Patent Publication Number: US-10761461-B2

Title: Fixing device and image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese Patent application No. 2018-090845, filed on May 9, 2018, which is incorporated by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates to a fixing device configured to fix a toner image on a sheet and an image forming apparatus provided with the fixing device. 
     As one of techniques to heat a fixing belt of a fixing device, a heating technique using a flat heater is known. In a fixing device of the flat heater heating technique, a cylindrical fixing belt is put between the flat heater and a pressing roller. When the pressing roller is driven to be rotated, the fixing belt is driven by the pressing roller to be rotated. Then, a sheet on which a toner is transferred is put between the fixing belt and the pressing roller to be conveyed, and the toner is fixed on the sheet. 
     In the flat heater heating technique, the fixing belt is slid with respect to the flat heater. Although a lubricant is applied on an inner circumferential face of the fixing belt in order to reduce a friction to the flat heater, if the lubricant is leaked from between the fixing belt and the flat heater, the lubrication effect may be deteriorated. Then, a technique to inhibit the leakage of the lubricant from between the fixing belt and the flat heater is discussed. For example, by abrading a surface of the flat heater in a movement direction of the fixing belt, recesses are formed on the surface of the flat heater. Alternatively, grooves in an axial direction of the pressing roller or mesh-shaped grooves are formed on a sliding face of the flat heater. 
     However, it is difficult to inhibit the leakage of the lubricant from an end portion of an inner circumferential face of the fixing belt completely by using the above technique. When the lubricant is leaked from the end portion of the inner circumferential face of the fixing belt, the lubricant may go around an outer circumferential face of the fixing belt and be adhered on the pressing roller. Then, a grip force of the pressing roller to convey the sheet is decreased and it becomes difficult to convey the sheet. 
     SUMMARY 
     In accordance with an aspect of the present disclosure, a fixing device includes a fixing belt, a flat heater, a holder and a pressing roller. The fixing belt is rotatable and cylindrical. The flat heater has a holding body and a heating resistor. The holding body has a longitudinal length longer than a longitudinal length of the fixing belt. The heating resistor has a longitudinal length shorter than the longitudinal length of the fixing belt and is embedded in the holding body such that both longitudinal end portions of the heating resistor are positioned inside longitudinal end portions of the holding body. The holder is configured to hold the flat heater such that a surface in which the heating resistor is embedded comes into contact with an inner circumferential face of the fixing belt via a lubricant, the longitudinal end portions of the holding body are positioned outside the longitudinal end portions of the fixing belt and the longitudinal end portions of the heating resistor are positioned inside the longitudinal end portions of the fixing belt. The pressing roller is configured to hold the fixing belt between the pressing roller and the flat heater to form a pressing area between the pressing roller and the fixing belt. A sheet is conveyed through the pressing area. The fixing belt is driven by the pressing roller to be rotated. Of a contact area of the flat heater with the inner circumferential face of the fixing belt, in an area outside the longitudinal end portion of the holding body, a groove is formed such that the lubricant flows to a center side of the holding body toward a downstream side in a rotational direction of the fixing belt. 
     In accordance with an aspect of the present disclosure, an image forming apparatus includes an image forming part and the fixing device. The image forming part is configured to form a toner image on the sheet. The fixing device is configured to fix the toner image on the sheet. 
     The above and other objects, features, and advantages of the present disclosure will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present disclosure is shown byway of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view schematically showing an inner structure of a printer according to an embodiment of the present disclosure. 
         FIG. 2  is a cross sectional view showing a fixing device according to the embodiment of the present disclosure. 
         FIG. 3  is a bottom view showing a flat heater according to the embodiment of the present disclosure. 
         FIG. 4  is a bottom view showing the flat heater according to the embodiment of the present disclosure. 
         FIG. 5  is a cross sectional view along a I-I line in  FIG. 4 . 
         FIG. 6  is a bottom view showing the flat heater according to the embodiment of the present disclosure. 
         FIG. 7  is a bottom view showing the flat heater according to the embodiment of the present disclosure. 
         FIG. 8  is cross sectional views along a II-II line and a III-III line in  FIG. 7 . 
         FIG. 9  is a bottom view showing the flat heater according to the embodiment of the present disclosure. 
         FIG. 10  is a cross sectional view along a I-I line in  FIG. 4 . 
