Patent Grant
10908537
The present invention relates to an image forming apparatus of an electrophotographic system, by which an image is formed using a liquid developer.
Image forming apparatuses configured to develop an electrostatic latent image formed on a photosensitive drum into a toner image using a liquid developer including a toner and a carrier liquid; to subject the developed toner image to primary transfer onto a transfer drum; and to subject the toner image that has been primary transferred onto the transfer drum, to secondary transfer onto a recording material, have been hitherto suggested. In the apparatus that uses a liquid developer as described in JP-A-2011-158905, a cleaning roller is in close contact with a transfer drum in order to remove the toner remaining on the transfer drum after secondary transfer. The toner moves by means of a liquid developer from the transfer drum to the cleaning roller along with an electric field formed concomitantly to the application of voltage to the cleaning roller in a nip portion formed by the cleaning roller and the transfer drum closely contacting with each other (so-called electrophoresis). Then, the toner that has moved to the cleaning roller is removed by a cleaning blade that rubs the cleaning roller.
Furthermore, in JP-A-2002-318493, an image forming apparatus that uses a dry developer instead of a liquid developer, the image forming apparatus having a pair of rollers arranged therein in order to remove any toner remaining on an intermediate transfer belt, such as an opposed roller on the inner side of the belt and a cleaning roller on the outer side of the belt, has been suggested. In this apparatus, the opposed roller and the cleaning roller are arranged so as to project the intermediate transfer belt interposed between the opposed roller and the cleaning roller, on the inner side of the belt.
Meanwhile, in the case of an apparatus equipped with an intermediate transfer belt as an image forming apparatus that uses a liquid developer, a nip portion should be secured in order to clean the toner by causing the toner to move from the intermediate transfer belt to the cleaning roller by electrophoresis by means of a liquid developer. Thus, also in the case of an image forming apparatus that uses a liquid developer, it may be considered to project the intermediate transfer belt on the inner side of the belt by means of the cleaning roller. However, in such a case, the front and rear surface velocities of the intermediate transfer belt differ from each other at the nip portion due to the difference in the curvature of the belt. Therefore, in a case in which the cleaning roller is driven at the same velocity as that of a driving roller that drives the intermediate transfer belt, the belt portion between the cleaning roller and the driving roller becomes loose, and there is a risk that running of the belt may become unstable, or the belt may be disengaged. Particularly, in the case of an intermediate transfer belt having an elastic layer in which the outer peripheral side (front side) is more elastic than the inner peripheral side (rear side), with the elastic layer having some thickness, a difference in the moving direction length is therefore very highly likely to occur between the outer periphery and the inner periphery of the belt, due to the expansion and contraction concomitant to elastic deformation of the elastic layer.
According to one aspect of the present invention, an image forming apparatus includes a belt member configured to rotate while carrying a liquid developer including a toner and a carrier liquid, the belt member being nipped respectively at a first transfer position for transferring a toner image onto the belt member and at a second transfer position for transferring the transferred toner image onto a recording material, a cleaning roller disposed upstream of the first transfer position and downstream of the second transfer position with respect to a moving direction of the belt member, the cleaning roller being configured to contact with an outer peripheral surface of the belt member at a contact portion and clean the belt member, the cleaning roller being driven in the same direction as the moving direction of the belt member at the contact portion, an opposed roller configured to oppose the cleaning roller, with the belt member being interposed therebetween, a driving roller disposed upstream of the first transfer position and downstream of the contact portion with respect to the moving direction of the belt member, the driving roller contacting with an inner peripheral surface of the belt member and driving the belt member, a first driving source configured to drive the cleaning roller, and a second driving source configured to drive the driving roller. When a roller disposed downstream of the contact portion and upstream of the first transfer position and adjacent to the cleaning roller or the opposed roller on a downstream side in the moving direction of the belt member and configured to tension the belt member is designated as a first roller, and a roller disposed upstream of the contact portion and downstream of the first transfer position and adjacent to the cleaning roller or the opposed roller on an upstream side in the moving direction of the belt member and configured to tension the belt member is designated as a second roller, at least a part of the cleaning roller is provided on the same side as the opposed roller with respect to an external common tangent of the first roller and the second roller. In a case in which a surface velocity when the driving roller is driven is designated as v1; a surface velocity when the cleaning roller is driven is designated as v2; a radius of the cleaning roller is designated as r; a radius of the opposed roller is designated as s; and a distance between the center of rotation of the cleaning roller and the center of rotation of the opposed roller is designated as x, the cleaning roller is rotated so as to satisfy a relationship: v2/v1<r/(x−s).
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Image Forming Apparatus
A first embodiment will be described. First, the configuration of an image forming apparatus of the present embodiment will be described using
The intermediate transfer unit 20 includes an endless intermediate transfer belt 21 serving as a belt member, primary transfer rollers 22Y to 22K, a driving roller 23, a tension roller 24, and a secondary transfer inner roller 25. The intermediate transfer belt 21 is supported so as to bridge over rollers such as the driving roller 23, the tension roller 24, and the secondary transfer inner roller 25, and is driven by the driving roller 23 to rotate in the direction of arrow R2 shown in
The image forming units UY to UK are arranged along the moving direction (direction of arrow R2 shown in
The image forming units UY to UK are configured to be almost the same, except that the colors of the toners used at the developing units 4Y, 4M, 4C, and 4K serving as supply-receiving units vary into yellow, magenta, cyan, and black. Thus, in the following description, the configuration and operation of the image forming units UY to UK will be described by omitting the characters Y, M, C, and K at the end of the reference symbols representing the distinction of the image forming units UY, UM, UC, and UK.
