Patent Publication Number: US-10315879-B2

Title: Sheet processing apparatus, control method therefor and storage medium

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sheet processing apparatus that has a function of aligning sheets stacked on a stack tray, and a control method for the sheet processing apparatus, and a storage medium. 
     Description of the Related Art 
     For sheet processing apparatuses that stack a large number of sheets, there has been demand for the ability to discharge and align the sheets with a high degree of accuracy. Japanese Patent Laid-Open No. 2006-206331 suggests a sheet alignment process in which alignment members (alignment mechanisms) are provided on a stack tray, and sheets are piled up in such a manner that the positions of edge surfaces of the sheets parallel to a sheet discharge direction are lined up by the alignment members coming into and out of contact with the edge surfaces of the sheets. 
     However, this conventional technique has the following problem. For example, when applying an alignment process to sheets newly stacked on printed materials (sheets) that are already stacked on a stack tray and have a different width from the newly-stacked sheets, the toner or ink on the already-stacked sheets could possibly be removed by the above alignment mechanisms coming into contact with the already-stacked sheets. This may lead to a reduction in the image quality of the printed materials. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problem. The present invention provides a technique to execute a sheet alignment process on a sheet processing apparatus and to maintain the image quality of sheets. 
     According to one aspect of the present invention, there is provided a sheet processing apparatus comprising: a first stack tray and, at least, a second stack tray on which a sheet is stacked; an alignment unit including, at least, an alignment member provided on the first stack tray, which is configured to align sheets stacked on the first stack tray by coming into contact with side surfaces of the stacked sheet; a determination unit configured, in a case where a second sheet is already stacked on the first stack tray that has been decided on as a discharge destination for a first sheet, to determine whether or not the width of the first sheet matches the width of the second sheet; and a control unit configured, in a case where the determination unit determines that the width of the first sheet does not match the width of the second sheet, to control the sheet processing apparatus not to discharge the first sheet to the first stack tray. 
     According to another aspect of the present invention, there is provided a control method for a sheet processing apparatus including a first stack tray and a second stack tray on which a sheet is stacked and an alignment unit including an alignment member that is provided on the first stack tray and aligns sheets stacked on the first stack tray by coming into contact with side surfaces of the stacked sheet, the control method comprising the steps of: determining, in a case where a second sheet is already stacked on the first stack tray that has been decided on as a discharge destination for a first sheet, whether or not a width of the first sheet matches a width of the second sheet; and performing, in a case where it is determined that the width of the first sheet does not match the width of the second sheets, control of the sheet processing apparatus not to discharge the first sheet to the first stack tray. 
     According to still another aspect of the present invention, there is provided a computer-readable storage medium storing a program that, when executed by a sheet processing apparatus including a first stack tray and a second stack tray on which a sheet is stacked and an alignment unit including an alignment member that is provided on the first stack tray and aligns sheets stacked on the first stack tray by coming into contact with side surfaces of the stacked sheet, causes the sheet processing apparatus to perform a control method comprising the steps of: determining, in a case where a second sheet is already stacked on the first stack tray that has been decided on as a discharge destination for a first sheet, whether or not a width of the first sheet matches a width of the second sheet; and performing, in a case where it is determined that the width of the first sheet does not match the width of the second sheets, control of the sheet processing apparatus not to discharge the first sheet to the first stack tray. 
     The present invention can provide a technique to execute a sheet alignment process on a sheet processing apparatus without interrupting a sheet process while maintaining the image quality of sheets. 
     Further features of the present invention will become apparent from the following description of embodiments (with reference to the attached drawings). Each of the embodiments of the present invention described below can be implemented solely or as a combination of a plurality of the embodiments or features thereof where necessary or where the combination of elements or features from individual embodiments in a single embodiment is beneficial. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a network including a printing system  100  and a host computer  101  according to the first and second embodiments. 
         FIG. 2  shows a configuration of the printing system  100  according to the first and second embodiments. 
         FIG. 3  is a diagram for describing the operations of a discharge unit  115  according to the first and second embodiments. 
         FIG. 4  is a diagram for describing the operations of the discharge unit  115  according to the first and second embodiments. 
         FIG. 5  is a diagram for describing the operations of the discharge unit  115  according to the first and second embodiments. 
         FIG. 6  is a diagram for describing the operations of the discharge unit  115  according to the first and second embodiments. 
         FIG. 7  is a flowchart of a procedure for a sheet process executed by the printing system  100  according to the first embodiment. 
         FIG. 8  shows an example of a configuration of an operation unit  113  according to the second embodiment. 
         FIG. 9  is a flowchart of a procedure for a sheet process executed by the printing system  100  according to the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the following embodiments are not intended to limit the scope of the appended claims, and that not all the combinations of features described in the embodiments are necessarily essential to the solving means of the present invention. 
