JP4676287B2 - Image forming apparatus - Google Patents

Description

The present invention copier using a band electrical location for uniformly charging the image bearing member such as sensitive light body, a facsimile, an image forming apparatus such as a printer.

  2. Description of the Related Art Conventionally, as such a charging device, a device that uniformly charges a photosensitive member by discharging from a charging roller that rotates while being in contact with the photosensitive member that is a charged member is known. The charging device described in Patent Document 1 is one of them. This charging device uniformly charges the photosensitive member by discharging from a charging roller to which a superimposed voltage in which an AC voltage is superimposed on a DC voltage is applied. After the electrostatic latent image is formed on the charged photoconductor, the electrostatic latent image is developed by adhesion of toner. The toner image obtained by the development is transferred from the surface of the photosensitive member to a transfer paper or an intermediate transfer member. The charging device described in Patent Document 1 serves to neutralize the surface of the photoconductor after the transfer process by switching the voltage applied to the charging roller from the above superimposed voltage to an AC voltage including only an AC component. Also bears. In such a configuration, discharge products such as ozone and nitrogen oxides can be reduced as compared with a charging device that uniformly charges an object to be charged by a charger method such as Scorotron. Further, the photoconductor can be uniformly charged as compared with a configuration in which only a DC voltage is applied to the charging roller.

  However, in this charging device, a transfer roller that adheres to the surface of the photosensitive member after passing through a transfer position for transferring the toner image to a transfer member such as transfer paper is charged with a charging roller that is in contact with the photosensitive member. It will be dislocated to. This dislocation easily causes toner contamination on the charging roller.

  On the other hand, conventionally, as described in Patent Document 2, for example, charging that uniformly charges a photosensitive member by discharging from a charging roller opposed to a charged member such as a photosensitive member through a predetermined gap. Devices are also known. Also in this charging device, discharge products such as ozone and nitrogen oxide can be reduced as in the charging device described in Patent Document 1. In addition, since the charging roller is disposed in a non-contact manner with respect to the photoconductor, toner contamination on the charging roller due to transfer of transfer residual toner from the photoconductor can be suppressed.

  In this charging device, a discharge member such as a discharge roller portion of a charging roller that faces a member to be charged such as a photosensitive member with a predetermined gap needs to be made of a material that exhibits a certain degree of conductivity. . Such materials are broadly divided into materials in which an electric resistance adjusting material made of carbon powder is dispersed in a material such as resin or rubber, and materials in which an ion conductive material such as a polyolefin polymer compound is contained in the material. Can be separated. Among these, the discharge member made of the former material requires the material to be hardened or plasticized in a state where the carbon powder is uniformly dispersed in the material. is there. On the other hand, the discharge member made of the latter material uses an ionic conductive material that is easy to adjust to the material, and therefore, there are few restrictions on the timing for curing or plasticizing the material. For this reason, cost reduction can be attained compared with the discharge member by the former material.

Japanese Patent Publication No. 5-24515 JP 2004-198621 A

  Therefore, from the viewpoint of cost reduction, it is desirable to employ a discharge member made of the latter material. However, such a discharge member has a large fluctuation in electrical resistance over time as compared with the discharge member made of the former material, and thus it has been difficult to stabilize the charged potential of the member to be charged for a long period of time. This became a bottleneck, and conventionally, an expensive discharge member made of the former material had to be adopted.

The present invention has been made in view of the above background, and has as its object to provide a following image imaging apparatus. That is, the image forming apparatus can charge the latent image carrier at a stable potential over a long period of time while using a discharge member that is less expensive than a discharge member made of a material in which carbon powder is dispersed.

In order to achieve the above object, the invention of claim 1 includes a latent image carrier that carries a latent image on a surface that moves endlessly, a charging device that uniformly charges the surface, and the surface after uniform charging. A latent image forming means for forming a latent image on the surface, and a developing means for developing the latent image on the surface. A discharge member that discharges from the surface toward the latent image carrier while facing the gap, a power source that supplies a voltage to the discharge member, and a power source control unit that controls output of the voltage from the power source In the image forming apparatus, in which the latent image carrier is uniformly charged by discharge from the discharge member, the discharge member is made of a material containing an ion conductive electrical resistance adjusting material. A DC voltage and an AC voltage as the power source. When possible, and when at least the latent image formation target area of the entire area of the surface of the latent image bearing member enters the position facing the discharge member, an AC voltage is superimposed on the DC voltage. When the superimposed voltage is output from the power source, and the formation target region is not entered into the facing position, an AC voltage consisting only of an AC component having a frequency higher than the AC voltage of the superimposed voltage is output from the power source. The power supply control means is configured so as to execute the processing.
According to a second aspect of the present invention, a latent image carrier that carries a latent image on a surface that moves endlessly, a charging device that uniformly charges the surface, and a latent image formed on the surface after uniform charging. A latent image forming unit; and a developing unit that develops the latent image on the surface, and the charging device has a surface on which the endless movement is opposed to the latent image carrier with a predetermined gap. A discharge member that discharges from the surface toward the latent image carrier, a power source that supplies a voltage to the discharge member, and a power source control unit that controls the output of the voltage from the power source. In the image forming apparatus for uniformly charging the latent image carrier by discharge from a member, the discharge member is made of a material containing an ion conductive electrical resistance adjusting material, and the power source As that which can output DC voltage and AC voltage, When at least the latent image formation target area of the entire area of the surface of the latent image carrier is made to enter a position facing the discharge member, a superimposed voltage obtained by superimposing an AC voltage on a DC voltage is applied to the power source. On the other hand, when the formation target region has not entered the facing position, an AC voltage consisting only of an AC component having a peak-to-peak value larger than the AC voltage of the superimposed voltage is output from the power source. The power supply control means is configured to perform processing .
Also, the invention of claim 3 is the image forming apparatus according to claim 1 or 2, provided a current detecting means for detecting an output current value from the power supply, based on the detection result of said current detecting means, the AC component The power supply control means is configured to perform control to change at least one of the frequency and peak-to-peak value of the AC voltage consisting of only the above.
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the charging device is used as a neutralizing unit that neutralizes the latent image carrier.

  In these inventions, the object to be charged can be charged with a stable potential over a long period of time while using a discharge member made of a material containing an ionic conductive material for the reason described below. That is, the present inventor has prepared a charging device testing machine having a charging roller that is opposed to a photoreceptor to be charged with a predetermined gap. In addition, a printer testing machine was also prepared in which a toner image was formed on a photoconductor uniformly charged by the charging device testing machine and transferred to transfer paper. Then, a test image is printed on the transfer paper while applying a superimposed voltage obtained by superimposing an alternating voltage on a direct current voltage to the discharge roller portion, which is a discharge member provided on the charging roller, to uniformly charge the photosensitive member. The experiment was conducted over a long period of 96 hours, and the change in the electrical resistance value of the discharge roller during that period was observed. From this experiment, it was confirmed that the electrical resistance value of the discharge roller portion increased with time. On the other hand, the present inventor interrupts the printout operation at a predetermined time interval, and only at that time, the voltage applied to the discharge roller unit is switched from the superimposed voltage to the AC voltage including only the AC component. In the same way as the above experiment, an experiment for printing out a test image was also performed. Surprisingly, in this experiment, the electrical resistance of the discharge roller portion was hardly changed over a long period of 96 hours. That is, the present inventor, based on these experiments, applied the superimposed bias to the discharge member such as the discharge roller unit to uniformly charge the charged body, and applied voltage to the charged body from the superimposed voltage. It has been found that fluctuations in electrical resistance over time of a discharge member made of a material containing an ionic conductive material can be suppressed by periodically switching to an AC component consisting of only an AC component. Therefore, in the present invention capable of switching the supply voltage for the discharge member from the superimposed voltage to the AC voltage, by periodically switching the supply voltage for the discharge member containing the ionic conductive material from the superimposed voltage to the AC voltage, The fluctuation of electric resistance of the discharge member over time is suppressed. As a result, the object to be charged can be charged at a stable potential over a long period of time while using a discharge member that is less expensive than a discharge member made of a material in which carbon powder is dispersed.

Next, an image forming apparatus according to the present invention, a color laser printer of electrophotographic system (hereinafter, simply referred to as a printer) will be described first embodiment.
First, a description will be given of the basic configuration of the printer according to the first embodiment. FIG. 1 is a schematic configuration diagram illustrating a printer according to the present embodiment. This printer includes four sets of toner image forming portions 1Y, M, C, and K for forming toner images of respective colors of yellow (Y), magenta (M), cyan (C), and black (K). Yes. Further, the optical writing unit 50, the first paper feeding cassette 51, the second paper feeding cassette 52, the paper feeding path 53, the registration roller pair 54, the transfer unit 60, the belt fixing type fixing device 80, the switchback device 90, A sheet feeding device 100, a stack unit 110, and the like are also provided. Furthermore, a waste toner bottle 111, a toner supply container (not shown), a power supply unit, and the like are also provided. Hereinafter, the subscripts Y, M, C, and K of the respective symbols indicate members for yellow, magenta, cyan, and black, respectively.