At the image forming unit U, a primary charger 12, an exposing unit 13, a developing unit 4, and a drum cleaning unit 14 are disposed so as to surround the photosensitive drum 11 serving as a photosensitive member. At the image forming unit U, a primary transfer unit T1 (transfer nip) is formed as a first transfer position at which a toner image is primarily transferred between the photosensitive drum 11 and the intermediate transfer belt 21 by the primary transfer roller 22. That is, the photosensitive drum 11 is disposed at a position facing the primary transfer roller 22, with the intermediate transfer belt 21 interposed therebetween. The photosensitive drum 11 has a photoconductive layer formed on the outer peripheral surface of an aluminum cylinder and is rotated in the direction of arrow R1 shown in
The primary charger 12 irradiates the photosensitive drum 11 with, for example, electrically charged particles associated with corona discharge, and the primary charger 12 charges the photosensitive drum 11 to a uniform negative dark potential. The exposing unit 13 uses a rotating mirror to scan a laser beam generated by on-off modulating scanning line image data obtained by expanding a decomposed color image of each color, and thereby writes in an electrostatic latent image of the image on the surface of a charged photosensitive drum 11. This electrostatic latent image is developed as a toner image by the developing unit 4.
The developing unit 4 accommodates a liquid developer obtained by dispersing a powdery toner as a dispersoid in a carrier liquid as a dispersing medium. To the developing unit 4, a liquid developer is supplied from a mixer that is not shown in the diagram. The liquid developer supplied from the mixer to the developing unit 4 is employed to coat (supply) a developing roller 4b by a coating roller 4a in the developing unit 4 and is used for development. The developing roller 4b carries the liquid developer on the surface and conveys the liquid developer, and the electrostatic latent image formed on the photosensitive drum 11 is developed with the toner. Such 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 respectively carried out using an electric field. Meanwhile, the liquid developer that has not been supplied to the development is returned to the mixer from the developing unit 4 and reused.
At the primary transfer unit T1 formed by the primary transfer roller 22, the toner image formed on the photosensitive drum 11 is subjected to primary transfer onto the intermediate transfer belt 21 using the electric field. After the primary transfer, the liquid developer (toner and carrier liquid) remaining on the photosensitive drum 11 is collected by a drum cleaning apparatus 14.
The secondary transfer unit T2 serving as a second transfer position is a toner image transfer nip toward a recording material P, which is formed by a secondary transfer outer roller 26 closely contacting with the intermediate transfer belt 21 supported by a secondary transfer inner roller 25. At the secondary transfer unit T2, a secondary transfer voltage is applied to the secondary transfer outer roller 26 serving as a transfer member, and thereby the toner image is subjected to secondary transfer from the intermediate transfer belt 21 to the recording material P that is conveyed to the secondary transfer unit T2. The toner remaining on the intermediate transfer belt 21 after the secondary transfer (residual toner) is removed together with the carrier liquid by a belt cleaning apparatus 30. The belt cleaning apparatus 30 will be described below (see
The recording material P having the four-colored toner image secondarily transferred thereon at the secondary transfer unit T2 is conveyed to a fixing unit or the like, which is not shown in the diagram, and the toner image transferred onto the recording material P is fixed by the fixing unit or the like. The recording material P having the toner image fixed thereon is discharged out of the apparatus body (out of the machine).
Liquid Developer
Next, the liquid developer used in the developing units 4Y to 4K will be described. Regarding the liquid developer, any liquid developer that has been hitherto used may be used; however, in the present embodiment, an ultraviolet-curable liquid developer is used.
The liquid developer is an ultraviolet-curable liquid developer including a cationically polymerizable liquid monomer, a photopolymerization initiator, and toner particles that are insoluble in the cationically polymerizable liquid monomer. Furthermore, the cationically polymerizable liquid monomer is a vinyl ether compound, and the photopolymerization initiator is a compound represented by the following General Formula (Chem 1).
The present invention will be described more specifically. First, the toner particles have a coloring material that gives a color, enclosed with a toner resin. Furthermore, the toner particles may also contain other materials such as a charge control agent, together with the toner resin and the coloring material. Regarding a method for producing toner particles, known technologies such as coacervation of dispersing a coloring material and gradually polymerizing a resin to enclose the coloring material; and an internal pulverization method of melting a resin or the like and enclosing a coloring material inside the resin, may be used. For the toner resin, an epoxy resin, a styrene-acrylic resin, or the like is used. The coloring material that gives a color may be a general organic or inorganic pigment. Furthermore, in view of production, a dispersant is used in order to increase toner dispersibility; however, a synergist can also be used.
A curable liquid serving as a carrier liquid is composed of a charge control agent that charges the toner surface, a photopolymerization agent generating acid when irradiated with ultraviolet radiation (UV), and a monomer that is bonded by acid. The monomer is a vinyl ether compound that is polymerized by a cationic polymerization reaction. Furthermore, apart from the photopolymerization agent, the curable liquid may also contain a sensitizer. Since preservability is decreased by photopolymerization, a cation polymerization inhibitor may be incorporated in an amount of 10 to 5,000 ppm. In addition, a charge control aid, other additive materials, and the like may also be used.
The ultraviolet curing agent (monomer) of this developer is a mixture of about 10% (% by weight) of a monofunctional monomer having one vinyl ether group represented by Chemical Formula (Chem 2) and about 90% of a bifunctional monomer having two vinyl ether groups represented by Chemical Formula (Chem 3).
The photopolymerization initiator has 0.1% of a compound represented by the following (Chem 4) mixed therein. When this photopolymerization initiator is used, satisfactory fixing is enabled, and a highly resistant liquid developer is obtained, unlike the case of using an ionic photo-acid generator.
It is desirable that the cationically polymerizable liquid monomer is a compound selected from the group consisting of dicyclopentadiene vinyl ether, cyclohexanedimethanol 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.
