JP4937573B2 - Electrophotographic image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or a composite machine using an electrophotographic method, and more particularly, is charged by using a two-component developer that charges toner using a magnetic carrier. The present invention relates to an electrophotographic image forming apparatus to be used in a non-contact developing system that develops an electrostatic latent image by holding only the toner on a developing roll and flying the toner onto the electrostatic latent image.

  A developing method generally called a non-contact developing method uses a two-component developer that charges a toner using a magnetic carrier, holds only the charged toner on a developing roll, and causes the toner to fly to an electrostatic latent image. In this way, the electrostatic latent image is developed. At that time, a uniform thin layer of toner is formed on the surface of the developing roll, the developing roll is brought close to the electrostatic latent image surface, and an AC voltage is biased and applied to the gap while causing the toner to fly and vibrate. The toner is selectively attached to the electrostatic latent image portion having a predetermined potential or less. However, the development characteristics depend not only on the AC voltage waveform but also on the variation in toner charge amount and particle size. Therefore, if an AC voltage waveform is set to a predetermined waveform and an image is formed with a predetermined image quality, The variation in charge amount and particle size of the toner had to be kept within an extremely narrow range.

  Therefore, in the developing device disclosed in Patent Document 1, the frequency of the alternating voltage applied to the developing roll is temporally changed. Specifically, three types of frequencies are periodically applied, and the electrostatic latent image is repeated at least once or more times while passing through the development effective area between the developing roll and the photosensitive drum. Add. As a result, development unevenness is eliminated and an image having excellent gradation and fineness is output.

Japanese Patent Publication No. 5-68694 (FIG. 5, column 7, lines 30 to 42)

  By the way, the so-called hybrid development system is a development system that adopts a non-contact development system (one-component jumping development system), and forms a magnetic brush composed of toner and carrier on a magnetic roll to which a DC alternating current superimposed voltage is applied, Only the toner in the magnetic brush is transferred to a developing roll to which another DC / AC superimposed voltage is applied, and the electrostatic latent image is developed as a toner image by a thin toner layer on the developing roll.

  However, in the hybrid development system, when the diameter of the developing roll or the magnetic roll is reduced with the miniaturization of the image forming apparatus, the recording medium such as a copy sheet is developed when developing the electrostatic latent image with a high printing rate. Since the development is performed by rotating the developing roll a plurality of times in one sheet, there may be a problem that the image density is lowered as the development time becomes the second half. This is particularly likely to occur when printing image data having a printing rate of 70% or more with respect to the sheet area. In addition, such a decrease in density in the latter half of the image is caused by the vertical feeding with a large paper length in the transport direction in which the number of rotations of the developing roll is increased even when an image is formed on the same size paper. This is likely to occur, particularly when the paper length size is A4 paper lengthwise, legal paper lengthwise, or larger.

  When images with a high printing rate are continuous, it is possible to keep the image density constant in the first half of the paper by using a method such as a process of forming a thin layer between images. .

  However, in the latter half, the toner layer on the developing roll tends to decrease, and even if the development conditions are varied, the development dependency due to the thickness of the toner layer increases, so it is difficult to stabilize the image density.

  Accordingly, an object of the present invention is to prevent a decrease in image density in the latter half of a recording medium such as paper (the portion developed later in time than the first half).

