Abstract:
An image forming device includes a first medium container for a first printing medium; a second medium container for a second printing medium; a medium feeder for feeding one of the first and second media from one of the first and second medium containers; a special process unit that performs a special process on the first printing medium fed by the medium feeder when image data, including instruction information instructing the special process, is received; a detector that detects a characteristic of the first printing medium; a special process capability determination unit for determining whether or not the special process can be performed based on the characteristic; and a control unit that ejects the printing medium and that causes the second medium to be fed from the second medium container when the special process capability determination unit determines that the special process cannot be performed.

Description:
CROSS REFERENCE 
       [0001]    The present application is related to, claims priority from and incorporates by reference Japanese patent application number 2009-002618, filed on Jan. 8, 2009. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an image forming device that has a special process functions, such as a duplex printing function, a staple function, and a hole punch function. Also, the present invention relates to an image forming method. 
       BACKGROUND 
       [0003]    An image forming device that has a duplex printing function was proposed in JP Laid-Open Publication No. 2002-167077. When the image forming device receives image data from a user terminal through a network, the image forming device determines whether or not the image data has instruction information for duplex printing first. 
         [0004]    When the image data has the instruction information for duplex printing, the device turns on a thickness detection sensor located in a conveying path and drives a sheet feeding roller so that the device feeds a sheet stored in a sheet tray by rotation of the sheet feeding roller. The fed sheet is carried to the conveying path, and the thickness of the sheet is detected by the thickness detection sensor. A judgment unit judges whether the detected thickness is greater than or equal to the threshold value. The threshold value represents a predetermined thickness at or above which duplex printing cannot be performed. 
         [0005]    The threshold value is set based on the following reasons. When a sheet on which a print process is performed on one side is thick and has large stiffness, a large curl occurs on the sheet because the sheet is carried through the conveying path while receiving a large bending force. When the sheet with the large curl is carried to the conveying path again to perform the print process on a reverse side, there is a high possibility that a paper jam will occur. Based on the reasons discussed above, the image forming device cannot perform duplex printing when the sheet thickness is greater than or equal to a predetermined threshold value in order to prevent the duplex printing process for a sheet that has an inappropriate thickness for duplex printing. 
         [0006]    When the detected thickness of the sheet is less than the threshold value, the image forming device judges that the sheet is satisfactory for duplex printing and performs an image forming process and an image fusing process on a first side of the sheet first. The image forming device carries the sheet to a sheet inversion mechanism that is used at the time of the duplex printing process for turning the sheet with reverse-side up. Then, the image forming device performs the image forming process and the image fusing process on the reverse side, or second side, as was done on the first side and ejects the sheet from an outlet. 
         [0007]    On the other hand, when the detected thickness is equal to or more than the threshold value, the image forming device judges that the sheet does not qualify for the duplex printing and drives a transferring belt and so on to carry the sheet, which is fed to a conveying path, by the belt rotation for ejecting the sheet from the outlet without printing. 
         [0008]    When the image forming device performs the duplex printing process, and when the thickness of the sheet, which is fed from the sheet tray, is equal to or more than the threshold value, the print process is forcibly stopped as discussed above. Therefore, there is a problem that the device may fail to meet the user&#39;s needs. 
         [0009]    In view of the problem mentioned above, an object of the present invention is to provide another option for a user through outputting selection information that is whether or not the duplex printing process is performed by using another sheet stored in another sheet tray and that is to be selected by the user when the thickness of the sheet fed from the sheet tray is equal to or more than the threshold value. 
       SUMMARY 
       [0010]    In order to solve the above problems, the image forming device includes a first medium container, which contains a first printing medium; a second medium container, which contains a second printing medium; a medium feeder for feeding one of the first and second media from one of the first and second medium containers; a special process unit that performs a special process on the first printing medium fed by the medium feeder when image data, which includes instruction information instructing the special process, is received; a detector that detects a characteristic of the first printing medium; a special process capability determination unit for determining whether or not the special process instructed by the instruction information can be performed based on the characteristic; and a control unit that ejects the printing medium and that causes the second medium to be fed from the second medium container when the special process capability determination unit determines that the special process cannot be performed. In the following embodiments, the medium container is performed as sheet trays. The printing medium is performed as a sheet or OHP. The medium feeders are as sheet feeding rollers. The special process unit is configured with a special process instruction status determination unit, or the detector is performed by an inlet sensor. 
         [0011]    Also, an image forming method of the application includes receiving image data, wherein the image data includes instruction information for instructing that a special process be performed in addition to image formation; feeding a sheet from one of first and second medium containers to a conveying path; detecting a characteristic of the sheet; determining whether or not the sheet qualifies for the special process based on the characteristic. If the sheet qualifies for the special process, the method includes performing the special process on the sheet. If the sheet fails to qualify for the special process, the method includes presenting a list of options on a display, and one of the options corresponds to an instruction for the image forming device to feed a sheet from the other of the first and second medium containers for the image forming process. 
         [0012]    With the above configuration, the user&#39;s expecting duplex printing are realized 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a block diagram of a control configuration of a printer  100  of a first embodiment. 
           [0014]      FIG. 2  is a schematic view of the printer  100  of the first embodiment. 
           [0015]      FIG. 3  is a schematic view of a display of print selection information of the first embodiment. 
           [0016]      FIG. 4  is a flow diagram of operation of the printer  100  of the first embodiment. 
           [0017]      FIG. 5  is a flow diagram of operation of the printer  100  of the first embodiment. 
           [0018]      FIG. 6  is a block diagram of a control configuration of a printer  100   a  of a second embodiment. 
           [0019]      FIG. 7  is a schematic view of the printer  100   a  of the second embodiment. 
           [0020]      FIGS. 8A ,  8 B and  8 C are explanatory diagrams of a configuration of a sheet thickness information memory unit  108  of the second embodiment. 
           [0021]      FIG. 9  is an explanatory diagram of a configuration of a threshold value information memory unit  105   a  of the second embodiment. 
           [0022]      FIG. 10  is a schematic view of a display of post-process selection information of the second embodiment. 
           [0023]      FIG. 11  is a flow diagram of operation of the printer  100   a  of the second embodiment. 
           [0024]      FIG. 12  is a flow diagram of operation of the printer  100   a  of the second embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    An embodiment according to the present invention is explained in detail below with reference to drawings. In the embodiment, a printer that is an example of an image forming device is explained. 
       First Embodiment 
       [0026]    As shown in  FIG. 2 , the printer  100  has the following structures: a first sheet tray  11  that loads and holds a sheet  1 ; a second sheet tray  12  that loads and holds another sheet  2 ; a conveying unit  13  having conveying paths and conveying rollers; an inlet sensor  14 ; a pair of registration rollers  15 ; a writing sensor  16 ; a sheet thickness sensor  17 ; an image forming unit  18 ; an image transferring unit  19 ; an image fusing unit  20 ; a duplex separator  21 ; an inversion unit  22 ; an eject unit  23 ; and a stacker unit  24  in which an ejected sheet from the printer  100  is loaded. The printer  100  in the first embodiment is configured with two sheet trays, the first sheet tray  11  and the second sheet tray  12 . However, the present invention is not limited to the printer  100  with two sheet trays and can be applied to a printer with more than two sheet trays. 
         [0027]    The printer  100  connects a host device  200 , such as a personal computer (PC) operated by a user, through a network  300  (see  FIG. 1 ). When the printer  100  receives print data from the host device  200 , the printer  100  converts the print data to image data that is usable for a print process through a rasterize transformation unit, which is not shown. The printer  100  performs the image forming process based on the image data. In this embodiment, the printer  100  performs the image forming process based on the print data received from the host device  200 . However, the present invention is not limited to the structure discussed above and can be applied to a printer that performs the image forming process based on image data created by scanning a manuscript through a reader (scanner) that is not shown. 
         [0028]    As shown in  FIG. 2 , the first sheet tray  11 , which loads and holds a sheet  1 , and the second sheet tray  12 , which loads and holds another sheet  2 , are detachable from the lower part of the printer  100 . 
         [0029]    A first sheet feeding roller  111  that feeds the sheet  1  loaded in the tray one by one from the top is located at the feeding side of the first sheet tray  11 . The first sheet feeding roller  111  is located and arranged to press the top of the sheet  1  loaded in the first sheet tray  11 . When the first sheet feeding roller  111  is driven to rotate by a drive unit, which is not shown and which is driven based on control of a control unit  101  shown in  FIG. 1 , the first sheet feeding roller  111  feeds the sheet  1  from the tray based on the rotation. The sheet  1  fed by the first sheet feeding roller  111  is supplied to a conveying unit  13  (conveying path A). 
         [0030]    A second sheet feeding roller  121  that feeds the sheet  2  loaded in the tray one by one from the top is located at the feeding side of the second sheet tray  12 . The second sheet feeding roller  121  is located and arranged to press the top of the sheet  2  loaded in the second sheet tray  12 . When the second sheet feeding roller  121  is driven to rotate by the drive unit, which is driven based on control of the control unit  101 , the second sheet feeding roller  121  feeds the sheet  2  from the tray based on the rotation. The sheet  2  fed by the second sheet feeding roller  121  is supplied to the conveying path A through a conveying path G. 
