Patent Publication Number: US-8126345-B2

Title: Image forming device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of prior U.S. application Ser. No. 12/390,791, filed Feb. 23, 2009, which claims priority from Japanese Patent Applications No. 2008-058009 filed Mar. 7, 2008, and No. 2009-034776 filed Feb. 18, 2009. The entire content of each of these priority applications is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming device, an image forming method, and a computer-readable recording medium. 
     BACKGROUND 
     A conventional developing device incorporated in an electrophotographic image forming apparatus includes a developing roller and a supply roller. The developing roller supplies a developer to the photosensitive body. The supply roller supplies the developer to the developing roller. The developer moves in accordance with a voltage between supply roller and developing roller. Therefore, in order to move the developer from the supply roller to the developing roller, a predetermined voltage is set between the developing roller and the supply roller corresponding to type of the apparatus and a developer used in the apparatus. 
     If the image forming apparatus has been used for a long time, the image density may decrease with time. In view of this, the apparatus disclosed in Japanese unexamined patent application publication No. HEI 07-005765 changes a voltage between the developing roller and the supply roller in accordance with a remaining amount of the developer. 
     The image density decreases not only due to temporal change, but also due to a characteristic of image to be printed. If the image has a high-density, such as a black image, the developer needs to move in large amount from the developing roller to the photosensitive body. In some cases, the developer may not be supplied from the supply roller to the developing roller in a sufficient amount, inevitably lowering the image density. Even if image data is a specific pattern having low density, the uneven density may be conspicuous in the specific pattern. This uneven density may be insufficiently recognized as uneven printing when the developer is supplied insufficiently. 
     If the developer used is a non-magnetic two-component developer, strongly-charged component is more consumed than weakly-charged component, as the voltage is kept applied at a high voltage. This phenomenon is so-called “selective development.” Consequently, a ratio of the weakly-charged component to the strongly-charged component will increase, as the developer is consumed. Finally, images can no longer be developed even if the bias applied is increased. 
     SUMMARY 
     In view of the foregoing, it is an object of the invention to provide an image forming device that can suppress a decrease in image density in printing an image at a high density. 
     In order to attain the above and other objects, the invention provides an image-forming device including an image acquiring unit, an image forming unit, a determining unit, and a controller. The image acquiring unit acquires image data for one page. The image forming unit forms an image based on the image data. The image forming unit includes a developing roller, a supply roller, and a bias applying unit. The developing roller carries a developer. The supply roller supplies the developer to the developing roller. The bias applying unit applies a first voltage between the developing roller and the supply roller in a first mode, and applies a second voltage between the developing roller and the supply roller in a second mode. The determining unit determines whether the image data includes a part having density greater than or equal to a prescribed density. The controller controls the bias applying unit to apply the first voltage when the determining unit determines that the image data does not include the part having density greater than or equal to a prescribed density, and to apply the second voltage when the determining unit determines that the image data includes the part having density greater than or equal to a prescribed density. The developer is more readily supplied from the supply roller to the developing roller when the bias applying unit applies the second voltage than when bias applying unit applies the first voltage. 
     According to another aspect, the present invention provides an image forming method including: acquiring image data for one page; determining whether the image data includes a part having density greater than or equal to a prescribed density; and controlling a bias applying unit to apply a first voltage between a developing roller and a supply roller when that the image data does not include the part having density greater than or equal to a prescribed density is determined in the determining step, and to apply a second voltage between the developing roller and the supply roller when that the image data includes the part having density greater than or equal to a prescribed density is determined in the determining step, the developing roller carrying a developer, the supply roller supplying the developer to the developing roller. 
     According to another aspect, the present invention provides a computer-readable recording medium that stores an image forming program, the image forming program comprising instructions for: acquiring image data for one page; determining whether the image data includes a part having density greater than or equal to a prescribed density; and controlling a bias applying unit to apply a first voltage between a developing roller and a supply roller when that the image data does not include the part having density greater than or equal to a prescribed density is determined in the determining step, and to apply a second voltage between the developing roller and the supply roller when that the image data includes the part having density greater than or equal to a prescribed density is determined in the determining step, the developing roller carrying a developer, the supply roller supplying the developer to the developing roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a sectional side view of an example of an image forming device according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram of configuration of the image forming device; 
         FIG. 3  is a block diagram explaining how a controller in the image forming device and a PC perform a printing process; 
         FIG. 4A  is a diagram showing a recording sheet in which image data will be printed; 
         FIG. 4B  is a diagram illustrating a storage area for using during an image determining process; 
         FIG. 5  is a flowchart explaining a main process according to the first embodiment; 
         FIG. 6  is a flowchart explaining an image determining process according to the first embodiment; 
         FIG. 7  is a flowchart explaining a recording control process according to the first embodiment; 
         FIG. 8  is flowchart explaining a main process according to a second embodiment; 
         FIG. 9A  is a flowchart explaining a recording control process according to the second embodiment; 
         FIG. 9B  is a flowchart explaining an interruption process; 
         FIG. 10  is a flowchart explaining an image determining process according to the second embodiment: 
         FIG. 11  is a flowchart explaining a recording control process according to a third embodiment; 
         FIG. 12  is a diagram showing an example of the data configuration of tables; 
         FIG. 13  is a flowchart explaining a recording control process according to a fourth embodiment; and 
         FIG. 14  is a diagram explaining another method for determining whether image data includes a specific pattern. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming device according to embodiments of the invention will be described while referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals to avoid duplicating description.  FIG. 1  is a sectional side view of a color laser printer  1  as an example of image forming device according to a first embodiment. 
