JP2012037706A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily detect a transfer history, and to suppress the occurrence of image failures due to the transfer history with a simple structure.SOLUTION: An image forming device comprises: an image holding body 15; charging means 16; exposure means 14; developing means 17; transfer means 26; density detecting means S for detecting the image density of a density detecting image P; and controlling means forming a high density image TB extending in an axial direction and including a non-image portion TBwhere an image is not formed, on the image holding body 15 in an image forming step immediately before forming the density detecting image P, forming the density detecting image P in an area including the non-image portion TBof the high density image TB, on the image holding body 15 in the next image forming step, and changing image forming conditions on the basis of the detection result of the image density of the density detecting image P.

Description

  The present invention relates to an image forming apparatus.

  Conventionally, image forming apparatuses such as copying machines and printers using an electrophotographic method are known to have an image failure called “ghost” due to a phenomenon in which a past transfer history occurs on a photoreceptor after transfer. An image forming apparatus for preventing such an image failure has been proposed. (For example, see Patent Documents 1 to 3).

  Here, Patent Document 1 discloses an image forming apparatus that detects a transfer current and controls a charging condition of a charging unit based on an image forming mode and a transfer current result at that time.

  Patent Document 2 discloses an image forming apparatus that changes a transfer current value and a pre-exposure amount / cleaner exposure amount in a charge removal process based on use history information of the image forming apparatus.

  Further, Patent Document 3 discloses a first charge eliminating unit that removes electricity by exposing the surface of the photoreceptor after transfer, a downstream side from the transfer unit, and an upstream side from the first charge eliminating unit. Discloses an image forming apparatus that includes a second static elimination unit that eliminates static electricity by performing predetermined discharge, and controls the second static elimination unit according to a transfer current value.

JP 2007-148165 A JP 2008-191554 A JP 2000-267530 A

  An object of the present invention is to provide an image forming apparatus capable of easily detecting a transfer history and suppressing the occurrence of an image failure associated with the transfer history with a simple configuration.

  In order to achieve the above object, an image forming apparatus according to a first aspect of the present invention includes a rotatable image carrier, charging means for charging the surface of the image carrier, and a static image on the image carrier. An exposure unit that forms an electrostatic latent image, a developing unit that visualizes the electrostatic latent image formed on the image carrier, and an image that is visualized on the image carrier is transferred onto the transfer member. A non-image portion in which an image is not formed in an image forming process immediately before forming the density detection image, and a density detection unit for detecting the image density based on the density detection image. A high-density image extending in the axial direction is formed on the image carrier, and the image for density detection is formed in a region including a non-image portion of the high-density image in the next image forming step. Based on the detection result of the image density of the density detection image And it is characterized in that it comprises a control means for changing the matter.

  According to a second aspect of the present invention, in the configuration of the first aspect, the high-density image is transferred onto a rotatable transfer body and is located upstream of the high-density image. The image for density detection is transferred, and the control means uses a density difference between a predetermined reference density and the image for density detection as a detection result.

  According to a third aspect of the present invention, there is provided the image forming apparatus according to the first aspect, wherein a plurality of the density detection images are transferred and formed on a rotatable transfer member, and the first downstream image is formed. The high density image is transferred between the first density detection image and the upstream second density detection image, and the first density detection image and the second density detection image This is characterized in that the difference in density is the detection result.

  According to a fourth aspect of the present invention, in the configuration according to any one of the first to third aspects, the non-image portion of the high-density image is formed linearly along the detection direction of the density detection unit. It is characterized by that.

  According to a fifth aspect of the present invention, there is provided the image forming apparatus according to any one of the second to fourth aspects, wherein the control unit has a density difference when the film thickness of the image carrier is equal to or greater than a predetermined film thickness. Is detected.

  According to a sixth aspect of the present invention, there is provided the image forming apparatus according to any one of the second to fourth aspects, wherein the control unit has a density difference when the temperature becomes higher or higher than a predetermined temperature / humidity. Is detected.

  The image forming apparatus according to claim 7 is the configuration according to any one of claims 2 to 6, wherein the control unit changes the image forming condition when the density difference is equal to or greater than a threshold value. It is characterized by.

  According to an eighth aspect of the present invention, in the configuration according to the seventh aspect, the control unit increases the charging bias and / or decreases the transfer bias when the density difference is equal to or greater than a threshold value. It is characterized by this.

  According to a ninth aspect of the present invention, in the configuration according to the eighth aspect, when the image holding member is replaced, the control unit sets the previously set charging bias and / or transfer bias setting values. It is characterized by resetting.

