Patent Publication Number: US-2016238979-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an image forming apparatus for forming an image. 
     A conventional image forming apparatus may be configured to display a printable sheet number according to a remaining amount of toner in a toner cartridge. For example, Patent Reference has disclosed a conventional image forming apparatus that calculates a printable sheet number under an assumption that a printing operation is continued with a predetermined coverage rate. 
     Patent Reference: Japanese Patent Publication No. 2012-252180 
     In the conventional image forming apparatus disclosed in Patent Reference, it has been desired to calculate a printable sheet number with higher accuracy, and it is necessary to improve calculation accuracy. 
     In order to solve the problems of the conventional image forming apparatus described above, an object of the present invention is to provide an image forming apparatus capable of calculating a printable sheet number with higher accuracy. 
     Further objects and advantages of the invention will be apparent from the following description of the invention. 
     SUMMARY OF THE INVENTION 
     In order to attain the objects described above, according to a first aspect of the present invention, an image forming apparatus includes an image forming portion; a usage amount calculating portion; a usage rate calculating portion; and a printable sheet number calculating portion. 
     According to the first aspect of the present invention, the image forming portion is configured to form an image on a printing medium using a color agent. The usage calculating portion is configured to calculate a usage amount of the color agent in the image forming portion. The usage rate calculating portion is configured to calculate a usage rate of the color agent on the printing medium according to the usage amount, and a medium size and a printed sheet number of the printing medium on which the image forming portion forms the image. The printable sheet number calculating portion is configured to calculate a printable sheet number that the image forming portion is capable of printing using a remaining color agent according to the usage amount and the usage rate. 
     According to a second aspect of the present invention, an image forming apparatus includes an image forming portion and a calculating portion. 
     According to the second aspect of the present invention, the image forming portion is configured to use a color agent to form an image on a printing medium. The calculating portion is configured to calculate a usage amount of the color agent in the image forming portion. The calculating portion is further configured to calculate a usage rate of the color agent on the printing medium according to the usage amount, and a medium size and a printed sheet number of the printing medium on which the image forming portion forms the image. The calculating portion is further configured to calculate a printable sheet number that the image forming portion is capable of printing using a remaining color agent according to the usage amount and the usage rate. 
     According to the first aspect and the second aspect of the present invention, the image forming apparatus is configured to calculate the printable sheet number according to the usage amount of the color agent and the usage rate of the color agent on the printing medium. Accordingly, it is possible to calculate the printable sheet number with higher accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view showing a configuration of an image forming apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a schematic sectional view showing a configuration of a developing unit of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 3  is a block diagram showing a control system of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 4  is a view showing a table including correlation coefficients of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 5  is a schematic view sowing a display portion and an operation portion of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 6  is a flow chart showing an operation of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 7  is a view showing a table including printing conditions of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 8  is a view showing a table including calculation results under the printing conditions of the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 9  is a block diagram showing a control system of an image forming apparatus according to a second embodiment of the present invention; and 
         FIG. 10  is a flow chart showing an operation of the image forming apparatus according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereunder, embodiments of the present invention will be explained with reference to the accompanying drawings. 
     First Embodiment 
     A first embodiment of the present invention will be explained.  FIG. 1  is a schematic sectional view showing a configuration of an image forming apparatus  1  according to the first embodiment of the present invention. It should be noted that the image forming apparatus  1  is a tandem type printer capable of forming an image using a photo-electric method. 
     As shown in  FIG. 1 , the image forming apparatus  1  includes a hopping roller  11 ; a register roller  12 ; a pinch roller  13 ; a transportation roller  14 ; a pinch roller  15 ; developing units  20  ( 20 B,  20 Y,  20 M, and  20 C); light sources  29  ( 29 B,  29 Y,  29 M, and  29 C); a transfer portion  30 ; a fixing portion  40 ; and a discharge roller  16 . These components are arranged along a transportation path  10  for transporting a printing medium  9 . 
     In the first embodiment, the hopping roller  11  is configured to pick up the printing medium  9  at the upper most position retaining in a sheet supply tray  9 . Further, the hopping roller  11  is configured to feed the printing medium  9  to the transportation path  10 . 
     In the first embodiment, the register roller  12  is arranged to face the register roller  12  with the transportation path  10  in between. The pinch roller  13  is arranged to face the register roller  12  with the transportation path  10  in between. The register roller  12  and the pinch roller  13  are configured to correct skew of the printing medium  9  supplied from the hopping roller  11 . 
     In the first embodiment, the transportation roller  14  is arranged to face the pinch roller  15  with the transportation path  10  in between. The pinch roller  15  is arranged to face the transportation roller  14  with the transportation path  10  in between. The transportation roller  14  and the pinch roller  15  are configured to transport the printing medium  9  supplied from the register roller  12  and the pinch roller  13  along the transportation path  10 , so that the printing medium  9  is guided to the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C). 
     In the first embodiment, the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C) are configured to form a toner image. More specifically, the developing unit  20 B is configured to form a toner image in black (B). The developing unit  20 Y is configured to form a toner image in yellow (Y). The developing unit  20 M is configured to form a toner image in magenta (M). The developing unit  20 C is configured to form a toner image in cyan (C). The developing units  20  ( 20 B,  20 Y,  20 M, and  20 C) are arranged in this order along a transportation direction F of the printing medium  9 . Further, each of the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C) is configured to be detachable. 
       FIG. 2  is a schematic sectional view showing a configuration of the developing units  20  of the image forming apparatus  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 2 , each of the developing units  20  includes a photosensitive drum  21 ; a charging roller  22 ; a developing roller  23 ; a supplying roller  24 ; a toner cartridge  25 ; a developing blade  26 ; a cleaning portion  27 ; and a data storage portion  28 . 
     In the first embodiment, the photosensitive drum  21  is a component configured to support a static latent image on a surface (a surface layer portion) thereof. Further, the photosensitive drum  21  is formed of a photosensitive member. The photosensitive drum  21  is arranged to rotate in a right direction with a driving force transmitted from a photosensitive drum motor (not shown). The charging roller  22  is provided for charging the photosensitive drum  21 . 
     In the first embodiment, the light source  29 B is configured to expose the photosensitive drum  21  of the developing unit  20 B. The light source  29 Y is configured to expose the photosensitive drum  21  of the developing unit  20 Y. The light source  29 M is configured to expose the photosensitive drum  21  of the developing unit  20 M. The light source  29 C is configured to expose the photosensitive drum  21  of the developing unit  20 C. Accordingly, the static latent image is formed on the surface of each of the photosensitive drums  21 . 
