Abstract:
An image heating apparatus includes a heating roller; a belt forming a heating nip; a heating device for heating the heating roller; a controller for controlling a temperature of the heating roller at temperature depending on thickness of sheet; an air feeding device for feeding air to the belt; an executing portion capable of executing an operation in a mode in which the air feeding device feeds the air into between the belt and the heating roller while the belt is spaced from the heating roller with the belt and the heating member being rotating. When a thin sheet is fed following a thick sheet, the executing portion executes the operation in the mode after the thick sheet passes through the nip and before the thin sheet is fed into the nip.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image heating apparatus for heating a toner image on a sheet of recording medium with the use of a nip. In particular, it relates to an image heating apparatus which has a pair of rotational heating members, a pair of circularly movable heating belts, and an air blowing device, and is structured so that both the rotational heating members and circularly movable belts can be separately controlled in temperature from each other, and also, so that the air blowing device is used for preventing the belts from excessively increasing in temperature. 
         [0002]    Image heating apparatuses having a combination of a pair of rotational heating members and a pair of circularly movable belts, which forms a nip for heating a toner image on recording medium have been in use for quite some time. In the case of the image heating apparatuses structured as described above, it is desired that the belt temperature is kept below the temperature of the rotational heating member, in order to prevent the problem that recording medium is given an excessive amount of heat. One of the methods for keeping the belt temperature below the temperature of the rotational heating member is to provide an image heating apparatus with an air blowing device, which is positioned so that it faces the outward surface of the belt, in terms of the loop which the belt forms. With the image heating apparatus being provided with the air blowing device, it is possible to keep the belt temperature below the temperature of the rotational heating member, by blowing air at the belt, during an image forming operation (Japanese Laid-open Patent Application 2006-119430). 
         [0003]    The belt temperature and rotational heating member temperature are desired to be adjusted according to the thickness of recording medium. In other words, in a case where a substantial number of sheets of recording medium, which are different in thickness, are successively conveyed through an image heating apparatus, the belt temperature and rotational heating member temperature have to be changed in temperature according to the recording medium thickness. For example, in a case where a sheet of thin paper is conveyed immediately following a sheet of thick paper, both the belt temperature and rotational heating member temperature have to be lowered. 
         [0004]    Japanese Laid-open Patent Application 2006-119430 discloses a method which quickly reduces a rotational member in temperature range by placing an air blowing device so that the air blowing device faces a belt. More specifically, this method transfers heat from the rotational heating member to the belt by blowing air at the belt with the use of an air blowing device while the rotational heating member and belt are kept in contact with each other. This method, however, increase the belt temperature while reducing the rotational heating member in temperature. Thus, it cannot quickly reduce both the rotational heating member and belt, and therefore, it is possible that the use of this method will increase the length of time it takes to change in temperature both the rotational heating member and belt. 
       SUMMARY OF THE INVENTION 
       [0005]    Thus, the primary object of the present invention is to provide an image heating apparatus which is provided with an air blowing device facing the belt of the image heating apparatus to cool the belt, and is structured so that it can quickly reduce in temperature both its rotational heating member and belt. 
         [0006]    According to an aspect of the present invention, there is provided an image heating apparatus comprising a heating rotatable member; a belt cooperating with said heating rotatable member to form a nip for heating an image on the recording material; a heating device for heating said heating rotatable member; a controller for controlling a temperature of said heating rotatable member at a first temperature when the recording material has a first thickness, and for controlling the temperature at a second temperature which is lower from the first temperature when the recording material has a second thickness which is smaller than the first thickness; an air feeding device for feeding air to said belt during an image heating operation; a moving mechanism for spacing said belt from said heating rotatable member; and an executing portion capable of executing an operation in a mode in which said air feeding device feeds the air into between said belt and said heating rotatable member while said belt is spaced from said heating rotatable member with said belt and said heating member being rotating, wherein when the recording material having the second thickness is fed following the recording material having the first thickness, said executing portion executes the operation in said mode after the recording material having the first thickness passes through the nip and before the recording material having the second thickness is fed into the nip. 
         [0000]    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 
         [0007]      FIG. 1  is a drawing for describing the structure of a typical image forming apparatus to which the present invention is applicable. 
           [0008]      FIG. 2  is a block diagram of the control system of the image forming apparatus. 
           [0009]      FIG. 3  is a drawing for describing the structure of the fixing device. 
           [0010]      FIG. 4  is a drawing for describing the structure of the belt cooling system in the first embodiment of the present invention. 
           [0011]      FIG. 5  is a flowchart of the control sequence for the belt cooling system in the first embodiment. 
           [0012]      FIG. 6  is a drawing for describing the difference in terms of cooling performance (changes in temperature of fixation belt and pressure belt) among the image heating device in the first embodiment, comparative image heating device, and conventional image heating device, which occurred as recording medium was switched from thick paper (cardboard) to thin paper (coated paper). 
           [0013]      FIG. 7  is a drawing for describing the difference in terms of cooling performance (changes in temperature of fixation belt and pressure belt) among the image heating device in the first embodiment, comparative image heating device, and conventional image heating device, which occurred as thin paper was selected as recording medium while the image forming apparatus was kept on standby. 
           [0014]      FIG. 8  is a drawing for describing the structure of the belt cooling system in the first embodiment. 
           [0015]      FIG. 9  is a drawing for describing how the belt cooling system is changed in the cooling area by an airflow direction changing member. 
           [0016]      FIG. 10  is a drawing for describing the cooling effect of the belt cooling system in the second embodiment of the present invention. 
           [0017]      FIG. 11  is a drawing for describing the cooling mode of the fixing device having a fixation roller, instead of a combination of a rotational heating member and a heating belt. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Hereinafter, the embodiments of the present invention are described in detail with reference to the appended drawings. The present invention is applicable to any image heating apparatus (device) as long as the apparatus (device) is structured so that the airflow generated by the air blowing device positioned on the pressure belt side can be made to cool both the rotational heating member and pressure belt, or only the pressure belt, with the use of a mechanism for pivotally moving the belt, even if the apparatus is partially or entirely different in structure from the image heating apparatus in the following embodiments of the present invention. 
         [0019]    The following embodiments of the present invention are described with reference to image heating apparatuses (device) which employ a combination of a heat applying belt (which hereafter will be referred to simply as heat belt), and a pressure applying belt (which hereafter may be referred to simply as pressure belt). However, the present invention is also applicable to image heating apparatuses (devices) which employs a combination of a heat belt and a pressure applying roller (which hereafter may be referred to simply as a pressure roller), and image heating apparatuses (devices) which employ a heat roller and a pressure belt. Further, not only is the present invention applicable to image heating apparatuses (device) as a fixing device, but also, gloss altering apparatuses or the like which are independent from an image forming apparatus. 
         [0020]    The image forming apparatuses in which the image heating apparatus (device) in accordance with the present invention is installable are not limited to those which employ an intermediary transfer belt. That is, the image heating apparatus (device) in accordance with the present invention is also installable in those which directly transfer a toner image onto a sheet of recording medium, those which employ an intermediary transfer drum, those which employ a recording medium conveying belt, or the like image forming apparatus. In the description of the image heating apparatuses (devices) in the following embodiments of the present invention, only the portions of the image forming apparatus, which relate to the primary sections of the apparatus, that is, the sections which relate to the formation and transfer of a toner image, are described. However, the present invention is also applicable to image forming apparatuses other than those in the following embodiments of the present invention. For example, it is also applicable to various printing machines, copying machines, facsimile machines, and the like, which are the combination of one of the image forming apparatuses in the following embodiments of the present invention, and additional devices, equipments, a case (container). 
       &lt;Image Forming Apparatus&gt; 
       [0021]      FIG. 1  is a drawing for describing the structure of a typical image forming apparatus to which the present invention is applicable.  FIG. 2  is a block diagram of the control system of the image forming apparatus. Referring to  FIG. 1 , the image forming apparatus  100  has image formation stations Pa, Pb, Pc and Pd which form yellow, magenta, cyan and black monochromatic toner images, respectively, which are aligned in tandem in the recording medium conveyance direction, and an intermediary transferring member. That is, it is a full-color printer of the tandem type, and also, of the intermediary transfer type. 
         [0022]    In the image formation station Pa, a yellow toner image is formed on a photosensitive drum  3   a , and is transferred onto an intermediary transfer belt  130 . In the image formation station Pb, a magenta toner image is formed on a photosensitive drum  3   b , and is transferred onto the intermediary transfer belt  130 . In the image formation station Pc, a cyan toner image is formed on a photosensitive drum  3   c , and is transferred onto an intermediary transfer belt  130 . In the image formation station Pd, a black toner image is formed on a photosensitive drum  3   d , and is transferred onto the intermediary transfer belt  130 . 
         [0023]    A secondary transfer roller  11  forms a secondary transfer station T 2 , by being placed in contact with the intermediary transfer belt  130  which is backed up by a belt backing roller  14  from within the loop which the belt  130  forms. As a sheet P of recording medium is pulled out of a recording medium cassette  10  ( 10   a  or  10   b ) while being separated from the rest of the sheets P in the cassette  10 , it is sent to a pair of registration rollers  12 , which conveys the sheet P to the secondary transfer station T 2  with such a timing that the sheet P arrives at the secondary transfer station T 2  at the same time as the toner image(s) on the intermediary transfer belt  130 . In the secondary transfer station T 2 , the toner images and sheet P are conveyed in layers while remaining pinched by the secondary transfer roller  11  and intermediary transfer belt  130 . While the combination of the toner images and sheet P is conveyed through the secondary transfer station T 2 , a preset positive DC voltage is applied to the secondary transfer roller  11 . Thus, a full-color toner image (made up of four monochromatic toner images different in color) is transferred (secondary transfer) from the intermediary transfer belt  130  onto the sheet P. A belt cleaning device  19  recovers the transfer residual toner, that is, the toner which failed to be transferred onto the sheet P, and therefore, is remaining on the intermediary transfer belt  130 . 
         [0024]    After the transfer of the four monochromatic toner images, different in color, onto the sheet P of recording medium, the sheet P is separated from the intermediary transfer belt  130  with the utilization of the curvature of the portion of the intermediary transfer belt  130 , which corresponds in position to the belt backing roller  14 . Then, the sheet P is sent into a fixing device  9 , which melts the toner by applying heat to the toner while applying pressure to the toner. Thus, the toner images are fixed to the sheet P. Thereafter, the sheet P is discharged from the main assembly of the image forming apparatus  100  by way of a pair of discharge rollers  73 . 
         [0025]    The image formation stations Pa, Pb, Pc and Pd are practically the same in structure, although they are different in the color (yellow, magenta, cyan and black) of the toner they use. Hereafter, therefore, only the image formation station Pa is described. The description of the image formation stations Pb, Pc and Pd is the same as that of the image formation station Pa except for the suffixes b, c and d which indicate to which image formation station P each component belongs. 
         [0026]    The image formation station Pa comprises a photosensitive drum  3   a , a charge roller  2   a , an exposing device  5   a , a developing device la, a transfer roller  24   a , and a drum cleaning device  4   a . The charge roller  2   a , exposing device  5   a , developing device la, transfer roller  24   a , and drum cleaning device  4   a  are in the adjacencies of the peripheral surface of the photosensitive drum  3   a . The photosensitive drum  3   a  has a negatively chargeable photosensitive layer. It rotates in the direction indicated by an arrow mark at a process speed of 200 mm/sec. 
         [0027]    The charge roller  2   a  uniformly and negatively charges the peripheral surface of the photosensitive drum  3   a  to a preset level VD (which hereafter may be referred to as “dark potential level”). The exposing device  5   a  writes an electrostatic image of the image to be formed. More concretely, it scans the uniformly charged area of the peripheral surface of the photosensitive drum  3   a  with a beam of laser light it outputs while deflecting the beam with its rotational mirror. Thus, the exposed points of the uniformly charged area of the photosensitive drum  3   a  reduce in potential level to a level VL (which hereafter may be referred to as “light potential level”). Consequently, an electrostatic image of the image to be formed is effected on the peripheral surface of the photosensitive drum  3   a . The developing device  1   a  develops the electrostatic image on the photosensitive drum  3   a , into a visible image, that is, an image formed of toner, with the use of two-component developer made up of toner and carrier. 
