JP2017125992A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of sufficiently suppressing increment in an electric resistance of a secondary transfer outer roller.SOLUTION: In the image forming apparatus, a secondary transfer inner roller 24 is connected to a secondary transfer power supply 40 and receives a voltage having the same polarity as the charged polarity of a toner image. A secondary transfer outer roller 25 includes an elastic layer 25b containing a conducting agent and a shaft core 25a connected to the ground potential through a grounding part. A feeder roller 26 in contact with the secondary transfer outer roller 25 is connected to an external power supply 50 and receives a voltage having an opposite polarity to a charged polarity of a toner image. Herein, absolute maximum values of voltages supplied by the secondary transfer power supply 40 and by the external power supply 50 are respectively defined as Vm1 and Vm2, and a thickness of the elastic layer 25b is defined as L1. The feeder roller 26 is arranged to be in contact with the secondary transfer outer roller 25 at a position farther by L1×(Vm1+Vm2)/Vm1 from the contact position between an intermediate transfer belt 12 and the secondary transfer outer roller 25.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus using electrophotographic technology, such as a printer, a copying machine, a facsimile machine, or a multifunction machine.

  Conventionally, there has been an intermediate transfer type image forming apparatus in which a toner image formed on a photosensitive drum is primarily transferred to an intermediate transfer belt (intermediate transfer member) at a primary transfer portion, and further secondary transferred onto a recording material at a secondary transfer portion. Are known. Such an image forming apparatus is provided with a secondary transfer outer roller that is in contact with the secondary transfer inner roller with the intermediate transfer belt interposed therebetween and forms a secondary transfer nip portion between the intermediate transfer belt. In the secondary transfer nip portion, for example, an electric field is formed by applying a bias voltage (secondary transfer voltage) to the secondary transfer inner roller, and the toner is transferred to the recording material according to the electric field, so that the secondary transfer is performed. Done.

  By the way, the secondary transfer outer roller constituting the secondary transfer portion has a conductive shaft core and an outer peripheral layer provided on the outer periphery of the shaft core, and an ion conductive conductive agent (ion conductive) is provided on the outer peripheral layer. Some agents are imparted with appropriate conductivity by being dispersed. In this case, it is conceivable that the electrical resistance increases as the energization time elapses due to the ionic conductive agent being biased (polarized) to the surface side or the axial core side of the outer peripheral layer by the secondary transfer voltage. When the secondary transfer voltage is increased in accordance with the increase in electrical resistance, it is necessary to replace the secondary transfer outer roller before the power capacity is exceeded. It was a factor that reduced the lifespan.

  Therefore, conventionally, in order to prevent an increase in electrical resistance due to the polarization of the conductive agent, a power supply roller that contacts the surface of the secondary transfer outer roller is provided, and the secondary transfer voltage is applied to the secondary transfer outer roller via the power supply roller. The structure to apply is proposed (refer patent document 1). In this configuration, in order to prevent the polarization of the ionic conductive agent, the electric charge supplied from the power supply roller reaches the secondary transfer nip portion via the shaft core.

JP-A-2005-316200

  By the way, in the configuration described in Patent Document 1, a secondary transfer nip portion and a power supply roller are arranged on one side and the other side in the radial direction with respect to the secondary transfer outer roller, and one piece connected to the power supply roller. The circuit configuration is such that electric power is supplied to the secondary transfer nip portion by the power source. For this reason, it is necessary to use a power source having a large capacity in consideration of the electrical resistance of the path that passes through the outer peripheral layer of the secondary transfer outer roller twice, which causes an increase in the cost of the power supply system. Therefore, the shaft core of the secondary transfer outer roller is substantially grounded, and a power source for applying the secondary transfer voltage and a power source for applying a voltage to the power supply roller are separately provided, thereby reducing the required capacity of each power source. It is possible to keep it small.

  However, in such a configuration, an increase in the electric resistance of the secondary transfer outer roller may not be sufficiently suppressed depending on the arrangement of the power supply roller. That is, when the contact position between the power supply roller and the secondary transfer outer roller is close to the secondary transfer nip portion, the electric charge supplied from the power supply roller directly passes through the axis of the secondary transfer outer roller. It was easy to flow toward the secondary transfer nip. As a result, the effect of the current supplied from the power supply roller canceling out the polarization of the ionic conductive agent has been impaired.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of sufficiently suppressing an increase in electrical resistance of a secondary transfer outer roller.

  An image forming apparatus according to the present invention includes an intermediate transfer belt that carries and moves a toner image transferred from an image forming unit, and a conductive shaft and an outer peripheral layer that is formed on the outer peripheral side of the conductive shaft and includes a conductive agent. And a transfer roller that forms a transfer portion on which the toner image is transferred to the recording material and the intermediate transfer belt, and an inner peripheral surface of the intermediate transfer belt, and the transfer portion sandwiching the intermediate transfer belt. A counter roller facing the roller, a power supply member capable of supplying a charge to a power supply position different from the position of the transfer portion in the circumferential direction of the outer peripheral layer, and a voltage having the same polarity as the charging polarity of the toner. A first power source for applying a bias electric field to the transfer portion, a second power source for applying a voltage having a polarity opposite to the charging polarity of the toner to the power supply member, the conductive shaft of the transfer roller and the ground potential Current between The power supply member has a maximum absolute value of the voltage applied by the first power source as Vm1, a maximum absolute value of the voltage applied by the second power source as Vm2, and the outer peripheral layer. Is set so that L2> L1 × (Vm1 + Vm2) / Vm1, where L2 is the distance from the contact position between the intermediate transfer belt and the transfer roller to the power feeding position. It is characterized by.

