Abstract:
An image forming apparatus for forming an unfixed toner image on a sheet includes a fixing unit including a fixing roller, a back-up member for cooperating with the fixing roller to form a nip therebetween, and a rotatable heating member contactable to a surface of the fixing roller to heat the fixing roller; wherein the sheet is outputted from the apparatus after the unfixed toner image is fixed on the sheet, wherein the apparatus is operable in a cleaning mode for cleaning the fixing unit, wherein when the cleaning mode is selected, a predetermined unfixed toner image is formed on the sheet, and the sheet is fed to the fixing unit, and the toner is transferred from the sheet onto the fixing roller in the fixing nip, and then the toner transferred onto the fixing roller is brought into contact to the heating member, and thereafter, the toner on the fixing roller is transferred onto the sheet in the fixing nip.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a laser beam printer, etc., which has a fixation unit for thermally fixing a toner image (image formed of toner) to a sheet of recording medium. 
         [0002]    A fixation unit having its heat source outside its fixation roller has been proposed as a thermal fixation unit for a copying machine, a printer, and the like. 
         [0003]    This type of fixation unit has: a fixation roller; a pressure roller which is pressed against the fixation roller to form a nip through which a sheet of recording medium is conveyed; and a heating member for heating the peripheral surface of the fixation roller. There are various types of heating member, for example, a heating roller having an internal halogen heater, an endless belt placed in contact with a ceramic heater in such a manner that its inward surface is in contact with the heater, etc. The fixation roller of this type of fixation unit is provided with an elastic layer, which is for ensuring that the fixation unit is provided with a nip (fixation nip) which is wide enough to ensure that the fixation unit is satisfactory in performance in terms of fixation. Further, in order to quickly increase the surface temperature of the fixation roller to a level high enough for satisfactory fixation, the fixation roller is heated from the outward side of its peripheral surface. 
         [0004]    However, a fixation unit, such as the above-described one, the fixation roller of which is heated from the outward side of its peripheral surface, has been problematic in that its heating member (external heating member) is contaminated. 
         [0005]    More specifically, in the case of a fixation unit which employs an external heating member, the offset toner resulting from jamming of recording medium or the like sometimes transfers onto the external heating member. Once the offset toner transfers onto the external heating member, the offset toner on the external heating member is not going to be completely removed by the first sheet of recording medium conveyed after the removal of the jammed sheet, because it is on the external heating member, that is, a member which does not directly come into contact with a sheet of recording medium. Therefore, the transferred toner on the external heating roller irregularly transfers back onto the fixation roller, soiling thereby the image on a sheet of recording medium. 
         [0006]    As one of the means for preventing the occurrence of the above-described problem, it has been proposed to make the external heater slipperier than the fixation roller (Japanese Laid-open Patent Application 2004-15752). 
         [0007]    Making the external heating member slipperier than the fixation roller makes the adhesion between the fixation roller and toner greater than the adhesion between the external heating member and toner. Therefore, even if toner adheres to the fixation roller in the fixation nip, and then, is made to reach the external heating member by the rotation of the fixation roller, it does not adhere to the external heating member, remaining therefore on the fixation roller. Then, the toner remaining of the fixation roller is returned by further rotation of the fixation roller, to the fixation nip, in which it is fixed to a sheet of recording medium. Then, it is discharged with the sheet P. 
         [0008]    However, if the external heating member is made slipperier than the fixation roller as it was according to the prior arts, it is possible that toner and the like will collect on the external heating member. 
         [0009]    When a sheet of recording medium is heated for the fixation by the fixation roller, contaminants such as the paper dust and/or the filler in the sheet of paper adheres to the fixation roller, although it is only by a minute amount. Then, as the fixation roller rotates further, the contaminants come into contact with the external heating member. Even if the external heating member is made slipperier than the fixation roller, paper dust and/or filler in the sheet of paper electrostatically and/or mechanically adheres to the external heating member. If the toner having adhered to the fixation roller adheres to the paper dusts having adhered to the external heating member, the toner mixes with the paper dusts. As the toner mixes with the paper dusts, it loses its adhesiveness. Thus, it fails to transfer the contaminants on the external heating member onto the fixation roller. Consequently, the mixture collects on the external heating member. 
         [0010]    The residual toner on the external heating member, that is, the toner having collected on the external heating member as described above, irregularly transfers back onto the fixation roller, and as it transfers back, it soils the image on the sheet. This has been the problem to be solved. 
         [0011]    The present invention was made in consideration of the above-described problem. Thus, its primary object is to provide an image forming apparatus capable of completely removing the toner having collected on the heating member for heating the fixation roller. 
       SUMMARY OF THE INVENTION 
       [0012]    According to an aspect of the present invention, there is provided an image forming apparatus for forming an unfixed toner image on a sheet; a fixing unit including a fixing roller, a back-up member for cooperating with said fixing roller to form a nip therebetween, and a rotatable heating member contactable to a surface of said fixing roller to heat said fixing roller; wherein the sheet is outputted from said apparatus after the unfixed toner image is fixed on the sheet, wherein said apparatus is operable in a cleaning mode for cleaning said fixing unit, wherein when the cleaning mode is selected, a predetermined unfixed toner image is formed on the sheet, and the sheet is fed to said fixing unit, and the toner is transferred from the sheet onto said fixing roller in said fixing nip, and then the toner transferred onto said fixing roller is brought into contact to said heating member, and thereafter, the toner on said fixing roller is transferred onto the sheet in said fixing nip. 
         [0013]    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 
         [0014]      FIG. 1  is a sectional view of the image forming apparatus having a fixation unit, in the first embodiment of the present invention, and shows the general structure of the apparatus. 
           [0015]      FIG. 2  is a sectional view of the fixation unit in the first embodiment of the present invention, and shows the structure of the unit. 
           [0016]      FIG. 3  is a sectional view of the ceramic heater in the first embodiment of the present invention, and shows the general structure of the heater. 
           [0017]      FIG. 4  is a block diagram which shows the connection among the ceramic heater and the means adjacent thereto. 
           [0018]      FIG. 5  is a schematic drawing for describing the “hot offset” of toner, which occurs in the fixation nip. 
           [0019]      FIG. 6  is a flowchart of the cleaning operation in the first embodiment of the present invention. 
           [0020]      FIG. 7  is a graphic time chart for showing the toner movements or the like in the first embodiment. 
           [0021]      FIG. 8  is an example of the pattern of the toner image for cleaning in the first embodiment. 
           [0022]      FIG. 9  is an example of the modification of the toner image for cleaning, in the first embodiment. 
           [0023]      FIG. 10  is a schematic drawing for describing the process for transferring the contaminant toner on the heating film, onto the fixation roller. 
           [0024]      FIG. 11  is a schematic drawing for describing the process for transferring the contaminant toner on the fixation roller, onto a sheet of recording medium. 
           [0025]      FIG. 12  is a drawing of an image on a sheet of recording medium after the completion of the cleaning of the fixation unit in the first embodiment. 
           [0026]      FIG. 13  is a drawing which shows the relationship between the temperature of an ordinary toner and the viscosity of the toner when the toner is in the melted state. 
           [0027]      FIG. 14  is a flowchart of the operation for cleaning the fixation unit in the second embodiment of the present invention. 
           [0028]      FIG. 15  is a graphical time table which shows the toner movements and the like in the second embodiment. 
           [0029]      FIG. 16  is a drawing of the pattern of the image for cleaning the fixation unit in the third embodiment of the present invention. 
           [0030]      FIG. 17  is a flowchart of the operation for cleaning the fixation unit in the third embodiment. 
           [0031]      FIG. 18  is a graphic time table which shows the toner movements and the like in the third embodiment. 
           [0032]      FIG. 19  is a drawing of the pattern of the image for cleaning the fixation unit in the fourth embodiment of the present invention. 
           [0033]      FIG. 20  is a flowchart of the operation for cleaning the fixation unit in the fourth embodiment. 
           [0034]      FIG. 21  is a graphic time table which shows the toner movements and the like which occur while a sheet of recording medium is conveyed through the fixation unit in the fourth embodiment. 
           [0035]      FIG. 22  is a graphic time table which shows the toner movements and the like, which occur while a sheet of recording medium is conveyed through the fixation unit in the fourth embodiment. 
           [0036]      FIG. 23  is a flowchart of the cleaning operation in the fifth embodiment of the present invention. 
           [0037]      FIG. 24  is a graphic time table which shows the toner movements and the like on the first surface of a sheet of recording medium, which occur when the sheet of recording medium is conveyed through the fixation nip in the fifth embodiment. 
           [0038]      FIG. 25  is a graphic time table which shows the toner movements and the like on the second surface of the sheet of recording medium in the fifth embodiment. 
           [0039]      FIG. 26  is a flowchart of the cleaning operation in the sixth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    Hereinafter, the preferred embodiments of the present invention will be described with reference to the appended drawings. First, one of the image forming apparatuses in the preferred embodiments will be described. 
       Embodiment 1 
       [0041]      FIG. 1  is a schematic drawing of the image forming apparatus in the first embodiment of the present invention. The apparatus has a fixation unit. Incidentally, the drawing is a vertical sectional view of a laser beam printer as an example of an image forming apparatus in accordance with the present invention, and shows the general structure thereof. 
       (Overall Structure of Image Forming Apparatus) 
       [0042]    The main assembly  101  of the laser beam printer (which hereafter will be simply referred to as main assembly  101 ) has a sheet feeder cassette  102 , a pair of feeding-and-conveying rollers  105 , etc. The sheet feeder cassette  102  is a cassette in which sheets of recording medium P are stored. The feeding-and-conveying rollers  105  are the rollers which pull out the sheets of recording medium P, one by one, from the sheet feeder cassette  102 . 
         [0043]    The main assembly  101  has also a laser scanner portion  107  and an image forming portion  108 , which are on the downstream side of the feeding-and-conveying rollers  105 . The image forming portion  108  forms a toner image (unfixed) on a sheet of recording medium P (which hereafter will be referred to simply as sheet P), with the use of a beam of laser light from the laser scanner portion  107 . That is, the image forming means of the laser printer in this embodiment is made up of the laser scanner portion  107  and image forming portion  108 . The image forming portion  108  is made up of a photosensitive drum  117 , a charge roller  119 , a developing device  120 , a first transfer roller  124 , a second transfer roller  121 , a cleaner  122 , an intermediary transfer member  123 , etc., which are necessary for an electrophotographic process. 
       [Fixation Unit] 
       [0044]    Next, referring to  FIG. 2 , the fixation unit  109  of the printer will be described.  FIG. 2  is a schematic sectional view of the fixation unit  109 , and shows the structure thereof. 
         [0045]    The fixation unit  109  has a fixation roller  30  and a pressure roller  17 . The fixation roller  30  functions as a pressing member as well as a heating member. It is a cylindrical, or roughly cylindrical, member, and is freely rotatably. The pressure roller  17  is a pressing member. It is cylindrical, or roughly cylindrical, and is freely rotatable. The fixation unit  109  fixes an unfixed toner image on a sheet of recording medium to the sheet of recording medium by applying heat and pressure to the unfixed toner image while the sheet P and the unfixed toner image thereon are moved through the nip formed by the fixation roller  30  and pressure roller  17 . Further, the fixation unit  109  has an endless film  16  and a ceramic heater  15 . The endless film  16  is a heating member for heating the fixation roller  30  by being placed in contact with the peripheral surface of the fixation roller  30 . It is circularly driven. More specifically, heat is applied from the ceramic heater  15  as a heating means to the fixation roller  30  through the film  16 . The temperature of the ceramic heater  15  is detected by a thermistor as a temperature sensing member (which hereafter will be referred to simply as thermistor  18 ). 
         [0046]    The ceramic heater  15  and fixation roller  30  form a fixation roller heating nip N 2  by being pressed against each other by an unshown pressing means, with the presence of the film  16  between the heater  15  and fixation roller  30 . Further, the fixation roller  30  and pressure roller  17  form a fixation nip N 1  by being pressed against each other by an unshown pressing member. 
         [0047]    The ceramic heater  15  of the fixation unit  109  is supported by a holder  19  attached to the main assembly of the fixation unit  109 . 
         [0048]    Referring to  FIG. 3 , the ceramic heater  15  is made up of a substrate  15 A and a heat generating resistor  15 B. The substrate  15 A is a piece of thin plate, the primary ingredient of which is a ceramic such as alumina, aluminum nitride, or the like. The heat generating resistor  15 B is on one of the primary surfaces of the substrate  15 A. Its primary ingredient is silver, palladium or the like. The aforementioned thermistor  18  is in contact with the opposite surface of the substrate  15 A from the surface on which the heat generating resistor  15 B is present. The first surface of the substrate  15 A is covered with a protective layer  15 C to protect the surface from the film  16 , which slides on the surface. The main ingredient of the protective layer  15 C is glass, or a heat resistance resin such as fluorinated resin, polyimide, or the like. Incidentally,  FIG. 3  is a schematic sectional view of the ceramic heater  15 , and shows the general structure thereof. 
         [0049]    Referring to  FIG. 4 , the fixation unit  109  is structured so that the heat generating resistor  15 B, which makes up the ceramic heater  15 , receives electric power from a commercial electric power source  21  through a triac  20 . As the heat generating resistor  15 B receives electric power from the commercial electric power source  21 , it generates heat, heating thereby the fixation roller  30  through the film  16 . 
         [0050]    Next, referring to  FIG. 2 , the film  16  of the fixation unit  109  is made so that in terms of its rotational direction, the dimension of its inward surface is slightly greater by a preset value than the peripheral surface of the film holder  19 . Thus, the film  16  is allowed to remain fitted around the holder  19  without being tensioned by the holder  19 . The fixation unit  109  is structured so that as the fixation roller  30  is rotationally driven by a driving mechanism (unshown), which is outside the main to assembly of the fixation unit  109 , the film  16  is circularly driven by the rotation of the fixation roller  30  while being guided by the holder  19  in a preset direction. 
         [0051]    Further, the film  16  is laminated. That is, it is made of two endless films laminated to each other. The main ingredient of one of the endless film is polyimide, and the main ingredient of the other is PFA. The latter is on the outward surface of the former. 
