Patent Application
20080175625
The present invention relates to an electrophotographic image forming apparatus and a developing device for use in the image forming apparatus. The present invention also relates to a method of charging a developer in an electrophotographic image forming apparatus.
Conventionally, there has been known two types of developing processes for use in the electrophotographic image forming apparatus; a single-component developing method using only toner as a principal component of the developer and a two-component developing method using toner and carrier in combination as principal components of the developer.
The developing device of the single-component developing method has a toner transporting member which transports toner and a frictional charging member which contacts with a toner transporting surface of the toner transporting member. With this arrangement, the toner transported by the toner transporting member is brought into the contact region of the frictional charging member, where it is formed into a thinned layer of toner charged with a certain polarity. Since the single-component development system charges the toner through the contact with the frictional charging member, the developing device has a simple and compact structure and therefore may be constructed economically. Contrarily, the toner is subjected to a strong stress in the contact region of the frictional charging member, which can damage the toner and deteriorate its charging ability in a short period of time. Also, the toner can adhere to the toner transporting member and/or the frictional charging member due to the contact pressure, which reduces charging abilities of those members and, as a result, shortens the life of the developing device.
Typically, the developing device using the two-component developing method causes the toner and the carrier to be charged into opposite polarities through the frictional contacts thereof. Therefore, the stress exerted on the toner is less than that in the developing device of the single-component developing method. Also, the carrier has a larger surface area than the toner, which provides less contamination to the carrier by the possible adhesion of the toner. However, an amount of toner fixed on the surface of the carrier, called toner-spent, increases with a mixing time, which in turn reduces the charging ability against the toner and causes problems such as unwanted fog-like toner adhesion onto the resultant image and/or scattering of the insufficiently charged toner into the air. It may be thought that the life of the two-component developing device be extended by increasing an amount of carrier contained in the developing device. Disadvantageously, this results in a considerable increase in size of the developing device.
In order to solve the problems described for the two-component developing device, JP 59-100472 A discloses a developing device in which an increase of deteriorated carrier is restricted by supplying new carrier only or new carrier and toner in combination to the developer while discharging a part of the deteriorated developer, intermittently. This, indeed, extends the life of the developer without increasing the size of the developing device but, disadvantageously, needs a mechanism for collecting the discharged carrier and also uneconomically wastes a considerable amount of carrier, which can cause an environmental problem. Further, a considerable number of printings are required to establish a predetermined ratio between not-deteriorated and deteriorated carriers.
JP 09-269614 A discloses a carrier and an image forming method using the carrier, in which each particle of carrier has a core material and a resin layer covering the core, the layer being made of a matrix resin and electrically conductive small particles dispersed in the matrix resin. In use, even if a portion or portions of the surface of this carrier are chipped off by the contact with other particles such as carrier particles and toner particles and/or components such as roller and screw, the underlying resin particles are alternatively exposed at the surface of the carrier, so that they make contacts with the toner to provide the toner with a necessary electric charge. However, a thickness of a resin cover layer is limited and then, once the layer is consumed, the carrier reaches the end of its life.
JP 2003-215855 A discloses a two-component developer comprising a carrier and a toner which carries chargeable particles on the surface thereof and an image forming method using the developer. The charging particles function as polishing material for removing the toner-spent which is the adhesion of the toner onto the carrier and thereby extending the life of the carrier. JP 2003-215855 Also discloses that the charging particle polishes the surface of an electrostatic latent image bearing member in the region for cleaning the bearing surface of the electrostatic latent image bearing member. Disadvantageously, the charging particle tends to be electrically charged into a polarity which is different from the polarity provided to the toner. This results in that the charging particle is likely to adhere to a region outside the image forming region, i.e., non-image forming region, of the electrostatic latent image bearing member and then consumed in a short period of time. In particular, a production of a number of images each with smaller image portions such as text images wastes a great amount of charging particles to restrict the polishing and the resultant reproduction characteristics of the carrier.
JP 2006-308687 A discloses an image forming apparatus having a developing device with a magnetic roller and a developing roller. The magnetic roller supports a developing material including toner and carrier. The carrier is magnetically retained on the magnetic roller and electrically holds the toner which is selectively provided to the developing roller for the development of the electrostatic latent images on the electrostatic latent image bearing member into visualized images. In particular, the developer also includes the charging particles existing but not electrically or magnetically retained between toner and carrier particles to prevent the unwanted adhesion of the toner and thereby the generation of the toner-spent on the carrier. The charging particles, however, are contained only in the toner initially introduced into the developing device and also consumed gradually with the consumption of the toner, due to the electrical connection with the toner, and supplied through the developing roller to the non-image forming region of the electrostatic latent image bearing member. As a result, a mass production of the image with a smaller image region or black-to-white ratio such as text image causes a considerable amount of charging particles to be wasted and thereby fails to ensure a long term, stable electric charging of the toner.
