Patent Publication Number: US-2021181663-A1

Title: Device including rotator and belt, such as a fixing device for an image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/729,632, filed Dec. 30, 2019, which claims priority from Japanese Patent Application No. 2019-062927 filed on Mar. 28, 2019, and Japanese Patent Application No. 2019-135777 filed Jul. 24, 2019. The contents the aforementioned applications is incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Aspects of the disclosure relate to a fixing device including a rotator and a belt, and an image forming apparatus including the fixing device. 
     BACKGROUND 
     A known fixing device includes a belt, a heat roller and a rubber pad that sandwich therebetween the belt, and an upstream guide surface located upstream of the rubber pad in a sheet conveying direction to guide the belt. In a direction where the rubber pad is pressed by the heat roller, a downstream end of the upstream guide surface is spaced from the rotation center of the heat roller further than an upstream end of the rubber pad. 
     SUMMARY 
     According to one or more aspects of the disclosure, a device includes a rotator, a belt, a nip forming member, an urging member, an upstream guide, and a downstream guide. The rotator has a rotation axis. The nip forming member is surrounded by the belt and configured to, with the rotator, pinch the belt to form a nip. The urging member is configured to urge one of the rotator and the nip forming member towards the other in a particular direction perpendicular to the rotation axis. The upstream guide includes an upstream guide surface configured to guide an inner peripheral surface of the belt. The upstream guide surface is positioned entirely upstream of the nip in a moving direction of the belt perpendicular to the particular direction and the rotation axis. The upstream guide does not form the nip. The downstream guide includes a downstream guide surface configured to guide the inner peripheral surface of the belt. The downstream guide surface is positioned entirely downstream of the nip in the moving direction. The downstream guide does not form the nip. The nip forming member includes a facing surface which faces the rotator. An upstream edge of the facing surface in the moving direction is located at a position farther from the rotation axis, in the particular direction, than a downstream edge of the upstream guide surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of a laser printer according to an illustrative embodiment of the disclosure. 
         FIG. 2  is a cross sectional view of a fixing device of the image forming apparatus. 
         FIG. 3  is an enlarged cross sectional view of a pressure unit of the fixing device at a pressed position. 
         FIG. 4  is an enlarged cross sectional view of the pressure unit at a nip release position. 
         FIG. 5  illustrates that a stay and a support surface of a holder are convex. 
         FIG. 6A  is a cross sectional view of an upstream guide according to an alternative embodiment of the disclosure. 
         FIG. 6B  is an enlarged cross sectional view of the upstream guide. 
         FIG. 7  is a perspective view of the upstream guide illustrated in  FIGS. 6A and 6B . 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative embodiment will be described with reference to the accompany drawings. 
     As illustrated in  FIG. 1 , an image forming apparatus  1  (e.g., a laser printer) includes a casing  2 , a sheet supply unit  3 , an exposure device  4 , an image forming unit  5 , and a fixing device  8 . 
     The sheet supply unit  3  is disposed in a lower portion of the casing  2 . The sheet supply unit  3  includes a sheet tray  31  for accommodating sheets S (e.g., sheets of paper), and a sheet supply mechanism  32 . The sheet supply mechanism  32  supplies a sheet S from the sheet tray  31  toward the image forming unit  5 . 
     The exposure device  4  is disposed in an upper portion of the casing  2 . The exposure device  4  includes a laser emitter, a polygon mirror, lenses, and reflecting mirrors. The exposure device  4  is configured to expose a surface of a photosensitive drum  61  by scanning thereon at high speed a laser beam (indicated by a dot-and-dash line) emitted from the laser emitter based on image data. 
     The image forming unit  5  is disposed below the exposure device  4 . The image forming unit  5  is constituted as a process cartridge. The image forming unit  5  is removable from the casing  2  through an opening formed when a front cover  21  disposed at a front of the casing  2  is open. The image forming unit  5  includes a photosensitive drum  61 , a charger  62 , a transfer roller  63 , a developing roller  64 , a supply roller  65 , and a developer chamber  66  configured to store therein developer, for example, dry toner. 
     In the image forming unit  5 , the charger  62  uniformly charges the surface of the photosensitive drum  61 . Thereafter, the exposure device  4  exposes the surface of the photosensitive drum  61  to a laser beam, and the surface of the photosensitive drum  61  carries an electrostatic latent image corresponding to image data. The supply roller  65  supplies developer in the developer chamber  66  to the developing roller  64 . 
