Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent occurrence of transfer failure with a simple configuration.SOLUTION: An image forming apparatus comprises an image forming unit, a transfer member, a density detection device, and a control unit. The image forming unit includes an image carrier, a charging device, an exposure device, and a developing device. The control unit can execute a printing mode and a calibration mode. The printing mode is to perform image formation according to a predetermined image formation condition. The calibration mode is to maintain image quality of a toner image. During the execution of the calibration mode, the control unit detects a change in the density of the toner image when a developing voltage to be applied is changed, for each of the different transfer voltages, and adjusts the developing voltage set in the image formation condition on the basis of a correlation between each of the developing voltages to be applied and the density of the toner image.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile, or a combination machine thereof, that is equipped with an image carrier. [Background technology]

[0002] A conventional image forming apparatus is disclosed in Patent Document 1. This image forming apparatus includes an image forming unit, a density detection device, and a transfer member. The image forming unit includes an image carrier, a charging device, an exposure device, and a developing device.

[0003] The image carrier has a photosensitive layer formed on its surface. The charging device charges the image carrier. The exposure device forms an electrostatic latent image by exposing the image carrier charged by the charging device. The developing device has a developer carrier, and forms a toner image by applying a predetermined development voltage to the developer carrier and attaching toner to the electrostatic latent image formed on the image carrier. The developer carrier is disposed opposite the image carrier, and carries a two-component developer containing a magnetic carrier and toner.

[0004] The transfer member is disposed opposite the image carrier, and applies a predetermined transfer voltage to the transferee member to perform primary transfer of the toner image formed on the image carrier. The toner image transferred to the transferee member is then secondary transferred to the transfer paper. The concentration detection device detects the concentration of the toner image transferred to the transferee member and the concentration of the toner image transferred to the transfer paper.

[0005] The developing voltage during print mode execution is adjusted based on the density of the primary transferred toner image detected by the density detection device and the density of the secondary transferred toner image, thereby preventing toner aggregation on the image carrier and suppressing the occurrence of white spots on the transfer target. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2011-59357 A Summary of the Invention [Problem to be solved by the invention]

[0007] In the configuration of Patent Document 1, the density detection device requires multiple sensors to detect the density of the primary transferred toner image and the density of the secondary transferred toner image, and there is a problem that the complex structure increases manufacturing costs.

[0008] SUMMARY OF THE PRESENT DISCLOSURE In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of preventing the occurrence of transfer defects with a simple configuration. [Means for solving the problem]

[0009] In order to achieve the above object, a first configuration of the present invention is an image forming apparatus including an image forming section, a transfer member, a concentration detection device, and a control section. The image forming section includes an image carrier, a charging device, an exposure device, and a developing device. The image carrier has a photosensitive layer formed on its surface. The charging device charges the image carrier. The exposure device exposes the image carrier charged by the charging device to form an electrostatic latent image. The developing device has a developer carrier, and forms a toner image by applying a predetermined development voltage to the developer carrier and attaching toner to the electrostatic latent image formed on the image carrier. The developer carrier is disposed opposite the image carrier and carries a two-component developer containing a magnetic carrier and a toner. The transfer member is disposed opposite the image carrier and transfers the toner image formed on the image carrier to a transferee by applying a predetermined transfer voltage. The concentration detection device detects the concentration of the toner image formed by the developing device. The control section controls the image forming section, the transfer member, and the concentration detection device. The control unit can execute a print mode and a calibration mode. In the print mode, an image is formed under predetermined image forming conditions. In the calibration mode, the image quality of the toner image is maintained. When executing the calibration mode, the control unit detects a change in density of the toner image at different transfer voltages when the development voltage is changed, and adjusts the development voltage set in the image forming conditions based on the correlation between the development voltage and the density of the toner image. Effect of the Invention

