Image forming device

Abstract

To predict the concentration of a charge control agent in consideration of an image ratio and supply the charge control agent to stabilize image density.SOLUTION: An image forming apparatus comprises: a first container that stores liquid developer to be supplied to a developer container; a second container that stores a charge control agent to be supplied to the first container; a driving unit that is driven to supply the charge control agent stored in the second container to the first container; and a control unit that controls the driving unit based on the image ratio of an output image so that the concentration of the charge control agent in the liquid developer stored in the first container becomes a predetermined value.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a developing device that develops an electrostatic latent image carried on a latent image carrier by a wet development method using a liquid developer in which toner is dispersed in a medium liquid, and to an image forming apparatus that forms an image using a developing device that uses the wet development method. [Background technology]

[0002] Electrophotography, which forms images by developing an electrostatic latent image formed on an image carrier such as a photoreceptor with charged particles (toner), can be broadly divided into two methods: dry development, which directly uses powder toner, and wet development, which uses a liquid developer in which toner is dispersed in a liquid. Of these, wet development is a promising development method in terms of high image quality and high definition, as it makes it possible to form images by controlling the particle size of the submicron order because the toner is dispersed in a medium (carrier) liquid.

[0003] In wet development, toner particles contained in a liquid developer are electrophoretically transferred onto a medium to form an image. Specifically, a developer containing an appropriate amount of toner is first deposited on a developing roller facing a deposition electrode, and a developer layer of an appropriate thickness is then formed on the developing roller by a squeezing roller. In the development process, the toner is electrophoretically transferred onto the photosensitive drum by an electric field in the development nip where the developing roller and photosensitive drum abut, according to the electrostatic latent image formed on the photosensitive drum. In the primary and secondary transfer processes following development, the image formation principle is that all toner is essentially transferred by an electric field.

[0004] To ensure the toner charge required for migration in the developing unit, it is necessary to maintain the charge control agent concentration in the liquid developer at an appropriate level. However, it is believed that charge control agents are often charged with the opposite polarity to the toner. As a result, when image formation continues and many non-image areas are drawn, the charge control agent migrates to the photosensitive drum and is collected by the photosensitive drum cleaning member. As a result, the charge control agent concentration decreases, resulting in an insufficient toner charge, which prevents a sufficient amount of toner from migrating to the photosensitive drum in the image area in the developing unit, resulting in a decrease in the density of the output image. Furthermore, in the non-image areas, the toner cannot be sufficiently pressed against the developing roller, resulting in the problem of fogging.

[0005] To address this issue, Patent Document 1 adjusts the charge control agent concentration in the developer by replenishing charge control agent from a charge control agent tank based on the optical reflection density of the toner image formed on the photosensitive member or recording paper. Patent Document 2 also measures the surface potential of the toner adhesion surface of the roller electrode in the detection device, and determines the charge control agent concentration from the measured value. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-65295 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-19852 Summary of the Invention [Problem to be solved by the invention]

[0007] The proposed method periodically forms a toner image for detecting the charge control agent concentration to estimate the charge control agent concentration, and then compensates for image degradation due to a decrease in the charge control agent concentration by adjusting the concentration by replenishing the charge control agent from the charge control agent tank. However, the charge control agent concentration changes with each image formation, and this method cannot address the decrease in the charge control agent concentration between periodic concentration control. Furthermore, because it takes time to form and measure the detection toner image, frequent charge control agent concentration control can lead to reduced productivity.

[0008] An object of the present invention is to predict the concentration of the charge control agent in consideration of the image ratio and to stabilize the image density by replenishing the charge control agent. [Means for solving the problem]

[0009] One aspect of the present invention is a developing device including an image carrier on which an electrostatic image is formed, an exposure device that exposes the image carrier to light in order to form an electrostatic image on the image carrier, a developing container that contains a liquid developer containing toner and a carrier liquid, and a developer carrier that carries and transports the liquid developer in order to develop the electrostatic image formed on the image carrier, a first container that contains the liquid developer to be replenished to the developing container, and a charge control agent to be replenished to the first container. Carrier liquid containing a second container for accommodating the charge control agent; a drive unit that is driven to replenish the first container with the charge control agent accommodated in the second container; and a control unit that controls the drive unit based on an image ratio of an output image so that the concentration of the charge control agent in the liquid developer accommodated in the first container becomes a predetermined value. The carrier liquid contained in the second container has a charge control agent concentration of 10 wt% or more and 20 wt% or less. The image forming apparatus is characterized by the above.

