Patent Publication Number: US-9423733-B2

Title: Sheet processing apparatus and image forming apparatus having the same

Description:
FIELD 
     Embodiments described herein relate generally to a sheet processing apparatus and an image forming apparatus having the sheet processing apparatus. 
     BACKGROUND 
     An image forming apparatus is capable of carrying out an image forming process with an erasable coloring material and a non-erasable coloring material. Another image forming apparatus is capable of carrying out an image erasing process of a fixed image formed with an erasable coloring material, using a fixing unit used for the image forming process. Such an image forming apparatus may be a printer, a multi function peripheral (MFP), and the like. A fixing temperature of an unfixed image may be different depending on the coloring material of the unfixed image, and an erasing temperature of a fixed image may also be different depending on the erasable coloring material of the fixed image. For example, when a fixing temperature of an unfixed image formed with an erasable coloring material is different from a fixing temperature of an unfixed image formed with a non-erasable coloring material, the image forming apparatus needs to change the fixing temperature according to the fixing target. However, it may take a significant amount of time for the image forming apparatus to increase and decrease the fixing or the erasing temperature when the target is switched. This process time may decrease usability of the image forming apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an image forming apparatus according to one embodiment. 
         FIG. 2  is a cross-sectional view of the image forming apparatus. 
         FIG. 3  illustrates a heat source mechanism in a fixing section of the image forming apparatus. 
         FIG. 4  is a block diagram of the heat source mechanism. 
         FIG. 5  is a flowchart of a temperature control carried out by a temperature controller of the heat source mechanism. 
         FIG. 6  is a graph showing a relationship between a time required to change the surface temperatures of a heating roller  51  and a pressing belt  52  from an ordinary temperature to a control temperature. 
         FIG. 7  is a graph showing a relationship between a time required to change the surface temperatures of the heating roller  51  and the pressing belt  52  in a case of switching from a first image forming mode to a second image forming mode. 
         FIG. 8  is a graph showing a relationship between a time required to change the surface temperatures of the heating roller  51  and the pressing belt  52  in a case of switching from the second image forming mode to the first image forming mode. 
         FIG. 9  illustrates time required to switch the image forming mode under two different conditions. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with one embodiment, a sheet processing apparatus includes a rotating unit, a pressing unit, a nip through which a sheet passes being formed between the rotating unit and the pressing unit, a first heating unit configured to heat the rotating unit, a second heating unit configured to heat the pressing unit, and a control unit. The control unit is configured to control the first and the second heating units according to a mode of operation including a first mode and a second mode, such that, in the first mode, the rotating unit at the nip is at a first temperature and the pressing unit at the nip is at a second temperature, and in the second mode, the rotating unit at the nip is at a third temperature that is equal to or higher than the first temperature and the pressing unit at the nip is at a fourth temperature that is higher than the second temperature. A difference between the second and the fourth temperatures being smaller than a difference between the first and the third temperatures. 
     Embodiments for implementing the present invention are described hereinafter with reference to the accompanying drawings. Same components in each figure are shown with the same reference numerals, and therefore repeated description is omitted. The “color-erasing” in the present embodiment refers to making the image, which is formed in a color different from the ground color of the paper, invisible. Herein, the “color different from the ground color” includes not only chromatic color but also achromatic color such as white color, black color, and the like. 
       FIG. 1  is a schematic view of an image forming apparatus  1  according to an embodiment. The image forming apparatus  1  includes a paper feed section  3  arranged at the lower portion of a housing  2 , an image forming section  4 , a fixing section  5 , an input panel  15 , and an image reading section  19  arranged at the upper portion of the housing  2 . 