Furthermore, regarding the charge control agent, known agents can be used. Specific examples of the compound include oils and fats such as linseed oil and soybean oil; alkyd resins, halogen polymers, aromatic polycarboxylic acids, acidic group-containing water-soluble dyes, oxidative condensates of aromatic polyamines, metal soaps such as cobalt naphthenate, nickel naphthenate, iron naphthenate, zinc naphthenate, cobalt octylate, nickel octylate, zinc octylate, cobalt dodecylate, nickel dodecylate, zinc dodecylate, aluminum stearate, and cobalt 2-ethylhexanoate; sulfonic acid metal salts such as petroleum-based sulfonic acid metal salts, and metal salts of sulfosuccinic acid esters; phospholipids such as lecithin; salicylic acid metal salts such as t-butyl salicylic acid metal complexes; polyvinylpyrrolidone resins, polyamide resins, sulfonic acid-containing resins, and hydroxybenzoic acid derivatives.
Belt Cleaning Apparatus
The configuration of a belt cleaning apparatus 30 of the present embodiment will be described using
The opposed roller 40 is provided to be freely rotatable on the inner peripheral side of the intermediate transfer belt 21, and is in close contact with the inner peripheral surface (rear surface) of the intermediate transfer belt 21 between a secondary transfer inner roller 25 and a driving roller 23 (see
The present embodiment is configured such that the physical nip T3b is secured by the opposed roller 40 and the cleaning roller 31. The reason for this is that discharge that is prone to occur in the vicinity of the nip portion T3 should be suppressed as much as possible. That is, when discharge occurs in the vicinity of the nip portion T3, a stronger electric field is needed in order to perform electrostatic cleaning of the toner on the intermediate transfer belt 21, and accordingly, there are concerns about the damage to the intermediate transfer belt 21 being increased. In the present embodiment, as will be described below, an elastic belt having an elastic layer is used as the intermediate transfer belt 21. Such an intermediate transfer belt 21 has high electrical resistance, and therefore, discharge is prone to occur particularly in the vicinity of the nip portion T3.
The cleaning roller 31 is driven to rotate in the same direction (direction of arrow R3) as the moving direction of the intermediate transfer belt 21 at the nip portion T3 with the intermediate transfer belt 21, by a motor 35 serving as a first driving source. As will be described below in detail, in the case of the present embodiment, the cleaning roller 31 is rotated such that the surface velocity of the roller is slower than the surface velocity of the intermediate transfer belt 21 at the nip portion T3. Then, the cleaning roller 31 electrically removes the toner remaining on the intermediate transfer belt 21 without being secondarily transferred by means of the action of an electric field (so-called electrophoresis). In the case of the present embodiment, the opposed roller 40 is earthed, while the cleaning roller 31 is connected to a power supply 36, and a voltage of opposite polarity to the polarity of 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 intermediate transfer belt) is moved from the intermediate transfer belt 21 to the cleaning roller 31 by means of a liquid layer of the liquid developer formed between the intermediate transfer belt 21 and the cleaning roller 31 at the nip portion T3. Meanwhile, since the cleanability achieved by electrophoresis increases proportionally to the passage time of the nip portion T3, it is desirable that the surface velocity of the intermediate transfer belt 21 and the surface velocity of the cleaning roller 31 at the nip portion T3 are the same. However, in order to satisfy the cleaning performance by electrophoresis at the nip portion T3, it has been verified by experiments of the inventors that the relative velocity between the surface velocity of the intermediate transfer belt 21 and the surface velocity of the cleaning roller 31 is desirably within ±10%.
The toner that has moved to the cleaning roller 31 is removed by a cleaning blade 32 together with the liquid developer. The cleaning blade 32 is, for example, a plate-shaped member made of a metal such as stainless steel, and is in close contact with the cleaning roller 31 on the downstream side in the moving direction from the nip portion T3 in relation to the moving direction of the cleaning roller 31. The toner removed by the cleaning blade 32 flows inside the cleaning container 33 along the direction of gravity together with the liquid developer and falls. The bottom face of the cleaning container 33 is formed in an inclined shape, and a sheet discharge port 34 is formed at the lowest part of the inclined bottom face. Therefore, the liquid developer including the toner removed by the cleaning blade 32 is delivered to the sheet discharge port 34 along the bottom face of the cleaning container 33 and is discharged out of the cleaning container 33 through the sheet discharge port 34.
Cleaning Roller
The above-described cleaning roller 31 will be explained. In an image forming apparatus that uses a liquid developer, it is desirable to use a cleaning roller 31 formed from a material that does not easily react with the organic solvent or the like included in the liquid developer. This is because the durability of the roller is increased by making it difficult to induce deterioration caused by dissolution or denaturing attributed to the compounds used for the carrier liquid. Generally, when the difference between the respective solubility parameter (SP) values of the roller and the organic solvent is 2 or more, the roller is more likely to deteriorate than in the case in which the difference of the SP value is less than 2 (that is, deterioration of the roller is accelerated). In the present embodiment, from the viewpoint of slowing down the deterioration of the roller, for example, a metal roller made of stainless steel or aluminum is used as the cleaning roller 31. As the metal roller, a metal roller having its surface thinly coated with a fluororesin or the like to the extent that the shape conformity depending on deformation will not change, may be used. Meanwhile, since there is less chance for the opposed roller 40 to be exposed to the liquid developer compared to the cleaning roller 31, it is not necessarily essential to use a metal roller, and a rubber roller may be used. However, when deterioration of the roller is taken into consideration, it is preferable to use a metal roller also for the opposed roller 40.
