JPH1184913A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of suppressing the generation of hollow characters by the concentration of the pressing force of a biasing roll. SOLUTION: This image forming device has an image carrying member 1 which is formed with the electrostatic latent image meeting image information, a developing device 3 which visualizes the electrostatic latent image as a toner image, an intermediate transfer belt 7 which is primarily transferred with the toner image and the biasing roll 10 which secondarily transfers the toner image on the belt 7 to a recording medium P. This intermediate transfer belt 7 consists of at least a two-layered structure consisting of a base material and a surface layer having dynamic hardness of <=10. This base material consists of a polyimide resin dispersed with carbon black or conductive metal oxide. The Young's moduli of these materials are respectively preferably >=30000 kg/cm<2> . Polyester resins and fluororesins denatured by various kids of rubber materials are adequately used as the component of the surface layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrophotographic copying machine,
Laser printer, facsimile, composite OA of these
The present invention relates to an image forming apparatus using an electrophotographic method, such as a device. More specifically, image formation is performed in which a toner image formed on an image carrier is temporarily transferred to an intermediate transfer belt and then transferred to a recording medium such as copy paper or an OHP sheet to obtain a reproduced image. Related to the device.

[0002]

2. Description of the Related Art An image forming apparatus using an electrophotographic method is a laser in which a uniform charge is formed on an image carrier composed of a photoreceptor made of an inorganic or organic photoconductive material, and an image signal is modulated. After forming an electrostatic latent image with light or the like, the electrostatic latent image is developed with charged toner to obtain a visualized toner image. Then, the toner image is transferred onto a recording medium such as paper directly or via an intermediate transfer member to obtain a required reproduced image. An image forming apparatus adopting a system in which a toner image formed on an image carrier is primarily transferred to an intermediate transfer member and a toner image on the intermediate transfer member is secondarily transferred to a recording medium is disclosed in, for example, No. 206567 discloses this. Examples of a belt material used in an image forming apparatus employing an intermediate transfer member system include polyvinylidene fluoride (PVDF) (JP-A-5-200904,
JP-A-6-228335), polycarbonate (P
C) (JP-A-6-95521), polyalkylene terephthalate (PAT) (JP-A-6-149081), and a blended material of PAT and PC (JP-A 6-1481)
No. 49083), a blend material of ethylene-tetrafluoroethylene copolymer (ETFE) and PC, ET
Blended material of FE and PAT, ETFE, PC and PA
Blend material with T (JP-A-6-149079)
There has been proposed a conductive endless belt in which a conductive agent such as carbon black is dispersed in a thermoplastic resin such as. The above PVD
Conductive materials composed of thermoplastic resins such as F and PC
Since the Young's modulus is inferior to 24000 kg / cm ^2, which is poor in mechanical properties, the belt is greatly deformed by the stress applied to the belt during driving, and when applied to an intermediate transfer belt, a high quality transfer image can be obtained stably. In addition, there is a problem that the durability of the belt is inferior because a crack is generated at the end of the belt during driving.

[0003]

As a material having excellent mechanical properties, polyimide resin and the like can be mentioned.
For example, Japanese Patent No. 2560727 proposes a seamless belt made of carbon black-dispersed polyimide resin. When a belt material having excellent mechanical properties such as a polyimide resin is used as the intermediate transfer member, when the recording medium is pressed against the intermediate transfer belt by the bias roll, deformation of the belt due to the pressing force is extremely small. When the toner image is electrostatically transferred to the recording medium by applying an electric field in the secondary transfer unit in such a state, the state shown in FIG.
As shown in (1), the load of the pressing force by the bias roll concentrates on the toner image on the intermediate transfer belt. In particular, since a multi-toner image having a high pile height passes through the primary transfer portion a plurality of times, the physical and electrostatic adhesion between the intermediate transfer belt and the toner image is further increased. When the pressing force of the bias roll is concentrated on such a toner image, the toner image on the intermediate transfer belt is further aggregated and remains on the intermediate transfer belt without being secondary-transferred. When the toner image is a line image, the pressing force by the bias roll is dispersed near the line edge due to toner collapse or the like, but there is no dispersion of the pressing force at the center of the line. Therefore, when a polyimide resin having excellent mechanical properties and a small deformation of the belt is used as the intermediate transfer belt, for example,
Due to the concentration of the pressing force, the toner image on the inner side of the line is not secondarily transferred while adhering to the intermediate transfer belt, and an image quality defect of a hollow character in which the line image falls out occurs (FIG. 6)
B). In particular, when a recording medium having high smoothness such as an OHP sheet is used, the occurrence of hollow characters is remarkable.

As a countermeasure for suppressing the occurrence of the image quality defect, there is a method of deforming the surface of the intermediate transfer member by the pressing force of the bias roll, but this reduces the concentration of the load on the toner image (image portion). It can be said that this is an effective means for causing the problem. For example, to improve overall transfer efficiency,
In order to obtain a high quality image without partial image loss, an intermediate transfer type image forming apparatus in which the surface hardness of the intermediate transfer drum is set to 40 ° or less according to JIS A is disclosed in
No. 2,262. However, the hardness used in the hardness test described in the above publication (JIS A)
When a conductive rubber material having a temperature of 40 ° or less is used as the single-layer belt, there is a problem in that elongation occurs with time between the stretching rolls, and color misregistration, banding, and poor control due to elongation occur. Further, when a rubber material having the above hardness is used as the surface coating layer, there is a disadvantage that the coating layer is too hard and the image quality defect of the hollow character is not improved. Therefore, an object of the present invention is to solve the above-mentioned problem, and it is possible to suppress the occurrence of an image quality defect of a hollow character due to concentration of the pressing force of a bias roll, and an image of an intermediate transfer belt type capable of suppressing the occurrence of image quality defects. An object of the present invention is to provide a forming apparatus.