In general, a toner thin layer of a hybrid developing roll is formed by a potential difference between a magnetic roller holding a magnetic brush and a developing roller via a magnetic brush having toner and a carrier. This thin toner layer is required to be always in a constant state during printing. Thereby, the uniformity of the image is maintained. When forming a toner thin layer, it is possible to instantaneously transfer the toner to the developing roll by applying a DC / AC superimposed voltage to the magnetic roll, and the DC component of the DC / AC superimposed voltage determines the thickness of the thin layer. To do. Further, the potential difference between the DC voltage of the magnetic roll and the developing roll becomes the potential of the surface of the toner thin layer of the developing roll, and the development is executed by the potential difference between this potential and the latent image portion of the photosensitive member. This is a parameter for determining the image density.
In the present invention, in such a hybrid development system, the potential difference between the photosensitive member latent image portion and the developing roller and the potential difference between the magnetic roll and the developing roll are kept constant, and the thickness of the toner thin layer is controlled to be constant. When an image is formed on one sheet of paper, the toner layer in the latter half (the portion developed later in time than the first half) in which the toner thin layer on the developing roller decreases with the rotation of the developing roller In order to maintain the toner layer determined by the potential difference, the AC voltage frequency applied to the magnetic roll and the developing roll (both frequencies are the same) is set to the number of development roll cycles in image formation (when an image is formed on one sheet of paper). The number of rotations of the developing roll).
Here, the formation of the toner thin layer is accelerated by the magnetic roll applied AC voltage frequency, and when the frequency is increased from the normal time, the toner thin layer formation is stabilized. This is because there is an effect of increasing the vibration of the magnetic brush, the toner and the carrier are easily loosened, and the toner is peeled off from the carrier. On the other hand, if the frequency of the AC voltage applied to the magnetic roll is increased more than necessary, the toner cannot vibrate, the toner and carrier peeling action is weakened, and the toner thin layer formation becomes unstable. In the present invention, this phenomenon is used to prevent a decrease in image density in the latter half of the sheet. Note that since this image density reduction tends to depend on the image printing rate, it is desirable to provide a printing rate detection means.

Based on such an action, the first means for solving the above-mentioned problem is to form a magnetic brush composed of toner and magnetic carrier on a magnetic roll to which a DC / AC superimposed voltage is applied, and in the magnetic brush The toner was transferred to a developing roll to which another DC / AC superimposed voltage was applied, and the electrostatic latent image was developed as a toner image by the toner thin layer on the developing roll, and remained on the developing roll when not developed. The alternating current applied to the magnetic roll in an electrophotographic image forming apparatus having a hybrid developing device that peels off toner and re-forms a thin toner layer, and transfers and fixes the toner image to a recording medium to form an image. voltage frequency is equal to the AC voltage frequency applied to the developing roll, when performing the image formation, the developing roller surface to the AC voltage frequency, performs advance the image forming So that the frequency set according to the number of times is to change.

  Thereby, the thickness of the toner thin layer on the developing roller corresponding to the latter half of the recording medium such as copy paper can be controlled to be constant, and the lowering of the image density in the latter half can be prevented.

  The second means is that in the first means, the AC voltage frequency is increased with an increase in the number of image formations since the number of image formations exceeds a predetermined number.

  As a result, even if image formation is performed many times and the toner of the toner thin layer is consumed, the thickness of the toner thin layer on the developing roller corresponding to the latter half of the recording medium such as copy paper is controlled to be constant. Therefore, it is possible to prevent the image density from decreasing in the latter half.

  The third means is to increase the AC voltage frequency in the first means as the printing rate of the image to be formed increases.

  As a result, even if image formation is performed many times and the toner of the toner thin layer is consumed, the thickness of the toner thin layer on the developing roller corresponding to the second half of the recording medium such as a copy sheet, depending on the printing rate. The thickness can be controlled to be constant, and a decrease in image density in the latter half can be prevented.

  The fourth means is that the third means is provided with a printing rate detection unit for detecting the printing rate.

  Thereby, the toner thin layer control according to the printing rate can be automatically performed.

  According to the present invention, it is possible to prevent the image density from being lowered in the second half of the recording material such as paper (the portion developed later in time than the first half).

  Embodiments of the present invention will be described below with reference to the drawings. However, even if there is a specific description about the dimensions, materials, shapes, relative arrangements, and the like of the components described in the present embodiment, the present invention is not intended to be limited thereto.

  [Configuration of this embodiment]

  FIG. 1 shows a front view of an example of a color copying machine (tandem machine) having a hybrid developing device as an example of an electrophotographic image forming apparatus. Note that the present invention can also be applied to a monochrome image forming apparatus. In the main body 1 of the tandem machine, for each color of yellow, magenta, cyan, and black (in no particular order), an image forming unit (a charger 70, an exposure unit 20, a developer 40, a transfer unit 90, It has a cleaning device 80. It also has a transfer belt 50 and a fixing device 120 for conveying paper.

