Abstract:
A developing device has a developer container, a first stirring/transporting member, a second stirring/transporting member, a developer carrying member, a developer supply port, a developer discharge port, a regulating portion, and a height adjustment opening. The developer container has a partition partitioning between first and second transport chambers, and communication portions through which the first and second transport chambers mutually communicate. The first and second stirring/transporting members stir and transport developer in the first and second transport chambers in opposite directions respectively. The regulating portion regulates movement of the developer toward a developer discharge port. The height adjustment opening is formed in the partition, and when the height of the developer in the second transport chamber is equal to or higher than a predetermined height, part of the developer passes through the height adjustment opening and moves to the first transport chamber.

Description:
INCORPORATION BY REFERENCE 
       [0001]    This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2015-245824 filed on Dec. 17, 2015, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    The present disclosure relates to a developing device incorporated in an image forming apparatus exploiting electrophotography, such as a copier, a printer, a facsimile machine, or a multifunction peripheral thereof, and to an image forming apparatus incorporating the developing device. More particularly, the present disclosure relates to a developing device which can be replenished with new two-component developer containing toner and carrier and meanwhile discharge surplus developer, and to an image forming apparatus incorporating such a developing device. 
         [0003]    In an image forming apparatus, an electrostatic latent image formed on an image carrying member comprising a photosensitive member or the like is made visible by being developed into a toner image by a developing device. Some such developing devices adopt a two-component developing system that uses two-component developer. In this type of developing device, two-component developer containing carrier and toner is stored in a developer container, a developing roller is arranged for feeding the developer to the image carrying member, and a stirring member is arranged for feeding, while stirring and transporting, the developer in the developer container to the developing roller. 
         [0004]    In a developing device adopting a two-component developing system, toner is consumed in developing operation, whereas carrier is left unconsumed in the developing device. Thus, carrier stirred together with toner inside the developer container deteriorates under mechanical stress as the carrier is stirred repeatedly, gradually diminishing the toner charging performance of the carrier. 
         [0005]    As a solution, developing devices have been proposed that supply fresh developer containing carrier and toner into a developer container while discharging surplus developer so as to suppress degradation in charging performance. 
         [0006]    For example, a developing device is known that includes a transport screw (first transporting portion) for transporting developer, a return screw (second transporting portion) arranged on the downstream side of the transport screw with respect to the transport direction of the transport screw, and a discharge screw (third transporting portion) arranged on the upstream side of the return screw with respect to the transport direction of the return screw for transporting the developer toward a discharge port, wherein a disk is provided between the return screw (second transporting portion) and the discharge screw (third transporting portion). The disk serves as a circular wall to push back a large part of the developer moving toward the discharge port so that no excessive developer is discharged through the discharge port. 
       SUMMARY 
       [0007]    According to one aspect of the present disclosure, a developing device has a developer container, a first stirring/transporting member, a second stirring/transporting member, a developer carrying member, a developer supply port, a developer discharge port, a regulating portion, and a height adjustment opening. The developer container has a plurality of transport chambers including a first transport chamber and a second transport chamber arranged parallel to each other, a partition partitioning between the first transport chamber and the second transport chamber, and communication portions through which the first and second transport chambers communicate with each other at opposite side end parts of the partition in the longitudinal direction thereof, and stores two-component developer containing magnetic carrier and toner. The first stirring/transporting member is composed of a rotary shaft and a first transport blade formed on the circumferential surface of the rotary shaft, and stirs and transports the developer in the first transport chamber in the axial direction of the rotary shaft. The second stirring/transporting member is composed of a rotary shaft and a second transport blade formed on the circumferential surface of the rotary shaft, and stirs and transports the developer in the second transport chamber in the opposite direction to the first stirring/transporting member. The developer carrying member is rotatably supported on the developer container, and carries on the surface thereof the developer in the second transport chamber. Through the developer supply port, the developer is supplied into the developer container. The developer discharge port is arranged in a downstream-side end part of the second transport chamber with respect to the transport direction of the developer in the second transport chamber, and through the developer discharge port, surplus developer in the developer container is discharged. The regulating portion is arranged opposite the developer discharge port on the downstream side of the second transport blade of the second stirring/transporting member with respect to the transport direction of the developer in the second transport chamber, and regulates movement of the developer toward the developer discharge port. The height adjustment opening is formed in the vicinity of the downstream end of the partition with respect to the transport direction of the developer in the second transport chamber, and when the height of the developer in the second transport chamber is equal to or higher than a predetermined height, part of the developer passes through the height adjustment opening and moves to the first transport chamber. 
