JP2007279277A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form an image with good image quality while surely preventing discharge from occurring by appropriately setting a reverse contrast potential difference. <P>SOLUTION: By controlling a potential difference (reverse contrast potential difference) between the average potential of developing bias and potential at the non-exposure part of a photoreceptor for carrying an electrostatic latent image, the image is formed with the good image quality while preventing the discharge from occurring. The reverse contrast potential difference is set to the minimum, first (step S101), and increased by one step so as to detect the discharge (steps S102 to S104), and the minimum potential difference (discharge start reverse contrast potential difference) at which the discharge occurs is obtained and stored (step S105). The output of a temperature and humidity sensor is read in (step S106) and a value obtained by subtracting voltage margin in accordance with the read result from the discharge start reverse contrast potential difference is set as the control target of the reverse contrast potential difference (step S107). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  In the present invention, a charging means to which a charging bias is applied is brought into contact to expose the surface of a photosensitive member charged to a predetermined surface potential to form an electrostatic latent image. The present invention relates to an image forming apparatus and an image forming method for forming an image by developing an electrostatic latent image with toner by applying a developing bias.

  A photosensitive member charged to a predetermined surface potential is exposed to form an electrostatic latent image, and an AC developing bias is applied to a toner carrier disposed opposite to the photosensitive member to visualize the electrostatic latent image with toner. In an image forming apparatus for forming an image, the relationship between the average potential of the developing bias and the surface potential of the photosensitive member has a great influence on the image quality. More specifically, the difference between the potential of the exposed exposed portion of the photoreceptor surface and the average potential of the developing bias (hereinafter referred to as “contrast potential difference”) affects the image density. The difference between the potential of the unexposed area that has not been exposed and the average potential of the developing bias (hereinafter referred to as “reverse contrast potential difference”) affects the amount of toner fogging on the image, as well as the amount of toner scattered inside the apparatus. Also related to quantity.

  Thus, for example, in the image forming apparatus described in Patent Document 1, a patch image is formed while changing and setting the development bias in multiple stages, and development is performed that gives a contrast potential difference for obtaining a desired image density based on the density detection result. By obtaining the optimal value of the bias and setting the charging bias so that the reverse contrast potential difference becomes a constant value, both the contrast potential difference and the reverse contrast potential difference are maintained in a preferable state so as to obtain a stable image quality. Yes.

Japanese Patent Laying-Open No. 2003-215863 (FIG. 8)

  The above prior art has the following problems to be solved. That is, in this type of image forming apparatus, in particular, an apparatus in which the photosensitive member and the toner carrying member are opposed to each other with a gap therebetween, a discharge occurs between the photosensitive member and the toner carrying member, thereby reducing the image quality. It may end up. Therefore, in order to perform stable image formation, it is first necessary to reliably prevent such discharge from occurring. From the viewpoint of preventing discharge, it is preferable to reduce the reverse contrast potential difference. On the other hand, in terms of image quality, it is preferable to take the reverse contrast potential difference as large as possible. In the above prior art, since the reverse contrast potential difference is always a constant value, these conflicting requirements have not been achieved at a high level.

  The present invention has been made in view of the above problems, and provides a technique capable of forming an image with excellent image quality while reliably preventing the occurrence of discharge by appropriately setting the reverse contrast potential difference. For the purpose.

  Hereinafter, in this specification, the potential difference between the potential of the unexposed portion of the surface of the photoconductor that is not exposed and the average potential of the developing bias at the position where the photoconductor and the toner carrier are opposed to each other is referred to as “reverse contrast potential difference”. Is defined.

  As a result of various experiments, the inventor of the present application has found that the discharge starting voltage between the photosensitive member and the toner carrier is not constant, as will be described in detail later in the section “Best Mode for Carrying Out the Invention”. We obtained the knowledge that it varies greatly depending on the operating amount of the photoreceptor and the surrounding environment.

  In view of the above knowledge, in order to achieve the above object, an image forming apparatus according to the present invention is applied with a photoconductor and a DC voltage as a charging bias, and contacts the photoconductor to bring the photoconductor to a predetermined surface potential. A charging means for charging the surface of the photosensitive member, an exposure means for exposing the surface of the charged photosensitive member to form an electrostatic latent image, and a toner carrying member on the surface and facing the photosensitive member with a predetermined gap therebetween. And an alternating voltage as a developing bias is applied, whereby the toner is applied to the surface of the photoreceptor to visualize the electrostatic latent image, and the reverse contrast potential difference is a predetermined target potential difference. And a control means for controlling the charging bias and the developing bias, and the control means controls the non-dew exposure of the photoconductor when the reverse contrast potential difference is changed. A reverse contrast potential difference setting process for obtaining a discharge start reverse contrast potential difference for starting discharge between the toner carrier to which the developing bias is applied and setting a value smaller than the discharge start reverse contrast potential difference as the target potential difference It is characterized by performing.

  The image forming method according to the present invention forms an electrostatic latent image by exposing the surface of a photoconductor charged to a predetermined surface potential by contacting a charging means to which a DC voltage as a charging bias is applied. The toner is applied to the surface of the photoconductor by applying an alternating voltage as a developing bias by disposing a toner carrier on the surface facing the photoconductor with a predetermined gap therebetween. An image forming method for visualizing the electrostatic latent image, wherein the non-exposed portion of the photoconductor and the developing bias are applied when the reverse contrast potential difference is changed in order to achieve the above object. Obtaining a discharge start reverse contrast potential difference for starting discharge with the toner carrier, setting a value smaller than the discharge start reverse contrast potential difference as a target potential difference; and Forming the electrostatic latent image while setting the charging bias and the developing bias so that a strike potential difference becomes the target potential difference and forming the electrostatic latent image with the toner to form an image; It is characterized by having.

  In these inventions, since the reverse contrast potential difference at the start of discharge is actually obtained, the development bias and the charging bias are set so that the reverse contrast potential difference does not exceed this, so that there is a difference between the photosensitive member and the toner carrier. Can be reliably prevented. Further, desired image quality can be obtained by appropriately setting the reverse contrast potential difference within a range in which no discharge is generated. For example, by setting the reverse contrast potential difference as large as possible within a range where no discharge occurs, it is possible to effectively prevent fogging and toner scattering while reliably preventing the occurrence of discharge.

