Abstract:
A developing apparatus includes a frame supporting the developing roller; a sealing member provided at each of one and the other ends of the developing device frame and contacted to the developing roller to prevent leakage of the developer; and a developing blade contacted to the developing roller to regulate an amount of the developer carried on the developing roller, the developing blade having a free end in a downstream side with respect to a rotational moving direction of the developing roller, wherein each of one and the other ends of the developing blade is provided with a projected portion projecting in the rotational axis direction between the developing roller and the sealing member.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image forming apparatus, a developing device for an image forming apparatus, and a process cartridge removably installable in an image forming apparatus. 
         [0002]    Generally, developing devices are provided with a development roller and a blade (which hereafter may be referred to as development blade). The development blade regulates the amount by which developer is borne on the development roller, by being placed in contact with the peripheral surface of the development roller. Further, some of these developing devices are structured so that the free edge (regulating edge) of the development blade is on the downstream side, in terms of the rotational direction of the development roller, relative to the base portion of the blade, by which the blade is anchored. This structural arrangement for a developing device, however, requires means for preventing toner from leaking from a developing device through the portions of the device, which are in the adjacencies of the lengthwise ends of the development roller and/or development blade. Thus, a substantial number of inventions have been made to solve this problem. A couple of these inventions are disclosed in Japanese Laid-open Patent Applications S62-192770 and H07-333981. 
         [0003]    According to the invention disclosed in Japanese Laid-open Patent Application S62-192770, the developing device is provided with a pair of seals for preventing the leak. More specifically, the seals are shaped so that they perfectly fill the wedge-shaped gaps, one for one, which are between the development blade and development roller, at the lengthwise ends of the device. 
         [0004]    In the case of the invention disclosed in Japanese Laid-open Patent Application H07-333981, the developing device is provided with a pair of seals which are in the form of a brush. Each seal fills the wedge-shaped gap between the development blade and development roller, at one of the lengthwise ends of the development device. 
         [0005]    The structural arrangements disclosed in the aforementioned patent applications, however, are problematic in that unless the shape of the seal perfectly matches the shape of the gap, and/or if the seal is erroneously installed, it is impossible for the presence of the seal to ensure that the toner does not leak (and scatter) from the developing device. 
         [0006]    More specifically, if the seal happens to be smaller than the gap between the development blade and development roller, there remain gaps between the sealing member and development blade, and/or between the sealing member and development roller. On the other hand, if the seal is larger than the wedge-shaped gap between the development blade and development roller, the sealing member lifts the development blade, creating thereby a gap between the development blade and development roller. That is, the inaccuracy in sealing member size (whether sealing member is too small or too large) leaves and/or creates gaps between the development blade and development roller, at the lengthwise ends of the developing device, and these gaps allow the developer in the developing device to leak (scatter) from the lengthwise ends of the device. 
       SUMMARY OF THE INVENTION 
       [0007]    Thus, the primary object of the present invention which was made in consideration of the above described problem is to provide a developing device which is structured so that the free edge (developer regulating edge) of its development blade is on the downstream side, in terms of the rotational direction of the development roller, relative to the base portion of the development blade, by which the blade is anchored, and yet, does not allow to leak toner from its areas corresponding to the lengthwise ends of the development roller. 
         [0008]    According to an aspect of the present invention, there is provided a developing apparatus comprising a developing device frame supporting said developing roller; a sealing member provided at each of one and the other ends of said developing device frame with respect to a rotational axis direction of said developing roller and contacted to said developing roller to prevent leakage of the developer; and a developing blade contacted to said developing roller to regulate an amount of the developer carried on said developing roller, said developing blade having a free end in a downstream side with respect to a rotational moving direction of said developing roller, wherein each of one and the other ends of said developing blade with respect to the rotational axis direction is provided with a projected portion projecting in the rotational axis direction between said developing roller and said sealing member. 
         [0009]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic sectional view of the image forming apparatus equipped with the development unit in one of preferred embodiments of the present invention, and shows the general structure of the apparatus. 
           [0011]      FIG. 2  is an enlarged sectional view of one of the cartridges shown in  FIG. 1 , and shows the general structure of the cartridge. 
           [0012]      FIG. 3  is a schematic perspective view of one of the lengthwise ends of the development unit of the cartridge shown in  FIG. 2 . 
           [0013]      FIGS. 4(   a ),  4 ( b ), and  4 ( c ) are: a plan view of the lengthwise ends of the development unit, as seen from the direction of the photosensitive member unit  13 ; a sectional view of the combination of the development roller  17 , seal  25 , development blade  21 , development unit frame  18 , and development blade supporting metallic plate  22 , at a plane J-J in  FIG. 4(   a ); and a sectional view of the combination of the development roller  17 , seal  25 , development blade  21 , development unit frame  18 , and development blade supporting metallic plate  22 , at a plane K-K in  FIG. 4(   a ), respectively. 
           [0014]      FIG. 5  is an enlarged schematic sectional view of the development unit  4  at a plane perpendicular to the lengthwise direction of the development unit  4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Hereinafter, one of the preferred embodiments of the present invention is described in detail with reference to the appended drawings. However, the measurements, materials, and shapes of the structural components of the developing device, and the positional relationship among the structural components, in the following embodiment of the present invention, are not intended to limit the present invention in scope, unless specifically noted. 
