Patent Publication Number: US-2023159295-A1

Title: Sheet post-processing device and sheet post-processing method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-189808, filed Nov. 24, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a sheet post-processing device and a sheet post-processing method. 
     BACKGROUND 
     In related art, an image forming device can be equipped with a sheet post-processing device that processes (“post-processes”) sheets after printing. Examples of post processing of the sheets include sorting and dividing sheets into a plurality of document copies and stapling processing of stapling each copy. After the sheets are post-processed in this manner, the post-processed sheets can be discharged as bundles (bundle-discharged) to a sheet discharge tray as a unit (bundled unit). For example, in a bundle discharge process, an upper surface of the sheet bundle on the sheet discharge tray is detected, and when no upper surface is detected, the sheet discharge tray is moved upward. 
     However, in the related art, when the sheets are to be bundle-discharged, the sheets might not be normally discharged depending on presence or absence of other sheets on the sheet discharge tray, and misalignment may occur with the sheets to be bundle-discharged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an external view of an image forming device according to an embodiment. 
         FIG.  2    is a diagram of an internal configuration of an image forming device. 
         FIG.  3    depicts a hardware configuration of an image forming device. 
         FIG.  4    depicts aspects of a sheet post-processing device. 
         FIG.  5    depicts aspects of a sheet post-processing device. 
         FIG.  6    depicts aspects of a sheet post-processing device. 
         FIG.  7    depicts aspects of a sheet post-processing device. 
         FIG.  8 A  depicts an example of a case where misalignment occurs in sheets to be bundle-discharged. 
         FIG.  8 B  depicts an example of a case where misalignment occurs in sheets to be bundle-discharged. 
         FIG.  9    illustrating an example when a position of a movable tray is changed. 
         FIG.  10    depicts an operation example of bundle discharge when a movable tray is moved to another discharge position. 
         FIG.  11    depicts a functional configuration of a sheet post-processing device. 
         FIG.  12    is a flowchart illustrating an example of discharge processing in a post-processing mode. 
         FIG.  13    depicts aspects related to a modification example when the position of a movable tray is changed. 
         FIG.  14    depicts an operation example of bundle discharge when the movable tray is moved to another discharge position in a modification example. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a sheet post-processing device and a sheet post-processing method capable of preventing misalignment in sheets to be bundle-discharged are provided. 
     A sheet post-processing device according to an embodiment, a sheet post-processing device includes a first tray to receive sheets from an image forming unit, a second tray to receive sheets from the first tray, and a third tray to receive sheets from the second tray. The third tray is movable relative to the second tray. A bundle claw is configured to move sheets in a first direction from the second tray to the third tray. A controller is configured to move the third tray relative to the second tray according to the sheets on the second tray. 
     A sheet post-processing device and a sheet post-processing method according to certain example embodiments will be described with reference to the drawings. In the following, components, elements, or aspects having the same or substantially similar functions are denoted by the same reference symbols. In addition, description of a duplicated component, element, or aspect may be omitted after an initial description. 
     First, an overall configuration of an image forming device  100  will be described with reference to  FIG.  1   . 
       FIG.  1    is an external view illustrating an overall configuration example of the image forming device  100  according to an embodiment. The image forming device  100  is, for example, a multifunction peripheral. The image forming device  100  includes a display  110 , a control panel  120 , a printer  130 , a sheet accommodating unit  140 , and an image reading unit  150 . 
     The display  110  is, for example, a liquid crystal display of a touch panel type. The display  110  displays various kinds of information. In addition, the display  110  receives an input operation from a user. 
     The control panel  120  includes various operation keys such as a numeric keypad and a start key. The control panel  120  receives various input operations from the user. For example, the control panel  120  receives an input operation relating to post processing. Examples of post processing include a sorting processing for sorting sheets into a plurality of copies, stapling processing for stapling each copy, and punching processing for punching holes in predetermined positions of sheets. In addition, the control panel  120  outputs operation signals corresponding to the various input operations received from the user to a control unit. 
     The printer  130  performs a series of printing operations based on various kinds of information output from the display  110 , the control panel  120 , the image reading unit  150 , and the like. The series of printing operations include receiving image information, forming an image, transferring the formed image to a sheet, conveying a sheet, and the like. 
     The sheet accommodating unit  140  includes a plurality of sheet cassettes. Each sheet cassette accommodates sheets therein. These sheets are usually normal copy sheets (plain paper), but may also sheets of other types such as a photographic sheet, a label sheet, a polyester film sheet, and the like. 
     The image reading unit  150  includes an automatic document sheet feeder and a scanner device. The automatic document sheet feeder feeds a document placed on a document tray to the scanner device. The scanner device optically scans a document on a document glass stand, and forms an image of reflected light from the document on a light-receiving surface of a charge coupled device (CCD) sensor. Accordingly, the scanner device reads a document image on the document glass stand. The image reading unit  150  generates image information (image data) by using a result of reading by the scanner device. 
     In  FIG.  1   , the image forming device  100  includes a fixed tray  56  and a movable tray  430 . The fixed tray  56  is a tray at a fixed position to which the sheets can be discharged. The sheets can be discharged to the fixed tray  56  in the following cases. 
     A case in which the number of the sheets to be printed is less than a predetermined number. 
     A case in which just one copy is to be printed. 
     A case in which two or more copies are to be printed, but are to be printed page by page. 
     The movable tray  430  is a tray that moves up and down. The sheets can be discharged to the movable tray  430  in the following cases. 