     First Embodiment 
     &lt;Configuration of Printing System  100 &gt; 
     The following describes a configuration of a network according to the first embodiment of the present invention with reference to  FIGS. 1 and 2 . As shown in  FIG. 1 , a printing system  100  is connected to a host computer  101  via a network  103 . The printing system  100  can communicate with external apparatuses such as the host computer  101  connected to the network  103 . Examples of the network  103  include a local area network (LAN), a wide area network (WAN), and a telephone line network. 
     As shown in  FIG. 2 , the printing system  100  includes a printing apparatus  200 , a discharge unit  115  that is a sheet stacker, and a sheet feeding unit  205  (sheet feeding unit E). The present embodiment describes an example where a sheet processing apparatus of the present invention is applied to a printing system in which the discharge unit  115  and the sheet feeding unit  205  are attached to the printing apparatus  200 . It should be noted, however, that the present invention is not limited in this way, and is applicable to any sheet processing apparatus with a mechanism for discharging and stacking sheets. That is to say, not only the printing system  100  but also the printing apparatus  200  and the discharge unit (sheet stacker)  115  can each serve as an example of the sheet processing apparatus. 
     In the present embodiment, it is assumed that the host computer  101  is a PC (personal computer). In general, the host computer  101  (PC) includes a CPU (central processing unit), an HDD (hard disk drive), a RAM (random-access memory), various types of input/output apparatuses, and the like. The host computer  101  also includes a network I/F  105 , and is connected to the network  103  via the network I/F  105 . The network I/F  105  performs communication control for establishing connection and communicating with external apparatuses via the network  103 . For example, the network I/F  105  can perform the Internet communication using the Transmission Control Protocol and Internet Protocol (TCP/IP), and transmit/receive data to/from the printing system  100 . 
     A printer driver  104  is installed in the host computer  101 . The printer driver  104  is software that runs on an operating system (OS) for controlling the host computer  101 . In accordance with a print instruction from an application running on the OS, the printer driver  104  generates a print job in which print data is represented using a page description language (PDL), and transmits the generated print job to the printing system  100 . 
     The printing system  100  includes a network I/F  106 , a CPU  107 , a RAM  108 , a storage unit  109 , a reading unit  110 , a print job processing unit  111 , an image forming unit  112 , an operation unit  113 , a sheet feeding unit  114 , and a discharge unit  115 . These units in the printing system  100  are connected to one another via a system bus  117 . The discharge unit  115  has an alignment unit  116  as its subsystem. In the present embodiment, the printing system  100  is described as a multi-function peripheral (MFP) that has functions of a copier, a printer, a facsimile (FAX) machine, and the like. However, the present invention is not limited in this way. Alternatively, the printing system  100  may be a single-function peripheral (SFP) that only has a function of a printer. 
     The network I/F  106  is connected to and communicates with external apparatuses such as the host computer  101  via the network  103 , and controls communication via the network  103 . For example, the network I/F  106  can perform the Internet communication using TCP/IP, and transmit/receive data to/from the host computer  101 . 
     The CPU  107  executes various types of calculation processing, information processing, apparatus control and the like by loading numerous programs stored in the storage unit  109  to the RAM  108  and executing the loaded programs. The RAM  108  is a general volatile storage device that can be accessed directly from the CPU  107 , and is used as a working area for the CPU  107  or as a temporary data storage area. The storage unit  109 , which is an HDD in the present embodiment, is used to temporarily or permanently store a print job accepted (received) from the host computer  101  via the network  103 . 
     The reading unit  110  is a scanner that optically reads a paper original. The reading unit  110  includes an original illumination lamp and a scan mirror, and optically scans an original placed on a glass platen. Reflected light from the original is directed to a lens by the scan mirror and a reflection mirror. Light that has passed through this lens is directed to a solid-state image sensor. The light directed to the solid-state image sensor is converted into electrical signals by the solid-state image sensor and recognized as image signals. Examples of the solid-state image sensor include an image sensor using a charge-coupled device (CCD) method or a complementary metal-oxide-semiconductor (CMOS) method. 
     The print job processing unit  111  generates image data for printing by processing a print job such as copying, PDL and FAX, and transfers the generated image data to the later-described image forming unit  112 . Note that a page description language is a language used to describe instructions for controlling a page printer. Representative examples of a page description language are PostScript (PS) and Printer Control Language (PCL). Upon receiving a print job, the print job processing unit  111  generates image data for printing by applying image processing to this image data in accordance with printing attributes of the received print job so as to convert image data included in the received print job into raster data on a per-page basis. 
