Abstract:
An image forming apparatus includes a toner image former; a fixing portion including a fixing roller and a pressing roller to fix the image; a feeder for reversing a facing orientation of the paper and refeed it to the image former; an executing portion for executing a cleaning mode in which a predetermined toner image is fixed on one side of paper, and the paper is refed to the image former with the reversed orientation to clean the pressing roller; a first integrator for integrating a number of the simplex mode executions; a notifying portion for prompting the cleaning mode operation upon a predetermined first integrated value; a second integrator for integrating a number of executions of a duplex mode in which toner images are formed on both sides of paper, wherein a corrector corrects the integrated value of the first integrator depending on the integrated value of the second integrator.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus which forms an image on a sheet of recording paper with the use of a combination of toner, and an electrophotographic image forming method, an electrophotographic image recording method, or the like. 
     An electrophotographic image forming apparatus has a fixing apparatus (device) which fixes (thermally welds) a toner image on a sheet of recording paper to the sheet of recording paper by conveying the sheet of recording paper and the toner image thereon while pinching the sheet and the toner image thereon and applying heat and pressure to the sheet and the toner image thereon. As a fixing device such as the one described above which is employable by an image forming apparatus, a fixing device which is made up of a fixation roller as a rotational fixing member, and a pressure roller as a rotational pressure applying member to be pressed upon the fixation roller, has been widely used. 
     In a case where a sheet of paper which contains a large amount of CaCO 3  (calcium carbonate) which is used to make recording paper appear whiter is used as recording medium, particles of CaCO 3  tend to adhere to the fixation roller and/or pressure roller little by little. Therefore, if a large number of sheets of recording paper containing a large amount of CaCO 3  are conveyed through a fixing device, it is possible that a substantial amount of CaCO 3  will adhere to the fixation roller and/or pressure roller. 
     The CaCO 3  particles having adhered to the fixation roller transfer onto the next sheet of recording paper by way of the toner on the next sheet of recording paper, although the amount by which they transfer is minuscule. Because the amount by which CaCO 3  particles transfer is minuscule, they are unlikely to reduce the toner image on the next sheet of recording paper in quality. Thus, it does not occur that CaCO 3  particles continuously accumulate on a fixation roller. 
     However, while an electrophotographic image forming apparatus is operated in the one-sided printing mode (simplex mode), the pressure roller does not come into the toner on a sheet of recording paper, which functions as the intermediary which transfers CaCO 3  particles back onto a sheet of recording paper. Therefore, CaCO 3  particles gradually accumulate on the pressure roller. Eventually, the amount of CaCO 3  particles becomes substantial, that is, unignorable. On the other hand, in a case where an electrophotographic image forming apparatus is operated in the two-sided printing mode (duplex mode) while an unignorable amount of CaCO 3  particles is on the pressure roller, it is possible that the CaCO 3  particles on the pressure roller will adhere to the back surface (first image formation surface of sheet of recording paper used in two-sided printing mode) by a substantial (unignorable) amount, which in turn derogatorily affects the apparatus in image quality. 
     In order to deal with the above-described problems, the apparatus (device) disclosed in Japanese Laid-open Patent Application Hei 6-83230 carries out a cleaning sequence for removing the contaminants on its pressure roller. More concretely, it forms an image, which is specifically designed for cleaning, on a sheet of recording paper, and fixes the image. Then, it places the sheet of recording paper upside down, and conveys the sheet through the fixing device for the second time. Thus, the image for cleaning comes into contact with the pressure roller. As a result, the CaCO 3  particles having accumulated on the pressure roller transfer onto the cleaning image on the sheet of paper; they are removed from the pressure roller. 
     As for the conventional timing with which such a cleaning sequence as the one described above, as the number of single-sided prints formed by an electrophotographic image forming apparatus reaches a preset value, the sequence is automatically carried out, or the user of the apparatus is informed that the cleaning sequence needs to be carried out. 
     However, an electrophotographic image forming apparatus structured to carry out the cleaning sequence with the above-described timing suffers from the following problem: 
     That is, as the apparatus is operated in the two-sided printing mode, the CaCO 3  particles having accumulated on the pressure roller are reduced or eliminated because of the above-described mechanism. Therefore, carrying out the cleaning sequence based on the number of single-sided prints outputted by the apparatus may result in excessive cleaning of the pressure roller, which cannot be said to be desirable for the following reason. That is, while the cleaning sequence is carried out by the apparatus, the apparatus cannot be used by the user for a printing operation. In other words, structuring an electrophotographic image forming apparatus so that it automatically carries out the cleaning sequence for every preset number of single-sided prints unnecessarily increases the apparatus in downtime. 
     SUMMARY OF THE INVENTION 
     Thus, the primary object of the present invention is to provide an image forming apparatus which is capable of informing, with an optimal timing, a user (operator) that it needs to be operated in the cleaning mode for cleaning its pressure applying rotational member. 
     Another object of the present invention is to provide an image forming apparatus which is capable of automatically operating, with an optimal timing, in the cleaning mode for cleaning its pressure applying rotational member. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising a toner image forming portion configured to form a toner image on recording paper; a fixing portion including a fixing rotatable member and a pressing rotatable member to fix the toner image on the recording paper while nipping and feeding the recording paper carrying the toner image formed by said toner image forming portion; a feeding portion configured and positioned to reverse a facing orientation of the recording paper on which the toner image is fixed by said fixing device and refeed the recording paper to said toner image forming portion and said fixing device; an executing portion configured to execute an operation in a cleaning mode in which a predetermined toner image is formed and fixed on one side of recording paper by said toner image forming portion and said fixing portion, and thereafter, the recording paper being refed by said feeding means to said toner image forming portion and then to said fixing portion with the reversed facing orientation, so that said pressing rotatable member is cleaned; a first integrating portion configured to integrate a number of the executions of the operations of the simplex mode only on the one side of the recording paper; a notifying portion configured to notify for promoting execution of the cleaning mode when the integrated value of said first integration is not less than a predetermined value; a second integrating portion configured to integrate a number of executions of operations of a duplex mode in which toner images are formed on both sides of recording paper; and a correcting portion configured to correct the integrated value of said first integrating portion in accordance with the integrated value of said second integrating portion. 
     According to another aspect of the present invention, there is provided an image forming apparatus comprising a toner image forming portion configured to form a toner image on recording paper; a fixing portion including a fixing rotatable member and a pressing rotatable member to fix the toner image on the recording paper while nipping and feeding the recording paper carrying the toner image formed by said toner image forming portion; a feeding portion configured and positioned to reverse a facing orientation of the recording paper on which the toner image is fixed by said fixing device and refeed the recording paper to said toner image forming portion and said fixing device; a first integrating portion configured to integrate a number of the executions of the operations of the simplex mode only on the one side of the recording paper; an executing portion configured to execute, when the integrated value of said first integrating portion is not less than a predetermined value, an operation in a cleaning mode in which a predetermined toner image is formed and fixed on one side of recording paper by said toner image forming portion and said fixing portion, and thereafter, the recording paper being refed by said feeding means to said toner image forming portion and then to said fixing portion with the reversed facing orientation, so that said pressing rotatable member is cleaned; a second integrating portion configured to integrate a number of executions of operations of a duplex mode in which toner images are formed on both sides of recording paper; and a resetting portion configured to reset the integrated value of said first integrating portion when the integrated value of said second integrating portion reaches a predetermined value. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of a typical image forming apparatus to which the present invention is applicable. 
