JP2006195143A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which only one electric supply source is provided for primary transfer bias applying members, thus realizing the low cost unit constitution, and occurrence of an abnormal image due to toner spattering which occurs in the most downstream side color can be prevented. <P>SOLUTION: The image forming apparatus is provided which comprises an intermediate transfer belt 10 having a volume resistivity of 1E+12 Ωcm, four photoreceptor drums 1 arranged parallel to the intermediate transfer belt 10, and primary transfer bias rollers 11-14 corresponding to the respective photoreceptor drums 1, wherein electric supply to the primary transfer bias applying members 11-14 is carried out using one power source, the respective primary transfer bias applying members are subjected to common constant-voltage control, and the resistance of the primary transfer bias applying roller 14 disposed on the most downstream side in the moving direction of the intermediate transfer belt 10 is made different from the resistance of the upstream primary transfer bias applying rollers 11-13. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to a color image forming apparatus.

  As a typical method for forming a color image, there is an intermediate transfer system in which toner images of different colors formed on a plurality of photoconductors are transferred while being superimposed on an intermediate transfer member, and then transferred onto a transfer sheet in a lump. This system is called a tandem system because a plurality of photoconductors are arranged side by side facing a transfer paper or intermediate transfer body. For each photoconductor, yellow (Y), magenta (M), cyan (C), For each color of black (K), an electrophotographic process such as formation and development of an electrostatic latent image is executed. On the transfer paper in the direct transfer method, on the running intermediate transfer member in the intermediate transfer method. Transcript. The tandem method has an advantage that color copying can be performed at a higher speed than the one-drum method.

  However, in general, the tandem system is more expensive than the one-drum system. One of the causes is that the tandem method requires a power supply to the primary transfer bias applying member for each color.

  In order to solve such a point, for example, in Patent Document 1, a single power supply is supplied to the tandem primary transfer bias applying member. In this way, the unit can be made at a lower cost than the configuration using four conventional power supplies, and the unit cost can be reduced to one drum system while ensuring a higher speed than the one drum system. Can be approached.

  On the other hand, in the tandem system, there is a problem peculiar to the most downstream color that dust is likely to be generated particularly in the most downstream color toner. This is because the charge amount increases when the upstream toner passes through the downstream transfer nip, and the adhesion between the transfer belt and the toner can be secured, whereas the most downstream color is only the charge amount at the time of transfer. Therefore, a sufficient charge amount cannot be obtained, and the toner flies after the transfer nip, and a dust image tends to be formed (see FIG. 8). It is also known that the higher the transfer belt potential just before the final color transfer nip, the more likely the dust (image disturbance) occurs before the final color transfer nip. Lower the transfer bias of the upstream three colors as much as possible, lower the transfer belt potential immediately before the final color transfer nip, and further increase the final color transfer bias as much as possible to increase the toner charge immediately after the final color transfer nip. Is the most effective (see FIG. 7).

  Therefore, when the current and voltage can be set for each color transfer unit, each transfer unit can be set to an optimum value. However, since four power supplies are used, the unit becomes an expensive unit. End up. In addition, with a method of using a single low-cost power supply, it is difficult to set process conditions that avoid this abnormal image of only the final color.

  In response to these problems, for example, in Patent Document 1, the downstream discharge amount is increased by lowering the downstream transfer roller resistance.

Conventionally, there is a technique described in Patent Document 2 as a technique for changing the resistance of the transfer portion of only the final color. According to this technique, as is clear from the drawing of Patent Document 2, each of the four colors has a separate primary transfer unit supply power, and after the optimum transfer electric field of the final color can be freely set, It is trying to improve Chile.
JP-A-6-289686 JP 2003-241545 A

  However, in the technique disclosed in Patent Document 1, the resistance values of the transfer rollers for all colors are changed as described above, which requires strict control of the transfer roller resistance values, and the transfer roller unit price increases. Problems may arise.

  In addition, the technique disclosed in Patent Document 2 has separate primary transfer unit power supplies for each of the four colors, and the optimum transfer electric field for the final color can be set individually and freely with a low-cost configuration. It is a configuration that cannot.

  The present invention is a conventional tandem system that realizes a low-cost unit of one power supply for the primary transfer bias application unit, and prevents the occurrence of abnormal images due to toner dust occurring in the most downstream color. An object of the present invention is to provide an image forming apparatus.

