Charge element, process cartridge, and image forming apparatus

A charge element includes an element body in parallel to a photoreceptor element and biased to the photoreceptor element by an elastic element, and a gap retainer unit provided on the element body, contacting with an outer face of the photoreceptor element to constantly retain a gap between an outer face of the element body and the outer face of the photoreceptor element, and comprising a first gap retainer element provided on the element body and a second gap retainer element provided on the first gap retainer element and made of a material softer than that of the first gap retainer element and that of the outer face of the photoreceptor element.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority from Japanese Patent Application No. 2010-115432, flied on May 19, 2010, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a charge element which evenly charges the outer face of a photoreceptor element as a photoreceptor drum used in a copier, a facsimile machine, a printer or a combined machine as well as to a process cartridge including the charge element, and an image forming apparatus including the process cartridge.

2. Description of the Prior Art

In prior art an electrophotographic image forming apparatus such as a copier, a facsimile machine, a printer or a combined machine comprises a photoreceptor drum on which an electrostatic latent image is formed, a charge roller evenly charging the outer face of the photoreceptor drum, an optical write unit exposing the evenly charged outer face of the photoreceptor element to form an electrostatic latent image, a develop unit developing the electrostatic latent image to form a toner image, a transfer unit transferring the toner image onto a paper sheet, and a cleaning unit removing remnant toner from the outer face of the photoreceptor drum after the transfer.

A contact type charge unit is generally used, comprising a charge roller contacting with the surface of the photoreceptor drum and a power source to apply direct voltage or alternating voltage to the charge roller. In this charge unit the charge roller is applied with the direct or alternating voltage to evenly charge the photoreceptor drum at a high voltage. It is known that the charge mechanism of the charge roller is an electric discharge according to Paschen's law in a finite space between the charge roller and the photoreceptor drum.

However, the contact type charge unit has various problems that due to the contact between the charge roller and the photoreceptor drum, a material composing the charge unit may exude and attach to the surface of the photoreceptor drum or a trace of the contacting charge unit may remain thereon. Moreover, other problems are noise from a vibrating charge unit when applied with an alternating voltage, a decrease in chargeability of the charge roller due to attached toner caused by the exuding material, and permanent deformation of the charge roller when the photoreceptor drum stops operating in a long period of time. In view of solving the problems, Japanese Patent Application Publication No. 2006-162646 (Reference 1), No. 2007-121480 (Reference 2), No. 2009-271131 (Reference 3) disclose a charge unit in which a charge unit and a photoreceptor drum are arranged with a distance between them, for example.

The charge roller in this charge unit comprises a body in parallel to the photoreceptor drum, gap retainer elements at both ends of the body and whose diameter is larger than that of the body, and a bias element biasing the body to the photoreceptor drum to allow the gap retainer elements to closely contact with the surface of the photoreceptor drum. The body and the photoreceptor drum are disposed with a distance and the distance may change owing to smudge or smear on the body and the photoreceptor drum, causing unevenness in charging. In the charge unit disclosed in the above references, the photoreceptor drum is evenly charged by superposing direct voltage and high alternating voltage on the body of the charge roller.

However, there still remains a problem that the appliance of the alternating voltage to the roller body may cause contaminant as a discharge product on the photoreceptor drum to disperse between the body and the photoreceptor drum and accumulate on the surface of the body over time. This causes unevenness in resistance of the surface of the body and uneven discharge, resulting in generation of a defective image.

In order to reduce the amount of contaminant on the photoreceptor drum, the charge roller can be disposed as far as possible from the photoreceptor drum. However, this requires an increase in the alternating voltage which is likely to cause anomalous discharge due to leak current, resulting in generation of a defective image including white dots.

In the meantime, the surface layer of the above charge roller (roller body) is made of conductive materials such as Ketjen Black®EC, conductive carbon as acetylene black, conductive polymer as polyaniline, polypyrrole, polyacetylene, and inorganic ion-conducting materials as sodium perchlorate, lithium perchlorate, calcium perchlorate, lithium chloride. By use of the conductive carbon, anomalous discharge due to the leak current is likely to occur, which may likely generate defective images including white dots. By use of the ion-conducting materials, since they are dispersible (soluble) in molecule level, anomalous discharge due to the leak current is unlikely to occur, preventing generation of defective images. The surface layer of the roller body disclosed in Reference 3 is made of ion-conducting materials with low resistance so that the charge roller can exert good chargeability even with a change in the distance from the photoconductor drum.

However, the charge unit in Reference 3 still faces a problem that at the beginning of usage (hereinafter, initial usage period), the surface of the roller element gets contaminated gradually over time. The contamination on the roller surface prevents the photoreceptor drum from being properly charged and causes anomalous discharge.

SUMMARY OF THE INVENTION

The present invention aims to provide a charge unit which can prevent anomalous electric discharge especially in the initial usage period as well as a process cartridge incorporating such a charge unit and an image forming apparatus incorporating such a process cartridge.

According to one aspect of the present invention, a charge unit comprises an element body in parallel to a photoreceptor element and biased to the photoreceptor element by an elastic element, and a gap retainer unit provided on the element body, contacting with an outer face of the photoreceptor element to constantly retain a gap between an outer face of the element body and the outer face of the photoreceptor element, and comprising a first gap retainer element provided on the element body and a second gap retainer element provided on the first gap retainer element and made of a material softer than that of the first gap retainer element and that of the outer face of the photoreceptor element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, one embodiment of the present invention will be described in detail with reference toFIG. 1toFIG. 5.FIG. 1shows the structure of an image forming apparatus according to one embodiment of the present invention.FIG. 2is a cross-sectional view of a process cartridge of the image forming apparatus inFIG. 1according to one embodiment of the present invention.

InFIG. 1an image forming apparatus1is configured to generate a full color image of yellow (Y), magenta (M), cyan (C), black (K) on a sheet of paper2. Herein, units associated with these colors are given numeric codes with Y, M, C, K at the end.

