Patent Publication Number: US-10324401-B1

Title: Image forming apparatus, image recoloring apparatus and image forming method

Description:
FIELD 
     Embodiments described herein relate generally to an image forming apparatus, an image recoloring apparatus and an image forming method. 
     BACKGROUND 
     Conventionally, in an image forming apparatus, in a case of performing printing on one sheet using both of a decolorable toner and a non-decolorable toner, it is necessary to perform the printing twice due to the difference in a fixing temperature. As a result, it takes extra time to perform the printing using the decolorable toner and the non-decolorable toner. Such a problem may occur not only when printing using a toner but also when printing using color material which can be decolored by heat and a non-decolorable color material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to an embodiment; 
         FIG. 2  is a block diagram illustrating an example of hardware components of the image forming apparatus; 
         FIG. 3  is a diagram illustrating a specific example of a temperature setting table; 
         FIG. 4  is a schematic block diagram illustrating an example of functional components of a processor; 
         FIG. 5  is a flowchart depicting the flow of a processing by the image forming apparatus; and 
         FIG. 6  is a flowchart depicting the flow of a processing in a modification of the image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with an embodiment, an image forming apparatus comprises a first image forming section configured to form an image on a sheet with a decolorable color material decolorable, a second image forming section configured to form an image on the sheet with a non-decolorable color material, a fixing device configured to heat the sheet on which the images are formed with the decolorable color material and the non-decolorable color material, and a cooling section configured to cool the heated sheet. 
     Hereinafter, an image forming apparatus, an image recoloring apparatus, and an image forming method of an embodiment are described with reference to the accompanying drawings. 
       FIG. 1  is a schematic cross-sectional view of the image forming apparatus of the embodiment. 
     An image forming apparatus  1  of the embodiment is a multifunction peripheral (MFP). The image forming apparatus  1  performs an image forming process. The image forming process is a process of forming an image on a sheet. The sheet is, for example, a paper sheet. The image forming apparatus  1  reads an image shown on the sheet and generates digital data to generate an image file. The sheet subjected to the image reading can be any type of sheet. 
     The image forming apparatus  1  includes an image reading section  10 , a control panel  20 , an image forming section  30 , a sheet housing section  40 , a fixing device  50 , a cooling section  60 , a control device  70  and a discharge section  90 . 
     The image reading section  10  reads an image which is a reading object as intensity of light. For example, the image reading section  10  reads an image printed on the sheet which is the reading object placed on a document reading table which is provided at the upper surface of the image reading section  10 . The image reading section  10  records the read image data. The recorded image data may be transmitted to another information processing apparatus via a network. The recorded image data may be used to form an image on the sheet by the image forming section  30 . 
     The control panel  20  includes a display section and an operation section. The display section is a display device such as a liquid crystal display and an organic EL (Electro Luminescence) display. The display section displays various information about processing in the image forming apparatus  1  under the control of the control device  70 . The operation section includes a plurality of buttons and the like. The operation section receives an operation inputs by a user. The operation section outputs a signal corresponding to the operation inputs by the user on the control device  70 . The display section and the operation section may be integrally provided as a touch panel. 
     The image forming section  30  executes the image forming process. In the image forming process, the image forming section  30  forms an image on the sheet based on the image data generated by the image reading section  10  or image data received via a communication path. The image forming section  30  according to the present embodiment uses a decolorable toner that can be decolored by heating and a non-decolorable toner which is not decolored by heating. “Decoloring” means making an image formed with a color (including not only chromatic color but also achromatic color such as white and black) different from a base color of the sheet so that the color is difficult to visually recognize. 
     The non-decolorable toner is, for example, a black (K) toner. In a case in which the image forming apparatus  1  is a device capable of performing printing using multiple colors, the non-decolorable toner may be toner of yellow (Y), magenta (M), cyan (C), black (K) or the like. Similar to the non-decolorable toner, the decolorable toner is a colored toner, for example, blue. The decolorable toner is decolored at a temperature higher than a temperature at which the non-decolorable toner is fixed on the sheet. The decolorable toner used in the present embodiment is a toner which is decolored at high temperature and develops a color again if maintained at a temperature lower than a predetermined recoloring temperature after decoloring. As such a decolorable toner, a toner disclosed in a known technology (for example, U.S. Pat. No. 8,252,496) may be used. 