[0010] One aspect of the present invention is an image forming apparatus comprising: an image carrier on which an electrostatic image is formed; an exposure device that exposes the image carrier to light to form an electrostatic image on the image carrier; a developer container that contains a liquid developer including toner and a carrier liquid; and a developer carrier that carries and transports the liquid developer to develop the electrostatic image formed on the image carrier; a first container that contains the liquid developer to be replenished to the developer container; a second container that contains the carrier liquid including a charge control agent to be replenished to the first container; a drive unit that is driven to replenish the charge control agent contained in the second container to the first container; and a control unit that controls the drive unit so that the amount of charge control agent contained in the second container replenished to the first container is greater when the image ratio of an output image is a first ratio than when the image ratio of an output image is a second ratio lower than the first ratio, wherein the concentration of the charge control agent in the carrier liquid contained in the second container is 10 wt % or more and 20 wt % or less. 。 One aspect of the present invention is a developing device including an image carrier on which an electrostatic image is formed, an exposure device that exposes the image carrier to light in order to form an electrostatic image on the image carrier, a developing container that contains a liquid developer containing toner and a carrier liquid, and a developer carrier that carries and transports the liquid developer in order to develop the electrostatic image formed on the image carrier, a first container that contains the liquid developer to be replenished to the developing container, a second container that contains a charge control agent to be replenished to the first container, a third container that contains the carrier liquid to be replenished to the first container, a fourth container that contains the toner to be replenished to the first container, and a third container that contains the carrier liquid to be replenished to the first container. and a control unit that controls the drive unit so that the amount of charge control agent contained in the second container replenished to the first container when the image ratio of the output image is a second ratio lower than the first ratio is greater than the amount of charge control agent contained in the second container replenished to the first container when the image ratio of the output image is a first ratio. [Effects of the Invention]

[0011] According to the present invention, the image density can be stabilized by predicting the charge control agent concentration in consideration of the image ratio and replenishing the charge control agent. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] 1 is a diagram showing a configuration of a developing device according to a first embodiment of the present invention. [Figure 3] FIG. 1 is a block diagram showing a control system according to a first embodiment of the present invention. [Figure 4] FIG. 10 is a diagram showing the configuration of a developing device according to a second embodiment of the present invention. [Figure 5] FIG. 10 is a block diagram showing a control system according to a second embodiment of the present invention. [Figure 6] FIG. 2 is a control flowchart of an embodiment of the present invention. [Figure 7] FIG. 2 is a control flowchart of an embodiment of the present invention. [Figure 8] FIG. 2 is a schematic diagram of a control time chart according to the embodiment of the present invention. [Figure 9] FIG. 2 is a schematic diagram of a control time chart according to the embodiment of the present invention. [Figure 10] FIG. 10 is a diagram showing the peeling rate of the charge control agent when an electric field is applied to the nip portion. [Figure 11] FIG. 10 is a diagram showing the dependency of the charge control agent reduction rate on the image ratio. [Figure 12] 1A is a graph showing a change in the concentration of a charge control agent in Example 1 of the present invention, and FIG. 1B is a graph showing a change in development efficiency in Example 1 of the present invention. [Figure 13] 10(a) is a graph showing a change in the concentration of a charge control agent in Example 2 of the present invention, and FIG. 10(b) is a graph showing a change in development efficiency in Example 2 of the present invention. [Figure 14]FIG. 10 is a diagram showing the transition of the charge control agent replenishment interval with respect to the durability status of the liquid developer. [Figure 15] FIG. 10 is a graph showing the change in the concentration of a charge control agent in Example 3 of the present invention. [Figure 16] FIG. 10 is a diagram showing the relationship between the predicted concentration of charge control agent and the replenishment amount ratio of charge control agent in Example 4 of the present invention. [Figure 17] FIG. 10 is a graph showing the change in the concentration of the charge control agent in Example 4 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] An example of the image forming apparatus of the present invention will be described below. [Example]

[0014] [Image forming equipment] First, the configuration of an image forming apparatus 100 in this embodiment will be described with reference to FIG.

[0015] The intermediate transfer belt 70 is driven to rotate while in contact with the photosensitive drums 20Y, 20M, 20C, and 20K and the secondary transfer unit 80. Four color toners are sequentially transferred onto the intermediate transfer belt 70 in an overlapping manner by primary transfer units 60Y, 60M, 60C, and 60K, which are made up of the intermediate transfer belt 70, primary transfer backup rollers 61Y, 61M, 61C, and 61K, and photosensitive drums 20Y, 20M, 20C, and 20K.

[0016] The secondary transfer unit 80 transfers the toner image formed on the intermediate transfer belt 70 onto a recording medium such as paper. In a fixing unit (not shown), the toner image transferred onto the recording medium is fixed onto the recording medium.

[0017] The developing units 50Y, 50M, 50C, and 50K have the function of developing the latent images with liquid developers containing toner particles of yellow (Y), magenta (M), cyan (C), and black (K) colors, respectively.

[0018] Since the developing units 50Y, 50M, 50C, and 50K of each color and their peripheral configurations are similar, the following will describe in detail one developing unit 50K and its periphery, and will omit descriptions of the other developing units 50Y, 50M, and 50C.

[0019] 1, around photoreceptor 20K, along the direction of rotation, are arranged a charging unit 30K that charges the photoreceptor, an exposure unit 40K that forms an electrostatic latent image on the charged photoreceptor 20K, and a primary transfer unit 60K. Photoreceptor 20K has a cylindrical substrate and a photosensitive layer formed on its outer peripheral surface, is rotatable around its central axis, and in this embodiment, rotates counterclockwise as indicated by the arrow in FIG.

[0020] The charging unit 30K is a device for charging the photoconductor 20K. The exposure unit 40K has a semiconductor laser, a polygon mirror, an F-θ lens, etc., and irradiates the charged photoconductor 20K with a modulated laser to form a latent image.