  A recording body (paper or the like) (not shown) is supplied onto the transfer belt 50 from the right in the drawing, the paper contacts the photoconductor 30, and the toner image on the photoconductor is transferred onto the paper by the transfer means 9. After the toner images of the respective colors are transferred, the toner images are fixed on the paper by the fixing device 120, and the paper is discharged out of the main body 11 as a copy. In a tandem type printer, it is important to design them compactly.

  FIG. 2 is a conceptual diagram of the hybrid developer. 3 is an electrostatic latent image carrier (photoconductor), 2 is a developing roll, 1 is a magnetic roll, 5 is toner, 4 is a magnetic carrier, and a bias is applied between the photosensitive body 3 and the developing roll 2. The The power supplies 7a and 7b output a DC bias voltage Vdc2 and an AC voltage applied to the developing roll 2, respectively. The power supplies 8a and 8b output a DC bias voltage Vdc1 and an AC voltage applied to the magnetic roll 1, respectively. 6 is a thin toner layer on the developing roll 2, 10 is a magnetic brush, and 9 is a regulating blade for controlling the thickness of the magnetic brush.

  For the exposure light source (not shown) for exposing the electrostatic latent image carrier 3, a semiconductor laser or LED can also be used. As for the wavelength of the exposure light, a wavelength around 770 nm is effective for a positively charged organic photosensitive material (positive OPC), and a wavelength around 685 nm is effective for an a-Si photosensitive member. Hereinafter, an example in the case of using positive OPC will be described. The positive OPC 3 that is the electrostatic latent image carrier is charged to 400 V by a charger (not shown). Thereafter, when exposure is performed with an LED having a wavelength of 770 nm, the post-exposure potential becomes 70V. The positive OPC photosensitive member is disposed with a space of about 250 μm with respect to the developing roll 2. No wire electrode or the like is used in this space. When positive OPC is used as the photosensitive material for the photoconductor 3, the generation of ozone and the like is small and charging is stable. In particular, even when the positive OPC having a single layer structure is used over a long period of time and the film thickness changes It is ideal for long-life systems because it has little change in characteristics and stable image quality. In addition, it is also possible to use an a-Si photoconductor. When a long-life system is used, the film thickness of the positive OPC is set to about 20 μm to 40 μm. In the case of 20 μm or less, when the film decreases and reaches 10 μm, black spots are noticeably generated due to dielectric breakdown. On the other hand, if the film thickness is thicker than 40 μm, the sensitivity is lowered and the image quality is lowered.

