Patent Publication Number: US-6909860-B2

Title: Image forming apparatus having a gap between toner-supplying and developing rollers

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus. 
   2. Description of the Related Art 
   Conventional electrophotographic image forming apparatus such as printers, copying machines, and facsimile machines employ electrophotographic processes. A charging roller applies a high voltage to a photoconductive drum to uniformly charge the surface of the photoconductive drum. An exposing unit illuminates the charged surface of the photoconductive drum to form an electrostatic latent image on the photoconductive drum. Then, a developing unit develops the electrostatic latent image into a toner image. The toner image is then transferred onto a print medium, e.g., print paper. 
   The developing unit can be of non-magnetic one component developing method. This type of developing unit uses a non-magnetic toner. A thin layer of toner is formed on the developing roller that rotates in contact with or in non-contact with a photoconductive drum. The toner on the developing roller is attracted to an electrostatic latent image formed on the photoconductive drum, thereby forming a visible image or toner image on the photoconductive drum. 
   In such a developing unit, the toner is charged by the use of the friction between the toner particles, the friction between the toner particles and the developing roller, and the friction between toner particles and the developing blade. 
   With a developing unit in which a developing roller rotates in contact with a photoconductive drum, in order to form a thin layer of toner on the developing roller, the toner-supplying roller rotates in the same direction as the developing roller. The toner is first applied to the toner-supplying roller, which in turn deposits on the developing roller. 
     FIG. 7  illustrates a conventional image forming apparatus. 
   Referring to  FIG. 7 , reference  10  denotes a casing of a developing unit. A photoconductive drum  11  rotates in a direction shown by arrow A. A charging roller  12  rotates in a direction shown by arrow B to uniformly charge the photoconductive drum  11 . An LED head  13  illustrates the charged surface of the photoconductive drum  11 , thereby dissipating the charges on the photoconductive drum in accordance with an image to be printed. The areas in which the charges are dissipated have a potential of substantially 0 volts. 
   A toner cartridge  15  is removably attached to the case  10  of the developing unit. A developing roller  17  rotates in contact with the photoconductive drum in a direction shown by arrow C. A toner-supplying roller  16  rotates in contact with the developing roller  17  in a direction shown by arrow D. The toner falls from the toner cartridge  15  into the case  10  of the developing unit, is then supplied by the toner-supplying roller  16  to the developing roller  17 , and is finally formed by the developing blade  14  into a thin layer on the developing roller  17 . 
   The toner supplied to the developing roller  17  is deposited to the electrostatic latent image, thereby developing the electrostatic latent image into a toner image. 
   The toner image on the photoconductive drum  11  is transferred onto recording paper  19  by a transfer roller  18  that rotates in a direction shown by arrow E. After transferring, a cleaning blade scrapes residual toner off the photoconductive drum  11 , thereby collecting the residual toner into a waste toner reservoir  15   a  provided in the toner cartridge  15 . An agitator  21  agitates the toner fallen from the toner cartridge  15  into the case  10  of the developing unit, and supplies the toner to the toner-supplying roller  16 . 
     FIG. 8  is a schematic view of a pertinent portion of the conventional image forming apparatus of FIG.  7 . 
   A description will be given of the toner-supplying roller  16 , developing roller  17 , and developing blade  14 . 
   Referring to  FIG. 8 , the toner-supplying roller  16  is surface-treated so that the surface of the toner-supplying roller  16  has a plurality of cells. The toner-supplying roller  16  is in contact with the developing roller  17 , made of a rubber material, under a predetermined pressure (Japanese Patent Laid-Open No. 2001-242701). 
   The photoconductive drum  11  rotates at a circumferential speed of 150 mm/s in a direction shown by arrow. The developing roller  17  rotates at a circumferential speed of 192 mm/s in the C direction. The toner-supplying roller  16  rotates at a circumferential speed of 99 mm/s in the D direction. 
   A metal developing blade  14  has a thickness of 0.08 mm. The tip portion of the developing blade  14  is pressed against the developing roller  17 . A power supply E 1  applies a voltage of −330 volts to the toner-supplying roller  16 . A power supply E 2  applies a voltage of −200 volts to the developing roller  17 . As the toner-supplying roller  16  rotates, the toner deposited on the toner-supplying roller  16  moves into frictional contact between the toner-supplying roller  16  and the developing roller  17 , so that the toner is negatively charged and supplied to the developing roller  17 . 
   The developing blade  14  forms a layer of toner having a uniform thickness on the developing roller  14 . Then, the toner on the developing roller  17  is deposited on areas on the photoconductive drum  11  in which the electrostatic latent image is formed, thereby developing the electrostatic latent image with the toner into a toner image. 
