Abstract:
An image forming apparatus is provided with an image forming section to form an image on a sheet and to discharge the sheet on which the image is formed; a stacking section to stack the sheet discharged from the image forming section; a setting section to set a condition whether or not to allow a mixed stack mode on which plural sheets related to at least two different jobs are mixed and stacked in the stacking section; and a discharge control section to control the image forming section to discharge the sheet to the stacking section in accordance with the setting condition whether or not to allow the mixed stack mode.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an image forming apparatus for forming an image on paper and outputting the same, particularly to a paper ejection control for ejection to a stacking section. 
   Some of the image forming apparatuses for forming an image on paper in a printer, copying machine and printer, and for outputting it are provided with a stacker for loading a large amount of output paper with an image formed thereon. The stacker proper incorporates a stacking section equipped with an elevation function for elevating the stacked surface in response to the amount of loading. It has a capacity of loading, for example, 5,000 sheets of the paper with an image formed thereon. 
   Incidentally, in shortrun printing industry, post-processing of binding by drilling and stitching is performed by an offline post-processor  504 , as shown in  FIG. 8 , in order to increase the operation efficiency. For example, when the image forming apparatus  501  is requested to execute a job of creating 100 copies of printed matter, each copy made up of 30 sheets, 3,000 sheets are loaded on the stacker  502  as a result. The loaded paper is taken out from the stacker  502  for each stacking section  503  and is carried to the offline post-processor  504 , where a booklet is produced. 
   [Patent Document 1] 
   Official Gazette of Japanese Patent Tokkaihei 9-278271 
   Two jobs can be together subjected to post-processing; however, no mention is made of the problem that may be caused when various sheets of recording paper having been recorded in a plurality of jobs are present on the stacking section. 
   As shown in  FIG. 8 , when paper loaded on the stacker is bound in a book form by an online processor, only the paper related to one job for book binding has to be loaded on the stacking section. In an image forming apparatus wherein the next job can be registered by reservation during the output of the previous job, however, there is already no operation in many cases where the output of the reserved job has started upon termination of the previous job, with the result that the paper of the reserved job is often loaded on the paper of the previous job, according to the prior art. If paper has been loaded in this manner, the operator is required to sort out the paper related to a different job, before paper is fed to the offline post-processor. This has required much trouble and time. 
   In the meantime, a large number of small jobs where each job consists of 1 page or a few pages are executed continuously, and the outputs of these jobs are loaded collectively. Such a configuration of data-center form is also practiced. For example, as shown in  FIG. 9 , when a great number of direct mail documents having the same text and different addresses are to be printed, one job corresponds to one address. After image formation, the printed matter is put into envelopes by the offline post-processor or manually. Thus, the capability of ejecting the outputs of many jobs, one on top of another, into one and the same stacking section is essential to ensure a high level of work efficiency. 
   SUMMARY OF THE INVENTION 
   In view of the prior art problems and requirements described above, it is an object of the present invention is to provide an image forming apparatus capable of loading a large volume of paper in the form conforming to the subsequent processing requirements. 
   The above object can be attained by the following structure. 
   An image forming apparatus, comprises: 
   an image forming section to form an image on a sheet and to discharge the sheet on which the image is formed; 
   a stacking section to stack the sheet discharged from the image forming section; 
   a setting section to set a condition whether or not to allow a mixed stack mode on which plural sheets related to at least two different jobs are mixed and stacked in the stacking section; and 
   a discharge control section to control the image forming section to discharge the sheet to the stacking section in accordance with the setting condition whether or not to allow the mixed stack mode. 
   Further, the above object may be attained by the following preferable structure. 
   The structure of item  1  is an image forming apparatus for forming an image on paper and outputting it, comprising: 
   a stacking section  80  for stacking the sheets of paper having been ejected; 
   a residual paper detecting section  241  for detecting whether or not the sheets of paper are loaded on the stacking section  80 ; 
   setting acceptance sections  220 ,  110 ,  120  and  3  for accepting the setting of whether or not the mixed stacking should be enabled, wherein the sheets of paper of two or more jobs are loaded on the stacking section  80  in a mixed form; 
   a paper ejection control section  111  for disabling the paper of the next job to be ejected to the stacking section  80 , in response to the state of detection by the residual paper detecting section  241 , and the state of the setting of whether the mixed stacking should be enabled or disabled. 
