JP2006215083A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which a charging member is disposed in pressure contact with an image carrier and which adopts a contact charging system in which a surface of the image carrier is uniformly charged by the charging member to a predetermined polarity and potential, the image forming apparatus which prevents occurrence of an abnormal image due to a deterioration in the charging ability of a charging member which rise of its resistance causes. <P>SOLUTION: The charging member undergoes a change in its resistance by pressing force to the charging member. The image forming apparatus has: a current detecting means to detect current flowing in the charging member; and a pressing force regulating means to regulate the pressing force of the charging member to the image carrier. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using a contact charging method in which a charging member is pressed against the surface of an image carrier, and the surface of the image carrier is uniformly charged to a predetermined polarity and potential by the charging member.

  Conventionally, transfer type image forming apparatuses such as copying machines, printers, facsimiles and the like using an electrophotographic method are photosensitive drums, which are general image bearing members of a rotating drum type, and the photosensitive members are uniform in predetermined polarity and potential. A charging device (this process is referred to as a charging process), an exposure device as an information writing means for forming an electrostatic latent image on the charged photosensitive body (this process is referred to as an exposure process), A developing device that visualizes the electrostatic latent image formed on the toner with toner as a developer (this process is referred to as a developing step), and a transfer device that transfers the toner image onto a transfer material such as paper from the surface of the photoreceptor. This process is called a transfer process), a cleaning device that removes some residual toner on the photoreceptor after the transfer process and cleans the surface of the photoreceptor (this process is called a cleaning process), and a toner image on the transfer material Constant And a like fixing device for (call this process fixing step). The photoreceptor repeats such a series of electrophotographic processes (charging process, exposure process, development process, transfer process, and cleaning process), and an image is formed on the transfer material.

  Until now, a charging device of a non-contact type charging method (corona charging method) utilizing a corona discharge phenomenon such as a scorotron charger has been mainly used as the charging device. A corona charging system charging device is effective as means for uniformly charging the surface of a charged body such as an image carrier to a predetermined potential. However, since a high-voltage power supply is required, there are problems such as an increase in cost and an increase in the size of the apparatus. There were also environmental problems such as the generation of ozone by corona discharge.

  In contrast to such a corona charging method, in recent years, a conductive charging member (roller type, blade type) is brought into direct contact with or close to a photoconductor such as a photoconductor, and a charging voltage is applied to the charging member. Contact charging is becoming mainstream.

  For example, in a contact charging type charging device in which a conductive rubber charging roller is pressed against a photosensitive drum as an image carrier, and the charging roller is driven to rotate as the photosensitive drum rotates, a cored bar serving as a shaft of the charging roller is used. By supplying a voltage to the photosensitive drum, the surface of the photosensitive drum can be uniformly charged by utilizing a discharge phenomenon that occurs in a small gap between the photosensitive drum and the photosensitive drum.

  Compared with the non-contact charging method, the contact charging method can lower the applied voltage required to obtain a desired potential on the surface of the object to be charged, and the amount of ozone generated in the charging process is very small. In addition, there is an advantage that the necessity of the ozone removing filter is eliminated, and the configuration of the exhaust system of the apparatus is simplified.

  FIG. 2 shows a conventional example of a schematic configuration of an image forming apparatus using a charging roller which is a contact charging type charging device.

  Reference numeral 1 denotes a member to be charged, which is a photosensitive drum as an image carrier. The photosensitive drum is driven to rotate in the direction of the arrow at a predetermined speed (so-called process speed).

  Reference numeral 2 denotes a charging member (charging roller) disposed in pressure contact with the photosensitive drum (image carrier) 1. The charging roller 2 is rotatably held at both ends of a cored bar (supporting member) 2a by bearing members (not shown), and is pressed against the photosensitive drum 1 by a pressing spring 2e with a predetermined pressing force. ing. As a result, the charging roller 2 rotates following the photosensitive drum 1. A pressure contact portion between the photosensitive drum 1 and the charging roller 2 is a charging portion (charging nip) a.

  A charging bias voltage of a predetermined condition is applied to the cored bar 2a of the charging roller 2 from the power source S1. As a result, the surface of the rotating photosensitive drum 1 is contact-charged to a predetermined polarity and potential by the charging roller 2.

  Here, FIG. 3 is an example of a layer configuration model diagram of the charging roller 2 conventionally used. This charging roller has a three-layer structure in which a lower layer 2b, an intermediate layer 2c, and a surface layer 2d are laminated in this order from the core metal around the core metal 2a. The lower layer 2b is a foamed sponge layer for reducing charging noise, and for example, foamed EPDM with carbon dispersion is used. The intermediate layer 2c is a resistance adjustment layer for adjusting the resistance value of the charging roller 2, and for example, NBR rubber dispersed in carbon is used. The surface layer 2d is a protective layer provided to prevent leakage even when the photosensitive drum 1 has a defect such as a pinhole. For example, a 6-nylon resin is used as a raw material to react with methanol or the like. Tolesin, a soft nylon that has been chemically modified and dissolved in alcohol, fluororesin to improve surface releasability, and conductivity such as carbon black and tin oxide to properly adjust the resistivity What added the appropriate amount of agent is used.

  The surface of the photosensitive drum 1 charged by the charging roller 2 is exposed according to target image information by an exposure device 3 as information writing means, and the electrostatic latent image corresponding to the target image information is exposed on the surface of the photosensitive drum 1. Is formed. The electrostatic latent image formed on the surface of the photosensitive drum 1 is developed as a toner image by the developing device 4.

  On the other hand, the transfer material P is fed and conveyed one by one from a paper feed mechanism (not shown), and the transfer material P is introduced into the transfer device 5 at a predetermined timing, so that the photosensitive drum is transferred to the transfer material P. The toner images formed on one surface are sequentially transferred.

