Abstract:
An image heating apparatus includes a heating rotatable member for heating an image on a recording material at a nip; a belt cooperative with the heating rotatable member to form the nip; an elastic pad and a rigid pad, disposed in the order named along a feeding direction for the recording material, for pressing the belt toward the heating rotatable member at the nip; and urging means for urging the elastic pad and the rigid pad toward the belt, wherein the urging means contacts the rigid pad to the belt earlier than the elastic pad.

Description:
FIELD OF THE INVENTION AND RELATED ART  
       [0001]     The present invention relates to an image heating apparatus employed by an image forming apparatus such as a copying machine, a printer, etc.  
         [0002]     In the field of an image forming apparatus such as an electrophotographic copying machine, a laser beam printer, etc., which is for forming an image, it has been common practice to obtain a fixed image by forming a toner image on the top surface of a recording medium, and then, thermally fixing this toner image to the recording medium. As a fixing means used for above described purpose, there has been available a fixing unit structured to fix an image to a recording medium while conveying the recording medium, convey the recording medium to which the image has just been fixed, and discharge the recording medium into an external delivery tray so that it is laid on top of the recording mediums having accumulated therein.  
         [0003]     Japanese Laid-open Patent Application 2001-818544, for example, discloses a fixing apparatus which comprises a fixing roller, a fixing pad, and a pressure application belt. The fixing apparatus is structured so that the pressure application belt is kept pressed against the fixing roller by the pressure application pad which presses on the rear surface of the pressure application belt. As for its fixing operation, while a recording sheet, on which toner (toner image) is present, is moved through the fixing area, that is, the interface effected by the pressing of the pressure application belt against the fixing roller by the fixing pad, the toner (toner image) is fixed. Generally, a fixing apparatus such as the above described one is structured so that it can switched in operational mode with the use of a lever; it can be put in the normal mode, that is, the pressure application mode, or the no pressure mode. The no pressure mode is the mode for making it easier for a user to remove the jammed paper. Obviously, it is used if a paper jam or the like occurs. More specifically, if a paper jam or the like occurs, a user is to remove the pressure by operating the lever, remove the jammed paper, and then, return the lever to the normal position to put the fixing apparatus back into the original mode, or the pressure application mode.  
         [0004]     However, a fixing apparatus such as the above described one, which is based on the prior art, suffers from the following problem. That is, referring to  FIG. 8 , when a user switches the fixing apparatus in operational mode by operating the unshown lever, from the mode in which the endless belt  118  is not under pressure, to the mode in which the endless belt  118  is under pressure, the fixing pad  113  presses the endless belt  118  toward the fixing roller  112  from inward side of the loop which the endless belt  118  forms. As a result, the fixing pad  113  first presses the endless belt  118  against the fixing roller  112 , generating pressure at the belt contacting surface  113   a , and then, the high rigidity pad  115  presses the endless belt  118  against the fixing roller  112 , generating higher pressure than the pressure generated by the fixing pad  113 .  
         [0005]     Therefore, the leading edge portion  113   b  of the fixing pad  113 , which is located very close to the high rigidity pad  115 , is pinched between the high rigidity pad  115  and endless belt  118  and is pressed while remaining pinched between the high rigidity pad  115  and endless belt  118 , sometimes preventing the belt contacting surface  115   a  of the high rigidity pad  115 , which is expected to come into contact with the endless belt  118  and press the endless belt  118  against the fixing roller  112 , from pressing the endless belt  118  against the fixing roller  112 .  
         [0006]     If the belt contacting surface  115   a  of the high rigidity pad  115  is prevented from pressing the endless belt  118  against the fixing roller  112 , it is impossible for a desired fixing nip to be formed. If the desired fixing nip is not formed, fixation failure occurs. Further, if the belt contacting surface  115   a  is prevented from pressing the endless belt  118  against the fixing roller  112 , the high pressure portion of the desired fixing nip is not formed. Without the high pressure portion, the recording sheet, on which toner is borne, fails to separate from the fixing roller  112 , wrapping itself around the fixing roller  112 . These are the problems which an image heating apparatus based on the prior art suffers.  
