Patent Publication Number: US-7899344-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2007-047472 filed on Feb. 27, 2007, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the present invention relates to an image forming apparatus. 
     BACKGROUND 
     There has been provided a laser printer employing a toner replenishment method, in which a toner cartridge for housing a toner is attached detachably from an inside of an apparatus body. The toner housed in the toner cartridge is supplied to the inside of a case for holding a developing roller or a photoconductive drum. When forming an image, the toner of the inside of the case is supplied to a surface of the photoconductive drum with the toner electrified positively or negatively by the developing roller. An electrostatic latent image formed on the surface of the photoconductive drum is developed by the toner and a toner image is carried on the surface of the photoconductive drum. The toner image is transferred to a sheet, and the image is formed on the sheet. In such a toner replenishment method, when the toner in the toner cartridge runs out, the toner cartridge is replaced with a new toner cartridge. 
     JP-A-11-282240 discloses a laser printer using suspension polymerization toner (for example, see JP-A-11-282240). 
     SUMMARY 
     Aspects of the invention provide an image forming apparatus capable of preventing occurrence of an image forming defect resulting from mixture of old and new suspension polymerization toners. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exemplary side sectional view showing an image forming apparatus according to an aspect of the present invention; 
         FIG. 2  is an exemplary side sectional view of a developing unit according to an aspect of the present invention wherein the developing unit is in a state in which a toner cartridge according to an aspect of the present invention is attached to a process frame; 
         FIG. 3  is an exemplary side sectional view of the developing unit showing a state in which the toner cartridge is detached from the process frame; 
         FIG. 4  is a schematic perspective view of the developing unit of  FIG. 2  as viewed from a front right side of the developing unit; 
         FIG. 5  is an exemplary perspective view, as viewed from the front right side of the developing unit of  FIG. 3 , showing the developing unit in a state in which the toner cartridge is detached from the process frame; 
         FIG. 6  is a schematic perspective view of the toner cartridge of  FIG. 5 , as viewed from a back left side, in a state in which an inside cabinet is in an opened position; 
         FIG. 7  is an exemplary perspective view of the toner cartridge of  FIG. 5 , as viewed from the back left side, in a state in which the inside cabinet is in a closed position; 
         FIG. 8  is an exemplary perspective view, as viewed from a back left side, of the inside cabinet of the toner cartridge of  FIG. 5 ; 
         FIG. 9  is an exemplary front view of the developing unit in a state of detaching the toner cartridge from the process frame; 
         FIG. 10  is an exemplary block diagram showing an electrical configuration of the image forming apparatus; 
         FIG. 11  is an exemplary flowchart showing a flow of empty processing; and 
         FIG. 12  is an exemplary flowchart showing modified example of the empty processing. 
     
    
    
     DETAILED DESCRIPTION 
     General Overview 
     According to an aspect of the invention, there is provided an image forming apparatus an image forming apparatus comprising: a first cabinet; a toner carrier that is supported in the first cabinet and carries suspension polymerization toner; an image carrier that forms an electrostatic latent image and carries a toner image obtained by imaging the electrostatic latent image by supply of the suspension polymerization toner from the toner carrier; a second cabinet that is disposed adjacently to the first cabinet in a horizontal direction orthogonal to a longitudinal direction of the toner carrier and houses the suspension polymerization toner; a toner amount detector that detects an amount of suspension polymerization toner in the second cabinet; and an empty decision unit that decides whether or not a remaining state of the suspension polymerization toner in the second cabinet is an empty state based on a detection result of the toner amount detector, wherein an inside of the first cabinet is communicated to an inside of the second cabinet through a plurality of openings, the plurality of openings including at least a supply port for supplying the suspension polymerization toner from the second cabinet to the first cabinet and a return port for returning the suspension polymerization toner from the first cabinet to the second cabinet, and the plurality of openings being arranged in the longitudinal direction of the toner carrier, wherein a spacing between one outer edge of one opening provided in one end side in a longitudinal direction of the plurality of openings and another outer edge in a direction opposite to a longitudinal direction of another opening provided in another end side in the longitudinal direction of the plurality of openings has a width not less than a width of an image forming region where the toner image is formed in the image carrier, and wherein the empty decision unit decides that the remaining state of the suspension polymerization toner in the second cabinet is the empty state before the amount of suspension polymerization toner in the second cabinet becomes smaller than the amount of suspension polymerization toner in the first cabinet. 
     Illustrative Aspects 
     Illustrative aspects of the invention will be described with reference to the drawings. 
     The suspension polymerization toner deteriorates as the image is formed. Due to the deterioration of the suspension polymerization toner, charging properties of the suspension polymerization toner deteriorate. Thus, when the toner cartridge is replaced with a new toner cartridge while the extremely deteriorating suspension polymerization toner remains inside the case, the deteriorated suspension polymerization toner is mixed with a new suspension polymerization toner in the new toner cartridge, and charged state of the mixed suspension polymerization toner becomes unequal. When such a mixed suspension polymerization toner is supplied to the surface of the photoconductive drum, an image forming defect (print defect) may be caused. Further, the deteriorating suspension polymerization toner and the new suspension polymerization toner may be aggregated due to a difference of the amount of electrification. Thus, the image forming defect may be caused. 
     Aspects of the invention provide an image forming apparatus capable of preventing occurrence of an image forming defect resulting from mixture of old and new suspension polymerization toners. 
     (Image Forming Apparatus) 
       FIG. 1  is an exemplary side sectional view showing an image forming apparatus according to an aspect of the present invention. In  FIG. 1 , the image forming apparatus is embodied in the form of a laser printer by way of an example. However, one of ordinary skill in the art will appreciate that the present inventive concept will apply equally to any apparatus which uses a toner for producing images on a recording medium. 
     An image forming apparatus  1  comprises a feeder unit  4  for feeding a sheet  3 , an image forming unit  5  for forming an image on the sheet  3 , a sheet discharge part  6  for discharging the sheet  3 , and a body casing  2  for housing the feeder unit  4 , the image forming unit  5 , and the sheet discharge part  6 . The body casing  2  is one example of an apparatus body. 
     (1) Body Casing 
     The body casing  2  has a substantially box shape. An opening is provided in a sidewall of one side of the body casing  2 . A front cover  7  for opening and closing the opening is provided in the sidewall of one side of the casing  2 . 