     The image forming section  30  includes a transfer belt  31 , an exposure section  32 , a developing device  33 D, a developing device  33 K, a photoconductive drum  34 D, a photoconductive drum  34 K, and a transfer section  35 . 
     The transfer belt  31  is an intermediate transfer body. The transfer belt  31  rotates in a direction indicated by an arrow. 
     The exposure section  32  is provided at a position facing the photoconductive drums  34 K and  34 D of the developing devices  33 K and  33 D. The exposure section  32  irradiates outer circumferential surfaces (which are photoconductive layers) of the photoconductive drums  34 K and  34 D of the developing devices  33 K and  33 D with laser light. The exposure section  32  is controlled to emit light based on the image data under the control of the control device  70 . The exposure section  32  emits the laser light based on the image data. In this way, the negative charge on the outer circumferential surfaces of the photoconductive drums  34 K and  34 D of the developing devices  33 K and  33 D is eliminated. As a result, a static electricity pattern is formed at a position irradiated with the laser light on the surfaces of the photoconductive drums  34 K and  34 D. Electrostatic latent images are formed on the surfaces of the photoconductive drums  34 K and  34 D by irradiation of the laser light by the exposure section  32 . In the exposure section  32 , LED (Light Emitting Diode) light may be used in place of the laser light. 
     The developing device  33 D and the developing device  33 K supply toner to the photoconductive drum  34 D and the photoconductive drum  34 K. For example, the developing device  33 D develops the electrostatic latent image on the outer circumferential surface of the photoconductive drum  34 D with the decolorable toner. The developing device  33 K develops the electrostatic latent image on the surface of the photoconductive drum  34 K with black (K) toner. The developing device  33 D and the developing device  33 K form toner images as visible toner images on the photoconductive drum  34 D and the photoconductive drum  34 K. The toner images formed on the photoconductive drum  34 D and the photoconductive drum  34 K are transferred (primary transfer) onto an outer circumferential surface of the transfer belt  31 . The developing device  33 D and the photoconductive drum  34 D are an example of a first image forming section which forms an image on the sheet with the decolorable toner. The developing device  33 K and the photoconductive drum  34 K are an example of a second image forming section which forms an image on the sheet with the non-decolorable toner. 
     The transfer section  35  includes a support roller  35   a  and a secondary transfer roller  35   b . The transfer section  35  transfers the toner image on the transfer belt  31  onto the sheet at a secondary transfer position U. The secondary transfer position U is a position where the support roller  35   a  and the secondary transfer roller  35   b  face each other across the transfer belt  31 . The transfer section  35  applies a transfer bias controlled by a transfer current to the transfer belt  31 . The transfer section  35  transfers the toner image on the transfer belt  31  onto the sheet by the transfer bias. The control device  70  controls the transfer current used for the secondary transfer processing. 
     The sheet housing section  40  includes one or more sheet feed cassettes. The sheet feed cassette houses a sheet  41  of a predetermined size and a predetermined type. The sheet feed cassette has a pickup roller (not illustrated). The pickup roller picks up the sheets  41  one by one from the uppermost sheet in the sheet feed cassette. The pickup roller supplies the sheet  41  taken out to a conveyance section  80 . 
     The fixing device  50  fixes the toner image on the sheet  41  by applying heat and pressure to the sheet  41 . The fixing device  50  includes a heating source  51 , a heat roller  52 , and a pressure roller  53 . The heating source  51  is a heater having a halogen lamp or an induction heating (IH) system heater or the like. The heating source  51  is turned on or turned off according to presence or absence of energization from the control device  70 . The heat roller  52  is warmed by the heat generated at the time the heating source  51  is energized. The heat roller  52  applies the heat to the sheet  41 . The pressure roller  53  is arranged to face the heat roller  52  across a sheet conveying path. The pressure roller  53  presses the sheet  41  against the heat roller  52 . The fixing device  50  is provided with a temperature sensor (not shown). The temperature sensor measures the temperature of the heat roller  52 . The temperature sensor transmits the measured temperature of the heat roller  52  to the control device  70 . 
