JP6836737B2 - Image forming device and image forming method - Google Patents

Description

The present invention relates to an image forming apparatus and an image forming method.

Conventionally, an image forming apparatus that transfers a toner image on an image carrier by each of a plurality of transfer nips to an endless belt member that sequentially contacts a plurality of image carriers and forms a transfer nip at each contact position. It has been known.

For example, the image forming apparatus described in Patent Document 1 includes an endless intermediate transfer belt that sequentially contacts a plurality of photoconductors that are image carriers and moves so as to form a primary transfer nip at each contact position. There is. It also includes a plurality of primary transfer bias rollers to which a transfer bias is applied with the intermediate transfer belts individually sandwiched between the plurality of photoconductors. Then, the toner image on the photoconductor is superposed on the surface of the intermediate transfer belt at each of the plurality of primary transfer nips to perform the primary transfer. After that, the superimposed toner image on the intermediate transfer belt is secondarily transferred to the recording sheet sandwiched between the secondary transfer nips due to the contact between the intermediate transfer belt and the secondary transfer bias roller.

In such a configuration, in order to realize a good primary transfer rate of the toner image and a long belt life, as an intermediate transfer belt, a flexible layer ensures high adhesion to the photoconductor while maintaining rigidity. It is desirable to use a multi-layer structure in which elongation is suppressed by a certain layer. However, it has been found by the experiments of the present inventors that the use of an intermediate transfer belt having a multi-layer structure tends to cause uneven image density.

In order to solve the above-mentioned problems, the present invention includes an endless belt member that sequentially contacts a plurality of image carriers and moves so as to form a transfer nip at each contact position, and the plurality of image carriers. A plurality of transfer bias members to which a transfer bias is applied with the belt members individually sandwiched between the and the belt member are provided, and a toner image on the image carrier is displayed on each of the plurality of transfer nips of the belt member. In the image forming apparatus for transferring to the surface, the belt member has a multilayer structure in which at least one layer is laminated on the front surface side of the belt substrate, and among the plurality of the transfer bias members, the transfer step. the electrical resistance of the transfer bias member disposed most downstream in the order is less than 10 7 ^[Omega], the resistance of all the other of the transfer bias member is 10 7 ^[Omega] or higher It is a feature.

According to the present invention, it is possible to suppress the occurrence of uneven image density while realizing good transfer efficiency of a toner image from an image carrier to a belt member and extending the life of the belt member. is there.

The schematic block diagram which shows the main part of the printer which concerns on embodiment. The schematic diagram which shows the transfer nip of an indirect transfer method and its surrounding structure. The schematic diagram which shows the transfer nip of a direct transfer method and its surrounding structure. The figure for demonstrating the belt manufacturing method by the rotary coating method. An enlarged schematic view showing an enlarged image portion of a high gradation (for example, a gradation in which the dot area gradation rate due to dither is 80%) in which image density unevenness is generated due to unevenness of the toner charge amount. Enlarged schematic diagram showing an enlarged image portion of a low gradation (for example, a gradation in which the dot area gradation rate due to dither is 10%) that causes uneven image density due to the enhancement of the afterimage on the photoconductor. .. The block diagram which shows the secondary transfer nip and the surrounding structure in the printer which concerns on the modification. The figure for demonstrating the measuring method of a roller resistance. The block diagram which shows a part of the electric circuit of the printer which concerns on 5th Embodiment.

Hereinafter, as an embodiment of the image forming apparatus to which the present invention is applied, a so-called tandem type intermediate transfer type printer (hereinafter, simply referred to as a printer) will be described.
First, the basic configuration of the printer according to the embodiment will be described. FIG. 1 is a schematic configuration diagram showing a main part of a printer according to an embodiment. This printer is equipped with four process units 6Y, 6M, 6C, and 6K for generating toner images of yellow, magenta, cyan, and black (hereinafter referred to as Y, M, C, and K).

The four process units 6Y, 6M, 6C, 6K have drum-shaped photoconductors 1Y, 1M, 1C, 1K which are image carriers. Around the photoconductors 1Y, 1M, 1C, 1K, charging devices 2Y, 2M, 2C, 2K, developing devices 5Y, 5C, 5M, 5K, drum cleaning devices 4Y, 4M, 4C, 4K, static elimination devices, etc. are arranged. It is installed. The process units 6Y, 6M, 6C, and 6K having such a configuration use Y, M, C, and K toners having different colors, but have the same configuration except for the above.

Above the process units 6Y, 6M, 6C, 6K, an optical writing unit for irradiating the surface of the photoconductors 1Y, 1M, 1C, 1K with laser light L to write an electrostatic latent image is arranged. Has been done.

Below the process units 6Y, 6M, 6C, and 6K, a transfer unit 7 including an endless intermediate transfer belt 8 as a belt member is arranged. In addition to the intermediate transfer belt 8, a plurality of tension rollers arranged inside the loop, a secondary transfer roller 18 arranged outside the loop, a tension roller 16, a belt cleaning device 100, a lubricant applying device 200, and the like. have.

Inside the loop of the intermediate transfer belt 8, four primary transfer rollers 9Y, 9M, 9C, 9K, a driven roller 10, a driving roller 11, a secondary transfer opposing roller 12, and three cleaning opposing rollers 13, 14 , 15 and a coating brush facing roller 17 are arranged. All of these rollers function as tension rollers for belt tensioning by hanging the intermediate transfer belt 8 around a part of its peripheral surface. It should be noted that the cleaning opposing rollers 13, 14 and 15 do not necessarily have to have a function of applying a constant tension as a necessary condition, and may be driven to rotate with the rotation of the intermediate transfer belt 8. The intermediate transfer belt 8 is endlessly moved in the counterclockwise direction in the drawing by the rotation of the drive roller 11 which is rotationally driven in the counterclockwise direction in the drawing by the driving means.

The primary transfer rollers 9Y, 9M, 9C, 9K as transfer bias members arranged inside the belt loop sandwich the intermediate transfer belt 8 between the photoconductors 1Y, M, C, and K. As a result, the primary transfer nips for Y, M, C, and K in which the front surface of the intermediate transfer belt 8 and the photoconductors 1Y, 1M, 1C, and 1K come into contact with each other are formed. A primary transfer bias having a polarity opposite to that of the toner is applied to the primary transfer rollers 9Y, 9M, 9C, and 9K by a power source.

Further, the secondary transfer opposing roller 12 arranged inside the belt loop sandwiches the intermediate transfer belt 8 with the secondary transfer roller 18 arranged outside the belt loop. As a result, a secondary transfer nip is formed in which the front surface of the intermediate transfer belt 8 and the secondary transfer roller 18 come into contact with each other. A secondary transfer bias having the same polarity as the normal charging polarity of the toner is applied to the secondary transfer opposing roller 12 by a power source. Further, the paper transfer belt is bridged by the secondary transfer roller 18, several support rollers, and the drive roller, and the intermediate transfer belt 8 and the paper transfer belt are placed between the secondary transfer roller 18 and the secondary transfer opposed roller 12. It may be configured to sandwich.

Further, the three cleaning opposing rollers 13, 14 and 15 arranged inside the belt loop are between the three cleaning brush rollers (details will be described later) of the belt cleaning device 100 arranged outside the belt loop. The intermediate transfer belt 8 is sandwiched between the two. As a result, a cleaning nip is formed in which the front surface of the intermediate transfer belt 8 and the three cleaning brush rollers come into contact with each other. The belt cleaning device 100 can be integrally replaced with the transfer unit 7. It is also possible to remove the belt cleaning device 100 from the transfer unit 7 removed from the printer body. Further, with the transfer unit 7 and the belt cleaning device 100 attached to the printer body, the cleaning subunit described later in the belt cleaning device 100 can be attached to and detached from the belt cleaning device 100. Details of the belt cleaning device 100 will be described in detail later.

This printer includes a paper feed cassette including a paper feed cassette for accommodating the recording paper P, a paper feed roller for feeding the recording paper P from the paper feed cassette to the paper feed path, and the like. Further, a resist roller pair that receives the recording paper sent from the paper feed unit and sends it toward the secondary transfer nip at a predetermined timing is provided on the right side in the drawing of the above-mentioned secondary transfer nip. Further, a fixing device that receives the recording paper P sent out from the secondary transfer nip and applies the toner image fixing process to the recording paper P is provided on the left side of the above-described secondary transfer nip in the drawing. Further, if necessary, a toner replenishing device for Y, M, C, K that replenishes Y, M, C, K toner to the developing devices 5Y, 5M, 5C, 5K is also provided.

When image information is sent from a personal computer or the like, the printer rotationally drives the drive roller 11 to move the intermediate transfer belt 8 endlessly. The tension rollers other than the drive roller 11 are driven to rotate by the belt. At the same time, the photoconductors 1Y, 1M, 1C, 1K of the process units 6Y, 6M, 6C, 6K are rotationally driven. Further, while the surfaces of the photoconductors 1Y, 1M, 1C, and 1K are uniformly charged by the charging devices 2Y, M, C, and K, an electrostatic latent image is formed by irradiating the charged surface with laser light L. To do. Then, by developing the electrostatic latent image formed on the surface of the photoconductors 1Y, 1M, 1C, 1K with the developing apparatus 5Y, 5M, 5C, 5K, Y, M on the photoconductors 1Y, 1M, 1C, 1K. , C, K toner image is obtained. The Y, M, C, K toner image is primarily transferred by being superimposed on the front surface of the intermediate transfer belt 8 by the above-mentioned primary transfer nip for Y, M, C, K. As a result, a four-color superimposed toner image is formed on the front surface of the intermediate transfer belt 8.