     Incidentally, in the following description, a side (right side in  FIG. 1 ) where the front cover  7  is provided is referred to as a front side (front face side) and an opposite side (left side in  FIG. 1 ) is referred to as a back side (rear side). Also, in the description that follows, references to upper, lower, left, and right sides of the image forming apparatus  1  are the upper, lower, left and right sides as viewed from the front side of the image forming apparatus  1 . Also, a developing unit  13  and a toner cartridge  22  (both described in more detail below) are similarly described using directions in reference to the front side of the image forming apparatus  1 . 
     (2) Feeder Unit 
     The feeder unit  4  includes a sheet feeding tray  8 , a sheet feeding roller  9 , a pair of resist rollers  10 , and a sheet feeding path  11 . An uppermost sheet  3  from an inside of the sheet feeding tray  8  is fed to the sheet feeding path  11  by rotation of the sheet feeding roller  9 . The fed sheet  3  is conveyed to the resist rollers  10  through the sheet feeding path  11 . The resist rollers  10  feed the sheet  3  toward the image forming unit  5  at proper timing. 
     (3) Image Forming Unit 
     The image forming unit  5  includes a scanner unit  12 , the developing unit  13  and a fixing part  14 . In  FIG. 1 , the developing unit  13  is embodied in the form of a process cartridge by way of an example. However, one of ordinary skill in the art will appreciate that the present inventive concept will apply equally to any unit which may be used for developing. 
     (3-1) Scanner Unit 
     The scanner unit  12  is arranged in an upper part of the inside of the body casing  2 . The scanner unit  12  includes a laser light emitting unit (not shown), a polygon mirror  15 , a lens  16  and a reflecting mirror  17 . A laser beam based on data of an image to be formed on the sheet  3  is emitted from the scanner unit  12  and a surface of a photoconductive drum  26  described below is irradiated with the laser beam. 
     (3-2) Developing Unit 
     The developing unit  13  is arranged under the scanner unit  12 . The developing unit  13  is attachable to and detachable from the body casing  2 . 
     The developing unit  13  includes a process frame  21 , and the toner cartridge  22  as one example of a second cabinet is detachably attached to the process frame  21 . 
       FIG. 2  is an exemplary side sectional view of the developing unit  13  showing a state of attaching the toner cartridge  22  to the process frame  21 .  FIG. 3  is an exemplary side sectional view of the developing unit  13  and shows a state of detaching the toner cartridge  22  from the process frame  21 . 
     The process frame  21  is formed in a substantially box shape longitudinally in a width direction. A transfer path  23  for guiding the sheet  3  between a transfer roller  28  and the photoconductive drum  26  described below is formed in a lower part of the process frame  21 . 
     In the process frame  21 , a bulkhead  24  extending in upper and lower directions and a width direction (left and right directions) is formed in the middle part of front and back directions. A frame side opening  25  as one example of an opening is formed in the bulkhead  24 . 
     In the process frame  21 , the portion of the back side from the bulkhead  24  is a process part  33  as one example of a first cabinet for receiving the photoconductive drum  26  as one example of an image carrier, an electrification device  27 , the transfer roller  28  as one example of a transfer unit, a toner transport member  29  as one example of a toner transport member, a supply roller  30 , a developing roller  31  as one example of a toner carrier, and a layer thickness regulating blade  32 . In  FIG. 2 , the toner transport member  29  is embodied in the form of an auger by way of example. However, one of ordinary skill in the art will appreciate that other structures are possible for transferring the toner. 
     The photoconductive drum  26  and the transfer roller  28  are rotatably supported by the process frame  21 . The electrification device  27  is arranged over the photoconductive drum  26 . The electrification device  27  may be, for example, a scorotron type electrification device for positive electrification. The transfer roller  28  is arranged under the photoconductive drum  26 . 
     A developing chamber  34  is provided inside the portion of about a half of front side of the process part  33 . The toner transport member  29 , the supply roller  30 , the developing roller  31  and the layer thickness regulating blade  32  are arranged inside the developing chamber  34 . 
     The toner transport member  29  is arranged in a position opposed to the frame side opening  25  in the back side of the frame side opening  25  as shown in  FIGS. 2 and 3 . The supply roller  30  is arranged under the toner transport member  29 . The developing roller  31  is arranged at the back side of the supply roller  30 . The toner transport member  29 , the supply roller  30  and the developing roller  31  are rotatably supported by the process frame  21 . 
     The layer thickness regulating blade  32  includes a flexible leaf spring member  41  formed in a thin plate shape, and a pressure contact rubber  42  disposed in the lower end of the leaf spring member  41 . The upper end of the leaf spring member  41  is fixed to the process frame  21  at the upper side of the developing roller  31 . The pressure contact rubber  42  presses a surface of a rubber roller  40  from the front side by an elastic force of the leaf spring member  41 . 
     In the process frame  21 , a portion of the front side from the bulkhead  24  forms a cartridge attachment part  42  to which the toner cartridge  22  is attached. 
     The toner cartridge  22  includes an inside cabinet  43  and an outside cabinet  44 . The inside cabinet  43  and the outside cabinet  44  form a double cylindrical shape in which both ends are closed. 
     The inside cabinet  43  provides a toner reception chamber  45  for housing a toner. In the image forming apparatus  1  of the aspect, the toner comprises a suspension polymerization toner which is a nonmagnetic one-component toner with positive electrification. 
     An agitator  46  is disposed inside the toner reception chamber  45 . The agitator  46  includes an agitator shaft  47  and an agitating blade  48  supported by the agitator shaft  47 . The agitator shaft  47  extends in the width direction and is rotatably supported in the inside cabinet  43 . 
     A cartridge side opening  49  is provided in a surface of the inside cabinet  43  and a surface of the outside cabinet  44  as shown in  FIG. 2 . The cartridge side opening  49  allows an inside of the toner reception chamber  45  to communicate with an outside of the toner reception chamber  45  in a state in which the toner cartridge is attached to the cartridge attachment part  42 . 
     Toner the inside of the toner reception chamber  45  is agitated by rotation of the agitator  46 . Also, by rotation of the agitator  46 , the toner of the inside of the toner reception chamber  45  is transported to the cartridge side opening  49  and is discharged from the cartridge side opening  49 . The toner discharged from the cartridge side opening  49  is supplied to the inside of the developing chamber  34  through the frame side opening  25 . The toner supplied to the inside of the developing chamber  34  is supplied to the supply roller  30  while being transported in a width direction by rotation of the toner transport member  29 . 
