Abstract:
A sheet erasing method according to an embodiment includes controlling a printer unit that forms an image on sheets, a finishing unit that performs post-processing on sheets, and a conveying unit that conveys sheets to the printer unit and to the finish unit. When a print job includes a selection of a reused sheet mode and an instruction to perform post-processing, a first reused sheet is conveyed to the printer unit for forming an image and to the finishing unit for performing post-processing. When the print job includes a selection of the reused sheet mode and not an instruction to perform post-processing, the second reused sheet is conveyed to the printer unit for forming an image without performing post processing in the finishing unit. When the print job includes a selection of the new sheet mode, the new sheet is conveyed to the printer unit for forming an image.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of co-pending U.S. patent application Ser. No. 14/796,952, filed on Jul. 10, 2015, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an image forming apparatus that reuses a sheet on which an image is formed with a decolorable colorant and a method for processing a reused sheet. 
     BACKGROUND 
     In the related art, image forming apparatuses such as a multifunction peripheral (MFP) are used to form images on a sheet. In order to reuse a sheet, it is possible to form images with a decolorable colorant on a sheet. When a decolorable colorant is used in forming images, the sheet may be reused by erasing images later with an erasing apparatus. 
     The decolorable colorant is erased by applying high temperature heat thereto. Therefore, the erasing apparatus is used to heat the sheet and erase images formed on the sheet when reusing the sheet. The erasing apparatus heats the sheet by transporting the sheet through a press roller and a heat source. In this manner, the decolorable colorant is erased. Reusing sheets leads to saving sheets and may contribute to environmental protection. Erasing images formed on the sheet may be called “decolorization” in the description below. 
     Erasing apparatuses in the related art determine whether a sheet is reusable after erasing images printed with a decolorable colorant. Determination of whether a sheet is reusable depends on the presence or absence of a residue of an erased image and/or the state (deformation, damage, stains, and the like) of a sheet. For example, when images are formed, and post-processing such as punching and stapling is performed by a finisher, the sheet may be determined as nonreusable because of the damage. For this reason, post-processing such as punching and stapling is avoided. 
     In addition, in the erasing apparatus, there may be a limit on the number of times of reuse. For example, a sheet that is reused for N times (where N is the maximum allowable number of reuse) may be transported to a reject cassette as a nonreusable sheet. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example configuration of an erasing apparatus. 
         FIG. 2  illustrates an example of an image and a mark, both of which are formed on a reused sheet. 
         FIG. 3  is a block diagram illustrating a control system of the erasing apparatus. 
         FIG. 4  is a flowchart illustrating an example sequence of operations of transporting a sheet in the erasing apparatus. 
         FIG. 5  illustrates an example configuration of an image forming apparatus and a finisher according to a first embodiment. 
         FIG. 6  is a block diagram illustrating a control system of the image forming apparatus. 
         FIG. 7  is a flowchart illustrating an example sequence of operations performed by the image forming apparatus and the finisher. 
         FIG. 8  illustrates an example of a display screen on an operating unit of the finisher. 
         FIG. 9  illustrates another example of a display screen on the operating unit. 
         FIG. 10  illustrates an example configuration of an image forming apparatus and a finisher according to a second embodiment. 
         FIG. 11  is a flowchart illustrating an example sequence of operations performed by the image forming apparatus and the finisher in the second embodiment. 
         FIG. 12  illustrates a modified example configuration of the image forming apparatus and the finisher. 
     
    
    
     DETAILED DESCRIPTION 
     A sheet erasing apparatus according to an embodiment includes an erasing unit, a scanning unit, and first, second and third sheet cassettes. A processing unit determines a number of times of reuse, and whether the sheet is reusable. When the sheet is determined to be reusable and the number of times reuse is less than a predetermined number, the sheet is conveyed to the first sheet cassette. When the sheet is determined to be reusable and the number of times of reuse is equal to a predetermined number, the sheet is conveyed to the second sheet cassette. When the sheet is determined to not be reusable, the sheet is conveyed to the third sheet cassette. 
     Hereinafter, exemplary embodiments will be described with reference to the appended drawings. The same places in each drawing will be given the same sign. 
     First Embodiment 
       FIG. 1  illustrates an example configuration of an erasing apparatus  10 . The erasing apparatus  10  is capable of making a sheet reusable by erasing an image that is formed with a decolorable colorant on the sheet. The sheet from which an image is erased is then used by a later-described image forming apparatus. 
     An erasing apparatus  10  is provided with an operating unit  11  that includes an operating panel and a display, a sheet feeding unit  12 , and a scanner  13 . In addition, the erasing apparatus  10  includes a first to a sixth transport path  141  to  146  and a plurality of sheet discharge cassettes  15 ,  16 , and  17 . 
     Each of the transport paths  141  to  146  includes a plurality of transport rollers  18  for transporting a sheet. Each of the plurality of transport rollers  18  is driven by a motor. A gate  19  is positioned to divide the sheet transport path  141  into the transport path  142  and the transport path  144 . An erasing unit  20  is disposed on the transport path  142 . 
     The first transport path  141  transports a sheet S from the sheet feeding unit  12  to the scanner  13 . The second transport path  142  transports the sheet S from the scanner  13  toward the erasing unit  20  in a direction of an arrow A. The third transport path  143  transports the sheet S from the erasing unit  20  to the scanner  13  again. The fourth transport path  144  transports the sheet S to the sheet discharge cassette  15 . The fifth transport path  145  transports the sheet S to the sheet discharge cassette  16 . The sixth transport path  146  transports the sheet S to the sheet discharge cassette  17 . 