  The optical writing unit 50 includes a light source composed of four laser diodes corresponding to each color of Y, M, C, and K, a regular hexahedral polygon mirror, a polygon motor, an fθ lens, a lens, and a reflection for rotating the light. It has a mirror. The laser light L emitted from the laser diode reaches any one of four photoconductors described later while being reflected by any one surface of the polygon mirror and deflected as the polygon mirror rotates. The surfaces of the four photosensitive members are optically scanned by the laser beams L emitted from the four laser diodes, respectively.

  The toner image forming portions 1Y, 1M, 1C, and 1K correspond to process units 2Y, 2M, 2C, and 3K having drum-shaped photoreceptors 3Y, 3M, 3C, and 3K as image carriers, respectively. Developing devices 40Y, 40M, 40C, and 40K are provided. The photoreceptors 3Y, 3M, 3C, and 3K are drums having a diameter of about 60 [mm] in which an organic photosensitive layer is coated on a base tube made of aluminum or the like, and are rotated clockwise in the drawing at a predetermined linear velocity by a driving unit (not shown). It can be driven to rotate. The optical writing unit 50 that emits laser light L modulated based on image information sent from a personal computer (not shown) is optically scanned in the dark for Y, M, C, and K. Carries an electrostatic latent image.

  FIG. 2 is an enlarged configuration diagram illustrating the toner image forming unit 1Y for Y of the four toner image forming units 1Y, M, C, and K together with a part of the transfer unit (60 in FIG. 1). FIG. 3 is an enlarged configuration diagram showing the Y process unit 2Y among the four process units 2Y, 2M, 2C, and 2K. The other color toner image forming portions (1M, C, K) and the process units (2M, C, K) have the same configuration as that for Y except that the toner colors used are different. Therefore, these descriptions are omitted.

  In FIG. 2, the Y toner image forming unit 1Y includes a process unit 2Y and a developing device 40Y. In addition to the photoreceptor 3Y, the process unit 2Y has a brush roller 4Y for applying a lubricant, a swingable counter blade 5Y for performing a cleaning process, a charge removal lamp 6Y for performing a charge removal process, and the like. Yes. Further, a charging device 10Y for uniformly charging the photoreceptor 3Y is also provided.

As the photoconductor 3Y, a photoconductor having a two-layer structure in which a charge generation layer and a charge transport layer are sequentially stacked on a conductive support can be used. As the conductive support, a metal tube such as aluminum or stainless steel having a volume resistivity of 10 ⁴ [Ω · cm] or less, or a metal such as nickel processed into an endless belt shape can be used.

  An undercoat layer may be formed between the charge generation layer and the conductive support as necessary. As an undercoat layer, a resin-based material is generally used. However, considering that the resin is dissolved in a solvent and applied, a material having high resistance to general organic solvents should be used. Is desirable. Examples of such resins include water-soluble resins such as polyvinyl alcohol, alcohol-soluble resins such as copolymer nylon, and curable resins that form a three-dimensional network structure such as polyurethane, alkyd-melamine, and epoxy. .

  For the purpose of preventing moire and reducing residual potential, fine powders of metal oxides such as titanium oxide, silica, and alumina may be added to the undercoat layer. Such an undercoat layer can be formed using an appropriate solvent and coating method. The thickness of the undercoat layer is suitably from 0 to 5 [μm].

  The charge generation layer is a layer mainly composed of a charge generation material. Examples of the charge generation material include monoazo pigments, disazo pigments, trisazo pigments, and phthalocyanine pigments. The charge generation layer can be formed by dispersing these charge generation materials in a solvent such as tetrahydrofuran and cyclohexanone together with a binder resin such as polycarbonate and applying the dispersion. About application | coating, it is possible to carry out by the dip coating method, spray coating, etc. The film thickness of the charge generation layer is suitably about 0.01 to 5 [μm].

  The charge transport layer is a layer mainly composed of a charge transport material and a binder resin. The charge transport layer can be formed by dissolving or dispersing these in a suitable solvent such as tetrahydrofuran, toluene, dichloroethane, and applying and drying the solution. Charge transport materials are roughly classified into electron transport materials and hole transport materials. Examples of the electron transport material include chloroanil, bromoanil, tetracyanoethylene, tetracyanoquinodimethane, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 1, An electron accepting substance such as 3,7-trinitrodibenzothiophene-5,5-dioxide can be exemplified. Examples of hole transport materials include oxazole derivatives, oxadiazole derivatives, imidazole derivatives, triphenylamine derivatives, phenylhydrazones, α-phenylstilbene derivatives, thiazole derivatives, triazole derivatives, phenazine derivatives, acridine derivatives, thiophene derivatives, etc. Examples of the electron donating substance can be exemplified. The binder resin used in the charge transport layer together with the charge transport material is thermoplastic such as polystyrene, styrene-acrylonitrile copolymer, styrene-butadiene copolymer, polyester, polyarylate, polycarbonate, acrylic, epoxy, melamine, phenol, etc. Or a thermosetting resin can be illustrated. The thickness of the charge transport layer is suitably in the range of 15 to 30 [μm], and an arbitrary thickness may be selected from this range as appropriate in accordance with desired photoreceptor characteristics.

  A protective layer can be formed on the surface of the Y photoreceptor 3Y for the purpose of protecting the photosensitive layer and improving durability. Examples of the configuration of the protective layer include a configuration in which metal oxide fine particles are dispersed in a binder resin, a configuration using a heat or photocrosslinkable resin, and the like.

  The Y photoconductor 3Y is uniformly charged to a negative polarity whose peripheral surface is the same as that of the toner by the Y charging device 10Y while being rotated in the clockwise direction in the drawing by a driving means (not shown). It is done. An electrostatic latent image for Y is formed on the surface of the Y photoreceptor 3Y that is uniformly charged in this manner by optical scanning by the optical writing unit (50) described above. Is developed into a Y toner image by the Y developing device 40Y.

  The developing device 40Y for Y has a developing roll 42Y that exposes a part of the peripheral surface from an opening provided in the casing 41Y. Further, it also includes a first transport screw 43Y, a second transport screw 44Y, a developing doctor 45Y, a toner concentration sensor (hereinafter referred to as T sensor) 46Y, and the like. The developing roll 42Y, the first transport screw 43Y, and the second transport screw 44Y are each driven to rotate in the direction of the arrow in the drawing by a driving unit (not shown). The developing roll 42Y includes a developing sleeve made of a non-magnetic pipe that is driven to rotate by a driving unit (not shown), and a magnet roller (not shown) that is included so as not to rotate.

  The casing 41Y contains a Y developer containing a magnetic carrier and negatively chargeable Y toner. The Y developer is frictionally charged while being agitated and conveyed by the first conveying screw 43Y and the second conveying screw 44Y, and is then adsorbed on the surface of the developing sleeve rotated by the developing roller 42Y by the magnetic force of the magnet roller in the developing roller 42Y. And pumped up. Then, after the layer thickness is regulated by the developing doctor 45Y, the layer is conveyed to a developing area facing the photoreceptor 3Y, where Y toner is attached to the electrostatic latent image on the photoreceptor 3Y. By this adhesion, the electrostatic latent image on the photoreceptor 3Y is developed into a Y toner image. The Y developer that has consumed Y toner by the development is returned to the casing 41Y as the surface of the developing roll 42Y (developing sleeve) rotates. The Y toner image on the photoreceptor 3Y is transferred to a transfer paper P that is conveyed by a paper conveyance belt 61 of a transfer unit described later.

  The T sensor 46Y composed of a magnetic permeability sensor is fixed to the bottom plate of the casing 41Y, and outputs a voltage having a value corresponding to the magnetic permeability of the Y developer conveyed by the first conveying screw 43Y. Since the magnetic permeability of the developer shows a good correlation with the toner density of the developer, the T sensor 46Y outputs a voltage having a value corresponding to the Y toner density. The value of the output voltage is sent to a toner supply control unit (not shown). This toner replenishment control unit is provided with storage means such as a RAM, in which Y Vtref, which is a target value of the output voltage from the Y T sensor 46Y, and a T sensor mounted in another developing device. The data of Vtref for M, C and K, which is the target value of the output voltage from is stored. For the developing device 40Y for Y, the value of the output voltage from the T sensor 46Y is compared with the Vtref for Y, and the Y powder pump 120Y connected to the Y toner cartridge (not shown) is driven for a time corresponding to the comparison result. Let As a result, the Y toner in the Y toner cartridge is supplied from the powder pump 120Y into the developing device 40Y. By controlling the driving of the powder pump (toner replenishment control) in this way, an appropriate amount of Y toner is replenished to the Y developer whose Y toner density has been reduced as a result of development, and development in the developing device 50Y is performed. The Y toner concentration of the agent is maintained within a predetermined range. Similar toner replenishment control is performed for other developing devices.

