Patent Publication Number: US-9835979-B2

Title: Developing device and image forming apparatus therewith

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2015-176805 filed on Sep. 8, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to developing devices used in image forming apparatuses adopting an electrophotographic system, such as copiers, printers, facsimile machines, and multifunction peripherals having combined functions of those, and to image forming apparatuses provided with such developing devices. More particularly, the present disclosure relates to developing devices that, while being supplied with fresh two-component developer containing toner and carrier, discharge surplus developer, and to image forming apparatuses provided with such developing devices. 
     In an image forming apparatus, an electrostatic latent image formed on an image carrier comprising a photosensitive body or the like is developed by a developing device into a toner image so as to be made visible. There is a type of developing device that adopts a two-component development system using two-component developer. In this type of developing device, two-component developer containing carrier and toner is stored in a developing container, a developing roller for feeding the developer to the image carrier is arranged, and a stirring member for feeding, while stirring and transporting, the developer inside the developing container to the developing roller is arranged. 
     In this developing device, as toner is consumed in developing operation, carrier remains unconsumed inside the developing container. Thus, the carrier, which is stirred with the toner inside the developing container, deteriorates due to mechanical stress from being stirred repeatedly. As a result, the carrier&#39;s ability to electrostatically charge the toner gradually diminishes. 
     As a solution, there have been proposed developing devices in which, while fresh developer containing carrier and toner is supplied into a developing container, surplus developer is discharged with a view to suppressing a drop in electrostatic charging performance. 
     For example, in a known developing device, two stirring members each having a rotary shaft and a helical blade formed in a helical shape on its circumference are arranged parallel to each other in transport chambers respectively. The transport chambers are divided by a partition portion, and in opposite end parts of the partition portion, communicating portions for delivering developer are provided. A developer discharge port is formed downstream of one of the transport chambers with respect to the developer transport direction, and between the stirring member there and the developer discharge port, an opposite helical blade formed in a helical shape wound in the direction opposite to the helical blade on the stirring member is provided, as a restricting portion, integrally with the rotary shaft. 
     With this developing device, when developer is supplied into the developing container, as the stirring member rotates, the developer is, while being stirred, transported to downstream of the transport chamber. As the opposite helical blade rotates in the same direction as the stirring member, the opposite helical blade applies to the developer a transporting force that acts in the direction opposite to the direction of developer transport by the stirring member. With this opposite transporting force, on the downstream side of the transport chamber, developer is blocked to have an increased height. Thus, surplus developer passes over the opposite helical blade (restricting portion) to move to the developer discharge port, and is discharged. In this way, developer can be replaced while the height of the developer inside the developing container is kept substantially constant. 
     SUMMARY 
     According to one aspect of the present disclosure, a developing device includes a developing container, a first stirring member, a second stirring member, a developer carrier, a developer supply port, and a developer discharge port. The developing container has a plurality of transport chambers including a first transport chamber and a second transport chamber arranged side by side, and communicating portions through which the first and second transport chambers communicate with each other at opposite longitudinal-direction end parts thereof. The developing container stores two-component developer containing carrier and toner. The first stirring member is composed of a rotary shaft and a first helical blade formed on the circumferential face of the rotary shaft, and stirs and transports developer inside the first transport chamber in the rotary-shaft direction. The second stirring member is composed of a rotary shaft and a second helical blade formed on the circumferential face of the rotary shaft, and stirs and transports developer inside the second transport chamber in the direction opposite to the first stirring member. The developer carrier is rotatably supported on the developing container, and carries on the surface thereof the developer inside the second transport chamber. Through the developer supply port, developer is supplied into the developing container. The developer discharge port is provided in a downstream-side end part of the second transport chamber with respect to the developer transport direction inside the second transport chamber, and through it, surplus developer inside the developing container is discharged. On the second stirring member, a restricting portion which restricts the movement of developer toward the developer discharge port is formed so as to be opposed to the developer discharge port downstream of the second helical blade with respect to the developer transport direction inside the second transport chamber. The restricting portion is composed of two or more turns of a restricting blade wound in the direction opposite to the second helical blade. The second transport chamber has a uniform inside diameter in the part thereof opposed to the restricting blade. The restricting blade has an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber. 
     Further features and advantages of the present disclosure will become apparent from the description of embodiments given below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically showing an overall construction of an image forming apparatus  1  incorporating developing devices  2   a  to  2   d  according to the present disclosure; 
