JP6708425B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus used in an image forming apparatus such as an electrophotographic type or an electrostatic recording type.

2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus has been widely applied as a copying machine, a printer, a facsimile, and a composite machine having a plurality of functions of these. In such an image forming apparatus, the photoconductor film formed on the surface layer of the photoconductor drum, which is an image carrier, is charged by a charging unit such as a charging roller and exposed by a laser scanner to form an electrostatic latent image. Further, the electrostatic latent image on the surface of the photosensitive drum is developed as a toner image with toner by a developing device, transferred onto a sheet, and visualized.

As an image forming apparatus adopting such a process, in order to continuously form a good image, a photosensitive drum, a charging roller, etc. are housed in an image carrier unit (for example, a drum cartridge), and are attached to and detached from the apparatus body. Those that can be exchanged by doing have become popular. As this image carrier unit, there is one that accommodates a photosensitive drum in which the thickness of the photosensitive film is changed according to the expected number of prints. That is, for example, when printing a large number of sheets, an image carrier unit containing a photosensitive drum having a thick photosensitive film is set, and when printing a small number of sheets, a photosensitive drum having a thin photosensitive film is stored. The image carrier unit. As a result, the user can select an image carrier unit suitable for the desired number of printed sheets and mount it on the apparatus main body, and can secure a proper life of the photosensitive drum and form a good image. ..

When the thickness of the photosensitive film of the photosensitive drum is different, in order to charge the surface of the photosensitive drum to an appropriate potential, it is necessary to adjust the charging bias output from the charging unit according to the thickness of the photosensitive film. .. For this reason, an image carrier unit has been developed that includes a storage unit that stores the initial setting value of the film thickness of the photoconductor film (see Patent Document 1). The image forming apparatus to which the image carrier unit is mounted reads the initial setting value of the film thickness of the photoconductor film from the storage unit of the image carrier unit, and adjusts the charging bias output from the charging unit according to the film thickness. By doing so, the charging potential of the surface of the photosensitive drum is optimized.

On the other hand, an image forming apparatus has been developed which is capable of detecting the film thickness of the photosensitive film of the photosensitive drum after being mounted on the apparatus body (see Patent Document 2). In this image forming apparatus, the film thickness of the photoconductor film can be detected based on the relationship between the charging voltage applied to the charging unit and the charging current of the charging unit. In this image forming apparatus, the film thickness of the photoconductor film can be detected even after the photoconductor drum is attached to the apparatus main body. Therefore, by adjusting the charging bias output from the charging unit in accordance with the film thickness, the photoconductor drum is adjusted. The charging potential of the surface of the can be optimized. When detecting the film thickness of the photoconductor film by this method, it is necessary to apply the charging voltage after setting the surface potential of the photoconductor drum to a predetermined value.

In this image forming apparatus, the surface potential of the photosensitive drum can be converged to 0V if the AC charging method is used, but if the film thickness is detected by the DC charging method, the surface potential cannot be converged to 0V like AC charging. Therefore, a static eliminator such as a pre-exposure device or a static eliminator is used. Further, in an image forming apparatus of the DC charging type and having no static eliminator such as a pre-exposure device or a static eliminator, the film thickness of the photoconductor film is detected by setting the surface potential of the photosensitive drum to 0 V by the exposing means, for example. be able to.

JP, 10-133545, A JP-A-5-223513

However, in the image forming apparatus of Patent Document 1 described above, since the film thickness stored in the storage unit is an initial setting value based on the design value of the product, an error occurs in the film thickness of the photoconductor film during actual manufacturing. Therefore, the initial set value may be different from the actual film thickness. If the initial setting value is different from the actual film thickness, the image forming apparatus sets the charging bias according to the wrong initial setting value, and thus the charging potential of the surface of the photosensitive drum cannot be optimized. However, the formation of a good image may be hindered.

Further, in the image forming apparatus of Patent Document 2 described above, when the surface of the photosensitive drum is discharged by using the exposing means by the DC charging method and without the discharging portion such as the pre-exposure device and the discharging device, the photosensitive drum Passes through the developing device from the exposing means to the charging roller. That is, the portion of the photosensitive drum where the charge is removed by the exposing means passes through the developing device before reaching the charging roller. Therefore, in the case of the two-component developing method, the magnetic carrier carried on the developing sleeve is mechanically attached to the surface of the photosensitive drum, which may adversely affect the photosensitive drum. For this reason, it is not preferable to detect the film thickness of the photosensitive drum in the image device in the case of the DC charging method without the charge eliminating portion, and the charging bias is set according to an incorrect initial setting value. The charge potential on the surface of the drum cannot be optimized. That is, in the case of the DC charging method, the discharge start voltage changes depending on the film thickness of the photoconductor film, so that the surface potential of the photosensitive drum changes. In particular, in an image forming apparatus using a magnetic carrier such as dry two-component development, the appropriate range of surface potential that does not cause fogging or carrier adhesion is narrowed. As a result, image defects such as fogging and carrier adhesion tend to occur, which may hinder the formation of good images.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus capable of acquiring information regarding an accurate initial value of the film thickness of a photoconductor film of an image carrier even when it does not have a charge eliminating section.

