Patent Publication Number: US-2018052410-A1

Title: Image forming apparatus and erasing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/246,591, filed on Aug. 25, 2016, which is a continuation of U.S. patent application Ser. No. 15/010,416, filed on Jan. 29, 2016, now U.S. Pat. No. 9,452,629, issued on Sep. 27, 2016, which is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2015-049495, filed on Mar. 12, 2015, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described here generally relate to an image forming apparatus and an erasing apparatus. 
     BACKGROUND 
     In recent years, image forming apparatuses having image forming functions and image erasing functions are being developed. The temperature necessary to erase images is higher than the temperature necessary to form images. Because of this, the temperature of ejected sheets is high when erasing images, and therefore the sheets may adhere to each other with toner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external view showing an image forming apparatus of an embodiment. 
         FIG. 2  is a block diagram schematically showing the image forming apparatus of this embodiment. 
         FIG. 3  is a diagram schematically showing a fusing device of the image forming apparatus of  FIG. 1 . 
         FIG. 4  is a flowchart showing an example of the control behavior of a processor of this embodiment. 
         FIG. 5  is a graph showing the relation between the processed time and the surface temperature of a heat roller of this embodiment. 
         FIG. 6  is a graph showing the relation between the number of ejected sheets stacked and the temperature of ejected sheets being ejected of this embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, an image forming apparatus performs an image forming process to form an image on a sheet, and performs an image erasing process to erase an image formed on a sheet. The image forming apparatus includes a fusing device, a cooling device, and a processor. The fusing device includes a fusing member and a pressure member, the fusing member being heated by a heat source. The fusing device conveys a sheet, the sheet being heated and pressed between the fusing member and the pressure member. The fusing device fuses the image into the sheet when the image forming process is performed, and erases an image formed on the sheet when the image erasing process is performed. The cooling device performs a cooling process to cool the fusing member of the fusing device. The processor causes the cooling device to perform the cooling process every predetermined timing when the image erasing process is performed. 
     First, an image-forming-and-erasing apparatus of this embodiment will be described schematically. In the following description, the image-forming-and-erasing apparatus will be simply referred to as an “image forming apparatus”. The image forming apparatus of this embodiment cools a fusing device at predetermined timing when the fusing device erases images-to-be-erased. The image-to-be-erased is an image to be erased. The image-to-be-erased is an image formed on a sheet by using an erasable recording material such as erasable toner. The erasable recording material is a recording material that is erasable at a predetermined erasing temperature or more. When the cooling process is performed, the image erasing process is suspended. The cooling process is repeatedly performed every predetermined timing. Since the image erasing process is suspended and the cooling process is performed every predetermined timing, the temperature of ejected sheets, from which images are erased, can be lower. As a result, it is possible to prevent sheets from adhering to each other with toner. 
     Hereinafter, an embodiment will be further described in detail with reference to the drawings. In the drawings, the same reference symbols show the same or similar parts.  FIG. 1  is an external view showing an example of the entire structure of the image forming apparatus  100  of this embodiment. The image forming apparatus  100  is, for example, a multi-function peripheral. The image forming apparatus  100  includes the display  110 , the control panel  120 , the printer  130 , the sheet storing device  140 , and the scanner  200 . In  FIG. 1 , the scanner  200  includes, for example, an automatic document feeder that conveys documents to the image-reading position of the scanner  200 . 
     The image forming apparatus  100  forms images on sheets by using developer such as toner as the above-mentioned recording material to form images. The sheets are, for example, sheets such as paper and label sheets. The sheets may be any recording media on which the image forming apparatus  100  can form images. 
     The display  110  is an image display apparatus such as a liquid crystal display and an organic EL (Electro Luminescence) display. The display  110  displays various kinds of information on the image forming apparatus  100 . 
     The control panel  120  includes buttons. The control panel  120  receives user&#39;s operations. Examples of the user&#39;s operation include an image erasing instruction (described later). The control panel  120  outputs signals corresponding to the user&#39;s operations to the processor  300  (see  FIG. 2  described below) of the image forming apparatus  100 . Note that an all-in-one touch panel, which includes the display  110  and the control panel  120 , may be provided. 