  In the image forming apparatus configured as described above, in the reverse contrast potential difference setting process, the control unit changes the reverse contrast potential difference by, for example, changing and setting at least one of the charging bias and the developing bias. While detecting whether or not a discharge occurs between the non-exposed portion of the photoreceptor and the toner carrier to which the developing bias is applied, the discharge start reverse contrast potential difference is obtained based on the result. Can do. Thus, by actually detecting the presence or absence of discharge while changing the reverse contrast potential difference, the discharge start reverse contrast potential difference can be obtained reliably.

  As specific means for detecting the presence or absence of discharge, for example, a current detection means for detecting a current flowing between the photosensitive member and the toner carrier is further provided, and the detection is performed based on the detection result of the current detection means. be able to. Since a discharge current flows between the photosensitive member and the toner carrying member due to the occurrence of the discharge, the presence or absence of the discharge can be determined by detecting this.

  Further, for example, toner detection means for detecting the toner adhesion amount on the photoreceptor can be further provided, and the detection can be performed based on the detection result by the toner detection means. Further, a transfer medium to which toner is transferred from the photoconductor and a toner detection unit for detecting the toner adhesion amount on the transfer medium may be further provided, and the detection may be performed based on the detection result by the toner detection unit. When using the toner detection means, the control means can determine that a discharge has occurred when the toner detection amount detected by the toner detection means is greater than a predetermined value. This is because when the discharge occurs, the toner on the toner carrier is scattered and a part of the toner adheres to the photosensitive member. When such toner adhesion is observed, it can be determined that a discharge has occurred.

  In addition, after executing the reverse contrast potential difference setting process, the control unit forms a patch image with each bias value while changing and setting the development bias in multiple stages, and optimizes the development bias based on the density detection result. The development bias optimizing process may be further executed. After the optimal reverse contrast potential difference is set by the reverse contrast potential difference setting process, it is possible to optimize the development bias and the charging bias by further optimizing the development bias to obtain better image quality. is there.

  In this case, in the development bias optimization process, the control unit forms the patch image while changing the charging bias according to the development bias so as to maintain the reverse contrast potential difference at the target potential difference. preferable. By doing so, the reverse contrast potential difference is maintained at the target potential difference even during patch image formation, and as a result, the developing bias and the charging bias can be controlled with higher accuracy.

  Further, the control means can execute the reverse contrast potential difference setting process accordingly when the apparatus is turned on. This is because when the power is turned on, there is a possibility that the surrounding environment has changed greatly from the time of the previous execution of the reverse contrast potential difference setting process. In addition, when any of the photosensitive member, the charging unit, and the toner carrier is replaced, the reverse contrast potential difference setting process may be executed accordingly. This is because, when these are exchanged, the likelihood of discharge between the photosensitive member and the toner carrying member changes due to the characteristic variation.

  Further, an environment detection unit that detects an operating environment of the apparatus may be further provided, and the control unit may determine the execution timing of the reverse contrast potential difference setting process based on a detection result by the environment detection unit. This is because the reverse contrast potential difference setting process is performed at an appropriate timing in response to changes in the ease of occurrence of discharge depending on the surrounding environment, particularly temperature and humidity.

  Further, the control unit may execute the reverse contrast potential difference setting process when an operation amount of the photoconductor or the charging unit reaches a predetermined threshold value. This is because the reverse contrast potential difference is always set to an optimum value in response to the change in the characteristics accompanying the increase in the operation amount. In particular, a plurality of different threshold values are set in advance, and the control means executes the inverse contrast potential difference setting process every time the operating amount reaches any of those threshold values, and the threshold interval is: It is preferable that the setting is made so as to increase as the operating amount increases. As will be described later, while the operation amount of the photosensitive member or the charging unit is small, the discharge start reverse contrast potential difference between the photosensitive member and the toner carrying member greatly fluctuates. On the other hand, the fluctuation decreases as the operation amount increases. Therefore, by increasing the threshold value as the operating amount increases, the reverse contrast potential difference setting process is executed at a high frequency in the initial stage and at a lower frequency in the late stage. It is also possible to cope with fluctuations in the starting reverse contrast potential difference.

  The control means preferably sets the amplitude of the developing bias to a constant value. This is because, if the amplitude of the developing bias is changed, the amount of toner supplied from the toner carrier greatly fluctuates, making it difficult to control the image density. Therefore, in order to obtain a desired image density while properly maintaining the reverse contrast potential difference and suppressing fog and toner scattering, the charging potential is not related to the amplitude of the developing bias, but to the non-exposed portion potential of the photoreceptor. It is desirable to make the bias variable.

  In the present invention, the above-described effect is particularly remarkable in an apparatus in which the surface of the toner carrier is formed of a conductive material. It has been found through experiments conducted by the inventors of the present application that the discharge occurring between the toner carrying member and the photosensitive member is particularly problematic. That is, even in an apparatus having such a configuration, by applying the present invention, it is possible to form an image with excellent image quality while reliably preventing the occurrence of discharge.

(1) Configuration example of image forming apparatus to which the present invention is applicable FIG. 1 is a diagram showing an embodiment of an image forming apparatus to which the present invention is applied. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This apparatus forms a full color image by superposing four color toners (developers) of yellow (Y), cyan (C), magenta (M), and black (K), or only black (K) toner. An image forming apparatus that forms a monochrome image using In this image forming apparatus, when an image signal is given to the main controller 11 from an external device such as a host computer, the CPU 101 provided in the engine controller 10 controls each part of the engine unit EG in response to a command from the main controller 11. Then, a predetermined image forming operation is executed, and an image corresponding to the image signal is formed on the sheet S.

  In the engine unit EG, the photosensitive member 22 is provided to be rotatable in the arrow direction D1 in FIG. A charging roller 23, a rotary developing unit 4 and a cleaning unit 25 are arranged around the photosensitive member 22 along the rotation direction D1. The charging roller 23 is applied with a predetermined charging bias, and charges the outer peripheral surface of the photosensitive member 22 to a predetermined surface potential. The cleaning unit 25 removes the toner remaining on the surface of the photosensitive member 22 after the primary transfer, and collects it in a waste toner tank provided inside. The photosensitive member 22, the charging roller 23, and the cleaning unit 25 integrally constitute the photosensitive member cartridge 2, and the photosensitive member cartridge 2 is detachably attached to the apparatus main body as a whole.