         [0008]    Further features and advantages of the present disclosure will become apparent from the description of embodiments given below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      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; 
           [0010]      FIG. 2  is a side sectional view of a developing device  2  according to one embodiment of the present disclosure; 
           [0011]      FIG. 3  is a sectional plan view of a stirring portion of the developing device  2  according to the embodiment; 
           [0012]      FIG. 4  is a sectional plan view of and around a developer discharge port  22   h  in the developing device  2  according to the embodiment; 
           [0013]      FIG. 5  is a side sectional view of and around the developer discharge port  22   h  in the developing device  2  according to the embodiment; 
           [0014]      FIG. 6  is a side sectional view of and around the developer discharge port  22   h  in the developing device  2  according to the embodiment, showing a modified example in which a height adjustment opening  65  in an inverted trapezoidal shape as seen in a side view is provided in a partition  22   b;    
           [0015]      FIG. 7  is a side sectional view of and around the developer discharge port  22   h  in the developing device  2  according to the embodiment, showing a modified example in which a step-shaped height adjustment opening  65  is provided so as to communicate with the downstream-side communication portion  22   h;  and 
           [0016]      FIG. 8  is a diagram comparing the relationship of the developer transport speed with the stable weight of developer inside the developer container  22  between two examples, namely one including a developing device  2  (present disclosure) provided with a height adjustment opening  65  and another including a developing device  2  (comparative example) provided with no height adjustment opening  65 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Hereinafter, an embodiment of the present disclosure will be described 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. Around photosensitive drums  11   a  to  11   d  which are rotatable, there are respectively arranged developing devices  2   a  to  2   d,  an exposure unit  12 , charging devices  13   a  to  13   d,  and cleaning devices  14   a  to  14   d.    
         [0018]    The charging devices  13   a  to  13   d  are arranged on the upstream side of the developing devices  2   a  to  2   d  with respect to the rotation direction (the counter-clockwise direction in  FIG. 1 ) 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  is for scanning the photosensitive drums  11   a  to  11   d  to expose them to light based on the image data entered in an image input unit (unillustrated) from a personal computer or the like. By laser light shone from the exposure unit  12 , electrostatic latent images are formed 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.    
         [0019]    An intermediate transfer belt  17  is wound, under tension, around a tension roller  6 , a driving roller  25 , and a following roller  27 . The intermediate transfer belt  17  is driven to rotate in the clockwise direction in  FIG. 1  by the rotation of the driving roller  25 . 
         [0020]    Primary transfer rollers  26   a  to  26   d  are arranged opposite the photosensitive drums  11   a  to  11   d  respectively across the intermediate transfer belt  17 , and are kept in pressed contact with the intermediate transfer belt  17 , thereby forming a primary transfer portion. In the primary transfer portion, as the intermediate transfer belt  17  rotates, the toner images of four colors, namely cyan, magenta, yellow, and black, on the photosensitive drums  11   a  to  11   d  are transferred sequentially to the intermediate transfer belt  17  with predetermined timing. After the transfer, toner left behind on the photosensitive drums  11   a  to  11   d  is removed by cleaning devices  14   a  to  14   d.    
         [0021]    A secondary transfer roller  34  is arranged opposite the driving roller  25  across the intermediate transfer belt  17 , and is kept in pressed contact with the intermediate transfer belt  17 , thereby forming a secondary transfer portion. In the secondary transfer portion, the toner images on the surface of the intermediate transfer belt  17  are transferred to a sheet P. After the toner images are transferred, a belt cleaning device  31  removes toner left behind on the surface of the intermediate transfer belt  17 . 