   According to the aforementioned structure, the setting of whether or not the mixed stacking should be enabled is accepted from a user, wherein the sheets of paper of two or more jobs are loaded on the stacking section  80  in a mixed form; then, control is provided to determine whether or not ejection of paper of the next job to the stacking section  80  is disabled or not, in response to the state of detection on whether or not paper remains in the stacking section  80 , and the state of the setting of whether or not the mixed stacking should be enabled. 
   For example, when starting the image formation of the next job, evaluation is made to determine whether or not the paper of the preceding job remains in the stacking section  80 . If paper still remains in the stacking section  80  and the setting has been made not to enable the mixed stacking, execution of the image formation processing of the next job is postponed, or the destination of paper ejection is switched over to another stacking section or paper ejection tray, thereby disabling the paper to be ejected to the stacking section  80  with paper remaining therein. When execution of the image formation processing of the next job is postponed, image formation processing of the next job should be started upon removal of the paper from the stacking section  80 . 
   When the output paper of the preceding job is not remaining in the stacking section  80  or a setting has been made to enable mixed stacking, the ejection of paper of the next job into the stacking section  80  with paper remaining therein is not disabled; ejection of paper to the stacking section  80  is enabled. 
   It is possible to arrange such a configuration that, when there are a plurality of stacking sections  80 , setting of whether the mixed stacking is enabled or disabled is provided for each stacking section  80 . In this case, arrangements should be made in such a way as to determine if the ejection of paper to the stacking section  80  is disabled or not, based on the setting of the mixed stacking for the stacking section  80  specified as the destination of output in the next job and the stacking of sheets in the stacking section  80 . It should be noted that the job means a set of work serving as a unit when a user requests the image forming apparatus to perform processing. 
   The structure of item  2  is an image forming apparatus for forming an image on paper and outputting it, comprising: 
   a stacking section  80  for stacking the sheets of paper having been ejected; 
   a residual paper detecting section  241  for detecting whether or not the sheets of paper are loaded on the stacking section  80 ; 
   setting acceptance sections  220 ,  110 ,  120  and  3  for accepting, for each job, the setting of whether or not the mixed stacking should be enabled, wherein the sheets of paper of the current job and those of other jobs are loaded on the stacking section  80  in a mixed form; 
   a paper ejection control section  111  for disabling ejection of paper of the next job to the stacking section  80 , in response to the state of setting of whether the mixed stacking in the preceding job is enabled or disabled, and the state of the setting of whether the mixed stacking in the next job is enabled or disabled, when paper of the preceding job is loaded in the stacking section  80 . 
   According to the aforementioned structure, the setting of whether the mixed stacking is enabled or disabled can be provided for each job. When the paper of the preceding job is stacked on the stacking section  8 , control is provided in such a way as to determine if ejection of the paper of the next job to the stacking section  80  is disabled or not, in response to the setting of the mixed stacking in the preceding job and the setting of the mixed stacking in the next job. 
   The setting for each job should be provided, for example, when registering or reserving the job. When there are a plurality of stacking section, evaluation is made to determine if ejection of paper to the stacking section is disabled or not, based on the setting of the mixed stacking in the next job; the stacking of paper in the stacking section specified as the output destination by this job; and the setting of mixed stacking in the destination job where paper is loaded in the stacking section. 
   The structure of item  3  is an image forming apparatus described in item  2 , wherein, when the aforementioned mixed stacking is enabled by both the preceding job and next job, the paper ejection control section  111  enables paper of the next job to be ejected to the stacking section  80 . 
   According to the aforementioned structure, even if the paper of the preceding job remains in the stacking section  80 , the paper of the next job can be ejected to the same stacking section  80  as that of the preceding job, if mixed stacking is enabled by both the preceding job and the next job. Since the paper can be ejected if mixed stacking is enabled by both the preceding job and the next job, the current setting of the mixed stacking in any of the jobs is observed. 