[0005]

Means for Solving the Problems The inventors of the present invention have been conducting intensive studies in order to solve the problem of hollow characters occurring in an image forming apparatus of an intermediate transfer system. The surface of the belt carrying the toner image is deformed in a concave shape following the pressing force of the bias roll by setting the dynamic hardness adopted as a hardness indicating method to a predetermined value or less by forming the material from a flexible material. As a result, the inventor has found that the above-mentioned object is achieved, and has accomplished the present invention. That is, the present invention relates to an image carrier on which an electrostatic latent image corresponding to image information is formed, a developing device for visualizing the electrostatic latent image formed on the image carrier with a toner as a toner image, and an image carrier. An intermediate transfer belt on which the toner image carried on the intermediate transfer belt is primarily transferred; and a bias roll for secondary transferring the unfixed toner image on the intermediate transfer belt to a recording medium. The intermediate transfer belt has at least a layer structure. The present invention relates to an image forming apparatus comprising a plurality of layers of a belt material having a substrate and a surface layer, wherein the surface layer has a dynamic hardness of 10 or less.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The present invention is not particularly limited as long as it is an intermediate transfer belt type image forming apparatus. For example, a normal mono-color image forming apparatus in which only a single-color toner is stored in a developing device, or a color image forming in which a toner image carried on an image carrier such as a photosensitive drum is repeatedly primary-transferred sequentially to an intermediate transfer belt The present invention is applied to a tandem-type color image forming apparatus in which a plurality of image carriers having developing devices for respective colors are arranged in series on an intermediate transfer belt. As an example, FIG. 1 shows an outline of a color image forming apparatus that repeats primary transfer. In FIG. 1, a charger 2, a developing device 3, a primary transfer device 4, a cleaning device 5, and the like are arranged around an image carrier 1 composed of a photosensitive drum along a rotation direction thereof.
Image writing means 6 for writing an electrostatic latent image on the surface of the image carrier 1 between the charger 2 and the developing device 3 is disposed in the image forming apparatus. An intermediate transfer belt 7 that travels between the image carrier 1 and the primary transfer device 4 in the direction of the arrow while abutting on the surface of the image carrier 1 is stretched around tension rolls 8a, 8b, 8c and a backup roll 9. ing. At positions facing the backup roll 9 and the tension roll 8a, a bias roll 10 and a belt cleaner 11 are disposed via the intermediate transfer belt 7, respectively. The portion of the image carrier 1 that faces the primary transfer device 4 via the intermediate transfer belt 7 serves as a primary transfer portion, and a primary transfer voltage is applied to the primary transfer device 4. In a secondary transfer section where the bias roll 10 presses against the backup roll 9, an electrode member 12 for applying a secondary transfer voltage to the bias roll 10 is pressed against the surface of the backup roll 9.

In the color image forming apparatus shown in FIG.
After the surface of the image carrier 1 rotating in the direction of the arrow is uniformly charged by the charger 2, the first color electrostatic latent image is formed by the image writing means 6 such as an image-processed laser beam. . The electrostatic latent image is visualized by the developing device 3 containing the toner corresponding to the color, and a toner image is formed. When the toner image passes through the primary transfer section, it is primary-transferred electrostatically onto the intermediate transfer belt 7 by the primary transfer device 4. Similarly, the second color, the third color, and the fourth color toner images are primarily transferred onto the intermediate transfer belt 7 carrying the first color toner image so as to be sequentially superimposed, and finally full color. A multiple toner image is formed. The developing device 3
Includes a plurality of developing units 3 ₁ to 3 ₄ containing the toner corresponding to the electrostatic latent image of each color. That is, black (K), yellow (Y), magenta (M), cyan
(C) Each color toner is stored.

The multi-toner image is electrostatically and collectively transferred to a recording medium P such as a sheet supplied at a predetermined timing from the sheet feeding section 13 when passing through the secondary transfer section. The recording medium P to which the toner image has been transferred is conveyed to the fixing device 14 and subjected to a fixing process, and then discharged outside the apparatus. After the primary transfer, the residual toner and the charge are removed from the image carrier 1 by the cleaning device 5 and the like, and the residual toner is removed from the intermediate transfer belt 7 after the secondary transfer by the belt cleaner 11 to prepare for the next image forming process. In the case of forming a multicolor image other than a full color image, two or three developing units store toner corresponding to the multicolor image. Further, an electrostatic latent image is formed on the image carrier 1 by the image writing means 6 which has been subjected to image processing so as to form a monochromatic electrostatic latent image, and only the toner corresponding to the color is developed by the developing device 3. When the image forming apparatus is accommodated in the image forming apparatus, the image forming apparatus shown in FIG. 1 can be applied to a monocolor image forming apparatus. Further, the photosensitive drum (1) can be replaced with a known belt photosensitive member.

As the primary transfer device 4, a corona transfer device such as a corotron, a transfer roll, or the like is used. A voltage of about 1 to 4 kV, opposite to the polarity of the toner, is applied to the primary transfer unit 4, and the primary transfer unit 4 carries the voltage on the image carrier 1 by the action of an electric field generated between the image carrier 1 and the primary transfer unit 4. The transferred toner image is primarily transferred to the intermediate transfer belt 7. The backup roll 9 forms a counter electrode of the bias roll 10. The layer structure of the backup roll 9 may be a single layer or a multilayer. For example, in the case of a single-layer structure,
It is composed of a roll in which an appropriate amount of conductive fine powder such as carbon black is mixed with silicone rubber, urethane rubber, EPDM (ethylene propylene diene rubber) or the like. In the case of a two-layer structure, a roll in the case of a single layer whose volume resistivity is appropriately adjusted is used as a lower layer, and the outer peripheral surface is formed of a roll having a conductive surface layer made of, for example, EPDM or fluororesin. As the fluororesin, tetrafluoroethylene (T
FE) -hexafluoropropylene copolymer (FE
P), TFE-perfluoroalkyl vinyl ether copolymer (PFA) and the like. The backup roll 9 preferably has a hardness of 65 to 75 ° in Asker C.