         [0022]    In a lower part of the image forming apparatus  1 , a sheet feed cassette  32  is arranged for storing sheets P, and at the right side of the sheet feed cassette  32 , a stack tray (manual sheet feed tray)  35  is arranged. A sheet P fed from the sheet feed cassette  32  by a pick-up roller  33   b  and a separating roller  33   a  is transported through a first sheet transport passage  33  to a registration roller pair  33   c.  A sheet P fed from the stack tray  35  is transported through a second sheet transport passage  36  to the registration roller pair  33   c.  The registration roller pair  33   c  transports those sheets P to the secondary transfer portion while adjusting the timing of image formation on the intermediate transfer belt  17  and sheet feeding. To the sheet P transported to the secondary transfer portion, a full color toner image on the intermediate transfer belt  17  is secondarily transferred by the secondary transfer roller  34  to which a transfer bias is applied, and the sheet P is then transported to a fixing portion  18 . 
         [0023]    The fixing portion  18  includes a fixing belt which is heated by a heat roller, a fixing roller which makes contact with the fixing belt from inside, a pressure roller which is arranged in pressed contact with the fixing roller across the fixing belt, etc. The fixing portion  18  applies heat and pressure to the sheet P having the toner images transferred to it, and thereby achieves fixing. After the toner images are fixed to the sheet P in the fixing portion  18 , the sheet P is reversed as necessary in a fourth sheet transport passage  40  so that toner images are secondarily transferred also to the reversed side of the sheet P by the second transfer roller  34  and are then fixed in the fixing portion  18 . The sheet P having the toner images fixed to it is discharged through a third sheet transport passage  39  onto a sheet discharging portion  37  by a discharge roller  19   a.    
         [0024]    In the image forming apparatus  1  according to the present disclosure, the driving speed of the apparatus can be switched between two levels according to the thickness and kind of the sheet (recording medium) that is transported. That is, when plain paper is used as the sheet, image formation is performed at an ordinary driving speed (hereinafter, referred to as a full speed mode); when thick paper is used as the sheet, image formation is performed at a speed lower than the ordinary speed (hereinafter, referred to as a reduced-speed mode). With this configuration, when thick paper is used, it is possible to secure a sufficient fixing time to improve image quality. 
         [0025]      FIG. 2  is a sectional plan view showing a structure of a developing device  2  incorporated in the above-described image forming apparatus  1 . While the following description deals with the structure and operation of the developing device  2   a  corresponding to the photosensitive drum  11   a  shown in  FIG. 1 , the structure and operation of the developing devices  2   b  to  2   d  are similar to those of the developing device  2   a,  and therefore no overlapping description will be repeated. Moreover, the suffixes “a” to “d” distinguishing the developing devices and the photosensitive members for different colors will be omitted. 
         [0026]    As shown in  FIG. 2 , the developing device  2  is composed of a developing roller  20 , a magnetic roller  21 , a regulating blade  24 , a stirring member  42 , a developer container  22 , etc. 
         [0027]    The developer container  22  forms the housing of the developing device  2 , and is divided, in a lower part of it, into a first transport chamber  22   c  and a second transport chamber  22   d  by a partition  22   b.  In the first and second transport chambers  22   c  and  22   d,  two-component developer (hereinafter, also referred to simply as developer) containing magnetic carrier and toner (here, positively charged toner) is stored. The developer container  22  rotatably holds the stirring member  42 , the magnetic roller  21 , and the developing roller  20 . In the developer container  22 , an opening  22   a  is formed through which the developing roller  20  is exposed toward the photosensitive drum  11 . 
         [0028]    The developing roller  20  is arranged opposite the photosensitive drum  11  across a predetermined interval, on the right side of the photosensitive drum  11 . The developing roller  20  forms, at a position opposite and close to the photosensitive drum  11 , a developing region D where toner is fed to the photosensitive drum  11 . The magnetic roller  21  is arranged opposite the developing roller  20  across a predetermined interval, obliquely on the lower right side of the developing roller  20 . The magnetic roller  21  feeds toner to the developing roller  20  at a position opposite and close to the developing roller  20 . The stirring member  42  is arranged largely under the magnetic roller  21 . The regulating blade  24  is fixedly held on the developer container  22 , obliquely on the lower left side of the magnetic roller  21 . 
         [0029]    The stirring member  42  is composed of two spirals, namely a first spiral  43  and a second spiral  44 . The second spiral  44  is arranged under the magnetic roller  21 , in the second transport chamber  22   d.  The first spiral  43  is arranged next to, on the right side of, the second spiral  44 , in the first transport chamber  22   c.    