     The printer  130  performs image forming process and image erasing process. In the image forming process, the printer  130  forms images on sheets based on obtained image information. The printer  130  heats the sheets, on which the images are formed, to thereby fuse the images into the sheets. The fusing device  50  (see  FIG. 2  and  FIG. 3  described below) performs heating for the image forming process. The fusing device  50  heats the sheets at a temperature (fusing temperature) necessary to fuse the developer and thereby fuses the images. The image information used to form images may be image information generated by the scanner  200  or image information received via a communication path such as a network. 
     In the image erasing process, the printer  130  heats the sheets on which images-to-be-erased are formed, and thereby erases the image-to-be-erased. For example, the fusing device  50  of the printer  130  performs heating in the image erasing process. The fusing device  50  heats the sheets at an erasing temperature or more to thereby erase the images-to-be-erased. The erasing temperature is higher than the fusing temperature in the image forming process. For example, the heat roller  501  (see  FIG. 3  described later) of the fusing device  50  is heated at 100 degrees (fusing temperature) when performing the image forming process. For example, the heat roller  501  of the fusing device  50  is heated at 130 degrees (erasing temperature) or more when performing the image erasing process. Further, the belt heating roller  515  (see  FIG. 3  described later) of the fusing device  50  is heated at, for example, 90 degrees when performing the image forming process. Further, the belt heating roller  515  (see  FIG. 3  described later) of the fusing device  50  is heated at, for example, 120 degrees or more when performing the image erasing process. 
     The sheet storing device  140  includes a cassette that stores sheets on which the printer  130  forms images. Further, the sheet storing device  140  includes a cassette that stores sheets from which the printer  130  erases images. 
     The scanner  200  reads image information of a document-to-be-read as lightness contrast. The scanner  200  stores the read image information. The stored image information may be sent to another information processing apparatus via the network. The printer  130  may form images on sheets based on the stored image information. 
       FIG. 2  is a block diagram schematically showing the image forming apparatus  100 . The image forming apparatus  100  includes the control panel  120 , the printer  130 , the scanner  200 , the processor  300 , the ROM  310 , and the DRAM  320 . The structural units are connected with each other via the system bus  330  such that they are capable of sending/receiving data to/from each other. 
     As shown in  FIG. 2 , the printer  130  includes the conveying device  20 , the image forming device  30 , the transferring device  40 , and the fusing device  50 . The printer  130  performs the image forming process as follows. The conveying device  20  of the printer  130  draws a sheet, on which an image is to be formed, from the sheet storing device  140 . The conveying device  20  conveys the drawn sheet to the transferring device  40  and to the fusing device  50 . The image forming device  30  of the printer  130  forms an electrostatic latent image on a photosensitive drum (not shown), for example, based on the image information of the document. The image forming device  30  attaches the developer on the electrostatic latent image, and thereby forms a visible image (image of developer). Toner is a specific example of the developer. The transferring device  40  of the printer  130  transfers the image of the image developer to the sheet. The fusing device  50  of the printer  130  heats the sheet at the fusing temperature, presses the sheet, and thereby fuses the image of the developer into the sheet. Note that a user may manually feed a sheet on a manual-feed tray (not shown), and an image may be formed on the manually-fed sheet. 
     The printer  130  performs the image erasing process as follows. The conveying device  20  of the printer  130  draws a sheet (sheet-to-be-erased) for the image erasing process from the sheet storing device  140 . The conveying device  20  conveys the drawn sheet to the fusing device  50 . The fusing device  50  of the printer  130  heats the sheet at the erasing temperature or more, presses the sheet, and thereby erases the image-to-be-erased. Note that a user may manually feed a sheet on a manual-feed tray (not shown), and an image may be erased from the manually-fed sheet. 