     The cooling section  60  recovers the color of the image formed with the decolorable toner on the sheet  41  decolored by the fixing device  50  by cooling the sheet  41  heated by the fixing device  50 . In other words, the decolored image becomes visible again as a result of “recoloring” the image by cooling the sheet  41 . The image decolored by the fixing device  50  becomes visible by cooling at the recoloring temperature lower than the predetermined temperature. The recoloring temperature is a temperature at which the color of the decolored image is recovered, and for example, it is −20 degrees centigrade or less. The cooling section  60  includes a plurality of rollers  61 ,  62  and  63  and a coolant  64 . The rollers  61 ,  62  and  63  convey the sheet  41 . The coolant  64  cools the inside of the cooling section  60 . The coolant  64  is, for example, a Peltier element. The coolant  64  lowers the temperature in the cooling section  60  according to the current applied from the control device  70 . The cooling section  60  is provided at the downstream side of the fixing device  50  in a conveyance direction of the sheet  41 . The cooling section  60  may be provided at any position on a route between the fixing device  50  and the discharge section  90  as long as it is a position where the sheet  41  heated by the fixing device  50  can be cooled. The cooling section  60  cools the sheet  41  heated through the image formation by the decolorable toner and the non-decolorable toner. As described above, the cooling section  60  has a function of recoloring the image. 
     The control device  70  controls each functional section of the image forming apparatus  1 . 
     The conveyance section  80  conveys the sheet  41 . The conveyance section  80  includes a conveyance path and a plurality of rollers (not shown). The conveyance path is a route along which the sheet  41  is conveyed. The rollers convey the sheet  41  by rotating under the control of the control device  70 . 
     The discharge section  90  receives the sheet  41  conveyed by the conveyance section  80  and discharged onto the discharge section  90 . 
       FIG. 2  is a block diagram illustrating an example of a hardware components of the image forming apparatus  1 . 
     The image forming apparatus  1  includes the image reading section  10 , the control panel  20 , the image forming section  30 , the sheet housing section  40 , the fixing device  50 , the cooling section  60 , the control device  70 , an auxiliary storage device  120 , and a network interface  130 . 
     The image reading section  10 , the control panel  20 , the image forming section  30 , the sheet housing section  40 , the fixing device  50 , and the cooling section  60  are the same as those explained above, and therefore the description thereof is omitted. The control device  70 , the auxiliary storage device  120 , and the network interface  130  are described below. Each of components is connected in a data communicable manner via a system bus line  2 . 
     The control device  70  includes a processor  71 , a ROM (Read Only Memory)  72 , and a RAM (Random Access Memory)  73 . The processor  71  is, for example, a processor such as a CPU (Central Processing Unit). The processor  71  controls the operation of each functional section of the image forming apparatus  1 . The processor  71  executes various processing by copying or developing a program stored in the ROM  72  in the RAM  73  and executing it. The ROM  72  stores the program for operating the processor  71 . The RAM  73  temporarily stores data used by each functional section of the image forming apparatus  1 . The RAM  73  may temporarily store the digital data generated by the image reading section  10 . Further, the RAM  73  may temporarily store jobs to be executed in the image forming apparatus  1  and job logs. 
     The network interface  130  transmits and receives data to and from other devices. Here, the other device is an information processing apparatus such as a personal computer, for example. The network interface  130  operates as an input interface to receive data or instructions transmitted from other devices. The instruction transmitted from another device is a printing execution instruction or the like. The network interface  130  operates as an output interface to transmit the data to another device. 
     The auxiliary storage device  120  is, for example, a hard disk or an SSD (solid state drive) and stores various data. The various data include, for example, a temperature setting table, digital data, jobs and job logs. The temperature setting table is used to register information on temperature setting of the fixing device  50 . 
