Patent Publication Number: US-7221890-B2

Title: Image forming apparatus

Description:
The present application is based on Japanese Patent Application No. 2003-344654 filed on Oct. 2, 2003, the content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to an image forming such as a color laser printer. 
   2. Discussion of Related Art 
   A conventional image forming apparatus such as a color laser printer is arranged to form a static latent image on a surface of an image carrier such as a photosensitive or photoconductive drum, and apply a toner to the surface of the image carrier, for thereby forming a toner image on the image carrier. This process is repeated for each of four toner images of different colors, which are formed on the respective image carriers, using respective yellow (Y), magenta (M), cyan (C) and black (K) toners, so that a color image is formed on a recording medium such as a sheet of paper, with the toner images of different colors being sequentially transferred from the respective image carriers to the recording medium. 
   The toner image of each color is transmitted from the corresponding image carrier directly to the recording medium while the recording medium is fed by a conveyor belt, or indirectly to the recording medium via an intermediate image transfer belt. In the latter method, the toner images of different colors are first transferred from the image carriers to the intermediate image transfer belt, to form the color image on the transfer belt, and the thus formed color image is transferred from the transfer belt to the recording medium. In either of these two methods, the toners inevitably remain on the surfaces of the image carriers after the toner images are transferred the surfaces. 
   To remove the remaining toner from each image carrier, the conventional image forming apparatus is provided with a cleaner mechanism such as a blade device, which is arranged to remove the toner by scraping the surface of the image carrier. Alternatively, a developing device of the image forming apparatus which includes a developing roller operable to apply the toner to the surface of the image carrier is electrically controlled such that a portion of the toner which remains on the image carrier and which does not form the toner image is adsorbed onto the surface of the developing roller. In the latter case, the image forming apparatus which is not provided with a cleaner mechanism may be referred to as a “cleanerless type” image forming apparatus”. An example of this cleanerless type is disclosed in JP-3154757B2 (JP-5053482A). 
   The image forming apparatus of the cleanerless type is advantageous in that the size of the apparatus can be reduced in the absence of a cleaner mechanism for removing the remaining toner from the surface of the image carrier. However, the apparatus still suffers from difficulty to remove paper dust which tends to adhere to the surface of the image carrier during a transfer of the toner image from the image carrier to the recording medium. In the presence of the paper dust on the image carrier, the toner may adhere to the paper dust and may be transferred together with the paper dust to the recording medium, giving rise to a risk of undesirable formation of unintended colored spots (generally called “paper dust spots”) on the recording medium. 
   That is, the paper dust produced by the recording medium in the form of paper sheets tends to adhere directly to the image carrier, or indirectly to the image carrier via the intermediate image transfer belt. In the image forming apparatus of the cleanerless type, the paper dust cannot be removed from the image carrier, leading to the risk of formation of the paper dust spots on the recording medium. 
   SUMMARY OF THE INVENTION 
   The present invention was made in view of the problems discussed above. It is therefore an object of the present invention to provide a multiple-color image forming apparatus of a cleanerless type, which is arranged to efficiently remove a dust from image carriers. 
   The above object may be achieved according to the principle of the present invention, which provides an image forming apparatus comprising a plurality of image forming units operable to form respective toner images of respective different colors on a toner-image receiving medium, each of the plurality of image forming units including (a) an image carrier, (b) a static-latent-image forming portion operable to form a static latent image on a surface of the image carrier, (c) a developing portion operable to apply a toner of a corresponding one of the different colors to the surface of the image carrier, according to the static latent image, for thereby forming the toner image of the corresponding color on the image carrier, and (d) a toner transferring portion operable to transfer the toner image from the image carrier to the toner-image receiving medium, the plurality of image forming units being disposed so as to be opposed to the toner-image receiving medium being fed in one direction, and operable to sequentially transfer the toner images from the image carriers to the toner-image receiving medium, wherein an improvement comprises: 
   the developing portion of each of the plurality of image forming units is operable to adsorb the toner remaining on the surface of the image carrier after the toner image formed last on the image carrier is transferred to the toner-image receiving medium; and 
   each of at least one of the plurality of image forming units is provided with a dust removing portion operable to remove, from the surface of the image carrier, a dust which has been transferred to the surface of the image carrier during a transfer of the toner image from the image carrier to the toner-image receiving medium, the above-indicated at least one of the plurality of image forming units including the most upstream one of the image forming units which is located at the most upstream position of the image forming apparatus and which is operated to transfer the toner image to the toner-image receiving medium before each of the other of the plurality of image forming units is operated to transfer the toner image to the toner-image receiving medium. 
   In the image forming apparatus of the present invention constructed as described above, at least the most upstream image forming unit is provided with the dust removing portion, so that the amount of the dust adhering to the surface of the image carrier of each image forming portion is reduced to improve the quality of a toner image formed on the toner-image receiving medium and the quality of a toner image eventually formed on a recording medium, even in the absence of a cleaner mechanism arranged to scrape the surface of the image carrier for removing the dust. 
