Patent Publication Number: US-7587154-B2

Title: Developing unit and image forming apparatus using the same

Description:
This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2007-3954 filed in Japan on 12 Jan. 2007, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE TECHNOLOGY 
   1. Field of the Technology 
   The present technology relates to a developing unit used in an image forming apparatus such as a copier, printer, facsimile machine or the like using electrophotography as well as relating to an image forming apparatus including this developing unit. The technology in particular relates to a developing unit including a cooling mechanism of the developing vessel and a suctioning mechanism of scattering toner around the developing area using ducts. 
   2. Description of the Prior Art 
   With recent development of copiers, facsimile machines, printers and multi-functional machines having these functions into high-speed and high-resolution configurations, down-sizing of the carrier and toner used as the elemental technology for the electrophotographic process has been in progress. However, as the carrier and toner become smaller in size, in the developing area inside the image forming apparatus the carrier particles are prone to transfer to the image latent bearer, i.e., a photoreceptor drum during development, also toner is prone to scatter, causing image degradation and defects. Further, increase in processing speed brings about temperature increase in the developing unit, changes the developer characteristics and degrades the image quality, hence gives rise to the problem that the developer becomes solidified inside the developing unit and completely locks the developing unit itself, especially when the machine is used under a high temperature and high humidity environment. 
   In order to solve the above deficiencies, patent document 1 (Japanese Patent Application Laid-open Hei No. 10-274883) discloses a developing unit including: a developing vessel for holding a developer; a magnet roller disposed inside this developing vessel for supplying the developer to a photoreceptor drum; and a sanctioning fan with its suctioning ports arranged at both ends of the magnet roller so as to suction the developer scattering from the magnet roller through the suctioning ports. This developing unit enables efficient suctioning of scattering developer using a compact suctioning device and also enables reduction of the photoreceptor drum in diameter. 
   Another patent document 2 (Japanese Patent Application Laid-open 2006-23413) discloses an image forming apparatus which includes a developing unit having an integrally layered structure of a cooling duct and a scattering toner suctioning duct. That is, in this image forming apparatus, a cooling duct is provided in a space defined by the developing unit and a paper feed path for feeding recording paper to a transfer unit so as to cool the developing unit by sending cooling air into the duct while the cooling duct is formed with an opening that opposes the paper feed path so as to blow the cooling air to the recording paper that passes through the paper feed path. Further, a scattering toner suctioning duct for suctioning the toner scattering is formed under the cooling duct. With this image forming apparatus, it is possible to inhibit increase in the temperature inside the apparatus due to its heat-fixing process without hindering downsizing of the apparatus and reduction in the number of parts. 
   There is also another disclosure of an image forming apparatus in patent document 3 (Japanese Patent Application Laid-open 2004-361869), which includes a developer conveyer that has the developer electrostatically attracted to the electrostatic latent image formed on an image bearer; a housing that has a space for accommodating the developer conveyer and the developer and is formed with an opening parallel with the image bearer; a suctioning duct for suctioning floating toner and paper particulates in this housing; and a cooling duct for cooling the developing unit, wherein air is suctioned from the suctioning duct and cooling duct by a common suctioning means. In this image forming apparatus, it is possible to prevent increase in temperature while reducing toner scattering in the developing unit. 
   In the developing unit disclosed in patent document 1, a suctioning path for drawing scattering toner from below, and at both ends of, magnet (developing) roller and exhausting air through a filter located at the center is provided. However, there is no reference to a carrier collecting member. Further the suctioning means cannot suction the carrier and toner separately, so it is impossible to avoid carrier loss and filter clogging. Also, there is no mention of inclusion of any cooling means, so it is not clear whether cooling effect can be obtained or not. 
   In the image forming apparatus of patent document 2, the suctioning means and the cooling means are structured in two layers. However, there is no reference to a carrier collecting means, so it is impossible for the suctioning means to suction the carrier and toner separately from each other. Accordingly, reduction of the carrier in the developer and suctioning of toner together with the carrier cause the filter to be clogged up with the carrier. Further, since the cooling means is constructed to send air to the paper feed path but is not laid out to cool the entire bottom of the developing unit, it cannot be said that the cooling effect is sufficient. 
   Moreover, in the image forming apparatus of patent document 3, since the suctioning means and the cooling means are laid out in layers, but the cooling means is not constructed so as to cool the entire bottom of the developing unit, the cooling of the developing vessel is not effective enough. Also, there is no reference to a carrier collecting means, so it is impossible to suction the carrier and toner separately from each other. That is, this configuration has not reached to such a level as to solve the problems of carrier reduction, filter clogging and the like. 
   In sum, the developing units and image forming apparatuses disclosed in the above patent documents have yet to reach to such a level as to be able to prevent toner scattering as well as to realize effective enough cooling of the developing vessel and inhibit increase of the developing vessel in temperature. Also, since carrier and toner cannot be suctioned separately, any of the above configurations has not reached to so far as to solve the problems of carrier reduction and filter clogging. 
   SUMMARY OF THE TECHNOLOGY 
   The present technology has been devised in view of the above problems entailed with the conventional developing units, it is therefore an object of the present technology to provide a new and improved developing unit and image forming apparatus which can inhibit toner scattering and temperature increase of the developing vessel and which can suction toner and carrier separately from the developer. 
   In order to solve the above problem, a developing unit for developing an electrostatic latent image formed on the outer peripheral surface of a latent image bearing member rotating in one direction with an electrified developer that is prepared by mixing two components, or an electrostatically chargeable toner and magnetic carrier in a developing vessel, includes: a developing roller for conveying the developer to the latent image bearing member; a carrier collecting roller disposed on the downstream side of the developing roller with respect to the rotational direction of the latent image bearing member; a cooling portion including a forced draft duct for airflow from a fan and disposed on the bottom side of the developing vessel; a suctioning portion including a suctioning duct for airflow from suctioning ports disposed on the lower side of the carrier collecting roller for drawing scattering toner inside the developing unit, disposed under the forced draft duct; a filter for removing the scattering toner from air including the scattering toner suctioned from the suctioning ports; and, an exhaust portion including an airflow exhaust duct for discharging the air having passed through the filter and the air having passed through the forced draft duct to an exhaust port. 
   With this configuration, it is possible to collect the scattering toner that is sufficiently separated from the carrier by suctioning the toner scattering around the developing area where the photoreceptor drum as the latent image bearing member and the developing roller abut each other, from the lower side of the carrier collecting roller. Accordingly, it is possible to prevent filter clogging and reduction of the carrier, which would occur when the toner together with carrier particles is suctioned. Further, it is possible to obtain a cooling effect of the developing unit by applying the air from the duct onto the bottom of the developing vessel through the forced draft duct. 
   In addition to the above configuration, in the cooling portion, a forced draft entrance as an inlet of the forced draft duct may be arranged on one end side with respect to the axial direction of the developing roller and a forced draft exist as an outlet of the forced draft duct may be arranged on the other end side, and the forced draft duct may be formed so as to cover substantially the whole surface of the bottom of the developing vessel. 
