Abstract:
A sheet post-processing apparatus of the present invention comprises: a sheet discharge section that feeds a sheet supplied from an image forming apparatus to a post-processing section and leads the sheet that has passed through the post-processing section to a sheet discharge port; a sheet discharge tray that receives a sheet discharged from the sheet discharge port and can move to a first standby position and a second standby position higher than the first standby position and nearer to the sheet discharge port; and a controller that controls the height position of the sheet discharge tray by identifying the type of the sheet. The controller changes the height position of the sheet discharge tray to allow the sheet discharge tray to receive the discharged sheet at the first standby position in the case where the sheet is a first type while to receive the discharged sheet at the second standby position in the case where the sheet is a second type.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 11/616,453 filed on Dec. 27, 2006, the entire contents of which are incorporated herein by reference. 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-380565 filed on Dec. 29, 2005, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet post-processing apparatus and a sheet post-processing method that perform post-processing for a sheet discharged from an image forming apparatus such as a copier, a printer, or a composite device. 
     2. Description of the Related Art 
     In recent years, there is developed a sheet post-processing apparatus which is disposed adjacent to the sheet discharge section of an image forming apparatus main body for the purpose of performing post-processing, such as sorting and stapling, for a sheet on which an image has been formed in the image forming apparatus. For example, in a post-processing apparatus that performs stapling processing, a plurality of sheets (sheet bundle) are aligned by an alignment means and stapled and, after that, fed to a sheet discharge tray to sequentially be loaded thereonto. 
     Such a sheet post-processing apparatus performs post-processing for a succeeding sheet after completion of the post-processing for a preceding sheet. Although the stapled sheet bundle is discharged onto the sheet discharge tray, the sheet alignment performance may deteriorate or the sheet may be folded at the time of discharge in some cases depending on the size, quality, stiffness or thickness of the sheet to be processed. 
     Jpn. Pat. Appln. Laid-open Publication No. 2004-155551 discloses a sheet discharge apparatus. The sheet discharge apparatus in this disclosure is featured in the configuration of a sheet discharge table, in which ingenuity has been applied thereto to prevent the discharged sheets from being disturbed. However, this disclosure does not cope with the deterioration of the sheet alignment performance due to a difference in the sheet type. 
     An object of the present invention is to provide a sheet post-processing apparatus and a sheet-post processing method capable of preventing the sheets discharged onto a sheet discharge tray from being disturbed to thereby enhancing the sheet alignment performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a sheet post-processing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a top view of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 3  is a view schematically showing a configuration of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 4  is a perspective view showing a stapler of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 5  is a perspective view showing vertical alignment rollers of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 6  is an explanatory view showing a paddle mechanism of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 7  is a perspective view schematically showing a standby tray and a processing tray of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 8  is a top view of the standby tray and processing tray of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 9  is a perspective view schematically showing horizontal alignment plates and a conveyer belt of the sheet post-processing apparatus according to the embodiment of the present invention: 
         FIG. 10  is a block diagram showing a control system of the sheet post-processing apparatus according to the embodiment of the present invention; 
         FIG. 11  is a flowchart explaining the operation of a sheet discharge section in the sheet post-processing apparatus according to the embodiment of the present invention: 
         FIGS. 12A to 12C  are explanatory views each showing the operation of the sheet discharge section in the sheet post-processing apparatus according to the embodiment of the present invention; and 
         FIGS. 13A to 13C  are explanatory views each showing the operation of the sheet discharge section in the sheet post-processing apparatus according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus of the present invention. 
     Hereinafter, an embodiment of the present invention will be described in detail. In the following description, the same reference numerals denote the same parts through the drawings, and the overlapped description is omitted. 
       FIG. 1  is a perspective view showing the main part of a sheet post-processing apparatus according to an embodiment of the present invention.  FIG. 2  is a top view of the main part of the sheet post-processing apparatus according to the embodiment of the present invention.  FIG. 3  is a view schematically showing a configuration of the sheet post-processing apparatus according to the embodiment of the present invention.  FIGS. 4 to 9  are views each showing a configuration of each component of the sheet post-processing apparatus. 