Therefore, it is an object of the present invention to provide a developing device and an image forming apparatus capable of providing a long-term stable toner-charging property to the carrier of the two-component developer.
In order to achieve the foregoing object, the present invention provides a developing device. The developing apparatus has a developer including toner and carrier particles, a first transport member, a second transport member positioned to oppose to the first transport member across a first region and to the electrostatic latent image bearing member across a second region. A first electric field forming unit forms a first electric field between the first and second transport members to move the toner particles from the first transport member to the second transport member. A second electric field forming unit forms a second electric field between the second transport member and the electrostatic latent image bearing member to move the toner particles from the second transport member to the electrostatic latent image bearing member and thereby to visualize an electrostatic latent image on the image into a visible image. The developer further includes charging particles releasably retained on surfaces of the toner particles. Once released from the toner particles and then held on surfaces of the carrier particles, the third particles function to provide an electric charge of the first polarity to the toner particles by the contact therewith.
According to the present invention, the first particles in the developing apparatus is provided with a stable amount of electric charge through frictional contacts with the third particles held on the second particles. Accordingly, the developing device and the image forming apparatus that incorporate the present invention can form images having an elevated quality over the long term.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following descriptions of the preferred embodiments are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses.
Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, terms referring to specific directions (for example, “up”, “down”, “left”, “right”, and other terms including one of those terms in combination, and “clockwise” and “counterclockwise”) will be used. It should be noted that these terms are used for the purpose of facilitating the understanding of the invention taken in conjunction with the drawings and meanings of these terms should not be construed as restricting the present invention. Also, in an image forming apparatus and developing device described below, like reference numerals indicated like parts throughout the specification.
1. Image Forming Apparatus
The charging station 16 includes charging device 26 which charges a photosensitive layer, or a peripheral surface of the photosensitive member 12, to a predetermined voltage. In the embodiment, the charging device 26 is shown in the form of a cylindrical roller. Alternatively, another charging devices in the form of, for example, rotatable or unrotatable roll brush and wire, can also be used. The exposure station 18 has a passage 32 through which an exposure device 28 provided adjacent to or away from the photosensitive member 12 projects image light 30 onto the outer peripheral surface of the photosensitive member 12 to form an electrostatic latent image. The electrostatic latent image has a portion or portion exposed to the light and thereby decreased in voltage and a portion or portions not exposed to light and thereby maintaining substantially the charged voltage. In the embodiment, the voltage-decreased portion is the imaging portion and the voltage-maintained portion is the non-imaging portion. The development station 20 has developing device 34 which visualizes the electrostatic latent image using a powder developer. The details of the developing device 34 will be described later. The transfer station 22 has transfer device 36 which transfers the visualized images formed on the peripheral surface of the photosensitive member 12 onto a sheet 38 such as paper and films. In the embodiment, the transfer device 36 is shown in the form of a cylindrical roller. Alternatively, the transfer device using a wire may be used. The cleaning station 24 has cleaning device 40 which collects the untransferred toner remaining on the peripheral surface of the photosensitive member 12 without being transferred to the sheet 38 at the transfer station 22. In the embodiment, the cleaning device 40 is shown in the form of blade made of plate. Alternatively, another type of cleaning device in the form of, for example, rotatable or unrotatable brush roll may be used.
In operation of the image forming apparatus 1 so constructed, the photosensitive member 12 rotates in the clockwise direction by the driving of the motor (not shown) Incremental portions of the photosensitive member passing through the charging station 16 are electrically charged to a predetermined voltage by the charging device 26. The charged peripheral portion of the photosensitive member is exposed to the image light 30 at the exposure station 18 to form an electrostatic latent image. The electrostatic latent image is transported to the development station 20 with the rotation of the photosensitive member 12, where it is visualized into a visual image using powder developer material by the developing device 34. The visualized developer image is transported to the transfer station 22 with the rotation of the photosensitive member 12, where it is transferred to the sheet 38 by the transfer device 36. The sheet 38 supporting the developer image is transported to a fixing station (not shown) where the developer image is permanently fixed to the sheet 38. The peripheral portion of the photosensitive member passing through the transfer station 22 is transported to the cleaning station 24 where the untransferred developer remaining on the peripheral surface of the photosensitive member 12 is collected.
