JP6319183B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that can execute density correction of an output image.

  In an image forming apparatus using an electrophotographic process, the density is corrected by transferring the toner directly onto the toner carrier when the apparatus is activated or when a mode (calibration mode) for setting the image density appropriately is set. It is common to form a pattern (reference image), detect the density, and perform density correction. For example, in the case of an image forming apparatus, the reference image of each color is formed on the image carrier by the image forming units of cyan, magenta, yellow, and black, and the density of the reference image transferred onto the toner carrier such as an intermediate transfer belt Is detected by the density detection sensor. Then, density correction is performed by changing the potential difference between the developing roller and the magnetic roller based on the detection result and setting the toner on the developing roller to the target layer thickness. A reflection type optical sensor is generally used as a density detection sensor for detecting the density of the reference image.

  An image forming apparatus that forms a density correction pattern (reference image) by transferring toner onto a conveyance belt (intermediate transfer member), detects the density, and corrects the density is disclosed in, for example, Patent Document 1. Yes.

JP 2005-308821 A

  By the way, the reflection type optical sensor has lower toner density detection accuracy when the toner density (toner adhesion amount) of the image is higher than when it is low density to intermediate density. For this reason, it is conceivable to perform density correction by forming a density correction pattern with a low toner density and detecting the toner density with a reflective optical sensor.

  However, in a developing device having a developing roller and a magnetic roller, if the potential difference between the developing roller and the magnetic roller is reduced in order to reduce the toner density of the density correction pattern, it is affected by the negative potential of the AC bias. The force that moves the toner from the developing roller to the magnetic roller works, and the amount of toner on the magnetic roller becomes excessive. If the development is continued in this state, the toner moved on the magnetic roller has a potential, so that excess toner on the magnetic roller moves to the developing roller. As a result, the amount of toner on the developing roller becomes excessive, and linearity is lost in the relationship between the potential difference between the developing roller and the magnetic roller and the toner layer thickness of the density correction pattern. As a result, there is a problem that the toner density detection accuracy is lowered when the toner density of the density correction pattern is low.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an image forming apparatus capable of improving the accuracy of density correction.

  In order to achieve the above object, an image forming apparatus according to a first configuration of the present invention is disposed to face an image carrier on which an electrostatic latent image is formed, and the image carrier in a region facing the image carrier. A developing roller for supplying toner to the developing roller, a developing roller disposed opposite to the developing roller, and a magnetic roller for supplying toner to the developing roller in a region facing the developing roller, and a DC voltage to the developing roller and the magnetic roller A voltage application unit that applies a developing bias with an alternating voltage superimposed thereon, a density detection sensor that detects the toner density of three or more density correction patterns formed using the development device, and controls the development device and the voltage application unit A control unit. The control unit determines that the first potential difference between the developing roller and the magnetic roller immediately before forming the first density correction pattern is greater than the second potential difference between the developing roller and the magnetic roller when forming the density correction pattern. The second potential difference when forming the first density correction pattern is increased to a minimum level value, the second potential difference when forming the second density correction pattern is set to the maximum level value, and the third density correction pattern Is set to an intermediate value between the minimum level value and the maximum level value.

  According to the first configuration of the present invention, the control unit determines the first potential difference between the developing roller and the magnetic roller immediately before forming the first density correction pattern as the developing roller when forming the density correction pattern. It is larger than the second potential difference between the magnetic roller. Thus, even when the second potential difference between the developing roller and the magnetic roller is reduced in order to form the density correction pattern with a low toner density, the first potential difference immediately before the density correction pattern is formed is increased. As a result, it is possible to suppress the toner from moving from the developing roller to the magnetic roller under the influence of the negative potential of the AC bias, and thus it is possible to suppress the toner amount on the magnetic roller from becoming excessive. . For this reason, the excess toner that has moved onto the magnetic roller moves again to the developing roller, the amount of toner on the developing roller becomes excessive, the second potential difference between the developing roller and the magnetic roller, and the toner of the density correction pattern Loss of linearity in relation to the layer thickness can be suppressed. As a result, even if the toner density of the density correction pattern is low, it is possible to prevent the toner density detection accuracy from being lowered, and thus the density correction precision can be improved.

