JP2019109418A - Image forming apparatus - Google Patents

Abstract

To ensure a nip part that is long enough to sufficiently move a toner on an intermediate transfer belt by electrophoresis, and prevents a change in contact surface according to belt drive.SOLUTION: An intermediate transfer belt 21 is pushed from an outside to an inside with a cleaning roller 31 to bend the intermediate transfer belt 21 in order to ensure a nip part T3 long enough to form a liquid layer of a liquid developer that can move a toner by electrophoresis. Doing this increases an amount of winding of the intermediate transfer belt 21 around the cleaning roller 31, and causes the nip length of the nip part T3 to be a length allowing the toner on the intermediate transfer belt 21 to be moved by electrophoresis. The cleaning roller 31 is fixed to an outer peripheral side of the intermediate transfer belt 21 so as not to be displaced with respect to an idler roller 80 and a driving roller 23 that stretch the intermediate transfer belt 21. Since the intermediate transfer roller 21 is bent by these fixed rollers, a contact area of the nip part T3 is not easily changed during belt drive.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an electrophotographic image forming apparatus that forms an image using a liquid developer.

  Conventionally, an electrostatic latent image formed on a photosensitive drum is developed into a toner image by using a liquid developer containing toner and carrier liquid, and the developed toner image is primarily transferred onto a transfer drum, and further onto the transfer drum. There is known an image forming apparatus for secondarily transferring a primarily transferred toner image onto a recording material. In an apparatus using a liquid developer, a cleaning roller is in contact with the transfer drum in order to remove toner remaining on the transfer drum after the secondary transfer (Japanese Patent Application Laid-Open No. 2001-147118). The toner moves through the liquid developer from the transfer drum to the cleaning roller in accordance with an electric field formed as a voltage is applied to the cleaning roller at a nip portion formed by the cleaning roller and the transfer drum coming into contact with each other (so-called , Electrophoresis). Then, the toner moved to the cleaning roller is removed together with the liquid developer by a cleaning blade which rubs against the cleaning roller.

  The image forming apparatus uses a dry developer instead of a liquid developer, but in order to remove toner from the intermediate transfer belt, an image in which an opposing roller is disposed inside the belt as a pair of rollers and a cleaning roller is disposed outside the belt A forming apparatus has been proposed (Patent Document 2). In this apparatus, the intermediate transfer belt held between the opposing roller and the cleaning roller is stretched so as to bend inward so that the intermediate transfer belt is wound around the cleaning roller, and the cleaning roller is pressed to the opposing roller side ing.

JP, 2011-158905, A JP, 2002-318493, A

  By the way, in the case of an image forming apparatus using a liquid developer, the toner may not be sufficiently removed from the intermediate transfer belt. Therefore, in order to improve the cleaning performance, it is conceivable to stretch the intermediate transfer belt sandwiched between the opposing roller and the cleaning roller so as to bend the inside of the belt. However, when the intermediate transfer belt is bent to the inner side of the belt as in the prior art, the size (contact area) of the nip portion fluctuates with the driving of the intermediate transfer belt, and the toner can be removed stably. There was a risk of disappearing. Therefore, in an image forming apparatus using a liquid developer, a device capable of suppressing the fluctuation of the contact area between the intermediate transfer belt and the cleaning roller is desired even when the intermediate transfer belt is stretched around the cleaning roller. However, such a device has not been proposed yet.

  In view of the above problems, the present invention can suppress the variation in the contact area of the intermediate transfer belt and the cleaning roller even when the intermediate transfer belt is stretched around the cleaning roller in an image forming apparatus using a liquid developer. It is an object of the present invention to provide an image forming apparatus.

  The image forming apparatus according to the present invention forms on the outer peripheral surface an endless belt member that carries and rotates a liquid developer containing toner and carrier liquid, and a contact portion that contacts the outer peripheral surface of the belt member. An outer roller through which toner is moved from the belt member via a liquid developer according to a potential difference generated between the belt member, an inner roller contacting the outer roller through the belt member, and the belt member The first roller, which first stretches the belt member downstream of the outer roller and the inner roller, and the upstream direction of the outer roller and the inner roller with respect to the movement direction of the belt member. A second roller for stretching the belt member first, and the outer roller, the first roller, and the second roller are rotatably fixed, and the first roller and the second roller are provided. The at least one roller of the second roller is provided on the same side as the outer roller with respect to a tangent at an intersection point of a straight line connecting the rotation center of the outer roller and the rotation center of the inner roller and the inner roller. Are characterized.

  The image forming apparatus according to the present invention forms on the outer peripheral surface an endless belt member that carries and rotates a liquid developer containing toner and carrier liquid, and a contact portion that contacts the outer peripheral surface of the belt member. The contact portion with respect to the moving direction of the belt member, the outer roller on which the toner is moved from the belt member via the liquid developer according to the potential difference between the belt member and the contact position with respect to the moving direction of the belt member And the first roller that stretches the belt member first on the downstream side of the contact portion, and the position where the first roller contacts the belt member with respect to the moving direction of the belt member. And a second roller which is disposed at a position not overlapping the belt and which first stretches the belt member upstream of the contact portion, the outer roller, the first roller, and the second roller The first roller and at least one of the second roller are rotatably fixed, and the outer roller is at the center of the contact portion with respect to the moving direction of the belt member. It is characterized in that it is provided on the same side as the roller.

  According to the present invention, in the image forming apparatus using the liquid developer, the nip member is formed by winding the belt member around the outer roller by the plurality of rollers including the rotatably fixed outer roller. Thus, even when the intermediate transfer belt is stretched around the cleaning roller, it is possible to easily suppress the fluctuation of the contact area of the belt member and the cleaning roller.

FIG. 1 is a schematic view showing a configuration of an image forming apparatus of the present embodiment. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the outline | summary of a belt cleaning apparatus, (a) is the schematic which shows the structure of a belt cleaning apparatus, (b) is an enlarged view which shows a nip part. The graph which shows the relation between the thickness of a belt, and the nip length. FIG. 7 illustrates electrophoresis of toner. BRIEF DESCRIPTION OF THE DRAWINGS Schematic which shows the belt cleaning apparatus of 1st embodiment. The graph which shows the relation between the electric field strength and the nip length. It is a figure explaining the belt cleaning apparatus of 2nd embodiment, (a) is a schematic diagram which shows arrangement | positioning of an opposing roller and a cleaning roller, (b) is an enlarged view which shows a nip part. FIG. 7 is a schematic view illustrating offset of the opposing roller and the cleaning roller. FIG. 7 is a schematic view showing a secondary transfer unit of the third embodiment. FIG. 14 is a schematic view showing a secondary transfer unit of the fourth embodiment. FIG. 13 is a schematic view showing a secondary transfer unit of the fifth embodiment. FIG. 13 is a schematic view showing a secondary transfer unit of the sixth embodiment. It is a figure explaining the belt cleaning apparatus of other embodiment, (a) is a schematic diagram which shows arrangement | positioning of an opposing roller and a cleaning roller, (b) is an enlarged view which shows a nip part.

First Embodiment
<Image forming apparatus>
The first embodiment will be described. First, the configuration of the image forming apparatus of the present embodiment will be described with reference to FIG. An image forming apparatus 10 shown in FIG. 1 is a full-color printer of an intermediate transfer type having a tandem configuration in which a plurality of image forming units UY, UM, UC, and UK are arranged. In the case of the present embodiment, the intermediate transfer unit 20 is disposed below the plurality of image forming units UY to UK in the direction of gravity.

  The intermediate transfer unit 20 includes an endless intermediate transfer belt 21 as primary belt members, primary transfer rollers 22Y to 22K, a drive roller 23, a tension roller 24, a secondary transfer inner roller 25, and an idler roller 80. The intermediate transfer belt 21 is stretched around and supported by the drive roller 23, the tension roller 24, the secondary transfer inner roller 25, the idler roller 80 and the like, and is driven by the drive roller 23 to rotate in the arrow R2 direction in FIG. In the present embodiment, the idler roller 80 is rotatably fixed to the inner peripheral side of the intermediate transfer belt 21 and stretches the intermediate transfer belt 21. On the other hand, the drive roller 23 is rotatably fixed on the inner peripheral side of the intermediate transfer belt 21, and stretches the intermediate transfer belt 21 downstream of the secondary transfer inner roller 25 in the moving direction of the intermediate transfer belt 21.