Next, the operation of the hybrid developer will be described. First, the developer composed of the toner 5 and the magnetic carrier 4 held on the surface of the magnetic roll 1 is held, and the toner 5 is charged to an appropriate level while stirring. The developer passes through the regulating blade 9 and contacts the developing roll 2 with a constant layer thickness. The gap between the regulating blade 9 and the magnetic roll 2 is, for example, 0.3 mm to 1.5 mm, and the gap between the magnetic roll 1 and the developing roll 2 is, for example, 0.3 mm to 0.5 mm, preferably 0.2 mm to 0.4 mm. Degree. The toner thin layer 6 on the developing roll 2 is set to a thickness of, for example, 20 μm to 100 μm, preferably 30 μm to 70 μm. This thickness is a value corresponding to about 5 to 10 layers of the toner 5 when the average particle size of the toner 5 is 7 μm. The gap between the developing roll 2 and the electrostatic latent image carrier 3 is, for example, 150 μm to 400 μm, preferably 200 μm to 300 μm. If it is smaller than 150 μm, it causes fogging. If it is larger than 400 μm, it becomes difficult to cause the toner 5 to fly to the photoreceptor 3, and a sufficient image density cannot be obtained. It also becomes a factor that causes selective development. Here, the selective development means that a highly developable toner (a toner having a relatively large particle diameter and a low charge amount) is selectively developed on the electrostatic latent image carrier 3, and the charge amount is increased when continuous printing is performed. This is a phenomenon in which high toner (toner having a relatively small particle diameter) accumulates on the developing roll 2 and remains, and when toner having a large charge amount and a relatively small particle diameter accumulates on the developing roll 2, Since these toners are not developed, a sufficient image density cannot be obtained even after development, which is an undesirable phenomenon.
Then, by applying an AC voltage to the conductive developing roll 2 and the magnetic roll 1 with the power supplies 7b and 8b, the development on the photoconductor 3 can be performed accurately, and the development residual toner on the magnetic roll 1 can be easily collected. It becomes. The surface of the developing roll 2 is a rotating body (conductive sleeve) made of conductive aluminum. The material of the conductive sleeve may be a uniform conductor, and SUS, conductive resin coating, etc. can be applied. The DC voltage and AC voltage from the power supplies 7 a and 7 b are transmitted to the conductive sleeve via the shaft of the developing roll 2. The output of the DC power supply 7a is 100V, the frequency of the AC voltage output from the AC power supply 7b is 2.7 kHz, Duty 27%, and Vpp is 1.6 kV. The magnetic roll rotating body receives the DC voltage and the AC voltage from the power supplies 8a and 8b on the shaft of the magnetic roll 1 and transmits them to the magnetic roll rotating body. The output of the power source 8a is 350 to 500v (variable by calibration), and the power source 8b has a Vpp of 300V, a frequency of 2.7 kHz, and a duty of 73%. The waveform of the AC component is preferably a rectangular wave. By applying these superimposed biases to the developing roll and the magnetic roll, it is possible to develop the electrostatic latent image on the electrostatic latent image carrier 3 with good developability and to form the toner thin layer 6 on the magnetic roll 1. The recoverability is improved and the stability of continuous printing is improved. In order to stabilize the image density in continuous printing, it is necessary to periodically remove the toner from the developing roll 2 according to the print data and refresh the developing roll 2. The toner thin layer is always refreshed if the toner is peeled off from the developing roll 2 at the end of development, but it takes time to form a stable toner layer again, and a sufficient printing speed cannot be achieved. In order to supply sufficient toner to the latent image on the electrostatic latent image carrier 3 without increasing the sheet interval, the peripheral speed of the developing roll 2 is 1.5 times or more that of the electrostatic latent image carrier 3. When set to, toner can be taken in and out in a short time. Further, when the magnetic roll 1 is set at a speed 1 to 2 times that of the developing roll 2, toner replacement is promoted. At this time, it is preferable that the rotating direction of the magnetic roll is opposite to the developing roll. In order to replace the toner thin layer 6 on the developing roll 2, the toner thin layer 6 of the developing sleeve is recovered by the magnetic brush 10 by changing the DC voltage in a state where AC is applied at the end of development.

The saturated toner amount of the toner thin layer 6 is determined by the difference between Vdc1 of the power supply 8a applied to the magnetic roll 1 and Vdc2 of the power supply 7a applied to the developing roll. When Vdc2 is set to 150 V and the value of Vdc1 is set to 400 V, a toner layer of about 1.0 mg / cm ² is obtained on the second turn of the developing roll. The adjustment of the toner layer is basically obtained by a potential difference of (Vdc2−Vdc1), but factors such as the toner charge amount and the magnetic pole magnetic pole strength may also contribute.

If Vdc2 is controlled based on an actually obtained image in order to control the toner layer thickness, a uniform and good image can be obtained at a target density. When continuously performing high density printing, it is advantageous to set the value of (Vdc2−Vdc1) slightly larger because the amount of toner transferred from the magnetic roll 1 to the developing roll 2 increases. If the toner layer is too thin at 0.5 mg / cm ² or less, the followability of the density when a high density image is continuous is lowered and image unevenness is likely to occur. On the other hand, if the toner layer exceeds 1.5 mg / cm ² and is too thick, the development ghost tends to be noticeable and the toner scattering tends to be noticeable. The toner layer thickness also depends on the charge amount of the toner. When the toner charge amount is as low as 10 μC / g or less, particularly 5 μC / g or less, the toner layer thickness increases and scattering increases. Also, development ghosts become prominent. On the other hand, when the toner charge amount is 20 μC / g or more, the toner layer thickness becomes thin, the charge increases, and the developability of the toner decreases.

  The toner thin layer 6 of the developing roll 2 is collected by the magnetic brush 10 held on the magnetic roll 1, and new developer is carried to the developing roll 2 through the regulating blade 9.

