JP6984337B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus.

Conventionally, as an image forming apparatus of this kind, for example, those described in Patent Documents 1 to 3 are already known.
In Patent Document 1, in order to prevent the occurrence of point-like transfer omission even for recording materials of different sizes, the recording material is sandwiched by the rollers of the adsorption charger before it rushes into the transfer portion, and the rollers are connected from the high voltage power supply. Disclosed is an image forming apparatus that detects the width of the recording material in advance and controls the output of the high-voltage power supply according to the width of the recording material when the recording material is backcharged by the bias applied to the recording material.
In Patent Document 2, the tip of the recording medium is grounded in advance in order to obtain stable image quality performance by correcting and equalizing the decrease in the electrostatic binding force between the paper and the toner image in the rear end region of the paper. When the toner image reaches the fixing means, the transfer charge of the recording medium is easily released through the fixing means. Therefore, when the toner image is transferred to the region on the rear end side of the predetermined position on the recording medium, the transfer means is used. An image forming apparatus including a transfer voltage control means for increasing an applied voltage is disclosed.
In Patent Document 3, the surface of the transfer material guide member that comes into contact with the paper is formed of a conductive member, and the conductive member has environmental characteristics that contradict the resistance of the paper, that is, a feeling that the resistance increases according to the amount of moisture absorbed. It is grounded via a wet resistor, which allows the sum of the resistance component of the paper that is present between the transfer site and the transfer material guide member and the resistance component of the transfer material guide member that is in contact with the paper, regardless of the environment. An image forming apparatus is disclosed in which the amount of transfer current leaking to the transfer material guide member side is made constant and the environment dependence is eliminated.

Japanese Unexamined Patent Publication No. 2000-162897 (Embodiments of the Invention, FIG. 1) Japanese Unexamined Patent Publication No. 10-97147 (Embodiments of the Invention, FIG. 9) Japanese Unexamined Patent Publication No. 6-124004 (Example, FIG. 1)

The technical problem to be solved by the present invention is that even if different types of recording media pass through the transfer area of the image holder, the grounding condition of the guide member installed on the inlet side of the transfer area is uniformly set. The purpose is to suppress uneven density of the transferred image in the transfer region.

The invention according to claim 1 has an image holder that holds and moves an image, and a transfer member that is arranged in contact with the image holding surface of the image holder, and the image holder and the transfer member. A transfer electric voltage is applied to the transfer region between the image holder and the transfer member by applying a transfer voltage from the side opposite to the image holding surface of the image holder while sandwiching and transporting the recording medium. A transfer device for electrostatically transferring the image held by the image holder to the recording medium and a recording medium provided in a state of being grounded to the inlet side of the transfer area of the transfer device and directed toward the transfer area. The guide member for guiding the guide member, the discriminator for discriminating the type of the recording medium traveling toward the transfer region, and the recording medium discriminated by the discriminator have a predetermined surface resistance value or less or the surface of the medium base material. Under the condition of low resistance having a conductive layer along the above, a switch for switching to low resistance grounding having a resistance value lower than the resistance value of high resistance grounding predetermined as the grounding condition of the guide member is provided. It is a characteristic image forming apparatus.

The invention according to claim 2 is an image forming apparatus according to claim 1, wherein the switching device employs a high resistance grounding using a high resistance larger than the resistance value of the transfer member. Is.
The invention according to claim 3 is an image forming apparatus according to claim 2, wherein the switching device includes a low resistance grounding having a resistance value of 0.
The invention according to claim 4 is a condition in which in the image forming apparatus according to any one of claims 1 to 3, the discriminator discriminates a recording medium having a high insulating property having a resistance value or more predetermined as a type of recording medium. Then, the switch is an image forming apparatus characterized by switching to bias grounding via static elimination bias instead of the high resistance grounding.
The invention according to claim 5 is the image forming apparatus according to any one of claims 1 to 4, wherein the switching device automatically switches the grounding condition based on the discrimination signal of the type of the recording medium by the discriminator. It is a characteristic image forming apparatus.
The invention according to claim 6 is an image forming apparatus according to any one of claims 1 to 5, wherein the switching device uses a variable resistor whose resistance value changes.
The invention according to claim 7 is the type of recording medium in the image forming apparatus according to any one of claims 1 to 6, wherein the discriminator is based on a detection signal from a detector that detects the surface resistance of the traveling recording medium. It is an image forming apparatus characterized by discriminating.
The invention according to claim 8 has an image holder that holds and moves an image, and a transfer member that is arranged in contact with the image holding surface of the image holder, and the image holder and the transfer member. In addition to sandwiching and transporting the recording medium, a transfer electric field is applied to the transfer region between the image holder and the transfer member to electrostatically transfer the image held by the image holder to the recording medium. The type of the transfer device, the guide member provided in a state of being grounded to the inlet side of the transfer area of the transfer device and guiding the recording medium toward the transfer area, and the recording medium traveling toward the transfer area. Under the condition that the discriminator for discriminating and the recording medium discriminated by the discriminator have a predetermined surface resistance value or less or a low resistance having a conductive layer along the surface of the medium base material, the grounding condition of the guide member is used. A switch for switching to a low resistance grounding having a resistance value lower than a predetermined high resistance grounding resistance value is provided, and the guide member includes a first guide member provided at a portion away from the transfer region. and a second guide member provided between said first guide member and said transfer zone, together with the the second guide member adds the switch which is switched ground conditions, the first The image forming apparatus is characterized in that the guide member of the above second guide member has a low resistance grounding lower than the resistance value of the high resistance grounding of the second guide member.
The invention according to claim 9 is the image forming apparatus according to any one of claims 1 to 8, wherein the transfer apparatus has a recording medium determined by the discriminator having a surface resistance value or less or a medium group determined in advance. The image forming apparatus is characterized by having a constant current control unit that controls the transfer current supplied to the transfer region to a constant current under the condition of low resistance having a conductive layer along the material surface.
The invention according to claim 10 is the image forming apparatus according to any one of claims 1 to 9, wherein the image holding body is intermediately transferred before transferring the image on the image forming holding body to a recording medium. It is an intermediate transfer body to be held, and the transfer device is an image forming device characterized in that an image on the intermediate transfer body is transferred to a recording medium.