     In the first embodiment, the charging roller  22  is a component configured to charge the surface (the surface layer portion) of the photosensitive drum  21 . The charging roller  22  is arranged to contact with the surface (a circumferential surface) of the photosensitive drum  21 , and is pressed against the photosensitive drum  21  with a specific press amount. When the photosensitive drum  21  is rotated, the charging roller  22  is rotated in a left direction. As described later, a power source  63  is provided for applying a charging voltage to the charging roller  22 . 
     In the first embodiment, the developing roller  23  is a component configured to support toner on a surface thereof. Further, the developing roller  23  is formed of, for example, a metal shaft with a cylindrical shape and a semi-conductive elastic layer formed on a surface of the metal shaft. The elastic layer may be formed of, for example, a urethane rubber. It is desirable to apply an isocyanate processing to a surface of the elastic layer to enhance conductivity. 
     In the first embodiment, the developing roller  23  is arranged to contact with the surface (the circumferential surface) of the photosensitive drum  21 , and is pressed against the photosensitive drum  21  with a specific press amount. The developing roller  23  is configured to rotate in the left direction with the photosensitive drum motor and a gear (not shown). It should be noted that a gear ratio is set such that a friction is generated between the surface of the developing roller  23  and the surface of the photosensitive drum  21  in each of the developing units  20 . The developing roller  23  is provided for supplying toner to the photosensitive drum  21 , so that the toner image is formed (developed) according to the static latent image. As described later, a power source  64  is provided for applying a developing voltage to the developing roller  23 . 
     In the first embodiment, the supplying roller  24  is a component configured to supply toner retained in the toner cartridge  25  to the developing roller  23 . Further, the supplying roller  24  is formed of, for example, a metal shaft with a cylindrical shape and a semi-conductive foamed elastic layer formed on a surface of the metal shaft. The foamed elastic layer may be formed of, for example, a silicone rubber with high wear resistance. Further, the supplying roller  24  is arranged to contact with the surface (a circumferential surface) of the developing roller  23 , and is pressed against the developing roller  23  with a specific press amount. The supplying roller  24  is configured to rotate in the left direction with the photosensitive drum motor and a gear (not shown). 
     In the first embodiment, when the supplying roller  24  is rotated, a friction is generated between the surface of the supplying roller  24  and the surface of the developing roller  23  in each of the developing units  20 . Accordingly, in each of the developing units  20 , toner is charged through so-called friction charging. As described later, a power source  65  is provided for applying a supplying voltage to the supplying roller  24 . 
     In the first embodiment, the toner cartridge  25  is a configured to retain toner. More specifically, the toner cartridge  25  of the developing unit  20 B is configured to retain toner in black (B). The toner cartridge  25  of the developing unit  20 Y is configured to retain toner in yellow (Y). The toner cartridge  25  of the developing unit  20 M is configured to retain toner in magenta (M). The toner cartridge  25  of the developing unit  20 C is configured to retain toner in cyan (C). 
     In the first embodiment, the developing blade  26  is a component arranged to abut against the surface of the developing roller  23 . Accordingly, the developing blade  26  forms a layer of toner (a toner layer) on the surface of the developing roller  23 , and regulates (controls or adjusts) a thickness of the toner layer. The developing blade  26  is formed of, for example, a plate shape elastic member formed of stainless and the like and formed in an L character shape. The developing blade  26  is arranged such that a curved portion thereof abuts against the surface of the developing roller  23 , and the developing blade  26  is pressed against the developing roller  23  with a specific pressing amount. Similar to the supplying roller  24 , the power source  65  is provided for applying the supplying voltage to the developing blade  26 . 
     In the first embodiment, the cleaning portion  27  is a component configured to scrape off toner remaining on the surface (the surface layer) of the photosensitive drum  21  for cleaning. The cleaning portion  27  is arranged to abut against the surface of the photosensitive drum  21  with a counter force (protruding in a direction opposite to the rotational direction of the photosensitive drum  21 ), and is pressed against the photosensitive drum  21  with a specific press amount. 
     In the first embodiment, the data storage portion  28  is configured to store various data including a filled dot counter value Dt. The filled dot counter value Dt is defined as a value that a toner amount filled in the toner cartridge  25  (an initial amount immediately after filling up) is converted to a value per dot as an exposure unit of the developing unit  20 . The data storage portion  28  may be formed of, for example, a so-called RFID (Radio Frequency Identifier). It is configured such that data stored in the data storage portion  28  is retrieved through a reading portion  66  (described later) using wired communication or wireless communication. 
     In the first embodiment, the light source  29 B (refer to  FIG. 1 ) is a component configured to irradiate light relative to the photosensitive drum  21  of the developing unit  20 B. The light source  29 Y is a component configured to irradiate light relative to the photosensitive drum  21  of the developing unit  20 Y. The light source  29 M is a component configured to irradiate light relative to the photosensitive drum  21  of the developing unit  20 M. The light source  29 C is a component configured to irradiate light relative to the photosensitive drum  21  of the developing unit  20 C. Each of the light sources  29  may be formed of, for example, a plurality of LEDs (Light Emitting Diodes) arranged in a main scanning direction (a direction toward a backside in  FIG. 1 ). It should be noted that the configuration of the light sources  29  is not limited thereto, and the light sources  29  may be formed of, for example, a laser light source. Each of the light sources  29  ( 29 B,  29 Y,  29 M, and  29 C) exposes the photosensitive drum  21  per dot. Accordingly, the static latent image is formed on the surface of the photosensitive drum  21 . 
     In the first embodiment, the transfer portion  30  is a component configured to transfer the toner images formed with the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C) to a transferred surface of the printing medium  9 . The transfer portion  30  includes transfer rollers  31 B,  31 Y,  31 M, and  31 C. The transfer roller  31 B is arranged to face the photosensitive drum  21  of the developing unit  20 B with the transportation path  10  in between. The transfer roller  31 Y is arranged to face the photosensitive drum  21  of the developing unit  20 Y with the transportation path  10  in between. The transfer roller  31 M is arranged to face the photosensitive drum  21  of the developing unit  20 M with the transportation path  10  in between. The transfer roller  31 C is arranged to face the photosensitive drum  21  of the developing unit  20 C with the transportation path  10  in between. A power source  62  (described later) is provided for supplying a transfer voltage to each of the transfer rollers  31 B,  31 Y,  31 M, and  31 C. Accordingly, in the image forming apparatus  1 , the toner images formed with the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C) are transferred to the transferred surface of the printing medium  9 . 
