JP7483546B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus having a developing device that develops an electrostatic latent image formed on an image carrier using a liquid developer containing toner and a carrier liquid.

Electrophotographic image forming devices (copy machines, printers, fax machines, etc., and all-in-one machines) that develop an electrostatic latent image formed on a latent image carrier such as a photoconductor with charged particles (toner) to form an image are widely used.

In electrophotography, the development process, which is the process of forming an image on a latent image carrier using toner particles, can be broadly divided into dry development, in which the toner particles are used in their powder form, and wet development, in which the toner particles are dispersed in a liquid and used to form an image. Generally, dry developers are used in most electrophotography systems used in offices, homes, or for light printing.

To obtain high-resolution prints, it is advantageous to use a developer with a smaller particle size. However, the smaller the particle size, the stronger the cohesive force between the particles, making it difficult to maintain adequate fluidity, and so the particle size of dry developers is limited to around 5 μm.

In contrast, liquid developers, which control the behavior of toner particles within the liquid carrier, do not scatter and maintain sufficient fluidity because the particles are dispersed in the liquid. For this reason, liquid developers can be made with particle diameters smaller than 1 μm, making it easier to obtain high-quality images (see Patent Document 1). Therefore, in fields such as digital commercial printing, which require higher image quality, the wet development method is a promising image formation process.

Here, we will outline the principles of image formation using liquid developer in an image forming device that uses a wet development method.

In a wet development type image forming device, the liquid developer used for image formation is in a developer mixing tank where toner particles and carrier liquid are present at an appropriate ratio (expressed as the weight percentage concentration (wt%) of toner particles in the entire developer, hereafter referred to as T/D), and the T/D is adjusted by supplying concentrated toner, in which toner particles are stored at a high filling rate (usually T/D is 20 wt% or more), and liquid carrier. The liquid developer in the developer mixing tank is supplied from a developer supply port to a developer supply tank, which supplies developer to the developer carrier by a supply pump.

In the developing device, the toner particles contained in the liquid developer are electrophoretically deposited on the developer carrier, and then transported to the image forming section facing the latent image carrier. Specifically, the liquid developer is first supplied to the developer supply tank, and is carried on the surface of the developer carrier by the rotation of the roller-shaped developer carrier. Then, as it passes through the opposing portion between the developer carrier and the film-forming electrode (gap of 100 μm), the toner is attracted to the surface of the developer carrier by the action of the electric field generated by the potential difference between the film-forming electrode and the surface of the developer carrier.

The liquid developer near the surface of the developer carrier is then transported to the throttling section, a gap of several microns formed by the developer carrier and the throttling member, which is mainly a roller member. When the developing device is not in operation and no liquid developer is present, the developer carrier and the throttling member are in contact with each other at an appropriate pressure, and when the developing device is in operation and liquid developer is supplied from upstream, the liquid developer flows between the two members due to their mutual rotational motion, forming a gap. In the throttling section, the toner particles electrophores due to the action of an electric field generated by the potential difference between the throttling member and the surface of the developer carrier when passing through the gap, and the toner is pressed against the surface of the developer carrier. As a result, a layer of toner particles about 1 to 3 times the diameter of the toner particles is formed closest to the surface of the developer carrier, and a layer of liquid carrier with a thickness of sub-micrometer to micrometer order is formed on top of that.

The liquid developer, which is formed into a film on the developer carrier by the film-forming electrode and the diaphragm member, is transported to the developing section, which faces the latent image carrier, where image formation is carried out. In the developing section, toner particles on the developer carrier move onto the latent image carrier by electrophoresis in the image section in response to the potential difference formed by the latent image drawn on the latent image carrier, and in the non-image section, the toner particles pass through the image forming section while being attracted to the developer carrier. Through this process, the electrostatic latent image drawn on the latent image carrier is made visible by the toner particles. In the developing section, the developer carrier and the latent image carrier are abutted with an appropriate pressure, and the rotation of both causes the developer layer on the developer carrier to flow in while forming a gap between the two carriers.

The toner image formed on the latent image carrier is then transferred to the media by electrophoresis through the processes of primary transfer and secondary transfer, and is fixed onto the media by the fixing means.

On the other hand, toner particles that do not contribute to image formation and remain on the developer carrier are collected by a cleaning member installed downstream. The cleaning member is mainly a roller member facing the developer carrier, and the toner particles transported to the gap between the two members, i.e., the cleaning section, are electrophoresed from the developer carrier to the cleaning member due to the action of the electric field between the two members, and are separated from the developer carrier. Here, as with the throttling section, the developer carrier and the cleaning member are in contact with each other with an appropriate pressure when the developing device is not in operation and no liquid developer is present, and when the developing device is in operation and liquid developer is supplied from upstream, the liquid developer flows between the two members due to their mutual rotational movements, forming a gap. The toner particles carried by the cleaning member are collected by the action of further cleaning members, etc., and returned to the developer mixing tank from the developer discharge port for reuse.

After the toner particles are removed by the cleaning member as described above, the developer carrier surface carries the liquid developer supplied from the developer supply tank again and advances to the area facing the film-forming electrode.

In the wet development device described above, the surface layer of the developer carrier is made of rubber or resin material with elastic mechanical properties. The reason for this is that in order to electrophorese the toner particles in the desired direction in the gaps formed at the locations where the developer carrier and roller-shaped members face each other, such as the squeeze section, development section, and cleaning section, it is necessary to ensure a wide contact width at each location to ensure sufficient time for the toner particles to electrophorese. Therefore, at each facing section, the developer carrier and each member are abutted with strong pressure within a range that does not interfere with operation.

At each point where the developer carrier faces a member, the difference in the moving speed of the member facing the developer carrier, i.e., the difference in peripheral speed, is set to an appropriate value according to each purpose. In a normal development section, the difference in peripheral speed between the developer carrier and the latent image carrier is set to approximately 0% within a range of ± several percent in order to faithfully visualize the latent image formed on the latent image carrier with toner particles. On the other hand, in the throttle section, the difference is often adjusted within a range of ± several hundred percent (-100% or less indicates reverse rotation) with the aim of optimizing the amount of developer carried on the developer carrier.