         [0052]    The fixation roller  30  of the fixation unit  109  is made of a metallic core  30 A, an elastic layer  30 B, and a slippery layer  30 C. The metallic core  30 A is cylindrical or roughly cylindrical, and is formed of iron, SUS, aluminum, or the like. The elastic layer  30 B is on the peripheral surface of the metallic core  30 A, and its main ingredient is silicone rubber, or the like. The slippery layer  30 C, which is the outermost layer, is on the outward surface of the elastic layer  30 B, and its main ingredient is PTFE, PFA, FEP, or the like. The fixation unit  109  is structured so that the fixation roller  30  is rotationally driven by the driving force transmitted to one of the lengthwise ends of the shaft of the metallic core  30 A from the driving mechanism. 
         [0053]    The pressure roller  17  of the fixation unit  109 , which functions as a member for backing up recording medium, is made up of a metallic core  17 A, an elastic layer  17 B, and a slippery layer  17 C. The metallic core  17 A is cylindrical or roughly cylindrical, and is formed of aluminum, or the like. The elastic layer  17 B is on the peripheral surface of the metallic core  17 A, and its main ingredient is silicone rubber, or the like. The slippery layer  17 C, which is the outermost layer, is on the outward surface of the elastic layer  17 B, and its main ingredient is PTFE, PFA, FEP, or the like. The fixation unit  109  is structured so that the pressure roller  17  is rotationally driven by the rotation of the fixation roller  30 . 
         [0054]    At this time, referring to  FIGS. 1 and 4 , the operation of the fixation unit  109  and that of the image forming apparatus will be described. As the unshown controller of the main assembly  101  receives a print signal, the fixation roller  30  is rotationally driven, and therefore, the film  16  and pressure roller  17  are rotated by the rotation of the fixation roller  30 . 
         [0055]    Further, electric power is sent to the ceramic heater  15  while being controlled so that the temperature of the ceramic heater  15 , which is detected by the thermistor  18 , reaches a preset target level. 
         [0056]    The peripheral surface of the fixation roller  30  is heated by the ceramic heater  15  through the film  16  until its temperature reaches a preset level. The surface temperature of the film  16  remains lower than the temperature of the ceramic heater  15 . Further, the surface temperature of the fixation roller  30  remains lower than that of the film  16 . The surface temperature of the pressure roller  17  remains even lower than that of the fixation roller  30 . As long as the ceramic heater  15  is being supplied with electric power, this thermal relationship, in terms of temperature level, among the abovementioned components, always remains the same, because the fixation unit  109  is structured so that the ceramic heater  15  is the primary heat source. 
         [0057]    In a case where the image forming apparatus is started after the fixation unit has cooled down, the temperature difference among these components are greater than in a case where the image forming apparatus is started before the fixation unit has not cooled down. That is, after the fixation nip of the fixation unit  109  became hot enough for fixation, the temperature difference is not as much as immediately after the starting of the image forming apparatus after the fixation unit has cooled down. 
         [0058]    The sheets P in the sheet feeder cassette  102  are pulled out one by one from the sheet feeder cassette  102  by the pair of conveyance rollers  105 , and are sent to a pair of registration rollers  106 . Then, each sheet P is conveyed to the image forming portion  108  by the registration rollers  106 . 
         [0059]    Then, an unfixed toner image is transferred onto the sheet P in the image forming portion  108 . Then, the sheet P is introduced into the fixation nip N 1  of the fixation unit  109 . 
       (Hot Offset) 
       [0060]    After the introduction of the sheet P into the fixation nip N 1 , the sheet P is conveyed between the fixation roller  30  and pressure roller  17 , which have increased in temperature to a preset fixation level by being heated by the heat from the ceramic heater  15  through the film  16 , while remaining pinched by the two rollers  30  and  17 . While the sheet P is conveyed through the fixation nip N 1 , the abovementioned unfixed toner is melted, and becomes permanently fixed to the sheet P. After the fixation of the unfixed image to the sheet P in the fixation nip N 1 , the sheet P is conveyed further by a pair of discharge rollers  114 , and discharged from the main assembly of the image forming apparatus into an external delivery tray  112  by a pair of discharge rollers  140 . 
         [0061]    In a case where an excessive amount of heat is given to the toner (image) on the sheet P in the fixation nip N 1 , some toner particles are excessively melted, and some of the excessively melted toner particles transfer onto the fixation roller  30 . Hereafter, this phenomenon will be referred to as “hot offset”. 
         [0062]    Next, referring to  FIG. 5 , as toner is excessively melted, it reduces in agglutinability. Thus, the toner layer on the sheet P separates into a sub layer of toner which is in contact with the sheet P, and a sub layer of toner which is in contact with the fixation roller  30 . The former sublayer tends to adhere to the sheet P, whereas the latter sublayer tends to adhere to the fixation roller  30 . Consequently, the toner image on the sheet P is partially fixed to the sheet P, and the portion of the toner image, which failed to be fixed to the sheet P, transfers onto the fixation roller  30 . In other words, “hot offset” occurs. As “hot offset” occurs, the toner having transferred onto the fixation roller  30  transfers back onto the sheet P after a full rotation of the fixation roller  30 . That is, the image bearing surface of the sheet P becomes contaminated with the toner having transferred back onto the sheet P. 
         [0063]    On the other hand, if the amount by which heat is applied to the toner image on the sheet P in the fixation nip N is insufficient, the toner in the toner image fails to be fully melted, failing therefore to be satisfactorily fixed to the sheet P. Thus, some toner in the toner image separates from the sheet P, reducing thereby the toner image in quality and/or soiling the hands of a user, after the discharging of the sheet P from the apparatus. 
         [0064]    When the image forming apparatus is in the ordinary fixation mode, the target level for fixation temperature is set so that the sheet P and the toner image thereon are given heat by a proper amount, that is, an amount which is smaller than an amount Qh beyond which “hot offset” occurs, but greater than an amount Q 1  below which the toner image is insufficiently fixed. With the target level for the fixation temperature set as described above, the toner in the toner image on the sheet P is given a proper amount Qn of heat, and therefore, the toner is satisfactorily fixed to the sheet P. 
         [0065]    The amount Qn, which does not cause “hot offset” nor under-fixation, and the target level for fixation temperature, are affected by the amount of the toner on the sheet P, and the toner type. Further, how the toner on the sheet P melts at the interface between the body of toner on the sheet P and the sheet P is affected by the basis weight and surface properties of the sheet P. They are also affected by the environment in which the image forming apparatus is used. Thus, the image forming apparatus (fixation unit  109 ) is designed so that it is switched in the target level for fixation temperature according to the type of the sheet P, toner type, and environment, in order to give heat to the toner image on the sheet P by the amount Qn, that is, the proper amount. 
         [0066]    As described above, when the image forming apparatus is in the ordinary fixation, the apparatus is controlled in fixation temperature to prevent the occurrence of “hot offset”. However, in such a case as where “hot offset” has occurred due to the usage of improper sheet (P) for the image forming apparatus, it is possible that as the fixation roller  30  rotates, the toner having adhered to the fixation roller  30  will transfer onto the film  16 . 
         [0067]    In this embodiment, therefore, it is desired that the strength of adhesion between slippery layer of the film  16  and toner is less than that between the slippery layer  30 C of the fixation roller  30  and toner. 
         [0068]    One of the methods for adjusting a slippery layer in slipperiness is as follows: a substance such as inorganic filler, which is inferior in terms of slipperiness, is dispersed in a material such as PTFE, PFA, FEP, or the like, which is the main ingredient of the slippery layer, to chemically adjust the material for the slippery layer in slipperiness. Further, the slippery layer may be adjusted in surface roughness by controlling the condition under which the slippery layer is formed. 
         [0069]    The slipperiness of each of the slippery layers is expressed in terms of the likeliness with which toner adheres to the slippery layer after the fixation unit has fully warmed up for fixation and the toner has been melted enough to “hot offset”. 
       (Method for Testing Slippery Layer in Effectiveness) 
       [0070]    Next, a method for testing the slippery layer in effectiveness in terms of the prevention of toner adhesion will be described. First, the heater  15  of the fixation unit  109  is increased in temperature to a preset level. Then, the fixation roller  30  and pressure roller  17  are rotated. The target level is a level high enough to cause the toner on the sheet P to “hot offset” in the fixation nip N 1 . The fixation unit  109  is warmed up by keeping the temperature of the heater  15  at this target level for five minutes. 
         [0071]    As soon as the fixation unit is sufficiently warmed up, the electric power supply to the heater  15 , and the rotational driving of the fixation roller  30  and pressure roller  17  are temporarily stopped. Then, while the fixation unit is remaining sufficiently warm for fixation, unfixed toner is moved into the nip N 1 , that is, the nip for fixation, and the nip N 2 , that is, the nip for heating the fixation roller  30 . It is desired that the length of time they are temporarily stopped is no more than 30 seconds, preferably, no more than 10 seconds. Then, the electric power supply to the heater  15 , and the rotation of the fixation roller  30  and pressure roller  17 , are restarted. As they are restarted, the toner which were in the fixation nip N 1  transfers onto the fixation roller  30  or the pressure roller  17 . The toner which was in the fixation roller heating portion N 2  transfers from the fixation roller  30  onto the film  16 . Here, if a component is greater in the amount by which toner has transferred thereto and adhered thereto than another component, the first component is considered to be inferior in terms of slipperiness than the second component. 
         [0072]    Generally speaking, if an object is smaller in surface energy and less rough across its surface than another object, toner is less likely to adhere to the first object than the second object. Further, if melted toner is sandwiched between two objects which are different in temperature, the melted toner is likely to transfer onto the object which is lower in temperature, because the portion of the body of melted toner, which is in contact with the object which is lower in temperature increases in viscosity, increasing therefore in adhesiveness. 
         [0073]    The amount of slipperiness between two objects, which is obtained through the above described slipperiness test, is affected by the material of each object and the temperature of each object. 
         [0074]    However, the above described slipperiness test is carried out after the fixation unit has been warmed up for a long time, and therefore, has reached its thermal equilibrium. In other words, it is carried out when the temperature differences among the aforementioned components are relatively small. However, when the image forming apparatus in this embodiment is in the cleaning mode, ordinary fixation mode, or the like, the fixation unit is not warmed up for a long time, and therefore, the difference in surface temperature among the film  16 , fixation roller  30  and pressure roller  17  is greater than when the slipperiness test is carried out. If the film  16  is slipperier than the fixation roller  30  in the abovementioned slipperiness test, the film  16  will be even more slipperier than the fixation roller, when the image forming apparatus is in the cleaning mode or ordinary fixation mode. 
       (Cleaning Mode) 
       [0075]    Next, referring to  FIGS. 6 and 7 , the cleaning of the fixation unit will be described.  FIG. 6  is a flowchart which shows the operation of the image forming apparatus.  FIG. 7  is a graphical time table which shows the toner movements on the sheet P, fixation roller  30 , and film  16 , and the changes in the temperature of the fixation roller  30 . 
         [0076]    Hereinafter, the ordinary operational mode of the fixation unit is referred to as “fixation mode” and the operational mode of the fixation unit, which is for cleaning, is referred to as “cleaning mode”. 
         [0077]    In the cleaning mode, the following three steps are consecutively carried out. 
         [0078]    1) First step: the unfixed toner on the sheet P is transferred onto the fixation roller  30 . 
         [0079]    2) Second step: the toner on the film  16  is removed with the use of the toner on the fixation roller  30 . 
         [0080]    3) Third step: the toner on the fixation roller  30  is transferred onto the sheet P, and then, is fixed to the sheet P. 
         [0081]    The toner on the film  16  is transferred onto the sheet P through the above described three steps, and then, is discharged with the sheet P. Next, each of the abovementioned three steps will be concretely described. 
       (First Step) 
       [0082]    First, the step for transferring the unfixed toner on the sheet P, onto the fixation roller  30  will be described. 
         [0083]    As a user determines that the fixation unit needs cleaning, the user is to switch the image forming apparatus in operational mode from the fixation mode to the cleaning mode, by inputting an operational mode switching signal through the control panel of the main assembly  101 , or an unshown host computer, while the image forming apparatus is kept on standby (Step  1 - a ). Incidentally, the image forming apparatus may be designed so that print count is kept by the apparatus main assembly  101 , and as the print count reaches a preset value, the control portion of the apparatus main assembly  101  determines that the cleaning is necessary, and automatically switches the image forming apparatus from the fixation mode to the cleaning mode. 
         [0084]    As the image forming apparatus begins to be operated in the cleaning mode, the target level for the temperature of the ceramic heater  15  of the fixation unit  109  is set to the temperature level for the cleaning mode. Further, the process speed of the image forming apparatus is set for the cleaning mode (Step  1 - b ). 
         [0085]    Then, the image forming apparatus begins to be operated at the preset speed for the cleaning, and the fixation roller  30  begins to be driven, with its temperature set to the target level for cleaning. Further, electric power begins to be sent to the heater  15  (Step  1 - c ). In the cleaning mode in this embodiment, the target temperature level for the fixation unit, and the process speed of the image forming apparatus, are set so that it is ensured that “hot offset” will occur. 
         [0086]    In other words, in the cleaning mode, the target temperature is set higher than in the fixation mode so that heat is given to the toner on the sheet P by the amount Qh (second amount) which is large enough to cause “hot offset” and is greater than the amount Qn, that is, the amount by which heat is given to the toner on the sheet P when the image forming apparatus (fixation unit) is in the fixation mode. It is desired that in the cleaning mode, the target temperature level is changed according to the sheet type (recording medium type), environment in which the apparatus is operated, and/or the like factors, so that even if the condition under which the image forming apparatus is operated changes, it is ensured that “hot offset” will occur. Instead, the image forming apparatus may be limited in the type of sheet of recording medium which is to be used in the cleaning mode. 
         [0087]    In the cleaning mode, the image forming apparatus may be controlled so that its process speed is switched to the one for the cleaning mode, and the toner on the sheet P can be given heat by the amount Qh which is large enough to cause “hot offset”. The slower the process speed, the longer the time available for transmitting heat from the fixation roller  30  to the toner on the sheet P in the fixation nip N 1 . Therefore, even if the fixation unit is kept unchanged in the target temperature, the amount by which heat is given to the toner increases. 
