Patent Application
20140140739
This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-253506 filed Nov. 19, 2012.
The present invention relates to developing devices and image forming apparatuses.
According to an aspect of the invention, a developing device includes a first developer holder disposed so as to face an image holding member that holds an electrostatic latent image, the first developer holder rotating while holding a developer on a surface thereof so as to move in a direction opposite to a direction of movement of the image holding member at a first facing portion at which the first developer holder faces the image holding member; and a second developer holder disposed downstream from the first developer holder in the direction of movement of the image holding member so as to face the image holding member, the second developer holder rotating while holding the developer on a surface thereof so as to move in a direction the same as the direction of movement of the image holding member at a second facing portion at which the second developer holder faces the image holding member. A value obtained by dividing an amount of developer per unit area held on the first developer holder by a shortest distance between the image holding member and the first developer holder is approximately in a range from 1.00×103 kg/m3 to 1.60×103 kg/m3.
Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein:
Referring now to the drawings, exemplary embodiments of the present invention will be described below.
The image forming apparatus 1 according to the first exemplary embodiment is, for example, a color printer. The image forming apparatus 1 includes multiple image forming devices 10, an intermediate transfer device 20, a sheet supply device 50, and a fixing device 40. The image forming devices 10 each form a toner image by developing a latent image with a toner included in a developer 4. The intermediate transfer device 20 holds the toner images formed by the image forming devices 10 and transports the toner images to a second transfer position at which the toner images are finally second-transferred to a recording sheet 5, which is an example of a recording medium. The sheet supply device 50 contains predetermined recording sheets 5 that are to be fed to the second transfer position of the intermediate transfer device 20 and transports the recording sheets 5 to the second transfer position. The fixing device 40 fixes the toner images that have been second transferred to a recording sheet 5 by the intermediate transfer device 20 to the recording sheet 5.
When, for example, the image forming apparatus 1 additionally includes an image input device 60 through which a document image that is to be formed on a recording sheet 5 is input, the image forming apparatus 1 is capable of functioning as a color copying machine. A housing 1a of the image forming apparatus 1 illustrated in
The image forming devices 10 are six image forming devices 10Y, 10M, 10C, 10K, 10S1, and 10S2, which individually form toner images of yellow (Y), magenta (M), cyan (C), black (K), a spot color (S1), and another spot color (S2). Here, yellow (Y), magenta (M), cyan (C), and black (K) are four standard colors. The six image forming devices 10 (10S1, 10S2, 10Y, 10M, 10C, and 10K) are arranged in a nearly straight line inside the housing 1a. Examples of the developers 4 of the spot colors S1 and S2 include materials having colors that are not or hardly be expressed with the four standard colors. Specifically, examples of the developers 4 of the spot colors S1 and S2 include toners having colors other than the four standard colors, toners having the same colors as the four standard colors but different chroma, a transparent toner for improving a gloss, a foam toner for Braille, and toners having fluorescent colors. As described below, the image forming devices 10 (10S1, 10S2, 10Y, 10M, 10C, and 10K) have substantially the same configuration, except that they use different types of developers.
As illustrated in
The photoconductor drum 11 has a cylindrical base member having an image holding surface at the circumferential surface of the cylindrical base member. The cylindrical base member is grounded. The image holding surface has a photoconductive layer (photosensitive layer) made of a photosensitive material. The photoconductor drum 11 is supported so as to be rotatable in a direction indicated by the arrow A in
A contactless charging device, such as a corona discharging device, disposed without contacting the photoconductor drum 11 is used as an example of the charging device 12. A charging voltage is supplied to a charging device in the charging device 12. In the case where the developing device 14 performs reversal development, the charging voltage that the charging device 12 supplies is a voltage or current having the same polarity as the polarity to which the toner supplied from the developing device 14 is charged.
The exposure device 13 irradiates the charged circumferential surface of the photoconductor drum 11 with light (indicated by the dotted arrow) LB formed on the basis of image information input to the image forming apparatus 1 so as to form an electrostatic latent image. When an electrostatic latent image is to be formed, image information (signal) input to the image forming apparatus 1 via any appropriate way is transmitted to the exposure device 13.
