JP2018116203A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device that forms an image using carrier liquid and liquid developer containing toner particles, with which it is possible to secure the uniformity of image densities.SOLUTION: The image formation device comprises: a rotatable development roller 54 that carries developer supplied to a photoreceptor drum; a liquid developer tank for storing developer supplied to the development roller 54; a filmmaking electrode 51, arranged so as to face the development roller 54 with a prescribed gap therebetween, for forming an electric field between the development roller 54 and itself by application of bias and supplying the developer stored in the liquid developer tank to the development roller 54; and a CPU for controlling a first power supply and a second power supply for applying bias to the filmmaking electrode 51. The CPU applies a larger bias to a portion of the filmmaking electrode 51 where a gap in the rotation axis direction of the development roller 54 is relatively large than for portions where the gap is relatively small, so that electric field intensity between the filmmaking electrode 51 and the development roller 54 becomes approximately constant in the rotation axis direction of the development roller 54.SELECTED DRAWING: Figure 3

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus that forms an image using a liquid developer including carrier liquid and toner particles, among image forming apparatuses such as an electrophotographic copying machine and an electrophotographic printer.

  In an electrophotographic image forming apparatus, a developing method for forming a toner image is roughly classified into a dry developing method using powder toner directly and a wet developing method using a liquid developer in which toner is dispersed in a liquid. The Among these, the wet development method disperses the toner in the carrier liquid, so that it is possible to control the toner particles having a particle size on the order of submicron to form an image, which is promising in terms of high image quality and high definition. Development method.

  Here, in the wet development method, a configuration for improving the supply efficiency of the developer to the developing roller as a developer carrying member for carrying the developer has been conventionally proposed. For example, in Patent Document 1, a film-forming electrode that is disposed to face a developing roller with a predetermined gap and forms an electric field with the developing roller by applying a bias to supply developer to the developing roller. A configuration having

JP-A-10-282895

  In the configuration having the film forming electrode as in the configuration of Patent Document 1, the toner concentration in the developer carried on the developing roller depends on the electric field strength of the electric field formed between the film forming electrode and the developing roller. . The electric field strength depends on the gap between the film forming electrode and the developing roller.

  In order to make the gap between the film forming electrode and the developing roller constant, the film forming electrode needs to have a shape having the same curvature as that of the developing roller. However, it is not easy to give high precision to the curvature of the film-forming electrode, and it is not easy to give a developing roller, which is generally formed of an elastic body, with a highly accurate diameter and roundness.

  Here, due to a problem in accuracy, when a fluctuation occurs in the gap between the film forming electrode and the developing roller in the rotation axis direction of the developing roller, the electric field strength between the developing roller and the film forming electrode fluctuates, and the developing roller The toner density in the developer above also fluctuates. As described above, when the toner density in the developer carried on the developing roller varies in the rotation axis direction of the developing roller, the final image density also varies.

  Accordingly, the present invention has been made in view of such a current situation, and in an image forming apparatus that forms an image using a liquid developer containing carrier liquid and toner particles, ensuring uniformity of image density. An object of the present invention is to provide an image forming apparatus capable of performing the above.

  In order to achieve the above object, a typical configuration of an image forming apparatus according to the present invention is an image forming apparatus for forming an image on a recording material using a liquid developer containing toner particles and a carrier liquid. A rotatable developer carrier that carries the developer supplied to the image carrier, a storage unit that contains the developer supplied to the developer carrier, the developer carrier and a predetermined Electrodes that are arranged to face each other with a gap and supply a developer to the developer carrier by forming an electric field with the developer carrier by applying a bias. And control means for controlling an application means for applying a bias to the electrode, wherein the control means has an electric field strength between the electrode and the developer carrier in the direction of the rotation axis of the developer carrier. For the electrode to be substantially constant The gap in the rotational axis direction is a relatively large portion, and applying a larger bias than the relatively small portion.

  According to the present invention, it is possible to ensure uniformity of image density in an image forming apparatus that forms an image using a liquid developer containing carrier liquid and toner particles.

1 is a schematic cross-sectional view of an image forming apparatus. 1 is a schematic cross-sectional view of a developing device. It is a schematic diagram which shows the structure of a film forming electrode typically. It is a block diagram which shows the control system used by bias adjustment control. It is a flowchart of bias adjustment control. FIG. 6 is a diagram illustrating a test toner image. It is a schematic diagram which shows the structure of a film forming electrode typically.