     The toner supplied to the supply roller  30  is supplied to the developing roller  31  by rotation of the supply roller  30 . The toner is frictionally electrified in positive polarity between the supply roller  30  and the developing roller  31 . In the toner supplied to the developing roller  31 , a layer thickness of the toner is regulated by the pressure contact rubber  42  of the layer thickness regulating blade  32 . Consequently, a thin layer of the toner is carried on a surface of the developing roller  31 . 
     In contrast, a surface of the photoconductive drum  26  is positively electrified uniformly by the electrification device  27  with rotation of the photoconductive drum  26 . Then, an electrostatic latent image corresponding to an image to be formed on the sheet  3  is formed by irradiating the positively electrified surface of the photoconductive drum  26  with a laser beam from the scanner unit  12  (see  FIG. 1 ). 
     When the electrostatic latent image formed on the surface of the photoconductive drum  26  is opposed to the developing roller  31  by rotation of the photoconductive drum  26 , the positively electrified toner carried on the surface of the developing roller  31  is supplied to the electrostatic latent image (that is, the low electric potential portion exposed by a laser beam among the surface of the photoconductive drum  26  positively electrified uniformly). Consequently, the electrostatic latent image is imaged and transferred and a toner image is carried on the surface of the photoconductive drum  26 . The toner image carried on the surface of the photoconductive drum  26  is transferred to the sheet  3  when the sheet  3  conveyed from the resister rollers  10  passes between the photoconductive drum  26  and the transfer roller  28 . 
     (3-3) Fixing Part 
     The fixing part  14  is disposed at the back side of the developing unit  13  as shown in  FIG. 1 . The fixing part  14  comprises a heating roller  50 , a pressurizing roller  51  brought into pressure contact with the heating roller  50 , and a pair of conveying rollers  52  arranged in the back of their rollers. 
     The toner image transferred to the sheet  3  is fixed to the sheet  3  by heating and pressurizing from the heating roller  50  and the pressurizing roller  51  while the sheet  3  passes between the heating roller  50  and the pressurizing roller  51 . The sheet  3  to which the toner image is fixed is conveyed to the sheet discharge part  6  by the pair of conveying rollers  52 . 
     (4) Sheet Discharge Part 
     The sheet discharge part  6  comprises a sheet discharge path  53 , a pair of sheet discharge rollers  54 , and a sheet discharge tray  55 . The sheet  3  conveyed from the fixing part  14  is conveyed to the sheet discharge path  53  and is discharged on the sheet discharge tray  55  by the sheet discharge rollers  54 . 
     (Developing Unit) 
       FIG. 4  is an exemplary perspective view as viewed from front right side of the developing unit  13  and shows a state of notching a part.  FIG. 5  is an exemplary perspective view as viewed from the front right side of the developing unit  13  and shows a state of detaching the toner cartridge  22  from the process frame  21 .  FIGS. 6 and 7  are exemplary perspective views as viewed from the back left side of the toner cartridge  22 .  FIG. 8  is an exemplary perspective view of the inside cabinet  43  of the toner cartridge  22  as viewed from the back left side.  FIG. 9  is an exemplary front view of the developing unit  13  in a state of detaching the toner cartridge  22  from the process frame  21 . 
     (1) Process Frame 
     The process frame  21  integrally includes the bulkhead  24 , the process part  33  at the back side of the bulkhead  24 , and the cartridge attachment part  42  at the front side of the bulkhead  24  as described above. 
     (1-1) Bulkhead 
     The bulkhead  24  comprises a curved portion in the middle of the upper and lower directions, as shown in  FIG. 2 . The curved portion of the bulkhead  24  has a radius of curvature that matches an outer peripheral surface of the toner cartridge  22  as shown in  FIGS. 2 and 3 . 
     Three rectangular frame side openings  25  are formed in the curved portion of the bulkhead  24  at a spacing in the width direction as shown in  FIG. 9 . 
     The frame side opening  25  at the center is a frame side supply port  61  as one example of a supply port for supplying toner from the inside of the toner cartridge  22  (i.e., toner reception chamber  45 ) to the inside of the process part  33  (i.e., developing chamber  34 ). The frame side supply port  61  is formed in a rectangular shape longer in a width direction (i.e., a left and right direction) than a width direction of the frame side openings  25  on either side of the frame side supply port  61 . 
     The frame side openings  25  of both sides of the frame side supply port  61  are frame side return ports  62  as one example of return ports for returning toner from the inside of the process part  33  (i.e., the developing chamber  34 ) to the inside of the toner cartridge  22  (i.e., the toner reception chamber  45 ). In these frame side return ports  62 , the lengths of the width direction are respectively formed in proper lengths at proper spacing between the frame side supply port  61  and the return ports  62  so that a spacing between the right edge of the right frame side return port  62  and the left edge of the left frame side return port  62  becomes longer than a width W of an image forming region in which a toner image is formed on a surface of the photoconductive drum  26 . 
     (1-2) Process Part 
     The process part  33  integrally includes an upper wall  63  backward extending from the upper edge of the bulkhead  24 , a bottom wall  64  backward extending from the lower edge of the bulkhead  24 , and side walls  65  respectively backward extending from both edges of the width direction of the bulkhead  24 . 
     The toner transport member  29  arranged in the process part  33  includes an toner transport member shaft  35 , and a screw  36  disposed around the toner transport member shaft  35 . The toner transport member shaft  35  extends in the width direction and both ends of the toner transport member shaft  35  are rotatably supported in the process frame  21 . The toner transport member shaft  35  is rotated by driving force from a motor (not shown) at the time of forming an image. The screws  36  are respectively disposed to both sides with respect to the center of the width direction in the toner transport member shaft  35  and are formed in a spiral shape for transporting toner from the center of the width direction to both outsides of the width direction as shown in  FIG. 9 . 
     (1-3) Cartridge Attachment Part 
     The cartridge attachment part  42  integrally includes a bottom plate  66  forward extending from the lower edge of the bulkhead  24 , and side plates  67  respectively forward extending from both edges of the width direction of the bulkhead  24  as shown in  FIG. 5 . 
     The bottom plate  66  is integrally formed continuously with the bottom wall  64  of the process part  33 . 
     A lower side fixed part  68  slightly protruding to the front side is provided in the center of the width direction of the front edge as shown in  FIGS. 2 ,  3  and  5 . 
     Each of the side plates  67  is integrally formed continuously with both the side walls  65  of the process part  33 . 
     A shutter support part  69  is disposed on an inside surface of the width direction of the back end of each of the side plates  67  as shown in  FIG. 5 . Each of the shutter support parts  69  has a substantially rectangular shape extending in the upper and lower directions and extends inwardly from the inside surface of the width direction of both the side plates  67 . 
