Abstract:
A sheet ejecting device for ejecting printed sheets to a stacker one by one, the sheet ejecting device including a pair of ejection rollers; a strengthener, located in a sheet ejection route, for providing a force for a sheet; a bender, arranged to be capable of advancing into and retreating from the sheet ejection route, for canceling out the force provided for the sheet by the strengthener; a driver for moving the bender to advance into and retreat from the sheet ejection route, a sensor for detecting a currently ejected sheet, and a controller for controlling the driver based on a signal sent from the sensor so that the bender can cancel out the force provided for the sheet by the strengthener.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application is a divisional of U.S. application Ser. No. 12/370,776 filed on Feb. 13, 2009 which claims the benefit of Japanese Application No. 2008-033681 filed on Feb. 14, 2008, the entire contents of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet ejecting device, and more particularly to a sheet ejecting device that is suited to be employed in an image forming apparatus such as an electrophotographic copying machine or a printer, or in an after-processing device such as a finisher connected to the image forming apparatus. 
     2. Description of Related Art 
     When a printed sheet is ejected from an image forming apparatus onto a sheet tray through a pair of ejection rollers, if the sheet is soft, trouble will occur. Specifically, the leading part of the soft sheet bends downward, and the sheet interferes with sheets already ejected and stacked on the tray. Thereby, the sheets stacked on the tray will be out of alignment. 
     In order to avoid this trouble, as disclosed by JP58-38641A and JP2005-263418A, conventionally, a currently ejected sheet is curved in a direction perpendicular to the sheet ejecting direction by use of a spring, an elastic member, a roller or the like so that the leading edge of the currently ejected sheet will be in contact with a stack of sheets on the tray at a point farther from the pair of ejection rollers. Thereby, the leading part of the currently ejected sheet is prevented from interfering with the stack of sheets on the tray and from pushing the stack of sheets on the tray out of alignment. However, since the currently ejected sheet is curved and strengthened evenly from the leading edge to the trailing edge, the sheet may jump out of the pair of ejection rollers with great force, and consequently, the sheet may push the stack of sheets on the tray out of alignment. 
       FIG. 17   a  shows a case of ejecting a sheet S onto a sheet tray  110  via a pair of ejection rollers  101  while not strengthening the sheet S. If the sheet S is very soft, the leading part of the sheet S bends downward after coming out of the ejection rollers  101  and interferes with a stack of sheets S′ at a position near the ejection rollers  101 , and the stack of sheets S′ is pushed. 
     As  FIG. 17   b  shows, when the sheet S is strengthened and ejected, the leading edge of the sheet S comes into contact with the stack of sheets S′ at a point farther from the ejection rollers  101 , and the interference between the sheet S and the stack of sheets S′ is eased. Thereby, the sheet S is ejected onto the sheet tray  110  in alignment with the stack of sheets S′.  FIG. 17   c  shows a case of ejecting a sheet S 1  of a relatively large size while strengthening the sheet S 1  from the leading edge to the trailing edge evenly. In this case, before the trailing edge of the sheet S 1  comes out of the ejection rollers  101 , the strengthened sheet S 1  becomes soft again, and it does not occur that the sheet S 1  is tense between the ejection rollers  101  and the stack of sheets S′. Thereby, the sheet S 1  is ejected onto the stack of sheets S′ smoothly, and there is no possibility that the sheet S 1  and the stack of sheets S′ on the sheet tray  110  will be out of alignment. 
       FIG. 17   d  shows a case of ejecting a sheet S 2  of a relatively small size while strengthening the sheet S 2 . In this case, the leading edge of the strengthened sheet S 2  comes into contact with the stack of sheets S′, and while the sheet S 2  keeps strong, the trailing edge of the sheet S 2  comes out of the ejection rollers  101 . Therefore, the sheet S 2  is tense between the ejection rollers  101  and the stack of sheets S′, and the sheet S 2  comes out of the ejection rollers  101  with great force. Thereby, the sheet S 2  cannot be in alignment with the stack of sheets S′. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a sheet ejecting device for ejecting a sheet onto a tray while strengthening the sheet so that the sheet can be ejected in alignment with a stack of sheets on the tray without pushing the stack of sheets out of alignment. 
