JP2015118179A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to easily control stains on a charging rotating body as compared with a case where a charging system is fixed.SOLUTION: An image forming apparatus having a charging roller, which comes into contact with a surface of a photoreceptor to charge it, includes transfer setting means which sets a transfer voltage to be applied in transferring a toner image formed on the surface of the photoreceptor. When the transfer voltage is less than 1000 V, a DC voltage is set as a charging voltage. When the transfer voltage is equal to or higher than 1000 V, a voltage formed by superimposing a DC voltage on an AC voltage is set as a charging voltage.

Description

  The present invention relates to an image forming apparatus using an electrophotographic process such as a copying machine, a printer, a facsimile machine, and a multifunction machine having a plurality of these functions.

  A contact charging method has been put to practical use as a method of charging a photoreceptor in an image forming apparatus using an electrophotographic process. In this system, a charging roller is used in which a conductive elastic layer is provided on the outer periphery of a conductive support (core metal) and a resistance layer is provided on the outer periphery of the conductive elastic layer as a charging member. The charging roller is disposed in contact with the surface of the photosensitive member so as to be rotatable, a charging voltage is applied to the cored bar, and a small discharge is generated in the vicinity of the contact nip between the charging roller and the photosensitive member. Charge the surface of the body.

  There are a DC charging method in which the charging voltage applied to the core metal is only a DC voltage, and an AC charging method in which an AC voltage is superimposed on the DC voltage. In the case of the AC charging method, the AC voltage to be superimposed in order to obtain charging uniformity is a voltage value at which discharge between the charging roller and the photoconductor starts when a DC voltage is applied to the charging roller. An AC voltage having a peak-to-peak voltage Vpp that is at least twice the absolute value of the starting voltage is used.

  In recent years, it has been desired to extend the life of a charging roller as the life of an image forming apparatus increases and the speed thereof increases. As a factor that affects the life of the charging roller, contamination due to adhesion of foreign matters such as toner and external additives remaining on the photoreceptor after transfer to the charging roller can be mentioned. A cleaning member such as a cleaning blade is provided to remove foreign matter such as toner and external additives remaining on the surface of the photoconductor after transfer, but foreign matter such as toner and external additives slipping through the cleaning member is generated. Therefore, the charging roller is contaminated. Since the charging roller is a member that rotates in contact with the photoreceptor, the toner image formed on the photoreceptor remains on the surface of the photoreceptor after being transferred to a transfer medium such as an intermediate transfer belt or a recording material. Such foreign matters are easily transferred to the charging roller and become dirty. As the charging roller is contaminated in this manner, the charging ability is reduced and charging unevenness occurs. Charging roller degradation and charging unevenness due to contamination of the charging roller is one of the factors that hinder the life of the charging roller.

  Therefore, the charging roller is charged by bringing the sponge roller as a charging cleaning member into contact with the outer peripheral surface of the charging roller, transferring the foreign matter adhering to the surface of the charging roller to the sponge roller, and removing the foreign matter from the surface of the charging roller. Patent Document 1 discloses that nonuniformity is improved.

JP-A-5-265307

  However, the method of bringing the sponge roller into contact with the charging roller as the charging rotator as in Patent Document 1 also has a limit such as a decrease in cleaning ability due to contamination of the sponge roller itself, and further extension of the life is required. .

  By the way, there is a difference in the degree of progress of dirt on the charging roller between the above-described DC charging method and AC charging method.

  For example, in a system using a negative charging method, both the DC charging method and the AC charging method charge a foreign material such as toner that remains on the photosensitive member after transfer and passes through a cleaning member such as a cleaning blade and reaches a charging roller. When the polarity is the same as the negative polarity of the charging roller, it is less likely to adhere to the charging roller due to the influence of the electric field, and is less likely to get dirty.

  However, in the AC charging method, the charging polarity of a foreign substance such as toner may be reversed depending on the amplitude of the AC voltage, and a relatively constant foreign substance is applied to the charging roller regardless of the distribution of the charging polarity of the foreign substance reaching the charging roller. Adhere to.

  On the other hand, in the DC charging method, foreign matters having a positive charge polarity tend to adhere and foreign matters having a negative charge polarity tend not to adhere, and the distribution of the charge polarity of the foreign matter reaching the charging roller is positive. When the distribution is large in the polarity charged foreign matter, the dirt level of the charging roller is remarkably deteriorated. When the distribution is large in the negative polarity charged foreign matter, the charge roller dirt level is compared with the AC charging method. Sometimes it is okay.

  Therefore, the charging system with a good dirt level of the charging roller differs depending on the distribution of the charging polarity of the foreign matter. If the charging system is fixed to the DC charging system or the AC charging system as in the prior art, the charging roller has a higher level. It was difficult to suppress the dirt.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of suppressing contamination of a charging rotator as compared with a case where a charging method is fixed.

  Therefore, an image forming apparatus according to the present invention includes a rotatable photosensitive member, a charging rotating member that contacts the surface of the photosensitive member and charges the photosensitive member, and a charging member that applies a charging voltage to the charging rotating member. Applying means; exposure means for exposing the surface of the photoreceptor charged by the charging rotator to form an electrostatic image on the photoreceptor; and an electrostatic image formed on the photoreceptor by the exposure means Developing means for developing with toner to form a toner image on the surface of the photoreceptor, transfer means for transferring the toner image formed on the surface of the photoreceptor by the developing means to a transfer medium at the transfer portion, and the transfer A transfer applying unit configured to apply a transfer voltage to the transfer unit when the unit transfers a toner image; and disposed downstream of the transfer unit and upstream of the charging rotator in the rotation direction of the photoconductor. Clean by touching Cleaning means; transfer setting means for setting the voltage value of the transfer voltage; and if the absolute value of the transfer voltage set by the transfer setting means is less than a predetermined value, a DC voltage is set as the charging voltage. And charging setting means for setting, as a charging voltage, a voltage obtained by superimposing a DC voltage and an AC voltage when the absolute value of the transfer voltage is equal to or greater than the predetermined value.