   With the above arrangement, since the bottom of the developing unit can be cooled as a whole by arranging the cooling portion and the suction portion separately in two layers, it is possible to markedly enhance the cooling effect of the developing unit. 
   In addition to the above configuration, the suctioning ports may be arranged on the lower side at both ends of the carrier collecting roller. 
   With the above configuration, suctioning can be performed selectively from both ends of the carrier collecting roller, at which toner scattering is most prone to occur, it is hence possible to prevent any wasteful increase in the suctioning load on the suctioning portion. 
   In addition to the above configuration, it is possible to provide a configuration such that the air streams containing the scattering toner suctioned from the suctioning ports are converged to one at the previous stage of the filter and led through the filter, and the filter has a structure that can be inserted into and removed from the suctioning portion. 
   With the above configuration, since the suctioning duct is formed so as to suction air from both ends towards the center to thereby realize more smooth suction of scattering toner, this configuration makes it possible to prevent toner and the like from building up inside the suctioning duct as well as to make filter maintenance easy. 
   In addition to the above configuration, the suctioning duct and the forced draft duct may be joined to a single airflow passage, and the fan may be arranged at only one place inside the single airflow passage. 
   With this configuration, provision of a single fan at the exhaust port makes it possible for the fan to be used for both cooling and suction. 
   In addition to the above configuration, the turning parts of the suctioning duct, the cooling duct and the exhaust duct may be formed with rounded corners. 
   With the above configuration, flow of air in each duct passage can be smoothened so that the effect of suctioning, cooling and exhausting can be enhanced more distinctively. 
   In addition to the above configuration, the bottom of the developing vessel may be formed of aluminum. 
   Since this configuration enhances the cooling effect, it is possible to prevent change of the developer characteristics, change of image quality and troubles of the developing unit due to solidification of the developer inside the developing unit, which would be caused by change of the temperature around the developing unit. 
   In addition to the above configuration, a pressure adjustment filter for adjusting the pressure inside the developing vessel may be provided in the developing vessel. 
   Since the filter works as a pressure reliever when the pressure inside the developing unit has increased, this configuration can suppress toner scattering. 
   In order to solve the above problems, the present technology resides in an image forming apparatus including the above described developing unit. 
   With the above configuration, the occurrence of toner scattering around the developing area in the image forming apparatus during development can be suppressed even with use of the carrier and developer that are downsized in diameter, hence it is possible to reduce the printout degradation of the images formed on recording paper etc. in the image forming apparatus. Further, since a sufficient cooling effect against temperature increase of the developing unit due to high-speed operations can be obtained, it is possible to prevent change of developer characteristics and change of image quality and other problems due to change of the surrounding temperature of the developing unit. 
   As has been described above, it is possible to achieve sufficient cooling by performing suctioning of scattering toner and cooling of the developing vessel using separate channels. Further, since scattering toner inside the image forming apparatus is suctioned from the lower side at both ends of the carrier collecting roller, it is possible to suction the toner particles only after the sufficient separation of the carrier from the toner, which was previously not sufficient. As a result, it is possible to prevent filter clogging and carrier reduction which would occur when the toner is suctioned with inclusion of the carrier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustrative view showing an overall configuration of an image forming apparatus in which a developing unit is used; 
       FIG. 2  is a partial detailed view showing the configuration of the apparatus body of the image forming apparatus of the same embodiment; 
       FIG. 3  is a schematic configurational side view showing a developing unit and toner feed device that are arranged in the image forming apparatus of the same embodiment; 
       FIG. 4  is a sectional view showing the configuration of the developing unit of the same embodiment; 
       FIG. 5A  is a sectional side view showing a configuration of a mixing roller that constitutes the developing unit of the same embodiment; 
       FIG. 5B  is a sectional view cut along a plane B 1 -B 1 ′ in  FIG. 5A ; 
       FIG. 5C  is a sectional view cut along a plane B 2 -B 2 ′ in  FIG. 5A ; 
       FIG. 5D  is a sectional view cut along a plane B 3 -B 3 ′ in  FIG. 5A ; 
       FIG. 5E  is a sectional view cut along a plane B 4 -B 4 ′ in  FIG. 5A ; 
       FIG. 6  is a perspective view showing a configuration of an exterior wall  260   b  that defines a suctioning duct  250  in a base structure of the developing unit of the same embodiment; 
       FIG. 7  is a plan view schematically showing a base structure of a developing unit of the same embodiment when viewed from top; 
       FIG. 8  is a table showing various setting conditions in an aging test with a developing unit of the same embodiment; 
       FIG. 9  is a graph showing temperature increase of developing vessels in aging tests, making a comparison between when air cooling is performed, and when not performed, by a cooling portion; and, 
       FIG. 10  is a table showing comparative results between a developing unit incorporating the disclosed technology and a conventional developing unit. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments will hereinafter be described with reference to the accompanying drawings. In the specification and drawings herein, components having substantially the same functions and configurations are allotted with the same reference numerals so that repeated description will be omitted. 
   The configuration of the first embodiment of an image forming apparatus in which a developing unit is used will be described with reference to the drawings.  FIG. 1  is an illustrative view showing an overall configuration of the first embodiment of an image forming apparatus in which a developing unit of the present invention is used.  FIG. 2  is a partial detailed view showing the configuration of the apparatus body of the same image forming apparatus. 
   As shown in  FIGS. 1 and 2  an image forming apparatus  1 A is an image forming apparatus that processes image data captured by a scanner etc., or image data transmitted from without to output a monochrome (single color) image, based on the electrophotography, by forming an electrostatic latent image on a rotationally driven, cylindrical photoreceptor drum (latent image bearing member)  3 , developing the electrostatic latent image into a visualized developer image with an electrified developer prepared by mixing two components, or an electrostatically chargeable toner and magnetic carrier, and transferring the developed image to a predetermined sheet of recording paper (to be referred to as paper hereinbelow) as a recording medium. This image forming apparatus  1 A adopts, as its developing device for visualizing the electrostatic latent image on photoreceptor drum  3 , a developing unit  2 , which is distinctive in the present technology, including a carrier collecting roller  220  ( FIG. 2 ) for collecting the magnetic carrier having adhered on photoreceptor drum  3  and a carrier removing device (carrier remover)  230  ( FIG. 2 ) for removing the magnetic carrier collected by carrier collecting roller  220  therefrom. 
   This image forming apparatus  1 A includes a paper feed tray  8  which can stack multiple sheets of paper P ( FIG. 1 ) thereon; a paper conveying portion  59  for conveying paper P fed from this paper feed tray  8  to an image forming portion  14 ; and a paper conveyor system  7  for conveying the paper P with an unfixed toner image printed thereon by image forming portion  14  to a fixing unit  6  where the unfixed toner is fused and fixed onto the paper. The image forming apparatus, based on the conveying speeds of paper P corresponding to a multiple number of preset printout processing modes, can select and control the conveying speed of paper P in accordance with a print request and automatically convey paper P from paper feed tray  8  to a paper output tray  9 . 