   In a first preferred form of the image forming apparatus of the invention, the dust removing portion is not provided for at least one of the plurality of image forming units, which includes the most downstream one of the image forming units which is located at the most downstream position of the image forming apparatus and which is operated to transfer the toner image to the toner-image receiving medium after each of the other of the plurality of image forming units is operated to transfer the toner image to the toner-image receiving medium. This arrangement is based on a fact that the amount of the dust to be removed by the dust removing portion of the relatively upstream image forming unit is larger than that to be removed by the dust removing portion of the relatively upstream image forming unit. The present arrangement is effective to reduce or prevent an adverse influence of the dust removing portion on the image carrier of the relatively downstream image forming unit or units not provided with the dust removing portion. Accordingly, each image forming unit not provided with the dust removing portion has an increased service life, and a reduced number of components, leading to a reduced size of the image forming apparatus as a whole. 
   In a second preferred form of the image forming apparatus of the invention, the plurality of image forming units comprise four image forming units operable to form respective toner images of respective yellow, magenta, cyan and black colors on the toner-image receiving medium, and the image forming unit operable to form the toner image of the yellow color is the most upstream one of the image forming units. 
   In the second preferred form of the image forming apparatus of the invention described above, the yellow toner image is first formed by the yellow image forming unit which is located at the most upstream position and which is most easily influenced by the dust transferred from the toner-image receiving medium. However, dust spots such as paper dust spots of the yellow color, if produced in the process of formation of the yellow toner image, are less likely to be perceived, so that the yellow toner image formed on the toner-image receiving medium is less likely to be deteriorated by the yellow dust spots even if they appear on the color image due to some small amount of the dust on the image carrier of the yellow image forming unit. 
   In a third preferred form of the image forming apparatus of the invention, the plurality of image forming units comprise an image forming unit operable to form a toner image of a black color, which is located at the most downstream position of the image forming unit and which is operated to transfer the toner image of the black toner to the toner-image receiving medium after each of the other of the plurality of image forming units is operated to transfer the toner image to the toner-image receiving medium. 
   The image forming unit operable to form the black toner image is used not only when the plurality of image forming units are operated to form a color image, but also when only the black image forming unit is operated to form a monochrome image, that is, a black image. According the frequency of use of the black image forming unit is considerably higher than those of the other image forming units, so that the service life of the image carrier of this black image forming unit tends to be shorter than those of the image carriers of the other image forming units. In the third preferred form of the image forming apparatus of the invention wherein the black image forming unit is located at the most downstream position, as described above, it is possible to reduce the dust removing capacity of the dust removing portion of the black image forming unit, with respect to those of the dust removing portions of the other image forming units, or to eliminate the dust removing portion for the black image forming unit, so that the influence of the dust removing portion on the image carrier of the black image forming unit can be reduced or prevented, whereby the service life of this image carrier can be increased. 
   In a fourth preferred form of the image forming apparatus of the invention, the plurality of image forming units comprise an image forming unit operable to form a toner image of a black color, which is the above-indicated most upstream one of the image forming units. 
   In the fourth preferred form of the image forming apparatus described above, the dust can be removed from the image carrier of the black image forming unit when only this black image forming unit is operated to form a monochrome image, namely, a black image. Accordingly, the black image can be efficiently formed with a high degree of quality. 
   In a fifth preferred form of the image forming apparatus, the dust removing portion comprises a base member and a brush of fibers planted in said base member. In a sixth preferred form the image forming apparatus, the dust removing portion comprises a non-woven fabric. The dust, such as paper dust, which is to be removed by the dust removing portion is generally amorphous, and can be relatively easily caught and removed by the fibers or non-woven fabric. 
   In a seventh preferred form of the image forming apparatus of the invention, the dust removing portion comprises an electrically conductive body, and a bias-voltage applying portion operable to apply a bias voltage between the electrically conductive body and the image carrier, to facilitate removal of the dust from the image carrier. In this respect, it is noted that the dust on the surface of the image carrier, such as a paper dust, has a polarity opposite to that of the image carrier which is electrostatically charged. Therefore, the application of the bias voltage to the electrically conductive body of the dust removing portion permits efficiency removal of the dust by electrostatic adsorption of the dust by the dust removing portion. 
   In a eighth preferred form of the image forming apparatus of the invention, the above-indicated at least one of the plurality of image forming units each of which is provided with the dust removing portion consists of the above-indicated most upstream one of the image forming units, and at least one downstream image forming unit which is located adjacent to and downstream of the most upstream one of the image forming units, and where the dust removing portions of the most upstream one of the image forming units and the above-indicated at least one downstream image forming units have dust removing capacities which are determined such that the dust removing capacity of the dust removing portion of one of two adjacent ones of the image forming units which is located downstream of the other of these two adjacent ones is not larger than that of the other of the two adjacent ones. In this eighth form of the image forming apparatus wherein all of the image forming units may be provided with the respective dust removing portions, the dust removing capacities of the image forming units may decrease in the downstream direction. In this case, the service life of the relatively downstream image forming unit or units each provided with the dust removing portion can be increased. 
   In a first advantageous arrangement of the eighth preferred form of the image forming apparatus, the dust removing portion of each of the most upstream image forming unit and the above-indicated at least one downstream image forming unit is held in contact with the surface of the image carrier, to remove the dust from the surface of the image carrier, and the dust removing portions of the most upstream image forming unit and the above-indicated at least one downstream image forming units have pressures of contact with the surfaces of the corresponding image carriers, which pressures of contact are determined such that the pressure of contact of the dust removing portion of the above-indicated downstream one of the two adjacent ones of the image forming units is not higher than that of the other of the two adjacent ones. In a second advantageous arrangement of the eighth preferred form of the image forming apparatus, the dust removing portion of each of the most upstream image forming unit and the above-indicated at least one downstream image forming unit is held in contact with the surface of the image carrier, to remove the dust from the surface of the image carrier, and the dust removing portions of said most upstream image forming unit and the above-indicated at least one downstream image forming units have dimensions of contact with the surfaces of the corresponding image carriers in a direction of movement of the surfaces, which dimensions of contact are determined such that the dimension of contact of the dust removing portion of the above-indicated downstream one of the two adjacent ones of the image forming units is not larger than that of the other of the two adjacent ones. 
