JP2019060993A - Image forming apparatus - Google Patents

Abstract

To prevent, when a high-resistance recording medium passes through a transfer area of an image holding body, a situation in which an image on the image holding body is electrostatically transferred to the recording medium before a transfer area of the image holding body, even if the recording medium is charged due to rubbing during the travel before passing through the transfer area of the image holding body.SOLUTION: An image forming apparatus comprises: a transfer device 2 that sandwiches and conveys a recording medium S with an image holding body 1 and a transfer member 2a, and electrostatically transfers an image G held on the image holding body 1 to the recording medium S; contact members 3 that are provided upstream of a transfer area TR of the transfer device 2 in a conveyance direction of the recording medium S, and in contact with the recording medium S; and a charging element 4 that supplies a charging voltage Vc having the same polarity as that of charged image forming particles to at least one of one or more contact members 3 located nearest to the transfer area TR of the transfer device 2 among the contact members 3, so that the recording medium S reaches the transfer area TR while maintaining a charging state having the same polarity as that of the charged image forming particles.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an image forming apparatus.

Conventionally, as an image forming apparatus of this type, for example, those described in Patent Documents 1 and 2 are already known.
In Patent Document 1, the toner is brought into contact with the toner image bearing surface of the intermediate transfer member through a recording medium so that high image quality can be obtained even when using an OHP sheet or thick transfer paper under low temperature and low humidity conditions. A secondary transfer roll for secondary transfer of a toner image to a recording medium by applying a voltage having a polarity opposite to the charge polarity of the image, and a toner image bearing surface of an intermediate transfer member at a position facing the secondary transfer roll And a backup roll in contact with the back surface of the intermediate transfer member, the transfer roller being disposed close to the back surface of the intermediate transfer member and upstream of the contact portion between the intermediate transfer member and backup roller An image forming apparatus is disclosed which is provided with reverse polarity electric field application means for applying an electric field of the opposite polarity to the above.
In Patent Document 2, when the transfer material is a high resistance (for example, OHP sheet), the OHP before transfer of the toner image is performed by the pre-transfer charging roller in order to prevent the transfer body bias in the downstream image forming station from becoming high. There is an image forming apparatus that applies charges opposite in polarity to toner to paper and sets the effective surface length on the pre-transfer charging roller shorter than the sheet width of OHP sheet to prevent the escape current from the non-sheet passing portion. It is disclosed.

JP-A-9-80925 (Embodiment of the invention, FIG. 2) JP 2003-223037 A (embodiment of the invention, FIG. 2)

  The technical problem to be solved by the present invention is that, when the high resistance recording medium passes through the transfer area of the image carrier, the recording medium rubs during traveling before the recording medium passes through the transfer area of the image carrier Even if charged by rubbing (hereinafter referred to as "sliding"), the object of the present invention is to suppress electrostatic transfer of the image on the image carrier onto the recording medium before the transfer area of the image carrier.

  The invention according to claim 1 has an image carrier which movably holds an image by charged image forming particles, and a transfer member which is disposed in contact with the image holding surface of the image carrier, the image carrier And the transfer member while holding and conveying the recording medium, and the transfer member held by the image support by applying a predetermined transfer voltage to the transfer area between the image support and the transfer member A transfer device for electrostatically transferring an image onto the recording medium, a contact member provided upstream of the transfer area of the transfer device in the conveyance direction of the recording medium and contacting the recording medium, and the recording medium being charged Of the contact members, at least one of the one or more contact members positioned in the immediate vicinity of the transfer region of the transfer device, so as to reach the transfer region while maintaining the charge state of the same polarity as the image particles. Provides charging voltage of the same polarity as the imaging particle A charging element that is an image forming apparatus comprising the.

According to a second aspect of the invention, in the image forming apparatus according to the first aspect, the contact member includes an alignment conveyance member for aligning and conveying the recording medium, and the charging element is provided at least on the alignment conveyance member. An image forming apparatus characterized in that
According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, the contact member includes a guide member for guiding the recording medium toward the transfer area, and the charging element is provided at least in the guide member. An image forming apparatus characterized in that
According to a fourth aspect of the present invention, in the image forming apparatus according to the first aspect, the contact member is provided between an alignment conveyance member for aligning and conveying the recording medium, and between the alignment conveyance member and the transfer area. An image forming apparatus comprising: a guide member for guiding the recording medium toward the transfer area, wherein the charging element is provided on the alignment conveyance member and the guide member.
The invention according to claim 5 is the image forming apparatus according to any one of claims 1 to 4, wherein the charging voltage by the charging element starts discharging between the contact member to which the charging voltage is supplied and the recording medium. The image forming apparatus is characterized by having a discharge start voltage or higher.
The invention according to claim 6 is the image forming apparatus according to any one of claims 1 to 5, wherein one or more contacts located upstream of the contact member provided with the charging element in the conveyance direction of the recording medium The member is an image forming apparatus characterized in that it is grounded via a predetermined high resistance.
According to a seventh aspect of the present invention, in the image forming apparatus according to the sixth aspect, the distance between the contact member to which high resistance is grounded and the contact member provided with the charging element is the conveyance direction of the recording medium. An image forming apparatus characterized by being within a range of length.
The invention according to claim 8 is the image forming apparatus according to any one of claims 1 to 7, wherein, when the recording medium has a high resistance equal to or higher than a predetermined resistance value, an operation of supplying a charging voltage by the charging element. An image forming apparatus characterized in that
The invention according to claim 9 is the image forming apparatus according to claim 8, the discriminator for discriminating whether or not the recording medium has a high resistance equal to or greater than a predetermined resistance value, and the discriminator using the discriminator. And a switch configured to switch and select the charging element so as to supply the charging voltage by the charging element when it is determined that the recording medium has high resistance.
The invention according to claim 10 is the image forming apparatus according to any one of claims 1 to 9, wherein the image carrier is intermediately transferred before the image on the image forming carrier is transferred to a recording medium. The image forming apparatus is an intermediate transfer member to be held, and the transfer device is to transfer an image on the intermediate transfer member to a recording medium.