A bias roll 10 for forming a transfer electrode
Is separated from the transfer belt 7 while the toner image carried on the image carrier 1 is primarily transferred onto the intermediate transfer belt 7, and the toner image carried on the transfer belt 7 is At the time of transfer, the transfer belt 7 is pressed against the transfer belt 7 supported by the backup roll 9 via the recording medium P. The pressing force is not particularly limited, but is usually in the range of 3 to 6 kgf (total load). The layer structure of the bias roll 10 may be either a single layer or a multilayer. For example, in the case of a single-layer structure,
It is composed of a rubber roll in which an appropriate amount of a conductive agent such as carbon black is blended with silicone rubber, urethane rubber, EPDM, fluorine rubber or the like. In the case of a two-layer structure, a single-layer roll whose volume resistivity is appropriately adjusted is used as a lower layer, and the outer peripheral surface of the roll is coated with a conductive surface layer of, for example, a fluororesin. FEP, PF as fluororesin
A and the like. Further, a three-layer structure in which an intermediate layer made of an appropriate rubber material is interposed between the lower layer and the surface layer may be employed. The bias roll 10 preferably has a hardness of 10 to 30 ° in Asker C. The electrode member 12 is not particularly limited as long as it is a member having good electrical conductivity. For example, a metal roll made of aluminum, stainless steel (SUS), copper, or the like, a conductive rubber roll, a conductive brush, a metal plate, A conductive resin plate or the like is used. A transfer voltage of −1 to −5 kV is applied to the bias roll 10 from the electrode member 12 through the backup roll 9. The polarity of the voltage applied to the electrode member 12 may be reversed (+) according to the charging polarity of the toner. In the above-described secondary transfer section, the electrode member 12 is not necessarily a necessary member. For example, the transfer voltage may be applied to the conductive shaft of the backup roll 9 or the bias roll 10.

In the present invention, the intermediate transfer belt 7 is composed of a base material and a plurality of belt materials having at least a surface layer having a hardness (dynamic hardness) of a predetermined value or less. As examples of the plurality of layers, as shown in FIG. 2, in addition to the two-layer structure of the base material 7a and the surface layer 7b, a three-layer structure in which an intermediate layer 7c is interposed between the base material 7a and the surface layer 7b Can be mentioned. In the case of a two-layer structure, it is formed of a base material 7a containing a resin material and a conductive agent as components, and a surface layer 7b containing a flexible material and a conductive agent. Also, 3
In the case of a layer structure, the intermediate layer 7c is made of, for example, an elastic material in which a conductive agent is dispersed or non-dispersed. Examples of the resin material constituting the base material include resins having excellent mechanical properties such as polyimide, polyether sulfone, and polyether ketone (including polyether ether ketone). Among them, a polyimide resin is preferably used from the viewpoint of availability. These resins have a feature that the deformation of the belt during driving is smaller than that of a thermoplastic resin used conventionally.

Polyimide is generally a polymer synthesized by condensation polymerization using tetracarboxylic dianhydride and diamine or diisocyanate as monomer components. Examples of the tetracarboxylic acid component of the dianhydride include pyromellitic acid, naphthalene-1,4,5,8-tetracarboxylic acid, naphthalene-2,3,6,7-tetracarboxylic acid,
2,3,5,6-biphenyltetracarboxylic acid, 2,
2 ', 3,3'-biphenyltetracarboxylic acid, 3,
3 ', 4,4'-biphenyltetracarboxylic acid, 3,
3 ', 4,4'-diphenylethertetracarboxylic acid, 3,3', 4,4'-benzophenonetetracarboxylic acid, 3,3 ', 4,4'-diphenylsulfonetetracarboxylic acid, 3,3', 4 4,4'-azobenzenetetracarboxylic acid, bis (2,3-dicarboxyphenyl) methane, bis (3,4-dicarboxyphenyl) methane,
β, β-bis (3,4-dicarboxyphenyl) propane, β, β-bis (3,4-dicarboxyphenyl) hexafluoropropane and the like.

As the diamine component, m-phenylenediamine, p-phenylenediamine, 2,4-diaminotoluene, 2,6-diaminotoluene, 2,4-diaminochlorobenzene, m-xylylenediamine, p-xylylenediamine , 1,4-diaminonaphthalene, 1,5-
Diaminonaphthalene, 2,6-diaminonaphthalene,
2,4'-diaminobiphenyl, benzidine, 3,3'-
Dimethylbenzidine, 3,3'-dimethoxybenzidine, 3,4'-diaminodiphenyl ether, 4,4'-
Diaminodiphenyl ether (oxy-p, p'-dianiline; ODA), 4,4'-diaminodiphenyl sulfide, 3,3'-diaminobenzophenone, 4,4'-
Diaminodiphenyl sulfone, 4,4'-diaminoazobenzene, 4,4'-diaminodiphenylmethane, β,
β-bis (4-aminophenyl) propane and the like. Examples of the diisocyanate component include compounds in which an amino group in the above diamine component is substituted with an isocyanate group. Commercial products of these polyimides include, for example, pyromellitic acid-based polyimides containing ODA as a diamine component (Kapton HA: manufactured by DuPont),
3,3 ', 4,4'-biphenyltetracarboxylic acid-based polyimide (UPILEX S: manufactured by Ube Industries, Ltd.)
3,3 ', 4,4'-benzophenonetetracarboxylic acid-based thermoplastic polyimide containing 3'-diaminobenzophenone as a diamine component (LARC-TPI: Mitsui Toatsu Chemicals, Ltd.)
Co., Ltd.).

