JP2013054110A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a structure capable of reducing the occurrence of an image defect even when image deletion is likely to occur in a black image formation part.SOLUTION: In order to detect image deletion of a photoconductor drum of a black image formation part, a voltage of -500 V is applied to a charging roller and a current value Idc flowing from the charging roller to the photoconductor drum is detected. Then, when the absolute value of the current value Idc is 2 μA or more, a ratio (UCR ratio) α' of a black image formed by the black image formation part to a process black image formed by a plurality of color image formation parts is set to 0 and a black image is formed only by a process black image. That is to say, when the absolute value of the current value Idc is 2 μA or more, which is a condition in which image deletion is likely to occur, the black image is formed by using the plurality of color image formation parts by which image deletion is hard to occur, without using the black image formation part. Thereby, generation of black image defects can be reduced.

Description

  The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, a facsimile, or a composite machine using the electrophotographic method or the electrostatic recording method.

  Tandem color image forming apparatus with multiple image forming units has excellent characteristics that there are many types of recording materials that can be used, the quality of full color images is high, and full color images can be obtained at high speed. Is provided. However, it is considered that a black image (monochrome image) occupies most of the uses in image formation. In this case, in the photosensitive member of the black image forming unit that forms a black image with a black developer, other image forming units Scratching increases compared to the photoconductor. In addition, image defects are likely to occur due to wear of the photosensitive member of the black image forming unit.

  For this reason, it is required that the photoconductor of the black image forming unit be made highly durable, for example, by making it harder than an OPC photoconductor generally used conventionally. For example, a structure is known in which only the photosensitive member of the black image forming unit is an amorphous silicon-based photosensitive member, and the durability of the photosensitive member of the black image forming unit is increased (see Patent Documents 1 and 2).

JP 10-333393 A Japanese Patent Laid-Open No. 11-24358

  However, as described in Patent Documents 1 and 2 above, when a photoconductor having high hardness such as an amorphous silicon photoconductor is used, blurring of an electrostatic latent image called “image flow” is likely to occur particularly in a high humidity environment. . That is, in the black image forming unit using a photoconductor with high hardness, image flow is more likely to occur than in other image forming units. The cause of the image flow is considered as follows.

  Discharge products such as ozone and NOx are generated mainly by the charging means and adhere to the surface of the photoreceptor. The discharge product adhering to the surface of the photoconductor is scraped off by a cleaning blade for removing the developer remaining on the surface of the photoconductor after transfer, but the coefficient of friction on the surface of the photoconductor is low. This amount of shaving is reduced. In particular, when the surface hardness of the photoreceptor is high, the amount of shaving is further reduced, and the discharge products attached to the surface are difficult to remove.

  If the discharge product remains on the surface of the photoconductor without being removed, the discharge product absorbs moisture in a high humidity environment, reducing the charge holding ability of the surface of the photoconductor and causing static discharge called “image flow”. Causes blurring of the latent image. Therefore, particularly when the hardness of the photosensitive member is high, the attached discharge products are more difficult to be removed, and the image flow is likely to occur.

  In general, the black image forming unit is often arranged at a position farthest from the fixing device. In this case, the photosensitive member of the black image forming unit is most difficult to warm. Therefore, the discharge product adhering to the photosensitive member of the black image forming portion easily adsorbs moisture, and from this point, the image flow is likely to occur.

  Furthermore, since the black image formation ratio in image formation is high, the frequency of use of the black image forming unit is higher than that of other image forming units. For this reason, more discharge products are accumulated in the photosensitive member of the black image forming unit than in the other image forming units, and the image flow tends to easily occur in the black image forming unit.

  As a countermeasure against image flow, a heater is generally installed in the vicinity of the photosensitive member or in the vicinity of the photosensitive member, and the surface of the photosensitive member is dried by raising the temperature of the surface of the photosensitive member. However, if an image is formed at a stage where the effects of these means cannot be obtained sufficiently, such as immediately after the power is turned on, an image flow may occur. When the image flow occurs, the image may be blurred or disappear, and an image defect may occur in the output.

  In view of such circumstances, the present invention was invented to realize a structure capable of reducing the occurrence of image defects even when an image flow is likely to occur in a black image forming unit.

  In the present invention, a black image forming unit that forms a black image with a black developer, an image is formed with a developer of a color other than black, and a process black image of black or a color close to black is formed by combining each color. A plurality of color image forming units that can be formed, and a control unit that controls the black image forming unit and the plurality of color image forming units, the black image forming unit and the plurality of color image forming units, An image carrier, charging means for charging the image carrier, latent image forming means for forming an electrostatic latent image on the charged image carrier, and an electrostatic latent image formed on the image carrier. An image forming apparatus having a developing means for developing with a developer, the image forming apparatus having a detecting means for detecting information relating to the image flow of the black image forming portion, the control means based on the detection result of the detecting means, Black One of a first mode in which an image is formed using only the black image forming unit and a second mode in which a black image is formed using the plurality of color image forming units is executed. It is in the image forming apparatus.

  According to the present invention, on the basis of the detection result of the detection means, either the first mode or the second mode is selected to form a black image. The occurrence of image defects can be reduced.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention. FIG. 3 is a block diagram illustrating image signal processing in the image forming apparatus according to the first embodiment. In order to explain UCR processing of an image signal, (a) is a schematic diagram showing a case where the UCR ratio is 100%, and (b) is a schematic diagram showing a case where the UCR ratio is 50%. In order to explain UCR processing of an image signal of a black image portion, (a) is a schematic diagram showing a case where the UCR ratio is 100%, and (b) is a schematic diagram showing a case where the UCR ratio is 50%. FIG. 3 is a block diagram illustrating a configuration of charging voltage application to the charging roller according to the first embodiment, where (a) illustrates a common part for all colors and (b) illustrates a characteristic part of a black image forming unit. 6 is a graph showing an example of a relationship between a charging DC voltage and a surface potential of a photosensitive drum. The graph which shows an example with respect to the case where the image flow has generate | occur | produced and the case where it has not generate | occur | produced as an example of the relationship between a charging DC voltage and the direct current value which flowed into the measurement circuit. FIG. 3 is a schematic diagram for explaining a mechanism in which charges are placed on a photosensitive drum when a charging DC voltage lower than a discharge start voltage is applied. The figure which shows the relationship between the direct current value which flows into a measurement circuit, and the density fall by an image flow. FIG. 4 is a diagram for explaining an operation sequence of the image forming apparatus. The control flowchart in 1st Embodiment. The figure which shows the relationship between the direct current value which flows into the measurement circuit in the 2nd Embodiment of this invention, and the UCR ratio of a black image part. The control flowchart in 2nd Embodiment. The figure which shows the threshold value of the direct current value in which the density fall by image flow in each relative humidity occurs. The figure which shows the relationship between the direct current value which flows into the measurement circuit in each relative humidity in the 3rd Embodiment of this invention, and the UCR ratio of a black image part. The control flowchart in 3rd Embodiment. The schematic diagram explaining the influence on a latent image in (a) patch part and (b) character part when image flow generate | occur | produces. FIG. 10 is a block diagram showing image signal processing in an image forming apparatus according to a fourth embodiment of the present invention. The figure which shows the relationship between the direct-current value which flows into the measurement circuit in 4th Embodiment, and the black image part other than a black character, and the UCR ratio of a black character part. The control flowchart in 4th Embodiment.

<First Embodiment>
A first embodiment of the present invention will be described with reference to FIGS. First, the overall configuration of the image forming apparatus of the present embodiment will be described with reference to FIG.

[Image forming apparatus]
The printer unit 20 of the image forming apparatus includes a plurality of image forming units P (Pa, Pb, Pc, Pd). An image forming unit that forms yellow in yellow, Pb in magenta, Pc in cyan, and Pd in black (black). Therefore, Pd corresponds to a black image forming unit, and Pa, Pb, and Pc correspond to a plurality of color image forming units. The image forming unit Pd forms a black image with a black developer (black toner).

