect in external heat fixing type fixing device)
There is a problem that paper wrinkles occur in the fixing device of the external heating fixing system using the open-cell sponge rubber for the elastic layer of the fixing roller as described above. In order to prevent paper wrinkles, it is effective to make the paper transport speed vs at the left and right ends in the rotation axis direction of the fixing roller faster than the paper transport speed vc at the center, as in the first embodiment. Also in the present embodiment, a method of satisfying vs> vc with a pressure film and a method of satisfying vs> vc with a fixing roller are exemplified.

  Although the fixing roller of this embodiment has a heat storage rubber layer between the elastic layer and the release layer, the elastic layer is an open-cell sponge rubber layer as in the first embodiment, and the shape of the fixing roller is an inverted crown shape. It is difficult to make the paper transport speed satisfy the relationship of vs> vc by the method described in the above. Therefore, conventionally, the paper wrinkle prevention in the fixing device of the external heat fixing method has been performed by making the shape of the pressure film 120 into an inverted crown shape.

  The principle is the same as the paper wrinkle prevention method in which the shape of the fixing film shown in the first embodiment is inverted crown shape. Further, if the amount of the reverse crown of the pressure film 120 is too large, the running of the pressure film 120 becomes unstable, and the problem of image displacement also occurs.

  Table 4 shows, as a conventional example of the external heat fixing method, the results of examining the occurrence of a paper transport speed difference Δv and the effect of preventing paper wrinkles when the pressure film is formed into an inverted crown shape. The measurement was performed in the same manner as in the first embodiment when the occurrence of the paper transport speed difference Δv and the effect of preventing paper wrinkles were investigated by the conventional method.

  The measured paper transport speed vs. the right and left end portions, the central portion paper transport speed vc, and the paper transport speed difference Δv are shown. Regarding the effect of preventing paper wrinkles, during continuous paper passing of 10 sheets, x indicates that paper wrinkles occurred, and o indicates that no wrinkles occurred. The reverse crown amount of the pressurized film was checked for 0 μm, 25 μm, and 80 μm, and the image shift was also evaluated. The case where image displacement occurred was indicated by x, and the case where image displacement did not occur was indicated by ○.

  When the pressurized film has a straight shape with a reverse crown amount of 0 μm, the difference in paper conveyance speed Δv is almost 0 mm / s, and at this time, paper wrinkles are generated even with a paper having a basis weight of 64 g. When the pressure film was formed into an inverted crown shape having an inverted crown amount of 25 μm, the paper transport speed difference Δv was 0.5 mm / s, and it was possible to prevent paper wrinkles of paper having a basis weight of 64 g.

  When the reverse crown amount of the pressure film was 80 μm, the difference Δv in the paper transport speed was increased to 1.4 mm / s, and the generation of paper wrinkles up to a paper weight of 52 g could be prevented. However, an image shift has occurred.

  As described above, in the fixing device of the external heat fixing method, the pressure film has a shape in which the outer diameters at the left and right end portions are larger than the outer diameter at the center portion in the rotation axis direction, that is, the inverted crown shape, vc. In addition, it was found that the difference Δv between the outer diameter at the center in the rotation axis direction and the outer diameter at the left and right ends, that is, the amount of inverted crown should be increased in order to increase Δv.

  However, if the reverse crown shape of the pressure film is too large, the running of the pressure film becomes unstable, and the unfixed paper on the paper pressed against the distorted pressure film and the fixing roller due to the unstable running. The toner is disturbed and an image shift occurs.

(Fixing nip configuration based on pressurized film guide shape of this embodiment)
As described above, if the conventional method of forming the pressurized film 120 into an inverted crown shape is used to prevent the generation of wrinkles on thin paper, an image shift occurs. However, as in the first embodiment, if the fixing nip width Nc at the central portion in the longitudinal direction is made larger than the fixing nip width Ns at the left and right end portions, image displacement does not occur, and thin paper wrinkles occur. Can be prevented.