     The processor  300  controls the respective structural units connected thereto via the system bus  330 . The ROM  310  stores various control programs necessary to operate the processor  300 . The ROM  310  stores, for example, programs that control the image forming process and the image erasing process. The DRAM  320  is used as a storage area that temporarily stores data when the processor  300  executes programs. 
       FIG. 3  is a diagram schematically showing an example of the structure of the fusing device  50  of the printer  130 . The fusing device  50  is a fusing-and-erasing device. The fusing device  50  includes the heat roller  501  as a fusing member when it functions as a fusing device and as a heating member when it functions as an erasing device. The fusing device  50  includes, as pressure members, the pressure belt  510 , the pressure pad  511 , the pad holder  512 , the pressure roller  513 , the tension roller  514 , and the belt heating roller  516 . 
     Further, the fusing device  50  includes the HR lamp  502 , the HR thermistor  503 , the pressure belt lamp  516 , the pressure thermistor  517 , and the cooling device  520 . Note that the fusing device  50  may include the cooling device  520  alternatively. 
     The heat roller  501  is a member having a cylindrical shape. The HR lamp  502  is provided in the heat roller  501 . The HR lamp  502  generates heat, and thereby heats the heat roller  501  at the fusing temperature or at the erasing temperature or more. The HR thermistor  503  measures the surface temperature of the heat roller  501 . 
     The pressure belt  510  is held by the pressure roller  513 , the tension roller  514 , and the belt heating roller  515 . The pressure pad  511  and the pressure roller  513  cause the pressure belt  510  to pressure-contact the heat roller  501 . Thanks to the pressure-contact, a nipped part is formed between the pressure belt  510  and the heat roller  501 . 
     The pressure pad  511  pressure-contacts the heat roller  501 , the pressure belt  510  being interposed therebetween, and is held. The pad holder  512  holds the pressure pad  511 , the pressure pad  511  pressure-contacting the heat roller  501 . 
     The pressure roller  513  is arranged at the downstream in the sheet-conveying direction. The pressure roller  513  causes the pressure belt  510  to pressure-contact the heat roller  501 . The nipped part ends at the pressure roller  513 . The tension roller  514  is arranged apart from the pressure roller  513  and the belt heating roller  515 , and thereby provides the tension of the pressure belt  510 . The belt heating roller  515  is arranged at the upstream in the sheet-conveying direction. The belt heating roller  515  is a member having a hollow cylindrical shape. The pressure belt lamp  516  is provided in the belt heating roller  515 . The pressure belt lamp  516  generates heat, and thereby heats the belt heating roller  515 . 
     As the pressure belt lamp  516 , for example, a halogen lamp is used. The pressure thermistor  517  measures the surface temperature of the pressure belt  510  near the belt heating roller  515 . The cooling device  520  cools the heat roller  501 . Since the cooling device  520  cools the heat roller  501 , the surface temperature of the heat roller  501  drops faster than that of an uncooled heat roller. The cooling device  520  may have any structure that can cool the heat roller  501 . The cooling device  520  may be, for example, an air-blower that blows air into the heat roller  501  by using a fan rotated by a motor. As shown in  FIG. 3 , for example, the cooling device  520  is arranged at the downstream side in the sheet-conveying direction of the heat roller  501 . Further, the cooling device  520  is arranged above the heat roller  501 , for example. 