     The image forming unit  112  is a printer engine that executes printing on a sheet based on the raster data, i.e. the image data for printing transmitted from the print job processing unit  111 . The image forming unit  112  outputs a printed material by forming, on a sheet, an image based on the raster data that was generated by the print job processing unit  111  through processing of a raster image processor (RIP). While the image forming unit  112  is a printer engine that forms an image using electrophotography in the present embodiment, the image forming unit  112  may alternatively be a printer engine that forms an image using other methods such as an inkjet method. 
     The operation unit  113  is a user interface (UI) used to, for example, operate and set the printing apparatus. In the present embodiment, the operation unit  113  is a UI utilizing a liquid crystal display (LCD) touchscreen method. 
     The sheet feeding unit  114  includes a plurality of cassettes or decks that contain sheets used in the printing system  100 . The sheet feeding unit to be used in feeding sheets changes depending on sheet attributes of print jobs and the settings or states of the printing system  100  itself, and the print job processing unit  111  decides which unit to use. Sheets fed from the sheet feeding unit  114  are conveyed to the image forming unit  112 , and images are printed on these sheet. As a result, printed materials are generated. 
     The discharge unit  115  is a finisher for discharging sheets on which images have been formed (printed) by the image forming unit  112  (printed materials) to the outside of the printing system  100 . The discharge unit  115  includes a plurality of stack trays as discharge destinations of sheets. The print job processing unit  111  executes discharge control as follows. After deciding on a discharge destination for sheets in accordance with attributes of a print job, the print job processing unit  111  instructs the discharge unit  115  to discharge sheets to the discharge destination that was decided on. It should be noted that the discharge unit  115  may be another discharge apparatus, such as a stacker, instead of a finisher. 
     The alignment unit  116  is a subsystem of the discharge unit  115 , and applies an alignment process to a plurality of sheets stacked on the stack trays included in the discharge unit  115 . The alignment unit  116  includes a plurality of alignment members. As will be described later, each alignment member is composed of a pair of members, and aligns a plurality of sheets stacked on a corresponding stack tray by coming into contact with (both) side surfaces of the stacked sheets parallel to a conveyance direction, with the pair of members holding the side surfaces therebetween. In this way, each alignment member aligns the plurality of stacked sheets in a bundle with their edges lined up. 
     The print job processing unit  111  according to the present embodiment confirms whether or not previously-discharged sheets (second sheets) are still placed on a stack tray in the discharge unit  115  that is designated as a discharge destination for sheets designated by a print job to be processed (first sheets). If sheets are still placed on the designated stack tray, the print job processing unit  111  determines whether or not the width of the designated sheets (a main scanning width that is orthogonal to the conveyance direction) matches the width of the already-stacked sheets. When the print job processing unit  111  determines that the width of the sheets designated by the print job to be processed does not match the width of the already-stacked sheets, the print job processing unit  111  changes the discharge destination for sheets to another stack tray that is included among the plurality of stack trays in the discharge unit  115  and is different from the designated stack tray. Thereafter, the print job processing unit  111  performs control to stack the sheets on the other stack tray and cause the alignment unit  116  to execute the alignment process. 
     A configuration of the printing system  100  is described in more detail below with reference to  FIG. 2 . As described above, the printing system  100  includes the printing apparatus  200  as well as the discharge unit  115  and the sheet feeding unit  205  connected to the printing apparatus  200 . The printing apparatus  200  includes the storage unit  109 , the reading unit  110 , the print job processing unit  111 , the image forming unit  112 , and the operation unit  113  shown in  FIG. 1 . 
     Sheet feeding units  201  to  204  (sheet feeding units A to D) provided inside the printing apparatus  200 , and the sheet feeding unit  205  (sheet feeding unit E) provided outside the printing apparatus  200 , all correspond to the sheet feeding unit  114  ( FIG. 1 ). In the present embodiment, the sheet feeding units  201  to  204  are made up of a plurality of sheet cassettes, and the sheet feeding unit  205  is made up of a sheet deck. The discharge unit  115  is provided with stack units  206  to  208  (stack units A to C) as a plurality of stack trays on which discharged sheets are stacked. A sheet conveyance path  209  represents a path along which, after print control has been started by the print job processing unit  111 , sheets are fed from one of the sheet feeding units  201  to  205  and arrive at the discharge unit  115  (one of the stack units  206  to  208 ) via the image forming unit  112 . 
     &lt;Configuration of Discharge Unit  115 &gt; 
       FIG. 3  shows the discharge unit  115  according to the present embodiment as viewed in a direction intersecting with the conveyance direction of sheets. As described above, the discharge unit  115  includes three stack units (stack trays), namely the stack units  206  to  208  (stack units A to C). The stack units  206  to  208  include alignment units  301  to  303  (alignment units A to C), respectively. Although the discharge unit  115  includes the stack units  206  to  208  as three stack trays in the present embodiment, the number of stack trays included in the discharge unit  115  is not limited to three, as long as the discharge unit  115  includes more than one stack trays. Alternatively, the stack units  206  to  208  may be stackers. 
