Patent Publication Number: US-8977178-B2

Title: Fixing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2012-018022 filed on Jan. 31, 2012, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the invention relate to a fixing device for heat-fixing a developer image transferred to a recording sheet. 
     BACKGROUND 
     In related art, a fixing device that is used for an image forming apparatus of an electrophotographic type has been known which has a cylindrical member, a ceramic heater, a heater frame and a backup roller (for example, refer to U.S. Pat. No. 6,266,510). The ceramic heater is disposed to slidingly contact an inner peripheral surface of the cylindrical member and is configured to heat the cylindrical member. The backup roller has a straight shape and pinches the cylindrical member between the ceramic heater and the backup roller. Also, the heater frame is disposed at one side of the backup roller opposite to another side of the nip plate where the backup roller is provided and supports the heater frame. 
     In the fixing device, a part of the heater frame facing the backup roller has a curved concave shape so that a central portion thereof in an axial direction of the cylindrical member is more spaced from the backup roller than both end portions thereof in the axial direction. Thereby, upon the fixing, when both end portions of the heater frame in the axial direction are pressed towards the backup roller, a pressure that is applied between the ceramic heater and the backup roller becomes higher at both end portions in the axial direction than at the central portion. Therefore, a conveying speed of a recording sheet, which is conveyed through between the cylindrical member and the backup roller, becomes faster at both end portions of the recording sheet, so that a central portion of the recording sheet is tensioned towards both end portions thereof. Thereby, it is possible to reduce wrinkles being generated on the recording sheet. 
     According to the above-described fixing device, however, only the heat frame has the concave shape, that is, the backup roller has the straight shape. Therefore, in order to make the above-described difference in pressures applied between the ceramic heater and the backup roller appropriate, an amount of depression of the heater frame needs to be made large. However, when the amount of depression of the heater frame is large, it is difficult to keep the cylindrical member straight during operation. 
     SUMMARY 
     Accordingly, an object of the invention is to provide a fixing device capable of reducing wrinkles from being generated on a recording sheet while keeping a cylindrical member straight. 
     According to an aspect of the present invention, there is provided a fixing device including: a flexible cylindrical member; a heater; a nip plate; and a roller. The heater is disposed inside the cylindrical member. The nip plate is configured to be in contact with an inner peripheral surface of the cylindrical member and is configured to be heated by the heater. The roller is configured to pinch the cylindrical member between the roller and the nip plate. At least during fixing, the nip plate has a curved shape where a central portion thereof in an axial direction of the cylindrical member is more spaced from the roller than both end portions thereof in the axial direction. The roller is a concave roller where a diameter of a central portion thereof in the axial direction is smaller than diameters of both end portions thereof respectively. 
     According to the fixing device configured as described above, compared to a configuration where only the nip plate is formed to have a concave shape and the backup roller has a straight shape, it is possible to reduce an amount of depression of the nip plate. Thereby, it becomes more easy to keep the cylindrical member, which is pinched between the nip plate and the backup roller, straight. Also, during fixing, a pressure that is applied between the nip plate and the backup roller becomes higher at both end portions in the axial direction than at the central portion. Therefore, the recording sheet is tensioned towards both end portions thereof in the axial direction, so that wrinkles are difficult to be generated on a central portion of the recording sheet in the axial direction. 
     According to another aspect of the invention, there is provided a thermal fixing device including: an endless belt; a nip plate; and a roller. The endless belt defines a first axial line extending in a first direction. The nip plate is disposed inside the endless belt and includes a central portion in the first direction and first and second end portions in the first direction, the first end portion of the nip plate being an end portion of the nip plate in the first direction and the second end portion of the nip plate being an opposite end portion of the first end portion in the first direction. The roller defines a second axial line and is opposed to an outer surface of the endless belt in a second direction perpendicular to the first direction. The roller is configured to pinch the endless belt between the roller and the nip plate and includes a central portion in the first direction and first and second end portions in the first direction, the first end portion of the roller being an end portion of the roller in the first direction and the second end portion of the roller being an opposite end portion of the first end portion in the first direction. The nip plate is configured have a curved shape such that a distance between the central portion of the nip plate and the second axial line in the second direction is greater than a distance between the first end portion of the nip plate and the second axial line in the second direction, and the distance between the central portion of the nip plate and the second axial line in the second direction is greater than a distance between the second end portion of the nip plate and the second axial line in the second direction. The roller is a concave roller where a diameter of a central portion of the roller is smaller than a diameter of the first end portion of the roller, and the diameter of the central portion of the roller is smaller than a diameter of the second end portion of the roller. 
