Abstract:
A rotatable image heating member for pressing against a pressure to form a nip for heating an image on a recording material; a heat generation layer for generating heat by electric power supply thereto, the heat generation layer being disposed at a position inside an end of the nip with respect to a rotational axis direction of the image heating member; an electric energy supply layer for electric energy supplying to the heat generation layer, the electric energy supply layer being provided at an end of the heat generation layer and electrically connected with the heat generation layer; an elastic layer provided outside the heat generation layer at a position inside an end of the nip with respect to the rotational axis direction; a surface layer provided outside the elastic layer and extending to an outside of the end of the nip so as to cover a part of the electric energy supply layer.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image heating device to be mounted in an image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus to heat an image formed on recording medium. 
         [0002]    As examples of an image heating apparatus, a heating device for fixing an unfixed image on recording medium to the recording medium, a heating device for heating an image on recording medium to enhance the image in gloss. 
         [0003]    There have been known various structures and heating methods for an image heating apparatus. In Japanese Laid-open Patent Application H9-006166, a method for supplying a heat generation roller (fixing member) made up of a substrate layer and a heat generation layer, with electric power is disclosed. The object of this proposal is to simplify a fixing device in structure, and also, to improve a fixing apparatus in durability. More concretely, the heat generation roller in this patent application is made up of a substrate, a heat generation layer, an insulation layer, and a power delivery layer. Electrical power is delivered to the heat generation layer by placing an electric power delivery member in contact with the power delivery layer. 
         [0004]    The nip in which the image on recording medium is heated is formed between a heat generation roller and a pressure applying member by pressing the heat generation roller and pressure applying member against each other. 
         [0005]    If a fixing apparatus is structured so that the just pressure applying portion of its pressure applying member is less in dimension in terms of the direction parallel to the axial line of its heat generation roller than the heat generation layer, heat is not robbed from the portions of heat generation layer, which are not pressed by the pressure applying portions. Therefore, the portion of the heat generation layer, which does not come into contact with the pressing portions, substantially increases in temperature. 
         [0006]    Therefore, it seems to be reasonable to structure an image heating device so that the length of the pressure applying portion becomes greater than the heat generation layer in terms of the above mentioned direction. However, even if the apparatus is structured as described above, the power delivery layer for supplying the heat generation layer with electric power is at each end of the heat generation layer. 
         [0007]    Thus, if an image heating device is structured so that the pressure applying portion becomes longer than the heat generation layer in terms of the above described direction, the pressure applying portion presses on each power delivery layer. Since the power delivery layer is not for heating the image on recording medium, it does not need to be pressed. Thus, from the standpoint of wear and the like attributable to usage, the mechanical load to which the power delivery layer is subjected is desired to be as small as possible. 
       SUMMARY OF THE INVENTION 
       [0008]    Thus, the primary object of the present invention is to provide an image heating device which is smaller than any image heating device in accordance with the prior art, in terms of the amount of mechanical pressure to which the power delivery layer is subjected by the pressure applying member. 
         [0009]    According to an aspect of the present invention, there is provided a rotatable image heating member for pressing against a pressure to form a nip for heating an image on a recording material, said image heating member comprising a heat generation layer for generating heat by electric power supply thereto, said heat generation layer being disposed at a position inside an end of the nip with respect to a rotational axis direction of the image heating member; an electric energy supply layer for electric energy supplying to said heat generation layer, said electric energy supply layer being provided at an end of said heat generation layer and electrically connected with said heat generation layer; an elastic layer provided outside said heat generation layer at a position inside an end of said nip with respect to the rotational axis direction; a surface layer provided outside said elastic layer and extending to an outside of the end of said nip so as to cover a part of said electric energy supply layer. 
         [0010]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic sectional view of the image forming apparatus in the first preferred embodiment of the present invention. 
           [0012]      FIG. 2  is an enlarged schematic cross-sectional view of the essential portions of the fixing device in the first preferred embodiment. 
           [0013]      FIG. 3(   a ) is a schematic front view of the essential portions of the fixing device in the first preferred embodiment, and  FIG. 3(   b ) is a schematic vertical sectional view of the fixing device, at a plane which coincides with the axial line of the heating belt (stay) and the axial line of the pressure roller. 