     As shown in  FIG. 1 , the laser printer  1  includes a main casing  2  and, within the main casing  2 , a feeding unit  4  for supplying a recording sheet  3  such as a paper, an image forming unit  5  for forming images on the recording sheet  3  supplied by the feeding unit  4 . In the front side wall of the main casing  2 , a front cover  11  capable of opening and closing over is provided. When the front cover  11  is open, a process cartridge  30  described later is detachably mounted in the main casing  2 . 
     The feeding unit  4  includes a sheet supply tray  12 , a sheet pressing plate  13 , a sheet supply roller  14 , a sheet-supplying pad  15 , paper dust removing rollers  16  and  17 , and registration rollers  18 . The sheet supply tray  12  is removably mounted on the bottom of the main casing  2 . The sheet pressing plate  13  is provided in the sheet supply tray  12 . The sheet supply roller  14  and the sheet supplying pad  15  are provided above front end of the sheet supply tray  12 . The paper dust removing rollers  16  and  17  are disposed downstream of the sheet supply roller  14  in a supplying direction of recording sheets  3 . The registration rollers  18  are provided downstream of the paper dust removing rollers  16  and  17  in the supply direction. 
     The sheet pressing plate  13  presses and conveys the recording sheets  3  stacked on the sheet supply tray  12  to the sheet supplying pad  15 . The sheet supply roller  14  and the sheet supplying pad  15  feed the topmost of recording sheets  3  forwards, one by one. The recording sheets  3  passes over the rollers  16  to  18  to the image-forming unit  5 . 
     The image forming unit  5  includes a scanning unit  20 , the process cartridge  30 , and a fixing unit  40 . The scanning unit  20  is disposed in the top section of the main casing  2  and includes a laser light source (not shown), a polygon mirror  21  that can be driven to rotate, lenses  22  and  23 , reflecting mirrors  24 ,  25 , and  26 , and the like. The laser light source emits a laser beam based on image data. As illustrated by a dotted line in  FIG. 1 , the laser beam is deflected by the polygon mirror  22 , passes through the fθ lens  23 , is reflected by the reflecting mirror  24 , passes through the lens  25 , and is reflected downward by the reflecting mirror  26  to be irradiated on the surface of the photosensitive drum  32  described later in the process cartridge  20 . 
     The process cartridge  30  is detachably mounted in the main casing  2  beneath the scanning unit  20 . Within the main casing  2 , the process cartridge  30  is also provided with the photosensitive cartridge  31 , a developing cartridge  33  as an example of a developing apparatus. The photosensitive cartridge  31  has a photosensitive body frame  31 A that is a hollow outer frame. 
     The developing cartridge  33  is removably mounted to the photosensitive-body frame  31 A. The developing cartridge  33  has a developing frame  33 A that is a hollow outer frame, and includes a developing roller  36 , a layer-thickness regulating blade  37 , a supply roller  38 , and a toner hopper  39 . The developing roller  36  and supply roller  38  are supported by the developing frame  33 A and can rotate around their axes. The toner hopper  39  holds toner that is used as developer. The toner is supplied to the developing roller  36  as the supply roller  38  rotates in the counterclockwise direction (in a direction shown by arrow in  FIG. 1 ). The toner is positively charged by friction, between the developing roller  36  and the supply roller  38 . The toner supplied to the developing roller  36  enters a gap between the developing roller  36  and the layer-thickness regulating blade  37  as the developing roller  36  rotates in the counterclockwise direction (in a direction shown by arrow in  FIG. 1 ). The toner is carried on the developing roller  36 , in the form of a thin layer having a prescribed thickness. In the first embodiment, the developing roller  36  and the supply roller  38  are applied with voltages based on a supply bias mode, as will be described later. 
     The photosensitive cartridge  31  includes a photosensitive drum  32 , a scorotron-type charger  34 , and a transfer roller  35 . The photosensitive drum  32  is supported by the photosensitive-body frame  31 A. The photosensitive drum  32  can rotate in the clockwise direction (in a direction shown by arrow in  FIG. 1 ). The photosensitive drum  32  is connected to the ground and has a positively charged photosensitive layer on the circumferential surface of the photosensitive drum  32 . 
     The charger  34  is disposed in opposition to but separates a prescribed distance from the photosensitive drum  32  so as not to contact the same. The charger  34  is configured of a positively charging Scorotron charger that generates a corona discharge from a wire formed of tungsten or the like, and can form a uniform charge of positive polarity over the surface of photosensitive drum  32 . 
     The transfer roller  35  is rotatably supported on the photosensitive-body frame  31 A and opposes and contacts the photosensitive drum  32  in a vertical direction from the bottom of the photosensitive drum  32 . The transfer roller  35  is configured of a metal roller shaft that is covered with a roller formed of a conductive rubber material. During a transfer operation, a transfer bias is applied to the transfer roller  35 . The transfer roller  35  is driven to rotate in a direction opposite the rotational direction of the photosensitive drum  32  by a motive force inputted from a motor (not shown), as shown in  FIG. 1 . 
     As the photosensitive drum  32  rotates, the charger  34  charges the surface of photosensitive drum  32  with a uniform positive polarity. Subsequently, a laser beam emitted from the scanning unit  20  is scanned at a high speed over the surface of photosensitive drum  32 , forming an electrostatic latent image corresponding to an image to be formed on the recording sheet  3 . Next, positively charged toner carried on the surface of developing roller  36  is transferred from the photosensitive drum  32  in contact with that when the developing roller  36  rotates, to areas on the surface of positively charged photosensitive drum  32  that were exposed to the laser beam and, therefore, have a lower potential. In this way, the latent image on the photosensitive drum  32  is transformed into a visible image according to a reverse developing process so that a toner image is carried on the surface of photosensitive drum  32 . 