The surface of the developer carrier is constantly supplied with liquid developer during image formation operation, but when operation stops, the toner particles are removed from the developer carrier and only the carrier liquid remains. Immediately after operation stops, the surface of the developer carrier is wet with carrier liquid, but over time the carrier liquid evaporates from the surface of the developer carrier. Therefore, after a long period of operation stoppage (for example, when the image forming device is turned from OFF to ON), the surface of the developer carrier becomes dry.

In wet development, a predetermined potential difference is created between the developer carrier and each component to electrophoretically migrate the toner. The required potential difference is roughly calculated from the nip passing time calculated from the width of each nip and the printing speed described above, and the migration ability of the toner. Typically, the potential difference applied to each nip during operation is several hundred volts to several kilovolts.

The material used for the surface layer of a conductive developer carrier is usually an elastic body (e.g., rubber) containing ion-conductive or carbon-conductive material. It is generally known that conductive rollers containing these materials deteriorate over time (resistance fluctuations) due to an electric field.

As mentioned above, after a long period of inactivity (when the image forming device is turned from OFF to ON), the surface of the developer carrier is dry, i.e., after a long period of inactivity, no liquid developer is present in the opposing areas between the developer carrier and each contact member.

During start-up of the developing device, if a potential difference is formed when no liquid developer is present in the opposing portions between the developer carrier and each contact member, an excessive current flows between the developer carrier and each contact member compared to normal image formation, in which a potential difference is formed when liquid developer is present in the opposing portions between the developer carrier and each contact member. As a result, the resistance of the elastic layer formed on the surface of the conductive developer carrier increases, and there is a risk of the life of the developer carrier being shortened.

JP 2018-091961 A

The present invention has been made in consideration of the above problems. In a start-up operation of the developing device, which is performed after the power supply of the image forming apparatus is switched from OFF to ON and before the exposure device starts exposing the image carrier, a liquid developer is placed between the developer carrier and each contact member and then a bias is applied to each of the developer carrier and each contact member. This provides an image forming apparatus that can prevent the life of the developer carrier from being shortened.

In order to achieve the above object, an image forming apparatus according to one aspect of the present invention has the following configuration: That is, the image forming apparatus includes an image carrier, an exposure device that exposes the image carrier to light in order to form an electrostatic latent image on the image carrier, a conductive developing roller that carries and transports a liquid developer containing toner and carrier liquid to a development position where the electrostatic latent image formed on the image carrier is developed and has an elastic layer formed on its surface, a development container that contains the liquid developer to be supplied to the developing roller, a first conductive roller that contacts the developing roller and is disposed downstream of a supply position on the developing roller where the liquid developer is supplied from the development container and upstream of the development position in terms of the rotation direction of the developing roller, and a second conductive roller that contacts the developing roller and is disposed upstream of the development position in terms of the rotation direction of the developing roller. a developing device having a developing roller disposed further downstream than the supply position and in contact with the developing roller; a supply device for supplying liquid developer to the developing container; and a developing roller, a first conductive roller, and a second conductive roller disposed upstream of the supply position, the developing roller, the first conductive roller, and the second conductive roller, arranged so that an electric field is formed in which the normally charged toner in the liquid developer is directed from the first conductive roller to the developing roller at a first contact position where the developing roller and the first conductive roller contact, and an electric field is formed in which the normally charged toner in the liquid developer is directed from the developing roller to the second conductive roller at a second contact position where the developing roller and the second conductive roller contact. a bias application unit that applies a bias to each of the developing roller, the first conductive roller, and the second conductive roller, and a rotation drive unit that rotationally drives each of the developing roller, the first conductive roller, and the second conductive roller, and in a start-up operation of the developing device that is performed after a power source of the image forming apparatus is switched from OFF to ON and before the exposure device starts exposing the image carrier, when the liquid developer supplied to the developing container by the supply device reaches the supply position, the rotation drive unit rotationally drives each of the developing roller, the first conductive roller, and the second conductive roller, and the liquid developer supplied to the developing container by the supply device reaches the supply position. The present invention is characterized in that, from the time when the rotation drive unit starts to rotate the developing roller when the liquid developer supplied to the developing container by the supply device reaches the supply position until the developing roller rotates at least one revolution, the bias application unit does not apply a bias to any of the developing roller, the first conductive roller, and the second conductive roller, and after the developing roller rotates at least one revolution after the rotation drive unit starts to rotate the developing roller when the liquid developer supplied to the developing container by the supply device reaches the supply position, the bias application unit applies a bias to each of the developing roller, the first conductive roller, and the second conductive roller.

According to the present invention, in the start-up operation of the developing device , which is performed after the power supply of the image forming device is switched from OFF to ON and before the exposure device starts to expose the image carrier , a liquid developer is interposed between the opposing portions of the developer carrier and each abutting member, and then a bias is applied to each of the developer carrier and each abutting member, thereby preventing the life of the developer carrier from being shortened.

2 is a cross-sectional view showing main components of a developing device 50 and a liquid developer supply/circulation system 200 according to an embodiment of the present invention. 1 is a cross-sectional view showing main components of an image forming apparatus 100 according to an embodiment of the present invention. FIG. 2 is a block diagram showing a control system according to an embodiment of the present invention. 6 is a chart showing the timing of developer supply, voltage application, and driving and stopping of each member in Example 1 (start-up operation). 6 is a chart showing the timing of developer supply, voltage application, and driving stop of each member in the first embodiment (shutdown operation). 10A to 10C are diagrams illustrating the effects of the first embodiment. FIG. 11 is a diagram for explaining the effect of the first embodiment.

The following describes the embodiment of the present invention with reference to the drawings.

In this embodiment, we will explain one form of a developing device and an image forming device to which the present invention is applied.

[Developing device]
First, the configuration of the developing device in this embodiment will be described.

Figure 1 is a cross-sectional view showing the main components of the development device 50.

The developing device 50 is centered around the developing roller 54, which is a developer carrier that transports liquid developer to the photosensitive drum 20, which is a latent image carrier. On the upstream side of the photosensitive drum 20, there are a developer supply tank 53 that stores the developer, a film-forming electrode 51 that carries the liquid developer from the developer supply tank 53 on the developing roller 54 and attracts the toner particles to the developing roller 54 side by the action of an electric field, a squeeze roller 52, which is a squeeze member that further presses the toner particles contained in the liquid developer carried on the developing roller 54 to the developing roller 54 side by the action of an electric field and at the same time squeezes and collects excess carrier liquid, and on the downstream side, there is a developing cleaning roller 58, which is a first cleaning member that collects toner particles on the developing roller 54 that did not contribute to image formation on the photosensitive drum 20 by the action of an electric field. These members form the developing unit 500.