[0021] The developing unit 50K is a device for developing the latent image formed on the photoreceptor 20K with black (K) liquid toner. Details of the developing unit 50K will be described later.

[0022] The primary transfer unit 60K is a device for transferring the toner image formed on the photosensitive member 20K onto the intermediate transfer belt .

[0023] [Developing device] Next, the configuration of the developing device in this embodiment will be described with reference to FIG.

[0024] The developing unit 50K is centered around a developing roller 51 as a developer carrier that carries and transports (carries and transports) liquid developer to the photoconductor 20K, with a developer supply tank 55, a film-forming electrode 52, and a squeezing roller 53 arranged upstream of the photoconductor 20K, and a developer cleaning roller 54 arranged downstream of the photoconductor 20K. Here, the film-forming electrode 52 serves to attract toner in the liquid developer supplied from the developer supply tank to the developing roller 51 by using an electric field, the squeezing roller 53 serves to pack the toner particles by using an electric field and at the same time squeeze out excess carrier liquid to form a developer layer of several μm on the developing roller 51, and the developer cleaning roller 54 serves to collect remaining toner in non-image areas from the developing roller 51 by using an electric field.

[0025] The developer supply tank 55 temporarily stores liquid developer for developing the latent image formed on the photoreceptor 20K, and supplies it to the developing roller 51. The developer supply tank 55 receives liquid developer from the developer agitation tank 57, with the toner particle mass concentration adjusted to approximately 3 wt% and the charge control agent mass concentration adjusted to approximately 0.1 wt%. The average particle size of the toner in the liquid developer is generally 0.5 to 2.0 μm. However, the liquid developer used in this embodiment is a mixture of toner particles with an average particle size of 0.8 μm, in which a colorant such as a pigment is dispersed in a polyester resin, and toner dispersants and charge control agents are added to a liquid carrier such as an organic solvent, and the surfaces of the toner particles are negatively charged. The amount of movement and pressure of the toner particles is controlled by adjusting the potential difference between each component. In this embodiment, the specific gravities of the toner particles and the carrier liquid are 1.3 g / cm. 3 , 0.9g / cm 3 Lipidure-S was used as a charge control agent.

[0026] The developer tank 581 contains developer containing toner, and serves to replenish developer to the developer agitation tank 57. The toner mass concentration of the developer in the developer tank 581 is generally between 15 wt% and 25 wt% but is set to approximately 20 wt% in this embodiment. One developer tank 581 is provided for each of the developing units 50Y to 50K of the Y, M, C, and K colors, and developer is replenished from the developer tank 581 of each color to the developer agitation tank 57 of each color. In this embodiment, the mass concentration of the charge control agent in the developer contained in the developer tank 581 is zero (0 wt%).

[0027] The carrier tank 582 is a location for storing carrier liquid, and serves to replenish the carrier liquid to the developer stirring tank 57. In this embodiment, the mass concentration of the charge control agent in the carrier liquid stored in the carrier tank 582 is zero.

[0028] Charge control agent tank 583 contains carrier liquid containing a specified amount or more of charge control agent. Charge control agent tank 583 replenishes charge control agent to developer agitation tank 57 when the control unit of image forming apparatus 100 predicts a decrease in the concentration of charge control agent in the liquid developer inside developer agitation tank 57. The concentration of charge control agent in the carrier liquid contained in charge control agent tank 583 is generally 10 wt% to 20 wt%, but in this embodiment it is set to 15 wt%. Developer agitation tank 57 is provided with a stirring member for stirring the developer (toner) supplied from developer tank (fourth container) 581, the carrier liquid supplied from carrier tank (third container) 582, and the carrier liquid (charge control agent) supplied from charge control agent tank 583.

[0029] There is one carrier tank 582, and the developing units 50Y to 50K of each color are supplied with carrier liquid from the same carrier tank 582. There is also one charge control agent tank 583, and the developing units 50Y to 50K of each color are supplied with carrier liquid containing a specified amount or more of charge control agent (the charge control agent concentration in the carrier liquid is 10 wt% or more and 20 wt% or less) from the same charge control agent tank 583.

[0030] Generally, the image formation process speed is 500 mm / s to 2000 mm / s, but in this embodiment, the image formation process speed is set to 800 mm / s, and the roller-shaped members that contribute to image formation are rotated so that the surface peripheral speed is 800 mm / s.

[0031] The surface of film-forming electrode 52 facing developing roller 51 has a circumferential length of 24 mm, and forms a gap of 400±30 μm with developing roller 51. Liquid developer supplied to developer supply tank 55 from developer stirring tank 57 is drawn into the gap between film-forming electrode 52 and developing roller 51 by the rotation of developing roller 51. As the toner in the liquid developer passes through the gap formed by developing roller 51 and film-forming electrode 52, it is attracted toward developing roller 51 by an electric field generated by the potential difference between developing roller 51 and film-forming electrode 52.