         [0031]    As shown in  FIG. 2 , the conveying unit  13  has the conveying path A, a conveying path B, a pair of conveying rollers  131 , a conveying path C, a pair of conveying rollers  132 , a pair of conveying rollers  133 , the conveying path G, and a pair of conveying rollers  134 . Each pair of conveying rollers is driven to rotate by a drive unit, which is not shown. The sheet  1  or another sheet  2  is carried along the conveying paths. 
         [0032]    A leading edge of the fed sheet abuts a nipping part that is formed between rollers of a pair of registration rollers  15 , which is suspended. This operation corrects a misalignment of the sheet. 
         [0033]    The inlet sensor  14  is used to detect the timing for driving the pair of registration rollers  15 , which is suspended, through controlling the drive unit by the control unit  101 . When the inlet sensor  14  detects the leading edge of the sheet, the inlet sensor generates an inlet detection signal and sends it to the control unit  101 . 
         [0034]    When the control unit  101  receives the inlet detection signal, the control unit  101  controls the drive unit and rotates the pair of registration rollers  15 . Therefore, the aligned sheet is carried toward the writing sensor  16  along the conveying path B by the pair of registration rollers  15  that started to rotate. 
         [0035]    The writing sensor  16  is used to adjust the positions between the starting position for forming a toner image on a photoreceptor drum  182  and the starting position for writing the toner image on the sheet in order to form the toner image that is formed on the photoreceptor drum  182  to the sheet. When the writing sensor  16  detects the leading edge of the sheet that is carried from the pair of registration rollers  15 , the writing sensor  16  generates a writing detection signal and sends it to the control unit  101 . When the control unit  101  receives the writing detection signal, the control unit  101  instructs an exposure head  186  (see  FIG. 2 ) to perform the exposure. 
         [0036]    The sheet that passes the writing sensor  16  is carried to the sheet thickness sensor  17  along the conveying path B. 
         [0037]    The sheet thickness sensor  17  has a light emitting part that emits light to the sheet and a light receiving part that receive reflected light from the sheet. The sheet thickness sensor  17  is a displacement sensor that compares a frequency of the received reflected light and a frequency of a reflected light that results when light is irradiated on a standard sheet. The frequency information is stored in a buffer memory that is not illustrated. The sheet thickness sensor further calculates the thickness of the sheet. When the sheet thickness sensor  17  is turned on by the control unit  101 , the sheet thickness sensor calculates the thickness of the sheet that is carried to the image forming unit  18  along the conveying path B. When the sheet thickness sensor  17  calculates the thickness, the sheet thickness sensor  17  generates a thickness detection signal showing the thickness of the sheet and sends the signal to the control unit  101 . The sheet thickness sensor  17  is used to detect the thickness of a first page corresponding to each data image. After the sheet thickness sensor  17  detects the thickness, the sheet thickness sensor  17  is turned off by a control of the control unit  101 . The control of the control unit  101  based on the thickness detection signal is explained in detail when the control configuration is discussed below. The sheet thickness sensor is a detector to detect characteristics (sheet thickness) of the sheet  1  or another sheet  2 . 
         [0038]    The image forming unit  18  is a structure to form a toner image in black color (K). As shown in  FIG. 2 , the image forming unit  18  has a toner cartridge  181  storing black color toner, the photoreceptor drum  182 , a charge roller  183 , a supply roller  184 , a developing roller  185 , and the exposure head  186 . In this embodiment, the image forming unit  18  forms only the toner image in black color. However, the present invention is not limited to the structure discussed above and can be applied to a printer (full color) that forms toner images in black color (K), yellow color (Y), magenta color (M) and cyan color (C). 
         [0039]    The photoreceptor drum  182  is an electrostatic latent image carrier in which a toner image is formed on the surface. 
         [0040]    The charge roller  183  is a charger to uniformly charge the surface of the photoreceptor drum  182  when the photoreceptor drum  182  rotates. 
         [0041]    The exposure head  186  is configured with an light emitting diode (LED) allay and is a exposing device to form an electrostatic latent image that corresponds to image data on the surface of the charged photoreceptor drum  182  based on the instruction of the exposure from the control unit  101 . 
         [0042]    The supply roller  184  is used to supply toner in black color stored in the toner cartridge  181  to the developing roller  185 . 
         [0043]    When the electrostatic latent image is formed on the surface of the photoreceptor drum  182 , the toner in black color supplied from the toner cartridge  181  adheres to the surface of the developing roller  185  through the supply roller  184  based on the control by the control unit  101 . Then, the toner is formed in the uniform thickness by a layer forming blade that is not shown and is developed in the electrostatic latent image on the drum. As a result, the toner image in black color that corresponds to the electrostatic latent image is formed on the surface of the photoreceptor drum  182 . 
         [0044]    The image transferring unit  19  is configured with a transferring roller  191  that locates in an opposite position to the photoreceptor drum  182  through a conveying belt that is not shown as the conveying path B. 
         [0045]    The image forming unit  18 , the image transferring unit  19 , and the conveying belt are driven synchronically by the control of the control unit  101 . First, a sheet that is adhered to the conveying belt by electrostatic adsorption and that is carried in accordance with the rotatable running of the belt is carried to the position between the photoreceptor drum  182  and the transferring roller  191 . Because a transferring voltage is applied to the transferring roller  191  by the control of the control unit  101 , the toner image in black color that is formed on the surface of the photoreceptor drum  182  is transferred on the surface of the sheet. The Sheet in which the toner image in black color is transferred is further carried to the image fusing unit  20  in accordance with the running of the conveying belt. 
         [0046]    Transferring residue toner remaining on the surface of the photoreceptor drum  182  is scraped off by, for example, a cleaning blade that is not shown and is collected into a waste toner container. 
         [0047]    The image fusing unit  20  is a structure to fuse the toner image transferred on the sheet to the sheet. As shown in  FIG. 2 , the image fusing unit  20  has a heat application roller  201 , a pressure application roller  202  that locate to contact the heat application roller  201  through a conveying belt. The heat application roller  201  has a heater that is not shown inside the roller to heat the heat application roller  201 . 
         [0048]    The sheet in which the toner image is transferred is carried to sandwich between the heat application roller  201  and the pressure application roller  202 . The transferred toner image in black color is fused. Then, the sheet is carried to a pair of transferring rollers  131 . 
         [0049]    The duplex separator  21  rotates from the initial position to the lower direction by the control of the control unit  101  at the time of the duplex printing process and changes over a conveying path of the sheet in which the image fusing process is finished on one side from a direction of the eject unit  23  (conveying path C) to a direction of the inversion unit  22  (conveying path D). When the duplex separator  21  rotates to the lower direction by the control of the control unit  101 , the duplex separator  21  rotates to the upper direction and returns to the initial position after a certain time elapses. 
         [0050]    When the duplex separator  21  is in the initial position, the sheet that is carried between the pair of transferring rollers  131  is diverted in the upper direction (conveying path C) of the device where the eject unit  23  is located. 
         [0051]    The sheet that is diverted in the upper direction of the device is ejected to the eject unit  23  along the conveying path C by the rotation of pairs of transferring rollers  132  and  133 . 
         [0052]    As shown in  FIG. 2 , the eject unit  23  has a pair of ejecting rollers  231  and ejects the sheet for which the print process is finished to the stacker unit  24  outside the device. 
         [0053]    On the other hand, when the duplex separator  21  rotates in the lower direction, the sheet is diverted in the lower direction (conveying path D) of the device where the inversion unit  22  is located. 
         [0054]    The inversion unit  22  is used at the time of the duplex printing process for turning the sheet reverse-side up after the print process for one side of the sheet is finished. As shown in  FIG. 2 , the inversion unit  22  has the conveying path D, an inversion path change lever  221 , a change position sensor  222 , a pair of inversion rollers  223 , a conveying path E, a conveying path F, and a plurality of pairs of conveying rollers  224 ,  225 , and  226 . The inversion unit  22  is a special process unit for performing the duplex printing process. 
         [0055]    As shown in  FIG. 2 , the inversion path change lever  221  rotates in a clockwise direction from the initial position, and the sheet, which is carried from the conveying path D to the conveying path E, is carried in a direction from the conveying path E to the conveying path F. When the inversion path lever does not rotate and is in the initial position, the sheet, which is carried along the conveying path D, is carried to a direction of the conveying path E by the pair of inversion rollers  223  that is rotated by drive from the drive unit based on the control of the control unit  101 . 
         [0056]    The change position sensor  222  locates in the vicinity of the inversion path change lever  221  as shown in  FIG. 2  and is used to detect the change timing of the rotation of the pair of inversion rollers  223  from the forward direction to the backward direction. Namely, when the change position sensor  222  detects the trailing edge of the sheet that is carried by the pair of inversion rollers  223 , the change position sensor  222  creates a trailing edge detection signal and sends it to the control unit  101 . 
         [0057]    When the control unit  101  receives the trailing edge signal, the control unit  101  stops the rotation of the pair of inversion rollers  223  by controlling the drive unit and rotates the inversion path change lever  221  in the clockwise direction. As a result, the pair of inversion rollers  223  stops while holding the trailing edge of the sheet. 
