JP2011128373A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can achieve prolongation of the life of a photoreceptor by suppressing wear of a photoreceptive layer to the utmost without deteriorating image quality when a step of electrifying a surface of the photoreceptor is performed by contact type DC electrification. <P>SOLUTION: A CPU 91 reads out image data stored in a temporary storage part 94 from an image input part 40 through an I/F 96, and determines whether a photographic area is included in each color image data. When the photographic area is included therein, the CPU 91 sets surface potential of the photoreceptor drum corresponding to the color including the photographic area to high potential. Meanwhile, when the photographic area is not included therein, the CPU 91 sets the surface potential of the photoreceptor drum corresponding to the color to low potential. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a laser printer, and a facsimile, and more particularly to an image forming apparatus provided with a charging member that charges a surface of a photosensitive drum in a contact or non-contact state. It is.

  In an image forming apparatus using an electrophotographic process, suitable means for uniformly charging the surface of the photosensitive drum include a corona charging method including a corona discharger and a conductive charging member typified by a charging roller. There is a contact charging system. Since many corona products such as ozone are generated in the corona discharger, components in the air are decomposed by ozone, and ion products such as NOx and SOx are generated. For this reason, in recent years, it has been recognized that the contact charging method is adopted instead of the corona charging method. This contact charging method can suppress generation of ozone, NOx, SOx, and the like by applying a DC voltage or a voltage obtained by superimposing an AC voltage on a DC voltage.

  In particular, in a tandem type color image forming apparatus having a plurality of charging devices, the amount of ozone generation increases, so that the merit of employing the contact charging method is great. In addition, charging uniformity is ensured when comparing a method in which only a DC voltage is applied (hereinafter also referred to as DC charging) and a method in which a voltage in which an AC voltage is superimposed on the DC voltage (hereinafter also referred to as AC + DC charging) is compared. In order to achieve this, AC + DC charging is superior, but DC charging is less likely to generate ozone and the cost of the power supply (high voltage transformer) can be reduced. In recent years, DC charging is becoming mainstream.

  However, in DC charging, there is no effect of uniformizing charging due to AC voltage, and therefore discharge tends to be non-uniform due to contamination of the charging member, unevenness of the surface of the photosensitive drum, and the like. Further, when the toner image formed on the photosensitive drum is transferred to the recording medium, a charge opposite to the charge is applied to the drum, and the drum surface potential is lowered. The surface potential gap at this time is called the transfer memory potential. If the transfer memory potential exceeds the allowable value, the image density changes at the leading and trailing edges of the recording medium, or the previously printed image appears on the next image. Although it may appear as an afterimage, this transfer memory also tends to occur with DC charging.

  In order to improve the uniformity of charging in DC charging, it is only necessary to increase the applied DC voltage to increase the discharge generated in the minute gap between the charging member and the photosensitive drum. On the other hand, it is known that in the contact charging method, since the charging process on the surface of the photosensitive drum is performed by discharge near the surface, the abrasion (film abrasion) of the photosensitive layer is promoted. The reason for this is considered to be that the energy of ions colliding with the drum surface is high, and the photosensitive layer on the drum surface is deteriorated and is easily scraped by the frictional force of a cleaning blade or the like.

  In particular, when using a photoreceptor having an organic photosensitive layer (OPC), the electrostatic capacity increases as the photosensitive layer wears, and the amount of charge to be applied to the surface of the photoreceptor in order to form an electrostatic latent image optimal for image formation is reduced. Change. Further, when the wear of the photosensitive layer progresses, the pressure resistance characteristic of the photoreceptor decreases, so that when the high voltage is applied to the charging member, the photosensitive layer is liable to cause local dielectric breakdown called a pinhole. This pinhole appears as a black spot (color spot) on the image, which is a big problem in image quality.

  In the AC + DC charging, various methods for suppressing the wear of the photosensitive layer have been proposed. For example, Patent Document 1 discloses that the AC voltage peak-to-peak voltage or current value during transfer of the transfer material is lower than that during image formation. Is disclosed. Patent Document 2 discloses an image forming apparatus that switches between AC + DC charging and DC charging based on the ratio of the intermediate density portion in the image.