         FIG. 2  is a side sectional view of a developing device  2  according to one embodiment of the present disclosure; 
         FIG. 3  is a sectional plan view of a stirring portion in the developing device  2  according to the embodiment; 
         FIG. 4  is an enlarged view around a developer discharge port  22   h  in the developing device  2  according to the embodiment; and 
         FIG. 5  is an enlarged view around the regulating portion  52  in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will be described below with reference to the accompanying drawings.  FIG. 1  is a sectional view schematically showing a construction of an image forming apparatus  1  incorporating developing devices  2   a  to  2   d  according to the present disclosure. The image forming apparatus  1  is a tandem-type color printer. There are arranged rotatable photosensitive drums (image carriers)  11   a  to  11   d  that are, for example, organic photosensitive bodies (OPC photosensitive bodies) having an organic photosensitive layer formed on them or amorphous silicon photosensitive bodies having an amorphous silicon layer formed on them and that are arranged to correspond to different colors, namely black, yellow, cyan, and magenta. Around the photosensitive drums  11   a  to  11   d , there are arranged developing devices  2   a  to  2   d , an exposure unit  12 , electrostatic chargers  13   a  to  13   d , and cleaning devices  14   a  to  14   d.    
     The developing devices  2   a  to  2   d  are arranged beside (to the right of) the photosensitive drums  11   a  to  11   d , respectively, so as to be opposed to them, and feed toner to the photosensitive drums  11   a  to  11   d . The electrostatic chargers  13   a  to  13   d  are arranged upstream of the developing devices  2   a  to  2   d  with respect to the rotation direction of the photosensitive drums  11   a  to  11   d  (in  FIG. 1 , the counter-clockwise direction) so as to be opposed to the surfaces of the photosensitive drums  11   a  to  11   d , and electrostatically charge the surfaces of the photosensitive drums  11   a  to  11   d  uniformly. 
     The exposure unit  12  scans the photosensitive drums  11   a  to  11   d  to expose them to light based on image data, conveying characters and figures, fed to an image input portion (unillustrated) from a personal computer or the like, and is arranged under the developing devices  2   a  to  2   d . Inside the exposure unit  12 , there are arranged a laser light source and a polygon mirror, and there are also arranged reflective mirrors and lenses to correspond to the photosensitive drums  11   a  to  11   d . The laser light emitted from the laser light source is shone, via the polygon mirror, reflective mirrors, and lenses, onto the surfaces of the photosensitive drums  11   a  to  11   d  from downstream of the electrostatic chargers  13   a  to  13   d  with respect to the rotation direction of the photosensitive drums  11   a  to  11   d . The laser light thus shone forms electrostatic latent images on the surfaces of the photosensitive drums  11   a  to  11   d , and these electrostatic latent images are developed into toner images by the developing devices  2   a  to  2   d.    
     An endless intermediary transfer belt  17  is wound around a tension roller  6 , a driving roller  25 , and a following roller  27 . The driving roller  25  is driven to rotate by an unillustrated motor, and as the driving roller  25  rotates, the intermediary transfer belt  17  is driven to circulate in the clockwise direction in  FIG. 1 . 
     The photosensitive drums  11   a  to  11   d  are arranged next to each other along the belt movement direction (the direction indicated by arrows in  FIG. 1 ) under the intermediary transfer belt  17  so as to remain in contact with the intermediary transfer belt  17 . Primary transfer rollers  26   a  to  26   d  are arranged so as to be opposed to the photosensitive drums  11   a  to  11   d  respectively across the intermediary transfer belt  17 , and are kept in pressed contact with the intermediary transfer belt  17  to form a primary transfer portion. At this primary transfer portion, as the intermediary transfer belt  17  rotates, the toner images on the photosensitive drums  11   a  to  11   d  are successively transferred to the intermediary transfer belt  17  with predetermined timing. As a result, on the surface of the intermediary transfer belt  17 , a full-color toner image is formed that has toner images of four colors, namely cyan, magenta, yellow, and black, superimposed together. 
     A secondary transfer roller  34  is opposed to the driving roller  25  across the intermediary transfer belt  17 , and is kept in pressed contact with the intermediary transfer belt  17  to form a secondary transfer portion. At this secondary transfer portion, the toner image on the surface of the intermediary transfer belt  17  is transferred to a paper or other sheet P. After the transfer, a belt cleaning device  31  clears the surface of the intermediary transfer belt  17  of toner that is left unused. 
     In a lower part inside the image forming apparatus  1 , a sheet feed cassette  32  for storing sheets P is arranged, and to the right of the sheet feed cassette  32 , a stack tray  35  for manual feeding of sheets is arranged. To the left of the sheet feed cassette  32 , there is arranged a first sheet transfer passage  33  through which a sheet P fed out of the sheet feed cassette  32  is transferred to the secondary transfer portion on the intermediary transfer belt  17 . To the left of the stack tray  35 , there is arranged a second sheet transfer passage  36  through which a sheet fed out of the stack tray  35  is transported to the secondary transfer portion. Furthermore, in an upper left part of the image forming apparatus  1 , there are arranged a fixing portion  18  which performs fixing operation on a sheet P having an image formed on it and a third sheet transport passage  39  through which a sheet having undergone fixing operation is transported to a sheet discharge portion  37 . 
     The sheet feed cassette  32 , when drawn out of the apparatus (toward the near side in  FIG. 1 ), can be replenished with sheets, and feeds out sheets P stored in it into the first sheet transfer passage  33 , one sheet after another, by the action of a pickup roller  33   b  and a separating roller  33   a.    
