Image forming apparatus

Abstract

To provide an image forming apparatus that can reduce a variation in the density of toner images formed by respective image developing means, despite an inexpensive configuration in which a common developing bias is supplied from a developing power supply.SOLUTION: A control unit is configured to, during execution of process condition adjustment processing, cause respective image developing means to form a toner patch group including a plurality of toner images for testing corresponding to the combinations of a plurality of different voltage values and a plurality of different dither values, on the basis of a print image signal corresponding to a reference density, and determine a voltage value and a dither value that a dither processing unit should apply to the respective image developing means during execution of image forming processing, on the basis of the densities of the plurality of toner images for testing detected by a density sensor.SELECTED DRAWING: Figure 13

Description

[Technical field]

[0001] The present disclosure relates to an image forming apparatus including a plurality of image developing means each including an image carrier and a developing means for developing a toner image on the image carrier, a transfer means including a transfer member to which each of the toner images developed on each of the image carriers is transferred, and a development power source for supplying a common development bias to each of the developing means. [Background technology]

[0002] Patent Document 1 discloses an image forming apparatus having an image density control means for a high density portion in a configuration in which a charging device and a developing device share a common development power source. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2004-163502 A Summary of the Invention [Problem to be solved by the invention]

[0004] In this image forming apparatus, it is assumed that a common developing bias voltage value is obtained from the developing power supply so that the densities of all four color toner images are within the target density range. However, since the developability of each toner differs due to the temperature and humidity of the environment around each developing unit, the remaining amount of each toner, the chargeability, etc., in a configuration in which a common developing bias is supplied from the developing power supply, the densities of all four color toner images may not be within the target density range (they may be darker or lighter than the desired density).

[0005] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide an image forming apparatus that can reduce the variation in density of each toner image formed by each image developing means, while having an inexpensive configuration in which a common developing bias is supplied from a developing power supply. [Means for solving the problem]

[0006] In order to achieve the above object, the image forming apparatus according to the present disclosure includes a plurality of image developing means each including an image carrier and a developing means for developing a toner image on the image carrier, a transfer means including a transfer member to which each of the toner images developed on each of the image carriers is transferred, and a developing power source for supplying a common developing bias to each of the developing means, the image forming apparatus including a control unit configured to be capable of executing an image forming process for forming the toner image based on an arbitrary print image signal by controlling each of the image developing means and to be capable of executing a process condition adjustment process for adjusting a voltage value of the developing bias by controlling the developing power source, and a control unit configured to be capable of executing a process condition adjustment process for adjusting a voltage value of the developing bias by controlling each of the image developing means, and a control unit configured to be capable of executing a process condition adjustment process for adjusting a voltage value of the developing bias by controlling each of the image developing means, the control unit configured to be capable of executing a process condition adjustment process for adjusting a voltage value of the developing bias by controlling each of the image developing means, the control unit configured to be capable of executing a process condition adjustment process for adjusting a voltage value of the developing bias by controlling each of the image developing means, and the control unit configured to be capable of executing a process condition adjustment process for adjusting a voltage value of the developing bias by controlling each of the image developing means, ... and a density sensor that detects the density of each of the toner images transferred to the transfer member, wherein the control unit is configured to, when the process condition adjustment process is performed, cause each of the image developing means to form a toner patch group including a plurality of test toner images corresponding to combinations of a plurality of different voltage values ​​and a plurality of different dither values ​​based on a print image signal corresponding to a reference density, and to determine the voltage value at the time of execution of the image formation process and the dither value to be applied to each of the image developing means by the dither processing unit based on the respective densities of the plurality of test toner images detected by the density sensor.

[0007] According to the above-described configuration, even if there are variations in the developability of each toner due to the temperature and humidity of the environment surrounding each developing means, the remaining amount of each toner, chargeability, etc., the dither value applied during the image formation process can be adjusted individually for each image developing means, thereby reducing the variation in density of each toner image formed by each image developing means, despite the use of an inexpensive configuration in which a common developing bias is supplied from the developing power supply.

[0008] In the image forming apparatus according to the present disclosure, the dither processing unit is configured to perform image processing on the test toner image using a plurality of dither values ​​ranging from a predetermined maximum dither value to a predetermined minimum dither value, and the control unit is configured to, when executing the process condition adjustment process, select a combination of the voltage value and the dither value in the toner patch group corresponding to the maximum dither value, at which the density of the test toner image detected by the density sensor falls within a predetermined target density range, and if there is a voltage value among the selected combinations at which the density of the image developing means at which the density of the test toner image is detected to be the lowest falls within the target density range, determine the voltage value with the smallest absolute value among the voltage values ​​as the voltage value at the time of execution of the image formation process, and determine, among the dither values ​​corresponding to the combination with the determined voltage value, the dither value at which the density of the formed test toner image is highest within the target density range as the dither value at the time of execution of the image formation process.

[0009] According to the above-mentioned configuration, even if the image developing means detects the lowest density of the test toner image, a voltage value that satisfies the target density is set as a common developing bias for each image developing means, and the dither value is adjusted individually for each image developing means, so that all image developing means can develop a toner image with an appropriate density. Note that the predetermined maximum dither value is a dither value that develops a toner image in the entire area constituting the test toner image, and the predetermined minimum dither value is a dither value that develops a toner image in the area constituting the test toner image excluding a predetermined area to the extent that jaggies are not noticeable.

[0010] In the image forming apparatus according to the present disclosure, the toner constituting the toner image is non-magnetic, the developing means contains a magnetic carrier, and the control unit is configured to, for an image developing means in which the density of the formed test toner image exceeds the target density range even at a minimum dither value corresponding to a combination with the determined voltage value, cause the image developing means to spit the toner image for a predetermined time and then execute the process condition adjustment process again.

[0011] When the toner concentration inside the developing means is high, the toner may not obtain a predetermined charge amount because of sufficient friction opportunities with the carrier, and the density of the resulting toner image may become too high. For such image developing means, as described above, the amount of toner inside the developing means is reduced by discharging the toner, and sufficient friction opportunities with the carrier inside the developing means are obtained, so that the toner obtains a predetermined charge amount, and as a result, the density of the toner image can be reduced.