       Embodiment 
       [0016]      FIG. 1  is a schematic sectional view of the image forming apparatus  100  in one of the preferred embodiments of the present invention, and shows the general structure of the apparatus  100 . The apparatus  100  has development units  4 Y- 4 K which are in accordance with the present invention. It uses one of the known electrophotographic image formation processes. As is evident from  FIG. 1 , the image forming apparatus  100  has the main assembly  100 A (which hereafter is referred to simply as “apparatus main assembly”). It has also the image formation stations  51 Y- 51 K which are in the main assembly  100 A. The image formation stations  51 Y- 51 K include photosensitive drums  1 Y- 1 K, primary transfer rollers  8 Y- 8 K, etc., respectively. Each of the photosensitive drums  1 Y- 1 K is an “image bearing member”, and each of the primary transfer roller  8 Y- 8 K is a “toner image transferring device”. 
         [0017]    The image forming apparatus  100  is a full-color laser printer of the so-called inline type. It employs one of the known intermediary transfer mediums. It can form a full-color image on a sheet P of recording medium (for example, ordinary recoding paper, plastic sheet, fabric, and the like), based on the information of an image to be formed. The information of an image to be formed is inputted into the apparatus main assembly  100 A from an image reading device which is connected to the apparatus main assembly  100 . The information is inputted into the apparatus main assembly  100 A also from a host device, such as a personal computer, which is in connected with the apparatus main assembly  100 A so that information can be exchanged between the image forming apparatus  100  and the host device. 
         [0018]    The image forming apparatus  100  has multiple image formation stations, more specifically, the first, second, third, and fourth image formation stations  51 Y,  51 M,  51 C, and  51 K for forming yellow (Y), magenta (M), cyan (C), and black (K) monochromatic images, respectively. The first to fourth image formation stations  51 Y- 51 K are sequentially aligned in parallel in the direction perpendicular to the bottom wall of the external shell of the image forming apparatus  100 . 
         [0019]    The first to fourth image formation stations  51 Y- 51 K are different in the color of the images they form, but are practically the same in structure and operation. Thus, they are going to be described together unless differentiation is necessary for specific reasons. That is, the suffixes Y, M, C, and K assigned to the referential codes to indicate colors may be eliminated. 
         [0020]    The image forming apparatus  100  has multiple photosensitive drums  1 , more specifically, four photosensitive drums  1 Y- 1 K, which are electrophotographic photosensitive members. The photosensitive drums  1  are aligned in parallel in the direction perpendicular to the bottom wall of the external shell of the apparatus main assembly  100 A. The photosensitive drums  1 Y- 1 K are rotated in the direction (counterclockwise direction) indicated by an arrow mark A ( FIG. 2 ) by an unshown driving means (power source). 
         [0021]    Further, the image forming apparatus  100  has a scanner unit  3  (exposing device) as an exposing means which forms an electrostatic latent image on the peripheral surface of each of the four photosensitive drums  1  by scanning the peripheral surface of the photosensitive drum  1  with a beam of laser light which it projects while modulating the beam with electrical signals which reflect the information of the image to be formed. The exposing means is in the top portion of the apparatus main assembly  100 A. 
         [0022]    Each image formation station has a charge roller  2 , a development unit  4  ( 4 Y- 4 K), a cleaning member  6  ( 6 Y- 6 K), which are in the adjacencies of the peripheral surface of the photosensitive drum  1 . The charge roller  2  is a charging means for uniformly charging the peripheral surface of the photosensitive drum  1 . The development unit  4  is for developing an electrostatic latent image into a visible image, that is, an image formed of toner (developer) (which hereafter may be referred to simply as toner image). The cleaning member  6  is for removing the transfer residual toner, that is, the toner remaining on the peripheral surface of the photosensitive drum  1  after the toner image transfer. 
         [0023]    The image forming apparatus  100  has also an intermediary transfer belt  5  for transferring the toner image from the photosensitive drum  1  onto the sheet P of recording medium. The intermediary transfer belt  5  is positioned so that it faces all of the four photosensitive drums  1 . It is going to be described later in more detail. 
         [0024]    In terms of the rotational direction of the photosensitive drum  1 , the point at which the photosensitive drum  1  is charged by the charge roller  2 , point at which the peripheral surface of the photosensitive drum  1  is exposed by the scanner unit  3 , point at which an electrostatic latent image is developed, point at which a toner image is transferred onto the intermediary transfer belt  5 , and point at which the peripheral surface of the photosensitive drum  1  is cleaned by the cleaning member  6  are in the listed order. 
         [0025]    The development unit  4  uses nonmagnetic single-component developer. That is, it uses plain toner. It develops an electrostatic latent image in reverse, by placing its development roller  17  ( FIG. 2 ), as a developer bearing member, in contact with the peripheral surface of the photosensitive drum  1 . More concretely, it develops an electrostatic image by adhering charged toner to the portions of the exposed portion of the peripheral surface of the photosensitive drum  1 , which were made to attenuate in electrical charge by the exposure. In the case of this image forming apparatus, the toner polarity (negative in this embodiment) is the same as the polarity to which the peripheral surface of the photosensitive drum  1  is charged. 
         [0026]    The photosensitive drum  1  and drum processing means, that is, charge roller  2 , development unit  4 , and cleaning member  6 , are integrated in the form of a cartridge (which hereafter may be referred to as process cartridge  7 , or simply as cartridge  7 ). The cartridge  7  is removably mountable in the apparatus main assembly  100 A by being assisted by cartridge mounting (removing) means, such as cartridge insertion guides and cartridge positioning members, etc., with which the apparatus main assembly  100 A is provided. The cartridges  7  for the four image formation stations  51 Y˜ 51 K are the same in shape, and contain yellow (Y), magenta (M), cyan (C), and black (K) toners, respectively. 
         [0027]    The intermediary transfer belt  5  as an intermediary transferring member is an endless belt. It is in contact with all the photosensitive drums  1 , and is circularly moved (rotated) in the direction (clockwise direction) indicated by an arrow mark B. It is supported and kept stretched (tensioned) by multiple belt supporting members, that is, a belt driving roller  51 , a roller  52 , and a roller  53 . 
