Abstract:
A development agent circulation unit includes a development unit, an agitation unit, and a rotary feeder. The development unit develops a latent image on an image carrier using a developing agent. The agitation unit agitates developing agent recovered from the development unit. The rotary feeder receives the developing agent from the agitation unit and discharges the developing agent in predetermined discrete amounts. The discharged developing agent is transported to the development unit using a gas stream. The rotary feeder includes a rotor and a stator having a clearance “t” between the rotor and the stator. The clearance “t” satisfies a relation “t&lt;2D” where D denotes a developing agent particle diameter, and a toner particle diameter dt of a toner particle of the developing agent and a carrier particle diameter dc of a carrier particle of the developing agent satisfy a relation D=dc+2dt.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese patent application No. 2007-147305, filed on Jun. 1, 2007 in the Japan Patent Office, the entire contents of which are hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure generally relates to a development unit for developing an electrostatic latent image on an image carrier, and an image forming apparatus employing the development unit. 
     2. Description of the Background Art 
     Typically, an image forming apparatus using electrophotography employs a development unit to develop an electrostatic latent image formed on an image carrier using a developing agent, such as a two-component developing agent mainly composed of toner and carrier. The development unit has an internal configuration designed to recover the developing agent, which consumes toner component at a development area for a development process, to mix and agitate the recovered developing agent and refilled toner, and to use such agitated developing agent for another developing process. The developing agent used in such configured development unit needs to maintain toner concentration and toner charge at a given level so as to produce a good toner images consistently over time. 
     The toner concentration in the development unit is maintained at a given level by adjusting a refill toner amount so as to exactly offset or balance an amount of toner consumed by a developing process. The toner charging amount can be generated by a frictional electrification effect produced between carrier and toner when the carrier and the toner are mixed. In such development unit, a two-component developing agent is sufficiently agitated to evenly disperse the toner and the carrier to uniformly distribute toner concentration in the development unit and to charge the toner to a given level so as to enable toner images to be reliably formed. 
     In one type of conventional development unit, two rotating screws are used to agitate the refilled toner, and to diffuse and charge the toner before the refilled toner is carried up to a developing sleeve, so that such agitation may be conducted within a short period of time. A drawback of such conventional development unit is that there is a possibility that too much toner may be refilled because such agitation is conducted in a relatively short time. If the refilled toner is carried up to the developing sleeve when not effectively dispersed, fogging and toner scattering may occur, degrading image quality. 
     In light of such drawback, in one known arrangement, the development unit is connected to a separate agitation unit, disposed separately from the development unit, and the development unit and the agitation unit are connected by a developing agent circulation system. In the agitation unit, the developing agent is agitated based on a condition of the developing agent so as to supply developing agent having a toner concentration and charge adjusted to a preferable level to the development unit. Such adjusted developing agent is transported to the development unit using air pressure while a rotary feeder of the agitation unit regulates the amount of the developing agent discharged to the development unit. 
     In such configuration, an agent storage unit, an agent supply unit, a transport tube, and an air supply source are provided to continuously transport the developing agent using air pressure through the tube. 
     Because the developing agent is transported using a stream of gas (e.g., an air stream) having positive pressure, a pressure difference occurs between the air supply source and the development unit that is the transport destination at atmospheric pressure. Because the developing agent in the developing unit is transported (or circulated) to the agent storage unit, the agent storage unit is also at atmospheric pressure. Accordingly, to transport the developing agent to the developing unit from the agitation unit, air leakage to the agent supply unit needs to be suppressed by sealing the agent supply unit, by which air leakage from the air supply source to the agent storage unit is also prevented. 
     Any leakage of air reduces the air pressure used for transporting the developing agent, which can cause the amount of developing agent transported to be insufficient. Further, if the air backflows to the agent storage unit (i.e., pressure is applied to the agent storage unit), discharge of the developing agent from the agent storage unit to the agent supply unit is blocked by such backflowing air, again reducing the amount of developing agent discharged as well as causing that amount to fluctuate uncontrollably. 