       FIG. 2  is a schematic cross-sectional view of the image forming apparatus  1 . The paper feed section  3 , the image forming section  4 , the fixing section  5 , and the image reading section  19  are arranged in the image forming apparatus  1  in sequence from the lower portion to the upper portion of the housing  2 . The paper feed section  3  has a plurality of paper feed cassettes  9  arranged in the vertical direction. Only the top paper feed cassette  9  is shown in  FIG. 2 . In each paper feed cassette  9 , unused (new) paper serving as a sheet to be subjected to image fixing processing or paper to be reused is stacked. The paper to be reused is paper to be subjected to color erasing processing for erasing an erasable toner image fixed thereon. The paper stacked in the paper feed cassette  9  is picked up one by one by a pickup roller  10 . The paper picked up by the pickup roller  10  is conveyed towards a separation/conveyance roller pair  11 . The separation/conveyance roller pair  11  conveys the paper conveyed by the pickup roller  10  towards a register roller pair  6  through a paper feed conveyance section  7 . The paper from a double-sided paper feed section  8  for printing on both sides of the paper is also conveyed to the register roller pair  6 , in addition to the paper conveyed through the paper feed conveyance section  7 . The double-sided paper feed section  8  extends from a position above the fixing section  5  via a space between the fixing section  5  and the lateral side of the housing  2  towards the register roller pair  6 . 
     The image forming section  4  includes a photoconductive drum  12 . The photoconductive drum  12  is formed into a columnar shape and has organic photo conductors (OPC) around the outer peripheral surface thereof. The photoconductive drum  12  is rotated at a predetermined circumferential speed. The image forming section  4  further includes a corona charger  13 , a laser exposing unit  14 , a first developing device  30 , a second developing device  32 , a transfer roller  18 , a cleaner  16 , and a charge removing lamp  17 . 
     The corona charger  13  is a scorotron-type charger for uniformly charging the outer peripheral surface of the photoconductive drum  12  to a negative polarity potential. The laser exposing unit  14  exposes the outer peripheral surface of the photoconductive drum  12 . The first developing device  30  stores erasable toner as the erasable recording agent. The first developing device  30  develops an electrostatic latent image formed on the outer peripheral surface of the photoconductive drum  12  with the erasable toner when it is selected to form an image with the erasable toner. The development with the erasable toner is carried out through the reversal development by a developing roller  30   a  of the first developing device  30 . The second developing device  32  stores non-erasable toner as the non-erasable recording agent. The second developing device  32  develops the electrostatic latent image formed on the outer peripheral surface of the photoconductive drum  12  with the non-erasable toner when it is selected to form an image with the non-erasable toner. The development with the non-erasable toner is carried out through the reversal development by a developing roller  32   a  of the second developing device  32 . Whether to form an image with the erasable toner or the non-erasable toner is selected based on, for example, selection information input by a user through the input panel  15 . The transfer roller  18  is arranged in such a manner that a constant load is applied to the photoconductive drum  12 . The paper is conveyed by the register roller pair  6  to a transfer nip where the transfer roller  18  is contacted with the outer peripheral surface of the photoconductive drum  12  in synchronization with the formation of the toner image. The transfer roller  18  presses the paper against the photoconductive drum  12  in the transfer nip to transfer the toner image developed on the outer peripheral surface of the photoconductive drum  12  to the paper. When transferring the toner image to the paper, the transfer roller  18  applies a positive polarity transfer bias through a high-voltage source. The cleaner  16  removes and collects the toner that is not transferred to the paper and is left on the outer peripheral surface of the photoconductive drum  12 . The charge removing lamp  17  removes the charge left on the outer peripheral surface of the photoconductive drum  12 . 
     An image signal received by the image forming apparatus  1  from an external computer or an image signal obtained through digital conversion of the document image read by the image reading section  19  is input to an image processing circuit  41 . The image processing circuit  41  carries out a predetermined image processing based on the image signal and output the image processing result to a laser driving circuit  42 . The laser driving circuit  42  controls the laser exposing unit  14  based on the image processing result. The laser exposing unit  14  carries out, with a semiconductor laser, scanning of laser light on the outer peripheral surface of the photoconductive drum  12  with a predetermined resolution to form an electrostatic latent image on the outer peripheral surface of the photoconductive drum  12 . 
