Image forming device

Abstract

To prevent transfer current from flowing to an image carrier through a non-image area on a medium, thereby preventing the image carrier and the medium from becoming dirty.SOLUTION: An image forming apparatus has: a plurality of image forming units that each include an image carrier, an electrifying member, and a developing section; exposure devices that each form a latent image on the image carrier; a transfer device that transfers developer images to a medium; a developer area ratio calculation processing section that, in each line in a predetermined image forming unit, calculates a developer area ratio representing the ratio at which a developer image in a color of one of the predetermined image forming unit and an upstream side image forming unit is formed; and an electrification voltage control processing section Pr5 that corrects electrification voltage applied to the electrifying members. In the image forming apparatus, printing with high printing duty is performed, and when the developer images are transferred to the medium in the predetermined image forming unit and the upstream side image forming unit, a large amount of transfer current can be prevented from flowing into non-print areas of the image carriers. The image carriers and the medium are prevented from becoming dirty.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] In conventional image forming devices such as printers, copiers, facsimile machines, and multifunction machines, for example, electrophotographic color printers, in the image forming units of each color, the surface of a photosensitive drum as an image carrier is charged by a charging roller as a charging member, and exposed by an LED head as an exposure device to form an electrostatic latent image on the photosensitive drum, which is then developed by toner as a developer on a developing roller as a developer carrier to form a toner image as a developer image. The toner images of each color are then transferred by a transfer device to paper as a medium transported along with the running of a transfer belt as a belt member, and fixed by a fixing device as a fixing device to form an image on the paper, thereby printing.

[0003] In the printer, when the toner on the developing roller deteriorates, the electrostatic latent image cannot be developed smoothly, and the image quality decreases.

[0004] In view of this, printers have been provided that correct the voltage applied to the charging roller in accordance with the print duty of the image, thereby preventing the deterioration of image quality (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2007-34087 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the conventional printers described above, when printing is performed at a high print duty and toner images formed in a specified image forming unit and other image forming units are transferred to paper, because toner is an insulator, in the specified image forming unit, the transfer current of the transfer device does not flow easily through the image area on the paper where the toner image has been transferred, and instead flows more into the non-printing area of ​​the photosensitive drum acting as an image carrier via the non-image area on the paper where the toner image has not been transferred, which can cause both ends of the photosensitive drum to be charged to a positive polarity.

[0007] In this case, the toner adheres to both ends of the photosensitive drum, causing the photosensitive drum to become dirty, or the toner adhered to both ends of the photosensitive drum adheres to paper, causing the paper to become dirty.

[0008] The present invention aims to provide an image forming apparatus that solves the problems of conventional printers, prevents transfer current from flowing to the image carrier through non-image areas on the medium, and prevents the image carrier and medium from becoming dirty. [Means for solving the problem]

[0009] To this end, the image forming apparatus of the present invention includes a plurality of image forming units arranged from the upstream side to the downstream side in the medium transport direction, each including an image carrier, a charging member for charging the image carrier, and a developing section for developing a latent image formed on the image carrier to form a developer image of each color on the image carrier; an exposure device arranged opposite the image carrier of each image forming unit, including a plurality of light-emitting elements arranged in the scanning direction, and forming the latent image on the image carrier by causing the light-emitting elements to emit light based on dot data; a transfer device for transferring the developer image formed on each image carrier to a medium; a developer area ratio calculation processing section for calculating a developer area ratio representing the ratio at which a developer image of a color of a given image forming unit, which is a given image forming unit, and a developer image of any color of an upstream image forming unit arranged upstream of the given image forming unit are overlapped in each line in which the plurality of light-emitting elements are arranged in the scanning direction; and a charging voltage control processing section for correcting a charging voltage applied to the charging member based on the developer area ratio. When the number of light-emitting elements arranged in the scanning direction is N, and the number of light-emitting elements that are caused to emit light in the exposure device of either the image forming unit or the upstream image forming unit is nj, the developer area ratio εj is given by εj=(nj / N)×100 to be. The developer area ratio calculation processing section calculates a maximum value of the developer area ratio for each predetermined range in the sub-scanning direction in the image forming unit. When the maximum value of the developer area ratio is equal to or greater than a threshold value, the charging voltage control processing section corrects and increases the charging voltage. Effect of the Invention

[0010] According to the present invention, the image forming apparatus of the present invention includes a plurality of image forming units arranged from the upstream side to the downstream side in a medium transport direction, each including an image carrier, a charging member for charging the image carrier, and a developing unit for developing a latent image formed on the image carrier and forming a developer image of each color on the image carrier; an exposure device arranged opposite the image carrier of each image forming unit, including a plurality of light-emitting elements arranged in a scanning direction, and forming the latent image on the image carrier by causing the light-emitting elements to emit light based on dot data; a transfer device for transferring the developer image formed on each image carrier to a medium; a developer area ratio calculation processing unit for calculating a developer area ratio representing the ratio at which a developer image of a color of a given image forming unit, which is a given image forming unit, and a developer image of any color of an upstream image forming unit arranged upstream of the given image forming unit are overlapped in each line in which the plurality of light-emitting elements are arranged in the scanning direction; and a charging voltage control processing unit for correcting a charging voltage applied to the charging member based on the developer area ratio. When the number of light-emitting elements arranged in the scanning direction is N, and the number of light-emitting elements that are caused to emit light in the exposure device of either the image forming unit or the upstream image forming unit is nj, the developer area ratio εj is given by εj=(nj / N)×100 to be. The developer area ratio calculation processing section calculates a maximum value of the developer area ratio for each predetermined range in the sub-scanning direction in the image forming unit. When the maximum value of the developer area ratio is equal to or greater than a threshold value, the charging voltage control processing section corrects and increases the charging voltage.