A voltage is applied to each of the developing roller 54, the film-forming electrode 51, the squeezing roller 52, and the developing cleaning roller 58 from a voltage application means (not shown). The movement of the toner particles in the liquid developer is caused by electrophoresis according to the potential difference of the voltage applied to each member, and the amount of movement and pressing of the toner particles is controlled by adjusting the potential difference between each member. In this embodiment, the voltages applied to each member of the developing roller 54, the film-forming electrode 51, and the squeezing roller 52 are all negative voltages, and the voltage applied to the developing cleaning roller 58 is a positive or negative voltage.

The liquid developer used in this embodiment is mainly composed of particles with an average particle size of 0.7 μm, in which a coloring agent such as a pigment is dispersed in a polyester resin, and is added to a liquid carrier such as an organic solvent together with a dispersant, a toner charge control agent, and a charge directing agent, and the surfaces of the toner particles are negatively charged to a certain extent. The specific gravities of the toner particles and the carrier liquid are 1.35 g/ ^cm3 and 0.83 g/ ^cm3 , respectively.

In this embodiment, the image formation process speed is 785 mm/s, and the roller-shaped members that contribute to image formation are each rotated so that the surface peripheral speed is 785 mm/s.

The developing roller 54 is a cylindrical member with a diameter of 42.6 mm, and rotates clockwise around the central axis as shown in FIG. 1. The developing roller 54 has an elastic layer of 4.3 mm thickness made of conductive polymer or the like on the outer periphery of an inner core made of a metal such as stainless steel. That is, an elastic layer is formed on the surface layer of the conductive developing roller 54, and the elastic material constituting the elastic layer is preferably based on a dispersion type resistance adjustment resin in which a resin selected from EPDM, urethane, silicone, nitrile butadiene rubber, chloroprene rubber, styrene butadiene rubber, butadiene rubber, etc. is dispersed and mixed with one or more conductive particles such as carbon, titanium oxide, etc. as an electrical resistance adjustment material, or based on an electrical resistance adjustment resin using one or more ionic conductive materials such as inorganic ionic conductive agents such as sodium perchlorate, calcium perchlorate, sodium chloride, etc. in the above-mentioned resin.

In general, the volume resistivity of the elastic layer is used in the range of 1×10 ² to 1×10 ¹² Ω·cm, including variations, and more preferably in the range of 1×10 ⁵ to 1×10 ⁹ Ω·cm. When a foaming agent is used in the foaming and mixing process to obtain elasticity, a silicon-based surfactant (polydialkylsiloxane, polysiloxane-polyalkylenoxide block copolymer) is appropriate. In general, the hardness of the elastic layer is used in the range of 20° to 50°, more preferably in the range of 25° to 45°, as measured using a JIS standard durometer type A. In this embodiment, the surface layer of the developing roller 54 is made of conductive urethane rubber, and an ion conductive agent is dispersed inside the surface layer of the developing roller, and the volume resistivity is adjusted to 5×10 ⁶ to 1×10 ⁸ Ω·cm in the initial state. Furthermore, the hardness is adjusted to 25° to 30° as measured using a JIS standard durometer type A in the initial state.

The developer supply tank 53 is a place where liquid developer for developing the latent image formed on the photosensitive drum 20 is stored in order to supply it to the developing roller 54. The liquid developer is supplied with a properly adjusted toner particle concentration (hereinafter, indicated as the weight percentage concentration [wt%] of the toner particles in the developer, abbreviated as T/D) in the developer mixing tank 101. In this embodiment, the T/D of the liquid developer in the developer mixing tank 101 is adjusted to 3.0±0.5 wt%, and the liquid developer is supplied from the developer supply port 531 to the developer supply tank 53 by the operation of the developer circulation pump 110. The configuration and operation of the liquid developer supply circulation system 200 will be described in detail separately.

The developer supply tank 53 is provided with a developer surface detection sensor 59 for detecting the developer surface of the liquid developer. Assume that liquid developer is supplied to the developer supply tank 53 from the developer supply port 531 by the operation of the developer circulation pump 110, and the height of the developer surface of the liquid developer in the developer supply tank 53 reaches the height of the detection surface of the developer surface detection sensor 59. In this case, the developer surface detection sensor 59 detects the presence of liquid developer, and the output of the developer surface detection sensor 59 changes from OFF to ON.

The developer supply tank 53 is also provided with a flushing flow path 57 and a developer discharge hole 532, which will be described in detail later. When the supply of liquid developer to the developer supply tank 53 is stopped, the liquid developer leaks out from the developer discharge hole provided in the bottom of the developer supply tank 53, so the amount of liquid developer contained therein gradually decreases and the developer supply tank 53 eventually becomes empty. It is assumed that the liquid developer leaks out from the developer discharge hole provided in the bottom of the developer supply tank 53 and the height of the developer surface in the developer supply tank 53 falls below the height of the detection surface of the developer surface detection sensor 59. In this case, the developer surface detection sensor 59 detects that there is no liquid developer, and the output of the developer surface detection sensor 59 changes from ON to OFF.

The film-forming electrode 51 has a circumferential length of 24 mm on the surface facing the developing roller 54, and forms a gap of 400±80 μm with the developing roller 54. When the liquid developer supplied to the developer supply tank 53 reaches the supply position on the developing roller 54 where the liquid developer is supplied from the developer supply tank 53, it is drawn into the gap between the film-forming electrode 51 and the developing roller 54 by the rotation of the developing roller 54 (arrow A in FIG. 1), and the toner particles are attracted to the developing roller 54 by the electric field generated in the gap due to the difference in applied voltage between the film-forming electrode 51 and the developing roller 54. That is, at the opposing position where the developing roller 54 and the film-forming electrode 51 face each other, an electric field is formed in which the normally charged toner in the liquid developer is directed from the film-forming electrode 51 to the developing roller 54.

