JP2011128345A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can form an image with high image quality over a long period of time by predicting a wear amount of a photosensitive layer on a surface of a photoreceptor accurately and controlling a charged voltage based on the predicted wear amount. <P>SOLUTION: When printing is executed, a driving time period t1 of each of photosensitive drums 1a to 1d and a charging bias applied time period t2 are counted for each of image forming sections Pa to Pd. Next, when a cumulative count value of t1 reaches ten minutes, a charging current (i) is detected by a charging current sensor 45. Then, an wear amount M of the photosensitive layer for every ten minutes driving of the drum is counted using an equation (1), and a cumulative wear amount ΣM which is obtained by accumulating the wear amount M is calculated and stored in a RAM 93. In accordance with an increase of the ΣM, a charging bias is corrected by referring to a shift voltage (correction value) with respect to a reference value of the charging bias from a charging bias correction table. <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.

  However, in the contact charging method, since the charging process on the surface of the photoreceptor is performed by discharge near the surface, the wear of the photosensitive layer is promoted. 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. In addition, when the wear of the photoconductor progresses, the pressure resistance characteristics of the photoconductor deteriorates. Therefore, when a high voltage is applied to the charging member, the photoconductive layer tends to cause local dielectric breakdown called a pinhole. This pinhole appears as a black spot on the image, which is a big problem in image quality. Therefore, in order to output a stable image over a long period of time, it is necessary to control the charging voltage according to the layer thickness (film thickness) of the photosensitive layer.

  Therefore, various methods for detecting the film thickness of the photosensitive layer during the use period of the image forming apparatus and controlling the process setting conditions have been proposed. For example, Patent Document 1 discloses a charging current value and a capacitance of the photosensitive member. The photosensitive layer thickness is calculated from the detected charging current value or the wear amount of the photosensitive layer is estimated from the usage time of the apparatus, and the thickness of the photosensitive layer is estimated. An image forming apparatus that controls a developing bias and a charging bias is disclosed.

  Patent Document 2 discloses an image forming apparatus that corrects a detection current amount based on a time transition such as a use state of an image forming apparatus or a change in installation environment, and controls process setting conditions according to the corrected detection result. Is disclosed. In Patent Document 3, data relating to the film thickness of the photosensitive layer determined in accordance with the temperature and inflow current of the photoconductor is stored in a storage unit, and the inflow current to the photoconductor and the photoconductor to be actually detected are stored. An image forming apparatus is disclosed that calculates the film thickness of the photosensitive layer based on the temperature and the data stored in the storage means.

  Further, in Patent Document 4, the coefficient K is selected on the condition of the discharge current value, the wear value W of the image carrier is calculated by a function using K as a coefficient, and the calculated sum is calculated by integrating the wear value W. An image carrier lifetime detection device and an image forming apparatus that determine the lifetime of an image carrier based on a wear value S are disclosed.

JP-A-10-246994 JP 2004-334063 A JP 2006-208477 A JP 2005-128150 A

  However, in the method of Patent Document 1, if the charging current can be detected, the film thickness is required. However, in actuality, the film thickness is the same due to the environmental temperature or light fatigue of the photosensitive layer during continuous printing. However, it was difficult to predict the film thickness because the charging current changed greatly. In addition, regarding the relationship between the usage time and the amount of wear of the photosensitive layer, if the voltage at the time of image formation is always applied during driving of the photosensitive member, the film thickness can be predicted by referring to the detection value of the charging current in the table. However, since the time during which the photosensitive member is rotated in a state where the charging voltage is not applied differs depending on the use state, it is difficult to predict the film thickness only by the use time.

  Further, in the method of Patent Document 2, in order to derive an approximate line from a large number of film thickness data calculated by the detected current and correct the film thickness, the memory capacity is increased and a large number of data is stored. There was a problem that it was necessary and led to the cost increase of an apparatus. Further, the method of Patent Document 3 requires temperature detection means and surface potential detection means for accurately detecting the temperature and surface potential of the photoreceptor, and particularly in a color image forming apparatus using a plurality of photoreceptors, the cost is increased. It leads to.

  Further, in Patent Document 4, since a voltage obtained by superimposing an AC voltage on a DC voltage is applied to the charging roller, the value of the discharge current flowing between the image carrier and the charging roller, the frequency of the AC voltage (charging frequency), and the like. Based on the above, the coefficient K is determined. A table for determining K is stored in advance in the storage element (paragraph [0085], FIG. 6). For this reason, the discharge current value corresponding to the value of K has a predetermined width (range), and the optimum value of the coefficient K according to a minute change in the discharge current value cannot be determined. The value W could not be calculated with high accuracy.