2. Developing Device
The developing device 34 includes a container or housing 42 which accommodates a two-component developer containing a first component or nonmagnetic toner particles and a second component or magnetic carrier particles. The housing accommodates various members which will be described below. For the simplification of the drawing, a part of the housing 42 is omitted therefrom. The housing 42 has an opening 44 opened toward the photosensitive member 12. A developing roller 48 serving as a toner transport member (a second transport member) is provided in a space 46 formed adjacent the opening 44. The developing roller 48, which is made of a cylindrical member (a second cylindrical member), is rotatably disposed in parallel with the photosensitive member 12 so that a constant development gap 50 is defined between the peripheral surface of the photosensitive member 12 and the opposed peripheral surface of the developing roller 48.
Provided on the back (right side in the drawing) of the developing roller 48 is another space 52 for receiving a transporting roller 54 or developer transport member (a first transport member) in parallel with the developing roller 48 so that a certain supplying and collecting gap 56 is defined between the opposed peripheral surfaces of the developing roller 48 and the transporting roller 54. The transporting roller 54 has a magnet 58 unrotatably fixed and a cylindrical sleeve 60 (a first cylindrical member) supported for rotation around the magnet 58. A regulating plate 62 is secured to the housing 42 so that it extends substantially in parallel with a longitudinal central axis of the sleeve 60 to define a regulating gap 64 between the peripheral surface of the sleeve 60 and the distal end of the regulating plate 62 opposing thereto.
The magnet 58 has a plurality of magnetic poles each opposing to the inner peripheral surface of the transporting roller 54 and extending radially outwardly from the central axis of the transporting roller 54. In the embodiment, the magnet 58 includes a magnetic pole S1 opposed to the upper inner surface portion of the transporting roller 54 adjacent the regulating plate 62, a magnetic pole N1 opposed to the left side inner surface portion of the transporting roller 54 adjacent the supplying and collecting gap 56, and a magnetic pole S2 opposed to the lower inner surface portion of the transporting roller 54, and two neighboring magnetic poles N2 and N3 having the same polarity and opposed to the right side inner surface portion of the transporting roller 54.
A developer mixing chamber 66 is provided on the back of the transporting roller 54. The mixing chamber 66 has a front chamber 68 positioned adjacent the transporting roller 54 and a rear chamber 70 positioned away from the transporting roller 54. A front screw 72 or front mixing and transporting member for mixing and transporting the developer in a direction extending from the front surface to the rear surface of the drawing sheet is rotatably mounted in the front chamber 68, and a rear screw 74 or a rear mixing and transporting member for mixing and transporting the developer in the opposite direction is also rotatably mounted in the rear chamber 70. As shown in drawing, the front chamber 68 and the rear chamber 70 may be separated by a partition 76 provided between the front and rear chambers. In this embodiment, portions of the partition in the vicinity of the opposite ends of the front chamber 68 and the rear chamber 70 are removed to form passages (not shown) so that the chambers 68 and 70 are communicated through the passages with each other to feed the developer reached the downstream end of one chamber into the upstream end of the other chamber and vice versa.
In operation of the developing device 34 so constructed, the developing roller 48 and the sleeve 60 rotate in the directions indicated by arrows 78, 80, respectively, by the driving of the motor (not shown). The front screw 72 rotates in the direction of arrow 82 and the rear screw 74 rotates in the direction of arrow 84. This allows that developer 2 accommodated in the developer mixing chamber 66 is mixed and circulated between the front and rear chambers 68 and 70. Consequently, the toner and carrier particles included in the developer make frictional contacts with each other to be electrically charged with different polarities. In the embodiment, it is assumed that the carrier particles are positively charged and the toner particles are negatively charged. As shown in
Referring back to
The toner particles 6 held on the developing roller 48 in the supplying sub-region 90 is transported in the counterclockwise direction with the rotation of the developing roller 48 into a region (development region) 96 defined between the photosensitive member 12 and the opposing developing roller 48, where they are transferred to the electrostatic latent image region on the peripheral surface of the photosensitive member 12.
