JP2005195890A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prolong the life time of a photoreceptor and moreover, to obtain a stable image at the initial time and the life end of the photoreceptor. <P>SOLUTION: In the image forming apparatus of a system, in which the photoreceptor 1 is electrified by applying voltage on which AC voltage whose peak-to-peak voltage is twice or less times as high as discharge start voltage, is superposed or DC voltage to an electrifier 2 arranged in a state where it is brought into contact with or in close location to the turning photoreceptor 1, potential rise caused by the wear of film of the photoreceptor and the sensitivity degradation of the photoreceptor are simultaneously corrected by performing control to drop applied voltage based on the wear of film of the photoreceptor and integrating operation time and control to drop the applied voltage to the electrifier 2, by an amount equivalent to a controlled variable as which [(potential rise amount caused by the wear of film of the photoreceptor) - (potential rise amount of the exposure part of the photoreceptor)] is set. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrophotographic image forming apparatus used in a copying machine, a laser printer, a facsimile, and the like, and more particularly, a method of charging a photosensitive member surface by bringing a charging member into contact with or in proximity to a rotating photosensitive member. The present invention relates to an image forming apparatus.

  Conventionally, as a charger for an electrophotographic image forming apparatus used for a copying machine, a laser printer, a facsimile, etc., a corotron charger using a wire and a case, a wire and a case, and a surface potential of the photosensitive member using a grid electrode. Many scorotron chargers are used to stabilize the temperature. In particular, the scorotron charger has an advantage that the surface potential of the photoconductor can be stably controlled by using a grid disposed between the wire and the photoconductor surface.

  However, these chargers have a drawback that a high voltage is required and a large amount of ozone is generated. In order to solve this drawback, in recent years, a charger in which a charging member is in contact with or close to a photosensitive member has been invented. Some examples include contact roller chargers, non-contact roller chargers, brush chargers, magnetic brush chargers, and the like. These chargers use a charging method based on micro-gap discharge except for some injection charging methods. Also, the mainstream is to superimpose the voltage applied to the charger on the DC bias, and in this charging method, as a means for improving spotted charging unevenness, the AC bias has a peak more than twice the discharge start voltage. In general, an inter-voltage (Vpp) is applied (see, for example, Patent Document 1).

  Such an AC superposed type charger is very effective in stabilizing the surface potential of the photoreceptor, but many problems have been reported that the film thickness of the photoreceptor (photosensitive layer scraping) increases. Yes.

  By the way, in recent years, the variation in resistance (rotor resistance, etc.) has become very small due to the improvement in the performance of the charging member, and the application of a peak-to-peak voltage (Vpp) that is twice or less the discharge start voltage (hereinafter referred to as “ Stable charging can be realized by applying a voltage of only a DC bias. For this reason, machines equipped with a charger with low Vpp application or DC bias application are on the market. At present, there is little film loss on the photoreceptor, which is almost the same as machines equipped with conventional scorotron chargers. Or, the situation is such that only a small amount of film reduction is required.

Another drawback of the charger by applying low Vpp or applying a DC bias is that the charged potential of the surface changes as the film thickness of the photoconductor decreases. To eliminate this, the film thickness of the photoconductor is reduced. There has been proposed an image forming apparatus configured to detect and correct that amount to keep the initial charged potential constant (see, for example, Patent Document 2). However, according to this proposed technique, there is a decrease in the surface potential of the photosensitive member due to a change in the resistance value of the charging roller, and therefore the surface potential of the photosensitive member is gradually decreased only by correction based on film reduction of the photosensitive member. In addition, no change over time due to deterioration of the sensitivity of the photosensitive member or increase in the potential (V _L ) of the exposed portion of the photosensitive member is performed.

  Here, conventionally, a machine equipped with a charger with low Vpp application or DC bias application is often mounted on a small printer, and the life of the photoconductor is short, so that the potential of the photoconductor due to film reduction of the photoconductor is short. Because the rise did not become a problem level, and the roller resistance value increased due to the change over time of the charging roller, and the phenomenon that the charging potential decreased was larger and dominant. It didn't matter so much.

In general, in an image forming apparatus, in order to correct the sensitivity deterioration of the photosensitive member and the increase in the potential (V _L ) of the photosensitive member exposed portion, after charging a predetermined number of sheets, the charging potential (non-exposed portion of the photosensitive member ( V ₀ ) is controlled so as to be higher than the initial charging potential (V ₀ ), and the conventional image forming apparatus cannot completely cancel out the photosensitive film due to the decrease in the charging potential due to the increase in the roller resistance value. Since the increase in the charging potential due to the decrease was within the range of these controls, there was no big problem.
JP-A 63-149668 JP-A-5-27557

  However, recently, the performance of the charging roller has been improved, and the resistance fluctuation due to the change with time of the charging roller has been reduced. Therefore, the potential increase due to the decrease in the film thickness of the photosensitive member cannot be offset by the potential decrease due to the increase in the resistance of the roller. The charged potential of the photosensitive member is increased, and there is a problem that the charged potential increasing range of normal aging correction is exceeded. Therefore, there is a problem that the density difference of the image appears remarkably in the initial stage and the life end of the photoreceptor.