  On the other hand, the other image forming portions Pa, Pb, and Pc form images of the respective colors using developers of colors other than black (yellow, magenta, and cyan toners). Further, the image forming units Pa, Pb, and Pc can form a process black image of black or a color close to black by combining the colors. That is, a substantially black image is formed by superimposing all of the yellow, magenta, and cyan toner images. In the present embodiment, a substantially black image formed by the image forming portions Pa, Pb, and Pc is referred to as a process black image.

  In addition, when describing the common part of each image forming unit, it is simply referred to as an image forming unit P, and when distinguishing colors, it is appropriately described as image forming units Pa, Pb, Pc, Pd. The description method of a code | symbol is the same also in another apparatus and member.

  Each of the image forming portions Pa, Pb, Pc, and Pd includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 that is an image carrier. That is, a yellow photosensitive drum 1a, a magenta photosensitive drum 1b, a cyan photosensitive drum 1c, and a black photosensitive drum 1d are provided. The yellow photosensitive drum 1a, the magenta photosensitive drum 1b, the cyan photosensitive drum 1c, and the black photosensitive drum 1d rotate in the directions of the arrows, respectively, during image formation.

  In the present embodiment, the yellow photosensitive drum 1a, the magenta photosensitive drum 1b, and the cyan photosensitive drum 1c are organic photoconductor (OPC) photosensitive members having negative charging characteristics. On the other hand, the black photosensitive drum 1d is an a-Si (amorphous silicon) photosensitive member having negative charging characteristics.

  The a-Si photoreceptor is more expensive than the OPC photoreceptor, but has a high hardness and excellent durability. From this characteristic, in the present embodiment, an a-Si drum is used as a black photosensitive drum that has a relatively large amount of image output by monochrome output. On the other hand, an inexpensive OPC drum is used as a photosensitive drum for colors (yellow, magenta, cyan) with a relatively small amount of image output. Therefore, the surface hardness of the photosensitive drum 1d of the image forming unit Pd which is a black image forming unit is larger than the surface hardness of the photosensitive drums 1a, 1b and 1c of the image forming units Pa, Pb and Pc which are a plurality of color image forming units. high.

  The surface of each photosensitive drum 1a, 1b, 1c, 1d is uniformly charged by a charging roller 2 (2a, 2b, 2c, 2d) as a charging means. Then, the surface of each charged photosensitive drum 1 is irradiated with an optical image 3 (3a, 3b, 3c, 3d) for each separated color by an exposure device 30A, which is a latent image forming unit, and electrostatically is applied to each photosensitive drum 1 respectively. A latent image is formed.

  Each electrostatic latent image formed on each photosensitive drum 1 is developed by each color developing device 4, that is, yellow developing device 4a, magenta developing device 4b, cyan developing device 4c, and black developing device 4d. Reversal developed. Each developing device 4 is filled with toner based on resin and pigment as a developer for each color, and each electrostatic latent image on the photosensitive drums 1a, 1b, 1c, and 1d is developed as a toner image. The A developing bias is applied to each developing device 4 during development of the electrostatic latent image.

  In this embodiment, each of the developing devices 4a, 4b, 4c, and 4d is loaded with a two-component developer that uses a mixture of non-magnetic toner and magnetic carrier. Absent. Further, the toner in the developing devices 4a, 4b, 4c, and 4d is maintained at a constant toner ratio (or toner amount) in the developing device 4 from a toner storage unit (hopper) (not shown) for each color. It is replenished at any time at a desired timing.

  The toner images formed on the photosensitive drums 1a, 1b, 1c, and 1d are primarily transferred at the primary transfer portion T1 (T1a, T1b, T1c, and T1d). Primary transfer rollers 5a, 5b, 5c, and 5d, which are primary transfer units, are arranged on an intermediate transfer body (intermediate transfer belt) 12 that uses toner images formed on the photosensitive drums 1a, 1b, 1c, and 1d as transfer media. Transfer in layers. At this time, a primary transfer bias is applied to the primary transfer rollers 5a, 5b, 5c, and 5d. As a result, the respective toner images are sequentially superimposed on the intermediate transfer belt 12 to form a full color toner image.

  Thereafter, the full-color toner image on the intermediate transfer belt 12 as a transfer medium is secondarily transferred to the recording material S at a time at the secondary transfer portion T2. The recording material S is conveyed one by one from the storage unit 13, and the secondary transfer roller 11 and the intermediate transfer belt 12 which are secondary transfer means for transferring the toner image on the intermediate transfer belt 12 to the recording material at a desired timing. To the secondary transfer portion T2 between the two. Then, as described above, the toner image is transferred onto the recording material S at the secondary transfer portion T2. Next, the recording material S passes through the conveyance section, and the toner image is fixed by a fixing device (heat roller fixing device) 9 and discharged to a paper discharge tray or a paper post-processing device (not shown).

  On the other hand, the transfer residual toner adhering to the photosensitive drums 1a, 1b, 1c, and 1d after the transfer of the toner image to the intermediate transfer belt 12 is rubbed by the cleaning blades of the cleaning devices 6a, 6b, 6c, and 6d, respectively. Removed. As a result, the surfaces of the photosensitive drums 1a, 1b, 1c, and 1d are cleaned and repeatedly used for image formation.

  Further, in the present embodiment, the printer unit 20 places the pre-exposure device 7 at a position downstream of the cleaning device 6 and upstream of the charging roller 2 in the rotation direction (surface movement direction) of each photosensitive drum 1. It is arranged. Pre-exposure devices 7a, 7b, 7c and 7d as pre-exposure means for irradiating the surface of each photosensitive drum 1 irradiate residual charges remaining on the surfaces of the photosensitive drums 1a, 1b, 1c and 1d after the transfer process. To remove static electricity. Then, the surface potentials of the photosensitive drums 1a, 1b, 1c, and 1d before the charging process are made constant near zero.

  An image signal is sent to the printer unit 20 from a reader unit 30 that is a document reading unit that reads a document or an external device 40 such as a computer or a facsimile. These image signals are subjected to predetermined image processing (color conversion, color separation) in the image processing unit 50, and then are irradiated with optical images 3a, 3b, 3c, and 3d from the exposure device 30A for each separated color, An electrostatic image is formed on each photosensitive drum 1. Further, the control circuit 100 serving as a control unit controls the overall image formation related to the printer unit 20 such as controlling each image forming unit P.

[Image signal processing]
Next, image signal processing will be described in detail. FIG. 2 is a block diagram schematically showing signal processing in the image processing unit 50. RGB image signal information sent from the reader unit 30 or an external device 40 such as a computer or a facsimile is received by the image processing unit 50 at C (cyan), M (magenta), Y (yellow), K (black). ) Signal.

  First, the LOG converter 51 performs luminance density conversion for converting the input RGB image signal into a YMC image signal (Y1, M1, C1 in the figure) based on the set parameters. On the other hand, the black image determination unit 52 performs saturation determination based on the RGB image signal. Here, the region determined to be an achromatic color is determined as a black image (black image portion), and the black image determination portion 52 outputs a signal for controlling the coefficient of under color removal processing (UCR) described below. To do.

Thereafter, in the UCR unit 53, under color removal processing (hereinafter referred to as “UCR processing”) using the CMY three-color signals as K (black) signals is performed according to the following equation.
Y2 = Y1- [alpha] * min (Y1, M1, C1)
M2 = M1- [alpha] * min (Y1, M1, C1)
C2 = C1-α × min (Y1, M1, C1)
K2 = α × min (Y1, M1, C1)
Here, α is a UCR ratio, that is, a process black image (black image formed by the image forming unit Pd) constituting the black image unit (substantially black formed by the image forming units Pa, Pb, and Pc). Is a ratio (black image ratio). Further, 0 ≦ α ≦ 1.