On the other hand, in the paper feed unit, the paper feed roller 27 feeds out the recording paper P one by one from the paper feed cassette and conveys it to the resist roller pair. Then, at a timing that can be synchronized with the four-color superposed toner image on the intermediate transfer belt 8, the register roller pair is driven to send the recording paper P to the secondary transfer nip to display the four-color superposed toner image on the belt. Batch secondary transfer to recording paper P. As a result, a full-color image is formed on the surface of the recording paper P. After the full-color image is formed, the recording paper P is conveyed from the secondary transfer nip to the fixing device to perform the fixing process of the toner image.

For the photoconductors 1Y, 1M, 1C, 1K after the Y, M, C, K toner image is first transferred to the intermediate transfer belt 8, the transfer residual toner is cleaned by the drum cleaning device 4Y, 4M, 4C, 4K. Give. Then, after static elimination with the static elimination lamp, the charging device 2Y, 2M, 2C, 2K uniformly charges the charge to prepare for the next image formation. Further, the intermediate transfer belt 8 after the primary transfer to the recording paper P is subjected to a cleaning process of the transfer residual toner by the belt cleaning device 100.

On the right side of the process unit 6K for K in the drawing, the optical sensor unit 90 is arranged so as to face the front surface of the intermediate transfer belt 8 with a predetermined gap. The optical sensor unit 90 reflects the light emitted from the light emitting element on the front surface of the intermediate transfer belt 8 or the toner image on the belt, and detects the amount of the reflected light by the light receiving element. The control unit can detect the toner image on the intermediate transfer belt 8 and detect the image density (toner adhesion amount per unit area) based on the output voltage values from these sensors.

The surface of the intermediate transfer belt 8 is coated with a lubricant by the lubricant coating device 200 in order to protect the belt surface. The lubricant coating device 200 applies a solid lubricant 202 such as zinc stearate and a lubricant powder obtained by abutting the solid lubricant and scraping it from the solid lubricant by rotation to the surface of the intermediate transfer belt 8. It includes a coating brush roller 201 as a member. Although this printer is provided with a lubricant coating device 200, it may not be necessary depending on the toner used, the material of the intermediate transfer belt 8, and the surface friction coefficient, and it is not always necessary to apply the lubricant.

In the image forming apparatus described in Patent Document 1, an intermediate transfer belt having a thickness of 100 [μm] made of PI (polyimide) is used. In such a configuration, by constructing the belt with polyimide having a relatively high rigidity, it is possible to suppress the elongation of the belt over time and realize a long life. However, since the adhesion between the intermediate transfer belt and the photoconductor is inferior, it is difficult to realize efficient primary transfer of the toner image. It is also difficult to satisfactorily transfer the toner image to a recording sheet having poor surface smoothness.

In recent years, in addition to plain paper that has been widely used as a recording paper, special paper having an uneven surface and special recording paper used for thermal transfer such as iron printing are increasingly used as a design. When such a special paper is used in the image forming apparatus described in Patent Document 1, when the toner image on the intermediate transfer belt on which the color toner is superposed is secondarily transferred to the paper as compared with the case of the conventional plain paper. Transfer defects are likely to occur.

The intermediate transfer belt in the image forming apparatus described in Patent Document 1 adjusts the electrical resistance of the belt by using a material in which carbon black is added to polyimide. Patent Document 1, the volume resistivity of the intermediate transfer ^belt, is described that a ^{10 8 ~10 12 [Ω · cm} ]. However, in order to apply the primary transfer bias to the intermediate transfer belt having such a relatively high electric resistance and to pass a desired primary transfer current through the primary transfer nip, it is necessary to apply a high voltage primary transfer bias. Then, when such a high voltage primary transfer bias is applied to the intermediate transfer belt whose electrical resistance is adjusted by adding carbon black, the substantial electrical resistance of the entire belt is irregularly lowered to reduce the primary transfer current. Stabilization becomes difficult. Therefore, it is considered necessary to make the volume resistivity lower than the value described in Patent Document 1.

There are an indirect transfer method and a direct transfer method as a method of transferring the toner image at the contact portion between the belt and the photoconductor. As shown in FIG. 2, the indirect transfer method is a method in which the transfer roller is brought into contact with the belt back surface region deviated from the back surface of the contact portion between the photoconductor 1 and the belt. In this method, the electric charge transmitted from the transfer roller to the belt needs to be transmitted in the circumferential direction of the belt and flows to the primary transfer nip, so that the electrical resistance of the belt needs to be relatively low. As the transfer roller, one made of a metal roller having almost no electric resistance can be used.

As shown in FIG. 3, the direct transfer method is a method in which a belt is sandwiched between the transfer roller and the photoconductor 1 to form a transfer nip. In this method, the image forming apparatus described in Patent Document 1 employs this direct transfer method. A single-layer belt is used as the intermediate transfer belt. In such a configuration, if a metal roller is used as the primary transfer roller, a violent electric discharge is generated between the photoconductor 1 and the primary transfer roller, so that the primary transfer roller made of the metal roller cannot be used. .. It is necessary to use a primary transfer roller consisting of a roller that exhibits a certain degree of electrical resistance, such as a sponge roller or a rubber roller. The volume resistivity of the primary transfer roller is about 6 to 8 [Log (Ω · cm)], and the range of 6.5 to 7.5 [Log (Ω · cm)] is preferred. If this level of electrical resistance is not exhibited, the voltage value of the primary transfer bias will fluctuate significantly under constant current control conditions, depending on conditions such as the charging potential of the photoconductor 1 and the image writing area, resulting in stable image quality. It becomes difficult to obtain.

In the indirect transfer method, the resistance on the back surface of the belt needs to be lower than that in the direct transfer method. For example, in the measured value by Mitsubishi Chemical Analytech's High Resta UP MCP-HT450 type, in the direct transfer method, good transfer efficiency can be obtained when the belt back surface resistance is set to 11 to 12 [LogΩ / □], whereas indirect transfer is obtained. Good primary transfer efficiency can be obtained in methods 9 to 10 [LogΩ / □]. In the indirect transfer method, the transfer bias is transmitted in the circumferential direction of the belt and easily escapes from the earth roller, resulting in poor power saving efficiency.

Next, a characteristic configuration of the printer according to the embodiment will be described.
In the printer according to the embodiment, as the intermediate transfer belt 8, a printer having a multi-layer structure in which at least one layer is laminated on the front surface side of a base layer (belt substrate) made of a material having relatively high rigidity is used. At least one of the layers laminated on the base layer is an elastic layer made of a material having excellent elasticity. By flexibly deforming this elastic layer, the intermediate transfer belt 8 and the photoconductor 1 are in good contact with each other in the primary transfer nip, so that good primary transfer efficiency of the toner image can be realized. Further, in the secondary transfer nip, the intermediate transfer belt 8 and the delicate unevenness on the surface of the recording sheet are brought into good contact with each other by the deformation of the elastic layer of the belt, so that the toner image can be obtained even on the recording sheet having low surface smoothness. Can be satisfactorily secondarily transferred.

In addition, by forming the base layer with a material having a relatively high rigidity such as polyimide, it is possible to suppress the elongation of the belt over time and extend the life of the intermediate transfer belt 8. In addition to the base layer and the elastic layer, the intermediate transfer belt 8 may be provided with a surface coat layer made of a material having excellent toner releasability (made of a material having better toner releasability than the elastic layer). desirable.

Examples of the material used for the elastic layer of the intermediate transfer belt 8 include elastic materials such as elastic rubber and elastomer. Specifically, butyl rubber, fluororubber, acrylic rubber, EPDM, NBR, acrylonitrile-butadiene-styrene rubber, natural rubber, isoprene rubber, styrene-butadiene rubber, butadiene rubber, urethane rubber and the like. Syndiotactic 1,2-polybutadiene, epichlorohydrin-based rubber, polysulfide rubber, polynorbornene rubber and the like may be used. Further, a thermoplastic elastomer (for example, polystyrene-based, polyolefin-based, polyvinyl chloride-based, polyurethane-based, polyamide-based, polyurea, polyester-based, fluororesin-based) or the like may be used. One or more of these can be used. However, it is not limited to these materials.

The thickness of the elastic layer depends on the hardness and the layer structure, but is preferably in the range of 0.07 to 1.0 [mm]. More preferably, the range is 0.25 to 0.5 [mm]. If the thickness of the elastic layer is smaller than 0.07 [mm], the pressure on the toner on the intermediate transfer belt 8 increases at the secondary transfer nip portion, and transfer hollowing out is likely to occur. The next transcription rate decreases.

