JP2018124408A - Image forming apparatus - Google Patents

Abstract

To provide a configuration capable of maintaining the transferability of a toner image in a secondary transfer portion T2 even when the voltage applied from a secondary transfer high voltage power supply 320 reaches a predetermined voltage. A power feeding roller 68 rotates in contact with the secondary transfer outer roller 64 at a position different from the secondary transfer portion T2, and can supply a current to the secondary transfer outer roller 64. The external high voltage power source 420 can apply a voltage to the power supply roller 68. The CPU 610 performs the following control when the secondary transfer current does not reach a predetermined value when a predetermined voltage is applied to the secondary transfer high-voltage power supply 320. That is, even if the voltage applied to the secondary transfer high-voltage power supply 320 is a predetermined voltage, the power is supplied from the power supply roller 68 via the secondary transfer outer roller 64 so that a predetermined current flows through the secondary transfer portion T2. The current flowing through the next transfer portion T2 is controlled. [Selection] Figure 2

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine having a plurality of these functions.

  Conventionally, there is an intermediate transfer type image forming apparatus that primarily transfers a toner image formed on a photosensitive drum to an intermediate transfer belt as an image carrier or a belt member, and secondarily transfers the toner image from the intermediate transfer belt to a recording material. Are known. A transfer roller (secondary transfer outer roller) that is in contact with the outer peripheral surface of the intermediate transfer belt is disposed in the secondary transfer portion for secondary transfer of the toner image to the recording material, and a transfer voltage is applied to the transfer roller. Secondary transfer is performed.

  In the transfer roller, an elastic layer is provided on the peripheral surface of the conductive shaft portion, and a conductive agent such as an ionic conductive agent is dispersed in the elastic layer to impart conductivity. Therefore, when the voltage application time to the transfer roller increases due to use, ions in the ionic conductive agent are polarized so as to be biased to one of the roller surface side or the shaft portion side, and the electrical resistance tends to increase. Therefore, in order to suppress an increase in electrical resistance caused by polarization, a voltage is applied from the power supply roller as the power supply rotating body in contact with the surface of the transfer roller to the transfer roller, and the toner is transferred from the intermediate transfer belt to the recording material. An apparatus for transferring an image has been proposed (Patent Document 1).

JP 2005-316200 A

  However, in the case of the configuration described in Patent Document 1, the transfer current flows from the power supply roller to the secondary transfer portion via the shaft portion of the transfer roller (that is, the shaft portion is electrically floated). The flow path is long. For this reason, a power source having a large high voltage capacity is used. Therefore, a first power source that stretches an intermediate transfer belt in a secondary transfer portion (transfer portion) and applies a voltage to a secondary transfer inner roller that faces the secondary transfer outer roller via the intermediate transfer belt, and a power supply roller A configuration having a second power supply for applying a voltage to the power supply can be considered.

  In this configuration, the secondary transfer outer roller is electrically grounded, a transfer current is caused to flow through the secondary transfer portion by the voltage applied to the secondary transfer inner roller, and the secondary transfer outer roller is driven by the voltage applied to the power supply roller. Suppresses the polarization. Further, the polarization of the secondary transfer outer roller can be suppressed without a large high-voltage capacity power source.

  In such a configuration, the absolute value of the current value flowing from the first power source on the secondary transfer inner roller side to the secondary transfer outer roller and the current value flowing from the second power source on the power supply roller side to the secondary transfer outer roller are: Each power supply is controlled to be equal. However, it is difficult to make these current values exactly the same, and it is inevitable that the resistance value of the secondary transfer outer roller increases due to use.

  For example, when the resistance value of the secondary transfer outer roller increases, the voltage applied by the first power supply on the secondary transfer inner roller side is increased so that a desired transfer current flows through the secondary transfer portion. However, since it is inevitable that the resistance value of the secondary transfer outer roller increases as described above, the voltage applied from the first power supply may reach a predetermined voltage (for example, an upper limit value). . As a result, if the current value (transfer current) flowing through the secondary transfer portion does not reach a predetermined value even when a predetermined voltage is applied from the first power source, it is difficult to maintain good toner image transferability. .

  The present invention provides a configuration capable of maintaining the transferability of a toner image at a transfer portion even when a current value flowing through the transfer portion does not reach a predetermined value even when a predetermined voltage is applied from a first power source. .

  The present invention relates to an image bearing member that carries and moves a toner image, an electrically conductive shaft portion that is electrically grounded or electrically grounded through an electric circuit element or a power source, and the shaft portion. A transfer roller that forms a transfer portion that transfers a toner image carried on the image carrier to a recording material, and a transfer roller that forms a transfer portion that transfers the toner image carried on the image carrier to a recording material. A power supply rotating body that rotates in contact with the transfer roller and can supply a current to the transfer roller; a first power source that can apply a voltage to the image carrier in order to transfer a toner image at the transfer unit; A second power source capable of applying a voltage to the feeding rotating body; a detecting unit capable of detecting a current flowing through the transfer portion; and a current value detected by the detecting unit when a predetermined voltage is applied to the first power source. Is not reached the predetermined value, the first power supply Control means for controlling the current flowing from the power supply rotating body to the transfer section via the transfer roller so that the current of the predetermined value flows to the transfer section even if the applied voltage is the predetermined voltage; And an image forming apparatus.

  The present invention also provides an endless belt member that carries and moves a toner image, and an electrically conductive shaft portion that is electrically grounded or electrically grounded via an electric circuit element or a power source, A first roller that forms a transfer portion that transfers a toner image carried on the belt member to a recording material, and an outer peripheral portion that includes a conductive agent formed on the outer periphery of the shaft portion; A second roller disposed so as to sandwich the belt member, a feeding rotating body that rotates in contact with the first roller at a position different from the transfer unit, and that can supply current to the first roller; A first power source capable of applying a voltage to the second roller, a second power source capable of applying a voltage to the feeding rotating body, and a current flowing through the transfer portion can be detected in order to transfer the toner image at the transfer portion. When a predetermined voltage is applied to the detection means and the first power source When the current value detected by the detection means does not reach a predetermined value, the voltage applied to the feeding rotating body is set higher than when the detected current value has reached the predetermined value. Thus, an image forming apparatus comprising: a control unit that controls the second power source.

  The present invention also provides an endless belt member that carries and moves a toner image, and an electrically conductive shaft portion that is electrically grounded or electrically grounded via an electric circuit element or a power source, A first roller that forms a transfer portion that transfers a toner image carried on the belt member to a recording material, and an outer peripheral portion that includes a conductive agent formed on the outer periphery of the shaft portion; A second roller disposed so as to sandwich the belt member, a feeding rotating body that rotates in contact with the first roller at a position different from the transfer unit, and that can supply current to the first roller; A first power source capable of applying a voltage to the second roller, a second power source capable of applying a voltage to the feeding rotating body, and a current flowing through the transfer portion can be detected in order to transfer the toner image at the transfer portion. A detecting means, a center of the feeding rotating body and the first roller; A change means capable of changing a distance; and when the current value detected by the detection means when a predetermined voltage is applied to the first power supply does not reach a predetermined value, the detected current value is The image forming apparatus includes: a control unit that controls the changing unit so that the center-to-center distance is smaller than when the predetermined value is reached.

  The present invention also provides an endless belt member that carries and moves a toner image, and an electrically conductive shaft portion that is electrically grounded or electrically grounded via an electric circuit element or a power source, A first roller that forms a transfer portion that transfers a toner image carried on the belt member to a recording material, and an outer peripheral portion that includes a conductive agent formed on the outer periphery of the shaft portion; A second roller disposed so as to sandwich the belt member, a feeding rotating body that rotates in contact with the first roller at a position different from the transfer unit, and that can supply current to the first roller; A first power source capable of applying a voltage to the second roller, a second power source capable of applying a voltage to the feeding rotating body, and a current flowing through the transfer portion can be detected in order to transfer the toner image at the transfer portion. Detecting means and the circumferential direction of the first roller. Detected by the detecting means when a predetermined voltage is applied to the first power source, and a moving means capable of moving the feeding rotating body so as to change a contact position between the rotating body and the first roller. When the current value does not reach the predetermined value, the contact position between the power feeding rotating body and the first roller is set in the circumferential direction as compared with the case where the detected current value has reached the predetermined value. And an image forming apparatus comprising: a control unit that controls the moving unit so as to approach the transfer unit.