     Here, “the second axial line” may be a central axial line of the roller. Further, “the distance in the second direction” may be a component of the distance in the second direction. 
     According to another aspect of the invention, there is provided a thermal fixing device including: an endless belt; a nip plate; a heater; and a roller. The endless belt defines a first axial line extending in a first direction. The nip plate is disposed inside the endless belt and includes a central portion in the first direction and an end portion in the first direction. The heater is configured to heat the endless belt. The roller defines a second axial line and is opposed to an outer surface of the endless belt in a second direction perpendicular to the first direction. The roller is configured to pinch the endless belt between the roller and the nip plate and includes a central portion in the first direction and an end portion in the first direction. The nip plate is configured have a curved shape such that a distance between the central portion of the nip plate and the second axial line in the second direction is greater than a distance between the end portion of the nip plate and the second axial line in the second direction. The roller is a concave roller where a diameter of a central portion of the roller is smaller than a diameter of the end portion of the roller. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a schematic configuration of a laser printer having a fixing device according to an illustrative embodiment of the invention; 
         FIG. 2  is a sectional view of the fixing device; 
         FIG. 3  is a perspective view showing a halogen lamp, a nip plate, a reflection plate, a pressing roller and a stay; 
         FIG. 4  shows the reflection plate and nip plate mounted on the stay, the pressing roller and urging members; 
         FIG. 5  illustrates a sheet that is conveyed in the fixing device; and 
         FIG. 6  is a sectional view showing a fixing device according to a modified embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an illustrative embodiment of the invention will be described in detail with reference to the drawings. Meanwhile, in the following descriptions, a schematic configuration of a laser printer  1  (image forming apparatus) having a fixing device  100  according to an illustrative embodiment of the invention will be first briefly described and then a detailed configuration of the fixing device  100  will be described. 
     Schematic Configurations of Laser Printer 
     As shown in  FIG. 1 , the laser printer  1  mainly has, in a body housing  2 , a feeder unit  3  that feeds a sheet P, which is an example of the recording sheet, an exposure device  4 , a process cartridge  5  that transfers a toner image (developer image) onto the sheet P and a fixing device  100  that heat-fixes the toner image on the sheet P. 
     Meanwhile, in the following descriptions, the directions are described on the basis of a user who uses the laser printer  1 . That is, the right side of  FIG. 1  is referred to as the “front side”, the left side of  FIG. 1  is referred to as the “rear side”, the front side of  FIG. 1  is referred to as the “left side” and the inner side of  FIG. 1  is referred to as the “right side.” Also, the upper-lower direction of  FIG. 1  is referred to as the “upper-lower direction”. 
     The feeder unit  3  is provided at a lower part in the body housing  2  and mainly has a sheet feeding tray  31  that accommodates therein sheets P, a sheet pressing plate  32  that lifts up a front side of the sheets P, a feeder roller  33 , a feeder pad  34 , paper dust removing rollers  35 ,  36  and register rollers  37 . The sheets P in the sheet feeding tray  31  are obliquely inclined towards the feeder roller  33  by the sheet pressing plate  32  and are separated one by one by the feeder roller  33  and the feeder pad  34 , which is then conveyed towards the process cartridge  5  through the paper dust removing rollers  35 ,  36  and the register rollers  37 . 
     The exposure device  4  is disposed at an upper part in the body housing  2  and mainly has a laser emitting unit (not shown), a polygon mirror  41  that is rotary driven, lenses  42 ,  43  and reflectors  44 ,  45 ,  46 . In the exposure device  4 , a laser light (refer to the dotted-dashed line) based on image data, which is emitted from the laser emitting unit, is reflected or passes in order of the polygon mirror  41 , the lens  42 , the reflectors  44 ,  45 , the lens  43  and the reflector  46  and is then scanned on a surface of a photosensitive drum  61  at high speed. 