  In addition, the control unit sets the second potential difference when forming the first density correction pattern to a minimum level value, sets the second potential difference when forming the second density correction pattern to a maximum level value, and sets the third potential difference to the third level. The second potential difference when forming the density correction pattern is set to an intermediate value between the minimum level value and the maximum level value. As a result, the linearity is more lost in the relationship between the second potential difference and the toner layer thickness of the density correction pattern than when the second potential difference when forming the second density correction pattern is an intermediate value. Therefore, the density correction accuracy can be further improved.

1 is a cross-sectional view schematically showing the structure of an image forming apparatus according to an embodiment of the present invention. 1 is a side sectional view showing a structure of a developing device according to an embodiment of the present invention. It is a block diagram which shows the control path | route of one Embodiment of this invention. FIG. 3 is a diagram illustrating a potential difference (first potential difference, second potential difference) between a developing roller and a magnetic roller when forming a density correction pattern in the image forming apparatus according to the embodiment of the present invention. It is a figure which shows the electric potential difference (1st electric potential difference, 2nd electric potential difference) between the developing roller at the time of forming a density | concentration correction pattern in the comparative example 1, and a magnetic roller. It is a figure which shows the relationship between the 2nd electric potential difference between a developing roller and a magnetic roller in Example 1 and Comparative Example 1, and the transmission density of a density | concentration correction pattern. It is a figure which shows the potential difference (1st potential difference, 2nd potential difference) when making the 1st potential difference just before forming each density correction pattern larger than the 2nd potential difference.

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

  An image forming apparatus 100 according to an embodiment of the present invention will be described with reference to FIGS. In the main body of the image forming apparatus 100 (here, a color printer), four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, and yellow are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  These image forming units Pa to Pd are provided with photosensitive drums (image carriers) 1a, 1b, 1c and 1d for carrying visible images (toner images) of the respective colors, and further, driving means (FIG. An intermediate transfer belt (intermediate transfer body, print medium) 8 that rotates clockwise in FIG. 1 is provided adjacent to each image forming portion Pa to Pd. The toner images formed on the photosensitive drums 1a to 1d are sequentially primary-transferred and superimposed on the intermediate transfer belt 8 that moves while contacting the photosensitive drums 1a to 1d. Thereafter, the toner image primarily transferred onto the intermediate transfer belt 8 is secondarily transferred onto a transfer paper P as an example of a recording medium by the action of the secondary transfer roller 9. Further, the transfer paper P on which the toner image is secondarily transferred is discharged from the main body of the image forming apparatus 100 after the toner image is fixed in the fixing unit 13. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG. Here, as the photosensitive drums 1a to 1d, amorphous silicon photosensitive members in which an amorphous silicon photosensitive layer is laminated on the outer peripheral surface of an aluminum drum tube are used.

  The transfer paper P on which the toner image is secondarily transferred is housed in a paper cassette 16 disposed at the lower part of the main body of the image forming apparatus 100, and is transferred to the secondary transfer roller via the paper feed roller 12a and the registration roller pair 12b. 9 and a nip portion between a later-described intermediate transfer belt 8 and a driving roller 11. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a (seamless) belt having no seam is mainly used. Further, a blade-like belt cleaner 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. Around and below the photosensitive drums 1a to 1d arranged to be rotatable, chargers 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d, and image information to each of the photosensitive drums 1a to 1d. The exposure device 5 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning parts 7a, 7b, 7c and 7d are provided.

  When image data is input from a host device such as a personal computer, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated according to the image data by the exposure device 5. The electrostatic latent images corresponding to the image data are formed on the respective photosensitive drums 1a to 1d. Each of the developing devices 3a to 3d is filled with a predetermined amount of a two-component developer containing toner of each color of cyan, magenta, yellow, and black. In addition, when the ratio of the toner in the two-component developer filled in each of the developing devices 3a to 3d is less than a predetermined value due to the formation of a toner image, which will be described later, each of the toner containers (toner storage containers) 4a to 4d The toner is supplied to the developing devices 3a to 3d. The toner in the developer is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and electrostatically adheres to the electrostatic latent image formed by the exposure from the exposure device 5. A toner image is formed.

  The primary transfer rollers 6a to 6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d, and cyan, magenta, yellow, and yellow on the photosensitive drums 1a to 1d. A black toner image is primarily transferred onto the intermediate transfer belt 8. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, in preparation for the subsequent formation of a new electrostatic latent image, the toner remaining on the surfaces of the photosensitive drums 1a to 1d after the primary transfer is removed by the cleaning units 7a to 7d.