         FIG. 2  is a block diagram of the essential portions of the image forming apparatus, and shows the relationship among the essential portions in terms of their control. 
         FIG. 3  is a schematic drawing of the control panel of the image forming apparatus. 
         FIG. 4  is a schematic sectional view of the fixing device in the first embodiment of the present invention. 
         FIG. 5  is a flowchart of the cleaning operation for cleaning the pressure roller. 
         FIG. 6  is a flowchart of the sequence for deciding whether or not the cleaning operation for cleaning the pressure roller needs to be carried out by the image forming apparatus in the first embodiment. 
         FIG. 7  is a block diagram of the essential portions of the image forming apparatus in the second embodiment of the present invention, and shows the relationship among the essential portions in terms of their control. 
         FIG. 8  is a flowchart of the sequence for deciding whether or not the cleaning operation for cleaning the pressure roller needs to be carried out by the image forming apparatus in the second embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention is described in detail with reference to image forming apparatuses which are in accordance with the present invention. 
     (Embodiment 1) 
     1. General Structure and Operation of Image Forming Apparatus 
       FIG. 1  is a schematic sectional view of the image forming apparatus  100  in this embodiment of the present invention. This image forming apparatus  100  uses an electrophotographic image forming method, and can form a full-color image on a sheet S of recording paper (recording medium) in response to the image formation signals transmitted thereto from an external device. It is of the so-called intermediary transfer type, and also, of the so-called tandem type. 
     The image forming apparatus  100  has image formation stations  1 Y,  1 M,  1 C and  1 K, and an intermediary transferring member (intermediary transfer belt, hereafter)  31 . The image formation stations  1 Y,  1 M,  1 C and  1 K are the toner image forming portions of the apparatus  100 . They form a toner image on a sheet of recording paper. They are aligned in parallel along the horizontal portion of the intermediary transfer belt  31 , in the direction in which the horizontal portion of the intermediary transfer belt  31  moves. 
     In this embodiment, the image formation stations  1 Y,  1 M,  1 C and  1 K are practically the same in structure and operation, although they are different in the color of the toner they use. Thus, unless they need to be differentiated, their components will be described together, without showing the suffixes Y, M, C and K which indicate the color of the image they form. 
     The image formation station  1  has a photosensitive drum  11 , as an image bearing member, which is an electrophotographic photosensitive member (photosensitive member) and is in the form of a drum. The photosensitive drum  11  is rotationally driven in the direction indicated by an arrow mark R 1  in the drawing. The image formation station  1  has also the following processing means, more specifically, a charge roller  12 , an exposing device  13 , a developing device  14 , a primary transfer roller  35 , and a cleaning device  15 , which are disposed in the adjacencies of the peripheral surface of the photosensitive drum  11 , in the listed order in terms of the rotational direction of the photosensitive drum  11 . The first one is the charge roller  12  as a charging means for uniformly charging the photosensitive drum  11 . It is a charging member which is in the form of a roller. The next one is the exposing device  13  as an exposing means for forming an electrostatic latent image (electrostatic image) on the photosensitive drum  11  by projecting a beam of light upon the photosensitive drum  11  while modulating the beam of light with the image formation signals. The third one is the developing device  14  as a developing means which forms a toner image, on the photosensitive drum  11 , by transferring toner onto the electrostatic latent image on the photosensitive drum  11 . The next one is the primary transfer roller  35  as the primary transferring means for transferring the toner image on the photosensitive drum  11 , onto the intermediary transfer belt  31  (primary transfer). It is a transferring member which is in the form of a roller. The last one is the cleaning device  15  as a cleaning means which removes the toner (primary transfer residual toner) remaining on the photosensitive drum  11  after the primary transfer, and stores the removed transfer residual toner. 
     The intermediary transfer belt  31  is an endless belt formed of film. It is suspended and kept stretched by multiple members, more specifically, a driver roller  33 , a tension roller  34 , and a secondary transfer roller opposing roller  32 . The intermediary transfer belt  31  is circularly driven by the driver roller  33  in the direction indicated by an arrow mark R 2  in the drawing. The image forming apparatus  100  is provided with four primary transfer rollers  35 Y,  35 M,  35 C and  35 K, which are disposed on the inward side of the loop (belt loop) which the intermediary transfer belt  31  forms, being positioned so that they oppose the photosensitive drums  11 Y,  11 M,  11 C and  11 K, respectively. Each primary transfer roller  35  is pressed upon the corresponding photosensitive drum  11  with the presence of the intermediary transfer belt  31  between itself and photosensitive drum  11 , forming the primary transfer area (primary transfer nip) N 1 , which is the area of contact between the intermediary transfer belt  31  and photosensitive drum  11 . The image forming apparatus  100  has also a secondary transfer roller  41  as the secondary transferring means which is disposed on the outward side of the belt loop, being positioned so that it opposes the secondary transfer roller opposing roller  32 . The secondary transfer roller  41  is in the form of a roller, and is kept pressed against the secondary transfer roller opposing roller  31 , with the presence of the intermediary transfer belt  31  between itself and secondary transfer roller opposing roller  32 . Further, there is disposed a belt cleaning device  36  as an intermediary transferring member cleaning means, which removes the toner (secondary transfer residual toner) remaining on the intermediary transfer belt  31  after the secondary transfer, and recovers the removed secondary transfer residual toner. The belt cleaning device  36  is positioned on the opposite side of the intermediary transfer belt  31  from the driver roller  33 . 
     When the image forming apparatus  100  is in the full-color mode, for example, yellow, magenta, cyan and black monochromatic toner images are formed on the photosensitive drums  11 Y,  11 M,  11 C and  11 K of the image formation stations  1 Y,  1 M,  1 C and  1 K, respectively. Then, the four monochromatic toner images, different in color, on the photosensitive drums  11 Y,  11 M,  11 C and  11 K are transferred in layers onto the intermediary transfer belt  31  (primary transfer), in the primary transfer areas (nips) N 1  of the image formation stations  1 Y,  1 M,  1 C and  1 K, respectively. 
     Meanwhile, a sheet S of recording paper (recording medium) in one of multiple recording paper cassettes  61 ,  62 ,  63  and  64  as recording medium storage, is conveyed into the recording medium conveyance passage  81 , by the rotation of recording medium supplying rollers  71 ,  72 ,  73  or  74 . Then, the sheet S of recording paper is conveyed further by a pair of registration rollers  75  to the secondary transfer area (nip) N 2 , with the same timing as the timing with which the layered toner images on the intermediary transfer belt  31  arrive at the secondary transfer area N 2 . 
     Then, in the secondary transfer area N 2 , the layered toner images on the intermediary transfer belt  31  are transferred together (secondary transfer) onto the sheet S of recording paper. 
     In this embodiment, the various components of the image formation station  1 , intermediary transfer belt  31 , and secondary transfer roller  41  make up the toner image formation station  1  for forming a toner image on a sheet S of recording paper. 
     After the transfer of the layered toner images onto the sheet S of recording paper, the sheet S is conveyed by the conveyer belt  42  to a fixing device  50  as the image fixing portion of the image forming apparatus  100 . The fixing device  50  has a pair of rotational members, more specifically, a fixation belt  51  as a rotational fixing member, and a pressure roller  52  as a rotational pressure applying member. The fixation belt  51  generates heat in itself by electromagnetic induction. The fixing device  50  conveys the sheet S through itself while keeping the sheet S pinched between its fixation belt  51  and pressure roller  52  and applying heat and pressure to the layered toner images on the sheet S. Consequently, the layered toner images on the sheet S are welded (fixed) to the sheet S. The fixing device  50  will be described later in detail. 