  According to the present invention, it is possible to suppress contamination of the charging rotating body as compared with the case where the charging method is fixed.

1 is a schematic cross-sectional configuration diagram of an image forming apparatus. FIG. 3 is a schematic cross-sectional configuration diagram of a charging roller. It is a figure explaining a process cartridge structure. It is a figure explaining the structure of charging voltage control. It is a figure explaining the structure of a charging roller cleaning member. It is a figure explaining the structure of a charging roller cleaning member. FIG. 6 is a diagram illustrating a distribution of charge amount of untransferred toner. It is a schematic diagram explaining charging roller contamination transition. It is a schematic diagram explaining charging roller contamination transition. FIG. 4 is an explanatory diagram for explaining a developing device and a toner supply device. It is a figure which shows the control flow in Example 1. FIG. It is a figure which shows the control flow in Example 2. FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, what attached | subjected the same code | symbol in each drawing has the same structure or effect | action, The duplication description about these is abbreviate | omitted suitably. Note that the dimensions, materials, shapes, relative positions, and the like of the component parts are not intended to limit the scope of application of this technical idea only to those unless otherwise specified.

[Overall Configuration and Operation of Image Forming Apparatus]
First, the overall configuration and operation of the image forming apparatus 100 of this embodiment will be described with reference to FIG.

  The image forming apparatus 100 according to this embodiment includes four image forming units (first, second, third, and fourth image forming units) 1Y provided corresponding to four colors of yellow, magenta, cyan, and black. This is a tandem color printer having 1M, 1C, and 1Bk. The image forming apparatus 100 can form a color image on a recording material (recording paper, plastic film, cloth, etc.) in accordance with image signals from the following devices connected to the image forming apparatus main body. The device is a document reading device (not shown), a host device such as a personal computer, or an external device such as a digital camera.

  The image forming apparatus 100 is formed on a cylindrical photosensitive member as an image carrier in the first to fourth image forming units 1Y, 1M, 1C, and 1Bk, that is, on the photosensitive drums 2Y, 2M, 2C, and 2Bk. The toner image is transferred onto an intermediate transfer belt 8 as an intermediate transfer member that is a transfer medium. Then, the toner image on the intermediate transfer belt 8 is transferred onto the recording material P to form a recorded image.

  In the following description, elements provided in common in each of the four image forming units 1Y, 1M, 1C, and 1Bk are denoted by the same reference numerals with subscripts Y, M, C, and Bk. When it is necessary to distinguish them from each other, the subscripts Y, M, C, and Bk added to the reference numerals to indicate that the elements are provided for any color are omitted, and generally explain.

  The image forming unit 1 is provided with a cylindrical photosensitive member, that is, a photosensitive drum 2 as an image carrier. The photosensitive drum 2 is rotationally driven in the arrow direction in the figure.

  Around the photosensitive drum 2 as a photosensitive member, a charging roller 3 as a charging rotator that is charged in contact with the photosensitive drum 2, and an electrostatic image formed on the photosensitive drum 2 is developed with toner to form a toner image. A developing device 4 as a developing means, a primary transfer roller 5 as a transfer means for transferring a toner image formed on the surface of the photosensitive drum 2 to a transfer medium at a transfer portion, downstream of the transfer portion in the rotation direction of the photosensitive drum 2 and A cleaning device 6 as a cleaning unit is disposed upstream of the charging roller 3. Further, a laser scanner (exposure device) 7 as an exposure means is disposed above the photosensitive drum 2 in the drawing. Further, an intermediate transfer belt 8 as an intermediate transfer member is disposed so as to face the photosensitive drum 2 of each image forming unit 1. The intermediate transfer belt 8 serving as a transfer medium is wound around a driving roller 9, a secondary transfer counter roller 10, and a driven roller 11, and rotates in the direction of the arrow in the figure by the driving force transmitted to the driving roller 9. The intermediate transfer belt 8 and the photosensitive drum 2 come into contact with each other at a position where the primary transfer roller 5 and the photosensitive drum 2 face each other, thereby forming a primary transfer portion (primary transfer nip) N1 as a transfer portion. A secondary transfer roller 12 as a secondary transfer unit is provided at a position facing the secondary transfer counter roller 10 via the intermediate transfer belt 8. The secondary transfer roller 15 contacts the intermediate transfer belt 8 at a position facing the secondary transfer counter roller 10 to form a secondary transfer portion (secondary transfer nip) N2.

  In this embodiment, the image forming apparatus 100 includes a color image forming mode in which a color image can be formed using all of the first to fourth image forming units 1Y, 1M, 1C, and 1Bk, and a fourth image. And a monochrome image forming mode for forming a black monochrome image using only the forming unit 1Bk.

  First, an image forming operation in the color image forming mode will be described. When the image forming operation is started, the surfaces of the photosensitive drums 2Y, 2M, 2C, and 2Bk rotating in the respective image forming units 1Y, 1M, 1C, and 1Bk are uniformly charged by the charging rollers 3Y, 3M, 3C, and 3Bk. The At this time, a charging bias as a charging voltage is applied to the charging rollers 3Y, 3M, 3C, and 3Bk from a charging bias power source as a charging application unit.