[0011] In this case, a developer area ratio is calculated for each line, which represents the ratio at which a developer image of the color of a specified image forming unit is overlapped with a developer image of any color of an upstream image forming unit arranged upstream of the specified image forming unit, and the charging voltage applied to the charging member is corrected based on the developer area ratio.Therefore, when printing is performed with a high printing duty and a developer image is transferred to a medium in the specified image forming unit and the upstream image forming unit, it is possible to prevent a large transfer current from flowing to the non-printing area of ​​the image carrier. Therefore, the developer does not adhere to both ends of the image carrier and soil the image carrier, and the developer adhered to both ends of the image carrier does not adhere to the medium and soil the medium. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a control block diagram of the printer according to the embodiment of the present invention. [Diagram 2] 1 is a schematic diagram of a printer according to an embodiment of the present invention. [Diagram 3] 4 is a flowchart showing the operation of a control device of the printer. [Figure 4] 5A to 5C are diagrams showing an example of dot data for one line and a determination result by a dot determination processing unit according to the embodiment of the present invention. [Diagram 5] 11 is a diagram showing an example of the transition of the surface potential of a photosensitive drum when a solid white image is formed on paper with a print duty of 0%. FIG. [Figure 6] FIG. 13 is a diagram showing the state of paper when a solid cyan image is formed with a print duty of 100%. [Figure 7] 13 is a diagram showing an example of the transition of the surface potential of a photosensitive drum when a solid cyan image is formed on paper with a print duty of 100%. FIG. [Figure 8]FIG. 13 is a diagram showing an example of the change in surface potential of a photosensitive drum when a cyan image with a print duty of 30% and a magenta image with a print duty of 70% are formed on paper in an embodiment of the present invention without overlapping the cyan toner image and the magenta toner image. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings, in which a printer as an image forming apparatus will be described.

[0014] FIG. 2 is a schematic diagram of a printer according to an embodiment of the present invention.

[0015] In the figure, 10 is the printer, Cs is a housing which is the exterior of the printer 10, Bd is the main body of the printer 10, i.e., the device main body, Wt is the top wall of the housing Cs, and sk1 is a stacker as a first loading section formed in part of the top wall Wt.

[0016] A paper cassette 11 is disposed at the bottom of the device main body Bd as a paper feed section and a medium storage section, and paper P as a medium is stored in a stacked state in a stacker sk2 as a second stacking section of the paper cassette 11. A hopping roller 12 as a feed member is disposed adjacent to the front end of the paper cassette 11, and a separation member (not shown) is disposed opposite the hopping roller 12. The paper P is separated one by one by the hopping roller 12 and the separation member and fed to a paper transport path Rt1 as a medium transport path, transported on the paper transport path Rt1 by a paper feed roller 13 of a paper feed roller pair m1 as a first transport member, and sent to the image forming section Q1 after skew is corrected by a registration roller pair m2 as a second transport member disposed downstream of the paper feed roller pair m1.

[0017] The image forming section Q1 is composed of four image forming units 16Bk, 16Y, 16M, 16C of black, yellow, magenta and cyan arranged from upstream to downstream in the transport direction of the paper P, an LED head Hd as an exposure device, a transfer unit u1 as a transfer device, etc.

[0018] Each of the image forming units 16Bk, 16Y, 16M, and 16C is detachably attached to the main body of the image forming units 16Bk, 16Y, 16M, and 16C, and includes a toner cartridge Ct as a developer container that contains toner as a developer having a non-magnetic single component negative charging characteristic, a photosensitive drum 21 as an image carrier arranged to be rotatable in the direction of arrow A, a charging roller 22 as a charging member arranged to be rotatable in the direction of arrow B in contact with the photosensitive drum 21, and a developer carrier arranged to be rotatable in the direction of arrow C in contact with the photosensitive drum 21. the developing roller 23 as a developer recovery device, a supply roller 24 as a supply member arranged in contact with the developing roller 23 and rotatable in the direction of arrow D, a developing blade 25 as a regulating member arranged with a predetermined location in contact with the developing roller 23, a toner recovery section Bx1 as a first developer recovery device arranged opposite the photosensitive drum 21, a cleaning blade 26 as a first cleaning member arranged in the toner recovery section Bx1 and arranged with its tip in contact with the photosensitive drum 21, and a static elimination device 27 arranged opposite the photosensitive drum 21.

[0019] A developing portion is formed by the developing roller 23, the supply roller 24, and the developing blade 25. Further, a toner receiving portion Rm serving as a developer receiving portion for receiving the toner supplied from the toner cartridge Ct is formed above the developing portion in the main body of each of the image forming units 16Bk, 16Y, 16M, and 16C.

[0020] The toner of each color uses a polyester resin as a binder resin, and internal additives such as charge control agents, release agents, and colorants are added to the inside of the toner particles, and external additives such as silica and titanium oxide for maintaining the fluidity and chargeability of the toner are added to the outside of the toner particles. Copper phthalocyanine pigments (CI Pigment, Blue 15, etc.) are used as the colorant. The specific surface area of ​​the toner is 2.7 [m 2 / g), sphericity is 0.95, and volume average particle size is 6.1 μm.

[0021] The photoconductor drum 21 is made of an organic photoconductor and includes a conductive support and a photoconductive layer formed on the surface of the conductive support. The conductive support is formed of an aluminum metal pipe, and the photoconductive layer is formed by laminating a charge generating layer and a charge transport layer.

[0022] The charging roller 22 includes a metal shaft and a semiconductive rubber layer made of epichlorohydrin rubber or the like formed on the outer periphery of the metal shaft, and uniformly charges the surface of the photoconductor drum 21 .

[0023] The developing roller 23 includes a metal shaft and a semiconductive silicone rubber elastic body having a thickness of 6 mm and a rubber hardness of 58° (Asker C) formed on the outer periphery of the metal shaft, and electrostatically attaches toner to an electrostatic latent image formed on the surface of the photosensitive drum 21, thereby developing the electrostatic latent image.

[0024] Supply roller 24 comprises a metal shaft, a semiconductive silicone sponge foam with a thickness of 5 mm and a rubber hardness of 55° (Asker F) formed on the outer periphery of the metal shaft, and is rotated in the same direction as developing roller 23 to supply toner supplied from toner cartridge Ct to toner receiver Rm to developing roller 23 and charge the toner by friction. A large number of cells consisting of recesses are formed on the surface of the foam.