     In order to achieve the object, according to a first aspect of the present invention, a sheet ejecting device for ejecting printed sheets one by one comprises: a pair of ejection rollers for ejecting a sheet; a strengthener, arranged to be capable of advancing into and retreating from a sheet ejection route, for strengthening a sheet; a driver for moving the strengthener to advance into and retreat from the sheet ejection route; a sensor for detecting a currently ejected sheet; and a controller for controlling the driver based on a signal sent from the sensor so that the strengthener can operate to strengthen a predetermined part of a sheet. 
     In the sheet ejecting device, the strengthener is moved to advance into and retreat from the sheet ejection route so that only a necessary part of a sheet can be strengthened. Therefore, there is no possibility that the leading part of a currently ejected sheet may bend downward and push a stack of sheets on the tray out of alignment. Further, even if the currently ejected sheet is of a small size, there is no possibility that the sheet may jump out of the ejection rollers with great force, and the sheet can be ejected onto the tray in alignment with the stack of sheets. 
     According to a second aspect of the present invention, a sheet ejecting device for ejecting printed sheets one by one comprises: a pair of ejection rollers; a strengthener, located in a sheet ejection route, for providing a force for a sheet; a bender, arranged to be capable of advancing into and retreating from the sheet ejection route, for canceling out the force provided for the sheet by the strengthener; a driver for moving the bender to advance into and retreat from the sheet ejection route; a sensor for detecting a currently ejected sheet; and a controller for controlling the driver based on a signal sent from the sensor so that the bender can cancel out the force provided for the sheet by the strengthener. 
     In the sheet ejecting device, the currently ejected sheet is provided with a force by the strengthener, and there is no possibility that the leading part of the sheet may downward and push a stack of sheets on the tray out of alignment. Further, the bender is driven at a good time to cancel out the force provided for the sheet by the strengthener. Thereby, even if the sheet is of a small size, there is no possibility that the sheet may jump out of the ejection rollers with great force, and the sheet can be ejected onto the tray in alignment with the stack of sheets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic elevation view of a sheet ejecting device according to a first embodiment of the present invention; 
         FIG. 2  is a schematic plan view of the sheet ejecting device according to the first embodiment; 
         FIG. 3  is a schematic elevation view of a sheet ejecting device according to a second embodiment of the present invention; 
         FIG. 4  is a schematic plan view of the sheet ejecting device according to the second embodiment; 
         FIG. 5  is a flowchart showing a control procedure carried out in each of the sheet ejecting devices according to the first and the second embodiments; 
         FIG. 6  is an illustration showing the relationship between a currently ejected sheet and the components of the sheet ejecting device according to the first embodiment; 
         FIG. 7  is an illustration showing the relationship between a currently ejected sheet and the components of the sheet ejecting device according to the second embodiment; 
         FIG. 8  is a time chart showing a first basic sequence for operating a strengthener in each of the sheet ejecting devices according to the first and the second embodiments; 
         FIG. 9  is a time chart showing a second basic sequence for operating the strengthener in each of the sheet ejecting devices according to the first and the second embodiments; 
         FIG. 10  is a flowchart showing another control procedure that may be carried out in each of the sheet ejecting devices according to the first and the second embodiments; 
         FIG. 11  is a schematic elevation view of a sheet ejecting device according to a third embodiment of the present invention; 