In the embodiment, a negative voltage VH is imparted to the peripheral surface of the photosensitive member 12 by the charging device 26. Typically, the charged region of the photosensitive member 12 is partially exposed to the image light 30 to form the electrostatic latent image including the image region with a surface voltage VL decreased by the exposure to the image light 30 and the non-image region with a surface voltage VH substatially identical to the original charged voltage. This results in that, in the development region 96, the negatively charged toner particles 6 adhere to the electrostatic latent image region with an aid of the electric field formed between the photosensitive member 12 and the developing roller 48 to visualize the electrostatic latent image.
The toner particles 6 are supplied to the developer 2. Preferably, the amount of toner particles 6 to be supplied substantially corresponds to that consumed by the development. To this end, the developing device 34 includes means for measuring a mixed ratio between amounts of toner and carrier particles in the housing 42. Also, a toner supply unit 98 is mounted above the rear chamber 70. The toner supply unit 98 has a container 100 for accommodating the toner particles. An opening 102 is formed at the bottom of the container 100, in which a supply roller 104 is disposed. The supply roller 104 is drivingly connected to a motor (not shown) so that the motor drives the supply roller 104 when the measurement of the mixing ratio between the toner and carrier particles to supply supplemental toner particles into the rear chamber 70.
3. Electrical Field Forming Unit
In order to make an effective transfer of the toner particles 6 from the sleeve 60 to the developing roller 48 in the supplying sub-region 90, the developing roller 48 and the sleeve 60 are electrically connected to an electric field forming unit 110 which is illustrated in
An embodiment of the electric field forming unit 110 in
As shown in
Accordingly, in the supplying sub-region 90, the negatively charged toner particles 6 are subjected to the pulsative electric field formed between the developing roller 48 and the sleeve 60 and are electrically attracted from the sleeve 60 to the developing roller 48. Also, in the development region 96, the negatively charged toner particles 6 held on the developing roller 48 adhere to the electrostatic latent image region due to the voltage difference between the developing roller 48 (VDC1: −200 V) and the electrostatic latent image region (VL: −80 V).
4. Developer
One of the disadvantages in using two-component developer including toner and carrier particles is that the toner particle or particles permanently adhere to the surface of the carrier particles to cause so-called toner-spent which would reduce the life of the carrier particles. In order to overcome this problem, charging particles (implant particles) are added in the two-component developer as a third component of the developer in the embodiments of the image forming apparatus of the present invention.
As best shown in
In the image forming operation, the charging particles 8 are transported together with the toner and carrier particles 6 and 4 within the housing 42. Also, the charging particles 8 are provided onto the sleeve 60 and transported through the regulating region 86, the supplying and collecting region 88, and the discharge region 94. In this transportation process, the charging particle 8 are positively charged by the contacts with toner particles 8 and then supported on the negatively charged toner particles 6. When the positively charged charging particles 8 are placed in the electric field formed in the supplying and collecting region 88, they are electrically forced in a direction opposite to the electric force acting on the toner particles 6 to separate from the toner particles 6. Subsequently, the separated charging particle 8 are in turn held on or implanted into the peripheral surface of the carrier particles 4 by the stress acting between the separated charging particle 8 and the carrier particles 4. In particular, as shown in
As described above, the charging particles 8 are electrically charged in the polarity opposite to that of the toner particles 6. Therefore, as shown in
If the charging particles were used for the developing device 34′ free from a developing roller as shown in
Further, in the developing device disclosed in JP 2006-308687 A, the charging particles are substantially not retained on toner or carrier particles, i.e., behaving freely, among toner and carrier particles. Also, the charging particles initially introduced into the developing device are charged in a polarity opposite to that of toner particles. This results in that the charging particles are electrically combined with the toner particles and then supplied to the developing roller together with the toner particles and then consumed onto the electrostatic latent non-image region on the photosensitive member, which deteriorates the charging ability of the toner particles. Contrarily, according to the developing device of the present invention, the charging particle 8 separated from the toner particles 6 in the supplying and collecting region 88 are quickly held onto the carrier particles 4 and then stay on the peripheral surface of the sleeve 60, rather than being supplied and consumed onto the photosensitive member 12 through the developing roller 48, which ensures the carrier particles to have a long term, stable toner-charging ability. Unavoidably, a small number of charging particles tend to be consumed onto the developing roller 48, together with toner particles 6, even in the developing device of the present invention. This is not a serious problem because the charging particles 8 are supplied from the supply unit 98 together with the toner as necessary to retain a necessary amount of charging particles in the developing device, which ensures a stable toner-charging property over the long term.