[0025] The developing blade 25 is made of a plate-like member such as stainless steel having a spring property and a thickness of 0.08 mm. The bent portion curved at a predetermined curvature is brought into contact with the developing roller 23, thereby regulating the thickness of the toner layer on the developing roller 23 and forming a toner layer as a developer layer.

[0026] In each of the image forming units 16Bk, 16Y, 16M, and 16C, the toner supplied from the toner cartridge Ct to the toner receiving section Rm is charged while being supplied to the developing roller 23 by the supply roller 24, and the layer thickness is regulated by the developing blade 25, and then the toner is attached to the photosensitive drum 21 to form a toner image. The toner not used in forming the toner image on the developing roller 23 is collected in the toner receiving section Rm as the developing roller 23 rotates, scraped off by the supply roller 24, and mixed with new toner supplied from the toner cartridge Ct.

[0027] The cleaning blade 26 is made of a rubber-like elastic material such as polyurethane rubber, and is brought into counter-contact with the photosensitive drum 21 so that its tip abuts against the photosensitive drum 21 in the direction of rotation to scrape off and remove toner remaining on the photosensitive drum 21 after the toner image is transferred to the paper P, as well as external additives that have peeled off from the toner and adhered to the photosensitive drum 21.

[0028] The static elimination device 27 is provided with an electrostatic elimination plate (not shown) arranged in the axial direction of the photosensitive drum 21 facing the photosensitive drum 21, and a plurality of LEDs (not shown) as electrostatic elimination light-emitting elements arranged at equal intervals along the electrostatic elimination plate, and when an electrostatic elimination voltage is applied, each of the LEDs is caused to emit light, and the electrostatic elimination light is irradiated onto the photosensitive drum 21 to eliminate static electricity on the surface of the photosensitive drum 21 after the toner image has been transferred to the paper P, thereby eliminating variations in the surface potential.

[0029] The LED head Hd consists of an LED array consisting of multiple LEDi (i = 1, 2, ..., N) arranged in the scanning direction as light-emitting elements for exposure, a drive element that causes each LEDi to emit light, and a lens array. Each LEDi irradiates light on a dot basis based on dot data described below onto the surface of the photosensitive drum 21 charged by the charging roller 22, and reduces the potential of the irradiated area by optical attenuation, thereby forming an electrostatic latent image.

[0030] The transfer unit u1 also includes a drive roller r1 as a first roller arranged to be rotatable in the direction of arrow E, a driven roller r2 as a second roller arranged to be rotatable in the direction of arrow F, a transfer belt Bt as a belt member stretched between the drive roller r1 and the driven roller r2 to be able to run in the direction of arrow G, a transfer roller 28 as a transfer member arranged to be rotatable in the direction of arrow H facing each of the photosensitive drums 21 via the transfer belt Bt, a toner recovery section Bx2 as a second developer recovery device arranged to face the lower running portion of the transfer belt Bt, and a cleaning blade 29 as a second cleaning member arranged in the toner recovery section Bx2 with its tip in contact with the transfer belt Bt.

[0031] The transfer belt Bt is pressed against the photosensitive drums 21 by the transfer rollers 28, forming nip portions between the transfer belt Bt and the photosensitive drums 21. Each nip portion constitutes a transfer portion.

[0032] A fixing device 31 is disposed downstream of the image forming unit Q1 on the paper transport path Rt1. The fixing device 31 includes a heat roller 32 as a first fixing member disposed rotatably in the direction of arrow I, and a backup roller 33 as a second fixing member and an opposing member disposed rotatably in the direction of arrow J opposite the heat roller 32.

[0033] Further, downstream of the fixing unit 31 in the paper transport path Rt1, a pair of discharge rollers m3 serving as a third transport member is disposed.

[0034] Next, the operation of the printer 10 will be described.

[0035] When the print processing unit Pr2 (Figure 1), which will be described later, drives the drive motor M1 by the drive control unit 45, which will be described later, the photosensitive drum 21, charging roller 22, developing roller 23 and supply roller 24 are rotated in each image forming unit 16Bk, 16Y, 16M, 16C, and each transfer roller 28 is rotated in the transfer unit u1, and the charging voltage Vch, developing voltage, supply voltage, regulating voltage, discharge voltage and transfer voltage are applied to the charging roller 22, developing roller 23, supply roller 24, developing blade 25, discharge device 27 and transfer roller 28 by the voltage control unit 43, which will be described later.

[0036] Next, the print processing unit Pr2 sends dot data to each LED head Hd by the exposure control unit 44 described later, and drives the LED head Hd to expose the surface of each photoconductor drum 21 to light, forming an electrostatic latent image. Then, due to the potential difference between each developing roller 23 and the electrostatic latent image, the toner on the developing roller 23 is attached to the electrostatic latent image, the electrostatic latent image is developed, and a toner image of each color is formed on the surface of the photoconductor drum 21.

[0037] At this time, the toner is negatively charged due to the potential difference and friction between the developing roller 23 and the supply roller 24, and the toner is supplied from the supply roller 24 to the developing roller 23. In addition, the toner is further negatively charged due to the potential difference and friction between the developing blade 25 and the developing roller 23, and excess toner on the developing roller 23 is scraped off, forming a uniform toner layer on the developing roller 23.

[0038] The toner that has not been supplied from the supply roller 24 to the developing roller 23 and the toner that has been scraped off by the developing blade 25 are returned to the toner storage section Rm.

[0039] When the print processing unit Pr2 drives the conveying motor M2 via the drive control unit 45 and rotates the hopping roller 12, the paper P in the paper cassette 11 is fed to the paper conveying path Rt1 by the hopping roller 12, conveyed by the conveying roller pair m1, and has its skew corrected by the registration roller pair m2 before being supplied to the image forming unit Q1.

[0040] Then, in the image forming section Q1, when the paper P is transported with the movement of the transfer belt Bt, the black, yellow, magenta and cyan toner images are transferred in order and superimposed on top of each other onto the paper P in accordance with the potential difference between each transfer roller 28 and the toner image on the photosensitive drum 21, forming a color toner image on the paper P. At this time, a transfer current flows from each transfer roller 28 to the photosensitive drum 21.

