JP6455223B2 - Image forming apparatus - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly, to image forming such as a copying machine, a printer, a facsimile machine, and the like provided with an image forming unit that forms an image using an electrophotographic method regardless of the type such as color / monochrome. Relates to the device.

  In general, in an image forming apparatus provided with an image forming unit that forms an image using an electrophotographic method, a photosensitive member is used as an electrostatic latent image carrier capable of carrying an electrostatic latent image. . The photoconductor is composed of a rotatable drum and a photoconductor film that covers the surface of the drum. When toner is supplied from a developing unit such as a developing roller, a toner image corresponding to the electrostatic latent image is obtained. Are transferred to an intermediate transfer belt or paper.

  Usually, a cleaning member such as a cleaner blade is disposed in contact with the photoconductor, and the residue on the surface of the photoconductor after the toner image is transferred is removed by the cleaning member. Therefore, each time the image forming apparatus is used repeatedly, the surface of the photoreceptor film is slightly scraped, and the film thickness of the photoreceptor film gradually decreases.

  When the film thickness of the photoconductor film decreases, the electrostatic capacity of the photoconductor film increases, so the amount of charge injected into the photoconductor by the charging device increases, and toner adheres during development. It becomes easy to do. For this reason, the developability is improved more than necessary, which causes a reduction in image quality such as image unevenness. In particular, when the film thickness of the photosensitive film is designed to be high in order to increase durability, the amount of decrease in film thickness from the beginning of the product life becomes very large at the end of the product life, and the image quality is significantly reduced. Will end up.

  In order to solve such a problem, for example, in Japanese Patent Application Laid-Open No. 11-15214 (Patent Document 1), when the film thickness of the photoconductor film is reduced, the potential of the photoconductor and the potential of the developing AC bias are described. A technique for suppressing a decrease in image quality by reducing the difference between the two and reducing developability is disclosed.

  Japanese Patent Laid-Open No. 2010-250051 (Patent Document 2) discloses that when the film thickness of the photosensitive film is reduced, the peak-to-peak voltage (Vpp) of the developing AC bias is reduced or the AC of the developing AC bias is changed. A technique is disclosed that suppresses the deterioration in image density and gradation by reducing developability by increasing the period during which components are cut.

Japanese Patent Laid-Open No. 11-15214 JP 2010-250051 A

  However, the change in developability that occurs when the film thickness of the photosensitive film is reduced does not change uniformly at the same magnification from the low density area to the entire high density area of the image density, particularly in the low density area. The developability is remarkably improved.

  On the other hand, since the techniques disclosed in Patent Documents 1 and 2 both reduce the developability equally from the low density area to the high density area of the image density, the technique is adopted. In particular, this is insufficient to lower the developability in the low density region. Therefore, even if the image quality in the high density region can be improved, the image quality in the low density region cannot be sufficiently improved.

  Therefore, the present invention has been made in view of the above-described problems, and can maintain stable image quality over the entire range from the low density area to the high density area of the image density from the beginning to the end of the product life. It is an object of the present invention to provide an image forming apparatus that can be used.

  An image forming apparatus according to the present invention includes a photosensitive member having a surface defined by a photosensitive film and capable of carrying an electrostatic latent image, a developing unit that supplies toner to the photosensitive member, and a developing AC bias applied to the developing unit. A bias applying unit to be applied, a film thickness detecting unit for detecting film thickness information of the photosensitive film, and a control unit for controlling the operation of the bias applying unit are provided. When the control unit determines that the film thickness of the photosensitive film is smaller than a predetermined value based on the film thickness information, the duty ratio on the development potential side of the development AC bias is increased and the development is performed. The operation of the bias applying unit is controlled so that the duty ratio on the recovery potential side of the AC bias becomes small.

  In the image forming apparatus according to the present invention, when the control unit determines that the film thickness of the photosensitive film is smaller than a predetermined value based on the film thickness information, the development is further performed. The operation of the bias applying unit is performed so that the difference between the development potential and the area center voltage is reduced and the difference between the recovery potential and the area center voltage is increased while the area center voltage of the AC bias is maintained. You may control.