     The first sheet transfer passage  33  and the second sheet transfer passage  36  meet just downstream of a registration roller pair  33   c , and by the registration roller pair  33   c , a sheet P is transported to the secondary transfer portion with appropriate timing with regard to image forming operation on the intermediary transfer belt  17  and sheet feeding operation. The sheet P thus transported to the secondary transfer portion then has the full-color toner image on the intermediary transfer belt  17  secondarily transferred to it by a secondary transfer roller  34  to which a bias potential is applied, and is then transported to the fixing portion  18 . 
     The fixing portion  18  includes, among others, a fixing belt which is heated by a heating roller, a fixing roller which is kept in contact with the fixing belt from inside, and a pressing roller which is arranged in pressed contact with the fixing roller, and performs fixing operation by applying heat and pressure to the sheet P having the toner image transferred to it. After the fixing of the toner image by the fixing portion  18 , the sheet P is, as necessary, reversed in a fourth sheet transport passage  40  so that a toner image is secondarily transferred to the reverse side of the sheet as well and is fixed in the fixing portion  18 . The sheet having the toner image or images fixed to it passes through the third sheet transport passage  39 , and is discharged onto the sheet discharge portion  37  by discharge rollers  19   a.    
       FIG. 2  is a side sectional view showing a construction of a developing device  2  that is used in the image forming apparatus  1  described above. The following description deals with the construction and operation of the developing device  2   a  corresponding to the photosensitive drum  11   a  shown in  FIG. 1 . Since the construction and operation of the developing devices  2   b  to  2   d  are similar to those of the developing device  2   a , no overlapping description will be repeated, and the suffixes “a” to “d” distinguishing the developing devices and the photosensitive bodies for different colors will be omitted. 
     As shown in  FIG. 2 , the developing device  2  is composed of a developing roller  20 , a magnetic roller (developer carrier)  21 , a restricting blade  24 , stirring members  42 , a developing container  22 , etc. 
     The developing container  22  forms a casing of the developing device  2 , and a lower part of the developing container  22  is divided into a first transport chamber  22   c  and a second transport chamber  22   d  by a partition portion  22   b . In the first and second transport chambers  22   c  and  22   d , developer containing carrier and toner (here, positive-charge toner) is stored. The developing container  22  rotatably holds the stirring members  42 , the magnetic roller  21 , and the developing roller  20 . The developing device  2  has an opening  22   a  formed in it through which the developing roller  20  is exposed toward the photosensitive drum  11 . 
     The developing roller  20  is opposed to the photosensitive drum  11 , and is arranged to the right of the photosensitive drum  11  across a predetermined distance. In the developing roller  20 , at a position close to and opposed to the photosensitive drum  11 , a developing region D is formed through in toner is fed to the photosensitive drum  11 . The magnetic roller  21  is opposed to the developing roller  20  across a predetermined distance, and is arranged obliquely to the lower right of the developing roller  20 . At a position close to and opposed to the developing roller  20 , the magnetic roller  21  feeds toner to the developing roller  20 . The stirring members  42  are arranged substantially under the magnetic roller  21 . The restricting blade  24  is fixedly held by the developing container  22  obliquely to the lower left of the magnetic roller  21 . 
     The stirring member  42  is composed of two members, namely a first spiral (first stirring member)  43  and a second spiral (second stirring member)  44 . The second spiral  44  is arranged under the magnetic roller  21 , inside the second transport chamber  22   d , and the stirring member  42  is arranged next to, to the right of, the second spiral  44 , inside the first transport chamber  22   c.    
     The first and second spirals  43  and  44  stir developer to electrostatically charge the toner in the developer to a predetermined level. This permits the toner to be held by the carrier. In opposite end parts of the partition portion  22   b , which divides between the first and second transport chambers  22   c  and  22   d , in its longitudinal direction (in  FIG. 2 , the direction perpendicular to the plane of the figure), communicating portions (unillustrated) are provided; thus, as the first spiral  43  rotates, electrostatically charged developer is transported to the second spiral  44  via one of the communicating portions formed in the partition portion  22   b  so that the developer circulates through the first and second transport chambers  22   c  and  22   d . The developer is then fed from the second spiral  44  to the magnetic roller  21 . 
     The magnetic roller  21  includes a magnetic pole member M and a non-magnetic sleeve  21   b  formed of a non-magnetic material, carries developer fed from the stirring member  42 , and feeds, out of the developer it carries, only the toner to the developing roller  20 . In the magnetic pole member M, a plurality of magnets having a fan-shaped cross-section and having different polarities in circumferential parts are arranged alternately, and are fixed to a roller shaft  21   a  with adhesive or otherwise. The roller shaft  21   a  is, inside the non-magnetic sleeve  21   b , non-rotatably supported on the developing container  22 , with a predetermined distance secured between the magnetic pole member M and the non-magnetic sleeve  21   b . The non-magnetic sleeve  21   b  rotates in the same direction (in  FIG. 2 , the clockwise direction) as the developing roller  20  by being driven by an unillustrated driving mechanism composed of a motor and gears, and a bias  56  that has an alternating-current voltage  56   b  superposed on a direct-current voltage  56   a  is applied to the non-magnetic sleeve  21   b . On the surface of the non-magnetic sleeve  21   b , electrostatically charged developer is carried while forming a magnetic brush under the magnetic force of the magnetic pole member M, and the height of the magnetic brush is adjusted to a predetermined height by the restricting blade  24 . 