         FIG. 11  is a cross sectional views along a II-II line and a III-III line in  FIG. 7 . 
         FIG. 12  is a view showing a result of an experiment according to the embodiment of the present disclosure. 
         FIG. 13  is a bottom view showing the flat heater according to the embodiment of the present disclosure. 
         FIG. 14  is a bottom view showing the flat heater according to the embodiment of the present disclosure. 
         FIG. 15  is cross sectional views along a II-II line and a III-III line in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, with reference to the attached drawings, an image forming apparatus and a fixing device of the present disclosure will be described. 
     First, with reference to  FIG. 1 , an entire structure of a printer  1  as the image forming apparatus will be described.  FIG. 1  is a front view schematically showing an internal structure of the printer  1 . In the following description, a near side of a paper surface of  FIG. 1  is defined to be a front side of the printer  1 , and left and right directions are based on a direction in which the printer  1  is viewed from the front side. “U”, “Lo”, “L”, “R”, “Fr” and “Rr” in each figure respectively indicates “an upper side”, “a lower side”, “a left side”, “a right side”, “a front side”, and “a rear side” of the printer  1 . 
     An apparatus main body  2  of the printer  1  is provided with a sheet feeding cassette  3  storing a sheet S, a sheet feeding device  5  feeding the sheet S from the sheet feeding cassette  3 , an image forming part  7  forming a toner image on the sheet S, a fixing device  9  fixing the toner image on the sheet S, an ejecting device  11  ejecting the sheet S and an ejected sheet tray  13  on which the ejected sheet S is stacked. In the apparatus main body  2 , a conveyance path  15  for the sheet S is formed to extend from the sheet feeding device  5  to the ejecting device  11  through the image forming part  7  and the fixing device  9 . 
     The sheet S fed from the sheet feeding cassette  3  by the sheet feeding device  5  is conveyed along the conveyance path  15  to the image forming part  7 . At the image forming part  7 , the toner image is formed on the sheet S. The sheet S is conveyed along the sheet conveyance path  15  to the fixing device  9 . At the fixing device  9 , the toner image is fixed on the sheet S. The sheet S on which the toner image is fixed is ejected by the ejecting device  11  on the ejected sheet tray  13 . 
     Next, with reference to  FIG. 2  and  FIG. 3 , a structure of the fixing device  9  will be described.  FIG. 2  is a cross sectional view showing the fixing device  9  and  FIG. 3  is a bottom view showing a flat heater  23 . The fixing device  9  includes a rotatable cylindrical fixing belt  21 , a flat heater  23 , a holder  25  and a pressing roller  27 . The flat heater  23  heats the fixing belt  21 . The holder  25  holds the flat heater  23  to come into contact with an inner circumferential face of the fixing belt  21 . The fixing belt  21  is put between the pressing roller  27  and the flat heater  23 , and a pressing area N through which the sheet S is conveyed is formed between the pressing roller  27  and the fixing belt  21 . The pressing roller  27  drives the fixing belt  21  to rotate it. Hereinafter, an axial direction X shows an axial direction (the front-and-rear direction) of the pressing roller  27 . The present embodiment shows an example where the fixing device  9  is arranged in a posture that the pressing roller  27  is positioned below the fixing belt  21 ; however, the fixing device  9  may be arranged in any posture. 
     The fixing belt  21  is a cylindrical endless belt long in the axial direction X, and has a predetermined inner diameter and a longitudinal length longer than a width of the sheet S. The fixing belt  21  is made of flexible material, and has a base layer, an elastic layer provided around an outer circumferential face of the base layer and a release layer provided around an outer circumferential face of the elastic layer. The base layer is made of metal, such as stainless steel or Ni alloy. The elastic layer is made of silicon rubber. The release layer is made of PFA tube. On an inner circumferential face of the base layer, a sliding layer may be provided. The sliding layer is made of PTFE or polyimide-amid resin. 
     A stay  24  is penetrated through a hollow space of the fixing belt  21 , and its end portions are fixed to a housing (not shown) of the fixing device  9 . The stay  24  is made of metal, such as stainless steel or Al alloy. The fixing belt  21  is supported by an arc-shaped belt guide (not shown) supported by the stay  24  and rotatable around the belt guide. 