An image forming apparatus of the present invention includes an apparatus main body, an image carrier, and a charging roller that is in contact with the image carrier and applies a DC voltage only to charge the image carrier. An image carrier unit that is attachable to and detachable from the main body, a memory that is provided in the image carrier unit, in which information regarding the initial discharge start voltage value of a new image carrier unit is recorded, and the memory that is stored in the memory. The information about the initial discharge start voltage value and the amount of change in the discharge start voltage value with respect to the amount of use of the image carrier set in advance, the discharge start voltage value is obtained by using the charge roller at the time of image formation. And a control unit that determines a DC voltage value to be applied.

According to the present invention, before the image carrier unit is attached to the apparatus main body, information regarding the film thickness of the photoconductor film of the image carrier is measured, and the measured value is stored in the storage means. Therefore, the control unit of the image forming apparatus can read the measured value from the storage unit of the image carrier unit mounted on the apparatus main body and control the control target such as the charging unit based on the measured value. .. As a result, the image forming apparatus can obtain the information regarding the accurate initial value of the film thickness of the photoconductor film of the image carrier, even when the image carrier does not have the charge removing unit for discharging the surface of the image carrier. ..

FIG. 1 is a sectional view showing a schematic configuration of an image forming apparatus according to an embodiment. FIG. 3 is a cross-sectional view showing a schematic configuration of the image forming unit according to the embodiment. It is a sectional view showing a schematic structure of a drum cartridge concerning an embodiment. It is a block diagram which shows the schematic structure of the control system which concerns on embodiment. 6 is a flowchart showing a procedure for measuring the film thickness of the photosensitive film of the drum cartridge according to the embodiment. 6 is a timing chart for measuring the film thickness of the photoconductor film of the drum cartridge according to the exemplary embodiment. 6 is a flowchart illustrating a procedure of mounting a drum cartridge on the image forming apparatus according to the embodiment and reading a measured value. 6 is a graph showing the relationship between the distance traveled by the photosensitive drum and the amount of change in the discharge start voltage according to the embodiment.

Hereinafter, the image carrier unit according to the embodiment of the present invention will be described in detail with reference to FIGS. In this embodiment, the case where the image carrier unit is applied to a tandem type full color printer as an example of the image forming apparatus is described. However, the present invention is not limited to an image carrier unit of a tandem type image forming apparatus, and may be an image carrier unit of an image forming apparatus of another system, and is not limited to being a full color and a monochrome. Or it may be monochrome. Alternatively, it can be implemented in various applications such as a printer, various printing machines, copiers, fax machines, and multi-function machines by adding necessary equipment, equipment, and housing structure. Further, in the present embodiment, the image forming apparatus 1 has the intermediate transfer belt 44b, and after the toner image of each color is primarily transferred from the photosensitive drum 51 to the intermediate transfer belt 44b, the composite toner image of each color is transferred to the sheet S. Secondary transfer is performed collectively. However, the present invention is not limited to this, and a method of directly transferring from the photosensitive drum to the sheet conveyed by the sheet conveying belt may be adopted.

As shown in FIG. 1, the image forming apparatus 1 includes an apparatus main body 10, a sheet feeding unit 30, an image forming unit 40, a sheet conveying unit and a sheet discharging unit (not shown), and a control unit 20. There is. The sheet S, which is a recording material, is one on which a toner image is formed, and specific examples thereof include plain paper, a synthetic resin sheet that is a substitute for plain paper, a thick paper, and a sheet for an overhead projector.

The apparatus main body 10 is a housing that accommodates the sheet feeding unit 30, the image forming unit 40, the control unit 20, and the like. For example, a door (not shown) that can be opened and closed is provided on the front side surface. By opening this door, the drum cartridge 57 described later can be attached to and detached from the apparatus main body 10. Further, the apparatus body 10 is provided with a door sensor (not shown) that detects opening/closing of the door.

The sheet feeding unit 30 is disposed in the lower portion of the apparatus main body 10, includes a sheet cassette 31 that stacks and stores the sheets S, and a feeding roller 32, and feeds the sheets S to the image forming unit 40. ..