[0012] In the image forming apparatus according to the present disclosure, each of the image developing means may be provided with an exposure means for exposing each of the image carriers, and the control unit may be configured to, for an image developing means in which the density of the test toner image formed by the image developing means exceeds the target density range even at a minimum dither value corresponding to a combination with the determined voltage value, reduce the amount of exposure light to the image carrier by the exposure means provided in the image developing means, and then execute the process condition adjustment process again.

[0013] For image forming devices in which the density of the resulting toner image is too high, as described above, the amount of exposure light can be reduced, which reduces the potential difference between the electrostatic latent image portion of the image carrier and the developing roller, thereby reducing the amount of toner adhering to the electrostatic latent image portion, and ultimately lowering the density of the toner image.

[0014] In the image forming apparatus according to the present disclosure, the control unit may be configured to, for an image developing means in which the density of the image developing means detected as having the lowest density of the test toner image among the selected combination is below the target density range even if the density is the maximum dither value, perform the process condition adjustment process again after toner is replenished to the image developing means.

[0015] It is considered that the image developing means where the density of the test toner image is insufficient has a lack of toner. With the above-mentioned configuration, toner is replenished to the image developing means where toner is considered to be insufficient, and it becomes possible to determine the voltage value and dither value in a state where there is a sufficient amount of toner. Note that the configuration may be such that the toner is replenished automatically or manually. Effect of the Invention

[0016] The image forming apparatus of the present disclosure has an advantage that it is possible to reduce the variation in density of each toner image formed by each image developing unit, while having an inexpensive configuration in which a common developing bias is supplied from a developing power supply. [Brief description of the drawings]

[0017] [Figure 1] 1 is an explanatory diagram illustrating a schematic configuration of a color image forming apparatus according to the present disclosure. [Diagram 2] FIG. 2 is an explanatory diagram of the inside of the color image forming apparatus shown in FIG. [Diagram 3] FIG. [Figure 4] 4 is an explanatory diagram of a group of toner patches formed on an intermediate transfer belt. FIG. [Diagram 5] FIG. 4 is an explanatory diagram of dither values. [Figure 6] 11 is an explanatory diagram of voltage values ​​and dither values ​​in a toner patch group related to black toner. FIG. [Figure 7] 5 is an explanatory diagram of the density of a toner image in a toner patch group of each color. FIG. [Figure 8]8 is a graph showing the density of each toner image shown in FIG. 7. [Figure 9] 5 is an explanatory diagram of the density of a toner image in a toner patch group of each color. FIG. [Figure 10] 10 is a graph showing the density of each toner image shown in FIG. [Figure 11] 5 is an explanatory diagram of the density of a toner image in a toner patch group of each color. FIG. [Figure 12] 12 is a graph showing the density of each toner image shown in FIG. 11. [Figure 13] 4 is a flowchart illustrating a process condition adjustment process. [Figure 14] 4 is a flowchart illustrating a process condition adjustment process. [Figure 15] 4 is a flowchart illustrating a process condition adjustment process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] [Embodiment 1] Hereinafter, an embodiment of a tandem type color image forming apparatus according to the present disclosure will be described. As shown in FIG. 1 and FIG. 2, the color image forming apparatus 1 includes four image developing devices 2 (2K, 2C, 2M, 2Y) that form toner images of each color of black (K), cyan (C), magenta (M), and yellow (Y), a developing power source 3 (see FIG. 2), a transfer device 4, a fixing device 5, a paper feed device 6, a conveying device 7, a paper discharge tray 8, and a control unit 9 (see FIG. 3) that controls the color image forming apparatus 1. The letters K, C, M, and Y of the image developing devices 2K, 2C, 2M, and 2Y are used to distinguish between the image developing devices corresponding to black (K), cyan (C), magenta (M), and yellow (Y), and may be omitted when the distinction is not necessary. The same applies to the configuration of each part below. P is a printing paper. The color image forming apparatus 1 corresponds to the "image forming apparatus" in the claims, and the image developing apparatuses 2K, 2C, 2M, and 2Y correspond to the "image developing means" in the claims.

[0019] Since the image developing devices 2K, 2C, 2M, and 2Y are basically configured in the same way, the image developing device 2K that forms a black (K) toner image will be described. As shown in Fig. 2, the image developing device 2K includes a photoconductor drum 20K, a charger 21K that charges the photoconductor drum 20K, an exposure device 22K that exposes the charged photoconductor drum 20K, a developing device 23K that develops a toner image on the exposed photoconductor drum 20K, and a cleaner device (not shown) that cleans the surface of the photoconductor drum 20K. The photoconductor drum 20K is the "image carrier" in the claims, and the exposure device 22K is the "exposure means" in the claims.

[0020] The photoconductor drum 20K includes a grounded conductive cylindrical member and a photosensitive layer formed on the outside of the cylindrical member. The photosensitive layer is made of a photoconductor that is insulating in the dark and becomes conductive in the irradiated area when irradiated with light. The photoconductor drum is rotated in a predetermined direction by a drive source (not shown).

[0021] The charger 21K includes a charging roller having a conductive shaft and a conductive elastic layer formed on the outside of the shaft. The charging roller is disposed so that the surface of the conductive elastic layer contacts the surface of the photoconductor drum 20K, and is supported so as to rotate with the rotation of the photoconductor drum 20K. A power source (not shown) is connected to the conductive shaft, and a predetermined voltage or current is supplied from the power source to charge the surface of the photoconductor drum 20K to a predetermined potential (for example, about -600V).

[0022] The exposure device 22K has a light-emitting unit (e.g., a laser diode) and exposes the surface (photosensitive layer) of the photoconductor drum 20K based on the image data for printing. The exposed area, which is the area exposed to the light emitted from the light-emitting unit, has a lower resistance value and a lower surface potential, and an electrostatic latent image according to the image data for printing is formed. The surface potential of the exposed area is closer to the ground potential (e.g., about -100V) than the non-exposed area, which is the other area not exposed to light. The exposure device 22 of the present disclosure is a scanning type exposure device that has multiple light-emitting units corresponding to each color and scans the light beams emitted from each light-emitting unit in the longitudinal direction of each of the photoconductor drums 20K, 20C, 20M, and 20Y.