   With the conventional developing unit of  FIG. 7 , the toner-supplying roller  16  is in pressure contact with the developing roller  17 . These two rollers  16  and  17  rotate in the same direction. Therefore, a large torque load is exerted on the toner-supplying roller  16 . This also exerts a large torque load on the developing roller  17 . Moreover, because the toner-supplying roller  16  is made of sponge, the toner-supplying roller  16  wears easily, and therefore the electrical properties of the toner-supplying roller  16  deteriorate accordingly. 
   When the toner-supplying roller  16  and developing roller  17  rotate, the frictional force developed between these two rollers causes the toner to wear and agglomerate, resulting in deteriorated electrical properties of the toner. 
   As a result, the conventional image forming apparatus fails to form an image with high contrast. 
   SUMMARY OF THE INVENTION 
   The present invention was made in view of the aforementioned drawbacks of the conventional image forming apparatus. 
   An object of the invention is to provide an image forming apparatus in which image quality is improved. 
   An image forming apparatus includes an image bearing body, a developing member, a developer-supplying member, and a controller. The developing member causes toner to adhere to an electrostatic latent image formed on the image bearing body to form the electrostatic latent image into a visible image. The developer-supplying member is spaced a predetermined distance from the developing member and supplying toner to the developing member. The controller applies a first voltage to the developing member and a second voltage to the developer-supplying member. 
   The predetermined distance is in the range of 0.05 to 1.0 mm. 
   The developing member and the developer-supplying member rotate in a same direction. 
   The absolute value of a difference between the first voltage and the second voltage is greater than 130 volts and lower than a voltage above which electrical discharge occurs across the developing member and said developer-supplying member. 
   The second voltage has an absolute value of voltage in the range of 330 to 600 volts. 
   The developing member and said developer-supplying member rotate in a same direction. 
   The developer has a degree of cohesion equal to or lower than 25%. 
   Another image forming apparatus includes an image bearing body, a developing member, and a developer-supplying member. The developing member causes toner to adhere to an electrostatic latent image formed on the image bearing body to form the electrostatic latent image into a visible image. The developer-supplying member is spaced a predetermined distance from the developing member and supplying toner to the developing member. The developer-supplying member spaced a predetermined distance from said developing member and supplying the developer to said developing member, the developer-supplying member having a surface with ridges and valleys formed therein. 
   The developer-supplying member is made of an electrically conductive material. 
   The electrically conductive material is a metal. 
   The developer-supplying member is made of a mixture of a resin and an electrically conductive material. 
   The ridges and valleys extend in a direction parallel to a longitudinal axis of said developer-supplying member. 
   The distance between the ridges and the valleys is in the range of 10 to 1000 μm and ridges are formed at a pitch in the range of 10 to 1500 μm. 
   The developer-supplying member has a surface with a straight knurl. 
   The developer-supplying member has a surface with a diamond knurl. 
   The image forming apparatus according to claim 8, wherein said developer has a degree of cohesion equal to or lower than 25%. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limiting the present invention, and wherein: 
       FIG. 1  illustrates a general configuration of an image forming apparatus according to a first embodiment of the invention; 
       FIG. 2  is a schematic view of a pertinent portion of an image forming apparatus according to the present invention; 
       FIG. 3  illustrates the relation between the developing roller and toner-supplying roller according to a second embodiment; 
       FIG. 4  is a fragmentary view, illustrating ridges and valleys formed in the surface of the toner-supplying roller according to the second embodiment; 
       FIG. 5A  illustrates the relation between a developing roller and a toner-supplying roller according to a third embodiment; 
       FIG. 5B  is a fragmentary enlarged view of the toner-supplying roller of  FIG. 5A ; 
       FIG. 5C  is a fragmentary enlarged view of the toner-supplying roller of FIG.  5 B;. 
       FIG. 6  illustrates the relation between the number of printed pages and the fluidity of toner according to the third embodiment; 
       FIG. 7  illustrates a conventional image forming apparatus; and 
       FIG. 8  is a schematic view of a pertinent portion of the conventional image forming apparatus of FIG.  7 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the invention will be described in detail with reference to the accompanying drawings. 
   First Embodiment 
   {Construction} 
     FIG. 1  illustrates a general configuration of an image forming apparatus according to a first embodiment of the invention. 