  According to the present invention, an increase in electrical resistance of the secondary transfer outer roller can be sufficiently suppressed.

1 is a schematic diagram illustrating a configuration of an image forming apparatus according to a first embodiment. The front view which shows the secondary transfer part which concerns on 1st Embodiment. The schematic diagram which shows arrangement | positioning of a feed roller. The front view which shows the secondary transfer part which concerns on 2nd Embodiment. The front view which shows the secondary transfer part which concerns on 3rd Embodiment. The front view which shows the secondary transfer part which concerns on 4th Embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the image forming apparatus according to the present embodiment will be described with reference to FIG. FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus. An image forming apparatus 100 shown in FIG. 1 is a tandem intermediate transfer type full-color printer in which yellow, magenta, cyan, and black image forming units UY, UM, UC, and UK are arranged along an intermediate transfer belt 12.

[Image forming apparatus]
In the image forming unit UY, a yellow toner image is formed on the photosensitive drum 1Y and transferred to the intermediate transfer belt 12. In the image forming unit UM, a magenta toner image is formed on the photosensitive drum 1M and transferred to the intermediate transfer belt 12. In the image forming units UC and UK, a cyan toner image and a black toner image are formed on the photosensitive drums 1C and 1K, respectively, and transferred (primary transfer) to the intermediate transfer belt 12. The four-color toner images transferred onto the intermediate transfer belt 12 are transferred to the secondary transfer unit 20 and collectively transferred (secondary transfer) onto the recording material S (sheet material such as paper or OHP sheet). The

  The image forming units UY, UM, UC, and UK are substantially the same except that the toner colors used in the developing devices 4Y, 4M, 4C, and 4K are yellow, magenta, cyan, and black, respectively. In the following, the configuration and operation of the image forming unit U will be described with reference numerals indicating the distinction between the image forming units UY, UM, UC, and UK, with the reference numerals omitting Y, M, C, and K at the end. .

  The image forming unit U includes a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 5, and a drum cleaning device 6 that are disposed around the photosensitive drum 1. The photosensitive drum 1 is a drum-shaped photosensitive member formed by forming a photosensitive layer on the peripheral surface of an aluminum cylinder, for example, and rotates in the direction of arrow R1 in the drawing at a predetermined process speed.

  The charging roller 2 is charged with a charging voltage in contact with the photosensitive drum 1 to charge the photosensitive drum 1 to a uniform dark portion potential having the same polarity as the charging polarity of the toner. The exposure device 3 generates a laser beam, which is ON / OFF modulated in accordance with the scanning line image data obtained by developing the separation color image of each color, from the laser light emitting element and scans it with a rotating mirror to convert the image data onto the surface of the photosensitive drum 1. Write the electrostatic image based. The developing device 4 contains a two-component developer containing toner and carrier, and agitates the developer inside the container, so that the toner and the carrier have opposite polarities (in this embodiment, the toner is negative and the carrier is positive). Is triboelectrically charged. The developing device 4 applies a developing voltage to the developing sleeve carrying the developer, thereby supplying toner to the photosensitive drum 1 and developing the electrostatic image into a toner image.

  The primary transfer roller 5 is disposed to face the photosensitive drum 1 with the intermediate transfer belt 12 interposed therebetween, and forms a primary transfer portion T1 as a nip portion between the photosensitive drum 1 and the intermediate transfer belt 12. The primary transfer roller 5 is applied with a primary transfer voltage having a polarity opposite to the toner charging polarity, for example, by a high voltage power source (not shown). As a result, the toner image carried on the photosensitive drum 1 is electrostatically attracted to the intermediate transfer belt 12 and primary transfer is performed. The drum cleaning device 6 has a cleaning blade that rubs against the photosensitive drum 1 and collects the primary transfer residual toner that remains slightly on the photosensitive drum 1 after the primary transfer.

  The intermediate transfer belt 12 as an intermediate transfer member is wound around a driving roller 23, a tension roller 28, idler rollers 22, 29, a secondary transfer inner roller 24, and the like, and is a photosensitive drum in the primary transfer portion T1 of each image forming portion. 1 abuts. The drive roller 23 is connected to a motor (not shown) and is rotationally driven to move (rotate) the intermediate transfer belt 12 in the direction of arrow R2 in the drawing. A force that pushes the back surface of the intermediate transfer belt 12 toward the front surface by an elastic member (not shown) such as a spring is applied to the tension roller 28, and a tension of about 3 to 12 kgf is applied to the intermediate transfer belt 12. The idler rollers 22 and 29 are arranged on the upstream side and the downstream side of the four primary transfer portions T1 arranged in the rotation direction of the intermediate transfer belt 12, and regulate the rotation trajectory of the intermediate transfer belt 12.

