JP2014052573A - Image forming apparatus - Google Patents

Abstract

The present invention provides a new method for reducing image density unevenness due to a development gap variation or the like.
Toner images formed on photoreceptors 20Y, 20C, 20M, and 20K by toner image forming means such as a charging device, a laser writing device, and a developing device are applied by a transfer bias applied by a primary transfer device. Then, in the image forming apparatus that forms an image on the paper by transferring it to the paper via the intermediate transfer belt 10, the density of the test toner image for density unevenness detection formed by the toner image forming means is measured by the optical sensor 110. In order to change the transfer bias so that the density unevenness information of the test toner image is detected based on the detection result and the density unevenness of the test toner image is reduced based on the detected density unevenness information. Adjust the condition settings.
[Selection] Figure 2

Description

  In the present invention, a toner image formed on an image carrier such as a latent image carrier or an intermediate transfer member by a toner image forming unit is transferred to another image carrier by the action of a transfer bias applied by the transfer unit or The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile, which forms an image on a recording material by directly transferring it to the recording material.

  As this type of image forming apparatus, an electrophotographic image forming apparatus is widely known. In general, in an electrophotographic image forming apparatus, a potential setting value adjustment that adjusts a developing potential by adjusting various potentials such as a target charging potential and a developing bias at the timing immediately after power-on or when the number of printed sheets reaches a predetermined number. Control is performed to adjust image quality such as image density (Patent Document 1). In this potential set value adjustment control, usually, after a plurality of reference toner images, which are predetermined adjustment toner patterns, are formed on the surface of a latent image carrier such as a photoconductor, the toner adhesion amount of the reference toner image is determined. Detect. Then, from the linear approximation formula of the development potential at the time of forming the reference toner image and the toner adhesion amount of the reference toner image, the development potential that can obtain the desired toner adhesion amount is obtained, and various potentials that can obtain the development potential are obtained. . The development potential means a potential difference between the potential of the electrostatic latent image portion on the surface of the latent image carrier and the surface potential of the developer carrier to which a development bias is applied.

  Patent Document 2 discloses an image forming apparatus for improving image density unevenness that occurs in one output image due to fluctuations in the development gap. In this image forming apparatus, the fluctuation of the development gap is detected by detecting the fluctuation amount of the outer periphery of the latent image carrier and the fluctuation amount of the outer periphery of the developer carrier of the developing device, and the development electric field caused by the fluctuation of the development gap is detected. Calculate the variation. Based on the calculation result, the exposure intensity value at the time of exposure for forming the electrostatic latent image on the latent image carrier is adjusted so as to cancel the fluctuation of the developing electric field.

  In an electrophotographic image forming apparatus, generally, a cylindrical developer carrier and a cylindrical latent image carrier face each other with a predetermined gap (hereinafter referred to as “development gap”), and each is independently driven to rotate. To do. Development is performed by the toner on the developer carrying member moving to the electrostatic latent image on the latent image carrying member by the developing electric field corresponding to the developing potential. In such a configuration, the developer carrier and the latent image carrier usually have a shake at the outer periphery with respect to each rotational drive shaft. In this case, when the developer carrier and the latent image carrier are rotationally driven, the development gap fluctuates due to the shake of the outer periphery of each, and the development electric field strength fluctuates as described above. When the developing electric field intensity varies, the amount of toner that moves to the electrostatic latent image on the latent image carrier also varies accordingly. Therefore, unevenness in image density occurs in one output image due to fluctuations in the development gap caused by the fluctuations in the outer circumference of the developer carrier and the latent image carrier.

  In order to eliminate this uneven image density, it is necessary to improve the shake accuracy of the developer carrier and the latent image carrier. However, in recent years, with the increase in the speed of electrophotographic image forming apparatuses, the developer carrier The body and the latent image carrier tend to increase in diameter, and it is not easy to improve the shake accuracy. Further, even if the runout accuracy can be improved, there is a problem that the cost of the apparatus is increased due to the improved accuracy.

  The image forming apparatus disclosed in Patent Document 1 adjusts the image development potential by adjusting the development potential at the timing immediately after the power is turned on, etc., but forms one output image so that the fluctuation range of the development electric field is reduced. It does not fluctuate the development potential during the process. Accordingly, when the fluctuation range of the developing electric field is large, image density unevenness due to such fluctuation of the developing electric field cannot be within an allowable range.

  On the other hand, the development potential change conditions such as the charging bias, exposure intensity, and development bias that can change the development potential are equivalent to the above-described image density unevenness (image density change that occurs in one output image). It is possible to change (time change) in a short cycle. Even in the image forming apparatus disclosed in Patent Document 2, the exposure intensity at the time of exposure for forming an electrostatic latent image on a latent image carrier is as short as an image density change occurring in one output image. By varying the cycle, control is performed to cancel the variation in the development electric field and keep the development electric field within an allowable range.

  However, as a result of the study by the present inventors, it is difficult to accurately adjust the image density unevenness caused by the fluctuation of the development gap within the allowable range only by adjusting the development potential by changing the development potential change condition. It turned out to be. In particular, the image density unevenness caused by the fluctuation of the development gap has an irregular waveform due to two factors, namely, the fluctuation of the outer periphery of the latent image carrier and the fluctuation of the outer periphery of the developer carrier of the developing device. It is difficult to keep such uneven image density with an irregular waveform within an allowable range with high accuracy only by adjusting the development potential. Therefore, apart from the method of reducing the image density unevenness by adjusting the development potential, a new method that can reduce the image density unevenness by adjusting the variable density change condition so that the density change occurs in the image is desired. .

  The present invention has been made in view of the above-described background, and an object of the present invention is to reduce image density unevenness generated in one output image due to a change in the development gap by a new method. It is an object of the present invention to provide an image forming apparatus capable of performing the above.

