Abstract:
An image forming apparatus includes an image carrier rotatable in a predetermined rotative direction, a charging unit for charging a surface of the image carrier, a precharging device disposed upstream from the charging unit with respect to the predetermined rotative direction of the image carrier for precharging the surface of the image carrier; and a voltage supply unit for supplying a voltage to the precharging device.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an image forming apparatus such as an electrophotographic printer, a facsimile machine and a copier, and specifically relates to a structure of a cleaning unit of such an image forming apparatus for scraping residual toner from a surface of a photoconductor after completion of an electrification process on the surface of the photoconductor and a toner transference process. 
   BACKGROUND OF THE INVENTION 
   The invention overcomes problems as explained below. 
   CONVENTIONAL PARTS OF THE INVENTION 
     FIG. 9  is a schematic side view of an image forming apparatus  50  having a conventional cleaning unit. A photoconductive drum  51  serving as an image carrier is installed in a not-illustrated chassis as well as a not-illustrated driving unit that drives the photoconductive drum  51  to rotate in the direction of the arrow A. A charging roller  52  for charging the periphery  51   a  of the photoconductive drum  51 , an LED head  53  for irradiating the charged periphery  51   a  with an image beam to form an electrostatic latent image, and a developing roller  55  for causing the toner adhere to the latent image are disposed around the photoconductive drum  51 . 
   A developing unit  56  is comprised of the developing roller  55 , a toner supply roller  57  for supplying the developing roller  55  with the toner  54 , and a not-illustrated toner cartridge storing the toner  54  to be supplied to the toner supply roller  57  and the developing roller  55 . A transferring roller  58  and a cleaning unit  59  are disposed downstream form the developing roller  55 . The transferring roller  58  receives at its rotation axis a certain pressure from a not-illustrated spring, so that its periphery is pressed against the periphery  51   a  of the photoconductive drum  51 . 
   The charging roller  52  forming a charging unit  61  is comprised of a metal shaft  52   a  and a roller  52   b  that includes an elastic semiconductive rubber layer formed on the metal shaft  52   a , and a resin coat or a surface-modifying layer formed on the elastic semiconductive rubber layer. The charging unit  61  presses the charging roller  52  against the photoconductive drum  51  so that the charging roller  52  rotates following the photoconductive drum  51 , or rotates at a different speed. The charging unit  61  is powered by a dedicated power source  62  and applies the metal shaft  52   a  with a d.c. (or a.c+d.c.) voltage. 
   The operation of the image forming apparatus having the above-described structure will be explained. 
   When the photoconductive drum  51  is driven at a constant speed in the direction of the arrow A by the not-illustrated driving unit, it receives charge from the charging roller  52  so that the periphery  51   a  of the photoconductive drum  51  becomes charged uniformly. 
   When the LED head  53  irradiates the charged periphery  51   a  of the photoconductive drum  51  with an image beam, a portion that has been applied with this image beam is discharged. Accordingly there arises a potential difference between the portion that has been applied with the image beam and a portion that has not been applied with the image beam, so an electrostatic latent image is formed. 
   This electrostatic latent image comes around to a position where it faces the developing roller  55  of the developing unit  56  with further rotation of the photoconductive drum  51  in the direction of the arrow A. At this position, the toner  54  charged in the same polarity as the charged surface of the photoconductive drum  51  adheres to the electrostatic latent image corresponding to the portion that has been applied with the image beam by coulombic attraction, so that the latent image becomes a visible image. 
   At that time, the transferring roller  58  rotates at the same peripheral velocity as the photoconductive drum  51  in the direction shown by the arrow B. As previously stated, the transferring roller  58  and the photoconductive drum  51  are disposed such that they press each other so that good contact can be kept between them. 
   On the other hand, a not-illustrated paper-supply unit carries recording paper  60  to a position in between the photoconductive drum  51  and the transferring roller  58  in synchronization with the rotation of the photoconductive drum  51 . When the front end of a recording paper  60  reaches the contact point of the photoconductive drum  51  and the transferring roller  58 , it is pinched between them and conveyed in the direction shown by the arrow C to meet the electrostatic latent image on the photoconductive drum  51 . At this time, timing adjustment is made in order that a desired part of the recording paper  60  meets this latent image. 
   The transferring roller  58  gives charge of the polarity opposite to that of the toner  54  to the back of the recording paper  60  while it is conveyed. As a result, when the recording paper  60  meets the latent image in the photoconductive drum  51 , the toner  54  that has adhered to the latent image is attracted by the charge of the opposite polarity, and accordingly, the toner image on the periphery  51   a  of the photoconductive drum  51  is transferred to the recording paper  60 . After that, the recording paper  60  bearing the toner image is applied with pressure and heat in a not-illustrate fixing unit to fix the toner image for completing a series of printing operations. 
   The conventional cleaning unit  59  will be explained with reference to FIG.  9 . 
   The cleaning unit  59  includes a rubber piece  59   b  having a thickness of 1 mm to 3 mm fixed to a metal holder  59   a . By pressing the edge of the rubber piece  59   b  (referred to as a cleaning blade hereinafter) against the periphery  51   a  of the photoconductive drum  51  at a certain angle, residual toner can be scraped off the periphery  51   a . For the cleaning blade  59   b , urethane rubber having excellent properties in elasticity, friction, and endurance can be used. 
   This cleaning blade  59   b  extends in the direction of the axis of the photoconductive drum  51  to cover the whole width of the photoconductive drum  51 . A toner receiver  59   c  integral with the metal holder  59   a  is disposed under the cleaning blade  59   b . A toner carrying spiral  63  having a rotation axis in parallel with that of the photoconductive drum  51  is disposed inside the toner receiver  59   c . Waste toner within the toner receiver  59   c  can be carried in a desired direction by rotating the toner carrying spiral  63 . 
   The conventional image forming apparatus described above is configured to charge the periphery of the photoconductive drum  51  to raise its potential that has bee lowered to nearly 0 V after exposing and transferring processes to a certain value only by use of the charging device  61 . Accordingly, the potential difference between the charging roller  52  and the periphery  51   a  of the photoconductive drum  51  is large, and therefore a large current flows between them. As a result, there arises a problem that a voltage drop in the rubber layer of the charging roller  52  is not negligible and the charging voltage therefore lowers, so the periphery  51   a  of the photoconductive drum  51  cannot be charged to a desired potential. 
   This problem invites the so-called ghost phenomenon of a photoconductive drum cycle, in which beam-exposed parts appear as density-increased parts of an image one cycle after (referred to as “OPC (Organic Photo Conductor) cyclic incidental image” hereinafter). 
   The reason why the OPC cyclic incidental image occurs will be explained with reference to FIGS.  10 ( a ) to  10 ( c ). FIG.  10 ( a ) schematically shows potentials of exposed areas and unexposed areas on the periphery  51   a  of the photoconductive drum  51  after execution of the exposing process. As shown in FIG.  10 ( a ), not only the unexposed areas but the exposed areas bear charge in some degree. 