     A shutter guide part  70  is disposed on an inside surface of the width direction of each of the shutter support parts  69 . Each of the shutter guide parts  70  forms a protrusion stripe extending inwardly from the inside surface of the width direction of the shutter support part  69 . Also, each of the shutter guide parts  70  has a substantially curved shape with substantially the same curvature as that of the curved portion of the bulk head  24 . Each of the shutter guide parts  70  is arranged in a position opposed to the curved portion of the bulkhead  24  at a slight spacing from the bulkhead  24  in the front and back directions. 
     An upper end surface of each of the shutter support parts  69  is arranged in a position slightly lower than the upper edge of each of the side plates  67 . The upper end surface of each of the shutter support parts  69  forms an upper side fixed part  71 . 
     Also, a shutter  72  for opening and closing the frame side openings  25  is disposed in the cartridge attachment part  42 . 
     The shutter  72  has a substantially rectangular shape extending in the width direction and has a substantially curved shape with substantially the same curvature as that of the curved portion of the bulkhead  24 . The shutter  72  is formed slightly shorter than the shutter guide parts  70  in the upper and lower directions. Also, three shutter opening parts  69 , corresponding to each of the frame side openings  25 , are provided in the shutter  72 . 
     The shutter  72  is arranged in a position opposed to the curved portion of the bulkhead  24 , and both ends of a width direction of the shutter  72  are slidably pinched between the bulkhead  24  and each of the shutter guide parts  70  as shown in  FIG. 3 . Accordingly, the shutter  72  can slide along each of the shutter guide parts  70  in the upper and lower directions between an opened position (see  FIG. 2 ) in which the frame side openings  25  are opened and a closed position (see  FIGS. 3 and 5 ) in which the frame side openings  25  are closed. When the shutter  72  is in the opened position, the shutter opening parts  69  in a position corresponding respectively to the frame side openings  25  and each of the frame side openings  25  is opened. When the shutter  72  is in the closed position, the portions of the shutter  72  below the shutter opening parts  69  are in a position corresponding respectively to the frame side openings  25  and each of the frame side openings  25  is closed. 
     Also, a swing arm  73  is disposed in the cartridge attachment part  42  as shown in  FIGS. 4 and 5 . The swing arm  73  has a substantially U shape in the case of being viewed from the plane. The swing arm  73  integrally comprises a grasp bar  74  extending in the width direction, and arm side plates  75  extending backward from both ends of the width direction of the grasp bar  74 . 
     A boss  76  protruding outward in the width direction is disposed at the back end of each of the arm side plates  75 . Each of the bosses  76  is turnably supported by a circular hole  77  provided in the center of each of the side plates  67 . 
     Also, a receiving recessed part  78  notched so as to be recessed to the lower side is provided in the upper side edge at the back end of each of the arm side plates  75 . 
     A grasp part  79  recessed to the upper side is provided in the center of the width direction of the grasp bar  74 . Also, backward extending press protrusion parts  80  with rectangular shapes in the case of being viewed from the rear are disposed in both ends of the width direction of the grasp bar  74 . 
     Using the boss  76  of each of the arm side plates  75  as a fulcrum, the swing arm  73  swings between a pressing release position (see  FIGS. 3 and 5 ) in which the lower edge of each of the arm side plates  75  makes contact with the front edge of the bottom plate  66  and a pressing position (see FIGS.  2  and  4 ) in which the toner cartridge  22  is pressed from the front side if the toner cartridge  22  is received in the cartridge attachment part  42 . 
     A light passage hole  81  for passing detection light of a toner sensor  130  described below is formed in the lower front side of the circular hole  77  in each of the side plates  67  as shown in  FIGS. 3 and 4 . The light passage holes  81  of each of the side plates  67  are mutually opposed in the width direction. 
     (2) Toner Cartridge 
     The toner cartridge  22  includes the inside cabinet  43  for housing a toner, and the outside cabinet  44  for receiving the inside cabinet  43  as described above. 
     (2-1) Inside Cabinet 
     The inside cabinet  43  integrally includes a cylindrical inside peripheral wall  91  extending in the width direction, and circular plate-shaped inside side walls  92  for closing both ends of the width direction of the inside peripheral wall  91  as shown in  FIG. 8 . 
     A boss part  93  with a circular thick plate shape outward extending in the width direction is disposed in the center of each of the inside side walls  92 . 
     An inside window  94  opposed to an outside hole  112  of the outside cabinet  44  described below in the width direction is formed in the lower back side of the boss part  93  in each of the inside side walls  92 . Both the inside windows  94  are mutually opposed in the width direction. 
     A slide protrusion  95  is disposed in an upper side portion of each of the inside side walls  92 . The slide protrusion  95  is formed in a circular arc shape (e.g., a circular arc shape with a center angle of about 45°) in the case of being viewed from the side along an outer peripheral surface of the inside side wall  92 , and is disposed so as to protrude from the inside side wall  92  to the outside of the width direction. 
     A pair of pinch protrusions  96  radially protruding from a peripheral end surface are disposed in a back side portion of each of the inside side walls  92 . The pair of pinch protrusions  96  are arranged at a spacing (i.e., spacing corresponding to a circumferential length of the shutter  72 ) in a circumferential direction mutually in the peripheral end surface of each of the inside side walls  92 . 
     In the inside peripheral wall  91 , three inside passage ports  97  are formed at a spacing in the width direction in a surrounded portion surrounded by a rectangle with the corners of the pair of pinch protrusions  96  (i.e., there are four pinch protrusions  96 ) arranged in both sides of the width direction. 
     Each of the inside passage ports  97  is formed in a substantially rectangular shape elongated in the width direction. 
     The inside passage port  97  of the center is an inside supply port  98  as one example of a supply port for supplying toner from the inside of the toner cartridge  22  (i.e, the toner reception chamber  45 ) to the inside of the process part  33  (i.e., the developing chamber  34 ). 
     The inside passage ports  97  of both sides of the inside supply port  98  are inside return ports  99  as one example of return ports for returning toner from the inside of the process part  33  (i.e., developing chamber  34 ) to the inside of the toner cartridge  22  (i.e., toner reception chamber  45 ). In these inside return ports  99 , the lengths of the width direction are respectively formed in proper lengths at proper spacings between the inside supply port  98  and the return ports  99  so that a spacing between the right edge of the right inside return port  99  and the left edge of the left inside return port  99  becomes longer than a width W (see  FIG. 9 ) of an image forming region in which a toner image is formed on a surface of the photoconductive drum  26 . 