  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 accurately predicts the wear amount of the photosensitive layer on the surface of the photoreceptor and controls the charging voltage based on the predicted wear amount to form a high-quality image over a long period of time. An object of the present invention is to provide an image forming apparatus capable of performing the above.

  In order to achieve the above object, the present invention provides an image carrier having a photosensitive layer formed on the surface thereof, and applying a charging bias consisting of only a DC voltage in a contact or non-contact state to the surface of the image carrier. A charging member that charges the surface of the image carrier, a voltage applying unit that applies a DC voltage to the charging member, and a charge that flows between the image carrier and the charging member when a charging bias is applied by the voltage applying unit. Current detection means for detecting current, drive time t1 of the image carrier, storage means for storing charging bias application time t2 to the charging member, and drive time of the image carrier stored in the storage means The wear amount M of the photosensitive layer is calculated on the basis of t1, the charging bias application time t2 to the charging member, and the charging current i detected by the current detecting means, and the wear amount M is accumulated. Ma And control means for correcting the reference value charging bias applied to the charging member according to the amount ΣM an image forming apparatus having a.

  According to the present invention, in the image forming apparatus configured as described above, the control unit stores the calculated integrated wear amount ΣM in the storage unit, and the integrated wear amount ΣM increases by a predetermined amount from the previous correction of the charging bias. It is characterized in that the charging bias is corrected every time.

In the image forming apparatus having the above-described configuration, the wear amount M is calculated by the following equation (1).
M = r × t1 + s × i × t2 (1)
However,
r: wear rate per unit time when the image bearing member is driven at a predetermined linear velocity when no charging bias is applied s: unit time when the image carrier is driven at a predetermined linear velocity, This is the wear rate per unit charging current.

  According to the present invention, in the image forming apparatus configured as described above, the storage unit stores a plurality of wear rates s corresponding to environmental conditions inside the apparatus.

  According to the present invention, in the image forming apparatus configured as described above, the storage unit stores a plurality of wear rates r and s corresponding to the linear velocity of the image carrier.

  According to the present invention, in the image forming apparatus having the above-described configuration, a plurality of sets of the image carrier and the charging member are provided, and each charge is determined based on the wear amount M and the accumulated wear amount ΣM calculated for each image carrier. The charging bias applied to the member is individually controlled.

  According to the present invention, in the image forming apparatus having the above-described configuration, the charging bias is set lower than a dielectric breakdown occurrence voltage determined according to an initial layer thickness of the photosensitive layer and the accumulated wear amount ΣM. .

  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, when a charging bias consisting of only a DC voltage is applied to the charging member, the wear amount is based on the driving time t1 of the image carrier, the charging bias application time t2, and the charging current i. By calculating M and integrating the calculated wear amount M to calculate the integrated wear amount ΣM, in addition to the wear amount during printing that is affected by the charging current, non-printing that eliminates the effect of the charging current The amount of wear can be calculated. Further, by controlling the charging bias based on the calculated integrated wear amount ΣM, it is possible to determine an appropriate charging bias according to the wear amount of the photosensitive layer.

  Further, according to the second configuration of the present invention, in the image forming apparatus having the first configuration, the calculated accumulated wear amount ΣM is stored in the storage unit, and the accumulated wear amount ΣM is equal to the previous charging bias. By correcting the charging bias every time it increases by a predetermined amount from the time of correction, correction is entered at an appropriate interval according to the change in layer thickness, so the waiting time increases due to the correction interval being too short or the correction interval is too long It is possible to suppress charging problems caused by the above.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the physical wear amount represented by the product of the wear rate r and the drive time t1 of the image carrier is obtained. , Wear rate s, charging current i, charging bias application time t2 represented by the product of wear amount increase by charging bias application separately calculated and added, for example, a large amount of printing in a short time The wear amount M can be calculated with high accuracy both in the case of performing continuously and in the case of performing a small amount of printing many times.

  According to the fourth configuration of the present invention, in the image forming apparatus having the third configuration, a plurality of wear rates s corresponding to the environmental conditions inside the apparatus are stored, so that the environmental conditions inside the apparatus are stored. Regardless of this, the prediction accuracy of the wear amount becomes higher.