  The present invention has been made to solve such a conventional problem, and can extend the life of the photoreceptor, and can obtain a stable image at the initial stage and the life end of the photoreceptor. An object is to provide an image forming apparatus.

  An image forming apparatus according to the present invention is an apparatus that forms an image by developing an electrostatic latent image formed on a photosensitive member by charging and exposing the photosensitive member, and contacts or rotates the rotating photosensitive member. A method of charging a photosensitive member by applying a voltage or a DC voltage superposed with an alternating current having a peak-to-peak voltage equal to or less than twice a discharge start voltage, provided with a charger arranged in a close state This image forming apparatus is characterized by comprising means for controlling the voltage applied to the charger based on the film thickness reduction of the photoreceptor and the accumulated operation time.

  In the present invention, “photoreceptor film thickness reduction” refers to a change in film thickness at which the film thickness of the photosensitive receiving layer of the photoconductor decreases.

  The operation of the image forming apparatus of the present invention will be described below.

  First, in an image forming apparatus that charges a photosensitive member by applying a DC voltage or a voltage superimposed with an alternating current whose peak-to-peak voltage (Vpp) is not more than twice the discharge start voltage to the charger, the photosensitive member is a film. When it decreases, there is a phenomenon in which the surface potential of the photoreceptor increases. This is because when the air gap between the charger (charging roller) and the surface of the photosensitive member and the photosensitive member are regarded as a capacitor, the air gap voltage increases due to the reduction of the film of the photosensitive member.

  If the increase in potential due to such film reduction (film change) of the photoconductor is within the control amount range based on the sensitivity deterioration of the photoconductor due to change over time, that is, within the charge potential increase range of normal time-dependent correction, the above-described case. Although there is no problem as described above, when the charged potential increase range of the normal aging correction is exceeded, the density difference of the image is greatly affected.

  Therefore, in the image forming apparatus according to the present invention, when the potential increase due to the reduction in the film thickness of the photoconductor exceeds a control amount range based on the sensitivity deterioration of the photoconductor, the applied voltage is lowered to correct the surface potential. By adopting such a configuration, the life of the photosensitive member can be extended. In addition, it is possible to prevent a phenomenon in which a difference in toner density of an image occurs at the initial stage and life end of the photoconductor.

  In the image forming apparatus according to the present invention, the information regarding the film thickness reduction of the photoconductor and the sensitivity deterioration of the photoconductor is obtained from the accumulated operation time of the photoconductor such as the cumulative exposure time of the photoconductor, the number of printed sheets, or the rotation amount of the photoconductor. Can be obtained.

  Further, if the decrease in the thickness of the photoconductor is detected by the rotation amount of the photoconductor, optimum control can be performed with a simple device, and a good image can be obtained.

An image forming apparatus according to the present invention is an apparatus that forms an image by developing an electrostatic latent image formed on a photosensitive member by charging and exposing the photosensitive member, and contacts or rotates the rotating photosensitive member. A method of charging a photosensitive member by applying a voltage or a DC voltage superposed with an alternating current having a peak-to-peak voltage equal to or less than twice a discharge start voltage, provided with a charger arranged in a close state In this image forming apparatus, [(amount of increase in potential due to decrease in film thickness of photoconductor) − (amount of increase in potential (V _L ) of the photosensitive member exposure portion)] is set as a control amount, and the control amount is applied to the charger. It is characterized by having means for reducing the voltage.

  The operation of the image forming apparatus of the present invention will be described below.

  First, in an image forming apparatus that charges a photosensitive member by applying a DC voltage or a voltage superimposed with an alternating current whose peak-to-peak voltage (Vpp) is not more than twice the discharge start voltage to the charger, the photosensitive member is a film. When it decreases, there is a phenomenon in which the surface potential of the photoreceptor increases. This is because when the air gap between the charger (charging roller) and the surface of the photosensitive member and the photosensitive member are regarded as a capacitor, the air gap voltage increases due to the reduction of the film of the photosensitive member.

  If the increase in potential due to such film reduction (film change) of the photoconductor is within the control amount range based on the sensitivity deterioration of the photoconductor due to change over time, that is, within the charge potential increase range of normal time-dependent correction, the above-described case. Although there is no problem as described above, when the charged potential increase range of the normal aging correction is exceeded, the density difference of the image is greatly affected.