According to the first aspect of the present invention, when the high resistance recording medium passes through the transfer area of the image carrier, charging is performed by rubbing during traveling before the recording medium passes through the transfer area of the image carrier Even if this is done, it is possible to suppress electrostatic transfer of the image on the image carrier onto the recording medium before the transfer area of the image carrier.
According to the second aspect of the present invention, when the recording medium is aligned and conveyed by the alignment conveyance member, the entire area of the recording medium is charged substantially uniformly to the same polarity as the charged image forming particles. it can.
According to the third aspect of the present invention, the recording medium can be charged to the same polarity as the charged image forming particles immediately before the recording medium goes to the transfer area of the transfer device via the guide member.
According to the fourth aspect of the present invention, the charged state of the charging member is stabilized with respect to the recording member traveling the alignment conveying member and the guiding member, as compared to the case where the charging member is provided on the alignment conveying member or the guiding member be able to.
According to the fifth aspect of the present invention, the charging voltage level of the charging element can be easily adjusted, and the charging operation by the discharge can be surely realized between the contact member and the recording medium.
According to the invention of claim 6, when the recording medium is charged by the charging element to the same polarity as the charged image forming particles, the leakage current from the contact member located on the upstream side in the conveyance direction of the recording medium is suppressed. Can.
According to the seventh aspect of the present invention, when the recording medium is charged by the charging element to the same polarity as the charged image forming particles, the height is higher than the minimum necessary contact member located upstream of the recording medium in the conveyance direction. By performing the resistance grounding, it is possible to suppress the leakage current from the contact member located on the upstream side in the conveyance direction of the recording medium.
According to the eighth aspect of the present invention, even if a high resistance recording medium is used as the recording medium, the recording medium is charged to the same polarity as the charged image forming particles when the recording medium passes the transfer area. it can.
According to the ninth aspect of the present invention, even if a high resistance recording medium is used as the recording medium, the recording medium is charged to the same polarity as the charged image forming particles when the recording medium passes the transfer area. It can be easily built.
According to the invention as set forth in claim 10, when the recording medium passes through the transfer area of the intermediate transfer body in the image forming apparatus of the intermediate transfer type, during traveling before the recording medium passes through the transfer area of the image carrier Even when charged by rubbing, it is possible to suppress electrostatic transfer of the image on the image carrier onto the recording medium before the transfer area of the image carrier.

FIG. 1 is an explanatory view showing an outline of an embodiment of an image forming apparatus to which the present invention is applied. FIG. 1 is an explanatory view showing an entire configuration of an image forming apparatus according to Embodiment 1; FIG. 3 is an explanatory view showing a configuration example around a secondary transfer portion of the image forming apparatus shown in FIG. 2; (A) is an explanatory view showing an image forming example 1 on a high resistance sheet by the image forming apparatus according to Embodiment 1, (b) is an explanatory view showing the same image forming example 2, (c) is shown in FIG. It is an explanatory view showing an example of a distinction machine. FIG. 6 is an explanatory view showing details of main elements around a secondary transfer portion used in the present embodiment. FIG. 6 is a flowchart showing a sheet type image forming sequence used in the image forming apparatus according to Embodiment 1. FIG. (A) is an explanatory view schematically showing the secondary transfer operation process when paper other than high resistance paper is used, (b) schematically shows the secondary transfer operation process when high resistance paper is used FIG. (A) is an explanatory view schematically showing a transfer operation in a secondary transfer area when using a high resistance sheet in the image forming apparatus according to Embodiment 1, (b) is an image according to comparison form 1 FIG. 14 is an explanatory view schematically showing a transfer operation in a secondary transfer area when using a high resistance sheet in the forming apparatus. (A) is an explanatory view showing a configuration example around a secondary transfer portion of the image forming apparatus according to the first modification, and (b) is a configuration example around the second transfer portion of the image forming apparatus according to the second modification FIG.

Outline of Embodiment FIG. 1 shows an outline of an embodiment of an image forming apparatus to which the present invention is applied.
In the figure, the image forming apparatus includes an image carrier 1 movably holding an image G formed by charged image forming particles, and a transfer member 2a disposed in contact with the image holding surface of the image carrier 1; The recording medium S is nipped and conveyed by the image carrier 1 and the transfer member 2a, and a predetermined transfer voltage _VTR is applied to the transfer area TR between the image carrier 1 and the transfer member 2a. A transfer device 2 for electrostatically transferring the image G held by the image carrier 1 onto the recording medium S, and a transfer area upstream of the transfer area TR of the transfer device 2 in the transport direction of the recording medium S The contact member 3 and the recording medium S are in the vicinity of the transfer region TR of the transfer device 2 in the contact member 3 so as to reach the transfer region TR while maintaining the charged state of the same polarity as the charged image forming particles. For at least one of the plurality of contact members 3, the same as the charged imaging particles And a charging element 4 for supplying a charging voltage Vc of a polarity.
In FIG. 1, the transfer device 2, transfer by the opposing member 2b is placed on the back surface of the image carrier 1 facing the transfer member 2a, applies a transfer voltage V _TR from the transfer power source 2c to the opposite member 2b A transfer electric field for transferring the image G to the area TR is formed.

In such technical means, as long as the image carrier 1 holds the image G, the intermediate transfer body of the intermediate transfer system includes, of course, the photosensitive body of the direct transfer system and the dielectric.
Further, the contact member 3 to which the charging element 4 can be added may be at least one of one or more contact members 3 located in the immediate vicinity of the transfer area TR.
Here, with regard to the installation location of the charging element 4, a mode in which the charging element 4 is added to one contact member 3 located in the immediate vicinity of the transfer area TR is representative, but it is not limited thereto. It may be added to both of the plurality of contact members 3 located or may be added to either one. However, when charging element 4 is added to contact member 3 on the side away from transfer region TR in a mode having a plurality of contact members 3 in the immediate vicinity of transfer region TR, contact member 3 located downstream thereof The surface of the downstream contact member 3 is covered with a covering layer that does not charge in reverse polarity so that the recording medium S charged at an angle by the upstream contact member 3 does not return to reverse polarity due to the rubbing. It is preferable to take measures to prevent recharging to the reverse polarity.
Furthermore, the charging element 4 may be any mode that supplies the charging voltage Vc, and basically, the charging voltage Vc may be a direct current bias, but if the polarity does not change, even if an alternating current bias is superimposed. Good.