           [0014]      FIG. 4  is a schematic drawing of the essential portions of the lengthwise end portions of the fixing device. 
           [0015]      FIG. 5  is a graph which shows the changes in the temperature of the fixation belt of the fixing device in the first embodiment, and those of comparative (conventional) fixing devices. 
           [0016]      FIG. 6  is a longitudinal cross-section of an apparatus according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Hereafter, the present invention is concretely described with reference to the preferred embodiments of the present invention. Incidentally, even thought the present invention is described with reference to the preferred embodiments of the present invention, the embodiments are not intended to limit the present invention in scope in terms of structure. That is, the present invention is also applicable to image heating devices other than those in the preferred embodiments, as long they are compatible with the gist of the present invention in terms of structure. 
       Embodiment 1 
     (1) Image Forming Portion 
       [0018]      FIG. 1  is a schematic vertical sectional view of an electrophotographic full-color printer as an example of an image forming apparatus having a fixing device  20  as an image heating device in accordance with the present invention. First, its image forming portion is roughly described. This printer is capable of forming (and outputting) a full-color image on a sheet of recording medium, according to the information of the image to be formed. The information of the image to be formed is inputted from an external host apparatus  200  which is in connection with the control portion  100  (control circuit: CPU) of the image forming apparatus, being therefore capable of exchanging information with the control portion  100 . The apparatus  200  is a computer, an image reader, or the like. The circuit portion  100  (control) exchanges information with the apparatus  200  and the control panel  300  of the image forming apparatus. It exchanges electrical signals with various image processing devices of the image forming apparatus to control the image formation sequence. 
         [0019]    Designated by a referential code  8  is an intermediary transfer belt, which is flexible and endless. The belt  8  is suspended and kept stretched between a belt-backing second transfer roller  9  and a tension roller  10 . As a roller  9  is driven, the belt  8  is circularly moved at a preset speed by the rotation of the roller  9  in the counterclockwise direction indicated by an arrow mark. Designated by a referential code  11  is a second transfer roller, which is kept pressed against the roller  9  with the presence of the intermediary transfer belt  8  between the two rollers  11  and  9 . The area of contact between the intermediary transfer belt  8  and roller  11  is the second transfer station. 
         [0020]    Designated by referential codes  1 Y,  1 M,  1 C, and  1 Bk are the first to fourth image forming stations, respectively. They are under the intermediary transfer belt  8 , and are in alignment in a straight line in the belt movement direction with preset intervals. Each image forming station is an electrophotographic image forming station, which uses a beam of laser light as its exposing means. It has an electrophotographic photosensitive member  2  which is the form of a drum (and therefore is referred to simply as drum  2 , hereafter) as an image bearing member. The drum  2  is rotated in the clockwise direction indicated by an arrow mark, at a preset peripheral velocity. Each image forming station has also a primary charging device  3 , a developing apparatus  4 , a primary transfer roller  5 , and a drum cleaning device  6 , which are in the adjacencies of the peripheral surface of the drum  2 . The roller  5  is on the inward side of the loop which the intermediary transfer belt  8  forms. It is kept pressed against the drum  2 , with the presence of the bottom portion of the intermediary transfer belt  8 , in terms of the belt loop, between the roller  5  and drum  2 . The area of contact between the drum  2  and intermediary transfer belt  8  is the primary transfer station. Designated by a referential code  7  is an exposing device which uses a beam of laser light as its exposing means. The exposing device means  7  exposes the drum  2  of each image forming station. It comprises: a laser light emitting means, a polygon mirror, a deflection mirror, etc. The laser light emitting means emits a beam of laser light while modulating the beam with electrical digital signals which correspond to the pixels, one for one, of the image to be formed, and are in accordance with the information of the image to be formed. 
         [0021]    The image forming operation performed by this image forming apparatus is as follows: An image forming operation is started after the information about the image forming operation to be performed, for example, recording medium size, data about the image to be formed, number of prints to be made, etc, which are set by a user, is transferred to the control  100  from the apparatus  200  and/or control panel  300 . The control  100  activates and controls each image forming station in response to the image formation signals inputted from the apparatus  200 . The image formation signals for forming a copy of an original color image (including black-and-white image) are obtained by separating the color image into monochromatic images of the primary colors of which the original color image is formed. As the four image forming stations  1 Y,  1 M,  1 C, and  1 Bk are activated, the