   First, the overall configuration of image forming apparatus  1 A will be described. Image forming apparatus  1 A is essentially composed of, as shown in  FIG. 1 , an apparatus body  1 A 1  including a light exposure unit  1 , developing unit  2 , a toner feed device  30 , photoreceptor drum  3 , a charger  4 , a charge erasing device  41 , a cleaner unit  5 , a fixing unit  6 , paper conveyor system  7 , a paper feed path  7   a , paper feed tray  8 , paper output tray  9 , a transfer device  10  and the like, and an automatic document processor  1 A 2 . 
   Provided at the top of apparatus body  1 A 1  is an original placement table  21  made of transparent glass on which a document is placed. Automatic document processor  1 A 2  is arranged on the top of this original placement table  21  so that it can pivotally open upwards, while a scanner portion  22  as a document reader for reading image information of originals is arranged under this original placement table  21 . 
   Arranged below scanner portion  22  are light exposure unit  1 , developing unit  2 , photoreceptor drum  3 , charger  4 , charge erasing device  41 , cleaner unit  5 , fixing unit  6 , paper conveyor system  7 , paper feed path  7   a , paper output tray  9  and transfer device  10 . Further, paper feed tray  8  that accommodates paper P therein is arranged under these. 
   Light exposure unit  1  provides a function of emitting laser beam in accordance with the image data output from an unillustrated image processor to irradiate the photoreceptor drum  3  surface that has been uniformly electrified by charger  4  to thereby write and form an electrostatic latent image corresponding to the image data on the photoreceptor drum  3  surface. This light exposure unit  1  is arranged directly under scanner portion  22  and above photoreceptor drum  3 , and includes laser scanning units (LSUs)  13   a  and  13   b  including laser emitters  11  and a reflection mirror  12 . In the present embodiment, in order to support high-speed printing operation, multiple laser beams from multiple laser emitters  11  are used to reduce the irradiation frequency of each laser beam (the processing load of each laser beam per unit time is reduced). More specifically, a two-beam technique using a pair of laser emitters  11  to emit two laser beams is adopted. Here, in the present embodiment laser scanning units (LSUs)  13   a  and  13   b  are used for light exposure unit  1 , but an array of light emitting elements, e.g., an EL (electroluminescence) or LED (light-emitting diode) writing head may also be used. 
   Photoreceptor drum  3  has an approximately cylindrical shape, is arranged under light exposure unit  1  and is controlled so as to rotate in a predetermined direction (in the direction of arrow A in the drawing) by an unillustrated driver and controller. Arranged along the peripheral surface of this photoreceptor drum  3 , starting from the position at which image transfer ends downstream in the rotational direction of the photoreceptor drum are, as shown in  FIG. 2 , a paper separation claw  31 , cleaner unit  5 , charger  4  as an electric field generator, developing unit  2  and charge erasing device  41  in the order mentioned. 
   Paper separation claw  31  is disposed so as to be moveable into and out of contact with the outer peripheral surface of photoreceptor drum  3  by means of a solenoid  32 . When this paper separation claw  31  is put in abutment with the outer peripheral surface of photoreceptor drum  3 , it functions to peel off the paper P that has adhered to the photoreceptor drum  3  surface during the unfixed toner image on photoreceptor drum  3  being transferred to the paper P. Here, as a driver for paper separation claw  31 , a drive motor or the like may be used instead of solenoid  32 , or any other driver may also be selected. 
   Developing unit  2  visualizes the electrostatic latent image formed on photoreceptor drum  3  with black toner, and is arranged at approximately the same level at the side (on the right side in the drawing) of photoreceptor drum  3  downstream of charger  4  with respect to the rotational direction of the photoreceptor drum (in the direction of arrow A in the drawing). A registration roller  15  is disposed under this developing unit  2  on the upstream side with respect to the recording medium&#39;s direction of conveyance. This developing unit  2  will be detailed later. 
   Carrier collecting roller  220  is arranged under developing unit  2  to collect magnetic carrier particles adhering on photoreceptor drum  3 . Carrier removing device  230  has the function of removing magnetic carrier collected by carrier collecting roller  220  from carrier collecting roller  220 . 
   Toner feed device  30  temporarily holds the toner discharged from a toner container  300  filled with toner, in an intermediate hopper  33  and then supplies it to developing unit  2 . This toner feed device is arranged adjacent to developing unit  2 . Provided under this toner feed device  30  is a duct unit  50  which includes an unillustrated air blowing fan for sending air to a developing vessel  200  of developing unit  2  in order to forcibly cool or remove operational heat that arises while developing unit  2  is operated. The detail of duct unit  50  will be described later. 
   Registration roller  15  is operated and controlled by an unillustrated driver and controller so as to convey the paper P delivered from paper feed tray  8  into and between photoreceptor drum  3  and a transfer belt  103  whilst making the leading end of the paper P register with the toner image on the photoreceptor drum  3 . 
   Charger  4  is a charging device for uniformly charging the photoreceptor drum  3  surface at a predetermined potential, and is arranged over photoreceptor drum  3  and close to the outer peripheral surface thereof. Here, a discharge type charger  4  is used in the present embodiment, but a contact roller type or a brush type may be used instead. 
   Charge erasing device  41  is a pre-transfer erasing device for lowering the surface potential of the photoreceptor drum  3  in order to facilitate the toner image formed on the photoreceptor drum  3  surface to transfer to paper P, and is laid out on the downstream side of developing unit  2  with respect to the photoreceptor drum&#39;s direction of rotation and under photoreceptor drum  3  and close to the outer peripheral surface of the same. Though in the present embodiment, charge erasing device  41  is configured using a charge erasing electrode, a charge erasing lamp or any other method can be used instead of the charge erasing electrode. 
   Cleaner unit  5  removes and collects the toner left on the surface of photoreceptor drum  3  after development and image transfer, and is disposed at approximately the same level at the side of photoreceptor drum  3  (on the left side in the drawing), on the approximately opposite side across photoreceptor drum  3  from developing unit  2 . 
   As described above, the visualized electrostatic image on photoreceptor drum  3  is transferred to the paper P being conveyed whilst the paper is being applied by transfer device  10  with an electric field having an opposite polarity to that of the electric charge of the electrostatic image. For example, when the electrostatic image bears negative (−) charge, the applied polarity of transfer device  10  should be positive (+). 
   Transfer device  10  is provided as a transfer belt unit in which transfer belt  103  having a predetermined resistivity (ranging from 1×10 9  to 1×10 13  Ω·cm in the embodiment) is wound and tensioned on a drive roller  101 , a driven roller  102  and other rollers, and is disposed under photoreceptor drum  3  with the transfer belt  103  surface put in contact with part of the outer peripheral surface of photoreceptor drum  3 . This transfer belt  103  conveys paper P while pressing the paper against photoreceptor drum  3 . An elastic conductive roller  105  having a conductivity different from that of drive roller  101  and driven roller  102  and capable of applying a transfer electric field is laid out at a contact point  104  where transfer belt  103  comes into contact with photoreceptor drum  3 . 