The first transfer device 15 is a contact transfer device that rotates while contacting the circumferential surface of the photoconductor drum 11 and that includes a first transfer roller to which a first transfer voltage is applied. A DC voltage that has the polarity opposite to the polarity to which the toner is charged is supplied as the first transfer voltage from a power source, not illustrated.
As illustrated in
As illustrated in
As an example of the intermediate transfer belt 21, an endless belt made of a material obtained by dispersing a resistance adjustment substance, such as carbon black, in a synthetic resin, such as a polyimide resin or polyamide resin, is used. The belt supporting roller 22 is a driving roller, while the belt supporting rollers 23, 25, and 27, are driven rollers used to keep the intermediate transfer belt 21 in a traveling position or other conditions. The belt supporting roller 24 functions as a tension roller. The belt supporting roller 26 functions as a backup roller for second transfer.
A second transfer device 30 is disposed on the outer peripheral surface (image holding surface) side of the intermediate transfer belt 21 at a position at which the intermediate transfer belt 21 is supported by the belt supporting roller 26. The second transfer device 30 second transfers toner images formed on the intermediate transfer belt 21 to a recording sheet 5. As illustrated in
The fixing device 40 includes a housing 41, a heating rotor 42, and a pressure applying rotor 43. The housing 41 has an insertion opening and an ejection opening through which a recording sheet 5 is inserted and ejected. The heating rotor 42 and the pressure applying rotor 43 are disposed inside the housing 41. The heating rotor 42 rotates in the direction of the arrow illustrated in
The sheet supply device 50 is disposed below the intermediate transfer device 20 and the second transfer device 30. The sheet supply device 50 includes one sheet container 51 (or more) and a feeding device 52. The sheet container 51 contains recording sheets 5 of a predetermined size or type in a stacked manner. The feeding device 52 feeds the recording sheets 5 one by one from the sheet container 51. The sheet container 51 is attached to the housing 1a so as to be drawn to, for example, the front of the housing 1a (to the side that a user faces during operation). Examples of the recording sheets 5 fed from the sheet supply device 50 include, in addition to plain paper and thick paper, a recording sheet having projections and depressions on its surface such as an embossed sheet.
The image forming apparatus 1 is appropriately switchable between a plain paper mode, in which the image forming apparatus 1 forms an image on plain paper, and an embossed paper mode, in which the image forming apparatus 1 forms an image on a recording sheet having projections and depressions on its surface such as embossed paper.
A sheet-feeding transport path extends between the sheet supply device 50 and the second transfer device 30. The sheet-feeding transport path includes multiple pairs of sheet transport rollers 53 to 57 and a transport guide member, not illustrated. The multiple pairs of sheet transport rollers 53 to 57 transport the recording sheet 5 fed by the sheet supply device 50 toward the second transfer position. The pair of sheet transport rollers 57, disposed at a position immediately preceding the second transfer position in the sheet-feeding transport path, function as, for example, rollers (registration rollers) that adjust the timing at which the recording sheet 5 is transported. In addition, a sheet transporting device 58 is disposed between the second transfer device 30 and the fixing device 40. The sheet transporting device 58 is in the form of, for example, a belt and transports a recording sheet 5 that has been fed from the second transfer belt 31 of the second transfer device 30 after undergoing second transfer to the fixing device 40. Furthermore, a pair of sheet ejecting rollers 59 are disposed near a sheet ejection opening formed in the housing 1a. The pair of sheet ejecting rollers 59 eject a recording sheet 5 that has been fed from the fixing device 40 after undergoing a fixing operation to the outside of the housing 1a.
The above-described image input device 60, which is included in the image forming apparatus 1 when the image forming apparatus 1 functions as a color copying machine, is an image reading device that reads an image of a document having image information that is to be printed. As illustrated in
Image information of a document read by and input to the image input device 60 is subjected to appropriate image processing by an image processing device 70. First, the image input device 60 transmits the read document image information to the image processing device 70 as image data (for example, 8-bit data) of three colors of red (R), green (G), and blue (B). The image processing device 70 performs predetermined image processing on image data transmitted from the image input device 60, such as shading correction, misregistration correction, lightness/color space conversion, gamma correction, frame erasure, or color/movement edition. The image processing device 70 converts signals of the image that has been image-processed into image signals of the above-described four standard colors of Y, M, C, and K and then transmits the image signals to the exposure devices 13. The image processing device 70 also generates image signals of the above-described two spot colors S1 and S2.