     The developing roller  64  supplies developer to the electrostatic latent image formed on the surface of the photosensitive drum  61 . The electrostatic latent image on the surface of the photosensitive drum  61  is thus visually developed as a developer image. Thereafter, when a sheet S supplied from the sheet supply unit  3  passes through between the photosensitive drum  61  and the transfer roller  63 , the developer image is transferred from the photosensitive drum  61  onto the sheet S. 
     The fixing device  8  is disposed at the rear of the image forming unit  5 . An overall structure of the fixing device  8  will be described in detail later. The fixing device  8  thermally fixes the developer image transferred onto a sheet S passing through the fixing device  8 . The image forming apparatus  1  uses conveying rollers  23  and discharge rollers  24  to discharge the sheet S having the developer image fixed thereto onto a discharge tray  22 . 
     As illustrated in  FIG. 2 , the fixing device  8  includes a heating unit  81 , a pressure unit  82 , and an urging member SP. The pressure unit  82  is urged toward the heating unit  81  by the urging member SP. In the following description, a direction in which the urging member SP urges the pressure unit  82  toward the heating unit  81  is referred to as “a particular direction”. The particular direction is orthogonal to a width direction and a moving direction which will be described later. The heating unit  81  and the pressure unit  82  face to each other in the particular direction. 
     In this embodiment, the urging member SP is simply illustrated as, but is not limited to, a helical compression spring. The urging member may be a helical tension spring that pulls an end of an arm rotatably supported by a frame of the fixing device  8 . In this case, the helical tension spring may urge the pressure unit  82  toward the heating unit  81  via the arm. 
     The heating unit  81  includes a heater  110  and a rotator  120 . The pressure unit  82  includes a belt  130 , a nip forming member including an upstream pad P 1  and a downstream pad P 2 , a holder  140 , a stay  150 , an upstream guide  160 , a downstream guide  170 , and a sliding sheet  180 . In the following description, a width direction of the belt  130  is referred to as just “a width direction”. The width direction extends in an axial direction of the rotator  120 . The width direction is orthogonal to the particular direction. 
     In this embodiment, the holder  140 , the upstream guide  160 , and the downstream guide  170  are assembled to the stay  150 . Instead of using the stay  150 , side guides (not illustrated) may support both end portions, in the width direction, of the holder  140 , the upstream guide  160 , and the downstream guide  170 . 
     The heater  110  is a halogen lamp and, when turned on, produces light for radiant heat to heat the rotator  120 . The heater  110  is disposed within an interior space of the rotator  120  along a rotation axis of the rotator  120 . 
     The rotator  120  is a cylindrical roller extending in the width direction to receive heat from the heater  110 . The rotator  120  includes a metal-made tube  121  and an elastic layer  122  covering an outer peripheral surface of the tube  121 . The elastic layer  122  is made of rubber such as silicone rubber. The rotator  120  has an outside diameter greater at its both ends in the width direction than its central portion. In other words, the rotator  120  has a concave shape with its outside diameter gradually greater from its central portion toward its both ends. The rotator may have a different shape. For example, the rotator may be cylindrical with a uniform outside diameter in the width direction. Alternatively, the rotator may have a crown shape having its outside diameter smaller from its central portion toward its both ends in the width direction. 
     The rotator  120  is rotatably supported by the frame of the fixing device  8 . The rotator  120  receives a driving force from a motor disposed in the casing  2  to rotate counterclockwise in  FIG. 2 . 
     The belt  130  is a flexible, long tubular member. The belt  130  has a base made of, for example, metal and resin, and a releasable layer covering an outer peripheral surface of the base. The belt  130  is in frictional contact with the rotator  120  or a sheet S and rotates clockwise in  FIG. 2  with the rotation of the rotator  120 . A lubricant, such as grease, is applied to an inner peripheral surface  131  of the belt  130 . The upstream pad P 1 , the downstream pad P 2 , the holder  140 , the stay  150 , the upstream guide  160 , the downstream guide  170 , and the sliding sheet  180  are disposed within an interior space of the belt  130 . 