   Elastic conductive roller  105  is composed of a soft material such as elastic rubber, foamed resin etc. Since this elasticity of elastic conductive roller  105  permits photoreceptor drum  3  and transfer belt  103  to come into, not line contact, but area contact of a predetermined width (called a transfer nip) with each other, it is possible to improve the efficiency of transfer to the paper P being conveyed. 
   Further, a charge erasing roller  106  for erasing the electric field that has been applied to the paper P as it was being conveyed through the transfer area so as to achieve smooth conveyance of the paper to the subsequent stage is disposed on the interior side of transfer belt  103 , on the downstream side, with respect to the direction of paper conveyance, of the transfer area of transfer belt  103 . 
   As shown in  FIG. 2 , transfer device  10  also includes a cleaning unit  107  for removing dirt due to leftover toner on transfer belt  103  and a plurality of charge erasing devices  108  for erasing electricity on transfer belt  103 . Erasure of charge by erasing devices  108  may be performed by grounding via the apparatus or by positively applying charge of a polarity opposite to that of the transfer field. 
   The paper P with the static image (unfixed toner) transferred thereon by transfer device  10  is conveyed to fixing unit  6 , where it is pressed and heated so as to fuse the unfixed toner and fix it to the paper P. This fixing unit  6  includes a heat roller  6   a  and a pressing roller  6   b  as shown in  FIG. 2  and fuses and fixes the toner image transferred on paper P by rotating heat roller  6   a  so as to convey the paper P held between heat roller  6   a  and pressing roller  6   b  through the nip therebetween. Arranged on the downstream side of fixing unit  6  with respect to the direction of paper feed is a conveyance roller  16  for conveying paper P. Also, a paper discharge roller  17  for discharging paper P to paper output tray  9  is arranged on the downstream side of this conveyance roller  16  with respect to the direction of paper feed. 
   Heat roller  6   a  has a sheet separation claw  611 , a thermistor  612  as a roller surface temperature detector and a roller surface cleaning member  613 , all arranged on the outer periphery thereof and also includes a heat source  614  for heating the heat roller surface at a predetermined temperature (set fixing temperature: approximately 160 to 200 deg. C.) in the interior part thereof. 
   Pressing roller  6   b  is provided at its each end with a pressing element  621  capable of abutting the pressing roller  6   b  with a predetermined pressure against heat roller  6   a . In addition a sheet separation claw  622  and a roller surface cleaning element  623  are provided on the outer periphery of pressing roller  6   b , similarly to the outer periphery of heat roller  6   a.    
   In this fixing unit  6 , as shown in  FIG. 2  the unfixed toner on the paper P being conveyed is heated and fused by heat roller  6   a , at the pressed contact (so-called fixing nip portion)  600  between heat roller  6   a  and pressing roller  6   b , so that the unfixed toner is fixed to the paper P by the anchoring effect to the paper P by the pressing force from heat roller  6   a  and pressing roller  6   b.    
   As shown in  FIG. 1 , paper feed tray  8  stacks a plurality of sheets (paper) to which image information will be output (printed), and is arranged under image forming portion  14  made up of light exposure unit  1 , developing unit  2 , photoreceptor drum  3 , charger  4 , charge erasing device  41 , cleaner unit  5 , fixing unit  6  etc. A paper pickup roller  8   a  is disposed at an upper part on the paper output side of this paper feed tray  8 . 
   This paper pickup roller  8   a  picks up paper P, sheet by sheet, from the topmost of a stack of paper stored in paper feed tray  8 , and conveys the paper downstream (for convenience&#39; sake, the supply side of paper P (the cassette side) is referred to as upstream and the paper output side is referred to as downstream) to the registration roller (also called “idle roller”)  15  side in paper feed path  7   a.    
   Since the image forming apparatus  1 A according to the present embodiment is aimed at performing high-speed printing operations, a multiple number of paper feed trays  8  each capable of stacking 500 to 1500 sheets of standard-sized paper P are arranged under image forming portion  14 . Further, a large-capacity paper feed cassette  81  capable of storing multiple kinds of paper in large volumes is arranged at the side of the apparatus while a manual feed tray  82  for essentially supporting printing etc. for irregular sized paper is arranged over the large-capacity paper feed cassette  81 . 
   Paper output tray  9  is arranged on the opposite side across the apparatus from that of manual feed tray  82 . It is also possible to configure such a system that instead of paper output tray  9 , a post-processing machine for stapling, punching of output paper and the like and/or a multi-bin paper output tray etc., may be arranged as an option. 
   Paper conveyor system  7  is laid out between the aforementioned photoreceptor drum  3  and paper feed tray  8 , and conveys paper P supplied from paper feed tray  8 , sheet by sheet, by way of paper feed path  7   a  included in paper conveyor system  7 , to transfer device  10 , where a toner image is transferred from photoreceptor drum  3  to the paper, further conveying it to fixing unit  6  where the unfixed toner image is fixed to the paper, then conveys the sheet as it is being guided by paper feed paths and branch guides, in accordance with the designated paper output processing mode. 
   In the image forming apparatus  1 A according to the present embodiment, two predetermined output processing modes, namely, one-sided printing mode and two-sided printing mode are prepared. In one-sided printing mode, there are two ways of paper output, i.e., the faceup output by which the paper is discharged with its printed surface faceup and the facedown output by which the paper is discharged with its printed surface facedown. 
   Next, developing unit  2  and its peripheral components that constitute image forming apparatus  1 A according to the present embodiment will be described with reference to the drawings.  FIG. 3  is a schematic configurational side view showing the developing unit and toner feed device that are included in the image forming apparatus according to the present embodiment. 
   In this embodiment, as shown in  FIG. 3 , toner feed device  30  is arranged adjacent to developing unit  2 . Duct unit  50  having an air blowing fan or the like for sending air to a developing vessel  200  that forms the exterior of developing unit  2  is provided under this toner feed device  30  in order to forcibly remove operational heat that arises while developing unit  2  is operated. 
   As shown in  FIG. 3 , in developing unit  2  a toner input port  201  for leading toner is formed, at a position where opening  30   a  for supplying toner from toner feed device  30  abuts the developing vessel  200  that forms its exterior. This developing vessel  200  incorporates developer roller  202 , a paddle roller  203 , a mixing roller  204 , a conveying roller  205 , a partitioning plate  206  and a regulating member  207 . Developing unit  2  is mounted inside image forming apparatus  1 A in such a manner that the peripheral surface (the developer adhering on the peripheral area) of developing roller  202  opposes in contact with the peripheral surface of photoreceptor drum  3 . That is, the peripheral surface area of developing roller  202  opposing in contact with photoreceptor drum  3  forms the developing position. Further, arranged adjacent to and under developing roller  202  in the opening, designated at  200   a  of developing vessel  200  is carrier removing device  230  including carrier collecting roller  220  for collecting carrier adhering on photoreceptor drum  3 . 