Now, a basic image forming operation performed by the image forming apparatus 1 is described below.
First, a description is given of an image forming operation performed when a full-color image is formed by using the four image forming devices 10 (10Y, 10M, 10C, and 10K) and by combining toner images of the four standard colors of Y, M, C, and K.
When the image forming apparatus 1 receives a command to start an image forming operation (printing), the four image forming devices 10 (10Y, 10M, 10C, and 10K), the intermediate transfer device 20, the second transfer device 30, the fixing device 40, and other related devices are actuated.
In each image forming device 10 (10Y, 10M, 10C, or 10K), firstly, the photoconductor drum 11 rotates in the direction of arrow A and the charging device 12 charges the surface of the photoconductor drum 11 to a predetermined polarity (negative polarity in the first exemplary embodiment) and to a predetermined potential. Subsequently, the exposure device 13 irradiates the charged surface of the photoconductor drum 11 with a light beam LB. Here, the light beam LB is emitted based on an image signal obtained by converting image information input to the image forming apparatus 1 into a corresponding color component (of the color of Y, M, C, or K). Thus, an electrostatic latent image for the corresponding color component is formed on the surface of the photoconductor drum 11 by using a predetermined potential difference.
Then, each developing device 14 (corresponding to Y, M, C, or K) develops the electrostatic latent image of the corresponding color formed on the corresponding photoconductor drum 11 by electrostatically attaching a toner of the corresponding color of Y, M, C, or K that has been charged to a predetermined polarity (negative polarity) to the electrostatic latent image. With this development with the toner of the corresponding one of the four standard colors of Y, M, C, and K, the electrostatic latent image of the corresponding color formed on the photoconductor drum 11 is rendered visible as a toner image of the corresponding color.
Subsequently, the toner images of the four standard colors of Y, M, C, and K formed on the photoconductor drums 11 of the image forming devices 10 (10Y, 10M, 10C, and 10K) are transported to the corresponding first transfer positions. Then, the first transfer devices 15 first transfer the toner images of the four standard colors such that the toner images are sequentially superposed on the intermediate transfer belt 21 of the intermediate transfer device 20 that rotates in the direction of arrow B.
After the first transfer is finished, in each image forming device 10, the pre-cleaning charging device 16 recharges the remnants such as a toner remaining on the surface of the photoconductor drum 11 that has undergone the first transfer. Then, the drum cleaning device 17 scrapes the recharged remnants off the surface of the photoconductor drum 11 to clean the surface of the photoconductor drum 11. Finally, the static eliminator 18 eliminates static on the cleaned surface of the photoconductor drum 11. Consequently, each image forming device 10 is restored ready for a subsequent image forming operation.
Subsequently, the intermediate transfer device 20 carries the first-transferred toner images to the second transfer position by using rotation of the intermediate transfer belt 21. The sheet supply device 50, meanwhile, feeds a predetermined recording sheet 5 to the sheet-feeding transport path in accordance with the image forming operation. In the sheet-feeding transport path, the pair of sheet transport rollers 57, serving as registration rollers, feed the recording sheet 5 to the second transfer position at appropriate timing for transfer.
At the second transfer position, the second transfer device 30 collectively second transfers the toner images on the intermediate transfer belt 21 to the recording sheet 5. After second transfer is finished, in the intermediate transfer device 20, the belt cleaning device 28 removes remnants such as a toner remaining on the surface of the intermediate transfer belt 21 that has undergone the second transfer to clean the surface of the intermediate transfer belt 21.