     Thereafter, after the registration rollers  18  rotates and conveys the recording sheet  3  through a transfer position between the photosensitive drum  32  and transfer roller  35 , the toner image carried on the surface of the photosensitive drum  32  is transferred onto the recording sheet  3 . 
     The fixing unit  40  is disposed on the rear side of the process cartridge  30  (downstream in the conveying direction of recording sheet  3 ). The fixing unit  40  includes a heating roller  41 , a pressure roller  42 , and a pair of conveying rollers  43 , and a flapper  49 . The pressure roller  42  is disposed below and in opposition to the heating roller  41  and cooperates with the heating roller  41  to nip the recording sheet  3 . The conveying rollers  43  are provided on the rear side of the heating roller  41  and pressure roller  42 . 
     The fixing unit  40  fixes the toner transferred onto the recording sheet  3  at the transfer position, to the recording sheet  3  by heat, when the recording sheet  3  passes between the heating roller  41  and pressure roller  42 . After the toner is fixed to the recording sheet  3 , the conveying roller  43  and flapper  49  convey the recording sheet  3  along a discharge path  44  that leads upward toward the top surface of main casing  2 . 
     The laser printer  1  further includes discharger rollers  45  at the top of discharge path  44 , and a discharge tray  46  formed on the top surface of the main casing  2 . The discharger rollers  45  receive the recording sheet  3  conveyed along the discharge path  44  and discharge the recording sheet  3  onto the discharge tray  46 . 
     As shown in  FIG. 2 , the laser printer  1  has a controller  100 , a printing unit  120 , a display unit  130 , and an operation unit  140 . The printing unit  120  includes mainly the image forming unit  5 . The display unit  130  is, for example, a liquid crystal display panel that can display images. The operation unit  140  is, for example, an operation panel that the user may operate to input instructions. 
     The controller  100  includes a CPU  110 , a RAM  111 , a ROM  112 , an EEPROM  113 , and an input/output interface  114 . The CPU  110  executes the various programs stored in the ROM  112  and EEPROM  113 , thereby to perform various functions, which will be described below. 
     Next, how to print image will be described with reference to  FIG. 3 . As shown in  FIG. 3 , the controller  100  has an image determining unit  101  and a printing performing unit  102 . The printing performing unit  102  includes a bias applying unit  103  for applying voltages to the developing roller  36  and supply roller  38 , the voltages according with a condition of image data described later. 
     The controller  100  receives a print job from an apparatus for instructing printing, such as a personal computer (PC)  200 . Note that the print job is data that includes image data and a print instruction. The controller  100  controls the printing unit  120  to form an image on the recording sheet  3 , in accordance with the print job. 
     The image determining unit  101  determines, based on the print job acquired from the PC  200 , whether a ratio of pixels at a prescribed area in one-page print image is greater than or equal to a prescribed value. The prescribed area is an area having a preset length in the sub-scanning direction of the image data. The image determining unit  101  outputs the decision to the printing performing unit  102 . A method for making this decision will be explained later. 
     The printing performing unit  102  forms an image on the recording sheet  3  in accordance with the decision made by the image determining unit  101 . More specifically, if the image determining unit  101  determines that a ratio of pixels to at a prescribed area is smaller than the preset ratio, the printing performing unit  102  forms an image in a first supply bias mode. In the first supply bias mode, a first voltage is applied between the developing roller  36  and the supply roller  38 . If the image determining unit  101  determines that a ratio of pixels at a prescribed area is greater than or equal to the preset ratio, the printing performing unit  102  forms an image in a second supply bias mode. In the second supply bias mode, a second voltage between the developing roller  36  and the supply roller  38  is applied. In the second supply bias mode, the toner is more readily moved from the supply roller  38  to the developing roller  36  than in the first supply bias mode. 
     The bias applying unit  103  applies, to the supply roller  38 , a voltage higher than to the developing roller  36 , for moving the toner more readily from the supply roller  38  to the developing roller  36 . The toner can readily move when the potential decreases from the supply roller  38  toward the developing roller  36 , and the toner is positively charged. If the supply roller  38  is applied a voltage higher than that of the developing roller  36  even in the first supply bias mode, the supply roller  38  should be applied, in the second supply bias mode, a voltage higher than that in the first supply bias mode. 
     The toner may be negatively charged. In this case, the supply roller  38  may be applied a voltage lower than the developing roller  36  for moving readily the toner from the supply roller  38  to the developing roller  36 . 
     In the first supply bias mode, the bias applying unit  103  may apply 0[V] as first voltage between the developing roller  36  and the supply roller  38 . That is, the first and second voltage may any values, even if the second voltage is set for moving the toner from the supply roller  38  to the developing roller  36  more readily than the first voltage. 
     In the first supply bias mode, the second voltage may be set to 0 [V] if the developing roller  36  is applied a voltage higher than the supply roller  38 . 
     Next, a method for controlling the printing unit  120  by the controller  100  will be explained with reference to  FIGS. 4-7 . In  FIGS. 4A and 4B , a main-scanning direction is an orthogonal direction to a conveying direction in which the recording sheet  3  is conveyed, and is identical to a widthwise direction of recording sheet  3  and an axial direction of the photosensitive drum  32 . In other words, the main-scanning direction is identical to the direction in which the surface of the photosensitive drum  32  is scanned with a laser beam. A sub-scanning direction is an orthogonal direction to the main-scanning direction, and is the conveying direction of recording sheet  3  shown by an arrow in  FIG. 4A . The main-scanning direction and sub-scanning direction for the image to be printed correspond to the main-scanning direction and sub-scanning direction for the recording sheet  3 , respectively. The recording sheet  3  has a margin area MA and a printing area PA as one-printing page. The margin area MA surrounds the printing area PA and corresponds to an area in which no data is to be printed. 