         [0028]    The roller  52  is a roller for backing up the intermediary transfer belt  5  against a second transfer roller. It opposes the second transfer roller, with the presence of the intermediary transfer belt  5  between itself and second transfer roller. The rollers  52  and  53  are rotated by the movement of the intermediary transfer belt  5  while supporting the belt  5 . 
         [0029]    There are four primary transfer rollers  8 Y- 8 K as primary transferring means on the inward side of the loop of the intermediary transfer belt  5 . The rollers  8 Y- 8 K are parallel to each other, and oppose the photosensitive drums  1 Y- 1 K, one for one. More specifically, they keep the intermediary transfer belt  5  pressed upon the photosensitive drums  1 Y- 1 K, one for one, forming thereby primary transfer nips N 1 , in which the intermediary transfer belt  5  contacts the photosensitive drums  1 Y- 1 K. Further, the image forming apparatus  100  is provided with an unshown primary transfer bias power source (high voltage source) as the primary transfer bias applying means for applying bias to the primary transfer rollers  8 Y- 8 K. The bias applied to the primary transfer rollers  8 Y- 8 K is opposite in polarity to the normal polarity to which toner becomes naturally charged. As the voltage is applied to the primary transfer rollers  8 , the toner images on the photosensitive drums  1 Y- 1 K are transferred (primary transfer) onto the intermediary transfer belt  5 . 
         [0030]    Further, the image forming apparatus  100  is provided with a secondary transfer roller  9  as the second transferring means, which is positioned on the outward side of the loop of the intermediary transfer belt  5  so that it opposes the belt backing roller  52 . More specifically, the secondary transfer roller  9  is pressed on the belt backing roller  52 , with the presence of the intermediary transfer belt  5  between itself and belt backing roller  52 , forming thereby the secondary transfer nip N 2 , in which the intermediary transfer belt  5  is in contact with the secondary transfer roller  9 . Further, the image forming apparatus  100  is provided with an unshown second transfer bias power source (high voltage power source) as the second transfer bias applying means, which is for applying bias to the secondary transfer roller  9 . As the secondary transfer bias is applied to the secondary transfer roller  9 , the toner image(s) on the intermediary transfer belt  5  is transferred (secondary transfer) onto the sheet P of recording medium. Incidentally, the primary transfer rollers  8  and secondary transfer roller  9  are the same in structure. 
         [0031]    The image formation sequence carried out by the image forming apparatus  100  is as follows: First, the peripheral surface of the photosensitive drum  1  is uniformly charged by the charge roller  2 . Then, the uniformly charged area of the peripheral surface of the photosensitive drum  1  is scanned by (exposed to) the beam of laser light projected from the scanner unit  3  while being modulated with electrical signals which reflect the information of the image to be formed. As a result, an electrostatic latent image which reflects the information of the image to be formed is effected on the peripheral surface of the photosensitive drum  1 . Then, the electrostatic latent image is developed by the development unit  4  into a visible image, that is, an image formed of toner. Then, the toner image is transferred (primary transfer) from the photosensitive drum  1  onto the intermediary transfer belt  5  by the function of the primary transfer roller  8 . 
         [0032]    When the image forming apparatus  100  is in the full-color mode, the above described process is sequentially carried out four times in the first to fourth image formation stations  51 Y- 51 K, once in each station. Thus, four monochromatic toner images, different in color, are sequentially transferred (primary transfer) in layers onto the intermediary transfer belt  5 . 
         [0033]    Meanwhile, the sheet P of recording medium is conveyed to the secondary transfer nip N 2 , in synchronism with the movement of the intermediary transfer belt  5 . Then, as the sheet P is conveyed through the secondary transfer nip N 2 , remaining in contact with the intermediary transfer belt  5 , the four monochromatic toner images, different in color, are transferred together onto the sheet P by the function of the secondary transfer roller  9  which is kept pressed against the intermediary transfer belt  5  (and belt backup roller  52 ) with the presence of the sheet P between itself and intermediary transfer belt  5 . After the transfer (secondary transfer) of the toner images onto the sheet P, the sheet P is conveyed to the fixing device  10  as the fixing means, and is conveyed through the fixing device  10 . As the sheet P is conveyed through the fixing device  10 , the fixing device  10  applies heat and pressure to the sheet P and the toner images thereon. Consequently, the toner images become fixed to the sheet P. 
         [0034]    The toner which is on the peripheral surface of the photosensitive drum  1  after the primary transfer is removed and recovered by the cleaning members  6  ( 6 Y- 6 K). The toner which is on the intermediary transfer belt  5  after the secondary transfer is removed by a cleaning device  11  for cleaning the intermediary transfer belt  5 . Incidentally, the image forming apparatus  100  is enabled to form a monochromatic image and a multicolor image, with the use of one or more of its four image formation stations  51 Y- 51 K. 
         [0035]      FIG. 2  is an enlarged schematic sectional view of the cartridge  7 , at a plane perpendicular to the lengthwise direction (axial line) of the photosensitive drum  1 . It shows the general structure of the cartridge  7 . Next, the cartridge  7 , which comprises the development unit  4  and is removably mountable in the apparatus main assembly  100 A, is described about its structure, with reference to  FIG. 2 . 
         [0036]    In the following description of the structures and operations of the development unit (developing device) and cartridge  7  in this embodiment of the present invention, terms “up”, “down”, “vertical”, “horizontal”, etc., which indicate directions, are used with the assumption that the development unit  4  and cartridge  7  are in their normal attitude in which they are when they are in use, unless specifically noted. That is, the normal attitudes in which the development unit  4  (developing device) or cartridge  7  are when they are in use are the attitudes in which they are after their proper mounting into their proper positions in the apparatus main assembly  100 A and are literally ready for image formation. All cartridges  7 Y- 7 K are virtually the same in structure and operation, although they are different in the type (color) of the toner therein. 
