Patent Publication Number: US-2011052290-A1

Title: Image forming apparatus, method of controlling image forming apparatus and recording medium

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, a method of controlling an image forming apparatus and a recording medium. 
     2. Description of the Related Art 
     Recently, there is known an image forming apparatus such as a copying machine which forms an image using a special toner such as a transparent (CL) toner in addition to four, cyan (C), magenta (M), yellow (Y), and black (K) color toners. An image forming apparatus of this type can adjust the glossiness of the printed surface of a sheet by further forming an image using the transparent toner on a sheet bearing an image formed using the four, C, M, Y, and K color toners. 
     As a method of performing an image forming process using a transparent toner in an image forming apparatus, for example, Japanese Patent Laid-Open No. 2004-128911 discloses a method of printing by one image forming process using five, C, M, Y, K, and CL toners when executing an image forming process using the transparent toner. More specifically, a feeding unit feeds a sheet, an image is formed using the five toners on the fed sheet, and the sheet is discharged. This image forming process is executed once, obtaining an output result. 
     Another method forms an image by two image forming processes. First, the user causes an image forming apparatus to execute the first image forming process using the four, C, M, Y, and K color toners. Then, he sets, again in the feeding unit, the sheet bearing the image formed in the first image forming process. The user sets to execute the second image forming process using the transparent toner, and causes the image forming apparatus to execute the image forming process. 
     The amount of toner applicable to a sheet by the first image forming process is restricted by the amount of toner fixable at once by the fixing unit of the image forming apparatus. Hence, the amount of toner applicable to a sheet can be increased by performing two image forming processes, compared to applying the five toners to a sheet in one image forming process. 
     However, the following problem arises when forming an image in two image forming processes. A sheet bearing an image formed in the first image forming process is set again in the feeding unit. However, till the start of the second image forming process, another image forming process may use the sheet set in the feeding unit. For example, if an image corresponding to another print job is formed on a sheet printed in the first process, a printed product the user does not want is output. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above problems. The present invention provides a mechanism for preventing, when executing the second image forming process for a sheet having undergone the first image forming process, the use of the sheet for an image forming process different from the second image forming process. 
     One aspect of the present invention provides an image forming apparatus comprising: a control unit that executes a first image forming process to form an image on a sheet fed from one of a plurality of feeding units; and a designation unit that designates, from the plurality of feeding units, a feeding unit used for a second image forming process to be executed for the sheet having an image formed in the first image forming process executed by the control unit, wherein when the sheet is set in the feeding unit designated by the designation unit, the control unit restricts feeding of the set sheet for an image forming process different from the second image forming process. 
     Another aspect of the present invention provides a method of controlling an image forming apparatus, comprising: causing a control unit to execute a first image forming process to form an image on a sheet fed from one of a plurality of feeding units; and causing a designation unit to designate, from the plurality of feeding units, a feeding unit used for a second image forming process to be executed for the sheet having an image formed in the first image forming process executed in the causing a control unit to execute a first image forming process, wherein in the causing a control unit to execute a first image forming process, when the sheet is set in the feeding unit designated in the causing a designation unit to designate a feeding unit, feeding of the set sheet for an image forming process different from the second image forming process is restricted. 
     When executing the second image forming process for a sheet having undergone the first image forming process, the present invention can prevent the use of the sheet for an image forming process different from the second image forming process. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram exemplifying a network configuration including an image forming apparatus  100  according to an embodiment of the present invention; 
         FIG. 2  is a sectional view exemplifying the hardware arrangement of the image forming apparatus  100  according to the embodiment of the present invention; 
         FIG. 3  is a sectional view exemplifying the hardware arrangement of a printer unit  130  in the image forming apparatus  100  according to the embodiment of the present invention; 
         FIG. 4  is a block diagram exemplifying the arrangement of a control unit  110  in the image forming apparatus  100  according to the embodiment of the present invention; 
         FIG. 5  is a view showing an image data processing sequence in the image forming apparatus  100  according to the embodiment of the present invention; 
         FIG. 6  is a view exemplifying a setting screen  600  regarding image formation using a transparent toner according to the embodiment of the present invention; 
         FIG. 7  is a table exemplifying setting data  700  regarding image formation using the transparent toner according to the embodiment of the present invention; 
         FIG. 8  is a view exemplifying a printed product in image formation using the transparent toner according to the embodiment of the present invention; 
         FIG. 9  is a flowchart showing the sequence of a printing process using the transparent toner according to the embodiment of the present invention; 
         FIG. 10  is a view exemplifying a setting screen displayed on an operation unit  140  according to the embodiment of the present invention; 
         FIG. 11  is a view exemplifying a setting screen displayed on the operation unit  140  according to the embodiment of the present invention; 
         FIG. 12  is a view exemplifying an instruction screen displayed on the operation unit  140  according to the embodiment of the present invention; and 
         FIG. 13  is a view exemplifying a screen which displays the status of a job input to the image forming apparatus  100  according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A preferred embodiment of the present invention will be described below. The embodiment to be described below would help understand various concepts such as superordinate, intermediate, and subordinate concepts of the invention. The technical scope of the present invention is defined by the scope of the claims, and is not limited by the following embodiment. 
