Patent Publication Number: US-9423730-B2

Title: Cooling device and image forming apparatus including same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/243,561, filed Apr. 2, 2014, which is a continuation of U.S. application Ser. No. 13/463,081 (now U.S. Pat. No. 8,725,026), filed May 3, 2012, and is based on and claims priority pursuant to 35 U.S.C. §119 from Japanese Patent Application Nos. 2011-129927, filed on Jun. 10, 2011 and 2011-159165, filed on Jul. 20, 2011, both in the Japan Patent Office, and the entire contents of each of the above are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention generally relate to a cooling device for an image forming apparatus such as a printer, a facsimile machine, and a copier, and an image forming apparatus including the cooling device. 
     2. Description of the Related Art 
     Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile capabilities, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus. 
     Although differing depending on types of toner and types and speed of conveyance of the sheet, the fixing device is generally controlled to have a temperature of about 180 C.° to 200 C.° so as to instantly melt toner and fix the toner image onto the sheet. Therefore, the temperature of the sheet immediately after passing through the fixing device is high, typically about 100 C.° to 130 C.° depending on the thermal capacity of each sheet such as specific heat and density. Because the melting point of toner is lower than the temperature of the sheet heated by the fixing device, the toner on the sheet is still slightly soft immediately after the sheet has passed through the fixing device, and remains adhesive until the sheet is sufficiently cooled. Consequently, in a case in which multiple sheets discharged from the fixing device are sequentially stacked one atop the other on a discharge tray during continuous image formation, such soft toner on one sheet may adhere to the next sheet, resulting in blocking and considerable image degradation. 
     In addition, when multiple sheets that are still warm are sequentially stacked one atop the other on the discharge tray after being discharged from the fixing device, the heat retained by the stacked sheets softens the toner on the sheets and the weight of the stacked sheets compresses the sheet and possibly causing them to stick together. If stuck sheets are forcibly separated, the toner images formed on the sheets may be damaged or destroyed. For these reasons, the sheets after the fixing process need to be sufficiently cooled. 
     There is known a cooling device including a single cooling member that contacts an inner circumference of an endless conveyance belt that conveys the sheet. The cooling member absorbs heat via the conveyance belt from the sheet conveyed by the conveyance belt to cool the sheet discharged from the fixing device. The sheet heated by the fixing device is cooled by the cooling member while being conveyed by the conveyance belt. Therefore, the temperature of the sheet is lowered as the sheet approaches a downstream portion of the cooling member in a direction of conveyance of the sheet. 
     With such a configuration, the amount of heat absorbed by the cooling member is also decreased toward the downstream portion of the cooling member. Therefore, an upstream portion of the cooling member is hotter than a downstream portion thereof. However, because a single cooling member is used to cool the sheet from upstream to downstream in the direction of conveyance of the sheet, heat from the hotter upstream portion of the cooling member is transmitted to the downstream portion. Consequently, the downstream end of the cooling member cannot be kept low, thereby degrading cooling efficiency and possibly preventing sufficient cooling of the sheet. 
     In another approach, an image forming apparatus includes a cooling device having a block-type cooling member provided downstream from the fixing device in the direction of conveyance of the sheet. A channel through which liquid coolant flows from downstream to upstream is formed inside the cooling member, and the cooling member contacts the sheet to cool the sheet while the sheet is conveyed past the cooling device. Thus, the sheet discharged from the fixing device is cooled by the cooling member included in the cooling device. Accordingly, toner on the sheet is also cooled and cured, thereby preventing blocking. The liquid coolant enters the cooling member from an inlet provided at a downstream end of the cooling member and flows through the channel to an outlet provided at an upstream end of the cooling member. Accordingly, the cooling member heated by heat absorbed from the sheet is cooled by the liquid coolant. 
     In a case in which the liquid coolant flows through the cooling member from upstream to downstream so as to cool the sheet, upstream and downstream portions of the cooling member sequentially absorb heat from the sheet. Consequently, the temperature of the liquid coolant flowing through the cooling member increases toward the downstream portion of the cooling member. As a result, a difference in temperature between the sheet and the liquid coolant flowing through the downstream portion of the cooling member also decreases, thereby degrading cooling efficiency. 
     By contrast, when the liquid coolant flows through the cooling member from downstream to upstream as described in the above example, the sheet can be cooled by the cooler liquid coolant at the downstream portion of the cooling member compared to the case in which the liquid coolant flows through the cooling member from upstream to downstream. As a result, the difference in temperature between the sheet and the liquid coolant flowing through the downstream portion of the cooling member can be increased, thereby efficiently cooling the sheet at the downstream portion of the cooling member. 
     However, again, because heat absorbed from the sheet by the upstream portion of the cooling member is transmitted to the downstream portion, the temperature of the liquid coolant flowing through the downstream portion of the cooling member is increased. Therefore, even in a configuration in which the liquid coolant flows through the cooling member from downstream to upstream, thermal transmission within the cooling member increases the temperature of the liquid coolant flowing through the downstream portion of the cooling member, thereby degrading cooling efficiency at the downstream portion of the cooling member. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the foregoing, illustrative embodiments of the present invention provide a novel cooling device using a plurality of cooling members that efficiently cool a recording medium even at a downstream end of each of the cooling members in a direction of conveyance of the recording medium. In the cooling device, the cooling members are disposed such that heat-absorbing surfaces of the respective cooling members together form a single stepless plane. Illustrative embodiments of the present invention further provide an image forming apparatus including the cooling device. 
     In one illustrative embodiment, a cooling device includes at least two cooling members to cool a recording medium passing thereover, a coolant circulation unit to circulate a coolant, and tubing that connects the coolant circulation unit to the cooling members and through which the coolant circulates. Each of the cooling members includes a heat-absorbing surface that directly contacts the recording medium or indirectly contacts the recording medium via a thermal transmission member, an internal channel provided within each of the cooling members through which the coolant circulates, and a channel inlet and outlet formed at downstream and upstream ends of each of the cooling members in a direction of conveyance of the recording medium, respectively. One of an interval and a thermal insulator is provided between the cooling members. 
     In another illustrative embodiment, an image forming apparatus includes a fixing device to fix an image formed on a recording medium onto the recording medium using heat and the cooling device described above. The cooling device is provided downstream from the fixing device in the direction of conveyance of the recording medium to cool the recording medium onto which the image is fixed by the fixing device. 
     In yet another illustrative embodiment, a cooling device includes an endless belt to convey a recording medium contacting an outer circumference of the belt by movement of the belt, at least two cooling members arranged side by side at an interval therebetween in a direction of movement of the belt, and a positioning member to position the cooling members flush with each other to form a single plane. The cooling members respectively include heat-absorbing surfaces each contacting an inner circumference of the belt within a range in which the outer circumference of the belt contacts the recording medium to cool the recording medium by absorbing heat from the recording medium via the belt. 
     In still yet another example, an image forming apparatus includes a fixing device to fix an image formed on a recording medium onto the recording medium using heat and the cooling device described above. The cooling device is provided downstream from the fixing device in a direction of conveyance of the recording medium to cool the recording medium onto which the image is fixed by the fixing device. 