         [0007]      FIG. 9  is a graph showing the pressure distribution in the fixing nip. The axis of abscissas of the graph represents the position in the fixing nip in terms of the direction in which recording medium is conveyed, and the axis of ordinates represents the pressure in the fixing nip. The solid line represents the ideal pressure distribution pattern, in which the internal pressure of the fixing nip continuously increases from the entrance of the fixing nip, at which the internal pressure is P 1  (low pressure), toward the exit of the fixing nip, at which the internal pressure is P 2  (high pressure). It should be noted here that P 2  is the amount of pressure necessary to make the recording sheet to separate from the fixing roller  112 ; the high rigidity pad  115  causes the rubber layer of the fixing roller  112  to partially deform, causing thereby the recording sheet to separate from the fixing roller  112 . The double-dot chain line in  FIG. 9  represents the pressure distribution in the fixing nip, which occurs when the leading edge portion  113   b  of the fixing pad  113  remains pinched between the endless belt  118  and high rigidity pad  115 . In this case, the high rigidity pad  115  does not press the endless belt  118  against the fixing roller  112 , failing to generate P 2 , which is necessary to make the recording medium to separate from the fixing roller  112 . Further, with the high rigidity pad  115  prevented from pressing the endless belt  118  against the fixing roller  112 , the resultant fixing nip will be narrower than the desired fixing nip, failing to supply the recording medium with the amount of heat necessary for fixation. Therefore, it is possible that fixation failure will occur.  
       SUMMARY OF THE INVENTION  
       [0008]     The primary object of the present invention is to provide an image heating apparatus which does not suffer from the image heating deficiency attributable to the formation of an unsatisfactory nip.  
         [0009]     Another object of the present invention is to provide an image heating apparatus, the elastic pad of which is not pinched between its rotational heating member and rigid pad.  
         [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 sectional view of the fixing unit in one of the preferred embodiments of the present invention.  
         [0012]      FIG. 2  is a sectional view of the image forming apparatus in the preferred embodiment of the present invention.  
         [0013]      FIG. 3  is a sectional view of the pressure application unit and fixing roller in the preferred embodiment.  
         [0014]      FIG. 4  is a sectional view of the fixing roller, and the pressure application unit kept pressed against the fixing roller, in the preferred embodiment.  
         [0015]      FIG. 5  is a sectional view of the fixing roller, and the pressure application unit separated from the fixing roller.  
         [0016]      FIG. 6  is a side view of the high rigidity block, and its adjacencies, in the preferred embodiment.  
         [0017]      FIG. 7  is a side view of the fixing pad, and its adjacencies, in the preferred embodiment.  
         [0018]      FIG. 8  is a sectional view of the essential portions of the fixing apparatus in accordance with the prior art.  
         [0019]      FIG. 9  is a graph schematically showing the nip pressure distribution of the image heating apparatus. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     Hereinafter, the preferred embodiment of the present invention will be described with reference to the appended drawings. Incidentally, the measurements, materials, and shapes of the structural components, and the positional relationship among the components, which will be described hereafter, are not intended to limit the scope of the present invention, unless specifically noted. Further, if a given component is identical in material, shape, etc., to another component which has already been described, it will not be described, unless specifically noted.  
         [0000]     (Image Forming Apparatus)  
         [0021]     First, an example of a preferable image forming apparatus compatible with an image heating apparatus in accordance with the present invention will be described.  FIG. 2  is a sectional view of the image forming apparatus in the preferred embodiment of the present invention, showing the general structure of the apparatus.  
         [0022]     The image forming apparatus A in this embodiment is an electrophotographic printer (copying machine). Designated by a referential symbol  1  is the main assembly of the image forming apparatus, and designated by a referential symbol  2  is an electrophotographic image forming portion (which hereinafter will be referred to as image forming portion). Designated by a referential symbol  3  is a sheet (recording medium) cassette, as a sheet feeding and conveying portion (which hereinafter will be referred to as cassette). The mechanism of this image forming portion for carrying out the image formation process is the same as the publicly known mechanism of an image forming portion. Therefore, its structure is illustrated in a simplified form.  
         [0023]     The image forming portion  2  carries out an image forming operation based on the image formation information and the print start signal, which are inputted into the control portion (unshown) of the apparatus main assembly  1  from a host apparatus (unshown) such as a computer. As the image forming operation is started, the feeder/conveyer roller  4  of the sheet feeding-and-conveying portion is driven with preset control timing. As a result, recording mediums S are fed from the cassette  3  into the apparatus main assembly  1 , while being separated one by one, and is conveyed further into the main assembly  1 , being guided upward by a conveyance path  5 . Then, each recording medium S is introduced into the image transferring portion of the image forming portion  2  by a pair of registration rollers with a preset timing. In the image transferring portion, a toner image is transferred onto the recording medium  2 . The image transferring portion will be described later.  