     &lt;Arrangement of Image Forming Apparatus&gt; 
     As an example of an image forming apparatus, the embodiment will explain an MFP (Multi Function Peripheral) having a plurality of functions such as a copy function and printer function. Note that the image forming apparatus may be an SFP (Single Function Peripheral) having only a copy function or printer function. 
     The arrangement of an MFP  100  will be exemplified with reference to  FIG. 1 . A controller unit (control unit)  110  is electrically connected to a reader unit  120  and printer unit  130 . The control unit  110  receives data from the reader unit  120  and printer unit  130 . The control unit  110  transmits various commands to the reader unit  120  and printer unit  130 . Further, the control unit  110  is connected to PCs  161  and  162  via a network  160 , and receives image data and control commands from them. The network  160  is formed from, for example, Ethernet®. 
     The reader unit  120  optically reads a document image, and converts it into image data. The reader unit  120  includes a scanner unit  121  having a function of reading a document, and a document feeding unit  122  which conveys a document to a position where the scanner unit  121  can read it. A scanner controller  123  of the scanner unit  121  controls the scanner unit  121  and document feeding unit  122  based on instructions from the control unit  110 . 
     The printer unit  130  includes a feeding unit  131  which stores sheets (paper or printing materials) for image formation (printing), a marking unit  132  which transfers and fixes image data onto a sheet, and a discharge unit  134  which discharges a printed sheet. Based on an instruction from the control unit  110 , the printer unit  130  feeds a sheet from the feeding unit  131  to the marking unit  132 . The marking unit  132  prints image data on the sheet, and discharges the sheet to the discharge unit  134 . The discharge unit  134  can perform processes such as sorting and stapling for the sheet printed by the marking unit  132 . The feeding unit  131  includes a plurality of feeding units. Sheets are stored and set in each feeding unit. Each feeding unit can store a plurality of types of sheets such as plain paper and glossy paper. Each feeding unit can also store again a sheet printed by the printer unit  130  of the MFP  100 . Examples of the feeding unit are a feeding cassette, feeding deck, and manual feeding tray. The form of the feeding unit is not limited to them as long as a stored sheet can be conveyed to the marking unit  132 . 
     An operation unit  140  includes, for example, hard keys, a liquid crystal display, and a touch panel adhered to the surface of the liquid crystal display. The operation unit  140  accepts instructions from the user via these components. The operation unit  140  can display soft keys, and the functions and state of the MFP  100  on the liquid crystal display. The operation unit  140  transmits a command corresponding to an instruction from the user to the control unit  110 . An HDD (Hard Disk Drive)  150  stores various settings and image data of the MFP  100 . 
     With this arrangement, the MFP  100  implements a variety of functions such as a copy function, image data transmission function, and printer function. When implementing the copy function, the control unit  110  controls the reader unit  120  to read image data of a document, and the printer unit  130  to print on a sheet using the image data. When implementing the image data transmission function, the control unit  110  converts image data of a document read by the reader unit  120  into code data, and transmits the code data to the PC  161  or  162  via the network  160 . When implementing the printer function, the control unit  110  converts code data (printing data) received from the PC  161  or  162  via the network  160  into image data, and transmits the image data to the printer unit  130 . The printer unit  130  prints on a sheet using the received image data. 
     The hardware arrangement of the MFP  100  will be exemplified with reference to  FIG. 2 . As an example of the MFP  100 , the embodiment will describe a one-drum color printer that performs color printing using a single drum. In  FIG. 2 , a document feeding unit  201 , scanner unit  202 , printer unit  203 , and control unit  204  correspond to the document feeding unit  122 , scanner unit  121 , marking unit  132 , and control unit  110  in  FIG. 1 . 