  Then, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photosensitive member 22 charged by the charging unit 23. The exposure unit 6 exposes the light beam L onto the photosensitive member 22 in accordance with an image signal given from an external device, and forms an electrostatic latent image corresponding to the image signal.

  The electrostatic latent image thus formed is developed with toner by the developing unit 4. That is, in this embodiment, the developing unit 4 is configured as a support frame 40 that is rotatably provided about a rotation axis center orthogonal to the paper surface of FIG. A yellow developing device 4Y, a cyan developing device 4C, a magenta developing device 4M, and a black developing device 4K are provided. The developing unit 4 is controlled by the engine controller 10. Based on the control command from the engine controller 10, the developing unit 4 is driven to rotate, and the developing units 4Y, 4C, 4M, and 4K selectively face the photoconductor 22 with a predetermined gap therebetween. When positioned at a predetermined position, a developing roller 44 that is provided in the developing unit and carries toner of a selected color is disposed to face the photosensitive member 22, and the developing roller 44 to the surface of the photosensitive member 22 at the opposite position. Toner is applied. As a result, the electrostatic latent image on the photosensitive member 22 is visualized with the selected toner color.

  The toner image developed by the developing unit 4 as described above is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TR1. The transfer unit 7 includes an intermediate transfer belt 71 stretched between a plurality of rollers 72 to 75, and a drive unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined rotation direction D2 by rotationally driving the roller 73. It has. When a color image is transferred to the sheet S, each color toner image formed on the photosensitive member 22 is superimposed on the intermediate transfer belt 71 to form a color image and taken out from the cassette 8 one by one. The color image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2 along the conveyance path F.

  At this time, in order to correctly transfer the image on the intermediate transfer belt 71 to a predetermined position on the sheet S, the timing of feeding the sheet S to the secondary transfer region TR2 is managed. Specifically, a gate roller 81 is provided on the transport path F on the front side of the secondary transfer region TR2, and the gate roller 81 rotates in accordance with the timing of the circumferential movement of the intermediate transfer belt 71. S is sent to the secondary transfer region TR2 at a predetermined timing.

  Further, the sheet S on which the color image is formed in this manner is fixed with the toner image by the fixing unit 9 and is conveyed to the discharge tray portion 89 provided on the upper surface portion of the apparatus main body via the pre-discharge roller 82 and the discharge roller 83. The Further, when images are formed on both sides of the sheet S, when the rear end portion of the sheet S on which the image is formed on one side as described above is conveyed to the reversal position PR behind the pre-discharge roller 82. The rotation direction of the discharge roller 83 is reversed, whereby the sheet S is conveyed in the direction of the arrow D3 along the reverse conveyance path FR. Then, the sheet is again placed on the transport path F before the gate roller 81. At this time, the surface of the sheet S to which the image is transferred by contacting the intermediate transfer belt 71 in the secondary transfer region TR2 is first transferred. It is the opposite surface. In this way, images can be formed on both sides of the sheet S.

  A cleaner 76 is disposed in the vicinity of the roller 75. The cleaner 76 can be moved toward and away from the roller 75 by an electromagnetic clutch (not shown). Then, the blade of the cleaner 76 abuts on the surface of the intermediate transfer belt 71 that is stretched over the roller 75 while moving to the roller 75 side, and the toner that remains on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. Remove.

  The cleaner 76 is controlled to be separated and abutted so as to remove toner remaining on the intermediate transfer belt 71 during the same rotation as the transfer of the image onto the sheet S in the secondary transfer region TR2. . Therefore, for example, when the apparatus continuously forms monochrome images, the image transferred to the intermediate transfer belt 71 in the primary transfer region TR1 is immediately transferred to the sheet S in the secondary transfer region TR2, so that the cleaner 76 is The contact state is maintained. On the other hand, when forming a color image, the cleaner 76 needs to be separated from the intermediate transfer belt 71 while the toner images of the respective colors are superimposed on each other. Then, the toner images of the respective colors are superimposed on each other to complete a full-color image, and the cleaner 76 is brought into contact with the intermediate transfer belt 71 in order to remove residual toner in the same round as the secondary transfer onto the sheet S. It will be.

  Further, a density sensor 60 and a vertical synchronization sensor 77 are arranged in the vicinity of the roller 75. The density sensor 60 is provided so as to face the surface of the intermediate transfer belt 71 and measures the image density of the toner image formed on the outer peripheral surface of the intermediate transfer belt 71 as necessary. Based on the measurement result, this apparatus adjusts the operating conditions of each part of the apparatus that affects the image quality, for example, the developing bias applied to each developing cartridge, the intensity of the light beam L, and the like. The density sensor 60 is configured to output a signal corresponding to the image density of a region of a predetermined area on the intermediate transfer belt 71 using, for example, a reflection type photosensor. The CPU 101 can detect the image density of each part of the toner image on the intermediate transfer belt 71 by periodically sampling the output signal from the density sensor 60 while rotating the intermediate transfer belt 71.

  The vertical synchronization sensor 77 is a sensor for detecting the reference position of the intermediate transfer belt 71, and is used to obtain a synchronization signal output in association with the rotational driving of the intermediate transfer belt 71, that is, a vertical synchronization signal Vsync. Functions as a sensor. In this apparatus, the operation of each part of the apparatus is controlled based on the vertical synchronization signal Vsync in order to align the operation timing of each part and accurately superimpose the toner images formed in the respective colors.

  In addition, a temperature / humidity sensor 90 that detects the temperature and humidity in the apparatus and outputs the detected temperature and humidity to the CPU 101 is provided in the vicinity of the engine unit EG inside the apparatus casing.