     In the first embodiment, the fixing portion  40  is a component configured to apply heat and a pressure to the printing medium  9 , so that the toner images transferred to the printing medium  9  are fixed to the printing medium  9 . The fixing portion  40  includes a heat roller  41  and a pressing roller  42 . The heat roller  41  is a component configured to include a heating heater such as a halogen lump and the like disposed therein, so that the heat roller  41  applies heat to toner on the printing medium  9 . The pressing roller  42  is a component arranged to form a pressing portion between the heat roller  41 , so that the pressing roller  42  applies a pressure to toner on the printing medium  9 . Accordingly, in the fixing portion  40 , toner on the printing medium  9  is heated, pressed, and melted. As a result, the toner images are fixed to the printing medium  9 . 
     In the first embodiment, the discharge roller  16  is a component to discharge the printing medium  9  outside the image forming apparatus  1 . 
       FIG. 3  is a block diagram showing a control system of the image forming apparatus  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 3 , the image forming apparatus  1  includes an interface portion  61 ; a print control portion  50 ; the power source  62 ; the power source  63 ; the power source  64 ; the power source  65 ; the reading portion  66 ; a drive portion  67 ; a display portion  68 ; and an operation portion  69 . 
     In the first embodiment, the interface portion  61  is configured to receive print data from a personal computer (not shown), and to communicate with the personal computer to receive and transmit various control signals. 
     In the first embodiment, the print control portion  50  is configured to control a printing operation of the image forming apparatus  1 . The print control portion  50  includes a print area calculating portion  51 ; a dot count calculating portion  52 ; a print duty calculating portion  53 ; a printable sheet umber calculating portion  54 ; a storage portion  55 ; a high voltage control portion  56 ; a drive control portion  57 ; an exposure control portion  58 ; and a control portion  59 . It should be noted that the print control portion  50  is not limited to the configuration described above. For example, the print control portion  50  may include a CPU (Central Processing Unit), an RAM (Random Access Memory), an ROM (Read Only Memory), and the like, so that the CPU performs an operation similar to those performed with the portions described above according to a program. 
     In the first embodiment, the print area calculating portion  51  is configured to calculate a print area S per print job according to the print data received with the interface portion  61 . More specifically, first, the print area calculating portion  51  retrieves a medium size (for example, A4, B5, and the like) from the print data, and retrieves an area of a text region excluding a surrounding blank space. Then, the print area calculating portion  51  converts the area of the text region into a value (a region area S 1 ) per unit dot as an exposure unit of the developing unit  20 . At last, the print area calculating portion  51  calculates the print area S through multiplying the region area S 1  with a sheet number N of the recording medium necessary to print the print data (S=S 1 ×N). 
     In the first embodiment, the print area calculating portion  51  also has a function of updating an accumulated print area Ur through adding the print area S to the accumulated print area Ur stored in the storage portion  55 . 
     In the first embodiment, the dot count calculating portion  52  is configured to calculate dot counts Dn (DnB, DnY, DnM, and DnC) per print job according to the print data received with the interface portion  61 . More specifically, the dot count calculating portion  52  calculates a usage amount of toner in black (B), and converts the usage amount to a value per unit dot to calculate the dot count DnB. Similarly, the dot count calculating portion  52  calculates a usage amount of toner in black (Y), and converts the usage amount to a value per unit dot to calculate the dot count DnY; calculates a usage amount of toner in black (M), and converts the usage amount to a value per unit dot to calculate the dot count DnM; and calculates a usage amount of toner in black (C), and converts the usage amount to a value per unit dot to calculate the dot count DnC. 
     In the first embodiment, the dot count calculating portion  52  also has a function of updating accumulated dot counts Dr (DrB, DrY, DrM, and DrC) through adding the dot counts Dn (DnB, DnY, DnM, and DnC) to the accumulated dot counts Dr (described later) stored in the storage portion  55 . More specifically, the dot count calculating portion  52  updates the accumulated dot count DrB through adding the dot count DnB to the accumulated dot count DrB; updates the accumulated dot count DrY through adding the dot count DnY to the accumulated dot count DrY; updates the accumulated dot count DrM through adding the dot count DnM to the accumulated dot count DrM; and updates the accumulated dot count DrC through adding the dot count DnC to the accumulated dot count DrC. 
     In the first embodiment, the dot count calculating portion  52  also has a function of calculating toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC) through subtracting the accumulated dot counts Dr (DrB, DrY, DrM, and DrC) stored in the storage portion  55  from filled dot counts Dt (DtB, DtY, DtM, and DtC; described later) stored in the storage portion  55 . More specifically, the dot count calculating portion  52  calculates the toner remaining amount dot count DzB through subtracting the accumulated dot count DrB from the filled dot count DtB; calculates the toner remaining amount dot count DzY through subtracting the accumulated dot count DrY from the filled dot count DtY; calculates the toner remaining amount dot count DzM through subtracting the accumulated dot count DrM from the filled dot count DtM; and calculates the toner remaining amount dot count DzC through subtracting the accumulated dot count DrC from the filled dot count DtC. 
     In the first embodiment, the dot count calculating portion  52  also has a function of calculating toner remaining amounts T (TB, TY, TM, and TC; described later) in the toner cartridge  25  according to the toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC), and the filled dot counts Dt (DtB, DtY, DtM, and DtC); described later) stored in the storage portion  55 . More specifically, the dot count calculating portion  52  calculates the toner remaining amount TB through dividing the toner remaining amount dot count DzB by the filled dot count DtB; calculates the toner remaining amount TY through dividing the toner remaining amount dot count DzY by the filled dot count DtY; calculates the toner remaining amount TM through dividing the toner remaining amount dot count DzM by the filled dot count DtM; and calculates the toner remaining amount TC through dividing the toner remaining amount dot count DzC by the filled dot count DtC. 