According to the present embodiment, in the case of the recording medium S having high resistance, for example, among the recording medium S, the various contact members 3 are moved during traveling before the recording medium S passes the transfer area TR of the image carrier 1 And may be partially charged. Under such circumstances, when the partially charged recording medium S rushes into the transfer area TR, the image G by the charged image forming particles on the image carrier 1 approaches the recording medium S near the entrance of the transfer area TR. Be placed. At this time, assuming that a partially charged portion of the recording medium S is charged to the opposite polarity to the polarity of the charged image forming particles, a part of the image G on the image carrier 1 is recorded before the transfer area TR. There is a possibility that the image may be transferred to the medium S, which may lead to a transfer failure such as a missing image or a dirty image. In particular, in the case of highly conductive charged imaging particles, the electrostatic transfer to the image carrier 1 is weak, so that the above-mentioned transfer failure tends to be noticeable.
On the other hand, in the present embodiment, when the recording medium S travels in contact with the contact member 3, the charging element 4 charges the recording medium S to the same polarity as the charged image forming particles. Even when the recording medium S and the image G formed by the charged imaging particles on the image carrier 1 are arranged close to each other at the entrance of the transfer area TR when entering the TR, the charged imaging particles on the image carrier 1 The image G is pressed against the charge on the recording medium S, ie, in the direction of the image carrier 1, and the image G by the charged image forming particles on the image carrier 1 rushes into the transfer area TR. There is no concern to shift to the recording medium S side prior to recording.

Next, a representative aspect or a preferable aspect of the image forming apparatus according to the present embodiment will be described.
First, as a representative aspect of the contact member 3, the following aspects may be mentioned.
(1) A mode including the alignment conveyance member 3a for aligning and conveying the recording medium S, and the charging element 4 is provided at least on the alignment conveyance member 3a.
(2) A mode including the guide member 3 b for guiding the recording medium S toward the transfer area TR, and the charging element 4 is provided at least on the guide member 3 b.
(3) The alignment conveyance member 3a for aligning and conveying the recording medium S, and the guide member 3b provided between the alignment conveyance member 3a and the transfer area TR and guiding the recording medium S toward the transfer area TR The aspect in which the charging element 4 is provided in the alignment conveyance member 3a and the guide member 3b.

In addition, as a typical setting method of the charging voltage Vc of the charging element 4, the charging voltage Vc by the charging element 4 is a discharge that starts the discharge between the contact member 3 to which the charging voltage Vc is supplied and the recording medium S The aspect which is more than a start voltage is mentioned.
Furthermore, as an aspect in which the effectiveness of the charging action by the charging element 4 is enhanced, one or more contact members 3 (for example, the contact members 3 located in the conveying direction of the recording medium S with respect to the contact member 3 provided with the charging The conveying member 3 c) for conveying the recording medium S may be grounded via a predetermined high resistance 5. In this example, even when the recording medium S is in contact with the contact member 3 (for example, 3c) positioned on the upstream side in the conveyance direction of the recording medium S when the recording medium S is charged by the charging element 4, Since the contact member 3 (for example, 3c) is grounded via the high resistance 5, the charge that has charged the recording medium S is unlikely to leak from the contact member 3 (for example 3c) that is high resistance grounded. It is preferable in that the charged state is well maintained.

  Furthermore, as a preferred embodiment in order to enhance the effectiveness of the charging action by the charging element 4, the contact member 3 (for example 3c) to which high resistance is grounded is in contact with the contact member 3 (for example 3a) provided with the charging element 4. There is a mode in which the distance between them is within the range of the transport direction length of the recording medium S. In an embodiment in which the distance between the two is outside the range of the transport direction length of the recording medium S, the charge of the recording medium S is primarily transmitted from the contact member 3 (for example, 3c) located on the upstream side of the transport direction of the recording medium S There is no concern about leakage.

In the present embodiment, the charging action by the charging element 4 may be applied to all the recording media S, but in a particularly preferable embodiment, the recording media S have a high resistance higher than a predetermined resistance value. When the charging voltage Vc is supplied by the charging element 4.
As a preferable mode in this case, a discriminator 6 for discriminating whether the recording medium S has a high resistance equal to or more than a predetermined resistance value and a discriminator 6 discriminate that the recording medium S has a high resistance And a switch 7 for switching and selecting the charging element 4 so as to supply the charging voltage Vc by the charging element 4 when being turned on. In the present embodiment, the discriminator 6 widely includes a designation of the user, a method of detecting the recording medium S during traveling, and the like. In the control method of this aspect, for example, the control device 8 recognizes the discrimination signal from the discriminator 6, and the control device 8 sends the control signal for switching to the switch 7 based on the recognition result. The ones that

実 施 Embodiment 1
Hereinafter, the present invention will be described in more detail based on the embodiments shown in the attached drawings.
FIG. 2 shows the entire configuration of the image forming apparatus according to the first embodiment.
-Overall configuration of image forming apparatus-
In the figure, the image forming apparatus 20 forms an image for forming a plurality of color components (white # 1, yellow, magenta, cyan, black, white # 2 in this embodiment) in the image forming apparatus casing 21. A belt-shaped intermediate transfer member 30 for sequentially transferring (primary transfer) and holding the forming portion 22 (specifically, 22a to 22f) and each color component image formed by each image forming portion 22; A secondary transfer device (collective transfer device) 50 for secondary transfer (batch transfer) of each color component image transferred onto the sheet S as a recording medium, and a fixing device for fixing the secondary transferred image on the sheet S And a sheet conveyance system 80 for conveying the sheet S to the secondary transfer area. In this example, white # 1 and white # 2 use white materials of the same color, but different white materials depending on whether they are located in the lower layer or the upper layer of the other color component image on the paper S Of course, it may be used. In addition, for example, a transparent material may be used instead of one white # 1.