         [0058]    The control unit  101  rotates the pair of inversion rollers  223  in the reverse direction by controlling the drive unit after a certain time elapses. When the pair of inversion rollers  223  starts to rotate in the reverse direction, the sheet, which is suspended and is held by the pair of inversion rollers  223 , is carried to the conveying path F because the inversion path change lever  221  has rotated in the clockwise direction. As shown in  FIG. 2 , the sheet, which is carried to the conveying path F, is carried to the conveying path B while the sheet is held by a plurality of pairs of transferring rollers  224 ,  225 , and  226  that rotate based on the control of the drive unit by the control unit  101 . 
         [0059]    The sheet is carried to the conveying path B direction. When the leading edge of the sheet is detected by the inlet sensor  14  again, the reverse-side, or second side, of the sheet is the up-side. Then, printing is performed on the reverse side of the sheet through the image forming unit  18  and the image fusing unit  20  as was performed on the first side of the sheet. After the print process for the reverse side of the sheet is finished, the sheet passes the duplex separator  21  that is located in the initial position, is carried along the conveying path C to the eject unit  23  direction, ejects from the eject unit  23  to outside of the device, and is loaded on the stacker unit  24 . 
         [0060]    Control configurations that are features of the printer  100  of the first embodiment according to the present invention are explained below. The printer  100  has the following structures as shown in  FIG. 1 : the control unit  101  that controls each of units in the printer  100  and controls overall of the printer  100 ; an inter face (I/F) unit  102  that communicates with the host device  200 ; an image memory  103 ; a special process instruction status determination unit  104 ; a threshold value information memory unit  105 ; a selection information memory unit  106 ; and a special process capability determination unit  107 . 
         [0061]    When the control unit  101  receives print data from the host device  200  through the I/F unit  102 , the control unit  101  converts the print data to image data through a rasterize transformation unit, which is not shown and is in the printer  100 . In this embodiment, the print data contains print instruction information indicating one-side printing or duplex printing, sheet size information indicating a size of paper, such as A4, and sheet tray information indicating automatic sheet feeding (without sheet tray assignment). The image data keeps the print instruction information, the sheet size information, and the sheet tray information that are contained in the print data. 
         [0062]    The image memory  103  is a memory unit to store the image data based on the control of the control unit  101 . In this embodiment, the image memory  103  stores the image data that is obtained through the rasterize transformation of the print data from the host device  200 . However, the present invention is not limited to the structure discussed above, and the image memory  103  can store the image data that is obtained by scanning a manuscript through the reader (scanner). 
         [0063]    After the image data is stored in the image memory  103 , the control unit  101  controls the drive unit to rotate the photoreceptor drum  182  and, at the same time, instructs the special process instruction status determination unit  104  to determine existence or non-existence of the duplex printing instruction. 
         [0064]    The special process instruction status determination unit  104  determines existence or non-existence of the duplex printing instruction based on the print instruction information contained in the image data. When the print instruction information indicates the duplex printing, the special process instruction status determination unit  104  determines that the duplex printing instruction exists. 
         [0065]    On the other hand, when the print instruction information indicates the one-side printing, the special process instruction status determination unit  104  determines that the duplex printing instruction does not exist. 
         [0066]    When the special process instruction status determination unit  104  determines that the duplex printing instruction exists, the control unit  101  turns on the sheet thickness sensor  17 . In this embodiment, when the printer  100  performs the print process based on the sheet tray information (automatic sheet feeding), the control unit  101  rotates the first sheet feeding roller  111  by the control of the drive unit based on a program that is set in a memory (not shown) in advance. As a result, the first sheet feeding roller  111  that starts to rotate feeds the top of the sheet  1  loaded in the first sheet tray  11  to the conveying path A. 
         [0067]    On the other hand, when the special process instruction status determination unit  104  determines that the duplex printing instruction does not exist, the control unit  101  does not turn on the sheet thickness sensor  17  and performs the print process on only one side of the sheet  1  based on the image data. The sheet  1  in which the print process is performed on only one side is ejected from the eject unit  23  and is loaded on the stacker unit  24 . 
         [0068]    When the sheet  1 , which is carried to the conveying path A, passes the sheet thickness sensor  17 , the sheet thickness sensor  17  calculates the thickness of the sheet  1  and sends the thickness detection signal to the control unit  101  as discussed above. 
         [0069]    When the control unit  101  receives the thickness detection signal, the control unit  101  instructs the special process capability determination unit  107  to determine whether duplex printing can be performed and transfers the thickness detection signal to the special process capability determination unit  107 . 
         [0070]    The threshold value information memory unit  105  stores the threshold information showing the predetermined threshold value that indicates the sheet thickness at or above which duplex printing cannot be performed. In this embodiment, the threshold value is set to 160 μm. This threshold value is obtained through experimentation with the printer  100  in advance, which determined at what sheet thicknesses two hundred sheets could be continuously printed with the duplex printing. 
         [0071]    The selection information memory unit  106  stores predetermined print selection information such as “change to one-side printing process,” “perform duplex printing process with another sheet,” and “stop print process,” one of which a user selects when the duplex printing cannot be performed. The print selection information is explained in detail when  FIG. 3  is explained below. 
         [0072]    The special process capability determination unit  107  determines whether or not duplex printing is possible based on the thickness determination signal. When the special process capability determination unit  107  receives the thickness determination signal, the special process capability determination unit  107  compares the threshold value (160 μm) stored in the threshold value information memory unit  105  with the sheet thickness shown in the thickness determination signal. When the sheet thickness is less than the threshold value (160 μm), the special process capability determination unit  107  determines that duplex printing can be performed. When the special process capability determination unit  107  determines that duplex printing can be performed, the control unit  101  controls the duplex printing as discussed above. 
         [0073]    On the other hand, when the sheet thickness is equal to or more than the threshold value (160 μm), the special process capability determination unit  107  determines that duplex printing cannot be performed. When the special process capability determination unit  107  determines that the duplex printing cannot be performed, the control unit  101  sends the print selection information, which is stored in the selection information memory unit  106 , to the host device  200  and controls that the sheet is carried between the image forming unit  18  and the image fusing unit  20  (conveying path B) without the print process. Then, the sheet in which the print process is not performed is ejected from the eject unit  23  to outside of the device through the rotation of the pair of ejecting rollers  231  and is loaded on the stacker unit  24 . In this embodiment, the control unit  101  sends the print selection information to the host device  200  in order to show the information on a display (not shown) of the host device  200 . However, the present invention is not limited to the structure discussed above and can be applied to a structure in which the information is shown on a display (not shown) of the printer  100 . When the print selection information is sent to the host device  200 , the control unit  101  acts as the special process confirmation unit outputting the information. 
         [0074]    As shown in  FIG. 3  the host device  200  is a PC operated by a user and generates print data by using application software stored in a memory (not shown) in advance of the host device  200 . The host device  200  also has a display (not shown) to show information that is sent from the printer  100  and a input unit (not shown) that has an input button. Namely, when the host device  200  receives the print selection information from the printer  100 , the host device  200  shows the information on a display unit  30  in  FIG. 3 . 
         [0075]    As shown in  FIG. 3 , the form  30  has a message  31  notifying that the duplex printing process cannot be performed on the sheet assigned by a user, selection buttons  32 ,  33 , and  34  corresponding to three options, “1. Change to One-Side printing Process,” “2. Perform Duplex Printing Process with Another Sheet,” “3. Stop Print Process,” respectively, a “Complete Selection” button  35 , and a “Cancel” button  36 . 
         [0076]    When the user refers the form showing on the display unit  30  of the host device  200 , selects the selection button  32  through an input unit (not shown) corresponding to “1. Change to One-Side Printing Process,” and pushes the “Complete Selection” button  35 , a control unit (not shown) of the host device  200  generates response information containing the “Change to One-Side Printing Process” option and sends the information to the printer  100 . 
         [0077]    Alternatively, when the user selects the selection button  33  corresponding to “2. Perform. Duplex Printing Process with Another Sheet,” and pushes the “Complete Selection” button  35 , a control unit generates response information containing the “Perform Duplex Printing Process with Another Sheet” option and sends the information to the printer  100 . 
         [0078]    Further alternatively, when the user selects the selection button  34  corresponding to “3. Stop Print Process,” and pushes the “Complete Selection” button  35 , a control unit generates response information containing the “Stop Print Process” option and sends the information to the printer  100 . 
         [0079]    When the control unit  101  of the printer  100  receives the response information from the host device  200 , and when the response information shows the “Change to One-Side Printing Process” option, the control unit  101  rotates the first sheet feeding roller  111  by driving the drive unit and feeds the top of the sheet  1  loaded in the first sheet tray  11  to the conveying path A. Then, the one-side printing is performed on the sheet  1  based on image data stored in the image memory  103 . 
         [0080]    Alternatively, when the response information shows “Perform Duplex Printing Process with Another Sheet,” the control unit  101  rotates the second sheet feeding roller  121  by driving the drive unit and feeds the top of the sheet  2  loaded in the second sheet tray  12  to the conveying path A. And then, the sheet thickness sensor  17 , which is turned on by the control unit  101 , calculates the thickness of the sheet  2 . When the thickness of the sheet  2  is less than the predetermined threshold value (160 μm), the control unit  101  determines that the duplex printing can be performed and performs the duplex printing process on the sheet  2 . In this embodiment, because the printer  100  is able to perform the duplex printing, the sheet loaded and held in at least one of the first sheet tray  11  and the second sheet tray  12  should have the thickness that permits duplex printing. 