     After the fixation of the layered toner images to the sheet S of recording paper, the sheet S is discharged from the main assembly of the image forming apparatus  100  onto the delivery tray  65 , through the recording medium discharge passage  82 . 
     Incidentally, the image forming apparatus  100  in this embodiment is capable of operating in the one-sided printing mode, in which a toner image is fixed to only one of the two surfaces of the sheet S of recording paper. It is also capable of operating in the two-sided printing mode, in which a toner image is fixed to both surfaces of the sheet S. In the two-sided printing mode, after a toner image is fixed to one of the two surfaces of the sheet S of recording paper, another toner image is fixed to the second surface of the sheet S before the sheet S is discharged from the image forming apparatus  100 . Therefore, the image forming apparatus  100  is provided with a recording medium conveying means B for the two-sided printing mode, which turns over the sheet S of recording paper after the fixation of a toner image to one of the two surfaces of the sheet S by the fixing device  50 , and sends the sheet S to the toner image formation station  1  for the second time. In this embodiment, the recording medium conveying means B for the two-sided printing mode is made up of a sheet reversing passage  83 , a two-sided printing mode sheet passage  85 , a pair of switchback rollers  87 , a pair of two-sided printing mode sheet conveyance rollers, etc. 
     In the one-sided printing mode, after the fixation of a toner image to one of the two surfaces of the sheet S of recording paper, the sheet S is discharged from the fixing device  50 , is directed toward the sheet discharge passage  85  by a flapper  86 , and then, is discharged into the delivery tray  65 . 
     In the two-sided printing mode, the sheet S of recording paper is discharged from the fixing device  50  after the fixation of a toner image to one of the two surfaces of the sheet S. Then, the sheet S is directed toward the two-sided printing mode sheet conveyance passage  85 , and then, is reversed in a conveyance direction by the pair of switchback rollers  87 . Then, it is sent into the two-sided printing mode sheet passage  85 . Then, the sheet S is conveyed to the second transfer area (image transfer area) N 2 , being positioned so that its second surface faces the intermediary transfer belt  31 . After the fixation of a toner image to the second surface of the sheet S, the sheet S is sent to the fixing device  50  for the second time, so that the toner image on its second surface is fixed to the sheet S. After the fixation of the toner image to the second surface of the sheet S, that is, after the fixation of two toner images to the both surfaces of the sheet S, one for one, the sheet S is discharged from the fixing device  50 , is directed by the flapper  86  toward the sheet discharge passage  85 , and is discharged into the delivery tray  65 . 
     2. Control of Image Forming Apparatus 
       FIG. 2  is a block diagram of the essential portions of the image forming apparatus  100  in this embodiment. It shows the relationship among the essential portions in terms of their control. 
     The image forming apparatus  100  has a control section  204  which integrally controls the operations of various portions of the image forming apparatus  100 . The control section  204  has a CPU  201  as a controlling means (controller) for integrally controlling the operations carried out by various portions of the image forming apparatus  100 . Further, the control section  204  has a ROM  202  as a storing means in which the programs for operating the CPU  201  are stored. It has also a RAM  203  as a storing means which the CPU  2021  uses to temporarily store the data. The control section (CPU circuit portion)  204  is an integral combination of the CPU  201 , ROM  202 , and RAM  203 . 
     A conveying means driving section  211  drives motors, clutches, solenoids, etc., for supplying the main assembly of the image forming apparatus  100  with a sheet of recording paper, and conveying sheets S through the main assembly, in response to the control signals from the CPU  201 . The image formation station  1  electrophotographically forms a toner image on the sheet S of recording paper, based on the image formation data transmitted thereto from the external I/F  220  (which will be described later), in response to the control signals transmitted thereto from the CPU  201 . The external I/F section  220  transmits the image data inputted from the external apparatus such as a scanner (image reading apparatus) and a PC (personal computer), to the CPU  201 . The control panel  600  functions as the means through which the image forming apparatus  100  receives a print job (image formation sequence for forming the image on single or multiple sheets of recording paper in response to an image formation start instruction), and informs an operator of the state of the image forming apparatus  100  in response to the control signals from the CPU  201 . 
     In this embodiment, the CPU  201  functions as the means which sets the timing with which the rotational member of the fixing device  50  to be cleaned, as will be described later in detail. It also functions as the means which makes the cleaning device carry out the cleaning operation for cleaning the rotational member of the fixing device  50 . Further, the CPU  201  functions also as the section for adjusting the timing with which the image forming apparatus  100  (fixing device  50 ) is operated in the cleaning mode, which will be described later. 
     Also in this embodiment, the control panel  600  functions as the instructing means (informing section) for informing an operator of the timing with which the rotational member of the fixing device  50  is to be cleaned, and also, as an executing means which makes the image forming apparatus  100  clean the rotational member of the fixing device  50  (means which makes the image forming apparatus operate in cleaning mode). 
     3. Control Panel 
       FIG. 3  is a schematic drawing of the control panel  600  of the image forming apparatus  100  in this embodiment. 
     The control panel  600  is provided with a start key  602  for making the image forming apparatus  100  start an image forming operation, a stop key for making the image forming apparatus  100  interrupt an on-going image forming operation, etc. It is also provided with numerical keys  604 - 612  and  614  for inputting numerical values, an ID key  613 , a clear key  615 , a reset key  616 , etc. Further, the control panel  600  is provided with a display  620 , which functions as a touch panel, and across which soft keys or the like can be formed. 
     Further, the control panel  600  is provided an area for informing an operator of the state of the image forming apparatus  100 , which corresponds in position to the bottom area of the drawing of the display  620  in  FIG. 3 . For example, if it is determined by the CPU  201  that the pressure roller  52  needs cleaning, a message which prompts the operator to clean the pressure roller  52  is displayed at the bottom of the display  620 , in response to the control signals from the CPU  201 , as will be described later in detail. Further, this control panel  600  functions also as a means for informing the operator of the information that the cumulative value in the first cumulating means exceeded a preset value, and therefore, the fixing device  50  needs to be operated in the cleaning mode. 
     4. Fixing Device 
       FIG. 4  is a schematic sectional view of the fixing device  50  of the image forming apparatus  100  in this embodiment. 
     The fixing device  50  has the fixation belt  51  as a rotational fixing member (rotational heating member) which is in the form of an endless belt having a metallic layer. It has also the pressure roller  52  as a rotational pressure applying member, which is disposed so that its peripheral surface contacts the peripheral surface of the fixation belt  51 . The fixation belt  51  is one of the pair of rotational members of the fixing device  50 , with which an unfixed toner image comes into contact. The pressure roller  52  is the other of the pair of rotational members of the fixing device  50 . 
     The fixing device  50  has a pressure applying member  301 , against which the pressure roller  52  is pressed, with the presence of the fixation belt  51  between itself and pressure roller  52 , to form a fixation nip N between the fixation belt  51  and pressure roller  52 . The pressure applying member  301  is disposed on the inward side of the loop which the fixation belt  51  forms. It is held to a metallic stay  302 . Further, the fixing device  50  has a magnetism blocking core  305  as a magnetism blocking member for preventing the stay  302  and pressure applying member  301  from being increased in temperature by the heat generated therein by magnetic induction. The magnetism blocking member  305  is disposed on the excitation coil side (which will be described layer) of the stay  302 . 