     The agent supply unit usually employs a rotary feeder to supply the developing agent, and such rotary feeder usually includes a rotor having a plurality of vanes thereon, and a stator for encasing the rotor. Although the rotary feeder can reliably supply the developing agent, air backflow to the agent supply unit may occur due to insufficient sealing of the agent supply unit. The seal may be enhanced by making the vanes of the rotor elastic so that the vanes can be effectively pressed against the stator. However, such configuration may accelerate degradation of the rotor and the stator over time, through scraping of the rotor and the stator or the like, which is undesirable. Because the carrier component of the developing agent is made of harder material than the toner, such as iron, ferrite, or the like, such vane-impressing configuration does not provide adequate durability. 
     In light of the above-described drawbacks, an image forming apparatus that can continuously supply a developing agent to a developing unit efficiently and effectively is desired. 
     SUMMARY 
     In an aspect of the present disclosure, a development agent circulation unit includes a development unit, an agitation unit, and a rotary feeder. The development unit develops a latent image on an image carrier using a developing agent. The agitation unit, disposed separately from the development unit, agitates developing agent recovered from the development unit. The rotary feeder receives the developing agent from the agitation unit and discharges the developing agent in predetermined discrete amounts. The discharged developing agent is transported to the development unit using a gas stream under a given pressure. The rotary feeder includes a rotor and a stator and has a clearance “t” between an external diameter of the rotor and an internal diameter of the stator. The clearance “t” satisfies a relation “t&lt;2D” where D denotes a developing agent particle diameter, and a toner particle diameter dt of a toner particle of the developing agent and a carrier particle diameter dc of a carrier particle of the developing agent satisfy a relation D=dc+2dt. 
     In another aspect of the present disclosure, a development agent circulation unit includes a development unit, an agitation unit, and a rotary feeder. The development unit develops a latent image on an image carrier using a developing agent. The agitation unit, disposed separately from the development unit, agitates developing agent recovered from the development unit. The rotary feeder receives the developing agent from the agitation unit and discharges the developing agent in predetermined discrete amounts. The discharged developing agent is transported to the development unit using a gas stream under a given pressure. The rotary feeder includes a rotor and a stator and has a clearance “t” between an external diameter of the rotor and an internal diameter of the stator. The clearance “t” satisfies a relation “t&lt;D” where D denotes a developing agent particle diameter, and a toner particle diameter dt of a toner particle of the developing agent and a carrier particle diameter dc of a carrier particle of the developing agent satisfy a relation D=dc+2dt. 
     In still another aspect of the present disclosure, an image forming apparatus includes a development unit, an agitation unit, and a rotary feeder. The development unit develops a latent image on an image carrier using a developing agent. The agitation unit, disposed separately from the development unit, agitates developing agent recovered from the development unit. The rotary feeder receives the developing agent from the agitation unit and discharges the developing agent in predetermined discrete amounts. The discharged developing agent is transported to the development unit using a gas stream under a given pressure. The rotary feeder includes a rotor and a stator and has a clearance “t” between an external diameter of the rotor and an internal diameter of the stator. The clearance “t” satisfies a relation “t&lt;2D” where D denotes a developing agent particle diameter, and a toner particle diameter dt of a toner particle of the developing agent and a carrier particle diameter dc of a carrier particle of the developing agent satisfy a relation D=dc+2dt. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG. 1  illustrates a schematic cross-sectional view of an image forming apparatus according to an exemplary embodiment; 
         FIG. 2  illustrates a perspective view of a development unit and an agitation unit used in the image forming apparatus of  FIG. 1 ; 
         FIG. 3A  illustrates a cross-sectional view of the agitation unit used in the image forming apparatus of  FIG. 1 ; 
         FIG. 3B  illustrates a cross-sectional view of an agitation unit, cut in a horizontal direction at line C-C; 
         FIG. 4  illustrates a convection flow of a developing agent in the agitation unit; 
         FIG. 5  illustrates a cross-sectional view of the development unit in the image forming apparatus of  FIG. 1 ; 
         FIG. 6A  illustrates a relationship of a clearance of rotor/stator and sealing performance in a conventional art; 
         FIGS. 6B and 6C  illustrate relationships of a clearance of rotor/stator and sealing performance according to exemplary embodiments; and 
         FIG. 7  shows a graph indicating a relationship of a clearance of rotor/stator and transportation amount of developing agent, obtained by experiment. 