     The discharge unit  115 , which is a finisher as described above, sequentially receives sheets that have been discharged from the printing apparatus  200  via the image forming unit  112 . The discharge unit  115  generates printed materials by applying various post-processes to the plurality of received sheets, such as an alignment process applied to the received sheets using the alignment units  301  to  303 , a staple process for binding the received sheets using a stapler, and a punch process for creating holes in the received sheets. 
     The print job processing unit  111  performs discharge control for discharging sheets onto a stack tray designated by a print job, or onto a stack tray that the print job processing unit  111  has decided on at its own discretion. After print control has been started by the print job processing unit  111 , sheets which have been fed from one of the sheet feeding units  201  to  205  and on which printing has been performed by the image forming unit  112  are discharged onto one of the stack units  206  to  208  through discharge control by the print job processing unit  111 . 
     The alignment units  301  to  303  are provided in correspondence with the plurality of stack units  206  to  208 , respectively, and are equivalent to the plurality of alignment members that each align sheets stacked on the corresponding stack unit by coming into contact with side surfaces of the stacked sheets parallel to the conveyance direction. As described above, each alignment member is composed of a pair of members. The alignment units  301  to  303  correspond to the alignment unit  116  ( FIG. 1 ). 
       FIG. 4  shows the discharge unit  115  according to the present embodiment as viewed in a direction opposing the conveyance direction of sheets. As shown in  FIG. 4 , each of the alignment units (alignment members)  301  to  303  is composed of a pair of members.  FIG. 4  shows an example where a plurality of sheets have been discharged and stacked on the stack unit  206  as printed materials  401 . 
     In the state shown in  FIG. 4 , before the sheets are discharged, the two members of the alignment unit  301  move in a width direction orthogonal to a discharge direction of the sheets so that the interval therebetween is larger than the width of the sheets. Furthermore, after the sheets are discharged, the two members of the alignment unit  301  move toward the center of the sheets (so that the interval therebetween is reduced in the width direction orthogonal to the discharge direction of the sheets), and come into contact with both side surfaces of the plurality of stacked sheets; as a result, both edges of the plurality of stacked sheets are lined up. In the above manner, the alignment unit  301  executes an alignment process for aligning the plurality of sheets stacked on the stack unit  206 . 
     Meanwhile, the alignment units  302  and  303 , which are respectively provided in correspondence with the stack units  207  and  208  on which sheets are not stacked, wait at their respective home positions until sheets are discharged onto the corresponding stack units  207  and  208  via discharge control by the print job processing unit  111 . 
     As with  FIG. 4 ,  FIG. 5  shows the discharge unit  115  according to the present embodiment as viewed in a direction opposing the conveyance direction of sheets. In  FIG. 5 , a plurality of sheets have been discharged and stacked as printed materials  401  on the stack unit  206 . The alignment unit  301  has already applied an alignment process to the printed materials  401 . Moreover, a plurality of sheets that have a smaller width than the sheets composing the printed materials  401  have been discharged and stacked as printed materials  501  on the printed materials  401 . 
     Assume the case where the alignment unit  301  applies an alignment process to the printed materials  501  stacked on the printed materials  401 . In this case, it is necessary to cause the bottom surfaces of the two members of the alignment unit  301  to come into contact with the stacked printed materials  401 . When the two members of the alignment unit  301  are moved in the width direction of the sheets in this state, the two members are slid against the already-stacked printed materials  401 . This gives rise to the possibility that the toner on the printed materials  401  is removed, and the removed toner attaches to the alignment unit  301  and then to a blank portion of the printed materials  401 . This can lead to a reduction in the image quality of the printed materials  401 . 
     In view of the above, in the case where sheets are already stacked on a stack unit designated as a discharge destination for sheets (in the present example, the stack unit  206 ), the present embodiment applies discharge control and an alignment process to sheets as shown in  FIG. 6 . Similarly to  FIGS. 4 and 5 ,  FIG. 6  shows the discharge unit  115  according to the present embodiment as viewed in a direction opposing the conveyance direction of sheets. In  FIG. 6 , a plurality of sheets have been discharged and stacked as printed materials  401  on the stack unit  206 . The alignment unit  301  has already applied an alignment process to the printed materials  401 . It will be assumed that the stack unit  206  is designated as a discharge destination for sheets to be newly discharged. 
     In this case, as the printed materials  401  (second sheets) are already stacked on the designated stack unit  206  as shown in  FIG. 6 , the print job processing unit  111  determines whether or not the width of these printed materials  401  matches the width of the sheets to be newly discharged (first sheets). If the widths of the sheets do not match, the print job processing unit  111  changes the discharge destination for the sheets to be newly discharged to the stack unit  207  or  208  that is different from the designated stack unit  206 . In the case of  FIG. 6 , as the width of the sheets to be newly discharged (printed materials  601 ) is smaller than the width of the already-stacked printed material  401 , the print job processing unit  111  changes the discharge destination for the sheets to be newly discharged from the stack unit  206  to the stack unit  207 . 