[0032] The squeeze roller 53 is a metal roller; in this embodiment, a stainless steel roller with a diameter of 16 mm is used. The squeeze roller 53 is in contact with the developing roller 51 so that pressure is constant along its length (354 mm in this embodiment) and rotates counterclockwise as shown in Figure 2. The liquid developer that has passed through the film-forming electrode 52 passes through a nip formed by the developing roller 51 and the squeeze roller 53, with a gap thickness of approximately 6 μm and a width of approximately 3 mm. In the nip, the toner is pressed toward the developing roller by an electric field generated by the potential difference between the developing roller 51 and the squeeze roller 53, forming a toner layer and a carrier layer. At the nip exit, the carrier layer separates between the developing roller 51 and the squeeze roller 53. As a result, in this embodiment, the toner mass concentration of the liquid developer film formed on the developing roller 51 is 50±5 wt%.

[0033] On the other hand, after passing through the gap between the developing roller 51 and the film-forming electrode 52, the liquid developer that cannot flow into the nip between the developing roller 51 and the squeezing roller 53 is bounced off the squeezing roller 53 and flows through the back surface of the film-forming electrode 52 into the developer recovery tank 56.

[0034] 2, a developer cleaning blade 541 is in contact with the developer cleaning roller 54. The developer cleaning blade 541 is a stainless steel blade with a thickness of 0.2 mm and a free length of 20 mm, and its tip is abutted against the developer cleaning roller 54 in the counter direction to the rotation direction of the developer cleaning roller 54, at an angle of 30±3° from the vertical direction. The toner particles collected from the developer roller 51 onto the surface of the developer cleaning roller 54 are scraped off by the developer cleaning blade 541 and flow down the slope of the developer cleaning blade 541 into the developer collection tank 56.

[0035] The liquid developer that has flowed into developer recovery tank 56 is discharged from developer discharge port 561 and returns to developer agitation tank 57. As shown in FIG. 2, the liquid developer circulates between development unit 50K and developer agitation tank 57. Therefore, the concentration of the charge control agent in the liquid developer in development unit 50K and the concentration of the charge control agent in the liquid developer in developer agitation tank 57 are maintained at substantially the same concentration. Furthermore, the toner concentration in the liquid developer in development unit 50K and the toner concentration in the liquid developer in developer agitation tank 57 are maintained at substantially the same concentration.

[0036] The toner particles in the liquid developer layer on the developing roller 51 form a visible image in the opposing portion between the developing roller 51 and the photosensitive drum 20K, i.e., the developing portion, in accordance with the latent image drawn on the photosensitive drum 20K, as will be described in detail below.

[0037] The photosensitive drum 20K is a cylindrical member that is wider than the developing roller 51 and has a photosensitive layer formed on its outer circumferential surface, and rotates counterclockwise as shown in FIG. 2. The photosensitive layer of the photosensitive drum 20K is usually made of an organic photosensitive material or an amorphous silicon photosensitive material. In this embodiment, a photosensitive drum with a diameter of 84 mm is used, in which the photosensitive layer is formed of a mixture of amorphous silicon and amorphous carbon.

[0038] Around the photosensitive drum 20K, a charging unit 30K that charges the photosensitive drum 20K in the direction of rotation, and an exposure unit 40K that forms an electrostatic latent image (electrostatic image) on the charged photosensitive drum 20K are arranged upstream of the development unit.

[0039] The charging unit 30K is a device for charging the photosensitive drum 20K. In this embodiment, the charging unit 30K is configured as a corona charger, and by applying a voltage of approximately -4.5 kV to -5.5 kV to the charging wire, the photosensitive drum surface is charged to approximately -500 V. The exposure unit 40K has a semiconductor laser, a polygon mirror, an F-θ lens, etc., and forms an electrostatic latent image by irradiating the charged surface of the photosensitive drum 20K with a modulated laser. In this embodiment, the exposure unit 40K forms the latent image so that the potential of the image area is approximately -100 V.

[0040] In this embodiment, a bias of approximately -300V is applied to the developing roller 51, and in the image area, toner particles move by electrophoresis onto the photosensitive drum 20K in accordance with the electric field formed by the electrostatic latent image on the photosensitive drum 20K (image area: -100V, non-image area: -500V), while in the non-image area, the electric field acts in a direction that presses the toner particles onto the developing roller 51, causing them to remain on the developing roller 51. In this way, a visible image is formed by the toner particles on the photosensitive drum 20K. In order to migrate a sufficient amount of toner to the photosensitive drum in the image area in the developing section, it is desirable to always maintain an appropriate concentration of charge control agent in the liquid developer inside the developing unit 50K and the developer stirring tank 57.

[0041] [Means for predicting charge control agent concentration] Next, the charge control agent concentration predicting means for the liquid developer will be described in detail.

[0042] The charge control agent, which is added to control the charge amount of the toner, is thought to be charged with the opposite polarity to the toner in the liquid developer, as can be seen from its function of imparting a charge to the toner. Therefore, it is thought that the charge control agent moves in the opposite direction to the toner in the development nip where an electric field is applied to the toner, and as a result, more of the charge control agent is present on the roller opposite to the roller where the toner is present at the nip exit separation point.

[0043] Figure 10 shows the results of an actual experiment to confirm the above situation. The horizontal axis is the voltage applied to the development nip, and the vertical axis is the percentage of charge control agent that migrated to the roller opposite the roller to which the toner moved, i.e., the peeling rate of the charge control agent. The experimental method is described in detail below.