The image forming apparatus1comprises a body3, paper feeder units4, a resist roller pair5, a transfer unit6, a fuse unit7, four laser write units8Y,8M,8C,8K and four process cartridges9Y,9M,9C,9K.

A body3in a box shape for example is placed on the floor or the like and contains the paper feeder units4, resist roller pair5, transfer unit6, fuse unit7, laser write units8Y,8M,8C,8K, and process cartridges9Y,9M,9C,9K.

The paper feeder units4are provided at the bottom of the body3to contain a pile of paper sheets2, and comprise detachable paper cassettes10and feed rollers11. The feed rollers11feed the topmost paper sheets2to between the resist roller pairs.

The resist roller pair5, rollers5a,5b, is provided on a carrier path of the paper sheet2from the paper feeder units4to the transfer unit6. The rollers5a,5bhold a paper sheet2between them and transmit it to between the transfer unit6and the process cartridges9Y,9M,9C,9K at a timing when a toner image is formed.

The transfer unit6is provided above the paper feeder units4and comprises a drive roller12, a driven roller13, a transfer belt14, and transfer rollers15Y,15M,15C,15K. The drive roller12is placed downstream of a delivery direction of the paper sheet2and rotated by a motor or the like.

The driven roller13is rotatably supported by the body3and placed upstream of the delivery direction of the paper sheet2. The transfer belt14is a loop and extends around the drive roller12and the driven roller13. By rotation of the drive roller12, the transfer belt14rotates counterclockwise in the drawing.

The paper sheet2on the transfer belt14is carried between the transfer rollers15Y,15M,15C,15K and the photoreceptor drums18of the process cartridges9Y,9M,9C,9K.

Toner images on the photoreceptor drums18are transferred onto the paper sheet2by the transfer rollers15Y,15M,15C,15K. The transfer unit6transmits the paper sheet2having the toner image thereon to the fuse unit7.

The fuse unit7is provided downstream of the delivery direction of the paper sheet2, and comprises a roller pair7a,7bto press and apply heat to the paper sheet2sent from the transfer unit6to fuse the toner image on the paper sheet2.

The laser write units8Y,8M,8C,8K are provided above the body3in association with the process cartridges9Y,9M,9C,9K to irradiate with laser the photoreceptor drums18uniformly charged by the charge units17and generate an electrostatic latent image. In the present embodiment, in addition to the laser write units, an exposure unit comprising an LED array and an imaging portion can be used.

The process cartridges9Y,9M,9C,9K are arranged between the transfer unit6and the laser write units8Y,8M,8C,8K in the delivery direction of the paper sheet2. They are detachable from the body3.

As shown inFIG. 2, the process cartridges9Y,9M,9C,9K each comprise a cartridge case16, a charge unit17, the photoreceptor drum18, a cleaning blade19, a develop unit20and a lubricant coating unit21. The image forming apparatus1comprises charge rollers22as a charge element, the photoreceptor drums18, cleaning blades19, develop units20and lubricant coating units21.

The cartridge cases16detachable from the body3each contain the charge unit17, photoreceptor drum18, cleaning blade19, develop unit20and lubricant coating unit21.

The charge units17evenly charge the surfaces of the photoreceptor drums18. The structure of the charge unit17will be described later.

The photoreceptor drums18as photoreceptor element are each made of a conductive support element in diameter of about 30 mm to 100 mm on which a photoreceptive layer and a surface layer are overlaid. The photoreceptive layer is made of a photosensitive material. The support element is made of a conductive metal such as aluminum, aluminum alloy, nickel, stainless steel. The photoreceptive layer can be a single layer type in which charge generation material and charge transport material are integrated functionally, or a double layer, functional separation type of a charge generation layer and a charge transport layer.

In a typical functional separation type photoreceptor drum the charge generation layer is directly or via an intermediate layer overlaid on the support element, and the charge transport layer containing charge transport material is overlaid on the charge generation layer. The functional separation type photoreceptive layer is preferable for the photoreceptor drum18since it provides a higher degree of freedom in terms of photosensitivity. The surface layer entirely covers the photoreceptive layer, and is made of a synthetic resin such as polycarbonate, for example. The photoreceptor element can be a photoreceptive belt.

The photoreceptor drum18is a cylindrical or columnar rotatable element disposed with a gap from a develop roller30. The surface thereof contacts with the transfer belt14. Charged by the charge unit17uniformly, electrostatic latent images are generated by the optical write units8Y, M,8C,8K on areas R1of the surfaces of the photoreceptor drums excluding both longitudinal ends (FIG. 3; hereinafter, image formed area), respectively. Toner of a developer24is attracted onto the electrostatic latent images on the image formed areas R1to generate toner images. The toner images are transferred onto the paper sheet2on the transfer belt14.

The cleaning unit19comprises a cleaning roller25and a coil26. The cleaning roller25is columnar in parallel to and in contact with the photoreceptor drum18, and rotatably supported by a later-described coating case40.

The cleaning roller25rotates along with the photoreceptor drum18to remove remnant toner from the surface of the photoreceptor drum18after the transfer of the toner image to the paper sheet2. The coil26recovers the remnant toner removed by the cleaning roller25.

The develop unit20inFIG. 2comprises a developer supply unit28, a housing29, a develop roller30, and a not-shown doctor blade.

The developer supply unit28comprises a container32and a pair of agitation screws33. The container32is in a box shape in a length almost equal to the length of photoreceptor drum18in an axial direction and includes a partition34extending in a longitudinal direction to divide inside of the container32into a first area35and a second area36. The first and second areas35,36communicate with each other.

The container32contains developer including magnetic carrier and toner in the first and second areas35,36. Toner is supplied to one end of the first area in a longitudinal direction when needed and it is fine spherical particles manufactured by emulsion polymerization method or suspension polymerization method. It can be made by pulverizing a synthetic resin lump in which various dyes or pigments are mixed and dispersed or other pulverizations. The average particle size of the toner is 3 μm or more and 7 μm or less.