     Further, the fixing device  50  has a heating device  311  as a heat source (inductive heating means) for heating the fixation belt  51  based on electromagnetic induction. The inductive heating device  311  has an excitation coil  312  made up of electric wire, for example, Litz wire, wound in the shape of the bottom portion of a long and narrow boat, so that it conforms in shape to the portion of the fixation belt  51 , which corresponds in position to the top and side portions of the loop which the fixation belt  51  forms. The inductive heating device  311  has a pair of external magnetic cores  313  and  314 , which are shaped so that they cover the excitation coil  312  to make it virtually impossible for the magnetic field generated by the excitation coil  312  to leak, except toward the metallic layer (electrically conductive layer) of the fixation belt  51 . Further, the inductive heating device  311  has a coil holding member  315  which holds the excitation coil  312  and external magnetic cores  313  and  314 , with the use of electrically insulating resin. The inductive heating device  311  is disposed on the top side of the outward surface of the fixation belt  51 , with reference to  FIG. 4 , being positioned in such an attitude that a preset amount of gap is provided between the outward surface of the fixation belt  51  and inductive heating device  311 . To the excitation coil  312  of the inductive heating device  311 , high frequency electric current, the frequency of which is in a range of 20-60 kHz, is applied while the fixation belt  51  is rotated. Thus, the metallic layer (electrically conductive layer) of the fixation belt  51  is inductively heated by the magnetic field generated by the excitation coil  312 . 
     Further, the fixing device  50  is provided with a temperature sensor (temperature detection element) TH 1 , such as a thermistor, which is disposed on the inward side of the loop which the fixation belt  51  forms. In terms of the widthwise direction (direction parallel to rotational axis), the temperature sensor TH 1  positioned at the center of the fixation belt  51 , in contact with the inward surface of the fixation belt  51 . Also in terms of the widthwise direction of the fixation belt  51 , the temperature sensor TH 1  detects the temperature of the fixation belt  51 , within the area in which the fixation belt  51  can contact the sheet S of recording paper. The information regarding the detected temperature of the fixation belt  51  is fed back to the CPU  201  which functions as the fixation controlling section. The CPU  201  controls the electrical power to be inputted into the excitation coil  312 , so that the detected temperature of the fixation belt  51 , which is inputted into the CPU  201 , remains a preset target level (fixation temperature). That is, as the detected temperature of the fixation belt  51  rises to the preset level, the electric power supply to the excitation coil  312  is stopped. In this embodiment, in order to keep the temperature of the fixation belt  51  stable at 180° C., or the target level, the electric power to be inputted into the excitation coil  312  is controlled. More specifically, the high frequency electric current to be inputted into the excitation coil  12  is changed in frequency in response to the value of the temperature of the fixation belt  51  detected by the temperature sensor TH 1 . 
     5. Cleaning Operation 
     Next, the operation (cleaning mode) for cleaning one of the rotational members of the fixing device  50  in this embodiment is described. In this embodiment, the cleaning operation is carried out to clean the pressure roller  52 , which is one of the rotational members of the fixing device  50 . It is positioned so that when a sheet S of recording paper is conveyed through the fixing device  50 , it will be on the opposite side of the sheet S from the surface of the sheet S, with which a toner image comes into contact before its fixation. 
     For example, in a case where the image forming apparatus  100  is operated in the one-sided printing mode to output a large number of prints (one-sided prints) using sheets of recording paper which are substantial in the amount of CaCO 3  contents, particles of CaCO 3  sometimes adhere to the pressure roller  52 . It also occurs sometimes that the pressure roller  52  is contaminated by the toner particles which have failed to be completely welded to the sheet S of recording paper, and therefore, are remaining on the fixation belt  51 . In a case where the image forming apparatus  100  is operated in the two-sided printing mode, the pressure roller  52  comes into contact with the fixed toner image on the surface of the sheet S, on which a toner image was fixed first. Thus, when the unfixed toner image on the second surface is fixed, the fixed toner image on the first surface of the sheet S is sometimes soiled. 
     In order to prevent the occurrence of this phenomenon, the image forming apparatus  100  in this embodiment is enabled to carry out the operation for cleaning the peripheral surface of the pressure roller  52  as necessary. 
     Roughly describing, in the cleaning operation, a predetermined toner image is formed on one of the two surfaces of the sheet S of recording paper by the toner image forming means I, and the formed toner image is fixed to the sheet S by the fixing device  50 . Then, the sheet S is placed upside down by the two-sided printing mode sheet conveying means B, and is conveyed through the fixing device  50  for the second time. Thus, the pressure roller  52  as a rotational member which is on the opposite side of the sheet S from the surface of the sheet S, with which the unfixed toner image comes into contact, is cleaned by the fixed toner image on the first surface of the sheet S. More concretely, in the cleaning operation in this embodiment, a solid image which matches the sheet S of recording paper in size is formed on one (first) of the two surfaces of the sheet S of recording paper, and is fixed to the first surface. Then, the sheet S is placed upside down, and conveyed through the fixing device  50 , to transfer the contaminative toner and CaCO 3  particles, etc., having adhered to the pressure roller  52 , onto the sheet S. 
     More specifically, the aforementioned solid image means such an image which can cover the entirety of the image formation area (range) of the sheet S of recording paper and is preset in density (highest density level, typically). 
     Incidentally, in this embodiment, the cleaning operation is carried out by the image forming apparatus  100  as an operator gives the image forming apparatus  100  (control section  204 ) a command for starting the cleaning operation. 
       FIG. 5  is a flowchart of the cleaning operation for cleaning the pressure roller  52 . It is for describing the steps which the image forming apparatus  100  is made to carry out by the CPU  201 . 
     As the cleaning operation is started, the CPU  201  controls the recording medium conveying means driving section  211  to feed a sheet S of recording paper as a cleaning sheet into the main assembly of the image forming apparatus  100  (S 101 ). Then, the sheet S is conveyed to the secondary transfer area (image transfer section) N 2  through the recording medium conveyance passage  81 . 
     Next, the CPU  201  decides whether it is the first time an image is formed on the sheet S of recording paper or not (S 102 ). 
     If the CPU  201  determines in Step S 102  that it is the first time an image is formed on the sheet S, it controls the image formation station  1  to form a solid image for cleaning, on sheet S (S 103 ). 
     In this embodiment, this solid image for cleaning is formed of yellow and cyan color toners, because a solid toner image formed of two color toners is greater in the amount of toner per unit area than a solid toner image formed of only one toner. The greater the solid image for cleaning, in the amount of toner per unit area, the better the solid image in cleaning performance. Therefore, it is desired that the solid image is formed of the yellow and cyan toners. Further, the solid image formed of the yellow and cyan toners makes more conspicuous the contaminants (toner and the like) having adhered to the solid image, being advantageous in that it makes it easier to see the cleaning effects of the solid image. Thus, the toner image to be formed for the cleaning operation for cleaning the rotational member(s) of the fixing means is desired to be such a solid image that is formed of two or more color toners. 
