Patent Publication Number: US-8977151-B2

Title: Image forming apparatus

Description:
This application is based on Japanese Patent Application No. 2011-170879 filed on Aug. 4, 2011, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Filed of the Invention 
     The present invention relates to an image forming apparatus, particularly to an image forming apparatus that develops an electrostatic latent image with toner. 
     2. Description of Related Art 
     In a conventional image forming apparatus, a developing roller supports a developer composed of magnetic carriers and non-magnetic toner, and supplies the non-magnetic toner to a photosensitive drum with an electrostatic latent image formed thereon so as to develop the electrostatic latent image. In the image forming apparatus, a developing bias is applied to the developing roller so as to form an electric field for movement of the toner from the developing roller to the photosensitive drum. 
     With respect to the developing bias application, there are two methods, namely, a DC application method and an AC application method. In the DC application method, a DC voltage is applied as the developing bias, and in the AC application method, a superimposed voltage of a DC voltage and an AC voltage is applied as the developing bias. The AC application method permits more faithful development of the electrostatic latent image on the photosensitive drum than the DC application method. Accordingly, an even and smooth toner image (a toner image with favorable graininess) can be formed by the AC application method. 
       FIG. 5  is a graph showing the AC application method, the relationship between the frequency of the AC developing bias voltage and the graininess. The x-axis shows the frequency of the AC developing bias voltage, and the y-axis shows the graininess. The graininess represents unevenness of a toner image. High graininess means that the toner image has unevenness.  FIG. 5  shows cases wherein halftone patches with screen ruling of 210 lpi were formed. The screen ruling represents the halftone dot fineness (how many dots are in a square inch) and is measured in lines per inch (lpi). 
     As is apparent from  FIG. 5 , as the frequency of the AC developing bias voltage becomes higher, the graininess becomes lower, and the picture quality of the toner image becomes higher. Therefore, in terms of graininess, it is preferred that the frequency of the AC developing bias voltage is high. More specifically, by setting the frequency of the AC developing bias voltage to 5 kHz or higher, unevenness in a visible degree can be prevented. 
       FIG. 6  is a graph showing the relationship between the frequency of the AC developing bias voltage and the edge density. The x-axis shows the frequency of the AC developing bias voltage, and the y-axis shows the edge density. The edge density means the toner density at an edge of a toner image in the main-scanning direction. A high edge density means that the toner image has a large difference in toner density between the edge portions in the main-scanning direction and the center portion in the main-scanning direction. (The difference in toner density between the edge portions in the main-scanning direction and the center portion in the main-scanning direction will be hereinafter referred to as a density difference.) Accordingly, when the edge density is high, the picture quality of the toner image is low.  FIG. 6  shows cases wherein halftone patches with screen ruling of 210 lpi were formed. 
     As is apparent from  FIG. 6 , as the frequency of the AC developing bias voltage becomes lower, the edge density becomes lower, that is, the density difference becomes smaller, and accordingly, the picture quality of the toner image becomes higher. Therefore, in view of a density difference, it is preferred that the frequency of the AC developing bias voltage is low. More specifically, by setting the frequency of the AC developing bias voltage to 5 kHz or lower, a visible density difference can be prevented. 
     In terms of both the graininess and the density difference, as described above, the frequency of the AC developing bias voltage is preferably 5 kHz. 
     In conventional image forming apparatuses, generally, the screen ruling is set, depending on the kind of an image to be printed, such as a character image, a photo image or the like. Specifically, when a character image is to be printed, it is necessary to lay weight on reproduction of sharp edges of characters, and therefore, the screen ruling is set high. On the other hand, when a photo image is to be printed, it is necessary to lay weight on smooth gradation expression, and therefore, the screen ruling is set low. In conventional image forming apparatuses, when the screen ruling is changed, it is difficult to keep both the graininess and the density difference in favorable degrees. 
       FIG. 7  is a graph showing the relationship between the frequency of the AC developing bias voltage and the graininess. The x-axis shows the frequency of the AC developing bias voltage, and the y-axis shows the graininess.  FIG. 8  is a graph showing the relationship between the frequency of the AC developing bias voltage and the edge density. The x-axis shows the frequency of the AC developing bias voltage, and the y-axis shows the edge density.  FIGS. 7 and 8  show cases where halftone patches with screen ruling of 190 lpi were formed and cases where halftone patches with screen ruling of 210 lpi were formed. 
     As is apparent from  FIG. 7 , when a toner image is formed under the conditions that the screen ruling is 190 lpi and that the frequency of the AC developing bias voltage is 5 kHz, the graininess is high, and unevenness is observed in the toner image. In order to avoid this trouble, when the screen ruling 190 lpi, the frequency of the AC developing bias voltage shall be set to 8 kHz or higher. 