  According to the fifth configuration of the invention, in the image forming apparatus having the third or fourth configuration, a plurality of wear rates r and s corresponding to the linear velocity of the image carrier are stored. The amount of wear in various modes can be predicted with higher accuracy.

  According to the sixth configuration of the present invention, when the image forming apparatus having any one of the first to fifth configurations includes a plurality of sets of image carriers and charging members, the calculation is performed for each image carrier. By appropriately controlling the charging bias for each charging member based on the wear amount M and the accumulated wear amount ΣM, for example, an appropriate charging bias is applied according to the use state of each image carrier in the tandem type color image forming apparatus. be able to.

  According to the seventh configuration of the present invention, in the image forming apparatus having any one of the first to sixth configurations, the charging bias is determined according to the initial layer thickness of the photosensitive layer and the accumulated wear amount ΣM. By setting the voltage lower than the dielectric breakdown voltage, pinholes can be reliably prevented.

  According to the eighth configuration of the present invention, in the image forming apparatus having any one of the first to seventh configurations, an appropriate charging bias can be applied to an image carrier that is an organic photosensitive layer in which the photosensitive layer is easily worn. Can be applied to reduce the risk of pinholes.

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. A graph showing the relationship between the photosensitive drum drive time t1 and the photosensitive layer wear amount M during image formation and non-image formation A graph showing the relationship between the photosensitive drum driving time x1 and the photosensitive layer wear amount y1 when continuous printing is performed using a scorotron charging device. Wear due to discharge by subtracting the wear amount at 0 μA from the wear amount of the photosensitive layer per unit time (100 minutes) for each current value and the charging current x2 when continuous printing is performed using a contact charging type charging device. Graph showing the relationship with the amount of increase y2 7 is a flowchart showing an example of charging bias control in the image forming apparatus of the present invention. FIG. 7 is a graph showing the relationship between the driving time of the photosensitive drum and the wear amount of the photosensitive layer when continuous printing is performed with the charging bias ON and when the image forming apparatus is driven with the charging bias OFF.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an image forming apparatus equipped with the developing device of the present invention. Here, a tandem color image forming apparatus is shown. In the main body of the color printer 100, four image forming portions Pa, Pb, Pc, and Pd are arranged in order from the right side in FIG. 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 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 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 drives a secondary transfer roller 9 and an intermediate transfer belt 8 described later via a paper feed roller 12a and a registration roller pair 12b. It is conveyed to the nip between the rollers 11. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a (seamless) belt having no seam is mainly used. Further, a blade-like belt cleaner 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side in the moving direction of the intermediate transfer belt 8 when viewed from the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are charging devices 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d, which are rotatably arranged. The exposure unit 4 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning devices 5a, 5b, 5c and 5d are provided.

  When image data is input from a host device such as a personal computer, 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 according to the image data by the exposure unit 4. The electrostatic latent images corresponding to the image data are formed on the respective photosensitive drums 1a to 1d. Each of the developing devices 3a to 3d is filled with a predetermined amount of a two-component developer containing toner of each color of cyan, magenta, yellow, and black. In addition, when the ratio of the toner in the two-component developer filled in each of the developing devices 3a to 3d is less than a predetermined value due to the formation of a toner image, which will be described later, each developing device 3a to The toner is replenished in 3d. The toner in the developer is supplied onto the photosensitive drums 1 a to 1 d by the developing devices 3 a to 3 d 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 between an upstream conveyance roller 10 and a downstream drive roller 11, and the intermediate transfer belt 8 rotates clockwise as the drive roller 11 is rotated by a drive motor (not shown). When the rotation starts, the transfer paper P is conveyed from the registration roller pair 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the full color image is transferred. 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 by the fixing roller pair 13 so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full color 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 on only one side of the transfer paper P, the image is directly discharged onto the discharge tray 17 by the discharge roller pair 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 pair 15 to the outside of the apparatus. Thereafter, the transfer paper P is distributed to the paper conveyance path 18 by the branching section 14 by rotating the discharge roller pair 15 in the reverse direction, and is re-conveyed 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. The paper is discharged from the discharge roller pair 15 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 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. Specifically, the driving time t1 of each of the photosensitive drums 1a to 1d, the charging bias application time t2 to each of the charging devices 2a to 2d, the wear rates r and s of the photosensitive layer described later, and these are used to calculate. In addition to the photosensitive layer wear amount M and the cumulative wear amount ΣM, a charging bias correction table storing the relationship between the shift amount (correction value) with respect to the photosensitive layer wear amount (layer thickness) is also stored. The counter 95 integrates and counts the number of printed sheets, the driving time t1, and the charging bias application time t2.