  Further, as shown in FIG. 2, each of the developing cartridges 4Y, 4C, 4M and 4K is provided with memories 91 to 94 for storing data relating to the manufacturing lot and usage history of the developing cartridge, the remaining amount of the built-in toner, and the like. ing. Further, the developing cartridges 4Y, 4C, 4M, and 4K are provided with wireless communication devices 49Y, 49C, 49M, and 49K, respectively. Then, if necessary, these selectively communicate with the wireless communication device 109 provided on the main body side in a non-contact manner, and data is transmitted between the CPU 101 and each of the memories 91 to 94 via the interface 105. Various information such as consumable management for the developing cartridge is managed by sending and receiving. In this embodiment, non-contact data transmission / reception is performed using electromagnetic means such as wireless communication. However, a connector is provided on the main body side and each developing cartridge side, and the connector is mechanically fitted. By doing so, data transmission / reception may be performed mutually.

  Further, as shown in FIG. 2, this apparatus includes a display unit 12 that is controlled by the CPU 111 of the main controller 11. The display unit 12 is constituted by, for example, a liquid crystal display, and in accordance with a control command from the CPU 111, the operation guidance to the user, the progress of the image forming operation, the occurrence of an abnormality in the apparatus, the replacement timing of any unit, etc. A predetermined message for notification is displayed.

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image given from an external device such as a host computer via the interface 112. Reference numeral 106 is a ROM for storing a calculation program executed by the CPU 101, control data for controlling the engine unit EG, and the like. Reference numeral 107 is a RAM for temporarily storing calculation results in the CPU 101 and other data. is there.

(2) Development Bias / Charging Bias Setting Processing Next, development bias / charging bias setting processing in the image forming apparatus will be described. Prior to that, knowledge obtained by the present inventor and development bias / charging bias setting based thereon The basic concept of processing will be described.

(2-1) Basic Principle FIG. 3 is a diagram showing the main part of the image forming apparatus according to the present invention. FIG. 4 is a diagram showing the potential of each part of the apparatus. Since this apparatus uses negatively charged toner, the potential applied to each part is basically a negative potential. Therefore, in order to facilitate understanding, the vertical axis is represented by a negative potential in FIG.

  As shown in FIG. 3, the photosensitive member 22 includes an element tube 221 made of a metal such as iron or aluminum, a base coating layer 222 on the surface thereof, a charge generation layer 223 made of an organic photosensitive material, and an outermost charge transport layer. 224. The elementary tube 221 is electrically grounded via the current detector 225.

  The charging roller 23 is a roller formed of a resin such as rubber or urethane in which conductive particles such as metal whisker or carbon black are dispersed, and is in contact with the surface of the photoreceptor 22 configured as described above. Has been. A predetermined DC charging bias Va is applied to the charging roller 23 from a charging bias generator 231 provided in the engine controller 10. The surface potential Vs of the photosensitive member 22 charged by such a direct current contact charging method does not rise to the same potential as the charging bias Va, but is generally lower than the charging bias. For example, when the charging bias Va is set to (−1200) V, the photoreceptor surface potential is about (−600) V. Further, the surface potential of the photosensitive member 22 is further lowered by dark decay until the surface of the charged photosensitive member 22 moves to a position facing the developing roller 44. Therefore, the non-exposed portion potential Vo of the photosensitive member 22 at the position facing the developing roller 44 is a value considerably lower than the charging bias Va. Further, the potential VL of the exposed portion exposed by the light beam L from the exposure unit 6 on the surface of the photoconductor 22 is a value that is reduced to about the residual potential determined by the characteristics of the photoconductor 22.

  On the other hand, the developing roller 44 disposed opposite to the photosensitive member 22 with a predetermined gap GP is a metal roller made of iron or aluminum, and a developing bias voltage from a charging bias generator 441 provided in the engine controller 10. Vb is applied. As shown in FIG. 4, the development bias Vb is a rectangular wave AC voltage having an asymmetric duty, the peak-to-peak voltage is Vpp, and the average potential (DC component) is Vavg.

  In such a configuration, the potential difference Vcon between the exposed portion potential VL of the photosensitive member 22 and the development bias average potential Vavg is a “contrast potential difference”, and this value greatly affects the image density. In other words, the image density can be controlled by adjusting the contrast potential difference Vcon. Further, the potential difference Vr between the non-exposed portion potential Vo of the photosensitive member 22 and the developing bias average potential Vavg is the “reverse contrast potential difference”, and this value does not significantly affect the image density, but the image density varies depending on the magnitude. Since the amount of toner changes, the image quality is affected as a result.

  Further, the potential difference V1 between the negative maximum potential Vmax of the developing bias Vb and the exposed portion potential VL of the photosensitive member 22, or the potential difference V2 between the positive maximum potential Vmin of the developing bias Vb and the non-exposed portion potential Vo of the photosensitive member 22. If either exceeds the discharge start voltage in the gap GP, discharge occurs between the photosensitive member 22 and the developing roller 44. According to various experiments by the inventor of the present application, it has been clarified that electric discharge easily occurs particularly between the non-exposed portion of the photosensitive member 22 and the developing roller 44. That is, it has been found that even though the potential difference V2 is lower than the discharge start voltage in the gap GP, a discharge may occur between the non-exposed portion of the photoreceptor 22 and the developing roller 44. It has also been found that this tendency is remarkable particularly when the developing roller 44 is formed of a conductive material such as metal.

  FIG. 5 is a diagram showing a mechanism of discharge that occurs between the non-exposed portion of the photoreceptor and the developing roller. As described above, according to the DC contact charging method, the surface potential Vs of the photosensitive member 22 has a value that is greatly reduced from the charging bias Va, but the value is not uniform and has some variation depending on the position. It is normal. That is, the charging bias Va is set so that the potential of the non-exposed portion of the photoconductor 22 becomes the above-mentioned potential Vo on average, but the actual surface potential Vs of the photoconductor 22 is shown in FIG. As shown, it may be lower or higher than the average value Vo depending on the position. Therefore, whether or not a discharge occurs between the non-exposed portion of the photosensitive member 22 and the developing roller 44 is simply determined by determining the positive side maximum potential Vmin of the developing bias Vb and the non-exposed portion potential (average potential) Vo of the photosensitive member 22. Microscopically, it is determined not by the potential difference V2 but by the potential difference V3 between the positive side maximum potential Vmin of the developing bias Vb and the maximum potential on the negative potential side of the non-exposed portion potential of the photosensitive member 22. It seems to be. Although the potential difference V2 between the positive side maximum potential Vmin of the developing bias Vb and the non-exposed portion potential Vo of the photosensitive member 22 does not exceed the discharge start voltage in the gap GP, the non-exposed portion of the photosensitive member 22 and the developing roller 44 It is considered that this is one of the main causes of discharge between the two.