         [0024]     The image forming portion  2  is of the tandem type, and employs an intermediary transfer belt. More specifically, the image forming portion  2  is made up of multiple image forming portions  50 Y,  50 M,  50 C, and  50 K, which are aligned in parallel and form monochromatic toner images different in color, one for one. Here, Y, M, C, and K stand for yellow, magenta, cyan, and black colors, respectively.  
         [0025]     The image forming portions  50 Y- 50 K have charging apparatuses  51 Y,  51 M,  51 C, and  51 K, exposing apparatuses  52 Y,  52 M,  52 C, and  52 K, developing apparatuses  53 Y,  53 M,  53 C, and  53 K, and photosensitive members  54 Y,  54 M,  54 C, and  54 K, respectively. The intermediary transfer belt  55  is stretched around a driver roller  56 , a tension roller  57 , and a secondary transfer roller  58 , and is suspended by the rollers. The secondary roller  58  is disposed on the inward side of the loop which the intermediary transfer belt  55  forms. The intermediary transfer belt  55  circularly moves in the direction indicated by an arrow mark in the drawing. As the intermediary transfer belt  55  moves, the monochromatic toner images different in color are sequentially transferred in layers onto the intermediary transfer belt  55  by the primary image transferring apparatuses  59 Y,  59 M,  59 C, and  59 K. Incidentally,  FIG. 2  shows an image forming apparatus, in which the image forming portions for forming the monochromatic toner images are positioned in the order of Y, M, C, and K. However, the order of the image forming portions does not need to be limited to the abovementioned one.  
         [0026]     The multiple monochromatic color toner images on the intermediary transfer belt  55  are transferred all at once by a secondary image transferring apparatus  500  onto the recording medium S delivered thereto from a recording medium feeding portion. The secondary transferring apparatus  500  has a secondary transfer roller  501 , which is on the outward side of the intermediary transfer belt loop, and forms a transfer nip by being pressed against the secondary transfer roller  58 , which is on the inward side of the belt loop. In each transfer nip, the toner image is electrostatically adhered to the recording medium S.  
         [0027]     After the reception of all the toner images by the recording medium S, the recording medium S is introduced into the fixing nip, which is the compression nip between a fixing roller (rotational heating member)  12  and a pressure application unit  13 . Then, as the recording medium S is conveyed through the fixing nip while remaining pinched by the fixing roller  12  and pressure application unit  13 , the toner images are permanently fixed to the recording medium S by the heat from the fixing roller  12  and the pressure in the fixing nip; the toner images on the recording medium S are turned into a single permanent image.  
         [0028]     After coming out of the fixing nip, the recording medium S is guided by a discharge path  10  into an external delivery tray  114 , and accumulated therein. Incidentally, the guides and the like which make up the recording medium conveyance paths are not shown in the drawings to prevent the drawings from becoming complicated.  
       Embodiment  
       [0029]     Next, the fixing unit  11  as one of the examples of an image heating apparatus in accordance with the present invention will be described.  FIG. 1  is a sectional view of the fixing unit in this embodiment.  
         [0030]     The fixing unit  11  has a frame  18 , which supports the fixing roller  12  by its axle. The fixing roller  12  is a rotational heating member, which is heated by an unshown heater. The fixing roller  12  is pressed by the pressure application unit  13 , forming a fixing nip between itself and the pressure application unit  13 . In this fixing nip, the toner images on the recording medium S are subjected to heat and pressure. As a result, the toner images are fixed to the recording medium S.  
         [0031]     The pressure application unit  13  has a stay  23  (pressing means), which is a supporting member (supporting plate). The stay  23  is roughly U-shaped in cross-section, and extends in the direction parallel to the rotational axis of the fixing roller  12 . The stay  23  supports a high rigidity block  19  and a fixing pad  20 . The high rigidity block  19  functions as the high rigidity pad for forming the high pressure portion of the fixing nip, which constitutes the downstream end portion of the fixing nip in terms of the recording medium conveyance direction, whereas the fixing pad  20  is an elastic pad for forming the low pressure portion of the fixing nip, which is the upstream portion of the fixing nip. In other words, the fixing nip is made up of the low pressure portion effected by the elastic pad, and the high pressure portion effected by the rigid pad.  