     Concrete configurations and operations of respective components shown in  FIGS. 1 and 2  will be described using  FIG. 4  and subsequent drawings afterward. Firstly, processing for a sheet in the sheet post-processing apparatus will be described using mainly  FIG. 3 . 
     A sheet P on which an image has been formed in an image forming apparatus  5  such as copier is fed by a pair of sheet discharge rollers  6  to a sheet post-processing apparatus  7 . As shown in  FIG. 3 , the sheet post-processing apparatus  7  has a standby tray  10 , a processing tray  12 , a stapler  14 , a sheet discharge trays  16  and  18 , a gate G, and the like. 
     The sheet P fed by the pair of sheet discharge rollers  6  of the image forming apparatus  5  is received by a pair of entrance rollers  22  provided near the entrance of the sheet post-processing apparatus  7 . The entrance rollers  22  have an upper roller  22   a  and lower roller  22   b  and are driven by a motor  26  (refer to  FIG. 1 ). 
     A gate G for dividing the feeding path of the received sheet P into two feeding paths is provided on the downstream side of the entrance rollers  22 . The gate G has a wedge-shaped cross section and its sharpened end is directed to the entrance rollers  22  side. The gate G is pivotably fitted to the side wall portion of the sheet post-processing apparatus  7 , so that the sharpened end can be positioned at both first and second positions respectively pointing the upper entrance roller  22   a  and lower entrance roller  22   b . The first position is selected in the case where post-processing needs to be applied to the sheet P, while the second position is selected in the case where post-processing need not be applied to the sheet P. 
     When the gate G is positioned at the first position, the sheet P is fed by first sheet feed rollers  24  to the standby tray  10 . Between the entrance rollers  22  and standby tray  10 , a paper path ceiling  36  for leading the sheet P to the sheet feed rollers  24  is installed. The sheet feed rollers  24  have an upper sheet feed roller and lower sheet feed roller. The standby tray  10  temporarily receives and loads the sheet P until post-processing applied to a plurality of sheets P on the processing tray  12  has ended. 
     Under the standby tray  10 , a processing tray  12  for loading the sheets P dropped and supplied from the standby tray  10  is arranged. The processing tray  12 , while the sheets P are stapled by the stapler  14  which is a post-processing mechanism for performing post-processing, supports the loaded sheets P in an aligned state. 
     As shown in  FIG. 7 , when a predetermined number of sheets are stored on the standby tray  10 , tray members  10   a  and  10   b  are opened in the directions of the arrow n and arrow m, respectively by a motor  34  (refer to  FIG. 1 ). Then, the sheet P is dropped onto the processing tray  12  by its own weight and supplied to the stapler  14 . 
     As shown in  FIG. 4 , the stapler  14  is slid and positioned by a staple drive section  49  in u direction for performing stapling processing. The sheet post-processing apparatus  7  has only one stapler  14 , and  FIG. 4  shows the states before and after the slide operation of the stapler  14 . In order to align a plurality of sheets P dropped and supplied from the standby tray  10  in the vertical direction which is the sheet feeding direction, the processing tray  12  has a pair of upper and lower vertical alignment rollers  38   a  and  38   b , as shown in  FIGS. 5 and 6 . 
     The vertical alignment rollers  38   a  and  38   b  serve also as bundle conveyer rollers that hold and take out stapled sheet bundle T from the stapler  14 . The vertical alignment roller  38   a  is driven by a motor  40  and vertical alignment roller  38   b  is driven by a motor  42 . At the position where the rear end of the sheet P which is dropped and supplied onto the processing tray  12  is dropped, a paddle  44  is arranged. The paddle  44 , which is configured to be rotatable, aligns vertically the uppermost sheet P loaded on the processing tray  12 . 