four drums  2  are rotated, and yellow, magenta, cyan, and black toner images are formed on the four drums  2 , one for one, across their peripheral surface. Incidentally, the principle of the electrophotography, and the electrophotographic process for forming a toner image on the drum  2 , are public knowledge, and therefore, are not described here. After the formation of the four monochromatic toner images, different in color, on the four drums  2  in the four image forming stations, one for one, the four images are sequentially transferred (first transfer) in layers onto the intermediary transfer belt  8 , which is being circularly moved at the same velocity as the peripheral velocity of each drum  2 , in the same direction as the direction of the movement of the peripheral surface of the drum  1 , in the first transfer stations, one for one. As a result, an unfixed full-color toner image is synthetically effected by the four monochromatic toner images, different in color, layered in vertical alignment on the surface of the intermediary transfer belt  8 . 
         [0022]    Meanwhile, the control  100  causes one of the sheet feeder cassettes  13  to feed sheets of recording medium, the size of which corresponds to the signals inputted regarding the recording medium size from the apparatus  200 , or through the control panel  300 , into the main assembly of the image forming apparatus. More specifically, the main assembly of the image forming apparatus is provided with sheet feeder cassettes  13 A,  13 B, and  13 C, which are different in the size (length and width) and type of recording medium storable therein, and are vertically stacked. As the image forming operation is started, the control  100  drives the sheet feeding roller  14  of the sheet feeder cassette  13  in which sheets of recording medium of the chosen size are present. Thus, one of the sheets P of recording medium in the cassette  13  which contains the sheets of recording medium of the chosen size, is fed into the main assembly while being separated from the rest, and then, is conveyed to a pair of registration rollers  16 . When the selected recording medium feeding means is the manual feeder tray  17  (multipurpose tray), the control  100  drives the sheet feeder roller  18 , whereby one of the sheets P of recording medium in the manual feeder tray is fed, while being separated from the rest, into the apparatus main assembly, and then, is conveyed to the rollers  16  through a sheet conveyance path  15 . The rollers  16  convey the sheet P of recording medium with such a timing that the leading edge of the sheet P arrives at the second transfer station at the same time as the leading edge of the full-color toner image, made up of the four monochromatic toner images, on the rotating intermediary transfer belt  8 . Thus, as the sheet P is conveyed through the second transfer station, the four monochromatic toner images on the intermediary transfer belt  8  are transferred together (second transfer) onto the sheet P. After being conveyed out of the second transfer station, the sheet P is separated from the intermediary transfer belt  8 , and is introduced into a fixing device  20  while being guided by a vertical guide  19 . It is by the fixing device  20  that the layered four monochromatic toner images, different in color, on the sheet P are welded (fixed) to the sheet P. As a result, a fixed full-color image is effected on the surface of the sheet P. After being conveyed out of the fixing device  20 , the combination of the sheet P and the fixed full-color toner image thereon is conveyed as a full-color print through a recording medium conveyance path  21 , and then, is discharged into a delivery tray  23  by a pair of discharge rollers  22 . After the separation of the sheet P from the intermediary transfer belt  8  in the second transfer station, the image bearing surface of the intermediary transfer belt  8  is cleaned by a belt cleaning device  12 : the toner remaining on the image bearing surface of the intermediary transfer belt  8  after the second transfer is removed by the belt cleaning device  12 . Then, the image bearing surface of the intermediary transfer belt  8  is repeatedly used for image formation. 
         [0023]    When the apparatus is in the black-and-white mode, it is only the fourth image forming station Bk, which forms a black toner image, that is activated and controlled by the control  100 . If the apparatus is in the two-side print mode, after the completion of the formation and fixation of an image on one (first) of the surfaces of the sheet P of recording medium, the sheet P is almost completely conveyed into the tray  23 . That is, just before the trailing edge of the sheet P is moved past the roller  22 , the roller  22  is reversed in rotation. Thus, the sheet P is fed back into the apparatus main assembly, and then, is introduced into a reconveyance path  24 , through which the sheet P is conveyed into the sheet path  15 , and conveyed to the pair of registration rollers  16  for the second time. As the sheet P is conveyed into the sheet path  15 , the sheet P is positioned so that its second surface faces the intermediary transfer belt  8 : the sheet P is positioned upside-down. Thereafter, the sheet P is conveyed through the second transfer station and fixing device, and then, is discharged as a two-sided print into the tray  24 . 
       (2) Fixing Device  20   