[0023] The developing device 23K includes a developing tank 231K, an agitating member 232K, a toner supplying member 233K, a developing roller 234K, etc. The developing device 23K corresponds to the "developing means" in the claims.

[0024] The developer tank 231K is a container in which the developer is stored. The developer contains non-magnetic toner and magnetic carrier. The toner particle size is about 5 to 10 μm, and the carrier particle size is about 40 μm. The mixture ratio of the toner and the carrier is, for example, about 1:9.

[0025] The agitating member 232K rotates by receiving a rotational force from the driving device 11 (see FIG. 3) and agitates the developer stored in the developing tank 231K. The agitation by the agitating member 232K generates friction between the toner and carrier contained in the developer, so that the toner becomes negatively charged and the carrier becomes positively charged. The agitating member 232K is disposed near the toner supplying member 233K and conveys the developer to the toner supplying member 233K while stirring the developer. The toner supplying member 233K rotates by receiving a rotational force from the driving device 11 and supplies the developer conveyed from the agitating member 232K to the surface of the magnetic developing roller 234K.

[0026] The developing roller 234K has a conductive sleeve that rotates by receiving a rotational force from the driving device 11, and a number of magnets fixed inside the sleeve so as not to rotate. The carrier contained in the developer conveyed from the stirring member 232K is attracted together with the surrounding toner to the surface of the non-conductive sleeve by the magnetic force generated by the multiple magnets and is carried on it. The developer carried on the surface of the developing roller 234K, i.e., the surface of the sleeve, is conveyed to the portion facing the photosensitive drum 20K by the rotation of the sleeve. A developing bias DV, which is a predetermined voltage, is applied to the sleeve from the developing power source 3, and at the facing portion, only the toner is moved (developed) from the developing roller 234K to the photosensitive drum 20K due to the potential difference between the developing bias DV and the electrostatic latent image (exposed area), and a toner image is formed on the photosensitive drum 20K.

[0027] The developing power source 3 supplies a developing bias DV, which is a predetermined voltage, to the developing rollers 234K, 234C, 234M, and 234Y included in the developing devices 23K, 23C, 23M, and 23Y of the image developing devices 2K, 2C, 2M, and 2Y. The developing power source 3 supplies the developing bias DV, which is the same voltage, to the developing rollers 234K, 234C, 234M, and 234Y. By configuring the developing rollers 234K, 234C, 234M, and 234Y to supply the same voltage value instead of supplying different voltage values ​​to them, the number of transformers used can be reduced, thereby reducing power supply costs. The common voltage value set as the developing bias DV is set by a process condition adjustment process by the control unit 9, which will be described in detail later.

[0028] The transfer device 4 includes primary transfer rollers 41K, 41C, 41M, and 41Y, an intermediate transfer belt 42, a drive roller 43, a driven roller 44, and a secondary transfer roller 45. The intermediate transfer belt 42 is the "transfer member" in the claims. A predetermined transfer voltage is applied to the primary transfer rollers 41K, 41C, 41M, and 41Y and the secondary transfer roller 45.

[0029] The intermediate transfer belt 42 is stretched around a drive roller 43 and a driven roller 44, and rotates when the drive roller 43 rotates upon receiving a rotational force from the drive device 11. The primary transfer rollers 41K, 41C, 41M, and 41Y are disposed inside the intermediate transfer belt 42 at positions facing the photoconductor drums 20K, 20C, 20M, and 20Y of the image developing devices 2K, 2C, 2M, and 2Y, respectively, and bring the intermediate transfer belt 42 into contact with the corresponding photoconductor drums 20K, 20C, 20M, and 20Y. The secondary transfer roller 45 is disposed so as to come into contact with the surface of the intermediate transfer belt 42 at a position facing the drive roller 43, and rotates together with the rotation of the intermediate transfer belt 42. The toner images developed by the developing devices 23K, 23C, 23M, and 23Y and formed on the photoconductor drums 20K, 20C, 20M, and 20Y are sequentially (primarily) transferred onto the intermediate transfer belt 42 by the primary transfer rollers 41K, 41C, 41M, and 41Y. The toner images transferred onto the intermediate transfer belt 42 are then sent to a contact portion with the secondary transfer roller 45, and transferred (secondarily transferred) onto the printing paper P sent to the contact portion.

[0030] The fixing device 5 sandwiches the print paper P between a heating roller with a built-in heat source and a pressure roller, and heats and pressure-feeds the toner image transferred to the print paper P, thereby fixing the toner image to the print paper P. The paper feeder 6 has a tray for loading the print paper P. The transport device 7 has a transport path for the print paper P and transport rollers and the like provided along the transport path. The paper output tray 8 is a tray on which the print paper P with an image printed thereon is placed.

[0031] The cleaner device (not shown) has a cleaning blade that scrapes off any toner remaining on the photoconductor drum 20K that was not transferred to the intermediate transfer belt 42, and a charge-removing device that removes charge from the surface of the photoconductor drum 20K and erases any electrostatic latent images that have been formed. This makes the photoconductor drum 20K ready for the next printing. The toner scraped off by the cleaning blade is collected in a waste toner box (not shown).

[0032] In addition, an image sensor 10 (see FIG. 2) for detecting the density of each toner image transferred onto the intermediate transfer belt 42 is provided between the primary transfer roller 41K for black, which is closest to the secondary transfer roller 45 in the moving direction (rotation direction) of the intermediate transfer belt 42, and the secondary transfer roller 45. The image sensor 10 has a light emitting section and a light receiving section, and is configured as a reflective optical sensor that emits light from the light emitting section toward the intermediate transfer belt 42 and receives the light reflected from the intermediate transfer belt 42 at the light receiving section. An intensity signal of the light received by the light receiving section is input to the control section 9. Although details will be described later, in the image forming apparatus of the present disclosure, the image sensor 10 is provided to detect the density of each toner image developed by each developing device 23K, 23C, 23M, and 23Y and intermediately transferred onto the intermediate transfer belt 42, and to set development conditions such as a common development bias DV to be supplied to each developing device 23K, 23C, 23M, and 23Y based on the obtained detection result.