         [0030]    The first and second spirals  43  and  44 , while stirring developer, electrostatically charge the toner contained in the developer up to a predetermined level. This allows the toner to be held on the carrier. Communication portions (unillustrated) are provided in opposite longitudinal-direction (the direction perpendicular to the plane of  FIG. 2 ) end parts of the partition  22   b  that partitions between the first transport chamber  22   c  and the second transport chamber  22   d.  As the first spiral  43  rotates, the charged developer is transported to the second spiral  44  via one of the communication portions arranged in the partition  22   b  so that the developer circulates through the first transport chamber  22   c  and the second transport chamber  22   d.  Then, the developer is fed from the second spiral  44  to the magnetic roller  21 . 
         [0031]    The magnetic roller  21  includes a roller shaft  21   a,  a magnetic pole member M, and a non-magnetic sleeve  21   b  formed of a non-magnetic material. The magnetic roller  21  carries the developer fed from the stirring member  42 , and feeds, out of the developer carried, the toner alone to the developing roller  20 . The magnetic pole member M has a plurality of magnets, which are each formed to have a fan-shaped section and which have on their peripheral parts different magnetic polarities from one to the next, arranged alternately. The magnetic pole member M is adhered or otherwise fixed to the roller shaft  21   a.  The roller shaft  21   a  is unrotatably supported on the developer container  22 , in the non-magnetic sleeve  21   b,  with a predetermined interval between the magnetic pole member M and the non-magnetic sleeve  21   b.  The non-magnetic sleeve  21   b  rotates in the same direction (the clockwise direction in  FIG. 2 ) as the developing roller  20  by the action of a driving mechanism comprising a motor and gears, of which none is illustrated. To the non-magnetic sleeve  21   b,  a bias  56  having an AC voltage  56   b  superimposed on a DC voltage  56   a  is applied. On the surface of the non-magnetic sleeve  21   b,  the charged developer is carried in the form of a magnetic brush by the magnetic force of the magnetic pole member M, and the magnetic brush is adjusted to have a predetermined height by the regulating blade  24 . 
         [0032]    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 the magnetic brush makes contact with the developing roller  20 , the toner alone out of the magnetic brush is fed to the developing roller  20  according to the bias  56  applied to the non-magnetic sleeve  21   b.    
         [0033]    The developing roller  20  is composed of a fixed shaft  20   a,  a magnetic pole member  20   b,  a developing sleeve  20   c  formed in a cylindrical shape out of a non-magnetic metal material, etc. 
         [0034]    The fixed shaft  20   a  is unrotatably supported on the developer container  22 . Around the fixed shaft  20   a,  the developing sleeve  20   c  is rotatably held. Moreover, to the fixed shaft  20   a,  the magnetic pole member  20   b  comprising a magnet is adhered or otherwise fixed at a position opposite the magnetic roller  21 , at a predetermined distance from the developing sleeve  20   c.  The developing sleeve  20   c  rotates in the direction indicated by an arrow in  FIG. 2  (the clockwise direction) by the action of a driving mechanism comprising a motor and gears, of which none is illustrated. To the developing sleeve  20   c,  a developing bias  55  having an AC voltage  55   b  superimposed on a DC voltage  55   a  is applied. 
         [0035]    As the developing sleeve  20   c  to which the developing bias  55  is applied rotates in the clockwise direction in  FIG. 2 , in the developing region D, due to the potential difference between the developing bias and the exposed part of the photosensitive drum, toner carried on the surface of the developing sleeve  20   c  flies to the photosensitive drum  11 . The flying toner attaches, sequentially, to the exposed part on the photosensitive drum  11  rotating in the direction indicated by arrow A (the counter-clockwise direction), and thus the electrostatic latent image on the photosensitive drum  11  is developed. 
         [0036]    Now, a stirring portion in the developing device  2  will be described in detail with reference to  FIG. 3 .  FIG. 3  is a sectional plan view (as taken across line X-X′ in  FIG. 2 ) of the stirring portion in the developing device  2 . 
         [0037]    In the developer container  22 , as described previously, there are formed a first transport chamber  22   c,  a second transport chamber  22   d,  a partition  22   b,  an upstream-side communication portion  22   e,  and a downstream-side communication portion  22   f.  In the developer container  22 , 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.  With respect to the first transport chamber  22   c,  the left side in  FIG. 3  is the upstream side and the right side in  FIG. 3  is the downstream side; with respect to the second transport chamber  22   d,  the right side in  FIG. 3  is the upstream side and the left side in  FIG. 3  is the downstream side. Thus, the communication portions and the side wall portions are distinguished between the upstream-side and downstream-side ones relative to the second transport chamber  22   d.    