     The flat heater  23  has a holding body  231  and a heating resistor  232 . The holding body  231  is formed in an almost rectangular plate shape long in the axial direction X, and made of material having an electric insulating property and a low thermal conductivity, such as ceramic or glass. The heating resistor  232  is formed in an almost rectangular plate shape long in the axial direction X, and made of material having an electric conductivity, such as metal. The holding body  231  has a longitudinal length longer than the longitudinal length of the fixing belt  21  and the heating resistor  232  has a longitudinal length shorter than the longitudinal length of the fixing belt  21 . On a bottom face of the holding body  231 , a recess  233  having a shape corresponding to a shape of the heating resistor  232  is formed. The heating resistor  232  is embedded in the recess  233 . Both longitudinal end portion of the heating resistor  232  are positioned inside both longitudinal end portions of the holding body  231 . On the bottom face of the holding body  231 , a protect layer (not shown) is formed in order to protect the heating resistor  232  and the holding body  231 . The protect layer is made of glass. 
     The holder  25  is a member having almost the same longitudinal length as the longitudinal length of the fixing belt  21 , and is fixed to the stay  24 . The holder  25  is made of heat resistant resin, such as liquid crystal polymer. The flat heater  23  is supported by a lower portion of the holder  25  with the heating resistor  232  facing downward. The holder  25  holds the flat heater  23  such that the bottom face (the face in which the heating resistor  232  is embedded) brings contact with the inner circumferential face of the fixing belt  21 , both the longitudinal end portions of the holding body  231  are positioned outside both the longitudinal end portions of the fixing belt  21  and both the longitudinal end portions of the heating resistor  232  are inside both the longitudinal end portions of the fixing belt  21 . 
     The pressing roller  27  includes a core metal, an elastic layer provided around an outer circumferential face of the core metal and a release layer provided around an outer circumferential face of the elastic layer. The elastic layer is made of silicon rubber. The release layer is made of PFA tube. The pressing roller  27  is supported to be pressed against the flat heater  23  via the fixing belt  21 . The pressing roller  27  is driven by a drive source  28 , such as a motor, to be rotated. 
     A fixing operation of the fixing device  9  having the above configuration will be described. When the pressing roller  27  is driven to be rotated in a predetermined rotational direction Z, the fixing belt  21  is driven by the pressing roller  27  to be rotated in a rotational direction Y counter to the rotational direction of the pressing roller  27 , and the inner circumferential face of the fixing belt  21  is slid with respect to the flat heater  23 . When electric power is supplied to the flat heater  23 , the fixing belt  21  is heated. After the fixing belt  21  is heated to a predetermined temperature, the sheet S on which the toner is transferred is conveyed to the pressing area N. At the pressing area N, the sheet S is put between the fixing belt  21  and the pressing roller  27  and then conveyed. At this time, the toner is heated and pressed by the fixing belt  21  to be fixed on the sheet S. The sheet S on which the toner is fixed is separated from the fixing belt  21  and then conveyed along the conveyance path  15 . 
     Next, by employing a first to an eighth embodiments 1 to 8, a groove  235  formed in the flat heater  23  will be described. 
     A First Embodiment 
     With reference to  FIG. 3  to  FIG. 5 , the first embodiment will be described.  FIG. 4  is a bottom view showing the flat heater  23 .  FIG. 5  is a cross sectional view along a I-I line in  FIG. 4 . Here, a groove  235  formed in a front portion of the flat heater  23  will be described; the groove  235  formed in a rear portion has the same structure as that formed in the front portion, except whose front-and-rear direction is inverted. 
     In the first embodiment, of a contact area of the flat heater  23  with the inner circumferential face of the fixing belt  21 , an area (hereinafter, called an outside area A) outside the heating resistor  232  in the longitudinal direction is formed with the groove  235 . The groove  235  is formed to be closer to a center side of the holding body  231  toward a downstream side in the rotational direction Y of the fixing belt  21 . Thereby, a lubricant applied between the fixing belt  21  and the flat heater  23  flows toward the center side of the holding body  231  as it flows toward the downstream side in the rotational direction Y of the fixing belt  21 . 
     Specifically, as shown in  FIG. 4 , the outside area A is a rectangular area surrounded by a left end  231 L and a right end  231 R of the holding body  231 , a front end  21 Fr of the fixing belt  21  and a straight line containing a front end  232 Fr of the heating resistor  232 , of the bottom face of the holding body  231 . The groove  235  is formed to extend linearly from near a right front apex to near a left rear apex of the outside area A. An upstream side end portion and a downstream side end portion of the groove  235  in the rotational direction Y are opened to an upstream side face and a downstream side face of the holding body  231 . Hereinafter, a plan shape of the groove  235  shown in  FIG. 4  is called a 1 type. 