  The transfer material P that has received the transfer of the toner image is conveyed to the fixing device 6 and undergoes a fixing process of the toner image by the fixing device 6.

  Further, the surface of the photosensitive drum 1 after the transfer of the toner image to the transfer material P is cleaned by removing the remaining adhered contaminants such as transfer residual toner by the cleaning member 21b of the cleaning device 21, and the process proceeds to the next image forming process. .

  By the way, in the contact charging method using the charging member such as the charging roller 2 provided in the image forming apparatus, the resistance value increases due to deterioration of the conductive rubber constituting the charging member as the number of image formation increases. In addition, the resistance value increases due to contaminants such as toner adhering to the surface of the contact charging member. The increase in the resistance value of the charging member may reduce the ability to uniformly charge the surface of the photosensitive drum 1.

  Specifically, as the resistance value of the charging roller 2 increases, the amount of discharge generated in a minute gap between the charging roller 2 and the photosensitive drum 1 decreases, and the surface of the photosensitive drum 1 cannot be uniformly charged. For example, there may be an abnormal image in which minute dots due to poor charging occur on a white background. Further, when the surface of the charging roller 2 is unevenly contaminated and the resistance value of the charging roller 2 rises to a predetermined value or more, the surface of the photosensitive drum 1 is charged with the uncontaminated portion and the contaminated portion of the charging roller 2. In the contaminated portion, the potential of the surface of the photosensitive drum 1 cannot be charged to a predetermined value, and there may be image unevenness having a density different from that of other portions on the image.

  Here, the increase in the resistance value due to the deterioration of the former conductive rubber increases the number of times of image formation, that is, by adjusting the resistance value inside the conductive rubber by continuously applying a voltage to the charging roller 2. This is caused by segregation of added resistance adjusting agents such as carbon and reduction of electrical paths.

  As a solution to this problem, a voltage that can be used to uniformly charge the surface of the photosensitive drum 1 even with the charging roller 2 that has been used and deteriorated until a predetermined replacement time (life of the charging roller 2) is set to an initial value. (Method 1).

  Further, the relationship between the number of times of image formation or voltage application time and the increase in the resistance value of the conductive rubber is examined in advance, and the voltage value applied to the charging roller 2 is adjusted as the number of image formation times or voltage application time increases. A method (predictive control of charging applied voltage) or the like is used (method 2).

  Further, a method of sampling the amount of current flowing from the charging roller 2 to the photosensitive drum 1 at regular timings and adjusting the voltage applied to the charging roller 2 according to the result (control of charging application voltage) is used. (See method 3, patent document 1).

  On the other hand, the increase in resistance value due to the adherence of contaminants to the surface of the latter charging roller 2 is caused by the accumulation of fine particles of toner and toner external additives that could not be removed by the cleaning device 21 on the surface of the charging roller. It occurs by going.

As a solution to this problem, a method of bringing a cleaning member into contact with the charging roller 2 is used. As the cleaning member, a soft, high-capturing pad member such as a sponge, a brush member, or the like is used (refer to Method 4 and Patent Document 2).
JP-A-4-138467 Japanese Patent Laid-Open No. 3-126961

  In the above method 1, since a voltage higher than necessary is applied to the charging roller 2 in the initial stage, the problem that occurs due to the high applied voltage, specifically, the amount of abrasion of the photosensitive drum 1 is accelerated (the durability of the photosensitive drum). Etc.) may occur.

  The method 2 does not take into consideration that the rate of deterioration may vary greatly depending on conditions other than the number of image formations or voltage application time. That is, when the voltage applied to the charging roller 2 is predicted and controlled, in order to change the applied voltage depending on the number of times of image formation or the voltage application time even though the resistance value does not increase so much in practice. However, problems such as the above-mentioned drum scraping may occur although the degree is improved. On the other hand, when the actual increase in resistance value is significant, the photosensitive drum 1 may not be uniformly charged and image defects may occur.

  In the method 3, it is necessary to increase the voltage value applied to the charging roller 2 with the number of image formations, and the amount of current flowing into the charging roller 2 increases. For this reason, segregation of a resistance adjusting agent such as carbon added to the inside of the conductive rubber is accelerated, and an increase in resistance value due to a decrease in electrical paths may be accelerated.

  On the other hand, in the method 4 described above, when a pad member having a high collection property is used as the cleaning member, the ability to remove contaminants attached to the surface of the charging roller 2 is initially excellent. Since contaminants accumulate in the contact nip of the roller 2, the cleaning ability of the cleaning member is quickly reduced, and contaminants attached to the surface of the charging roller 2 cannot be completely removed.

  Further, in the method 4, when a brush member that plays a role of uniformly dispersing contaminants attached to the charging roller 2 over the length of the charging roller 2 is used as a cleaning member, the contamination attached to the surface of the charging roller 2 is uniform to some extent. However, since the contaminants on the charging roller 2 cannot be completely removed, the resistance value of the charging roller 2 cannot be prevented from increasing.

  As described above, when the resistance value of the charging member increases, the charging ability of the surface of the image bearing member by the charging member decreases, and image defects due to charging failure become a problem. It has not been.

  Accordingly, the present invention provides an image forming apparatus using a contact charging method in which a charging member is disposed in pressure contact with an image carrier, and the surface of the image carrier is uniformly charged to a predetermined polarity and potential by the charging member. An object of the present invention is to prevent the occurrence of an abnormal image due to a decrease in charging ability accompanying an increase in resistance value.

  The present invention is an image forming apparatus having the following configuration.