  According to the present invention, even when a predetermined voltage is applied from the first power source, the transferability of the toner image at the transfer portion can be maintained even when the value of the current flowing through the transfer portion does not reach the predetermined value.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment. (A) Schematic showing the configuration of the secondary transfer portion of the first embodiment, (b) another first example, (c) another second example, and (d) another third example, respectively. Diagram. The figure which shows the relationship between the contact position of a power feeding roller, and the electric current which flows into a secondary transfer inner roller from a power feeding roller. FIG. 3 is a block diagram of secondary transfer voltage control according to the first embodiment. 5 is a flowchart of secondary transfer voltage control according to the first embodiment. The figure which shows the relationship between the voltage applied to a feed roller, and the electric current which flows into a secondary transfer part. The figure which shows the electric current which flows into a secondary transfer outer roller in (a) 1 sheet printing and (b) continuous printing, respectively. (A) The figure which shows the relationship between the number of passages which a recording material passes a secondary transfer part, and the voltage of a secondary transfer high voltage power supply, (b) The figure which shows the relationship between the number of passages and the electric current which flows into a secondary transfer part. (C) The figure which shows the relationship between the voltage of an external high voltage power supply, and an advection current. The flowchart of control of the external high voltage power supply which concerns on 1st Embodiment. FIG. 9 is a schematic configuration diagram illustrating a configuration of a secondary transfer unit according to a second embodiment, in which (a) the position of the power supply roller is not changed and (b) the position of the power supply roller is changed. The block diagram of the voltage control and cam control which concern on 2nd Embodiment. The figure which shows the relationship between the voltage of an external high voltage power supply and an advection current before and behind the change of the position of a feed roller. The flowchart of control of the change of the position of the electric power feeding roller which concerns on 2nd Embodiment. FIG. 10 is a schematic configuration diagram illustrating a configuration of a secondary transfer unit according to a third embodiment, in which (a) the position of the power supply roller is not changed and (b) the position of the power supply roller is changed. The block diagram of the voltage control and gear control which concern on 2nd Embodiment. The figure which shows the relationship between the contact position of an electric power feeding roller, and an advection current. 10 is a flowchart of control for changing the position of a power supply roller according to the third embodiment.

  A first embodiment will be described with reference to FIGS. First, a schematic configuration of the image forming apparatus of the present embodiment will be described with reference to FIG.

[Image forming apparatus]
The image forming apparatus 100 is a so-called tandem type intermediate having four image forming portions Pa, Pb, Pc, and Pd provided corresponding to four colors of yellow, magenta, cyan, and black (Y, M, C, and K). It is a transfer type full-color printer. The image forming apparatus 100 is a toner image according to an image signal from a document reading device (not shown) connected to the image forming apparatus main body or a host device such as a personal computer connected to the image forming apparatus main body so as to be communicable. (Image) is formed on the recording material P. Examples of the recording material include sheet materials such as paper, plastic film, and cloth.

  The outline of such an image forming process will be described. First, in each of the image forming portions Pa, Pb, Pc, and Pd, toner images of respective colors are formed on the photosensitive drums 50a, 50b, 50c, and 50d as photosensitive members, respectively. To do. The toner images of the respective colors thus formed are transferred onto an intermediate transfer belt 56 as an image carrier or belt member, and subsequently transferred onto the recording material P from the intermediate transfer belt 56. The recording material to which the toner image has been transferred is conveyed to a fixing device (not shown), and the toner image is fixed to the recording material. This will be described in detail below.

  The four image forming units Pa, Pb, Pc, and Pd included in the image forming apparatus 100 have substantially the same configuration except that the development colors are different. Therefore, the image forming portion Pa will be described below as a representative.

  The image forming portion Pa is provided with a cylindrical photosensitive member, that is, a photosensitive drum 50a. The photosensitive drum 50a is an organic photosensitive member in which a photosensitive layer of an organic substance and a surface protective layer are sequentially laminated on a conductive support, and is driven to rotate in the direction of an arrow in the figure. Around the photosensitive drum 50a, a charging roller 51a as a charging device, a developing device 53a, a primary transfer roller 54a, and a drum cleaning device 55a are arranged. An exposure device 52a is disposed below the photosensitive drum 50a in the drawing.

  In addition, the intermediate transfer belt is configured such that the outer peripheral surface can be in contact with the photosensitive drums 50a, 50b, 50c, and 50d at the positions facing the photosensitive drums 50a, 50b, 50c, and 50d of the image forming units Pa, Pb, Pc, and Pd. 56 is arranged. The intermediate transfer belt 56 is stretched by a tension roller 60, a driving roller 63, a secondary transfer inner roller 62, and idler rollers 61 and 67, and rotates (rotates) in the direction of the arrow in the drawing by driving the driving roller 63. The intermediate transfer belt 56 is applied with a tension of about 29 to 118 N (≈3 to 12 kgf).

  A secondary transfer outer roller 64 serving as a transfer roller or a first roller is disposed at a position facing the secondary transfer inner roller 62 and the intermediate transfer belt 56, and records a toner image carried on the intermediate transfer belt 56. A secondary transfer portion T2 to be transferred to the material P is formed. A fixing device 7 is disposed downstream of the secondary transfer portion T2 in the recording material conveyance direction. Such an image forming apparatus 100 is controlled by a control unit 80 (FIG. 4) as a control unit. That is, the control unit 80 controls each configuration of the image forming apparatus 100 such as the image forming units Pa to Pd.

  A process for forming, for example, a four-color full-color image by the image forming apparatus 100 configured as described above will be described with reference to FIG. First, when an image forming operation start signal is issued, the photosensitive drum 50a is rotationally driven at a predetermined process speed (200 mm / sec in this embodiment). Then, the surface of the rotating photosensitive drum 50a is uniformly charged by the charging roller 51a to a predetermined negative potential (about −650 V in this embodiment). At this time, a charging bias obtained by superimposing an AC voltage on a DC voltage is applied to the charging roller 51a from a power source (not shown). More specifically, the charging bias is an oscillating voltage in which a DC voltage of −650 V and an AC voltage that is a sine wave having a frequency f of 1 kHz and a peak-to-peak voltage Vpp of 1.5 kV are superimposed.

  Next, the photosensitive drum 50a is exposed with a laser beam corresponding to an image signal emitted from the exposure device 52a. As a result, an electrostatic latent image corresponding to the image signal is formed on the photosensitive drum 50a. The electrostatic latent image on the photosensitive drum 50a is visualized by the toner accommodated in the developing device 53a and becomes a visible image. Specifically, a developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 50a is applied to the developing device 53a, whereby yellow toner is attached to the photosensitive drum 50a to perform reversal development, and a toner image. As a visible image. The developing bias in this embodiment is a bias composed of a DC component: −400 V, an AC component: 1.5 kVpp, a frequency: 3 kHz, a waveform: a rectangular wave and an AC voltage superimposed on a DC voltage.

  The toner image formed on the photosensitive drum 50a is primarily transferred to the intermediate transfer belt 56 at the primary transfer portion T1a formed between the toner image and the primary transfer roller 54a disposed on the inner peripheral surface side of the intermediate transfer belt 56. Is done. At this time, a positive primary transfer bias is applied to the primary transfer roller 54a from a primary transfer bias power source (not shown). In this embodiment, the primary transfer bias is constant current control of +1.5 μA. The toner (transfer residual toner) remaining on the surface of the photosensitive drum 50a after the primary transfer is removed by the drum cleaning device 55a.

  The portion of the intermediate transfer belt 56 to which the yellow toner image has been transferred is moved to the photosensitive drum 50 b side by driving the drive roller 63. Then, the magenta toner image formed on the photosensitive drum 50b in the same manner as the yellow toner image formation is transferred to the intermediate transfer belt 56 at the primary transfer portion T1b which is a contact portion between the photosensitive drum 50b and the intermediate transfer belt 56. The toner image is transferred to the upper yellow toner image.

  Similarly, cyan and black toner images formed on the photosensitive drums 50c and 50d are sequentially superimposed and primarily transferred onto yellow and magenta toner images superimposed and transferred onto the intermediate transfer belt 56 in the same manner. Then, a full color toner image is formed on the intermediate transfer belt 56.

  Thereafter, the recording material P stored in a recording material storage cassette (not shown) is conveyed from the registration roller 66 to the secondary transfer portion T2 in accordance with the toner image formation timing. Then, a secondary transfer bias having the same polarity as the toner is applied from the secondary transfer high-voltage power supply 320 (FIGS. 2 and 4) to the secondary transfer inner roller 62, so that a full-color toner image is batched on the recording material P. Is transferred (secondary transfer). In the present embodiment, as will be described later, the secondary transfer high-voltage power supply 320 is controlled so that the current value (secondary transfer current) flowing through the secondary transfer portion becomes −40 μA.

  Then, the recording material P on which the full-color toner image is formed is conveyed to the fixing device 7, and the full-color toner image is heated and pressed at the fixing nip portion between the fixing roller 71 and the pressure roller 72 of the fixing device 7. Then, it is fixed on the surface of the recording material P. Thereafter, the recording material on which the toner image is fixed is discharged to the outside of the apparatus, and a series of image forming operations is completed. The secondary transfer residual toner and paper dust remaining on the surface of the intermediate transfer belt 56 after the secondary transfer are removed and collected by the intermediate transfer belt cleaning device 65.