         FIG. 12  is a schematic plan view of the sheet ejecting device according to the third embodiment; 
         FIG. 13  is an illustration showing the relationship between a currently ejected sheet and the components of the sheet ejecting device according to the third embodiment; 
         FIG. 14  is a time chart showing a first basic sequence for operating a bender in the sheet ejecting device according to the third embodiment; 
         FIG. 15  is a time chart showing a second basic sequence for operating the bender in the sheet ejecting device according to the third embodiment; 
         FIG. 16  is a flowchart showing a control procedure carried out in the sheet ejecting device according to the third embodiment; and 
         FIGS. 17   a - 17   d  are illustrations of sheet ejection performed by a conventional sheet ejecting device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Sheet ejecting devices according to preferred embodiments of the present invention are hereinafter described with reference to the drawings. In the following embodiments, the same members and parts are provided with the same reference symbols, and repetitious descriptions are omitted. 
     First Embodiment 
     See FIGS.  1  and  2   
     As  FIGS. 1 and 2  show, a sheet ejecting device according to a first embodiment is used as a sheet ejecting section  10  of an image forming apparatus, such as a copying machine and a printer, or of an after-processing device, such as a finisher, connected to the image forming apparatus. The sheet ejecting device generally comprises a pair of ejection rollers  11  for ejecting a sheet S, a strengthener  15 , arranged to advance into and retreat from a sheet ejection route, for strengthening a sheet S, a sheet sensor  19  located in the sheet ejection route, and a tray  20  for receiving sheets S ejected via the pair of ejection rollers  11  and holding the sheets S in a stack. 
     The strengthener  15  comprises a plurality of levers (five levers, in the first embodiment), and the plural levers are aligned in a position immediately downstream from the ejection rollers  11 , in a direction perpendicular to a sheet ejecting direction “A”. Each of the levers  15  is pivoted on a pin  15   a , and a driver section  15 ′, which is composed of a motor, a solenoid, etc., drives the levers  15  to move between a position shown by the solid line in  FIG. 1  (an advance position) and a position shown by the dashed line in  FIG. 1  (a retreat position). 
     A printed sheet S is fed from the direction indicated by an arrow “A” into a nip portion of the ejection rollers  11  and is ejected onto the tray  20  by the rotation of the ejection rollers  11 . In this moment, the strengthening levers  15  advance into the sheet ejection route to curve the sheet S in a direction perpendicular to the ejecting direction “A” such that the curvature extends in the ejecting direction “A”, and the curved and strengthened sheet S is ejected onto the tray  20 . 
     More specifically, first, the leading edge of the sheet S is detected by the sensor  19 . Then, before the leading edge of the sheet S reaches the strengthener  15 , the levers  15  advance into the sheet ejection route. Thereby, the strengthening of the sheet S starts with the leading edge of the sheet S. The strengthened sheet S comes into contact with a stack of sheets S′ on the tray  20  at a point farther from the ejection rollers  11  than a point at which a non-strengthened sheet comes into contact with the stack of sheets S′ on the tray  20 . Therefore, there is no possibility that the sheet S will push the stack of sheets S′ out of alignment. 
     When (or a specified time after) the trailing edge of the sheet S is detected by the sensor  19  or a specified time after the leading edge of the sheet S is detected by the sensor  19 , the strengthening levers  15  retreat upward from the sheet ejection route and stop strengthening the sheet S. In other words, at a time after the leading edge of the sheet S comes into contact with the stack of sheets S′ and before the sheet S passes through the nip portion of the ejection rollers  11 , the levers  15  retreat from the sheet ejection route. Thereafter, the sheet S is not strengthened. Thus, when the sheet S passes through the ejection rollers  11 , the sheet S is no longer strengthened, and there is no possibility that the sheet S may jump out of the ejection rollers  11  with great force. Consequently, the sheet S can be ejected in alignment with the stack of sheets S′ on the tray  20 . 