  In an image forming apparatus using a contact charging method in which a charging member is disposed in pressure contact with an image carrier, and the surface of the image carrier is uniformly charged to a predetermined polarity and potential by the charging member, the charging member includes: The resistance value changes depending on the pressure applied to the charging member, current detection means for measuring the current value flowing through the charging member, and pressure applied to adjust the pressure applied to the image carrier of the charging member And an adjustment unit.

  According to the present invention, in an image forming apparatus using a contact charging method in which a charging member is disposed in pressure contact with an image carrier, and the surface of the image carrier is uniformly charged to a predetermined polarity and potential by the charging member. It is possible to prevent the occurrence of an abnormal image due to an increase in the resistance value of the member.

Example 1
Hereinafter, the image forming apparatus (image recording apparatus) of Example 1 will be described.

  FIG. 1 is a schematic configuration model diagram of an image forming apparatus according to Embodiment 1 of the present invention. The image forming apparatus of this example is an electrophotographic laser beam printer using a roller-type charging roller as a charging member.

(A) Image carrier 1 in FIG. 1 is a photosensitive drum as an image carrier. The surface of the photosensitive drum (image carrier) 1 is formed of a negatively charged organic photoconductor (OPC), and has an outer diameter of 60 mm and a process speed (peripheral speed) of 100 mm / sec centered on the central support shaft. ) To rotate in the counterclockwise direction indicated by the arrow.

  As shown in the layer configuration model diagram of FIG. 3, the photosensitive drum 1 has an undercoat layer 1b that suppresses light interference and improves the adhesion of the upper layer on the surface of an aluminum cylinder (conductive drum base) 1a. The charge generation layer 1c and the charge transport layer 1d are coated in order from the bottom.

  The charge generation layer can be formed by coating a coating solution in which an azo pigment is dispersed together with a binder resin in a suitable solvent on a conductive support (conductive drum substrate) by a known method. The thickness is desirably a thin film layer of, for example, 5 μm or less, preferably 0.1 to 1 μm. The binder resin used in this case is selected from a wide range of insulating resins or organic photoconductive polymers. Polyvinyl butyral, polyvinyl benzal, polyarylate, polycarbonate, polyester, phenoxy resin, cellulose resin, acrylic resin, Polyurethane and the like are preferable, and the amount used is 80% by mass or less, preferably 40% by mass or less, in terms of the content in the charge generation layer. The solvent to be used is preferably selected from those which dissolve the resin and do not dissolve the charge transport layer and the undercoat layer described below. Specifically, ethers such as tetrahydrofuran and 1,4-dioxane, ketones such as cyclohexanone and methyl ethyl ketone, amides such as N, N-dimethylformamide, esters such as methyl acetate and ethyl acetate, toluene, Aromatics such as xylene and chlorobenzene, alcohols such as methanol, ethanol and 2-propanol, and aliphatic halogenated hydrocarbons such as chloroform, methylene chloride, dichloroethylene, carbon tetrachloride and trichloroethylene.

  The charge transport layer is stacked on or under the charge generation layer, and has a function of receiving charge carriers from the charge generation layer and transporting them in the presence of an electric field. The charge transport layer is formed by dissolving and applying a charge transport material in a solvent together with an appropriate binder resin as required, and the film thickness is generally 5 to 40 μm, preferably 15 to 30 μm. Charge transport materials include electron transport materials and hole transport materials. Examples of electron transport materials include 2,4,7-trinitrofluorenone, 2,4,5,7-tetranitrofluorenone, chloranil, tetra Examples include electron-withdrawing substances such as cyanoquinodimethane and those obtained by polymerizing these electron-withdrawing substances. Examples of hole transporting substances include polycyclic aromatic compounds such as pyrene and anthracene, carbazole-based, indo- Heterocyclic compounds such as benzene, imidazole, oxazole, thiazol, oxadiazol, pyrazole, pyrazoline, thiadiazol, triazole, etc., p-diethylamino Hydra such as benzaldehyde-N, N-diphenylhydrazone, N, N-diphenylhydrazino-3-methylidene-9-ethylcarbazole Compounds, styryl compounds such as α-phenyl-4′-N, N-diphenylaminostilbene, 5- [4- (di-p-tolylamino) benzylidene] -5H-dibenzo [a, d] cycloheptene, benzidine And a polymer having a main chain or a side chain having a group composed of these compounds in the main chain or side chain (for example, poly-N-vinylcarbazole, polyvinylanthracene, etc.). In addition to these organic charge transport materials, inorganic materials such as selenium, selenium-tellurium, amorphous silicon and cadmium sulfide can also be used. These charge transport materials can be used alone or in combination of two or more. When the charge transport material does not have film-forming properties, a suitable binder can be used. Specifically, insulating resin such as acrylic resin, polyarylate, polyester, polycarbonate, polystyrene, acrylonitrile-styrene copolymer, polyacrylamide, polyamide, chlorinated rubber, or organic photoconductivity such as poly-N-vinylcarbazole and polyvinylanthracene. For example, a conductive polymer.

  Examples of the conductive support on which the charge generation layer and the charge transport layer are formed include aluminum, aluminum alloy, copper, zinc, stainless steel, vanadium, molybdenum, chromium, titanium, nickel, indium, gold and platinum. In addition, plastics (for example, polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, acrylic resin, etc.) or conductive particles (for example, carbon black, silver particles, etc.), which are formed by coating these metals or alloys by vacuum deposition, are used as appropriate binder resins. In addition, a support coated on a plastic or metal substrate or a support impregnated with plastic or paper with conductive particles can be used.