As described above, in the previous embodiment, the toner particles 6 are negatively charged and the carrier particles 4 are positively charged by frictional contacts therebetween. The charging particles 8 provide negative charge to the toner particles by the frictional contacts therewith. In turn, the charging particles are charged into the positive polarity by the contact with toner particles.
The charged polarities of the particles are not restrictive to the present invention. For example, the toner and carrier particles may be charged positively and negatively, respectively, by the frictional contacts therewith, and the charging particles may be charged into the negatively charged by the frictional contacts with the toner particles while imparting positive charge to the toner particles.
5. Materials for Particles
Materials of constituting toner, carrier and charging particles, respectively, will be described below.
Charging Particles
The charging particles used are selected in accordance with the polarity of the charge to be provided to the toner particles. A number-average particle diameter of the charging particle ranges, for example, from 100 to 1,000 nm. When the toner particles to be negatively charged through the frictional contact with the carrier are used, particles to be positively charged through the frictional contact with the toner particles are used as charging particles. Such particles may be composed of inorganic particles such as strontium titanate, barium titanate, and almina, or thermoplastic resins or thermosetting resins such as acrylic resin, benzoguanamine resin, nylon resin, polyimide resin and polyamide resin. A resin composing the particle may contain a positively charged control agent to be positively charged by contact with the toner particles. For a positively charged control agent, for example, nigrosine dyes and quaternary ammonium salt may be used. The charging particle may be composed of a nitrogen-containing monomer. Examples of materials composing the nitrogen-containing monomer include, for example, 2-dimethylaminoethyl acrylate, 2-diethylaminoethyl acrylate, 2-dimethylaminoethyl methacrylate, 2-diethylaminoethyl methacrylate, vinylpyridine, N-vinylcarbazole, and vinylimidazole.
When using toner particles to be positively charged by contact with the carrier, particles to be negatively charged by contact with the toner are used for the charging particles. For those particles, for example, inorganic particles such as silicand titanium oxide, or particles composed of thermoplastic resins or thermosetting resins such as fluororesin, polyolefin resin, silicone resin, polyester resin may be used. A resin composing the charging particle may contain a negatively charged control agent to be negatively charged by contact with the toner particles. For a negatively charged control agent, for example, chromium complex, aluminum complex, iron complex and zinc complex of salicylic acid or naphthol may be used. The charging particle may be copolymers of fluorine-containing acrylic monomers or fluorine-containing methacrylic monomers.
In order to control the electrostatic propensity or the hydrophobicity of the charging particles, the surface of the inorganic particle may be surface treated with, for example, a silane coupling agent, a titanium coupling agent, and a silicon oil. Particularly for providing positive electrostatic propensity for the inorganic particle, the inorganic particle is preferably surface-treated with an amino group-containing coupling agent. For providing negative electrostatic propensity for the particle, the inorganic particle is preferably surface-treated with an fluorine group-containing coupling agent.
Toner Particles
Conventional, commercially available toner particles may be used for the image forming apparatus. Each of the toner particles has a diameter ranging, for example, from about 3 to 15 μm. Other toner particles such as toner particles containing binder resin and coloring agent contained therein, toner particles containing charge control agent or release agent, and toner particles retaining additives thereon may be employed.
The toner particles may be produced by the conventional methods such as a pulverization method, an emulsion polymerization method, and a suspension polymerization method.
Binder Resin
Although not limited, the binder resin used for the toner may be styrenic resins (styrene or homopolymer or copolymer including styrene substitution), polyester resins, epoxy resins, vinyl chloride resin, phenol resin, polyethylene resin, polypropylene resin, polyurethane resin, silicone resin, and any combinations thereof. The binder resin preferably have a softening point of about 80 to 160° C. and a glass transition point of about 50 to 75° C.
Coloring Agent
Conventional, commercially available coloring agents such as carbon black, aniline black, activated carbon, magnetite, benzine yellow, permanent yellow, naphthol yellow, phthalocyanine blue, first sky blue, ultramarine blue, rose bengal, and lake red may be used. Typically, an amount of the coloring agent is preferably 2 to 20 parts by weight for 100 parts by weight of the binder resin.