  In order to achieve the object, the invention according to claim 1 is directed to a transfer belt having a volume resistivity of 1E + 12 Ωcm or less, four or more image carriers in contact with the transfer belt, and primary transfer corresponding to each of the image carriers. An electrophotographic system having a bias applying member, supplying power to the primary transfer bias applying member corresponding to each image carrier by a single power source, and performing common constant voltage control for each primary transfer bias applying member In this image forming apparatus, the resistance value of the primary transfer bias applying member corresponding to the image carrier disposed on the most downstream side with respect to the moving direction of the transfer belt is different from the resistance value of the other primary transfer bias applying members. It is characterized by setting.

  In an ordinary quadruple tandem image forming apparatus, four power supplies to the transfer bias applying member corresponding to the image carrier are used in order to individually set the bias of each color. However, this increases the cost. In order to reduce the cost, there is a method in which a single power supply is supplied to the transfer bias applying member corresponding to the image carrier. However, since it is not possible to set individual biases for each subtle color, there is a means for avoiding abnormal images. It will be limited. Therefore, it is possible to change the resistance value of the primary bias applying member on the most downstream side by configuring as described above, paying attention to the fact that the abnormal color is caused by concentration in the final color in the quadruple tandem machine. Thus, it is possible to adjust the amount of current flowing to the transfer member at the most downstream side and set a set value for reducing abnormal images. As a result, low cost can be realized and abnormal images can be avoided.

  The invention according to claim 2 is the invention according to claim 1, wherein the volume resistivity of the primary transfer bias applying member corresponding to the most downstream image carrier is set lower than that of the other primary transfer bias applying members. It is characterized by. The invention according to claim 3 is the invention according to claim 1, wherein a foamed rubber transfer roller is used as the primary transfer bias application member, and the volume resistivity of the primary transfer bias application member is all the same, The thickness of the foam rubber layer of the transfer roller corresponding to the most downstream image carrier is made thinner than the thickness of the foam rubber layers of the other transfer rollers.

  Thus, by adopting a method of reducing the transfer roller resistance of only the most downstream color or reducing the thickness of the transfer roller, the cost is lower than the method of sequentially reducing the roller resistance of all colors as in Patent Document 1. Benefits arise. Here, regarding the decrease in the downstream transfer rate described in Patent Document 1 when a transfer roller having the same resistance is used, a transfer belt having a volume resistivity of 1E + 12 Ωcm or less is used. It is possible to prevent charge-up from occurring, and the transfer rate does not decrease on the downstream side. Therefore, according to this precondition, according to the present invention, it is possible to use transfer rollers having the same resistance for the three upstream colors.

  According to a fourth aspect of the present invention, in the first, second, or third aspect of the invention, in the four or more image carriers, the upstream side of the resistance value of the resistance bias applying member corresponding to the downstream image carrier. The resistance value of the resistance bias applying member corresponding to the image carrier on the side is relatively lowered.

  The invention according to claim 5 is the invention according to any one of claims 1 to 4, wherein, in the four or more image carriers, a resistor is inserted between a tube of the image carrier and the ground. Thus, the resistance of the downstream image carrier is relatively lower than that of the upstream image carrier.

  The invention according to claim 6 is the invention according to any one of claims 1 to 5, characterized in that the transfer belt has an intermediate transfer belt having a four-tandem configuration.

  The invention according to a seventh aspect is characterized in that in the invention according to any one of the first to sixth aspects, an image is formed by a polymerized toner produced by a polymerization method.

  The invention according to claim 8 is the invention according to any one of claims 7, wherein the toner has a shape factor SF1 in the range of 100 to 180 and a shape factor SF2 in the range of 100 to 180. Features.

  According to the present invention, it is possible to reduce the cost by using one power supply to the transfer bias applying member corresponding to the image carrier. Furthermore, by changing the resistance value of the primary bias applying member corresponding to the most downstream image carrier, it is possible to adjust the amount of current flowing to the most downstream transfer section and set a set value for reducing abnormal images. It becomes.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a side view showing a schematic configuration of a color image forming apparatus using an intermediate transfer belt as an intermediate transfer member in a first embodiment of the present invention. Reference numeral 1 denotes a cylindrical photosensitive drum. The photosensitive drum 1 rotates in the direction of the arrow at a peripheral speed of 150 mm / sec. A roller-shaped charger 4 serving as charging means is pressed against the surface of the photosensitive drum 1, and is rotated by the rotation of the photosensitive drum 1. The charger 4 is uniformly charged to a surface potential of -500 V by applying an AC and DC bias from a high voltage power source (not shown).