         [0038]    The partition  22   b  extends in the longitudinal direction of the developer container  22  to separate the first transport chamber  22   c  and the second transport chamber  22   d  parallel to each other. On one hand, the right side end part of the partition  22   b  in the longitudinal direction forms the upstream-side communication portion  22   e  together with the inner wall part of the upstream-side wall portion  22   i.  On the other hand, the left side end part of the partition  22   b  in the longitudinal direction forms the downstream-side communication portion  22   f  together with the inner wall part of the downstream-side wall portion  22   j.  Thus, developer can circulate through the first transport chamber  22   c,  the upstream-side communication portion  22   e,  the second transport chamber  22   d,  and the downstream-side communication portion  22   f.    
         [0039]    The developer supply port  22   g  is a port through which new toner and carrier are supplied from a developer supply container (unillustrated) provided over the developer container  22  into the developer container  22 . The developer supply port  22   g  is arranged on the upstream side (the left side in  FIG. 3 ) of the first transport chamber  22   c.    
         [0040]    The developer discharge port  22   h  is a port through which surplus developer in the first and second transport chambers  22   c  and  22   d  resulting from supply of new developer is discharged. The developer discharge port  22   h  is arranged continuous with the second transport chamber  22   d  in the longitudinal direction on the downstream side of the second transport chamber  22   d.    
         [0041]    In the first transport chamber  22   c,  the first spiral  43  is arranged, and in the second transport chamber  22   d,  the second spiral  44  is arranged. 
         [0042]    The first spiral  43  has a rotary shaft  43   b  and a first helical blade  43   a  provided integrally with the rotary shaft  43   b  and formed in 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 side end parts of the first transport chamber  22   c  in the longitudinal direction, and is arranged so as to face the upstream-side and downstream-side communication portions  22   e  and  22   f.  The rotary shaft  43   b  is rotatably supported on the upstream-side wall portion  22   i  and the downstream-side wall portion  22   j  of the developer container  22 . 
         [0043]    The second spiral  44  has a rotary shaft  44   b  and a second helical blade  44   a  provided integrally with the rotary shaft  44   b  and formed in a helical shape spiraling in the opposite direction (in the opposite phase) to the first helical blade  43   a  with the same pitch as 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 larger than that of the magnetic roller  21  in the axial direction, and is arranged so as to extend up to a position facing the upstream-side communication portion  22   e.  The rotary shaft  44   b  is arranged parallel to the rotary shaft  43   b  and is rotatably supported on the upstream-side wall portion  22   i  and the downstream-side wall portion  22   j  of the developer container  22 . 
         [0044]    On the rotary shaft  44   b,  a regulating portion  52  and a discharge blade  53  are integrally arranged together with the second helical blade  44   a.    
         [0045]    The regulating portion  52  blocks the developer transported to the downstream side inside the second transport chamber  22   d  and transports the developer to the developer discharge port  22   h  when the amount of developer is equal to or higher than a predetermined amount. The regulating portion  52  comprises a helical blade (regulating blade) provided on the rotary shaft  44   b,  and is formed in a helical shape spiraling in the opposite direction (in the opposite phase) to the second helical blade  44   a.  The regulating portion  52  is configured to have substantially the same outer diameter as, but a smaller pitch than, the second helical blade  44   a.  The circumferential portion of the regulating portion  52  has a predetermined gap (clearance) secured from the inner wall portion (the downstream-side wall portion  22   j ) of the developer container  22 . The surplus developer is discharged through the gap. 
         [0046]    The rotary shaft  44   b  extends into the developer discharge port  22   h.  On the rotary shaft  44   b  in the developer discharge port  22   h,  the discharge blade  53  is provided. The discharge blade  53  comprises a blade spiraling in the same direction as the second helical blade  44   a,  but has a smaller pitch and a smaller blade circumference than those of the second helical blade  44   a.  Thus, as the rotary shaft  44   b  rotates, the discharge blade  53  also rotates so that the surplus developer transported into the developer discharge port  22   h  over the regulating portion  52  is transported to the left side in  FIG. 3  to be discharged out from the developer container  22 . The discharge blade  53 , the regulating portion  52 , and the second helical blade  44   a  are formed integrally with the rotary shaft  44   b  out of synthetic resin. 