The hardness of the elastic layer is preferably 10 [°] ≤ HS ≤ 65 [°] (JIS-A). Although the optimum hardness differs depending on the thickness of the elastic layer, if the hardness is lower than 10 [°] JIS-A, transfer hollowing out is likely to occur. On the other hand, if the hardness is higher than 65 ° JIS-A, it becomes difficult to stretch the roller, and since it is stretched by a long-term stretching, it is not durable and requires early replacement.

Materials used for the base layer of the intermediate transfer belt 8 include polycarbonate, fluororesin (ETFE, PVDF, etc.), polystyrene, chloropolystyrene, poly-α-methylstyrene, styrene-butadiene copolymer, and styrene-vinyl chloride copolymer. And so on. Further, it may be a styrene-vinyl acetate copolymer, a styrene-maleic acid copolymer, or a styrene-acrylic acid ester copolymer. Examples of the styrene-acrylic acid ester copolymer include the following. That is, styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-butyl acrylate copolymer, styrene-octyl acrylate copolymer, styrene-phenyl acrylate copolymer and the like. Further, as the material of the base layer, a styrene-methacrylate copolymer (styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene-phenyl methacrylate copolymer, etc.) may be used. In addition, styrene-based resins (monopolymers or copolymers containing styrene or styrene substituents) such as styrene-α-methyl chloroacrylate copolymers and styrene-acrylonitrile-acrylic acid ester copolymers, methyl methacrylate resins. Etc. may be used. In addition, butyl methacrylate resin, ethyl acrylate resin, butyl acrylate resin, modified acrylic resin (silicone modified acrylic resin, vinyl chloride resin modified acrylic resin, acrylic / urethane resin, etc.), vinyl chloride resin, styrene-vinyl acetate common weight. You may use coalescence or the like. Further, a pinyl chloride-vinyl acetate copolymer, rosin-modified maleic acid resin, phenol resin, epoxy resin, polyester resin, polyester polyurethane resin, polyethylene, polypropylene, polybutadiene, polyvinylidene chloride and the like may be used. Further, ionomer resin, polyurethane resin, silicone resin, ketone resin, ethylene-ethyl acrylate copolymer, xylene resin and polyvinyl butyral resin, polyamide resin, modified polyphenylene oxide resin and the like may be used. One or more of these can be used. However, it is not limited to these materials.

The thickness of the base layer is, for example, 50 [μm] to 100 [μm].

In order to prevent the elastic layer made of a rubber material having a large elongation from stretching, a core layer made of a material such as canvas may be provided between the base layer and the elastic layer. Examples of the stretch-preventing material used for the core layer include natural fibers such as cotton and silk. Synthetic fibers such as polyester fiber, nylon fiber, acrylic fiber, polyolefin fiber, polyvinyl alcohol fiber, polyvinyl chloride fiber, polyvinylidene chloride fiber, polyurethane fiber, polyacetal fiber, polyfluoroethylene fiber, and phenol fiber may be used. Further, inorganic fibers such as carbon fiber and glass fiber, and metal fibers such as iron fiber and copper fiber may be used. One or more of these can be used in the form of threads or woven fabrics. Of course, it is not limited to these materials. The yarn for forming a filament or a fibrous yarn may be twisted in any way, such as twisting one or a plurality of filaments, single-twisted yarn, multi-twisted yarn, and twin yarn. Further, the above-mentioned fibers may be blended. Of course, the yarn can also be used by subjecting it to an appropriate conductive treatment. As the woven fabric, any woven fabric such as knitted fabric can be used, a mixed woven fabric can also be used, and a conductive treatment can be applied.

The surface coating layer of the intermediate transfer belt 8 is for coating the surface of the elastic layer, and is made of a layer having good smoothness. The material used for the coat layer is not particularly limited, but generally, a material that reduces the adhesion of toner to the surface of the intermediate transfer belt 8 and enhances the secondary transferability is used. For example, one type or two or more types such as polyurethane, polyester, and epoxy resin, or a material that reduces surface energy and enhances lubricity can be exemplified. Specifically, one kind or two or more kinds of particles such as fluorine material fat, fluorine compound, fluorocarbon, titanium oxide, and silicon carbide, or those having different particle sizes as needed are dispersed and used. Can be done. Further, it is also possible to use a material such as a fluorine-based rubber material in which a fluorine layer is formed on the surface by heat treatment to reduce the surface energy.

The base layer, elastic layer, and surface coat layer are made of a material containing a resistance adjusting material. In the embodiment, an ionic conductive agent is used as the resistance adjusting material. As the ionic conductive agent, an ammonium salt can be exemplified. Specific examples thereof include perchlorates such as tetraalkylammonium, trialkylbenzylammonium, hexadecyltrimethylammonium, dodecyltrimethylammonium, benzyltrimethylammonium, and modified fatty acid dimethylethylammonium. Further, a chlorate salt, a hydrochloride salt, a bromate salt salt, an iodate salt salt, a borohydrochloride salt salt, a sulfate salt, an ethyl sulfate salt salt, a carboxylate salt salt, a sulfonate salt salt and the like may be used. Examples of the ionic conductive agent different from the ammonium salt include alkali metals such as lithium, sodium, potassium, calcium and magnesium. Also, alkaline earth metal perchlorates, chlorates, hydrochlorides, bromates, iodates, borohydrochlorides, sulfates, trifluoromethylsulfates, sulfonates, high molecular weight ions. It may be a conductive agent or the like. One of these may be used alone, or two or more thereof may be used in combination. Among them, an ionic conductive agent containing a quaternary ammonium salt having high conductivity, a small increase in electrical resistance with time, and excellent durability is preferable. As a commercially available product of an ionic conductive agent containing a quaternary ammonium salt, "QAP-01" (tetrabutylammonium perchlorate) manufactured by Carlit Japan Co., Ltd. can be exemplified. In addition, the "1SX series" manufactured by Taisei Fine Chemicals Co., Ltd. and the "IL-A series" manufactured by Koei Chemical Co., Ltd. are also on the market.

The intermediate transfer belt 8 whose electrical resistance is adjusted with an ionic conductive agent has a high resistance and does not reduce the electrical resistance even when a high voltage primary transfer bias is applied. Therefore, the primary transfer has a stable value. Current can flow through the primary transfer nip.

Polyimide used as a kind of base layer material is obtained via a polyamic acid (polyimide precursor) by a reaction between a generally known aromatic polyvalent carboxylic acid anhydride or a derivative thereof and an aromatic diamine. Is something that can be done. Due to its rigid main chain structure, it is insoluble in solvents and has insoluble properties. As a synthesis method, first, a polyimide precursor (polyamic acid or polyamic acid) soluble in an organic solvent is synthesized from an acid anhydride and an aromatic diamine. At this stage, molding is performed by various methods, and then the polyamic acid is dehydrated by a heating or chemical method to be cyclized (imidized) to obtain a polyimide. This is a resin having a rigid imide group and an amide group that imparts flexibility in the molecular skeleton, and has a generally known structure.

The method for producing the intermediate transfer belt 8 is not particularly limited, but it can be suitably produced by, for example, a polyimide precursor solution by the rotary coating method shown in FIG. In this rotary coating method, a cylindrical molded tube 911 processed to an outer diameter corresponding to the length of the intermediate transfer belt 8 is prepared. Then, a nozzle 915 for discharging the coating liquid 916 onto the outer peripheral surface of the cylindrical molded tube 911 is arranged at a position along the outer peripheral surface of the cylindrical molded tube 911. In the figure, the nozzle 915 is connected to the coating liquid container 914 through a pipe. Further, the coating liquid container 914 is connected to the pressurizing device 917 through a pipe. Further, below the nozzle 915, a blade 918 for leveling the discharged coating liquid 916 on the outer peripheral surface of the cylindrical molded tube 911 is arranged.

The operator rotates the cylindrical forming tube 911 in the direction of rotation of the cylindrical forming tube (arrow D), discharges the coating liquid 916 from the nozzle 915 onto the outer peripheral surface of the cylindrical forming tube 911, and uses the blade 918 to discharge the outer periphery of the cylindrical forming tube 911. Level on the surface. At this time, the nozzle 915 and the blade 918 are moved at a constant speed in the direction of arrow E in the drawing, and the coating liquid 916 is applied on the outer peripheral surface of the cylindrical forming tube 911 with a constant thickness. A constant amount of the coating liquid 916 is discharged from the nozzle 915 by the pressurizing device 917. As a result, a coating liquid 916 coating film is formed on the outer peripheral surface of the cylindrical molded tube 911. The obtained coating liquid 916 coating film is heat-dried and cooled to form a base layer (belt substrate). An elastic layer or the like is formed on the front surface side of this base layer by the same procedure.

When a polyimide precursor is used as the resin material of the coating liquid 916, after forming a coating film of the coating liquid 916 on the outer peripheral surface of the cylindrical molded tube 911, it is dried at 80 [° C.] or higher and 170 [° C.] or lower. To remove the solvent (drying step). Then, by heating to 250 [° C.] or higher and 350 [° C.] or lower, imide conversion (calcination step) is performed to obtain a base layer made of a polyimide resin film.