[0044] The experimental setup consisted of a rubber roller with a rubber surface layer and a metal roller that were rotated without any difference in peripheral speed. The rest of the setup was the same as described above. In this state, a voltage was applied between the two rollers to the side that would draw the toner toward the rubber roller, and a liquid developer with a known concentration of charge control agent was dropped onto the rubber roller. The dropped liquid developer entered the nip between the rubber roller and the metal roller as the rollers rotated, and was subjected to the action of an electric field. The toner was drawn toward the rubber roller within the nip, leaving only the carrier liquid on the metal roller. This remaining carrier liquid contained charge control agent that had been removed by the action of the electric field. Therefore, by contacting a rubber blade with the metal roller and measuring the concentration of the charge control agent, the rate at which the charge control agent was removed when the electric field was applied could be calculated.

[0045] 10, which shows the results of the above experiment, it can be seen that when voltage is applied to the nip, approximately 60% of the charge control agent that flows in moves to the side opposite the toner. When non-image areas are continuously output, approximately 60% of the charge control agent that flows into the development nip is collected in the photoreceptor cleaning liquid collection section 22, and the more non-image areas are output, the lower the concentration of the charge control agent inside the development unit 50K and developer agitation tank 57 becomes.

[0046] The charge control agent concentration is calculated from the amount of charge control agent that moves with the toner in the image area and the amount that peels off from the toner and moves in the non-image area. Figure 11 is a graph showing the rate of charge control agent reduction in the developer stirring tank 57 versus the image ratio, calculated under conditions of a charge control agent concentration of 0.1 wt% and a charge control agent peeling rate of 70% in the developing section. The relative reduction rate is shown, with the amount of charge control agent reduction when the image ratio is 0 being set to 1. As shown in Figure 11, the charge control agent concentration can be predicted from the image ratio of the output image.

[0047] [Means for controlling the concentration of charge control agent] Next, a detailed description will be given of the charge control agent concentration control means that is implemented when the charge control agent concentration prediction means predicts a decrease in the charge control agent concentration.

[0048] Fig. 3 shows an excerpt of the parts necessary to embody the present invention in Example 1. A controller 110 in Fig. 3 is a control unit that controls the image forming device 120 that forms the toner image described above, and constitutes a charge control agent concentration prediction means. Specifically, the controller 110 includes a CPU 2, a prediction mechanism 3, a storage device 4, and input data 8.

[0049] The CPU 2 is configured to command the image forming engine 5 to form an image, and also functions as a counter that keeps track of the total number of images formed. The storage device 4 also stores data that records the correspondence between the image ratio and the amount of charge control agent consumed. The prediction mechanism 3 calculates a predicted value for the charge control agent concentration based on the image ratio of the output image. Furthermore, the CPU 2 is configured to start the motor (drive unit) 7 via the device controller 6, and to replenish the charge control agent from the charge control agent tank 583 to the developer agitation tank 57 based on the predicted value for the charge control agent concentration predicted by the prediction mechanism 3.

[0050] That is, in this embodiment, developer agitation tank 57 is a first container that contains liquid developer to be replenished to developer supply tank 55 as a developing container, charge control agent tank 583 is a second container that contains charge control agent to be replenished to the first container, and motor 7 is a drive unit that is driven to replenish the charge control agent contained in the second container to the first container. Controller 110 is configured to function as a control unit that controls the drive unit based on the image ratio of the output image so that the concentration of the charge control agent in the liquid developer contained in the first container becomes a predetermined value.

[0051] More specifically, when the power supply of the image forming apparatus 100 is turned on (step S1 in FIG. 6) and image formation on a sheet is started (step S2), first, the operation unit 1 transmits a command to start image formation to the prediction mechanism 3 of the present invention via the CPU 2. The CPU 2 also commands the image creation engine 5 to form an image.

[0052] When the start of image formation is notified, the prediction mechanism 3 obtains the image ratio of the output image from the input data 8, and refers to the corresponding data from the storage device 4 to predict the amount of charge control agent reduction during image formation (step S3).

[0053] More specifically, the prediction mechanism 3 first acquires the image ratio α (%) at the start of image formation. Then, it predicts the charge control agent concentration decrease rate X = β × (50 - α) for image formation from the previously stored charge control agent concentration change rate β when the image ratio changes by 1%. Furthermore, it calculates an updated charge control agent concentration predicted value Y = Y0 × (1 - X / 100) from the charge control agent concentration predicted value Y0 before the start of image formation and the charge control agent concentration decrease rate X, and updates the charge control agent concentration predicted value Y0 before the start of image formation with the calculated charge control agent concentration predicted value Y.

[0054] CPU 2 determines whether the newly calculated charge control agent concentration predicted value Y is below a specified value Z (for example, 0.08 wt %, which is a charge control agent concentration predicted value that can achieve sufficient image density) (step S4, decision 1). Then, every time the charge control agent concentration predicted value Y falls below the specified value Z, that is, when Y≦Z (YES in step S4), the driving of motor 7 described above is started (time t1 in FIG. 8), and charge control agent is replenished from charge control agent tank 583, thereby increasing the charge control agent concentration in developer agitation tank 57 and restoring the decreased charge control agent concentration (step S5).