         [0037]    The cartridge  7  has: a photosensitive member unit  13  which has the photosensitive drum  1 , etc.; and the development unit  4  which has the development roller  17 , etc. 
         [0038]    The photosensitive member unit  13  has a frame  14  which supports various internal components of the photosensitive member unit  13 . The frame  14  functions also as the storage for the residual toner. It rotatably supports the photosensitive drum  1 , with the presence of unshown bearings between itself and the photosensitive drum  1 . The photosensitive drum  1  is rotated in the direction indicated by an arrow mark A (counterclockwise direction), in synchronism with the progression of an image forming operation, by the driving force transmitted to the photosensitive member unit  13  from an unshown motor as the driving force providing means (power source). 
         [0039]    The photosensitive drum  1  plays the central role in the image formation process. It is an organic photosensitive drum. It is made up of an aluminum cylinder, and three functional layers, that is, an undercoat layer, a carrier generation layer, and a carrier transfer layer, which are coated in layers in the listed order on the peripheral surface of the aluminum cylinder. The photosensitive drum  1  is 200 mm/s in rotational speed. 
         [0040]    There are the cleaning member  6  and charge roller  2 , in the adjacencies of the peripheral surface of the photosensitive drum  1 , being in contact with the peripheral surface of the photosensitive drum  1 . The transfer residual toner, that is, the toner remaining on the peripheral surface of the photosensitive after the first transfer, is removed by the cleaning member  6 , and is stored in the frame  14 . That is, as the transfer residual toner on the peripheral surface of the photosensitive drum  1  is removed by the cleaning member  6 , it falls into the frame  14  and remains therein. 
         [0041]    The charge roller  2 , which is a charging means, is kept pressed upon the peripheral surface of the photosensitive drum  1  so that its electrically conductive elastic layer is pressed upon the peripheral surface of the photosensitive drum  1 . It is rotated by the rotation of the photosensitive drum  1 . During an image forming operation, −1100 V of DC voltage, relative to the photosensitive drum  1 , is applied to the metallic core of the charge roller  2 , whereby the peripheral surface of the photosensitive drum  1  is uniformly charged to a potential level of roughly −550 V, that is, the pre-exposure voltage. 
         [0042]    Next, the uniformly charged area of the peripheral surface of the photosensitive drum  1  is exposed to the beam of laser light projected from the above described scanner unit  3  while being modulated with the electrical signals which reflect the data of the image to be formed. As a given point of the uniformly charged area of the peripheral surface of the photosensitive drum  1  is exposed to the beam of laser light, it is made to lose its potential by the carrier from the carrier generation layer. That is, it reduces in potential. More specifically, its potential reduces to −100 V. In other words, various points of the uniformly charged area of the peripheral surface of the photosensitive drum  1  are reduced in potential to −100 V. Consequently, an electrostatic latent image is effected on the uniformly charged area of the photosensitive drum  1 . The points of the latent image, which correspond to the exposed points, are −100 V (V 1 ) in potential level, and the points of the latent image, which correspond to the unexposed points, are −550 V (Vd) in potential level. 
         [0043]    As for the development unit  4 , it has a frame  18  which supports various internal components of the development unit  4 . The development unit  4  has the development roller  17  which is rotated in the direction (clockwise direction) indicated by an arrow mark D while being kept in contact with the peripheral surface of the photosensitive drum  1 . The development roller  17 , which is a “developer bearing member”, is a roller that develops the electrostatic image on the image bearing member by rotating while bearing developer on its peripheral surface. The image forming apparatus  100  is structured so that as the development roller  17  and photosensitive drum  1  are rotated, the peripheral surface of the development roller  17  and the peripheral surface of the photosensitive drum  1  move in the same direction (downward in this embodiment) in the area of contact between the development roller  17  and photosensitive drum  1 . The development roller  17  is rotatably supported by the frame  18  (development unit frame), by its lengthwise ends (in terms of direction parallel to its axial line), with the presence of unshown development unit plates between the development roller  17  and frame  18 . 
         [0044]    In the case of the development unit  4  in this embodiment, the peripheral surface of development roller  17  is kept in contact with the peripheral surface of the photosensitive drum  1 . However, the image forming apparatus  100  may be structured so that a preset amount of gap is kept between the peripheral surface of the development roller  17  and the peripheral surface of the photosensitive drum  1 . 
         [0045]    To the development roller  17 , −350 V of DC voltage (bias) is applied, and the toner in the development unit  4  is frictionally charged to the negative potential. Thus, in the development station, that is, where the negative charged toner on the peripheral surface of the development roller  17  comes into contact with the peripheral surface of the photosensitive drum  1 , the difference in potential level between the toner and the development roller  17  causes the toner to transfer onto only the exposed points of the peripheral surface of the photosensitive drum  1 , that is, the points with the reduced potential. Consequently, the electrostatic image is developed into a visible image made of toner. In other words, the development system in this embodiment is one of the so-called reverse development systems. 
         [0046]    The development roller  17  is a so-called elastic development roller, which is made up of a metallic core, and a cylindrical elastic layer formed on the peripheral surface of the metallic core. More specifically, the development roller  17  is made up of a metallic core, the first layer (base layer), and the second layer (surface layer). The metallic core is formed of stainless steel, and is 6 mm in diameter. The second layer is formed of solid rubber made by dispersing carbon particles in silicone rubber. It is roughly 3 mm in thickness. The second layer is formed of urethane adjusted in electrical resistance by electrically conductive agent. It is formed on the peripheral surface of the first layer to a thickness of roughly 10 μm. The peripheral velocity of the development roller  17  is set to be to roughly 1.3 times faster than that of the photosensitive drum  1 . 