     The electrostatic latent image formed on the outer peripheral surface of the photoconductive drum  12  is developed with the erasable toner stored in the first developing device  30  or the non-erasable toner stored in the second developing device  32 . In a standby state during which image forming processing is not carried out, the first developing device  30  and the second developing device  32  are positioned at a certain distance away from the photoconductive drum  12 . In an operation state during which image forming processing is carried out, either of the first developing device  30  or the second developing device  32  selected according to the selection information is moved to a position nearby the photoconductive drum  12 . At this time, the developing roller  30   a  of the first developing device  30  or the developing roller  32   a  of the second developing device  32  is moved close to the photoconductive drum  12 . The moving of the first developing device  30  is carried out with a cam  31 . The moving of the second developing device  32  is carried out with a cam  33 . When selected in the operation state, the first developing device  30  or the second developing device  32  is moved to a position near the photoconductive drum  12  and is connected with a driving source through a clutch (not shown). After being connected with the driving source, the developing roller  30   a  or the developing roller  32   a  is driven to rotate so as to supply toner to the outer peripheral surface of the photoconductive drum  12 . 
     The first developing device  30  stores erasable two-component developing agent serving as a mixture of a magnetic carrier and the erasable toner charged to negative polarity. For example, the volume average particle diameter of the erasable toner is 10 μm, and the volume average particle diameter of the magnetic carrier is 40 μm. A toner concentration sensor is disposed in the first developing device  30 . The toner concentration sensor detects the concentration of the erasable toner in the first developing device  30 . When the concentration of the erasable toner is lower than a predetermined concentration, the erasable toner stored in an erasable toner cartridge in the image forming apparatus  1  is supplied to the first developing device  30 . 
     The second developing device  32  stores non-erasable two-component developing agent serving as a mixture of a magnetic carrier and the non-erasable toner charged to negative polarity. For example, the volume average particle diameter of the non-erasable toner is 8 μm, and the volume average particle diameter of the magnetic carrier is 40 μm. Similar to the first developing device  30 , the second developing device  32  has a toner concentration sensor. The toner concentration sensor detects the concentration of the non-erasable toner in the second developing device  32 . When the concentration of the non-erasable toner is lower than a predetermined concentration, the non-erasable toner stored in anon-erasable toner cartridge in the image forming apparatus  1  is supplied to the second developing device  32 . 
     The toner image formed on the outer peripheral surface of the photoconductive drum  12  with the erasable toner or the non-erasable toner is transferred to the paper passing through the transfer nip. The register roller pair  6  conveys the paper supplied from the paper feed conveyance section  7  or the double-sided paper feed section  8  to the transfer nip at predetermined timing. The paper to which the toner image formed with the erasable toner or the non-erasable toner is transferred is conveyed to the fixing section  5 . The toner image transferred to the paper as an unfixed recording agent image is fixed on the paper through a processing carried out by the fixing section  5 . 
     Toner that is not transferred to the paper and is left on the photoconductive drum  12  is removed by the cleaner  16 . The charge removing lamp  17  removes the charge left on the outer peripheral surface of the photoconductive drum  12  after the toner left on the photoconductive drum  12  is removed. After the charge removing processing is carried out by the charge removing lamp  17 , the corona charger  13  uniformly charges the outer peripheral surface of the photoconductive drum  12  for the next electrostatic latent image formation. 
     In addition, the image forming process for forming a toner image serving as an unfixed recording agent image on the paper described in the present embodiment is just exemplified as one example, and the present invention is not limited to this. 
     The paper on which the toner image is fixed at the fixing section  5  is conveyed towards a paper discharge roller pair  20 . In a case of one-side printing, the paper on which the toner image is fixed is discharged to a paper discharge tray  21  by the paper discharge roller pair  20 . In a case of duplex printing, the paper on the first surface of which the toner image is fixed is conveyed to the switchback type double-sided paper feed section  8  for printing on the second surface of the paper. 
     The paper conveyed to the double-sided paper feed section  8  is further conveyed to the register roller pair  6 . The paper conveyed to the register roller pair  6  through the double-sided paper feed section  8  is further conveyed to the transfer nip at the timing of transferring the toner image to the second surface. The paper on the second surface of which the toner image is transferred is conveyed towards the fixing section  5 . Then the paper of which the toner image on the second surface is fixed at the fixing section  5  is discharged to the paper discharge tray  21  by the paper discharge roller pair  20 . 