The image forming unit 40 includes image forming units 50y, 50m, 50c and 50k, a toner bottle (not shown), exposure devices 42y, 42m, 42c and 42k, an intermediate transfer unit 44, a secondary transfer unit 45, and a fixing unit. And a section 46. The image forming unit 40 can form an image on the sheet S based on the image information. The image forming apparatus 1 according to the present embodiment is compatible with full color, and the image forming units 50y, 50m, 50c and 50k include yellow (y), magenta (m), cyan (c) and black ( Each of the four colors k) is separately provided with the same configuration. Therefore, in FIG. 1, each of the four color components is shown with the same reference numeral followed by a color identifier, but in FIGS. 2 and 3 and the specification, only the reference numeral is used without a color identifier. In some cases.

In this embodiment, a two-component developer which is a mixture of non-magnetic toner and magnetic carrier is used as the developer. The toner is produced by encapsulating a colorant, a wax component, and the like in a resin such as polyester or styrene and pulverizing or polymerizing. The carrier is produced by applying a resin coating to the surface layer of a core made of resin particles obtained by kneading ferrite particles or magnetic powder.

The image forming unit 50 includes four image forming units 50y, 50m, 50c and 50k for forming four color toner images. As shown in FIG. 2, each image forming unit 50 includes a photosensitive drum (image carrier) 51 that forms a toner image, a charging device (charging means) 52, a developing device 53, and a cleaning device 54. There is. The image forming unit 50 according to the present embodiment does not have a charge removing unit that charges the surface of the photosensitive drum 51.

The photosensitive drum 51 is an organic photosensitive member including a cylindrical conductive support 51a and a photosensitive film 51b having a photosensitive layer of an organic material and a surface protective layer, which are sequentially stacked on the outer peripheral surface of the conductive support 51a. It consists of a body and can carry an electrostatic image on its surface. That is, the photosensitive drum 51 has a photosensitive film 51b capable of carrying an electrostatic image on the surface layer. The photosensitive layer is formed by applying a thin film of an organic photoconductor (OPC) having a negative charging polarity. The surface protective layer contains fine particles of fluororesin. In the present embodiment, a cylinder made of aluminum having a thickness of 1 mm is used as the conductive support 51a, and a photosensitive film 51b having a film thickness of 20 μm is formed on the outer peripheral surface of the conductive support 51a. A photosensitive drum 51 having a diameter of 30 mm is configured. The photosensitive drum 51 rotates in the arrow direction at a predetermined process speed (peripheral speed). The image forming apparatus 1 according to the present embodiment does not have a film thickness detecting means for detecting the film thickness of the photoconductor film 51b of the photosensitive drum 51.

The charging device 52 has a charging roller 55 and a power source 56, and can charge the surface of the photosensitive drum 51 by applying a charging voltage to the surface of the photosensitive film 51b. The charging roller 55 is arranged in contact with the photosensitive drum 51, and uniformly charges the surface of the photosensitive drum 51 to a predetermined potential. Both ends of the charging roller 55 in the longitudinal direction are supported by a pressing mechanism (not shown) toward the photosensitive drum 51 with a predetermined pressing force. Therefore, the charging roller 55 is driven to rotate as the photosensitive drum 51 rotates. The driving method of the charging roller 55 is not limited to the driven rotation of the photosensitive drum 51, and may be a driving method using an electric motor, for example.

The charging roller 55 has a conductive cored bar 55a that is a shaft and serves as a base body, and an elastic layer 55b provided around the conductive cored bar 55a. The conductive cored bar 55a is made of, for example, a metal material such as iron, copper, stainless steel, or aluminum, and is made of aluminum in the present embodiment. Incidentally, the conductive cored bar 55a may be subjected to a plating treatment in order to impart rust prevention and scratch resistance as long as the conductivity is not lost.

The elastic layer 55b is formed by dispersing rubber (EPDM (ethylene-propylene-diene rubber)) which is an elastic material and carbon black which is a conductive agent. In the present embodiment, the elastic layer 55b has a conductivity adjusted by the resistance adjustment treatment of less than 10 ¹⁰ Ωcm due to the dispersion of carbon black. In this embodiment, the diameter of the conductive cored bar 55a is 8 mm, the resistance of the elastic layer 55b is 1×10 ⁶ Ωcm, and the outer diameter of the charging roller 55 is 14 mm. As the elastic material, natural rubber, SBR, silicone rubber, urethane rubber, epichlorohydrin rubber, synthetic rubber such as IR, BR, NBR and CR, polyamide resin, polyurethane resin, silicone resin, etc. may be used. . Further, as the conductive agent, electron conductive materials such as graphite and conductive metal oxides, and ion conductive materials such as alkali metal salts may be used.