     Second Embodiment 
     See FIGS.  3  and  4   
     A second embodiment of the present invention, which is shown by  FIGS. 3 and 4 , basically has the same structure as the first embodiment. A sheet ejecting device according to the second embodiment has a strengthener  16  and a driver section  16 ′ for driving the strengthener  16  to advance into and retreat from the sheet ejection route. 
     The strengthener  16  comprises a plurality of plates (five plates, in the second embodiment), and the plural plates are aligned in a position immediately upstream from the ejection rollers  11 , in the direction perpendicular to the sheet ejecting direction “A”. Each of the strengthening plates  16  is movable up and down, and the driver section  16 ′, which is composed of a motor, solenoid, etc., moves the plates  16  between a position shown by the solid line in  FIG. 3  (an advance position) and a position shown by the dashed line in  FIG. 3  (a retreat position). 
     Like in the first embodiment, a printed sheet S is detected by the sensor  19 , strengthened by the strengthener  16  and ejected onto the tray  20 . The strengthened sheet S comes into contact with a stack of sheets S′ on the tray  20  at a point farther from the ejection rollers  11  than a point at which a non-strengthened sheet comes into contact with the stack of sheets S′ on the tray  20 . Therefore, there is no possibility that the sheet S will push the stack of sheets S′ out of alignment. 
     When (or a specified time after) the trailing edge of the sheet S is detected by the sensor  19  or a specified time after the leading edge of the sheet S is detected by the sensor  19 , the strengthening plates  16  retreat upward from the sheet ejection route and stop strengthening the sheet S. In other words, at a time after the leading edge of the sheet S comes into contact with the stack of sheets S′ and before the sheet S passes through the nip portion of the ejection rollers  11 , the plates  16  retreat from the sheet ejection route. Thereafter, the sheet S is not strengthened. Thus, when the sheet S passes through the ejection rollers  11 , the sheet S is no longer strengthened, and there is no possibility that the sheet S may jump out of the ejection rollers  11  with great force. Consequently, the sheet S can be ejected in alignment with the stack of sheets S′ on the tray  20 . 
     Strengthener 
     The strengthener comprises levers (in the first embodiment) or plates (in the second embodiment) made of metal, resin or an elastic material, and the levers or the plates are capable of advancing into and retreating from the sheet ejection route. As shown in  FIGS. 2 and 4 , the levers or the plates are arranged among wheels of the ejection rollers  11 . The strengthener may have only one lever or one plate, and in this case, the single lever or plate is arranged in the center with respect to the direction perpendicular to the sheet ejecting direction “A”. 
     When the strengthener comprises a plurality of levers  15  or a plurality of plates  16 , the plural levers or plates may be selectively driven in accordance with the size of the sheet. For example, when an A4-sized sheet is ejected with its shorter sides parallel to the sheet ejecting direction (hereinafter referred to as A4-lateral sheet ejection), all the levers  15  or all the plates  16  are driven. On the other hand, when an A4-sized sheet is ejected with its longer sides parallel to the sheet ejecting direction (hereinafter referred to as A4-vertical sheet ejection), the sheet does not pass both sides of the sheet ejection route, and it is not necessary to drive the levers  15  or the plates  16  located on both sides of the sheet ejection route. 
     Further, the amount by which the levers  15  and the plates  16  advance into the sheet ejection route may be variable. The levers  15  or the plates  16  provide a sheet with a force in proportion to the advancing amount, and by varying the advancing amount of the levers  15  or the plates  16 , it is possible to provide a sheet with a variable force. 
     Control Procedure 
     See FIG.  5   