Charge Control Agent
Conventional, commercially available charge control agents may be used. Specifically, nigrosine dyes, quaternary ammonium salt compounds, triphenylmethane compounds, imidazole compounds, and a polyamine resin, for example, may be used for positively charged toner particles. Azo dyes containing metals such as Cr, Co, Al and Fe, metal salicylate compounds, metal alkylsalicylate compounds, and calixarene compounds, for example, may be used for the negatively charged toner particles. It is preferable to use the charge control agent in an amount 0.1 to 10 parts by weight for 100 parts by weight of binder resin.
Conventional, commercially available release agents may be used. Polyethylene, polypropylene, carnauba wax, Sasol Wax, or mixtures in combination thereof may be used for the release agent. It is preferable to use the release agent in an amount 0.1 to 10 for 100 parts by weight of a binder resin.
Other Additives
A fluidizing agent which accelerates the fluidization of the developer may be added. For the fluidizing agent, inorganic particles such as silica, titanium oxide and aluminum oxide, or resin particles such as acrylic resin, a styrene resin, a silicone resin and a fluororesin may be used. Preferably, materials with hydrophobicity provided by the addition of, for example, silane coupling agent, titanium coupling agent, and silicon oil may be used. It is preferable to add the fluidizing agent in an amount 0.1 to 5 for 100 parts by weight of the toner. The number-average primary particle diameters of the additives are preferably 9 to 100 nm.
Carrier Particles
Conventional, commercially carrier particles such as binder carrier and coated carrier particles may be used. A particle diameter of the carrier is not limited. Preferably, the carrier particle has a diameter of about 15 to 100 μm.
Typically, the binder carrier is formed by dispersing magnetic substance particles in the binder resin, and substances having particles to be positively or negatively charged or the coating layer on the surface may be used for the binder carrier. Charging characteristics such as the polarity of the binder carrier may be controlled by selecting a material of a binder resin, particles having electrostatic propensity, and materials of the surface coating layer.
Examples of the binder resin used for the binder carrier include thermoplastic resins such as vinyl resins typified by polystyrene resin, polyester resins, nylon resins and polyolefin resins, and thermosetting resins such as phenolic resin.
For the magnetic substance particle of the binder carrier, magnetite, spinel ferrite such as g-iron oxide, spinel ferrite containing one or two or more materials of nonferrous metals (Mn, Ni, Mg, Cu, etc.), magnetoplumbite ferrite such as barium ferrite, and particles of iron or alloy, having a oxidation layer in the surface, may be used. The shape of the carrier may be any one of a grain, sphere, and needle. When it is required to highly magnetize the particle, iron base ferromagnetic particle is preferably used. To increase the chemical stability, it is preferable to use magnetite, spinel ferrite including g-iron oxide, and ferromagnetic particles of magnetoplumbite type ferrite such as barium ferrite. By appropriately selecting the materials or the content of ferromagnetic particles, a magnetic resin carrier having desired magnetization may be obtained. The magnetic substance particle is properly added to the magnetic resin carrier in an amount 50 to 90 percent by weight.
For the surface coating material of the binder carrier, for example, a silicone resin, an acrylic resin, an epoxy resin, and a fluororesin may be used. The ability of the carrier to provide charges may be improved by coating the surface of the carrier with these resins and curing these resins to form a coat layer.
The fixation of a particle having electrostatic propensity or a conductive particle to the surface of the binder carrier particle is performed by uniformly mixing, for example, magnetic resin carriers and particles to allow these particles to adhere to the surface of the magnetic resin carrier, and then implanting the particle into the magnetic resin carrier by giving mechanical or thermal shock. In this case, the particle is not completely embedded in the magnetic resin carrier and it is fixed to the surface in such a way that a part of the particle protrudes through the surface of the magnetic resin carrier. An organic insulating or an inorganic insulating material is used for the particle having electrostatic propensity. As a specific organic insulating material, organic insulating particles such as polystyrene, styrenic copolymer, acrylic resin, various acryl copolymers, nylon, polyethylene, polypropylene, fluororesin, and crosslinked products thereof may be used. The ability to provide charges and the charged polarity may be adjusted by, for example, a material of a particle having electrostatic propensity, a polymerization catalyst, and a surface treatment. As an inorganic insulating material, inorganic particles to be negatively charged such as silicand titanium dioxide, and inorganic particles to be positively charged such as strontium titanate and alumina may be used.
