JP2019020480A - Image forming apparatus - Google Patents

Abstract

To prevent an image on a medium from being transferred to an application unit, compared with a case where when an application unit holds a medium to which images are transferred and applies a voltage having a second polarity to the medium, the polarity of the images on the medium remains to be a first polarity.SOLUTION: An image forming apparatus comprises: a plurality of transfer units that sequentially transfer images held on holding bodies to a medium with application of a transfer voltage; a charging unit that is arranged between the plurality of transfer units and charges the images having a first polarity transferred to the medium to have a second polarity opposite to the first polarity in a non-contact manner; and an application unit that is arranged downstream of the charging unit between the transfer units and holds the medium to which the images charged to have the second polarity are transferred and applies a voltage having the second polarity to the medium.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an image forming apparatus.

  In Patent Document 1, a plurality of photosensitive drums are exposed to a color-separated light image of an original to form an electrostatic latent image, and a color-specific toner image obtained by developing the electrostatic latent image is formed of a dielectric belt. In a transfer method of a multicolor electrophotographic copying machine that sequentially superimposes and transfers onto a paper transported by a transport belt, the photoreceptor drum is subjected to corona discharge by a transfer charger disposed opposite to the photoreceptor drum with the transport belt and the paper interposed therebetween. Multicolor electrophotography characterized by removing residual charge on paper and belt by transferring toner image onto paper and corona discharge from both sides of paper and belt after transfer by means of static elimination charger applied with AC or DC voltage A copying machine transfer method is disclosed.

Japanese Examined Patent Publication No. 62-052296

  Here, in the configuration in which the image held on the holding body is sequentially transferred to the medium by applying a transfer voltage, the application unit sandwiches the medium on which the first polarity image is transferred between the plurality of transfer units, When a voltage having a second polarity opposite to the one polarity is applied, the developer on the medium may be transferred to the application unit.

  In the present invention, when the application unit applies the second polarity voltage across the medium on which the image is transferred, the image on the medium is compared with the case where the polarity of the image on the medium remains the first polarity. The purpose is to suppress the transfer to the application section.

  According to the first aspect of the present invention, a plurality of transfer portions that sequentially transfer the image held on the holding body to the medium by applying a transfer voltage, and the first transferred to the medium are disposed between the plurality of transfer portions. A charging unit that charges a polarity image in a non-contact manner to a second polarity opposite to the first polarity, and a downstream portion of the charging unit between the charging unit and the image charged to the second polarity is transferred. And an application unit that applies a second polarity voltage across the medium.

  According to a second aspect of the present invention, the image on the medium, which is disposed on the downstream side of the application unit and between which the voltage of the second polarity is applied by the application unit, is charged to the first polarity in a non-contact manner. A charging unit is provided.

  According to a third aspect of the invention, the application unit applies a voltage having an absolute value smaller than the absolute value of the voltage applied by the transfer unit immediately upstream of the application unit.

  According to a fourth aspect of the present invention, four transfer units are provided along the conveyance direction of the medium, and the application unit and the charging unit are located downstream of the four transfer units in the conveyance direction. And disposed between the second transfer portion and the third transfer portion.

  In the invention of claim 5, among the four transfer portions, the first transfer portion and the second transfer portion between the first transfer portion and the second transfer portion toward the downstream side in the transport direction, and the third transfer portion. The application unit and the charging unit are not arranged between the transfer unit and the fourth transfer unit.

  According to a sixth aspect of the present invention, the plurality of transfer units are configured, and the application unit and the charging unit are not arranged immediately upstream, and one image held on the holding body is stored in the medium on and after the second time. A transfer unit for transferring to the medium, a measuring unit for measuring the surface potential of one image transferred to the medium after the transfer unit by the transfer unit on the holder, and the medium before the transfer of the one image. Based on the other image transferred and the charge amount of the medium, the surface potential measured by the measurement unit, and the surface capacitance of the holding body and the combined capacitance of the one image, the one image And a setting unit for setting a transfer voltage value when the image is transferred.

  The invention according to claim 7 includes another measurement unit that measures the surface potential of the other image, and the setting unit includes the surface potential measured by the other measurement unit, the other image, and the medium. The charge amount is obtained based on the combined capacitance of

  The transfer unit transfers the other image outside the image area of the medium, and the other measurement unit transfers the surface of the other image transferred outside the image area of the medium. Measure the potential.

  According to the configuration of the first aspect of the present invention, when the application unit applies the second polarity voltage across the medium on which the image is transferred, the polarity of the image on the medium remains the first polarity. Compared with the case, it can suppress that the image on a medium transfers to an application part.

  According to the configuration of the second aspect of the present invention, when the polarity of the image on the holding member transferred to the medium downstream of the application unit is the first polarity, the voltage of the second polarity is applied by the application unit. Compared with the case where the polarity of the image on the medium remains the second polarity, the transfer of the image on the medium to the holding body is suppressed.

  According to the configuration of claim 3 of the present invention, the application unit applies the voltage at the application unit as compared to the case where the application unit applies a voltage having an absolute value equal to or greater than the absolute value of the voltage applied by the transfer unit immediately upstream of the application unit. The surface potential of the medium after being applied can be reduced.

  According to the configuration of claim 4 of the present invention, when the application unit and the charging unit are disposed only between the first transfer unit and the second transfer unit toward the downstream side in the transport direction, Compared to the case where the transfer unit and the charging unit are arranged only between the third transfer unit and the fourth transfer unit toward the downstream side in the conveyance direction, Variation in surface potential can be suppressed.

  According to the configuration of the fifth aspect of the present invention, when the application unit and the charging unit are also arranged between the first transfer unit and the second transfer unit toward the downstream side in the transport direction, Compared with the case where the charging unit and the charging unit are also arranged between the third transfer unit and the fourth transfer unit toward the downstream side in the transport direction, the transfer voltage of each transfer unit is reduced with a simple configuration. Variation in the surface potential of the medium after application can be suppressed.