         [0047]    The development unit  4  is provided with a developer supply roller  20  for supplying the peripheral surface of the development roller  17  with developer. The developer supply roller  20  is positioned so that its peripheral surface is in contact with the peripheral surface of the development roller  17 . It is rotated in the direction (clockwise direction) indicated by an arrow mark E, that is, the direction which makes the peripheral surface of the development roller  17  move in the opposite direction from the developer supply roller  20 , at the interface between the peripheral surface of the development roller  17  and the peripheral surface of the developer supply roller  20 . Not only does the developer supply roller  20  supply the peripheral surface of the development roller  17  with toner, but also, it strips the peripheral surface of the development roller  17  of the residual toner, that is, the toner which was not consumed for development, and therefore, are remaining on the peripheral surface of the development roller  17 . Further, the development unit  4  is provided with a development blade  21  which regulates in thickness the toner layer, by being placed virtually in contact with the peripheral surface of the development roller  17 , as the toner layer is formed on the peripheral surface of the development roller  17 , of the toner supplied to the peripheral surface of the development roller  17  by the developer supply roller  20 . 
         [0048]    The developer supply roller  20  is made up of an electrically conductive metallic core, and a cylindrical foamed layer (which hereafter may be referred to simply as foamed layer) formed on the peripheral surface of the metallic core, of a foamed substance, the cells of which are continuous. The foamed layer of the developer supply roller  20  plays two roles, that is, a role of supplying the development roller  17  with toner, and a role of stripping away the toner which did not contribute to the development. Regarding the stripping of the toner on the peripheral surface of the development roller  17 , the edge of each of the numerous cells of the foamed layer rubs the peripheral surface of the development roller  17 , whereby the residual toner is mechanically stripped away by the edge. 
         [0049]    More concretely, the metallic core of the developer supply roller  20  is 5 mm in external diameter. The cylindrical layer is an elastic sponge roller which is formed of relatively soft polyurethane foam. It is 5.5 mm in thickness (300-450 μm in cell diameter), and 16 mm in external diameter. Since the surface layer of the developer supply roller  20  is made of the foamed substance whose cells are continuous, the peripheral surface of the supply roller  20  is provided with peaks and valleys of the proper dimensions for the developer supply roller  20 . Therefore, it is unnecessary for a large amount of pressure to be applied to the developer supply roller  20  in order to satisfactorily strip away the toner remaining on the peripheral surface of the development roller  17 , that is, the toner which was not consumed for the development. 
         [0050]    The choices of the substance for providing the peripheral surface of the developer supply roller  20  with the scraping edges provided by the cellular structure of the substance do no need to be limited to urethane foam. That is, substances other than urethane foam can also be used as the material for the foamed layer. For example, NBR rubber, silicone rubber, acrylic rubber, hydrin rubber, ethylene propylene rubber (EPDM), compounds of the preceding substances, ordinary rubbers, etc., can be used as the material for the surface layer of the develop supply roller  20 . Further, the chloroprene rubber, styrene butadiene rubber, isoprene rubber, acrylnitrylbutadiene rubber, compounds of preceding substances, etc., are also usable as the material for the surface layer of the developer supply roller  20 . As for the substance for providing a proper amount of electrical resistance, one of the known electrically conductive ionic agents, inorganic microscopic particles, carbon black, or the like may be dispersed in the material for the surface layer of the developer supply roller  20 . 
         [0051]    Further, such bias that works in the direction to move the toner from the developer supply roller  20  to the development roller  17  may be applied to the supply roller  20  in order to assist the toner movement. That is, the amount by which toner is borne on the peripheral surface of the development roller  17 , on the upstream side of the development blade  21  in terms of the rotational direction of the development roller  17 , can be increased by applying to the developer supply roller  20  the bias that works in the direction to induce the negatively charged toner to move toward the development roller  17 . In addition, the application of such bias is likely to increase in toner density the toner layer as the toner layer is formed on the peripheral surface of the development roller  17 . In other words, even if the peripheral surface of the development roller  17  is relatively less rough, a developer layer is likely to become uniform in toner density as it is formed on the peripheral surface of the development roller  17 . 
         [0052]    The development unit  4  is structured so that in the area in which the peripheral surface of developer supply roller  20  is in contact with the peripheral surface of the development roller  17 , the peripheral surface of the supply roller  20  moves in the opposite direction from the peripheral surface of the development roller  17  (so-called counter rotation). The peripheral velocity of the developer supply roller  20  was 0.85 time that of the development roller  17 . 
         [0053]      FIG. 3  is a schematic perspective view of one of the lengthwise end portions of the development unit  4 , from which the development unit  4  is driven. It shows the structure of the lengthwise portion of the development unit  4 . As is evident from  FIG. 3 , the development unit  4  is provided with a pair of seal seating surface  18   a , which are at the ends of the development unit frame  18 , one for one, in terms of the direction parallel to the lengthwise direction of the development roller  17 . Further, the development unit  4  is provided with a pair of seal  25 , which are attached to the seal seating surfaces  18   a  (which hereafter will be referred to simply as seal seats  18   a ), one for one. The seal seats  18   a  are semi-cylindrical, and the development roller  17 , which is rotatable about its rotational axis, fits in the concaves which the seal seats  18   a  form; the lengthwise ends of the development roller  17  fit in the concaves which the two seal seats  18   a  form. The development roller  17  is rotated in the direction indicated by the arrow mark D. 