  Next, laser light is emitted from the exposure devices 7Y, 7M, 7C, and 7Bk in accordance with the separated color image signals corresponding to the respective image forming units. As a result, each of the photosensitive drums 2Y, 2M, 2C, and 2Bk is exposed according to the image information of the corresponding separated color, and an electrostatic image (latent image) corresponding to the image signal is formed thereon.

  The electrostatic images formed on the photosensitive drums 2Y, 2M, 2C, and 2Bk are developed as toner images by the toners stored in the developing devices 4Y, 4M, 4C, and 4Bk. In this embodiment, the reversal development method is adopted as the development method, and the toner adheres from the developing unit 4 to the exposed portion (bright portion potential portion) on the photosensitive drum 2.

  The toner images formed on the photosensitive drums 2Y, 2M, 2C, and 2Bk are sequentially transferred onto the intermediate transfer belt 8 at each primary transfer portion N1 so as to overlap on the intermediate transfer belt 8 as a transfer medium ( Primary transfer). When the toner image is transferred onto the intermediate transfer belt 8, the primary transfer rollers 5Y, 5M, 5C, and 5Bk serving as the transfer unit have a polarity opposite to the normal charging polarity of the toner from the primary transfer bias power source serving as the transfer application unit. A primary transfer bias as a transfer voltage is applied. Thus, a multiple toner image is formed on the intermediate transfer belt 8 by superimposing the four color toner images. Incidentally, toner (primary transfer residual toner) and external additives remaining on the surfaces of the photosensitive drums 2Y, 2M, 2C, and 2Bk after the primary transfer are transferred to the photosensitive drums 2Y, 2M, 2C, and 6Bk by the cleaning devices 6Y, 6M, 6C, and 6Bk. It is removed from the surface of 2Bk and recovered.

  On the other hand, in accordance with the movement and timing of the toner image on the intermediate transfer belt 8, the recording material P stored in a recording material storage cassette (not shown) is conveyed to the secondary transfer portion N2 by the supply roller 13 or the like. .

  The multiple toner images on the intermediate transfer belt 8 are collectively transferred (secondary transfer) onto the recording material P at the secondary transfer portion N2. At this time, a secondary transfer bias having a polarity opposite to the normal charging polarity of the toner is applied to the secondary transfer roller 12 from a secondary transfer bias power source.

  Next, the recording material P is conveyed to a fixing device 14 as a fixing unit by a conveying member or the like. By being heated and pressurized by the fixing device 14, the toner on the recording material P is melted and mixed, and is fixed on the recording material P to form a color fixed image. Thereafter, the recording material P is discharged out of the apparatus. Incidentally, the toner (secondary transfer residual toner) not transferred to the recording material P in the secondary transfer portion N2 but remaining on the intermediate transfer belt 8 is collected by the intermediate transfer belt cleaner 13.

  Next, an image forming operation in the single color image forming mode will be described. In the monochromatic image forming mode, a toner image is formed on the photosensitive drum 2Bk only in the fourth image forming unit 1Bk. The toner image is primarily transferred to the intermediate transfer belt 8 and then secondarily transferred to the recording material P. The toner image forming operation, primary transfer operation, and secondary transfer operation itself in the fourth image forming unit 1Bk are the same as those in the color image forming mode described above.

[Developer]
Next, the developing device 4 as developing means and the toner replenishing device 49 for replenishing toner will be described with reference to FIG. In this embodiment, the developing device 4 has a configuration shown in FIG. In the present embodiment, the configurations of the developing devices 4Y, 4M, 4C, and 4Bk are the same. In this embodiment, the configuration of the toner replenishing device 49 is the same for all the developing devices. 10, the developing device 4 is shown as a plan view as viewed from above in FIG. 1, and the toner replenishing device 49 is shown as a cross-sectional view along the axial direction of the photosensitive drum 2 (direction perpendicular to the surface movement direction).

  The developing device 4 includes a developing container (developing device main body) 44 in which a two-component developer (developer) including non-magnetic toner particles (toner) and magnetic carrier particles (carrier) as main components is accommodated. In the developing container 44, two screws, a first stirring and conveying screw 43a and a second stirring and conveying screw 43b, are arranged as developer stirring and conveying members. A portion of the developing container 44 facing the photosensitive drum 2 is partially opened, and a developing sleeve 41 as a developer carrying member is rotatably disposed so as to be partially exposed from the opening. Inside the developing sleeve 41, a magnet roll (not shown) as a magnetic field generating means is fixedly arranged. The magnet roll has a plurality of magnetic poles in the circumferential direction, attracts the developer in the developer container 44 by the magnetic force and carries it on the developing sleeve 41, and at the development position facing the photosensitive drum 2, the developer spikes ( Magnetic brush).

  The developing sleeve 41 and the first and second agitating and conveying screws 43a and 43b are arranged in parallel to each other. The developing sleeve 41 and the first and second agitating / conveying screws 43 a and 43 b are arranged in parallel to the axial direction of the photosensitive drum 2. The inside of the developing container 44 is divided into a first chamber (developing chamber) 44a and a second chamber (stirring chamber) 44b by a partition wall 44d. The developing chamber 44a and the stirring chamber 44b communicate with each other at both ends in the longitudinal direction of the developing container 44 (left end and right end in FIG. 10).