         [0028]    The transfer roller  24   a  forms, between the photosensitive drum  3   a  and intermediary transfer belt  130 , a transfer station, in which the toner image is transferred onto the intermediary transfer belt  130 . To the transfer roller  24   a , a preset transfer voltage, which is opposite in polarity from the polarity to which toner is charged is applied. As the portion of the peripheral surface of the photosensitive drum  3   a , on which toner is present, moves through the transfer station, the toner (which makes up visible image) is transferred onto the intermediary transfer belt  130 . The drum cleaning device  4   a  recovers the transfer residual toner, that is, the toner which failed to be transferred from the photosensitive drum  3   a , and therefore, is remaining on the photosensitive drum  3   a.    
         [0029]    The image forming apparatus  100  can continuously output prints by sequentially repeating the process of feeding a sheet of recording paper into the main assembly of the image forming apparatus  100 , process of forming an unfixed toner image, process of fixing an unfixed toner image, and process of discharging the sheet of recording medium. It can output  80  full-color prints per minute when recording medium is a sheet of ordinary paper, which is A4 in size. 
         [0030]    Referring to  FIG. 2 , the image forming apparatus  100  has: a control section  141  made up of a microcomputer; and a control panel which functions as an interface for a user to access the image forming apparatus  100 . The control section  141  oversees the image forming operation of the image forming apparatus  100  while observing and controlling the operation of each of the various sections of the image forming apparatus  100 . The control panel  142  is the section through which basic information of a print job (information, such as basis weight of recording medium, density of image to be formed, number of prints to be made, etc.), and/or the detailed setting for a so-called “serial job”, that is, a printing job made up of a serial combination of smaller jobs which are different in recording medium type. 
       &lt;Fixing Device&gt; 
       [0031]      FIG. 3  is a drawing for describing the structure of the fixing device  9 .  FIG. 3(   a ) shows the state of the fixing device  9 , in which the pressure belt is in contact with the heat belt (fixation belt).  FIG. 3(   b ) shows the state of the fixing device, in which a space is present between the heat belt (fixation belt) and pressure belt. 
         [0032]    Referring to  FIG. 3(   a ), the fixing device  9  is made up of a fixation belt  51 , and a pressure belt  52  which is pressed upon the fixation belt  51  to form a heating nip N. The fixation belt  51  is controlled in temperature so that its temperature remains above the melting point of the toner. A sheet P of recording medium, on which toner image(s) is present, is conveyed through the heating nip N while remaining pinched by the fixation belt  51  and pressure belt  52 . Consequently, the toner image(s) on the sheet P is fixed to the sheet P by the heat and pressure applied to the sheet P and toner image(s) thereon, by the fixing device  9 . The pressure belt  52 , which is an example of an endless belt, can form, between itself and fixation belt  51 , the heating hip N for heating the sheet P and the toner image(s) thereon. The fixation belt  51  is positioned above the pressure belt  52 , and its temperature is kept at a higher target level than that for the pressure belt  52 . 
         [0033]    The fixation belt  51  and pressure belt  52  of the fixing device  9  form the heating nip N, which is rectangular in shape, and the widthwise direction of which is parallel to the direction in which a sheet P of recording medium is conveyed. A combination of the fixation belt (heating belt) driving roller  101 , and a stay  105  which is in the form of a pad, and a combination of a pressure roller  102  and a pressure pad  106 , sandwich the portion of the fixation belt  51 , and the portion of the pressure belt  52 , which are within the heating nip N. A sheet P of recording medium is conveyed through the fixing device  9  in the right-to-left direction, while being subjected to heat and pressure, in the heating nip N. Consequently, the toner image(s) on the sheet P is fixed to the surface of the sheet P. 
         [0034]    The fixation belt  51  is supported by the fixation belt driving roller  101 , and a tension roller  103  which functions as a roller for providing the fixation belt  51  with a preset amount of tension, by being wrapped around the two rollers  101  and  103 . The substrate of the fixation belt  51  is an endless metallic belt formed of nickel, and is 50 μm in thickness, 380 mm in width, and 160 mm in length. The substrate is coated with a layer of silicon rubber, which is 400 μm in thickness. The silicon rubber layer is covered with a surface layer, which is made of PFA tube and is 40 μm in thickness. 
         [0035]    The fixation belt driving roller  101  is a hollow roller which is made of a piece of stainless steel pipe. It is 20 mm in external diameter. The tension roller  103  also is a hollow roller which is made of a piece of stainless steep pipe. It is 20 mm in external diameter, and 18 mm in internal diameter. Its lengthwise end portions are under the pressure from a pair of unshown tension springs, providing thereby the fixation belt  51  with a preset amount of tension. 
         [0036]    There is a pressure pad  105  on the inward side of the loop which the fixation belt  51  forms. The pressure pad  105  is formed of stainless steel, and is positioned on the entrance side of the heating nip N so that it opposes the pressure pad  106 . The pressure pad  105  doubles as a heat storing member for preventing the heating nip N from reducing in temperature when a sheet P of recording medium is conveyed through the heating nip N. 
         [0037]    The pressure belt  52  is supported by the pressure roller  102 , and a tension roller  104  which is given the function of providing the pressure belt  52  with a preset amount of tension, by being wrapped around the two rollers  102  and  104 . The substrate of the pressure belt  52  is an endless metallic belt formed of nickel, and is 50 μm in thickness, 380 mm in width, and 172 mm in length. The substrate is coated with a layer of silicon rubber, which is 350 μm in thickness. The silicon rubber layer is covered with a surface layer, which is made of PFA tube and is 40 μm in thickness. 
         [0038]    The pressure roller  102  is a hollow roller which is made of a piece of stainless steel pipe. It is 20 mm in external diameter. The tension roller  104  also is a hollow roller which is made of a piece of stainless steep pipe. It is 20 mm in external diameter, and 18 mm in internal diameter. Its lengthwise end portions are under the pressure from a pair of unshown tension springs, providing thereby the pressure belt  52  with a preset amount of tension. There is the pressure pad  106  on the inward side of the loop which the pressure belt  52  forms. The pressure pad  106  is formed of silicone rubber, and is on the entrance side of the heating nip N. Further, the pressure pad  106  is kept pressed upon the inward surface of the pressure belt  52  with the application of a total pressure of 400 N. 
         [0039]    There is a first heating element  201  in the hollow of the fixation belt driving roller  101 . The rated power of the first heating element  201  is 1,000 W. Further, there is a second heating element  202  in the hollow of the pressure roller  102 , the rated power of the second heating element is also 1,000 W. The fixation belt driving roller  101  and pressure roller  102  are in connection with each other through a pair of gears attached, one for one, to one of the lengthwise ends of the roller  101  and the same lengthwise end of the roller  102 . Thus, the two rollers  101  and  102  rotate with roughly the same peripheral velocity by being driving by an external force. Therefore, the fixation belt  51  and pressure belt  52  circularly move with roughly the same speed whether they are kept in contact with each other, or kept separated from each other. 
       &lt;Pressure Belt Pivoting Mechanism&gt; 
       [0040]    A pressure belt pivoting mechanism  207  can place the pressure belt  52  in contact with the fixation belt  51  or separate the pressure belt  52  from the fixation belt  51 . The mechanism  207  is such a mechanism that can pivotally move the tension roller  104  (which supports pressure belt  52 , on recording medium entrance side of heating nip N) about an axis  111 , which is on the recording medium exit side of the heating nip N. 
         [0041]    The pressure belt  52 , pressure roller  102 , pressure pad  106 , and tension roller  104  are attached to a plate  13 , which is pivotally movable about the axis  111 . Thus, they make up a pressure application unit which can be pivotally moved, along with the plate  113 , about the axis  111 . Further, the fixing device  9  is provided with a pair of pressure application arms  112  and a pair of pivotally movable pressure application plate  113 , the positions of which correspond to the lengthwise ends, one for one, of the pressure roller  102 , are independently and pivotally movable about the axis  111 . The pressure roller  102  and pressure pad  106  are supported by a pressure application plate  114 , and are kept pressed upward by a pair of compression springs  115 . 
         [0042]    The fixing device  9  is also provided with a pressure application cam  120 , which is in contact with the bottom surface of the pivotally movable pressure application plate  113 . Thus, as the pressure application cam  120  rotates, the plate  113  is moved upward or downward, causing the pressure belt  52  to be pressed upon the fixation belt  51  or to be separated from the belt  51 . The pressure application cam  120  is driven by the pressure belt pivoting mechanism  207 , thereby making the pressure belt  52  (supported by pressure pad  106 , and tension roller  104 ) pivot upward or downward about the axis  111 . 
         [0043]    The control section  141  can press the pressure belt  52  upon the fixation belt  51 , or separate the pressure belt  52  from the fixation belt  51 . The amount of distance provided between the pressure belt  52  and fixation belt  51  is optional. The total amount of load applied between the fixation belt  51  and pressure belt  52  as the pressure belt  52  is pressed upon the fixation belt  51  is roughly 800 N (80 kgf). As the pressure belt  52  is pressed upon the fixation belt  51 , the heating nip N is formed, which is rectangular, and the dimension of which in the recording medium conveyance direction is roughly 15 mm. The conventional objectives of the pressure belt pivoting mechanism  207  are to make it easier for a user to deal with paper jam or the like problems, to extend the fixation belt  51  in service life, to prevent the pressure belt  52  from increasing in temperature while no sheet of paper is conveyed through the heating nip N, or the like. 
         [0044]    However, if the temperature of the pressure belt  52  is excessively high when the pressure belt  52  is pressed upon the fixation belt  51 , the moisture in a sheet of coated paper evaporates into steam, and the steam breaks through the coated surface layer of the sheet and erupts from the sheet P. As the steam breaks through the coated surface layer, it disturbs the toner image(s) on the surface of the sheet P, causing a phenomenon called “blistering”. Further, if the temperature of the pressure belt  52  is excessively high, the moisture in a sheet P of recording medium evaporates into steam, and the steam reduces the amount of the friction between the pressure belt  52  and the bottom surface of the sheet P. The reduction in the amount of the friction between the pressure belt  52  and the bottom surface of the sheet P makes the pressure belt  52  and sheet P slip relative to each other, making it possible for the sheet P to be improperly conveyed. Further, if the steam attributable to the evaporation of the moisture in the sheet P settles between the fixation belt  51  and the image bearing surface of the sheet P, it is likely for the fixation belt  51  to float above the image bearing surface of the sheet P, and therefore, it is possible for the image forming apparatus  100  to output images which are nonuniform in gloss. 
         [0045]    Therefore, the image forming apparatus  100  in this embodiment is controlled so that the temperature of the pressure belt  52  is kept substantially lower than that of the fixation belt  51 . Further, while no sheet of recording medium is conveyed, the pressure belt  52  is kept separated from the fixation belt  51  by the pressure belt pivoting mechanism  207  in order to prevent the fixation belt  51  from being reduced in temperature. Therefore, it is ensured that the fixing device  9  can satisfactorily fix the toner image(s) on the sheet P of recording medium to the sheet P while minimizing the amount by which heat is applied to the sheet P. 
       &lt;Temperature Control of Fixing Device&gt; 
       [0046]    Next, referring to  FIG. 2  as well as  FIG. 3 , a temperature control section  200  adjusts the fixation belt  51  in surface temperature, by controlling the amount by which electric power is supplied to the first heating element  201  (heating device), based on the temperature of the fixation belt  51  detected by first temperature detection element  205  which is on the downstream side of the heating nip N and is in contact with the center of the fixation belt  51  in terms of the widthwise direction of the belt  51 . Further, the temperature control section  200  adjusts the pressure belt  52  in surface temperature by controlling the amount by which electric power is supplied to the second heating element  202  (heating device), based on the temperature of the pressure belt  52  detected by the second temperature detection element  206  which is on the downstream side of the heating nip N and is in contact with the center of the pressure belt  52  in terms of the widthwise direction of the belt  51 , and also, controls the air blowing fan  203 . The first and second heating elements  201  and  202  in this embodiment are halogen lamps. However, they may be replaced with heat generating resistors, induction heating elements, or the like. 
         [0047]    As a print job is started, the control section  141  selects a target temperature level for the temperature adjustment of the fixing device  9 , based on the information of a sheet P of recording medium inputted through the control panel  142 , and makes the temperature control section  200  control the fixation belt  51  and pressure belt  52  in temperature, based on the selected target temperature level. Table 1 is a target temperature table for the temperature control of the fixing device  9 . It is to be used when the image on a sheet P of recording medium is heated for fixation while the fixation belt  51  and pressure belt  52  are kept in contact with each other. That is, during an image forming operation, the heating device  9  and air blowing fan  203  are controlled so that the temperature of the fixation belt  51  and that of the pressure belt  52  remain at their target temperature levels, respectively. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                   
                 Job Start 
               