       FIG. 3  is a diagram illustrating a specific example of the temperature setting table. The temperature setting table has a plurality of records indicating information on the temperature setting of the fixing device  50 . Each record associates a set temperature with a printing instruction. The printing instruction indicates the printing instruction issued to the image forming apparatus  1 . The set temperature indicates the temperature to be set as the target temperature of the heat roller  52  of the fixing device  50 . The set temperature indicates a fixing temperature at which the fixing of at least one toner is enabled. 
     In an example shown in  FIG. 3 , a plurality of “printing instructions” is registered in the “temperature setting table”. In  FIG. 3 , in a record registered in the uppermost row of the temperature setting table, the printing instruction “printing by non-decolorable toner” is associated with the set temperature of “130 degrees centigrade”. This indicates that the temperature of the heat roller  52  of the fixing device  50  is set to 130 degrees centigrade if the instruction of printing by the non-decolorable toner is received by the control panel  20 . In the example in  FIG. 3 , 130 degrees centigrade is shown as the fixing temperature of the non-decolorable toner. 
     In a record registered in the second row of the temperature setting table, the printing instruction “printing by decolorable toner” is associated with the set temperature of “100 degrees centigrade”. This indicates that the temperature of the heat roller  52  of the fixing device  50  is set to 100 degrees centigrade if the instruction of printing by the decolorable toner is issued. In the example in  FIG. 3 , 100 degrees centigrade is shown as the fixing temperature of the decolorable toner. 
     In a record registered in the third row of the temperature setting table, the printing instruction “printing by decolorable toner and the non-decolorable toner” is associated with the set temperature of “130 degrees centigrade”. This indicates that the temperature of the heat roller  52  of the fixing device  50  is set to 130 degrees centigrade if an instruction of printing by the decolorable toner and the non-decolorable toner is issued. In the example in  FIG. 3 , 130 degrees centigrade is shown as the fixing temperature of the non-decolorable toner. 130 degrees centigrade is the fixing temperature of the non-decolorable toner, and is also the temperature at which the decolorable toner is decolored. 
     The set temperatures shown in the temperature setting table above are examples. 
       FIG. 4  is a schematic block diagram indicating an example of the functional components of the processor  71 . The processor  71  includes a conveyance controller  711 , an operation controller  712 , and a printing controller  713 . 
     The conveyance controller  711  controls conveyance of the sheet  41  by controlling various rollers of the conveyance section  80 . For example, if the printing instruction is made, the conveyance controller  711  controls the pickup roller to pick up the sheet  41  one by one from the sheet housing section  40 . 
     The operation controller  712  controls the operation of the cooling section  60 . For example, the operation controller  712  controls the coolant  64  to lower the temperature in the cooling section  60  to a prescribed value (e.g., −20 degrees centigrade) if the power of the image forming apparatus  1  is supplied to the cooling section  60 . For example, in the operation of the image forming apparatus  1 , the operation controller  712  periodically controls the coolant  64  to maintain the temperature in the cooling section  60  at the prescribed value. 
     The printing controller  713  executes the image forming processing in response to the printing execution instruction. For example, the printing controller  713  selects the toner to be used in the image forming section  30  and the temperature of the fixing device  50  by referring to the temperature setting table in response to the content of the printing execution instruction. 
       FIG. 5  is a flowchart depicting the flow of processing of the image forming apparatus  1 . 
     The operation controller  712  controls the operation of the cooling section  60  (ACT  101 ). For example, if the power source of the image forming apparatus  1  is turned on, the operation controller  712  controls the coolant  64  to lower the temperature in the cooling section  60  to the prescribed value. Specifically, the operation controller  712  causes the cooling section  60  to lower the temperature in the cooling section  60  to the prescribed value by applying the current needed to satisfy the prescribed value to the coolant  64 . In the operation of the image forming apparatus  1 , the operation controller  712  periodically controls the coolant  64  to maintain the temperature in the cooling section  60  at the prescribed value. 
     The printing controller  713  determines whether or not there is a printing instruction (ACT  102 ). If there is no printing instruction (No in ACT  102 ), the printing controller  713  waits for until the printing instruction is issued. 