  In order to achieve the above object, according to the present invention, a toner image formed on an image carrier by a toner image forming unit is transferred via another image carrier or directly by an action of a transfer bias applied by the transfer unit. In an image forming apparatus that forms an image on a recording material by transferring to a recording material, a density detecting unit that detects the density of a test toner image for detecting density unevenness formed by the toner image forming unit, and the density Based on the detection result of the detection means, the density for adjusting the setting of the transfer bias condition so as to vary the transfer bias during the image forming operation for one output image so as to reduce the density unevenness of the test toner image. And unevenness reducing means.

  Conventionally, the setting value of the transfer bias has been determined from the viewpoint of increasing the transfer rate as much as possible for the purpose of reducing waste of toner. As a result of reviewing the transfer bias from a new point of view, the present inventor changed the set value of the transfer bias acting to transfer the toner image from the image carrier to another image carrier or recording material. The inventors have found that it is possible to cause image density unevenness in one output image. Based on this knowledge, the present invention reduces the image density unevenness by a new method of reducing the image density unevenness generated in one output image by adjusting the setting of the transfer bias condition.

  In particular, the new method according to the present invention for reducing the image density unevenness in one output image by adjusting the transfer bias condition does not affect the development potential even when the transfer bias is changed by the adjustment. Or it is small. Therefore, the new method according to the present invention can be used independently of the method of reducing the image density unevenness in one output image by adjusting the development potential. As a result, it is possible to reduce the image density unevenness remaining after being reduced by the method of adjusting the development potential by the new method of the present invention. Therefore, even if the image density unevenness cannot be accurately reduced to an allowable range by just adjusting the developing potential by the new method according to the present invention, it can be used together with the method for adjusting the developing potential. It becomes feasible to accurately fit within the allowable range.

  As described above, according to the present invention, it is possible to obtain an excellent effect that the image density unevenness generated in one output image due to the development gap variation or the like can be reduced by a new method.

FIG. 2 is a schematic configuration diagram illustrating a schematic configuration of the copier. FIG. 2 is an enlarged configuration diagram showing an intermediate transfer unit and its peripheral configuration in the copier. 2 is an enlarged configuration diagram illustrating two of four image forming units in the copier. FIG. FIG. 2 is a block diagram showing a main part of an electric circuit of the copier. 6 is a graph showing image density unevenness in the sub-scanning direction (paper transport direction) generated in the copier. (A) is a graph showing an image density unevenness component having a peripheral period of the developing sleeve in the developing device of the copier. (B) is a graph showing an image density unevenness component having a peripheral period of the photoconductor of the copier. 3 is an explanatory diagram illustrating an example of a test toner image for detecting density unevenness formed on an intermediate transfer belt of the copier. FIG. It is a perspective view showing an installation location of an optical sensor for detecting the density of the test toner image. It is a flowchart which shows the flow of density nonuniformity condition adjustment control in embodiment. It is a graph which shows the waveform of the output value of the optical sensor which performs the density | concentration detection of a 1st test toner image. (A) is a graph showing a waveform having a developing sleeve circumference period extracted from the sensor output waveform shown in FIG. 10, and (b) is a photoreceptor circumference obtained from the sensor output waveform shown in FIG. It is a graph which shows the waveform which has a period. FIG. 5 is an explanatory diagram for explaining the contents of a control table such as a developing bias for reducing density unevenness components in a developing sleeve circumferential cycle and a photosensitive member circumferential cycle. It is a graph which shows the relationship between transfer current and transfer efficiency (transfer rate).

Hereinafter, as an image forming apparatus to which the present invention is applied, an embodiment of a so-called tandem type full-color electrophotographic copying machine (hereinafter simply referred to as “copying machine”) provided with a plurality of photosensitive members will be described.
First, a basic configuration of the copying machine according to the present embodiment will be described.

FIG. 1 is a schematic configuration diagram showing a schematic configuration of the copying machine according to the present embodiment.
In FIG. 1, a copying machine includes a printing unit 100 that forms an image, a paper feeding device 200 on which the printing unit 100 is mounted and that supplies paper 5 as a recording material to the printing unit 100, and a printing unit 100. A scanner 300 that is mounted on the scanner 300 for reading a document image and an automatic document feeder (ADF) 400 that is mounted on the scanner 300 are provided. The printing unit 100 is provided with a manual feed tray 6 for manually feeding the paper 5 and a paper discharge tray 7 for discharging the paper 5 on which an image has been formed.

FIG. 2 is an enlarged configuration diagram illustrating the configuration of the printing unit 100 in an enlarged manner.
The print unit 100 is provided with an endless intermediate transfer belt 10 as an intermediate transfer member that is an image carrier. As the material of the intermediate transfer belt 10, polyimide, which is a material excellent in mechanical characteristics, is used in order to prevent displacement due to belt elongation. In this polyimide, carbon is dispersed as an electric resistance adjusting agent in order to achieve high image quality and high stability, that is, to always obtain stable transfer performance regardless of the temperature and humidity environment. For this reason, the intermediate transfer belt 10 is black.

  The intermediate transfer belt 10 is rotationally driven in the clockwise direction in FIG. 2 while being stretched around the three support rollers 14, 15 and 16. As shown in FIG. 2, yellow (Y), cyan (C), and magenta are provided on the belt stretched portion between the first support roller 14 and the second support roller 15 among the support rollers 14, 15, and 16. Four image forming units 18Y, 18C, 18M, and 18K of (M) and black (K) are arranged side by side. Further, in order to detect a test toner image for density unevenness detection formed on the intermediate transfer belt 10 in a belt stretch portion stretched between the first support roller 14 and the second support roller 15. An optical sensor 110 is attached as a density detecting means.