         [0081]    Further alternatively, when the response information shows “Stop Print Process,” the control unit  101  deletes the image data stored in the image memory  103  by using an image delete unit (not shown). Then, the printer  100  is in a print data waiting state by the control of the control unit  101 . 
         [0082]      FIGS. 4 and 5  are flow diagrams of operation of the printer  100  of the first embodiment according to the present invention. Operation of the printer  100  of the first embodiment according to the present invention is explained with reference to the flow diagrams in  FIGS. 4 and 5 . The first sheet tray  11  loads and holds the sheet  1  that has the following characteristics: the quality of a sheet is normal (normal paper); the size of a sheet is A4; and the thickness of a sheet is 170 μm. Similarly, the second sheet tray  12  loads and holds the sheet  2  that has the following characteristics: the quality of a sheet is normal (normal paper); the size of a sheet is A4; and the thickness of a sheet is 80 μm. 
         [0083]    When the control unit  101  of the printer  100  receives print data from the host device  200  through the I/F unit  102  (S 101 ), the control unit  101  converts the print data to image data through a rasterize transformation unit that is not shown and is in the printer  100 . The print data contains print instruction information showing a one-side printing or a duplex printing, sheet size information showing a size of paper, such as A4, and sheet tray information showing automatic sheet feeding (without sheet tray assignment). The image data keeps the print instruction information, the sheet size information, and the sheet tray information that are contained in the print data. 
         [0084]    After the image data is stored in the image memory  103  (S 102 ), the control unit  101  controls the drive unit to rotate the photoreceptor drum  182  and, at the same time, instructs the special process instruction status determination unit  104  to determine existence or non-existence of the duplex printing instruction. 
         [0085]    When the special process instruction status determination unit  104  receives the instruction containing existence or non-existence of the duplex printing instruction, and when the print instruction information indicates the duplex printing, the special process instruction status determination unit  104  determines that the duplex printing instruction exists (S 103 ). 
         [0086]    On the other hand, when the print instruction information indicates the one-side printing, the special process instruction status determination unit  104  determines that the duplex printing instruction does not exist. 
         [0087]    When the special process instruction status determination unit  104  determines that the duplex printing instruction exists, the control unit  101  turns on the sheet thickness sensor  17  (S 104 ). When the printer  100  performs the print process based on the sheet tray information (automatic sheet feeding), the control unit  101  rotate the first sheet feeding roller  111  by the control of the drive unit based on a program that is set in a memory in advance that is not shown. As a result, the first sheet feeding roller  111  that starts to rotate feeds the top of the sheet  1  loaded in the first sheet tray  11  to the conveying path A. 
         [0088]    On the other hand, when the special process instruction status determination unit  104  determines that the duplex printing instruction does not exist, the control unit  101  does not turn on the sheet thickness sensor  17  and performs the print process on only one side of the sheet  1  based on the image data (S 105 ). The sheet  1  in which the print process is performed on only one side is ejected from the eject unit  23  and is loaded on the stacker unit  24 . 
         [0089]    When the sheet  1 , which is carried to the conveying path A, passes the sheet thickness sensor  17 , the sheet thickness sensor  17  calculates the thickness (170 μm) of the sheet  1  (S 106 ) and sends the thickness detection signal to the control unit  101 . 
         [0090]    When the control unit  101  receives the thickness detection signal, the control unit  101  instructs the special process capability determination unit  107  to determine capability of the duplex printing and transfers the thickness detection signal to the special process capability determination unit  107 . 
         [0091]    When the special process capability determination unit  107  receives the thickness determination signal, the special process capability determination unit  107  compares the threshold value (160 μm) stored in the threshold value information memory unit  105  with the sheet thickness (170 μm) shown in the thickness determination signal. Because the sheet thickness (170 μm) is more than the threshold value (160 μm) (S 107 ), the special process capability determination unit  107  determines that duplex printing cannot be performed (S 108 ). When the special process capability determination unit  107  determines that the duplex printing cannot be performed, the control unit  101  sends the print selection information, which is stored in the selection information memory unit  106 , to the host device  200  (S 109 ) and controls that the sheet is carried between the image forming unit  18  and the image fusing unit  20  (conveying path B) without the print process. Then, the sheet in which the print process is not performed is ejected from the eject unit  23  to outside of the device through the rotation of the pair of ejecting rollers  231  and is loaded on the stacker unit  24  (S 110 ). 
         [0092]    On the other hand, when the sheet thickness is less than the threshold value (160 μm), the special process capability determination unit  107  determines that the duplex printing can be performed (S 111 ). When the special process capability determination unit  107  determines that the duplex printing can be performed, the control unit  101  controls the duplex printing (S 112 ). 
         [0093]    When the control unit of the host device  200  receives the print selection information from the printer  100 , the control unit of the host device  200  shows the information on a display unit  30  as shown in  FIG. 3 . 
         [0094]    When the user refers the form appearing on the display unit  30  of the host device  200 , selects the selection button  32  through an input unit (not shown) corresponding to “1. Change to One-Side Printing Process,” and pushes the “Complete Selection” button  35 , the control unit of the host device  200  generates response information containing the “Change to One-Side Printing Process” option and sends the information to the printer  100 . 
         [0095]    Alternatively, when the user selects the selection button  33  corresponding to “2. Perform Duplex Printing Process with Another Sheet,” and pushes the “Complete Selection” button  35 , the control unit of the host device  200  generates response information containing the “Perform Duplex Printing Process with Another Sheet” option and sends the information to the printer  100 . 
         [0096]    Further alternatively, when the user selects the selection button  34  corresponding to “3. Stop Print Process,” and pushes the “Complete Selection” button  35 , the control unit of the host device  200  generates response information containing the “Stop Print Process” option and sends the information to the printer  100 . 
         [0097]    When the control unit  101  of the printer  100  receives the response information from the host device  200  (S 113 ), and when the response information shows the “Change to One-Side Printing Process” option (S 114 ), the control unit  101  rotates the first sheet feeding roller  111  by driving the drive unit and feeds the top of the sheet  1  loaded in the first sheet tray  11  to the conveying path A. Then, the one-side printing is performed on the sheet  1  based on image data stored in the image memory  103  (S 115 ). The sheet  1  in which the one-side printing process is finished is ejected from the eject unit  23  and is loaded on the stacker unit  24 . 
         [0098]    Alternatively, when the response information shows “Perform Duplex Printing Process with Another Sheet” (S 116 ), the control unit  101  rotates the second sheet feeding roller  121  by driving the drive unit and feeds the top of the sheet  2  loaded in the second sheet tray  12  to the conveying path A. And then, the sheet thickness sensor  17 , which is turned on by the control unit  101 , calculates the thickness (80 μm) of the sheet  2 . Because the thickness (80 μm) of the sheet  2  is less than the predetermined threshold value (160 μm), the control unit  101  determines that the duplex printing can be performed and performs the duplex printing process on the sheet  2  (S 117 ). The sheet  2  in which the duplex printing process is finished is ejected from the eject unit  23  and is loaded on the stacker unit  24 . 
         [0099]    Further alternatively, when the response information shows “Stop Print Process,” the control unit  101  stops the printing process and deletes the image data stored in the image memory  103  by using an image delete unit (not shown) (S 118 ). Then, the printer  100  is in a print data waiting state by the control of the control unit  101 . 
         [0100]    When the duplex printing is performed in the printer  100  of the first embodiment, and when the thickness of the sheet  1  is equal to or more than the threshold value, which corresponds that the duplex printing cannot be performed, the printer  100  sends the print selection information to the host device  200 . The user uses the print selection information for deciding whether or not the duplex printing is performed on another sheet  2 . The print selection information can be shown in a display unit in the printer  100 . Therefore, the first embodiment can provide another option of performing the duplex printing on another sheet  2  for the user. 
       Second Embodiment 
       [0101]    A printer  100   a  of a second embodiment according to the present invention has a structure that enables to perform one-side printing process in the printer  100  of the first embodiment (without duplex separator  21  and inversion unit  22 ) and structures that enable to perform a hole punch process and a staple process (see  FIG. 7 ). In this embodiment, the same reference numerals are used for the same structures of the first embodiment, and detailed explanation for them is omitted. 
         [0102]    As shown in  FIG. 7 , The printer  100   a  has the following structures as shown in  FIG. 7 : a first sheet tray  11  that loads and holds a sheet  1 ; a second sheet tray  12  that loads and holds another sheet  2 ; a conveying unit  13  having each of conveying paths and each of conveying rollers; an inlet sensor  14 ; a pair of registration rollers  15 ; a writing sensor  16 ; a sheet thickness sensor  17 ; an image forming unit  18 ; an image transferring unit  19 ; an image fusing unit  20 ; an eject unit  23 ; a stacker unit  24 ; an eject destination change separator  25 ; a post-process unit  26  in which the post-processes (hole punch process and staple process) are performed; and a post-process stacker unit  27  in which a sheet that performed a post-process is loaded. The printer  100   a  in this embodiment is configured with two sheet trays, a first sheet tray  11  and a second sheet tray  12 . However, the present invention is not limited to the printer  100   a  with two sheet trays and can be applied to a printer with a plurality of sheet trays. 