(First embodiment)
<Image forming apparatus>
First, the overall configuration of the image forming apparatus A according to the first embodiment of the present invention will be described with reference to the drawings together with the operation during image formation. Note that the dimensions, materials, shapes, relative arrangements, and the like of the described components are not intended to limit the scope of the present invention only to those unless otherwise specified.

  The image forming apparatus A transfers intermediate toners (developers) of yellow Y, magenta M, cyan C, and black K to an intermediate transfer belt, and then transfers the image to a sheet to form an image. The color image forming apparatus. In the following description, although the members using the toners of the respective colors are given subscripts Y, M, C, and K, the configuration and operation of each member are different except that the color of the toner used is different. Since they are substantially the same, subscripts are omitted as appropriate unless distinction is required.

  The image forming apparatus A includes an image forming unit that transfers a toner image onto a sheet that is a recording material, a sheet feeding unit that supplies the sheet to the image forming unit, and a fixing unit that fixes the toner image on the sheet.

  As shown in FIG. 1, the image forming unit includes a photosensitive drum 20 (20Y, 20M, 20C, 20K) as an image carrier and a charging wire 30 (30Y) that is a corona charger that charges the surface of the photosensitive drum 20. 30M, 30C, 30K). In addition, a photoconductor cleaner 21 (21Y, 21M, 21C, 21K), an exposure unit 40 (40Y, 40M, 40C, 40K), a developing device 50 (50Y, 50M, 50C, 50K), and an intermediate transfer unit 60 are provided.

  The photosensitive drum 20 is a cylindrical member having a photosensitive layer formed on the outer peripheral surface of the rotation shaft, and rotates in the direction of the arrow R1 shown in FIG. The photosensitive layer is composed of an organic photoreceptor or an amorphous silicon photoreceptor, and amorphous silicon is used in the present embodiment.

  The developing device 50 performs development by supplying toner to the electrostatic latent image formed on the photosensitive drum 20 using a developer. In this embodiment, particles having an average particle diameter of 0.8 μm, in which a colorant such as a pigment is dispersed in a resin as a developer, together with a dispersant, a toner charge control agent, and a charge directing agent in a liquid carrier such as an organic solvent. A liquid developer having a toner particle concentration of about 7 wt% is used. The surface of the toner particles is charged with a certain amount of negative polarity. The detailed configuration of the developing device 50 will be described later.

  The exposure unit 40 includes a semiconductor laser (not shown), a polygon mirror (not shown), an Fθ lens (not shown), etc., and irradiates the modulated laser onto the charged photosensitive drum 20 to form a latent image. Form.

  The intermediate transfer unit 60 includes an intermediate transfer belt 80, a primary transfer roller 61 (61Y, 61M, 61C, 61K), a driving roller 82, a driven roller 85, a secondary transfer roller 81, a secondary transfer counter roller 86, and a secondary transfer cleaner 83. Etc.

  The intermediate transfer belt 80 is an endless belt stretched around a primary transfer roller 61, a drive roller 82, a driven roller 85, and a secondary transfer counter roller 86. The intermediate transfer belt 80 is rotated by a driving roller 82 receiving a rotational driving force from a driving source, and is thereby rotated while being in contact with the photosensitive drum 20 and the secondary transfer roller 81.

  Next, an image forming operation will be described. First, when the CPU 71 shown in FIG. 4 receives an image forming job signal, sheets (not shown) stacked and stored in a sheet feeding cassette (not shown) are formed by a feeding roller (not shown) and a conveying roller (not shown). It is conveyed to the part.

On the other hand, in the image forming unit, first, a charging bias of about −4.5 kV to −5.5 kV is applied to the charging wire 30 to charge the surface of the photosensitive drum 20 to about −800V.
Thereafter, the exposure unit 40 irradiates the surface of the photosensitive drum 20 with laser light in accordance with an image signal transmitted from an external device (not shown) or the like, thereby setting the potential of the image portion to about −50 V and forming an electrostatic latent image. Form.

  Thereafter, a developing bias of −400 V is applied to the developing roller 54 at the developing nip portion formed from the developing roller 54 and the photosensitive drum 20 provided in the developing device 50. Thus, according to the electric field formed between the developing roller 54 and the photosensitive drum 20, the toner particles selectively move to the image portion on the photosensitive drum 20 to form a toner image. That is, the charging wire 30, the exposure unit 40, and the developing device 50 are toner image forming units that form a toner image on the surface of the photosensitive drum 20. Since the carrier liquid is not affected by the electric field, it is separated at the outlet of the developing nip and adheres to both the developing roller 54 and the photosensitive drum 20.