[0033] 3 is a block diagram showing the control unit 9. The control unit 9 has an arithmetic processing unit 91, a storage unit 92, a dither processing unit 93, an input unit 94, and an output unit 95, which are connected to each other by a bus line (not shown).

[0034] The arithmetic processing unit 91 includes a CPU (Central Processing Unit), and the storage unit 92 includes a non-volatile memory such as a ROM (Read Only Memory) and a RAM (Random Access Memory). The arithmetic processing unit 91 reads out a control program stored in the storage unit 92 and executes each function.

[0035] The dither processing unit 93 executes image processing with a plurality of different dither values ​​DT that provide a predetermined gradation for each of the image developing devices 2K, 2C, 2M, and 2Y. The dither processing unit 93 is the "dither processing unit" in the claims. The dither processing will be described in detail later.

[0036] The image sensor 10 is communicatively connected to the input unit 94, and the image developing devices 2K, 2C, 2M, 2Y, the developing power supply 3, the transfer device 4, the fixing device 5, the paper feeding device 6, the conveying device 7, the drive device 11, etc. are communicatively connected to the output unit 95.

[0037] The control unit 9 is configured to execute a process condition adjustment process for the color image forming apparatus 1 at predetermined times (e.g., developing a toner image on the photosensitive member 1000 times) so as to prevent the density (and gradation) of the toner image transferred to the paper from changing over time.

[0038] The process condition adjustment process will be described below. As shown in FIG. 4, the control unit 9 forms a plurality of toner images (patch images) for image density measurement during the process condition adjustment process. Here, the toner images for image density measurement are formed by changing the development bias condition and the dither value, which is one of the exposure conditions. That is, the control unit 9 forms a toner patch group G including a plurality of test toner images T (TK, TC, TM, TY) formed by combining a plurality of development biases DV with different voltages and a plurality of different dither values ​​DT, and detects the density of each of the toner images by the image sensor 10. Note that the arrow in FIG. 4 indicates the movement direction of the intermediate transfer belt 42.

[0039] Here, the number of test toner images T constituting the toner patch group G is determined by the number of toner colors, the number of settings of the developing bias DV, and the number of settings of the dither value DT. In this embodiment, the number of toner colors is four, the number of settings of the developing bias DV is three, and the number of settings of the dither value DT is three. In other words, the toner patch group G is composed of 36 (=4×3×3) test toner images T.

[0040] The value of the developing bias DV set when forming a test toner image will be described. In this embodiment, three values ​​of the developing bias DV are set, and the respective values, i.e., the voltage values ​​DV1, DV2, and DV3, are -450V, -420V, and -400V, respectively. As described above, the potential of the surface of the photoconductor drum 20 exposed by the exposure device 22 is about -100V, so the difference between the surface potential of the exposed photoconductor drum 20 and the developing bias DV increases in the order of the voltage value DV3, the voltage value DV2, and the voltage value DV1. Therefore, the amount of toner developed also increases in the order of the voltage value DV3, the voltage value DV2, and the voltage value DV1, and the density also increases.

[0041] The voltage values ​​DV1, DV2, and DV3 set as the value of this developing bias DV are determined in advance based on the results of experiments, etc. More specifically, a table determined according to the temperature and humidity of the environment around each of the developing devices 23K, 23C, 23M, and 23Y, the remaining amount of each toner, the cumulative number of rotations of each of the photoconductor drums 20K, 20C, 20M, and 20Y, etc. is stored in the storage unit 92. Therefore, the control unit 9 can set the value of the developing bias DV by referring to the storage unit 92 when performing the process condition adjustment process.

[0042] Next, the dither value DT set when forming a test toner image will be described. In this embodiment, as shown in FIG. 5, the dither value DT is a value in the range of 0 to 840 that is determined by which of 840 squares (29 dots vertical x 29 dots horizontal) toner is to be attached to. Here, a dot is a predetermined unit that allows the color image forming apparatus 1 to control the attachment of toner, and one dot refers to the diameter of the light beam irradiated from the exposure device 22. One dot of the image forming apparatus 1 of this embodiment has a diameter of 42 μm (resolution 600 DPI). Therefore, the dither value DT means the number of squares to be exposed out of 840 squares in the range of 42 μm x 29 dots = 1.22 mm square.

[0043] For example, when the dither value DT is 840, 840 of the 840 squares, or 100% of the squares, are exposed. The value of the dither value DT at this time, 840, is the maximum value of the dither value DT. When the dither value DT is 740, 740 of the 840 squares, or about 88% of the squares, are exposed. Similarly, when the dither value DT is 640, 640 of the 840 squares, or about 76% of the squares, are exposed.

[0044] Since toner is developed in the above-mentioned exposed squares, the larger the dither value, the higher the image density. However, depending on the relationship between the value of the developing bias DV and the charge amount of the toner, the amount of toner supplied to the exposed squares may be large, and the toner attached to the surface of the photosensitive drum 20 may extend beyond the exposed squares, reducing the area of ​​the unexposed squares, resulting in a high density toner image even with a smaller dither value. In this embodiment, by utilizing this characteristic, the density and gradation of the developed toner images of each color can be made the same even if the charge amount of the toner in the four developing devices 23K, 23C, 23M, and 23Y to which the same developing bias DV is supplied varies.

[0045] When the process condition adjustment process is performed, the control unit 9 changes the dither value DT by three values ​​to form a test toner image T. The values ​​of the dither value DT changed here, DT1, DT2, and DT3, are 840, 740, and 640, respectively. It has been experimentally confirmed that when the dither value DT falls below about 630, that is, when the number of exposed squares falls below about 75% of the 840 squares, the formed toner image has noticeable jaggies and is no longer recognized as a so-called solid image (an image in which a predetermined area is filled with toner so that the density is uniform) (=an image with uneven density). Therefore, in this embodiment, the lower limit of the dither value applied when the process condition adjustment process is performed is set to 640. In the dither method, for example, the order in which the squares are exposed as the density increases is predetermined. In this embodiment, the order of the exposed squares is determined so that they increase in the order of the symbols 0, 1, 2, 3 . . . 840 in FIG. 5, but a known dither pattern is used for the pattern of the order, etc.