The squeeze roller 52 is a cylindrical member (conductive roller) made of metal, and in this embodiment, a roller made of stainless steel with a diameter of 16.8 mm is used. The squeeze roller 52 is in contact with the developing roller 54 so that the pressure is constant (35±5 kPa in this embodiment) over the length (354 mm in this embodiment), and rotates counterclockwise as shown in FIG. 1. Of the liquid developer pumped up from the developer supply tank 53 and conveyed to the squeezing roller 52-developing roller 54 opposing portion at a specified speed through the gap between the film-forming electrode 51 and the developing roller 54, the portion present near the surface of the developing roller 54 enters between the squeeze roller 52 and the developing roller 54 and stably forms a nip with a gap of approximately 1.5 to 2.0 μm and a width of approximately 3 mm (arrow B in FIG. 1). In the nip, the toner particles are pressed against the developing roller 54 by the electric field generated by the difference in applied voltage between the squeeze roller 52 and the developing roller 54. That is, at the contact position where the developing roller 54 and the squeezing roller 52 come into contact, an electric field is formed that directs the normally charged toner in the liquid developer from the squeezing roller 52 to the developing roller 54.

Near the exit of the nip between the squeezing roller 52 and the developing roller 54, the liquid developer separates onto the surface of each roller, but since almost all of the toner particles and carrier liquid present in the nip are carried along to the developing roller 54 side, and only the carrier liquid is carried along to the squeezing roller 52 side, the T/D of the liquid developer layer formed on the developing roller 54 is more than 10 times higher than the T/D of the liquid developer in the developer supply tank.

In this embodiment, after passing through the nip between the squeeze roller 52 and the developing roller 54, a developer layer is formed on the surface of the developing roller 54, with a toner layer of approximately 60 vol% and a thickness of 1.0±0.05 μm, and the carrier liquid layer above that is 0.3±0.05 μm thick, and the T/D in the developer is 50±5 wt.

On the other hand, liquid developer that does not enter the gap between the squeeze roller 52 and the development roller 54 after passing through the gap between the film-forming electrode 51 and the development roller 54 is bounced off the squeeze roller 52 and flows to the back of the film-forming electrode 51 (arrow C in Figure 1) and is collected in the developer collection tank 55.

The liquid developer, which contains a layer of toner particles carried on the developing roller 54, forms a visible image at the opposing portion between the developing roller 54 and the photoreceptor drum 20, i.e., the developing portion, in accordance with the latent image drawn on the photoreceptor drum 20, as described in detail below.

The photoconductor drum 20 is a cylindrical member that is wider than the developing roller and has a photosensitive layer formed on its outer circumferential surface, and rotates counterclockwise as shown in FIG. 1. The photoconductor drum 20's photosensitive layer is usually made of an organic photoconductor or an amorphous silicon photoconductor. In this embodiment, a photoconductor drum 20 with a diameter of 84 mm is used, in which the photosensitive layer is formed from a mixture of amorphous silicon and amorphous carbon.

A charging unit 30 that charges the photoconductor drum 20 in the direction of rotation, and an exposure unit 40 that forms an electrostatic latent image on the charged photoconductor drum 20 are arranged around the photoconductor drum 20 upstream of the development unit.

The charging unit 30 is a device for charging the photoconductor drum 20. In this embodiment, the charging unit 30 is configured with a corona charger, and the surface of the photoconductor drum 20 is charged to approximately -500V by applying a voltage of approximately -4.5kV to -5.5kV to the charging wire. The exposure unit 40 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 photoconductor drum 20 with a modulated laser. In this embodiment, the exposure unit 40 forms a latent image so that the potential of the image area is approximately -100V.

The electrostatic latent image formed on the photoconductor drum 20 upstream of the developing section is developed into a visible image by toner particles in the developing section. In the developing section, in this embodiment, a developing bias of approximately -300V is applied to the developing roller 54, and in the image section, the toner particles move onto the photoconductor drum 20 by electrophoresis according to the electric field formed by the electrostatic latent image on the photoconductor drum 20 (image section: -100V, non-image section: -500V), while in the non-image section, the electric field acts in a direction that presses the toner particles onto the developing roller, so that the toner particles remain on the developing roller. As a result, a visible image by toner particles is formed on the photoconductor drum 20.

The toner particles that have moved onto the photoconductor drum 20 in the development section proceed to the downstream image formation process and are primarily transferred onto the intermediate transfer belt 70. In the primary transfer section, the photoconductor drum 20 and the intermediate transfer belt 70 face each other, and the primary transfer backup roller 61 is in contact with the back surface of the intermediate transfer belt 70. A voltage of the opposite polarity to the charging characteristics of the toner particles (+200 to +300 V in this embodiment) is applied to the primary transfer backup roller 61, and the toner particle image formed on the photoconductor drum 20 moves onto the intermediate transfer belt 70 by electrophoresis. Carrier liquid and a small amount of toner (a few percent) remain on the photoconductor drum 20, but this is scraped off by the photoconductor cleaning blade 21 located downstream of the primary transfer section and collected by the photoconductor cleaning liquid collection section 22.

On the other hand, the toner particles remaining on the developing roller 54 are collected and reused in the process. On the developing roller 54, downstream from the developing section, the developing cleaning roller 58, which is the first cleaning member, is in contact. The developing cleaning roller 58 is in contact with the developing roller 54 so that the pressure is constant (80±5 kPa in this embodiment) over the length (354 mm in this embodiment), and rotates counterclockwise in the cross section shown in FIG. 1. In this embodiment, a roller (conductive roller) with a diameter of 16.8 mm made of stainless steel (metal) is used as the cleaning roller. At the nip between the developing roller 54 and the developing cleaning roller 58, an electric field is generated due to the difference in voltage applied to each, and the toner particles on the developing roller 54 that did not contribute to image formation in the developing section enter the nip and move to the surface of the developing cleaning roller 58 by electrophoresis. That is, at the contact position where the developing roller 54 and the developing cleaning roller 58 come into contact, an electric field is formed in which the normally charged toner in the liquid developer moves from the developing roller 54 to the developing cleaning roller 58.

As shown in FIG. 1, the developer cleaning roller 58 is abutted against the developer cleaning roller cleaning blade (hereinafter simply referred to as the cleaning blade) 56, which is a second cleaning member. The cleaning blade 56 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 58 in the counter direction to the rotation of the developer cleaning roller 58, tilted 30±3° from the vertical direction. The liquid developer containing toner particles collected from the developer roller 54 onto the surface of the developer cleaning roller 58 is scraped off at the contact point between the tip of the cleaning blade 56 and the developer cleaning roller 56, and flows down the slope of the cleaning blade 56 into the developer collection tank 55.