     As the non-magnetic sleeve  21   b  rotates, the magnetic brush is transported while being carried on the surface of the non-magnetic sleeve  21   b  by the magnetic pole member M; when it makes contact with the developing roller  20 , out of the magnetic brush, only the toner is fed to the developing roller  20  in accordance with the bias  56  applied to the non-magnetic sleeve  21   b.    
     The developing roller  20  is composed of, among others, a fixed shaft  20   a , a magnetic pole member  20   b , and a developing sleeve  20   c  formed of a non-magnetic metal material in a cylindrical shape. 
     The fixed shaft  20   a  is non-rotatably supported on the developing container  22 . On the fixed shaft  20   a , the developing sleeve  20   c  is rotatably held, and the magnetic pole member  20   b , which is formed of magnets, is fixed with adhesive or otherwise, at a position opposed to the magnetic roller  21 , with a predetermined distance secured from the developing sleeve  20   c . The developing sleeve  20   c  is rotated in the direction indicated by an arrow in  FIG. 2  (the clockwise direction) by an unillustrated mechanism composed of a motor and gears. A developing bias  55  that has an alternating-current voltage  55   b  superposed on a direct-current voltage  55   a  is applied to the developing sleeve  20   c.    
     As the developing sleeve  20   c  having the developing bias  55  applied to it rotates in the clockwise direction in  FIG. 2 , in the developing region D, a potential difference between the developing bias potential and the potential in the exposed part of the photosensitive drum  11  causes the toner carried on the surface of the developing sleeve  20   c  to fly to the photosensitive drum  11 . The toner thus flown attaches to the exposed part of the photosensitive drum  11  rotating in the direction indicated by arrow A (the counter-clockwise direction), and thereby an electrostatic latent image on the photosensitive drum  11  is developed. 
     Next, with reference to  FIG. 3 , a stirring portion in the developing device  2  will be described in detail.  FIG. 3  is a sectional plan view (sectional view across line X-X′ in  FIG. 2  as seen from the direction indicated by arrows) showing the stirring portion in the developing device  2 . 
     As mentioned above, inside the developing container  22 , there are formed a first transport chamber  22   c , a second transport chamber  22   d , a partition portion  22   b , an upstream-side communicating portion  22   e , and a downstream-side communicating portion  22   f , and there are further formed a developer supply port  22   g , a developer discharge port  22   h , an upstream-side wall portion  22   i , and a downstream-side wall portion  22   j . Of the first transport chamber  22   c , the left side in  FIG. 3  are referred to as the upstream side, and the right side in  FIG. 3  as the downstream side. Of the second transport chamber  22   d , the right side in  FIG. 3  are referred to as the upstream side, and the left side in  FIG. 3  as the downstream side. Accordingly, with respect to the second transport chamber  22   d , the communicating portions are each referred to as either an upstream-side or downstream-side communicating portion, and the side wall portions are each referred to as either an upstream-side or downstream-side wall portion. 
     The partition portion  22   b  extends in the longitudinal direction of the developing container  22  to divide between the first and second transport chambers  22   c  and  22   d  such that these lie side by side. A right end part of the partition portion  22   b  in its longitudinal direction forms, together with an inner wall part of the upstream-side wall portion  22   i , the upstream-side communicating portion  22   e ; on the other hand, a left end part of the partition portion  22   b  in its longitudinal direction forms, together with an inner wall part of the downstream-side wall portion  22   j , the downstream-side communicating portion  22   f . Thus, the developer can circulate through the first transport chamber  22   c , the upstream-side communicating portion  22   e , the second transport chamber  22   d , and the downstream-side communicating portion  22   f.    
     The developer supply port  22   g  is an opening through which fresh toner and carrier are supplied from a developer supply container (unillustrated) provided over the developing container  22 , and is arranged in an upstream-side (in  FIG. 3 , left-side) part of the first transport chamber  22   c.    
     The developer discharge port  22   h  is an opening through which, as fresh developer is supplied, surplus developer inside the first and second transport chambers  22   c  and  22   d  is discharged, and is provided in a downstream-side part of the second transport chamber  22   d  so as to be continuous with the second transport chamber  22   d  in its longitudinal direction. 
     Inside the first transport chamber  22   c , the first spiral  43  is arranged, and inside the second transport chamber  22   d , the second spiral  44  is arranged. 
     The first spiral  43  has a rotary shaft  43   b  and a first helical blade  43   a  that is formed integrally with the rotary shaft  43   b  and that has a helical shape with a predetermined pitch in the axial direction of the rotary shaft  43   b . The first helical blade  43   a  extends up to opposite end parts of the first transport chamber  22   c  in its longitudinal direction so as to be opposed to the upstream- and downstream-side communicating portions  22   e  and  22   f . The rotary shaft  43   b  is rotatably pivoted on the upstream- and downstream-side wall portions  22   i  and  22   j.    