[0046] Fig. 6 explains the conditions for forming a test toner image T using black (K) toner as an example. The leftmost line indicates the toner patch number for the test toner image T, the center line indicates three conditions for the developing bias DV to be applied (DV1, DV2, DV3), and the rightmost line indicates three conditions for the dither value DT to be applied (DT1, DT2, DT3). Here, the values ​​of DV1, DV2, DV3, DT1, DT2, and DT3 are as described above. Nine toner patches that combine such conditions of the developing bias DV and dither value DT are formed for each color.

[0047] Returning now to Fig. 4, the explanation is given. Fig. 4 shows test toner images T (TK, TC, TM, TY) of each color formed with the dither value DT being DT1 (=840), DT2 (=740), and DT3 (=640) when the developing bias DV is at a voltage value DV1 (=-450V).

[0048] In detail, from the right, a black (K) test toner image TK formed with a dither value DT of DT1 (=840) when the developing bias DV is at a voltage value DV1 (=-450V). 11 , Cyan (C) test toner image TC 11 , magenta (M) test toner image TM 11 , Yellow (Y) test toner image TY 11 Next, a black (K) test toner image TK formed with a dither value DT of DT2 (=740) 12 , Cyan (C) test toner image TC 12 , magenta (M) test toner image TM 12 , Yellow (Y) test toner image TY 12 , and a black (K) test toner image TK formed with a dither value DT of DT3 (=640). 13 , Cyan (C) test toner image TC 13 , magenta (M) test toner image TM 13 , Yellow (Y) test toner image TY 13 is shown.

[0049] Note that FIG. 4 only shows 12 test toner images TK, TC, TM, and TY formed when the developing bias DV has a voltage value DV1 and the dither value DT has values ​​DT1, DT2, and DT3, but test toner images TK, TC, TM, and TY are also formed sequentially when the developing bias DV has voltage values ​​DV2 and DV3. Therefore, toner patch group G also includes test toner images TK, TC, TM, and TY of each color formed with dither value DT values ​​DT1 (=840), DT2 (=740), and DT3 (=640) when the developing bias DV is a voltage value DV2 (=-420V), and test toner images TK, TC, TM, and TY of each color formed with dither value DT values ​​DT1 (=840), DT2 (=740), and DT3 (=640) when the developing bias DV is a voltage value DV3 (=-400V).

[0050] Next, the density detection of the formed test toner image T (patch image) will be described. The density of the test toner image T is detected by the image sensor 10 described above. The image sensor 10 is an optical sensor having a light source and a light receiving unit. The image sensor 10 irradiates light from the light source onto the test toner image T transferred to the intermediate transfer belt 42, and detects the density ID of the test toner image T based on the amount of reflected light of the irradiated light detected by the light receiving unit. Here, the density ID is calculated by the common logarithm of the reciprocal of the reflectance (i.e., reflected light intensity / incident light intensity), and the value increases as the density becomes darker. More specifically, the density ID of a solid image is about 1.3 to 1.7, and the value of the density ID approaches 0 as the density becomes lighter. Note that the irradiation light of the image sensor 10 of this embodiment has an irradiation range of φ1 mm, so in order to accurately measure the density ID of the test toner image T, it is necessary to form the size of the test toner image T to be larger than φ1 mm. In this embodiment, the test toner image T is formed as a patch image having a square shape that is, for example, 10 mm in the circumferential direction of the photosensitive drum 20 and 10 mm in the direction intersecting the circumferential direction of the photosensitive drum 20, i.e., the longitudinal direction.

[0051] The control unit 9 detects the densities of all the test toner images T included in the toner patch group G with the image sensor 10, and based on these detection results, determines the developing bias DV to be applied when performing the image forming process and the dither value DT to be applied to each of the image developing devices 2K, 2C, 2M, and 2Y. More specifically, the control unit 9 determines the developing bias DV and the dither value DT to be applied to each of the image developing devices 2K, 2C, 2M, and 2Y so that the density of a solid image of each color (an image filled with toner so that a predetermined area has a uniform density) satisfies the reference density.

[0052] Here, the reference density specifies the density ID of an image that a solid image should satisfy, and a lower limit and an upper limit are set. In this embodiment, the lower limit is set to 1.3, and the upper limit is set to 1.5. If the detected density ID value falls below the lower limit, the image is determined to be light (insufficient) in density, and if it exceeds the upper limit, the image is determined to be too dark (excessive). In other words, the reference density is specified as a target density range that is an appropriate range determined by the lower limit and upper limit.

[0053] The control unit 9 selects a combination of the developing bias DV and the dither value DT that falls within a predetermined target density range based on the detection result of the density of the test toner image T.

[0054] First, the control unit 9 determines the value of the developing bias DV at which the detected density ID value exceeds the lower limit of the reference density and the absolute value of the developing bias DV is the lowest among the test toner images T created with the dither value DT being DT1, i.e., the maximum value 840. Then, the control unit 9 sets (determines) the value of the developing bias DV thus determined as the value of the developing bias DV during the execution of the image forming process.

[0055] Then, the control unit 9 refers to the detected density of the test toner image T formed with the obtained value of the developing bias DV, and finds the condition of the dither value DT (the value of the dither value DT) that is below the lower limit of the reference density for each color. Then, the control unit 9 sets the value of the found dither value DT as the maximum value of the dither value to be applied when the image forming process is executed.

[0056] The above-mentioned processing by the control unit 9 will be described with reference to Figs. 7 and 8. Figs. 7 and 8 show the forming conditions and detected density values ​​of test toner images TK, TC, TM, and TY formed during a certain process condition adjustment processing for each color of toner. From Fig. 7, for example, in the case of black (K) toner, it can be seen that the detected density value of the test toner image TK formed under the conditions of a developing bias DV value of DV2 and a dither value DT value of DT2 is 1.3, which is within the target density range (1.3 to 1.5). On the other hand, in the case of magenta (M) toner, it can be seen that the detected density value of the test toner image TM formed under the conditions of a developing bias DV value of DV2 and a dither value DT value of DT2 is 1.2, which is below the target density range (1.3 to 1.5).