         [0047]    On an outer wall of the developer container  22 , gears  61  to  64  are arranged. The gears  61  and  62  are fixed on the rotary shaft  43   b,  the gear  64  is fixed on the rotary shaft  44   b,  and the gear  63  is rotatably held on the developer container  22  to mesh with the gears  62  and  64 . 
         [0048]    During development, during which no new developer is supplied, as the gear  61  rotates by the action of a driving source such as a motor, the first helical blade  43   a  rotates together with the rotary shaft  43   b.  By the first helical blade  43   a,  developer in the first transport chamber  22   c  is transported in the direction indicated by arrow P, and the developer is then transported through the upstream-side communication portion  22   e  into the second transport chamber  22   d.  Moreover, as the second helical blade  44   a  rotates together with the rotary shaft  44   b  which follows the gear  64 , by the second helical blade  44   a,  the developer in the second transport chamber  22   d  is transported in the direction indicated by arrow Q. Thus, the developer is, while greatly varying its height, transported from the first transport chamber  22   c  through the upstream-side communication portion  22   e  into the second transport chamber  22   d,  and the developer is then, without going over the regulating portion  52 , transported through the downstream-side communication portion  22   f  to the first transport chamber  22   c.    
         [0049]    In this way, developer circulates through, while being stirred, the first transport chamber  22   c,  the upstream-side communication portion  22   e,  the second transport chamber  22   d,  and the downstream-side communication portion  22   f,  and the stirred developer is fed to the magnetic roller  21 . 
         [0050]    Now, how developer is supplied through the developer supply port  22   g  will be described. As toner is consumed in development, developer containing toner and carrier is supplied through the developer supply port  22   g  into the first transport chamber  22   c.    
         [0051]    The supplied developer is, as during development, transported in the direction indicated by arrow P inside the first transport chamber  22   c  by the first helical blade  43   a,  and the developer is then transported through the upstream-side communication portion  22   e  into the second transport chamber  22   d.  Moreover, by the second helical blade  44   a,  the developer in the second transport chamber  22   d  is transported in the direction indicated by arrow Q. As the regulating portion  52  rotates together with the rotary shaft  44   b,  a transporting force in the direction opposite to the developer transport direction ascribable to the second helical blade  44   a  is applied to the developer by the regulating portion  52 . The developer increases its height by being blocked by the regulating portion  52 , and the surplus developer is discharged over the regulating portion  52  via the developer discharge port  22   h  out of the developer container  22 . 
         [0052]      FIG. 4  is a sectional plan view of and around the developer discharge port  22   h  in the developing device  2  according to the embodiment, and  FIG. 5  is a side sectional view of and around the developer discharge port  22   h.  As shown in  FIGS. 4 and 5 , the regulating portion  52  is composed of two turns (two phases) of regulating blades spiraling in the opposite direction (the opposite phase) to the second helical blade  44   a.  In the partition  22   b,  a height adjustment opening  65  is formed at a position facing the downstream end of the second helical blade  44   a  with respect to the transport direction of developer inside the second transport chamber  22   d.    
         [0053]    As described above, switching between the full speed mode and the reduced-speed mode changes the rotation speed of the first spiral  43  and the second spiral  44  in the developing device  2 , thus abruptly changing the transport speed of developer inside the developer container  22 . As a result, uneven developer distribution occurs in the developer container  22 . Specifically, in the full speed mode, as a result of the rotation speed of the first and second spirals  43  and  44  being high, the height of the developer transported inside the second transport chamber  22   d  by the second helical blade  44   a  is larger than that in the reduced-speed mode. 
         [0054]    In the developing device  2  according to the embodiment, uneven developer distribution inside the developer container  22  can be suppressed by the height adjustment opening  65  formed in the partition  22   b.  That is, in the full speed mode, owing to the large height of the developer transported inside the second transport chamber  22   d,  the developer is transported through the downstream-side communication portion  22   f  and the height adjustment opening  65  (as indicated by a hollow arrow and a hatched arrow in  FIG. 4 ) to the first transport chamber  22   c.    