Magnetic carrier (magnetic powder) is contained in the first and second areas35,36and the average particle size thereof is 20 μm or more and 50 μm or less.

The agitation screws33are accommodated in the first and second areas35,36, respectively. A longitudinal direction of the agitation screws33is in parallel to that of the container32, the develop roller30and the photoreceptor drums18. The agitation screws33are rotated around the axis to deliver the developer24while agitating the toner and magnetic carrier.

InFIG. 2the agitation screw118in the first area35delivers the developer24from one end to the other in the longitudinal direction and the agitation screw33in the second area36delivers it oppositely.

Thus, the developer supply unit28agitates toner supplied from one end of the first area35with magnetic carrier and delivers it to the other end and to the second area36. It further agitates the toner and magnetic carrier in the second area36and supplies it to the surface of the develop roller30.

The housing29in a box shape is attached to the container32of the developer supply unit28to cover the container32, the develop roller30and else. It includes an opening29aat a portion facing the photoreceptor drum18.

The develop roller30being columnar is placed between the second area36and the photoreceptor drum18near the opening125ain parallel to the photoreceptor drum18and the container32. There is a gap between the develop roller30and the photoreceptor drum18facing each other. The gap forms a develop area37in which an electrostatic latent image is developed by attracting the toner in the developer24and a toner image is generated.

InFIG. 4the develop roller30comprises a develop sleeve39and a magnet roller38. The magnet roller38is parallel to the photoreceptor drum18in a longitudinal direction, supported by the housing29and does not rotate. It comprises a plurality of fixed magnetic poles extending straight longitudinally on an outer circumference. The fixed magnetic poles attract the developer24onto the surface of develop sleeve39.

The develop sleeve39contains the magnet roller38having a main magnetic pole and the fixed magnetic poles and rotates therearound. It is made of non-magnetic materials as aluminum alloy, brass, stainless steel (SUS) and conductive resin.

The doctor blade is provided at an end of the develop unit20closer to the photoreceptor drum18, and attached to the housing29with a distance from the outer face of the develop sleeve39. It adjusts an amount of the developer on the develop sleeve39to a desired amount by partially removing it in the container32.

In the develop unit20the developer supply unit28sufficiently agitates the toner and the magnetic carrier and the developer is attracted onto the outer face of the develop sleeve39by the fixed magnetic poles. Along with the rotation of the develop sleeve39, the developer attracted by the fixed magnetic poles are delivered to the develop area37. The developer of a desired amount adjusted by the doctor blade is attracted onto the photoreceptor drum18. Thus, the developer is held on the develop roller30and delivered to the develop area131to develop an electrostatic latent image on the image formed area R1of the photoreceptor drum18and generate a toner image.

Then, used developer24is dropped in the container32, accumulated and agitated with unused developer again in the second area36and used for developing an electrostatic latent image on the photoreceptor drum18. When a toner density sensor detects a decrease in toner density supplied to the photoreceptor drum18, the agitation screws33start rotating to deliver the toner to the develop roller30.

The lubricant coating unit21inFIG. 2comprises a case40, a coating roller41, a solid lubricant42, and a coating blade43.

The case40is accommodated in the cartridge case16in such a position that the photoreceptor drum18is placed between the case40and the develop unit20. The coating roller41is in parallel to the photoreceptor drum18and contacts therewith. It includes a core entirely covered with fibers and is rotatably supported by the case40. It rotates around the axis to scrape off a part of the solid lubricant42and coat the surface of the photoreceptor drum with the lubricant.

The solid lubricant42is formed in a cubic shape in parallel to the photoreceptor drum18and the coating roller41in the longitudinal direction. It is made of generally used lubricant materials. Preferably, it can be made of lubricant materials such as solid melamine cyanurate excelling in lubricity, polytetrafluoroethylene, boron nitride, metal salt of fatty acid excelling in film formation, zinc stearate. The solid lubricant42is biased by a not-shown spring to the coating roller41.

The coating blade43is in parallel to the photoreceptor drum in the longitudinal direction and can be made of elastic synthetic resins such as urethane resin, silicone resin, fluorine resin but urethane resin is preferable in terms of abrasion resistance and mechanical strength. Generally, it is produced by forming a sheet of elastic synthetic resin by centrifugal molding and cutting it into a blade.

The coating blade43is fixed on the sidewall of the coating case40closer to the charge unit17via a blade support element57made of materials selected from metal, plastic, and ceramic. The materials of the blade support element57are arbitrarily selectable. It is elastically deformed to contact with the surface of the photoreceptor drum18at a predetermined pressure. It is of a so-called counter type to contact with the photoreceptor drum18against the rotation thereof and thinly spread the solid lubricant on the photoreceptor drum18.

The charge unit17inFIG. 2is provided between the lubricant coating unit21and the develop unit20above the photoreceptor drum8, and comprises a charge roller22, springs45(FIG. 3A), a cleaning roller pair23, and a not-shown power source. The charge roller22is in parallel to the photoreceptor drum and is separable or approachable from/to the photoreceptor drums18and comprises a body46and a pair of gap retainer units47.

The body46inFIGS. 3A,3B comprises a metal core48, an electric resistant layer49surrounding the center of the metal core48, and a surface layer50.

The metal core48is columnar and made of a conductive metal. The electric resistant layer49is coaxial with the metal core48, surrounds the center of the metal core48in a longitudinal direction and integrally comprises a large diameter portion51in the center and a pair of small diameter portions52connecting with both ends of the large diameter portion51and whose diameter is smaller than that of the large diameter portion51. The diameters of the large and small diameter portions51,52are constant in the longitudinal direction. The large diameter portion51is longer than the image formed area R1on the photoreceptor drum18and both ends thereof are outside both ends of the image formed area R1.