     In the first embodiment, the dot count calculating portion  52  also has a function of updating consumed dot counts Ds (DsB, DsY, DsM, and DsC) through adding the dot counts Dn (DnB, DnY, DnM, and DnC) to the consumed dot counts Ds (described later) stored in the storage portion  55 . More specifically, the dot count calculating portion  52  updates the consumed dot count DsB through adding the dot count DnB to the consumed dot count DsB; updates the consumed dot count DsY through adding the dot count DnY to the consumed dot count DsY; updates the consumed dot count DsM through adding the dot count DnM to the consumed dot count DsM; and updates the consumed dot count DsC through adding the dot count DnC to the consumed dot count DsC. 
     In the first embodiment, the print duty calculating portion  53  is configured to calculate average print duties A (AB, AY, AM, and AC) according to the consumed dot counts Ds (DsB, DsY, DsM, and DsC) stored in the storage portion  55 , an accumulated print area Ur (described later), a medium size BS (described later), and a correction coefficient α using the following equations (1) to (4): 
         AB=DsB/Ur×α   (1)
 
         AY=DsY/Ur×α   (2)
 
         AM=DsM/Ur×α   (3)
 
         AC=DsC/Ur×α   (4)
 
       FIG. 4  is a view showing a table including the correlation coefficients α of the image forming apparatus  1  according to the first embodiment of the present invention. As shown in  FIG. 4 , the correlation coefficients α vary according to the medium size BS. Accordingly, the print duty calculating portion  53  calculates the average print duties A (AB, AY, AM, and AC) according to using the correlation coefficients α varying according to the medium size BS. 
     In the first embodiment, the printable sheet umber calculating portion  54  is configured to calculate printable sheet numbers M (MB, MY, MM, and MC) according to the toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC), the average print duties A (AB, AY, AM, and AC), the medium size BS, and the correction coefficient α using the following equations (5) to (8): 
         MB=DzB /( AB×B )×α  (5)
 
         MY=DsY /( AY×B )×α  (6)
 
         MM=DsM /( AM×B )×α  (7)
 
         MC=DsC /( AC×B )×α  (8)
 
     where B is a value (a text region area) obtained through converting the area of the text region excluding the surrounding blank space to a value per dot unit in the printing medium  9  with the medium size BS. 
     In the first embodiment, the storage portion  55  may be formed of, for example, a non-volatility memory. The storage portion  55  is configured to store the accumulated print area Ur, the medium size BS, the filled dot counts Dt (DtB, DtY, DtM, and DtC), the accumulated dot counts Dr (DrB, DrY, DrM, and DrC), and the consumed dot counts Ds (DsB, DsY, DsM, and DsC). 
     In the first embodiment, the accumulated print area Ur is obtained through accumulating the print area S per print job. It is configured such that the accumulated print area Ur is reset together with the consumed dot counts Ds (DsB, DsY, DsM, and DsC) at an arbitrary timing through an operation of a user. 
     In the first embodiment, the user operates the operation portion  69  to set the medium size BS in advance. More specifically, the user selects a standard size (for example, A4, B5, and the like), or operates a ten-key of the operation portion  69  to be able to set an arbitrary size (a custom size). 
     In the first embodiment, when the toner cartridge  25  is replaced, for example, the filled dot counts Dt (DtB, DtY, DtM, and DtC) are retrieved from the data storage portion  28  and are stored in the storage portion  55 . More specifically, for example, when the toner cartridge  25  of the developing unit  20 B, the filled dot count Dt retrieved from the data storage portion  28  of the developing unit  20 B is stored in the storage portion  55  as the filled dot count DtB. Similarly, when the toner cartridge  25  of the developing unit  20 Y, the filled dot count Dt retrieved from the data storage portion  28  of the developing unit  20 Y is stored in the storage portion  55  as the filled dot count DtY. When the toner cartridge  25  of the developing unit  20 M, the filled dot count Dt retrieved from the data storage portion  28  of the developing unit  20 M is stored in the storage portion  55  as the filled dot count DtM. When the toner cartridge  25  of the developing unit  20 C, the filled dot count Dt retrieved from the data storage portion  28  of the developing unit  20 C is stored in the storage portion  55  as the filled dot count DtC. 
     In the first embodiment, the accumulated dot counts Dr (DrB, DrY, DrM, and DrC) are obtained through accumulating the dot counts Dn (DnB, DnY, DnM, and DnC) per print job. Further, it is configured such that the accumulated dot counts Dr (DrB, DrY, DrM, and DrC) are reset when the toner cartridge  25  is replaced. More specifically, when the toner cartridge  25  of the developing unit  20 B is replaced, for example, the accumulated dot count DrB is reset. When the toner cartridge  25  of the developing unit  20 Y is replaced, the accumulated dot count DrY is reset. When the toner cartridge  25  of the developing unit  20 M is replaced, the accumulated dot count DrM is reset. When the toner cartridge  25  of the developing unit  20 C is replaced, the accumulated dot count DrC is reset. 
     In the first embodiment, the consumed dot counts Ds (DsB, DsY, DsM, and DsC) are obtained through accumulating the dot counts Dn (DnB, DnY, DnM, and DnC) per print job. Further, it is configured such that the consumed dot counts Ds (DsB, DsY, DsM, and DsC) are reset together with the accumulated print area Ur at an arbitrary timing through an operation of the user. 
     In the first embodiment, when the print duty calculating portion  53  calculates the average print duties A (AB, AY, AM, and AC), the printable sheet umber calculating portion  54  uses the correction coefficient α to calculate the printable sheet numbers M (MB, MY, MM, and MC). As shown in  FIG. 4 , the correction coefficient α has difference values according to the medium size BS. 
     In the first embodiment, the high voltage control portion  56  is configured to control the power source  62 , the power source  63 , the power source  63 , and the power source  65  to apply the voltage to each component in the transfer portion  30  and each of the developing units  20 . More specifically, the high voltage control portion  56  controls the power source  62  to generate the transfer voltage to be applied to the transfer portion  30 . Further, the high voltage control portion  56  controls the power source  63  to generate the charging voltage to be applied to the charging roller  22  of each of the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C). Further, the high voltage control portion  56  controls the power source  64  to generate the developing voltage to be applied to the developing roller  23  of each of the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C). Further, the high voltage control portion  56  controls the power source  65  to generate the voltage to be applied to the supplying roller  24  and the developing blade  26  of each of the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C). 
     In the first embodiment, the drive control portion  57  is configured to control the drive portion  67 , so that each motor in the image forming apparatus  1  is operated. Accordingly, the drive control portion  57  is capable of rotating the photosensitive drum  21  of each of the developing units  20 , the hopping roller  11 , the register roller  12 , and the like. Further, the exposure control portion  58  is configured to control the exposure operation of the light sources  29  ( 29 B,  29 Y,  29 M, and  29 C). 