  An undercoat layer having a barrier function and an adhesive function may be provided between the conductive support and the charge generation layer and the charge transport layer. The undercoat layer can be formed of casein, polyvinyl alcohol, nitrocellulose, polyamide (nylon 6, nylon 66, nylon 610, copolymer nylon, alkoxymethylated nylon, etc.), polyurethane, aluminum oxide, or the like. The thickness of the undercoat layer is 5 μm or less, preferably 0.1 to 3 μm.

  The image forming apparatus of the present invention is not limited to the form having the photosensitive drum having the above-described configuration, but functions as an image carrier of the image forming apparatus, that is, has high sensitivity characteristics and stable potential characteristics in repeated use. If so, an image carrier such as a photosensitive drum having another configuration may be provided.

(B) Charging means 2 in FIG. 1 is a charging member provided in the charging means, and in this example, a charging roller is used. The charging roller is disposed in pressure contact with the photosensitive drum 1 and has a function of uniformly charging the surface of the image carrier to a predetermined polarity and potential. In addition, a charging roller control unit includes a current detection unit that detects a current flowing through the charging member, and a pressing force adjusting unit that adjusts a pressing force of the charging member to the image carrier. Details of the charging roller and the charging roller control means in this embodiment are as follows.

(B-1) Charging roller The charging roller 2 is configured such that both ends of the cored bar (supporting member) 2a are rotatably held by bearing members (not shown), and one side toward the photosensitive drum 1 by the pressing spring 2e. A load of 45.0 gf (0.441 N) is applied, and the load rotates following the rotation of the photosensitive drum 1. Here, the longitudinal length of the charging roller 2 is 300 mm, and the linear pressure of the charging roller 2 on the photosensitive drum 1 is 3.0 gf / cm (2.94 N / m). A pressure contact portion between the photosensitive drum 1 and the charging roller 2 is a charging portion (charging nip) a.

  Further, a charging bias voltage of a predetermined condition is applied to the core 2a of the charging roller 2 from the power source S1, and the peripheral surface of the photosensitive drum 1 arranged in pressure contact with the charging roller is charged to a predetermined polarity and potential. It is processed. In this example, the charging bias voltage for the charging roller 2 is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, DC voltage: −500 V, AC voltage; frequency f1 kHz, peak-to-peak voltage Vpp 1.5 kV, and vibration voltage superimposed with a sine wave, and the peripheral surface of the photosensitive drum 1 is −500 V (dark potential Vd). Are uniformly charged.

  The outer diameter of the charging roller 2 is 16 mmΦ, and a three-layer structure in which a lower layer 2b, an intermediate layer 2c, and a surface layer 2d are laminated on the outer periphery of a cored bar (support member) 2a as shown in the layer configuration model diagram of FIG. It is. More specifically, the specification of the charging roller 2 of this example is as follows.

Metal core 2a; stainless steel round bar with a diameter of 8 mm Lower layer 2b; Foamed EPDM with carbon dispersion, specific gravity 0.5 g / cm ³ , volume resistivity 10 ² to 10 ⁶ Ωcm, layer thickness 3.2 mm, length 320 mm
Intermediate layer 2c; polymer nanocomposite material composed of silicone rubber / spherical carbon / alumina particles, volume resistivity 10 ⁵ to 10 ⁷ Ωcm, layer thickness 750 μm
Surface layer 2d; tin oxide and carbon dispersed in resin resin of fluorine compound, volume resistivity 10 ⁷ to 10 ⁹ Ωcm, surface roughness (JIS standard 10-point average surface roughness Ra) 1.5 μm, layer thickness 10 μm
The lower layer 2b has a foamed sponge layer for reducing charging noise, the intermediate layer 2c has a resistance adjusting layer for adjusting the resistance value of the charging roller 2, and the surface layer 2d has defects such as pinholes on the photosensitive drum 1. Even in this case, it functions as a protective layer provided to prevent leakage.

  Here, a characteristic of the charging roller 2 used in this embodiment is that a polymer nanocomposite material is used for the intermediate layer 2c. The charging member included in the image forming apparatus of the present invention needs to have a resistance value that changes depending on the pressure applied to the charging member. In this embodiment, the charging member 2 forms the intermediate layer 2c. A specific polymer nanocomposite material is used as the material.

  In general, a polymer nanocomposite material is a material in which ultrafine particles of nano-order ceramics are dispersed using a polymer as a matrix material. The mechanical and electrical properties of the polymer nanocomposite material are greatly changed without changing the specific gravity from the original polymer. It is a generic name for materials that can be improved and new properties can be added.

As described above, as the number of times of image formation performed in the image forming apparatus increases, the resistance value increases due to segregation of the resistance adjusting agent added to adjust the resistance value in the charging roller 2 or the charging roller 2 increases. The resistance value rises due to the adhesion of contaminants such as toner on the surface. When such an increase in the resistance value of the charging roller 2 occurs, the surface of the photosensitive drum 1 cannot be uniformly charged to a predetermined potential, and an image defect occurs due to a charging failure on the surface of the photosensitive drum 1. End up. FIG. 7 shows a transition diagram of a change in the resistance value of the charging roller 2 with an increase in the number of image formations when the conventional charging roller 2 is used. As shown in the figure, the resistance value of the charging roller 2 is 3 × 10 ⁶ Ω · cm or more when the number of image formations is 22k or more. In such a case, for example, a halftone front uniform image is caused by a charging failure. Unevenness may occur. Such unevenness cannot be tolerated particularly in a color printer or the like.