   In developing vessel  200 , the toner that was fed from toner feed device  30  and input through toner input port  201  is conveyed by conveying roller  205  to mixing roller  204 , where the toner is mixed with magnetic carrier to thereby prepare a dual-component developer. Mixing roller  204  mixes this newly formed dual-component developer with the surplus developer that is returned by the aforementioned partitioning plate  206 . The developer thus obtained by mixing with mixing roller  204  is tribo-electrified as it is agitated by paddle roller  203 , then supplied to developing roller  202  for developing electrostatic latent images and further conveyed to the electrostatic latent image formed on photoreceptor drum  3 . 
   The developer to be supplied to developing roller  202  is first rubbed and pre-charged by a rubbing member  211  that is integrally formed at one end side of partitioning plate  206  whilst being regulated as to the amount of conveyance (layer thickness) thereby. Then, the layer thickness of the developer being conveyed by developing roller  202  is further controlled by regulating member  207  that is supported by a supporting member  212  as a part of developing vessel  200 . In this way, the supplied amount of developer is regulated and the excluded, surplus developer is returned in directions going away from regulating member  207  by partitioning plate  206  that functions as a recirculating plate for returning surplus developer. These regulating member  207 , rubbing member  211  and partitioning plate  206  are formed to be as long as developing roller  20 . 
   Further, a plurality of rectifying plates  208  are formed on the upper side of partitioning plate  206  while a partitioning plate-side conveyor  209  which conveys surplus developer by a conveyor screw  210  is arranged on the lower side of partitioning plate  206 . Details of these components provided in developing unit  2  will be described later. 
   Toner feed device  30  is arranged adjacent to developing unit  2 , and temporarily reserves the toner discharged from toner container  300  filled with toner, in intermediate hopper  33  and then feeds the toner to developing unit  2 . In the present embodiment, toner container  300  is configured so that its container body  310  charged with toner is rotatably supported by a supporting structure  350 . 
   The toner thus sent out to intermediate hopper  33  is agitated therein by an agitator  34  first. Agitator  34  is comprised of an agitator shaft  34   a  and agitating vanes  34   b  attached thereto. As agitator shaft  34   a  turns, agitating vanes  34   b  rotate about agitator shaft  34   a  to thereby agitate the toner in intermediate hopper  33  that has been fed from toner container  300 . 
   The toner thus agitated by agitator  34  is sent by the agitating action of agitator  34  and conveyed to the feed roller  36  side via a conveying roller  35 . Feed roller  36  sends out the toner that has been conveyed from agitator  34  via conveying roller  35 , to opening  30   a  that is formed at the position where intermediate hopper  33  abuts developing unit  2 , to thereby supply the toner to developing unit  2 . 
   Provided on the bottom side (the underside when toner container  300  is mounted on image forming apparatus  1 A) of supporting structure  350  of toner container  300  is a shutter opening and closing mechanism  400  for opening and closing a toner feed aperture  300   a  through which toner supplied from toner container  300  is discharged out of supporting structure  350 , as shown in  FIG. 3 . Specifically, as toner feed aperture  300   a  of supporting structure  350  is released by shutter opening and closing mechanism  400 , passage between toner feed aperture  300   a  and opening  33   a  provided for intermediate hopper  33  is communicated, so that the toner discharged from toner container  300  is supplied to intermediate hopper  33 . 
   Next, the characteristic configuration of developing unit  2  according to the present embodiment will be described in detail with reference to the drawings.  FIG. 4  is a sectional view showing the configuration of the developing unit according to the present embodiment;  FIG. 5A  is a side sectional view showing a configuration of a mixing roller that constitutes the developing unit;  FIG. 5B  is a sectional view cut along a plane B 1 -B 1 ′ in  FIG. 5A ;  FIG. 5C  is a sectional view cut along a plane B 2 -B 2 ′ in  FIG. 5A ;  FIG. 5D  is a sectional view cut along a plane B 3 -B 3 ′ in  FIG. 5A ; and  FIG. 5E  is a sectional view cut along a plane B 4 -B 4 ′ in  FIG. 5A . 
   As shown in  FIG. 4 , developing unit  2  includes developing vessel  200  forming its exterior, and toner input port  201  for leading toner is formed in this developing vessel  200  at a position where opening  30   a  ( FIG. 3 ) provided for toner feed device  30  to deliver toner abuts the developing vessel  200 . This developing vessel  200  reserves the developer therein and incorporates developer roller  202 , paddle roller  203 , mixing roller  204 , conveying roller  205 , a regulating member  207  and collecting roller  220 . 
   Developing unit  2  is mounted inside image forming apparatus  1 A in such a manner that the peripheral surface (the developer adhering on the peripheral area) of developing roller  202  that is partly exposed from developing vessel  200  opposes in proximity to the peripheral surface of photoreceptor drum  3 . That is, the peripheral surface area of developing roller  202  opposing photoreceptor drum  3  forms the developing position (developing area). 
   In developing vessel  200 , the toner that was fed from toner feed device  30  ( FIG. 3 ) and input through toner input port  201  is conveyed by conveying roller  205  to mixing roller  204 , where the toner is mixed with the magnetic carrier to thereby prepare a dual-component developer. Mixing roller  204  mixes the aforementioned newly formed dual-component developer with the existing developer inside developing vessel  200 . The developer obtained by mixing with mixing roller  204  is tribo-electrified as it is agitated by paddle roller  203 , then supplied to developing roller  202  for developing electrostatic latent images, and conveyed by developing roller  202  to the electrostatic latent image formed on photoreceptor drum  3 . The developer supplied to developing roller  202  and conveyed thereby is controlled as to its layer thickness by regulating member  207  that is supported by supporting member  212  as a part of developing vessel  200 . In this way, the amount of developer to be supplied to photoreceptor drum  3  is regulated. 
   In order to make the forced air-cooling by duct unit  50  more efficient, developing vessel  200  is made of a metallic material having a high thermal conductivity such as aluminum or the like as a countermeasure against increase in temperature inside developing unit  2 , and has opening  200   a  ( FIG. 3 ) facing (opposing) the peripheral surface of photoreceptor drum  3 . 
   Provided on the upper outside part of supporting member  212  that forms the top of developing vessel  200  is a pressure relief mechanism (pressure adjustment filter)  217  for reducing the pressure inside developing vessel  200 . This pressure relief mechanism  217  is periodically operated to release the pressure inside developing unit  2  so that toner scattering inside the apparatus can be prevented. Here, the attached position of pressure relief mechanism  217  is not limited to the top of developing vessel  200 . For example, the mechanism may be arranged on the flank or at the bottom of developing vessel  200  as long as it can release the pressure inside developing vessel  200 . 
   Developing roller  202  is arranged at the position inside developing vessel  200  where opening  200   a  is formed while conveying roller  205  that conveys the developer (toner) supplied from toner input port  201  into developing vessel  200  to mixing roller  204  is disposed rotatably at a position that opposes toner input port  201 . 
   A toner concentration sensor  213  for detecting the toner concentration inside developing vessel  200  is provided at the bottom opposing the lower side of mixing roller  204  in developing vessel  200 . Image forming apparatus  1 A is configured so as to supply toner from toner input port  201  based on the measurement of toner concentration sensor  213  when the amount of toner being mixed and agitated by mixing roller  204  becomes lower than the proper amount. 