  Each color toner of Y, M, C, and K has a binder resin, a colorant, and a charge control agent as main components, and other additives are added as necessary. As the binder resin, polystyrene, styrene-acrylic acid ester copolymer, polyester resin, or the like can be used.

  As the coloring material (for example, yellow, magenta, cyan, and black), conventionally known materials can be used. The amount of the coloring material is suitably 0.1 to 15 parts by weight with respect to 100 parts by weight of the binder resin.

  As the charge control agent, a nigrosine dye, a chromium-containing complex, a quaternary ammonium salt, or the like can be used, and these are properly used depending on the polarity of the toner particles. The addition amount of the charge control agent is 0.1 to 10 parts by weight with respect to 100 parts by weight of the binder resin.

  It is desirable to add a fluidity imparting agent to the toner particles. Examples of the fluidity-imparting agent include fine particles of metal oxides such as silica, titania and alumina, and those obtained by surface-treating these fine particles with a silane coupling agent, titanate coupling agent, etc., polystyrene, polymethyl methacrylate, polyvinylidene fluoride. Polymer fine particles such as can be used. These fluidity imparting agents preferably have a particle size in the range of 0.01 to 3 μm. The addition amount of the fluidity imparting agent is preferably in the range of 0.1 to 7.0 parts by weight with respect to 100 parts by weight of the toner particles.

  As a method for producing the toner, various known methods and methods combining them can be used. For example, in the kneading and pulverization method, first, a dry mixture of a binder resin, a colorant (for example, carbon black) and other necessary additives is heated with an extruder, a two-roll, a three-roll, etc. Melt and knead. Then, after cooling and solidifying, it is pulverized by a pulverizer such as a jet mill, and then classified by an airflow classifier to obtain a toner. It is also possible to produce toner from monomers, colorants, additives, etc. by suspension polymerization or non-aqueous dispersion polymerization.

  As a developer carrier, a carrier made of only a core material or a core material provided with a coating layer is generally used. As the core material of the resin-coated carrier, ferrite, magnetite or the like having a particle size of about 20 to 60 μm can be used. Examples of the material for the coating layer on the core material include vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinyl ether substituted with fluorine atoms, vinyl ketone substituted with fluorine atoms, and the like. can do. Examples of the method for forming the coating layer include a method in which a resin is applied to the surface of the core material particles by a spraying method, a dipping method, or the like, as in the past.

  As shown in FIG. 3, the Y process unit 2Y includes a Y photoconductor 3Y, a brush roller 4Y, a counter blade 5Y, a static elimination lamp 6Y, a coiled auger 7Y, and the like as a unit on the printer body. On the other hand, it is supported by a common support so as to be detachable. The surface of the photoreceptor 3Y after the Y toner image is transferred to a transfer paper (not shown) enters a contact position with the brush roller 4Y that is rotated by a driving means (not shown). The brush roller 4Y is applied to the surface of the photoreceptor 3Y while scraping off the lubricant from a lubricant solid such as a zinc stearate lump (not shown). The surface of the photoreceptor 3Y after the lubricant is applied enters the contact position with the counter blade 5Y. Then, the transfer residual toner is scraped off by the counter blade 5Y. The transfer residual toner scraped off is conveyed toward the coiled auger 7Y by the brush roller 4Y, and is then orthogonally crossed in the process unit 2Y by the coiled auger 7Y rotated by a driving means (not shown). Conveyed in the direction. A waste toner discharge port (not shown) is formed at an end portion of the casing of the process unit 2Y in a direction orthogonal to the drawing surface. The transfer residual toner conveyed by the coiled auger 7Y is discharged as waste toner. It is discharged from the exit. Then, it falls into the waste toner bottle 111 shown in FIG.

  The surface of the photoreceptor 3Y from which the transfer residual toner has been scraped off by the counter blade 5Y is neutralized by light irradiation by the neutralizing lamp 6Y and then uniformly charged by the charging device 10Y. The charging device tail 10Y will be described in detail later.

  In the Y process unit 2Y configured to be detachable from the printer main body, the rotation shaft member 3aY of the photoreceptor 3Y is a main positioning portion in the printer main body. By engaging one end portion of the rotation shaft member 3aY in the axial direction with the engagement hole on the printer main body side, the process unit 2Y is positioned in the vertical and horizontal directions on the drawing sheet. Further, the casing of the process unit 2Y is provided with a first quasi-position positioning portion 8Y and a second quasi-positioning portion 9Y, which are brought into contact with or engaged with the printer body. Positioning in the direction perpendicular to the drawing surface in the printer main body is performed.

  The process unit is a unit that supports a latent image carrier such as a photosensitive member and at least one member or apparatus disposed around the latent image carrier with a single support. Regarding the process unit 3Y according to the present invention, it is desirable that the unit is provided with a charging device 10Y as at least one device. By doing so, a charging roller, which will be described later, is supported by a single support together with the photosensitive member, and a charging gap, which will be described later, can be maintained with high accuracy. Members and devices other than the photoreceptor 3Y and the charging device 10Y shown in FIG. 3 may or may not be included in the process unit 2Y. Further, a member or apparatus that is not provided in the process unit 2Y illustrated in FIG. 3 may be provided in the process unit 2Y. For example, the developing device 40Y shown in FIG. 2 may be provided in the process unit 2Y.

  Of the four toner image forming units 1Y, 1M, 1C, and 1K shown in FIG. 1, the toner image forming unit 1Y for Y has been described with reference to FIGS. Since the parts 1M, C, and K have the same configuration, the description thereof is omitted.

  In FIG. 1, a transfer unit 60 is disposed below the toner image forming portions 1Y, 1M, 1C, and 1K of each color in the drawing. The transfer unit 60 moves the endless paper transport belt 61 endlessly in the counterclockwise direction in the drawing while being stretched by a plurality of stretching rollers. Specifically, the plurality of stretching rollers are an entrance roller 62, a minute roller 63, a driving roller 64, a tension roller 65, and four transfer bias rollers 66Y, 66M, 66C, 66K. In addition to these, the transfer unit 60 also includes an electrostatic adsorption roller 67, a bracket (not shown), and the like.

  Examples of the material of the paper transport belt 61 include resin materials such as polyvinylidene fluoride, polyimide, polycarbonate, and polyethylene terephthalate. These resin materials are molded into a seamless belt and used. However, it is desirable not to use the resin material as it is, but to adjust the electrical resistance by dispersing a conductive material such as carbon black. Moreover, a surface layer or the like may be laminated on a belt base by molding these resin materials into a seamless belt shape by a method such as spraying or dipping.

  The entrance roller 62, the transfer bias rollers 66Y to 66K, the separation roller 63, the drive roller 64, and the tension roller 65 are all in contact with the back surface (loop inner peripheral surface) of the paper transport belt 61. Among these rollers, the entrance roller 62 disposed on the rightmost side in the drawing is configured such that the paper transport belt 61 is sandwiched between the entrance roller 62 and the electrostatic adsorption roller 67 disposed above the entrance roller 62. The electrostatic attraction roller 67 is fed from a registration roller pair 54 described later by applying a positive charge to the front surface of the paper transport belt 61 by an electrostatic attraction bias applied from a power source (not shown). The transfer paper P is electrostatically attracted to the belt front surface.

  The four transfer bias rollers 66Y, 66M, 66C, and 66K are rollers in which a metal cored bar is covered with an elastic body such as a sponge, and Y, M, C, and K photoconductors 3Y, M, C, and K, respectively. The paper conveying belt 61 is sandwiched by being pressed toward K. By this pressing, four transfer nips for Y, M, C, and K are formed in which the four photoreceptors 3Y, 3M, 3C, and 3K and the paper transport belt 61 are in contact with each other with a predetermined length in the belt moving direction. Yes. A transfer bias controlled at a constant current by a transfer bias power source (not shown) is applied to the cores of the four transfer bias rollers 66Y, 66M, 66C, 66K. As a result, transfer charges are applied to the back surface of the paper transport belt 61 via the four transfer bias rollers 66Y, 66M, 66C, 66K, and the paper transport belt 61 and the photoreceptors 3Y, 3M, 3C, and 3K at each transfer nip. A transfer electric field is formed between the two. In this printer, the transfer bias rollers 66Y, 66M, 66C, and 66K are provided as transfer means. However, a brush, a blade, or the like may be used instead of the rollers. A transfer charger or the like may be used.

  Of the four transfer bias rollers 66Y, 66M, 66C, and 66K, three for Y, M, and C are each supported by a swing bracket (not shown) via a bearing member (not shown). The swing bracket is disposed inside the loop of the paper transport belt 61 and can swing about a rotation shaft (not shown). By this swing, the three transfer bias rollers 66Y, 66M, 66C move, and the tension posture of the paper conveying belt 61 causes the belt to contact only the K photoconductor 3K among the four photoconductors. It may be in a posture or a posture in contact with each of the four photoconductors.