The solid content concentration of the coating liquid 916 is preferably 10 [mass%] or more and 40 [mass%] or less, and the viscosity is preferably 1 [Pa · s] or more and 100 [Pa · s] or less. Further, the coating liquid 916 contains the ionic conductive agent evenly according to the required logarithmic value of the surface resistivity of the intermediate transfer belt 8.

In the method for producing the intermediate transfer belt 8, it is desirable that the drying temperature is 100 [° C.] or higher and 170 [° C.] or lower. If the drying temperature is too low in the drying step, the solvent in the polyimide precursor does not dry easily, and it is difficult to reach a predetermined residual solvent amount even if the drying time is lengthened. If the drying temperature is too high, a dry film is formed on the surface of the polyimide precursor electrode at the very beginning of the drying process. As a result, the solvent inside the polyimide precursor may not be sufficiently dried, or the surface of the polyimide precursor film may be attracted after the drying step, and wrinkle-like defects are likely to occur on the surface of the coating film. .. By setting the drying temperature to 100 [° C.] or higher and 170 [° C.] or lower, a polyimide precursor film having a predetermined residual solvent amount and a better coating film can be obtained.

After forming an elastic layer and, if necessary, a surface coat layer, the coating and drying steps are repeated to form a belt having a multi-layer structure. This is peeled off from the cylindrical molded tube 911 and cut to a predetermined width to obtain an intermediate transfer belt 8.

The following can be exemplified as a first configuration example of the intermediate transfer belt 8. That is, the intermediate transfer belt 8 is a belt having a two-layer structure in which an elastic layer is laminated on the front surface side (loop outer surface side) of the base layer made of polyimide. The front surface resistance, which is the resistance on the front surface side, is 11.3 [logΩ / □], and the back surface resistance, which is the resistance on the back surface side, is 11.3 [logΩ / □]. The volume resistivity of the belt is 9.7 [logΩ / cm].

Further, as a second configuration example of the intermediate transfer belt 8, the following can be exemplified. That is, it is a belt having a two-layer structure in which an elastic layer is laminated on the front surface side of a base layer made of polyimide. Its front surface resistance is 12.5 [logΩ / □], back surface resistance is 11.0 [logΩ / □], and volume resistivity is 10.0 [logΩ / cm].

Further, as a third configuration example of the intermediate transfer belt 8, the following can be exemplified. That is, it is a belt having a two-layer structure in which an elastic layer is laminated on the front surface side of a base layer made of polyamide-imide. Its front surface resistance is 11.3 [logΩ / □], its back surface resistance is 11.3 [logΩ / □], and its volume resistivity is 9.7 [logΩ / cm].

Further, as a fourth configuration example of the intermediate transfer belt 8, the following can be exemplified. That is, it is a belt having a two-layer structure in which an elastic layer is laminated on the front surface side of a base layer made of polyamide-imide. Its front surface resistance is 11.3 [logΩ / □], its back surface resistance is 11.3 [logΩ / □], and its volume resistivity is 9.7 [logΩ / cm].

By using the intermediate transfer belt 8 having the above-mentioned multilayer structure, it is possible to realize good primary transfer efficiency of the toner image from the photoconductor 1 to the intermediate transfer belt 8 and to extend the life of the intermediate transfer belt 8. it can. However, it has been found by the experiments of the present inventors that there is a problem that image density unevenness is likely to occur. The image density unevenness tends to occur particularly easily in the K toner image among the Y toner image, the M toner image, and the C toner image and the K toner image.

As a result of diligent research on the cause, it was found that the cause was unevenness in the amount of charge of the toner. Specifically, the intermediate transfer belt 8 having a multi-layer structure inevitably has a large thickness and an interface between layers is generated, so that the electric resistance is relatively high. Then, the primary transfer bias becomes a high voltage in order to pass a required amount of the primary transfer current. When a metal roller is used as the primary transfer roller 9 and a high voltage primary transfer bias is applied to the primary transfer roller 9, a significant current leak due to a violent discharge occurs between the photoconductor 1 and the metal primary transfer roller 9. It will occur. Therefore, as the primary transfer roller 9, it is necessary to use a roller made of a non-metal. However, even if the roller is made of non-metal, a slight discharge is generated between the roller and the photoconductor 1 via the intermediate transfer belt 8. This discharge is a so-called peeling discharge that is intensively generated in a minute void near the outlet of the primary transfer nip where the intermediate transfer belt 8 is separated from the photoconductor 1. Then, the peeling discharge does not occur uniformly, but occurs according to the slight resistance unevenness of the non-metal primary transfer roller 9, so that the toner charge amount unevenness occurs in the toner layer on the intermediate transfer belt 8. It was found that this uneven charge amount causes a difference in the secondary transfer rate, which causes uneven image density. In the toner layer, the toner at the portion that has undergone the peeling discharge is charged up to the side of the normal charging polarity to increase the charging amount.

In the intermediate transfer belt 8 having a multi-layer structure, a small margin for peeling discharge also causes uneven image density. The margin for peeling discharge is the difference between the lowest primary transfer current value required for primary transfer of the toner image and the lowest primary transfer current value that causes peeling discharge. For example, if the minimum primary transfer current value required for primary transfer of a toner image is 40 [μA] and the minimum primary transfer current that causes peeling discharge is 80 [μA], the margins are both. The difference is 40 [μA]. In a multi-layered belt, this difference tends to be smaller than in a single-layered belt. If there is a minute defect inside the elastic layer in the intermediate transfer belt 8 or if there is a minute void at the time between the base layer and the elastic layer, that becomes the discharge starting point and peeling discharge is likely to occur. It is thought that it has become.

Image density unevenness caused by unevenness in the amount of charge of the toner mainly occurs in a high-gradation image area (an image area having a relatively high image density). FIG. 5 is an enlarged schematic view showing an enlarged image portion of a high gradation (for example, a gradation in which the dot area gradation rate due to dither is 80%) in which image density unevenness is generated due to unevenness of the toner charge amount. is there. Depending on the specifications of the device, the image density of the portion of the entire toner image that has undergone the peeling discharge may be higher or lower, but the figure shows an example in which the density is higher. In the K toner image unit, the image density of the portion subjected to the peeling discharge tends to be higher. When the secondary transfer current value is gradually lowered from this state, the secondary transfer rate of the portion subjected to the peeling discharge is gradually lowered, and the image density difference is gradually lowered. Eventually, the concentration difference becomes almost inconspicuous, but when the secondary transfer current value is further lowered, on the contrary, the portion that has undergone the peeling discharge becomes white with almost no secondary transfer. This is because when the secondary transfer current value is considerably low, a good secondary transfer rate can be obtained in the region of the low charge amount in the toner image, whereas the secondary transfer efficiency is remarkably deteriorated in the region of the high charge amount.

Although the image density unevenness caused by the unevenness of the toner charge amount caused by the peeling discharge has been described, the image density unevenness caused by the afterimage on the photoconductor 1 being emphasized by the peeling discharge also occurs depending on the device configuration. To do. The image density unevenness caused by the enhancement of the afterimage on the photoconductor 1 mainly occurs in the low-gradation image portion. FIG. 6 is an enlarged image portion of a low gradation (for example, a gradation in which the dot area gradation rate due to dither is 10%) that causes uneven image density due to the enhancement of the afterimage on the photoconductor 1. It is an enlarged schematic diagram shown by. When a peeling discharge occurs, the toner is charged up to the normal charge polarity side at the discharge part on the belt side, whereas the photoconductor 1 is charged up on the opposite polarity side to the toner at the discharge part on the photoconductor side. Occurs. As a result, the discharge portion in the electrostatic latent image of the photoconductor 1 is emphasized as an afterimage. When the emphasized afterimage adheres to the low-gradation image portion at the primary transfer nip of the next round, the portion of the afterimage is not satisfactorily transferred to the primary and becomes white. This causes uneven image density.

It was also found that among the toner images of each color, the K toner image is particularly likely to cause uneven image density due to the following reasons. That is, among the primary transfer nip for Y, M, C, and K, the primary transfer nip for K is arranged at the most downstream in the order of the primary transfer steps. The Y toner image that is primarily transferred onto the belt by the primary transfer nip for Y has an uneven charge amount due to the peeling discharge at the outlet of the primary transfer nip for Y. However, after that, almost all Y toners are charged up to the saturated charge amount by receiving peeling discharge at the outlet of the primary transfer nip for M, the outlet of the primary transfer nip for C, and the outlet of the primary transfer nip for K. To do. Therefore, when the secondary transfer is performed on the recording sheet by the secondary transfer nip, there is almost no unevenness in the charge amount.

The M toner image, which was subjected to peeling discharge at the outlet of the primary transfer nip for M and caused unevenness in the charge amount, is also repeatedly subjected to peeling discharge at the outlet of the primary transfer nip for C and the outlet of the primary transfer nip for K. Then, almost the entire area is charged up to the saturated charge amount. Therefore, when the secondary transfer is performed on the recording sheet by the secondary transfer nip, there is almost no unevenness in the charge amount.

The C toner image, which received a peeling discharge at the outlet of the primary transfer nip for C and caused unevenness in the charge amount, receives a peeling discharge at the outlet of the primary transfer nip for K and is further charged up to increase the charge amount. Unevenness is reduced. When the secondary transfer is performed on the recording sheet by the secondary transfer nip, the unevenness of the charge amount is reduced.