     The sheet discharge cassette  15  is a reuse cassette that retains a reusable sheet after an image is erased therefrom. The sheet discharge cassette  16  is a reject cassette that retains a sheet which is not reusable and is to be recycled (nonreusable sheet). The sheet discharge cassette  17  is a last reuse cassette that retains a sheet of which the number of times of reuse will soon reach an allowable number of times (N times). 
     Assuming that the allowable number of reuse is N, the sheet discharge cassette  17  retains a sheet (regular sheet) of which the number of reuse has reached N−1. That is to say, the sheet discharge cassette  17  retains a sheet that can be reused one last time in the next printing (image forming). The sheet discharge cassette  17  serves as a dedicated cassette for a sheet of which the number of reuse reaches N−1. Since a sheet that is transported to the sheet discharge cassette  17  will not be reused after an image is formed thereon by the image forming apparatus, post-processing such as punching and stapling may be performed for the sheet. 
     In the description below, the sheet discharge cassette  15 , the sheet discharge cassette  16 , and the sheet discharge cassette  17  are respectively called a reuse cassette  15 , a reject cassette  16 , and a last reuse cassette  17 . 
     The erasing apparatus  10  in  FIG. 1  generally performs an erasing process as follows. First, a mode of decolorization and reading for the sheet S is selected in the operating unit  11 . Afterward, the sheet S is transported from the sheet feeding unit  12  to the scanner  13  through the transport path  141 . The scanner  13  includes a first scanner  131  and a second scanner  132  and scans both sides of the sheet S. The scanner  13  scans an image of each side of the sheet and generates corresponding image data before the image on the sheet S is decolorized. In addition, the scanner  13  reads a mark (will be described later) that is printed on the sheet. The mark is printed by the image forming apparatus as will be described later. 
     The scanner  13  further reads the percentage of printing and the printing status on the sheet S. The state of the sheet S is determined from the read printing status. The sheet S is determined to be nonreusable when the state includes deformation such as ripples and crumples or damage such as punched holes on the sheet S. The sheet S that is determined as nonreusable is transported to the reject cassette  16  through the fifth transport path  145 . A sheet having a high percentage of printing is also determined to be nonreusable, because the sheet is likely to curl at the time of decolorization, and thus is transported to the reject cassette  16 . The sheet S having no ripples or crumples, or a sufficiently low percentage of printing is transported to the erasing unit  20  by the second transport path  142 . 
     The erasing unit  20  includes a first erasing unit and a second erasing unit. The first erasing unit includes a heating roller  21  and a press roller  22 . The second erasing unit includes a press roller  23  and a heating roller  24 . The sheet S is heated by being transported through the heating roller  21  and the press roller  22  and through the press roller  23  and the heating roller  24 . The heating rollers  21  and  24  are provided with heat sources therein. A lamp, for example, may be used as a heat source. 
     The sheet S that is transported to the erasing unit  20  is heated when passing through the erasing unit  20 , and the image formed on the sheet S is decolorized by heat. The erasing unit  20  applies heat and pressure to the sheet S at a comparatively high temperature of, for example, 175° C. to 200° C. and decolorizes the image on the sheet S. That is to say, a decolorable colorant is used in forming the image on the sheet S, and the decolorable colorant is decolorized when reaching a predetermined temperature. Therefore, the image on the sheet S may be decolorized by transporting the sheet S to the erasing unit  20  that heats the sheet S at the predetermined temperature. 
     The sheet S that passes through the erasing unit  20  is again transported to the scanner  13  by the third transport path  143 . The scanner  13  reads again the printing status, as previously described. It is then determined whether the image is completely decolorized. The sheet S that is determined as reusable from the result of reading by the scanner  13  after the image is erased is transported as a reuse sheet to the reuse cassette  15  through the fourth transport path  144 . 
     Based on the printing status that is read by the scanner  13 , the sheet S determined to have an image formed with a non-decolorable colorant or a handwritten image that remains in an image area is transported to the reject cassette  16  through the fifth transport path  145 . The sheet S that is determined to be rippled or crumpled is also transported to the reject cassette  16 . The scanner  13  further reads the mark that is printed on the sheet S and determines the number of reuse. A sheet having the number of reuse of N−1 is retained in the last reuse cassette  17 . 
       FIG. 2  illustrates an example of an image  25  and a mark  26 , both of which are formed on the reused sheet S. The image  25  is printed with a decolorable colorant (decolorable toner, decolorable ink, and the like). The mark  26  is printed with a non-decolorable colorant that is not erased by heat. 
     The image forming apparatus prints the mark on the sheet at each time of printing with a decolorable colorant. For example, after the sheet is reused for n times, n numbers of the marks  26  are printed on the sheet. The mark  26  is printed according to the number of reuse for each of the front side and the rear side of the sheet. That is to say, the mark  26  serves as information indicating the number of reuse of each side. 
     The number of reuse of the sheet S is determined based on the image of the mark  26  read by the scanner  13 . For example, the image data of both sides of the read sheet S (the image  25  and the mark  26 ) is stored in a storage unit in the erasing apparatus  10  on a per sheet basis. In addition, the number of reuse of the sheet is determined by counting the number of marks  26 . Assuming that N is the maximum allowable number of reuse, the regular sheet S of which the number of reuse reaches N−1 is transported to the last reuse cassette  17  through the sixth transport path  146 . The mark  26  is preferably printed at a corner of the sheet S in an unnoticeable size. 
     Instead of printing the mark  26  on the sheet S, an IC chip or the like may be affixed to the sheet S. When identifiable information of the same IC chip is read N−1 times, the corresponding sheet S may be desirably transported to the last reuse cassette  17 . 