  Patent Document 3 discloses switching between AC + DC charging and DC charging according to conditions such as an image forming mode, an image mode, and resolution, and even when AC + DC charging or DC charging is selected, even under the same environmental conditions. An image forming apparatus that controls the magnitude of an AC bias value or a DC bias value is disclosed.

JP-A-9-120197 Japanese Patent Laid-Open No. 11-174785 Japanese Patent Laying-Open No. 2004-347751 (paragraphs [0064] to [0068], FIG. 8)

  However, the methods disclosed in Patent Documents 1 and 2 are techniques for using only AC + DC charging or using both AC + DC charging and DC charging, and are not sufficient as a method for suppressing wear of the photosensitive layer in DC charging as described above. there were.

  Further, in a tandem type color image forming apparatus having a plurality of photosensitive drums and a plurality of charging members that charge the photosensitive drums, for example, a character area and a photographic area are mixed in an original image, and the character area is The image mode of each color formed using each photoconductor drum may be different, as in the case of printing a photographic region in color with black.

  In this regard, Patent Document 3 describes that the present invention can be applied to a color copying machine and a printer (paragraph [0072]). Usually, the image mode (character mode, photo mode, etc.) of the entire original image is set. For the determination, it is assumed that when the technique of Patent Document 3 is applied to a tandem color image forming apparatus, the charging bias applied to the plurality of charging members is uniformly controlled according to the image mode of the entire document image. It is thought that. That is, there has been no description of a specific method for individually determining the image mode of each color and individually controlling the charging bias applied to the charging member corresponding to each photosensitive drum according to the image mode.

  Even when the surface of the photosensitive member is charged in a non-contact state, if the charging member and the photosensitive member are close to each other, a problem similar to that of the contact charging method as described above occurs.

  In view of the above problems, the present invention has a long life of the photosensitive member by suppressing the abrasion of the photosensitive layer as much as possible without deteriorating the image quality when the charging process of the photosensitive member surface is performed by contact-type DC charging. An object of the present invention is to provide an image forming apparatus capable of achieving the above.

  In order to achieve the above object, the present invention applies a plurality of image carriers having a photosensitive layer formed on the surface, and a charging bias consisting of only a DC voltage in contact or non-contact with the surface of the image carrier. In the image forming apparatus comprising: a plurality of charging members that charge the surface of the image bearing member; and a voltage applying unit that applies a DC voltage to the charging member, a storage unit that stores image data of an output image; A photographic region including a halftone density of a predetermined number of gradations or more is detected from the image data of each color stored in the storage means, and a charging bias applied from the voltage applying means to the charging member is determined based on the detection result. And control means for individually setting the surface potential of each of the image carriers by control.

  According to the present invention, in the image forming apparatus configured as described above, the control unit sets the surface potential of the image carrier corresponding to a color including a photographic region in the image data to a first potential, The surface potential of the image carrier corresponding to a color that does not include a photographic area is set to a second potential lower than the first potential.

  According to the present invention, in the image forming apparatus having the above-described configuration, the control unit divides the image data stored in the storage unit into a plurality of regions, and detects a density change exceeding a predetermined value existing in each region as an edge. At the same time, it is characterized in that an area having an edge ratio smaller than a reference value is determined as a photographic area.

  According to the present invention, in the image forming apparatus configured as described above, the photosensitive layer is an organic photosensitive layer.

  According to the first configuration of the present invention, the photographic area is detected from the image data of each color stored in the storage unit, and the charging bias applied to the charging member from the voltage applying unit is controlled based on the detection result. By setting the surface potential of each image carrier individually, when outputting an image including a photographic area that requires charging uniformity, the DC voltage is increased to suppress transfer memory and abnormal charging, and to adjust the halftone. Tone reproducibility can be improved. In addition, when outputting only images of characters, lines, and solid areas that do not include a photographic area, it is sufficient to apply the minimum necessary DC voltage to obtain an image having no practical problem. be able to. Accordingly, it is possible to extend the life of the photosensitive drum while maintaining the quality of the image including the photographic region.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the surface potential of the image carrier is lower than the first potential and the first potential depending on the presence or absence of a photographic region. By performing the two-stage control to the second potential, it is possible to extend the life of the photosensitive drum while maintaining the quality of the image including the photographic region with simple control.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the image data stored in the storage unit is divided into a number of regions, and the predetermined number or more existing in each region. By detecting the change in density as an edge and discriminating the region having the edge ratio smaller than the reference value as the photo region, the photo region can be easily and accurately discriminated.

