Patent Publication Number: US-9850087-B2

Title: Sheet processing apparatus and image forming system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet processing apparatus and an image forming system. 
     2. Description of Related Art 
     In an electrophotographic image forming apparatus, when an image forming process includes a step of heating paper sheets, such as a fixing step, water evaporates from the heated sheets and causes an uneven distribution on the moisture content on the sheet, which may cause curling of the sheet such as waving. 
     A sheet processing apparatus is known that humidifies a sheet with an image formed thereon and passes the sheet through waving uneven parts to correct the curling of the sheet (For example, Japanese Patent Laid-Open Publication No. 2010-1089). 
       FIG. 9  illustrates an example of such a sheet processing apparatus  500  in this structure. 
     The sheet processing apparatus  500  includes, for example, a receiver  510  receiving a sheet from an image forming apparatus (not shown), a humidifier  520  humidifying the sheet, a decurler  530  having uneven parts  10 ,  20 ,  30  applying a pressing force to the sheet being conveyed, an ejector  540  discharging the sheet to the exterior of the apparatus, a first conveyor  550  conveying the sheet from the humidifier  520  to the decurler  530 , and a second conveyor  560  conveying the sheet from the decurler  530  to the ejector  540 . 
     However, such a sheet processing apparatus has a conveying path having a certain length between the humidifier  520  and the decurler  530 . Some of a proper amount of water added to the sheet at the humidifier  520  evaporates from the sheet passing through the conveying path. As a result, the moisture content of the sheet may become improper at the decurler  530 , which may preclude a desired decurling performance. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above problem of the conventional technology. An object of the present invention is to provide a sheet processing apparatus and an image forming system that achieves a stable decurling performance. 
     To achieve the object described above, according to an aspect of the present invention, there is provided a sheet processing apparatus, including: 
     a pair of belts defining a conveying path for holding and conveying a paper sheet; 
     a water supplying unit which supplies water to outer surfaces of the belts; 
     a curving unit which curves the conveying path defined by the belts; and 
     a controller which makes the water supplying unit supply water to the outer surfaces of the belts, and makes the belts convey the sheet between the belts in a state where the conveying path is curved by the curving unit to correct the curling of the sheet. 
     According to another aspect of the present invention, there is provided an image forming system, including: 
     an image forming apparatus which forms an image on a paper sheet; and 
     the sheet processing apparatus conveying the sheet having the image thereon which is formed by the image forming apparatus and thereby correcting the curling of the sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be fully understood with reference to the detailed description below and the accompanying drawings. It should be noted that the detailed description and accompanying drawings are not intended to limit the present invention, and wherein: 
         FIG. 1  is a schematic diagram illustrating the entire structure of an image forming system; 
         FIG. 2  is a block diagram illustrating the main structure for controlling operation of the image forming system; 
         FIG. 3  illustrates the main structure of a first sheet processing apparatus; 
         FIG. 4  is an enlarged diagram of an essential part illustrating the structure of a humidifying decurler; 
         FIG. 5A  is a schematic diagram explaining the operation of the humidifying decurler; 
         FIG. 5B  is a schematic diagram explaining the operation of the humidifying decurler; 
         FIG. 6  is a table for determining sheet conditions; 
         FIG. 7  is a flow chart illustrating a humidifying and decurling process performed by the humidifying decurler; 
         FIG. 8  explains the state of a humidified and decurled sheet; and 
         FIG. 9  explains a conventional sheet processing apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. It should be noted that the scope of the invention is not limited to the examples illustrated by the drawings. 
     [Image Forming System] 
       FIG. 1  is a schematic diagram illustrating the entire structure of an image forming system  1  of one embodiment.  FIG. 2  is a block diagram illustrating the main structure for controlling operation of the image forming system  1 . 
     As shown in  FIGS. 1 and 2 , the image forming system  1  includes an image forming apparatus  100 , a first sheet processing apparatus  200 , and a second sheet processing apparatus  300 . 
     The image forming apparatus  100  forms an image on a sheet P. 
     The first sheet processing apparatus  200  performs a humidifying and decurling process to humidify the sheet P on which an image has been formed at the image forming apparatus  100 , and to correct (decurl) the curling of the sheet P. 