-Image formation unit-
In the present embodiment, each of the image forming portions 22 (specifically, 22a to 22f) has a drum-shaped photosensitive member 23, and a corotron in which the photosensitive member 23 is charged around the photosensitive member 23. And a charging device 24 such as a transfer roll, an exposure device 25 such as a laser scanning device in which an electrostatic latent image is written on the charged photosensitive member 23, an electrostatic latent image written on the photosensitive member 23 to each color component toner Developing device 26, a primary transfer device 27 such as a transfer roll where the toner image on the photosensitive member 23 is transferred to the intermediate transfer member 30, and a photosensitive member cleaning device 28 where residual toner on the photosensitive member 23 is removed. Each one is provided.
Further, the intermediate transfer member 30 is stretched around a plurality (three in the present embodiment) of tension rolls 31 to 33. For example, a drive roll in which the tension roll 31 is driven by a drive motor not shown. It is used as a circular movement by the drive roll concerned. Further, an intermediate transfer member cleaning device 35 for removing residual toner on the intermediate transfer member 30 after the secondary transfer is provided between the tension rolls 31 and 33.

-Secondary transfer device (batch transfer device)-
Furthermore, as shown in FIGS. 2 and 3, the secondary transfer device (collective transfer device) 50 places the transfer roll 55 in pressure contact with the portion of the intermediate transfer body 30 facing the tension roll 33, and The thirty tension rolls 33 are used as the opposing roll 56 that forms the opposing electrode of the transfer roll 55. Here, in this example, the transfer roll 55 has a configuration in which a metal shaft is covered with an elastic layer in which foamed urethane rubber or EPDM is compounded with carbon black or the like, and the transfer roll 55 and the opposing roll 56 The nip area of the intermediate transfer member 30 nipped between them functions as a secondary transfer area (collective transfer area) TR.
Further, the transfer bias _VTR from the transfer power source 60 is applied to the facing roll 56 (in this example, also used as the tension roll 33) via the conductive feed roll 57, and between the transfer roll 55 and the facing roll 56. The predetermined transfer electric field is formed on the
In the present embodiment, the secondary transfer device 50 is a mode in which the transfer roller 55 is disposed in pressure contact with the intermediate transfer member 30. However, the present invention is not limited to this. As a matter of course, a belt transfer module or the like may be used to bridge the tension between the tension rolls.

-Fixing device-
As shown in FIG. 2, the fixing device 70 is in pressure contact with the heat fixing roll 71, which can be driven and rotated, disposed in contact with the image holding surface side of the sheet, and the heat fixing roll 71. A pressure fixing roll 72 rotating following the roll 71, passing an image held on the sheet to a transfer area between the fixing rolls 71 and 72, and heating and fixing the image is there.

-Paper transport system-
Further, as shown in FIGS. 2 and 3, the sheet conveyance system 80 has a plurality of (in this example, two) sheet supply containers 81 and 82, and is supplied from any of the sheet supply containers 81 and 82. Leading to the secondary transfer area TR through the horizontal conveyance path 84 extending in the substantially horizontal direction from the vertical conveyance path 83 extending substantially in the vertical direction, and thereafter conveying the sheet S on which the transferred image G is held, The sheet reaches a fixing portion by the fixing device 70 via the belt 85 and is discharged to a sheet discharge receiver 86 provided on the side of the image forming apparatus casing 21.
Further, the sheet conveyance system 80 further includes a reversible branch conveyance path 87 branched downward from a portion of the horizontal conveyance path 84 located on the downstream side of the fixing device 70 in the sheet conveyance direction, and the branch conveyance path The sheet S reversed at 87 is returned through the conveying path 88 and returned again from the vertical conveying path 83 to the horizontal conveying path 84, and the image G is transferred to the back surface of the sheet S in the secondary transfer area TR to fix the fixing device 70. Then, the sheet is discharged to the sheet discharge tray 86.
Further, the sheet conveyance system 80 is provided with an appropriate number of conveyance rolls 91 in each conveyance path 83, 84, 87, 88, in addition to the alignment rolls 90 which align the sheet S and supply the sheet S to the secondary transfer area TR. It is done.
Furthermore, on the opposite side of the paper discharge receptacle 86 of the image forming apparatus casing 21, a manual paper feeding device 92 capable of feeding manual paper toward the horizontal conveyance path 84 is provided.

Further, on the entrance side of the secondary transfer area TR of the horizontal conveyance path 84, a guide chute 93 for guiding the sheet S having passed through the alignment roll 90 to the secondary transfer area TR is provided. In this example, one guide chute 93 is provided between the alignment roll 90 and the secondary transfer area TR, and the sheet ch is guided by arranging a pair of metal chute members facing each other. It is designed to regulate the trajectory.
In the present embodiment, one guide chute 93 is provided between the alignment roll 90 and the secondary transfer area TR, but the invention is not limited to this, and a plurality (for example, two) may be provided. In the case where a plurality of guide chutes 93 are provided, it is possible to arrange them in different inclined postures, and the degree of freedom in adjusting the guide path of the sheet S is increased.

-Paper type-
Examples of the paper S usable in this embodiment include, for example, plain paper having a surface resistance of 10 ¹⁰ to 10 ¹² Ω / □, and high-resistance paper having a surface resistance higher than that of plain paper.
Here, there is a plastic film such as a PET film as a typical aspect of the high resistance paper, and in the present embodiment, an application target may be selected in advance for the threshold value of resistance for the high resistance paper. Although not present, in the present embodiment, a sheet having a surface resistance of 10 ¹⁵ Ω / □ or more is applied as a high resistance sheet.
It is possible to directly form a color image consisting of, for example, YMCK (yellow, magenta, cyan, black) on this type of high-resistance paper, but as shown in FIG. 4A, for example, on high-resistance paper. for example by using the image forming unit 22f shown in FIG. 2 to form a white image G _W of a background image by white (white) W, using the image forming unit 22b~22e shown in FIG. 2 on a white image G _W Color image G _YMCK may be formed by _YMCK , or, as shown in FIG. 4B, YMCK using image forming units 22b to 22e shown in FIG. to form a color image _{G YMCK} by, so as to form a white image _{G W} by white (white) W by using the image forming unit 22a shown in FIG. 2 on the color image _{G YMCK} It may be.
In particular, although the white image G _W by White (white) W would use the charged imaging particles, such as white toner, white toners often have the colorant of the metal, this type of white toner Since the conductivity is high and it is hard to be charged, and the component adhesion to the photosensitive member 23 and the intermediate transfer member 30 is weak, it is necessary to take care of handling at the time of imaging taking this into consideration.