         [0103]    The eject destination change separator  25  rotates from the initial position to the lower direction by the control of the control unit  101   a  at the time of the hole punch process or the staple process and changes over a conveying path of the sheet  1  or another sheet  2  from a direction of the eject unit  23  (conveying path C) to a direction of the post-process unit  26  (conveying path H). When the eject destination change separator  25  rotates to the upper direction and returns to the initial position by the control of the control unit  101   a  after a print process based on image data stored in an image memory is finished. 
         [0104]    When the eject destination change separator  25  does not rotate to the lower direction and is in the initial position, the sheet that is carried between the pair of transferring rollers  131  is forwarded to the upper direction (conveying path C) of the device where the eject unit  23  locates. 
         [0105]    On the other hand, when the eject destination change separator  25  rotates to the lower direction, the sheet is forwarded to the post-process unit  26  direction (conveying path H). 
         [0106]    The sheet that is forwarded to the upper direction of the device is ejected to the eject unit  23  along the conveying path C by the rotation of pairs of transferring rollers  132  and  133 . 
         [0107]    The post-process unit  26  is used for post-processes (hole punch process and staple process) and performs the hole punch process or the staple process for the sheet. As shown in  FIG. 7 , the post-process unit  26  has a pair of conveying rollers  261 , an intake sensor  262 , a hole punch unit  263 , the conveying path H, a pair of conveying rollers  264 , a conveying path I, a staple unit  265 , an eject belt  266 , an eject claw  267 , a drive roller  268 , and a driven roller  269 . 
         [0108]    The sheet that is carried to the post-process unit  26  direction is carried to the intake sensor  262  direction by the rotation of the pair of conveying rollers  261  that is driven to rotate by the control of the control unit  101   a.    
         [0109]    The intake sensor  262  is used to detect that the sheet is sucked into the post-process unit  26 . When the intake sensor  262  detects the leading edge of the sheet that is carried along the conveying path H, the intake sensor  262  generates an intake detection signal and sends the signal to the control unit  101   a.    
         [0110]    The hole punch unit  263  perforates through the sheet in which the print process is performed for filing. The hole punch unit  263  has a punch (not shown) for making a hole and a die (not shown) being a pair with the punch. Chips that are generated at the time of perforation are collected to a chip collection unit (not shown). In the hole punch unit  263 , when the hole punch process is performed for the sheet in which the thickness is more than a certain amount, burr (unevenness that is created at the time of paper cutting) may be created. Therefore, the hole punch unit  263  is performed for only the sheet in which the thickness permits accurate perforation based on the control of the control unit  101   a . In this embodiment, the hole punch unit  263  is performed for one sheet at a time. However, the present invention is not limited to the structure discussed above, and the perforation process can be applied to several sheets in which the print process is finished. 
         [0111]    When the control unit  101   a  receives the intake detection signal, and when the hole punch process is performed, a leading edge of the sheet that is carried between the pair of varying rollers  261  abuts a nipping part that is formed between rollers of the pair of registration rollers  264  that is suspended. Then, the control unit  101   a  drives the hole punch unit  263  to perforate certain positions of the sheet. After the hole punch process is finished, the pair of conveying rollers  264  starts to rotate. Therefore, the perforated sheet is carried to the eject belt  266  direction by the rotation of the pair of rollers. 
         [0112]    The eject belt is used to eject the perforated sheet or a group of sheets, which is stapled by the staple process discussed later, to outside of the device. As shown in  FIG. 7 , the eject belt  266  is an endless belt tensioned by the drive roller  268  and the driven roller  269 . As shown in  FIG. 7 , the eject belt  266  locate in the position where the driven roller  269  is located higher than the drive roller  268  to adjust to the angle of the post-process stacker unit  27 . 
         [0113]    As shown in  FIG. 7 , the eject claw  267  is fixed to the eject belt  266 . When the eject belt  266  is suspended, the eject claw  267  is in the initial position in the vicinity of the drive roller  268  by the control of the control unit  101   a  as shown in  FIG. 7 . As a result, when the sheet in which the hole punch process is performed is carried on the suspended eject belt  266  by the control of the control unit  101   a , the trailing edge of the sheet contacts the eject claw  267  on the belt. 
         [0114]    The suspended eject belt  266  and the eject claw  267  function as the staple stacker in which a group of sheets is loaded for stapling as one set of sheets at the time of the staple process. 
         [0115]    When the sheet in which the hole punch process is performed is placed on the eject belt  266 , the drive roller  268  rotates by the control of the control unit  101   a  and the belt starts to run. The eject claw  267  is moved to the post-process stacker unit  27  direction in accordance with the running of the eject belt  266 . As a result, the trailing edge of the sheet placed on the running eject belt  266  is pushed toward the post-process stacker unit  27  direction by the eject claw  267 . Then, the sheet ejected outside of the device is loaded on the post-process stacker unit  27 . 
         [0116]    The staple unit  265  has a stapler that staples a group of sheets in which the print process is performed. In the staple unit  265 , when the staple process is performed by a stapler for a group of sheets in which the thickness of the group of sheets is more than a certain amount, problems may be occurred such as the stapler is broken, and the accurate stapling cannot be performed. In the staple unit  265 , the staple process is performed based on the control of the control unit  101   a  discussed below so long as the thickness of the group of sheets is such that accurate stapling is possible. 
         [0117]    When the control unit  101   a  receives the intake detection signal, and when the staple process is performed, the sheet is carried on the suspended eject belt  266  (table stacker) by rotating the pair of conveying rollers  264  by the control unit  101   a . As a result, several sheets, which are subject to have the staple process, are loaded on the eject belt  266  (table stacker) one by one. Then, after all of sheets are loaded on the eject belt  266 , the control unit  101   a  drives the staple unit  265  to staple the certain position of the sheets. 
         [0118]    After the staple process is performed on the group of sheets, and the sheets are stapled as one set of sheets, the eject belt  266  starts to run by the rotation of the drive roller  268  through the control of the control unit  101   a . As a result, the trailing edge of the group of sheets placed on the running eject belt  266  is pushed toward the post-process stacker unit  27  direction by the eject claw  267 . Then, the group of sheets ejected outside of the device is loaded on the post-process stacker unit  27 . 
         [0119]    Control configurations that are features of the printer  100   a  of the second embodiment according to the present invention are explained referring to  FIG. 6 . The printer  100   a  has the following structures as shown in  FIG. 6 : the control unit  101   a  that controls each of units in the printer  100   a  and controls overall of the printer  100   a ; an inter face (I/F) unit  102 ; an image memory  103 ; a special process instruction status determination unit  104   a ; a threshold value information memory unit  105   a ; a selection information memory unit  106   a ; a special process capability determination unit  107   a ; a sheet thickness information memory unit  108 ; and a sheet thickness calculation determination unit  109 . 
         [0120]    When the control unit  101   a  receives print data from the host device  200  through the I/F unit  102 , the control unit  101   a  converts the print data to image data through a rasterize transformation unit that is not shown and is in the printer  100   a . The print data contains post-process instruction information showing existence or non-existence of instructions for the hole punch process and the staple process, number of sheets information showing the total number of printed sheets, sheet size information showing a size of paper, such as A4, and sheet tray information showing automatic sheet feeding. The image data keeps the post-process instruction information, the number of sheets information, the sheet size information, and the sheet tray information that are contained in the print data. Note that one of the hole punch process and the staple process can be assigned for the post-process instruction information. 
         [0121]    After the image data is stored in the image memory  103 , the control unit  101   a  controls the drive unit to rotate the photoreceptor drum  182  and, at the same time, instructs the special process instruction status determination unit  104   a  to determine existence or non-existence of the post-process instruction. 
         [0122]    The special process instruction status determination unit  104   a  determines existence or non-existence of the post-process instruction (hole punch process or staple process) based on the post-process instruction information contained in the image data. When the special process instruction status determination unit  104   a  receives the instruction of the special process instruction status determination, and when the post-process instruction information contained in the image data that is stored in the image memory  103  shows “existence of the hole punch process,” the special process instruction status determination unit  104   a  determines that the hole punch process instruction exists. 
         [0123]    When the post-process instruction information shows “existence of the staple process,” the special process instruction status determination unit  104   a  determines that the staple process instruction exists. 
         [0124]    On the other hand, when the post-process instruction information shows “non-existence of the hole punch process and non-existence of the staple process,” the special process instruction status determination unit  104   a  determines that the post-process instruction does not exist. 
         [0125]    As shown in  FIGS. 8A ,  8 B and  8 C the sheet thickness information memory unit  108  is a memory to store sheet thickness calculation value T (μm) showing thicknesses of the sheet  1  and/or the sheet  2  calculated by the sheet thickness sensor  17 . The sheet thickness information memory unit  108  is a volatile memory in which the memory contents are deleted and initialized by the control of the control unit  101   a  when the printer  100   a  is turned off or when any of the sheet trays is ejected. When the sheet thickness information memory unit  108  is initialized by the control of the control unit  101   a , “to be determined (TBD)” is stored in the sheet thickness information memory unit  108 . “To be determined (TBD)” shows that the sheet thickness calculation value T (μm) is not yet calculated for the trays as shown in  FIG. 8A . 