The elastic layer 55b has a so-called crown shape in which the central portion in the longitudinal direction is thicker than both end portions in the longitudinal direction. In this embodiment, the elastic layer 55b is formed into a crown shape by polishing the surface of the elastic layer 55b. As described above, since both ends of the charging roller 55 in the longitudinal direction are urged by the pressing mechanism toward the photosensitive drum 51 with a predetermined pressing force and supported, the charging roller 55 may slightly bend. For this reason, the contact pressure on the photosensitive drum 51 at the central portion in the longitudinal direction of the charging roller 55 tends to be smaller than that at the both end portions, but by forming the elastic layer 55b in a crown shape, the longitudinal direction of the charging roller 55 is increased. The contact pressure is uniform regardless of the position in the direction.

The power supply 56 is connected to the conductive cored bar 55a and can apply a charging bias. In this embodiment, the charging bias is a DC voltage of -1400V. When the DC charging method is adopted, the film thickness of the photoconductor film 51b is abraded due to long-term use, and the discharge start voltage is lowered. Therefore, when a constant voltage is continuously applied to the photoconductor film 51b, the surface potential increases as the film thickness of the photoconductor film 51b decreases. It is desirable to control the applied voltage to be lowered in accordance with the decrease of

The developing device 53 includes a developing container 53a, transport screws 53b and 53c, and a developing sleeve 53d. Toner is supplied to the developing container 53a from a toner bottle filled with toner. The transport screws 53b and 53c stir the developer inside the developing container 53a and supply the developer to the developing sleeve 53d. The developing sleeve 53d is provided so as to face the photosensitive drum 51, and develops the electrostatic image formed on the photosensitive drum 51 with the toner contained in the developing container 53a.

As a result, the charging roller 55 comes into contact with the surface of the photosensitive drum 51 and charges the surface of the photoconductor film 51b to, for example, a uniform negative dark part potential. After charging, the exposure device 42 forms an electrostatic image on the surface of the photosensitive drum 51 based on image information. The photosensitive drum 51 carries the formed electrostatic image, moves around, and is developed with toner by the developing device 53. The developed toner image is primarily transferred to an intermediate transfer belt 44b described later. The cleaning device 54 is arranged so that the blade 54a is in contact with the surface of the photosensitive drum 51, and cleans the transfer residual toner and the like remaining on the surface of the photosensitive drum 51 after the primary transfer.

In the present embodiment, as shown in FIG. 3, the photosensitive drum 51, the charging roller 55, and the cleaning device 54 are provided in a drum cartridge (image carrier unit) 57 and are unitized. The drum cartridge 57 is attachable to and detachable from (attachable to) the apparatus main body 10, and when attached, constitutes the image forming unit 50 together with the developing device 53 and the like. As shown in FIG. 4, the drum cartridges 57y, 57m, 57c, and 57k are separately provided with the same configuration for each of the four colors, and can be attached to and detached from the apparatus body 10 (see FIG. 1).

Each of the drum cartridges 57y, 57m, 57c, 57k has memory tags (storage means) 58y, 58m, 58c, 58k. Each of the memory tags 58y, 58m, 58c, 58k is composed of, for example, a non-volatile memory, and stores the measurement value measured before mounting on the apparatus main body 10 regarding the information about the film thickness of the photosensitive film 51b of each photosensitive drum 51. There is. In the present embodiment, each memory tag 58 sends the information about the film thickness of the photoconductor film 51b of each photoconductor drum 51 to the apparatus main body 10 about the discharge start voltage on the surface of the photoconductor film 51b of each photoconductor drum 51. It stores the measured values measured before mounting.

As shown in FIG. 1, the intermediate transfer unit 44 is arranged below the image forming units 50y, 50m, 50c and 50k. The intermediate transfer unit 44 includes a plurality of rollers such as a driving roller 44a, a driven roller 44d, primary transfer rollers 43y, 43m, 43c, and 43k, and an intermediate transfer belt 44b wound around these rollers. Each of the primary transfer rollers 43 is arranged so as to face the photosensitive drum 51, and abuts on the intermediate transfer belt 44b.

By applying a positive transfer bias from each power source 47 connected to each primary transfer roller 43 through each primary transfer roller 43, each negative polarity on each photosensitive drum 51y, 51m, 51c, 51k is changed. The toner images that it has are sequentially transferred in multiplex onto the intermediate transfer belt 44b. As a result, the intermediate transfer belt 44b transfers and moves the toner image obtained by developing the electrostatic image on the surface of the photosensitive drum 51.

The secondary transfer portion 45 includes a secondary transfer inner roller 45a and a secondary transfer outer roller 45b. By applying a positive secondary transfer bias to the outer secondary transfer roller 45b, the full-color image formed on the intermediate transfer belt 44b is transferred to the sheet S. The fixing unit 46 includes a fixing roller 46a and a pressure roller 46b. When the sheet S is sandwiched and conveyed between the fixing roller 46a and the pressure roller 46b, the toner image transferred to the sheet S is heated and pressed to be fixed on the sheet S. The sheet S on which the toner image is formed is conveyed by a sheet conveying unit (not shown) and discharged from a sheet discharging unit (not shown).