Therefore, in the image forming apparatus according to the present invention, when the potential increase due to the film reduction of the photoconductor exceeds a control amount range based on the sensitivity deterioration of the photoconductor, )-(Increased potential (V _L ) of the photosensitive member exposed portion)] is set as a control amount, and control is performed to correct the surface potential by lowering the voltage applied to the charger by the control amount. By adopting such a configuration, the life of the photoreceptor can be extended. In addition, it is possible to prevent a phenomenon in which a difference in toner density of an image occurs at the initial stage and life end of the photoconductor.

In the present invention, if the potential (V _L ) rise of the photosensitive member exposed portion is measured with a surface potentiometer and the applied voltage is controlled using the measurement result, a stable potential condition is always created. And a good image can be obtained.

  In addition, if the decrease in the thickness of the photosensitive member is detected by the rotation amount of the photosensitive member, optimum control can be performed with a simple apparatus configuration. Therefore, a good image can be obtained also in this case. .

  According to the present invention, a voltage or a direct current voltage superimposed with an alternating current whose peak-to-peak voltage is twice or less the discharge start voltage is applied to a charger arranged in contact with or close to the rotating photoreceptor. In the image forming apparatus of the type that charges the photoconductor, the surface potential is decreased by lowering the applied voltage when the potential increase due to the reduction in the film thickness of the photoconductor exceeds the control amount range based on the sensitivity deterioration of the photoconductor. Since the correction is made, the life of the photoreceptor can be extended. In addition, it is possible to prevent a phenomenon such as a difference in toner density between images at the initial stage and at the life end of the photoconductor, and a good image can be obtained at all times.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  First, an image forming apparatus of the present invention is a digital image forming apparatus, and an image carrier (photosensitive member) is obtained by an electrophotographic process having charging, exposure, development, transfer, cleaning, fixing, and static elimination processes. It has a function of forming a toner image thereon and transferring it to a recording sheet (sheet), and can be applied to electrophotographic apparatuses such as copying machines, laser printers, and facsimile machines.

  A specific example of the image forming apparatus of the present invention will be described below.

<Embodiment 1>
FIG. 1 is a diagram schematically showing a configuration of an embodiment of an image forming apparatus of the present invention.

  The image forming apparatus X in this example is a monochrome printing type image forming apparatus, and is provided with a cylindrical photosensitive drum 1 as an image carrier. A charger 2, an exposure device 3, a developing unit 4, a transfer unit 5, and a cleaning member 7 are arranged in this order around the photosensitive drum 1 as a center.

  Further, between the photosensitive drum 1 and the transfer unit 5, a paper conveyance path for conveying a recording paper (PPC paper or the like) P as a recording medium is disposed. A fixing unit 6 is disposed on the downstream side of the photosensitive drum 1 when viewed from the conveyance direction Y of the sheet conveyance path.

  The photosensitive drum 1 is obtained by coating a photosensitive drum such as selenium (Se) or organic photoconductor (OPC) in a thin film on the outer peripheral surface of a metal drum such as aluminum as a base material.

  The charger 2 is for uniformly charging the surface of the photosensitive drum 1. As the charger 2, a charger using a minute gap discharge such as a contact roller charger or a brush charger, or a charger using an injection charging method in which the electrostatic capacity of the photosensitive member affects the charging potential of the photosensitive member. Can be used. In this example, a contact roller type charger 2 having a contact type charging roller 21 is used. The charger 2 is applied with a voltage superimposed with an alternating current whose direct current voltage or peak-to-peak voltage (Vpp) is twice or less the discharge start voltage by a voltage applying means 15 described later. Further, the voltage applied to the charger 2 is changed by the control described later.

  The exposure device 3 has a function of forming an electrostatic latent image corresponding to image data on the surface of the photosensitive layer 1 by exposing the charged photosensitive drum 1 with a laser beam or LED.

  The developing unit 4 develops the electrostatic latent image on the photosensitive drum 1 with toner to generate a toner image. In this example, the developing unit 4 uses a two-component developer. The developing unit 4 includes a developing roller 41 that supplies toner to the photosensitive drum 1, a power source that supplies a developing voltage to the developing roller 41, and the like (not shown).

  The transfer unit 5 is a unit that transfers a toner image onto a recording sheet P that is supplied from a sheet feeding device (not shown) and is conveyed in the Y direction, and includes a charger method and a roller method. Thereafter, the fixing unit 6 fixes the toner image on the recording paper P by heat melting.

  The cleaning unit 7 scrapes and collects the toner remaining on the photosensitive drum 1 after the toner image is transferred.

  In the image forming apparatus X having the above configuration, one image formation is completed by the series of operations of the respective units described above.