  Such a variation in the potential of the non-exposed portion of the photosensitive member 22 is considered to be caused by a change in characteristics of the charging roller 23 and the photosensitive member 22 over time. According to the experiment by the present inventor, in-plane variation of the surface potential Vs of the photosensitive member 22 is large when charged by a relatively new charging roller 23. In addition, even when the deterioration of the photosensitive member 22 progresses, the in-plane variation of the non-exposed portion potential tends to increase.

  On the other hand, the exposure portion potential VL of the photosensitive member 22 does not decrease below the residual potential determined by the characteristics of the photosensitive member 22 even if the exposure power is changed greatly, for example, so that the variation is relatively small. In particular, there is almost no factor that causes the potential difference with the negative maximum voltage Vmax of the developing bias Vb to increase. Therefore, the discharge between the exposed portion of the photosensitive member 22 and the developing roller 44 can be predicted relatively easily.

  Note that the ease of discharge changes also by changing the amplitude of the developing bias Vb. However, if the amplitude of the developing bias is changed, the amount of toner flying in the gap GP changes, and the image density also changes at the same time. The possibility is high and density control becomes complicated. Therefore, in this apparatus, the amplitude of the developing bias is constant, and the average potential (DC component) Vavg and the charging bias Va of the developing bias are controlled as necessary.

  FIG. 6 is a diagram showing the relationship between the operation amount of the photosensitive cartridge and the discharge start reverse contrast potential difference. Here, the “discharge start reverse contrast potential difference” refers to the non-exposed portion of the photoconductor 22 when the reverse contrast potential difference is gradually increased by changing at least one of the charging bias Va and the average potential Vavg of the developing bias. This is the minimum potential difference that causes a discharge with the developing roller 44. As shown in FIG. 6, when the amount of operation (represented by the total number of rotations of the photosensitive member 22) is small, that is, when the photosensitive member cartridge 2 is relatively new, the mode of change varies, but the discharge start reverse contrast. The potential difference tends to increase. Then, the discharge start reverse contrast potential difference becomes maximum when the operation amount of the photosensitive cartridge 2 is a certain value, and after that, it tends to gradually decrease. Here, the change of the discharge start reverse contrast potential difference with respect to the operation amount of the photosensitive member 22 is shown. However, in this apparatus, the photosensitive member 22 and the charging roller 23 are incorporated in the photosensitive member cartridge 2, and these are integrated into the apparatus main body. However, the horizontal axis is substantially the same even when the horizontal axis is expressed as the operation amount of the charging roller 23.

  According to the experiment of the present inventor, the first variation of the reverse contrast potential difference at the start of discharge in sample 1 increases after becoming small, but the tendency of the initial variation in sample 2 does not tend to decrease. Although different for each sample, the point of increasing toward the threshold value Cth of the photosensitive member operating amount was commonly observed in all samples. In addition, the value of the threshold Cth at which the discharge start reverse contrast potential difference shifts from increase to decrease, and the tendency of the subsequent discharge start reverse contrast potential difference to decrease was small for each sample.

  In addition, the discharge start reverse contrast potential difference also changes depending on the operating environment of the apparatus. That is, in a high temperature and high humidity (HH) environment, discharge is more likely to occur than in a normal (NN) environment, and the reverse contrast potential difference at the start of discharge is small. On the other hand, since it is difficult for discharge to occur in a low temperature and low humidity (LL) environment, the reverse contrast potential difference at which discharge starts increases.

  When the reverse contrast potential difference is constant in the conventional image forming apparatus, the reverse contrast potential difference does not exceed the discharge start reverse contrast potential difference in order to reliably prevent the occurrence of discharge from the initial stage to the end of the photosensitive member cartridge. For this purpose, it is necessary to set the reverse contrast potential difference to a value indicated by a one-dot chain line in FIG. 6 in consideration of a margin for dealing with a change due to the environment. In this case, a sufficiently large reverse contrast potential difference cannot be secured, and there remains room for improvement in terms of image quality.

  Therefore, in this image forming apparatus, the reverse contrast potential difference is constantly changed according to the operation status of the apparatus, so that the reverse contrast potential difference as large as possible can be ensured while reliably preventing the occurrence of discharge. ing. Specifically, as described below, the discharge start reverse contrast potential difference is detected by detecting whether or not a discharge occurs between the photosensitive member 22 and the developing roller 44 each time the reverse contrast potential difference is variously changed. And set the target value of the reverse contrast potential difference as large as possible without exceeding that. Then, an image is formed while setting the developing bias and the charging bias so that the reverse contrast potential difference becomes the target value. By doing so, this image forming apparatus can form an image with excellent image quality while reliably preventing the occurrence of discharge.

  For example, when the operating amount of the photosensitive member is a certain value Ca in the sample 1 apparatus, a value obtained by subtracting a slight margin from the discharge start reverse contrast potential difference Vdis at that time is set as the target value Vtgt of the reverse contrast potential difference.

(2-2) Processing Flow FIG. 7 is a flowchart showing the flow of developing bias / charging bias setting in this image forming apparatus. In this image forming apparatus, when the CPU 101 executes the processing shown in FIG. 7, the operating conditions of the apparatus, more specifically, the charging bias, the developing bias, and the exposure power are controlled to the optimum conditions, and an image with a desired image quality is obtained. Can be formed.

  First, the target value Vtgt for the reverse contrast potential difference is determined (step S1). In this apparatus, the reverse contrast potential difference is maintained at a target value so as to keep the image quality stable while preventing discharge. How to determine the target value will be described later. Next, the development bias is optimized (step S2). This processing content is basically the same as the technique described in Patent Document 1 (Japanese Patent Laid-Open No. 2003-215863). In other words, a high density patch image is formed while changing the development bias in multiple stages, the density is detected by the density sensor 60, and an optimum value Vbopt of the development bias for obtaining a desired image density is calculated to obtain the development bias. Is set to its optimal value.