     The second spiral  44  has a rotary shaft  44   b  and a second helical blade  44   a  that is formed integrally with the rotary shaft  44   b  and that has a helical shape with a blade wound with the same pitch as but in the opposite direction (with the opposite phase) to the first helical blade  43   a  in the axial direction of the rotary shaft  44   b . The second helical blade  44   a  has a length that is larger than the length of the magnetic roller  21  in its axial direction, and extends to reach a position opposed to the upstream-side communicating portion  22   e . The rotary shaft  44   b  is arranged parallel to the rotary shaft  43   b , and is rotatably pivoted on the upstream- and downstream-side wall portions  22   i  and  22   j  of the developing container  22 . 
     On the rotary shaft  44   b , there are integrally arranged, in addition to the second helical blade  44   a , a restricting portion  52  and a discharge blade  53 . 
     The restricting portion  52  blocks the developer transported downstream inside the second transport chamber  22   d , but allows the developer exceeding a predetermined amount to be transported to the developer discharge port  22   h . The restricting portion  52  comprises a helical blade (restricting blade) formed on the rotary shaft  44   b , and has a helical shape with a blade wound in the opposite direction (with the opposite phase) to, and with a smaller pitch than, the second helical blade  44   a . The circumferential part of the restricting portion  52  has a predetermined distance (clearance) from the inner wall part of the developing container  22 . Through this gap, surplus developer is discharged. 
     The rotary shaft  44   b  extends to reach inside the developer discharge port  22   h . On a part of the rotary shaft  44   b  inside the developer discharge port  22   h , the discharge blade  53  is provided. The discharge blade  53  comprises a blade in a helical shape wound in the same direction as the second helical blade  44   a , but has a smaller pitch than the second helical blade  44   a , and the blade has a smaller circumference. Accordingly, as the rotary shaft  44   b  rotates, the discharge blade  53  rotates together so that the surplus developer that has been transported into the developer discharge port  22   h  over the restricting portion  52  is transported leftward in  FIG. 3  to be discharged out of the developing container  22 . The discharge blade  53 , the restricting portion  52 , and the second helical blade  44   a  are formed of synthetic resin integrally with the rotary shaft  44   b.    
     On the outer wall of the developing container  22 , gears  61  to  64  are arranged. The gears  61  and  62  are fixed to the rotary shaft  43   b , the gear  64  is fixed to the rotary shaft  44   b , and the gear  63  is rotatably held on the developing container  22  and meshes with the gears  62  and  64 . 
     During development in which no fresh developer is supplied, as the gear  61  is rotated by a driving source such as a motor, the first helical blade  43   a  rotates together with the rotary shaft  43   b ; thus, by the first helical blade  43   a , developer is transported in the direction indicated by arrow P inside the first transport chamber  22   c , and is then transported through the upstream-side communicating portion  22   e  into the second transport chamber  22   d . Then, as the second helical blade  44   a  rotates together with the rotary shaft  44   b  which is interlocked with the gear  64 , by the second helical blade  44   a , developer is transported in the direction indicated by arrow Q inside the second transport chamber  22   d . Thus, developer is, while greatly changing its height, transported from the first transport chamber  22   c  through the upstream-side communicating portion  22   e  into the second transport chamber  22   d , and is transported through the downstream-side communicating portion  22   f  into the first transport chamber  22   c  without passing over the restricting portion  52 . 
     As described above, as developer circulates through the first transport chamber  22   c , the upstream-side communicating portion  22   e , the second transport chamber  22   d , and the downstream-side communicating portion  22   f , it is stirred, and the stirred developer is fed to the magnetic roller  21 . 
     Next, a description will be given of a case where developer is supplied through the developer supply port  22   g . As development progresses and toner is consumed, developer containing toner and carrier is supplied into the first transport chamber  22   c  through the developer supply port  22   g.    
     As during development, by the first helical blade  43   a , the supplied developer is transported in the direction indicated by arrow P inside the first transport chamber  22   c , and is then transported through the upstream-side communicating portion  22   e  into the second transport chamber  22   d . Then, by the second helical blade  44   a , the developer is transported in the direction indicated by arrow Q inside the second transport chamber  22   d . As the rotary shaft  44   b  rotates and thus the restricting portion  52  rotates together, the restricting portion  52  applies to the developer a transporting force that acts in the direction opposite to the direction of developer transport by the second helical blade  44   a . The restricting portion  52  thus blocks the developer and increases its height; as a result, surplus developer passes over the restricting portion  52 , and is discharged through the developer discharge port  22   h  out of the developing container  22 . 
       FIG. 4  is an enlarged view around the developer discharge port  22   h  in the developing device  2  according to the embodiment, and  FIG. 5  is an enlarged view around the restricting portion  52  in  FIG. 4 . As shown in  FIGS. 4 and 5 , the restricting portion  52  is composed of one turn of an upstream-side restricting blade  52   a  which is located at the most upstream-side position with respect to the developer transport direction and which adjoins the second helical blade  44   a , a one turn of a downstream-side restricting blade  52   b  which is located at the most downstream-side position with respect to the developer transport direction and which adjoins the discharge blade  53 , and a one turn of a middle restricting blade  52   c  which is arranged between the upstream-side restricting blade  52   a  and the downstream-side restricting blade  52   b.    