  To avoid the selective developability as described above, the toner 5 is adjusted to have a particle diameter of, for example, 5.0 μm to 10.0 μm, and fine powder smaller than this range or coarse powder larger than this range should be removed. It is effective to define the particle size distribution. Generally, the particle size distribution of the toner is measured by a coal counter, and the spread of the particle size distribution is expressed by the ratio of the volume distribution average diameter to the number distribution average diameter. In order to prevent selective development, it is important to reduce the ratio. When the distribution is wide, toner having a relatively small particle size is deposited on the developing roll 2 in continuous printing, and developability is deteriorated.

  The two-component developer forms a magnetic brush 10 composed of toner 5 and carrier 4 on the magnetic roll 1, and the toner 5 is charged by stirring. The magnetic brush 10 on the magnetic roll 1 is layer-regulated by a regulating blade 9, and a thin layer 6 containing only toner is formed on the developing roll 2 by a potential difference between the magnetic roll 1 and the developing roll 2.

As the carrier 4, a ferrite carrier having a volume resistivity of 10 ⁸ Ωcm coated with a silicone resin, a saturation magnetization of 40 emu / g, and an average particle diameter of 35 μm may be used. When the average particle size exceeds 50 μm, the carrier stress increases, and the toner concentration in the developer cannot be increased, and the amount of toner supplied to the developing roll 2 decreases. As the carrier, a magnetite carrier, Mn ferrite, Mn-Mg ferrite, or the like can be used. These carriers may be used as they are, but they may be used after being surface-treated within an appropriate resistance range. The mixing ratio of the toner is 5 to 20% by weight, preferably 5 to 15% by weight, based on the total amount of the carrier and the toner. When the mixing ratio of the toner is less than 5% by weight, the toner charge amount becomes high and a sufficient image density cannot be obtained. When the toner content exceeds 20% by weight, a sufficient charge amount cannot be obtained. The toner is scattered from the inside and contaminates the inside of the image forming apparatus, or toner fog occurs on the image.

  FIG. 3 shows the bias between the developing roll 2 and the magnetic roll 1 between sheets (between images). After developing the electrostatic latent image, the DC voltage Vdc2 of the developing roll 2 is set to zero, the toner thin layer is peeled off while the developing roll rotates once, and then the development of the electrostatic latent image is started. Reform the toner thin layer. The offset voltage (DC) of the developing roll 2 is higher than the offset voltage (DC) of the magnetic roll 1, and the potential difference is preferably set to 50 to 200v. If it is 50 V or less, the peeling is insufficient. On the other hand, if it is 200 V or more, the peeling is too strong, and it is difficult to form the next thin layer during continuous printing, and the second image becomes thin. Occurs. In addition, by setting the duty ratio to 50% or more at the time of peeling, there is an effect of preventing the toner from adhering to the surface of the magnetic roll, and the adhesion of the toner on the surface of the developing roll can be reduced more effectively.

  FIG. 4 is a block diagram of a control unit that controls the frequency of the AC voltage applied to the developing roll 2 during image formation. The printing rate detection unit 51 recognizes the printing rate of the document. Therefore, for example, the image portion signal in the digital image signal obtained by using a CCD or the like is divided by the effective print area. The obtained printing rate is input to the CPU 53, and the CPU 53 extracts a frequency change corresponding to the printing rate from a frequency change lookup table 54 described later. Further, the CPU 53 receives the number of image formations from the image formation number counter 52, and the AC voltage applied to the magnetic roll 1 and the development roll 2 via the frequency controller 55 based on the printing rate and the number of image formations. The frequency (the frequency is the same for the magnetic roll 1 and the developing roll 2) is controlled.

  The AC voltage frequency is increased with an increase in the number of image formations from the time when the number of image formations exceeds a predetermined number based on a frequency change lookup table described later. Further, based on the frequency change lookup table, the AC voltage frequency is increased as the printing rate of the image to be formed increases. As a result, even if image formation is performed many times and the toner of the toner thin layer is consumed, the thickness of the toner thin layer on the developing roller corresponding to the second half of the recording medium such as a copy sheet, depending on the printing rate. The thickness can be automatically controlled to be constant, and a decrease in image density in the latter half can be prevented. However, in the present embodiment, it is assumed that the printing rate is constant over the entire surface of the paper.