     Once the printed materials  601  have been discharged and stacked on the stack unit  207 , the discharge unit  115  applies an alignment process to the stacked printed materials  601  using the alignment unit  302  that is provided in correspondence with the stack unit  207 . When sheets that have a smaller width than a plurality of sheets that are already stacked on the designated stack unit  206  are to be newly discharged, the discharge destination is changed to another stack unit, and an alignment process is applied to the sheets stacked on the other stack unit. This can prevent a reduction in the image quality of the already-stacked sheets caused by the alignment process applied to the newly-stacked sheets. Furthermore, as the discharge destination for sheets is changed to another discharge destination based on the result of determination regarding the sheet widths, the stacking and alignment processes for the sheets are not interrupted, and therefore the productivity of a sheet process is not reduced. 
       FIG. 6  shows an example where a discharge destination for a plurality of sheets to which an alignment process is to be applied is changed from the stack unit  206  to the stack unit  207 . When changing a discharge destination for a plurality of sheets, it is sufficient for the print job processing unit  111  to select, as appropriate, any of a plurality of stack units where an alignment process using the alignment members can be executed. For example, it is sufficient to decide, as a discharge destination, one of the stack units provided with the alignment units which is different from the designated stack unit and on which sheets are not currently stacked or sheets having the same width as the sheets to be discharged are currently stacked. That is to say, it is sufficient to perform control such that a stack unit on which sheets having a different width from the sheets to be discharged are stacked is not decided on as a discharge destination, even if that stack unit is different from the designated stack unit. When sheets having the same width as already-stacked sheets are newly stacked on the already-stacked sheets, the alignment unit (alignment member) does not come into contact with the front surface of the already-stacked sheets during the application of an alignment process to the newly-stacked sheets, and thus the image quality of the already-stacked sheets is not reduced. 
     &lt;Procedure for Sheet Process by Printing System  100 &gt; 
     The following describes a procedure for a sheet process executed by the printing system  100  according to the present embodiment with reference to  FIG. 7 . It should be noted that the processes of steps in the flowchart of  FIG. 7  are realized in the printing system  100  by the print job processing unit  111  reading programs stored in the storage unit  109  into the RAM  108  and executing the read programs. 
     First, in S 701 , the print job processing unit  111  executes an accepting process for accepting a print job and starts a process based on the accepted print job. Examples of the print job include a printing job transmitted from the host computer  101 , a copy job executed in the printing system  100 , and a FAX job transmitted from an external apparatus via the network  103  (a telephone line and the like). Another example of the print job is a stored data job for printing of data stored in the storage unit  109  in accordance with an instruction from the operation unit  113 . The print job processing unit  111  temporarily spools the accepted (received) print job in the storage unit  109 . 
     Next, in S 702 , the print job processing unit  111  acquires the type and the print settings of the accepted print job (in particular, discharge destination designation information that shows a designated discharge destination for sheets) by analyzing the accepted print job. When a discharge destination is not specifically designated by the accepted print job, the print job processing unit  111  may select a stack unit (stack tray) that is to serve as a discharge destination, at its own discretion, and generate the discharge destination designation information accordingly. For example, the print job processing unit  111  may select a stack unit that has been specified in advance in the printing system  100  as a discharge destination. In this case, in the later-described S 704 , the print job processing unit  111  executes a determination process with respect to this stack unit that has been specified in advance. 
     In S 703  that follows S 702 , the print job processing unit  111  applies image processing to image data to be printed included in the accepted print job, thereby acquiring raster data to which processing of RIP has been applied. The print job processing unit  111  then proceeds to the process of S 704 . 
     In S 704 , the print job processing unit  111  determines whether or not an alignment process can be executed at the designated stack unit based on the discharge destination designation information acquired in S 702 . More specifically, as described above, when sheets are not stacked on the designated stack unit, the print job processing unit  111  determines that the alignment process can be executed at the designated stack unit. On the other hand, when sheets are already stacked on the designated stack unit, the print job processing unit  111  determines whether or not the width of the already-stacked sheets matches the width of sheets to be discharged based on the print job. Information showing whether or not sheets are already stacked on each stack unit and information showing the widths of the stacked sheets may be stored in the storage unit  109  by the print job processing unit  111  ahead of time. The print job processing unit  111  can recognize whether or not sheets are stacked on each sheet stacking unit, as well as the widths of the stacked sheets, by referring to the pieces of information stored in the storage unit  109  afterward. 