  The intermediate transfer belt 8 is stretched over an upstream driven roller 10 and a downstream drive roller 11, and the intermediate transfer belt 8 rotates clockwise as the drive roller 11 is rotated by a drive motor (not shown). When the rotation starts, the transfer paper P is conveyed from the registration roller pair 12b to the nip portion (secondary transfer nip portion) between the drive roller 11 and the secondary transfer roller 9 provided adjacent thereto at a predetermined timing. The full color image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P. The transfer sheet P on which the toner image is secondarily transferred is conveyed to the fixing unit 13.

  The transfer paper P conveyed to the fixing unit 13 is heated and pressed by the fixing roller pair 13a to fix the toner image on 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 is formed is distributed in the transport direction by the branching portion 14 that branches in a plurality of directions. When an image is formed on only one side of the transfer paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller pair 15.

  On the other hand, when images are formed on both sides of the transfer paper P, a part of the transfer paper P that has passed through the fixing unit 13 is once protruded from the discharge roller pair 15 to the outside of the apparatus. Then, after the trailing edge of the transfer paper P has passed through the branch portion 14, the discharge roller pair 15 is rotated in the reverse direction and the transport direction of the branch portion 14 is switched. As a result, the transfer paper P is distributed from the rear end to the paper transport path 18 and is transported again to the secondary transfer nip portion with the image surface reversed. Then, the next toner image formed on the intermediate transfer belt 8 is secondarily transferred to the surface of the transfer paper P on which the image is not formed by the secondary transfer roller 9. The transfer paper P onto which the toner image has been secondarily transferred is conveyed to the fixing unit 13 where the toner image is fixed and then discharged onto the discharge tray 17.

  A density detection sensor 45 is disposed on the downstream side of the image forming unit Pd and the upstream side of the secondary transfer roller 9. The density detection sensor 45 irradiates the density correction pattern formed on the intermediate transfer belt 8 in the image forming units Pa to Pd with measurement light, receives the reflected light from the pattern, photoelectrically converts it, and outputs a light reception output signal The output value is A / D converted and then transmitted to the control unit 42 described later as a sensor output value (output value).

  As the density detection sensor 45, a reflection type optical sensor provided with a light emitting element composed of an LED or the like and a light receiving element composed of a photodiode or the like is generally used. When measuring the toner density of the density correction pattern, if the measurement light is sequentially irradiated from the light emitting element to each patch image on the intermediate transfer belt 8, the measurement light is reflected by the toner and the light reflected by the belt surface. Is incident on the light receiving element.

  When the toner adhesion amount is large, the reflected light from the belt surface is blocked by the toner, so that the light reception amount of the light receiving element is reduced. On the other hand, when the adhesion amount of toner is small, conversely, the amount of reflected light from the belt surface increases, resulting in an increase in the amount of light received by the light receiving element. Therefore, the toner adhesion amount (toner density) of the density correction pattern of each color is detected from the output value based on the received reflected light amount, and the density is adjusted for each color by adjusting the development bias and the like in comparison with a predetermined target density. Correction is performed.

  However, the reflection type optical sensor has lower toner density detection accuracy when the toner density of the image (density correction pattern) is higher than when the toner density is low to intermediate density. Therefore, as will be described later, in the image forming apparatus 100, in order to set the density correction pattern to a relatively low toner density, the potential difference (second potential difference described later) ΔV between the developing roller and the magnetic roller is relatively low. Then, a density correction pattern is formed, and the toner density is detected by a reflective optical sensor to perform density correction.

  Next, the detailed structure of the developing device 3a will be described with reference to FIG. In the following description, the developing device 3a disposed in the image forming unit Pa in FIG. 1 is illustrated, but the configuration of the developing devices 3b to 3d disposed in the image forming units Pb to Pd is basically the same. Therefore, explanation is omitted.

  As shown in FIG. 2, the developing device 3 a includes a developing container (casing) 20 in which a two-component developer containing a magnetic carrier and toner (hereinafter simply referred to as a developer) is stored. Is divided into an agitating and conveying chamber 21 and a supply and conveying chamber 22 by a partition wall 20a. In the agitating / conveying chamber 21 and the supply / conveying chamber 22, an agitating / conveying screw (agitating / conveying member) 25 a for mixing and charging the toner supplied from the toner container 4 a (see FIG. 1) with a magnetic carrier and charging the agitated conveying chamber 21 A conveying screw (stirring conveying member) 25b is rotatably arranged.