  Further, according to the fourth configuration of the present invention, in the image forming apparatus having any one of the first to third configurations, the image quality can be improved even in the image carrier that is an organic photosensitive layer in which the photosensitive layer is easily worn. The risk of pinholes can be reduced while maintaining.

1 is a schematic cross-sectional view showing the overall configuration of an image forming apparatus of the present invention. Partial enlarged view around the image forming unit Pa in FIG. 1 is a block diagram showing a control path of an image forming apparatus of the present invention. Graph showing change in surface potential of photoconductor during transfer of toner image A graph showing the relationship between the layer thickness of the photosensitive layer and the transfer memory potential ΔV1 when the surface potential of the photoreceptor is changed Graph showing the relationship between the DC voltage applied to the charging roller and the surface potential of the photoreceptor A graph showing the relationship between the charging current flowing from the charging roller to the surface of the photosensitive member when a DC voltage is applied and the surface potential of the photosensitive member Graph showing the relationship between the charging current and the amount of wear of the photosensitive layer per unit time 6 is a flowchart illustrating an example of control of an applied voltage applied to the charging roller in the image forming apparatus of the present invention. A graph showing the relationship between the number of printed sheets and the amount of shaving of the photosensitive layer when the surface potential setting is switched according to the presence or absence of a photographic area (invention) and when the surface potential setting is constant (comparative example)

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of the image forming apparatus of the present invention. In the main body of the color printer 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, and yellow are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  The image forming portions Pa to Pd are provided with photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors, and are further driven by a driving unit (not shown). The intermediate transfer belt 8 that rotates clockwise is provided adjacent to the image forming portions Pa to Pd. The toner images formed on the photosensitive drums 1a to 1d are sequentially primary-transferred and superimposed on the intermediate transfer belt 8 that moves while contacting the photosensitive drums 1a to 1d, and then the secondary transfer roller. 9 is secondarily transferred onto a transfer paper P as an example of a recording medium, and further fixed on the transfer paper P in the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The transfer paper P onto which the toner image is transferred is accommodated in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via a paper feed roller 12a and a registration roller pair 12b. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a (seamless) belt having no seam is mainly used. A blade-shaped belt cleaner 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. Around and below the photosensitive drums 1a to 1d having organic photosensitive layers (OPC) rotatably disposed, charging devices 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d, and respective photosensitive drums Remaining on the photosensitive drums 1a to 1d, the exposure unit 4 that exposes image information to the photosensitive drums 1a to 1d, the developing devices 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d. Cleaning devices 5a to 5d for removing the developer (toner) are provided.

  When an image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging devices 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing devices 3a to 3d includes a developing roller disposed so as to face the photosensitive drums 1a to 1d, and is filled with a predetermined amount of a two-component developer containing toner of each color of yellow, cyan, magenta, and black. This toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing devices 3a to 3d, and electrostatically adheres to the electrostatic latent image formed by exposure from the exposure unit 4. A toner image is formed.

  Then, the primary transfer rollers 6a to 6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d, and yellow, cyan, magenta, and yellow on the photosensitive drums 1a to 1d are applied. A black toner image is primarily transferred onto the intermediate transfer belt 8. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning devices 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched around a plurality of suspension rollers including a driven roller 10 and a drive roller 11, and the intermediate transfer belt 8 rotates clockwise as the drive roller 11 is rotated by a drive motor (not shown). Is started, the transfer paper P is transported from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the nip portion (secondary transfer nip portion) with the intermediate transfer belt 8 is transferred. ), The full color image is secondarily transferred onto the transfer paper P. The transfer paper P onto which the toner image is transferred is conveyed to the fixing unit 7.