  Here, the fixing roller of this embodiment has a heat storage rubber layer between the elastic layer and the release layer, but is sufficiently thinner than the elastic layer (open-cell sponge layer). Therefore, as in the first embodiment, as the pressing force is increased and crushed, bubbles in the open-cell sponge rubber layer are released and the outer peripheral length is reduced. That is, even when a fixing roller having a heat storage rubber layer as in the present embodiment is used, as in the first embodiment, in order to generate the paper conveyance speed difference Δv in order to prevent paper wrinkles, it is necessary to use the central portion. Is larger than the fixing nip width Ns at the left and right ends.

  In the present embodiment, the fixing nip width N is adjusted according to the shape of the pressure film guide 121. That is, the pressurized film guide 121 as the second pressurizing member has a shape (convex shape) that is closer to the fixing roller 111 at the central portion (first region) in the longitudinal direction than the end portion (second region). Have. As a result, as can be understood from FIG. 9, the fixing roller 111 has a higher pressing force at the central portion (first region) in the longitudinal direction than at the end portion (second region), and the central portion (second region) in the longitudinal direction. In the first region, the nip width is larger than in the end portion (the second region).

  FIG. 13A is a fixing device viewed from A1 in FIG. 12, and the shape of the pressure film guide 121 is indicated by a thick line. FIG. 13B shows the fixing nip shape at this time. In the present embodiment, the nip width at the left and right ends is 6.8 mm by setting H1 which is the protrusion amount of the central convex portion of the pressurized film guide 121 in FIG. The fixing nip width at the center is 7.2 mm.

(Paper wrinkle prevention effect in the present embodiment)
The results of evaluating the occurrence of paper wrinkles in the configuration of the present embodiment are shown. In the same manner as in the evaluation method described above, the presence or absence of paper wrinkles when 10 sheets of each basis weight were continuously passed was evaluated. The paper transport speed difference Δv was measured in the same manner. For comparison, the results of Experiment Nos. 12 and 13 which are the results obtained when only the countermeasures against paper wrinkles by making the pressurized film in Table 4 have an inverted crown shape are also shown. The image shift was also confirmed, and the results are shown in Table 5.

  In the configuration of this embodiment, the central portion of the fixing nip width in the longitudinal direction is set to be 0.4 mm larger than the fixing nip width of the left and right end portions, so that the central portion paper transport speed is 1. It could be slowed down by 8 mm / s. For this reason, paper wrinkles of a thin paper having a basis weight of 42 g, which could not be prevented by the conventional method for preventing paper wrinkles by using an inverted crown as a pressure film, could be prevented. Further, since the reverse crown amount of the pressure film 120 was 0 μm, no image shift occurred.

  As described above, according to the present embodiment, even when the elastic layer of the fixing roller is an open-cell sponge layer in a fixing device of an external heat-fixing method, even if the paper is thinner than the conventional one, it is possible to prevent paper wrinkles without causing image displacement. Was completed. Further, even when the elastic layer of the fixing roller is an open-cell sponge roller and partially includes a solid layer, if the nip width is larger at the center in the same direction than the left and right ends in the rotation direction, the paper is similarly formed. It was found that wrinkles can be suppressed.

<< 3rd Embodiment >>
In the third embodiment of the present invention, there is no fixing film according to the first embodiment or the pressure film according to the second embodiment, and paper wrinkling cannot be prevented by a method of making the film into an inverted crown shape. This is an external heating type heating device. Instead of a film, a non-fixed toner image on the paper P is fixed on the paper P by a fixing nip formed by pressing a fixing pad, which is a fixed pressing body, with a fixing roller. Therefore, it is called a pad system. As described above, even in the configuration of the present embodiment having no film, paper wrinkling can be prevented by using a configuration in which the fixing nip width is larger at the central portion than at the left and right ends in the longitudinal direction of the fixing roller.

  This embodiment will be described below. In the present embodiment, an image forming apparatus that forms an unfixed toner image is the same as in the first and second embodiments, and a description thereof is omitted.