  Next, the characteristic part of the control system of the image forming apparatus X of this example will be described with reference to FIG.

  In the control system of FIG. 2, it is assumed that the time-dependent change of the photosensitive drum 1 is the accumulated rotation (rotation) time, which is measured (detected) using a timer / counter. Specifically, in the next printing operation stage after the counter 11 detects a predetermined number of printed sheets, information (number of sheets detection information) is supplied to the temporal change detecting device 12 and the charging member resistance detecting means 13.

  The temporal change detection device 12 obtains the film thickness change (photoreceptor film reduction) of the photosensitive drum 1 based on the information from the counter 11 and supplies the film thickness change information to the control means 14. The charging member resistance detection unit 13 obtains a change in the roller resistance value of the charger 2 based on the information from the counter 11 and supplies information on the change in the roller resistance value to the control unit 14. The change in the roller resistance value of the charger 2 is obtained from a graph as shown in FIG. 3, for example.

  Based on the information from the temporal change detection device 12 and the charging member resistance detection means 13, the control means 14 increases the potential due to the decrease in the film thickness of the photoreceptor, and decreases the charging potential due to the increase in the roller resistance value of the charger 2. If it exceeds the charging potential increase range of the normal correction with time, the voltage applying means 15 is instructed to decrease the voltage applied to the charger 2 in order to set the surface potential to a normal value. By controlling the voltage applied to the charger 2 as described above, it is possible to simultaneously perform potential increase correction due to film loss of the photosensitive member and sensitivity deterioration correction of the photosensitive member.

  The control of the applied voltage by the control means 14 may be performed by providing a dedicated control device, or may be performed by a control unit of the image forming apparatus.

<Embodiment 2>
FIG. 4 is a diagram schematically showing the configuration of another example of the image forming apparatus of the present invention.

The image forming apparatus X of this example is characterized in that the probe 81 of the surface potentiometer 8 is arranged between the exposure and development of the photosensitive drum 1 and the potential (V _L ) of the photosensitive member exposure part is measured. The other configuration is the same as the example of FIG.

In this example, in the control system shown in FIG. 3, the control means 14 is based on the information from the time-change detection device 12 and the charging member resistance detection means 13 and the measurement result of the surface electrometer 8 [ A command for lowering the voltage applied to the charger 2 by the control amount is set as a control amount (amount of increase in potential due to film reduction of the photoconductor)-(amount of increase in potential (V _L ) of the photosensitive member exposed portion)]. The voltage is applied to the voltage applying means 15. Therefore, also in this example, it is possible to simultaneously perform the potential increase correction due to the decrease in the film thickness of the photoconductor and the sensitivity deterioration correction of the photoconductor.

  In this example as well, the control by the control unit 14 may be performed by providing a dedicated control device, or may be performed by the control unit of the image forming apparatus.

<Embodiment 3>
FIG. 5 is a diagram schematically showing the configuration of another embodiment of the image forming apparatus of the present invention.

  An image forming apparatus 100 shown in FIG. 5 is a color tandem type image forming apparatus that forms multicolor and single color images on a recording sheet (sheet) in accordance with image data transmitted from the outside. Units 101a to 101d, developing units 102a to 102d, photosensitive drums 103a to 103d, chargers 105a to 105d, cleaner units 104a to 104d, transfer conveyance belt unit 108, fixing unit 113, sheet conveyance path S, sheet feed tray 110, and The paper discharge trays 115 and 118 are configured.

  Image data handled in the image forming apparatus 100 corresponds to a color image using each color of black (K), cyan (C), magenta (M), and yellow (Y). Therefore, as shown in FIG. 5, exposure units 101a, 101b, 101c, 101d, developing units 102a, 102b, 102c, 102d, photosensitive drums 103a, 103b, 103c, 103d, chargers 105a, 105b, 105c, 105d, Four cleaner units 104a, 104b, 104c, and 104d are provided to form four types of latent images corresponding to the respective colors (K, C, M, and Y). , Y), four image stations STa, STb, STc, STd are configured. In each code, “a” corresponds to black, “b” corresponds to cyan, “c” corresponds to magenta, and “d” corresponds to yellow.

  The photoconductive drums 103 a to 103 d are disposed at substantially the center of the image forming apparatus 100.

  The chargers 105a to 105d are charging means for uniformly charging the surfaces of the photoconductive drums 103a to 103d to a predetermined potential. The charging units 105a to 105d are contact roller type chargers having a contact type charging roller as shown in FIG. A vessel is used. The chargers 105a to 105d are applied with a voltage on which a direct current voltage or an alternating current whose peak-to-peak voltage (Vpp) is less than or equal to twice the discharge start voltage is superimposed by a voltage applying unit 205 described later. Further, the voltage applied to the charger is changed by the control described later. As the charger, another contact / proximity charger may be used.