  Then, the developer is agitated by the agitating / conveying screw 25a and the supply / conveying screw 25b and conveyed in the axial direction (perpendicular to the paper surface of FIG. 2) to pass through the developer (not shown) formed at both ends of the partition wall 20a. It circulates between the stirring and conveying chamber 21 and the supply and conveying chamber 22 through the path. That is, a developer circulation path is formed in the developer container 20 by the agitating / conveying chamber 21, the supply / conveying chamber 22, and the developer passage.

  The developing container 20 extends obliquely upward to the left in FIG. 2. A magnetic roller 30 is disposed above the supply / conveying screw 25 b in the developing container 20, and a developing roller 31 is disposed obliquely above the left of the magnetic roller 30. Opposed. A part of the outer peripheral surface of the developing roller 31 is exposed from the opening 20b of the developing container 20 and faces the photosensitive drum 1a. The magnetic roller 30 and the developing roller 31 rotate in the clockwise direction in FIG.

  The magnetic roller 30 is composed of a non-magnetic rotating sleeve that rotates clockwise in FIG. 2 and a fixed magnet body having a plurality of magnetic poles enclosed in the rotating sleeve.

  The developing roller 31 includes a cylindrical developing sleeve that rotates in a clockwise direction in FIG. 2 and a developing roller side magnetic pole that is fixed in the developing sleeve. The magnetic roller 30 and the developing roller 31 are opposed to each other. Are opposed to each other with a predetermined gap. The developing roller side magnetic pole has a different polarity from the opposing magnetic pole of the fixed magnet body.

  Further, a spike cutting blade 35 is attached to the developing container 20 along the longitudinal direction of the magnetic roller 30 (perpendicular to the paper surface of FIG. 2), and the spike cutting blade 35 rotates in the rotational direction of the magnetic roller 30 (FIG. 2). (Clockwise direction) of the developing roller 31 and the magnetic roller 30 is positioned upstream of the facing region R. A slight gap (gap) is formed between the tip of the earbrushing blade 35 and the surface of the magnetic roller 30.

  A DC voltage (hereinafter also referred to as Vslv (DC)) and an AC voltage (hereinafter also referred to as Vslv (AC)) are applied to the developing roller 31, and a DC voltage (hereinafter referred to as Vmag (DC)) is applied to the magnetic roller 30. And an alternating voltage (hereinafter also referred to as Vmag (AC)) are applied. These DC voltage and AC voltage are applied to each sleeve of the developing roller 31 and the magnetic roller 30 from a developing bias power supply (voltage applying unit) 46 (see FIG. 3) via a bias control circuit (not shown). . The control unit 42, which will be described later, transmits a control signal to a bias control circuit (not shown) and supplies Vslv (DC), Vslv (AC), Vmag (DC), and Vmag (AC) applied from the developing bias power supply 46. Control.

  As described above, the toner is charged by circulating through the agitating and conveying chamber 21 and the supplying and conveying chamber 22 in the developing container 20 while the developer is agitated by the agitating and conveying screw 25a and the supply and conveying screw 25b. The developer is conveyed to the magnetic roller 30. Then, a magnetic brush (not shown) is formed on the magnetic roller 30 from the developer. After the layer thickness of the magnetic brush on the magnetic roller 30 is regulated by the ear cutting blade 35, the magnetic roller 30 and the developing roller 31 A thin toner layer is formed on the developing roller 31 by a potential difference ΔV between Vmag (DC) applied to the magnetic roller 30 and Vslv (DC) applied to the developing roller 31 and a magnetic field.

  The thickness of the toner layer on the developing roller 31 varies depending on the resistance of the developer, the rotational speed difference between the magnetic roller 30 and the developing roller 31, and the like, but the potential difference between the magnetic roller 30 and the developing roller 31 (second to be described later). It can be controlled by (potential difference) ΔV. When this potential difference ΔV is increased, the toner layer on the developing roller 31 becomes thicker, and when the potential difference ΔV is decreased, the toner layer becomes thinner. The range of potential difference ΔV during development is generally about 100V to 350V.

  The toner thin layer formed on the developing roller 31 by the magnetic brush is conveyed to a region (developing region) opposite to the photosensitive drum 1 a and the developing roller 31 by the rotation of the developing roller 31. Since a predetermined bias is applied to the developing roller 31, toner jumps from the developing roller 31 to the photosensitive drum 1a due to a potential difference with the photosensitive drum 1a, and an electrostatic latent image on the photosensitive drum 1a is formed. Developed.