  Thereafter, the toner image formed on the surface of the photosensitive drum 20 is sent to a primary transfer nip portion formed by the photosensitive drum 20 and the primary transfer roller 61. Each color toner image sent to the primary transfer nip is primarily transferred to the intermediate transfer belt 80 by applying to the primary transfer roller 61 a primary transfer bias of about +200 V opposite to the toner charging polarity. . As a result, the four color liquid toners are sequentially transferred onto the intermediate transfer belt 80 to form a full color liquid toner image.

  Thereafter, the toner image is sent to the secondary transfer portion formed by the secondary transfer roller 81 and the secondary transfer counter roller 86 by the rotation of the intermediate transfer belt 80. Then, a secondary transfer bias of +1000 V is applied to the secondary transfer roller 81 in the secondary transfer portion, whereby the toner image on the intermediate transfer belt 80 is transferred to the sheet.

  The sheet on which the toner image is transferred is sent to a fixing device (not shown), heated and pressed by the fixing device to fix the toner image on the sheet, and then discharged to the outside of the image forming apparatus A.

  The toner and carrier liquid remaining on the photosensitive drum 20 after the primary transfer are scraped and removed by the photosensitive cleaner 21. Further, the toner remaining on the intermediate transfer belt 80 after the secondary transfer is removed by a belt cleaner (not shown) in contact with the intermediate transfer belt 80. Further, the toner remaining on the secondary transfer roller 81 after the secondary transfer is removed by the secondary transfer cleaner 83.

  In the above image formation, the transfer efficiency in each toner particle transfer process is adjusted to be extremely high at about 95% or more. For this reason, in the developing device 50, it is important for stabilizing the image quality of the image output on the sheet to accurately stabilize the toner density, which is the amount of toner in the developer carried on the developing roller 54. .

<Developing device>
Next, the configuration of the developing device 50 will be described. Since the developing devices 50Y, 50M, 50C, and 50K and peripheral devices thereof have the same configuration except for the color of the toner to be used, the developing device 50K that uses black toner will be described as an example here.

  FIG. 2 is a schematic sectional view of the developing device 50K. As shown in FIG. 2, the developing device 50K has a developing solution tank 53K, a film forming electrode 51K, a squeezing roller 52K, and a developing cleaner 58K around the developing roller 54K.

  The developing roller 54K (developer carrying member) is a cylindrical member that is rotatably supported, and an elastic body such as conductive urethane rubber, a resin layer, and rubber on the outer periphery of a metal inner core such as stainless steel. With a layer. The developing roller 54K rotates about the inner core in the direction of the arrow R2 shown in FIG. 2, and supplies the developer carried on the surface to the photosensitive drum 20K. Note that the width of the developing roller 54K is narrower than the width of the photosensitive drum 20K.

  The developer tank 53K (container) is a developer carried on the developing roller 54K, and contains a developer for developing the electrostatic latent image formed on the photosensitive drum 20K.

  The film-forming electrode 51K is arranged to face the developing roller 54K with a predetermined gap, and an electric field is formed between the developing roller 54K by applying a bias to the developing roller 54K and stored in the developer tank 53K. The developer is supplied to the developing roller 54K to form the developer on the developing roller 54K. In this embodiment, the gap (interval) between the film forming electrode 51K and the developing roller 54K is set to 200 μm. When the developer is supplied from the film forming electrode 51K to the developing roller 54K, a bias of −400 V is applied to the developing roller 54K, and a voltage of −550 to −600 V is applied to the film forming electrode 51K. As a result, most of the toner particles are attracted to the surface of the developing roller 54K, and in the vicinity of the rear end portion of the film forming electrode 51K in the rotation direction of the developing roller 54K, the developer follows the surface of the developing roller 54K and the back of the film forming electrode 51K. Divide into things that run down. The bias applied to the film forming electrode 51K will be described in detail later.

  The squeezing roller 52K is a cylindrical member made of metal, and in this embodiment, a roller formed of stainless steel is used. The squeezing roller 52K is disposed in contact with the developing roller 54, and rotates in the arrow R3 direction shown in FIG.