     The second sheet processing apparatus  300  performs a punching process, a binding process, or a folding process on the sheets P on which images have been formed at the image forming apparatus  100 . 
     In the following description, the direction X indicates the coupling direction of the image forming apparatus  100 , the first sheet processing apparatus  200 , and the second sheet processing apparatus  300 , the direction Z indicates the vertical direction, and the direction Y indicates the direction perpendicular to the directions X and Z. 
     [Image Forming Apparatus} 
     The image forming apparatus  100  will be described first. 
     As shown in  FIGS. 1 and 2 , the image forming apparatus  100  includes a reader  110 , a sheet tray  120 , a sheet conveyor  130 , an image forming unit  140 , a fixing unit  150 , a controller  160 , a memory  170 , an operation displaying unit  180 , and a communication unit  190 . 
     The reader  110  optically reads a document and generates image data. 
     Specifically, the reader  110  includes a sensing device  111  which has an image sensor such as a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor, and outputs electric signals corresponding to the read image, a generator (not shown) generating image data based on the electric signals output from the sensing device  111 , a platen  113  having a transparent board-shaped member on which a document can be placed, the transparent board-shaped member being disposed on a side where the sensing device  111  reads a document, and an auto document feeder (ADF)  114  conveying a document such that the document is moved with respect to the sensing device  111 . The sensing device  111  reads the document placed on the platen  113 . The sensing device  111  also reads the document conveyed by the auto document feeder  114 . 
     The sheet tray  120  is a box-shaped member storing a stack of sheets P. The sheet tray  120  can be pulled out of the housing of the image forming apparatus  100  or pushed back into the housing. The sheet tray  120  stores sheets P to be processed for forming images thereon by the image forming apparatus  100 . As shown in  FIG. 1 , the image forming apparatus  100  includes a plurality of sheet trays  120 , each of which stores sheets P of a different size. These sheet trays  120  may store sheets P of the same size. 
     The sheet conveyor  130  conveys a sheet P among sections such as the sheet tray  120 , the image forming unit  140  and the fixing unit  150 . 
     Specifically, the sheet conveyor  130  includes a plurality of rollers disposed at intervals along the sheet conveying path disposed between or in the vicinity of sections such as the sheet tray  120 , the image forming unit  140 , and the fixing unit  150 , and driving units (not shown) driving these rollers. The sheet conveyor  130  draws sheets P stored in the sheet tray  120  one by one, and conveys them to the image forming unit  140 . After the image forming unit  140  forms an image on the sheet P, the sheet conveyor  130  conveys the sheet P with the image formed thereon to the fixing unit  150 . After the fixing unit  150  fixes the image onto the sheet P, the sheet conveyor  130  conveys the sheet P with the image fixed thereon to an ejector disposed on the side of the first sheet processing apparatus  200 . The sheet P is then discharged from the ejector. In printing on both sides of a sheet P, the sheet conveyor  130  conveys the sheet P to a turning point  131  disposed on the conveying path between the fixing unit  150  and the ejector. At the turning point  131 , the sheet P is flipped over. The sheet conveyor  130  then conveys the sheet P back to the image forming unit  140 . 
     The image forming unit  140  forms an image on a sheet P. 
     Specifically, the image forming unit  140  includes a photoconductor  141  the outer surface of which comes into contact with a sheet P being conveyed by the sheet conveyor  130 , a charging unit  142  electrifying the photoconductor  141 , an exposing unit  143  exposing the electrified photoconductor  141  to light based on the image data, a developing unit  144  forming a toner image on the photoconductor  141  corresponding to the image data (a primary transfer), a transfer unit  145  transferring the toner image on the photoconductor  141  to the sheet P (a secondary transfer), and a cleaner  146  removing the toner remaining on the photoconductor  141 . The image forming unit  140  of the present embodiment has such a structure for forming an electrophotographic image. This is merely an example and not limitative. The image forming unit  140  may have any other structure for forming an image in any other image forming mechanism such as ink-jet. 