  A first paper feed cassette 51 and a second paper feed cassette 52 are disposed below the printer main body so as to overlap in the vertical direction. These paper feed cassettes store a plurality of transfer papers P in a stack of transfer papers, and press the paper feed belts (51a, 52a) against the uppermost transfer paper P. Then, the sheet feeding belt is moved endlessly at a predetermined timing, and the transfer sheet P is sent out to the sheet feeding path 53. In the middle of the paper feed path 53, a plurality of transport roller pairs are arranged, and the transfer paper P is transported to the vicinity of the end of the paper feed path while sequentially passing between the rollers of the transport roller pairs. . A registration roller pair 54 is disposed near the end of the paper feed path 53. The registration roller pair 54 rotates both rollers in order to sandwich the transfer paper P sent from the paper feed cassette between the rollers. However, as soon as the leading edge of the transfer paper P is sandwiched, both rollers rotate. Stop. Then, the transfer paper P is sent out toward the transfer unit 60 at the timing when the transfer paper P can be synchronized with the Y toner image on the Y photoconductor 3Y at the Y transfer nip.

  This printer performs the following printing operation when printing out a full-color image. That is, when full-color image data sent from an external personal computer (not shown) or the like is received, all of the four photoconductors 3Y, 3M, 3C, and 3K with respect to the paper transport belt 61 are driven by the above-described swing bracket. These photoconductors are each driven to rotate in the clockwise direction in FIG. Then, toner images are formed on the photoreceptors by the toner image forming portions 1Y, 1M, 1C, and 1K, respectively, and the transfer paper P is sent out from the registration roller pair 54 at a predetermined timing. Next, while feeding the transferred transfer paper P to the front surface of the paper transport belt 61, the transfer paper P is transported from the lower right to the upper left in the drawing along with the endless movement of the belt, and is used for Y, M, C, and K. Feeds sequentially to the transfer nip. As a result, the Y, M, C, and K toner images on the photoreceptors 3Y, 3M, 3C, and 3K are transferred to the transfer paper P while being superimposed on the transfer paper P, and a full color image is obtained on the transfer paper P. Then, the transfer paper P after full-color image formation is transported to the belt stretching position by the separation roller 63 as the paper transport belt 61 moves endlessly. At this belt stretching position, the paper transport belt 61 is wound around the separation roller 63 at a steep winding angle that substantially reverses the belt moving direction. Due to this sudden change in the moving direction, the transfer paper P adsorbed on the paper transport belt 61 is separated from the paper transport belt 61 and sent to the fixing device 80.

  The fixing device 80 includes a pressure roller 81, a fixing belt 82, a heating roller 83, a driving roller 84, and the like. The fixing belt 82 is endlessly moved clockwise in the figure by a driving roller 84 that is rotated by a driving unit (not shown) while being stretched by a heating roller 83 and a driving roller 84. The heating roller 83 serving as a heating means includes a heat source such as a halogen lamp, and thereby heats the fixing belt 82 from the back surface. On the other hand, the pressure roller 81 as a contact roller rotates to move the surface in the same manner as the belt at the contact portion while making contact with the fixing belt 82 that is moved endlessly to form a fixing nip. The transfer paper P transferred from the paper transport belt 61 of the transfer unit 60 to the fixing device 80 is sandwiched between the fixing nips in such a posture that the image transfer surface is in contact with the fixing belt 82. Then, it passes through the fixing device 80 while a full-color image is fixed on the image transfer surface by heating or pressing.

  The transfer paper P discharged from the fixing device 80 enters the paper discharge path 55. The subsequent transport path of the transfer paper P differs depending on the print mode. Specifically, in the single-sided mode in which an image is formed only on one side of the transfer paper P, the transfer paper P is conveyed forward in the paper discharge path 55 and discharged at the end of the paper discharge path 55. Via a pair of paper rollers 56, the paper is stacked on a stack unit 110 provided on the upper surface of the printer housing. On the other hand, in the double-side mode in which images are formed on both sides of the transfer paper P and an image is still formed on only one side, the rear end of the transfer paper P is within the discharge path 55 to some extent. Until the paper enters, the transfer paper P is conveyed forward in the paper discharge path 55. Thereafter, a plurality of conveyance roller pairs disposed in the paper discharge path 55 respectively rotate the rollers in the reverse direction to convey the transfer paper P in the reverse direction in the paper discharge path 55. Then, the transfer paper P is fed into the switchback device 90 from the rear end side.

  The switchback device 90 includes a switchback path 91 extending in the vertical direction and a plurality of pairs of conveying rollers disposed in the switchback path 91. Then, the transfer paper P received from the paper discharge path 55 is conveyed in the forward direction from the upper side to the lower side in the switchback path 91. Near one end of the switchback path 91, one end of the retransmission branch path 112 is connected, and the other end of the retransmission branch path 112 is connected to a refeed device 100 described later. The transfer paper P passes between the rollers of the most downstream conveying roller pair 92 in the switchback path 91 until the trailing edge just passes between the rollers of the most downstream conveying roller pair 92. It is conveyed in the forward direction in the switchback path 91. Thereafter, the most downstream transport roller pair 92 in the switchback path 91 and the branch transport roller pair 93 disposed in the vicinity of the retransmission branch path 112 respectively rotate the rollers in reverse. As a result, the transfer paper P is conveyed in the reverse direction in the switchback path 91 with the leading end at the top, and enters the retransmission branch path 112 via the branch conveyance roller pair 93. Then, the paper is sent toward the paper refeed device 100.

  The refeed device 100 is disposed between the transfer unit 60 and the waste toner bottle 111 in the vertical direction. The sheet feeding path 101 extends from the upper left to the lower left in the figure, and has a plurality of conveying roller pairs disposed therein at a predetermined pitch, and the transfer paper P received from the switchback device 90 is fed. Re-carry toward the paper path 53. The transfer paper P that has been sent again into the paper feed path 53 by this re-conveyance is sent toward the transfer unit 60 by the registration roller pair 54 at a predetermined timing. At this time, the orientation in which the surface on which the full-color image has not yet been formed (hereinafter referred to as the second surface) is directed to each of the toner image forming portions 1Y, 1M, 1C, and 1K by the switchback in the previous switchback device 90. Sent by. Then, after the full color image is also transferred to the second surface by the transfer unit 60, it is discharged to the stack unit 110 via the fixing device 80, the paper discharge path 55, and the paper discharge roller pair 56 in order.

  In the continuous print mode in which images are continuously formed on a plurality of transfer papers P, a continuous print operation for forming a full color image on one or both sides of each of the plurality of transfer papers P by repeating the processes described so far. Is done.

  A manual feed tray 113 that can be opened and closed with respect to the housing is provided on the left side surface of the printer housing in the drawing. When this manual feed tray 113 is rotated in the direction of arrow A in the drawing to be opened with respect to the printer housing, the manual feed tray 113 directs the paper placement surface upward in the vertical direction. If the transfer paper P is placed on the paper placement surface, the transfer paper P can be fed from the manual feed tray 113 toward the paper feed path 53.

  This printer performs the following printing operation when printing out a monochrome image. That is, when monochrome image data sent from an external personal computer (not shown) or the like is received, among the four photosensitive members 3Y, 3M, 3C, and 3K with respect to the paper transport belt 61 by driving the swing bracket described above. Then, only the K photoconductor 3K is rotated in the clockwise direction in the drawing while taking the posture of contacting only the K photoconductor 3K. In the case of a monochrome image, the toner image is not transferred at the transfer nips for Y, M, and C, so that the transfer nips are not formed. As a result, a monochrome image can be printed out without applying extra load to the paper conveying belt 61, toner image forming units 1Y, 1M, 1C, and 1C (especially photoconductors) other than those for K and its driving system.

  When driving of the K photoconductor 3K and the paper conveyance belt 61 is started, a toner image is formed on the photoconductor 3K by the toner image forming unit 1K, and the transfer paper P is sent out from the registration roller pair 54 at a predetermined timing. The electrostatic attraction roller 67 electrostatically attracts the paper transport belt 61. Next, while the transferred transfer paper P is held on the front surface of the paper transport belt 61, it is transported from the lower right to the upper left in the figure along with the endless movement of the belt, and is sequentially fed to the transfer nip for K. As a result, the K toner image on the K photoconductor 3K is transferred to the transfer paper P at the K transfer nip, and a monochrome image is obtained on the transfer paper P. Then, after the transfer sheet P is sent to the fixing device 80, the transfer sheet P is discharged to the stack unit 110, and is switched back and fed again. In the continuous print mode, by repeating such a process, a continuous print operation for forming a monochrome image on one or both sides of the plurality of transfer papers P is performed.