  Specifically, the secondary transfer unit 20 described later includes a transfer nip T2 as a transfer unit that transfers (secondary transfer) a toner image carried on the intermediate transfer belt 12 to the recording material S. In the transfer nip portion T2, the full-color toner image carried on the intermediate transfer belt 12 is collectively transferred to the recording material S. Secondary transfer residual toner remaining on the intermediate transfer belt 12 after the secondary transfer is collected by a belt cleaning device 11 having a blade member that rubs the intermediate transfer belt 12.

  The recording material S to which the full color toner image is transferred in the secondary transfer unit 20 is conveyed to the fixing device 30. The fixing device 30 includes fixing rollers 31 and 32 that are in contact with each other to form a fixing nip portion T3. The fixing device 30 fixes the toner image onto the recording material S while conveying the recording material S at the fixing nip portion T3. The fixing device 30 is provided with a heating source such as a lamp heater that heats the fixing roller 31 from the inside, and an urging mechanism such as a spring that presses the fixing rollers 31 and 32 together. As a result, the recording material S sandwiched in the fixing nip T3 is heated and pressurized, and the toner image is melted and fixed on the recording material S. The recording material S on which the toner image has been fixed by passing through the fixing device 30 is discharged out of the machine body by a discharge mechanism (not shown).

  In addition, the image forming apparatus 100 includes a transport system including a tray (not shown) on which the recording material S is stacked, a feeding unit (not shown) for feeding the recording material S, and a registration roller pair 66. ing. The feeding unit includes a pickup roller that feeds out the recording material S stacked on the tray, and feeds the recording material S to the apparatus main body in accordance with image formation in the image forming units UY to UK. The registration roller pair 66 abuts against the front end of the recording material S fed from the feeding unit to correct skewing, and moves the recording material S to the transfer nip T2 in accordance with the transfer timing in the secondary transfer unit 20. And carry.

[Primary transfer roller]
The primary transfer roller 5 is a metal roller formed into a roller shape using a metal such as SUM (free cutting steel) or SUS (stainless steel). The primary transfer roller 5 is formed in a straight shape in the thrust direction (rotation axis direction), and its diameter (roller diameter) is about 6 to 10 mm.

[Intermediate transfer belt]
The intermediate transfer belt 12 is an endless belt member formed using a resin such as polyimide or polyamide or an alloy thereof, or various rubber materials containing an appropriate amount of an antistatic agent such as carbon black. . The intermediate transfer belt 12 is formed to a thickness of about 0.04 to 0.5 mm, and is given conductivity with a surface resistivity of 1 × 10 ^{9 to} 5 × 10 ¹³ (Ω / □).

  Hereinafter, the secondary transfer unit of the image forming apparatus according to the first to fourth embodiments will be described in detail. The configuration of the image forming apparatus 100 described above is common to the first to fourth embodiments except for the secondary transfer unit. Therefore, in the following description, members having the same configuration and function are denoted by the same reference numerals and description thereof is omitted.

<First Embodiment>
The secondary transfer unit 20 according to the first embodiment will be described with reference to FIG. The secondary transfer unit 20 includes a secondary transfer outer roller 25 that contacts the outer peripheral surface (toner image carrying surface) of the intermediate transfer belt 12, a secondary transfer inner roller 24 that contacts the inner peripheral surface of the intermediate transfer belt 12, And a power feeding roller 26 that contacts the secondary transfer outer roller 25. A transfer nip T2 (secondary transfer nip) is formed as a nip between the secondary transfer outer roller 25 and the intermediate transfer belt 12. The secondary transfer inner roller 24 and the secondary transfer outer roller 25 are disposed to face each other with the intermediate transfer belt 12 interposed therebetween, and are driven to rotate according to the rotation of the intermediate transfer belt 12. The power supply roller 26 arranged in parallel with the secondary transfer outer roller 25 and the shaft center is driven to rotate in accordance with the rotation of the secondary transfer outer roller 25 while being in contact with the outer peripheral portion of the secondary transfer outer roller 25.

[Secondary transfer outer roller]
The secondary transfer outer roller 25 (transfer roller) as a transfer rotator has a conductive shaft core 25a (conductive shaft) and an elastic layer 25b (outer peripheral layer) formed on the outer periphery of the shaft core 25a. The shaft core 25a is a columnar member made of a conductor such as stainless steel. The elastic layer 25b is formed of, for example, a sponge-like or solid elastic material having a thickness of 6 mm, and is configured such that the entire diameter (roller diameter) of the secondary transfer outer roller 25 is about 24 mm.