     In addition, there may be a case where the number of reuse is different for different sides of the same sheet. When the number of reuse is different for different sides of the sheet, the sheet is transported to the last reuse cassette  17  when the number of reuse of any side reaches N−1. 
       FIG. 3  is a block diagram illustrating a control system of the erasing apparatus  10 . The erasing apparatus  10  includes a system control unit  100  that controls each unit in whole. The system control unit  100 , for example, includes a CPU  101  that is a controller, a random access memory (RAM)  102 , a read-only memory (ROM)  103 , a hard disk drive (HDD)  104 , and a network interface (I/F)  105 . 
     The operating unit  11 , the sheet feeding unit  12 , the scanner  13 , the erasing unit  20 , and a transporting unit  140  are connected to the system control unit  100  through a bus  106 . The operating unit  11  includes an operating panel  111  and a display  112 . The transporting unit  140  includes motors (not illustrated) that rotate the transport rollers  18  which are disposed on each of the transport paths  141  to  146 . 
     The CPU  101  achieves various processing functions by executing a control program that is stored on the ROM  103 . The RAM  102  is a main memory that functions as a working memory. The ROM  103  stores the control program and control data for controlling operation of the erasing apparatus  10  and achieving various processing functions. 
     The HDD  104  is a large capacity memory for data storage. The HDD  104 , for example, stores the image data generated by the scanner  13  and data obtained from reading the mark  26 . The network interface (I/F)  105  communicates with the image forming apparatus and other external apparatuses (PC and the like) through, for example, a local area network (LAN). 
     Next, a description will be provided for an operation of erasing the image on the sheet S in the erasing apparatus  10  and an operation of transporting the sheet S out of the erasing apparatus  10 . The erasing apparatus  10  may obtain the number of reuse of the sheet by reading the mark  26 . Assuming the allowable number of reuse is N, the erasing apparatus  10  identifies a sheet of which the number of reuse is N−1 transports the sheet to the last reuse cassette  17 . Each sheet retained in the last reuse cassette  17  is treated as a sheet that may be post-processed (punched or stapled). 
     Specifically, when printing is performed on a sheet of which the number of reuse is N−1, the sheet is determined as nonreusable because the number of reuse has reached the allowable number of reuse N. Thus, after the Nth reuse, the sheet will be transported to the reject cassette  16  to be disused. Therefore, post-processing such as punching and stapling may be performed on the sheet of which the number of reuse is (N−1). 
       FIG. 4  is a flowchart illustrating an example sequence of operations of transporting the sheet S out of the erasing apparatus  10  under control of the CPU  101 . In ACT 1  in  FIG. 4 , the CPU  101  instructs the sheet feeding unit  12  to feed the sheet S. In ACT 2 , the CPU  101  instructs the scanner  13  to read the image (includes the mark  26 ) that is printed on the sheet S. The CPU  101  causes the read image data to be stored on the storage unit of the HDD  104  and the like. In ACT 2 , in addition, the CPU  101  reads the state of the sheet. 
     In ACT 3 , the CPU  101  determines whether the sheet S is deformed, damaged, or stained from the result of reading the state of the sheet. When the sheet S is determined as deformed, damaged, or stained (YES in ACT 3 ), the CPU  101  determines that the sheet S is nonreusable. In the case of the nonreusable sheet S, the CPU  101  transitions to ACT 9  and instructs the transporting unit  140  to transport the sheet S to the reject cassette  16 . 
     When the sheet S is determined to be not deformed, not damaged, or not stained in ACT 3  (NO in ACT 3 ), the CPU  101  determines whether the number of reuse of the sheet S is less than the allowable number of reuse (N times) ACT 4 . In other words, in ACT 4 , the CPU  101  reads the mark  26  that is included in the image read in ACT 2 , and when the same mark  26  is read N times or more, the CPU  101  transitions to ACT 9  and transports the sheet S to the reject cassette  16 . 
     The scanner  13  may read the sheet S that is fed from the sheet feeding unit  12  or may read the sheet S after the image on the sheet S is erased by the erasing unit  20 . Therefore, the CPU  101 , when counting the number of reuse of the sheet S, determines the number of reuse from the information that is read at the time of feeding of the sheet S by the sheet feeding unit  12 . 
     When the number of reuse is N or less in ACT 4 , the CPU  101  determines whether the number of reuse is N−1 in ACT 5 . In ACT 5 , when the number of reuse is determined to be N−1 (YES in ACT 5 ), the CPU  101  transitions to ACT 10  and instructs the transporting unit  140  to transport the sheet S to the last reuse cassette  17 . 
     When the number of reuse is determined to be less than N−1 in ACT 5  (NO in ACT 5 ), the CPU  101  transports the sheet S to the erasing unit  20  through the second transport path  142  in ACT 6 . The erasing unit  20  erases the image on the sheet that is transported to the erasing unit  20 . In ACT 7 , the CPU  101  reads again the sheet S with the scanner  13  and determines whether the sheet S reusable. If, for example, there is a residue that is not erased from the sheet S, or the sheet S is deformed, damaged, or stained, the sheet S is determined as nonreusable in ACT 7 . The CPU  101  then transitions to ACT 9  and instructs the transporting unit  140  to transport the sheet S to the reject cassette  16 . 
     When the sheet S is determined to be reusable in ACT 7  (YES in ACT 7 ), the CPU  101  proceeds to ACT 8 . In ACT 8 , the CPU  101  instructs the transporting unit  140  to transport the sheet S to the reuse cassette  15 , and then ends the operation. The mark  26  remains because the mark  26  is not erased. However, since the mark  26  is small, the mark  26  may be ignored by a user even though the mark  26  remains. 