  Above the image forming units 18Y, 18C, 18M, and 18K, as shown in FIG. 1, a laser writing device 21 is provided as a latent image forming unit constituting the toner image forming unit. The laser writing device 21 emits writing light by driving a semiconductor laser (not shown) by a laser control unit (not shown) based on the image information of the original read by the scanner 300. Then, the writing light exposes and scans the drum-shaped photoconductors 20Y, 20C, 20M, and 20K, which are latent image carriers provided in the image forming units 18Y, 18C, 18M, and 18K, and electrostatically scans the photoconductors. A latent image is formed. Note that the light source of the writing light is not limited to the laser diode, and may be an LED, for example.

FIG. 3 is an enlarged configuration diagram showing two of the four image forming units 18Y, 18C, 18M, and 18K.
The four image forming units 18Y, 18C, 18M, and 18K have the same configuration except that the colors of the toners used are different from each other. Therefore, in FIG. The subscripts Y, C, M, and K are omitted. In the following description, these subscripts are omitted as appropriate.

  In the image forming unit 18, a charging device 60 as a charging unit, a developing device 61 as a developing unit, a photoconductor cleaning device 63 as a cleaning unit, and a neutralizing device 64 as a neutralizing unit are provided around the photoconductor 20. ing. Further, a primary transfer device 62 as a transfer unit is provided at a position facing the photoconductor 20 via the intermediate transfer belt 10. Among these, the charging device 60 and the developing device 61 constitute a toner image forming unit.

  The charging device 60 is of a contact charging type employing a charging roller, and uniformly charges the surface of the photoconductor 20 by applying a voltage in contact with the photoconductor 20. As the charging device 60, a non-contact charging type using a non-contact scorotron charger or the like can be used.

  The developing device 61 uses a two-component developer composed of a magnetic carrier and a non-magnetic toner. As the developer, a one-component developer may be used. The developing device 61 can be broadly divided into a stirring unit 66 and a developing unit 67 provided in the developing case 70. In the stirring unit 66, a two-component developer (hereinafter simply referred to as “developer”) is conveyed while being stirred and supplied onto a developing sleeve 65, which will be described later, as a developer carrier. The stirring unit 66 is provided with two parallel screws 68, and a partition plate is provided between the two screws 68 for partitioning so that both ends communicate with each other. Further, a toner concentration sensor 71 for detecting the toner concentration of the developer in the developing device 61 is attached to the developing case 70. On the other hand, in the developing unit 67, the toner in the developer attached to the developing sleeve 65 is transferred to the photoconductor 20. The developing portion 67 is provided with a developing sleeve 65 that faces the photoreceptor 20 through the opening of the developing case 70, and a magnet (not shown) is fixedly disposed in the developing sleeve 65. Further, a doctor blade 73 as a developer regulating member is provided so that the tip approaches the developing sleeve 65.

  In the developing device 61, the developer is conveyed and circulated while being stirred by the two screws 68 and supplied to the developing sleeve 65. The developer supplied to the developing sleeve 65 is pumped up to the sleeve surface by the magnetic force generated by the magnet roller disposed in the developing sleeve 65. The developer pumped up by the developing sleeve 65 is conveyed along with the rotation of the developing sleeve 65 and is regulated to an appropriate amount by the doctor blade 73. The regulated developer is returned to the stirring unit 66. Thus, the developer conveyed to the developing area facing the photoconductor 20 becomes a spiked state by the magnetic force generated by the magnet roller, and forms a magnetic brush. In the developing region, a developing electric field that moves the toner in the developer to the electrostatic latent image portion on the photoreceptor 20 is formed by the developing bias applied to the developing sleeve 65. As a result, the toner in the developer is transferred to the electrostatic latent image portion on the photoreceptor 20, and the electrostatic latent image on the photoreceptor 20 is visualized to form a toner image. The developer that has passed through the developing region is transported to a portion where the magnetic force of the magnet is weak, and thus is separated from the developing sleeve 65 and returned to the stirring unit 66. When the toner concentration in the stirring unit 66 becomes light by repeating such an operation, the toner concentration sensor 71 detects this, and the toner is supplied to the stirring unit 66 based on the detection result.

  As the primary transfer device 62, a primary transfer roller is adopted, and is installed so as to be pressed against the photoconductor 20 with the intermediate transfer belt 10 interposed therebetween. The primary transfer device 62 may not be a roller shape, but may be a conductive brush shape, a non-contact corona charger, or the like.

The photoconductor cleaning device 63 includes a cleaning blade 75 made of, for example, polyurethane rubber, which is disposed so that the tip thereof is pressed against the photoconductor 20. In this embodiment, in order to improve the cleaning performance, a conductive fur brush 76 that contacts the photoconductor 20 is also used. The toner removed from the photoconductor 20 by the cleaning blade 75 and the fur brush 76 is accommodated in the photoconductor cleaning device 63.
The static eliminator 64 including a static eliminator lamp irradiates light to initialize the surface potential of the photoreceptor 20.