     The nip forming member (i.e. the upstream P 1  and the downstream pad P 2 ) is surrounded by the belt  130  and together with the rotator  120 , pinch the belt  130  to form a nip NP. In the illustrated examples, the nip NP has an upstream nip NP 1  and a downstream nip NP 2 . More specifically, the upstream pad P 1  is box-shaped and long in the width direction. The upstream pad P 1  is made of rubber, such as silicone rubber. The upstream pad P 1  and the rotator  120  pinch the belt  130  therebetween, forming the upstream nip NP 1 . 
     In the following description, a moving direction of the belt  130  at the upstream nip NP 1  and the nip NP is referred to as just “a moving direction”. The moving direction is a direction where the belt  130  moves along an outer peripheral surface of the rotator  120 . This direction is, however, along a direction substantially orthogonal to the particular direction and the width direction, and thus illustrated as being orthogonal to the particular direction and the width direction. The moving direction is substantially the same as a direction directed from an entrance to the nip NP toward an exit therefrom. 
     The downstream pad P 2  is box-shaped and long in the width direction. The downstream pad P 2  is made of rubber, such as silicone rubber. The downstream pad P 2  and the rotator  120  pinch the belt  130  therebetween, forming a downstream nip NP 2 . 
     The downstream pad P 2  is disposed downstream from the upstream pad P 1  in the moving direction. The downstream pad P 2  is spaced from the upstream pad P 1  in the moving direction. 
     This structure provides, between the upstream nip NP 1  and the downstream nip NP 2 , a middle nip NP 3  where no pressure from the pressure unit  82  directly acts. At the middle nip NP 3 , the belt  130  still contacts the rotator  120  but hardly receives pressure because there is nothing to pinch the belt  130  with the rotator  120 . Thus, the sheet S is heated by the rotator  120  under almost no pressure while passing the middle nip NP 3 . In this embodiment, the nip NP refers to a range from the upstream end of the upstream nip NP 1  to the downstream end of the downstream nip NP 2 , that is, the entire range where the outer peripheral surface of the belt  130  and the rotator  120  contact each other. In other words, the nip NP includes a portion not subjected to pressure from the upstream pad P 1  and the downstream pad P 2 . 
     The upstream pad P 1  has a higher hardness than the elastic layer  122  of the rotator  120 . The downstream pad P 2  has a higher hardness than the upstream pad P 1 . 
     The above hardness refers to a durometer hardness specified in ISO7619-1. The durometer hardness is a value that may be obtained from an amount of the penetration of a pin into a specimen under specified conditions. For example, when the durometer hardness of the elastic layer  122  is 5, that of the upstream pad P 1  is preferably 6 to 10, and that of the downstream pad P 2  is preferably 70 to 90. 
     The hardness of silicone rubber may be adjusted by changing the ratio of an additive (e.g., a silica filler and a carbon filler) to be added at the time of manufacture. Specifically, the hardness of silicone rubber increases with a higher ratio of an additive. The hardness decreases with the addition of silicone-based oil. As a rubber processing method, injection molding and extrusion may be adopted. Generally, injection molding is suitable for low hardness rubber and extrusion is suitable for high hardness rubber. 
     The upstream guide  160  has an upstream guide surface Fg to guide the inner peripheral surface  131  of the belt  130  at a position upstream from the nip NP in a rotation direction of the belt  130 , that is, in the moving direction at the nip NP. More specifically, the upstream guide surface Fg guides the inner peripheral surface  131  of the belt  130  via the sliding sheet  180 . The upstream guide  160  is spaced from the upstream pad P 1  in the moving direction, and as such, the upstream guide  160  is entirely upstream of the upstream pad P 1  and does not form part of the nip NP. The upstream guide  160  is made of a heat-resistant resin. 
     The downstream guide  170  has a downstream guide surface Fd to guide the inner peripheral surface  131  of the belt  130  at a position downstream from the nip NP in the rotation direction of the belt  130 , that is, in the moving direction. More specifically, the downstream guide surface Fd guides the inner peripheral surface  131  of the belt  130  via the sliding sheet  180 . The downstream guide  170  is spaced from the downstream pad P 2  in the moving direction, and as such, the downstream guide  170  is entirely downstream of the downstream pad P 2  and does not form part of the nip NP. The downstream guide  170  is spaced in the particular direction from a rotation center X 1  of the rotator  120  further than the downstream pad P 2 . More specifically, an upstream end Ed of the downstream guide surface Fd in the moving direction is located at a position farther from the rotation center X 1  of the rotator  120 , in the particular direction, than a facing surface Fp 2  of the downstream pad P 2 . The downstream guide  170  is made of a heat-resistant resin. 