As shown in FIG. 4, the control unit 20 is configured by a computer, for example, a CPU 21, a ROM 22 that stores a program that controls each unit, a RAM 23 that temporarily stores data, and an input/output that inputs and outputs signals to and from the outside. And a circuit (I/F) 24. The CPU 21 is a microprocessor that controls the entire control of the image forming apparatus 1, and is the main body of the system controller. The CPU 21 is connected to the sheet feeding unit 30, the image forming unit 40, the sheet conveying unit, the sheet discharging unit, and the operation unit via the input/output circuit 24, exchanges signals with each unit, and controls the operation. Also, the CPU 21 via the input/output circuit 24, memory tags 58y, 58m, 58c, 58k of the drum cartridges 57y, 57m, 57c, 57k and power sources 56y, 56m, 56c of the charging rollers 55y, 55m, 55c, 55k. , 56k. The control unit 20 executes the control using the measurement values stored in the memory tags 58y, 58m, 58c, 58k, and particularly controls the power supplies 56y, 56m, 56c, 56k based on the measurement values to control the charging voltage. Control. Further, the control unit 20 is connected to a door sensor that detects opening/closing of the door of the apparatus body 10.

Next, an image forming operation in the image forming apparatus 1 configured as above will be described.

As shown in FIG. 1, when the image forming operation is started, the photosensitive drum 51 rotates and the surface is charged by the charging roller 55. Then, the exposure device 42 emits laser light to the photosensitive drum 51 based on the image information, and an electrostatic latent image is formed on the surface of the photosensitive drum 51. By the toner adhering to the electrostatic latent image, it is developed and visualized as a toner image, and is transferred to the intermediate transfer belt 44b.

On the other hand, the feeding roller 32 is rotated in parallel with such a toner image forming operation, and the uppermost sheet S of the sheet cassette 31 is fed while being separated. Then, the sheet S is conveyed to the secondary transfer portion 45 via the conveying path in synchronization with the toner image on the intermediate transfer belt 44b. Further, the image is transferred from the intermediate transfer belt 44b to the sheet S, and the sheet S is conveyed to the fixing unit 46, where the unfixed toner image is heated and pressed to be fixed on the surface of the sheet S, and the sheet discharging unit. Discharged from.

Next, a procedure for detecting the discharge start voltage Vth of the photosensitive film 51b of the photosensitive drum 51 and writing it in the memory tags 58y, 58m, 58c, 58k will be described with reference to the flowchart of FIG. 5 and the time chart of FIG. To do.

An assembled drum cartridge 57 and a detection device for detecting the discharge start voltage Vth of the photosensitive drum 51 housed therein are prepared in advance. As the detection device here, for example, a driving unit capable of driving the photosensitive drum 51, a voltage applying unit capable of applying a DC voltage to the charging roller 55, and a surface potential vd at a position corresponding to the developing area of the photosensitive drum 51. It is assumed to be provided with a potential measuring unit capable of measuring and a charge eliminating unit. The charge removing unit is, for example, an AC voltage applying unit or a charge removing member, and is arranged in a charge removing region downstream of the developing region of the photosensitive drum 51 in the drum rotation direction and upstream of the charging roller 55 in the drum rotation direction. The surface of the drum 51 can be neutralized. In addition, in this Embodiment, although the detection device has the static elimination part, it is not limited to this, and the static elimination part may be omitted. Further, the detection device is not limited to the configuration including only the drive unit, the voltage application unit, the potential measurement unit, and the static elimination unit, and for example, an image forming apparatus modified for detection may be used. ..

Here, the principle of detecting the discharge start voltage Vth of the photoconductor film 51b by this detection device will be described. In the case of the DC charging method, the electrostatic capacity of the photoconductor film 51b of the photosensitive drum 51 is C1, and the electrostatic capacity of the air layer between the discharge regions between the charging roller 55 and the photosensitive drum 51 is C2. The charging phenomenon of the photosensitive drum 51 by 55 can be expressed by an electrical equivalent circuit. In a steady environment, the resistance of the charging roller 55 is smaller than that of the photosensitive drum 51 and the air layer, and can be ignored. When the DC voltage V is applied to this equivalent circuit, the voltage is proportionally distributed to the impedance of each capacitor, and the voltage Va applied to the air layer is calculated by the following formula (1).
Va=C1/(C1+C2)×V... (1)