     The image forming unit  140  shown in  FIG. 1  includes the single developing unit  144  and forms a monochromatic image. This is merely an example and not limitative. The image forming unit  140  may include developing units  144  for different colors such as cyan (C), magenta (M), yellow (Y), and black (K), and form a multicolor image by combining these colors. 
     The fixing unit  150  fixes an image formed on a sheet P by the image forming unit  140 . 
     Specifically, the fixing unit  150  includes a pair of fixing rollers  151 ,  151  the outer surfaces of which are in contact with each other so as to sandwich the conveying path for a sheet P, and a heater  152  heating one of the fixing rollers  151 ,  151 , which is on the image side of the sheet P which is formed by the image forming unit  140  (the upper side in  FIG. 1 , for example). In the fixing unit  150 , the rotating fixing rollers  151 ,  151  hold and convey a sheet P while the roller heated by the heater  152  heats the image side of the sheet P. As a result, the toner image transferred on the sheet P is fixed on the sheet P. 
     The controller  160  performs an overall control of the individual apparatuses of the image forming system  1  and the individual units of each of these apparatuses in response to instructions inputted via the operation displaying unit  180  or the communication unit  190 . The controller  160  includes a central processing unit (CPU) and a random access memory (RAM). The CPU reads the programs stored in the memory  170 , expands them in a work area in the RAM, and performs various processes in cooperation with the programs. 
     For example, the controller  160  controls the image forming apparatus  100  to form an image on a sheet P based on image data sent from the reader  110 . 
     The controller  160  also controls the first sheet processing apparatus  200  to humidify and decurl the sheet P based on preset sheet conditions. 
     The controller  160  also controls the second sheet processing apparatus  300  to perform a predetermined process on the sheet P based on the preset setting conditions. 
     Although, the image forming apparatus  100  includes the controller  160  which performs the overall control of the image forming system  1  in the present embodiment, each of the image forming apparatus  100 , the first sheet processing apparatus  200 , and the second sheet processing apparatus  300  may include a controller controlling the corresponding apparatus and communicating with the other apparatuses. 
     The memory  170  stores programs to be executed by the controller  160  and data necessary for execution of the programs. 
     For example, the memory  170  stores sheet conditions for controlling a humidifying and decurling process performed by the humidifying decurler  230  of the first sheet processing apparatus  200 . When a user sets sheet conditions via the operation displaying unit  180 , the memory  170  stores the set sheet conditions. The sheet conditions will be described in detail later. 
     In addition, the memory  170  stores settings on sheet processing at the second sheet processing apparatus  300 . 
     The operation displaying unit  180  displays various types of information on the operation of the image forming system  1 . The operation displaying unit  180  includes a touch panel detecting various input operations by a user in response to displayed content, and a plurality of switches for various input operations by the user. The controller  160  performs a process for controlling displayed contents on the touch panel of the operation displaying unit  180 . The controller  160  also controls the operation of the image forming system  1  based on various input operations by the user via the operation displaying unit  180 . 
     The operation displaying unit  180  is used, for example, for inputting sheet conditions on a humidifying and decurling process performed by the humidifying decurler  230  of the first sheet processing apparatus  200 . 
     The communication unit  190  includes a network interface card (NIC) or an equivalent structure, and establishes communication between the image forming system  1  and other computers. When a print job including image data is input via the communication unit  190 , the controller  160  forms an image based on the print job. 
     After the image forming apparatus  100  ejects a sheet P, the sheet P is conveyed to the first sheet processing apparatus  200 . 
     [First Sheet Processing Apparatus] 
     The first sheet processing apparatus  200  will be described next. 
       FIG. 3  illustrates the main structure of the first sheet processing apparatus  200 . 
     As shown in  FIGS. 2 and 3 , the first sheet processing apparatus  200  includes a receiver  210 , a first conveyor  220 , a humidifying decurler  230 , a second conveyor  240 , and an ejector  250 . 
     In the following description, the direction X has a right side and a left side, and the direction Z has an upper side and a lower side. The right side of the first sheet processing apparatus  200  is adjacent to the image forming apparatus  100 . The left side of the first sheet processing apparatus  200  is adjacent to the second sheet processing apparatus  300 . The upper side of the first sheet processing apparatus  200  is on the side of the ejector  250 . The lower side of the first sheet processing apparatus  200  is on the side of the humidifying decurler  230 . 