  Therefore, in this embodiment, the charging roller 2 using a polymer nanocomposite material in which silicone rubber is used as a matrix material, spherical carbon as a resistance adjusting agent, and alumina particles as ceramic ultrafine particles are added is used. Such a polymer nanocomposite material in which a resistance modifier and ceramic ultrafine particles are blended in a matrix material made of elastomer has a linear relationship between applied pressure resistance values, and controls the applied pressure. Thus, it has a characteristic that it can be changed to a desired resistance value. The polymer nanocomposite material having such a structure is generally used in the field of pressure-sensitive sensors, and the resistance value changes when pressure is applied, and the pressure is measured by monitoring the current value at that time. . In the present invention, this principle is used in reverse. That is, by using the charging roller 2 in which the intermediate layer 2 c is configured using the polymer nanocomposite material having such a configuration, the charging roller 2 is controlled by controlling the pressing force (pressing force) that presses the charging roller 2 against the photosensitive drum 1. It is possible to adjust the resistance value.

In addition to silicone rubber, elastomers such as ethylene propylene rubber, chloroprene rubber, urethane rubber, and nitrile rubber can be used as the matrix material constituting the polymer nanocomposite material having the above characteristics. Examples of the resistance adjusting agent include carbon nanotubes and metal fibers in addition to spherical carbon. The number average particle diameter is preferably 10 nm to 100 nm. Examples of the ceramic ultrafine particles include silica (SiO ₂ ), zinc oxide (ZnO), titania (TiO ₂ ) and the like in addition to alumina (Al ₂ O ₃ ) particles. The number average particle diameter is preferably 10 nm to 100 nm. By combining various matrix materials, resistance adjusting agents, and ceramic ultrafine particles in an appropriate ratio, the same characteristics as the polymer nanocomposite material used in this example can be obtained. Here, the reason why the above is preferable as the range of the number average particle size of the resistance adjuster and the ultrafine ceramic particles is that if the particle size is too small, the cohesiveness becomes high and cannot be dispersed well in the matrix material (is easily segregated) This is because the dispersibility (uniformity) in the matrix material may deteriorate if the particle size is too large.

  Further, when the polymer nanocomposite material having such a configuration is used for the charging roller 2, in addition to being used for the material constituting the intermediate layer 2c of the charging roller 2 as in this embodiment, it is used for the material constituting the lower layer 2b. The same characteristics can be obtained.

  Here, the principle that the resistance value of the polymer nanocomposite material changes due to the applied pressure will be described. First, as shown in the schematic diagram of FIG. 5, when only a small pressing force is applied to the polymer nanocomposite material, the conductive particles as the resistance adjusting agent in the elastomer as the matrix material are almost uniformly dispersed. Only a conductive path is formed at a part of the aggregated portion (FIG. 5A). However, when a large pressing force is applied, the volume fraction of the conductive particles in the matrix material increases and the proportion of contact between the conductive particles increases, so that the conductive path increases compared to when the pressing force is small ( FIG. 5B). According to such a principle, when the applied pressure is small, the resistance value of the polymer nanocomposite material is increased, and the resistance value is decreased as the applied pressure is increased. However, this phenomenon is considered to be the same in principle only by adding conductive particles to the elastomer, even if the polymer nanocomposite material is not mixed with the above-mentioned ceramic ultrafine particles.

  Therefore, the reason for using the polymer nanocomposite material containing the ultrafine ceramic particles as described above in Example 1 will be described. Table 1 summarizes the material for forming the intermediate layer 2c of the charging roller 2 and the result of the pressure dependency of the resistance value at that time. FIG. 4 summarizes the results of studies on how the resistance value changes when the pressure applied to each charging roller 2 is changed.

  First, the charging roller (1) using NBR as an intermediate layer forming material like a conventional charging roller has a substantially constant resistance value even when the pressing force is changed from 1.0 gf / cm to 10.0 gf / cm. There was almost no pressure dependency of the resistance value. In addition, when the charging rollers (2) to (4) using polymer nanocomposites obtained by adding spherical carbon to silicone rubber as an intermediate layer forming material are examined, spherical carbon (number average particle diameter: about 100 nm) is obtained. The charging roller (2) with an addition amount of 5 vol% has a tendency similar to that of the charging roller (1), and the resistance value is substantially constant even when the pressing force is changed from 1.0 gf / cm to 10.0 gf / cm. The pressure dependence of the resistance value was hardly observed. The charging roller (3) with an added amount of spherical carbon of 20 vol% has a resistance value that varies greatly with the applied pressure, but the resistance value decreases rapidly when the applied pressure is in the range of 1.0 gf / cm to 2.0 gf / cm. Thus, the resistance value was almost constant when the applied pressure was 2.0 gf / cm or more. The charging roller (4) in which the amount of spherical carbon added is 30 vol%, the resistance value decreases very rapidly when the pressing force is 1.0 gf / cm to 2.0 gf / cm. The resistance value has fallen to a range where it cannot be used. That is, in the charging rollers (3) and (4), although the pressure dependency of the resistance value is seen, it is considered that the pressure range for adjusting the resistance value is narrow and the resistance value is difficult to adjust. Further, it is considered that the function as the charging roller is not sufficiently exhibited when the applied pressure is 1.0 gf / cm to 2.0 gf / cm. Therefore, a charging roller (5) in which the amount of spherical carbon added is 20 vol% and alumina particles having a primary particle diameter of several tens of nm (number average particle diameter: about 20 nm) are added to spherical carbon by 5 wt% is manufactured. did. This charging roller (5) shows that the resistance value gradually decreases in the range of the applied pressure from 1.0 gf / cm to 10.0 gf / cm, and that the resistance value depends on the applied pressure in a wide range. It was.

  From the above, when silicone rubber is used as the matrix material, 15-25 vol% of spherical carbon is added as a resistance adjuster, and 2-10 mass% of alumina particles are added as the ultrafine ceramic particles to the spherical carbon. It is preferable to use a composite material.