     (2-2) Outside Cabinet 
     The outside cabinet  44  is formed slightly larger than the inside cabinet  43  in the width and radial directions in order that the outside cabinet  44  may turnably receive the inside cabinet  43 . The outside cabinet  44  integrally includes an outside peripheral wall  101  with a substantially cylindrical shape extending in the width direction, and circular plate-shaped outside side walls  102  for closing both ends of the width direction of the outside peripheral wall  101  as shown in  FIG. 7 . 
     A circular boss hole  103  for receiving the boss part  93  is formed in each of the outside side walls  102 . 
     A slide hole  104  into which the slide protrusion  95  is inserted is formed in each of the outside side walls  102 . The slide hole  104  has a circular arc shape (e.g., a circular arc shape with a center angle of about 90°) with a circumferential length longer than that of the slide protrusion  95  in the case of being viewed from the side, and extends between the upper front side and the upper back side of the boss hole  103 . 
     Further, the outside hole  112  opposed to the inside window  94  of the inside cabinet  43  is formed in each of the outside side walls  102 . 
     A backward protruding upper side part  105  to be fixed is formed on a peripheral end surface of each of the outside side walls  102 . A positioning boss  106  outward protruding in the width direction is disposed in the back end of each of the upper side parts  105  to be fixed. 
     A forward extending pressed part  107  is disposed on the peripheral end surface of each of the outside side walls  102  as shown in  FIG. 5 . Each of the pressed parts  107  is formed in a substantially rectangular shape in the case of being viewed from the front and a substantially circular arc shape in the case of being viewed from the side. 
     Four elongated holes  108  into which the pair of pinch protrusions  96  (i.e., there are four pinch protrusions  96 ) are respectively inserted are provided at both ends of the width direction in the outside peripheral wall  101  as shown in  FIG. 7 . Each of the elongated holes  108  is formed in a substantially rectangular shape extending in the upper and lower directions in the case of being viewed from the rear and is formed in a length corresponding to a swing range between the opened position and the closed position of the shutter  72 . 
     In the outside peripheral wall  101 , three outside passage ports  109  are formed at a spacing in the width direction in a portion of the outside peripheral wall  101  surrounded by a rectangle with corners at the four elongated holes  108 . 
     Each of the outside passage ports  109  is formed in a substantially rectangular shape elongated in the width direction. 
     The outside passage port  109  of the center is an outside supply port  110  as one example of a supply port for supplying toner from the inside of the toner cartridge  22  (i.e., the toner reception chamber  45 ) to the inside of the process part  33  (i.e., the developing chamber  34 ). 
     The outside passage ports  109  of both sides of the outside supply port  110  are outside return ports  111  as one example of return ports for returning toner from the inside of the process part  33  (i.e., the developing chamber  34 ) to the inside of the toner cartridge  22  (i.e., the toner reception chamber  45 ). 
     The outside supply port  110  corresponds the frame side supply port  61  and is located in a position opposite to the frame side supply port  61 , and each of the outside return ports  111  corresponds to respective frame side return ports  62  in a state in which the toner cartridge  22  is attached to the process frame  21  as shown in  FIG. 2 . 
     In the front side of the outside peripheral wall  101 , a grip part  113  is disposed in the center of the width direction as shown in  FIG. 5 . 
     The grip part  113  includes an upper grasp plate  114  with a substantially rectangular shape forward protruding from the outside peripheral wall  101 , and a locking arm  115  with a substantially J shape in the case of being viewed from the side downward extending from the upper grasp plate  114 . The upper end of the locking arm  115  is swingably supported by a support shaft  116  disposed in the upper grasp plate  114 . A locking claw  117  for locking in the lower side fixed part  68  is disposed in the lower end of the locking arm  115 . A lower grasp plate  118  with a substantially rectangular shape protruding forward is disposed in the vicinity of the upper end of the locking arm  115 . The lower grasp plate  118  is arranged so as to extend in a space parallel with the upper grasp plate  114 . 
     A compression spring (not shown) for urging the upper and lower grasp plates in a direction of separation is interposed between the upper grasp plate  114  and the lower grasp plate  118 . 
     (2-3) Relative Arrangement of Inside Cabinet and Outside Cabinet 
     The inside cabinet  43  is turnably received inside the outside cabinet  44 . 
     More specifically, an outer peripheral surface of the inside peripheral wall  91  is inward fitted slidably in a circumferential direction with respect to an inner peripheral surface of the outside peripheral wall  101 . Outer peripheral surfaces of both the inside side walls  92  are inward fitted slidably in a circumferential direction with respect to inner peripheral surfaces of both the outside side walls  102 . The boss part  93  is turnably supported in the boss hole  103  as shown in  FIGS. 6 and 7 . The slide protrusion  95  is inserted into the slide hole  104 . Each of the pinch protrusions  96  is inserted into each of the elongated holes  108 , and each of the pinch protrusions  96  is protruded from each of the elongated holes  108  to the outside of a radial direction. The inside window  94  of the inside cabinet  43  is opposed to the outside hole  112  of the outside cabinet  44 . 
     The inside cabinet  43  permits relative turns using the boss part  93  as a fulcrum between a closed position (see  FIG. 7 ) in which the inside passage ports  97  are not opposed to the outside passage ports  109  and an opened position (see  FIG. 6 ) in which the inside passage ports  97  are opposed to the outside passage ports  109  with respect to the outside cabinet  44 . 
     When the inside cabinet  43  is in the closed position, each of the slide protrusions  95  is arranged in the front end of each of the slide holes  104  as shown in  FIG. 7 . Each of the pinch protrusions  96  is arranged in the upper end of each of the elongated holes  108 . The inside passage ports  97  are arranged in a position above the outside passage ports  109 , and the outside passage ports  109  are closed by the inside peripheral wall  91  of the inside cabinet  43 . 
     The inside cabinet  43  is relatively turned in a direction (lower side) in which the inside passage ports  97  move toward the outside passage ports  109  with respect to the outside cabinet  44  using the boss part  93  as a fulcrum. Then, each of the slide protrusions  95  slides each of the slide holes  104  from the front end toward the back end. Each of the pinch protrusions  96  slides each of the elongated holes  108  from the upper end toward the lower end. 
     When each of the slide protrusions  95  abuts on the back edge of each of the slide holes  104  and each of the pinch protrusions  96  abuts on the lower edge of each of the elongated holes  108 , the inside cabinet  43  is arranged in the opened position as shown in  FIG. 6 . 