  The developer carried on the developing roller 54 stably forms a nip having a gap of about 6 μm and a width of about 5 mm with the squeezing roller 52K. Here, a bias 50V to 120V higher than the bias applied to the developing roller 54K is applied to the aperture roller 52K. That is, if the bias applied to the developing roller 54K is −400V, the bias applied to the aperture roller 52K is −450V to −520V. As a result, the toner particles in the developer carried on the developing roller 54K move toward the developing roller 54K. For this reason, the developer carried on the developing roller 54K is separated into respective rollers in the vicinity of the exit of the squeezing roller 52K and the developing roller 54K, but the toner concentration in the developer on the developing roller 54K after the separation is separated. Compared to twice. As a result, a thin layer coat having a toner concentration of about 35 wt% is formed on the developing roller 54K. The developer attached to the squeezing roller 52K is removed by the squeezing cleaner 56K.

  The developing cleaner 58K is a roller made of metal or the like, and abuts on the surface of the developing roller 54K to remove the developer remaining on the surface of the developing roller 54K.

<Applying means>
Next, application means for applying a bias to the film forming electrode 51K will be described.

  As described above, the film forming electrode 51K is disposed to face the developing roller 54K with a predetermined gap, and this gap is set to 200 μm in this embodiment. However, in practice, it is difficult to make the gap completely coincide with the entire area in the rotation axis direction (hereinafter referred to as “longitudinal direction”) of the developing roller 54K due to a problem in accuracy. When the gap fluctuates in this way, the toner density in the developer carried on the developing roller 54K fluctuates, and the final image density also fluctuates.

  Therefore, in this embodiment, a constant bias in the longitudinal direction is not applied to the film forming electrode 51K as in the prior art, but a different bias in the longitudinal direction is applied according to the gap by the configuration described below.

  FIG. 3 is a schematic diagram schematically showing the configuration of the film forming electrode 51K. As shown in FIG. 3, a first power supply 90K and a second power supply 91K are electrically connected to both ends in the longitudinal direction of the film forming electrode 51K, and the CPU 71 controls these power supplies to form a film. Different biases can be applied to both ends in the longitudinal direction of the electrode 51K. The film forming electrode 51K is formed of a resistor. Similarly, the first power sources 90Y, 90M, and 90C and the second power sources 91K, 91M, and 91C are connected to the other film forming electrodes 51Y, 51M, and 51C.

  Here, when assembling the developing device 50K in the factory, the gap between the film forming electrode 51K and the developing roller 54K is measured in advance with a laser length measuring machine or the like. With respect to the bias applied to the film forming electrode 51K, a bias value is stored in advance in the memory 74 (see FIG. 4) so that the electric field strength of the electric field formed between the two in accordance with the measured gap is substantially constant in the longitudinal direction. Let me. Specifically, a bias value is set such that a larger bias is applied to a portion having a relatively large gap between the two in the longitudinal direction than a relatively small portion. Further, the CPU 71 controls the first power supply 90K and the second power supply 91K when supplying the developer from the film forming electrode 51K to the developing roller 54K, and applies it to the film forming electrode 51K based on the set values stored in the memory 74. A predetermined bias is applied.

  Thus, when the image forming apparatus A is shipped, even when the gap between the film forming electrode 51K and the developing roller 54K is shaken in the longitudinal direction, it is formed when the developer is supplied from the film forming electrode 51K to the developing roller 54K. The electric field strength of the applied electric field can be made substantially constant in the longitudinal direction. Accordingly, the toner concentration in the developer carried on the developing roller 54K is substantially constant in the longitudinal direction, and the uniformity of the density of the output image can be ensured.

Further, the resistivity of the film forming electrode 51K is preferably about 1 Ωcm or more, and more preferably 100 Ωcm or more. Further, it is preferably lower than the resistivity of the developer. This is because, when the resistivity of the film forming electrode 51K is too high, the potential difference between the film forming electrode 51K and the developing roller 54K, particularly in the central portion in the longitudinal direction, does not depend on the bias applied to the film forming electrode 51K. The bias is applied to the roller 54K. Then, a desired electric field does not act on the developer. On the other hand, if the resistance of the resistor is too low, an excessive current flows in the film forming electrode 51K, which is not preferable because the burden on the first power supply 90K and the second power supply 91K increases. Further, it is not preferable that the current flowing through the resistor is too large than the current flowing from the film forming electrode 51K through the developer to the developing roller 54K. In this embodiment, since the resistivity of the developer is 1 × 10 ¹⁰ Ωcm, alumite having a resistivity of 100 Ωcm is used as the resistor.