     When the width of the already-stacked sheets matches the width of the sheets to be discharged based on the print job, the print job processing unit  111  determines that the alignment process can be executed at the designated stack unit in S 704 , and proceeds to the process of S 705 . The print job processing unit  111  decides on the stack unit designated by the print job as the discharge destination in S 705 , and proceeds to the process of S 708 . 
     On the other hand, if the width of the already-stacked sheets does not match the width of the sheets to be discharged based on the print job, the print job processing unit  111  determines that the alignment process cannot be executed at the designated stack unit in S 704 , and proceeds to the process of S 706 . In S 706 , the print job processing unit  111  determines whether or not the alignment process can be executed at another stack tray that is different from the stack unit designated by the print job. More specifically, as described above, the print job processing unit  111  selects, from among a plurality of stack units that are provided with alignment units (alignment members) and where the alignment process can be executed, another stack unit that is different from the stack unit designated by the print job as a discharge destination for sheets. For example, the discharge destination cannot be changed when the following conditions are both satisfied: sheets are already stacked on all stack units other than the designated stack unit, and none of the widths of these stacked sheets matches the width of sheets designated by the print job. When both of these conditions are satisfied, the print job processing unit  111  determines that the alignment process cannot be executed at another stack tray, and returns to the process of S 704 . When one of these conditions are not satisfied, the print job processing unit  111  determines that the alignment process can be executed at another stack tray, and proceeds to the process of S 707 . When returning to the process of S 704  from the process of S 706 , the print job processing unit  111  may notify a user of an instruction to remove the sheets stacked on the stack trays via the operation unit  113  without starting the printing. 
     The print job processing unit  111  decides on the stack unit selected in S 706  as the discharge destination for sheets in S 707 , and proceeds to the process of S 708 . 
     After proceeding to the process of S 708  from the process of S 705  or S 707 , the print job processing unit  111  transfers the raster data corresponding to the print job to the image forming unit  112  and instructs the image forming unit  112  to form images on the sheets based on the raster data in S 708 . Finally, in S 709 , the print job processing unit  111  causes the image forming unit  112  to convey the sheets on which images have been formed to the discharge unit  115 . The print job processing unit  111  also instructs the discharge unit  115  to discharge the sheets conveyed from the image forming unit  112  onto the stack unit corresponding to the discharge destination that was decided on, and to apply the alignment process to the sheets that have been discharged and stacked as a post-process. The print job processing unit  111  then finishes print operations based on the print job. 
     As described above, in the present embodiment, the printing system  100  includes a plurality of stack units (stack trays) that are each provided with an alignment member for applying an alignment process to sheets. At the time of discharging sheets, when sheets are already stacked on a stack tray that is designated as a discharge destination for sheets that are to be discharged, the printing system  100  determines whether or not the width of the sheets to be discharged matches the width of the already-stacked sheets. When the widths of the sheets do not match, the printing system  100  changes the discharge destination for the sheets to be discharged to another stack tray that is included among the plurality of stack trays and is different from the designated stack tray. The printing system  100  then stacks the sheets on the changed discharge destination and executes an alignment process. 
     The present embodiment can prevent a reduction in the image quality of already-stacked sheets caused by an alignment process applied to sheets that are newly stacked on a stack tray. Furthermore, as the stacking and alignment processes for the sheets are executed after changing a sheet discharge destination without interrupting a sheet process, the productivity of the sheet process is not reduced. 
     The above-described first embodiment is advantageous particularly when the width of sheets to be discharged is smaller than the width of sheets that are already stacked on a stack tray designated as a discharge destination for the sheets to be discharged. This is particularly because, when the alignment process is executed after stacking sheets having a small width on already-stacked sheets having a large width, the alignment member comes into contact with the already-stacked sheets during the alignment process. 
     Conversely, when the alignment process is executed after stacking sheets having a large width on already-stacked sheets having a small width, the alignment member does not come into contact with the already-stacked sheets during the alignment process. However, in this case also, when the alignment process has been applied to the already-stacked sheets, there is a possibility that the aligned state of the already-stacked sheets will degrade due to the alignment process applied to the newly-stacked sheets. For example, there is a possibility that the edge surfaces of the already-stacked sheets will be displaced by the shaking caused by the alignment member coming into and out of contact with the edge surfaces of the newly-stacked sheets. In this case, changing the discharge destination for sheets as in the above-described first embodiment prevents the aligned state of the already-stacked sheets from being degraded. 
     Second Embodiment 
     A description is now given of the second embodiment of the present invention with reference to  FIGS. 8 and 9 . It should be noted that the following description has been simplified by omitting a description of portions that are the same as the first embodiment whenever possible. 