[Intermediate transfer belt]
The intermediate transfer belt 56 is an endless belt member in which an appropriate amount of an antistatic agent such as carbon black is contained in a resin such as polyimide or polyamide or an alloy thereof, or various rubbers. The intermediate transfer belt 56 is formed so as to have a surface resistivity of 1E + 9 to 5E + 13Ω / □, and a thickness of the intermediate transfer belt 56 is, for example, about 0.04 to 0.5 mm.

[Primary transfer roller]
Next, the configuration of the primary transfer rollers 54a to 54d will be described in detail. Since the configurations of the primary transfer rollers 54a to 54d are the same, the primary transfer roller 54a will be described below as a representative. The primary transfer roller 54 a faces the photosensitive drum 50 a and is provided inside the intermediate transfer belt 56. The primary transfer roller 54a is made of a metal roller such as SUM (sulfur and sulfur composite free cutting steel) or SUS (stainless steel). A voltage having a polarity opposite to the charging polarity of the toner is applied to the primary transfer roller 54a. As a result, a primary transfer contrast which is a potential difference between the surface potential of the photosensitive drum 50a and the potential of the primary transfer roller 54a is formed. By forming a predetermined primary transfer contrast in each of the primary transfer portions T1a to T1d, the toner images on the respective photosensitive drums 50a to 50d are sequentially electrostatically attracted to the intermediate transfer belt 56, and then onto the intermediate transfer belt 56. A toner image superimposed on is formed. The primary transfer roller 54a has a straight shape in the thrust direction, and the roller diameter is about 6 to 10 mm.

[Secondary transfer section]
Next, the secondary transfer portion T2 will be described in detail with reference to FIG. 1 and FIG. The secondary transfer portion T <b> 2 is formed by a secondary transfer outer roller 64 as a transfer roller or a first roller contacting the toner image carrying surface (outer peripheral surface) of the intermediate transfer belt 56. Specifically, the secondary transfer inner roller 62 as the second roller is disposed so as to sandwich the intermediate transfer belt 56 with the secondary transfer outer roller 64, and between the intermediate transfer belt 56 and the secondary transfer outer roller 64. Thus, a nip portion (secondary transfer portion T2) for sandwiching the recording material is formed. Then, the toner image is transferred from the intermediate transfer belt 56 to the recording material passing through the nip portion.

  The secondary transfer inner roller 62 is configured by providing a rubber layer such as EPDM (ethylene / propylene / diene rubber) around the core metal. The secondary transfer inner roller 62 is formed to have a roller diameter of 20 mm and a rubber thickness of 0.5 mm, and the hardness is set to, for example, 70 ° (Asker C).

  On the other hand, the secondary transfer outer roller 64 has a cored bar 64a as a shaft part having conductivity, and an elastic layer 64b as an outer peripheral part including a conductive agent formed on the outer periphery of the cored bar 64a. That is, the secondary transfer outer roller 64 is configured by providing an elastic layer 64b made of NBR (nitrile rubber) or EPDM containing an ion conductive agent such as a metal complex or carbon around the core metal 64a. The secondary transfer outer roller 64 is formed so that the hardness is 30 ° (Asker C), the resistance when 2000 V is applied is 1E + 8Ω, and the roller diameter is 24 mm.

  As shown in FIG. 2A, secondary transfer as a first power source capable of applying a secondary transfer bias (voltage) to the secondary transfer inner roller 62 (image carrier side) to perform a secondary transfer process. A high voltage power source 320 is connected. In the present embodiment, the secondary transfer high voltage power source 320 uses a constant voltage power source. Further, the core metal 64a of the secondary transfer outer roller 64 is electrically grounded.

  In addition, the secondary transfer outer roller 64 is configured to relax the polarization of the conductive material by being supplied with a current from a power supply roller 68 as a power supply rotating body. The power supply roller 68 is a metal roller having a diameter of 10 mm. The power supply roller 68 rotates in contact with the secondary transfer outer roller 64 at a position different from the secondary transfer portion T <b> 2, and can supply current to the secondary transfer outer roller 64.

  In the present embodiment, the power supply roller 68 is positioned so as to contact the opposite side of the secondary transfer outer roller 64 with respect to the secondary transfer inner roller 62. More specifically, the contact position between the power supply roller 68 and the secondary transfer outer roller 64 is larger than the contact position between the secondary transfer outer roller 64 and the intermediate transfer belt 56. The power supply roller 68 is disposed so as to be displaced by approximately 180 ° in the direction. Note that the position of the power supply roller 68 may be another position as long as the position is different from the contact position between the secondary transfer outer roller 64 and the intermediate transfer belt 56. Specifically, the axis center of the secondary transfer outer roller 64 is the origin, the line connecting the secondary transfer portion T2 and the origin is 0 °, and the rotation direction of the secondary transfer outer roller 64 is a negative angle. In this case, the contact position between the power feeding roller 68 and the secondary transfer outer roller 64 is preferably in the range of 90 ° to 270 °, more preferably in the range of 150 ° to 210 °. .

  The power supply roller 68 is connected to an external high voltage power source 420 as a second power source. The external high-voltage power supply 420 can apply a voltage in a direction that relaxes the polarization of the conductive agent of the secondary transfer outer roller 64 generated in the secondary transfer process to the power supply roller 68. In the secondary transfer process, the positive conductive agent is biased outward in the radial direction of the elastic layer 64b of the secondary transfer outer roller 64, and the negative conductive agent is biased inward (core metal 64a side). The external high-voltage power supply 420 supplies a current to the secondary transfer outer roller 64 via the power supply roller 68 in a direction to cancel such a state. In the present embodiment, a constant current power source is used as the external high voltage power source 420.

  In principle, the combination of the constant voltage power source and the constant current power source used for the secondary transfer high voltage power source 320 and the external high voltage power source 420 is arbitrary. In FIG. 2A, the secondary transfer outer roller 64 is directly grounded, but the secondary transfer outer roller 64 may be electrically grounded via an electric circuit element or a power source. FIGS. 2B and 2C show a case where an electric circuit element is connected between the cored bar 64a of the secondary transfer outer roller 64 and the ground, and FIG. 2D shows the secondary transfer outer roller 64. The case where a high voltage power supply is connected between the cored bar 64a and the ground is shown.

  FIG. 2B shows a case where a Zener diode 640 is connected as an electric circuit element, and FIG. 2C shows a case where a varistor 641 is connected as an electric circuit element. In both cases, if the breakdown voltage and the varistor voltage are sufficiently small compared to the voltage applied to the secondary transfer high-voltage power supply 320 and the external high-voltage power supply 420, for example, both 40V, the current flows to the ground through the electric circuit. The above system is established without any problem in principle. In addition, even when the high voltage power supply 642 is connected as shown in FIG. 2D, the applied voltage may be a value sufficiently smaller than the voltage applied to the secondary transfer high voltage power supply 320 and the external high voltage power supply 420, for example, 20V. Current flows to ground via the high voltage power supply 642. For this reason, the above-mentioned system is established without any problem in principle.

  Here, FIG. 3 shows the relationship between the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 and the current flowing directly from the power supply roller 68 to the secondary transfer inner roller 62. As shown in FIG. 3, when the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 is not within the above range, a current flows directly from the power supply roller 68 to the secondary transfer inner roller 62. For this reason, the effect of relaxing the polarization of the conductive agent of the secondary transfer outer roller 64 in this configuration is reduced. Therefore, the contact position of the power supply roller 68 is preferably in the above-described range.

[Secondary transfer voltage control]
Next, the secondary transfer voltage control in the secondary transfer portion T2 will be described with reference to FIGS. As described above, the control unit 80 as a control unit constitutes a control unit that controls the entire image forming apparatus. The control unit 80 includes a CPU 610 and a memory such as a ROM 82 and a RAM 83. The CPU 610 executes a predetermined process while appropriately exchanging data with the RAM 83 in accordance with a program stored in the ROM 82. The control unit 80 is input with the start and end of an image forming operation and an image signal.

  When an image forming job is input and an image forming signal is issued, a secondary transfer voltage is applied during the pre-rotation so that a desired secondary transfer current value (a predetermined current value, -40 μA in the present embodiment) flows. (Secondary transfer bias) is set (S1). In this embodiment, since the secondary transfer voltage is applied from the secondary transfer high-voltage power supply 320 to the secondary transfer inner roller 62, the secondary transfer high-voltage power supply is used in order for the current of the secondary transfer portion to be −40 μA. A current of +40 μA flows from 320 to the secondary transfer inner roller 62.

  The image forming job is a period from the start of image formation to the completion of the image forming operation based on a print signal for forming an image on a recording material. Specifically, it refers to the period from the pre-rotation to the post-rotation after receiving the print signal (input of the image forming job), and includes the image forming period and the interval between sheets (during non-image forming). The pre-rotation is a period in which rotation of the photosensitive drum and the intermediate transfer belt is started as a preparatory operation before image formation, and various voltages are sequentially raised and various voltages are adjusted. The post-rotation is a period in which as the operation after image formation, various voltages are sequentially lowered while the rotation of the photosensitive drum and the intermediate transfer belt is continued, and finally the rotation of the photosensitive drum and the intermediate transfer belt is stopped. The interval between the sheets is a period corresponding to the interval between the recording material that continuously passes through the secondary transfer portion T2.