   The structure of item  4  is an image forming apparatus described in item  2 , wherein, when the mixed stacking is not enabled by the preceding job, the control section disables the ejection of the paper of the next job to the stacking section  80 , at least while the paper of the preceding job is loaded in the stacking section  80 . 
   According to the aforementioned structure, when the preceding job with paper remaining in the stacking section  80  does not enable mixed stacking, ejection of paper of the subsequent job to the stacking section  80  is disabled. In this case, ejection is disabled, regardless of whether the subsequent job enables mixed stacking or not. It is preferred to arrange such a configuration that paper ejection is enabled upon removal of the paper of the preceding job from the stacking section  80 . 
   The structure of item  5  is an image forming apparatus described in item  2 , wherein, when the mixed stacking is not enabled by the next job, the control section disables the ejection of the paper of the next job to the stacking section  80 , at least while the paper of the preceding job is loaded in the stacking section  80 . 
   According to the aforementioned structure, when paper of the preceding job remains in the stacking section  80  and the next job does not enable mixed stacking, ejection of paper of the subsequent job to the stacking section  80  is disabled. In this case, ejection is disabled, regardless of whether the preceding job enables mixed stacking or not. It is preferred to arrange such a configuration that paper ejection is enabled upon removal of the paper of the preceding job from the stacking section  80 . 
   The structure of item  6  is an image forming apparatus described in one of the items  1 ,  2 ,  3 ,  4  and  5 , comprising reporting sections ( 220  and  110 ) for reporting that the aforementioned control section has disabled ejection of the paper of the next job to the stacking section  80  when the control section has done so. 
   According to the aforementioned structure, a report is issued to notify that the paper of the next job cannot be ejected to the stacking section  80  because the paper of the preceding job remains in the stacking section  80 , when such inability of ejection has occurred. In this case, any method of reporting—display of a message, sounding of a buzzer or display of a problem solution (removal of paper from stacking section)—can be used. 
   The structure of item  7  is an image forming apparatus described in one of the items  1 ,  2 ,  3 ,  4 ,  5  and  6 , wherein the aforementioned setting acceptance sections  220 ,  110  and  120  receives from the external apparatus  3  the information on the setting of whether mixed stacking is enabled or not. 
   According to the aforementioned structure, the information on the setting of whether mixed stacking is enabled or not can be received from the external apparatus ( 3 ). In this case, the external apparatus  3  can be of any type. The setting can be made from an external computer or the like via the network such as LAN (Local Area Network), communications line and communications cable. For example, when the print job is received from an external computer, information denoting the enable/disable status of mixed stacking together with the print data is received from the printer driver of the computer apparatus, whereby the setting of mixed stacking is accepted. 
   The structure of item  8  is an image forming apparatus described in one of the items  1 ,  2 ,  3 ,  4 ,  5 ,  6  and  7 , wherein the image forming operation of the next job is disabled, thereby disabling the ejection of paper to the stacking section  80 . 
   According to the aforementioned structure, even when there is only one stacking section  80  or change of the stacking section  80  at the destination of ejection is disabled, ejection of paper to the relevant stacking section is disabled by disabling the image forming operation. 
   According to the image forming apparatus of the present invention, a user is allowed to select between enabling and disabling of the mixed stacking wherein the paper of the next job is ejected to the same stacking section when the paper of the preceding job remains in that stacking section. Thus, mixed stacking is disabled when the stacked sheets of paper is used for bookbinding by an offline post-processor. The mixed stacking is enabled, when continuous execution of many small jobs, each of which consists of one or a few pages, for example, in the case of creating a direct mail, and the outputs of these many jobs are collectively stacked. This arrangement provides suitable stacking of paper conforming to each of different forms of usage, and ensures compatibility with different stacking requirements. 