   Referring to  FIG. 1 , reference  10  denotes a casing of a developing unit. A photoconductive drum  11  rotates in a direction shown by arrow A. A charging roller  12  rotates in a direction shown by arrow B to uniformly charge the photoconductive drum  11 . An LED head  13  illuminates the charged surface of the photoconductive drum  11 , thereby dissipating the charges on the photoconductive drum in accordance with an image to be printed. The areas on the photoconductive drum in which the charges are dissipated have a potential of substantially 0 volts. 
   A toner cartridge  15  is removably attached to the casing  10  of the developing unit. A developing roller  17  rotates in contact with the photoconductive drum  11  in a direction shown by arrow C. A toner-supplying roller  46  rotates in contact with the developing roller  17  in a direction shown by arrow D. The toner falls from the toner cartridge  15  into the casing  10  of the developing unit, is then supplied from the toner-supplying roller  46  to the developing roller  17 , and is finally formed by the developing blade  14  into a thin layer on the developing roller  17 . 
   The toner supplied to the developing roller  17  is deposited to the electrostatic latent image to form a toner image. 
   The toner image on the photoconductive drum  11  is transferred onto recording paper  19  by a transfer roller  18  that rotates in a direction shown by arrow E. After transferring, a cleaning blade  20  scrapes residual toner off the photoconductive drum  11 , thereby collecting the residual toner into a waste toner reservoir  15   a  provided in the toner cartridge  15 . An agitator  21  agitates the toner fallen from the toner cartridge  15  into the casing  10  of the developing unit, and supplies the toner to the toner-supplying roller  46 . 
   A description will be given of the toner-supplying roller  46 , developing roller  17 , and developing blade  14 . 
     FIG. 2  is a schematic view of a pertinent portion of an image forming apparatus according to the present invention. 
   Referring to  FIG. 2 , the toner-supplying roller  46  has been surface-treated so that the surface of the toner-supplying roller  46  has a plurality of cells formed therein. The toner-supplying roller  46  and the developing roller  17  are spaced apart by a predetermined distance. The developing roller  17  is made of a rubber material and is in pressure contact with the photoconductive drum The photoconductive drum  11  rotates at a circumferential speed of 150 mm/s in the A direction. The developing roller  17  rotates at a circumferential speed of 192 mm/s in the C direction. The toner-supplying roller  16  rotates at a circumferential speed of 99 mm/s in the D direction. 
   A metal developing blade  14  has a thickness 0.08 mm. The tip portion of the developing blade  14  is pressed against the developing roller  17  under pressure. A power supply E 1  applies a voltage of −330 volts to the toner-supplying roller  46 . A power supply E 2  applies a voltage of −200 volts to the developing roller  17 . As the toner-supplying roller  46  rotates, the toner deposited on the toner-supplying roller  46  moves into frictional contact between the toner-supplying roller  46  and the developing roller  17 , so that the toner is negatively charged and supplied to the developing roller  17 . 
   The developing blade  14  forms a layer of toner on the developing roller  14 , the layer having a uniform thickness. Then, the toner on the developing roller  17  is deposited on areas of the photoconductive drum in which the electrostatic latent image is formed, thereby developing the electrostatic latent image with the toner into a toner image. 
   By setting the distance between the circumferential surfaces of the toner-supplying roller  46  and the developing roller  17  to a value in the range of 0.05 to 1 mm, the toner can be supplied efficiently from the toner-supplying roller  46  to the developing roller  17 . The distances larger than 1 mm do not allow the toner to be supplied efficiently, resulting in deteriorated image quality. When a shaft-to-shaft distance L 1  between the developing roller  17  and the toner-supplying roller  46  is 8.14 mm, the diameter D of the toner-supplying roller  46  is preferably in the range of 15.94 to 14.04 mm. 
   Because the toner-supplying roller  46  is not in contact with the developing roller  17 , the ability of the toner-supplying roller  46  to supply toner decreases if the toner-supplying bias is a conventional voltage of −330 volts. Thus, a sufficient amount of toner cannot be supplied to the developing roller  17 , so that blurring occurs to deteriorate image quality. 
   Therefore, a toner-supplying bias higher than the conventional bias is applied to the toner-supplying roller  46  to create a potential difference greater than 130 volts between the developing bias and the toner supplying bias. 
   Because the toner-supplying roller  46  and the developing roller  17  are not in contact with each other, the toner between these rollers  46  and  17  cannot be charged triboelectrically. For this reason, a toner having low cohesion (i.e., high fluidity) is used. The cohesion of toner can be determined as follows: A three-stage sieve is built by stacking three sieves: an upper sieve having a mesh size of 150 μm, a middle sieve having a mesh size of 75 μm, and a lower sieve having a mesh size of 40 μm. Four grams of toner is placed on the upper sieve and the three-stage sieve is subjected to vibration. By the use of Powder Tester (available from Hosokawa Micron), the cohesion E of the toner can be calculated by
 
