Patent Publication Number: US-6343200-B1

Title: Developing device having conveyance mixing units

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a developing device for use in an image forming apparatus such as copier and printer to supply a developing agent to a photoreceptor so as to develop a latent image into a toner image, and more particularly to a developing device which uses a two-component developing agent consisting of toner and carrier. 
     2. Description of the Related Art 
     Heretofore, there has been known a developing device for use in an image forming apparatus to develop an electrostatic latent image formed on the surface of a photosensitive drum into a toner image with use of a two-component developing agent consisting of toner and carrier. 
     Such a developing device is provided with a housing for accommodating a developing agent, developing agent supplier means for supplying the developing agent in the housing to the surface of the photosensitive drum to develop a latent image into a toner image, developing agent mixing means for mixing the developing agent in the housing while at the same time feeding the developing agent in a certain direction, and toner replenishing means for replenishing toner into the housing. 
     The developing agent supplier means supplies toner on a certain area (developing area) of the drum surface to develop the latent image into a toner image within the area. As the toner in the housing is used up in accordance with progress of the image formation, the toner replenishing means replenishes toner into the housing. The developing agent mixing means agitates the developing agent, namely mixes the toner particles with the carriers which are electrically charged to convey the toner particles together with the charged carriers owing to electrostatic friction. 
     A known technology relating to agitation and transport of developing agent is disclosed in, for example, Japanese Unexamined Patent Publication No. 10-142942. The publication discloses a four-membered tandem type developing device for use in a color copier comprising: developing agent supplier means for supplying a developing agent onto the surface of a photosensitive drum; first mixing/transporting means disposed near the developing agent supplier means to agitate the developing agent while transporting the developing agent in a first direction thereof; second mixing/transporting means disposed underneath the first mixing/transporting means to agitate the developing agent while transporting the developing agent in a second direction opposite to the first direction; and feeder means coaxially provided along with the longitudinal direction of the developing agent supplier means to feed the developing agent transported by the second mixing/transporting means toward the first mixing/transporting means in response to driving of the developing agent supplier means. 
     The above-mentioned conventional developing device has room for improvement in the aspect of agitation and transport of developing agent, prevention of undesired stay of developing agent, and reducing the dimensions of the developing device itself. Particularly, as for the dimensions of the device itself, since the feeder means is provided coaxially with the developing agent supplier means (magnet roller) to feed the developing agent from the second mixing means (lower mixing means) to the first mixing means (upper mixing means), the device itself is inevitably large in the axial direction of the magnet roller by a dimension corresponding to the axial length of the feeder means which axially protrudes beyond the developing area. 
     SUMMARY OF THE INVENTION 
     In view of the above problems residing in the prior art, it is an object of the present invention to provide a developing device of a compact size particularly in the axial length thereof that enables to ensure sufficient agitation of a developing agent and prevention of undesired stay of the developing agent. 
     In one aspect of this invention, a developing device for use in an image forming apparatus comprises: a developing agent supply unit which supplies a developing agent to a photoreceptor; a first mixing unit provided near the developing agent supply unit to convey the developing agent in a first conveyance direction while mixing the developing agent; a second mixing unit provided underneath the first mixing unit to receive the developing agent from the first mixing unit at a downstream end along the first conveyance direction so as to supply the developing agent to the developing agent supply unit while mixing and conveying the developing agent in a second conveyance direction; and a developing agent scoop-up unit provided substantially in the middle in a height direction between the first mixing unit and the second mixing unit and at a downstream end of the second mixing unit along the second conveyance direction to receive the developing agent from the second mixing unit and draw up the received developing agent to the first mixing unit. 
     With this arrangement, the developing agent is sufficiently mixed while conveyed in the first and second conveyance directions by the first and second mixing units. Further, this arrangement enables to provide an image forming apparatus of a compact size, particularly in the axial direction of the developing device while preventing stagnation of the developing agent. 
     These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing an overall construction of an image forming apparatus incorporated with a developing device according to this invention. 
     FIG. 2A is a diagram showing an internal mechanism of the developing device. 
     FIG. 2B is a cross-sectional view taken along the line  2 B— 2 B in FIG.  2 A. 
     FIG. 3 is a cross-sectional view taken along the line  3 — 3  in FIG.  2 A. 
     FIG. 4 is a cross-sectional view taken along the line  4 - 4  in FIG.  2 A. 
     FIG. 5 is a schematic diagram showing essential elements of the developing device. 
     FIG. 6 is a diagram showing an interlocked state of gears viewed from rear side of the image forming apparatus. 
     FIG. 7 is a diagram showing an interlocked state of gears viewed from front side of the image forming apparatus. 