     As described above, a regular sheet of which the number of reuse has reached N−1 is retained in the last reuse cassette  17 . Since the sheet that is retained in the last reuse cassette  17  will be reused for the last time, the sheet may be post-processed (punched or stapled). Therefore, the sheets retained in the last reuse cassette  17  are preferentially used as sheets for post-processing. 
     When there is a sheet transported to the last reuse cassette  17 , a message such as “Please use the sheets in the last reuse cassette for post-processing” may be displayed on the display  112  of the operating unit  11 . 
     Next, the configuration of the image forming apparatus according to the first embodiment will be described with reference to  FIG. 5 . In  FIG. 5 , an image forming apparatus  30  is, for example, a multifunction peripheral (MFP) that is a compound machine, a printer, a photocopier or the like. In the description below, the image forming apparatus  30  will be assumed to be an MFP. 
     A finisher  70  may be connected to the MFP  30 . Connecting the finisher  70  to the MFP  30  enables post-processing such as punching and stapling. A sheet on which an image is formed by the MFP  30  is transported to the finisher  70 . The finisher  70  performs post-processing such as punching and/or stapling on the sheet that is supplied from the MFP  30 . The finisher  70  includes a punching unit  71  and/or a stapling unit  75 . The punching unit  71  makes a punch hole on a sheet. The stapling unit  75  staples a bundle of sheets. 
     A document table  32  is arranged above a main body  31  of the MFP  30 . An automatic document feeder (ADF)  33  is disposed on the document table  32  in an openable and closable manner. A glass  34  that is a window for reading a document is fixed under the ADF  33 . A sheet feeding tray  35 , on which a document may be placed, is disposed in the ADF  33 . An operating panel  36  is positioned above the main body  31 . The operating panel  36  includes various operational keys  37  and a touch panel display unit  38 . 
     A scanner unit  40  that functions as an image reading apparatus is provided under the document table  32  of the MFP  30 . The scanner unit  40  scans a document that is transported by the ADF  33 . In addition, the scanner unit  40  scans a document by scanning a side of the document that is mounted on the document table  32 . The scanner unit  40  includes a first carriage  41  and a second carriage  42 . 
     The first carriage  41  incorporates a light source  43  for irradiating the side of the document. The light source  43  irradiates the document with light. The light source  43  uses, for example, an LED. Light reflected by the document is reflected off of a mirror that is disposed in the first carriage  41  and the second carriage  42 . The light reflected by the mirror is guided to an image sensor  45  through a lens  44 . The image sensor  45  includes, for example, a charge coupled device (CCD) line sensor. 
     The reflective light from the document is converted into electrons by the image sensor  45 . By the conversion of light into electrons, an electrical signal is output from the image sensor  45 . The electrical signal output from the image sensor  45  is subject to analog-processing and is converted into a digital signal. The digital signal is subject to image-processing, and the image data is generated. 
     A printer unit  50  is provided in the MFP  30 . The printer unit  50  includes a photoreceptor drum, a laser, and the like. The printer unit  50  processes the image data generated by the scanner unit  40  or transmitted by a personal computer (PC), or the like. The printer unit  50  forms an image on a recording medium based on the processed image data. Hereinafter, the sheet S will be described as a recording medium. 
     The printer unit  50  is provided with a loop transfer belt  51  that functions as an image carrier. An image forming unit  500  is disposed downstream of the transfer belt  51 . An electrostatic charger  53 , a developer  54 , a primary transfer roller  55 , a cleaner  56 , a blade  57 , and the like are suitably arranged around the photoreceptor drum  52 . 
     The electrostatic charger  53  uniformly electrifies the entire surface of the photoreceptor drum  52 . The image forming unit  500  exposes the surface of the photoreceptor drum  52  by scanning the surface of the photoreceptor drum  52  with a laser beam from a laser  60 . By the exposure, an electrostatic latent image is formed on the photoreceptor drum  52 . The laser  60  scans the laser beam based on the image data read by the scanner unit  40 . 
     The developer  54  includes a mixer and a developing roller. The developing roller supplies a dual component developer toner including a toner and a carrier to the photoreceptor drum  52 . 
     The toner image on the photoreceptor drum  52  is transferred to the transfer belt  51  by the primary transfer roller  55 . The cleaner  56  removes the remaining toner on the surface of the photoreceptor drum  52  with the blade  57 . The toner image that is on the transfer belt  51  is then transferred to the sheet S by a secondary transfer roller  58 . The transfer belt  51  is stretched over a drive roller  591  and a driven roller  592 . The transfer belt  51  is circularly moved by rotation of the drive roller  591 . 
     The toner image that is transferred to the sheet S is fixed on the sheet S by a fixer  61 . The fixer  61  includes a fixing roller and a press roller. The sheet S passes through between the fixing roller and the press roller. The sheet S is heated and pressed to fix the toner image on the sheet S. 
     The printer unit  50 , when forming a color image, includes a plurality of image forming units  500  for, for example, yellow (Y), magenta (M), cyan (C), and blue (B). Each of the plurality of image forming units  500  are arranged facing the transfer belt  51  from the upstream side to the downstream side of the transfer belt  51 . Each of the plurality of image forming units  500  has the same configuration. Thus, only one image forming unit  500  is illustrated in  FIG. 5 . 
