Patent Grant
6904244
The present application claims priority to Japanese Patent Application No. 2001-359098 filed in the Japanese Patent Office on Nov. 26, 2001, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a developing device and an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus including the developing devices, and more particularly relates to a developing device using a developer including toner and carrier.
2. Discussion of the Background
In an electrophotographic image forming method, an electrostatic latent image formed on a latent image carrier is developed with a developer containing a toner. The toner needs to be appropriately charged in the developer to develop the latent image. Generally, there are two methods of developing an electrostatic latent image: (1) a method of developing an electrostatic latent image with a two-component developer including a mixture of toner and carrier, and (2) a method of developing an electrostatic latent image with a one-component developer including toner as a main component.
The developing method using the one-component developer has a disadvantage such as unstable charging property of toner. In the developing method using the two-component developer, a relatively stable good quality image can be obtained. However, deterioration of carrier and variations of the mixing ratio of toner and carrier may tend to occur. When repeatedly developing electrostatic latent images with a two-component developer, a toner density (i.e., a weight ratio of toner to the developer) varies due to consumption of toner in the two-component developer. Therefore, the toner density needs to be controlled by supplying toner to the developer in order to obtain a stable good quality image.
In order to control the toner density, a toner supply control method has been proposed in which a toner supplying device controls the toner supply based on data of a toner density in a developing device. The density is detected by a toner density detecting device using a transmission sensor, a fluidity sensor, an image density sensor, a bulk density sensor, etc. As a recent trend, the image density sensor or a combination of the image density sensor and a magnetic permeability sensor (a kind of the bulk density sensor) is widely used.
In the toner supply control method using the image density sensor, an image pattern formed on a latent image carrier is developed with a two-component developer and exposed to light. A toner supply amount is controlled by detecting the image density of the developed image pattern based on the light reflected from the developed image pattern. In the toner supply control method using the combination of the image density sensor and the magnetic permeability sensor, a toner supply amount is controlled by changing a target value of the magnetic permeability sensor according to the image density of the developed image pattern.
The carrier in the two-component developer includes a core material covered with a resin coating layer. The resin coating layer is used for various purposes such as prevention of toner from forming films on the core material, provision of a uniform, non-abrasive surface, prevention of surface oxidation, prevention of moisture absorption, extension of useful lifetime, protection of a latent image carrier from damages or abrasion by carrier, control of charging polarity, and control of a charging amount. For example, a carrier core material may be coated with a resin material (for example, described in the published Japanese patent application No. 58-108548), or a resin coating layer to which various additives are added (for example, described in the published Japanese patent application Nos. 54-155048, 57-40267, 58-108549, 59-166968, 6-202381, and in the Japanese patent publication Nos. 1-19584, 3-628). Further, additives may be adhered onto a carrier surface (for example, described in the published Japanese patent application No. 5-273789), or a carrier core material may be covered with a resin coating layer containing a conductive powder in which the average particle diameter of the conductive powder is equal to the thickness of the resin coating layer or greater (for example, described in the published Japanese patent application No. 9-160304). Moreover, a carrier coating material may include benzoguanamines-n-butyl alcohol-formaldehyde copolymers as a main component (for example, described in the published Japanese patent application No. 8-6307), or a melamine resin crosslinked with an acrylic resin (for example, described in the Japanese Patent No. 2683624).
Even though a resin coating layer is provided with a core material of carrier, the following problem may arise. When an original document having a low image area (e.g., an occupation ratio of an image on the original document is 3% or less) which subjects a two-component developer to much stresses, is repeatedly printed or copied, the charging amount of carrier increases due to the frictional charging of toner and carrier. As a result, a phenomenon in which a bulk density of the developer decreases due to the repulsive force between carrier particles, may occur. This phenomenon is accelerated when the external agents of toner become embedded in the toner due to rubbing against the toner between the carrier particles, and the fluidity of the entire developer decreases.
The above-described magnetic permeability sensor detects a distance between the magnetic carrier and the sensor. The detected value of the magnetic permeability sensor decreases as the carrier is away from the sensor and as the carrier becomes sparse in the developer. Therefore, when the carrier is away from the sensor and is sparse in the developer due to the decrease of the bulk density of the developer, the detected value of the magnetic permeability sensor decreases, and therefore the sensor erroneously detects that the toner density has increased, although the toner density has not varied. Because the toner supplied to the developer is decreased based on the above detection output of the sensor, the toner density in the developer decreases, thereby deteriorating developing performance. As described above, when the two-component developer is used in a high-stress condition, the bulk density of the developer varies, thereby causing the toner density to be unstably controlled.