There is a breakdown voltage according to Paschen's law in the air layer, and assuming that the thickness of the air layer is d [μm], discharge occurs when the voltage Va applied to the air layer exceeds 312+6.2 d [V], and charging occurs. Is performed. Since the voltage at which discharge occurs for the first time is when the quadratic equation for d when the voltage Va in Formula (1) becomes equal to 312+6.2d has multiple solutions (electrostatic capacitance C2 is also a function of d), The voltage Va at this time corresponds to the discharge start voltage Vth. That is, the discharge start voltage Vth is calculated by the following mathematical expression (2).
Vth=312+6.2×10 ⁶ ×t/ε+(7736.7×10 ⁶ ×t/ε) ^0.5
(However, ε: dielectric constant of photosensitive layer, t: thickness of photosensitive layer) (2)

Therefore, when the film thickness t of the photoconductor film 51b of the photoconductor drum 51 decreases, the electrostatic capacitance C1 of the photoconductor film 51b of the photoconductor drum 51 increases and the discharge start voltage Vth decreases. For example, in the present embodiment, the photosensitive film 51b has a film thickness of 20 μm, the discharge start voltage Vth of the photosensitive drum 51 is about 580 V, and the change amount of the discharge start voltage Vth per film thickness 1 μm is about 8 V. is there. Therefore, the film thickness of the photoconductor film 51b can be calculated by measuring the discharge start voltage Vth.

When detecting the discharge start voltage Vth of the photoconductor film 51b of the photosensitive drum 51, the drum cartridge 57 is installed in the detecting device, and the driving unit of the detecting device starts the driving of the photosensitive drum 51 (step S1, FIG. 6). t1). Then, the charge removal unit of the detection device starts charge removal of the photosensitive drum 51 (step S2, t1 in FIG. 6). If the detection device does not have a static elimination unit, step S2 is not executed.

While the rotation speed of the photosensitive drum 51 is stable, the voltage application unit applies the first charging voltage V1 (for example, 1200 V) via the charging roller 55 (step S3, t2 to t4 in FIG. 6). After applying the first charging voltage V1, the surface potential vd1 of the photosensitive drum 51 is measured by the potential measuring unit in the developing area of the photosensitive drum 51 (step S4, t3 in FIG. 6). It is assumed that the measurement result here is 620 V, for example. The measurement of the first surface potential vd1 and the second and third surface potentials vd2 and vd3, which will be described later, by the potential measuring unit include at least the photosensitive drum 51 and the charging roller 55 in the circumferential direction. It is preferable to measure at least one revolution of the drum 51. However, of course, it is not limited to this.

Next, the voltage application unit applies the second charging voltage V2 (for example, 1400V) via the charging roller 55 (step S5, t5 to t7 in FIG. 6). After applying the second charging voltage V2, the potential measuring unit measures the second surface potential vd2 of the photosensitive drum 51 in the developing area of the photosensitive drum 51 (step S6, t6 in FIG. 6). It is assumed that the measurement result here is, for example, 820V.

Further, the voltage application unit applies the third charging voltage V3 (for example, 1600 V) via the charging roller 55 (step S7, t8 to t10 in FIG. 6). After applying the third charging voltage V3, the potential measuring unit measures the third surface potential vd3 of the photosensitive drum 51 in the developing area of the photosensitive drum 51 (step S8, t9 in FIG. 6). The measurement result here is assumed to be, for example, 1020V.

After three times of measurement by the potential measuring unit, the charge removing unit removes the charge, and then the charge removing is stopped (step S9, t11 in FIG. 6), and the drive of the photosensitive drum 51 is stopped (step S10, t11 in FIG. 6). ). If the detection device does not have a charge removal unit, step S9 is not executed. Then, the discharge start voltage Vth is calculated by averaging three times the difference between each charging voltage V1 to V3 and each surface potential vd1 to vd3 at that time (step S11). That is, the discharge start voltage Vth can be calculated by the following mathematical expression (3).
Vth=((V1-vd1)+(V2-vd2)+(V3-vd3))/3 (3)
The calculated discharge start voltage Vth is stored in the memory tag 58 provided in the drum cartridge 57 as a measured value of the discharge start voltage Vth (step S12). For example, in the present embodiment, as shown in FIG. 6, Vth=580 [V].

Next, FIG. 7 shows a procedure in which the image forming apparatus 1 reads the measured value of the discharge starting voltage Vth after the drum cartridge 57 having the memory tag 58 in which the discharge starting voltage Vth is written is attached to the image forming apparatus 1. A description will be given along the flow chart shown.