  The first stirring and conveying screw 43a is disposed in the developing chamber 44a, and the second stirring and conveying screw 43b is disposed in the stirring chamber 44b. These first and second agitating and conveying screws 43 a and 43 b are rotationally driven in the same direction via the gear train 54 by the rotation of the motor 52. By this rotation, the developer in the agitating chamber 44b moves to the left in FIG. 10 while being agitated by the second agitating and conveying screw 43b, and moves into the developing chamber 44a through the communicating portion. Further, the developer in the developing chamber 44a moves to the right in FIG. 10 while being stirred by the first stirring and conveying screw 43a, and then moves into the stirring chamber 44b through the communication portion. That is, the developer is circulated and conveyed in the developing container 44 while being agitated by the two screws of the first and second agitating and conveying screws 43a and 43b. The toner in the developer is charged by the agitation and conveyance as described above.

  The developing sleeve 41 is rotationally driven by a motor 51 in the direction indicated by an arrow (counterclockwise) in FIG. The developing sleeve 41 conveys the developer applied in a layered manner on the surface by a regulating blade (not shown) to the developing position facing the photosensitive drum 2 by its rotation. At the developing position, the developer on the developing sleeve 41 rises due to the magnetic force of the magnet roll to form a magnetic brush that contacts or approaches the surface of the photosensitive drum 2. The toner is supplied to the electrostatic image on the photosensitive drum 2 from the developer (two-component developer) thus transported to the development position. Thereby, toner selectively adheres to the image portion of the electrostatic image, and the electrostatic image is developed as a toner image. More specifically, when the electrostatic image on the photosensitive drum 2 reaches the developing position, a developing bias as a developing voltage in which an AC voltage and a DC voltage are superimposed by a developing bias power source (not shown) is applied to the developing sleeve 41. Applied. At this time, the developing sleeve 41 is rotationally driven by the motor 51 in the direction of the arrow in FIG. 1, and the toner in the developer is applied on the photosensitive drum 2 according to the electrostatic image on the surface of the photosensitive drum 2 by the above-described developing bias. To metastasize.

  Here, the toner has colored resin particles containing a binder resin, a colorant, and other additives as required, and colored particles to which an external additive such as colloidal silica fine powder is externally added. doing. The toner is a negatively chargeable polyester resin produced by a polymerization method, and the volume average particle diameter is preferably 5 μm or more and 8 μm or less. In this example, the volume average particle size of the toner was 6.2 μm.

As the carrier, for example, metal such as surface oxidized or unoxidized iron, nickel, cobalt, manganese, chromium, rare earth, alloys thereof, or oxide ferrite can be preferably used. The method for producing these magnetic particles is not particularly limited. The carrier has a weight average particle diameter of 20 to 50 μm, preferably 30 to 40 μm, and a resistivity of 107 Ω · cm or more, preferably 108 Ω · cm or more. In this example, a carrier having a resistivity of 108 Ω · cm was used. In this example, as a low specific gravity magnetic carrier, a resin magnetic carrier produced by mixing a phenolic binder resin with a magnetic metal oxide and a non-magnetic metal oxide at a predetermined ratio and using a polymerization method was used. The volume average particle size of the carrier used in this example is 35 μm, the true density is 3.6 to 3.7 g / cm ³ , and the magnetization is 53 A · m ² / kg.

[Charging device]
Next, the charging device will be described in detail.

  The charging device uses a rubber roller (hereinafter referred to as a charging roller) that is in contact with the photosensitive member and can be rotated by being driven by the photosensitive member.

  The longitudinal length of the charging roller 3 as the charging rotating body is 320 mm. As shown in FIG. 2, the lower layer 3b, the intermediate layer 3c, and the surface layer 3d are arranged from the bottom around the core metal (supporting member) 3a. It has a three-layer structure in which the layers are sequentially stacked. The lower layer 3b is a foamed sponge layer for reducing charging noise, and the surface layer 3d is a protective layer provided to prevent leakage even if there are defects such as pinholes on the photosensitive drum 2. is there.

More specifically, the specification of the charging roller 3 in the present embodiment is as follows.
Core metal 3a; stainless steel round bar lower layer 3b with a diameter of 6 mm; foamed EPDM with carbon dispersion, specific gravity 0.5 g / cm ³ , volume resistivity 10 ² to 10 ⁹ Ωcm, layer thickness 3.0 mm
Intermediate layer 3c; carbon-dispersed NBR rubber, volume resistivity 10 ² to 10 ⁵ Ωcm, layer thickness 700 μm
Surface layer 3d: tin oxide and carbon dispersed in a resin resin of fluorine compound, volume resistivity 10 ⁷ to 10 ¹⁰ Ωcm, surface roughness (JIS standard 10-point average surface roughness Ra) 1.5 μm, layer thickness 10 μm
FIG. 4 is a block circuit diagram of a charging bias application system for the charging roller 3.

  A predetermined vibration voltage (bias voltage Vdc + Vac) obtained by superimposing an AC voltage having a frequency f on a DC voltage from the power supply S1 is applied to the charging roller 3 through the cored bar 3a, whereby the peripheral surface of the rotating photoreceptor 1 is rotated. Is charged to a predetermined potential.

  A power source S 1 that is a charging application unit that applies a charging voltage to the charging roller 3 includes a direct current (DC) power source 11 and an alternating current (AC) power source 12.