  According to a tenth aspect of the present invention, there is provided the image forming apparatus according to the eighth aspect, wherein when the control unit becomes a temperature lower or a lower humidity than a predetermined temperature / humidity, In other words, the transfer bias setting value is reset.

  According to the first aspect of the present invention, it is possible to easily detect the transfer history, and it is possible to suppress the occurrence of an image failure accompanying the transfer history with a simple configuration.

  According to the second aspect of the present invention, the transfer history can be detected more easily with relatively few image forming steps.

  According to the third aspect of the present invention, it is possible to more effectively detect the transfer history with a plurality of density detection images.

  According to the fourth aspect of the present invention, the density difference detection accuracy can be further improved by detecting and averaging the density at a plurality of locations along the non-image portion.

  According to the fifth aspect of the present invention, the generation of the transfer history can be more effectively suppressed in an environment where the transfer history is likely to occur.

  According to the sixth aspect of the present invention, the generation of the transfer history can be more effectively suppressed in an environment where the transfer history is likely to occur.

  According to the seventh aspect of the present invention, it is possible to determine whether or not the image forming conditions have been changed according to the level of image failure.

  According to the eighth aspect of the present invention, the level of image disturbance (ghost) can be improved.

  According to the ninth aspect of the present invention, it is possible to set an appropriate charging bias and transfer bias for the image holding member as the environment is restored.

  According to the tenth aspect of the present invention, it is possible to set an appropriate charging bias and transfer bias for the image carrier along with the environmental recovery.

1 is a schematic configuration diagram illustrating an embodiment of an image forming apparatus according to the present embodiment. FIG. 3 is a schematic enlarged view for explaining the configuration of an image forming unit according to the present embodiment. It is a schematic diagram which shows the arrangement configuration of a density sensor. It is a schematic diagram for demonstrating the generation | occurrence | production mechanism of transcription | transfer history. It is a schematic diagram for clarifying the structure of the density detection image. It is a figure which shows the relationship between the level of a ghost, and a charging voltage value. It is a figure which shows the relationship between the level of a ghost, and a transfer electric current value.

  Embodiments of the present invention will be described below with reference to the drawings.

  First, the configuration of an image forming apparatus to which the present invention can be applied will be described with reference to FIG. Here, FIG. 1 is a configuration diagram showing a tandem type digital color copying machine as an example of an image forming apparatus to which the present invention is applicable. The tandem color electrophotographic copying machine includes an image reading device, but does not include the image reading device, and forms an image based on image data output from a personal computer (not shown) or the like. It may be a color printer or a facsimile.

  In FIG. 1, reference numeral 1 denotes a main body of a tandem type digital color copying machine as an example of an image forming apparatus to which the present invention can be applied. A document 2 is separated one by one at the top of the main body 1. An automatic document conveying device 3 that automatically conveys the document and a document reading device 4 that reads an image of the document 2 conveyed by the automatic document conveying device 3 are arranged. The document reader 4 illuminates a document 2 placed on a platen glass 5 with a light source 6, and reflects a reflected light image from the document 2 from a full-rate mirror 7, half-rate mirrors 8 and 9, and an imaging lens 10. The image reading element 11 made of a CCD or the like is scanned and exposed through a reduction optical system, and the image material 11 reflects the color material reflected light image of the original 2 by a predetermined dot density (for example, 16 dots / mm). ).

  The color material reflected light image of the document 2 read by the document reading device 4 is subjected to image processing, for example, as document reflectance data of three colors of red (R), green (G), and blue (B) (8 bits each). The image processing unit 12 performs image processing such as shading correction, position shift correction, brightness / color space conversion, gamma correction, frame deletion, color / moving editing, etc. on the reflectance data of the document 2. Is given. In addition, the image processing unit 12 performs predetermined image processing on image data sent from a personal computer or the like.

  The image data that has been subjected to predetermined image processing by the image processing unit 12 as described above is processed by the image processing unit 12 in the same manner as yellow (Y), magenta (M), cyan (C), and black (K). (8 bits each) is converted into four-color original reproduction color material gradation data, and as described below, image formation of each color of yellow (Y), magenta (M), cyan (C), and black (K) The images are sent to the exposure devices 14 of the units 13Y, 13M, 13C, and 13K, and the exposure device 14 performs image exposure with the laser beam LB in accordance with the original reproduction color material gradation data of each color.