The liquid developer collected on the developing cleaning roller 58 has a different concentration of toner particles (i.e., T/D) depending on the image formed in the developing section, but the maximum T/D is very high, at approximately 65 wt%. At this time, the apparent viscosity of the liquid developer collected on the developing cleaning roller 58 rises to approximately 140 mPa·s.

When liquid developer with such a high apparent viscosity is scraped off by the cleaning blade 56, it becomes difficult for the liquid developer to flow along the slope of the surface of the cleaning blade 56 into the developer recovery tank 55, and therefore the toner particles tend to remain at the tip of the cleaning blade 56 or on the stepped parts of the surface.

In order to alleviate the above situation, in this embodiment, the liquid developer with a low T/D (3.0±0.5 wt%) supplied to the developer supply tank 53 is flushed upstream of the contact area between the developer cleaning roller 58 and the cleaning blade 56 (flushing). Specifically, the flushing function is realized by flowing a part of the liquid developer supplied to the developer supply tank 53 into the flushing flow path 57 provided at the bottom of the film-forming electrode 51 (arrow D in FIG. 1). By flushing, the T/D of the liquid developer collected on the developer cleaning roller 58 is reduced to a maximum of approximately 10 wt%. As a result, the apparent viscosity of the liquid developer is reduced to approximately 8.0 mPa·s, so that the liquid developer scraped off by the cleaning blade 56 flows smoothly down to the developer recovery tank 55 without stagnating on the surface or steps.

The liquid developer that is bounced off the squeeze roller 52 and collected from the back of the film-forming electrode 51 into the developer recovery tank 55, the liquid developer that is collected by the developer cleaning roller 58 and collected into the developer recovery tank 55 by flushing and scraping by the cleaning blade 56, and the liquid developer that leaks from the developer discharge hole 532 into the developer recovery tank 55 are discharged from the developer discharge port 551 and are supplied again to the developer mixing tank 101 via a circulation flow path not shown.

In the liquid developer supply circulation system 200, as described above, the liquid developer whose concentration has been adjusted in the developer mixing tank 101 (3.0±0.5 wt% in this embodiment) is supplied to the developing device 50 by the developer circulation pump 110. Of the liquid developer supplied to the developer supply tank 53, the part that does not contribute to image formation is collected in the developer mixing tank 101 via the developer collection tank 55, and the T/D in the collected liquid developer depends on the amount of toner consumed in image formation. Therefore, the developer mixing tank 101 is provided with a liquid developer concentration detection means (not shown), and concentrated developer and carrier liquid are replenished based on the T/D information obtained by the detection means, and the T/D in the developer mixing tank 101 is controlled to be within the range of appropriate values. In this embodiment, as shown in FIG. 1, concentrated developer, which has a T/D of about 40 wt%, is supplied from concentrated developer container 102 via a buffer (not shown), and carrier liquid is supplied from carrier liquid tank 103 in the required amounts by metering pumps 111a and 111b, respectively.

[Image forming apparatus]
Next, the configuration of the image forming apparatus in this embodiment will be described.

Figure 2 is a diagram showing the main configuration of an image forming device 100 according to an embodiment of the present invention.

The image forming apparatus 100 is a full-color image forming apparatus that uses liquid developers of four colors: yellow (Y), magenta (M), cyan (C), and black (K). As described above in detail, a total of four developing devices 50 for each color are arranged above the intermediate transfer belt 70 in the order of developing devices 50Y, 50M, 50C, and 50K from upstream as shown in FIG. 2.

The intermediate transfer belt 70 is an endless belt stretched around a belt drive roller 82, a driven roller 85, and a secondary transfer inner roller 86, and is driven to rotate while in contact with the photoconductors 20Y, 20M, 20C, and 20K, and the secondary transfer outer roller 81. Four colors of liquid toner are transferred onto the intermediate transfer belt 70 in succession by the primary transfer units 60Y, 60M, 60C, and 60K, which are made up of the intermediate transfer belt 70, the primary transfer backup rollers 61Y, 61M, 61C, and 61K, and the photoconductors 20Y, 20M, 20C, and 20K, to form a full-color liquid toner image.

The primary transfer unit 60K is a device for transferring the black liquid toner image formed on the photoreceptor 20K to the intermediate transfer belt 70.

The toner image transferred onto the intermediate transfer belt 70 by the primary transfer unit 60K travels to the secondary transfer unit 80. In the secondary transfer unit 80, a voltage of +1000V is applied to the secondary transfer outer roller 81, and the secondary transfer outer roller 82 is kept at 0V, and the toner particles on the intermediate transfer belt 70 are secondarily transferred onto the surface of a medium such as paper. The developer remaining on the intermediate transfer belt 70 after the secondary transfer is collected by an intermediate transfer belt cleaning member (not shown).

The secondary transfer unit 80 is composed of a secondary transfer outer roller 81, an intermediate transfer belt drive roller 82, a secondary transfer outer roller blade 83, and a secondary transfer roller cleaning liquid recovery section 84, and transfers the single-color liquid toner image or full-color liquid toner image formed on the intermediate transfer belt 70 onto a recording medium such as paper.

Test images for monitoring the image density are periodically drawn on the intermediate transfer belt during image formation operations, and the density is detected by a toner image density sensor 87 provided upstream of the secondary transfer unit 80. In this embodiment, the toner image density sensor 87 is an optical sensor that detects the density of the toner image from the intensity of the regular reflection and diffuse reflection of the LED light irradiated onto the test image. Based on the information on the detected toner image density, the image density is optimized by feedback control. Specifically, the image density is adjusted by adjusting the voltage applied to the film deposition electrode 51.

In a fixing unit (not shown), the single-color liquid toner image or full-color liquid toner image transferred onto the recording medium is fixed onto the recording medium.

[Operation of Suppressing Increase in Resistance of Developer Carrier Surface Layer]
The following describes how the invention may be embodied in this example.