     Next, how the processor  300  controls the image erasing process will be described in detail. When an image-to-be-erased is erased controlled by the processor  300 , the fusing device  50  is cooled at predetermined timing controlled by the processor  300 . The predetermined timing contains timing at which images are erased from a predetermined threshold number of sheets, and timing at which the image erasing process based on an image erasing instruction from a user received by the control panel  120  is completed. The cooling process performed at timing at which images are erased from a predetermined threshold number of sheets will be referred to as the first cooling process in the following description. Meanwhile, the cooling process performed at timing at which the image erasing process is completed will be referred to as the second cooling process in the following description. Note that the simple “cooling process” means the first and/or second cooling processes/process. The first cooling process is completed when, for example, the surface temperature of the heat roller  501  reaches a predetermined threshold temperature (cooling-target temperature). In other words, the first cooling process is continued until, for example, the surface temperature of the heat roller  501  reaches the cooling-target temperature. Alternatively, the first cooling process may be completed after it continues for a predetermined time period, for example. The predetermined time period is determined arbitrarily based on the experimental results of  FIG. 5  and  FIG. 6  (described later). The cooling device  520  continues the first cooling process until the first cooling process is completed even if the control panel  120  receives an image erasing instruction from a user during the first cooling process. When the cooling device  520  completes the first cooling process, the fusing device  50  performs the image erasing process based on the received image erasing instruction from a user. To the contrary, the second cooling process is completed when the surface temperature of the heat roller  501  reaches the cooling-target temperature, or is finished based on the image erasing instruction from a user. So the cooling device  520  finishes the second cooling process when the control panel  120  receives an image erasing instruction from a user during the second cooling process. When the cooling device  520  completes the second cooling process, the fusing device  50  performs the image erasing process based on the received image erasing instruction from a user. 
     The processor  300  controls the cooling device  520  to perform the cooling process as follows. The processor  300  drives the cooling device  520  at the predetermined timing. Further, the processor  300  turns off the HR lamp  502 , and thereby stops heating the heat roller  501 . The processor  300  may not turn off the HR lamp  502  and reduce the electric power supplied to the HR lamp  502  instead, and thereby may heat the heat roller  501  less. As the result of this process, the surface temperature of the heat roller  501  is reduced. The processor  300  obtains the surface temperature of the heat roller  501  based on the output from the HR thermistor  503 . When the surface temperature of the heat roller  501  drops and reaches the cooling-target temperature or less, the processor  300  completes the cooling process by the cooling device  520 . The cooling-target temperature is lower than the erasing temperature. The cooling-target temperature is equal to or lower than the fusing temperature (for example 100 degrees), at which the heat roller  501  is heated during the image forming process, for example. In other words, the processor  300  controls the HR lamp  502  such that the surface temperature of the heat roller  501  drops below erasing temperature, and, at the same time, causes the cooling device  520  to perform the cooling process. The processor  300  suspends the image erasing process when the cooling device  520  performs the first cooling process. Meanwhile, if the control panel  120  receives an image erasing instruction when the cooling device  520  performs the second cooling process, the processor  300  causes the cooling device  520  to finish the cooling process and causes the fusing device  50  to perform the image erasing process. The processor  300  repeats that control every predetermined timing, and thereby causes the cooling device  520  to perform the first and second cooling processes. When the image erasing process is performed after the cooling process, the processor  300  increases the surface temperature of the heat roller  501 . The surface temperature of the heat roller  501  is increased to reach the erasing temperature (for example 130 degrees). The processor  300  controls the HR lamp  502  of the heat roller  501  based on the predetermined temperature depending on each process such that the surface temperature of the heat roller  501  reaches the temperature (erasing temperature) necessary for the image erasing process or the temperature (fusing temperature) necessary for the image forming process. 
     The processor  300  counts the number of sheets from which images are erased. The image forming apparatus  100  includes a sensor that detects sheets, the number of which is to be counted by the processor  300 . In this embodiment, for example, the sheet storing device  140  includes the sensor. The sensor detects sheets drawn from the sheet storing device  140  for the image erasing process. The processor  300  includes a counter that counts the number of sheets based on the result of detecting sheets by the sensor. The counter is a nonvolatile memory that stores the counted number, for example. In other words, the processor  300  detects, by using the sensor, sheets drawn from the sheet storing device  140  as the number of sheets from which images are erased. The processor  300  increments the counter every time the sensor detects the sheet, and thereby counts the number of sheets. The counter&#39;s value is held even if the image forming apparatus  100  is powered off. When the counter&#39;s value reaches a threshold number, that shows a predetermined sheet number, or more, the processor  300  suspends the image erasing process, and causes the cooling device  520  to execute the first cooling process. Further, when the image erasing process instructed by a user is completed, the processor  300  causes the cooling device  520  to perform the second cooling process. When the first cooling process is completed, the processor  300  resets the counter&#39;s value. The counter&#39;s value is reset and thereby returns to the default value (zero). Note that, for example, a sheet-ejecting unit (not shown) of the image forming apparatus  100  may include the sensor. If the sheet-ejecting unit of the image forming apparatus  100  includes the sensor, the sensor detects sheets, from which images are erased, ejected to the sheet-ejecting unit of the image forming apparatus  100 . 