In the case of an image forming apparatus that uses a dry developer, it is difficult to use a metal roller as the cleaning roller 31. That is, when the toner included in a dry developer is an insulator, and the cleaning roller 31 is a metal roller having low electrical resistance, the toner having reversed polarity may fuse together due to the discharge occurring at the nip portion or the gap in the vicinity thereof. Then, the cleaning performance is deteriorated. In contrast, in the case of an image forming apparatus that uses a liquid developer, even if discharge occurs, the polarity of the toner is substantially not reversed. Further, since the toner moves through the liquid layer of the liquid developer by means of electrophoresis, it is possible to use a metal roller. However, when a metal roller is compared with a rubber roller, the shape conformity caused by deformation is very low. Thus, in a case in which a metal roller is used as the cleaning roller 31, it is necessary to secure the nip length of the nip portion T3 (the moving direction length of the intermediate transfer belt 21) so that the toner can be reliably moved by electrophoresis. As will be described below in detail (see
Intermediate Transfer Belt
The intermediate transfer belt 21 will be explained. The intermediate transfer belt 21 is an elastic belt having a base layer 21a and an elastic layer 21b (see
Around the base layer 21a, an elastic layer 21b that is more elastic than the base layer 21a is formed. The elastic layer 21b is formed from a rubber having a Young's modulus (E2) lower than the Young's modulus (E1) of the base layer, or the like (E1>E2). Furthermore, the elastic layer 21b is such that its thickness (t2) is larger than the thickness (t1) of the base layer 21a (t1<t2). In a case in which the thickness of such an intermediate transfer belt 21 is 1 mm or less, if a metal roller having a diameter larger than 40 mm is not used as the cleaning roller 31, it is difficult to secure a sufficient nip length that is enough for removing most of the toner. There is a need to secure a nip length of, for example, 1.2 mm or more. However, from the viewpoint that when a metal roller has an increased diameter, the weight becomes heavier by the square, it is preferable to use a metal roller having a diameter of 40 mm or less as the cleaning roller 31.
Here, with regard to the nip length of the nip portion formed in a case in which the metal roller is pressed against an endless belt, an experiment of investigating the nip length by varying the thickness of the belt was carried out. The experiment results are presented in
As shown in
Meanwhile, in a case in which the contact pressure is too low, it may be considered that the distance between the opposed roller 40 or the cleaning roller 31 and the belt is increased by thickness unevenness of the belt or driving unevenness at the time of rotation. In this case, the physical nip width or the tension nip width disappears, and the cleaning ability is deteriorated. Therefore, it is desirable that the contact pressure is as low as possible (for example, 30 N), to the extent that variation does not occur in the nip width described above. In the present embodiment, the lower limit of the contact pressure is set to 30 N. As such, in the present embodiment, the contact pressure was adjusted to be from 30 N to 300 N.
Electrophoresis of Toner
Next, electrophoresis of the toner at the nip portion T3 will be described using
As described above, the belt cleaning apparatus 30 electrically removes the toner F on the intermediate transfer belt 21 by means of the action of an electric field (so-called electrophoresis). At that time, it is necessary to secure the nip length L at the nip portion T3 so that the toner F can be reliably moved from the intermediate transfer belt 21 to the cleaning roller 31 by electrophoresis, and the nip length L (m) is a length that satisfies the following Formula 1.
(μ×E)×(L/P)>d Formula 1
In Formula 1, μ (m2/(V×s)) represents the toner mobility; E (V/m) represents the strength of the electric field generated at the nip portion T3 concomitantly with application of a voltage to the cleaning roller 31; P (m/s) represents the rotational speed of the intermediate transfer belt 21; and d (μm) represents the liquid thickness of the liquid developer G at the nip portion T3. Meanwhile, the nip length L is the length over which, in a case in which a so-called solid image obtained by loading the toner over the entire surface of the recording material is subjected to secondary transfer as a toner image, the toner remaining on the intermediate transfer belt 21 after secondary transfer is cleaned by electrophoresis.
Then, the left-hand side of Formula 1 is the product of the moving velocity of the toner as represented by (μ×E) and the passage time pertaining to the passage of the nip portion T3 as represented by (L/P), that is, the distance over which the toner can move by electrophoresis from the intermediate transfer belt 21 toward the cleaning roller 31. On the other hand, the right-hand side of Formula 1 is, as described above, the liquid thickness of the liquid developer at the nip portion T3. That is, when a nip length L where the left-hand side of Formula 1 is larger than the right-hand side is secured, the toner can move from the intermediate transfer belt 21 to the cleaning roller 31 by means of the liquid thickness of the liquid developer while passing through the nip portion T3. For instance, the toner mobility is 1.00−10 to 1.00−11 (m2/(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 be 1.5 (mm) or more. However, when the nip length L is made long, the winding angle of the intermediate transfer belt 21 with respect to the cleaning roller 31 becomes large. In that case, since the intermediate transfer belt 21 is likely to bend repeatedly along with this rotation, it is not preferable from the viewpoint of the belt lifetime. 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°. The winding angle is more preferably less than 45°, and even more preferably less than 20°.
Here, the measurement of the toner mobility in Formula 1 described above, the electric field, the nip length, and the liquid thickness of the liquid developer will be described. The toner mobility μ can be represented by the following Formula 2.
μ=|v/E|=Q/(6π×η×α) Formula 2
In Formula 2, v (m/s) represents the moving velocity of the toner; and E (V/m) represents the strength of the electric field generated at the nip portion T3 concomitantly with application of a voltage to the cleaning roller 31. Furthermore, Q (C) represents the amount of charge carried by the toner in the liquid developer; π represents the ratio of the circumference of a circle to its diameter; n represents the viscosity (Pa·s) of the liquid developer; and α (μm) represents the diameter of the toner. For instance, 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. Furthermore, the moving velocity of the toner in the case of the present embodiment is about 9 to 90 (m/s). The amount of charge of the toner can be calculated from the above-described various parameters that have been quantitatively determined. Meanwhile, the toner mobility can be quantitatively determined by making measurement using a measuring device such as a zeta potentiometer measuring apparatus, Zeta-APS (manufactured by Matec Applied Sciences, Inc.).
The electric field can be generally determined by the following Formula 3. β (V) represents the voltage value applied to the cleaning roller 31; and d (μm) represents the liquid thickness of the liquid developer at the nip portion T3.