  In the block diagram of FIG. 4, a command is issued from the control unit 80, analog conversion is performed by the D / A converter 310, and then high voltage is output from the secondary transfer high voltage power supply 320. At this time, the value of the current flowing through the secondary transfer portion T2 is detected by a current detection portion 330 as detection means, digitally converted by an A / D converter 340, and fed back to the control portion 80. In this embodiment, the secondary transfer voltage is set by applying two or more different arbitrary voltage values from the secondary transfer high-voltage power supply 320 and detecting the current values at that time by the current detector 330. Then, the VI characteristic is calculated from the relationship between the applied voltage value and the current value detected at that time, and a voltage value is obtained such that the target current value flows through the secondary transfer portion T2.

  Further, a secondary transfer voltage is set so that a desired transfer current flows by adding a shared voltage corresponding to a recording material type such as plain paper or cardboard, which is stored in the ROM 82 in advance, and an image is obtained using this voltage value. Formation is carried out (S2). The applied current to the power supply roller 68 at that time is set to a value corresponding to the secondary transfer current value (transfer current that flows by adding the voltage shared by the recording material) having a polarity opposite to that of the toner. This is instructed from the control unit 80, converted to analog by the D / A converter 410, and then outputs a high voltage from the external high-voltage power supply 420. The voltage value at that time is detected by the voltage detection unit 430, is digitally converted by the A / D converter 440, and is fed back to the control unit 80. In this way, by applying a current having the same magnitude as the secondary transfer current from the external high-voltage power supply 420 via the power supply roller 68 during image formation, the polarization of the ionic conductive agent of the secondary transfer outer roller 64 is suppressed. , Resistance increase can be suppressed.

  After the image formation, a voltage between the sheets is applied to the secondary transfer inner roller 62 (S3), and then it is determined whether the image formation is completed (S4). If not completed, S2 and S3 are repeated, When the image formation is finished, the secondary transfer voltage control is finished.

  In the secondary transfer outer roller 64, when the transfer current flows, the ionic conductive agent is polarized, and the resistance value is increased. In order to maintain good toner image transferability in the secondary transfer portion T2 against such an increase in resistance value, as described above, the current flowing from the secondary transfer inner roller 62 and the current flowing from the power supply roller 68 are Is controlling. That is, control is performed to maintain the transfer current flowing from the secondary transfer inner roller 62 at an optimum value. Further, in order to suppress an increase in the resistance value of the secondary transfer outer roller 64, currents flowing from the secondary transfer inner roller 62 and the power supply roller 68 to the secondary transfer outer roller 64 when the toner image is transferred to the recording material. Control is performed so that the absolute values of are equal.

[About advection current]
Next, a current (convection current) that flows through the secondary transfer portion T2 due to voltage application to the power supply roller 68 will be described. FIG. 6 shows a current that flows through the secondary transfer portion T2 when a voltage is applied to the power supply roller 68 when no voltage is applied to the secondary transfer inner roller 62.

  As shown in FIG. 6, even when no voltage is applied to the secondary transfer inner roller 62, a current of several μA flows to the secondary transfer portion T <b> 2 according to the voltage applied to the power supply roller 68. This is because the surface of the secondary transfer outer roller 64 is charged up by voltage application from the power supply roller 68 and reaches the secondary transfer portion T2 with a certain potential. For this reason, even if the voltage of the secondary transfer inner roller 62 is set to 0 V between sheets, depending on the voltage applied to the power supply roller 68, a current flows through the secondary transfer portion T2. Such a current is called an advection current. That is, the advection current is a current that flows from the power supply roller 68 to the secondary transfer portion T2 via the secondary transfer outer roller 64.

  Even when the voltage of the secondary transfer inner roller 62 is set to a value other than 0 V, the current flowing through the secondary transfer portion T due to the voltage applied to the power supply roller 68 in the relationship shown in FIG. This is added to the current flowing by the voltage of the transfer inner roller 62. In the present embodiment, the voltage applied to the power supply roller 68 at the time of image formation is normally about 1600 V in order to flow a current corresponding to the secondary transfer current +40 μA.

[Increased resistance of secondary transfer outer roller]
Here, even when the current is supplied from the power supply roller 68 to the secondary transfer outer roller 64 as described above, the current values flowing from the power supply roller 68 and the secondary transfer inner roller 62 are slightly shifted, resulting in a second difference. Polarization of the ion conductive agent occurs in the next transfer outer roller 64. For example, depending on the shape of the toner image, current unevenness occurs in the axial direction of the roller on the secondary transfer inner roller 62 side, causing a deviation from the current value from the power supply roller 68 in which the current flows uniformly in the axial direction. As described above, it is difficult to make the current values flowing from the power supply roller 68 and the secondary transfer inner roller 62 to the secondary transfer outer roller 64 strictly equal. For this reason, the resistance value of the secondary transfer outer roller 64 continues to increase even though the energization time becomes longer as the image is formed.

  Further, the resistance value of the secondary transfer outer roller 64 also increases due to the following factors. FIG. 7A shows the relationship between the current flowing from the secondary transfer inner roller 62 to the secondary transfer outer roller 64 and the current flowing from the power supply roller 68 to the secondary transfer outer roller 64 in the case of printing one sheet. FIG. 7B shows a similar relationship in the case of continuous printing.

  In order to form a sufficient electric field for transferring the toner image on the intermediate transfer belt 56 onto the paper P as a recording material at the secondary transfer portion T2, a voltage is applied from the secondary transfer high voltage power source 320. At this time, a voltage is applied so that a current of about +40 μA flows from the secondary transfer high-voltage power supply 320 to the paper passing, including before and after the paper P passes through the secondary transfer portion T2 (non-paper passing period). . That is, in order to pass a current of −40 μA from the secondary transfer outer roller 64 to the secondary transfer inner roller 62 as a secondary transfer current, a current of +40 μA is applied from the secondary transfer high-voltage power supply 320 to the secondary transfer inner roller 62. .

  Similarly, the voltage applied from the external high-voltage power supply 420 is controlled so that a current having a polarity opposite to that of the secondary transfer current +40 μA flows from the secondary transfer outer roller 64 to the power supply roller 68 in the power supply roller 68 as well. . That is, a current of −40 μA is applied from the external high voltage power source 420 to the power supply roller 68 so that a current of +40 μA flows from the secondary transfer outer roller 64 to the power supply roller 68.

  In such a case, as shown in FIG. 7A, the secondary transfer high-voltage power supply 320 is excessively applied to the secondary transfer outer roller 64 during the non-sheet passing period before and after the paper P passes through the secondary transfer portion T2. Current flows. The cause of the excessive current flow is the same in the case of continuous paper feeding as shown in FIG. As a result, the current value flowing from the external high-voltage power source 420 to the secondary transfer outer roller 64 and the current value flowing from the secondary transfer high-voltage power source 320 to the secondary transfer outer roller 64 are different, and the resistance of the secondary transfer outer roller 64 due to polarization is different. An increase occurs. In this way, the resistance value of the secondary transfer outer roller 64 continues to increase with image formation.

  As described above, the resistance value of the secondary transfer outer roller 64 continues to increase as the image is formed. Therefore, the high voltage capacity of the secondary transfer high voltage power source 320 that applies a transfer current to the secondary transfer portion T2 is the upper limit value. (Predetermined voltage) is reached. As a result, the transfer current becomes insufficient, and it becomes difficult to maintain good toner image transferability.

[Phenomenon when the secondary transfer high-voltage power supply reaches the upper limit]
Next, a phenomenon when the secondary transfer high voltage power source 320 reaches the upper limit value of the high voltage capacity due to the increase in resistance of the secondary transfer outer roller 64 will be described. FIG. 8A shows the relationship between the number of sheets that the recording material passes through the secondary transfer portion T2 and the voltage of the secondary transfer high-voltage power source 320. FIG. 8B shows the relationship between the number of passing sheets and the current flowing through the secondary transfer portion T2.

  As shown in FIG. 8A, the absolute value of the voltage input from the secondary transfer high-voltage power supply 320 in order to maintain the current necessary for transferring the toner image against the increase in the resistance of the secondary transfer outer roller 64 is growing. In this embodiment, when the number of passing sheets reaches about 1,000,000, it reaches 6.5 kV which is the upper limit value of the high voltage capacity of the secondary transfer high voltage power source 320, and after that, it maintains about 6.5 kV. When the number of passing sheets is about 1,000,000 or more, as shown in FIG. 8B, the voltage does not change, but the resistance of the secondary transfer outer roller 64 continues to increase, so the current flowing through the secondary transfer portion T2 The value becomes smaller. Then, the transfer current becomes insufficient from 40 μA, which is the target current that flows to the secondary transfer portion T2. The transfer current at the secondary transfer portion T2 is preferably 40 μA ± 10 μA.