     A case in which the number of the sheets to be printed is more than a predetermined number. 
     A case two or more copies are to be printed, but are to be printed as separate copies. 
       FIG.  2    is a diagram illustrating an example of an internal configuration of the image forming device  100 . As illustrated in  FIG.  2   , the image forming device  100  (more particularly, the printer  130  portion of the image forming device  100 ) includes four image forming units  20   a  to  20   d.  The image forming device  100  is of a so-called quadruple tandem type in this example. The image forming device  100  includes an image processing unit  10 , an image forming unit  20  (including units  20   a  to  20   d ), an intermediate transfer unit  30 , a fixing unit  40 , a sheet conveying unit  50 , and a sheet post-processing device  60 . 
     The image processing unit  10  receives the image information (print data). The received image information can be generated by the image reading unit  150  or transmitted from another device. The image processing unit  10  performs digital image processing for processing the received image information according to an initial setting or a setting selected by the user. For example, the digital image processing includes gradation correction based on gradation correction data. In addition to the gradation correction, the digital image processing includes various correction processing that may be made to the image data such as color correction or shading correction as well as processing such as compression. 
     Next, the image forming unit  20  will be described. The image forming unit  20  includes the image forming unit  20   a  corresponding to Y (yellow), the image forming unit  20   b  corresponding to M (magenta), the image forming unit  20   c  corresponding to C (cyan), and the image forming unit  20   d  corresponding to K (black). The image forming units  20   a  to  20   d  respectively include photoconductor drums  21   a  to  21   d,  chargers  22   a  to  22   d,  an exposure device  23 , developing devices  24   a  to  24   d,  and a drum cleaning device. In the following, the reference numerals a to d will be omitted since the description applies to each image forming unit  20   a  to  20   d.    
     The photoconductor drum  21  is, for example, an organic photo-conductor (OPC) of a charge type in which an undercoat layer, a charge generation layer, and a charge transport layer are sequentially laminated on a circumferential surface of a conductive cylinder made of aluminum. The photoconductor drum  21  changes conductivity in response to light. 
     The charger  22  generates corona discharge. The charger  22  uniformly charges a surface of the photoconductor drum  21 . 
     The exposure device  23  is, for example, a semiconductor laser. The exposure device  23  irradiates the photoconductor drum  21  with laser light corresponding to an image of each color component. When the laser light is irradiated by the exposure device  23 , a potential of a region irradiated with the laser light is changed on the surface of the photoconductor drum  21 . Due to this change in conductivity, an electrostatic latent image is formed on the surface of the photoconductor drum  21 . 
     The developing device  24  accommodates a developer. The developing device  24  attaches a toner of a respective color component to the surface of the respective photoconductor drum  21 . Accordingly, a toner image of one color toner is formed on the photoconductor drum  21 . That is, the electrostatic latent image formed on the surface of the photoconductor drum  21  is developed. 
     Here, the developer will be described. As the developer, for example, a two-component developer is used. The two-component developer includes a non-magnetic toner and a carrier. For the carrier, for example, iron powder having a particle diameter of several tens of microns (μm) or a polymer ferrite particle is used. The carrier is mixed with the toner in the developing device  24 , and is frictionally charged, thereby giving a charge (for example, a negative charge) to the toner particles. In addition, the carrier conveys the toner to the electrostatic latent image by a magnetic force. 
     The drum cleaning device includes a cleaning blade in contact with the surface of the photoconductor drum  21 . The cleaning blade removes a residual toner remaining on the surface of the photoconductor drum  21  after primary transfer. The removed residual toner is collected in an accommodating unit of the drum cleaning device. 
     Next, the intermediate transfer unit  30  will be described. The intermediate transfer unit  30  includes an intermediate transfer body  31 , a primary transfer roller  32 , a plurality of support rollers  33 , a secondary transfer roller  34 , a belt cleaning device  35 , and the like. 
     The intermediate transfer body  31  is, for example, an endless belt (transfer belt). The intermediate transfer body  31  is not substantially conductive or elastic. Specifically, for example, the intermediate transfer body  31  is a belt made of polyimide. However, in other examples, the intermediate transfer body  31  may have conductivity and elasticity. 
     The support rollers  33   a  to  33   c  support the intermediate transfer body  31  such that tension is applied to the intermediate transfer body  31 . Accordingly, the intermediate transfer body  31  is formed in a loop shape. One of the plurality of support rollers  33   a  to  33   c  (for example, the support roller  33   a ) is a drive roller. The rollers other than the drive roller are driven rollers. When the driving roller is rotated (by a motor or the like), the intermediate transfer body  31  travels in the direction A depicted in  FIG.  2    at a predetermined speed and in a predetermined cycle. 
     The primary transfer roller  32  is disposed to face the photoconductor drum  21  with the intermediate transfer body  31  interposed therebetween. Specifically, the primary transfer roller  32  is disposed so as to apply a pressure on the photoconductor drum  21  with the intermediate transfer body  31  interposed therebetween. Accordingly, the primary transfer roller  32  and the photoconductor drum  21  form a primary transfer unit that nips the intermediate transfer body  31 . 
     When the intermediate transfer body  31  passes through the primary transfer unit, the toner image formed on the photoconductor drum  21  is transferred to the intermediate transfer body  31 . When the intermediate transfer body  31  passes through the primary transfer unit, a primary transfer bias is applied to the primary transfer roller  32 . Specifically, for example, a charge having a polarity (e.g., a positive polarity) opposite to that of the toner (is applied to the primary transfer roller  32 . Accordingly, the toner image formed on the photoconductor drum  21  is electrostatically transferred to the intermediate transfer body  31 . 