  According to the configuration of the sixth aspect of the present invention, the transfer voltage value when one image is transferred is set based only on the charge amount between the other image transferred to the one image and the medium. Compared to the case, transfer defects can be suppressed.

  According to the configuration of the seventh aspect of the present invention, transfer defects can be suppressed as compared to the case where the stored information stored in advance is used to determine the charge amount of other images and media.

  According to the configuration of the eighth aspect of the present invention, compared to the case where the surface potential of another image transferred to the image area of the medium is measured, the other image to be measured for the surface potential is adapted to the measurement. Can be.

1 is a schematic diagram illustrating a configuration of an image forming apparatus according to a first embodiment. 6 is a graph showing the surface potential of a recording medium after transfer of a toner image in each transfer unit. 7 is a graph showing the image density of a transferred image when the transfer voltage value of the transfer voltage in each transfer unit is constant (same) when the surface potential of the recording medium increases every time the toner image is transferred. (A) It is a schematic diagram which shows the electric potential of a transfer roll and a backup roll, (B) It is a schematic diagram which shows the surface electric potential of a recording medium. (A) It is a schematic diagram which shows the electric potential of a 1st charging part, (B) It is a schematic diagram which shows the electric potential of the 1st roll of an application part, and a 2nd roll. (A) It is a schematic diagram which shows the surface potential of a recording medium, (B) It is a schematic diagram which shows the potential of a 2nd charging part. (A) It is a schematic diagram which shows the surface potential of a recording medium, (B) It is a schematic diagram which shows the potential of a transfer roll and a backup roll. 6 is a graph showing a transfer voltage value when a transfer voltage with an increase in the surface potential of a recording medium is applied to each transfer unit. It is the schematic which shows the structure of the image forming apparatus which concerns on 2nd embodiment. FIG. 6 is a schematic diagram schematically showing the charge amount of each part when setting a transfer voltage value.

  Below, an example of an embodiment concerning the present invention is described based on a drawing.

<First embodiment>
(Image forming apparatus 10)
First, the configuration of the image forming apparatus 10 according to the present embodiment will be described. FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus 10 according to the present embodiment.

  As shown in FIG. 1, the image forming apparatus 10 includes a transport unit 17 that transports a recording medium P (an example of a medium) in the direction of arrow A, and a toner image (image image) on the recording medium P transported by the transport unit 17. An image forming unit 14 that forms an example), and a fixing device 16 that fixes the toner image on the recording medium P.

(Conveying unit 17)
As the recording medium P transported by the transport unit 17, a long recording medium P having a length in the transport direction is used. The recording medium P is not limited to paper, and may be a film formed of a resin material. The recording medium P may be a recording medium cut to a predetermined size, for example, a sheet (cut paper).

  The transport unit 17 includes an unwinding roll 72 for unwinding the recording medium P, a winding roll 74 for winding the recording medium P, and a plurality of winding rolls 77 around which the recording medium P is wound. . The winding roll 74 is rotationally driven by a drive unit (not shown). Accordingly, the take-up roll 74 takes up the recording medium P, and the unwind roll 72 unwinds the recording medium P.

  The plurality of winding rolls 77 are wound around the recording medium P between the winding roll 72 and the winding roll 74. Thereby, the conveyance path of the recording medium P from the unwinding roll 72 to the winding roll 74 is defined. The plurality of winding rolls 77 are driven to rotate by the recording medium P traveling toward the winding roll 74 when the winding roll 74 winds the recording medium P.

(Fixing device 16)
The fixing device 16 includes a heating roll 62 that heats the recording medium P and a pressure roll 64 that pressurizes the recording medium P. In the fixing device 16, the recording medium P is sandwiched between the heating roll 62 and the pressure roll 64, and the recording medium P is pressurized while being heated, so that the toner image on the recording medium P is fixed to the recording medium P.

(Image forming unit 14)
The image forming unit 14 includes toner image forming units 14Y, 14M, 14C, and 14K that form toner images of respective colors of yellow (Y), magenta (M), cyan (C), and black (K). The toner image forming units 14Y, 14M, 14C, and 14K (hereinafter referred to as 14Y to 14K) are arranged in this order from the upstream side to the downstream side in the conveyance direction of the recording medium P.

  Each of the toner image forming units 14 </ b> Y to 14 </ b> K includes an image forming unit 18 and a transfer unit 50. The toner image forming units 14Y to 14K are configured similarly except for the toner to be used. In FIG. 1, Y, M, C, and K are appropriately assigned to the image forming unit 18, the transfer unit 50, and the transfer units 50 </ b> Y to 50 </ b> K according to the color of the toner to be used.

(Image forming unit 18)
Each of the image forming units 18Y, 18M, 18C, and 18K (hereinafter referred to as 18Y to 18K) is a unit that forms a toner image using a liquid developer. Specifically, as shown in FIG. 1, each of the image forming units 18 </ b> Y to 18 </ b> K includes a rotating photosensitive drum 22, a charging device 24 that charges the photosensitive drum 22, and exposure light to the photosensitive drum 22. An exposure device 26 that forms an electrostatic latent image by irradiation and a developing device 40 that develops the electrostatic latent image on the photosensitive drum 22 to form a toner image are included.

  Since the image forming units 18Y to 18K are configured similarly except for the toner to be used, the reference numerals of the respective parts of the image forming units 18M, 18C, and 18K are omitted in FIG.

  The developing device 40 has a developing roll 42 (developing body). The developing roll 42 is a roll that develops the electrostatic latent image formed on the photosensitive drum 22 with a liquid developer in which toner (particles) is dispersed in a carrier liquid. Specifically, the developing roll 42 is supplied with a liquid developer from a supply roll (not shown) immersed in the liquid developer, and holds the liquid developer on the outer peripheral surface. The developing roll 42 rotates following the photosensitive drum 22.