On the other hand, the K toner image, which was subjected to peeling discharge at the outlet of the primary transfer nip for K and caused unevenness in the charge amount, entered the secondary transfer nip as it was, and was caused by the unevenness in the charge amount. The difference in secondary transferability causes uneven image density.

The present inventors conducted an experiment in which K test toner images were continuously printed and the image density unevenness was evaluated. The primary transfer roller 9K for K, those resistance is ^{10 6 [Ω], 10 6.5} [Ω] at which those using a three but is ^107, an image density irregularity in the respective .. The image density unevenness was evaluated on a three-point scale: none (◯), slightly observed but within the permissible range (Δ), and conspicuous density unevenness (x). The results of this experiment are shown in Table 1 below. In Table 1, k sheets mean 1000 sheets. For example, 100k sheets is 100,000 sheets.

As shown in Table 1, when using resistance = 10 7 a ^[Omega] the primary transfer roller 9K is, in the initial stage of printing do not cause uneven image density, when the continuous printing number is later 200k sheets, image Concentration unevenness is noticeable. This is because the electrical resistance of the primary transfer roller 9K increases with time, and the primary transfer bias increases accordingly. The primary transfer bias is controlled by a constant current.

When the primary transfer roller 9K having a resistance = 10 ^6.5 [Ω] is used, the degree of image density unevenness can be kept within an allowable range even when printing 1600 k sheets. This is because the electric resistance of the primary transfer roller 9K is made lower, so that the primary transfer bias output by the constant current control is made lower, and it becomes more difficult to generate a peeling discharge at the primary transfer nip outlet for K.

When using resistance = a 10 6 ^[Omega] the primary transfer roller 9K can be performed 1600k prints do not cause uneven image density. Thus, the resistance is less than 10 7 ^[Omega] By using the primary transfer roller 9K, it is possible to effectively suppress the occurrence of image density unevenness due to unevenness of charge amount of K toner due to separation discharge.

Therefore, in the printer according to the embodiment, as the primary transfer roller 9K for K, is used as the resistance is less than 10 7 ^[Ω]. In the other hand, Y, M, primary transfer rollers 9Y for C, 9M, as 9C, is used as the resistance is 10 ⁷ [Omega] or more. This is due to the reasons explained below. That is, when the resistance to less than 10 7 ^[Omega], since would rather aggravate the uneven image density. When the resistance is lowered to reduce the charge-up amount of the toner image due to the peeling discharge, when the toner image enters the primary transfer nip on the downstream side, a part of the toner image is transferred to the photoconductor 1 by the primary transfer nip on the downstream side. It is considered that the reverse transfer causes uneven image density. Since such reverse transfer does not occur in the K toner image in which the primary transfer step is the most downstream, suppressing the unevenness of the charge amount by reducing the peeling discharge is an effective means for suppressing the unevenness of the image density.

As described above, in the printer according to the embodiment, of the four primary transfer rollers 9Y, 9M, 9C, and 9K, the electrical resistance of the primary transfer roller 9K for K arranged at the most downstream in the order of the primary transfer steps. Is lower than the electrical resistance of the primary transfer rollers 9Y, 9M, 9C for Y, M, and C. As a result, it is possible to suppress the occurrence of uneven image density.

By the way, the characteristics of the toner affect the image density unevenness. Among the characteristics, charge retention, dielectric constant, and resistance have the greatest effect. The charge retention property is a property that indicates whether the toner itself easily retains or releases the charge. For example, the charged toner is left under certain conditions and then quantified by measuring the amount of charge. Is possible. When a toner having poor charge retention (easily releasing charges) receives uneven discharge, it tends to cause uneven image density during secondary transfer. Even if the toner having poor charge retention is rubbed in the developing apparatus, it is difficult to charge the toner, so that the charge amount is relatively low. Therefore, unevenness in the amount of charge due to peeling discharge is likely to occur at the outlet of the primary transfer nip. On the contrary, a toner having a good charge retention and a relatively high charge amount reaches a saturated charge amount immediately when it receives a peeling discharge at the outlet of the primary transfer nip, so that it is unlikely to cause unevenness in the charge amount.

The resistance and dielectric constant of the toner affect the ease of reflection when the unevenness of the charge amount is reflected in the image density unevenness. If the secondary transfer current is too small in the secondary transfer nip, the strength of the secondary transfer electric field is insufficient in the high-gradation image area (for example, all solids, especially the entire solid surface in which multiple colors are superimposed), and the secondary transfer Causes defects. At this time, the portion of the toner image in which the toner charge amount is small is secondarily transferred, but the portion where the toner charge amount is large is difficult to be secondarily transferred. On the contrary, when the secondary transfer current is excessive, the secondary transfer current becomes excessive in the low-gradation image portion (for example, a monochromatic halftone portion), and a phenomenon called over-transfer occurs, resulting in secondary transfer. Transcription is poor. At this time, in the toner image, the portion where the charge amount is small is not subjected to the secondary transfer due to the excessive strength of the secondary transfer electric field, while the portion where the charge amount is large is secondarily transferred. Overtransfer is considered to be a phenomenon in which a minute electric discharge is generated from the belt surface to the recording sheet due to a strong electric field in the transfer nip, and the electric discharge causes the toner to be backcharged to deteriorate the transferability. .. At this time, the lower the resistance of the toner, the more easily it is affected by the discharge and the more easily it is backcharged. When the original charge amount of the discharged toner is small, the toner is backcharged due to the influence of the discharge. On the other hand, when the amount of charge is large, the absolute value becomes small with normal charge, causing transfer failure.

In addition, the characteristics of the toner also affect the image density unevenness caused by the afterimage. If the toner has a low charge retention ability, the effect will be noticeable. When the amount of charge of the toner becomes small, the developing ability is improved. Therefore, the development bias and the charge bias are adjusted to be low, and the image density is adjusted around the target value. If the charging bias is small, it is difficult to erase the potential history of the circumference in front of the photoconductor, so that it tends to become an afterimage. Furthermore, since the developing ability is high in the first place, even a small potential difference increases the density difference. Therefore, for stations with low toner charge retention capacity, it is desirable to reduce discharge unevenness at the transfer section as much as possible, which is consistent with the above-mentioned direction of wanting to eliminate toner charge unevenness on the belt. ..
Therefore, in the printer according to the embodiment, as the K toner, one having a higher dielectric constant and a lower resistance than the Y toner, the M toner, and the C toner is used. With such a configuration, it is possible to further suppress the occurrence of image density unevenness due to unevenness in the amount of charge of the toner.

In the intermediate transfer belt 8 having a multi-layer structure, it is desirable to raise the secondary transfer nip pressure to some extent in order to apply a sufficient secondary transfer nip pressure to the elastic layer. Therefore, for example, a configuration may be adopted in which the pressing force of the secondary transfer roller 18 against the secondary transfer opposing roller 12 is changed so as to change the secondary transfer nip pressure. When the secondary transfer nip pressure is relatively high, it becomes difficult to separate the recording sheet from the intermediate transfer belt 8 at the outlet of the secondary transfer nip, and it becomes easy to cause poor separation of the recording sheet from the belt.

Therefore, a transfer transfer belt system as shown in FIG. 7 may be adopted. In the figure, a transfer transfer belt 51 is sandwiched between the secondary transfer opposed roller 12 and the secondary transfer roller 18 in addition to the intermediate transfer belt 8. The endless transfer transfer belt 51 is tensioned by being hung around the secondary transfer roller 18, the first support roller 52, the second support roller 53, and the third support roller 54 arranged inside the loop. In this state, it can be moved endlessly in the counterclockwise direction in the figure. The front surface of the transfer transfer belt 51 is cleaned by the cleaning blade 55. The third support roller 54, which sandwiches the belt between the cleaning blade 55 and the cleaning blade 55, also functions as a tension roller.

The transfer transfer belt 51 is a single-layer belt having a thickness of 80 [μm] made of PI, PAI, PVDF, or the like. The recording sheet that has passed through the secondary transfer nip due to the contact between the intermediate transfer belt 8 and the transfer transfer belt 51 is sucked by the electric charge of the transfer transfer belt 51 and is easily separated from the intermediate transfer belt 8. After that, it moves together with the transfer accompaniment belt 51 while being held on the surface of the transfer transfer belt 51, but when the transfer transfer belt 51 reaches the hanging portion with respect to the first support roller 52 and the traveling direction suddenly changes, the transfer transfer belt 51 is transferred. The curvature is separated from the surface of the belt 51. With such a configuration, various types of recording sheets such as thin paper and thick paper unevenness can be satisfactorily conveyed.

As shown in FIG. 8, the roller resistance is obtained from the voltage and current at that time by abutting the roller to be inspected against the metal roller 59 and applying a bias while applying a constant load. At this time, there are a method of measuring by rotating the roller and a method of measuring by fixing the roller. For resistance, while rotating the roller at a speed of 30 [rpm] with a special jig, pressurize the roller from both ends in the direction of the roller axis and apply a total load of 1 [N] to each end to apply a load of 1 [kV]. ] Is the value measured by applying the bias. The environment at the time of measurement is set to 23 [° C.] and 50 [% RH]. The volume resistivity is calculated from the rubber thickness (length) and the nip width as resistance x nip width / rubber thickness based on the definition using the value.