     Additional features and advantages of the present disclosure will become more fully apparent from the following detailed description of illustrative embodiments, the accompanying drawings, and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be more readily obtained as the same becomes better understood by reference to the following detailed description of illustrative embodiments when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic vertical cross-sectional view illustrating an example of a configuration of a tandem-type full-color image forming apparatus employing an intermediate transfer belt system, in which a cooling device according to illustrative embodiments is installed; 
         FIG. 2  is schematic view illustrating an example of an overall configuration of a cooling device according to a first illustrative embodiment; 
         FIG. 3  is a schematic view illustrating an example of a configuration around one of cooling plates included in the cooling device; 
         FIG. 4  is a schematic view illustrating an example of a configuration around the other one of the cooling plates included in the cooling device; 
         FIG. 5A  is a side view illustrating the configuration around the cooling plate; 
         FIG. 5B  is a graph showing temperature distribution corresponding to the configuration illustrated in  FIG. 5A ; 
         FIG. 6A  is a side view illustrating an example of a configuration of a single cooling plate provided to a cooling device according to a comparative example; 
         FIG. 6B  is a side view illustrating the configuration of the two separate cooling plates provided to the cooling device according to the first illustrative embodiment; 
         FIG. 6C  is a graph showing temperature distribution corresponding to the configurations respectively illustrated in  FIGS. 6A and 6B ; 
         FIG. 7  is schematic view illustrating an example of an overall configuration of the cooling device including a thermal insulator between the cooling plates; 
         FIG. 8  is a perspective view illustrating an example of a configuration around cooling plates included in a cooling device according to a second illustrative embodiment; 
         FIG. 9A  is a side view illustrating the configuration around the cooling plates in the cooling device according to the second illustrative embodiment; 
         FIG. 9B  is a graph showing temperature distribution corresponding to the configuration illustrated in  FIG. 9A ; 
         FIG. 10  is a vertical cross-sectional view illustrating an example of a configuration of the cooling device according to the second illustrative embodiment in a case in which the cooling plates are not appropriately disposed; 
         FIGS. 11A and 11B  are perspective views respectively illustrating positioning members provided to the cooling device according to the second illustrative embodiment; 
         FIG. 12  is a perspective view illustrating an example of a configuration of a positioning member having cutouts; 
         FIG. 13  is a vertical cross-sectional view illustrating an example of a configuration of a cooling device according to a first variation of the second illustrative embodiment; 
         FIG. 14  is a vertical cross-sectional view illustrating an example of a configuration of the cooling device illustrated in  FIG. 13  in which the cooling plates are not appropriately disposed; 
         FIG. 15  is a perspective view illustrating an example of a configuration of the cooling plates and the positioning members included in the cooling device according to the first variation of the second illustrative embodiment; 
         FIG. 16  is a perspective view illustrating replacement of a cooling belt included in the cooling device according to the first variation of the second illustrative embodiment; 
         FIG. 17  is a perspective view illustrating an example of a configuration of a cooling device according to a second variation of the second illustrative embodiment; 
         FIG. 18  is a vertical cross-sectional view illustrating an example of a configuration of a cooling device according to a third illustrative embodiment; 
         FIG. 19  is a schematic view illustrating a flow of liquid coolant in the cooling device illustrated in  FIG. 18 ; 
         FIG. 20  is a vertical cross-sectional view illustrating an example of a configuration of a cooling device according to a first variation of the third illustrative embodiment; 
         FIG. 21  is a schematic view illustrating an example of a configuration around cooling plates included in the cooling device illustrated in  FIG. 20 ; 
         FIG. 22  is a schematic view illustrating an example of a configuration of a cooling device according to a second variation of the third illustrative embodiment; 
         FIG. 23  is a vertical cross-sectional view illustrating an example of a configuration of a cooling device according to a third variation of the third illustrative embodiment; and 
         FIG. 24  is a schematic view illustrating an example of a configuration around cooling plates included in the cooling device illustrated in  FIG. 23 . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In describing illustrative embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Illustrative embodiments of the present invention are now described below with reference to the accompanying drawings. 
     In a later-described comparative example, illustrative embodiment, and exemplary variation, for the sake of simplicity the same reference numerals will be given to identical constituent elements such as parts and materials having the same functions, and redundant descriptions thereof omitted unless otherwise required. 
       FIG. 1  is a schematic vertical cross-sectional view illustrating an example of a configuration of a tandem-type full-color image forming apparatus  200  employing an intermediate transfer belt system, in which a cooling device  18  according to illustrative embodiments is included. 
     It is to be noted that the cooling device  18  is applicable to any device in which cooling of a sheet-type member is needed as well as to image forming apparatuses. In addition, although liquid is used as a coolant in illustrative embodiments, the coolant is not limited thereto but may be any fluid, such as air. 
     The image forming apparatus  200  includes an intermediate transfer belt  51  wound around multiple rollers such as first, second, and third rollers  52 ,  53 , and  55 . The intermediate transfer belt  51  is rotated by rotation of the rollers  52 ,  53 , and  55  in a clockwise direction as indicated by an arrow a in  FIG. 1 , and processing units for image formation are disposed around the intermediate transfer belt  51 . 
     Part of the processing units, that is, image forming units  54 Y,  54 C,  54 M, and  54 K (hereinafter collectively referred to as image forming units  54 ), are disposed above the intermediate transfer belt  51  between the first and second rollers  52  and  53 , in that order from upstream to downstream in the direction of rotation of the intermediate transfer belt  51 . Taking the image forming unit  54 Y as a representative example, a charger  10 Y, an optical writing device  12 Y, a developing device  13 Y, and a cleaning device  14 Y are provided around a drum-type photoconductor  111 Y. The image forming unit  54 Y further includes a primary transfer roller  15 Y provided opposite the photoconductor  111 Y with the intermediate transfer belt  51  interposed therebetween. It is to be noted that, the other three image forming units  54 C,  54 M, and  54 K have the same configuration as the image forming unit  54 Y, only differing in color of toner used. The image forming units  54  are arranged side by side at predetermined intervals. 
     Although each of optical writing devices  12 Y,  12 C,  12 M, and  12 K (hereinafter collectively referred to as optical writing devices  12 ) includes an LED as a light source, alternatively, a semiconductor laser may be used as the light source. The optical writing devices  12  irradiate photoconductors  111 Y,  111 C,  111 M, and  111 K (hereinafter collectively referred to as photoconductors  111 ) with light based on image data, respectively. 
     The image forming apparatus  200  further includes a sheet storage  19  that stores a sheet-type member such as a sheet P, a sheet feed roller  223 , a pair of registration rollers  221 , a secondary transfer roller  56 , a belt cleaning device  59 , a thermal fixing device  16 , the cooling device  18 , and a discharge storage  17 , each of which is disposed below the intermediate transfer belt  51 . The secondary transfer roller  56  is disposed opposite the third roller  55  with the intermediate transfer belt  51  interposed therebetween to transfer a toner image from the intermediate transfer belt  51  onto the sheet P. The belt cleaning device  59  that contacts an outer surface of the intermediate transfer belt  51  is provided opposite a roller  58  that contacts an inner surface of the intermediate transfer belt  51  so as to clean the outer surface of the intermediate transfer belt  51 . The cooling device  18  includes cooling plates  1   a  and  1   b , both of which cool the sheet P. The sheet P having a fixed toner image thereon is discharged to the discharge storage  17 . A sheet conveyance path  28  is extended within the image forming apparatus  200  from the sheet storage  19  to the discharge storage  17 . The image forming apparatus  200  further includes a sheet conveyance path  29  for duplex image formation that reverses the sheet P conveyed from the cooling device  18  and further conveys the sheet P to the pair of registration rollers  221  again when an image is formed also on a back side of the sheet P during duplex image formation. 
     The cooling device  18  includes the cooling plates  1   a  and  1   b , a pump  100 , a tank  101 , a radiator  103 , and a cooling fan  104 . Each of the cooling plates  1   a  and  1   b  is a heat absorber that absorbs heat from the sheet P. The tank  101  is a storage device that stores a liquid coolant. Tubing  105  consisting of subsections  105   a - 105   c  is connected to an inlet and outlet provided to each of the cooling plates  1   a  and  1   b , and connects the cooling plates  1   a  and  1   b , the radiator  103 , the tank  101 , and the pump  100  so that the liquid coolant is circulated in the cooling device  18 . The pump  100  is a coolant circulation unit that conveys the liquid coolant stored in the tank  101  through the tubing  105 . The radiator  103  is a heat releasing part that releases heat absorbed from the sheet P by the liquid coolant via the cooling plates  1   a  and  1   b  outside the image forming apparatus  200 . The cooling fan  104  is an air generator mounted on the radiator  103  to generate air flow around the radiator  103  to cool the radiator  103 . 
     As indicated by solid arrows in  FIG. 1  each representing the tubing  105 , the liquid coolant cooled by the radiator  103  is supplied to the cooling plates  1   b  and  1   a , flows through the cooling plates  1   b  and  1   a , and then is discharged from the cooling plates  1   b  and  1   a . The liquid coolant thus discharged is conveyed to the tank  101  and the pump  100  and is returned to the radiator  103  again to be cooled. The liquid coolant is circulated by rotational pressure from the pump  100 , and heat is released from the liquid coolant by the radiator  103 , which in turn cools the cooling plates  1   a  and  1   b . The capacity of the pump  100  to convey the liquid coolant and the size of the radiator  103  are determined by thermal design considerations such as an amount of cooling required of the cooling plates  1   a  and  1   b.    
     Taking the image forming unit  54 Y as a representative example, image forming processes performed in the image forming apparatus  200  are described in detail below. In the same way as the general electrophotographic method, first, the surface of the photoconductor  111 Y is evenly charged by the charger  10 Y. The optical writing unit  12 Y irradiates the charged surface of the photoconductor  111 Y with light to form an electrostatic latent image on the surface of the photoconductor  111 Y. Then, the developing device  13 Y develops the electrostatic latent image with toner so that a toner image is formed on the surface of the photoconductor  111 Y. The toner image is then primarily transferred from the surface of the photoconductor  111 Y onto the intermediate transfer belt  51  by the primary transfer roller  15 Y to which a transfer bias is supplied. Thereafter, the surface of the photoconductor  111 Y is cleaned by the cleaning device  14 Y. The above-described image forming processes are also performed in the other three image forming units  54 C,  54 M, and  54 K, differing only the color of toner used. 