         [0088]    In terms of size, the sheet P for cleaning is desired to be largest in width among various sheets of recording medium usable with the image forming apparatus. Here, the width of the sheet P means the dimension of the sheet P in terms of the direction perpendicular to the sheet conveyance direction. Further, the sheet P for cleaning is desired to be larger in width than the sheets P which have been used before the cleaning operation, because it is possible that the contaminants might have adhered to the film  16  across the entire portion of the film  16  which came in contact with the sheets P used before the cleaning operation. 
         [0089]    Next, the image forming apparatus is made to start an image forming operation. Thus, the sheet P is pulled out of the sheet feeder cassette  109 , and is conveyed to the image forming portion  108 , by the pair of feed-and-conveyance rollers  105  (Step  1 - d ). Then, an image having a pattern for the cleaning mode is formed on the sheet P by the image forming portion  108  (Step  1 - e ). Incidentally, an example of the image pattern for the cleaning mode is shown in shown in  FIG. 8 . 
         [0090]    The image to be formed on the sheet P for the cleaning mode is desired to be a solid image which is as wide as possible in terms of the direction perpendicular to the sheet conveyance direction. In terms of the direction parallel to the sheet conveyance direction, it is desired to be longer than the external circumference of the film  16 . In terms of print ratio, it is desired to be in a range of 50%-300%, preferably, 70%-200%. 
         [0091]    Here, “print ratio” is not simply the ratio between the portion of the sheet P covered with toner and the portion of the sheet P which is not covered with toner. It means such a ratio that reflects the density of each dot as well. That is, “print ratio” is the percentage of the number of the picture elements to be exposed, relative to the number of the entire picture elements of which an image is formed based on image formation data (exposed picture element/entire picture element)×100. It corresponds to the value obtained by integrating the density (toner amount) of each picture element, which is adjusted by pulse width modulation (PWM). 
         [0092]    Regarding the definition of “print ratio”, in the case of an image forming apparatus which reproduces 256 levels of gradation, for example, by PWM, the density of each picture element can be expressed in the form of a fraction, for example, x/256, or a percentage obtained by converting the fraction into percentage. For example, assuming that a monochromatic solid image is 100% in print ratio, if the optical density of an image is 50% relative to the solid image, the image is 50% in print ratio, and a solid image of the secondary color, such as blue (color of area where solid magenta image and solid cyan image overlap) is 200%. 
         [0093]    If an image on the sheet P is no more than 50% in print ratio, the portion of the sheet P, which is covered with toner, is smaller than the portion of the sheet P, which is not covered with toner. Therefore, the amount of the toner which can be adhered to the peripheral surface of the fixation roller  30  is smaller. If the amount of the toner adhered to the peripheral surface of the fixation roller  30  is smaller than a certain value, the slipperiness of the peripheral surface of the fixation roller  30  overwhelms the adhesion between the body of toner on the fixation roller  30  and the peripheral surface of the fixation roller  30 , making it therefore difficult to clean the fixation unit. On the other hand, if the print ratio is no less than 300%, the sheet P is excessive in the amount of the toner thereon, making it therefore difficult to cause “hot offset”. 
         [0094]    The image pattern for the cleaning sheet P is desired to be such that an image resulting from the pattern has a white black space across the leading edge portion of the sheet P, so that the portion of the image, from which toner is to be “hot offset”, will enters the fixation nip N 1  after the leading edge portion of the sheet P enters the nip between the pair of discharge rollers  111 . Creating the image pattern for the cleaning sheet P as described above can prevent the cleaning sheet P from wrapping around the fixation roller  30 . Further, the image pattern is desired to be such that the cleaning sheet P resulting from the pattern will have a blank space also across its trailing end portion, in order to prevent toner from “hot offsetting” onto the fixation roller  30  by an amount greater than the amount equivalent to the circumference (single full rotation) of the fixation roller  30 . 
         [0095]    Referring to  FIG. 9 , the image pattern for the formation of a cleaning sheet may be designed so that the unfixed toner image will be diagonally placed on the sheet P. In the case of this image pattern, as the cleaning sheet is conveyed through the fixation nip N 1 , the unfixed toner image gradually offsets. Therefore, the adhesion between the toner image and fixation roller  30  is less than in the case where the toner image offsets all at once across its entire range in terms of the width direction of the sheet P. Thus, it is even less likely for the cleaning sheet (P) to wrap around the fixation roller  30 . 
         [0096]    The cleaning sheet, or the sheet P on which the unfixed toner image is present, is introduced into the fixation unit  109  (Step  1 - f ). 
         [0097]    Then, the unfixed toner image on the sheet P reaches the fixation nip N 1 , and is conveyed through the fixation nip N 1 . As the sheet P is conveyed through the fixation nip N 1 , a part of the toner in the unfixed toner image on the sheet P is “hot-offset” onto the fixation roller  30  (Step  1 - g ). Hereafter, the toner having transferred onto the fixation roller  30  will be referred to as offset toner To. 
       (Second Step) 
       [0098]    Next, the process for cleaning the contaminative toner on the film  16  (toner having adhered to the film  16 , and will be referred to as contaminant toner Tc hereafter), with the utilization of the offset toner To on the fixation roller  30  will be described. 
         [0099]    As the fixation roller  30  rotates, the offset toner To, that is, the toner having transferred (“hot offset”) onto the fixation roller  30 , reaches the fixation roller heating nip N 2 , in which it comes into contact with the film  16 . Next, referring to  FIG. 10 , the contaminant toner Tc on the film  16  is transferred onto the fixation roller  30  by the adhesiveness of the offset toner To (Step  2 - g ). Incidentally,  FIG. 10  is a schematic sectional view of the fixation unit when it is performing Step  2 . 
         [0100]    Here, the mechanism of the transfer of the contaminant toner Tc on the film  16  onto the fixation roller  30  will be descried. 
         [0101]    The contaminant toner Tc on the film  16  is a mixture of resinous toner particles, and paper dusts consisting of pieces of fibers of which the sheet P is made, and fillers made of inorganic substances such as calcium carbonate, talc, etc. Therefore, it is difficult to thermally soften, and also, is low in adhesiveness. 
         [0102]    These paper dusts separate (fall) from the sheet P when the sheet P is heated for the fixation. As they separate, they adhere to the fixation roller  30 . Then, they come into contact with the film  16  in the fixation roller heating nip N 2 . Even if the film  16  is made slipperier than the fixation roller  30 , the paper dusts and the like electrostatically and mechanically adhere to the film  16 . To this small amount of paper dusts made up of inorganic substances, a small amount of toner adheres, turning thereby into the contaminant toner Tc. 
         [0103]    The contaminant toner Tc is a mixture of two types of toner particles. One type of toner particles are those which adhered to the surface of the film  16  and solidified thereon. The other type of toner particles are those which transferred onto the surface of the film  16 , but did not solidified, and yet, lack adhesiveness. Thus, they do not firmly adhere to the film  16 , neither to the fixation roller  30 , being therefore remaining on the film  16 . 
         [0104]    Those having solidified and firmly adhered to the film  16  do not easily separate (fall) from the film  16 , and therefore, do contribute to the formation of unsatisfactory image. On the other hand, those having not completely solidified and remaining on the film  16  because of the presence of small amount of adhesiveness between them and film  16 , irregularly transfers back onto the fixation roller  30 , contributing thereby to the formation of unsatisfactory images. 
         [0105]    The toner particles in the contaminant toner Tc, which are remaining on the film  16  because of the presence of the small amount of adhesiveness between them and film  16 , can be removed from the film  16  by operating the image forming apparatus in this embodiment in the cleaning mode. 
         [0106]    Immediately after the offsetting, the contaminants such as paper dusts have not adhered to the offset toner To by a large amount, and therefore, the offset toner To is adherent enough to adhere to fixation roller  30 , which is less slippery than the film  16 , in the fixation roller heating nip N 2 . Thus, the offset toner To remains adhered to the fixation roller  30 . In other words, the portion of the contaminant toner Tc, which are on the film  16  only because of the presence of the small amount of adhesiveness between this portion and film  16 , can be transferred onto the fixation roller  30  with the utilization of the adherence between the offset toner To on the fixation roller  30 , and the contaminant toner Tc on the film  16 . 
         [0107]    If the film  16  is slipperier than the fixation roller  30 , in the fixation roller heating nip N 2 , most of the offset toner To is retained by the fixation roller  30 . 
         [0108]    However, the offset toner To is not very agglutinative. Therefore, it sometimes occurs that a part of the offset toner To adheres to the film  16 . This phenomenon is likely to occur if the difference in slipperiness between the film  16  and fixation roller  30  is small. In such a case, as the film  16  rotates, the offset toner To having adhered to the film  16  returns to the fixation roller heating nip N 2 , and comes into contact with the fixation roller  30  again. In other words, most of the offset toner To having adhered to the film  16  returns to the fixation roller  30 . Moreover, as the fixation roller  30 , and the offset toner To on the film  16 , come into contact with each other the next time, most of the rest of the offset toner To returns to the fixation roller  30 . Eventually, therefore, the offset toner To having adhered to the film  16  returns to the fixation roller  30  almost entirely. 
       (Third Step) 
       [0109]    Next, the third step, or the step in which the toner on the fixation roller  30  is transferred onto the sheet P, and fixed to the sheet P, will be described. 
         [0110]      FIG. 11  is a schematic sectional view of the fixation unit when the fixation unit is in the third step. As will be evident from  FIG. 11 , as the fixation roller  30  rotates, the offset toner To and contaminant toner Tc on the fixation roller  30  return to the fixation nip N 1 , in which they come into contact with the sheet P, and are fixed to the sheet P (Step  3 - a ). 
         [0111]    If a large amount of the contaminant toner Tc is on the fixation roller  30 , and/or the sheet P is inferior in surface properties, the offset toner To and contaminant toner Tc on the fixation roller  30  fail to be fixed in entirety. Thus, it sometimes occurs that a part of the offset toner To and/or a part of the contaminant toner Tc remains on the fixation roller  30  and passes the fixation nip N 1  without being fixed. 
         [0112]    As the fixation roller  30  rotates further, the offset toner To remaining on the fixation roller  30  reaches the fixation roller heating nip N 2  again. However, the film  16  is slipperier than the fixation roller  30 . Therefore, it seldom occurs that the offset toner To on the fixation roller  30  transfers onto the film  16 . Besides, even if a part of the offset toner To on the fixation roller  30  transfers onto the film  16 , it transfers back onto the fixation  30  during the following several full rotations of the fixation roller  30 . As for the contaminant toner Tc, it remains adhered to the offset toner To on the fixation roller  30 . Therefore, it seldom transfers onto the film  16 . Then, the further rotation of the fixation roller  30  makes the toner on the fixation roller  30  return to the fixation nip N 1 , in which the toner on the fixation roller  30  is fixed to the sheet P. Even if it fails to be fixed all at once, it is eventually fixed during the following several full rotations of the fixation roller  30 . 
         [0113]    Thereafter, the sheet P is discharged into the delivery tray  112  by way of the pair of discharge rollers  111  and pair of discharge rollers  140 . As the contaminant toner Tc on the film  16  is put through the above described sequential steps, it is transferred onto the sheet P by way of the fixation roller  30 , and then, is discharged with the sheet P from the apparatus main assembly  101  (Step  3 - b ). Then, the driving of the image forming apparatus, and sending of electric power to the heater, are stopped to end the operation in the cleaning mode (Step  3 - c ). 
         [0114]    The steps described above are sequentially started. However, it takes a certain length of time from the starting of each step to the completion of the step. Therefore, the steps partially overlap. For example, Step  2 - a  begins in the fixation roller heating nip N 2  before Step  1 - g  ends in the fixation nip N 1 . Further, in a case where the length of the unfixed image is greater than the length of the external circumference of the fixation roller  30 , Step  3 - a  begins in the fixation nip N 1  before Step  1 - g  ends in the fixation nip N 1 . 
         [0115]    In this case, the unfixed toner on the sheet P will have hot-offset onto the fixation roller  30 , and at the same time, the offset toner To and contaminant toner Tc which were on the fixation roller  30  will be fixed to the sheet P. It is rather difficult to cause the entirety of the contaminant toner Tc on the fixation roller  30 , to be fixed to the sheet P during the first passage of them together through the fixation nip N 1 . However, the contaminant toner Tc which remained after the preceding rotation of the fixation roller  30  is eventually fixed to the sheet P during one of the following rotations of the fixation roller  30 . 
       (Test Results) 
       [0116]    Next, the effectiveness of the fixation unit in this embodiment, which was confirmed through a text, will be described. 
         [0117]    The image forming apparatus used for the test was a laser printer which is 90 mm/sec in process speed, and capable of outputting full-color images at a rate of 14 copies per minute. 
         [0118]    First, the fixation unit in this embodiment, which was used in the test, will be described about its structure. The ceramic heater  15  was made up of the substrate  15 A, a heat generating resistor  15 B, and a protective layer  15 C. The substrate  15 A was made of aluminum. It was 1.0 mm in thickness and 7.0 mm in width. The heat generating resistor  15 B was on one of the primary surfaces of the substrate  15 A. It was made of silver and palladium. It was 10 μm in thickness and 4.0 mm in width. The heat generating resistor  15 B, and the surface of the substrate  15 A, which was holding the heat generating resistor  15 B, were covered with the protective layer  15 C, which was a glass layer. It was 60 μm in thickness. The film  16  was laminated, being made of a substrate layer and a slippery layer. The substrate layer was formed of polyimide resin. It was 20 mm in inward circumference, and 30 μm in thickness. The slippery layer was formed of PFA resin, and was 20 μm in thickness. 
         [0119]    The fixation roller  30  was made of a metallic core  30 A, an elastic layer  30 B, and a slippery layer  30 C. The metallic core  30 A was made of aluminum, and 14 mm in external diameter. The elastic layer  30 B was on the peripheral surface of the metallic core  30 A, and was 3.0 mm in thickness. It was formed of silicone rubber, which was 0.2 W/m·K in thermal conductivity. The slippery layer  30 C, which was the outermost layer, was on the outward surface of the elastic layer  30 B. It was formed of PFE resin, and was 20 μm in thickness. 
         [0120]    The pressure roller  17  was made up of a metallic core  17 A, an elastic layer  17 B, and a slippery layer  17 C. The metallic core  17 A was formed of aluminum and 14 mm in external diameter. The elastic layer  17 B was on the peripheral surface of the metallic core  17 A, and was 3.0 mm in thickness. It was formed of silicone rubber, which was 0.2 W/m·K in thermal conductivity. 