     The fixing section  5  includes a cylindrical heating roller  51  serving as a fixing member and a pressing belt  52  serving as a pressing member, which is pressed against the heating roller  51  and rotated. The pressing belt  52  contacts the outer peripheral surface of the heating roller  51  over a given range to form a fixing nip. The heating roller  51  includes a heating roller lamp  53  serving as a second heat source therein. For example, a halogen lamp is used for the heating roller lamp  53 . The diameter of the heating roller  51  can be set to 45 mm and the peripheral length of the pressing belt  52  can be set to 47 mm. 
     The pressing belt  52  is stretched with a certain tension applied by a belt heating roller  54 , a pressing roller  55  and a tension roller  56 . The belt heating roller  54  is positioned upstream of a pressing roller  55  in the paper conveyance direction in the fixing section  5 . The belt heating roller  54  and the pressing roller  55  are positioned near the heating roller  51 . Part of the pressing belt  52  stretched between the belt heating roller  54  and the pressing roller  55  is pressed against the heating roller  51  to contact the heating roller  51  in pressure. The pressing belt  52  is in contacted with the heating roller  51  in pressure so as to form the fixing nip. The tension roller  56  is located at a position away from the heating roller  51  than the positions of the belt heating roller  54  and the pressing roller  55 . The tension roller  56  applies a force in a direction away from the tension roller  56  to the pressing belt  52  to cause a certain tension of the pressing belt  52 . A pressure pad folder  57  is arranged inside the loop of the pressing belt  52 . A pressure pad  58  is disposed on the pressure pad folder  57 . The pressure pad  58  presses the pressing belt  52  against the heating roller  51  from the inside of the loop of the pressing belt  52  so that the pressing belt  52  is in contact with the heating roller  51  in pressure. The belt heating roller  54  is formed in a cylindrical shape. The belt heating roller  54  includes a pressing belt lamp  59  serving as a first heat source therein. For example, a halogen lamp is used for the pressing belt lamp  59 . The diameter of the belt heating roller  54  can be set to 20 mm, the diameter of the pressing roller  55  can be set to 18 mm, and the width of the pressure pad  58  can be set to 10 mm. 
     A fixing member thermistor  61  is disposed in the fixing section  5  in the manner of contacting the outer peripheral surface of the heating roller  51 . The fixing member thermistor  61  detects the surface temperature of the outer peripheral surface of the heating roller  51 . A pressing member thermistor  62  is arranged in the manner of contacting a part of the pressing belt  52  that is in contact with the belt heating roller  54 . The pressing member thermistor  62  detects the temperature of the surface of the pressing belt  52  that is in contact with the paper. 
     The heating roller  51  serving as the fixing member contacts the unfixed toner image formed on the paper at the fixing nip. The heating roller  51  has a release layer on, for example, an aluminum roller substrate having a wall thickness of 1.0 mm. The release layer on the roller substrate is formed by, for example, fluororesin PFA (copolymer of tetrafluoroethylene and Perfluoro alkyl vinyl ether) having a thickness of about 25 μm. 
     The pressing belt  52  serving as the pressing member is formed by laminating a silicone rubber layer on a nickel belt substrate and then laminating a release layer on the silicone rubber layer. For example, the thickness of the nickel belt substrate is about 40 μm, and the thickness of the silicone rubber layer is 200 μm. The fluororesin PFA is used for the release layer. The thickness of the fluororesin PFA of the release layer is about 30 μm. 
     The heating roller  51  is driven by a driving source to rotate from the upstream side of the conveyance direction towards the downstream side of the conveyance direction. The pressing belt  52  is rotated from the upstream side of the conveyance direction towards the downstream side of the conveyance direction in accordance with the rotation of the heating roller  51 . 
       FIG. 3  illustrates heat source mechanism of the fixing section  5 . The heating roller lamp  53  arranged inside the heating roller  51  includes a heating roller center lamp  53 A and a heating roller side lamp  53 B. The heating roller center lamp  53 A heats the outer peripheral surface of the central part of the heating roller  51  in the center axis direction from inside. The heating roller side lamp  53 B heats the outer peripheral surfaces of the two ends of the heating roller  51  in the center axis direction from inside. The pressing belt lamp  59  arranged inside the belt heating roller  54  heats the whole outer peripheral surface of the belt heating roller  54  from inside. For example, when the heating roller  51  is heated over a range corresponding to the width of A4-sized paper, the heating roller center lamp  53 A is used. When the heating roller  51  is heated over a range corresponding to the length of A4-sized paper, both the heating roller center lamp  53 A and the heating roller side lamp  53 B are used. For example, a halogen lamp having an output of 300 W can be used for the heating roller center lamp  53 A, the heating roller side lamp  53 B, and the pressing belt lamp  59 . 