     Developing devices  13 Y,  13 C,  13 M, and  13 K (hereinafter collectively referred to as developing devices  13 ) included in the respective image forming units  54  develop electrostatic latent images formed on the surfaces of the photoconductors  111  with toner of specific colors, that is, yellow (Y), cyan (C), magenta (M), and black (K), respectively. Thus, a full-color toner image is formed using the four image forming units  54 . Specifically, the toner images formed on the surfaces of the photoconductors  111  are sequentially transferred onto the intermediate transfer belt  51  one atop the other by primary transfer rollers  15 Y,  15 C,  15 M, and  15 K (hereinafter collectively referred to as primary transfer rollers  15 ), each supplied with a transfer bias and provided opposite the respective photoconductors  111  with the intermediate transfer belt  51  interposed therebetween. Accordingly, a single full-color toner image is formed on the intermediate transfer belt  51 . 
     The full-color toner image formed on the intermediate transfer belt  51  is secondarily transferred onto the sheet P by the secondary transfer roller  56 . The intermediate transfer belt  51  is then cleaned by the belt cleaning device  59 . A transfer bias is supplied to the secondary transfer roller  56  to form a transfer electric field between the secondary transfer roller  56  and the third roller  55  with the intermediate transfer belt  51  interposed therebetween. Thus, the full-color toner image formed on the intermediate transfer belt  51  is secondarily transferred from the intermediate transfer belt  51  onto the sheet P conveyed to a nip formed between the secondary transfer roller  56  and the intermediate transfer belt  51 . After secondary transfer of the full-color toner image from the intermediate transfer belt  51  onto the sheet P, the sheet P having the full-color toner image thereon is conveyed to the fixing device  16  to fix the full-color toner image to the sheet P. Then, the sheet P having the fixed full-color image thereon is discharged to the discharge storage  17 . 
     In the image forming apparatus  200  according to illustrative embodiments, before being discharged to the discharge storage  17 , the sheet P having the fixed image thereon passes the cooling device  18  disposed immediately after the fixing device  16 . When passing the cooling device  18 , the sheet P heated by the fixing device  16  contacts the cooling plates  1   a  and  1   b . At this time, heat is absorbed from the sheet P by heat-absorbing surfaces of the cooling plates  1   a  and  1   b  that face the sheet P. The heat thus absorbed by the cooling plates  1   a  and  1   b  is transmitted to the liquid coolant flowing through the cooling plates  1   a  and  1   b . The liquid coolant heated by the heat transmitted from the cooling plates  1   a  and  1   b  is then discharged from the cooling plates  1   a  and  1   b  to be conveyed to the radiator  103  having the cooling fan  104  via the tank  101  and the pump  100 . The heat released from the liquid coolant by the radiator  103  is discharged outside the image forming apparatus  200 . After the heat is released from the liquid coolant by the radiator  103  and the temperature of the liquid coolant is lowered to room temperature, the liquid coolant is conveyed to the cooling plates  1   b  and  1   a  again. The above-described heat releasing cycle having good cooling capability using the liquid coolant can efficiently cool the sheet P heated by the fixing device  16 . 
     As a result, when the sheet P is stored in the discharge storage  17 , toner on the sheet securely hardens and is fixed onto the sheet P. In particular, blocking, which tends to occur during duplex image formation in which the fixing device  16  performs the fixing process twice for each sheet P, can be reliably prevented by use of the cooling device  18 . 
       FIG. 2  is a schematic view illustrating an example of an overall configuration of the cooling device  18  according to the first illustrative embodiment. 
     In the first illustrative embodiment, the pump  100 , the radiator  103 , the tank  101 , and cooling members, which, in the present illustrative embodiment, are the cooling plates  1   a  and  1   b , are connected to one another by the tubing  105  constructed of rubber tubes. A serpentine liquid circulation channel is formed within each of the cooling plates  1   a  and  1   b.    
       FIG. 3  is a schematic view illustrating an example of a configuration around the cooling plate  1   b  in the cooling device  18  according to the first illustrative embodiment. 
     An inlet  70   b  from which the liquid coolant enters the cooling plate  1   b  is provided at a downstream end on a lateral surface of the cooling plate  1   b  in a direction of conveyance of the sheet P. An outlet  71   b  from which the liquid coolant is discharged from the cooling plate  1   b  is provided at an upstream end on the lateral surface of the cooling plate  1   b . The inlet  70   b  and outlet  71   b  of the cooling plate  1   b  are connected to respective ends of a serpentine internal channel  73   b  formed within the cooling plate  1   b  in a width direction of the sheet P perpendicular to the direction of conveyance of the sheet P. One end of a tube  105   a  is connected to the pump  100 , and the other end thereof is connected to the inlet  70   b . One end of a tube  105   c  is connected to the outlet  71   b.    
       FIG. 4  is a schematic view illustrating an example of a configuration around the cooling plate  1   a  in the cooling device  18  according to the first illustrative embodiment. 
     An inlet  70   a  from which the liquid coolant enters the cooling plate  1   a  is provided at a downstream end on a lateral surface of the cooling plate  1   a  in the direction of conveyance of the sheet P. An outlet  71   a  from which the liquid coolant is discharged from the cooling plate  1   a  is provided at an upstream end on the lateral surface of the cooling plate  1   a . The inlet  70   a  and outlet  71   a  of the cooling plate  1   a  are connected to respective ends of a serpentine internal channel  73   a  formed within the cooling plate  1   a  in the width direction of the sheet P. The one end of the tube  105   c  is connected to the outlet  71   b  of the cooling plate  1   b , and the other end thereof is connected to the inlet  70   a  of the cooling plate  1   a . One end of a tube  105   b  is connected to the radiator  103 , and the other end thereof is connected to the outlet  71   a.    
     Thus, the inlet  70   a  and outlet  71   a  are provided on the same lateral surface of the cooling plate  1   a , and the inlet  70   b  and outlet  71   b  are provided on the same lateral surface of the cooling plate  1   b . Accordingly, all the tubes  105   a ,  105   b , and  105   c  can be disposed on one side of the cooling plates  1   a  and  1   b  in the width direction of the sheet P, thereby simplifying placement of the tubing  105  within the cooling device  18  and achieving a space-saving configuration. 
     The liquid coolant stored in the tank  101  is conveyed by the pump  100  so as to enter the cooling plate  1   b  from the inlet  70   b  via the tube  105   a . The liquid coolant absorbs heat while flowing through the cooling plate  1   b , and is discharged from the cooling plate  1   b  to the tube  105   c  via the outlet  71   b . The liquid coolant thus discharged then enters the cooling plate  1   a  from the inlet  70   a  via the tube  105   c . The liquid coolant absorbs heat while flowing through the cooling plate  1   a , and is discharged from the cooling plate  1   a  to the tube  105   b  via the outlet  71   a . The liquid coolant heated by heat absorbed from the cooling plates  1   a  and  1   b  while flowing through the cooling plates  1   a  and  1   b  is then conveyed to the radiator  103  so that the heat is released from the liquid coolant. Thereafter, the liquid coolant sufficiently cooled by the radiator  103  is returned to the tank  101 . 
     The fixing device  16  includes a pair of heat rollers  116  having a heater therein. The full-color toner image is fixed to the sheet P by heat supplied from the pair of heat rollers  116 . The sheet P thus heated is conveyed by a pair of conveyance rollers  60  to the cooling device  18 . In the cooling device  18 , the sheet P contacts an upper surface of each of the cooling plates  1   a  and  1   b , that is, heat-absorbing surfaces  11   a  and  11   b , while being conveyed. At this time, the cooling plates  1   a  and  1   b  absorb heat from the sheet P contacting the heat-absorbing surfaces  11   a  and  11   b  using thermal transmission to cool the sheet P. 
       FIG. 5A  is a side view illustrating the configuration around the cooling plate  1   a , and  FIG. 5B  is a graph showing temperature distribution in the direction of conveyance of the sheet P corresponding to the configuration illustrated in  FIG. 5A . It is to be noted that, in the graph shown in  FIG. 5B , the horizontal axis represents position in the direction of conveyance of the sheet P and the vertical axis represents temperature. 
     The sheet P heated by the pair of heat rollers  116  is conveyed by the pair of conveyance rollers  60  to the cooling plate  1   a  so that the sheet P is cooled by the cooling plate  1   a  while contacting the heat-absorbing surface  11   a  of the cooling plate  1   a . Accordingly, temperature distribution in the direction of conveyance of the sheet P occurs in the cooling plate  1   a  that absorbs heat from the sheet P. 
     Each of bold lines A, B, and C in  FIG. 5B  indicates temperature distribution in the case of the first illustrative embodiment as described above, in which the liquid coolant enters the cooling plate  1   a  from the inlet  70   a , flows through the cooling plate  1   a  through the internal channel  73   a , and then is discharged from the cooling plate  1   a  via the outlet  71   a . In other words, the liquid coolant flows through the cooling plate  1   a  from downstream to upstream in the direction of conveyance of the sheet P. 