     In the first embodiment, the control portion  59  is configured to control each unit in the print control portion  50 . Further, the print control portion  50  has a function of receiving the print data from the interface portion  61 , and controlling an operation of the interface portion  61 . Further, the control portion  59  retrieves the filled dot counts Dt (DtB, DtY, DtM, and DtC) from the data storage portion  28  of each of the developing units  20  through the reading portion  66 . Further, the control portion  59  controls an operation of the image forming apparatus  1  according to operation information of the user supplied from the operation portion  69 , and controls the display portion  68  to display various information. 
     In the first embodiment, the power source  62  is configured to generate the transfer voltage to be applied to the transfer portion  30  according to an instruction from the high voltage control portion  56 . Further, the power source  63  is configured to generate the charging voltage to be applied to the charging roller  22  of each of the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C) according to an instruction from the high voltage control portion  56 . Further, the power source  64  is configured to generate the developing voltage to be applied to the developing roller  23  of each of the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C) according to an instruction from the high voltage control portion  56 . Further, the power source  65  is configured to generate the voltage to be applied to the supplying roller  24  and the developing blade  26  of each of the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C) according to an instruction from the high voltage control portion  56 . 
     In the first embodiment, the reading portion  66  is configured to retrieve the filled dot counts Dt (DtB, DtY, DtM, and DtC) from the data storage portion  28  of each of the developing units  20 , and supplies the filled dot counts Dt (DtB, DtY, DtM, and DtC) to the control portion  59 . Further, the drive portion  67  is configured to drive each motor in the image forming apparatus  1  according to an instruction from the drive control portion  57 . 
     In the first embodiment, the display portion  68  is configured to display various types of information according to an instruction from the control portion  59 . The display portion  68  is formed of, for example, a liquid crystal display panel. 
     In the first embodiment, the operation portion  69  is configured to receive an operation of the user. More specifically, the user can reset the accumulated print area Ur and the consumed dot counts Ds (DsB, DsY, DsM, and DsC) through operating the operation portion  69 . 
       FIG. 5  is a schematic view sowing the display portion  68  and the operation portion  69  of the image forming apparatus  1  according to the first embodiment of the present invention. 
     As shown in  FIG. 5 , the operation portion  69  includes a plurality of buttons. The display portion  68  is configured to display a toner remaining amount T; an average print duty A; and a printable sheet number M per each color. 
     In the first embodiment, the developing units  20  ( 20 B,  20 Y,  20 M, and  20 C), the light sources  29  ( 29 B,  29 Y,  29 M, and  29 C), and the transfer portion  30  correspond to an image forming portion of the image forming apparatus  1 . Toner is a specific example of a color agent. The dot count calculating portion  52  corresponds to a usage amount calculating portion of the image forming apparatus  1 . The accumulated dot counts Dr (DrB, DrY, DrM, and DrC) and the consumed dot counts Ds (DsB, DsY, DsM, and DsC) correspond to a specific example of a usage amount. The print duty calculating portion  53  corresponds to a usage rate calculating portion of the image forming apparatus  1 . The average print duties A (AB, AY, AM, and AC) correspond to a specific example of a usage rate. The accumulated print area Ur corresponds to a specific example of a print area. The interface portion  61  corresponds to a data obtaining portion of the image forming apparatus  1 . The photosensitive drum  20  corresponds to an image supporting portion of the image forming apparatus  1 . The developing roller  23  corresponds to a developing portion of the image forming apparatus  1 . 
     An operation of the image forming apparatus  1  will be explained next. First, an overall operation of the image forming apparatus  1  will be explained with reference to  FIG. 3 . 
     In the first embodiment, the interface portion  61  is configured to receive the print data from a personal computer (not shown), and to communicate with the personal computer to receive and transmit various control signals. The print control portion  50  is configured to control the printing operation of the image forming apparatus  1 . 
     More specifically, the print area calculating portion  51  is configured to calculate the print area S per print job according to the print data received with the interface portion  61 . The dot count calculating portion  52  is configured to calculate the dot counts Dn (DnB, DnY, DnM, and DnC) per print job according to the print data received with the interface portion  61 . The print duty calculating portion  53  is configured to calculate the average print duties A (AB, AY, AM, and AC) according to the consumed dot counts Ds (DsB, DsY, DsM, and DsC) stored in the storage portion  55 , the accumulated print area Ur, the medium size BS (described later), and the correction coefficient α. 
     Further, the printable sheet umber calculating portion  54  is configured to calculate the printable sheet numbers M (MB, MY, MM, and MC) according to the toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC), the average print duties A (AB, AY, AM, and AC), the medium size BS, and the correction coefficient α. The high voltage control portion  56  is configured to control the power source  62 , the power source  63 , the power source  63 , and the power source  65  to apply the voltage to each component in the transfer portion  30  and each of the developing units  20 . The drive control portion  57  is configured to control the drive portion  67 , so that each motor in the image forming apparatus  1  is operated. Further, the exposure control portion  58  is configured to control the exposure operation of the light sources  29  ( 29 B,  29 Y,  29 M, and  29 C). 
       FIG. 6  is a flow chart showing the operation of the image forming apparatus  1  when the print control portion  50  calculates the printable sheet numbers M (MB, MY, MM, and MC) according to the first embodiment of the present invention. Every time when the interface portion  61  receives the print data, the print control portion  50  calculates the average print duties A (AB, AY, AM, and AC) and the printable sheet numbers M (MB, MY, MM, and MC). In the following, the operation of the print control portion  50  will be explained in detail. 
     In step S 1 , the control portion  59  of the print control portion  50  determines whether the user operates the operation portion  69  to reset the consumed dot counts Ds (DsB, DsY, DsM, and DsC) and the accumulated print area Ur. In step S 2 , when the control portion  59  determines the user resets the consumed dot counts Ds (DsB, DsY, DsM, and DsC) and the accumulated print area Ur, the control portion  59  resets the consumed dot counts Ds (DsB, DsY, DsM, and DsC) stored in the storage portion  55  and the accumulated print area Ur. 
     In step S 3 , the control portion  59  determines whether the interface portion  61  receives the print data. When the control portion  59  determines that the interface portion  61  does not receive the print data (N in step S 3 ), the process returns to step S 1 . Accordingly, the process between step S 1  and step S 3  is repeated until the interface portion  61  receives the print data. 