  As another method for dealing with the fluctuation of the gap, a method of adjusting before shipping by providing a gap correction mechanism for adjusting the attachment of the film forming electrode 51K to the developing roller 54K is conceivable. As a configuration of the gap correction mechanism, for example, screws at both ends holding the film forming electrode 51K are adjustment screws whose distance to the base is variable.

  However, with the long-term use of the developing device 50K, the surface layer of the developing roller 54K, particularly the rubber layer, may increase or decrease in diameter due to the influence of a chemical attack of the developer. This occurs due to the characteristics of the rubber layer or the carrier liquid itself, the charge control agent, the charge directing agent, or impurities contained in the developer. The diameter of the developing roller 54K also changes depending on the temperature of the environment where the image forming apparatus A is installed. Due to such a change in the diameter of the developing roller 54K, the gap between the film forming electrode 51K and the developing roller 54K changes, and the gap between the two in the longitudinal direction may be shaken. Note that such gap fluctuation can naturally occur not only in the developing device 50K using black toner but also in the other developing devices 50Y, 50M, and 50C.

  Therefore, in order to deal with such a gap fluctuation, the image forming apparatus A performs bias adjustment control for adjusting a bias applied to the film forming electrode 51 from the first power supply 90 and the second power supply 91. Hereinafter, the bias adjustment control will be described.

<Bias adjustment control>
First, a control system used for bias adjustment control will be described with reference to a block diagram shown in FIG. As shown in FIG. 4, the image forming apparatus A includes a CPU 71 as a control unit. Further, a memory 74 is connected to the CPU 71. The memory 74 includes a ROM and a RAM.

  A first power supply 90 and a second power supply 91 are connected to the CPU 71. The CPU 71 can control the bias applied to the film forming electrode 51 by controlling the first power supply 90 and the second power supply 91.

  An operation unit 72 and a reader 73 (reading unit) are connected to the CPU 71. The operation unit 72 outputs an operation instruction input by the user to the CPU 71. The reader 73 reads an image formed on the sheet.

  Next, the content of the bias adjustment control will be described using the flowchart shown in FIG. The bias adjustment control is started by a user instruction. The user starts the bias adjustment control by operating the operation unit 72 at an arbitrary timing, such as when the image forming apparatus A starts to be used or when density unevenness occurs. Note that this control may be automatically performed not at the user's instruction but at the timing when the image forming apparatus A is turned on or every predetermined number of image forming sheets.

  As shown in FIG. 5, when the bias adjustment control is started, a test toner image is first formed by the above-described image forming operation (S1). FIG. 6 is a diagram illustrating an example of a test toner image. As shown in FIG. 6, in the test toner image, an image pattern having the highest density in a single color with 100% image signal is arranged in the longitudinal direction. Note that the rotation axis direction and the longitudinal direction of the photosensitive drum 20 are the same direction.

  Next, the test toner image transferred to the sheet is read by the reader 73 (S2), and the density of the test toner image is detected (S3). That is, the reader 73 is a density detecting unit that reads an image and detects the density thereof. Here, when the gap between the developing roller 54 and the film forming electrode 51 fluctuates in the longitudinal direction, a density difference occurs in the longitudinal direction on the image pattern of the test toner image. That is, the density of the image pattern is relatively small in a portion where the gap is relatively large in the longitudinal direction, and the density of the image pattern is relatively large in a portion where the gap is relatively small.

  Therefore, the CPU 71 sets a bias for correcting the density difference in the longitudinal direction of the image pattern of the test toner image as a bias applied to the film forming electrode 51 (S4). That is, the CPU 71 applies a bias applied to a position corresponding to a portion where the density of the image pattern is relatively small in the longitudinal direction as a bias applied to the film forming electrode 51 to a position corresponding to a portion where the density is relatively high. Set to be larger than the bias to be applied. Note that such bias setting is performed by the developing devices 50Y, 50M, 50C, and 50K that use toner of each color.

  For a specific bias setting method, for example, a table determined in consideration of the characteristics of the image forming apparatus A is stored in the memory 74 in advance, and the table is referred to according to the detected density of the test toner image. To set the bias. Alternatively, it may be controlled to form a test toner image while changing several biases to be applied to the film forming electrode 51 and find a bias that achieves the target density.