   When the enable/disable status of mixed stacking can be set for each job, minute setting of the enable/disable status of mixed stacking is possible. Especially when the job is reserved, it is possible to set the enable/disable status of the mixed stacking in the reserved job, independently of the setting of the mixed stacking in other jobs during image formation at that time. The advantages of reservation can be utilized for the form of stacking. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flowchart representing paper ejection control by an image forming apparatus as an embodiment of the present invention; 
       FIG. 2  is a cross sectional view representing an image forming apparatus as an embodiment of the present invention; 
       FIG. 3  is a block diagram showing the configuration of an image forming apparatus as an embodiment of the present invention; 
       FIG. 4  is an explanatory diagram representing the operation display section of an image forming apparatus as an embodiment of the present invention; 
       FIG. 5  is an explanatory diagram of an example showing how an output setting screen pops up on the basic screen by operating the Output Setting button of  FIG. 4 ; 
       FIG. 6  is an explanatory diagram representing an example of the printer property screen displayed by the printer driver of an external apparatus for sending print data to an image forming apparatus as an embodiment of the present invention; 
       FIG. 7  is a list summarizing the result of evaluation when paper remains in the stacking section in paper ejection control of  FIG. 6 ; 
       FIG. 8  is an explanatory diagram representing an example of work flow when bookbinding is performed by an offline post-processor; 
       FIG. 9  is an explanatory diagram representing an example of the configuration of a job when printing a large volume of documents for direct mail; and 
       FIG. 10  is an explanatory diagram representing an example of work flow when creating a large volume of direct mail. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following describes the embodiments of the present invention with reference to drawings: 
     FIG. 2  shows an example of an image forming system with a high-volume sheet feed tray  90  connected to the image forming apparatus  10  of the present invention. The image forming apparatus  10  is a so-called digital multifunction device having a copying function of reading the document and forming its copied image on paper, and a printing function of receiving print data from such an external apparatus as a computer and forming the corresponding image on paper and outputting it. 
   The image forming apparatus  10  comprises an automatic document conveyance apparatus  20 , a reading section  30 , a printing section  40  and a stacker apparatus  80  capable of accommodating a large volume of paper with an image formed thereon. The automatic document conveyance apparatus  20  feeds each of the documents loaded on the document accommodation tray  21  to the reading position of the reading section  30 . For the double-sided document, one side is read first, and the document is then reversed and fed again into the reading section  30 . 
   The automatic document conveyance apparatus  20  comprises a sheet feed roller  22  for feeding the documents  2  placed on the document accommodation tray  21 , sequentially starting from the topmost one; a contact roller  23  for feeding the document kept in close contact with a contact glass  31 ; and a guide roller  24  for guiding the document fed by the sheet feed roller  22 , along the contact roller  23 . It further includes: 
   a switching claw  25  for switching the direction of the course of the document having passed through the contact glass  31 ; 
   a reversing roller  26  for reversing the double sided document; and 
   an ejection tray  27  for ejecting the documents having been read. 
   The reading section  30  reads the document fed in by the automatic document conveyance apparatus  20  and outputs the corresponding image data. The reading section  30  also includes: 
   an exposure scanning section  35  composed of a light source  33  and a mirror  34 ; 
   a line image sensor  36  for receiving the light reflected from the document and outputting the electric signal in response to the light intensity; 
   a condensing lens  37  for condensing the light reflected from the document to the line image sensor  36 ; and 
   various mirrors  38  forming an optical path for guiding the light reflected from the mirror  34  of the exposure scanning section  35 , to the line image sensor  36 . 
   When reading the document fed in by the automatic document conveyance apparatus  20 , the exposure scanning section  35  moves to the reading position below the contact glass  31  and stops there to read the document fed by the contact roller  23  thereover. When reading the document placed on a platen glass  32 , the exposure scanning section  35  travels from left to right along the bottom surface of the platen glass  32 , thereby reading the document remaining at rest. 
   The printing section  40  forms an image conforming to the image data on paper based on electrophotographic process. The printing section  40  has a laser unit  42  for outputting a laser beam that turns on and off in response to the image data. The printing section  40  is also provided with a photoconductor  43  for forming an electrostatic latent image, a charging device  44  arranged around it, a developing apparatus  45 , a transfer apparatus  46 , a separation apparatus  47  and a cleaning apparatus  48 . 
   The photoconductor  43  forms a cylindrical form and is rotated in a predetermined direction (marked by arrow A in the drawing) by the drive section (not illustrated). The charging device  44  uniformly charges the photoconductor  43  through corona discharging. The surface of the photoconductor  43  charged uniformly can be scanned by the laser beam that turns on and off in response to the image data, whereby an electrostatic latent image is formed on the surface of the photoconductor  43 . The electrostatic latent image formed on the surface of the photoconductor  43  is made visible as a toner image by the developing apparatus  45 . 