     The document feeding unit  201  conveys a document onto to a document glass  205  so that the document can be optically read. The scanner unit  202  transmits, to the control unit  204 , an optical signal obtained by scanning a document image using an image reading sensor. The control unit  204  generates an image signal using the received optical signal. Based on the image signal generated by the control unit  204 , the printer unit  203  prints on a sheet fed from the feeding unit. Note that the MFP  100  includes, as feeding units, feeding cassettes  206  to  209 , a feeding deck  210 , and a manual feeding tray  211 . The printer unit  203  can feed a sheet from any feeding unit. 
     (Arrangement of Printer Unit  130 ) 
     The hardware arrangement of the printer unit  130  will be exemplified with reference to  FIG. 3 . A printer controller  135  controls each unit of the printer unit  130 . An image forming process for a sheet under the control of the printer controller  135  will be described briefly. The printer controller  135  rotates a photosensitive member  302  counterclockwise in  FIG. 3  based on an instruction from the control unit  110 . A charger  319  charges the surface of the photosensitive member  302 . A laser driver  301  irradiates the photosensitive member  302  with a laser beam based on an image signal generated by the control unit  110 , forming an electrostatic latent image on the surface of the photosensitive member  302 . 
     Based on an instruction from the printer controller  135 , a developer  303  develops the electrostatic latent image formed on the surface of the photosensitive member  302  using a plurality of color developing materials (toners). The toner image developed on the surface of the photosensitive member  302  is transferred to an intermediate transfer member  305  which rotates clockwise in  FIG. 3 . Note that the developer  303  includes developers corresponding to five toners, that is, yellow (Y), magenta (M), cyan (C), and black (K) colored toners and a transparent (CL) toner. For color printing in the one-drum MFP  100  shown in  FIG. 3 , images are transferred to the intermediate transfer member  305  sequentially for the respective colors. More specifically, toner images sequentially formed on the surface of the photosensitive member  302  by the developer  303  are superimposed and transferred at the same position on the intermediate transfer member  305  in synchronism with rotation of the intermediate transfer member  305 . As a result, a 1-page color image is formed on the surface of the intermediate transfer member  305  using the Y, M, C, and K toners. When it is designated to print using the CL toner, a CL toner image may be transferred subsequently to transferring the Y, M, C, and K toner images. 
     While executing this image forming process, the printer controller  135  feeds a sheet from one of the feeding cassettes  206  to  209 , feeding deck  210 , and manual feeding tray  211 . For example, when feeding a sheet from one of the feeding cassettes  206  to  209 , the printer controller  135  operates a corresponding pickup roller  311 . Feeding rollers  313  convey the fed sheet to the position of a conveyance roller  315 . The conveyance roller  315  conveys the sheet up to a registration roller front position  316 . 
     The printer controller  135  conveys the sheet to a position between the intermediate transfer member  305  and a transfer belt  306  at the timing when a color image transfer process for the intermediate transfer member  305  is completed. At this position, the transfer belt  306  transfers, to the sheet, the 1-page image formed on the intermediate transfer member  305 . After the transfer process, the printer controller  135  conveys the sheet to fixing rollers  307   a  and  307   b.  The fixing rollers  307   a  and  307   b  heat and press the toner image to fix it onto the sheet. The printer controller  135  discharges the sheet to a face-up discharge port  317  or face-down discharge port  318 . 
     In double-sided printing, the printer controller  135  conveys a sheet having undergone the fixing process to a reverse path  320 . Reverse rollers  321  turn the sheet upside down, and the conveyance roller  315  conveys the sheet again to the registration roller front position  316  via a reverse surface path  322 . By the same method as image formation for the first page (obverse surface) on one surface of the sheet, the printer controller  135  executes image formation on the second page (reverse surface) on the other surface of the sheet. Then, the printer controller  135  discharges the sheet bearing images on the two surfaces to the face-up discharge port  317  or face-down discharge port  318 . By this operation, the image forming process (printing process) for the sheet is completed. 
     The control unit  110  performs this operation by executing a print job stored in the HDD  150 . The print job is obtained by associating image data to be printed with data that sets printing conditions (for example, single/double-sided setting and printing layout setting) used to print the image data. 
     (Arrangement of Control Unit  110 ) 
     The block arrangement of the control unit  110  will be exemplified with reference to  FIG. 4 . A main controller  411  includes a CPU  412 , bus controller  413 , and various I/F controller circuits. The CPU  412  and bus controller  413  comprehensively control the operation of the whole control unit  110 . The CPU  412  executes a variety of operations based on a program loaded from a ROM  414  via an ROM I/F  415 . For example, based on the loaded program, the CPU  412  interprets code data (for example, PDL (Page Description Language)) received from the PC  161  or PC  162  shown in  FIG. 1 . The bus controller  413  controls data transfer via each I/F, for example, controls bus arbitration and DMA data transfer. 