  Reference numeral 60 denotes a control circuit as a charge setting means, which controls on / off of the DC power supply 11 and the AC power supply 12 of the power supply S1 and applies a DC voltage, an AC voltage, or a superimposed voltage of both to the charging roller 3. A function for controlling to apply, a DC voltage value applied from the DC power source 11 to the charging roller 3, and a peak-to-peak voltage value of the AC voltage applied from the AC power source 12 to the charging roller 3, or an AC current value Have

  Further, the charging device in the present embodiment is a rotatable open-cell foam that takes in transfer residual toner and an external additive adhering to the surface of the charging roller 3 (mating member) by elastically compressing and contacting the charging roller 3. A sponge roller 20 as a body roller is provided. The uppermost vertex of the sponge roller 20 serving as a charging cleaning means is located below the rotation center of the charging roller 3 and downstream of the charging roller 3 in the rotation direction with respect to the charging nip where the charging roller 3 contacts the photosensitive drum 2. The charging roller 3 is driven to rotate as the charging roller 3 rotates. Details of the sponge roller 20 will be described later.

  As shown in FIG. 3, the photosensitive drum 2 and the charging device (the charging roller 3 and the sponge roller 20 excluding the charging bias power source) are integrated as a unit as a process cartridge 21 and can be attached to and detached from the image forming apparatus. Therefore, it is possible to easily replace the photosensitive drum 2 when the photosensitive drum 2 reaches the end of its life by forming a prescribed number of images. The process cartridge may include the developing device 4.

  Also in the present embodiment, an image forming operation is performed in the same manner as in the conventional image forming apparatus described above. In this embodiment, description of the image forming operation is omitted.

  Next, the sponge roller 20 as a charging cleaning unit of the above-described charging device will be described.

  As shown in FIGS. 5A and 5B, the sponge roller 20 includes a metal core 22 as a support shaft and a conductive material-dispersed urethane rubber in which the wall surface of the bubble portion communicates with the peripheral surface thereof. It is composed of a roller-like foam layer 23 made of silicon rubber, EPDM rubber, acrylic rubber or the like.

  The sponge roller 20 can be manufactured as follows, for example. First, an unvulcanized / unfoamed urethane rubber layer in which a compounding agent of a conductive material such as a foaming agent or carbon black is uniformly dispersed is formed on a metal cored bar 22. These are set in a cylindrical mold molding cabinet (not shown) and heated to vulcanize and foam the unvulcanized / unfoamed rubber layer, and the conductivity within the molding cabinet conforms to the mold. Molded as a sponge. And the sponge roller 20 of the desired outer diameter provided with the foam layer 23 on the surrounding surface of the metal core 22 is obtained by polishing the surface of the open-cell sponge and breaking the skin layer.

  In addition, as shown in FIGS. 6A and 6B, in the case of a sponge roller having a plurality of (two layers in the figure) foam layers 23a and 23b on the peripheral surface of the core metal 22, the single-layer configuration described above is used. The foam layer 23 can be formed by repeating the manufacturing process.

  Examples of the material for forming the foam layer 23 (23a, 23b) of the sponge roller 20 in the present invention include urethane rubber, silicon rubber, EPDM rubber, acrylic rubber, nitrile rubber, hydrin rubber, and fluorine rubber. I do not. Examples of the blowing agent generally include inorganic blowing agents such as sodium hydrogen carbonate, sodium carbonate, and ammonium carbonate, and organic blowing agents such as nitroso compounds, azo compounds, and sulfonyl hydrazide compounds, but are not particularly specified.

  Conductive agents can be broadly classified into electronic conductive agents and ionic conductive agents. Examples of electronic conductive agents include carbon black and metal oxides. Examples of ionic conductive agents include quaternary ammonium salts and aliphatic alcohol sulfate salts. Are not specified.

  The elastic body is not particularly limited to a single layer structure, and may be a plurality of two or more layers as shown in FIGS. 6A and 6B if necessary. Further, the layer configuration is such that the bubble diameter decreases from the cored bar 22 toward the roller surface. Thereby, the transfer residual toner peeled off in the vicinity of the contact portion with the charging roller 3 can be moved more smoothly to the central portion of the roller.

  The average cell diameter of the above-mentioned open-cell sponge depends on the particle diameter and shape of the toner, but is preferably 10 times or more the average particle diameter of the powder in order to sufficiently take the toner into the bubbles. Further, the amount of penetration into the charging roller 3 is less than the thickness of the foam layer 23, and although it depends on the thickness of the foam layer 23, it is more preferably half or less of the layer thickness.

  The contact pressure (linear pressure) of the sponge roller 20 with the charging roller 3 is preferably 0.049 to 0.98 N / cm. When the contact pressure with the charging roller 3 is 0.049 N / cm or less, a peeling failure occurs at the contact portion with the charging roller 3, and charging is performed when toner that has not been removed remains on the surface of the charging roller 3. It may cause defects. Conversely, if the contact pressure with the charging roller 3 is 0.98 N / cm or more, the rotational torque increases or the rubbing force increases, so that the toner taken into the sponge roller 20 is softened by rubbing. As a result, there is a risk of fusing to the surface of the charging roller 3.

  Incidentally, the contact pressure (linear pressure) was measured by the following method. Prepare a SUS thin plate with a length of 100 mm × width 15 mm × thickness 30 μm as a drawing plate, and a SUS thin plate with a length of 180 mm × width 30 mm × thickness 30 μm folded as a sandwich plate so that the length is halved, A drawing plate is inserted between the folded sandwich plates. In this state, the sandwiching plate is inserted between the charging roller 3 and the sponge roller 20. Then, only the spring attached to the extraction plate is pulled, the extraction plate is extracted at a constant speed, and the value indicated by the spring at that time is read. The contact pressure (linear pressure) between the charging roller 3 and the sponge roller 20 can be measured by dividing the value of the spring alone by 1.5 cm which is the width of the drawing plate.