     As described above, even if the image forming apparatus  100  is powered off, the counter&#39;s value at the time of power-off is held as it is. When the image erasing process is performed after power-on, the latest counter&#39;s value at the time of power-off is incremented. Because of this, even if the image forming apparatus  100  is powered off without being cooled at all and is powered on immediately after that, it is possible to prevent the fusing device  50  from being heated too much. 
     When the cooling device  520  starts the cooling process, the processor  300  changes cooling flag&#39;s value by recording a value showing the cooling status. The cooling flag is a flag showing if the cooling process is being performed or not. The cooling flag&#39;s value is stored in a nonvolatile storage unit (not shown). The cooling flag&#39;s value is held even if the image forming apparatus  100  is powered off. In the following description, “to set the cooling flag” means to change the cooling flag&#39;s value by recording the value showing the cooling status. “To reset the cooling flag” means to change the cooling flag&#39;s value by recording the value showing the uncooling status. The processor  300  sets the cooling flag when the cooling process is started. The processor  300  keeps the state where the cooling flag is set during the cooling process. The processor  300  resets the cooling flag when the cooling process is completed or finished. For example, if the image forming apparatus  100  is powered off without completing the cooling process after the cooling process is started, the cooling flag&#39;s value, i.e., the value showing the cooling status, is held as it is. When the image forming apparatus  100  is powered on after that, the processor  300  refers to the cooling flag&#39;s value and causes the cooling device  520  to perform the cooling process if necessary. In other words, if the cooling flag is set when the image forming apparatus  100  is powered on, the processor  300  causes the cooling device  520  to perform the cooling process. Because of this, even if the image forming apparatus  100  is powered off without being cooled completely and is powered on immediately after that, it is possible to prevent the fusing device  50  from being heated too much. 
       FIG. 4  is a flowchart showing an example of the control behavior of the processor  300 . In ACT 101 , the processor  300  detects if the control panel  120  receives an image erasing instruction from a user. In ACT 102 , the processor  300  detects if the cooling flag is set or not. In other words, the processor  300  determines if the cooling process is completed or not based on the fact that the cooling flag is set or not. If it is determined that the cooling process is uncompleted, i.e., the cooling flag is set (ACT 102 , YES), the processor  300  proceeds to ACT 103 . In ACT 103 , the processor  300  determines if the counter&#39;s value is smaller than the threshold number or not. In other words, the processor  300  determines if the total number of sheets, from which images are erased, reaches the threshold number or not. If the counter&#39;s value is smaller than the threshold number (ACT 103 , YES), the processor  300  determines that it is not time to perform the first cooling process. In other words, the processor  300  determines that the uncompleted cooling process is the second cooling process. Further, the processor  300  finishes the uncompleted second cooling process and resets the cooling flag. After that (ACT 103 , YES), the processor  300  proceeds to ACT 104 . In ACT 104 , the processor  300  causes the fusing device  50  to perform the image erasing process based on the image erasing instruction from a user. Specifically, the processor  300  causes the conveying device  20  to convey sheets, from which images are to be erased, from the sheet storing device  140  to the fusing device  50 . The fusing device  50  erases images from the conveyed sheets. As described above, if the image erasing instruction is received via the control panel  120  and if the uncompleted cooling process is the second cooling process, the processor  300  finishes the uncompleted cooling process without continuing. After the uncompleted second cooling process is finished, the processor  300  causes the fusing device  50  to perform the image erasing process based on the image erasing instruction from a user. Meanwhile, if the cooling process is completed in ACT 102  (ACT 102 , NO), the processor  300  proceeds to ACT 104 . In ACT 104 , the processor  300  causes the fusing device  50  to perform the image erasing process based on the image erasing instruction from a user. After that, in ACT 105 , the processor  300  increments the counter&#39;s value. In other words, the processor  300  adds one to the counter&#39;s value. 