         [0121]    The ceramic heater  15  was kept pressed against the fixation roller  30  with the application of 100 kg of pressure, so that the fixation roller heating nip N 2 , which was 5.0 mm in width, was created and maintained between the film  16  and fixation roller  30 . The pressure roller  17  was kept pressed upon the fixation roller  30 , with the application of 15 kg of pressure, so that the fixation nip N 1 , which was 6.0 mm in width, was created and maintained between the pressure roller  17  and fixation roller  30 . 
         [0122]    The image forming apparatus was provided with the pair of discharge rollers  111 , which were 60 mm downstream of the fixation nip N 1  in terms of the recording medium conveyance direction. After being conveyed through the fixation nip N 1 , the sheet P was sent to the pair of discharge rollers  111 , by which it was discharged from the apparatus main assembly  101 . 
         [0123]    The image forming apparatus used in this experiment was designed as follows: When it was in the fixation mode, in which toner images were fixed to a sheet of recording medium which was 80 g/m 2  in basis weight, the ceramic heater  15  was controlled so that when the apparatus was used to form full-color images in an environment in which temperature was 15° C., its temperature remained in a range of 180° C.-200° C. (target range). When the fixation roller  30  and pressure roller  17  were cold (had cooled down), the target temperature level for the ceramic heater  15  was set higher than when the fixation roller  30  and pressure roller  17  were warm. As long as the ceramic heater  15  was controlled as described above, with its target temperature level set in the above range, the toner on a sheet of recording medium was satisfactorily fixed; it was neither under-fixed, nor “hot offset”. 
         [0124]    This image forming apparatus was used in an environment which was 15° C. in temperature and 15% in humidity, to print 10,000 copies of a literal image which was 5% in print ratio, using ordinary sheets for a laser printer, which was 80 g/m 2  in basis weight and A4 in size. By the time this printing operation was finished, contaminants had begun to adhere to the sheets of recording medium. The observation of the interior of the fixation unit revealed that contaminants such as the contaminant toner Tc had adhered to the film  16 . 
         [0125]    Then, the image forming apparatus was set in the cleaning mode, and a cleaning test was conducted in which an attempt was made to remove the contaminant toner Tc on the above described film  16 . 
         [0126]    As the cleaning sheet P, a sheet of glossy paper, more specifically, a sheet of HP Presentation Paper LBP (product of Hewlet Packard Co., Ltd.), which was 130 g/m 2  in basis weight and of the letter size (216 mm in width and 279 mm in length) was used. The glossy paper, which is very flat, is likely to conform to the surface of the fixation roller with no gap, and therefore, makes it easier to recover the offset toner To and contaminant toner Tc after they are made to adhere to the fixation roller  30 . 
         [0127]    In an image forming operation in which sheets of above described glossy paper (HP Presentation Paper) are used, the image forming apparatus (fixation unit) is set for glossy paper with a basis weight of 130 g/m 2 . Further, the ceramic heater  15  is controlled so that when the process speed is 45 mm/sec, its temperature will be in a target temperature range of 180° C.-200° C. With this setup, an unfixed toner image on a sheet of the glossy paper is satisfactorily fixed; it is neither under-fixed nor “hot offset”. 
         [0128]    As the image forming apparatus was started in the cleaning mode, the process speed was set to 45 mm/sec. Then, an image forming operation was started, and also, the fixation roller  30  of the fixation unit began to be driven. The ceramic heater  15  was controlled so that its temperature remained in a range of 200° C.-220° C. Incidentally, like in the ordinary fixation mode, if the image forming apparatus begins to be operated in the cleaning mode when the fixation roller  30  and pressure roller  17  are cold (after they have cooled down), the target temperature of the ceramic heater  15  is set relatively high, whereas if the image forming apparatus begins to be operated in the cleaning mode when the fixation roller  30  and pressure roller  17  are warm, the target temperature of the ceramic heater  15  is set relatively low. In this test, in order to cause “hot offset” by giving an excessive amount of heat to the toner on the sheet P with a basis weight of 130 g/m 2 , the target temperature level of the ceramic heater  15  was set higher than in the ordinary fixation mode. 
         [0129]    Then, the sheet P was conveyed, and an unfixed toner image for the cleaning mode was formed on the sheet P in the image forming portion. Then, the sheet P was conveyed into the fixation nip N 1  of the fixation unit 20 seconds after electric power began to be sent to the ceramic heater  15 . 
         [0130]    At this point of the cleaning operation, the temperature of the film  16  had climbed to 190° C.-210° C. and the temperature of the fixation roller  30  had climbed to 180° C.-200° C. Further, the temperature of the pressure roller  17  had climbed to 100° C.-120° C. 
         [0131]    The pattern of the image for cleaning mode was the same as the one shown in  FIG. 8 . That is, the pattern was such that as it was formed on the sheet P, the portion of the sheet P, which was between the leading edge of the sheet P, in terms of the sheet conveyance direction, and a hypothetical straight line (first hypothetical line) on the sheet P, which is perpendicular to the leading edge and was 70 mm apart from the leading edge, was left as a blank (white) space; the portion of the sheet P, which was between the above described first hypothetical straight line and a hypothetical straight line (second hypothetical line), which was perpendicular to the leading edge and was 140 mm apart from the first hypothetical line, was covered with a 200 mm wide solid black image (100% in print ratio); and the portion of the sheet P, which was between the second hypothetical straight line and the trailing edge of the sheet P, remained as a blank (white) space. When the image for the cleaning mode, which has the above described pattern is formed, the amount of the toner on the solid black portion of the image is 5.0 g/m 2 . 
         [0132]    That is, in order to prevent the sheet P from wrapping around the fixation roller  30 , the leading end portion of the sheet P, which corresponds in length to the distance between the fixation nip N 1  to the nip between the pair of discharge roller  111 , was left as a blank space. Then, the solid toner image, the length of which equaled the circumference of the film  16 , and which was for transferring the contaminant toner Tc, and the like, on the film  16 , onto the cleaning sheet P, was formed on the upstream side of the blank space. Then, the rest of the cleaning sheet P, that is, the upstream side of the solid toner image on the sheet P, was left as a blank space, onto which the contaminant toner Tc and the like would be spit out from the fixation roller  30  after being transferred onto the fixation roller  30  from the film  16 . In other words, the offset toner To and contaminant toner Tc were recovered by the cleaning sheet P, that is, a sheet of recording medium having a solid image for causing “hot offset”. 
         [0133]    Toward the end of the operation in the cleaning mode, a sheet of paper, which looked as shown in  FIG. 12  was discharged from the fixation unit. The black toner in the solid image which covered the portion of the cleaning sheet P, which was between the hypothetical straight line which was perpendicular to the leading edge of the sheet P and was 70 mm apart from the leading edge of the sheet P, and the hypothetical straight line which was perpendicular to the leading edge and was 140 mm apart from the leading edge of the sheet P, had “hot offset” onto the film  16 . The offset toner To was fixed, along with the contaminant toner Tc having adhered to the film  16 , to the portion of the cleaning sheet P, which was between a hypothetical straight line which was perpendicular to the leading edge of the sheet P and was 133 mm apart from the leading edge of the sheet P, and a hypothetical straight line which was perpendicular to the leading edge of the sheet P and was 203 mm from the leaning edge of the sheet P, that is, the portion of the sheet P, which corresponds to the first full rotation of the fixation roller  30  after its “hot offset” rotation. The offset toner To adhered also to the portion of the cleaning sheet P, which was between the hypothetical straight line which was 203 mm from the leading edge of the cleaning sheet P, and a hypothetical straight line which was perpendicular to the leading edge of the sheet P and was 266 mm from the leading edge of the cleaning sheet P, although by only a minutes amount. However, virtually no offset toner To was visually detectable on the portion of the cleaning sheet P, which was between the hypothetical straight line which was 266 mm from the leading edge of the sheet P to the trailing edge of the sheet P, that is, the portion which corresponding to the third full rotation of the fixation roller  30  after its “hot offset” rotation. 
         [0134]    The results of this operation of the image forming apparatus, which was carried out in the cleaning mode, proved that the first embodiment of the present invention can remove the contaminants such as the contaminant toner Tc and the like which had not solidly adhered to the film  16 . As for the other contaminants, that is, those which had solidly adhered to the film  16 , it did not occur that they peel from the film  16  and transfers onto the sheet P when the image forming apparatus is in the ordinary fixation mode. In other words, this embodiment of the present invention was very effective to clean the fixation unit. 
       Embodiment 2 
       [0135]    Next, the apparatus in the second embodiment of the present invention will be described. The basic structure of the image forming apparatus in this embodiment is virtually the same as that in the first embodiment, and therefore, will not be described except for the portions different from the counterparts in the first embodiment, that is, the portions which characterize this embodiment. Further, the portions, components, etc., of the image forming apparatus in this embodiment, which are the same in function as the counterparts in the first embodiment, will be given the same referential codes as those given to the counterparts in the first embodiment. 
         [0136]    In this embodiment, the image forming apparatus is switched in the target temperature level of the fixation unit and process speed, when the sheet P is being conveyed through the fixation unit. Further, while the toner image on the sheet P, which is for generating the offset toner To, is moving through the fixation nip N 1 , the fixation roller  30  is given heat by the amount Qh, which is large enough to cause “hot offset”. Then, after the passage of the “hot offset” causing image on the sheet P through the fixation nip N 1 , the fixation roller  30  is given heat by an amount (third amount) which is smaller than the aforementioned amount Qh, which causes “hot offset”. Incidentally, the third amount in this embodiment is the same as the amount Qn by which heat is given to the fixation roller  30  in an ordinary fixation mode. 
         [0137]    As a means for causing the offset toner To to adhere to the fixation roller  30 , the toner is given the excessive amount Qh of heat to make “hot offset” occur. However, as the offset toner To is excessively melted, it becomes less likely to agglutinate, and weaker in adhesiveness. 
         [0138]      FIG. 13  shows the general relationship between toner temperature and the viscosity of melted toner. As toner is increased in temperature by the application of an excessively amount of heat, it is likely to excessively melt, excessively reducing thereby in viscosity, agglutinability, and adhesiveness. 
         [0139]    Reducing the amount by which heat is given to the toner after the completion of Step  1 , to a proper value, increases the offset toner To in agglunatability and adhesiveness; improves thereby the offset toner To in terms of its performance with which it cleans the film  16  in Step  2 ; and makes it easier for the offset toner To to be fixed to the sheet P in Step  3 . In other words, it makes it easier for the offset toner To to be recovered. 
         [0140]    Next, referring to  FIG. 14 , which is a flowchart, and  FIG. 15 , which is a time table, the cleaning mode in this embodiment will be described.  FIG. 14  is a flowchart of the operation of the image forming apparatus in this embodiment, when the apparatus is in the cleaning mode.  FIG. 15  is a graphical time table which shows the timing of the toner movement on the film  16  and the timing of the temperature changes of the fixation roller  30 . 
         [0141]    The image forming apparatus is started in the cleaning mode (Step  1 - a ). Then, a target temperature and process speed are set. The target temperature level is to be high enough to generate heat by the amount Qh, which is large enough to cause the toner on the sheet P to “hot offset”. The process speed is to be proper to allow the toner on the sheet P to satisfactorily “hot offset” (Step  1 - b ). 
         [0142]    Next, each of various components involved with the cleaning mode begin to be driven, and electric power begins to be sent to the heater  15 , whereby the temperature of the fixation roller  30  climbs to a preset level (Step  1 - c ). 
         [0143]    Next, the sheet P is fed into the apparatus main assembly  101  (Step  1 - d ), and an unfixed image for cleaning is formed on the sheet P (Step  1 - e ). This sheet P is conveyed to the fixation unit (Step  1 - f ). As the unfixed image on the sheet P conveyed through the fixation nip N 1 , the toner, of which the unfixed image is formed, turns into the offset toner To, and transfers onto the fixation roller  30  (Step  1 - g ). 
         [0144]    After the entirety of the image on the sheet P, which is for generating the offset toner To, passes the fixation nip N 1 , the heater  15  is changed in target temperature (Step  1 - h ), whereby the fixation temperature  30  is changed in temperature, so that the sheet P will be given heat by the amount Qn, which is the same amount by which heat is given to the sheet P in the ordinary fixation mode. 
         [0145]    As the fixation roller  30  rotates, the offset toner To reaches the fixation roller heating nip N 2 , in which it comes into contact with the film  16 , whereby it transfers the contaminant toner Tc on the film  16 , onto the fixation roller  30  (Step  2 - a ). 
         [0146]    Next, as the fixation roller  30  rotates further, the contaminant toner Tc and offset toner To reach the fixation nip N 1 , in which they are fixed to the sheet P (Step  3 - a ), and then, the sheet P is discharged (Step  2 - b ). Then, the electric power to the heater  15  is turned off, and the image forming apparatus is stopped, ending the image forming operation in the cleaning mode (Step  3 - c ). 
         [0147]    In this embodiment, the timing with which Step  1 - h  is started is set so that Step  1 - h  starts after the passage of the entirety of the unfixed image on the sheet P. Therefore, the timing with which Step  1 - h  starts is after the timing with which Step  2 - a  is started, and the timing with which Step  3 - a  started, although it depends on the length of the unfixed image. Thus, the fixation unit increases in film cleaning performance during Step  2 , and further, the contaminant toner Tc begins to be better fixed to the sheet P during Step  3 . 
         [0148]    That is, in this embodiment, the Step  1 - h  is not started until sometime in Step  2  and Step  3 . However, even before the starting of Step  1 - h , Step  2  and Step  3  work just as well as after the starting of Step  1 - h , as it does in the first embodiment. 
         [0149]    The timing with which Step  1 - h  is to be started may be set so that Step  1 - h  is started as the distance between the leading edge of the unfixed image on the sheet P and the fixation nip N 1  becomes equal to the external circumference of the film  16  after the passage of the leading edge of the unfixed image through the fixation nip N 1 . The offset toner To is adhered to the fixation roller  30  by the amount which is proportional to the external circumference of the film  16 . The rest of the toner of which the unfixed image is formed is ordinarily fixed to the sheet P. 