  The liquid developer held on the developing roll 42 is supplied from the developing roll 42 to the photosensitive drum 22. With the liquid developer, the electrostatic latent image on the photosensitive drum 22 is developed to form a toner image. Accordingly, the toner images of the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) are constituted by the developer layers of the respective colors of developer (toner).

(Transfer unit 50)
The transfer units 50Y, 50M, 50C, and 50K (hereinafter referred to as 50Y to 50K) are units that transfer the toner images formed by the image forming units 18Y to 18K to the recording medium P. Specifically, each of the transfer units 50 </ b> Y to 50 </ b> K includes a transfer roll 52 (an example of a holding body), a backup roll 54 (an example of a transfer unit), and an application unit 56.

  Each of the transfer rolls 52Y, 52M, 52C, and 52K (hereinafter referred to as 52Y to 52K) is configured such that a part of the outer peripheral surface rotates in contact with a part of the outer peripheral surface of each photosensitive drum 22. A transfer voltage is applied between the transfer rolls 52 </ b> Y to 52 </ b> K and the photosensitive drums 22 by an application unit (not shown). As a result, a transfer electric field is formed between each of the transfer rolls 52Y to 52K and each of the photosensitive drums 22, and an electrostatic force acts on the toner image formed on each of the photosensitive drums 22. It is transferred to each of the transfer rolls 52Y to 52K. The transfer rolls 52Y to 52K hold the transferred toner image on the outer peripheral surface.

  Each backup roll 54 is arranged on the opposite side to each transfer roll 52 with respect to the recording medium P conveyed by the conveyance unit 17. Each of the applying units 56Y, 56M, 56C, and 56K (hereinafter referred to as 56Y to 56K) applies a transfer voltage between the transfer rolls 52Y to 52K and the backup rolls 54.

  In this embodiment, since the image transferred to the recording medium P is charged with positive polarity (an example of the first polarity), each of the applying units 56Y to 56K has a transfer roller 52 side having a positive potential. In addition, a transfer voltage is applied between the transfer rolls 52 </ b> Y to 52 </ b> K and each backup roll 54.

  As a result, a transfer electric field is formed between each of the transfer rolls 52Y to 52K and each of the backup rolls 54, and an electrostatic force acts on the toner image held on each of the transfer rolls 52Y to 52K. The image is transferred to the recording medium P at the transfer position.

  As described above, in the image forming apparatus 10, the toner images held on the transfer rolls 52 </ b> Y to 52 </ b> K are applied to the recording medium P by applying the transfer voltage between the transfer rolls 52 </ b> Y to 52 </ b> K and the backup rolls 54. Sequentially transferred. Specifically, the toner images held on the transfer rolls 52Y to 52K are transferred onto the recording medium P in the order of yellow (Y), magenta (M), cyan (C), and black (K). The

  Here, when a transfer voltage is applied between the transfer rolls 52Y to 52K and the backup rolls 54 by the application means 56Y to 56K, depending on the type of the recording medium P, as shown in FIG. The recording medium P may be charged and the surface potential of the recording medium P may increase. Note that when a film formed of a resin material is used as the recording medium P, the surface potential of the recording medium P is likely to increase. Further, in this embodiment, a transfer voltage is applied between the transfer rolls 52Y to 52K and each backup roll 54 so that the transfer roll 52 side has a positive potential, so that an image charged to positive polarity is obtained. Since it is transferred to the recording medium P, the recording medium P is positively charged (see FIGS. 4A and 4B).

  When the surface potential of the recording medium P increases, the transfer efficiency decreases. Therefore, when the transfer voltage value of the transfer voltage applied by each of the applying units 56Y to 56K is constant (same), it is shown in FIG. As described above, the transfer units 50M, 50C, and 50K arranged on the downstream side in the conveyance direction of the recording medium P may have a lower image density and lower than the target value. In FIG. 3, for the purpose of explanation, the relative density values are shown relative to the density target values of the colors Y, M, C, and K, and the reflection density values of the colors Y, M, C, and K are shown. May be set individually.

  Therefore, the image forming apparatus 10 adjusts the surface potential of the recording medium P by applying a reverse polarity (negative polarity) charge to the charged recording medium P. Hereinafter, a specific configuration will be described.

  As shown in FIG. 1, the image forming apparatus 10 includes a first charging unit 81 (an example of a charging unit), an application unit 90, and a second charging unit 82 (an example of a charging unit) disposed between the transfer unit 50M and the transfer unit 50C. An example of another charging unit).

  The application unit 90 includes a first roll 91 that contacts an image surface (upper surface) of the recording medium P, a second roll 92 that contacts a non-image surface (lower surface) of the recording medium P, and an application unit 98 having a power source and the like. ,have.

  The first roll 91 and the second roll 92 have an axial length extending from one end to the other end in the width direction of the recording medium P (direction intersecting the transport direction and the thickness direction). Accordingly, the first roll 91 and the second roll 92 sandwich the recording medium P from one end portion to the other end portion in the width direction of the recording medium P.

  The applying means 98 applies a voltage between the first roll 91 and the second roll 92 so that the first roll 91 side has a negative potential (see FIG. 5B). That is, the application unit 98 applies a voltage between the first roll 91 and the second roll 92 with a polarity opposite to that of the toner image held on the transfer roll 52 (negative polarity) on the first roll 91 side. To do.

  In this manner, the application unit 90 applies a negative voltage (an example of the second polarity) across the recording medium P to which the toner image is transferred, and charges the toner image and the recording medium P to the negative polarity.