     FIG. 8 is an explanatory diagram showing as to how a developing agent is delivered through the developing device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram showing an overall construction of a color printer as an embodiment of an image forming apparatus according to this invention. 
     The construction of the color printer is described with reference to FIG.  1 . The color printer  2  includes a sheet feeder section  4  for feeding a sheet member  3  on which an image is to be copied one by one, a sheet transport section  6  for transporting the sheet member  3  fed from the sheet feeder section  4  in a certain direction, an imaging assembly  5  for forming an toner image which is to be transferred onto the sheet member  3 , an image transfer section  7  for transferring the toner image onto the sheet member  3 , an image fixing section  8  for fixing the transferred toner image on the sheet member  3 , a sheet discharge section  10  which receives the sheet member  3  after the image fixation, and an optical unit  11 . 
     Specifically, the sheet feeder section  4  includes a first feeder unit  15  and a second feeder unit  18 . The first feeder unit  15  is constructed in such a manner that sheet members  3  stacked on a first sheet tray  12  are fed one by one by a feed roller (drive roller)  13  and a retard roller (driven roller)  14  toward downstream of the sheet transport direction. The second feeder unit  18  is constructed in such a manner that sheet members  3  stacked on a manual insertion tray  16  are fed one by one by a feed roller  17  toward downstream of the sheet transport direction. 
     The sheet transport section  6  includes a transport path  20  along which the sheet member  3  is fed from the first sheet feeder unit  15  or the second sheet feeder unit  18  toward the imaging assembly  5 , a transport roller  21  disposed at a certain position on the transport path  20 , a registration roller pair  22  which temporarily suspends transport of the sheet member  3  as timed with nipping of the lead end of the sheet member  3  and resumes transport of the sheet  3  toward the imaging assembly  5  while correcting a skew transport of the sheet member  3 , and a manual transport path  23  which is connected to the transport path  20  at an upstream position from the registration roller pair  22  with respect to the sheet transport direction. 
     The imaging assembly  5  includes a first imaging section  24  adapted to form a black color image, a second imaging section  25  adapted to form a cyan color image, a third imaging section  26  adapted to form a magenta color image, and a fourth imaging section  27  adapted to form a yellow color image. The first, second, third, and fourth imaging sections  24 ,  25 ,  26 ,  27  each is provided with a cylindrical photosensitive drum  28 , a developing unit  30  for forming individual color toner image on the surface of the photosensitive drum  28 , a cleaning unit  33  provided with a brush  31  for removing toner residues on the surface of the photosensitive drum  28  and a cleaning blade  32  for scraping off toner residues on the surface of the photosensitive drum  28 , and a charger  34  which supplies electric charges on the photosensitive drum surface to uniformly charge the surface of the photosensitive drum  28 . 
     The image transfer section  7  includes an endless belt  37  composed of a resin material containing fluorine compound, a drive roller  38  and a tension roller (driven roller)  40  for circulating the belt  37 , and transfer rollers  41   a ,  41   b ,  41   c ,  41   d  respectively disposed at certain positions opposing the corresponding photosensitive drums  28 . The image transfer section  7  is constructed in such a manner that the sheet member  3  is conveyed further downstream of the sheet transport direction over the endless belt  37  which is driven between the transfer roller  41   a  ( 41   b ,  41   c ,  41   d ) and the corresponding photosensitive drum  28 . Guide rollers  42 ,  43  are provided at respective appropriate positions on an inner circumference of the transfer belt  37 . 
     In the case where the image transfer section  7  is adapted to form a monochromatic image, the transfer roller  41   a  is driven while the other transfer rollers  41   b ,  41   c ,  41   d  being set apart from the surface of the corresponding photosensitive drums  28 . 
     The image fixing section  8  includes a heater roller  44  built-in with a heater (not shown), a fixing roller  45 , a metallic endless belt  46  which is wound around the heater roller  44  and the fixing roller  45 , and a presser roller  47  which is pressed against the fixing roller  45 . The image transfer section  7  further includes an oil supply roller  48  for applying silicone oil onto the outer surface of the endless belt  46 , an oil replenish roller  50  for replenishing silicone oil to the oil supply roller  48 , and a cleaning roller  51  for wiping off oil residue on the surface of the oil replenish roller  50 . 
     The sheet discharge section  10  includes a first discharge unit  52  for receiving the sheet members  3  discharged from the fixing section  8  one by one to stack the sheet members  3  one over another with the surface thereof formed with an image facing upward, and a second discharge unit  54  for guiding the sheet members  3  discharged from the fixing section  8  one by one toward an upper part of a main body  36  of the image forming apparatus via a discharge path  53  to stack the sheet members  3  one over another with the surface thereof formed with an image facing downward. In the case where the first discharge unit  52  is not used, it is accommodated in the apparatus main body  36 . 