     The process cartridge  5  is disposed below the exposure device  4  and is detachably mounted to the body housing  2  through an opening that is formed when a front cover  21  provided to the body housing  2  is opened. The process cartridge  5  has a drum unit  6  and a developing unit  7 . 
     The drum unit  6  mainly has the photosensitive drum  61 , a charger  62  and a transfer roller  63 . Also, the developing unit  7  is detachably mounted to the drum unit  6  and mainly has a developing roller  71 , a supply roller  72 , a layer thickness regulation blade  73  and a toner accommodation unit  74  that accommodates therein toner (developer). 
     In the process cartridge  5 , the surface of the photosensitive drum  61  is uniformly charged by the charger  62  and then exposed by the high-speed scanning of the laser light emitted from the exposure device  4 , so that an electrostatic latent image based on the image data is formed on the photosensitive drum  61 . Also, the toner in the toner accommodation unit  74  is supplied to the developing roller  71  via the supply roller  72 , is introduced between the developing roller  71  and the layer thickness regulation blade  73  and is then carried on the developing roller  71  as a thin layer having a predetermined thickness. 
     The toner carried on the developing roller  71  is supplied from the developing roller  71  to the electrostatic latent image formed on the photosensitive drum  61 . Thereby, the electrostatic latent image becomes visible and a toner image is thus formed on the photosensitive drum  61 . Then, the sheet P is conveyed between the photosensitive drum  61  and the transfer roller  63 , so that the toner image on the photosensitive drum  61  is transferred onto the sheet P. 
     The fixing device  100  is arranged at the rear of the process cartridge  5 . The toner image (toner) transferred onto the sheet P is heat-fixed on the sheet P as the sheet passes through the fixing device  100 . The sheet P having the toner image heat-fixed thereon is discharged onto a sheet discharge tray  22  by conveyance rollers  23 ,  24 . 
     Detailed Configuration of Fixing Device 
     As shown in  FIGS. 2 and 3 , the fixing device  100  has a fixing belt  110  that is an example of the cylindrical member, a halogen lamp  120  that is an example of the heater, a nip plate  130 , a reflection plate  140 , a pressing roller  150  that is an example of the backup roller and a stay  160 . 
     The fixing belt  110  is a stainless steel belt of an endless shape (cylindrical shape). The fixing belt  110  has heat resistance and flexibility and rotation thereof is guided at both end portions by a guide member (not shown). The fixing belt  110  has a straight shape having a substantially constant outer diameter in an axial direction of the fixing belt  110 , i.e., in the left-right direction. 
     The halogen lamp  120  is a heating element that heats the nip plate  130  and the fixing belt  110  to thus generate radiation heat heating the toner on the sheet P and is disposed at a predetermined interval from inner sides of the fixing belt  110  and the nip plate  130  at an inside of the fixing belt  110 . 
     The nip plate  130  is a plate-shaped member to which the radiation heat from the halogen lamp  120  is applied, and is disposed to slidingly contact an inner surface of the cylindrical fixing belt  110 . The nip plate  130  transfers the radiation heat, which is applied from the halogen lamp  120 , to the toner on the sheet P through the fixing belt  110 . 
     The nip plate  130  has a planar shape made of a material having higher thermal conductivity that the steel stay  160 , for example, aluminum plate. The nip plate  130  has a thickness that can be deflection-deformed. 
     The reflection plate  140  is a member that reflects the radiation heat from the halogen lamp  120  (the radiation heat radiated mainly in the front-rear direction or upper direction) toward the nip plate  130  (an upper surface of the nip plate  130 ). The reflection plate  140  is disposed at a predetermined interval from the halogen lamp  120  so that it surrounds the halogen lamp  120  at the inside of the fixing belt  110 , more specifically, between the halogen lamp  120  and the stay  160  that will be described later. 