  As shown in FIG. 3, when forming chromatic images having different Y, M, and C signals (toner ratios), the UCR processable area corresponds to the black image portion. Therefore, as shown in FIG. 3A, when α = 1, it is called 100% UCR, and all process black images created with three colors of YMC are replaced with black images of K1 color (single color). Similarly, as shown in FIG. 3B, for example, when α = 0.5, it is called 50% UCR, and among process black images created with three colors of YMC, 50% is a monochrome black image. Replaced.

Next, UCR processing of an area (achromatic image) determined by the black image determination unit 52 as a black image portion will be described with reference to FIG. If the YMCK signals of the black image portion after UCR processing are Y2 ′, M2 ′, C2 ′, and K2 ′, respectively,
Y2 ′ = Y1−α ′ × min (Y1, M1, C1)
M2 ′ = M1−α ′ × min (Y1, M1, C1)
C2 ′ = C1−α ′ × min (Y1, M1, C1)
K2 ′ = α ′ × min (Y1, M1, C1)
Here, α ′ is the UCR ratio in the black image portion as in α described above, and 0 ≦ α ′ ≦ 1. Also, since the area recognized as a black image is achromatic,
Y1 = M1 = C1 = min (Y1, M1, C1)
It is. After rewriting
Y2 ′ = (1-α ′) Y1
M2 ′ = (1−α ′) Y1
C2 ′ = (1-α ′) Y1
K2 ′ = α ′ × Y1
Can be expressed.

  Therefore, as shown in FIG. 4A, when α ′ = 1, all the process black images created with the three colors YMC are replaced with a monochrome black image. At this time, all YMC signals are 0, and only K image signals are obtained. Similarly, as shown in FIG. 4B, for example, when α ′ = 0.5, 50% of the process black images created with the three colors YMC are replaced with a monochrome black image.

  Here, the effect of the UCR process in the black image portion will be described. As described above, in process color printing, black and gray can be expressed by mixing three colors of CMY (cyan, magenta, yellow) or four colors of CMYK (cyan, magenta, yellow, black). However, since it is possible to print only with K (black), a method of removing approximately equal amounts of three colors of CMY from the black (black) portion and increasing the amount of K toner accordingly is used. Also, the three colors CMY are removed from the gray portion and replaced with K toner.

  By performing such UCR processing, black characters and fine lines can be output with high quality. The UCR processing is used when it is desired to more neutrally represent the pattern gray. In addition, there is a merit that it can be used without worrying much about color balance, etc., rather than overlapping CMY.

  For this reason, in the present embodiment, in a normal state in which no image flow occurs, the black image portion performs processing of α ′ = 1 (α = 1), that is, 100% UCR, and is all a monochrome black image. Perform image formation. The signals Y2, M2, C2, and K2 (Y2 ′, M2 ′, C2 ′, and K2 ′ in the black image portion) that have been subjected to UCR processing by the UCR unit 53 are subjected to adjustment processing and sent to the printer unit 20. Then, each image forming unit P forms an image based on each signal.

[Configuration of charging voltage application]
Next, the configuration of charging voltage application in which each image forming unit P applies a charging voltage to the charging roller 2 to charge the photosensitive drum 1 will be described with reference to FIG. As shown in FIG. 5A, a predetermined vibration voltage (Vdc + Vac) obtained by superimposing an AC voltage of frequency f on a DC voltage from the charging power supply 60 is applied to the charging roller 2 via the core bar 201, thereby rotating. The peripheral surface of the photosensitive drum 1 is charged to a predetermined potential.

  For this purpose, the charging power source 60 which is a voltage application means for the charging roller 2 has a direct current (DC) power source 61 and an alternating current (AC) power source 62. The control circuit 100 can control the DC power supply 61 and the AC power supply 62 of the charging power supply S <b> 1 to apply a DC voltage, an AC voltage, or a superimposed voltage of both to the charging roller 2.

  Further, as shown in FIG. 5B, in the black image forming portion Pd, a direct current value measuring circuit (current) is a current detecting means for measuring the direct current value flowing from the charging roller 2d to the photosensitive drum 1d. (Hereinafter referred to simply as “measurement circuit”) 63. Information of the measured DC current value Idc is input from the measurement circuit 63 to the control circuit 100 as control means.

  The control circuit 100 as the control means determines whether or not an image flow can be caused from the information of the direct current value input from the measurement circuit 63, and has a function of controlling image signal processing (to be described later) accordingly. Have. In the present embodiment, the measurement circuit 63 is detection means for detecting information related to image flow, and the information related to image flow is a current value.

  The environment sensor 64 serving as an environment detection unit detects the relative humidity in (or around) the printer unit 20 and transmits information on the detected relative humidity to the control circuit 100. Further, a durable sheet number detecting means (counter) 65 as a usage amount detecting means detects the durable sheet number (image forming sheet number) converted to the number of A4 sheets from the start of use of the photosensitive drum 1 mounted on the printer unit 20. The number information is transmitted to the control circuit 100. The usage amount detecting means may detect the driving time or driving distance of the photosensitive drum, and further, of the driving time or driving distance of the photosensitive drum, the time when the bias such as charging is applied or the application time. Alternatively, the driving distance of the photosensitive drum may be detected.

[Image Flow Detection]
Next, a system for detecting image flow in the black image forming unit Pd will be described. In the following description, when referring to the magnitude relationship between the voltage value and the current value, the magnitude relationship with respect to the absolute value is referred to for convenience.

  FIG. 6 is a graph showing the results of measuring the relationship between the DC voltage applied to the charging roller 2d and the surface potential of the photosensitive drum 1d in an environment at a temperature of 23 ° C. and a relative humidity of 50%. When the DC voltage to be applied is increased, the surface potential on the photosensitive drum 1d does not increase at first, but the surface potential starts to increase from a certain voltage value. This is the discharge start voltage Vth. In the present embodiment, −550 V is the discharge start voltage Vth.

  The discharge start voltage Vth is determined from the gap between the charging roller 2d and the photosensitive drum 1d, the thickness of the photosensitive layer, and the relative dielectric constant of the photosensitive layer. When a voltage equal to or higher than the discharge start voltage Vth is applied to the charging roller 2d, discharge development occurs in the gap based on Paschen's law, and charges are placed on the photosensitive drum 1d.

  FIG. 7 shows the relationship between the DC voltage applied to the charging roller 2d and the DC current value flowing into the measurement circuit 63 in the black image forming portion Pd. FIG. 7 shows the results of measurement under the same conditions as in FIG. 6 for the photosensitive drum 1d where no image flow occurred and the photosensitive drum 1d where image flow occurred.

  From FIG. 7, it can be seen that the measurement circuit 63 hardly detects the direct current value of the photosensitive drum 1d in which no image flow has occurred at an applied voltage lower than the discharge start voltage Vth. On the other hand, in the photosensitive drum 1d in which image flow occurs, it can be seen that the DC current value is detected by the measurement circuit 63 even with an applied voltage lower than the discharge start voltage Vth.

  This phenomenon occurs by the following mechanism. That is, by performing the charging process, discharge products accumulate on the surface of the photosensitive drum 1d, and when the discharge products continue to be on the photosensitive drum 1d, they adsorb moisture in the air, and the surface of the photosensitive drum 1d. The electric resistance of the image is reduced, and image flow occurs. The photosensitive drum 1d in which such an image flow is generated is charged with a minute amount even when a DC voltage (predetermined voltage, −500 V in the present embodiment) less than the discharge start voltage Vth based on Paschen's law is applied. start. This is due to “injection charging” due to a decrease in the electrical resistance of the surface of the photosensitive drum 1d. FIG. 8 schematically shows this mechanism.

  In a normal state where no image flows, when a DC voltage of −500 V is applied to the charging roller 2d, as shown in FIG. 8A, the change in the surface potential of the photosensitive drum 1d before and after passing through the charging roller 2d. Does not happen. That is, no charge transfer from the charging roller 2d to the photosensitive drum 1d occurs. Therefore, no current flows in such a normal state, and the DC current value is not detected by the measurement circuit 63, or even if it is detected, it is a very small value. On the other hand, when a DC voltage equal to or higher than the discharge start voltage Vth is applied to the charging roller 2d even in a normal state in which such image flow does not occur, as shown in FIG. 8B, the photosensitive drum before and after passing through the charging roller 2d. Charge is placed on 1d.