  As shown in FIG. 1, in the image forming unit 18, as the photoconductor 20 rotates, the surface of the photoconductor 20 is first uniformly charged to − (minus) 700 V, for example, by the charging device 60. Next, based on the image information read by the scanner 300, laser writing light is emitted from the laser writing device 21 to form an electrostatic latent image on the photoconductor 20. At this time, the potential of the electrostatic latent image portion irradiated with the writing light by the laser writing device 21 is, for example, −120V. Thereafter, the electrostatic latent image is visualized by the developing device 61 to form a toner image. At this time, if the developing bias voltage is set to −470 V, for example, a developing potential of 350 V can be secured, and thus the negatively charged toner can be statically transferred from the developing device 61 side to the electrostatic latent image portion on the photosensitive member 20. The development process is performed by moving electrically. The toner image is primarily transferred onto the intermediate transfer belt 10 by applying a positive transfer bias between the photoconductor 20 and the intermediate transfer belt 10 by the primary transfer device 62. The transfer residual toner remaining on the surface of the photoconductor 20 after the primary transfer is removed by the photoconductor cleaning device 63, and then the surface of the photoconductor 20 is discharged by the charge removal device 64 and used for the next image formation. The

  As shown in FIG. 2, a secondary transfer roller 24, which is a secondary transfer device, is provided at a position facing the third support roller 16 among the support rollers. When the toner image on the intermediate transfer belt 10 is secondarily transferred onto the paper 5, the secondary transfer roller 24 is pressed against the intermediate transfer belt portion wound around the third support roller 16 to perform the secondary transfer. I do. The secondary transfer device may not be configured using the secondary transfer roller 24 but may be configured using, for example, a transfer belt or a non-contact transfer charger. The secondary transfer roller 24 is in contact with a roller cleaning unit 91 that cleans toner adhering to the secondary transfer roller 24.

  Further, on the downstream side of the secondary transfer roller 24 in the paper conveyance direction, an endless belt-like conveyance belt 22 is stretched between the two rollers 23a and 23b. Further, a fixing device 25 for fixing the toner image transferred onto the paper 5 is provided further downstream in the transport direction. The fixing device 25 has a configuration in which a pressure roller 27 is pressed against a heating roller 26. Further, a belt cleaning device 17 is provided at a position facing the second support roller 15 among the support rollers of the intermediate transfer belt 10. The belt cleaning device 17 is for removing residual toner remaining on the intermediate transfer belt 10 after the toner image on the intermediate transfer belt 10 is transferred to the paper 5.

  As shown in FIG. 1, the printing unit 100 is provided with a conveyance path 48 that guides the paper 5 fed from the paper feeding device 200 to the paper discharge tray 7 via the secondary transfer roller 24. . Along the conveyance path 48, a conveyance roller 49a, a registration roller 49b, a discharge roller 56, and the like are also provided. A switching claw 55 is provided on the downstream side of the conveyance path 48 to switch the conveyance direction of the sheet 5 after transfer to the sheet discharge tray 7 or the sheet reversing device 93. The paper reversing device 93 reverses the paper 5 and sends it again to the secondary transfer roller 24. Further, the print unit 100 is provided with a manual paper feed path 53 that joins from the manual feed tray 6 to the transport path 48, and the paper 5 set in the manual feed tray 6 is placed on the upstream side of the manual paper feed path 53. A paper feed roller 50 and a separation roller 51 are provided for feeding sheets one by one.

  The paper feeding device 200 feeds a plurality of paper feeding cassettes 44 for storing paper 5, a paper feeding roller 42 and a separation roller 45 for feeding the papers stored in these paper feeding cassettes 44 one by one, and feeding the fed paper. It is composed of a transport roller 47 that transports along the path 46. The paper feed path 46 is connected to the transport path 48 of the printing unit 100.

  In the scanner 300, the first and second traveling bodies 33, 34 mounted with a document illumination light source and a mirror are reciprocated in order to read and scan a document (not shown) placed on the contact glass 31. To do. The image information scanned by the traveling bodies 33 and 34 is collected by the imaging lens 35 on the imaging surface of the reading sensor 36 installed behind the imaging lens 35 and read by the reading sensor 36 as an image signal.

FIG. 4 is a block diagram showing a main part of the electric circuit of the copying machine according to the present embodiment.
As shown in the figure, the copying machine is provided with a main control unit 500 having a computer configuration. The main control unit 500 drives and controls each unit and executes various arithmetic processes. The main control unit 500 includes a ROM (Read Only Memory) 503 that stores in advance fixed data such as a computer program via a bus line 502 in a CPU (Central Processing Unit) 501 that executes various calculations and drive control of each unit. A RAM (Random Access Memory) 504 that functions as a work area for storing various data in a rewritable manner is connected. The ROM 503 stores a conversion table (not shown) that stores information related to conversion of the output value of the optical sensor 110 into toner adhesion amount per unit area. The main control unit 500 is connected to each unit of the printing unit 100, the paper feeding device 200, the scanner 300, and the automatic document feeder 400. Here, the optical sensor 110 of the printing unit 100 sends the detected information to the main control unit 500.

Next, density change condition adjustment control for suppressing image density unevenness, which is a characteristic part of the present invention, will be described.
In this specification, the toner image density unevenness is the time variation of the toner adhesion amount per unit area. FIG. 5 is a graph showing the image density unevenness in the sub-scanning direction (paper transport direction) generated in the copying machine. .
The graph shown in FIG. 5 shows an example of image density unevenness in the paper transport direction when an image having a single density on the entire surface is formed using this copying machine. Looking at this graph, it appears that the image density fluctuates irregularly, but this waveform can be decomposed mainly into the two waveforms shown in FIGS. 6 (a) and 6 (b). . The period of the waveform shown in FIG. 6A corresponds to the peripheral period of the developing sleeve 65 in the developing device 61 of the copying machine. The period of the waveform shown in FIG. 6B corresponds to the peripheral period of the photoconductor 20 of the copying machine. That is, the image density unevenness generated in this copying machine is a combination of the image density unevenness of the developing sleeve circumferential length and the image density unevenness of the photosensitive member circumferential length.

  Such image density unevenness is caused by the fluctuation of the outer circumference of the developing sleeve 65 caused by the manufacturing error due to the eccentricity of the developing sleeve 65 and the outer circumference of the developing sleeve 65 caused by the manufacturing error caused by the eccentricity of the photoreceptor 20. This is caused by a change in time (fluctuation) of the development gap in the development region where they face each other. When the development gap fluctuates, the development electric field strength formed in the development region also changes with time (fluctuates) accordingly. The amount of toner that moves to the electrostatic latent image portion on the photoconductor 20 also fluctuates in accordance with the fluctuation of the developing electric field strength, resulting in uneven image density in the output image.