     The receiver  210  receives a sheet P discharged from the image forming apparatus  100 , and feeds the sheet P to the interior of the first sheet processing apparatus  200 . 
     The first conveyor  220  conveys the sheet P fed into the first sheet processing apparatus  200 , through the receiver  210 , to the humidifying decurler  230  disposed below the receiver  210 . The first conveyor  220  includes a branch point Q 1  at which the conveying path is branched to introduce the sheet P to the second conveyor  240 . 
       FIG. 4  is an enlarged diagram of an essential part illustrating the structure of the humidifying decurler  230 . 
     The humidifying decurler  230  is a mechanism that humidifies the sheet P conveyed by the first conveyor  220  and corrects (decurls) the curling of the sheet P. 
     The humidifying decurler  230  includes a pair of humidifying belts  231 ,  231 , pressing rollers  232 ,  232  pressing and curving the humidifying belts  231 ,  231 , water supplying rollers  233 ,  233  supplying water to the humidifying belts  231 ,  231 , storages  234 ,  234  storing water to be supplied to the water supplying rollers  233 ,  233 , a tank  235  storing water, and fans  236 ,  236  blowing air on a sheet P downstream of the humidifying belts  231 ,  231 . 
     Each of the humidifying belts  231 ,  231  is stretched by two rollers  231   a ,  231   a  that are separated in a predetermined distance. 
     The right and left pairs of rollers  231   a ,  231   a  are rotated by a first driver  231   b  (see  FIG. 2 ) in the opposite directions to each other as shown by the arrows A in  FIG. 4 . The rotation of the pairs of rollers  231   a ,  231   a  allows the humidifying belts  231 ,  231  to hold and convey a sheet P. 
     The humidifying belts  231 ,  231  are composed of hydrophilic elastic material, such as rubber. The humidifying belts  231 ,  231  can retain water on their outer surfaces and are elastically deformable. 
     The rotating humidifying belts  231 ,  231  retaining water hold the sheet P and convey it downstream while humidifying the sheet P. 
     Each of the pressing rollers  232 ,  232  (a curving unit) is disposed in the middle of the inside of the corresponding humidifying belt  231 . 
     Each of the pressing rollers  232  is provided with a connector bar  232   b  coupled to the pressing roller  232  at one end and to an eccentric cam  232   a  at the other end. 
     The eccentric cam  232   a  is rotated clockwise or counterclockwise by a predetermined angle with a second driver  232   c  (see  FIG. 2 ). The rotation of the eccentric cam  232   a  moves the connector bar  232   b , which allows the pressing roller  232  to press the middle of the humidifying belt  231  and curve the humidifying belt  231  rightward or leftward. 
       FIGS. 5A and 5B  illustrate the humidifying belts  231 ,  231  being pressed and curved by the pressing rollers  232 ,  232 . 
       FIG. 5A  illustrates the humidifying belts  231 ,  231 , which are bent or protrude to the right, and  FIG. 5B  illustrates the humidifying belts  231 ,  231 , which are bent or protrude to the left. 
     As shown in  FIG. 5A , the clockwise rotation of the eccentric cam  232   a  moves the connector bar  232   b  rightward, which deforms the conveying path defined by the humidifying belts  231 ,  231  from a liner conveying path H 1  ( FIG. 4 ) to a rightward curved conveying path H 2  ( FIG. 5A ). 
     As shown in  FIG. 5B , the couterclockwise rotation of the eccentric cam  232   a  moves the connector bar  232   b  leftward, which deforms the conveying path defined by the humidifying belts  231 ,  231  from the liner conveying path H 1  ( FIG. 4 ) to a leftward curved conveying path H 3  ( FIG. 5B ). 
     The conveying path H 1  ( FIG. 4 ) is a conveying path for the case where the correction of the curling of a sheet P is not necessary. The conveying path H 2  ( FIG. 5A ) is a conveying path for the correction of the leftward curling of a sheet P. The conveying path H 3  ( FIG. 5B ) is a conveying path for the correction of the rightward curling of a sheet P. 