[Comparative Examples and Examples] Table 1 shows experimental conditions. In the experiment, A4 vertical length (about 297 mm) solid black image (printing rate 100%) and halftone printing (printing rate 75%, 50%, 25%) were printed. Since the diameter of the developing roller in the used developing device was 16 mm (however, the developing roller's peripheral speed was set to 1.3 times the photosensitive member's peripheral speed), the frequency was the fifth turn of the second half of the sheet. After that (development is completed in about 8 laps in the A4 vertical direction), it was decided to change.
This frequency was set by the printer control system so that the toner thin layer on the developing roll was automatically changed in timing so as to correspond to the latter half of the paper. In the comparative example, the frequency was not changed (Comparative Example 1) and the frequency was increased more than necessary (Comparative Example 2). Under the above conditions, the image density of one sheet was measured at five points in the circumferential direction of the developing roll 2 in the second half of the image for each developing roll cycle (for each rotation of the developing roll) and averaged. Are shown in Tables 2 to 5 for printing rates of 100%, 75%, 50%, and 25%, respectively.

[Comparative Example 1] In Comparative Example 1, it can be seen that the image density (uses a spectrophotometer SPM50 manufactured by Gretag Macbeth Co., Ltd.) is lowered particularly in the latter half of the paper. This is because the state of the toner thin layer has changed from the first half. In particular, it is considered that the toner amount is decreasing. In Comparative Example 2, the image density was increased by 0.3 KHz in the latter half, but a decrease in image density at the end of the paper was observed.
[Comparative Example 2] In Comparative Example 2, since the frequency was set higher than necessary, it can be seen that the developability is remarkably lowered at the rear end.

  [Example 1] In Example 1, the frequency 2 was set to 3 KHz until the number of rotations of the developing roll was 4, and the frequency was set to 3.3 KHz after 5 times. As a result, when the printing rate was 100%, the initial image density was 1.39, and after 1. 8 rotations of the developing roll, the density became 1.28. When the printing rate was 75%, the initial image density was 0.99, and after 8 rotations of the developing roll, it was 0.82, and the density decreased. When the printing rate was 50%, the initial image density was 0.70, and after 8 rotations of the developing roll, the density became 0.65, and the density reduction decreased.

  Note that when the printing rate is 25%, the initial image density becomes 0.44 after the eight rotations of the developing roll with respect to the initial image density of 0.44, and the density becomes higher. In Examples 2, 3, and 4 to be described later, the printing rate In all cases where the printing rate was 50%, 75% and 100%, including 25%, the decrease in printing density was suppressed.

[Example 2] In Example 2, the frequency was increased in steps of 0.1 KHz. As a result, when the printing rate was 100%, the initial image density 1.39 was 1.32 after 8 cycles (after 8 rotations) of the developing roll, and the density reduction was suppressed. When the printing rate was 75%, the initial image density was 0.99, and after 8 cycles (after 8 rotations) of the developing roll, the density was reduced to 0.83.
Further, when the printing rate was 50%, the initial image density was 0.70, and after 8 cycles (after 8 rotations) of the developing roll, the density was reduced to 0.70. Further, when the printing rate was 25%, the initial image density was 0.45, and after 8 cycles (after 8 rotations) of the developing roll, the density was reduced to 0.45.

  [Example 3] In Example 3, the frequency was increased in steps of 0.3 KHz. As a result, when the printing rate was 100%, the initial image density 1.39 was 1.34 after 8 cycles (after 8 rotations) of the developing roll, and density reduction was suppressed. Further, when the printing rate was 75%, the initial image density was 0.99, and after 8 cycles (after 8 rotations) of the developing roll, the density was reduced to 0.86. When the printing rate was 50%, the initial image density was 0.70, and after 8 cycles (after 8 rotations) of the developing roll, the image density became 0.72. Further, when the printing rate was 25%, the image density increased to 0.49 after 8 cycles (after 8 rotations) of the developing roll with respect to the initial image density of 0.45.