As a measure of the degree of openness of the foam layer 23, the air flow rate per 1000 cm ³ of the foam layer 23 is 500 to 5000 ml / (min · 1000 cm ³ ), more preferably 1000 to 4000 ml (min · 1000 cm). ³ ).

If it is 500 ml / (min · 1000 cm ³ ) or less, not only does the hardness increase and the contact pressure with the charging roller 3 increases, but the amount of toner that can be contained is small, so it must be replaced frequently. Will not be. Further, if it is 5000 ml / (min · 1000 cm ³ ) or more, the bubbles are excessively communicated and the desired average cell diameter cannot be maintained, and the collected toner cannot be held in the bubbles.

  In this embodiment, a sponge having a single foam layer 23 made of open-cell urethane sponge rubber having a layer thickness of 4 mm, in which carbon black is dispersed and blended as a conductive material on a metal core bar 22 having a diameter of 4 mm. A roller 20 was used. The amount of toner t that can be sucked into the foam layer 23 by the sponge roller 20 is about 30 g.

  Moreover, the conditions of the sponge roller 20 of this Embodiment were set as follows.

The resistance value is about 105Ω, the rubber hardness is about 15 ° in Asker CSC2 hardness, the average cell diameter in the surface layer of the foam layer 23 is 200 μm, the contact pressure (linear pressure) with the charging roller 3 is 0.196 N / cm, foam The air flow rate representing the openness of the body layer 23 is 1800 (min · 1000 cm ³ ).

[Charging voltage condition]
Next, the switching operation of the charging voltage applied to the charging roller used in this embodiment will be described in detail.

  The charge amount distribution of the residual toner after transfer is such that the negatively charged toner is developed on the photoconductor, and remains on the photoconductor without being transferred in a process in which the toner is transferred to the transfer body by a positive transfer bias. It is distributed near positive polarity. This is because negatively charged toner of normal polarity is easily transferred, while toner of opposite polarity is difficult to transfer, and even when a positive discharge is generated by a primary transfer bias as a transfer voltage, it is normally negative. This is because a toner whose polarity has been reversed by transfer discharge may remain on the photosensitive member. It is known that the primary transfer voltage is determined by a control called ATVC (Auto Transfer Voltage control) so that the transfer current becomes a predetermined value. In ATVC, a plurality of types of transfer voltages are applied to the primary transfer roller 5, the transfer current flowing through the primary transfer roller at that time is detected to determine the relationship between the transfer voltage and the transfer current, and the transfer current becomes a predetermined value. Control for setting the voltage as the transfer voltage is performed. In this embodiment, a control circuit as a transfer setting unit executes control by ATVC. When the transfer voltage set by the ATVC is high and the amount of discharge is large, the toner remaining after transfer tends to increase in the toner distribution having a positive charge polarity opposite to the normal polarity. The transfer residual toner is disposed downstream of the transfer portion in the rotation direction of the photosensitive drum 2 and upstream of the charging roller 3, and is removed by a cleaning blade as a cleaning unit that contacts and cleans the photosensitive drum 2, but is completely removed. The transfer residual toner that has passed through the cleaning blade without reaching the charging roller.

  FIG. 7 schematically shows the charge amount distribution of the transfer residual toner and the distribution of the toner adhering to the charging roller according to the distribution. FIG. 7A shows a case where the transfer voltage is small and the distribution of the transfer residual toner is biased toward the negative polarity that is relatively on the normal charging polarity side. In the case of the AC charging method, positive discharge and negative discharge are alternately repeated according to the amplitude of the alternating voltage superimposed on the charging voltage, and the transfer residual toner is originally charged with the positive charge polarity by receiving the discharge alternately. Even if it has a negative charge polarity, it will adhere to the charging roller at a fixed ratio.

  On the other hand, in the case of the DC charging method, only the negatively charged side DC discharge is performed. Therefore, the transfer residual toner having a negative charging polarity is not attracted to the charging roller in an electric field in the charging nip. Only a part of the transfer residual toner bearing the toner adheres to the charging roller. Therefore, in this case, the amount of dirt on the charging roller tends to be smaller in the DC charging method than in the AC charging method.

  Conversely, FIG. 7B shows a state in which the transfer voltage is large and the distribution of the transfer residual toner is relatively biased to the positive polarity side. Even in this case, the adhesion amount of the transfer residual toner to the charging roller in the AC charging method does not change greatly. However, in the DC charging method, a large amount of positive transfer polarity residual toner adheres to the charging roller. Rather than the charging method, the adhesion amount of the transfer residual toner increases. Accordingly, by appropriately switching between the AC charging method and the DC charging method according to the polarity distribution of the transfer residual toner, it is possible to suppress the amount of the transfer residual toner attached to the charging roller as compared with the case where the charging method is fixed. Become.