[0055] At this time, the replenishment of charge control agent is terminated when the charge control agent concentration of the liquid developer reaches a preset recovery judgment level (for example, the charge control agent concentration before use) (step S6, time t2 in FIG. 8). Then, when the replenishment of charge control agent is completed, image formation of the next image is started. Note that if the charge control agent concentration prediction value Y does not fall below the specified value Z in step S4 (NO in step S4), CPU 2 starts image formation of the next image without replenishment of charge control agent.

[0056] The graph in Figure 12(a) shows how the charge control agent concentration of the liquid developer in developer stirring tank 57 changes with the charge control agent concentration control means described above and a comparative example (a control means that detects the charge control agent concentration every 1000 sheets of image formation and replenishes the charge control agent). Here, the charge control agent peeling rate was set to 70%, the image ratio in normal printing mode to 10%, and the charge control agent concentration before use to 0.1 wt%, and the charge control agent concentration was controlled by the prediction means described above when the charge control agent concentration prediction value Y fell below the specified value Z = 0.08. The recovery judgment criterion was the charge control agent concentration before use. As shown in Figure 12(a), in the comparative example (dashed line), the charge control agent concentration decreases by a maximum of 62.0% from the initial concentration after 5,000 images have been formed. In contrast, when the charge control agent concentration control unit described above is used (solid line), the charge control agent concentration decrease is limited to a maximum of 23.9% from the initial concentration after 5,000 images have been formed. Furthermore, Figure 12(b) shows the development efficiency (the rate of toner movement in the development nip) over time. In the comparative example (dashed line), the development efficiency decrease is a maximum of 7.2% after 5,000 images have been formed. In contrast, when the charge control agent concentration control unit described above is used (solid line), the development efficiency decrease is a maximum of 2.8% after 5,000 images have been formed. The present invention can suppress the decrease in development efficiency, and therefore the decrease in image density. While the above description of charge control agent concentration control has been given using the development unit 50Y as an example, similar control is performed for development units 50M, 50C, and 50K.

[0057] In the present invention, the amount of charge control agent consumed is predicted from the image ratio of the output image, so there is no need to form a new toner image for detecting the charge control agent concentration.This makes it easier to control the charge control agent concentration at shorter time intervals than when a detection toner image is formed, and the image density can be stabilized compared to the comparative example. [Example]

[0058] Next, a different embodiment of the present invention will be described. Note that the configuration and operation of the image forming apparatus of this embodiment overlaps with those of the first embodiment in many respects, so only the differences from the first embodiment will be described.

[0059] [Image forming equipment] The image forming apparatus is the same as in the first embodiment, so a description thereof will be omitted.

[0060] [Developing device] The difference between the configuration of the developing device in this embodiment and that in the first embodiment will be described with reference to FIG.

[0061] As described in the first embodiment, a toner image is formed on the photosensitive drum 20K.

[0062] An optical reflection density measuring device 72 is disposed downstream of this photosensitive drum 20K. The optical reflection density measuring device 72 irradiates the formed toner image with light and can detect the optical reflection density of the developer from the reflected light. Furthermore, the charge control agent concentration of the developer can be predicted by referring to a correspondence table between the optical reflection density and the charge control agent concentration, which has been obtained by measuring in advance.

[0063] [Means for predicting charge control agent concentration] The charge control agent concentration predicting means for the liquid developer is the same as that in the first embodiment, and therefore the description thereof will be omitted.

[0064] [Means for controlling the concentration of charge control agent] Next, a detailed description will be given of the charge control agent concentration control means that is implemented when the charge control agent concentration prediction means predicts a decrease in the charge control agent concentration.

[0065] Fig. 5 shows an excerpt of the parts necessary to embody the method of the present invention in the second embodiment. The differences from block diagram 3 in the first embodiment will be explained with reference to Fig. 5. The prediction mechanism 3 executes the detection mechanism 10 based on the cumulative number of image formation sheets transmitted from the CPU 2, and receives charge control agent concentration data. The charge control agent concentration prediction value is changed according to the received charge control agent concentration data.

[0066] In the charge control agent concentration detecting means, whenever the number of image formation sheets A exceeds a certain specified number of sheets B (for example, 2000 sheets, which is twice the number of image formation sheets for which charge control agent concentration detection is performed in the comparative example), i.e., whenever A≧B (decision 2, NO in step S10 of FIG. 7), a detection toner image is formed and the charge control agent concentration is predicted from the optical reflection density detected by the optical reflection density measuring device 72 (step S11). The charge control agent concentration predicted value Y is changed according to the charge control agent concentration obtained by the charge control agent concentration detecting means, and charge control agent is replenished from the charge control agent tank 58, thereby increasing the charge control agent concentration in the developer stirring tank 57 and restoring the decreased charge control agent concentration.