  However, in a state where image flow occurs, the electrical resistance of the surface is lowered, so that even if it is less than the discharge start voltage Vth, it is slightly injected and charged. Therefore, as shown in FIG. 8C, a slight charge is placed on the surface of the photosensitive drum 1 on the downstream side of the charging roller 2d. As a result, a potential difference is generated before and after the charging roller 2d, and a direct current flows even when a predetermined voltage that is a direct current voltage less than the discharge start voltage Vth is applied to the charging roller 2d, and the direct current value is detected by the measurement circuit 63. Is done.

  In the present embodiment, using such a phenomenon, it is determined whether or not the photosensitive drum 1d is in a state capable of generating image flow (that is, image flow detection is performed). Specifically, in an environment with a relative humidity of 50%, −500 V (predetermined voltage), which is a DC voltage lower than the discharge start voltage Vth, is applied to the charging roller 2 d, and the DC current value Idc at that time is measured by the measurement circuit 63. taking measurement. Then, the degree of image flow on the photosensitive drum 1d is determined based on whether the current value Idc detected by the measurement circuit 63 is greater than or less than a predetermined value.

  FIG. 9 shows the relationship between the detected DC current value Idc (absolute value) and the occurrence of image flow on the output image. Here, a black monotone halftone (HT) image patch having a size of 1 cm × 1 cm is output, and the density reduction rate of the image patch is measured. That is, a halftone image having a reflection density of 0.5 was output by the black image forming unit Pd, the density of the actually formed image was measured, and the reduction rate was obtained. The reflection density was measured with a spectral densitometer X-Rite 504/508 (manufactured by X-Rite Co.).

  Then, when there is no image flow on the image and the density of the image patch having a reflection density of 0.5 is 0.4 or less, that is, the density reduction rate is 80% or less, an image defect (density) due to image flow. (Decrease) occurred. From FIG. 9, it can be seen that when the absolute value of the DC current value Idc measured by the measurement circuit 63 is 2 μA or more, there is a risk of density reduction due to image flow. For this reason, in this embodiment, the predetermined value for determining the degree of image flow is 2 μA. That is, when a predetermined voltage (−500 V) is applied to the charging roller, control is performed as described later depending on whether or not the absolute value of the current detected by the measurement circuit 63 is equal to or greater than a predetermined value (2 μA). The mode is changed.

[Operation sequence]
Here, the operation sequence of the image forming apparatus of the present embodiment will be described with reference to FIG.

a. Initial rotation operation (front multiple rotation process)
This is a start operation period (start operation period, warming period) when the printer unit 20 is started. When the power switch is turned on, the photosensitive drum 1 is driven to rotate, and a preparatory operation for a predetermined process device such as raising the fixing device 9 to a predetermined temperature is executed.

b. Print preparation rotation operation (pre-rotation process)
This is the preparatory rotation operation period before image formation from when the print signal is turned on until the actual image formation (printing) process operation is performed. When the print signal is input during the initial rotation operation, it is executed following the initial rotation operation. Is done. When the print signal is not input, the drive of the main motor is temporarily stopped after the initial rotation operation is completed, and the rotation of the photosensitive drum 1 is stopped. The printer unit 20 is kept in a standby (standby) state until the print signal is input. Be drunk. When the print signal is input, the print preparation rotation operation is executed.

  In the present embodiment, in this print preparation rotation operation period, it is determined whether or not an image flow can occur in the image forming unit Pd (black image forming unit). Change the image signal processing. This will be described later.

c. Printing process (image forming process, image forming process), transfer process When a predetermined printing preparation rotation operation is completed, an image forming process for the photosensitive drum 1 is subsequently performed. Then, the toner image formed on the surface of the photosensitive drum 1 is transferred to the intermediate transfer belt 12, transferred to the recording material S, and the toner image is fixed by the fixing device 9. Is done. In the continuous printing (continuous printing) mode, the above-described printing process is repeatedly executed for a predetermined set number n of prints.

d. Inter-sheet process In the continuous printing mode, after the trailing edge of one recording material S passes through the secondary transfer portion T2, the leading edge of the next recording material S reaches the secondary transfer portion T2. This is a non-passing state period of the recording material S in the secondary transfer portion T2.

e. Post-rotation operation This is a period in which after the printing process of the last recording material S is completed, the main motor is continuously driven for a while to rotate the photosensitive drum 1 and execute a predetermined post-operation.

f. Standby When the predetermined post-rotation operation is completed, the drive of the main motor is stopped, the rotation of the photosensitive drum 1 is stopped, and the printer unit 20 is kept in a standby state until the next print start signal is input. In the case of printing only one sheet, after the printing is completed, the printer unit 20 enters a standby state through a post-rotation operation. When the print start signal is input in the standby state, the printer unit 20 proceeds to the pre-rotation process.

  Note that the printing process of c is the time of image formation, and the initial rotation operation of a, the pre-rotation operation of b, the paper gap process of d, and the post-rotation operation of e are non-image formation.

[Image formation mode for black image area]
In the case of the present embodiment, the control circuit 100 includes a first mode in which a black image is formed using only the image forming unit Pd (black image forming unit), and a black image is formed in the image forming units Pa, Pb, Pc (multiple images). The color image forming unit) is used to execute one of the second modes. That is, the first mode is a mode in which the UCR ratios α and α ′ are 1 and the black image portion is formed only by the image forming portion Pd. On the other hand, the second mode is a case where the UCR ratios α and α ′ are less than 1 or 0, and a black image portion is formed by the image forming portion Pd and the image forming portions Pa, Pb, and Pc, or image formation is performed. In this mode, only the portions Pa, Pb, and Pc are used.

  In the present embodiment, the first mode and the second mode are based on the information regarding the image flow, that is, the current value (detection result) detected by the measurement circuit 63 when a predetermined voltage is applied to the charging roller 2. One of the above is executed. Specifically, the first mode is executed when the absolute value of the current detected by the measurement circuit 63 is less than a predetermined value, and the second mode is executed when it is greater than or equal to the predetermined value. In the present embodiment, −500 V is applied to the charging roller 2d, and α and α ′ are 1 when the absolute value of the current detected by the measurement circuit 63 is less than 2 μA, and α and α ′ when 2 μA or more. Is set to 0.

[Control flow]
The control flow of this embodiment will be described with reference to FIG. FIG. 11 shows the detection of the image flow as described above, and changes the UCR ratio of the black image portion in the case of achromatic color (FIG. 4 described above) when there is a possibility that the image flow may occur in the black image forming portion Pd. It is an example of the flowchart of control to perform. In the case of a chromatic color (FIG. 3 described above), the same control is performed only by replacing α ′ with α.

  First, when an image formation signal is input, the control circuit 100 determines whether it is the timing of image flow detection (S01). For example, it is determined whether or not it is the aforementioned print preparation rotation operation period. Note that it is preferable that the image flow detection timing is always executed in the print preparation rotation operation in order to reduce the image flow more reliably. However, in consideration of the mass productivity of the output product of the image forming apparatus, it may be executed when any one or combination of the following conditions is satisfied. That is, it is always executed before image formation immediately after the power is turned on. This is executed when the environmental sensor 64 detects a predetermined relative humidity (for example, 30%) or more. This is executed after the number of image formations has rotated N or more from the previous detection of image flow (after the photosensitive drum has rotated M or more times), that is, when the durable number detection means 65 detects a predetermined value or more.