  A control unit (a combination of a CPU 501, a ROM 503, and a RAM 504) functioning as density unevenness information detecting means and density unevenness reducing means of the copier is at a predetermined timing such as immediately after a power switch (not shown) is turned on. It is configured to perform density change condition adjustment control for suppressing the above-described image density unevenness. In this density change condition adjustment control, test toner images for detecting density unevenness are formed on the surfaces of the photoreceptors 20Y, 20C, 20M, and 20K in the four image forming units 18Y, 18C, 18M, and 18K, respectively. The image is transferred onto the intermediate transfer belt 10. Each of the Y, M, C, and K test toner images is composed of three types of test toner images, which will be described later. For example, the test toner images are formed on the intermediate transfer belt 10 in the state shown in FIG.

FIG. 8 is a perspective view showing an installation location of the optical sensor 110 for detecting the density (toner adhesion amount per unit area) of the test toner image described above.
In the present embodiment, the optical sensor 110 is installed in the vicinity of the first support roller 14 in the belt stretch portion that is stretched between the first support roller 14 and the second support roller 15. The optical sensor 110 according to the present embodiment includes a single sensor head 111 on a sensor substrate 112, and the sensor head 111 is disposed substantially at the center in the belt width direction (main scanning direction) of the intermediate transfer belt 10. Is installed. As shown in FIG. 7, the three types of test toner images T1 to T3 (total 12 test toner images) for each color sequentially pass through the detection area of the sensor head 111. It is formed sequentially at the substantially central portion in the width direction.

  In this embodiment, the density of all test toner images is detected by one sensor head 111. However, for example, for each color or test toner image at different positions in the belt width direction of the intermediate transfer belt 10. A plurality of sensor heads 111 may be arranged for each type. In this case, a corresponding test image is formed on the intermediate transfer belt 10 so as to pass through the detection area of each sensor head 111.

  The three types of test toner images T1 to T3 in the present embodiment are in the form of a strip that is long along the belt conveyance direction, and the length thereof is the larger of the circumferential length of the photosensitive member or the circumferential length of the developing sleeve. It is longer than the length. This is to detect the image density unevenness of the photosensitive member circumferential length period or the developing sleeve circumferential length period from the density unevenness of the test toner images T1 to T3. The three types of test toner images T1 to T3 are each formed to have a uniform image density. However, the first test toner image T1 is formed at a high density, and the second test toner image. T2 and the third test toner image T3 are formed at a low density. The difference in density between the three types of test toner images T1 to T3 may be controlled by the area gradation method or by the density gradation method.

FIG. 9 is a flowchart showing the flow of density unevenness condition adjustment control in the present embodiment.
In the present embodiment, examples of the density changing condition that can periodically change the image density depending on the set value include a charging bias, a developing bias, an exposure intensity, and a transfer bias. First, the main controller 500 forms the first test toner image T1 on the intermediate transfer belt 10 while keeping these density change conditions at the current set values (S1). In the present embodiment, the first test toner image T1 is formed by a solid image that is a typical pattern of a high density image. In the case where image density irregularities of the developing sleeve circumferential period and the photoreceptor circumferential period occur in the output image, the same density irregularities are also applied to the first test toner image T1 along the longitudinal direction (sub-scanning direction). Appears.

  When the first test toner image T1 passes through the detection area of the optical sensor 110 as the intermediate transfer belt 10 moves, the density of the first test toner image T1 at each location in the longitudinal direction is continuously detected by the optical sensor 110. (S2). Output data of the optical sensor 110 is sent to the main controller 500. The main controller 500 functions as density unevenness information detection means, and detects density unevenness information of the first test toner image T1 based on the output data of the optical sensor 110 (S3).

FIG. 10 is a graph showing the waveform of the output value of the optical sensor 110 that detects the density of the first test toner image T1.
FIG. 11A is a graph showing a waveform having a circumferential period of the developing sleeve 65 extracted from the sensor output waveform shown in FIG. 10, and FIG. 11B is extracted from the sensor output waveform shown in FIG. 3 is a graph showing a waveform having a peripheral period of the photoreceptor 20 to be performed.
The extraction of each waveform shown in FIGS. 11A and 11B is performed by combining the waveform of the developing sleeve circumferential period and the waveform of the photosensitive member circumferential period, as shown in the following formula (1), Multivariate analysis is performed so that the sensor output waveform shown in FIG. 10 matches, and the optimum value of each parameter is determined.

Ｐ_０：光学センサ１１０の出力値
Ｐ_１：光学センサ１１０の出力値の現像スリーブ周長周期成分
Ｐ_２：光学センサ１１０の出力値の感光体周長周期成分
ｐ_１：現像スリーブ周長周期波形の振幅
λ_１：現像スリーブ周長周期波形の波長
ｑ_１：現像スリーブ周長周期波形の位相
ｐ_２：感光体周長周期波形の振幅
λ_２：感光体周長周期波形の波長
ｑ_２：感光体周長周期波形の位相
ｃｏｎｓｔ：センサ出力値の平均値

P ₀ : Output value of the optical sensor 110 P ₁ : Development sleeve circumference period component of the output value of the optical sensor 110 P ₂ : Photoconductor circumference period component of the output value of the optical sensor 110 p ₁ : Development sleeve circumference period waveform Λ ₁ : Wavelength of the developing sleeve circumference periodic waveform q ₁ : Phase of the developing sleeve circumference periodic waveform p ₂ : Amplitude of the photoreceptor circumference periodic waveform λ ₂ : Wavelength of the photoreceptor circumference periodic waveform q ₂ : Photosensitivity Phase of body circumference periodic waveform const: Average value of sensor output value

  Thereafter, the main control unit 500 functions as a density change condition adjusting unit, and adjusts the setting value of the developing bias, which is one of the density change conditions, based on the detected density unevenness information. Specifically, the development bias control table stored in the RAM 504 is updated so that the density unevenness of the test toner image indicated by the density unevenness information is reduced (S4).