  In the image forming apparatus according to the present embodiment, a plurality of image forming units 13Y, 13M, 13C, and 13K that form toner images of different colors are arranged in parallel, and the plurality of image forming units 13Y and 13M are arranged in parallel. , 13C, and 13K, a belt-like intermediate transfer body 25 to which the toner images of the respective colors formed by the plurality of image forming units 13Y, 13M, 13C, and 13K are transferred is disposed, and a plurality of images are further provided. Below the forming units 13Y, 13M, 13C, and 13K, an exposure device 14 that writes images on the image carriers 15Y, 15M, 15C, and 15K of the image forming units 13Y, 13M, 13C, and 13K is disposed.

  Next, constituent devices of the image forming units 13Y, 13M, 13C, and 13K will be described with reference to FIG. Here, FIG. 2 is a schematic enlarged view for explaining the configuration of each image forming unit.

  As shown in an enlarged view in FIG. 2, the four image forming units 13Y, 13M, 13C, and 13K corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) Along the moving direction of the endless intermediate transfer belt 25 stretched by the stretch rolls, they are arranged in parallel in the horizontal direction at a certain interval, and each of yellow, magenta, cyan and black The toner images are sequentially formed at a predetermined timing. Since these image forming units 13Y, 13M, 13C, and 13K are all configured in the same manner, hereinafter, the reference numerals of the respective constituent devices are generically expressed (for example, the photosensitive drum 15).

  Each of the image forming units 13Y, 13M, 13C, and 13K is roughly classified into a photosensitive drum 15 as an image holding member that is rotationally driven in a direction indicated by an arrow at a predetermined speed (for example, 200 mm / sec), and the photosensitive drum 15. A charging device 16 as a charging means for uniformly charging the surface of the photosensitive drum 15; an exposure device 14 as an exposure means for forming an electrostatic latent image by exposing an image corresponding to each color on the surface of the photosensitive drum 15; Writing is performed by a developing device 17 as a developing unit that develops the electrostatic latent image formed on the drum 15 with toner of each color, a drum cleaning device 18 as a cleaning unit that cleans the surface of the photosensitive drum 15, and an exposure unit. In order to eliminate the influence of the latent image, it comprises an erase lamp 50 or the like as a charge eliminating means for exposing the entire surface of the photosensitive drum 15 before charging.In the present embodiment, the photosensitive drum 15 and the components disposed around the photosensitive drum 15 are integrally formed as a unit and can be individually replaced from the main body 1.

  The photosensitive drum 15 has a configuration in which a functional layer (photosensitive layer) such as a charge generation layer and a charge transport layer made of an organic photoconductor is laminated on the surface (outer peripheral surface) of a conductive metal cylinder. It is driven to rotate in the direction of the arrow (in this example, counterclockwise in FIG. 1) by a driving means (not shown).

  The charging device 16 is formed, for example, as a charging roll in which a conductive layer made of a synthetic resin, rubber, or the like and having an adjusted electric resistance is coated on the surface of the core metal. A charging bias power source (not shown) is connected, and a predetermined charging bias is applied.

  The exposure device 14 is provided in common to the image forming units 13Y, 13M, 13C, and 13K for each color of yellow (Y), magenta (M), cyan (C), and black (K), and includes four semiconductors (not shown). The laser is modulated in accordance with the original material color reproduction gradation data for each color, and laser beams LB-Y, LB-M, LB-C, and LB-K are emitted from these semiconductor lasers in accordance with the gradation data. It is configured. The laser beams LB-Y, LB-M, LB-C, and LB-K emitted from the semiconductor laser are irradiated to the rotary polygon mirror 19 through an f-θ lens (not shown) and deflected by the rotary polygon mirror 19. Scanned. The laser beams LB-Y, LB-M, LB-C, and LB-K deflected and scanned by the rotary polygon mirror 19 are passed through a plurality of reflection mirrors (not shown), and the photosensitive drums 15Y, 15M, 15C, The surface of 15K is scan-exposed along the main scanning direction.

  In addition, as the exposure apparatus 14, an exposure apparatus configured by an LED light emitting element array or the like provided individually for each image forming unit may be used.

  From the image processing unit 12, an exposure apparatus 14 provided in common for the image forming units 13Y, 13M, 13C, and 13K for each color of yellow (Y), magenta (M), cyan (C), and black (K). In addition, the image data of each color is sequentially output, and the laser beams LB-Y, LB-M, LB-C, and LB-K emitted from the exposure device 14 according to the image data are the surfaces of the corresponding photosensitive drums 15. And an electrostatic latent image is formed. The electrostatic latent image formed on the photosensitive drum 15 is developed as a toner image of each color of yellow (Y), magenta (M), cyan (C), and black (K) by the developing device 17.