  Therefore, in the present embodiment, the control circuit 60 as the charge setting means determines the DC voltage based on the absolute value of the transfer voltage applied to the primary transfer roller as the transfer means set by the control circuit as the transfer setting means. The charging voltage is set by switching between the charging voltage superimposed with the AC voltage and the charging voltage of only the DC voltage. When the absolute value of the transfer voltage is less than the predetermined value, the DC voltage is set as the charging voltage. If the absolute value of the transfer voltage is greater than or equal to a predetermined value, a voltage obtained by superimposing the DC voltage and the AC voltage is set as the charging voltage. Specifically, in an environment where the absolute value of the transfer voltage is 1000 V or more, a voltage obtained by superimposing a DC voltage and an AC voltage is set as a charging voltage and charging is performed by an AC charging method, and the absolute value of the transfer voltage is less than 1000 V In this environment, only the DC voltage was set as the charging voltage, and control was performed so as to execute charging by the DC charging method. Here, when a discharge start voltage Vth, which is a voltage at which discharge is started between the charging roller and the photoconductor when a DC voltage is applied to the charging roller, is −600 V, the AC charging method is used, for example, −600 V. When charging is performed after superimposing a DC voltage of 1.5 kV and an AC voltage having an amplitude of 1.5 kV and then switching to the DC charging method, the charging potential of -1200 V can be obtained so that the same charging surface can be obtained. Switch to apply a DC voltage as the charging voltage. The value of the charging voltage is based on the environment and endurance table stored in advance in the ROM 18.

  Next, the effect of this control will be described. FIG. 8 schematically shows the durability transition of the contamination level of the charging roller and the allowable level of image unevenness due to contamination. In general, the AC charging method and the DC charging method have different levels of image unevenness due to defective charging of a contaminated portion when the amount of dirt is the same. That is, the AC charging method is superior in charging potential convergence, and the amount of stains at which the image quality of the formed image is NG level is twice or more that of the DC charging method. The AC charging system shows a constant contamination transition regardless of the environment as long as the amount of residual toner that leaves the cleaning device as the cleaning means and reaches the charging roller is constant, but in the case of the DC charging system, When the absolute value of the transfer voltage is relatively high, the contamination transition becomes remarkably fast and becomes NG when the durable number is a. On the other hand, when the absolute value of the transfer voltage is relatively low, the DC charging method can suppress the amount of contamination and can be used up to time point c. Accordingly, by appropriately changing to AC charging when the absolute value of the transfer high pressure is high and DC charging when the absolute value is low, the amount of contamination can be minimized and the life from b to c is guaranteed.

  FIG. 11 is a flowchart of control in this embodiment.

  At the time of initial rotation operation (pre-multi-rotation process) in which a predetermined preparatory operation is performed for raising the fixing temperature such as when the image forming apparatus is turned on or when returning from the sleep mode, At the time of printing preparation rotation operation (pre-rotation step) in which a predetermined preparation operation is executed from when the image is input to when the image corresponding to the image information is actually written out, every time a predetermined number of image formations are performed, for example, every 20 image formations The control is started at a predetermined timing during non-image formation such as. The control in the present embodiment is executed by a control circuit 60 as charge setting means.

  When this control is started, it is first determined whether or not the absolute value of the transfer voltage set by the transfer setting means is less than 1000 V (S1). If the absolute value of the transfer voltage is less than 1000 V, only the DC voltage is set as the charging voltage applied to the charging roller (S2). When the absolute value of the transfer voltage is 1000 V or more, a voltage obtained by superimposing the DC voltage and the AC voltage is set as a charging voltage to be applied to the charging roller (S3). In the charging voltage setting step in S2 and S3, the voltage value of the DC voltage, the peak-to-peak voltage of the AC voltage, and the frequency may be set together by a known method.

  In this embodiment, an example of a tandem type image forming apparatus including a plurality of image forming units and an intermediate transfer belt as a transfer medium has been described. A method of directly transferring to a recording material carried on a conveyance belt may be used. In this case, a configuration including one image forming unit may be employed. In this case, the recording material corresponds to the transfer medium.

  As described above, according to the present embodiment, the charging method is switched between the DC charging method and the AC charging method on the basis of the absolute value of the transfer voltage, so that the image forming is performed. In addition, it is possible to suppress the contamination of the charging rotating body.

  In this embodiment, as shown in FIG. 8, when a transfer voltage having a relatively low absolute value is set, for example, the life is reached at c by the DC charging method, but at this time, switching to the AC charging method is performed. Although the slope of the transition of the amount of dirt increases due to the above, it is different from the first embodiment in that the life can be extended to d.

  In this embodiment, there is provided a dirt amount detecting means for detecting the dirt amount by obtaining the dirt amount of the charging roller based on information corresponding to the accumulated charging time by the charging roller.

  When the amount of dirt detected by the dirt amount detection means exceeds a predetermined amount, the charging setting means sets a voltage obtained by superimposing the DC voltage and the AC voltage as the charging voltage.

  As a use history detecting means as means for acquiring information corresponding to the accumulated charging time, for example, a sheet passing number counter that counts the number of used sheets, which is the cumulative number of image formations, can be used. When it is determined by the contamination amount detection means that the contamination amount of the charging roller has reached the contamination amount of the DC charging allowable limit, the charging is forcibly switched to the AC charging method by the control circuit 60 as the charging setting means. The life of the roller can be extended.

  The control circuit 60 serving as a dirt amount detecting means determines the number of sheets used (cumulative number) in each of the DC charging method and AC charging method acquired by a sheet passing number counter as means for acquiring information corresponding to the accumulated charging time. The sum of the slope of the dirt amount transition with respect to the endurance number of sheets (the number of sheets used) in both charging systems as shown in FIG. 8, ie, the product of a predetermined coefficient corresponding to the absolute value of the transfer voltage is added. Obtain the amount of dirt.