  The first charging unit 81 is disposed on the upstream side in the transport direction with respect to the first roll 91 and the second roll 92 between the transfer unit 50M and the transfer unit 50C. The first charging unit 81 is constituted by a charger using corona discharge. In the first charging unit 81, the yellow and magenta toner images transferred to the recording medium P are negatively charged in a non-contact manner (see FIG. 5A).

  The second charging unit 82 is disposed on the downstream side in the transport direction with respect to the first roll 91 and the second roll 92 between the transfer unit 50M and the transfer unit 50C. The second charging unit 82 is composed of a charger using corona discharge. The second charging unit 82 charges the yellow and magenta toner images on the recording medium P to which the negative voltage has been applied by the applying unit 90 in a non-contact manner (see FIG. 6B).

  As described above, the first charging unit 81, the application unit 90, and the second charging unit 82 are downstream in the transport direction of the recording medium P among the four transfer units 50 provided along the transport direction of the recording medium P. The second transfer unit 50 </ b> M and the third transfer unit 50 </ b> C are arranged. The first charging unit 81, the application unit 90, and the second charging unit 82 are provided between the first transfer unit 50Y and the second transfer unit 50M, and the third transfer unit 50C. It is not arranged between the eye transfer unit 50K. Further, with respect to the first charging unit 81 and the second charging unit 82, an electrode for forming an electric field on the back surface of the recording medium P may be arranged.

(Operation according to the first embodiment)
According to the image forming apparatus 10 according to the present embodiment, in the transfer unit 50Y, the applying unit 56Y has a transfer voltage between the transfer roll 52Y and the backup roll 54 such that each transfer roll 52Y side has a positive potential. Is applied (see FIG. 1).

  As a result, the yellow toner image charged positively is transferred from the transfer roll 52Y to the recording medium P.

  Next, in the transfer unit 50M, the applying unit 56M applies a transfer voltage between the transfer roll 52M and the backup roll 54 so that the transfer roll 52M side has a positive potential. At this time, the potential of the transfer roll 52Y is set to +3000 V as an example, as shown in FIG.

  As a result, as shown in FIG. 4B, the magenta toner image charged positively is transferred from the transfer roll 52M to the recording medium P. As an example, the surface potential of the recording medium P to which the magenta toner image is transferred is + 1500V.

  Next, as shown in FIG. 5A, the first charging unit 81 charges the yellow and magenta toner images transferred to the recording medium P to the negative polarity without contact. At this time, the potential of the first charging unit 81 is set to −4000 V as an example.

  Next, as shown in FIG. 5 (B), in the application unit 90, the application means 98 is between the first roll 91 and the second roll 92 so that the first roll 91 side has a negative potential. Apply voltage to The potential of the first roll 91 at this time is set to −500 V as an example.

  As a result, a negative charge is applied to the recording medium P that has been charged positively, and the polarity of the recording medium P is reversed. As shown in FIG. 6A, the surface potential of the recording medium P is −250 V as an example. The absolute value (500) of the voltage applied by the application unit 90 is set to be smaller than the absolute value (3000) of the transfer voltage applied in the transfer unit 50M disposed immediately upstream of the application unit 90.

  Next, as shown in FIG. 6B, the second charging unit 82 charges the yellow and magenta toner images on the recording medium P to positive polarity without contact. At this time, the potential of the second charging unit 82 is set to +4000 V as an example.

  As a result, as shown in FIG. 7A, the surface potential of the recording medium P is, for example, + 200V. As described above, the surface potential of the recording medium P after the magenta toner image is transferred is lowered by the action of the first charging unit 81, the applying unit 90, and the second charging unit 82 (+ 1500V → + 200V).

  Next, in the transfer unit 50C, the applying unit 56C applies a transfer voltage between the transfer roll 52C and the backup roll 54 so that the transfer roll 52C side has a positive potential. At this time, as shown in FIG. 7B, the potential of the transfer roll 52C is, for example, + 3200V.

  As a result, a cyan toner image charged positively is transferred from the transfer roll 52C to the recording medium P.

  Further, in the transfer unit 50K, the applying unit 56K applies a transfer voltage between the transfer roll 52K and the backup roll 54 so that each transfer roll 52K side has a positive potential (see FIG. 1).

  As a result, the black toner image charged to the positive polarity is transferred from the transfer roll 52K to the recording medium P.

  In the fixing device 16, the recording medium P is sandwiched between the heating roll 62 and the pressure roll 64, and the recording medium P is pressurized while being heated, so that the toner image on the recording medium P is fixed to the recording medium P. . As described above, a toner image is formed on the recording medium P.

  In the present embodiment, as described above, the surface potential of the recording medium P after the magenta toner image is transferred is reduced by the action of the first charging unit 81, the application unit 90, and the second charging unit 82 (+ 1500V). → + 200V).

  Here, in the configuration (comparative example) in which the surface potential of the recording medium P is maintained without being lowered, and the surface potential of the recording medium P is maintained after the magenta toner image is transferred, each transfer unit 50M. , 50C, 50K, as shown in FIG. 8, in order to maintain the density of the transferred image, it is necessary to apply a transfer voltage in which the increase in the surface potential of the recording medium P is added. In FIG. 8, the transfer voltage value of the added transfer voltage is indicated by an arrow.

  In each of the transfer units 50M, 50C, and 50K, when the transfer voltage to be applied becomes a high voltage, a high-voltage power source is required, and a discharge is generated between the unit and the grounding portion. In particular, there is a limit to applying a high transfer voltage by further increasing the transfer unit 50 by the number of colors exceeding four colors.

  On the other hand, in the present embodiment, the surface potential of the recording medium P after the magenta toner image is transferred is reduced, so that the transfer voltage applied by the applying unit 56C is reduced compared to the comparative example described above. Also, the density of the cyan toner image is maintained.