     A DRAM  416  is connected to the main controller  411  via a DRAM I/F  417 . The DRAM  416  is used as a work area for the operation of the CPU  412 , and an area for accumulating image data. A codec  418  compresses raster image data accumulated in the DRAM  416  according to a scheme such as MH, MR, MMR, JBIG, or JPEG. Also, the codec  418  decompresses compressed/accumulated code data into raster image data. An SRAM  419  is used as the temporary work area of the codec  418 . The codec  418  is connected to the main controller  411  via an I/F  420 . Data transfer between the SRAM  419  and the DRAM  416  is controlled by the bus controller  413  and implemented by DMA transfer. 
     A graphic processor  435  performs processes such as image rotation, image scaling, color space conversion, and binarization for raster image data accumulated in the DRAM  416 . An SRAM  436  is used as the temporary work area of the graphic processor  435 . The graphic processor  435  is connected to the main controller  411  via an I/F  437 . Data transfer between the graphic processor  435  and the DRAM  416  is controlled by the bus controller  413  and achieved by DMA transfer. 
     A network controller  421  is connected to the main controller  411  via an I/F  423  and an external network (for example, the network  160 ) via a connector  422 . An expansion connector  424  for connecting an expansion board, and an I/O control unit  426  are connected to a general-purpose high-speed bus  425 . The general-purpose high-speed bus  425  is, for example, a PCI bus. In the I/O control unit  426 , an asynchronous serial communication controller  427  has two channels to transmit/receive control commands to/from the CPUs of the reader unit  120  and printer unit  130 . 
     The I/O control unit  426  is connected to a scanner I/F  440  and printer I/F  445  via an I/O bus  428 . A panel I/F  432  is used to transmit/receive data to/from the operation unit  140 . The panel I/F  432  transfers image data from an LCD controller  431  to the operation unit  140 . The panel I/F  432  transfers, to the I/O control unit  426  via a key input I/F  430 , a key input signal input via the hard key or touch panel of the operation unit  140 . 
     A real-time clock module  433  receives power from a backup battery  434 , and updates and saves the date and time managed in the MFP  100 . An E-IDE I/F  461  connects the HDD  150 . The CPU  412  stores image data in the HDD  150  via the E-IDE I/F, and reads image data from the HDD  150 . 
     Connectors  442  and  447  are connected to the reader unit  120  and printer unit  130 , respectively. The connectors  442  and  447  are connected to the scanner I/F  440  and printer I/F  445  via asynchronous serial I/Fs  443  and  448 , and video I/Fs  444  and  449 , respectively. 
     The scanner I/F  440  is connected to the reader unit  120  via the connector  442 , and the main controller  411  via a scanner bus  441 . The scanner I/F  440  performs a predetermined process for an image received from the reader unit  120 . The scanner I/F  440  outputs, to the scanner bus  441 , a control signal generated based on a video control signal received from the reader unit  120 . The bus controller  413  controls data transfer from the scanner bus  441  to the DRAM  416 . 
     The printer I/F  445  is connected to the printer unit  130  via the connector  447 , and the main controller  411  via a printer bus  446 . The printer I/F  445  performs a predetermined process for image data output from the main controller  411 , and outputs the processed image data to the printer unit  130 . The bus controller  413  controls transfer of raster image data rasterized in the DRAM  416  to the printer unit  130 . The raster image data is DMA-transferred to the printer unit  130  via the printer bus  446 , printer I/F  445 , and video I/F  449 . 
     An SRAM  451  receives power from a backup battery and can keep holding stored contents even when the entire MFP  100  is turned off. The SRAM  451  is connected to the I/O control unit  426  via a bus  450 . An EEPROM  452  is a memory that is also connected to the I/O control unit  426  via the bus  450 . 
     &lt;Image Forming Process Using Transparent Toner&gt; 
     An image data processing sequence will be described with reference to  FIG. 5 .  FIG. 5  shows functional blocks implemented by executing a program stored in the ROM  414  by the CPU  412  of the control unit  110 , and a data flow.  FIG. 5  shows an image data input portion  501 , color conversion portion  502 , tone correction portion  503 , halftone processing portion  504 , transparent toner image generation portion  506 , line delay portion  510 , and mask processing portion  511 . 
     The image data input portion  501  receives image data obtained by reading a document by the reader unit  120 , or image data received from the PC  161  or  162  via the network  160 . The CPU  412  temporarily accumulates, for each page in the DRAM  416 , the image data obtained by the reader unit  120 . Then, the CPU  412  reads out the data for each pixel and inputs it to the image data input portion  501 . 