     On the other hand, if there is the printing instruction (Yes in ACT  102 ), the printing controller  713  determines the content of the printing instruction (ACT  103 ). In a case in which the content of the printing instruction is the non-decolorable toner printing (ACT  103 : printing by the non-decolorable toner), the printing controller  713  performs printing with the non-decolorable toner (ACT  104 ). Specifically, the printing controller  713  first enables the conveyance controller  711  to convey the sheet  41 . 
     The conveyance controller  711  controls the rollers to convey the sheet  41  under the control of the printing controller  713 . The conveyance section  80  conveys the sheet  41 . Next, the printing controller  713  controls the image forming section  30  to execute the image formation by the non-decolorable toner. The printing controller  713  refers to the temperature setting table to acquire information on the fixing temperature of the non-decolorable toner. In the example in  FIG. 3 , the printing controller  713  acquires the information of 130 degrees centigrade as the information on the fixing temperature of the non-decolorable toner. 
     The printing controller  713  performs the control so that the temperature of the heat roller  52  becomes the acquired fixing temperature (130 degrees centigrade) by energizing the heating source  51  of the fixing device  50 . Under the control of the printing controller  713 , the image forming section  30  executes the image formation by the non-decolorable toner on the conveyed sheet  41 . The conveyance controller  711  controls the rollers to convey the sheet  41  on which the image is formed to the fixing device  50 . The conveyance section  80  conveys the sheet  41  to the fixing device  50 . 
     The fixing device  50  fixes the toner by heating the conveyed sheet  41 . As a result, the printing by the non-decolorable toner is executed. Thereafter, the conveyance controller  711  controls the rollers to discharge the heated sheet  41  to the discharge section  90  via the cooling section  60 . The sheet  41  subjected to the image formation by the non-decolorable toner does not change even if cooled by the cooling section  60 . 
     If the content of the printing instruction is the printing by the decolorable toner (ACT  103 : printing by the decolorable toner), the printing controller  713  performs printing with the decolorable toner (ACT  105 ). Specifically, the printing controller  713  first enables the conveyance controller  711  to convey the sheet  41 . 
     The conveyance controller  711  controls the rollers to convey the sheet  41 . The conveyance section  80  conveys the sheet  41 . Next, the printing controller  713  controls the image forming section  30  to execute the image formation by the decolorable toner. The printing controller  713  refers to the temperature setting table to acquire information on the fixing temperature of the decolorable toner. In the example in  FIG. 3 , the printing controller  713  acquires the information of 100 degrees centigrade as the information on the fixing temperature of the decolorable toner. 
     The printing controller  713  performs control so that the temperature of the heat roller  52  becomes the acquired fixing temperature (100 degrees centigrade) by energizing the heating source  51  of the fixing device  50 . Under the control of the printing controller  713 , the image forming section  30  executes the image formation by the decolorable toner on the conveyed sheet  41 . The conveyance controller  711  controls the rollers to convey the sheet  41  on which the image is formed to the fixing device  50 . The conveyance section  80  conveys the sheet  41  to the fixing device  50 . 
     The fixing device  50  fixes the toner by heating the conveyed sheet  41 . As a result, the printing by the decolorable toner is executed. Thereafter, the conveyance controller  711  controls the rollers to discharge the heated sheet  41  to the discharge section  90  via the cooling section  60 . The sheet  41  subjected to the image formation by the decolorable toner does not change even if it is cooled by the cooling section  60  because the image is not decolored. 
     If the content of the printing instruction is printing by both (ACT  103 : printing by both), the printing controller  713  performs printing with both toners (ACT  106 ). For example, the printing controller  713  performs the printing using both the decolorable toner and the non-decolorable toner. In this case, it is assumed that an area to be printed by the decolorable toner and an area to be printed by the non-decolorable toner are set in the print data. The setting of each print area may be set in advance by the user or may be dynamically set by the printing controller  713 . In a case in which the user sets the print area in advance, for example, it can be set using a known technology (for example, U.S. Pat. No. 9,348,546). 