(Image heating device)
FIG. 14 is a sectional view of a fixing device 300 as an image heating device according to the present embodiment. The fixing device 300 of the present embodiment is an example of a fixing device of an external heat fixing type similar to the second embodiment, and is used for achieving quick start and power saving. The fixing device according to the second embodiment has a pad fixing system in which the pressing film 120 and the pressing film guide 121 of the pressing unit 113 are eliminated. This pad fixing type fixing device does not have a pressurized film and has a small heat capacity, so that a quicker quick start is possible.

  A cross-sectional view of the fixing device 300 of the present embodiment is as shown in FIG. As described above, the configuration is almost the same as that of the second embodiment except that the pressure film and the pressure film guide are not provided, and the same members are denoted by the same reference numerals and description thereof is omitted.

  Similar to the fixing roller described in the second embodiment, the fixing roller 142 uses an open-cell sponge rubber having a low heat capacity and a low heat conductivity for the elastic layer 115b in order to enable a quick quick start. . The heat storage rubber layer 115a having a high thermal conductivity is provided on the outside. However, in the present embodiment, the paper P and the pressure pad 141 slide while the paper P is passing. Unless the frictional force between the fixing roller 142 and the paper P is set to be stronger than the frictional force between the paper P and the pressure pad 141, the paper is not conveyed.

  Therefore, the fixing roller 142 of the present embodiment does not have a release layer on its surface. Other configurations such as the outer diameter of the fixing roller, the core metal diameter, the thermal conductivity of the open-cell sponge rubber layer, and the specific gravity are the same.

The pressing pad 141 is pressed against the rotating fixing roller 142 and forms a fixing nip Nip1 while rubbing against the fixing roller 142. In the figure, the total pressure contact force in the direction of arrow W is 15 kgf as in the first embodiment, and the width of the fixing nip Nip is 6 to 8 mm. The width of the axial direction of the elastic layer of the continuous foam sponge roller is 225 mm, the contact pressure per nip unit area is ^{0.82 × 10 5 Pa~1.1 × 10 5} Pa.

  The heat is supplied from the heater 117 and transmitted to the surface of the fixing roller 142 via the heating film 118 in the heating nip Nip2, as in the second embodiment. The unfixed toner image on the paper P is melted by the heat received from the heated fixing roller 142 and the pressure in the fixing nip Nip1, and is fixed on the paper P. The material of the pressure pad 141 needs to have heat resistance and durability, and is preferably made of the same material as the sliding member 122 of the second embodiment.

  In particular, a member having a heat insulating property is suitable so as not to deprive the fixing roller 142 of heat. Further, the surface 141a may have a sliding layer such as a glass coat or a fluorine resin for reducing frictional resistance. As the pressure pad 141 of the present embodiment, a liquid crystal polymer having high heat insulation properties coated with 10 μm of PTFE having high slidability was used.

(Paper wrinkling in pad fixing type fixing device)
As described above, in the fixing device of the pad fixing system using the open-cell sponge rubber for the elastic layer of the fixing roller, there is a problem that paper wrinkles occur. Since there is no pressure film as in the second embodiment, paper wrinkling cannot be prevented by a method in which the pressure film has an inverted crown shape.

(Fixing nip configuration based on pressure pad shape of this embodiment)
The fixing roller of this embodiment has substantially the same configuration as that of the second embodiment except that it does not have a release layer, and the elastic layer is an open-cell sponge rubber layer. Therefore, in this embodiment, as in the first and second embodiments, in order to generate the paper conveyance speed difference Δv in order to prevent paper wrinkles, the fixing nip width Nc at the central portion in the longitudinal direction is set to the left and right ends. The fixing nip width Ns may be larger than the fixing nip width Ns.

  In the present embodiment, the fixing nip width N1 is adjusted according to the shape of the pressure pad 141. That is, the pressure pad 141 has a shape (convex shape) that is closer to the fixing roller 142 at the central portion (first region) in the longitudinal direction than at the end portion (second region). As a result, as can be understood from FIG. 9, the fixing roller 142 has a higher pressing force at the longitudinal central portion (first region) than at the end portion (second region), and the longitudinal central portion (second region). In the first region, the nip width is larger than in the end portion (the second region).