     A temperature control section  60  controls the heating roller center lamp  53 A, the heating roller side lamp  53 B, and the pressing belt lamp  59 . The temperature control section  60  includes a first, a second, and a third switching elements  63 A,  63 B, and  64  and a temperature controller  65 . The first switching element  63 A is arranged between the heating roller center lamp  53 A and the power source. The second switching element  63 B is arranged between the heating roller side lamp  53 B and the power source. The third switching element  64  is arranged between the pressing belt lamp  59  and the power source. Each of the first, the second, and the third switching elements  63 A,  63 B, and  64  is controlled by the temperature controller  65 . The temperature controller  65  respectively turns on or turns off the first, the second, and the third switching elements  63 A,  63 B, and  64  to control the heating. For example, bidirectional thyristors are used for the first, the second, and the third switching elements  63 A,  63 B, and  64 . The power source for supplying power to the heating roller center lamp  53 A, the heating roller side lamp  53 B, and the pressing belt lamp  59  may be, for example, a commercial-use AC power source. 
     The fixing member thermistor  61  includes a center thermistor  61 A and a side thermistor  61 B. The center thermistor  61 A detects the temperature of the outer peripheral surface of the central part of the heating roller  51  in the center axis direction. The side thermistor  61 B detects the temperature of the outer peripheral surface of either a first end or a second end of the heating roller  51  in the center axis direction. Each of the center thermistor  61 A and the side thermistor  61 B sends the detected temperature to the temperature controller  65 . The temperature controller  65  inputs the temperatures detected by the center thermistor  61 A and the side thermistor  61 B as temperature information. For example, when the paper to be subjected to toner image fixing processing is A4-sized paper, the temperature controller  65  reduces the energization time of the heating roller side lamp  53 B. In this way, the temperature of the outer peripheral surface at the two ends of the heating roller  51  will not rise to a temperature above a predetermined temperature. 
     The pressing member thermistor  62  detects the surface temperature of the central part of the pressing belt  52  in the width direction. The pressing member thermistor  62  sends the detected temperature to the temperature controller  65 . The temperature controller  65  inputs the temperature detected by the pressing member thermistor  62  as temperature information. 
       FIG. 4  is a block diagram of the heat source mechanism of the fixing section  5 . As shown in  FIG. 4 , the center thermistor  61 A is connected with the temperature controller  65  through an A/D converter  66 . The temperature detected by the center thermistor  61 A is converted into temperature information as a digital signal by the A/D converter  66 . The side thermistor  61 B is connected with the temperature controller  65  through an A/D converter  67 . The temperature detected by the side thermistor  61 B is converted into temperature information as a digital signal by the A/D converter  67 . The pressing member thermistor  62  is connected with the temperature controller  65  through an A/D converter  68 . The temperature detected by the pressing member thermistor  62  is converted into temperature information as a digital signal by the A/D converter  68 . The temperature controller  65  inputs the temperature information converted by the A/D converters  66 ,  67 , and  68 . The temperature controller  65  is connected with an ROM  69  and an RAM  70 . Programs according to which the temperature controller  65  executes the temperature control are stored in the ROM  69 . A plurality of control parameters including the control temperature of each of the belt heating roller  54  and the pressing belt  52  in the temperature control is stored in the ROM  69  in advance. The RAM  70  temporarily stores the parameters used by the temperature controller  65  in the temperature control. The parameters temporarily stored by the RAM  70  includes, for example, the temperature information indicating the temperatures of the belt heating roller  54  and the pressing belt  52  acquired by the temperature controller  65  and the like. 