     The sliding sheet  180  is rectangular and reduces frictional resistance between each pad P 1 , P 2  and the belt  130 . The sliding sheet  180  is pinched at the nip between the inner peripheral surface  131  of the belt  130  and each pad P 1 , P 2 . The sliding sheet  180  has one end fixed to an inner wall surface of the upstream guide  160 . The inner wall surface of the upstream guide  160  is opposite to the guide surface Fg and spaced from the inner peripheral surface  131  of the belt  130  further than the guide surface Fg. The sliding sheet  180  is located covering the guide surface Fg of the upstream guide  160 , with its other end located between the downstream guide  170  and the inner peripheral surface  131  of the belt  130 . 
     The embodiment shows but is not limited to that the other end of the sliding sheet  180  is a free end. The other end of the sliding sheet  180  may be fixed to the downstream guide  170 . The sliding sheet  180  may be made of any material. In this embodiment, a polyimide-containing resin sheet is used. 
     The holder  140  holds the upstream pad P 1  and the downstream pad P 2 . The holder  140  is made of a heat-resistant resin. The holder  140  is long in the width direction. The holder  140  includes a support wall  141 , an upstream wall  142 , a middle wall  143 , and a downstream wall  144 . 
     The support wall  141  has an upstream support surface F 1  for supporting the upstream pad P 1  and a downstream support surface F 2  for supporting the downstream pad P 2 . When viewed in cross section orthogonal to the width direction, the upstream support surface F 1  and the downstream support surface F 2  are orthogonal to the particular direction. The upstream support surface F 1  and the downstream support surface F 2  are at the same positions in the particular direction. 
     The upstream wall  142 , the middle wall  143 , and the downstream wall  144  extend from the support wall  141  toward the rotator  120 . The upstream wall  142  is disposed at an upstream end of the support wall  141 . 
     The downstream wall  144  is disposed at a downstream end of the support wall  141 . The middle wall  143  is disposed between and spaced from the upstream wall  142  and the downstream wall  144 . 
     The upstream support surface F 1  is located between the upstream wall  142  and the middle wall  143 . The downstream support surface F 2  is located between the middle wall  143  and the downstream wall  144 . 
     The upstream pad P 1  is located in contact with the upstream wall  142  and spaced from the middle wall  143 . The downstream pad P 2  is located in contact with the downstream wall  144  and spaced from the middle wall  143 . 
     The stay  150  transmits a force from the urging member SP to the holder  140 . The stay  150  is made of metal. The stay  150  is long in the width direction. The stay  150  has a contact surface Ft that contacts a surface F 3  of the support wall  141  opposite to each support surface F 1 , F 2 . 
     The stay  150  is disposed to the downstream pad P 2  in the moving direction. As illustrated in  FIG. 4 , a distance D 2  is smaller than a distance D 1  in the moving direction. The distance D 2  is a distance from a center of the contact surface Ft of the stay in the moving direction to an upstream end of the downstream pad P 2  in the moving direction. The distance D 1  is a distance from the center of the contact surface Ft to a downstream end of the upstream pad P 1  in the moving direction. The stay  150  is disposed such that the stay  150  projected in the particular direction overlaps the downstream pad P 2 . 
     As illustrated in  FIG. 5 , the contact surface Ft of the stay  150  is convex toward the rotator  120  when viewed in the moving direction, with its center in the width direction protruding further than its ends. One urging member SP is disposed at each of both ends of the stay  150  in the width direction. 
     While each urging member SP urges a corresponding end of the stay  150  toward the rotator  120 , a central portion of the stay  150  in the width direction receives a reaction force from the rotator  120  and thus becomes deformed in a direction away from the rotator  120 . In this case, if the contact surface Ft of the stay  150  is flat, the nip pressure at the central portion of the stay  150  in the width direction may become too low. In this embodiment, however, the contact surface Ft is convex as described above. This prevents the nip pressure at the central portion of the stay  150  from becoming too low. 
     While each urging member SP urges a corresponding end of the stay  150  toward the rotator  120 , the support wall  141  of the holder  140  is deformed following the shape of the contact surface Ft of the stay  150 . In this state, when viewed in the moving direction, the center of the upstream support surface F 1  in the width direction is located closer to the rotator  120  than the ends of the upstream support surface F 1  in the width direction. Similarly, when viewed in the moving direction, the center of the downstream support surface F 2  in the width direction is located closer to the rotator  120  than the ends of the downstream support surface F 2  in the width direction. 