   In a third advantageous arrangement of the eighth preferred form of the image forming apparatus, the dust removing portion of each of the most upstream image forming unit and the above-indicated at least one downstream image forming unit comprises a base member and a brush of fibers planted in the base member, and the brushes of fibers of the dust removing portions of the most upstream image forming unit and the above-indicated at least one downstream image forming units have densities which are determined such that the density of the brush of fibers of the dust removing portion of the above-indicated downstream one of the two adjacent ones of the image forming units is not higher than that of the other of the two adjacent ones. 
   In a fourth advantageous arrangement of the eighth preferred form of the image forming apparatus, the dust removing portion of each of the most upstream image forming unit and the above-indicated at least one downstream image forming unit comprises an electrically conductive body, and a bias-voltage applying portion operable to apply a bias voltage between the electrically conductive body and the image carrier, to facilitate removal of the dust from the image carrier, and the bias voltages applied by the bias-voltage applying portions of the dust removing portions of the most upstream image forming unit and the above-indicated at least one downstream image forming units are determined such that the bias voltage applied by the bias-voltage applying portion of the dust removing portion of the downstream one of the two adjacent ones of the image forming units is not higher than that of the other of the two adjacent ones. 
   In a fifth advantageous arrangement of the eighth preferred form of the image forming apparatus, the dust removing portion of each of the most upstream image forming unit and the above-indicated at least one downstream image forming unit comprises a dust container for storing the dust removed from said image carrier, and the dust removing portions of the most upstream image forming unit and the above-indicated at least one downstream image forming units have storage capacities which are determined such that the storage capacity of the dust container of the dust removing portion of the above-indicated downstream one of the two adjacent ones of the image forming units is not larger than that of the other of the two adjacent ones. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, advantages and technical and industrial significance of the present invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: 
       FIG. 1  is an elevational view in cross section showing a basic arrangement of a color laser printer constructed according to a first embodiment of this invention; 
       FIG. 2  is an enlarged elevational view showing in detail one of four image forming units of the color laser printer of  FIG. 1 ; 
       FIG. 3  is an elevational view for explaining a decrease in the density of fibers of brushes of paper dust removing devices of the four image forming units, to reduce the paper dust removing capacities of the units of the color laser printer of  FIG. 1  in a feeding direction of a paper sheet; 
       FIG. 4  is a fragmentary elevational view showing a modification of the paper dust removing device shown in  FIG. 3 ; 
       FIG. 5  is an elevational view corresponding to that of  FIG. 3 , showing a color laser printer according to a second embodiment of this invention wherein the brushes of the four paper dust removing devices have the same density of fibers, and the paper dust removing capacities of the four image forming units are reduced in the feeding direction of the paper sheet by reducing the bias voltage to be applied to the brushes, in the feeding direction; 
       FIG. 6  is an elevational view corresponding to that of  FIG. 3 , showing a color laser printer according to a third embodiment of this invention wherein the most downstream image forming unit is not provided with the paper dust removing device; 
       FIG. 7  is an elevational view corresponding to that of  FIG. 3 , showing a color laser printer according to a fourth embodiment of this invention wherein the black image forming unit is provided as the most upstream unit; and 
       FIG. 8  is an elevational view corresponding to that of  FIG. 3 , showing a color laser printer according to a fifth embodiment of this invention wherein a toner image is transferred from an intermediate image transfer belt to a sheet of paper. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIGS. 1–3 , a basic arrangement of a color laser printer  1  constructed according to a first embodiment of the present invention will be described. As shown in  FIG. 1 , the present color laser printer  1  has a main body  2 , which houses a paper supplying portion  4  arranged to supply a recording medium in the form of sheets of paper  3 , an image forming portion  5  arranged to form a color image on the paper sheets  3  supplied from the paper supplying portion  4 , and a paper ejecting portion  6  arranged to eject the paper sheets  3  on which the color image has been formed by the image forming portion  5 . The image forming portion  5  includes four image forming units  16  which are arranged in tandem in the horizontal direction. 
   The paper supplying portion  4  includes a paper supply tray  12 , a paper supply roller  13 , and a pair of paper feed rollers  14 . The paper supply tray  12  is removably installed in the bottom portion of the main body  2 , such that the paper supply tray  12  can be inserted into and removed from the main body  2  through an opening provided at the front end portion of the main body  2 . The paper supply roller  13  is located adjacent to the front and upper end of the paper supply tray  12 , and the paper feed rollers  14  are located above the paper supply roller  13 , and downstream of the paper supply roller  13  as viewed in a direction of feeding of the paper sheets  3  from the paper supply tray  12 . 
   The paper supply tray  12  is arranged to accommodate a stack of the paper sheets  3  such that the uppermost paper sheet  3  of the stack is supplied from the paper supply tray  12  to the paper feed rollers  14 , by rotation of the paper supply roller  13 . The feed rollers  14  advance the paper sheet  3  onto a conveyor belt  67 . 