  The development bias control table stored in the RAM 504 describes the control contents of the development bias applied to the development sleeve 65. The main control unit 500 controls a developing power source (not shown) of the developing device 61 according to the description of the developing bias control table, so that the developing bias that reduces the density unevenness of the first test toner image T1 indicated by the density unevenness information. Can be applied.

  Specifically, as shown in FIG. 12, for the developing sleeve circumference periodic waveform and the photoreceptor circumference periodic waveform extracted by using the above-described multivariate analysis, the sensor output (phase is 180 °) with respect to each other. A development bias control table for controlling the development bias that causes the sensor output to be shifted) is created. The main control unit 500 controls a developing power source (not shown) of the developing device 61 according to the description contents of the developing bias control table for the developing sleeve circumferential cycle waveform and the developing bias control table for the photosensitive member circumferential cycle waveform. A developing bias that cancels the combined wave of these waveforms is applied, and the density unevenness of the first test toner image T1 indicated by the density unevenness information can be reduced.

  Examples of density unevenness conditions effective for density control for a high density image include the exposure intensity (output power of writing light) of the laser writing device 21 in addition to the developing bias. Therefore, as the density unevenness condition effective for the density control for the first test high density image, in addition to the developing bias, the exposure intensity of the laser writing device 21 (the output power of the writing light) can be mentioned. Therefore, as the density unevenness condition to be adjusted based on the density unevenness information according to the density detection result of the high-density first test toner image T1, the laser writing device 21 may be used instead of the development bias or together with the development bias. The exposure intensity may be adopted. In this case, the exposure power control table stored in the RAM 504 is updated so that the density unevenness of the first test toner image T1 indicated by the density unevenness information is reduced.

  Here, when the development bias control table is updated so as to reduce density unevenness, the development bias periodically fluctuates. As a result, the development potential fluctuates periodically, and thereby the ratio between the development potential and the background potential fluctuates. . The background potential means a potential difference between the potential of the non-electrostatic latent image portion (non-exposed portion) on the surface of the photoreceptor and the surface potential of the developer carrying member to which a developing bias is applied. Therefore, such development bias control reduces the image density unevenness of the solid image, but causes the image density unevenness in the halftone image. This is the same even when the exposure intensity is adjusted instead of the developing bias.

  Therefore, in this embodiment, the second test toner image T2 is then formed on the intermediate transfer belt 10 using the updated development bias control table while maintaining other density change conditions at the current set values. (S5). In the present embodiment, the second test toner image T2 employs a halftone image formed with a uniform halftone density. When there is image density unevenness of the developing sleeve circumference period and photosensitive member circumference period that could not be reduced even by the adjustment of the developing bias described above, or image density unevenness of the same period caused by the adjustment of the developing bias described above. In the second test toner image T2, similar density unevenness appears along the longitudinal direction (sub-scanning direction).

  When the second test toner image T2 passes through the detection area of the optical sensor 110 as the intermediate transfer belt 10 moves, the density thereof is continuously detected by the optical sensor 110 (S6). Density unevenness information of the test toner image T2 is detected (S7). The method for detecting density unevenness information is the same as the method for detecting density unevenness information of the first test toner image T1 described above. The main control unit 500 adjusts the setting value of the charging bias, which is one of the density change conditions, based on the detected density unevenness information. Specifically, the charging bias control table stored in the RAM 504 is updated so that the density unevenness of the second test toner image T2 indicated by the density unevenness information is reduced (S8).

  The charging bias control table stored in the RAM 504 describes the control content of the charging bias applied to the charging roller of the charging device 60. The main control unit 500 controls the charging bias power source (not shown) of the charging device 60 in accordance with the description content of the charging bias control table so as to reduce the density unevenness of the second test toner image T2 indicated by the density unevenness information. A bias can be applied. Specifically, the second bias is applied by applying a charging bias that causes density unevenness opposite to the density unevenness of the second test toner image T2 indicated by the density unevenness information (density unevenness whose phase is shifted by 180 °). Control is performed to cancel density unevenness of the test toner image T2.

  In order to control the density of the halftone image, it is effective to change the background potential. In this embodiment, the charging bias is used as the density unevenness condition. However, the present invention is not limited to this, and the background potential may be changed. If possible, for example, the intensity of the static elimination light of the static elimination device 64 may be adopted.

  In this embodiment, after the density unevenness of the high density image is first reduced using the high density first test toner image T1, the low density image (halftone) is used using the low density second test toner image T2. In this example, the density unevenness of the image is reduced, but this order may be reversed. That is, after reducing the density unevenness of the low density image (halftone image) using the low density second test toner image T2, the density density unevenness of the high density image using the high density first test toner image T1 is reduced. May be reduced. However, the effect of updating the development bias control table and the exposure power control table on the density unevenness of the halftone image is greater than the effect of updating the charging bias control table on the density unevenness of the high density image (solid image). Therefore, the order of this embodiment in which the density unevenness of the low density image (halftone image) is reduced later is easier to control. Even in the example of reducing the density unevenness of the low density image (halftone image) first, if the gain at the time of creating the control table is appropriate, the same degree of control effect can be obtained.