     If the printing controller  713  dynamically sets the print area, the printing controller  713  sets a predetermined text string and image in a text of the print data as the area printed with the decolorable toner. In this case, the printing controller  713  sets text strings and images other than the predetermined text string and image as an area printed with the non-decolorable toner. Or, the printing controller  713  sets a predetermined text string and image among the text of the print data as the area printed with the non-decolorable toner. In this case, the printing controller  713  sets the text strings and images other than the predetermined text string and image as the area printed with the decolorable toner. 
     Thereafter, the printing controller  713  enables the conveyance controller  711  to convey the sheet  41 . The conveyance controller  711  controls the rollers to convey the sheet  41 . The conveyance section  80  conveys the sheet  41 . Next, the printing controller  713  controls the image forming section  30  to execute the image formation by the decolorable toner and the non-decolorable toner. The printing controller  713  transmits information on the area printed with the decolorable toner and information on the area printed with the non-decolorable toner to the image forming section  30 . 
     In addition, the printing controller  713  refers to the temperature setting table to acquire the information on the fixing temperatures of the decolorable toner and the non-decolorable toner. In the example in  FIG. 3 , the printing controller  713  acquires the information of 130 degrees centigrade as the information on the fixing temperatures of the decolorable toner and the non-decolorable toner. 
     The printing controller  713  performs control so that the temperature of the heat roller  52  becomes the acquired fixing temperature (130 degrees centigrade) by energizing the heating source  51  of the fixing device  50 . Under the control of the printing controller  713 , the image forming section  30  executes the image formation by the decolorable toner and the non-decolorable toner on the conveyed sheet  41 . The conveyance controller  711  controls the rollers to convey the sheet  41  on which the image is formed to the fixing device  50 . The conveyance section  80  conveys the sheet  41  to the fixing device  50 . 
     The fixing device  50  fixes the toner by heating the conveyed sheet  41 . As a result, the printing by the decolorable toner and the non-decolorable toner is executed. The temperature of the heating roller  52  of the fixing device  50  is the fixing temperature of the non-decolorable toner and is the decoloring temperature of the decolorable toner. Therefore, the image formed by the decolorable toner is decolored on the sheet  41  subjected to the image formation. Thereafter, the conveyance controller  711  controls the rollers to discharge the heated sheet  41  to the discharge section  90  via the cooling section  60 . On the sheet  41  subjected to the image formation by the decolorable toner and the non-decolorable toner, the image formed by the decolorable toner has been decolored. The temperature in the cooling section  60  is a temperature at which the color of the image formed by the decolorable toner is recovered. Therefore, the color of the image formed by the decolorable toner decolored in the fixing device  50  is recovered. 
     According to the image forming apparatus  1  constituted as described above, the sheet on which an image is formed by the decolorable toner and the non-decolorable toner is cooled after heating. Here, the cooling of the sheet is carried out, for example, by conveying the sheet  41  to the cooling section  60  or stopping the sheet  41  at the cooling section  60  for a predetermined time. Here, preferably, a conveyance speed of the sheet  41  at the time of being conveyed to the cooling section  60  is lower than a conveyance speed before conveyance to the cooling section  60 . Therefore, the image formed by the decolorable toner which is already decolored at the fixing temperature of the non-decolorable toner is restored. In this way, the image forming apparatus  1  can perform the printing using the decolorable toner and the non-decolorable toner in one printing. Therefore, it is possible to shorten the time required for printing using both the decolorable toner and the non-decolorable toner. 
     Below, modifications of the image forming apparatus  1  are described. 
     In the present embodiment, the image forming section  30  forms an image using both the decolorable toner and the non-decolorable toner. The image forming section  30  may form an image using a color material other than the toner. For example, the image forming section  30  may form an image using ink as the color material. In such a case, the image forming section  30  forms an image using decolorable ink decolorable by heating and non-decolorable ink which is not decolored by heating. 
     The heating source  51  may be composed of a plurality of heater lamps. The heating source  51  may be composed of a plurality of induction heating (IH) type heaters. The heating source  51  may be composed of a combination of the heater lamp and the heater. 
     The temperature setting table may be stored in the ROM  72  or may be transmitted from another device via the network interface  130  and copied or developed in the RAM  73 . 