   Between the paper supply roller  13  and the paper feed rollers  14 , there is provided a guide member  15  which extends in an almost vertical direction and which is arranged to guide the paper sheets  3  successively supplied from the paper supply tray  12  by the paper supply roller  13 , to a nip between the two paper feed rollers  14 . 
   Each of the four image forming units  16  of the image forming portion  5  includes an image carrier in the form of a photoconductive or photosensitive drum  56 , and components disposed around or adjacent to the outer circumference of the photosensitive drum  56 . These components include a charger  21 , a static-latent-image forming portion in the form of a scanner unit  19 , and a developing portion in the form of a developing unit  20 . The charger  21  is arranged to electrostatically charge the outer circumferential surface of the photosensitive drum  56 , and the scanner unit  19  is arranged to form a static latent image on the outer circumferential surface of the photosensitive drum  56 . The developing unit  20  is arranged to apply a developing material in the form of a toner to the outer circumferential surface of the photosensitive drum  56 , for thereby forming a toner image on the photosensitive drum  56 . 
   The photosensitive drum  56  is a hollow cylindrical body consisting of an aluminum cylindrical sleeve having an outer circumferential surface on which a photoconductive layer is formed of an organic photosensitive composition a major component of which is a positively chargeable material such as a positively chargeable polycarbonate. The photosensitive drum  56  is rotatably supported by the main body  2 , with its cylindrical sleeve being grounded. 
   The charger  21  is a scorotron type charger including charging wires formed of tungsten, for example, which are energized to effect a corona discharge for positively charging the photoconductive layer of the photosensitive drum  56  evenly or uniformly over its entire surface. 
   The scanner unit  19  has a scanner casing  22  in the form of a generally box construction, and includes a laser generator (not shown), a polygon mirror  23 , a pair of lenses  24 ,  25  and three reflecting mirrors  26 ,  27 ,  28 , which are housed within the scanner casing  22 . The laser generator is arranged to generate a laser radiation for forming a static latent image on the outer circumferential surface of the photosensitive drum  56 . 
   In operation of the scanner unit  19 , the laser radiation (indicated by one-dot chain line in  FIG. 2 ) generated by the laser generator is reflected by the polygon mirror  23 , and the reflected laser radiation is transmitted through the lens  24 , reflected by the reflecting mirrors  26  and  27 , transmitted through the lens  25  and reflected by the reflecting mirror  28 . The laser radiation reflected by the mirror  28  is emitted out of the scanner casing  22  through an emission window  29 . The outer circumferential surface of the photosensitive drum  56  is scanned according to image data, by irradiation with the laser radiation emitted through the emission window  29 , so that a static latent image is formed on the surface of the photosensitive drum  56 . 
   The developing unit  20  has a developer casing  43 , and includes a toner container in the form of a toner hopper  31 , a toner supplying portion in the form of a toner supply roller  32 , and a toner carrier in the form of a developing roller  33 , which are housed within the developer casing  43 . 
   The toner hopper  31  is defined by the developer casing  43 , as an interior space within the casing  43 . Within the toner hopper  31 , there is disposed an agitator  48 . The toner hopper  31  of each image forming unit  16  accommodate a toner of the corresponding color, that is, a yellow (Y), magenta (M), cyan (C), or black (K) toner. 
   Described more specifically, the four image forming units  16  consist of a yellow image forming unit  16 Y provided with the toner hopper  31  accommodating the yellow (Y) toner, a magenta image forming unit  16 M provided with the toner hopper  31  accommodating the magenta (M) toner, a cyan image forming unit  16 C provided with the toner hopper  31  accommodating the cyan (C) toner, and a black image forming unit  16 K provided with the toner hopper  31  accommodating the black (K) toner. 
   The toner accommodated in the toner hopper  31  is a one-component type positively chargeable non-magnetic developing agent, which is produced by suspension polymerization or emulsion polymerization. This developing agent is a powder consisting of generally spherical fine particles and having an extremely high degree of fluidity. 
   The toner supply roller  32  is located in a lower portion of the toner hopper  31 , and consists of a metallic shaft portion and a roller portion which is formed, on the shaft portion, of an electrically conductive spongy material. This toner supply roller  32  and the developing roller  33  are disposed adjacent to each other so as to define a nip therebetween, and are rotatably supported by the casing  43  such that the two rollers  32 ,  33  are rotatable in the opposite directions. 
   The developing roller  33 , which is located under the toner supply roller  32  and held in rolling contact with the roller  32 , consists of a metallic shaft portion and a roller portion which is formed, on the shaft portion, of an electrically conductive elastic material such as an electrically conductive rubber material. 
   The toner transferring portion  17 , which is also housed within the main body  2 , is located on side of the photosensitive drum  56  which is opposite to the developing unit  20  diametrically of the photosensitive drum  56 . Thus, each image forming unit  16  is disposed so as to be opposed to the paper sheet  3  which is fed together with the conveyor belt  67 . The toner transferring portion  17  includes a drive roller  65 , a driven roller  66 , an endless conveyor belt  67 , and a toner transfer roller  68 . 