The electric resistant layer49is formed of a thermoplastic resin composition in which ionic conductive polymer is dispersed. It is preferable to form the electric resistant layer49with materials having volume resistivity of 10−5to 10−9Ωcm. This is because materials with volume resistivity of less than 10−9Ωcm cannot exert sufficient chargeability or transferability while materials with volume resistivity of more than 10−5Ωcm causes leak currents due to voltage convergence on the photoreceptor drum18.

An arbitrary thermoplastic resin composition can be used for the electric resistant layer49, however, general resins such as polyethylene (PE), polypropylene (PP), polymethyl metacrylate (PMMA), polysthylene (PS) and their copolymer (AS, ABS), polyamide, polycarbonate (PC) are preferable owing to their manageability for molding. Polymer compound containing polyetherester amide is preferable for the ionic conductive polymer dispersed in the thermoplastic resin composition. Polyetherester amide is an ionic conductive material and evenly dispersed and immobilized in matrix polymer in molecular level, therefore, there is no deviation in resistance value due to insufficient dispersion unlike a compound in which electron-conductive agent as metal oxide or carbon black is dispersed.

Applied with a high voltage, paths through which electricity easily flow are locally formed in an electron-conductive agent. This may cause a leak current to the photoreceptor drum18and generation of a defective image including white or black dots with the charge roller22. Polyetherester amide is a polymer and unlikely to bleed out. Compound rate of the electric resistant layer49need be 20 to 70 weight % of thermoplastic resin and 80 to 20 weight % of ionic conductive polymer in order to set a desired resistance value.

Further, it is possible to add electrolyte salt in the electric resistant layer49for the purpose of adjusting the resistance value. Salt such as alkali metal salt including sodium perchlorate, quaternary phosphonium salt including lithium perchlorate, ethyl triphenyl phosphoniumtetra fluoroborate can be used. The plurality of conductive agents can be mixed as long as physicality is not impaired.

To evenly microdisperse a conductive material in matrix polymer at molecule level, a compatibilizing agent is added in the ionic conductive polymer. Additionally, additives as antioxidant can be also used as long as physicality is not impaired.

The resin compound can be produced arbitrarily. It is easy to produce the resin compound by mixing materials and melting and kneading them by a twin screw compounding extruder or a twin screw kneading extruder. Also, it is easy to form the electric resistant layer49on the metal core48by covering it with the ionic conductive resin compound by extrusion or injection molding.

The surface layer50is formed on the electric resistant layer49of the metal core48in order to prevent toner and toner additives from attaching to the electric resistant layer49, reducing chargeability.

The surface layer50is formed in uniform thickness in both of longitudinal and circumferential directions to cover the large diameter portion51of the electric resistant layer49. It is preferably made of fluorite resin, silicon resin, polyamide resin, polyester resin or the like since they can prevent toner attachment owing to their good non-adherence. The surface layer50can be formed by dissolving materials in organic solvent to produce paint and coating the paint by spraying, dipping, roll coating or the like. The thickness of the layer is preferably about 10 to 30 μm.

The surface layer50made of two-component paint together with a curing agent excels in environment resistance, non-adherence and mold releasability. Two component paint of a base agent containing hydroxyl in molecules and isocyanate resin producing cross-linking reaction with the hydroxyl is effective. Use of isocyanate resin induces cross-linking and curing reaction at a relatively low temperature of 100 degrees or less. Cross-linking density of the surface can be freely adjusted by changing the amount of curing agent relative to 1 functional group (OH group) equivalent.

Silicon resin, silicon, and grafted fluorine resin are good materials for the surface layer50with toner non-adherence taken into consideration.

Since electric property (resistance) of the charge roller22is important, the surface layer50needs to be conductive. It can be made ionic conductive by dispersing electrolyte salt in resin materials. Alkali metal chloride such as sodium perchlorate, lithium perchlorate, calcium perchlorate, lithium chloride, fluorine containing organic anion salt as quaternary phosphonium salt such as alkaline-earth metal salt, lithium bis(trifluoromethanesulfonyl)imide, lithium tris(trifluoromethanesulfonyl)methane, lithium trifluoromethanesulfonate, tyltriphenylphosphonium tetrafluoroborate, tetraphenylphosphonium bromide, modified aliphatic ethyldimethyl ammonium ethyl sulfate, stearic acid ammonium acetate, lauryl ammonium acetate can be used. Especially, lithium bis(trifluoromethanesulfonyl)imide, lithium tris(trifluoromethanesulfonyl), and methane lithium trifluoromethanesulfonate realize low resistance of the charge roller22.

To realize low resistance of the ionic conductive materials, polyether polyol type (ether oxygen) comprising polyethylene oxide, polypropylene oxide, or copolymer thereof is also needed. Polyether polyol preferably accounts for 20 to 70 weight % of the entire resin forming the surface layer (coating film), most preferably 35 to 55 weight %. Amount of polyether oxygen in polyether polyol is preferably 40 weight % or more in ethylene oxide amount (EO). A product of rate of polyether polyol resin and EO need be 25 weight % or more, preferably 45 weight % or more.

Additive amount of electrolyte salt is preferably 1 to 15 weight % of the entire resin forming the coating film, and most preferably 1.5 to 10 weight %. Plural kinds of electrolyte salt can be mixed as long as physicality is not impaired.

The gap retainer units47are provided at ends of the metal core48and around the small diameter portion52of the electric resistant layer49, that is, on both ends of the roller body46. The gap retainer units47are in an opposite position to non-image formed areas R2(FIG. 3A) which are continuous with the image formed area R1and on which no electrostatic latent image is formed.