  Further, if these conditions are satisfied, during non-image formation other than the print preparation rotation operation period, for example, during initial rotation operation, during a sheet interval process, during post-rotation operation, and more than one of these non-image formation It may be executed sometimes. As described above, when executing at any time during non-image formation, for example, when the temperature and humidity of a room in which an image forming apparatus is placed during standby change greatly due to ON or OFF of the air conditioning of the room. There is.

  Next, if it is not the timing of image flow detection (NO in S01), image formation is started as it is (S07). On the other hand, if it is the timing of image flow detection (YES in S01), the photosensitive drum 1d is rotated, and a DC voltage (predetermined voltage) less than the discharge start voltage Vth is applied to the charging roller 2d. Apply. At this time, the pre-exposure device 7d is turned on, the exposure device 3d is turned off, and both the development voltage and the transfer voltage are turned off (S02). With such a voltage setting, if the photosensitive drum 1d can generate image flow, even when a DC voltage lower than the discharge start voltage Vth is applied, charge is injected from the charging roller 2d to the photosensitive drum 1d. . In this state, the measurement circuit 63 detects the direct current value Idc (S03).

  Next, the control circuit 100 determines whether or not the absolute value of the DC current value Idc detected by the measurement circuit 63 is less than 2 μA (S04). If the absolute value of the direct current value Idc is less than 2 μA (YES in S04), the control circuit 100 sets the UCR ratio α ′ = 1 in the black image portion (S05) and executes image formation (S07). On the other hand, if the absolute value of the direct current value Idc is 2 μA or more (NO in S04), the control circuit 100 sets the UCR ratio α ′ = 0 in the black image portion (S06) and executes image formation (S07). The value of α ′ determined here is recorded in a recording unit (not shown) by the control circuit 100 and stored until image flow detection control is performed again. The control circuit 100 controls various devices in this control flow.

  According to the present embodiment, when the black image is formed by selecting either the first mode or the second mode based on the detection result of the measurement circuit 63, the image flow is likely to occur in the black image forming unit. However, the occurrence of image defects can be reduced. That is, when the current value detected by the measurement circuit 63 is equal to or greater than a predetermined value, the image formation is likely to occur in the image forming unit Pd that is the black image forming unit. This is because in the image forming portion Pd, the photosensitive drum 1d is made of amorphous silicon having a high surface hardness, and thus more discharge products are attached than in the other image forming portions. On the other hand, the photosensitive drums of other image forming units are less likely to cause image flow than the image forming unit Pd because there is less adhesion of discharge products even in situations where image flow is likely to occur in the image forming unit Pd.

  For this reason, in this embodiment, when it is determined from the detection result of the measurement circuit 63 that an image flow is likely to occur on the photosensitive drum 1d of the image forming unit Pd, the black image portion is formed in the second mode. In the second mode, a process black image is formed by the other image forming units Pa, Pb, and Pc, and the image flow does not easily occur in these other image forming units Pa, Pb, and Pc. Even when it is likely to occur, the occurrence of image defects can be reduced.

  Even if the photosensitive drum 1d of the image forming unit Pd is configured in the same manner as the photosensitive drums of the other image forming units and the surface hardness is the same, the photosensitive drum 1d of the image forming unit Pd has an image flow more than that of the other image forming units. Is likely to occur. This is because, as described above, the frequency of use of the image forming unit Pd that forms a black image is higher than that of other image forming units, and thus discharge products are likely to adhere. Further, when the image forming unit Pd is arranged at the position farthest from the fixing device 9 as in the present embodiment, the moisture in the image forming unit Pd is less likely to evaporate than the other image forming units. Image flow due to the discharge product is more likely to occur than in the image forming unit. Even in such a case, if it is determined whether or not an image flow is likely to occur on the photosensitive drum 1d based on the detection result of the measurement circuit 63 as in the present embodiment, and the above-described control is executed, an image defect will occur. Can be reduced.

[Confirmation of effect of this embodiment]
Table 1 shows the results of experiments conducted to confirm the effects of the present embodiment as described above as Example 1 when the image was formed by performing the above-described control, and Comparative Example 1 when the image was formed without performing the above-described control. Show.

  Here, the photosensitive drum 1d in a state where the DC current value Idc measured by the measuring circuit 63 is 0 to −2.5 μA is used. That is, a plurality of photosensitive drums 1d having different usage conditions such as a usage period and a discharge amount during use were prepared. For example, a photosensitive drum having a long period of use has many discharge products adhering to it, and a flowing current value increases. Further, when the voltage applied from the charging roller in use is high, the discharge product is increased and the flowing current is increased. In the present embodiment, a plurality of discharge products adhering to the surface are varied in this manner so that the DC current value Idc measured by the measurement circuit 63 is 0 to −2.5 μA. A photosensitive drum 1d was prepared.

  A black single-tone halftone (HT) image patch having a size of 1 cm × 1 cm was output on each photosensitive drum 1 d thus prepared under the condition that the reflection density was 0.5 and there was no image flow on the image. . Then, the density of the actually formed image was measured, and the reduction rate was obtained. The reflection density was measured with a spectral densitometer X-Rite 504/508 (manufactured by X-Rite). In addition, the circle mark in Table 1 indicates a case where there is almost no decrease in density, the triangle mark indicates a case where the concentration decrease rate is suppressed to a state larger than 80%, and the cross mark indicates that the concentration decrease rate is 80% or less. Each case is shown.

  As shown in Table 1, in Comparative Example 1, when the absolute value of Idc is 2 μA or more, the density of the image patch decreases to 0.4 or less, and an image defect (density decrease) due to image flow occurs. It became the level recognized as doing. On the other hand, in Example 1, even when the absolute value of Idc was 2 μA or more, the patch density reduction was suppressed to a state larger than 80%, and image defects due to image flow could be reduced.

  That is, in the first embodiment, when the absolute value of Idc is 2 μA or more, the UCR ratio of the black image portion is changed to α ′ = 0 by the above control, and the black image portion is entirely replaced with the process black image. In the yellow, magenta, and cyan image forming portions Pa, Pb, and Pc that form a process black image, an OPC drum that does not easily cause image flow is used as a photosensitive drum. Therefore, the replaced process black image can output a patch image (density 0.5 to 0.45 in the first embodiment) in which the occurrence of density reduction due to image flow is small.

<Second Embodiment>
A second embodiment of the present invention will be described with reference to FIGS. 12 and 13 with reference to FIGS. In the first embodiment described above, whether or not a direct current value Idc having an absolute value of 2 μA or more is detected when -500 V, which is a direct current voltage less than the discharge start voltage Vth, is applied to the charging roller 2d. The UCR ratio α ′ was switched between 0 and 1.

  In the case of such a first embodiment, there is an advantage that the black image is not replaced with a process black image more than necessary. On the other hand, the UCR ratio α ′ is suddenly changed before and after the threshold. For example, the UCR ratio α ′ may be changed from 1 to 0 in a certain image forming job and the subsequent image forming job. In such a case, when comparing the images formed by the preceding and subsequent image forming jobs, the difference between the color of the image formed with a single black color and the color of the image formed with process black is easily noticeable. . Therefore, in terms of image quality, a configuration in which the value of α ′ is changed stepwise is preferable.

  Therefore, in the present embodiment, the control circuit 100 reduces the black image ratio of the black image forming the black image to the process black image as the absolute value of the current detected by the measurement circuit 63 increases in the second mode. ing. Specifically, as shown by the solid line in FIG. 12, the value of α ′ is changed stepwise according to the detected direct current value Idc. That is, the value of α ′ is set to decrease from 1 to 0 as the absolute value of the detected direct current value Idc increases. In FIG. 12, the direct current value Idc is shown as an absolute value.

  Such a control flow of the present embodiment will be described with reference to FIG. First, when an image formation signal is input, the control circuit 100 determines whether it is the timing of image flow detection (S11). This timing is the same as S01 in FIG. If it is not the timing of image flow detection (NO in S11), image formation is started as it is (S15).