   The transfer apparatus  46  transfers the toner image of the surface of the photoconductor onto paper by applying an electric field. The separation apparatus  47  separates paper from the photoconductor  43  by electric charge elimination. After transfer, the cleaning apparatus  48  removes and collects the toner remaining on the photoconductor  43  by scraping with a blade or the like. The collected toner is fed back to the developing apparatus  45  through the path (not illustrated). The fixing apparatus  49  fixes the toner image onto paper by applying pressure and heat. 
   The toner separation section  60  has a plurality of sheet feed cassettes  61 . They normally accommodate sheets of paper having different size and type. The first sheet feed roller  62  feeds the sheets of paper held in the sheet feed cassette  61 , sequentially starting from the topmost one, and sends them to the sheet conveyance section  70 . The sheet feed path from the high-volume sheet feed tray  90  is omitted from the drawing. 
   The sheet conveyance section  70  comprises: 
   a normal path  70   a  for ensuring that the paper fed out of the sheet feed cassettes  61  passes through the transfer position between the photoconductor  43  and transfer apparatus  46 , and is ejected to the succeeding stacking apparatus  80  through the fixing apparatus  49  located downstream; and 
   a reversing path  70   b  for reversing the paper having passed through the fixing apparatus  49  and again meeting the normal path  70   a  upstream from the transfer position thereafter. When the paper passes through the normal path  70   a , an image is formed on the paper. When paper passes through the normal path  70   a  again after having passed through the reversing path  70   b , an image is formed on the back of paper. Each of the paths  70   a  and  70   b  contains many conveyance rollers  71  arranged at an interval smaller than the size of the minimum-sized paper in the feed direction. 
   The stacking apparatus  80  incorporates a stacking section having an elevation function of elevating the stacked surface in response to the amount of stacking. For example, it has a capacity of stacking 5,000 sheets of paper with an image formed thereon. When paper is not loaded, the stacking section  81  is located at the topmost position, and is lowered as sheets of paper are loaded. When the loaded paper is removed, the stacking section  81  goes upward. The bottom of the stacking section  81  is provided with a wheel. When the door on the side of the stacking apparatus  80  is opened, the stacking section  81  can be pulled out to carry paper to the post-processor or the like. 
   A sub-plate  82  that allows stacking of the sheets of paper with an image formed thereon is provided on the top of the stacking apparatus  80 . The stacking apparatus  80  is capable of selecting whether paper should be stacked on the stacking section  81  or sub-plate  82 . A user is allowed to select the stacking section  81  or sub-plate  82  as a destination of paper ejection for each job. 
     FIG. 3  is a block diagram showing the electrical configuration of the image forming apparatus  10 . The automatic document conveyance apparatus  20  has an ADF control section  200  for controlling the drive section (not illustrated). The reading section  30  has a line image sensor  36  and scanner control section  210 . The scanner control section  210  controls the lighting of the light source  33  and the traveling of the exposure scanning section  35 . 
   The potential detecting means  220  receives various operations of a user and displays various type of formation to the user. It also has a function of setting acceptance section. An operation display section  220  includes: 
   a display section  221  consisting of a liquid crystal display; 
   an operation section  220  consisting of a touch switch arranged on the screen and other switches; and 
   an operation control section  223  for controlling the operation of the display section  221  and operation section  222 . 
   The printing section  40  has a laser unit  42  and a printer control section  230 . The printer control section  230  has a function of administrative control of the rotation of a polygon mirror; application of power to the charging device  44 , transfer apparatus  46  and separation apparatus  47 ; rotation of the photoconductor  43 ; and the operations of the developing apparatus  45 , cleaning apparatus  48 , fixing apparatus  49 , sheet feed section  60  and sheet conveyance section  70 . 