[0041] Any toner remaining on each photoconductor drum 21 after transfer is scraped off and removed by a cleaning blade 26. In addition, a charge-removing voltage is applied to a charge-removing device 27, whereby charge-removing light is irradiated onto the photoconductor drum 21, thereby eliminating variations in the surface potential on the photoconductor drum 21.

[0042] Thereafter, the paper P is sent to the fixing device 31, where the color toner image on the paper P is melted by the heat roller 32 and fixed to the paper P by pressure applied by the backup roller 33, forming a color image on the paper P. The paper P on which the color image has been formed is discharged outside the device body Bd by the discharge roller pair m3, and is stacked on a stacker sk2.

[0043] The toner remaining on the transfer belt Bt is scraped off by a cleaning blade 29 and collected in a toner collection section Bx2.

[0044] Incidentally, when a specific image forming unit among the image forming units 16Bk, 16Y, 16M, and 16C, for example, the image forming unit 16C is defined as the image forming unit, and the image forming units 16Bk, 16Y, and 16M upstream of the image forming unit in the conveying direction of the paper P (the running direction of the transfer belt Bt) are defined as the upstream image forming units, in addition to the cyan toner image, the black, yellow, and magenta toner images formed in the upstream image forming units are transferred to the paper P at the nip portion between the photoconductor drum 21 and the transfer belt Bt in the image forming unit. Since the toner of each toner image is an insulator, when printing with a high print duty is performed, in the image forming unit, the transfer current of the transfer roller 28 does not easily flow through the image area on the paper P where the toner images of each color are transferred, and flows to the photoconductor drum 21 through the non-image area on the paper P where the toner image is not transferred. As a result, both ends of the photosensitive drum 21 are charged to a positive polarity, and the toner adheres to both ends of the photosensitive drum 21, soiling the photosensitive drum 21, or the toner adhered to both ends of the photosensitive drum 21 adheres to the paper P, soiling the paper P.

[0045] Therefore, in this embodiment, it is determined whether the toner image will make it difficult for the transfer current to flow in the nip portion between the transfer belt Bt and the photosensitive drum 21, and if it is determined that the transfer current will not flow easily, both ends of the photosensitive drum 21 are prevented from being charged to a positive polarity.

[0046] Next, the control device of the printer 10 will be described.

[0047] FIG. 1 is a control block diagram of a printer according to an embodiment of the present invention.

[0048] In the figure, reference numeral 10 denotes a printer, and a PC denotes a host computer as a higher-level device connected to the printer 10 via a wired or wireless LAN or the like.

[0049] Further, 50 denotes a control section, IF denotes an interface section as a communication section that connects the printer 10 to a host computer PC and receives print commands, print data, etc. from the host computer PC and transmits status information of the printer 10 to the host computer PC, 53 denotes a ROM as a first storage section, and 54 denotes a RAM as a second storage section. In this embodiment, a rewritable flash ROM or the like is used as the ROM 53.

[0050] Reference numeral 55 denotes a print counting section which counts the number of prints, which is the number of sheets of paper P printed in the printer 10, 56 denotes an operation panel as an operation / display section, s1 denotes a temperature sensor as a first environmental variable detection section, and s2 denotes a humidity sensor as a second environmental variable detection section. The temperature sensor s1 is disposed at a position which is not affected by the temperature of the fixing unit 31.

[0051] Further, 43 denotes a voltage control unit which applies a charging voltage Vch, a developing voltage, a supply voltage, a regulating voltage, a static elimination voltage and a transfer voltage to the charging roller 22, the developing roller 23, the supply roller 24, the developing blade 25, the static elimination device 27 and the transfer roller 28, respectively; 44 denotes an exposure control unit which controls the light emission of each LEDi of the LED head Hd; M1 denotes a drive motor (ID motor) as an image formation drive unit which rotates the photosensitive drum 21 (FIG. 2) etc.; M2 denotes a conveying motor as a conveying drive unit which rotates the hopping roller 12, the paper feed roller pair m1, the registration roller pair m2, the discharge roller pair m3 etc.; M3 denotes a transfer motor as a transfer drive unit which rotates the driven roller r2 by rotating the drive roller r1 and runs the transfer belt Bt; and 45 denotes a drive control unit which sends drive signals to the drive motor M1, the conveying motor M2 and the transfer motor M3 to drive the drive motor M1, the conveying motor M2 and the transfer motor M3. When the drive control unit 45 drives the drive motor M1 to rotate the photosensitive drum 21, the charging roller 22 is rotated in response to the rotation of the photosensitive drum 21, and the developing roller 23, the supply roller 24, the transfer roller 28, etc. are rotated via a gear train (not shown) connected to the photosensitive drum 21.

[0052] The control unit 50 includes a CPU as an arithmetic unit (not shown), an input / output port, a timer, etc., and controls the entire printer 10 while also performing various processes based on programs recorded in a ROM 53 .

[0053] In addition to the programs, various set values, threshold values, and the like are recorded in the ROM 53 .

[0054] The RAM 54 temporarily stores the print data, dot data (image data) generated based on the print data for causing each LEDi of the LED head Hd to emit light, various control data, etc. The RAM 54 functions as a work area when the CPU performs calculations.

[0055] The operation panel 56 includes an operation unit 58 consisting of switches, keys, etc. (not shown) through which the operator inputs instructions to the printer 10, and a display unit 59 consisting of an LED screen, etc. (not shown) for displaying the status of the printer 10. When the operation panel 56 is formed of a touch panel, the display unit 59 also functions as the operation unit.

[0056] The temperature sensor s1 detects the temperature around the printer 10 as a first environmental variable, and the humidity sensor s2 detects the humidity around the printer 10 as a second environmental variable. The temperature detected by the temperature sensor s1 and the humidity detected by the humidity sensor s2 are sent to a control unit 50, and a voltage control unit 43 receives instructions from the control unit 50 and controls the charging voltage Vch, the developing voltage, the supply voltage, the regulating voltage, the neutralization voltage, and the transfer voltage applied to the charging roller 22, the developing roller 23, the supply roller 24, the developing blade 25, the neutralization device 27, and the transfer roller 28 in accordance with the temperature and humidity sent to the control unit 50.