[Control of external high-voltage power supply]
Therefore, in this embodiment, even when the secondary transfer high-voltage power supply 320 reaches the upper limit of the high-voltage capacity with the above-described configuration, the voltage applied by the external high-voltage power supply 420 is controlled to generate a good toner image. The transferability is maintained. In other words, when the secondary transfer outer roller 64 containing a conductive agent is used and the polarization of the conductive agent is relaxed by applying a current from the power supply roller 68, the secondary transfer high voltage power source 320 reaches the upper limit of the high voltage capacity. In addition, the advection current from the power supply roller 68 is controlled.

  For this reason, when the control unit 80 applies a predetermined voltage (upper limit value) to the secondary transfer high-voltage power supply 320, the current value detected by the current detection unit 330 does not reach the predetermined value (target current). In addition, the advection current is controlled. That is, the control unit 80 causes the secondary transfer outer roller from the power supply roller 68 so that a predetermined value of current flows through the secondary transfer unit T2 even if the voltage applied to the secondary transfer high-voltage power supply 320 is a predetermined voltage. The current (advection current) flowing through the secondary transfer portion T2 via 64 is controlled. In the present embodiment, the control unit 80 controls the voltage applied by the external high voltage power supply 420.

  Specifically, the control unit 80 applies a predetermined voltage to the secondary transfer high-voltage power supply 320, and the current detection unit 330 detects the current value flowing through the secondary transfer unit T2 at this time. Then, when the current value detected by the current detection unit 330 has not reached the predetermined value, the control unit 80 applies the power to the power supply roller 68 more than when the detected current value has reached the predetermined value. The external high-voltage power supply 420 is controlled to increase the voltage to be applied.

  Such control is performed at a timing other than the time of image formation, such as the above-described pre-rotation or paper interval. That is, the control unit 80 applies a voltage to the secondary transfer high-voltage power source 320 at a timing other than the time of image formation, and applies a voltage to the secondary transfer high-voltage power source 320 during image formation from the current value detected by the current detection unit 330. It is possible to execute a mode for determining Normally, in this mode, the secondary transfer voltage is determined as described above with reference to FIG. In the present embodiment, in this mode, the control unit 80, in the case where the current value detected by the current detection unit 330 when a predetermined voltage is applied to the secondary transfer high-voltage power supply 320 does not reach the predetermined value, The voltage applied by the external high voltage power supply 420 is determined as follows. That is, the voltage to be applied by the external high voltage power source 420 is determined so that a predetermined value of current flows through the secondary transfer portion T2 when a predetermined voltage is applied to the secondary transfer high voltage power source 320.

  FIG. 8C shows the relationship between the voltage applied by the external high-voltage power supply 420 and the advection current. FIG. 8C shows that the advection current can be increased by increasing the voltage applied by the external high-voltage power supply 420. For this reason, in the present embodiment, when a predetermined voltage is applied to the secondary transfer high-voltage power supply 320, if the current value detected by the current detection unit 330 does not reach the predetermined value, it is applied by the external high-voltage power supply 420. The voltage is increased to increase the advection current. The voltage applied by the external high-voltage power supply 420 may be determined based on the detection result of the current detection unit 330 based on a table as shown in FIG. 8C, or a predetermined constant voltage may be applied. May be.

  Hereinafter, a specific description will be given using FIG. 9 with reference to FIG. When the image forming operation start signal is issued, the secondary transfer voltage applied by the secondary transfer high-voltage power supply 320 during the previous rotation is set so that a desired secondary transfer current value (predetermined value, 40 μA in this embodiment) flows. (S21). At this time, the current value flowing through the secondary transfer portion T2 is detected by the current detection portion 330 (S22). The detection result (1) is fed back to the CPU 610 (S23). If the detection result is 40 μA (Y in S24), the CPU 610 applies a voltage according to the detection result from the secondary transfer high-voltage power supply 320. Further, an output is performed so that a current of −40 μA flows from the external high-voltage power supply 420 that performs constant current control (S25). Thereafter, printing is started (S26). When printing is completed (S27), the control is also terminated.

  On the other hand, if the detection result (1) is not 40 μA in S24 (N in S24), the voltage applied by the secondary transfer high-voltage power supply 320 is increased (S28). Thereafter, the secondary transfer current is detected by the current detector 330 (S29), and the detection result (2) is fed back to the CPU 610 (S30). If the detection result is 40 μA (Y in S31), a voltage is applied from the secondary transfer high-voltage power supply 320 at a voltage setting corresponding to the detection result (2), and a current of −40 μA is simultaneously applied from the external high-voltage power supply 420 ( S25) and printing is started (S26, 27).

  If the detection result (2) is not 40 μA in S31 (N in S31), the CPU 610 determines whether or not the high-voltage capacity of the secondary transfer high-voltage power supply 320 has reached the upper limit (S32). If the high voltage capacity of the secondary transfer high voltage power source 320 has not reached the upper limit (N in S32), the process returns to S28, and if the detection result (2) becomes 40 μA in S31, the process proceeds to S25.

  When the CPU 610 determines that the secondary transfer high voltage power supply 320 has reached the upper limit value in S32 (Y in S32), the CPU 610 performs control to change the voltage applied by the external high voltage power supply 420 (S33). Specifically, for example, by applying a constant voltage, the voltage applied by the external high-voltage power supply 420 is made higher than before. As a result, the advection current flowing through the secondary transfer portion T2 is changed.

  After the voltage applied by the external high-voltage power source 420 is changed, the process returns to S29, and the current value flowing through the secondary transfer unit T2 is detected again by the current detection unit 330 and fed back to the CPU 610 (S30). In S31, if the detection result (2) is 40 μA, the process proceeds to S25, and outputs are performed with the voltage setting of the secondary transfer high-voltage power supply 320 and the current setting of the external high-voltage power supply 420 according to the detection result (2).

  On the other hand, if the detection result (2) is not 40 μA in S31, the process proceeds to S32 and S33, and the voltage applied by the external high-voltage power supply 420 is changed again. For example, a constant voltage is applied again to the voltage immediately before it. Then, the process returns to S29, and this control is repeated until the detection result (2) becomes 40 μA in S31. When the upper limit of the high-voltage capacity of the external high-voltage power supply 420 is reached, the control is terminated and, for example, a signal indicating that the secondary transfer outer roller 64 has reached the end of its life is output to a display unit (not shown). To do.

  In this embodiment, even when a predetermined voltage is applied from the secondary transfer high-voltage power source 320, even if the current value flowing through the secondary transfer portion T2 does not reach the predetermined value, the toner in the secondary transfer portion T2 Image transferability can be maintained. That is, when the high-voltage capacity of the secondary transfer high-voltage power supply 320 reaches the upper limit, the voltage applied from the external high-voltage power supply 420 is increased to supplement the secondary transfer current that is insufficient due to the advection current. For this reason, even when the secondary transfer current cannot be sufficiently secured only by the voltage from the secondary transfer high-voltage power supply 320, the secondary transfer current can be secured by adding the advection current, and the toner image can be transferred with a good transfer property. Can be maintained.

[Experimental result]
Next, the results of experiments conducted to confirm the effects of this embodiment will be described. In the experiment, the occurrence of transfer omission when an image with an image ratio of 100% is printed using the secondary transfer outer roller 64 when the number of passing sheets reaches about 1100000 in FIG. It investigated on two conditions. The transfer omission is a phenomenon in which a part of the toner image is not transferred to the recording material when the toner image is transferred to the recording material at the secondary transfer portion T2.

  First, in the condition (1), the external high voltage power supply 420 is controlled so that the detection result of the current detection unit 330 and the current applied from the external high voltage power supply 420 are equal without performing the control of the advection current as described above. did. In condition (2), the external high-voltage power supply 420 was controlled with respect to the detection result of the current detection unit 330 in consideration of the advection current as shown in FIG. Under each condition, the radius of the secondary transfer outer roller 64 was 12 mm, the rotation speed was 200 mm / s, and the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 was 180 °.

  As a result of the experiment, under condition (1), transfer omission occurred due to insufficient transfer current. That is, it was found that transferability was lowered. On the other hand, under the condition (2) which is the condition of the present embodiment, it was found that the transfer current sufficiently flows, so that transfer omission does not occur and good transferability is maintained.

<Second Embodiment>
A second embodiment will be described with reference to FIGS. In this embodiment, in order to control the advection current, not the voltage applied by the external high-voltage power supply 420 but the center-to-center distance between the power supply roller 68 and the secondary transfer outer roller 64 is changed. Since other configurations are the same as those of the first embodiment described above, the same components are denoted by the same reference numerals, the description thereof is omitted or simplified, and the following description is centered on differences from the first embodiment. explain.