               
                   
                 Target Temp. 
                 Discriminating Temp. 
               
             
          
           
               
                   
                 Basis Wt. 
                 Fixing 
                 Pressing 
                 Fixing 
                   
               
               
                 Materials 
                 (g/m{circumflex over ( )}2) 
                 Belt 
                 Belt 
                 Belt 
                 Pressing Belt 
               
               
                   
               
               
                 Thick 2 
                 181-256 
                 190° C. 
                 100° C. 
                 190° C. 
                 100° C.-120° C. 
               
               
                 Thick 1 
                 106-180 
                 185° C. 
                 100° C. 
                 185° C. 
                 100° C.-120° C. 
               
               
                 Plain 2 
                  91-105 
                 180° C. 
                 100° C. 
                 180° C. 
                 100° C.-120° C. 
               
               
                 Plain 1 
                 64-90 
                 175° C. 
                 100° C. 
                 175° C. 
                 100° C.-110° C. 
               
               
                 Thin 
                 52-63 
                 165° C. 
                 100° C. 
                 165° C. 
                 100° C.-110° C. 
               
               
                 Coated 
                 106-180 
                 170° C. 
                 100° C. 
                 170° C. 
                 100° C.-110° C. 
               
               
                   
               
             
          
         
       
     
         [0048]    Referring to Table 1, the control section  141  controls in temperature the fixation belt  51  and pressure belt  52  by selecting a proper target level for each belt, from among the several levels, according to the recording medium type (basis weight, surface properties, etc.). For paper which is not coated, for example, ordinary printing paper or the like, the target temperature is set to a level which can satisfy both the conveyablility of recording medium (wrinkle prevention, ease of separation, etc.) and image quality (fixation, toner-offset, surface gloss, etc.). In other words, the greater in basis weigh the recording medium, the higher the level to which the target temperature is set. In comparison, for coated paper, that is, paper, the surface layer of which is formed of resin, the target temperature is set to a level which is specific for satisfying not only the basis requirements (conveyability, image quality), but also, for the prevention of the occurrence of such problems as recording medium conveyance error and formation of defective images that are peculiar to coated paper. That is, in order to prevent the amount by which heat is applied to recording medium to heat the image on the recording medium, from becoming excessive, the target temperature for the pressure belt  52  is set lower than that for the fixation belt  51  as shown in Table 1. In order to keep the temperature of the pressure belt  52  at one of its target levels, the control section  141  controls the air blowing fan  203  according to the selected temperature level for the pressure belt  52 . That is, as the temperature of the pressure belt  52  becomes higher than the selected level, the control section  141  operates the air blowing fan  203 , and as the temperature of the pressure belt  52  becomes lower than the selected level, the control section  141  stops the air blowing fan  203 . 
         [0049]    From the standpoint of both the conveyability of recording medium, and image quality, the target temperature for the fixation belt  51  and the job start temperature are set so that the greater in basis weight a sheet P of recording medium, the higher. 
         [0050]    Basically, the target temperature for the pressure belt  52  is set to 100° C. regardless of recording medium type. However, as a substantial number of sheets of recording medium are continuously conveyed through the fixing device  9 , the pressure belt  52  increases in temperature, because the fixation belt  51  comes into contact with the pressure belt  52  during the sheet intervals. Therefore, print job interruption temperature is provided for the pressure belt  52 . If the temperature of the pressure belt  52  reaches the print job interruption temperature, the ongoing image forming operation is interrupted to reduce the pressure belt  52  in temperature, and the image forming apparatus  100  is idled until the pressure belt temperature falls below the print job interruption level. 
         [0051]    Referring to Table 1, in an image forming operation in which recording medium is an ordinary paper  1  (which is small in basis weight) or thin paper, the heat of the pressure belt  52  is likely to transmit to the toner layer through a sheet P of recording medium, and excessively melt the toner layer, because of the thinness of the recording medium. As the toner layer excessively melts, the melted toner is likely to flow along the microscopic hills and valleys of the surface of the recording medium, making thereby the toner image nonuniform in density as the toner image becomes fixed. Therefore, for ordinary paper  1  or thin paper the print job interruption temperature is set to 110° C. Further, as the toner layer on the paper whose surface has numerous microscopic hills and valleys is excessively melted, the toner which is on the microscopic hill portions of the paper flows down to microscopic valley portions of the paper, because the excessively melted toner is very low in viscosity. Consequently, the toner image becomes conspicuously nonuniform in density and gloss, compared to a toner image, the portions of which on the microscopic hills of the sheet of recording medium are the same in the amount of the toner as the portions of which in the microscopic valleys of the sheet of recording medium. Therefore, for coated paper, the print interruption temperature for the pressure belt  52  is set to 110° C., in order to prevent the occurrence of the above described blistering. For recording mediums other than coated paper, the print interruption temperature for the pressure belt  52  is set to 120° C. in order to prioritize the conveyability of the recording medium (wrinkling prevention, ease of separation). 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Target temperature 
               
             
          
           
               
                   
                 Fixing roller 
                 Pressing roller 
               
               
                   
                   
               
               
                   
                 180° C. 
                 100° C. 
               
               
                   
                   
               
             
          
         
       