       FIG. 5  is a flowchart of the temperature control carried out by the temperature controller  65 . The temperature controller  65  carries out the temperature control for each of the heating roller center lamp  53 A, the heating roller side lamp  53 B, and the pressing belt lamp  59 . When the temperature control is started, the temperature controller  65  acquires temperature information (ACT 1). The temperature controller  65  determines whether or not the temperature indicated by the acquired temperature information is higher than a control temperature (ACT 2). When the temperature is higher than the control temperature (YES in ACT 2), the temperature controller  65  turns off the switching element and stops the power supply for the lamp to turn off the lamp (ACT 3). After turning off the switching element, the temperature controller  65  returns to ACT 1 to repeat the process following ACT 1. When the temperature is not higher than the control temperature (NO in ACT 2), the temperature controller  65  turns on the switching element and supplies power for the lamp to turn on the lamp (ACT 4). After turning on the switching element, the temperature controller  65  returns to ACT 1 to repeat the process following ACT 1. 
     Through the temperature control described above, the temperature controller  65  maintains the surface temperature of each of the heating roller  51  and the pressing belt  52  at the control temperature. The control temperature for the heating roller  51  and the control temperature for the pressing belt  52  are determined separately. In addition, different control temperatures are determined when the toner image is formed with erasable toner and when the toner image is formed with non-erasable toner. 
     For example, when an image is formed with the erasable toner, the process speed of the image forming apparatus  1  is 136 mm/sec. The process speed refers to a moving speed of the outer peripheral surface of the photoconductive drum  12  and a paper conveyance speed when the paper passes through the transfer nip and the fixing nip. When the process speed is 136 mm/sec, the image forming apparatus  1  can process thirty sheets of A4-sized paper per minute. Further, when an image is formed with the non-erasable toner, the process speed of the image forming apparatus  1  is 210 mm/sec. When the process speed is 210 mm/sec, the image forming apparatus  1  can process forty five sheets of A4-sized paper per minute. The print speed in a case of using non-erasable toner is 1.5 times as fast as the print speed in a case of using erasable toner. When color erasing processing is carried out on the paper on which the erasable toner is fixed, the process speed of the image forming apparatus  1  is 136 mm/sec. When the color erasing process speed is 136 mm/sec, the image forming apparatus  1  can process thirty sheets of A4-sized paper per minute. When the length of the fixing nip of the fixing section  5  in the conveyance direction is about 21 mm, and the process speed is 136 mm/sec, the paper can pass through the fixing nip in 0.154 sec. When the length of the fixing nip in the conveyance direction is about 21 mm, and the process speed is 210 mm/sec, the paper can pass through the fixing nip in 0.1 sec. 
     The toner image formed on the paper is heated and pressed at the fixing section  5  to be fixed on the paper. In order to fix the toner image formed with the erasable toner on the paper without erasing the toner image, it is necessary that the fixing section  5  heats the toner image at a temperature lower than a color erasing temperature. That is, in a case of fixing the erasable toner, the temperature controller  65  needs to control the surface temperature of each of the heating roller  51  and the pressing belt  52  below the color erasing temperature. Further, in order to uniformly heat the toner image formed on the paper, the surface temperature of the heating roller  51  may be controlled to be the same as or approximate to the surface temperature of the pressing belt  52 . 
     The toner image formed with the non-erasable toner is not erased, and therefore, there is no limit on the temperature when fixing the toner image formed with the non-erasable toner. When the surface temperature of the outer peripheral surface of the heating roller  51  is sufficiently high, the toner image can be sufficiently fixed on the paper even if the print speed is high. 
     Operational modes of the image forming apparatus  1  according to the present embodiment include a first image forming mode for forming an image with the erasable toner and a second image forming mode for forming an image with the non-erasable toner. The user of the image forming apparatus  1  operates, for example, the input panel  15  to switch between the first image forming mode and the second image forming mode. The temperature control of the fixing section  5  in the first image forming mode is different from that in the second image forming mode. Specifically, the surface temperature of the outer peripheral surface of the heating roller  51  and the surface temperature of the pressing belt  52  in the first image forming mode is different from the surface temperature of the outer peripheral surface of the heating roller  51  and the surface temperature of the pressing belt  52  in the second image forming mode. When the image forming mode is switched, certain time is needed before the surface temperatures of the heating roller  51  and the pressing belt  52  are changed to the corresponding control temperature. 