According to an aspect of the present invention, a developing device includes a developer including toner having a coloring agent dispersed in a binder resin, and carrier having a core material, and a coating layer covering the core material and containing a binder resin and a powder, a toner density detecting device configured to detect a toner density of the developer by use of a bulk density sensor, and a control device configured to control the toner density based on a detection result of the toner density detecting device. The toner density is controlled such that a ratio (D/h) of an average particle diameter (D) of the powder to a thickness of the coating layer is greater than 1 and less than 10.
Objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.
A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Preferred embodiments of the present invention are described in detail referring to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views.
In the preferred embodiment, the present invention is applied to an electrophotographic color laser printer (hereafter referred to as a laser printer) as an example of an image forming apparatus.
The laser printer of
The laser writing unit 2 includes a power supply, a polygonal mirror, an f-θ lens, and reflection mirrors. The laser writing unit 2 irradiates the surfaces of the photoconductive drums 11M, 11C, 11Y, and 11BK with a laser beam based on image data of original documents.
Referring to
With the above-described construction and operation of the laser printer of
Referring to
In the photoreceptor unit 10M, the charging roller 15M, to which a voltage is applied, uniformly charges the surface of the photoconductive drum 11M. Subsequently, the surface of the photoconductive drum 11M is exposed to a laser beam modulated and deflected in the laser writing unit 2, and thereby an electrostatic latent image is formed on the surface of the photoconductive drum 11M. The electrostatic latent image formed on the photoconductive drum 11M is developed with magenta toner by the developing device 20M and formed into a magenta toner image. At a transfer section (Pt) where the transfer sheet 100 carried on the transfer belt 60 passes through, the magenta toner image on the photoconductive drum 11M is transferred onto the transfer sheet 100. After the magenta toner image is transferred from the photoconductive drum 11M onto the transfer sheet 100, the lubricant applying/discharging brush roller 12M applies a predetermined amount of lubricant onto the surface of the photoconductive drum 11M, and discharges the surface of the photoconductive drum 11M. The residual toner remaining on the surface of the photoconductive drum 11M is removed by the cleaning blade 13M. As a result, the surface of the photoconductive drum 11M is prepared for a next image forming operation.
The developing device 20M uses a two-component developer 28M (hereafter simply referred to as a “developer”) including magnetic carrier and negatively charged magenta toner to develop an electrostatic latent image formed on the photoconductive drum 11M. The developing device 20M includes a case 21M, a developing sleeve 22M serving as a developer carrier formed from a non-magnetic material, and a magnet roller (not shown) serving as a magnetic field generating device fixed inside of the developing sleeve 22M. The developing sleeve 22M is arranged such that a part of the developing sleeve 22M is exposed to outside through an opening of the case 21M to face the photoconductive drum 11M. The developing device 20M further includes developer conveying screws 23M and 24M, a doctor blade 25M, a magnetic permeability sensor 26M serving as a toner density detecting device that detects the magnetic permeability of the developer 28M, a toner cartridge 29M that contains magenta toner, and a powder pump 27M. A developing bias voltage, in which an alternating current (AC) voltage is superimposed on a negative direct current (DC) voltage, is applied from a developing bias power supply (not shown), serving as a developing electric field generating device, to the developing sleeve 22M. Thereby, the developing sleeve 22M is biased with a predetermined voltage relative to a substrate layer of the photoconductive drum 11M.
Referring to
Because the density of magenta toner in the developer 28M contained in the case 21M decreases due to the consumption of the developer in the image forming operation, the magenta toner is supplied from the toner cartridge 29M into the case 21M through the powder pump 27M according to an image area and a detected value (Vt) of the magnetic permeability sensor 26M. Thereby, the density of magenta toner is maintained at a predetermined value. The developing device 20M includes a control device 30M including a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM), and an input/output (I/O) interface, so as to control the toner density.
Specifically, the control device 30M calculates a difference (ΔT) between a target value (Vref) of toner density and the detected value (Vt) of the magnetic permeability sensor 26M. When the difference (ΔT) is positive, the control device 30M judges that the toner density is sufficiently high and controls the toner cartridge 29M to reduce the supply of magenta toner sent into the case 21M. When the difference (ΔT) is negative, the control device 30M judges that the toner density is too low and controls the toner cartridge 29M to increase the supply of magenta toner sent into the case 21M relative to greater the absolute value of the difference (ΔT). The amount of toner supplied into the case 21M is controlled to increase such that the detected value (Vt) of the magnetic permeability sensor 26M approaches the target value (Vref). The target value (Vref), the charging potential, and the laser amount are preferably set by a process control performed one time for every 10 copies (about 5 to 200 copies depending on a copying speed). For example, each toner density of a plurality of halftone and solid filled pattern images formed on the photoconductive drum 11M is detected by a reflection toner density sensor, and an adhesion amount of toner is calculated. Then, the target value (Vref), the charging potential, and the laser amount are set such that a target adhesion amount of toner can be obtained.