     Through the conveying path H 1 , the conveying path H 2 , or the conveying path H 3 , a sheet P is conveyed downstream. 
     The water supplying rollers  233 ,  233  (a water supplying unit) are provided for the respective humidifying belts  231 ,  231 . 
     The water supplying rollers  233 ,  233  are rotated in the opposite directions to each other by a fourth driver  233   b  (see  FIG. 2 ) as shown by the arrows B in  FIG. 4 . Each of the water supplying rollers  233 ,  233  rotates while being in contact with the outer surface of the humidifying belt  231  and water stored in the storage  234 . The water supplying roller  233  thus can retain water from the storage  234  on its outer surface to supply water to the humidifying belt  231 . 
     The water supplying rollers  233 ,  233  can be brought into or out of contact with the humidifying belts  231 ,  231  by a third driver  233   a  (see  FIG. 2 ). This allows the water supplying rollers  233 ,  233  to adjust the pressing force applied to the humidifying belts  231 ,  231 , thereby adjusting the amount of water supplied to the humidifying belts  231 ,  231 . 
     The storage  234  has a convex bottom surface that conforms to the outer surface of the water supplying roller  233 , and stores water on the convex bottom surface. Part of the outer surface of the water supplying roller  233  is kept below the surface of water stored in the storage  234  and is out of contact with the convex bottom surface of the storage  234 . This allows the rotating water supplying roller  233  to soak in the water stored in the storage  234  and retain the water on its outer surface. The storage  234  is coupled to the tank  235  below with a delivering path  234   a.    
     The tank  235  is a container storing water. Water in the tank  235  is delivered by a pump (not shown) via the delivering path  234   a  to the storages  234 . This allows the storages  234  to store a proper volume of water. 
     The fans  236 ,  236  (a drying unit) are disposed on both sides of the sheet conveying path downstream of the humidifying belts  231 ,  231 . The fans  236 ,  236  blow air against both sides of a sheet P immediately after the release from the humidifying belts  231 ,  231  to dry the sheet P. 
     The fans  236 ,  236  dry the sheet P conveyed in a humidified state. The dried sheet P will thus not be curled again even if the sheet P passes through a curved conveying path downstream of the humidifying belts  231 ,  231  (and the fans  236 ,  236 ). 
     In the present embodiment, the humidifying decurler  230  described above humidifies a sheet P and corrects (decurls) the curling of the sheet P under the control of the controller  160  based on the sheet conditions stored in the memory  170 . The humidifying and decurling process will be described in detail later. 
     Referring back to  FIG. 3 , the second conveyor  240  conveys the sheet P fed to the interior of the first sheet processing apparatus  200  through the receiver  210 , bypassing the humidifying decurler  230 , downstream of the humidifying decurler  230 . In other words, the second conveyor  240  conveys the sheet P conveyed via the branch point Q 1  of the first conveyor  220 , bypassing the humidifying decurler  230 , to a junction point Q 2  disposed downstream of the humidifying decurler  230 . 
     The ejector  250  discharges the sheet P conveyed from the humidifying decurler  230  or the second conveyor  240 , to the second sheet processing apparatus  300  disposed on the outside of the first sheet processing apparatus  200 . 
     The first sheet processing apparatus  200  may include any other structure such as a cooling and conveying path cooling a sheet P other than the structures of the units described above. 
     The humidifying and decurling process performed by the humidifying decurler  230  of the first sheet processing apparatus  200  will be described now. 
       FIG. 6  is an example of a setting table for determining sheet conditions. 
     As shown in  FIG. 6 , a setting table T 1  includes the sheet conditions on the types of sheets P (thin, plain, thick, and coated) and their coverage rates (small, medium, and large), and the amounts of water per unit area to be added to the sheets P based on the combinations of the sheet types and the coverage rates. In the setting table T 1  of  FIG. 6 , the coverage rate indicates the rate of the printed area to the area on both sides of a sheet. The same amount of water is set so as to be added to both sides of the sheet P. The setting table T 1  does not include the amounts of water for sheets not subject to the humidifying and decurling process. 