         [0150]    The length of time the offset toner To is adhered to the fixation roller  30  in Step  1  becomes shorter, that is, it becomes equal to the length of time it takes for the film  16  to fully rotate once in the ordinary. However, the period in which the fixation unit is higher in cleaning performance in Step  2 , and the period in which it is easier for the offset toner To and contaminant toner Tc to be fixed in Step  3 , are longer. Therefore, if the sheet P is unsatisfactory in terms of flatness and smoothness, and therefore, it is feared that the offset toner To and contaminant toner Tc on the fixation roller  30  may not be fully recovered, Step  1 - h  had better be started with the above described timing. 
       (Test Results) 
       [0151]    A test is carried out to confirm the effectiveness of the fixation unit in this embodiment in the cleaning mode. The image forming apparatus and fixation unit used for the experiment are the same as those in the first embodiment. 
         [0152]    The fixation unit in this embodiment was tested in terms of the effectiveness with which it can remove the contaminant toner To having adhered to the film  16 , in the same manner as the fixation unit in the first embodiment was tested. 
         [0153]    The cleaning sheet P used to test the fixation unit in this embodiment was a sheet of ordinary LBP printing paper (CLC 80 g: product of Canon), which was 82 g/m 2  in basis weight, and A4 (210 mm in width and 297 mm in length). In terms of flatness, recording medium CLC 80 g, which is ordinary paper for color printing, is relatively high, although it is inferior to glossy paper. The image forming apparatus (fixation unit) in this embodiment can more easily recover the offset toner To on the fixation roller  30  than the image forming apparatus (fixation unit) in the preceding embodiment. Therefore, it is unnecessary to use glossy paper, which usually is more expensive than ordinary paper; the fixation unit can be cleaned with the use of ordinary flat paper. 
         [0154]    When this paper (CLC 80 g) was used as recording medium in an ordinary image forming operation, the apparatus was operated in the fixation mode for ordinary paper which is 80 g/m 2 . The process speed was set to 70 mm/sec, and the ceramic heater  15  was controlled so that its temperature was in a range of 180° C.-200° C., in order to ensure that an ordinary image was satisfactorily fixed, that is, to prevent the problem that the image is under-fixed and/or “hot offset”. 
         [0155]    As the image forming apparatus was started in the cleaning mode, the process speed was set to 45 mm/sec, which was half the process speed in the ordinary fixation mode. Then, an image forming operation was started, and also, the fixation roller  30  of the fixation unit began to be driven. The ceramic heater  15  was controlled so that its temperature remained in a range of 200° C.-220° C. 
         [0156]    Then, the sheet P was conveyed, and an unfixed toner image for the cleaning mode was formed on the sheet P in the image formation portion. Then, the sheet P was conveyed to the fixation nip N 1  of the fixation unit with such a timing that it entered the fixation nip N 1  20 seconds after electric power began to be sent to the ceramic heater  15 . 
         [0157]    At this point of the cleaning operation, the temperature of the film  16  had climbed to 190° C.-210° C., and the temperature of the fixation roller  30  had climbed to 180° C.-200° C. Further, the temperature of the pressure roller  17  had climbed to 100° C.-120° C. The pattern of the image for cleaning mode was the same as the one in the first embodiment. 
         [0158]    As the portion of the sheet P, which was between the leading edge of the sheet P and a hypothetical line on the sheet P, which was 140 mm from the leading edge, passed through the fixation nip N 1 , the target temperature range for the ceramic neater  15  was changed to 160° C.-180° C. while the process speed was kept at 45 mm/sec. This setup was for giving a proper amount of heat to the toner on the sheet P so that neither the under-fixation nor hot “offset” would occur when the process speed was 45 mm/sec. 
         [0159]    Also at this point of the cleaning operation, the temperature of the film  16  was in a range of 150° C.-170° C., and the temperature of the fixation roller  30  was in a range of 140° C.-160° C. Further, the temperature of the pressure roller  17  was in a range of 70° C.-90° C. 
         [0160]    The results of this test in which the image forming apparatus was operated in the cleaning mode were as follows: Among the contaminants such as the contaminant toner Tc having adhered to the film  16 , those which had not solidly adhered to the film  16  was removed. Further, the problem that the rest of the contaminants peels away from the film  16  and appear on the sheet P while the apparatus is operated in the ordinary fixation mode did not occur. In other words, this embodiment of the present invention also was very effective to clean the fixation unit. 
         [0161]    In this embodiment, the offset toner To and contaminant toner Tc which were adhered to the peripheral surface of the fixation roller  30  were entirely recovered by being adhered to the surface of the sheet P, in spite of the use of a sheet of paper, which was ordinary in terms of flatness, as the cleaning sheet P. 
       Embodiment 3 
       [0162]    Next, the image forming apparatus (fixation unit) in the third embodiment of the present invention will be described. The basic structure of the apparatus in this embodiment is the same as those of the apparatuses in the preceding embodiments. Therefore, the portions of the structure of the apparatus in this embodiment, which are the same in function as those in the preceding embodiments will not be described. Further, the components, portions, etc., which are the same in function as the counterparts in the preceding embodiments will be given the same referential codes as those given to the counterparts in the preceding embodiments. 
         [0163]    In this embodiment, the cleaning sheet P is formed so that the image from which toner is “hot offset” is on the leading end portion of the sheet P, and the image for recovering the offset toner To, that is, the portion of the sheet P onto which the toner on the fixation roller  30  is transferred, is on the trailing end portion of the sheet P. 
         [0164]      FIG. 16  is a drawing of an example of the pattern in which an image for the cleaning mode is formed. That is, the portion of the sheet P, which is between the leading edge of the sheet P and a hypothetical straight line (first hypothetical line) which is perpendicular to the leading edge of the sheet P, and the distance of which from the leading edge is no less than the distance between the fixation nip N 1  and the pair of discharge rollers  111 , is left as a blank; toner is not transferred onto this portion of the sheet P. The portion of the sheet P, which is on the upstream side of the first hypothetical straight line is covered with a solid image which is no less in length (in terms of lengthwise direction of sheet P) than the circumference of the film  16 , and is 50%-300%, preferably 70%-200%, in print ratio. This solid image is used as the source of the offset toner To. On the portion of the sheet P, which is between this solid image and the trailing edge of the sheet P, a solid image which is longer than the circumference of the film  16 , and is no less than 50% preferably, 100%, in print ratio, is formed. This image is used as the image for recovering the offset toner To. 
         [0165]    In this embodiment, the image forming apparatus is changed in the target temperature of its fixation roller and process speed while the sheet P is being moved through the fixation unit. More concretely, while the image on the sheet P, which is the source of the offset toner To, is being moved through the fixation nip N 1 , heat is given by the amount Qh which is large enough to cause “hot offset”. Then, after the passage of the image on the sheet P, which is the source of the offset toner To, through the fixation nip N 1 , heat is given by the amount Qn, which is the same as the amount by which heat is given in the ordinary fixation mode, that is, the fixation mode which does not cause “hot offset”. 
         [0166]    While the toner image on the sheet P, which is for the recovery of the offset toner To, is being moved through the fixation nip N 1 , heat is given by the amount Qn, which is the same as the amount by which heat is given in the ordinary fixation mode, that is, the fixation mode in which “hot offset” does not occur. At the same time as the image for recovering the offset toner To is fixed to the sheet P, it peels the offset toner To and contaminant toner Tc having adhered to the fixation roller  30 , from the fixation roller  30 , by its adhesiveness, making it easier for the offset toner To and contaminant toner Tc to be recovered by the cleaning sheet P. The higher the image for the contaminant recovery in print ratio, the more adhesive it becomes, and therefore, the easier it makes for the offset toner To and contaminant toner Tc to be recovered by the cleaning sheet P. 
         [0167]    At this time, referring to  FIG. 17  (flowchart) and  FIG. 18  (time table), the process carried out when the image forming apparatus (fixation unit) is in the cleaning mode will be described.  FIG. 17  is a flowchart of the operation of the image forming apparatus in this embodiment when the apparatus is in its cleaning mode.  FIG. 18  is a graphical time table which shows the timing of the toner movement on the film  16  and the timing of the temperature changes of the fixation roller  30 . 
         [0168]    The image forming apparatus is started in the cleaning mode (Step  1 - a ). Then, a target temperature and process speed are set. The target temperature level is to be high enough to generate heat by the amount Qh, which is large enough to cause the toner on the sheet P to “hot offset”. The process speed is to be proper to allow the toner on the sheet P to satisfactorily “hot offset” (Step  1 - b ). 
         [0169]    Next, each of various components involved with the cleaning mode begin to be driven, and electric power begins to be sent to the heater  15 , whereby the temperature of the fixation roller  30  climbs to a preset level (Step  1 - c ). Next, the sheet P is fed into the apparatus main assembly  101  (Step  1 - d ). Then, an unfixed image for generating the offset toner To, and an unfixed image for recovering the offset toner To and contaminant toner Tc, are formed on the sheet P (Step  1 - e ). 
         [0170]    This sheet P is conveyed to the fixation unit, and the unfixed image on the sheet P, which is for the generation of the offset toner To, reaches the fixation nip N 1 . As the unfixed image on the sheet P, which is for generating the offset toner To, reaches the fixation nip N 1  (Step  1 - f ), the toner, of which the unfixed image is formed, turns into the offset toner To, and transfers (“hot offset”) onto the fixation roller  30  (Step  1 - g ). 
         [0171]    After the image on the sheet P, which is for generating the offset toner To, passes the fixation nip N 1 , the heater  15  is changed in target temperature, changing thereby the fixation roller  30  in temperature, so that the sheet P will be given heat by the amount Qn, which is the same amount by which heat is given to the sheet P in the ordinary fixation mode (Step  1 - h ). 
         [0172]    As the fixation roller  30  rotates, the offset toner To on the fixation roller  30  reaches the fixation roller heating nip N 2 , in which it comes into contact with the film  16 , whereby it transfers the contaminant toner Tc on the film  16 , onto the fixation roller  30  (Step  2 - a ). 
         [0173]    Next, as the fixation roller  30  rotates further, the contaminant toner Tc and offset toner To on the fixation roller  30  reach the fixation nip N 1 , in which they are fixed to the sheet P (Step  3 - a ). Thereafter, the image on the sheet P, which is for recovering the offset toner To, reaches the fixation nip N 1 , in which the contaminant toner Tc and offset toner To are fixed, along with the toner image for recovery, to the sheet P (Step  3 - b ). Then, the sheet P is discharged (Step  3 - c ). Then, the electric power to the heater  15  is turned off, and the image forming apparatus is stopped, ending the image forming operation in the cleaning mode (Step  3 - d ). 
       (Test Results) 
       [0174]    A test is carried out to confirm the effectiveness of the fixation unit in this embodiment in the cleaning mode. The image forming apparatus and fixation unit used for the test are the same as those in the first embodiment. The fixation unit in this embodiment was tested in terms of the effectiveness with which it can remove the contaminant toner To having adhered to the film  16 , in the same manner as the manner in which the fixation unit in the first embodiment was tested. 
         [0175]    As the image forming apparatus is started in the cleaning mode, the process speed was set to 40 mm/sec, which was half the process speed in the ordinary fixation mode. Then, an image forming operation was started, and the driving of the fixation roller  30  of the fixation unit was also started. The ceramic heater  15  was controlled so that its temperature remained in a range of 200° C.-220° C. 
         [0176]    The cleaning sheet P used to test the fixation unit in this embodiment was a sheet of ordinary LBP printing paper (product of), which was 80 g/m 2  in basis weight, and A4 in size. The sheet of paper used as the cleaning sheet in this test was not a sheet of paper dedicated to color printing. It was a sheet of ordinary LBP printing paper. It was not special in terms of flatness. However, when the image forming apparatus (fixation unit) in this embodiment was operated in the fixation mode, an image for recovering the offset toner To and contaminant toner Tc was formed on the trailing end portion of the cleaning sheet P. Therefore, the fixation unit (film  16 ) was satisfactorily cleaned. 
         [0177]    In the test, the sheet P was conveyed to the image forming portion, and an unfixed toner image for the cleaning mode was formed on the sheet P in the image formation portion. Then, the sheet P was conveyed to the fixation nip N 1  of the fixation unit with such a timing that it entered the fixation nip N 1  20 seconds after electric power began to be sent to the ceramic heater  15 . 
         [0178]    At this point of the cleaning operation, the temperature of the film  16  had climbed to 190° C.-210° C., and the temperature of the fixation roller  30  had climbed to 180° C.-200° C. Further, the temperature of the pressure roller  17  had climbed to 100° C.-120° C. 
         [0179]    As for the pattern of the image formed on the sheet P for the cleaning mode, the portion of the sheet P, which was between the leading edge of the sheet P to a hypothetical straight line (first line) on the sheet P, which was 70 mm apart from the leading edge, was left as a blank (white) space; the portion of the sheet P, which was between the aforementioned first hypothetical straight line on the sheet P to a hypothetical straight line (second line) which was 140 mm from the leading edge of the sheet P, was covered with a monochromatic solid image, which was formed of black toner and was 200 mm in width; the portion of the sheet P, which was between the hypothetical second straight line which was 140 mm apart from the leading edge of the sheet P, and a hypothetical straight line (third line) which was 210 mm apart from the leading edge of the sheet P, was left as a blank (white) space; and the portion of the sheet P, which was between the hypothetical third straight line which was 210 mm apart from the leading edge of the sheet P and the trailing edge of the sheet P, was covered with a solid image, which was formed of cyan toner and magenta toner, was 200% in print ratio, and was 210 mm in width. 
         [0180]    The image formed on the portion of the sheet P, which was between the first hypothetical line which was 70 mm apart from the leading edge and the second hypothetical line which was 140 mm apart from the leading edge, was for generating the offset toner To, and the image formed on the portion of the sheet P, which was between the aforementioned two hypothetical straight lines which were 140 mm and 210, respectively, apart from the leading edge of the sheet P, was for recovering the offset toner To. 
         [0181]    The portion of the sheet P, which was between the leading edge of the sheet P and the first hypothetical straight line which was 70 mm apart from the leading edge, was left as a blank (white) space in consideration of the length of time it takes for the fixation unit to change in the fixation temperature. This portion of the sheet P also functioned to recover the offset toner To having adhered to the fixation roller  30 . 