     As illustrated in  FIGS. 3 and 4 , the upstream pad P 1  has a facing surface Fp 1  that faces the rotator  120 . The facing surface Fp 1  faces the rotator  120  via the sliding sheet  180  and the inner peripheral surface  131  of the belt  130 . 
     An upstream end Ep of the facing surface Fp 1  in the moving direction is located at a position farther from the rotation center X 1  of the rotator  120 , in the particular direction, than a downstream end Eg of the guide surface Fg of the upstream guide  160 . In other words, when the pressure unit  82  is at a pressed position illustrated in  FIG. 3 , a distance Dp is greater than a distance Dg 1 . When the pressure unit  82  is at a nip release position illustrated in  FIG. 4 , a distance Ds is greater than the distance Dg 2 . The distance Dp is a distance from the rotation center X 1  to the upstream end Ep of the upstream pad P 1  in the particular direction when the pressure unit  82  is at the pressed position illustrated in  FIG. 3 . The distance Dg 1  is a distance from the rotation center X 1  to the downstream end Eg of the upstream guide  160  in the particular direction when the pressure unit  82  is at the pressed position illustrated in  FIG. 3 . The distance Ds is a distance from the rotation center X 1  to the upstream end Ep of the upstream pad P 1  in the particular direction when the pressure unit  82  is at the nip release position illustrated in  FIG. 4 . The distance Dg 2  is a distance from the rotation center X 1  to the downstream end Eg of the upstream guide  160  when the pressure unit  82  is at the nip release position illustrated in  FIG. 4 . 
     The facing surface Fp 1  has an upstream portion Fp 11  and a downstream portion Fp 12 . 
     The upstream portion Fp 11  includes the upstream end Ep of the facing surface Fp 1 . The upstream portion Fp 11  is spaced from the sliding sheet  180 . In other words, the upstream portion Fp 11  and the rotator  120  do not pinch the belt  130  and the sliding sheet  180  therebetween. 
     The downstream portion Fp 12  is located downstream of the upstream portion Fp 11  in the moving direction. The downstream portion Fp 12  and the rotator  120  pinch the belt  130  and the sliding sheet  180  therebetween, thus forming the upstream nip NP 1 . 
     The upper surface of the upstream wall  142  of the holder  140  is spaced in the particular direction from the rotation center X 1  further than the downstream end Eg of the upstream guide  160  and the facing surface Fp 1  of the upstream pad P 1 . At least when each pad P 1 , P 2  is under no pressure ( FIG. 4 ), the upstream guide  160  is spaced from the upstream pad P 1  in the moving direction by a distance greater than or equal to the dimension of the upstream wall  142  in the moving direction. 
     The downstream pad P 2  has the facing surface Fp 2  located to the rotator  120  and facing the inner peripheral surface  131  of the belt  130 . The facing surface Fp 2  faces the rotator  120  via the sliding sheet  180  and the inner peripheral surface  131  of the belt  130 . 
     The facing surface Fp 2  has an upstream portion Fp 21  and a downstream portion Fp 22 . The upstream portion Fp 21  includes an upstream end of the facing surface Fp 2 . The upstream portion Fp 21  and the rotator  120  pinch the belt  130  and the sliding sheet  180  therebetween, thus forming the downstream nip NP 2 . 
     The downstream portion Fp 22  is located downstream of the upstream portion Fp 21  in the moving direction. The downstream portion Fp 22  is spaced from the sliding sheet  180 . In other words, the downstream portion Fp 22  and the rotator  120  do not pinch the belt  130  and the sliding sheet  180  therebetween. 
     The upper surface of the downstream wall  144  of the holder  140  is spaced in the particular direction from the rotation center X 1  further than the upstream end Ed of the downstream guide surface Fd of the downstream guide  170  in the moving direction and the facing surface Fp 2  of the downstream pad P 2 . At least when each pad P 1 , P 2  is under no pressure ( FIG. 4 ), the downstream guide  170  is spaced from the downstream pad P 2  in the moving direction by a distance greater than or equal to the dimension of the downstream wall  144  in the moving direction. 