  In the image forming apparatus according to the present invention, when the control unit determines that the film thickness of the photosensitive film is smaller than a predetermined value based on the film thickness information, the development is further performed. The operation of the bias application unit may be controlled so that the peak-to-peak voltage, which is the difference between the development potential of the AC bias and the recovery potential, becomes small.

  The image forming apparatus according to the present invention may further include a photosensitive member driving mechanism that drives the photosensitive member and a developing unit driving mechanism that drives the developing unit. However, the operation of the photosensitive member driving mechanism and the operation of the developing unit driving mechanism may be further controlled. In this case, when the control unit determines that the film thickness of the photoconductor film is smaller than a predetermined value based on the film thickness information, the controller further moves the surface speed of the photoconductor and the development. The operation of the photosensitive member driving mechanism and the operation of the developing unit driving mechanism may be controlled so that the speed ratio with the moving speed of the surface of the unit becomes small.

  The image forming apparatus according to the present invention may further include a toner supply unit that supplies the toner to the developing unit. In this case, the control unit further controls the operation of the toner supply unit. May be. In this case, when the control unit determines that the film thickness of the photosensitive film is smaller than a predetermined value based on the film thickness information, the density of the toner supplied to the developing unit is further increased. You may control the operation | movement of the said toner supply part so that it may become low.

  In the image forming apparatus according to the present invention, the film thickness detection unit may be constituted by a travel distance detection unit that detects a travel distance of the surface of the photoreceptor.

  According to the present invention, it is possible to provide an image forming apparatus capable of maintaining stable image quality over the entire region from the low density region to the high density region of the image density from the beginning to the end of the product life.

1 is a schematic diagram of an image forming apparatus in an embodiment of the present invention. It is a principal part expansion schematic diagram of the image formation unit shown in FIG. 6 is a graph schematically showing the relationship between ΔV, which is the difference between the DC component voltage Vdc of the development AC bias and the photoreceptor potential Vd, and the image density ID. FIG. 5 is a flowchart showing a control flow of a control unit of the image forming apparatus in the embodiment of the present invention. FIG. 4 is a diagram illustrating a waveform of a developing AC bias applied to a developing roller in the image forming apparatus according to the embodiment of the present invention. It is a graph which shows the test result of a 1st verification test. It is a table | surface which shows the test conditions of a 2nd verification test. It is a table | surface which shows the test result of a 2nd verification test.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, the same or common parts are denoted by the same reference numerals in the drawings, and description thereof will not be repeated.

  FIG. 1 is a schematic diagram of an image forming apparatus according to an embodiment of the present invention. First, a schematic configuration of the image forming apparatus 1 in the present embodiment will be described with reference to FIG. The image forming apparatus 1 illustrated in the present embodiment is a so-called tandem type color copying machine that employs an electrophotographic system.

  As shown in FIG. 1, the image forming apparatus 1 mainly includes an apparatus main body 2 and a paper feeding unit 5. The apparatus main body 2 includes an image forming unit 3 that is a part for forming an image on a sheet, and a paper feeding unit 4 that is a part for supplying the sheet to the image forming unit 3. The paper supply unit 5 stores paper to be supplied to the image forming unit 3 and is detachably provided in the paper supply unit 4.

  Various rollers are installed inside the image forming apparatus 1 so as to straddle the image forming unit 3 and the paper feeding unit 4 described above, whereby the sheet is conveyed along a predetermined direction. Further, the paper feed unit 4 may be separately provided with a manual feed tray 6 for supplying paper to the image forming unit 3. In addition, a so-called paper jam is eliminated at a predetermined position of the apparatus main body 2. An open / close door 7 for maintenance may be installed.

  The image forming unit 3 includes image forming units 10Y, 10M, 10C, and 10K, an intermediate transfer belt 11 suspended on the image forming units 10Y, 10M, 10C, and 10K, and image forming units 10Y, 10M, 10C, and 10K. The exposure apparatus 12 and the transfer unit 13 provided on the running path of the intermediate transfer belt 11 are mainly provided.

  The image forming units 10Y, 10M, 10C, and 10K receive the exposure L (see FIG. 2) from the exposure apparatus, and toner images of each color of yellow (Y), magenta (M), cyan (C), and black (K). Alternatively, a toner image composed only of black (K) is formed and transferred to the intermediate transfer belt 11 (so-called primary transfer). As a result, a color toner image or a monochrome toner image is formed on the intermediate transfer belt 11.