     The paddle  44 , as shown in  FIG. 6 , has a receiving portion  44   a  which receives the rear end of the sheets P in the standby tray  10 , a beating portion  44   b  for beating down the sheets P on the processing tray  12 , and a feeding portion  44   c  for aligning the sheets P on the processing tray  12 . The puddle  44  is driven by a motor  46  (refer to  FIG. 5 ). The paddle  44  is composed of a rubber material and has elasticity. 
     At the end of the processing tray  12  on the side of the stapler  14 , a stopper  45  that is brought into contact with the rear end of each of the sheets P and thereby restricts the rear end position is arranged. Almost at the center of the processing tray  12 , a conveyor belt  50  is installed. The conveyer belt  50  feeds the sheet bundle T, which is stapled and taken out from the stapler  14  by the upper and lower vertical alignment rollers  38   a  and  38   b , up to first or second sheet discharge tray  16  or  18 . To the conveyer belt  50 , a feed pawl  50   a  for hooking the rear end of the sheet bundle T is attached. 
     The standby tray  10  can drop and supply the sheet P onto the processing tray  12  and further can be used to feed the sheet P to the first or second sheet discharge tray  16  or  18 . When the sheet P is to be fed to the sheet discharge tray  16  or  18 , rotating rollers  28  for aligning the sheet P is brought into contact with the sheet P on the standby tray  10 . The rotating rollers  28  are controlled by a standby tray roller drive source  30  in terms of their vertical movement and rotated by a motor  32  (refer to  FIG. 2 ). 
     As shown in  FIG. 3 , the standby tray  10 , to support the sheets P in a state that the front ends of the sheets P are positioned higher than the rear ends thereof, is arranged in a tilt angle θ 1 . The first or second sheet discharge tray  16  or  18  is moved up and down by a sheet discharge drive section  52  and either of them is selected. The first or second sheet discharge tray  16  or  18  is moved up and down up to almost the same height as that of the standby tray  10  or the processing tray  12  when loading the sheets P to improve the consistency of the sheets P to be discharged. Further, the first or second sheet discharge tray  16  or  18 , to support the sheets P in a state that the front ends of the sheets P are positioned higher than the rear ends thereof, is arranged in a tilt angle θ 2 . 
     As shown in  FIGS. 7 and 8 , the standby tray  10  has a pair of tray members  10   a  and  10   b . The tray members  10   a  and  10   b  receive the sheets P in a state that the members slide in the width of the sheet P and support both sides of the sheet P. On the tray members  10   a  and  10   b , standby stoppers  10   c  and  10   d  for restricting the rear ends of the sheets P are installed. 
     The standby tray  10  slides and moves by the motor  34  (refer to  FIG. 2 ). Between the standby tray  10  and the processing tray  12 , horizontal alignment plates  47   a  and  47   b  shown in  FIG. 9  are installed. When the sheets P are dropped and supplied from the standby tray  10  onto the processing tray  12 , the horizontal alignment plates  47   a  and  47   b  prevent the sheets P from turning away in the horizontal direction perpendicular to the feeding direction and horizontally align them. The horizontal alignment plates  47   a  and  47   b  are formed slidably in v direction so as to fit to the width of the sheet P by a motor  48  and can change the position where the sheet alignment is achieved. 
     The motors  26 ,  32 ,  34 ,  40 ,  42 ,  46 , and  48  that drive the abovementioned mechanisms and the drive sections  49  and  52  are controlled by a control circuit. 
     Next, the operation of the sheet post-processing apparatus  7  will be described in line with the flow of the sheet. Although the sheet to which post-processing has been applied can be discharged onto the first or second sheet discharge trays  16  or  18 , it is assumed hereinafter that the sheet is discharged onto the first sheet discharge tray  16  for the sake of simplification. 