     The radiation heat from the halogen lamp  120  is collected to the nip plate  130  by the reflection plate  140 . Thereby, it is possible to effectively use the radiation heat from the halogen lamp  120 , thereby rapidly heating the nip plate  130  and the fixing belt  110 . 
     The reflection plate  140  is formed by bending a plate such as aluminum plate and the like, which have high reflectance of the infrared light and far-infrared light, into a substantially U shaped section. More specifically, the reflection plate  140  mainly has a reflection part  141  having a bent shape (substantially U shape in a cross-sectional-view) and flange parts  142  that extend from both end portions of the reflection part  131  towards the outside in the front-rear direction and are an example of the supported part. In the meantime, in order to increase the heat reflectivity, the reflection plate  140  may be formed using an aluminum plate and the like for which a mirror finishing has been performed. Although the flange parts  142  have a planar shape, it is possible to perform deflection-deformation, so that the entire reflection plate  140  can be deformed. 
     The pressing roller  150  is an elastically deformable member and is disposed below the nip plate  130 . The pressing roller  150  pinches the fixing belt  110  between the nip plate  130  and the pressing roller  150  at an elastically deformed state. 
     The pressing roller  150  is a concave roller in which a diameter of a central portion thereof in the left-right direction is smaller than diameters of both end portions thereof. Specifically, as shown in  FIG. 4 , the pressing roller  150  has a metal shaft  151  and a rubber layer  152  that covers a periphery of the shaft  151 . The shaft  151  has a straight shape having a substantially constant diameter in the left-right direction. The rubber layer  152  has both end portions thicker than a central portion in the left-right direction so that it has a concave shape in which outer diameters of both end portions thereof in the left-right direction is larger than an outer diameter of the central portion thereof. 
     In the meantime, for example, the rubber layer  152  is manufactured using a mold having a hollow cylindrical cavity. Since the rubber layer  152  can be elastically deformed, it is possible to pull out the rubber layer  152  from the mold even though the rubber layer has the concave shape. 
     The pressing roller  150  is configured to rotate as a driving force is transferred thereto from a motor (not shown) provided in the body housing  2 . As the pressing roller  150  rotates, it rotates the fixing belt  110  by a frictional force with the fixing belt  110  (or sheet P). Thereby, the sheet P having the toner image transferred thereto is conveyed between the pressing roller  150  and the heated fixing belt  110 , so that the toner image (toner) is heat-fixed thereon. 
     Referring back to  FIGS. 2 and 3 , the stay  160  is a member that supports both end portions of the nip plate  130  in the front-rear direction via the flange parts  142  of the reflection plate  140 , thereby securing the rigidity of the nip plate  130 . The stay  160  has a shape (substantially U-shaped section) following an outward shape of the reflection plate  140  (reflection part  141 ) and is disposed to cover the reflection plate  140 . The stay  160  is formed by bending a steel plate and the like having relatively high rigidity into a substantially U shape when seen in a cross-sectional view, which is symmetric in the left-right direction. 
     The stay  160  is disposed above the reflection plate  140  and has an upper wall  161  having a planar shape, a front wall  162  extending downwards from a front end of the upper wall  161  and a rear wall  163  extending downwards from a rear end of the upper wall  161 . End faces of lower ends of the front wall  162  and rear wall  163  are support surfaces  164  that support the nip plate  130  and the flange parts  142  of the reflection plate  140 . 
     As shown in  FIG. 3 , the front wall  162  and the rear wall  163  of the stay  160  are configured so that distances of central portions thereof in the left-right direction from the upper wall  161  to the support surfaces  164  are smaller than distances of both end portions thereof in the left-right direction from the upper wall  161  to the support surfaces  164 . That is, the support surface  164  has a curved concave shape in which a central portion thereof in the left-right direction is more spaced from the pressing roller  150  than both end portions thereof in the left-right direction. Here, as an example of the central portion of the support surface  164  in the left-right direction being more spaced from the pressing roller  150  than both end portions thereof in the left-right direction, a distance between the central portion of the support surface  164  and a central axial line of the roller  150  may be greater than distances between both end portions of the support surface and the central axial line of the roller  150  respectively. 