  The charging device 10Y includes a cleaning roller 11Y and a charging roller 12Y as shown in FIG. In addition, a charging power supply circuit and a charging control circuit (not shown) are also provided. As shown in FIG. 4, a charging roller 12Y to which a superimposed voltage obtained by superimposing an AC voltage on a DC voltage is applied by a charging power supply circuit (not shown). The charging roller 12Y includes a shaft member 13Y, two butting rollers 15Y, and a discharge roller portion 16Y as a discharge member. Etc.

  A gear (not shown) is fixed to one end portion of the shaft member 13Y of the charging roller 12Y, which meshes with a gear formed on a flange (not shown) of the photoreceptor 3Y. When the photoconductor 3Y is rotationally driven by a photoconductor drive motor (not shown), the charging roller 12Y rotates at a linear speed substantially equal to that of the photoconductor 3Y.

  The shaft member 13Y serves as a core of the charging roller 12Y, and shaft protruding portions 14Y formed at both ends in the axial direction of the shaft member 13Y are rotatably supported by bearings (not shown). A superimposed voltage obtained by superimposing an AC voltage on a DC voltage is applied to the shaft member 13Y by a power source (not shown). On the surface of the central portion in the axial direction of the shaft member 13Y, a discharge roller portion 16Y made of an elastic and conductive material is formed over the entire circumference in the axial direction. Ring-shaped abutting rollers 15Y made of an insulating material are fixed by press-fitting and adhesion near both ends of the shaft member 13Y so as to sandwich the discharge roller portion 16Y. The outer diameter of these abutting rollers 15Y is larger by several tens to several hundreds [μm] than the outer diameter of the discharge roller portion 16Y. As shown in FIG. 5, the charging roller 12Y makes the discharge roller portion 16Y face the photoconductor 3Y with a predetermined charging gap while the abutting roller 15Y is in contact with the photoconductor 3Y. Then, the surface of the photosensitive member 3Y is uniformly charged by the discharge from the discharge roller portion 16Y while being rotated so as to move the surface thereof in the direction opposite to the surface movement of the photosensitive member 3Y by a driving means (not shown). If the charging gap becomes too wide, abnormal discharge occurs and it becomes difficult to uniformly charge the photoreceptor 3Y. Therefore, both the photoreceptor 3Y and the charging roller 12Y require high level dimensional accuracy and assembly accuracy. The

  A metal such as stainless steel is used as the material of the shaft member 13Y. If the shaft member 13Y is too thin, the amount of deflection at the time of cutting the discharge roller portion 16Y or when it is pressed toward the photoreceptor 3Y will increase, making it difficult to obtain the required charging gap accuracy. On the other hand, when the shaft member 13Y is too thick, the charging roller 12Y becomes larger or heavier. For these reasons, the diameter of the central portion of the shaft member 13Y is preferably about 6 to 10 [mm].

As a material for the discharge roller portion 16Y, it is desirable to use a material that exhibits a volume resistance of 10 ⁴ to 10 ⁹ [Ω · cm]. If the electric resistance of the discharge roller portion 16Y is too low, the discharge becomes non-uniform due to minute resistance unevenness of the discharge roller portion 16Y and it becomes easy to generate uneven charging on the photoreceptor 3Y. On the other hand, if the electric resistance is too high, a sufficient amount of discharge does not occur and a uniform charged potential cannot be obtained.

  About the discharge roller part 16Y, a desired volume resistance is obtained by mix | blending a conductive material with resin used as a base material. Examples of the resin serving as the base material include resins such as polyethylene, polypropylene, polymethyl methacrylate, polystyrene, acrylonitrile-butadiene-styrene copolymer (ABS), and polycarbonate. Since these resins have good moldability, they can be molded easily.

  A typical example of the conductive material for adjusting the electric resistance of the base material of the discharge roller portion 16Y is carbon black powder. However, since it is difficult to uniformly disperse a powdery conductive material such as carbon black in the resin, uneven resistance is likely to occur. In particular, when the discharge roller portion 16Y is disposed in a non-contact manner with respect to the photoreceptor 3Y as in this printer, uneven charging due to uneven resistance is likely to occur.

  As a conductive material, an ion conductive electrical resistance adjusting material (hereinafter referred to as an ion conductive material) made of an electrolyte salt such as a Li salt is also known. Since the ion conductive material can be easily dispersed uniformly in the resin, the production cost of the charging roller 12Y can be reduced by using the ion conductive material. Examples of the ion conductive material include polyether ester amides described in Patent Document 2. Such an ion conductive material is uniformly dispersed in a resin as a base material by means of a biaxial kneader, a kneader or the like. Then, the dispersed resin is coated on the shaft member 13Y by injection molding or extrusion molding to obtain a roller-shaped discharge roller portion 16Y.

  About the compounding ratio of an ion conductive material and resin, it is desirable to mix an ion conductive material in the ratio of 30-80 weight part with respect to 100 weight part resin. By adding a compatibilizing agent that improves the dispersion state of the ion conductive material in the resin, the dispersion state of the base resin and the ion conductive material can be made more uniform and dense.

  The thickness of the discharge roller portion 16Y on the shaft member 13Y is preferably about 0.5 to 3 [mm]. If the discharge roller portion 16Y is too thin, it is difficult to mold, and there is a problem in terms of strength. On the other hand, if the thickness is too large, the charging roller 12Y is enlarged, and the resistance of the discharge roller portion 16Y is increased to lower the charging efficiency.

  As the resin or ion conductive material used as the material of the discharge roller portion 16Y, it is not always necessary to use a material that is difficult to fix the toner. This is because, if necessary, a surface layer made of a fluororesin or the like having excellent toner releasability may be formed on the substrate with a thickness of about several tens of μm by coating or the like.

  After the discharge roller portion 16Y is formed on the surface of the central portion of the shaft member 13Y by injection molding or the like, the abutting rollers 15Y previously formed in a ring shape are press-fitted, bonded, or bonded to the surfaces near both ends of the shaft member 13Y, respectively. Fix both of them together. The method of fixing the abutting roller 15Y on the shaft member 12Y is not limited to press-fitting or bonding, but the layer for the abutting roller 15Y is molded together with the layer for the discharge roller portion Y on the shaft member 12Y by two-color molding. It is also possible to do.

  After the discharge roller portion 16Y and the two abutment rollers 15Y are fixed on the shaft member 13Y, the discharge roller portion 16Y and the abutment roller 15Y are cut while rotating the charging roller 12Y around the shaft member 12Y. Or grinding. As a result, the outer diameter of the discharge roller portion 16Y and the outer diameter of the abutting roller 15Y with the shaft member 12Y as the center are simultaneously adjusted to reduce the variation in the charging gap accompanying the rotation of the charging roller 12Y. be able to.

  The material of the butting roller 15Y is propylene, polybutene, polyisoprene, ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, ethylene-san vinyl acetate copolymer, ethylene-propylene copolymer, ethylene-hexene copolymer Resin such as coalescence can be exemplified. Further, similarly to the base material of the discharge roller portion 16Y, a resin such as polyethylene, polypropylene, polymethyl methacrylate, polystyrene, acrylonitrile-butadiene-styrene copolymer, and polycarbonate may be used. Examples of resin materials that do not easily damage the photosensitive layer of the photoreceptor 3Y due to excellent slidability include polyvinylidene fluoride, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer. Resin can also be used.

  For the purpose of processing the charging roller 12Y with high accuracy or reducing fluctuations in the charging gap due to changes in temperature and humidity in the usage environment, a material having a high hardness should be used as the material for the discharge roller portion 16Y and the abutting roller 15Y. Is desirable. However, if the hardness of the butting roller 15Y is too high, the photosensitive layer of the photoreceptor 3Y is likely to be deteriorated at the contact position with the roller, and the durability of the photoreceptor 3Y is reduced. For this reason, as a material for the discharge roller portion 16Y and the abutting roller 15Y, it is desirable to use a material having a JIS D hardness of about 45 to 75 degrees.

  As the material of the abutting roller 15Y, a conductive resin can be used similarly to the discharge roller portion 16Y. However, in order to prevent the photosensitive layer from being deteriorated by the discharge from the roller and the toner from adhering to the roller, insulation is used. It is desirable to use a functional material.

  When the discharge roller unit 16Y is opposed to the photoreceptor 3Y through a small charging gap as in this printer, the charging gap is generally changed slightly with the rotation of the charging roller 12Y. In order to charge the photosensitive member 3Y as uniformly as possible under such a condition in which such fluctuations occur, the voltage applied to the discharge roller unit 16Y is set to a DC voltage, and a discharge start voltage between the discharge roller unit 16Y and the photosensitive member 3Y. It is desirable to employ a superimposed voltage obtained by superimposing an AC voltage having an amplitude (peak-to-peak voltage) that is at least twice as large as the above. If the frequency of the alternating current component in this superposed voltage is too low, stripe-shaped charging unevenness becomes conspicuous, and therefore, the frequency [Hz] must be set to at least 6 times the linear velocity [mm / s] of the photoreceptor 3Y. desirable. Further, when the frequency of the AC component is too high, excessive discharge is generated to promote wear of the photoreceptor 3Y, or filming of toner or toner external additive is easily generated on the photoreceptor 3Y. Therefore, it is desirable to set the frequency [Hz] which is 12 times or less of the photosensitive member linear velocity [mm / s].