[0057] The control unit 50 includes an image conversion processing unit Pr1, a print processing unit Pr2, a dot determination processing unit Pr3, a toner area ratio calculation processing unit Pr4 as a developer area ratio calculation processing unit, a charging voltage control processing unit Pr5, and the like.

[0058] The image conversion processing unit Pr1 performs image conversion processing, converting the print data received by the printer 10 from the host computer PC into black, yellow, magenta, and cyan dot data, and records it in the RAM 54.

[0059] The print processing unit Pr2 performs print processing, reads out dot data for each color from RAM 54, drives each LED head Hd based on each dot data, forms an image on paper P, and performs printing. The dot data consists of binary data (0, 1), and the print processing unit Pr2 selectively causes LEDi on one line in the scanning direction of the LED head Hd to emit light based on the dot data. That is, when the dot data is 0, the LEDi is not emitted, and when the dot data is 1, the LEDi is emitted.

[0060] The dot determination processing unit Pr3 performs dot determination processing, reads out dot data of each color from the RAM 54, and determines, for each line of LEDi, whether the LEDi of the LED head Hd can be made to emit light in either the image forming unit or the upstream image forming unit based on the dot data. If the LEDi can be made to emit light in either the image forming unit or the upstream image forming unit, the dot determination processing unit Pr3 sets the determination result to 1, and if the LEDi cannot be made to emit light in any image forming unit, the dot determination processing unit Pr3 sets the determination result to 0 and records the determination result of each LEDi in the ROM 53.

[0061] The toner area ratio calculation processing unit Pr4 performs a toner area ratio calculation process as a developer area ratio calculation process, reads out the judgment results by the dot judgment processing unit Pr3 for each line from ROM 53, and calculates the toner area ratio εj (j=bk, y, m, c) as the developer area ratio of each color for each line based on the judgment results, and calculates the maximum value εmax of the toner area ratio εj of each line for each specified range in the sub-scanning direction, in this embodiment, the width of the nip portion formed between the photosensitive drum 21 of the image forming unit and the transfer belt Bt, i.e., for each range of the nip width.

[0062] The toner area ratio εj is expressed as a percentage of the number nj of LEDs of each color for which the dot determination processing unit Pr3 determines that the result is 1, where nj (j=bk, y, m, c) is the number of LEDs of each color for each line. εj=(nj / N)×100 become.

[0063] The toner area ratio εj is the proportion of a toner image formed in either the image forming unit in question or an upstream image forming unit and transferred to paper P, and represents the degree to which the toner of the toner image makes it difficult for the transfer current to flow.

[0064] The charging voltage control processing unit Pr5 performs charging voltage control processing, determines whether the maximum value εmax of the toner area ratio εj for each nip width range is equal to or greater than the threshold value εth, and if the maximum value εmax of the toner area ratio εj is equal to or greater than the threshold value εth, corrects and increases the charging voltage Vch, and if it is less than the threshold value εth, does not correct the charging voltage Vch.

[0065] Next, the operation of the control device of the printer 10 will be described.

[0066] FIG. 3 is a flowchart showing the operation of the control device of the printer in the embodiment of the present invention, and FIG. 4 is a diagram showing an example of one line of dot data and a judgment result by the dot judgment processing unit in the embodiment of the present invention.

[0067] When the printer 10 receives a print command, print data, etc. from the host computer PC (step S1), the image conversion processing unit Pr1 converts the print data into dot data of each color (step S2) and records it in the RAM 54. The dot data of each color takes a value of 0 or 1 for each line and for each LEDi of each color, as shown in Fig. 4. For example, in the LED head Hd of the black image forming unit 16Bk, the dot data of LEDs 1 to 10 takes values ​​of 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, and in the LED head Hd of the yellow image forming unit 16Y, the dot data of LEDs 1 to 10 takes values ​​of 0, 1, 0, 1, 0, 1, 0, 0, 0, 1.

[0068] Next, the dot determination processing unit Pr3 reads out the dot data of each color from RAM 54, and for each line, determines whether the LEDi of the LED head Hd can be illuminated in either the image forming unit or the upstream image forming unit, and if the LEDi can be illuminated in either image forming unit, sets the determination result to 1, and if the LEDi cannot be illuminated in any image forming unit, sets the determination result to 0 and records the determination result in ROM 53 (step S3).

[0069] For example, if the image forming unit is image forming unit 16Bk, the dot judgment processing unit Pr3 judges for each line whether each LEDi of the LED head Hd in image forming unit 16Bk can be illuminated, and sets the judgment results of LED1 to LED10 to 1, 0, 0, 0, 0, 0, 0, 1, 0, 0.

[0070] In addition, if the image forming unit in question is image forming unit 16C, the dot judgment processing unit Pr3 judges for each line whether each LEDi can be illuminated in any of the image forming units 16Bk, 16Y, and 16M, and sets the judgment results of LED1 to LED10 to 1, 1, 1, 1, 0, 1, 1, 1, 1, 1.

[0071] Next, the toner area ratio calculation processing unit Pr4 reads out the determination results of each line by the dot determination processing unit Pr3 from the ROM 53, and calculates the toner area ratio εj of each color for each line based on the determination results (step S4).

[0072] For example, when the image forming unit is image forming unit 16Bk, the number nbk of LEDs whose determination result is 1 among the 10 black LEDi is 2, so the toner area ratio εbk is 20[%].

[0073] Furthermore, when the image forming unit is the image forming unit 16Y, the number ny of the LEDs for which the determination result is 1 among the 10 yellow LEDi is 6, so the toner area ratio εy is 60[%].

[0074] When the image forming unit is image forming unit 16M, the toner area ratio εm is 80% and when the image forming unit is image forming unit 16C, the toner area ratio εc is 90%. Note that the toner area ratio εj is 100% when a toner image with a print duty of 100% is formed, regardless of whether the image forming unit is image forming unit 16Bk, 16Y, 16M, or 16C.