     
         [0052]    Referring to Table 2, the default setting for the standby target temperature is 180° C. for the fixation belt  51 , and 100° C. for the pressure belt  52 , in order to make it possible to immediately (without any waiting period) start a pending image forming operation when ordinary paper  2 , which is more frequently used than the other type of recording medium, is used as the recording medium. Incidentally, the default setting for the standby target temperature may be named as “target temperature level for default paper”, and displayed as such on the display of the control panel  142 . 
         [0053]    The fixing device  9  is provided with multiple levels of target temperature. Therefore, each time it is switched in the target temperature, a waiting period occurs. The print start temperature is affected by the type and basis weight of recording medium. Thus, as the recording medium is switched in type and/or basis weight, the fixation belt  51  and pressure belt  52  have to be heated or cooled so that their temperatures settle at their print start temperatures. 
         [0054]    In particular, in a case where the fixing device  9  is large in thermal capacity, it takes a substantial length of time for the fixing device  9  to be cooled. Thus, when it becomes necessary for the device  9  to be cooled, a substantial length of waiting time is required after the switching of the target temperature. An image heating device for a high-speed image forming apparatus is structured to be large in thermal capacity in order to be prevented from reducing in temperature while a substantial number of sheets of recording medium are continuously conveyed through the fixing device. Therefore, if the new target temperature level is lower than the immediately preceding target temperature level, it takes more time for the fixing device to reach the new target temperature level, affecting thereby the image forming apparatus  100  in overall productivity, than if the new target temperature level is higher than the immediately preceding target temperature level. 
         [0055]    For example, in a case where the fixing device  9  is switched in target temperature to the level for thin paper from the standby period level shown in Table 2, it takes a substantial length of time for the temperatures of the fixation belt  51  and pressure belt  52  to settle at their new target temperature levels. In other words, a substantial length of downtime occurs, and therefore, the image forming apparatus  100  reduces in productivity. Further, in the case of a “serial job”, that is, a job in which an image formation sequence in which a substantial number of sheets of thin paper are continuously conveyed, and an image formation sequence in which a substantial number of sheets of thick paper are continuously conveyed are alternately carried out, the downtime for cooling occurs each time the image forming apparatus  100  is switched in image formation sequence (recording medium is switched from thick paper to thin paper). Therefore, in the case of a “serial job”, the image forming apparatus  100  is significantly lower in productivity than in a case of a job in which only the ordinary paper  2  is used as the recording medium. The frequent occurrence of the downtime is not desirable from the standpoint of usability. 
         [0056]    One of the conventional methods for cooling the fixation belt  51  is to press the pressure belt  52  upon the fixation belt  51 . 
         [0057]    However, in a case where an image formation sequence in which a substantially number of sheets of thick paper are continuously conveyed is replaced with an image formation sequence in which a substantial number of sheets of thin paper are continuously conveyed, in a “serial job”, for example, it is necessary to cool both the fixation belt  51  and pressure belt  52  as shown in Table 2. When the recording medium is thick paper  2 , the target temperature for the fixation belt  51  is 190° C. (first level), whereas when the recording medium is thin paper  1 , it is 165° C. (second level, which is lower than first level). In a situation such as the above-described one, the conventional method is effective as the method for quickly lowering the temperature of the fixation belt  51 . However, the conventional method increases the temperature of the pressure belt  52  as well. Thus, the overall length of time required to reduce in temperature both the fixation belt  51  and pressure belt  52  to their target levels is rather long. In recent years, from the standpoint of reducing energy consumption, it has been desired to reduce the amount of toner consumption by an image forming apparatus as much as possible while ensuring that image quality is maintained at a conventional level or higher. One of the methods for maintaining image quality while reducing the amount of toner consumption compared to the conventional method is to increase toner in pigment ratio. Because of the recent trend in which it is desired to reduce an image forming apparatus in toner consumption, it has become very important to control the fixation belt  51  and pressure belt  52  in temperature, in particular, to prevent the pressure belt  52  from excessively increasing in temperature. Further, from the standpoint of preventing the problem attributable to the excessive melting of the toner layer, it has become very important to prevent the pressure belt  52  from excessively increasing in temperature, in order to prevent the toner layer from being supplied with an excessive amount of heat from the portions of the pressure belt  52 , which are outside the recording medium path in terms of the lengthwise direction of the belt  52 . 
         [0058]    In this embodiment, therefore, air is blown upon the pressure belt  52  during an image forming operation. Further, while the fixation belt  51  and pressure belt  52  are changed in temperature, air is blown into the space between the fixation belt  51  and pressure belt  52 . That is, the fixing device  9  is provided with an air blowing device for cooling the belts  51  and  52 . Therefore, both the rotational heating members and belts can be quickly reduced in temperature immediately after the switching of the target temperature for the belts  51  and  52 . 
       Embodiment 1 
       [0059]      FIG. 4  is a drawing for describing the structure of the belt cooling system in the first embodiment of the present invention.  FIG. 5  is a flowchart of the operational sequence for controlling the fixation belt  51  and pressure belt  52  in the first embodiment.  FIG. 6  is a drawing for describing how the fixation belt  51  and pressure belt  52  are cooled after the recording medium is switched from thick paper to thin paper. 
         [0060]    Referring to  FIG. 4 , the air blowing fan  203 , which is an example of an air blowing device, is on the pressure belt side of the recording medium passage of the heating nip N. In terms of the recording medium conveyance direction, the air blowing device is on the upstream side of the heating nip N as shown in  FIG. 4 . Moreover, the position of the air blowing device corresponds to the upstream side of the pressure belt in terms of the recording medium conveyance direction. The air blowing fan  203  can be made to cool, with air, the portion of the pressure belt  52 , which is facing opposite from the fixation belt  51 , or the portion of the pressure belt  52 , which is facing the fixation belt  51 , as the tension roller  104  is pivotally moved about the axis  111 . More concretely, as the pressure belt pivoting mechanism  207  separates the pressure belt  52  from the fixation belt  51 , the airflow generated by the air blowing fan  203  moves along the upstream portion of the top portion of the pressure belt  52 , which is on the upstream side of the heating nip N, and reaches fixation belt  51 . That is, the air blowing device sends air through the space between the fixation belt  51  and pressure belt  52 . On the other hand, as the pressure belt pivoting mechanism  207  places the pressure belt  52  in contact with the fixation belt  51 , the airflow which is generated by the air blowing fan  203  and would have reached the fixation belt  51 , is blocked by the pressure belt  52 . Thus, the air blowing fan  203  sends air only to the pressure belt  52 . Further, the fixing device  9  in this embodiment is provided with a member  210  for changing the direction in which the airflow generated by the air blowing fan  203  moves. That is, the member  210  can direct the airflow generated by the air blowing fan  203  toward the pressure belt  52 , or the space between the fixation belt  51  and pressure belt  52 . 
         [0061]    As described above, the fixing device  9  in this embodiment is structured so that (a) not only can the pressure belt  52  be changed in attitude by the pressure belt pivoting mechanism  207 , but also, (b) the airflow generated by the air blowing fan  203  can be changed in direction by an airflow direction changing member  210 , that is, a member for changing the direction of the airflow. 
         [0062]    The control section  141 , which is an example of a controlling means, functions as the section for operating the fixing device  9  in a cooling mode (first cooling mode) in which both the fixation belt  51  and pressure belt  52  are cooled. Further, the control section  141  functions also as the section for operating the fixing device  9  in a cooling mode (second cooling mode) in which only the pressure belt  52  is cooled. In this embodiment, as the target temperature for the fixation belt  51  is lowered, the control section  141  operates the fixing device  9  in the first cooling mode first, and then, operates the fixing device  9  in the second cooling mode. 
         [0063]    Referring to  FIG. 4(   a ), in the first cooling mode, the pressure belt  52  is separated from the fixation belt  51  in order to make the airflow generated by the air blowing fan  203  to be guided to the fixation belt  51  by the upwardly facing portion of the pressure belt  52 , so that both the fixation belt  51  and pressure belt  52  are cooled. That is, in the first cooling mode, air is blown through the space between the fixation belt  51  and pressure belt  52  by the air blowing fan  203 . More concretely, the airflow generated by the air blowing fan  203  flows along the portion of the pressure belt  52 , which faces toward the fixation belt  51 , and moves through the space between the fixation belt  51  and pressure belt  52 . 
         [0064]    Next, referring to  FIG. 4(   b ), in the second cooling mode, the distance by which the pressure belt  52  is separated from the fixation belt  51  is made less than that in the first cooling mode, so that the pressure belt  52  becomes the primary object to be cooled by the airflow generated by the air blowing fan  203 . 