     The outside diameter of the middle restricting blade  52   c  is larger than the outside diameter of the upstream-side restricting blade  52   a , and the outside diameter of the downstream-side restricting blade  52   b  is larger than the outside diameter of the middle restricting blade  52   c . That is, the restricting portion  52  has a stepwise increasing outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber  22   d . The outside diameter of the upstream-side restricting blade  52   a  is equal to or larger than the outside diameter of the discharge blade  53 . The outside diameter of the downstream-side restricting blade  52   b  is equal to or smaller than the outside diameter of the second helical blade  44   a.    
     The inside diameter of the second transport chamber  22   d  is uniform. Thus, the inside diameter of the part of the second transport chamber  22   d  opposed to the second helical blade  44   a  and that of the part of the second transport chamber  22   d  opposed to the restricting portion  52  are equal. 
     With the construction described above, when the distance (clearance) between the second helical blade  44   a  and the inner wall face of the second transport chamber  22   d  is represented by La, and the distance (clearance) between the downstream-side restricting blade  52   b  and the inner wall face of the second transport chamber  22   d  is represented by Lb, the relationship Lb≧La is fulfilled. When the distance (clearance) between the discharge blade  53  and the inner wall face of the developer discharge port  22   h  is represented by Lc, the relationship Lb≧Lc is fulfilled. 
     Owing to the outside diameter of the upstream-side restricting blade  52   a  being smaller than the outside diameter of the downstream-side restricting blade  52   b , the upstream-side restricting blade  52   a  exerts a weaker restricting force on the developer that has been transported through the second transport chamber  22   d  by the second helical blade  44   a . As a result, while the developer continues to be transported in the main developer transport direction (the direction indicated by arrow Q), the transport speed drops; thus, the developer stagnates and thereby suppresses ruffling (fluctuation) on the surface of the developer moving toward the developer discharge port  22   h  and the downstream-side communicating portion  22   f . Thus, the upstream-side restricting blade  52   a  serves to slacken the flow of developer in the restricting portion  52 . 
     Owing to the outside diameter of the downstream-side restricting blade  52   b  being larger than the outside diameter of the upstream-side restricting blade  52   a , the developer that has been transported through the second transport chamber  22   d  is acted on by a strong restricting force exerted by the downstream-side restricting blade  52   b . As a result, the developer is acted on by a transporting force that acts in the direction opposite to the main transport direction. Thus, by applying the opposite transporting force to the developer whose flow has been slackened by the upstream-side restricting blade  52   a , the downstream-side restricting blade  52   b  serves to increase the height of the developer in the restricting portion  52  and thereby adjust the amount of developer fed to the developer discharge port  22   h  over the downstream-side restricting blade  52   b  (i.e., the amount of developer discharged). 
     In the embodiment, as described above, the restricting portion  52  has an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber  22   d . That is, the outside diameter of the upstream-side restricting blade  52   a  is smaller than the outside diameters of the middle regulating blade  52   c  and the downstream-side restricting blade  52   b . Thus, even when the transport speed of developer is low (the stirring speed is low) or the fluidity of developer is low (the toner concentration is high, or the absolute humidity is high), it is possible to alleviate the difficulty of developer reaching the downstream-side restricting blade  52   b , and thus to alleviate the difficulty of developer being discharged to the developer discharge port  22   h . Moreover, the outside diameter of the downstream-side restricting blade  52   b  is larger than outside diameters of the upstream-side restricting blade  52   a  and the middle restricting blade  52   c . Thus, even when the transport speed of developer is high (the stirring speed is high) or the fluidity of developer is high (the toner concentration is low, or the absolute humidity is low), it is possible to reduce the ease with which developer is discharged to the developer discharge port  22   h . As described above, it is possible to stabilize the amount of developer discharged to the developer discharge port  22   h  against changes in the transport condition of the developer in the developing container  22  resulting from variations in the fluidity of developer due to variations in environmental conditions (humidity), variations in the toner concentration during development, variations in the developer transport speed, etc. It is thus possible to obtain a developing device  2  that can suppress variation of the amount of developer inside the developing container  22  and that thus provides stable developing performance. 
     Moreover, as described above, the outside diameter of the downstream-side restricting blade  52   b  is equal to or smaller than the outside diameter of the second helical blade  44   a . This helps alleviate the difficulty of developer being discharged to the developer discharge port  22   h.    
     Moreover, as described above, the outside diameter of the upstream-side restricting blade  52   a  is equal to or larger than the outside diameter of the discharge blade  53 . This helps prevent the upstream-side restricting blade  52   a  from having an excessively small outside diameter, and it is thus possible, when the fluidity of developer is high or the transport speed of developer is high, to prevent an excessive amount of developer from reaching the downstream-side restricting blade  52   b.    
     Moreover, as described above, by fulfilling Lb≧La, it is possible to alleviate the difficulty of developer being discharged to the developer discharge port  22   h.    