     The image determining unit  101  divides, in the main-scanning direction, the printing area PA into four sub-printing areas AX 1 , AX 2 , AX 3  and AX 4 . Each sub-printing area has a determining area having a length C 1  measured from position Y 0 , to all pixel positions available over the length C 1 . The image determining unit  101  determines whether the image data includes a part having density greater than or equal to a prescribed density. Specifically, the image determining unit  101  determines, each sample part, whether a ratio of pixels to be printed to total pixel positions available in the subject sample part is greater than or equal to a prescribed ratio. More precisely, the image determining unit  101  determines whether a ratio of pixels to be exist at a sample part of the image data corresponding to each determining area is greater than or equal to the prescribed ratio. For example, the image determining unit  101  determines whether the sample part of image data corresponding to the determining area have density greater than or equal to a preset density, the determining area which has length C 1  measured from position Y 0  in the sub-printing area AX 1 . The sample part used to determine type of image at once corresponds to a hatched area shown in  FIG. 4A . The controller  100  selects a desirable supply bias mode for one page data in accordance with the determination result of the sample parts determined sequentially along the sub-scanning direction. 
     In the first embodiment, the length C 1  is equivalent to entire outer-circumference-length of the developing roller  36 . The number of pixels “DL” corresponding to the length “C 1 ” on the recording sheet  3  can be given from outer-circumference-length “L”, print resolution “PR”, rotation speed “Sp” and “Sd” of the photosensitive drum  32  and the developing roller  36  respectively as follows:
 
 DL (dots)= PR (dpi)× L (inch)× Sp (rpm)÷ Sd (rpm)
 
     In this embodiment, when the resolution is 600 (dpi), the number DL corresponding to the length C 1  is 898 (dots). 
     The length C 1  is set equivalent to entire circumference-length of the developing roller  36 , because the inventors found that uneven density occurs if an image includes a filled image longer than a prescribed length in the sub-scanning direction. The filled image is image filled with a certain pattern or with one color. As the cause is closely investigated, if the image data has a high density, the toner moves in a large amount to the photosensitive drum  32  from the developing roller  36 . When a part of the photosensitive drum  32 , which holds a large amount of toner, comes to face again the supply roller  38 , the toner must be supplied in a large amount from the supply roller  38  to the developing roller  36 . In other words, the developing roller  36  must hold at all times much toner that should be supplied to the photosensitive drum  32 . Hence, toner must be readily moved from the supply roller  38  to the developing roller  36  if a high-density printed area extends longer than entire outer-circumference-length of the developing roller  36 . 
     In view of the above, the length C 1  can be equivalent to the entire outer circumference of the supply roller  38 . If the toner moves in a large amount from the supply roller  38  to the developing roller  36 , the toner on the supply roller  38  will likely decrease in amount. It is possible to suppress the decrease in the printing density, by providing a condition that enables the toner to move more readily to the developing roller  36 . 
       FIG. 4B  shows a determining buffer that is provided in the RAM  111  and stores a part of image data (determining data) for use in determining the type of image data. The determining buffer includes, for each of the sub-printing areas AX 1  to AX 4 , a storage area that can store the sample part for a distance equivalent to 898 pixels in the sub-scanning direction. If the resolution is set to 600 dpi and if the recording sheet  3  is a letter-size sheet, the number of pixels arranged in the main-scanning direction (i.e., total number of pixels arranged in the horizontal direction for all sub-printing areas AX 1  to AX 4 ) is 5100 pixels. Thus, the four storage areas can store 5100×898 pixels. Note that the numbers appended, at a left part in  FIG. 4B , indicates number “n” of lines in the determining buffer. Further, the each storage area includes a dot count area Sum(AXn). Each of the dot count areas Sum(AXn) stores the number of pixels existing at the nth line at the sample part. If the image is a monochromic one, the dot count area Sum(AXn) stores number of black pixels in the sample part. 
     A main process will be explained with reference to  FIGS. 5-7 . The process shown in  FIGS. 5-7  is performed by the CPU  110  ( FIG. 3 ) by executing the program stored in the ROM  112 . In the flowcharts of  FIGS. 5-7 , “Y” is a line number at the one page image in the sub-scanning direction shown in  FIG. 4A , “n” is a position in the determining buffer, “P” is page number, and “M(P)” is a supply bias mode data for the page corresponding to number P. The number “n” corresponds to position Y in the sub-scanning direction at the print area PA, when Y=n+898×a (a=0, 1, 2 . . . ). When M( 10 ) is 1 and 2, M( 10 ) denotes that the supply bias for page 10 of the image data is set to the first supply bias mode and to the second supply bias mode, respectively. The CPU  110  initializes the position Y, the supply bias mode data M, and the page number P are set to initial values, i.e., 0, 1 and 1, respectively before starting the main process. 
     As shown in  FIG. 5 , in S 101  as a beginning step of the main process, the CPU  110  first acquires a print job from the PC  200 . Then, the CPU  110  instructs the activation of a recording control process described later (S 102 ). The recording control process is a process for controlling the transport of the recording sheet  3  and the voltage between the developing roller  36  and supply roller  38 . The recording control process is performed independently from the main process. 
     Next, the CPU  110  determines whether the print job includes the supply bias mode M(P) for each page corresponding to the page number P (S 103 ). If the print job includes the supply bias mode data M(P), as a determination result, determined by PC  200  (S 103 : Yes), the CPU  110  skips an image determining process of S 300  described later and advances to S 110 . On the other hand, if the print job does not include the supply bias mode data M(P) (S 103 : NO), the CPU  110  performs the image determining process described later (S 300 ). 