   FIG.  10 ( b ) shows potentials of the exposed areas and unexposed areas after execution of the transferring process. It is apparent from FIG.  10 ( b ) that the potential of the exposed areas falls to nearly 0 V, while the potential of the unexposed areas does not fall to that extent and they still bear 20% of the supplied charge. 
   FIG.  10 ( c ) shows potentials of the exposed areas and unexposed areas after the periphery  51   a  is charged again by the charging unit  61 . At this time, the potential of the unexposed areas rises to a desired level. However, the potential of the last exposed areas does not rise to the desired value for the reason described above. This becomes a cause of the OPC cyclic incidental image. 
   Recently, the problem of the OPC cyclic incidental image is becoming serious, because the OPC cyclic incidental image becomes conspicuous as the rotation speeds of the photoconductive drum  51  and the charging roller  52  increase, and accordingly a large charging current flows causing the periphery of the photoconductive  51  drum to be charged incompletely due to the aforementioned voltage drop. This problem is also exacerbated by increase of resolution of such an image forming apparatus  50  to, for example, 1200 dpi. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to solve the above-described problem of the OPC cyclic incidental image in the image forming apparatus by eliminating incomplete electrification on the periphery of a photoconductive drum. 
   This object is achieved by an image forming apparatus including an image carrier rotatable in a predetermined rotative direction, a charging unit for charging a surface of the image carrier, a precharging device disposed upstream from the charging unit with respect to the predetermined rotative direction of the image carrier for precharging the surface of the image carrier, and a voltage supply unit for supplying a voltage to the precharging device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which: 
       FIG. 1  is a section view schematically showing a structure of a cleaning unit for use in an image forming apparatus of a first embodiment according to the invention; 
     FIG.  2 ( a ) is a section view schematically showing a structure of a cleaning unit for use in an image forming apparatus of a second embodiment according to the invention; 
     FIG.  2 ( b ) shows a principal part of the cleaning unit of the first embodiment on large scale; 
       FIG. 3  is a section view schematically showing a structure of a cleaning unit for use in an image forming apparatus of a third embodiment according to the invention; 
     FIG.  4 ( a ) is a section view schematically showing a structure of a cleaning unit for use in an image forming apparatus of a fourth embodiment according to the invention; 
     FIG.  4 ( b ) shows a principal part of the cleaning unit of the fourth embodiment on large scale; 
     FIG.  5 ( a ) is a section view schematically showing a structure of a cleaning unit for use in a variant of the fourth embodiment; 
     FIG.  5 ( b ) shows a principal part of the cleaning unit of the variant of the fourth embodiment on large scale; 
       FIG. 6  is a section view schematically showing a structure of a cleaning unit for use in an image forming apparatus of a fifth embodiment according to the invention; 
     FIG.  7 ( a ) is a section view schematically showing a structure of a cleaning unit for use in an image forming apparatus of a sixth embodiment according to the invention; 
     FIG.  7 ( b ) is a perspective view of a metal plate of the cleaning unit of the sixth embodiment; 
     FIGS.  8 ( a ) to  8 ( c ) are explanatory diagrams explaining the process for charging the periphery of a photoconductive drum without causing the OPC cyclic incidental image. 
       FIG. 9  is a section view schematically showing a structure of an image forming apparatus having a conventional cleaning unit; and 
     FIGS.  10 ( a ) to  10  ( c ) are explanatory diagrams explaining the process for charging the periphery of a photoconductive drum in which the OPC cyclic incidental image is caused. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A First Embodiment. 
     FIG. 1  is a section view schematically showing a structure of a cleaning unit  1  for use in an image forming apparatus of a first embodiment according to the invention. 
   This cleaning unit  1  can replace the cleaning unit  59  of the image forming apparatus  50  described above with reference to FIG.  9 . without changing other units and elements. Accordingly, the cleaning unit  1  will be described supposing that it is disposed in the image forming apparatus  50  shown in  FIG. 9 , and detailed explanation of each of the parts of the image forming apparatus  50  will be omitted except the cleaning unit  1  and other parts associated with this cleaning unit  1  in the interest of simplicity. 
   As shown in  FIG. 1 , the cleaning unit  1  includes a metal holder  2  disposed in the vicinity of the photoconductive drum  51  serving as an image carrier so as to extend in the direction of the axis of the photoconductive drum  51  to cover the whole width of the photoconductive drum  51  (as in the case of the cleaning unit  59  shown in FIG.  9 ). Fixed to an incline  2   a  of the metal holder  2 , is an edge  3   a  of a cleaning blade  3  having a thickness of 1 mm to 3 mm made of semiconductive urethane rubber that extends substantially for the whole width of the photoconductive drum  51 . The other edge  3   b  presses the periphery  51   a  of the photoconductive drum  51 . This cleaning blade  3  serves as an auxiliary charging unit. 
   A toner receiver  4  of L-shape cross section integral with the metal holder  2  is disposed under a contact point where the edge  3   b  and the periphery  51   a  of the photoconductive drum  51  press each other for receiving the toner falling from the contact point. A toner-leakage preventing film  6  extends from an edge of the bottom of the toner receiver  4 . The front end of the toner-leakage preventing film  6  is tangent to the periphery  51   a  of the photoconductive drum  51 . 
   Like the cleaning unit  59  shown in  FIG. 9 , the toner carrying spiral  63  is disposed inside the toner receiver  4  for carrying waste toner accumulated within the toner receiver  4  in a desired direction. 
   The semiconductive urethane rubber forming the cleaning blade  3  contains carbon black to exhibit resistivity of 106 Ω cm-1010 Ω cm. The metal holder  2  and the cleaning blade  3  made of the semiconductive urethane rubber are fixed to each other in an electrically interconnected state. A dedicated power supply  5  of the cleaning unit  1  that serves as a voltage supplying unit applies a d.c. voltge (or d.c.+a.c. voltage) to the cleaning blade  3  made of the semiconductive urethane rubber through a wiring connected to the metal holder  2 . Since the d.c. voltge (or d.c.+a.c. voltage) is supplied to the cleaning blade  3  through the metal holder plate  2 , the cleaning blade  3  is applied with the voltage uniformly. 
   The operation of the image forming apparatus of the first embodiment having the above-described structure will be explained. 
   The toner that has adhered to the periphery  51   a  of the photoconductive drum  51  is scraped by the edge  3   b  of the cleaning blade  3  and falls into the toner receiver  4  when it reaches the contact point where the periphery  51   a  of the photoconductive drum  51  and the edge  3   b  of the cleaning blade  3  press each other. The waste toner accumulated within the toner receiver  4  is carried in a certain direction by the toner spiral  63  rotating inside the toner receiver  4  and discharged from the toner receiver  4 . 
   On the other hand, the edge  3   b  of the cleaning blade  3  applied with the voltage by the dedicated power supply  5  charges the periphery  51   a  of the photoconductive drum  51  in a contact manner at the contact point between the periphery  51   a  of the photoconductive drum  51  and the edge  3   b  of the cleaning blade  3 . 
   This preliminary electrification on the peripheral  51   a  of the photoconductive drum  51  performed by the edge  3   b  of the cleaning blade  3  satisfies the following expression (1).
 