         [0032]     The pressure application unit  13  has a member  34 , which was welded to the stay  23  in a manner to cover the open side of the stay  23 . The stay  23  is formed by bending a flat piece of metallic plate so that the resultant product has a roughly U-shaped cross section. The stay  23  constitutes the backbone of the pressure application unit  13 .  
         [0033]     The pressure application unit  13  also has an endless belt  27  (which hereafter will be referred to as belt), which is fitted around the aforementioned stay  23 . The pressure application unit  13  is structured so that the endless belt  27  is rotated by the rotation of the fixing roller  12 . The pressure application unit  13  is provided with a pair of belt guides  22  and  33  ( FIG. 3 ), which are disposed on the inward side of the belt loop to ensure that the belt  27  smoothly rotates. Also disposed on the inward side of the belt loop is a piece of felt (unshown) impregnated with silicon oil, as lubricant, to be supplied to the inward surface of the belt  27  to improve the belt  27  in terms of the slipperiness relative to the block  19  and fixing pad  20 .  
         [0034]     Further, the pressure application unit  13  is provided with multiple compression springs  24  as pressure applying means, which are aligned in parallel in the lengthwise direction (which is direction parallel to the rotational axis of fixing roller). The compression springs  24  keeps the fixing pad  20  pressured toward the belt  27 , causing the fixing pad  20  to form the low pressure portion of the fixing nip, which will be described later.  
         [0035]     Further, the pressure application unit  13  is provided with multiple compression springs  25  for keeping the block  19  pressured toward the belt  27 . The compression springs  25  raise the block  19  toward the fixing roller  12  as the fixing pressure is removed. This removal of the fixing pressure will be described later.  
         [0036]     The pressure application unit  13  is supported by a pair of lateral plates  14  (pressure applying means), which are pivotally movable about an axle  15 . The lateral plates  14  keep the pressure application unit  13  pressured toward the fixing roller  12  by being kept pressured by fixing springs  17  (pressure applying means). The lateral plates  14  are located at the lengthwise ends of the pressure application unit  13 , one for one. In other words, the pressure application unit  13  is structured so that not only can it be made to keep the belt  27  pressured upon the fixing roller  12 , but also, it can be made not to pressure the belt  27  upon the fixing roller  12 .  
         [0037]     The fixings springs  17  are strong springs, which are strong enough to generate roughly 500 N of pressure in the fixing nip. They are for forming the fixing nip, and are adjustable in pressure by an adjustment screw  16  to adjust the internal pressure of the fixing nip to a desired value.  
         [0038]     The fixing roller  12  is made up of a cylindrical metallic core, an elastic layer coated on the peripheral surface of the metallic core, and a release layer, as a surface layer, coated on the elastic layer. The metallic core is formed of iron (SUS), aluminum, or the like, and has a thin wall (roughly 1 mm thick). The elastic layer is formed of silicon rubber or the like, and is roughly 0.5 mm in thickness. The release layer is formed of PFA or the like, and is roughly 30 μm in thickness. Within the hollow of the fixing roller  12 , a halogen lamp (unshown) is disposed as a heat source, which is controlled in temperature so that the temperature of the fixing roller  12  remains in the adjacencies of 200 degrees.  
         [0039]     The belt  27  is formed of polyimide or the like resin, and is roughly 90 μm in thickness. It is provided with a roughly 30 μm thick release layer formed of PFA or the like.  
         [0040]     The fixing pad  20  is a first pressure applying portion, which keep the belt  27  pressured toward the fixing roller  12 , and is relatively low in hardness; its hardness is in the range of 15-40 degrees in the rubber hardness scale (HS). The pad  20  is formed of such heat resistant rubber that is capable of withstanding a temperature of roughly 200° so that the pad  20  can satisfactorily performs at this level of temperature. It is integrally attached to a base  21  through the molding process used to form the pad  20 .  