     
    
    
     The accompanying drawings are intended to depict exemplary embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted, and identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A description is now given of exemplary embodiments of the present invention. It should be noted that although such terms as first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that such elements, components, regions, layers and/or sections are not limited thereby because such terms are relative, that is, used only to distinguish one element, component, region, layer or section from another region, layer or section. Thus, for example, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     In addition, it should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. Thus, for example, as used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Furthermore, although in describing expanded views shown in the drawings, specific terminology is employed for the sake of clarity, the present disclosure is not limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
     Referring now to the drawings, an image forming apparatus employing a development unit according to an exemplary embodiment is described with reference to  FIGS. 1 to 7 . The image forming apparatus may employ electrophotography, for example, but not limited thereto. 
     As illustrated in  FIG. 1 , an image forming apparatus  100  according to an exemplary embodiment includes image forming engines  6 Y,  6 M,  6 C,  6 K, and an intermediate transfer unit  10 , for example. The intermediate transfer unit  10  includes an intermediate transfer belt  8  as an image carrying member for carrying an unfixed toner image thereon. The image forming engines  6 Y,  6 M,  6 C, and  6 K are arranged in a tandem manner below the intermediate transfer belt  8 . The image forming engines  6 Y,  6 M,  6 C, and  6 K have a similar configuration one another except toner color used for image forming process of each of colors of yellow, magenta, cyan, and black, respectively. Hereinafter, the image forming engine  6  may be used to indicate each one of the image forming engines  6 Y,  6 M,  6 C, and  6 K. The suffixes of Y, M, C, and K respectively indicate color of yellow, magenta, cyan, and black in this disclosure. The image forming engine  6  includes a photoconductor drum  1  as an image carrier, surrounded by a charging device (not shown), a development unit  50 , and a cleaning device (not shown), for example. 
     An image forming process is conducted on the photoconductor drum  1  to form a desired toner image thereon, wherein the image forming process includes a charging process, an exposure process, a developing process, a transfer process, and a cleaning process, for example. The photoconductor drum  1  is rotated in a clockwise direction in  FIG. 1  by a driving unit (not shown), and then the charging device uniformly charges a surface of the photoconductor drum  1  (charging process). An optical writing unit (not shown) emits a laser beam to form an electrostatic latent image on the photoconductor drum  1  (exposure process). The electrostatic latent image is then developed by the development unit  50  to form a desired toner image on the photoconductor drum  1  (developing process). The toner image is primary transferred from the photoconductor drum  1  to the intermediate transfer belt  8  when the surface the photoconductor drum  1  comes to a position of the intermediate transfer belt  8  and a primary transfer roller  9  (primary transfer process). After transferring the toner image, the surface of the photoconductor drum  1  is cleaned by the cleaning device to recover toner remaining on the photoconductor drum  1  (cleaning process). After such cleaning process, the surface of the photoconductor drum  1  is de-charged by a de-charge roller (not shown) to prepare the photoconductor drum  1  for another image forming process. With such processes, one cycle of image forming process on the photoconductor drum  1  completes. 
     Such image forming process is conducted on each one of the image forming engines  6 Y,  6 M,  6 C, and  6 K. The optical writing unit (not shown), disposed below the image forming engines  6 Y,  6 M,  6 C, and  6 K, emits laser beams corresponding to each of color image data to the photoconductor drum  1  of the respective image forming engines  6 Y,  6 M,  6 C, and  6 K. The toner images formed on the photoconductor drum  1  in the developing process are superimposingly transferred onto the intermediate transfer belt  8  to form a color image on the intermediate transfer belt  8 . 
     The primary transfer rollers  9 Y,  9 M,  9 C, and  9 K and the photoconductor drums  1 Y,  1 M,  1 C, and  1 K sandwiches the intermediate transfer belt  8  therebetween to form a primary transfer nip. The primary transfer rollers  9 Y,  9 M,  9 C, and  9 K are supplied with a transfer bias voltage having a polarity opposite to a toner polarity. The intermediate transfer belt  8  travels in a direction shown by an arrow, and sequentially passes through the primary transfer nip. At the primary transfer nip, the toner images on the photoconductor drums  1 Y,  1 M,  1 C, and  1 K are superimposingly transferred to the intermediate transfer belt  8  by the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K. 
     Then, the intermediate transfer belt  8  having the superimposed toner images comes to a position of a secondary transfer nip, set by a secondary transfer roller  19  used as a secondary transfer device. At the secondary transfer nip, the toner image formed on the intermediate transfer belt  8  is transferred to a transfer sheet P used as a recording medium. With such processes, one cycle of transfer process for the intermediate transfer belt  8  completes. 