     The optical unit  11  is constructed in such a manner that a set of a polygonal mirror  55 , lenses  56 ,  57 , and a mirror  58  is arranged at a certain position in correspondence with the corresponding photosensitive drum  28 . A laser beam is irradiated onto the surfaces of the photosensitive drums  28  via slits  61  each of which is formed in an optical unit support plate  60  at a certain position corresponding to the photosensitive drum  28 . 
     In the color printer  2  having the above construction, when a sheet member  3  is fed from the first feeder unit  15  or the second feeder unit  18 , the sheet member  3  is guided first between the photosensitive drum  28  of the first imaging unit  24  and the transfer belt  37  via the sheet transport section  6 . Wile the sheet member  3  is conveyed over the transfer belt  37  between the transfer rollers  41   a ,  41   b ,  41   c ,  41   d  and the corresponding photosensitive drums  28  in this order sequentially, toner images of respective colors (black, magenta, cyan, yellow) formed by the respective photosensitive drums  28  are sequentially transferred onto the sheet member  3 . 
     When the sequential color image transfer is completed, the sheet member  3  is guided between the fixing roller  45  and the presser roller  47  while conveyed by the transfer belt  37  to thereby fix the color image. After the color image fixation, the sheet member  3  is discharged onto the first discharge unit  52  or the second discharge unit  54 . 
     Next, the developing device for use in an image forming apparatus according to this invention is described in detail with reference to FIGS. 2 to  4 . 
     FIG. 2A is a diagram showing an internal mechanism of the developing unit  30  viewed from top of the apparatus main body  36 , FIG. 3 is a cross-sectional view taken along the line  3 — 3  in FIG. 2A, and FIG. 4 is a cross-sectional view taken along the line  4 — 4  in FIG.  2 A. 
     The developing unit  30  has a developing housing  144 . The housing  144  is made of a synthetic resin material, and generally includes a bottom wall  132 , a first side wall  134  generally extending upward perpendicularly from the bottom wall  132  over the entire length thereof (axial direction of the developing unit  30 ), a second side wall  136  formed on left side of the first side wall  134  in FIG. 2A, a front wall  138  formed at a left end of the first side wall  134  in FIG. 2A, a rear wall  140  formed at a right end of the first side wall  134  in FIG. 2A, and a top wall  142  formed at a top part of the housing  144  as shown in FIGS. 3 and 4. The second side wall  136  constitutes part of a developing agent scoop-up section of the developing unit  30  which is described below. 
     Note that the axial direction of the developing unit  30  corresponds to the depthwise direction of the apparatus main body  36 , and the left side and the right side of the developing unit  30  in FIGS. 2A and 5 respectively correspond to front side and rear side of the apparatus main body  36 . Further, left and right directions in FIG. 1 corresponds to widthwise directions of the apparatus main body  36 . 
     As shown in FIG. 3, the housing  144  is provided with a developing agent supply unit  146  at a left end thereof. The developing agent supply unit  146  includes a cylindrical sleeve  148  composed of a non-magnetized material such as aluminum and extending axially to generally cover a developing area by the developing device for image formation, and an inner member  150  provided inside the developing sleeve  148  and composed of a magnetic material. 
     The sleeve  148  is rotatably connected to a driver system which is described below, and the inner member  150  is fixed to the sleeve  148 . As shown in FIG. 2A, a rotary shaft  151  fixed to the developing sleeve  148  protrudes in depthwise directions of the apparatus main body  36  and passes through the rear wall  140 . An input gear  152  is fixed to a protruding end of the rotary shaft  151 . When a drive source (not shown) of the driver system is driven, a driving force thereof is transmitted to the input gear  152  by way of an output gear  154  and an idle gear  156 , thereby rotating the rotary shaft  151  in a direction shown by the arrow R 1  in FIG. 3. A developing agent  100  consisting of toner particles and carriers is conveyed on the outer surface of the developing sleeve  148  due to magnetic attraction of the carriers toward the magnetic member  150  provided inside the developing sleeve  148 . The manner of conveying the developing agent  100  is described below in detail. 
     Hereinafter, specifications of toner and carrier as main components of a developing agent used in the present invention, but not limited thereto, are described. 
     (1) Carrier: 
     Carrier medium grain size (diameter): 0.6 μm; 
     Shape: Spherical; 
     Material: Ferrite; 
     Saturation Magnetization: 65 emu; 
     (2) Toner: 
     Toner medium grain size: 8.5 μm; 
     Resin composition: Polyester; 
     Toner Density: fluctuates in a range of 4-5 wt % under normal condition and fluctuates in a range of 3.5 to 6 wt % under non-ordinary condition such as a shortage of toner or a change in environmental factor. 