  Image information is exposed to the charged photosensitive drum 1 by an exposure unit 5 which is a latent image forming unit, and an electrostatic latent image is formed. This exposure process is performed by a laser beam scanner or LED using a laser diode.

  Reference numeral 3 denotes a photosensitive member cleaning unit. The photosensitive member cleaning unit 3 includes a blade 2. The blade 2 cleans the transfer residual toner on the surface of the photosensitive drum 1, and the transfer residual toner is stored in the photosensitive member. It is collected in the cleaning unit 3.

  An image forming unit is configured by arranging the charger 4, the developing unit 6 or the developing units 7 to 9, and the photosensitive member cleaning unit 3 around the photosensitive drum 1, and four sets of the image forming units are circulated and moved. The photosensitive drums 1 are juxtaposed so as to contact the intermediate transfer belt 10. The developing units 6 to 9 are of the developing type by two-component non-magnetic contact development. In the four sets of image forming units, the yellow developing unit 6 and the cyan developing unit are arranged in the moving direction of the intermediate transfer belt 10 from the upstream side to the downstream side. A device 7, a magenta developing device 8, and a black developing device 9 are provided. The developing units 6 to 9 visualize the electrostatic latent image on the photosensitive drum 1 as a toner image by a predetermined developing bias supplied from one high voltage power source (not shown). The toner used in this embodiment is a polymerized toner generated by a polymerization method.

  When a full color image is formed, a visible image is formed in the order of yellow, cyan, magenta, and black, and a visible image of each color is sequentially transferred onto the intermediate transfer belt 10 to form a full color image.

  The intermediate transfer belt 10 is stretched by a drive roller 21, primary transfer bias rollers 11 to 14, a secondary transfer counter roller 19, and a belt cleaning counter roller 20. The intermediate transfer belt 10 is driven to rotate in the direction of the arrow in the figure by a drive motor (not shown). Is done. The primary transfer bias roller 14 is held by a primary transfer bias roller holding member 15, and the primary transfer bias roller holding member 15 is pressed toward the photosensitive drum 1 by a contact / separation cam 16. In the normal state, the contact / separation cam 16 pushes the primary transfer bias roller 14 toward the photosensitive drum 1 by lifting the primary transfer bias roller holding member 15, and only when the photosensitive drum 1 or the intermediate transfer belt 10 is attached or detached. The contact / separation cam 16 rotates, and the primary transfer bias roller 14 is separated by lowering the primary transfer bias roller holding member 15.

  Reference numeral 24 denotes a belt cleaning unit, which performs cleaning by scraping off the transfer residual toner on the intermediate transfer belt 10 with a blade 23 provided in the belt cleaning unit 24. Each roller that stretches the intermediate transfer belt 10 is supported from both sides of the intermediate transfer belt 10 by an intermediate transfer belt unit side plate (not shown). As a material used for the intermediate transfer belt 10, a conductive material such as carbon black is dispersed in PVDF (vinylidene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PI (polyimide), PC (polycarbonate), and the like. A resin film-like endless belt can be used. The intermediate transfer belt 10 in the present embodiment has a single layer structure in which carbon black is added to PI (polyimide), and the thickness thereof is adjusted to 100 μm.

  Next, a method for measuring the resistance of the intermediate transfer belt 10 will be described. In this embodiment, a probe (inner electrode diameter 50 mm, ring electrode inner diameter 60 mm: conforming to JIS-K6911) is connected to a digital ultrahigh resistance microammeter (manufactured by Advantest: R8340A), and the front and back sides of the intermediate transfer belt 10 A voltage of 1000 V (surface resistivity of 500 V) was applied to the electrode and measurement was performed at discharge 5 sec and charge 10 sec. The environment during the measurement was fixed at 22 ° C. and 55% RH.

Here, the volume resistivity and the surface resistivity of the intermediate transfer belt 10 are desirably in the range of 10 ⁸ to 10 ¹² Ωcm and the surface resistivity of 10 ⁹ to 10 ¹² Ω / □. . When the volume resistivity and surface resistivity of the intermediate transfer belt 10 exceed the above-described ranges, the intermediate transfer belt 10 is charged. Therefore, it is necessary to take a measure such as setting the set voltage value higher as it goes downstream in the image forming order. Thus, it becomes difficult to use a single power supply for the primary transfer unit. This is because the charge generated on the surface of the intermediate transfer belt 10 becomes high due to the discharge generated in the transfer process, the transfer material peeling process, and the like, and self-discharge becomes difficult. Need arises. Also, if the volume resistivity and surface resistivity are below the above ranges, the charge potential decays faster, which is advantageous for static elimination by self-discharge, but toner scatter occurs because the current during transfer flows in the surface direction. Resulting in. Therefore, the volume resistivity and the surface resistivity of the intermediate transfer belt 10 must be within the ranges described above.