  FIG. 15 shows the configuration of the present embodiment, and FIG. 15A shows the fixing device viewed from A1 in FIG. 14, and the shape of the pressure pad 141 is indicated by a thick line. FIG. 15B shows the fixing nip shape at this time. In the present embodiment, the fixing nip width at the left and right ends is 6.8 mm, while H1 which is the protrusion amount of the convex portion at the center of the pressure pad 141 in FIG. Has a fixing nip width of 7.2 mm.

(Paper wrinkle prevention effect in the present embodiment)
Table 6 shows the results of evaluating whether paper wrinkles occurred in the configuration of the present embodiment. In the same manner as in the evaluation method described above, the presence or absence of paper wrinkles when 10 sheets of each basis weight were continuously passed was evaluated. The paper transport speed difference Δv was measured in the same manner. For comparison, the results before the measures are also shown.

  Also in this embodiment, by increasing the fixing nip width by 0.4 mm at the center from the left and right ends in the longitudinal direction, the paper conveyance speed becomes 1.9 mm / s faster at the left and right ends than at the center. Wrinkling of 42 g of thin paper was prevented. That is, even in a pad-type fixing device without a pressurized film as in the present embodiment, even if a fixing roller in which the elastic layer of the fixing roller is open-cell sponge rubber is used, the fixing nip is adjusted in accordance with a necessary paper conveyance speed difference. By making the width wider at the left and right ends than at the center, paper wrinkles could be prevented.

(Modification)
In the above-described embodiment, a preferred embodiment of the present invention has been described, but the present invention is not limited to this, and various modifications can be made within the scope of the present invention.

(Modification 1)
Both the fixing film 22 according to the first embodiment and the pressure film 120 according to the second embodiment have been described in the straight shape in which the running is stable and the image is not shifted. However, the fixing film 22 and the pressure film 120 may have an inverted crown shape as long as no image shift occurs.

  That is, the effect of increasing the fixing nip width difference ΔN and the effect of increasing the reverse crown amount of the fixing film or the pressure film, which is a conventional paper wrinkle prevention method, are combined to reduce the fixing nip width difference ΔN. It may be smaller. The fixing nip width relates to the toner fixing property, and the uniformity of the toner fixing property can be improved by reducing the fixing nip width difference ΔN and making the fixing nip structure close to a straight shape.

  Further, in the third embodiment, the fixing roller 142 has a straight shape with an inverted crown amount of 0 μm, but may have an inverted crown shape. Even if the fixing roller 142 has an inverted crown shape, the paper conveyance speed at the left and right ends does not change much. This is because when the outer peripheral length of the open-cell sponge roller is reduced by the pressing force, the effect of increasing the paper conveying speed due to the increase in the outer diameter of the left and right end portions is reduced.

  However, even in this state, the paper transport speed at the central portion can be made slower than the paper transport speed at the left and right ends by arranging the central portion to be crushed from the left and right ends. That is, even if the fixing roller 142 has an inverted crown shape, if the fixing nip width is adjusted so as to satisfy Nc> Ns, the effect of preventing paper wrinkles can be similarly obtained.

  As for the elastic layer of the fixing roller, in the second and third embodiments, the configuration in which the heat storage rubber layer (solid layer) is provided outside the open-cell sponge rubber layer has been described. The position of the solid layer may be any position as long as it is a thin solid layer. For example, as shown in FIG. 16, a structure in which an adhesive solid layer 160 as an adhesive rubber layer may be provided inside the open cell sponge rubber layer 24a in order to improve the adhesion between the core metal 26 and the elastic layer 24a. Even if a solid layer is sufficiently thinner than the elastic layer of the fixing roller, if the nip width is larger at the center in the same direction than at the left and right ends in the rotation direction, paper wrinkling can be suppressed, and the same operation and effect as described above can be achieved. Can be obtained.