     The image forming apparatus  100  includes a sheet feed unit  26  at its lower part. The sheet feed unit  26  stackingly stores a given volume of transfer sheet P, from which a feed roller  27  feeds the transfer sheet P one by one to a registration roller  28 , at which the transfer sheet P is temporarily stopped. After correcting the orientation of the transfer sheet P, such as orientation of slanted sheet, the registration roller  28  transports the transfer sheet P to the secondary transfer nip at a given timing. At the secondary transfer nip, a desired color image is transferred on the transfer sheet P by the secondary transfer roller  19 . 
     After transferring the color image to the transfer sheet P at the secondary transfer nip, the transfer sheet P is transported to a fixing unit  20 , in which a fixing roller and a pressure roller apply heat and pressure to the transfer sheet P to fix the color image on the transfer sheet P. After fixing the color image on the transfer sheet P, the transfer sheet P is ejected to and stacked on an ejection tray  30  by an ejection roller  29 . With such processes, one cycle of image forming process of the image forming apparatus  100  completes. The image forming apparatus  100  may also include a scanning unit  32  as shown in  FIG. 1 . 
     A description is now given to a configuration of a developing agent agitation/circulation system including the development unit  50  with reference to  FIGS. 2 to 5 .  FIG. 1  shows the development unit  50  of the developing agent agitation/circulation system. 
     As illustrated in  FIG. 2 , the developing agent agitation/circulation system includes the development unit  50 , an agitation unit  51 , a toner cartridge  52 , a rotary feeder  53 , and an air pump  54 , for example. The development unit  50  develops an electrostatic latent image on the photoconductor drum  1 . The agitation unit  51  agitates the developing agent (hereinafter, the developing agent may be referred as “agent”) based on a condition of the developing agent. The agitation unit  51  is separated and distanced from the development unit  50 . The toner cartridge  52  stores toner to be refilled to the agitation unit  51 . The rotary feeder  53  is disposed below the agitation unit  51 . The air pump  54  generates an air pressure used for transporting or circulating the developing agent, in which gas other than air may be used as required. 
     The development unit  50  and the agitation unit  51  are connected by a circulation tube  55 . The rotary feeder  53  and the development unit  50  are connected by a circulation tube  56 . The toner cartridge  52  and the agitation unit  51  are connected by a toner supply route  57 . The air pump  54  and the rotary feeder  53  are connected by a tube  58 . In  FIG. 2 , a motor  59  drives the toner cartridge  52 , a motor  60  drives the agitation unit  51 , and a motor  61  drives the rotary feeder  53 . 
     As illustrated in  FIG. 5 , the development unit  50  includes a casing  62 , transport screws  63  and  64 , and a developing roller  65 . The transport screws  63  and  64  having spiral fins are rotatably supported in the casing  62 . The casing  62  includes a two-component developing agent mainly composed of toner and carrier. The transport screws  63  and  64  circulate and transport the developing agent in the casing  62 . The transport screw  63  transports the developing agent in one direction, and some of the developing agent is carried up to the developing roller  65  with an effect of magnetic force of the developing roller  65 . The developing agent is then leveled to a uniform thickness on the developing roller  65  by a doctor blade  66 . Such developing agent is used to develop an electrostatic latent image on the photoconductor drum  1  as a toner image. 
     The developing agent used for a developing process is ejected from the development unit  50  via an ejection port  67  (see  FIG. 2 ), disposed at one end of the transport screw  64 , to the agitation unit  51  through the circulation tube  55 . 
     A toner concentration sensor (not shown) may be disposed at a most downstream of the transport screw  64 . Based on signals of the toner concentration sensor, the toner cartridge  52  is activated to refill toner. The toner cartridge  52  is driven by the motor  59 , wherein the motor  59  rotates a screw (not shown) in a toner supply route  57  to feed fresh refill toner to the agitation unit  51 . The toner is refilled from the toner cartridge  52  to the agitation unit  51  at a portion disposed at an upper portion of the agitation unit  51 . In  FIG. 2 , the toner supply route  57  is connected to the circulation tube  55  which is used for transporting the used developing agent to the agitation unit  51 , for example. 