  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 the personal computer or the like by 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. A predetermined bias is applied to the charging devices 2a to 2d, the magnetic roller 27, the developing roller 29, the primary transfer rollers 6a to 6d, and the secondary transfer roller 9 in accordance with a control signal from the control circuit 41. The charging current sensor 45 detects a current value flowing between the charging roller 21 and the photosensitive drums 1 a to 1 d when a charging bias is applied to the charging roller 21, and the detection result is transmitted to the control unit 90.

  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.

  Next, a method for predicting the wear amount of the photosensitive layer in the image forming apparatus of the present invention will be described. In the image forming apparatus provided with the contact-type charging devices 2 a to 2 d as shown in FIG. 1, the image forming apparatus drives the apparatus with the charging bias applied to the charging roller 21, and the charging bias is applied to the charging roller 21. The amount of wear (wear rate) per unit time of the photosensitive layer formed on the surface of each of the photosensitive drums 1a to 1d differs depending on the non-image formation in which the apparatus is driven in a state where no toner is applied.

  The main factor that the photosensitive drums 1a to 1d wear is physical wear caused by the intermediate transfer belt 8 and the cleaning blade 31 that are in contact with the photosensitive drums 1a to 1d. The physical wear includes high-hardness fine particles such as titanium oxide as a toner external additive by the contact members such as the intermediate transfer belt 8 and the cleaning blade 31 being pressed against the photosensitive drums 1a to 1d with a predetermined pressure. The photosensitive drums 1a to 1d rotate with a difference in linear velocity with respect to the contact member in a state where the developer is interposed, and friction occurs between the contact member and the photosensitive drums 1a to 1d by the rotation of the photosensitive drums 1a to 1d. Occurs.

  During image formation, in addition to the above physical wear, wear is promoted by a charging current flowing from the charging roller 21 to the photosensitive drums 1a to 1d. Therefore, the wear rate of the photosensitive layer during image formation to which a charging bias is applied is larger than the wear rate of the photosensitive layer during non-image formation without applying a charging bias. As will be described later, the wear rate when a charging bias consisting of only a DC voltage is applied is proportional to the charging current i flowing between the photosensitive drum and the charging device.

  Therefore, the wear rate of the photosensitive layer per unit time without applying a charging bias (hereinafter referred to as wear rate r), and the wear rate per unit time and unit charging current when applied with a charging bias (hereinafter referred to as wear). (Referred to as rate s), the physical wear amount calculated by the product of the photosensitive drum driving time and the wear rate r, the charging bias application time, the charging current i, and the wear rate s. The wear amount of the photosensitive layer can be predicted by adding the increase in the wear amount due to the application of the charging bias calculated by the product.

That is, assuming that the photosensitive drum driving time is t1 and the charging bias application time is t2, the wear amount M of the photosensitive layer is expressed by the following equation (1).
M = r × t1 + s × i × t2 (1)

  Since t1 = t2 at the time of image formation, the wear amount M of the photosensitive layer is equal to the physical wear amount (r × t1) and the increase in wear due to discharge (s) as shown in FIG. Xi * t2). On the other hand, since t2 = 0 at the time of non-image formation, the wear amount M of the photosensitive layer is only the physical wear amount (r × t1) as shown in FIG.

  Then, the wear amount M for each arbitrary time is calculated a plurality of times (M1, M2,...) During the use period of the image forming apparatus, and M is added to calculate the integrated wear amount ΣM. A calculation example of the integrated wear amount ΣM is shown in Table 1. In Table 1, the wear amount M is calculated for 10 times M1 to M10 after completion of the job in which the integrated value of the drive time t1 exceeds 10 minutes, and the integrated wear amount ΣM is obtained by integrating the calculation results of M1 to M10. ing.