(Modification 2)
In the above-described heating method, a ceramic heater is used as a heating element, and a film is heated by contacting the heater. However, the present invention is not limited to this. Other heating methods such as a non-contact heating method using a heating body separated from a film such as a halogen heater 171 as shown in FIG. 17 may be used.

(Modification 3)
In the above-described embodiment, the fixing nip width is set to Nc> Ns by adjusting the shape (height) of the heater holder 21, the pressure film guide 121, and the pressure pad 141 as shown in FIGS. 11, 13, and 15. However, the present invention is not limited to this. For example, if the shape of the stay 25 is adjusted and the fixing nip width is set to Nc> Ns, the effect of preventing paper wrinkles can be obtained similarly.

  Further, the paper transport speed Δv required for the paper for which paper wrinkles are to be prevented is determined, and the paper transport speed with respect to the fixing nip width in the configuration is measured. From the relationship between ΔN and Δv, the fixing nip widths at the left and right ends and the central portion are determined. Is adjusted, the same effect of preventing paper wrinkles as described above can be obtained.

(Modification 4)
In the above-described embodiment, the case where the open-cell foam sponge rubber in which bubbles are adjacent to each other is used as the fixing roller has been described. .

(Modification 5)
In the above-described embodiment, a fixing device that fixes an unfixed toner image to a sheet has been described as an example. However, the present invention is not limited to this, and in order to improve the gloss of an image, a toner image assumed to be The present invention is similarly applicable to a device for heating and pressurizing (also called an image heating device in this case).
is there.

(Modification 6)
In the embodiment described above, the recording paper is described as the recording material, but the recording material in the present invention is not limited to paper. Generally, a recording material is a sheet-like member on which a toner image is formed by an image forming apparatus, and includes, for example, regular or irregular regular paper, thick paper, thin paper, an envelope, a postcard, a seal, a resin sheet, an OHP sheet, Includes glossy paper and the like. In the above-described embodiment, for the sake of convenience, the handling of the recording material (sheet) P has been described using terms such as paper passing, paper discharging, paper feeding, a paper passing unit, and a non-paper passing unit. Is not limited to paper.

22 fixing film, 24 fixing roller, 111 fixing roller, 120 pressure film, 141 pressure pad, 142 fixing roller, Nip, Nip1, fixing nip portion (nip portion)

Claims (6)

A core metal serving as a rotating shaft, provided on the outer periphery of the core metal, a roller having a elastic layer containing therein the bubbles, the with continuous with each other,
A cylindrical film whose outer surface is in contact with the outer peripheral surface of the roller,
A pressure member that is in contact with the inner surface of the film and forms a nip portion with the roller through the film ,
An image heating apparatus for heating an image on a recording material while nipping and conveying the recording material at the nip portion,
Since the pressure for forming the nip portion is applied between the roller and the pressing member, the elastic layer in the central region in the axial direction of the rotation shaft of the roller is the elastic layer in the end region. The outer peripheral length of the roller in the central region is shorter than the outer peripheral length of the roller in the end region, and
When the nip of the serial Rokuzai the width of the conveying direction Nc corresponding to the central area, the front Symbol width of the nip portion corresponding to the end regions and Ns,
Nc> Ns
Image heating apparatus characterized in that it meets the condition that. It said pressure member before a heater in contact with the inner surface of the notated Irumu, image heating apparatus according to claim 1, characterized in that it comprises a and a heater holder for holding said heater. The image heating apparatus according to claim 2 , wherein the heater holder has a shape closer to the roller in the central area than in the end area. Before notated Irumu image heating apparatus according to any one of claims 1 to 3, characterized in that an inverse crown shape.   The image heating apparatus according to any one of claims 1 to 4, wherein the specific gravity of the elastic layer is 0.40 to 0.72.   The image heating apparatus according to claim 5, wherein the hardness of the elastic layer is 30 ° to 60 ° in Asker C hardness (500 g load).