100V|Vo|−|Va|50V  (1)
 
   where V 0  is the potential of the periphery  51   a  developed by the electrification performed by the charging unit  61  shown in  FIG. 9 , and Va is the potential of the periphery  51   a  developed by the electrification performed by the cleaning blade  3 . 
   If the above condition is satisfied, the difference between the potential of the exposed areas of the periphery  51   a  of the photoconductive drum  51  that has fallen to nearly 0 V after an overall exposure and the potential of those areas developed by being subject to the electrification by the charging unit  61  can be within 100 V. As a result, the downstream charging unit  61  can charge the periphery  51   a  of the photoconductive drum  51  to a desired potential without causing the above-described OPC cyclic incidental image. 
   The process in which the periphery  51   a  of the photoconductive drum  51  is charged without causing the OPC cyclic incidental image will be explained with reference to FIGS.  8 ( a ) to  8 ( c ). 
   FIG.  8 ( a ) schematically shows potentials of the exposed areas and unexposed areas of the periphery  51   a  of the photoconductive drum  51  after execution of the transferring process. As shown in FIG.  8 ( a ), the potential of the exposed areas falls to nearly 0 V, while the potential of the unexposed areas does not fall to that extent and they still bear 20% of the supplied charge. 
   FIG.  8 ( b ) shows the potentials of the exposed areas and unexposed areas after execution of the preliminary electrification by the cleaning blade  3 . It is desirable that the surface potentials of the exposed areas and unexposed areas are within the range satisfying the expression (1). 
   FIG.  8 ( c ) shows the potentials of the exposed areas and unexposed areas after execution of a second electrification by the charging unit  61 . Not only the unexposed areas but the last exposed areas as well are charged to the desired potential on the strength of the preliminary electrification, thereby avoiding the OPC cyclic incidental image. 
   It has been found through experiment that if the value of |Vo|−|Va| exceeds 100 V, the OPC cyclic incidental image occurs. 
   On the other hand, for enabling the charging unit  61  to charge the periphery of the photoconductor drum  51  uniformly and stably, it is desirable that the surface potential Va of the photoconductor drum  51  before reaching the charging unit  61  is lower than the surface voltage V 0  after passing the charging unit  61  by at least 50 V. 
   The reason for that will be explained below. The charging unit  61  performs electrification by use of the charging roller  52  in a desirable environment where temperature and humidity are controlled. Accordingly, in the charging process by the charging unit  61 , electrification is carried out stably and uniformly. On the other hand, the cleaning unit  1  is strictly for the purpose of scraping the toner, and therefore it uses a specific member (the cleaning blade  3 ) that serves this purpose. Accordingly, it is difficult for the cleaning unit  1  to perform electrification stably and uniformly. 
   So, in this embodiment, the charging unit  61  operates as a workhorse for the electrification on the photoconductive drum  51 , and the cleaning unit  1  is no more than an auxiliary charging unit that operates for reducing the potential difference between the exposed areas and the unexposed areas to satisfy the expression (1). 
   It has been found through experiment that if the value of |Vo|−|Va| is smaller than 50 V, the photoconductive drum  51  cannot be charged uniformly. 
   Accordingly, in this embodiment, V 0  is set to −620 V and Va is set to −570 V. 
   Table 1 shows performances of the conventional image forming apparatus shown in FIG.  9  and the image forming apparatus of this first embodiment according to the invention in each of the items “Cleaning Capability”, “Independent Charging Capability”, “Rubber Durability, and “Resistivity to OPC cyclic Incidental Image”. 
   The item “Cleaning Capability” is for making an assessment of the capability of scraping the toner off the photoconductive drum  51  when the toner-transferring process is skipped for having all the toner supplied to the photoconductive drum remain. The item “Independent Charging Capability” is for making an assessment of the capability of charging the photoconductive drum by the charging unit  61  for the conventional image forming apparatus, and the capability of charging the photoconductive drum  51  by the cleaning unit  1  alone for the first embodiment. The item “Rubber Durability” is for making an assessment of the degradation of the cleaning blade  3  such as wear, chip, or settling. 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               Conventional Image 
                 
             
             
                 
               Item 
               Forming Apparatus 
               Embodiment 1 
             
             
                 
                 
             
           
           
             
                 
               Cleaning Capability 
               ⊚ 
               ∘ 
             
             
                 
                 
               (excellent) 
               (satisfying) 
             
             
                 
               Independent Charging 
               ⊚ 
               ∘ 
             
             
                 
               Capability 
               (stable) 
               (good though not 
             
             
                 
                 
                 
               uniform) 
             
             
                 
               Rubber Durability 
               ∘ 
               Δ 
             
             
                 
               (chipping off etc.) 
               (good) 
               (acceptable) 
             
             
                 
               Resistivity to OPC 
               x 
               ⊚ 
             
             
                 
               Cyclic Incidental 
               (no good) 
               (excellent) 
             
             
                 
               Image 
             
             
                 
                 
             
           
        
       
     