  The exposure units 101a to 101d expose the charged photosensitive drums 103a to 103d in accordance with the input image data, whereby electrostatic charges corresponding to the image data are formed on the surfaces of the photosensitive drums 103a to 103d. It has a function of forming a latent image. For the exposure units 101a to 101d, for example, an EL or LED writing head in which light emitting elements are arranged in an array, or a laser scanning unit (LSU) including a laser irradiation unit and a reflection mirror is used.

  The developing units 102a to 102d are to visualize the electrostatic latent images formed on the respective photosensitive drums 103a to 103d with toners of respective colors (K, C, M, Y). The cleaner units 104a to 104d remove and collect toner remaining on the surfaces of the photosensitive drums 103a to 103d after development and image transfer.

  The transfer / conveying belt unit 108 is disposed below the photosensitive drums 103a to 103d. The transfer conveyance belt unit 108 includes a transfer belt 107, a transfer belt driving roller 171, a transfer belt tension roller 173, transfer belt driven rollers 172 and 174, transfer rollers 106a, 106b, 106c, and 106d, and a transfer belt cleaning unit 109. ing. These transfer belt drive roller 171, transfer belt tension roller 173, transfer rollers 106a to 106d, and transfer belt driven rollers 172 and 174 stretch the transfer belt 107 and rotate the transfer belt 107 in the direction of arrow B. It is.

  The transfer rollers 106 a to 106 d are rotatably supported by a transfer roller mounting portion (not shown) of the housing of the transfer belt unit 108, and attract the toner images on the photosensitive drums 103 a to 103 d onto the transfer belt 107. Thus, a transfer bias for transferring to the recording paper conveyed is provided.

  The transfer belt 107 is provided so as to be in contact with the photosensitive drums 103a to 103d. The toner images of the respective colors formed on the photosensitive drums 103a to 103d are sequentially transferred onto the recording paper, thereby transferring the color. A toner image (multicolor toner image) is formed. The transfer belt 107 is formed endlessly using a film having a thickness of about 100 to 150 μm.

  Transfer of the toner image from the photosensitive drums 103 a to 103 d to the recording paper is performed by transfer rollers 106 a to 106 d that are in contact with the back side of the transfer belt 107. To the transfer rollers 106a to 106d, a high voltage transfer bias (a high voltage having a polarity (+) opposite to the charging polarity (−) of the toner) is applied to transfer the toner image.

  The transfer rollers 106a to 106d are based on a metal (for example, stainless steel) shaft having a diameter of 8 to 10 mm, and the surface thereof is covered with a conductive elastic material (for example, EPDM, urethane foam, or the like). With this conductive elastic material, a high voltage can be uniformly applied to the recording paper. In the present embodiment, the transfer rollers 106a to 106d are used as the transfer electrodes, but a brush or the like may be used in addition thereto.

  Further, the toner adhering to the transfer belt 107 due to contact with each of the photoconductive drums 103a to 103d causes the back surface of the recording paper to become dirty, and is set to be removed and collected by the transfer belt cleaning unit 109. .

  The transfer belt cleaning unit 109 is provided with a cleaning blade 109a that comes into contact with the transfer belt 107, and a transfer of a portion (a lower portion between the image forming station STc and the image forming station STd) with which the cleaning blade 109a comes into contact. The belt 107 is supported by a transfer belt driven roller 174 from the back side.

  The paper feed tray 110 is a tray for storing recording paper used for image formation, and is provided below the image forming unit of the image forming apparatus 100. A paper discharge tray 115 provided on the side of the image forming apparatus 100 is a tray for placing image-formed recording paper face up. A paper discharge tray 118 provided at the top of the image forming apparatus 100 is a tray for placing printed recording paper face down.

  The image forming apparatus 100 is provided with a substantially “S” -shaped paper conveyance path S for sending the recording paper in the paper feed tray 110 to the paper discharge tray 118 via the transfer conveyance unit 108 and the fixing unit 113. ing. Further, a pickup roller 111, a registration roller 112, a fixing unit 113, a conveyance direction switching guide 114, and a conveyance roller for conveying a recording sheet are provided in a sheet conveyance path S from the paper feed tray 110 to the paper discharge tray 115 and the paper discharge tray 118. 117 etc. are arranged.

  The transport roller 117 is a small roller that promotes and assists transport of the recording paper, and a plurality of transport rollers 117 are provided along the paper transport path S. The pickup roller 111 is a calling roller that supplies recording sheets from the sheet feed tray 110 to the sheet conveyance path S one by one, and is provided at the end of the sheet feed tray 110.