         [0126]    After the sheet thickness information memory unit  108  is initialized, and when the sheet thickness sensor  17  calculates the thickness of the sheet  1  (the thickness of another sheet  2  is not calculated), the thickness of the sheet  1  is stored in the sheet thickness information memory unit  108  by the control of the control unit  101   a  as the sheet thickness calculation value T (μm) that corresponds to the first sheet tray  11  as shown in  FIG. 8B . 
         [0127]    After the sheet thickness calculation value T (μm) that corresponds to the first sheet tray  11  is stored in the sheet thickness information memory unit  108 , and when the sheet thickness sensor  17  calculates the thickness of the sheet  2 , the thicknesses of the sheet  1  and the sheet  2  are stored in the sheet thickness information memory unit  108  by the control of the control unit  101   a  as the sheet thickness calculation value T (μm) that corresponds to each of the trays as shown in  FIG. 8C . 
         [0128]    In  FIG. 9  the threshold value information memory unit  105   a  stores the predetermined threshold value of sheet thickness at or above which the hole punch process cannot be performed. Also, the memory unit  105   a  stores the predetermined threshold value of the thickness of a group of sheets (total thickness of all sheets in the group) at which the staple process cannot be performed. In this embodiment, the threshold value at or above which the hole punch process cannot be performed is set to 190 μm (per sheet). This threshold value is obtained through experimentation with the printer  100   a  in advance to determine at what thicknesses 200 sheets could be continuously perforated in the hole punch process without any problems. 
         [0129]    In this embodiment, the threshold value at or above with the staple process cannot be performed is set to 8000 μm (total thickness of the sheets). This threshold value is obtained through experimentation with the printer  100   a  in advance to determine at what thicknesses a set of fifty sheets could be continuously stapled two hundred times without any problems. Note that the threshold value indicates the total thickness of a set of fifty sheets (160 μm×50 sheets=8000 μm). 
         [0130]    When the special process instruction status determination unit  104   a  determines that the hole punch process instruction exists, the control unit  101   a  instructs the sheet thickness calculation determination unit  109  to determine whether or not the thickness of the sheet  1  is calculated and forwards the type of the post-process (the hole punch process). 
         [0131]    On the other hand, when the special process instruction status determination unit  104   a  determines that the staple process instruction exists, the control unit  101   a  instructs the sheet thickness calculation determination unit  109  to determine whether or not the thickness of the sheet  1  is calculated and forwards the type of the post-process (the staple process). 
         [0132]    When the special process instruction status determination unit  104   a  determines that the post-process instruction does not exist, the control unit  101   a  performs the print process on the sheet  1  based on the image data. The sheet  1  in which the print process is performed is ejected from the eject unit  23  and is loaded on the stacker unit  24 . 
         [0133]    The sheet thickness calculation determination unit  109  determines whether or not the sheet thickness sensor  17  is driven based on sheet thickness information stored in the sheet thickness information memory unit  108 . Namely, when the sheet thickness calculation determination unit  109  receives instruction for determination of the thickness of the sheet  1  and another sheet  2  discussed later, the sheet thickness calculation determination unit  109  searches the sheet thickness information memory unit  108  to obtain the sheet thickness calculation value T (μm) that corresponds to the thickness of the sheets loaded and held in the first sheet tray  11  or the second sheet tray  12 . 
         [0134]    When the sheet thickness calculation value T (μm) is “TBD,” the sheet thickness calculation determination unit  109  determines that the drive of the sheet thickness sensor  17  is necessary. When the sheet thickness calculation determination unit  109  determines that the drive of the sheet thickness sensor  17  is necessary, the control unit  101   a  turns on the sheet thickness sensor  17 . In this embodiment, when the printer  100   a  performs the print process based on the automatic sheet feeding, the control unit  101   a  rotates the first sheet feeding roller  111  by the control of the drive unit based on a program that is set in a memory (not shown) in advance. As a result, the first sheet feeding roller  111  that starts to rotate feeds the top of the sheet  1  loaded in the first sheet tray  11  to the conveying path A. 
         [0135]    On the other hand, when the sheet thickness calculation value T (μm) is not “TBD,” the sheet thickness calculation determination unit  109  determines that the drive of the sheet thickness sensor  17  is not necessary. When the sheet thickness calculation determination unit  109  determines that the drive of the sheet thickness sensor  17  is not necessary, the control unit  101   a  instructs the special process capability determination unit  107   a  to determine whether or not the special process is performed and at the same time forwards the name of sheet feeding tray (first sheet tray  11  or second sheet tray  12 ) in which the sheet to be determined is loaded and held and the type of the post-process (hole punch process or staple process). 
         [0136]    When the sheet, which is carried to the conveying path A, passes the sheet thickness sensor  17 , the sheet thickness sensor  17  calculates the thickness of the sheet and sends the thickness detection signal to the control unit  101   a  as discussed in the first embodiment. The thickness detection signal contains the name of sheet feeding tray and the type of the post-process. 
         [0137]    When the control unit  101   a  receives the thickness detection signal, the sheet thickness indicated by the signal is stored as the sheet thickness calculation value T (μm) in the sheet thickness information memory unit  108  corresponding to the name of sheet feeding tray contained in the signal. When the memory process is finished, the control unit  101   a  instructs the special process capability determination unit  107   a  to determine whether or not the special process is capable and forwards the name of sheet feeding tray and the type of the post-process. 
         [0138]    The selection information memory unit  106   a  stores predetermined post-process selection information such as “do not perform post-process, but perform only print process,” “perform print process and post-process with another sheet,” and “stop print process,” one of which is selected by a user selects when the hole punch process or the staple process cannot be performed. The post-print selection information is explained in detail when  FIG. 10  is explained later. 
         [0139]    The special process capability determination unit  107   a  determines whether or not the hole punch process or the staple process can be performed based on the sheet thickness information stored in the sheet thickness information memory unit  108 . Namely, when the special process capability determination unit  107   a  receives the name of the sheet feeding tray and the type of the post-process, and when the type of the post-process is the hole punch process, the special process capability determination unit  107   a  obtains the sheet thickness calculation value T (μm) that corresponds to the sheet tray through searching the sheet thickness information stored in the sheet thickness information memory unit  108  based on the name of the sheet feeding tray. The special process capability determination unit  107   a  compares the sheet thickness calculation value T (μm) that is obtained with the threshold value (160 μm), which is stored in the threshold information memory unit  105   a , for the hole punch process. When the sheet thickness calculation value T (μm) is less than the threshold value (160 μm), the special process capability determination unit  107   a  determines that the hole punch process is possible. When the special process capability determination unit  107   a  determines that the hole punch process is possible, the control unit  101   a  controls the print process and then controls the hole punch process as discussed above. 
         [0140]    On the other hand, when the sheet thickness calculation value T (μm) is equal to or more than the threshold value (160 μm), the special process capability determination unit  107   a  determines that the hole punch process is not possible. When the special process capability determination unit  107   a  determines that the hole punch process is not possible, the control unit  101   a  sends the post-process selection information that is stored in the selection information memory unit  106   a  to the host device  200 . The control unit  101   a  also controls that the sheet  1  is carried between the image forming unit  18  and the image fusing unit  20  (conveying path B) without the print process when the sheet  1  is carried through the conveying unit  13 . Then, the sheet in which the print process is not performed is ejected from the eject unit  23  to outside of the device through the rotation of the pair of ejecting rollers  231  and is loaded on the stacker unit  24 . 
         [0141]    When the type of the post-process is the staple process, the special process capability determination unit  107   a  search the sheet thickness information stored in the sheet thickness information memory unit  108  based on the name of the sheet feeding tray and obtains the sheet thickness calculation value T (μm) that corresponds to the sheet tray. The special process capability determination unit  107   a  compares the obtained sheet thickness calculation value T (μm) and total sheet thickness calculated by the total number of sheets information with the threshold value (8000 μm) for the staple process that is stored in the threshold value information memory unit  105   a . When the total thickness of all of the sheets is less than the threshold value (8000 μm), the special process capability determination unit  107   a  determines that the staple process is possible. When the special process capability determination unit  107   a  determines that the staple process is possible, the control unit  101   a  controls the print process as discussed above, and then controls the staple process. 
         [0142]    On the other hand, when the total thickness of all of the sheets is equal to or more than the threshold value (8000 μm), the special process capability determination unit  107   a  determines that the staple process is not possible. When the special process capability determination unit  107   a  determines that the staple process is not possible, the control unit  101   a  sends the post-process selection information, which is stored in the selection information memory unit  106   a , to the host device  200  and controls that the sheet  1  is carried between the image forming unit  18  and the image fusing unit  20  (conveying path B) without the print process when the sheet  1  is carried through the conveying unit  13 . Then, the sheet in which the print process is not performed is ejected from the eject unit  23  to outside of the device through the rotation of the pair of ejecting rollers  231  and is loaded on the stacker unit  24 . 