  As shown in FIGS. 10A and 10B, in the case of the present embodiment, a change unit 700 is provided as a change unit that can change the center-to-center distance between the power supply roller 68 and the secondary transfer outer roller 64. The changing unit 700 includes a cam 720 that moves the power supply roller 68 in contact with the power supply roller 68, and a cam motor 710 as a driving unit that drives the cam 720. In the present embodiment, the power supply roller 68 is supported so as to be movable so that the center-to-center distance with the secondary transfer outer roller 64 is changed. For example, both ends of the rotation shaft of the secondary transfer outer roller 64 are supported on the frame of the apparatus main body via bearings. In this frame, a long hole parallel to a line connecting the central axes of the secondary transfer outer roller 64 and the power feeding roller 68 is formed, and the rotation shaft is movable along this long hole.

  The cam 720 has a shape in which the outer peripheral surface is decentered with respect to the rotation center, and is formed so that the distance between the rotation center and the outer peripheral surface in contact with the secondary transfer outer roller 64 is changed according to the phase of the cam. ing. The cam 720 supports the rotating shaft of the power supply roller 68 and is in contact with a bearing that can move together with the power supply roller 68. Further, the bearing is biased toward the cam 720 by biasing means such as a spring. Thereby, when the cam 720 rotates, the power supply roller 68 moves along the long hole through the bearing.

  As shown in FIG. 11, the cam motor 710 is controlled by the control unit 80 to rotationally drive the cam 720. Note that a phase detector such as an encoder capable of detecting the phase of the cam 720 is provided on the rotating shaft of the cam motor 710 or the rotating shaft of the cam 720 so as to detect the phase of the cam 720. However, the rotation of the cam 720 may be controlled by performing pulse control using, for example, the cam motor 710 as a pulse motor without providing the phase detection unit.

  The configuration for moving the power supply roller 68 may be a configuration other than the cam and the motor. For example, the power supply roller 68 may be moved by a solenoid, or the power supply roller 68 may be moved by a gear mechanism such as a rack and pinion.

  In any case, the control unit 80 applies a predetermined voltage to the secondary transfer high-voltage power supply 320, and the current detection unit 330 detects the current value flowing through the secondary transfer unit T2 at this time. When the current value detected by the current detection unit 330 has not reached a predetermined value, the control unit 80 performs the secondary transfer and the feeding roller 68 more than when the detected current value has reached a predetermined value. The changing unit 700 is controlled so that the center-to-center distance with the outer roller 64 is reduced. That is, the cam motor 710 is driven, the phase of the cam 720 is changed, and the position of the power supply roller 68 is changed.

  Here, since the outer peripheral portion of the secondary transfer outer roller 64 is the elastic layer 64b, the amount of the power supply roller 68 entering the elastic layer 64b changes as the power supply roller 68 moves. That is, the power supply roller 68 is in contact with the outer peripheral surface of the secondary transfer outer roller 64 even when it is farthest from the secondary transfer outer roller 64. Therefore, when the power supply roller 68 is moved by the changing unit 700, the distance between the power supply roller 68 and the secondary transfer outer roller 64 changes due to a change in the amount of penetration of the power supply roller 68 into the elastic layer 64b. To do.

  Also in this embodiment, in the mode in which the secondary transfer voltage is determined at a timing other than the time of image formation such as the pre-rotation and the paper interval as described above, the center between the feeding roller 68 and the secondary transfer outer roller 64 is determined. The distance, that is, the amount of penetration of the power supply roller 68 is determined. That is, in this mode, when the current value detected by the current detection unit 330 when the predetermined voltage is applied to the secondary transfer high voltage power source 320 does not reach the predetermined value in this mode, the following is performed. In addition, the amount of penetration of the power supply roller 68 is determined. That is, the power supply roller 68 and the secondary transfer outer roller 64 changed by the changing unit 700 so that a predetermined value of current flows through the secondary transfer unit T2 when a predetermined voltage is applied to the secondary transfer high-voltage power source 320. Determine the center-to-center distance.

  FIG. 10A shows a normal state in which the amount of penetration of the power supply roller 68 into the secondary transfer outer roller 64 is not changed. That is, in this normal state, even if the voltage applied by the secondary transfer high-voltage power supply 320 does not reach the predetermined voltage or reaches the predetermined voltage, a current having a predetermined value is supplied to the secondary transfer portion T2. When it is flowing.

  On the other hand, FIG. 10B shows a state in which the power supply roller 68 is inserted into the secondary transfer outer roller 64 with respect to the normal state. That is, in this state, the voltage applied by the secondary transfer high-voltage power supply 320 reaches a predetermined voltage, and even when the predetermined voltage is applied, a current having a predetermined value does not flow through the secondary transfer portion T2. is there. In this case, by increasing the penetration amount S of the power supply roller 68, a predetermined value of current flows through the secondary transfer portion T2 due to the advection current. In this embodiment, the difference in the penetration amount S from the normal state in FIG. 10A to the state in FIG. 10B is 0.8 mm.

  FIG. 12 shows the voltage of the external high-voltage power supply 420 in the normal state (before change) of FIG. 10A and the state of FIG. 10B (after change) when the voltage of the external high-voltage power supply 420 is constant. The relationship with advection current is shown. As is apparent from FIG. 12, the advection current can be increased by increasing the penetration amount S of the power supply roller 68 without changing the voltage of the external high-voltage power supply 420.

  The control of this embodiment will be specifically described with reference to FIG. Note that S41 to S52 are the same as S21 to S32 of FIG. 9 described in the first embodiment, and therefore, here, differences from FIG. 9 will be mainly described.

  When the CPU 610 determines that the secondary transfer high-voltage power supply 320 has reached the upper limit value in S52 (Y in S52), the CPU 610 performs control to change the intrusion amount S of the power supply roller 68 (S53). Specifically, the cam motor 710 is driven to rotate the cam 720, and the intrusion amount S of the power supply roller 68 is set to the state shown in FIG. As a result, the advection current flowing through the secondary transfer portion T2 is changed.

  In this embodiment, even when a predetermined voltage is applied from the secondary transfer high-voltage power source 320, even if the current value flowing through the secondary transfer portion T2 does not reach the predetermined value, the toner in the secondary transfer portion T2 Image transferability can be maintained. That is, when the high-voltage capacity of the secondary transfer high-voltage power supply 320 reaches the upper limit value, the penetration amount S of the power supply roller 68 is increased to supplement the secondary transfer current that is insufficient due to the advection current. For this reason, even when the secondary transfer current cannot be sufficiently secured only by the voltage from the secondary transfer high-voltage power supply 320, the secondary transfer current can be secured by adding the advection current, and the toner image can be transferred with a good transfer property. Can be maintained.

  In the above description, the phase of the cam 720 is changed in two stages, but the phase may be changed in three or more stages, and the intrusion amount S of the power supply roller 68 may be changed in each phase. . Thereby, the advection current can be controlled more finely. In the above description, the advection current is controlled by changing the intrusion amount of the power supply roller 68 without changing the voltage applied by the external high voltage power source 420. However, these may be combined. For example, when the voltage of the external high voltage power supply 420 reaches the upper limit as a result of changing the voltage of the external high voltage power supply 420 and controlling the advection current, control is performed to change the intrusion amount of the power supply roller 68. Also good.

[Experimental result]
Next, the results of experiments conducted to confirm the effects of this embodiment will be described. In the experiment, the occurrence of transfer omission when an image with an image ratio of 100% is printed using the secondary transfer outer roller 64 when the number of passing sheets reaches about 1100000 in FIG. It investigated on two conditions.

  First, in the condition (1), the detection result of the current detection unit 330 and the current applied from the external high-voltage power source 420 are equal without controlling the advection current by changing the penetration amount of the power supply roller 68 as described above. Thus, the external high voltage power supply 420 was controlled. In the condition (2), the change in the amount of intrusion of the power supply roller 68 was controlled for the detection result of the current detection unit 330 in consideration of the advection current as shown in FIG. Under each condition, the radius of the secondary transfer outer roller 64 was 12 mm, the rotation speed was 200 mm / s, and the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 was 180 °.

  As a result of the experiment, under condition (1), transfer omission occurred due to insufficient transfer current. That is, it was found that transferability was lowered. On the other hand, under the condition (2) which is the condition of the present embodiment, it was found that the transfer current sufficiently flows, so that transfer omission does not occur and good transferability is maintained.

<Third Embodiment>
A third embodiment will be described with reference to FIGS. In the present embodiment, in order to control the advection current, not the voltage applied by the external high-voltage power supply 420 but the contact position between the power supply roller 68 and the secondary transfer outer roller 64 in the circumferential direction of the secondary transfer outer roller 64. changed. Since other configurations are the same as those of the first embodiment described above, the same components are denoted by the same reference numerals, the description thereof is omitted or simplified, and the following description is centered on differences from the first embodiment. explain.