[0010] According to the first aspect of the present invention, it is possible to provide an image forming apparatus capable of preventing the occurrence of transfer defects with a simple configuration. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a side cross-sectional view showing an internal configuration of an image forming apparatus 100 according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a side cross-sectional view of a developing device 3a mounted in an image forming apparatus 100 according to an embodiment of the present invention. [Diagram 3]FIG. 1 is a partial enlarged view of the periphery of an image forming unit Pa including a control path in a developing device 3a of an image forming apparatus 100 according to an embodiment of the present invention; [Figure 4] Graph showing the correlation between the applied developing voltage and the density of the toner image transferred to the intermediate transfer belt 8 at different transfer voltages. [Diagram 5] Graph showing the correlation between the applied developing voltage and the density of the toner image transferred to the intermediate transfer belt 8 at different transfer voltages. [Figure 6] 1 is a flowchart showing an example of execution of a calibration mode in an image forming apparatus 100 according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view showing the internal structure of an image forming apparatus 100 according to an embodiment of the present invention.

[0013] The image forming apparatus 100 includes image forming sections Pa to Pd, primary transfer rollers (transfer members) 6a to 6d, an image density sensor (density detection device) 40, and a main control section (control section) 80.

[0014] The four image forming units Pa, Pb, Pc, and Pd are disposed in order from the upstream side in the transport direction (the left side in FIG. 1) inside the main body of image forming apparatus 100 (here, a color printer). The image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, magenta, cyan, and black), and sequentially form images of yellow, magenta, cyan, and black through the processes of charging, exposing, developing, and transferring, respectively.

[0015] The image forming units Pa to Pd each include photoconductor drums (image carriers) 1a to 1d, charging devices 2a to 2d, an exposure device 5, and developing devices 3a to 3d. The image forming units Pa to Pd each include photoconductor drums (image carriers) 1a, 1b, 1c, and 1d that carry visible images (toner images) of each color.

[0016] The intermediate transfer belt (transferee) 8 is rotated counterclockwise in FIG. 1 by a driving means (not shown). The intermediate transfer belt (transferee) 8 is provided adjacent to each of the image forming units Pa to Pd. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is mainly used.

[0017] The primary transfer rollers (transfer members) 6a to 6d are disposed opposite the photosensitive drums (image carriers) 1a to 1d. A predetermined transfer voltage is applied to the primary transfer rollers (transfer members) 6a to 6d, whereby the visible images (toner images) of each color formed on the photosensitive drums (image carriers) 1a to 1d are transferred to the intermediate transfer belt (transferee) 8. As a result, the toner images formed on the photosensitive drums 1a to 1d are sequentially primarily transferred and superimposed on the intermediate transfer belt 8, which moves while contacting the photosensitive drums 1a to 1d.

[0018] Thereafter, the toner image primarily transferred onto the intermediate transfer belt 8 is secondarily transferred onto a transfer paper P, which is an example of a recording medium, by a secondary transfer roller 9. Furthermore, the transfer paper P onto which the toner image has been secondarily transferred is discharged from the main body of the image forming apparatus 100 after the toner image has been fixed in a fixing unit 13. The photoconductor drums 1a to 1d rotate in a clockwise direction in FIG. 1. This causes an image forming process to be performed on each of the photoconductor drums 1a to 1d.

[0019] The transfer paper P onto which the toner image is secondarily transferred is stored in a paper cassette 16 arranged at the bottom of the main body of the image forming apparatus 100. The transfer paper P is transported to a nip portion between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8 via a paper feed roller 12a and a pair of registration rollers 12b. In addition, a blade-shaped belt cleaner 19 is arranged downstream of the secondary transfer roller 9 to remove toner and the like remaining on the surface of the intermediate transfer belt 8.

[0020] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photoconductor drums 1a to 1d, charging devices 2a to 2d, exposure device 5, developing devices 3a to 3d, and cleaning devices 7a to 7d are provided.

[0021] The charging devices 2a to 2d charge the photoconductor drums (image carriers) 1a to 1d. The exposure device 5 exposes the photoconductor drums (image carriers) 1a to 1d charged by the charging devices 2a to 2d to light, thereby forming electrostatic latent images.