 E ={(1/ T ){ W   1   +W   2 (3/5)+ W   3 (1/5)}×100%
 
where T is a total amount of toner initially placed on the upper sieve (4 grams in this case) , W 1  is a weight of toner particles remaining on the upper sieve, W 2  is a weight of toner remaining on the middle sieve, and W 3  is a weight of toner remaining on the lower sieve. The Powder Tester was set to calibration “5” and subjected to vibration for 30 seconds. A toner having low cohesion has high fluidity and therefore there is less chance of toner agglomerating. Thus, a large number of toner particles can escape through the meshes of the respective sieves.
 
   For the developing unit where the developing roller  17  is not in contact with the toner-supplying toner  46 , the toner cannot be charged sufficiently and blurring occurs in printed images unless the toner has cohesion of less than 25%. Because the developing roller  17  is not in contact with the toner-supplying roller  46 , the toner-supplying roller  46  rotates at a speed 1.5 times that of the conventional toner-supplying roller  16  ( FIG. 7 ) so as to improve the ability of the roller  46  to supply the toner. For this purpose, a first gear, not shown, attached to an end of the developing roller  17 , a second gear, not shown, attached to an end of the toner-supplying roller  46 , and an idle gear  25  in mesh with the first and second gears are designed to have a predetermined number of teeth, respectively. 
   With the aforementioned image forming apparatus, the toner having a potential of 0 volts falls from the toner cartridge  15  ( FIG. 1 ) into the casing of the developing unit. The toner is directed to the toner-supplying roller  46 , which in turn delivers the toner at a speed of 1.5 times that of the conventional apparatus. The difference in potential between the toner-supplying roller  46  and the developing roller  17  is in the range of 130 to 600 volts. This potential difference allows the toner to be supplied from the toner-supplying roller  46  to the developing roller  17  despite the gaps G between these two rollers  46  and  17 . The toner supplied to the developing roller  17  is negatively charged before being deposited to the electrostatic latent image formed on the photoconductive drum  11 . 
   {Operation} 
   Table 1 lists the results of experiment conducted for different gaps G in the range of 0.1 to 1.4 mm. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
                 
                 
               supply of 
             
             
                 
                 
               toner to 
             
             
                 
                 
               developing 
             
             
               gap G (mm) 
               Blurring 
               roller 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               0.01 
               not occurred 
                 
             
             
               0.05 
               not occurred 
             
             
               0.1 
               not occurred 
             
             
               0.2 
               not occurred 
             
             
               0.4 
               not occurred 
             
             
               0.6 
               not occurred 
             
             
               0.8 
               not occurred 
             
             
               1.0 
               not occurred 
             
             
               1.2. 
               occurred 
               insufficient 
             
             
               1.4 
               occurred 
               insufficient 
             
             
                 
             
          
         
       
     
   