[0057] The control unit 9 sets the developing bias VD to be applied when performing the image forming process in the following order. First, the value of the developing bias DV at which the detected density ID value exceeds the lower limit of the reference density is confirmed for each color among the test toner images T created with the dither value DT set to DT1, i.e., the maximum dither value DT of 840. For example, in the case of black toner, the values ​​of the developing bias DV at which the detected density is equal to or greater than the lower limit of the target density are DV1, DV2, and DV3. Here, the values ​​of the developing bias DV, DV1, DV2, and DV3, are -450V, -420V, and -400V, respectively. Since these values ​​are the same as those described above, the specific voltages will not be shown and the following description will be given by symbols. In the case of cyan toner and yellow toner, the values ​​of the developing bias DV at which the detected density is equal to or greater than the lower limit of the target density are DV1, DV2, and DV3. On the other hand, in the case of magenta toner, the values ​​of the developing bias DV at which the detected density is equal to or greater than the lower limit of the target density are DV1 and DV2. In other words, the value of the developing bias DV required to satisfy the lower limit of the reference density is DV2 (-420V), which needs to have a larger voltage value (absolute value) than DV3 (-400V) for the other toners. By checking in this way, it is possible to identify (confirm) that the image developing device 2M corresponding to the magenta toner is the unit that is least likely to produce a density. It is also clear that even for the image developing device 2M, which is the unit that is least likely to produce a density, it is possible to form a toner image that satisfies the lower limit of the target density by setting the value of the developing bias DV to DT2.

[0058] Therefore, the control unit 9 sets the voltage value of the developing bias DV applied to each of the image developing devices 2K, 2C, 2M, and 2Y during the execution of the image forming process to DT2, which has the smallest absolute value.

[0059] Next, when the value of the developing bias DV applied during the image forming process is set to DV2, in the case of yellow toner (when the value of the dither value DT is DT1), the detected density ID value becomes 1.6, which exceeds the upper limit of the target density of 1.5. (It can be seen that the image developing device 2Y with yellow toner is more likely to produce a high density than other units.) Therefore, the control unit 9 checks the detected density of the test toner image T formed under the condition that the value of the developing bias DV is DV2, and obtains the value of the dither value DT for each color of toner, which makes the detected density equal to or lower than the upper limit of the target density. For example, in the case of yellow toner, it can be seen that when the value of the dither value DT is DT2, the density becomes the same value as the upper limit of the target value. Therefore, the control unit 9 sets the maximum value of the dither value DT applied to the image developing device 2Y for yellow toner during the image forming process to the value of DT2. The value of the dither value DT set for the image developing devices 2K, 2C, and 2M corresponding to the black toner, cyan toner, and magenta toner is DT1. This is because the density when the developing bias DV has a value of DV2 and the dither value DT has a value of DT2 falls within the target density range.

[0060] In this manner, the value of the developing bias DV supplied to each image developing device 2K, 2C, 2M, and 2Y and the individual dither values ​​DT (maximum values) applied to each image developing device 2K, 2C, 2M, and 2Y during image formation processing are set.

[0061] Even if there is variation in the developability of each toner due to the temperature and humidity of the environment around the developing devices 23K, 23C, 23M, and 23Y, the remaining amount of each toner, and the like, by setting the value of the developing bias DV and the value of the applied dither value DT (the maximum value), as described above, it is possible to prevent the density and gradation of the toner image transferred to the paper from varying depending on the color. In other words, even with an inexpensive configuration in which a common developing bias is supplied from the developing power source 3, it is possible to print with reduced variation in the density of each toner image formed for each image developing device 2K, 2C, 2M, and 2Y.

[0062] As shown in Figures 9 and 10, for an image developing device 2Y in which the density of the formed test toner image TY exceeds the target density range (1.3 to 1.5) even if the minimum dither value DT3 corresponding to the combination with the determined voltage value DV2 is used, the control unit 9 is configured to form a solid image (toner image) on the photosensitive drum 20Y by the developing device 23Y for a predetermined time, and then to send the solid image (toner image) formed on the photosensitive drum 20Y to a cleaning device having a cleaning member not shown in the figure that removes any adhering matter (toner image) on the photosensitive drum 20Y without transferring it to the intermediate transfer belt 42, and then toner image discharging, which is a process of removing the adhering matter (toner image) on the photosensitive drum 20Y, and then execute the process condition adjustment process again.

[0063] In each of the developing devices 23K, 23C, 23M, and 23Y, if the toner concentration inside is high, the toner may not be able to obtain a predetermined charge amount because sufficient friction opportunities with the carrier are obtained. In such a case, the density of the toner image obtained is high, so as described above, the amount of toner inside the corresponding developing device 23K, 23C, 23M, and 23Y can be reduced by discharging the toner. This allows sufficient friction opportunities with the toner carrier inside the developing device 23K, 23C, 23M, and 23Y, so that the toner can obtain a predetermined charge amount, and as a result, the density of the toner image can be reduced. Then, by executing the process condition adjustment process again, the appropriate voltage value DV2 and dither value DT3 are determined also in the image developing device 2Y, for example, as shown on the right side of the arrow in Figures 9 and 10.

[0064] 11 and 12, the control unit 9 is configured to prompt the image developing device 2M to replenish toner because the density of the image developing device 2M in which the density of the test toner image TM is lowest among the selected combinations is below the target density range (1.3 to 1.5) even at the maximum dither value DT1. Then, by executing the process condition adjustment process again after the toner is replenished, an appropriate voltage value DV3 and dither value DT1 are determined for the image developing device 2M as well, for example, as shown to the right of the arrows in FIGS. 9 and 10.

[0065] The process condition adjustment process of the control unit 9 configured as above will be described with reference to a flowchart.

[0066] As shown in FIG. 13, in step S1, the control unit 9 detects the density ID values ​​of the test toner images TK, TC, TM, and TY transferred to the intermediate transfer belt 42 using the image sensor 10, and stores them in the storage unit 92.