  The intermediate transfer belt 11 transports a color toner image or a monochrome toner image formed on the surface thereof to the transfer unit 13, and a pair of sheets in the transfer unit 13 together with the paper conveyed from the paper supply unit 4 to the transfer unit 13. It is pressed by the transfer roller. As a result, a color toner image or a monochrome toner image formed on the surface of the intermediate transfer belt 11 is transferred onto a sheet (so-called secondary transfer). The paper on which the color toner image or monochrome toner image is transferred is then pressed and heated by a fixing device, thereby forming an image on the paper.

  FIG. 2 is an enlarged schematic view of the main part of the image forming unit shown in FIG. Next, the configuration of the image forming unit 10 and the control block that controls the image forming unit 10 will be described in detail with reference to FIG. Since the image forming units 10Y, 10M, 10C, and 10K have basically the same configuration, the following description will be made as the image forming unit 10 without particularly distinguishing them.

  As shown in FIG. 2, the image forming unit 10 includes a photosensitive member 21, a charging device 22 arranged in order along the periphery of the photosensitive member 21, a developing roller 23 as a developing unit, a primary transfer roller 25, and a cleaning device 26. have. The image forming apparatus 1 includes a control unit 30, a photosensitive member driving mechanism 31, a developing roller driving mechanism 32 as a developing unit driving mechanism, a bias applying unit 33, and a control block related to control of the operation of the image forming unit 10. A film thickness detection unit 34 is provided, and a toner supply unit 24 that supplies toner to the developing roller 23 is further provided.

  The photoconductor 21 includes a support shaft 21a serving as a rotation center, a drum 21b held by the support shaft 21a, and a photoconductor film 21c formed so as to cover the outer peripheral surface of the drum 21b. Thereby, the surface (outer peripheral surface) of the photoconductor 21 is defined by the photoconductor film 21c. The drum 21b is made of, for example, aluminum, and the photoreceptor film 21c is a film made of a negatively charged or positively charged organic photoreceptor material. The photoconductor 21 is rotationally driven in the direction of arrow B in the figure by the photoconductor drive mechanism 31. In the present embodiment, a film made of a negatively charged organic photoreceptor material is used as the photoreceptor film 21c.

  The charging device 22 is configured by a roller-type charging device, and charges the photoreceptor film 21c formed on the surface of the photoreceptor 21 to a predetermined potential. The charging device 22 includes a cored bar, a conductive cylindrical part integrally formed with the cored bar, and a highly conductive layer having elasticity formed on the surface (outer peripheral surface) of the cylindrical part. The charging roller includes a charging roller. The charging roller is arranged in contact with the photosensitive member 21 and is driven to rotate in the direction of arrow C in the figure. A charging bias is applied to the charging roller via a cored bar, whereby the photosensitive film 21c is contact-charged. Although a roller type charging device is illustrated here, various devices such as a wire discharge type charging device and a brush type charging device can be used instead.

  The developing roller 23 is provided adjacent to the toner supply unit 24 and holds the toner supplied from the toner supply unit 24 on the surface thereof. The developing roller 23 is driven in the direction of arrow D in the figure by the developing roller driving mechanism 32.

  The toner supply unit 24 supplies toner to the developing roller 23 described above, and contains a two-component developer composed of, for example, a nonmagnetic toner and a magnetic carrier. The toner supply unit 24 includes an agitation roller for agitating the developer, and can adjust the concentration of toner supplied to the development roller 23. The operation of the toner supply unit 24 is controlled by the control unit 30.

  A developing AC bias is applied to the developing roller 23 by a bias applying unit 33. By applying a developing AC bias to the developing roller 23, the toner held on the surface of the developing roller 23 is supplied to the surface of the photosensitive member 21, and accordingly, according to the carried electrostatic latent image. A toner image is formed on the surface of the photoreceptor 21.