   The experiment was conducted with the following conditions. The developing bias was −200 volts and the toner-supplying bias was −470 volts. The developing roller  17  has an electrically conductive shaft on which a layer of rubber (urethane rubber) is formed. The material of developing blade  14  is SUS304B-TA, and has a thickness of 0.08 mm and a rounded tip having a radius of 0.275 mm. The tip portion is bent and placed in contact with the developing roller  17 . 
   By using a pattern having lateral stripes, continuous printing of 20000 pages was performed at a duty cycle of 5%. Subsequently, printing was performed for a solid black (i.e., duty cycle of 100%) pattern, a 2×2 pattern, (i.e., duty cycle of 50%) , and a plurality of ruled lines of single dots, to determine whether blurring occurs in printed images. 
   For the solid black pattern and the 2×2 pattern, it is determined that blurring has occurred if white lines are observed in a printed image. For the plural 1-dot ruled lines, it is determined that blurring has occurred if the absence of dot is observed in plural 1-dot ruled lines. It is determined that blurring has not occurred if dots are absent only in one dotted line. 
   Experiment revealed that gaps G larger than 1.0 mm cause blurring and reduce the supply of toner to the developing roller  17  while gaps G equal to or smaller than 1.0 mm do not cause blurring. Thus, gap G should be equal to or smaller than 1.0 mm. Considering manufacturing variations and assembly accuracy of the toner-supplying roller  46 , it can be said that gaps G larger than 0.05 mm do not cause contact between the toner-supplying roller  46  and the developing roller  17  and gaps G smaller than 0.05 mm may cause contact. If the toner-supplying roller  46  is made with high accuracy, a gap G of 0.01 mm still prevents the toner-supplying roller  46  from contacting the developing roller  17  but the cost of the image forming apparatus will increase correspondingly. 
   The gap G is preferably in the range of 0.05≦G≦1.0 mm. 
   The toner bias will be described. Table 2 lists the results of experiment conducted for different toner biases in the range of −310 to −850 volts. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
                 
               Toner 
                 
             
             
                 
               Gap 
               supplying 
             
             
                 
               (mm) 
               bias (V) 
               Blurring 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
                 
               0.05 
               −310 
               occurred 
             
             
                 
                 
               −320 
               occurred 
             
             
                 
                 
               −330 
               not occurred 
             
             
                 
                 
               −340 
               not occurred 
             
             
                 
                 
               −350 
               not occurred 
             
             
                 
                 
               −400 
               not occurred 
             
             
                 
                 
               −450 
               not occurred 
             
             
                 
                 
               −500 
               not occurred 
             
             
                 
                 
               −600 
               not occurred 
             
             
                 
                 
               −650 
               not occurred 
             
             
                 
                 
               −700 
               not occurred 
             
             
                 
                 
               −750 
               not occurred 
             
             
                 
                 
               −800 
               not occurred 
             
             
                 
                 
               −850 
               Not occurred 
             
             
                 
                 
                 
               (discharge occurred 
             
             
                 
                 
                 
               across toner- 
             
             
                 
                 
                 
               supplying roller and 
             
             
                 
                 
                 
               developing roller) 
             
             
                 
               1.0 
               −310 
               occurred 
             
             
                 
                 
               −320 
               occurred 
             
             
                 
                 
               −330 
               not occurred 
             
             
                 
                 
               −340 
               not occurred 
             
             
                 
                 
               −350 
               not occurred 
             
             
                 
                 
               −400 
               not occurred 
             
             
                 
                 
               −450 
               not occurred 
             
             
                 
                 
               −500 
               not occurred 
             
             
                 
                 
               −600 
               not occurred 
             
             
                 
                 
               −650 
               not occurred 
             
             
                 
                 
               −700 
               not occurred 
             
             
                 
                 
               −750 
               not occurred 
             
             
                 
                 
               −800 
               not occurred 
             
             
                 
                 
               −850 
               not occurred 
             
             
                 
                 
             
          
         
       
     
   
   The experiment was conducted with the previously mentioned conditions. The gap G was selected in two values, 0.05 mm and 1.0 mm: a minimum value and a maximum value of the aforementioned range. 
   No blurring was observed for gaps of 0.05 mm and 1.0 mm, provided that the toner-supplying bias was higher than −330 volts (i.e., the absolute value of toner-supplying bias is greater than 330) and the potential difference between the toner-supplying bias and the developing bias was larger than 130 volts. When the toner-supplying bias was lower than −330 volts (i.e., the absolute value of toner-supplying bias is smaller than 330) volts and the potential difference in volts was less than an absolute value of 130, blurring was observed. 
   When the gap G was 0.05 mm, if the toner-supplying bias is higher than −850 volts (i.e., the absolute value of toner-supplying bias is greater than 850) and the potential difference was higher than 650 volts above which a discharge can occur, then a discharge occurred actually. 
   Therefore, the toner-supplying bias is preferably between −330 volts and −600 volts, and the difference in potential between the toner-supplying roller  46  and the developing roller  17  should be such that a breakdown voltage exceeds the difference by at least 130 volts (in absolute value) . Breakdown voltage is a voltage above which an electrical discharge occurs. That is, the absolute value of the potential difference is larger than a sum of 130 and the absolute value of breakdown voltage. The cohesion of toner will be described. Table 3 lists the results of experiment conducted for different toners in degree of cohesion ranging from 15 to 35%. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
                 