The coated carrier particle is made by coating a carrier core particle formed by a magnetic substance with resin, and particles having electrostatic propensity to be positively or negatively charged may be fixed to the surface of the carrier as with the binder carrier. Charging characteristics such as the polarity of the coated carrier may be controlled by selecting materials of a surface coating layer or particles having electrostatic propensity. As a coating resin, the same resin as a binder resin of the binder carrier may be used.
A mixing ratio of the toner and carrier particles may be adjusted so as to attain a desired amount of charge of toner particles, and a rate of toner is 3 to 50 percent by weight with respect to the total amount of the toner and the carrier, and preferably 6 to 30 percent by weight.
Experiment a
Advantages obtained by using the charging particle were examined using an image forming apparatus having developing device of
Toner “A”
The method of producing the toner “A” is as follows. To 100 parts by weight of a toner base material, prepared by a wet granulating method, having a volume average particle diameter of about 6.5 μm, a plurality of additives (0.2 parts by weight of first hydrophobic silica, 0.5 parts by weight of second hydrophobic silicand 0.5 parts by weight of hydrophobic titanium oxide) were added. Next, the toner base material to which the additives had been added was stirred with Henschel mixer manufactured by Mitsui Mining Co., Ltd. to cause the additives to adhere to the surface of the toner base material to obtain a negatively charged toner “A”. A rotation speed of the mixer was 40 m/sec and a stirring duration was 3 minutes. The first hydrophobic silica was obtained by surface treating silica (No. 130, produced by Nippon Aerosil Co., Ltd.) having a number-average primary particle diameter of 16 nm with hexamethyldisilazane (HMDS) of a hydrophobizing agent. The second hydrophobic silica was obtained by surface treating silica (No. 90, produced by Nippon Aerosil Co., Ltd.) having a number-average primary particle diameter of 20 nm with HMDS. Hydrophobic titanium oxide was obtained by surface treating anatase type titanium oxide having a number-average primary particle diameter of 30 nm with isobutyltrimethoxysilane, a hydrophobizing agent, in a water base wet ambient.
Toner “B”
A producing method of toner “B” is as follows. Strontium titanate having a number-average particle diameter of 350 nm was added to the toner “A” as a charging particle. An amount of the charging particle to be added was 2 for 100 parts by weight of the toner base material particle contained in the toner “A”. Next, toner “A” to which the charging particles had been added was stirred with Henschel mixer manufactured by Mitsui Mining Co., Ltd. to cause the charging particle to adhere to the surface of the toner to prepare toner “B”. A rotation speed of the mixer was 40 m/sec and a stirring duration was 3 minutes.
Carrier
A carrier (carrier particles) for Bizhub C350 manufactured by Konica Minolta Business Technologies, Inc. was used for experiments. This carrier is a coated carrier formed by coating a carrier core particle formed of a magnetic substance with an acrylic resin.
As developing device, developing device of constitution shown in
As a developing roller, an aluminum roller, the surface of which is anodized, was used. A supply collection gap between the developing roller and the sleeve was set at 0.3 mm. Thereby, a toner supply electric field formed between the developing roller and the sleeve was 3.3×106 V/m (=1000 V/0.3 mm). The regulating gap between the regulating plate and the sleeve was set at 0.4 mm so that the magnetic brush on the sleeve comes into contact with the peripheral surface of the developing roller. The developing device was constructed in such a way that the developing roller and the sleeve rotate in the same direction, and that the toner on the developing roller moves in the direction opposite to the direction of transportation of the developer on the sleeve in the supplying and collecting region. A charged voltage of the photosensitive member was −550 volts and a voltage of a non-image region and an image region of the electrostatic latent image formed on the photosensitive member was −60 volts. The development gap between the photosensitive member and the developing roller was set at 0.15 mm.
The same developer as that in Example 1 was used. As developing device, developing device of the constitution shown in
The toner “A” not carrying the charging particles was used in place of the toner “B” having the charging particles. Other conditions were the same as those for Example 1.
Evaluation
Using an image forming apparatus formed by modifying a copying machine Bizhub C350 manufactured by Konica Minolta Business Technologies, Inc., an original image having an image region percentage of 5 percent was printed in 50,000 sheets in different conditions. The developer in the equipment was sampled every 10000 sheets of printing and an amount of charge of the toner was measured. The results of measurements are plotted on a graph of
After the completion of printing of 50,000 sheets, the carrier was separated from the developer and the surface of the carrier was observed with a scanning electron microscope.