The gap retainer units47are coaxial with the metal core48and the electric resistant layer49and each comprise first and second gap retainer elements53,54coaxial with each other. The first gap retainer element53is provided at end of the metal core48and on the small diameter portion52(roller body) of the electric resistant layer49and the diameter thereof is slightly larger than that of the surface layer50on the large diameter portion51. That is, a step D1in height of 40 to 60 μm is formed between the first gap retainer element53and the surface layer50as shown inFIG. 3B.

The second gap retainer element54is provided on the first gap retainer element53and the diameter thereof is slightly larger than that of the first gap retainer element53. That is, a step D2in height about 100 μm (preferably 60 μm or more 100 μm or less) is formed between the second gap retainer element54and the surface layer50as shown inFIG. 3B.

The gap retainer units47configured as above contact with the non-image formed areas R2on the photoreceptor drum18when the metal core48is biased to the photoreceptor drum18. Thereby, a gap between the surface layer50of the roller body46and the photoreceptor drum18coincides with the height of the step D2(about 100 μm for example) and is constantly maintained when the charge roller22rotates together with the photoreceptor drum18. The second gap retainer element54is made of a materials softer than those (polycarbonate for example) of the first gap retainer element53and the surface layer of the photoreceptor drum18as later described. Therefore, it is gradually abraded over time as the number of prints increases. When the second gap retainer element54is completely abraded, the first gap retainer element53contacts with the photoreceptor drum18and the gap between the surface layer50of the roller body46and the photoreceptor drum18is maintained at the height of the step D1, 40 to 60 μm, for example.

Thus, either of the first and second gap retainer elements53,54contacts with the non-image formed areas R2on the photoreceptor drum18to maintain the gap between the surface layer50of the roller body46and the photoreceptor drum18to about 100 μm, specifically 40 to 80 μm. This can prevent generation of defective images during operation of the charge unit17under ambient temperature and humidity. With the gap being over 100 μm, a discharge start voltage by the Paschen's law increases and local discharge breakdown or anomalous discharge is likely to occur.

Meanwhile, with the gap being below 40 μm, the photoreceptor drum18can be charged with a small discharge power. However, an air flow deteriorates in a narrow space between the charge roller22and the photoreceptor drum18. Because of this, a large amount of discharge product formed in a discharge area is accumulated in this space even after completion of image generation, and contaminates or adheres to the roller22and the photoreceptor drum18, causing a charge failure. Accordingly, the gap between the surface layer50of the roller body46and the photoreceptor drum18need be 40 μm or more.

In the initial use period, the surface of the charge roller22is clean and free from contaminant matter so that anomalous discharge due to leak currents does not occur even with a relatively large gap. However, as the number of paper sheets on which images are generated increases over time, contaminant matter is gradually accumulated on the charge roller22, which decreases the gap between the charge roller22and the photoreceptor drum18accordingly.

FIG. 4shows a relation indicated by a dashed-dotted line X between the number of paper sheets2printed and the gap when no anomalous discharge due to leak current occurs and a relation indicated by a dashed-two-dotted line between the same and the gap when no anomalous discharge due to accumulated contaminant matter occurs. In the graph, discharge due to leak current occurs in the area above the dash-dotted line X while no discharge occurs in the area below the dash-dotted line X. Similarly, no discharge due to accumulated contaminant matter occurs in the area above the dash-two-dotted line Y while the discharge occurs in the area below the dash-two-dotted line Y.

Thus, the gap between the charge roller22and the photoreceptor drum18has to be maintained to a value in the area between the lines X and Y. Thereby, the charge roller22can properly charge the photoreceptor drum18. After the initial use period, a change in the lines X and Y becomes gradual since contamination on the charge roller22does not increase much over time.

With the above taken into account, in the present embodiment the second gap retainer element54is made of easily abraded materials and the gap between the charge roller22and the photoreceptor drum18is set to be wide during the initial use period in which the chargeability of the charge roller22is highest. Over time the chargeability gradually decreases, the second gap retainer element54is abraded and the gap decreases. When the second gap retainer element54is completely abraded, the first gap retainer element53then contacts with the photoreceptor drum18. The first gap retainer element53is not abraded since it is made of materials with higher hardness than those of the surface layer of the photoreceptor drum18. Therefore, the gap between the charge roller22and the photoreceptor drum18is constantly maintained even with an increase in the number of paper sheets2printed over time. The gap can be constantly maintained in a state that a decrease in the chargeability of the charge roller slows down.

The gap retainer units47are required to stably form the gap without influenced by environment over a long period time. For this reason, it is preferable that the first and second gap retainer elements53,54are made of materials of low hygroscopic nature which are free from attachment of toner and toner additive and do not abrade the photoreceptor drum. Also, it is preferable that materials of the first and second gap retainer elements53,54are arbitrarily selected depending on various conditions.

Specifically, the gap retainer units47or the first second gap retainer elements53,54are preferably made of one or more materials selected from general resin such as polyethylene (PE), polypropylene (PP), polyacetal (POM), methyl metacrylate (PMMA), polysthylene (PS) and their copolymers (AS, ABS), polycarbonate (PC), urethane, and fluorine (PTFE). Preferably, the materials of the first gap retainer element53are ones harder than those of the surface layer50of the photoreceptor drum18among the above-mentioned resin materials. The materials of the second gap retainer element54are ones softer than those of the surface layer50and the first gap retainer element53.

Further, in view of preventing leak current to the photoreceptor drum18from occurring, the first and second gap retainer element53,54are preferably made of insulating materials with volume specific resistance of 10 to 13 Ωcm or more.

The gap retainer unit47is formed in the following manner. First, the first gap retainer element53made by molding is mounted on both ends of the electric resistant layer49and then covered with the second gap retainer element54. Alternatively, the second gap retainer element54is placed adjacent to the first gap retainer element53or it can be formed by coating the first gap retainer element53with solvent-soluble resin materials. Moreover, it is possible to increase precision of the first and second gap retainer elements53,54by cutting or polishing. The gap between the charge roller22and the photoreceptor drum18can be more precisely set by concurrently processing the first and second gap retainer elements53,54and the electric resistant layer49.