     After the image determining process is performed, the CPU  110  increases the page number P to be determined by one (S 105 ). In S 110 , the CPU  110  stores the print job containing the supply bias mode data M(P) in the RAM  111 . In other words, the main process transfers the print job containing the supply bias mode data M(P) to the recording control process described later. 
     After transferring the print job to the recording control process, the CPU  110  determines whether the next page exists in S 111 . If the next page exists (S 111 : Yes), the CPU  110  returns to S 103 . If the next page does not exist (S 111 : No), the CPU  110  determines whether the printing process has been completed (S 112 ). More precisely, the CPU  110  determines whether a flag is 0 or not (1). The flag indicates whether data is being printed or not. The flag is 0 indicating that the printing process has not been performed, and 1 indicating that the printing process is performed. If the printing process has not been completed (S 112 : No), the CPU  110  waits for the completion of printing process. If the printing process has been completed (S 112 : Yes), the CPU  110  instructs the termination of the recording control process (S 113 ) and ends the main process. 
     Next, the image determining process of S 300  will be described with reference to  FIG. 7 . In the image determining process, the CPU  110  first transfers and stores, to corresponding nth line of the determining buffer, one-line image data for the position Y (=n+898×a) (S 301 ). For example, when the CPU  110  determines the image at the first time, the CPU  110  transfers, to the first line of determining buffer, the print job representing the uppermost line (first line) in the image data for one page. In  5302 , the CPU  110  sets a variable number AX to 1. The variable number 1, 2, 3, or 4 is a value representing that the sample part corresponds to the determining area belonging to the sub-printing areas AX 1 , AX 2 , AX 3 , or AX 4 , respectively. In S 303 , the CPU  110  calculates the number Sum(AX) of pixels to be printed in the sample part corresponding to the determining area of sub-printing area AX, and stores the number Sum(AX) in the associated area provided in the determining buffer shown in  FIG. 4B . 
     Next, in S 310  the CPU  110  determines whether number Sum(AX) is greater than or equal to a prescribed threshold value Rth. If Sum(AX) is greater than or equal to the threshold value Rth (Sum(AX)≧Rth) (S 310 : Yes), the CPU  110  sets the supply bias mode M to 2 (S 311 ) and then ends the image determining process. On the other hand, if Sum(AX) is smaller than the threshold value Rth (Sum(AX)&lt;Rth) (S 310 : No), the CPU  110  increments “AX” by one (S 312 ) and then determines whether AX exceeds 4 (S 313 ). 
     If AX does not exceed 4 (S 313 : No), the CPU  110  returns to S 303 . For example, the CPU  110  calculates and stores the number Sum(AX) for the sub-printing area AX 2  in the second loop. On the other hand, if AX exceeds 4, in other words, each sub-printing areas AX 1  to AX 4  at the position Y has low density (S 313 : Yes), the CPU  110  increases number “n” by one in order to determine (n+1)th line (S 314 ). 
     In S 315 , the CPU  110  determines whether n is smaller than or equal to 898. If n is greater than 898 (S 315 : No), the CPU  110  sets n to 1 in S 316 . That is, the destination, to which the next line data should be transferred, is reset to the first line since the storage area has been used to the last line in the determining buffer. If n is smaller than or equal to 898 (S 315 : Yes), the CPU  110  does not change the value “n” and increases “Y” by one (S 317 ). 
     The CPU  110  determines whether Y is greater than or equal to a threshold value Y th  (S 318 ). If Y is smaller than Y th  (S 318 : No), the CPU  110  returns to S 301 . Note that Y th  is a threshold for Y, which is used to determine whether the entire of one page has undergone the image determining process. As shown in  FIG. 4A , Y th  is equivalent to the position of the last line in the printing area PA. The CPU  110  sets the threshold value Y th  appropriately in accordance with the size of recording sheet  3  designated in the print job. 
     If Y is greater than or equal to Y th , in other words, the entire subject page has low density (S 318 : Yes), the CPU  110  sets the supply bias mode M(P) to 1 in S 319 , and then ends the image determining process. The value of the supply bias mode data M(P) is stored for each one page data. 
     Next, the recording control process will be described with reference to  FIG. 7 . The recording control process is performed in parallel with the main process of  FIG. 5 . In S 201 , the CPU  110  sets the flag to 0. In S 202 , the CPU  110  determines whether print job has been stored into a storage device (e.g., RAM  111 ) by the main process shown in  FIG. 5 . If no print job is stored (S 202 : No), the CPU  110  determines whether a termination instruction has received from the main process (S 203 ). If a termination instruction has received (S 203 : Yes), the CPU  110  terminates the recording control process. On the other hand, if no termination instruction has been received (S 203 : No), the CPU  110  returns to S 202 . 
     If print job is stored (S 202 : Yes), the CPU  110  sets the flag “I” to 1 in S 205 . In S 206 , the CPU  110  determines whether the supply bias mode data M(P) is 2 or not (1). If M(P) is 1 (in a first supply bias mode) (S 206 : No), the CPU  110  instructs the bias applying unit  103  to turn on (apply) the supplying bias and the developing bias, setting supply voltage Vsr applied to the supply roller to 0 V and developing voltage Vdr applied to the developing roller to 400 V (S 207 ). On the other hand, if M(P) is 2 (in a second supply bias mode)(S 206 : Yes), the CPU  110  instructs the bias applying unit  103  to turn on the supplying bias and the developing bias, setting the supply voltage Vsr applied to the supply roller to 700 V to 0 V and developing voltage Vdr applied to the developing roller to 400 V (S 208 ). Then, the CPU  110  performs a printing process for one page (S 209 ), and instructs the bias applying unit  103  to turn off the supplying bias and the developing bias (S 210 ). Then, the CPU  110  returns to S 201  and repeats the process. Thus, every time when the print job is transferred from the main process, the printing process is performed at the supplying bias in accordance with the supply bias mode data M(P) determined in the control process. 