       [0024]    The fixing device  20  in this embodiment is an image heating device which uses an endless belt which has a heat generation layer made of an electrically resistant substance. The belt is not tensioned. An unfixed toner image is heated by the heat generated by the endless belt while the belt is circularly moved. In the following description of the fixing device  20 , the “lengthwise” direction of the fixing device  20  and that of each member of the fixing device  20  are the directions perpendicular to the direction in which a sheet of recording medium is conveyed through recording medium conveyance paths. That is, the “lengthwise” direction is parallel to the rotational axis of the belt. The “front” side of the fixing device  20  is the side from which a sheet of recording medium is introduced into the fixing device  20 , and the “left” and “right” sides of the apparatus  20  are the left and right sides as seen from the “front” side of the apparatus  20 . 
         [0025]      FIG. 2  is an enlarged schematic cross-sectional view of the essential portions of the fixing device  20 .  FIG. 3(   a ) is a schematic front view of the essential portions of the fixing apparatus  20 , and  FIG. 3(   b ) is a schematic vertical sectional view of the essential portions of the apparatus  20 , at a plane which coincides with the axial line of the belt  12 . Designated by referential codes  10  and  18  are a fixation belt assembly, and a pressure applying elastic roller. The fixation nip N is formed by pressing the assembly  10  and pressure applying roller  18  against each other. A portion of the assembly  10 , which is designated by a referential code  12 , is the fixation belt for heating the image on the sheet P of recording medium. The belt  12 , which is flexible, is cylindrical (in the form of a cylinder). Designated by a referential code  11  is a belt backing member, around which the belt  12  is loosely fitted. The belt backing member  11  is roughly semi-cylindrical in cross-section, and is formed of heat resistant resin. Designated by a referential code  14  is a rigid pressure application stay, which is roughly U-shaped in cross section and is in the hollow of the belt-backing member  11 . Designated by a pair of referential codes  15  are a pair of stay holders, one for one, which fit with the arms  14   a  which extend from the left and right ends of the stay  14 , one for one. Designated by a pair of referential codes  15   a  are flange portions of the holders  15 , which are integral parts of the holders  15 , one for one. 
         [0026]    Referring to  FIG. 4  which is a schematic drawing of the essential portions of the fixing device  20 , the belt  12  has multiple layers, more specifically, a substrate layer  12   a , a heat generation layer  12   b , and a parting layer  12   d . The substrate layer  12   a  is roughly cylindrical. The heat generation layer  12   b  is on the outward surface of the substrate layer  12   a , and is formed of an electrically resistive substance. The heat generation layer  12   b  covers the entirety of the surface of the substrate layer  12   a . If necessary, another functional layer may be placed between the heat generation layer  12   b  and parting layer  12   d . In this embodiment, the belt  12  is provided with an elastic layer  12   c , which is between the heat generation layer  12   b  and parting layer  12   d . The elastic layer  12   c  is for controlling the toner when the toner is in the melted state. Further, the belt  12  is provided with a pair of electric power delivery layer  12   e , which are in the form of a narrow ring and cover the lengthwise end portions of the substrate layer  12   e . Each power delivery layer  12   e  functions as a low resistance electrode for supplying the heat generation layer  12   b  with electric power. It is electrically in contact with the corresponding lengthwise end of the heat generation layer  12 . More specifically, each power delivery layer  12   e  is in the form of a narrow ring, and covers the substrate layer  12   a , across the area on the outward side of the substrate layer  12   a  in terms of the direction parallel to the lengthwise direction of the fixing device  20 , and is electrically in connection with the heat generation layer  12   b.    
         [0027]    The substrate layer  12   a  is made of a heat resistant, electrically insulative, and mechanically strong substance. More concretely, the substrate  12   a  is cylindrical and is formed of polyimide. It is 30 μm in thickness, and 30 mm in internal diameter. Polyimide is such a resinous substance that is heat resistant, electrically insulative, and mechanically very strong. From the standpoint of rigidity, the thickness of the substrate layer  12   a  is desired to be no less than 15 μm. In order to ensure that the belt  12  quickly heats up, the belt  12  is desired to be small in thermal capacity, and therefore, the thickness of the belt  12  is desired to be no more than 100 μm. The heat generation layer  12   b  is made up of a combination of epoxy resin, and additives, such as carbon black powder and graphite powder, and metallic power (silver powder, for example) mixed into the epoxy resin. The additives in this embodiment were carbon black powder and silver powder. As for the elastic layer  12   c , it is formed of silicon rubber or fluorinated rubber, for example. The elastic layer  12   c  in this embodiment was formed of silicon rubber, and was 300 μm in thickness. The parting layer  12   d  is for facilitating the toner separation from the belt  12 , and is formed of fluorinated resin. More concretely, it is a piece of PFA tube which is 30 μm in thickness. 