Next, the printer of each embodiment to which a characteristic configuration is added by the printer according to the embodiment will be described. Unless otherwise specified below, the configuration of the printer according to each embodiment is the same as that of the embodiment.
[First Example]
In the printer according to the first embodiment, each of the primary transfer rollers 9Y, 9M, 9C, and 9K for Y, M, C, and K is made of a rubber roller provided with a rubber roller portion. In the embodiment, each of the primary transfer rollers 9Y, 9M, 9C, and 9K is made of a sponge roller provided with a roller portion made of foamed sponge. However, in this case, the roller surface is rich in unevenness. It is easy to cause peeling discharge at the outlet of the primary transfer nip. In addition, unevenness in electrical resistance due to the unevenness is also a factor that easily causes peeling discharge. On the other hand, the primary transfer rollers 9Y, 9M, 9C, and 9K made of rubber rollers have a smooth surface and no air bubbles, so that resistance unevenness in the surface direction is small. Therefore, as compared with the sponge roller, it is possible to suppress the occurrence of peeling discharge at the outlet of the primary transfer nip. Table 2 below shows the results of a continuous print test using the primary transfer roller 9K for K, which consists of a rubber roller.

Compared with Table 1, which shows the experimental results using the primary transfer roller 9K made of a sponge roller, the margin is improved. This is because there is no spatial defect that causes a non-uniform discharge like the bubbles of a sponge, so that a uniform discharge occurs. However, if the resistance is equal to 10 7 ^[Omega], thereby causing the image density unevenness unacceptable. It is considered that the rubber roller generates a non-uniform discharge starting point due to foreign matter adhesion, toner filming, discharge product adhesion, etc. when a large discharge is generated.

[Second Example]
In the printer according to the second embodiment, the primary transfer rollers 9Y, 9M, 9C, 9K for Y, M, C, and K are made of metal rollers. Then, as the primary transfer method of Y, M, C, and K, an indirect transfer method is adopted instead of the direct transfer method. As the intermediate transfer belt 8, a belt having a back surface resistance of 10 [Log (Ω / □)] is used. Table 3 below shows the results of an experiment in which K test toner images were continuously printed in such a configuration.

Since the metal roller does not cause an increase in resistance over time, as shown in Table 3, it is possible to avoid the occurrence of image density unevenness due to uneven charging amount of K toner over a long period of time. is made of. Even if it is made of the same metal, the electrical resistance of the metal material is different, so that the most downstream K of the primary transfer rollers 9Y, 9M, 9C, 9K for Y, M, C, K The electrical resistance of the primary transfer roller 9K is lower than that of the other primary transfer rollers 9Y, 9M, 9C.

Only the most downstream primary transfer roller 9K for K is made of metal rollers, and the other primary transfer rollers 9Y, 9M, 9C are made of sponge rollers and rubber rollers, and Y, M, C are three. Only the color may be directly transferred.

[Third Example]
In the printer according to the third embodiment, the primary transfer rollers 9Y, 9M, 9C, 9K for Y, M, C, and K are made of metal rollers. Then, the direct transfer method is adopted as the primary transfer method of Y, M, C, and K. As the intermediate transfer belt 8, a belt having a considerably large thickness and a high resistance is used. By increasing the resistance of the intermediate transfer belt 8 considerably, even if the metal primary transfer rollers 9Y, 9M, 9C, and 9K are used by the direct transfer method, a violent discharge is generated between the primary transfer rollers and the photoconductor 1. It is possible to prevent the occurrence of an abnormal image due to an excessive current leak without generating it. By adopting the direct transfer method, vibration and banding of the intermediate transfer belt 8 can be suppressed as compared with the second embodiment which is the indirect transfer method. In such a configuration, the result of the experiment in which the K test toner image was continuously printed was the same as in Table 3.

Even if it is made of the same metal, the electrical resistance of the metal material is different, so that the most downstream K of the primary transfer rollers 9Y, 9M, 9C, 9K for Y, M, C, K The electrical resistance of the primary transfer roller 9K is lower than that of the other primary transfer rollers 9Y, 9M, 9C.

[Fourth Example]
The printer according to the fourth embodiment is the printer according to the first embodiment, the second embodiment, or the third embodiment with the following improvements. That is, any one of the three colors Y, M, and C is arranged at the most downstream of the four colors, and K is arranged at a position other than the most downstream. Then, among the primary transfer rollers 9Y, 9M, 9C, and 9K for Y, M, C, and K, the primary transfer rollers (9Y, 9M, or 9C) of Y, M, or C that are the most downstream in the order of the primary transfer steps. ) Is lower than the electrical resistance of other primary transfer rollers.

[Fifth Example]
FIG. 9 is a block diagram showing a part of the electric circuit of the printer according to the fifth embodiment. In the figure, the control unit 900 is composed of a well-known CPU, RAM, ROM, non-volatile memory, etc., and controls the drive of each device in the printer and performs a predetermined enzyme process.

The power supply 901 outputs each of the primary transfer biases for Y, M, C, and K for applying to the primary transfer rollers 9Y, 9M, 9C, and 9K for Y, M, C, and K by individual constant current control. To do. In order to control each of the primary transfer biases for Y, M, C, and K individually with a constant current, the output target values of the primary transfer currents for Y, M, C, and K are stored. Then, for example, in the case of the primary transfer bias for Y, the voltage output value of the primary transfer bias is adjusted so that the output target value for Y and the output current value of the primary transfer bias are the same value.

The environment sensor 902 detects the temperature and relative humidity in the printer and outputs the respective detection results to the control unit 900. The control unit 900 calculates the absolute humidity using the temperature detection result sent from the environment sensor 902 and the relative humidity detection result. Then, based on the calculation result, it is specified whether the in-flight environment is an LL (low temperature / low humidity) environment, an MM (medium temperature / medium humidity) environment, or an HH (high temperature / high humidity) environment. The identification is done according to the following rules.
・ When the absolute humidity is 8 [kg / kg] or less → LL environment ・ When the absolute humidity exceeds 8 [kg / kg] and is 15 [kg / kg] → MM environment ・ Absolute humidity is 15 [kg] / Kg] → HH environment

When the environment is specified, the control unit 900 newly determines the output target value in the constant current control of the primary transfer bias for Y, M, C, and K according to Table 4 below. Then, each of the newly determined output target values for Y, M, C, and K is output to the power supply 901. The power supply 901 updates each of the output target values for Y, M, C, and K stored up to that point to the same values as the new values sent from the control unit 900. After that, the output of the primary transfer bias for Y, M, C, and K is controlled by a constant current based on the updated output target values for Y, M, C, and K.

For the three colors Y, M, and C, which have relatively high electrical resistance of the primary transfer rollers 9Y, 9M, and 9C, the margin of peel discharge at the outlet of the primary transfer nip for Y, M, and C is greater than K. Is also getting smaller. The margin becomes smaller as the environment becomes colder and less humid. Therefore, as shown in Table 4, for Y, M, and C, the output target value of the primary transfer current is reduced in the order of HH environment, MM environment, and LL environment.

On the other hand, for K, in which the electrical resistance of the primary transfer roller 9K is relatively low and the margin of peel discharge at the outlet of the primary transfer nip is relatively large, the degree of peel discharge does not increase so much even in the LL environment. .. Therefore, as shown in Table 4, the output target value of the primary transfer current in the LL environment is set to the same value as in the MM environment. As a result, even in the LL environment, the primary transfer current can be satisfactorily passed through the primary transfer nip for K. In K, since the electrical resistance of the primary transfer roller 9K is relatively low, the rise time of the primary transferability is delayed. Therefore, if the primary transfer current is reduced, a good primary transfer rate is likely to be obtained. , Such a problem can be made less likely to occur. Further, by generating a small uniform small peeling discharge to some extent without lowering the output target value, it is possible to give a good charge to the K toner on the belt and increase the average charge amount of the toner.

The output target values in the HH environment are also different between Y, M, C and K. In K, it is preferable to generate a uniform peeling discharge to some extent and slightly increase the charge amount of the K toner. Therefore, the output target value is made larger than that of Y, M, and C. In other words, in Y, M, and C, the output target value is smaller than that in K. This is because the primary transferability rise rate is relatively high in the HH environment, and the primary transferability rise in Y, M, and C is due to the relatively high electrical resistance of the primary transfer rollers 9Y, 9M, and 9C. This is because the speed becomes even faster. In K, since the electric resistance of the primary transfer roller 9K is relatively low, the rising speed of the primary transferability is not so fast, but rather the output target value is relatively large in order to obtain a uniform peeling discharge. There is. In this way, by individually adjusting the electrical resistance of the primary transfer roller and the output target value of the primary transfer current for each color, good image quality can be obtained for each color.