               Cohesion 
                 
             
             
                 
               Gap (mm) 
               (%) 
               Blurring 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
                 
               0.05 
               15 
               not occurred 
             
             
                 
                 
               20 
               not occurred 
             
             
                 
                 
               25 
               not occurred 
             
             
                 
                 
               30 
               occurred 
             
             
                 
                 
               35 
               occurred 
             
             
                 
               1.0 
               15 
               not occurred 
             
             
                 
                 
               20 
               not occurred 
             
             
                 
                 
               25 
               not occurred 
             
             
                 
                 
               30 
               occurred 
             
             
                 
                 
               35 
               occurred 
             
             
                 
                 
             
          
         
       
     
   
   The experiment was conducted with the previously mentioned conditions. Two values of gap G were selected, 0.05 mm and 1.0 mm: a minimum value and a maximum value of the aforementioned optimum range. 
   Experiment revealed that gaps of 0.05 mm and 1.0 mm did not cause blurring if the toner has a degree of cohesion less than 25%. This is because the lower the cohesion, the higher the fluidity, so that a sufficient amount of toner is supplied from the toner-supplying roller  46  to the developing roller  17 . 
   As mentioned above, the toner-supplying roller  46  and developing roller  17  are not in contact with each other. Thus, even when the rollers  46  and  17  are rotated in the same direction, the toner-supplying roller  46  is free from torque load and the toner-supplying roller  46  does not exert a significant torque load on the photoconductive drum  11 . The average torque load on the photoconductive drum  11  was in the range of 4 to 4.5 kg in the conventional art but in the range of 1.8 to 2 kg in the embodiment. Consequently, the embodiment reduces the load exerted on a drive motor, not shown, that drives the photoconductive drum  11  in rotation. Thus, fluctuation in the rotation of the photoconductive drum  11  can be prevented. 
   The toner-supplying roller  46  is not subject to wear because the toner-supplying roller  46  is not in contact engagement with the developing roller  17 . This prevents the electrical properties of the toner-supplying roller  46  from being deteriorated. Moreover, even if the toner-supplying roller  46  and developing roller  17  are rotated in the same direction, a large force is not exerted on the toner between the two rollers  46  and  17 . Thus, wear and cohesion of toner can be prevented so that the electrical properties of toner are prevented from being deteriorated. 
   As a result, an original image having high contrast can be reproduced, thereby improving image quality. 
   Second Embodiment 
   {Construction} 
     FIG. 3  illustrates the relation between a developing roller and a toner-supplying roller according to a second embodiment. 
     FIG. 4  illustrates ridges and valleys formed in the surface of the toner-supplying roller according to the second embodiment. 
   Referring to  FIG. 3 , the toner-supplying roller  56  is not in contact engagement with developing roller  17 . The toner-supplying roller  56  has the same rotational speed, diameter D, toner-supplying bias, and gap G as the toner-supplying roller  46  in the first embodiment. 
   The toner-supplying roller according to the second embodiment is made of an electrically conductive material (e.g., metal) having a surface with straight knurls, thereby ensuring as good an ability to supply toner as the toner-supplying roller  46  according to the first embodiment. In other words, there are provided projections  56  on the surface of the toner-supplying roller, the projections  56  extending in directions parallel to a longitudinal axis of the toner-supplying roller. The projection  56   a  has a height H in the range from 10 to 1000 μm, a pitch P in the range of 10 to 1500 μm m, an angle θ of about 90°, and a rounded top end having a radius in the range of 0.1 to 0.15 mm. 
   The projections  56   a  improve the ability of the toner-supplying roller  56  to supply toner to the developing roller  17 . Just as in the first embodiment, experiment revealed that the toner having a degree of cohesion higher than 25% can cause blurring. 
   {Operation} 
   By using the aforementioned toner-supplying roller  56  with straight knurl, experiment was conducted for different heights H of projection in the range of 0 to 1200 μm. Table 4 lists the results of the experiment. 
   
