JP2010250051A - Image forming apparatus - Google Patents

Abstract

An object of the present invention is to provide an image forming apparatus capable of suppressing a change in image density and a reduction in gradation even when the film thickness of an image carrier changes.
A typical configuration of an image forming apparatus according to the present invention includes a photosensitive drum 1 and a sleeve 20 that carries a developer, and the electrostatic latent image formed on the photosensitive drum 1 is developed as a developer. A developing device 4 for developing the toner image, a bias applying device 19 for applying a developing bias to the sleeve 20,
A film thickness detection circuit 31 for detecting information on the film thickness of the photosensitive drum 1;
The AC bias of the developing bias applied to the sleeve 20 is controlled based on the detection result of the film thickness detecting circuit 31 so that the toner density to be developed decreases as the film thickness of the photosensitive drum 1 decreases. And a control circuit 33.
[Selection] Figure 6

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a recorded image display apparatus, and a facsimile using an electrophotographic system, an electrostatic recording system, and the like.

  In general, the surface of an organic photoreceptor drum (image bearing member) used in an electrophotographic image forming apparatus is actively polished by a cleaning blade, and a good image is maintained by refreshing the surface. As a result, the surface layer thickness of the photosensitive drum decreases with image formation. Thus, when the photosensitive drum is scraped and the film thickness is reduced, a phenomenon in which the capacitance of the photosensitive drum increases is caused.

  That is, when the electrostatic capacity C of the photosensitive drum, the dielectric constant ε, the area S contributing to charging, and the film thickness d are set, the film thickness d in C = ε · (S / d) (1) is reduced. The capacitance C increases. As the electrostatic capacitance C in the formula (1) increases, the amount of charge that can be held on the photosensitive drum increases.

  That is, when the charge amount Q that can be held on the photosensitive drum, the electrostatic capacitance C, and the potential difference V between the developing sleeve and the photosensitive drum, the electrostatic capacitance C in the relational expression of Q = CV (2) increases. Therefore, the value of the charge amount Q increases accordingly. When the amount of charge Q that can be held on the photosensitive drum in Formula (2) increases, the amount of toner that can be placed on the photosensitive drum increases, so that the image density increases. That is, as the surface layer thickness d of the photosensitive drum decreases, the amount of toner carried by the photosensitive drum 1 increases and the image density increases.

  On the other hand, there are cases where the image density fluctuates during image formation due to environmental fluctuations, deterioration of the developer, and the like, in addition to the surface layer of the photosensitive drum. With respect to these image density fluctuations, control for keeping the density constant is performed during image formation. As control for keeping the image density constant, there are a method for controlling the development contrast and a method for controlling the mixing ratio (toner density) of the toner and the carrier. As is well known, the former controls the amount of toner loaded on the photosensitive drum by changing the development contrast. In the latter, the charge amount applied to the toner is optimized by correcting the mixing ratio of the toner and the carrier, and the loading amount of the toner placed on the photosensitive drum is controlled.

(Method for controlling development contrast)
The above-described image density control is also proposed for the photoconductor drum scraping. In Patent Document 1 (Japanese Patent Laid-Open No. 10-232523), development contrast is corrected according to the usage state of the photosensitive drum, and the image density is controlled.

JP-A-10-232523

(Method for controlling development contrast)
However, Japanese Patent Laid-Open No. 2004-228561 performs the density increase accompanying the decrease in the drum film thickness by reducing the maximum development contrast (the maximum value of the potential difference between the drum potential and the development potential). Here, if the maximum development contrast is reduced, there are the following problems. Normally, the gradation of the image density is realized by changing the contrast stepwise with respect to the maximum development contrast. At this time, if the maximum development contrast is reduced, the potential width Vw of the development contrast that can be assigned to one gradation is reduced. For this reason, when the drum surface potential fluctuates for some reason, the development contrast fluctuates, and the image density tends to fluctuate.

  Therefore, in order to suppress fluctuations in image density, it is conceivable to maintain the development contrast potential width Vw assigned to one gradation as it is in the initial state. However, the number of gradations decreases (hereinafter referred to as a decrease in gradation).

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of suppressing a change in image density and a reduction in gradation even when the film thickness of an image carrier changes.