Next, the manufacture of the charge roller22is described with reference toFIGS. 5A to 5F. First, the electric resistant layer49is formed around the metal core48by injection molding inFIG. 5Aso that outer diameter thereof is constant. InFIG. 5Bboth ends of the electric resistant layer49are cut off to create the small diameter portions52. InFIG. 5Cboth ends of the metal core48and the small diameter portions52are pressed into ring portions55which are formed to include a hole in the same diameter as the outer diameter of the metal core48and step portions in the same diameter as that of the small diameter portions52. InFIG. 5Dthe outer circumferences of the ring portions55are cut off to form the first gap retainer elements53. Also, the outer circumference of the electric resistant layer49is cut off to form the large diameter portion51. InFIG. 5Ethe first gap retainer elements53are pressed into another ring portions56. InFIG. 5Fthe outer circumferences of the ring portions56are cut off to form the second gap retainer elements54. Then, the surface layer50is formed around the large diameter portion51of the electric resistant layer49to complete the charge roller22.

The springs45are provided for both ends of the metal core48of the charge roller22to bias them to both ends of the photoreceptor drum18.

The cleaning roller pair23is arranged in parallel with an interval and parallel to the charge roller22. It is rotatable around the axis and contacts with the surface of the charge roller22. It rotates together with the charge roller22to remove contaminant matter from the charge roller22.

The charge roller22is applied with a predetermined voltage from the power source. The applied voltage can be a direct voltage and preferably a superimposed direct and alternating voltage. This is because applied with a direct voltage, uneven potentials may occur on the surface of the photoreceptor drum18due to an uneven thickness of the electric resistant layer49and the surface layer50. On the other hand, applied with the superimposed voltage, potentials on the surface of the charge roller22are even and it can stably discharge and charge the photoreceptor drum18evenly. A peak voltage of the direct voltage is preferably set to be twice or more the charge start voltage of the photoreceptor drum18. The charge start voltage refers to an absolute value of a voltage at which the charge of the photoreceptor drum18starts when the charge roller22is applied with a direct voltage only. This causes a back discharge from the photoreceptor drum18to the charge roller22and a smoothing effect to evenly stably charge the photoreceptor drum18. The frequency of the alternating voltage is preferably 7 times or more the rotation speed of the photoreceptor drum18, thereby preventing viewable moire images.

The charge unit17is configured that the charge roller22can continuously contact with the photoreceptor drum18by the bias force of the springs45even though the second gap retainer elements54are abraded as the number of prints increases over time. InFIG. 4when the number of paper sheets2printed (number of images generated) is over the point S (at which a change in the lines X, Y gets gradual; the number of prints is 200K in the drawing), the second gap retainer element54is completely abraded and does not exist anymore. Instead, the first gap retainer element53contacts with the photoreceptor drum18to constantly maintain the gap. InFIG. 4the gap is 70 μm indicated by the solid line when an image is generated on a first sheet of paper2, and it gradually decreases over time and becomes constant at about 50 μm after the point S.

Next, image generation of the image forming apparatus1is described. First, the photoreceptor drum18is rotated and evenly charged with the charge roller22at −700V. Then, the photoreceptor drum18is exposed with laser and a voltage of an image portion thereon turns to −150V to generate an electrostatic latent image. The electrostatic latent image is applied with a bias voltage of −550V and developed in the develop area37by attracting toner of the developer24from the develop roller30of the develop unit20. Thus, a toner image is generated on the photoreceptor drum18.

The toner image is transferred onto the paper sheet2fed by the feed roller11and else between each photoreceptor drum18and the transfer belt14. The fuse unit7fuses the toner image to generate a color image on the paper sheet2.

Remnant toner T on the photoreceptor drum18is recovered by the cleaning roller. The toner-free photoreceptor drum18is neutralized by a not-shown neutralizer and coated with a part of the solid lubricant42by the lubricant coating unit21. Then, it is charged with the charge unit17again for the next image generation.

The image forming apparatus1performs a process control to prevent a variation in image quality due to environmental or temporal change. Specifically, it comprises a not-shown optical sensor detecting image density of a toner pattern which is formed on the photoreceptor drum under a condition that a bias voltage is constant, to detect develop performance of the develop unit20from a density change. A target toner density is changed to adjust the develop performance to a preset target performance, thereby maintaining constant image quality. For example, when the detected image density of a toner pattern is lower than a target toner density, a not-shown controller (CPU) controls a drive circuit for a motor agitating the developer to increase the toner density. When the detected image density is higher than the target toner density, the CPU controls the drive circuit to decrease the toner density. The toner density is detected by a not-shown toner density sensor. The image density of the toner pattern on the photoreceptor drum18may slightly vary because of a periodic unevenness in the image density caused by the develop roller30.

According to the image forming apparatus1in the present embodiment, the second gap retainer element is made of a material softer than those of the first gap retainer element53and the surface layer of the photoreceptor drum18and configured to be gradually abraded especially in the initial use period as the rotation of the charge roller22increases. The gap between the roller body46and the second gap retainer element54is decreased accordingly. This makes it possible to prevent occurrence of anomalous discharge due to leak current even when the roller body gradually gets contaminated as the number of prints increases in the initial use period.

Furthermore, the first gap retainer element53is made of a material harder than that of the surface layer of the photoreceptor drum18. Therefore, it is never abraded by contacting with the photoreceptor drum18after the second gap retainer element54is entirely abraded. After the initial use period in which the second gap retainer element54has contacted with the photoreceptor drum18, once the first gap retainer element53contacts with the photoreceptor drum18, the gap between the roller body46and the photoreceptor drum18is constantly maintained irrespective of an increase in the number of paper sheets2printed. In addition, contamination on the roller body46is not increased after the initial use period so that it is possible for the charge roller to stably charge the photoreceptor drum18without failure.