     In step S 4 , when the control portion  59  determines that the interface portion  61  receives the print data (Y in step S 3 ), the print area calculating portion  51  calculates the accumulated print area Ur. More specifically, the print area calculating portion  51  retrieves the medium size of the printing medium  9  from the print data, and retrieves the area of the test region excluding the surrounding blank space of the printing medium  9 . Then, the print area calculating portion  51  converts the area of the text region into the value per dot (the region area S 1 ). In the next step, the print area calculating portion  51  multiplies the region area S 1  by the sheet number N of the print medium necessary for printing the print data, so that the print area S is calculated (S=S 1 ×N). Lastly, the print area calculating portion  51  adds the print area S to the accumulated print area Ur stored in the storage portion  55 , so that the accumulated print area Ur is updated. 
     In step S 5 , the dot count calculating portion  52  calculates the accumulated dot counts Dr (DrB, DrY, DrM, and DrC), the toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC), the toner remaining amounts T (TB, TY, TM, and TC), and the consumed dot counts Ds (DsB, DsY, DsM, and DsC). 
     More specifically, the dot count calculating portion  52  calculates the usage amount of toner in black (B), and converts the usage amount to the value per unit dot to calculate the dot count DnB. Similarly, the dot count calculating portion  52  calculates the usage amount of toner in black (Y), and converts the usage amount to the value per unit dot to calculate the dot count DnY; calculates the usage amount of toner in black (M), and converts the usage amount to the value per unit dot to calculate the dot count DnM; and calculates the usage amount of toner in black (C), and converts the usage amount to the value per unit dot to calculate the dot count DnC. 
     Further, the dot count calculating portion  52  updates the accumulated dot count DrB through adding the dot count DnB to the accumulated dot count DrB; updates the accumulated dot count DrY through adding the dot count DnY to the accumulated dot count DrY; updates the accumulated dot count DrM through adding the dot count DnM to the accumulated dot count DrM; and updates the accumulated dot count DrC through adding the dot count DnC to the accumulated dot count DrC. 
     Further, the dot count calculating portion  52  calculates the toner remaining amount dot count DzB through subtracting the accumulated dot count DrB from the filled dot count DtB; calculates the toner remaining amount dot count DzY through subtracting the accumulated dot count DrY from the filled dot count DtY; calculates the toner remaining amount dot count DzM through subtracting the accumulated dot count DrM from the filled dot count DtM; and calculates the toner remaining amount dot count DzC through subtracting the accumulated dot count DrC from the filled dot count DtC. 
     Further, the dot count calculating portion  52  calculates the toner remaining amount TB through dividing the toner remaining amount dot count DzB by the filled dot count DtB; calculates the toner remaining amount TY through dividing the toner remaining amount dot count DzY by the filled dot count DtY; calculates the toner remaining amount TM through dividing the toner remaining amount dot count DzM by the filled dot count DtM; and calculates the toner remaining amount TC through dividing the toner remaining amount dot count DzC by the filled dot count DtC. 
     Further, the dot count calculating portion  52  updates the consumed dot count DsB through adding the dot count DnB to the consumed dot count DsB; updates the consumed dot count DsY through adding the dot count DnY to the consumed dot count DsY; updates the consumed dot count DsM through adding the dot count DnM to the consumed dot count DsM; and updates the consumed dot count DsC through adding the dot count DnC to the consumed dot count DsC. 
     In step S 6 , the print duty calculating portion  53  calculates the average print duties A (AB, AY, AM, and AC). More specifically, the print duty calculating portion  53  calculates the average print duties A (AB, AY, AM, and AC) according to the correction coefficient α corresponding to the medium size BS using the equations (1) to (4). 
     In step S 7 , the printable sheet umber calculating portion  54  calculates the printable sheet numbers M (MB, MY, MM, and MC). More specifically, the printable sheet umber calculating portion  54  calculates the text region area B according to the medium size BS, and calculates the printable sheet numbers M (MB, MY, MM, and MC) according to the text region area B using the equations (5) to (8). 
     In step S 8 , the control portion  59  determines whether any one of the printable sheet numbers M (MB, MY, MM, and MC) is equal to or less than zero. 
     In step S 9 , when the control portion  59  determines that all of the printable sheet numbers M (MB, MY, MM, and MC) is greater than zero (N in step S 8 ), the display portion  68  displays the toner remaining amounts T (TB, TY, TM, and TC), the average print duties A (AB, AY, AM, and AC), and the printable sheet numbers M (MB, MY, MM, and MC). 
     In step S 10 , when the control portion  59  determines that one of the printable sheet numbers M (MB, MY, MM, and MC) is equal to or less than zero, the control portion  59  controls the display portion  68  to display information indicating that the remaining toner amount is zero. 
     In the first embodiment, after step S 10 , the process is complete. Afterward, the print control portion  50  repeats the process described above. 
     An effect of the image forming apparatus  1  will be explained next as compared to an image forming apparatus  1 R of a comparative example. In the following description, it is assumed that the image forming apparatus  1  and the image forming apparatus  1 R are capable of forming an image on the printing medium  9  having the medium size in a range between A6 and A3, and the image forming apparatus  1  and the image forming apparatus  1 R form an image using only black toner. 
     According to the comparative example, it should be noted that the image forming apparatus  1 R is configured to determine the printable sheet numbers M (MB, MY, MM, and MC) at a predetermined print duty (5%) and a predetermined medium size (A4) according to the toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC). 
     In an evaluation, the image forming apparatus  1  and the image forming apparatus  1 R perform the printing operation under the following nine printing conditions C 1  to C 9 . In the printing operation, the dot count DnB is 792 dots when the image forming apparatus  1 R performs the printing operation on one sheet of the printing medium  9  with the size A4 at the print duty of 5%. Further, the region area S 1  of one sheet of the printing medium  9  with the size A4 is 16,993 dots. Further, the filled dot count DtB is 7,920,000. 
       FIG. 7  is a view showing a table including the printing conditions C 1  to C 9  of the image forming apparatus  1 R according to the comparative example and the image forming apparatus  1  according to the first embodiment of the present invention. It should be noted that one print job is performed under the printing conditions C 1  to C 6 , and two print jobs are performed under the printing conditions C 7  to C 9 . 