[0022] The developing devices 3a to 3d each have a developing roller (developer carrier) 31, and form a toner image by applying a predetermined developing voltage to the developing roller (developer carrier) 31 to cause toner to adhere to the electrostatic latent image formed on the photosensitive drum (image carrier) 1a to 1d. The developing roller (developer carrier) 31 is disposed opposite the photosensitive drum (image carrier) 1a to 1d, and carries a two-component developer containing a magnetic carrier and toner.

[0023] The cleaning devices 7a to 7d remove residual developer (toner) and the like remaining on the photosensitive drums 1a to 1d.

[0024] When image data is input from a host device such as a personal computer, first, the surfaces of the photoconductor drums 1a-1d are uniformly charged by the charging devices 2a-2d. Next, the exposure device 5 irradiates light according to the image data, and an electrostatic latent image according to the image data is formed on each of the photoconductor drums 1a-1d. The developing devices 3a-3d are filled with a predetermined amount of two-component developer containing toner of each color, yellow, magenta, cyan, and black.

[0025] When the ratio of toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a specified value due to the formation of a toner image described later, toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in the developer is supplied onto the photoconductor drums 1a to 1d by the developing devices 3a to 3d, and electrostatically adheres to the photoconductor drums 1a to 1d, forming a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5.

[0026] An electric field is applied between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d by the primary transfer rollers 6a-6d at a predetermined transfer voltage. As a result, the yellow, magenta, cyan and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed with a predetermined positional relationship that is determined in advance for forming a predetermined full-color image. After that, in preparation for the subsequent formation of a new electrostatic latent image, the toner and the like remaining on the surfaces of the photosensitive drums 1a-1d after the primary transfer are removed by the cleaning devices 7a-7d.

[0027] The intermediate transfer belt 8 is stretched over a driven roller 10 on the upstream side and a driving roller 11 on the downstream side, and when the intermediate transfer belt 8 starts to rotate counterclockwise with the rotation of the driving roller 11 by a driving motor (not shown), the transfer paper P is transported from the registration roller pair 12b to a nip portion (secondary transfer nip portion) between the driving roller 11 and a secondary transfer roller 9 provided adjacent thereto at a predetermined timing, and the full-color image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P. The transfer paper P to which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0028] The transfer paper P transported to the fixing section 13 is heated and pressurized by the fixing roller pair 13a, so that the toner image is fixed to the surface of the transfer paper P, and a predetermined full-color image is formed. The transfer paper P on which the full-color image has been formed is then transported in a different direction by the branching section 14, which branches into multiple directions, and is discharged directly (or after being sent to the double-sided transport path 18 and having images formed on both sides) onto the discharge tray 17 by the discharge roller pair 15.

[0029] Furthermore, an image density sensor (density detection device) 40 is disposed at a position facing the drive roller 11 with the intermediate transfer belt 8 in between. As the image density sensor 40, an optical sensor equipped with a light-emitting element such as an LED and a light-receiving element such as a photodiode is generally used. When measuring the amount of toner adhesion on the intermediate transfer belt 8, when the light-emitting element irradiates each reference image formed on the intermediate transfer belt 8 with measurement light, the measurement light is incident on the light-receiving element as light reflected by the toner and light reflected by the belt surface.

[0030] The light reflected from the toner and the belt surface includes specularly reflected light and diffusely reflected light. This specularly reflected light and diffusely reflected light are separated by a polarizing separation prism and then enter separate light receiving elements. Each light receiving element photoelectrically converts the received specularly reflected light and diffusely reflected light and outputs an output signal to the main control unit 80 (see FIG. 3). The density of the toner image is then detected from the characteristic changes of the output signals of the specularly reflected light and diffusely reflected light.