  The toner that is not used for development and remains on the developing roller 31 is conveyed again to the facing region R between the developing roller 31 and the magnetic roller 30 by the rotation of the developing roller 31 and is collected by the magnetic brush on the magnetic roller 30. The magnetic brush is peeled off from the magnetic roller 30 at the same polarity portion of the fixed magnet body of the magnetic roller 30 and then falls into the supply transfer chamber 22.

  FIG. 3 is a block diagram showing a control path of the image forming apparatus 100 of the present invention. Portions common to FIGS. 1 and 2 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image forming unit Pa to Pd, an image input unit 40, an AD conversion unit 41, a control unit 42, a storage unit 43, an operation panel 44, a fixing unit 13, an intermediate transfer belt 8, a density detection sensor 45, and a development. The configuration includes a bias power supply 46 and the like.

  When the image forming apparatus 100 is a color copying machine, the image input unit 40 is a scanning optical system equipped with a scanner lamp that illuminates the document during copying, a mirror that changes the optical path of reflected light from the document, and reflection from the document. 1 is an image reading unit that includes a condenser lens that collects light to form an image and a CCD that converts the formed image light into an electrical signal. In the case of a printer, the receiving unit receives image data transmitted from a personal computer or the like. The image signal input from the image input unit 40 is converted into a digital signal by the AD conversion unit 41 and then sent to the image memory 50 in the storage unit 43.

  The storage unit 43 stores print image data input from the image input unit 40 and digitally converted by the AD conversion unit 41 in units of pages, and necessary data and image formation generated during the control of the image forming apparatus 100. A readable / writable RAM (Random Access Memory) 51 in which data and the like temporarily required for the control of the apparatus 100 are stored, and the image forming apparatus 100 such as a control program for the image forming apparatus 100 and numerical values necessary for the control. A read-only ROM (Read Only Memory) 52 that stores data that is not changed during use is provided.

  The RAM 51 stores a target value (target density) of the sensor output value at the time of density correction.

  The operation panel 44 displays the state of the image forming apparatus 100, the image forming status, and the number of copies, and as a touch panel, functions such as double-sided printing and black-and-white reversal, and various settings such as magnification setting and density setting, the number of printing copies When the image forming apparatus 100 has a FAX function, the numeric keypad for inputting the other party's FAX number, a start button for instructing the user to start image formation, and when stopping image formation. There are provided a stop / clear button, a reset button used when various settings of the image forming apparatus 100 are set to a default state, and the user operates the operation panel 44 to input an instruction, whereby the image forming apparatus 100 is operated. Are set to execute various functions such as image formation.

  The control unit 42 is, for example, a central processing unit (CPU), and the transfer paper P from the image input unit 40, the image forming units Pa to Pd, the fixing unit 13, and the paper cassette 16 (see FIG. 1) according to a set program. The image signal input from the image input unit 40 is converted into image data by scaling processing or gradation processing as necessary. The exposure device 5 irradiates laser light based on the processed image data to form latent images on the photosensitive drums 1a to 1d.

  Further, when the calibration mode is set, the control unit 42 receives the sensor output value from the density detection sensor 45 and compares the sensor output value of each patch image of the density correction pattern (reference image) with the target value. And a function of performing density correction for setting a developing bias in accordance with the comparison result.

  Next, density correction control in the image forming apparatus 100 according to an embodiment of the present invention will be described.

  First, when the calibration mode is set by a user operation or after printing a predetermined number of sheets, the control unit 42 instructs the formation of a density correction pattern. In the present invention, for each color of cyan, magenta, yellow and black, a plurality of sets of density correction patterns having a predetermined density (here, 100%) are obtained by changing the developing bias application conditions applied to the developing devices 3a to 3d ( Here, 4 sets) are formed.

  Vslv (DC) of 50V is applied to the developing roller 31 of the developing devices 3a to 3d, and the magnetic roller 30 is changed to a total of four stages of Vmag (DC) of 90V, 130V, 170V, and 210V. That is, the second potential difference ΔV between the developing roller 31 and the magnetic roller 30 is changed to a total of four levels (four levels) of 40 V, 80 V, 120 V, and 160 V as the second potential difference ΔV during development.