     The secondary transfer roller  34  is disposed to face the support roller  33   a  with the intermediate transfer body  31  interposed therebetween. Specifically, the secondary transfer roller  34  is disposed so as to apply a pressure on the support roller  33   a  with the intermediate transfer body  31  interposed therebetween. Accordingly, the secondary transfer roller  34  and the support roller  33   a  form a secondary transfer unit  38  that brings the intermediate transfer body  31  into contact with the sheets being printed. 
     When a sheet pass through the secondary transfer unit  38 , the toner image formed on the intermediate transfer body  31  is transferred to the sheet. When the sheets pass through the secondary transfer unit  38 , a secondary transfer bias is applied to the support roller  33   a.  Specifically, a charge having the same polarity (e.g., a negative polarity) as that of the toner is applied to the support roller  33   a.  Accordingly, the toner image on the intermediate transfer body  31  is electrostatically transferred to the sheets. 
     The secondary transfer roller  34  and the support roller  33   a  can be separated from each other. Accordingly, when the sheets are jammed in the secondary transfer unit  38 , the user can remove the sheets. 
     The belt cleaning device  35  includes a cleaning blade in contact with a surface of the intermediate transfer body  31 . The cleaning blade removes any residual toner remaining on the surface of the intermediate transfer body  31  after the secondary transfer. The removed residual toner is collected in an accommodating unit of the belt cleaning device  35 . 
     The fixing unit  40  heats and presses the sheets to which the toner image has been transferred. The fixing unit  40  is, for example, a roller-type fixing unit including a heated roller that heats the sheets and a pressing roller that is pressed against the heated roller. Accordingly, the fixing unit  40  fixes the toner image onto the sheets. The fixing unit  40  can also adopt a method of fixing toner images to the sheets by heating of a film member interposed therebetween. 
     Next, the sheet conveying unit  50  will be described. The sheet conveying unit  50  includes a sheet feed unit  51 , a registration unit  52 , a first guide unit  53 , a second guide unit  54 , and a sheet discharge unit  55 . 
     The sheet feed unit  51  conveys the sheets accommodated in the sheet accommodating unit  140  one by one to the registration unit  52 . The registration unit  52  stops each sheets being conveyed from the sheet feed unit  51  and then feeds the sheet toward the secondary transfer unit  38  at a timing appropriate for toner image transfer to the sheet. The appropriate timing in this context is the timing at which the toner image formed on the intermediate transfer body  31  can be secondarily transferred by the secondary transfer unit  38  to the sheet at a correct position. The first guide unit  53  restricts movement of the sheets fed from the registration unit  52  to the secondary transfer unit  38 . 
     The secondary transfer unit  38  transfers the toner image to the sheets after the first guide unit  53 . The secondary transfer unit  38  then feeds the sheets to which the toner image has been transferred toward the fixing unit  40 . 
     The second guide unit  54  restricts movement of the sheets fed from the secondary transfer unit  38  to the fixing unit  40 . The fixing unit  40  heats and presses the sheets after the second guide unit  54 , and then feeds the sheets to the sheet discharge unit  55  or the sheet post-processing device  60 . The sheet discharge unit  55  simply discharges the sheets to the fixed tray  56  (see  FIG.  1   ). 
     The sheet post-processing device  60  post-processes the sheets fed from the fixing unit  40 , and discharges sheets to the movable tray  430  (see  FIG.  1   ). After post processing, such as the sorting processing and the stapling processing, is performed, the sheet post-processing device  60  discharges the sheets as a group (bundle) to the movable tray  430 . The post processing for sorting includes a process of aligning the sheets with respect to one another. The post processing for stapling includes a process of aligning the sheets as well as stapling of the aligned sheets. 
     Next, a hardware configuration of the image forming device  100  will be described with reference to  FIG.  3   . 
       FIG.  3    is an explanatory diagram illustrating an example of the hardware configuration of the image forming device  100 . As illustrated in  FIG.  3   , the image forming device  100  includes, in addition to the already described configurations, a central processing unit (CPU)  201 , a memory  202 , a communication unit  203 , and a speaker  204 . These components can communicate with each other via a bus. 
     The CPU  201  is a central processing unit, and controls each unit illustrated in  FIG.  3    by reading and executing various programs stored in the memory  202 . The various programs include a sheet post-processing program according to the present embodiment. 
     The memory is a ROM, a RAM, a hard disk, or the like. The ROM is a read-only memory, and stores various kinds of information used by the CPU, such as a program. The RAM is a memory that can be read and written, and stores various kinds of information. For example, the RAM stores information acquired from the outside and information generated in various processing. The hard disk stores various kinds of information. 
     The communication unit  203  is an interface that transmits and receives information to and from other devices. 
     The speaker  204  outputs sound. Next, configuration examples and general operation examples of the sheet post-processing device  60  will be described with reference to  FIGS.  4  to  7   . 
       FIGS.  4  to  7    are explanatory diagrams illustrating examples of configurational aspects and general operations of the sheet post-processing device  60 . As depicted in  FIG.  4   , the sheet post-processing device  60  includes a standby tray  410 , a processing tray  420 , and the movable tray  430 . The standby tray  410  is a fixed tray that stands by (buffers) until the number of sheets St fed from a conveying roller  411  reaches a predetermined number (hereinafter referred to as a “buffer number”). The buffer number is, for example, three. When the number of the sheets St on the standby tray  410  reaches the buffer number then, as illustrated in  FIG.  5   , the sheets St on the standby tray  410  are automatically moved to the processing tray  420 . 