  The transfer paper P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13, and the toner image is fixed on the surface of the transfer paper P, so that a predetermined full color is obtained. An image is formed. The transfer paper P on which the full-color image is formed is distributed in the transport direction by the branching portion 14 that branches in a plurality of directions. When an image is formed only on one side of the transfer paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when images are formed on both sides of the transfer paper P, a part of the transfer paper P that has passed through the fixing unit 7 is once protruded from the discharge roller 15 to the outside of the apparatus. Thereafter, the transfer paper P is distributed to the paper transport path 18 by the branching section 14 by rotating the discharge roller 15 in the reverse direction, and is transported again to the registration roller pair 12b with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the transfer paper P on which the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image. , And discharged to the discharge tray 17.

  FIG. 2 is an enlarged view of the vicinity of the image forming portion Pa in FIG. Since the image forming units Pb to Pd have basically the same configuration, description thereof is omitted. Around the photosensitive drum 1a, a charging device 2a, a developing device 3a, a cleaning device 5a, and a charge eliminating lamp 20 are disposed along the drum rotation direction (counterclockwise in FIG. 2), and the intermediate transfer belt 8 is interposed therebetween. The primary transfer roller 6a is arranged.

  The charging device 2 a includes a charging roller 21 that contacts the photosensitive drum 1 a and applies a charging bias (DC voltage) to the drum surface, and a charging cleaning roller 23 for cleaning the charging roller 21. In the present invention, in order to reduce the amount of generated ozone and reduce the cost of the charging bias power source 42 (see FIG. 3), a charging bias consisting only of a DC voltage is applied to the charging roller 21.

  The developing device 3a is a two-component developing type having two agitating and conveying screws 25, a magnetic roller 27, and a developing roller 29, and a toner thin film is applied to the developing roller 29 using a magnetic brush standing on the surface of the magnetic roller 27. A layer is formed, and a developing bias having the same polarity (positive) as the toner is applied to the developing roller 29 to cause the toner to fly on the drum surface.

  The cleaning device 5 a has a cleaning blade 31 and a recovery screw 33.

  A cleaning blade 31 is fixed in contact with the photosensitive drum 1a on the photosensitive drum 1a surface downstream of the contact surface with the intermediate transfer belt 8 in the rotation direction. As the cleaning blade 31, for example, a polyurethane rubber blade having a JIS hardness of 78 ° is used, and is attached at a predetermined angle with respect to the tangential direction of the photosensitive member at the contact point. The material, hardness, dimensions, the amount of biting into the photosensitive drum 1a, the pressure contact force, and the like of the cleaning blade 31 are appropriately set according to the specifications of the photosensitive drum 1a. Residual toner removed from the surface of the photosensitive drum 1 a by the cleaning blade 31 is discharged to the outside of the cleaning device 5 a as the recovery screw 33 rotates.

  A neutralizing lamp 20 is disposed between the cleaning device 5a and the charging device 2a. The neutralization lamp 20 removes residual charges on the surface of the drum by irradiating the surface of the photosensitive drum 1a with light.

  Next, the control path of the image forming apparatus of the present invention will be described. FIG. 3 is a block diagram showing an example of a control path used in the image forming apparatus of the present invention. In addition, since various controls of each part of the apparatus are performed when the color printer 100 is used, the control path of the entire color printer 100 becomes complicated. Therefore, here, a portion of the control path that is necessary for the implementation of the present invention will be mainly described.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 that is a read-only storage unit, a random access memory (RAM) 93 that is a read / write storage unit, A plurality of (two in this case) that transmit control signals to and receive input signals from the operation unit 50, such as a temporary storage unit 94 that stores image data and the like, a counter 95, and each device in the color printer 100. At least an I / F (interface) 96 is provided. Further, the control unit 90 can be arranged at an arbitrary location inside the apparatus main body.