     The stay  160  may be manufactured, for example, by punching a material so that parts corresponding to the support surfaces  164  of the lower ends of the front wall  162  and rear wall  163  of the stay  160  become desired concave curved surfaces and bending the obtained blank material into a U shape. 
     Hereinafter, the concave shape of this illustrative embodiment is described. 
     As shown in  FIG. 4 , an amount of depression X of the concave shape of the pressing roller  150  is defined as X=(ΦA+ΦB)/2−ΦC where ΦA and ΦB are outer diameters of both right and left end portions of the pressing roller  150  and ΦC is an outer diameter of the central portion of the pressing roller  150 . Further, an amount of depression Y of the concave shape of the stay  160  is defined as a distance D between a flat surface and central portions of the front wall  162  and rear wall  163  in the left-right direction when both left and right end portions of the front wall  162  and rear wall  163  of the stay  160  in the left-right direction are placed on the flat surface. 
     For example, at a state where the pressing roller  150  and the nip plate  130  are spaced from each other, the amount of depression X of the pressing roller  150  is 0.1 to 0.2 mm and the amount of depression Y of the stay  160  is 0 to 0.1 mm. That is, the amount of depression X of the pressing roller  150  is larger than the amount of depression Y of the stay  160 . 
     Note that, in this illustrative embodiment, regarding the pressing roller  150 , the straight shape is a shape in which the amount of depression X is smaller than 0.050 mm and the concave shape is a shape in which the amount of depression X is 0.050 mm or larger. Also, in this illustrative embodiment, regarding the stay  160 , the straight shape is a shape in which the amount of depression Y is smaller than 0.001 mm and the concave shape is a shape in which the amount of depression Y is 0.001 mm or larger, preferably 0.010 mm or larger, and more preferably 0.050 mm or larger. 
     According to the above configuration, it is possible to obtain the following operations and effects in this illustrative embodiment. 
     As shown in  FIG. 2 , the nip plate  130  and the reflection plate  140  are mounted to the stay  160  so that the flange parts  142  of the reflection plate  140  are pinched by the support surfaces  164  of the stay  160  and the nip plate  130 . During the fixing, as shown in  FIG. 4 , the stay  160  is urged towards the pressing roller  150  by urging members  200  that are provided at both left and right end portions. 
     At this state, when the halogen lamp  120  becomes ON during the fixing, the nip plate  130  and the flange parts  142  of the reflection plate  140  are deformed to follow the support surfaces  164  of the stay  160  and the central portions thereof in the left-right direction are curved so that they are more spaced from the pressing roller  150  than both end portions thereof. 
     Thereby, the pressure that is applied between the nip plate  130  and the pressing roller  150  becomes higher at both end portions in the left-right direction than at the central portion in the left-right direction. At this state, when the sheet P is conveyed, a nip area NP of the sheet P, which is positioned between the fixing belt  110  and the pressing roller  150 , is more widened in the conveyance direction (front-rear direction) at both end portions than at the central portion in the width direction (left-right direction) of the sheet P. Thereby, the conveying speed of the sheet P becomes faster at both end portions in the left-right direction of the sheet P than at the central portion in the left-right direction of the sheet P. Accompanied by this, a part (an upstream part with respect to the nip area NP) of the sheet P, which enters the nip area NP (between the fixing belt  110  and the pressing roller  150 ) from now, is tensioned towards both end portions in the left-right direction of the nip area NP, so that the central portion is tightened. 
     As described above, since the sheet P enters the nip area NP (between the fixing belt  110  and the pressing roller  150 ) with the central portion thereof in the width direction being tightened, it is possible to reduce wrinkles from being generated on the sheet P. 
     In addition, the nip plate  130  is curved, and further, the pressing roller  150  is also made to have the concave shape. Thereby, it is possible to reduce the amounts of depression of both the nip plate  130  and the pressing roller  150 , compared to a configuration where only the nip plate  130  is curved. As a result, the fixing belt  110  that is pinched by the nip plate  130  and the pressing roller  150  does not deform so much, and can be rotated while keeping the straight state thereof. 