     The processing tray  420  is an example of a processing placement unit. In the stapling processing, the processing tray  420  stores the sheets St as needed. The processing tray  420  is a fixed tray on which the sheets St are aligned and wait until the number of sheets St reaches the number for bundle discharge (hereinafter referred to as a “bundle discharge number”), and a post processing is performed after the bundle discharge number is reached. In the stapling processing, the bundle discharge number is the number of pages in a document copy to be stapled together as a unit. In the sorting processing, the bundle discharge number can be equal to the buffer number. That is, in the sorting processing, the processing tray  420  aligns the sheets St each time the sheets St reach the buffer number (the bundle discharge number) in the standby tray  410  and thus are moved to the processing tray  420 , and then discharges the aligned sheets St to the movable tray  530 . 
     A bundle claw  421  is disposed on one end (a main body side) of the processing tray  420 , and a discharge roller  422  is disposed on the other end (a discharge side). The bundle claw  421  includes a moving mechanism that moves from a standby position illustrated in  FIG.  5    to an advanced position illustrated in  FIG.  7   . When the bundle discharge number is reached on the processing tray  420  and the post processing has been performed as needed, the bundle claw  421  pushes out the bundled sheets St toward the movable tray  430 . 
     The discharge roller  422  feeds the sheets St pushed out by the bundle claw  421  to the movable tray  430 . The discharge roller  422  may be a driven roller, or may be a passive roller that rotates freely as the sheets St are discharged. When the number of the sheets St on the processing tray  420  reaches the bundle discharge number, the bundle claw  421  pushes out the bundled sheets St as illustrated in  FIG.  6   . As illustrated, the sheet post-processing device  60  according to the present embodiment is not provided with a pinch roller or the like that faces the discharge roller  422 . Therefore, when the sheets St are discharged from the processing tray  420  to the movable tray  430  there is no pressing on the bundle from above (such as would be the case with a pinch roller opposite the discharge roller  422 ). 
     As illustrated in  FIG.  7   , when the bundle claw  421  reaches the advanced position, the sheets St pushed out by the bundle claw  421  are placed on the movable tray  430  with the discharge roller  422  interposed therebetween. The movable tray  430  is an example of a movable placement unit. The movable tray  430  receives the sheets St discharged from the processing tray  420 . The movable tray  430  includes a mechanism that can move up and down. A height of the movable tray  430  can be appropriately changed according to an amount of the sheets St to be placed thereon, for example. 
     Here, an example of a case where misalignment occurs in the sheets St to be bundle-discharged will be described with reference to  FIGS.  8 A and  8 B . 
       FIGS.  8 A and  8 B  are explanatory diagrams illustrating an example of a case where misalignment occurs in the sheets St to be bundle-discharged. In  FIG.  8 A , it is assumed that no sheets St are initially on the movable tray  430 . For example, if a large step  800  is present between the processing tray  420  and the movable tray  430 , the sheets St stand by in a deflected state (that is, the sheets St bend or bow downward). When the bundle discharge is performed in this situation, the relationship between the force (conveying force) moving the sheets St onto the movable tray  430  and the resistance force (friction force) due to contact between the tip portions of the sheets St being moved and the movable tray  430 , the resistance force may be larger than the conveying force. Accordingly, as illustrated in  FIG.  8 B , the sheets St being moved onto the movable tray  430  are likely to be additionally deflected and may not be discharged normally, resulting in misalignment of the sheets St. 
     Such deflection when the sheets St are discharged tends to occur more easily as the step  800  becomes larger. In addition, such deflection can be more significant when longer type sheets St are used, or when a total weight of the sheets St being transferred is low. 
     When the sheets St are already on the movable tray  430  when additional sheets St are being moved on the movable tray  430 , the relevant contact resistance is that between the different sheets St. However, since an air layer may be initially formed between the individual sheets St, the contact resistance between such individual sheets St can be lower than the resistance force due between sheets St and the movable tray  430 . That is, when the sheets St are already present on the movable tray  430 , issues with sheet deflection when additional sheets St are discharged are considered less likely to occur. 
     Therefore, the sheet post-processing device  60  according to the present embodiment adjusts a position of the movable tray  430  based on presence or absence of the sheets St already on the movable tray  430  when a bundle discharge is performed. 
       FIG.  9    is an explanatory diagram illustrating an example of a case where the position of the movable tray  430  is changed in the present embodiment. When a bundle discharge is performed, the movable tray  430  is disposed at a first position (hereinafter referred to as a “first discharge position P 1 ”) higher than a standby position in a normal control. In addition, as illustrated in  FIG.  9   , when sheets St are not already on the movable tray  430 , the movable tray  430  is disposed at a second position (hereinafter referred to as a “second discharge position P 2 ”) higher than the first discharge position P 1 . At the second discharge position P 2 , a difference H 2  in height between a surface of the processing tray  420  and a surface of the movable tray  430  is smaller than a difference H 1  at the first discharge position P 1 . 