In the laser printer of
In a multi-color image formation mode, the four photoconductive drums 11M, 11C, 11Y, and 11BK are brought in contact with the transfer belt 60. An adsorbing bias applying roller 61 applies an electric charge having a polarity equal to that of the toner to the transfer sheet 100 to adsorb the transfer sheet 100 to the transfer belt 60. The transfer sheet 100 is conveyed while being adsorbed to the transfer belt 60. The magenta, cyan, and yellow toner images respectively formed on the photoconductive drums 11M, 11C, and 11Y are sequentially transferred onto the transfer sheet 100 while being superimposed upon each other. Lastly, the black toner image formed on the photoconductive drum 11BK is transferred onto the superimposed color toner image on the transfer sheet 100. Subsequently, the transferred multi-color toner image on the transfer sheet 100 is fixed thereonto in the fixing unit 7.
In a single color image formation mode in which a black image is formed on the transfer sheet 100, the photoconductive drums 11M, 11C, and 11Y are separated from the transfer belt 60 and only the photoconductive drum 11BK is brought in contact with the transfer belt 60. The transfer sheet 100 is conveyed to a transfer section formed between the photoconductive drum 11BK and the transfer belt 60, and the black toner image formed on the photoconductive drum 11BK is transferred onto the transfer sheet 100. The transferred black toner image is fixed onto the transfer sheet 100 in the fixing unit 7.
Having generally described this invention, further understanding can be obtained by reference to certain specific examples which are provided herein for the purpose of illustration only and are not intended to be limiting. In each of the examples and comparative examples described below, the mechanical conditions and toner conditions are maintained as shown in Table 1, while the carrier conditions are changed among the examples. Parts and percentages are determined by weight.
The carrier conditions for example 1 were as follows:
The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K.K.) and dried to form a resin coating layer of 0.15 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thickness of the resin coating layer of the carrier was found by measurement of cross-sections of the carrier with a transmission electron microscope, and was defined by the mean value of the measured carrier. The carrier core material preferably has an average particle diameter of at least about 20 μm to prevent the carrier from adhering onto the photoconductive drum as the image carrier, and preferably has an average particle diameter of not greater than about 100 μm to prevent image deterioration caused by, for example, carrier streak. Specific examples of the core material include materials known as electrophotographic two-component carrier such as ferrite, magnetite, iron, nickel, and the like.
The thus obtained carrier was subjected to a running test in which 900 copies were continuously produced using a digital full color copier (Ipsio Color 8000 manufactured by Ricoh Company, Ltd.) using a single black color toner. Specifically, 900 copies of an original document having no image were continuously produced to subject a two-component developer to extreme stresses. The results are shown in
The carrier conditions for Example 2 were as follows:
The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K.K.) and dried to form a resin coating layer of 0.15 μm in thickness. The coated particles were then calcined at 300° C. for two hours in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in
The carrier conditions for Example 3 were as follows:
The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K.K.) and dried to form a resin coating layer of 0.10 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in
The carrier conditions for Example 4 were as follows:
The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPWRA COTA (manufactured by Okada Seiko K.K.) and dried to form a resin coating layer of 0.08 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in
The carrier conditions for Example 5 were as follows:
The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K. K.) and dried to form a resin coating layer of 0.03 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in
The carrier conditions for comparative Example 1 were as follows:
The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K. K.) and dried to form a resin coating layer of 0.15 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in
The carrier conditions for comparative Example 2 were as follows:
The above-described components of carrier were mixed with a homomixer for 10 minutes to prepare a resin layer coating liquid. The resin layer coating liquid was applied to ferrite particles as a carrier core material by SPIRA COTA (manufactured by Okada Seiko K. K.) and dried to form a resin coating layer of 0.15 μm in thickness. The coated particles were then calcined at 150° C. for one hour in an electric oven and the resulting bulk of the ferrite particles were crushed and sieved with a sieve having a sieve opening of 100 μm to obtain a carrier. The thus obtained carrier was subjected to a running test in the same manner as that in Example 1. The results are shown in
As seen from the results in
On the other hand, as seen from the results in
Thus, as a result of the investigations described above, the present inventors found that when the ratio (D/h) of an average particle diameter (D) of the powder in the coating layer of the carrier to a thickness (h) of the coating layer is greater than 1 and less than 10, preferably greater than 1 and less than 5, a good effect of suppressing the variations in the bulk density of the developer is obtained, even though the developer is subjected to much stresses. It is considered that because the powder protrudes through the surface of the coating layer of the carrier, a contact area of carrier particles while being agitated is reduced, thereby decreasing the charging amount of the carrier. Further, it is considered that because the protrusion of the powder from the surface of the coating layer provides space between carrier particles, the extent of rubbing against toner while being agitated is reduced, thereby preventing external agents of the toner from being embedded in the toner (hereinafter referred to as a space effect).