Reference numeral 22 denotes a secondary transfer bias roller. The secondary transfer bias roller 22 is configured by covering a metal core such as SUS with an elastic body such as urethane adjusted to a resistance value of 10 ⁶ to 10 ¹⁰ Ω with a conductive material. Here, if the resistance value of the secondary transfer bias roller 22 exceeds the above-described range, it becomes difficult for the current to flow. Therefore, a higher voltage must be applied in order to obtain a required transfer property, and the power supply cost increases. Invite. In addition, since it is necessary to apply a high voltage, discharge occurs in the gaps before and after the transfer portion nip, and white spots may be lost due to the discharge on the halftone image. This is conspicuous in a low-temperature and low-humidity environment (for example, 10 ° C. and 15% RH). Conversely, when the resistance value of the secondary transfer bias roller 22 falls below the above-described range, a multi-color image portion (for example, 3 The transferability between the color superimposed image and the monochrome image portion cannot be achieved. This is because the resistance value of the secondary transfer bias roller 22 is low, so that a sufficient current flows to transfer the monochrome image portion at a relatively low voltage, but it is optimal for the monochrome image portion to transfer the multiple color image portion. This is because a voltage value higher than the required voltage is required, and if the voltage is set to a voltage capable of transferring a plurality of color image portions, the transfer current becomes excessive in a single color image, resulting in a reduction in transfer efficiency. Therefore, the resistance value of the secondary transfer bias roller 22 must be within the above-described range.

  The resistance value of the secondary transfer bias roller 22 is measured by installing the secondary transfer bias roller 22 on a conductive metal plate and applying a load of 4.9 N on one side (total of 9.8 N on both sides) to both ends of the core metal. In the state which applied, it computed from the electric current value which flows when a voltage of 1000V is applied between a metal core and the said metal plates. Note that the environment was fixed to 22 ° C. and 55% RH when measuring the resistance of the secondary transfer bias roller 22. In the present embodiment, the resistance of the secondary transfer bias roller 22 is adjusted to be 7.8 LogΩ when measured by the method described above.

  Here, the primary transfer bias rollers 11 to 14 have the same configuration as the secondary transfer bias roller described above. This is because the intermediate transfer belt 10 is in contact with the photosensitive drum 1 and the primary transfer bias rollers 11 to 14 cannot secure a primary transfer nip without appropriate elasticity. However, the resistance range is not as strict as the secondary transfer bias roller 22 by the amount of the intermediate transfer belt 10 layers, but in this embodiment, when the resistance of the primary transfer bias rollers 11 to 14 is measured by the above-described method, it is 7. Adjustment was made to be 0 LogΩ.

  The transfer material 29 is fed by the pickup roller 28, the paper feeding / conveying roller 27, and the registration roller 26 in accordance with the timing when the leading edge of the toner image on the surface of the intermediate transfer belt 10 reaches the secondary transfer position. The toner image on the intermediate transfer belt 10 is transferred to the transfer material 29 by applying a predetermined transfer bias. The transfer material 29 is separated from the intermediate transfer belt 10 by the curvature of the secondary transfer counter roller 19 and a predetermined separation bias applied by the separation means 30, and the toner image transferred to the transfer material 29 is fixed by the fixing means 25. Is discharged after being discharged.

  In the present embodiment, a single-color mode for forming an image of any one color of yellow, magenta, cyan, and black, and a two-color mode for forming an image of any two colors of yellow, magenta, cyan, and black in an overlapping manner. , Yellow, magenta, cyan, and black, a three-color mode that forms images by overlapping them, and a full-color mode that forms four-color images as described above. These modes are specified on the operation unit. Is possible.

  Furthermore, in the present embodiment, the process speed at the time of fixing can be changed depending on the type of the transfer material 29. Specifically, when a transfer material with a continuous weight of 110 kg or more is used, the process speed is set to half speed, and the transfer material takes twice the normal process speed over the fixing nip formed by the fixing roller pair. The toner image can be secured. Here, the ream of the ream is a unit that collectively represents 1000 sheets of paper finished to the same specified size. In the case of 4/6 size paper, the 4/6 size is defined as the specified size, and the weight of the 1000 sheets Is called “ream” and the unit is displayed in <kg>.