     As is apparent from  FIG. 8 , when a toner image is formed under the conditions that the screen ruling is 210 lpi and that the frequency of the AC developing bias voltage is 8 kHz, a density difference is observed in the toner image. 
     As described above, in conventional image forming apparatuses, it is difficult to control both the graininess and the density difference. 
     As a conventional image forming apparatus, for example, an image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 7-325468 is known. The image forming apparatus changes the frequency of the developing AC bias voltage in accordance with the resolution and the screen ruling so as to perform stable image formation less affected by environmental changes and the usage history. Specifically, in the image forming apparatus, when the screen ruling is high, the frequency of the AC developing bias voltage is set high, and when the screen ruling is low, the frequency of the AC developing bias voltage is low. Therefore, the image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 7-325468 cannot control both the graininess and the density difference. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, an image forming apparatus comprises: a latent image support member; a latent image forming unit for forming an electrostatic latent image on the latent image support member in accordance with image data; a developer support member for supporting a developer including toner and for supplying the toner to the latent image support member to develop the electrostatic latent image; a voltage applying device for applying a developing bias, which is a superimposed voltage of a DC voltage and an AC voltage, to the developer support member; a first selecting device for selecting a screen ruling in accordance with the image data; and a second selecting device for selecting a frequency of the AC voltage, wherein the second selecting device selects a first frequency when the first selecting device selects a first screen ruling, and selects a second frequency lower than the first frequency when the first selecting device selects a second screen ruling higher than the first screen ruling. 
     According to a second aspect of the present invention, an image forming method comprises the steps of: forming an electrostatic latent image on a latent image support member in accordance with image data; supplying toner from a developer support member to the latent image support member to develop the electrostatic latent image; applying a developing bias, which is a superimposed voltage of a DC voltage and an AC voltage, to the developer support member; selecting a screen ruling in accordance with the image data; and selecting a frequency of the AC voltage, wherein a first frequency is selected when a first screen ruling is selected, and a second frequency lower than the first frequency is selected when a second screen ruling higher than the first screen ruling is selected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a skeleton framework of an image forming apparatus; 
         FIG. 2  is a transparent view of a developing device, viewed from above; 
         FIG. 3  is a flowchart showing procedures carried out by a control unit of the image forming apparatus for forming a toner image; 
         FIG. 4  is a flowchart showing procedures carried out by a control unit of a modified image forming apparatus for forming a toner image; 
         FIG. 5  is a graph showing the relationship between the frequency of an AC developing bias voltage and the graininess; 
         FIG. 6  is a graph showing the relationship between the frequency of an AC developing bias voltage and the edge density; 
         FIG. 7  is a graph showing the relationship between the frequency of an AC developing bias voltage and the graininess; and 
         FIG. 8  is a graph showing the relationship between the frequency of an AC developing bias voltage and the edge density. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An image forming apparatus according to an embodiment of the present invention will be hereinafter described. 
     Structure of the Image Forming Apparatus 
     The structure of an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings.  FIG. 1  shows the overall structure of the image forming apparatus  1 . 
     The image forming apparatus  1  is an electrophotographic color printer, and forms and combines images of four colors, namely, yellow (Y), magenta (M), cyan (C) and black (K) by a tandem method. The image forming apparatus  1  forms a toner image on a sheet of a print medium P in accordance with image data read with a scanner. As shown in  FIG. 1 , the image forming apparatus  1  comprises a printing section  2 , a feeding section  15 , a pair of timing rollers  19 , a fixing device  20 , a pair of ejection rollers  21 , a printed-sheet tray  23 , a control unit  30  and a voltage applying device  32 . 
     The control unit  30  is to control the whole image forming apparatus  1 , and the control unit  30  is, for example, a CPU. The control unit  30  also functions as a first selecting device for selecting a screen ruling depending on the kind of image data inputted thereto. Specifically, the control unit  30  selects a high screen ruling (for example, 210 lpi) for character data and selects a low screen ruling (for example, 190 lpi) for photo data. The control unit  30  may be configured to recognize the kind of image data by analyzing the image read with the scanner or to recognize the kind of image data based on the print mode set by a user. 