  The yellow (Y), magenta (M), cyan (C), and black (K) toner images sequentially formed on the photosensitive drums 15 of the image forming units 13Y, 13M, 13C, and 13K are as follows. On the intermediate transfer belt (belt-shaped intermediate transfer member) 25 of the transfer unit 22 arranged over the image forming units 13Y, 13M, 13C, and 13K, four primary transfer rolls 26Y, 26M, 26C, and 26K are used. Primary transfer is sequentially performed in multiples. These primary transfer rolls 26Y, 26M, 26C, and 26K are disposed on the back side of the intermediate transfer belt 25 corresponding to the photosensitive drums 15 of the image forming units 13Y, 13M, 13C, and 13K. Each primary transfer roll 26Y, 26M, 26C, and 26K is adjusted to a desired volume resistance value, and a transfer bias power source (not shown) is connected to these primary transfer rolls 26Y, 26M, 26C, and 26K. A transfer bias having a polarity opposite to the toner polarity (positive polarity in this example) is applied at a predetermined timing.

  The intermediate transfer belt 25 is wound around the drive roll 27, the tension roll 24, and the backup roll 28 with a constant tension, and is rotated by a dedicated drive motor with excellent constant speed (not shown). The drive roll 27 is driven to circulate at a predetermined speed in the direction of the arrow. As the intermediate transfer belt 25, for example, a flexible synthetic resin film such as PET is formed in a band shape, and both ends of the synthetic resin film formed in a band shape are connected by means such as welding, thereby endless A belt-shaped one is used.

  The yellow (Y), magenta (M), cyan (C), and black (K) toner images transferred onto the intermediate transfer belt 25 in multiple layers are backed up on the side of the intermediate transfer belt 25. The recording sheet 30 that has been secondarily transferred onto a recording sheet 30 as a recording medium by a pressing force and electrostatic attraction force by a secondary transfer roll 29 that is in pressure contact with the roll 28, and onto which the toner images of these colors are transferred, is directed upward. It is conveyed to the fixing device 31 located.

  Residual toner, paper dust, and the like are removed from the surface of the intermediate transfer belt 25 after the toner image transfer process is completed by a belt cleaning device 43 provided at the position of the drive roll 27 to prepare for the next image forming process. . The belt cleaning device 43 includes a cleaning brush 43a and a cleaning blade 43b, and residual toner and paper dust on the intermediate transfer belt 25 are removed by the cleaning brush 43a and the blade 43b.

  The secondary transfer roll 29 is in pressure contact with the side of the backup roll 28, and secondary-transfers each color toner image onto a recording paper 30 conveyed upward from below. As the secondary transfer roll 29, for example, an elastic body layer made of a conductive elastic body such as a rubber material to which a conductive agent is added is coated on the outer periphery of a metal core made of a metal such as stainless steel to a predetermined thickness. Used. The recording paper 30 on which the toner images of the respective colors are transferred is subjected to a fixing process by heat and pressure by a fixing device 31 and then discharged onto a discharge tray 33 provided on the upper portion of the main body 1 by a discharge roll 32. The

  The recording paper 30 having a predetermined size is separated one by one by a nudger roll 35 and a paper separating / feeding feed roll 36 from a paper feeding device 34 disposed inside the apparatus main body 1. In this state, the paper is once transported to a registration roll 38 provided in the paper transport path 37 and stopped. The recording paper 30 supplied from the paper feeding device 34 is sent to the secondary transfer position of the intermediate transfer belt 25 by a registration roll 38 that rotates at a predetermined timing.

  In the digital color image forming apparatus according to the present embodiment, when full-color double-sided copying is performed, the recording paper 30 having an image fixed on one side is directly discharged onto the discharge tray 33 by the discharge roll 32. Instead, the conveyance direction is switched by a switching gate (not shown), and the sheet is conveyed to the duplex conveyance unit 40 via the sheet conveyance roller pair 39. In the double-sided conveyance unit 40, the recording paper 30 is conveyed again to the registration roll 38 with the front and back sides of the recording paper 30 reversed by a pair of conveyance rollers (not shown) provided along the conveyance path 41. Is discharged onto the discharge tray 33 after the image is transferred and fixed on the back surface of the recording paper 30. In the drawing, 44Y, 44M, 44C, and 44K are the colors corresponding to the developing devices 17Y, 17M, 17C, and 17K of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Each of the toner cartridges supplying the toner is shown.