   
   As seen from Table 1, the first embodiment is somewhat inferior to the conventional image forming apparatus in the cleaning capability and the rubber durability, since the cleaning blade  3  of the cleaning unit  1  of the first embodiment is made of semiconductive urethane rubber, though the first embodiment is at an acceptable in these items. In the case of having the cleaning unit  1  perform electrification alone, slight unevenness occurs in a formed image after repetition of image forming cycles. However, it has been confirmed that in the case of having the cleaning unit  1  and the charging unit  61  perform electrification jointly, such uneveness becomes imperceptible, and the OPC cyclic incidental image does not occur as well. 
   Also, it has been confirmed through experiment that when the resistivity of the semiconductive urethane rubber is between 106 Ωcm-1010 Ωcm, electrification is performed uniformly, when the resistivity of the semiconductive urethane rubber is smaller than 106 Ωcm, nonuniformity arises in the electrification, and when the resistivity of the semiconductive urethane rubber exceeds 1010 Ωcm, the surface potential does not rise to the desired value and therefore the OPC cyclic incidental image does not disappear. 
   As described above, according to the first embodiment, since the cleaning unit  1  precharges the periphery  51   a  of the photoconductive drum  51  whose potential has been lowered to nearly 0 V after an overall exposure to an appropriate level while preserving its cleaning ability, it is possible to charge the periphery  51   a  of the photoconductive drum  51  to a desired potential by the charging unit  61  at once. 
   Thus, a high quality image free from the OPC cyclic incidental image can be obtained. 
   In the first embodiment, although carbon is added to the urethane rubber to provide the urethane rubber with conductivity, metal oxide, conductive filler, ions, or a mixture of them may be added instead. For the material of the cleaning blade  3 , urethane rubber is used in the first embodiment, the present invention is not limited thereto, and different kinds of rubber may be used. 
   Furthermore, the first embodiment has the photoconductive drum  51  as an image carrier, it is needless to say that the present invention is applicable to any image forming apparatus that forms electrostatic latent image. 
   A Second Embodiment 
   FIG.  2 ( a ) is a section view schematically showing a structure of a cleaning unit  10  for use in an image forming apparatus of a second embodiment according to the invention. 
   This cleaning unit  10  can replace the cleaning unit  59  of the image forming apparatus  50  described above with reference to  FIG. 9  without any change in other units and elements of the image forming apparatus  50 . This cleaning unit  10  differs from the cleaning unit  1  of the first embodiment in that the material of the cleaning blade  11  of the cleaning unit  10  is different from that of the cleaning unit  1 , and the cleaning unit  10  is additionally provided with a semiconductive resin tape  12 . 
   Accordingly, the cleaning unit  10  will be described supposing that it is disposed in the image forming apparatus  50  shown in  FIG. 9 , and detailed explanation of each of the parts of the image forming apparatus  50  will be omitted except the cleaning unit  10  and other parts common to the cleaning unit  1  of the first embodiment in the interest of simplicity. 
   In  FIG. 2 , to the incline  2   a  of the metal holder  2  fixed is the edge  11   a  of a cleaning blade  11  having a thickness of 1 mm to 3 mm made of semiconductive urethane rubber that extends substantially for the whole width of the photoconductive drum  51 . The other edge  11   b  presses the periphery  51   a  of the photoconductive drum  51 . 
   The semiconductive resin tape  12  is stuck on the surface of the cleaning blade  11  as a precharging device so as to extend from the metal holder  2  to the edge  11   b  of the cleaning blade  11 . The semiconductive resin tape  12  is preferably made of a material having resistivity of 106 Ωcm-1010 Ωcm such as nylon resin, urethane resin, or fluorine resin. The metal holder  2  and the semiconductive resin tape  12  are therefore in an electrically interconnected state. 
   FIG.  2 ( b ) is a partially enlarged view of the edge  11   b  of the cleaning blade  11  and the parts adjoining thereto. The distance L between the edge  11   b  of the cleaning blade  11  and the front edge of the semiconductive resin tape  12  is preferably shorter than 1 mm. If the distance L is too long, electrification becomes impossible. 
   The operation of the image forming apparatus of the second embodiment having the above-described structure will be explained. 
   The toner that has adhered to the periphery  51   a  of the photoconductive drum  51  is scraped by the edge  11   b  of the cleaning blade  11  and falls into the toner receiver  4  when it reaches the contact point where the periphery  51   a  of the photoconductive drum  51  and the edge  11   b  of the cleaning blade  11  press each other. The waste toner accumulated within the toner receiver  4  is carried in a certain direction by the toner spiral  63  rotating inside the toner receiver  4  and discharged from the toner receiver  4 . 
   On the other hand, in the vicinity of the contact point between the periphery  51   a  of the photoconductive drum  51  and the edge  11   b  of the cleaning blade  11 , the semiconductive resin tape  12  applied with the voltage supplied from the dedicated power supply  5  precharges the periphery  51   a  of the photoconductive drum  51  in a non-contact manner. At this time, it is preferable that the above-described expression (1) is satisfied for the previously described reason. 
   Table 2 shows performances of the conventional image forming apparatus shown in  FIG. 9 , the image forming apparatus of the previously described first embodiment, and the image forming apparatus of the second embodiment in each of the items explained below. 
   The items include “Resistivity to Damage to Photoconductive Drum” in addition to “Cleaning Capability”, “Independent Charging Capability”, “Rubber Durability, and “Resistivity to OPC Cyclic Incidental Image” explained in the foregoing first embodiment. 
   
     
       
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
               Conventional 
                 
                 
             
             
                 
               Image Forming 
                 
                 
             
             
               Item 
               Apparatus 
               Embodiment 1 
               Embodiment 2 
             
             
                 
             
           
           
             
               Cleaning 
               ⊚ 
               ∘ 
               ⊚ 
             
             
               Capability 
               (excellent) 
               (satisfying) 
               (as excellent as 
             
             
                 
                 
                 
               previously) 
             
             
               Independent 
               ⊚ 
               ∘ 
               ∘ 
             
             
               Charging 
               (stable) 
               (good though 
               (good though 
             
             
               Capability 
                 
               not uniform) 
               not uniform) 
             
             
               Rubber Durability 
               ∘ 
               Δ 
               ∘ 
             
             
               (chipping off 
               (good) 
               (acceptable) 
               (as good as 
             
             
               etc.) 
                 
                 
               previously) 
             
             
               Resistivity to OPC 
               x 
               ⊚ 
               ⊚ 
             
             
               Cyclic Incidental 
               (no good) 
               (excellent) 
               (excellent) 
             
             
               Image 
             
             
               Resistivity to 
               ∘ 
               ∘ 
               Δ 
             
             
               Damage to 
               (good) 
               (good) 
               (acceptable) 
             
             
               Photoconductive 
             
             
               Drum 
             
             
                 
             
           
        
       
     
   