       FIG.  10    is an explanatory diagram illustrating an operation example of the bundle discharge when the movable tray  430  is moved to the second discharge position P 2 . In  FIG.  10   , it is assumed that no sheets St are on the movable tray  430  initially. As illustrated in  FIG.  10   , since the height (the difference H 2 ) between the surface of the processing tray  420  and the surface of the movable tray  430  is reduced, the resistance force due to the contact between the sheets St and the movable tray  430  can be made smaller than the conveying force of the tip portions of the sheets St on the movable tray  430  side. Therefore, the deflection of the sheets St can be made less likely to occur even when no sheets St are on the movable tray  430  in the bundle-discharge. 
     Next, a functional configuration of the sheet post-processing device  60  included in the image forming device will be described with reference to  FIG.  11   . 
       FIG.  11    is an explanatory diagram illustrating an example of the functional configuration of the sheet post-processing device  60 . The sheet post-processing device  60  includes a post-processing unit  501 , a discharge processing unit  502 , and a placement control unit  503 . Each of the units  501  to  503  is implemented by the CPU  201 . That is, the CPU  201  implements functions of the units  501  to  503  by executing a sheet post-processing program stored in memory. The CPU  201  is not limited to performing the processing according to the present embodiment by executing programs, and can also perform the processing according to the present embodiment by using, for example, hardware (a circuit unit; including circuitry) such as a large scale integration (LSI) circuit, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a graphics processing unit (GPU), and can also perform the processing according to the present embodiment by a cooperation of software and hardware. 
     The post-processing unit  501 , the discharge processing unit  502 , and the placement control unit  503  are not limited to being implemented by the CPU  201  of the image forming device  100 . For example, when the sheet post-processing device  60  itself includes a CPU, each of the units  501  to  503  may be implemented by a CPU included in the sheet post-processing device  60 . 
     The post-processing unit  501  causes the processing tray  420  to stand by until the number of the conveyed sheets St reaches the bundle discharge number (a predetermined number). Then, the post-processing unit  501  post-processes the sheets St of the bundle discharge number as a bundle. Examples of the post processing include sorting processing, stapling processing, and punching processing. The post-processing unit  501  performs the post processing whenever the post processing is set to be performed. The post processing can be selected based on an operation input by a user using the control panel  120  or based on setting information received from another device (a user terminal such as a personal computer) by using the communication unit  203 . Hereinafter, printing for which the post processing is set may be referred to as a “post-processing mode” printing. 
     The discharge processing unit  502  discharges the bundle of sheets St post-processed by the post-processing unit  501  to the movable tray  430 . Specifically, the discharge processing unit  502  controls the bundle claw  421  to discharge (bundle-discharge) the bundle of sheets St to the movable tray  430 . 
     The placement control unit  503  performs control to change the position of the movable tray  430  with respect to the processing tray  420  based on the presence or absence of the sheets St on the movable tray  430  before the sheets St are discharged to the movable tray  430  by the discharge processing unit  502 . The presence or absence of the sheets St on the movable tray  430  can be detected by a sensor included in the image forming device  100 . The sensor is provided proximate to the movable tray  430 . 
     Specifically, the placement control unit  503  disposes the movable tray  430  at the first discharge position P 1  (see  FIG.  9   ) when the post-processing mode is started. The first discharge position P 1  is higher than a normal disposing position (normal position) (In this context, the normal position is the movable tray  430  position when post-processing is not being performed on the sheets St being printed). In addition, when sheets have not yet been placed on the movable tray  430  in the post-processing mode, the placement control unit  503  adjusts the position of the movable tray  430  to the second discharge position P 2 , which is different from the first discharge position P 1 . In particular, the second discharge position P 2  is higher than the first discharge position P 1 . 
     As described above, the deflection in a bundle discharge can be more significant when longer sheets St are being printed. Therefore, in some examples, when sheets St are not yet on the movable tray  430 , the placement control unit  503  may change the position of the movable tray  430  in view of the size of the sheets St to be printed. For example, when the sheets St are long in a sub-scanning direction, the placement control unit  503  adjusts the position of the movable tray  430  accordingly. 
     Specifically, if A 3  size sheets (which are longer than in usual sub-scanning direction than a A 4  size sheet or a plain letter page sheet) are being printed, then when the sheets St are discharged the height of the movable tray  430  is adjusted. For example, when at least one long-type sheet St is present in the sheets St to be bundle-discharged, then such a long-type sheet St may be deflected. Therefore, in the present embodiment, when at least one sheet St having a long size is present in the sheets St to be bundle-discharged, the placement control unit  503  changes the position of the movable tray  430 . However, the position of the movable tray  430  in some examples need not be changed if the post processing is being performed on mixed A 3  size and A 4  size sheets but an A 4  size sheet is positioned at a lowest position in the bundle (stack). 
     As described above, the deflection in a bundle discharge can be more significant when the weight of the sheets St is lower. Therefore, when sheets St are not presently on the movable tray  430 , the placement control unit  503  may change the position of the movable tray  430  based on a weight of the sheets St. For example, the placement control unit  503  changes the position of the movable tray  430  when the sheets St are expected to be of a weak stiffness, such as when the weight of the sheets St being printed is equal to or less than some predetermined value. 
     In this context, the matter of the weight of the sheets St being equal to or less than a predetermined value can be based on the per sheet weight of the sheets being printed being equal to or less than 90 g/m 2 . For example, when at least one sheet St having a weak stiffness is present in the sheets St to be bundle-discharged, this sheet St having a weak stiffness may be deflected. Therefore, in the present embodiment, the placement control unit  503  changes the position of the movable tray  430  when at least one sheet St having a weak stiffness is known to be present in the sheets to be bundle-discharged. 