In order to solve the above problems, a typical configuration of an image forming apparatus according to the present invention includes an image carrier,
A developing device that includes a developer carrying member carrying the developer, and that develops the electrostatic latent image formed on the image carrier with the developer; and a bias applying device that applies a developing bias to the developer carrying member. ,
A sensor for detecting information relating to the film thickness of the image carrier, and development applied to the developer carrier so that the developed toner density with respect to a predetermined development contrast potential decreases as the film thickness decreases. And a controller for controlling an AC bias of the bias.

  According to the present invention, it is possible to provide an image forming apparatus capable of suppressing a change in image density and a reduction in gradation even when the film thickness of the image carrier changes.

1 is a configuration diagram of an image forming apparatus according to a first embodiment. It is a block diagram of a developing device. (A) It is a figure which shows the waveform of the developing bias which concerns on 1st embodiment. (B) (c) It is a figure which shows the waveform of the blank pulse bias which concerns on 2nd embodiment. It is a block diagram which shows a detection means and a control means. It is a figure which shows the relationship between the detection electric current amount (DC electric current value) which flows into a photosensitive drum, and the surface layer film thickness of a photosensitive drum. (A) It is a figure which shows the correction | amendment flow of the amplitude of the alternating current component of the developing bias which concerns on 1st embodiment. (B) It is a figure which shows the correction | amendment flow of the blank time which concerns on 2nd embodiment. It is a control sequence diagram.

[First embodiment]
A first embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings.

(Image forming device)
FIG. 1 is a configuration diagram of an image forming apparatus according to the present embodiment. As shown in FIG. 1, the image forming apparatus 100 according to the present embodiment includes four image forming units (from the upstream side to the downstream side) along the rotation direction (arrow R17 direction) of the intermediate transfer belt 17 as an intermediate transfer member. Image forming stations Pa, Pb, Pc, and Pd are provided. Each of the image forming units Pa to Pd is an image forming unit for forming toner images of yellow, magenta, cyan, and black in this order, and each of the image forming units Pa to Pd is a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive member”). It is called a “drum”.) 1Y, 1M, 1C, 1K.

  The photosensitive drums 1Y to 1K are each driven to rotate in the direction of arrow R1. Around each of the photosensitive drums 1Y to 1K, the charging rollers 2Y to 2K, the exposure devices 3Y to 3K, the developing devices 4Y to 4K, the primary transfer rollers 5Y to 5K, and the drum cleaner 6Y to approximately the order along the rotation direction. 6K is provided.

  A transfer conveyance belt 18 is disposed below the intermediate transfer belt 17 serving as an intermediate transfer member, and a fixing device (fixing means) 16 is provided downstream of the transfer material S in the conveyance direction (the direction of arrow R18 in FIG. 1). Is arranged.

  A photosensitive drum 1 having a diameter of 30 mm is used. The photosensitive drum 1 is obtained by forming and coating a photosensitive layer made of a normal organic photoconductive layer (OPC) on the outer peripheral surface of a grounded drum base 1a made of a conductive material such as aluminum. In this photoreceptor layer, an undercoat layer (UCL) 1b, a charge carrier generation layer (CGL) 1c, and a charge carrier transfer layer (CTL) 1d are laminated. The photoreceptor layer is usually an insulating layer and has a property of becoming a conductor when irradiated with light of a specific wavelength. This is because, by irradiating with light, holes are generated in the charge carrier generation layer 1c, and they become the charge carriers. The charge carrier generation layer 1c is made of a phthalocyanine compound having a thickness of 0.2 μm, and the charge carrier transfer layer 1d is made of polycarbonate in which a hydrazone compound having a thickness of about 25 μm is dispersed.

  The charging roller (charging member) 2 is disposed so as to contact the surface of the photosensitive drum 1. The charging roller 2 has a conductive metal core 2a in the center, and a conductive elastic layer 2b, a medium resistance conductive layer 2c, and a low resistance conductive layer 2d are formed on the outer periphery of the metal core 2a. The charging roller 2 is rotatably supported at both ends by bearings (not shown), and is arranged in parallel to the rotation axis of the photosensitive drum 1. Bearings at both ends of the charging roller 2 are pressed against the photosensitive drum 1 with an appropriate pressing force by an elastic member (not shown) such as a spring. The charging roller 2 is rotated by the rotation of the photosensitive drum 1 by the pressure contact force.