     In order to ensure that the entirety of the peripheral surface of the pressure roller  52  is cleaned, the sheet S of recording paper which is to be used as the recording medium on which the solid image for cleaning is formed is desired to be a sheet of recording paper, the dimension of which in terms of the recording medium conveyance direction is greater than the circumference of the pressure roller  52 . That is, the solid toner image for cleaning is desired to be such that its dimension in terms of the recording medium conveyance direction is greater than the circumference of the rotational member, which is on the opposite side of the sheet S from the surface of the sheet S, with which the solid toner image for cleaning comes into contact before it is fixed. Further, the sheet S of recording paper which is to be used as the recording medium on which the solid image for cleaning is formed is desired to be such a sheet of recording paper that in terms of the direction which is roughly perpendicular to the recording medium conveyance direction, its dimension is greater than the dimension of the largest image which the image forming apparatus  100  can form. Therefore, it is desired that a sheet S of recording paper which is largest in dimension in terms of the direction which is roughly perpendicular to the recording medium conveyance direction, that is, the longest sheet S of recording paper among the sheets S of recording medium conveyable through the fixing device  50 , is used as the sheet S of recording paper used for cleaning. With the use of such a sheet of recording paper as the sheet S of recording paper for cleaning, the problem that the image on the second surface of the sheet S of recording paper is soiled by the contaminants such as the CaCO 3  particles can be prevented regardless of the size of the image on the surface of the sheet S on which an image is formed first when the image forming apparatus  100  is in the two-sided printing mode. In this embodiment, the dimension of the sheet S of recording paper for cleaning, in terms of the recording medium conveyance direction, and that in terms of the direction which is roughly perpendicular to the recording medium conveyance direction, are set to the values in the above-described range. 
     Next, the CPU  201  controls the fixing device  50  to make the fixing device  50  fix the solid image for cleaning, to the sheet S of recording paper (S 104 ). 
     Then, the CPU  201  controls the recording medium conveying means driving section  211  so that the sheet S of recording to which the solid image for cleaning has just been fixed, into the recording medium reversing passage  83 , and changes the sheet S in direction to send the sheet S into the recording medium conveyance passage  83  for the two-sided printing mode (S 105 ). Thus, the sheet S is placed upside down. 
     Next, the CPU  201  controls the recording medium conveying means driving section  211  to feed the sheet S of recording paper, which is bearing the fixed solid image for cleaning, into the secondary transfer area (nip) N 2  for the second time (S 106 ). Then, the CPU  201  returns to Step S 102 . 
     If the CPU  201  determines in Step S 102  that the next image to be formed on the sheet S of recording paper is not the first image to be formed on the sheet S, that is, if it determines that the next image to be formed is the second image to be formed on the sheet S of recording paper, the CPU  201  controls the image formation station  1  to form a solid white image (S 107 ). That is, no image is formed on the second surface of the sheet S. 
     Next, the CPU  201  controls the fixing device  50  to fix the “white” image to the sheet S (S 108 ). That is, the CPU  201  controls the fixing device  50  so that the sheet S is conveyed through the fixing device  50  in such an attitude that the surface (first surface) which is bearing the fixed solid image for cleaning faces the pressure roller  52 , and the surface (second surface) on which no toner image was formed faces the fixation belt  51 . While the sheet S is conveyed through the fixing device  50 , the fixed solid image on the first surface of the sheet S is melted again. That is, the melted solid image for cleaning comes into contact with the pressure roller  52 . Thus, the contaminative toner, CaCO 3  particles, and the like contaminants having adhered to the peripheral surface of the pressure roller  52  are adhered to the melted toner (toner image) on the sheet S, being thereby transferred onto the sheet S. In other words, the peripheral surface of the pressure roller  52  is cleaned by the cleaning sheet S, that is, the sheet S of recording paper having the solid image form cleaning. 
     Incidentally, in this embodiment, the temperature of the fixing device  50  is kept at 180° C., or the target level, as described above, either while the sheet S of recording paper for cleaning is conveyed through the fixing device  50  for the first time (fixation on first surface), or for the second time (fixation on second surface). 
     Lastly, the CPU  201  controls the recording medium conveying means driving section  211  to conveys the sheet S for cleaning to the sheet discharge passage  82  and discharge the sheet S into the delivery tray  65  (S 109 ). 
     6. Setting of Cleaning Timing 
     Next, the means, in this embodiment, which decides whether or not the pressure roller  52  needs to be cleaned, is described. 
     The primary object of the present invention is to provide an image forming apparatus which is simpler in structure, and yet, can easily determine the timing with which the rotational member of its fixing means to be cleaned, than any image forming apparatus in accordance with the prior art. To describe the object in further detail, it is to provide an image forming apparatus which does not require a sensor such as an optical sensor dedicated to the operation for deciding whether or not its pressure roller  52  needs to be cleaned, being therefore lower in cost, and smaller in size, and yet, can prevent the image contamination attributable to the contamination of the pressure roller  52 , as effectively as, or more effectively, than any image forming apparatus in accordance with the prior art. 
     The contaminative toner and CaCO 3  particles, which is on the peripheral surface of the pressure roller  52 , are removed little by little while the image forming apparatus  100  is operated in the two-sided printing mode, because the contaminative toner and CaCO 3  particles transfer onto the toner image on the first surface of the sheet S of recording paper, by a minute amount, and are discharged with the sheet S, each time the sheet S is conveyed through the fixing device for the second time in the two-sided printing mode. Therefore, it is reasonable to think that after a certain number of two-sided prints are outputted by the image forming apparatus  100 , the pressure roller  52  is clean, and therefore, does not need to be cleaned. However, it is also reasonable to think that in a case where a certain number of one-sided prints are outputted by the image forming apparatus  100 , without being interrupted by an image forming operation for outputting two-sided prints, a substantial amount of the contaminative toner and CaCO 3  particles will have accumulated on the peripheral surface of the pressure roller  52 , and therefore, it will be necessary for the pressure roller  52  to be cleaned. 
     In the case of the image forming apparatus  100  in this embodiment, therefore, it is provided with each of the following means. The first one is the first accumulating means for accumulating the information related to the number by which one-sided prints are outputted by the image forming apparatus  100 . The second means is the second accumulating means for accumulating the information related to the number of the two-sided prints outputted by the image forming apparatus  100 . The third is an altering means which reduces the value in the first accumulating means as the value in the second accumulating means reaches a preset value. The fourth is an informing means which informs an operator (user) of the timing with the pressure roller  52  is to be cleaned, that is, as the value in the first accumulating means exceeds the preset value. In particular, in this embodiment, the above-described altering means functions also as a resetting means for resetting the value in the first accumulating means as the number of prints outputted by the image forming apparatus  100  in the two-sided printing mode reaches the preset value. 
     The typical information related to the amount by which the image forming apparatus  100  is operated in the one-sided printing mode is the information related to the number of the sheets S of recording paper discharged from the image forming apparatus  100  in the one-sided printing mode. However, it does not need to be limited to this information. That is, all that is necessary is that the information is related to the amount by which the image forming apparatus  100  is operated in the one-sided printing mode. For example, it may be the number of times one of the rotational members of the fixing device  50  is rotated in a given image forming operation carried out in the one-sided printing mode. This is true with the information related to the amount by which the image forming apparatus  100  is operated in the two-sided printing mode. As for the altering means, it clears the first accumulating means as the value in the second accumulating means exceeds the preset value, as will be described later in detail. However, this embodiment is not intended to limit the present invention in scope. For example, the image forming apparatus  100  may be designed so that the value in the first accumulating means is reduced by a preset amount in proportion to the degree by which the rotational member is cleaned by the image forming operation carried out in the two-sided printing mode. 