         [0182]    As the portion of the sheet P, which was between the leading edge of the sheet P and the second hypothetical line on the sheet P, which was 140 mm apart from the leading edge, passed through the fixation nip N 1 , the target temperature range for the ceramic heater  15  was changed to 160° C.-180° C. while the process speed was kept at 45 mm/sec. This temperature setup was for giving a proper amount of heat to the toner on the sheet P so that neither the under-fixation nor “hot offset” would occur when the process speed was 45 mm/sec. 
         [0183]    When the fixation roller  30  and pressure roller  17  were cold (had cooled down), the target temperature for the ceramic heater  15  was set higher than when the fixation roller  30  and pressure roller  17  were warm. By the time the portion of the sheet P, which was 210 mm apart from the leading edge of the sheet P, reached the fixation nip N 1 , the temperature of the ceramic heater  15  and the temperature of the fixation roller  30 , changed to the target temperature level for the ordinary fixation mode. Thus, the image for recovering the offset toner To was fixed, without being “hot offset” at all. 
         [0184]    At this point in time, the temperature of the film  16  was 150° C.-170° C., and the temperature of the fixation roller  30  was 140° C.-160° C. Further, the temperature of the pressure roller  17  was 70° C.-90° C. 
         [0185]    The results of this operation in the cleaning mode were as follows: Among the contaminants such as the contaminant toner Tc having had adhered to the film  16 , those which had not solidly adhered to the film  16  were removed. Further, it did not occur that the rest of the contaminants peels from the film  16  and transferred onto the sheet P when the image forming apparatus is in the normal fixation mode. In other words, this embodiment of the present invention also was excellent in terms of effectiveness in the fixation mode. 
         [0186]    Although the sheet of paper used as the cleaning sheet P in this embodiment was no flatter than a sheet of ordinary paper, the offset toner To and contaminant toner Tc, which were adhered to the peripheral surface of the fixation roller  30 , were entirely recovered by being fixed to the surface of the sheet P. 
       Embodiment 4 
       [0187]    Next, the image forming apparatus in the fourth embodiment of the present invention will be described. The basic structure of the apparatus in this embodiment is the same as those of the apparatuses in the preceding embodiments. Therefore, the portions of the structure of the apparatus in this embodiment, which are the same in function as those in the preceding embodiments will not be described. Further, the components, portions, etc., which are the same in function as the counterparts in the preceding embodiments will be given the same referential codes as those given to the counterparts in the preceding embodiments. 
         [0188]    In this embodiment, the cleaning operation is carried out using sheets P 1  and P 2 . The sheet P 1  is for generating the offset toner To. The sheet P 2  is for recovering the offset toner To. 
         [0189]    The image for generating the offset toner To is formed on the sheet P 1  for generating the offset toner To, which is the first sheet of paper to be fed into the image forming apparatus when the apparatus is in the fixation mode. The sheet P 1  is conveyed to the fixation nip N 1 . In the fixation nip N 1 , the toner of which the image on the sheet P 1  is formed is adhered to the fixation roller  30  by giving heat to the sheet P 1  and the image thereon by the amount Qh, which is large enough to cause “hot offset”. Then, the apparatus is changed in the target temperature and process speed. 
         [0190]    Then, an image for recovering the offset toner To is formed on the sheet P 2 , that is, the sheet for recovering the offset toner To. Then, the sheet P 2  is conveyed to the fixation nip N 1 . In the fixation nip N 1 , the offset toner To and contaminant toner Tc having been adhered to the fixation roller  30  are recovered by giving heat to the toner on the sheet P 2  by the amount Qn, which does not cause the under-fixation nor “hot offset”, that is, the same amount as the amount by which heat is given to the toner on a sheet of recording medium when the apparatus is in the ordinary fixation mode. 
         [0191]    It is desired that the fixation roller  30 , film  16 , and pressure roller  17  fully rotate several times between when the sheet P 1  comes out of the fixation nip N 1  and when the sheet P 2  is introduced into the fixation nip N 1 . 
         [0192]    Regarding the relationship between the adhesiveness between the surface layer (slippery layer) of the pressure roller  17  (backup member) and toner, and the adhesiveness between the surface layer (slippery layer) of the surface layer  30 C (slippery layer) of the fixation roller  30  and toner, the former is higher than the latter; the surface layer of the pressure roller  17  is slipperier than the surface layer  30 C (slippery layer) of the fixation roller  30 . 
         [0193]    The method used for testing the apparatus in this embodiment was the same as that used for testing the apparatus in the first embodiment. However, it is desired that when the apparatus in this embodiment is in the cleaning mode, the length of time between when electric power begins to be sent to the ceramic heater  15  and when the sheet P 1  begins to be conveyed through the fixation nip N 1  is made as long as possible, in order for the fixation unit to become warm enough to make the difference in temperature between the fixation roller  30  and pressure roller  17  relatively small. 
         [0194]    This is desired to prevent the toner on the fixation roller  30  from transferring onto the pressure roller  17  during the period between when the sheet P 1  comes out of the fixation nip N 1  and when the sheet P 2  begins to be conveyed through the fixation nip N 1 . 
         [0195]    Next, referring to  FIGS. 19(   a ) and  19 ( b ), the image for the cleaning mode in this embodiment will be described. 
         [0196]    The pattern in which the image for the cleaning mode is formed on the sheet P 1  (sheet for generating offset toner To) is as follows: As the pattern is printed on the sheet P 1 , the portion of the sheet P 1 , which is between the leading edge of the sheet P 1  and a hypothetical straight line which is perpendicular to the leading edge, and the distance of which from the leading edge is no less than the distance from the fixation nip N 1  to the pair of discharge rollers  111 , is left as a blank (white) space (no toner is placed on this area); and the portion of the sheet P 1 , which is between the abovementioned hypothetical straight line and the trailing edge of the sheet P 1  or another hypothetical straight line which is perpendicular to the leading edge, and the distance of which from the first hypothetical straight line is no less than the circumference of the film  16 , is covered with a solid image, the print ratio of which is in a range of 50% -300%, preferably, 70%-200%. The sheet P 1  across which the image is formed in the above-described pattern, is used as the sheet P for generating the offset toner To. 
         [0197]    The offset toner To which came onto the fixation roller  30  from the trailing edge portion of the sheet P 1  remains on the fixation roller  30 , and keeps on cleaning the film  16 , until the second sheet P 2  is introduced into the fixation nip N 1 . 
         [0198]    The image pattern for the sheet P 2  (sheet for recovering offset toner To) is as follows: As the pattern is printed on the sheet P 2 , the portion of the sheet P 2 , which is between the leading edge of the sheet P 2  and a hypothetical straight line on the sheet P 2 , which is perpendicular to the leading edge, and the distance of which from the leading edge is no less than the distance from the fixation nip N 1  to the pair of discharge rollers  111 , is left as a blank (white) space (no toner is placed on this area); and the portion of the sheet P 2 , which is between the abovementioned hypothetical straight line and the trailing edge of the sheet P 2  or another hypothetical straight line on the sheet P 2 , which is perpendicular to the leading edge, and the distance of which from the first hypothetical straight line is no less than the circumference of the film  16 , is covered with a solid image, the print ratio of which is no less than 50%, preferably, 100%. The sheet P across which the image is formed of toner in the above described pattern, is used as the sheet P 2 , that is, the sheet P for recovering the offset toner To. 
         [0199]    When the apparatus in this embodiment is in the cleaning mode, the offset toner To on the fixation roller  30  can be moved through the fixation roller heating nip N 2  two or more times. Thus, it is more effective in terms of the cleaning of the film  16  than those in the preceding embodiments. Further, the contaminant toner Tc on the pressure roller  17  also can be recovered by placing the offset toner To on the fixation roller  30 , in contact with the pressure roller  17 , in the fixation nip N 1 . 
         [0200]    Next, referring to  FIGS. 20  (flowchart),  21  (time table), and  22  (time table), the process carried out when the image forming apparatus (fixation unit) is in the cleaning mode will be described.  FIG. 20  is a flowchart of the operation of the image forming apparatus in this embodiment when the apparatus is in its cleaning mode.  FIGS. 21 and 22  are graphical time tables which show the timing of the toner movements on the sheets P 1  and P 2 , fixation roller  30 , the film  16 , and pressure roller  17 , and the timing of the temperature changes of the fixation roller  30 . Shown in  FIG. 21  are the operational steps in the period from when the sheet P 1  is conveyed into the fixation unit to when the sheet P 1  is discharged from the fixation unit. Shown in  FIG. 22  are the operations steps in the period from when the sheet P 2  is conveyed into the fixation unit to when the sheet P 2  is discharged from the fixation unit. 
         [0201]    The image forming apparatus is started in the cleaning mode (Step  1 - a ). Then, a target temperature and process speed are set (Step  1 - b ). The target temperature level is to be high enough to generate heat by the amount Qh, which is large enough to cause the toner on the sheet P 1  to “hot offset”. The process speed is to be proper to allow the toner on the sheet P to satisfactorily “hot offset”. 
         [0202]    Next, each of various components involved with the cleaning mode begin to be driven, and electric power begins to be sent to the heater  15  (Step  1 - c ). As the temperature of the fixation roller  30  climbs to a preset level, the sheet P 1  is fed into the apparatus main assembly  101  (Step  1 - d ). Then, an unfixed image for cleaning is formed on the sheet P 1  (Step  1 - e ). 
         [0203]    The sheet P 1  is conveyed to the fixation unit (Step  1 - f ), and the unfixed image on the sheet P, which is for the generation of the offset toner To, reaches the fixation nip N 1 . As the unfixed image on the sheet P 1  is conveyed through the fixation nip N 1 , the toner, of which the unfixed image is formed, turns into the offset toner To, and transfers onto the fixation roller  30  (Step  1 - g ). 
         [0204]    Then, as the fixation roller  30  is rotated, the offset toner To on the fixation roller  30  reaches the fixation roller heating nip N 2 , in which it comes into contact with the film  16 , causing thereby the contaminant toner Tc on the film  16  to transfer onto the fixation roller  30  (Step  2 - a ). 
         [0205]    Next, as the fixation roller  30  rotates further, the combination of the contaminant toner Tc and offset toner To reach the fixation nip N 1 . 
         [0206]    As the combination of the contaminant toner Tc and offset toner To reaches the fixation nip N 1 , a part of the combination is fixed to the sheet P 1 , and the rest remains on the fixation roller  30 , along with the fresh supply of offset toner To having just been generated from the unfixed image on the sheet P 1 , which is for the generation of the offset toner To, and been transferred onto the fixation roller  30 . Thus, after the passage of the sheet P 1  through the fixation nip N 1 , at least the offset toner To which is traceable to the unfixed toner image formed across the trailing end portion of the sheet P 1  is present on the fixation roller  30 . 
         [0207]    As soon as the sheet P 1  comes out of the fixation nip N 1  (Step  2 - b ), the heater  15  is changed in target temperature (Step  2 - c ), whereby the fixation roller  30  is changed in temperature. The new target temperature level is such that gives the sheet P 2  heat by the amount Qn, which is the same amount by which heat is given to a sheet of recording medium when the image forming apparatus (fixation unit) is in the ordinary fixation mode. 
         [0208]    As the fixation roller  30  rotates further, the offset toner To reaches the fixation nip N 1 , in which it comes into contact with the pressure roller  17 , causing thereby the contaminant toner Tc having adhered to the pressure roller  17 , to transfer onto the fixation roller  30  (Step  2 - d ). 
         [0209]    Until the sheet P 2  is conveyed to the fixation unit, the offset toner To on the fixation roller  30  remains on the peripheral surface of the fixation roller  30 , and therefore, is repeatedly moved through the fixation nip N 1  and fixation roller heating nip N 2 , while continuously cleaning the pressure roller  17  and film  16 , respectively. The slippery layer of the film  16  and the slippery layer of the pressure roller  17  are made slipperier than the slippery layer of the fixation roller  30 . Therefore, the offset toner To can clean the pressure roller  17  and film  16  while remaining on the peripheral surface of the fixation roller  30 . 
         [0210]    It is possible that if, for some reasons, the temperature of the pressure roller  17  does not climb high enough, a part of the offset toner To and the like on the fixation roller  30  will transfer onto the pressure roller  17 . However, even if a part of the offset toner To and the like on the fixation roller  30  transfers onto the pressure roller  17 , it is discharged from the fixation unit by being transferred from the pressure roller  17  onto the back side of the sheet P 2  in Step  3 - a  and the steps thereafter. 
         [0211]    During the period from when the sheet P 1  is discharged from the fixation unit to when the sheet P 2  is conveyed to the fixation unit, the fixation unit lessens in the film cleaning performance. However, the contaminant toner Tc removed from the film  16  while the sheet P 1  is conveyed through the fixation unit can be discharged can be discharged even during this period. 
         [0212]    The contaminant toner Tc having had adhered to the pressure roller  17  can also be discharged by fixing it to the back side of the sheet P 2  with the utilization of the adhesiveness of the offset toner To having adhered to the pressure roller  17 , through Step  3 - a  and the steps thereafter. 
         [0213]    Next, the sheet P 2  is fed into the apparatus main assembly  101  (Step  3 - a ). Then, an unfixed image for recovery is formed on the sheet P 2  (Step  3 - b ). This sheet P 2  is conveyed to the fixation unit (Step  3 - c ). 
         [0214]    The contaminant toner Tc and offset toner To on the fixation roller  30  are fixed to the sheet P 2  in the fixation nip N 1  (Step  3 - d ). 
         [0215]    The image for recovery, which is on the sheet P 2 , reaches the fixation nip N 1 , in which the contaminant toner Tc and offset toner To are fixed to the sheet P 2 , along with the image for recovery (Step  3 - e ). 
         [0216]    Then, this sheet P 2  is discharged (Step  3 - f ). Then, the electric power to the heater  15  is turned off, and the image forming apparatus is stopped, ending thereby the operation in the cleaning mode (Step  3 - g ). 
       (Test Results) 
       [0217]    A test is carried out to confirm the effectiveness of the fixation unit in this embodiment in the cleaning mode. The image forming apparatus and fixation unit used for the test are the same as those in the first embodiment. 
         [0218]    The fixation unit in this embodiment was tested in terms of the effectiveness with which it can remove the contaminant toner Tc having adhered to the film  16 , and the contaminant toner Tc having adhered to the pressure roller  17 , in the same manner as the manner in which the fixation unit in the first embodiment was tested. 