     As shown in  FIG. 5 , the groove  235  has edge portions  235   a  and  235   b  each formed in a wall shape perpendicular to the bottom face of the holding body  231  and a bottom face  235   c  parallel to the bottom face of the holding body  231 . Hereinafter, a cross sectional shape of the groove  235  shown in  FIG. 5  is called a box type. 
     According to the first embodiment, the lubricant applied between the fixing belt  21  and the flat heater  23  in the outside area A flows toward the downstream side in the rotational direction Y by the rotation of the fixing belt  21 . At this time, a part of the lubricant enters the groove  235  and then flows toward the center side of the holding body  231  along the groove  235 . Accordingly, compared with a case having no groove  235 , an amount of the lubricant which flows on an area outside the groove  235  in the longitudinal direction of the holding body  231  is decreased. Then, according to the first embodiment, compared with a case having no groove  235 , it becomes possible to inhibit a leakage of the lubricant from the end portion of the inner circumferential face of the fixing belt  21 . As a result, a situation hardly occurs, where the leaked lubricant goes around the outer circumferential face of the fixing belt  21  and it becomes difficult to convey the sheet S. 
     Because the groove  235  is formed in the area outside the heating resistor  232  in the longitudinal direction of the holding body  231 , of the contact area of the flat heater  23  with the inner circumferential face of the fixing belt  21 , a temperature unevenness hardy occurs. On the other hand, when the groove  235  is formed on the protect layer provided on the surface of the heating resistor  232 , the temperature unevenness may occur. 
     A Second Embodiment 
     With reference to  FIG. 6 , the second embodiment will be described.  FIG. 6  is a bottom view showing the flat heater  23 . In the second embodiment, a plurality of grooves  235  and  236  is disposed side by side in the longitudinal direction of the holding body  231 . Specifically, a plurality of grooves  236  each having the same configuration as that of the groove  235  is formed in the outside area A, in addition to the groove  235  of the first embodiment. The plurality of grooves  236  is formed to be closer to the center side of the holding body  231  than the groove  235 . The grooves  235  and  236  are formed in parallel to each other. The upstream side end portion of the groove  236  in the rotational direction Y is opened to the upstream side face of the holding body  231 ; the downstream side end portion of the groove  236  in the rotational direction Y is not opened to the downstream side face of the holding body  231 . Hereinafter, a plan shape of the groove shown in  FIG. 6  is called a 2 type. A cross sectional shape of the groove  236  is the same box type as the groove  235 . 
     According to the second embodiment, a part of the lubricant enters the grooves  235  and  236  and then flows toward the center side of the holding body  231  along the grooves  235  and  236 . Accordingly, compared with the first embodiment, an amount of the lubricant which flows on the area outside the groove  235  in the longitudinal direction of the holding body  231  is decreased. Then, according to the second embodiment, compared with the first embodiment, it becomes possible to inhibit the leakage of the lubricant from the end portion of the inner circumferential face of the fixing belt  21 . 
     A Third Embodiment 
     With reference to  FIG. 7  and  FIG. 8 , the third embodiment will be described.  FIG. 7  is a bottom view showing the flat heater  23 .  FIG. 8  is cross sectional views along a II-II line and a line in  FIG. 7 . A groove  237  in the third embodiment is formed in a right angled triangle whose width becomes narrower toward the downstream side in the rotational direction Y of the fixing belt  21 . Of an inside edge portion  237   a  close to the heating resistor  232  and an outside edge portion  237   b  far from the heating resistor  232  of the groove  237 , the outside edge portion  237   b  is formed to be closer to the center side of the holding body  231  toward the downstream side in the rotational direction Y of the fixing belt  21 . 
     Specifically, as shown in  FIG. 7 , the outside edge portion  237   b  is formed to extend linearly from near the right front apex to near the left rear apex of the outside area A. On the other hand, the inside edge portion  237   a  is formed to be linearly parallel to the longitudinal end face of the heating resistor  232 . Hereinafter, a plan shape of the groove shown in  FIG. 7  is called a 3 type. 