Subsequently, the recording sheet 5 to which the toner images have been second transferred is separated from the intermediate transfer belt 21 and the second transfer belt 31 and then transported to the fixing device 40 by the sheet transporting device 58. At the fixing device 40, the second-transferred recording sheet 5 is inserted into and caused to pass through a contact portion between the rotating heating rotor 42 and the rotating pressure applying rotor 43 so that the toner images that have not been fixed to the recording sheet 5 are fixed to the recording sheet 5 by undergoing an appropriate fixing operation (application of heat and pressure). In the case where the image forming operation is performed to form an image on a single side of the recording sheet 5, the recording sheet 5 that has undergone the fixing operation is finally ejected by the pair of sheet ejecting rollers 59 toward, for example, an ejected-sheet container, not illustrated, disposed outside the housing 100.
With the above operation, the recording sheet 5 on which a full-color image is formed by combining toner images of four colors is output.
Now, a description is given of the case where the image forming apparatus 1 performs the above-described normal image forming operation and also an operation of forming spot color toner images using developers of the spot colors S1 and S2.
In this case, firstly, the image forming devices 10S1 and 10S2 perform an image forming operation that is similar to the operation performed by the image forming devices 10 (10Y, 10M, 10C, and 10K). Thus, toner images of spot colors S1 and S2 are formed on the photoconductor drums 11 of the image forming devices 10S1 and 10S2. Subsequently, as in the case of the image forming operation of the toner images of the four standard colors, the spot color toner images formed by the image forming devices 10S1 and 10S2 are first transferred to the intermediate transfer belt 21 of the intermediate transfer device 20 and then second transferred (together with the toner images of the four standard colors) to the recording sheet 5 from the intermediate transfer belt 21 by the second transfer device 30. Finally, the recording sheet 5 to which the toner images of the spot colors and the four standard colors have been second transferred is subjected to a fixing operation by the fixing device 40 and ejected to the outside of the housing 1a.
With the above operation, the recording sheet 5 on which the two spot color toner images are superposed on the entirety or part of the full-color image formed by combining the toner images of four colors is output.
In the case where the image forming apparatus 1 is a color copying machine including the image input device 60, a basic image forming operation is performed in the following manner.
When a document 6 is set on the image input device 60 and the image input device 60 receives a command to start an image forming operation (copying), the image input device 60 reads a document image of the document 6. Then, the image processing device 70 performs the above-described image processing on information of the read document image and generates image signals. Thereafter, the image signals are transmitted to the exposure devices 13 of the image forming devices 10 (10S1, 10S2, 10Y, 10M, 10C, and 10K). Thus, each image forming device 10 forms an electrostatic latent image on the basis of the image information of the document 6 and forms a toner image. Each image forming device 10 then operates similarly as in the case of the above-described image forming operation (printing). Finally, an image formed of the toner images is formed on the recording sheet 5 and the recording sheet 5 is output.
Here, the image forming apparatus 1 may directly transfer the toner images formed by the image forming devices 10 (10S1, 10S2, 10Y, 10M, 10C, and 10K) to the recording sheet 5 without using the intermediate transfer belt 21 of the intermediate transfer device 20.
As illustrated in
The first development roller 141 includes a first magnetic roller 141a and a first development sleeve 141b. The first magnetic roller 141a is stationarily disposed at the inner side of the first development roller 141 and the first development sleeve 141b is disposed on the outer circumference of the first magnetic roller 141a. The second development roller 142 includes a second magnetic roller 142a and a second development sleeve 142b. The second magnetic roller 142a is stationarily disposed at the inner side of the second development roller 142 and the second development sleeve 142b is disposed on the outer circumference of the second magnetic roller 142a.
As illustrated in
The second development sleeve 142b, on the other hand, is driven to rotate in the direction opposite to the direction of rotation of the photoconductor drum 11 (the direction of the arrow illustrated in
As illustrated in
As illustrated in
As illustrated in
As illustrated in
In the first exemplary embodiment, the case where seven poles are arranged along the circumference of each of the first and second magnetic rollers 141a and 142a is described. However, the number of poles arranged along the circumference of each magnetic roller 141a or 142a is not limited to seven and may be five, for example.