The first gap retainer element53is made of a material whose durometer hardness is larger than 55 selected from general resin as high density polyethylene, polypropylene (PP), polyacetal (POM), polymethylmethacrylate (PMMA), polysthylene and their copolymers (AS, ABS) and polycarbonate (PC). This makes it possible to surely maintain the gap between the roller body46and the photoreceptor drum18constantly irrespective of an increase in the number of paper sheets2printed.

The second gap retainer element54is made of a material whose durometer hardness is smaller than 50 selected from low density polyethylene, soft polypropylene, a fluoric resin layer (fluorocarbon polymer layer) containing FEP, PTFE. Because of this, during the initial use period it is gradually abraded contacting with the photoreceptor drum18as the number of prints increases so that the gap between the roller body46and the photoreceptor drum18is surely decreased accordingly.

The process cartridges9Y,9M,9C,9K and the image forming apparatus1according to the present embodiment comprise the above charge unit17. Therefore, it can prevent occurrence of anomalous discharge due to leak current even with a gradual contamination on the roller body46as the number of prints increases since the roller body46gradually approaches the photoreceptor drum18.

In the present embodiment described above the second gap retainer element54is formed by cutting the outer circumference of the ring portions56, for example. However, the present invention should not be limited to such an example. Alternatively, it can be formed by adhering a sheet onto the outer circumference of the first gap retainer element53or coating it with resin.

The inventors of the present invention produced several samples of the charge roller22according to the present embodiment and conducted experiment to confirm their effects. The results are shown in the following Table.

In this experiment the inventors generated images on the paper sheets2, using the sample rollers A to G and checked occurrence of white dots, anomalous discharge due to leak current, image quality on the 500,000thpaper sheet2and the state of the charge roller22(gap retainer element) after 500,000 paper sheets are printed. The sample rollers F, G are not according to the present invention and produced for comparison. Results are indicated by “Good”, “Poor” in the Table. The sample rollers were applied with a direct voltage of −700V and an alternating voltage of 2.2 kVpp (at frequency of 2 kHz) under ambient condition of temperature 23 degrees, humidity 60% RH.

The sample A was produced in the following manner. First, 25 weight % of ABS resin (GR-3000 by Denki Kagaku Kogyo Kabushiki Kaisha) and 75 weight % of polyether ester amide (IRGASTAT, P18 by BASF) were blended to acquire a resin compound. Polycarbonate glycidyl methacrylate acrylonitrile copolymer (Modiper CL440-G by NOF Corporation) of 4 pts·wt. was added to 100 pts·wt. of the resin compound and melted and kneaded to acquire a molten resin compound. The electric resistant layer49in outer diameter 13 mm was produced by injection molding of the molten resin compound on the metal core48in outer diameter 10 mm made of Ni coated free cutting steels (SUM). Then, the small diameter portions52were formed at both ends of the electric resistant layer49and pressed into the ring portions55together with both ends of the metal core48. The ring portions were in outer diameter 13 mm, inner diameter 12 mm and made of high density polyethylene (NOVATEC HD, HY540 by Japan Polychem Co.). By cutting the ring portions55, the first gap retainer elements53in outer diameter 12.60 mm and the large diameter portion51in outer diameter 12.46 mm of the electric resistant layer49were obtained. The first gap retainer elements53were pressed into the ring portions56in outer diameter 13 mm, inner diameter 12.6 mm made of low density polyethylene (NOVATEC LD, LJ902 by Japan Polychem Co.). By cutting the ring portions56, the second gap retainer elements54in outer diameter 12.64 mm were obtained. The surface of the large diameter portion51was spray-coated in thickness of 20 μm with paint including 20 pts·wt. of toner non-adherent acrylic modified silicone resin (3000VH-P by Kawakai Paint MFG Co., Ltd.), 50 pts·wt. of polyetherpolyol resin (E504 by Asahi Glass Co., Ltd), 24 pts·wt. of isocyanate curing agent (T4 by Kawakami Paint MFG Co., Ltd.), 5.5 pts·wt. of lithium bis(trifluoromethanesulfonyl)imide butyl acetate solvent (by Sanko Chemical Ind. Co., Ltd) as electrolyte salt, 0.5 pts·wt. of organochloride catalyser (U-CAT-SA1 by San-Apro Ltd.), 0.5 pts·wt. of butyl acetate molten carbon black dispersion (REC-SM23 by Resino Color Industry Co. Ltd.) and whose solid components had been conditioned in a diluted solvent of butyl acetate and methyl ethyl ketone (MEK). Then, it was heated at temperature of 100 degrees in 1.5 hours to form the surface layer50by hardening the coating. Thus, the sample A of the charge roller22with a step of 70 μm between the second gap retainer elements54and the electric resistant layer49and a step of 50 μm between the first gap retainer elements53and the electric resistant layer49was obtained.

The sample B was produced in the following manner. First, as in the sample A, the electric resistant layer49in outer diameter 13 mm was formed on the metal core48of Ni coated SUM in outer diameter 10 mm. The small diameter portions52were formed at both ends of the electric resistant layer49and pressed into the ring portions55in outer diameter 13 mm, inner diameter 12 mm of high density polyethylene (NOVATEC HD, HY540 by Japan Polychem Co.) together with both ends of the metal core48. By cutting the ring portions55, the first gap retainer elements53in outer diameter 12.60 mm and the large diameter portion51in outer diameter 12.46 mm were obtained. A low density polypropylene (NOVATEC LL, UF240 by Japan Polychem Co.) film (containing adhesive component) in thickness 20 μm was adhered onto the outer circumferences of the first gap retainer elements53to form the second gap retainer elements54. The surface layer50was then formed in the same manner as that of the sample A. Thus, the sample B of the charge roller22with a step of 70 μm between the second gap retainer elements54and the electric resistant layer49and a step of 50 μm between the first gap retainer elements53and the electric resistant layer49was obtained.