  FIG. 9 is a diagram schematically illustrating the change in the amount of dirt for each transfer voltage. The stain amount transition in the DC charging method in which the transfer voltage is 500 V to 1000 V and the stain amount transition in the AC charging when the transfer voltage is 1000 V or more are shown.

b and c are 150k sheets and 200k sheets, respectively, and when the allowable amount of dirt in the AC charging method is 100, and the allowable amount of dirt in the DC charging method is 50, the AC charging method is a crossed X sheet, DC The contamination amount K of the charging roller at the time when the charged Y-pass sheet is passed is expressed by the following equation.
K = 100 / 150,000 × X + 50 / (100 + 0.2 × (1000−T)) × Y
... (1)
The value K corresponding to the amount of dirt is calculated by the control circuit 60 as the dirt amount detecting means using the above equation (1), and when the dirt amount K exceeds 50, the control circuit 60 as the charge setting means is used. The AC charging method is forcibly set. Here, T in the expression (1) is a transfer voltage (V) set by the transfer setting means, and a coefficient to be multiplied by Y (the slope of the stain amount transition with respect to the durable number (the number of used sheets)) varies depending on the transfer setting. It is The change in the amount of dirt in the transfer voltage setting does not necessarily coincide with this, and the above formula (1) can be appropriately corrected according to characteristics such as the ease of dirt on the charging roller due to the surface shape of the charging roller. It is. In the above formula (1), the coefficient applied to X is 100 / 150,000, and the coefficient applied to Y is (100 + 0.2 × (1000−T)).

  FIG. 12 is a flowchart of control in this embodiment.

  Also in the present embodiment, at the time of initial rotation operation in which a predetermined preparatory operation is performed for raising the fixing temperature such as when the image forming apparatus is turned on or when returning from the sleep mode (pre-multi-rotation process) Also, during a print preparation rotation operation (pre-rotation step) in which a predetermined preparation operation is executed from when an image formation signal is input to when an image corresponding to image information is actually written out, every time a predetermined number of image formation operations are performed, for example Control is started at a predetermined timing at the time of non-image formation, such as every 20 images are formed. The control in the present embodiment is also executed by the control circuit 60 as the charge setting means as in the first embodiment.

  When this control is started, first, the contamination amount K of the charging roller is obtained by the contamination amount detection means by the method described above. When the determined dirt amount K is 50 or more, the control circuit 60 as the charge setting means forcibly switches to the AC charging method (S0). When the dirt amount K is less than 50, the same control as that in the first embodiment is performed (S1 to S4).

  With the configuration described above, it has become possible to extend the life of the charging roller, which was 100 k when the AC charging method is fixed, to a life exceeding 200 k.

  According to the present invention, in addition to the effects of the first embodiment, it is possible to further extend the life of the charging roller.

  In the first embodiment, an example in which the transfer voltage is set by control by ATVC has been described. However, the present invention is not limited to this.

  In the first embodiment, an example in which the sheet passing number counter is used as a means for acquiring information corresponding to the accumulated charging time has been described. However, the present invention is not limited thereto, and the accumulated time in which charging is actually performed by each charging method is obtained. The configuration may be acceptable, and the cumulative number of rotations of the photosensitive drum 2 may be obtained.

DESCRIPTION OF SYMBOLS 1 Image formation part 2 Photosensitive drum (image carrier)
DESCRIPTION OF SYMBOLS 3 Charging roller 4 Developing device 5 Primary transfer roller 6 Cleaning device 7 Exposure device 8 Intermediate transfer belt 14 Fixing device 15 Secondary transfer roller 20 Sponge roller (charging roller cleaning member)
44 Developing container 46 Toner container 49 Toner supply device 60 Control circuit

Claims (7)

A rotatable photoreceptor,
A charging rotator that contacts the surface of the photoreceptor and charges the photoreceptor;
Charging application means for applying a charging voltage to the charging rotating body;
Exposure means for exposing the surface of the photoreceptor charged by the charging rotator to form an electrostatic image on the photoreceptor;
Developing means for developing an electrostatic image formed on the photoreceptor by the exposure means with toner to form a toner image on the surface of the photoreceptor;
Transfer means for transferring a toner image formed on the surface of the photoreceptor by the developing means to a transfer medium at a transfer portion;
A transfer applying means for applying a transfer voltage to the transfer means when the transfer means transfers a toner image;
A cleaning unit that is disposed downstream of the transfer unit and upstream of the charging rotator in the rotation direction of the photoconductor, and performs cleaning in contact with the photoconductor;
Transfer setting means for setting a voltage value of the transfer voltage;
When the absolute value of the transfer voltage set by the transfer setting means is less than a predetermined value, a DC voltage is set as the charging voltage, and when the absolute value of the transfer voltage is greater than or equal to the predetermined value, a DC voltage is set. And charging setting means for setting a voltage on which an alternating voltage is superimposed as a charging voltage. Comprising a dirt amount detecting means for detecting the dirt amount of the charging rotating body;
2. The charge setting unit according to claim 1, wherein when the amount of dirt detected by the dirt amount detection unit exceeds a predetermined amount, a voltage obtained by superimposing a DC voltage and an AC voltage is set as a charging voltage. Image forming apparatus.   The image forming apparatus according to claim 2, wherein the stain amount detection unit obtains a stain amount of the charging rotator based on information corresponding to an accumulated charging time by the charging rotator.   The image forming apparatus according to claim 3, wherein the information corresponding to the cumulative charging time is a cumulative number of images formed.   5. The transfer medium according to claim 1, wherein the transfer medium is an intermediate transfer member for conveying a toner image transferred from the photosensitive member and transferring the toner image to a recording material at a second transfer portion. 2. The image forming apparatus according to item 1.   The image forming apparatus according to claim 1, wherein the transfer medium is a recording material. The image forming apparatus according to claim 1, further comprising a charging cleaning unit that contacts the charging rotator and cleans the charging rotator.

Images