  When forming a patch image, it is desirable to change the charging bias Va in conjunction with the change in the development bias Vb so that the reverse contrast potential difference Vr is maintained at the target value Vtgt determined above. By doing so, the patch image is formed under conditions close to those in the actual image forming operation, and it becomes possible to obtain a high image quality by controlling the image density with higher accuracy.

  Subsequently, the charging bias Va is set based on the target value Vtgt of the reverse contrast potential difference thus obtained and the optimum value Vbopt of the developing bias (step S3). That is, a charging bias value is obtained such that the difference (reverse contrast potential difference) between the obtained optimum value of the developing bias and the non-exposed portion potential Vo of the photosensitive member 22 becomes the target value, and the charging bias is set to that value. To do.

  Then, the exposure power is optimized under the charging bias and the developing bias set in this way (step S4). Here, as in the technique described in Patent Document 1, the exposure power is optimized to obtain a desired image density by forming a low density patch image while changing and setting the exposure power in multiple stages and detecting the density. The value Eopt is calculated and the exposure power is set to the optimum value. As a result, the charging bias, the developing bias, and the exposure power are all set to optimum values according to the operation status and environment of the apparatus at that time, and the subsequent image forming operation stably forms an image with good image quality. can do.

(2-3) Setting of Reverse Contrast Potential Difference Target Value FIG. 8 is a flowchart showing processing for setting the reverse contrast potential difference target value. First, the reverse contrast potential difference Vr is set to the minimum value that can be set by appropriately setting at least one of the DC component Vavg and the charging bias Va of the developing bias (step S101). This minimum value may be a value that is not used in a normal image forming operation. Then, the developing bias and the charging bias set in this way are applied to the developing roller 44 and the photosensitive member 22 to detect the presence or absence of discharge between them (step S102).

  The presence / absence of discharge can be detected by detecting the current flowing through the current detector 225 connected to the photoconductor 22, for example. When a discharge occurs between the photoconductor 22 and the developing roller 44, a discharge current flows through the photoconductor 22, so that the discharge can be detected by detecting this current.

  It is also possible to detect discharge using the density sensor 60. When a discharge occurs between the photosensitive member 22 and the developing roller 44, the toner carried on the surface of the developing roller 44 is ejected, and a part of the toner adheres to the photosensitive member 22. If the surface of the photosensitive member 22 is not exposed, no toner is attached to the surface of the photosensitive member 22, or only a small amount of toner due to fogging is attached. Much bigger than. Therefore, the presence or absence of discharge can also be detected by detecting the toner adhesion amount on the photosensitive member 22 or on the intermediate transfer belt 71 to which the toner is transferred.

  If no occurrence of discharge is detected (step S103), the reverse contrast potential difference Vr is increased by one step (for example, 20V) by changing and setting at least one of the DC component Vavg and the charging bias Va of the developing bias (step 20) (step S103). S104), discharge detection in step S102 is performed again. On the other hand, when the occurrence of discharge is detected, the set value of the reverse contrast potential difference at that time is regarded as the “discharge start reverse contrast potential difference” and stored in the RAM 107 (step S105).

  Subsequently, the output of the temperature / humidity sensor 90 is read (step S106), the ambient environment of the apparatus is determined based on the output, and the target value of the reverse contrast potential difference is determined (step S107). Under a normal (NN) environment, a value 30 V lower than the discharge start reverse contrast potential difference obtained above is set as the target value Vtgt of the reverse contrast potential difference. This 30 V is a discharge margin voltage in consideration of device variations and environmental variations. On the other hand, when the apparatus is in a high-temperature and high-humidity (HH) environment, discharge is likely to occur, so a wide margin is required. Therefore, in this case, a value that is 10V smaller, that is, a value that is 40V lower than the discharge start reverse contrast potential difference obtained above is set as the target value. On the contrary, since the discharge hardly occurs in a low temperature / low humidity (LL) environment, the margin may be small. Therefore, a value 20 V lower than the discharge start reverse contrast potential difference obtained above is set as a target value.

  In the subsequent operation, the developing bias Vb is maintained so that the potential difference (reverse contrast potential difference Vr) between the DC component Vavg of the developing bias and the non-exposed portion potential Vo of the photosensitive member 22 is always maintained at the target value Vtgt thus set. In addition, the charging bias Va is controlled. It should be noted that the ease of occurrence of discharge in the gap GP, the optimum value of the developing bias for obtaining a desired image density, and the like are different for each developing device, and thus the above processing needs to be performed for each developing device. is there.

  Further, the non-exposed portion potential Vo of the photosensitive member 22 is not directly set, but indirectly controlled by controlling the charging bias Va. In other words, it is necessary to set the charging bias Va so that the non-exposed portion potential Vo becomes a desired value in consideration of a loss when transferring the charge from the charging roller 23 to the photosensitive member 22 and a potential decrease due to dark decay. In the DC contact charging type apparatus such as this apparatus, it is necessary to consider the following. In such a charging system device, the charging efficiency may increase as the film thickness of the photosensitive member 22 decreases. That is, when the photosensitive member 22 is worn and becomes thinner as it is used, the surface potential of the photosensitive member gradually increases even when the same charging bias is applied. Therefore, in determining the charging bias, it is necessary to consider the increase in the charging potential due to the reduction in the film thickness of the photoreceptor. If this is not taken into consideration, the surface potential of the photoconductor becomes higher than expected, and as a result, the reverse contrast potential difference becomes larger than the target value, which may increase the risk of causing discharge.

(2-4) Execution Timing Next, the timing at which the development bias / charging bias setting process should be executed will be examined. First, immediately after the device is turned on, there is a possibility that the surrounding environment has changed greatly after a long time has passed since the previous setting process. In addition, there is a possibility that maintenance work is performed while the power is off and the state of the apparatus is changed. Therefore, in order to form an image with stable quality, it is desirable to optimize the developing bias and the charging bias by executing the developing bias / charging bias setting process described above immediately after the power is turned on.