-Configuration example of discriminator-
In this example, as shown in FIG. 3, a discriminator 110 for discriminating the sheet type is provided in a part of the vertical conveyance path 83 or the horizontal conveyance path 84 of the sheet conveyance system 80. For example, as shown in FIG. 4C, the discriminator 110 has a pair of discrimination rolls 111 and 112 arranged in parallel along the conveyance direction of the sheet S, and is disposed on the upstream side of the conveyance direction of the sheet S. The discrimination power supply 113 is connected to one of the discrimination rolls 111, and the other is grounded via the resistor 114, and between one of the discrimination rolls 112 of the pair configuration located on the downstream side in the conveyance direction of the sheet S and the ground. The ammeter 115 is provided to the Note that, as the determination rolls 111 and 112, the conveyance member (the alignment roll 90 and the conveyance roll 91) of the sheet S may be combined, or may be provided separately from the conveyance members.

In this example, assuming that plain paper is used as the paper S, for example, the surface resistance of the plain paper is about 10 ¹⁰ to 10 ¹² Ω / □. In the case where the discrimination current from the discrimination power supply 113 is a component that flows so as to cross the discrimination roll 111 of the pair configuration and a component that travels through the sheet S and reaches the ammeter 115 on the discrimination roll 112 side. It divides and flows.
On the other hand, assuming that a high resistance sheet having a surface resistance of 10 ¹⁵ Ω / □ or more is used as the sheet S, the surface resistance of the high resistance sheet is larger than that of the plain sheet. , 112, the discriminating current from the discriminating power supply 113 is reduced by the impedance and flows across the discriminating roll 111 of the pair configuration, but the sheet S is transmitted along the sheet S. It hardly reaches the ammeter 115 on the determination roll 112 side. As a result, the surface resistance of the sheet S is calculated by the measurement current measured by the ammeter 115 and the applied voltage of the discrimination power supply 113, and the sheet type is discriminated.
In this embodiment, the discriminator 110 discriminates the sheet type by measuring the surface resistance of the sheet S being conveyed, but for example, based on a designation signal when the user designates the sheet type to be used. The type of sheet may be determined.

-Contact member with the paper positioned on the inlet side of the secondary transfer area-
In the present embodiment, as the contact member with the sheet S positioned on the inlet side of the secondary transfer area TR, as shown in FIGS. 3 and 5, the guide chute 93 is provided in order from the side close to the secondary transfer area TR. There are an alignment roll 90 and a transport roll 91.
In the present example, the alignment roll 90 is constituted by a pair of metal roll members and is directly grounded. Further, the guide chute 93 has a metal chute member grounded via a resistor 94, and the resistor 94 in this example is selected to be higher than the resistance value of the transfer roll 55 (volume resistivity in this example). There is. In this example, the resistance 94 is selected in contrast to the resistance value of the transfer roll 55, but when the secondary transfer device 50 uses, for example, a belt transfer module, the resistance to the ground of the belt transfer module is reached. It may be selected in comparison with the value.
Here, the reason that the alignment roll 90 is directly grounded is to prevent the situation where the alignment roll 90 is charged up when it is charged due to rubbing when the sheet S passes. The guide chute 93 is grounded via the resistor 94 in order to prevent leakage of part of the transfer current in the secondary transfer area TR.
Although the alignment roll 90 is directly grounded in this embodiment, it is needless to say that the alignment roll 90 may be grounded appropriately through a resistor. In addition, although the guide chute 93 adopts a mode of grounding via the resistor 94, it is also possible to adopt a mode of direct grounding, and in a mode in which the guide chute 93 is divided into a plurality and used. For this, either one may be selected as appropriate, such as direct grounding.
Further, in the present embodiment, the conveyance roll 91 is provided at an adjacent location on the upstream side of the conveyance direction of the sheet S with respect to the alignment roll 90, and the sheet conveyance path length between the alignment roll 90 and the conveyance roll 91 The d is set shorter than the transport direction length ds of the sheet S.

-Charging mechanism-
In the present embodiment, as shown in FIGS. 3 and 5, a charging mechanism 130 for charging the alignment roll 90 and the guide chute 93 is provided.
In the present embodiment, the charging mechanism 130 is directly grounded to the charging power source 131 for applying a charging voltage Vc including at least a DC component having the same polarity as the toner (in the present embodiment, negative polarity). The first changeover switch 132 as a switch for switching whether to apply the charging voltage Vc from the charging power supply 131 and grounding the guide chute 93 via the resistor 94 or the charging voltage from the charging power supply 131 And a second changeover switch 133 as a changeover switch for switching whether to apply Vc.
In this example, the charging power source 131 is shared with the alignment roll 90 and the guide chute 93. However, it is needless to say that the charging power source 131 may be separately provided.

<Method of selecting charging voltage>
Here, supplementing to the method of selecting the charging voltage Vc of the charging power source 131, as shown in FIG. 5, for example, discharge can be started at the minute gap g between the metal roll member of the alignment roll 90 and the sheet S. A voltage higher than the discharge start voltage Vs may be selected as the charging voltage Vc.
Further, as shown in FIG. 3, when the sheet S is determined to be a high resistance sheet by the discriminator 110, the first and second changeover switches 132 and 133 receive the discrimination signal, and A switching signal is sent to the first and second changeover switches 132 and 133, and the charging voltage Vc is applied to the alignment roll 90 and the guide chute 93 by switching between the changeover switches 132 and 133.