     In the first embodiment, the CPU  110  sets the supply bias mode to the second supply bias mode if the ratio of print pixels is greater than or equal to the prescribed value in at least one of the sub-printing areas AX 1  to AX 4 . Accordingly, it is possible to prevent degradation of printing density. 
     Next, a second embodiment will be described. In the first embodiment, the CPU  110  determines the type of image data by using software (various program executed by CPU  110 ). In the second embodiment, the print job processing hardware (as ASIC in this embodiment) performs both the image determining process and the printing process at the same time. Note that the ASIC (not shown) is provided in the printing unit  120  shown in  FIG. 2 . In the second embodiment, the ASIC can perform printing process, as a part of the controller  100 . The processes shown in the flowcharts of  FIGS. 9B and 10  are performed by the ASIC. The processes shown in the flowcharts of  FIGS. 8 and 9A  are performed by the CPU  110 . 
       FIG. 8  is a flowchart of a main process according to the second embodiment. The main process according to the second embodiment has the processes same as that in the first embodiment shown in  FIG. 5 , wherein is designated by the same step numbers to avoid duplicating description. Thus, only process of S 403  will be described. In the main process performed by software, the CPU  110  transfers the image data for one page contained in the print job to the recording control process shown in  FIG. 9A  (S 403 ), without determining the type of image data represented by the print job in the main process. 
       FIG. 9A  is a flowchart of the recording control process according to the second embodiment. The recording control process according to the second embodiment has the same processes of S 202 , S 203 , S 207 , S 209 , and S 210  as that in the first embodiment shown in  FIG. 7 . Thus, processes of S 510  and  513  will be specifically described. 
     As shown in  FIG. 9A , the CPU  110  determines whether the print job has been transferred from the main process (S 202 ). If the print job has not been transferred (S 202 : No), the CPU  110  determines whether the termination instruction has been generated in the main process (S 203 ). If the termination instruction has been generated in the main process (S 203 : Yes), the CPU  110  terminates the recording control process. 
     If print job exists (S 202 : Yes), the CPU  110  sets the threshold value Y th , and initializes Y to 0, flag I to 0, and n to 1, all being the initially set values, and clears the determining buffer (S 510 ). Thereafter, the CPU  110  activates the ASIC for starting the image determining process. Note that the flag I indicates that the ASIC has not performed an interruption process ( FIG. 9B ) described later when I is 0, and the ASIC has performed an interruption process when I is 1. 
     Then, the CPU  110  instructs the bias applying unit  103  to turn on the first supplying bias (Vsr=0 [V]) and the developing bias (S 207 ) and performs a printing process (S 209 ). While the CPU  110  is performing the printing process, an interruption process may be performed to set the second supplying bias (Vsr=700 [V]) as requested by the ASIC. The interruption process will be described later. 
     When the printing process is completed in the recording control process, the CPU  110  stops the image determining process (S 513 ). Then, the CPU  110  instructs the bias applying unit  103  to turn off the supplying bias (S 210 ) and returns to S 202 . 
       FIG. 10  is a flowchart of an image determining process according to the second embodiment. The image determining process according to the second embodiment has the same processes of S 301 -S 303 , S 310 , and S 312 -S 317  as that in the first embodiment shown in  FIG. 6 . Thus, processes of S 601 , S 603 , S 606 , S 608 , and S 616  will be specifically described. If the image data has density greater than or equal to a preset value, the ASIC requests to perform an interruption process shown in  FIG. 9B  so that the supplying bias may increase (S 608 ). As described above, the image determining process is a part of the printing process performed by the ASIC. 
     As shown in  FIG. 10 , the ASIC determines whether Y is greater than or equal to the prescribed value Y th  (S 601 ). If the ASIC determines Y is greater than or equal to Y th  (S 601 : Yes), the ASIC writes a blank data representing no print job in the nth line of the determining buffer (S 603 ). On the other hand, if the ASIC determines that Y is smaller than Y th  (S 601 : No), the ASIC transfers one-line image data (line data) to the nth line of the determining buffer (S 301 ). Then, the ASIC sets the variable number AX to 1 (S 302 ). The ASIC calculates and stores the number Sum(AX) in the determining buffer (S 303 ). 
     Next, in  5606 , the ASIC determines whether the flag I is 0 or not (1). If the ASIC determines that I is not 0 (1) (S 606 : No), the interruption process for the page data has been performed in the printing process instead of S 207 . That is, the supplying bias Vsr has been set to 700 [V] (S 208  in  FIG. 9B ). Hence, the ASIC advances to S 312 . If the ASIC determines that I is 0 (S 606 : Yes), the ASIC determines whether Sum(AX) is greater than or equal to the threshold value R th  (S 310 ). If Sum(AX) is greater than or equal to the threshold value R th  (Sum(AX)≧Rth), in other words, the ratio of print pixels is high (S 310 : Yes), the ASIC requests for the interruption process for switching the supply bias mode to the second supplying bias (S 608 ). When the ASIC sends a request for the interruption process, the ASIC sets the supplying bias Vsr to 700 V in S 208  as shown in  FIG. 9B . After the ASIC ends the interruption process of  FIG. 9B , the ASIC sets I to 1 (S 608 ). 