Examples of the conductive agent to be dispersed in the base material include conductive carbon-based substances such as carbon black and graphite; metals and alloys such as aluminum and copper alloys;
Zinc oxide, antimony oxide, indium oxide, potassium titanate, antimony oxide-tin oxide composite oxide (AT
O), conductive metal oxides such as indium oxide-tin oxide composite oxide (ITO), and electrolytes such as lithium perchlorate, quaternary ammonium perchlorate, quaternary ammonium chloride, and sodium trifluoromethanesulfonate. One or two or more kinds of fine powders are used. The metal oxide may be coated with insulating fine particles such as barium sulfate, calcium carbonate, and magnesium silicate. Among these conductive agents, carbon black is preferred in view of price and environmental stability. Further, from the viewpoint of dispersibility, a tin oxide composite oxide (product name: UF) having an average particle diameter of 0.1 μm, a zinc oxide (Pastran Type-II) having an average particle diameter of 0.1 μm, manufactured by Mitsui Kinzoku Co., Ltd., Barium sulfate having an average particle diameter of 0.4 μm (Pastran Type-IV) coated with a tin-based oxide, 0.2 μm ATO, 0.2 μm I
Metal oxides such as TO having an average particle diameter of 1 μm or less are also preferably used. The conductive metal oxide is preferably surface-treated with one or more of various silane coupling agents. Since the surface-treated metal oxide has improved compatibility with the resin constituting the base material, its dispersion becomes uniform,
Variation in the resistance value of the substrate is suppressed.

By the way, it is known that the elongation and contraction (displacement) of the belt due to the load at the time of driving the belt are inversely proportional to the Young's modulus of the belt material. That is, the relationship between the Young's modulus of the belt material and the amount of displacement of the belt due to the load when the belt is driven can be expressed by the following equation (1). Δl = α · P · l / (t · w · E) (1) Δl: belt displacement (μm) α: coefficient P: load (N) l: belt length between two tension rolls ( mm) t: Belt thickness (mm) w: Belt width (mm) E: Young's modulus of belt material (N / mm ² ) To reduce the elongation / shrinkage of the belt during driving, The Young's modulus is preferably 30,000 kg / cm ² or more. In this case, since the thermoplastic resin materials conventionally used, such as PC and PVDF, have a Young's modulus of 24000 kg / cm ² or less when carbon black is dispersed, the disturbance (load fluctuation) during belt driving is the same. At some point, the elongation and shrinkage of the belt is reduced by 20% or more compared to the conventional case. Therefore, a high quality transfer image can be stably obtained by forming the surface layer above the base material and, in some cases, the intermediate layer with a material having flexibility or elasticity.
In order to reduce the amount of displacement of the belt due to disturbance at the time of driving the belt and to obtain a high-quality transferred image, the thickness of the base material must be 50.
It is preferably at least μm. On the other hand, if the base material is too thick, the deformation of the belt surface becomes large, and when forming a color image, the position of the multiple toner image is shifted and color misregistration occurs. ~ 150μ
m, particularly preferably in the range of 70 to 100 μm.

As described above, the surface layer is made of a flexible material in which a conductive agent is dispersed. As the conductive agent, those described above are used. For the same reason, carbon black is preferable. Examples of carbon black include furnace black, acetylene black,
Ketjen black, channel black and the like. As the flexible material, a flexible resin material, a fluorine resin modified with various rubber materials, or the like is used. As the former flexible resin material, an aliphatic polyester resin in which polymer segments are linearly bonded, a polyurethane resin having a soft segment in a molecule, and the like are preferable. For example, commercially available polyester resins include Byron 30SS, Byron 200, Byron 300, and Byron GM800 manufactured by Toyobo Co., Ltd.

Further, in the latter fluororesin modified with a rubber material, since the fluororesin has a small surface energy, the fluororesin has a characteristic that the toner hardly adheres to the surface of the intermediate transfer belt. For this reason, not only the secondary transfer from the belt material to the recording medium is facilitated, but also a hollow character due to toner adhesion is less likely to occur. Further, since the rubber-modified fluororesin has elasticity, there is no possibility that the surface layer will be cracked due to repeated deformation at the belt roller curvature portion. TFE resin, PVDF,
Examples include FEP, ETFE, and PFA. The rubber material that modifies the fluororesin is not particularly limited, but urethane rubber or fluororubber is preferably used. As a commercially available rubber-modified fluororesin, Daikin Latex NF- manufactured by Daikin Industries, Ltd., in which carbon black is dispersed in a water emulsion paint (Diel Latex GLS-213) containing fluororubber and FEP.
915, the same DIAL latex DPS-231RD in which the fluororesin is PFA, urethane rubber and TFE
Water emulsion paint containing resin (Emuralon 34
5, EYLALON 345 ESD, JYL manufactured by Acheson Japan Ltd. in which carbon black is dispersed in JYL-601).
-601 ESD and the like. Further, for example, a conductive urethane paint in which fine powder of TFE resin and carbon black are dispersed can be used. As the TFE resin, KTL-5 manufactured by Kitamura Corporation having a particle size of 0.3 to 0.7 μm is used.
00F.

In order to prevent the occurrence of hollow characters, the surface layer must serve as a so-called toner image cushion that is deformed following the pressing force of the bias roll, and the thickness of the surface layer is 10 μm or more. Is preferred. In a color image forming apparatus, the average particle diameter is preferably three times or more. Here, the toner average particle diameter means its volume average particle diameter, usually 3 to 13 μm.
m toner particles are used. As an example,
When a toner having a volume average particle diameter of 6.5 μm is used, the thickness of the surface layer is preferably about 20 μm or more.
Also, if the surface layer becomes too thick, the tension roll
Since the difference in the amount of deformation between the belt front surface and the belt back surface at (8a to 8c; 9) becomes large, the thickness of the surface layer is generally 80%.
It is set to less than μm. Further, when the thickness is more than 80 μm, liquid dripping occurs at the time of forming a coating film by a coating method generally adopted as a thin film coating forming method, and it is possible to stably form a smooth and uniform coating film. It becomes difficult. A more preferable thickness range is 30 to 65 μm.
m.