     In addition, since the pressing roller  150  is heated, the central portion thereof in the left-right direction is expanded and becomes near to the straight shape. Therefore, the amount of depression X of the pressing roller  150  is set to be larger than the amount of depression Y of the stay  160 , so that the fixing belt  110 , which is pinched between the pressing roller  150  and the nip plate  130 , can be rotated without being deformed so much while keeping the straight state, compared to a configuration where the amount of depression Y of the stay  160  is set to be larger than the amount of depression X of the pressing roller  150 . 
     Since the nip plate  130  is deformed and curved while following the shape of the support surfaces  164  of the stay  160 , it is possible to easily manufacture the nip plate  130 , compared to a configuration where the nip plate  130  is made to have a curved shape in advance. 
     Although the illustrative embodiment of the invention has been described, the invention is not limited to the illustrative embodiment. The specific configurations can be appropriately changed without departing from the gist of the invention. 
     In the above-described illustrative embodiment, the stay  160  has the substantially U shape that is symmetric in the left-right direction. However, the invention is not limited thereto. For example, as shown in  FIG. 6 , the stay  160  may have a substantially U shape that is asymmetric in the left-right direction. 
     Specifically, the stay  160  is configured so that an upper-lower length of a front wall  262  extending downwards from the front end of the upper wall  161  is smaller than an upper-lower length of a rear wall  263  extending downwards from the rear end of the upper wall  161 . A lower end portion of the front wall  262  is formed with a flange  264  extending forwards. 
     The stay  160  is formed with a support surface  265 , which is curved so that a central portion thereof in the left-right direction is closer to the upper wall  161  than both end portions thereof in the left-right direction, by bending the flange  264 . Further, the stay  160  is formed with a support surface  266 , which is curved so that a central portion thereof in the left-right direction is closer to the upper wall  161  than both end portions thereof in the left-right direction, by pressing an end face of a lower end of the rear wall  263 . 
     As described above, the stay  160  is formed to have the substantially U shape that is asymmetric in the left-right direction. Thereby, it is possible to bend upwards the front end portion of a nip plate  230 . Specifically, the nip plate  230  has a base part  231  having a planar shape and a curved part  232  that is curved upwards from a front end of the base part  231 . 
     The nip plate  230  is configured so that a rear end portion  231 A of the base part  231  is supported to the support surface  266  of the stay  260  via the flange part  142  of the reflection plate  140  and an end face  232 A of the curved part  232  is supported to the support surface  265  of the stay  260  viva the flange part  142  of the reflection plate  140 . 
     As described above, the front end portion of the nip plate  230  is curved, so that the stay  160  is formed to have the substantially U shape which is asymmetric in the left-right direction. Thus, compared to a configuration where the nip plate  230  is not provided with the curved part  232  (refer to the dashed-two dotted line), distance between the fixing belt  110  and the pressing roller  150  at the front of the pressing-contact part of the fixing belt  110  and the pressing roller  150 , i.e., at the upstream side in the conveyance direction of the sheet P, are increased. Thereby, even when the sheet P is conveyed at any angle, it is easy to guide the sheet between the fixing belt  110  and the pressing roller  150 . 
     In the above-described illustrative embodiment, the nip plate  130  and the flange parts  142  of the reflection plate  140  are configured to be deformed to follow the shape of the support surfaces  164  of the stay  160  and are curved at least during the fixing. However, the invention is not limited thereto. For example, the flange parts of the reflection plate and the nip plate may be formed to have a curved shape where the central portions thereof in the left-right direction are more spaced from the pressing roller  150  than both end portions thereof in the left-right direction in advance. 
     Further, in the above-described illustrative embodiment, the cylindrical member is made of stainless steel, i.e., metal. However, the invention is not limited thereto. For example, the cylindrical member may be formed of resin. 
     Further, in the above-described illustrative embodiment, the halogen lamp  120  has been exemplified as the heater. However, the invention is not limited thereto. For example, a carbon heater, an IH heater and the like may be also adopted. 
     Further, in the above-described illustrative embodiment, the sheet P such as normal sheet, postcard and the like has been exemplified as the recording sheet. However, the invention is not limited thereto. For example, an OHP sheet may be also used.