  By applying the bias set in this way to the film forming electrode 51 when supplying the developer to the developing roller 54, the electric field strength of the electric field formed between the film forming electrode 51 and the developing roller 54 is increased in the longitudinal direction. It becomes almost constant. That is, the CPU 71 determines that the position corresponding to the portion where the density of the test toner image is relatively low in the longitudinal direction is relatively larger than the position corresponding to the portion where the density is relatively high. In order to make the electric field strength formed between the film forming electrode 51 and the developing roller 54 substantially constant in the longitudinal direction, relative to the portion determined to have a relatively large gap with respect to the film forming electrode 51, A bias larger than the portion determined to be small is applied. As a result, even when a fluctuation occurs in the gap between the developing roller 54 and the film-forming electrode 51 due to durability or environmental fluctuations, it is possible to suppress the occurrence of fluctuation in the toner concentration in the developer carried on the developing roller 54, The uniformity of the density of the output image can be ensured.

  Conventionally, due to the shake of ± 10% with respect to the 200 μm gap between the film forming electrode 51 and the developing roller 54, the density difference of the image on the sheet has a shake of about ± 10%. On the other hand, by executing this control, it is possible to suppress the density difference of the image on the sheet to a difference of ± 2%, and the shake can be suppressed to the accuracy of the scanner of the reader 73. .

  The following effects can also be obtained by the bias adjustment control. That is, not only the fluctuation of the gap between the developing roller 54 and the film forming electrode 51 but also the contact fluctuation of the developing roller 54 and the squeezing roller 52, the contact fluctuation of the developing roller 54 and the photosensitive drum 20, and the light quantity of the exposure unit 40. It is also possible to suppress the density fluctuation of the output image based on the shake or the like.

  Further, the following effects can be obtained by forming an image pattern of a test toner image with a 100% image signal. That is, as described above, the fluctuation of the electric field strength due to the fluctuation of the gap between the film forming electrode 51 and the developing roller 54 is manifested as the fluctuation of the toner density of the developer on the developing roller 54. In order to suppress such a shake, ideally, it is desirable to set a bias to be applied to the film forming electrode 51 by directly looking at a difference in toner density on the developing roller 54 and a difference in toner amount. However, there are many difficulties in terms of cost and space, such as sensor placement. Therefore, in order to see the fluctuation of the toner density in the developer on the developing roller 54 as close as possible on the developing roller 54, the toner movement during the subsequent development process and transfer process, which is an image forming process, is almost Perform with 100% efficiency. By looking at the density of the image pattern formed in this way, it is possible to obtain the same detection accuracy as the toner density on the developing roller 54. In this developing process, the image signal is set to 100% in order to achieve almost 100% toner movement.

  In the present embodiment, seven image patterns are arranged in the longitudinal direction as the image pattern of the test toner image, but the present invention is not limited to this. That is, for example, in this embodiment, the number of image patterns may be two in order to determine the bias to be applied to both ends in the longitudinal direction of the film forming electrode 51. However, since it is desirable to determine the bias to be applied in consideration of the entire longitudinal direction, it is preferable to arrange many patterns.

  In the present embodiment, the image on the sheet is read to detect the density, but the present invention is not limited to this. In other words, the toner image density may be detected on the photosensitive drum 20, or the density may be detected by another method.

(Second Embodiment)
Next, a second embodiment of the image forming apparatus according to the present invention will be described. About the part which overlaps with the said 1st Embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  In the first embodiment, among the fluctuations in the longitudinal gap between the film forming electrode 51 and the developing roller 54, an output image based on the fluctuation of the gap caused by the fluctuation in the inclination accuracy of the film forming electrode 51 and the developing roller 54 in particular. The fluctuation of the concentration is suppressed. However, it is also conceivable that complicated fluctuations such as the gap being reduced only in the central part in the longitudinal direction may occur due to deformation due to durability. Therefore, in the present embodiment, the bias applied to the film forming electrode 51 can be set more finely in order to suppress the density fluctuation of the output image based on the more complicated gap fluctuation.

  FIG. 7 is a schematic diagram schematically showing the configuration of the film forming electrode 51 according to the present embodiment. As shown in FIG. 7, a plurality of longitudinal electrodes 99 (control terminals) electrically connected to a plurality of power sources (not shown) are attached to the film forming electrode 51 side by side in the longitudinal direction. The CPU 71 can control a plurality of power sources (not shown) to apply different biases to the plurality of longitudinal electrodes 99, thereby applying different biases to the film forming electrodes 51 in the longitudinal direction. That is, the plurality of longitudinal electrodes 99 and the power source (not shown) connected to the plurality of longitudinal electrodes 99 are bias applying means for applying a bias to the film forming electrode 51.