     On the other hand, if Sum(AX) is smaller than the value Rth (Sum(AX)&lt;Rth) (S 310 : No), the ASIC advances to S 312 . The processes of S 312 -S 317  performed by the ASIC is same as that performed by the CPU  110  in the image determining process according to the first embodiment (see  FIG. 6 ). 
     Then, the ASIC determines whether a stop instruction has been received from the main process in S 616 . If a stop instruction has been received (S 616 : Yes), the ASIC terminates the image determining process. On the other hand, if no stop instruction has been received (S 616 : No), the ASIC returns to S 601 . 
     As described above, the hardware (ASIC) performs the printing process and the image determining process. If the ratio of print pixels is greater than or equal to the prescribed value, the supply bias mode is set to the second supply bias mode (Vsr=700 V). Accordingly, it is possible to prevent degradation of printing density. Further, the hardware (ASIC) performs the image determining process involving a large amount of data in the second embodiment, whereby it is possible to perform the processes quickly. Further, since the print job are sequentially transferred to the hardware (ASIC) as soon as the main process acquires the print job for one page in S 403 , it is possible to decrease time for waiting for generation of next print job based on the print job. That is, the software (CPU  110 ) can efficiently perform the process. 
     Next, a third embodiment of this invention will be described with reference to  FIGS. 11 and 12 . The third embodiment is different from the second embodiment in that the supplying bias Vsr is changed to various values, between the first supplying bias (0V) and the second supplying bias (700V). The RAM  111  stores a relation (Tup table) that bias level Rp and bias value V when the supplying bias is increased, and a relation (Tdown table) that the bias level Rp and bias value V when the supplying bias is decreased shown in  FIG. 12 . Both the printing control process and the image determining process performed in the third embodiment are identical to those performed in the second embodiment shown in  FIGS. 8-9B . Accordingly, a recording control process in the third embodiment will be described. 
     As shown in  FIG. 11 , the CPU  110  determines in S 202  whether any print job has been transferred from the main process. If no print job has been transferred (S 202 : No), the CPU  110  determines whether a termination instruction has been received from the main process (S 203 ). If the CPU  110  determines that a termination instruction has not been received (S 203 : No), the CPU  110  returns to S 202 . If the CPU  110  determines that a termination instruction has been received (S 203 : Yes), the CPU  110  terminates the recording control process. 
     If print job has been transferred from the main process (S 701 : Yes), the CPU  110  initializes Y th , Y to 0, I to 1, and n to 1, clears the determining buffer, and then activates the image determining unit  101  (S 710 ). The CPU  110  further sets the flag I to 1, indicating that an interruption process has come from the ASIC in S 710 . The supplying bias is changed in the recording control process, not when an interruption process is performed by the ASIC. 
     In  5711 , the CPU  110  sets a previous bias level “Ro” to 0, Ro representing a bias level of the supplying bias set in a previous loop, for the sample part determined in the previous loop and being adjacent to a sample part to be determined in the present loop. Then, the CPU  110  instructs the bias applying unit  103  to turn the supplying bias on, setting the supplying bias Vsr to 0 [V] (first supplying voltage) (S 207 ). 
     Next, the CPU  110  calculates a present bias level “Rp” of the supplying bias (S 713 ). Specifically, the CPU  110  reads the four values of Sum(AX) for the sample part corresponding to the sub-printing areas AX 1  to AX 4  that have been sequentially rewritten by the ASIC (see S 303  shown in  FIG. 10 ), and determines maximum value max(Sum(AX)) among the Sum(AX) for the sample part. Then, the CPU  110  calculates the bias level Rp by dividing the maximum value max(Sum(AX)) by a coefficient C. Rp is an integral variable, and the coefficient C is a conversion coefficient for changing Sum(AX) to one of ten values, i.e., 0 to 9. 
     In S 720 , the CPU  110  compares the present bias level Rp with the previous bias level Ro. If Rp is greater than Ro, in other words, the ratio of print pixels tends to be high (S 270 : Rp&gt;Ro), the CPU  110  acquires a bias value “V” corresponding to Rp by referring to the Tup table shown in  FIG. 12  (S 721 ). On the other hand, if Rp is smaller than Ro, in other words, the ratio of print pixels tends to be low (S 270 : Rp&lt;Ro), the CPU  110  acquires the bias value “V” corresponding to Rp by referring to the Tdown table shown in  FIG. 12  (S 722 ). If Rp is equal to Ro (S 270 : Rp=Ro), the CPU  110  advances to S 725  without changing the bias value V. 
     In S 723 , the CPU  110  compares the bias value “V” acquired with the supplying bias Vsr, and determines whether V is equal to Vsr. If the CPU  110  determines that V is equal to Vsr (S 723 : Yes), the CPU  110  advances to S 725  without changing the supplying bias Vsr and the previous bias level Ro. If the CPU  110  determines that V is not equal to Vsr (S 723 : No), the CPU  110  substitutes V into Vsr, and Rp into Ro (S 724 ). That is, the CPU  110  changes the supplying bias “Vsr”, and updates the previous level “Ro” to the present bias level “Rp” in S 724 . 
     In S 725 , the CPU  110  determines whether the printing process for one page has completed or not. If the printing process for one page has not been completed (S 725 : No), the CPU  110  returns to S 713  and repeatedly changes the supplying bias Vsr in accordance with the ratio of print pixels. On the other hand, if the printing process for one page has been completed (S 725 : Yes), the CPU  110  sends a stop instruction to the image determining process (S 513 ). The CPU  110  then turns off the supplying bias (S 210 ). 
     In the recording control process according to the third embodiment, the CPU  110  changes, during the printing process for one page, the first supply bias mode from the first supply bias mode to the second supply bias mode if the ratio of print pixels is equal to or lower than the prescribed value. Accordingly, it is possible to prevent degradation of printing density. Further, it is also possible to prevent abruptly changing density of image, because the supplying bias is gradually increased or decreased. 