[0031] That is, an image density sensor (density detection device) 40 detects the density of the toner image transferred to the intermediate transfer belt (transfer receiving body) 8. The density of the toner image detected by the image density sensor (density detection device) 40 is compared with a predetermined reference density to adjust the characteristic value of the developing voltage, etc. This performs correction (calibration) of the toner density for each color. Note that the calibration will be described in detail later.

[0032] 2 is a side cross-sectional view of the developing device 3a mounted on the image forming apparatus 100. In the following description, the developing device 3a arranged in the image forming section Pa in FIG. 1 is illustrated as an example, but the configurations of the developing devices 3b to 3d arranged in the image forming sections Pb to Pd are basically the same, so description thereof will be omitted.

[0033] The developing device 3a includes a developing container 20 that contains a two-component developer (hereinafter, simply referred to as developer) containing a magnetic carrier and a toner, and the developing container 20 is partitioned by a partition wall 20a into a stirring and conveying chamber 21 and a supply and conveying chamber 22. In the stirring and conveying chamber 21 and the supply and conveying chamber 22, a stirring and conveying screw 25a and a supply and conveying screw 25b are rotatably disposed, respectively, for mixing and stirring the toner supplied from a toner container 4a (see FIG. 1) with the magnetic carrier, and charging the toner.

[0034] The developer is then stirred and transported in the axial direction (perpendicular to the paper surface of Figure 2) by the stirring and transporting screw 25a and the supply transporting screw 25b, and circulates between the stirring and transporting chamber 21 and the supply transporting chamber 22 via developer passages (not shown) formed at both ends of the partition wall 20a.

[0035] 2, a developing roller (developer carrier) 31 is disposed diagonally above and to the right of the supply transport screw 25b within the developing container 20. A part of the outer circumferential surface of the developing roller 31 is exposed from an opening 20b of the developing container 20 and faces the photoconductor drum 1a. The developing roller 31 rotates counterclockwise in FIG.

[0036] The developing roller 31 is composed of a cylindrical developing sleeve that rotates counterclockwise in Fig. 2, and a magnet (not shown) having multiple magnetic poles fixed inside the developing sleeve. Note that, although a developing sleeve with a knurled surface is used here, it is also possible to use a developing sleeve with a large number of concave shapes (dimples) formed on the surface, a developing sleeve with a blasted surface, or even a developing sleeve that has been blasted in addition to being knurled or having concave shapes formed, or a developing sleeve that has been plated.

[0037] In addition, a regulating blade 27 is attached to the developing container 20 along the longitudinal direction of the developing roller 31 (perpendicular to the paper surface of FIG. 2). A small gap is formed between the tip of the regulating blade 27 and the surface of the developing roller 31.

[0038] A developing voltage made up of a direct current voltage Vslv (DC) (hereinafter also referred to as Vdc) and an alternating current voltage Vslv (AC) is applied to the developing roller 31 by a developing voltage power supply 43 (see FIG. 3).

[0039] 3 is a partially enlarged view of the periphery of image forming unit Pa, including the control path of developing unit 3a. In the following description, the configuration of image forming unit Pa and the control path of developing unit 3a will be described, but the configurations of image forming units Pb to Pd and the control paths of developing units 3b to 3d are similar, so their description will be omitted.

[0040] The developing roller 31 is connected to a developing voltage power supply 43 that generates an oscillating voltage in which a DC voltage and an AC voltage are superimposed. The developing voltage power supply 43 includes an AC constant voltage power supply 43a and a DC constant voltage power supply 43b. The AC constant voltage power supply 43a outputs a sine wave AC voltage generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer (not shown). The DC constant voltage power supply 43b outputs a DC voltage obtained by rectifying a sine wave AC voltage generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer.

[0041] During image formation, the developing voltage power supply 43 outputs a developing voltage in which an AC voltage is superimposed on a DC voltage from the AC constant voltage power supply 43a and the DC constant voltage power supply 43b. The current detection unit 44 detects the value of a DC current flowing between the developing roller 31 and the photosensitive drum 1a.