   Arranged within opening  200   a  of developing vessel  200 , adjacent to and below developing roller  202  is carrier collecting roller  220  for collecting the magnetic carrier that has transferred to photoreceptor drum  3 . More specifically, carrier collecting roller  220  is arranged at a position downstream of developing roller  202  with respect to the rotational direction of photoreceptor drum  3  and positioned a small gap of about 1 mm apart from photoreceptor drum  3 . Also, carrier removing device  230  which removes the magnetic carrier collected by this carrier collecting roller  220  therefrom is disposed at a position on the upstream side with respect to the carrier collecting roller  220 &#39;s direction of rotation. 
   In the present embodiment, as shown in  FIGS. 4 and 6 , a forced draft duct  240  for airflow from the blowing fan of duct unit  50  is arranged under the bottom, designated at  200   b , of developing vessel  200 , as a cooling portion for forcibly cooling the operating heat that arises while developing unit  2  is being operated. Formed under this forced draft duct  240  is a suctioning duct  250  for airflow from suctioning ports  252  disposed at both ends and on the lower side of carrier collecting roller  220  to a suctioning fan  53  ( FIG. 6 ) as a suctioning portion for suctioning scattering toner near the developing area of developing unit  2 . 
   Though in the present embodiment, these forced draft duct  240  and suctioning duct  250  are integrally formed with base structure  260  that supports developing vessel  200  of developing unit  2 , forced draft duct  240  and suctioning duct  250  may be formed separately as long as they are formed in a double-layered structure in which forced draft duct  240  is formed on the upper layer side in proximity with bottom  200   b  of developing vessel  200  and suctioning duct  250  is formed on the lower layer side under forced draft duct  240 . The detailed configuration of base structure  260  that is integrally formed with forced draft duct  240  and suctioning duct  250  will be described later. 
   As shown in  FIG. 4 , developing roller  202  is arranged a development gap (about 0.5 to 1.5 mm) apart from photoreceptor drum  3 . Developing roller  202  is formed of a magnet roller  214  with multiple magnetic poles and a non-magnetic sleeve  215  that is approximately cylindrically formed of an aluminum alloy, brass and the like and is arranged rotatably over, and relative to the magnet roller  214 . In this magnetic roller  214 , a plurality of bar magnets having rectangular sections, specifically magnetic pole elements N 1 , N 2 , N 3  and N 4  providing N-pole magnetic fields and magnetic pole elements S 1 , S 2  and S 3  providing S-pole magnetic fields, are radially arranged apart one from another in the order shown in  FIG. 4 . 
   Magnet roller  214  is unrotatably supported and fixed at its both ends by the side walls of developing vessel  200 . Magnetic pole element N 1  is disposed at a position opposing the peripheral surface of photoreceptor drum  3 . Each of the chained lines designated at P 1 , P 2 , P 3  and P 4  of magnetic pole elements N 1 , N 2 , N 3  and N 4  represents the center of the width of the associated magnetic pole element or the central axis of the associated magnetic pole, with respect to the circumferential direction of developing roller  202 . These magnetic pole&#39;s center axes P 1 , P 2 , P 3  and P 4  are radially extended from the developing roller&#39;s central axis O 2  and formed across the full length of the magnet elements (across the length of sleeve  215 ). The magnetic pole element N 1  that opposes the peripheral surface of photoreceptor drum  3  is positioned so that the magnetic pole&#39;s center axis P 1  substantially coincides with the line (plane) that passes through both the center axis (outside the area of  FIG. 4 ) of photoreceptor drum  3  and the center axis O 2  of developing roller  202 . 
   The above magnetic pole elements are laid out in the order of N 1 , S 3 , N 4 , N 3 , S 2 , N 2  and S 1  in the rotational direction of developing roller  202 . The magnetic field created by the thus arranged magnetic pole elements N 1 , N 2 , N 3 , N 4 , S 1 , S 2  and S 3 , attracts the dual-component developer particles made of toner and carrier to the peripheral surface of rotating sleeve  215  so as to form brush-like spikes (to be referred as magnetic brush) extending in the circumferential direction of the sleeve. As the photoreceptor drum rotates, the photoreceptor drum  3  surface is rubbed in the above-mentioned development gap area by the magnetic brush created on rotating developing roller  202  to thereby achieve development. 
   Regulating member  207  controls the amount of the developer conveyed between itself and developing roller  202  while performing principal electrification of the developer, and is formed of a non-magnetic metal plate having an approximately rectangular section. One end of regulating member  207  opposes the outer peripheral surface of developing roller  202  (sleeve  215 ) with a predetermined gap in between. Regulating member  207  is fixed to a cover element  216  and disposed inside opening  200   a  ( FIG. 3 ). This regulating member  207  is formed of a non-magnetic metal plate such as aluminum, stainless steel or the like. 
   Mixing roller  204  agitates and conveys the toner supplied from toner feed device  30  ( FIG. 3 ) as shown in  FIG. 5A  and is comprised of a rotary shaft  204   a  arranged substantially parallel to developing roller  202  ( FIG. 4 ) and a plurality of separate plate-like agitating elements  204   b  ( 204   b   1  to  204   b   4 ). 
   Agitating elements  204   b  are arranged inclined at an angle of approximately 45 degrees with the direction in which the axis of rotary shaft  204   a  extends (to be referred to as the axial direction). Agitating elements  204   b  include agitating element  204   b   3  disposed at the approximate center, with respect to the axial direction, of rotary shaft  204   a , a group  204 B 1  of an agitating element  204   b   1  and multiple agitating elements  204   b   2  arranged on the right side in the drawing and a group  204 B 2  of an agitating element  204   b   4  and multiple agitating elements  204   b   2  arranged on the left side in the drawing. Here, agitating elements  204   b   1  and  204   b   4  are disposed at both ends with respect to the axial direction of rotary shaft  204   a.    
   In the present embodiment, group  204 B 1  includes as many agitating elements  204   b   2  as group  204 B 2  does. That is, mixing roller  204  has an odd number of agitating elements  204   b.    
   As shown in  FIGS. 5B and 5E , agitating elements  204   b   1  and  204   b   4  arranged at both ends of rotary shaft  204   a  have approximately semicircular shapes which are point symmetrical with respect to rotary axis  204   a . Detailedly, agitating elements  204   b   1  and  204   b   4  each have a hemi-elliptic shape by cutting an elliptic shape having a major axis L 1  passing through rotary axis  204   a  in half along the line that is substantially perpendicular to the major axis L 1 . 
   A plurality of agitating elements  204   b   2  are provided between agitating element  204   b   3  and agitating element  204   b   1  and between agitating element  204   b   3  and agitating element  204   b   4 , each being inclined with the axial direction of rotary shaft  204   a  and having a substantially elliptic shape, as shown in  FIG. 5C . This configuration makes it possible for each agitating element to produce a stronger conveying force in the direction of the rotary axis. 