  The primary transfer roller 25 is disposed so as to sandwich the intermediate transfer belt 11 with the photosensitive member 21, and the arrow E in the drawing corresponds to the movement of the intermediate transfer belt 11 in the direction of arrow A in the drawing. Rotate along direction. A primary transfer bias having a polarity opposite to that of the toner is applied to the primary transfer roller 25, whereby the toner image formed on the surface of the photoreceptor 21 is transferred to the intermediate transfer belt 11.

  The cleaning device 26 has a cleaner blade 26a, and the cleaner blade 26a is disposed in contact with the surface of the photoreceptor 21 (that is, the surface of the photoreceptor film 21c). As a result, the residue on the surface of the photoconductor 21 after the toner image is transferred is removed by the cleaner blade 26 a and collected by the cleaning device 26.

  The photosensitive member driving mechanism 31 includes, for example, a driving source such as a motor and a power transmission member such as a gear that transmits power from the driving source to the photosensitive member 21, and its operation is controlled by the control unit 30. As a result, the photosensitive member driving mechanism 31 rotationally drives the photosensitive member 21.

  The developing roller driving mechanism 32 includes, for example, a driving source such as a motor and a power transmission member such as a gear that transmits power from the driving source to the developing roller 23, and its operation is controlled by the control unit 30. As a result, the developing roller driving mechanism 32 rotationally drives the developing roller 23.

  The operation of the bias applying unit 33 is controlled by the control unit 30, and the developing AC bias is applied to the developing roller 23 by receiving power from a power source (not shown). The development AC bias is a bias in which a DC component is superimposed on the AC bias, and is adjusted by the control unit 30.

  The film thickness detector 34 detects the film thickness information of the photoreceptor film 21c, and indirectly or directly detects the film thickness of the photoreceptor film 21c. As a method of indirectly detecting the film thickness information, the film thickness detection unit 34 can be configured by a travel distance detection unit that detects the travel distance of the surface of the photoconductor 21, and the film thickness information is directly detected. As a technique to do this, the film thickness detector 34 can be configured by a detection sensor or the like that detects the displacement of the surface of the photoreceptor 21 in the radial direction. The film thickness detector 34 outputs the film thickness information of the photoreceptor film 21 c to the controller 30.

  The control unit 30 mainly controls the operation of the image forming unit 10 by controlling the operations of the photosensitive member driving mechanism 31, the developing roller driving mechanism 32, the bias applying unit 33, and the toner supplying unit 24 described above. In particular, the developability is adjusted by controlling the operation of the bias application unit 33 based on the film thickness information of the photosensitive film 21c input from the film thickness detection unit 34 described above.

  FIG. 3 is a graph schematically showing the relationship between ΔV, which is the difference between the DC component voltage Vdc of the development AC bias and the photoreceptor potential Vd, and the image density ID. Next, referring to FIG. 3, the relationship between ΔV, which is the difference between the DC component voltage Vdc of the developing AC bias and the photosensitive member potential Vd, and the image density ID is determined through repeated use of the image forming apparatus. How it changes will be described.

  As shown in FIG. 3, at the beginning of the product life, a linear relationship is obtained between ΔV, which is the difference between the DC component voltage Vdc of the development AC bias and the photoreceptor potential Vd, and the image density ID. As described above, the thickness of the photoconductor is generally set. In this case, desired developability can be obtained from the low density region to the high density region of the image density ID, and a stable image quality image can be obtained regardless of the image density of the image to be formed. .

  On the other hand, if no image is taken and image formation is performed under the same conditions as the beginning of the product life even after lapse of time (that is, at the end of the product life), the film thickness of the photoreceptor film is reduced. Accordingly, the relationship between ΔV and the image density ID as described as “after time (before improvement)” in FIG. 3 changes. Specifically, after a lapse of time, the relationship between ΔV and the image density ID in the low density region changes so as to rise remarkably (that is, the slope increases significantly) in the low density region, and ΔV and the image density in the high density region. The relationship with the ID changes so as to rise slightly (that is, the slope slightly increases) compared to the initial value.

  Thus, the change in developability that occurs when the film thickness of the photosensitive film is reduced does not change uniformly at the same magnification from the low density area to the entire high density area of the image density. In particular, developability is remarkably improved, and there is a demand for returning this to a state closer to the initial state.