     In the present embodiment, the placement control unit  503  may change the position of the movable tray  430  based on both the size of the sheets St and the weight of the sheets St. However, the present embodiment is not limited thereto, and the placement control unit  503  may change the position of the movable tray  430  based on either the size of the sheets St or the weight of the sheets St alone. 
     Here, since the post-processed sheets St on the movable tray  430  are printed in groups corresponding to a complete document, some of the sheets St may be removed by the user even though not all the printing is ended (that is, some copies still remain to be printed). Thus, sheets St on the movable tray  430  may be removed even during the printing process. Therefore, in the present embodiment, when a plurality of copies are to be printed, the presence or absence of sheets St on the movable tray  430  can determined for each copy to be discharged to the movable tray  430 . 
     Specifically, the placement control unit  503  determines the presence or absence of the sheets St on the movable tray  430  each time a sheet St reaches the processing tray  420  up until the bundle discharge number. In the present embodiment, the placement control unit  503  also performs a similar determination in a process of buffering the sheets St fed from the conveying roller  411  in the standby tray  410 . 
     The placement control unit  503  changes the position of the movable tray  430  whenever it is determined there are no sheets already on the movable tray  430  but sheets still remain to be discharged to the movable tray  430 . In the present embodiment, the placement control unit  503  changes the position of the movable tray  430  whenever it is determined that no sheets St are on the movable tray  430 . However, the determination of the presence or absence of the sheets St on the movable tray  430  is not limited to being performed in the conveying process of every one of the sheets St, and in other examples may be performed in a conveying process of just a last sheet St in the bundle discharge. 
     In addition, the deflection of the sheets St tends to be less likely to occur as the bundle discharge number becomes larger, that is, as the bundle of sheets St gets heavier. Therefore, in the present embodiment, when the bundle discharge number is equal to or greater than a threshold value, the placement control unit  503  does not change the position of the movable tray  430  regardless of the presence or absence of the sheets St on the movable tray  430 . The threshold value can be set as appropriate. In the present embodiment, the threshold value is, for example, 10. 
     The placement control unit  503  can perform control of changing the position of the movable tray  430  in a mode other than the post-processing mode. For example, even in a normal control (no post-processing) mode. In normal control the movable tray  430  would generally only be raised or lowered according to the number of discharged sheets St whenever a predetermined number or more of sheets St is to be printed. However, the movable tray  430  may be raised excessively if such normal control was to be performed when the movable tray  430  is disposed at the second discharge position P 2 . Therefore, in the present embodiment, the placement control unit  503  is configured to not perform normal control when the movable tray  430  is disposed at the second discharge position P 2 . 
     Next, discharge processing in the post-processing mode performed by the image forming device  100  will be described with reference to  FIG.  12   . 
       FIG.  12    is a flowchart illustrating an example of the discharge processing in the post-processing mode as performed by the image forming device  100 . As illustrated in  FIG.  12   , the image forming device  100  (more particularly, the sheet post-processing device  60 ) stands by (ACT  701 : NO) until a printing is started. When the printing is started (ACT  701 : YES), the image forming device  100  determines whether the current printing is in the post-processing mode (ACT  702 ). When the current printing is not in the post-processing mode (ACT  702 : NO), the image forming device  100  returns to ACT  701 . 
     On the other hand, when the current printing is in the post-processing mode (ACT  702 : YES), the image forming device  100  moves the movable tray  430  from the normal position to the first discharge position P 1  (ACT  703 ). Then, the image forming device  100  buffers the printed and conveyed sheets St in the standby tray  410  (ACT  704 ). Next, the image forming device  100  determines whether the presently set bundle discharge number is equal to or greater than a threshold value (for example, 10) (ACT  705 ). In the sorting processing, since just three sheets are bundle-discharged in this example, ACT  705  is determined as NO. On the other hand, in the stapling processing, 10 or more sheets might be bundle-discharged according to a setting by the user. When the set bundle discharge number is equal to or larger than the threshold value (10) (ACT  705 : YES), the image forming device  100  proceeds to ACT  711  without moving the movable tray  430  to the second discharge position P 2 . 
     On the other hand, when the set bundle discharge number is less than the threshold value ( 10 ) (ACT  705 : NO), the image forming device  100  next determines whether the movable tray  430  is at the first discharge position P 1  (ACT  706 ). When the movable tray  430  is not at the first discharge position P 1  (ACT  706 : No), that is, when the movable tray  430  is already disposed at the second discharge position P 2 , the image forming device  100  proceeds to ACT  711 . On the other hand, when the movable tray  430  is at the first discharge position P 1  (ACT  706 : YES), the image forming device  100  next determines the presence or absence of the sheets St already on the movable tray  430  (ACT  707 ). 
     When the sheets St are already on the movable tray  430  (ACT  707 : YES), the image forming device  100  proceeds to ACT  711  without moving the movable tray  430  to the second discharge position P 2 . If the movable tray  430  is moved to the second discharge position P 2  in a state in which the sheets are already on the movable tray  430 , some sheets St may flow back to the processing tray  420 . Therefore, when the sheets St are already on the movable tray  430 , the movable tray  430  is held at the first discharge position P 1 . 
     On the other hand, when sheets St are not already on the movable tray  430  (ACT  707 : NO), the image forming device  100  next determines whether the size of the sheets is A 3  or larger (ACT  708 ). When the size of the sheets is smaller than A 3  (ACT  708 : NO), the image forming device  100  proceeds to ACT  711  without moving the movable tray  430  to the second discharge position P 2 . 