         [0055]    On the other hand, in the reduced-speed mode, owing to the small height of the developer transported inside the second transport chamber  22   d,  the developer is transported through the downstream-side communication portion  22   f  alone (as indicated by the white arrow in  FIG. 4 ) to the first transport chamber  22   c.  This helps reduce the amount of developer moving to the regulating portion  52  in the full speed mode in which the height of developer is large, and thereby reduce the difference between the amounts (discharge amounts) of developer transported over the regulating portion  52  to the developer discharge port  22   h  in the full speed mode and in the reduced-speed mode respectively. 
         [0056]    As described above, even when the height of the developer inside the developer container  22  varies, no excessive developer is transported over the regulating portion  52  to the developer discharge port  22   h,  and it is thus possible to stabilize the amount of developer discharged through the developer discharge port  22   h.  Thus, the stable weight of developer inside the developer container  22  can be kept substantially constant when the process speed of the image forming apparatus  1  is changed. Moreover, the stable weight of developer inside the developer container  22  can be kept substantially constant when a developing device  2  with the same specifications is incorporated in a plurality of kinds of image forming apparatuses  1  having different process speeds. 
         [0057]    In this embodiment, the opening dimension X of the height adjustment opening  65  in the developer transport direction (the horizontal direction) equals a range from the most downstream end of the second helical blade  44   a  to an end part of the circumferential surface (developer carriable region) of the magnetic roller  21  facing the developable region of the developing roller  20  (the region where toner can be fed to the image forming region of the photosensitive drums  11   a  to  11   d ). This permits the developer having an increased height inside the second transport chamber  22   d  to be efficiently transported into the first transport chamber  22   c  and to be stably fed to the developer carriable region of the magnetic roller  21  facing the developable region of the developing roller  20 . 
         [0058]    The opening dimension Y of the height adjustment opening  65  in the vertical direction equals a range from the top end of the partition  22   b  to the position of the top surface of the developer present in the reduced-speed mode. This allows the height of the developer inside the second transport chamber  22   d  in the full speed mode to be substantially the same as that in the reduced-speed mode. 
         [0059]    It is not always necessary to form the height adjustment opening  65  over the entire range of the above-described opening dimensions X and Y, and thus the height adjustment opening  65  may be an arbitrary size within the range of the opening dimensions X and Y. That is, the maximum opening dimension of the height adjustment opening  65  in the horizontal direction equals X, and the maximum opening dimension of the height adjustment opening  65  in the vertical direction equals Y. The shape of the height adjustment opening  65  is not limited to a rectangular shape; for example, the height adjustment opening  65  may be an inverted trapezoidal shape as shown in  FIG. 6  or may be step shaped such that a downstream-side end part of the height adjustment opening  65  communicates with the downstream-side communication portion  22   f  as shown in  FIG. 7 . 
         [0060]    The embodiment described above is in no way meant to limit the present disclosure, which thus allows for many modifications and variations within the spirit of the present disclosure. For example, the present disclosure is not limited to developing devices provided with a magnetic roller  21  and a developing roller  20  like those shown in  FIG. 2 ; it is applicable to various developing devices that use two-component developer containing toner and carrier. For example, the present disclosure is applicable exactly in the same manner also to developing devices adopting a system in which toner in a magnetic brush formed on a magnetic roller (developer carrying member) is attached to an electrostatic latent image on a photosensitive member. 
         [0061]    Although the above-described embodiment has dealt with a two-shaft transport type developing device provided with a first transport chamber  22   c  and a second transport chamber  22   d  arranged parallel to each other as developer circulation passages in a developer container  22 , the present disclosure is applicable also to a three-shaft transport type developing device provided additionally with a collection transport chamber in which developer removed from a magnetic roller  21  is collected to be fed back to the second transport chamber  22   d.    