  In order to realize this, in the image forming apparatus 1 in the present embodiment, the duty ratio on the development potential side of the development AC bias is increased after the time when the film thickness of the photosensitive film decreases. At the same time, the initial condition is changed so as to reduce the duty ratio on the recovery potential side of the development AC bias. The details will be described below.

  FIG. 4 is a flowchart showing a control flow of the control unit of the image forming apparatus according to the present embodiment. FIG. 5A is a diagram illustrating a waveform of the developing AC bias applied to the developing roller in a state where the reduction in the thickness of the photosensitive film is not progressing in the image forming apparatus according to the present embodiment. FIG. 5B is a diagram showing a waveform of the developing AC bias applied to the developing roller in a state in which the reduction in the thickness of the photosensitive film has progressed.

  In the image forming apparatus 1 according to the present embodiment, the control unit 30 determines whether the film thickness of the photosensitive film 21c is smaller than a predetermined value based on the film thickness information input from the film thickness detection unit 34. Based on this, the operation of the bias applying unit 33 is controlled to change the developing AC bias applied to the developing roller 23.

  Specifically, as shown in FIG. 4, after receiving an image formation instruction from the user in step S <b> 1, the control unit 30 acquires the film thickness information of the photosensitive film 21 c from the film thickness detection unit 34 in step S <b> 2. In step S3, it is determined whether the film thickness of the photosensitive film 21c is smaller than a predetermined value based on the film thickness information.

  When it is determined that the film thickness of the photoconductor film 21c is larger than the predetermined value (NO in step S3), the control unit 30 proceeds to step S4A and maintains the initially set duty ratio. In step S5, an image is formed.

  The developing AC bias applied to the developing roller 23 at this time is, for example, as shown in FIG. 5A, and the developing AC bias is between the developing potential and the recovery potential based on a predetermined duty ratio. It is a rectangular wave shape that changes periodically.

  Here, in the image forming apparatus 1 according to the present embodiment, since a film made of a negatively charged organic photosensitive material is used as the photosensitive film 21c, the developing potential is a DC component voltage of a developing AC bias. Vmin1 is a negative peak value from Vdc, and the recovery potential is Vmax1 which is a positive peak value from the DC component voltage Vdc of the development AC bias. The voltage application time on the development potential side per cycle based on the above-described duty ratio is dm1, and the voltage application time on the recovery potential side is dp1.

  On the other hand, when it is determined that the film thickness of the photoreceptor film 21c is smaller than the predetermined value (YES in step S3), the control unit 30 proceeds to step S4B and is on the development potential side from the initial setting. The duty ratio is set large and an image is formed in step S5.

  The developing AC bias applied to the developing roller 23 at that time is, for example, as shown in FIG. That is, the voltage application time on the development potential side per cycle is dm2 longer than dm1 described above, and the voltage application time on the recovery potential side per cycle is dp2 shorter than dp1 described above.

  More preferably, the DC component voltage Vdc of the development AC bias is maintained at the same value as the initial setting, and the development potential is set to Vmin2 higher than the initial development potential Vmin1, and the recovery potential is determined. Is set to Vmax2 lower than the initial development potential Vmax1. At this time, the area center voltage of the developing AC bias is maintained between the initial setting and the changed setting (that is, the sum of the areas Sm1 and Sp1 shown in FIG. 5A), It is more preferable that the area Sm2 and the area Sp2 shown in FIG.

  As described above, in the state in which the film thickness of the photoconductor film 21c has progressed, the development AC bias on the development potential side of the developing AC bias at the time of image formation is smaller than in the state in which the film thickness of the photoconductor film 21c has not progressed. By increasing the duty ratio and decreasing the duty ratio on the recovery potential side, it is possible to obtain the relationship between ΔV and the image density ID as described as “after time (after improvement)” in FIG.

  Specifically, referring to FIG. 3, in image forming apparatus 1 according to the present embodiment, the duty ratio is changed after the passage of time to change between the low density region and the high density region. Thus, the relationship between ΔV and the image density ID can be returned to a more linear relationship (that is, a state closer to the initial state).

  Therefore, by using the image forming apparatus 1 according to the present embodiment, it is possible to maintain a stable image quality over the entire range from the low density region to the high density region of the image density from the beginning to the end of the product life. Become.