     Next, in ACT 106 , the processor  300  determines if the condition to finish is satisfied or not. The “condition to finish” is a condition to finish the image erasing process based on the image erasing instruction of ACT 101 . The image erasing instruction includes the condition to finish. The condition to finish may be, for example, the number of sheets from which images are to be erased, the number of sheets being preset by a user. Alternatively, for example, the condition to finish may be the number of sheets from which images are to be erased, the number of sheets being preset for the image forming apparatus  100 . In those cases, in ACT 106 , the processor  300  determines if the number of sheets, from which images are erased, reaches the preset number or not. Further, for example, the condition to finish may be the fact that the sheet storing device  140  does not store anymore sheets, from which images are to be erased. 
     If the condition to finish is not satisfied (ACT 106 , NO), the processor  300  proceeds to ACT 107 . In ACT 107 , the processor  300  determines if the counter&#39;s value is smaller than the threshold number or not. In other words, the processor  300  determines if the total number of sheets, from which images are erased, reaches the threshold number or not. If the counter&#39;s value is smaller than the threshold number (ACT 107 , YES), the processor  300  returns to ACT 104 . In ACT 104 , as described above, the processor  300  causes the fusing device  50  to perform the image erasing process. In other words, if the condition to finish is not satisfied and if the counter&#39;s value is smaller than the threshold number, the processor  300  repeats the processes of ACT 104  to ACT 107 . 
     Further, in ACT 103 , if it is determined that the counter&#39;s value is equal to or larger than the threshold number, i.e., the total number of sheets from which images are erased reaches the threshold number (ACT 103 , NO), the processor  300  determines that it is time to perform the first cooling process. In other words, the processor  300  determines that the uncompleted cooling process is the first cooling process. Next, in ACT 108 , the processor  300  causes the cooling device  520  to perform the first cooling process. In other words, if the uncompleted cooling process is the first cooling process, the processor  300  causes the cooling device  520  to continue the first cooling process without causing the cooling device  520  to finish the uncompleted first cooling process. Further, in ACT 109 , the processor  300  sets the cooling flag. In other words, the processor  300  maintains the state where the cooling flag is set. Meanwhile, in ACT 107 , if it is determined that the counter&#39;s value is equal to or larger than the threshold number, i.e., the total number of sheets from which images are erased reaches the threshold number (ACT 107 , NO), the processor  300  determines that it is time to perform the first cooling process. Next, in ACT 108 , the processor  300  causes the cooling device  520  to perform the first cooling process. When the first cooling process is started, in ACT 109 , the processor  300  sets the cooling flag. After that, in ACT 110 , the processor  300  determines that the surface temperature of the heat roller  501  reaches the cooling-target temperature, and then completes the first cooling process. Further, in ACT 111 , the processor  300  resets the cooling flag. Further, in ACT 112 , the processor  300  resets the counter&#39;s value. When the counter&#39;s value is reset, the processor  300  returns to ACT 104 . As described above, if the first cooling process, which was started immediately before performing the image erasing process, is not completed, the processor causes the cooling device to continue the first cooling process. After the cooling process is completed, the processor causes the fusing device to perform the image erasing process. 