     Firstly, a case where the post-processing is not to be performed will be described. When the sheet P on which an image has been formed in the image forming apparatus  5  is supplied by the sheet discharge roller  6 , the first sheet discharge tray  16  slides and moves to the position indicated by a dotted line shown in  FIG. 3  and can load the sheets P discharged from the standby tray  10  in a good alignment state. 
     In this case, the gate G is positioned at the first position, and the sheet P fed from the entrance rollers  22  through the paper path ceiling  36  is fed toward the standby tray  10  by the sheet feed rollers  24 . The sheet P is then dropped down onto the standby tray  10 , fed by the rotating rollers  28  rotated in the direction of the arrow f, and is discharged onto the sheet discharge tray  16 . 
     Next, a case where the post-processing (stapling processing) is to be performed and no preceding sheets P in execution of the staple processing remain on the processing tray  12  will be described. The gate G is positioned at the first position, and the standby tray  10  slides and moves the tray members  10   a  and  10   b  respectively in the directions of arrows m and n of  FIG. 8  to open the dropping and supplying path of the sheet P. The horizontal alignment plates  47   a  and  47   b , to align the sheet P dropping from the sheet feed rollers  24  in the horizontal direction, are arranged so that the gap between the horizontal alignment plates  47   a  and  47   b  is made almost equal to the width of the sheet P. By doing this, the sheet P fed by the sheet feed rollers  24 , without the feeding being obstructed by the standby tray  10 , is dropped and supplied directly onto the processing tray  12 . 
     At the time of dropping and supplying the sheet P, the upper vertical alignment roller  38   a  is shifted upward, and the receiving portion  44   a  of the paddle  44  receives the rear end of the sheet P. Both sides of the sheet P drop in contact with the horizontal alignment plates  47   a  and  47   b  and are aligned in the horizontal direction. Then, the paddle  44  rotates in the direction of the arrow o in  FIG. 6 , drops the rear end of the sheet P from the receiving portion  44   a , and beats down it onto the processing tray  12  by the beating portion  44   b . Furthermore, the paddle  44  feeds the sheet P in the direction of the arrow q by the feeding portion  44   c  and brings the rear end of the sheet P into contact with stopper  45  and thereby the vertical alignment of the sheet P is accomplished. 
     In this way, the sheet P on which an image has been formed is loaded directly on the processing tray  12  from the sheet feed rollers  24  while sequentially being aligned in the horizontal direction and vertical direction. When the sheets P reach a predetermined number, the stapler  14  staples the sheets P on the processing tray  12  at a desired position and bundles them to form the sheet bundle T. Thereafter, as shown in  FIG. 6 , the sheet bundle T is then held between the upper vertical alignment roller  38   a  rotated in the direction of the arrow r and the lower vertical alignment roller  38   b  rotated in the direction of the arrow s and is fed toward the sheet discharge tray  16 . 
     When the rear end of the sheet bundle T passes the upper and lower vertical alignment rollers  38   a  and  38   b , it is hooked by the feed pawl  50   a  of the conveyor belt  50  rotated in the direction of the arrow t in  FIG. 5  and is sent to the first sheet discharge tray  16 . Thereafter, the sheet bundle T is discharged onto the sheet discharge tray  16  by discharge rollers  38   c . At this time, the sheet discharge tray  16  has been slid from the position denoted by the dotted line in  FIG. 3  to the position denoted by the solid line. 
     The first sheet discharge tray  16  is arranged in a tilt angle θ 2  and therefore the front end of the sheet P is positioned higher than the rear end thereof, so that the sheet P is sequentially loaded on the sheet discharge tray  16  unless the order is disturbed. 
     Next, a case where the staple processing is to be performed and preceding sheets P in execution of the staple processing remain on the processing tray  12  will be described. At this time, the standby tray  10  slides and moves the tray members  10   a  and  10   b  respectively in the opposite direction of the direction of the arrow m and in the opposite direction of the direction of the arrow n in  FIG. 8 , and can support the sheet P. The rotating rollers  28  are shifted above the standby tray  10  so as not to disturb the sheets P. The sheets P discharged from the image forming apparatus  5  and fed by the sheet feed rollers  24  are loaded once on the standby tray  10  to wait for the processing tray  12  to be free. 