     To describe in further detail, the image forming apparatus  100  in this embodiment is provided with a cleaning counter (one-sided print counter) C, as the first accumulating means, which is a storing means and cumulatively counts the number of one-sided prints ( FIG. 2 ). Further, the image forming apparatus  100  is provided with a two-sided print counter D, as the second accumulating means which is a storing means and cumulatively counts the number of the two-sided prints ( FIG. 2 ). As the value in the cleaning counter exceeds a preset value, the CPU  201 , as the prompting means, determines that the rotational member needs to be cleaned. Then, it outputs a signal that informs an operator of the timing with which the rotational member is to be cleaned. Further, as the value in the two-sided print counter D exceeds the preset value, the CPU  201  as the prompting means clears the cleaning counter C. 
     Here, it is desired that the second accumulating means accumulates the information related to the amount by which the image forming apparatus  100  is operated in the two-sided printing mode, based on the sum of all the sheets S of recording paper, in terms of their dimension in the recording medium conveyance direction, outputted by the image forming apparatus  100  in the two-sided printing mode. Thus, it is desired that the image forming apparatus  100  is provided with a means for obtaining the information related to the dimension of each sheets S of recording medium in terms of the direction in which the sheet S is conveyed. Further, it is desired that the value to be added to the value in the second accumulating means each time a two-sided print is outputted by the image forming apparatus  100 , is set based on the information related to the dimension of the sheet S of recording paper used to output the two-sided print, in terms of the recording medium conveyance direction, obtained by the recording medium dimension obtaining means. 
     For example, in a case where the dimension, in terms of the recording medium conveyance direction, of the sheet S of recording paper to be used for a two-sided print is greater than 215.9 mm, its effectiveness in terms of the removal of the contaminative toner and CaCO 3  particles on the peripheral surface of the pressure roller  52  is equivalent to twice that of the sheet S of recording paper to be used for the two-sided print, which is no more than 215.9 mm. In this embodiment, therefore, in a case where the sheet S of recording paper to be used to output a two-sided print is greater in dimension than 215.9 mm in terms of the recording medium conveyance direction, 2 is added to the value in the two-sided print counter D. 
     However, this embodiment is not intended to limit the present invention in terms of the method for counting the prints outputted from the image forming apparatus  100 . That is, the method may be such that the print count is weighted in proportion to the length of each print. That is, the image forming apparatus  100  may be designed so that the greater the length indicated by the information which is related to the dimension of the sheet S of recording paper used to output two-sided prints, and which is obtained by the sheet dimension obtaining means, the greater the value to be added to the value in the second accumulating means each time a two-sided print is outputted by the image forming apparatus  100 . 
       FIG. 6  is a flowchart for describing the process carried out by the CPU  201  to decide whether or not the pressure roller  52  in this embodiment needs to be cleaned. 
     As a printing job is started, the CPU  201  remains on standby until a sheet S of recording paper moves through the fixing device  50  (S 201 ). 
     As the CPU  201  determines in Step S 201  that a sheet S of recording paper has moved through the fixing device  50 , it decides whether or not the sheet S is for a one-sided print (S 202 ). 
     In a case where the CPU  201  determines, in Step  202 , that the sheet S of recording paper is for a one-sided print, it adds 1 to the cleaning counter (S 203 ). 
     Then, the CPU  201  clears the two-sided print counter D (S 204 ), and proceeds to Step S 212 . 
     On the other hand, in a case where the CPU  201  determines in Step S 202  that the sheet S of recording paper is not for a one-sided print, that is, the sheet S is for a two-sided print, it increases the value in the two-sided print counter D in the following manner. 
     First, the CPU  201  obtains the length of the sheet S of recording paper in terms of the recording medium conveyance direction, from the information regarding the sheet S, of which the CPU  201  is informed through the external I/F  220  (S 205 ). 
     Next, the CPU  201  decides whether or not the length of the sheet S of recording paper in terms of the recording medium conveyance direction is greater than 215.9 mm (S 206 ). 
     If the CPU  201  determines, in Step S 206 , that the sheet S of recording paper is longer than 215.9 mm, it adds 2 to the value in the two-sided print counter (S 207 ). 
     On the other hand, if the CPU  201  determines in Step S 206  that the length of the sheet S of recording paper in terms of the recording medium conveyance direction is no more than 215.9 mm, it adds 1 to the value in the two-sided print counter D (S 208 ). 
     As the CPU  201  finishes the process for increasing the value in the two-sided print counter D as described above, it decides whether or not the value in the two-sided print counter D is no less than 5 (S 209 ). 
     If the CPU  201  determines in Step S 209  that the value in the two-sided print counter D is no less than 5, it clears the cleaning counter C (S 210 ). 
     Next, the CPU  201  clears the two-sided print counter D (S 211 ). 
     On the other hand, if the CPU  201  determines in Step S 209  that the value in the two-sided print counter D is no more than 5, it proceeds to Step S 212 . 
     Incidentally, to what degree the image forming apparatus  100  needs to be operated in the two-sided printing mode to remove the contaminative toner and CaCO 3  particles on the pressure roller  52  to such a degree that they become negligible can be confirmed in advance by experiments. In this embodiment, it was confirmed in advance through experiments that in a case where sheets S of recording medium used for a given image forming operation in the two-sided printing mode is no more than 215.9 mm in length in terms of the recording medium conveyance direction, as long as no less than five of them are conveyed through the fixing device  50 , the pressure roller  52  is satisfactorily cleaned, whereas in a case where sheets S of recording medium used for a given image forming operation in the two-sided printing mode is no less than 215.9 mm in terms of the recording medium conveyance direction, as long as no less than 2.5 of them is used, the pressure roller  52  is satisfactorily cleaned. In this embodiment, therefore, it is decided in Step S 209  whether or not the value in the two-sided print counter D is no less than 5. 
     Next, the CPU  201  decides, as follows, whether or not the pressure roller  52  needs to be cleaned. That is, the CPU  201  decides whether or not the value in the cleaning counter C is no less than 2,000 (S 212 ). 
     If the CPU  201  determines in Step S 212  that the value in the cleaning counter C is no less than 2,000, it outputs a message, on the display  302  of the control panel  600 , which indicates that the pressure roller  52  needs to be cleaned ( FIG. 3 ) (S 213 ). 
     On the other hand, if the CPU  201  determines in Step S 212  that the value in the cleaning counter C is no more than 2,000, it proceeds to Step S 214 . 
     The threshold value to be used to decide whether or not the pressure roller  52  is in the state in which it needs to be cleaned may be set in advance through experiments. In this embodiment, it was confirmed through experiments that as roughly 2,000 one-sided prints were continuously outputted, the contaminative toner and CaCO 3  particles began to adhere to the pressure roller  52 . This is why  2 , 000  is set as the threshold value for the cleaning counter C, in Step S 212 , in this embodiment. 
     Incidentally, the image forming apparatus  100  may be structured so that this threshold value is optional. That is, it can be changed to change the timing with which the above-described message is shown on the display portion  302  of the control panel  600 . 
     Lastly, the CPU  201  decides whether or not all the prints to be outputted by the on-going printing operation have been outputted (S 214 ). If it determines that they have not been, it returns to Step S 201 , and remains on standby until the next sheet S of recording paper moves through the fixing device  50 . 