     
       
         
             
             
             
           
             
               TABLE 4 
             
             
                 
             
             
                 
                 
               Supply of 
             
             
                 
                 
               toner to 
             
             
                 
                 
               developing 
             
             
               Height (μm) 
               Blurring 
               roller 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               0 
               occurred 
               insufficient 
             
             
               5 
               occurred 
               insufficient 
             
             
               10 
               not occurred 
             
             
               100 
               not occurred 
             
             
               200 
               not occurred 
             
             
               400 
               not occurred 
             
             
               600 
               not occurred 
             
             
               800 
               not occurred 
             
             
               1000 
               not occurred 
             
             
               1100 
               occurred 
               toner is 
             
             
                 
                 
               clogged in 
             
             
                 
                 
               recess 
             
             
               1200 
               occurred 
               toner is 
             
             
                 
                 
               clogged in 
             
             
                 
                 
               recess 
             
             
                 
             
          
         
       
     
   
   The conditions of the experiment are the same as for the first embodiment. The pitch P is changed in increments of 250 μm from 500 to 3000 μm. 
   When the experiment was conducted with the aforementioned conditions, toner particles slipped through the valleys between adjacent ridges if the height of the ridges is less than 5 μm. This causes insufficient delivery of toner to the developing roller  17 , resulting in blurring in printed image. Heights greater than 1100 μm cause the toner to enter the valleys to be trapped therein. The toner agglomerates in the shallow valleys to make the valleys even shallower, thereby allowing the toner particles to slip through the valleys. This causes insufficient delivery of toner to the developing roller  17 , resulting in blurring. 
   For this reason, the height of the ridges is preferably in the range of 10≦ H ≦1000 μm. The results were the same for pitches P in the range of 500 to 3000 μm. 
   By using the aforementioned toner-supplying roller with straight knurl, experiment was conducted for different heights of projection in the range of 0 to 1200 μm. Table 5 lists the results of the experiment. 
   
     
       
         
             
             
             
           
             
               TABLE 5 
             
             
                 
             
             
                 
                 
               Supply of toner to 
             
             
               Pitch (μm) 
               Blurring 
               developing roller 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               5 
               occurred 
               toner is clogged in 
             
             
                 
                 
               recess 
             
             
               10 
               not occurred 
             
             
               100 
               not occurred 
             
             
               200 
               not occurred 
             
             
               400 
               not occurred 
             
             
               600 
               not occurred 
             
             
               800 
               not occurred 
             
             
               1000 
               not occurred 
             
             
               1100 
               not occurred 
             
             
               1200 
               not occurred 
             
             
               1300 
               not occurred 
             
             
               1400 
               not occurred 
             
             
               1500 
               not occurred 
             
             
               1600 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
               1800 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
               2000 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
               2500 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
               3000 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
                 
             
          
         
       
     
   
   The conditions of the experiment are the same as for the first embodiment. The experiment was conducted for different values of height H in increments of 250 μm from 0 to 1200 μm. 
   Pitches P smaller than 5 μm make the recesses too narrow so that toner particles are trapped therein. The narrow recesses cause the toner particles to agglomerate, making the recesses too shallow so that the toner particles slip through the valleys defined between the ridges. This causes insufficient delivery of toner to the developing roller  17 , resulting in blurring in printed images. 
   Heights greater than 1600 μm make the recesses too wide so that the change in surface height is rather too gentle. This gentle change in surface height causes the toner to slip through the recesses, failing to deliver a sufficient amount of toner to the developing roller  17  and thus resulting in blurring. 
   As described above, the second embodiment prevents occurrence of blurring and improves image quality. The toner-supplying roller  56  made of metal is more effective in reducing the cost of an image forming apparatus than the toner-supplying roller  46  made of sponge (FIG.  1 ). 
   In the second embodiment, the toner-supplying roller  56  is made of a metal, but may be formed of a material in which a mold resin such as ABS, PC/PS is mixed with an electrically conductive material such as carbon and titanium oxide. The toner-supplying roller yet may be made of plastics materials and acrylic materials, in which case, the image forming apparatus can be reduced in cost and weight. 
   Third Embodiment 
   {Construction} 
   In the second embodiment, when a large number of pages is printed at a high duty cycle, a sufficient amount of toner may not be delivered from the toner-supplying roller  56  to the developing roller  17 , in which case, print density may become low. The third embodiment can supply a sufficient amount of toner from the toner-supplying roller  56  to the developing roller  17  for proper print density even when the printing duty cycle is high. 
     FIG. 5A  illustrates the relation between a developing roller and a toner-supplying roller according to the third embodiment. 
     FIG. 5B  is a fragmentary enlarged view of the toner-supplying roller of  FIG. 5A ; 
     FIG. 5C  is a fragmentary enlarged view of a modification to the surface of the toner-supplying roller of  FIG. 5B ; 
   A toner-supplying roller  66  is made of an electrically conductive material, for example, metal, and is diamond-knurled. Alternatively, the diamond knurls shown in  FIG. 5B  may be replaced by substantially square knurls or rectangular knurls.  FIG. 5C  illustrates substantially rectangular projections aligned in matrix form. Just as in the second embodiment, the projection has a height H in the range from 10 to 1000 μm, a pitch P in the range of 10 to 1500 μm, an angle θ of about 90°, and a rounded end R having a radius in the range of 0.1 to 0.15 mm. The projections or lines of projections extend in a longitudinal direction thereof. The toner-supplying bias is the same as for the second embodiment. Experiment revealed that the toner having a degree of cohesion lower than 25% can cause blurring. 
   Because the surface of the toner-supplying roller  66  is diamond-knurled, the surface has a large area in contact with toner. This further improves the ability of the toner-supplying roller  66  to deliver the toner. 
   {Operation} 
   Tables 6 and 7 list the results of experiment conducted in the same way as the second embodiment. 
   