[0067] The graph in Figure 13(a) shows how the charge control agent concentration of the liquid developer in the developer stirring tank 57 changes over time for the charge control agent concentration control means described above and a comparative example (a control means that detects the charge control agent concentration every 1000 sheets of image formation and replenishes the charge control agent). The charge control agent peeling rate was set to 70%, the image ratio in normal printing mode to 10%, and the charge control agent concentration before use to 0.1 wt%. The charge control agent concentration was controlled by the prediction means described above when the charge control agent concentration prediction value Y fell below the specified value Z = 0.08, and the charge control agent concentration was detected every specified number of sheets B = 2000. The recovery judgment criterion was the charge control agent concentration before use. As shown in Figure 13(a), in the comparative example (dashed line), the charge control agent concentration decreases by a maximum of 62.0% from the initial concentration after 5,000 images have been formed, whereas when the charge control agent concentration control means is used (solid line), the charge control agent concentration decrease is limited to a maximum of 21.6% from the initial concentration after 5,000 images have been formed. Figure 13(b) shows the change in development efficiency. When the charge control agent concentration control means is used (solid line), the decrease in development efficiency is a maximum of 2.5% after 5,000 images have been formed. The present invention can further suppress the decrease in development efficiency than Example 1, and therefore can further suppress the decrease in image density.

[0068] Although it is necessary to form a detection toner image periodically as in the conventional method, the present invention makes it possible to make the detection interval longer than in the conventional method while maintaining the stability of the image density.

[0069] FIG. 7 shows a control flowchart of the second embodiment, and FIG. 9 shows a schematic diagram of a time chart after the second determination. [Example]

[0070] [Image forming equipment] The image forming apparatus is the same as in the second embodiment, and therefore the description thereof will be omitted.

[0071] [Developing device] The configuration of the developing device in this embodiment is the same as that in the second embodiment, and therefore the description thereof will be omitted.

[0072] [Means for predicting charge control agent concentration] The charge control agent concentration predicting means for the liquid developer is the same as that in the second embodiment, and therefore the description thereof will be omitted.

[0073] [Means for controlling the concentration of charge control agent] Next, the difference from the second embodiment will be described in detail with respect to the means for controlling the charge control agent concentration, which is implemented when the charge control agent concentration prediction means predicts a decrease in the charge control agent concentration.

[0074] Liquid developers deteriorate over time, and the charge control agent peeling rate also changes. Therefore, in Example 3, the prediction mechanism 3 in Figure 5 changes Y according to the liquid developer's durability, and the charge control agent replenishment interval changes accordingly. Figure 14 shows the change in charge control agent replenishment interval over the course of liquid developer durability. The graph shows the change in charge control agent replenishment interval over the course of 30,000 images, assuming an initial charge control agent peeling rate of 70%, an image ratio of 10% in the normal print mode, and a charge control agent concentration of 0.1 wt% before use. The charge control agent is replenished when the predicted charge control agent concentration value Y falls below the specified value Z = 0.08 (the charge control agent concentration detection means does not change the replenishment amount). Because the charge control agent peeling rate increases over time, it is necessary to shorten the charge control agent replenishment interval and control the charge control agent replenishment amount.

[0075] The graph in Figure 15 shows how the charge control agent concentration of the liquid developer in the developer agitation tank 57 changes with the charge control agent concentration control means described above and the charge control agent concentration control means of Example 2. Here, the charge control agent peeling rate was 70%, the image ratio in the normal print mode was 10%, and the charge control agent concentration before use was 0.1 wt%. The charge control agent concentration was controlled by the prediction means when the charge control agent concentration prediction value Y fell below the specified value Z = 0.08, and the charge control agent concentration was detected every specified number B = 2,000 sheets. As shown in Figure 15, when the charge control agent concentration control means of Example 2 was used (dashed line), the charge control agent concentration decreased by a maximum of 31.2% from the initial concentration after 30,000 sheets of image formation. When the charge control agent concentration control means described above was used (solid line), the charge control agent concentration decreased by a maximum of 20.9% from the initial concentration after 30,000 sheets of image formation. That is, compared with Example 2, the decrease in the concentration of the charge control agent can be suppressed, and the image density can be stabilized. [Example]

[0076] [Image forming equipment] The image forming apparatus is the same as in the first embodiment, so a description thereof will be omitted.

[0077] [Developing device] The configuration of the developing device in this embodiment is the same as that in the first embodiment, so a description thereof will be omitted.

[0078] [Means for predicting charge control agent concentration] The charge control agent concentration predicting means for the liquid developer is the same as that in the first embodiment, and therefore the description thereof will be omitted.

[0079] [Means for controlling the concentration of charge control agent] Next, differences from the first embodiment will be described in detail regarding the charge control agent concentration control means implemented when the charge control agent concentration prediction means predicts a decrease in the charge control agent concentration. In the first embodiment, when the charge control agent concentration prediction value Y falls below a certain specified value Z, charge control agent is replenished up to a predetermined recovery judgment level for the charge control agent concentration of the liquid developer (for example, the charge control agent concentration before use). However, the charge control agent concentration may be maintained by changing the replenishment amount per time depending on the image ratio. Therefore, in the fourth embodiment, the replenishment amount of charge control agent is changed according to the charge control agent concentration prediction value Y calculated from the image ratio data at every specified replenishment interval γ (for example, 200 sheets). Figure 16 shows the ratio of the charge control agent replenishment amount to the predicted charge control agent concentration when the charge control agent peeling rate is 70%, the charge control agent concentration before use is 0.1 wt% and the charge control agent replenishment amount when the predicted charge control agent concentration is 0 is set to 1. That is, the charge control agent concentration control means controls the motor 7 to change the amount of charge control agent replenished so that the amount of charge control agent replenished from the charge control agent tank 583 when the image ratio of the output image is a second ratio lower than the first ratio is greater than the amount of charge control agent replenished from the charge control agent tank 583 when the image ratio of the output image is a first ratio.