  As the polymer nanocomposite material used as a material whose resistance value changes with pressure, a composition suitable for adjusting the resistance value can be appropriately selected. For example, a composition capable of obtaining desired characteristics from a polymer nanocomposite material in which 15 to 25 vol% of a resistance modifier is added to the matrix material and 2 to 10 mass% of ceramic ultrafine particles are added to the resistance regulator. Can be appropriately selected.

The resistance value of the charging member is preferably variable in the range of 5 × 10 ^{5 to} 5 × 10 ⁶ Ω · cm. The pressure range for realizing the variable resistance value in the above range is 1.0 to 5.0 gf / cm (0.98 to 4.9 N / m) for a new charging roller, and 50k sheets are used. In the case of the charged roller, it is preferably 3.5 to 6.0 gf / cm (3.43 to 5.88 N / m).

  The image forming apparatus of the present invention is not limited to the configuration including the charging roller having the above-described configuration, and includes a charging roller having another configuration as long as the resistance value has a characteristic that changes according to the applied pressure. It can also be in the form.

(B-2) Charging Roller Control Unit Next, the charging roller 2 control unit will be described. Reference numeral 2g in FIG. 1 denotes current detection means for measuring a current value flowing through the charging roller 2. Reference numeral 2f denotes a pressure adjusting means for adjusting the pressure applied to the photosensitive drum 1 by the charging roller 2. The image forming apparatus of the present invention has the current detection unit and the pressure adjusting unit as the charging roller control unit, and the resistance value of the charging roller can be controlled by these units.

  Here, a method for controlling the resistance value of the charging roller 2 by the charging roller control means will be described. A flowchart of the control method is shown in FIG. First, (a) the pressing force of the charging roller 2 is changed to three levels centering on the pressing force before the control is executed. For example, when the applied pressure before control is 3.0 gf / cm (2.94 N / m), 2.75 gf / cm (2.70 N / m), 3.0 gf / cm (2.94 N / m), 3 Change to 3 levels of .25 gf / cm (3.19 N / m). (B) A constant voltage (−100 V in this embodiment) is applied to the charging roller 2 at each applied pressure, and the amount of current flowing through the charging roller is measured by the current detection means 2g. The amount of current at this time is preferably measured and averaged at 16 points for one rotation of the charging roller. Next, (c) three points of pressure applied to the charging roller 2 and three points of resistance values calculated from the amount of current measured by the current detecting means 2g are plotted, and the pressure of the charging roller 2 is calculated using the least square method. A relational expression of the resistance value is derived (see FIG. 8). Then, (d) this relational expression is compared with the image defect upper limit threshold value of the resistance value that is likely to cause an image defect, and the minimum pressure of the charging roller 2 is read (see FIG. 8). Here, it is preferable to change the image defect upper limit threshold value of the resistance value that is concerned that an image defect occurs depending on the use environment (absolute water content). In this example, the following five levels were used. (E) The pressing force of the charging roller 2 is adjusted by the pressing force adjusting means 2f. The pressing force at this time may be equal to or higher than the minimum pressing force read above, but it is preferable to take a margin and multiply the value of the minimum pressing force by 1.1 to 1.3.

Here, if the relative humidity is Ψ (%), the absolute humidity x is
x = 0.622 × (Ψ × ps / 100) / {P− (Ψ × ps / 100)} (1)
And relative humidity Ψ (%) is determined by the partial pressure of water vapor Ψ = p / ps × 100 (2)
However,
p: Water vapor partial pressure in humid air (mmHg)
ps: Water vapor partial pressure of saturated humid air (mmHg)
P: Total pressure of wet air (mmHg) (constant at 760 mmHg)
Is required. This absolute humidity means the amount of water vapor (kg) in 1 m ^{3 of} humid air, and the absolute water content can be determined from this value.

  The above control is performed when the power of the image forming apparatus is turned on and when the number of times of image formation reaches a predetermined number (in this embodiment, the number of times of image formation after the previous control is 1000 times). It is preferable to carry out at each timing when the absolute water content changes.

  FIG. 6 shows the relationship between the pressure applied to the charging roller 2 and the resistance value of the charging roller 2 at the initial stage of use and the charging roller 2 in which the number of image formations is 50k. It can be seen that in the charging roller 2 in which the number of image formations is 50k, when the pressure applied by the charging roller is low, the resistance value is too high and an image defect may occur. The applied pressure of the charging roller of this embodiment is 3.0 gf / cm (2.94 N / m) in the initial stage. If the charging roller 2 after 50 k times is used with the initial applied pressure without control, image defects may occur. It turns out that there is a possibility of becoming.

FIG. 7 shows the transition of the number of image formations and the resistance value of the charging roller 2 in the image forming apparatus with and without the control (the absolute water content in the usage environment is 7. 0 g / m ³ ). When the pressure applied to the charging roller 2 was kept constant at 3.0 gf / cm (2.94 N / m) without changing from the initial stage, the resistance value caused an image defect at about 22 k. On the other hand, the resistance value in the case of performing control did not exceed the threshold value for causing image defects. The transition of the applied pressure at this time became 4.0 gf / cm after 50 k from the initial 3.0 gf / cm.

(C) Information writing means 3 in FIG. 1 is an exposure apparatus as information writing means for forming an electrostatic latent image on the surface of the photosensitive drum 1 that has been charged, and this example is a laser beam scanner using a semiconductor laser. It is. A laser beam modulated in response to an image signal sent from a host process such as an image reading device (not shown) to the printer side is output, and the uniformly charged surface of the rotating photosensitive drum 1 is exposed at the exposure position b. Laser scanning exposure L (image exposure) is performed. The laser scanning exposure L lowers the potential of the photosensitive drum 1 surface irradiated with the laser beam, so that electrostatic latent images corresponding to the scanned and exposed image information are sequentially formed on the rotating photosensitive drum 1 surface. It will be done.