     The second and subsequent sheets P loaded on the standby tray  10  are fed to the standby stoppers  10   c  and  10   d  side by the rotating rollers  28  rotated in the opposite direction of the direction of the arrow f in  FIG. 3 . As a result, the sheets P are vertically aligned with the rear end of the sheets P brought into contact with the standby stoppers  10   c  and  10   d . Since the standby tray  10  is arranged in a tilt angle θ 1  and therefore the front end of the sheet P is positioned higher than the rear end thereof, the sheet P drops by its own weight down to the position where the rear end thereof is brought into contact with the standby stoppers  10   c  and  10   d  and the vertical alignment is accomplished. 
     During this period, when the preceding sheet P on the processing tray  12  is discharged on the side of the sheet discharge tray  16  and the processing tray  12  becomes free, the standby tray  10  slides and moves the tray members  10   a  and  10   b  respectively in the directions of the arrows m and n in  FIG. 8 . 
     By doing this, for example, two sheets P standing by on the standby tray  10  are dropped and supplied onto the processing tray  12  from between the tray members  10   a  and  10   b . Thereafter, the sheets P dropped from the standby tray  10  are controlled on both sides by the horizontal alignment plates  47   a  and  47   b  and are aligned horizontally. Then, as described in  FIG. 6 , the sheets P are fed in the direction of the arrow q by the vertical alignment roller  38   b , and the rear ends thereof are brought into contact with the stopper  45 , thereby accomplishing the vertical alignment of the sheets P. In the same manner as described above, the third and subsequent sheets P are directly dropped and supplied onto the processing tray  12  from between the tray members  10   a  and  10   b . Thereafter, the third and subsequent sheets P are sequentially aligned on the sheets P loaded earlier on the processing tray  12  by the paddle  44 . 
     When the sheets P loaded on the processing tray  12  reach a predetermined number, the sheets P are stapled by the stapler  14  to form a sheet bundle T. Thereafter, the sheet bundle T is fed toward the sheet discharge tray  16  by the upper and lower vertical alignment rollers  38   a  and  38   b . Further, the rear end of the sheet bundle T is hooked by the feed pawl  50   a  of the conveyor belt  50  and is sent to the sheet discharge tray  16 . Thereafter, the sheet bundle T is discharged onto the sheet discharge tray  16  by the discharge rollers  38   c.    
     The entire operation of the sheet post-processing apparatus  7  has been described. Next, a configuration of the sheet discharge section for discharging the sheet to the sheet discharge tray  16 , which is the feature of the present invention will be described. 
     As shown in  FIG. 3 , the sheet discharge section has a sensor  71  near a sheet discharge port  70  of the sheet post-processing apparatus  7 . The sensor  71  detects the upper most surface of the sheets discharged onto the sheet discharge tray  16 . In response to the detection result of the sensor  71 , the sheet discharge section controls the height position of the sheet discharge tray  16 . Further, the sheet discharge section detects an input operation made to an operation section  72  provided on the image forming apparatus  5  in order to determine the type of a sheet to be processed. 
       FIG. 10  is a block diagram showing a control system for controlling the sheet discharge section. In  FIG. 10 , a reference numeral  81  denotes a control circuit that controls the image forming apparatus  5 . The control circuit  81  is constituted by a microprocessor including, e.g., a CPU and controls respective sections for completing image forming processing in response to an input made to the operation section  72 . The operation section  72  has a selection key  73  for a user to select sheet type and sheet size when he or she performs printing. 
     A reference numeral  82  denotes a control circuit for controlling the sheet post-processing apparatus  7 . The control circuit  82  is constituted by a microprocessor including, e.g., a CPU and communicates with the control circuit  81  of the image forming apparatus  5  so as to allow operations of the image forming apparatus  5  and sheet post-processing apparatus  7  to work together. 