A. Control system related to start of operation of developing device In the method of the present invention, the invention is realized by appropriately controlling the rotational drive of the developer circulation pump 110 for supplying liquid developer to the developer supply tank 53, the developing roller 54, the squeezing roller 52, and the developing cleaning roller 58, and the voltages applied to the developing roller 54, the film deposition electrode 51, the squeezing roller 52, and the developing cleaning roller 58. Fig. 3 shows an excerpt of the control system of the image forming apparatus 100 of this embodiment that is necessary for realizing the method of the present invention.

In this embodiment, liquid developer is continuously supplied from the developer mixing tank 101 to the developer supply tank 53 during image formation. At that time, the supplied liquid developer advances between the film-forming electrode 51 and the developing roller 54 and is carried on the developing roller 54, or advances to the flushing passage 57 and contributes to flushing on the developing cleaning roller 58. In addition, a part of the liquid developer leaks from the developer supply tank 53 to the developer recovery tank 55 through the developer discharge hole 532. When the supply of liquid developer to the developer supply tank 53 is stopped, the supply of liquid developer to the developing roller 54 and the brushing passage 57 stops, and then the liquid developer gradually leaks out from the developer discharge hole 532 until the developer supply tank 53 is finally empty.

During image formation, voltages are applied to the developing roller 54, the film-forming electrode 51, the squeezing roller 52, and the developing cleaning roller 58, respectively, which act as driving forces for the electrophoresis of the toner particles. In this embodiment, the voltages applied to the developing roller 54, the squeezing roller 52, and the developing cleaning roller 58 during image formation are -350V to -300V, -750V to -350V, and -150V to +700V, respectively, and are appropriately controlled according to the resistivity of the liquid developer or the resistivity of the surface layer of the developing roller 54. In this embodiment, detection means (not shown in Figures 1 and 2) are provided in the developer mixing tank 101 to detect the resistivity of the liquid developer, and around the developing roller 54 to detect the resistivity of the surface layer of the developing roller 54. The voltage applied to the film-forming electrode 51 is controlled by the image density detected by the toner image density sensor 87 installed on the intermediate transfer belt 70. This is because the mobility (movement speed relative to the electric field strength) of the toner particles in the liquid developer that contributes to image formation changes depending on the consumption status of the toner particles, etc. In a typical situation, the voltage applied to the film-forming electrode 51 is -600 to -900 V.

During image formation, the developing roller 54, squeezing roller 52, and developing cleaning roller 58 each rotate at approximately the same surface peripheral speed. The driving force for rotation is provided to the developing roller 54 by a motor (not shown), and the driving force is shared from the developing roller 54 to the squeezing roller 52 and developing cleaning roller 58 via gears. Therefore, in this embodiment, these three roller members start and stop rotating at the same time.

The developing unit 500, which includes the developing roller 54, operates so that the developing roller 54 makes contact with and separates from the photosensitive drum 20. In this embodiment, during image formation, the developing roller 54 and the photosensitive drum 20 make contact with each other at a contact pressure of 80±5 kPa. Before and after the image formation, the developing roller 54 is separated from the photosensitive drum 20 and both operations are stopped.

B. Start-up Sequence of the Developing Device Next, the start-up sequence of the developing device 50 in this embodiment will be described.

Specifically, the timing at which the supply of liquid developer from the developer mixing tank 101 to the developer supply tank 53, the rotational drive of the developing roller 54, the squeezing roller 52, and the developing cleaning roller 58, and the application of voltage to each of the developing roller 54, the film-forming electrode 51, the squeezing roller 52, and the developing cleaning roller 58 are controlled to start the image formation operation will be described in detail below using the timing chart shown in Figure 4.

When the developing device 50 is not in operation, as described above, the liquid developer leaks from the developer discharge hole on the bottom of the developer supply tank 53, so the liquid developer is drained out and the tank is empty.

The surface of the developing roller 54 is always supplied with liquid developer during image formation operation, whereas when operation is stopped, the toner particles are removed from the developing roller 54 and only the carrier liquid is attached. Immediately after operation is stopped, the surface of the developing roller 54 is wet with carrier liquid, but over time the carrier liquid evaporates from the surface of the developing roller 54. Therefore, after a long period of operation stoppage (for example, when the power of the image forming device 100 is turned from OFF to ON), the surface of the developing roller 54 is in a dry state. In other words, after a long period of operation stoppage, no liquid developer is present in the opposing portion between the developing roller 54 and the squeezing roller 52, and no liquid developer is present in the opposing portion between the developing roller 54 and the developing cleaning roller 58.

After a long period of inactivity (when the power supply of the image forming apparatus 100 is turned from OFF to ON), the start-up operation of the developing device 50 is started in this state. Let us assume that a bias is applied to each of the developing roller 54, the squeezing roller 52, and the developing cleaning roller 58 in a state in which no liquid developer is present in either the opposing portion between the developing roller 54 and the squeezing roller 52 or the opposing portion between the developing roller 54 and the developing cleaning roller 58. In this case, a potential difference is formed between the developing roller 54 and the squeezing roller 52 in a state in which no liquid developer is present, and a potential difference is formed between the developing roller 54 and the developing cleaning roller 58 in a state in which no liquid developer is present.

During the start-up operation of the developing device 50, if a potential difference is formed between the developing roller 54 and the squeezing roller 52 without the liquid developer being present between them, an excessive current flows between the developing roller 54 and the squeezing roller 52, compared to normal image formation in which a potential difference is formed between the developing roller 54 and the squeezing roller 52 with the liquid developer being present between them. Similarly, if a potential difference is formed between the developing roller 54 and the developing cleaning roller 58 without the liquid developer being present between them, an excessive current flows between the developing roller 54 and the developing cleaning roller 58, compared to normal image formation in which a potential difference is formed between the developing roller 54 and the developing cleaning roller 58 with the liquid developer being present between them. As a result, the resistance of the elastic layer formed on the surface layer of the conductive developing roller 54 increases, and there is a risk of the life of the developing roller 54 being shortened.

The start-up operation of the developing device 50 starts from this state, so first the developer circulation pump 110 is operated to supply the liquid developer contained in the developer mixing tank 101 to the developer supply tank 53 of the developing device 50 (T1).