[0075] Next, the toner area ratio calculation processor Pr4 calculates the maximum value εmax of the toner area ratio εj of each line for each range of the nip width in the sub-scanning direction (Step S5).

[0076] Next, the charging voltage control processor Pr5 judges whether the maximum value εmax of the toner area ratio εj is equal to or greater than the threshold value εth, or 90% or more in this embodiment (step S6).

[0077] When the maximum value εmax of the toner area ratio εj is 90% or more, the charging voltage control processor Pr5 corrects the charging voltage Vch applied to the charging roller 22, and increases it by the correction value ΔVch (in the negative direction) (step S7).

[0078] The correction value ΔVch is a ratio that indicates the extent to which the maximum value εmax of the toner area ratio εj exceeds 90%. γ=(εmax-90) / (100-90) and the value of the positive polarity surface potential formed at both ends of the photosensitive drum 21 when the toner area ratio εj is 100 [%] is taken as a reference value Vst, the value obtained by multiplying the reference value Vst by the ratio γ is: δVch=Vst×γ =Vst×((εmax-90) / (100-90)) to be.

[0079] Then, the print processing unit Pr2 performs the print processing (step S9).

[0080] Moreover, when the maximum value εmax of the toner area ratio εj is less than 90[%], the charging voltage control processing unit Pr5 does not correct the charging voltage Vch (step S8).

[0081] Next, the print processing unit Pr2 performs print processing (step S9).

[0082] Next, the flowchart will be described. In step S1, the printer 10 receives a print command, print data, etc. from the host computer PC. In step S2, the image conversion processing unit Pr1 converts the print data into dot data of each color. Step S3: The dot determination process unit Pr3 records the determination result in the ROM 53. Step S4: The toner area ratio calculation processor Pr4 calculates the toner area ratio εj of each color for each line. In step S5, the toner area ratio calculation processor Pr4 calculates the maximum value εmax of the toner area ratio εj of each line. In step S6, the charging voltage control processor Pr5 judges whether the maximum value εmax of the toner area ratio εj is equal to or greater than the threshold value εth. If the maximum value εmax of the toner area ratio εj is equal to or greater than the threshold value εth, the process proceeds to step S7. If the maximum value εmax of the toner area ratio εj is less than the threshold value εth, the process proceeds to step S8. In step S7, the charging voltage control processor Pr5 corrects the charging voltage Vch applied to the charging roller 22, increasing it by the correction value ΔVch, and the process proceeds to step S9. In step S8, the charging voltage control processor Pr5 does not correct the charging voltage Vch. In step S9, the print processing unit Pr2 performs the print processing and ends the process.

[0083] In this manner, in the present embodiment, the toner area ratio εj, which indicates the ratio of a toner image of any color formed in the image forming unit and the upstream image forming unit, is calculated for each line in the image forming unit, and the charging voltage Vch applied to the charging roller 22 is corrected based on the toner area ratio εj, so that printing with a high print duty is performed, and when a toner image is transferred to the paper P in the image forming unit and the upstream image forming unit, it is possible to prevent a large transfer current from flowing to the non-printing area of ​​the photosensitive drum 21. Therefore, toner does not adhere to both ends of the photosensitive drum 21 and soil the photosensitive drum 21, and toner adhered to both ends of the photosensitive drum 21 does not adhere to the paper P and soil the paper P.

[0084] Next, a description will be given of the transition of the surface potential of the photosensitive drum 21 when an image is formed on the paper P in the printer of the reference example.

[0085] Fig. 5 is a diagram showing an example of the transition of the surface potential of the photoconductor drum when a solid white image is formed on paper with a print duty of 0%, Fig. 6 is a diagram showing the state of paper when a solid cyan image is formed on paper with a print duty of 100%, and Fig. 7 is a diagram showing an example of the transition of the surface potential of the photoconductor drum when a solid cyan image is formed on paper with a print duty of 100%. In Figs. 5 and 7, the horizontal axis represents time and the vertical axis represents the surface potential of the photoconductor drum 21. Also, on the horizontal axis, charging, exposure, transfer, and de-electrification represent the charging process, exposure process, transfer process, and de-electrification process, respectively.

[0086] In FIG. 5, L1 is a line representing the transition of the surface potential of the photosensitive drum 21 during the first and second revolutions when a solid white image is formed on the paper P with the print duty set to 0%.

[0087] When printing starts, in the charging process of the first rotation of the photosensitive drum 21, the photosensitive drum 21 is charged by the charging roller 22, and the surface potential of the photosensitive drum 21 becomes -500 [V]. Next, in the exposure process, since the print duty is 0 [%], each LEDi of the LED head Hd is not emitted, and light is not irradiated onto the photosensitive drum 21. Therefore, the surface potential of the photosensitive drum 21 remains at -500 [V]. Next, in the transfer process, a transfer voltage of +3000 [V] is applied to the transfer roller 28, and the toner image on the photosensitive drum 21 is transferred to the paper P due to the potential difference between the transfer roller 28 and the photosensitive drum 21. At this time, a transfer current flows from the transfer roller 28 to the photosensitive drum 21, and the surface potential of the photosensitive drum 21 decreases to -200 [V]. Next, in the charge removal process, the LED is illuminated by the charge removal device 27, charge removal light is irradiated onto the photoconductor drum 21, and the surface potential of the photoconductor drum 21 becomes -50 [V]. Then, in the charging process for the second rotation of the photoconductor drum 21, the photoconductor drum 21 is charged again by the charging roller 22, and the surface potential of the photoconductor drum 21 becomes -500 [V].

[0088] Also, in Figure 6, P is paper, TC is a solid cyan toner image formed on paper P with a print duty of 100%, Ar1 is an image area on paper P where a toner image is formed, which corresponds to the printing area of ​​photosensitive drum 21, and Ar2 is a non-image area on paper P where a toner image is not formed, which corresponds to the non-printing area of ​​photosensitive drum 21.