     The image forming unit for yellow (Y) forms an image with a non-decolorable yellow colorant. The image forming unit for magenta (M) forms an image with a non-decolorable magenta colorant. The image forming unit for cyan (C) forms an image with a non-decolorable cyan colorant. The image forming unit for blue (B) forms an image with a decolorable blue-based colorant (may be called a B toner hereinafter). 
     The image forming units for yellow, magenta, and cyan may form a monochrome (black) image by mixing each color thereof. In a case of reusing a sheet, an image may be formed with the decolorable B toner by the image forming unit for blue (B). When an image is formed on a sheet with the decolorable colorant, the sheet may be reused by erasing the image with the erasing apparatus  10 . 
     The mark  26  that indicates the number of reuse is printed with a non-decolorable toner at an unnoticeable position (a corner or the like) on the sheet S. When the sheet is reused, the number of marks  26  is increased for each time of reuse. 
     A sheet feeding cassette that accommodates various sizes of sheets is provided in the lower portion of the main body  31 . The sheet feeding cassette includes a plurality of cassettes  62 ,  63 , and  64 . The number of cassettes is not limited to three. The cassette  62  accommodates, for example, a new sheet. The cassette  63  accommodates a sheet that was retained in the reuse cassette  15  of the erasing apparatus  10 . The cassette  64  accommodates a sheet that was retained in the last reuse cassette  17  of the erasing apparatus  10 . 
     In the description below, a sheet that the cassette  63  accommodates is called a reuse sheet. A sheet that the cassette  64  accommodates is called a last reuse sheet. A last reuse sheet is a regular sheet of which the number of reuse has reached N−1. 
     A transport roller  65  and a resist roller  66  are arranged between the cassette  62  and the secondary transfer roller  58 . The transport roller  65  transports the sheet S that is taken out of each of the cassettes  62 ,  63 , and  64  to the resist roller  66 . The resist roller  66  transports the sheet S to the secondary transfer roller  58 . The secondary transfer roller  58  transfers the toner image to the sheet S. The sheet S on which the toner image is fixed by the fixer  61  is discharged by a sheet discharge roller  67 . 
     When the finisher  70  is connected to the rear of the MFP  30 , a sheet can be transported to the finisher  70  through a transport path  68  and a transport roller  69 . The sheet S that does not need to be post-processed is discharged to a fixed tray  93 . A gate  91  and a discharge roller  92  are positioned to guide the sheet S to the fixed tray  93 . The gate  91  is positioned between the punching unit  71  and the transport roller  69  in the main body  31 . By switching the gate  91 , the sheet S is selectively transported to the finisher  70  or the fixed tray  93 . 
     In the embodiment, the sheet S is transported from one of the sheet feeding cassettes  62 ,  63 , and  64  to the sheet discharge roller  67 . Therefore, a side of the sheet feeding cassettes  62 ,  63 , and  64  is defined as the upstream direction of transport of the sheet. In addition, a side of the sheet discharge roller  67  is defined as the downstream direction of transport of the sheet. 
     Next, an example of the finisher  70  will be described. The finisher  70  includes the punching unit  71  and/or the stapling unit  75 . The punching unit  71  includes a punching blade  72  and a dust box  73 . The punching unit  71  is disposed between the MFP  30  and the stapling unit  75 . 
     The punching blade  72  of the punching unit  71  goes down in order to make a punch hole on the sheet. Punch wastes produced by punching fall into the dust box  73 . The sheet S that is discharged from the punching unit  71  is transported to the stapling unit  75 . 
     The stapling unit  75  includes a waiting tray  76 , a processing tray  77 , and a stapler  78 . An entrance roller  79  that is disposed in a transport entrance of the stapling unit  75  receives the sheet S that is discharged from the punching unit  71 . The entrance roller  79  is driven by a motor (not illustrated). 
     A sheet feeding roller  80  is positioned on the downstream side of the entrance roller  79 . The sheet S that the entrance roller  79  receives is transported to the waiting tray  76  through the sheet feeding roller  80 . The sheet feeding roller  80  is driven by a motor (not illustrated). The processing tray  77  is positioned below the waiting tray  76 . The sheet S that falls from the waiting tray  76  may be stacked on the processing tray  77 . 
     The waiting tray  76  has an openable structure while the sheet S is stacked thereon. When a predetermined number of sheets S are stacked on the waiting tray  76 , the waiting tray  76  is opened, and the sheet S falls onto the processing tray  77 . The sheet that falls onto the processing tray  77  is guided to the stapler  78  by a transport belt  81  and is stapled. The transport belt  81  is driven by a motor (not illustrated). 
     The transport belt  81  rotates in one direction when guiding the sheet S to the stapler  78  and in an opposite direction when discharging the stapled sheet S. The stapled sheet S is transported to a storage tray  82  by the transport belt  81 . The storage tray  82  may be raised or lowered. 
     When stapling is not necessary, and only punching is performed, the sheet S is not guided to the stapler  78  and is discharged to the storage tray  82  by a rotating roller. When only stapling is performed without punching, the punching blade  72  of the punching unit  71  is placed at a standby position from the transport path of the sheet. The sheet S is transported to the stapling unit  75  without being punched and is stapled by the stapler  78 . 
       FIG. 6  is a block diagram illustrating a control system of the MFP  30  according to the first embodiment. In  FIG. 6 , the MFP  30  includes a main control unit  300 , the operating panel  36 , the scanner unit  40 , and the printer unit  50 . The control system of the MFP  30  includes a plurality of CPUs of a main CPU  301  in the main control unit  300 , a panel CPU  361  of the operating panel  36 , a scanner CPU  401  of the scanner unit  40 , and a printer CPU  501  of the printer unit  50 . The control system performs communication between each CPU. 