  The conveyance direction switching guide 114 is rotatably provided on the side cover 116 of the image forming apparatus 100, and the recording paper is fed from the middle of the conveyance path S by changing the state shown by the solid line in FIG. The recording paper can be discharged to the paper discharge tray 115 after being separated. On the other hand, in the state indicated by the solid line in FIG. 5, the recording paper is conveyed between the fixing unit 113, the side cover 116, and the conveyance direction switching guide 114 (conveyance portion S ′ (part of the paper conveyance path S). ) To the upper discharge tray 118.

  The registration roller 112 temporarily holds the recording paper conveyed through the paper conveyance path S, and the photosensitive drum 103a is capable of satisfactorily transferring the toner images on the photosensitive drums 103a to 103d onto the recording paper. It has a function of conveying the recording paper in a timely manner in accordance with the rotation of ˜103d. That is, the registration roller 112 aligns the leading edge of the toner image on each of the photosensitive drums 103a to 103d with the leading edge of the image forming range on the recording sheet based on a detection signal output from a pre-registration detection switch (not shown). In addition, the recording paper is set to be conveyed.

  The fixing unit 113 includes a heat roller 113a and a pressure roller 113b. The heat roller 113a and the pressure roller 113b are configured to rotate with a recording sheet interposed therebetween.

  The heat roller 113a is set by a control unit (not shown) based on a signal from a temperature detector (not shown) to reach a predetermined fixing temperature, and heats the recording paper together with the pressure roller 113b. By pressure bonding, the multicolor toner image transferred to the recording paper is melted, mixed, and pressed, and thermally fixed to the recording paper.

  The recording paper after the fixing of the multicolor toner image is conveyed to the reverse paper discharge path of the paper conveyance path S by the conveyance roller 117 and is discharged in a reversed state (the multicolor toner image is directed downward). The paper is discharged onto the paper tray 118.

  In the image forming apparatus 100 described above, density patches (also referred to as “patch images”, “density patch images”, and “reference patch images”) are based on image data configured with a plurality of densities determined in advance for each color. Are formed on the photosensitive drums 103a to 103d. Thereafter, the image is transferred onto the transfer belt 107 separately.

  Here, the density patch transferred onto the transfer belt 107 is detected by a density sensor (not shown) of the toner density detection unit. The density sensor has a light emitting element and a light receiving element, irradiates the transfer belt 107 with light from the light emitting element, receives the diffuse reflected light with the light receiving element, and measures the amount of light. . When there is a density patch on the transfer belt 107, the light from the light emitting element is used to change the amount of diffuse reflected light received by the light receiving element according to the density of the density patch. Thus, the density of the density patch on the transfer belt 107 is detected. Based on the detection result, the control conditions (surface potential of the photosensitive drum, developing bias potential, transfer bias potential, power of the semiconductor laser light source, etc.) of each part (process part) of the image forming unit are adjusted. Furthermore, since the toner density control is performed after the charging potential of the photosensitive drum is adjusted in advance by a time-change detecting means or the like (details will be described later), the adjustment time can be shortened.

  Next, features of the control system of the image forming apparatus of this example will be described.

  In the image forming apparatus 100 of this example, the control system shown in FIG. 6 is provided in each image station STa, STb, STc, STd. In the following description, the control content of one image station (image forming unit) will be described. The photosensitive drums 103a to 103d are referred to as the photosensitive drum 103, and the chargers 105a to 105d are referred to as the charger 105.

  The control system of FIG. 6 has basically the same configuration as the example of FIG. 2, and the change with time of the photosensitive drum 103 is regarded as its accumulated rotation (rotation) time, which is measured (detected) using a timer / counter. It shall be. Specifically, in the next printing operation stage after the counter 201 detects a predetermined number of printed sheets, information (sheet number detection information) is supplied to the temporal change detection device 202 and the charging member resistance detection unit 203.

  The temporal change detection device 202 obtains a change in the film thickness of the photosensitive drum 103 (reduction in the film thickness of the photosensitive member) based on information from the counter 201 and supplies information on the change in film thickness to the control unit 204. Further, the charging member resistance detection unit 203 obtains a change in the roller resistance value of the charger 105 based on the information from the counter 201, and supplies information on the change in the roller resistance value to the control unit 204. The change in the roller resistance value of the charger 105 is obtained from a graph as shown in FIG. 3, for example.

  Based on the information from the time-change detecting device 202 and the charging member resistance detecting unit 203, the control unit 204 causes a potential increase due to a decrease in the film thickness of the photosensitive member to decrease a charging potential due to an increase in the roller resistance value of the charger 105. If it exceeds the charging potential increase range of the normal correction with time, the voltage applying means 205 is instructed to decrease the applied voltage to the charger 2 in order to set the surface potential to a normal value. By controlling the voltage applied to the charger 105 as described above, it is possible to simultaneously perform potential increase correction due to film loss of the photosensitive member and sensitivity deterioration correction of the photosensitive member.