     Note that the above specifications for the carrier and the toner are one of the examples, thus inducing no limitations to the application of the present invention. 
     As shown in FIG. 3, a developing agent scraper  160  is provided above the developing agent supply unit  146 . The scraper  160  includes a blade  62  with a pointed edge portion at a lowermost part thereof and a holder  64  for holding the blade  62 . The holder  64  is fixedly supported at a certain position on an upper front part of a partition wall  66  which is adapted to accommodate a first mixing unit  68 . The edge portion of the blade  62  extends substantially over an entire axial length of the developing unit  30  at a slightly downstream position from an uppermost part of the developing sleeve  148  with respect to the rotating direction R 1  of the developing sleeve  148 . 
     The edge portion of the blade  62  is set generally in the middle between a magnetic field of north pole and a magnetic field of south pole at an upper part of the magnetic member  150  (see FIG.  3 ). Residue of the developing agent  100  which has deposited on the circumferential surface of the developing sleeve  148  is scraped off by the edge portion of the blade  62 , subjected to agitation/mixing by a second mixing unit  70 , and then reused for next image formation by magnetic attraction of the carriers of the developing agent  100  toward the surface of the developing sleeve  148  again. 
     Next, an operation of a developing agent mixing device of the developing unit  30  is described in detail with reference to FIG.  5 . FIG. 5 is an expansion view showing a view of the developing unit  30  taken along a line extending through a center of the developing agent supply unit  146 , a center of the second mixing unit  70 , a center of the first mixing unit  68 , and a center of a scoop-up unit. The first and second mixing units  68 ,  70  constitute the developing agent mixing device. 
     The first mixing unit  68  is constructed in such a manner that a rotary shaft  72  is rotatably mounted between the front wall  138  and an inner wall  141  formed on the inner side (left side in FIG. 5) of the rear wall  140 . The rotary shaft  72  is formed with an impeller  74  generally over the axial length thereof except an area A 1  at a downstream end with respect to a first conveyance direction of the developing agent  100  shown by the arrow C 1  in FIG.  5 . The rotary shaft  72  extends outwardly from the front wall  138  by a certain length, and a gear  80  is attached to the distal end thereof. A gear  75  is fixed to the rotary shaft  72  between the rear wall  140  and the inner wall  141 . 
     The second mixing unit  70  is constructed in such a manner that a rotary shaft  76  is rotatably mounted between the front wall  138  and the rear wall  140 . Similar to the rotary shaft  72 , the rotary shaft  76  is formed with an impeller  78  generally over the axial length thereof except an area A 2  at a downstream end with respect to a second conveyance direction of the developing agent  100  shown by the arrow C 2  in FIG.  5 . The output gear  154  is fixed to a right end of the rotary shaft  76 , and a gear  81  is mounted to the rotary shaft  76  between the inner wall  141  and the rear wall  140 . The gear  81  is interlocked with the gear  75 . 
     Next, the developing agent scoop-up unit  82  is described in detail. As shown in FIG. 4, the developing agent scoop-up unit  82  is provided at a substantially middle position vertically between the first mixing unit  68  and the second mixing unit  70  on the right side of the first and second mixing units  68 ,  70 . 
     Referring back to FIG. 5, the developing agent scoop-up unit  82  is provided on the front side (left side in FIG. 5) of the housing  144 , namely, beyond the developing area of the developing sleeve  148  in the axial direction of the developing unit  30 . The developing agent scoop-up unit  82  is constructed in such a manner that a rotary shaft  84  is rotatably mounted between the front wall  138  and a second rear wall  83 . The rotary shaft  84  is fixedly mounted with a scoop-up roller  86  composed of a magnetic material generally over the axial length thereof. The rotary shaft  84  protrudes forward (leftward in FIG. 5) from the front wall  138 , and a gear  88  is fixed at the protruding end. The gear  88  is interlocked with the gear  80 . The scoop-up roller  86  is in the shape of a cylinder with a continued surface and made of a ferrite material. The scoop-up roller  86  is magnetized in such a manner that a magnetic field generates in eight equi-sectioned areas circumferentially with each field having a magnetic force of about 800 gauss and that north pole and south pole appear alternately in the eight fields. 