   As seen from Table 2, the second embodiment having the cleaning unit  10  shows as good performances in the cleaning capability and the rubber durability as the first embodiment since the cleaning blade  11  is made of urethane rubber as previously. Although toner adhesion and streaks of nonuniformity due to scratches caused by friction occur slightly to the periphery of the photoconductive drum  51  after repetition of image forming cycles in the case of having the cleaning unit  10  perform electrification alone, it has been confirmed that a formed image is at a satisfying level and no OPC cyclic incidental image occurs in the case of having the cleaning unit  10  and the charging unit  61  perform electrification jointly. 
   As described above, the second embodiment has, not only the same advantage as the first embodiment, the different advantage that the cleaning unit  10  thereof shows good performance in the cleaning capability and the rubber durability since it is made of the same material as the conventional cleaning unit, thereby allowing a good and stable cleaning performance over a long period of time. 
   Although the second embodiment uses the semiconductive resin tape  12 , any appropriate tape coated with a semiconductive resin film maybe used. If such a tape is coated with nylon resin, urethane resin, or fluororesin which is used as a material of a charging roller because of their resistivity to the toner adhesion and good addhesiveness with the urethane rubber which is used as the material of the cleaning blade  11 , the cleaning unit  19  can perform stable electrification over a long period of time. 
   A Third Embodiment. 
     FIG. 3  is a section view schematically showing a structure of a cleaning unit  15  for use in an image forming apparatus of a third embodiment according to the invention. 
   This cleaning unit  15  can replace the cleaning unit  59  in the image forming apparatus  50  described above with reference to  FIG. 9  as is the case with the cleaning unit  1  of the first embodiment without any change in other units and elements of the image forming apparatus  50 . This cleaning unit  15  differs from the cleaning unit  10  of the second embodiment in that the shape of the metal holder  16  is different from that of the metal holder  2 , and a semiconductive rubber layer  17  is used in place of the semiconductive resin tape  12 . 
   Accordingly, the cleaning unit  15  will be described supposing that it is disposed in the image forming apparatus  50  shown in  FIG. 9 , and detailed explanation of each of the parts of the image forming apparatus  50  will be omitted except the cleaning unit  15  and other parts common to the cleaning unit  10  of the second embodiment in the interest of simplicity. 
   In  FIG. 3 , to an incline  16   a  of the metal holder  16  fixed is the edge  11   a  of the cleaning blade  11  having a thickness of 1 mm to 3 mm made of semiconductive urethane rubber that extends substantially for the whole width of the photoconductive drum  51 . The other edge  11   b  presses the periphery  51   a  of the photoconductive drum  51 . The metal holder  16  is provided with an L-shape holding member  16   b  extending from the incline  16   a  for fixing the cleaning blade  11  therein. The cleaning blade  11  is held within the L-shape holding member  16   b  in a slightly compressed state. 
   The semiconductive rubber layer  17  made of the same material (for example, rubber made of silicon polymer containing carbon as an additive to achieve conductivity) as the elastic layer of the charging roller  52  ( FIG. 9 ) is stuck as a precharging device on the surface of the L-shape holding member  16   b  and the surface of the cleaning blade  11  which are flush with each other. The resistivity of this semiconductive rubber layer  17  is preferably between 106 Ωcm-1010 Ωcm so that the metal holder  16  and the semiconductive rubber layer  17  are in an electrically interconnected state. 
   The rubber layer  17  is disposed such that the tip thereof is slightly behind the edge  11   b  of the cleaning blade  11  keeping a certain distance appropriate for non-contact electrification from the periphery  51   a  of the photoconductive drum  51 . 
   The operation of the third embodiment having the above structure will be explained. 
   The toner that has adhered to the periphery  51   a  of the photoconductive drum  51  is scraped by the edge  11   b  of the cleaning blade  11  and falls into the toner receiver  4  when it reaches the contact point where the periphery  51   a  of the photoconductive drum  51  and the edge  11   b  of the cleaning blade  11  press each other. The waste toner accumulated within the toner receiver  4  is carried in a certain direction by the toner carrying spiral  63  rotating inside the toner receiver  4  and discharged from the toner receiver  4 . 
   On the other hand, in the vicinity of the contact point between the periphery  51   a  of the photoconductive drum  51  and the edge  11   b  of the cleaning blade  11 , the semiconductive rubber layer  17  applied with the voltage supplied from the dedicated power supply  5  precharges the periphery  51   a  of the photoconductive drum  51  in a non-contact manner. At this time, it is preferable that the above-described expression (1) is satisfied for the previously described reason. 
   If any resin layer or surface modification layer is provided on the surface of the elastic rubber layer of the charging roller  52  (FIG.  9 ), it is preferable that a similar resin layer or a similar surface modification layer is provided on the surface of the rubber layer  17 . 
   Table 3 shows performances of the image forming apparatus of the foregoing first and second embodiments, and the image forming apparatus of this third embodiment in each of the items explained below. 
   The items include “Cleaning Capability”, “Independent Charging Capability”, “Rubber Durability, “Resistivity to OPC Cyclic Incidental Image”, and “Resistivity to Damage to Photoconductive Drum ” as in the case of the second embodiment. 
   
     
       
             
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
               Item 
               Embodiment 1 
               Embodiment 2 
               Embodiment 3 
             
             
                 
             
           
           
             
               Cleaning 
               ∘ 
               ⊚ 
               ⊚ 
             
             
               Capability 
               (satisfying) 
               (as excellent as 
               (as excellent as 
             
             
                 
                 
               previously) 
               previously) 
             
             
               Independent 
               ∘ 
               ∘ 
               ⊚ 
             
             
               Charging 
               (good though 
               (good though 
               (excellent) 
             
             
               Capability 
               not uniform) 
               not uniform) 
             
             
               Rubber Durability 
               Δ 
               ∘ 
               ∘ 
             
             
               (chipping off 
               (acceptable) 
               (as good as 
               (as good as 
             
             
               etc.) 
                 
               previously) 
               previously) 
             
             
               Resistivity to OPC 
               ⊚ 
               ⊚ 
               ⊚ 
             
             
               Cyclic Incidental 
               (excellent) 
               (excellent) 
               (excellent) 
             
             
               Image 
             
             
               Resistivity to 
               ∘ 
               Δ 
               ∘ 
             
             
               Damage to 
               (as good as 
               (acceptable) 
               (as good as 
             
             
               Photoconductive 
               previously) 
                 
               previously) 
             
             
               Drum 
             
             
                 
             
           
        
       
     
   