  On the other hand, since the secondary transfer process of transferring the toner image on the intermediate transfer belt 10 to the transfer material 29 at this time is also performed at half speed, the bias value applied to the secondary transfer bias roller 22 is “thick paper”. Mode ”is applied. In the present embodiment, the type of the transfer material 29 can be designated by an operation unit (not shown), and each of them is “plain paper mode” (normal process speed), “thick paper mode” (half speed), and “OHP mode” (half speed). ).

Next, the toner will be described. The toner has a shape factor SF1 of 100 to 180 and a shape factor SF2 of 100 to 180. 2 and 3 are diagrams schematically showing the shape of the toner in order to explain the shape factor SF1 and the shape factor SF2. The shape factor SF1 indicates the ratio of the roundness of the toner shape, and is expressed by the equation (Equation 1). A value obtained by dividing the square of the maximum length MXLNG of the shape formed by projecting the toner on a two-dimensional plane by the figure area AREA and multiplying by 100π / 4.
(Equation 1)
SF1 = {(MXLNG) ² / AREA} × (100π / 4)
When the value of SF1 is 100, the shape of the toner becomes a true sphere, and becomes larger as the value of SF1 increases.

Further, the shape factor SF2 indicates the ratio of the unevenness of the toner shape, and is expressed by the equation (Equation 2). A value obtained by dividing the square of the perimeter PERI of the figure formed by projecting the toner on the two-dimensional plane by the figure area AREA and multiplying by 100π / 4.
(Equation 2)
SF2 = {(PERI) ² / AREA} × (100π / 4)
When the value of SF2 is 100, there is no unevenness on the toner surface, and as the SF2 value increases, the unevenness of the toner surface becomes more prominent. Specifically, the shape factor in this embodiment is measured by taking a photograph of the toner with a scanning electron microscope (S-800: manufactured by Hitachi, Ltd.) and introducing it into an image analyzer (LUSEX3: manufactured by Nireco). Analyzed and calculated.

  When the shape of the toner is close to a spherical shape, the contact state between the toner and the toner or the toner and the photosensitive drum 1 becomes a point contact, so that the adsorbing force between the toners is weakened, and thus the fluidity is increased. The attracting force between the toner and the photosensitive drum 1 is also weakened, and the transfer rate is increased. If either of the shape factors SF1 and SF2 exceeds 180, the transfer rate decreases, which is not preferable.

  Next, details of each primary transfer bias roller will be described.

  FIG. 4 is an explanatory diagram showing a main configuration of the image forming apparatus according to the first embodiment of the present invention. In each of the primary transfer bias rollers 11 to 14, the most downstream primary transfer bias roller 14 is a low resistance roller having an increased volume resistivity of 1E + 02 Ωcm to 1E + 03 Ωcm by increasing the carbon amount of the EPDM foam rubber. Further, the primary transfer bias rollers 11, 12, and 13 on the upstream side of the primary transfer bias roller 14 are medium resistance rollers of about 1E + 06 to 1E + 07 Ωcm by adjusting the carbon amount of the same EPDM foam rubber. Here, regarding the material of the primary transfer bias rollers 11 to 14, not only the material selected this time but also an ion conductive material can be used. The diameters of the primary transfer bias rollers 11 to 14 were φ14, the cored bar was φ8, and all the colors had the same diameter.

  In this way, by providing one power supply to the primary transfer bias rollers 11 to 14 corresponding to the four photosensitive drums 1, it is possible to reduce the cost. Further, by changing the resistance value of the primary transfer bias roller 14 corresponding to the most downstream photosensitive drum 1, the set value is set so as to adjust the amount of current flowing to the most downstream transfer portion and reduce abnormal images. It becomes possible to do.

  FIG. 5 is an explanatory diagram showing the configuration of the main part of the image forming apparatus according to the second embodiment of the present invention. The same members or members having the same functions as those shown in FIG. Detailed description will be omitted.

  In the second embodiment, the most downstream primary transfer bias roller 14 has a diameter φ12 and a core metal φ8, and the upstream three-color primary transfer bias rollers 11, 12, and 13 have a diameter φ14 and a core metal φ8.