     As shown in FIGS. 2A,  5  and  8 , a toner supply unit TS having a toner supply opening TSO is provided in an area vertically above the merging area between the first mixing unit  68  and the scoop-up roller  86  so that the toner can be directly supplied onto the merging area. More specifically, the toner supply opening TSO is preferably provided on a downstream side in the merging area along the developing agent delivering direction by the first mixing unit  68 . By positioning the toner supply opening TSO as described in the above, the supplied toner from the toner supply opening TSO is subject to both a scoop-up force derived by the scoop-up roller  86  and a delivering (propelling) force derived by the first mixing unit  68  so that the supplied toner is well blended with the existing developing agent. Furthermore, there is a slight clearance between a lead end  112 e of a separation wall  112  and an external surface of the scoop-up roller  87 , thus the supplied toner from the toner supply opening TSO may not easily fall through the clearance. Note that the lead end  112   e  of the separation wall is also referred to as a scraping portion that scrapes off the excessive amount of the developing agent attracted to around the scoop-up roller  86 . 
     FIG. 6 is a diagram showing an interlocked state of the gears viewed from the rear side of the developing unit  30 , and FIG. 7 is a diagram showing an interlocked state of the gears viewed from the front side of the developing unit  30 . The developing unit  30  is described further with reference to FIGS. 6 and 7. 
     When a driving force of the drive source (not shown) is transmitted to the output gear  154 , the driving force is transmitted to the gears  81 ,  75  via the rotary shaft  72 . The driving force which has been transmitted to the gear  75  is then transmitted to the gears  80 ,  88  via the rotary shaft  72 . 
     On the other hand, when the driving force of the drive source is transmitted to the output shaft  154 , the driving force is also transmitted to the idle gear  156  and the input gear  152 . Thus, as the driving force is transmitted to the gears, the first mixing unit  68  rotates in the direction shown by the arrow R 2  in FIGS. 3 and 4, and the second mixing unit  70  rotates in the direction shown by the arrow R 3  in FIGS. 3 and 4. The developing agent scoop-up unit  82  rotates in the direction shown by the arrow R 4  in FIG.  4 . 
     Referring back to FIG. 3, the developing agent  100  is stored in the developing housing  144 . More specifically, the developing agent  100  is distributed in a first mixing section  102 , a second mixing section  104 , and a third mixing section  106  of the developing unit  30 . The first mixing section  102  is defined by the partition wall  66  and the top wall  142  in which part of the first mixing unit  68  is accommodated. The second mixing section  104  is defined by a lower surface of the partition wall  66 , the first side wall  134 , and the bottom wall  132  in which the developing agent supply unit  146  and part of the second mixing unit  70  are accommodated. The third mixing section  106  is defined by the second side wall  136  (see FIG.  4 ), the bottom wall  132 , and the top wall  142  in which part of the first mixing unit  68 , part of the second mixing unit  70  and the scoop-up unit  82  are accommodated. 
     In the above construction where the developing agent  100  is distributed in each section, as the rotary shaft  72  of the first mixing unit  68  rotates in the direction of arrow R 2 , the developing agent  100  in the first mixing section  102  is conveyed along the axial direction of the rotary shaft  72 , namely, in the first conveyance direction C 1  shown in FIG. 5 while being mixed well homogeneously during its conveyance. 
     The developing agent  100  in the second mixing section  104  is conveyed in the second conveyance direction C 2  as the rotary shaft  76  of the second mixing unit  70  rotates in the direction of arrow R 3 . The developing agent  100  is mixed well homogeneously during its conveyance. At this time, part of the developing agent  100  is supplied from the second mixing unit  70  to the developing agent supply unit  146 , and then supplied to the developing area of the surface of the photosensitive drum  28  to develop a latent image into a toner image. The photosensitive drum  28  rotates in the direction of arrow R 5  in FIGS. 3 and 4. 
     Next, with reference to FIGS. 5 and 8, how the developing agent  100  circulates while mixed and conveyed along the predetermined directions is described. First, the developing agent  100  in the first mixing section  102  is conveyed in the first conveyance direction C 1 , namely, from front to rear of the apparatus main body  36  by the operation of the first mixing unit  68 . At this time, since the first mixing section  102  is not formed with the partition wall  66  at the area A 1  and the impeller  74  is not formed on the first mixing unit  68  at the area A 1 , the developing agent  100  that has reached the area A 1  is restrained from being conveyed further rearward in the first conveyance direction C 1  and falls down from the first mixing section  102  into the second mixing section  104  by its weight at the area A 1  (see an arrow D in FIG.  8 ). 
     Then, the developing agent  100  that has fallen off from the first mixing section  102  into the second mixing section  104  is conveyed in the second conveyance direction C 2  in FIG.  5  and FIG. 8 by the operation of the second mixing unit  70  toward the area A 2  at the left end in FIG.  5 . Since the impeller  78  is not formed on the second mixing unit  70  at the area A 2 , there is no possibility that the developing agent  100  is conveyed further forward in the second conveyance direction C 2 . In addition, there is no possibility that the developing agent  100  stagnates on the area A 2  and agglomerates thereat for the following reason. 