E=β/d Formula 3
With regard to the electric field, the route including from the cleaning roller 31 to the opposed roller 40 via the liquid developer and the resistor of the intermediate transfer belt 21 is subjected to modeling using a series circuit, and the electric field can be determined by the circuit calculation. For instance, the voltage value applied to the cleaning roller 31 is 1,000 (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). Furthermore, 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).
Regarding the nip length, a momentary stop is induced by cutting the main power supply in the middle of image formation or the like, 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 opposed roller 40, and the amount of deformation of the intermediate transfer belt 21. According to the present embodiment, the diameter of the cleaning roller 31 is 28 mm, and the diameter of the opposed roller 40 is 21 mm. Meanwhile, the surface roughness of the cleaning roller 31 and the opposed roller 40 is less than 0.2 μm according to the standard of JIS B 0031:2003. The surface roughness of these rollers can be measured using PU-OS400 (manufactured by Kosaka Laboratory, Ltd.).
Regarding the liquid thickness of the liquid developer, a portion of the liquid developer is peeled off from the surface of the intermediate transfer belt 21 that has passed through the nip portion T3 using a scraper or the like, and the difference of elevation between a site at which the liquid developer has been peeled off, and a site at which the liquid developer has not been peeled off, is actually measured using a confocal microscope or the like. Further, a value equivalent to twice the elevation difference thus measured is designated as the liquid thickness of the liquid developer. That is, the liquid developer at the nip portion T3 is separated into the intermediate transfer belt 21 and the cleaning roller 31 after passing through the nip portion T3. 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, becomes a half of the liquid thickness of the liquid developer at the nip portion T3. Thus, since the elevation difference actually measured as described above becomes twice, the liquid thickness of the liquid developer at the nip portion T3 can be determined. Meanwhile, regarding the confocal microscope, for example, confocal microscope VK8700 (manufactured by Keyence Corporation) may be used.
Cleaning Nip Portion
As described above, in the present embodiment, in order to cause the toner to move from the intermediate transfer belt 21 to the cleaning roller 31 by electrophoresis by means of the liquid developer, it is necessary to form the nip portion T3 at the nip length L that satisfies the above-described Formula 1. In order to do so, in the present embodiment, the cleaning roller 31 and the opposed roller 40 are disposed such that the intermediate transfer belt 21 is wound around the cleaning roller 31. This will be explained using
As shown in
The cleaning roller 31 is fixed to be freely rotatable so as to compress the intermediate transfer belt 21 from the outer side toward the inner side. On the other hand, with regard to the opposed roller 40, the bearings (not shown in the diagram) supporting the opposed roller 40 at the two ends are energized by a pressing spring 41 such that the opposed roller 40 compresses the intermediate transfer belt 21 from the inner side toward the outer side by means of the pressing spring 41.
In a case in which the intermediate transfer belt 21 is bent, the winding amount of the intermediate transfer belt 21 about the cleaning roller 31 increases, compared to the case in which the intermediate transfer belt 21 is not bent. As described above, the nip portion T3 has a physical nip T3b and a tension nip T3a (see
According to the present embodiment, the relationship between the nip length L required for subjecting the toner to electrophoresis and the electric field E can be represented by Formula 4, which is obtained by modifying the above-described Formula 1.
E>(d×P/μ)/L Formula 4
Here, the relationship between the nip length and the strength of the electric field (electric field intensity) is shown in
Meanwhile, in the case of the present embodiment, the toner mobility may be adjusted to be 1.00−11 (m2/(V×s)). It is because the toner mobility can be decreased concomitantly with use; however, in a case in which the toner mobility satisfies the lower limit, the cleanability achieved by the belt cleaning apparatus 30 is guaranteed.
Next, an experiment of comparing the cleaning performance in relation to the case in which the intermediate transfer belt 21 was bent as described above and the case in which the intermediate transfer belt 21 was not bent, was carried out using either a metal roller or a rubber roller for the cleaning roller 31 and the opposed roller 40. The experiment results are presented in Table 1. The cleaning roller 31 used for the experiment has a diameter of 28 mm, and the opposed roller 40 has a diameter of 21 mm. Furthermore, the rubber roller is such that the thickness of the elastic layer formed from urethane rubber is 2 mm, and its Young's modulus is 0.3 (MPa). Meanwhile, a first example, a second example, and a fourth example that will be described below are Comparative Examples, and a third example and a fifth example correspond to the present embodiments.
As the first example, in a case in which the cleaning roller 31 and the opposed roller 40 are both rubber rollers, and the projection quantity of the cleaning roller 31 is “0 mm”, a nip portion T3 having a nip length of “1.5 mm” is formed. Here, the projection quantity of the cleaning roller 31 is the distance between the external common tangent Z on the intermediate transfer belt side and the farthest close contact position from the external common tangent Z among the close contact positions between the cleaning roller 31 and the intermediate transfer belt 21 in the nip portion T3 (represented by symbol W in
As the second example, in a case in which the cleaning roller 31 is a metal roller, the opposed roller 40 is a rubber roller, and the projection quantity of the cleaning roller 31 is “0”, a 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 intensity was increased (115 (V/μm)) relative to the first example, satisfactory cleaning performance could not be obtained. This is because a metal roller is not easily deformed compared to a rubber roller, only a short nip length is obtained compared to the first example, and a nip portion T3 cannot be secured to the extent that can form a liquid layer of the liquid developer sufficient for moving the toner by electrophoresis. Thus, as a third example, the projection quantity of the cleaning roller 31 was set to “5 mm”, in other words, the intermediate transfer belt 21 was bent, a nip length of “1.5 mm” that was equal to the first example was secured. By securing a nip length of “1.5 mm”, as shown in Table 1, satisfactory cleaning performance is obtained at an electric field intensity of 85 (V/μm).