(Developing device 4)
Next, the developing device 4 for each color will be described with reference to FIG. The developing device (developing unit) 4 includes a developing container 30 in which a two-component developer containing toner and a carrier as a developer is accommodated. Further, the developing container 30 has a developing sleeve (developing member) 20 as a developer carrying member, and a panning member (developer regulating blade) 22 that regulates the ears of the developer carried on the developing sleeve 20. is doing. The regulating blade 22 regulates the layer thickness of the developer on the developing sleeve 20.

  Inside the developing container 30, a divided developing chamber 25 and a stirring chamber 26 are provided, and conveying screws 23 and 24 as developer agitating / conveying means are arranged in the respective dividing chambers.

  There is an opening at a position corresponding to the developing region of the developing container 30 facing the photosensitive drum 1, and the developing sleeve 20 is rotatably disposed in the opening so as to be partially exposed in the direction of the photosensitive drum. Yes. The developing sleeve 20 is made of a non-magnetic material such as aluminum or stainless steel, and a fixed magnet roller 21 serving as a magnetic field generating means is included therein. During the developing operation, the developing sleeve 20 rotates in the direction of arrow R20 (counterclockwise) shown in FIG. 2, and is applied with a developing bias by a bias applying device 19 as a developing bias applying means, and carries a two-component developer. The developing sleeve 20 has the layer thickness of the two-component developer regulated by the cutting of the magnetic brush by the regulating blade 22, transports the two-component developer to the developing area facing the photosensitive drum 1, and is placed on the photosensitive drum 1. A developer is supplied to the formed electrostatic latent image to develop the latent image. The developer conveyed to the development area by the rotation of the developing sleeve 20 is conveyed as it is by the developing sleeve 20 after the development is completed, and is collected in the developing container 30.

  On the other hand, a detachable toner container 27 containing replenishing toner is provided above the developing device 4. Toner consumed by development is supplied from a supply port (not shown) provided in the toner container 27 through a supply conveyance path 28 and supplied from the supply port (not shown) provided in the developer container 30 into the developing container 30. The A replenishment screw (toner replenishing means) 29 is provided in the replenishment conveyance path 28, and the amount of toner replenished into the developing container 30 is adjusted by controlling the rotation time of the replenishment screw 29. ing.

(Two-component developer)
Next, the two-component developer used in this embodiment will be described. The two-component developer is composed mainly of a nonmagnetic toner and a low magnetization high resistance carrier.

  The non-magnetic toner is configured by using an appropriate amount of a binder resin such as a styrene resin or a polyester resin, a colorant such as carbon black, a dye or a pigment, a release agent such as wax, a charge control agent, or the like. Such a non-magnetic toner can be produced by a method such as a pulverization method or a polymerization method.

The non-magnetic toner (negative charging characteristics) preferably has a triboelectric charge amount of about −1 × 10 ⁻² to −5.0 × 10 ⁻² C / kg. If the triboelectric charge amount of the non-magnetic toner is out of the above range, the counter charge amount generated in the magnetic carrier becomes large and the white level is deteriorated, which may cause image defects. The triboelectric charge amount of the non-magnetic toner may be adjusted depending on the type of material used, or may be adjusted by adding an external additive.

  The triboelectric charge amount of the non-magnetic toner is the charge induced in the measuring container by using a general blow-off method, with the developer amount being about 0.5 to 1.5 g, and sucking the toner from the developer by air suction. It can be measured by measuring the amount.

  Moreover, a conventionally well-known thing can be used as a magnetic carrier. For example, a resin carrier formed by dispersing magnetite as a magnetic material in a resin and dispersing carbon black for conductivity and resistance adjustment is also used. Moreover, what adjusted resistance by oxidizing and reducing the surface of magnetite simple substance, such as a ferrite, is also used. Moreover, what coated with the magnetite single-piece | unit surface resin, such as a ferrite, and adjusted resistance, etc. are used. The method for producing these magnetic carriers is not particularly limited.

The magnetic carrier preferably has a magnetization of 3.0 × 10 ⁴ A / m to 2.0 × 10 ⁵ A / m in a magnetic field of 0.1T. Reducing the amount of magnetization of the magnetic carrier has the effect of suppressing the scavenging by the magnetic brush, but it becomes difficult to adhere to the non-magnetic cylinder by the magnetic field generating means, and image defects such as adhesion of the magnetic carrier to the photosensitive drum An image defect such as a close-up may occur. If the magnetization of the magnetic carrier is larger than the above range, an image defect may occur due to the pressure of the magnetic brush as described above.