   As seen from Table 3, the third embodiment having the cleaning unit  15  shows as good performances in the cleaning capability and the rubber durability as the second embodiment since the cleaning blade  11  is made of urethane rubber as previously. In addition, it has been confirmed that a high quality image with very little nonuniformity can be formed even when electrification is performed by the cleaning unit  15  alone, since the photoconductive drum  51  is charged by the semiconductive rubber layer  17  made of the same material as the charging roller  52  and having the same resistance as the charging roller  52 . 
   As described above, the third embodiment has, in addition to the same advantage as the second embodiment, the different advantage that the probability of damaging the periphery  51   a  of the photoconductive drum  51  is less since the rubber layer  17  that performs electrification is resistant to toner adhesion, allowing stable electrification over a long period of time. 
   A fourth embodiment 
   FIG.  4 ( a ) is a section view schematically showing a structure of a cleaning unit  20  for use in an image forming apparatus of a fourth embodiment according to the invention. 
   This cleaning unit  20  can replace the cleaning unit  59  in the image forming apparatus  50  described above with reference to  FIG. 9  as is the case with the cleaning unit  1  of the first embodiment without any change in other units and elements. This cleaning unit  20  differs from the cleaning unit  1  of the first embodiment in that the material of the cleaning blade  11  of the cleaning unit  20  is different from that of the cleaning unit  1 , and the cleaning unit  20  is additionally provided with a semiconductive resin tape  21 . 
   Accordingly, the cleaning unit  20  will be described supposing that it is disposed in the image forming apparatus  50  shown in  FIG. 9 , and detailed explanation of each of the parts of the image forming apparatus  50  will be omitted except the cleaning unit  20  and other parts common to the cleaning unit  1  of the first embodiment in the interest of simplicity. 
   The cleaning blade  11  of this embodiment is made of insulative urethane rubber having a thickness of 1 mm to 3 mm as in the case of the second embodiment. 
   FIG.  4 ( b ) is a partially enlarged view showing the toner-leakage preventing film  6  and the parts adjoining thereto. As shown in FIG.  4 ( b ), the semiconductive resin tape  21  serving as a precharging device is stuck to the tip of the toner-leakage preventing film  6  so as to make contact with the periphery  51   a  of the photoconductive drum  51 . This semiconductive resin tape  21  is applied with the d.c. (or a.c+d.c.) voltage by the dedicated power supply  5 . 
   The operation of the fourth embodiment having the above structure will be explained. 
   The toner that has adhered to the periphery  51   a  of the photoconductive drum  51  is scraped by the edge  11   b  of the cleaning blade  11  and falls into the toner receiver  4  when it reaches the contact point where the periphery  51   a  of the photoconductive drum  51  and the edge  11   b  of the cleaning blade  11  press each other. The waste toner accumulated within the toner receiver  4  is carried in a certain direction by the toner spiral  63  rotating inside the toner receiver  4  and discharged from the toner receiver  4 . 
   The semiconductive resin tape  21  upstream from the cleaning blade  11  negatively charges the periphery  51   a  of the photoconductive drum  51 . At this time, it is preferable that the above-described expression (1) is satisfied for the previously described reason. In this fourth embodiment, the periphery  51   a  of the photoconductive drum  51  is negatively charged before it is cleaned by the toner-scraping operation of the cleaning blade  11  so that the residual negatively charged toner (most of the residual toner except fog toner is negatively charged) can be easily removed from the periphery  51   a  of the photoconductive drum  51  by electrostatic repulsion. 
   FIG.  5 ( a ) is a section view schematically showing a structure of a cleaning unit  25  for use in a variant of the fourth embodiment according to the invention. In this variant, a semiconductive resin tape  27  is stuck as a precharging device to the tip of a diselectrification plate  26  provided as necessary at the bottom  4   a  of the toner receiver  4 . This diselectrification plate  26  is applied with the d.c. (or a.c+d.c.) voltage by the dedicated power supply  5 , and the semiconductive resin tape  27  is applied with the same voltage. 
   The cleaning operation and the electrification operation of the cleaning unit  25  are much the same as those of the cleaning unit  20  shown in  FIG. 4 , explanation thereof will be omitted. 
   Table 4 shows performances of the image forming apparatus of the foregoing first, second and third embodiments, and the image forming apparatus of this fourth embodiment in each of the items explained below. 
   The items include “Resistivity to Drum-Filming” in addition to “Cleaning Capability”, “Independent Charging Capability”, “Rubber Durability”, “Resistivity to OPC Cyclic Incidental Image”, and “Resistivity to Damage to Photoconductive drum ” explained in the foregoing third embodiment. 
   
     
       
             
             
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
                 
               Embodiment 
               Embodiment 
               Embodiment 
               Embodiment 
             
             
               Item 
               1 
               2 
               3 
               4 
             
             
                 
             
           
           
             
               Cleaning 
               ∘ 
               ⊚ 
               ⊚ 
               ⊚ 
             
             
               Capability 
               (satisfying) 
               (as excellent 
               (as excellent 
               (as excellent 
             
             
                 
                 
               as 
               as 
               as 
             
             
                 
                 
               previously) 
               previously) 
               previously) 
             
             
               Independent 
               ∘ 
               ∘ 
               ⊚ 
               Δ 
             
             
               Charging 
               (good though 
               (good though 
               (excellent) 
               (acceptable) 
             
             
               Capability 
               not uniform) 
               not uniform) 
             
             
               Rubber 
               Δ 
               ∘ 
               ∘ 
               ∘ 
             
             
               Durability 
               (acceptable) 
               (as good as 
               (as good as 
               (as good as 
             
             
               (chipping off 
                 
               previously) 
               previously) 
               previously) 
             
             
               etc.) 
             
             
               Resistivity to 
               ⊚ 
               ⊚ 
               ⊚ 
               ⊚ 
             
             
               OPC Cyclic 
               (excellent) 
               (excellent) 
               (excellent) 
               (excellent) 
             
             
               Incidental 
             
             
               Image 
             
             
               Resistivity to 
               ∘ 
               Δ 
               ∘ 
               Δ 
             
             
               Damage to 
               (as good as 
               (acceptable) 
               (as good as 
               (acceptable) 
             
             
               Photo- 
               previously) 
                 
               previously) 
             
             
               conductive 
             
             
               Drum 
             
             
               Resistivity to 
               ∘ 
               ∘ 
               ∘ 
               ⊚ 
             
             
               Drum-Filming 
               (good) 
               (good) 
               (good) 
               (excellent) 
             
             
                 
             
           
        
       
     
   
   As seen from Table 4, it has been confirmed that the OPC cyclic incidental image does not occur in the fourth embodiment having the cleaning unit  20  or  25  as is the case with the preceding embodiments. In addition, this fourth embodiment is superior to other embodiments in resistivity to the occurrence of the drum-filming phenomena because of the improvement in the cleaning capability. The item “Resistivity to drum-cleaning capability” is for making an assessment of the capability of preventing the drum-filming phenomena that may occur on the periphery  51   a  of the photoconductive drum  51  after repetitions of image forming cycles. 
   As described above, the fourth embodiment has, in addition to the same advantage as the second embodiment, the different advantage that the toner-scraping capability of the cleaning blade  11  is improved since the periphery  51   a  of the photoconductive drum  51  is charged upstream from the cleaning blade  11 , and accordingly the drum-filming phenomenon can be avoided, thereby enabling forming high quality images for a long period of time. 
   A Fifth Embodiment 
     FIG. 6  is a section view schematically showing a structure of a cleaning unit  30  for use in an image forming apparatus of a fifth embodiment according to the invention. 
   This cleaning unit  30  can replace the cleaning unit  59  in the image forming apparatus  50  described above with reference to  FIG. 9  as is the case with the cleaning unit  1  of the first embodiment without any change in other units and elements. This cleaning unit  30  differs from the cleaning unit  1  of the first embodiment in that the material of the cleaning blade  11  of the cleaning unit  30  is different from that of the cleaning unit  1 , and the cleaning unit  30  is additionally provided with a brush roller  31 . 
   Accordingly, the cleaning unit  30  will be described supposing that it is disposed in the image forming apparatus  50  shown in  FIG. 9 , and detailed explanation of each of the parts of the image forming apparatus  50  will be omitted except the cleaning unit  30  and other parts common to the cleaning unit  1  of the first embodiment in the interest of simplicity. 
   The cleaning blade  11  of this embodiment is made of insulative urethane rubber having a thickness of 1 mm to 3 mm as in the case of the cleaning unit  10  shown in  FIG. 2  of the second embodiment. 
   The brush roller  31  serving as the precharging device is comprised of a metal shaft  32  disposed within the toner receiver  4  so as to extend in parallel with the rotation axis of the photoconductive drum  51  in the vicinity of the photoconductive drum  51  and semiconductive fibers  33  made of nylon provided so as to extend radially from the metal shaft  32 . The metal shaft  32  is set in such a position that the tips of the semiconductive fibers  33  contact with and slide on the periphery  51   a  of the photoconductive drum  51 . 
   The metal shaft  32  is driven by a not-illustrated driving unit to have a peripheral speed different from that of the photoconductive drum  51  so that the tips of the semiconductive fibers  33  brush the periphery  51   a  of the photoconductive drum  51 . The metal shaft  32  is also applied with the d.c. (or a.c+d.c.) voltage by the dedicate power supply  5 . 
   The operation of the fifth embodiment having the above structure will be explained. 
   The toner and toner additive that have adhered to the periphery  51   a  of the photoconductive drum  51  are scraped therefrom preparatively by the brush roller  31  before they are scraped by the cleaning blade  11 . Concurrently, the periphery  51   a  of the photoconductive drum  51  is precharged in a contact manner by the brush roller  31  applied with the voltage by the dedicated power supply  5 . At this time, it is preferable that the above-described expression (1) is satisfied for the previously described reason. 
   Table 5 shows performances of the image forming apparatus of the foregoing second, third and fourth embodiments, and the image forming apparatus of this fourth embodiment in each of the items explained below. 
   The items include “Resistivity to Contamination of the Charging Roller” in addition to “Cleaning Capability”, “Independent Charging Capability”, “Rubber Durability”, “Resistivity to OPC Cyclic Incidental Image”, “Resistivity to Damage to Photoconductive Drum ”, and “Resistivity to Drum-Filming” explained in the foregoing third embodiment. 
   