  [Example 4] In Example 4, the frequency was increased in steps of 0.4 KHz. As a result, when the printing rate was 100%, the initial image density 1.39 was 1.36 after 8 cycles (after 8 rotations) of the developing roll, and density reduction was suppressed. When the printing rate was 75%, the initial image density was 0.99, and after 8 cycles (after 8 rotations) of the developing roll, the density was 0.87. When the printing rate was 50%, the initial image density was 0.70, and after 8 cycles (after 8 rotations) of the developing roll, the image density was 0.74. Further, at a printing rate of 25%, the initial image density was 0.45, and after 8 cycles (after 8 rotations) of the developing roll, it was 0.53, and the image density was high.

  FIG. 5 is a graph showing the relationship between the printing rate and the frequency. The frequency was not changed from the first cycle to the fourth cycle of the developing roller 2, but was changed from the fifth cycle to the eighth cycle according to Examples 1 to 4 as described above. FIG. 5 is a graph in which the average frequency change is plotted in a straight line from the fifth to the eighth lap. This straight line plot is stored in the memory as a frequency change lookup table, and the CPU 53 reads it. As above the thickness of the toner thin layer, it can be confirmed that the image density can be controlled by changing the frequency of the DC / AC superimposed voltage applied to the developing device according to the printing rate, and there is an appropriate value depending on the conditions. It was found that the image density of one sheet can be stably maintained.

  INDUSTRIAL APPLICABILITY The present invention can be used in image forming apparatuses such as copying machines, printers, facsimiles, and complex machines using electrophotography, and in particular, a two-component developer that charges non-magnetic toner using a magnetic carrier. And developing a latent image by forming a uniform thin layer of only the charged toner on the developing roll and causing the toner on the developing roll to fly to the electrostatic latent image by the AC superimposed DC voltage of the developing roll. The present invention can be used for an image forming apparatus having a hybrid developing device.

1 is an overall view of an example of a color copying machine. It is a conceptual diagram of a hybrid developing device. 4 is a timing chart of toner thin layer peeling and thin layer formation on the developing roller. FIG. 4 is a block diagram of a control unit that controls a developing roller applied AC voltage frequency based on a printing rate and the number of image formations. It is a graph which shows the relationship between a printing rate and the developing roller application AC voltage frequency.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetic roll 2 Developing roll 3 Electrostatic latent image carrier (photoreceptor)
4 Carrier 5 Toner 6 Toner thin layer 9 Regulating blade 10 Magnetic brush 11 Main body 20 of tandem type color image forming apparatus Exposure unit 30 Photoconductor 40 Developer 50 Transfer belt 60 Density sensor 70 Charger 80 Cleaning device 90 Transfer means 120 Fixing device 51 Print Rate Detection Unit 52 Image Forming Count Counter 53 CPU
54 Frequency change look-up table 55 Frequency controller

Claims (4)

A magnetic brush composed of toner and a magnetic carrier is formed on a magnetic roll to which a DC / AC superimposed voltage is applied, the toner in the magnetic brush is transferred to a developing roll to which another DC / AC superimposed voltage is applied, and the development A hybrid developing device that develops an electrostatic latent image as a toner image with a thin toner layer on a roll, and peels off the toner remaining on the developing roll during non-development to re-form the thin toner layer; In an electrophotographic image forming apparatus in which an image is formed by transferring and fixing to a recording material,
The AC voltage frequency applied to the magnetic roll is equal to the AC voltage frequency applied to the developing roll,
An electrophotographic image forming apparatus, wherein when the image is formed, the AC voltage frequency is changed so as to be a frequency set in advance according to the number of developing roll cycles in which the image is formed.   The electrophotographic image forming apparatus according to claim 1, wherein the AC voltage frequency is increased with an increase in the number of times of image formation after the number of times of the developing roll exceeds a predetermined number.   2. The electrophotographic image forming apparatus according to claim 1, wherein the AC voltage frequency is increased with an increase in a printing rate of the image to be formed.   The electrophotographic image forming apparatus according to claim 3, further comprising a printing rate detection unit that detects the printing rate.

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