   The driven roller  66  is located frontwards of the photosensitive drum  56  of the yellow image forming unit  16 Y which is the most upstream image forming unit as viewed in the direction of feeding of the paper sheets  3 . Further, the driven roller  66  is located upwards and rearwards of the paper supply roller  13 . On the other hand, the drive roller  65  is located rearwards of the photosensitive drum  56  of the black image forming unit  16 K which is the most downstream image forming unit as viewed in the feeding direction of the paper sheets  3 . Further, the drive roller is located obliquely downwards and frontwards of the image fixing portion  18 . 
   The conveyor belt  67  is formed of a resinous material such as polycarbonate or polyimide, which contains an electrically conductive powder such as a carbon powder such that the electrically conductive powder is evenly dispersed within the resinous material. The conveyor belt  67  connects the drive roller  65  and the driven roller  66  such that the outer surface of an upper span of the conveyor belt  67  between the drive and driven rollers  65 ,  66  is held in contact with the photosensitive drums  56  of the four image forming units  16 . 
   The driven roller  66  and the conveyor belt  67  are rotated by the drive roller  65  in the counterclockwise direction such that the above-indicated outer surface of the upper span of the conveyor belt  67  between the drive and driven rollers  65 ,  66  is held in contact with the photosensitive drums  56  of the image forming units  16  while the photosensitive drums  56  are rotated in the clockwise direction. 
   The toner transfer roller  68  is located within the conveyor belt  67 , such that the toner transfer roller  68  cooperates with the photosensitive drum  56  of each image forming unit  16  to define a nip therebetween. This toner transfer roller  68  consists of a metallic shaft portion and a roller portion which is formed, on the shaft portion, of an electrically conductive elastic material such as an electrically conductive rubber material. 
   The toner transfer roller  68  is supported such that the roller  68  is rotatable in the counterclockwise direction, in rolling contact with an inner surface of the upper span of the conveyor belt  67  rotated in the counterclockwise direction. In operation of the color laser printer  1 , the paper sheet  3  supported by the conveyor belt  67  is passed through the nip between each photosensitive drum  56  and the corresponding toner transfer roller  68 , and a predetermined image-transfer bias voltage is applied between the toner transfer roller  68  and the photosensitive drum  56 , in a direction that permits the toner image to be transferred from the photosensitive drum  56  onto the paper sheet  3  on the conveyor belt  67 , when the paper sheet  3  is passed through the above-indicated nip. 
   The image fixing portion  18  is located rearwards and downstream of the image forming units  16  and the toner transferring portion  17 , and includes a presser roller  69  and a heater roller  70 . The heater roller  70  consists of a metallic tube which has an outer circumferential surface coated with a releasing layer and which incorporates a halogen lamp extending in the axial direction. With the halogen lamp being energized, the heater roller  70  is heated to a predetermined image fixing temperature at its releasing layer. The presser roller  69  cooperates with the heater roller  70  to define a pressure nip therebetween. 
   The pair of paper ejector rollers  11  is located downstream of the image fixing portion  18 , and the paper ejector tray  10  is located downstream of the paper ejector rollers  11 . In the present color laser printer  1 , a portion of the toner not forming the toner image does not remain on the photosensitive drum  56  of each image forming unit  16 , but is held adsorbed on the surface of the developing roller  33 . That is, each image forming unit  16  of the present laser printer  1  is of the so-called “cleanerless type” without a cleaner mechanism. 
   Described in detail, the outer circumferential surface of the photosensitive drum  56  on which a given amount of the toner remains after the transfer of the last toner image to the paper sheet  3  is uniformly charged by the charger  21  of scorotron type at a predetermined charging position as the photosensitive drum  56  is rotated. At a predetermined exposing position, the surface of the photosensitive drum  56  is locally selectively or imagewise exposed to the laser radiation emitted from the scanner unit  19 . A portion of the toner remaining in the non-exposed local areas of the photosensitive drum  56  is adsorbed onto the developing roller  33  the static polarity of which is opposite to that of the non-exposed portions. On the other hand, a portion of the toner remaining in the exposed local areas of the photosensitive drum  56  and a portion of the toner transferred from the developing roller  33  remain on the exposed local areas, and cooperate to form a toner image. 
   In operation of the color laser printer  1  constructed as described above, the outer circumferential surface (photoconductive layer) of the photosensitive drum  56  of each image forming unit  16  is uniformly electrostatically charged by the charger  21 , and is imagewise exposed to the laser radiation (indicated by the one-dot chain line in  FIG. 2 ) which is emitted from the scanner unit  19  and modulated according to the image data, so that a static latent image is formed on the surface of the photosensitive drum  56 . This static latent image is developed by the developing unit  20  into a visible toner image of the corresponding color, with the toner being transferred from the developing roller  33  to the exposed local areas of the photosensitive drum  56 . 
   At a predetermined toner transferring position, the visible toner image is transferred from the photosensitive drum  56  onto the paper sheet  3 , by application of the bias voltage between the toner transfer roller  68  and the photosensitive drum  56 , when the paper sheet  3  supplied from the paper supply tray  12  and fed by the paper feed roller  14  is passed together with the upper span of the conveyor belt  67 , through the nip between the photosensitive drum  56  and the toner transfer roller  68 . A color image is formed on the paper sheet  3 , as a result of transfer of the four toner images of the respective four colors (Y, M, C, K) from the four image forming units  16 Y,  16 M,  16 C and  16 K. 