[0067] In step S2, the control unit 9 obtains the lower limit value of the developing bias DV (=DVB “DVB_K_IDlow”, “DVB_C_IDlow”, “DVB_M_IDlow”, “DVB_Y_IDlow”) for each color, where the density of the test toner image T formed with the dither value DT being DT1 is equal to or higher than the lower limit value “ID lower thresh” of the target value. In the example of FIG. 8, the lower limit value “DVB_K_IDlow” of the developing bias DV for black is DV3, the lower limit value “DVB_C_IDlow” of the developing bias DV for cyan is DV3, the lower limit value “DVB_M_IDlow” of the developing bias DV for magenta is DV2, and the lower limit value “DVB_Y_IDlow” of the developing bias DV for yellow is DV3. In other words, it can be seen that the density of the test toner image T formed with the dither value DT of DT1 can be made to exceed the lower limit of the target density for all colors by setting the value of the development bias DV to DV2 or greater.

[0068] Next, in step S3, the control unit 9 checks whether the value of the developing bias DV for each color obtained in step S2 ("DVB_K_IDlow", "DVB_C_IDlow", "DVB_M_IDlow", "DVB_Y_IDlow") is equal to or less than DV1, which is the maximum value of the developing bias DV. In other words, it checks whether the value of the developing bias DV does not reach the lower limit of the target density even if it is DV1, which is the maximum value.

[0069] In step S3, if the values ​​of the developing bias DV for each color obtained in step S2 ("DVB_K_IDlow", "DVB_C_IDlow", "DVB_M_IDlow", "DVB_Y_IDlow") are all below DV1 (Y in S3), the control unit 9 proceeds to step S4, and if there is a color that does not reach the lower limit of the target density even if the value of the developing bias DV is DV1 (N in S3), the control unit 9 proceeds to process A. In step 4, the control unit 9 determines the developing bias DV (=DVB_MIN_IDlow) to be used during image formation with the smallest absolute value among the developing biases DV for each color obtained in step S2 ("DVB_K_IDlow", "DVB_C_IDlow", "DVB_M_IDlow", "DVB_Y_IDlow").

[0070] After that, in step S5, the control unit 9 checks the density of the test toner image T formed with the smallest dither value DT value "DITHlower thresh" under the bias condition (=DVB_MIN_IDlow) with the smallest absolute value among the developing biases DV for each color obtained in step S2, and stores them as "ID_K_Dlt", "ID_C_Dlt", "ID_M_Dlt", and "ID_Y_Dlt" in the storage unit 92. Explaining with the example of FIG. 8, the densities of the test toner image T formed under the condition of the developing bias DV value of DV2 and the dither value DT value of DT3 are 1.2, 1.3, 1.1, and 1.4 for black, cyan, magenta, and yellow, respectively.

[0071] When the control unit 9 confirms that all of "ID_K_Dlt", "ID_C_Dlt", "ID_M_Dlt", and "ID_Y_Dlt" calculated in step S6 are equal to or less than the upper limit of the target density (ID upper thresh) of 1.5 (Y in S6), it proceeds to step 7, and if even one of them exceeds the upper limit of the target density, it proceeds to process B. Then, in step S7, it calculates the dither values ​​DT "ID_K_Dlt", "ID_C_Dlt", "ID_M_Dlt", and "ID_Y_Dlt" that bring the density closest to the maximum value of 1.5 for each color.

[0072] Then, in step S8, the values ​​of the dither value DT, "ID_K_Dlt", "ID_C_Dlt", "ID_M_Dlt", and "ID_Y_Dlt", calculated for each color in step S6, are set to "DT_K_IDup", "DT_C_IDup", "DT_M_IDup", and "DT_Y_IDup", which are the values ​​of the dither value DT to be applied to each color during image formation. In the example of FIG. 8, the values ​​of "DT_K_IDup", "DT_C_IDup", "DT_M_IDup", and "DT_Y_IDup" are 840, 840, 840, and 740, respectively. Here, the value of the developing bias DV applied during image formation is set to the value of "DVB=DVB_MIN_IDlow" calculated in step S4, which is DV2 (=-420V) in FIG. 8. As described above, by individually setting the developing bias DV and the dither value DT applied to each color, the density of the solid image of each color can be kept within a predetermined range (density ID 1.3 to 1.5). In other words, the density variation can be controlled within an allowable range.

[0073] If there are image developing devices 2K, 2C, 2M, and 2Y that do not satisfy the conditions in step S3 of FIG. 13 (N in S3), the process proceeds to process A. In step S9 of process A shown in FIG. 14, an image developing device that falls below the lower limit of the target density of 1.3 is identified. Then, in the next step S10, toner is replenished to the identified image developing device, and the process returns to step S1 of FIG. 13. In the developing device 23 of an image developing device that falls below the lower limit of the target density of 1.3, the amount of toner is often reduced below the appropriate value. Therefore, by replenishing toner to the target developing device 23, the required density can be achieved.

[0074] Also, if there are image developing devices 2K, 2C, 2M, and 2Y that do not satisfy the conditions in step S6 of FIG. 13 (N in S6), the process proceeds to process B. In step S11 shown in FIG. 15, image developing devices that exceed the upper limit value 1.5 of the target density are identified. Then, in the next step S12, a solid image having a predetermined length in the circumferential direction of the photosensitive drum 20, for example a length of 100 mm, is developed once or multiple times for the identified image developing device, and the developed toner image is collected by a cleaner device without intermediate transfer to the intermediate transfer belt 42, that is, a so-called solid discharge is performed. After the execution of step S12, the process returns to step S1 of FIG. 13. In the developing device 23 of the image developing device that exceeds the upper limit value 1.5 of the target density, the amount of toner increases more than the appropriate value, and the charge amount of the toner is often lower than the appropriate value due to the decrease in the frequency of friction with the carrier. Therefore, by performing solid discharge, the toner in the target developing device 23 is forcibly consumed and reduced. This increases the frequency of friction between the carrier and each toner particle, making it possible to adjust the charge amount of the toner to an appropriate value, and thus making it possible to prevent the density from exceeding the upper limit of the target density.