  In the image forming apparatus according to the present embodiment, toner patches for density detection (hereinafter also referred to as density detection images) are formed on the intermediate transfer belt 25 at a predetermined timing. By detecting the density of the patch with a density sensor as a density detecting means, the amount of toner replenished to the developing device 17 and the like are controlled.

  In the present embodiment, as schematically shown in FIG. 3, a plurality (two in this example) of density sensors S are arranged along the direction intersecting the moving direction of the intermediate transfer belt 25. These density sensors S are configured to detect the toner density of toner patches (density detection images) formed on the front side and the back side of the intermediate transfer belt 25. The density sensor S according to the present embodiment has, for example, a circular detection area having a diameter of about 6 mm, and this detection area is sampled at, for example, 20 points every 5 msec along the moving direction of the intermediate transfer belt 25. Each time, the sampling data of 18 points excluding the maximum value and the minimum value are averaged, and the toner density of the density detection image is detected.

  As the density sensor S, a conventionally known optical density which irradiates light emitted from a light emitting element such as an LED onto the surface of the intermediate transfer belt 25 on which a density detection image is formed and detects reflected light by a light receiving element. A sensor can be used. The details of the density detection image according to the present embodiment will be described later.

  In the image forming apparatus configured as described above, a generation mechanism of a transfer history called “ghost” in the image forming process will be described with reference to FIG.

  When forming a color or black-and-white image, first, the surface of the photosensitive drum 15 is uniformly charged to a predetermined potential having a negative polarity by the charging roll 16, as shown in FIG. Thereafter, for example, based on image information read by the image reading device 4, the image portion is scanned and exposed by the laser beam LB output from the exposure device 14, and an electrostatic latent image corresponding to the image is formed on the surface of the photosensitive drum 15. Is formed. At this time, as the photosensitive drum 15, the one using the organic photoconductor (OPC) as described above is used, and the photosensitive drum 15 using the organic photoconductor is an organic photoconductor (OPC). Since a charge transport medium that transports positive charges among positive and negative charges generated by irradiating light is used, it was exposed by exposing the image area after being uniformly charged to a negative polarity. The portion of the organic photoconductor becomes conductive, neutralizing negative charges and forming an electrostatic latent image (see FIG. 4B).

  The electrostatic latent image formed on the photosensitive drum 15 is transferred to the developing device 17 by a reversal developing method in which a region exposed by the exposure device 14, that is, a region where the negative potential is lowered is developed with toner charged to a negative polarity. And developed into a toner image. The toner image formed on the photosensitive drum 15 is primarily transferred onto the intermediate transfer belt 25 by being charged with a predetermined amount of positive polarity by a primary transfer unit including a primary transfer roll 26 and the like. . At this time, since the photosensitive drum 15 and the intermediate transfer belt 25 are in direct contact with each other in the region other than the toner image on the photosensitive drum 15, the positive flow that flows from the primary transfer roll 26 via the intermediate transfer belt 25 that is a resistor. The electric charge also flows into the surface of the photosensitive drum 15, and the surface potential of the photosensitive drum 15 is biased to a positive polarity. On the other hand, in the area where the toner image of the photosensitive drum 15 is present, since the toner is a dielectric, almost no electric charge flows, and remains in a negative polarity potential reduced by image exposure (FIG. 4C). )reference). Thereafter, the surface of the photosensitive drum 15 is removed by removing the residual toner and the like by the drum cleaning device 18 and then uniformly exposed by the charge removing means 50 including an erase lamp and the like to prepare for the next image forming process. .

  By the way, in the case of the image forming apparatus configured as described above, when the surface of the photosensitive drum 15 is uniformly exposed by the neutralizing means including the erase lamp 50 and the like, Since an organic photoconductor (OPC) using a charge transport medium that transports the charge is used, the negatively charged region on the surface of the photosensitive drum 15 has a positive charge generated by exposure. By attracting and neutralizing, the potential approaches 0V, but the region charged with a positive polarity is charged with a positive polarity without being removed even if the organic photoconductor (OPC) exhibits conductivity. The state remains as it is (see FIG. 4D). Therefore, after the charge removal by the erase lamp 50, if the surface of the photosensitive drum 15 is uniformly charged to a predetermined negative potential by the charging roll 16 for the next image forming process, the charge becomes positive before the charge removal. The image is recharged with the effect of the potential difference between the charged region and the negatively charged region remaining (see FIG. 4E), and the previous image is the next image as the transfer history. A phenomenon called “ghost” that appears in

  Therefore, in the image forming apparatus according to the present embodiment, the density detection image is formed after the high-density image (high-density toner image) as follows, and the variation in the surface potential of the photosensitive drum 15 after the transfer is performed. By changing the image forming condition based on the toner density of the density detection image so as to reduce (smooth), it is possible to suppress the occurrence of ghost with a simple configuration.