     When the inside cabinet  43  is in the opened position, each of the slide protrusions  95  is arranged in the back end of each of the slide holes  104 . Also, each of the pinch protrusions  96  is arranged in the lower end of each of the elongated holes  108 . Then, the inside passage ports  97  are opposed to the outside passage ports  109  and these passage ports communicate with each other and are opened. 
     In addition, when the inside cabinet  43  is returned to the closed position, the inside cabinet  43  is relatively turned in a direction in which the inside passage ports  97  rise from the outside passage ports  109  with respect to the outside cabinet  44  using the boss part  93  as the fulcrum. 
     (3) Attachment and Detachment of Toner Cartridge to and from Process Frame 
     (3-1) Attachment of Toner Cartridge to Process Frame 
     Referring to  FIG. 5 , when the toner cartridge  22  is attached to the process frame  21 , the upper grasp plate  114  and the lower grasp plate  118  of the grip part  113  are pinched in a direction in which the grasp plates move near to each other. Then, the toner cartridge  22  is attached to the cartridge attachment part  42  of the process frame  21 . The inside cabinet  43  of the toner cartridge  22  is arranged in the closed position. Also, the shutter  72  of the cartridge attachment part  42  is arranged in the closed position. The swing arm  73  is arranged in a pressing release position. 
     When the toner cartridge  22  is attached to the cartridge attachment part  42 , each of the positioning bosses  106  is placed on each of the upper side fixed parts  71  and a pair of the pinch protrusions  96  (see  FIGS. 6 and 7 ) on both sides in the width direction respectively pinch the upper edge and the lower edge on both ends in the width direction of the shutter  72  and each of the slide protrusions  95  is fitted into each of the receiving recessed parts  78 . 
     Thereafter, when the pinch of the grip part  113  is released, the locking claw  117  is locked in the lower side fixed part  68  as shown in  FIG. 2 . 
     The outside cabinet  44  is fixed to the cartridge attachment part  42  since the positioning bosses  106  are placed to the upper side fixed parts  71  (see  FIG. 5 ) and the locking claw  117  is locked to the lower side fixed part  68 . 
     The swing arm  73  is swung from the pressing release position to a pressing position. Each of the press protrusion parts  80  presses each of the pressed parts  107  toward the back side as shown in  FIGS. 2 and 4 . Consequently, the toner cartridge  22  is pressed toward the process part  33 . 
     Also, when the swing arm  73  is swung from the pressing release position to the pressing position, with swing of each of the arm side plates  75 , each of the slide protrusions  95  fitted into each of the receiving recessed parts  78  slides each of the slide holes  104  to the back side and is arranged in the back end of each of the slide holes  104 . Further, with the shutter  72  pinched, a pair of the pinch protrusions  96  on both sides in the width direction slides each of the elongated holes  108  to the lower side and is arranged in the lower end of each of the elongated holes  108  (see  FIG. 6 ). 
     Accordingly, the inside cabinet  43  is arranged in the opened position as shown in  FIG. 2 . Then, the inside passage ports  97  are opposed to the outside passage ports  109  and these passage ports are communicated and opened. Thus, the cartridge side opening  49  as one example of an opening is formed. 
     At the time of forming an image, by the agitating blade  48  of the agitator  46 , toner of the inside of the toner reception chamber  45  (i.e., in the inside cabinet  43 ) is transported to the cartridge side opening  49  made of the inside supply port  98  and the outside supply port  110  and is discharged from the cartridge side opening  49 . The toner discharged from the cartridge side opening  49  is supplied to the inside of the developing chamber  34  (i.e., process part  33 ) through the frame side supply port  61 . 
     By the toner transport member  29 , the toner supplied from the frame side supply port  61  to the inside of the process part  33  is transported from the center of the width direction to both sides of the width direction and is supplied to the supply roller  30  on the way. The toner supplied to the supply roller  30  is supplied to the developing roller  31  as described above. The toner which has not been supplied to the supply roller  30  and the toner scraped down from the developing roller  31  by the layer thickness regulating blade  32  or the supply roller  30  are transported to the frame side return ports  62  and pass through the cartridge side opening  49  made of the inside return ports  99  and the outside return ports  111  and are returned to the inside of the toner reception chamber  45 . Accordingly, the toner is circulated between the developing chamber  34  and the toner reception chamber  45 . 
     In a state of attaching the toner cartridge  22  to the process frame  21 , the outside hole  112  and the inside window  94  opposed in the toner cartridge  22  are opposed to the light passage hole  81  in the width direction. 
     (3-2) Detachment of Toner Cartridge from Process Frame 
     When the toner cartridge  22  is detached from the process frame  21 , the swing arm  73  is first swung from the pressing position to the pressing release position. Then, each of the press protrusion parts  80  is separated from each of the pressed parts  107  and a press of the toner cartridge  22  on the process part  33  is released. 
     Also, when the swing arm  73  is swung from the pressing position to the pressing release position, with swing of each of the arm side plates  75 , each of the slide protrusions  95  fitted into each of the receiving recessed parts  78  slides each of the slide holes  104  to the front side and is arranged in the front end of each of the slide holes  104  as shown in  FIG. 7 . Further, with the shutter  72  pinched, a pair of the pinch protrusions  96  on both sides in the width direction slides each of the elongated holes  108  to the upper side and is arranged in the upper end of each of the elongated holes  108 . 
     Consequently, the inside cabinet  43  is arranged in the closed position, and the inside peripheral wall  91  of the inside cabinet  43  is opposed to the outside passage ports  109 , and the outside passage ports  109  are closed. Also, the shutter  72  is arranged in the closed position, and the frame side openings  25  are opposed to the shutter  72  and are closed. 
     When the upper grasp plate  114  and the lower grasp plate  118  of the grip part  113  are pinched in a direction in which the grasp plates move near to each other, the locking mechanism of the locking claw  117  to the lower side fixed part  68  is released, so that when the toner cartridge  22  is pulled from the cartridge attachment part  42  to the front side, the toner cartridge  22  is detached from the process frame  21  as shown in  FIG. 5 . 
     (Control Unit of Image Forming Apparatus) 
       FIG. 10  is an exemplary block diagram showing an electrical configuration of the image forming apparatus  1 . 
     The image forming apparatus  1  includes a control unit  126  of a configuration including a CPU  121 , RAM  122 , ROM  123 , a timer  124  and an I/O port  125 . The CPU  121  as one example of an empty decision unit, a toner supply stop unit, a toner return unit, an empty notification unit and toner consumption means is electrically connected to the RAM  122 , the ROM  123 , the timer  124  and the I/O port  125 . 