     The image data input portion  501  divides the image data of each pixel input by the CPU  412  into R, G, and B image signals, and outputs them to the color conversion portion  502 . The color conversion portion  502  converts the received R, G, and B image signals into C, M, Y, and K image signals corresponding to a printout color space, and outputs them to the tone correction portion  503 . The tone correction portion  503  performs tone correction for the received image signals so as to obtain a normal tone characteristic, and outputs the resultant image signals to the halftone processing portion  504 . The halftone processing portion  504  performs a pseudo halftone process for the image signals having undergone tone correction, and outputs the image signals to the printer unit  130 . 
     In contrast, a transparent toner pattern designation portion  505  is arranged in the operation unit  140  or the PC  161  or  162 . The transparent toner pattern designation portion  505  creates setting data regarding image formation using a transparent toner based on data input from the user, and inputs the setting data to a transparent toner image generation portion  506 . In accordance with the setting data, the transparent toner image generation portion  506  generates bitmap image data to be output in image formation using a transparent toner. The transparent toner image generation portion  506  outputs the image data as an image signal of each pixel to the line delay portion  510 . The line delay portion  510  delays an image signal contained in the received image data, and outputs the delayed image signal to the mask processing portion  511 . The mask processing portion  511  performs a mask process for the received image signal, and outputs the resultant image signal to an image forming portion  512  (corresponding to the printer unit  130  in  FIG. 1 ). 
     Based on the image signal received from the halftone processing portion  504  and that received from the mask processing portion  511 , the image forming portion  512  prints C, M, Y, and K color images and a transparent toner image on a sheet. 
     A setting screen  600  regarding image formation using the transparent toner will be exemplified with reference to  FIG. 6 . The transparent toner pattern designation portion  505  implemented by the CPU  412  displays the setting screen  600  on the operation unit  140 . This screen appears on the operation unit  140  when the user presses a transparent toner print setting key (not shown) arranged on the operation unit  140 . The CPU  412  accepts an input from the user via the touch panel attached to the operation unit  140 . Note that the setting screen  600  may be displayed on the display device of the PC  161  or  162  under the control of a printer driver installed in the PC  161  or  162 . 
     A check box  601  is used to set whether to print a specific character string using the transparent toner. When the user checks the check box  601 , the CPU  412  sets to print a character string selected from a pull-down menu  602  using the transparent toner. Candidates selectable by the user using the pull-down menu  602  are character strings such as “Eyes-only” and “Copy Protected”, in addition to “Confidential” shown in  FIG. 6 . 
     A check box  603  is used to set whether to print an arbitrary character string using the transparent toner. When the user checks the check box  603 , the CPU  412  sets to print an arbitrary character string input to a text box  604  using the transparent toner. The user can input a character string to the text box  604  using a software keyboard displayed on the operation unit  140 . When the user presses an input button  605 , the software keyboard appears on the operation unit  140 . 
     A check box  606  is used to set whether to print the date using the transparent toner. When the user checks the check box  606 , the CPU  412  sets to print, using the transparent toner, the date when printing is executed. Note that the CPU  412  acquires date data from the real-time clock module  433 , and displays it on a date data display field  607 . 
     A check box  608  is used to set whether to print the user ID of the user of the MFP  100  using the transparent toner. The CPU  412  stores, in the HDD  150 , a user ID input when the user logs in to the MFP  100 , and displays his user ID in a user ID display field  609 . 
     A check box  610  is used to set whether to print an image using the transparent toner. A reference button  611  is used to select an image to be printed using the transparent toner when the user checks the check box  610 . When the user presses the reference button  611 , the CPU  412  causes the operation unit  140  to display a list of image data stored in the HDD  150 . The user selects arbitrary image data from the displayed list. When the user selects image data, the CPU  412  displays the file name of the selected image data and the like in a text box  612 . 
     Pull-down menus  613  and  614  are used to designate the printing format (size and inclination of a character string) of a character string when printing the character string using the transparent toner. When the user presses the pull-down menus  613  and  614 , the CPU  412  displays candidates he can select. 
     A preview display screen  616  displays the preview of a printing result when printing using the transparent toner in accordance with contents set in the setting screen  600 . The CPU  412  presents this display. The user visually checks the preview display screen  616  and can confirm the output result of a printed sheet before the start of actually printing on a sheet. 
     When the user finally presses an OK button  615 , the CPU  412  finalizes the contents set in the respective items of the setting screen  600 . Based on the setting contents, the CPU  412  generates setting data to be output to the transparent toner image generation portion  506  in  FIG. 5 . 