     The feeding section  15  feeds sheets P one by one, and the feeding section  15  comprises a sheet tray  16  and a feed roller  17 . In the sheet tray  16 , sheets P to be subjected to printing are stacked. The feed roller  17  picks one from the stack of sheets and feeds the sheet P out of the tray  16 . The pair of timing rollers  19  feeds the sheet P in synchronized timing so that a toner image can be transferred onto the sheet P at the printing section  2 . 
     The printing section  2  forms a toner image on the sheet P fed from the feeding section  15 . The printing section  2  comprises an optical scanning device  6 , transfer devices  8  ( 8 Y,  8 M,  8 C,  8 K), an intermediate transfer belt  11 , a driving roller  12 , a driven roller  13 , a secondary transfer roller  14 , a cleaning device  18 , image forming units  22  ( 22 Y,  22 M,  22 C,  22 K) and toner bottles  24  ( 24 Y,  24 M,  24 C,  24 K). Each of the image forming units  22  ( 22 Y,  22 M,  22 C,  22 K) comprises a photosensitive drum  4  ( 4 Y,  4 M,  4 C,  4 K), a charger  5  ( 5 Y,  5 M,  5 C,  5 K), a developing device  7  ( 7 Y,  7 M,  7 C,  7 K), a cleaner  9  ( 9 Y,  9 M,  9 C,  9 K) and an eraser  10  ( 10 Y,  10 M,  10 C,  10 K). 
     Each of the photosensitive drums  4  is cylindrical, and as shown in  FIG. 1 , rotates clockwise. Each of the photosensitive drums  4  functions as an electrostatic latent image support member that supports an electrostatic latent image on its peripheral surface. Each of the chargers  5  charges the peripheral surface of the corresponding photosensitive drum  4  to a negative potential. The optical scanning device  6  is controlled by the control unit  30  to scan the peripheral surfaces of the photosensitive drums  4  with respective beams BY, BM, BC and BK. In this moment, the control unit  30  controls the performance of the optical scanning device  6  based on the selected screen ruling. The potentials of the portions scanned with the beams BY, BM, BC and BK become almost 0V. Accordingly, an electrostatic latent image is formed on each of the photosensitive drums  4 . Thus, the optical scanning device  6 , in cooperation with the control unit  30 , functions as an electrostatic latent image forming device for forming an electrostatic latent image on each of the photosensitive drums  4  in accordance with image data. 
     Each of the developing devices  7  develops the electrostatic latent image on the corresponding photosensitive drum  4  into a toner image with a two-component developer composed of non-magnetic toner and magnetic carriers. The developing devices  7  will be described below referring to the drawings.  FIG. 2  is a transparent view of one of the developing devices  7 , viewed from above. The vertical direction on the paper of  FIG. 2  is the lateral (right-left) direction of the image forming apparatus  1 , and the lateral direction on the paper of  FIG. 2  is the longitudinal (front-back) direction of the image forming apparatus  1 . The direction perpendicular to the paper surface of  FIG. 2  is the vertical (upper-lower) direction of the image forming apparatus  1 . In the following paragraphs, the vertical direction on the paper of  FIG. 2  is referred to merely as the lateral (right-left) direction, the lateral direction on the paper of  FIG. 2  is referred to merely as the longitudinal (front-back) direction, and the direction perpendicular to the paper surface of  FIG. 2  is referred to merely as the vertical (upper-lower) direction. 
     Each of the developing devices  7 , as shown by  FIG. 2 , comprises a body  72 , a stirring screw  74 , a supply screw  76 , a developing roller  78 , a sensor  79  and a motor  80 . 
     The body  72  is a case wherein a developer, the stirring screw  74 , the supply screw  76  and the developing roller  78  are set. The body  72  extends in the longitudinal direction and incorporates a stirring space Sp 1  and a supply space Sp 2  that are adjacent to each other in the lateral direction. The stirring space Sp 1  is formed in the left side of the body  72  from the supply space Sp 2 . The stirring space Sp 1  and the supply space Sp 2  lead to each other at both ends in the longitudinal direction. 
     The stirring screw  74  is provided in the stirring space Sp 1  and extends in the longitudinal direction. The stirring screw  74  is rotated, thereby feeding the developer from rear to front while stirring the developer. Thereby, the toner is charged negative, and the carriers are charged positive. The developer fed by the stirring screw  74  flows into the supply space Sp 2  through the front end of the stirring space Sp 1 . 
     The supply screw  76  is provided in the supply space Sp 2  and extends in the longitudinal direction. The supply screw  76  is rotated, thereby feeding the developer from front to rear. The developer fed by the supply screw  76  flows into the stirring space Sp  1  through the rear end of the supply space Sp 2 . Hence, the developer circulates in the stirring space Sp 1  and the supply space Sp 2 . 
     The developing roller  78  is provided in the supply space Sp 2  and extends in the longitudinal direction. The developing roller  78  is opposed to the supply screw  76 . The developing roller  78  protrudes from the body  72  and is opposed to the corresponding photosensitive drum  4 . The developing roller  78  incorporates a magnet and attracts the magnetic carriers together with the non-magnetic toner by the magnetic force. In this way, the developing roller  78  supports the developer fed by the supply screw  76 . 
     The sensor  79  is fitted to the body  72 . The sensor  79  is a magnetic sensor that detects the toner concentration of the developer, which represents the ratio of the non-magnetic toner to the magnetic carriers, by detecting the magnetic permeability of the developer. When the toner concentration detected by the sensor  79  is lower than a predetermined reference value, the control unit  30  makes the toner bottle  24  replenish the body  72  with toner. 