  In the above-described embodiment, when it is determined that the film thickness of the photoreceptor film 21c is smaller than a predetermined value, the control unit 30 increases the duty ratio on the development potential side of the development AC bias and collects it. Although the case where the bias applying unit 33 is configured to be controlled so as to reduce the duty ratio on the potential side is exemplified, in addition to this, at least one of the following controls may be performed.

  In the first additional control, when it is determined that the film thickness of the photosensitive film 21c is smaller than a predetermined value, the control unit 30 further determines a peak-to-peak voltage that is a difference between the development potential of the development AC bias and the recovery potential. The operation of the bias applying unit 33 is controlled so that Vpp (see FIG. 5) becomes small.

  In the second additional control, when it is determined that the film thickness of the photoconductor film 21c is smaller than a predetermined value, the control unit 30 further moves the moving speed of the surface of the photoconductor 21 and the moving speed of the surface of the developing roller 23. The operation of the photosensitive member driving mechanism 31 and the operation of the developing unit driving mechanism 32 are controlled so that the speed ratio Θ is small.

  In the third additional control, when it is determined that the film thickness of the photoreceptor film 21c is smaller than a predetermined value, the control unit 30 further reduces the toner concentration supplied to the developing roller 23. The operation of the supply unit 24 is controlled.

  Since these first to third additional controls all reduce the developability, at least one of the first to third additional controls is simultaneously performed in accordance with the control for changing the duty ratio described above. This makes it possible to maintain more stable image quality.

  Below, the 1st verification test and 2nd verification test which were performed in order to verify the effect of this invention are demonstrated. FIG. 6 is a graph showing test results of the first verification test, and FIGS. 7 and 8 are tables showing test conditions and test results of the second verification test, respectively.

  In the first verification test, when a photoconductor having a different photoconductor film thickness is prepared and the duty ratio of the development AC bias is changed using the photoconductor, the DC component voltage Vdc of the development AC bias and the photoconductor potential described above are used. This is a verification of how the relationship between ΔV, which is the difference from Vd, and the image density ID actually changes.

  Here, in the first verification test, two types of photoreceptors were prepared: a photoreceptor having a film thickness t of 35 [μm] and a photoreceptor having a film thickness of 20 [μm]. For a photoconductor having a film thickness t of 35 [μm], an image is formed under the condition that the duty ratio drm on the development potential side of the development AC bias is 40 [%]. For a photoconductor having a film thickness t of 20 [μm], an image is formed under conditions where the duty ratio drm on the development potential side of the development AC bias is 30 [%], 40 [%], and 50 [%], respectively. Was done.

  At that time, the above-mentioned ΔV was variously adjusted to collect a plurality of image samples, and the image density ID was measured by measuring the reflection density of the image for each of them.

  Note that the image forming conditions other than those described above were all set to be the same. Here, as in the above-described embodiment, a film made of a negatively charged organic photoreceptor material was used as the photoreceptor film, and the DC voltage component Vdc of the development AC bias was set to 400 [V]. As the charging device, a roller type was used.

  As shown in FIG. 6, when an image is formed under image forming conditions with a duty ratio drm of 40 [%] using a photoconductor having a film thickness t of 35 [μm]. As shown in the figure, it was confirmed that ΔV and the image density ID described above have a linear relationship, and that stable image quality can be obtained from a low density area to a high density area of the image density.

  Further, when an image is formed under an image forming condition with a duty ratio drm of 40 [%] using a photoreceptor having a film thickness t of 20 [μm], as shown in the figure. When the relationship between ΔV and the image density ID is obtained and taken into consideration in conjunction with the above results, the image quality that was good at the beginning of the product life is the end of the product life (that is, the film thickness t of the photoreceptor film is reduced). It can be said that even when the image is formed under the same image forming conditions, it is confirmed that the image quality drops significantly.

  On the other hand, when an image is formed under image forming conditions with a duty ratio drm of 50 [%] using a photoconductor with a film thickness t of 20 [μm], as shown in the figure. It was confirmed that the relationship between ΔV and the image density ID is closer to a linear relationship, and stable image quality can be obtained from the low density region to the high density region of the image density. Considering this together with the above results, at the end of the product life (that is, after the film thickness t of the photosensitive film is decreased), the duty ratio on the development potential side is increased to improve the developability of the product life. It can be said that it has been confirmed that it is possible to approach the initial state.