After the discharge start voltage Vth is stored in the memory tag 58 (step S21, step S12 in FIG. 5), the drum cartridge 57 is sealed and packaged and shipped as a product (step S22). When using a new drum cartridge 57, for example, a user of the image forming apparatus 1 unpacks the drum cartridge 57 and mounts it on the image forming apparatus 1 (step S23). The control unit 20 determines whether or not the main body power supply of the image forming apparatus 1 is turned on (step S24), and when the main body power supply is turned on, the door on the front side of the apparatus main body 10 is closed. It is determined whether or not it is present based on the detection result of the door sensor (step S25). When the control unit 20 determines that the door of the apparatus body 10 is closed, it is determined that the drum cartridge 57 is attached and the image formation is possible, and the discharge start voltage Vth is output from the memory tag 58 of the drum cartridge 57. The measured value of is read (step S26). If the control unit 20 determines that the main body power supply is not turned on, or if the door of the main body 10 of the apparatus is not closed, the control unit 20 does not access the memory tag 58, and then the image forming apparatus 1 is restarted. It is determined whether or not the main body power source of is on (step S24).

In step S26, if the control unit 20 cannot read the information of the memory tag 58 or if the read value exceeds the allowable range, the change amount of the discharge start voltage Vth is set to be small. That is, in the table of FIG. 8 which will be described later, control is performed so as to be on the side of the reduction lower limit value 64. As a result, it is possible to more reliably suppress the occurrence of carrier adhesion, so that it is possible to prevent deterioration of the photosensitive drum 51 due to carrier adhesion.

The control unit 20 writes the discharge start voltage Vth read from the memory tag 58 in the RAM 23 as an offset value of the charging DC voltage applied during image formation. Here, the ROM 22 stores in advance a discharge start voltage table according to the traveling distance of the photosensitive drum 51 and a discharge start voltage table according to the operating environment of the main body, and the control unit 20 charges the charging applied during image formation. The DC voltage is determined from the mileage and the operating environment of the main body. The control unit 20 adds the offset value obtained from the memory tag 58 to the determined charging DC voltage to perform actual control.

FIG. 8 shows changes in the allowable range (latitude) for fogging or carrier adhesion with respect to the amount of change in the discharge start voltage Vth. As described above, the discharge start voltage Vth is lowered by reducing the film thickness of the photoconductor film 51b of the photosensitive drum 51 according to the traveling distance of the photosensitive drum 51. When a constant voltage is continuously applied, the surface potential rises as the film thickness decreases, so it is necessary to control the applied voltage according to the film thickness of the photoconductor film 51b. It is widely known that the decreasing transition of the film thickness of the photoconductor film 51b is influenced by the usage condition of the image forming apparatus 1, the usage environment, the charging DC current amount, the contact pressure of the cleaning device 54, and the like. In the present embodiment, since the image forming apparatus 1 does not have the film thickness detecting means, as shown in FIG. 8, the relationship between the traveling distance of the photosensitive drum 51 and the variation amount of the discharge start voltage Vth is obtained in advance. It is acquired as a table.

In this table, when the film thickness of the photoconductor film 51b is drastically reduced from the allowable range and the discharge start voltage Vth is greatly reduced from the allowable range, the carrier adhesion limit value 61 is set as the occurrence of carrier adhesion. ing. A region where the discharge start voltage Vth exceeds the carrier adhesion limit value 61 is defined as a carrier adhesion region R2 where carrier adhesion occurs. Further, in this table, when the film thickness of the photoconductor film 51b is smaller than the permissible range and the discharge start voltage Vth is lower than the permissible range, the fog limit value 62 is set as the fog is generated. ing. A region where the discharge start voltage Vth exceeds the fogging limit value 62 is defined as a fogging region R3 in which fogging occurs. Therefore, the center value 60 of the discharge start voltage Vth is set to the center of the limit values 61 and 62 in the allowable region R1 between the carrier adhesion limit value 61 and the fogging limit value 62 so as to avoid carrier adhesion and fogging as much as possible. It is set to be near.

It is preferable to design the upper and lower limits of the variation in variation with respect to the center value 60 to have an allowable range for carrier adhesion and fogging. Therefore, with respect to the center value 60, the reduction upper limit value 63 and the reduction lower limit value 64 are set in accordance with the traveling distance of the photosensitive drum 51 so that neither of them reaches the limit values 61 and 62. ..

However, when the initial value of the film thickness of the photoconductor film 51b is different from the actual film thickness, the center value 60 deviates from the actual value. Therefore, for example, when the initial value of the film thickness is thinner than the actual film thickness by the amount of error and reaches the reduction upper limit value 63, the actual reduction upper limit 63 There is a relationship of value 65. In this case, when the traveling distance of the photosensitive drum 51 becomes long, the actual reduction upper limit value 65 exceeds the carrier adhesion limit value 61, so that there is a possibility that carrier adhesion may occur during long-term use. For example, when the initial value of the film thickness is thicker than the actual film thickness by an error amount and reaches the reduction lower limit value 64, the actual reduction lower limit value 66 obtained by subtracting the error amount of the film thickness from the reduction lower limit value 64 is used. It becomes a relationship. In this case, when the traveling distance of the photosensitive drum 51 becomes long, the actual reduction lower limit value 66 exceeds the fogging limit value 62, so that there is a possibility that fog may occur during long-term use.