     The scanner unit  12  and the developing unit  13  are connected to the I/O port  125 . 
     A main motor  127  arranged inside the body casing  2  (see  FIG. 1 ) is connected to the I/O port  125 . Driving force of the main motor  127  is used in driving of various rollers of the feeder unit  4  and the sheet discharge unit  6  and driving of the photoconductive drum  26 , the electrification device  27 , the transfer roller  28 , the toner transport member  29 , the supply roller  30 , the developing roller  31  and the agitator  46  mounted in the developing unit  13 . 
     A clutch  128  is interposed between the main motor  127  and the agitator  46  (agitator shaft  47 ). By on/off of the clutch  128 , the driving force from the main motor  127  can be transmitted and broken with respect to the agitator  46 . The clutch  128  is connected to the I/O port  125 . 
     Further, a display panel  129  for performing various display is connected to the I/O port  125 . The display panel  129  is arranged on, for example, an upper surface of the body casing  2 . 
     The image forming apparatus  1  includes a toner sensor  130  for detecting the remaining amount of toner of the inside of the toner reception chamber  45 . The toner sensor  130  is constructed of a light emitting unit  131  for emitting detection light made of a laser beam, and a light receiving unit  132  for receiving the detection light from the light emitting unit  131 . 
     The light emitting unit  131  and the light receiving unit  132  are arranged in both sides of the width direction in a state of sandwiching the process frame  21  between the light emitting unit  131  and the light receiving unit  132 , and are respectively opposed to the light passage holes  81  of the side plates  67  from the outside of the width direction. In a state of attaching the toner cartridge  22  (see  FIG. 6 ) to the process frame  21 , the outside holes  112  and the inside windows  94  of the toner cartridge  22  are opposed to the light passage holes  81  in the width direction as described above, so that the light passage holes  81 , the inside windows  94  and the outside holes  112  line on an optical path of the detection light emitted from the light emitting unit  131  toward the light receiving unit  132 . Therefore, when the toner is not present between both the inside windows  94 , the detection light from the light emitting unit  131  passes through the inside of the toner reception chamber  45  and is received by the light receiving unit  132 . 
     In addition, illustration of the toner cartridge  22  is omitted in  FIG. 10 . 
     The light emitting unit  131  and the light receiving unit  132  are connected to the I/O port  125 . For example, while the light receiving unit  132  receives the detection light from the light emitting unit  131 , a low level signal is outputted from the light receiving unit  132  and the low level signal is inputted to the CPU  121  through the I/O port  125 . In contrast, while the light receiving unit  132  does not receive the detection light from the light emitting unit  131 , a high level signal is outputted from the light receiving unit  132  and the high level signal is inputted to the CPU  121  through the I/O port  125 . 
     When the clutch  128  is turned on by a command from the CPU  121 , driving force of the main motor  127  is transmitted to the agitator  46  and the agitator  46  is driven at a constant rotational speed. When the agitating blade  48  (see  FIG. 2 ) of the agitator  46  passes between both the inside windows  94 , toner between both the inside windows  94  is pushed away by the agitating blade  48  and it changes to a state in which the toner is not present between both the inside windows  94 . As a result, the detection light from the light emitting unit  131  is received by the light receiving unit  132 , a low level signal from the light receiving unit  132  is inputted to the CPU  121 . In a state in which the toner is sufficiently present inside the toner reception chamber  45 , immediately after the agitating blade  48  passes between both the inside windows  94 , the toner returns to a position opposed to the inside windows  94  and the detection light from the light emitting unit  131  is blocked by the toner. As a result, a signal outputted from the light receiving unit  132  is immediately shifted from the low level signal to a high level signal. As the amount of toner of the inside of the toner reception chamber  45  decreases, the return of the toner becomes slow and accordingly, time for which the low level signal is outputted from the light receiving unit  132  becomes long. Therefore, the CPU  121  can detect the amount of toner remaining inside the toner reception chamber  45  based on the output time of the low level signal from the light receiving unit  132 . 
     In addition, in this aspect, toner amount detector is constructed of the CPU  121  and the toner sensor  130 . The empty notification unit is constructed of the CPU  121  and the display panel  129 . 
     At the time of forming an image, toner is circulated between the developing chamber  34  and the toner reception chamber  45  and a certain amount of toner is present inside the developing chamber  34 . When the same amount of toner as the certain amount of toner remains inside the toner reception chamber  45 , the output time of the low level signal from the light receiving unit  132  becomes a substantially constant time and the constant time can be obtained by measurement previously. The CPU  121  decides that a remaining state of the toner in the inside of the toner reception chamber  45  is an empty state when the output time of the low level signal from the light receiving unit  132  matches with time in which a predetermined margin time is added to the constant time obtained by measurement. That is, it can be decided that the remaining state of the toner in the inside of the toner reception chamber  45  is the empty state before the amount of toner of the inside of the toner reception chamber  45  becomes smaller than the amount of toner of the inside of the developing chamber  34 . 
     When the CPU  121  decides that the remaining state of the toner in the inside of the toner reception chamber  45  is the empty state, the following empty processing is executed. 
     (Empty Processing) 
       FIG. 11  is a flowchart showing a flow of empty processing. 
     In the empty processing, the clutch  128  is first turned off. Accordingly, transmission of driving force from the main motor  127  to the agitator  46  is broken and the agitator  46  is stopped (S 1 ). 
     Next, the timer  124  is reset (S 2 ). Then, measurement of time elapsed since the clutch  128  was turned off is started by the timer  124 . 
     Thereafter, a driving state (ON) of the main motor  127  is maintained until a predetermined time (for example, three minutes) is measured by the timer  124  (S 3 , S 4 ). During this period, in a state in which the agitator  46  stops, the driving force of the main motor  127  is transmitted to the toner transport member  29  and the toner transport member  29  is driven. The agitator  46  stops, so that toner is not supplied from the inside of the toner reception chamber  45  to the developing chamber  34 . In contrast, the toner transport member  29  is driven, so that the toner of the inside of the developing chamber  34  is transported from the center of the width direction to both sides of the width direction by the toner transport member  29  and is returned from the frame side return ports  62  to the inside of the toner reception chamber  45 . As a result, the toner of the inside of the developing chamber  34  is returned to the toner reception chamber  45  and the amount of toner present inside the developing chamber  34  decreases. 