     The bold solid line A in  FIG. 5B  indicates temperature distribution in the cooling plate  1   a  in the direction of conveyance of the sheet P. The bold broken line B in  FIG. 5B  indicates temperature distribution in the liquid coolant flowing through the cooling plate  1   a  in the direction of conveyance of the sheet P. The bold broken line C in  FIG. 5B  indicates temperature distribution in the sheet P in the direction of conveyance thereof. 
     Meanwhile, each of fine lines a, b, and c in  FIG. 5B  indicates temperature distribution in a configuration according to a comparative example, in which the liquid coolant enters the cooling plate  1   a  from the outlet  71   a , flows through the cooling plate  1   a  through the internal channel  73   a , and is then discharged from the cooling plate  1   a  via the inlet  70   a . Thus, in the comparative example, the liquid coolant flows through the cooling plate  1   a  from upstream to downstream in the direction of conveyance of the sheet P, which is the reverse of the configuration employed in the first illustrative embodiment. 
     The fine solid line a in  FIG. 5B  indicates temperature distribution in the cooling plate  1   a  in the direction of conveyance of the sheet P according to the comparative example. The fine broken line b in  FIG. 5B  indicates temperature distribution in the liquid coolant flowing through the cooling plate  1   a  in the direction of conveyance of the sheet P according to the comparative example. The fine broken line c in  FIG. 5B  indicates temperature distribution in the sheet P in the direction of conveyance thereof according to the comparative example. 
     As is clear from  FIG. 5B , at the upstream end of the cooling plate  1   a , the temperature of the cooling plate  1   a  according to the first illustrative embodiment indicated by the bold solid line A is higher than that according to the comparative example indicated by the fine solid line a. By contrast, at the downstream end of the cooling plate  1   a , the temperature of the cooling plate  1   a  according to the first illustrative embodiment is lower than that according to the comparative example. The above difference in temperature distribution in the cooling plate  1   a  between the first illustrative embodiment and the comparative example reflects the temperature of the liquid coolant flowing through the cooling plate  1   a.    
     When the liquid coolant enters the cooling plate  1   a  from the inlet  70   a  provided at the downstream end of the cooling plate  1   a , liquid coolant at its coolest flows around the downstream end of the cooling plate  1   a  as indicated by the bold broken line B. Then, the liquid coolant absorbs heat while flowing through the cooling plate  1   a  from downstream to upstream so that the temperature of the liquid coolant is gradually increased toward the upstream end of the cooling plate  1   a . When hottest, the liquid coolant is discharged from the outlet  71   a  provided at the upstream end of the cooling plate  1   a.    
     By contrast, when the liquid coolant enters the cooling plate  1   a  from the outlet  71   a  provided at the upstream end of the cooling plate  1   a , liquid coolant at its coolest flows around the upstream end of the cooling plate  1   a  as indicated by the fine broken line b. Then, the liquid coolant absorbs heat while flowing through the cooling plate  71   a  from upstream to downstream so that the temperature of the liquid coolant is gradually increased toward the downstream end of the cooling plate  71   a . When hottest, the liquid coolant is discharged from the inlet  70   a  provided at the downstream end of the cooling plate  1   a.    
     Thus, in the case of the first illustrative embodiment, in which the liquid coolant flows through the cooling plate  1   a  from downstream to upstream, the downstream end of the cooling plate  1   a  has a lower temperature and the upstream end thereof has a higher temperature compared to the case of the comparative example, in which the liquid coolant flows through the cooling plate  1   a  from upstream to downstream. 
     The above difference in temperature distribution in the cooling plate  1   a  between the first illustrative embodiment and the comparative example affects cooling efficiency. Comparing the bold broken line C to the fine broken line c, at the upstream portion of the cooling plate  1   a , that is, at the start of cooling of the sheet P, the temperature of the sheet P according to the comparative example indicated by the fine broken line c is lower than that according to the first illustrative embodiment indicated by the bold broken line C. However, at the downstream portion of the cooling plate  1   a , that is, at the end of cooling of the sheet P, a temperature of the sheet P according to the first illustrative embodiment is lower than that according to the comparative example. The reason for the lower temperature of the sheet P at the downstream portion of the cooling plate  1   a  according to the first illustrative embodiment is that the sheet P contacts a portion of the heat-absorbing surface  11   a  having the lower temperature at the downstream end of the cooling plate  1   a.    
     In order to prevent blocking, the sheet P needs to be cooled as low as possible by the cooling device  18  before being discharged to the discharge storage  17 . Therefore, it is preferable that the downstream end of the cooling plate  1   a , which cools the sheet P in the last stage of cooling operation performed by the cooling plate  1   a , have a lower temperature even if the upstream end of the cooling plate  1   a  has a rather higher temperature. 
     Thus, in the first illustrative embodiment, the liquid coolant enters the cooling plate  1   a  from the inlet  70   a  provided at the downstream end of the cooling plate  1   a  and flows through the cooling plate  1   a  through the internal channel  73   a  in a direction opposite the direction of conveyance of the sheet P. Thereafter, the liquid coolant is discharged from the cooling plate  1   a  via the outlet  71   a  provided at the upstream end of the cooling plate  1   a . As a result, a decrease in cooling efficiency at the downstream end of the cooling plate  1   a  can be prevented, thereby efficiently cooling the sheet P. 
     In the first illustrative embodiment, in a manner similar to the cooling plate  1   a , the liquid coolant enters the cooling plate  1   b  from the inlet  70   b  provided at the downstream end of the cooling plate  1   b  and flows through the cooling plate  1   b  through the internal channel  73   b  in the direction opposite the direction of conveyance of the sheet P. Thereafter, the liquid coolant is discharged from the cooling plate  1   b  via the outlet  71   b  provided at the upstream end of the cooling plate  1   b . As a result, a decrease in cooling efficiency at the downstream end of the cooling plate  1   b  can be also prevented, thereby efficiently cooling the sheet P. 
     Because the fixing device  16  melts the toner by heat from the pair of heat rollers  116  to fix the toner image to the sheet P, moisture contained in the sheet P is evaporated, resulting in an increase in humidity around the fixing device  16 . Consequently, if the upstream end of the cooling plate  1   a  provided near the pair of heat rollers  116  is too cool, a difference in temperature between the cooling plate  1   a  and the pair of heat rollers  116  is increased too much, thereby easily causing condensation on the surface of the cooling plate  1   a  at the upstream end thereof. 
     By contrast, when the liquid coolant flows through the cooling plate  1   a  from downstream to upstream as in the case of the first illustrative embodiment, the temperature at the upstream end of the cooling plate  1   a  is increased, thereby reducing the difference in temperature between the pair of heat rollers  116  and the cooling plate  1   a . Accordingly, condensation on the surface of the cooling plate  1   a  at the upstream end thereof can be prevented. 
     In addition, the split configuration incorporating an interval between the cooling plates  1   a  and  1   b  provides further cooling efficiency, particularly compared to a configuration employing a single continuous cooling plate, as is described below with reference to  FIG. 6 . 
       FIG. 6A  is a side view illustrating an example of a configuration of a single cooling plate  1  provided to a cooling device according to a second comparative example. The cooling plate  1  has a length of X mm in the direction of conveyance of the sheet P.  FIG. 6B  is a side view illustrating the configuration of the cooling plates  1   a  and  1   b  arranged side by side at an interval therebetween in the direction of conveyance of the sheet P according to the first illustrative embodiment. The cooling plates  1   a  and  1   b  are respectively disposed in two separate ranges obtained by dividing a single range having the length of X mm into the two ranges.  FIG. 6C  is a graph showing temperature distribution corresponding to the configurations respectively illustrated in  FIGS. 6A and 6B . It is to be noted that in the graph shown in  FIG. 6C , the horizontal axis represents position in the direction of conveyance of the sheet P and the vertical axis represents temperature. 
     In the case of the second comparative example in which the single cooling plate  1  is provided as illustrated in  FIG. 6A , the liquid coolant enters the cooling plate  1  from an inlet  70  provided at a downstream end on a lateral surface of the cooling plate  1 , flows through the cooling plate  1  through an internal channel  73 , and is then discharged from the cooling plate  1  via an outlet  71  provided at an upstream end on the lateral surface of the cooling plate  1 . 
     In the case of the first illustrative embodiment, in which the two separate cooling plates  1   a  and  1   b  are provided side by side at an interval therebetween in the direction of conveyance of the sheet P as illustrated in  FIG. 6B , first, the liquid coolant enters the cooling plate  1   b  from the inlet  70   b  provided at the downstream end of the cooling plate  1   b , flows through the cooling plate  1   b  through the internal channel  73   b , and is then discharged from the cooling plate  1   b  via the outlet  71   b  provided at the upstream end of the cooling plate  1   b  to the tube  105   c . Next, the liquid coolant discharged to the tube  105   c  enters the cooling plate  1   a  from the inlet  70   a  provided at the downstream end of the cooling plate  1   a , flows through the cooling plate  1   a  through the internal channel  73   a , and is then discharged from the cooling plate  1   a  via the outlet  71   a  provided at the upstream end of the cooling plate  1   a  to the tube  105   b.    