  In this case, the same material is used for the foam rubber layer of the primary transfer bias rollers 11-14. In this embodiment, a medium resistance roller of about 1E + 06 to 1E + 07 Ωcm of hydrin rubber was used.

  With this configuration, it is possible to adjust the amount of current flowing to the most downstream transfer unit and set a setting value that can reduce abnormal images.

  FIG. 6 is an explanatory view showing the main configuration of the image forming apparatus according to the third embodiment of the present invention. The same members or members having the same functions as those shown in FIG. Detailed description will be omitted.

  In the third embodiment, the same primary transfer bias rollers 11 to 14 are used, and further, a resistor 30 is inserted into the upstream three colors between the base tube of each photosensitive drum 1 and the ground. Is. In this embodiment, the primary transfer bias rollers 11, 12, and 13 are medium resistance rollers of about 1E + 06 to 1E + 07 Ωcm.

  With this configuration, the resistance of the most downstream transfer portion can be relatively lower than that of the upstream three colors, and the diameters and resistances of the primary transfer bias rollers 11 to 14 are the same. Thus, there is a merit that an increase in the number of parts can be suppressed.

  The present invention is useful in the field of tandem type color image forming apparatuses using electrophotography.

1 is a side view showing a schematic configuration of a color image forming apparatus using an intermediate transfer belt as an intermediate transfer member in a first embodiment of the present invention. The figure which represented the shape of the toner typically in order to demonstrate shape factor SF1 The figure which represented the shape of the toner typically in order to explain shape factor SF2 Explanatory drawing which shows the principal part structure in the image forming apparatus of the 1st Embodiment of this invention. Explanatory drawing which shows the principal part structure in the image forming apparatus of the 2nd Embodiment of this invention. Explanatory drawing which shows the principal part structure in the image forming apparatus of the 3rd Embodiment of this invention. Characteristic diagram showing the relationship between the potential on the intermediate transfer belt before the final color transfer nip and the abnormal image level in a tandem color image forming apparatus Characteristic diagram showing the relationship between the toner charge amount on the intermediate transfer belt after the final color transfer nip and the abnormal image level in the tandem color image forming apparatus

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photoconductor drum 3 Photoconductor cleaning unit 4 Charger 6 Yellow developing device 7 Cyan developing device 8 Magenta developing device 9 Black developing device 10 Intermediate transfer belt 11-14 Primary transfer bias roller 22 Secondary transfer bias roller 30 Resistor

Claims (8)

A transfer belt having a volume resistivity of 1E + 12 Ωcm or less, four or more image carriers in contact with the transfer belt, and a primary transfer bias applying member corresponding to each of the image carriers, and corresponding to each of the image carriers. In an electrophotographic image forming apparatus in which power is supplied to a primary transfer bias applying member by a single power source and common primary voltage control is performed on each primary transfer bias applying member.
The resistance value of the primary transfer bias applying member corresponding to the image carrier disposed on the most downstream side with respect to the moving direction of the transfer belt is set to be different from the resistance value of the other primary transfer bias applying members. An image forming apparatus.   2. The image forming apparatus according to claim 1, wherein a volume resistivity of a primary transfer bias applying member corresponding to the most downstream image carrier is set lower than that of other primary transfer bias applying members.   A foamed rubber transfer roller is used as the primary transfer bias applying member, the volume resistivity of the primary transfer bias applying member is all the same, and the foamed rubber layer of the transfer roller corresponding to the most downstream image bearing member 2. The image forming apparatus according to claim 1, wherein the thickness is made thinner than the thickness of the foamed rubber layer of another transfer roller.   In the four or more image carriers, the resistance value of the resistance bias applying member corresponding to the upstream image carrier is relatively lower than the resistance value of the resistance bias applying member corresponding to the downstream image carrier. The image forming apparatus according to claim 1, 2, or 3.   In the four or more image carriers, by inserting a resistor between the tube of the image carrier and the ground, the resistance of the image carrier on the downstream side is relatively higher than the resistance of the image carrier on the upstream side. The image forming apparatus according to claim 1, wherein the image forming apparatus is low.   The image forming apparatus according to claim 1, wherein the transfer belt includes an intermediate transfer belt having a quadruple tandem configuration.   The image forming apparatus according to claim 1, wherein image formation is performed by using a polymerized toner produced by a polymerization method.   8. The image forming apparatus according to claim 7, wherein the toner has a shape factor SF1 in the range of 100 to 180 and a shape factor SF2 in the range of 100 to 180.

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