       FIG. 7  is a view exemplifying finalized setting data  700 . The setting data shown in  FIG. 7  includes items such as “transparent printing” and “character string printing”, and their parameters. The items shown in  FIG. 7  correspond to those the user can set in  FIG. 6 . 
     An item  701  is information indicating whether to print using the transparent toner. “YES” is set when it is set in the screen shown in  FIG. 6  to print using the transparent toner, and “NO is set when it is set not to print using the transparent toner. If item  701  is “YES”, the CPU  412  executes printing using the transparent toner in accordance with the settings of items  702  to  708 . If item  701  is “NO”, the CPU  412  does not execute printing using the transparent toner, and items  702  to  708  need not be set. 
     Item  702  is information indicating whether to print a character string using the transparent toner, and correspond to the settings of the check boxes  601  and  603  in  FIG. 6 . “YES” is set when the check box  601  or  603  is checked in the screen shown in  FIG. 6 , and “NO is set when neither the check box  601  nor  603  is checked. A character string is set in item  703  in correspondence with the pull-down menu  602  and text box  604  in  FIG. 6 . The size and inclination of the character string are set in items  704  and  705  in correspondence with pull-down menus  613  and  614 . 
     Items  706  and  707  are pieces of information indicating whether to print a date and user ID in correspondence with the check boxes  606  and  608  in  FIG. 6 . Item  708  is information indicating whether to designate an image in correspondence with the check box  610  and reference button  611 . An item  709  is an image ID indicating an image to be used when the image is designated. In item  709 , a file path for specifying an image may be set in place of the image ID. The setting data  700  may include an item not shown in  FIG. 7 . 
     The generated setting data  700  is transmitted to the transparent toner image generation portion  506 . The CPU  412  forms a toner image for the transparent toner based on the setting data, and causes the printer unit  130  to execute a printing process for the sheet.  FIG. 8  is a view showing an example of a product printed by the MFP  100 . In  FIG. 8 , an image is formed with the Y, M, C, and K color toners using image data generated by reading a document  801  by the reader unit  120 . In addition, an image is formed with the transparent toner in accordance with the setting data  700 . As shown in  FIG. 8 , a specific character string “Copy Protected” selected from the pull-down menu  602  in  FIG. 6  is printed with the transparent toner on a printed product  802  output from the printer unit  130 . Also, a date “2004.10.10” displayed in the date data display field in  FIG. 6  is printed on the printed product  802 . Note that the output result shown in  FIG. 8  is merely an example, and another character string and image can also be printed in accordance with settings in the setting screen  600  shown in  FIG. 6 . 
     The following effects can be obtained by creating a printed product as described above using printing toners. For example, a product printed using the transparent toner can be guaranteed as an original. If the product (original) printed using the transparent toner is copied, the image of the transparent toner is not copied, so the copy and original can be discriminated from each other. By printing an image using the transparent toner in a specific area of an original image, the glossiness of the printing result can be adjusted at the portion where the transparent toner is attached. 
     Printing methods using the transparent toner are roughly classified into the following two: 
     (1) Method of Printing Using Transparent Toner in One Printing Process 
     The MFP  100  transfers and fixes an image onto a sheet by one printing process using the five, C, M, Y, K, and CL toners. 
     (2) Method of Printing Using Transparent Toner in Two Printing Processes 
     The MFP  100  transfers and fixes an image onto a sheet using the four, C, M, Y, and K color toners as the first printing process (called the first process, first printing process, or first image forming process). After the user sets again the temporarily discharged sheet in the feeding unit and inputs a printing start instruction, the MFP  100  feeds the sheet, and transfers and fixes an image onto the sheet using the CL toner as the second printing process (second process). The second process will also be called the second printing process or second image forming process. 
     According to method (1), all five toner images are transferred by one printing process, shortening the time taken for the printing process. However, the performance of a fixing unit restricts the amount of toner fixable at once onto one sheet. Thus, method (1) of fixing even an image of the CL toner together with those of the four, C, M, Y, and K color toners by one process has the restriction on the amount of fixable transparent toner. To the contrary, according to method (2), only an image of the transparent toner is fixed in the second process. Compared to method (1), method (2) can increase the amount of fixable transparent toner. 
     However, in method (2), until a sheet discharged after the first process is set in a specific feeding unit and the MFP  100  starts the second process, another print job may use the feeding unit. In this case, erroneous printing may be done by the other print job for the sheet printed in the first process. Hence, a user who executes a print job according to method (2), and one who executes another print job have to execute printing again. 