[0042] The charging voltage power supply 45 applies a charging voltage in the form of an AC voltage superimposed on a DC voltage to the charging roller 34 of the charging device 2a. The configuration of the charging voltage power supply 45 is similar to that of the developing voltage power supply 43. The transfer voltage power supply 47 applies a primary transfer voltage (transfer voltage) and a secondary transfer voltage to the primary transfer rollers (transfer members) 6a to 6d and the secondary transfer roller 9 (see FIG. 1), respectively.

[0043] The cleaning device 7a includes a cleaning blade 32 that removes residual toner from the surface of the photosensitive drum 1a, a rubbing roller 33 that removes residual toner from the surface of the photosensitive drum 1a and also rubs and polishes the surface of the photosensitive drum 1a, and a transport spiral 35 that discharges the residual toner removed from the photosensitive drum 1a by the cleaning blade 32 and the rubbing roller 33 to the outside of the cleaning device 7a.

[0044] Next, the control system of the image forming apparatus 100 will be described with reference to Fig. 3. The image forming apparatus 100 is provided with a main control unit (control unit) 80 composed of a CPU and the like. The main control unit 80 is connected to a storage unit 70 composed of a ROM, RAM and the like. The storage unit 70 stores in advance programs for executing a normal print mode and a calibration mode.

[0045] The main control unit 80 controls each unit of the image forming apparatus 100 (such as the charging devices 2a-2d, the developing devices 3a-3d, the exposure device 5, the primary transfer rollers 6a-6d, the cleaning devices 7a-7d, the secondary transfer roller 9, the fixing unit 13, the developing voltage power supply 43, the current detection unit 44, the charging voltage power supply 45, the transfer voltage power supply 47, and the voltage control unit 50) based on the control program and the control data stored in the storage unit 70. That is, the main control unit (control unit) 80 controls the image forming units Pa-Pd, the primary transfer rollers (transfer members) 6a-6d, and the image density sensor (density detection device) 40.

[0046] The voltage control unit 50 controls a development voltage power supply 43 that applies a development voltage to the development roller 31, a charging voltage power supply 45 that applies a charging voltage to the charging roller 34, and a transfer voltage power supply 47 that applies a transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9. The voltage control unit 50 may be configured by a control program stored in the storage unit 70.

[0047] A liquid crystal display unit 90 and a transmission / reception unit 91 are connected to the main control unit 80. The liquid crystal display unit 90 functions as a touch panel for the user to perform various settings of the image forming apparatus 100, and also displays the state of the image forming apparatus 100, the image formation status, the number of printed sheets, etc. The transmission / reception unit 91 communicates with the outside world using a telephone line or an Internet line.

[0048] The main control unit 80 can execute a print mode and a calibration mode. In the print mode, normal image formation is performed under predetermined image formation conditions (predetermined charging voltage, developing voltage, and transfer voltage). The calibration mode is a mode that is performed separately from the print mode in order to maintain the image quality of the toner image.

[0049] In the image forming apparatus 100, when the calibration mode is executed, the developing voltage is changed while the transfer voltage applied to the primary transfer rollers (transfer members) 6a to 6d is kept constant. At this time, the density of the toner image detected by the image density sensor (density detection device) 40 increases in proportion to the increase in the developing voltage.

[0050] In addition, when the development voltage is increased to a predetermined value, coagulated toner is likely to occur on the photoconductor drums (image carriers) 1a to 1d. As a result, the toner around the coagulated toner is not transferred during transfer, and white spots are likely to occur in the toner image transferred to the intermediate transfer belt 8. As a result, when the development voltage exceeds a predetermined value, the rate of increase in the density of the toner image relative to the increase in the development voltage decreases.

[0051] In this embodiment, the main control unit (control unit) 80 detects the change in density of the toner image at different transfer voltages when the development voltage is changed during execution of the calibration mode. In addition, the main control unit (control unit) 80 adjusts the development voltage set in the image formation conditions of the print mode based on the correlation between the development voltage and the density of the toner image. Hereinafter, a method for adjusting the development voltage set in the image formation conditions of the print mode, which is a characteristic part of the present invention, will be described.