<Method of selecting resistance of transport roll>
Furthermore, in the present embodiment, the transport roll 91 disposed adjacent to the upstream side of the sheet S in the transport direction with respect to the alignment roll 90 is grounded via the resistor 95. Here, as the resistance value of the resistor 95, for example, when the sheet S is determined to be a high-resistance sheet, the charging voltage Vc is applied to the alignment roll 90 and the guide chute 93, and the high-resistance sheet is charged negatively. However, since the high resistance sheet is disposed straddling between the alignment roll 90 and the conveyance roll 91, the high resistance sheet is charged when the conveyance roll 91 is directly grounded. Even if that is the case, there is a high possibility that the charge will leak through the path from the transport roll 91 to the ground. Therefore, from the viewpoint of minimizing the leakage current, the resistance 95 of the transport roll 91 is selected to have a high resistance value so that the leakage current falls within the allowable level.

-Drive control system of image forming apparatus-
In the present embodiment, as shown in FIG. 3, reference numeral 120 is a control device for controlling the image forming process of the image forming apparatus, and this control device 120 is a microcomputer including a CPU, a ROM, a RAM and an input / output interface. Switch signals such as a start switch (not shown) and a mode selection switch for selecting an image forming mode through an input / output interface, and various sensor signals, and further, a sheet discrimination signal from the discriminator 110 that discriminates a sheet type The CPU executes various image forming control programs (see FIG. 6) stored in advance in the ROM and generates control signals for the drive control object, and then the drive control objects (switching switches 132 and 133). Etc.) are sent out.

-Operation of the image forming apparatus-
Now, in the image forming apparatus shown in FIG. 2, assuming that sheets S having different surface resistances are used together, as shown in FIG. 6, the image forming apparatus is turned on by operating the start switch (not shown). Printing (image formation processing) is started.
At this time, the sheet S is supplied from any of the sheet supply containers 81 and 82 or the manual sheet supply device 92, and is conveyed toward the secondary transfer area TR through a predetermined conveyance path, but is transferred to the secondary transfer area TR. In the middle of conveyance before reaching, measurement of the surface resistance of the sheet S by the discriminator 110 (discrimination processing of the sheet type) is performed.
The control device 120 determines whether or not the sheet S is a high resistance sheet based on the determination result of the discriminator 110. If the sheet S is a high resistance sheet, the changeover switches 132 and 133 are switched to align the registration roll 90 and the guide chute. The charging voltage Vc is applied to 93.
On the other hand, when the control device 120 determines that the sheet S is not a high resistance sheet, unlike the case of the high resistance sheet, the changeover switches 132 and 133 are switched to directly ground the alignment roll 90 and the guide chute 93 is resistance Ground via 94.
Thereafter, when the sheet S reaches the secondary transfer area TR, the image G formed on each of the image forming portions 22 (22a to 22f) and primarily transferred to the intermediate transfer member 30 is secondarily transferred to the sheet S, After being subjected to a fixing process by the fixing device 70, the sheet is discharged to the sheet discharge receiver 86, and a series of printing (image forming process) is completed.

-Secondary transfer operation process-
<Plain paper>
Now, when the sheet S is the plain sheet Sa, as shown in FIG. 3, FIG. 6 and FIG. 7A, the alignment roll 90 is directly grounded by the changeover switches 132 and 133, and the guide chute 93 is resisted. In this state, the transfer voltage _VTR by the transfer power supply 60 is applied from the feed roll 57 to the opposing roll 56 in this state, and the transfer electric field acts from the intermediate transfer member 30 side. The transfer current flows to the transfer roll 55 side.
In this state, the plain paper Sa passes through the alignment roll 90 and the guide chute 93 and reaches the secondary transfer area TR, and the image G on the intermediate transfer member 30 is secondarily transferred onto the sheet S in the secondary transfer area TR. . At this time, since the plain paper Sa contacts the alignment roll 90 and the guide chute 93 before reaching the secondary transfer area TR, the plain paper Sa may be partially charged due to the rubbing of both, but it is normal. Since the surface resistance of the paper Sa is smaller than that of the high resistance paper St, the charge on the plain paper Sa is removed through the path to the ground of the alignment roll 90 or the path to the ground of the guide chute 93 . Therefore, even if the plain paper Sa rushes into the secondary transfer area TR, it hardly reaches the secondary transfer area TR in a state where the plain paper Sa is partially charged in the reverse polarity to the toner, and the secondary transfer area There is no concern that the image G by the toner on the intermediate transfer member 30 is transferred to the side of the plain paper Sa before transfer near the entrance of the TR.
Therefore, the transfer operation to the plain paper Sa in the secondary transfer area TR is stably performed, and there is little concern that a transfer failure such as an image omission or an image contamination may occur.

<High resistance paper>
Next, the case where the sheet S is a high resistance sheet (for example, a plastic film) St will be described.
In this case, as shown in FIG. 3, FIG. 6 and FIG. 7B, the control device 120 switches the changeover switches 132 and 133 to charge the registration roller 90 and the guide chute 93 with the charging voltage Vc of the same polarity as the toner. In this state, the transfer voltage _V.sub.TR by the transfer power source 60 is applied from the feed roll 57 to the opposing roll 56 in the secondary transfer area TR, and a transfer electric field acts from the intermediate transfer member 30 side.
At this time, the high resistance sheet St passes through the transport roll 91, the alignment roll 90, and the guide chute 93, reaches the secondary transfer area TR, passes through the secondary transfer area TR, and is transported to the transport belt 85 side. .
Now, at the stage before the high resistance sheet St passes through the transport roll 91 and reaches the alignment roll 90, the high resistance sheet St is transferred to the transport roll 91 (see FIG. 5) as shown in FIG. Since partial charging is achieved by rubbing against a guide chute (not shown), it is presumed that positive (+) and negative (-) charges are mixed on the surface of the high-resistance sheet St.
In FIG. 8A, the changeover switches 132 and 133 (see FIG. 7) of the charging mechanism 130 are omitted.