  On the other hand, when an image is formed under image forming conditions with a duty ratio drm of 30 [%] using a photoconductor with a film thickness t of 20 [μm], as shown in the figure. When the relationship between ΔV and image density ID is obtained and taken into consideration in conjunction with the above results, the duty ratio on the developing potential side is reduced at the end of the product life (that is, after the film thickness t of the photosensitive film is reduced). In this case, it can be said that it has been confirmed that the image quality is further deteriorated.

  In the second verification test, the image forming apparatuses according to verification examples 1 to 5 having different image forming conditions are actually prepared, each of which is used for a long period of time, and the traveling distance of the surface of the photoreceptor increases. At this time, a sample of the formed image is extracted step by step and the image quality is confirmed to verify the difference in image quality due to the difference in image forming conditions.

  As shown in FIG. 7, the image forming apparatus according to the first verification example has an initial image forming condition (duty ratio drm on the developing potential side of the developing AC bias) regardless of the travel distance D on the surface of the photosensitive member. 40 [%], development AC bias peak-to-peak voltage Vpp: 1400 [V], speed ratio Θ: 1.85 [−] between the moving speed of the surface of the developing roller and the moving speed of the photosensitive member, toner density Ct : 6.5 [%]), and the image was formed.

  The image forming apparatus according to the verification example 2 changes the duty ratio drm on the developing potential side to 50 [%] among the initial image forming conditions when the travel distance D of the photoreceptor exceeds 30 [km]. Thus, an image is formed.

  In the image forming apparatus according to the verification example 3, when the travel distance D of the photosensitive member exceeds 30 [km], the duty ratio drm on the development potential side is changed to 50 [%] among the initial image forming conditions. In addition, the peak-to-peak voltage Vpp is changed to 1200 [V] to form an image.

  The image forming apparatus according to the verification example 4 changes the duty ratio drm on the developing potential side to 50 [%] among the default image forming conditions when the travel distance D of the photoreceptor exceeds 30 [km]. In addition, the image is formed by changing the speed ratio Θ between the developing roller and the photosensitive member to 1.5 [−].

  In the image forming apparatus according to the verification example 5, the duty ratio drm on the developing potential side is changed to 50 [%] among the initial image forming conditions when the travel distance D of the photoreceptor exceeds 30 [km]. In addition, the toner density Ct was changed to 5 [%] to form an image.

  The timing at which the sample of the image is extracted stepwise is every 10 [km] where the travel distance D of the photoreceptor is in the range from 0 [km] to 60 [km]. In addition, as image samples, there are two types of images: a 1-dot halftone image (image 1) and an image (image 2) with an exposure potential of 25% of the reference value. Confirmed. Here, in the evaluation, the case where the uniformity of the image density is sufficiently secured is defined as “good”, and the case where the uniformity of the image density is ensured to an acceptable level although it is insufficient is defined as “possible”. The case where the uniformity of the image density was not ensured to an acceptable level was determined as “impossible”.

  Note that the image forming conditions other than those described above were all set to be the same. Here, as in the above-described embodiment, a film made of a negatively charged organic photoreceptor material was used as the photoreceptor film, and the DC voltage component Vdc of the development AC bias was set to 400 [V]. As the charging device, a roller type was used.

  As shown in FIG. 8, in any of the image forming apparatuses according to the first to fifth verification examples, both the image 1 and the image 2 are “when the travel distance D of the photoconductor is in the range of 0 [km] to 30 [km]. The result “good” was obtained, and it was confirmed that the uniformity of the image density could be secured satisfactorily.

  However, in the image forming apparatus according to the verification example 1, when the travel distance D of the photosensitive member reaches 40 [km] and 50 [km], both the image 1 and the image 2 are evaluated as “good”. Furthermore, when reaching 60 [km], both the image 1 and the image 2 were evaluated as “impossible”.

  On the other hand, in the image forming apparatuses according to the verification examples 2 to 5, both the image 1 and the image 2 are “good” in the range of 30 [km] to 60 [km], and the image density is excellent. It was confirmed that uniformity was secured.