     As illustrated in  FIG. 4 , when the rotator  120  is spaced from the belt  130  or when each pad P 1 , P 2  is under no pressure, the upstream pad P 1  has a dimension greater in the particular direction than that of the downstream pad P 2 . In other words, when the rotator  120  is spaced from the belt  130 , the downstream portion Fp 12  of the upstream pad P 1  is located closer to the rotation center X 1  of the rotator  120  than the upstream portion Fp 21  of the downstream pad P 2  in the particular direction. 
     The pressure unit  82  is movable between the pressed position illustrated in  FIG. 3  and the nip release position illustrated in  FIG. 4  by cams and urging members SP. The cams are each located at a position to press a corresponding end of the stay  150  in the width direction against the urging force of the urging member SP. 
     Technical advantages of the fixing device  8  according to the illustrative embodiment will now be described. 
     When the pressure unit  82  moves from the nip release position illustrated in  FIG. 4  to the pressed position illustrated in  FIG. 3 , the downstream portion Fp 12  of the upstream pad P 1  is pressed more than the upstream portion Fp 21  of the downstream pad P 2 . The downstream portion Fp 12  of the upstream pad P 1  and the rotator  120  thus form the upstream nip NP 1  therebetween with stability. 
     As illustrated in  FIG. 2 , when the fixing device  8  is driven, the rotator  120  rotates counterclockwise and the belt  130  rotated clockwise. The upstream end Ep of the upstream pad P 1  is spaced from the rotation center X 1  further than the downstream end Eg of the guide surface Fg of the upstream guide  160 , and the belt  130  and the sliding sheet  180  are not pinched between the upstream end Ep of the upstream pad P 1  and the rotator  120 . Thus, the belt  130  does not press the upstream end Ep of the upstream pad P 1  via the sliding sheet  180 . This prevents unintended deformation of the upstream pad P 1 . 
     From the above description, the illustrative embodiment may have the following advantages. 
     The upstream pad P 1  is prevented from being deformed into an unintended shape. This prevents fluctuations of the width (the dimension in the moving direction) of the nip NP. The upstream nip NP 1  is formed without the use of the upstream portion Fp 11  of the upstream pad P 1 , thus improving durability of the upstream pad P 1 , unlike, for example, a structure forming an upstream nip with the use of the entire upstream pad. The downstream nip NP 2  is formed with the rubber-made downstream pad P 2 . Unlike a pad made of a hard material, for example, resin, the rubber-made downstream pad P 2  may provide correct nip pressure for the downstream nip NP 2  without the need to be shaped accurately. 
     When the rotator  120  and each pad P 1 , P 2  are pressed in contact with each other, the downstream portion Fp 12  of the upstream pad P 1  can be pressed before the upstream portion Fp 21  of the downstream pad P 2  is pressed, thus forming the upstream nip NP 1  with stability. 
     The upstream pad P 1  is spaced from the downstream pad P 2  in the moving direction. This allows for widening of the width of the nip NP without the need to use a wider pad. The pads P 1 , P 2  are insusceptible to each other&#39;s heat. 
     The upstream guide  160  is spaced from the upstream pad P 1  in the moving direction. This reduces heat transmission from the upstream pad P 1  to the upstream guide  160 . 
     The downstream guide  170  is spaced from the downstream pad P 2  in the moving direction. This reduces heat transmission from the downstream pad P 2  to the downstream guide  170 . 
     The downstream guide  170  is spaced in the particular direction from the rotation center X 1  of the rotator  120  further than the downstream pad P 2 . This reduces the belt  130  having passed the downstream pad P 2  from being caught and worn by the downstream guide  170 . 
     When viewed in cross section orthogonal to the width direction, the upstream support surface F 1  and the downstream support surface F 2  are orthogonal to the particular direction. This structure provides a great angle between a tangent to the rotator  120  at the downstream end of the downstream nip NP 2  and the facing surface Fp 2  of the downstream pad P 2 , when compared to a structure where, for example, the support surface for the downstream pad is inclined relative to the support surface for the upstream pad and each pad is entirely pressed in contact with the rotator. A sheet S having passed the downstream nip NP 2  can thus separate from the rotator  120  easily. 
     When viewed in the moving direction, the upstream support surface F 1  and the downstream support surface F 2  each have a central portion in the width direction, which is convex toward the rotator  120 . This convex shape prevents the nip pressure at the central portion from becoming low, unlike a structure that each support surface F 1 , F 2  may be flat. 