         [0054]    To the development unit frame  18 , the development blade  21  is solidly attached. The development blade  21 , which is a “regulating member”, is placed in contact with the development roller  17  to control the amount by which the developer is allowed to remain on the peripheral surface of the development roller  17  after being borne by the development roller  17 . The development blade  21  is made of an elastic substance, and is attached to a blade supporting metallic plate  22 , which is supported like a cantilever, by the development unit frame  18 . More specifically, the development blade  21  is supported by the development unit frame  18 , with the presence of the blade supporting metallic plate  22  between the blade  21  and development unit frame  18 , so that in terms of the rotational direction of the development roller  17  indicated by the arrow mark D, the free edge of the development blade  21 , that is, the developer controlling edge of the development blade  21 , is on the downstream side relative to the base portion of the development blade  21 , by which the development blade  21  is attached to the blade supporting metallic plate  22 , and also, so that the closer to the free edge of the development blade  21 , the smaller the distance between the development blade  21  and the peripheral surface of the development roller  17 . Also in terms of the rotational direction of the development roller  17  indicated by the arrow mark D, the development blade  21  is positioned so that its free edge (controlling edge) is in contact with the development roller  17 , on the downstream side of the developer supply roller  20  ( FIG. 2 ). 
         [0055]    As the development roller  17  rubs the development blade  21 , the toner in the developer layer on the peripheral surface of the development roller  17  is frictionally charged, and the developer layer is regulated in thickness. Further, the development blade  21  is being supplied with a preset voltage (−550 V) from an unshown blade bias power source. 
         [0056]    The development blade  21  has the main portion and a pair of small tabs  21   c . The main portion is roughly in the form of a thin rectangular plate. The pair of small tabs  21   c  project from the lengthwise ends of the main portion, one for one, outward of the development unit  4 , in the lengthwise direction M which is parallel to the rotational axis  16  of the development roller  17 . Although  FIG. 3  shows only one of the small tabs (projection)  21   c , the other small tabs  21   c  (projection) is the same in shape. 
         [0057]    As described above, the development blade  21  has the pair of small tabs  21   c  (projections) which extend outward of the development unit  4  from the lengthwise ends of the main portion of the development blade  21  in the lengthwise direction M of the development roller  17 . The development unit  4  is structured so that the tabs  21   c  are between the development roller  17  and seal  25 . That is, listing from the inward side of the development unit  4  in terms of the radius direction of the development roller  17 , the development roller  17 , tab  21   c  (projection), and seal  25  are in the listed order. The seal  25  and tab  21   c  are in contact with each other. Further, the tab  21   c  and the peripheral surface of the development roller  17  are hermetically in contact with each other, at least at contact nips  26  between the two ( FIG. 4(   c )). 
         [0058]    Referring to  FIG. 3 , designated by a referential code  21   f  is one of the lengthwise ends of the base side of the development blade  21 , and designated by a referential code  21   d  is one of the lengthwise ends of the free edge side of the development blade  21 , that is, the outward edge of the aforementioned outwardly extending small tab  21   c . The outward edge  21   d  may be defined as the outermost edge of the outwardly extending tab  21   c  in terms of the lengthwise direction M. Also in terms of the lengthwise direction M, the outward edge  21   d  is on the inward side relative to the outward surface  25   a  of the seal  25 . 
         [0059]    Further, the development blade  21  has an edge  21   e  which extends outward, in terms of the lengthwise direction M, from the outward edge  21   f  to the outermost edge  21   d . That is, the edge  21   e  may be defined as the upstream edge of the tab  21   c  in terms of the rotational direction (indicated by arrow mark D) of the development roller  17 . Also in terms of the lengthwise direction M, the upstream edge  21   e  is within the nip  26  ( FIG. 4(   c )) between the development roller  17  and development blade  21 . That is, the upstream edge  21   e  is sandwiched between the development roller  17  and development blade  21 , being in contact with both the development roller  17  and development blade  21 . 
         [0060]    Designated by a referential code  25   c  is the “downstream end” of the seal  25  in terms of the rotational direction of the development roller  17 . The downstream end  25   c  is on the downstream side relative to the upstream edge  21   e  of the tab  21   c  of the development blade  21  in terms of the rotational direction of the development roller  17 . 
         [0061]    The material for the development blade  21  in this embodiment is a piece of thin plate of copper phosphate, which is 0.1 mm in thickness. However, a thin plate of stainless steel, a rubbery substance such as urethane, or the like can also be used as the material for the development blade  21 . 
         [0062]    Next, the seal  25 , which is placed in the gap between the development unit frame  18  and development roller  17  is described about its structure. The development unit  4  is provided with the pair of seals  25  (which are for sealing development unit at lengthwise ends of development unit  4 ), which are placed in the spaces, one for one, surrounded by the inward surfaces (seal seating surfaces  18   a ) of the development unit frame  18  and the peripheral surface of the development roller  17 . More concretely, the development unit  4  is provided with the pair of seals  25 , which are at the ends of the development unit  4 , one for one, in terms of the lengthwise direction of the development roller  17 . The seals  25  are the members for preventing the toner (developer) in the development unit  4  from leaking out of the unit  4 , through the gaps between the development roller  17  and development unit frame  18 . 
         [0063]    The development unit frame  18  is provided with the pair of seal seats  18   a  as described above. The development unit frame  18  is structured so that there will be a preset amount of gap  60  between each seal seat  18   a  and the peripheral surface of the development roller  17 . The seal  25  is pasted to the seal seat  18   a . The development unit frame  18  is structured so that the amount of the gap  60  is less than the thickness of the seal  25 . Thus, as the development unit frame  18  is assembled, the seal  25  is compressed by the peripheral surface of the development roller  17  and the seal seat  18   a  (to which seal  25  is pasted), being thereby enabled to prevent the toner from leaking out of the development unit  4  along the peripheral surface of the development roller  17 . 