  The image forming apparatus according to the present invention is not limited to the form having the laser beam scanner using the above-described semiconductor laser, and any other apparatus can be used as long as it can form an electrostatic latent image on the surface of the charged image carrier. An information writing unit such as an exposure apparatus may be provided.

(D) Developing means 4 in FIG. 1 is a developing device (developing device) as a developing means for supplying a developer (toner) to the electrostatic latent image on the photosensitive drum 1 to visualize the electrostatic latent image. An example is a two-component magnetic brush developing type reversal developing device.

  4a is a developing container, 4b is a non-magnetic developing sleeve, and this developing sleeve 4b is rotatably arranged in the developing container 4a with a part of its outer peripheral surface exposed to the outside. 4c is a non-rotating fixed magnet roller inserted in the developing sleeve 4b, 4d is a developer coating blade, 4e is a two-component developer contained in the developing container 4a, and 4f is disposed on the bottom side in the developing container 4a. The provided developer agitating member, 4g, is a toner hopper and contains replenishing toner.

The two-component developer 4e in the developing container 4a is a mixture of toner and a magnetic carrier and is stirred by the developer stirring member 4f. In this example, the resistance value of the magnetic carrier is about 10 ¹³ Ωcm, and the particle size is 40 μm. The toner is triboelectrically charged to negative polarity by rubbing with the magnetic carrier.

  The developing sleeve 4b is disposed in close proximity to the photosensitive drum 1 while maintaining a closest distance (referred to as S-Dgap) to the photosensitive drum 1 at 350 μm. A facing portion between the photosensitive drum 1 and the developing sleeve 4a is a developing portion c. The developing sleeve 4b is rotationally driven in the developing portion c so as to travel in the direction opposite to the traveling direction of the photosensitive drum 1. A part of the two-component developer 4e in the developing container 4a is adsorbed and held as a magnetic brush layer on the outer peripheral surface of the developing sleeve 4b by the magnetic force of the magnet roller 4c in the sleeve, and is rotated and conveyed as the sleeve rotates. A predetermined thin layer is formed by the developer coating blade 4d, and in contact with the surface of the photosensitive drum 1 at the developing portion c, the surface of the photosensitive drum is rubbed appropriately. A predetermined developing bias is applied to the developing sleeve 4b from the power source S4. In this example, the developing bias voltage for the developing sleeve 4b is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, it is a DC voltage; -350V, an AC voltage; a frequency f8.0 kHz, a peak-to-peak voltage 1.8 kV, and an oscillating voltage in which a rectangular wave is superimposed.

  Thus, the toner in the developer coated as a thin layer on the surface of the rotating developing sleeve 4b and transported to the developing section c corresponds to the electrostatic latent image on the surface of the photosensitive drum 1 by the electric field due to the developing bias. By selectively adhering, the electrostatic latent image is developed as a toner image. In the case of this example, the toner adheres to the exposed bright portion of the surface of the photosensitive drum 1 and the electrostatic latent image is reversely developed.

  The developer thin layer on the developing sleeve 4b that has passed through the developing section c is returned to the developer reservoir in the developing container 4a with the subsequent rotation of the developing sleeve.

  In order to maintain the toner concentration of the two-component developer 4e in the developing container 4a within a predetermined substantially constant range, the toner concentration of the two-component developer 4e in the developing container 4a is adjusted by, for example, an optical toner concentration sensor (not shown). The toner hopper 4g is driven and controlled according to the detected information, and the toner in the toner hopper is supplied to the two-component developer 4e in the developing container 4a. The toner supplied to the two-component developer 4e is stirred by the stirring member 4f.

  The image forming apparatus according to the present invention is not limited to the above two-component magnetic brush developing type reversal developing apparatus, and can develop an electrostatic latent image formed on the surface of the image carrier as a toner image. If it exists, it can also be set as the form which comprises developing means, such as a developing device of another system.

(E) Transfer Means / Fixing Means 5a in FIG. 1 is a transfer device, and this example is a transfer roller. The transfer roller 5a is brought into pressure contact with the photosensitive drum 1 with a predetermined pressing force, and the pressure nip portion is a transfer portion d. A transfer material (a member to be transferred, a recording material) P is fed to the transfer portion d from a paper feeding mechanism portion (not shown) at a predetermined control timing.

  The transfer material P fed to the transfer portion d is nipped and conveyed between the rotating photosensitive drum 1 and the transfer roller 5a. During this time, the negative polarity that is the normal charging polarity of the toner from the power source S3 is transferred to the transfer roller 5a. By applying a positive polarity transfer bias (in this example, +2 kV) having a reverse polarity, the toner image on the surface of the photosensitive drum 1 is sequentially electrostatically charged onto the surface of the transfer material P that is nipped and conveyed by the transfer portion d. It will be transcribed.

  The image forming apparatus according to the present invention is not limited to the form having the transfer roller as described above, and other transfer means may be used as long as the toner image formed on the surface of the image carrier can be transferred to the transfer material. It can also be set as the form to comprise.

  The transfer material P that has received the transfer of the toner image through the transfer portion d is sequentially separated from the surface of the rotary photosensitive drum 1 and is conveyed to the fixing device 6 (in this example, a heat roller fixing device) for fixing the toner image. Upon receipt, it is output as an image formed product (print, copy).

(F) Cleaning Means 21 in FIG. 1 is a cleaning device, and in this example, includes a cleaning container 21a, a cleaning member 21b, and a developer stirring member 21c. Residual adhering contaminants such as toner remaining on the surface of the photosensitive drum 1 after the transfer of the toner image to the transfer material P are removed and cleaned by the cleaning member 21b of the cleaning device 21, and the process proceeds to the next image forming process. . The removed residual adhering contaminant 21d is accumulated in the cleaning container 21a.