  As shown in FIGS. 14A and 14B, in the present embodiment, the contact position between the power supply roller 68 and the secondary transfer outer roller 64 is changed in the circumferential direction of the secondary transfer outer roller 64. The moving unit 800 is a moving unit that can move the power supply roller 68. The moving unit 800 includes an arm 92 as a rotating member that rotatably supports the power feeding roller 68 around the central axis of the secondary transfer outer roller 64, and a driving unit 820 as a driving unit that drives the arm 92. Have.

  The drive unit 820 includes a gear motor 810 and a gear 91 that is supported concentrically with the central axis of the secondary transfer outer roller 64 and is rotationally driven by the gear motor 810. A gear train is provided between the gear motor 810 and the gear 91, and the gear 91 rotates around the central axis of the secondary transfer outer roller 64 by driving the gear motor 810. Further, the base end portion of the arm 92 is fixed to the gear 91 so as to rotate together with the gear 91. A feeding roller 68 is rotatably supported at the tip of the arm 92 via a bearing.

  As shown in FIG. 15, the gear motor 810 is controlled by the control unit 80 to rotate the gear 91. A phase detector such as an encoder capable of detecting the phase of the gear 91 is provided on the rotary shaft of the gear motor 810 or the rotary shaft of the gear 91 so as to detect the phase of the gear 91. However, the rotation of the gear 91 may be controlled by performing pulse control using, for example, the gear motor 810 as a pulse motor without providing the phase detection unit. The configuration for moving the power supply roller 68 may be a configuration other than such a gear mechanism. For example, a mechanism that transmits drive by a belt may be used.

  In any case, the control unit 80 applies a predetermined voltage to the secondary transfer high-voltage power supply 320, and the current detection unit 330 detects the current value flowing through the secondary transfer unit T2 at this time. When the current value detected by the current detection unit 330 has not reached the predetermined value, the control unit 80 is more in contact with the power supply roller 68 than when the detected current value has reached the predetermined value. Is controlled to move closer to the secondary transfer portion T2. That is, the gear motor 810 is driven, the gear 91 and the arm 92 are rotated, and the power supply roller 68 is moved along the circumferential direction of the secondary transfer outer roller 64. As a result, the contact position between the power supply roller 68 and the secondary transfer outer roller 64 is brought close to the secondary transfer portion T2.

  Also in the present embodiment, the position of the power supply roller 68 is determined in a mode in which the secondary transfer voltage is determined at a timing other than the time of image formation such as the pre-rotation or the sheet interval as described above. That is, in this mode, when the current value detected by the current detection unit 330 when the predetermined voltage is applied to the secondary transfer high voltage power source 320 does not reach the predetermined value in this mode, the following is performed. Next, the position of the power supply roller 68 is determined. That is, the position of the power supply roller 68 moved by the moving unit 800 is determined so that a predetermined value of current flows through the secondary transfer unit T2 when a predetermined voltage is applied to the secondary transfer high-voltage power source 320.

  FIG. 14A shows a normal state where the position of the power supply roller 68 is not changed. That is, in this normal state, even if the voltage applied by the secondary transfer high-voltage power supply 320 does not reach the predetermined voltage or reaches the predetermined voltage, a current having a predetermined value is supplied to the secondary transfer portion T2. When it is flowing. In a normal state, the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 is 180 °. In the case of this embodiment as well, the angle of the contact position is defined by a line connecting the axis center of the secondary transfer outer roller 64 as the origin and the secondary transfer portion T2 and the origin, as in the first embodiment. The rotation direction of the secondary transfer outer roller 64 is 0 ° and a negative angle.

  On the other hand, FIG. 14B shows a state in which the contact position between the power supply roller 68 and the secondary transfer outer roller 64 is close to the secondary transfer portion T2 with respect to the normal state. That is, in this state, the voltage applied by the secondary transfer high-voltage power supply 320 reaches a predetermined voltage, and even when the predetermined voltage is applied, a current having a predetermined value does not flow through the secondary transfer portion T2. is there. In this case, by bringing the contact position of the power supply roller 68 close to the secondary transfer portion T2, a current having a predetermined value flows through the secondary transfer portion T2 by the advection current. In the state of FIG. 14B, the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 is 90 °.

  In the present embodiment, unlike the second embodiment, control for changing the amount of penetration of the power supply roller 68 into the secondary transfer outer roller 64 is not performed. That is, if the configuration of the second embodiment is not provided and the accuracy of each member such as the gear 91 and the arm 92 is not taken into consideration, the intrusion amount is substantially constant regardless of the position of the power supply roller 68. It is trying to become.

  FIG. 16 shows the relationship between the contact position between the power supply roller 68 and the secondary transfer outer roller 64 and the advection current when the voltage of the external high-voltage power supply 420 is constant. As is apparent from FIG. 16, the advection current can be increased as the contact position angle decreases, that is, as the contact position of the power supply roller 68 approaches the secondary transfer portion T2. Therefore, the advection current can be controlled by changing the position of the power supply roller 68 without changing the voltage of the external high-voltage power supply 420.

  The control of this embodiment will be specifically described with reference to FIG. Note that S61 to S72 are the same as S21 to S32 of FIG. 9 described in the first embodiment, and therefore, here, differences from FIG. 9 will be mainly described.

  If the CPU 610 determines that the secondary transfer high-voltage power source 320 has reached the upper limit value in S72 (Y in S72), the CPU 610 performs control to change the position of the power supply roller 68 (S73). Specifically, the gear 91 and the arm 92 are rotated by driving the gear motor 810, and the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 is set to the state shown in FIG. As a result, the advection current flowing through the secondary transfer portion T2 is changed.

  In this embodiment, even when a predetermined voltage is applied from the secondary transfer high-voltage power source 320, even if the current value flowing through the secondary transfer portion T2 does not reach the predetermined value, the toner in the secondary transfer portion T2 Image transferability can be maintained. That is, when the high-voltage capacity of the secondary transfer high-voltage power supply 320 reaches the upper limit value, the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 is brought close to the secondary transfer portion T2, and the shortage caused by the advection current. The next transfer current is supplemented. For this reason, even when the secondary transfer current cannot be sufficiently secured only by the voltage from the secondary transfer high-voltage power supply 320, the secondary transfer current can be secured by adding the advection current, and the toner image can be transferred with a good transfer property. Can be maintained.

  In the above description, the position of the power supply roller 68 is changed in two stages, but the position of the power supply roller 68 may be changed in a plurality of stages of three or more stages. Thereby, the advection current can be controlled more finely. Here, if the contact position of the power supply roller 68 is too close to the secondary transfer portion T2, the advection current increases, but the effect of relaxing the polarization of the conductive material of the secondary transfer outer roller 64 is reduced. For this reason, it is preferable to move the contact position between the power supply roller 68 and the secondary transfer outer roller 64 in the range of 90 ° to 270 °.

  In the above description, the advection current is controlled by changing the position of the power supply roller 68 without changing the voltage applied by the external high-voltage power supply 420. However, these may be combined. For example, when the voltage of the external high voltage power supply 420 reaches the upper limit as a result of changing the voltage of the external high voltage power supply 420 and controlling the advection current, control for changing the position of the power supply roller 68 is executed. good.

[Experimental result]
Next, the results of experiments conducted to confirm the effects of this embodiment will be described. In the experiment, the occurrence of transfer omission when an image with an image ratio of 100% is printed using the secondary transfer outer roller 64 when the number of passing sheets reaches about 1100000 in FIG. It investigated on two conditions.

  First, in condition (1), the detection result of the current detection unit 330 and the current applied from the external high-voltage power source 420 are not changed without changing the contact position of the power supply roller 68 with respect to the secondary transfer outer roller 64 at 180 °. The external high voltage power source 420 was controlled so as to be equal. That is, in the condition (1), the control of the advection current by changing the position of the power supply roller 68 as described above was not performed. In condition (2), the detection result of the current detection unit 330 was controlled to change the position of the feed roller 68 in consideration of the advection current as shown in FIG. Under each condition, the radius of the secondary transfer outer roller 64 was 12 mm, and the rotation speed was 200 mm / s.

  As a result of the experiment, under condition (1), transfer omission occurred due to insufficient transfer current. That is, it was found that transferability was lowered. On the other hand, under the condition (2) which is the condition of the present embodiment, it was found that the transfer current sufficiently flows, so that transfer omission does not occur and good transferability is maintained.

<Other embodiments>
In each of the above-described embodiments, the predetermined voltage is the upper limit value that can be applied by the secondary transfer high-voltage power supply 320 (first power supply), but this predetermined voltage is, for example, an arbitrary value lower than the upper limit value of the power supply. It may be a voltage. Alternatively, when the voltage of the first power supply is constant, this voltage may be a predetermined voltage.