Further, the volume resistivity of the magnetic carrier is preferably 10 ⁷ to 10 ¹⁴ Ωcm in consideration of leakage and developability.

The magnetization of the carrier was measured using BHV-30, which is an oscillating magnetic field type automatic magnetic recording device manufactured by Riken Denshi Co., Ltd. As the magnetic characteristic value of the carrier powder, an external magnetic field of 0.1 T is created, and the strength of magnetization at that time is obtained. The carrier is packed in a cylindrical plastic container so as to be sufficiently dense. In this state, the magnetization moment is measured, the actual weight when the sample is put is measured, and the strength of magnetization is obtained (Am ² / kg). Next, the true specific gravity of the carrier particles is obtained by a dry automatic density Accupick 1330 (manufactured by Shimadzu Corporation), and the true specific gravity is applied to the strength of magnetization (Am ² / kg) to be used in this embodiment. The intensity of magnetization (A / m) per unit volume can be determined.

(Operation of image forming apparatus)
Next, the operation of the image forming apparatus having the above configuration will be described. In FIG. 1, the surface of the photosensitive drum 1 uniformly charged by the charging roller 2 is scanned and exposed by an exposure device (latent image forming means) 3 to form an electrostatic latent image on the photosensitive drum 1. The electrostatic latent image formed on the photosensitive drum 1 is developed as a toner image of each color of yellow, magenta, cyan, and black by the developing device 4. These four color toner images are sequentially primary transferred onto the intermediate transfer belt 17 by applying a primary transfer bias to the primary transfer roller (primary transfer means) 5 in the primary transfer nip. Thus, the four color toner images are superimposed on the intermediate transfer belt 17. At the time of primary transfer, toner (residual toner) that is not transferred to the intermediate transfer belt 17 and remains on the photosensitive drum 1 is removed by a drum cleaner (cleaning means) 6. The photosensitive drum 1 from which the residual toner has been removed is used for the next image formation.

  The four color toner images superimposed on the intermediate transfer belt 17 as described above are secondarily transferred to the transfer material S. The transfer material S conveyed from the feeding cassette (not shown) by the feeding / conveying device is supplied to the secondary transfer nip by the registration roller so as to be synchronized with the toner image on the intermediate transfer belt 17. To the supplied transfer material S, a secondary transfer bias is applied to the secondary transfer roller 11 at the secondary transfer nip, whereby the four color toner images on the intermediate transfer belt 17 are secondarily transferred collectively. .

  The transfer material S on which the unfixed toner image is secondarily transferred is heated and pressed by the fixing roller 14 and the pressure roller 15 of the fixing device 16 to fix the toner image on the surface. The transfer material S after the toner image is fixed is discharged onto a discharge tray. The four-color full-color image formation on one surface (front surface) of one transfer material S is thus completed. After the secondary transfer, toner remaining on the intermediate transfer belt 17 without being transferred (transfer residual toner) is removed by the belt cleaner 10.

(Development bias)
Here, the developing bias applied to the developing sleeve 20 will be described. FIG. 3A is a diagram showing a time waveform of an alternating current component of the developing bias applied to the developing sleeve 20. In FIG. 3A, the horizontal axis represents time, and the vertical axis represents voltage.

  As shown in FIG. 3A, in the present embodiment, a waveform in which an AC voltage and a DC voltage are superimposed is used. This is a bias that repeats this cycle, with the entire period of the AC voltage and DC voltage (vibration unit) A and the subsequent period of only DC voltage (blank time) B applied as one cycle. (Hereinafter referred to as blank pulse bias). The amplitude part A has a frequency of 15 kHz, and the time required for one cycle is 100 ms. Since this is repeated two cycles, it is applied for 200 ms. Blank time B is also applied for 200 ms. The amplitude value width Vpp of the AC voltage is set to 2.0 kV in the initial state.

  The image forming apparatus 100 includes a control circuit (controller, control unit) 33 that controls a developing bias applied to the developing sleeve 20. The control circuit 33 is connected to a film thickness detection circuit (sensor, film thickness detection means) 31 and has a backup memory 32 (see FIG. 4).