The surface layer is desirably formed by the above-described coating method using the former resin material, a conductive agent and, if necessary, a conductive paint containing a crosslinking agent, an inorganic filler and the like. As the crosslinking agent, for example, when the resin component is a polyester resin, a melamine resin or the like is preferable, and when the resin component is a polyurethane resin, a polyisocyanate compound is suitably used. As the inorganic filler, silica, whisker-like potassium titanate and the like are preferably used. Hexane, benzene, toluene, and diluents for conductive paint
Examples include hydrocarbons such as xylene, ketones such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone, and esters such as ethyl acetate, butyl acetate, and ethyl butyrate. These diluents may be used alone or in combination of two or more. As the coating method, a brush coating, a dipping method, a spray method, a roll coater method, an electrostatic coating method, or the like can be employed. The resin component is cured by heating the coating film formed on the base material at 140 to 180 ° C. for 10 to 60 minutes. When a rubber-modified fluororesin is used, the conductive coating is applied onto a substrate by the above-mentioned coating method and then heated to form a surface layer. The heating temperature range is, for example, N
17 in case of conductive fluoro rubber modified paint such as F-940
0 to 300 ° C, and 80 to 150 ° C when a conductive urethane-modified paint is used.

When the intermediate transfer belt is a belt material having a three-layer structure, as described above, the intermediate layer is made of, for example, the conductive material dispersed or non-dispersed elastic material. The elastic material is not particularly limited, and any rubber material can be used. Specifically, isoprene rubber, chloroprene rubber, butyl rubber, norbornene rubber, fluorine rubber, silicone rubber, urethane rubber, acrylic rubber, SBR (styrene-butadiene rubber), NB
One or more of R (acrylonitrile-butadiene rubber) and EPDM are used. As the conductive agent,
As described above, the conductive agent is not necessarily required to be dispersed when a highly polar rubber material such as urethane rubber or NBR is used. Further, the thickness of the intermediate layer is preferably in the range of 30 to 100 μm,
In that case, the thickness of the surface layer is preferably in the range of 10 to 40 μm.

In the present invention, the hardness of the surface layer of the intermediate transfer belt is set to 10 or less in dynamic hardness.
Dynamic hardness is a measure of the average length of the diagonal line of the indentation formed on the sample when the load on the indenter is removed, such as Vickers hardness, which is widely used as a method for measuring the hardness of metallic materials. Rather, it depends on how much the indenter penetrated the sample. In other words, the dynamic hardness is the hardness obtained from the load and the indentation depth of the process of pushing the tip of the indenter into the sample, and is a material that includes not only plastic deformation but also elastic deformation of the sample. Represents the strength characteristics of Here, the load of the indenter applied to the sample is P (mN), and the penetration amount (indentation depth) of the indenter into the sample is d.
(Μm), the dynamic hardness DH is given by the following equation:
Defined in (2). DH≡α · P / d ² (2) α in the above equation (2) is a constant depending on the shape of the indenter, and is 3.8584 in the case of a regular triangular pyramid with a facing angle of 115 °.

Specifically, the dynamic hardness of the surface layer was determined according to the following measurement method. The polyimide sheet on which the surface layer was formed was cut into a 5 mm square, and this sample piece was fixed to a glass plate with an instant adhesive. Next, an ultra-micro hardness tester (DU)
H-201S: manufactured by Shimadzu Corporation) to measure the penetration amount (d) of the indenter pushed into the sample piece,
Dynamic hardness was calculated. The measurement conditions are as follows. Measurement environment: 23 ° C., 55% RH Working indenter: regular triangular pyramid made of diamond with 115 ° facing angle (tip) Test mode: 3 (soft material test) Test load: 0.70 gf Loading speed: 0.0145 gf / sec Holding time : 5 sec Here, the soft material test in test mode 3 will be briefly described. In the dynamic hardness measuring device, a load is applied to the indenter by an electromagnetic force, and some of the generated force (60 to
80 μN / μm) is spent on displacement of the indenter. Therefore, since the load (electromagnetic force) actually applied to the sample includes the force required for displacement of the indenter, it is necessary to correct the load of the indenter. This correction is performed in the test mode 3, and as shown in FIG. 3, an actual load (P) obtained by subtracting a force for applying a displacement to the indenter from a load on the indenter is obtained, and according to the above equation (2). Dynamic hardness is calculated.

It has been found that there is an extremely accurate correlation between the dynamic hardness obtained as described above and the occurrence level of hollow characters. That is, as shown in Table 1 below, when the dynamic hardness of the surface layer of the intermediate transfer belt is 10, hollow characters hardly occur. Further, when the dynamic hardness is 5, the generation level of the hollow character is further improved. In particular, when the dynamic hardness is 3 or less, the hollow character is not generated at all. This is because the dynamic hardness of the surface layer (7b) is 1
4A, the surface of the intermediate transfer belt (7) is sufficiently deformed by the pressing force of the bias roll (10) in the secondary transfer portion as shown in FIG. 4A, and the surface is curved concavely. This is because the pressing force that tends to concentrate on the toner image on the belt (7) is dispersed. As a result, the toner is transferred onto the recording medium (P) without agglomeration, and even if the toner image is a line image, a hollow character in which the image falls through is suppressed, and the occurrence of image quality defects is prevented. (FIG. 4B). As described above, in the image forming apparatus of the present invention, since the toner does not remain on the intermediate transfer belt during the secondary transfer, the transfer efficiency is greatly improved. Therefore, the amount of toner consumption can be kept low, and the running cost can be reduced. In addition to this, not only the life of the belt cleaner is significantly extended, but also the residual toner on the belt does not adhere to the image carrier, so that the life of the cleaning device and the like can be significantly extended. In addition, since the toner which has conventionally remained on the belt does not scatter inside the image forming apparatus, the labor required for maintenance and inspection due to contamination inside the apparatus is reduced, and the reliability of the apparatus is improved.