  In the present embodiment, before the voltage application by the first roll 91 and the second roll 92 sandwiching the recording medium P, the first charging unit 81 removes the yellow and magenta toner images transferred to the recording medium P. Charge to negative polarity by contact.

  For this reason, when the voltage is applied by the first roll 91 and the second roll 92 sandwiching the recording medium P, the toner on the recording medium P is compared with the case where the polarity of the toner image on the recording medium P remains positive. Transfer of the toner of the image to the first roll 91 and the second roll 92 is suppressed.

  Further, in the present embodiment, the second charging unit 82 charges the yellow and magenta toner images on the recording medium P to positive polarity without contact.

  This suppresses the transfer of the toner image on the recording medium P to the transfer roll 52 </ b> C as compared with the case where the polarity of the toner image on the recording medium P remains negative.

  In the present embodiment, the absolute value (500) of the voltage applied in the application unit 90 is smaller than the absolute value (3000) of the transfer voltage applied in the transfer unit 50M disposed immediately upstream of the application unit 90. Has been.

  Thereby, the surface potential of the recording medium P after the voltage is applied by the application unit 90 is smaller than when the application unit 90 applies a voltage having an absolute value equal to or greater than the absolute value of the voltage applied by the transfer unit 50M. Become.

  In the present embodiment, the first charging unit 81, the application unit 90, and the second charging unit 82 include four transfer units 50 provided along the conveyance direction of the recording medium P, and the conveyance direction of the recording medium P. It is arranged between the second transfer unit 50M and the third transfer unit 50C toward the downstream side.

  For this reason, in the middle of the four transfer units 50Y to 50K, the surface potential of the recording medium P whose surface potential has increased due to the application of the transfer voltage in the two transfer units 50Y and 50M is lowered. Accordingly, the first charging unit 81, the application unit 90, and the second charging unit 82 are disposed only between the transfer unit 50Y and the transfer unit 50M, or the first charging unit 81, the application unit 90, and the second charging unit. Compared to the case where the portion 82 is disposed only between the transfer unit 50C and the transfer unit 50K, the variation in the surface potential of the recording medium P after the transfer voltage is applied in each of the transfer units 50Y to 50K is suppressed.

  In the present embodiment, the recording medium P is transported downstream in the conveyance direction between the first transfer unit 50Y and the second transfer unit 50M, and the third transfer unit 50C and the fourth transfer unit 50C. The first charging unit 81, the application unit 90, and the second charging unit 82 are not disposed between the eye transfer unit 50K.

  Therefore, when the first charging unit 81, the application unit 90, and the second charging unit 82 are also disposed between the transfer unit 50Y and the transfer unit 50M, or when the first charging unit 81, the application unit 90, and the second charging unit 82 are arranged. Compared with the case where the charging unit 82 is also arranged between the transfer unit 50C and the transfer unit 50K, the surface potential of the recording medium P after the transfer voltage is applied in each of the transfer units 50Y to 50K with a simple configuration. Variations are suppressed.

(Modification)
In the image forming apparatus 10, the first charging unit 81, the application unit 90, and the second charging unit 82 are downstream in the transport direction of the recording medium P among the four transfer units 50 provided along the transport direction of the recording medium P. Although it is arranged between the second transfer unit 50M and the third transfer unit 50C toward the side, the present invention is not limited to this. For example, the first charging unit 81, the application unit 90, and the second charging unit 82 may be added between the transfer unit 50Y and the transfer unit 50M, in addition to or instead of the transfer unit 50M and the transfer unit 50C. It may be arranged between the unit 50C and the transfer unit 50K. That is, the first charging unit 81, the application unit 90, and the second charging unit 82 may be arranged in at least one of the transfer units 50Y to 50K.

  In addition, the image forming apparatus 10 includes the second charging unit 82, but is not limited thereto. For example, when the cyan toner image on the transfer roll 52 </ b> C is negatively charged, the image forming apparatus 10 may be configured not to include the second charging unit 82.

  In the image forming apparatus 10, the liquid developer is used, but the present invention is not limited to this. The image forming apparatus may be, for example, a dry image forming apparatus using a powdery developer.

  In the image forming apparatus 10, the toner image transferred from the photosensitive drum 22 to the transfer roll 52 is transferred to the recording medium P. However, the present invention is not limited to this. The toner image formed on the photosensitive drum 22 may be directly transferred to the recording medium P. In this case, the photosensitive drum 22 functions as an example of a holding body that holds an image.

<Second embodiment>
Next, the image forming apparatus 200 according to the second embodiment will be described. In addition, about the part which has the same function as 1st embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted suitably.

  In the image forming apparatus 200, as shown in FIG. 9, the transfer units 50M and 50K include a setting unit 58, a first surface electrometer 11 (an example of a measuring unit), and a second surface electrometer 12 (other measurement). An example).

  The transfer units 50M and 50K are transfer units in which the first charging unit 81, the application unit 90, and the second charging unit 82 are not arranged immediately upstream of the transfer units 50Y to 50K, and the recording medium. This is the second and subsequent transfer units toward the downstream side in the P transport direction. That is, the transfer units 50 </ b> M and 50 </ b> K are transfer units that transfer the toner image held on the transfer roll 52 to the recording medium P after the second time.

  Each first surface potential meter 11M, 11K has a function of measuring the surface potential of the toner image held on each transfer roll 52M, 52K. That is, each of the first surface potential meters 11M and 11K uses the surface potential of the magenta (M) and black (K) toner images (an example of one image) transferred to the recording medium P after the second transfer roller. It has a function to measure on 52M and 52K.

  Each of the second surface potential meters 12M and 12K has a function of measuring the surface potential of the yellow (Y), magenta (M), and cyan (C) toner images transferred to the recording medium P on the recording medium P. ing. Specifically, each of the second surface electrometers 12M and 12K is yellow (Y) transferred to the recording medium P until the transfer of the magenta (M) and black (K) toner images (an example of one image). , Magenta (M), and cyan (C) toner images (an example of other images).