  Hereinafter, specific control contents in the image forming apparatus according to the present embodiment will be described with reference to the drawings.

As shown schematically in FIG. 5, first, the photosensitive drum 15 to the first density detection image P ₀ for detecting image density are formed on, the density detection image P ₀ of the intermediate transfer belt 25 above Transcript to. In the present embodiment, the first density detection image P ₀ is formed at both axial ends of the intermediate transfer belt.

In the next image forming step, in a corresponding region on the photosensitive drum 15 on which the first density detection image P ₀ is formed, a discontinuous strip-shaped high-density toner image of each color extending in the axial direction (for example, a photosensitive drum) 15 is a so-called solid image having a toner coverage of 90 to 100% on the surface 15). Here, the image forming process refers to a series of image forming processes (charging, exposure, development, transfer, etc.) corresponding to one rotation of the photosensitive drum 15.

Specifically, the high-density toner image TB has a non-image portion TB ₀ formed in a part of an area corresponding to the width (length along the axial direction) of the first density detection image P _0. The width of the non-image portion TB ₀ is set to be narrower than the width of the first density detection image P ₀ . That is, a non-image portion TB ₀ (a portion where no toner image is formed) and a high-density image portion TB ₁ are formed in a region corresponding to the density detection image P ₀ of the high-density toner image TB. . Then, the high-density toner image TB is transferred onto the intermediate transfer belt 25, the high-density toner (upstream side in the movement direction of the intermediate transfer belt 25) upstream of the first density detection image P ₀ An image TB is formed.

In this way, by providing the non-image portion TB ₀ that does not form an image in the portion corresponding to the density detection image P ₀ of the high-density toner image TB as a high-density image extending in the axial direction, the subsequent image forming process is performed. In FIG. ₅ , a potential difference (step) between the non-image portion TB ₀ and the high-density image portion TB ₁ on the photosensitive drum 15 that causes ghost is easily generated. The formation area (axial length) of the high-density toner image TB may be interrupted in the middle of the axial direction as long as it exceeds the width of the density detection image P (for example, From the viewpoint of concentrating charges on the non-image portion TB ₀ and easily generating a potential difference that causes a transfer history, the pattern is schematically shown in FIG. As shown specifically, the intermediate transfer belt 25 is preferably formed over the entire width direction (axial direction).

Subsequently, in the next image forming step, a similar second density detection image P ₁ is formed in an area corresponding to the first density detection image P ₀ on the photosensitive drum 15, and the second density detection image P ₁ is formed. The density detection image P ₁ is transferred onto the intermediate transfer belt 25, and a second density detection image P ₁ similar to the first density detection image P ₀ is upstream of the high-density toner image TB. Form.

In the second density detection image P ₁ , since the high-density toner image TB including the non-image portion TB ₀ is formed in the immediately preceding image forming process, the non-image portion of the density detection image P ₁ is formed. image density portion (region) corresponding to the TB ₀ is more likely to thicken by the transfer history (the potential difference between the non-image portion TB ₀ on the photosensitive drum 15 and the high-density image portion TB _1).

Therefore, in the image forming apparatus according to the present embodiment, the conventionally known optical density sensor S detects the density difference between the first density detection image P ₀ and the second density detection image P _1. Thus, the ghost grade (level) is determined, and the charging condition and / or the transfer condition are changed. That is, a normal density detection image P ₀ and the stress state density detection image P ₁ (non-image portion TB ₀ on the photosensitive drum 15 and the ghost is generated easily state by a potential difference between the high-density image portion TB ₁₎ in , And the density difference grade is determined by detecting the density difference between the two, and the charging potential and transfer current (voltage) value of the photosensitive drum 15 are controlled based on the grade.

  Specifically, as schematically shown in FIG. 6, when the above-described density difference is larger than a predetermined threshold value (when the grade value set stepwise according to the density difference is lowered). For example, the charging bias of the charging roll 16 is controlled so that the charging potential of the photosensitive drum 15 becomes higher (in this example, the absolute value of the negative potential of the photosensitive drum 15 is increased).