     Hereinafter, the temperature control for reducing the time required to change the surface temperatures of the heating roller  51  and the pressing belt  52  to the control temperature is described.  FIG. 6  is a graph showing a relationship between the time required to change the surface temperatures of the heating roller  51  and the pressing belt  52  from an ordinary temperature to the control temperature. Herein, the ordinary temperature is set to 23 degrees centigrade. In the graph shown in  FIG. 6 , the abscissa indicates the control temperature/warming-up temperature, and the ordinate indicates the warming-up time. As shown in the graph in  FIG. 6 , the higher the control temperature is, the longer the warming-up time serving as the time required for the rise of the surface temperature is. Particularly, the warming-up time of the pressing belt  52  increases exponentially as the control temperature increases. That is because the heat transferred from the belt heating roller  54  to the pressing belt  52  diffuses to the pressure pad  58 , the pressing roller  55 , and the tension roller  56 . 
     In a case of fixing the erasable toner, an evaluation result indicates that the proper control temperature of the heating roller  51  is 100 degrees centigrade and the proper control temperature of the pressing belt  52  is 90 degrees centigrade. The color erasing starting temperature of the erasable toner in the evaluation is 109 degrees centigrade. The print speed in the evaluation is thirty sheets of paper per minute. 
       FIG. 7  is a graph showing a relationship between the time required for the change of the surface temperatures of the heating roller  51  and the pressing belt  52  in a case of switching from the first image forming mode to the second image forming mode. In the graph shown in  FIG. 7 , the abscissa indicates the control temperature/warming-up temperature, and the ordinate indicates the warming-up time. As shown in the graph in  FIG. 7 , similar to the case of changing from the ordinary temperature to the control temperature, the higher the control temperature is, the longer the time required for the rise of the surface temperature is. Further, the warming-up time of the pressing belt  52  increases exponentially as the control temperature increases. 
     In a case of fixing the non-erasable toner, an evaluation result indicates that the proper value of the control temperature of the heating roller  51  is 120 degrees centigrade when the control temperature of the pressing belt  52  is 110 degrees centigrade. And an evaluation result indicates that the proper value of the control temperature of the heating roller  51  is 130 degrees centigrade when the control temperature of the pressing belt  52  is 90 degrees centigrade. 
       FIG. 8  is a graph showing a relationship between the time required for the change of the surface temperatures of the heating roller  51  and the pressing belt  52  in a case of switching from the second image forming mode to the first image forming mode. In the graph shown in  FIG. 8 , the abscissa indicates the control temperature/fixing temperature, and the ordinate indicates the cool down time. After the image forming apparatus  1  continuously carries out image forming processing on 500 sheets of A4-sized paper, the time required for the change of the surface temperature is measured. The control temperature in the second image forming mode is 100 degrees centigrade. The cool down time serving as the time required for the decrease of the surface temperature refers to the time required to decrease the surface temperature from the control temperature in the first image forming mode to 100 degrees centigrade. As shown in the graph in  FIG. 8 , the higher the control temperature in the first image forming mode is, the longer the cool down time is. Particularly, the cool down time of the pressing belt  52  increases exponentially as the control temperature in the first image forming mode increases. That is because much heat is stored in the pressure pad  58 , the pressing roller  55 , and the tension roller  56  through the processing carried out on the 500 sheets of paper. The heat of the pressing belt  52  can hardly diffuse due to the heat stored in the pressure pad  58 , the pressing roller  55 , and the tension roller  56 ; thus, a long time is required to decrease the surface temperature of the pressing belt  52 . 
     As shown in the graphs in  FIGS. 6-8 , the time required to increase or decrease the surface temperature of the pressing belt  52  is longer than the time required to increase or decrease the surface temperature of the heating roller  51 . It is known that the time required for the switch of the image forming mode can be reduced if the control temperatures of the pressing belt  52  in the first image forming mode and the second image forming mode are the same.  FIG. 9  illustrates results obtained by measuring the time required for the switch of the image forming mode under two different conditions. In a first condition, the control temperatures of the pressing belt  52  in the first image forming mode and the second image forming mode are set to 90 and 110 degrees centigrade, respectively. In a second condition, the control temperatures of the pressing belt  52  in the first image forming mode and the second image forming mode are set to be the same, that is, 90 degrees centigrade.  FIG. 9  shows that the time required for the switch of the image forming mode is reduced if the control temperatures of the pressing belt  52  in the first and the second image forming modes are the same. 