  As the elastic material constituting the elastic layer 25b, an elastomer such as NBR (acrylonitrile butadiene rubber) or EPDM (ethylene propylene rubber), or other synthetic resin can be used. An ionic conductive agent such as a metal complex is added to the elastic layer 25b to impart moderate conductivity (semiconductive). Note that a semiconductive polymer such as epichlorohydrin rubber may be kneaded with the base material of the elastic layer 25b as an ion conductive conductive agent, or a semiconductive polymer and a metal complex may be used in combination. Further, an electroconductive conductive agent such as carbon or metal oxide and an ionic conductive agent may be dispersed in the elastic layer 25b. In short, this embodiment can be applied as long as the outer peripheral layer of the secondary transfer outer roller 25 includes an ionic conductive agent.

[Secondary transfer inner roller]
The secondary transfer inner roller 24 as an opposing roller facing the secondary transfer outer roller 25 with the intermediate transfer belt 12 interposed therebetween supports an elastic layer made of an elastic material such as EPDM and a thickness of about 0.5 mm, and the elastic layer. And having a diameter (roller diameter) of about 20 mm. Appropriate conductivity is imparted to the elastic layer by adding a conductive agent such as the above-described ionic conductive agent or carbon. The hardness of the elastic layer is preferably set to be 70 ° using, for example, an Asker C type measuring instrument.

[Power supply roller]
The power supply roller 26 as a power supply member capable of supplying electric charges to the secondary transfer outer roller 25 is a metal roller formed in a roller shape using a metal such as SUM or SUS. The power supply roller 26 has a diameter (roller diameter) of about 10 mm, for example, and is provided so as to be driven to rotate by the secondary transfer outer roller 25 in a state of being in contact with the surface of the elastic layer 25 b of the secondary transfer outer roller 25.

[Secondary transfer power supply and external power supply]
Next, the electrical configuration of the secondary transfer unit 20 will be described. As shown in FIG. 2, the image forming apparatus 100 is provided with a secondary transfer power supply 40 and an external power supply 50 as high-voltage power supplies that supply power to the secondary transfer unit 20. However, “external” of the external power supply 50 refers to supplying power to members other than the secondary transfer inner roller 24 and the secondary transfer outer roller 25 that form the transfer nip portion T2, and the image forming apparatus 100. It does not refer to the outside of the housing.

  A secondary transfer power source 40 as a first power source that forms a bias electric field in the transfer nip portion T2 applies a voltage having the same polarity (negative polarity in the present embodiment) as the charging polarity of the toner image to the secondary transfer inner roller 24. It is a constant voltage source. The applied voltage of the secondary transfer power source 40 is controlled so that the magnitude of the current (transfer current) flowing between the intermediate transfer belt 12 and the secondary transfer outer roller 25 at the transfer nip portion T2 approaches the target value (constant current). Controlled).

  The control device provided in the apparatus main body is configured such that the magnitude of the transfer current can be measured by an ammeter 80 interposed between the secondary transfer power supply 40 and the secondary transfer inner roller 24, for example. For example, the control device refers to the transfer current measured under the condition that the recording material S does not intervene in the transfer nip T2 before the image forming operation, according to the type (basis weight, material, etc.) of the recording material S. Sets the applied voltage during image formation. Note that the maximum output (maximum absolute value of the voltage that can be applied) Vm1 of the secondary transfer power supply 40 is set to 6000 [V]. In other words, the minimum applied voltage of the secondary transfer power supply 40 in this embodiment is −6000 [V].

  The secondary transfer outer roller 25 is in contact with the intermediate transfer belt 12 at the transfer nip portion T2, and is in contact with the power supply roller 26 at a position (power supply position) different from the transfer nip portion T2 in the circumferential direction. The shaft core 25a of the secondary transfer outer roller 25 is configured to be able to pass a current to the ground potential by being electrically connected to the ground potential via a conductive member 60 as a current path. However, the conductive member 60 is not limited to a conductive wire, and may be any member that has conductivity and grounds the shaft core 25a. For example, the bearing portion of the shaft core 25a may be formed of a conductive material.

  With the configuration described above, the secondary transfer outer roller 25 side is opposite to the charging polarity of the toner image by the secondary transfer voltage applied from the secondary transfer power supply 40 to the intermediate transfer belt 12 side at the transfer nip T2. A polarity bias electric field is formed. As a result, the toner image is electrostatically attracted to the recording material S sandwiched by the transfer nip T2, and the secondary transfer process is achieved.

  Here, when a transfer current flows through the transfer nip portion T <b> 2, an amount of charge corresponding to the transfer current passes through the elastic layer 25 b of the secondary transfer outer roller 25. That is, in the present embodiment, the negative ionic conductive agent moves inward in the radial direction of the elastic layer 25b, and the positive ionic conductive agent moves outward in the radial direction of the elastic layer 25b. If the ionic conductive agent is biased (polarized) toward the inner or outer peripheral side of the elastic layer 25b, the resistance of the elastic layer 25b increases with respect to the current in the direction of increasing the degree of polarization of the ionic conductive agent. End up. Then, in order to maintain the transfer current, it is necessary to set the secondary transfer voltage to a higher voltage. In this case, since it is necessary to replace the secondary transfer outer roller 25 before exceeding the maximum output of the secondary transfer power supply 40, the polarization of the ionic conductive agent becomes a factor that reduces the life of the secondary transfer outer roller 25. End up. In addition, when the secondary transfer voltage is set to a high voltage in order to maintain the transfer current, the toner image is easily transferred to the recording material S on the upstream side of the transfer nip portion T2, and there is a possibility that the image quality is deteriorated. .