     Referring to  FIG. 5 , a control procedure for each of the sheet ejecting devices according to the first and the second embodiments is described. The control procedure is carried out by a CPU provided in the image forming apparatus or in the after-processing device. Ejection of a sheet S is started (step S 1 ), and when the leading edge of the sheet S is detected by the sensor  19  (step S 2 ), the strengthener  15  or  16  is moved to advance into the sheet ejection route (step S 3 ). Thereby, the sheet S is strengthened. Thereafter, when the trailing edge of the sheet S is detected by the sensor  19  (step S 4 ), the strengthener  15  or  16  retreats from the sheet ejection route and stops strengthening the sheet S (step S 5 ). Then, ejection of one sheet S is completed (step S 6 ). 
     Time to Operate the Strengthener 
     See FIGS.  6 - 9   
       FIG. 6  shows the relationship between the position of the sheet S and the time to operate the strengthener  15  in the first embodiment.  FIG. 7  shows the relationship between the position of the sheet S and the time to operate the strengthener  16  in the second embodiment. The reference symbol “L 1 ” denotes a predetermined distance (approximately 50 mm) by which the sheet S is further conveyed after the leading edge of the sheet S comes into contact with the stack of sheets S′ on the tray  20 . The reference symbol “L 2 ” denotes a distance (approximately 100 mm) between the nip portion of the ejection rollers  11  and a contact point of the sheet S with the stack of sheets S′. The reference symbol “L 3 ” denotes a distance between the nip portion of the ejection rollers  11  and the trailing edge of the sheet S. The reference symbol “L 4 ” denotes a distance between the nip portion of the ejection rollers  11  and the sensor  19 . The reference symbol “L 3 ′” denotes a distance calculated by subtracting 30 mm from the distance L 3 . The distance L 3  should be at least 30 mm, and the strengthener  15  or  16  retreats from the sheet S at a time while the part L 3 ′ of the sheet S passes through the nip portion of the ejection rollers  11 . 
       FIG. 8  shows a first basic sequence for moving the strengthener  15  or  16 , and  FIG. 9  shows a second basic sequence for moving the strengthener  15  or  16 . In the first basic sequence shown by  FIG. 8 , a time T 1  after the leading edge of the sheet S is detected by the sensor  19 , the strengthener  15  or  16  is moved to advance into the sheet ejection route. Thereafter, a time T 2  after the trailing edge of the sheet S is detected by the sensor  19 , the strengthener  15  or  16  is moved to retreat from the sheet ejection route. Thus, in the first basic sequence, the time to move the strengthener to advance into the sheet ejection route and the time to move the strengthener to retreat from the sheet ejection route are based on a detection signal of the leading edge of the sheet S and a detection signal of the trailing edge of the sheet S, respectively. 
     In the second basic sequence shown by  FIG. 9 , the time T 1  after the leading edge of the sheet S is detected by the sensor  19 , the strengthener  15  or  16  is moved to advance into the sheet ejection route, and a time T 3  after the leading edge of the sheet S is detected by the sensor  19 , the strengthener  15  or  16  is moved to retreat from the sheet ejection route. The time to move the strengthener to retreat from the sheet ejection route, that is, the time T 3  varies depending on the size of the sheet S. 
     Setting of Strengthener Retreating Time 
     In the first and the second embodiments, for example, the time T 3  to move the strengthener  15  or  16  to retreat from the sheet ejection route in the second basic sequence may be changed in accordance with the size of the sheet S (the dimension of the sheet S in the sheet ejecting direction “A”), the kind of the sheet S, the environmental conditions, etc. Referring to  FIGS. 6 and 7 , the distance of travel of the sheet S between the time when the sensor  19  detects the leading edge of the sheet S and the time when the strengthener  15  or  16  is moved to retreat from the sheet ejection route shall be within a range from (L 4 +L 2 +L 1 ) to (the length of the sheet S+L 4 −L 3 ). 
     If the distance L 4  is 60 mm and if the speed of conveying the sheet S is 300 mm/s, the time T 3  shall be within a range as shown by Table 1 below. In table 1, “A4-vertical” means the case of ejecting an A4-sized sheet with its longer sides parallel to the sheet ejecting direction, and “A4-lateral” means the case of ejecting an A4-sized sheet with its shorter sides parallel to the sheet ejecting direction. 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 T3 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A4-Vertical 
                 0.7 sec.-1.09 sec. 
               