     Under the printing condition C 1 , for example, the image forming apparatus  1 R performs the printing operation in black on 8,000 sheets of the printing medium  9  with the size A4 at the print duty of 5%. Under the printing condition C 1 , the dot count DnB is 6,336,000 (=792×8,000), and the print area S is 135,944,000 (=16,993×8,000). As a result, the accumulated dot count DrB is 6,336,000 dots, and the accumulated print area Ur is 135,944,000 dots. 
     Further, under the printing condition C 4 , for example, the image forming apparatus  1 R performs the printing operation in black on 4,000 sheets of the printing medium  9  with the size A3 at the print duty of 5%. Under the printing condition C 4 , the dot count DnB is 6,336,000 (=792×2×4,000), and the print area S is 135,944,000 (=16,993×2×4,000). As a result, the accumulated dot count DrB is 6,336,000 dots, and the accumulated print area Ur is 135,944,000 dots. 
     Further, under the printing condition C 7 , for example, the image forming apparatus  1 R performs the printing operation in black on 1,000 sheets of the printing medium  9  with the size A4 at the print duty of 20% for the print job J 1 , and on 2,000 sheets of the printing medium  9  with the size A3 at the print duty of 5% for the print job J 2 . For the print job J 1 , the dot count DnB is 3,168,000 (=792×(20/5)×1,000), and the print area S is 16,993,000 (=16,993×1,000). For the print job J 2 , the dot count DnB is 3,168,000 (=792×2×2,000), and the print area S is 67,972,000 (=16,993×2×2,000). As a result, after the image forming apparatus  1 R performs the printing operation for the print job J 1  and the print job J 2 , the accumulated dot count DrB is 6,336,000 dots, and the accumulated print area Ur is 84,965,000 dots. 
     In the printing operations described above, it should be noted that the accumulated dot count DrB becomes the same level of 6,336,000 dots under the printing conditions C 1  to C 9 . 
       FIG. 8  is a view showing a table including calculation results of the printable sheet numbers M (MB, MY, MM, and MC) under the printing conditions C 1  to C 9  of the image forming apparatus  1 R according to the comparative example and the image forming apparatus  1  according to the first embodiment of the present invention. 
     In the evaluation, the accumulated dot count DrB becomes the same level of 6,336,000 dots under the printing conditions C 1  to C 9 . Accordingly, the toner remaining amount TB becomes the same level of 20% under the printing conditions C 1  to C 9 . 
     According to the comparative example, the image forming apparatus  1 R is configured to determine the printable sheet numbers M (MB, MY, MM, and MC) at the predetermined print duty (5%) and the predetermined medium size (A4) according to the toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC). As a result, the printable sheet number M is determined to be 2,000 sheets under the printing conditions C 1  to C 9 . 
     As described above, in the image forming apparatus  1 R according to the comparative example, the printable sheet numbers M (MB, MY, MM, and MC) are determines using the predetermined print duty (5%) and the predetermined medium size (A4). Accordingly, as shown in  FIGS. 7 and 8 , even when the medium size BS, the print duty, or the print number is varied, as long as the accumulated dot count is the same, the printable sheet number is determined at the same level. As a result, it is difficult to determine the printable sheet number with high accuracy. 
     In general, various users use an image forming apparatus under various conditions. For example, some of them print mostly texts, some of them print mostly images, some of them print many print media with the size A4, or some of them print many print media with the size B5. However, in the image forming apparatus  1 R according to the comparative example, only the print duty and the medium size BS are variables. Accordingly, the calculation of the printable sheet number does not reflect actual print conditions, and it is difficult to determine the printable sheet number with high accuracy. 
     On the other hand, in the first embodiment, the image forming apparatus  1  first determines the average print duties A (AB, AY, AM, and AC), and then determines the printable sheet numbers M (MB, MY, MM, and MC) according to the average print duties A (AB, AY, AM, and AC). It should be noted that, when the toner cartridge  25  is replaced, the user operates the operation portion  69  to reset the consumed dot counts Ds (DsB, DsY, DsM, and DsC) and the accumulated print area Ur. In other words, the consumed dot counts Ds (DsB, DsY, DsM, and DsC) have the same values as those of the accumulated dot counts Dr (DrB, DrY, DrM, and DrC). 
     As shown in  FIG. 8 , in the image forming apparatus  1  in the first embodiment, even when the accumulated dot count DrB is the same, the average print duty AB is decreased as the accumulated print area Ur increases. Further, the image forming apparatus  1  determines the printable sheet number MB according to the average print duty AB. When the average print duty AB is increased, the printable sheet number MB is decreased. Accordingly, in the image forming apparatus  1 , it is possible to determine the printable sheet number MB according to the printing conditions performed previously. Further, when the user sets the medium size BS in advance, it is possible to determine the printable sheet number MB according to the medium size BS that the user actually uses. Accordingly, the calculation of the printable sheet number reflects actual print conditions, and it is possible to determine the printable sheet number with high accuracy. 
     As described above, in the first embodiment, the printable sheet numbers M (MB, MY, MM, and MC) are determined according to the average print duties A (AB, AY, AM, and AC). Accordingly, it is possible to determine the printable sheet number with high accuracy. 
     In the first embodiment, the display portion  68  is configured to display the toner remaining amounts T (TB, TY, TM, and TC), the average print duties A (AB, AY, AM, and AC), the printable sheet numbers M (MB, MY, MM, and MC) for all four colors. It should be noted that the present invention is not limited to the configuration. Alternatively, the display portion  68  may be configured to display only the printable sheet numbers M (MB, MY, MM, and MC) for all four colors. Further, the display portion  68  may be configured to display only one of the printable sheet numbers M (MB, MY, MM, and MC) with the least number. 
     Second Embodiment 
     A second embodiment of the present invention will be explained next. In the second embodiment, it is configured to obtain the medium size BS through a different process from that in the first embodiment. In the second embodiment, components of an image forming apparatus  2  similar to those of the image forming apparatus  1  in the first embodiment are designated with the same reference numerals, and explanations thereof are omitted. 
       FIG. 9  is a block diagram showing a control system of the image forming apparatus  2  according to the second embodiment of the present invention. 
     As shown in  FIG. 9 , the image forming apparatus  2  includes a print control portion  70 . The print control portion  70  includes a print area calculating portion  71 ; a print duty calculating portion  73 ; a printable sheet umber calculating portion  74 ; a storage portion  75 ; and a control portion  79 . 