  The discharge roller portion 16Y of the charging roller 12Y is not in contact with the photoreceptor 3Y, but the toner on the photoreceptor 3Y may fly and adhere due to electrostatic force. The adhered toner is cleaned from the surface of the discharge roller portion 16Y by a cleaning roller 11Y (see FIG. 3) that rotates while contacting the discharge roller portion 16Y. The cleaning roller 11Y is a brush roller in which conductive fibers are electrostatically flocked on a metal core, and is in contact with the discharge roller portion 16Y of the charging roller 12Y by its own weight, and rotates with the rotation of the charging roller 12Y. .

  In the present printer having the above basic configuration, each of the four photoconductors 3Y, 3M, C, and K functions as a latent image carrier that carries a latent image on a surface that moves endlessly by rotation. Further, the optical writing unit 50 functions as a latent image forming unit that forms a latent image on the surface of the photoreceptor after being uniformly charged. In addition, each color developing device functions as a developing unit that develops a latent image on the surface of the photoreceptor.

Next, various experiments conducted by the inventor will be described.
Prior to the experiment, a charging roller for K was prepared as follows. That is, a resin obtained by dispersing 60 parts by weight of polyetheresteramide as an ion conductive material on a base resin made of 40 parts by weight of an ABS resin on a shaft member made of stainless steel having an outer diameter of 9 [mm]. The composition was injection-molded into a roller shape on a shaft member made of stainless steel and having an outer diameter of 9 [mm]. A surface layer composed of an acrylic silicon resin, an isocyanate curing agent, and carbon black was applied to the molded resin surface to produce a discharge roller portion having an outer diameter of 11.7 [mm]. Abutting rollers made of high molecular polyethylene were fixed to both ends of the discharge roller portion on the shaft member so that the charging gap was about 50 [μm]. The K charging roller manufactured in this way was attached to the K process unit 2K in the printer testing machine having the same configuration as the printer shown in FIG.

  In addition, prior to the experiment, the present inventor made a photoconductor 3K for K as follows. That is, an undercoat layer of 3.5 [μm], a charge generation layer of 0.15 [μm], a charge transport layer of 22 [μm], and a 5 [μm] layer on an aluminum tube having a diameter of 60 [mm]. A surface layer was sequentially laminated. The surface layer was formed by spray coating the material, and the other layers were formed by a dip coating method. Polycarbonate was used as the base resin for both the charge transport layer and the surface layer, and alumina particles having an average particle size of 0.3 [μm] were added to the surface layer in a proportion of 10% by weight based on the total solid content of the surface layer. The K photoconductor 3K manufactured in this way was attached to the K process unit 2K of the printer testing machine.

  The experiment described below was performed using a printer tester equipped with the process unit 2K including the K charging roller and the K photoconductor 3K described above. While driving the K process unit 2K including the charging roller and the paper conveying belt 61, an operation of continuously applying a voltage to the charging roller was performed. The paper transport belt 61 was driven in such a posture as to contact only the K photoconductor 3K among the four photoconductors 3Y, 3M, C, and K. For the four toner image forming units 1Y, 1M, 1C, and 1K, only the K toner image forming unit 1K is driven as in the case of the monochrome printout. In such an operation, the electric resistance of the discharge roller portion of the K charging roller was measured before the start of operation, 24 hours, 48 hours, 72 hours, and 96 hours after the start. For the measurement of electrical resistance, instead of the K photoconductor 3K, a photosensitive element tube with a 0.5 [mm] conductive rubber sheet attached to the surface is set, and this conductive rubber sheet is used for K. It contacts with the discharge roller part in the anti-static roller. In this state, a predetermined voltage is applied to the shaft member of the charging roller without rotating the photosensitive member 3K and the charging roller, respectively, and based on the voltage value and the current value flowing through the base tube of the photosensitive member 3Y connected to the ground. The electrical resistance of the discharge roller part was determined. As the conductive rubber sheet affixed to the photoreceptor 3Y, a sheet having an electric resistance sufficiently lower than that of the discharge roller portion was used. The length of the conductive rubber sheet in the charging roller axial direction was set to 12 [cm]. As the applied voltage at the time of resistance measurement, a DC voltage of −100 [V] was adopted, and the electric resistance was obtained based on the current value 30 seconds after the voltage application. This measuring method is merely an example, and other methods may be used as long as the volume resistance from the shaft member of the charging roller to the surface direction can be measured.

As the superimposed voltage applied to the charging roller for K during the operation of the printer testing machine, the conditions listed below were adopted.
・ DC voltage = -700 [V]
・ AC voltage amplitude voltage (Vpp) = 2200 [V]
-AC voltage frequency (f) = 2.2 [kHz] sine wave

  Of the plurality of experiments described below, a voltage applied only to the DC component is used as the voltage applied to the charging roller. In this case, a DC voltage of −700 [V] is used. . There is also an experiment using an AC voltage consisting only of AC components. In this case, an AC voltage of Vpp = 2200 [V] and f = 2.2 [kHz] was used.

[Experiment 1]
While continuously applying the above-mentioned superimposed voltage to the K charging roller, the electrical resistance in the discharge roller portion of the K charging roller was measured at each of the five timings described above. The measurement results were as shown in Table 1 below.

As shown in Table 1, the electric resistance of the discharge roller portion made of the base resin containing the ion conductive material increased with time. The electrical resistance value (4.71210 ⁶ Ω) after 96 hours was 16 times or more that before the start of operation (0.287 × 10 ⁶ Ω). If such a significant change in electrical resistance is caused, the K photoconductor 3Y cannot be charged at a stable potential over a long period of time.

For reference, the charging potential of the photosensitive member 3K at each timing is as shown in Table 2 below.

  As shown in Table 2, it can be seen that the charging potential of the photoreceptor decreases with time. It can be seen that the charging potential (−523V) after 96 hours was only about 0.8 times that before the printout (−636V).

表３に示すように、放電ローラ部の経時的な抵抗上昇は認められなかった。９６時間経過までにおいて、抵抗変動は最大でも０．０８６×１０^６Ωであり（０．３４４×１０^６Ω−０．２５８×１０^６Ω）は、最大変動量は最大値の０．２５倍程度に留まっている。このように抵抗変動が小さい条件下では、Ｋ用の感光体３Ｋを長期に渡って安定して帯電させることができる。但し、帯電ローラに印加する電圧に交流成分を含めていないため、帯電ローラの回転に伴う微妙な帯電ギャップ変動による帯電ムラが大きくなる。よって、感光体３Ｋ全体としては長期に渡って安定して帯電させることができるが、感光体３Ｋに部分的な帯電ムラを発生させ、それによる画像濃度ムラを引き起こし易くなってしまう。 [Experiment 2]
While continuously applying a DC voltage consisting of only a DC component to the charging roller, the electrical resistance at the discharge roller portion of the K charging roller was measured at the above-described five timings. The measurement results were as shown in Table 3 below.

As shown in Table 3, there was no increase in resistance of the discharge roller portion with time. Until 96 hours, the resistance variation is 0.086 × 10 ⁶ Ω at the maximum (0.344 × 10 ⁶ Ω−0.258 × 10 ⁶ Ω), and the maximum variation is 0.25 times the maximum value. It remains to the extent. Under such a small resistance fluctuation condition, the K photoconductor 3K can be stably charged over a long period of time. However, since the AC component is not included in the voltage applied to the charging roller, charging unevenness due to slight charging gap fluctuation accompanying rotation of the charging roller increases. Therefore, the entire photoconductor 3K can be stably charged over a long period of time. However, partial charging unevenness is generated in the photoconductor 3K, thereby easily causing image density unevenness.

表４に示すように、放電ローラ部の経時的な抵抗上昇は認められなかった。９６時間経過までにおいて、抵抗変動は最大でも０．１１３×１０^６Ωであり（０．２７１×１０^６Ω−０．１５８×１０^６Ω）は、最大変動量は最大値の０．５８倍程度に留まっている。しかしながら、帯電ローラに印加する電圧が交流成分だけからなるので、帯電極性の反転が繰り返されてしまい、感光体３Ｋがマイナス極性に十分に帯電されなくなる。よって、正常なプリントアウトを行うことはできない。 [Experiment 3]
While continuously applying an AC voltage consisting of only an AC component to the charging roller, the electrical resistance in the discharge roller portion of the K charging roller was measured at the above-described five timings. The measurement results were as shown in Table 4 below.