         [0219]    As the image forming apparatus was started in the cleaning mode, the process speed was set to 45 mm/sec, which was half the process speed in the ordinary fixation mode. Then, an image forming operation was started, and also, the fixation roller  30  of the fixation unit began to be driven. The ceramic heater  15  was controlled so that its temperature remained in a range of 200° C.-220° C. 
         [0220]    Then, sending of electric power to the ceramic heater  15  was started. Then, the image forming apparatus was kept on standby, for 10 seconds˜5 minutes, while continuously sending electric power to the ceramic heater  15 . Then, the sheet P 1  was conveyed into the fixation nip N 1  of the fixation unit. 
         [0221]    Incidentally, if the image forming apparatus begins to be operated in the cleaning mode after the fixation unit cools down, the apparatus is to be kept on standby longer than when the apparatus is started in the cleaning mode while the fixation unit is still warm. While the apparatus is kept on standby, the temperature of the film  16  will have climbed to 190° C. -210° C., and the temperature of the fixation roller  30  will have climbed to 180° C.-200° C. Further, the temperature of the pressure roller  17  will have climbed to 140° C.-150° C. The pattern of the image for cleaning mode was the same as the one in the first embodiment. 
         [0222]    As the cleaning sheet P 1  for generating the offset toner To, a sheet of LBP printing paper (product of) which was 80 g/m 2  in basis weight and A4 in size was conveyed into the image forming portion, in which an unfixed toner image for the cleaning mode was formed on the cleaning sheet P 1 . Then, the sheet P 1  was conveyed into the fixation nip N 1 . As for the pattern of the image formed on the sheet P 1  for the cleaning mode, the portion of the sheet P 1 , which was between the leading edge of the sheet P 1  to a hypothetical straight line on the sheet P 1 , which was perpendicular to the sheet conveyance direction and is 70 mm apart from the leading edge, was left as a blank (white) space, and the portion of the sheet P 1 , which was between the abovementioned hypothetical straight line which was 70 mm apart from the leading edge and the trailing edge of the sheet P 1 , was covered with a solid monochromatic image which was formed of black toner in a width of 200 mm and was 100% in print ratio. The toner on the sheet P 1  was “hot offset” onto the fixation roller  30 , becoming the offset toner To, in the fixation nip N 1 . 
         [0223]    As the sheet P 1  came out of the fixation nip N 1 , the process speed was changed to 90 mm/sec, and the target temperature of the ceramic heater  15  was changed to 180° C.-200° C. This target temperature range was such a range that was able to provide the toner on the sheet P 2  with a proper amount of heat, that is, such an amount of heat that did not cause neither the under-fixation nor “hot offset”. When the fixation roller  30  and pressure roller  17  were cold (having cooled down), the target temperature for the ceramic heater  15  was set relatively high, whereas when they were warm, it was set relatively low, as it was in the ordinary fixation mode. 
         [0224]    At this point of the cleaning operation, the temperature of the film  16  was in a range of 170° C.-190° C., and the temperature of the fixation roller  30  was in a range of 160° C.-180° C. Further, the temperature of the pressure roller  17  was in a range of 90° C.-110° C. 
         [0225]    Incidentally, as the contaminant toner Tc collects on the film  16 , it occurs sometimes that the contaminant toner Tc peels away from the film  16 , adheres to the fixation roller  30 , transfers from the fixation roller  30  to the pressure roller  17 , and adhere to the pressure roller  17 . It also occurs sometimes that the contaminant toner Tc collects on the pressure roller  17 . 
         [0226]    By the time the cleaning sheet P 2  was introduced into the fixation nip N 1  after the passage of the fixation nip N 1  of the cleaning sheet P 1  through the fixation nip N 1 , the fixation roller  30 , pressure roller  17 , and film  16  were fully rotated five times. During this period, the contaminant toner Tc having adhered to the film  16  and the contaminant toner Tc having adhered to the pressure roller  17  were transferred onto the fixation roller  30 . 
         [0227]    Next, as the cleaning sheet P 2  for recovering the offset toner To, a sheet of LBP printing paper (product of) which was 80 g/m 2  in basis weight and A4 in size was conveyed into the image forming portion, in which an unfixed toner image for the cleaning mode was formed on the cleaning sheet P 2 . Then, the sheet P 2  was conveyed into the fixation nip N 1 . As for the pattern of the image formed on the sheet P 2  for the cleaning mode, the portion of the sheet P 2 , which was between the leading edge of the sheet P 2  and a hypothetical straight line on the sheet P 2 , which was perpendicular to the sheet conveyance direction and is 70 mm apart from the leading edge, was left as a blank (white) space, and the portion of the sheet P 2 , which was between the abovementioned hypothetical straight line which was 70 mm apart from the leading edge and the trailing edge of the sheet P 2 , was covered with a solid monochromatic image which was formed of a combination of cyan toner and magenta toner in a width of 200 mm and was 200% in print ratio. 
         [0228]    Then, the offset toner To and contaminant toner To on the fixation roller  30  were recovered onto the sheet P 2  while fixing the image on the sheet P 2 , which was for recovering the offset toner To, without allowing the image to “hot offset”, in the fixation nip N 1 . 
         [0229]    The results of this operation in the cleaning mode were as follows: Among the contaminants such as the contaminant toner Tc having had adhered to the film  16 , those which had not solidly adhered to the film  16  were removed. Further, it did not occur that the rest of the contaminants peels from the film  16  and transfers onto the sheet P 2  when the image forming apparatus is in the normal fixation mode. In other words, this embodiment of the present invention also was excellent in effectiveness in the fixation mode. Further, among the contaminants having had adhered to the pressure roller  17 , those which had not have solidly adhered to the pressure roller  17  were removed. 
       Embodiment 5 
       [0230]    Next, the image forming apparatus (fixation unit) in the fifth embodiment of the present invention will be described. The basic structure of the apparatus in this embodiment is the same as those of the apparatuses in the preceding embodiments. Therefore, the portions of the structure of the apparatus in this embodiment, which are the same in function as those in the preceding embodiments will not be described. Further, the components, portions, etc., which are the same in function as the counterparts in the preceding embodiments will be given the same referential codes as those given to the counterparts in the preceding embodiments. 
         [0231]    In the case of the image forming apparatus in the fourth embodiments described above, which was capable of printing on both surfaces of a sheet of recording medium, the cleaning operation was carried out using the sheet P 1 , which is for generating the offset toner To, and the sheet P 2 , which is for recovering the offset toner To. In this embodiment, one of the two surfaces of the sheet P is used to adhere the offset toner To to the fixation roller  30 . Then, as the sheet P is discharged from the fixation unit, it is turned over, and an image for recovering the offset toner To is formed on the other surface of the sheet P. In other words, the sheet P used for adhering the offset toner To to the fixation roller  30  is also used for recovering the offset toner To and contaminant toner Tc on the fixation roller  30 . 
         [0232]    An image for generating the offset toner To is formed on the sheet P in the image forming portion. Then, this sheet P is conveyed to the fixation nip N 1 , in which heat is given the toner on the sheet P by the amount Qh, which is large enough to cause the toner on the sheet P to “hot offset”, adhering thereby toner to the fixation roller  30 . Then, the apparatus is changed in the target temperature for the fixation unit, and the process speed. While the sheet P is conveyed through the fixation nip N 1 , a part of the unfixed toner on the sheet P is transferred (“hot offset”) onto the fixation roller  30 , and the rest is fixed, as “residual toner image”, to the sheet P. After the adhering of the offset toner To to the fixation roller  30 , the sheet P is discharged from the fixation unit, and then, is turned over by an automatic sheet turning mechanism with which the image forming apparatus is provided, or is manually turned over by a user. Then, the sheet P is conveyed again into the image forming portion, with its surface having “residual toner image” facing opposite from the direction in which it was facing when the sheet P is conveyed through the image forming portion for the first time. 
         [0233]    Then, an image for recovering the offset toner To is formed on the second surface of the sheet P in the image forming portion. Then, this sheet P is conveyed to the fixation nip N 1 . In the fixation nip N 1 , the offset toner To and contaminant toner Tc adhered to the fixation roller  30  are recovered by the sheet P while the image for recovering the offset toner To is fixed by giving heat to the toner on the sheet P by the amount Qn, which is the same as the amount by which heat is given to the toner on a sheet of recording medium when the apparatus is in the ordinary fixation mode, that is, the amount which does not cause neither the under-fixation nor “hot offset”. During this operational period, the fixed “residual toner image” on the first surface of the sheet P is melted, becoming thereby adhesive, and removes the contaminant toner To on the pressure roller  17  by coming into contact with the pressure roller  17 . 
         [0234]    Also in this embodiment, it is desired that by the time the sheet P is introduced into the fixation nip N 1  for the second time after the passage of the sheet P through the fixation nip N 1 , the fixation roller  30 , film  16 , and pressure roller  17  are fully rotated two or more times, as in the fourth embodiment. 
         [0235]    This embodiment was able to highly effectively clean the fixation unit with the use of only a single sheet of recording medium. Further, it was able to clean the pressure roller  17  more effectively than the preceding embodiments. 
         [0236]    Next, referring to  FIG. 23  (flowchart),  24  (time table), and  25  (time table), the process carried out in the cleaning mode in this embodiment will be described.  FIG. 23  is a flowchart of the operation of the image forming apparatus in the cleaning mode in this embodiment.  FIGS. 24 and 25  are graphic time tables which show the toner movements on the sheet P, fixation roller  30 , film  16 , and pressure roller  17 , and the temperature changes of the fixation roller  30 . 
         [0237]      FIG. 24  shows the operational period in which the sheet P is introduced first time into the fixation unit, is discharged therefrom, is turned over, and then, is introduced into the fixation unit for the second time.  FIG. 25  shows the operational period from when the turned sheet P is introduced into the fixation unit to when the turned sheet P is discharged from the fixation unit. 
         [0238]    The image forming apparatus is started in the cleaning mode (Step  1 - a ). Then, the target temperature and process speed are set (Step  1 - b ). The target temperature level is to be high enough to generate heat by the amount Qh, which is large enough to cause the toner on the sheet P to “hot offset”. The process speed is to be proper to allow the toner on the sheet P to satisfactorily “hot offset”. 
         [0239]    Next, each of various components involved with the cleaning mode begin to be driven, and electric power begins to be sent to the heater  15 , whereby the temperature of the fixation roller  30  climbs to a preset level (Step  1 - c ). Next, the sheet P is fed into the apparatus main assembly  101  (Step  1 - d ). Then, an unfixed image for cleaning is formed on the sheet P (Step  1 - e ). 
         [0240]    This sheet P is conveyed to the fixation unit (Step  1 - f ), and the unfixed image on the sheet P, which is for the generation of the offset toner To, reaches the fixation nip N 1 . As the unfixed image on the sheet P is conveyed through the fixation nip N 1 , the toner, of which the unfixed image is formed, turns into the offset toner To, and transfers onto the fixation roller  30 . The toner which does not “hot offset” is fixed to the sheet P (Step  1 - g ). Hereafter, the fixed image formed of the toner which did not “hot offset” will be referred to as a residual image. 
         [0241]    As the fixation roller  30  rotates, the offset toner To on the fixation roller  30  reaches the fixation roller heating nip N 2 , in which it comes into contact with the film  16 , whereby it transfers the contaminant toner Tc on the film  16 , onto the fixation roller  30  (Step  2 - a ). 
         [0242]    Next, as the fixation roller  30  rotates further, the combination of the contaminant toner Tc and offset toner To reaches the fixation nip N 1 . 
         [0243]    A part of the combination of the contaminant toner Tc and offset toner To is fixed to the sheet P, and the rest remains on the fixation roller  30 , along with the new supply of offset toner To generated by the unfixed image for offset toner generation, which is on the sheet P. After the passage of the sheet P through the fixation nip N 1 , at least the offset toner To, which is traceable to the portion of the unfixed image, which is formed on the trailing end portion of the sheet P, will have adhered to the fixation roller  30 . 
         [0244]    After the passage of the sheet P through the fixation nip N 1 , there will be the abovementioned residual image, offset toner To, contaminant toner Tc, etc., on the sheet P. 
         [0245]    After the passage of the sheet P through the fixation nip N 1  (Step  2 - b ), the fixation roller  30  is changed in temperature by changing the target temperature of the heater  15  (Step  2 - c ). The temperature level to which the target temperature of the heater  15  is changed is such a level that heat is given to the toner by the amount Qn, which is the same as the amount by which heat is given to the toner when the apparatus is in the ordinary fixation mode. 
         [0246]    As the fixation roller  30  rotates, the offset toner To reaches the fixation nip N 1 , in which it comes into contact with the pressure roller  17 , causing thereby the contaminant toner Tc to transfer onto the fixation roller  30  (Step  2 - d ). 
         [0247]    Until the sheet P is conveyed to the fixation unit, the offset toner To on the fixation roller  30  remains on the peripheral surface of the rotating fixation roller  30 , being therefore repeatedly put through the film heating nip N 2  and fixation nip N 1  while cleaning the film  16  and pressure roller  17 , respectively. The peripheral layer (slippery layer) of the film  16 , and the peripheral layer (slippery layer) of the pressure roller  17 , are slipperier than that of the fixation roller  30 . Thus, the offset toner To on the fixation roller  30  can clean the film  16  and pressure roller  17  while remaining on the peripheral surface of the fixation roller  30 . 
         [0248]    Even if the offset toner To having adhered to the fixation roller  30  transfers onto the pressure roller  17 , it is removed from the pressure roller  17  as it was in the fourth embodiment. 
         [0249]    As the sheet P comes out of the fixation unit, it is turned over by the automatic sheet turning mechanism, and is re-fed into the apparatus main assembly  101  (Step  3 - a ). Then, an unfixed image for contaminant recovery is formed on the second surface of the sheet P (Step  3 - b ). This sheet P is conveyed to the fixation unit (Step  3 - c ). 
         [0250]    The contaminant toner Tc and offset toner To on the fixation roller  30  are fixed to the second surface of the sheet p in the fixation nip N 1  (Step  3 - d ). Then, the contaminant recovery image on the sheet P reaches the fixation nip N 1 , in which the contaminant toner Tc and offset toner To are fixed to the second surface of the sheet P, along with the contaminant recovery image (Step  3 - e ). 