[Sixth Example]
In the printer according to the sixth embodiment, the diameter of the photoconductor 1K for K is made larger than the diameter of the photoconductors 1Y, 1M, 1C for Y, M, C. The diameter of the photoconductor 1K for K is set to 100 [mm], while the diameter for Y, M, and C is set to 60 [mm], which is the same as in the embodiment. In general, by increasing the diameter of the photoconductor 1K of K, which has the highest output frequency, among the colors, the life of the photoconductor 1K can be extended, and the photosensitizers for Y, M, and C, which have low output frequency, can be extended. With the bodies 1Y, 1M, 1C and the photoconductor 1K, the replacement time can be balanced and the maintainability can be improved. However, for K, the distance of the minute gap is made longer than Y, M, C, and K near the outlet of the primary transfer nip due to the contact between the intermediate transfer belt 8 and the photoconductor 1K, and the generation time of peeling discharge occurs. It's getting longer. For this reason, a peeling discharge is more likely to occur at the outlet of the primary transfer nip for K as compared with the case where the diameter of the photoconductor 1K is not made larger, but the electric resistance of the primary transfer roller 9K is still relatively low. Therefore, it is possible to suppress the occurrence of uneven image density.

So far, an example using a power supply that outputs the primary transfer bias under constant current control has been described, but the present invention can also be applied to a configuration using a power supply that outputs the transfer bias under constant voltage control. In the case of such a configuration, the output target value in the constant voltage control of the transfer bias for applying to the most downstream transfer bias member is made lower than the output target value of the transfer bias for applying to the other transfer bias member. A desired amount of transfer current is passed through each transfer nip. As a result, the potential of the most downstream transfer bias member can be made lower, and the occurrence of peeling discharge at the outlet of the most downstream transfer nip can be suppressed.

Further, an example in which the toner image primaryly transferred from the image carrier to the intermediate transfer belt 8 is secondarily transferred to the recording sheet has been described, but the toner image is transferred from the image carrier to the recording sheet held on the surface of the belt member. The present invention can also be applied to a configuration for transferring. In such a configuration, since the secondary transfer is not performed, the image density unevenness due to the secondary transfer property unevenness caused by the unevenness of the toner charge amount due to the peeling discharge does not occur. However, image density unevenness occurs by emphasizing the afterimage of the image carrier due to the peeling discharge. By making the resistance of the most downstream transfer bias member lower than the resistance of other transfer bias members, it is possible to suppress the occurrence of image density unevenness.

The above description is an example, and the effect peculiar to each of the following aspects is exhibited.
[Aspect A]
Aspect A is an endless belt member (for example, an intermediate transfer belt) that sequentially contacts a plurality of image carriers (for example, photoconductors 1Y, 1M, 1C, 1K) and moves so as to form a transfer nip at each contact position. 8) and a plurality of transfer bias members (for example, primary transfer rollers 9Y, 9M, 9C, 9K) to which a transfer bias is applied with the belt members individually sandwiched between the plurality of image carriers. In an image forming apparatus that transfers a toner image on the image carrier to the surface of the belt member at each of the plurality of transfer nips, at least one layer of the belt member on the front surface side of the belt substrate is provided. The electric resistance of the transfer bias member (for example, the primary transfer roller 9K) arranged at the most downstream in the order of the transfer steps among the plurality of transfer bias members is the highest. It is characterized in that it is lower than the electric resistance of the transfer bias member (for example, the primary transfer roller 9Y) arranged upstream.

In such a configuration, the belt member has a multi-layer structure in which the flexible layer ensures high adhesion to the image carrier and the rigid layer suppresses the elongation of the toner image. Good transfer efficiency and long life of the belt member can be realized.

Since the belt member having such a multi-layer structure has a relatively large thickness and an interface is formed between the layers, the electric resistance becomes relatively high. Then, in order to pass the transfer current required for the transfer nip using such a belt member having high electrical resistance, it is necessary to apply a high voltage transfer bias to the transfer bias member, which causes the transfer bias member. Raises the potential of. Then, a peeling discharge is generated in the minute gap between the image carrier and the belt member formed near the outlet of the transfer nip, which tends to cause unevenness in the charge amount of the toner image on the belt member. However, of the plurality of transfer nip, the toner image that caused unevenness in the charge amount at the outlet of the transfer nip on the upstream side of the most downstream side receives the peeling discharge again at the outlet of the transfer nip on the downstream side. Is increased to near the saturated charge amount, so that the unevenness of the charge amount is reduced. On the other hand, when the toner image transferred to the belt member by the most downstream transfer nip receives a peeling discharge at the outlet of the most downstream transfer nip and causes unevenness in the charge amount, the toner image is transferred to the recording sheet as it is. It is sent to the transfer process. Therefore, the image density unevenness occurs remarkably mainly in the toner image transferred to the belt member in the most downstream transfer process.

Therefore, in the aspect A, the electric resistance of the transfer bias member arranged at the most downstream in the order of the transfer steps is made lower than the electric resistance of the transfer bias member arranged at the most upstream. As a result, the transfer bias applied to the most downstream transfer bias member is made lower than the transfer bias applied to the most upstream transfer bias member, so that the occurrence of peeling discharge is suppressed at the outlet of the most downstream transfer nip. Then, with respect to the toner image transferred to the belt member at the most downstream transfer nip, it becomes possible to suppress the occurrence of unevenness of the charge amount due to the peeling discharge at the outlet of the most downstream transfer nip. It is possible to suppress the occurrence of image density unevenness due to unevenness.

If the electrical resistance of the most upstream transfer bias member is also relatively low, the outlet of the most upstream transfer nip and the downstream side of the image of the most upstream toner transferred to the belt member by the most upstream transfer nip. The outlet of the transfer nip is repeatedly subjected to a relatively weak peeling discharge. As a result, the upstream toner image is gradually charged up with a charge amount lower than the saturated charge amount, and the unevenness of the charge amount is gradually deteriorated, so that the image density unevenness of the upstream toner image is rather deteriorated. ..

[Aspect B]
Aspect B is characterized in that, in Aspect A, the electric resistance of the transfer bias member arranged at the most downstream is made lower than the electric resistance of the other transfer bias member. In such a configuration, the toner images other than the most downstream toner image are charged with a relatively strong peeling discharge in the process of repeatedly passing through the outlet of the transfer nip to be charged up to near the saturated charge amount. It is possible to suppress the occurrence of image density unevenness caused by the unevenness of the image. In addition to this, the most downstream toner image is subjected to a relatively weak peeling discharge to reduce unevenness in the amount of charge. Thereby, the image density unevenness caused by the unevenness of the charge amount can be suppressed.

[Aspect C]
Aspect C is characterized in that, in Aspect A or B, at least one of the plurality of layers in the belt member is made of a material containing an ionic conductive material. In such a configuration, even if a high voltage transfer bias is applied, it is unlikely that the electrical resistance of the belt member is lowered due to the application, so that the instability of the image quality due to the fluctuation of the electrical resistance can be suppressed.

[Aspect D]
Aspect D is a transfer bias for applying to the transfer bias member disposed at the most downstream in any one of aspects A to C, and for applying to at least one of the other transfer bias members. A power supply for transfer bias (for example, power supply 901) is configured so as to output the transfer bias individually by constant current control, and an environment detection means (for example, environment sensor 902) for detecting the environment and constant current control for the power supply are used. A control means (for example, control unit 900) that corrects the target value based on the detection result by the environment detection means is provided, and the target value of the former transfer bias and the target value of the latter transfer bias are mutually set. It is characterized in that the control means is configured so as to make corrections according to different algorithms. In such a configuration, the transfer current can be controlled to a value suitable for the combination of the electrical resistance of the transfer bias member and the environment, and the occurrence of image quality deterioration due to improper transfer current can be suppressed.

[Aspect E]
Aspect E is any one of aspects A to D, for forming a toner image on the surface of the image carrier arranged at the most downstream in the order of the transfer steps among the plurality of image carriers. It is characterized in that the volume resistivity of the toner is lower than the volume resistivity of the toner for forming a toner image on the surface of the other image carrier. In such a configuration, the volume resistivity of the toner transferred to the belt member at the most downstream transfer nip is lower than the volume resistivity of the toner transferred to the belt member at the other transfer nip, so that the volume resistivity of the toner transferred to the belt member at the most downstream transfer nip is lower. The occurrence of peeling discharge at the outlet of the transfer nip is further suppressed. As a result, it is possible to further suppress the occurrence of image density unevenness due to the unevenness of the toner charge amount of the toner image transferred to the belt member at the most downstream transfer nip due to the peeling discharge at the outlet of the most downstream transfer nip. Can be done.

[Aspect F]
Aspect F is any one of aspects A to E, for forming a toner image on the surface of the image carrier arranged on the most downstream side in the order of the transfer steps among the plurality of image carriers. It is characterized in that the dielectric constant of the toner is higher than the dielectric constant of the toner for forming a toner image on the surface of the other image carrier. In such a configuration, the permittivity of the toner transferred to the belt member at the most downstream transfer nip is higher than the permittivity of the toner transferred to the belt member at the other transfer nip, so that the most downstream transfer nip The occurrence of peeling discharge at the outlet of is further suppressed. As a result, it is possible to further suppress the occurrence of image density unevenness due to the unevenness of the toner charge amount of the toner image transferred to the belt member at the most downstream transfer nip due to the peeling discharge at the outlet of the most downstream transfer nip. Can be done.