  The ROM 92 stores control programs for the color printer 100, data necessary for control, and the like that are not changed during use of the color printer 100. The RAM 93 stores necessary data generated during the control of the color printer 100, data temporarily required for the control of the color printer 100, and the like. The ROM 92 (or RAM 93) also stores the cumulative number of printed sheets, the relationship between the cumulative number of printed sheets and the amount of charged charge used to determine the amount of static elimination and charging bias described below, and the relationship between the cumulative number of printed sheets and the charging bias. ing. The counter 95 adds up the number of printed sheets and counts it. Note that the number of times may be stored in the RAM 93, for example, without providing the counter 95 separately.

  The control unit 90 transmits a control signal from the CPU 91 to the respective units and devices in the color printer 100 through the I / F 96. In addition, a signal indicating the state and an input signal are transmitted from each part or device to the CPU 91 through the I / F 96. For example, the image forming units Pa to Pd, the exposure unit 4, the primary transfer rollers 6 a to 6 d, the fixing unit 7, the secondary transfer roller 9, the image input unit 40, and bias control are included in each part and device controlled by the control unit 90. Examples include the circuit 41, the operation unit 50, and the like.

  The image input unit 40 is a receiving unit that receives image data transmitted from a personal computer or the like to the color printer 100. The image signal input from the image input unit 40 is converted into a digital signal and then sent to the temporary storage unit 94.

  The bias control circuit 41 is connected to the charging bias power source 42, the developing bias power source 43, and the transfer bias power source 44, and operates each of these power sources in accordance with an output signal from the control unit 90. In accordance with a control signal from the control circuit 41, the charging bias power source 42 is applied to the charging roller 21 in the charging devices 2a to 2d, and the developing bias power source 43 is applied to the magnetic roller 27 and the developing roller 29 in the developing devices 3a to 3d. A predetermined bias 44 is applied to the primary transfer rollers 6a to 6d and the secondary transfer roller 9, respectively.

  The operation unit 50 is provided with a liquid crystal display unit 51 and LEDs 52 that indicate various states. The user operates the operation unit 50 to input instructions, thereby performing various settings of the color printer 100 and image. Various functions such as forming are executed. The liquid crystal display unit 51 displays the state of the color printer 100, displays the image formation status and the number of copies, and can be used as a touch panel to perform various settings such as functions such as double-sided printing and black-and-white reversal, magnification setting, and density setting. It has become.

  In addition, the operation unit 50 includes a start button for instructing the user to start image formation, a stop / clear button used when the image formation is stopped, and various settings of the color printer 100 when making the default settings. A reset button or the like to be used is provided.

  Since the color printer 100 shown in FIG. 1 employs a DC charging method in which only a DC voltage is applied to the charging roller 21, the transfer memory can be compared with an AC + DC charging method that can expect a uniform charge effect due to the AC voltage. Is likely to occur. FIG. 4 is a graph showing changes in the surface potential of the photoconductor during transfer of the toner image. When the transfer bias is turned ON, a charge (transfer current) opposite to the charge (opposite polarity with toner, here negative) flows on the surface of the photoconductor, so that the surface potential of the photoconductor is partially lowered. The surface potential gap ΔV1 at this time is called a transfer memory potential, and as shown in FIG. 4, the surface potential changes immediately after the transfer bias is turned on and immediately before the transfer bias is turned off, and the image at the leading edge and the trailing edge of the image. The concentration will change.

  In addition, the transfer current flowing on the surface of the photosensitive member is different between the exposed portion (image portion) to which the toner adheres and the unexposed portion (white background portion) to which the toner does not adhere. That is, since the transfer current is taken by the toner in the exposed portion, the decrease in the surface potential is smaller than that in the unexposed portion. As a result, when the transfer bias is turned off and the surface potential rises again, the surface potential of the exposed portion becomes higher than that of the unexposed portion. For example, after printing a high density image, a halftone image of the entire image area is printed. When printing, the image printed last time becomes an afterimage (ghost) and appears whitish.