     In the present embodiment, a case in which the conveyance speed of the sheet  41  is constant from the time at which the printing instruction is issued until the sheet  41  is discharged is described as an example, but the present invention is not limited thereto. For example, the conveyance speed of the sheet  41  from the issuance of the printing instruction to the execution of the fixing may be controlled to be different from the conveyance speed of the sheet  41  from the fixing to the discharge of the sheet  41 . The conveyance speed of the sheet  41  from the issuance of the printing instruction to the execution of the fixing is set as a first conveyance speed. The conveyance speed of the sheet  41  from just after a completion of the fixing to the discharge of the sheet  41  is set as a second conveyance speed. 
     The conveyance controller  711  controls the second conveyance speed to be lower than the first conveyance speed. Whether the fixing is already completed may be determined based on whether a predetermined time elapses after the printing instruction. If the predetermined time elapses since the printing instruction, the conveyance controller  711  determines that the fixing is already completed. In this case, the conveyance controller  711  controls the conveyance speed of the sheet  41  to be a conveyance speed lower than the first conveyance speed. If the predetermined time has not elapsed since the reception of the printing instruction, the conveyance controller  711  determines that the fixing is not completed yet. In this case, the conveyance controller  711  controls the conveyance speed of the sheet  41  to be the first conveyance speed. 
     With such control, it is possible to lengthen the time to cool the sheet  41 . Therefore, it is possible to increase the probability for restoring the decolored image. 
     The conveyance controller  711  may control the conveyance speed of the sheet  41  according to a printing ratio of the data which is a printing object. The printing ratio of the data which is the printing object may be the percentage of the total area of the sheet that is printed. The printing ratio of the data which is the printing object may be the percentage of the total area of the sheet  41  that is printed using decolorable toner. The conveyance controller  711  controls the conveyance speed of the sheet  41  to be lower than a preset conveyance speed if the printing ratio of the data which is the printing object is equal to or higher than a threshold value. Specifically, the conveyance controller  711  lowers the conveyance speed of the sheet  41  to a speed lower than the preset conveyance speed by lowering rotational speed of the various rollers. There may be plural threshold values. 
     The operation controller  712  may control the temperature in the cooling section  60  according to the printing ratio of the data which is the printing object. The operation controller  712  controls the temperature in the cooling section  60  to be lower than a prescribed value if the printing ratio of the decolorable toner of the data which is the printing object is equal to or greater than a threshold value. In particular, the operation controller  712  causes the cooling section  60  to lower the temperature in the cooling section  60  to be lower than the prescribed value by applying more current to the coolant  64 . There may be plural threshold values. 
     In the present embodiment, the cooling section  60  is always operated. However, if the printing by the non-decolorable toner and the printing by the decolorable toner are instructed, there is no need to cool the sheet  41 . The reason is that it is not necessary to cool the sheet  41  for recoloring since the image on the sheet  41  is not decolored at the time of passing through the fixing device  50 . Therefore, if the cooling section  60  is always operated, even if there is no need to cool the sheet  41 , it is necessary to supply electric power to operate the cooling section  60 . Therefore, the cost of electric power increases. Thus, the image forming apparatus  1  may operate the cooling section  60  only when the printing by the non-decolorable toner and the decolorable toner is instructed. The operation is described in detail with reference to  FIG. 6 . 
       FIG. 6  is a flowchart depicting the flow of a processing in the modification of the image forming apparatus  1 . The processing in  FIG. 6  similar to that in  FIG. 5  is donated with the same reference numerals as those in  FIG. 5 , and the description thereof is omitted. 
     If the content of the printing instruction is printing by the non-decolorable toner and the decolorable toner (ACT  103 : printing by both), the printing controller  713  instructs the operation controller  712  to operate the cooling section  60 . The operation controller  712  activates the cooling section  60  according to the instruction of the printing controller  713  (ACT  201 ). In particular, the operation controller  712  causes the cooling section  60  to lower the temperature in the cooling section  60  to the prescribed value by applying a current required to satisfy the prescribed value to the coolant  64 . The printing controller  713  performs the printing by both toner (ACT  202 ). For example, the printing controller  713  performs the printing using both the decolorable toner and the non-decolorable toner. If the printing is completed, an image formed by the decolorable toner on the sheet  41  on which the image formation is performed by the decolorable toner and the non-decolorable toner is decolored. The conveyance controller  711  controls the rollers to convey the sheet  41  heated by the fixing device  50  to the cooling section  60  (ACT  203 ). The conveyance section  80  conveys the sheet  41 . 