         [0143]    As shown in  FIG. 10 , when the host device  200  receives the post-process selection information from the printer  100   a , the host device  200  shows the information on a display unit shown as a form  40  in  FIG. 10 . 
         [0144]    As shown in  FIG. 10 , the form  40  has a message  41  notifying that the post-process (hole punch process and staple process) cannot be performed on the sheet assigned by a user, selection buttons  42 ,  43 , and  44  corresponding to three options, “1. Do Not Perform Pos-Process, but Perform Only Print Process,” “2. Perform Print Process and Post-Print Process with Another Sheet,” “3. Stop Print Process,” respectively, a “Complete Selection” button  45 , and a “Cancel” button  46 . 
         [0145]    When the user refers the form  40  showing on the display unit of the host device  200 , selects the selection button  42  through an input unit (not shown) corresponding to “1. Do Not Perform Pos-Process, but Perform Only Print Process,” and pushes the “Complete Selection” button  45 , a control unit (not shown) of the host device  200  generates response information containing the “Perform Only Print Process” option and sends the information to the printer  100   a.    
         [0146]    Alternatively, when the user selects the selection button  43  corresponding to “2. Perform Print Process and Post-Print Process with Another Sheet,” and pushes the “Complete Selection” button  45 , the control unit of the host device  200  generates response information containing the “Perform Print Process and Post-Print Process with Another Sheet” option and sends the information to the printer  100   a.    
         [0147]    Further alternatively, when the user selects the selection button  44  corresponding to “3. Stop Print Process,” and pushes the “Complete Selection” button  45 , the control unit of the host device  200  generates response information containing the “Stop Print Process” option and sends the information to the printer  100   a.    
         [0148]    When the control unit  101   a  of the printer  100   a  receives the response information from the host device  200 , and when the response information shows the “Perform Only Print Process” option, the control unit  101   a  rotates the first sheet feeding roller  111  by driving the drive unit and feeds the top of the sheet  1  loaded in the first sheet tray  11  to the conveying path A. Then, the print process is performed on the sheet  1  based on image data stored in the image memory  103 . The sheet  1  in which the print process is finished is ejected from the eject unit  23  to outside of the device by the control of the control unit  101   a  and is loaded on the stacker unit  24 . 
         [0149]    Alternatively, when the response information shows “Perform Print Process and Post-Print Process with Another Sheet,” the control unit  101   a  instructs the sheet thickness calculation determination unit  109  to determine whether or not the calculation of the thickness of another sheet  2  is possible and forwards the type of the post-process (hole punch process or staple process). When the sheet thickness calculation determination unit  109  determines that the calculation of the thickness of another sheet  2  is possible, the sheet thickness sensor  17  turns on by the control of the control unit  101   a  and calculates the thickness of another sheet  2 . The calculated thickness of the other sheet  2  is stored into the sheet thickness information memory unit  108  as the sheet thickness calculation value T (μm) that corresponds to the sheet thickness information of the second sheet tray  12  by the control of the control unit  101   a . Then, when the special process capability determination unit  107   a  determines that the hole punch process or the staple process is possible, the hole punch process or the staple process is performed by using the other sheet  2 . The sheet or the group of sheets for which the above described process is finished is ejected to outside of the device through the eject belt  266  by the control of the control unit  101   a  and is loaded on the post-process stacker unit  27 . In this embodiment, because the printer  100   a  is capable of performing the post-process, the sheet loaded and held in at least one of the first sheet tray  11  and the second sheet tray  12  should have a thickness that qualifies for the post-process. 
         [0150]    Further alternatively, when the response information shows “Stop Print Process,” the control unit  101   a  deletes the image data stored in the image memory  103  by using an image delete unit (not shown). Then, the printer  100   a  is in a print data waiting state by the control of the control unit  101   a.    
         [0151]      FIGS. 11 and 12  are flow diagrams of operation of the printer  100   a  of the second embodiment according to the present invention. Operation of the printer  100   a  of the second embodiment according to the present invention is explained with reference to the flow diagrams in  FIGS. 11 and 12 . The first sheet tray  11  loads and holds the sheet  1  that has the following characteristics: the quality of a sheet is normal (normal paper); the size of a sheet is A4; and the thickness of a sheet is 190 μm. Similarly, the second sheet tray  12  loads and holds the sheet  2  that has the following characteristics: the quality of a sheet is normal (normal paper); the size of a sheet is A4; and the thickness of a sheet is 80 μm. 
         [0152]    When the control unit  101   a  of the printer  100   a  receives print data from the host device  200  through the I/F unit  102  (S 201 ), the control unit  101   a  converts the print data to image data through a rasterize transformation unit that is not shown and is in the printer  100   a . The print data contains the post-process instruction information showing existence or non-existence of the hole punch process and the staple process, number of sheets information showing total number of paper for printing (50 sheets), sheet size information showing a size of paper, such as A4, and sheet tray information showing automatic sheet feeding. The image data keeps the post-process instruction information, the number of sheet information, the sheet size information, and the sheet tray information that are contained in the print data. 
         [0153]    After the image data is stored in the image memory  103  (S 202 ), the control unit  101   a  controls the drive unit to rotate the photoreceptor drum  182  and, at the same time, instructs the special process instruction status determination unit  104   a  to determine existence or non-existence of the post-process instruction. 
         [0154]    When the special process instruction status determination unit  104   a  receives the instruction containing existence or non-existence of the special process instruction, and when the post-process instruction information indicates the “existence of the hole punch process,” the special process instruction status determination unit  104   a  determines that the hole punch process instruction exists (S 203 ). 
         [0155]    When the post-process instruction information indicates the “existence of the staple process,” the special process instruction status determination unit  104   a  determines that the staple process instruction exists (S 204 ). 
         [0156]    On the other hand, when the post-process instruction information indicates the “non-existence of the hole punch process and non-existence of the staple process,” the special process instruction status determination unit  104   a  determines that the post-process instruction does not exist. 
         [0157]    When the special process instruction status determination unit  104   a  determines that the hole punch process instruction exists, the control unit  101   a  instructs the sheet thickness calculation determination unit  109  to determine whether or not the thickness of the sheet  1  is calculated and forwards the type of the post-process (the hole punch process). 
         [0158]    On the other hand, when the special process instruction status determination unit  104   a  determines that the staple process instruction exists, the control unit  101   a  instructs the sheet thickness calculation determination unit  109  to determine whether or not the thickness of the sheet  1  is calculated and forwards the type of the post-process (the staple process). 
         [0159]    When the special process instruction status determination unit  104   a  determines that the post-process instruction does not exist, the control unit  101   a  performs the print process on the sheet  1  based on the image data (S 205 ). The sheet  1  in which the print process is performed is ejected from the eject unit  23  and is loaded on the stacker unit  24 . 
         [0160]    When the sheet thickness calculation determination unit  109  receives instruction for determination of the thickness of the sheet  1  and another sheet  2  discussed later, the sheet thickness calculation determination unit  109  searches the sheet thickness information memory unit  108  to obtain the sheet thickness calculation value T (μm) that corresponds to the thickness of the sheets loaded and held in the first sheet tray  11  or the second sheet tray  12  (S 206 ). 
         [0161]    When the sheet thickness calculation value T (μm) is “TBD” (S 207 ), the sheet thickness calculation determination unit  109  determines that the drive of the sheet thickness sensor  17  is necessary. When the sheet thickness calculation determination unit  109  determines that the drive of the sheet thickness sensor  17  is necessary, the control unit  101   a  turns on the sheet thickness sensor  17  (S 208 ). When the printer  100   a  performs the print process based on the automatic sheet feeding, the control unit  101   a  rotates the first sheet feeding roller  111  by the control of the drive unit based on a program that is set in a memory (not shown) in advance. As a result, the first sheet feeding roller  111  that starts to rotate feeds the top of the sheet  1  loaded in the first sheet tray  11  to the conveying path A. 
         [0162]    On the other hand, when the sheet thickness calculation value T (μm) is not “TBD,” the sheet thickness calculation determination unit  109  determines that the drive of the sheet thickness sensor  17  is not necessary. When the sheet thickness calculation determination unit  109  determines that the drive of the sheet thickness sensor  17  is not necessary, the control unit  101   a  instructs the special process capability determination unit  107   a  to determine whether or not the special process is performed and at the same time forwards the name of sheet feeding tray (first sheet tray  11 ) and the type of the post-process (hole punch process or staple process). 
         [0163]    When the sheet, which is carried to the conveying path A, passes the sheet thickness sensor  17 , the sheet thickness sensor  17  calculates the thickness (190 μm) of the sheet (S 209 ) and sends the thickness detection signal to the control unit  101   a . The thickness detection signal contains the name of sheet feeding tray (first sheet tray  11 ) and the type of the post-process. 
         [0164]    When the control unit  101   a  receives the thickness detection signal, the sheet thickness (190 μm) indicated by the signal is stored as the sheet thickness calculation value T (μm) in the sheet thickness information memory unit  108  corresponding to the name of sheet feeding tray contained in the signal (S 210 ). When the memory process is finished, the control unit  101   a  instructs the special process capability determination unit  107   a  to determine whether or not the special process is possible and forwards the name of sheet feeding tray (first sheet tray  11 ) and the type of the post-process. 