  Note that the specification of the initial layer thickness of the photosensitive layer is determined (for example, 36 ± 2 μm), and the reference value of the charging bias is set so that the surface of the photosensitive drums 1a to 1d has a target surface potential under predetermined environmental conditions. It is set in the formation portions Pa to Pd. Then, the charging bias applied to the charging roller 21 is controlled so as not to exceed the dielectric breakdown occurrence voltage even when the initial layer thickness is the lower limit value of the specification in accordance with the obtained cumulative wear amount ΣM of the photosensitive layer. By doing so, it is possible to prevent the occurrence of pinholes in the photosensitive layer and to output a stable image over a long period of time. Hereinafter, a specific method for obtaining the wear rates r and s will be described.

(How to find the wear rate r)
The contact type charging devices 2a to 2d in the image forming apparatus in FIG. 1 are changed to scorotron type charging devices, and the amount of wear with respect to the printing time when continuous printing is performed is plotted. When a scorotron type charging device is used, an increase in the amount of wear due to discharge is almost negligible. Therefore, the wear amount of the photosensitive layer is considered to be due to physical wear by the contact member (intermediate transfer belt 8, cleaning blade 31), and if there is no significant change in the contact state of each member, the wear amount is driven by the photosensitive drum. Proportional to time.

  FIG. 5 shows the photosensitive drum drive time x1 (min) on the horizontal axis and the photosensitive layer wear amount y1 (μm) on the vertical axis, and the photosensitive drum is rotated at a predetermined linear velocity (for example, 168 mm / s). This is a graph when continuous printing is performed. Since the graph shown in FIG. 5 is a straight line represented by the linear expression y1 = 0.0005 × 1, the inclination 0.0005 is a wear rate r (one minute when the photosensitive drum is driven at a linear speed of 168 mm / s). Equivalent to the amount of abrasion of the photosensitive layer).

(How to find the wear rate s)
In the image forming apparatus of FIG. 1, a constant current (for example, 0 μA, 30 μA, 50 μA, and 70 μA) is applied between the charging roller 21 of the charging devices 2 a to 2 d and the photosensitive drums 1 a to 1 d to photosensitive drums 1 a to 1 d. Is rotated at the same speed as that during image formation. In the middle, in order to prevent the cleaning blade 31 from being turned over, toner is replenished as a lubricant as necessary. Then, the thickness of the photosensitive layer is measured in the initial state and after a predetermined time, and the amount of wear per unit time for each current value is calculated.

  FIG. 6 shows an increase y2 in wear amount due to discharge obtained by subtracting the wear amount at 0 μA from the wear amount of the photosensitive layer per unit time (100 minutes) for each current value with the charging current x2 (μA) on the horizontal axis. (μm / 100 minutes) is a graph in which the vertical axis indicates the photosensitive drum and the photosensitive drum is rotated at a predetermined linear velocity (for example, 168 mm / s) for continuous printing. Since the graph shown in FIG. 6 is a straight line represented by the linear expression y2 = 0.003 × 2 + 0.0056, the unit charge when the photosensitive drum is driven at a linear speed of 168 mm / s for 100 minutes with an inclination of 0.003. This corresponds to an increase in the amount of wear per current (1 μA). Therefore, the wear rate s (the increase in wear per minute / μA when the photosensitive drum is driven at a linear speed of 168 mm / s) is 0.003 / 100 = 0.00003.

  By calculating the wear amount of the photosensitive layer by the above-described method, in the color printer as shown in FIG. 1, for example, during monochrome printing operation, a charging bias is applied only to the charging device 2d of the image forming unit Pd, and the image forming unit Even when the charging bias is not applied to the charging devices 2a to 2c of Pa to Pc, the wear amount of the photosensitive layer of the photosensitive drums 1a to 1c excluding the influence of the charging current can be accurately calculated.

  Further, when a large amount of printing is continuously performed in a short time (drum driving time t1≈charging bias application time t2) and when a small amount of printing is performed many times (drum driving time t1> charging bias application time t2). In either case, the wear amount can be calculated with high accuracy. Further, since the wear amount can be calculated if the discharge amount (≈charging current) from the charging roller 21 to the photosensitive drums 1a to 1d is known, the wear amount can be calculated accurately regardless of the use environment.

  In particular, in a high humidity environment, if the surface of the charging roller 21 absorbs moisture, the rate of charge injection from the contact portions with the photosensitive drums 1a to 1d increases, so the wear rate s with respect to the detected current value decreases. Therefore, a plurality of wear rates s are stored in advance corresponding to the use environment of the apparatus, particularly humidity conditions, and the wear rate is changed by changing the wear rate s according to the humidity inside the apparatus detected by the humidity detection sensor. If calculated, the prediction accuracy of the wear amount becomes higher.