         [0028]    The pressure roller  18  is a multilayer roller made up of a metallic core  18   a , an elastic layer  18   b , and a parting layer  18   c , listing from the inward side of the roller  18 . More specifically, the roller  18  is 30 mm in external diameter. The metallic core  18   a  is a solid cylindrical member made of SUS. The elastic layer  18   b  is made of silicon rubber and is 3.0 μm in thickness. The parting layer  18   c  is a piece of PFA tube, which is 30 μm in thickness. The belt pressing portion of the pressure roller  18  is made up of the elastic layer  18   b  and PFA tube  18   c . The roller  18  is between the left and right lateral plates  16 L and  16 R of the apparatus frame  16 , and is rotatably supported by a pair of bearings  17  positioned between the left and right end portions of the roller  18  and the left and right plates  16 L and  16 R, respectively. The aforementioned assembly  10  is positioned in parallel to the roller  18  in such an attitude that the downwardly facing side of the belt backing member  11  faces the roller  18 . Further, the fixing device  20  is provided with a pair of compression springs  19  (pressure application mechanism), which keep the left and right end portions of the holder  15  pressed toward the axial line of the roller  18  by a preset amount of pressure. That is, the stay  14  is kept pressed toward the axial line of the roller  18 . Thus, the downwardly facing surface of the belt backing member  11  is kept pressed upon the pressure applying portion of the roller  18  against the elasticity of the elastic layer  18   b . Thus, a fixation nip N, which has a preset width (dimension in terms of recording medium conveyance direction c) and is necessary for the thermal fixation, is formed between the belt  12  and roller  18 . More concretely, the amount of the pressure by which the assembly  10  is kept pressed against the roller  18  is 300 N, and the dimension (width) of the nip N in terms of the recording medium conveyance direction c is 8 mm. Designated by referential codes  21  and  22  are the entrance and exit guides, respectively, attached to the apparatus frame  16 . 
         [0029]    Designated by a referential code G in  FIG. 3  is a drive gear solidly attached to one of the lengthwise ends of the metallic core  18   a  of the roller  18 . The rotational force of a fixation motor M is transmitted to this gear G through an unshown power transmitting mechanism. As the force is transmitted, the roller  18  is rotated in the counterclockwise direction, indicated by an arrow mark in  FIG. 2 . Thus, the belt  12  is rotated by the force transmitted to the roller  18  and the friction between the roller  18  and belt  12  in the fixation nip N. Thus, the belt  12  rotates around the belt backing member  11  in the clockwise direction, indicated by an arrow mark, sliding on the belt backing member  11  by its inward surface, in the fixation nip N. The speed of the belt  12  is roughly the same as the peripheral velocity of the roller  18 . The left and right flanges  15   a  and  15   a  play the role of catching the belt  12  as the belt  12  shifts leftward or rightward in terms of the direction parallel to the lengthwise direction of the belt backing member  11  while being circularly moved. That is, as the belt shifts leftward or rightward, it comes into contact with the left or right flange  15   a , being thereby prevented from becoming excessively off-centered. The inward surface of the belt  12  is coated with grease (lubricant) to ensure that the belt easily slides on the belt backing member  11 . 
         [0030]    After being introduced into the nip N, the sheet P of recording medium is conveyed through the nip N by the rotation of the roller  18  and belt  12 , while remaining pinched between the roller  18  and belt  12 . In this embodiment, the sheet P is conveyed through the nip N in such a manner that in terms of the lengthwise direction of the nip N, the center of the sheet P coincides with the center of the nip N. Thus, when a sheet P of recording medium is conveyed through the fixing device  20  (fixing nip), it is aligned relative to the fixing device  20  (fixation nip N) so that in terms of the lengthwise direction of the nip N, the center of the sheet P coincides with that of the fixation nip N regardless of its size and the attitude in which it is conveyed. Designated by a referential code S is the referential line (theoretical line) for the “central conveyance”. A referential code WP stands for the width of the recording medium path of the fixing device  20 , that is, the dimension of the widest sheet P of recording medium (in terms of direction perpendicular to recording medium conveyance direction) conveyable through the fixing device  20  (usable with apparatus). 
         [0031]    The fixing device  20  is provided with a thermistor TH as a temperature detecting means for detecting the belt temperature to control the belt  12  in temperature. The thermistor TH is on the inward side of the belt loop. More concretely, in order to detect the temperature of the belt portion which falls within the recording medium path, regardless of recording medium size in terms of the lengthwise direction of the fixing device  20 , the thermistor TH is placed in such a manner that it contacts roughly the center of the inward surface of the belt  12  in terms of its widthwise direction (lengthwise direction of fixing device), and is enabled to remain in contact with the belt  12  regardless of anomalies in the belt movement. More concretely, an elastic member  13  is solidly attached to the stay  14 , and the thermistor TH is attached to the tip of the elastic member  13  so that the thermistor TH is kept in contact with the inward surface of the belt  12  regardless of the anomalies in the movement of the belt  12 . 