Next, the developing roller 54, squeezing roller 52, and developing cleaning roller 58 start rotating (T2). In order to suppress the addition of frictional force that may be applied to the elastic layer on the surface of the developing roller 54, it is desirable that the developing roller 54, the squeezing roller 52 that is in contact with the surface of the developing roller 54, and the developing cleaning roller 58 start rotating at the same time. As described above, in this embodiment, the rotating motion of the squeezing roller 52 and the developing cleaning roller 58 is configured such that a driving force is provided from the developing roller 54 via gears, and the above-mentioned purpose is satisfied. Note that when the rotating motion of each roller member is provided by a separate drive, the timing at which their rotating motion starts is synchronized as much as possible.

In addition, in order to prevent excessive current from flowing between the rollers in the nip portions between the developing roller 54 and the squeezing roller 52 and between the developing roller 54 and the developing cleaning roller 58, a state is created in which the liquid developer is supplied to each nip portion when the rotational operation of each roller member begins. Therefore, in this embodiment, the developer supply tank 53 is filled with liquid developer, and the rotational operation of the developing roller 54, the squeezing roller 52, and the developing cleaning roller 58 begins in response to the output of the developer surface detection sensor 59 provided in the developer supply tank 53 changing from OFF to ON (i.e., the developer surface detection sensor 59 detecting the presence of liquid developer). Alternatively, in this embodiment, the rotational operation of the developing roller 54, the squeezing roller 52, and the developing cleaning roller 58 begins after a predetermined time has elapsed since the output of the developer surface detection sensor 59 provided in the developer supply tank 53 changing from OFF to ON (i.e., the developer surface detection sensor 59 detecting the presence of liquid developer).

In the example sequence shown in Figure 4, the time when the output of the developer level detection sensor 59 changes from OFF to ON is defined as T1', and the developing roller 54, squeezing roller 52, and developing cleaning roller 58 start rotating after a predetermined time (T2-T1') has elapsed from T1'.

Then, by rotating the developing roller 54 at least once after the developing roller 54 starts to rotate, liquid developer is present in the nip between the developing roller 54 and the squeezing roller 52 and between the developing roller 54 and the developing cleaning roller 58.

A modified example may be used in which the developing roller 54, squeeze roller 52, and developer cleaning roller 58 start rotating at the same time as the developer circulating pump 110 starts operating during startup of the developing device 50. However, in this modified example, the developing roller 54, squeeze roller 52, and developer cleaning roller 58 rotate for a long time without liquid developer being present in the nip portions between the developing roller 54 and squeeze roller 52 and between the developing roller 54 and developer cleaning roller 58. Therefore, the amount of accumulated friction force applied to the elastic layer on the surface of the developing roller 54 increases until the liquid developer reaches the nip portions between the developing roller 54 and squeeze roller 52 and between the developing roller 54 and developer cleaning roller 58.

Therefore, from the viewpoint of suppressing wear of the elastic layer on the surface of the developing roller 54, it is preferable to delay the start timing of the rotational operation of the developing roller 54, squeezing roller 52, and developing cleaning roller 58 with respect to the start timing of the operation of the developer circulation pump 110, in the start-up operation of the developing device 50, rather than starting the rotational operation of the developing roller 54, squeezing roller 52, and developing cleaning roller 58 at the same time as the operation of the developer circulation pump 110 starts. In this embodiment, the start timing (T2) of the rotational operation of the developing roller 54, squeezing roller 52, and developing cleaning roller 58 is set 6.0 s after the start timing (T1) of the operation of the developer circulation pump 110.

The application of voltage to each of the developing roller 54, the film-forming electrode 51, the squeeze roller 52, and the developing cleaning roller 58 starts after the developing roller 54 rotates at least once (preferably after the developing roller 54 rotates at least once and the rotational movements of the developing roller 54, the squeeze roller 52, and the developing cleaning roller 58 have stabilized) in a state where the liquid developer reaches the supply position on the developing roller 54 where the liquid developer is supplied from the developer supply tank 53 to the developing roller 54 (T3. In this embodiment, T3 is 2.0 s after T2). In addition, it is desirable to apply a slight time difference to the application timing of the voltage to each member so that the magnitude relationship of the potential is not reversed, taking into account the speed of the voltage rise. Although this embodiment is not described in detail in FIG. 4, the voltage is applied in the order of the film-forming electrode 51, the squeeze roller 52, the developing cleaning roller 58, and the developing roller 54, with a 0.5 s difference in timing.

After the developer state on the developing roller 54 has stabilized, the exposure unit 40 creates a latent image on the photoconductor drum 20, and the image is actually formed (T4. In this embodiment, T4 is 3.0 s after T3).

Next, Figure 5 shows the sequence at the end of the operation (when the operation is stopped). Basically, the timing is the opposite to that at the start (when the operation is started). After T5 when image formation is completed, 1.0 s later, T6, the high voltages are stopped. Then, 2.0 s later, T7, the drive is stopped. Finally, T8, the circulation is stopped.

Here, in order to confirm the effect of this embodiment, the resistance fluctuation of the developing roller 54 was compared when (1) the operation of the developing device 50 was started and ended using the method of this embodiment, (2) the operation of the developing device 50 was started and ended at the same timing as the roller drive and high voltage application, and (3) the timing of the roller drive and high voltage application was reversed from this embodiment. Specifically, no image was formed on the photosensitive drum 20, and one part of an A4 size image was formed 10,000 times, and the volume fluctuation of the developing roller 54 during this period was measured.

First, in both sequences, the initial volume resistivity of the developing roller 54 is 1.0E7 Ωcm. In sequence (1) of this embodiment, the volume resistivity after 10,000 times is 7.2E7 Ωcm, which is not much different. On the other hand, in sequence (2), in which the roller is driven and high voltage is applied at the same time, there is a single-digit increase to 1.0E8 Ωcm. Also, in sequence 3, in which the roller is driven after high voltage is applied, there is a nearly double-digit increase to 1.0E9 Ωcm.

As mentioned above, it is desirable to use the developing roller 54 with a volume resistivity of less than 1.0E9 Ωcm, and the effect of this embodiment is much greater than that shown in FIG. 6.

In this embodiment, high voltage is applied after the drive of the developer carrier is stabilized, but it is also possible to apply high voltage in stages at the same time as driving, thereby shortening the time for applying high voltage and shortening the time until an image is created.