     
       
             
             
             
             
             
           
         
             
               TABLE 5 
             
             
                 
             
             
                 
               Embodiment 
               Embodiment 
               Embodiment 
               Embodiment 
             
             
               Item 
               2 
               3 
               4 
               5 
             
             
                 
             
           
           
             
               Cleaning 
               ⊚ 
               ⊚ 
               ⊚ 
               ⊚ 
             
             
               Capability 
               (as excellent 
               (as excellent 
               (as excellent 
               (as excellent 
             
             
                 
               as 
               as 
               as 
               as 
             
             
                 
               previously) 
               previously) 
               previously) 
               previously) 
             
             
               Independent 
               ∘ 
               ⊚ 
               Δ 
               Δ 
             
             
               Charging 
               (good though 
               (excellent) 
               (acceptable) 
               (acceptable) 
             
             
               Capability 
               not uniform) 
             
             
               Rubber 
               ∘ 
               ∘ 
               ∘ 
               ∘ 
             
             
               Durability 
               (as good as 
               (as good as 
               (as good as 
               (as good as 
             
             
               (chipping off 
               previously) 
               previously) 
               previously) 
               previously) 
             
             
               etc.) 
             
             
               Resistivity to 
               ⊚ 
               ⊚ 
               ⊚ 
               ⊚ 
             
             
               OPC Cyclic 
               (excellent) 
               (excellent) 
               (excellent) 
               (excellent) 
             
             
               Incidental 
             
             
               Image 
             
             
               Resistivity to 
               Δ 
               ∘ 
               Δ 
               ∘ 
             
             
               Damage to 
               (acceptable) 
               (as good as 
               (acceptable) 
               (as good as 
             
             
               Photo- 
                 
               previously) 
                 
               previously) 
             
             
               conductive 
             
             
               Drum 
             
             
               Resistivity to 
               ∘ 
               ∘ 
               ⊚ 
               ⊚ 
             
             
               Drum-Filming 
               (good) 
               (good) 
               (excellent) 
               (excellent) 
             
             
               Resistivity to 
               Δ 
               ∘ 
               ∘ 
               ⊚ 
             
             
               Contamination 
               (acceptable) 
               (good) 
               (good) 
               (excellent) 
             
             
               of the Charging 
             
             
               Roller 
             
             
                 
             
           
        
       
     
   