  Here, in the present embodiment, as shown in FIG. 4, the control unit 42 has the first potential difference ΔV1 immediately before forming the first density correction pattern larger than the second potential difference ΔV2 when forming the density correction pattern. For example, it is set to 300 V or more (here, 300 V). Further, the control unit 42 sets the second potential difference ΔV21 when forming the first density correction pattern to a minimum level value (here, 40V), and maximizes the second potential difference ΔV22 when forming the second density correction pattern. The level value (here, 160V) is set, and the second potential difference ΔV23 when the third density correction pattern is formed is an intermediate value (here, the value between the minimum level (ΔV21) and the maximum level (ΔV22)). 80V), and the second potential difference ΔV24 when the fourth density correction pattern is formed is set to an intermediate value (120V in this case) larger than the value (ΔV23) when the third density correction pattern is formed.

  Although the DC bias value is changed here, the development bias level may be set by changing at least one of the peak-to-peak value, frequency, and duty ratio of the AC bias instead of or together with the DC bias value. good.

  The intermediate transfer belt 8 is formed with a total of four 100% density correction patterns for four colors of cyan, magenta, yellow, and black at a predetermined developing bias (for example, the second potential difference ΔV2 = 40 V). Since the same density correction pattern is formed when the second potential difference ΔV2 is 160V, 80V, and 120V, a total of 4 × 4 = 16 patterns are formed on the intermediate transfer belt 8.

  Next, the toner density of the density correction pattern formed on the intermediate transfer belt 8 is detected by the density detection sensor 45, and a development bias is set for each color so that the sensor output value becomes a target value. At this time, the density of the density correction pattern at each developing bias is stored in the storage unit 43. Then, the density correction control of the image forming apparatus 100 ends.

  In the present embodiment, as described above, the control unit 42 uses the first potential difference ΔV1 between the developing roller 31 and the magnetic roller 30 immediately before forming the first density correction pattern as the density correction pattern. The second potential difference ΔV2 between the developing roller 31 and the magnetic roller 30 is set larger. As a result, even when the second potential difference ΔV2 is reduced in order to form a density correction pattern with a low toner density, the negative AC bias can be reduced by increasing the first potential difference ΔV1 immediately before the density correction pattern is formed. Since the toner can be prevented from moving from the developing roller 31 to the magnetic roller 30 due to the influence of the potential, it is possible to suppress the toner amount on the magnetic roller 30 from becoming excessive. For this reason, the excess toner that has moved onto the magnetic roller 30 moves again to the developing roller 31 and the amount of toner on the developing roller 31 becomes excessive, and there is a relationship between the second potential difference ΔV2 and the toner layer thickness of the density correction pattern. Loss of linearity can be suppressed. As a result, even if the toner density of the density correction pattern is low, it is possible to prevent the toner density detection accuracy from being lowered, and thus the density correction precision can be improved.

  Further, the control unit 42 sets the second potential difference ΔV21 when forming the first density correction pattern to a minimum level value, and sets the second potential difference ΔV22 when forming the second density correction pattern to a maximum level value. The second potential difference ΔV23 when forming the third density correction pattern is set to an intermediate value between the minimum level value and the maximum level value. As a result, linearity is lost in the relationship between the second potential difference ΔV2 and the toner layer thickness of the density correction pattern as compared with the case where the second potential difference ΔV22 when forming the second density correction pattern is set to an intermediate value. Therefore, the accuracy of density correction can be further improved.

  Further, as will be described later, the time required to form the density correction pattern is longer than when the first potential difference ΔV1 immediately before the formation of each (all) density correction pattern is set to be larger than the second potential difference ΔV2. Can be suppressed.

  Further, as will be described later, when the value of the minimum level of the second potential difference ΔV2 is 100 V or less, linearity tends to be lost in the relationship between the second potential difference ΔV2 and the toner layer thickness, so that the minimum level of the second potential difference ΔV2 is reduced. It is particularly effective to make the first potential difference ΔV1 larger than the second potential difference ΔV2 when the value is 100 V or less.

  Further, as described above, even when a reflection type optical sensor generally used as the density detection sensor 45 for detecting the toner density of the image is used, the accuracy of density correction can be improved. It is valid.

  Further, as described above, the control unit 42 sets the first potential difference ΔV1 to 300 V or more. As a result, the loss of linearity in the relationship between the second potential difference ΔV2 and the toner layer thickness can be sufficiently suppressed, so that the density correction accuracy can be sufficiently improved.