     With such configuration, the developing agent used for the developing process and the fresh refill toner are mixed, and thereby a developing agent having a good level of toner concentration and charging amount can be supplied to the agitation unit  51 . Such developing agent passes through an agent exit port  70  disposed at the bottom of the agitation unit  51 , and enters the rotary feeder  53 . 
     The rotary feeder  53  includes a rotor  75 , which rotates to discharge the developing agent in predetermined discrete amounts to a downward direction. The discharged developing agent passes through the circulation tube  56 , and is then supplied to the development unit  50  again via an inlet port  68 . 
       FIG. 3A  illustrates a cross-sectional view of the agitation unit  51 . The agitation unit  51  includes an agitation vessel  51   a  having an agent supply port  69  at its upper face and an agent exit port  70  at its bottom face. The agitation vessel  51   a  has an inverted cone shape, for example. Specifically, the closer to the agent exit port  70 , the diameter of the agitation vessel  51   a  becomes smaller. The agitation vessel  51   a  includes a screw  71 , and an agitation member  72 , for example. As illustrated in  FIGS. 3A and 3B , the screw  71  is disposed at a center portion of the agitation vessel  51   a , and the agitation member  72  is disposed near an internal periphery of the agitation vessel  51   a . In an exemplary embodiment, two agitation members  72  are disposed, for example. The screw  71  transports the developing agent from lower side to upper side, and the two agitation members  72  rotate around the screw  71 . Such screw  71  and agitation members  72  rotate to agitate and mix the developing agent in the agitation vessel  51   a . The motor  60  rotates the agitation members  72  and the screw  71 . The screw  71  is directly coupled to the motor  60 , and the agitation members  72  are rotated using speed-reduction gears  73   a  to  73   d . As illustrated in  FIGS. 3A and 3B , the agitation members  72  is fixed to a support base  74  with setting some angle, in which the support base  74  is directly coupled to the speed-reduction gears  73   a  to  73   d.    
     The developing agent is transported from the agent supply port  69  to the agent exit port  70  in the agitation unit  51  using gravity force. Because the agitation unit  51  may not become empty (i.e., some developing agent exists in the agitation unit  51 ), a developing agent not mixed with fresh refill toner is not discharged from the agent exit port  70 . 
     The rotary feeder  53  includes a rotor  75  and a stator  76 . The rotor  75  has a plurality of vanes  75   a  extending in a radial direction, and the stator  76  encases the rotor  75 , which is rotated by the motor  61 . A joint tube  77  connects the rotary feeder  53 , the circulation tube  56 , and the tube  58 . 
       FIG. 4  illustrates a schematic view for describing a flow stream of developing agent in the agitation unit  51  when the developing agent is agitated. The screw  71  rotates to push up the developing agent from the lower side to the upper side in a direction shown by an arrow A. Such pushed-up developing agent then moves to a downward direction shown by an arrow B with a rotation of the agitation members  72 , and then accumulates again around the screw  71 . As such, the developing agent is consistently convecting in the agitation unit  51  to evenly mix the developing agent in the agitation vessel  51   a . Because electrical charging of toner can be generated by friction of toner and carrier, it is better to increase contact probability of toner and carrier to increase charging speed or charging amount of toner. Based on the research for this disclosure, it was confirmed that convecting the developing agent in the agitation unit  51  can increase contact probability of toner and carrier, and damages to the developing agent can be reduced. 
     A description is now given to a configuration of the rotary feeder  53  with reference to  FIG. 6 . As illustrated in              6 , a leading edge of the vane  75   a  of the rotor  75  and an interior surface (or interior wall) of the stator  76  face each other across a clearance “t.” When a diameter of the developing agent is set to “D,” the clearance “t” is preferably set in a relationship of “t&lt;2D” as shown in  FIG. 6B , wherein the diameter D of the developing agent is defined as below.
 
 D=dc+ 2 dt,  
 
in which a toner particle diameter is “dt,” and a carrier particle diameter is “dc,” and “dt” is an average particle diameter of toner and “dc” is average particle diameter of carrier, and the average particle diameter is a volume average particle diameter.

     If a clearance exists between the rotor  75  and the stator  76 , some of the air generated by the air pump  54  may pass through the clearance “t” and enter the agitation unit  51  in a direction from a lower side to a upper side in  FIG. 6 , by which an air amount used for transporting the developing agent to the development unit  50  is decreased (i.e., air pressure is decreased). The greater the clearance “t,” the more air can pass through the clearance “t.” 