The developing device body 140 includes a first agitating chamber 145 and a second agitating chamber 146. The first agitating chamber 145 is disposed at the rear side of the second development roller 142. The first agitating chamber 145 contains the developer 4 and the developer 4 is agitated therein. The second agitating chamber 146 is adjacent to the first agitating chamber 145. The developing device body 140 includes, in the first agitating chamber 145, a first agitating member 147 that transports the developer 4 while agitating the developer 4. The developing device body 140 includes, in the second agitating chamber 146, a second agitating member 148 that transports the developer 4 while agitating the developer 4. The first agitating member 147 and the second agitating member 148 transport the developer 4 in opposite directions. Each of the first and second agitating members 147 and 148 includes a rotation shaft and a transportation vane helically attached to the outer circumference of the rotation shaft. The first and second agitating members 147 and 148 are mounted on the developing device body 140 so as to be rotatable.
The first agitating chamber 145 and the second agitating chamber 146 are separated by a partition member 149. The partition member 149 has openings at both end portions in the longitudinal direction of the partition member 149 to connect the first agitating chamber 145 and the second agitating chamber 146 together. The developer 4 contained in the first and second agitating chambers 145 and 146 is transported by the first and second agitating members 147 and 148 so as to circulate throughout the first and agitating chambers 145 and 146 via the openings.
At an upstream end portion of the second agitating member 148 in the direction of transporting the developer, a developer replenishing device, not illustrated, is provided that replenishes the second agitating chamber 146 with a new lot of developer 4 including at least a toner.
The developer 4 contained in the developing device body 140 is a two-component developer including a toner and a carrier. The toner includes toner particles and external additives whose mean volume particle diameter is in a range from 50 nm to 400 nm, inclusive. The toner particles include, for example, a binder resin, a coloring agent, and, if needed, other additives such as a release agent.
The properties of the toner particles are described now. Preferably, the toner particles have a mean shape factor in a range from 100 to 150. The mean shape factor is the mean of shape factors calculated by (ML2/A)×(π/4)×100, where ML denotes the maximum length of a particle and A denotes the projected area of the particle. More preferably, the toner particles have a mean shape factor in a range from 105 to 145, or most preferably, 110 to 140.
Preferably, the toner particles have a mean volume particle diameter (D50) in a range from 2.0 μm to 6.5 μm, inclusive, or more preferably 2.0 μm to 6.0 μm, inclusive. When the mean volume particle diameter (D50) of the toner particles falls within the above range, a streak-like background fog is prevented from occurring. Reduction of the particle diameter of toner particles leads to improvement of graininess of an image (image quality). On the other hand, if the particle diameter falls below 2.0 μm, the electric charge per toner particle becomes too small, thereby causing a background fog or transfer failure.
Preferably, the mean volume particle diameter of the additive falls within the range from 50 nm to 400 nm, more preferably, 60 nm to 300 nm, and further preferably, 80 nm to 200 nm. If the mean volume particle diameter of the additive falls below 50 nm, the additive is highly likely to adhere to a toner particle and to be buried in the toner particle. Thus, the toner particle is less likely to be removed from an electrophotographic photoconductor. On the other hand, if the particle diameter of the additive exceeds 400 nm, the additive is less likely to adhere to a toner particle and more likely to come off the toner particle. Thus, the effect of the additive is less likely to be sustained.
Examples of a carrier of the two-component developer includes a coated ferrite core having a specific gravity exactly or approximately in a range between 2 to 7×103 kg/m3 and a mean volume particle diameter exactly or approximately in a range between 10 to 40 μm. Here, the toner density falls within a range, for example, between 5 to 15%.
A thickness regulating member 150 is also disposed inside the developing device body 140. The thickness regulating member 150 regulates the thickness of a layer of the developer 4 supplied by the first agitating member 147 to the surface of the second development roller 142. The thickness regulating member 150 is made of, for example, a nonmagnetic-metal flat plate. The thickness regulating member 150 is disposed downstream from the second north pole N2 of the second magnetic roller 142a in the direction of rotation of the second development sleeve 142b so as to face the surface of the second development sleeve 142a with a predetermined gap therebetween. The thickness regulating member 150 regulates the amount of developer 4 that has been supplied to the surface of the first development roller 141.