  Further, as described above, when the photoconductor drums 1a to 1d are amorphous silicon photoconductors, the relative permittivity of the photoconductor is high, and the development amount greatly varies due to the gap variation between the developing roller 31 and the photoconductor. Therefore, a method of developing all the toner layers on the developing roller 31 is usually employed. For this reason, it is important to appropriately control the amount of toner on the developing roller 31, and therefore it is particularly effective to apply the present invention.

  Next, a confirmation experiment performed to confirm the above effect will be described. This confirmation experiment was conducted for Example 1 corresponding to this embodiment and Comparative Example 1 not corresponding to this embodiment.

  In Example 1, the density correction pattern was formed by setting the second potential difference ΔV2 from 40V to 160V every 40V using the developing device 3a. At this time, as shown in FIG. 4, the first potential difference ΔV1 (here, 300 V) immediately before forming the first density correction pattern was set larger than the second potential difference ΔV2. The second potential difference ΔV2 was set in the order of 40V (minimum level value), 160V (maximum level value), 80V (intermediate value), and 120V (intermediate value) to form a density correction pattern.

  In Comparative Example 1, the density correction pattern was formed by setting the second potential difference ΔV2 from 40V to 160V every 40V using the developing device 3a. At this time, as shown in FIG. 5, the density correction pattern was formed by increasing the voltage in increments of 40V. Other conditions in Comparative Example 1 were the same as in Example 1.

  In this confirmation experiment (Example 1 and Comparative Example 1), the voltage application conditions to the developing roller 31 were Vslv (DC) = 50 V and Vpp of Vslv (AC) was 1500 V. Further, Vmag (AC) Vpp was set to 2200 V to the magnetic roller 30 in an opposite phase. The other configurations of Example 1 and Comparative Example 1 were the same as in the above embodiment.

  The transmission density of the density correction patterns of Example 1 and Comparative Example 1 was detected by a toner transmission density detection sensor (not shown). The result is shown in FIG. Since the toner transmission density and the toner layer thickness (toner density) are correlated (proportional), the toner transmission density can be substituted for the toner layer thickness (toner density).

  Referring to FIG. 6, in Example 1 and Comparative Example 1, the first potential difference ΔV1 immediately before forming the density correction pattern is made larger than the second potential difference ΔV2, so that the second potential difference ΔV2 and the density correction pattern are transmitted. It has been found that loss of linearity can be suppressed in relation to the density (toner layer thickness). This is thought to be due to the following reasons. That is, even in a region where the second potential difference ΔV2 is small (about 100 V or less), the developing roller is affected by the negative potential of the AC bias by increasing the first potential difference ΔV1 immediately before forming the density correction pattern. Since it was possible to suppress the toner from moving from 31 to the magnetic roller 30, it was possible to suppress an excessive amount of toner on the magnetic roller 30. For this reason, the excess toner that has moved onto the magnetic roller 30 moves again to the developing roller 31 and the amount of toner on the developing roller 31 becomes excessive, and the second potential difference ΔV2 and the transmission density (toner layer thickness) of the density correction pattern. In the relationship, the loss of linearity could be suppressed.

  Further, in Example 1, it was found that the loss of linearity can be further suppressed in the relationship between the second potential difference ΔV2 and the transmission density (toner layer thickness) of the density correction pattern, as compared with Comparative Example 1. did. This is thought to be due to the following reasons. That is, the second potential difference ΔV2 when the second density correction pattern is formed is set to the maximum value (160V), so that the second value difference is higher than that when the intermediate value (80V) is set (Comparative Example 1). By applying the potential difference ΔV 2, the toner that has started to accumulate excessively on the magnetic roller 30 when the value reaches the minimum level (40 V) is circulated between the developing roller 31 and the magnetic roller 30, so that the magnetic roller The toner accumulated on the surface 30 could be eliminated. For this reason, it was possible to form a density correction pattern with an intermediate value from a state in which excessive toner on the developing roller 31 was suppressed. As a result, the loss of linearity in the relationship between the second potential difference ΔV2 and the transmission density (toner layer thickness) of the density correction pattern can be further suppressed.