   As seen from Table 5, the fifth embodiment having the cleaning unit  30  is superior in points of the cleaning capability and the resistivity to the drum-filming since the periphery  51   a  of the photoconductive drum  51  is charged upstream from the cleaning blade  11  as is the case with the fourth embodiment having the cleaning unit  25 . Furthermore, since the brush roller  31  brushes the periphery  51   a  of the photoconductive drum  51  with its relatively soft semiconductive fibers  33 , the photoconductive drum  51  suffers less damage compared with the fourth embodiment. In the fifth embodiment, since the brush roller  31  is applied with a high negative voltage, the external toner additive is kept within the brush roller  31  by electrostatic suction power. 
   In addition, since it is possible to negatively charge the external toner additive as a result of improvement in capability of frictional electrification owing to the rotation of the brush roller  31  and the voltage application to the brush roller  31 , and it is possible to negatively charge the photoconductive drum  51  upstream from the cleaning blade  11 , the fifth embodiment has excellent toner and external toner additive removing capability as in the case of the fourth embodiment. Accordingly, the quantity of the external additive that is not scraped down by the cleaning blade  11  but passes the cleaning blade  11  can be reduced. 
   Furthermore, since the quantity of the external additive adhering to the charging roller  52  ( FIG. 9 ) is small by virtue of the cleaning blade  30 , the fifth embodiment can avoid the contamination of the charging roller  52  with a large quantity of the external additive that can form an insulation film that brings about poor electrification. 
   The contamination of the charging roller  52  can be detected quantitatively from the quantity of the external additive caught in the charging roller  52 . Accordingly, the assessment on the item “contamination of the charging roller  52 ” in Table 5 is based on the quantity of the external additive adhered to the charging roller  52 . It has been confirmed that the quantity of the external additive caught in the charging roller  52  of the fifth embodiment is far smaller than that of any other foregoing embodiments. 
   As described above, the fifth embodiment provided with the cleaning blade  30  has, in addition to the same advantages as those of the fourth embodiment, the different advantages that occurrence of the drum-filming phenomena can be avoided since the photoconductive drum  51  is charged to the desired potential by the brush roller  31  upstream from the cleaning blade  11  and the contamination of the charging roller  52  with a large quantity of the external additive passing through the cleaning blade  11  can be avoided. Accordingly, with the fifth embodiment, it is possible to form high quality images stably for a long period of time. 
   A Sixth Embodiment 
   FIG.  7 ( a ) is a section view schematically showing a structure of a cleaning unit  35  for use in a sixth embodiment according to the invention, and FIG.  7 ( b ) is a partially enlarged perspective view of a metal plate  36  when viewed in the direction of the arrow D shown in FIG.  7 ( a ) trending from the reverse side to the obverse side of the drawing sheet. 
   This cleaning unit  35  can replace the cleaning unit  59  in the image forming apparatus  50  described above with reference to  FIG. 9  as is the case with the cleaning unit  1  of the first embodiment without any change in other units and elements. This cleaning unit  35  differs from the cleaning unit  1  of the first embodiment in that the material of the cleaning blade  11  of the cleaning unit  35  is different from that of the cleaning unit  1 , and the metal plate  36  and some incidental members are additionally provided. 
   Accordingly, the cleaning unit  35  will be described supposing that it is disposed in the image forming apparatus  50  shown in  FIG. 9 , and detailed explanation of each of the parts of the image forming apparatus  50  will be omitted except the cleaning unit  35  and other parts common to the cleaning unit  1  of the first embodiment in the interest of simplicity. 
   The cleaning blade  11  of this embodiment is made of insulative urethane rubber having a thickness of 1 mm to 3 mm as in the case of the cleaning unit  10  shown in  FIG. 2  of the second embodiment. 
   As shown in FIG.  7 ( a ), the metal plate  36  that is resilient and is 0.05 mm to 0.5 mm in thickness is provided on the obverse side (the surface not in contact with the metal holder  2 ) of the cleaning blade  11 . The metal plate  36  is fixed to one end portion  11   a  of the cleaning blade  11  keeping a curved state so that its surface in contact with the cleaning blade  11  (referred to as a contact surface hereinafter) faces the periphery  51   a  of the photoconductive drum  51 . As shown in FIG.  7 ( b ), a semiconductive resin coat layer  37  having resistivity of 106 Ωcm-1010 Ωcm is provided on the contact surface of the metal plate  36  as a precharging device. Insulation films  38  having thickness between 20 μm-70 μm, preferably 50 μm are stuck to both corners of the end of the semiconductive resin coat layer  37  in order to leave a clearance of about 20 μm-70 μm between the semiconductive resin coat layer  37  and the periphery  51   a  of the photoconductive drum  51 . It has been confirmed through experiment that the periphery  51   a  of the photoconductive drum  51  and the semiconductive resin coat layer  37  can stably keep a non-contact state with each other when 50 μm of clearance is left between them. 
   Accordingly, the semiconductive resin coat layer  37  urged towards the periphery  51   a  of the photoconductive drum  51  by the metal plate  36  can keep a distance between 20 μm-70 μm from the periphery  51   a  of the photoconductive drum  51  by use of the insulation films  38  having a thickness of about 50 μm. This semiconductive resin coat layer  37  is applied with a d.c. (or a.c+d.c.) voltage thorugh the metal plate  36  by the dedicated power supply  5 . 
   The operation of the sixth embodiment having the above structure will be explained. 
   The toner that has adhered to the periphery  51   a  of the photoconductive drum  51  is scraped by the cleaning blade  11  as is the case with the foregoing embodiments. In this embodiment, the semiconductive resin coat layer  37  charges the periphery  51   a  of the photoconductive drum  51  in a non-contact manner. At this time, it is preferable that the above-described expression (1) is satisfied for the previously described reason. 
   According to the sixth embodiment, since a certain clearance is kept between the periphery  51   a  of the photoconductive drum  51  and the semiconductive resin coat layer  37 , and accordingly the semiconductive resin coat layer  37  is not broken by the toner that has adhered to the photoconductive drum  51 , the occurrence of non-uniform electrification and scratches on the periphery  51   a  of the photoconductive drum  51  can be avoided. 
   In this sixth embodiment, since the cleaning unit  35  performs electrification in a non-contact manner, it is desirable that the dedicated power supply  5  generates the (a.c+d.c.) voltage higher than the voltage to be generated in the case of performing electrification in a contact manner. 
   Table 6 shows performances of the image forming apparatus of the foregoing second, fourth and fifth embodiments, and the image forming apparatus of this sixth embodiment in each of the items explained below. 
   The items include “Cleaning Capability”, “Independent Charging Capability”, “Rubber Durability”, “Resistivity to OPC Cyclic Incidental Image”, “Resistivity to Damage to Photoconductive Drum”, “Resistivity to Drum-Filming”, and “Resistivity to Contamination of the Charging Roller” as in the case of the fifth embodiment. 
   
     
       
             
             
             
             
             
           
         
             
               TABLE 6 
             
             
                 
             
             
                 
               Embodiment 
               Embodiment 
               Embodiment 
               Embodiment 
             
             
               Item 
               2 
               4 
               5 
               6 
             
             
                 
             
           
           
             
               Cleaning 
               ⊚ 
               ⊚ 
               ⊚ 
               ⊚ 
             
             
               Capability 
               (as excellent 
               (as excellent 
               (as excellent 
               (excellent) 
             
             
                 
               as 
               as 
               as 
             
             
                 
               previously) 
               previously) 
               previously) 
             
             
               Independent 
               ∘ 
               Δ 
               Δ 
               ⊚ 
             
             
               Charging 
               (good though 
               (acceptable) 
               (acceptable) 
               (excellent) 
             
             
               Capability 
               not uniform) 
             
             
               Rubber 
               ∘ 
               ∘ 
               ∘ 
               ∘ 
             
             
               Durability 
               (as good as 
               (as good as 
               (as good as 
               (good) 
             
             
               (chipping off 
               previously) 
               previously) 
               previously) 
             
             
               etc.) 
             
             
               Resistivity to 
               ⊚ 
               ⊚ 
               ⊚ 
               ⊚ 
             
             
               OPC Cyclic 
               (excellent) 
               (excellent) 
               (excellent) 
               (excellent) 
             
             
               Incidental 
             
             
               Image 
             
             
               Resistivity to 
               Δ 
               Δ 
               ∘ 
               ∘ 
             
             
               Damage to 
               (acceptable) 
               (acceptable) 
               (as good as 
               (excellent) 
             
             
               Photo- 
                 
                 
               previously) 
             
             
               conductive 
             
             
               Drum 
             
             
               Resistivity to 
               ∘ 
               ⊚ 
               ⊚ 
               ∘ 
             
             
               Drum-Filming 
               (good) 
               (excellent) 
               (excellent) 
               (good) 
             
             
               Resistivity to 
               Δ 
               ∘ 
               ⊚ 
               Δ 
             
             
               Contamination 
               (acceptable) 
               (good) 
               (excellent) 
               (acceptable) 
             
             
               of the Charging 
             
             
               Roller 
             
             
                 
             
           
        
       
     
   
   As seen from Table 6, it has been confirmed that the damage to the photoconductive drum  51  in the sixth embodiment having the cleaning unit  35  is small compared to other embodiments. 
   As described above, the sixth embodiment has, in addition to the same advantages as those of the second embodiment, the different advantage that the photoconductive drum  51  is not damaged, since the non-contact state between the periphery  51   a  of the photoconductive drum  51  and the semiconductive resin coat layer  37  is secured, whereby the photoconductive drum  51  is not damaged and accordingly high quality images can be formed over a long period of time. Furthermore, since the preliminary electrification is performed downstream from the cleaning unit  35 , the precharging device  36  resists being contaminated by the toner, so that the preliminary electrification can be performed stably. 
   Although the semiconductive resin coat layer  37  is provided on the contact surface of the metal plate  36  in the sixth embodiment, it is permissible to use any appropriate semiconductive tape instead of the semiconductive resin coat layer  37 . 
   The above explained preferred embodiments are exemplary of the invention of the present application which is described solely by the claims appended below. It should be understood that modifications of the preferred embodiments may be made as would occur to one of skill in the art.