     Moreover, as described above, the restricting portion  52  is arranged between the upstream-side restricting blade  52   a  and the downstream-side restricting blade  52   b , and includes a middle restricting blade  52   c  of which the outside diameter is larger than the outside diameter of the upstream-side regulating blade  52   a  but smaller than the outside diameter of the downstream-side restricting blade  52   b . It is thus possible to gradually vary the transport speed of the developer moving from the upstream-side restricting blade  52   a  to the downstream-side restricting blade  52   b.    
     Although the embodiment deals with a case where the restricting portion  52  is composed of three turns of an opposite helical blade, specifically an upstream-side restricting blade  52   a , a downstream-side restricting blade  52   b , a the middle restricting blade  52   c , the restricting portion  52  may be composed of any number of turns of an opposite helical blade other than three, for example, two turns or four or more turns. Although the embodiment deals with a case where the restricting portion  52  has a stepwise increasing outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber  22   d , it may instead have a gradually (continuously) increasing outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber  22   d.    
     The present disclosure can be implemented in any manner other than specifically described by way of an embodiment above, and allows for many modifications without departing from the spirit of the present disclosure. For example, the application of the present disclosure is not limited to developing devices incorporating a magnetic roller  21  and a developing roller  20  as shown in  FIG. 2 ; the present disclosure finds application in various developing devices that use two-component developer containing toner and carrier. For example, although the embodiment described above deals with a developing device adopting a two-axis transport system including, as a developer circulation passage inside the developing container  22 , a first transport chamber  22   c  and a second transport chamber  22   d  that are arranged side by side, application is also possible to developing devices adopting a three-axis transport system including a collection transport chamber for collecting developer separated from the magnetic roller  21  for gathering it into the second transport chamber  22   d.    
     The application of the present disclosure is not limited to tandem-type color printers like the one shown in  FIG. 1 ; the present disclosure finds application in various image forming apparatuses adopting a two-component development system, such as digital or analog monochrome copiers, monochrome printers, color copiers, and facsimile machines. Now, the effects of the present disclosure will be described in more detail by way of a practical example. 
     With the developing device  2  described above, experiments were conducted to see how the amount of developer inside the developing container  22  changed when the restricting portion  52  had an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber  22   d . The experiments were conducted in the black image formation portion that included the photosensitive drum  11   a  and the developing device  2   a.    
     The experiments were conducted for each of Practical Example and Comparative Example. Practical Example incorporated a developing device  2  in which, as shown in  FIGS. 4 and 5 , the upstream-side restricting blade  52   a  had an outside diameter of 10 mm, the middle restricting blade  52   c  had an outside diameter of 11 mm, and the downstream-side restricting blade  52   b  had an outside diameter of 12 mm. Comparative Example incorporated a developing device  2  in which, the upstream-side restricting blade  52   a , the middle restricting blade  52   c , and the downstream-side restricting blade  52   b  all had an outside diameter of 12 mm. In Practical Example, the distance between the upstream-side restricting blade  52   a  and the inner wall face of the second transport chamber  22   d  was 3.5 mm, the distance between the middle restricting blade  52   c  and the inner wall face of the second transport chamber  22   d  was 3.0 mm, and the distance Lb between the downstream-side restricting blade  52   b  and the inner wall face of the second transport chamber  22   d  was 2.5 mm. In Comparative Example, the distances from the upstream-side restricting blade  52   a , the middle restricting blade  52   c , and the downstream-side restricting blade  52   b  to the inner wall face of the second transport chamber  22   d  all were 2.5 mm. 
     In the second spiral  44  used in both Practical Example and Comparative Example, the second helical blade  44   a  had an outside diameter of 14 mm and a pitch of 30 mm, and the distance La between the second helical blade  44   a  and the inner wall face of the second transport chamber  22   d  was 1.5 mm. The upstream-side restricting blade  52   a , the middle restricting blade  52   c , and the downstream-side restricting blade  52   b  had a pitch of 5 mm. The discharge blade  53  had an outside diameter of 8 mm and a pitch of 5 mm, and the distance Lc between the discharge blade  53  and the inner wall face of the developer discharge port  22   h  was 1.5 mm. 150 cm 3  of developer was stored in the developing container  22  (the first and second transport chambers  22   c  and  22   d ). 
     The reference conditions were as follows: the stirring speed (rotation speed) of the first spiral  43  and the second spiral  44  was 300 rpm; the toner concentration in the developer stored in the developing container  22  (the mass ratio of toner to carrier, T/C) was 10%; the absolute humidity was 10 g/m 3 . With respect to these reference conditions, the stirring speed (rotation speed) of the first spiral  43  and the second spiral  44  was changed among three different levels of 200 rpm, 300 rpm, and 400 rpm; the toner concentration in the developer stored in the developing container  22  was changed among three different levels of 8%, 10% and 12%; the absolute humidity was changed among three level of 5 g/m 3 , 10 g/m 3 , and 20 g/m 3 . Under these different conditions, the amount of developer (stable volume) inside the developing device  2  at the time that the discharge of developer from the developing container  22  ceased was measured. The results are shown in Tables 1, 2, and 3. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Stirring  
                 Toner 
                 Absolute 
                 Stable Volume (cm 3 ) 
               