  As described above, the development bias and the charging bias are adjusted to reduce the density unevenness of the high density image (solid image) and the density unevenness of the low density image (halftone image). In some cases, image density unevenness exceeding an allowable range may occur. Therefore, in the present embodiment, a process for reducing remaining density unevenness that cannot be reduced by adjusting the developing bias and the charging bias is further performed.

  Here, the remaining density unevenness that cannot be reduced by adjusting the developing bias and the charging bias, that is, the remaining that cannot be reduced by adjusting the developing potential by the developing bias and adjusting the background potential by the charging bias. Density unevenness. For this reason, if it is attempted to reduce the remaining density unevenness by a density change condition that fluctuates the development potential and background potential such as exposure power and static elimination light intensity, the density unevenness that has already been reduced by adjusting the development bias and the charging bias is reduced. Some of them appear again, and eventually, the remaining density unevenness cannot be reduced, and conversely, the density unevenness may be increased.

  Therefore, in this embodiment, the remaining density unevenness that cannot be reduced by adjusting the development bias and the charging bias is reduced by adjusting density change conditions that do not affect or have little influence on the development potential and the background potential. To do.

  Specifically, using the updated development bias control table and charging bias control table, the third test toner image T3 is formed on the intermediate transfer belt 10 while maintaining other density change conditions at the current set values. (S9). In the present embodiment, the third test toner image T3 employs a halftone image formed with a uniform halftone density. This is because the third test toner image T3 is for detecting a residue of density unevenness that cannot be reduced by adjusting the developing bias and the charging bias. That is, the halftone image is affected by both the density unevenness of the high density image and the density unevenness of the low density image, but the solid image is hardly affected by the density unevenness of the low density image. This is because if T3 is a solid image, the residue of density unevenness due to the adjustment of the charging bias cannot be detected with high accuracy.

  When the third test toner image T3 passes through the detection area of the optical sensor 110 as the intermediate transfer belt 10 moves, the density is continuously detected by the optical sensor 110 (S10). Then, density unevenness information of the third test toner image T3 is detected (S11). The method for detecting density unevenness information is the same as the method for detecting density unevenness information of the first test toner image T1 described above. This density unevenness information indicates the remaining density unevenness that could not be reduced by adjusting the developing bias and the charging bias. Then, the main control unit 500 adjusts the setting value of the transfer bias, which is one of the density change conditions, based on the detected density unevenness information. Specifically, the transfer bias control table stored in the RAM 504 is updated so that the density unevenness of the third test toner image T3 indicated by the density unevenness information is reduced (S12).

  The transfer bias control table stored in the RAM 504 describes the control contents of the primary transfer bias (transfer current in this embodiment) applied to the primary transfer roller of the primary transfer device 62. The main control unit 500 controls the transfer power source (not shown) of the primary transfer device 62 according to the description of the transfer bias control table, thereby reducing the density unevenness of the third test toner image T3 indicated by the density unevenness information. A primary transfer bias can be applied. Specifically, a primary transfer bias that causes density unevenness (density unevenness whose phase is shifted by 180 °) opposite to the density unevenness of the third test toner image T3 indicated by the density unevenness information is applied, Control is performed to cancel density unevenness of the third test toner image T3.

FIG. 13 is a graph showing the relationship between transfer current and transfer efficiency (transfer rate).
When the transfer bias control table is created, the fluctuation range of the transfer bias (transfer current) that fluctuates according to the control based on the transfer bias control table is the use area shown in FIG. A transfer bias control table is created so that the transfer bias is in a linear relationship. By controlling the transfer bias (transfer current) in this region, it is possible to easily control image density unevenness due to adjustment of the transfer bias.

  In the present embodiment, the primary transfer bias is used as the density change condition to be adjusted using the third test toner image T3. However, the density change condition that does not affect the development potential or the background potential or has little effect. For example, not only the primary transfer bias but also a secondary transfer bias can be employed.

In this embodiment, the configuration of the intermediate transfer method in which the toner image on the photoconductor 20 is transferred to the paper 5 via the intermediate transfer belt 10 has been described as an example. For example, the following configuration can also be adopted. In this system, a paper transport belt is disposed at a position facing the photoconductor, and the toner image on the photoconductor is directly transferred to the paper being transported while being held on the surface of the paper transport belt. In such a system, the density detection of the test toner image for detecting density unevenness may be performed by detecting the test toner image transferred onto the paper held on the surface of the paper transport belt or transferring it to the surface of the paper transport belt. The test toner image may be detected. In either method, the density of the test toner image may be detected on the photoreceptor.
Further, although a color type copying machine that forms a multicolor toner image by superimposing transfer has been described, the present invention can also be applied to a single color type image forming apparatus that forms only a single color toner image.

What has been described above is merely an example, and the present invention has a specific effect for each of the following modes.
(Aspect A)
Transfer bias applied by a transfer unit such as a primary transfer unit 62 to a toner image formed on an image carrier such as the photosensitive member 20 by a toner image forming unit such as a charging device 60, a laser writing unit 21 and a developing unit 61. In the image forming apparatus for forming an image on the recording material by transferring the recording material to the recording material such as the sheet 5 through another image carrier such as the intermediate transfer belt 10 by the action of the above, the toner image forming unit forms the image. Density detection means such as an optical sensor 110 for detecting the density of the test toner image for density unevenness detection, and one sheet so as to reduce the density unevenness of the test toner image based on the detection result of the density detection means. Density unevenness reducing means such as a controller 500 that adjusts the setting of the transfer bias condition in order to vary the transfer bias during the image forming operation for the output image. The features.
According to this, the image density unevenness can be reduced by a new method of reducing the image density unevenness by adjusting the transfer bias condition.