     The control circuit  82  further controls a drive motor  83  provided in a sheet discharge tray drive section  52  and a drive motor  84  that rotates the conveyer belt  50 . To the conveyer belt  50 , the feed pawl  50   a  for hooking the rear end of the sheet bundle T is attached. When the conveyer belt  50  is rotated, the feed pawl  50   a  feeds the sheet bundle T to the sheet discharge port  70 . To the control circuit  82 , a detection result from the sensor  71  is input. 
       FIG. 11  is a flowchart for explaining operation of the sheet discharge section performed under control of the control circuits  81  and  82 . In step S 1  of  FIG. 11 , when a user operates the operation key  73  to select sheet type for performing printing, the sheet type (sheet thickness in this case) is determined in step S 2 . It is assumed here that an A4 sheet having a normal thickness or A4 sheet having a reduced thickness is selected and that stapled sheet bundle is discharged. 
     In the case where the A4 sheet having a normal thickness is selected, the flow then advances to step S 3  where discharge operation of the sheet bundle T is started to rotate the conveyer belt  50 . Accordingly, the feed pawl  50   a  feeds the sheet bundle T to the discharge port  70 , and the sheet discharge operation is completed in step S 4 . At this time, the sheet discharge tray  16  is positioned at a previously set first height position H 1  for receiving the A4 sheet having a normal thickness and then receives the discharged sheet bundle T and stores it. 
     Subsequently, the sheet discharge tray  16  starts moving to go down once in step S 5  and then goes up in step S 6 . Then, in step S 7 , the sensor  71  detects the upper surface of the sheet bundle T discharged onto the sheet discharge tray  16 , and, in step S 8 , the sheet discharge tray  16  is moved down such that the upper surface of the sheet bundle T is positioned at the first height position H 1 . The sheet discharge tray  16  stands by at this position. This standby position is set as A. 
     In the case where the A4 sheet having a reduced thickness has been detected in step S 2 , the flow advances to step S 9 , where the sheet discharge tray  16  is moved up to a second height position H 2 . The second height position H 2  is higher than the first height position H 1  and positions immediately below the sensor  71 . 
     In step S 10 , discharge operation of the sheet bundle T is started to rotate the conveyer belt  50 . Accordingly, the feed pawl  50   a  starts feeding the sheet bundle T to the sheet discharge port  70 . After that, in step S 11 , the sheet discharge tray  16  is moved down so that the feed pawl  50   a  does not collide with the sheet discharge tray  16 . 
     Subsequently, in step S 12 , the sheet discharge tray  16  is moved up once again and receives the discharged sheet bundle T and stores. it. After the discharge operation of the sheet bundle T has been completed in step S 13 , the sensor  71  detects the upper surface of the sheet bundle T in step S 14 . Thus, in step S 15 , the sheet discharge tray  16  is stopped at the position where the upper position of the sheet bundle T is positioned at the second height position H 2  and stands by at this position. This standby position is set as B. 
     Although a case where the sheet bundle T is stored in the sheet discharge tray  16  has been described in this example, the same operation is applied also to a case where the sheet is discharged one by one. 
     The above operation of the sheet discharge section shown in the flowchart is shown in  FIGS. 12A to 12C  and  FIGS. 13A to 13C .  FIGS. 12A to 12C  show a case where the A4 sheet having a normal thickness is selected.  FIGS. 13A to 13C  show a case where the A4 sheet having a reduced thickness is selected. 
       FIG. 12A , which is the operation from step S 2  to step S 4 , shows a state immediately before the discharge operation of the sheet has been completed.  FIG. 12B , which is the operation from step S 5  to step S 7 , shows the operation up to the detection of the sheet upper surface made by the sensor  71 .  FIG. 12C , which is the operation of step S 8 , shows a state where the sheet discharge tray  16  stands by at the position where the uppermost surface of the discharged sheet corresponds to the first height position H 1 . This position is the first standby position. 