         [0062]    Although in the above-described embodiment, a configuration is adopted where a regulating portion  52  comprises a helical blade spiraling in the opposite phase to a second helical blade  44   a,  this is in no way meant to limit the present disclosure; as the regulating portion  52 , a disk having a larger size than the opening of a developer discharge port  22   h  may be formed on a rotary shaft  44   b,  and may be arranged next to the developer discharge port  22   h.  Although in the above-described embodiment, use is made of a first stirring screw  43  having a first helical blade  43   a  continuously arranged on the circumferential surface of a rotary shaft  43   b  and a second stirring screw  44  having a second helical blade  44   a  continuously arranged on the circumferential surface of a rotary shaft  44   b,  the transport blade that transports developer is not limited to a helical blade; instead, use may also be made of, for example, a stirring/transporting member having a plurality of semicircular disks (circular disks divided in halves) alternatively arranged with a predetermined inclination angle on the circumferential surfaces of the rotary shafts  43   b  and  44   b.    
         [0063]    The present disclosure is applicable, not only to tandem-type color printers like the one shown in  FIG. 1 , but to various image forming apparatuses adopting a two-component developing system, such as digital and analog monochrome copiers, monochrome printers, color copiers, and facsimile machines. Below, by way of practical examples, the effect of the present disclosure will be described more specifically. 
       PRACTICAL EXAMPLES 
       [0064]    With a developing device  2  as shown in  FIG. 2 , experiments were conducted to see how the stable weight of developer inside the developer container  22  varied as the transport speed of developer was varied. The experiments were performed with respect to the image forming portion for cyan that included the photosensitive drum  11   d  and the developing device  2   d.    
         [0065]    In the experiments, as shown in  FIGS. 4 and 5 , a developing device  2  in which a rectangular height adjustment opening  65  was provided in the partition  22   b  with an opening dimension X of 15 mm in the transport direction and an opening dimension Y of 3.5 mm in the vertical direction was taken as a practical example of the present disclosure; a developing device  2  which had the same structure as the practical example of the present disclosure except that no height adjustment opening  65  was provided was taken as a comparative example. The developing devices  2  according to the present disclosure and the comparative example both employed a second spiral  44  that had a second helical blade  44   a  with an outer diameter of 14 mm and that had formed on it a regulating portion  52  composed of two turns of reverse helical blades with an outer diameter of 11 mm spiraling in the opposite direction to the second helical blade  44   a.  The second spiral  44  also had formed on it a discharge blade  53  with an outer diameter of 8 mm spiraling in the same direction as the second helical blade  44   a.    
         [0066]    The developer containers  22  (the first and second transport chambers  22   c  and  22   d ) in the developing devices  2  according to the present disclosure and the comparative example were each charged with 250 g of two-component developer containing positively charged toner having an average particle diameter of 6.7 μm and ferrite carrier. This amount was a predetermined amount with no surplus developer stored in the developer container  22 . The toner concentration (the weight ratio of toner to carrier, T/C) in the developer was 8%. Then, with the developing devices  2  driven, measurements were taken of the amounts (stable weights) of the developer present while the rotation rate of the first and second spiral  43  and  44  was increased stepwise. The  FIG. 8  shows the results. 
         [0067]    As will be clear from  FIG. 8 , in the developing device  2  according to the present disclosure (the data series indicated by hollow circular symbols in  FIG. 8 ), variation in the stable weight of developer in the developer container  22  was within 10 g as the rotation rate of the first and second spirals  43  and  44  was varied from 220 rpm to 640 rpm. 
         [0068]    By contrast, in the developing device  2  according to the comparative example (the data series indicated by hollow rhombic symbols in  FIG. 8 ), variation in the stable weight of developer in the developer container  22  increased to 30 g as the rotation rate of the first and second spirals  43  and  44  was varied from 220 rpm to 640 rpm. 
         [0069]    The above results confirm that in the developing device  2  according to the present disclosure, where the height adjustment opening  65  is provided in the partition  22   b,  as compared with the developing device  2  according to the comparative example, where no height adjustment opening  65  is provided, the stable weight of developer varies quite stably irrespective of the stirring speed. Thus, by use of the developing device according to the present disclosure, it is possible to obtain stabilized developing performance and also to effectively suppress image defects and unnecessary discharge of developer. 
         [0070]    The present disclosure finds application in developing devices incorporated in image forming apparatuses exploiting electrophotography, such as copiers, printers, facsimile machines, multifunction peripherals thereof, etc., and in image forming apparatuses provided with such developing devices. In particular, the present disclosure finds application in developing devices which can be replenished with new two-component developer containing magnetic carrier and toner and meanwhile discharge surplus developer, and in image forming apparatuses provided with such developing devices.