  FIG. 5 is a graph showing the relationship between the layer thickness of the photosensitive layer and the transfer memory potential ΔV1 when the surface potential of the photoconductor is changed, and FIG. 6 is a graph showing the DC voltage applied to the charging roller and the photoconductor. It is a graph which shows the relationship with the surface potential. As shown in FIG. 5, the transfer memory potential ΔV1 decreases as the surface potential of the photosensitive member increases to 300 V (indicated by ◆ in the figure), 600 V (indicated by ▪ in the figure), and 900 V (indicated by ● in the figure). . Further, as shown in FIG. 6, the DC voltage applied to the charging roller and the surface potential of the photoconductor are in a proportional relationship, and the surface potential of the photoconductor increases as the applied DC voltage increases.

  Therefore, it can be seen that in order to reduce the transfer memory potential to be lower than the allowable value and maintain the uniformity of charging on the surface of the photoreceptor, it is necessary to increase the DC voltage and set the surface potential higher.

  FIG. 7 is a graph showing the relationship between the charging current flowing from the charging roller to the surface of the photosensitive member when a DC voltage is applied, and the surface potential of the photosensitive member. FIG. 8 is a graph showing the amount of abrasion of the photosensitive layer per unit time (FIG. Hereinafter, it is a graph showing the relationship with the wear rate. As shown in FIG. 7, the charging current and the surface potential of the photosensitive member are in a proportional relationship. As shown in FIG. 8, the charging current and the wear rate are also in a proportional relationship. It can be seen that the wear rate of the photosensitive layer increases when it is set higher.

  By the way, in the color printer 100 as shown in FIG. 1, each of the image forming units Pa to Pd is based on the image data input from the image input unit 40, for example, a halftone of 256 gradations from 0 to 255. (Halftone) density can be output. In order to output the gradation of the halftone density uniformly with good reproducibility, the charging uniformity of the surface of the photoconductive drums 1a to 1d is required. Therefore, the surface potential needs to be set high.

  However, general output images include not only multi-tone halftone densities such as photographic originals, but also many originals containing only characters, lines, and solid (solid) images that do not include multi-tones. Characters, lines, and solid originals do not require strict gradation reproducibility compared to photographic originals, so that even if the surface potential of the photoconductor is slightly nonuniform, an image having no practical problem can be obtained.

  In view of the above, in the present invention, whether or not there is a region (hereinafter referred to as a photographic region) containing a halftone density of a predetermined number of gradations or more that requires gradation reproducibility in one image data. Accordingly, the DC voltage applied to the charging roller 21 is controlled. Specifically, the image data of each color input from the image input unit 40 and stored in the temporary storage unit 94 is read by the CPU 91 and divided into a large number of minute areas, and density changes (edges) exceeding a predetermined value existing in each area. ) Is detected. Then, an area where the edge ratio is smaller than a predetermined reference value is determined as a photographic area where halftones are frequently used. An area where edges exist at a ratio equal to or higher than the reference value is determined as a character, line, or solid area (hereinafter referred to as a character area).

  Note that the discrimination between the character area and the photographic area does not necessarily match the actual image. For example, even if the actual image is a photograph or a figure, if the rise (contrast) of the density change from the ambient density is greater than or equal to a predetermined value, it is detected as an edge, and the edge is included at a ratio greater than the reference value Is identified as a character area. On the contrary, even if the actual image is a character, it is a character like a photograph with gradation changes such as a decorative character, and if the rise in density change from the surrounding density is smaller than a predetermined value, it is not detected as an edge, When the edge ratio is less than the reference value, it is determined as a photo area.

  Using this discrimination result, the DC voltage applied to the charging device (for example, the charging device 2a for cyan image data) corresponding to the color in which the photographic region is present in the image data is set high to make the surface potential uniform. Ensure sex. On the other hand, the DC voltage applied to the charging device corresponding to the color having no photographic area is set low to reduce the charging current flowing on the surface of the photoreceptor.

  As a result, when outputting an image including a photographic region that requires charging uniformity, the DC voltage can be increased to suppress transfer memory and abnormal charging, thereby improving halftone gradation reproducibility. In addition, when outputting only images of characters, lines, and solid areas that do not include a photographic area, it is sufficient to apply the minimum necessary DC voltage to obtain an image having no practical problem. be able to. Therefore, it is possible to extend the life of the photosensitive drums 1a to 1d while maintaining the quality of an image including a photographic region.