  As shown in FIG. 7, even when the second potential difference ΔV2 is increased by 40V in order, the first potential difference ΔV1 immediately before each (all) density correction pattern is formed is larger than the second potential difference ΔV2. If set, it can be considered that the loss of linearity can be suppressed in the relationship between the second potential difference ΔV2 and the transmission density (toner layer thickness) of the density correction pattern, as in the first embodiment. However, in this case, the time required to form the density correction pattern is increased by applying the first potential difference ΔV1 three times.

  The embodiments and examples disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments and examples but by the scope of claims for patent, and includes all modifications within the meaning and scope equivalent to the scope of claims for patent.

  For example, although an example in which the present invention is applied to a color printer has been described, the present invention is not limited to this. Needless to say, the present invention can be applied to various image forming apparatuses including a developing device having a developing roller and a magnetic roller, such as a monochrome printer, a monochrome copying machine, a digital multifunction peripheral, and a facsimile machine.

  Alternatively, the toner carried on the developing roller 31 may be discharged to the photosensitive drums 1a to 1d immediately before forming the density correction pattern by increasing the first potential difference ΔV1 immediately before forming the density correction pattern. With this configuration, even when the undeveloped toner on the developing roller 31 is collected by the magnetic roller 30 at a low printing rate and the toner adheres excessively to the magnetic roller 30, the excess toner is removed from the developing roller. 31 to the photosensitive drums 1a to 1d. For this reason, it is possible to suppress the influence of excess toner due to the low printing rate when forming the density correction pattern, and therefore it is possible to further improve the density correction accuracy.

  In the above embodiment, the second potential difference ΔV24 when forming the fourth density correction pattern is shown as an intermediate value larger than the value (ΔV23) when forming the third density correction pattern. The present invention is not limited to this. The second potential difference ΔV24 when forming the fourth density correction pattern may be an intermediate value smaller than the value (ΔV23) when forming the third density correction pattern.

  In the above embodiment, the density correction pattern formed on the intermediate transfer belt 8 is detected. However, in the case of using a direct transfer type tandem image forming apparatus, the conveyance belt that conveys the sheet is used as a print medium. The density correction pattern formed on the conveyor belt can also be detected.

  The calibration mode may be set when the apparatus is turned on or when the apparatus is returned from the sleep mode (power saving mode).

  Further, a configuration obtained by appropriately combining the configurations of the above-described embodiment and modification examples is also included in the technical scope of the present invention.

1a to 1d Photosensitive drum (image carrier)
3a to 3d Developing device 30 Magnetic roller 31 Developing roller 42 Control unit 45 Concentration detection sensor 46 Developing bias power supply (voltage applying unit)
100 Image forming apparatus

Claims (6)

A developing roller that is disposed opposite to an image carrier on which an electrostatic latent image is formed, and that is disposed opposite to the developing roller to supply toner to the image carrier in a region facing the image carrier; A developing device having a magnetic roller for supplying toner to the developing roller in a region facing the developing roller;
A voltage applying unit that applies a developing bias in which an AC voltage is superimposed on a DC voltage to the developing roller and the magnetic roller;
A density detection sensor for detecting the toner density of three or more density correction patterns formed using the developing device;
A control unit for controlling the developing device and the voltage application unit;
With
The controller is
A first potential difference between the developing roller and the magnetic roller just before forming the first of the density correction pattern, a plurality of first between the developing roller and the magnetic roller when forming the density correction pattern Of the two potential differences , larger than the second potential difference when forming the first density correction pattern ,
The second potential difference when forming the first density correction pattern is set to a minimum level value, the second potential difference when forming the second density correction pattern is set to a maximum level value, and the third density correction pattern is set. The image forming apparatus is characterized in that the second potential difference when forming the image is set to an intermediate value between the minimum level value and the maximum level value. 2. The control unit according to claim 1, wherein the control unit makes the first potential difference when the minimum level value is 100 V or less larger than the second potential difference when forming the first density correction pattern. Image forming apparatus. The control unit increases the first potential difference larger than the second potential difference when forming the first density correction pattern, so that the toner carried on the developing roller is immediately before forming the density correction pattern. The image forming apparatus according to claim 1, wherein the image forming apparatus discharges the image onto the image carrier.   The image forming apparatus according to claim 1, wherein the density detection sensor is a reflective optical sensor.   The image forming apparatus according to claim 1, wherein the control unit sets the first potential difference to 300 V or more.   The image forming apparatus according to claim 1, wherein the image carrier is an amorphous silicon photoconductor.

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