  The image forming units UY to UK are arranged along the moving direction of the intermediate transfer belt 21 (the direction of the arrow R2 in FIG. 1). In the image forming unit UY, a yellow toner image is formed on the photosensitive drum 11 Y and transferred to the intermediate transfer belt 21. In the image forming unit UM, a magenta toner image is formed on the photosensitive drum 11M and transferred to the intermediate transfer belt 21. In the image forming units UC and UK, cyan toner images and black toner images are formed on the photosensitive drums 11C and 11K, respectively, and are transferred to the intermediate transfer belt 21. The four color toner images transferred to the intermediate transfer belt 21 are conveyed to the secondary transfer portion T2 and collectively transferred to the recording material P (paper, sheet material such as OHP sheet, etc.). In the present embodiment, the intermediate transfer belt 21 carries and rotates a liquid developer including toner and carrier liquid.

  The image forming units UY to UK are configured substantially the same except that the colors of toners used in the developing devices 4Y, 4M, 4C, and 4K as supplied portions are different from yellow, magenta, cyan, and black. Therefore, in the following, the configurations and operations of the image forming units UY to UK will be described by omitting Y, M, C, and K at the end of the reference numerals indicating the distinction between the image forming units UY, UM, UC, and UK.

  In the image forming unit U, a primary charger 12, an exposure device 13, a developing device 4, and a drum cleaning device 14 are disposed so as to surround a photosensitive drum 11 as a photosensitive member. In the image forming unit U, the primary transfer roller sandwiches the intermediate transfer belt 21 so that the primary transfer portion T1 of the toner image is formed between the photosensitive drum 11 and the intermediate transfer belt 21 by the primary transfer roller 22. It is disposed at a position opposed to 22. A photosensitive layer is formed on the outer peripheral surface of an aluminum cylinder, and the photosensitive drum 11 is rotated in the direction of arrow R1 of FIG. 1 at a predetermined process speed.

  The primary charger 12 irradiates, for example, charged particles associated with corona discharge to charge the photosensitive drum 11 to a uniform negative dark potential. The exposure device 13 scans, with a rotating mirror, a laser beam in which scanning line image data obtained by developing separated color images of each color is ON-OFF modulated, and writes an electrostatic latent image of the image on the charged photosensitive drum 11 surface. The electrostatic latent image is developed as a toner image by the developing device 4.

  The developing device 4 contains a liquid developer in which powder toner, which is a dispersoid, is dispersed in a carrier liquid, which is a dispersion medium. The liquid developer is supplied to the developing device 4 from a mixer (not shown). The liquid developer supplied from the mixer to the developing device 4 is coated (supplied) on the developing roller 4 b by the coat roller 4 a in the developing device 4 and used for development. The developing roller 4 b carries and transports a liquid developer on the surface, and develops the electrostatic latent image formed on the photosensitive drum 11 with toner. The coating of the liquid developer from the coating roller 4a to the developing roller 4b and the development of the electrostatic latent image on the photosensitive drum 11 from the developing roller 4b are performed using an electric field. The liquid developer that has not been used for development is returned from the developing device 4 to the mixer and reused.

  At the primary transfer portion T1 formed by the primary transfer roller 22, the toner image formed on the photosensitive drum 11 is primarily transferred to the intermediate transfer belt 21 using an electric field. The liquid developer (toner and carrier liquid) remaining on the photosensitive drum 11 after the primary transfer is collected by the drum cleaning device 14.

  The four color toner images transferred to the intermediate transfer belt 21 are conveyed to the secondary transfer portion T2 and collectively secondarily transferred to the recording material P. In the secondary transfer portion T2, a secondary transfer voltage is applied to the secondary transfer outer roller 26 as a transfer member, whereby the toner image is secondarily transferred from the intermediate transfer belt 21 to the recording material P. The recording material P is conveyed by the secondary transfer unit 50 to the secondary transfer portion T2. The toner (residual toner) remaining on the intermediate transfer belt 21 after the secondary transfer is removed together with the carrier liquid by the belt cleaning device 30. The belt cleaning device 30 will be described later (see FIG. 2A).

<Secondary transfer unit>
The secondary transfer unit 50 includes a secondary transfer belt 51, a secondary transfer outer roller 26, a separation roller 53, an idler roller 54, a belt drive roller 56, and a two-turn cleaning device 30A. The secondary transfer unit 50 secondarily transfers the toner image when the recording material P carried on the secondary transfer belt 51 passes through the secondary transfer portion T2.

  The secondary transfer belt 51 is formed in an endless belt shape, and is stretched around the secondary transfer outer roller 26, the separation roller 53, the idler roller 54, and the belt drive roller 56. The secondary transfer belt 51 is rotated in the direction of arrow R4 in the figure in synchronization with the intermediate transfer belt 21 by the belt drive roller 56, passes the recording material P through the secondary transfer portion T2, and conveys it to the fixing device (not shown). Do. The belt drive roller 56 stretches the secondary transfer belt 51 downstream of the idler roller 54 in the moving direction of the secondary transfer belt 51. A resin belt is employed for the secondary transfer belt 51. Specifically, it is made of polyimide.

  The secondary transfer outer roller 26 is in pressure contact with the secondary transfer inner roller 25 via the intermediate transfer belt 21 and the secondary transfer belt 51, and a secondary transfer portion between the intermediate transfer belt 21 and the secondary transfer belt 51. Form T2. A secondary transfer voltage having a polarity opposite to that of the toner is applied to the secondary transfer outer roller 26 by a high voltage power supply (not shown), and a transfer electric field is generated in the secondary transfer portion T2 accordingly. In response to the transfer electric field, the yellow, magenta, cyan and black toner images carried on the intermediate transfer belt 21 are collectively secondarily transferred onto the recording material P.

  The separation roller 53 separates the recording material P from the secondary transfer belt 51 downstream of the secondary transfer portion T <b> 2 in the moving direction of the secondary transfer belt 51. Specifically, after the recording material P on the secondary transfer belt 51 reaches the separation roller 53, the curvature separation from the secondary transfer belt 51 on the curved surface of the secondary transfer belt 51 along the circumferential surface of the separation roller 53 Do. In the case of this embodiment, the separation roller 53 applies a predetermined tension to the secondary transfer belt 51 by urging the secondary transfer belt 51 from the inside to the outside by the urging force of a pressure spring (not shown). ing. That is, the separation roller 53 doubles as a tension roller.

  The recording material P whose curvature is separated from the secondary transfer belt 51 is conveyed to a fixing device (not shown), and the toner image is fixed on the recording material P by the fixing device. The recording material P on which the toner image is fixed is discharged to the outside of the apparatus main body (outside of the apparatus). Then, after passing through the secondary transfer portion T2, the toner on the secondary transfer belt 51 is removed by the secondary transfer cleaning device 30A. The second roll cleaning device 30A will be described later (see FIGS. 9 to 12).

<Liquid developer>
Next, liquid developers used in the developing devices 4Y to 4K will be described. Although a conventionally used liquid developer may be used as the liquid developer, a UV-curable liquid developer is used in the present embodiment.

  The liquid developer is an ultraviolet curable liquid developer containing a cationically polymerizable liquid monomer, a photopolymerization initiator, and toner particles insoluble in the cationically polymerizable liquid monomer. The cationically polymerizable liquid monomer is a vinyl ether compound, and the photopolymerization initiator is a compound represented by the following general formula (Formula 1).

  It will be described more specifically. First, toner particles contain a coloring material that emits a color in toner resin. In addition to the toner resin and the colorant, other materials such as a charge control agent may be contained. As a method for producing toner particles, known techniques such as core shelving in which a coloring material is dispersed, polymerizing and encapsulating resin gradually, melting of resin and the like, and incorporating coloring material in resin may be used. Good. As the toner resin, epoxy, styrene acrylic, etc. are used. The colorant that emits a color may be a general organic-inorganic pigment. In addition, although a dispersant is used to improve toner dispersibility in production, a synergist is also possible.

  The curable liquid, which is a carrier liquid, is composed of a charge control agent that gives a charge on the toner surface, a photopolymerization agent that generates an acid by UV irradiation that is ultraviolet light, and a monomer that is bonded by an acid. The monomer is a vinyl ether compound that polymerizes by a cationic polymerization reaction. In addition to the photopolymerization agent, a sensitizer may be contained. Since the preservability is reduced by photopolymerization, 10 to 5000 ppm of a cationic polymerization inhibitor may be added. In addition, charge control aids, other additives, etc. may be used.

  The UV curing agent (monomer) of this developer is a vinyl ether represented by the chemical formula (Chem. 3) with about 10% (% by weight) of a monofunctional monomer having one vinyl ether group represented by the chemical formula (Chem. 2) About 90% of bifunctional monomers having two groups are mixed.