     When the developing agent  100  reaches the area A 2 , the scoop-up roller  86  of the developing agent scoop-up unit  82  magnetically attracts the developing agent  100  (carriers along with the toner particles) toward the circumferential surface thereof by its magnetic force and guides the developing agent  100  into the third mixing section (scoop-up section)  106  (see an arrow U in FIG.  8 ). 
     In the scoop-up section  106 , the partition wall  112  (see FIG. 4) extends transversely from a side wall of the housing  144  in FIG. 4 at a vertically middle position between the first mixing unit  68  and the second mixing unit  70 . The lead end  112   e  (or scraping portion) of the partition wall  112  is formed into an acute angular shape with a clearance of e.g., about 1.0 mm from the outer surface of the scoop-up roller  86 . 
     The developing agent  100  magnetically attracted to the surface of the scoop-up roller  86  constitutes a magnetic heap as shown in FIG.  4 . More specifically, the magnetic heap of the developing agent  100  deposits on the surface of the scoop-up roller  86  in such a manner that the deposited developing agent  100  forms a sharp peak corresponding to a high-magnetized region of the magnetic field of N- or S- pole and forms a moderate recess corresponding to a low-magnetized region defined circumferentially in the middle between the magnetic fields of N- and S- poles. 
     More specifically, the scoop-up roller  86  has the following specifications. 
     The length of the scoop-up roller  86  can be set in a range of 15 mm to 50 mm and is preferably set in a range of 20 mm to 35 mm, which is more than a pitch of a blade of an impeller  78  for the second mixing unit  70 . Setting the length longer than the upper limit of the specified range will cause an unnecessary enlargement of the apparatus as a whole whereas lowering the length beyond the lower limit of the specified range will result in an insufficient scoop-up force derived by the scoop-up roller  86 . The peripheral speed of the scoop-up roller  86  can be set in a range of 100 to 250 mm/min and is preferably set in a range of 150 to 200 mm/min. Raising the peripheral speed beyond the upper limit of the specified range will rapidly scrape off the developing agent around the scoop-up roller  86  by the lead end  112   e  of the partition wall, causing the developing agent to deteriorate more easily. On the other hand, lowering the peripheral speed beyond the lower limit of the specified range will result in an insufficient scoop-up capability derived by the scoop-up roller  86 . Furthermore, the magnetic force for the scoop-up roller  86  is set in a range of 500 to 1200 Gauss. Exceeding the upper limit of the specified range may increase the possibility to deteriorate the developing agent whereas lowering beyond the lower limit of the specified range will result in an insufficient scoop-up capability of the scoop-up roller  86 . The clearance between the circumference of the scoop-up roller  86  and the lead end (scraping portion)  112   e  of the separation wall  112  is set to be in a range of 0.3 mm-3 mm, and preferably in a range of 0.5 mm to 2 mm. Having the clearance larger than the upper limit of the specified range will cause an insufficient supply of the developing agent to the first mixing unit  68 . On the other hand, the clearance smaller than the lower limit of the specified range will increase the chance of deteriorating the developing agent. 
     In this embodiment, since the magnetic field generates in such a state that the field of N-pole and the field of S-pole generate alternately in the circumferential direction of the scoop-up roller  86 , a field having a magnetic force of 0 gauss does not exist in the scoop-up roller  86 . Therefore, the scoop-up force (conveyance force) of the developing agent  100  by the scoop-up unit  82  can be varied according to needs. 
     As the scoop-up roller  86  rotates in the direction of arrow R 4 , the magnetic heap of the developing agent  100  deposited in the aforementioned manner on the surface of the scoop-up roller  86  is scraped off therefrom by the lead end  112   e  of the separation wall  112 . The thus collected developing agent  100  is conveyed to the first mixing section  102  while carried over the transversely extending upper surface of the separation wall  112 . When the developing agent  100  reaches the first mixing section  102 , the developing agent  100  is conveyed in the first conveyance direction C 1  by the rotation of the rotary shaft  72  of the first mixing unit  68  while mixed well. In this way, the developing agent  100  circulates while conveyed back and forth in the axial direction of the developing unit  30  in such a manner that the developing agent  100  is homogeneously mixed during the back and forth conveyance. 