         [0064]    In this embodiment, the amount of the gap between the peripheral surface of the development roller  17  and the seal seat  18   a  of the development unit frame  18  was set to 2 mm. The seal  25  was a piece of felt, which was 2.5 mm in thickness. Thus, after the assembly of the development unit  4 , the seal  25  remains compressed by 0.5 mm by the peripheral surface of the development roller  17  and the seal seat  18   a  of the development unit frame  18 . 
         [0065]    Next, referring to  FIG. 3 , the seal  25  is not between the development blade  21  and development roller  17 . In terms of the lengthwise direction M, the seal  25  is adjacent to the space between the development blade  21  and development roller  17 . Also in terms of the lengthwise direction M, the protruding portion  18   b  of the development unit frame  18 , which has the seal seat  18   a , is in the adjacencies of the space between the development blade  21  and development roller  17 . In terms of the radius direction of the development roller  17 , the protruding portion  18   b  is in the adjacencies of the outward surface of the seal  25 . 
         [0066]      FIG. 4(   a ) is a schematic plan view of the lengthwise end portions of the development unit  4  as seen from the direction of the photosensitive member unit  13 .  FIG. 4  does not show the portions of the development unit  4 , which are on the outward side of the development blade  21  in terms of the radius direction of the development roller  17 . Next, referring to  FIG. 4 , the positional relationship between the development blade  21  and seal  25  in terms of the lengthwise direction M is described.  FIG. 4(   b ) is a sectional view of the combination of the development roller  17 , seal  25 , development blade  21 , development unit frame  18 , and development blade supporting metallic plate  22 , at a plane J-J in  FIG. 4(   a ).  FIG. 4(   c ) is a sectional view of the combination of the development roller  17 , seal  25 , development blade  21 , development unit frame  18 , and development blade supporting metallic plate  22 , at a plane K-K in  FIG. 4(   a ). 
         [0067]    Referring to  FIG. 4(   a ), the development blade  21  has the small tabs  21   c  which extend outward from the lengthwise ends of the main portion of the blade  21 , one for one, in terms of the lengthwise direction of the development blade  21 . Further, these tabs  21   c  are on the free edge side of the development blade  21 . 
         [0068]    In terms of the lengthwise direction M, the outward edge  21   d  of each tab  21   c  is on the inward side of the outward surface  25   a  of the seal  25 . That is, the seal  25  has an area which is preset in dimension in terms of the lengthwise direction M, is on the outward side of the outward edge  21   d , in terms of the lengthwise direction M, and is hermetically in contact with the peripheral surface of the development roller  17 . In this embodiment, the dimension of the seal  25  in terms of the lengthwise direction M is 6 mm, and the length by which the tab  21   c  and seal  25  overlap with each other in the lengthwise direction M is 3 mm. That is, the seal  25  is hermetically in contact with the peripheral surface of the development roller  17 , on the outward side of the outward edge  21   d  in terms of the lengthwise direction M, and the length of the physical contact between the seal  25  and the peripheral surface of the development roller  17  in terms of the lengthwise direction M is 3 mm. This is how the seals  25  are kept hermetically in contact with the peripheral surface of the development roller  17 . 
         [0069]    Next, referring to  FIGS. 4(   c ) and  5 , the state of contact between the seal  25  and other members of the development unit  4  is described, starting from the upstream side of the rotational direction of the development roller  17  indicated by the arrow mark D.  FIG. 5  is an enlarged schematic sectional view of the development unit  4  at a plane perpendicular to the lengthwise direction M. In terms of the radius direction of the development roller  17 , the outward surface of the seal  25  is hermetically in contact with the development unit frame  18 , between the upstream and downstream edges of the seal seat  18   a  of the development unit frame  18  in terms of the rotational direction of the development roller  17 . Further, the upstream portion of the inward surface of the seal  25  is directly in contact with the peripheral surface of the development roller  17 , whereas the downstream portion of the seal  25  is hermetically in contact with the upstream edge  21   e  and its adjacencies of the outward surface of the tab  21   c . In other words, the tab  21   c  is sandwiched between the seal  25  and development roller  17 . 
         [0070]    Further, the development unit  4  is structured so that the amount of the gap between the upstream edge  21   e  of the tab  21   c  and the seal seat  18   a  of the development unit frame  18  is less than the thickness of the seal  25 . Therefore, the seal  25  is kept hermetically in contact with the upstream edge  21   e  and its adjacencies of the tab  21   c , and keeps the tab  21   c  pressed upon the peripheral surface of the development roller  17 . 
         [0071]    Further, the development unit  4  is structured so that in terms of the rotational direction of the development roller  17 , the seal  25  extends downstream beyond at least the upstream edge  21   e  of the tab  21   c , for the following reason. That is, if the size and shape of the seal  25  are such that in terms of the rotational direction of the development roller  17 , the downstream end of the seal  25  is on the upstream side of the upstream edge  21   e  of the tab  21   c  of the development blade  21 , the tab  21   c  is not sandwiched by the seal  25  and development roller  17 . 
         [0072]    As described above, the seal  25  is seamlessly in contact with the peripheral surface of the development roller  17  and the tab  21   c  of the development blade  21 . Thus, the gaps  60 , which are between the seal seat  18   a  of the development unit frame  18  and the peripheral surface of the development roller  17  at the lengthwise ends of the seal  25 , one for one, in terms of the lengthwise direction M, remain satisfactorily sealed by the pair of seals  25  alone. Thus, the toner in the development unit  4  is unlikely to leak out of the development unit  4  and scatter, at the lengthwise ends of the development unit  4  along the peripheral surface of the development roller  17 . 