  As the photopolymerization initiator, 0.1% of one represented by the following (chemical formula 4) is mixed. By using this photopolymerization initiator, a high resistance liquid developer can be obtained unlike the case where an ionic photoacid generator is used, while enabling good fixing.

  The cationically polymerizable liquid monomer is dicyclopentadiene vinyl ether, cyclohexane dimethanol divinyl ether, tricyclodecane vinyl ether, trimethylolpropane trivinyl ether, 2-ethyl-1,3-hexanediol divinyl ether, 2,4-diethyl- 1,5-Pentanediol divinyl ether, 2-butyl-2-ethyl-1,3-propanediol divinyl ether, neopentyl glycol divinyl ether, pentaerythritol tetravinyl ether and 1,2-decanediol divinyl ether It is desirable that the compound be

  Furthermore, as the charge control agent, known ones can be used. Specific compounds include oils and fats such as linseed oil and soybean oil; alkyd resins, halogen polymers, aromatic polycarboxylic acids, acid group-containing water-soluble dyes, oxidation condensates of aromatic polyamines, cobalt naphthenate, naphthenic acid Metallic soaps such as nickel, iron naphthenate, zinc naphthenate, cobalt octylate, nickel octylate, zinc octylate, cobalt dodecylate, nickel dodecylate, zinc dodecylate, aluminum stearate, cobalt 2-ethylhexanoate; Metal salts of petroleum based sulfonic acid metal salt, metal salt of sulfosuccinic acid ester; phospholipids such as lecithin; salicylic acid metal salts such as t-butylsalicylic acid metal complex; polyvinyl pyrrolidone resin, polyamide resin, sulfonic acid-containing resin, Hydroxybenzoic acid derivatives etc. .

<Belt cleaning device>
The configuration of the belt cleaning device 30 according to the present embodiment will be described with reference to FIGS. 2 (a) and 2 (b). As shown in FIG. 2A, the belt cleaning device 30 includes a cleaning container 33 forming a casing, a cleaning roller 31, a cleaning blade 32, an opposing roller 40, and the like.

  The opposing roller 40 as an inner roller is rotatably provided on the inner peripheral side of the intermediate transfer belt 21, and the idler roller 80 and the driving roller 23 (see FIG. 1) with respect to the movement direction (arrow R 2 direction) of the intermediate transfer belt 21. Contact with the inner peripheral surface (rear surface) of the intermediate transfer belt 21. The opposing roller 40 is driven to rotate by the intermediate transfer belt 21. A portion of the cleaning container 33 facing the intermediate transfer belt 21 is open, and the cleaning roller 31 is rotatably provided so as to be exposed to the outside from this portion. The cleaning roller 31 as an outer roller is disposed on the opposite roller 40 with the intermediate transfer belt 21 interposed therebetween, and is in contact with the outer peripheral surface (surface) of the intermediate transfer belt 21. The opposing roller 40 and the cleaning roller 31 abut on the inner peripheral surface and the outer peripheral surface of the intermediate transfer belt 21, respectively, to form a cleaning nip portion (hereinafter simply referred to as a nip portion T3) as an abutting portion. In the present embodiment, the nip portion T3 is formed by winding the intermediate transfer belt 21 around the cleaning roller 31 so as to have the physical nip T3b and the tension nip T3a shown in FIG. 2B. In the present specification, the physical nip T3b as the first contact portion refers to a region (contact region) in which the opposing roller 40 and the cleaning roller 31 simultaneously contact the intermediate transfer belt 21 on the front and back. The cleaning roller 31 contacts the intermediate transfer belt 21 on the surface of the contact area, and the opposing roller 40 contacts the back surface of the contact area. The tension nip T3a as the second contact portion indicates a region where only the cleaning roller 31 abuts, with the opposing roller 40 not in contact.

  In the present embodiment, the physical nip T3b is secured by the opposing roller 40 and the cleaning roller 31. The reason for this is to suppress the discharge that is likely to occur near the nip portion T3 as much as possible. That is, if a discharge occurs near the nip portion T3, a stronger electric field is required to clean the toner on the intermediate transfer belt 21, and there is a concern that the damage to the intermediate transfer belt 21 will be increased accordingly. . In the present embodiment, an elastic belt having an elastic layer is used as the intermediate transfer belt 21 as described later. Since such an intermediate transfer belt 21 has a high electrical resistance, discharge is particularly likely to occur near the nip portion T3.

  The cleaning roller 31 is driven by a motor 35 as a driving means at a speed substantially equal to that of the intermediate transfer belt 21 in the same direction (arrow R3 direction) as the moving direction of the intermediate transfer belt 21 at the nip portion T3 with the intermediate transfer belt 21. It is rotationally driven. Then, the cleaning roller 31 electrically removes the toner remaining on the intermediate transfer belt 21 without being secondarily transferred, by the action of the electric field (so-called electrophoresis). In the case of this embodiment, the opposing roller 40 is connected to the ground, the cleaning roller 31 is connected to the power supply 36, and a voltage having a reverse polarity to the toner is applied to the cleaning roller 31 by the power supply 36. Then, the toner remaining on the intermediate transfer belt 21 (on the intermediate transfer belt) passes through the liquid developer liquid layer formed between the secondary transfer belt 51 and the cleaning roller 31 at the nip portion T3 to form an intermediate transfer belt. 21 is moved to the cleaning roller 31.

  The toner moved to the cleaning roller 31 is removed by the cleaning blade 32 together with the liquid developer. The cleaning blade 32 is a plate member made of metal such as stainless steel, for example, and abuts the cleaning roller 31 on the downstream side of the moving direction of the cleaning roller 31 than the nip portion T3. The toner removed by the cleaning blade 32 flows down in the cleaning container 33 by gravity along with the liquid developer. The bottom surface of the cleaning container 33 is formed to be inclined, and the discharge port 34 is formed at the lowest part of the inclined bottom surface. Therefore, the liquid developer containing the toner removed by the cleaning blade 32 is transmitted from the bottom of the cleaning container 33 to the discharge port 34 and is discharged from the discharge port 34 to the outside of the cleaning container 33.

<Cleaning roller>
The above cleaning roller 31 will be described. In an image forming apparatus using a liquid developer, it is desirable to use a cleaning roller 31 formed of a material that hardly reacts with an organic solvent or the like contained in the liquid developer. This is for the purpose of enhancing the durability of the roller by making it difficult to cause deterioration due to dissolution or deterioration caused by the compound used for the carrier liquid. In general, when the difference between the solubility parameter (SP) value of each of the roller and the organic solvent is 2 or more, the roller is more easily deteriorated than when the difference of the SP value is less than 2 (that is, deterioration of the roller) Will be accelerated). In the present embodiment, a metal roller made of, for example, stainless steel or aluminum is used as the cleaning roller 31 from the viewpoint of delaying deterioration of the roller. As the metal roller, a metal roller whose surface is thinly coated with a fluorine resin or the like may be used to such an extent that the shape following property by deformation does not change. The opposing roller 40 has a smaller chance of touching the liquid developer compared to the cleaning roller 31, and therefore, it is not necessary to use a metal roller, and a rubber roller may be used. However, in consideration of deterioration of the roller, it is preferable to use a metal roller as the opposing roller 40 as well.

  In the case of an image forming apparatus using a dry developer, it is difficult to use a metal roller as the cleaning roller 31. That is, when the toner contained in the dry type developer is an insulator, and the cleaning roller 31 is a metal roller having a small electric resistance, the toner whose polarity is reversed is melted by the discharge occurring at the nip portion and the gap in the vicinity thereof. I can wear it. If this happens, the cleaning performance will deteriorate. On the other hand, in the case of an image forming apparatus using a liquid developer, the polarity of the toner is not substantially reversed even if discharge occurs. Since the toner moves in the liquid layer of the liquid developer by electrophoresis, it is possible to use a metal roller. However, the metal roller is much less conformable to deformation due to deformation than a rubber roller. Therefore, when using a metal roller for the cleaning roller 31, the nip length (the length in the moving direction of the intermediate transfer belt 21) of the nip portion T3 is secured to increase the contact area in order to move the toner reliably by electrophoresis. There is a need to. As described in detail later (refer to FIG. 5), in the present embodiment, the cleaning roller 31 is projected to the inside of the intermediate transfer belt 21 so that the intermediate transfer belt 21 is wound around the cleaning roller 31. The nip length is secured.