  On the other hand, if it is the timing of image flow detection (YES in S11), the photosensitive drum 1d is rotated, and a DC voltage (predetermined voltage) less than the discharge start voltage Vth is applied to the charging roller 2d. Is applied. At this time, the pre-exposure device 7d is turned on, the exposure device 3d is turned off, and both the development voltage and the transfer voltage are turned off (S12). With such a voltage setting, if the photosensitive drum 1d can generate image flow, even when a DC voltage lower than the discharge start voltage Vth is applied, charge is injected from the charging roller 2d to the photosensitive drum 1d. . In this state, the measurement circuit 63 detects the direct current value Idc (S13).

  Here, the control circuit 100 changes the value of α ′ as shown in FIG. 12 according to the absolute value of the direct current value Idc measured in the measurement circuit 63 (S14), and executes image formation (S15). . The value of α ′ determined here is recorded in a recording unit (not shown) by the control circuit 100 and stored until image flow detection control is performed again. The control circuit 100 controls various devices in this control flow.

  According to the present embodiment, since the black image ratio is changed according to the detection result of the measurement circuit 63, control according to the degree of image flow on the photosensitive drum 1d is possible. For this reason, the image quality can be further improved. In particular, when image forming jobs are continuously performed, a change in color can be suppressed in the preceding and succeeding image forming jobs. Other structures and operations are the same as those in the first embodiment.

[Confirmation of effect of this embodiment]
The results of the experiment performed to confirm the effects of the present embodiment as described above are shown in Example 2 when the image is formed by performing the above-described control, and in Comparative Example 2 (Comparative Example 1) when the image is formed without performing the above-described control. Are shown in Table 2.

  Here, a plurality of photosensitive drums 1d in a state in which the DC current value Idc measured in the measurement circuit 63 is 0 to −2.5 μA are used as in the experiment of Table 1 described above. Further, a black single-tone black halftone (HT) image patch having a size of 1 cm × 1 cm is output on each photosensitive drum 1d under such a condition that a reflection density of 0.5 can be output without any image flow on the image. .

  Then, the density of the actually formed image was measured, and the reduction rate was obtained. The reflection density was measured with a spectral densitometer X-Rite 504/508 (manufactured by X-Rite). The round marks in Table 2 indicate the case where there is almost no decrease in density, the triangular marks indicate the case where the concentration decrease rate is suppressed to a state larger than 80%, and the cross marks indicate that the concentration decrease rate is 80% or less. Each case is shown.

  As shown in Table 2, in Comparative Example 2, when the absolute value of Idc is 2 μA or more, the density of the image patch decreases to 0.4 or less, and an image defect (density decrease) occurs due to image flow. It became the level recognized as doing. On the other hand, in Example 2, a good image with almost no decrease in density was obtained regardless of the value of Idc.

  That is, in the second embodiment, when the Idc is detected, the UCR ratio α ′ of the black image portion is changed stepwise by the above control, and the black image portion is replaced with the process black image according to the ratio. In the second embodiment, α ′ decreases as Idc increases, the ratio of black images decreases, and the ratio of process black images increases.

  In the yellow, magenta, and cyan image forming portions Pa, Pb, and Pc that form a process black image, an OPC drum that does not easily cause image flow is used as a photosensitive drum. For this reason, the replaced process black image can output a patch image (density 0.5) in which density reduction due to image flow does not occur.

<Third Embodiment>
A third embodiment of the present invention will be described with reference to FIGS. 14 to 16 with reference to FIGS. In the first and second embodiments described above, the image flow detection and the control of the UCR ratio α ′ based on the detection are described as an example in an environment where the relative humidity is 50%. However, when the environment changes, the electrical resistance of the charging roller 2d and the photosensitive drum 1d itself also changes. Therefore, for example, when the relative temperature rises, the DC current value Idc detected by the measurement circuit 63 when the image flow can be generated increases.

  FIG. 14 shows the relative humidity in the printer unit 20 detected by the environment sensor 64 and the direct current value Idc (absolute value) flowing through the photosensitive drum 1d when an image defect (density reduction) due to image flow occurs. It is the figure which showed the relationship. Here, the determination as to whether or not density reduction has occurred is made according to the following criteria, as in the first and second embodiments. That is, a black monochromatic halftone (HT) image patch having a size of 1 cm × 1 cm was output under the condition that a reflection density of 0.5 can be output without any image flow on the image. Then, when the density of the image patch decreased to 0.4 or less, it was determined that an image defect (density decrease) due to image flow occurred.

  As is clear from FIG. 14, as the relative humidity increases, an image defect occurs due to image flow, and thus the absolute value of the current value detected by the measurement circuit 63 increases. Therefore, for more precise control, the setting of the direct current value Idc, which is a threshold value for determining whether or not the photosensitive drum 1d can generate image flow, can be made variable according to the environment. preferable.

  Therefore, in the present embodiment, the control circuit 100 decreases the reduction rate of the black image ratio with respect to the increase in the absolute value of the current detected by the measurement circuit 63 as the relative humidity detected by the environment sensor 64 is higher. Specifically, as shown in FIG. 15, the α ′ reduction rate accompanying the increase in the absolute value of the current value of the measurement circuit 63 is changed stepwise in accordance with the detected relative temperature. That is, as the detected direct current value Idc increases, the value of α ′ decreases from 1 to 0 at any relative humidity. However, the higher the detected relative humidity, the greater the direct current value. The reduction rate of the value of α ′ with respect to the increase in Idc is set to be small. In FIG. 15, the direct current value Idc is shown as an absolute value.

  Such a control flow of the present embodiment will be described with reference to FIG. First, when an image formation signal is input, the control circuit 100 determines whether it is the timing of image flow detection (S21). This timing is the same as S01 in FIG. If it is not the timing of image flow detection (NO in S21), image formation is started as it is (S26).

  On the other hand, if it is the timing of image flow detection (YES in S21), first the relative humidity in the printer unit 20 is detected by the environmental sensor 64, and the information is transmitted to the control circuit 100 (S22). Thereafter, the control circuit 100 rotates the photosensitive drum 1d to apply a DC voltage (predetermined voltage) lower than the discharge start voltage Vth, in this embodiment, a DC voltage of −500 V, to the charging roller 2d. At this time, the pre-exposure device 7d is turned on, the exposure device 3d is turned off, and both the development voltage and the transfer voltage are turned off (S23). With such a voltage setting, if the photosensitive drum 1d can generate image flow, even when a DC voltage lower than the discharge start voltage Vth is applied, charge is injected from the charging roller 2d to the photosensitive drum 1d. . In this state, the measurement circuit 63 detects the direct current value Idc (S24).

  Here, the control circuit 100 changes the value of α ′ as shown in FIG. 15 according to the direct current value Idc measured by the measurement circuit 63 and the relative humidity detected by the environmental sensor 64 (S25). ), Image formation is executed (S26). The value of α ′ determined here is recorded in a recording unit (not shown) by the control circuit 100 and stored until image flow detection control is performed again. The control circuit 100 controls various devices in this control flow.

  According to the present embodiment, the reduction rate of the black image ratio, that is, the UCR ratio α ′ with respect to the increase in the absolute value of the current value of the measurement circuit 63 is changed according to the relative humidity detected by the environment sensor 64. Control according to the environment becomes possible. Therefore, even if the environment where the image forming apparatus is placed changes, a high-quality image can be stably obtained. Other structures and operations are the same as those in the second embodiment.

[Confirmation of effect of this embodiment]
In the case where an image is formed by performing the control according to the detection result on the environmental sensor 64 as described above, the experiment result performed for confirming the effect of the present embodiment as described above is performed without performing the image in the third embodiment. The case where it is formed is shown in Table 3 as Comparative Example 3. In the environment where this experiment was performed, the relative humidity detected by the environment sensor 64 was 20%. In Comparative Example 3, an experiment was performed with a decrease rate of the UCR ratio α ′ with respect to an increase in the absolute value of the current value of the measurement circuit 63 when the relative humidity was 50%.