With the above-described conditions, when the toner density is constant, the phenomenon in which the bulk density of the developer decreases can be suppressed, thereby reducing the variations in the bulk density of the developer. Thus, in the image forming apparatus according to the present embodiment, variations in the bulk density of the developer due to causes other than the toner density can be suppressed, thereby preventing the detection error of the bulk density sensor. Therefore, the toner density can be stably controlled.
When the ratio (D/h) is 1 or less, the powder is buried within the coating layer, and the above-described good effect is hard to be obtained. When the ratio (D/h) is 10 or greater, the powder cannot be tightly secured by the coating layer because the contact area of the powder and the binder resin in the coating layer is small. As a result, the powder is easily detached from the coating layer. In order to prevent the powder from being detached from the coating layer, it is preferable that the ratio (D/h) is 5 or less.
In the above-described embodiment, the magnetic permeability sensor as a kind of the bulk density sensor is used as a toner density detecting device to control the toner density based on the detected value of the magnetic permeability sensor in the developing device. With use of the above-described carrier of the present invention in this developing device, a stable toner density control can be performed even though the developer is used in a high-stress giving condition.
Further, in the above-described embodiment, the resistivity of the powder of the carrier is 1012 Ω-cm or greater. Because of the high resistivity, even when the powder secured to the core material by the binder is exposed on the surface of the carrier, leakage of charges does not occur. Thus, throughout its long service period, the carrier exhibits a satisfactory charging amount and a stable chargeability. When the resistivity of the powder is less than 1012 Ω-cm, leakage of the charge on the carrier occurs through the powder. In the present embodiment, the powder is used not as a resistivity controlling agent, but as a protecting agent for the coating layer and as an agent for controlling the shape of the surface of the coating layer. Any powder may be used so long as the resistivity of the powder is at least 1012 Ω-cm.
Further, in the above-described embodiment, the amount of the powder in the coating layer is preferably 50-95% by weight, more preferably 70-90% by weight. When the amount of the powder in the coating layer is less than 50% by weight, the sufficient stable bulk density of the developer cannot be obtained because the carrier does not provide the above-described effects such as the decrease of charging amount of the carrier and the space effect. Too large an amount of the powder, in excess of 95% by weight, causes reduction of chargeability of the carrier. In addition, as the amount of the carrier is much greater than that of the binder resin in the coating layer, the binder resin cannot securely hold the powder. Therefore, the powder tends to be detached from the coating layer, thereby decreasing the durability of the carrier. Any binder resin generally used for coating a core material of carrier may be employed in the present embodiment.
In the present invention, the powder may be alumina, silica, or a mixture of alumina and silica. In the case of using alumina powder, it is preferable that an average particle diameter of the alumina powder is 10 μm or less. Surface-treated or non-treated alumina powder may be used. The surface treatment may be to impart hydrophobicity to the alumina powder. Alternatively, surface-treated or non-treated silica powder may be used. The surface treatment may be to impart hydrophobicity to the silica powder.
The coating layer of the carrier may include one or more additives as a charging or resistivity controlling agent such as carbon black, an acid catalyst, and a combination of carbon black and acid catalyst. The carbon black may be one generally used for carrier and toner. The acid catalyst, which may be, for example, a compound having an alkyl group or a reactive group such as a methylol group, an imino group or both methylol and imino groups, serves to catalyze. The above-described examples of the acid catalyst are not limited thereto.
In the above-described image forming apparatus according to the embodiment of the present invention, even when the developer is used in a high-stress condition, for example, when an original document having a low image area (e.g., an occupation ratio of an image on the original document is 3% or less) is repeatedly printed or copied, variations in the bulk density of the developer can be suppressed and a toner density can be stably controlled. As a result, a high quality image can be obtained.
The present invention has been described with respect to the embodiments as illustrated in the figures. However, the present invention is not limited to the embodiment and may be practiced otherwise. For example, in the above-described embodiment, a stable toner density control can be performed by use of the bulk density sensor other than the magnetic permeability sensor. Moreover, the present invention has been described with respect to an electrophotographic color laser printer as an example of an image forming apparatus. However, the present invention may be applied to other image forming apparatuses such as a copying machine or a facsimile machine.
In the above-described color image forming apparatus, the order of forming images of respective colors and/or the arrangement of the image forming devices for respective colors are not limited to the ones described above and can be practiced otherwise. In addition, the above-described image forming apparatus may form single-color images instead of multi-color images.
Numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.
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| 2001-359098 | Nov 2001 | JP | national |
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| Number | Date | Country | |
|---|---|---|---|
| 20030161645 A1 | Aug 2003 | US |