<Intermediate transfer belt>
The intermediate transfer belt 21 will be described. The intermediate transfer belt 21 is formed into a film with a predetermined thickness by using a resin such as polyimide or polyamide, or an alloy thereof and an antistatic agent such as carbon black in an appropriate amount. For example, the intermediate transfer belt 21 is a resin belt having a surface resistivity of 1E + 9 to 1E + 13 Ω / □ and a thickness of 0.04 to 0.1 mm.

  The intermediate transfer belt 21 is formed using a resin having a high Young's modulus (for example, 300 MPa). In the case of the intermediate transfer belt 21 having a low Young's modulus and easily deformed, even if a metal roller is used for the cleaning roller 31, the nip length of the physical nip T3b can be increased. However, when the thickness of the intermediate transfer belt 21 is 1 mm or less, if a metal roller having a diameter larger than 40 mm is not used, a sufficient nip length sufficient to remove most toner can be secured. difficult. The nip length needs to be, for example, 1.2 mm or more. However, it is preferable to use a metal roller having a diameter of 40 mm or less as the cleaning roller 31 in view of the fact that when the diameter of the metal roller is increased, the weight becomes twice as large as the square.

  Here, with respect to the nip length of the nip portion formed when the metal roller is pressed against the endless belt, an experiment was conducted to check the nip length by changing the thickness of the belt. The experimental results are shown in FIG. In FIG. 3, the abscissa represents the thickness of the belt, and the ordinate represents the nip length. Using a resin belt made of polyimide having a thickness of 0.1 mm as a belt and an elastic belt having an elastic layer made of a urethane sponge having a thickness of 0.8 mm and 1.2 mm on the resin belt, respectively, the nip length I measured it. The contact pressure between the belt and the metal roller was 80 N, and the length in the moving direction (longitudinal direction) of the belt was 400 mm. The Young's modulus of the elastic layer was 0.3 (MPa). The Young's modulus can be measured by "FISCHERSCOPE HM2000S" (manufactured by Fisher).

  As shown in FIG. 3, if the thickness of the belt is not 1 mm or more, the nip length (for example, 1.2 mm) sufficient to remove the toner can not be secured at the contact pressure 80 N. Therefore, in order to secure a nip length sufficient for removing toner, it is conceivable to make the contact pressure 80 N or more. However, if the contact pressure is too high, the belt is likely to break. In the present embodiment, the upper limit of the contact pressure is 300N. In particular, when an elastic belt is used for an intermediate transfer belt carrying a liquid developer, micro cracks easily occur as the belt expands and contracts due to the pressing of a metal roller, and the liquid developer enters the belt and the belt It may swell or change the electrical resistivity of the belt. In order to avoid this, it is desirable to use a thin belt of an elastic layer having a thickness of 1 mm or less as the intermediate transfer belt 21. When the contact pressure is too low, it is conceivable that the distance between the opposite roller 40 or the cleaning roller 31 and the belt is increased due to the unevenness of thickness of the belt and the unevenness of driving during rotation. In this case, the physical nip width or the tension nip width is lost, and the cleaning ability is reduced. Therefore, it is desirable that the contact pressure be as low as possible (for example, 30 N) to the extent that blurring does not occur in the above-described nip width. In the present embodiment, the lower limit value of the contact pressure is 30N. As described above, in the present embodiment, the contact pressure is set to 30 N or more and 300 N or less.

<Electrophoresis of Toner>
Next, electrophoresis of the toner in the nip portion T3 will be described with reference to FIG. FIG. 4 is a diagram for explaining modeling of the electrophoresis of the toner, and here, for convenience of illustration, the intermediate transfer belt 21 is shown in a straight line.

As described above, the belt cleaning device 30 electrically removes the toner F on the intermediate transfer belt 21 by the action of the electric field (so-called electrophoresis). At that time, in order to reliably move the toner F from the intermediate transfer belt 21 to the cleaning roller 31 by electrophoresis, it is necessary to secure the nip length L of the nip portion T3, and the nip length L (m) is It is a length that satisfies Expression 1 shown below.
(Μ × E) × (L / P)> d ··· Formula 1

In Equation 1, μ (m ² / (V × s)) is the toner mobility, and E (V / m) is the strength of the electric field generated in the nip portion T3 with the application of the voltage to the cleaning roller 31, P (m / S) is the rotational speed of the intermediate transfer belt 21, and d (μm) is the liquid thickness of the liquid developer G at the nip portion T3. The nip length L allows the toner remaining on the intermediate transfer belt 21 after the secondary transfer to be cleaned by electrophoresis when a so-called solid image with the toner placed on the entire surface of the recording material as the toner image is secondarily transferred. It is a length.

The left side of Equation 1 is the product of the moving speed of the toner represented by (μ × E) and the transit time for passing through the nip portion T3 represented by (L / P), ie, intermediate transfer It is a distance that the toner can move from the belt 21 toward the cleaning roller 31 by electrophoresis. On the other hand, the right side of Formula 1 is the liquid thickness of the liquid developer at the nip portion T3 as described above. That is, if the nip length L in which the left side of Expression 1 is larger than the right side is secured, the toner is transferred from the intermediate transfer belt 21 to the cleaning roller 31 via the liquid developer liquid thickness while passing through the nip portion T3. It is movable. As an example, the toner mobility is 1.00 ^-10 to 1.00 ^-11 (m ² / (V × s)). The electric field is 90 (V / μm). The rotational speed of the intermediate transfer belt 21 is 600 (mm / s). The liquid thickness d of the liquid developer at the nip portion T3 is 2 (μm). In such a case, the nip length L may be secured to 1.5 (mm) or more. However, when the nip length L is increased, the winding angle of the intermediate transfer belt 21 with respect to the cleaning roller 31 is increased. In that case, since the intermediate transfer belt 21 is easily bent repeatedly as it rotates, it is not preferable from the viewpoint of the belt life. In view of this point, the winding angle of the intermediate transfer belt 21 with respect to the cleaning roller 31 is preferably less than 90 °. More preferably, it is less than 45 °, more preferably less than 20 °.

Here, measurement of the toner mobility, the electric field, the nip length, and the liquid thickness of the liquid developer in the above-described Formula 1 will be described. The toner mobility μ can be expressed by Equation 2 below.
μ = | v / E | = Q / (6π × η × α) ··· Formula 2

  In Equation 2, v (m / s) is the moving speed of the toner, and E (V / m) is the strength of the electric field generated in the nip portion T3 with the application of the voltage to the cleaning roller 31. Further, Q (C) is the charge amount of the toner in the liquid developer, π is the circumference ratio, η is the viscosity (Pa · s) of the liquid developer, and α (μm) is the diameter of the toner. As an example, the viscosity of the liquid developer is 4.0 (Pa · s), the outer diameter of the toner is 1.0 (μm), and the toner mobility can be calculated from these parameters. Also, the moving speed of the toner is about 9 to 90 (m / s) in the case of the present embodiment. The charge amount of the toner can be calculated from the various quantified parameters described above. The toner mobility can be quantified by measuring it using a measuring instrument such as a Zeta electrometer measuring device Zeta-APS (manufactured by Matec applied science).

The electric field is generally determined by Equation 3 below. β (V) is a voltage value applied to the cleaning roller 31, and d (μm) is a liquid thickness of the liquid developer in the nip portion T3.
E = β / d · · · Equation 3

  As for the electric field, the path from the cleaning roller 31 to the opposing roller 40 via the liquid developer and the resistor of the intermediate transfer belt 21 can be modeled by a series circuit, and the circuit can be calculated. As an example, the voltage value applied to the cleaning roller 31 is 1000 (V), the electrical resistivity of the liquid developer is 6.0E + 6 (Ω · cm), and the liquid thickness of the liquid developer is 2 (μm). The electrical resistivity of the intermediate transfer belt 21 is 1.0E + 10 (Ω · cm), and the thickness of the intermediate transfer belt 21 is 100 (μm). In this case, the electric field is calculated to be about 90 (V / μm).

  As for the nip length, a momentary power failure may be caused by turning off the main power supply during image formation, and the length of the nip portion T3 may be measured in a stopped state. Here, the nip length is determined by the diameters of the cleaning roller 31 and the opposing roller 40 and the amount of deformation of the intermediate transfer belt 21. In the present embodiment, the diameter of the cleaning roller 31 is 28 mm, and the diameter of the opposing roller 40 is 21 mm. The surface roughness of the cleaning roller 31 and the opposing roller 40 is smaller than 0.2 μm according to JIS B 0031: 2003 standard. The surface roughness of these rollers can be measured by PU-OS 400 (manufactured by Kosaka Laboratory).