  Therefore, in the present embodiment, a power supply roller 26 that can supply charges to the secondary transfer outer roller 25 and an external power supply 50 as a second power source that applies a voltage to the power supply roller 26 are provided. The external power supply 50 applies a voltage having a polarity opposite to the charging polarity of the toner image (positive polarity in this embodiment) to the power supply roller 26. Accordingly, the current supplied by the external power supply 50 is in the opposite direction to the direction of the transfer current in the radial direction of the elastic layer 25b. In this embodiment, the transfer current flows radially outward in the elastic layer 25b, while a radially inward current flows between the power supply roller 26 and the shaft core 25a.

  The external power supply 50 is a constant current source, and supplies a current having a magnitude corresponding to the transfer current to the power supply roller 26. In other words, the target value of the current output from the external power supply 50 is controlled to be equal to the target value in the constant current control of the transfer current. Thereby, the ionic conductive agent is moved in the radial direction to the opposite side of the moving direction by the transfer current, so that the polarization of the ionic conductive material is relaxed (cancelled) and the performance of the elastic layer 25b is maintained over a long period of time. can do. Hereinafter, of the current flowing through the elastic layer 25b, the current flowing between the feeding roller 26 and the shaft core 25a is referred to as “relaxation current”. Note that the maximum output Vm2 of the external power supply 50 (absolute value of the applicable voltage) Vm2 is set to 5000 [V]. That is, in this embodiment, the maximum applied voltage of the external power supply 50 is +5000 [V].

  For example, the conductive member 60 is removed to cut off the secondary transfer outer roller from the ground potential, and the secondary transfer inner roller 24 is grounded, whereby the transfer current and the relaxation current are generated by one high-voltage power source (external power source 50). The structure which supplies can be considered. However, in such a configuration, a path through which a current flows is longer than in the present embodiment, and one high-voltage power source bears the resistance of the entire path from the power supply roller 26 to the secondary transfer inner roller 24. Become. Therefore, a large-capacity high-voltage power supply is required to form a sufficiently strong bias electric field at the transfer nip T2, which causes an increase in cost. On the other hand, in this embodiment, by dispersing the load to the two high-voltage power supplies, it is possible to reduce the polarization of the ion conductive agent while suppressing the cost of the power supply system.

[Short-circuit current]
By the way, in the configuration of the secondary transfer unit 20 described above, when the contact position between the power supply roller 26 and the secondary transfer outer roller 25 is too close to the transfer nip T2, there is a possibility that the polarization of the ionic conductive agent may not be relaxed well. There is. That is, a part of the current supplied from the power supply roller 26 to the elastic layer 25b passes through the elastic layer 25b without passing through the shaft core 25a (hereinafter referred to as a short-circuit current). ). Then, the closer the position of the power supply roller 26 is to the position of the transfer nip portion T2, the greater the value of such a short-circuit current.

  When the currents supplied from the external power supply 50 to the power supply roller 26 are equal, the relaxation current value decreases as the short-circuit current increases, so the effect of relaxing the polarization of the ionic conductive agent decreases. Therefore, it is preferable that the power supply roller 26 be disposed apart from the transfer nip portion T2. However, depending on the positional relationship with other members, it may be difficult to dispose the power supply roller 26 at a position that is 180 degrees out of phase with the transfer nip T2, so the range in which the power supply roller 26 can be disposed is clarified. It will be necessary.

  Hereinafter, the arrangement conditions of the secondary transfer unit 20 will be specifically described with reference to the schematic diagram of FIG. In FIG. 3, the thickness of the elastic layer 25b of the secondary transfer outer roller 25 is L1 [mm], and from the contact position (point P) between the intermediate transfer belt 12 and the secondary transfer outer roller 25, the power supply roller 26 and the secondary transfer roller 26 are secondary. The distance to the contact position (point Q) with the outer transfer roller 25 is L2 [mm]. However, the position of the point P is located on a line connecting the axial centers of the rollers (24, 25) in the transfer nip portion T2 which is a contact portion between the secondary transfer outer roller 25 and the intermediate transfer belt 12. To do. The position of the point Q indicating the feeding position is assumed to be on a line connecting the axes of the rollers (25, 26).

  When the applied voltage to the secondary transfer inner roller 24 of the secondary transfer power supply 40 is V1 [V], a potential gradient of | V1 | / L1 [V / mm] is provided at a position overlapping the line segment OP in the elastic layer 25b. Is formed. However, the thickness of the elastic layer of the secondary transfer inner roller 24 is negligible. Further, when the voltage applied to the power supply roller 26 of the external power supply 50 is V2 [V], a potential gradient of | V2−V1 | / L2 [V / mm] is present at a position overlapping the line segment PQ in the elastic layer 25b. It is formed.