     Further, in ACT 106 , if the condition to finish is satisfied (ACT 106 , YES), the processor  300  proceeds to ACT 113 . In ACT 113 , the processor  300  causes the cooling device  520  to perform the second cooling process. When the second cooling process is started, in ACT 114 , the processor  300  sets the cooling flag. After that, in ACT 115 , the processor  300  determines that the surface temperature of the heat roller  501  reaches the cooling-target temperature, and then completes the second cooling process. When the second cooling process is completed, in ACT 116 , the processor  300  resets the cooling flag. When the cooling flag is reset, in ACT 117 , the processor  300  resets the counter&#39;s value. After the above-mentioned processes, the processor  300  finishes the image erasing process based on the image erasing instruction of ACT 101 . 
     As described above, the image forming apparatus  100  erases images-to-be-erased by using the fusing device  50 . Further, the image forming apparatus  100  cools the fusing device  50  by using the cooling device  520  at predetermined timing. Because of this, the temperature of ejected sheets, from which images-to-be-erased are erased, can be lower. As a result, it is possible to prevent sheets from adhering to each other with toner. 
     Hereinafter, effects of the image forming apparatus  100  will be described in detail.  FIG. 5  is a graph showing the relation between the processed time (sec) and the surface temperature of the heat roller  501  of the fusing device  50 . The processed time shows the elapsed time after the fusing device  50  starts the image erasing process. In  FIG. 5 , the horizontal axis shows the processed time. In  FIG. 5 , the vertical axis shows the surface temperature of the heat roller  501  of the fusing device  50 . In the graph, the dashed line (uncooled) shows the change of the temperature of the surface of the heat roller  501  when the image erasing process is continued without cooling. In the graph, the solid line (cooled) shows the change of the temperature of the surface of the heat roller  501  in the image erasing process of the image forming apparatus  100  of this embodiment. In other words, in the graph, the solid line shows the change of the temperature of the surface of the heat roller  501  when the cooling process is performed every predetermined timing. 
     In this embodiment, stability control is employed to keep the temperature of the surface of the heat roller  501  near about 130° C. According to the stability control, the predetermined temperature, at which the fusing device  50  is cooled, is decreased depending on a time period in which sheets are continuously supplied to the fusing device  50 . As a result, the temperature of the surface of the heat roller  501  is to be changed within a target temperature range. According to the stability control of this embodiment, timing at which the predetermined temperature is decreased is controlled based on time. According to the stability control of this embodiment, for example, the predetermined temperature is decreased by about 2° C. when the processed time reaches about 140 sec, and the predetermined temperature is decreased by about 3° C. when the processed time reaches about 230 sec. In  FIG. 5 , each of the points A and B shows timing at which the predetermined temperature is decreased. The predetermined temperature is decreased, i.e., controlled, based on a prepared table, that stores the relation between processed time and drop of temperature. In other words, the processor  300  reads the drop of temperature in relation to the processed time from the table, and controls the temperature of the heat roller  501  (the HR lamp  502 ). Thanks to the stability control, the temperature of the surface of the heat roller  501  of the fusing device  50  is kept around 130° C. 
     However, even if the above-mentioned stability control is performed, the temperature of the surface of the heat roller  501  of the fusing device  50  tends to increase slightly (see  FIG. 5 , point C). In order to prevent the temperature of the surface of the heat roller  501  from increasing, the above-mentioned first cooling process is performed at timing when the processed time reaches about 400 sec. Thanks to the cooling process, the temperature of the surface of the heat roller  501  drops and reaches about 100 degrees as shown in the solid line of  FIG. 5 . After that, the temperature of the surface of the heat roller  501  is increased again and reaches the erasing temperature (for example 130 degrees) necessary to perform the image erasing process. Then the image forming apparatus  100  proceeds with the image erasing process. 
     If the cooling process is not performed, the temperature of the surface of the heat roller  501  increases, and thereby ejected sheets adheres to each other when the image erasing process is performed for a long period of time without stopping, which is problematic. According to this embodiment, even if about 300 sheets, from which images are to be erased, are supplied to the fusing device  50  continuously, it is possible to previously prevent the temperature of the surface of the heat roller  501  from increasing. Because of this, ejected sheets do not adhere to each other, and the image erasing process can be performed for a long period of time without stopping. 