  FIG. 7 is a flowchart illustrating an example of charging bias control in the image forming apparatus of the present invention. The charging bias control procedure based on the predicted value of the layer thickness of the photosensitive layer will be described along the steps of FIG. 7 with reference to FIGS. When printing is executed in the color printer 100 shown in FIG. 1, the driving time t1 and the charging bias application time t2 of the photosensitive drums 1a to 1d are counted for each of the image forming portions Pa to Pd (step S1). The counting of t1 and t2 is continued until the integrated count value of t1 reaches 10 minutes (step S2), and when it reaches 10 minutes, the charging current in the full speed mode (for example, 168 mm / s) is detected by the charging current sensor 45. (Step S3).

  The charging current detected in step S3 is regarded as the charging current i during the charging ON time for 10 minutes of drum driving. In addition, when image formation is performed in a mode different from the full speed mode, such as 1/2 line speed and 3/4 line speed, current detection is performed only in the full speed mode, and the integrated count value is 1/2, 3/4, etc. The drum driving time t1 and the charging bias application time t2 are corrected by multiplying by the coefficient. t1 and t2 are stored in the ROM 93.

  Then, the wear amount M of the photosensitive layer every 10 minutes of drum driving is calculated using the equation (1) (step S4), and t1 and t2 are reset (step S5). Further, the calculated wear amount M is integrated to calculate an integrated wear amount ΣM (step S6), and ΣM is stored in the RAM 93 (step S7). Next, it is determined whether or not ΣM has increased by 1 μm (step S8). If ΣM has increased by 1 μm, the charging bias is corrected by referring to the charging bias correction table with reference to the shift voltage (correction value) for the charging bias reference value. (Step S8), and this is reflected in the application of the charging bias at the next and subsequent image formation. Specifically, the charging bias is set to a voltage value that is lower than the dielectric breakdown occurrence voltage determined according to the layer thickness of the photosensitive layer and that can obtain a desired drum surface potential.

  After correcting the charging bias, the process returns to step S1, and the wear amount M is calculated in the same procedure, added to the accumulated wear amount ΣM stored in the RAM 93, and overwritten and stored. The charging bias is corrected every time 1 μm is increased. Note that the optimum value of the charging bias varies depending on factors other than the layer thickness of the photosensitive layer, such as temperature and humidity conditions inside the apparatus, so the actual charging bias is determined by adding shift voltages determined by a plurality of factors.

  By controlling the charging bias as described above, a necessary and sufficient charging bias is applied to maintain the surface potential of the photosensitive drum constant according to the layer thickness of the photosensitive layer even when the photosensitive layer is worn. be able to. Therefore, an image forming apparatus capable of stably outputting a high-quality image free from black spots due to pinholes in the photosensitive layer and image fogging due to a decrease in drum surface potential over a long period of time. Moreover, since the occurrence of pinholes in the photosensitive layer is also suppressed, it contributes to the extension of the life of the photosensitive drum.

  Further, if the drum driving time t1, the charging bias application time t2, the accumulated wear amount ΣM, and the wear rates r and s are stored, the charging bias control based on the wear amount of the photosensitive layer becomes possible, and these are stored. The capacity of the RAM 93 can be reduced.

  In addition, when the line speed is different from the normal line speed (full speed mode) such as 1/2 line speed, 3/4 line speed, etc., when the charging condition is largely changed from the normal line speed, or depending on the use mode of the user, the normal line It is also conceivable that the frequency of use is high at a linear speed different from the speed. In such a case, by setting a plurality of values of the wear rates r and s for each linear velocity, the wear amount M is calculated separately for each job with a different linear velocity. Can be predicted with higher accuracy.

  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 addition, the cumulative count value of the driving time when calculating the wear amount M of the photosensitive layer and the increase width of the cumulative wear amount ΣM that becomes a correction trigger for the charging bias are merely examples, and the characteristics of the photosensitive layer, the initial layer thickness, etc. It can be appropriately changed depending on the situation.

  In the above-described embodiment, the charging roller 21 is configured to contact the surfaces of the photosensitive drums 1a to 1d to charge the surfaces of the photosensitive drums 1a to 1d, but the charging roller 21 is configured to charge the photosensitive drum 1a. In the embodiment arranged to be close to the surface of 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, and various image forming apparatuses such as an analog or digital monochrome copying machine, a monochrome printer, a rotary or tandem color copying machine, a facsimile, and the like. Of course, it can be applied to. Hereinafter, the effects of the present invention will be described in more detail with reference to examples.