     Moreover, in the third embodiment, the CPU  110  acquires the bias value “V” corresponding to the present bias level “Rp” by referring to the two tables as shown in  FIG. 12 , the Tup table referred when the supplying bias is increased (when the ratio of print pixels increases), and the Tdown table referred when the supplying bias is decreased (when the ratio of print pixels decreases). The Tup table has greater number of present bias levels Rp corresponding to 0 [V] as the supplying bias V than other levels Rp corresponding to other supplying bias. On other hands, the Tdown table has greater number of levels Rp corresponding to 700 [V] as the supplying bias V than present bias levels Rp corresponding to other supplying bias. Therefore, once set to the first supplying bias or the second supplying bias, the supplying bias will not immediately change when the ratio of print pixels fluctuates a little. Accordingly, it is possible to prevent the supplying bias Vsr from changing unnecessarily even if the ratio of print pixels varies a little. That is, unnecessary fluctuation of printing density can be eliminated. 
     Next, a fourth embodiment will be described with reference to  FIG. 13 . The main process and recording control process according to the fourth embodiment is same as that according to the first embodiment. An image determining process according to the fourth embodiment is different from that in the first embodiment. The image determining process will be described with reference to  FIG. 13 . 
     In the fourth embodiment, the print job transferred from the PC  200  is data structure similar to device context rather than image data of dot-matrix type. The device context includes instruction designating an image to be printed on a page, for example, as “black image printed in a designated area” and “text printed at a designated position.” 
     In the image determining process in  FIG. 13 , the CPU  110  first determines whether the print job includes an instruction designating a black pattern (S 801 ). The black pattern includes continuing pixels to be printed consecutively. If the CPU  110  determines that print job does not include an instruction designating a black pattern (S 801 : No), the CPU  110  sets the supply bias mode M(P) to 1 (S 804 ). 
     If the print job includes an instruction designating a black pattern (S 801 : Yes), the CPU  110  determines whether the black pattern has a size value “Yp” measured in the sub-scanning direction is larger than or equal to the prescribed length C 1  (S 802 ). If the CPU  110  determines that Yp is shorter than C 1  (S 802 : No), the CPU  110  sets the supply bias data M(P) to 1 (S 804 ). 
     If the CPU  110  determines that Yp is larger than or equal to C 1  (S 802 : Yes), the CPU  110  determines whether density “D” set for the black pattern is higher than or equal to a prescribed threshold value D th  (S 803 ). 
     If the CPU  110  determines that D is lower than D th  (S 803 : No), the CPU  110  sets the supply bias data M(P) to 1 (S 804 ). If the CPU  110  determines that D is higher than or equal to D th  (S 803 : Yes), the CPU  110  sets the supply bias mode data M(P) to 2 (S 805 ). 
     In the image determining process according to the fourth embodiment, the CPU  110  sets the second supply bias mode (M(P)=2) when the print job includes a specific black pattern, the black pattern having a size in the sub-scanning direction larger than the prescribed size, and having a density higher than the threshold density. Accordingly, it is possible to prevent degradation of printing density by performing a simple determining method. 
     While the invention has been described in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     In the first embodiment described above, the one page is divided into the four areas for determining the ratio of pixels in the main-scanning direction. Instead, the one page may be divided into a plurality of area corresponding to each row of pixels, i.e., each line spaced part from any adjacent line in the sub-scanning direction. Because the human eye can not recognize image that has uneven density and has no relatively large size, image data for one page should better be determined for each of the areas divided in the main-scanning direction rather than line by line. 
     In the fourth embodiment, the CPU  110  determines whether the density designated for a specific pattern, such as a filling image in black, exceeds a prescribed threshold value. Instead, the CPU  110  may determine whether identical text characters are continuously arranged in the sub-scanning direction, for a distance longer than a threshold value. 
     Alternatively, dot-matrix image data may be divided into a plurality of pixel blocks, for example, pixel block including 8×8 pixel block. In the case, the CPU  110  may determine whether these pixel blocks are identical in terms of pattern of the divided block though a great amount of data must be processed. Accordingly, the CPU  110  can determine whether identical patterns are arranged in the sub-scanning direction for a prescribed distance. 
     Further, another method for determining whether identical patterns are arranged will be described with reference to  FIG. 14 . As shown in  FIG. 14 , when the dot-matrix image data is input to the controller  100 , the CPU  110  may compare the image data corresponding to the first line L 1  of the area AX 1  with that of the second line downstream of the first line L 1  in the sub-scanning direction, with that of third line, with that of forth line, and with that of fifth line immediately. For example, when the pattern of image data corresponding to first line L 1  is identical to that corresponding to the fifth line L 2 , the CPU may determine whether same line pattern is formed, at five lines, within the sample part corresponding to length C 1  in the area AX 1 , for determining whether identical patterns are continuously arranged in the sub-scanning direction. 
     In the embodiments described above, monochrome laser printer is provided as an example of device applied the present invention, but the present invention can be applied to color printer. In this case, the CPU may determine, each color of toners, whether print pixels are arranged in the sub-scanning direction at a ratio higher than or equal to a prescribed ratio. Further, a printer in which the photosensitive drum is exposed to LED beams may be provided. Moreover, since this invention is not limited to printers, the invention can be applied to other types of image forming apparatuses such as copiers and multi-function peripheries. 
     Further, the present invention can be reduced to practice, in the form of a computer program for use in computers that serve to form images. That is, a printer driver installed in a computer may cause the computer to determine the type of any image to be formed and to instruct the components to form images in the first or second supply bias mode.