  In particular, in the tandem color printer 100 shown in FIG. 1, there are also image forming portions Pa to Pd that do not output halftones depending on the output image. For example, in the case of a user who has a high ratio of text documents, the black image forming unit Pd has a very low ratio of outputting halftones. Therefore, by individually setting the surface potentials of the photosensitive drums 1a to 1d in the image forming portions Pa to Pd as in the present invention, the photosensitive layers of the photosensitive drums 1a to 1d are not wasted. The service life can be extended.

  FIG. 9 is a flowchart showing a control example of the applied voltage applied to the charging roller in the image forming apparatus of the present invention. The procedure for setting the DC voltage applied to the charging roller according to the steps of FIG. 9 will be described with reference to FIGS. 1 to 3 as necessary. First, when a print command and image data are transmitted from a personal computer or the like and printing is started (step S1), the CPU 91 reads the image data stored in the temporary storage unit 94 from the image input unit 40 via the I / F 96. (Step S2), it is determined whether or not a photo area is included in the image data of each color (Step S3).

  If a photographic area is included, the CPU 91 sets the surface potential of the photosensitive drum corresponding to the color including the photographic area to a high potential (here, 600 V) (step S4). On the other hand, when the photographic area is not included, the surface potential of the photosensitive drum corresponding to the color is set to a low potential (300 V in this case) (step S5).

  Next, the CPU 91 transmits a control signal to the bias control circuit 41, and applies a predetermined DC to the charging roller 21 of the charging devices 2a to 2d so that the photosensitive drums 1a to 1d have the surface potential set in step S4 or S5. A voltage is applied (step S6). Then, an electrostatic latent image corresponding to the image data is formed on each of the photosensitive drums 1a to 1d by the exposure unit 4, developed into a toner image by the developing devices 3a to 3d, and then primarily transferred onto the intermediate transfer belt 8. Then, an image to be secondarily transferred onto the transfer paper P is output (step S7). Thereafter, if the printing of a predetermined number of sheets has been completed, the process is terminated. If the printing is being continued, the process returns to step S2, and the same control is repeated thereafter (steps S3 to S7).

  In the control example shown in FIG. 9, the DC voltage is set in two stages according to the presence / absence of a photographic area, but the DC voltage may be set in three or more stages according to the ratio of the photographic area. Further, the set value of the surface potential of the photosensitive member may be appropriately changed according to the layer thickness and charging characteristics of the photosensitive layer.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, the materials, shapes, relative arrangements, and the like of the component parts described in the above embodiments are not intended to limit the scope of the present invention, but merely illustrative examples, unless otherwise specified. .

  In the above-described embodiment, the charging roller 21 is configured to charge the surfaces of the photosensitive drums 1a to 1d while being in contact with the surfaces of the photosensitive drums 1a to 1d. Even in the embodiment that is arranged close to the surfaces of the drums 1a to 1d and configured to be charged in a non-contact state, substantially the same effect can be obtained by substantially the same action.

  The present invention is not limited to the tandem type color printer shown in FIG. 1, but can be applied to various color image forming apparatuses such as a monochrome printer, a rotary type or a tandem type color copying machine.

  A test machine (FC-C5300DN modified machine manufactured by Kyocera Mita Co., Ltd.) as shown in FIG. 1 was used to investigate the image quality and the durability of the photosensitive drum when the control of the present invention was performed. The test machine conditions were a printing speed of 160 mm / sec, a photosensitive drum with a diameter of 30 mm having a positively charged single-layer OPC photosensitive layer with an initial layer thickness of 35 μm, and a charging roller made of epichlorohydrin rubber with a diameter of 12 mm. .

  The test method was to perform durable printing of test images (one set of 10 sheets, 5 out of 10 originals with mixed photo areas, and the remaining 5 sheets of text originals) at 23 ° C and 65% RH. When the DC voltage applied to the roller is switched to 600 V for a photo region mixed original and 300 V for a character original (invention), and when the DC voltage is maintained at 600 V or 300 V (Comparative Examples 1 and 2). Then, the layer thickness of the photosensitive layer was measured, and the image quality was visually observed. Further, the allowable abrasion amount (wear limit) of the photosensitive layer was 25 μm, and the target life of the photosensitive drum was 200,000 sheets. The results are shown in FIG.