A flattening member 152 is disposed downstream from a facing portion 151 at which the first development roller 141 and the second development roller 142 face each other. The flattening member 152 flattens the surfaces of layers of the developer 4 divided between the first development roller 141 and the second development roller 142. The flattening member 152 has a cylindrical shape or a shape having a triangular cross section. The flattening member 152 is disposed so as to face the surfaces of the first development roller 141 and the second development roller 142 with gaps interposed therebetween.
As illustrated in
As illustrated in
Now, an operation of the developing device 14 according to the exemplary embodiment is described.
As illustrated in
The developer 4 supplied to the surface of the second development roller 142 is transported counterclockwise as the second development sleeve 142b rotates while the thickness of a layer of the developer 4 is regulated by the thickness regulating member 150. Thereafter, at the facing portion 151 at which the first development roller 141 and the second development roller 142 face each other, the developer 4 is divided between the first development roller 141 and the second development roller 142 by the second north pole N2 of the first magnetic roller 141a and the third south pole S3 of the second magnetic roller 142a, which serve as dividing poles. At this time, the proportions of the developer 4 divided between the first development roller 141 and the second development roller 142 are changeable by changing the widths of the second north pole N2 and third south pole S3, serving as dividing poles, or the magnitudes of the magnetic force of the poles N2 and S3. The developer 4 allotted to the first development roller 141 is transported clockwise as the development sleeve 141b rotates. After the surface of a layer of the developer 4 is flattened by the flattening member 152, the developer 4 arrives the first development region 143 at which the developer 4 faces the surface of the photoconductor drum 11. Here, an electrostatic latent image formed on the surface of the photoconductor drum 11 is developed by the third north pole N3, serving as the development pole, with the developer 4 that is in a form of a magnetic brush by the toner adhering to the carrier. The developer 4 held on the surface of the first development roller 141 is transported clockwise as the first development sleeve 141b rotates further and then removed from the surface of the development sleeve 141b by the second south pole S2 and third south pole S3, serving as removal poles that form a repulsive magnetic field. Thereafter, a new lot of developer 4 is supplied to the surface of the first development roller 141 by the second development roller 142.
On the other hand, the developer 4 allotted to the second development roller 142 is transported counterclockwise as the second development sleeve 142b rotates. After the surface of a layer of the developer 4 is flattened by the flattening member 152, the developer 4 arrives the second development region 144 at which the developer 4 faces the surface of the photoconductor drum 11. Here, an electrostatic latent image formed on the surface of the photoconductor drum 11 is developed by the fourth south pole S4, serving as a development pole, with the developer 4 that is in the form of a magnetic brush. The developer 4 held on the surface of the second development roller 142 is transported counterclockwise as the second development sleeve 142b rotates further and then removed from the surface of the development sleeve 142b by the first south poles S1 and second south pole S2, serving as removal poles that form a repulsive magnetic field. Thereafter, a new lot of developer 4 is supplied to the surface of the development sleeve 142b by the first agitating member 147.
As illustrated in
As illustrated in
Here, the transfer efficiency with which a toner image is transferred from the intermediate transfer belt 21 to a recording medium 5 at the second transfer position varies with the type of the recording medium 5. When a recording sheet 5 having projections and depressions on its surface, such as an embossed sheet, is used as an example of a recording medium, the transfer efficiency with which a toner image is transferred from the intermediate transfer belt 21 to the recording sheet 5 is low: part of the toner image might not be transferred to a depression of the sheet 5 from the intermediate transfer belt 21.
In the image forming apparatus 1, a toner having a small particle diameter is often used for improving the image quality. Thus, the diameter of the toner is relatively smaller than the size of the depressions of the recording sheet 5, thereby causing a problem in transfer of the toner. In view of this, for improvement of the transfer efficiency, an additive (transfer support additive) having a relatively large particle diameter is added to the toner of the two-component developer 4.