     The temperature controller  65  carried out a temperature control so that the difference between the control temperatures of the pressing belt  52  in the first and the second image forming modes is smaller than the difference between the control temperatures of the heating roller  51 . For example, the temperature controller  65  carries out a temperature control to set the control temperature of the heating roller  51  to 100 degrees centigrade and set the control temperature of the pressing belt  52  to 90 degrees centigrade in the first image forming mode. Further, the temperature controller  65  carries out a temperature control to set the control temperature of the heating roller  51  to 130 degrees centigrade and set the control temperature of the pressing belt  52  to 90 degrees centigrade in the second image forming mode. That is, the difference between the control temperatures of the pressing belt  52  in the first and the second image forming modes is 0 degrees centigrade, and the difference between the control temperatures of the heating roller  51  in the first and the second image forming modes is 30 degrees centigrade. As the temperature controller  65  carried out the temperature control described above, in this way, the time required for the switch of the image forming mode is reduced, and usability of the image forming apparatus is improved. 
     The temperature controller  65  may set the control temperature of the pressing belt  52  in each of the first and the second image forming modes to a constant temperature. Then the temperature controller  65  may switch the control temperature of the heating roller  51  in each of the first and the second image forming modes. 
     The temperature controller  65  may operates to increase the difference between the surface temperatures of the heating roller  51  contacting the unfixed toner image in the first and the second image forming modes. The time required for fixing processing can be reduced by setting the surface temperature of the heating roller  51  to a sufficiently high temperature in the second image forming mode for fixing the toner image formed with the non-erasable toner. In this way, the image forming apparatus  1  can improve the print speed in the second image forming mode. 
     Further, the temperature control section  65  may carry out the same temperature control on the surface temperatures of the pressing belt  52  in the first image forming mode and the second image forming mode. As the control temperature of the pressing belt  52  is made not to be changed, it is possible to reduce the time required for the change of the surface temperature of the pressing belt  52  when the image forming mode is switched. As the time required for the switch of the image forming mode is reduced, usability of the image forming apparatus is improved. 
     In addition, in the present embodiment, the control temperatures of the pressing belt  52  in the first and the second image forming modes are the same. However, the present invention is not limited to this. The difference between the control temperatures of the pressing belt  52  is allowed as long as it is in such a range that the time required for the change of the surface temperature of the pressing belt  52  is shorter than the time required for the change of the surface temperature of the heating roller  51 . 
     In the present embodiment, the selection of the toner in the image forming section  4  is carried out by determining whether to use the first developing device  30  or the second developing device  32  in the development. However, the present invention is not limited to this. As to the selection of the toner in the image forming section  4 , the electrostatic latent image may be developed for each color with a well-known image forming apparatus for carrying out color printing. Specifically, it may be determined whether to use a rotary development system or a revolver development system in which a plurality of developing devices are selected in sequence to develop the electrostatic latent image on the photoconductive drum  12 . Further, it may be determined to use a tandem development system in which the toner image is formed on the intermediate transfer belt nearby the photoconductive drum of each of the plurality of developing devices and then transferred to the paper. Moreover, in the present embodiment, the image forming apparatus  1  forms the toner image with one kind of non-erasable toner. However, the present invention is not limited to this. The image forming apparatus  1  may form the toner image with a plurality of kinds of non-erasable toner. For example, the image forming apparatus  1  may use the cyan, yellow, magenta, and black toner used in the color printing as the non-erasable toner to form the toner image. 
     Further, in the present embodiment, the surface temperature of each of the heating roller  51  and the pressing belt  52  is detected by the thermistors. However, the present invention is not limited to this. The center thermistor  61 A, the side thermistor  61 B, and the pressing member thermistor  62  may be temperature sensors other than the thermistors. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.