  As shown in FIG. 10, in the present invention (indicated by ● in the figure) in which the surface potential setting is switched to 600 V for a photographic region mixed original and 300 V for a character original, the photosensitive member lifetime is about 23 to 200,000, which is the target lifetime. It was possible to extend to 10,000 sheets. In addition, since the surface potential was set high in the photographic region mixed original, abnormal images such as spots and unevenness were not seen. In the case of a text document, the surface potential was lowered to 300 V, but even if the charging was slightly uneven, it was not recognized as a spot or uneven image, and was output as a good image with no practical problems.

  On the other hand, in Comparative Example 1 (indicated by x in the figure) printed with the surface potential set at 600 V, the abrasion amount of the photosensitive layer reached 25 μm, which is the wear limit per 180,000 sheets. At the target life of 200,000 sheets, the amount of abrasion of the photosensitive layer exceeded the allowable value of 25 μm, and pinholes were generated on the surface. However, the photo area mixed originals were reproduced uniformly without any unevenness.

  On the other hand, in Comparative Example 2 (printed with ◇ in the figure) printed with the surface potential set at 300 V, the amount of abrasion of the photosensitive layer was reduced and about 240,000 sheets could be printed until the wear limit reached 25 μm. The number exceeded 200,000. However, since the surface potential setting is low, spots and unevenness occur in the photographic region mixed original, resulting in a significant loss in image uniformity.

  The present invention can be used in an image forming apparatus including a charging member that charges the surface of a photosensitive drum in a contact or non-contact state. By utilizing the present invention, it is possible to provide an image forming apparatus that can promote the extension of the life of the photosensitive drum while maintaining the quality of an image including a photographic region that requires gradation reproducibility.

  In addition, the image data is divided into a large number of areas, a density change exceeding a predetermined value in each area is detected as an edge, and an area having a ratio of edges smaller than the reference value is determined as a photographic area, so the photographic area can be easily In addition, it becomes possible to determine with high accuracy.

  In particular, when applied to an image forming apparatus equipped with an organic photoconductor such as OPC where the photosensitive layer is easily worn, pinholes associated with wear of the photosensitive layer are maintained while maintaining image quality when printing a character document and a photographic document. It is possible to reduce the risk of occurrence of the problem and to extend the life of the apparatus and improve the maintainability.

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
2a to 2d charging device 3a to 3d developing device 4 exposure unit 6a to 6d primary transfer roller 8 intermediate transfer belt 9 secondary transfer roller 20 static elimination lamp 21 charging roller (charging member)
41 Bias control circuit (voltage application means)
42 Charging bias power supply (voltage application means)
90 Control unit (control means)
94 Temporary storage (storage means)
100 color printer

Claims (4)

A plurality of image carriers having a photosensitive layer formed on the surface;
A plurality of charging members for charging the surface of the image carrier by applying a charging bias consisting of only a direct current voltage in contact or non-contact with the surface of the image carrier;
Voltage applying means for applying a DC voltage to the charging member;
In an image forming apparatus comprising:
A storage unit that stores image data of an output image; and a photographic area including a halftone density equal to or greater than a predetermined number of gradations is detected from the image data of each color stored in the storage unit, and the voltage based on the detection result An image forming apparatus comprising: control means for individually setting a surface potential of each image carrier by controlling a charging bias applied to the charging member from an applying means.   The control means sets the surface potential of the image carrier corresponding to a color including a photographic region in image data to a first potential and the image carrier corresponding to a color not including a photographic region in the image data. The image forming apparatus according to claim 1, wherein the surface potential of the body is set to a second potential lower than the first potential.   The control means divides the image data stored in the storage means into a large number of areas, detects a change in density exceeding a predetermined value existing in each area as an edge, and detects an area where the ratio of edges is smaller than a reference value. The image forming apparatus according to claim 1, wherein the image forming apparatus is identified as a photographic area.   4. The image forming apparatus according to claim 1, wherein the photosensitive layer is an organic photosensitive layer.

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