[0089] In Figure 7, L2 is a line that represents the change in surface potential of the printing area of ​​the photosensitive drum 21 on the first and second rotations when a solid cyan image is formed on paper P with a print duty of 100%, and L3 is a line that represents the change in surface potential of the non-printing area of ​​the photosensitive drum 21 on the first and second rotations when a solid cyan image is formed on paper P with a print duty of 100%.

[0090] In the printing area of ​​the photoconductor drum 21, when printing starts, in the charging process, the photoconductor drum 21 is charged by the charging roller 22, and the surface potential of the photoconductor drum 21 becomes -500 [V]. Next, in the exposure process, since the print duty is 100 [%], each LEDi of the LED head Hd is caused to emit light, and the light is irradiated onto the photoconductor drum 21. Therefore, the surface potential of the photoconductor drum 21 becomes -100 [V]. Next, in the transfer process, a transfer voltage of +3000 [V] is applied to the transfer roller 28, but since the toner image has been transferred to the image area Ar1 on the paper P, it becomes difficult for a transfer current to flow from the transfer roller 28 to the photoconductor drum 21, and the surface potential of the photoconductor drum 21 does not decrease and remains at -100 [V]. Next, in the charge removal process, the LED is illuminated by the charge removal device 27, charge removal light is irradiated onto the photoconductor drum 21, and the surface potential of the photoconductor drum 21 becomes -50 [V]. Then, in the charging process for the second rotation of the photoconductor drum 21, the photoconductor drum 21 is charged again by the charging roller 22, and the surface potential of the photoconductor drum 21 becomes -500 [V].

[0091] Also, in the non-printing area of ​​the photoconductor drum 21, when printing starts, the photoconductor drum 21 is charged by the charging roller 22 in the charging process, and the surface potential of the photoconductor drum 21 becomes -500 [V]. Next, in the exposure process, each LEDi of the LED head Hd is not emitted, so the surface potential of the photoconductor drum 21 remains at -500 [V]. Next, in the transfer process, a transfer voltage of +3000 [V] is applied to the transfer roller 28, but since a toner image is not transferred to the non-image area Ar2 on the paper P, a transfer current flows from the transfer roller 28 to the photoconductor drum 21 by an amount that does not flow through the image area Ar1, and the surface potential of the photoconductor drum 21 becomes low, becoming +50 [V]. In order to make the positive surface potential of +50 [V] at this time 0 [V], the reference value Vst is set to -50 [V]. Next, in the charge eliminating process, the portion that has been positively charged cannot be eliminated by the charge eliminating device 27, so the surface potential of the photoconductor drum 21 remains at +50 [V].

[0092] Then, in the charging process of the second rotation of the photoconductor drum 21, the photoconductor drum 21 is charged again by the charging roller 22, but because the photoconductor drum 21 is charged with a surface potential of +50 [V], the surface potential of the photoconductor drum 21 becomes lower than that of the first rotation, becoming -400 [V]. Thereafter, the photoconductor drum 21 continues to rotate, and the surface potential of the non-printing area of ​​the photoconductor drum 21 continues to decrease, and when the surface potential becomes equal to or lower than the developing voltage applied to the developing roller 23, the cyan toner on the developing roller 23 adheres to the non-printing area, and both ends of the photoconductor drum 21 become dirty.

[0093] 6, not only when a solid cyan image is formed on paper P with a print duty of 100%, but also when a cyan image with a print duty of 30% and a magenta image with a print duty of 70% are formed on paper P without overlapping the cyan toner image and the magenta toner image, the surface potential of the non-printing area of ​​the photoconductor drum 21 of the image forming unit 16C continues to decrease, and when the surface potential becomes equal to or lower than the development voltage applied to the development roller 23, the cyan toner on the development roller 23 in the image forming unit 16C adheres to the non-printing area of ​​the photoconductor drum 21, soiling both ends of the photoconductor drum 21. That is, even when images of different colors are formed without overlapping toner images so that the print duty is 100% in total in the image forming unit and the upstream image forming unit, the toner adheres to the non-printing area of ​​the photoconductor drum 21 in the image forming unit, soiling both ends of the photoconductor drum 21.

[0094] In contrast, when a cyan image with a print duty of 30% and a magenta image with a print duty of 70% are formed on paper P by overlapping a cyan toner image and a magenta toner image, the non-image printing area Ar2 on paper P where the toner image is not transferred becomes wider, and a transfer current easily flows from transfer roller 28 to photoconductor drum 21, so that the non-printing area of ​​photoconductor drum 21 is not positively charged. Therefore, toner does not adhere to the non-printing area of ​​photoconductor drum 21 in the image forming unit, and both ends of photoconductor drum 21 do not become dirty.

[0095] Furthermore, it has been found through experiments that even if the nip width is only 5 mm, if the maximum value εmax of the toner area ratio εj within the range of the nip width in either the image forming unit in question or the upstream image forming unit is 90% or more, the surface potential of the photosensitive drum 21 will be a positive value in the transfer process.

[0096] Next, a transition of the surface potential of the photoconductor drum 21 when an image is formed on the paper P in this embodiment will be described.

[0097] 8 is a diagram showing an example of the transition of the surface potential of the photoconductor drum when a cyan image with a print duty of 30% and a magenta image with a print duty of 70% are formed on a sheet of paper without overlapping the cyan toner image and the magenta toner image in an embodiment of the present invention. In the diagram, the horizontal axis represents time and the vertical axis represents the surface potential of the photoconductor drum. Also, on the horizontal axis, charging, exposure, transfer, and de-electrification represent the charging process, exposure process, transfer process, and de-electrification process, respectively.

[0098] In the figure, L4 is a line representing the change in surface potential of the printing area of ​​the photoconductor drum 21 on the first and second revolutions when a cyan image with a print duty of 30% and a magenta image with a print duty of 70% are formed on paper P (FIG. 2) without overlapping the cyan toner image and the magenta toner image, and L5 is a line representing the change in surface potential of the non-printing area of ​​the photoconductor drum 21 on the first and second revolutions when a cyan image with a print duty of 30% and a magenta image with a print duty of 70% are formed on paper P without overlapping the toner images. In this embodiment, the environmental conditions of the printer 10 are a temperature of 22° C. and a humidity of 55%.