         [0032]    Further, the fixing device  20  is provided with a pair of power delivery members  23 , which are held so that they remain elastically in contact with the pair of power delivery portions  12   e  of the belt  12 , one for one, which are on the widthwise end portions of the belt  12 . The power delivery member  23  is formed of carbon black. As the belt  12  rotates, the power delivery member  23  slides on the power delivery portion  12   e  of the belt  12 . Since it is elastically in contact with the power delivery portion  12   e  of the belt  12 , it is capable of maintaining electrical contact with the power delivery portion  12   e  regardless of the movement of the belt  12 . As electric power is supplied between the left and right power delivery members  23  from an electric power source  24  (AC power source), heat is generated by the heat generation layer  12   b  across the entire range of the heat generation layer  12   b , whereby the portion of the belt  12  having the heat generation layer  12   b  is heated. Then, the temperature of the belt  12  is detected by the thermistor TH, and the electrical information outputted by the thermistor TH regarding the temperature of the belt  12  is inputted into the control  26  (CPU) by way of an A/D converter  25 . The control  26  controls a triac  27 , based on the output (information in the form of electrical signals) of the thermistor TH so that the belt temperature remains at a preset level (fixation level). That is, the control  26  controls the electric power which is to be supplied from the AC power source  24  to the power deliver portions  12   e  (heat generation layer  12   b ). 
         [0033]    Then, the control  26  begins to rotate the roller  18  by controlling a fixation motor drive circuit  28  in response to a preset control signal. Further, it starts heating the belt  12  by controlling the triac  27 . The belt backing member  11  adiabatically holds and guides the belt  12  from the inward side of the belt loop. As the belt  12  becomes stable in speed, and its temperature reaches the preset level, a sheet P of recording medium, on which an unfixed toner image t is present, is introduced into the nip N from the direction of the image forming stations while being guided by the entrance guide  21 , in such an attitude that the image bearing surface of the sheet P faces the belt  12 . Then, the sheet P is conveyed, along with the belt  12 , through the nip N while being kept in contact with the belt  12 . While the sheet P is conveyed through the nip N, heat is applied to the sheet P and the unfixed toner image t thereon, by the heated belt  12 , whereby the unfixed toner image t is thermally fixed to the surface of the sheet P. After being conveyed through the nip N, the sheet P is separated from the belt  12  by the curvature of the belt  12 , and is further conveyed while being guided by the exit guide  22  to be discharged into the delivery tray  23 . 
         [0034]    Referring to  FIG. 4 , referential code L 12 , L 12   b , and L 18  stands for the dimension of the belt  12 , dimension of the heat generation layer  12   b , and dimension of the roller  18  (=elastic layer  18   b  (pressure applying portion)), in terms of the lengthwise direction of the fixing device  20 . Further, a referential code L 19  stands for the dimension of the parting layer  12   d  in terms of the lengthwise direction of the fixing device  20 . In the case of the fixing device in this embodiment, there is the following dimensional relationship: L 12 &gt;L 19 ≧L 18 &gt;L 12   b &gt;WP. A referential code C stands for the position of one of the edges of the parting layer  12   d . The fixing device  20  is structured so that the point C coincides with a point B, or is on the outward side of the point B. Since the fixing device  20  is structured as described above, it does not occur that the roller  18  comes directly in contact with the power delivery layer  12   e , within the range between the points A and B. Therefore, it does not occur that the power delivery layer  12   e  is frictionally worn by the roller  18 . Therefore, the fixing device  20  is more stable in terms of the power supply to the heat generation layer  12   e  than any fixing device  20  in accordance with the prior art. Further, the fixing device  20  has no elastic layer between the points A and B, being therefore significantly smaller in the amount of pressure applied to the power delivery layer  12   e  by the pressure applying portion. Incidentally,  FIG. 4  (schematic drawing of fixing device  20 ) is drawn as if a space is present between the peripheral surface of the roller  18  and the elastic layer  12   c  by the parting layer  12   d , in the range between the points A and B of the lengthwise end of the roller  18 . In reality, however, the elastic layer  12   c  and parting layer  12   d  are very thin, and further, the elastic layer  12   c  of the belt  12  and the elastic layer  18   b  of the roller  18  are compressed by the pressure applied to keep the assembly  11  in contact with the roller  18 . Therefore, as the roller  18  rotates, the portions of the roller  18 , which are between the points A and B, remain in contact with the parting layer  12   d.    
         [0035]    In comparison, in the cases of examples of a comparative fixing device, the point A is on the outward side of the point B in terms of the lengthwise direction of the fixing device. Table 1 shows the distance between the points A and B in the fixing devices in the embodiments 1-1-1-3, and examples 1-1-1-4 of a comparative fixing device. In Table 1, if the value (mm) which indicates the distance between the points A and B is positive, it means that the point B is on the outward side of the point A, whereas if it is negative, it means that the point A is on the outward side of the point B. 
         [0000]    
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 A-B distance (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Embodiment 1-1 
                 10 
               