            
           
           
               
               
               
               
               
            
               
                 Speed 
                 Concentration 
                 Humidity 
                 Practical 
                 Comparative 
               
               
                 (rpm) 
                 (%) 
                 (g/m 3 ) 
                 Example 
                 Example 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 200 
                 10 
                 10 
                 117 
                 123 
               
               
                 300 
                 10 
                 10 
                 115 
                 118 
               
               
                 400 
                 10 
                 10 
                 113 
                 115 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Stirring  
                 Toner 
                 Absolute 
                 Stable Volume (cm 3 ) 
               
            
           
           
               
               
               
               
               
            
               
                 Speed 
                 Concentration 
                 Humidity 
                 Practical 
                 Comparative 
               
               
                 (rpm) 
                 (%) 
                 (g/m 3 ) 
                 Example 
                 Example 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 300 
                 8 
                 10 
                 115 
                 115 
               
               
                 300 
                 10 
                 10 
                 115 
                 118 
               
               
                 300 
                 12 
                 10 
                 114 
                 120 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Stirring  
                 Toner 
                 Absolute 
                 Stable Volume (cm 3 ) 
               
            
           
           
               
               
               
               
               
            
               
                 Speed 
                 Concentration 
                 Humidity 
                 Practical 
                 Comparative 
               
               
                 (rpm) 
                 (%) 
                 (g/m 3 ) 
                 Example 
                 Example 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 300 
                 10 
                 5 
                 116 
                 115 
               
               
                 300 
                 10 
                 10 
                 115 
                 118 
               
               
                 300 
                 10 
                 20 
                 116 
                 119 
               
               
                   
               
            
           
         
       
     
     Table 1 reveals the following. With the developing device  2  of Practical Example, a change in the stirring speed from 200 rpm to 400 rpm brought a 4 cm 3  variation in the volume of the developer inside developing container  22 . In contrast, with the developing device  2  of Comparative Example, the change brought a 8 cm 3  variation in the volume of the developer inside developing container  22 . 
     Table 2 reveals the following. With the developing device  2  of Practical Example, a change in the toner concentration from 8% to 12% brought a 1 cm 3  variation in the volume of the developer inside developing container  22 . In contrast, with the developing device  2  of Comparative Example, the change brought a 5 cm 3  variation in the volume of the developer inside developing container  22 . 
     Table 3 reveals the following. With the developing device  2  of Practical Example, a change in the absolute humidity from 5 g/m 3  to 20 g/m 3  brought a 1 cm 3  variation in the volume of the developer inside developing container  22 . In contrast, with the developing device  2  of Comparative Example, the change brought a 4 cm 3  variation in the volume of the developer inside developing container  22 . 
     Based of the results above, it is confirmed that, in the developing device according to the present disclosure where the three turns of a restricting blade constituting the restricting portion  52  has an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction, the stable volume of developer exhibits a stable transition irrespective of the stirring speed or the developer fluidity (toner concentration, absolute humidity) as compared with that in a conventional construction where the restricting blade has a uniform outside diameter. Thus, by use of the developing device according to the present disclosure, it is possible to obtain stable developing performance, and to effectively suppress image defects and unnecessary discharge of developer. 
     The comparison of Practical Example with Comparative Example shows that the stable volume of developer was slightly lower in the Practical Example than in the Comparative Example. This resulted from the restricting portion  52  having a smaller outside diameter on the upstream side (the upstream-side restricting blade  52   a ), and the stable volume of developer can be adjusted by modifying the outside diameter of or the number of turns in the restricting blade. 
     The present disclosure finds applications in developing devices used in image forming apparatuses adopting an electrophotographic system, such as copiers, printers, facsimile machines, and multifunction peripherals having combined functions of those, and in image forming apparatuses provided with such developing devices. More particularly, the present disclosure finds applications in developing devices that, while being supplied with fresh two-component developer containing toner and carrier, discharge surplus developer, and in image forming apparatuses provided with such developing devices.