  Here, a plurality of photosensitive drums 1d in a state in which the DC current value Idc measured in the measurement circuit 63 is 0 to −2.5 μA are used as in the experiment of Table 1 described above. Further, a black single-tone black halftone (HT) image patch having a size of 1 cm × 1 cm is output on each photosensitive drum 1d under such a condition that a reflection density of 0.5 can be output without any image flow on the image. .

  Then, the density of the actually formed image was measured, and the reduction rate was obtained. The reflection density was measured with a spectral densitometer X-Rite 504/508 (manufactured by X-Rite). The circles in Table 3 indicate the case where there is almost no decrease in density, and the triangles indicate the case where the concentration decrease rate is suppressed to a state larger than 80%.

  As shown in Table 3, in Comparative Example 3, when the DC current value Idc is in the range of −0.5 to −1.5 μA, the image does not reach a level that is recognized as an image defect (density reduction). The patch density has decreased. On the other hand, in Example 3, a good image with almost no decrease in density was obtained regardless of the value of Idc.

  That is, in the third embodiment, when the Idc is detected, an appropriate UCR ratio α ′ of the black image portion is calculated by the above control in consideration of the environment, and the black image portion is converted into the process black image according to the ratio. Is replaced. The replaced process black image can stably output a patch image (density 0.5) that does not cause density reduction due to image flow.

<Fourth Embodiment>
A fourth embodiment of the present invention will be described with reference to FIGS. 17 to 20 with reference to FIGS. In the first to third embodiments described above, image flow detection in the black image portion and control of the UCR ratio α ′ based thereon are described as an example, and in particular, the density of the image patch as a black image is used as a determination criterion. However, compared to image patches and photographic images, which generally have a wide latent image, characters and lines (lines) with a narrow latent image are more susceptible to image flow, resulting in density reduction and image disappearance. It tends to occur. The mechanism will be described with reference to FIG.

  In the image formation, as described above, the light image 3 is irradiated on the photosensitive drum 1 charged to a predetermined potential (VD potential) by the exposure device 30A based on the image signal sent to the printer unit 20. Then, an electrostatic latent image as shown by a solid line in FIG. 17 is formed. The potential (VL potential) of the portion irradiated with the light image 3 has a smaller absolute value than the developing potential of the developing device, and negatively charged toner is supplied from the developing device at the irradiated position. Is finally formed and output as an output product.

  Here, in a normal state in which no image flow occurs, an image is formed according to the shape irradiated with the light image 3. On the other hand, in a state where image flow occurs, the latent image is dulled from the edge portion to the non-image area as indicated by the dotted line in FIG.

  As shown in FIG. 17A, if the latent image has a relatively large area such as a patch, the center of the latent image is relatively unaffected, so image blur occurs slightly at the edge portion. Is less affected by image flow. On the other hand, as shown in FIG. 17B, in the case of a latent image having a small area (width) such as a character or a line, it is easily affected by the latent image blur from the edge portion, and the image is blurred or the image is blurred. Disappear.

  Therefore, in the present embodiment, the control circuit 100 changes the value of the black image ratio between the portion having the predetermined image pattern and the other portion of the black image. Here, the predetermined image pattern is at least one of a character and a line. In the present embodiment, the reduction rate of the black image ratio with respect to the increase in the absolute value of the current detected by the measurement circuit 63 is larger in the case of the predetermined image pattern than in the other portions.

  For this purpose, in the present embodiment, as shown in FIG. 18, a black character determination unit 54 for detecting a black character or a line (line) in the black image portion is provided. After the area determined to be achromatic by the black image determination unit 52 is determined as a black image portion, the black character determination unit 54 performs character thickness determination and edge detection based on the RGB image signal. Here, the area determined to be a character or a line is determined as a black character portion, and the black character determination portion 54 outputs a signal for controlling the UCR coefficient.

  As described above, the character portion tends to be more susceptible to image flow compared to a portion such as a patch or a photographic image. Therefore, in the present embodiment, a character other than black characters is used depending on the detected direct current value Idc. The value of α ′ of the black image portion and the black character portion is changed.

  More specifically, as shown in FIG. 19, in the environment where the relative humidity is 50%, the α ′ value in the black image portion other than the black character portion and the α ′ value in the black character portion are changed stepwise. As the DC current value detected for both the black image portion and the black character portion other than the black character increases, the value of α ′ decreases from 1 to 0, but the DC current value Idc increases in the black character portion. The reduction rate of the value of α ′ with respect to is set to be large. In FIG. 19, the direct current value Idc is shown as an absolute value.

  Such a control flow of the present embodiment will be described with reference to FIG. First, when an image formation signal is input, the control circuit 100 determines whether it is the timing of image flow detection (S31). This timing is the same as S01 in FIG. If it is not the timing of image flow detection (NO in S31), image formation is started as it is (S35).

  On the other hand, if it is the timing of image flow detection (YES in S31), the photosensitive drum 1d is rotated, and a DC voltage (predetermined voltage) less than the discharge start voltage Vth is applied to the charging roller 2d. Is applied. At this time, the pre-exposure device 7d is turned on, the exposure device 3d is turned off, and both the development voltage and the transfer voltage are turned off (S32). With such a voltage setting, if the photosensitive drum 1d can generate image flow, even when a DC voltage lower than the discharge start voltage Vth is applied, charge is injected from the charging roller 2d to the photosensitive drum 1d. . In this state, the measurement circuit 63 detects the direct current value Idc (S33).

  Here, the control circuit 100 changes the black image portion other than the black character and the α ′ value of the black character portion, respectively, as shown in FIG. 19 in accordance with the DC current value Idc measured by the measurement circuit 63 (S34). ), Image formation is executed (S35). The value of α ′ determined here is recorded in a recording unit (not shown) by the control circuit 100 and stored until image flow detection control is performed again. The control circuit 100 controls various devices in this control flow.

  According to the present embodiment, in the case of a character or line image that is greatly affected by image flow, the reduction rate of the black image ratio, that is, the UCR ratio α ′ with respect to the increase in the absolute value of the current value of the measurement circuit 63 is increased. Therefore, the influence of image flow can be reduced. That is, the ratio of the process black image is increased at an early stage with respect to the increase in the absolute value of the current value of the measurement circuit 63 as compared with the other images, twice as large as the image of characters and lines. For this reason, when the same control is performed for lines and characters and other images, the image blur is not noticeable in other images, but the characters and lines are blurred or disappear. However, it is possible to reduce the blurring of characters and lines and to prevent this image from disappearing. Other structures and operations are the same as those of the second embodiment described above.

[Confirmation of effect of this embodiment]
The results of experiments conducted to confirm the effects of the present embodiment as described above are the cases where images are formed by performing control to change the black image portion other than black characters and α ′ of the black character portions as described above. Table 4 shows a fourth comparative example in which an image was formed without performing the fourth embodiment. Note that Comparative Example 4 is equivalent to Example 2 of the experiment of Table 2 described above.

  Here, a plurality of photosensitive drums 1d in a state in which the DC current value Idc measured in the measurement circuit 63 is 0 to −2.5 μA are used as in the experiment of Table 1 described above. Further, a black single-tone black halftone (HT) image patch having a size of 1 cm × 1 cm is output on each photosensitive drum 1d under such a condition that a reflection density of 0.5 can be output without any image flow on the image. .

  Then, the density of the image of the black image portion other than the actually formed black characters was measured, and the reduction rate was obtained (the upper part of Table 4). The reflection density was measured with a spectral densitometer X-Rite 504/508 (manufactured by X-Rite). In the upper part of Table 4, the circle mark indicates a case where there is almost no decrease in density, the triangle mark indicates a case where the concentration decrease rate is suppressed to a state larger than 80%, and the cross mark indicates that the concentration decrease rate is 80% or less. Each case is shown.