  Here, when | V2−V1 | / L2> | V1 | / L1, the potential gradient in the direction along the line segment PQ is superior to the potential gradient in the direction along the line segment OP inside the elastic layer 25b. It becomes a state. That is, the potential gradient of the path directly toward the power feeding roller 26 is larger than the path toward the shaft core 25a from the transfer nip T2, and the short-circuit current is likely to flow. Therefore, the inequality sign is inverted and L2 may be determined so as to satisfy the following expression (1a).

      | V2-V1 | / L2 <| V1 | / L1 (1a)

  The condition of the expression (1a) is that the following expressions are used by using the maximum outputs Vm1 and Vm2 of the secondary transfer power supply 40 and the external power supply 50, considering that V1 and V2 are opposite signs and the fluctuations of V1 and V2. It is automatically satisfied when (1b) is satisfied.

      (Vm1 + Vm2) / L2 <Vm1 / L1 (1b)

  Therefore, the power supply roller 26 may be arranged so as to satisfy the following formula (1c) by modifying the formula (1b).

L2> L1 × (Vm1 + Vm2) / Vm1 (1c)
This means that Lmin = L1 × (Vm1 + Vm2) / Vm1, where Lmin [mm] is the minimum distance (arrangement prohibition distance) necessary for the arrangement of the power supply roller 26. In other words, the power feeding roller 26 may be arranged so that the point Q (power feeding position) in FIG. 3 is located outside the circle having the radius of Lmin centered on the point P.

  In the configuration of the present embodiment described above, since Vm1 = 6000, Vm2 = 5000, and L1 = 6, the value of Lmin is as follows.

Lmin = 6 × (5000 + 6000) / 6000
= 11 [mm]
Then, the magnitude δ of ∠POQ when PQ = Lmin can be obtained by the following equation.

δ = ± Arcsin ((Lmin / 2) / L1)
= ± Arcsin ((11/2) / 6)
= ± 54.6 [degree]
Therefore, in this embodiment, the feed roller 26 is set so that the angle is larger than 54.6 degrees with reference to a straight line extending from the center of the secondary transfer outer roller 25 in the direction of the transfer nip T2 (point P). Deploy. With such a configuration, it is possible to suppress the short-circuit current and effectively relax the polarization of the ionic conductive agent.

[Comparison experiment]
Hereinafter, the result of the comparative experiment in which the magnitude of the short-circuit current is compared between the case where the arrangement of the present embodiment is applied and the case where the arrangement is not. As an experimental method, first, two secondary transfer units having the same configuration as the above-described secondary transfer unit 20 and different only in the arrangement angle of the power supply roller 26 were prepared. However, the arrangement angle of the power supply roller 26 is a phase difference between the contact positions of the intermediate transfer belt 12 and the power supply roller 26 with respect to the secondary transfer outer roller 25 (the magnitude of ∠POQ in FIG. 3). The arrangement angle of the power supply roller 26 to which the present embodiment is applied is 180 degrees, and the arrangement angle of the other power supply roller 26 as a comparative example is 45 degrees.

  Then, current flows from the secondary transfer power source 40 to the secondary transfer inner roller 24, and current flows from the external power source 50 to the power supply roller 26, and then flows from the power supply roller 26 to the secondary transfer inner roller 24. The value of the short circuit current was measured. Specifically, first, a transfer current of a set value (left column in Tables 1 and 2) flows to the transfer nip portion T2 in a state where the power supply roller 26 is separated (insulated) from the secondary transfer outer roller 25. Thus, the applied voltage of the secondary transfer power supply 40 was set. Next, without changing the applied voltage of the secondary transfer power supply 40, the power supply roller 26 is brought into contact with the secondary transfer outer roller 25, and a current (middle row of Tables 1 and 2) having the same magnitude as the set value is applied. The power supply roller 26 was supplied from the external power source 50. In this state, when the current flowing from the secondary transfer power supply 40 to the inner secondary transfer roller 24 is measured, a transfer path directly flowing from the intermediate transfer belt 12 to the power supply roller 26 via the elastic layer 25b is added. The amount of current flowing through the nip portion T2 increases compared to the set value. Therefore, the amount of increase in the current amount with respect to the set value (the right column in Tables 1 and 2) was defined as the short-circuit current.

  As shown in Table 1, in the comparative example (arrangement angle 45 degrees), 10% or more of the current supplied from the power supply roller 26 flows directly to the secondary transfer inner roller 24 as a short-circuit current. It was. Further, the larger the current of the power supply roller 26, the greater the magnitude of the short circuit current. That is, in this comparative example, a short-circuit current having a magnitude that cannot be ignored flows, so that the effect of relaxing the polarization of the ionic conductive agent is impaired.

  On the other hand, as shown in Table 2, the short-circuit current is always less than 1 [μA] and less than 2% of the total current supplied to the power supply roller in the case of the arrangement (arrangement angle 180 degrees) to which the present embodiment is applied. It was suppressed to. That is, the short-circuit current is suppressed by disposing the power supply roller 26 at a position farther than the disposition prohibiting distance Lmin. Thus, according to the configuration of the secondary transfer unit 20 according to the present embodiment, it is possible to effectively relax the polarization of the ionic conductive agent.