         [0041]     Since the hardness of the fixing pad  20  is in the abovementioned low range, it is easy for the fixing pad  20  to elastically deform. Therefore, as it is pressured against the fixing roller  12 , it generates relatively low pressure (P 1 , which will be described later) across the contact area between itself and fixing roller  12 , perfectly conforming to the curvature of the fixing roller  12 . Further, for the purpose of preventing the pressure applied to the fixing pad  20  to keep the belt  27  pressed upon the fixing roller  12  from the inward side of the belt loop, from escaping, the fixing pad  20  is shaped so that its belt contacting surface matches in shape the peripheral surface of the fixing roller  12 .  
         [0042]     The fixing pad  20  is with a surface layer formed of fluorinated latex film to improve the fixing pad  20  in terms of the slipperiness relative to the belt  27 . Incidentally, providing the fixing pad  20  with the surface layer formed of fluorinated latex prevents the silicon oil from seeping into the rubber portion of the fixing pad  20 , preventing thereby the rubber portion from being made to swell by the silicon oil. Further, providing the fixing pad  20  with the surface layer formed of fluorinated latex improves the fixing pad  20  in terms of the slipperiness relative to the lateral surface  19   a  of the block  19 .  
         [0043]     A pad mount  26 , which supports the fixing pad  20  and base  21 , is provided with a guide  29  ( FIG. 3 ), being rendered slidable in the direction to remove the pressure applied to the fixing pad  20 . As the pressure application unit  13  is moved into the position in which it does not apply pressure to the fixing roller  12 , the pad mount  26  is pressured upward, that is, toward the fixing roller  12 , by the compression springs  24 . However, the movement of the pad mount  26  toward the fixing roller  12  is regulated by a stopper  30 .  
         [0044]     The block  19  is disposed in contact with the fixing pad  20 . It is a second pressure applying member which generates higher pressure in the fixing nip than the pressure which the fixing pad  20  generated in the fixing nip. The block  19  is formed of a metallic substance such as aluminum, stainless steel, or the like, and is preferred to be formed in a single-piece. The metallic surface of the block  19  may be covered with resin such as liquid polymer that is highly rigid and highly heat resistant. In particular, the high rigidity block  19  (high rigidity pad) is required to remain sufficiently rigid and hard even at the fixing temperature (roughly 200° C.). Therefore, when aluminum alloy (#5,000), for example, is used as the material for the block  19 , an aluminum alloy, the hardness of which is greater than 60 HB (in the case of stainless steel, no less than 100 HB) is selected to ensure that the high pressure (P 3  which will be described later) can be generated. In order to ensure the generation of P 3 , the metallic material for the block  19  is required of the above described level of rigidity. In other words, the block  19  is harder than the fixing pad  20 .  
         [0045]      FIG. 7  is a side view of the fixing pad  20 . As described above, the pressure application unit  13  is provided with the multiple compression springs  24 , which are aligned in parallel in the lengthwise direction which is parallel to the lengthwise direction of the fixing pad  20 , with preset intervals, so that the pressure applied to the fixing pad  20  by the compression springs  24  becomes uniform in distribution in terms of the lengthwise direction of the fixing pad  20 . The two sets of guide  29  and stopper  30  are disposed at two locations, one for one, in terms of the lengthwise direction of the fixing pad  20 , so that the fixing pad  20  is kept stable in attitude when the pressure is removed.  
         [0046]      FIG. 6  is a side view of the high rigidity block  19 . The block  19  is provided with a pair of recesses  36 , which are located in the lengthwise end portions of the block  19 , one for one, and in which one end of the compression spring  25  is fitted. The block  19  is also provided with a pair of guides  32 , which are integral parts of the block  19 , enabling the block  19  to slide independently from the fixing pad  20 . The stopper  31  regulates the movement of the block  19  in the pressure applying direction. The only role which the compression springs  25  play is to lift the block  19  when the pressure is removed. While the fixing pressure is applied, the bottom surface  19   d  of the block  19  remains in contact with the top surface of the member  34 . Therefore, the block  19  bears the pressure from the fixing springs  17 , through the stay  23 .  
         [0047]     In other words, according to this embodiment, when the pressure application unit  13  is in such a state that the fixing springs  17  keep the belt  27  pressed upon the fixing roller  12 , the fixing pad  20  generates the aforementioned low pressure by being pressured by the compression springs  24  through the stay  23 , and the block  19  is pressured by the stay  23 . Therefore, the block  19 , which is required to generate the pressure higher than the pressure generated by the fixing pad  20 , can be pressured by stronger fixing springs which are located outside the pressure application unit  13 , being thereby enabled to apply the higher pressure. Therefore, the pressure application unit  13  can be reduced in size.  