  The thickness of the liquid developer is determined by peeling off part of the liquid developer from the surface of the intermediate transfer belt 21 which has passed through the nip portion T3 with a scraper or the like, and peeling off the liquid developer using a confocal microscope or the like. Measure the difference in height from the unstripped area. Then, a value twice as high as the measured height difference is taken as the liquid thickness of the liquid developer. That is, when the liquid developer in the nip portion T <b> 3 passes through the nip portion T <b> 3, it separates between the intermediate transfer belt 21 and the cleaning roller 31. Therefore, the liquid thickness of the liquid developer on the surface of the intermediate transfer belt 21 which has passed through the nip portion T3 is half the liquid thickness of the liquid developer in the nip portion T3. Then, the liquid thickness of the liquid developer in the nip portion T3 can be obtained by doubling the height difference actually measured as described above. As a confocal microscope, for example, a confocal microscope VK8700 (manufactured by Keyence Corporation) may be used.

<Cleaning nip part>
As described above, in the present embodiment, in order to move the toner from the intermediate transfer belt 21 to the cleaning roller 31 via the liquid developer by electrophoresis, the nip portion T3 is set to a nip length L that satisfies the above equation 1 It needs to be formed. In order to do so, in the present embodiment, the cleaning roller 31 is disposed at a position where the intermediate transfer belt 21 can be wound around the cleaning roller 31. This will be described with reference to FIG.

  As shown in FIG. 5, the cleaning roller 31 presses the stretching portion 21 a of the intermediate transfer belt 21 stretched between the idler roller 80 and the driving roller 23 from the outer circumferential side to the inner circumferential side. The cleaning roller 31 is rotatably fixed on the outer peripheral side of the intermediate transfer belt 21 so as not to be displaced with respect to the idler roller 80 and the drive roller 23 that stretch the intermediate transfer belt 21. In the present embodiment, the intermediate transfer belt 21 projects inward beyond the common external tangent Z on the intermediate transfer belt side (belt member side) among the common external tangents of the idler roller 80 and the drive roller 23 as described above. The positional relationship of the two rollers is determined. At least one of the drive roller 23 and the idler roller 80 is a cleaning roller 31 with respect to a tangent line I passing through an intersection J of the opposing roller 40 and a straight line H connecting the rotation center of the opposing roller 40 and the rotation center of the cleaning roller 31. Provided on the same side as In other words, it is provided at a position where it intrudes into the area Y opposite to the opposing roller 40 with reference to the tangent line I. Here, both the drive roller 23 and the idler roller 80 are provided to intrude into the area Y. In the case of the present embodiment, the driving roller 23 is a first roller that first stretches the intermediate transfer belt 21 downstream in the moving direction relative to the cleaning roller 31 and the opposing roller 40. On the other hand, the idler roller 80 is a second roller which first stretches the intermediate transfer belt 21 upstream of the cleaning roller 31 and the opposing roller 40 in the moving direction. The drive roller 23 and the idler roller 80 are fixed so as not to move from the arrangement position. On the other hand, the opposing roller 40 is not fixed but biased (that is, movable) so as to press the intermediate transfer belt 21 from the inside to the outside by the pressure spring 41.

  The intermediate transfer belt 21 is pressed by the cleaning roller 31 at a predetermined contact pressure. That is, if the contact pressure is too high, not only the above-described deterioration of the intermediate transfer belt 21 is accelerated but also the load on the rollers for stretching the intermediate transfer belt 21 increases, thereby increasing the drive load of the drive roller 23 or There is a possibility that the wear of the bearings for supporting each roller may be accelerated. However, if the contact pressure is too small, the cleaning roller 31 is likely to fluctuate when the intermediate transfer belt 21 is driven, and the winding amount of the intermediate transfer belt 21 wound around the cleaning roller 31 may fluctuate. That is, the size (contact area) of the nip portion may fluctuate when the belt is driven. Therefore, in the present embodiment, the positional relationship of the three rollers is determined so that the contact pressure between the intermediate transfer belt 21 and the cleaning roller 31 is set to 30 N or more and 300 N or less as described above. It is fixed at each position. The adjustment of the contact pressure between the intermediate transfer belt 21 and the cleaning roller 31 can be made by the tension roller 24 (see FIG. 1) in addition to the positional relationship of the three rollers.

  By the intermediate transfer belt 21 being bent inward by the above three rollers (here, the cleaning roller 31, the idler roller 80, and the drive roller 23), the contact pressure is stabilized, and the size of the nip portion T3 at the time of belt drive Fluctuation is less likely to occur. That is, when the intermediate transfer belt 21 is driven by fixing the three rollers, the intermediate transfer belt 21 returns to the linear state even if a force to return from the substantially U-shaped bent state to the linear state is applied. Because of the difficulty, the size of the nip portion T3 hardly changes. For example, the fluctuation (such as blurring) of the intermediate transfer belt 21 caused by the intrusion of the recording material P into the secondary transfer portion T2 is suppressed by the three rollers, and the size of the nip portion T3 hardly changes. Further, in the case of the present embodiment, since the fluctuation of the intermediate transfer belt 21 is also suppressed by the tension roller 24 (see FIG. 1), the size of the nip portion T3 is obtained by cooperation of the three rollers and the tension roller 24. Is less likely to fluctuate.

  When the intermediate transfer belt 21 is bent in this manner, the amount of winding of the intermediate transfer belt 21 with the cleaning roller 31 is increased as compared with the case where the intermediate transfer belt 21 is not bent. As described above, the nip portion T3 has the physical nip T3b and the tension nip T3a (see FIG. 2B), and as the winding amount of the intermediate transfer belt 21 increases, the tension nip T3a becomes longer. By lengthening the tension nip T3a, the nip length of the nip portion T3 can be made a length satisfying the above-mentioned equation 1, and the toner on the intermediate transfer belt 21 can be sufficiently removed by electrophoresis.

In the present embodiment, the relationship between the nip length L and the electric field E necessary for the toner to electrophorese can be expressed by Equation 4 obtained by modifying Equation 1 above.
E> (d × P / μ) / L formula 4

Here, the relationship between the nip length and the strength of the electric field (electric field strength) is shown in FIG. In FIG. 6, the case where the toner mobility is 1.00 ⁻¹¹ (m ² / (V × s)) is indicated by a solid line, and the toner mobility is 1.00 ⁻¹⁰ (m ² / (V × s)) The case is shown by a dotted line. This graph shows the lowest field strength required for each nip length. For example, when the toner mobility is ^{1.00 -10 (m 2 / (V} × s)), if the nip length is 1.5 mm, about 1.0E + 1 (V / μm) larger than the electric field intensity Can not be moved by electrophoresis. Therefore, if an electric field higher than the electric field strength indicated by each line is obtained, the toner can move by electrophoresis in the nip portion T3. However, there is an upper limit to the electric field strength. This is because when the electric field strength is too strong, discharge occurs in the vicinity of the nip portion T3 and the cleaning performance is reduced. In the case of this embodiment, since the discharge occurs at an electric field strength higher than 1.0E + 2 (V / μm), the electric field strength is 1.0E + 2 (V / μm) or less.

In the case of the present embodiment, the toner mobility may be 1.00 ⁻¹¹ (m ² / (V × s)). This is because the toner mobility may decrease with use, but when the toner mobility satisfies the lower limit value, the cleaning property by the belt cleaning device 30 is guaranteed.

Next, using either the metal roller or the rubber roller for the cleaning roller 31 and the opposing roller 40, the cleaning performance is determined with respect to the case where the intermediate transfer belt 21 is bent as described above and the case where the intermediate transfer belt 21 is not bent. An experiment was conducted to compare. The experimental results are shown in Table 1. The cleaning roller 31 used in the experiment has a diameter of 28 mm, and the opposing roller 40 has a diameter of 21 mm. Further, in the rubber roller, the thickness of the elastic layer made of urethane rubber is 2 mm, and the Young's modulus thereof is 0.3 (MPa). The first example, the second example, and the fourth example described below are comparative examples, and the third example and the fifth example correspond to the present embodiment.

  As a first example, when the cleaning roller 31 and the opposing roller 40 are both rubber rollers, and the overhang amount of the cleaning roller 31 is "0 mm", a nip portion T3 having a nip length of "1.5 mm" is formed. Ru. Here, the amount of extension of the cleaning roller 31 is the farthest from the common external tangent Z among the contact positions of the cleaning roller 31 and the intermediate transfer belt 21 in the nip portion T3 and the common external tangent Z on the intermediate transfer belt side. It is the distance to the contact position (indicated by the symbol W in FIG. 5). That is, when the intermediate transfer belt 21 is not bent, the amount of extension of the cleaning roller 31 is “0”. In this case, as shown in Table 1, good cleaning performance can be obtained at an electric field strength of 85 (V / μm).