[Aspect G]
Aspect G is a mode E or F, characterized in that the toner for forming a toner image on the image carrier arranged at the most downstream is a black toner. In such a configuration, the following can be achieved by using a K toner containing a coloring material made of carbon black having excellent conductivity. That is, the toner transferred to the belt member by the most downstream transfer nip can easily have a lower electric resistance or a higher dielectric constant than the toner transferred to the belt member by another transfer nip. can do.

[Aspect H]
Embodiments H is intended to be any of embodiments A-G, the electrical resistance of the transfer bias member disposed most downstream in the order of the transfer process and less than 10 7 ^[Ω] is there. With such a configuration, as revealed by the present inventors in an experiment, it is possible to effectively suppress the occurrence of peeling discharge at the outlet of the most downstream transfer nip.

[Aspect I]
Aspect I is Aspect H, characterized in that the surfaces of the plurality of transfer bias members are all made of foamed sponge. In such a configuration, even if a foreign substance is caught in the transfer nip, the transfer bias member made of the foamed sponge is flexibly deformed, so that damage to the image carrier due to the foreign substance being caught can be suppressed.

[Aspect J]
Aspect J is Aspect H, characterized in that the surfaces of the plurality of transfer bias members are all made of rubber. In such a configuration, since the surface of the transfer bias member is made of rubber having better smoothness than the foamed sponge, the generation of peeling discharge at the outlet of the transfer nip is suppressed as compared with the case where the transfer bias member made of the foamed sponge is used. be able to.

[Aspect K]
Aspect K is a mode I or J, in which the volume resistivity of the transfer bias member disposed most downstream in the order of the transfer process and less than 10 7 ^[Ω · cm] is there.

[Aspect L]
Aspect L is any one of aspects A to G, characterized in that the transfer bias member arranged at the most downstream in the order of the transfer steps is made of metal. In such a configuration, since the electric resistance of the most downstream transfer bias member does not increase with time, it is possible to avoid the deterioration of the peeling discharge due to the increase of the electric resistance over a long period of time.

[Aspect M]
Aspect L is any one of the embodiments A to L, characterized in that the electrical resistance of the transfer bias member disposed most upstream in the order of the transfer step is 10 7 ^[Omega] or higher is there. In such a configuration, the most upstream transfer nip promotes the peeling discharge, thereby promoting the charge-up of the toner due to the peeling discharge. As a result, the toner transferred to the belt member by the most upstream transfer nip is charged up to the saturated charge amount by the peeling discharge at the outlet of the transfer nip on the downstream side, and the charge amount of the toner is made uniform. The occurrence of image density unevenness can be suppressed.

[Aspect N]
In aspect N, in any of aspects A to M, the diameter of the image carrier arranged on the most downstream side in the order of the transfer steps among the plurality of image carriers is set to the diameter of the other image carrier. It is characterized by being larger than. With such a configuration, the life of the most downstream image carrier can be extended.

[Aspect O]
Aspect O is a step of moving the endless belt member endlessly so as to sequentially contact a plurality of image carriers and form a transfer nip at each contact position, and the belt between the plurality of image carriers. A step of applying a transfer bias to each of a plurality of transfer bias members individually sandwiching the members, and a toner image on the image carrier is held on the surface of the belt member or on each of the plurality of transfer nips. In the image forming method for carrying out the step of transferring to a recording sheet, a belt member having a multi-layer structure in which at least one layer is laminated on the front surface side of the belt substrate is used, and a plurality of the transfers are performed. Among the bias members, as the transfer bias member arranged at the most downstream in the order of the transfer steps, a member having a lower electric resistance than the transfer bias member arranged at the most upstream is used. is there.

1Y, 1M, 1C, 1K: Photoreceptor (image carrier)
8: Intermediate transfer belt (belt member)
9Y, 9M, 9C, 9K: Primary transfer roller (transfer bias member)
9Y: Primary transfer roller for Y (uppermost transfer bias member)
9K: Primary transfer roller for K (downstream transfer bias member)
900: Control unit (control means)
901: Power supply 902: Environmental sensor (environmental detection means)

Japanese Unexamined Patent Publication No. 2006-195143

Claims (15)

A state in which the belt member is individually sandwiched between an endless belt member that sequentially contacts a plurality of image carriers and moves so as to form a transfer nip at each contact position, and the plurality of image carriers. In an image forming apparatus that includes a plurality of transfer bias members to which a transfer bias is applied, and transfers a toner image on the image carrier to the surface of the belt member at each of the plurality of transfer nips.
As the belt member, a multi-layer structure in which at least one layer is laminated on the front surface side of the belt substrate is used.
And, among the plurality of the transfer bias member, the electrical resistance of the transfer bias member disposed most downstream in the order of the transfer step is 10 7 less than ^[Omega], of all the other of the transfer bias member image forming apparatus, wherein the electrical resistance of 10 7 ^[Omega] or more. In the image forming apparatus of claim 1,
An image forming apparatus characterized in that at least one of the plurality of layers in the belt member is made of a material containing an ionic conductive material. In the image forming apparatus of claim 1 or 2.
A transfer bias for applying to the transfer bias member arranged at the most downstream and a transfer bias for applying to at least one of the other transfer bias members are individually output by constant current control. Configure a power supply for transfer bias,
An environment detecting means for detecting the environment and a control means for correcting the target value of the constant current control in the power supply based on the detection result by the environment detecting means are provided.
Moreover, the image forming apparatus is characterized in that the control means is configured so as to correct the target value of the former transfer bias and the target value of the latter transfer bias according to different algorithms. The image forming apparatus according to claim 3.
In a high temperature and high humidity environment and a low temperature and low humidity environment, the target value of the transfer current flowing through the transfer bias member arranged at the most downstream is set to be equal to or higher than the target value of the transfer current flowing through the other transfer bias member. Image forming device. The image forming apparatus according to claim 4.
The target value of the transfer current flowing through the transfer bias member arranged at the most downstream in the low humidity and low temperature environment is the same as the target value of the transfer current flowing through the transfer bias member arranged at the most downstream in the medium temperature and medium humidity environment. An image forming apparatus characterized in that it has been used. The image forming apparatus according to claim 1 or 2.
The transfer bias for applying to the transfer bias member arranged at the most downstream and the transfer bias for applying to at least one of the other transfer bias members are individually output by constant voltage control. Configure a power supply for transfer bias,
The feature is that the output target value in the constant voltage control of the transfer bias for applying to the transfer bias member arranged at the most downstream is lower than the output target value of the transfer bias for applying to the other transfer bias member. Image forming apparatus. The image forming apparatus according to any one of claims 1 to 6.
Among the plurality of the image carriers, the volume resistivity of the toner for forming the toner image on the surface of the image carrier arranged at the most downstream in the order of the transfer step is the surface of the other image carrier. An image forming apparatus characterized in that the volume resistivity of the toner for forming a toner image is lower than that of the toner. The image forming apparatus according to any one of claims 1 to 7.
Among the plurality of the image carriers, the dielectric constant of the toner for forming the toner image on the surface of the image carrier arranged at the most downstream in the order of the transfer step is set on the surface of the other image carriers. An image forming apparatus characterized in that it has a higher dielectric constant than the toner for forming a toner image. The image forming apparatus according to claim 7 or 8.
An image forming apparatus, characterized in that the toner for forming a toner image on the image carrier arranged at the most downstream is black toner. The image forming apparatus according to any one of claims 1 to 9.
An image forming apparatus characterized in that the surfaces of the plurality of transfer bias members are all made of foamed sponge. The image forming apparatus according to any one of claims 1 to 9.
An image forming apparatus characterized in that the surfaces of the plurality of transfer bias members are all made of rubber. The image forming apparatus according to claim 10 or 11.
An image forming apparatus, wherein the volume resistivity of the transfer bias member disposed most downstream in the order of the transfer step is less than 10 7 ^[Ω · cm]. The image forming apparatus according to any one of claims 1 to 9.
An image forming apparatus characterized in that the transfer bias member arranged at the most downstream in the order of transfer steps is made of metal. In the image forming apparatus according to any one of claims 1 to 13.
An image forming apparatus characterized in that the diameter of the image carrier arranged at the most downstream of the plurality of image carriers in the order of transfer steps is made larger than the diameter of the other image carriers. The step of moving the endless belt member endlessly so as to sequentially contact a plurality of image carriers and form a transfer nip at each contact position, and the belt member individually between the plurality of image carriers. An image of performing a step of applying a transfer bias to each of a plurality of sandwiched transfer bias members and a step of transferring a toner image on the image carrier to the surface of the belt member at each of the plurality of transfer nips. In the forming method
As the belt member, a multi-layer structure in which at least one layer is laminated on the front surface side of the belt substrate is used.
And, among the plurality of transfer bias member, as the transfer bias member disposed most downstream in the order of the transfer process, the electric resistance is used of less than 10 7 ^[Omega], all other of the transfer as a bias member, the image forming method of electrical resistance, characterized in that used as the 10 7 ^[Omega] or more.