   The paper sheet  3  is further advanced to the image fixing portion  18 , and passed through the nip between the presser roller  69  and the heater roller  70 , so that the visible toner image on the paper sheet  3  is heated and pressed onto the paper sheet  3 , whereby the toner image is fixed on the paper sheet  3 . The paper sheet  3  is further advanced by the paper ejector rollers  11  into the paper ejector tray  10 . 
   The toner remaining on the photosensitive drum  56  is removed from the drum  56  by the developing unit  20 , and re-used for developing the next latent image into a visible toner image. Thus, the present color laser printer  1  is not provided a cleaner mechanism for scraping off the toner remaining on the photosensitive drum  56 , and does not require a waste toner container which would be required to accommodate the toner removed by the cleaner mechanism. Accordingly, the present laser printer  1  can be made small-sized, and the toner can be used with a relatively high ratio of utilization. 
   Thus, the toner remaining on the photosensitive drum  56  of each image forming unit  16  can be removed by the developing roller  33  of the developing unit  20  and returned back to the developing roller  33 . The present laser printer  1  is further required to remove paper dust which may be transferred from the paper sheet  3  to the photosensitive drum  56  during the transfer of the visible toner image from the drum  56  to the paper sheet  3 . 
   To meet the above-described requirement, each image forming unit  16  is provided with a dust removing portion in the form of a paper dust removing device  80 , as shown in  FIG. 2 . The paper dust removing device  80  is arranged to remove, from the photosensitive drum  56 , the paper dust which has been transferred from the paper sheet  3 . 
   As shown in  FIG. 2 , the paper dust removing device  80  includes a paper dust container  84  which is L-shaped in cross section so as to cover a left lower portion of the outer circumference of the photosensitive drum  56 , and to store or accommodate the paper dust removed from the photosensitive drum  56 . The paper dust removing device  80  further includes a base member  81  fixed to the paper dust container  84  such that the base member  81  faces an upper part of the above-indicated left lower portion of the outer circumferential surface of the photosensitive drum  56 . The base member  81  carries a brush of electrically conductive fibers  82 . The paper dust removing device  80  further includes a constant-voltage source  90  provided to apply a bias voltage to the base member  81  and the brush of fibers  82 . 
   The fibers  82  are planted in the base member  81  such that the fibers  82  are held in pressing contact with the outer circumferential surface of the photosensitive drum  56 , so that the brush of fibers  82  scrapes the paper dust off the surface of the photosensitive drum  56  during rotation of the drum  56 . 
   The constant-voltage source  90  is arranged to apply, between the base member  81  (brush of fibers  82 ) and the photosensitive drum  56 , a bias voltage the polarity of which is opposite to that of the paper dust, such that the paper dust is removed from the photosensitive drum  56  and adsorbed by the fibers  82 . 
   In the present first embodiment, the yellow (Y), magenta (M), cyan (C) and black (K) image forming units  16 Y,  16 M,  16 C and  16 K are arranged in this order of description along a feeding path of the paper sheet  3 , in the feeding direction of the paper sheet  3 , as shown in  FIG. 3 . The paper dust removing devices  80  of the four image forming unit  16  are arranged such that the density of the fibers  82  of the paper dust removing devices  80  of the four image forming units  16  decreases in the feeding direction of the paper sheet  3 , to reduce the paper dust removing capacities of the paper dust removing devices  80  in the feeding direction, so that the paper dust removing device  80  of the yellow image forming unit  16 Y has the highest density of the fibers  82 , while that of the black image forming unit  16 K has the lowest density. Further, the volume or storage capacity of the paper dust containers  84  of the paper dust removing devices  80  of the four image forming devices  16  decreases in the feeding direction of the paper sheet  3  as the density of the fibers  82  decreases in the feeding direction. 
   In the color laser printer  1  according to the present first embodiment of this invention, the four image forming units  16 Y,  16 M,  16 C and  16 K for forming the respective toner images of yellow, magenta, cyan and black colors are provided with the respective paper dust removing devices  80  which are arranged to remove the paper dust from the photosensitive drums  56 , even if the paper dust is transferred from the paper sheet  3  to the photosensitive drums  56 . 
   Accordingly, the present color laser printer  1  having the image forming units  16  of the cleanerless type permits an improved quality of a visible color image printed on the paper sheet  3 , without deterioration of the image quality due to the paper dust spots. 
   Further, the paper dust removing devices  80  provided for the respective image forming units  16  are arranged such that the density of the fibers  82  planted in the base members  81  of the four paper dust removing devices  80  decreases in the feeding direction of the paper sheet  3 , such that the brush of fibers  82  of the paper dust removing device  80  of a downstream one of the two adjacent image forming units  16  has a lower density of the fibers  82  than that of the paper dust removing device  80  of the other unit  16 . Further, the dimension of contact of the brushes of fibers  82  with the photosensitive drums  56  in the rotating direction of the drums  56  also decreases in the feeding direction of the paper sheet  3 , such that the brush of fibers  82  of the device  80  of the most upstream image forming unit  16 Y has the largest dimension of contact, while that of the most downstream image forming unit  16 K has the smallest dimension of contact. In this case, the device  80  of the most upstream image forming unit  16 Y has the largest number of the fibers  82 , while that of the most downstream image forming units  16 K has the smallest number of the fibers  82 . 