As a fourth example, in a case in which the cleaning roller 31 and the opposed roller 40 are both metal rollers, and the projection quantity of the cleaning roller 31 is “0”, a nip portion T3 having a nip length of “0.3 mm” is formed. In this case, as shown in Table 1, even if the electric field intensity was increased to a large extent (300 (V/μm)) compared to the first example, satisfactory cleaning performance was not obtained. This is because when metal rollers are used in combination, only a shorter nip length is obtained, and a nip portion T3 cannot be secured to the extent that can form a liquid layer of the liquid developer sufficient for moving the toner by electrophoresis. Furthermore, it is because the electric field intensity is too high, and therefore, discharge may occur. Thus, as a fifth example, the projection quantity of the cleaning roller 31 is adjusted to be “7 mm” that is larger than the third example. In this way, a nip length of “1.5 mm” that is equal to the first example can be secured, and as shown in Table 1, satisfactory cleaning performance is obtained at an electric field intensity of 85 (V/μm).
However, as described above, in a case in which the intermediate transfer belt 21 is bent, the surface velocity on the outer peripheral side (front surface) of the intermediate transfer belt 21 can become less than the surface velocity on the inner peripheral side (rear surface), as the elastic layer 21b is compressed in the moving direction of the intermediate transfer belt 21 at the nip portion T3. In this case, due to the relative velocity difference with the cleaning roller 31, strain occurs in the elastic layer 21b of the intermediate transfer belt 21 at the nip portion T3, and when this strain exceeds the limit, the intermediate transfer belt 21 may instantaneously change. At that time, winding of the intermediate transfer belt 21 by means of the driving roller 23 is likely to become loose. Thus, in a case in which the intermediate transfer belt 21 is caused to project on the inner side, it is important to prevent the intermediate transfer belt 21 from loosening. Therefore, in the present embodiment, the cleaning roller 31 is rotated such that the surface velocity thereof becomes slower than the surface velocity of the intermediate transfer belt 21 at the nip portion T3, and thus the relative velocity difference between the intermediate transfer belt 21 and the cleaning roller 31 is reduced. In the following description, this will be described with reference to
The elastic layer 21b of the intermediate transfer belt 21 is compressed in the thickness direction of the belt by means of the cleaning roller 31 and the opposed roller 40 at the nip portion T3. Here, the thickness t of the intermediate transfer belt 21 at the nip portion T3 can be represented by the following Formula 5, in a case in which the radius of the cleaning roller 31 is designated as r, the radius of the opposed roller 40 is designated as s, and the distance between the center of rotation of the cleaning roller 31 and the center of rotation of the opposed roller 40 is designated as x.
t=x−r−s Formula 5
The surface velocity u on the outer peripheral side of the intermediate transfer belt 21 at the nip portion T3 can be represented by the following Formula 6, in a case in which the surface velocity of the driving roller 23 is v1, from the relationship of the radius ratio based on the winding of the intermediate transfer belt 21 around the cleaning roller 31.
u=r/(r+t)×v1 Formula 6
When Formula 5 is substituted into the above-described Formula 6, Formula 7 is obtained.
u=r/(x−s)×v1 Formula 7
When the cleaning roller 31 is rotated so as to satisfy the relationship (v2<u) that the surface velocity of the cleaning roller 31 (designated as v2) is slower than the surface velocity u of the intermediate transfer belt 21, the intermediate transfer belt 21 receives frictional force on the opposite side in the moving direction at the nip portion T3. In this case, a state in which the intermediate transfer belt 21 is pulled toward the driving roller 23 is achieved, and the relative velocity difference between the intermediate transfer belt 21 and the cleaning roller 31 is reduced at the nip portion T3. Then, winding of the intermediate transfer belt 21 by the driving roller 23 is not easily loosened. Furthermore, in order to satisfy the cleaning performance by electrophoresis, as described above, it is desirable that the relative velocity between the surface velocity of the intermediate transfer belt 21 and the surface velocity of the cleaning roller 31 is within ±10%. From the above description, in the case of the present embodiment, it is preferable that the cleaning roller 31 is rotated so as to satisfy the following Formula 8. That is, when the cleaning roller 31 is rotated so as to satisfy the relationship of Formula 8, stable cleaning performance and belt running performance are attained.
r/(x−s)−0.1<v2/v1<r/(x−s) Formula 8
As described above, in the present embodiment, the intermediate transfer belt 21 is bent by pushing in the intermediate transfer belt 21 from the outer side to the inner side by the cleaning roller 31 so that a nip portion T3 can be secured to the extent that a liquid layer of the liquid developer sufficient for moving the toner by electrophoresis can be formed. Thereby, the nip length of the nip portion T3 can be adjusted to a length over which the toner on the intermediate transfer belt 21 can be sufficiently moved by electrophoresis (see the above-described Formula 1). Furthermore, in the present embodiment, the cleaning roller 31 is rotated such that the surface velocity thereof becomes slower than the surface velocity of the intermediate transfer belt 21 at the nip portion T3. Thereby, in a case in which a nip portion T3 sufficient for sufficiently removing the toner on the intermediate transfer belt 21 by electrophoresis is secured, loosening of the winding of the intermediate transfer belt 21 concomitant to rotary driving of the cleaning roller 31 and the driving roller 23 can be suppressed. That is, according to the present embodiment, an image forming apparatus that includes a cleaning roller and a driving roller that are respectively subjected to rotary driving and forms an image using a liquid developer, is provided, and loosening of the belt member to which tension is applied between the driving roller and the cleaning roller can be suppressed, while a nip amount between the cleaning roller and the belt member is secured. Furthermore, in the present embodiment, the driving roller 23 is a roller that initially applies tension to the intermediate transfer belt 21 on the downstream side of the cleaning roller 31 in the belt moving direction; however, the driving roller is not limited to this. For example, another tension-applying roller may be provided between the driving roller 23 and the cleaning roller 31.