     Under the circumstance, the MFP  100  according to the embodiment controls to prevent, when executing a print job including two printing processes, that is, the first and second processes mentioned above, erroneous use of a sheet in the feeding unit by another print job after executing the first process. The sequence of a printing process according to the embodiment will be explained with reference to  FIG. 9 . Note that the CPU  412  reads out a program stored in the ROM  414  and executing it, thereby achieving the processes of steps shown in  FIG. 9 . 
     In S 901 , the CPU  412  displays a setting screen regarding a print job on the operation unit  140 . The user makes settings regarding the print job via the setting screen.  FIG. 10  shows a setting screen  1000  regarding a copy job, which is one of print jobs, as an example of the setting screen displayed on the operation unit  140 . In the setting screen  1000 , a button  1001  (“transparent toner+1 pass”) corresponds to method (1), and a button  1002  (“transparent toner+2 passes”) corresponds to method (2). When the CPU  412  detects that the user has pressed the button  1002 , it displays, for example, a screen shown in  FIG. 11  on the operation unit  140  as a setting screen for designating a feeding unit to be used in the second process out of two printing processes. In  FIG. 11 , buttons  1101  to  1106  corresponding to the respective feeding units of the MFP  100  are displayed. For example, the buttons  1101  to  1104  correspond to the feeding cassettes  206  to  209 , the button  1105  corresponds to the feeding deck  210 , and the button  1106  corresponds to the manual feeding tray  211 . The user can designate a feeding unit to be used in the second process by pressing one of the buttons displayed in the setting screen. After that, the process shifts to S 902 . In S 901 , as shown in  FIG. 10 , the user can make various settings such as the size and type of sheet, the copy ratio, density, and monochrome/color printing as settings regarding the print job, in addition to the feeding unit to be used in the second process. 
     In S 902 , the CPU  412  executes a printing process for an image on a sheet as the first process using the four, C, M, Y, and K color toners, other than the transparent toner (CL). For example, when the CPU  412  detects that the user has pressed a start button arranged on the operation unit  140 , it starts the process. After the end of the printing process, the process shifts to S 903 . 
     In S 903 , the CPU  412  displays an instruction screen  1200  in  FIG. 12  on the operation unit  140  in order to instruct the user to set a sheet in the feeding unit designated in S 901 . From this, the user can easily confirm the feeding unit in which he is to set the sheet having the image formed in the first process. For example, when the number of pages to be processed in the first process is large, a long time is taken till the completion of the first process after the start. Especially in this case, a display as shown in  FIG. 12  is effective. This display allows the user to easily recognize the feeding unit in which he is to set the sheet having the image formed in the first process, without remembering the feeding unit. In S 904 , the CPU  412  shifts the state of the MFP  100  to a feeding standby state, and then the process shifts to S 905 . In S 904 , the CPU  412  saves information of the print job in the HDD  150 . When the user sets the sheet in the feeding unit designated for the second process of the print job, the CPU  412  can execute the saved print job. Although not shown in  FIG. 12 , the instruction screen  1200  may display a guidance indicating how to set the sheet (for example, face-up, face-down, or top-and-bottom direction). 
     In S 905 , the CPU  412  determines whether the user has set the sheet in the designated feeding unit. For example, when an opening/closing sensor arranged in each of the feeding cassettes  206  to  209  and feeding deck  210  detects that a corresponding feeding unit has been opened and then closed, the CPU  412  determines that the user has set the sheet in the feeding unit. As for the manual feeding tray  211 , when a sheet sensor arranged in the manual feeding tray  211  detects a sheet, the CPU  412  determines that the user has set the sheet in the feeding unit. 
     If no sheet has been set in S 905 , the process shifts to S 906 . In S 906 , the CPU  412  determines whether another print job different from one in progress has been input. If no other print job has been input, the process returns to S 905 ; if another print job has been input, shifts to S 907 . 
     In S 907 , the CPU  412  determines whether a feeding unit designated by the other print job is one to be used in the second process of the job in progress. If a feeding unit designated by the other print job is one to be used in the second process, the CPU  412  inhibits the other print job from performing a printing process using the feeding unit, and the process returns to S 905 . If a feeding unit designated by the other print job is not one to be used in the second process, the process shifts to S 908 . In S 908 , the CPU  412  allows the use of the feeding unit designated by the other print job, and executes the other print job. Upon completion of executing the job, the process returns to S 905 . 