[0052] Specifically, the different transfer voltages applied when the calibration mode is executed are a transfer voltage set in the image forming conditions (hereinafter referred to as the print transfer voltage) and a transfer voltage lower than the print transfer voltage (hereinafter referred to as the low transfer voltage). In this embodiment, the print transfer voltage is a standard transfer voltage set in accordance with the image forming conditions stored in advance in the storage unit 70.

[0053] The change in density of the toner image transferred to the intermediate transfer belt 8 when the transfer voltage is fixed to the print transfer voltage and the development voltage is changed is detected by an image density sensor (density detection device) 40. Similarly, the change in density of the toner image transferred to the intermediate transfer belt 8 when the transfer voltage is fixed to the low transfer voltage and the development voltage is changed is detected by the image density sensor (density detection device) 40.

[0054] 4 and 5 are graphs showing the correlation between the developing voltage and the density of the toner image transferred to the intermediate transfer belt 8 at different transfer voltages. The first graph A is obtained from the correlation between the developing voltage and the density of the toner image at the print transfer voltage. The second graph B is obtained from the correlation between the developing voltage and the density of the toner image at the low transfer voltage.

[0055] The first graph A and the second graph B have singular points Sa and Sb, respectively. The singular points Sa and Sb are points at which the rate of increase in density of the toner image decreases with an increase in the development voltage. When a development voltage higher than the singular points Sa and Sb is applied, coagulated toner is likely to occur on the photoconductor drums (image carriers) 1a to 1d, and white spots are likely to occur in the toner image transferred to the intermediate transfer belt 8.

[0056] The target density D of the toner image is the density of the toner image that can be determined to be properly formed in the print mode, and if a density equal to or higher than the target density D is detected, it can be determined that the print mode is being executed properly.

[0057] In this embodiment, the main control unit (control unit) 80 acquires a first graph A and a second graph B by executing the calibration mode, and estimates a developing voltage Vd corresponding to a target density D based on the first graph A. It also estimates a developing voltage Vsb at a singular point Sb of the second graph B. It also compares the developing voltage Vd with the developing voltage Vsb to adjust the developing voltage set in the image formation conditions.

[0058] Specifically, when the developing voltage Vd is equal to or lower than the developing voltage Vsb, the developing voltage Vd is set as the image forming condition for the print mode. On the other hand, when the developing voltage Vd is higher than the developing voltage Vsb, the developing voltage Vsb is set as the image forming condition for the print mode. This makes it possible to adjust the developing voltage during execution of the print mode to an optimal value based on the change in density of the toner image detected by one image density sensor (density detection device) 40. Therefore, it is possible to provide an image forming apparatus 100 that can prevent transfer defects from occurring with a simple configuration. This makes it possible to maintain the image quality of the toner image by correcting the density of the toner image by executing the calibration mode.

[0059] 6 is a flowchart showing an example of the execution of the calibration mode in the image forming apparatus 100. The main control unit (control unit) 80 can execute the calibration mode. The calibration mode is executed, for example, when the cumulative number of printed sheets since the previous execution of the calibration mode is equal to or exceeds a predetermined number (50k sheets to 100k sheets).

[0060] When the calibration mode is executed, in step S1, the surfaces of the photoconductor drums 1a to 1d are charged by the charging devices 2a to 2d, and then the exposure device 5 exposes the surfaces of the photoconductor drums 1a to 1d to form an electrostatic latent image pattern of a reference image. Then, a predetermined development voltage is applied to the development roller 31 by the development voltage power source 43 to develop the electrostatic latent image into a toner image. As a result, a toner image is formed on the photoconductor drums 1a to 1d. At this time, the development voltage applied is set according to the image formation conditions previously stored in the storage unit 70.