  In the present embodiment, apart from the toner image transferred to the image area of the recording medium P, a dedicated toner image for measuring the surface potential (hereinafter referred to as a measurement image) is outside the image area of the recording medium P. Transcribed. That is, in each of the image forming units 18M and 18K, measurement is performed at a predetermined position on the photosensitive drum 22 separately from a toner image (hereinafter referred to as a request image) requested by an operator (user) of the image forming apparatus 10. An image is formed. The requested image and the measurement image are transferred to the transfer roll 52, held on the transfer roll 52, and then transferred from the transfer roll 52 to the image area and the outside of the image area of the recording medium P, respectively. For example, the measurement image is transferred to the outside of at least one of the one end portion and the other end portion in the width direction (direction intersecting the transport direction and the thickness direction) with respect to the image forming region of the recording medium P. Each of the second surface potential meters 12M and 12K measures the surface potential of the measurement image transferred outside the image forming area of the recording medium P.

  Then, the setting unit 58M reads the yellow (Y) toner image transferred to the recording medium P and the charge amount of the recording medium P, the surface potential Vd measured by the first surface potential meter 11M, and the surface of the transfer roll 52M. The transfer voltage value Vb of the applying unit 56M is set based on the combined electrostatic capacity Cd of the layer and the magenta (M) toner image. Specifically, the setting unit 58M sets the transfer voltage value Vb of the applying unit 56M as follows.

  The charge amount of the yellow (Y) toner image and the recording medium P is the charge amount of the developer layer (hereinafter referred to as the yellow developer layer) and the recording medium P constituting the yellow (Y) toner image. . The yellow developer layer and the recording medium P constitute a transfer layer to which a magenta (M) toner image is transferred.

  The combined capacitance of the surface layer of the transfer roll 52M and the magenta (M) toner image is the developer layer (hereinafter referred to as magenta developer layer) constituting the surface layer of the transfer roll 52M and the magenta (M) toner image. It is a synthetic capacitance.

  The charge amount Qm of the yellow developer layer and the recording medium P is obtained based on the surface potential Vm measured by the second surface potentiometer 12M and the synthetic capacitance Cm of the yellow developer layer and the recording medium P. (See FIG. 10). Specifically, the charge amount Qm is obtained by Expression (1).

  Qm = Cm × Vm (1)

  The combined capacitance Cm is obtained based on the capacitance Cf of the recording medium P and the capacitance Ct1 of the yellow developer layer (see FIG. 10). Specifically, the combined electrostatic capacitance Cm is obtained by Expression (2).

  1 / Cm = 1 / Cf + 1 / Ct1 (2)

  The electrostatic capacity Ct1 of the yellow developer layer is stored in a storage unit (not shown) and is known. The setting unit 58M acquires information on the electrostatic capacity Ct1 of the yellow developer layer from the storage unit.

  The electrostatic capacitance Cf of the recording medium P is associated with the type of the recording medium P and is stored in a storage unit (not shown) in advance as a table. The capacitance Cf corresponding to the type of the recording medium P is read from the table by inputting the type of the recording medium P (manual input, reading of the identification code attached on the recording medium P, etc.), and the setting unit 58M Acquires information on the capacitance Cf.

  In addition, the electrostatic capacity Cf is measured by arranging a metal plate on the front and back surfaces of the conveyance path of the recording medium P, applying a predetermined voltage across the recording medium P, and detecting the amount of electric charge that has flowed. Thus, the setting unit 58M may acquire the capacitance Cf. In this case, the electrostatic capacity Cf of the recording medium P is obtained by sandwiching the recording medium P between electrode pairs having a known area and dividing the detected charge amount by the applied voltage.

  Further, based on the surface potential Vd measured by the first surface potential meter 11M and the combined capacitance Cd of the surface layer of the transfer roll 52M and the magenta developer layer, magenta development to be transferred to the recording medium P next. The charge amount Qd of the agent layer (magenta (M) toner image) is obtained (see FIG. 10). Specifically, the charge amount Qd is obtained by the equation (3).

  Qd = Cd × Vd (3)

  The synthetic capacitance Cd is obtained based on the capacitance Ct of the magenta developer layer and the capacitance Cc of the surface layer of the transfer roll 52M (see FIG. 10). Specifically, the combined electrostatic capacitance Cd is obtained by Expression (4).

  1 / Cd = 1 / Ct + 1 / Cc (4)

  The electrostatic capacity Ct of the magenta developer layer and the electrostatic capacity Cc of the surface layer of the transfer roll 52M are stored in a storage unit (not shown) and are already known. The setting unit 58M acquires information on the capacitance Ct of the magenta developer layer and the capacitance Cc of the surface layer of the transfer roll 52M from the storage unit.

  The induced charge amount Qp induced on the surface of the transfer layer composed of the yellow developer layer and the recording medium P is the charge amount Qm of the transfer layer and the magenta developer layer transferred to the transfer layer. The transfer voltage value Vb applied between the transfer roll 52M and the backup roll 54 is set so as to be equal to or greater than the sum of the charge amounts Qd.

  Specifically, the induced charge amount Qp is obtained by the equation (5). Specifically, the transfer voltage value Vb is obtained by Expression (6). In addition, Vb in Formula (6) is a lower limit. The following Vp is a potential in the recording medium P due to partial pressure, and is a potential induced on the surface of the transfer layer in the transfer portion by application of the transfer voltage value Vb.

  Qp = Cm × Vp = Qm + Qd (5)

Vb = Vp / (Cd / (Cm + Cd))
= (Qm + Qd) × (Cm + Cd) / (Cm × Cd) (6)

  Then, the transfer voltage value Vb is set so that Vb ≧ (Qm + Qd) × (Cm + Cd) / (Cm × Cd).