               
                   
                 A4-Lateral 
                 0.7 sec.-0.8 sec.  
               
               
                   
                   
               
             
          
         
       
     
     When the sheet S is ejected via the ejection rollers  11 , the sheet S bends downward by its own weight. The degree of the bend-down (the strength) of the sheet S depends on the kind of the sheet S (the weight per square meter, that is, whether to be thin paper, ordinary paper or thick paper, and whether to be recycled paper) and the environmental conditions (temperature and humidity). Therefore, it is preferred that the time T 3  is determined based on the kind of the sheet S and the environmental conditions. 
     Table 2 below shows an exemplary setting of the time T 3  in accordance with whether the sheet S is thin paper, ordinary paper or thick paper. Here, thin paper is paper with a weight per square meter smaller than 60 g/m 2 . Ordinary paper is paper with a weight per square meter within a range from 60 g/m 2  to 90 g/m 2 . Thick paper is paper with a weight per square meter greater than 90 g/m 2 . 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 A4-Vertical 
                 A4-Lateral 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Thin Paper 
                 1.0 sec. 
                 0.8 sec. 
               
               
                   
                 Ordinary Paper 
                 0.9 sec. 
                 0.75 sec.  
               
               
                   
                 Thick Paper 
                 0.8 sec. 
                 0.7 sec. 
               
               
                   
                   
               
             
          
         
       
     
     Further, recycled paper is relatively soft, and the time T 3  in the case of ejecting a sheet of recycled paper may be set longer than the time T 3  in the case of ejecting a sheet of non-recycled paper with the same weight per square meter. Table 3 shows an exemplary setting of the time T 3  in accordance with whether the sheet S is recycled paper. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 A4-Vertical 
                 A4-Lateral 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Recycled Paper 
                 1.0 sec. 
                 0.8 sec. 
               
               
                   
                 Non-recycled Paper 
                 0.9 sec. 
                 0.75 sec.  
               
               
                   
                   
               
             
          
         
       
     
     Furthermore, it is preferred that the time T 3  is set longer as the temperature and the humidity become higher. Table 4 shows an exemplary setting of the time T 3  in accordance with whether the sheet ejecting device is under high temperature and high humidity, under ordinary temperature and ordinary humidity, or low temperature and low humidity. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 A4-Vertical 
                 A4-Lateral 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 High-Temp/High-Humid 
                 1.0 sec. 
                 0.8 sec. 
               
               
                   
                 Ordinary-Temp/Ordinary Humid 
                 0.9 sec. 
                 0.75 sec.  
               
               
                   
                 Low-Temp/Low-Humid 
                 0.8 sec. 
                 0.7 sec. 
               
               
                   
                   
               
             
          
         
       