     As shown in  FIG. 8 , the inside edge portion  237   a  and the outside edge portion  237   b  each are formed in a wall shape perpendicular to the bottom face of the holding body  231 . The groove  237  has a bottom face  237   c  parallel to the bottom face of the holding body  231 . A distance between the inside edge portion  237   a  and the outside edge portion  237   b  becomes shorter toward the downstream side in the rotational direction Y of the fixing belt  21 . Hereinafter, a cross sectional shape of the groove shown in  FIG. 8  is called a box type (a 3 type). An upstream side end portion and a downstream side end portion of the groove  237  in the rotational direction Y are opened to the upstream side face and the downstream side face of the holding body  231 . 
     According to the third embodiment, a part of the lubricant enters the groove  237  and then flows to the center side of the holding body  231  along the groove  237 . Because the groove  237  is formed in an area including the grooves  235  and  236  in the second embodiment, according to the third embodiment, it becomes possible to inhibit the leakage of the lubricant from the end portion of the inner circumferential face of the fixing belt  21  more than the second embodiment. 
     A Fourth Embodiment 
     With reference to  FIG. 9 , the forth embodiment will be described.  FIG. 9  is a bottom view showing the flat heater  23 . In the fourth embodiment, a plurality of grooves  238  each curved toward the center side of the holding body  231  is disposed side by side in the longitudinal direction of the holding body  231 . 
     Specifically, the groove  238  is formed to be closer to the center side of the holding body  231  toward the downstream side in the rotational direction Y of the fixing belt  21 . The groove  238  is formed in a curve line curved toward the center side of the holding body  231 , and its curvature becomes large toward the downstream side in the rotational direction Y of the fixing belt  21 . Upstream side end portions of the grooves  238  in the rotational direction Y penetrate through the holding body  231 . Of the grooves  238 , the downstream side end portion of the outermost groove  238  in the rotational direction Y is opened to the downstream side face of the holding body  231 , and the downstream side end portions of the other grooves  238  in the rotational direction Y are not opened to the downstream side face of the holding body  231 . Hereinafter, a plan view of the groove  238  shown in  FIG. 9  is called a 4 type. A cross section of the groove  238  is the same box type as that of the groove of the first embodiment 1. 
     According to the fourth embodiment, because the groove  238  is curved toward the center side of the holding body  231 , the flowing of the lubricant entered the groove  238  toward the center side of the holding body  231  is promoted compared with the second embodiment. Furthermore, because the groove  238  has a length longer than the length of the grooves  235  and  236 , an amount of the lubricant entered the groove  238  is larger than that in the second embodiment. Accordingly, it becomes possible to inhibit the leakage of the lubricant from the end portion of the inner circumferential face of the fixing belt  21  more than the second embodiment. 
     A Fifth Embodiment 
     With reference to  FIG. 4  and  FIG. 10 , the fifth embodiment will be described.  FIG. 10  is a cross sectional view along a I-I line in  FIG. 4 . In the fifth embodiment, the cross section of the groove  235  in the first embodiment is changed into a cross section shown in  FIG. 10 . In the fifth embodiment, the groove  235  is formed to become shallower toward the center side of the holding body  231  in a direction crossing to the longitudinal direction of the groove  235 . 
     Specifically, of the inside edge portion  235   a  close to the heating resistor  232  and the outside edge portion  235   b  far from the heating resistor  232  of the groove  235 , the outside edge portion  235   b  is formed to a wall shape perpendicular to the bottom face of the holding body  231 . From a lower end of the outside edge portion  235   b , an inclined face  235   d  upwardly to the center side of the holding body  231  is formed. The inclined face  235   d  reaches the bottom face of the holding body  231 , and the inside edge portion  235   a  has no step. Hereinafter, a cross sectional shape of the groove  235  shown in  FIG. 10  is called a wedge type. 
     According to the fifth embodiment, the lubricant applied between the fixing belt  21  and the flat heater  23  in the outside area A flows toward the downstream side in the rotational direction Y by the rotation of the fixing belt  21 . At this time, a part of the lubricant enters the groove  235  and then flows toward the center side of the holding body  231 . Because the groove  235  is formed to become shallower toward the center side of the holding body  231  in the direction crossing to the longitudinal direction of the groove  235 , as compared with the first embodiment, the lubricant entered the groove  235  easily overflows to the center side of the holding body  231  and the flowing of the lubricant toward the center side of the holding body  231  is promoted. Then according to the fifth embodiment, compared with the first embodiment, it becomes possible to inhibit the leakage of the lubricant from the end portion of the inner circumferential face of the fixing belt  21 . 