[0061] In step S2, a predetermined primary transfer voltage (transfer voltage) is applied to each of the primary transfer rollers (transfer members) 6a to 6d by the charging voltage power supply 45, and the toner images formed on the photosensitive drums 1a to 1d are transferred to the intermediate transfer belt (transferee) 8. At this time, two different transfer voltages are applied as the predetermined primary transfer voltage. For one transfer voltage, a print transfer voltage set under the same image formation conditions as when the print mode is executed is used. For the other transfer voltage, a low transfer voltage lower than the print transfer voltage is used. As a result, a toner image applied with two different transfer voltages is formed on the intermediate transfer belt 8. The toner images are formed in the colors of yellow, magenta, cyan, and black under the same image formation conditions (e.g., charging conditions, exposure conditions) as when the print mode is executed.

[0062] In step S3, the density of the toner image formed on the intermediate transfer belt 8 is detected by an image density sensor (density detection device) 40.

[0063] In step S4, steps S1 to S3 are repeated multiple times by changing the development voltage applied to the development roller 31. At this time, for two different transfer voltages, the change in density of the toner image when the development voltage is changed is detected, and a first graph A and a second graph B are obtained from the correlation between the development voltage and the density of the toner image (see FIGS. 4 and 5).

[0064] In step S5, the main control unit 80 adjusts the developing voltage based on the first graph A and the second graph B acquired in step S4. In this embodiment, the developing voltage Vd corresponding to the target density D of the toner image transferred to the intermediate transfer belt 8 is estimated based on the first graph A. In addition, the developing voltage Vsb at the singular point Sb of the second graph B is estimated.

[0065] Next, the developing voltage Vd is compared with the developing voltage Vsb, and if the developing voltage Vd is equal to or lower than the developing voltage Vsb (see FIG. 4), it is determined that transfer defects such as white spots will not occur, and the developing voltage Vd is set as the image forming condition for the print mode.

[0066] Furthermore, when the developing voltage Vd is greater than the developing voltage Vsb (see FIG. 5), it is determined that there is a high possibility of transfer defects such as white spots occurring, and the developing voltage Vsb is set as the image forming condition for the print mode. At this time, an adjustment is made to lower the developing voltage during normal printing (when the print mode is being executed). As a result, the toner image transferred to the intermediate transfer belt 8 will have a lower density than the target density D, but it is possible to prevent transfer defects such as white spots from occurring. The main control unit 80 may also correct the density of the toner image by changing the image forming conditions other than the developing voltage. This allows the density of the toner image to be corrected, enabling stable output of high-quality images.

[0067] According to this embodiment, when the calibration mode is executed, the correlations (first graph A, second graph B) between the development voltage and the density of the toner image are detected at different transfer voltages, and the development voltage set in the image formation conditions is adjusted based on each correlation (first graph A, second graph B). This makes it possible to provide an image forming apparatus 100 that can prevent transfer defects with a simple configuration using an image density sensor (density detection device) 40 without the need to use multiple sensors.

[0068] In addition, the developing voltage during execution of the print mode is adjusted based on the first graph A and the second graph B. Specifically, the developing voltage Vd corresponding to the target density of the toner image estimated based on the first graph A is compared with the developing voltage Vsb at the singular point Sb of the second graph B, and when the developing voltage Vd is equal to or lower than the developing voltage Vsb, the developing voltage Vd is set as the image forming condition for the print mode. In addition, when the developing voltage Vd is greater than the developing voltage Vsb, the developing voltage Vsb is set as the image forming condition for the print mode. When the printing condition during normal printing (when the print mode is executed) is set to the developing voltage Vsb, an adjustment is made to lower the developing voltage during normal printing (when the print mode is executed). This makes it possible to more reliably prevent the occurrence of transfer defects such as white spots.