[0024]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to the following Examples. (Image Forming Apparatus) FIG. 5 is an overall view of a digital color copying machine provided with an intermediate transfer belt as an image forming apparatus of the present invention. In FIG. 5, light emitted from a document illumination lamp 22 moving along the lower surface of a document (not shown) placed on a platen 21 and reflected by the document is reflected by a moving mirror unit 23, a lens 24, and a fixed mirror. The image is converged on the CCD of the image reading unit via the line 25. The CCD converts the original image into an electric signal for each color by using a large number of photoelectric conversion elements. The electric signal is input to the image processing circuit 26, and the image processing circuit 26 has an image memory that converts a document image reading signal input for each color into a digital signal and stores it. The optical writing control device 27 reads out the image data of the image processing circuit 26 at a predetermined timing and outputs it to the light beam writing device 28. The light beam writing device 28 writes an electrostatic latent image corresponding to each color on the image carrier 1 composed of a photosensitive drum rotating in the arrow direction A. These numbers 22 to
28 constitutes the image writing means 6.

Around the image carrier 1, a charger 2 for uniformly charging the surface, a developing unit 3 for developing the electrostatic latent image written on the image carrier 1 into a toner image of each color, A primary transfer roll 4 for transferring the toner image to the intermediate transfer belt 7, a cleaning unit 5 having a cleaning blade and a static eliminator are arranged. The developing unit 3 has a developing device containing toner of each color of K, Y, M, and C, and develops and visualizes the electrostatic latent image with the toner of each color. The intermediate transfer belt 7 is stretched around tension rolls 8 a, 8 b, 8 c and a backup roll 9, and runs in a tangential direction while contacting the surface of the image carrier 1. A bias roll 10 and a belt cleaner 11 are disposed on the front side of the transfer belt 7 that carries the unfixed toner image, facing the backup roll 9 and the tension roll 8a. An electrode roll 12 to which a secondary transfer voltage having the same polarity as the toner is applied is pressed against the backup roll 9. In addition, between the bias roll 10 and the belt cleaner 11, a peeling claw 29 for peeling the paper P carrying the secondary-transferred toner image from the transfer belt 7 is arranged. A cleaning blade 3 made of polyurethane is provided on the surface of the bias roll 10.
0 is always in contact, and foreign matters such as toner particles and paper dust adhered in the transfer step and the like are removed.

An extractable paper feed tray 13 is provided below the main body of the image forming apparatus U, and a pickup roller 31 is arranged above the paper feed tray 13. This pickup roller 3
A pair of feed rolls 32 for preventing double feeding of the paper P, a paper transporting roll 33, a guide member 34 for guiding the paper P, and a registration roll 35 are sequentially arranged downstream of 1. On the downstream side of the secondary transfer section, a transport belt 3 for transporting a sheet P carrying the secondary-transferred toner image
6. A fixing device 14 for fixing an unfixed toner image on the paper P, a pair of discharge rolls 37 for discharging the paper P on which the fixed image has been formed to the outside of the apparatus, and a paper discharge on which the discharged paper P is placed. The trays 38 are sequentially arranged.

(Operation of the Image Forming Apparatus) The surface of the image carrier 1 rotating in the direction of arrow A is charged to a predetermined potential by the charger 2, and an electrostatic latent image is written by the light beam writing device 28. The electrostatic latent image on the image carrier 1 is developed into a toner image by the developing unit 3. In the formation of the toner image, the first color toner image is formed first, and thereafter, every time the image carrier 1 makes one rotation, the second color to the fourth color toner images are formed. In this embodiment, K, Y, M, and C toner images are sequentially formed. After the toner image is transferred to the intermediate transfer belt 7 on the surface of the image carrier 1, the residual toner and the charge are removed by the cleaning unit 5. Here, the optical writing control device 27 first reads out the digital signal image-processed to the first K color and outputs it to the light beam writing device 28. This writing device 2
Reference numeral 8 writes an electrostatic latent image corresponding to the K color on the surface of the image carrier 1. The electrostatic latent image corresponding to the color K is developed by the developing unit K in the developing unit 3 into a visualized toner image of the color K, and moves to the primary transfer unit. In the primary transfer section, an electric field having a polarity opposite to the charging polarity is applied to the toner image from the primary transfer roll 4 disposed on the back surface side of the intermediate transfer belt 7 so that the K color toner image reaches the primary transfer section. Is electrostatically adsorbed to the transfer belt 7, and primary transfer is performed by traveling the transfer belt 7 in the arrow direction B.

The intermediate transfer belt 7 runs in the same cycle as the image carrier 1 while adsorbing and carrying the K toner image. When the transfer of the K toner image of the first color is completed, writing of the electrostatic latent image corresponding to the light image color-separated by the blue filter is started by the output from the optical writing control device 27. Then, when the transfer start position of the transfer belt 7 carrying the K toner image reaches the primary transfer portion, the primary transfer roll 4 causes the Y transfer of the second color.
The transfer of the toner image is performed. Subsequently, the electrostatic latent images corresponding to the light images separated by the green and red filters are visualized by the developing devices M and C, and the transfer of the M toner image and the C toner image is performed in the same manner as the transfer of the Y toner image. Done in
In this way, a multiple toner image superimposed on each color is formed on the intermediate transfer belt 7. Until the toner images of each color are primarily transferred onto the transfer belt 7, the transfer belt 7
The bias roll 10, the peeling claw 29, and the belt cleaner 11, which are disposed on the surface side of the transfer belt 7, are held at a retracted position separated from the transfer belt 7.

On the other hand, the paper P stored in the paper feed tray 13
Are taken out one by one by a pickup roller 31 at a predetermined timing, fed by a pair of feed rolls 32 and a paper transport roll 33, and temporarily stopped by a pair of registration rolls 35. The sheet P is then transferred from the registration roll 35 to the secondary transfer section in synchronization with the transfer of the multiple toner images of each color (K, Y, M, C) transferred onto the intermediate transfer belt 7 to the secondary transfer section. It is transported to the transfer section. In the secondary transfer section, the bias roll 10 is in pressure contact with the backup roll 9 via the intermediate transfer belt 7. Then, the conveyed sheet P passes through the secondary transfer section by the press-contact conveyance between the rolls 9 and 10 and the traveling of the transfer belt 7. At this time, by applying a transfer voltage having the same polarity as the charging polarity of the toner image to the electrode roll 12, the multiple toner image adsorbed and carried on the transfer belt 7 is secondarily transferred to the sheet P from the surface of the transfer belt 7. .