   The density and dimension of contact of the brushes of fibers  82  of the paper dust removing devices  80  are reduced in the feeding direction of the paper sheet  3 , to reduce the paper dust removing capacities of the devices  80  in the feeding direction, since the amount of the paper dust which is transferred from the paper sheet  3  to the photosensitive drum  56  of the most upstream image forming unit  16 Y is the largest so that the amount of the paper dust removed from the photosensitive drum  56  of the most upstream unit  16 Y is the largest, while on the other hand, the amount of the paper dust transferred to and removed from the photosensitive drum  56  of the relatively downstream unit  16 M,  16 C is smaller, and the amount of the paper dust transferred to and removed from the photosensitive drum  56  of the most downstream unit  16 K is the smallest. Accordingly, the pressures of contact of the brushes of fibers  82  with the photosensitive drums  56  are lowered in the feeding direction of the paper sheet  3 , whereby the degrees of deterioration of the photosensitive drums  56  are reduced in the feeding direction, so that the photosensitive drums  56  of the relatively downstream image forming units  16  have a relatively long service life. 
   It is noted that the paper dust transferred from the paper sheet  3  would have the largest influence on the yellow image forming unit  16 Y which is the most upstream unit  16  as viewed in the feeding direction of the paper sheet  3 . However, the paper dust spots of the yellow (Y) color are less likely to be perceived than the paper dust spots of the other colors, magenta (M), cyan (C) and black (K). Therefore, the color image formed on the paper sheet  3  is less likely to be deteriorated by the yellow paper dust spots even if they appear on the color image due to some small amount of the paper dust on the photosensitive drum  56  of the yellow image forming unit  16 Y. 
   In this respect, the paper dust removing device  80  of the yellow image forming unit  16 Y need not have an extremely large paper dust removing capacity to assure complete removal of the paper dust from the corresponding photosensitive drum  56 . Accordingly, the service life of this photosensitive drum  56  can be improved. 
   In the present embodiment, the paper dust is removed by not only the contact of the brush of fibers  82  with the photosensitive drum  56 , but also the application of a bias voltage from the constant-voltage source  90  to the brush of fibers  82 . This arrangement ensures a high degree of stability of removal of the paper dust by the paper dust removing device  80 . 
   Accordingly, the paper dust removing capacity of the paper dust removing device  80  can be adjusted by controlling the bias voltage to be applied from the constant-voltage source  90  to the brush of fibers  82 , as well as the contact pressure of the brush of fibers  82  with respect to the photosensitive drum  56 . 
   Namely, the required pressure of contact of the brush of fibers  82  with the photosensitive drum  56  can be reduced by increasing the bias voltage to be applied to the brush of fibers  82 , while maintaining the required paper dust removing capacity of the paper dust removing device  80 . In this respect, too, the service life of the photosensitive drum  56  can be increased. 
   While the first embodiment of the present invention has been described in detail, it is to be understood that the invention is not limited to the illustrated first embodiment, but may be embodied with various changes and modifications. For instance, the brush of fibers  82  planted in the base member  81  of the paper dust removing device  80  may be replaced by an unwoven fabric  83 , as shown in  FIG. 4 . 
   The non-woven fabric  83  employed for the modified paper dust removing device is preferably mixed with an electrically conductive material, so that the paper dust can be adsorbed by the non-woven fabric  83  by application of a bias voltage from the constant-voltage source  90  to the non-woven fabric. Although the first embodiment is arranged such that both the density and the dimension of contact of the brushes of fibers  82  of the paper removing devices  80  of the four image forming units  16  are reduced in the feeding direction of the paper sheet  3  from the upstream side to the downstream side, only one of the density and the dimension of contact of the brushes of fibers  82  with the photosensitive drums  56  in the rotating direction of the drums  56  (in the direction of movement of the surfaces of the drums  56 ) may be reduced in the feeding direction of the paper sheet  3 . In this case where the devices  80  have the different densities and the same dimension of contact, or the different dimensions of contact and the same density, the device  80  of the most upstream image forming unit  16 Y having the highest density or dimension of contact has the largest number of the fibers  82 , while that of the most downstream image forming units  16 K having the lowest density has the smallest number of the fibers  82 . 
   Referring next to  FIG. 5 , there will be described a color laser printer constructed according to a second embodiment of the present invention, wherein the brushes of fibers  82  of the paper dust removing devices  80  of the four image forming units  16  have the same density, and the same dimension of contact with the photosensitive drums  56 , but the bias voltage to be applied to the brushes of fibers  82  of the paper dust removing devices  80  of the image forming units  16  is reduced in the feeding direction of the paper sheet  3 , to reduce the paper dust removing capacity of the paper dust removing devices  80 , for increasing the service life of the photosensitive drums  56  of the relatively downstream image forming units  16 M,  16 C,  16 K. 
   In the second embodiment described above, the amounts of removal of the paper dust from the photosensitive drums  56  of the relatively upstream image forming units  16 Y,  16 M are relatively large owing to the application of the relatively high bias voltage to the brushes of fibers  82  of the paper dust removing devices  80  of those relatively upstream image forming units. Further, the service life of the photosensitive drums  56  of the relatively downstream image forming units  16 C,  16 K can be increased owing to the application of the relatively low bias voltage to the brushes of fibers  82  of the paper dust removing devices  80  of those relatively downstream image forming units. 
     FIG. 6  shows a color laser printer constructed according to a third embodiment of this invention, which is different from the color laser printer  1  of the first embodiment, in that each of the yellow, magenta and cyan image forming units  16 Y,  16 M and  16 C is provided with the paper dust removing device  80 , but the most downstream image forming unit, that is, the black image forming unit  16 K is not provided with the paper dust removing device  80 , in the third embodiment. 