         [0251]    Then, the fixed residual image, etc., on the first surface of the sheet P reaches the fixation nip N 1 , come into contact with the pressure roller  17 , whereby it removes the contaminant toner Tc on the pressure roller  17  (Step  3 - f ). 
         [0252]    Then, the sheet P is discharged (Step  3 - g ). Then, heater  15  is turned off, and the image forming apparatus is stopped, ending the operation in the cleaning mode (Step  3 - h ). 
       (Test Results) 
       [0253]    A test is carried out to confirm the effectiveness of the fixation unit in this embodiment in the cleaning mode. The image forming apparatus and fixation unit used for the test are the same as those in the first embodiment. However, the image forming apparatus in this embodiment was provided with an automatic sheet turning mechanism so that after an image is fixed to one (first) of the two surfaces of the sheet P, the sheet P is automatically turned over, is introduced into the image forming apparatus for the second time, and an image is formed on the second surface of the sheet P. 
         [0254]    The performance of the image forming apparatus (fixation unit) in this embodiment is tested in terms of the removal of the contaminant toner Tc having adhered to the film  16 , and the removal of the contaminant toner Tc having adhered to the pressure roller  17 , as the performance of the image forming apparatus in the first embodiment was tested. 
         [0255]    As the image forming apparatus was started in the cleaning mode, the process speed was set to 45 mm/sec, which was half the process speed in the ordinary fixation mode. Then, an image forming operation was started, and also, the fixation roller  30  of the fixation unit began to be driven. The ceramic heater  15  was controlled so that its temperature remained in a range of 200° C.-220° C. 
         [0256]    Then, sending of electric power to the ceramic heater  15  was started. Then, the image forming apparatus was kept on standby for 10 seconds-5 minutes, while continuously sending electric power to the ceramic heater  15 . Then, the sheet P was conveyed into the fixation nip N 1  of the fixation unit. 
         [0257]    Incidentally, if the image forming apparatus begins to be operated in the cleaning mode after the fixation unit cools down, the apparatus is to be kept on standby longer than when the apparatus is started in the cleaning mode while the fixation unit is still warm. While the apparatus is kept on standby, the temperature of the film  16  will have climbed to 190° C. -210° C., and the temperature of the fixation roller  30  will have climbed to 180° C.-200° C. Further, the temperature of the pressure roller  17  will have climbed to 140° C.-150° C. The pattern of the image for cleaning mode was the same as the one in the first embodiment. 
         [0258]    As the cleaning sheet P for generating the offset toner To, a sheet of LBP printing paper (product of) which was 80 g/m 2  in basis weight and A4 in size was conveyed into the image forming portion, in which an unfixed toner image for the cleaning mode was formed on one of the two surfaces of the cleaning sheet P. Then, the sheet P was conveyed into the fixation nip N 1 . As for the pattern of the image formed on the sheet P for the cleaning mode, the portion of the sheet P, which was between the leading edge of the sheet P 1  and a hypothetical straight line on the sheet P, which is perpendicular to the sheet conveyance direction and is 70 mm apart from the leading edge, was left as a blank (white) space, and the portion of the sheet P, which was between the abovementioned hypothetical straight line which was 70 mm apart from the leading edge and the trailing edge of the sheet P, was covered with a solid monochromatic image which was formed of black toner in a width of 200 mm and was 100% in print ratio. The toner on the sheet P was “hot offset” onto the fixation roller  30 , becoming the offset toner To, in the fixation nip N 1 . 
         [0259]    As the sheet P came out of the fixation nip N 1 , the process speed was changed to 90 mm/sec, and the target temperature of the ceramic heater  15  was changed to 180° C.-200° C. This target temperature range was such a range that was able to provide the toner on the sheet P with a proper amount of heat, that is, such an amount of heat that did not cause neither the under-fixation nor “hot offset” when the process speed was 90 mm/sec. When the fixation roller  30  and pressure roller  17  were cold (having cooled down), the target temperature for the ceramic heater  15  was set relatively high, whereas when they were warm, it was set relatively low, as it was in the ordinary fixation mode. At this point of the cleaning operation, the temperature of the film  16  was in a range of 170° C.-190° C., and the temperature of the fixation roller  30  was in a range of 160° C.-180° C. Further, the temperature of the pressure roller  17  was in a range of 90° C.-110° C. 
         [0260]    After the sheet P was discharged from the fixation unit, it was turned over by the automatic sheet turning mechanism, and was sent to the image forming portion for the second time. 
         [0261]    In the image forming portion, an unfixed toner image for contaminant recovery was formed on the sheet P. Then, the sheet P was conveyed into the fixation nip N 1  of the fixation unit. As for the pattern of the image for the cleaning mode, the portion of the sheet P, which was between the leading edge of the sheet P and a hypothetical straight line on the sheet P, which was perpendicular to the sheet conveyance direction and is 70 mm apart from the leading edge, was left as a blank (white) space, and the portion of the sheet P, which was between the abovementioned hypothetical straight line which was 70 mm apart from the leading edge and the trailing edge of the sheet P, was covered with a solid monochromatic image which was formed of a combination of cyan toner and magenta toner in a width of 200 mm and was 200% in print ratio. 
         [0262]    The results of this operation in the cleaning mode were as follows: Among the contaminants such as the contaminant toner Tc having had adhered to the film  16 , those which had not solidly adhered to the film  16  were removed. Further, it did not occur that the rest of the contaminants peels from the film  16  and transfers onto the sheet P when the image forming apparatus is in the ordinary fixation mode. In other words, this embodiment of the present invention also was very effective to clean the fixation unit. 
         [0263]    Further, it was also possible to remove the contaminant toner Tc having had adhered to the pressure roller  17 . 
       Embodiment 6 
       [0264]    Next, the image forming apparatus (fixation unit) in the sixth embodiment of the present invention will be described. The basic structure of the apparatus in this embodiment is the same as those of the apparatuses in the preceding embodiments. Therefore, the portions of the structure of the apparatus in this embodiment, which are the same in function as those in the preceding embodiments will not be described. Further, the components, portions, etc., which are the same in function as the counterparts in the preceding embodiments will be given the same referential codes as those given to the counterparts in the preceding embodiments. 
         [0265]    In this embodiment, the fixation unit is cleaned by intermittently conveying the cleaning sheet P through the fixation unit. 
         [0266]    To describe more concretely the operation carried out by the image forming apparatus in this embodiment when the apparatus is in the cleaning mode, as the apparatus is started in the cleaning mode, the target temperature of the ceramic heater  15  of the fixation unit  109  is set to the temperature level for the cleaning mode, and the process speed of the apparatus is set to a preset value. Then, the driving of the fixation roller  30  is started, and the ceramic heater is turned on. 
         [0267]    Then, the image forming apparatus begins an image forming operation. As the image forming operation is started, the sheet P is fed into the apparatus main assembly  101  from the sheet feeder cassette  102 , and is sent to the image forming portion  108 , by the pair of sheet feeding-and-conveying roller  105 . In the image forming portion  108 , an image for the cleaning mode is formed on the sheet P. The image for the cleaning mode in this embodiment is the same in pattern as that in the first embodiment. 
         [0268]    As the unfixed image on the sheet P enters the fixation nip N 1  by a preset length, the fixation roller  30  is stopped from rotating, and is kept stationary for a preset length of time. Then, the sheet P is conveyed again by rotating the fixation roller  30  by an angle which is equivalent to the width of the fixation nip N 1  in terms of the sheet conveyance direction. Then, the fixation roller  30  is stopped again. That is, the sheet P is intermittently conveyed by repeating the alternate rotation and stopping of the fixation roller  30 . Then, as the trailing edge of the sheet P approaches the fixation nip N 1 , the fixation roller  30  is continuously rotated to discharge the sheet P. 
         [0269]    While the fixation roller  30  is kept stationary, it gives heat to the toner on the sheet P by an amount which is large enough to cause the unfixed toner to “hot offset” onto the fixation roller  30 . 
         [0270]    The offset toner To on the fixation roller  30  is intermittently moved through the fixation roller heating nip N 2  by the repetitive and alternate rotation and stopping of the fixation roller  30 , while transferring the contaminant toner Tc on the film  16  onto the fixation roller  30 . Then, the combination of the offset toner To and transferred contaminant toner Tc on the fixation roller  30  reach the fixation nip N 1 , while being intermittently moved by the repetitive and alternate rotation and stopping of the fixation roller  30 . Then, the combination is recovered by being fixed to the sheet P in the fixation nip N 1 . 
         [0271]    In this embodiment, the cleaning sheet P is intermittently moved through the fixation nip N 1 . Therefore, the length of time the fixation roller  30  and sheet P remain in contact with each other in the fixation nip N 1 , and the length of time the fixation roller  30  and film  16  remain in contact with each other, are longer than those in the preceding embodiments. 
         [0272]    Thus, this embodiment can more effectively transfer the contaminant toner Tc from the film  16  onto the fixation roller  30 , and recover the offset toner To and contaminant toner Tc having adhered to the fixation roller  30  onto the sheet P. In other words, this embodiment can more effectively clean the fixation unit. 
         [0273]    Further, in a case where the contaminant toner Tc had adhered to the pressure roller  17 , more time is available to melt the contaminant toner Tc to clean the film  16  by welding the contaminant toner Tc to the reverse surface of the sheet P. 
         [0274]    Next, referring to  FIG. 16  (flowchart), the process carried out to test this embodiment will be described. 
         [0275]    The image forming apparatus is started in the cleaning mode (Step  1 - a ). Then, target temperature and process speed are set (Step  1 - b ). Next, each of various components involved with the cleaning mode begin to be driven, and electric power begins to be sent to the heater  15 , whereby the temperature of the fixation roller  30  climbs to a preset level (Step  1 - c ). 
         [0276]    Next, the sheet P is fed into the apparatus main assembly  101  (Step  1 - d ). Then, an unfixed image for cleaning is formed on the sheet P (Step  1 - e ). This sheet P is conveyed to the fixation unit (Step  1 - f ). 
         [0277]    As the unfixed image on the sheet P reaches the fixation nip N 1  (Step  1 - g ), the intermittent conveyance of the sheet P is started (Step  1 - h ). That is, as the unfixed image reaches the fixation nip N 1 , the fixation roller  30  is stopped, and is kept stationary for a preset length Ts of time. Then, the fixation roller  30  is rotated for a length Td of time, which is equivalent to the width of the fixation nip N 1  in terms of the sheet conveyance direction, to convey the sheet P. Then, the fixation roller  30  is stopped again. This combination of the alternate stopping and rotation of the fixation roller  30  is repeated to intermittently convey the sheet P through the fixation nip N 1 . 
         [0278]    In the fixation nip N 1 , the unfixed image on the sheet P is turned into the offset toner To, and transfers onto the fixation roller  30  (Step  1 - i ). The, as the fixation roller  30  rotates further, the offset toner Tc on the fixation roller  30  reaches the fixation roller heating nip N 2 , in which it comes into contact with the film  16 , causing thereby the contaminant toner Tc on the film  16 , to transfer onto the fixation roller  30  (Step  2 - a ). 
         [0279]    Then, as the fixation roller  30  rotates further, the contaminant toner Tc and offset toner To reach the fixation nip N 1 , in which they are fixed to the sheet P (Step  3 - a ). 
         [0280]    Then, as the trailing edge of the sheet P approaches the fixation nip N 1 , and a preset hypothetical line on the sheet P, which is perpendicular to the sheet conveyance direction, comes out of the fixation nip N 1 , the rotation of the fixation roller  30  is continued until the sheet P is discharged (Step  3 - b ), discharging thereby the sheet P (Step  3 - c ). Then, the electric power to the heater  15  is turned off, and the image forming apparatus is stopped (Step  3 - d ), ending the image forming operation in the cleaning mode. 
       (Test Results) 
       [0281]    A test is carried out to confirm the effectiveness of the fixation unit in this embodiment in the cleaning mode. The image forming apparatus and fixation unit used for the test are the same as those in the first embodiment. 
         [0282]    The fixation unit in this embodiment was tested in terms of the effectiveness with which it can remove the contaminant toner To having adhered to the film  16 , in the same manner as the manner in which the fixation unit in the first embodiment was tested. 
         [0283]    As the image forming apparatus in this embodiment began to be operated in the cleaning mode, the image forming apparatus began an image forming operation, and the fixation roller  30  of the fixation unit  30  began to be driven. The ceramic heater  15  was controlled so that its temperature remained at a target level of 200° C. The cleaning sheet P used to test the fixation unit in this embodiment was a sheet of ordinary LBP printing paper (product of), which was 80 g/m 2  in basis weight, and A4 in size. The sheet P was conveyed to the image forming portion, and an unfixed toner image for the cleaning mode was formed on the sheet P in the image formation portion. Then, the sheet P was conveyed into the fixation nip N 1  of the fixation unit. The pattern of the image for cleaning was the same as that in the first embodiment. 
         [0284]    In this test, the rotation of the fixation roller  30  was stopped at the moment when a hypothetical line on the sheet P, which was parallel to the leading edge of the sheet P and is 76 mm apart from the leading edge, that is, the leading edge of the image for generating the offset toner To, entered the fixation nip N 1 . Then, the fixation roller  30  was kept stationary for three seconds. Then, it was rotated again to convey the sheet P by a distance equal to the width of the fixation nip N 1  in terms of the sheet conveyance direction, and was stopped again. This rotation and stopping of the fixation roller  30  was repeated 24 times to convey the sheet P until a hypothetical line on the sheet P, which was parallel to the leading edge of the sheet P and was 220 mm apart from the leading edge entered the fixation nip N 1 . Thereafter, the fixation roller  30  was continuously rotated to discharge the sheet P. 
         [0285]    The results of this test operation in the cleaning mode were as follows: Among the contaminants such as the contaminant toner Tc having had adhered to the film  16 , those which had not solidly adhered to the film  16  were removed. Further, it did not occur that the rest of the contaminants peels from the film  16  and transfers onto the sheet P when the image forming apparatus is in the normal fixation mode. In other words, this embodiment of the present invention also was excellent in effectiveness in the fixation mode. Further, it was possible to remove the contaminant toner Tc having had adhered to the pressure roller  17 . 
         [0286]    While the invention has been described with reference to the structures disclosed herein, it was 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. 
         [0287]    This application claims priority from Japanese Patent Application No. 097636/2009 filed Apr. 14, 2009 which is hereby incorporated by reference.