[0099] In the printing area of ​​the photosensitive drum 21, when printing begins, the photosensitive drum 21 is charged by the charging roller 22 in a charging process, and at this time, a cyan image with a print duty of 30% and a magenta image with a print duty of 70% are formed without overlapping each toner image.

[0100] In this case, as described above, the reference value Vst is set to -50 [V], and a cyan image with a print duty of 30 [%] and a magenta image with a print duty of 70 [%] are formed on the paper P without overlapping each toner image, so the maximum value εmax of the toner area ratio εj is 100 [%]. Therefore, the correction value ΔVch of the charging voltage Vch is δVch=Vst×((εmax-90) / (100-90)) =-50×((100-90) / (100-90)) =-50 [V] After the correction, the charge voltage Vch is Vch=-500-50 =-550 [V] The surface potential of the photoconductor drum 21 becomes −550 [V]. Next, in the exposure process, each LEDi of the LED head Hd is caused to emit light, and the photoconductor drum 21 is irradiated with light. Therefore, the surface potential of the photoconductor drum 21 becomes −100 [V]. Next, in the transfer process, a transfer voltage of +3000 [V] is applied to the transfer roller 28. However, since the cyan and magenta toner images are transferred to the image area Ar1 (FIG. 6) on the paper P, it becomes difficult for the transfer current to flow from the transfer roller 28 to the photoconductor drum 21, and the surface potential of the photoconductor drum 21 does not decrease and remains at −100 [V]. Next, in the charge removal process, the LED is caused to emit light by the charge removal device 27, the photoconductor drum 21 is irradiated with charge removal light, and the surface potential of the photoconductor drum 21 becomes −50 [V]. Then, in the charging process of the second rotation of the photosensitive drum 21, the photosensitive drum 21 is charged again by the charging roller 22, and the surface potential of the photosensitive drum 21 becomes −550 [V].

[0101] On the other hand, in the non-printing area of ​​the photoconductor drum 21, when printing starts, the photoconductor drum 21 is charged by the charging roller 22 in the charging process, and the surface potential of the photoconductor drum 21 becomes -550 [V]. Next, in the exposure process, each LEDi of the LED head Hd is not emitted, so the surface potential of the photoconductor drum 21 remains at -550 [V]. Next, in the transfer process, a transfer voltage of +3000 [V] is applied to the transfer roller 28, but since a toner image is not transferred to the non-image area Ar2 on the paper P, a transfer current flows from the transfer roller 28 to the photoconductor drum 21 by an amount that does not flow through the image area Ar1, and the surface potential of the photoconductor drum 21 becomes lower. As a result, since the surface potential before the decrease was -550 [V], it does not become positive polarity and becomes 0 [V]. Next, in the charge eliminating process, since the charge eliminating device 27 cannot eliminate the 0 [V] portion, the surface potential of the photoconductor drum 21 remains at +0 [V].

[0102] Then, in the charging process of the second rotation of the photosensitive drum 21, the photosensitive drum 21 is charged again with the charging voltage Vch by the charging roller 22, so that the surface potential of the photosensitive drum 21 becomes −550 [V].

[0103] Although the printer 10 has been described in this embodiment, the present invention can be applied to image forming apparatuses such as copying machines, facsimile machines, and multifunction machines.

[0104] The present invention is not limited to the above-described embodiment, and various modifications are possible based on the spirit of the present invention, and these modifications are not excluded from the scope of the present invention. [Explanation of symbols]

[0105] 10 Printers 16Bk, 16Y, 16M, 16C Image forming unit 21 Photoconductor drum 22 Charging roller 23 Developing roller 24 Supply roller 25 Development blade Hd LED Head P paper Pr4 Toner area ratio calculation processing unit Pr5 Charge voltage control processing section u1 transcription unit Vhc Charge voltage εj Toner area ratio

Claims

1. (a) a plurality of image forming units arranged from the upstream side to the downstream side in a medium transport direction, each of which includes an image carrier, a charging member for charging the image carrier, and a developing section for developing a latent image formed on the image carrier and forming a developer image of each color on the image carrier; (b) an exposure device that is disposed opposite the image carrier of each of the image forming units, includes a plurality of light-emitting elements arranged in a scanning direction, and forms the latent image on the image carrier by illuminating the light-emitting elements based on dot data; (c) a transfer device for transferring the developer image formed on each of the image carriers onto a medium; (d) a developer area ratio calculation processing section that calculates a developer area ratio that represents a ratio of a developer image of a color of a predetermined image forming unit that is a predetermined image forming unit and a developer image of any color of an upstream image forming unit disposed upstream of the predetermined image forming unit that is a predetermined image forming unit, the developer image being formed in such a manner that the developer image is overlapped with the developer image of the color of the predetermined image forming unit. (e) a charging voltage control processing unit that corrects a charging voltage applied to the charging member based on the developer area ratio, (f) When the number of light-emitting elements arranged in the scanning direction is N and the number of light-emitting elements that are caused to emit light in the exposure device of either the image forming unit or the upstream image forming unit is nj, the developer area ratio εj is given by εj=(nj / N)×100 To (g) the developer area ratio calculation processing unit calculates a maximum value of the developer area ratio for each predetermined range in the sub-scanning direction in the image forming unit; (h) The image forming apparatus, wherein the charging voltage control processing section corrects and increases the charging voltage when the maximum value of the developer area ratio is equal to or greater than a threshold value.

2. The image forming apparatus according to claim 1 , wherein the developer area ratio calculation processing unit calculates the developer area ratio based on dot data.

3. In the image forming unit, when a transfer current flows from a transfer member of a transfer device to an image carrier in a transfer step, the reference value of the surface potential when the image carrier becomes positive is Vst, the maximum value of the developer area ratio is εmax, and the threshold value of the developer area ratio is εth, the correction value δVch for correcting the charging voltage is expressed as follows: δVch=Vst×((εmax-εth) / (100-εth)) 2. The image forming apparatus according to claim 1 .

Images