     The stay  150  that receives a force from the urging member SP is disposed to the downstream pad P 2 , thus maintaining the nip pressure of the downstream nip NP 2  appropriately. 
     While the disclosure has been described in detail with reference to the specific embodiment thereof, various changes, arrangements and modifications may be applied therein as will be described below. In the following description, elements similar to or identical with those illustrated in the above embodiment are designated by similar numerals, and thus the description thereof can be omitted for the sake of brevity. 
     The upstream guide is shaped as illustrated in the above embodiment, but may have any other shape. In an alternative embodiment illustrated in  FIGS. 6A and 7 , an upstream guide  260  includes an outer peripheral wall  261  and ribs  262 . The outer peripheral wall  261  has a guide surface Fg similar to that of the above embodiment. The outer peripheral wall  261  has an inner peripheral surface Fb opposite to the guide surface Fg. 
     As illustrated in  FIG. 7 , the ribs  262  protrude from the inner peripheral surface Fb of the outer peripheral wall  261 . The ribs  262  are spaced from one another in the width direction. As illustrated in  FIGS. 6A and 6B , each rib  262  has an upper portion  262 A and a lower portion  262 B. The upper portion  262 A faces the pad P 1  in the moving direction. The upper portion  262 A connects the outer peripheral wall  261  and the lower portion  262 B. The lower portion  262 B faces the support wall  141  and the stay  150  in the moving direction. The lower portion  262 B is located at a position farther from the rotation center X 1  of the rotator  120 , in the particular direction, than the upper portion  262 A. 
     Returning to  FIG. 6A , the outer peripheral wall  261  includes a downstream end portion  261 A in the moving direction and a base portion  261 B extending from an upstream end of a rib  262  to a downstream end of the rib  262  in the rotation direction of the belt  130 . The downstream end portion  261 A is located further downstream than the upper portion  262 A of the rib  262  in the moving direction. 
     As illustrated in  FIG. 6B , the downstream end portion  261 A has a thickness T 1 . The thickness T 1  may be smaller than or equal to a thickness T 2  of the base portion  261 B. In this alternative embodiment, the thickness T 1  is smaller than the thickness T 2  of the base portion  261 B. The downstream end portion  261 A tapers downstream in the moving direction. 
     The downstream end portion  261 A has an outer surface Fo facing the heater  110  and an inner surface Fi facing the holder  140 . The outer surface Fo is arcuate in cross section and adjacent to a downstream end of the guide surface Fg in the moving direction and a downstream end of the inner surface Fi in the moving direction. 
     The inner surface Fi is a flat surface orthogonal to the particular direction. The inner surface Fi is spaced in the particular direction from the rotation center X 1  of the rotator  120  further than the upstream end Ep of the facing surface Fp 1  of the upstream pad P 1 . 
     In this alternative embodiment, the tapering downstream end portion  261 A reduces physical contact with the sliding sheet  180 , thus reducing the possibility of a wearing out of the sliding sheet  180 , unlike, for example, a non-tapering downstream end portion. 
     The sliding sheet  180  may be omitted. Even in this case, the belt  130  rarely contacts the downstream end portion  261 A as the downstream end portion  216 A tapers, that is, the outer surface Fo of the downstream end portion  216 A extends away from the belt  130 . Thus, the belt  130  is prevented from being strongly pressed against and worn by the downstream end portion  261 A. 
     The above embodiment shows but is not limited to that the urging members SP urge the holder  140  toward the rotator  120 . The urging members may urge the rotator toward the holder. The urging members SP are not limited to helical compression springs. Examples of the urging members include a helical compression spring, a leaf spring, and a torsion spring. 
     In the illustrative embodiment, the halogen lamp is illustrated as a heater. Examples of the heater include a carbon heater. 
     In the illustrative embodiment, a cylindrical roller having the heater  110  therein is illustrated as a rotator. Examples of the rotator may include a belt whose inner peripheral surface may be heated by a heater. An outer peripheral surface of the rotator may be heated by a heater disposed outside of the rotator or using an induction heating (“IH”) element. A heater may be disposed within an interior space of a belt to indirectly heat the rotator contacting an outer peripheral surface of the belt. A heater may be disposed within an interior space of each of the rotator and the belt. 
     Each of the elements or components which have been described in the illustrative embodiment and modifications may be used in any combination.