As shown in Table 4, there was no increase in resistance of the discharge roller portion over time. By 96 hours, the resistance fluctuation is 0.113 × 10 ⁶ Ω at the maximum (0.271 × 10 ⁶ Ω−0.158 × 10 ⁶ Ω), and the maximum fluctuation amount is 0.58 times the maximum value. It remains to the extent. However, since the voltage applied to the charging roller is composed only of an AC component, the reversal of the charging polarity is repeated, and the photoreceptor 3K is not sufficiently charged to the negative polarity. Therefore, normal printout cannot be performed.

[Experiment 4]
The above-described superimposed bias is continuously applied to the charging roller for K until the lapse of 48 hours from the start of printout, and at the three timings before the start of operation, 24 hours after the start and 48 hours after the start, The electrical resistance was measured. Thereafter, the voltage applied to the charging roller was switched to an AC voltage composed only of an AC component, and the operation was resumed. Then, the electrical resistance of the discharge roller part was measured after 4 hours (52 hours in total) and 20 hours (72 hours in total) after restarting. The results are shown in Table 5 below.

  As shown in Table 5, in the operation of applying the superimposed voltage to the charging roller, the electric resistance of the discharge roller portion increases with time. On the other hand, when the voltage applied to the charging roller is switched to an AC voltage, the electrical resistance of the discharge roller portion suddenly decreases toward the initial value (refer to the resistance value after 52 hours in total). When the time elapses (after 72 hours in total), the value returns to the same value as the initial value. From this, it was found that even if the electrical resistance of the discharge roller portion is increased by applying the superimposed voltage, the electrical resistance of the discharge roller portion can be reduced to the initial value by switching the voltage from the superimposed voltage to the AC voltage. . However, in the voltage switching method as in Experiment 4, it takes 24 hours for the electrical resistance to return, and during that time, it is impossible to perform printout.

[Experiment 5]
Measure the electrical resistance of the discharge roller at each of the five timings described above while repeatedly switching the voltage after applying the above superimposed bias to the charging roller for 20 seconds and then applying the AC voltage consisting of only the AC component for 2 seconds. did. The results are shown in Table 6 below.

  As shown in Table 6, it was possible to stabilize the electric resistance value of the discharge roller portion over a long period by switching the voltage at a relatively short cycle such as several tens of seconds. This is because the process of applying the AC voltage to the discharge roller part where the electrical resistance is slightly increased by applying the superimposed voltage and returning the electrical resistance to the original state was repeated at a relatively short cycle. The reason why the electrical resistance of the discharge roller portion can be restored by applying an AC voltage is considered as follows. In other words, in the superimposed bias in which the center value of the amplitude is closer to the minus side than 0 [V] due to the DC component, the fluctuation of the AC component occurs at the boundary of the minus side center value. It is considered that a deviation occurs in the orientation. This is considered to be because the deviation of the orientation of ions was eliminated by applying an alternating voltage consisting only of an alternating current component having 0 [V] as the center of amplitude to the discharge roller part that caused such a deviation. .

Next, a characteristic configuration of the printer according to the first embodiment will be described.
FIG. 6 is a block diagram showing a part of the electric circuit of the printer. In the drawing, four charging devices 10Y, 10M, 10C, and 10K of the printer have charging rollers 12Y, 12M, 12C, and 12K. In addition, it also includes charging power supply circuits 20Y, 20M, 20C, and 20K as power sources, power supply control circuits 21Y, M, C, and K as power supply control means, and ammeters 22Y, M, C, and K as current detection means. . The power supply circuits 20Y, 20M, 20C, and 20K output a superimposed voltage obtained by superimposing an AC voltage on a DC voltage or an AC voltage composed only of an AC component. The output voltage is applied to the charging rollers 12Y, M, C, and K through ammeters 22Y, M, C, and K. The ammeters 22Y, 22M, 22C, and 22K detect the current value flowing from the charging power supply circuits 20Y, 20M, 20C, and 20K to the charging rollers 12Y, 12M, 12C, and 12K, and control the power according to the detection signal. Output to circuits 21Y, M, C, K.

  The main control unit 200 shown in FIG. 1 controls the entire printer, and outputs reference timing signals to the power supply control circuits 21Y of the charging devices 10Y, 10M, 10C, and 10K, respectively. The power supply control circuits 21Y, 21M, 21C, and 21K charge the latent image formation target area in the circumferential direction of the photoconductor (3Y, M, C, K) based on the reference timing signal sent from the main control unit 200. The timing of entering the charging processing position by the rollers 10Y, M, C, and K can be obtained. It is also possible to determine the timing at which the latent image non-formation target region in the circumferential direction of the photoconductor (3Y, M, C, K) enters the charging processing position by the charging rollers 10Y, M, C, K.

  The charging power supply circuits 20Y, 20M, 20C, 20K, and 20K switch the output voltage from the superimposed voltage to the AC voltage based on the control signals sent from the power supply control circuits 21Y, M, C, and K, or the AC voltage to the superimposed voltage. It is configured to switch to. Further, it is configured to change at least one of the amplitude (Vpp) and the frequency (f) of the AC voltage based on the control signals sent from the power supply control circuits 21Y, 21M, 21C, 21K.

  The power supply control circuits 21Y, 21M, 21C, and 21K control the charging power supply circuits 20Y, 20M, 20C, and 20K as follows based on the reference timing signal sent from the main control unit 200 during the printing operation. It is configured as follows. That is, at least the latent image formation target area in the circumferential direction of all the peripheral areas of the photoreceptors (3Y, M, C, K) is opposed to the discharge roller portions of the charging rollers 12Y, M, C, K. When the charging processing position is entered, a superimposed bias is output. As a result, the superimposed voltage is supplied to the discharge roller portions of the charging rollers 12Y, 12M, 12C, 12K, and the electrophotographic process can be realized on the latent image formation target area of the photoreceptor (3Y, M, C, K). Charge uniformly to the negative polarity side to the level. On the other hand, when the latent image formation target area of the photoconductor (3Y, M, C, K) is not entered into the charging processing position, that is, the latent image in the circumferential direction of the photoconductor (3Y, M, C, K). The following control is performed during the period in which the non-image forming area enters the charging processing position. That is, during a part or all of the period, the charging power supply circuits 20Y, 20M, 20C, and 20K are made to output an AC voltage consisting only of an AC component. As a result, AC voltage is supplied to the discharge roller portions of the charging rollers 12Y, 12M, 12C, and 12K, and the resistance reduction of the discharge roller portion is restored. In this printer, the photosensitive member (3Y, M, C, K) can be charged with a stable potential over a long period of time by such resistance reduction recovery.

  The latent image formation target area in the circumferential direction of the photoconductor (3Y, M, C, K) is specifically an area of the peripheral surface of the photoconductor that is in close contact with the transfer paper P at the transfer nip. That is. Further, the latent image non-formation region is a region of the peripheral surface of the photoconductor that cannot be brought into close contact with the transfer paper P at the transfer nip. Even in the continuous print mode in which a plurality of transfer papers P are continuously passed, a gap is always provided between the preceding transfer paper P and the subsequent transfer paper P. Therefore, even in the continuous print mode, an inter-paper area is always provided on the peripheral surface of the photoreceptor (3Y, M, C, K), and this inter-paper area, which is a latent image non-formation area, is set as the charging processing position. When entering, resistance reduction return processing by application of AC voltage is performed.

  In addition, the main control unit 200 determines that the paper transport belt (61) when any one of the four charging power supply circuits 20Y, 20M, 20C, 20K outputs an AC voltage, regardless of whether or not the print job is finished. And control to drive the process unit.

  Further, as a method for stabilizing the charging potential of the photoconductor for a long time, there is a method of changing the value of the superimposed voltage applied to the charging roller according to the detection result of the potential sensor for detecting the surface potential of the photoconductor. is there. However, in this method, an expensive potential sensor is mounted, thereby increasing the cost or increasing the size of the apparatus.

A static-rollers 12Y, M, C, as a method for improving the resistance reduction effect of the discharge roller portion of K, lengthening the application time of the AC voltage, to increase the AC voltage amplitude (Vpp), the AC voltage frequency ( There are methods such as increasing f). However, if the AC voltage application time is lengthened, the printing speed is reduced by that amount. In particular, in the continuous printout mode, the feeling of low speed given to the user is further promoted by increasing the inter-paper area.

Therefore, according to the printer according to the first embodiment, the frequency of the AC voltage as if only AC component, to perform a control to be larger than the frequency of the AC component in the superimposed voltage, the power supply control circuit 21Y, M, C, K is configured. Thereby, the electrical resistance of the discharge roller portion can be reduced and restored in a shorter time. However, if the frequency of the alternating voltage is increased, the photoreceptor is further deteriorated. Since the rate of increase of the electrical resistance of the discharge roller portion increases when a superimposed voltage is applied for a relatively long time, it is desirable to do the following. In other words, if a continuous printout of several sheets is applied, an alternating current voltage having the same frequency as the superimposed voltage is applied to the discharge roller unit, and an alternating current component of the superimposed voltage is produced when a large number of continuous printouts of several tens of sheets are performed. An AC voltage having a higher frequency than that is applied to the discharge roller portion. By such control, it is possible to suppress deterioration of the photoreceptor due to higher frequency.