         [0065]    In the first embodiment, the object to which air is sent by the air blowing fan  203  is changed by changing the position of the tension roller  104  (attitude of pressure belt  52 ) with the use of the pressure belt pivoting mechanism  207 . That is, as the mechanism  207  separates the pressure belt  52  from the fixation belt  51 , it becomes possible for the air blowing fan  203  to sent air to the space between the fixation belt  51  and pressure belt  52 . 
         [0066]    Thereafter, the control section  141  controls the movement of the airflow direction changing member  210 . That is, the control section  141  controls the airflow direction changing member  210  in such a manner that as the pressure belt  52  is separated from the fixation belt  51 , the airflow is directed toward the space between the fixation belt  51  and pressure belt  52  by the member  210 . 
         [0067]    Next, referring to  FIG. 5  along with  FIG. 4 , if the target temperature for the pressure belt  52  after the switching of recording medium is different from the actually measured current temperature of the pressure belt  52  (S 1 ), the control section  141  decides whether it is necessary to cool the pressure belt  52  or not (S 2 ). If it is unnecessary to cool the pressure belt  52  (No in S 2 ), the control section  141  does not activate the air blowing fan  203  (S 9 ), and controls the temperature control section  200  to activate the first heating element  201  and second heating element  202  (S 3 ). Then, as soon as the temperature of the fixation belt  51  and the temperature of the pressure belt  52  reach their target level, the control section  141  makes the image forming apparatus  100  start a printing job (S 4 ). This is a temperature increasing process that does not require cooling. Therefore, it takes a relatively short length of time to start the job. 
         [0068]    If it is necessary to cool the pressure belt  52  (Yes in S 2 ), the control section  141  decides whether it is necessary to cool the fixation belt  51  (S 5 ). 
         [0069]    If it is necessary to cool the fixation belt  51  (Yes in S 5 ), the control section  141  pivotally moves the tension roller  104  of the pressure belt  52  about the axis  111  to change the pressure belt  52  in attitude so that a space large enough for the airflow generated by the air blowing fan  203  to flow through is created between the fixation belt  51  and pressure belt  52  as shown in  FIG. 4(   a ) (S 6 ). Then, the control section  141  turns on the air blowing fan  203  (S 8 ) to simultaneously cool both the fixation belt  51  and pressure belt  52 . That is, the pressure belt  52  is kept separated from the fixation belt  51  (presence of large distance between two belts  51  and  52 ), and the airflow generated by the air blowing fan  203  moves between the fixation belt  51  and pressure belt  52 . 
         [0070]    As the fixation belt  51  is cooled enough, that is, it becomes unnecessary to cool the fixation belt  51  (No in S 5 ), the control section  141  pivots the pressure belt  52  about the axis  111  toward the fixation belt  51 , and stops the pressure belt  52  right before the pressure belt  52  comes into contact with the fixation belt  51 , as shown in  FIG. 4(   b ) so that the pressure belt  52  is prevented from being directly heated by the fixation belt  51  (S 7 ). Then, the control section  141  cools only the pressure belt  52  by the air blowing fan  203  while keeping the pressure belt  52  separated from the fixation belt  51  by such a distance (small distance) that can prevent the pressure belt  52  from being heated by the fixation belt  51  (S 8 ). 
         [0071]    Then, as the fixation belt  51  is cooled enough, that is, as it becomes unnecessary to cool the pressure belt  52  (No in S 2 ), the control section  141  stops sending air to the pressure belt  52  (S 9 ), and goes back to the normal temperature control process (S 3 ). Then, it makes the image forming apparatus  100  start the print job (S 4 ). 
         [0072]    The first embodiment is described with reference to a “serial job”, in which a substantial number of prints are continuously outputted with the use of sheets of thick paper, and then, the recording medium is switched to coated paper. Referring to Table 1, the target temperatures for thick paper  2  were 190° C./118° C. (fixation belt/pressure belt). However, as the sheets of thick paper were continuously conveyed through the fixing device  9 , the pressure belt  52  increased in temperature. 
         [0073]    In the case of the fixing device in the first embodiment, the temperatures of the fixation belt  51  and pressure belt  52  right after 200 sheets of thick paper  2  were continuously conveyed through the fixing device  9  were 190° C./118° C. (fixation belt/pressure belt). Referring to Table 1, when the recording medium is coated paper, the target temperatures for the fixation belt  51  and pressure belt  52  are 170° C./110° C. (fixation belt/pressure belt). Therefore, both the fixation belt  51  and pressure belt  52  had to be cooled before it became possible for coated paper to be used as recording medium. 
         [0074]    In the experiment carried out to test the above described fixing apparatus in the first embodiment, a substantial number of sheets of thick paper  2  were continuously conveyed through the fixing device  9  up to a point in time of 0 minute 0 second as indicated by round black dots (bold line) in  FIG. 6 . Then, both the fixation belt  51  and pressure belt  52  began to be cooled at 0 minute 0 second, with the presence of a space between the fixation belt  51  and pressure belt  52  as shown in  FIG. 4(   a ). The temperature of the fixation belt  51  reduced to a target level of 170° C. with the elapse of 21 seconds. Then, the pressure belt  52  was pivotally moved back toward the fixation belt  51  until the distance between the fixation belt  51  and pressure belt  52  became the preset minimum, as shown in  FIG. 4(   b ), and the cooling of the pressure belt  52  was immediately started. At this point in time, however, the temperature of the pressure belt  52  had already reduced to 110° C. Therefore, the image forming operation which uses sheets of coated paper was started at the same time as the temperature of the fixation belt  51  came down to 170° C. 
         [0075]    In this experiment, the conventional method for cooling the fixation belt  51  and pressure belt  52  was also studied. That is, the temperature of the fixation belt  51  was reduced to 170° C. while the pressure belt  52  was kept pressed upon the fixation belt  51 . Then, the pressure belt  52  was separated from the fixation belt  51 , and the temperature of the pressure belt  52  was reduced to its target level of 110° C. 
         [0076]    The result of the conventional method is indicated by multiplication signs in  FIG. 6 . In the case of the conventional method, a substantial number of sheets of thick paper  2  were continuously conveyed through the fixing device  9  up to 0 minute 0 second, and then, the air blowing fan  203  was activated at 0 minute 0 second while the pressure belt  52  was kept in contact with the fixation belt  51 . Thus, heat was removed from the fixation belt  51  by the pressure belt  52  which was being cooled while remaining in contact with the fixation belt  51 . The temperature of the fixation belt  51  reduced to its target level of 170° C. in 11 seconds. However, while the fixation belt  51  was cooled, the pressure belt  52  was kept in contact with the fixation belt  51 , being thereby increased in temperature to 140° C. Consequently, it took additional 20 seconds to reduce the temperature of the pressure belt  52  to its target level. In other words, a total downtime of 30 seconds was necessary to ready the fixing device  9  for fixation. 
         [0077]    If recording medium is switched from thick paper to thin paper immediately after a substantial number of sheets of thick paper are continuously conveyed through the fixing device  9 , it is necessary to cool both the fixation belt  51  and pressure belt  52 . If the conventional method is used in this situation, the pressure belt  52  is increased in temperature while the fixation belt  51  is cooled by the pressure belt  52  which is kept pressed upon the fixation belt  51 . Thus, the amount of time it takes for the temperature of the pressure belt  52  to reach its target level becomes longer, even though the conventional method reduces the amount of time it takes to cool the fixation belt  51 . 
         [0078]    In this experiment, a comparative method for cooling the fixation belt  51  and pressure belt  52  was studied. In the case of the comparative method, the pressure belt  52  was cooled while it was kept separated by a minute distance from the fixation belt  51 . In other words, the fixation belt  51  was naturally cooled through the heat radiation therefrom, for the following reason. That is, in the case of the comparative method, only the pressure belt  52  is cooled, with the presence of a minutes distance between the fixation belt  51  and pressure belt  52 . However, the fixation belt  51  is higher in temperature than the pressure belt  52 . Thus, as the supply of electric power to the first heating element  201  is stopped, the fixation belt  51  relatively quickly reduces in temperature. 
         [0079]    The result of the usage of the conventional method is indicated by rhombic signs in  FIG. 6 . In the case of the comparative method, a substantial number of sheets of thick paper  2  were continuously conveyed through the fixing device  9  from 59 minute 30 second to 0 minute 0 second. Then, the air blowing fan  203  was activated at 0 minute 0 second, to remove heat only from the pressure belt  52  while keeping a small distance between the fixation belt  51  and pressure belt  52  as shown in  FIG. 4(   b ). In this case, it took only three seconds for the temperature of the pressure belt  52  to reduced to its target level of 110° C. However, it took 43 seconds for the fixation belt  51  to be cooled to its target temperature level by the natural heat radiation. 
         [0080]    Table 3 is a summary of  FIG. 6 , regarding the lengths of time required for the fixing device  9  (image forming apparatus  100 ) to become ready for an image forming operation in which recording medium is thin paper, immediately after 200 sheets of thick paper  2  were continuously conveyed for image formation. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Cooling method 
                 Cooling durations 
                   