On the other hand, according to the drum cartridge 57 of the present embodiment, the film thickness of the photoconductor film 51b is measured before the drum cartridge 57 is attached to the apparatus body 10, and the control unit 20 measures the film thickness during image formation. It uses the value. Therefore, the relationship between the travel distance of the photosensitive drum 51 and the change amount of the discharge start voltage Vth can be corrected so as to approach the center value 60. Therefore, the amount of change in the discharge start voltage Vth can be controlled so that carrier adhesion and fogging do not occur.

As described above, according to the drum cartridge 57 of the present embodiment, the discharge start voltage Vth of the photoconductor film 51b of the photosensitive drum 51 is measured before the drum cartridge 57 is attached to the apparatus main body 10, and the measured value is It is stored in the memory tag 58. Therefore, the control unit 20 can read the measured value from the memory tag 58 of the drum cartridge 57 mounted on the apparatus body 10 and control the charging device 52, for example, based on the measured value. As a result, the image forming apparatus 1 can obtain the information regarding the accurate initial value of the film thickness of the photoconductor film 51b of the photosensitive drum 51, even when the image forming apparatus 1 does not have the charge removing unit for removing the charge on the surface of the photosensitive drum 51. Becomes As a result, it is possible to provide a stable and good image for a longer period of time even in the image forming apparatus 1 that employs the two-component development method without using the DC charging method and without the charge removal section.

Further, according to the image forming apparatus 1 of the present embodiment, the discharge start voltage Vth of the photoconductor film 51b is applied as the information regarding the film thickness of the photoconductor film 51b. Therefore, when the control unit 20 controls the charging voltage applied to the charging roller 55, the change amount of the discharge start voltage Vth read from the table (see FIG. 8) is changed to the discharge start voltage Vth read from the memory tag 58. It can be easily corrected using the measurement value of. Therefore, control can be simplified.

In the image forming apparatus 1 of the above-described embodiment, the case where the discharge start voltage Vth of the photoconductor film 51b is applied as the information regarding the film thickness of the photoconductor film 51b has been described, but the present invention is not limited to this. The information regarding the film thickness of the photoconductor film 51b may be, for example, the film thickness of the photoconductor film 51b or the charging direct current of the photoconductor film 51b. Also in these cases, the control unit 20 can read the measured value from the memory tag 58 of the drum cartridge 57 mounted on the apparatus body 10 and control the charging device 52, for example, based on the measured value.

When the information about the film thickness of the photoconductor film 51b is the film thickness of the photoconductor film 51b, the film thickness can be calculated as follows. In the present embodiment, as described above, the discharge start voltage Vth of the photoconductor film 51b having a film thickness of 20 μm is 580 [V], and the change amount of the discharge start voltage Vth per 1 μm of the film thickness of the photoconductor film 51b. Is about 8V. Therefore, when the calculated discharge start voltage Vth is, for example, 572 [V], the film thickness of the photoconductor film 51b can be calculated to be 19 μm.

DESCRIPTION OF SYMBOLS 1... Image forming apparatus, 10... Device main body, 20... Control part, 51, 51c, 51k, 51m, 51y... Photosensitive drum (image carrier), 51b... Photosensitive film, 52, 52c, 52k, 52m, 52y... Charging device (charging means), 57, 57c, 57k, 57m, 57y... Drum cartridge (image carrier unit), 58, 58c, 58k, 58m, 58y... Memory tag (storage means).

Claims (2)

The device body,
An image carrier unit that includes an image carrier and a charging roller that is in contact with the image carrier and that applies only a DC voltage to charge the image carrier, and is detachable from the apparatus body. ,
A memory provided in the image carrier unit, in which information about the initial discharge start voltage value of a new image carrier unit is recorded,
Information about the initial discharge start voltage value stored in the memory, and a change amount of the discharge start voltage value with respect to the preset usage amount of the image carrier, to obtain the discharge start voltage value, the image A controller for determining a DC voltage value applied to the charging roller at the time of formation,
An image forming apparatus characterized by the above. The control unit uses the information about the initial discharge start voltage value, the amount of change in the discharge start voltage value with respect to the preset usage amount of the image carrier, and the information about the use environment, and the discharge start voltage value is used. To determine the DC voltage value applied to the charging roller during image formation,
The image forming apparatus according to claim 1, wherein:

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