     
       
         
             
             
             
           
             
               TABLE 6 
             
             
                 
             
             
                 
                 
               Supply of toner to 
             
             
               Height (μm) 
               Blurring 
               developing roller 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               0 
               occurred 
               insufficient 
             
             
               5 
               occurred 
               insufficient 
             
             
               10 
               not occurred 
             
             
               100 
               not occurred 
             
             
               200 
               not occurred 
             
             
               400 
               not occurred 
             
             
               600 
               not occurred 
             
             
               800 
               not occurred 
             
             
               1000 
               not occurred 
             
             
               1100 
               occurred 
               toner is clogged in 
             
             
                 
                 
               recess 
             
             
               1200 
               occurred 
               toner is clogged in 
             
             
                 
                 
               recess 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
           
             
               TABLE 7 
             
             
                 
             
             
                 
                 
               Supply of toner to 
             
             
               Pitch (μm) 
               Blurring 
               developing roller 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               5 
               occurred 
               toner is clogged in 
             
             
                 
                 
               recess 
             
             
               10 
               not occurred 
             
             
               100 
               not occurred 
             
             
               200 
               not occurred 
             
             
               400 
               not occurred 
             
             
               600 
               not occurred 
             
             
               800 
               not occurred 
             
             
               1000 
               not occurred 
             
             
               1100 
               not occurred 
             
             
               1200 
               not occurred 
             
             
               1300 
               not occurred 
             
             
               1400 
               not occurred 
             
             
               1500 
               not occurred 
             
             
               1600 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
               1800 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
               2000 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
               2500 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
               3000 
               occurred 
               toner slips through 
             
             
                 
                 
               wide recess 
             
             
                 
             
          
         
       
     
   
   Experiment reveals that the toner-supplying roller  66  having a surface with diamond knurl provides the same results as the toner-supplying roller  56  ( FIG. 3 ) having a surface with straight knurl. 
   The height H of the projection of the toner-supplying roller  66  should be in the range of 10≦ H ≦1000 μm and the pitch should be in the range of 10≦ H ≦1500 μm. 
   Because the toner is agitated during printing, the larger the number of pages, the higher the cohesion of toner, so that the fluidity of toner deteriorates correspondingly. 
     FIG. 6  illustrates the relation between the number of printed pages and the fluidity of toner according to the third embodiment. 
   Referring to  FIG. 6 , fluidity less than f % causes blurring in printed image. Symbol “A ”denotes a region in which blurring occurs. Line L 1  shows the fluidity of toner when the toner-supplying roller  56  has straight knurls is used. Line L 2  shows the fluidity of toner when the toner-supplying roller  66  with diamond knurls is used. 
   As is clear from  FIG. 6 , the fluidity of toner decreases with increasing number of printed pages. The toner fluidity is higher when the toner-supplying roller  66  diamond-knurled is used than when the toner-supplying roller  56  with straight knurls is used. 
   The print density in solid black printing can be improved by 5% by using the toner-supplying roller  66  instead of the toner-supplying roller  56 . 
   As described above, the diamond-knurled surface of the toner-supplying roller  66   a  allows a sufficient amount of toner to be supplied from the toner-supplying roller  66  to the developing roller  17 , preventing blurring in solid black printing as well as providing sufficient print density. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art intended to be included within the scope of the following claims.