     Fine lines  1 A and  7 A in  FIG. 6C  indicate temperature distribution in the case of the second comparative example in which the single cooling plate  1  is provided as illustrated in  FIG. 6A . Specifically, the fine solid line  1 A indicates temperature distribution in the cooling plate  1  in the direction of conveyance of the sheet P. The fine broken line  7 A indicates temperature distribution in the sheet P in the direction of conveyance thereof. 
     Bold lines  1 B and  7 B in  FIG. 6C  indicate temperature distribution in the case of the first illustrative embodiment in which the cooling plates  1   a  and  1   b  are arranged side by side at an interval therebetween in the direction of conveyance of the sheet P as illustrated in  FIG. 6B . Specifically, the bold solid line  1 B indicates temperature distribution in the cooling plates  1   a  and  1   b  in the direction of conveyance of the sheet P. The bold broken line  7 B indicates temperature distribution in the sheet P in the direction of conveyance thereof. 
     Compared to the temperature of the cooling plate  1  indicated by the fine solid line  1 A, the temperature of the cooling plate  1   a  indicated by the bold solid line  1 B is higher overall and the temperature of the cooling plate  1   b  also indicated by the bold solid line  1 B is lower overall. 
     The reason for the lower temperature of the cooling plate  1   b  is that the interval provided between the cooling plates  1   a  and  1   b  prevents thermal transmission between the cooling plates  1   a  and  1   b . Assuming that the cooling plates  1   a  and  1   b  are that contacts with each other without an interval therebetween, thermal transmission between the cooling plates  1   a  and  1   b  occurs. Consequently, temperature distribution is equalized between the cooling plates  1   a  and  1   b , resulting in the similar temperature distribution obtained in the case of the second comparative example in which the single cooling plate  1  is provided as illustrated in  FIG. 6A . 
     As described above, in order to reduce the temperature of the sheet P discharged to the discharge storage  17 , it is more effective that a portion which cools the sheet P at the last stage of cooling operation has a lower temperature. The two separate cooling plates  1   a  and  1   b  according to the first illustrative embodiment, which are arranged side by side at an interval therebetween in the direction of conveyance of the sheet P, can prevent thermal transmission from the upstream cooling plate  1   a  to the downstream cooling plate  1   b  and the temperature increase at the downstream end of the cooling plate  1   b . Accordingly, the cooling plates  1   a  and  1   b  can more effectively cool the sheet P compared to the case in which the sheet P is cooled by the single cooling plate  1 . As a result, a temperature increase in the liquid coolant flowing through the downstream end of the cooling plate  1   b  can also be prevented, thereby efficiently and effectively cooling the sheet P even at the downstream end of the cooling plate  1   b.    
     Alternatively, in a variation illustrated in  FIG. 7 , a thermal insulator  80  may be provided between the cooling plates  1   a  and  1   b  to prevent thermal transmission between the cooling plates  1   a  and  1   b . In such a case, the same effects as those obtained by the first illustrative embodiment described above can be achieved. 
     A description is now given of a second illustrative embodiment of the present invention.  FIG. 8  is a perspective view illustrating an example of a configuration around the cooling plates  1   a  and  1   b  provided to the cooling device  18  according to the second illustrative embodiment. 
     In the second illustrative embodiment, a polyimide cooling belt  45  is rotatably wound around a drive roller  61  and multiple driven rollers  62 ,  63 , and  64 . In addition, a conveyance belt  46  would around driven rollers  65  and  66  is provided opposite the cooling belt  45 . The conveyance belt  46  is formed of an elastic material such as acrylic rubber or polyimide, or has a multi-layered structure formed of the elastic material and polyimide. The sheet P is conveyed, while sandwiched between the cooling belt  45  and the conveyance belt  46 , by the cooling belt  45  rotated by a drive force from the drive roller  61  and the conveyance belt  46  rotated as the cooling belt  45  rotates. 
     The two separate cooling plates  1   a  and  1   b  arranged side by side at an interval therebetween in the direction of conveyance of the sheet P and connected with each other by the tube  105   c  are fixed to contact an inner circumference of the cooling belt  45 . The cooling plates  1   a  and  1   b  contact the inner circumference of the cooling belt  45  rotated by the drive roller  61  to absorb heat, via the cooling belt  45 , from the sheet P conveyed by the cooling belt  45  and the conveyance belt  46 . 
     The inlet  70   b  from which the liquid coolant enters the cooling plate  1   b  is provided at the downstream end on the lateral surface of the cooling plate  1   b . The outlet  71   b  from which the liquid coolant is discharged from the cooling plate  1   b  is provided at the upstream end on the lateral surface of the cooling plate  1   b . The inlet  70   b  and outlet  71   b  of the cooling plate  1   b  are connected to the respective ends of the serpentine internal channel  73   b  formed within the cooling plate  1   b  in the width direction of the sheet P. One end of the tube  105   a  is connected to the pump  100 , and the other end thereof is connected to the inlet  70   b . One end of the tube  105   c  is connected to the outlet  71   b.    
     The inlet  70   a  from which the liquid coolant enters the cooling plate  1   a  is provided at the downstream end on the lateral surface of the cooling plate  1   a . The outlet  71   a  from which the liquid coolant is discharged from the cooling plate  1   a  is provided at the upstream end on the lateral surface of the cooling plate  1   a . The inlet  70   a  and outlet  71   a  of the cooling plate  1   a  are connected to the respective ends of the serpentine internal channel  73   a  formed within the cooling plate  1   a  in the width direction of the sheet P. One end of the tube  105   c  is connected to the outlet  71   b  of the cooling plate  1   b , and the other end thereof is connected to the inlet  70   a  of the cooling plate  1   a . One end of the tube  105   b  is connected to the radiator  103 , and the other end thereof is connected to the outlet  71   a  of the cooling plate  1   a.    
     The liquid coolant enters the cooling plate  1   b  via the tube  105   a  and is discharged from the cooling plate  1   b  to the tube  105   c . Then, the liquid coolant thus discharged from the cooling plate  1   b  enters the cooling plate  1   a  via the tube  105   c  and is discharged from the cooling plate  1   a  to the tube  105   b.    
     Multiple pressing rollers  26 , each contacting an inner circumference of the conveyance belt  46  to press the conveyance belt  46  against the cooling plates  1   a  and  1   b , are provided inside the loop of the conveyance belt  46 . Accordingly, an outer circumference of the cooling belt  45  more reliably contacts the sheet P and the cooling plates  1   a  and  1   b  more reliably contact the inner circumference of the cooling belt  45 . Further, the cooling belt  45  and the conveyance belt  46  more securely convey the sheet P. 
     The sheet P sandwiched and conveyed by the cooling belt  45  and the conveyance belt  46  is cooled by the cooling plates  1   a  and  1   b  via a thermal transmission member, which, in the present illustrative embodiment, is the cooling belt  45 . As a result, the sheet P does not slide against the cooling plates  1   a  and  1   b , thereby preventing blots or blurs on the sheet P caused by sliding against the cooling plates  1   a  and  1   b.    
     In a manner similar to the first illustrative embodiment, in the second illustrative embodiment the liquid coolant flows through the two separate cooling plates  1   b  and  1   a  from downstream to upstream, that is, the liquid coolant flows from the cooling plate  1   b  to the cooling plate  1   a , so as to cool the sheet P by the cooling plates  1   a  and  1   b  using the liquid coolant. As a result, the downstream end of the cooling plate  1   b  which cools the sheet P in the last stage of cooling operation has a lower temperature, thereby efficiently cooling the sheet P. In addition, as described previously in the first illustrative embodiment, use of the two separate cooling plates  1   a  and  1   b  arranged side by side at an interval therebetween can more effectively cool the sheet P compared to the case in which the single cooling plate  1  is used. 
       FIG. 9A  is a side view illustrating the configuration around the cooling plates  1   a  and  1   b  in the cooling device  18  according to the second illustrative embodiment, and  FIG. 9B  is a graph showing temperature distribution corresponding to the configuration illustrated in  FIG. 9A . 
     While the sheet P having a higher temperature heated by the fixing device  16  is conveyed by the cooling belt  45  and the conveyance belt  46 , the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b  slidably contact the inner circumference of the cooling belt  45  and absorb heat from the sheet P via the cooling belt  45 . 