  As a second example, when the cleaning roller 31 is a metal roller, the opposing roller 40 is a rubber roller, and the overhang amount of the cleaning roller 31 is “0”, the nip portion T3 having a nip length of “0.8 mm” Is formed. In this case, as shown in Table 1, even if the electric field strength was increased (115 (V / μm)) as compared with the first example, a good cleaning performance was not obtained. This is because metal rollers are less likely to deform than rubber rollers, and only a short nip length can be obtained as compared with the first example, and it is possible to form a liquid developer liquid layer sufficient to move toner by electrophoresis. This is because the nip portion T3 can not be secured. Therefore, as a third example, the extension length of the cleaning roller 31 is “5 mm”, that is, the intermediate transfer belt 21 is bent to secure a nip length of “1.5 mm” equivalent to that of the first example. By securing a nip length of “1.5 mm”, as shown in Table 1, good cleaning performance can be obtained with an electric field strength of 85 (V / μm).

  As a fourth example, when the cleaning roller 31 and the opposing roller 40 are both metal rollers and the overhang amount of the cleaning roller 31 is “0”, the nip portion T3 having a nip length of “0.3 mm” is formed. Be done. In this case, as shown in Table 1, good cleaning performance was not obtained even if the electric field strength was increased significantly (300 (V / μm)) as compared with the first example. This is because if the rollers are metal rollers, only a shorter nip length can be obtained, and the nip portion T3 can not be secured enough to form a liquid developer liquid layer sufficient to move the toner by electrophoresis. . In addition, the electric field strength is too high, and discharge may occur. Therefore, as a fifth example, the protrusion amount of the cleaning roller 31 is set to “7 mm” which is larger than the third example. By doing this, a nip length of “1.5 mm” equivalent to that of the first example can be secured, and as shown in Table 1, good cleaning performance with an electric field strength of 85 (V / μm) Is obtained.

  As described above, in the present embodiment, the intermediate transfer belt 21 is viewed from the outside by the cleaning roller 31 in order to secure the nip portion T3 that can form a liquid layer of the liquid developer sufficient to move the toner by electrophoresis. The intermediate transfer belt 21 is bent by being pushed inward. As a result, the amount of winding of the intermediate transfer belt 21 around the cleaning roller 31 is increased, so the tension nip T3a can be made longer. By making the tension nip T3a longer, the nip length of the nip portion T3 can be set to a length (see the above equation 1) which can sufficiently move the toner on the intermediate transfer belt 21 by electrophoresis. Further, the cleaning roller 31 is fixed to the outer peripheral side of the intermediate transfer belt 21 so as not to be displaced with respect to the idler roller 80 and the driving roller 23 that stretch the intermediate transfer belt 21. Since the intermediate transfer belt 21 is bent by these three fixed rollers, the size (contact area) of the nip portion T3 hardly changes when the belt is driven. As described above, in the present embodiment, it is sufficient to sufficiently move the toner on the intermediate transfer belt 21 by electrophoresis, and it is easy to secure the nip portion T3 in which the fluctuation of the contact area according to the belt drive is suppressed. You can

Second Embodiment
The second embodiment will be described with reference to FIGS. 7A to 8. The second embodiment shown here is different from the first embodiment described above, in that the cleaning roller 31 and the opposing roller 40 are disposed in an offset manner, so that the nip length is longer than in the first embodiment. It is intended to form the nip portion T3. Hereinafter, the same components as those in the first embodiment described above are denoted by the same reference numerals, and description and illustration thereof will be omitted or simplified, and parts different from the first embodiment will be mainly described.

  As shown in FIG. 7A, in the present embodiment as well as in the first embodiment, the cleaning roller 31 is a common external tangent Z on the intermediate transfer belt side among the common external tangents of the drive roller 23 and the idler roller 80. The intermediate transfer belt 21 is protruded more inside than the other. At least one of the drive roller 23 and the idler roller 80 is a cleaning roller 31 with respect to a tangent line I passing through an intersection J of the opposing roller 40 and a straight line H connecting the rotation center of the opposing roller 40 and the rotation center of the cleaning roller 31. And Y is provided in the same area Y. Here, the idler roller 80 is provided on the same side as the cleaning roller 31.

  Further, unlike the first embodiment, the cleaning roller 31 and the opposing roller 40 are disposed offset. That is, the cleaning roller 31 has a first intersection point N between the common circumscribed tangent Z and the vertical line passing through the rotation center M of the cleaning roller 31, and a second intersection point Q Are arranged to shift in the moving direction. However, in the case of the present embodiment, the movement direction central position of the physical nip T3b is disposed downstream of the movement direction central position of the nip portion T3 (see FIG. 7B). The cleaning roller 31 and the opposing roller 40 are offset in the range in which the physical nip T3b is formed. By forming the physical nip T3b, the discharge can be suppressed.

  By disposing the cleaning roller 31 and the opposing roller 40 in an offset manner, the nip portion T3 of the nip length satisfying the above-described equation 1 without increasing the protrusion amount of the cleaning roller 31 compared to the first embodiment described above. Can be formed. That is, as shown in FIG. 7B, the amount of winding of the intermediate transfer belt 21 around the cleaning roller 31 is increased by the offset arrangement, and the tension nip T3a can be made longer. The toner on the intermediate transfer belt 21 can be sufficiently removed by electrophoresis by lengthening the tension nip T3a and setting the nip length of the nip portion T3 to a length that satisfies the above equation 1. Such a second embodiment is particularly effective when both the cleaning roller 31 and the opposing roller 40 are metal rollers.

  Then, as shown in FIGS. 7A and 7B, in the case of the present embodiment, the cleaning roller 31 abuts on the intermediate transfer belt 21 upstream of the opposing roller 40 in the moving direction of the intermediate transfer belt 21. It is preferred to be located at That is, the cleaning roller 31 is preferably disposed such that the first intersection point N is located upstream of the second intersection point Q in the moving direction of the intermediate transfer belt 21 with respect to the opposing roller 40. This is to suppress the decrease in the cleaning performance due to the occurrence of the discharge.

  The above discharge will be described using FIG. 8 with reference to FIG. 7 (b). FIG. 8 also illustrates the case where the cleaning roller 31 is not offset from the opposing roller 40 (see FIG. 5). In the present embodiment, as described above, by arranging the cleaning roller 31 and the opposing roller 40 in an offset manner, the tension nip T3a is made longer. Then, although the path of the current flowing from the cleaning roller 31 to the intermediate transfer belt 21 is expanded, the path of the current is narrowed depending on the position of the opposing roller 40. Then, as shown in FIG. 8, when the cleaning roller 31 is offset to the downstream side of the opposing roller 40a in the moving direction, discharge tends to occur. That is, in this case, the current is concentrated from the intermediate transfer belt 21 toward the opposing roller 40 a in a state where the charge injection from the cleaning roller 31 to the intermediate transfer belt 21 is small. In particular, at the physical nip T3b on the upstream side in the moving direction of the intermediate transfer belt 21, the potential difference between the surface of the cleaning roller 31 and the surface of the intermediate transfer belt 21 increases, and discharge may occur on the upstream side in the moving direction. In this case, since a large amount of toner still remains on the cleaning roller 31 side on the upstream side in the movement direction, the influence of the discharge on the cleaning performance is large.

  On the other hand, when the cleaning roller 31 is offset from the opposing roller 40b in the moving direction, discharge is less likely to occur on the upstream side in the moving direction. That is, in this case, after charge injection from the cleaning roller 31 to the intermediate transfer belt 21 occurs at the tension nip T3a, current concentrates from the intermediate transfer belt 21 toward the opposing roller 40b at the physical nip T3b. In such a case, the potential difference between the surface of the cleaning roller 31 and the surface of the intermediate transfer belt 21 does not increase on the upstream side in the moving direction of the intermediate transfer belt 21, so that the discharge hardly occurs. Further, in this case, even if a discharge occurs in the physical nip T3b on the downstream side in the moving direction of the intermediate transfer belt 21, most of the toner has already moved to the cleaning roller 31 side, which gives the cleaning performance. The impact is minor.