     The developing agent mixing device is described in detail. The developing agent conveyance forces of the impeller  74  of the first mixing unit  68  and that of the impeller  78  of the second mixing unit  70  are set generally at the same level. Parameters to adjust the conveyance force include sizes of the impellers  74 ,  78 , and shape thereof (twisting direction), spiral pitch thereof, and rotational speed, which enables to set the conveyance forces of the first and second mixing units  68 ,  70  in a desired relation. As one example for the present invention, an external diameter of the impeller of the second mixing unit  70  is set smaller than that of the first mixing unit  68 . On the other hand, the rotational speed of the second mixing unit  70  is set greater than that of the first mixing unit  68 . As a result, the conveyance capability (amount of developing agent per a given time) of the developing agent by the second mixing unit  68  is set substantially equal to that by the first mixing unit  68 . Note that in this embodiment, a diameter of a shaft on which impeller wings are mounted for the first mixing unit  68  and that for the second mixing unit  70  are substantially the same so that when the external diameter of the impeller is greater than the other, the radial length of the impeller is longer than that of the other. With this feature, the developing agent supply unit  146  can be positioned further rightward direction (in FIG.  3 ). As a result, the width (a left to right dimension in FIG. 3) of the developing device as a whole can be made smaller, thereby enabling to make a space between the imaging sections  24 ,  25 ,  26 ,  27  small as well, which eventually contributes downsizing the printer  2  as a whole. 
     Furthermore, as shown in FIGS. 2A,  2 B, and  5 , a separation plate  190  having a U-shape cross section is provided in the downstream portion of the first mixing unit  68 . More specifically, the separation plate  190  is provided on the downstream side of the impeller  74  at the most downstream side along the first mixing unit  68  so that the developing agent accumulates in an area immediately upstream side of the separation plate  190 . With this construction, the developing agent accumulates beyond the amount held by the separation plate  190  overflows from the separation plate  190 . Accordingly, the overflowed developing agent from the separation plate  190  is supplied to the second mixing unit  70 . 
     The upper edge of the separation plate  190  is so set as to come to a position as high as approximately eighty percent (0.8φ) of the diameter (φ) of the impeller  74  as shown in FIG.  2 B. 
     In addition, the conveyance capability (amount of developing agent per a given time) derived by the scoop-up roller  86  is set larger than that by the impeller  74  of the first mixing unit  68  and that by the impeller  78  of the second mixing unit  70 . Specifically, the conveyance capability of the developing agent by the scoop-up roller  86  is set in a range of 1.0 to 1.5 times of the conveyance capability by the second mixing unit  70 . 
     More specifically, the conveyance force of the first and second mixing units  68 ,  70  is set at 1,120 g/min, and that of the developing agent scoop-up unit  82  is set at 1,350 g/min. Note that the present invention is not necessarily limited to the specified numerals. 
     When the image forming apparatus stops its operation, the developing agent tends to accumulate in downstream portion of the second mixing unit  70  and if the usual amount of the developing agent is delivered upon resuming the operation when the accumulated agent was not yet removed, it may adversely affect the conveyance performance of the developing agent. However, since the conveyance capability by the scoop-up roller  86  in the present invention is set greater than that by the second mixing unit  70 , the agent accumulation in the downstream area of the second mixing unit  70  is scooped up to the first mixing unit  68  faster than the developing agent being transferred from the upstream side of second mixing unit  70 . As a result, with the above described arrangement, the likely occurring problem, i.e., excessive accumulation of the agent in the downstream side of the second mixing unit  70 , will be prevented. Furthermore, the amount of the developing agent delivered from the downstream side of the first mixing unit  68  to the upstream side of the second mixing unit  70  may fluctuate because of the presence of the separation plate  190 . However, even if it fluctuates, conveyance of the developing agent throughout the entire passage will be well balanced because of the greater conveyance ability given to the scoop-up roller  86  than the second mixing unit  70 . Moreover, the greater conveyance ability of the scoop-up roller  86  functions as a buffer to cope with the changes in conveyance capability of the each component of the developing agent due to the variation of the toner density or other environmental factors such as temperature and humidity. With this arrangement, when the developing agent  100  reaches the downstream end with respect to the second conveyance direction C 2 , namely, the area A 2  in FIG. 5 after conveyed in the second conveyance direction C 2  by the second mixing unit  70  while being homogeneously mixed in the second mixing section  104 , the scoop-up roller  86  magnetically draws up the developing agent  100  that has reached the area A 2  of the second mixing unit  70  into the third mixing section  106  by the operation of the scoop-up unit  82 . Then, the developing agent  100  is securely and speedily supplied to the leftmost end of the first mixing unit  68  (see FIG. 5) located above the area A 2  of the second mixing unit  70  after carried over the upper surface of the separation wall  112 . With this arrangement, there can be prevented stagnation and agglomeration of the developing agent  100  in the developing unit  30  because the developing agent  100  is conveyed in a well-mixed manner in the predetermined directions back and forth, and a desirable image formation is carried out. 