         [0048]     The fixing nip formed between the fixing roller  12  and belt  27  has the low pressure portion, that is, the portion which is relatively low in internal pressure, which is formed by the fixing pad  20 , and the high pressure portion, that is, the portion which is relatively high in internal pressure, which is formed by the block  19 . Further, the low and high pressure portions are contiguous. In other words, the pressure application unit  13  is structured so that, in terms of the recording medium conveyance direction, the internal pressure of the fixing nip is lowest at the upstream end, and also, so that the closer to the downstream end, the higher the internal pressure, being at its peak near the downstream end. It should be noted here that for the purpose of forming a wide nip within a limited space, it is effective to employ a fixing pad to apply fixing pressure to form a fixing nip.  
         [0049]     In this embodiment, a wider nip is formed by pressuring the fixing pad  20  against the fixing roller  12  with the belt  27  pinched between the fixing pad  20  and fixing roller  12 . This type of structural arrangement causes the entirety of the fixing roller facing surface of the fixing pad  20  to be pressed against the fixing roller  12 , making it possible to form a wider fixing nip while minimizing the space necessary to form the nip.  
         [0050]      FIG. 9  is a graph schematically showing the pressure distribution in the fixing nip of the image heating apparatus. The axis of abscissas represents the position in the fixing nip in terms of the recording medium conveyance direction, and the axis of ordinates represents the internal pressure of the fixing nip at a given point. The solid line represents the ideal distribution pattern for the internal pressure of the fixing nip. In other words, the distribution of the internal pressure of the fixing nip is desired to be such that the internal pressure is no lower than the low pressure P 1  (0.05-0.2 MPa) and no higher than the high pressure P 3  (0.3-0.5 MPa), and also, such that the closer to the exit of the fixing nip, the higher the internal pressure of the fixing nip, for the following reason. That is, if a given portion of the fixing nip, in terms of the recording medium conveyance direction, is lower in internal pressure than the upstream portion, the pressure applied to the recording medium to fix the toner images temporarily falls while the recording medium is conveyed through the fixing nip. Consequently, a copy suffering from image deviation and/or nonuniformity in glossiness is yielded. Incidentally, one pascal is the SI unit of pressure equal to one newton per square meter.  
         [0051]     The heating of the toner images on the recording medium begins at the entrance of the fixing nip, and the heating temperature is highest at the exit of the nip. Applying high pressure while the toner is in the fully melted condition is an effective pressure application method for better fixation. The pressure P 2  (roughly 0.2 MPa) is the amount of pressure necessary to cause the recording medium S to separate from the fixing roller  12 , that is, the amount of pressure necessary for the high rigidity block  19  to partially deform the rubber layer of the fixing roller  12 . Therefore, in order to enable the image heating apparatus in this embodiment to display the above described image fixing performance, the apparatus is structured so that its fixing nip is provided with the low pressure portion and high pressure portions, which are contiguous.  
         [0052]     If the recording medium S becomes stuck at the fixing unit  11 , an unshown lever is to be rotated to eliminate the fixing nip, in order to make it possible for the recording medium S to be removed. The rotation of the lever causes the pair of lateral plates  14  to rotate in the direction opposite to the direction in which the plates  14  are rotated for the pressure application. As a result, the fixing nip is eliminated against the force generated by the fixing springs  17 .  
         [0053]      FIGS. 3 and 4  are sectional views of the fixing roller  12 , and the pressure application unit  13  kept pressed against the fixing roller  12 . The two drawings are different in the position of sectional plane, in terms of the lengthwise direction of the pressure application unit  13 .  
         [0054]     The block  19  forms the high pressure portion of the fixing nip by being pressed against the fixing roller  12  by the fixing springs  17 , which pressures the entirety of the pressure application unit  13 , through the stay  23 . The compression springs  25  are compressed by the force generated by the fixing springs  17 , so that the bottom surface  19   d  of the block  19  comes into contact with the top surface of the member  34  of the stay  23 . With the bottom surface  19   d  remaining in contact with the stay  23 , the high pressure portion of the fixation remains stable in internal pressure. As for the fixing pad  20 , it is made to form the low pressure portion of the fixing nip, by being pressured by the compression springs  24 .  