  In each of the above-described embodiments, the advection current is controlled so that the current value flowing through the secondary transfer portion is 40 μA. However, the advection current is controlled so that the current value falls within a range of 40 μ ± 10 μA, for example. You may do it.

  Moreover, you may combine each above-mentioned embodiment suitably. That is, the first to third embodiments may be combined, or the second and third embodiments may be combined. When combining the first to third embodiments, the voltage of the external high-voltage power source 420 is changed, the distance between the centers of the power supply roller 68 and the secondary transfer outer roller 64 is changed, and the secondary transfer outer roller of the power supply roller 68 is changed. The contact position with respect to 64 is changed as appropriate. For example, when the voltage of the external high-voltage power supply 420 reaches the upper limit as a result of changing the voltage of the external high-voltage power supply 420 and controlling the advection current, the control for changing the center-to-center distance of the second embodiment Any one of the control for changing the contact position of the third embodiment is executed. If the secondary transfer current cannot be secured even by this, the other control is executed.

  When combining the second and third embodiments, execute one of the control for changing the center-to-center distance of the second embodiment and the control for changing the contact position of the third embodiment, Control the advection current. If the secondary transfer current cannot be secured even by this, the other control is executed.

  In the above description, an image bearing member using an intermediate transfer belt is targeted, but the image bearing member may be a photosensitive member such as a photosensitive drum. That is, the present invention can be applied even to a direct transfer system in which a toner image is directly transferred from a photosensitive drum to a recording material. In addition to the intermediate transfer belt, the image carrier may be, for example, an intermediate transfer drum.

  The image forming apparatus of the present invention can be applied to a copying machine, a printer, a facsimile machine, a multifunction machine having a plurality of these functions, and the like.

  56 ... Intermediate transfer belt (image carrier, belt member) / 62 ... Secondary transfer inner roller (second roller) / 64 ... Secondary transfer outer roller (transfer roller, first roller) / 64a ... Core (shaft part) / 64b ... Elastic layer (outer peripheral part) / 68 ... Power feeding roller (power feeding rotator) / 80 ... Control part (control means) / 92 ... Arm ( Rotating member) / 320... Secondary transfer high voltage power source (first power source) / 330... Current detector (detecting means) / 420... External high voltage power source (second power source) / 640. Diode (electric circuit element) / 641... Varistor (electric circuit element) / 642... High voltage power supply (power supply) / 700... Changing portion (changing means) / 710... Cam motor (driving means) / 720 ... Cam / 800 ... Moving part (moving means) / 820 ... Drive part Drive means)

Claims (13)

An image carrier that carries and moves a toner image;
Electrically grounded, or a conductive shaft part that is electrically grounded via an electric circuit element or a power source, and an outer peripheral part including a conductive agent formed on the outer periphery of the shaft part, A transfer roller that forms a transfer portion for transferring the toner image carried on the image carrier to a recording material;
A feeding rotator that rotates in contact with the transfer roller at a position different from the transfer unit, and that can supply current to the transfer roller;
A first power source capable of applying a voltage to the image carrier in order to transfer a toner image at the transfer unit;
A second power source capable of applying a voltage to the feeding rotating body;
Detection means capable of detecting a current flowing through the transfer portion;
Even if the voltage applied to the first power supply is the predetermined voltage when the current value detected by the detecting means when the predetermined voltage is applied to the first power supply does not reach the predetermined value, Control means for controlling a current flowing from the power feeding rotating body through the transfer roller to the transfer unit so that a current of the predetermined value flows through the transfer unit;
An image forming apparatus. The control means controls a voltage applied by the second power source;
The image forming apparatus according to claim 1, wherein: A changing means capable of changing a distance between the centers of the feeding rotating body and the transfer roller;
The control means controls the changing means;
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. Moving means capable of moving the power feeding rotator so as to change the contact position between the power feeding rotator and the transfer roller with respect to the circumferential direction of the transfer roller;
The control means controls the moving means;
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. An endless belt member that carries and moves a toner image;
Electrically grounded, or a conductive shaft part that is electrically grounded via an electric circuit element or a power source, and an outer peripheral part including a conductive agent formed on the outer periphery of the shaft part, A first roller that forms a transfer portion for transferring a toner image carried on a belt member to a recording material;
A second roller disposed so as to sandwich the first roller and the belt member;
A feeding rotator that rotates in contact with the first roller at a position different from the transfer portion and can supply current to the first roller;
A first power source capable of applying a voltage to the second roller to transfer a toner image at the transfer unit;
A second power source capable of applying a voltage to the feeding rotating body;
Detection means capable of detecting a current flowing through the transfer portion;
When the current value detected by the detection means when the predetermined voltage is applied to the first power supply does not reach the predetermined value, the detected current value reaches the predetermined value. And a control means for controlling the second power supply so as to increase the voltage applied to the power feeding rotating body,
An image forming apparatus. The control means includes
It is possible to execute a mode in which a voltage is applied to the first power source at a timing other than the time of image formation and a voltage applied to the first power source at the time of image formation is determined from a current value detected by the detection unit.
In the mode, when the predetermined voltage is applied to the first power supply, the predetermined voltage is applied to the first power supply when the current value detected by the detecting means does not reach the predetermined value. Determining a voltage to be applied by the second power source so that the current of the predetermined value flows through the transfer portion when
The image forming apparatus according to claim 5, wherein: An endless belt member that carries and moves a toner image;
Electrically grounded, or a conductive shaft part that is electrically grounded via an electric circuit element or a power source, and an outer peripheral part including a conductive agent formed on the outer periphery of the shaft part, A first roller that forms a transfer portion for transferring a toner image carried on a belt member to a recording material;
A second roller disposed so as to sandwich the first roller and the belt member;
A feeding rotator that rotates in contact with the first roller at a position different from the transfer portion and can supply current to the first roller;
A first power source capable of applying a voltage to the second roller to transfer a toner image at the transfer unit;
A second power source capable of applying a voltage to the feeding rotating body;
Detection means capable of detecting a current flowing through the transfer portion;
Changing means capable of changing a center-to-center distance between the power feeding rotating body and the first roller;
When the current value detected by the detection means when the predetermined voltage is applied to the first power supply does not reach the predetermined value, the detected current value reaches the predetermined value. And a control means for controlling the changing means so that the center-to-center distance is reduced.
An image forming apparatus. The control means includes
It is possible to execute a mode in which a voltage is applied to the first power source at a timing other than the time of image formation and a voltage applied to the first power source at the time of image formation is determined from a current value detected by the detection unit.
In the mode, when the predetermined voltage is applied to the first power supply, the predetermined voltage is applied to the first power supply when the current value detected by the detecting means does not reach the predetermined value. Determining the center-to-center distance to be changed by the changing means so that the current of the predetermined value flows through the transfer portion when
The image forming apparatus according to claim 7, wherein: The feeding rotating body is supported so as to be movable so that the center-to-center distance is changed,
The changing means includes a cam for moving the power feeding rotating body and a driving means for driving the cam.
The image forming apparatus according to claim 7, wherein the image forming apparatus is an image forming apparatus. An endless belt member that carries and moves a toner image;
Electrically grounded, or a conductive shaft part that is electrically grounded via an electric circuit element or a power source, and an outer peripheral part including a conductive agent formed on the outer periphery of the shaft part, A first roller that forms a transfer portion for transferring a toner image carried on a belt member to a recording material;
A second roller disposed so as to sandwich the first roller and the belt member;
A feeding rotator that rotates in contact with the first roller at a position different from the transfer portion and can supply current to the first roller;
A first power source capable of applying a voltage to the second roller to transfer a toner image at the transfer unit;
A second power source capable of applying a voltage to the feeding rotating body;
Detection means capable of detecting a current flowing through the transfer portion;
Moving means capable of moving the power feeding rotator so as to change a contact position between the power feeding rotator and the first roller in the circumferential direction of the first roller;
When the current value detected by the detection means when the predetermined voltage is applied to the first power supply does not reach the predetermined value, the detected current value reaches the predetermined value. And a control means for controlling the moving means so that a contact position between the power feeding rotator and the first roller is brought close to the transfer portion in the circumferential direction.
An image forming apparatus. The control means includes
It is possible to execute a mode in which a voltage is applied to the first power source at a timing other than the time of image formation and a voltage applied to the first power source at the time of image formation is determined from a current value detected by the detection unit.
In the mode, when the predetermined voltage is applied to the first power supply, the predetermined voltage is applied to the first power supply when the current value detected by the detecting means does not reach the predetermined value. Determining the position of the feeding rotating body to be moved by the moving means so that the current of the predetermined value flows to the transfer portion when
The image forming apparatus according to claim 10. The moving means includes a rotating member that rotatably supports the power feeding rotating body around a central axis of the first roller, and a driving means that drives the rotating member.
The image forming apparatus according to claim 10, wherein the image forming apparatus is an image forming apparatus. The predetermined voltage is an upper limit value that can be applied by the first power source.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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