Charged Polarity of Charging Particle
The charged polarity of the charging particle was confirmed using an experimental apparatus 170 shown in
Conclusion
From the experiment described above, it was found that strontium titanate to be charged in an opposite polarity to that of the toner adheres to the surface of the carrier in the developing device, and thereby the reduction in the ability of the carrier to cause the toner to be charged is supplemented and the amount of charge of the toner is kept at a value required for a long period of time.
Experiment B
Using the toner “B” used in Example 1 and developing device of
As is apparent from Table 1, when a low toner supply electric field, a direct electric field, of 1×106 V/m or less is formed in the supplying and collecting region (experiments 1, 13), relatively large reductions in amount of charge were found. However, even in such a condition, image quality which has practically no problem was obtained. The reason for the reduction in amount of charge may be that, since an electric field to separate the charging particle from the toner is low, the charging particle moves with the toner and therefore an amount of the charging particle adhering to the surface of the carrier is small and adequate toner-charging performance was not attained. Therefore, when the direct electric field is employed, it is preferable to form a toner supply electric field of 1×106 V/m or more in the supplying and collecting region.
As is apparent from the comparisons of the results between experiments 1, 13 and experiment 4, and between experiment 2 and experiments 8, 9, it is evident that, when the pulsating field acts on the supplying and collecting region, higher toner-charging performance may be attained even though its mean electric field is equal to that of direct electric field. The reason for this is that two biasing forces, acting in the direction opposite to each other, alternately act on the toner and the charging particle with the biasing force on the toner opposite to that on the charging particle by the pulsating field, and thereby the charging particle is separated from the toner efficiently. Accordingly, it may be the that the formation of the pulsating field is more preferable than the formation of the direct electric field in the supplying and collecting region. When the pulsating field is used, it is thought that a toner supply electric field is preferably about 2.5×106 V/m or more.
Experiment C
Using the developing device of
As shown in Table 3, it was verified that the developing device of
Thus, there are large difference in the replacement of the toner on the developing roller, that is, the movement of the toner in the supplying and collecting region, between the developing device of
Experiment D
A plurality of toners “C” to “G” were prepared in addition to toner “B”, and changes in an amount of charge were investigated. The toners “C” to “G” were obtained by adding charging particles formed of strontium titanate having a number-average particle diameter of 210 nm, 140 nm, 70 nm, 850 nm and 1000 nm to the toner “A” and stirring the resulting mixture. An amount of the charging particle to be added was set at 2 for 100 parts by weight of the toner base material particle. Stirring was performed at a stirring speed of 40 m/sec for 3 minutes using Henschel mixer. Other conditions were the same as Example 1.
The developing device of
Experiment E
A plurality of toners “H” to “K” were prepared in addition to toner “B”, and changes in an amount of charge of toner were investigated. The toners “H” to “K” were obtained by adding charging particles formed of titanium oxide having a number-average particle diameter of 150 nm, alumina having a number-average particle diameter of 200 nm, barium titanate having a number-average particle diameter of 500 nm, melamine resin beads having a number-average particle diameter of 200 nm to the toner “A” and stirring the resulting mixture. An amount of the charging particle to be added was set at 2 for 100 parts by weight of the toner base material particle. Stirring was performed at a stirring speed of 40 m/sec for 3 minutes using Henschel mixer. Other conditions were the same as Example 1.
The developing device of
A saturation amount of charge of a material itself is thought to have caused the variations in the amounts of charge thus produced due to the difference between materials composing the charging particle. That is, when a charge amount which the particle may bear is small, the particle is saturated early and its saturated amount of charge is insufficient for causing the toner to be charged to the required extent, but when a charge amount which the particle may bear is large, the particle charged to a saturation state can cause the toner to be charged to the required extent.
An amount of charge which the particle may bear is proportional to the dielectric constant of a material composing the particle. The dielectric constants of the materials of the charging particle described above are shown in the following Table 4 together with the results of evaluations of the electrostatic propensity obtained in experiment E. From this Table, it is evident that the dielectric constant of the charging particle to be added to the toner is preferably 8.5 or more.
Experiment F
The developing device was loaded with a developer including the charging particles and a container of a toner supply section was loaded with the toner not including the charging particles, and an image having an image region percentage of 5 percent was printed in 50,000 sheets, and an amount of charge of the toner held on the developing roller was measured every 10,000 sheets. The results of measurements are plotted on a graph in
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-006720 | Jan 2007 | JP | national |