Although the transfer of a full-color image has been described above, when a mono-color image is formed, for example, when a K-color toner image primarily transferred on the intermediate transfer belt 7 moves to a secondary transfer portion, The toner image is immediately transferred to the paper P. When a two-color or three-color image is formed, a desired hue may be selected, and when the multicolor toner image moves to the secondary transfer unit, the toner image may be transferred to the paper P. As described above, the paper P on which the toner image has been transferred to the desired hue is peeled by the operation of the peeling claw 29, placed on the transport belt 36, and transported to the fixing device 14. After fixing the unfixed toner image to the permanent image in the fixing device 14, the sheet P is transferred to a pair of discharge rolls 3.
7, the paper is discharged to the paper discharge tray 38. When the secondary transfer is completed, the intermediate transfer belt 7 is cleaned by a belt cleaner 11 provided on the downstream side of the secondary transfer unit,
Prepare for the next transfer.

(Production of Intermediate Transfer Belt) In the following composition, "parts" indicating the ratio means "parts by weight". Example 1 A polyimide varnish (U varnish for heat-resistant film-S: manufactured by Ube Industries, Ltd.) using Iupirex S as a resin component and N-methylpyrrolidone as a solvent, and 18 parts of carbon black with respect to 100 parts of the resin component Was added and mixed with a mixer. The obtained membrane-forming stock solution was 168 mm in diameter and 5 in height.
Inject into a 00 mm stainless steel cylindrical mold,
It was centrifuged while drying with hot air at 20 ° C. for 120 minutes. Then, the cylindrical film removed from the semi-cured state is covered with an iron core, and the temperature is increased from 120 ° C. to 350 ° C. over 30 minutes to evaporate the solvent. The main curing of dehydrating and condensing the acid was performed. The obtained carbon black-dispersed polyimide film having a thickness of 80 μm was cut into a width of 320 mm to form a seamless belt substrate (7a). On the other hand, as a component for forming a surface layer,
Linear polyester resin (byron 30SS) 8
0 parts, melamine resin (Super Beckamine G-821)
60: manufactured by Toyobo Co., Ltd.) and 5 parts of carbon black (FW200: manufactured by Tegusa) were mixed by a sand mill. 100 parts of the obtained surface layer forming component was xylene 10
The viscosity was adjusted by diluting to 0 part to prepare a conductive paint. This conductive paint is coated on the surface of the above-mentioned belt base material (7a) by a bell-type electrostatic coating machine (manufactured by Ransberg Industries), and then heated at 160 ° C. for 30 minutes to form a 50 μm thick surface layer (7b). Formed. When the hardness of the surface layer (7b) was determined according to the measurement method described above, the dynamic hardness was 1.6.

Example 2 An FEP-containing fluororubber-based coating material in which carbon black was dispersed (the above-mentioned Daiel Latex NF-915) was spray-coated on the surface of the belt base material formed in the same manner as in Example 1, and then 250 Heat at 20 ° C for 20 minutes,
A surface layer having a thickness of 0 μm was formed. This surface layer had a dynamic hardness of 1.3, and a 2 μm thick FEP resin layer was formed on the surface.

Example 3 As a conductive metal oxide, barium sulfate coated with a tin oxide-based conductive agent and having an average particle diameter of 0.4 μm (the above-mentioned pastelane) was used.
Type-IV) whose surface was treated with γ-aminopropyltriethoxysilane was used. 37 parts of this conductive metal oxide was added to 100 parts of the resin component of the polyimide varnish used in Example 1, and the mixture was sufficiently mixed with a mixer. The obtained film-forming stock solution was uniformly cast on a stainless steel sheet to a thickness of 300 μm,
And dried at 150 ° C for 30 minutes and 200 ° C for 3 minutes.
0 minutes, 250 ° C for 60 minutes, 350 ° C for 30 minutes, 420 ° C
For 30 minutes to obtain a 75 μm-thick polyimide sheet. Molded polyimide sheet with length 54
After cutting to 0 mm and width 320 mm, one end 1 of the sheet
A one-component elastic adhesive (Super X-8008; manufactured by Cemedine Co., Ltd.) was applied to 0 mm, and both ends were joined to form a seamless belt base material. Thereafter, a surface layer having a dynamic hardness of 1.6 was formed in the same manner as in Example 1.

Example 4 The procedure of Example 2 was repeated, except that the thickness was changed to 20 μm.
A surface layer was formed on the same belt substrate as in Example 1. Its dynamic hardness was 2.3. Example 5 A PFA-containing fluororubber-based coating material (Daily Latex DPS-231RD: manufactured by Daikin Industries, Ltd.) in which carbon black was dispersed was spray-coated on the same belt substrate surface as in Example 3. Thereafter, a surface layer having a thickness of 20 μm was formed in the same manner as in Example 2. Its dynamic hardness was 5. Example 6 Byron GM800 (Toyobo) as a polyester resin
A surface layer having a thickness of 20 μm was formed on the same belt base material as in Example 3 in the same manner as in Example 1 except that the above-mentioned method was used. Its dynamic hardness was 10.

Comparative Example 1 An FEVE-based paint (Duflon K) in which carbon black is dispersed
300: Nippon Paint Co., Ltd.) was applied to the same belt substrate surface as in Example 3 by the spray coating method.
Heating was performed at 0 ° C. for 20 minutes to form a surface layer having a thickness of 20 μm. Its dynamic hardness was 15. Comparative Example 2 The same seamless belt base material as in Example 1 was used as an intermediate transfer belt as it was in an image quality evaluation test described below. The dynamic hardness of the belt was 22. Comparative Example 3 PVDF-based coating material dispersed with carbon black (Duflon K
500: Nippon Paint Co., Ltd.) was coated on the same belt substrate surface as in Example 1, and then a 50 μm thick surface layer was formed in the same manner as in Comparative Example 1. Its dynamic hardness was 24.5.