  The control of the applied voltage by the control means 204 described above may be performed by providing a dedicated control device, or may be performed by a control unit of the image forming apparatus.

Here, the image forming apparatus 100 of FIG. 5 also includes a surface potential meter as shown in FIG. 4, that is, a surface potential meter 8 for measuring the potential (V _L ) of the photosensitive member exposure portion of each photosensitive drum 103, Based on the measurement result of the surface potential meter 8 and each information from the above-described time-change detecting device 202 and the charging member resistance detecting unit 203, the control unit 204 [[potential increase due to film reduction of the photoreceptor] − (Increased potential (V _L ) of photosensitive member exposed portion)] is set as a control amount, and a command is given to the voltage applying means 205 to decrease the voltage applied to the charger 105 by the control amount. Also good. By adopting such a configuration, it is possible to simultaneously perform a potential increase correction due to a decrease in the film thickness of the photoconductor and a sensitivity deterioration correction of the photoconductor.

<Example 1>
In this example, an example in which an increase in surface potential due to film reduction of the photosensitive member is controlled by lowering an applied voltage is shown.

  In this embodiment, an experiment was conducted using the monochrome image forming apparatus X shown in FIG. Specifically, DM-2515 manufactured by Sharp was used as the image forming apparatus X. The experiment was conducted under the basic conditions of DM-2515 except for the following process conditions such as bias.

  For the charger, a contact type charging roller was used. NBR and EPDM were used for the base layer of the charging roller, and the surface layer was a urethane-coated roller having an outer diameter of φ14 mm. The hardness was Asker C: 77 degrees, and the 10-point average roughness (Rz) of the charging roller was 5.6 μm. The roller resistance value of the charging roller was 7.0 power Ω when an applied voltage of −500 V was applied to the metal shaft in a state where the roller was pressed against the metal plate with a linear pressure of 0.313 N (32.0 gf) / cm. .

  In the above experimental apparatus, the above-mentioned charging roller was pressed against the photosensitive drum (linear pressure 0.313 N / cm), and the charging roller was rotated around the photosensitive drum.

  As for the bias applied to the charging roller, a DC voltage of −1180 V was applied to the charging roller shaft from a high voltage power source (not shown). At that time, the photoreceptor surface potential was -600 V, and the inflow current was -34 μA.

  In this experimental apparatus, when the copy count is 30000 copies, the applied voltage correction is performed to obtain the results shown in Table 1 below, which is as good as the initial state at the end of life (50000 copies). I was able to get an image.

<Comparative Example 1>
In Example 1, an aging test was performed using the same experimental apparatus and conditions as in Example 1 except that applied voltage correction due to changes over time (applied voltage correction at 30000 copy counts) was not performed. The results as shown in 1 were obtained. In the results of the aging test, when the initial image and 50,000 images were compared, it was noticeable that the overall image density was lowered.

＜実施例２＞
この例では、［（感光体の膜減りによる電位上昇分）−（感光体露光部の電位（Ｖ_L）上昇分）］を制御量として、その制御量分の印加電圧を下げる場合の例を示す。

<Example 2>
In this example, [(amount of increase in potential due to decrease in photosensitive member film) − (amount of increase in potential (V _L ) of the photosensitive member exposure portion)] is used as a control amount, and an example of a case where the applied voltage is decreased by the control amount. Show.

  In this embodiment, an experiment was performed using the color tandem type image forming apparatus 100 shown in FIG. Specifically, AR-C260 manufactured by Sharp was used as the image forming apparatus 100. In addition, as experimental conditions, the experiment was performed under the basic conditions of AR-C260 except for the following process conditions such as bias.

  For the charger, a contact type charging roller was used. NBR and EPDM were used for the base layer of the charging roller, and the surface layer was a urethane-coated roller having an outer diameter of φ14 mm. The hardness was Asker C: 80 degrees, and the 10-point average roughness (Rz) of the charging roller was 7.0 μm. The roller resistance value of the charging roller was 5.39 Ω when an applied voltage of −300 V was applied to the metal shaft in a state where the metal plate was pressed at a linear pressure of 0.29 N (29.6 gf) / cm. .

  In the above experimental apparatus, the above-mentioned charging roller was pressed against the photosensitive drum (linear pressure 0.087 N (8.9 gf) / cm), and the charging roller was rotated around the photosensitive drum.

  Regarding the bias applied to the charging roller, a DC voltage of -1230 V was applied to the shaft of the charging roller from a high voltage power source (not shown). At that time, the surface potential of the photosensitive member was −620 V, and the flowing current was −30 μA.