  Using the image forming apparatus shown in FIG. 1, the wear amount of the photosensitive layer when the continuous printing is actually performed, or when the driving is performed with the charging bias OFF, and the calculation is performed using the equation (1). The predicted amount of wear was compared. As a test method, the charging current and the thickness of the photosensitive layer were measured every predetermined time, and the transition of the measured value of the wear amount with respect to the driving time of the photosensitive drum was plotted. On the other hand, a predicted wear amount is calculated using Equation (1) from the predetermined wear rates r and s (here, r = 0.0005, s = 0.00003) and the measured charging current i. And plotted in the same manner as the actually measured values. The results are shown in FIG.

  As shown in FIG. 8, the predicted value of the amount of wear of the photosensitive layer (indicated by □ in the figure) calculated by the equation (1) and the actual amount of wear of the photosensitive layer (indicated by ● in the figure) are continuous. It can be seen that there is a good match both when printing (shown with a solid line in the figure) and when charging bias is OFF (shown with a broken line in the figure). From this result, it was confirmed that the wear amount of the photosensitive layer can be accurately predicted using the method of the present invention, and the charging bias can be appropriately controlled based on the predicted value of the wear amount.

  The present invention can be used in an image forming apparatus including a charging member that charges a drum surface by applying a charging bias including only a DC voltage to the surface of the photosensitive drum in a contact or non-contact state. By utilizing the present invention, the amount of wear of the photosensitive layer can be accurately predicted, and the occurrence of pinholes can be effectively suppressed by applying an appropriate charging bias according to the layer thickness of the photosensitive layer. It is possible to provide an image forming apparatus capable of promoting the life extension.

  In particular, when applied to an image forming apparatus equipped with an organic photoconductor such as OPC where the photosensitive layer is subject to wear, the risk of pinholes associated with wear of the photosensitive layer is reduced, the life of the apparatus is extended, and the output image quality is improved. In addition, it is possible to 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 5a to 5d cleaning device 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)
45 Charging current sensor (current detection means)
90 Control unit (control means)
93 RAM (storage means)
95 counter 100 color printer

Claims (8)

An image carrier having a photosensitive layer formed on the surface;
A charging member that charges 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;
Current detecting means for detecting a charging current flowing between the image carrier and the charging member when a charging bias is applied by the voltage applying means;
Storage means for storing a driving time t1 of the image carrier and a charging bias application time t2 to the charging member;
The wear amount M of the photosensitive layer is calculated based on the driving time t1 of the image carrier stored in the storage means, the charging bias application time t2 to the charging member, and the charging current i detected by the current detection means. An image forming apparatus comprising: control means for correcting a charging bias applied to the charging member from a reference value according to an accumulated wear amount ΣM obtained by calculating and accumulating the wear amount M.   The control means stores the calculated integrated wear amount ΣM in the storage means, and corrects the charging bias every time the integrated wear amount ΣM increases by a predetermined amount from the previous correction of the charging bias. The image forming apparatus according to claim 1. The image forming apparatus according to claim 1, wherein the wear amount M is calculated by the following formula (1).
M = r × t1 + s × i × t2 (1)
However,
r: wear rate per unit time when the image bearing member is driven at a predetermined linear velocity when no charging bias is applied s: unit time when the image carrier is driven at a predetermined linear velocity, This is the wear rate per unit charging current.   The image forming apparatus according to claim 3, wherein the storage unit stores a plurality of wear rates s corresponding to environmental conditions inside the apparatus.   5. The image forming apparatus according to claim 3, wherein the storage unit stores a plurality of wear rates r and s corresponding to a linear velocity of the image carrier.   A plurality of sets of the image carrier and the charging member are provided, and the charging bias applied to each charging member is individually controlled based on the wear amount M and the accumulated wear amount ΣM calculated for each image carrier. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   7. The charging bias according to claim 1, wherein the charging bias is set lower than a dielectric breakdown generation voltage determined according to an initial layer thickness of the photosensitive layer and the accumulated wear amount ΣM. Image forming apparatus.   The image forming apparatus according to claim 1, wherein the photosensitive layer is an organic photosensitive layer.

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