  In the discussion of the disposition prohibiting distance Lmin described above, the potential gradient direction (see FIG. 3) is used with reference to the contact position (point P) between the intermediate transfer belt 12 and the secondary transfer outer roller 25. The power feeding position (point Q) may be used as a reference. In this case, in the vicinity of the feeding position, the potential gradient in the direction along the line segment OQ is | V2 | / L1, and the potential gradient in the direction along the line segment PQ is | V2-V1 | / L2. Therefore, L2 may be determined so as to satisfy the following expression (2a).

      | V2-V1 | / L2 <| V2 | / L1 (2a)

  The condition of the expression (2a) is that V1 and V2 are opposite signs, and the following expressions (2b) and (2c) equivalent to each other are satisfied, taking into account the fluctuations of V1 and V2, It is filled.

(Vm1 + Vm2) / L2 <Vm2 / L1 (2b)
L2> L1 × (Vm1 + Vm2) / Vm2 (2c)
That is, by arranging the power supply roller so as to satisfy the condition of the expression (2c), the polarization of the ionic conductive agent can be effectively reduced. Further, it is more preferable that the power supply roller 26 is disposed so as to satisfy the expressions (1c) and (2c) at the same time.

Second Embodiment
Next, the secondary transfer portion 20A according to the second embodiment will be described with reference to FIG. The secondary transfer unit 20A according to the present embodiment is different from the secondary transfer unit 20 according to the first embodiment (see FIG. 2) in that a Zener diode 61 is disposed in the grounding unit 60A, and other configurations. Are the same. About the same structure, the same code | symbol as 1st Embodiment is attached | subjected and description is abbreviate | omitted.

  As shown in FIG. 4, the secondary transfer outer roller 25 according to the present embodiment is connected to the ground potential by a ground portion 60 </ b> A having a conductive member 60 and a Zener diode 61. The conductive member 60 is made of a conductive material such as a conductive wire. The Zener diode 61 is interposed in the conductive path formed by the conductive member 60, and has a breakdown voltage set to a sufficiently small value (for example, 50V) compared to the applied voltage of the secondary transfer power supply 40 and the external power supply 50. Therefore, the Zener diode 61 causes a current between the shaft core 25a and the ground potential when the potential difference between the shaft core 25a of the secondary transfer outer roller 25 and the ground potential becomes a predetermined value or more (breakdown voltage or more). It is an example of the electrical element which flows.

  Even in such a configuration, the polarization of the ionic conductive agent contained in the secondary transfer outer roller 25 can be reduced by supplying a current corresponding to the transfer current from the external power supply 50 to the power supply roller 26. . The same effect as that of the first embodiment described above can be obtained by disposing the power supply roller 26 at a position farther from the transfer nip T2 than the disposition prohibiting distance Lmin.

<Third Embodiment>
Next, the secondary transfer unit 20B according to the third embodiment will be described with reference to FIG. The secondary transfer unit 20B according to the present embodiment is different from the secondary transfer unit 20 of the first embodiment (see FIG. 2) in that a varistor 62 is disposed on the grounding unit 60B. Are the same. About the same structure, the same code | symbol as 1st Embodiment is attached | subjected and description is abbreviate | omitted.

  As shown in FIG. 5, the secondary transfer outer roller 25 according to the present embodiment is connected to a ground potential by a ground portion 60 </ b> B having a conductive member 60 and a varistor 62. The conductive member 60 is made of a conductive material such as a conductive wire. The varistor 62 is interposed in the conductive path formed by the conductive member 60, and has a varistor voltage set to a sufficiently small value (for example, 50 V) compared to the applied voltage of the secondary transfer power supply 40 and the external power supply 50. However, the varistor voltage refers to a voltage when a current equal to or larger than the transfer current (for example, 10 [μA]) flows as a reference current. Accordingly, the varistor 62 allows a current to flow between the shaft core 25a and the ground potential when the potential difference between the shaft core 25a of the secondary transfer outer roller 25 and the ground potential exceeds a predetermined value (varistor voltage). It is an example of an electrical element.

  Even in such a configuration, the polarization of the ionic conductive agent contained in the secondary transfer outer roller 25 can be reduced by supplying a current corresponding to the transfer current from the external power supply 50 to the power supply roller 26. . The same effect as that of the first embodiment described above can be obtained by disposing the power supply roller 26 at a position farther from the transfer nip T2 than the disposition prohibiting distance Lmin.

<Fourth embodiment>
Next, the secondary transfer unit 20C according to the fourth embodiment will be described with reference to FIG. The secondary transfer unit 20C according to the present embodiment is different from the secondary transfer unit 20 according to the first embodiment (see FIG. 2) in that a low-voltage power source 63 is provided in the grounding unit 60C, and other configurations. Are the same. About the same structure, the same code | symbol as 1st Embodiment is attached | subjected and description is abbreviate | omitted.