     The main control unit  300  includes the main CPU  301 , a ROM  302 , a RAM  303 , an image processing unit  304 , an image memory unit  305  such as an HDD, a communication interface (I/F)  306 , and the like. The main CPU  301  controls the entire operation of the MFP  30 . In addition, the main CPU  301  may control the finisher  70 . The ROM  302  stores a control program and the like. The RAM  303  temporarily stores data when the main CPU  301  performs various processes. 
     The image processing unit  304  processes the image data generated by the scanner unit  40  and the image data transmitted from a PC and the like. As a process for the image data, for example, an image conversion process of enlarging or shrinking the image may be performed. The image memory unit  305  compresses and stores the image data generated by the scanner unit  40  and the image data transmitted from a PC (document data, drawing image data, and the like). 
     The image data that is stored in the image memory unit  305  is input into the image processing unit  304 . The image processing unit  304  performs various types of image processing. The printer unit  50  prints the image data subject to image-processing on the sheet. The communication I/F  306  may be connected to the erasing apparatus  10  and the like through a network NW such as a LAN. 
     The operating panel  36  includes the panel CPU  361  that is connected to the main CPU  301 , the various operational keys  37 , and the display unit  38  which may be configured of a liquid crystal display or the like. The operational keys  37  include a ten key and the like for, for example, instructing the number of prints. The display unit  38  has a function of a touch panel. Instructions for a sheet size, printing magnification, simplex printing, and duplex printing are input on the display unit  38 . In addition, selecting a type of a sheet, specifying a post-processing mode, specifying a printing mode, and the like are performed on the display unit  38  (will be described in detail later). 
     The scanner unit  40  includes the scanner CPU  401  and a CCD driver  402  that drives the image sensor  45 . The CCD driver  402  scans the image of the document by driving the image sensor  45  and converts the image into image data. 
     The printer unit  50  includes the printer CPU  501 , a ROM  502 , a RAM  503 , an image forming unit  504 , a transport control unit  505 , and the like. The printer unit  50  performs printing on a sheet in cooperation with the main CPU  301 . The ROM  502  stores a program and the like for controlling the printer unit  50 . The RAM  503  is a storage unit and temporarily stores data when the printer CPU  501  performs various processes. 
     The printer CPU  501  controls the image forming unit  504 . The image forming unit  504  is a generic term for a plurality of image forming units for yellow (Y), magenta (M), cyan (C), and blue (B). The image forming unit  504  forms an image by controlling the photoreceptor drum  52 , the electrostatic charger  53 , the developer  54 , the secondary transfer roller  58 , and the like. In addition, the image forming unit  504  controls the temperature and rotation of a heating roller of the fixer  61 . 
     The gate  91  and a motor  506  are connected to the transport control unit  505 . The transport control unit  505  controls transport of the sheet S by driving the motor  506 . In addition, the transport control unit  505  switches the gate  91  and guides the sheet S to the finisher  70  or the fixed tray  93 . 
     Next, a description will be provided for an image forming operation performed by the MFP  30  and a post-processing operation performed by the finisher  70  according to the embodiment. 
       FIG. 7  is a flowchart illustrating an example sequence of operations performed by the MFP  30  and the post-processing operation performed by the finisher  70  according to the embodiment. Processes in  FIG. 7  are performed under control of a control unit that mainly includes the main CPU  301  and the printer CPU  501 .  FIG. 8  illustrates an example of a display screen that is displayed on the display unit  38  of the operating panel  36  in the MFP  30 . 
     First, operations on the menu screen will be described with reference to  FIG. 8 . Displaying choices for a type of a sheet, a post-processing mode and a printing mode may be performed in the operating panel  36  by displaying the menu screen in  FIG. 8  on the display unit  38 . 
     On the menu screen in  FIG. 8 , a type of a sheet may be selected by operating a standard button  1  or an eco button  2 . That is to say, a selection may be made between printing of an image on a new sheet by touching the standard button  1  and printing of an image on a reuse sheet or a last reuse sheet by touching the eco button  2 . 
     Regarding designations of a post-processing mode, a designation of stapling a sheet with the finisher  70  may be made by touching a staple button  3  on the menu screen. A designation of punching a sheet may be made by touching a punch button  4 . When the staple button  3  and the punch button  4  are not touched, post-processing is not performed. 
     Regarding designations of a printing mode, a designation of printing an image on a sheet with a non-decolorable colorant may be made by touching a normal button  5  on the menu screen. A designation of printing an image on a sheet with a decolorable colorant (B toner) may be made by touching a decolorize button  6 . 
     When a perform button  7  is touched after selection of a type of a sheet, a post-processing mode and a printing mode is finished, the MFP  30  starts forming an image, and the finisher  70  starts the post-processing operation according to the selections. 
     In ACT 11  in  FIG. 7 , the main CPU  301  determines the selected type of a sheet. For example, when a user touches the eco button  2  on the menu screen in  FIG. 8 , the main CPU  301  determines that an image is printed on a reuse sheet or a last reuse sheet (YES in ACT 11 ). In ACT 12 , the main CPU  301  determines whether post-processing is to be performed. 
     That is to say, when the user touches at least one of the staple button  3  and the punch button  4  on the menu screen in  FIG. 8  (YES in ACT 12 ), the main CPU  301  controls the finisher  70 . In ACT 13 , the main CPU  301  sets the finisher  70  into an operable state (ON). 