         [0055]      FIG. 5  is a sectional view of the fixing roller, and the pressure application unit which is not being pressed against the fixing roller. The fixing nip is eliminated by separating the pressure application unit  13  from the fixing roller  12 . Incidentally, as long as the image heating apparatus is structured to allow the jammed recording medium S to be easily pulled out, it does not need to be structured to allow the pressure application unit  13  to be completely separated from the fixing roller  12 . As the pressure application unit  13  is separated from the fixing roller  12 , the fixing pad  20 , which is on the inward side of the belt loop, is moved upward by the pressure from the compression springs  24 , causing the stopper  30  of the pad mount  26  to come into contact with the bottom surface  23   a  of the stay  23 . At the same time, the block  19  is moved upward by the pressure from the compression springs  25 , causing the stopper  31 , which is an integral part of the block  19 , to come into contact with the bottom surface  23   a  of the stay  23 .  
         [0056]     As for the positional relationship between the leading edge portion  20   a  of the fixing pad  20  and the edge  19   b  of the block  19 , the edge  19   b  remains positioned higher than the leading edge portion  20   a ; the fixing pad  20  never protrudes above the block  19 . That is, the belt contacting surface  20   b  of the fixing pad  20 , as the first pressure application surface, by which the fixing pad  20  presses the belt  27 , the belt contacting surface  19   c  of the block  19 , as the second pressure application surface, by which the block  19  presses the belt  27 , are positioned next to each other, with the presence of a step between the two belt contacting surfaces  20   b  and  19   c . Thus, after the removal of the pressure applied to the belt  27 , the belt contacting surface  19   c  of the block  19  is closer to the fixing roller  12  than the belt contacting surface  20   b  of the fixing pad  20 . In other words, the pressure application unit  13  is structured so that when pressure is applied to the belt  27 , the block  19  comes into contact with the belt  27  before the fixing pad  20 . Therefore, when the pressure application unit  13  is moved from the position in which it is not pressed against the fixing roller  12 , to the position in which it is pressed against the fixing roller  12 , the belt contacting surface  19   c  of the block  19  begins to press the belt  27  toward the fixing roller  12  before the belt contacting surface  20  of the fixing pad  20  does. Then, the belt contacting surface  20   b  of the fixing pad  20  begins to press the belt  27  toward the fixing roller  12  to complete the fixing nip.  
         [0057]     Therefore, the above described problem which an image heating apparatus in accordance with the prior art suffers, that is, the problem that the leading edge portion  20   a  of the fixing pad  20  is pinched by the block  19 , does not occur. Therefore, each time the pressure applying operation is carried out, a pressure nip which is identical to the desired fixing nip, which was initially formed, is formed, regardless of the number of times the combination of the pressure removing operation and pressure applying operation is carried out. In other words, the present invention is effective to improve an image heating apparatus in terms of image fixing performance endurance. Further, the present invention prevents the leading edge portion  20   a  of the fixing pad  20  from being subjected to an excessive amount of pressure, preventing thereby the coating of the fixing pad  20  from peeling. In other words, the present invention extends the service life of the fixing pad  20 ; it can prevent the problem that the amount of pressure which the fixing pad  20  generates is changed by the swelling of the fixing pad  20 , which is caused by the silicon oil.  
         [0058]     Further, as the pressure application unit  13  is moved from the position in which it is not pressed against on the fixing roller  12 , to the position in which it is pressed against the fixing roller  12 , the block  19 , which is harder than the fixing pad  20  and generates higher pressure than the fixing pad  20 , begins to generate pressure before the fixing pad  20 , which is softer than the block  19  and generates low pressure than the block  19  applies, does. Therefore, the problem that the fixing pad  20 , which is softer than the block  19 , is partially pinched between the block  19  and belt  27  does not occur. Therefore, the problem that the desired fixing is not reproduced does not occur. Further, it does not occur that a part or parts of the fixing pad  20  is subjected to an excessive amount of pressure. Therefore, the fixing pad  20  does not deform nor break. In other words, the present invention is effective to extend the service life of an image heating apparatus.  
         [0059]     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.  
         [0060]     This application claims priority from Japanese Patent Application No. 142419/2005 filed May 16, 2005 which is hereby incorporated by reference.