The inventors have studied and examined the reason why the transfer efficiency with which a toner image is transferred to the recording sheet 5, such as an embossed sheet, having projections and depressions on its surface is low from various aspects and found the following fact by investigating the transfer efficiency of a toner image by using a toner having a mean volume particle diameter of approximately 3.8 μm. Specifically, the amount of transfer support additive adhering to the toner varies between the cases where the toner image is successfully transferred to a depression of the recording sheet 5 and where the toner image fails to be transferred to a depression of the recording sheet 5.
Specifically, the surfaces of the toner particles used when toner images are successfully transferred to the depression of the recording sheet 5 and the surfaces of the toner particles used when toner images are not sufficiently transferred to the depression have been observed using a scanning electron microscope (SEM) and the additives on the toner particles for both cases have been compared. As a result, the additive adhering to the toner particles used when toner images are successfully transferred to the depression of the recording sheets 5 remain substantially unchanged from the initial state or only a little amount of additive comes off or is buried in the toner. On the other hand, on the surfaces of the toner particles used when toner images are not sufficiently transferred to the depression of the recording sheet 5, a larger amount of additive that have adhered to the toner particles comes off or is buried in the toner.
In other words, when a sufficiently large amount of additive is interposed between the intermediate transfer belt 21 and the toner at the second transfer position, the transfer efficiency with which a toner image is transferred from the surface of the intermediate transfer belt 21 to a recording medium 5 such as an embossed sheet is preferably high. On the other hand, when the amount of additive interposed between the surface of the intermediate transfer belt 21 and the toner particles is small, the transfer efficiency with which a toner image is transferred from the surface of the intermediate transfer belt 21 to the recording medium 5 such as an embossed sheet is low.
The toner image formed on the surface of the photoconductor drum 11 is first transferred to the intermediate transfer belt 21 at the first transfer position and then is transported to the second transfer position as it is. Thus, an amount of additive interposed between the intermediate transfer belt 21 and the toner is determined when the toner image is formed on the surface of the photoconductor drum 11, that is, determined by conditions under which the developing device 14 renders the electrostatic latent image formed on the surface of the photoconductor drum 11 visible.
Thus, how well the additive adhering to the surfaces of the toner particles is maintained while the developing device 14 develops an electrostatic latent image formed on the photoconductor drum 11 into a toner image markedly affects the degree of improvement of the transfer efficiency with which the toner image is transferred to the recording medium 5 having projections and depressions on its surface such as an embossed sheet.
The developing device 14 includes the first development roller 141 and the second development roller 142 to develop an electrostatic latent image formed on the surface of the photoconductor drum 11. In consideration of maintaining the amount of additive adhering to the surface of the toner particles, it seems that sliding friction between the surface of the photoconductor drum 11 and the developer 4 held on the surfaces of the first and second development rollers 141 and 142 significantly affects the amount of additive. At the facing portion 143 at which the first development roller 141 faces the photoconductor drum 11, the first development roller 141 moves in the direction opposite to the direction of movement of the surface of the photoconductor drum 11. On the other hand, at the facing portion 144 at which the second development roller 142 faces the photoconductor drum 11, the second development roller 142 moves in the same direction as the direction of movement of the surface of the photoconductor drum 11. Thus, the sliding friction between the surface of the photoconductor drum 11 and the developer 4 held on the surface of the first development roller 141 is larger than the sliding friction between the surface of the photoconductor drum 11 and the developer 4 held on the surface of the second development roller 142.
In view of this, the inventors have made a prototype of the image forming apparatus 1 illustrated in
Among the development conditions under which the first development roller 141 and the second development roller 142 of the developing device 14 perform development, the amount of developer per unit area (hereinafter also called “MOS”) held on the surface of each of the development rollers 141 and 142 and the shortest distance (hereinafter also called “DRS”) between the photoconductor drum 1 and each of the development rollers 141 and 142 are changed for the experiments. Here, each MOS and each DRS are set so as to be different between the first development roller 141 and the second development roller 142.