As described above, by increasing the surface potential of the photosensitive drum 15, it is possible to reduce the amount of deviation of the surface potential of the photosensitive drum 15 to the opposite polarity at the time of transfer, and as a result, the non-image area on the photosensitive drum 15. The potential difference (step) between the image area and the image area can be reduced, and the ghost grade can be improved. In addition, by detecting the density difference between the first density detection image P ₀ and the second density detection image P ₁ , both are similarly affected by the environmental change and the like. The influence of disturbance can be eliminated (cancelled), and the density change accompanying the transfer history can be detected with high accuracy.

  Note that when the surface of the photosensitive drum 15 is uniformly charged to a negative polarity, the extent to which the area (non-image portion) where the toner image is not formed on the photosensitive drum 15 is biased to the positive side is the primary transfer. Since it also changes depending on the transfer current (transfer voltage) flowing through the roll 26, as schematically shown in FIG. 7, when the above-mentioned density difference becomes larger than a predetermined threshold value (stepwise in advance according to the density difference). The transfer bias may be controlled so as to reduce the transfer current or transfer voltage. Thus, since the amount of deviation of the surface potential of the photosensitive drum 15 from the reverse polarity to the reverse polarity can be reduced also by reducing the transfer current value and the transfer voltage value, the ghost level can be improved. it can.

  Naturally, both the charging condition (charging bias) and the transfer condition (transfer bias) may be changed according to the density difference (grade). Further, the charging bias and the transfer bias are exemplified as the image forming conditions to be changed based on the density difference. However, the control conditions (for example, developing bias) having a correlation between the image density and the surface potential of the photosensitive drum 15 after the transfer are exemplified. As a matter of course, this condition may be controlled as a primary or secondary image forming condition.

It should be noted that conventionally known image patches used for detecting the image density can be used for the first density detection image P ₀ and the second density detection image P ₁ described above.

The non-image portion TB ₀ in the high-density toner image TB may be formed in an area corresponding to the density detection image P, and the number and shape of the non-image portions TB ₀ can be arbitrarily set as appropriate. . For example, not only a linear shape but also an arbitrary shape such as an inclined line, a circle, a polygon, radiation, and a combination thereof, and the number of installations can be adopted.

However, from the viewpoint of more accurately detecting the density change associated with the transfer history, as schematically shown in FIG. 5, in the detection direction of the density sensor S (in this example, the movement direction of the intermediate transfer belt 25). it is preferable to form the non-image portion TB ₀ along. In this way, by forming the non-image portion TB ₀ linearly across the detection direction (detection region) of the density sensor S, the density corresponding to the non-image portion TB ₀ in which the density change associated with the transfer history is likely to occur. Since the toner density of the detection image P can be continuously detected at a plurality of points (a plurality of points) in the detection direction of the density sensor S, the density detection data at the plurality of points is averaged to obtain a disturbance or the like. It is possible to detect the occurrence of the transfer history with higher accuracy by eliminating the influence.

  Note that the above-described control contents all use existing components such as the density sensor S, the charging roll 16, and the transfer roll 26. Therefore, the control function of the existing control device that controls such components is diverted. And can be easily realized.

  In the image forming apparatus according to the present embodiment configured as described above, a simple configuration that diverts existing components without separately providing a special charge eliminating unit or the like for removing the transfer history on the photosensitive drum 15. Generation of transfer history can be suppressed.

In the above-described embodiment, the ghost grade is determined based on the density difference between the first density detection image P ₀ and the second density detection image P ₁ , but the first density detection image P _The ghost grade is determined by omitting ₀ and using the density detection image P ₁ formed on the high density toner image TB and the upstream side thereof (upstream side along the moving direction of the intermediate transfer belt 25). May be.

Specifically, first, a high-density toner image TB including the non-image portion TB ₀ is formed on the photosensitive drum 15, and the high-density toner image TB is transferred onto the intermediate transfer belt 25. In the next image forming step, the density detection image P ₁ is formed in an area on the photosensitive drum 15 including the non-image portion TB ₀ , and the density detection image P ₁ is transferred onto the intermediate transfer belt 25. Then, the ghost grade may be determined by comparing the density of the density detection image P ₁ with a predetermined reference density. When configured in this manner, it is possible to easily determine the ghost grade with fewer image forming steps. Further, the density of the area corresponding to the non-image part TB ₀ and the area corresponding to the high-density image part TB ₁ in the density detection image P _{1 is} detected by the single or plural density sensors S, respectively. You may comprise so that the grade of a ghost may be determined based on both density differences.