(Thickness detection means)
Next, the film thickness detection means of the photosensitive drum 1 will be described. FIG. 4 is an explanatory diagram of detection means and control means. As shown in FIG. 4, the film thickness detection circuit (film thickness detection means) 31 is disposed in the image forming apparatus main body (hereinafter simply referred to as “apparatus main body”) and detects information relating to the film thickness of the photosensitive drum 1. To do. The film thickness detection circuit 31 is a current detection unit.

  The charging roller 2 is also used as an electrode member for detecting the film thickness of the photosensitive drum 1. The film thickness detection circuit 31 measures the charging voltage applied to the charging roller 2 and the charging current flowing at that time, and detects the film thickness of the photosensitive drum 1. The charging voltage applied to the charging roller 2 is an oscillating voltage obtained by superimposing an AC voltage on a DC voltage, and a constant voltage of −800 V is used as the DC voltage and 1800 V is used as the AC voltage.

  The film thickness detection circuit 31 is charged from the current flowing through the photosensitive drum 1 when charged from the state where the charge of the photosensitive drum 1 is removed (reversely, when the charge is removed from the charged state). This is a method for detecting the film thickness (current detection method). Pre-exposure devices 7Y to 7K (see FIG. 1) were used as means for removing charges, and current detection was performed with the potential of the photosensitive drum 1 set to zero.

The principle of the film thickness detection method of the photosensitive drum 1 will be briefly described. When the surface potential of the photosensitive drum 1 is increased from 0 to Vd (V) or decreased from Vd to 0 (V), the DC current I _DC flowing through the photosensitive drum 1 is I _DC = ε · ε0. • L · vp · Vd / d (3) The film thickness of the photosensitive drum 1 is d, the relative dielectric constant is ε, the dielectric constant in vacuum is ε0, the effective charging width of the primary charging roller is L, and the process speed is vp.

Here, epsilon, .epsilon.0, L, vp, since Vd can be regarded as a constant, DC current _{I DC} is inversely proportional to the thickness d of the photosensitive drum 1. Therefore, it is possible to detect the film thickness d of the photosensitive drum 1 by measuring the DC current I _DC.

In this embodiment, since ε = 3, ε0 = 8.85 × 10 ⁻¹² , L = 300 mm, vp = 120 [mm / sec], and Vd = −800 [V], for example, the DC current is I _DC When 25.5 μA, the film thickness of the photosensitive drum 1 is 30 μm. The current detection method is described in detail in Japanese Patent Laid-Open No. 05-223513.

FIG. 5 shows the relationship (current amount-surface layer thickness table) between the surface layer thickness of the photosensitive drum 1 and the detected current amount (DC current amount) in the present embodiment. Table 1 shows the relationship between the surface layer thickness of the photosensitive drum 1 and the DC current value _IDC (a numerical value of FIG. 5). In addition, each of the elements required to output a constant image density is shown. The value of Vpp at the film thickness is shown. The relationship between the surface layer thickness and the detected current amount does not depend on the environment. The image forming apparatus 100 holds the current amount-surface layer thickness table of FIG. 5 and the current value-Vpp table of Table 1 in the backup memory 32 (see FIG. 4).

(Development bias Vpp correction)
Hereinafter, correction of Vpp in the present embodiment will be described. FIG. 6A is a flowchart of the Vpp correction procedure in this embodiment. The present invention controls the AC bias of the developing bias applied to the developing sleeve so that the density of the toner image to be developed decreases as the film thickness of the photosensitive drum 1 decreases. In this embodiment, as shown in FIG. 6A, the peak-to-peak Vpp (a value twice the amplitude of the AC bias) of the AC voltage of the developing bias according to the amount of DC current flowing through the photosensitive drum 1. It was decided to make corrections. Specifically, control is performed so that the peak-to-peak Vpp of the AC voltage of the developing bias is reduced in accordance with the increase in the amount of DC current flowing through the photosensitive drum 1. That is, in this embodiment, as described above, when the amount of DC current flowing through the photosensitive drum 1 increases, the film thickness of the photosensitive drum 1 decreases.

  When the image forming apparatus is initially installed or when the photosensitive drum 1 is replaced, the DC current flowing through the photosensitive drum 1 is detected and determined to be Vpp corresponding to the initial current value.

Further, when the image forming apparatus is used, when the power of the image forming apparatus is turned on, the photosensitive drum 1 is rotated as shown in the control sequence of FIG. detecting a DC current _{I DC} of rotation between). Vpp corresponding to the detected current value is corrected.