     When the developing agent is discharged from the agitation unit  51 , the developing agent enters the clearance, by which a sealing effect (or performance) can be generated, and the air leakage can be reduced. However, if the clearance becomes too great, such sealing effect cannot be attained, and the air leakage cannot be prevented. Accordingly, in order to efficiently transport the developing agent discharged from the rotary feeder  53 , an air intrusion to the agitation unit  51  is required to be set as low as possible, wherein the air is generated by the air pump  54 . Accordingly, the aforementioned clearance “t” needs to be set to a given level to effectively transport or circulate the developing agent. 
     An experiment was conducted to evaluate a relationship between the clearance “t” and transportation amount of developing agent, which is shown in  FIG. 7 . As shown in  FIG. 7 , when the clearance “t” becomes 0.08 mm (80 μm) or greater, the transportation amount of developing agent decreases rapidly, which may mean that an air leakage to the agitation unit  51  increases. The average particle diameter of toner and carrier used in the experiment was 5 μm and 35 μm, respectively, and thereby the developing agent particle had a particle diameter D of 45 μm, for example. Therefore, if the clearance “t” becomes greater than about two particles of developing agent (90 μm), the air leakage becomes greater, and the transportation amount of developing agent decreases. If the clearance “t” becomes greater than about two particles of developing agent, more than two particles can exist in the clearance “t” between the rotor  75  and the stator  76  (see  FIG. 6A ). In such a case, the developing agent particles in the clearance “t” can be moved easily by air pressure. Especially, a developing agent particle in the middle of the developing agent particles can be moved easily by air pressure. The developing particle agent is composed of carrier and toner, coated on the carrier. Because such toner on the carrier may function as a spacer (or roller), the developing agent particle sandwiched by other developing agent particles can be moved easily. Accordingly, under such condition, an air leakage may occur easily, by which a transportation amount of developing agent decreases. 
     If the clearance “t” is less than 2D (t&lt;2D) as shown in  FIG. 6B , a developing agent particle is not sandwiched by other developing agent particles, and the developing agent particle is not moved easily, and thereby an air leakage can be decreased. Further, if the clearance “t” is less than D (t&lt;D) as shown in  FIG. 6C , an air leakage can be further decreased as indicated by the experiment result shown in  FIG. 7 , and if the clearance “t” is less than D (t&lt;D), damages to the developing agent can be decreased. The toner sandwiched in the clearance “t” between the rotor  75  and the stator  76  may be degraded by friction with the rotor  75  and the stator  76 . However, if the clearance “t” is less than D (t&lt;D), probability of such toner sandwiching phenomenon between the rotor  75  and the stator  76  may be reduced significantly. 
     As illustrated in  FIG. 6 , if the leading edge of the vane  75   a  has a rounded leading edge (i.e., rounded shape) in cross-section along an axial direction of rotation of the rotor  75 , the developing agent may be less likely sandwiched in the clearance “t,” by which damages to the developing agent can be reduced. Further, at least one of the rotor  75  and the stator  76  can be made of a material softer than carrier, preferably softer than toner, such as resin material, elastic material to reduce damages to the developing agent. Further, to reduce damages to the developing agent, a surface roughness Rmax of the interior surface of the stator  76  can be set to the diameter dt of toner particle or less (Rmax&lt;dt). The surface roughness is arithmetic mean deviation of the profile defined by JIS B 0601-2001, which is one of the standards of Japan Industrial Standard. With such setting for surface roughness of the stator  76 , toner may not adhere and accumulate on the interior surface of the stator  76  easily over time, by which the toner may not receive stress from the rotary feeder  53 . Further, an adhesion of the rotor  75  and the stator  76  can be also prevented. 
     As above described, in an exemplary embodiment, the developing agent can be agitated with lesser stress, and the toner can be preferably charged, by which the image forming apparatus can produce a higher quality images. 
     Further as above described, in an exemplary embodiment, because an intrusion of air, used for transportation of developing agent, to the agitation unit can be effectively prevented, the developing agent can be discharged from the agitation unit reliably, and the developing agent can be effectively transported to the development unit, by which the image forming apparatus can produce a higher quality images. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different examples and illustrative embodiments may be combined each other and/or substituted for each other within the scope of this disclosure and appended claims.