         [0073]    In this embodiment, the development unit  4  is structured so that the development blade  21  is held, like a cantilever, to the blade supporting plate  22  in such a manner that its free edge (developer regulating edge) is on the downstream side relative to its base portion in terms of the rotational direction of the development roller  17 . That is, the free edge faces downstream in terms of the rotational direction of the development roller  17 . Since the development unit  4  is structured as described above, there is a wedge-shaped space O between the development blade  21  and the peripheral surface of the development roller  17 , as shown in  FIG. 4(   b ). More specifically, the space O between the development blade  21  and development roller  17  is shaped so that the more upstream from the contact nip  26  between the development blade  21  and the peripheral surface of the development roller  17 , the greater the distance between the development blade  21  and the peripheral surface of the development roller  17 . 
         [0074]    If the development blade  21  is shaped so that after the assembly of the development unit  4 , the position of the tab  21   c  of the development blade  21  corresponds to where the gap between the development blade  21  and development roller  17  is relatively large, the tab  21   c  is on the base side of the development blade  21  relative to the position of the tab  21   c  shown in  FIG. 3 . Thus, the portion of the seal  25 , which is on the outward side of the outward edge  21   d  of the tab  21   c , in terms of the lengthwise direction M, and comes hermetically in contact with the peripheral surface of the development roller  17 , has to be increased in its dimension in terms of the lengthwise direction M. Thus, if it is gently that the tab  21   c  of the development blade  21  has to be pressed upon the peripheral surface of the development roller  17 , from its base portion to the outward edge  21   d , the seal  25  has to be increased in its dimension in terms of the lengthwise direction M. 
         [0075]    Thus, from the standpoint of reducing in size the aforementioned wedge-shaped space O, that is, from the standpoint of toner leak prevention, it is advantageous to shape the development blade  21  so that after the assembly of the development unit  4 , the upstream edge  21   e  of its tab  21   c  is near the contact nip  26 . Incidentally, if the development blade  21  is shaped so that after the assembly of the development unit  4 , the upstream edge  21   e  of its tab  21   c  is near the contact nip  26 , the seal  25  does not need to be as wide, in terms of the lengthwise direction M, as it has to be if the development blade  21  is shaped otherwise. 
         [0076]    The aforementioned component measurements are nothing but examples, and are not intended to limit the present invention in scope. 
         [0077]    As described above, according to the structural arrangements in this embodiment, even if the development unit  4  suffers from assembly errors related to the sealing of the development unit  4 , the phenomenon that toner leaks out of the development unit  4  at the ends of the development roller  17  in terms of the lengthwise direction M is prevented. In particular, unlike the prior art which places the seal  25  in the space between the development blade  21  and development roller  17 , the seal  25  blocks the space between the development blade  21  and development roller  17  in the lengthwise direction M, while pressing on the tabs  21  of the development blade  21 . Therefore, even if a given development unit  4  suffers from assembly errors related to sealing, it does not suffer from the phenomenon that the space between its development blade  21  and development roller  17  becomes greater than a preset dimension, and/or the phenomenon that the development blade  21  separates from the peripheral surface of the development roller  17 . 
         [0078]    Further, according to the structural arrangement in this embodiment, the outward edge  21   d  is on the inward side of the outward surface  25   a  in terms of the lengthwise direction M. Thus, the tab  21   c , which extends in the lengthwise direction M, is placed hermetically in contact with the peripheral surface of the development roller  17 , preventing thereby toner from scattering, and the seal  25  is hermetically in contact with the peripheral surface of the development roller  17 , on the outward side of the tab  21   c , preventing thereby toner from scattering. 
         [0079]    Further, according to the structural arrangement in this embodiment, the development blade  21  is made up of the roughly rectangular main portion, and the pair of small tabs  21   c  which extend outward from the lengthwise ends of the main portion, one for one, in the lengthwise direction M. Further, the tabs  21   c  are on the free edge (toner layer regulating edge) side of the development blade  21 . Further, referring to  FIG. 4(   c ), the development unit  4  is structured so that after its assembly, the upstream edge  21   e  of each tab  21   c  will be in the contact nip  26 . Thus, the wedge-shaped spaces can be reduced in size by the tabs  21   c . Further, the seal  25  is seamlessly in contact with both the peripheral surface of the development roller  17  and the tab  21   c  of the development blade  21 . Thus, the gaps which are present adjacent to the peripheral surface of the development roller  17  at the ends of the development unit  4  in terms of the lengthwise direction M remain satisfactorily sealed by the seal  25  alone. 
         [0080]    Further, the seal  25  is provided with the main portion which hermetically contacts the peripheral surface of the development roller  17 , and a portion which extends upstream from the upstream end of the main portion in terms of the rotational direction of the development roller  17 . The extending portion sandwiches the upstream edge  21   e  and its adjacencies of the tab  21   c , between itself and the peripheral surface of the development roller  17 . Further, the development unit frame  18  is structured so that the amount of the gap  60  between the portion of the development unit frame  18 , which corresponds in position to the upstream edge  21   e  of the tab  21   c , and the seal seat  18   a , is less than the thickness of the seal  25 . Therefore, the seal  25  is made to contact with the upstream edge  21   e  of the tab  21   c , and press the tab  21   c  upon the peripheral surface of the development roller  17 . As described above, the development unit  4  is not structured to place the seal  25  in the gap  60  between the development blade  21  and development roller  17  so that the seal  25  is sandwiched by the development blade  21  and development roller  17 . Therefore, it is possible to prevent toner from leaking out of the development unit  4  through the gap  60  which occurs as the sealing member  25  lifts the development blade  21 . 
         [0081]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0082]    This application claims priority from Japanese Patent Applications Nos. 002776/2011 and 260274/2011 filed Jan. 11, 2011 and Nov. 29, 2011, respectively which are hereby incorporated by reference.