         [0165]    When the special process capability determination unit  107   a  receives the name of sheet feeding tray (first sheet tray  11 ) and the type of the post-process, and when the type of the post-process is the hole punch process, the special process capability determination unit  107   a  obtains the sheet thickness calculation value T (190 μm) that corresponds to the sheet tray through searching the sheet thickness information stored in the sheet thickness information memory unit  108  based on the name of the sheet feeding tray. The special process capability determination unit  107   a  compares the obtained sheet thickness calculation value T (190 μm) with the threshold value (160 μm), which is stored in the threshold information memory unit  105   a , for the hole punch process. Because the sheet thickness calculation value T (190 μm) is more than the threshold value (160 μm) (S 211 ), the special process capability determination unit  107   a  determines that the hole punch process is not possible (S 212 ). When the special process capability determination unit  107   a  determines that the hole punch process is not possible, the control unit  101   a  sends the post-process selection information that is stored in the selection information memory unit  106   a  to the host device  200  (S 213 ). The control unit  101   a  also controls that the sheet  1  is carried between the image forming unit  18  and the image fusing unit  20  (conveying path B) without the print process when the sheet  1  is carried through the conveying unit  13  (S 214 ). Then, the sheet in which the print process is not performed is ejected from the eject unit  23  to outside of the device through the rotation of the pair of ejecting rollers  231  and is loaded on the stacker unit  24  (S 215 ). 
         [0166]    On the other hand, when the sheet thickness calculation value T (μm) is less than the threshold value (160 μm), the special process capability determination unit  107   a  determines that the hole punch process is possible (S 216 ). When the special process capability determination unit  107   a  determines that the hole punch process is possible, the control unit  101   a  controls the print process for the sheet  1  and then controls the hole punch process (S 217 ). 
         [0167]    When the type of the post-process is the staple process, the special process capability determination unit  107   a  search the sheet thickness information stored in the sheet thickness information memory unit  108  based on the name of the sheet feeding tray (first sheet tray  11 ) and obtains the sheet thickness calculation value T (190 μm) that corresponds to the first sheet tray. The special process capability determination unit  107   a  compares the obtained sheet thickness calculation value T (190 μm) and total sheet thickness (9500 μm) calculated by the total number (fifty sheets) of sheets information with the threshold value (8000 μm) for the staple process that is stored in the threshold value information memory unit  105   a . Because the total thickness (9500 μm) of all of the sheets is more than the threshold value (8000 μm), the special process capability determination unit  107   a  determines that the staple process is not possible. When the special process capability determination unit  107   a  determines that the staple process is not possible, the control unit  101   a  sends the post-process selection information, which is stored in the selection information memory unit  106   a , to the host device  200  and controls that the sheet  1  is carried between the image forming unit  18  and the image fusing unit  20  (conveying path B) without the print process when the sheet  1  is carried through the conveying unit  13 . Then, the sheet in which the print process is not performed is ejected from the eject unit  23  to outside of the device through the rotation of the pair of ejecting rollers  231  and is loaded on the stacker unit  24 . 
         [0168]    On the other hand, when the total thickness of all of the sheets is less than the threshold value (8000 μm), the special process capability determination unit  107   a  determines that the staple process is possible. When the special process capability determination unit  107   a  determines that the staple process is possible, the control unit  101   a  controls the print process for the sheet  1  as discussed above, and then controls the staple process. 
         [0169]    When the host device  200  receives the post-process selection information from the printer  100   a , the host device  200  shows the information on a display unit shown as the form  40  in  FIG. 10 . 
         [0170]    When the user refers the form  40  showing on the display unit of the host device  200 , selects the selection button  42  through an input unit (not shown) corresponding to “1. Do Not Perform Pos-Process, but Perform Only Print Process,” and pushes the “Complete Selection” button  45 , a control unit (not shown) of the host device  200  generates response information containing the “Perform Only Print Process” option and sends the information to the printer  100   a.    
         [0171]    Alternatively, when the user selects the selection button  43  corresponding to “2. Perform Print Process and Post-Print Process with Another Sheet,” and pushes the “Complete Selection” button  45 , the control unit of the host device  200  generates response information containing the “Perform Print Process and Post-Print Process with Another Sheet” option and sends the information to the printer  100   a.    
         [0172]    Further alternatively, when the user selects the selection button  44  corresponding to “3. Stop Print Process,” and pushes the “Complete Selection” button  45 , the control unit of the host device  200  generates response information containing the “Stop Print Process” option and sends the information to the printer  100   a.    
         [0173]    When the control unit  101   a  of the printer  100   a  receives the response information from the host device  200  (S 218 ), and when the response information shows the “Perform Only Print Process” option (S 219 ), the control unit  101   a  rotates the first sheet feeding roller  111  by driving the drive unit and feeds the top of the sheet  1  loaded in the first sheet tray  11  to the conveying path A. Then, the print process is performed on the sheet  1  based on image data stored in the image memory  103  (S 220 ). The sheet  1  in which the print process is finished is ejected from the eject unit  23  to outside of the device by the control of the control unit  101   a  and is loaded on the stacker unit  24 . 
         [0174]    Alternatively, when the response information shows “Perform Print Process and Post-Print Process with Another Sheet,” the control unit  101   a  instructs the sheet thickness calculation determination unit  109  to determine whether or not the calculation of the thickness of another sheet  2  can be performed and forwards the type of the post-process (hole punch process or staple process). When the sheet thickness calculation determination unit  109  determines that the calculation of the thickness of another sheet  2  is possible, the sheet thickness sensor  17  turns on by the control of the control unit  101   a  and calculates the thickness (80 μm) of another sheet  2  (S 221 ). The calculated thickness (80 μm) of another sheet  2  is stored into the sheet thickness information memory unit  108  as the sheet thickness calculation value T (μm) that corresponds to the sheet thickness information of the second sheet tray  12  by the control of the control unit  101   a . And then, when the special process capability determination unit  107   a  determines that the hole punch process or the staple process is possible, the hole punch process or the staple process is performed by using another sheet  2  (S 222 ). The sheet or the group of sheets in which the above described process is finished is ejected to outside of the device through the eject belt  266  by the control of the control unit  101   a  and is loaded on the post-process stacker unit  27 . 
         [0175]    Further alternatively, when the response information shows “Stop Print Process,” the control unit  101   a  stops the print process and deletes the image data stored in the image memory  103  by using an image delete unit (not shown) (S 223 ). Then, the printer  100   a  is in a print data waiting state by the control of the control unit  101   a.    
         [0176]    Because in the printer  100   a  of the second embodiment, the thickness of each sheet in each sheet tray that is calculated through the sheet thickness sensor  17  is stored in the sheet thickness information memory unit  108 , the sheet thickness that qualifies for the special process is determined without calculation for each sheet at each print process. Therefore, the time required to determine whether or not the special process is possible can be shortened. Number of sheets that is supplied and that is loaded on the stacker unit  24  without print process can be decreased. 
         [0177]    In the second embodiment, the printer  100   a  performs the one-side printing. However, the present invention is not limited to the structure discussed above and is applied to duplex printing. In this case, the printer  100   a  can perform duplex printing and the post-process (hole punch process and staple process). 
         [0178]    In the first and second embodiments, because the printers  100  and  100   a  need to determine whether or not duplex printing, the hole punch process and the staple process can be performed on the sheets  1  and  2 , the sheet thickness sensor  17  is used to obtain the thickness of the sheets. However, the present invention is not limited to the structure discussed above and is applied to structures in which the hardness of the sheet is obtained as the characteristics of the sheet, or a reflective photo sensor is used to determine whether or not the sheet is for OHP. 
         [0179]    In this case, when the hardness of the sheet is equal to or more than the threshold value that is a certain value of the hardness or the sheet is for OHP, the printers  100  and  100   a  determines that the duplex printing, the hole punch process and the staple process cannot be performed. On the other hand, when the hardness of the sheet is less than the threshold value that is a certain value of the hardness or the sheet is not for OHP, the printers  100  and  100   a  determines that the above each process can be performed. 
         [0180]    When the printer  100  determines whether or not the sheet  1  or another sheet  2  qualifies for duplex printing, the size of each sheet is determined by the relationship between the period of time that the sensor turns on and the sheet feeding speed, or the reflection ratio of a photo sensor determines whether or not each sheet is the reverse-side up. 
         [0181]    In the structure discussed above, the printer  100  determines that duplex printing is not possible because of structural restrictions when the size of the sheet is less than A6. The printer  100  also determines that duplex printing is not possible when the sheet is already printed on one side and is ready to be printed on the reverse side. On the other hand, the printer  100  determines that duplex printing is possible when the size of the sheet is equal to or larger than A6, and both sides of the sheet are yet not printed. 
         [0182]    In the embodiments discussed above, the printer as the image forming device is explained. However, the present invention is not limited to the structure discussed above and may be applicable to a multifunction peripheral (MFP) that incorporates one of the following functions: a printer function; a copier function; a facsimile function; and a scanner function. The image forming device being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be apparent to one of ordinary skill in the art are intended to be included within the scope of the following claims.