   The stacking apparatus  80  comprises a stacker control section  240  for controlling the operation of the elevator of the stacking section  81 ; and a stacker residual paper detecting section  241  for detecting if paper is stacked on the stacking section  81  or not. The stacker residual paper detecting section  241  consists of, for example, a reflection type light sensor and an actuator for blocking light of this light sensor by displacement, depending on whether paper is stacked or not. The stacker residual paper detecting section  241  can adopt any detection method so long as stacking of paper can be detected. 
   The aforementioned ADF control section  200 , scanner control section  210 , operation section  220 , printer control section  230  and stacker control section  240  consist of the circuits where the CPU, ROM and RAM are the major components. Various forms of control are executed according to the program stored in the ROM. 
   The main control section  100  provides administrative control of the operation of the image forming apparatus  10 . The main control section  100  comprises a read processing section  101 , DRAM control section  102 , compression/extension section  103 , image memory  104 , write processing section  105 , program memory  106 , system memory  107 , nonvolatile memory  108  and I/O port  109  and image control CPU  110 . 
   The read processing section  101  applies processing of enlargement and mirror imaging, and binary processing by error diffusion to the image data outputted by the reading section  30 . The compression/extension section  103  compresses the binarized image data and decompressed the once compressed data to restore the original image data. The image memory  104  performs the function as a page memory  104   a  capable of storing the non-compressed image data in units of page and the function as a compressed data memory  104   b  for storing the compressed image data. 
   The write processing section  105  sends the image data, read from the image memory  104  and decompressed thereafter, to the laser unit  42  at timed intervals conforming to the operation of the printer  40 . The DRAM control section  102  controls the read/write and refresh timing with respect to the image memory  104  consisting of a dynamic RAM. It also controls the timing of compressing the image data and storing it in the image memory  104 , and reading the compressed data from the image memory  104  and decompressing it. 
   The image control CPU  110  is a central processor unit for controlling the entire operation of the copying apparatus  10 . It controls the image data flow and performs the functions of managing the reservation, registration and execution of a job. The program memory  106  stores the program executed by the image control CPU  110 . The system memory  107  is a work memory for various data items on a temporary basis during the execution of the program. The nonvolatile memory  108  stores the user data and system data that should be stored even after the power has been turned off. The I/O port  109  is connected with various sensors and LEDs. 
   A print controller  120  receives print data from an external apparatus  3  such as a personal computer through the LAN and network, and performs the function of expanding it into a raster image data. 
   The image forming apparatus  10  provides a copying function by a combination of: 
   the processing of compressing the image data obtained by reading through the document reading section  30 ; and 
   the processing of image formation wherein the image data is read from the compressed data memory  104   b  and is decompressed, and an image is formed on paper by the printing section  40 . 
   In the processing of scanning the document, the data is read from the line image sensor  36  of the reading section  30  and is sent to the control section  101 , DRAM control section  102 , compression/extension section  103  (compress), DRAM control section  102  and compressed data memory  104   b , in that order. In the processing of image formation, the data is sent from the compressed data memory  104   b  to the DRAM control section  102 , compression/extension section  103 , DRAM control section  102 , write processing section  105  and laser unit  42 , in that order. Paper is fed under the control of the printer control section  230  and is ejected to the stacking apparatus  80  after the image has been transferred and fixed in position. 
   The printer function is achieved by a combination of the processing of document input wherein the print data is received and expanded into image data, and is then compressed and stored into the compressed data memory  104   b ; and the processing of image formation described above. In the processing of document input, data is sent to the application program and printer driver of the external apparatus  3 , DRAM control section  102  compression/extension section  103  (compress), DRAM control section  102  and compressed data memory  104   b , in that order. 
   Use of the compressed data memory  104   b  serving an intermediary allows the image forming apparatus  10  to read or input the document of the next job during execution of image formation of the current job, and to reserve and register the job. For example, when there are many copies to be taken, the document is read once for each page of the document, but processing of image formation is repeated by the number of times equivalent to the copies required. This arrangement allows the reading section  30  to be unoccupied during execution of the image formation, so that the next job can be reserved and registered. 
   The image forming apparatus  10  is capable of setting, for each job, the information on whether mixed stacking is enabled or not, the mixed stacking being the mode of stacking wherein sheets of paper of the current job and other jobs are loaded on the stacking apparatus  80  in a mixed form. The image forming apparatus  10  has the control function of determining if the ejection of the paper of the next job to the stacking apparatus  80  should be disabled or not, in conformity to the setting of mixed stacking in each of that job and the preceding job and the presence/absence of paper on the stacking apparatus  80 , when starting the processing of image formation of the next job. 