     If the size of the sheets is A 3  or larger (ACT  708 : YES), the image forming device  100  next determines whether the per sheet weight (basis weight) of the sheets is equal to or less than 90 g/m 2  (ACT  709 ). When the basis weight of the sheets is not equal to or less than 90 g/m 2  (ACT  709 : NO), that is, when the basis weight of the sheets exceeds 90 g/m 2 , the image forming device  100  proceeds to ACT  711  without moving the movable tray  430  to the second discharge position P 2 . On the other hand, if the basis weight of the sheets is equal to or less than 90 g/m 2  (ACT  709 : YES), the image forming device  100  then moves the movable tray  430  to the second discharge position P 2  (ACT  710 ). 
     Then, the image forming device  100  determines whether the number of the sheets standing on the standby tray  410  reaches the buffer number (for example, three) (ACT  711 ). The buffer number is not always three. For example, when eight sheets are to be post-processed into a bundle or the like, the buffer number is three at the initial buffering and again at the next buffering, but is only two at the last buffering for the  8  sheet bundle. 
     When the number of the sheets standing by on the standby tray  410  has not yet reached the buffer number (ACT  711 : NO), the image forming device  100  returns to ACT  704 . On the other hand, if the number of the sheets standing on the standby tray  410  reaches the buffer number (ACT  711 : YES), the image forming device  100  moves the sheets St on the standby tray  410  to the processing tray  420  (ACT  712 ). 
     Then, the image forming device  100  determines whether the number of the sheets St on the processing tray  420  has reached the bundle discharge number (ACT  713 ). In a sorting processing, since the bundle discharge number and the buffer number are the same, if ACT  711  is determined as YES, then ACT  713  will also be determined as YES. When the number of the sheets St on the processing tray  420  does not yet reach the bundle discharge number (ACT  713 : NO), the image forming device  100  returns to ACT  704 . On the other hand, when the number of the sheets St on the processing tray  420  has reached the bundle discharge number (ACT  713 : YES), the image forming device  100  post-processes the aligned sheet bundle (ACT  714 ). 
     Then, the image forming device  100  bundle-discharges the sheets St to the movable tray  430  by moving the bundle claw  421  from the standby position to the advanced position (ACT  715 ). Next, the image forming device  100  determines whether the number of copies has been reached (ACT  716 ). When the number of copies is not yet reached (ACT  716 : NO), the image forming device  100  returns to ACT  703 . When the number of copies has been reached (ACT  716 : YES), the image forming device  100  returns the movable tray  430  to the normal position (ACT  717 ), and ends the series of processing. 
     The flowchart description illustrated a case including each of ACT  705 , ACT  708 , and ACT  709 . However, these processes need be not all included, and one or more may be excluded. In addition, the movable tray  430  may be moved to the second discharge position P 2  only if all of these processes are satisfied (when all are YES), or if at least one processing is satisfied (when at least one is YES). 
     As described above, the image forming device  100  changes the position of the movable tray  430  with respect to the processing tray  420  based on the presence or absence of the sheets St on the movable tray  430  to which sheets St are to be discharged from the processing tray  420  for bundling. Accordingly, the resistance force generated by the contact between the sheets St and the movable tray  430  can be made smaller than the conveying force of the tip portions of the sheets St on the movable tray  430  side. Therefore, the sheets St being bundle-discharged from the processing tray  420  can be prevented from being misaligned on the movable tray  430 . 
     The image forming device  100  moves the movable tray  430  to the first discharge position P 1  when the post-processing mode is started, and can move the movable tray  430  from the first discharge position P 1  to the second discharge position P 2  if no sheets St are on the movable tray  430 . Accordingly, since the movable tray  430  can stand by at the first discharge position P 1 , which is higher than the normal position, the movable tray  430  can be immediately moved to the second discharge position P 2 . Therefore, the speed of the bundle discharging of the sheets St can be prevented from being slowed. Therefore, the speed of outputting of the sheets St can be prevented from dropping and occurrence of misalignment on the movable tray  430  when bundle-discharged can be prevented. 
     In an embodiment, the difference in height between the processing tray  420  and the movable tray  430  at the connection position thereof is the first difference H 1  at the first discharge position P 1 , and the difference is the second difference H 2  (less than the first difference H 1 ) at the second discharge position P 2 . Accordingly, the resistance force generated by the contact between the sheets St and the movable tray  430  can be made smaller than the conveying force of the tip portions of the sheets St on the movable tray  430  side. Therefore, the sheets St bundle-discharged from the processing tray  420  can be prevented from being deflected on the movable tray  430 . 
     When no sheets St are on the movable tray  430 , the image forming device  100  according to an embodiment may change the position of the movable tray  430  based on at least one of the size of the sheets St and the weight of the sheets St. Accordingly, even when sheets St having a long size or sheets St having a weak stiffness are included in the bundle-discharged sheets St, the sheets St bundle-discharged from the processing tray  420  can be efficiently prevented from being deflected on the movable tray  430 . 
     The image forming device  100  determines the presence or absence of the sheets St on the movable tray  430  for each sheet St conveyed until the number of the sheets on the processing tray  420  reaches the bundle discharge number, and changes the position of the movable tray  430  based on the determination result (Sheets St on movable tray  430 , YES/NO?) of each sheet conveyed to the processing tray  420 . Accordingly, even if the sheets St on the movable tray  430  are removed by a user during the printing, the sheets St bundle-discharged from the processing tray  420  can still be prevented from being misaligned on the movable tray  430 . 