Table 2 shows the results of comparison of the cleaning performance in the case where the cleaning roller 31 and the opposing roller 40 are offset on the upstream side in the moving direction and on the downstream side in the moving direction. Further, for reference, the cleaning performance was also shown in the case where the cleaning roller 31 was not offset from the opposing roller 40 (center in Table 2).

  As can be understood from Table 2, when the cleaning roller 31 and the opposing roller 40 are disposed offset, the cleaning performance is better if the cleaning roller 31 is offset and disposed upstream of the opposing roller 40b in the moving direction. is there. On the other hand, when the cleaning roller 31 is offset to the downstream side of the opposing roller 40b in the moving direction, discharge occurs on the upstream side in the moving direction as described above, and some residual toner is generated. The density of the remaining toner was measured with a densitometer of X-rite, and was about 0.008. This indicates that the cleaning performance is lowered compared to the case where the density of the toner residue is about 0.003 or less and the cleaning roller 31 is offset-arranged on the upstream side of the opposing roller 40b in the moving direction. There is.

  As described above, in the second embodiment, by arranging the cleaning roller 31 and the opposing roller 40b in an offset manner, the nip portion T3 sufficient to move the toner on the intermediate transfer belt 21 by electrophoresis is secured. Can be done easily. In particular, when the cleaning roller 31 is offset to the upstream side of the opposing roller 40b in the moving direction, the cleaning performance can be further improved.

  The above-described belt cleaning device 30 according to the first and second embodiments can be applied to a double-turn cleaning device 30A (see FIG. 1) that cleans the endless secondary transfer belt 51 as a belt member. Therefore, a secondary transfer unit having such a secondary transfer cleaning device 30A will be described with reference to FIGS. In addition, below, the same code | symbol is attached | subjected about the structure same in each embodiment including 1st, 2nd embodiment mentioned above, description and illustration are abbreviate | omitted or simplified, and it demonstrates focusing on a different part.

Third Embodiment
First, the third embodiment will be described with reference to FIG. As shown in FIG. 9, in the case of the secondary transfer unit 50 of this embodiment, the opposing roller 40 and the cleaning roller 31 abut on the inner peripheral surface and the outer peripheral surface of the secondary transfer belt 51, respectively, and the nip portion T4. Form The idler roller 54 (second roller) and the belt drive roller 56 (first roller) are fixed to the inner peripheral side of the secondary transfer belt 51 and stretch the secondary transfer belt 51. The cleaning roller 31 presses the stretching portion 51 a of the secondary transfer belt 51 stretched between the idler roller 54 and the belt driving roller 56 from the outer peripheral side to the inner peripheral side. The cleaning roller 31 is rotatably fixed to the outer peripheral side of the secondary transfer belt 51 so as not to be displaced with respect to the idler roller 54 and the belt drive roller 56.

  The secondary transfer belt 51 is extended inward of the common external tangent Z on the secondary transfer belt side among the common external tangents of the idler roller 54 and the belt drive roller 56. At least one of the idler roller 54 and the belt drive roller 56 is a cleaning roller relative to a tangent I passing through the intersection J of the opposing roller 40 and a straight line H connecting the rotational center of the opposing roller 40 and the rotational center of the cleaning roller 31. It is provided on the same side as 31. Here, the positional relationship of the cleaning roller 31, the idler roller 54, and the belt drive roller 56 is determined such that both the idler roller 54 and the belt drive roller 56 are provided on the same side as the cleaning roller 31. The cleaning roller 31, the idler roller 54, and the belt driving roller 56 are fixed so as not to move from the arrangement position. As a result, the toner on the secondary transfer belt can be sufficiently moved by electrophoresis, and the nip portion T4 can be easily secured in which the fluctuation of the contact area according to the belt drive is suppressed.

Fourth Embodiment
A fourth embodiment will be described using FIG. The secondary transfer unit 50A of the present embodiment further includes a secondary transfer between the idler roller 54 and the belt drive roller 56 as compared to the secondary transfer unit 50 (see FIG. 9) of the third embodiment described above. The difference is that a tension roller 55 for tensioning the belt 51 is disposed. In this case, as shown in FIG. 10, the cleaning roller 31 has a tension portion 51a of the secondary transfer belt 51 stretched between the idler roller 54 (second roller) and the tension roller 55 (first roller). Press from the outer side to the inner side. The cleaning roller 31 is rotatably fixed to the outer peripheral side of the secondary transfer belt 51 so as not to be displaced with respect to the idler roller 54 and the stretching roller 55. As described above, it is sufficient to sufficiently move the toner on the secondary transfer belt by electrophoresis, and it is easy to secure the nip portion T4 in which the variation of the contact area according to the belt drive is suppressed. The same effect as the third embodiment can be obtained.

Fifth Embodiment
The fifth embodiment will be described with reference to FIG. As compared with the secondary transfer unit 50A (see FIG. 10) of the fourth embodiment described above, the secondary transfer unit 50A according to the present embodiment has a plurality (two in this case) of the second transfer cleaning devices 30A as the secondary transfer unit. The point of being provided along the belt 51 is different. As shown in FIG. 11, in the two second-turn cleaning devices 30A, a tensioning portion 51a of the secondary transfer belt 51 in which the cleaning rollers 31a and 31b are tensioned between the idler roller 54 and the tension roller 55 is used. Press from the outer side to the inner side. The cleaning roller 31a as an outer roller presses the stretching portion 51a to form a first nip portion T4a. The cleaning roller 31b as a first roller presses the stretching portion 51a downstream of the cleaning roller 31a in the moving direction of the secondary transfer belt 51 to form a second nip portion T4b. For the cleaning roller 31a, the opposing roller 40a as the first inner roller is in contact with the inner peripheral surface of the secondary transfer belt 51, and for the cleaning roller 31b, the opposing roller 40b as the second inner roller is secondary It is in contact with the inner circumferential surface of the transfer belt 51. Also in the cleaning roller 31b and the opposing roller 40b, similarly to the cleaning roller 31a and the opposing roller 40a, the toner is moved from the secondary transfer belt 51 via the liquid developer due to the potential difference between them.

  The cleaning rollers 31 a and 31 b are rotatably fixed on the outer peripheral side of the secondary transfer belt 51 so as not to be displaced with respect to the idler roller 54 and the stretching roller 55. In the present embodiment, the four rollers of the cleaning rollers 31a and 31b, the idler roller 54, and the stretching roller 55 are provided so that the secondary transfer belt 51 protrudes inward from the common external tangent Z on the secondary transfer belt side. The positional relationship is determined. And each roller is being fixed so that it may not move from an arrangement position. As described above, it is sufficient to sufficiently move the toner on the secondary transfer belt by electrophoresis, and it is easy to secure the nip portion T4 in which the variation of the contact area according to the belt drive is suppressed. The same effect as the third and fourth embodiments can be obtained.

Sixth Embodiment
A sixth embodiment will be described using FIG. The secondary transfer unit 50B of the present embodiment presses the secondary transfer belt 51 to the outer peripheral side between the plurality of secondary transfer cleaning devices 30A as compared with the above-described secondary transfer unit 50A (see FIG. 10). The difference is that the pressing roller 57 is disposed. In the case of the present embodiment, in the two-turn cleaning device 30A on the upstream side in the moving direction, the idler roller 54 corresponds to a second roller, and the pressing roller 57 corresponds to a first roller. On the other hand, in the two-roll cleaning device 30A on the downstream side in the moving direction, the pressing roller 57 corresponds to a second roller, and the stretching roller 55 corresponds to a first roller.

  In the case of the present embodiment, as shown in FIG. 12, the pressing roller 57 presses the stretching portion 51b of the secondary transfer belt 51 stretched between the cleaning roller 31a and the cleaning roller 31b from the inner peripheral side to the outer peripheral side. Do. The pressure roller 57 is rotatably fixed on the inner peripheral side of the secondary transfer belt 51 so as not to be displaced with respect to the cleaning roller 31 a and the cleaning roller 31 b (more specifically, the idler roller 54 and the tension roller 55). . The pressing roller 57 projects the secondary transfer belt 51 outside the common external tangent Z2 on the secondary transfer belt side among the common external tangents of the cleaning roller 31a and the cleaning roller 31b. Thus, when the movement path of the secondary transfer belt 51 is formed in a substantially W shape by further providing the pressing roller 57, compared with the fifth embodiment (see FIG. 10) described above, the first nip portion T4a and the second nip portion The nip length of the nip portion T4b can be made longer. As such, in the present embodiment, the positional relationship of the five rollers, that is, the cleaning rollers 31a and 31b, the idler roller 54, the stretching roller 55, and the pressing roller 57, is determined. Thus, it is sufficient to move the toner on the secondary transfer belt sufficiently by electrophoresis, and it is easy to secure the nip portion (T4a, T4b) in which the fluctuation of the contact area according to the belt drive is suppressed. The same effect as the third to fifth embodiments described above can be obtained.