     As shown in  FIG. 6 , a sheet with a larger coverage rate requires a larger amount of water if the sheet type is the same. At a larger coverage rate, toner on the sheet surface precludes the precise correction of the curling of the sheet. Therefore, a larger amount of water for humidifying is set for a sheet with a larger coverage rate to satisfactorily correct the curling of the sheet. In addition, a thicker sheet (a sheet with a larger basis weight) requires a larger amount of water to be added to the sheet as a whole. A thicker sheet with a larger basis weight has a higher rigidity, which precludes the precise correction of the curling of the sheet. Therefore, a larger amount of water for humidifying is set for a thicker sheet with a larger basis weight to satisfactorily correct the curling of the sheet. The amounts of water shown in  FIG. 6  are merely examples and may be changed appropriately changeable. 
       FIG. 7  is a flow chart illustrating the humidifying and decurling process performed by the humidifying decurler  230 . As a premise, the sheet P is conveyed at a constant speed. 
       FIG. 8  illustrates an example of change of a sheet P due to the humidifying and decurling process.  FIG. 8  illustrates the sheet P the front side of which is printed and curled outward. 
     The controller  160  obtains the sheet conditions (the type of the sheet P and its coverage rate) included in the job information (Step S 11 ). 
     The controller  160  then refers to the setting table T 1  ( FIG. 6 ) stored in the memory  170  and determines whether the type of sheet P included in the obtained sheet conditions falls under the types of sheet for which the humidifying and decurling process is performed, and thereby determining whether the humidifying and decurling process should be performed or not (Step S 12 ). 
     If the controller  160  determines that the humidifying and decurling process does not need to be performed (Step S 12 : NO), the controller  160  ends the process. 
     If the controller  160  determines that the humidifying and decurling process needs to be performed (Step S 12 : YES), the controller  160  determines the amount of water to be supplied to the sheet P (Step S 13 ). 
     Specifically, the controller  160  refers to the setting table T 1  ( FIG. 6 ) stored in the memory  170  and determines the amount of water to be added to both sides of the sheet P based on the sheet type and coverage rate in the obtained sheet conditions. 
     Based on the determined water amount, the pressing force of the water supplying rollers  233 ,  233  to be applied to the humidifying belts  231 ,  231  is determined. Specifically, if the amount of water to be added to the sheet is large, the pressing force of the water supplying roller  233 ,  233  to be applied to the humidifying belts  231 ,  231  is determined to be high. The detailed relationships between the amounts of water to be added to the sheet and the pressing forces of the water supplying rollers  233 ,  233  to be applied to the humidifying belts  231 ,  231  are determined by experiments or simulations. 
     The controller  160  then controls the third driver  233   a  to drive the water supplying rollers  233 ,  233  such that the water supplying rollers  233 ,  233  come into contact with the humidifying belts  231 ,  231  under the pressing force which supplies the amount of water determined at Step S 13  (Step S 14 ). 
     The controller  160  then determines the curving direction of the humidifying belts  231 ,  231  based on the sheet conditions obtained at Step S 11  (Step S 15 ). 
     Specifically, the controller  160  determines the curving direction of the humidifying belts  231 ,  231  such that the conveying path is curved in the direction that corrects the curling of the sheet P (the direction opposite to the curling of the sheet P). For example, the memory  170  may store a table (not shown) on the sheet types and the directions in which the sheets tend to be curled, and the controller  160  may refer to the table to determine the curving direction of the humidifying belts  231 ,  231 , which is the direction opposite to the curling of the sheet. 
     In the example shown in  FIG. 8 , the conveying path H 2  which is convex rightward is selected. 
     The controller  160  then controls the second driver  232   c  to rotate the eccentric cam  232   a . The rotation of the eccentric cam  232   a  moves the connector bar  232   b , which allows the pressing rollers  232 ,  232  to press and curve the humidifying belts  231 ,  231  in the direction determined at Step S 15 . As a result, the conveying path H 2  or the conveying path H 3  is defined (Step S 16 ). 