  As shown in FIG. 6, the secondary transfer outer roller 25 according to the present embodiment is connected to the ground potential by a ground portion 60 </ b> C having a conductive member 60 and a low-voltage power source 63. The low-voltage power supply 63 applies a voltage whose absolute value is sufficiently smaller (for example, 20 V) than the applied voltage of the secondary transfer power supply 40 and the external power supply 50. Therefore, the low-voltage power supply 63 is connected between the shaft core 25a and the ground potential when the potential difference between the shaft core 25a of the secondary transfer outer roller 25 and the ground potential exceeds a predetermined value (voltage applied to the low-voltage power supply). Current is passed through.

  Even in such a configuration, the polarization of the ionic conductive agent contained in the secondary transfer outer roller 25 can be reduced by supplying a current corresponding to the transfer current from the external power supply 50 to the power supply roller 26. . The same effect as that of the first embodiment described above can be obtained by disposing the power supply roller 26 at a position farther from the transfer nip T2 than the disposition prohibiting distance Lmin.

<Other embodiments>
In the first to fourth embodiments described above, the intermediate transfer member is described as being configured by the endless belt member (12). However, the present invention is not limited to this, and for example, a drum-shaped intermediate transfer member (intermediate transfer drum). May be used. In this case, the first power supply (secondary transfer power supply 40) may be connected to the intermediate transfer drum to apply a voltage.

  Further, an endless belt member (transfer conveyance belt) may be stretched around the transfer roller. Even in this case, the same effect as that of the above-described embodiment can be obtained by arranging the power supply roller (power supply member) so as to satisfy the above-described condition (formula (1c)).

  The power feeding member is not limited to a roller member that contacts the secondary transfer outer roller, but may be a brush shape, a blade, a wire (corona discharge type), or the like. In short, any configuration may be used as long as the charge can be supplied to a position different from the transfer nip T2 in the circumferential direction in the outer peripheral portion of the transfer rotator.

  12 ... Intermediate transfer belt / 24 ... Opposite roller (secondary transfer inner roller) / 25 ... Transfer roller (secondary transfer outer roller) / 25a ... Conductive shaft (axial core) / 25b ... Outer peripheral layer (elastic layer) / 26 ... Power supply member (power supply roller) / 40 ... 1st power supply (secondary transfer power supply) / 50 ... 2nd power supply (external power supply) / 60A, 60B, 60C ... Current path / 60 ... Current path, conductive member / 61, 62 ... Electrical element (Zener diode, varistor) / 63 ... Power source (low voltage power source) / 100 ... Image forming apparatus / Q ... Power feeding position / S ... Recording material / UY, UM, UC, UK ... Image forming unit

Claims (9)

An intermediate transfer belt that carries and moves the toner image transferred from the image forming unit;
A transfer roller having a conductive shaft and an outer peripheral layer formed on an outer peripheral side of the conductive shaft and including a conductive agent, and forming a transfer portion between the intermediate transfer belt and the toner image transferred to a recording material; ,
A counter roller that contacts the inner peripheral surface of the intermediate transfer belt and faces the transfer roller across the intermediate transfer belt;
A power supply member capable of supplying a charge to a power supply position different from the position of the transfer portion in the circumferential direction of the outer peripheral layer;
A first power source configured to apply a voltage having the same polarity as the charging polarity of the toner to the opposite roller to form a bias electric field in the transfer portion;
A second power source for applying a voltage having a polarity opposite to the charging polarity of the toner to the power supply member;
A current path for passing a current between the conductive shaft of the transfer roller and a ground potential,
In the case where the power supply member has Vm1 as the maximum absolute value of the voltage applied by the first power supply, Vm2 as the maximum absolute value of the voltage applied by the second power supply, and L1 as the thickness of the outer peripheral layer. In addition, the distance from the contact position between the intermediate transfer belt and the transfer roller to the power feeding position is L2,
L2> L1 × (Vm1 + Vm2) / Vm1
Arranged to meet,
An image forming apparatus. The power supply member is a power supply roller rotatable at the power supply position.
The image forming apparatus according to claim 1. The power supply member has a distance L2 from the contact position between the intermediate transfer belt and the transfer roller to the power supply position L2> L1 × (Vm1 + Vm2) / Vm2.
Arranged to meet,
The image forming apparatus according to claim 1. The current path is made of a conductive member having conductivity.
The image forming apparatus according to claim 1. The current path is electrically conductive and connects the conductive axis to a ground potential, and the current path is interposed in the conductive member, and the potential difference between the potential of the conductive axis and the ground potential is a predetermined value or more. An electrical element through which a current flows,
The image forming apparatus according to claim 1. The electrical element is a Zener diode or a varistor.
The image forming apparatus according to claim 5. The current path is electrically conductive and connects the conductive shaft to a ground potential, and is interposed in the conductive member, and has an absolute value compared to the applied voltage of the first power source and the second power source. A power source for applying a small voltage to the conductive axis,
The image forming apparatus according to claim 1. The first power source is a constant voltage source;
The second power source is a constant current source.
The image forming apparatus according to claim 1. Setting the absolute value of the target value of the current output by the constant current source so as to be equal to the absolute value of the current flowing between the intermediate transfer belt and the transfer roller;
The image forming apparatus according to claim 8.

Images