     In general, the setting of a designated discharge destination for sheets in a print job reflects some sort of intention of a creator (user) of the print job. Therefore, there are cases where it is not desirable to automatically change a discharge destination for sheets designated by a print job to another discharge destination in the printing system  100 . In such cases, it would be desirable to confirm with the user in advance whether or not the discharge destination for sheets designated by the print job can be changed to another discharge destination. 
     In the present embodiment, when the width of sheets to be newly discharged does not match the width of sheets that are stacked on a stack unit designated as a discharge destination for the sheets that are to be newly discharged, the print job processing unit  111  accepts, from the user, a selection as to whether or not to change the discharge destination for the sheets to be newly discharged to a different stack unit. This selection is accepted via the operation unit  113 . Furthermore, when the user has made the selection to the effect that the discharge destination for the sheets to be newly discharged is changed to a different stack unit, the print job processing unit  111  performs control to change the discharge destination for the sheets to be newly discharged to a different stack unit, stack the sheets on the different stack unit, and execute an alignment process as with the first embodiment. On the other hand, when the user has made the selection to the effect that the discharge destination for the sheets to be newly discharged is not changed to a different stack unit, the print job processing unit  111  performs control to leave the discharge destination unchanged, stack the sheets on the designated stack unit, and execute an alignment process. 
     First, a description is given of a configuration of the operation unit  113  and a user operation to the operation unit  113  in the present embodiment with reference to  FIG. 8 . The operation unit  113  includes a display unit  801 , input keys  802 , a start key  803 , a stop key  804 , and a reset key  805 . In  FIG. 8 , the display unit  801  displays a warning message  806 , an OK button  807 , and a wait button  808 . The warning message  806  shown in  FIG. 8  is displayed on the display unit  801  in the later-described S 907  of  FIG. 9 . In the present embodiment, this warning message  806  is used to prompt the user to select whether or not to change the discharge destination for sheets to a different stack unit. In the present embodiment, the start key  803 , the stop key  804  and the reset key  805  are all constituted as hardware keys. 
     The display unit  801 , which is a touchscreen-equipped LCD, is a UI used to, for example, operate and set the printing system  100 . The input keys  802  include numeric keys for inputting numeric values, a clear key for deleting input, and the like. The start key  803  is a UI used to issue an instruction for starting print operations. The stop key  804  is a UI used to issue an instruction for interrupting print operations. The reset key  805  is a UI used to issue an instruction for resetting input values and setting values to default values. 
     &lt;Procedure for Sheet Process by Printing System  100 &gt; 
     The following describes a procedure for a sheet process executed by the printing system  100  according to the present embodiment with reference to  FIG. 9 . It should be noted that the processes of steps in the flowchart of  FIG. 9  are realized in the printing system  100  by the print job processing unit  111  reading programs stored in the storage unit  109  into the RAM  108  and executing the read programs. 
     First, S 901  to S 906  are similar to S 701  to S 706 . In the present embodiment, the print job processing unit  111  proceeds to the process of S 906  from the process of S 904 . When the print job processing unit  111  determines in S 906  that the alignment process can be executed at another stack tray different from the stack unit designated by the print job, the print job processing unit  111  proceeds to the process of S 907 . On the other hand, if the print job processing unit  111  determines in S 906  that the alignment process cannot be executed at another stack tray different from the stack unit designated by the print job, the print job processing unit  111  returns to the process of S 904 . When returning to the process of S 904 , the print job processing unit  111  may notify the user of an instruction to remove the sheets stacked on the stack trays via the operation unit  113  without starting the printing. 
     In S 907 , the print job processing unit  111  displays a presentation (warning message  806 ) related to options for a discharge destination on the operation unit  113 . In this way, the print job processing unit  111  prompts the user of the printing system  100  to select a discharge destination for sheets via the operation unit  113 . In S 907 , the warning message  806  is used to prompt the user to select whether to change the discharge destination for sheets to another stack unit that is different from the stack unit designated by the print job and where the alignment process can be executed as determined in S 906 , or to leave the discharge destination unchanged and discharge sheets to the designated stack unit. 
     Next, if the print job processing unit  111  determines in S 908  that the user has made the selection via the operation unit  113  to the effect that the discharge destination is changed to another stack unit (that is to say, if the OK button  807  has been pressed), the print job processing unit  111  proceeds to the process of S 909 . As with S 707 , the print job processing unit  111  decides on the stack unit selected in S 906  as the discharge destination for sheets in S 909 , and proceeds to the process of S 708 . 
     On the other hand, if the print job processing unit  111  determines in S 908  that the user has made the selection via the operation unit  113  to the effect that the discharge destination is left unchanged and sheets are to be discharged onto the designated stack unit (that is to say, if the wait button  808  has been pressed), the print job processing unit  111  proceeds to the process of S 910 . In S 910 , the print job processing unit  111  determines again whether or not the alignment process can be executed at the designated stack unit. In this case, unless the sheets that are already stacked on the designated stack unit are removed, the print job processing unit  111  determines that the alignment process cannot be executed at the designated stack unit and interrupts the execution of the print job. On the other hand, upon removal of the sheets stacked on the designated stack unit, the print job processing unit  111  determines that the alignment process can be executed at the designated stack unit and proceeds to the process of S 911 . 