     In the example shown in  FIG. 8 , the conveying path H 2  which is convex rightward is defined. 
     The controller  160  then controls the first driver  231   b  to rotate the right and left pairs of rollers  231   a ,  231   a , which allows the humidifying belts  231 ,  231  to rotate while holding and conveying the sheet P (Step S 17 ). 
     As shown in  FIG. 8 , the humidifying belts  231 ,  231  hold and convey the sheet P while correcting the curling of the sheet P. 
     The controller  160  then controls the fans  236 ,  236  to blow air on the sheet P after a predetermined time, that is, when the advancing front end of the sheet P is released from the humidifying belts  231 ,  231  (Step S 18 ). 
     The controller  160  then determines whether the humidifying and decurling process should be finished or not (Step S 19 ). If the process should not be finished (Step S 19 : NO), Step S 19  will be repeated. 
     If the process should be finished (Step S 19 : YES), the controller  160  controls the humidifying decurler  230  to stop the operations of their parts and restore them to the original positions (Step S 20 ) to finish the process. 
     In the humidifying and decurling process described above, the curling of the sheet P is corrected immediately after the sheet P is humidified. The generally flattened sheet without the curling is conveyed downstream as shown in  FIG. 8 . 
     [Second Sheet Processing Apparatus] 
     Referring back to  FIG. 1 , the second sheet processing apparatus  300  includes a first conveying path  310 , a second conveying path  320 , a guide  330 , and ejector trays  340 A,  340 B. The first conveying path  310  includes a puncher  311 , for example. The second conveying path  320  includes a stacker  321 , an aligner  322 , a binder  323 , and a folder  324 . 
     The first conveying path  310  is called a straight path and includes a plurality of rollers, and drivers (not shown) driving the rollers to convey a sheet P. 
     The puncher  311  includes a punch rod and performs a punching process on a sheet P conveyed from the first sheet processing apparatus  200 . 
     The second conveying path  320  is called a stack path and includes a plurality of rollers, and drivers (not shown) driving the rollers to convey a sheet P as in the first conveying path  310 . 
     The stacker  321  receives sheets P from the second conveying path  320  and stores the stack of sheets P temporarily. 
     The aligner  322  is a pair of aligning members aligning sheets in the width direction by jogging the longitudinal edges of the sheets. The aligning members are movably disposed on both sides of the stacker  321 . 
     The binder  323  includes a stapler and performs a binding process with staples on the stack of sheets P on the stacker  321 . 
     The folder  324  performs a folding process on the sheets P on the stacker  321 . The stack of folded sheets is discharged to the ejector tray  340 B by a conveyor belt (not shown). 
     The guide  330  guides the conveying direction of a sheet and switches the conveying direction between the first conveying path  310  and the second conveying path  320 . 
     The ejector tray  340 A receives the stack of sheets discharged from the first conveying path  310 . The ejector tray  340 B receives the stack of sheets discharged from the second conveying path  320 . 
     The structure of the second sheet processing apparatus  300  is not limited to the above structure. The second sheet processing apparatus  300  may have any other structure such as a structure for cutting sheets, for example. 
     As described above, the present embodiment includes the pair of humidifying belts  231 ,  231  defining the conveying path along which a sheet P is held and conveyed, the water supplying rollers  233 ,  233  supplying water to the outer surfaces of the humidifying belts  231 ,  231 , the pressing rollers  232 ,  232  pressing and curving the conveying path H 1  defined by the humidifying belts  231 ,  231 , and the controller  160  controlling the water supplying rollers  233 ,  233  to supply water to the outer surfaces of the humidifying belts  231 ,  231 , and controlling the humidifying belts  231 ,  231  to correct the curling of the sheet P by conveying the sheet P between the humidifying belts  231 ,  231  while the conveying path H 1  is curved by the pressing rollers  232 ,  232 . 
     This allows a proper amount of water to be added to the sheet P for the correction of the curling of the sheet P, which achieves a stable decurling performance. 
     In the humidifying and decurling process performed by the humidifying belts  231 ,  231 , a stable holding pressure is applied to the sheet P, which keeps the amount of water to be added to the sheet P constant. 