Next, a printer according to the second embodiment will be described. The printer according to the second embodiment, the frequency of the AC voltage as if only AC component, instead of carrying out the control to be larger than the frequency of the AC component in the superimposed voltage, implement the following control. That is, the power supply control circuits 21Y, 21M, 21C, and 21K are configured so as to perform control for making the amplitude (Vpp) of the AC voltage that is composed of only the AC component larger than the amplitude of the AC component in the superimposed voltage. Thereby, the electrical resistance of the discharge roller portion can be reduced and restored in a shorter time. However, if the amplitude of the AC voltage is increased, the photoreceptor is further deteriorated. Therefore, it is desirable to do the following. That is, if the continuous printout is about several sheets, an alternating current voltage having the same amplitude as the superimposed voltage is applied to the discharge roller unit, and the alternating voltage component of the superimposed voltage is applied when a large number of continuous printouts of several tens of sheets are performed. An AC voltage having a larger amplitude than that is applied to the discharge roller portion. By such control, it is possible to suppress the deterioration of the photoreceptor due to the larger amplitude.

  From the viewpoint of reducing the resistance of the discharge roller portion, a reverse polarity superimposed voltage obtained by superimposing an AC voltage on a positive polarity DC voltage having a polarity opposite to that at the time of uniform charging is applied to the discharge roller portion. If the reduction is achieved, the efficiency is improved. However, this is not preferable because the photoconductor is charged to a polarity opposite to the normal polarity, and the photoconductor characteristics are greatly deteriorated due to an increase in the residual potential.

Next, a printer according to a third embodiment will be described. In the printer according to the third embodiment, conductive Nagarekei 22Y, M, C, based on the detection result of the K, perform control to change at least one of the frequency and amplitude of the AC voltage consisting of only AC components Thus, the power supply control circuits 21Y, M, C, and K are configured. As a result, the resistance change of the discharge roller due to the change in temperature and humidity, not the effect of the application of the superimposed voltage, is absorbed by the change in the resistance reduction rate due to the application of the AC voltage, and the photosensitive member (3Y, 3 The charging potential of M, C, K) can be further stabilized.

Next, a printer according to a fourth embodiment will be described. The printer according to the fourth embodiment performs the following control. That is, when an AC voltage consisting only of an AC component is applied to the charging rollers 12Y, 12M, 12C, and 12K, the photoconductor (3Y, M, C, K) can be neutralized at a position facing the roller. The present in the fourth embodiment, is omitted eliminating lamp of the process units, so as to neutralize the photoreceptor charging device 10Y, M, C, by K. In such a configuration, it is possible to reduce the size and cost of the apparatus by not mounting the static elimination lamp.

  So far, a so-called tandem printer in which a plurality of photoconductors 3Y, 3M, 3C, and 3K are provided to form a full color image has been described. However, the present invention is applicable to a single full color type or monochrome type image forming apparatus. Can be applied. In the single full color system, only one latent image carrier such as a photoconductor is provided, and a plurality of developing devices are opposed to the latent image carrier, and Y, M, and C are sequentially formed by one latent image carrier. , K toner images are superimposed on an intermediate transfer member to obtain a full color image.

As described above, in the printer according to the first embodiment, the power supply control circuit 21Y, which is the power supply control unit, is controlled so that the frequency of the AC voltage including only the AC component is larger than the frequency of the AC component in the superimposed voltage. M, C, and K are configured. In such a configuration, for the reason described above, it is possible to perform the resistance reduction recovery of the discharge roller portion in a shorter time.

In the printer according to the second embodiment, control is performed so that the peak-to-peak value of the AC voltage composed of only the AC component is larger than the peak-to-peak value of the AC component in the superimposed voltage. The power control circuits 21Y, 21M, 21C, 21C, 21K, and 21K, which are power control means, are configured. Even in such a configuration, it is possible to perform resistance reduction recovery of the discharge roller portion in a shorter time for the reason described above.

In the printer according to the third embodiment, ammeters 22Y, 22M, 22C, and 22K are provided as current detection means for detecting output current values from the charging power supply circuits 20Y, 20M, 20C, and 20K as power sources. Based on the detection result, the power supply control circuits 21Y, 21M, 21C, 21C, 21C, 21C, 21C, 21C, and 21C, which are power supply control means, are implemented so as to perform control to change at least one of the frequency and peak-to-peak value K is configured. In such a configuration, the change in resistance of the discharge roller due to the change in temperature and humidity, not the influence of the application of the superposed voltage, is absorbed by the change in the resistance reduction rate due to the application of the AC voltage, so that the photosensitive member (3Y , M, C, K) can be further stabilized.

Further, in the printer according to the fourth embodiment, by using the charging devices 10Y, 10M, 10C, and 10K as a charge eliminating unit that removes the photosensitive member (3Y, M, C, K) as a latent image carrier, It is possible to reduce the size and cost of the apparatus by omitting a dedicated neutralizing unit for neutralizing the photosensitive member such as a neutralizing lamp.

1 is a schematic configuration diagram illustrating a printer according to a first embodiment. FIG. 3 is an enlarged configuration diagram illustrating a toner image forming unit for Y in the printer together with a part of a transfer unit. FIG. 3 is an enlarged configuration diagram illustrating a process unit of the toner image forming unit. The longitudinal cross-sectional view which shows the charging roller of the charging device of the process unit. FIG. 3 is a perspective view showing a photosensitive member and a charging roller of the process unit. FIG. 2 is a block diagram showing a part of an electric circuit in the printer.

Explanation of symbols

3Y, M, C, K: photoconductor (charged body, latent image carrier)
10Y: charging device 12Y: charging roller 16Y: discharge roller portion (discharge member)
20Y, M, C, K: Charging power supply circuit (power supply)
21Y, M, C, K: power control circuit (power control means)
22Y, M, C, K: Ammeter (current detection means)
40Y: developing device (developing means)
50: Optical writing unit (latent image forming means)

Claims (4)

A latent image carrier that carries a latent image on a surface that moves endlessly, a charging device that uniformly charges the surface, a latent image forming unit that forms a latent image on the surface after uniform charging, Developing means for developing the latent image of
And a discharge member that discharges from the surface toward the latent image carrier while the charging device faces the endlessly moving surface with a predetermined gap to the latent image carrier, and the discharge member An image forming apparatus having a power supply for supplying a voltage to the power supply and a power supply control means for controlling the output of the voltage from the power supply, and uniformly charging the latent image carrier by a discharge from the discharge member. In the device
As the discharge member, one made of a material containing an ion conductive electrical resistance adjusting material is used,
As the power source, one that can output a DC voltage and an AC voltage is used.
When at least the latent image formation target area of the entire area of the surface of the latent image carrier is made to enter the position facing the discharge member, a superimposed voltage obtained by superimposing an AC voltage on a DC voltage is applied to the power source. On the other hand, when the formation target region has not entered the facing position, a process of outputting an AC voltage consisting only of an AC component having a frequency higher than the AC voltage of the superimposed voltage from the power source is performed. An image forming apparatus comprising the power supply control means. A latent image carrier that carries a latent image on a surface that moves endlessly, a charging device that uniformly charges the surface, a latent image forming unit that forms a latent image on the surface after uniform charging, Developing means for developing the latent image of
And a discharge member that discharges from the surface toward the latent image carrier while the charging device faces the endlessly moving surface with a predetermined gap to the latent image carrier, and the discharge member An image forming apparatus having a power supply for supplying a voltage to the power supply and a power supply control means for controlling the output of the voltage from the power supply, and uniformly charging the latent image carrier by a discharge from the discharge member. In the device
As the discharge member, one made of a material containing an ion conductive electrical resistance adjusting material is used,
As the power source, one that can output a DC voltage and an AC voltage is used.
When at least the area where the latent image is to be formed is made to enter the position facing the discharge member among the entire area of the surface of the latent image carrier, the superimposed voltage obtained by superimposing the alternating voltage on the direct current voltage is applied to the power source. On the other hand, when the formation target region has not entered the facing position, an AC voltage consisting only of an AC component having a peak-to-peak value larger than the AC voltage of the superimposed voltage is output from the power source. An image forming apparatus characterized in that the power control means is configured to perform processing . The image forming apparatus according to claim 1 or 2 ,
Current detection means for detecting the output current value from the power source is provided, and based on the detection result by the current detection means, at least one of the frequency and peak-to-peak value of the AC voltage consisting only of AC components An image forming apparatus characterized in that the power supply control means is configured to perform control to be changed. The image forming apparatus according to any one of claims 1 to 3 ,
An image forming apparatus characterized in that the charging device is used as a charge eliminating means for discharging the latent image carrier.

Images