   Agitating element  204   b   3  arranged at the substantially center of rotary shaft  204   a  has a substantially elliptic shape having a cutout portion  204   c  at the position opposing the aforementioned toner concentration sensor  213  as shown in  FIG. 5D , so that light for detection from toner concentration sensor  213  is permitted to pass through. Another cutout portion  204   c  is formed in the agitating element at a position point symmetrical, with respect to the center of rotary axis  204   a , to the position of the aforementioned cutout. That is, a pair of cutout portions  204   c  are formed at positions point symmetrical to each other with respect to the center of rotary axis  204   a . Thus, this configuration of agitating element  204   b   3  makes it possible to prevent output ripples from occurring at toner concentration sensor  213  due to developer&#39;s volume density change which would occur as agitating element  204   b  of mixing roller  204  rotates. 
   Arranged between developing roller  202  and mixing roller  204 , as shown in  FIG. 4  is paddle roller  203 , which agitates and electrifies the developer that was prepared by mixing of mixing roller  204  to supply the developer to developing roller  202 . 
   Paddle roller  203  is formed with a supporting shaft extending longitudinally and a plurality of flat plate-like blades radially extending from the supporting shaft so that the blades can rotate about the supporting shaft. As paddle roller  203  rotates about the supporting shaft, the developer can be agitated. 
   As described above, carrier collecting roller  220  for collecting the magnetic carrier having adhered to photoreceptor drum  3  is arranged below developing roller  202  so as to abut photoreceptor drum  3 . Carrier removing device  230  which removes the magnetic carrier collected by this carrier collecting roller  220  therefrom is arranged at a position on the downstream side with respect to the rotational direction of carrier collecting roller  220 . 
   Next, the base structure  260  which is the characteristic part of developing unit  2  according to the present embodiment will be described in detail with reference to the drawings.  FIG. 6  is a perspective view showing the configuration of an exterior wall  260   b  that defines a suctioning duct  250  in the base structure of the developing unit of this embodiment.  FIG. 7  is a plan view schematically showing the base structure of the developing unit of the same embodiment when viewed from top. 
   Base structure  260  is integrally formed of a cooling portion that is formed on the bottom  200   b  ( FIG. 4 ) side of developing vessel  200  and includes forced draft duct  240  ( FIG. 4 ) for airflow sent from blowing fan  51  provided in duct unit  50 , and a suctioning portion which is formed under forced draft duct  240  and includes suctioning duct  250  ( FIG. 4 ) for airflow from suctioning ports through which scattering toner around the developing area of developing unit  2  is drawn in. Here, as mentioned already, the cooling portion and the suctioning portion may be formed separately in base structure  260 . 
   In order to define forced draft duct  240  and suctioning duct  250 , base structure  260  includes partitioning wall  260   a  and an exterior wall  260   b  as shown in  FIG. 4 . 
   Partitioning wall  260   a  is formed so as to cover substantially the whole area of bottom  200   b  while defining forced draft duct  240  between itself and bottom  200   b  when base structure  260  is mounted to bottom  200   b  of developing vessel  200 . 
   Exterior wall  260   b  is formed so as to cover partitioning wall  260   a  while defining suctioning duct  250  between itself and partitioning wall  260   a . Though exterior wall  260   b  of the present embodiment is constructed so as to cover partitioning wall  260   a  as a whole, the present technology should not be limited to this. That is, the exterior wall will achieve its expected function as long as it covers part of partitioning wall  260   a  that can form suctioning duct  250 . 
   Partitioning wall  260   a  has a first side that opposes bottom  200   b  of developing vessel  200  and a second side that opposes exterior wall  260   b.    
   This base structure  260  ( FIG. 6 ) is connected to duct unit  50 . 
   Duct unit  50  includes blowing fan  51  for blowing air for cooling developing vessel  200 , a suctioning fan  53  as a suctioning source for drawing scattering toner and an exhaust duct  54  for leading air from forced draft duct  240  and suctioning duct  250  to an exhaust port  55 . 
   Blowing fan  51  blows air into forced draft duct  240 . Suctioning fan  53  draws air from suctioning duct  250 . 
   Forced draft duct  240  has, as shown in  FIG. 7 , a forced draft entrance  240   a  as an inlet of air from blowing fan  51  and a forced draft exit  240   b  as an outlet. 
   Forced draft entrance  240   a  and forced draft exit  240   b  are formed at both ends (they may be formed near both ends at each end side with) respect to the longitudinal direction (axial direction) of developing roller  202  with a predetermined distance (the width of a combined suctioning port  253  in this embodiment) apart from each other. However, its layout should not be limited to this. 
   Forced draft duct  240  is formed in a bracket shape ([) or U-shape from forced draft entrance  240   a  toward forced draft exit  240   b , but its layout should not be limited to this. 
   The opening at forced draft entrance  240   a  and the opening at forced draft exit  240   b  are oriented in the same direction, but their layout should not be limited to this. The two openings may be oriented in different directions. 
   In sum, forced draft duct  240  may take any configuration as long as it can cool bottom  200   b.    
   In the present embodiment, in order to be able to cool entire part of bottom  200   b  of developing unit  2 , forced draft duct  240  is constructed so as to cover substantially whole surface of bottom  200   b  of developing vessel  200  as shown in  FIG. 7 . However, its layout should not be limited to this. 
   Air passing through forced draft duct  240  is sent out from forced draft exit  240   b , then discharged from exhaust port  55 , passing through a forced draft exhaust duct  54   b  and common exhaust duct  54 . 
   On the other hand, suctioning duct  250  has two suctioning ports  252  for suctioning scattering toner on the upstream side with respect to the direction of airflow and one combined suctioning port  253  on the downstream side, forming an approximately Y-shaped passage. However, the shape of suctioning duct  250  is not limited to this approximate Y-shape. 
   The two suctioning ports  252  as the entrance for scattering toner are arranged near both ends of carrier collecting roller  200  where toner scattering is prone to occur, for example, at both lower ends of the carrier collecting roller only. Accordingly, it is possible to reduce the suctioning load on suctioning fan  53  and realize efficient suction. 
   The ratio between the open width of suctioning port  252  and the length of the non-opening portion between two suctioning ports  252  and  252  with regard to the longitudinal direction (axial direction) of carrier collecting roller  220  can be experimentally determined so as to maximize the above operational effect. 
   Though the openings of suctioning ports  252  are oriented toward photoreceptor drum  3 , their layout should not be limited to this. That is, the openings may be oriented toward, for example carrier collecting roller  220 , toward the opposing portion between photoreceptor drum  3  and carrier collecting roller  220  or the like, so that scattering toner can be efficiently collected. 
   Air streams containing scattering toner that have been suctioned from two suctioning ports  252  provided at both ends of carrier collecting roller  220  are converged so that the combined airflow passes through a filter  52  for removing the scattering toner from the air that contains scattering toner and is suctioned from combined suctioning port  253  by suctioning fan  53 . 