       FIG. 13  exemplifies a screen that displays the status of a job input to the MFP  100 . A screen  1300  displays three jobs A to C. Job A is a print job which executes two printing processes using the transparent toner according to the embodiment. Jobs B and C are normal print jobs which do not execute a printing process using the transparent toner. In  FIG. 13 , job A designates the feeding cassette  206  as a feeding unit to be used in the second process using the transparent toner, and stands by (“suspended”) until the user sets a sheet in the feeding cassette  206 . For example, when job B designates the feeding cassette  206  and job C designates the feeding cassette  207 , the CPU  412  controls to inhibit execution of job B in S 907 , but execute job C in S 908 . Instead of unconditionally inhibiting execution of job B, the CPU  412  may prompt the user to confirm whether to execute job B, and upon accepting an instruction from the user to execute job B, allow execution of job B. In this case, when the CPU  412  has not accepted an instruction from the user to execute job B, it inhibits execution of job B. In this way, the CPU  412  restricts execution of job B in S 907 . 
     If the CPU  412  determines in S 905  that the user has set the sheet, the process advances to S 909 . The CPU  412  cancels the standby state in S 909 , and in S 910 , executes a printing process for an image on the sheet using the transparent toner (CL) as the second process. If job C is in progress, the CPU  412  controls to execute the second process after the completion of executing job C. Note that the CPU  412  restricts the use of the feeding unit, which is to be used in the second process, by another print job (for example, job B) till the start of the second process after determining in S 905  that the user has set the sheet. In this example, it is controlled to automatically execute the second process when the CPU  412  determines that the user has set the sheet. However, the present invention is not limited to this, and the second process may start when the CPU  412  detects in S 910  that the user has pressed the start button arranged on the operation unit  140 . 
     After the end of the printing process in S 910 , the CPU  412  discharges the sheet to the discharge unit  134 . When the second process starts upon detecting that the user has pressed the start button, the CPU  412  restricts the use of the feeding unit, which is to be used in the second process, by another print job till the start of the second process after the user has pressed the start button. 
     If another job is inhibited in S 907  from using the feeding unit to be used in the second process, the time of inhibition may be limited to a predetermined one. For example, when a predetermined time has elapsed after executing the first process, the CPU  412  cancels inhibition of the use of the feeding unit by another job and allows the use of the feeding unit designated for the second process, to execute the other job. Upon the lapse of the predetermined time, if there is a running job or there is a job which has been accepted before the other job, the CPU  412  executes the other job upon completion of executing such a job. In this way, the time-out time can be set for the time by which the user needs to set a sheet in a designated feeding unit. This can prevent, for example, no execution of another job using the same feeding unit as one to be used in the second process of a given print job upon completion of executing the first process of the print job. Note that the predetermined time may be set in shipment of an image forming apparatus from the factory or by the user via the operation unit  140 . 
     The predetermined time may be set to the time till the end of the second process. In this case, after the end of the second process, another job can quickly use the feeding unit which has been used in the second process, increasing the job processing efficiency. 
     As described above, the image forming apparatus according to the embodiment can obtain the following effects. When a sheet printed in the first printing process of a given print job is set in a feeding unit, the image forming apparatus can prevent the use of the sheet set in the feeding unit by another print job different from the second process of the print job. 
     More specifically, after the end of the first printing process, the user sets a sheet having undergone the first printing process in a feeding unit designated in advance. The image forming apparatus prevents the use of the sheet set in the feeding unit for another print job. When executing two printing processes for the same sheet, the image forming apparatus can prevent another job from erroneously using the sheet bearing an image formed in the first printing process. 
     In the embodiment, the first process is a printing process using the four, C, M, Y, and K colored toners, and the second process is one using the transparent toner (CL). However, the present invention is not limited to this, and is applicable to an image forming apparatus using a print job including a plurality of printing processes. For example, the present invention is applicable to a case in which the image forming apparatus performs double-sided printing as a plurality of printing processes. That is, the first process is a printing process for one surface of a sheet, and the second process is one for its other surface. 
     The second process is not limited to an image forming process of forming an image, and suffices to be a process to be executed for a sheet bearing an image formed in the first image forming process. The second process may be a work process such as a gloss process. 
     In the above-described embodiment, another print job has been exemplified as an image forming process, execution of which is restricted, but the present invention is not limited to this. For example, execution of an image forming process to form a predetermined test pattern for calibration may be restricted for a sheet fed from one of feeding units. The condition under which execution of the image forming process to form the predetermined test pattern is restricted is that a feeding unit set as the feeding source of a sheet on which the predetermined test pattern is to be formed is the same as that of the second process. 
     OTHER EMBODIMENTS 
     Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment(s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (for example, computer-readable medium). 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2009-201081, filed Aug. 31, 2009, which is hereby incorporated by reference herein in its entirety.