     The developing roller  78  supplies toner to the photosensitive drum  4  so as to develop the electrostatic latent image on the photosensitive drum  4  into a visible image. The development is described below. The voltage applying device  32  applies a developing bias, which is a superimposed voltage of a DC voltage and an AC voltage, to the developing roller  78 . With the application of the developing bias, the potential on the peripheral surface of the developing roller  78  becomes lower than the potentials on the portions of the photosensitive drum  4  that were irradiated with the beam BY, BM, BC or BK (about 0V) and higher than the potentials on the portions that were not irradiated with the beam BY, BM, BC or BK. Since the non-magnetic toner of the developer supported by the developing roller  78  is negatively charged, the toner adheres to the portions of the photosensitive drum  4  that were irradiated with the beam BY, BM, BC or BK. Hence, a toner image with a negative potential is formed on the peripheral surface of each of the photosensitive drums  4 . 
     The motor  80  rotates the stirring screw  74 , the supply screw  76  and the developing roller  78 . 
     The intermediate transfer belt  11  is stretched between the driving roller  12  and the driven roller  13 . Toner images formed on the respective photosensitive drums  4  are transferred to the intermediate transfer belt  11  and combined with each other into a composite image (primary transfer). 
     The transfer devices  8  are located in positions to face the inner peripheral surface of the intermediate transfer belt  11 . A primary transfer voltage is applied to the transfer devices  8  so that the toner images on the respective photosensitive drums  4  will be transferred to the intermediate transfer belt  11 . Thus, the transfer devices  8 Y,  8 M,  8 C and  8 K, and the image forming units  22 Y,  22 M,  22 C and  22 K function as toner image forming sections for forming toner images of the respective colors on the intermediate transfer belt  11 . The cleaners  9  collect residual toner from the peripheral surfaces of the photosensitive drums  4  after the primary transfer. The erasers  10  eliminate the charges from the peripheral surfaces of the photosensitive drums  4 . The driving roller  12  is rotated by an intermediate transfer belt driving section (not shown), thereby driving the intermediate transfer belt  11  in a direction shown by arrow a. Thereby, the intermediate transfer belt  11  conveys the composite toner image to the secondary transfer roller  14 . 
     The secondary transfer roller  14 , which is cylindrical, is in contact with the intermediate transfer belt  11 . In the following paragraphs, the area between the intermediate transfer belt  11  and the secondary transfer roller  14  will be referred to as a nip portion N. A positive bias voltage is applied to the secondary transfer roller  14 , and thereby, the secondary transfer roller  14  transfers the toner image supported by the intermediate transfer belt  11  to a sheet P passing through the nip portion N (secondary transfer). More specifically, the driving roller  12  keeps the grounding potential, and the intermediate transfer belt  11 , which is in contact with the driving roller  12 , keeps a positive potential near the grounding potential. A positive bias voltage is applied to the secondary transfer roller  14  so that the potential of the secondary transfer roller  14  will be higher than the potential of the driving roller  12  and the potential of the intermediate transfer belt  11 . Since the toner image is charged negative, the toner image is transferred from the intermediate transfer belt  11  to the sheet P by the effect of an electric field generated between the driving roller  12  and the secondary transfer roller  14 . 
     The cleaning device  18  has a blade that is in contact with the intermediate transfer belt  11 , and after the secondary transfer of the toner image to the sheet P, the cleaning device  18  removes residual toner from the intermediate transfer belt  11 . 
     The sheet P with a toner image transferred thereon is fed to the fixing device  20 . In the fixing device  20 , the sheet P is subjected to a heating treatment and a pressure treatment, whereby the toner image is fixed on the sheet P. The pair of ejection rollers  21  ejects the sheet P to the printed-sheet tray  23 . On the printed sheet tray  23 , printed sheets P are stacked. 
     In the image forming apparatus  1  of the structure above, the control unit  30  also functions as a second selecting device for selecting the frequency of the AC developing bias voltage. As the screen ruling is set higher, the number of reproducible tone rows becomes smaller, and the resolution becomes higher. Therefore, when the image data is character data, the control unit  30  selects a high screen ruling (for example, 210 lpi) to achieve a high resolution. On the other hand, when the image data are photo data, the control unit  30  selects a low screen ruling (for example, 190 lpi) to permit more tone rows. 
     However, when the screen ruling is changed, it is difficult to keep both the graininess and the density difference in favorable degrees. Table 1 was prepared based on the graph of  FIG. 7  to show the relationship among the frequency of the AC developing bias voltage, the screen ruling and the graininess. Table 2 was prepared based on the graph of  FIG. 8  to show the relationship among the frequency of the AC developing bias voltage, the screen ruling and the density difference. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 5 kHz 
                 7 kHZ 
                 9 kHz 
               