(Aspect B)
In the above aspect A, the toner image forming means includes one or more density changes such as a charging bias, a developing bias, and an exposure intensity that can be varied so as to cause a density change in the toner image formed on the image carrier. The toner image is formed on the image carrier in accordance with the setting contents of the conditions, and the density unevenness reducing unit is performing an image forming operation on one output image so as to reduce the density unevenness of the test toner image. In order to change the density change condition, the setting of the density change condition is also adjusted.
According to this, even for image density unevenness that cannot be accurately reduced to within the allowable range only by adjusting the transfer bias condition, the method for adjusting the setting of the density change condition is also used in combination, and this is within the allowable range. Can be accurately stored.

(Aspect C)
In the above aspect B, the toner image forming unit attaches toner to the electrostatic latent image after the electrostatic latent image is formed on the image carrier by the action of the developing potential of the developing unit such as the developing device 61. To form a toner image on the image carrier,
The density change condition includes a development potential change condition such as a charging bias, a development bias, and an exposure intensity that causes a time change in the development potential.
According to this, even for image density unevenness that cannot be accurately reduced to within the allowable range only by the method of adjusting the development potential, it is possible to accurately set this within the allowable range.

(Aspect D)
In the above-described aspect B or C, the density unevenness reducing unit is based on the density detection results of test toner images having different densities between two or more of the conditions whose settings are adjusted by the density unevenness reducing unit. The setting is adjusted.
According to this, it is possible to accurately store the image density unevenness within the allowable range over a wide image density range.

(Aspect E)
In any one of the above aspects B to D, the density unevenness reducing unit is configured to develop a developing bias with respect to some conditions such as a charging bias and a transfer bias among the conditions in which the setting is adjusted by the density unevenness reducing unit. The setting is adjusted based on the density detection result of the test toner image after adjusting the setting of other conditions such as the above.
According to this, since the remaining image density unevenness that could not be reduced by setting adjustment of other conditions can be reduced by setting adjustment of some conditions, the image density unevenness within the allowable range in total. It can be accurately stored.

(Aspect F)
In any one of the above aspects B to E, the density unevenness reducing means is a relationship between a transfer bias value applied by the transfer means and a transfer rate from the image carrier to another image carrier or recording material. The transfer bias condition is adjusted within the range of the transfer bias in which is substantially linear.
This facilitates the process of setting and adjusting the transfer bias condition.

(Aspect G)
In any one of the above aspects B to F, the density unevenness reducing unit detects density unevenness information for each frequency by multivariate analysis based on the detection result of the density detection unit, and detects a plurality of detected density unevennesses. The transfer bias condition setting is adjusted based on the information.
According to this, density unevenness information for each frequency can be easily extracted.

(Aspect H)
In any one of the above aspects B to G, the density unevenness reducing unit cancels the density unevenness of the test toner image for some or all of the conditions for which the setting is adjusted by the density unevenness reducing unit. As described above, the setting is adjusted.
According to this, it is possible to further reduce the image density unevenness and accurately fit within the allowable range.

DESCRIPTION OF SYMBOLS 10 Intermediate transfer belt 18 Image forming unit 20 Photoconductor 21 Laser writing device 60 Charging device 61 Developing device 62 Primary transfer device 65 Developing sleeve 100 Print unit 110 Optical sensor 500 Main control unit

JP 2007-33770 A JP 2010-191364 A

Claims (8)

The toner image formed on the image carrier by the toner image forming unit is transferred onto the recording material through another image carrier or directly onto the recording material by the action of a transfer bias applied by the transfer unit. In an image forming apparatus for forming an image,
Density detecting means for detecting the density of a test toner image for detecting density unevenness formed by the toner image forming means;
Based on the detection result of the density detector, the setting of the transfer bias condition is adjusted so as to vary the transfer bias during the image forming operation for one output image so as to reduce the density unevenness of the test toner image. An image forming apparatus comprising: a density non-uniformity reducing unit. The image forming apparatus according to claim 1.
The toner image forming means forms a toner image on the image carrier according to the setting contents of one or more density change conditions that can be varied so as to cause a density change in the toner image formed on the image carrier. Is what
The density unevenness reducing means adjusts the setting of the density change condition so as to vary the density change condition during the image forming operation for one output image so that the density unevenness of the test toner image is reduced. An image forming apparatus. The image forming apparatus according to claim 2.
The toner image forming means forms an electrostatic latent image on the image carrier, and then attaches toner to the electrostatic latent image by the action of a developing potential by the developing means, thereby allowing the toner image to be formed on the image carrier. To form a toner image,
The image forming apparatus, wherein the density change condition includes a development potential change condition that causes a time change in the development potential. The image forming apparatus according to claim 2 or 3,
The density unevenness reducing means adjusts the setting based on the density detection results of test toner images having different densities between two or more of the conditions whose settings are adjusted by the density unevenness reducing means. An image forming apparatus. The image forming apparatus according to any one of claims 2 to 4,
The density unevenness reducing means, for some of the conditions whose settings are adjusted by the density unevenness reducing means, based on the density detection result of the test toner image after adjusting the settings of other conditions, An image forming apparatus characterized by adjusting the setting. The image forming apparatus according to any one of claims 1 to 5,
The density unevenness reducing means is within a transfer bias range in which the relationship between the transfer bias value applied by the transfer means and the transfer rate from the image carrier to another image carrier or recording material is substantially linear. And adjusting the setting of the transfer bias condition. The image forming apparatus according to any one of claims 1 to 6,
The density unevenness reducing means detects density unevenness information for each frequency by multivariate analysis based on the detection result of the density detection means, and adjusts the setting of the transfer bias condition based on the detected plurality of density unevenness information. An image forming apparatus. The image forming apparatus according to any one of claims 1 to 7,
The density unevenness reducing means adjusts the setting so that the density unevenness of the test toner image is canceled for a part or all of the conditions for which the setting is adjusted by the density unevenness reducing means. Image forming apparatus.

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