   Combined suctioning port  253  is formed between forced draft entrance  240   a  and forced draft exit  240   b  with respect to the longitudinal direction (axial direction) of developing roller  202  and the opening of combined suctioning port  253  is oriented in the same direction as that of forced draft entrance  240   a  and forced draft exit  240   b . However, its layout should not be limited to this. 
   Air having passed through the filter  52  and combined suctioning port  253  is discharged from exhaust port  55  through suctioning exhaust duct  54   a  and common exhaust duct  54 . 
   In the above way, formation of suctioning duct  250  that includes a pair of suctioning ports  252  arranged at both ends and on the lower side of carrier collecting roller  220  and one combined suctioning port  253  formed on the upstream of suctioning fan  53 , makes it possible to achieve smooth suction of scattering toner without causing any buildup of toner etc. inside suctioning duct  250 . 
   In order to facilitate maintenance, filter  52  is adapted to be able to be inserted into and removed from a filter holder  52   a  ( FIG. 6 ) from above, which is formed at a position close to suctioning fan  53  in base structure  260 . Further, in the present embodiment, in order to achieve more efficient suctioning, cooling and exhausting by smoothing air flow in each duct, all the corners in suctioning duct  250 , cooling duct  240  and exhaust duct  54  at which the direction of airflow turns are formed with curved surfaces (round surfaces) as shown in  FIG. 7 . 
   As has been described, in the present embodiment, scattering toner around the developing area where photoreceptor drum  3  and developing roller  202  abut each other is suctioned from the lower side of carrier collecting roller  220 , so that it is possible to collect the scattering toner that is sufficiently separated from the carrier. Accordingly, it is possible to prevent filter clogging and carrier reduction, which would occur when the toner together with carrier particles is suctioned. Further, since the airflow from blowing fan  51  is applied through forced draft duct  240  onto approximately the whole surface of bottom  200   b  of developing vessel  200 , this arrangement brings about a remarkable cooling effect on developing vessel  200  of developing unit  2 . 
   Though in the above first embodiment, forced draft duct  240  and suctioning duct  250  are constructed with two layered separate channels so as to be connected to blowing fan  51  and suctioning fan  53 , respectively, forced draft duct  240  and suctioning duct  250  may be joined to be a single air passage in which only a single fan is arranged at a site therein. This configuration enables use of a single fan in common for both cooling and suction by arranging it at the exhaust port, for example. 
   (Aging Test) 
   Next, the effect obtained from the developing unit configuration will be explained by comparing the developing unit of the first embodiment with a conventional developing unit.  FIG. 8  is a table showing various setting conditions in aging tests.  FIG. 9  is a graph showing temperature increase of developing vessels in the aging tests, making a comparison between when air cooling is performed, and when not performed, by a cooling portion of the first embodiment, specifically showing the increase in temperature from the ambient temperature in relation to whether the cooling is performed or not.  FIG. 10  is a table showing comparative results of the aging test between the developing unit of the first embodiment and a conventional developing unit. This table shows the amounts of carrier reduction after an aging run of 500 K (500×10 3 ) printouts, the amounts of filter&#39;s weight increase (the sum of weight increase; each filter was replaced every 100 K (100×10 3 ) printouts) and the number of times the operation failures occurred due to solidification of the developer during the 500 K aging run. Here, the conventional developing unit has no cooling mechanism as the cooling portion provided for the developing unit in the first embodiment and no carrier collecting roller. Also, the conventional developing unit is configured so as to perform suctioning from the lower part at both ends of the developing roller, in the same manner as in the developing unit according to the first embodiment. 
   In the aging test, developing unit  2  was designed so as to meet the conditions as shown in  FIG. 8  and was operated. As described above, the developing unit  2  according to the first embodiment is constructed such that forced draft duct  240  functioning as a cooling portion is arranged at the top of suctioning duct  250  functioning as suctioning portion so as to blow air across the whole part of bottom  200   b  of developing vessel  200 . As a result, as shown in  FIG. 9 , with no air cooling performed by the cooling portion, the temperature increase from the ambient air temperature was 24.1 deg. In contrast, when the cooling portion was used, the temperature increase from the ambient air temperature was 15.5 deg. That is, as the forced cooling effect of the cooling portion including suctioning duct  250  and blowing fan  51  of the present technology, a temperature reduction of 8.6 deg. (=24.1 deg.−15.5 deg.) was achieved. That is, the temperature increase from the ambient air temperature can be reduced to equal to or lower than 18 degree, which is the target value of the temperature increase, with which preferable result can be obtained without causing any operation failure and other problems with regards to the developing unit. Thus, it is possible to prevent developing unit  2  from failing to operate from solidification of the developer inside the developing unit as a result of temperature increase of developing unit  2  due to high-speed operations. 
   Further, in developing unit  2  suctioning ports  252  as the entrance of suctioning duct  250  are formed at both ends on the lower side of carrier collecting roller  220 . The air streams containing scattering toner suctioned from these suctioning ports  252  pass through suctioning duct  25  and are collected into combined suctioning port  253  with filter  52  that is arranged at the center on the opposite side of developing vessel  200  from its side with developing roller  202 . The thus filtrated airflow is further drawn toward exhaust port  55  by way of suctioning fan  53 , suctioning exhaust duct  54   a  and common exhaust duct  54 . As a result, in the developing unit, it is possible to collect the scattering toner that is sufficiently separated from the carrier by performing suction from the lower side of carrier collecting roller  220 , hence it is possible to suppress clogging in the filter etc. and reduction of the carrier, compared to the conventional developing unit, as shown in  FIG. 10 . 
   Further, since suction is performed positively and selectively from both ends of carrier collecting roller  200 , at which toner scattering is most prone to occur, it is possible to realize suctioning without causing any increase in waste suctioning load. Further, since suctioning duct  250  is formed so as to suction air from both ends towards the center to thereby realize smooth suction, no buildup of toner etc., will occur inside suctioning duct  250 . Accordingly, it is possible to reduce the number of operation failures of developing unit  2  occurring due to solidified developer or the like inside the developing unit, as shown in  FIG. 10 . 
   That is, since, in developing unit  2 , toner can be suctioned by sufficiently separating the carrier from the toner, it is possible to collect toner without capturing carrier in filter  52 , reduce the filter&#39;s weight increase, or more explicitly inhibit reduction of the amount of carrier in the developer. Further, since cooling of developing vessel  200  in developing unit  2  is performed by cooling bottom  200   b  of developing unit  2  as a whole by providing a cooling passage that is separate from the suctioning passage, operation failures hardly occur. 
   Having described the preferred embodiment with reference to the attached drawings, it goes without saying that the present technology should not be limited to the above-described examples, and it is obvious that various changes and modifications will occur to those skilled in the art within the scope of the appended claims. Such variations are therefore understood to be within the technical scope of the present technology. 
   For example, though, in the above-described first embodiment, the present technology is applied to a developing unit that is mounted to a monochrome image forming apparatus including a single toner container, it is also possible to apply the cooling mechanism of the developing vessel and the suctioning mechanism of scattering toner using the ducts of the developing unit, to a developing unit for color printing including a plurality of toner containers.