In addition, in order to confirm the effect of this embodiment, the degree of damage to the surface elastic layer of the developing roller 54 was compared between a case in which the developing device 50 was started to operate according to the method of this embodiment and a case in which the developing device 50 was started to operate according to another method. Specifically, no image was formed on the photosensitive drum 20, and liquid developer was supplied to the developer supply tank 53, the developing roller 54, the squeezing roller 52, and the developing cleaning roller 58 were rotated, and voltage was applied and stopped to the developing roller 54, the film-forming electrode 51, the squeezing roller 52, and the developing cleaning roller 58 every time equivalent to the formation of 150 A4-size images, and the change in the surface roughness Ra of the elastic layer on the surface of the developing roller 54 was confirmed. In both cases, the initial surface roughness Ra of the developing roller 54 was 0.20 μm. The results are shown in FIG. 7.

First, when the developing device 50 was started to operate using the method of this embodiment, as shown by the solid line in Figure 7, the surface roughness Ra of the elastic layer on the surface of the developing roller 54 increased very slightly from the initial value of 0.20 μm to 0.38 μm over an operating time equivalent to 2,000 k sheets.

As another operation start sequence for the developing device 50, the supply of liquid developer to the developer supply tank 53 and the rotational driving of the developing roller 54, squeezing roller 52, and developing cleaning roller 58 were started simultaneously (corresponding to T1 = T2 in Figure 4), and the durability change of the surface roughness Ra of the elastic layer on the surface of the developing roller 54 was confirmed. As a result, as shown by the dotted line in Figure 7, the surface roughness Ra of the elastic layer on the surface of the developing roller 54 increased to 0.79 μm after an operation time equivalent to 2000 k sheets.

From the above, it has been shown that by using the method of this embodiment, the mechanical stress (mainly friction) applied to the surface of the elastic layer on the surface of the developing roller 54 is suppressed, and the life of the developing roller 54 is extended.

REFERENCE SIGNS LIST 100 Image forming apparatus 20 Photoconductor drum 40 Exposure unit 50 Developing device 52 Squeezing roller 53 Developer supply tank 54 Developing roller 58 Developing cleaning roller 110 Developer circulating pump

Claims (8)

An image forming apparatus,
An image carrier;
an exposure device that exposes the image carrier to light in order to form an electrostatic latent image on the image carrier;
a developing device including: a conductive developing roller which carries and transports a liquid developer containing toner and a carrier liquid to a developing position where an electrostatic latent image formed on the image carrier is developed, and which has an elastic layer formed on its surface; a developing container which contains the liquid developer to be supplied to the developing roller; a first conductive roller which contacts the developing roller and is disposed downstream of a supply position on the developing roller to which the liquid developer is supplied from the developing container and upstream of the developing position in relation to a rotational direction of the developing roller; and a second conductive roller which contacts the developing roller and is disposed downstream of the developing position and upstream of the supply position in relation to the rotational direction of the developing roller;
a supply device for supplying a liquid developer to the developing container;
a bias application unit that applies a bias to each of the developing roller, the first conductive roller, and the second conductive roller so that an electric field is formed in which the normally charged toner in the liquid developer is directed from the first conductive roller to the developing roller at a first contact position where the developing roller and the first conductive roller are in contact with each other, and an electric field is formed in which the normally charged toner in the liquid developer is directed from the developing roller to the second conductive roller at a second contact position where the developing roller and the second conductive roller are in contact with each other;
a rotation drive unit that drives and rotates each of the developing roller, the first conductive roller, and the second conductive roller;
Equipped with
an exposure device that exposes an image carrier to light, the exposure device being configured to expose the image carrier to light when the liquid developer supplied to the developing container by the supply device reaches the supply position, the rotation drive unit rotationally drives each of the developing roller, the first conductive roller, and the second conductive roller, and the bias application unit does not apply a bias to any of the developing roller, the first conductive roller, and the second conductive roller during a period from when the rotation drive unit starts to rotate the developing roller when the liquid developer supplied to the developing container by the supply device reaches the supply position until the developing roller rotates at least one time, and the bias application unit applies a bias to each of the developing roller, the first conductive roller, and the second conductive roller after the developing roller rotates at least one time after the rotation drive unit starts to rotate the developing roller when the liquid developer supplied to the developing container by the supply device reaches the supply position. a sensor for detecting the liquid developer contained in the developing container,
The image forming apparatus according to claim 1, characterized in that, during a start-up operation of the developing device performed after the power of the image forming apparatus is transitioned from OFF to ON and before the exposure device starts exposing the image carrier, the rotational drive unit rotates and drives each of the developing roller, the first conductive roller, and the second conductive roller in response to the sensor detecting liquid developer supplied to the developing container by the supply device. a sensor for detecting the liquid developer contained in the developing container,
The image forming apparatus according to claim 1, characterized in that, during a start-up operation of the developing device that is performed after the power of the image forming apparatus is transitioned from OFF to ON and before the exposure device starts exposing the image carrier, after a predetermined time has elapsed since the sensor detected the liquid developer supplied to the developing container by the supply device, the rotational drive unit rotates and drives each of the developing roller, the first conductive roller, and the second conductive roller. 2. The image forming apparatus according to claim 1, characterized in that, during a start-up operation of the developing device performed after the power supply of the image forming apparatus is switched from OFF to ON and before the exposure device starts exposing the image carrier, the rotation drive unit rotates and drives each of the developing roller, the first conductive roller, and the second conductive roller simultaneously with the supply of liquid developer to the developing container by the supply device. 5. The image forming apparatus according to claim 1, wherein the bias application section applies a bias to the first conductive roller, the second conductive roller, and the developing roller in this order. the developing device further includes an electrode that is disposed downstream of the supply position and upstream of the first contact position in a rotational direction of the developing roller, and that is disposed opposite the developing roller with a predetermined gap therebetween;
6. The image forming apparatus according to claim 1, wherein the bias application unit applies a bias to each of the developing roller and the electrode so that an electric field is formed in which the normally charged toner in the liquid developer is directed from the electrode to the developing roller at an opposing position where the developing roller and the electrode face each other. 7. The image forming apparatus according to claim 1, wherein the first conductive roller is a squeeze roller for regulating an amount of liquid developer carried on the developing roller. 8. The image forming apparatus according to claim 1, wherein the second conductive roller is a cleaning roller for removing toner from the liquid developer carried on the developing roller.

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