               
             
          
           
               
                   
                 Fixing 
                 Pressing 
                 Fixing 
                 Pressing 
                 Waiting time 
               
               
                   
                   
               
             
          
           
               
                 Emb 1 
                 Fan cooling 
                 21 sec 
                 5 sec 
                 21 sec 
               
             
          
           
               
                 Comp. Ex 
                 No 
                 Fan cooling 
                 43 sec 
                 3 sec 
                 43 sec 
               
               
                 Prior art 
                 Press-contact 
                 Fan cooling 
                 11 sec 
                 30 sec  
                 30 sec 
               
               
                   
               
             
          
         
       
     
         [0081]    Referring to Table 3, in the case of the conventional cooling method, the air blowing fan  203  was used to cool only the pressure belt  52 , whereas in the case of the cooling method in this embodiment, the air blowing fan  203  was combined with the mechanism  207  for pivotally moving the pressure belt  52 , to make it possible to cool both the fixation belt  51  and pressure belt  52 . In the case of the cooling method in the first embodiment, therefore, the fixation belt  51  and pressure belt  52  were simultaneously cooled, which made the cooling method in this embodiment shorter in the total amount of time necessary to reduce the temperatures of the fixation belt  51  and pressure belt  52  to their target levels than the conventional cooling method, and the comparative cooling method in which either the fixation belt  51  or pressure belt  52  is cooled through natural heat radiation. 
         [0082]      FIG. 7  is a drawing for describing the results of an experiment in which the cooling method in this embodiment, comparative cooling method, and conventional cooling method were tested in effectiveness after the thin paper was selected as recording medium while the image forming apparatus was kept on standby. In the case of the cooling method in this embodiment, the pressure belt  52  was pivotally moved as shown in  FIG. 4  to confirm the effectiveness of the cooling method in this embodiment after thin paper was selected as recording medium. 
         [0083]    Referring to Table 2, the default setting for the standby target temperature is 180° C. for the fixation belt  51 , and 100° C. for the pressure belt  52 . Next, referring to Table 1, the referential values for the highest temperature levels at which an image forming operation in which thin paper is recording medium can be started is 165° C./110° C. (fixation belt/pressure belt). Therefore, both the fixation belt  51  and pressure belt  52  had to be cooled before it became possible for thin paper to be used as recording medium. 
         [0084]    The results of the controlling (cooling) method in this embodiment are indicated by round black dots (bold line) in  FIG. 7 . In the case of the control in this embodiment, the image forming apparatus  100  was kept on standby until 0 minute 0 second, and the fixation belt  51  and pressure belt  52  began to be cooled at 0 minute 0 second, with the pressure belt  52  kept separated from the fixation belt  51  as shown in  FIG. 4(   a ). The temperature of the fixation belt  51  reduced to a target level of 165° C. with the elapse of 13 seconds. Then, the pressure belt  52  was placed close to the fixation belt  51  as shown in  FIG. 4(   b ), and the pressure belt  52  was cooled. At this point in time, however, the temperature of the pressure belt  52  had reduced to 100° C. Therefore, the job in which thin paper was used as recording medium was started at the same time as the cooling of the fixation belt  51  was completed. 
         [0085]    The result of the conventional control is represented by the multiplication signs in  FIG. 7 . In the case of the conventional control, the image forming apparatus  100  was kept on standby until 0 minute 0 second, and the cooling of the fixation belt  51  was started at 0 minute 0 second through the pressure belt  52  which was in contact with the fixation belt  51 . As a result, the pressure belt  52  was increased in temperature, requiring no less than 10 seconds to cool the pressure belt  52 . 
         [0086]    The comparative control is represented by rhombic dots. The image forming apparatus  100  was kept on standby until 0 minute 0 second, and only the pressure belt  52  began to be cooled at 0 minute 0 second, with the presence of a small distance between the fixation belt  51  and pressure belt  52 . As for the fixation belt  51 , the electric power supply to the first heating element  201  for the fixation belt  51  was stopped so that the fixation belt  51  was cooled by natural heat radiation. As a result, it took 40 seconds to cool the fixation belt  51 . 
         [0087]    Table 4 is a summary of the lengths of time it took for the controls in this embodiment, comparative control, and conventional control to ready the image forming apparatus  100 , which was kept on standby, for a printing operation which used thin paper as recording medium. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 Cooling method 
                 Cooling durations 
                   
               
             
          
           
               
                   
                 Fixing 
                 Pressing 
                 Fixing 
                 Pressing 
                 Waiting time 
               
               
                   
                   
               
             
          
           
               
                 Emb 2 
                 Fan cooling 
                 13 sec 
                 No 
                 13 sec 
               
             
          
           
               
                 Comp. Ex 
                 No 
                 Fan cooling 
                 40 sec 
                 No 
                 40 sec 
               
               
                 Prior art 
                 Press-contact 
                 Fan cooling 
                  8 sec 
                 20 sec 
                 20 sec 
               
               
                   
               
             
          
         
       
     