     In S 911 , the print job processing unit  111  resumes the execution of the print job, and decides on the stack unit designated by the print job directly as the discharge destination as with S 705  or S 905 . 
     After proceeding to the process of S 912  from the process of S 905 , S 909  or S 911 , the print job processing unit  111  executes the processes that are similar to the processes of S 708  and S 709  in S 912  and S 913 , respectively. 
     The present embodiment has described the example where the warning message  806  is displayed on the operation unit  113  to prompt the user to select a discharge destination in S 907 . The user&#39;s selection may be accepted not only via the operation unit  113 , but also via the host computer  101  (external apparatus) operated by the user. In this case, in S 907 , the print job processing unit  111  inquires the host computer  101  that has transmitted the print job as to whether or not to change the discharge destination for sheets to a different stack unit. At this time, it is sufficient for the printer driver  104  in the host computer  101  to display a message similar to the warning message  806  on a display unit of the host computer  101  in response to the inquiry from the print job processing unit  111  in the printing system  100 . It is sufficient for the host computer  101  to send, to the printing system  100 , a response showing the user&#39;s selection accepted via the display unit. The print job processing unit  111  in the printing system  100  accepts the user&#39;s selection in accordance with the result of this response. The above method also allows the print job processing unit  111  to accept the user&#39;s selection and execute discharge control in accordance with the user&#39;s intentions. 
     As with the first embodiment, the present embodiment can prevent a reduction in the image quality of already-stacked sheets caused by an alignment process applied to sheets that are newly stacked on a stack tray. Furthermore, as the stacking and alignment processes for the sheets are executed after changing a sheet discharge destination without interrupting a sheet process, the productivity of the sheet process is not reduced. The present embodiment further allows the execution of discharge control that reflects the user&#39;s intentions. 
     Other Embodiments 
     In the example described in the above embodiments, if the width of sheets already stacked on a stack unit designated by a print job does not match the width of sheets to be discharged based on the print job, a determination is made as to whether or not an alignment process can be executed at another stack tray different from the designated stack tray. Alternatively, if the width of the sheets already stacked on the designated stack unit is smaller than the width of the sheets to be discharged based on the print job, the sheets to be discharged based on the print job may be permitted to be stacked on the designated stack unit. 
     In the example described in the above embodiments, a determination is made as to whether or not the alignment process can be executed at another stack tray different from the designated stack tray, and the discharge destination is not changed when both of the following conditions are satisfied: sheets are already stacked on all of stack trays other than the designated stack tray, and none of the widths of these stacked sheets matches the width of sheets designated by the print job. However, the present invention is not limited in this way. When the width of sheets that are already stacked on another stack tray different from the designated stack tray is smaller than the width of sheets to be discharged based on the print job, the sheets to be discharged based on the print job may be permitted to be stacked on the other stack tray. This makes it even more possible to execute the alignment process for sheets without interrupting the sheet process so as to maintain the image quality of sheets. 
     In the example described in the above embodiments, the processes shown in  FIGS. 7 and 9  are executed when executing a print job. These processes may be executed for each page included in a print job. In this way, a reduction in the image quality of sheets can be suppressed in the case where a print job is composed of a plurality of pages and a sheet for a printed page stacked first has a larger width than a sheet for a printed page stacked next. 
     In the example described in the above embodiments, whether or not the stack tray is to be changed is controlled based on the difference between the width of sheets already stacked on the stack tray and the width of sheets to be printed. However, the present invention is not limited in this way. Alternatively, whether or not the stack tray is to be changed may be controlled based on information showing the size of sheets already stacked on the stack tray and on information showing the size of sheets to be printed. For example, in the case where the size of the printed sheets already stacked on the stack tray and the size of the sheets to be printed are both A4, control may be performed to leave the stack tray unchanged and stack the sheets to be printed on the stack tray on which the A4 sheets are already stacked. On the other hand, in the case where the size of the printed sheets already stacked on the stack tray is A4R and the size of the sheets to be printed is A4, control may be performed to change the stack tray. In addition, in the case where the size of the printed sheets already stacked on the stack tray is A4 and the size of the sheets to be printed is A5, control may be performed to change the stack tray. 
     Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiments, and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiments. For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (e.g., computer-readable medium). 
     While the present invention has been described with reference to embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. 
     This application claims the benefit of Japanese Patent Application No. 2012-243849, filed Nov. 5, 2012, which is hereby incorporated by reference herein in its entirety.