     A Sixth Embodiment 
     With reference to  FIG. 6  and  FIG. 10 , the sixth embodiment will be described. In the sixth embodiment, the cross sectional shape of each of the grooves  235  and  236  in the second embodiment is changed into the cross sectional shape shown in  FIG. 10 . The groove  235  is formed to become shallower toward the center side of the holding body  231  in the direction crossing to the longitudinal direction of the groove  235 . The groove  236  is formed to become shallower toward the center side of the holding body  231  in the direction crossing to the longitudinal direction of the groove  236 . According to the sixth embodiment, compared with the second embodiment, the flowing of the lubricant toward the center side of the holding body  231  is promoted. Then, according to the sixth embodiment, compared with the second embodiment, it becomes possible to inhibit the leakage of the lubricant from the end portion of the inner circumferential face of the fixing belt  21 . 
     A Seventh Embodiment 
     With reference to  FIG. 7  and  FIG. 11 , the seventh embodiment will be described.  FIG. 11  is cross sectional views along a II-II line and along a III-III line in  FIG. 7 . In the seventh embodiment, the cross sectional shape of the groove  237  in the third embodiment is changed into a cross sectional shape shown in  FIG. 11 . In the seventh embodiment, the groove  237  is formed to become shallower toward the center side of the holding body  231  in a direction crossing to a longitudinal direction of the groove  237 . 
     Specifically, of the inside edge portion  237   a  close to the heating resistor  232  and the outside edge portion  237   b  far from the heating resistor  232  of the groove  237 , the outside edge portion  237   b  is formed to a wall shape perpendicular to the bottom face of the holding body  231 . From a lower end of the outside edge portion  237   b , an inclined face  237   d  upwardly to the center side of the holding body  231  is formed. The inclined face  237   d  reaches the bottom face of the holding body  231 , and the inside edge portion  237   a  has no step. Hereinafter, a cross sectional shape of the groove  237  shown in  FIG. 11  is called a wedge type (a 3 type). According to the seventh embodiment, as compared with the third embodiment, the flowing of the lubricant toward the center side of the holding body  231  is promoted. Then, according to the seventh embodiment, compared with the third embodiment, it becomes possible to inhibit the leakage of the lubricant from the end portion of the inner circumferential face of the fixing belt  21 . 
     A Eighth Embodiment 
     With reference to  FIG. 9  and  FIG. 10 , the eighth embodiment will be described. In the eighth embodiment, the cross sectional shape of the groove  238  in the fourth embodiment is changed into the cross sectional shape shown in  FIG. 10 . The groove  238  is formed to become shallower toward the center side of the holding body  231  in the direction crossing to the longitudinal direction of the groove  238 . According to the eighth embodiment, compared with the fourth embodiment, the flowing of the lubricant toward the center side of the holding body  231  is promoted. Then, according to the eighth embodiment, compared with the fourth embodiment, it becomes possible to inhibit the leakage of the lubricant from the end portion of the inner circumferential face of the fixing belt  21 . 
     &lt;Result of Experiment&gt; Next, with reference to  FIG. 12 , a result of experiment of the first to the eighth embodiments will be described.  FIG. 12  is a table showing the result of the experiment. A comparative example shown in  FIG. 12  shows a result of an experiment in a case where the groove is not formed in the outside area A. A circumferential speed of the fixing belt  21  is set to 400 mm/sec, and after continuous driving of the fixing belt  21  for 5, 10 and 20 hours, a degree of the leakage of the lubricant from the end portion of the fixing belt  21  is judged. A surface temperature of the fixing belt  21  during the continuous driving is set to about 175° c. At an initial state before the continuous driving, a lubricant of 0.7 g is applied on the flat heater  23  uniformly. As the lubricant, fluorine-based grease is employed. 