         [0088]    Referring to Table 4, even in the case in which the target temperatures were switched while the image forming apparatus  100  was kept on standby, the control in this embodiment simultaneously cooled both the fixation belt  51  and pressure belt  52 . Therefore, the control in this embodiment was substantially shorter in downtime than the comparative and conventional controls which left the cooling of either the fixation belt  51  or pressure belt  52  to natural heat radiation. 
         [0089]    As described above, in the case in which the target temperatures for the fixation belt  51  and/or pressure belt  52  are switched during the execution of a “serial job”, that is, a job made up of a serial combination of small jobs which are different in recording medium, or while the image forming apparatus  100  is kept on standby, the control in this embodiment operates the image forming apparatus  100  in the first cooling mode. Therefore, it is very effectively to cool the fixation belt  51  while preventing the excessive increase in the temperature of the pressure belt  52 , which is one of the causes of the formation of unsatisfactory images by the image forming apparatus  100 . 
         [0090]    In the first cooling mode, the pressure belt  52  is kept separated from the fixation belt  51  by a substantial distance. Therefore, the airflow generated toward the fixing device  9  by the air blowing fan  203  can simultaneously cool both the fixation belt  51  and pressure belt  52  by flowing between the fixation belt  51  and pressure belt  52 . In the second cooling mode, the pressure belt  52  is kept separated from the fixation belt  51   by  only a small distance. Therefore, the airflow generated toward the fixing device  9  by the air blowing fan  203  is concentrated upon the pressure belt  52 , cooling therefore only the pressure belt  52 . That is, in this embodiment, the pressure belt  52  can be changed in attitude to control the distance between the fixation belt  51  and pressure belt  52 . Therefore, both the fixation belt  51  and pressure belt  52 , or only the pressure belt  52 , can be cooled by the air blowing fan  203  without requiring the air blowing fan  203  to be changed in the direction in which the air blowing fan  203  generates airflow. 
         [0091]    Further, in this embodiment, the temperature of the center portion of the fixation belt  51  in terms of the lengthwise direction of the fixation belt  51 , and the temperature of the center portion of the pressure belt  52  in terms of the lengthwise direction of the pressure belt  52 , are detected, and the distance between the fixation belt  51  and pressure belt  52  is controlled by changing the pressure belt  52  in attitude according to the target temperatures of the fixation belt  51  and pressure belt  52 . Therefore, both the fixation belt  51  and pressure belt  52 , or only the pressure belt  52 , can be selectively cooled. Therefore, the control in this embodiment can make the temperature of the fixation belt  51  and that of the pressure belt  52  reach their target levels within the least amount of time, that is, as quickly as possible, within the range of the cooling capacity of the air blowing fan  203 . 
         [0092]    Incidentally, the fixing device  9  in this embodiment is structured so that, first, (a) the pressure belt  52  is changed in attitude by the pressure belt pivoting mechanism  200 , and then, (b) the airflow generated by the air blowing fan  203  is changed in direction with the use of the aforementioned airflow direction changing member  21 . However, this embodiment is not intended to limit the present invention in terms of the structure of the fixing device  9 . For example, the fixing device  9  may be structured so that the airflow can be simply changed in direction, that is, toward the pressure belt  52 , or the space between the fixation belt  51  and pressure belt  52 , (b) by changing the airflow direction by the airflow direction changing member  210 , or (a) by changing the surface of the pressure belt  52  in position with the use of the pressure belt pivoting mechanism  200 . 
       Embodiment 2 
       [0093]    Next, the second embodiment of the present invention is described. However, the features of the fixing device  9  in this embodiment, which are the same in description as the counterparts in the first embodiment, are not described; only the differences of the second embodiment from the first embodiment are described.  FIG. 8  is a drawing for describing the belt cooling system in the second embodiment.  FIG. 9  is a drawing for describing how the airflow generated by the air blowing fan  203  is changed in cooling area.  FIG. 10  is a drawing for describing the cooling effect of the belt cooling system in the second embodiment. The second embodiment can prevent the problem that the portions of the fixation belt  51 , which are out of the recording medium path, from increasing in temperature. Therefore, it can reduce the length of time a user has to wait until the portions of the fixation belt  51 , which are out of the recording medium path cool down. 
         [0094]    During a printing operation, the control section  141  keeps the pressure belt  52  in contact with the fixation belt  51 , and controls the fixation belt  51  in temperature based on Table 1 which shows the target temperature levels for the fixation belt  51  and pressure belt  52  according to recording medium type, with the use of the temperature control section  200 . The temperature control section  200  controls the temperature of the fixation belt  51  based on the temperature level detected by the first temperature detection which is positioned at the center of the fixation belt  51  in terms of the widthwise direction of the fixation belt  51 . Therefore, as a substantial number of sheets of recording medium are continuously conveyed through the fixing device  9 , the widthwise edge portions of the fixation belt  51 , that is, the portions of the fixation belt  51 , which are outside the recording medium path, gradually increase in temperature. As described above, the temperature control section  200  controls the first heating element so that the amount by which the fixation belt  51  is supplied with heat by the first heating element equals the amount by which heat is robbed from the recording medium path portion (center portion) of the fixation belt  51  by the recording medium. Therefore, the widthwise edge portions of the fixation belt  51 , or the out-of-sheet-path portions of the fixation belt  51 , which are not robbed of heat by the sheets of recording medium, are made to increase in temperature by the heat supplied by the first heating element  201 . 
         [0095]    In the second embodiment, therefore, the fixing device  9  is provided with an airflow direction controlling member  208  in addition to the air blowing fan  203  so that while sheets of recording medium are conveyed through the fixing device  9 , the widthwise edge portions of the pressure belt  52  are cooled by the combination of the air blowing fan  203  and airflow direction controlling member  208 , to indirectly cool the out-of-sheet-path portions of the fixation belt  51 , which are in contact with the widthwise edge portions of the pressure belt  52 , in order to prevent the out-of-sheet-path portions of the fixation belt  51  from excessively increasing in temperature. 
         [0096]    Referring to  FIG. 8 , the fixing device  9  in the second embodiment is provided with the airflow direction controlling member  208 , which is fixed in its positional relationship to the pressure belt  52 . The airflow direction controlling member  208 , which is an example of an airflow blocking member, is positioned so that it faces the center range of the pressure belt  52  to reduce the air blowing fan  203  in the ratio of the amount of the air blown toward the center range of the pressure belt  52  in the second cooling mode. That is, the airflow direction controlling member  208  is a structural component of the fixing device  9  in this embodiment, which is for removing heat from the out-of-sheet-path portions of the pressure belt  52 , in the second cooling mode, that is, the cooling mode in which the pressure belt  52  is kept in contact with the fixation belt  51 . 
         [0097]    The airflow direction controlling member  208  is solidly positioned so that its positional relationship relative to the aforementioned pair of pivotally movable plates  113  which are at the lengthwise ends of the rotational axis of the pressure belt  52 , one for one, does not change. The pressure belt  52 , pressure roller  102 , pressure pad  106 , tension roller  104 , and airflow direction controlling member are attached to the pivotally movable plate  113 , making up a pressure application unit which is can be pivoted together with the pivotally movable pressure application plate  113  about the axis  111 . 
         [0098]    Like the pressure belt  52 , pressure roller  102 , second heating element  202  in the pressure roller  52 , and second temperature detection element  206 , the airflow direction controlling member  208  also is attached to the pivotally movable pressure application plate  113 , with the unshown frame formed of metallic plate, making up an integral part of the pressure application unit. A user can place the pressure belt  52  in contact with the fixation belt  51 , or optionally set the distance between the fixation belt  51  and pressure belt  52  by pivotally moving the pivotally movable pressure application plate  113 , with the use of the pressure belt pivoting mechanism  207 . 
         [0099]    Referring to  FIG. 8(   a ), the airflow direction controlling member  208  is positioned so that when the pressure belt  52  is kept separated from the fixation belt  51 , the airflow direction controlling member  208  does not block the airflow generated by the air blowing fan  203  in the direction to flow along the pressure belt  52  to cool the fixation belt  51 . Therefore, the airflow is not blocked by the airflow direction controlling member  208 , reaching thereby both the fixation belt  51  and pressure belt  52  as shown in  FIG. 9(   a ). The cooling effect of the first cooling mode, that is, the mode in which the image forming apparatus  100  is operated in a case where the target temperatures for the fixation belt  51  and/or pressure belt  52  are switched during the “serial job” described in the description of the first embodiment, or while the image forming apparatus  100  is kept on standby, is not lost. 
         [0100]    Next, referring to  FIG. 8(   b ), the airflow direction controlling member  208  is positioned so that while the pressure belt  52  is kept in contact with the fixation belt  51 , the airflow generated by the air blowing fan  203  in the direction of the pressure belt  52  is prevented from hitting the central range of the pressure belt  52  in terms of the widthwise direction of the pressure belt  52 . That is, the airflow direction controlling member  208  is positioned so that while the pressure belt  52  is kept in contact with the fixation belt  51 , the airflow generated by the air blowing fan  203  in the direction of the pressure belt  52  is made to flow on the outward side of the airflow direction controlling member  208  in terms of the widthwise direction of the member  208 , and cools the widthwise edge portions of the pressure belt  52  as shown in  FIG. 9(   b ). Therefore, the problem that the widthwise edge portions of the fixation belt  51 , more specifically, the portions of the fixation belt  51 , which are outside the recording medium path, excessively increase in temperature, can be prevented by operating the image forming apparatus  100  in the second cooling mode, with the air blowing fan  203  activated, to cool the edge portions of the pressure belt  52  in terms of the widthwise direction of the pressure belt  52 . 
         [0101]      FIG. 10(   a ) is a drawing for describing the cooling effect of the second cooling mode in the second embodiment. It shows how effectively the widthwise end portions of the pressure belt  52  were cooled. In the experiment performed to test the effect of the second cooling mode, the temperature distribution of the fixation belt  51  in terms of the direction parallel to the axial line of the fixation belt driving roller  101  was obtained immediately after 1,000 sheets of ordinary paper, which were A4 in size and 200 g in basis weight, were continuously conveyed through the fixing device  9  in the portrait position. Incidentally, when a sheet of ordinary paper, which is A4 in size is conveyed in the portrait position, the portions of the fixation belt  51 , which are outside the recording medium path, are larger, and therefore, are more likely to excessively increase in temperature, than when the sheet is conveyed in the landscape position. 
         [0102]    The conditions under which the experiment was carried out were 350 mm in the width of the opening of the air blowing fan  203 , 140 mm in the width of the airflow direction controlling member  208 , 400 mm in the width of the fixation belt  51 , and 185 mm in the length of the fixation belt  51 . Further, the target temperature for the fixation belt  51  was set to 190° C., and the rated highest temperature level for the fixation belt  51 , which was set based on the expected durability of the fixation belt  51 , was 220° C. Ordinarily, as the detected temperature level of the fixation belt  51  reaches 220° C., the recording medium conveyance is temporarily stopped. Then, it is started as the detected temperature level of the fixation belt  51  falls below 220° C. However, the experiment was for testing the temperature control in the second embodiment. Therefore, in order to accurately evaluate the temperature increase of the fixation belt  51  across its out-of-sheet-path portions, 1,000 sheets of paper were continuously conveyed without temporarily stopping the recording medium conveyance, even when the detected temperature level of the fixation belt  51  exceeded 220° C. 
         [0103]    Given in Table 5 are the results of the experiment in which the control in the second embodiment (which operates the image forming apparatus  100  in the second cooling mode during an image forming operation in which sheets of paper are continuously conveyed), conventional control (which does not activate the air blowing fan  203  during an image forming operation in which sheets of paper are continuously conveyed), and comparative control (which cools the entire surface of the pressure belt  52  by activating the air blowing fan  203 , during an image forming operation in which sheets of paper are continuously conveyed), were tested. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                   
                 Temperature rise 
                   