  Further, in the setting unit 58K, similarly to the setting unit 58M, yellow (Y), magenta (M) and cyan (C) toner images transferred to the recording medium P (yellow developer layer, magenta developer) Layer and cyan developer layer) and the charge amount Qm of the recording medium P, and the charge amount Qd of the black (K) toner image (black developer layer) to be transferred to the recording medium P, and the application means 56K. Set the transfer voltage value. This amount of charge Qd is determined by the surface potential Vd measured by the first surface potential meter 11K and the combined electrostatic capacitance Cd of the surface layer of the transfer roll 52K and the black (K) toner image (black developer layer). Is required.

  The transfer layer to which the black (K) toner image is transferred is composed of a yellow developer layer, a magenta developer layer, a cyan developer layer, and the recording medium P. Therefore, in the setting unit 58K, yellow development is performed. The charge amount Qm of the recording medium P with the developer layer, the magenta developer layer and the cyan developer layer is obtained.

  The charge amount Qm between the yellow developer layer, the magenta developer layer, the cyan developer layer, and the recording medium P is the surface potential Vm measured by the second surface potential meter 12K, the yellow developer layer, the magenta developer layer, It is obtained based on the combined electrostatic capacity Cm of the cyan developer layer and the recording medium P. Specifically, the charge amount Qm is obtained by Expression (1).

  Qm = Cm × Vm (1)

  The combined electrostatic capacity Cm is obtained based on the electrostatic capacity Cf of the recording medium P, the electrostatic capacity Ct1 of the yellow developer layer, the electrostatic capacity Ct2 of the magenta developer layer, and the electrostatic capacity Ct3 of the cyan developer layer. . Specifically, the synthetic capacitance Cm is obtained by the equation (2-1).

  1 / Cm = 1 / Cf + 1 / Ct1 + 1 / Ct2 + 1 / Ct3 Formula (2-1)

  The electrostatic capacity Ct3 of the cyan developer layer is stored in a storage unit (not shown) and is known in the same manner as the electrostatic capacity Ct1 of the yellow developer layer and the electrostatic capacity Ct2 of the magenta developer layer. Yes. The setting unit 58K acquires information on the electrostatic capacity Ct1 of the yellow developer layer, the electrostatic capacity Ct2 of the magenta developer layer, and the electrostatic capacity Ct3 of the cyan developer layer from the storage unit.

  The induced charge amount Qp induced on the surface of the transfer layer composed of the yellow developer layer, the magenta developer layer, the cyan developer layer and the recording medium P is the charge amount Qm of the transfer layer, The transfer voltage value Vb is set so as to be not less than the sum of the charge amount Qd of the black developer layer transferred to the transfer layer.

(Operation according to the second embodiment)
According to the image forming apparatus 200 according to the present embodiment, as described above, the setting unit 58M includes the yellow (Y) toner image transferred to the recording medium P, the charge amount of the recording medium P, and the first surface potential. The transfer voltage value of the applying unit 56M is set based on the surface potential measured by the total 11M and the combined capacitance of the surface layer of the transfer roll 52M and the magenta (M) toner image.

  Specifically, the induced charge amount Qp induced on the surface of the transfer layer composed of the yellow developer layer and the recording medium P is the charge amount Qm of the transfer layer and magenta transferred to the transfer layer. The transfer voltage value Vb is set so as to be equal to or greater than the sum of the charge amount Qd of the developer layer.

  Further, in the setting unit 58K, similarly, the induced charge amount Qp induced on the surface of the transfer layer composed of the yellow developer layer, the magenta developer layer, the cyan developer layer, and the recording medium P is set to The transfer voltage value Vb is set so as to be equal to or greater than the sum of the charge amount Qm of the transfer layer and the charge amount Qd of the black developer layer transferred to the transfer layer.

  As described above, in each of the transfer units 50M and 50K on the downstream side with respect to the transfer unit 50Y, the setting units 58M and 58K set the transfer voltage value Vb in consideration of the charge amount Qm of the transfer layer. A transfer voltage on which the surface potential of the recording medium P charged by application is added is applied.

  As a result, even in the transfer units 50M and 50K arranged on the downstream side in the transport direction of the recording medium P with respect to the transfer unit 50Y, the image density exceeds the target value without decreasing the image density. As a result, transfer failure of the toner image transferred to the recording medium P is suppressed.

  In this way, in the transfer units 50M and 50K, the setting units 58M and 58K set the transfer voltage value Vb, so that the setting unit 58 sets the transfer voltage value in only one of the transfer units 50M and 50K. Transfer defects in each transfer unit 50 are suppressed.

  In the image forming apparatus 10, the setting units 58 </ b> M and 58 </ b> K set the transfer voltage value Vb in consideration of the charge amount Qd of the developer layer of the toner image transferred to the recording medium P. The optimum transfer voltage value Vb is set as compared with the case where the transfer voltage value Vb is set based only on the charge amount Qm. As a result, transfer failure of the toner image transferred to the recording medium P is suppressed.

  Further, in the image forming apparatus 10, as described above, the charge amount Qm of the transfer layer is obtained using the surface potential Vm measured by the second surface potential meters 12M and 12K. As described above, since the charge amount Qm of the transferred layer is obtained based on the actually measured surface potential Vm, the optimum transfer can be performed as compared with the case where the charge amount Qm of the transferred layer is obtained using only stored information stored in advance. A voltage value Vb is set. As a result, transfer failure of the toner image transferred to the recording medium P is suppressed.