     When the predetermined time is measured by the timer  124  (S 4 : YES), the main motor  127  is stopped (i.e., turned off) (S 5 ). Then, display to the effect that it is necessary to replace the toner cartridge  22  with a new toner cartridge is performed on the display panel  129  (S 6 ). 
     In addition, display to the effect that toner runs out inside the toner reception chamber  45  may be performed and the display suggests that it is necessary to replace the toner cartridge  22  with a new toner cartridge, so that the display can be identified with the display to the effect that it is necessary to replace the toner cartridge  22  with the new toner cartridge. 
     As described above, the CPU  121  decides that a remaining state of toner in the toner cartridge  22  is an empty state before the amount of toner of the inside of the toner cartridge  22  for housing the toner becomes smaller than the amount of toner of the inside of the process part  33 . At the time of forming an image, a certain amount of toner is housed inside the process part  33 , so that it is decided that a state in which the certain amount or more of toner remains inside the toner cartridge  22  is the empty state, and a normal image forming action (i.e., an image forming action by user&#39;s intention) is inhibited. Thus, extreme deterioration of the toner of the insides of the process part  33  and the toner cartridge  22  can be prevented. Accordingly, when the toner cartridge  22  is replaced with a toner cartridge in which new toner is housed and the new toner is supplied from the toner cartridge  22  to the inside of the process part  33 , the new toner can be prevented from being mixed with the extremely deteriorating toner. As a result, occurrence of an image forming defect resulting from mixture of the old and new toners can be prevented. 
     A spacing between the right edge of the right frame side return port  62  and the left edge of the left frame side return port  62  becomes longer than the width W of an image forming region in which a toner image is formed on a surface of the photoconductive drum  26 . A spacing between the right edge of the cartridge side opening  49  made of the right inside return port  99  and outside return port  111  and the left edge of the cartridge side opening  49  made of the left inside return port  99  and outside return port  111  becomes longer than the width W of the image forming region in which the toner image is formed on the surface of the photoconductive drum  26 . As a result, toner can be fluidized in the whole width of the image forming region in the inside of the process part  33  by supply of the toner from the inside of the toner cartridge  22  to the inside of the process part  33  and return of the toner from the inside of the process part  33  to the inside of the toner cartridge  22 . As a result, a good toner image can be formed in the image forming region of the photoconductive drum  26 . 
     When the CPU  121  decides that the remaining state is the empty state, driving of the agitator  46  disposed inside the toner cartridge  22  is stopped. Accordingly, supply of toner from the toner cartridge  22  to the process part  33  can be stopped. As a result, in a state in which a large amount of deteriorating toner remains inside the process part  33 , the toner cartridge  22  can be prevented from being replaced with a toner cartridge in which new toner is housed. 
     When the CPU  121  decides that the remaining state is the empty state, supply of toner from the toner cartridge  22  to the process part  33  is stopped. In this state, the toner transport member  29  is driven, and the toner of the inside of the process part  33  is returned from a return port to the inside of the toner cartridge  22 . Accordingly, the amount of toner remaining inside the process part  33  can be reduced. As a result, occurrence of an image forming defect resulting from mixture of the old and new toners can be prevented further. 
     Then, the display panel  129  displays on the display an indication that toner of the inside of the toner cartridge  22  runs out after the toner transport member  29  is driven for a predetermined time. Accordingly, in a state in which the amount of toner remaining inside the process part  33  reduces, the toner cartridge  22  is replaced with a toner cartridge in which new toner is housed, so that occurrence of an image forming defect resulting from mixture of the old and new toners can be prevented further. 
     6. Another Example of Empty Processing 
       FIG. 12  is a flowchart describing another example of empty processing. 
     In the empty processing, the clutch  128  is first turned off. Accordingly, transmission of driving force from the main motor  127  to the agitator  46  is broken and the agitator  46  is stopped (S 11 ). 
     Next, a value n of a counter set in the RAM  122  is set at zero (S 12 ). 
     Thereafter, in a state of stopping the agitator  46 , an image forming action is performed and one surface of the sheet  3  is painted out by a solid black color, the so-called solid printing is done (S 13 ). The agitator  46  stops, so that toner is not supplied from the inside of the toner reception chamber  45  to the developing chamber  34 . In contrast, the toner of the inside of the developing chamber  34  is consumed by the solid printing. As a result, the amount of toner present inside the developing chamber  34  decreases. 
     When the solid printing to one sheet  3  is done, the value n of the counter is updated to n+1 (S 14 ). 
     When the value n of the counter reaches a predetermined number of sheets (S 15 : YES), that is, when the solid printing to the sheets  3  of the predetermined number of sheets is achieved, the main motor  127  is stopped (i.e., turned off) (S 15 ). Then, display to the effect that it is necessary to replace the toner cartridge  22  with a new toner cartridge is performed on the display panel  129  (S 16 ). 
     After supply of the toner from the toner cartridge  22  to the process part  33  is stopped, the solid printing is executed in order to consume the toner of the inside of the process part  33 . Accordingly, the amount of toner remaining inside the process part  33  can be reduced. As a result, occurrence of an image forming defect resulting from mixture of old and new toners can be further prevented. 
     Then, the display panel  129  displays on the display an indication that the toner of the inside of the toner cartridge  22  runs out after the solid printing to the sheets  3  of the predetermined number of sheets is done. Accordingly, in a state in which the amount of toner remaining inside the process part  33  reduces, the toner cartridge  22  is replaced with a toner cartridge in which new toner is housed, so that occurrence of an image forming defect resulting from mixture of the old and new toners can be prevented further. 
     MODIFIED EXAMPLES 
     In the above-described aspects, the photoconductive drum  26 , the electrification device  27 , the transfer roller  28 , the toner transport member  29 , the supply roller  30 , the developing roller  31  and the layer thickness regulating blade  32  are integrally held in the developing unit  13 . However, the developing unit  13  may include a drum cartridge which is constructed attachably to and detachably from the body casing  2  and integrally holds the photoconductive drum  26 , the electrification device  27  and the transfer roller  28 , and a development cartridge which is constructed attachably to and detachably from the drum cartridge and integrally holds the toner transport member  29 , the supply roller  30 , the developing roller  31  and the layer thickness regulating blade  32 . 
     Further, the photoconductive drum  26 , the electrification device  27  and the transfer roller  28  may be mounted inside the body casing  2  rather than a form of the drum cartridge. 
     Still further, although aspects of the present inventive concept have been described in relation to a laser printer, the present inventive concept is not limited to a monochrome laser printer. Rather, the present inventive concept can also be applied to a color laser printer including a tandem type and an intermediate transfer type printer.