The determination and correction of Vpp are performed as follows. First, the surface potential of the photosensitive drum 1 is set to 0 V by the pre-exposure device 7 (S1). Then, the charging voltage Vd is applied to the charging roller 2, and the surface potential of the photosensitive drum 1 is increased from 0 to Vd (V) (S2).
Averaged current measured during one rotation of the photosensitive drum 1 in order to eliminate noise influence, the average value (average current value) is obtained I _DC (S3). The average current value I _DC is the initial current value and the correction current value of the photosensitive drum 1, and the initial current value and the correction current value are corresponded from the table of the DC current value I (μA) and Vpp (kV) shown in Table 1. Vpp is determined (S4).

  As a result, when the film thickness d of the photosensitive drum 1 is reduced and the detected DC current value I is increased, the amplitude value width of the AC component of the developing bias applied to the developing sleeve 20 by the control circuit (control means) 33. Control to lower Vpp is performed. As a result, the amount of developer carried by the developing sleeve 20 to the photosensitive drum 1 is reduced, and an increase in image density due to a reduction in the film thickness d of the photosensitive drum 1 can be suppressed.

  In this embodiment, the number of threshold values of the amplitude value width Vpp is six, but the number of threshold values is not limited to this, and the numerical values in Table 1 are also examples, and are limited to these numerical values. is not. The control circuit 33 only needs to have one or more threshold values.

In the present embodiment, the amplitude value width Vpp is corrected when the average current value _IDC is equal to or greater than the threshold value. The amplitude value width Vpp may be corrected when the average value of the detection results obtained by the latest multiple times of the current value detected by the film thickness detection circuit 31 is equal to or greater than a predetermined threshold value.

  As described above, the amplitude value width Vpp of the AC component of the developing bias is corrected according to the detected current value flowing through the photosensitive drum 1. As a result, even when the film thickness of the photosensitive drum fluctuates, a stable image can be output over a long period without correcting the development contrast and without greatly changing the toner density. That is, it is possible to suppress image density variation and gradation deterioration that occur when correcting the development contrast. In addition, it is possible to suppress image defects at the end of the life of the photosensitive drum that occur when the toner density is greatly changed, and to output a stable image over a long period of time.

  In this embodiment, the Vpp is controlled based on the current detection result once. However, the Vpp is controlled when the threshold value is continuously determined at least twice or more. Also good. Thereby, erroneous detection can be prevented.

  The current detection operation can be performed during the start-up operation after the image forming apparatus is turned on, before the start-up operation is started, or after the start-up operation is completed.

[Second Embodiment]
Next, a second embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. About the part which overlaps with said 1st embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted. FIG. 6B is a flowchart of the blank time correction procedure in the present embodiment. Table 2 shows the relationship between the surface layer thickness d (μm) of the photosensitive drum 1 and the DC current value I _DC (μA) (a numerical value of FIG. 5). In addition, a constant image density is output. The value of the necessary blank time B (ms) at each film thickness d (μm) required for this is shown.

  As shown in FIG. 6B, the image forming apparatus of the present embodiment corrects the blank time B of the AC component of the developing bias in place of the amplitude value width Vpp of the AC component of the developing bias. That is, the developing bias blank time B is corrected according to the amount of current flowing through the photosensitive drum 1. The image forming apparatus according to the present embodiment holds the current value-blank time table in Table 2 in the backup memory 32.

  When the image forming apparatus is initially installed or when the photosensitive drum 1 is replaced, the DC current flowing through the photosensitive drum 1 is detected, and the blank time corresponding to the initial current value is determined from the current value and blank time table in Table 2. B is determined.

Further, when the image forming apparatus is used, when the power of the image forming apparatus is turned on, the photosensitive drum 1 is rotated as shown in the control sequence of FIG. detecting a DC current _{I DC} of rotation between). From the current value and blank time table in Table 2, the blank time B is corrected corresponding to the current value.

The determination and correction of Vpp are performed as follows. First, the average current value _{I DC} in the same manner as in the first embodiment (S1 to S3). Then, the average current value I _DC is used as the initial current value and the correction current value of the photosensitive drum 1, and the initial current value and the correction are calculated from the table of the DC current value I (μA) and the blank time B (ms) shown in Table 2. The blank time B corresponding to the current value is determined (S5).