  Moreover, it is preferable to perform the control based on the above-described density difference at the time of an environmental change in which a ghost is likely to occur. For example, when the image forming apparatus is turned on, for example, when setting up image forming conditions, or when the environmental temperature / humidity is higher than a predetermined temperature / humidity, the film thickness of the photosensitive drum 15 is increased. When the film thickness is larger than a predetermined thickness (when the photosensitive drum 15 is new), it is preferable to perform the above-described control.

  Furthermore, from the viewpoint of setting an appropriate charging bias / transfer bias for the photosensitive drum 15 when returning from an environment in which ghosts are likely to occur, the environmental temperature / humidity is changed when the photosensitive drum 15 or the like is replaced. When the temperature / humidity is lower than a predetermined temperature, it is preferable to reset the previous control conditions (charging bias and transfer bias setting values) based on the density difference to return to the initial setting values.

  The technical scope of the present invention is not limited to the above-described embodiments, and various changes or improvements can be added without departing from the scope of the present invention described in the claims. . For example, in the above-described embodiment, the tandem type digital color copying machine as an example of the image forming apparatus has been exemplified. However, the present invention may naturally be applied to a monochrome or black and white copying machine or printer. In the above-described embodiment, the density difference of the density detection image transferred onto the intermediate transfer belt 25 is detected. However, the density difference of the density detection image formed on the photosensitive drum 15 is detected. You may comprise as follows.

13Y, 13M, 13C, 13K: image forming unit, 14: exposure device, 15Y, 15M, 15C, 15K: photosensitive drum, 16Y, 16M, 16C, 16K: charging roll, 17Y, 17M, 17C, 17K: developing device, 18Y, 18M, 18C, 18K: drum cleaning device, 25: intermediate transfer belt, 26Y, 26M, 26C, 26K: primary transfer roll, 28: backup roll, 29: secondary transfer roll, 30: recording paper, 31: fixing Device: 34: paper feeding device, 40: duplex conveying unit, 43: belt cleaning device, 50: erase lamp, P: density detection image, S: density sensor, TB: high density toner image, TB ₀ : non-image Part, TB ₁ : High-density image part

Claims (10)

An image carrier formed to be freely rotatable;
Charging means for charging the surface of the image carrier;
Exposure means for forming an electrostatic latent image on the image carrier;
Developing means for visualizing the electrostatic latent image formed on the image carrier;
Transfer means for transferring an image visualized on the image carrier onto a transfer body;
Density detecting means for detecting the image density based on the density detection image;
In the image forming process immediately before forming the density detection image, a high-density image extending in the axial direction including a non-image portion where no image is formed is formed on the image carrier, and the next image formation In the step, the density detection image is formed on an image carrier in an area including the non-image portion of the high-density image, and image forming conditions are set based on the detection result of the image density of the density detection image. An image forming apparatus comprising: a control means for changing.   The high-density image is transferred onto a freely formed transfer body, and the density detection image is transferred to the upstream side of the high-density image. 2. The image forming apparatus according to claim 1, wherein a density difference between the obtained reference density and the density detection image is used as a detection result.   A plurality of the density detection images are transferred and formed on a freely formed transfer body, and between the downstream first density detection image and the upstream second density detection image. 2. The image according to claim 1, wherein the high-density image is transferred, and a density difference between the first density detection image and the second density detection image is used as a detection result. Forming equipment.   The image forming apparatus according to claim 1, wherein the non-image portion of the high-density image is formed in a straight line along a detection direction of the density detection unit.   5. The image forming apparatus according to claim 2, wherein the control unit detects a density difference when a film thickness of the image carrier is equal to or greater than a predetermined film thickness. 6.   5. The image forming apparatus according to claim 2, wherein the control unit detects a density difference when the temperature is higher or higher than a predetermined temperature / humidity.   The image forming apparatus according to claim 2, wherein the control unit changes the image forming condition when the density difference is equal to or greater than a threshold value.   The image forming apparatus according to claim 7, wherein the control unit increases the charging bias and / or reduces the transfer bias when the density difference is equal to or greater than a threshold value.   The image forming apparatus according to claim 8, wherein the control unit resets the previously set charging bias and / or transfer bias when the image carrier is replaced.   9. The control unit according to claim 8, wherein when the temperature becomes lower or lower than a predetermined temperature / humidity, the control unit resets the previously set charging bias and / or transfer bias setting values. Image forming apparatus.

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