  In this experimental apparatus, when the copy count was 30000 sheets, the applied voltage correction was performed to obtain the results shown in Table 2 below, and a good image that was the same as the initial value could be obtained. Also, the first copy time was equivalent to the conventional machine.

<Comparative example 2>
In Example 2, an aging test was performed using the same experimental apparatus and conditions as in Example 2 except that the applied voltage correction due to changes over time (applied voltage correction when the copy count was 30,000) was not performed. Results as shown in 2 were obtained. As a result of the aging test, when the initial image and 50,000 images were compared with each other, it was confirmed that the entire image had shifted to a lighter density. Moreover, the result that the first copy time was also increased compared with the conventional machine was obtained.

＜実施例３＞
この例では、図４に示したモノクロの画像形成装置Ｘを用いた。具体的には、画像形成装置Ｘとして実施例１と同じ装置つまりシャープ製：ＤＭ−２５１５を使用し、感光体ドラムの露光−現像間に表面電位計８のプローブ８１（Ｔｒｅｋ社製 Ｍｏｄｅｌ５５５Ｐ−ＩＶ）を設置したこと以外は、実施例１と同じプロセス条件及びバイアス条件で検討を行った。

<Example 3>
In this example, the monochrome image forming apparatus X shown in FIG. 4 was used. Specifically, as the image forming apparatus X, the same apparatus as in Example 1, that is, Sharp: DM-2515 is used, and the probe 81 of the surface potential meter 8 (Model 555P-IV manufactured by Trek) is used between exposure and development of the photosensitive drum. ) Was examined under the same process conditions and bias conditions as in Example 1.

Table 3 below shows the measurement results of the transition of the photoreceptor surface potential and the increase in the potential (V _L ) of the exposed area of the photoreceptor. Then, the time-dependent potential correction control was performed for every 10,000 sheets, with [(amount of increase in potential due to decrease in film thickness of photoconductor) − (amount of increase in potential (V _L ) of the photosensitive member exposed portion)]] Even in the life end image (50000 sheets), a good image having the same copy density as the initial image could be obtained.

  The present invention relates to an electrophotographic image forming apparatus used for a copying machine, a laser printer, a facsimile, and the like, and more particularly, an image of a system in which a charging member is brought into contact with or close to a rotating photosensitive member to charge the surface of the photosensitive member. It can be effectively used for a forming apparatus.

1 is a diagram schematically showing a structure of an embodiment of an image forming apparatus of the present invention. FIG. 2 is a block diagram illustrating a configuration of a control system of the image forming apparatus in FIG. 1. It is a graph which shows a time-dependent change of roller resistance value. It is a figure which shows typically the structure of other embodiment of the image forming apparatus of this invention. It is a figure which shows typically the structure of another embodiment of the image forming apparatus of this invention. FIG. 6 is a block diagram illustrating a configuration of a control system of the image forming apparatus in FIG. 5.

Explanation of symbols

X image forming apparatus 1 photoconductor drum 2 charger 21 charging roller 3 exposure device 4 developing unit 5 transfer unit 6 fixing unit 7 cleaning member 8 surface potential meter 81 probe 100 image forming apparatus 101a to 101d exposure unit 102a to 102d developing unit 103a 103d Photoconductor drum 104a to 104d Cleaner unit 105a to 105d Charger 106a to 106d Transfer roller 107 Transfer belt 108 Transfer conveyance belt unit

Claims (4)

  An apparatus for developing an electrostatic latent image formed on a photoconductor by charging and exposing the photoconductor to form an image, the charging being arranged in contact with or close to the rotating photoconductor In the image forming apparatus of the type in which the photoconductor is charged by applying a voltage or a DC voltage on which an alternating current whose peak-to-peak voltage is not more than twice the discharge start voltage is applied to the charger. An image forming apparatus comprising: means for controlling a voltage applied to the charger based on the film thickness reduction and the accumulated operation time.   An apparatus for developing an electrostatic latent image formed on a photoconductor by charging and exposing the photoconductor to form an image, the charging being arranged in contact with or close to the rotating photoconductor In the image forming apparatus of the type in which the photosensitive member is charged by applying to the charger a voltage or a DC voltage on which an alternating current whose peak-to-peak voltage is twice or less the discharge start voltage is applied. And a means for lowering the voltage applied to the charger by the control amount. Image forming apparatus.   3. The image forming apparatus according to claim 2, further comprising a surface potentiometer that detects a potential of the photosensitive member exposure unit, wherein the applied voltage is controlled based on a detection result of the surface potentiometer. The image forming apparatus according to claim 1, wherein a film thickness reduction of the photoconductor is detected by a rotation amount of the photoconductor.

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