       FIG. 6  is a graph showing the relation between the number of ejected sheets stacked and the temperature of ejected sheets being ejected. The “ejected sheet” means an ejected sheet, from which images are erased by the image forming apparatus  100 . The “number of stacked sheets” means the number of ejected sheets stacked on an ejecting unit (not shown) of the image forming apparatus  100 , the ejected sheets having been ejected into the ejecting unit without stopping. In the graph of  FIG. 6 , the horizontal axis shows the number of stacked sheets. In the graph of  FIG. 6 , the vertical axis shows the temperature of ejected sheets being ejected. In the graph of  FIG. 6 , the dashed line shows the change of the temperature of ejected sheets being ejected, the image erasing process being performed without stopping and without performing the cooling process. In the graph of  FIG. 6 , the solid line shows the change of the temperature of ejected sheets being ejected, the image erasing process being performed by the image forming apparatus  100  of this embodiment. In other words, in the graph, the solid line shows the change of the temperature of ejected sheets being ejected, the cooling process being performed every predetermined timing. 
     The larger the number of stacked sheets, the longer the elapsed time after the image erasing process is started. The larger the number of stacked sheets, the higher the temperature of the surface of the heat roller  501  of the fusing device  50 . As the temperature of the surface of the heat roller  501  of the fusing device  50  increases, the temperature of ejected sheets being ejected increases. Note that the temperature of ejected sheets being ejected when the cooling process is not performed is much higher than the temperature of ejected sheets being ejected when the cooling process is performed. In the experiment, the result of which is shown in the graph of  FIG. 6 , when the temperature of ejected sheets being ejected exceeds about 70 degrees, a small number of sheets adhered to each other (little adherence of sheets). Further, in the experiment, the result of which is shown in the graph of  FIG. 6 , when the temperature of ejected sheets being ejected exceeds about 80 degrees, a larger number of sheets adhered to each other (more adherence of sheets). According to this embodiment, the image forming apparatus  100  performs the cooling process every predetermined timing. Because of this, the temperature of ejected sheets being ejected, i.e., the temperature at which a less number of sheets adhered to each other, can be kept. 
     A designer of the image forming apparatus  100 , for example, conducts the experiments shown in  FIG. 5  and  FIG. 6 , and arbitrarily determines when to perform the cooling process and how long the cooling process is to be continued. 
     If the fusing device  50  as a unit is mounted on the image forming apparatus  100 , the fusing device  50  may include a controller that behaves similar to the processor  300 . 
     The cooling process may be performed after a predetermined time period passes after the erasing process is started, for example, which can be the predetermined timing. Further, the cooling process may be performed when the temperature of the surface of the heat roller  501  of the fusing device  50  that performs the erasing process reaches a predetermined temperature, for example, which can be the predetermined timing. Alternatively, the cooling process may be performed when all the conditions of both the types of the timing are satisfied, which can be the predetermined timing. In other words, the predetermined timing is determined based on at least one of the number of sheets from which images are erased, the elapsed time after the image erasing process is started, and the temperature of the fusing member of the fusing device. 
     The timing at which the cooling process is completed is not necessarily limited to the timing at which the temperature of the surface of the heat roller  501  drops below the cooling-target temperature. For example, as described above, the processor  300  may complete the cooling process after a predetermined time period passes after the cooling process is started. 
     The specific value of the predetermined timing (the threshold number) at which the first cooling process is performed may be determined based on the size of sheets from which images are to be erased. For example, the processor  300  may have a table that prestores the predetermined threshold number of sheets from which images are erased in relation with each sheet size. In this case, the processor  300  obtains, with reference to the table, the predetermined threshold number depending on the size of sheets from which images are to be erased. 
     According to at least one of the above-mentioned embodiments, the image erasing process is suspended every predetermined timing and the cooling process is performed. Because of this, the temperature of ejected sheets from which images are erased can be lower. As a result, it is possible to prevent sheets from adhering to each other with toner. 
     While certain this embodiments have been described, these this embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel this 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 inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.