       FIG. 13A , which is the operation from step S 9  to step S 10 , shows a state where the feed pawl  50   a  is moved to start the sheet discharge operation.  FIG. 13B , which is the operation of step S 11 , shows a state where the sheet discharge tray  16  has been moved down.  FIG. 13C , which is the operation from step S 12  to step S 15 , shows a state where the upper surface of the sheet is detected by the sensor  71  and the sheet discharge tray  16  stands by at the position where the uppermost surface of the discharged sheet corresponds to the second height position H 2 . This position is the second standby position. 
     As shown in  FIG. 12C , the A4 sheet having a normal thickness is discharged from the sheet discharge port  70  with comparatively a large height difference onto the sheet discharge tray  16 . This positional relationship is a normal state, and the sheet discharge tray  16  is moved down as the thickness of the sheet to be loaded increases. The discharged sheets are sequentially loaded up to the height position H 1 . 
     As described above, the A4 sheet having a normal thickness has comparatively a large stiffness and therefore difficult to be curled. Since the sheet is difficult to be curled even if it is received at the first standby position H 1 , the sheets P loaded on the sheet discharge tray  16  are less disturbed. 
     Also in the case where a sheet having a larger size in the sheet discharge direction, such as an A4-R, A3, or LT-R size sheet is to be processed, the sheet may be received at the first standby position H 1 . Such a sheet has a large length and the front end of the discharged sheet reaches the surface of the sheet discharge tray  16  before it is curled (refer to  FIG. 12A ). Thus, the curl of the sheet hardly occurs. 
     On the other hand, as shown in  FIG. 13C , the A4 sheet having a reduced thickness is discharged from the sheet discharge port  70  with a small height difference onto the sheet discharge tray  16 . The reduced thickness sheet is easier to be curled at the front end thereof as compared to the sheet having a normal thickness. Therefore, the reduced thickness sheet becomes much easier to be curled in the case where it is received at the first height position H 1 . However, when the sheet is received at the second height position H 2  which means that the sheet is discharged in a near-linear state onto the sheet discharge tray  16 , it is possible to prevent the sheet from being curled. 
     The sheet curling characteristics are determined not only depending on the sheet thickness but also sheet quality. For example, an LT (letter) size sheet which is commonly used in United States is easy to be curled. Therefore, when the LT has been selected on the operation section  72 , it is determined that the sheet easy to be curled has been selected, and the sheet is received at the second height position H 2 , thereby reducing the curl of the sheet. Further, sheet alignment performance on the sheet discharge tray  16  can be enhanced. 
     Further, a small-size sheet, such as a B5 sheet is easy to be curled. Therefore, in the case where the small-size sheet is to be processed, the sheet should be received at the second standby position H 2 . 
     Although the sheet type is determined based on the operation made to the operation section  72  in the above embodiment, in the case where the image forming apparatus  5  and a PC (Personal Computer) are connected to each other through a network and the image forming apparatus  5  is used to print a document or the like created on the PC, a configuration may be adopted in which the specification state of the sheet from the PC is sent to the image forming apparatus  5  and, based on it, the sheet type is determined. 
     As described above, according to the present invention, the position of the sheet discharge tray is controlled depending on the sheet type when the sheet is discharged onto the sheet discharge tray and thereby it is possible to provide a sheet post-processing apparatus capable of preventing the sheet from being curled, preventing the sheets loaded on the sheet discharge tray from being disturbed, and achieving good sheet alignment performance. 
     The present invention is not limited to the above embodiment and various modifications are possible within the scope of the invention. For example, the post-processing is not limited to the staple processing, but may be other processing such as hole-punching processing in sheets. 
     Although an exemplary embodiment of the present invention has been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications, or alterations to the invention as described herein may be made, none of which depart from the spirit of the present invention. All such changes, modifications, and alternations should therefore be seen as within the scope of the present invention.