               
                   
                   
               
             
            
               
                   
                   
                 210lpi 
                 Unevenness 
                 Unevenness 
                 Unevenness 
               
               
                   
                   
                   
                 Not Observed 
                 Not Observed 
                 Not Observed 
               
               
                   
                   
                 190lpi 
                 Unevenness 
                 Unevenness 
                 Unevenness 
               
               
                   
                   
                   
                 Observed 
                 Observed 
                 Not Observed 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 5 kHz 
                 7 kHZ 
                 9 kHz 
               
               
                   
               
             
            
               
                 210lpi 
                 Density Difference 
                 Density Difference 
                 Density Difference 
               
               
                   
                 Not Observed 
                 Observed 
                 Observed 
               
               
                 190lpi 
                 Density Difference 
                 Density Difference 
                 Density Difference 
               
               
                   
                 Not Observed 
                 Not Observed 
                 Not Observed 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, as long as the screen ruling is 210 lpi, when the frequency of the AC developing bias voltage is set to any value within the range of 5 kHz to 9 kHz, unevenness is not observed, and the graininess is kept sufficiently low. On the other hand, when the screen ruling is 190 lpi and when the frequency of the AC developing bias voltage is set within the range of 5 kHz to 7 kHz, unevenness is observed, and the graininess cannot be kept low. Thus, in order to keep the graininess sufficiently low, the frequency of the AC developing bias voltage shall be set relatively high when the screen ruling is relatively low. 
     As shown in Table 2, as long as the screen ruling is 190 lpi, when the frequency of the AC developing bias voltage is set to any value within the range of 5 kHz to 9 kHz, there occurs no density difference. On the other hand, when the screen ruling is 210 lpi and when the frequency of the AC developing bias voltage is set within the range of 7 kHz to 9 kHz, a density difference is observed. Thus, in order to keep the density difference sufficiently low, the frequency of the AC developing bias voltage shall be set relatively low when the screen ruling is relatively high. 
     For the reasons described above, in the image forming apparatus  1 , the control unit  30  selects a relatively high frequency (for example, 9 kHz) as the frequency of the AC developing bias voltage when selecting a relatively low screen ruling (for example, 190 lpi), and selects a relatively low frequency (for example, 5 kHz) as the frequency of the AC developing bias voltage when selecting a relatively high screen ruling (for example, 210 lpi). In this way, in the image forming apparatus  1 , both the graininess and the density difference can be kept sufficiently low. 
     Operation of the Image Forming Apparatus 
     Next, the operation of the image forming apparatus  1  will be described.  FIG. 3  is a flowchart showing procedures carried out by the control unit  30  for forming toner images. 
     The procedures are started when the control unit  30  takes in image data. The image data may be sent from the scanner or may be read out from a storage (not shown). 
     The control unit  30  selects a screen ruling in accordance with image data sent thereto (step S 1 ). In this embodiment, when the image data are character data, the control unit  30  selects 210 lpi as the screen ruling. When the image data are photo data, the control unit selects 190 lpi as the screen ruling. 