  According to the test results of the first and second verification tests described above, by using the image forming apparatus as in the embodiment of the present invention described above, the image density is low from the beginning to the end of the product life. It can be said that it has been experimentally confirmed that stable image quality can be maintained over the entire range from the density range to the high density range.

  In the above-described embodiment of the present invention, when the film thickness of the photoreceptor film falls below a predetermined value (that is, a predetermined threshold value), the developing potential side of the developing AC bias at the time of image formation In this example, the plurality of threshold values are set in stages, and each time the value falls below one of the plurality of threshold values set in stages, development at the time of image formation is illustrated. Of course, it is possible to increase the duty ratio on the developing potential side of the AC bias.

  Further, in the above-described embodiment of the present invention, the case where the film thickness information of the photosensitive film is acquired is illustrated as an example after the image forming instruction is received from the user. May be at any timing such as immediately after the image forming apparatus is turned on, immediately before or after image formation, or during image formation.

  Thus, the above-described embodiment disclosed herein is illustrative in all respects and is not restrictive. The technical scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Image forming apparatus, 2 Housing | casing, 3 Image forming part, 4 Paper feed part, 5 Paper feed unit, 6 Manual feed tray, 7 Open / close door, 10Y, 10M, 10G, 10K Image forming unit, 11 Intermediate transfer belt, 12 Exposure Device, 13 Transfer section, 21 Photoconductor, 21a Support shaft, 21b Drum, 21c Photoconductor film, 22 Charging device, 23 Developing roller, 24 Toner supply section, 25 Primary transfer roller, 26 Cleaning device, 26a Cleaner blade, 30 Control , 31 Bias applying unit, 32 Photosensitive member driving mechanism, 33 Developing roller driving mechanism, 34 Film thickness detecting unit.

Claims (6)

A photoreceptor whose surface is defined by a photoreceptor film and capable of carrying an electrostatic latent image;
A developing unit for supplying toner to the photoreceptor;
A bias applying unit for applying a developing AC bias to the developing unit;
A film thickness detector for detecting film thickness information of the photosensitive film;
A control unit for controlling the operation of the bias applying unit,
When the control unit determines that the film thickness of the photosensitive film is smaller than a predetermined value based on the film thickness information, the duty ratio on the development potential side of the development AC bias is increased and the development is performed. An image forming apparatus that controls the operation of the bias applying unit so that the duty ratio on the recovery potential side of the AC bias becomes small.   When the control unit determines that the film thickness of the photosensitive film is smaller than a predetermined value based on the film thickness information, the developing AC bias is further maintained while maintaining the area center voltage of the developing AC bias. The image forming apparatus according to claim 1, wherein the operation of the bias applying unit is controlled such that a difference between the potential and the area center voltage is reduced and a difference between the recovery potential and the area center voltage is increased.   When the control unit determines that the film thickness of the photosensitive film is smaller than a predetermined value based on the film thickness information, the difference between the development potential of the development AC bias and the recovery potential is further increased. The image forming apparatus according to claim 1, wherein the operation of the bias applying unit is controlled so that the peak-to-peak voltage is smaller. A photosensitive member driving mechanism for driving the photosensitive member;
A developing unit driving mechanism for driving the developing unit;
The controller further controls the operation of the photosensitive member driving mechanism and the operation of the developing unit driving mechanism;
When the control unit determines that the film thickness of the photoconductor film is smaller than a predetermined value based on the film thickness information, the moving speed of the surface of the photoconductor and the surface of the developing unit are further increased. 4. The image forming apparatus according to claim 1, wherein the operation of the photosensitive member driving mechanism and the operation of the developing unit driving mechanism are controlled so that a speed ratio with respect to the moving speed of the developing unit becomes small. A toner supply unit for supplying the toner to the developing unit;
The control unit further controls the operation of the toner supply unit;
When the control unit determines that the film thickness of the photosensitive film is smaller than a predetermined value based on the film thickness information, the density of the toner supplied to the developing unit is further reduced. The image forming apparatus according to claim 1, wherein the operation of the toner supply unit is controlled.   6. The image forming apparatus according to claim 1, wherein the film thickness detection unit includes a travel distance detection unit that detects a travel distance of the surface of the photoconductor.