(Mechanical Property Test of Intermediate Transfer Belt Material) The Young's modulus (tensile modulus) of the base material in the belt materials manufactured in Examples 1 and 3 was determined in accordance with JIS K7127.
Using a strip test piece of 25 × 250 mm, pulling speed 20 m
It was measured in m / min. As a result, the Young's modulus was 62
000 kg / cm ² . (Image Quality Evaluation Test) Test Image Forming Apparatus Each intermediate transfer belt of Examples 1 to 6 and Comparative Examples 1 to 3 was mounted on the above-described image forming apparatus shown in FIG. 5 and a copy test was performed. Intermediate transfer belt in this image forming apparatus
The specific configuration of the secondary transfer member other than (7) is as follows. As the backup roll (9), a 5.3 mm thick foamed EPDM rubber roll formed by coating a 1.2 mm thick EPDM rubber material on a SUS shaft having an outer diameter of 15 mm was used. EPDM rubber materials on the surface include NBR and EPD in which acetylene black (manufactured by Denki Kagaku) and thermal black (manufactured by Asahi Carbon Co., Ltd.) are dispersed.
Rubber material (NE4) in which M is blended in a weight ratio of 4: 6.
0: manufactured by Japan Synthetic Rubber Co., Ltd.). The rubber hardness of the backup roll (9) is 70 ° for Asker C. As a bias roll (10), a 20 μm-thick conductive fluororesin coat 6.5 on a SUS shaft having an outer diameter of 15 mm.
A conductive urethane foam rubber roll molded to a thickness of mm was used. The rubber hardness of Asker C is 20 °. The outer diameter of each of the rolls (9) and (10) is 28.0 mm, and the pressing force of the bias roll (10) on the intermediate transfer belt (7) is 5.0 kgf (total load). A SUS metal roll having an outer diameter of 14 mm is used as the electrode roll (12), and a voltage of -2 kV is applied to this roll.

Copy Test A toner having an average particle diameter of 6.5 μm and O as a recording medium
Using an HP sheet, the above image forming apparatus was operated at a process speed of 160 mm / sec. The obtained line image was enlarged with a loupe, and the state of occurrence of a hollow character (DH) was visually evaluated according to the following criteria. In this copy test, toners of Y and M colors (two colors)
And toners of three colors (Y, M, and C) were used, and these multiple toner images were transferred onto the OHP sheet in the secondary transfer portion. Table 1 shows the evaluation results. ◎: There is no occurrence of hollow characters ホ: There is almost no occurrence of hollow characters △: There are some occurrences of hollow characters ×: There is remarkable occurrence of hollow characters

[0038]

[Table 1]

As shown in Table 1, as the dynamic hardness of the surface of the intermediate transfer belt decreases, the level of occurrence of hollow characters decreases. That is, there is a correlation between the hardness of the belt surface and the obtained image quality. In Table 1,
In each of the examples in which the dynamic hardness of the surface layer was 10 or less, hollow characters hardly occurred, and the level was not a problem in practical use. It can be seen that the preferred dynamic hardness is 5 or less, particularly 3 or less. on the other hand,
Hollow characters were generated in Comparative Examples in which the dynamic hardness of the surface layer was higher than 10, and in particular, Comparative Example 3 in which the dynamic hardness of the surface layer was 24.5 showed remarkable hollow characters. When two color toners and three color toners are used, the former having a lower pile height has a lower level of occurrence of hollow characters, but by ensuring a sufficient thickness of the surface layer, Therefore, the present invention is suitably applied to a full-color image forming apparatus.

[0040]

According to the present invention, since the intermediate transfer belt of the present invention has a flexible surface layer having a dynamic hardness of 10 or less, the belt surface is deformed following the pressing force of the bias roll. Therefore, there is no occurrence of image quality defects due to the occurrence of hollow characters on the recording medium, and a high-quality transferred image can be stably obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an image forming apparatus of an intermediate transfer belt type having main components.

FIG. 2 is an explanatory diagram illustrating a cross-sectional structure of an intermediate transfer belt according to the present invention.

FIG. 3 is an explanatory diagram of a load corrected at the time of calculating a dynamic hardness.

FIG. 4 is an explanatory view showing a mechanism for suppressing generation of hollow characters in an intermediate transfer belt of the present invention and a toner image secondarily transferred onto a recording medium.

FIG. 5 is an overall view of an image forming apparatus shown as one embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a conventional mechanism of generating a hollow character and a state of the generation.

[Explanation of symbols]

U: image forming apparatus, P: recording medium, 1: image carrier, 3 ...
Developing device, 7: intermediate transfer belt, 7a: base material, 7b: surface layer, 10: bias roll.

Claims (6)

[Claims] An image carrier on which an electrostatic latent image corresponding to image information is formed; a developing device for visualizing the electrostatic latent image formed on the image carrier as a toner image with toner; An intermediate transfer belt on which the toner image carried on the intermediate transfer belt is primarily transferred; and a bias roll for secondary transferring the unfixed toner image on the intermediate transfer belt to a recording medium. The intermediate transfer belt has at least a layer structure. An image forming apparatus comprising a plurality of layers of a belt material having a material and a surface layer, wherein the hardness of the surface layer is 10 or less in dynamic hardness. 2. The image forming apparatus according to claim 1, wherein said surface layer comprises a polyester resin as a constituent. 3. The image forming apparatus according to claim 1, wherein said surface layer comprises a fluorine resin modified with a rubber material as a constituent. 4. The image forming apparatus according to claim 1, wherein said substrate is made of a polyimide resin in which carbon black is dispersed. 5. The image forming apparatus according to claim 1, wherein the base is made of a polyimide resin in which a conductive metal oxide is dispersed. 6. The base material has a Young's modulus of 30,000.
The image forming apparatus according to claim 4, wherein the weight is not less than kg / cm ² .