     As described above, the image forming apparatus  100  in this embodiment has a toner image forming means which form a toner image on a sheet S of recording paper. It has also the fixing means  50  equipped with the rotational fixing member  51  and rotational pressure applying member  52 , which fix the toner image formed on the sheet S of recording paper by the toner image forming means, to the sheet S, while conveying the sheet S with the sheet S pinched between the rotational fixing member  51  and rotational pressure applying member  52 . It has also the recording medium conveying means B, which places upside down the sheet S of recording paper which is bearing the toner image fixed thereto by the fixing means  50 , and conveys the sheet S to the toner image forming means  1  and fixing means  50  for the second time. Further, it has the executing means  201  which makes the image forming apparatus  100  operate in the cleaning mode. In the cleaning mode, a preset toner image is formed on one of the two surfaces of a sheet S of recording paper by the toner image forming means  1  and is fixed to the sheet S by the fixing means  50 . Then, the sheet S is placed upside down by the recording medium conveying means B, and is delivered to the toner image forming means  1  and fixing means  50 , in the listed order, for the second time, to clean the rotational pressure applying member  52 . Further, the image forming apparatus  100  has the accumulating means C which cumulatively stores the number of one-sided prints which were continuously outputted by the image forming apparatus. Further, the image forming apparatus  100  has the informing means  600  which prompts an operator (user) that the fixing means  50  needs to be operated in the cleaning mode, as the value in the accumulating means C exceeds a preset value. Further, the image forming apparatus  100  has the resetting means  201  which resets the value in the accumulating means C as the number of the continuously outputted two-sided prints reaches a preset value. In particular, in this embodiment, the toner image to be formed on the sheet S of recording paper by the toner image forming means  1  to clean the rotational pressure applying member  52 , is longer in terms of the recoding medium conveyance direction than the circumferential length of the rotational pressure applying member  52 . Also in this embodiment, the image forming apparatus  100  has multiple recording medium storing portions  61 - 64  which store sheets S of recording medium. Thus, in the cleaning mode, a sheet S of recording paper which is largest in size (width, or dimension in terms of direction perpendicular to recording medium conveyance direction) among the sheets S of recording paper stored in the multiple recording medium storing portions  61 - 64  is fed to the toner image forming means I. Further, in this embodiment, in the cleaning mode, it is on only one of the two surfaces of a sheet S of recording paper that the above-described toner image for cleaning is formed by the toner image forming means I. 
     As will be evident from the detailed description of this embodiment of the present invention, the present invention makes it possible for the image forming apparatus  100  to decide whether or not the pressure roller  52  needs to be cleaned, without employing a dedicated sensor such as an optical sensor. Thus, it can provide a fixing device (apparatus) which is substantially less in the amount of the image contamination attributable to the contamination of the pressure roller  52 , and also, less in cost and size than a fixing device (apparatus) in accordance with the prior art. That is, the present invention makes it possible to provide a fixing device which is significantly simpler in the structure of the rotational member of its fixing means, and also, can simply determines the timing with which the rotational member of the fixing means is to be cleaned, than any fixing device (apparatus) in accordance with the prior art. 
     (Embodiment 2) 
     Next, another embodiment of the present invention is described. In terms of the basic structure and operation, this embodiment is the same as the first embodiment. Thus, the elements of the image forming apparatus in this embodiment which are the same as, or equivalent to, the counterparts in the first embodiment are given the same referential codes as those given to the counterparts, one for one, and are not described here. 
     The difference of this embodiment from the first embodiment is as follows. That is, in this embodiment, the image forming apparatus  100  is provided with the second accumulating means which is different from the second accumulating means in the first embodiment, in that it accumulates the information related to the amount of the toner used to form a toner image on one (first) of the two surfaces of a sheet S of recording paper in the two-sided printing mode. Thus, the altering means, in this embodiment, for altering the value in the first accumulating means which accumulates the information related to the amount by which the image forming apparatus  100  is continuously operated in the one-sided printing mode, reduces the value in the first accumulating means, as the value in the second accumulating means exceeds a preset value. 
     The typical information related to the amount of the toner used to form a toner image on the first surface of the sheet S of recording paper in the two-sided printing mode is such information as pixel count. However, the information does not need to be limited to pixel count. For example, it may be such information as the cumulative value of the pixel density, for example. In other words, the information has only to be such information that is related to the amount of the toner used to form a toner image on the first surface of a sheet S of recording paper. 
     More concretely, in the first embodiment, as the image forming apparatus  100  is operated in the two-sided printing mode, the value in the two-side-print counter D by a preset value, based on the length of each sheets of recording paper in terms of the recording medium conveyance direction. If the value in the two-sided print counter D exceeds a preset threshold value, the CPU  201  determines that the pressure roller  52  does not need to be cleaned, and clears the cleaning counter C. 
     In comparison, in this embodiment, as the image forming apparatus  100  is operated in the two-sided printing mode, the CPU  201  obtains the video count V for each of the monochromatic toner images of the primary color on the first surface of a sheet S of recording paper, and then, obtains the sum Vsum of the video counts V as will be described later in detail. Then, if the sum Vsum of the video counts V exceeds a preset threshold value, the CPU  201  determines that the pressure roller  52  does not need to cleaned, and clears the cleaning count C. 
     That is, the contaminative toner and CaCO 3  particles on the peripheral surface of the pressure roller  52  are discharged from the image forming apparatus  100  by being transferred onto the toner image on the first surface of a sheet S of recording paper. Therefore, the smaller the amount of the toner in the toner image on the first surface of the sheet S is, the smaller the amount of the contaminative toner and CaCO 3  particles which have to be removed from the peripheral surface of the pressure roller  52 . In other words, the greater the amount of the toner in the toner image formed on the first surface of the sheet S is, the greater the amount of the contaminative toner and CaCO 3  particles which have to be removed from the peripheral surface of the pressure roller  52 . Therefore, to what degree the contaminative toner and CaCO 3  particles on the peripheral surface of the pressure roller  52  were removed can be estimated by counting the amount of the toner in the toner image formed on the first surface of the sheet S in the two-sided printing mode. 
       FIG. 7  is a block diagram of the essential portions of the image forming apparatus  100  in this embodiment, and shows how the essential portions are controlled. 
     The image processing section  250  processes the image data inputted from the external I/F  220 , and sends the processed image data to the exposing device  13  of the image forming portion I. The image processing section  250  has a video counting internal circuit  251  as a video counting means. It is capable of counting the pixels of the toner image formed on a sheet S of recording paper per page (it is capable of obtaining video count value). The CPU  201  obtains the sum Vsum of the video count value V of the toner image formed on the first surface of a sheet S of recording paper in the two-sided printing mode, per primary color, based on the video count value V per primary color. Then, it stores the sum Vsum of this video count, as the information related to the amount of the toner in the toner image on the first surface of the sheet S in the two-sided printing mode, in a video counter E for the two-sided printing mode, which is a storing means as the second accumulating means. 
       FIG. 8  is a flowchart for describing the operational sequence, in this embodiment, to be carried out by the CPU  201  to decide whether or not the pressure roller  52  needs to be cleaned. 
     As a printing job is started, the CPU  201  remains on standby until a sheet S of recording paper moves through the fixing device  50  (S 301 ). 
     If the CPU  201  determines in Step S 301  that a sheet S of recording paper moved through the fixing device  50 , it decides whether or not the sheet S is for a one-sided print (S 302 ). 
     If the CPU  201  determines in Step S 302  that the sheet S is for a one-sided print, it adds 1 to the cleaning counter C (S 303 ). 
     Then, the CPU  201  clears the two-sided printing mode video counter E; it reduces the value Vsum in the video counter E to zero (S 304 ). Then, it proceeds to Step S 310 . 