     The degree of the leakage of the lubricant is evaluated based on three steps. A step “0” shows no leakage of the lubricant or a degree where the leakage of the lubricant is not affected on the conveyance of the sheet. A step “A” shows a degree where the leakage of the lubricant begins to be affected on the conveyance of the sheet. A step “x” shows a degree where the conveyance of the sheet becomes difficult owing to the leakage of the lubricant. In Comparative Example, the steps of the degrees of the lubricant after 5, 10 and 20 hours are judged as “x”. In the first embodiment, the step of the degree of the leakage of the lubricant after 5 hours is judged as “0”; however, that after 10 and 20 hours are judged as “x”. In the second to the fourth embodiments, the steps of the degrees of the leakage of the lubricant after 5 and 10 hours are judged as “0”; however, that after 20 hours is judged as “A”. In the sixth to the eighth embodiments, the steps of the degrees of the leakage of the lubricant after 5, 10 and 20 hours are judged as “o”. 
     Compared the first embodiment with Comparative Example, it was confirmed that the groove of the present embodiment has an effect to inhibit the leakage of the lubricant. Next, compared the first to the fourth embodiments each other, each having the cross sectional shape of the box type, it was confirmed that the 2 to 4 types have the effect to inhibit the leakage of the lubricant higher than the 1 type. Next, compared the fifth to the eighth embodiments each other, each having the cross sectional shape of the wedge type, it was confirmed that the 2 to 4 types have the effect to inhibit the leakage of the lubricant higher than the 1 type. Next, compared with the first embodiment with the fifth embodiment, each having the plan shape of the 1 type, it was confirmed that the wedge type has the effect to inhibit the leakage of the lubricant higher than the box type. Next, compared the second embodiment with the sixth embodiment, each having the plan shape of the 2 type, compared the third embodiment with the seventh embodiment, each having the plan shape of the 3 type, and compared the fourth embodiment with the eighth embodiment, each have the plan shape of the 4 type, it was confirmed that the wedge type has the effect to inhibit the leakage of the lubricant higher than the box type. As described above, it was confirmed that the 2 to 4 types have the effect to inhibit the leakage of the lubricant higher than the 1 type and the wedge type has the effect to inhibit the leakage of the lubricant higher than the box type. 
     Modified Example 
     The first, the second, the fifth and the sixth embodiments show an example that the upstream and the downstream end portions of the groove  235  in the rotational direction Y are opened to the upstream and the downstream side faces of the holding body  231 . However, as shown in  FIG. 13  (the bottom view showing the flat heater  23 ), the upstream and the downstream end portions of the groove  235  in the rotational direction Y are not always opened to the upstream and the downstream side faces of the holding body  231 . Alternatively, one of the upstream and the downstream end portions of the groove  235  in the rotational direction Y may be opened to one of the upstream and the downstream side faces of the holding body  231 . The grooves  237  of the third embodiment and the seventh embodiment and the grooves  238  closest to the end portion of the fixing belt  21  of the fourth and the eighth embodiments are the same as the above. 
     In the third and the seventh embodiments, of the inside edge portion  237   a  close to the heating resistor  232  and the outside edge portions  237   b  far from the heating resistor  232  of the groove  237 , the outside edge portion  237   b  is formed to be closer to the center side of the holding body  231  toward the downstream side in the rotational direction Y of the fixing belt  21 . However, as shown in  FIG. 14  (the bottom view showing the flat heater  23 ), in addition to the outside edge portion  237   b , the inside edge portion  237   a  may be formed to be closer to the center side of the holding body toward the downstream side in the rotational direction Y of the fixing belt  21 . That is, of the inside edge portion  237   a  and the outside edge portions  237   b  of the groove  237 , at least the outside edge portion  237   b  may be formed to be closer to the center side of the holding body  231  toward the downstream side in the rotational direction Y of the fixing belt  21 . 
     The seventh embodiment shows that the inclined face  237   d  reaches the bottom face of the holding body  231  and the inside edge portion  237   a  has not step. However, as shown in  FIG. 15  (cross sectional views along a II-II line and a line in  FIG. 7 ), the inside edge portion  237   a  may be formed to be a wall shape perpendicular to the bottom face of the holding body  231 . 
     In the above embodiments, as a cross sectional shape of the groove  235 , the box type and the wedge type are described; however, the groove  235  may have a cross section of a U-shape, a V-shape or an arc-shape. 
     The second embodiment shows that the groves  236  and  235  are parallel to each other; however, the grooves  236  and  235  may not be parallel to each other. 
     While the above description has been described with reference to the particular illustrative embodiments, the present disclosure is not limited to the above embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present disclosure.