  As shown in the upper part of Table 4, in Comparative Example 4 and Example 4, good images were obtained in the black image portion other than the black character portion. That is, in both the comparative example 4 and the example 4, when the Idc is detected, the UCR ratio α ′ of the black image portion is changed stepwise by the above control, and the black image portion is processed black according to the ratio. Replaced with an image. In Comparative Example 4 and Example 4, α ′ decreases as Idc increases, the ratio of black images decreases, and the ratio of process black images increases.

  In the yellow, magenta, and cyan image forming portions Pa, Pb, and Pc that form a process black image, an OPC drum that does not easily cause image flow is used as a photosensitive drum. For this reason, the replaced process black image can output a patch image (density 0.5) in which density reduction due to image flow does not occur.

  Next, a plurality of photosensitive drums 1d in a state in which the DC current value Idc measured in the measurement circuit 63 is 0 to −2.5 μA are used as in the experiment of Table 1 described above. Black characters having a size of 3 points (pt) and 5 points (pt) were output on each photosensitive drum 1d thus prepared under a condition that a reflection density of 1.2 can be output without any image flow on the image. .

  And the image state of the black character part was determined visually. In the lower part of Table 4, the circle mark indicates that the image has not disappeared or the image is not blurred, the triangle mark indicates that the image has not been erased, but the image blur can be recognized, and the cross mark indicates that the image is not blurred. Each case has disappeared.

  As shown in the lower part of Table 4, in Comparative Example 4, when the size of the black character portion was 3 points (pt), the image was blurred or the image disappeared. On the other hand, in Example 4, the image did not disappear regardless of the character size. That is, in Comparative Example 4, the value of α ′ corresponding to the direct current value Idc is the same in the black character portion and the black image portion other than the black character. For this reason, when the direct current value Idc is in the range of −0.5 to 1.0 μA, the three-point black characters are unreadable and image defects (character disappearances) due to image flow occur.

  On the other hand, in the fourth embodiment, an appropriate black character portion UCR ratio α ′ is calculated by the above control in consideration of the character portion, and the replaced process black image stabilizes the black character in which character disappearance due to image flow does not occur. Could be output.

  In the above description, control is performed to change the UCR ratio between the black image portion other than black characters and the black character portion. However, as the character size is smaller, image blur and character disappearance are more likely to occur due to image flow. Therefore, the control may be such that the UCR ratio is further changed depending on the character size of the black character portion. That is, in the above description, the decrease rate of α ′ is changed by distinguishing between characters and lines as a predetermined image pattern and other images. On the other hand, the predetermined image pattern may be at least one of a character having a predetermined size (for example, 5 points) or less and a line having a predetermined thickness (2 points) or less.

  Specifically, the decrease rate of α ′ may be changed by distinguishing between a character having a predetermined size or less, or a line having a predetermined thickness or less and another character or line. In this case, the reduction rate of α ′ may be changed by distinguishing between other characters or lines and images other than the characters and lines, or may not be changed. Further, the reduction rate of α ′ may be changed depending on the typeface of the character. For example, even at a character at the same point, a decrease rate of α ′ is larger for a Mincho character than for a Gothic character.

  In the above description, the reduction rate of α ′ is changed between the predetermined image pattern and the other image patterns. However, in the case of the predetermined image pattern, the process black image is uniformly formed, Other images may be formed as black images. That is, α ′ may be 0 for a predetermined image pattern, and α ′ may be 1 for other images.

<Other embodiments>
As mentioned above, although this invention was demonstrated according to specific embodiment, this invention is not limited to each above-mentioned embodiment. Further, the embodiments described above can be implemented by appropriately combining them. For example, the control in consideration of the relative humidity detected by the environmental sensor 64 as in the third embodiment can be applied to other embodiments. good. In the above description, α ′ is mainly used for explanation, but the same can be said for α.

  In each of the above-described embodiments, the predetermined voltage less than the discharge start voltage Vth applied at the time of image flow detection is set to −500 V. However, the DC voltage to be applied may be less than the discharge start voltage Vth. However, at that time, the image flow generation current value also changes.

  Further, in each of the above-described embodiments, it has been described that the pre-exposure device is turned on in order to make the potential before charging low and constant when detecting a direct current value in image flow detection. However, in addition to exposure, it is also possible to use a static eliminating device that applies a voltage on the photosensitive drum after the transfer portion.

  In each of the above-described embodiments, the DC current value that flows into the photosensitive drum is used as information relating to image flow. However, image flow detection is not limited to the above-described means, and image flow occurrence prediction based on the environment and the number of durable sheets. May be performed on the basis of For example, when the relative humidity detected by the environmental sensor 64 is a predetermined value (for example, 30% or less), the control of the present invention is performed. If the relative humidity exceeds the predetermined value, α, Increase the ratio of α ′. The durable number is the number of photosensitive drums from the start of use, that is, the usage amount of the photosensitive drum. For example, α and α ′ are changed according to the number of sheets detected by the counter 65. The usage amount of the photosensitive drum may be detected by the driving time or driving distance of the photosensitive drum. Further, the photosensitive drum driving time or driving distance may be detected based on the time during which a bias such as charging is applied or the photosensitive drum driving distance during which the bias is applied. Furthermore, in determining α and α ′, any or all of the above-described current value, relative humidity, and durable number may be combined.

  1 (1a, 1b, 1c, 1d)... Photosensitive drum (image carrier), 2 (2a, 2b, 2c, 2d)... Charging roller (charging means), 20. ..Exposure device (latent image forming means), 52... Black image determination unit, 53... UCR unit, 54... Black character determination unit, 63 .. DC current value measurement circuit (current detection unit) 64... Environmental sensor (environment detection means), 100... Control circuit (control means), Pa, Pb, Pc... Image forming section (color image forming section), Pd. Image forming unit)

Claims (9)

A black image forming unit that forms a black image with a black developer, and a plurality of process black images that can form black or a color close to black by forming an image with a developer other than black and combining each color A color image forming unit, and a control means for controlling the black image forming unit and the plurality of color image forming units,
The black image forming unit and the plurality of color image forming units each include an image carrier, a charging unit that charges the image carrier, and a latent image formation that forms an electrostatic latent image on the charged image carrier. An image forming apparatus comprising: a developing unit that develops the electrostatic latent image formed on the image carrier with a developer;
Detecting means for detecting information on the image flow of the black image forming unit;
The control unit forms a black image using only the black image forming unit based on the detection result of the detecting unit, and forms a black image using the plurality of color image forming units. To perform any one of the second modes,
An image forming apparatus. In the second mode, the control unit forms a black image using the black image forming unit and the plurality of color image forming units, and generates a black image based on a detection result of the detection unit. Changing a black image ratio of the black image to the process black image to be configured;
The image forming apparatus according to claim 1, wherein: The control means changes the value of the black image ratio between a portion having a predetermined image pattern and a portion other than the black image,
The image forming apparatus according to claim 2, wherein: The predetermined image pattern is at least one of a character and a line;
The image forming apparatus according to claim 3, wherein: The predetermined image pattern is at least one of a character having a predetermined size or less and a line having a predetermined thickness or less.
The image forming apparatus according to claim 4, wherein: The detecting means is a current detecting means for detecting a current flowing from the charging means to the image carrier when a predetermined voltage is applied to the charging means;
Executing the first mode when the absolute value of the current detected by the current detection means is less than a predetermined value, and executing the second mode when the absolute value is greater than or equal to a predetermined value;
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. In the second mode, the control unit forms a black image using the black image forming unit and the plurality of color image forming units, and the larger the absolute value of the current detected by the current detecting unit is, Reducing the black image ratio of the black image constituting the black image to the process black image;
The image forming apparatus according to claim 6. Having an environment detection means for detecting the relative humidity in or around the image forming apparatus;
The control means reduces the decrease rate of the black image ratio with respect to the increase in the absolute value of the current detected by the current detection means, as the relative humidity detected by the environment detection means is higher.
The image forming apparatus according to claim 7, wherein: The surface hardness of the image carrier of the black image forming unit is higher than the surface hardness of the image carrier of the plurality of color image forming units,
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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