     Referring back to the arrangement of the housing  144 , a toner density detector (not shown) is provided at an appropriate position inside the housing  144  to detect the toner density of the developing agent  100 . The toner density detector detects the toner density by sensing permeability of the developing agent  100 . 
     Further, the toner supply unit TS is provided in the housing  144  which is operable in response to a detection output from the toner density detector  180 . The toner supply unit TS automatically replenishes toner particles when the output of the toner density detector  180  falls below a predetermined level and suspends the replenishment when the output reaches the predetermined level. With this arrangement, the toner density in the housing  144  can be maintained in a certain range to develop a latent image into a toner image at a desirable toner density. 
     As an embodiment of this invention, but not limited thereto, the toner density detector  180  is provided near the separation plate  190  on its upstream side as shown in FIG.  2 A. More specifically speaking, the toner density detector  180  has a sensing portion  181 , generally a cylindrical form, which is provided at substantially the same level as the center axis of the first rotary shaft  72  and immediately upstream of the separation plate  190  such that the sensing portion can be maintained in contact with the accumulated developing agent in the downstream area of the first mixing unit  68 . Furthermore, the sensing portion  181  has a sensing surface  181   s  which is cleaned up by a horizontally extended edge  74   s  of the impeller  74  as it rotates over the sensing surface  181   s . As a result, the sensor  181  does not usually surface over the accumulated developing agent so that the sensor  181  does not likely sense the density of something other than the developing agent, thereby improving an accuracy in measurement. Furthermore, the place immediately upstream side of the separation plate  190  is where the developing agent has been well-mixed. Thus the sensor  181  can detect the toner density accurately. 
     As mentioned above, in one aspect of this invention, the developing device is constructed in such a manner that the developing agent is conveyed from upstream to downstream with respect to the first conveyance direction while being agitated by the first mixing unit and reaches the downstream end. Then, the developing agent is conveyed from upstream to downstream with respect to the second conveyance direction while being agitated by the second mixing unit. When the developing agent reaches the downstream end with respect to the second conveyance direction, the developing agent scoop-up unit magnetically draws up the developing agent to carry the developing agent up to the first mixing unit. In this way, the developing agent can be sufficiently mixed/agitated by the first and second mixing units aided by a speedy draw-up operation of the scoop-up unit. 
     This invention is advantageous in preventing stagnation of the developing agent in the downstream end with respect to the first and second conveyance directions. Therefore, an agglomeration-free developing device is provided to ensure a clear image formation supplied with well-mixed and sufficiently-charged toner particles. 
     Further, in another aspect of this invention, the developing device is constructed in such a manner that the first and second mixing units are vertically stacked one over the other with the second mixing unit located below the first mixing unit, and the developing agent scoop-up unit is provided with its axial direction aligned with the axial direction of the first and second mixing units at a substantially middle position vertically between the first and second mixing units. Furthermore, the rotary member (scoop-up roller) of the developing agent scoop-up unit is magnetized to magnetically draw up the developing agent. With this arrangement, scoop-up unit can be made smaller and the axial length of the developing device itself can be shortened, thereby enabling to provide a developing device and image forming apparatus of a compact size. 
     An embodiment of the developing device for use in an image forming apparatus according to this invention has been described in the above section. The developing device is not limited to the above embodiment, and the following modifications and alterations are applicable. 
     (1) In the embodiment, the developing agent scoop-up unit  82  is provided on right side of the first and second mixing units  68 ,  70  on the plane of FIG.  4 . Alternatively, the scoop-up unit  82  may be provided on left side of the first and second mixing units  68 ,  70 . In the altered arrangement, the scoop-up unit  82  can be installed in a space axially extending from the developing sleeve  148  where the developing sleeve  148  is not provided. Thereby, the transverse length (left and right directions in FIG. 4) of the developing unit  30  can be reduced. This altered arrangement makes it possible to shorten the overall width of the apparatus main body  36  even if the developing device is utilized in an image forming apparatus provided with a four-membered tandem type developing device as a color printer. 
     (2) In the embodiment, a sleeve is not provided on the scoop-up roller  84  of the developing agent scoop-up unit  82 , which reduces the transverse size of the developing unit  30  (namely, the width of the apparatus main body  36 ). A sleeve may be mounted on the scoop-up roller  84  since mounting of the sleeve does not substantially influence the overall width of the apparatus main body  36 . 
     (3) In the embodiment, the photosensitive drum  28  is used as a photoreceptor. A photosensitive belt may be used as a photoreceptor. 
     This application is based on patent application No. 11-216466 filed in Japan, the contents of which are hereby incorporated by references. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative an not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to embraced by the claims.