  Further, it is desirable to execute the developing bias / charging bias setting process even when the combination of the developing roller 44, the charging roller 23, and the photosensitive member 22 is changed. In this apparatus, the charging roller 23 and the photosensitive member 22 are configured as one photosensitive member cartridge 2, so that substantially any one of the photosensitive member cartridge 2, the developing device 4Y, etc. is replaced or newly mounted. It is desirable to execute development bias / charging bias setting processing. In this case, for example, when the photosensitive member cartridge 2 is replaced, it is necessary to perform processing for all the developing units. However, when only a part of the developing unit is replaced, the processing is not necessarily performed for all the developing units. It is not necessary to perform the processing, and the processing may be performed using only the replaced developing device.

  Also, when it is known from the output of the temperature / humidity sensor 90 that the development bias / charge bias setting process has been executed and the environment of the apparatus has changed significantly, the development bias / charge bias setting process is executed again. It is desirable. As described above, by determining the execution timing of the development bias / charging bias setting process based on the output of the temperature / humidity sensor 90, the fluctuation of the image quality due to the environmental change and the occurrence of discharge are suppressed, and the image quality is good. An image can be formed stably.

  In addition to the above, it is preferable to execute the developing bias / charging bias setting process every time the number of formed images (or the operating amount of the apparatus) reaches a predetermined value. This is because the optimum operating condition for the apparatus changes from moment to moment due to factors such as changes in the remaining amount of toner in the developing unit. In this case, as shown in FIG. 6, it is already known that the change in reverse contrast potential difference at the discharge start is large when the photosensitive member 22 and the charging roller 23 are relatively new. While the development bias / charging bias setting process is executed at a high frequency, if the operating amount exceeds a certain level, the execution frequency of the developing bias / charging bias setting process is reduced, so that unnecessary processing can be omitted and efficient control can be performed. It becomes.

  That is, several threshold values are set for the operation amount of the photosensitive cartridge, and when the operation amount reaches the threshold value, the developing bias / charging bias setting process is executed. Just make it bigger. For example, the development bias described above is applied every 200 sheets until the total number of image formations after the newly installed photoconductor cartridge 2 reaches 1000 sheets (corresponding to the value Cth in FIG. 6) and every 500 sheets thereafter. A charging bias setting process can be performed.

(3) Summary As described above, in this image forming apparatus, the reverse contrast potential difference, that is, the potential difference between the DC component of the developing bias and the non-exposed portion potential of the photosensitive member is not fixed, but is set according to the state of the apparatus. Therefore, it is possible to form an image with excellent image quality while reliably preventing the occurrence of discharge regardless of changes in the operating status of the apparatus. Also, the reverse contrast potential difference is set to the highest possible value within the range where no discharge occurs. Specifically, the reverse contrast potential difference is reversed to a value obtained by subtracting the margin determined according to the environment from the discharge start reverse contrast potential difference obtained by discharge detection. By determining the target value Vtgt of the contrast potential difference, a high fog suppression effect can be obtained.

  As described above, according to an embodiment of the image forming apparatus to which the present invention is applied, a desired image density is obtained and fog is prevented while reliably preventing the occurrence of discharge in the gap GP between the photosensitive member 22 and the developing roller 44. Thus, it is possible to stably form an image with excellent image quality.

  As described above, in this embodiment, the charging roller 23 and the developing roller 44 function as the “charging unit” and the “toner carrier” of the present invention, respectively. Further, the exposure unit 6 and the engine controller 10 function as “exposure means” and “control means” of the present invention, respectively. In this embodiment, the current detection unit 225, the concentration sensor 60, and the temperature / humidity sensor 90 correspond to the “current detection unit”, “toner detection unit”, and “environment detection unit” of the present invention, respectively.

  The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above embodiment, the development bias / charging bias setting process for optimizing the development bias, the charging bias, and the exposure power is executed following the determination of the target value of the reverse contrast potential difference. The purpose of this is to determine the target value of the reverse contrast potential difference according to the situation at that time, and to form an image while controlling the development bias and the charging bias so that the reverse contrast potential difference maintains the target value. The reverse contrast potential difference only needs to maintain the target value, and it is not always necessary to successively set the target value and optimize the development bias, the charging bias, and the exposure power. Therefore, for example, after obtaining the target value of the reverse contrast potential difference, the setting value of the development bias is not changed to maintain the image density, and only the setting value of the charging bias is set to a value corresponding to the obtained target value. It may be changed. However, from the viewpoint of obtaining good image quality, it is more preferable to reset the operating conditions subsequently as described above.

  In the above embodiment, the density sensor 60 is provided to face the intermediate transfer belt 71, and the toner transferred from the photosensitive member 22 to the intermediate transfer belt 71 is detected by the density sensor 60. A sensor may be disposed opposite the photoconductor to detect toner on the photoconductor.

  Further, in the above embodiment, the temperature / humidity sensor 90 is provided as the “environment detection means” of the present invention so as to detect the temperature and humidity in the machine relating to the ease of discharge, but instead of this, In addition to this, other environmental elements may be detected. For example, since the atmospheric pressure in the gap GP affects the ease of discharge, a configuration for detecting the atmospheric pressure may be further provided.

  In the above embodiment, the presence or absence of discharge is detected by variously changing the reverse contrast potential difference, and the target value of the reverse contrast potential difference is determined based on the result. . That is, as shown in FIG. 6, the reverse contrast potential difference at the start of discharge has a large change with time and individual variation in the initial stage of the photoconductor cartridge 2, while the variation is relatively slow and there is little individual variation in the later stage. In other words, in the latter period, a change in the discharge start reverse contrast potential difference can be predicted to some extent. Therefore, until the operation amount of the photosensitive cartridge reaches a certain threshold value, the discharge start reverse contrast potential difference is obtained by the process involving the discharge detection described above, and thereafter, the reverse discharge start reverse is performed according to the change characteristic predicted in advance. Assuming that the contrast potential difference changes, the target value of the reverse contrast potential difference may be determined without performing discharge detection.

  In the above embodiment, a rectangular wave voltage with asymmetric duty and a constant peak-to-peak voltage Vpp is used as the developing bias Vb. However, the developing bias waveform is not limited to this. Further, the waveform duty and amplitude may be variable.

  Furthermore, the present invention can be applied not only to an apparatus having a rotary developing unit as in the above-described embodiment, but also to a so-called tandem image forming apparatus and a monochrome image forming apparatus having only one developing device. Is possible.