     
     Control Procedure 
     See FIG.  10   
     Referring to  FIG. 10 , in each of the sheet ejecting devices according to the first and second embodiments, a procedure for changing the time T 3  in the second basic sequence in accordance with the conditions is described. Ejection of a sheet S is started (step S 11 ), and the times to move the strengthener  15  or  16  (the times T 1  and T 3 ) are determined based on the size and the kind of the sheet S and the environmental conditions (step S 12 ). When the leading edge of the sheet S is detected by the sensor  19  (step S 13 ), a timer is started (step S 14 ). When the timer counts the time T 1  (step S 15 ), the strengthener  15  or  16  is moved to advance into the sheet ejection route to strengthen the sheet S (step S 16 ). The timer counts the time T 3  (step S 17 ), the strengthener  15  or  16  is moved to retreat from the sheet ejection route (step S 18 ) to stop strengthening the sheet S. Then, ejection of one sheet S is completed (step S 19 ). 
     Third Embodiment 
     See FIGS.  11  and  12   
     As  FIGS. 11 and 12  show, a sheet ejecting device according to a third embodiment comprises a bender  17  in a position downstream from the ejection rollers  11  as well as the strengthener  16  located in a position upstream from the ejection rollers  11 . The strengthener  16  comprises a plurality of components located fixedly in the sheet ejection route. The bender  17  comprises a plurality of levers (five levers, in the third embodiment) aligned in a direction perpendicular to the sheet ejecting direction “A”. Each of the levers  17  is pivoted on a pin  17   a , and the levers  17  are driven by a driver  17 ′ (a motor, a solenoid, etc.) to pivot between a retreat position shown by the solid line and an advance position shown by the dashed line. When the bender  17  advances into the sheet ejection route at the position downstream from the ejection rollers  11  and pushes the strengthened sheet S, the force provided for the sheet S by the strengthener  16  is cancelled out. 
     In the third embodiment, the strengthener  16  that is fixed in the sheet ejection route curves the sheet S in the direction perpendicular to the sheet ejecting direction “A” with the curvature extending in the sheet ejecting direction “A”, such that the sheet S is strengthened from the leading edge to the trailing edge. The strengthened sheet S comes into contact with a stack of sheets S′ on the tray  20  at a point farther from the ejection rollers  11  than a point at which a non-strengthened sheet comes into contact with the stack of sheets S′ on the tray  20 . Therefore, there is no possibility that the sheet S will push the stack of sheets S′ out of alignment. 
     At a time after the leading edge of the sheet S comes into contact with the stack of sheets S′ and before the trailing edge of the sheet S passes through the ejection rollers  11 , the bender  17  is driven to advance into the sheet ejection route so as to cancel out the force provided for the sheet S by the strengthener  16 . Therefore, when the sheet S passes through the ejection rollers  11 , the sheet S is no longer strengthened, and there is no possibility that the sheet S may jump out of the ejection rollers  11  with great force. Consequently, the sheets S′ can be kept in alignment on the tray  20 . 
     In the third embodiment, as the bender  17 , a plurality of levers are aligned in the direction perpendicular to the sheet ejecting direction “A”, and the levers are selectively driven in accordance with the size of the sheet S. However, the bender  17  may have only one lever located in the center with respect to the direction perpendicular to the sheet ejecting direction “A”. 
     Time to Operate the Bender 
     See FIGS.  13 - 15   
       FIG. 13  shows the relationship between the position of the sheet S and the time to operate the bender  17  in the third embodiment. The reference symbol “L 1 ” denotes a predetermined distance (approximately 50 mm) by which the sheet S is further conveyed after the leading edge thereof comes into contact with the stack of sheets S′. The reference symbol “L 2 ” denotes a distance (approximately 100 mm) between the nip portion of the ejection rollers  11  and the contact point of the leading edge of the sheet S with the stack of sheets S′. The reference symbol “L 3 ” denotes a distance between the nip portion of the ejection rollers  11  and the trailing edge of the sheet S. The reference symbol “L 4 ” denotes a distance between the nip portion of the ejection rollers  11  and the sensor  19 . The reference symbol “L 3 ′” denotes a distance calculated by subtracting 30 mm from the distance L 3 . The distance L 3  should be at least 30 mm, and operation of the bender  17  is started while the part L 3 ′ of the sheet S passes through the nip portion of the ejection rollers  11 . 
       FIG. 14  shows a first basic sequence for operating the bender  17 , and  FIG. 15  shows a second basic sequence for operating the bender  17 . In the first basic sequence shown by  FIG. 14 , a time T 1  after the trailing edge of the sheet S is detected by the sensor  19 , the bender  17  is moved to advance into the sheet ejection route. Then, a time T 2  after the trailing edge of the sheet S is detected by the sensor  19 , the bender  17  is moved to retreat from the sheet ejection route. Thus, in this case, the times to move the bender  17  to advance into and to retreat from the sheet ejection route are based on the detection signal of the trailing edge of the sheet S. 
     In the second base sequence shown by  FIG. 15 , a time T 1 ′ after the leading edge of the sheet S is detected by the sensor  19 , the bender  17  is moved to advance into the sheet ejection route. Then, a time T 3  after the leading edge of the sheet S is detected by the sensor  19 , the bender  17  is moved to retreat from the sheet ejection route. The times to move the bender  17  (the times T 1 ′ and T 3 ) depend on the size of the sheet S. 
     Setting of Bender Advancing Time 
     In the third embodiment, in the case in which the bender  17  is operated in accordance with the second base sequence shown by  FIG. 15 , the time T 1 ′ to move the bender  17  to advance into the sheet ejection route (to start canceling out the force provided for the sheet S by the strengthener  16 ) may be determined based on the size of the sheet S (the length of the sheet S in the sheet ejecting direction “A”), the kind of the sheet S, the environmental conditions, etc. Referring to  FIG. 13 , the distance of travel of the sheet S between the time when the sensor  19  detects the leading edge of the sheet S and the time when the bender  17  is moved to advance into the sheet ejection route shall be within a range from (L 4 +L 2 +L 1 ) to (the length of the sheet S+L 4 −L 3 ). 
     If the distance L 4  is 60 mm and if the speed of conveying the sheet S is 300 mm/s, the time T 1 ′ shall be within a range as shown by Table 5 below. 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 T1′ 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A4-Vertical 
                 0.7 sec.-1.09 sec. 
               
               
                   
                 A4-Lateral 
                 0.7 sec.-0.8 sec.  
               