               
                   
                 Embodiment 1-2 
                 5 
               
               
                   
                 Embodiment 1-3 
                 1 
               
               
                   
                 Comp. Example 1-1 
                 0 
               
               
                   
                 Comp. Example 1-2 
                 −1 
               
               
                   
                 Comp. Example 1-3 
                 −5 
               
               
                   
                 Comp. Example 1-4 
                 −10 
               
               
                   
                   
               
             
          
         
       
     
         [0036]    In the case of the fixing device  20  in this embodiment, the roller  18  was rotated at a peripheral velocity of 246 mm/s, and the belt  12  was moved by the movement of the roller  18 . The sheet P of recording medium used for the test was A4 in size, and 105 g/m 2  in basis weight. The sheets P were continuously fed at a rate of 50 sheets per minute, in such an attitude that the lengthwise edges of the sheet P become parallel to the recording medium conveyance direction. The overall resistance of the heat generation layer  12   b  was 10Ω. Thus, 1,000 W of electric power was delivered to the heat generation layer  12   b  by applying 100 V of AC voltage. The temperature changes which occurred to the areas of the belt  12 , which were outside the recording medium path in terms of the lengthwise direction of the fixing device  20 , are given in  FIG. 5 . 
         [0037]    For the first thirty seconds, the belt  12  was increased in temperature, while being rotated, so that the center portion of the belt  12  in terms of the lengthwise direction of the fixing device  20  reached and remained at 190° C. Then, the sheet conveyance was started 30 seconds after the starting of the image forming operation (starting of heating of belt  12 ). A line a in  FIG. 5  stands for the temperature of the center portion of the belt  12  in terms of the lengthwise direction of the fixing device  20 . The changes in the center portion of the belt  12  were roughly the same regardless of the position of the point A and that of the point B. Other lines in  FIG. 5  stand for the temperature changes which occurred to the portions of the belt  12 , which were outside the sheet P path in terms of the lengthwise direction of the fixing device  20 , under various conditions (in terms of position of point A and that of point B, and distance between points A and B). In the cases of the fixing devices in the embodiments 1-1-1-3, the temperature of the out-of-sheet-path portion of the belt  12  did not increase beyond 230° C. (which is the highest temperature which PFA tube can withstand) for 60 seconds after the starting of the feeding of a sheet P of recording medium into the fixing device  20 , although they were slightly different in the temperature level they reached. In comparison, in the cases of the comparative fixing devices 1-1-1-4, the out-of-sheet-path portion of the belt  12  exceeded 230° C. virtually immediately, that is, within roughly 10 seconds, after the starting of the feeding of the sheet P into the fixing device  20 . 
         [0038]    Thus, it is evident that from the standpoint of the prevention of the excessive temperature increase of the out-of-sheet-path portions of the belt  12 , the fixing device  20  is desired to be structured so that the point B is on the outward side of the point A in terms of the lengthwise direction of the fixing device  20 . Referring to  FIG. 4 , if the fixing device  20  is structured so that the point B is on the inward side of the point A, the heat generated by the portions of the heat generation layer  12   b , which are between the point which corresponds to the edge of the largest sheet of recording medium conveyable through the fixing device  20  (usable by image forming apparatus) and the point A cannot be transferred to the roller  18 , causing thereby the out-of-sheet-path portions of the belt  12  to excessively increase in temperature. As is evident from  FIG. 5 , this phenomenon, that is, the excessive temperature increase of the out-of-sheet-path portions of the belt  12 , occurs also in a case where the fixing device  20  is structured so that the point A coincides with the point B (example 1-1 of comparative fixing device), because the heat transfer from the belt  12  to the roller  18  is insufficient also in this case. As for the distance between the points A and B, it may be reasonable to say that the distance between the points A and B is desired to be greater by no less than 1 mm (fixing device in Embodiments 1-1-1-3) than the distance in  FIG. 5 . In other words, structuring the fixing device  20  so that the distance between the points A and B is no less than zero is effective to minimize (prevent) the excessive temperature increase of the out-of-sheet-path portions of the belt  12 . Further, it is preferable that the fixing device  20  is structured so that the distance between the points A and B is no less than 1 mm. 
         [0039]    Next, the positional relationship between the end of the elastic layer  12   c  and the corresponding end of the heat generation layer  12   b  in terms of the lengthwise direction of the fixing device  20  is described. The position of the end of the elastic layer and that of the corresponding end of the heat generation layer  12   b  practically coincide with each other. In comparison, in a case where the end of the elastic layer  12   c  is on the outward side of the corresponding end of the heat generation layer  12   b , the pressure from the pressing portion presses on the elastic layer  12   c , whereby the power delivery layer  12   e  is pressed by the elastic layer  12   c . Therefore, this setup is not desirable. On the other hand, in a case where the end of the elastic layer  12   c  is on the outward side of the corresponding end of the heat generation layer  12   b , the portions of the heat generation layer  12   b , which are not covered with the elastic layer  12   c , become higher in temperature than the covered portion. However, as long as the distance between the end of the elastic layer  12   c  and the corresponding end of the heat generation layer  12   b  is small, the amount of the temperature increase of the portions of the heat generation layer  12   b , which are not covered with the elastic layer  12   c , is relatively small. Therefore, it is desired that the position of the end of the elastic layer  12   c  and that of the heat generation layer  12   b  practically coincide with each other, or the latter is on the outward side of the former. Next, the relationship between the points B and C is described. In a case where the end (point C) of the parting layer  12   d  is on the inward side of the end (point B) of the pressure applying portion, the pressure applying portion directly presses on the power delivery layer  12   e , which possibly increases the amount by which the power delivery layer  12   e  is frictionally worn. Therefore, it is desired that the position of the end of the parting layer is the same as, or on the outward side, of the position of the corresponding end of the pressure applying portion. 