Amount of developer (MOS) on first development roller: 250 g/m2 (comparisons between 250 g/m2 and 310 g/m2)
Amount of developer (MOS) on second development roller: 240 g/m2 (comparisons ranging from 240 g/m2 to 310 g/m2)
Center of shortest distance (DRS) between first development roller and photoconductor drum: 180 μm (comparisons ranging from 140 μm to 240 μm)
Center of shortest distance (DRS) between second development roller and photoconductor drum: 200 μm (comparisons ranging from 130 μl to 250 μm)
Rotation speed (process speed) of photoconductor drum: 370 mm/sec
Rotation direction (MRS) of first development roller: same as rotation direction of photoconductor drum (or against direction) at peripheral velocity ratio of 1.75
Rotation direction (MRS) of second development roller: opposite to rotation direction of photoconductor drum (or with direction) at peripheral velocity ratio in a range from 1.5 to 2.0
Surface shape and surface roughness of development rollers: sleeves grooved at pitch of 0.8 mm
Diameter Φ of development rollers: 25 mm
Magnitude of magnetic force of development poles on development rollers: 100 to 159 mT
DC component voltage included in voltage applied to first and second development rollers: −300 to −500 V
Wave form of AC component voltage (development AC bias) superimposed on DC component voltage applied to first and second development rollers: sine wave (square wave)
Amplitude of development AC bias (Vp-p or peak to peak voltage): 500 to 1200 V
Frequency of development AC bias: 5 to 20 kHz
Good: above 95%
Poor: below 90%
Here, “good” denotes preferable, “fair” denotes acceptable, and “poor” denotes unacceptable.
As is clear from the table of
When the MOS/DRS ratio obtained by dividing the amount of developer per unit area (MOS) held on the surface of the first development roller 141 by the shortest distance (DRS) between the photoconductor drum 1 and the first development roller 141 is 1.79 or 1.88×103 kg/m3, transfer failure occurs. On the other hand, when the MOS/DRS ratio falls within a range from 1.04 to 1.56×103 kg/m3, the transfer efficiency is preferable or acceptable.
As described above, when the MOS/DRS ratio obtained by dividing the amount of developer per unit area (MOS) held on the surface of the first development roller 141 by the shortest distance (DRS) between the photoconductor drum 1 and the first development roller 141 is in a range from 1.00×103 kg/m3 to 1.60×103 kg/m3, inclusive, it is found that the transfer efficiency with which the toner image is transferred to an embossed sheet having projections and depressions on its surface is capable of being maintained at a preferable or acceptable level.
More specifically, when the MOS/DRS ratio obtained by dividing the amount of developer per unit area (MOS) held on the surface of the first development roller 141 by the shortest distance (DRS) between the photoconductor drum 1 and the first development roller 141 is in a range from 1.00×103 kg/m3 to 1.40×103 kg/m3, inclusive, it is found that the transfer efficiency with which the toner image is transferred to an embossed sheet having projections and depressions on its surface is capable of being maintained at a preferable level.
In the developing device 14, when the amount of developer per unit area (MOS) held on the surface of the first development roller 141 falls within the range from 200 to 400 g/m2, the shortest distance (DRS) between the photoconductor drum 1 and the first development roller 141 falls within the range from 100 to 400 μm, and a gap between the thickness regulating member 150 and the second development roller 142 falls within the range from 0.4 to 0.8 mm, it has been configured that results that are similar to those illustrated in
As illustrated in
As is clear from
On the other hand, when the peripheral velocity ratio (V3/V1) of the second development roller 142 to the photoconductor drum 11 is 4.00 or lower, the transfer efficiency is at an acceptable or preferable level since the second development roller 142 and the photoconductor drum 11 move in the same direction at the facing portion. Thus, in order to prevent an additive from adhering to the surface of the photoconductor drum 11, it is only required to set the peripheral velocity ratio of the second development roller 142 to the photoconductor drum 11 to 4.00 or lower.
In addition, when the peripheral velocity ratio (V3/V1) of the second development roller 142 to the photoconductor drum 11 is set larger than the peripheral velocity ratio (V2/V1) of the first development roller 141 to the photoconductor drum 11, the development performance of the second development roller 142 is improved while sliding friction between the developer 4 held on the surface of the first development roller 141 and the photoconductor drum 11 is kept low.
The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-253506 | Nov 2012 | JP | national |