1 is a diagram showing an embodiment of an image forming apparatus to which the present invention is applied. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. 1 is a diagram showing a main part of an image forming apparatus according to the present invention. The figure which shows the electric potential of each part of an apparatus. FIG. 3 is a diagram illustrating a mechanism of electric discharge that occurs between a non-exposed portion of a photoreceptor and a developing roller. The figure which shows the relationship between the operating amount of a photoreceptor, and a discharge start reverse contrast potential difference. 6 is a flowchart showing a flow of setting a developing bias and a charging bias. The flowchart which shows the setting process of a reverse contrast potential difference target value.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 2 ... Photoconductor cartridge, 6 ... Exposure unit (exposure means), 10 ... Engine controller (control means), 22 ... Photoconductor, 23 ... Charging roller (charging means), 44 ... Developing roller (toner carrier), 60 ... Density sensor (toner detection means), 90 ... temperature / humidity sensor (environment detection means), 225 ... current detection part (current detection means), Vavg ... development bias average potential, Vo ... non-exposure part potential, Vr ... reverse contrast potential difference

Claims (16)

A photoreceptor,
A charging unit that is applied with a DC voltage as a charging bias and contacts the photoconductor to charge the photoconductor to a predetermined surface potential;
Exposure means for exposing the surface of the charged photoreceptor to form an electrostatic latent image;
The toner is carried on the surface and opposed to the photosensitive member with a predetermined gap therebetween, and an alternating voltage as a developing bias is applied to apply the toner to the surface of the photosensitive member and thereby the electrostatic latent image. A toner carrier for visualizing an image;
When the potential difference between the potential of the unexposed portion of the surface of the photoconductor that is not exposed and the average potential of the developing bias at the position where the photoconductor and the toner carrier are opposed to each other is defined as a reverse contrast potential difference, Control means for controlling the charging bias and the developing bias so that the reverse contrast potential difference becomes a predetermined target potential difference,
The control means obtains a discharge start reverse contrast potential difference that starts discharge between the non-exposed portion of the photoconductor and the toner carrier to which the developing bias is applied when the reverse contrast potential difference is changed; An image forming apparatus that executes reverse contrast potential difference setting processing for setting a value smaller than the discharge start reverse contrast potential difference as the target potential difference.   In the reverse contrast potential difference setting process, the control unit applies the non-exposed portion of the photoreceptor and the development bias while changing the reverse contrast potential difference by changing and setting at least one of the charging bias and the development bias. 2. The image forming apparatus according to claim 1, wherein whether or not a discharge occurs with the toner carrying member is detected, and the discharge start reverse contrast potential difference is obtained based on the result. A current detecting means for detecting a current flowing between the photosensitive member and the toner carrier;
The image forming apparatus according to claim 2, wherein the control unit detects the presence or absence of discharge based on a detection result by the current detection unit. A toner detecting means for detecting a toner adhesion amount on the photoreceptor;
The image forming apparatus according to claim 2, wherein the control unit detects the presence or absence of discharge based on a detection result by the toner detection unit. A transfer medium to which toner is transferred from the photoreceptor;
A toner detecting means for detecting a toner adhesion amount on the transfer medium;
The image forming apparatus according to claim 2, wherein the control unit detects the presence or absence of discharge based on a detection result by the toner detection unit.   The image forming apparatus according to claim 4, wherein the control unit determines that a discharge has occurred when a toner adhesion amount greater than a predetermined value is detected by the toner detection unit.   After the execution of the reverse contrast potential difference setting process, the control unit forms a patch image with each bias value while changing and setting the development bias in multiple stages, and optimizes the development bias based on the density detection result. The image forming apparatus according to claim 1, further performing a developing bias optimization process.   8. The development bias optimization process, wherein the control unit forms the patch image while changing the charging bias according to the development bias so as to maintain the reverse contrast potential difference at the target potential difference. Image forming apparatus.   The image forming apparatus according to claim 1, wherein the control unit executes the inverse contrast potential difference setting process in response to power-on of the apparatus.   9. The image according to claim 1, wherein the control unit executes the reverse contrast potential difference setting process in response to any of the photoconductor, the charging unit, and the toner carrying member being replaced. Forming equipment. It further comprises environment detection means for detecting the operating environment of the device,
The image forming apparatus according to claim 1, wherein the control unit determines execution timing of the inverse contrast potential difference setting process based on a detection result by the environment detection unit.   The image forming apparatus according to claim 1, wherein the control unit executes the reverse contrast potential difference setting process when an operation amount of the photoconductor or the charging unit reaches a predetermined threshold value. A plurality of different threshold values are set, and the control means executes the reverse contrast potential difference setting process every time the operating amount reaches any one of the threshold values,
The image forming apparatus according to claim 12, wherein the interval between the thresholds is set so as to increase as the operating amount increases.   The image forming apparatus according to claim 1, wherein the control unit sets the amplitude of the developing bias to a constant value.   The image forming apparatus according to claim 1, wherein a surface of the toner carrier is formed of a conductive material. A toner carrier that exposes the surface of a photosensitive member charged to a predetermined surface potential by contacting a charging means to which a DC voltage as a charging bias is contacted to form an electrostatic latent image and carries toner on the surface. Is arranged opposite to the photoconductor with a predetermined gap and an alternating voltage is applied as a developing bias to apply the toner to the surface of the photoconductor to visualize the electrostatic latent image. In the forming method,
When the potential difference between the potential of the unexposed portion of the surface of the photoconductor that is not exposed and the average potential of the developing bias at the position where the photoconductor and the toner carrier are opposed to each other is defined as a reverse contrast potential difference,
When the reverse contrast potential difference is changed, a discharge start reverse contrast potential difference for starting discharge between the non-exposed portion of the photoconductor and the toner carrier to which the developing bias is applied is obtained, and the discharge start reverse contrast is obtained. Setting a value smaller than the potential difference as the target potential difference;
While setting the charging bias and the developing bias so that the reverse contrast potential difference becomes the target potential difference, the electrostatic latent image is formed and the electrostatic latent image is visualized with the toner to form an image. And an image forming method.

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