[0075] [Embodiment 2] In the above-described embodiment, the control unit 9 adjusts the charge amount of the toner by discharging the toner, but the present invention is not limited to such a configuration. For example, in the example shown in FIG. 9 and FIG. 10, even if the dither value DT3 is the minimum, the density of the test toner image T formed by the image developing device 2Y exceeds the target density upper limit value 1.5, but the amount of exposure light to the photoconductor drum 20Y of the exposure device 22Y corresponding to the image developing device 2Y may be reduced. This reduction in the amount of exposure light can be achieved by reducing the light intensity of the light emitting unit of the exposure device 22. This is because the surface potential of the exposed area of ​​the photoconductor drum 20Y becomes, for example, from −100V to −150V, and the difference with the development bias DV applied to the development roller 234Y becomes smaller, and the amount of toner adhering to the exposed area decreases, so that the density of the toner image can be reduced. In this way, the light amount of the light emitting unit of the corresponding exposure device 22 is adjusted, and then the process condition adjustment process is executed again, so that the appropriate voltage value DV2 and dither value DT3 may be set in the image developing device 2Y as well.

[0076] In the above-mentioned embodiments, the surfaces of the photoconductor drums 20K, 20C, 20M, and 20Y are charged to a potential of -600V, but this value is an example. In addition, three voltage values ​​DV1, DV2, and DV3, that is, -450V, -420V, and -400V of the developing bias DV are applied to the developing rollers 234K, 234C, 234M, and 234Y, respectively, but this value is also an example. These potentials and developing bias values ​​may be variably determined according to the temperature and humidity of the surrounding environment of the developing devices 23K, 23C, 23M, and 23Y, the remaining amount of each toner, the charging property, and the like. In any case, it is preferable to set the potential difference between the photoconductor drums 20K, 20C, 20M, and 20Y and the developing rollers 234K, 234C, 234M, and 234Y to be about 100V to 200V.

[0077] The configurations disclosed in any of the above-described embodiments may be applied in combination with configurations disclosed in other embodiments, provided that no inconsistencies arise. Furthermore, the embodiments disclosed in this specification are merely examples, and the embodiments of the present disclosure are not limited thereto, and may be modified as appropriate within the scope of the present disclosure. [Industrial Applicability]

[0078] The present disclosure can be used as a tandem type color image forming apparatus in a copying machine, a multifunction machine, a printer, a facsimile machine, and the like. [Explanation of symbols]

[0079] 1: Color image forming device 2: Image developing device (image developing means) 20: Photoconductor drum (image carrier) 21: Charger 22: Exposure device (exposure means) 23: Developing device 231:Developer tank 232 : Stirring member 233: Toner supply member 234: Developing roller 3: Development power supply 4: Transfer device (transfer means) 41: Primary transfer roller 42: Intermediate transfer belt 43: Drive roller 44: Driven roller 45: Secondary transfer roller 9: Control section 10: Image sensor 91: Processing unit 92: Storage section 93: Dither processing section 94: Input section 95: Output section DT: Dither value DT1: Dither value DT2: Dither value DT3: Dither value DV: Voltage value DV1: Voltage value DV2: Voltage value DV3: Voltage value G: Toner patch group P:Printing paper T: Test toner image

Claims

1. a plurality of image developing means each including an image carrier and a developing means for developing a toner image on the image carrier; a transfer means including a transfer member to which each of the toner images developed on each of the image carriers is transferred; a developing power source for supplying a common developing bias to each of the developing means, a control unit configured to be capable of executing an image forming process for forming the toner image based on an arbitrary print image signal by controlling each of the image developing means, and to execute a process condition adjustment process for adjusting a voltage value of the developing bias by controlling the developing power source; a dither processing unit capable of executing image processing with a plurality of different dither values ​​having a predetermined gradation for each of the image developing units; a density sensor for detecting the density of each of the toner images transferred to the transfer member, The control unit, when performing the process condition adjustment process, an image forming apparatus configured to cause each of the image developing means to form a toner patch group including a plurality of test toner images corresponding to combinations of a plurality of different voltage values ​​and a plurality of different dither values ​​based on a print image signal corresponding to a reference density, and to determine the voltage value at the time of execution of the image formation process and the dither value to be applied to each of the image developing means by the dither processing unit based on the density of each of the plurality of test toner images detected by the density sensor.

2. the dither processing unit is configured to perform image processing on the test toner image with a plurality of dither values ​​in a range from a predetermined maximum dither value to a predetermined minimum dither value; the control unit, when performing the process condition adjustment process, selects a combination of the voltage value and the dither value in the toner patch group corresponding to the maximum dither value, such that the density of the test toner image detected by the density sensor falls within a predetermined target density range; If there is a voltage value among the selected combinations that causes the density of the image developing means to be the lowest and fall within the target density range, the voltage value having the smallest absolute value among the voltage values ​​is determined as the voltage value during execution of the image forming process, and 2. The image forming apparatus according to claim 1, further comprising: a dither value that, among the dither values ​​corresponding to the combinations with the determined voltage values, results in the highest density of the test toner image formed within the target density range; and a dither value that results in the highest density of the test toner image formed within the target density range.

3. the toner constituting the toner image is non-magnetic, and the developing means contains a magnetic carrier; The control unit, for an image developing unit in which the density of the formed test toner image exceeds the target density range even if the minimum dither value corresponding to the combination with the determined voltage value is used, 3. The image forming apparatus according to claim 2, further comprising: a step of causing the image developing means to discharge the toner image for a predetermined period of time, and then executing the process condition adjustment process again.

4. Each of the image developing units includes an exposure unit for exposing each of the image carriers to light, The control unit, for an image developing unit in which the density of the formed test toner image exceeds the target density range even if the minimum dither value corresponding to the combination with the determined voltage value is used, 3. The image forming apparatus according to claim 2, further comprising: a step of reducing an amount of exposure light to the image carrier from the exposure means provided in the image developing means, and then executing the process condition adjustment process again.

5. The control unit, for an image developing unit in which the density of the test toner image of the image developing unit detected as the lowest among the selected combinations is below the target density range even if the density is a maximum dither value, 5. The image forming apparatus according to claim 2, further comprising: a step of executing the process condition adjustment process again after the image developing means is replenished with toner.

Images