               
               
                   
                   
                   
                 prevention at end 
                 No. of processed 
               
               
                   
                 Cooling 
                 Max. 
                 portions (1000 
                 sheets up to design 
               
               
                   
                 range 
                 temp. 
                 sheets) 
                 temp. 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Emb. 2 
                 Opposite 
                 212° C. 
                 G 
                 ≧1000 
               
               
                   
                 ends 
               
               
                 Comp. 
                 Whole 
                 226° C. 
                 NG 
                 500 
               
               
                 Ex. 
                 surface 
               
               
                 Prior art 
                 No 
                 224° C. 
                 NG 
                 700 
               
               
                   
               
             
          
         
       
     
         [0104]    Referring to  FIG. 10(   b ), in the case of the second embodiment, the image forming apparatus  100  was operated in the second cooling mode in an image forming operation in which a substantial number of sheets of paper were continuously conveyed. As a result, even in an image forming operation in which 1,000 sheets of paper were continuously conveyed, the temperature of the out-of-sheet-path portions of the fixation belt  51  was prevented from exceeding 212° C. In comparison, in the case of the conventional control which does not activate the air blowing fan  203  during an image forming operation in which a substantial number of sheets of paper are continuously conveyed, the temperature of the out-of-sheet-path portions of the fixation belt  51  reached as high as 224° C. 
         [0105]    Further, in the case of the comparative fixing device which does not have the airflow direction controlling member  208 , and cooled the entirety of the pressure belt  52  by activating the air blowing fan  203 , during an image forming operation in which a substantial number of sheets of paper were continuously conveyed, the out-of-sheet-path portions of the fixation belt  51  became higher in temperature than those of the conventional fixing device, for the following reason. That is, in the case of the comparative fixing device, the central portion of the fixation belt  51 , that is, the portion of the fixation belt  51 , which was being controlled in temperature, was cooled. Thus, the first heating element  201  was increased in load, being thereby made to generate more heat. Consequently, the amount of the heat which the out-of-sheet-path portions of the fixation belt  51  were given also increased. Also in the case of the comparative fixing device, both the temperatures of the fixation belt  51  and pressure belt  52  had to be kept at their target levels. Thus, the comparative fixing device was greater in the amount of electric power consumption than the fixing device in the second embodiment; electrical power was wastefully consumed. 
         [0106]    By the way, in reality, the fixing device  9  is provided with a third temperature detecting element (thermistor), which is positioned in contact with one of the out-of-sheet-path portions of the fixation belt  51 , so that as the detected temperature of the out-of-sheet-path portion reaches 220°, which is the highest level in terms of the temperature rating of the fixation belt  51 , the sheet conveyance is temporarily stopped to idle the image forming apparatus  100  until the out-of-sheet-path portions of the fixation belt  51  cool down to 200° C. When the temperature of the out-of-sheet-path portions of the fixation belt  51  is 220° C., it takes roughly three minutes for the out-of-sheet-path portions of the fixation belt  51  to cool down to 200° C. In other words, roughly 3 minutes are wasted. 
         [0107]    In the case where the image forming apparatus  100  was operated in the second cooling mode, the out-of-sheet-path portions of the fixation belt  51  did not reach 220° C., or the highest temperature level which the fixation belt  51  can withstand from the standpoint of design. Thus, even during an image forming operation in which 1,000 sheets of paper were continuously conveyed through the fixing device, the apparatus  100  was not idled even once for cooling. In comparison, in the case of the conventional fixing device, the temperature of the out-of-sheet-path portions of the fixation belt  51  reached once to 220° C., or the highest temperature level which the fixation belt  51  can with stand from the stand point of its design, and the apparatus  100  had to be idled for roughly 3 minutes for cooling. In the case of the comparative fixing device, the temperature of the out-of-sheet-path portions of the fixation belt  51  reached twice 220° C., or the highest temperature level which the fixation belt  51  can with stand from the standpoint of its design, and the apparatus  100  had to be idled for roughly six minutes; a user had to wait roughly 6 minutes. 
         [0108]    As described above, in the case of the fixing device in the second embodiment, it is provided with the airflow direction controlling member  208 , and the excessive increase in the temperature of the out-of-sheet-path portions of the fixation belt  51  is prevented by operating the image forming apparatus  100  in the second cooling mode, that is, the cooling mode in which the pressure belt  52  is kept in contact with the fixation belt  51 . Thus, the out-of-sheet-path portions of the fixation belt  51  are very effectively prevented from excessively increasing in temperature even during a job in which a substantial number of sheets of recording medium are continuously conveyed. In other words, the second embodiment of the present invention can improve a fixing device (image forming apparatus) in terms of the length of time the image forming apparatus has to be idled (user has to wait) to cool the out-of-sheet-path portions of the fixation belt  51 . 
         [0109]    The above-described experiment proved the effectiveness of the second embodiment of the present invention, that is, the second embodiment can eliminate various problems attributable to the excessive temperature increase which occurs to the out-of-sheet-path portions of the fixation belt  51  during the execution of an image forming apparatus in which a substantial number of sheets of recording paper are continuously conveyed through the fixing device. 
         [0110]    In the case of the fixing devices in the first and second embodiments, the heating nip, in which a sheet of recording medium is heated, is formed by placing the pressure belt  52  in contact with the fixation belt  51  (heating belt). However, the first and second embodiments are not intended to limit the present invention in terms of the structure of a fixing device. For example, the present invention is also effectively applicable to a fixing device structured so that a pressure belt  52 A is placed in contact with a heat roller  51 A ( FIG. 11 ). 
         [0111]    Referring to  FIG. 11(   b ), a heat nip N is formed by pressing the pressure belt  52 A upon the fixation roller  51 A. The fixing device  9 A is structured so that the pressure belt  52 A can be pivotally moved, like the pressure belt  52  in the first embodiment, by the pressure belt pivoting mechanism  207 . In the first cooling mode, the pressure belt  52 A is kept separated from the fixation roller  51 A, and the airflow which moves along the pressure belt  52 A cools the fixation roller  51 A, as shown in  FIG. 11(   a ). In the second cooling mode, the pressure belt  52 A is kept a minute distance away from the fixation roller  51 A, and therefore, the airflow generated by the air blowing fan  203  in the direction of the fixation roller  51 A is blocked by the pressure belt  52 A. 
         [0112]    In the preceding embodiments of the present invention, the image forming apparatus was a color printer of the tandem type, and also, of the intermediary transfer type. That is, the image forming apparatus was structured so that image forming stations were aligned in tandem along the intermediary image bearing member. However, these embodiments are not intended to limit the present invention in terms of the structure of an image forming apparatus. For example, the present invention is also applicable to a color printer of the intermediary transfer/single drum type, which sequentially forms multiple monochromatic images, different in color, on its single image bearing member, and transfers the toner images onto its intermediary transfer member, a color printer of the tandem/direct transfer type, which does not have an intermediary transfer member, and directly transfers multiple monochromatic toner images, different in color, from its image bearing member onto a sheet of recording medium. Moreover, the present invention is also applicable to an image forming apparatus other than a printer. That is, it is applicable to a copying machine, a facsimile machine, etc. 
         [0113]    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. 
         [0114]    This application claims priority from Japanese Patent Application No. 184295/2011 filed Aug. 26, 2011, which is hereby incorporated by reference.