     In the second embodiment, similar to the print area calculating portion  51  in the first embodiment, the print area calculating portion  71  is configured to calculate the print area S per print job according to the print data received with the interface portion  61 . Further, the print area calculating portion  71  is configured to update the accumulated print area Ur. Further, the print area calculating portion  71  has a function of updating medium history data DD (described later) stored in the storage portion  75  according to the medium size of the printing medium  9 . 
     In the second embodiment, the print duty calculating portion  73  is configured to calculate the average print duties A (AB, AY, AM, and AC) according to the consumed dot counts Ds (DsB, DsY, DsM, and DsC) stored in the storage portion  75 , the accumulated print area Ur (described later), the medium history data DD (described later), and the correction coefficient α using the equations (1) to (4). More specifically, first, the print duty calculating portion  73  determines the medium size BS according to the medium history data DD. In this process, the print duty calculating portion  73  estimates the medium size that the user will likely use in the future according to, for example, the medium history data DD, so that the print duty calculating portion  73  determines the medium size as the medium size BS. Then, the print duty calculating portion  73  calculates the average print duties A (AB, AY, AM, and AC) using the correction coefficient α according to the medium size BS thus determined. 
     In the second embodiment, the printable sheet umber calculating portion  74  is configured to calculate the printable sheet numbers M (MB, MY, MM, and MC) according to the toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC), the average print duties A (AB, AY, AM, and AC), the medium size BS determined with the print duty calculating portion  73 , and the correction coefficient α using the equations (5) to (8). 
     In the second embodiment, the storage portion  75  is configured to store the accumulated print area Ur, the medium history data DD, the filled dot counts Dt (DtB, DtY, DtM, and DtC), the accumulated dot counts Dr (DrB, DrY, DrM, and DrC), the consumed dot counts Ds (DsB, DsY, DsM, and DsC), and the correction coefficient α. 
     In the second embodiment, the medium history data DD represents a history of the medium sizes of the printing medium  9  that the image forming apparatus  2  printed. The medium history data DD may, for example, a print number per medium size. It is configured such that the medium history data DD is reset at an arbitrary timing through an operation of the user. 
     In the second embodiment, similar to the control portion  59  in the first embodiment, the control portion  79  is configured to control each unit in the print control portion  70 . 
       FIG. 10  is a flow chart showing an operation of the image forming apparatus  2  when the print control portion  70  calculates the printable sheet numbers M (MB, MY, MM, and MC) according to the second embodiment of the present invention. 
     In step S 1 , similar to the control portion  59  of the print control portion  50  in the first embodiment, the control portion  79  of the print control portion  70  determines whether the user operates the operation portion  69  to reset the consumed dot counts Ds (DsB, DsY, DsM, and DsC) and the accumulated print area Ur. In step S 2 , when the control portion  79  determines the user resets the consumed dot counts Ds (DsB, DsY, DsM, and DsC) and the accumulated print area Ur, the control portion  79  resets the consumed dot counts Ds (DsB, DsY, DsM, and DsC) stored in the storage portion  75  and the accumulated print area Ur. 
     In step S 3 , the control portion  79  determines whether the interface portion  61  receives the print data. When the control portion  79  determines that the interface portion  61  does not receive the print data (N in step S 3 ), the process returns to step S 1 . Accordingly, the process between step S 1  and step S 3  is repeated until the interface portion  61  receives the print data. 
     In step S 4 , when the control portion  79  determines that the interface portion  61  receives the print data (Y in step S 3 ), the print area calculating portion  71  calculates the accumulated print area Ur similarly to the print area calculating portion  51  in the first embodiment. Further, the print area calculating portion  71  adds the print area S to the accumulated print area Ur stored in the storage portion  75 , so that the accumulated print area Ur is updated. 
     In step S 14 , the print area calculating portion  71  updates the medium history data DD stored in the storage portion  75  according to the medium size of the printing medium  9  obtained from the print data. 
     In step S 5 , the dot count calculating portion  52  calculates the accumulated dot counts Dr (DrB, DrY, DrM, and DrC), the toner remaining amount dot counts Dz (DzB, DzY, DzM, and DzC), the toner remaining amounts T (TB, TY, TM, and TC), and the consumed dot counts Ds (DsB, DsY, DsM, and DsC). 
     In step S 15 , the print duty calculating portion  73  determines the medium size BS according to the medium history data DD. More specifically, the print duty calculating portion  73  estimates the medium size that the user will likely use in the future according to, for example, the medium history data DD, so that the print duty calculating portion  73  determines the medium size as the medium size BS. 
     In step S 6 , the print duty calculating portion  73  calculates the average print duties A (AB, AY, AM, and AC) according to the correction coefficient α corresponding to the medium size BS. 
     In step S 7 , the printable sheet umber calculating portion  74  calculates the text region area B according to the medium size BS, and calculates the printable sheet numbers M (MB, MY, MM, and MC) according to the text region area B. The process after step S 7  is the same as that in the first embodiment. 
     As described above, in the image forming apparatus  2  in the second embodiment, the medium size BS is determined according to the medium history data DD, and the average print duties A (AB, AY, AM, and AC) and the printable sheet numbers M (MB, MY, MM, and MC) are determined according to the medium size BS. Accordingly, it is possible to reflect the printing trend to the medium size, and to determine the printable sheet number with high accuracy. 
     As described above, in the second embodiment, the medium size BS is determined according to the medium history data DD. Accordingly, it is possible to determine the printable sheet number with high accuracy. 
     In the first embodiment and the second embodiment, the image forming apparatus  1  and the image forming apparatus  2  have the configurations described above. It should be noted that the present invention is not limited to the configurations, and may be applicable to modified configurations. 
     For example, in the first embodiment and the second embodiment, the present invention is applied to the printers of the photoelectric type. The present invention is not limited thereto, and may be applicable to various types of printers. For example, the present invention may be applicable to a printer of an inkjet type. 
     Further, in the first embodiment and the second embodiment, the present invention is applied to the color printers. The present invention is not limited thereto, and may be applicable to, for example, a monochrome printer. 
     Further, in the first embodiment and the second embodiment, the present invention is applied to the printers. The present invention is not limited thereto, and may be applicable to, for example, a multi function peripheral having functions of a printer, a facsimile, a scanner and the like. 
     The disclosure of Japanese Patent Application No. 2014-027453, filed on Nov. 20, 2014, is incorporated in the application. 
     While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.