[0069] In addition, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. In the above-described embodiment, a color printer as shown in Fig. 1 was used as an example of image forming apparatus 100, but the image forming apparatus may be other image forming apparatuses such as monochrome and color copiers, digital multifunction machines, and facsimiles. [Industrial Applicability]

[0070] The present invention can be applied to an image forming apparatus that uses a two-component developer containing a toner and a carrier. [Explanation of symbols]

[0071] 1a to 1d Photoconductor drum (image carrier) 2a~2d Charging device 3a~3d developing device 4a~4d Toner container 5 Exposure equipment 6a to 6d Primary transfer roller (transfer member) 7a~7d Cleaning device 8 Intermediate transfer belt (transfer receiving material) 9 Secondary transfer roller 10 Driven roller 11 Drive roller 12a Paper feed roller 12b Registration roller pair 13 Fixing section 13a Fixing roller pair 14 Branch 15 Discharge Roller Pair 16 Paper Cassette 17 Output Tray 18-sided conveyor 19 Belt cleaner 20 Developing container 20a Partition wall 20b opening 21 Mixing and transport chamber 22 Supply and transport room 25a Mixing and conveying screw 25b Supply conveying screw 27 Regulatory Blade 31 Developing roller (developer carrier) 32 Cleaning blade 33 Rubbing roller 34 Charging roller 35 Conveying Spiral 40 Image density sensor (density detection device) 43 Development voltage power supply 43a AC constant voltage power supply 43b DC constant voltage power supply 44 Current detection section 45 Charge voltage power supply 47 Transfer voltage power supply 50 Voltage control section 70 Storage section 80 Main control unit (control unit) 90 LCD display section 91 Transmitter / receiver 100 Image forming device A Graph 1 B 2nd graph D Target concentration P Transfer paper Pa~Pd Image forming section Sa, Sb singularity Vd Development voltage Vsp Development voltage

Claims

1. an image carrier having a photosensitive layer formed on its surface; a charging device for charging the image carrier; an exposure device for forming an electrostatic latent image by exposing the image carrier charged by the charging device; a developing device that is disposed opposite to the image carrier and has a developer carrier that carries a two-component developer containing a magnetic carrier and a toner, and that applies a predetermined developing voltage to the developer carrier to cause the toner to adhere to the electrostatic latent image formed on the image carrier, thereby forming a toner image; an image forming unit including: a transfer member disposed opposite the image carrier and configured to transfer the toner image formed on the image carrier to a transfer target by application of a predetermined transfer voltage; a density detection device that detects the density of the toner image transferred to the transfer medium; a control unit that controls the image forming unit, the transfer member, and the concentration detection device; In an image forming apparatus comprising: the control unit is capable of executing a print mode in which an image is formed under a predetermined image forming condition and a calibration mode in which an image quality of the toner image is maintained; The control unit, when executing the calibration mode, detects the change in density of the toner image when the developing voltage is changed at different transfer voltages, and adjusts the developing voltage set in the image forming conditions based on the correlation between the developing voltage and the density of the toner image.

2. the control unit, during execution of the calibration mode, obtains a first graph from a correlation between the developing voltage and the density of the toner image transferred to the transfer medium at a predetermined transfer voltage set in the image forming conditions, and obtains a second graph from a correlation between the developing voltage and the density of the toner image transferred to the transfer medium at a transfer voltage lower than the predetermined transfer voltage set in the image forming conditions, 2. The image forming apparatus according to claim 1, wherein the developing voltage set in the image forming conditions is adjusted by comparing the developing voltage corresponding to the target density estimated based on the first graph with the developing voltage at a singular point of the second graph.

3. When the developing voltage corresponding to the target density of the toner image is equal to or lower than the developing voltage at a singular point of the second graph, the developing voltage corresponding to the target density of the toner image is set as the image forming condition during execution of the print mode; 3. The image forming apparatus according to claim 2, wherein when the developing voltage corresponding to the target density of the toner image is higher than the developing voltage at a singular point of the second graph, the developing voltage at the singular point of the second graph is set as the image forming condition.

Images