  In the image forming apparatus 10, yellow (Y), magenta (M), and cyan (C) toners transferred to the recording medium P before the black (K) toner image is transferred by the setting unit 58 </ b> K. The transfer voltage value Vb is set based on the charge amount of the image (yellow developer layer, magenta developer layer, and cyan developer layer). As described above, since the transfer voltage value is set based on the charge amounts of the plurality of toner images transferred to the recording medium P, cyan (C) transferred to the recording medium P immediately before the transfer of the black (K) toner image is obtained. ) Based on the charge amount of only the toner image (cyan developer layer), transfer defects are suppressed as compared with the case where the transfer voltage value Vb is set.

  In the image forming apparatus 10, the second surface potentiometers 12 </ b> M and 12 </ b> K measure the surface potential Vm of the measurement image transferred outside the image area of the recording medium P. For this reason, irrespective of the required image transferred to the image area of the recording medium P, a measurement image to be measured by the second surface potentiometers 12M and 12K is formed. Therefore, compared with the case where the surface potential of the required image transferred to the image area of the recording medium P is measured, the image (measurement image) to be measured for the surface potential is, for example, in size, position and density. The image is formed on the recording medium P as an image suitable for measurement. As a result, the optimum transfer voltage value Vb is set, and the transfer failure of the toner image transferred to the recording medium P is suppressed.

(Modification)
In the image forming apparatus 200, each of the setting units 58M and 58K uses the surface potential Vm measured by the second surface potentiometers 12M and 12K to determine the charge amount Qm of the transferred layer. A configuration may be used in which the charge amount Qm of the transfer layer is obtained without using the potential Vm. Specifically, for example, the configuration is as follows. That is, the charge amount Qm of the transferred layer is made to correspond to the type of the recording medium P, and is previously tabulated and stored in a storage unit (not shown). Then, the charge amount Qm corresponding to the type of the recording medium P is read from the table by inputting the type of the recording medium P (manual input, reading of the identification code attached on the recording medium P, etc.), and each setting unit 58M 58K, the setting unit 58M acquires information on the charge amount Qm.

  In the image forming apparatus 200, each of the second surface potentiometers 12M and 12K measures the surface potential Vm of the measurement image transferred outside the image area of the recording medium P. However, the present invention is not limited to this. For example, each of the second surface potential meters 12M and 12K may be configured to measure the surface potential of the requested image transferred to the image area of the recording medium P. In this case, the measurement position on the recording medium P is specified with reference to the image data of the requested image, and the second surface electrometers 12M and 12K measure the surface potential.

  Further, the image forming apparatus 200 includes the four color toner image forming units 14Y to 14K, but is not limited thereto. The image forming apparatus may be configured to have toner image forming units of two colors, three colors, or five colors or more. In this case, the above-described combined capacitance Cm is equal to the capacitance Cf of the recording medium P and the capacitances Ct1,... Ctn of images (developer layers) for n colors transferred to the recording medium P so far. Based on the above. Specifically, the synthetic capacitance Cm is obtained by the equation (2-2).

  1 / Cm = 1 / Cf + 1 / Ct1 +... + 1 / Ctn Formula (2-2)

  The present invention is not limited to the above-described embodiment, and various modifications, changes, and improvements can be made without departing from the spirit of the present invention. For example, the modification examples described above may be appropriately combined. Moreover, you may apply the modification in 1st embodiment and 2nd embodiment mutually suitably.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 11 1st surface electrometer (an example of a measurement part)
12 Second surface potentiometer (an example of another measuring unit)
52 Transfer roll (an example of a holder)
54 Backup roll (example of transfer section)
58 Setting unit 81 First charging unit (an example of charging unit)
82 Second charging unit (an example of another charging unit)
90 Application section

Claims (8)

A plurality of transfer sections that sequentially transfer the image held on the holding body to the medium by applying a transfer voltage;
A charging unit that is disposed between the plurality of transfer units and charges the first polarity image transferred to the medium in a non-contact manner to a second polarity opposite to the first polarity;
An application unit that is disposed on the downstream side of the charging unit in the middle and that applies a voltage of the second polarity across the medium on which the image charged with the second polarity is transferred;
An image forming apparatus comprising: 2. Another charging unit that is arranged on the downstream side of the application unit between the two and charges the image on the medium to which the second polarity voltage is applied by the application unit to the first polarity in a non-contact manner. The image forming apparatus described in 1. The image forming apparatus according to claim 1, wherein the application unit applies a voltage having an absolute value smaller than an absolute value of a voltage applied by the transfer unit immediately upstream of the application unit. Four transfer sections are provided along the conveyance direction of the medium,
The application unit and the charging unit are arranged between the second transfer unit and the third transfer unit toward the downstream side in the transport direction among the four transfer units. Item 4. The image forming apparatus according to any one of Items 1 to 3. Out of the four transfer portions, the first transfer portion and the second transfer portion between the first transfer portion and the second transfer portion toward the downstream side in the transport direction, and the third transfer portion and the fourth transfer portion. The image forming apparatus according to claim 4, wherein the application unit and the charging unit are not disposed between the transfer unit and the transfer unit. A plurality of transfer units, the application unit and the charging unit are not arranged immediately upstream, and a transfer unit that transfers the first image held on the holding body to the medium after the second,
A measurement unit for measuring the surface potential of one image transferred to the medium after the transfer unit by the transfer unit on the holder;
The amount of electric charges of the other image and the medium transferred to the medium before the transfer of the one image, the surface potential measured by the measurement unit, the surface layer of the holding body, and the composite static of the one image A setting unit for setting a transfer voltage value when the one image is transferred based on the electric capacity;
An image forming apparatus according to any one of claims 1 to 5. Another measurement unit for measuring the surface potential of the other image,
The image forming apparatus according to claim 6, wherein the setting unit obtains the charge amount based on a surface potential measured by the other measurement unit and a combined capacitance of the other image and the medium. The transfer unit transfers the other image outside the image area of the medium,
The image forming apparatus according to claim 7, wherein the other measurement unit measures a surface potential of the other image transferred outside the image area of the medium.