     Next, the control unit  30  judges whether the selected screen ruling is equal to or higher than 200 lpi (step S 2 ). That is, at step S 2 , the control unit  30  judges whether the selected screen ruling is relatively high (210 lpi) or relatively low (190 lpi). When the screen ruling is equal to or higher than 200 lpi, the processing goes to step S 3 . When the screen ruling is lower than 200 lpi, the processing goes to step S 4 . 
     When the selected screen ruling is equal to or higher than 200 lpi, the control unit  30  recognizes that 210 lpi has been selected as the screen ruling, and the control unit  30  selects 5 kHz as the frequency of the AC developing bias voltage (step S 3 ). Thereafter, the processing goes to step S 5 . 
     When the selected screen ruling is lower than 200 lpi, the control unit  30  recognizes that 190 lpi has been selected as the screen ruling, and the control unit  30  selects 9 kHz as the frequency of the AC developing bias voltage (step S 4 ). Thereafter, the processing goes to step S 5 . 
     At step S 5 , the control unit  30  makes the printing section  2  form toner images (step S 5 ). The toner image formation is carried out according to a conventional procedure, and a description of the procedure is omitted. 
     When the toner image formation is finished, the control unit  30  stops the voltage applying device  32  from outputting the developing bias (step S 6 ). In this way, the toner image forming procedures are completed. 
     Advantages 
     The image forming apparatus  1  of the structure above can form high-quality images, regardless of changes in the screen ruling. As shown in Table 1 above, it is preferred, in view of graininess, that the frequency of the AC developing bias voltage is set relatively high when the screen ruling is relatively low (190 lpi). Also, as shown in Table 2 above, it is preferred, in view of density difference, that the frequency of the AC developing bias voltage is set relatively low when the screen ruling is relatively high (210 lpi). 
     Therefore, the control unit  30  selects a relatively high frequency (9 kHz) as the frequency of the AC developing bias voltage when selecting a relatively low screen ruling (190 lpi) and selects a relatively low frequency (5 kHz) as the frequency of the AC developing bias voltage when selecting a relatively high screen ruling (210 lpi). Thereby, in the image forming apparatus  1 , both the graininess and the density difference can be controlled. Consequently, the image forming apparatus  1  can form high-quality images, regardless of changes in the screen ruling. 
     In the image forming apparatus  1  according to this embodiment, the control unit  30  selects a proper one from two kinds of screen ruling in accordance with image data. However, the control unit  30  may be configured to select a proper one from three or more kinds of screen ruling in accordance with image data. In this case, as the control unit  30  selects a higher screen ruling, the control unit  30  selects a lower frequency as the frequency of the AC developing bias voltage. A modified image forming apparatus  1  will be described below. 
     Modification 
     In a modified image forming apparatus  1 , the control unit  30  selects one from three kinds of screen ruling, namely, 210 lpi, 200 lpi and 190 lpi. Table 3 shows the relationship among the frequency of the AC developing bias voltage, the screen ruling and the graininess. Table 4 shows the relationship among the frequency of the AC developing bias voltage, the screen ruling and the density difference. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 5 kHz 
                 7 kHZ 
                 9 kHz 
               
               
                   
                   
               
             
            
               
                   
                   
                 210lpi 
                 Unevenness 
                 Unevenness 
                 Unevenness 
               
               
                   
                   
                   
                 Not Observed 
                 Not Observed 
                 Not Observed 
               
               
                   
                   
                 200lpi 
                 Unevenness 
                 Unevenness 
                 Unevenness 
               