  As a result, when the film thickness d of the photosensitive drum 1 decreases and the detected DC current value I increases, the control circuit (control unit) 33 blanks the AC component of the developing bias applied to the developing sleeve 20. Control to lengthen B is performed. As a result, the amount of developer carried by the developing sleeve 20 to the photosensitive drum 1 is reduced, and an increase in image density due to a reduction in the film thickness d of the photosensitive drum 1 can be suppressed.

  FIG. 3B and FIG. 3C are diagrams showing the development bias waveform after controlling the blank time B of the blank pulse bias in the present embodiment. FIG. 3B shows a waveform with a blank time of 300 ms. FIG. 3C shows a waveform with a blank time of 400 ms.

  In the present embodiment, the number of threshold values for the blank time is three points, but the threshold score is not limited to this, and the numerical values in Table 2 are also examples, and are not limited to these numerical values. Absent. The control circuit 33 only needs to have one or more threshold values.

  As described above, the blank time B of the AC component of the developing bias is corrected in accordance with the detected current value flowing through the photosensitive drum 1. As a result, even when the film thickness of the photosensitive drum fluctuates, a stable image can be output over a long period without correcting the development contrast and without greatly changing the toner density. That is, it is possible to suppress image density variation and gradation deterioration that occur when correcting the development contrast. In addition, it is possible to suppress image defects at the end of the life of the photosensitive drum that occur when the toner density is greatly changed, and to output a stable image over a long period of time.

  In the present embodiment, the blank time B is controlled based on the current detection result of one time. However, when the value exceeds the threshold value determined continuously at least twice or more, the blank time B is controlled. Control may be performed. Thereby, erroneous detection can be prevented.

That is, the blank time B may be corrected when the average value of the detection results obtained by the latest multiple times of the current value detected by the film thickness detection circuit 31 is equal to or greater than a predetermined threshold value.

  Further, the control circuit 33 detects the amount of current flowing through the photosensitive drum 1 and, according to the detection result, at least one of the amplitude value width Vpp of the AC component of the developing bias and the blank time B of the AC component of the developing bias. You may make it control one.

  The current detection operation can be performed during the start-up operation after the image forming apparatus is turned on, before the start-up operation is started, or after the start-up operation is completed.

A ... Vibrating part B ... Blank time P ... Image forming part S ... Transfer material Vpp ... Amplitude value width d ... Film thickness 1 ... Photosensitive drum (image bearing member)
2 ... Charging roller 3 ... Exposure device 4 ... Developing device 5 ... Primary transfer roller 6 ... Drum cleaner 7 ... Pre-exposure device 11 ... Secondary transfer counter roller 14 ... Fixing roller 15 ... Pressure roller 16 ... Fixing device 17 ... Intermediate transfer Belt 18 ... transfer conveyance belt 19 ... developing bias applying means 20 ... developing sleeve (developing member)
DESCRIPTION OF SYMBOLS 21 ... Magnet roller 22 ... Developer control blade 23, 24 ... Conveying screw 25, 26 ... Stirring chamber 27 ... Toner container 28 ... Replenishment conveyance path 29 ... Replenishment screw 30 ... Developer container 31 ... Film thickness detection circuit (detection means)
32 ... Backup memory 33 ... Control circuit (control means)
100: Image forming apparatus

Claims (4)

An image carrier;
A developing device that includes a developer carrying member for carrying the developer, and that develops the electrostatic latent image formed on the image carrier with the developer;
A bias applying device for applying a developing bias to the developer carrying member;
A sensor for detecting information on the film thickness of the image carrier;
A controller for controlling the AC bias of the developing bias applied to the developer carrier so that the toner density to be developed decreases as the film thickness of the image carrier decreases according to the detection result of the sensor; An image forming apparatus comprising: The image forming apparatus according to claim 1, wherein the controller reduces an amplitude of an AC bias of a developing bias applied to the developer carrying member in accordance with the decrease in the film thickness. 3. The controller according to claim 1, wherein the controller performs control so that a blank time of an alternating current component of a developing bias applied to the developer carrying member becomes longer as the film thickness decreases. Image forming apparatus. The sensor includes a charging member that contacts the surface of the image carrier and charges the image carrier, and a circuit that detects a current flowing through the image carrier when a voltage is applied to the charging member. 4. The image forming apparatus according to claim 1, wherein information relating to a film thickness of the image carrier is detected based on a current flowing through the circuit.

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