     At this time, temperature distribution occurs in both the cooling plates  1   a  and  1   b . A fine solid line T 11  in  FIG. 9B  indicates temperature distribution in a target surface of the sheet P to be cooled, that is, an upper surface of the sheet P. A bold solid line T 1   a  indicates temperature distribution in the heat-absorbing surface  11   a  (the lower surface) of the cooling plate  1   a , and the bold solid line T 1   b  indicates temperature distribution in the heat-absorbing surface  11   b  (the lower surface) of the cooling plate  1   b.    
     A fine broken line T 11 ′ indicates temperature distribution in the target surface of the sheet P in a case of a comparative example in which the cooling plates  1   a  and  1   b  are arranged side by side to contact each other without an interval therebetween. A bold broken line T 1  indicates temperature distribution in the heat-absorbing surfaces (the lower surfaces)  11   a  and  11   b  of the cooling plates  1   a  and  1   b  in the case of the comparative example. 
     As described previously in the first illustrative embodiment, thermal transmission between the cooling plates  1   a  and  1   b  does not occur when the cooling plates  1   a  and  1   b  are disposed in upstream and downstream sides within the cooling device  18  in the direction of conveyance of the sheet P, respectively, with an interval therebetween. Therefore, compared to the case of the comparative example, the upstream cooling plate  1   a  has a higher temperature and the downstream cooling plate  1   b  has a lower temperature in the second illustrative embodiment. 
     The temperature of the downstream end of the cooling plate  1   b  considerably affects the temperature of the sheet P discharged from the cooling device  18 . Therefore, the cooling plate  1   b  having a lower temperature can more effectively cool the sheet P even if the temperature of the cooling plate  1   a  is somewhat higher. 
     After the sheet P passes the cooling plate  1   a , the temperature of the sheet P is increased by heat retained by the sheet P while the sheet P passes through the interval between the cooling plates  1   a  and  1   b  because the sheet P is not cooled in that interval. The higher the temperature of the sheet P, the cooling members such as the cooling plates  1   a  and  1   b  more easily absorb heat from the sheet P. Therefore, the temperature increase in the sheet P at the interval between the cooling plates  1   a  and  1   b  is advantageous for the cooling device  18  to cool the sheet P. 
     Thus, the sheet P is more effectively cooled by the cooling plates  1   a  and  1   b  disposed at an interval therebetween compared to the case in which the cooling plates  1   a  and  1   b  are disposed to contact with each other without an interval therebetween. 
     It is preferable that the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b  be disposed on the same level with a difference in height of not greater than 100 μm. 
       FIG. 10  is a vertical cross-sectional view illustrating an example of a configuration of the cooling device  18  according to the second illustrative embodiment in a case in which the cooling plates  1   a  and  1   b  are not appropriately disposed but instead are vertically offset from each other. When the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b  are disposed with a difference in height and do not together form a single flush surface as illustrated in  FIG. 10 , a gap is generated between the cooling belt  45  and the cooling plate  1   a  or  1   b . In the example illustrated in  FIG. 10 , there is a gap between the cooling belt  45  and the downstream portion of the cooling plate  1   a . Consequently, the sheet P cannot be cooled by the cooling plate  1   a  at that portion where the gap exists. In addition, a step between the cooling plates  1   a  and  1   b  causes large loads on the cooling belt  45 , resulting in rapid deterioration of the cooling belt  45 . 
       FIGS. 11A and 11B  are perspective views illustrating an example of a configuration of positioning members  102   a  and  102   b  provided to the cooling device  18 . Specifically,  FIG. 11A  is a perspective view illustrating a state in which the cooling plates  1   a  and  1   b  are not yet placed on the positioning members  102   a  and  102   b , and  FIG. 11B  is a perspective view illustrating a state in which the cooling plates  1   a  and  1   b  are placed on the positioning members  102   a  and  102   b.    
     Both the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b  are placed on the same surface of each of the positioning members  102   a  and  102   b  so as to dispose the heat-absorbing surfaces  11   a  and  11   b  at substantially the same height. 
     Each of the positioning members  102   a  and  102   b  has an L-shape in cross-section and includes a positioning surface  121   a  or  121   b  on which the cooling plates  1   a  and  1   b  are placed. As illustrated in  FIG. 11B , both the positioning surfaces  121   a  and  121   b  are positioned outside the both edges of the cooling belt  45  in a width direction of the cooling belt  45 . 
     Alternatively, although only the positioning member  102   b  is shown as a representative example in  FIG. 12 , each of the positioning surfaces  121   a  and  121   b  of the positioning member  102   a  and  102   b  may have cutouts, as long as a desired flatness is obtained at a contact surface in which the positioning surface  121   a  or  121   b  contacts the cooling plates  1   a  and  1   b . As a result, the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b  are disposed on the same level with a difference in height of not greater than 100 μm. 
     As described above, in the second illustrative embodiment, the sheet P is sandwiched and conveyed by the cooling belt  45  and the conveyance belt  46 , each of which is wound around the multiple rollers. The cooling plates  1   a  and  1   b  are arranged side by side at an interval therebetween in the direction of conveyance of the sheet P to slidably contact the inner circumference of the cooling belt  45 . Alternatively, the cooling plates  1   a  and  1   b  may be disposed to contact the inner circumferences of the cooling belt  45  and the conveyance belt  46 , respectively. Such a configuration is described in detail later in a third illustrative embodiment. 
     A description is now given of a first variation of the second illustrative embodiment.  FIG. 13  is a vertical cross-sectional view illustrating an example of a configuration of the cooling device  18  according to the first variation of the second illustrative embodiment. 
     As illustrated in  FIG. 13 , each of the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b  are convexly curved. Accordingly, the heat-absorbing surfaces  11   a  and  11   b  more evenly contact the inner circumference of the cooling belt  45 . 
     The cooling plates  1   a  and  1   b  have the same shape, and each of the heat-absorbing surfaces  11   a  and  11   b  has an even curvature radius. Thus, the heat-absorbing surfaces  11   a  and  11   b  can more easily be disposed to together form a single flat stepless plane, and such a configuration can be easily achieved even when number of cooling members is increased to three, four, and so on. 
     In addition to the driven rollers  65  and  66 , driven rollers  67  and  68  are provided so that the conveyance belt  46  is wound around the four rollers  65 ,  66 ,  67 , and  68 . Thus, both the cooling belt  45  and the conveyance belt  46  more evenly contact the sheet P. As a result, the cooling device  18  can be more effectively cool the sheet P. 
     The following problems occur when the cooling plates  1   a  and  1   b  are not optimally arranged inside the loop of the cooling belt  45  and the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b  do not together form a single flat plane. In a manner similar to the example illustrated in  FIG. 10 , a gap is generated between the cooling belt  45  and the cooling plate  1   a  or  1   b  around the interval between the cooling plates  1   a  and  1   b . In the example illustrated in  FIG. 14 , there is a gap between the cooling belt  45  and the downstream portion of the cooling plate  1   a . Because the cooling plate  1   a  does not contact the cooling belt  45  at the downstream portion where the gap exists, the sheet P cannot be cooled at that portion. In addition, a step between the cooling plates  1   a  and  1   b  causes large loads on the cooling belt  45 , resulting in rapid deterioration of the cooling belt  45 . 
     To solve the above problems, the cooling device  18  according to the first variation of the second illustrative embodiment includes the positioning members  102   a  and  102   b  as illustrated in  FIG. 15 . The positioning members  102   a  and  102   b  have the positioning surfaces  121   a  and  121   b , respectively, each of which has the same curvature as the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b . The cooling plates  1   a  and  1   b  are placed on the positioning surfaces  121   a  and  121   b  of the positioning members  102   a  and  102   b . As a result, the cooling plates  1   a  and  1   b  are appropriately disposed such that the heat-absorbing surfaces  11   a  and  11   b  together form a single curved stepless plane. 
     Alternatively, each of the curved positioning surfaces  121   a  and  121   b  may have cutouts in a manner similar to the example illustrated in  FIG. 12  as long as a desired outline is obtained at a contact surface in which the positioning surface  121   a  or  121   b  contacts the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b . Further alternatively, the positioning member  102   a  may be detachably installed in the cooling device  18  as illustrated in  FIG. 16  such that the positioning member  102   a  is detached from the cooling device  18  upon replacement of the cooling belt  45 , thereby facilitating attachment and detachment of the cooling belt  45  to and from the cooling device  18 . In the example illustrated in  FIG. 16 , each of the positioning member  102   a  and the cooling belt  45  is detached from the cooling device  18  in a direction indicated by arrows, that is, a direction opposite a drive motor  8  in an axial direction of a drive roller  8   a.    
     A description is now given of a second variation of the second illustrative embodiment with reference to  FIG. 17 .  FIG. 17  is a schematic view illustrating how to fix the cooling plates  1   a  and  1   b  to the cooling device  18 . 
     As described previously, when the cooling plates  1   a  and  1   b  are not appropriately positioned inside the loop of the cooling belt  45 , there may be a gap between the cooling belt  45  and the cooling plate  1   a  or  1   b . Consequently, the sheet P cannot be effectively cooled by the cooling plate  1   a  or  1   b  and the cooling belt  45  may be damaged. 