   Execution of the image formation of the next job is postponed until the paper of the preceding job is removed from the stacking apparatus  80 , whereby ejection of paper to the stacking apparatus  80  is disabled. 
   The function of a setting acceptance section for accepting the setting of whether mixed stacking is enabled or not is achieved by the operation display section  220  and image control CPU  110 . When accepting the setting from the external apparatus  3 , the print controller  120  also performs the function as a setting acceptance section partly. 
   The image control CPU  110  has a function of the paper ejection control section  111  for determining if ejection of paper of the next job to the stacking apparatus  80  is disabled or not. The image control CPU  110  of the operation display section  220  provides the function of reporting means for reporting that ejection of paper of the next job to the stacking apparatus  80  is disabled, whenever this ejection has been actually disabled. 
     FIG. 4  shows a basic screen  300  for receiving the request from a copy shop. The basic screen  300  contains various letters and patterns for reporting the settings to the user, and various operation keys for modifying the current settings. In the drawing, the hatched operation keys and display columns indicate the settings selected for the item. A message display column  301  indicating various information guides and warnings are displayed is arranged on the upper portion of the basic screen  300 . 
   When the Output Setting button  302  on the basic screen  300  is pressed, the output setting shown in  FIG. 5  is popped up for display. The output setting screen  310  has an output destination selection column  311  and a mixed stacking selection column  312 . The output destination selection column  311  allows selection between the stacking section  81  and sub-plate  82  as a paper ejection destination. The mixed stacking selection column  312  makes it possible to select if the mixed stacking should be enabled or not, for the jot to be registered. Immediately when the OK button  313  is pressed, the selection is determined and the setting is established. 
     FIG. 6  shows an example of the printer property screen  320  indicated by the printer driver of the external apparatus  3  on the display of the apparatus thereof, when request for printing job is given from the external apparatus  3  to the image forming apparatus  10 . The printer property screen  320  shown in  FIG. 6  indicates the status when the POD tag  321  has been selected. When the POD tag  321  has been selected, settings for post-processing such as folding and binding can be made, and paper ejection mode, and selection of the enable/disable status of the mixed stacking can be made, using the paper ejection mode selection column  322  of pull down menu format. 
   The information on the setting of mixed stacking status (enable/disable status) is added to the print data sent from the printer driver to the image forming apparatus  10 . The image forming apparatus  10  identifies the current setting of stacking in the current print job, based on this additional information. 
   It is also possible to make such arrangements that setting of mixed stacking enable/disable status is provided as a default. In this case, “Mixed stacking: Disabled” is displayed as a default setting, when the select screen given in  FIG. 5  or  6  has appeared. This setting can be kept unchanged if the user is satisfied. If not, the setting can be changed. This arrangement saves the time of setting. The default setting can be changed as required, depending on the method of use or the time of use. 
     FIG. 1  shows the flow of the processing of paper ejection carried out when starting the processing of image formation for the next job (prior to processing of image formation of the first page). In this case, the destination of ejection for each job is assumed to be set on the stacking section  81  of the stacking apparatus  80 . In the first place, prior to processing of image formation of the current job (or the next job as viewed from the preceding job), evaluation is made to see if paper of the preceding job remains in the stacking section  81  of the stacking apparatus  80  (Step S 401 ). If not (N in Step S 401 ), processing of the image formation for the current job starts. The paper with an image formed thereon is ejected to the stacking section  81  (Step S 406 ). 
   If paper still remains in the stacking section  81  of the stacking apparatus  80  (Y in Step S 401 ), evaluation is made to see if the setting the mixed stacking status for the current job (next job) is “Enabled” or not (Step S 402 ). If the status is “Disabled” (N in Step S 402 ), the start of the image formation for this job is postponed, thereby disabling ejection of paper for this job to the stacking section  81  (Step S 404 ). Further, the message of removing the paper of the preceding job from the stacking section  81  of the stacking apparatus  80  is displayed on the operation display section  220  (Step S 405 ). This arrangement allows the user to recognize that execution of the job is being suspended in order to prevent the sheets of paper involved in two or more jobs, from being stacked in one and the same stacking section. The user is also informed of what action should be taken. For example, an action message such as “Remove paper from the stacker” appears on the screen. 