  As described above, in the image forming apparatus in which the charging roller 2 is brought into contact with the surface of the photosensitive drum 1 to charge the surface of the photosensitive drum 1, as described in the present embodiment, silicone rubber is used as the matrix material for the intermediate layer 2c of the charging roller 2. In addition, using a polymer nanocomposite material in which spherical carbon as a resistance adjusting agent and alumina nanoparticles as ceramic fine particles are added, a current detecting means 2g for detecting a current flowing through the charging roller 2; The pressure adjusting means 2f for controlling the pressure is provided, and the control for adjusting the resistance value of the charging roller 2 by changing the pressing force is performed in real time, thereby generating an abnormal image due to the increase in the resistance value of the charging roller 2. Therefore, it is possible to provide an image forming apparatus that is stable over a long period of time.

(Example 2)
FIG. 10 is a schematic configuration of the second embodiment of the image forming apparatus according to the present invention.

  The basic configuration of the image forming apparatus (printer) of the present embodiment is the same as that of the first embodiment except that a charging blade is used as the charging member 2. Accordingly, elements having the same functions and configurations as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

In the charging blade 2, a conductive elastic member 2h having a resistance value of about 10 ⁶ Ω · cm is attached to the support member 2a, and a constant pressing force (3.0 gf / cm (2. 94 N / m)) against the photosensitive drum 1. The surface of the photosensitive drum 1 is charged at the contact portion a between the conductive elastic member 2 h of the charging blade 2 and the photosensitive drum 1 formed at this time.

  Here, the conductive elastic member 2h is a polymer nanocomposite material in which silicone rubber is used as a matrix material, spherical carbon as a resistance adjusting agent, and alumina particles as ceramic fine particles are added (intermediate layer of the charging roller of Example 1). Is the same as the material that constitutes.

  A charging bias voltage of a predetermined condition is applied to the support metal plate 2a of the charging blade 2 from the power source S1. In this example, the charging bias voltage for the charging roller 2 is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac). More specifically, a DC voltage; −500 V, an AC voltage; a frequency f0.8 kHz, a peak-to-peak voltage Vpp 1.5 kV, and an oscillating voltage superimposed with a sine wave, and the peripheral surface of the photosensitive drum 1 is −500 V (dark potential). Vd) is uniformly charged.

  Between the charging blade 2 and the power source S1, charging blade current detecting means 2g for measuring the current flowing through the charging blade 2, and the charging blade 2 according to the resistance value obtained from the current value detected by the current detecting means. The pressure adjusting means 2f for adjusting the pressure applied to the photosensitive drum 1 is controlled, and the same control as that performed in the first embodiment is performed to adjust the pressure applied to the charging blade 2.

  As described above, according to the present embodiment, by adopting such a configuration, even when the charging blade is employed as the charging member, the same effect as that of the first embodiment can be obtained.

1 is a schematic configuration model diagram of an image forming apparatus of Embodiment 1. FIG. It is a schematic structure model diagram of a conventional image forming apparatus. It is a layer structure model figure of a photosensitive drum and a charging roller. It is a figure which shows the applied pressure dependence of the resistance value of charging roller (1)-(5). It is the figure which represented the principle in which a polymer nanocomposite material changes a resistance value with an applied pressure, (a) shows the state when the applied pressure is small, and (b) shows the state when the applied pressure is large. FIG. 3 is a diagram illustrating the pressure dependency of the resistance value of the charging roller included in the image forming apparatus according to the first exemplary embodiment (initially and after 50k times of image formation). FIG. 6 is a diagram illustrating a change in resistance value of a charging roller in the image forming apparatus according to the first exemplary embodiment. It is the figure which derived | led-out the relational expression of the applied pressure of a charging roller, and resistance value using the least squares method. 3 is a flowchart of a control method implemented in an implementation die 1. FIG. 3 is a schematic configuration model diagram of an image forming apparatus according to a second embodiment.

Explanation of symbols

1 ... Photosensitive drum (image carrier)
2... Charging member 2 g... Current detection means 2 f... Applied pressure adjustment means 3... Laser beam scanner 4... Development device 5. S1-S5 ... Power supply

Claims (7)

  In an image forming apparatus using a contact charging method in which a charging member is disposed in pressure contact with an image carrier, and the surface of the image carrier is uniformly charged to a predetermined polarity and potential by the charging member, the charging member includes: The resistance value changes depending on the pressure applied to the charging member, current detection means for measuring the current value flowing through the charging member, and pressure applied to adjust the pressure applied to the image carrier of the charging member And an adjustment unit.   The image forming apparatus according to claim 1, wherein the charging member has a polymer nanocomposite material in which a resistance adjusting agent and ceramic ultrafine particles are blended in a matrix material made of an elastomer.   The nanocomposite material is silicone rubber as the matrix material, spherical carbon having a number average particle size of 10 nm to 100 nm as the resistance adjuster, and alumina particles having a number average particle size of 10 nm to 100 nm as the ceramic ultrafine particles, The image forming apparatus according to claim 2, further comprising:   4. A pressing force applied to the image carrier by the charging member is controlled by the pressing force adjusting unit according to a resistance value obtained from a current value detected by the current detecting unit. The image forming apparatus according to any one of the above.   5. The control is performed when the power of the image forming apparatus is turned on, when the number of image formation after the control reaches a predetermined number, and when the absolute moisture content in the use environment atmosphere changes. The image forming apparatus described in 1.   The image forming apparatus according to claim 1, wherein the charging member is a roller-shaped charging roller.   The image forming apparatus according to claim 1, wherein the charging member is a charging blade having a plate shape.

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