               
                   
                   
               
             
          
         
       
     
     When the sheet S is ejected via the ejection rollers  11 , the sheet S bends downward by its own weight. The degree of the bend-down (the strength) of the sheet S depends on the kind of the sheet S (the weight per square meter, that is, whether to be thin paper, ordinary paper or thick paper, and whether to be recycled paper) and the environmental conditions (temperature and humidity). Therefore, it is preferred that the time T 1 ′ is determined based on the kind of the sheet S and the environmental conditions. 
     Table 6 below shows an exemplary setting of the time T 1 ′ in accordance with whether the sheet S is thin paper, ordinary paper or thick paper. Here, thin paper is paper with a weight per square meter smaller than 60 g/m 2 . Ordinary paper is a paper with a weight per square meter within a range from 60 g/m 2  to 90 g/m 2 . Thick paper is paper with a weight per square meter greater than 90 g/m 2 . 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 A4-Vertical 
                 A4-Lateral 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Thin Paper 
                 1.0 sec. 
                 0.8 sec. 
               
               
                   
                 Ordinary Paper 
                 0.9 sec. 
                 0.75 sec.  
               
               
                   
                 Thick Paper 
                 0.8 sec. 
                 0.7 sec. 
               
               
                   
                   
               
             
          
         
       
     
     Further, recycled paper is relatively soft, and the time T 1 ′ in the case of ejecting a sheet of recycled paper may be set longer than the time T 1 ′ in the case of ejecting a sheet of non-recycled paper with the same weight per square meter. Table 8 shows an exemplary setting of the time T 1 ′ in accordance with whether the sheet S is recycled paper. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 A4-Vertical 
                 A4-Lateral 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Recycled Paper 
                 1.0 sec. 
                 0.8 sec. 
               
               
                   
                 Non-recycled Paper 
                 0.9 sec. 
                 0.75 sec.  
               
               
                   
                   
               
             
          
         
       
     
     Furthermore, it is preferred that the time T 1 ′ is set longer as the temperature and the humidity become higher. Table 8 shows an exemplary setting of the time T 1 ′ in accordance with whether the sheet ejecting device is under high temperature and high humidity, under ordinary temperature and ordinary humidity, or low temperature and low humidity. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 A4-Vertical 
                 A4-Lateral 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 High-Temp/High-Humid 
                 1.0 sec. 
                 0.8 sec. 
               
               
                   
                 Ordinary-Temp/Ordinary Humid 
                 0.9 sec. 
                 0.75 sec.  
               
               
                   
                 Low-Temp/Low-Humid 
                 0.8 sec. 
                 0.7 sec. 
               
               
                   
                   
               
             
          
         
       
     
     Further, there may be a case in which the bender  17  is not operated depending on the size, the kind and the material of the sheet S and/or the environmental conditions. 
     Control Procedure 
     See FIG.  16   
     Referring to  FIG. 16 , in the sheet ejecting device according to the third embodiment, a procedure for changing the time T 1 ′ in the second basic sequence in accordance with the conditions is described. Ejection of a sheet S is started (step S 21 ). Then, after confirming that operation of the bender  17  is necessary (“YES” at step S 22 ), the times T 1 ′ and T 3  to move the bender  17  to advance into and to retreat from the sheet ejection route are determined based on the size and the kind of the sheet S and the environmental conditions (step S 23 ). When the leading edge of the sheet S is detected by the sensor  19  (step S 24 ), a timer is started (step S 25 ). Then, the timer counts the time T 1 ′ (step S 26 ), the bender  17  is moved to advance into the sheet ejection route (step S 27 ) to cancel out the force provided for the sheet S by the strengthener  16 . Further, when the timer counts the time T 3  (step S 28 ), the bender  17  is moved to retreat from the sheet ejection route (step S 29 ). Then, ejection of the sheet S onto the tray  20  is completed (step S 30 ). 
     Other Embodiments 
     The detailed constructions of the tray  20 , the ejection rollers  11 , the strengtheners  15 ,  16  and the bender  17  may be arbitrarily designed. 
     Although the present invention has been described in connection with the preferred embodiments above, various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.