[0080] Figure 17 shows how the charge control agent concentration of the liquid developer in the developer agitation tank 57 changes over time using the charge control agent concentration control unit described above and a comparative example (a control unit that detects the charge control agent concentration and replenishes the charge control agent every 1,000 sheets of image formation). The charge control agent peeling rate was 70%, the image ratio was randomly set between 0 and 20%, and the charge control agent concentration before use was 0.1 wt%. In the comparative example (dashed line), the charge control agent concentration decreased by a maximum of 62.0% from the initial concentration after 5,000 sheets of image formation. However, when the charge control agent concentration control unit described above is used (solid line), the charge control agent concentration decreased by a maximum of 23.0% from the initial concentration after 5,000 sheets of image formation. In other words, the present invention can stabilize image density compared to the comparative example.

[0081] Although the embodiments of the present invention have been described above, the present invention is not limited to the above. [Explanation of symbols]

[0082] 100: image forming apparatus / 20: image carrier (photosensitive drum) / 40: exposure device (exposure unit) / 50: developing device / 51: developer carrier (developing roller) / 55: developer container (developer supply tank) / 57: first container (developer stirring tank) / 58: second container (charge control agent tank) / 7: drive unit (motor) / 110: control unit (controller) / 582: third container (carrier tank) / 581: fourth container (developer tank)

Claims

1. an image carrier on which an electrostatic image is formed; an exposure device that exposes the image carrier to light in order to form an electrostatic image on the image carrier; a developing device including a developer container that contains a liquid developer containing toner and a carrier liquid, and a developer carrier that carries and transports the liquid developer to develop an electrostatic image formed on the image carrier; a first container for accommodating a liquid developer to be replenished to the developing container; a second container containing a carrier liquid containing a charge control agent to be replenished to the first container; a driving unit that is driven to replenish the charge control agent contained in the second container to the first container; a control unit that controls the drive unit based on an image ratio of an output image so that the concentration of the charge control agent in the liquid developer contained in the first container becomes a predetermined value, The carrier liquid contained in the second container has a charge control agent concentration of 10 wt % or more and 20 wt % or less. An image forming apparatus characterized by:

2. an image carrier on which an electrostatic image is formed; an exposure device that exposes the image carrier to light to form an electrostatic image on the image carrier; a developing device including a developer container that contains a liquid developer containing toner and a carrier liquid, and a developer carrier that carries and transports the liquid developer to develop an electrostatic image formed on the image carrier; a first container for accommodating a liquid developer to be replenished to the developing container; a second container containing a carrier liquid containing a charge control agent to be replenished to the first container; a driving unit that is driven to replenish the charge control agent contained in the second container to the first container; a control unit that controls the drive unit so that the amount of charge control agent contained in the second container replenished to the first container when the image ratio of the output image is a first ratio is greater than the amount of charge control agent replenished when the image ratio of the output image is a second ratio that is lower than the first ratio, The carrier liquid contained in the second container has a charge control agent concentration of 10 wt % or more and 20 wt % or less. An image forming apparatus characterized by:

3. a third container containing a carrier liquid to replenish the first container; a fourth container for containing toner to be replenished to the first container; a stirring member provided in the first container for stirring the carrier liquid containing the charge control agent supplied from the second container, the carrier liquid supplied from the third container, and the toner supplied from the fourth container; The carrier liquid contained in the third container has a charge control agent concentration of 0 wt %, The concentration of the charge control agent in the toner contained in the fourth container is 0 wt %.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. a third container containing a carrier liquid to replenish the first container; a fourth container for containing toner to be replenished to the first container; a stirring member provided in the first container for stirring the carrier liquid containing the charge control agent supplied from the second container, the carrier liquid supplied from the third container, and the toner supplied from the fourth container; The carrier liquid contained in the third container has a charge control agent concentration of 0 wt %, The concentration of the charge control agent in the toner contained in the fourth container is 0 wt %.

3. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium.

5. an image carrier on which an electrostatic image is formed; an exposure device that exposes the image carrier to light to form an electrostatic image on the image carrier; a developing device including a developer container that contains a liquid developer containing toner and a carrier liquid, and a developer carrier that carries and transports the liquid developer to develop an electrostatic image formed on the image carrier; a first container for accommodating a liquid developer to be replenished to the developing container; a second container containing a charge control agent to be replenished to the first container; a third container containing a carrier liquid to replenish the first container; a fourth container for containing toner to be replenished to the first container; a stirring member provided in the first container for stirring the charge control agent supplied from the second container, the carrier liquid supplied from the third container, and the toner supplied from the fourth container; a driving unit that is driven to replenish the charge control agent contained in the second container to the first container; a control unit that controls the drive unit so that the amount of charge control agent contained in the second container replenished to the first container when the image ratio of the output image is a first ratio is greater than the amount of charge control agent replenished when the image ratio of the output image is a second ratio that is lower than the first ratio, An image forming apparatus characterized by:

Images