Then, when the high resistance sheet St reaches the alignment roll 90 and further passes through the alignment roll 90 to reach the guide chute 93, a charging voltage Vc having the same polarity as that of the toner is applied to the alignment roll 90 and the guide chute 93. As shown in FIG. 8A, the high-resistance sheet St in which positive and negative charges (+,-) are mixed is a charge of the same polarity as the toner (in this example, negative). It is uniformly charged to (-).
At this time, since the transport direction length ds of the high resistance sheet St is in a dimensional relationship longer than the sheet transport path length d between the alignment roll 90 and the transport roll 91, the high resistance sheet St is aligned roll 90. Even if there is a state of contact across the transport roller 91 (see FIG. 3 and FIG. 5) at a position adjacent to the alignment roller 90, the transport roller 91 has a high resistance value 95 Because it is grounded, the negative charge (-) of the high resistance sheet St does not easily flow through the path leading to the ground of the transport roll 91, and the charged state by the negative charge (-) on the high resistance sheet St is impaired There are few concerns.

After this, the leading edge of the high resistance sheet St rushes into the secondary transfer area TR, but when the high resistance sheet St rushes into the secondary transfer area TR, it is near the entrance of the secondary transfer area TR. Even if the high resistance sheet St and the image G by the negative polarity toner on the intermediate transfer member 30 are disposed close to each other, the image G (-) by the toner on the intermediate transfer member 30 is charged on the high resistance sheet St (-) The image G (-) by the toner on the intermediate transfer member 30 is pushed to the secondary transfer area TR side, that is, the side of the high There is no concern to move to In particular, in the case of using a highly conductive charged imaging material such as white toner, it is difficult to be charged and electrostatic adhesion of the toner is weak, but a charged charge of the same polarity as the toner on the high resistance paper St side (- ), So that the white toner is effectively prevented from peeling off.
Therefore, a part of the image G (-) by the toner on the intermediate transfer member 30 does not scatter to the high resistance sheet St side near the entrance of the secondary transfer area TR, and a transfer failure such as an image omission or an image contamination There is no concern that

形態 Form of comparison 1
Next, in order to evaluate the transfer operation performance in the secondary transfer portion of the image forming apparatus according to the present embodiment, the toner at the time of the secondary transfer operation around the secondary transfer portion of the image forming apparatus according to the comparison form 1 The behavior of the image G according to.
The structure around the secondary transfer portion in the present comparative embodiment is provided with a charging mechanism 140 for charging the alignment roll 90 and the guide chute 93 substantially as in the first embodiment, as shown in FIG. 8B. However, unlike the first embodiment, the charging mechanism 140 of this example applies the charging voltage Vc 'of the opposite polarity to the toner to the alignment roll 90 and the guide chute 93 when the high resistance sheet St is used. A possible charging power supply 141 is connected. Also in the present comparative embodiment, the changeover switches corresponding to the changeover switches 132 and 133 similar to those of the first embodiment are provided, but these changeover switches are omitted in FIG.
In the form of this comparison, the high-resistance sheet St is charged with positive and negative charges (+, −, etc.) by rubbing with the transport roll 91 and other guide chutes not shown before reaching the alignment roll 90. However, when the high resistance paper St reaches the alignment roll 90 and passes through the alignment roll 90 to reach the guide chute 93, the alignment roll 90 and Since a charging voltage Vc 'having a polarity opposite to that of the toner is applied to the guide chute 93, as shown in FIG. 8B, the high resistance sheet St in which positive and negative charges (+,-) are mixed is obtained. Is uniformly charged to a charge (+) of the opposite polarity (positive in this example) to the toner.

  After that, when the high resistance sheet St rushes into the secondary transfer area TR, the high resistance sheet St and the image G by the negative polarity toner on the intermediate transfer member 30 are disposed close to each other near the entrance of the secondary transfer area TR. Then, since the image G (-) by the toner on the intermediate transfer member 30 and the charge (+) on the high-resistance sheet St are opposite in polarity, the image G (-) by the toner on the intermediate transfer member 30 Is attracted to the charge (+) on the high resistance sheet St, that is, attracted to the high resistance sheet St side, and a part of the image G (−) by the toner on the intermediate transfer member 30 Before rushing into the secondary transfer area TR, it tends to scatter on the high resistance sheet St side. Therefore, in the form of this comparison, a part of the image G (-) by the toner on the intermediate transfer member 30 scatters to the high resistance sheet St side near the entrance of the secondary transfer area TR, and the image is missing There is a concern that transfer defects such as image contamination may occur.

In the present embodiment, the charging mechanism 130 uses a mode in which the charging voltage Vc is applied to both the alignment roll 90 and the guide chute 93. However, the charging mechanism 130 is not necessarily limited to this. For example, FIG. The charging mechanism 130 according to the first and second modified embodiments as shown in (b) may be employed.
形態 Modification 1
In FIG. 9A, the charging mechanism 130 is obtained by adding the charging mechanism 130 only to the guide chute 93 at a position closest to the secondary transfer area TR on the entrance side of the secondary transfer area TR. In this example, the charging mechanism 130 switches between the charging power source 131 for applying the charging voltage Vc and the conduction path from the guiding chute 93 to the ground via the resistor 94 with the changeover switch 135 to select the high resistance sheet St. The charging voltage Vc is applied to the guide chute 93 when it is used.
And, in this example, the alignment roll 90 adjacent to the guide chute 93 is grounded via the resistance 96, and the transport roll 91 adjacent to the alignment roll 90 is the resistance 95 as in the first embodiment. It is grounded through. Here, as the resistor 96, one having a high resistance value to the extent that the charges charged on the high resistance sheet St are unlikely to leak is used.

  According to this modification, the high-resistance sheet St travels toward the secondary transfer area TR while contacting the guide chute 93 before reaching the secondary transfer area TR. Since the high-resistance sheet St rushes into the secondary transfer area TR because it is uniformly charged to the charge (−) having the same polarity as that of the toner at the passing stage, the intermediate transfer member 30 is formed as in the first embodiment. The image G with toner on the intermediate transfer member 30 is charged with the charge (-) of the high resistance sheet St, even if the image G with toner is arranged close to the high resistance sheet St near the entrance of the secondary transfer area TR. The secondary transfer area TR is reached while being pressed against the intermediate transfer body 30 by repulsion. Therefore, the image G by the toner on the intermediate transfer member 30 is not scattered to the high resistance sheet St side near the entrance of the secondary transfer area TR, and is transferred to the high resistance sheet St after reaching the secondary transfer area TR. The operation is performed stably.