   In the third embodiment, therefore, the paper dust is removed from the photosensitive drums  56  of the first three image forming units  16 Y,  16 M and  16 C as counted from the most upstream unit  16 Y, and the photosensitive drum  56  of the fourth or most downstream image forming unit  16 K which is not provided with the paper dust removing device  80  is free from a contact pressure which would receive from the device  80  if provided for the unit  16 K, so that the service life of the photosensitive drum  56  of the unit  16 K can be increased. 
   The color laser printer according to the third embodiment of  FIG. 6  is further advantageous in that the fourth image forming unit  16  which is not provided with the paper dust removing device  80  is the black image forming unit  16 K which is generally operated most frequently and tends to have a comparatively short service life. In this respect, the elimination of the paper dust removing device  80  for this black image forming unit  16 K results in an increase in its service life, and a consequent saving of the cost of maintenance of the color laser printer. 
   In the color laser printers of the first, second and third embodiments of  FIGS. 3–6 , a monochrome printing operation by only the black image forming unit  16 K is usually performed while the photosensitive drums  56  of the other yellow, magenta and cyan image forming units  16 Y,  16 M and  16 C which are not in operation are held apart from the conveyor belt  67  (paper sheet  3 ). In the monochrome printing operation, it is preferable to hold the photosensitive drum  56  of one of those yellow, magenta and cyan image forming units  16 Y,  16 M,  16 C in contact with the paper sheet  3  on the conveyor belt  67 , so that the paper dust is removed through this photosensitive drum  56 , before the monochrome printing operation is performed by the black image forming unit  16 K. This arrangement assures an improved quality of a black image formed on the paper sheet  3 . Where the paper dust removing devices  80  of the three image forming units  16 Y,  16 M and  16 C have the same paper dust removing capacities, the photosensitive drum  56  of any desired one of those three units  16 Y,  16 M,  16 C is held in contact with the paper sheet  3 . Where the paper dust removing devices  80  of those three image forming units have different paper removing capacities, the photosensitive drum  56  of the relatively upstream image forming unit  16 Y or  16 C having the relatively large paper dust removing capacity is preferably held in contact with the paper sheet  3 . In the former case in which the photosensitive drum  56  of any desired one of the three units  16 Y,  16 M,  16 C having the same paper dust removing capacity is held in contact with the paper sheet  3 , it is preferable to alternately bring the photosensitive drums  56  of those units  16 Y,  16 M,  16 C into contact with the paper sheet  3 , in the monochrome printing operations, so that the overall service life of the color laser printer can be increased, with all of the image forming units  16  having almost equal service lives. 
   Reference is now made to  FIG. 7  showing a color laser printer according to a fourth embodiment of the present invention, which is different from the color laser printers of the first, second and third embodiments, in that the black image forming unit  16 K is provided as the first image forming unit as counted in the feeding direction of the paper sheet  3 , that is, located at the most upstream position, in the fourth embodiment. 
   In the color laser printer of the fourth embodiment of  FIG. 7 , the paper dust removing device  80  of the black image forming unit  16 K has the highest density of the brush of fibers  82 , of the paper dust removing devices  80  of all of the four image forming units  16 . This arrangement permits a monochrome printing operation with an improved quality of a black image, with an operation of the paper dust removing device  80  of the black image forming unit  16 K, even while the photosensitive drums  56  of the other image forming units  16 Y,  16 M,  16 C are held apart from the paper sheet  3 . 
   In the fourth embodiment, the yellow image forming unit  16 Y is provided as the second image forming unit located adjacent to the first or most upstream black image forming unit  16 K. As described above with respect to the first embodiment, the yellow paper dust spots are less likely to deteriorate the color image, so that the yellow image forming unit  16 Y provided as the second image forming unit assures a comparatively high quality of the color image formed by the four image forming units  16 . 
   Referring further to  FIG. 8 , there will be described a color laser printer according to a fifth embodiment of this invention, which is different from the color laser printers of the preceding embodiments which are arranged such that the toner images are transferred from the photosensitive drums  56  directly to the paper sheet  3  fed by the conveyor belt  67 , to form a color image. Namely, the color laser printer of the fifth embodiment of  FIG. 8  is provided with an intermediate image transfer conveyor belt  72 , which is different from the conveyor belt  67  provided to advance or feed the paper sheet  3 . The intermediate image transfer conveyor belt  72  is provided to receive the yellow, magenta, cyan and black toner images from the photosensitive drums  56 , in cooperation with the primary toner transfer rollers  68  held in contact with the inner surface of the upper span of the conveyor belt  72 . The toner images thus received by the intermediate image transfer belt  72  are then transferred to the paper sheet  3 , in cooperation with a secondary toner transfer roller  74  which cooperates with the driven roller  66  to pinch the paper sheet  3  fed by the paper feed rollers  14 . Thus, the yellow, magenta, cyan and black toner images are sequentially transferred to the paper sheet  3  via the intermediate transfer conveyor belt  72 , to form the color image on the paper sheet  3 . 
   Although the color laser printers capable of performing a full color printing operation have been described above, the principle of the present invention is equally applicable to any image forming apparatus capable of forming toner images of a plurality of colors, in the form of a laser printer, an LED printer arranged to form a static latent image with light emitting diodes, a color copier, and a color facsimile machine, for example.