               
                   
                   
                   
                 Observed 
                 Not Observed 
                 Not Observed 
               
               
                   
                   
                 190lpi 
                 Unevenness 
                 Unevenness 
                 Unevenness 
               
               
                   
                   
                   
                 Observed 
                 Observed 
                 Not Observed 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                 5 kHz 
                 7 kHZ 
                 9 kHz 
               
               
                   
               
             
            
               
                 210lpi 
                 Density Difference 
                 Density Difference 
                 Density Difference 
               
               
                   
                 Not Observed 
                 Observed 
                 Observed 
               
               
                 200lpi 
                 Density Difference 
                 Density Difference 
                 Density Difference 
               
               
                   
                 Not Observed 
                 Not Observed 
                 Observed 
               
               
                 190lpi 
                 Density Difference 
                 Density Difference 
                 Density Difference 
               
               
                   
                 Not Observed 
                 Not Observed 
                 Not Observed 
               
               
                   
               
            
           
         
       
     
     As is shown in Table 3, when the screen ruling is 200 lpi and when the frequency of the AC developing bias voltage is set to a value within a range of 7 kHz to 9 kHz, unevenness is not observed, and the graininess is kept sufficiently low. As shown in Table 4, when the screen ruling is 200 lpi and when the frequency of the AC developing bias voltage is set to a value within a range of 5 kHz to 7 kHz, a density difference is not observed. 
     Therefore, the control unit  30  selects 9 kHz as the frequency of the AC developing bias voltage when selecting 190 lpi as the screen ruling, selects 7 kHz as the frequency of the AC developing bias voltage when selecting 200 lpi as the screen ruling, and selects 5 kHz as the frequency of the AC developing bias voltage when selecting 210 lpi as the screen ruling. 
     The operation of the modified image forming apparatus  1  will be described below.  FIG. 4  is a flowchart showing procedures carried out by the control unit  30  of the modified image forming apparatus  1  for forming toner images. 
     The procedures are started when the control unit takes in image data. The image data may be sent from the scanner or may be read out from a storage (not shown). 
     The control unit  30  selects 210 lpi, 200 lpi or 190 lpi as the screen ruling in accordance with image data inputted thereto (step S 11 ). 
     Next, the control unit  30  judges whether the selected screen ruling is equal to or higher than 205 lpi (step S 12 ). When the selected screen ruling is equal to or higher than 205 lpi, the processing goes to step S 13 . When the selected screen ruling is lower than 205 lpi, the processing goes to step S 14 . 
     When the selected screen ruling is equal to or higher than 205 lpi, the control unit  30  recognizes that 210 lpi has been selected as the screen ruling, and the control unit  30  selects 5 kHz as the frequency of the AC developing bias voltage (step S 13 ). Thereafter, the processing goes to step S 17 . 
     When the selected screen ruling is lower than 205 lpi, the control unit  30  judges whether the selected screen ruling is equal to or higher than 195 lpi (step S 14 ). When the selected screen ruling is equal to or higher than 195 lpi, the processing goes to step S 15 . When the selected screen ruling is lower than 195 lpi, the processing goes to step S 16 . 
     When the selected screen ruling is equal to or higher than 195 lpi, the control unit  30  recognizes that 200 lpi has been selected as the screen ruling, and the control unit  30  selects 7 kHz as the frequency of the AC developing bias voltage (step S 15 ). Thereafter, the processing goes to step S 17 . 
     When the selected screen ruling is lower than 195 lpi, the control unit  30  recognizes that 190 lpi has been selected as the screen ruling, and the control unit  30  selects 9 kHz as the frequency of the AC developing bias voltage (step S 16 ). Thereafter, the processing goes to step S 17 . 
     At step S 17 , the control unit  30  makes the printing section  2  form toner images. The toner image formation is carried out according to a conventional procedure, and a description of the procedure is omitted. 
     When the toner image formation is finished, the control unit  30  stops the voltage applying device  32  from outputting the developing bias (step S 18 ). In this way, the toner image forming procedures are completed. 
     Like the image forming apparatus  1 , the modified image forming apparatus  1  can form high-quality images, regardless of changes in the screen ruling. 
     Although the present invention has been described in connection with the preferred embodiment above, it is to be noted that various changes and modifications are possible for those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.