形態 Modification 2
In FIG. 9B, the charging mechanism 130 is obtained by adding the charging mechanism 130 to the alignment roll 90 positioned on the inlet side of the secondary transfer area TR. In this example, when the charging mechanism 130 switches and selects the charging power source 131 for applying the charging voltage Vc and the conduction path from the alignment roll 90 to the direct grounding by the changeover switch 136, and uses the high resistance paper St. The charging voltage Vc is applied to the guide chute 93.
And in this example, the guide chute 93 located in the immediate vicinity of the entrance side of the secondary transfer area TR is grounded via the resistor 94, but the contact surface of the guide chute 93 with the high resistance paper St, that is, metal A covering layer 137 made of an antistatic film for preventing charging due to rubbing with the high resistance sheet St is formed on the opposing surfaces of the chute member.
The transport roll 91 adjacent to the alignment roll 90 is grounded via the resistor 95 as in the first embodiment.

According to this modification, since the charging voltage Vc is applied to the alignment roll 90 when the high-resistance sheet St passes through the alignment roll 90 before reaching the secondary transfer area TR, the high-resistance sheet St is high. The resistance sheet St is uniformly charged to a charge (−) of the same polarity as that of the toner when it passes through the registration roll 90. Thereafter, the high resistance sheet St travels to the secondary transfer area TR via the guide chute 93 to which the charging voltage Vc is not applied, but the contact surface of the guide chute 93 with the high resistance sheet St is from the antistatic film. Since the covering layer 137 is formed, the uniform chargeability due to the charge of the same polarity as the toner of the high-resistance sheet St is hardly impaired, and the high-resistance sheet St reaches the secondary transfer area TR.
Then, when the high resistance sheet St rushes into the secondary transfer area TR, as in the first embodiment, the image G with the toner on the intermediate transfer member 30 is in the vicinity of the entrance of the secondary transfer area TR. The image G by the toner on the intermediate transfer member 30 is pressed to the intermediate transfer member 30 side by repulsion with the charged charge (−) of the high resistance paper St, and the image G is transferred to the secondary transfer region TR. Through. Therefore, the image G by the toner on the intermediate transfer member 30 is not scattered to the high resistance sheet St side near the entrance of the secondary transfer area TR, and is transferred to the high resistance sheet St after reaching the secondary transfer area TR. The operation is performed stably.

  DESCRIPTION OF SYMBOLS 1 image carrier 2 transfer device 2a transfer member 2b opposing member 2c transfer power source 3 contact member 3a alignment conveyance member 3b guide member 3c conveyance member 4 Charging element, 5 ... high resistance, 6 ... discriminator, 7 ... switching device, 8 ... control device, G ... image, S ... recording medium, TR ... transfer area

Claims (10)

An image carrier which movably holds an image formed by charged imaging particles;
The image carrier has a transfer member disposed in contact with the image holding surface of the image carrier, and the recording medium is nipped and conveyed by the image carrier and the transfer member, and the image carrier and the transfer member A transfer device for electrostatically transferring the image held by the image carrier onto the recording medium by applying a predetermined transfer voltage to the transfer area between them;
A contact member provided upstream of the transfer area of the transfer device in the conveyance direction of the recording medium and in contact with the recording medium;
At least one of the contact members of the contact member positioned in the immediate vicinity of the transfer region of the transfer device so that the recording medium maintains a charged state of the same polarity as the charged image forming particles and reaches the transfer region. And a charging element for supplying a charging voltage of the same polarity as the charging image forming particles.
An image forming apparatus comprising: In the image forming apparatus according to claim 1,
The contact member includes an alignment conveyance member for aligning and conveying the recording medium,
The image forming apparatus, wherein the charging element is provided at least on the alignment conveyance member. In the image forming apparatus according to claim 1,
The contact member includes a guide member for guiding the recording medium toward the transfer area,
The image forming apparatus, wherein the charging element is provided at least on the guide member. In the image forming apparatus according to claim 1,
The contact member includes an alignment conveyance member for aligning and conveying the recording medium, and a guide member provided between the alignment conveyance member and the transfer area and guiding the recording medium toward the transfer area. ,
The image forming apparatus, wherein the charging element is provided on the registration conveyance member and the guide member. The image forming apparatus according to any one of claims 1 to 4.
An image forming apparatus, wherein a charging voltage by the charging element is equal to or higher than a discharge start voltage at which a discharge is started between a contact member to which the charging voltage is supplied and the recording medium. The image forming apparatus according to any one of claims 1 to 5,
1. The image forming apparatus according to claim 1, wherein one or more contact members positioned on the upstream side of the recording medium in the conveyance direction with respect to the contact member on which the charging element is provided are grounded via a predetermined high resistance. In the image forming apparatus according to claim 6,
2. The image forming apparatus according to claim 1, wherein the distance between the contact member to which the high resistance is grounded and the contact member provided with the charging element is within the range of the transport direction length of the recording medium. The image forming apparatus according to any one of claims 1 to 7.
An image forming apparatus characterized in that the charging element is operated to supply a charging voltage when the recording medium has a high resistance equal to or higher than a predetermined resistance value. In the image forming apparatus according to claim 8,
A discriminator that determines whether the recording medium has a high resistance equal to or higher than a predetermined resistance value;
And a switch for switching and selecting the charging element so as to supply the charging voltage by the charging element when the discriminator determines that the recording medium has high resistance. Forming device. The image forming apparatus according to any one of claims 1 to 9.
The image carrier is an intermediate transfer member which transfers and holds the image on the image forming holder intermediately before transferring the image onto the recording medium, and the transfer device transfers the image on the intermediate transfer member to the recording medium An image forming apparatus characterized by transferring.

Images