Patent Application
20060078359
This application is based on Japanese Patent Application NO. 2004-296006 filed on Oct. 8, 2004 in Japanese Patent Office, the entire content of which is hereby incorporated by reference.
The present invention relates to an image forming apparatus, and specifically relates to a color image forming apparatus having an intermediate transfer member onto which a toner image is transferred.
As a color image forming apparatus, which employs an intermediate transfer member, there has been well known such an apparatus in which a toner image formed on an image bearing member is transferred onto an intermediate transfer member, and then, the toner image transferred onto the intermediate transfer member is further transferred onto a transfer material. In the color image forming apparatus of this type, the toner image, formed on the image bearing member and charged at a predetermined polarity, is transferred onto the intermediate transfer member by using an electrostatic force, and then, the toner image residing on the intermediate transfer member is further transferred onto a transfer material by using an electrostatic force.
Specifically, since it is possible for the abovementioned color image forming apparatus to make plural toner images, formed on the image bearing member, overlap with each other on the intermediate transfer member, the apparatus is widely employed as a color image forming apparatus for forming a color image on the transfer material. In the color image forming apparatus of this type, unicolor toner images, each of which is formed on an image bearing member corresponding to each of the primary colors, are sequentially transferred one by one while overlapping with each other on the intermediate transfer member so as to form a full color toner image thereon, and then, the full color toner image is further transferred onto the transfer material at a time by means of a electrostatic force.
Since an amount of electronic charge per one toner. particle is substantially uniform, an electronic potential in the toner image residing on the intermediate transfer member is determined by an amount of toner adhered within a predetermined area. Accordingly, in the color image forming apparatus, within the full color toner image formed on the intermediate transfer member, a charged potential of an area at which plural color toners overlap with each other is greater than that of another area at which only a single color toner resides. Further, for instance, when the abovementioned full color toner image formed on the intermediate transfer member includes both a high density area and a halftone area, a charged potential of the high density area is greater than that of the halftone area.
Further, sometimes, variations of the charged potential within the toner image, passed through the primary transferring section for transferring the toner image from the image bearing member to the intermediate transfer member, would occur depending on environmental factors.
The fact that the variations of the charged potential widely vary within the toner image, as mentioned in the above, means that plural areas being different from each other in its transferring characteristics coexist within the same toner image. If it is tried to transfer all of such the plural areas, being different from each other in its transferring characteristics, onto the transfer material under the same transferring conditions, various kinds of image deficiencies are liable to occur at the time of the secondary transferring operation from the intermediate transfer-member to the transfer material.
In recent years, colorization trends have been proliferated in the filed of imaging apparatus including a copier, a facsimile, a compound image forming apparatus having such the functions of them, and further, associated with employments of the polymerized toner and small-sized toner particles, demands for high quality imaging have largely increased in the field of transferring process. In addition, speedup trends of the abovementioned image forming apparatus have been progressing as well. In order to produce a good image according to the abovementioned trends, it is necessary to compensate for the electronic potential of the toner image residing on the intermediate transfer member, which varies with a number of the primary transferring operations and the environmental factors, at a substantially a uniform potential, so as to improve the secondary transferring efficiency.
To overcome the abovementioned problem in conventional image forming apparatus, there has been proposed such a configuration that a conductive roller member, serving as a pre-charging device for charging a toner image primary-transferred on the intermediate transfer member before transferring it onto a transfer material, is disposed at a back side of the intermediate transfer member, which is opposed to a charging device and electrodes of the charging device, so as to work as an opposed electrode (for instance, set forth in Patent Document 1). According to this configuration, the charging operation conducted by means of a corona discharging action of AC (Alternate Current), DC (Direct Current), etc., is applied to the toner image transferred onto the intermediate transfer member, so as to make the charge amount over the toner image substantially uniform.
Further, it has been also proposed to provide a controlling apparatus for controlling a charging condition for the pre-charging device, in response to the surface moving velocity of the intermediate transfer member passing through the charging position of the pre-charging device (for instance, set forth in Patent Document 2).
[Patent Document 1]
[Patent Document 2]
According to the methods described in Patent Documents 1 and 2, since the charge amount of the toner image on the intermediate transfer member is set at a relatively large value as the uniform potential, when the resistivity of the paper, serving as the transfer material, is high due to a low humidity environment or a transferring operation of the second surface of the paper in the duplex copy mode, image. deficiencies, due to discharging actions caused by the electronic potential rise of the paper, are liable to occur. If the transferring voltage were set at a low value in order to prevent such the image deficiency, an area, where the total charging amount is relatively large, would be suffered by a lack of transferring electronic field, resulting in an occurrence of the density unevenness.
Further, with respect to the configuration that the conductive roller member, serving as the pre-charging device, is disposed at the back side of the intermediate transfer member, which is opposed to the charging device, so as to work as the opposed electrode, it becomes difficult to acquire a sufficient charging effect according as the line velocity of the intermediate transfer member is getting large in accordance with the speedup trend of the image forming apparatus. In addition, since it is necessary to control the charging condition in response to the line velocity of the intermediate transfer member, the total configuration is getting complicated.
In the image forming apparatus, which is provided with such a discharging device that makes it possible to conduct a secondary transferring operation well in such a manner that the toner image potential is made to be suppressed for an area of the intermediate transfer member at which the total charge amount of the toner image is relatively large, while the toner image potential is made to be maintained as it is for another area, such as a halftone area, etc., at which a small amount of toner is adhered, so as to acquire a good secondary transferred image without occurring a image roughening and a toner scattering, it is an object of the present invention to provide an image forming apparatus, which makes it possible to conduct a sufficient discharging operation with a simple structure, even if the line velocity of the intermediate transfer member becomes high as a result of speedup measures for its image forming operation, so as to improve the secondary transferring efficiency and to acquire good image.
Accordingly, to overcome the cited shortcomings, the abovementioned object of the present invention can be attained by image forming apparatus described as follow.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:
Referring to the drawings, an embodiment of the present invention will be detailed in the following. The scope of the present invention is not limited to the embodiment described in the following.
The color image forming apparatus shown in
The image forming section 20Y for forming the-unicolor image of color Y (Yellow) includes a charging device 2Y, an exposing section 3Y, a developing device 4Y, a primary transferring device SY and a cleaning device 6Y, which are arranged around the photoreceptor drum 1Y serving as an image bearing member. The image forming section 20M for forming the unicolor image of color M (Magenta) includes a charging device 2M, an exposing section 3M, a developing device 4M, a primary transferring device 5M and a cleaning device 6M, which are arranged around the photoreceptor drum 1M serving as an image bearing member. The image forming section 20C for forming the unicolor image of color C (Cyan) includes a charging device 2C, an exposing section 3C, a developing device 4C, a primary transferring device 5C and a cleaning device 6C, which are arranged around the photoreceptor drum 1C serving as an image bearing member. The image forming section 20K for forming the unicolor image of color K (Black) includes a charging device 2K, an exposing section 3K, a developing device 4K, a primary transferring device 5K and a cleaning device 6K, which are arranged around the photoreceptor drum 1K serving as an image bearing member.
The intermediate transfer member 7, having a semi-conductive property and formed in a belt shape, is-threaded on a plurality of rollers 701a, 701b, 701c, 701d, 71, etc., so as to rotatably move around them. In the present embodiment, the rollers 701d and 71 support the intermediate transfer member 7 in such a manner that the section of the intermediate transfer member 7 residing between the rollers 701d and 71 is maintained as a straight flat plane. Accordingly, the rollers 701d and 71 work as supporting member of the intermediate transfer member 7, in this embodiment.
The image forming section 20Y, including the charging device 2Y, the exposing section 3Y and the developing device 4Y, conducts the charging operation, the exposing operation and the developing operation for the photoreceptor drum 1Y, to form a yellow toner image on the photoreceptor drum 1Y. In the same way, the image forming section 20M, including the charging device 2M, the exposing section 3M and the developing device 4M, forms a magenta toner image on the photoreceptor drum 1M, and the image forming section 20C, including the charging device 2C, the exposing section 3C and the developing device 4C, forms a cyan toner image on the photoreceptor drum 1C, and the image forming section 20K, including the charging device 2K, the exposing section 3K and the developing device 4K, forms a cyan toner image on the photoreceptor drum 1K. The unicolor toner images formed on the photoreceptor drums 1Y, 1M, 1C, 1K are sequentially transferred onto the intermediate transfer member 7 by the primary transferring devices 5Y, 5M, 5C, 5K while overlapping with each other so as to form a multicolor toner image (hereinafter, also referred to as a full color toner image).
Although a well known material, such as an organic photosensitive material, an a-Silicon photosensitive material, etc., can be employed as the photoreceptor drum 1Y, the organic photosensitive material would be preferable, and specifically, it is preferable that the organic photosensitive material has a negative charging property. In the present embodiment, the organic photosensitive material having the negative charging property is employed.
Although a corona discharging device, such as a scorotron, a corotron, etc., is employed for each of charging devices 2Y, 2M, 2C, 2K, the scorotron discharging device is preferably employed.
A light emitting element, such as a laser, a LED array, etc., which emits light modulated according to image data, is employed for each of the exposing sections 3Y, 3M, 3C, 3K.
Although either a developing device, which employs two-component developer containing carrier and toner as main gradients, or another developing device, which employs single-component developer containing only toner as a main gradient without containing carrier, can be employed as each of the developing devices 4Y, 4M, 4C, 4K, it is preferable to employ the developing device, which employs the two-component developer. Further, although either a normal developing method or a reverse developing method can be employed for the developing device, it is preferable to employ the reverse developing method in which toner, charged at a polarity same as that of the charged photoreceptor drum by applying a developing bias having a polarity same as that of the photoreceptor drum to a developing sleeve 4a, are used for the developing operation. In the present embodiment, the reverse developing method using the negative charged toner is employed for the developing operation.
To maintain a high image quality and to prevent an occurrence of fogging, it is preferable that a volume average particle diameter of toner to be employed is in a range of 3-6 μm.
The value of the volume average particle diameter, being the average particle diameter referring to the volume, is measured by the “Coulter counter TA-II” (manufactured by Coulter-Co. LTD.), which is provided with a wet dispersion apparatus.
By employing the abovementioned toner, it becomes possible to form a high quality image with a high-resolution capability.
Further, in the present invention, it is preferable to employ spherical-shaped toner having a spherical degree in a range of 0.94-0.98, so as to hardly receive strong stresses in the developing device, so as to hardly cause the fogging and the scattering of the toner, and so as to maintain the cleaning efficiency at a high level.
The spherical degree can be calculated by employing the following equation.
SD=CLC/PCL
where SD: spherical degree,
To calculate the spherical degree, initially, photographic images enlarged up to 500 times of the original size with respect to 500 resin particles are captured by the scanning-type electron-microscope or the laser microscope, and then, the spherical degrees of them are measured by analyzing the photographic images by means of a “SCANNING IMAGE ANALYZER” (manufactured by Japan Electron Co. LTD.), and finally, the arithmetical mean of them is calculated as the spherical degree of the toner concerned. Alternatively, as a simplified method for measuring the spherical degree, a flow-type particle image analyzer (“FPIA-1000”, manufactured by Toa Iyou Denshi K.K.) can be applicable for this purpose.
It is desirable that polymerization toner are used for such the toner having the small-sized particles and the high spherical degree.
The polymerization toner are such toner that is acquired by forming the binder resin included in the toner and the shape of each toner particle through the polymerization process of raw material monomer or pre-polymer and successive chemical processing. Concretely speaking, the polymerization toner are acquired through a polymerizing reaction process, such as a suspension polymerization, an emulsification polymerization, etc., and through an adhesion process between particles to be successively conducted after the polymerizing reaction process as needed. Since the polymerization toner are manufactured by polymerizing the raw material monomer or pre-polymer after uniformly dispersing it in a certain water system, it is possible to produce the toner having a uniform distribution of the toner granularity and a uniform particle shape.
Concretely speaking, the polymerization toner can be manufactured by the method including the steps of manufacturing material created according to the suspension polymerization method, or manufacturing polymerized particles as fine particles by emulsion-polymerizing monomer in the liquid of water medium added with emulsifying liquid, and then, associating them by adding organic solvent, flocculating agent, etc. Further, a preparation method of mixing fluid dispersion, such as parting agent, coloring agent, etc., which is necessary as a component of the toner, when associating them, a method of emulsion-polymerizing the component of the toner, such as the parting agent, the coloring agent, etc., after dispersing them, etc. can be cited. Hereinafter, the term of the “associating” is to fuse plural resin particles and plural coloring particles with each other.
Numeral 5A indicates a secondary transferring device, which includes a transfer roller 5AR, having a conductive rubber roller, and a power source 5AE.
Numeral 6A indicates an intermediate-transfer member cleaning device for cleaning intermediate transfer member 7, while numeral 8 indicates a fixing device for fixing a toner image onto the transfer material P.
The intermediate transfer member 7 is either a single layer belt or a multi layer belt made of polyamide, polyimide, etc. The volume resistivity of intermediate transfer member 7 is a value in a range of 107-1012 Ω·cm. In the present embodiment, the volume resistivity is set at 109 Ω·cm.
On the other hand, after the secondary transferring operation for the transfer material P, conducted by the transfer roller 5AR, is completed, the intermediate transfer member 7 passes through a cleaning point. At this cleaning point, the intermediate-transfer member cleaning device 6A cleans the surface of the intermediate transfer member 7.
In the present embodiment, a scorotron discharging device 9, serving as a pre-discharging device, is disposed at such a position at which the intermediate transfer member is supported in a state of the straight flat plane between the primary transferring device 5K and the secondary transferring device 5A along the intermediate transfer member 7. Referring to
A wired grid, a plate-shaped grid having a pattern formed by etching a metal plate, etc., can be employed as the grid 92. In the present embodiment, a plate-shaped grid, onto which gold plating is applied, is employed as the grid 92. Further, the width of the grid 92 is set at L2 in the moving direction of the intermediate transfer member 7 (indicated by arrow X in
Although a wire material, such as a tungsten wire, a stainless wire, a gold wire, etc., whose diameter is in a range of 20-150 μm, can be employed for the discharging electrode 91, it is specifically preferable that the surface of the discharging electrode 91 is finished with a gold coating. It is applicable that the gold wire itself is employed as the discharging electrode, or the gold coating is applied onto the stainless wire, the tungsten wire, etc. for finishing the discharging electrode. It is desirable that the thickness of the gold coating is set at a value in a range of 1-5 μm as an average coating thickness, from view points of an efficiency for removing discharging products, such as ozone, etc., a manufacturing cost and a discharging efficiency.
A opposing electrode 12, constituted by a conductive brush 12a and a supporting member 12b for supporting the conductive brush 12a, is disposed at a back side of the intermediate transfer member 7 opposing to the scorotron discharging device 9 thereat. The conductive brush 12a contacts the back side of the intermediate transfer member 7, and the opposing electrode 12 is coupled to the ground through resister 12c.
The conductive brush 12a is made of conductive resin fibers, such as acrylic, nylon, polyester, etc. It is preferable that each diameter of the conductive resin fibers is in a range of 0.111-0.778 tex in the metric unit of yarn number proposed by ISO, and a fiber density of the conductive brush 12a is in a range of 12000-77000 fibers/cm2, and a resistivity of each fiber is in a range of 1-105 Ω·cm. Further, the width of the conductive brush 12a is set at width L1 in the moving direction of the intermediate transfer member 7 (indicated by arrow X shown in
The present embodiment is so constituted that a DC voltage in a range of 0-5 kV having a polarity inverse to that of the toner can be applied to the discharging electrode 91, while a voltage in a range of 0-−300 V can be applied to the grid 92.
In the present embodiment, a DC voltage of +4 kV is applied to the discharging electrode 91 of the scorotron discharging device 9, a voltage of −50 V is applied to the grid 92, length L1 of the grid 92 is set at 40 mm, gap L3 between the scorotron discharging device 9 and the intermediate transfer member 7 is set at 1 mm, and width L1 of the conductive brush 12a is set at 45 mm.
Referring to
An electric potential Vi of a full color area F1, at which an amount of adhered toner is relatively large and the electric potential is relatively high, is reduced to electric potential V3 after the full color area F1 has passed through the scorotron discharging device 9. On the other hand, an electric potential V2 of a halftone area H1, at which an amount of adhered toner is relatively small and the electric potential is relatively low, is substantially maintained as it is even after the halftone area H1 has passed through the scorotron discharging device 9.
To concretely describe the present invention, various embodiments will be detailed in the following. However, the scope of the present invention is not limited to the following embodiments.
<Embodiment 1>
In the embodiment 1, the color image forming apparatus, in which the scorotron discharging device 9 and the opposing electrode 12 are disposed in the manner shown in
The main specifications of the color image forming apparatus shown in
The image is formed under the conditions same as those of embodiment 1, except that width L1 of conductive brush 12a of the opposing electrode 12 is changed to 60 mm.
<Embodiment 3>
The image is formed under the conditions same as those of embodiment 1, except that width L1 of conductive brush 12a of the opposing electrode 12 is changed to 30 mm.
In the comparison example 1, instead of the color image forming apparatus employed in the embodiment 1 and shown in
Instead of the opposing electrode 12 including the conductive brush 12a; which is employed in the color image forming apparatus shown in
The overall structures, other than the configuration changed in the above, are common as those of the color image forming apparatus shown in
The image is formed under the conditions same as those of comparison example 1, except that the length of the grid is changed to 20 mm.
In the color image forming apparatus shown in
<Evaluation Method and Evaluation Reference>
High-density duplex images, each of which was formed by mixing color M (Magenta) and color C (Cyan) with each other under a low temperature and low humidity environment (10° C., 20% (ReH)), were outputted, and the transferring unevenness of the reverse sides were evaluated on the basis of visual observation of a viewer, so as to categorize them into the following ranks.
(Ranks of Unevenness)
Bad: transferring unevenness was clearly recognized to an extent that it would become a practical problem.
As shown in Table 1, in the embodiment 1 and the embodiment 2, by disposing the discharging device at such the position that the intermediate transfer member is supported in a state of the straight flat plane, by employing the conductive brush as the opposing electrode, by contacting the conductive brush onto the intermediate transfer member, and by making length L1 of the conductive brush 12a fulfill the condition of L1>L2+2×L3, the effect of discharging the toner image was secured even when the line velocity of intermediate transfer belt was relatively high, resulting in an acquisition of the good image.
In the embodiment 3, since length L1 of the conductive brush 12a was shorter than that of the discharging electrode without fulfilling the condition of L1>L2+2×L3, a little transferring unevenness was generated. However, the effect of discharging the toner image, which would cause practically no problem, could be achieved.
In the comparison example 1 and the comparison example 2, since the opposing electrode was the roller 71 having a curvature, it was impossible to maintain the gap between the grid 92 and the intermediate transfer member 7 within a certain range. Accordingly,. it was impossible to acquire the effect of sufficient discharging the toner image, and therefore, the transferring unevenness was generated on the acquired image to an extent that it would become a practical problem.
According to the present invention, the following effects can be attained.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-296006 | Oct 2004 | JP | national |