     In ACT 14 , the main CPU  301  selects the cassette  64 . The cassette  64  accommodates a last reuse sheet. Since a last reuse sheet becomes nonreusable after being reused once more, a last reuse sheet may be stapled or punched. Therefore, when the eco button  2  is selected, and post-processing is specified, the cassette  64  is preferentially selected. 
     When there is no last reuse sheet accommodated in the cassette  64 , a message  8  is displayed on the display unit  38  of the operating panel  36  as illustrated in  FIG. 9 . In  FIG. 9 , a message  8  such as “There is no sheet in cassette  3 . Please insert last reuse sheets.” is displayed. A sheet sensor that is disposed in the cassette  64  determines whether there is a sheet in the cassette  64 . 
     Next, in ACT 15 , the printer CPU  501  performs printing by controlling the printer unit  50 . That is to say, the printer CPU  501  prints an image on a last reuse sheet by controlling the image forming unit  504 . Printing on a last reuse sheet is performed in a printing mode that is specified in  FIG. 8 . For example, printing is performed with a non-decolorable colorant when the normal button  5  is touched, and printing is performed with a decolorable colorant (B toner) when the decolorize button  6  is touched. In ACT 15 , the main CPU  301  staples or punches a sheet by controlling the finisher  70 . In ACT 16 , the finisher  70  discharges a stapled or a punched sheet to the storage tray  82  (first tray). 
     Meanwhile, when punching and stapling are not specified in ACT 12  (NO in ACT 12 ), the main CPU  301  selects the cassette  63  in ACT 17 . The cassette  63  accommodates a reuse sheet. Since a reuse sheet may be reused for two or more times, a reuse sheet should not be selected for being stapled or punched. Therefore, when the eco button  2  is selected, and post-processing is not specified, the main CPU  301  selects the cassette  63 . 
     In ACT 18 , the printer CPU  501  performs printing by controlling the printer unit  50 . That is to say, the printer CPU  501  prints an image on a reuse sheet by controlling the image forming unit  504 . Printing on a reuse sheet is performed in the printing mode that is specified in  FIG. 8  (with a non-decolorable colorant or a decolorable colorant (B toner)). 
     In ACT 19 , the transport control unit  505  switches the gate  91 . In this case, the gate  91  is switched in a manner in which a sheet is transported to the fixed tray  93  (second tray). Therefore, a reuse sheet on which printing is performed is not transported to the finisher  70  and is discharged to the fixed tray  93  (second tray). 
     When the user touches the standard button  1  on the menu screen in  FIG. 8 , the main CPU  301  determines that an image is printed on a new sheet (NO in ACT 11 ). In ACT 20 , the main CPU  301  selects the cassette  62 . The cassette  62  accommodates a new sheet. 
     In ACT 21 , the main CPU  301  determines whether post-processing is to be performed. That is to say, when the user touches at least one of the staple button  3  and the punch button  4  on the menu screen in  FIG. 8  (YES in ACT 21 ), the main CPU  301  controls the finisher  70 . In ACT 22 , the main CPU  301  sets the finisher  70  into an operable state (ON). 
     After ACT 22 , the printer CPU  501  prints an image on a new sheet in ACT 15 . Printing on a new sheet is performed in a printing mode that is specified in  FIG. 8 . In ACT 15 , the finisher  70  staples or punches a sheet. In ACT 16 , the finisher  70  discharges a stapled or a punched sheet to the storage tray  82  (first tray). 
     When punching and stapling are not specified in ACT 21  (NO in ACT 21 ), the process flow proceeds to ACT 18 . In ACT 18 , the printer CPU  501  prints an image on a sheet. Printing on a new sheet is performed in a printing mode that is specified in  FIG. 8 . In ACT 19 , a sheet on which printing is performed is discharged to the fixed tray  93  (second tray). 
     When there is no sheet accommodated in the case where the cassette  63  is selected in ACT 17  or in the case where the cassette  62  is selected in ACT 20 , a message similar to that in  FIG. 9  may be displayed on the display unit  38 . 
     When the sheets in the cassette  62  are used up during the printing in which the cassette  62  is selected in ACT 20 , and punching or stapling is specified, printing may be continued by switching the cassette  62  automatically to the cassette  64  and using a last reuse sheet preferentially. 
     The image forming apparatus according to the embodiment, as described so far, may preferentially use a last reuse sheet when a last reuse sheet is present in a specified cassette (cassette  64 ), and post-processing such as punching and stapling is specified. 
     Types of sheets to insert into the cassette  62 ,  63 , and  64  may be desirably set in advance. Then, a corresponding cassette may be desirably selected according to the result of determinations in ACT 11  and ACT 12 . 
     Second Embodiment 
     Next, the configuration of an image forming apparatus according to a second embodiment will be described with reference to  FIG. 10 . The image forming apparatus  30  in  FIG. 10  includes two of the cassettes  62  and  63  as a sheet feeding cassette. The cassette  62  accommodates a new sheet, and the cassette  63  accommodates a reuse sheet and a last reuse sheet. 
     A scanner  507  that is a reading unit is disposed in front of the resist roller  66 . The scanner  507  reads the mark  26  that is printed on a reuse sheet (or a last reuse sheet) and determines the number of reuse. When the mark  26  is present on both sides of a sheet, the scanner  507  reads the mark  26  for each of the front side and the rear side of a sheet. Other configurations are the same as those in  FIG. 5  and thus will not be described in detail. 
     In the second embodiment, the scanner  507  that is disposed in the MFP  30  determines whether a sheet that is accommodated in the cassette  63  is a reuse sheet or a last reuse sheet. 