A second embodiment will be described using
As shown in
Furthermore, unlike the first embodiment, the cleaning roller 31 and the opposed roller 40 are disposed with an offset. That is, the cleaning roller 31 is disposed such that a first intersection point N of the external common tangent Z and a perpendicular line passing through the center of rotation M of the cleaning roller 31, and a second intersection point Q of the external common tangent Z and a perpendicular line passing through the center of rotation O of the opposed roller 40, are shifted in the moving direction. However, in the case of the present embodiment, the central position of a physical nip T3b in the moving direction is disposed on the downstream side of the central position of the nip portion T3 in the moving direction (see
When the cleaning roller 31 and the opposed roller 40 are disposed with an offset, a nip portion T3 having a nip length that satisfies the above-described Formula 1 can be formed even without making the projection quantity of the cleaning roller 31 larger compared to the above-described first embodiment. That is, as shown in
Then, as shown in
The above-described discharge will be described using
On the other hand, when the cleaning roller 31 is disposed with an offset on the upstream side of the opposed roller 40b in the moving direction, discharge does not easily occur on the upstream side in the moving direction. That is, in this case, since charge injection from the cleaning roller 31 to the intermediate transfer belt 21 occurs at the tension nip T3a, the electric current is concentrated from the intermediate transfer belt 21 toward the opposed roller 40b at the physical nip T3b. In that case, the potential difference between the surface of the cleaning roller 31 and the surface of the intermediate transfer belt 21 does not become large on the upstream side in the moving direction of the intermediate transfer belt 21, and discharge does not easily occur. Furthermore, in this case, for example, even if discharge occurs at the physical nip T3b on the downstream side in the moving direction of the intermediate transfer belt 21, since the occurrence of discharge comes after most of the toner has already moved to the cleaning roller 31 side, the influence exerted on the cleaning performance is negligible.
In Table 2, results obtained by comparing the cleaning performance in the case in which the cleaning roller 31 and the opposed roller 40 are disposed with an offset on the upstream side in the moving direction and on the downstream side in the moving direction, respectively. Furthermore, for reference, the cleaning performance is shown also with regard to the case in which the cleaning roller 31 is not disposed with an offset with respect to the opposed roller 40 (middle in Table 2).
As can be understood from Table 2, in a case in which the cleaning roller 31 and the opposed roller 40 are disposed with an offset, satisfactory cleaning performance is obtained when the cleaning roller 31 is disposed with an offset on the upstream side of the opposed roller 40b in the moving direction. In contrast, when the cleaning roller 31 is disposed with an offset on the downstream side of the opposed roller 40b in the moving direction, discharge occurs on the upstream side in the moving direction as described above, and a slight amount of toner residue is generated. The concentration of the toner residue was measured with a concentration meter manufactured by X-Rite, Inc., and the concentration was about 0.008. This shows that the cleaning performance is degraded compared to the case in which the concentration of the toner residue is about 0.003 or less, and the cleaning roller 31 is disposed with an offset on the upstream side of the opposed roller 40b in the moving direction.
As described above, in the second embodiment, a nip portion T3 sufficient for sufficiently moving the toner on the intermediate transfer belt 21 by electrophoresis can be easily secured by disposing the cleaning roller 31 and the opposed roller 40b with an offset. Particularly, when the cleaning roller 31 is disposed with an offset on the upstream side of the opposed roller 40b in the moving direction, more satisfactory cleaning performance can be obtained. Furthermore, even in this case, similarly to the above-described first embodiment, the cleaning roller 31 is rotated such that the surface velocity thereof becomes slower than the surface velocity of the intermediate transfer belt 21 at the nip portion T3. Therefore, in the second embodiment, even in a case in which a larger nip portion T3 for removing the toner on the intermediate transfer belt 21 by electrophoresis is secured, loosening of the intermediate transfer belt 21 concomitant to the rotary driving of the cleaning roller 31 and the driving roller 23 can be suppressed.
In the first embodiment and second embodiment described above, an example of bending the intermediate transfer belt 21 by pushing in the intermediate transfer belt 21 from the outer side to the inner side by means of the cleaning roller 31 is disclosed; however, the example is not limited to this. For example, the intermediate transfer belt 21 may be bent by pushing in the intermediate transfer belt 21 from the inner side to the outer side by means of the opposed roller 40. The third embodiment as such will be described using
As shown in
In the case of the present embodiment, compared to the first embodiment described above, a nip portion T3 is secured by subjecting the elastic layer 21b to greater elastic deformation by using a cleaning roller 31 having a larger diameter, making the welding pressure of the opposed roller 40 greater, or the like. In that case, since the elastic layer 21b is expanded as the intermediate transfer belt 21 is wound around the opposed roller 40 at the nip portion T3, the surface velocity on the outer peripheral side (front surface) of the intermediate transfer belt 21 may become greater than the surface velocity of the inner peripheral side (rear surface). In this case, due to the strain in the elastic layer 21b of the intermediate transfer belt 21 occurring at the nip portion T3 as a result of the velocity difference between the intermediate transfer belt 21 and the cleaning roller 31, the intermediate transfer belt 21 between the cleaning roller and the driving roller 23 may become loose. Thus, even in a case in which the intermediate transfer belt 21 is projected on the outer side, it is important not to make the intermediate transfer belt 21 loose. From this point of view, in the present embodiment, the cleaning roller 31 is rotated such that the surface velocity thereof is slower than the surface velocity of the intermediate transfer belt 21 at the nip portion T3. Hereinafter, this point will be explained with reference to
The elastic layer 21b of the intermediate transfer belt 21 is compressed in the thickness direction of the belt by the cleaning roller 31 and the opposed roller 40 at the nip portion T3. The thickness t of the intermediate transfer belt 21 at the nip portion T3 can be represented by the above-described Formula 5 (t=x−r−s), in a case in which the radius of the cleaning roller 31 is designated as r; the radius of the opposed roller 40 is designated as s; and the distance between the center of rotation of the cleaning roller 31 and the center of rotation of the opposed roller 40 is designated as x.