  In the case of the fifth and sixth embodiments described above, the voltage applied to the cleaning roller 31 by the power supply 36 in the second transfer cleaning device 30A disposed on the upstream side and the downstream side in the moving direction of the secondary transfer belt 51, respectively. Of the same polarity.

  In the third to sixth embodiments described above, as in the second embodiment described above, the cleaning roller 31 (31a, 31b) is offset from the opposing roller 40 in the moving direction. preferable.

[Other embodiments]
In the first to sixth embodiments described above, the present invention is applied to the belt cleaning device 30 and the second rotation cleaning device 30A. However, the present invention is not limited to this. The present invention is applicable to, for example, a primary transfer portion T1 capable of transferring toner from the photosensitive drums 11Y to 11K to the intermediate transfer belt 21. In this case, the photosensitive drums 11Y to 11K correspond to outer rollers, and the primary transfer rollers 22Y to 22K correspond to inner rollers. Further, the present invention is applicable to the secondary transfer portion T2 capable of transferring the toner image from the intermediate transfer belt 21 to the recording material P. In this case, the secondary transfer inner roller 25 corresponds to the inner roller, and the secondary transfer outer roller 26 corresponds to the outer roller.

  Although the first to sixth embodiments described above are described on the premise that the physical nip T3b is formed, the present invention is not limited to this. For example, the opposing roller 40 may be offset further downstream so that only the tension nip T3a is formed without forming the physical nip T3b between the cleaning roller 31 and the intermediate transfer belt 21. The belt cleaning device when the physical nip T3b is not formed will be described with reference to FIGS. 13 (a) and 13 (b). In FIGS. 13 (a) and 13 (b), the same components as those in the second embodiment described above are denoted by the same reference numerals (see FIGS. 7 (a) and 7 (b)), and description and illustration thereof are omitted. Or simplify it.

  As shown in FIG. 13A, the tangent I 'of the cleaning roller 31 at the center position S in the movement direction of the intermediate transfer belt 21 in the nip portion T3 will be considered. In the case of the present embodiment, as shown in FIG. 13B, only the tension nip T3a is formed between the cleaning roller 31 and the intermediate transfer belt 21, so that the movement direction central position S moves the intermediate transfer belt 21. It is in the middle of the tension nip T3a with respect to the direction.

  Here, assuming that a roller that first stretches the intermediate transfer belt 21 downstream of the cleaning roller 31 in the moving direction of the intermediate transfer belt 21 is the first roller, in the case of the present embodiment, the first roller Become. The opposing roller 40 is disposed at a position where the position in contact with the intermediate transfer belt 21 does not overlap with the tension nip T3a in the moving direction of the intermediate transfer belt 21 (see FIG. 13B). On the other hand, assuming that the second roller is a roller that first stretches the intermediate transfer belt 21 on the upstream side of the cleaning roller 31 in the moving direction of the intermediate transfer belt 21, the second roller is the secondary transfer inner roller in this embodiment. It becomes 25. The position of the secondary transfer inner roller 25 in contact with the intermediate transfer belt 21 is also disposed at a position not overlapping the tension nip T3a. Then, at least one of the opposing roller 40 (first roller) and the secondary transfer inner roller 25 (second roller) is provided on the same side as the cleaning roller 31 with respect to the tangent line I ′. Just do it. In the present embodiment, both the opposing roller 40 and the secondary transfer inner roller 25 are provided on the same side as the cleaning roller 31 with respect to the tangent I ′.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 11Y (11M, 11C, 11K) Photosensitive body (photosensitive drum) 21 Belt member (intermediate transfer belt) 23 First roller (drive roller) 26 Transfer member (secondary transfer Outer roller 31) Outer roller (cleaning roller) 40 Inner roller (counter roller) 40a First inner roller (counter roller) 40b Second inner roller (counter roller) 51 Belt member (2 Next transfer belt) 80: second roller (idler roller) T3 (T4) contact portion (nip portion) T3a second contact portion (tension nip) T3b first contact portion (physical nip)

Claims (12)

An endless belt member that rotates by carrying a liquid developer containing toner and carrier liquid on the outer peripheral surface thereof;
An outer roller that forms a contact portion that contacts the outer peripheral surface of the belt member, and the toner is moved from the belt member through the liquid developer according to the potential difference between the belt member and the belt;
An inner roller that contacts the outer roller via the belt member;
A first roller which first stretches the belt member downstream of the outer roller and the inner roller in the moving direction of the belt member;
And a second roller that first stretches the belt member upstream of the outer roller and the inner roller with respect to the moving direction of the belt member.
The outer roller and the first roller and the second roller are rotatably fixed.
At least one of the first roller and the second roller is connected to the outer roller with respect to a tangent at an intersection point of a straight line connecting the rotation center of the outer roller and the rotation center of the inner roller and the inner roller. Provided on the same side,
An image forming apparatus characterized by The contact portion has a first contact portion and a second contact portion at different positions in the moving direction of the belt member,
The inner roller contacts the first contact portion,
In the second contact portion, the inner roller is in non-contact,
The central position in the movement direction at the first contact portion is disposed downstream of the central position in the movement direction at the contact portion.
The image forming apparatus according to claim 1, The outer roller has a mobility of toner μ (m ² / (V x s)), the strength of the electric field generated at the contact portion with the application of a voltage E (V / m), and the rotational speed of the belt member Where P (m / s) and the liquid thickness of the liquid developer at the contact portion is d (μm), the moving direction length L (m) of the contact portion is
(Μ × E) × (L / P)> d
Winding the belt member so as to satisfy
The image forming apparatus according to claim 1 or 2, wherein The outer roller is a metal roller,
The image forming apparatus according to any one of claims 1 to 3, wherein The outer roller contacts the belt member at a contact pressure of 30 N or more and 300 N or less.
The image forming apparatus according to any one of claims 1 to 4, characterized in that: The outer roller has a diameter of 40 mm or less.
The image forming apparatus according to any one of claims 1 to 5, wherein The belt member wraps around the outer roller at a wrap angle of less than 45 °.
The image forming apparatus according to any one of claims 1 to 6, wherein The belt member has a thickness of 1 mm or less.
The image forming apparatus according to any one of claims 1 to 7, wherein A photoconductor,
An intermediate transfer belt on which the toner image formed on the photosensitive member is primarily transferred;
A transfer member for secondarily transferring the toner image transferred to the intermediate transfer belt to a recording material;
And a cleaning roller for removing toner remaining after secondary transfer on the intermediate transfer belt in response to application of a voltage.
The belt member is the intermediate transfer belt,
The outer roller is the cleaning roller.
The image forming apparatus according to any one of claims 1 to 8, wherein A photoconductor,
An intermediate transfer belt on which the toner image formed on the photosensitive member is primarily transferred;
A secondary transfer belt for secondarily transferring the toner image transferred to the intermediate transfer belt to a recording material;
And a cleaning roller for removing the toner on the secondary transfer belt in response to the application of a voltage.
The belt member is the secondary transfer belt,
The outer roller is the cleaning roller.
The image forming apparatus according to any one of claims 1 to 8, wherein The inner roller is a first inner roller,
The first roller is in contact with the outer peripheral surface of the belt member,
And a second inner roller that is in contact with the inner circumferential surface of the belt member and faces the first roller via the belt member.
The toner is moved from the belt member through the liquid developer by a potential difference between the first roller and the second inner roller.
The image forming apparatus according to any one of claims 1 to 8, wherein An endless belt member that rotates by carrying a liquid developer containing toner and carrier liquid on the outer peripheral surface thereof;
An outer roller that forms a contact portion that contacts the outer peripheral surface of the belt member, and the toner is moved from the belt member through the liquid developer according to the potential difference between the belt member and the belt;
A first roller which is disposed at a position not in contact with the contact portion in the moving direction of the belt member, and which first stretches the belt member downstream of the contact portion;
A second roller disposed at a position not in contact with the contact portion with respect to the moving direction of the belt member, wherein the second roller tensions the belt member first on the upstream side of the contact portion; Equipped with
The outer roller and the first roller and the second roller are rotatably fixed.
At least one of the first roller and the second roller is provided on the same side as the outer roller with respect to the tangent of the outer roller at the center position of the contact portion in the moving direction of the belt member. ,
An image forming apparatus characterized by

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