     Compared with the structure of the conventional apparatus in which the humidifier and the decurler are separated, the present embodiment enables the compact structure of the apparatus that saves space. 
     In the present embodiment, the controller  160  determines the amount of water to be supplied to the sheet P based on the sheet conditions including the type of the sheet P and its coverage rate. 
     This allows a proper amount of water to be supplied to the sheet P on a sheet-by-sheet basis, which achieves a stable decurling performance. 
     In the present embodiment, the water supplying rollers  233 ,  233  are provided in contact with the respective humidifying belts  231 ,  231 . The controller  160  adjusts the pressing force of the water supplying rollers  233 ,  233  applied to the humidifying belts  231 ,  231 , thereby controlling the amount of water to be supplied to the humidifying belts  231 ,  231 . 
     This enables the adjustment of the amount of water to be supplied to the humidifying belts  231 ,  231  without any complicate structure for adjusting the water amount. 
     In the present embodiment, the fans  236 ,  236  are disposed downstream of and in the vicinity of the humidifying belts  231 ,  231  to remove the moisture of the sheet P. 
     This allows the sheet P to be dried immediately after the release of the sheet P from the humidifying belts  231 ,  231 , which prevents the sheet P from being curled again due to the curve of the conveying path downstream. 
     In the above embodiment, the controller  160  determines the same amount of water to be added to both sides of the sheet P at Step S 13 . However, the different amounts of water may be determined for the front side of the sheet P and the back side of the sheet P. In other words, the controller  160  may determine different amounts of water respectively for the front side and back side of the sheet, and controls the water supplying rollers  233 ,  233  to supply the determined amounts of water to the outer surfaces of the humidifying belts  231 ,  231  which correspond to the front side and back side of the sheet, respectively. 
     This allows proper amounts of water to be supplied respectively to the front side and back side of the sheet P, which achieves a stable decurling performance. 
     For example, the controller  160  may determine the amounts of water to be supplied respectively to the front side and back side of the sheet based on the coverage rates of the front side and back side of the sheet. More specifically, if the sheet has a larger coverage rate on the front side than that of the back side, the water amount for the front side of the sheet is determined to be larger than that for the back side of the sheet. 
     This allows proper amounts of water to be supplied respectively to the front side and back side of the sheet P based on the coverage rates of the front side and back side of the sheet P, which achieves a stable decurling performance. 
     In the above embodiment, the controller  160  determines the curving direction of the humidifying belts  231 ,  231  at Step S 15  based on the sheet conditions obtained at Step S 11 . The controller  160  may determine the curving direction of the humidifying belts  231 ,  231  based on the result detected by a detector detecting the curling of the sheet P. The detector may include a plurality of sensors aligned across the conveying path defining the first conveyor  220  and in the width direction of the sheet P. The sensors face the sheet surface for detecting the distances to the sheet surface. This structure enables the detection of the curling of the sheet P based on the distances to the sheet surface detected by the sensors. Then, the controller  160  determines the curving direction of the humidifying belts  231 ,  231  to define the conveying path curved in the direction that corrects the curling of the sheet P (the direction opposite to the curling of the sheet P). 
     In the above embodiment, the humidifying belts  231 ,  231  define three conveying paths H 1  to H 3 . The humidifying belts  231 ,  231  may define more than three conveying paths by adjusting the pressing force of the pressing rollers  232 ,  232  at more than three levels. 
     In the above embodiment, the humidifying belts  231 ,  231  define a fixed conveying path while holding and conveying the sheet P at Step S 17 . The humidifying belts  231 ,  231  may deform the conveying path while holding and conveying the sheet P. In other words, the controller  160  may control the second driver  232   c  to curve the humidifying belts  231 ,  231  to deform the conveying path while the humidifying belts  231 ,  231  are holding and conveying the sheet P. 
     In the above embodiment, the drying unit includes the fans  236 ,  236 . The drying unit may include any other device for drying the sheet P such as a heater heating and drying the sheet P, for example. 
     The entire disclosure of Japanese Patent Application No. 2015-003122 filed on Jan. 9, 2015 is incorporated herein by reference in its entirety.