     To solve the above problems, in the second variation of the second illustrative embodiment, the cooling plates  1   a  and  1   b  are fixed to the cooling device  18  without the positioning members  102   a  and  102   b.    
     Specifically, each of the cooling plates  1   a  and  1   b  has a fastening point at each corner thereof into which an adjustment member, that is, a fastening screw  106 , is inserted to fix the cooling plates  1   a  and  1   b  to the cooling device  18 . The adjustment member can adjust a position and an angle of each of the cooling plates  1   a  and  1   b . A fastening depth of each of the screws  106  is adjusted at each fastening point such that a height and an angle of each of the cooling plates  1   a  and  1   b  relative to the cooling device  18  can be finely adjusted. As a result, the heat-absorbing surfaces  11   a  and  11   b  of the cooling plates  1   a  and  1   b  together form a single curved stepless plane. 
     A description is now given of a third illustrative embodiment of the present invention with reference to  FIG. 18 .  FIG. 18  is a vertical cross-sectional view illustrating an example of a configuration of the cooling device  18  according to the third illustrative embodiment. In the third illustrative embodiment, the cooling plates  1   a  and  1   b  are disposed vertically one above the other. 
     As illustrated in  FIG. 18 , the cooling belt  45  is rotatably wound around the drive roller  61  and the multiple driven rollers  62 ,  63 , and  64 . In addition, the conveyance belt  46  is rotatably wound around the drive roller  67  and the multiple driven rollers  65 ,  66 , and  68 . The cooling plate  1   a  is provided opposite the cooling plate  1   b  with both the cooling belt  45  and the conveyance belt  46  interposed therebetween so that both upper and lower surfaces of the sheet P can be cooled by the cooling plates  1   b  and  1   a , respectively, at the same time. 
     As a result, the sheet P heated by the fixing device  16  can be more efficiently cooled by the cooling plates  1   a  and  1   b  from both the upper and lower surfaces of the sheet P, thereby achieving good cooling efficiency in a shorter cooling path. 
       FIG. 19  is a schematic view illustrating an example of a flow of the liquid coolant in the cooling plates  1   a  and  1   b  provided to the cooling device  18  illustrated in  FIG. 18 . 
     The inlet  70   b  from which the liquid coolant enters the cooling plate  1   b  is provided at the downstream end on the lateral surface of the cooling plate  1   b  provided above the cooling plate  1   a . The outlet  71   b  from which the liquid coolant is discharged from the cooling plate  1   b  is provided at the upstream end on the lateral surface of the cooling plate  1   b . The inlet  70   b  and outlet  71   b  of the cooling plate  1   b  are connected to the respective ends of the serpentine internal channel  73   b  formed within the cooling plate  1   b  in the width direction of the sheet P. One end of the tube  105   a  is connected to the pump  100 , and the other end thereof is connected to the inlet  70   b . One end of the tube  105   c  is connected to the outlet  71   b.    
     The inlet  70   a  from which the liquid coolant enters the cooling plate  1   a  is provided at the downstream end on the lateral surface of the cooling plate  1   a  provided below the cooling plate  1   b . The outlet  71   a  from which the liquid coolant is discharged from the cooling plate  1   a  is provided at the upstream end on the lateral surface of the cooling plate  1   a . The inlet  70   a  and outlet  71   a  of the cooling plate  1   a  are connected to the respective ends of the serpentine internal channel  73   a  formed within the cooling plate  1   a  in the width direction of the sheet P. One end of the tube  105   c  is connected to the outlet  71   b  of the cooling plate  1   b , and the other end thereof is connected to the inlet  70   a  of the cooling plate  1   a . One end of the tube  105   b  is connected to the radiator  103 , and the other end thereof is connected to the outlet  71   a  of the cooling plate  1   a.    
     The liquid coolant enters the cooling plate  1   b  from the inlet  70   b  provided at the downstream end of the cooling plate  1   b , flows through the cooling plate  1   b  through the internal channel  73   b , and is then discharged from the cooling plate  1   b  via the outlet  71   b  provided at the upstream end of the cooling plate  1   b  to the tube  105   c . The liquid coolant thus discharged to the tube  105   c  then enters the cooling plate  1   a , which is provided below the cooling plate  1   b , from the inlet  70   a  provided at the downstream end of the cooling plate  1   a  and connected to the tube  105   c , flows through the cooling plate  1   a  through the internal channel  73   a , and is discharged from the cooling plate  1   a  via the outlet  71   a  provided at the upstream end of the cooling plate  1   a  to the tube  105   b . Thus, the liquid coolant sequentially flows through the cooling plates  1   b  and  1   a.    
     As illustrated in  FIG. 19 , when an image is formed only on an upper surface of the sheet P, a toner image T is fixed to the upper surface of the sheet P by the pair of fixing rollers  116 . Therefore, the liquid coolant having a lower temperature first flows through the cooling plate  1   b  which faces the upper surface of the sheet P having the fixed toner image T thereon. As a result, the temperature of the cooling plate  1   b  can be kept lower, thereby more efficiently cooling the toner image T formed on the upper surface of the sheet P. 
     In addition, because the sheet P is cooled by the cooling plates  1   a  and  1   b  from both the upper and lower surfaces thereof, an amount of heat absorbed from the sheet P by each of the cooling plates  1   a  and  1   b  at the upstream portions thereof is reduced compared to the case in which both the cooling plates  1   a  and  1   b  are disposed side by side on the single side of the sheet P, that is, either above or below the conveyance path of the sheet P. As a result, an amount of heat transmitted from upstream to downstream in each of the cooling plates  1   a  and  1   b  is also reduced, thereby preventing a temperature increase in the downstream end of each of the cooling plates  1   a  and  1   b . Accordingly, a temperature increase in the liquid coolant flowing at the downstream end of each of the cooling plates  1   a  and  1   b , which cools the sheet P in the last stage of cooling operation, can be prevented, thereby efficiently cooling the sheet P even at the downstream end of each of the cooling plates  1   a  and  1   b.    
     A description is now given of a first variation of the third illustrative embodiment.  FIG. 20  is a schematic view illustrating an example of a configuration of the cooling device  18  according to the first variation of the third illustrative embodiment. In the cooling device  18  illustrated in  FIG. 20 , the two separate cooling plates  1   a  and  1   b  arranged side by side at an interval therebetween in the direction of conveyance of the sheet P and connected with each other by a tube  105   c   1  are fixed to contact the inner circumference of the cooling belt  45 . In addition, a second pair of cooling plates  1   a ′ and  1   b ′ arranged side by side at an interval therebetween in the direction of conveyance of the sheet P and connected with each other by a tube  105   c   3  are fixed to contact an inner circumference of the conveyance belt  46 . 
     The liquid coolant first flows through the cooling plates  1   b  and  1   a  provided above the second pair of cooling plates  1   b ′ and  1   a ′, and then flows through the cooling plates  1   b ′ and  1   a′.    
     Specifically, as illustrated in  FIG. 21 , the liquid coolant enters the cooling plate  1   b  from the inlet  70   b  provided at the downstream end on the lateral surface of the cooling plate  1   b , flows through the cooling plate  1   b  through the internal channel  73   b , and then is discharged to the tube  105   c   1  from the cooling plate  1   b  via the outlet  71   b  provided at the upstream end on the lateral surface of the cooling plate  1   b . Next, the liquid coolant discharged to the tube  105   c   1  enters the cooling plate  1   a  from the inlet  70   a  provided at the downstream end on the lateral surface of the cooling plate  1   a , flows through the cooling plate  1   a  through the internal channel  73   a , and is then discharged to a tube  105   c   2  from the cooling plate  1   a  via the outlet  71   a  provided at the upstream end on the lateral surface of the cooling plate  1   a.    
     Subsequently, the liquid coolant discharged to the tube  105   c   2  enters the cooling plate  1   b ′ from an inlet  70   b ′ provided at a downstream end on a lateral surface of the cooling plate  1   b ′, flows through the cooling plate  1   b ′ through an internal channel  73   b ′, and is then discharged to the tube  105   c   3  from the cooling plate  1   b ′ via an outlet  71   b ′ provided at an upstream end on the lateral surface of the cooling plate  1   b ′. Thereafter, the liquid coolant discharged to the tube  105   c   3  enters the cooling plate  1   a ′ from an inlet  70   a ′ provided at a downstream end on a lateral surface of the cooling plate  1   a ′, flows through the cooling plate  1   a ′ through an internal channel  73   a ′, and is then discharged to the tube  105   b  from the cooling plate  1   a ′ via an outlet  71   a ′ provided at an upstream end on the lateral surface of the cooling plate  1   a′.    