     On the sheet  41  on which the image formation is performed by the decolorable toner and the non-decolorable toner, the image formed by the decolorable toner is decolored. The cooling section  60  cools the conveyed sheet  41 . As the sheet  41  is cooled, the color of the image formed by the decolorable toner which is decolored in the fixing device  50  is recovered. The sheet  41  passing through the cooling section  60  is discharged to the discharge section  90  by the rollers. The printing controller  713  enables the operation controller  712  to stop operation of the cooling section  60  after the sheet  41  passes through the cooling section  60 . 
     The printing controller  713  determines that the sheet  41  already passes through the cooling section  60  as follows. For example, the printing controller  713  may determine that the sheet  41  passes through the cooling section  60  in response to elapse of a first time since the cooling section  60  is started. The first time is equal to or longer than a time from a moment the control device  70  receives the printing instruction until the sheet  41  is discharged to the discharge section  90 . For example, the printing controller  713  may determine that the sheet  41  passes through the cooling section  60  at a timing at which a sensor provided inside the cooling section  60  or outside the cooling section  60  detects the sheet  41 . If the sensor is provided in the cooling section  60 , the sensor is provided at the downstream side of the coolant  64  in the conveyance direction in which the sheet is conveyed. If the sensor is provided outside the cooling section  60 , the sensor is provided on the conveyance path between the cooling section  60  and the discharge section  90 . The operation controller  712  stops the operation of the cooling section  60  according to the instruction of the printing controller  713  (ACT  204 ). Specifically, the operation controller  712  stops the operation of the cooling section  60  by stopping applying the current to the coolant  64 . 
     With such control, it is possible to suppress power consumption. Therefore, the cost of electric power can be suppressed. 
     The cooling section  60  and the conveyance section  80  may be constituted as an image recoloring apparatus. In this way, the conveyance section  80  conveys the sheet  41  on which the image is formed by the decolorable color material and the non-decolorable color material. The cooling section  60  cools the heated sheet  41  conveyed by the conveyance section  80 . The image recoloring apparatus may be provided with the sheet housing section  40 . In this case, the sheet housing section  40  houses the sheet  41  on which the image is formed by the decolorable color material and the non-decolorable color material. 
     The cooling section  60  may cool the sheet  41  other than the Peltier element. For example, the cooling section  60  may cool the sheet  41  using a refrigerator, a freezer, or the like that uses a reverse Carnot cycle as a cooling method. 
     According to at least one embodiment described above, it is possible to shorten the time required for printing using the decolorable toner and the non-decolorable toner. 
     A part of the functions of the image forming apparatus  1  according to the embodiment described above may be realized by a computer. In that case, programs for realizing those functions are recorded on a computer-readable recording medium. Then, the functions may be realized by enabling a computer system to read and execute the programs recorded on the recording medium in which the above-described program is recorded. 
     The “computer system” here includes an operating system and hardware such as peripheral devices. The “computer-readable recording medium” refers to a portable medium or a storage device. The portable medium is a flexible disk, a magneto-optical disk, a ROM, a CD-ROM or the like. Furthermore, the storage device is a hard disk built in the computer system. Furthermore, the “computer-readable recording medium” dynamically holds programs for a short time like a communication line in the case of transmitting the programs via the communication wire. The communication line is a network such as an Internet, a telephone line, or the like. The “computer readable recording medium” may be a volatile memory in the computer system which is a server or a client. The volatile memory holds a program for a certain period. The above program may realize a part of the above-described functions. Furthermore, the above-described functions may be realized by a combination of the above-described program and a program already recorded in the computer system. 
     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 inventions. 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 inventions. The accompanying claims and there equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.