  The bias applied to the longitudinal electrode 99 during image formation is such that the electric field strength of the electric field formed between the film forming electrode 51 and the developing roller 54 is substantially constant in the longitudinal direction according to the gap between the film forming electrode 51 and the developing roller 54. Is applied. That is, as in the first embodiment, at the time of shipment, the bias to be applied is set based on the memory 74 in which a bias value is stored such that the electric field strength between the two is substantially constant in the longitudinal direction. After bias adjustment control, the bias set by this control is applied. Note that the bias to be applied may be set only by bias adjustment control.

  With such a configuration, even when the fluctuation of the gap between the developing roller 54 and the film forming electrode 51 is complicated, the fluctuation of the toner concentration in the developer carried on the developing roller 54 due to the fluctuation is suppressed. The uniformity of the density of the output image can be ensured.

  In the present embodiment, 16 longitudinal electrodes 99 are arranged at a pitch of about 20 mm. This is because the configuration of the present embodiment is not for the purpose of correcting streaks of 10 mm or less, but for the purpose of correcting gentle density unevenness.

  Further, it is preferable that the image pattern of the test toner image formed during the bias adjustment control is equal to or more than the number of the longitudinal electrodes 99 because it is easy to set the bias strictly. However, it is not necessarily limited to this.

20 ... Photosensitive drum (image carrier)
51 ... Film-forming electrode (electrode)
54. Developing roller (developer carrier)
53 ... Developer tank (container)
71 ... CPU (control means)
73 ... Reader (reading means, density detection means)
74 ... Memory (storage means)
90 ... First power supply (applying means)
91 ... Second power source (applying means)
99 ... Longitudinal electrode (control terminal)

Claims (7)

In an image forming apparatus for forming an image on a recording material using a liquid developer containing toner particles and a carrier liquid,
An image carrier;
A rotatable developer carrier for carrying a developer supplied to the image carrier;
An accommodating portion for accommodating a developer supplied to the developer carrying member;
The developer is placed opposite to the developer carrier with a predetermined gap, and an electric field is formed between the developer carrier and the developer contained in the container by applying a bias. An electrode to be supplied to the developer carrier;
Control means for controlling application means for applying a bias to the electrodes;
With
The control means has the gap in the rotational axis direction with respect to the electrode so that the electric field strength between the electrode and the developer carrier is substantially constant in the rotational axis direction of the developer carrier. An image forming apparatus, wherein a bias larger than a relatively small portion is applied to a relatively large portion.   The storage unit stores a bias value set corresponding to the gap in advance, and the control unit applies a bias to the electrode based on the bias value stored in the storage unit. The image forming apparatus according to claim 1. Toner image forming means for supplying toner from the developer carrier to the image carrier to form a toner image on the surface of the image carrier;
Density detecting means for detecting the density of the toner image;
Further comprising
The control means forms a test toner image by the toner image forming means and detects the density thereof, and corresponds to a portion where the density of the test toner image is relatively small in the rotation axis direction with respect to the gap. The image forming apparatus according to claim 1, wherein a position is determined to be larger than a position corresponding to a portion having a relatively high density.   The image forming apparatus according to claim 3, wherein the density detecting unit detects the density of a portion in which an image signal when forming the test toner image is 100%.   The image forming apparatus according to claim 3, wherein the density detection unit is a reading unit that reads the toner image transferred to the recording material and detects the density of the toner image. The application means is a plurality of power sources respectively connected to both ends of the electrode in the rotation axis direction;
The control means controls the plurality of power supplies to apply different biases to the both end portions of the electrode, thereby relative to a portion where the gap is relatively large in the rotation axis direction with respect to the electrode. The image forming apparatus according to claim 1, wherein a bias larger than a smaller portion is applied. The application means has a plurality of control terminals attached to the electrodes in the rotation axis direction, and a plurality of power supplies respectively connected to the control terminals,
The control means controls the plurality of power supplies and applies different biases to the control terminals, so that the gap is relatively small in a portion where the gap is relatively large in the rotation axis direction with respect to the electrode. The image forming apparatus according to claim 1, wherein a bias larger than the portion is applied.