The surface velocity u on the outer peripheral side of the intermediate transfer belt 21 at the nip portion T3 can be represented by the following Formula 9, in a case in which the surface velocity of the driving roller 23 is V1, from the relationship of the radius ratio based on the winding of the intermediate transfer belt 21 around the opposed roller 40.
u=(s+t)/s×v1 Formula 9
When Formula 5 is substituted into the above-described Formula 9, Formula 10 is obtained.
u=(x−r)/s×v1 Formula 10
Then, the cleaning roller 31 is rotated so as to satisfy the relationship (v2<u) that the surface velocity thereof (designated as v2) is slower than the surface velocity u of the intermediate transfer belt 21. Hereupon, a state in which the intermediate transfer belt 21 is pulled by the driving roller 23 is achieved, and the relative velocity difference between the intermediate transfer belt 21 and the cleaning roller 31 at the nip portion T3 is reduced. Then, the winding of the intermediate transfer belt 21 by the driving roller 23 does not easily become loose. Furthermore, it is desirable that the relative velocity between the surface velocity of the intermediate transfer belt 21 and the surface velocity of the cleaning roller 31 is within ±10%, as described above. From the above description, in the case of the present embodiment, it is preferable that the cleaning roller 31 is rotated so as to satisfy the following Formula 11. That is, when the cleaning roller 31 is rotated so as to satisfy the relationship of Formula 11, stable cleaning performance and belt running performance are obtained.
(x−r)/s−0.1<v2/v1<(x−r)/s Formula 11
As described above, in the present embodiment, the intermediate transfer belt 21 is bent by pushing in the intermediate transfer belt 21 from the inner side to the outer side by means of the cleaning roller 31, so that a nip portion T3 can be secured to the extent that a liquid layer of the liquid developer sufficient for moving the toner by electrophoresis can be formed. In this case, the cleaning roller 31 is rotated such that the surface velocity thereof is slower than the surface velocity of the intermediate transfer belt 21 at the nip portion T3. Thereby, loosening of the intermediate transfer belt 21 concomitant to rotary driving of the cleaning roller 31 and the driving roller 23 can be suppressed.
Also in the third embodiment described above, similarly to the second embodiment described above, the cleaning roller 31 and the opposed roller 40b may be disposed with an offset in a state in which a physical nip T3b is formed (see
In the first to fourth embodiments described above, an example of subjecting the driving roller 23 to rotary driving by means of a motor 231 is disclosed (see
As shown in
r/(x−s)+0.1>v2/v1>r/(x−s) Formula 12
Thereby, even in a case in which the secondary transfer inner roller 25 also functions as a driving roller, loosening of the intermediate transfer belt 21 concomitant to the rotary driving of the cleaning roller 31 and the secondary transfer inner roller 25 can be easily suppressed.
The belt cleaning apparatuses 30 of the first to fifth embodiments described above can be applied to a two-roller belt cleaning apparatus for cleaning a secondary transfer belt. Hereinafter, the apparatus will be described using
Meanwhile, the first to fifth embodiments described above have been explained on the premise that a physical nip T3b is formed; however, the present invention is not limited to this. For example, the opposed roller 40 may be offset on a further downstream side so as to form only a tension nip T3a, without forming a physical nip T3b between the cleaning roller 31 and the intermediate transfer belt 21. A belt cleaning apparatus in the case in which a physical nip T3b is not formed will be described using
As shown in
Here, when a roller that initially applies tension to the intermediate transfer belt 21 on the downstream side of the cleaning roller 31 in the moving direction of the intermediate transfer belt 21 is designated as a first roller, in the case of the present embodiment, the first roller corresponds to the opposed roller 40. The opposed roller 40 is disposed at a position at which the position that is in close contact with the intermediate transfer belt 21 does not overlap with the tension nip T3a in relation to the moving direction of the intermediate transfer belt 21 (see
Meanwhile, in the respective embodiments described above, an example in which the cleaning roller 31 and the driving roller 23 or the secondary transfer inner roller 25 are driven by separate driving sources (motors) is disclosed; however, the present invention is not limited to this. For example, the cleaning roller 31, driving roller 23, and secondary transfer inner roller 25 may be configured such that the respective rollers are rotary-driven by transferring a rotary drive force produced by a single driving source (motor) by a gear unit including a number of gears.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
The present image forming apparatus is suitable to be used particularly for cases where a liquid developer is used.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-243645 | Dec 2017 | JP | national |
This application is a Continuation of International Patent Application No. PCT/JP2018/045749, filed Dec. 12, 2018, which claims the benefit of Japanese Patent Application No. 2017-243645, filed Dec. 20, 2017, both of which are hereby incorporated by reference herein in their entirety.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6151469 | Lee | Nov 2000 | A |
| 7590368 | Kamijo et al. | Sep 2009 | B2 |
| 20090232536 | Nishimura et al. | Sep 2009 | A1 |
| 20110182611 | Hofmann et al. | Jul 2011 | A1 |
| 20110262170 | Kozuma | Oct 2011 | A1 |
| Number | Date | Country |
|---|---|---|
| 2001-109346 | Apr 2001 | JP |
| 2002-318493 | Oct 2002 | JP |
| 2007-147973 | Jun 2007 | JP |
| 2009-244840 | Oct 2009 | JP |
| 2011-158905 | Aug 2011 | JP |
| Entry |
|---|
| International Search Report dated Feb. 12, 2019, in International Patent Application No. PCT/JP2018/045479. |
| Number | Date | Country | |
|---|---|---|---|
| 20200319577 A1 | Oct 2020 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2018/045749 | Dec 2018 | US |
| Child | 16906014 | US |