     Thus, the liquid coolant having a lower temperature first flows through the cooling plates  1   b  and  1   a , each of which faces the upper surface of the sheet P having the fixed toner image T thereon. As a result, the cooling plates  1   a ,  1   b ,  1   a ′ and  1   b ′ can efficiently absorb heat from both the upper and lower surfaces the sheet P to effectively cool the sheet P. In addition, the temperature of each of the cooling plates  1   a  and  1   b  provided above the cooling plates  1   a ′ and  1   b ′ can be kept lower, thereby more efficiently cooling the toner image T formed on the upper surface of the sheet P. 
     Further, thermal transmission from the cooling plate  1   a  or  1   a ′, each of which is provided upstream from the cooling plate  1   b  or  1   b ′, to the cooling plate  1   b  or  1   b ′ can be prevented. Accordingly, a temperature increase in the downstream end of the cooling plate  1   b  or  1   b ′ can be prevented. As a result, a temperature increase in the liquid coolant flowing through the downstream end of each of the cooling plates  1   b  and  1   b ′, which cools the sheet P in the last stage of cooling operation, can be prevented, thereby efficiently and effectively cooling the sheet P even at the downstream end of each of the cooling plates  1   b  and  1   b′.    
     A description is now given of a second variation of the third illustrative embodiment.  FIG. 22  is a schematic view illustrating an example of a flow of the liquid coolant in the cooling device  18  according to the second variation of the third illustrative embodiment. 
     In the cooling plate  1   b  provided above the cooling plate  1   a , multiple internal channels  73   b   1 ,  73   b   2 ,  73   b   3 , and  73   b   4  are provided, in that order, from downstream to upstream in the direction of conveyance of the sheet P. Each of the internal channels  73   b   1 ,  73   b   2 ,  73   b   3 , and  73   b   4  passes through the cooling plate  1   b  in the width direction of the sheet P perpendicular to the direction of conveyance of the sheet P. One end of each of the internal channels  73   b   1 ,  73   b   2 ,  73   b   3 , and  73   b   4  is connected to inlets  70   b   1 ,  70   b   2 ,  70   b   3 , and  70   b   4 , respectively, and the other end of each of the internal channels  73   b   1 ,  73   b   2 ,  73   b   3 , and  73   b   4  is connected to outlets  71   b   1 ,  71   b   2 ,  71   b   3 , and  71   b   4 , respectively. 
     In a manner similar to the cooling plate  1   b , in the cooling plate  1   a  provided below the cooling plate  1   b , multiple internal channels  73   a   1 ,  73   a   2 ,  73   a   3 , and  73   a   4  are provided, in that order, from downstream to upstream in the direction of conveyance of the sheet P, and each of the internal channels  73   a   1 ,  73   a   2 ,  73   a   3 , and  73   a   4  passes through the cooling plate  1   a  in the width direction of the sheet P. One end of each of the internal channels  73   a   1 ,  73   a   2 ,  73   a   3 , and  73   a   4  is connected to inlets  70   a   1 ,  70   a   2 ,  70   a   3 , and  70   a   4 , respectively, and the other end of each of the internal channels  73   a   1 ,  73   a   2 ,  73   a   3 , and  73   a   4  is connected to outlets  71   a   1 ,  71   a   2 ,  71   a   3 , and  71   a   4 , respectively. 
     One end of the tube  105   a  is connected to the pump  100 , and the other end thereof is connected to the inlet  70   b   1 . The outlet  71   b   1  and the inlet  70   a   1  are connected to the respective ends of the tube  105   c   1 , and the outlet  71   a   1  and the inlet  70   b   2  are connected to the respective ends of the tube  105   c   2 . The outlet  71   b   2  and the inlet  70   a   2  are connected to the respective ends of the tube  105   c   3 , and the outlet  71   a   2  and the inlet  70   b   3  are connected to the respective ends of a tube  105   c   4 . The outlet  71   b   3  and the inlet  70   a   3  are connected to the respective ends of a tube  105   c   5 , and the outlet  71   a   3  and the inlet  70   b   4  are connected to the respective ends of a tube  105   c   6 . The outlet  71   b   4  and the inlet  70   a   4  are connected to the respective ends of a tube  105   c   7 . One end of the tube  105   b  is connected to the radiator  103 , and the other end thereof is connected to the outlet  71   a   4 . 
     The liquid coolant enters the cooling plate  1   b  from the inlet  70   b   1  provided at the extreme downstream side on the lateral surface of the cooling plate  1   b , alternately flows between the cooling plates  1   b  and  1   a  in a spiral manner, and is ultimately discharged from the cooling plate  1   a  via the outlet  71   a   4  provided at the extreme upstream side on the lateral surface of the cooling plate  1   a.    
     As a result, the temperature of each of the cooling plates  1   a  and  1   b  is further reduced at the downstream end of each of the cooling plates  1   a  and  1   b , and a difference in temperature between the cooling plates  1   a  and  1   b  can be reduced, thereby evenly cooling both the upper and lower surfaces of the sheet P. 
     In addition, because the sheet P is cooled by the cooling plates  1   a  and  1   b  from both the upper and lower surfaces thereof, an amount of heat absorbed from the sheet P by each of the cooling plates  1   a  and  1   b  at the upstream portions thereof is reduced compared to the case in which both the cooling plates  1   a  and  1   b  are disposed side by side on the single side of the sheet P, that is, either above or below the conveyance path of the sheet P. As a result, an amount of heat transmitted from upstream to downstream in each of the cooling plates  1   a  and  1   b  is also reduced, thereby preventing a temperature increase in the downstream end of each of the cooling plates  1   a  and  1   b . Accordingly, a temperature increase in the liquid coolant flowing through the downstream end of each of the cooling plates  1   a  and  1   b , which cools the sheet P in the last stage of cooling operation, can be prevented, thereby efficiently and effectively cooling the sheet P even at the downstream end of each of the cooling plates  1   a  and  1   b.    
     A description is now given of a third variation of the third illustrative embodiment.  FIG. 23  is a vertical cross-sectional view illustrating an example of a configuration of the cooling device  18  according to the third variation of the third illustrative embodiment. 
     In the cooling device  18  illustrated in  FIG. 23 , the two separate cooling plates  1   b  and  1   b ′ arranged side by side at an interval therebetween in the direction of conveyance of the sheet P are fixed to contact the inner circumference of the cooling belt  45 . The cooling plate  1   b  is provided downstream from the cooling plate  1   b ′. In addition, the two separate cooling plates  1   a  and  1   a ′ arranged side by side at an interval therebetween in the direction of conveyance of the sheet P are fixed to contact the inner circumference of the conveyance belt  46  provided below the cooling belt  45 . The cooling plate  1   a  is provided downstream from the cooling plate  1   a′.    
     As illustrated in  FIG. 24 , the liquid coolant enters the cooling plate  1   b  through the tube  105   a  connected to the downstream end on the lateral surface of the cooling plate  1   b , and alternately flows between the cooling plates  1   b  and  1   a  in a spiral manner through the tubes  105   c   1  to  105   c   7  from downstream to upstream. Next, the liquid coolant discharged from the cooling plate  1   a  is conveyed to the cooling plate  1   b ′ via a tube  105   c   8 , one end of which is connected to the upstream end on the lateral surface of the cooling plate  1   a  and the other end of which is connected to the downstream end on the lateral surface of the cooling plate  1   b ′. Thereafter, the liquid coolant alternately flows between the cooling plates  1   b ′ and  1   a ′ in a spiral manner through tubes  105   c   9  to  105   c   15  from downstream to upstream, and is ultimately discharged from the cooling plate  1   a ′ to the tube  105   b  connected to the upstream end on the lateral surface of the cooling plate  1   a′.    
     As a result, the temperature of each of the cooling plates  1   a  and  1   b  is further reduced at the downstream end of each of the cooling plates  1   a  and  1   b . In addition, a difference in temperature between each of the cooling plates  1   a  and  1   b  and the cooling plates  1   a ′ and  1   b ′ can be reduced, thereby evenly cooling both the upper and lower surfaces of the sheet P. 
     Further, thermal transmission from the cooling plate  1   b ′ or  1   a ′ provided upstream from the cooling plate  1   b  or  1   a  to the cooling plate  1   b  or  1   a  can be prevented. Accordingly, a temperature increase in the downstream end of the cooling plate  1   a  or  1   b  can be prevented. As a result, a temperature increase in the liquid coolant flowing through the downstream end of each of the cooling plates  1   a  and  1   b , which cools the sheet P in the last stage of cooling operation, can be prevented, thereby efficiently and effectively cooling the sheet P even at the downstream end of each of the cooling plates  1   a  and  1   b.    
     Elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
     Illustrative embodiments being thus described, it will be apparent that the same may be varied in many ways. Such exemplary variations are not to be regarded as a departure from the scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 
     The number of constituent elements and their locations, shapes, and so forth are not limited to any of the structure for performing the methodology illustrated in the drawings.