       Embodiment 2 
       [0040]    In the first preferred embodiment, the fixing device  20  was structured so that the end of the pressure applying portion of the pressure applying member was on the outward side of the corresponding end of the elastic layer of the belt  12 . In comparison, in this embodiment, not only is the fixing device  20  structured so that the end of the parting layer  12   d  of the belt  12  is on the inward side of the corresponding end of the pressuring portion of the pressing member, but also, it is provided with an insulation layer f, the end of which is on the outward side of the corresponding end of the pressure applying portion of the pressure applying member. 
         [0041]      FIG. 6  shows the structure of the fixing device in this embodiment. In the following description of this embodiment, if a given component of the fixing device in this embodiment has the same referential code as the counterpart in the first embodiment, it is the same in structure as the counterpart in the first embodiment, unless specifically noted. 
         [0042]    In this embodiment, an insulation layer  12   f  is provided between the heat generation layer  12   b  and elastic layer  12   c . Further, the position of the end of the parting layer  12   d  and that of the elastic layer  12   c  practically coincide with each other. 
         [0043]    First, the insulation layer  12   f  is described. The insulation layer  12   f  is between the heat generation layer  12   b  and elastic layer  12   c . It is formed of polyimide, and is 10 μm in thickness. In terms of the direction parallel to the rotational axis of the belt  12 , its length L 20  is greater than the length L 12   b  of the heat generation layer  12   b . Further, the length L 18  of the pressure applying portion is greater than the length L 12   b  of the heat generation layer  12   b , but is less than the length L 20  of the insulation layer  12   f . That is, the dimensional relationship among these portions of the belt  12  is: WP&lt;L 12   b &lt;L 18 &lt;L 20 . Structuring the fixing device as described above prevents the roller  18  from coming into direct contact with the power delivery layer  12   e  within the range between the points A and B, preventing thereby the problem that the power delivery layer  12   e  is frictionally worn by the roller  18 . Therefore, it ensures that the heat generation layer  12   b  is continuously supplied with a proper amount of the electric power. Further, in this embodiment, no elastic layer is provided between the points A and B. Therefore, the fixing device in this embodiment is smaller in the amount of pressure applied to the power delivery layer by the pressing portion of the pressing member than the fixing device in the first embodiment. 
         [0044]    Next, the positional relationship between the end of the elastic layer  12   c  and the corresponding end of the heat generation layer  12   b  is described. The position of the end of the elastic layer  12   c  and that of the heat generation layer  12   b  practically coincide with each other. To elaborate, if the end of the elastic layer  12   c  is on the outward side of the end of the heat generation layer  12   b , the pressure from the pressure applying portion of the pressure applying member presses on the elastic layer  12   c , causing thereby the elastic layer  12   c  to press on the power delivery layer  12   e . Therefore, this setup is not desirable. On the other hand, if the end of the elastic layer  12   c  is on the outward side of the corresponding end of the heat generation layer  12   b , the portions of the belt  12 , which correspond to the portions of the heat generation layer  12   b , which are not covered with the elastic layer  12   c , become higher in temperature than the portion of the belt  12 , which corresponds to the portion of the heat generation layer  12   b , which are not covered with the elastic layer  12   c . However, as long as the distance between the end of the elastic layer  12   c  and that of the heat generation layer  12   b  is small, the amount of the temperature increase is relatively small. Therefore, it is desired that the position of the end of the elastic layer  12   c  and that of the heat generation layer  12   b  practically coincide with each other, or the latter is on the outward side of the former. In this embodiment, the position of the elastic layer  12   c  and the position of the corresponding end of the heat generation layer  12   b  are practically the same. Incidentally, it is desired that the fixing device is structured so that the position of the end of the elastic layer  12   c  and the position of the corresponding end of the parting layer  12   d  coincide with each other, or the latter is on the outward side of the former. Next, the relationship between the points B and C is described. If the end (point C) of the insulation layer  12   f  is on the inward side of the end (point B) of the pressure applying portion, the pressure applying portion directly presses on the power delivery layer  12   e , which possibly increases the amount by which the power delivery layer  12   e  is frictionally worn. Therefore, it is desired that the position of the end of the insulation layer  12   f  coincides with the position of the corresponding end of the pressure applying portion, or is on the outward side of the position of the corresponding end of the pressure applying portion. 
       [Miscellanies] 
       [0045]    1) The present invention is also applicable to a fixing device which has a flexible endless belt and multiple belt suspension rollers, inclusive of a belt driving roller, and is structured so that the endless belt is suspended and kept stretched by the belt suspension rollers, and is circularly driven by the belt driving roller. 
         [0046]    2) The present invention is also applicable to a fixing device which employs a heating roller comprising: a substrate  12   a  which is a rigid, dielectric, cylindrical, and hollow or solid roller; and a heat generating resistor layer  12   b , an elastic layer  12   c , a power delivery layer  12   e , etc., layered on the peripheral surface of the substrate  12   a.    
         [0047]    As described above, the present invention can reduce a fixing device in the amount of mechanical load applied to the power delivery layer of the heating member of the fixing device by the pressure applying member of the fixing device, even if the fixing device is structured so that in terms of the direction parallel to the lengthwise direction of the fixing device, the end of the pressure applying portion of the pressure applying member of the fixing device is on the outward side of the corresponding end of the heat generation layer of the image fixing (heating) member of the fixing device. 
         [0048]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0049]    This application claims priority from Japanese Patent Application No. 173662/2010 filed Aug. 2, 2010, which is hereby incorporated by reference.