   When the mixed stacking of the current job is “Disabled” (Y in Step S 402 ), a decision is made on whether or not the mixed stacking of the preceding job is set to “Enabled” (Step S 403 ). If the mixed stacking of the preceding job is set to “Disabled”, ejection of paper to the stacking section  81  in the image formation of the current job is disabled, in the similar manner to the above (Step S 404 ). This situation is displayed in the form of a message (Step S 405 ). 
   If the mixed stacking of the preceding job is set to “Enabled” (Y in Step S 403 ), the processing of image formation of this job is started, and the paper with the image formed thereon is ejected to the stacking section  81  (Step S 406 ). 
   If paper is removed from the stacking section  81  after paper ejection has been removed (N in Step S 401 ), processing of the image formation of the job having been suspended is started, and ejection of paper to the stacking section  81  starts (Step S 406 ). 
     FIG. 7  provides a list summarizing the result of evaluation when paper remains in the stacking section  81 . Paper ejection starts only when mixed stacking is enabled for both the preceding job and execution job (the current job or next job as viewed from the preceding job). If the mixed stacking is disabled for either the preceding job or execution job, paper ejection is disabled. 
   Incidentally, when it is possible to set to a stacking section of the ejection destination different for each job, the following paper ejection should be controlled: In Step S 401 , evaluation is made to determine whether or not paper remains in the stacking section designated as an ejection destination of the current job. In Step S 403 , evaluation should be made on the preceding job where the paper ejection destination is designated to the same stacking section as that of the current job. 
   The setting of the mixed stacking of the preceding job is stored in the system memory  107  at least until the start of processing of image formation in the next job where the same stacking section is set as the ejection destination, or until removal of paper from the stacking section as an ejection destination of the preceding job. 
   A plurality of stacking sections  81  can be provided. The following describes the case where the first and second stacking sections are provided: 
   The second stacking section is also provided with a stacker residual paper detecting section. 
   When the mixed stacking is disabled and there is recording paper of the preceding job in the first stacking section, the presence/absence of recording paper in the second stacking section is detected by the stacker residual paper detecting section. If there is no recording paper, paper is ejected to the second stacking section. In the meantime, if recording paper is present in the second stacking section as well, ejection is disabled. If there are a third and subsequent stacking sections, the same processing can be applied. 
   As shown in  FIG. 8 , when bookbinding is performed by the online post-processor, the mixed stacking is set to “Disabled” and job request is issued. This arrangement prevents sheets of paper of two or more jobs from being stacked on one and the same stacking section in a mixed form, an efficient printing work flow can be ensured, without the need of performing the sorting work. 
   When printing work as in a data center is to be performed, as shown in  FIGS. 9 and 10 , the mixed stacking is set to “Enabled” and the job request is issued. Then sheets of paper of a plurality of jobs are stacked continuously on the stacking section. This eliminates the trouble of removing paper from the stacking section for each job, and ensures a smooth printing work flow. 
   The above has described the embodiment of the present invention with reference to drawings. A specific configuration is not restricted to those shown in the embodiments. The present invention includes the modification and addition that have been made without departing from the spirit of the present invention. For example, in the aforementioned embodiment, the enable/disable status of the mixed stacking is set for each job. It is also possible to make such arrangements that the enable/disable status of the mixed stacking can be set for each operation of the apparatus in the configuration setting or initialization. In this case, assume that paper remains in the stacking section designated as the paper ejection destination of that job when starting the processing of image formation of a certain job, and the mixed stacking is set to “Disabled” in the apparatus. Then it is also possible to arrange such a configuration that ejection of paper to that stacking section is disabled in such a case; and otherwise, ejection of paper is enabled. 
   In the above description of the embodiment, an example of a digital multifunction device is used for explanation of an image forming apparatus. A printer or a printing press, for example, can be used only if an image is formed on paper and is outputted.