The sample C was produced in the following manner. First, as in the sample A, the electric resistant layer49in outer diameter 13 mm is formed on the metal core48of Ni coated SUM in outer diameter 10 mm. The small diameter portions52are formed at both ends of the electric resistant layer49and pressed into the ring portions55in outer diameter 13 mm, inner diameter 12 mm of high density polyethylene (NOVATEC HD, HY540 by Japan Polychem Co.) together with both ends of the metal core48. By cutting the ring portions55, the first gap retainer elements53in outer diameter 12.60 mm and the large diameter portion51in outer diameter 12.42 mm were obtained. The first gap retainer elements53were pressed into the ring portions56made of fluorinate resin (Neo Flon NP-20 by Daikin Industries Ltd.) in outer diameter 13 mm, inner diameter 12.4 mm. By cutting the ring portions56, the second gap retainer elements54in outer diameter 12.55 mm were formed. The surface layer50was then formed in the same manner as that of the sample A. Thus, the sample C of the charge roller22with a step of 70 μm between the second gap retainer elements54and the electric resistant layer49and a step of 50 μm between the first gap retainer elements53and the electric resistant layer49was obtained.

The sample D was produced in the following manner. First, as in the sample A, the electric resistant layer49in outer diameter 13 mm was formed on the metal core48of Ni coated SUM in outer diameter 10 mm. The small diameter portions52were formed at both ends of the electric resistant layer49and pressed into the ring portions55in outer diameter 13 mm, inner diameter 12 mm of high density polyethylene (NOVATEC HD, HY540 by Japan Polychem Co.) together with both ends of the metal core48. By cutting the ring portions55, the first gap retainer elements53in outer diameter 12.60 mm and the large diameter portion51in outer diameter 12.46 mm were obtained. A flexible polypropylene (Newcon R-type by Japan Polychem Co.) film (containing adhesive components) in thickness 20 μm was adhered onto the outer circumferences of the first gap retainer elements53to form the second gap retainer elements54. The surface layer50was then formed in the same manner as that of the sample A. Thus, the sample D of the charge roller22with a step of 70 μm between the second gap retainer elements54and the electric resistant layer49and a step of 50 μm between the first gap retainer elements53and the electric resistant layer49was obtained.

The sample E was produced in the following manner. First, as in the sample A, the electric resistant layer49in outer diameter 13 mm was formed on the metal core48of Ni coated SUM in outer diameter 10 mm. The small diameter portions52were formed at both ends of the electric resistant layer49and pressed into the ring portions55in outer diameter 13 mm, inner diameter 12 mm of high density polyethylene (NOVATEC HD, HY540 by Japan Polychem Co.) together with both ends of the metal core48. By cutting the ring portions55, the first gap retainer elements53in outer diameter 12.60 mm and the large diameter portion51in outer diameter 12.46 mm were obtained. The outer circumferences of the first gap retainer elements53were coated with a primer and a fluorinate resin (ZX-022 by Fuji Kasei Kogyo Co., Ltd.) in thickness 20 μm and then cross-linked with isocyanate to form the second gap retainer elements54. The surface layer50was then formed in the same manner as that of the sample A. Thus, the sample E of the charge roller22with a step of 70 μm between the second gap retainer elements54and the electric resistant layer49and a step of 50 μm between the first gap retainer elements53and the electric resistant layer49was obtained.

The sample F for comparison was produced in the following manner. First, as in the sample A, the electric resistant layer49in outer diameter 13 mm was formed on the metal core48of Ni coated SUM in outer diameter 10 mm. The small diameter portions52were formed at both ends of the electric resistant layer49and pressed into the ring portions55in outer diameter 13 mm, inner diameter 12 mm of high density polyethylene (NOVATEC HD, HY540 by Japan Polychem Co.) together with both ends of the metal core48. By cutting the ring portions55, the first gap retainer elements53in outer diameter 12.70 mm and the large diameter portion51in outer diameter 12.42 mm were obtained. The surface layer50was then formed in the same manner as that of the sample A. Thus, the sample F of the charge roller22with a step of 120 μm between the first gap retainer elements53and the electric resistant layer49was obtained.

The sample G for comparison was produced in the following manner. First, as in the sample A, the electric resistant layer49in outer diameter 13 mm was formed on the metal core48of Ni coated SUM in outer diameter 10 mm. The small diameter portions52were formed at both ends of the electric resistant layer49and pressed into the ring portions55in outer diameter 13 mm, inner diameter 12 mm of high density polyethylene (NOVATEC HD, HY540 by Japan Polychem Co.) together with both ends of the metal core48. By cutting the ring portions55, the first gap retainer elements53in outer diameter 12.2 mm and the large diameter portion51in outer diameter 12.42 mm were obtained. The surface layer50was then formed in the same manner as that of the sample A. Thus, the sample G of the charge roller22with a step of 30 μm between the first gap retainer elements53and the electric resistant layer49was obtained.

As shown in the Table, white dotted images occurred due to an anomalous discharge by leak current using the samples F, G while they did not using the samples A to E according to the present embodiment. Similarly, using the samples F, G, color and black dotted images and images with unintended lines occurred on the 500,000thsheet of paper2due to uneven discharge by accumulated contaminant while they did not at all, using the samples A to E according to the present embodiment.

The above embodiment has described an example of the image forming apparatus1comprising the process cartridges9Y,9M,9C,9K detachable from the body3and each including the cartridge case16, charge unit17, photoreceptor drum18, cleaning unit19, and develop unit20. However, the present invention should not be limited to such an example. The process cartridge has only to include the develop unit20. Moreover, the image forming apparatus1has only to include the charge unit17and can exclude the process cartridges9Y,9M,9C,9K.

Although the present invention has been described in terms of exemplary embodiments, it is not limited thereto. It should be appreciated that variations or modifications may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims.