JP4454806B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile using an electrophotographic method, and in particular, sequentially transfers a plurality of color toner images formed on a plurality of image carriers onto a transfer material on a transfer material conveying belt. The present invention relates to an image forming apparatus that transfers images in a superimposed manner or sequentially superimposes them on an intermediate transfer belt to perform primary transfer, and then collectively transfers them to a transfer material to obtain a color image on the transfer material.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus is known that transfers a toner image formed on an image carrier using electrophotography onto a transfer material and obtains an image on the transfer material. Under these circumstances, with the progress of the information society in recent years, needs for color image forming apparatuses are widening. Furthermore, in order to increase the speed of color image output, a plurality of image carriers are arranged in a line, and a toner image is sequentially formed on each image carrier, and the toner images are directly transferred to a transfer material or via an intermediate transfer member. In-line image forming apparatuses that transfer images are attracting attention.
[0003]
An example of an electrophotographic inline-type full-color image forming apparatus will be briefly described with reference to FIG. 1. The image forming apparatus has four color images of yellow (Y), magenta (M), cyan (C), and black (K). Forming units (image forming stations) 10Y, 10M, 10C, and 10K are provided, and each station has a photosensitive drum 70 (70Y to 70K) as an image carrier.
[0004]
  In order to form an image, first, the surface of the photosensitive drum 70 (70Y to 70K) of each station is primary.ElectrificationRoller 12 (12Y-12K)soThe image is charged uniformly and subjected to color exposure of the original by the laser exposure device 13 (13Y to 13K), and an electrostatic latent image corresponding to the separation color of the original is formed on the surface of the photosensitive drum 70. Is developed with minus toner by the developing device 14 (14Y to 14K), and a toner image of each color is formed on the surface of the photosensitive drum 70. The toner image of each color on the photosensitive drum 70 is formed on the intermediate transfer belt 80. Primary transfer is performed by sequentially superimposing by a primary transfer roller 54 (54Y to 54K) to which a primary transfer bias is applied from a constant voltage power supply 48 (48Y to 48K) of a primary transfer power supply.
[0005]
Thereafter, a secondary transfer roller 55 to which a secondary transfer bias is applied from the constant voltage power source 49 of the secondary transfer power source onto the transfer material P conveyed with the four color toner images on the intermediate transfer belt 80 to the intermediate transfer belt 80. Then, the transfer material P after the secondary transfer is conveyed to the fixing device 40, and the four colors of toner are melted and mixed to be fixed on the transfer material P, thus transferring. A full color image is formed on the material P.
[0006]
[Problems to be solved by the invention]
In the image forming apparatus having the plurality of image forming stations, in order to obtain a full color image having a sufficient density, the primary transfer efficiency of the toner image on the photosensitive drum 70 to the intermediate transfer belt 80 at each station 10Y to 10K. It is important to balance the retransfer rate to the photosensitive drum of the image forming station for the second and subsequent colors of the toner image on the intermediate transfer belt, such as the second and subsequent image forming stations for the yellow toner image of the first color. It is.
[0007]
It is important to optimize the surface potential of the photosensitive drum 70 and the primary transfer bias of the constant voltage power supply 48 (48Y to 48K) in each of the image forming stations 10Y to 10K. In particular, the primary transfer current is the density of each color of a full-color image. Must be controlled appropriately.
[0008]
The object of the present invention is to control the transfer current so as to balance the transfer efficiency of each color and the retransfer rate in the image forming section after the next color of each color, thereby obtaining a color image having a sufficient density of each color. An image forming apparatus is provided.
[0009]
[Means for Solving the Problems]
  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present inventionCarrying a toner imageA plurality of image carriers and rotation along the plurality of image carriersPossible endlessIntermediate transfer member and the plurality of image carriersAnd the plurality of image bearing members and transfer nip portions are formed respectively.Multiple transfer charging meansAnd forming a color toner image by transferring a toner image from the plurality of image bearing members to the intermediate transfer member by passing a transfer current to the transfer nip portion by the plurality of transfer charging units.In the image forming apparatus,
  The plurality of transfer charging meansBeforeRotation direction of intermediate transfer memberPlaced at the most upstream positionBestCurrentPhotographic charging meansFlows into the transfer nip formed byTransfer currentofvalueFlows to each transfer nip formed by a plurality of other transfer charging meansTransfer currentofGreater than valueIn addition, the value of the transfer current flowing through each transfer nip formed by the other plurality of transfer charging means is substantially the same.An image forming apparatus characterized by the above.
[0010]
  The present invention also carries a toner image.Multiple image carriersAnd rotating along the plurality of image carriersIt is possible to endlessly convey the transfer materialTransfer materialCarrierAnd the plurality of image carriersAnd a plurality of transfer charging means that respectively form the plurality of image carriers and a transfer nip portion, and each of the plurality of image charging bodies is formed on the transfer nip portion by the plurality of transfer charging means. By supplying a transfer current, a toner image is transferred from the plurality of image carriers to a transfer material conveyed by the transfer material carrier to form a color toner image.In the image forming apparatus,
  The plurality of transfer charging meansBeforeTransfer materialCarrierDirection of rotationPlaced at the most upstream positionBestCurrentPhotographic charging meansFlows into the transfer nip formed byTransfer currentofvalueFlows to each transfer nip formed by a plurality of other transfer charging meansTransfer currentThe value of theBigger,The value of the transfer current flowing through each transfer nip formed by the other plurality of transfer charging means is substantially the same.An image forming apparatus characterized by the above.
[0011]
  According to the present invention,Common constant voltage power supply for applying voltage to the other plurality of transfer charging means.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.
[0013]
Example 1
FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus of the present invention. This image forming apparatus is configured as a four-drum, intermediate transfer type full-color printer.
[0014]
As shown in FIG. 1, the image forming apparatus includes four color image forming units (image forming stations) 10Y, 10M, 10C, yellow (Y), magenta (M), cyan (C), and black (K). 10K, and further includes a transfer device including an intermediate transfer belt 80 and a fixing device 40.
[0015]
Each of the image forming stations 10Y, 10M, 10C, and 10K is configured as an image forming unit, and photosensitive drums (drum-shaped electrophotographic photosensitive members) 70Y, 70M, 70C, and 70K that are image carriers are rotatable in the direction of arrow a. is set up. Primary charging rollers 12Y, 12M, 12C, and 12K that uniformly charge the surface of the photosensitive drum are disposed on the outer peripheral surfaces of the photosensitive drums 70Y, 70M, 70C, and 70K, respectively. Laser exposure devices 13Y, 13M, 13C, and 13K for exposing the surface of the photosensitive drum with laser light modulated in accordance with the image signal are further provided on the downstream side of each surface of the photosensitive drum surface formed by laser exposure. Developing units 14Y, 14M, 14C, and 14K for developing the electrostatic latent image using corresponding colors of yellow, magenta, cyan, and black toners are arranged.
[0016]
  Photosensitive drum 70Y, 70M, 70C, and 70K sandwiching the intermediate transfer belt 80, primary transfer rollers 54Y, 54M, 54C, and 54K that form a primary transfer portion together with the photosensitive drum are opposed to each other. The primary transfer rollers 54Y, 54M, 54C and 54K are connected to constant voltage power supplies 48Y, 48M, 48C and 48K as primary transfer power supplies, respectively, and variable primary transfer biases Vy, Vm, Vc and Vk are respectively set. Applied.
[0017]
The intermediate transfer belt 80 is stretched over three rollers, that is, a driving roller 51, a tension roller 52, and a secondary transfer counter roller 53, and passes through the image forming stations 10Y to 10K to pass through the photosensitive drums 70Y to 70K. Is placed in contact. The intermediate transfer belt 80 is rotationally driven by the drive roller 51 in the direction of the arrow b in the figure.
[0018]
  Photosensitive drum 70Y, 70M, 70C, and 70K, drum cleaners 16Y, 16M, 16C, and 16K are installed downstream of the primary transfer rollers 54Y, 54M, 54C, and 54K. A belt cleaner 33 is disposed at the drive roller 51 of the intermediate transfer belt 80.
[0019]
The image forming operation of the image forming apparatus configured as described above will be described using the yellow image forming station 10Y as an example.
[0020]
The photosensitive drum 70Y of the yellow station 10Y has a photoconductive layer formed on the surface of an aluminum cylinder, and the surface is uniformly negatively charged by the primary transfer roller 12Y in the process of rotating in the direction of arrow a ( (Charge potential = −650V), and then image exposure is performed by the laser exposure device 13Y (surface potential after exposure = −250V), and an electrostatic latent image corresponding to the yellow image component of the original is formed on the surface of the photosensitive drum 70Y. Is done. This latent image is developed using negatively charged yellow toner by the developing device 14Y, and the latent image is visualized as a yellow toner image. The obtained yellow toner image is primarily transferred onto the intermediate transfer belt 80 by applying a primary transfer bias from the primary transfer power supply 48Y to the primary transfer roller 54Y. After the transfer, the untransferred toner adhering to the surface of the photosensitive drum 70Y is removed by the cleaner 16Y, and used for the next image formation.
[0021]
The above image forming operation is performed at a predetermined timing in each of the image forming stations 10Y to 10K, and the toner images on the photosensitive drums 70Y to 70K are sequentially overlapped on the intermediate transfer belt 80 at the respective primary transfer portions. Next transfer.
[0022]
In the full color mode, the toner images are sequentially transferred to the intermediate transfer belt 80 in the order of yellow, magenta, cyan, and black. In the single color or 2-3 color mode, the toner images of the necessary colors are as described above. Transcribed in the same order.
[0023]
Thereafter, the four color toner images on the intermediate transfer belt 80 are transferred to the secondary transfer roller with the grounded secondary transfer counter roller 53 and the intermediate transfer belt 8 interposed between them as the intermediate transfer belt 80 rotates in the direction of arrow b. 55 is moved to the contacted secondary transfer portion, and the transfer material P is supplied to the secondary transfer roller 55 from the constant voltage power source 49 of the secondary transfer power source on the transfer material P supplied thereto at a predetermined timing. Secondary transfer is collectively performed by applying a secondary transfer bias.
[0024]
The transfer material P on which the four-color toner images are secondarily transferred is conveyed to the fixing device 40, where it is pressurized and heated, and the four-color toners are melted and mixed to be fixed on the transfer material P. A full color image is formed. On the other hand, the intermediate transfer belt 80 that has finished the secondary transfer has the transfer residual toner remaining on the surface removed by the belt cleaner 33.
[0025]
In this embodiment, a negatively chargeable OPC drum having a diameter of 30.6 mm is used for the photosensitive drum 70 (70Y to 70K), and a charging bias in which an AC component is superimposed on a DC component is applied to the charging roller 12 (12Y to 12K). The surface of the photosensitive drum 70 was uniformly charged to about −650 V regardless of the environment. The exposure device 13 (13Y to 13K) has a near-infrared laser diode having a wavelength of 760 nm and a polygon mirror that scans the photosensitive drum 70 with laser light, and reduces the surface potential of the photosensitive drum 70 to −250 V at the exposure portion. Then, an electrostatic latent image is formed using this as an image portion.
[0026]
The black developing device 14K is a jumping developing type developing device using magnetic toner (magnetic one-component developer), and a sleeve-like developer carrying member (developing sleeve) containing a roller-like fixed magnetic member (magnet roller). ) Is loaded with a magnetic toner having a particle diameter of 6 μm, the thickness of the toner layer is regulated by an elastic blade as the developing sleeve rotates, and conveyed to the developing unit facing the photosensitive drum, and the AC component is applied to the DC component applied to the developing sleeve. The toner on the developing sleeve is jumped to the electrostatic latent image on the photosensitive drum by the developing bias superimposed on the photosensitive drum, and is adhered to the exposed portion of the latent image, and the latent image is reversely developed.
[0027]
The yellow developing unit 14Y, the magenta developing unit 14M, and the cyan developing unit 14C are jumping developing type developing units using non-magnetic toner (non-magnetic one-component developer), and have a core / shell structure including wax as the non-magnetic toner. Is used. The polymerized toner having a particle size of 6 μm is used and coated on the surface of the developing sleeve by a coating roller, and the toner layer thickness is regulated by an elastic blade as the developing sleeve rotates, and development facing the photosensitive drum is performed. In the same manner as the black developing device 14K, the electrostatic latent images on the photosensitive drums 70Y, 70M, and 70C are jumped to perform reverse development.
[0028]
The primary transfer roller 54 (54Y to 54K) is formed by covering an 8 mm diameter cored bar with a conductive rubber layer of EPDM over a longitudinal direction of 310 mm to have a diameter of 16 mm. Are connected to a high voltage power supply 48 (48Y to 48K). The roller hardness of the primary transfer roller 54 is 35 ° in Asker C, and the resistance value is applied to a 30 mm diameter aluminum cylinder rotated at a peripheral speed of 24 mm / sec. Measured under the condition that 50 V is applied between the cylinder and the primary transfer roller, 1 × 10⁶Ω.
[0029]
The secondary transfer roller 55 is formed by covering a core metal having a diameter of 8 mm with a urethane-based conductive rubber layer over a length of 310 mm and forming a diameter of 17 mm. The resistance measured by the same method as the roller is 1 × 10⁷Ω. The core of the secondary transfer roller 55 is also connected to a high voltage power source 49 through a power supply spring.
[0030]
The driving roller 51, the tension roller 52, and the secondary transfer counter roller 53 are all made of an aluminum conductive roller having a diameter of 32 mm, and the cored bar portion is grounded via a power supply spring.
[0031]
The intermediate transfer belt 80 is a single-layer seamless endless belt made of polyimide resin whose resistance is adjusted by carbon dispersion, and has a thickness of 75 μm, a circumferential length of 1115 mm, and a widthwise length of 310 mm perpendicular to the circumferential direction. . In accordance with JIS-K6911, in order to obtain good contact between the electrode and the surface of the belt, a conductive rubber is used as an electrode, and then the volume resistance of the intermediate transfer belt is measured using an R8340 ultrahigh resistance meter manufactured by Advantest. When the rate ρv and the surface resistivity ρs are measured, ρv = 5 × 10 when 100 V is applied for 10 seconds.⁸Ωcm, ρs> 1 × 10¹³A value of Ω / □ was obtained. Note that ρs has the same result regardless of whether it is measured on the front or back side of the belt.
[0032]
The tension of the intermediate transfer belt 80 stretched around the three rollers 51, 52 and 53 is 6 kgf. The distance between the drive roller 51 and the tension roller 52 is 500 mm, and is constituted by the photosensitive drum 70 (70Y to 70K) and the primary transfer roller 54 (54Y to 54K) of each image forming station 10 (10Y to 10K). The primary transfer portions are arranged at equal intervals on the intermediate transfer belt 80 between the rollers 51 and 52. Each primary transfer roller 54 is lifted by a spring having a load of 500 gf provided at both ends, and is in contact with the back surface of the intermediate transfer belt 80 by a force obtained by pulling 150 g of the primary transfer roller itself.
[0033]
In this image forming apparatus, the maximum size of the transfer material that can be used is A3. The process speed is 117 mm / second. By setting the primary transfer bias Vy to Vk to +300 V and the secondary transfer bias to +2.3 kV, good transferability on plain paper can be obtained for all colors.
[0034]
Next, the relationship between the transfer current, transfer efficiency, and retransfer rate in one image forming station of the image forming apparatus will be described. The relationship between the transfer current, transfer efficiency, and retransfer rate is as shown in FIG.
[0035]
The transfer efficiency indicates the ratio of the toner amount of the solid image transferred onto the intermediate transfer belt 80 to the total toner amount of the solid image developed on the photosensitive drum 70, and the retransfer rate indicates the intermediate transfer belt at a certain station. The ratio of the amount of toner that the solid image is reversely transferred to the photosensitive drum at the next downstream station with respect to the total amount of toner of the solid image transferred on 80 is shown. Therefore, retransfer is not considered in the yellow station 10Y of the first color existing at the most upstream part in the moving direction of the intermediate transfer belt.
[0036]
Here, the transfer current means a current flowing in the contact area (transfer nip) of the primary transfer portion where the photosensitive drum 70 and the primary transfer roller 54 are in contact with each other via the intermediate transfer belt 80, and between stations. As long as there is no current interference between the driving roller and the tension roller, the current flows in each constant voltage power supply 48 (48Y to 48K), and hence the current flowing through the transfer roller 54 (54Y to 54K). In this embodiment, the solid image on each photosensitive drum 70 has a toner tribo of −26 μC / g and a loading amount of 0.62 mg / cm.²It is.
[0037]
In general, the transfer efficiency shows a profile having a peak value near a certain transfer current, while the retransfer rate shows a monotonic increase with respect to the transfer current, the transfer current Ia giving the maximum transfer efficiency, the high transfer efficiency, The transfer current Ib in which the low retransfer rate is well compatible has a relationship of Ia> Ib.
[0038]
In this image forming apparatus, the maximum transfer efficiency can be obtained with a transfer current in the vicinity of Ia = 17.5 μA, and good coexistence of high transfer efficiency and low retransfer rate can be obtained with a transfer current in the vicinity of Ib = 11.5 μA. Therefore, in the first yellow station 10Y where no retransfer occurs, the transfer current is set to a current in the vicinity of Ia = 17.5 μA, and the transfer current is set in the second and subsequent magenta, cyan, and black stations 10M, 10C, and 10K. The current was set to be near Ib = 11.5 μA.
[0039]
In this embodiment, by setting the primary transfer bias of the constant voltage power supply 48Y to Vy = 300V, the transfer current in the vicinity of the above Ia = 17.5 μA is used, and the primary transfer bias of the constant voltage power supplies 48M, 48C, 48K is set to Vm. By setting = Vc = Vk = 170V, a transfer current in the vicinity of Ib = 11.5 μA was obtained. As a result, the first color is transferred with the maximum transfer efficiency, the second and subsequent colors are transferred with high transfer efficiency, and the re-transfer is kept low during the transfer of the second and subsequent colors, and a full color image with sufficient density of each color is output. We were able to.
[0040]
In the above, the absolute value of the transfer current varies depending on the characteristics of the components of the apparatus such as the toner, the photosensitive drum, the intermediate transfer belt, and the transfer roller. Further, the lower limit of the required transfer efficiency, the upper limit of the retransfer rate, and the balance thereof vary depending on the level required by the apparatus and the user. For example, when the image density of a high-order color image such as a line image or a secondary color is also important, it is necessary to set a transfer current value in consideration of the transfer efficiency and the transfer current of the re-transfer rate. In a so-called cleanerless type image forming apparatus in which the transfer residual toner remaining on the photosensitive drum is again collected and reused in the developing unit, there is a concern about the color change due to the mixing of other color toner in each developing unit. Therefore, it is necessary to set the transfer current with the retransfer rate suppressed as much as possible.
[0041]
Although the present invention does not prescribe the absolute value of the transfer current in each image forming station or the difference in transfer current between different image forming stations, sufficient density is satisfied while satisfying the requirements specific to each apparatus. Needless to say, setting the transfer current value as described above is effective for obtaining a full-color image.
[0042]
As described above, according to the present embodiment, since the transfer current of the first image forming station is larger than the transfer current of the second and subsequent image forming stations, the primary transfer efficiency of each color and the next of each color are transferred. It is possible to balance the re-transfer rate in the image forming unit after the color, and it is possible to obtain a full-color image having a sufficient image density of each color.
[0043]
Example 2
FIG. 3 is a schematic configuration diagram showing another embodiment of the image forming apparatus of the present invention.
[0044]
In this embodiment, in the image forming apparatus of Embodiment 1 shown in FIG. 1, among the constant voltage power supplies 48Y to 48K for applying the primary transfer bias, the power supplies 48M to 48C after the second image forming station are shown in FIG. As shown, the power supply is simplified by combining them into a common single constant voltage power supply 48X.
[0045]
The other configuration of the image forming apparatus of the present embodiment is basically the same as that of the image forming apparatus of the first embodiment shown in FIG. 1, and the same reference numerals as those in FIG. Show.
[0046]
Also in this embodiment, a primary transfer bias Vy = 300 V is applied to the primary transfer roller 54Y from the constant voltage power supply 48Y, and the same constant voltage is applied to the primary transfer rollers 54M, 54C, and 54K. The same primary transfer bias Vx (Vm, Vc, Vk) = 170 V as in Example 1 was applied in parallel by the power supply 48X.
[0047]
As described above, when the primary transfer bias in the second and subsequent stations may be the same value, the primary transfer bias may be applied in parallel by a single constant voltage power source, thereby realizing cost reduction of the apparatus. The
[0048]
Also in this embodiment, it is possible to balance the primary transfer efficiency of each color and the retransfer rate in the image forming unit after the next color of each color, and a full-color image having a sufficient image density of each color can be obtained.
[0049]
Example 3
FIG. 4 is a schematic configuration diagram showing still another embodiment of the image forming apparatus of the present invention.
[0050]
This embodiment uses a constant current power supply 50 (50Y to 50K) as shown in FIG. 4 in place of the constant voltage power supply 48 (48Y to 48K) in the image forming apparatus of the first embodiment shown in FIG. did.
[0051]
In this embodiment, a primary transfer bias is applied to the primary transfer roller 54Y by the constant current power supply 50Y by constant current control of the transfer current Iy = 17.5 μA, and the primary transfer roller is driven by the constant current power supplies 50M, 50C, and 50K. A primary transfer bias was applied to 54M, 54C, and 54K by constant current control of transfer current Im = Ic = Ik = 11.5 μA, respectively.
[0052]
As described above, stabilization of the transfer current is achieved by using the constant current power source as the primary transfer power source of each image forming station. When the intermediate transfer belt has a higher resistance and has a characteristic of gradually charging up at the time of transfer at each image forming station and further during continuous printing, when the charging potential and exposure potential of the photosensitive drum at each station change, or image formation Even when the charging potential and exposure potential of the photosensitive drum are different for each station, stable transfer current control is maintained.
[0053]
In the above description, the constant current power sources 50Y to 50K are provided for the transfer rollers 54Y to 54K of the first to fourth stations, respectively. However, according to the second embodiment, the second and subsequent ones having the same primary transfer bias value are used. The station's constant current power supplies 50M to 50K may be combined into a single constant current power supply, thereby reducing the cost of the apparatus.
[0054]
Also in this embodiment, it is possible to balance the primary transfer efficiency of each color and the retransfer rate in the image forming unit after the next color of each color, and a full-color image having a sufficient image density of each color can be obtained.
[0055]
Example 4
FIG. 5 is a schematic configuration diagram showing still another embodiment of the image forming apparatus of the present invention.
[0056]
The image forming apparatus of the present embodiment is a modification of the intermediate transfer type image forming apparatus of the third embodiment shown in FIG. 4 into a multiple transfer type image forming apparatus.
[0057]
  In this embodiment, instead of the intermediate transfer belt 80 of the image forming apparatus of FIG.(Ie, transfer material carrier)The transfer belt 90 is used, and the transfer material P supplied from the paper feed roller 20 is carried on the transfer belt 90 and conveyed, and the photosensitive drums 70 (7Y to 70K) of the image forming stations 10Y to 10K. Are transferred onto the transfer material P by a transfer roller 54 (54Y to 54K) to which a transfer bias is applied from a constant current power supply 50 (50Y to 50K). To do. Thereafter, the transfer material P is separated from the transfer belt 90 and conveyed to the fixing device 40, and four color toner images are pressurized and heated to form a full color fixed image.
[0058]
The transfer belt 90 has the same material and the same size as the intermediate transfer belt 80, but in order to have an action of electrostatically attracting the transfer material P, the resistance value adjusted by carbon dispersion is set higher. . When the volume resistivity ρv and the surface resistivity ρs of the transfer belt 90 are measured under the conditions of 100 V and 10 seconds applied by the measurement method described in Example 1, ρv = 2 × 10.¹⁵Ωcm, ρs> 1 × 10¹³A value of Ω / □ was obtained. Note that ρs has the same result regardless of whether it is measured on the front or back side of the belt.
[0059]
In this embodiment, as in the third embodiment, a transfer bias is applied to the transfer roller 54Y by the constant current power supply 50Y by constant current control of the transfer current Iy = 17.5 μA, and the transfer roller is supplied by the constant current power supplies 50M, 50C, and 50K. A transfer bias was applied to 54M, 54C, and 54K under constant current control of transfer current Im = Ic = Ik = 11.5 μA.
[0060]
As a result, even in the multiple transfer method, by making the transfer current of the first station larger than the transfer current of the second and subsequent stations, the primary transfer efficiency of each color and the retransfer rate in the image forming unit for the next and subsequent colors of each color Can be balanced, and a full color image with sufficient image density of each color can be obtained.
[0061]
In the above, the constant current power supplies 50Y to 50K are provided for the transfer rollers 54Y to 54K of the first to fourth stations, respectively, but the constant current power supplies 50M to 50M of the second and subsequent stations having the same primary transfer bias value. 50K may be combined into a single constant current power source, and the cost of the apparatus can be reduced.
[0062]
Also in the multiple transfer system, the transfer power supply can be a constant voltage power supply as in the first and second embodiments. Similarly, by making the transfer current of the first station larger than the transfer current of the second and subsequent stations. By balancing the primary transfer efficiency of each color and the retransfer rate in the image forming section after the next color of each color, it is possible to obtain a full color image with a sufficient image density of each color.
[0063]
【The invention's effect】
  As described above, according to the present invention, a plurality of color toner images on a plurality of image carriers in a plurality of image forming stations are transferred to an intermediate transfer member, or a plurality of color toners on a plurality of image carriers. Image transfer materialCarrierWhen transferring to the transfer material above,
(A) The value of the transfer current flowing in the transfer nip formed by the most upstream transfer charging means arranged at the most upstream position in the rotation direction of the intermediate transfer member among the plurality of transfer charging means is the other transfer charging The value of the transfer current flowing in each transfer nip formed by the plurality of transfer charging means is larger than the value of the transfer current flowing in each transfer nip formed by the means, or substantially the same, or
(B) The value of the transfer current flowing through the transfer nip formed by the most upstream transfer charging means arranged at the most upstream position in the rotation direction of the transfer material carrier among the plurality of transfer charging means is the other plurality of transfers. The value of the transfer current flowing through each transfer nip formed by the plurality of transfer charging units is larger than the value of the transfer current flowing through each transfer nip formed by the charging unit, and substantially the same.
Because it is configuredBy balancing the primary transfer efficiency of each color and the retransfer rate in the stations after the next color of each color, a full color image having a sufficient image density of each color can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus of the present invention.
2 is an explanatory diagram showing a relationship between a transfer current, a transfer efficiency, and a retransfer rate in the image forming station of the apparatus of FIG.
FIG. 3 is a schematic configuration diagram showing another embodiment of the image forming apparatus of the present invention.
FIG. 4 is a schematic configuration diagram showing still another embodiment of the image forming apparatus of the present invention.
FIG. 5 is a schematic configuration diagram showing still another embodiment of the image forming apparatus of the present invention.
[Explanation of symbols]
10Y-10K image forming station
48Y-48K constant voltage power supply
50Y-50K constant current power supply
54Y-54K transfer roller
70Y-70K photosensitive drum
80 Intermediate transfer belt
90 Transfer belt

Claims (4)

A plurality of image carriers that carry toner images ; an endless intermediate transfer member that can rotate along the plurality of image carriers; and a plurality of image carriers that are opposed to each other via the intermediate transfer member, A plurality of transfer charging means for forming the plurality of image carriers and a transfer nip portion, respectively, and by causing a transfer current to flow through the transfer nip portion by the plurality of transfer charging portions, from the plurality of image carriers. In an image forming apparatus for forming a color toner image by transferring a toner image to the intermediate transfer member ,
The value of the transfer current flowing to the transfer nip portion to the uppermost per pass shooting charging means disposed at the position of the most upstream formation in the rotational direction of the plurality of transfer charging means sac Chi before Symbol intermediate transfer member, a plurality of other transfer charging means rather greater than the value of the transfer current flowing to each of the transfer nip formed, the value of the transfer current flowing to each of the transfer nip portion in which the other of the plurality of transfer charging means is formed, and characterized in that substantially the same Image forming apparatus. The image forming apparatus according to claim 1, wherein a constant voltage power source for applying a voltage to the plurality of other transfer charging units is shared . A plurality of image bearing member for bearing a toner image, a transfer material carrying member for carrying the transfer material to a rotatable endless along said plurality of image bearing members, the transfer material to said plurality of image bearing members A plurality of transfer charging means that respectively face the plurality of image carriers and form transfer nip portions, respectively, and cause a transfer current to flow through the transfer nip portions by the plurality of transfer charging means. In the image forming apparatus for forming a color toner image by transferring a toner image from the plurality of image carriers to a transfer material conveyed by the transfer material carrier ,
The value of the transfer current flowing to the transfer nip portion to the uppermost per pass shooting charging means disposed at the position of the most upstream formation in the rotational direction of the plurality of transfer charging means sac Chi before Symbol transfer material bearing member, the other plurality It is larger than the value of the transfer current flowing through each transfer nip formed by the transfer charging unit, and the value of the transfer current flowing through each transfer nip formed by the plurality of other transfer charging units is substantially the same. Image forming apparatus. 4. The image forming apparatus according to claim 3, wherein a constant voltage power source for applying a voltage to the plurality of other transfer charging units is shared .

Images