     On the other hand, if the CPU  201  determines in Step S 202  that the sheet S of recording paper is not for a one-sided print, that is, if it determines that the sheet S is for a two-sided print, it increases the value Vsum in the video counter E in the following manner. 
     First, the CPU  201  obtains the video count value V of the image formed on the first surface of the sheet S of recording paper, per primary color (S 305 ). This video count value V is such a value that is obtained by the video counting circuit  251 , based on the image data transferred thereto through the external I/F  220 . 
     Then, the CPU  201  adds the video count value V of the image formed on the first surface of the sheet S, per primary color, to the value Vsum in the two-sided printing mode video counter E (S 306 ). 
     Next, the CPU  201  decides whether or not the value Vsum in the two-side video counter E is no less than 500 (S 307 ). 
     What kind of toner image in terms of toner consumption has to be formed on the first surface of a sheet S of recording paper for a two-sided print to reduce the contaminative toner and CaCO 3  particles on the peripheral surface of the pressure roller  52  to such a degree that they become ignorable can be determined in advance by experiments. In the case of this embodiment, it was confirmed by experiments that the amount of toner has only to be equal to the amount of the toner in the solid toner image which was formed on a sheet S of recording paper of A4 size, and which was large enough to cover virtually the entirety of one of the two surfaces of the sheet S. The video count of a solid monochromatic toner image which is large enough to cover the entirety of one of the two surfaces of a sheet S of recording paper of size A4 is roughly 500. In this embodiment, therefore, 500 is used as the threshold value in Step S 307 . 
     If the CPU  201  determines in Step S 307  that the value Vsum in the video counter E is no less than 500, it clears the cleaning counter C (S 308 ). 
     Then, the CPU  201  clears the two-sided printing mode video counter E; it reduces the value in the counter E to zero (S 309 ). 
     On the other hand, if the CPU  201  determines in Step S 309  that the sum Vsum of the video count is not more than 500, it proceeds to Step S 310 . 
     Thereafter, the CPU  201  decides whether or not the pressure roller  52  needs to be cleaned, based on the value in the cleaning counter C, as in the first embodiment. The processes to be carried out in Steps S 310 , S 311  and S 312  are the same as those to be carried out in Steps S 212 , S 213  and S 214  ( FIG. 6 ), respectively, in the first embodiment, and therefore, there are not described here. 
     As described above, the effects obtainable by this embodiment are the same as those obtainable by the first embodiment. Further, whether or not the pressure roller  52  needs to be cleaned can be decided based on the degree to which the pressure roller  52  is cleaned as the image forming apparatus is operated in the two-sided printing mode. 
     (Miscellanies) 
     Heretofore, the present invention was described with reference to the preferred embodiments of the present invention. However, these embodiments are not intended to limit the present invention in scope. 
     In the above-described embodiments, the operation for cleaning the rotational member, with which the fixing means is provided, is carried out by the image forming apparatus in response to the instruction given by the operator. However, these embodiments are not intended to limit the present invention in terms of how the cleaning operation is initiated. For example, an image forming apparatus may be designed so that as it is determined that the rotational member needs to be cleaned, and the cleaning timing indicating means indicates the timing with which the cleaning operation needs to be carried out, the operation for cleaning the rotational member is automatically carried out by the executing means for carrying the cleaning operation. In this case, the executing means is enabled to carry out the cleaning operation for cleaning the rotational member, with an optimal timing, while no image is formed, after it is determined that the rotational member needs to be cleaned. The examples of the period in which no image is formed during a given image forming operation are: the period immediately after an image forming apparatus is turned on, and the period immediately after an image forming apparatus is restarted while it is kept in the sleep mode, that is, the period in which the preparatory operation in which the rotational members are idly rotated multiple times; the period from when the a print signal (image formation start signal) is inputted and to when the image forming apparatus starts actually writing an image based on the information of the image to be formed, in which the preset preparatory operation to be carried out, and in which the rotational members are idly rotated multiple times; sheet intervals which correspond to the distance between the consecutively conveyed two sheets of recording paper; and period immediately after the completion of a given image forming operation, in which a preset post-image formation operation (preparatory operation for next image formation) is carried out. 
     For example, in a case where an image forming apparatus is designed so that as the cumulative value which shows the amount by which the image forming apparatus is actually operated in the one-sided printing mode reaches a preset value, the on-going printing job is automatically interrupted and the apparatus is operated in the cleaning mode (forced cleaning operation). In this case, as soon as the cleaning operation is ended, the interrupted printing job is restarted after the cumulative value obtained in the one-sided printing mode is reset. If the timing with which a print job is ended coincides with the timing with which the timing to start operating the image forming apparatus in the cleaning mode, it may be during the so-called post-rotation period that the image forming apparatus is operated in the cleaning mode. Further, in a case where the cumulative value obtained in the one-sided printing mode reaches the preset value during an image forming operation, and the number of prints which has yet to be outputted by the on-going printing job is relatively small, the on-going printing job does not need to be interrupted for cleaning the pressure roller; the image forming apparatus may be operated in the cleaning mode during the post-rotation period. In other words, instead of the above-described message which prompts an operator to put the image forming apparatus in the cleaning mode, a message which indicates that the image forming apparatus is being mandatorily operated in the cleaning mode, is shown on the display of the control panel. In this case, the image forming apparatus has: the accumulating means which cumulatively stores the number of times prints are continuously outputted by the image forming apparatus while the apparatus is operated in the one-sided printing mode, that is, the mode in which a toner image is formed only one of the two surfaces of a sheet of recording paper; and executing means which makes the image forming apparatus operate in the cleaning mode as the value in the accumulating means is no less than a preset value. Also in this case, the image forming apparatus may be provided with the resetting means for resetting the accumulating means as the number of prints outputted by the image forming apparatus which the apparatus is operated in the two-sided printing mode reaches a preset value. 
     Further, the image forming apparatus may be designed so that in a case where it is determined that the rotational member of the fixing means needs to be cleaned, and the prompting means shows the timing with which the image forming apparatus is to be operated in the cleaning mode, a message that prompts an operator to remove the rotational member of the fixing means and clean the rotational member, is given to the operator. 
     Further, the image forming apparatuses in the above-described embodiments of the present invention were described as image forming apparatuses of the so-called intermediary transfer type. However, the present invention is also applicable to an image forming apparatus of the so-called direct transfer type, which is well known among people in the business to which the present invention is related. The effects of such application of the present invention are the same as those of the preceding embodiments. An image forming apparatus of the so-called direct transfer type has an endless recording sheet conveyance belt, or the like, as a recording sheet bearing member, which bears and conveys a sheet of recording paper, in place of the intermediary transferring member which an image forming apparatus of the intermediary transfer type has. Thus, the toner images, different in color, formed on the photosensitive members of the image forming stations, one for one, are transferred in layers onto a sheet of recording paper borne by the recording medium bearing member, and then, are fixed to the sheet of recording paper. 
     The application of the present invention is not limited to a color image forming apparatus. That is, the present invention is also applicable to a monochromatic image forming apparatus, and the effects of the application of the present invention to a monochromatic image forming apparatus are the same as those obtainable by the application of the present invention to a color image forming apparatus. 
     Further, in the above-described embodiments, the pair of rotational members which constitute the fixing device include fixation belt and pressure roller. However, the pair may be constituted by a fixation roller and a pressing belt, or both of them may be rollers or belts. Further, both of the rotational members may be provided with respective heat sources. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims priority from Japanese Patent Application No. 239337/2012 filed Oct. 30, 2012 which is hereby incorporated by reference.