     When the bundle discharge number is equal to or greater than a threshold value, the image forming device  100  does not change the position of the movable tray  430  regardless of the presence or absence of the sheets St on the movable tray  430 . Accordingly, when the bundle discharge number is large, and the deflection of the sheets St is less likely to occur, the control to move the movable tray  430  to the second discharge position P 2  can be omitted. Therefore, a load of the processing relating to the bundle discharge can be reduced. 
     Next, modifications will be described. In each of the following, differences from the above-described embodiments will be mainly described. In addition, each of the described modifications may be combined with the above-described embodiment and each other to the extent technically feasible. 
     First, a first modification will be described. The above-described embodiment included a sheet post-processing device  60  that moves the position of the movable tray  430  in a height direction based on the presence or absence of the sheets St on the movable tray  430 . The first modification includes a sheet post-processing device  60  that changes an angle of the movable tray  430  based on the presence or absence of the sheets St on the movable tray  430 . 
       FIG.  13    is an explanatory diagram illustrating this modification when the position of the movable tray  430  is changed. In the modification, the movable tray  430  includes a swingable mechanism with a support unit  1300  as a support shaft. As illustrated in  FIG.  13   , at the second discharge position P 2 , the movable tray  430  is tilted so as to be closer to parallel with the support unit  1300  than at the first discharge position P 1 . Specifically, at the second discharge position P 2 , the movable tray  430  is tilted such that the surface of the movable tray  430  is closer to parallel to the surface of the processing tray  420  than at the first discharge position P 1 . 
       FIG.  14    is an explanatory diagram illustrating an operation example of the bundle discharge when the movable tray  430  is moved to the second discharge position P 2  in the modification. In  FIG.  14   , it is assumed that no sheets St are initially on the movable tray  430 . Since the surface of the processing tray  420  and the surface of the movable tray  430  are close to parallel, the resistance force generated by the contact between the sheets St and the movable tray  430  can be made smaller than the conveying force of the tip portions of the sheets St on the movable tray  430  side, even when no sheets St are present. Therefore, the deflection of the sheets St is less likely to occur, and the sheets St can be discharged to the movable tray  430 . 
     According to this modification, the sheets St bundle-discharged from the processing tray  420  can be prevented from being misaligned on the movable tray  430 . 
     Next, a second modification will be described. The above-described embodiment included a sheet post-processing device  60  in which the movable tray  430  can be disposed at two positions, that is, the first discharge position P 1  and the second discharge position P 2 . The second modification includes a sheet post-processing device  60  in which the movable tray  430  can be disposed at three positions, that is, the first discharge position P 1 , the second discharge position P 2 , and a third discharge position. 
     In the second modification, the placement control unit  503  can also dispose the movable tray  430  at the third discharge position, which is an intermediate position between the first discharge position P 1  and the second discharge position P 2 . Specifically, for example, the third discharge position is a position where the difference in height between the surface of the processing tray  420  and the surface of the movable tray  430  is smaller than the difference at the first discharge position P 1 , but larger than the difference at the second discharge position P 2 . 
     When the size of the sheets St is slightly larger than standard size, the placement control unit  503  moves the movable tray  430  to the third discharge position. For example, when the sheets are B 4  size being discharged in the longitudinal direction. The predetermined size may be, for example, a sheet size larger than A 4  but smaller than A 3 . 
     Further, when the stiffness of the sheets St is slightly weaker (in a case of a predetermined basis weight) than standard, the placement control unit  503  moves the movable tray  430  to the third discharge position. The predetermined basis weight (X g/m 2 ) in this context is, for example, in a range of 90 g/m 2  &lt;X g/m 2 100 g/m 2 . 
     When the bundle discharge number is slightly larger (when the predetermined number is Y), the placement control unit  503  moves the movable tray  430  to the third discharge position. The predetermined number in this context is, for example, in a range of 10≤Y&lt;15. 
     According to the second modification, even when sheets St having a slightly longer size or sheets St having a slightly weaker stiffness are included in the bundle-discharged sheets St, or when the bundle discharge number is slightly larger, the sheets St bundle-discharged from the processing tray  420  can be efficiently prevented from being deflected on the movable tray  430 . 
     In the second modification, the discharge position of the movable tray  430  is set to three different discharge positions. However, the modification is not limited thereto, and the discharge position of the movable tray  430  may be set to four or more different discharge positions. 
     Described functions of the image forming device  100  (and/or the sheet post-processing device  60 ) may be implemented by a computer executing instructions of a software program or the like. In this case, such a program may be recorded on a non-transitory, computer-readable recording medium. In this context, “computer-readable recording medium” refers to a storage device such as a portable medium such as a flexible disk, a magneto-optical disk, a ROM, and a CD-ROM, or a hard disk. Further, the “computer-readable recording medium” may be accessed or downloaded via a communication line such as a network such as the Internet a telephone line. The program may be a program for implementing a part of the functions, or may be a program capable of implementing the functions in combination with programs already recorded in the computer system, such as an operating system. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. These embodiments can be implemented in other various forms, and various omissions, replacements, and changes can be made without departing from the spirit of the disclosure. These embodiments and modifications thereof are included in the scope and spirit of the invention, as well as in the scope of the invention described in the scope of claims and the equivalent scope thereof.