       FIG. 11  is a flowchart illustrating an example sequence of operations performed by the MFP  30  and the finisher  70  according to the second embodiment. In ACT 31  in  FIG. 11 , the main CPU  301  determines the selected type of a sheet. For example, when the user touches the eco button  2  on the menu screen in  FIG. 8  (YES in ACT 31 ), the main CPU  301  determines that an image is to be printed on a reuse sheet or a last reuse sheet. In ACT 32 , the main CPU  301  selects the cassette  63 . In ACT 33 , the main CPU  301  determines whether post-processing is to be performed. 
     That is to say, when the user touches at least one of the staple button  3  and the punch button  4  on the menu screen in  FIG. 8  (YES in ACT 33 ), the main CPU  301  controls the finisher  70 . In ACT 34 , the main CPU  301  sets the finisher  70  into an operable state (ON). 
     In ACT 35 , since the cassette  63  accommodates a reuse sheet and a last reuse sheet, the printer CPU  501  determines the number of reuse of a sheet from the result of reading of the mark  26  by the scanner  507 . That is to say, in ACT 35 , the printer CPU  501  determines whether the number of reuse of a sheet in the cassette  63  is N−1. When there is a sheet of which the number of reuse is not N−1, printing is not performed on the sheet, and the sheet is discharged to the fixed tray  93  (second tray) in ACT 36 . 
     When the sheet in the cassette  63  is determined to be a last reuse sheet in ACT 35  (YES in ACT 35 ), the printer CPU  501  prints an image on a last reuse sheet by controlling the image forming unit  504  in ACT 37 . Printing on a last reuse sheet is performed in the printing mode that is specified in  FIG. 8 . For example, printing is performed with a non-decolorable colorant when the normal button  5  is touched, and printing is performed with a decolorable colorant (B toner) when the decolorize button  6  is touched. 
     In ACT 37 , the transport control unit  505  switches the gate  91 . The gate  91  is switched in a manner in which a sheet is transported to the finisher  70 . Therefore, a last reuse sheet on which printing is performed is transported to the finisher  70 . The main CPU  301  controls the finisher  70  to be subject to processes of staple or punch on a sheet. In ACT 38 , the finisher  70  discharges a sheet subject to the processes of staple or punch to the storage tray  82  (first tray). 
     Meanwhile, when punching and stapling are not specified in ACT 33  (NO in ACT 33 ), the process flow proceeds to ACT 39 . In ACT 39 , the printer CPU  501  prints an image on a reuse sheet or a last reuse sheet by controlling the image forming unit  504 . Printing on a reuse sheet or a last reuse sheet is performed in the printing mode that is specified in  FIG. 8  (with a non-decolorable colorant or a decolorable colorant (B toner)). 
     In ACT 40 , the transport control unit  505  switches the gate  91 . The gate  91  is switched in a manner in which a sheet is transported to the fixed tray  93  (second tray). Therefore, a sheet on which printing is performed is discharged to the fixed tray  93  (second tray). 
     When the user touches the standard button  1  on the menu screen in  FIG. 8  (NO in ACT 31 ), the main CPU  301  determines that an image is printed on a new sheet. In ACT 41 , the main CPU  301  selects the cassette  62 . The cassette  62  accommodates a new sheet. 
     In ACT 42 , the main CPU  301  determines whether post-processing is to be performed. That is to say, when the user touches at least one of the staple button  3  and the punch button  4  on the menu screen in  FIG. 8  (YES in ACT 42 ), the main CPU  301  controls the finisher  70 . In ACT 43 , the main CPU  301  sets the finisher  70  into an operable state (ON). 
     After ACT 43 , the process flow proceeds to ACT 37 , and the printer CPU  501  prints an image on a new sheet. Printing on a new sheet is performed in a printing mode that is specified in  FIG. 8 . In ACT 37 , the finisher  70  staples or punches a sheet. In ACT 38 , the finisher  70  discharges a sheet subject to the processes of staple or punch to the storage tray  82  (first tray). 
     When punching and stapling are not specified in ACT 42  (NO in ACT 42 ), the process flow proceeds to ACT 39 . In ACT 39 , the printer CPU  501  prints an image on a new sheet. Printing on a new sheet is performed in the printing mode that is specified in  FIG. 8 . In ACT 40 , a sheet on which printing is performed is discharged to the fixed tray  93  (second tray). 
     In the image forming apparatus according to the second embodiment, as described so far, a reuse sheet and a last reuse sheet are accommodated in a specified cassette (the cassette  63  in the present example). In addition, when post-processing such as punching and stapling is specified, printing is preferentially performed on a last reuse sheet based on the result of reading by the scanner  507 , and post-processing is performed. When post-processing is not specified, printing may be performed by using a reuse sheet or a last reuse sheet. 
     Not limited to the embodiments described so far, various applications may be carried out. For example, as illustrated in  FIG. 12 , the gate  91 , the discharge roller  92 , and the fixed tray  93  may be disposed in the finisher  70 . 
     In  FIG. 12 , the gate  91  and the discharge roller  92  are disposed in front of the punching unit  71  in the finisher  70 . Therefore, a sheet for which post-processing is not necessary may be discharged to the fixed tray  93  of the finisher  70 , by switching the gate  91 . When post-processing is to be performed, a sheet is transported to the finisher  70 . 
     In  FIG. 5 ,  FIG. 10 , and  FIG. 12 , the first carriage  41  may be replaced by a scan head that includes an LED element. Although each unit is described as being controlled by the main CPU  301  of the main control unit  300  and the printer CPU  501  of the printer unit  50  in  FIG. 6 , the main CPU  301  and the printer CPU  501  may be configured as a single CPU. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.