The invention according to claim 11 has an image holder that holds and moves an image, and a transfer member that is arranged in contact with the image holding surface of the image holder, and the image holder and the transfer member. By sandwiching and transporting the recording medium and applying a transfer voltage from the side opposite to the image holding surface of the image holder , a transfer electric field is applied to the transfer region between the image holder and the transfer member. A transfer device that acts to electrostatically transfer the image held by the image holder to the recording medium, and a transfer device that is provided in a state of being grounded to the inlet side of the transfer area of the transfer device, and the recording toward the transfer area. The guide member that guides the medium, the discriminator that discriminates the type of the recording medium traveling toward the transfer region, and the recording medium discriminated by the discriminator have high insulation properties equal to or higher than a predetermined surface resistance value. In the condition of the recording medium, the image forming apparatus is provided with a switch for switching to bias grounding via static electricity elimination bias instead of the predetermined high resistance grounding as the grounding condition of the guide member. be.

According to the first aspect of the present invention, even if different types of recording media pass through the transfer area of the image holder, as compared with the case where the grounding condition of the guide member installed on the inlet side of the transfer area is uniformly set. Therefore, it is possible to suppress uneven density of the transferred image in the transfer region.
According to the invention of claim 2, when a high-resistance recording medium having a high surface resistance value is used, the guide member is connected to a guide member as compared with the case where a high-resistance grounding having a resistance value equal to or lower than that of the transfer member is adopted. It is possible to suppress the leakage of the transfer current and secure the transfer current to the transfer site.
According to the third aspect of the present invention, it is possible to suppress the density unevenness of the transferred image in the transfer region to be less than that in the case of adopting the low resistance grounding having a resistance value larger than 0.
According to the invention of claim 4, even if a recording medium having a particularly high insulating property passes through the transfer region of the image holder, the charging unevenness of the recording medium can be reduced before reaching the transfer region, and the transfer image in the transfer region. It is possible to suppress uneven concentration of.
According to the invention of claim 5, even if any kind of recording medium is used, the grounding condition of the guide member can be appropriately selected.
According to the invention of claim 6, switching between high resistance grounding and low resistance grounding can be easily realized.
According to the invention of claim 7, even if any kind of recording medium is used, recording media having different surface resistances can be discriminated during traveling.
According to the invention of claim 8, even if a recording medium having a low resistance is used and the rear end portion of the recording medium in the transport direction passes through the first guide member and reaches the second guide member, the transfer area is reached. It is possible to suppress uneven density of the transferred image in.
According to the invention of claim 9, the system resistance of the transfer region changes when the recording medium having low resistance passes through the guide member, as compared with the case where the transfer current supplied to the transfer region is controlled by a constant voltage. However, the transfer electric field acting on the transfer region can be secured.
According to the invention of claim 10, in the image forming apparatus of the intermediate transfer method, even if the recording medium having low resistance passes through the transfer region of the intermediate transfer body, it is possible to suppress the density unevenness of the transferred image in the transfer region. can.
According to the invention of claim 11, among the predetermined high resistance recording media, even if the recording medium having particularly high insulating property passes through the transfer region of the image holder, the recording medium is recorded before reaching the transfer region. It is possible to reduce the uneven charging of the image and suppress the uneven density of the transferred image in the transfer area.

It is explanatory drawing which shows the outline of embodiment of the image forming apparatus to which this invention was applied. It is explanatory drawing which shows the whole structure of the image forming apparatus which concerns on Embodiment 1. FIG. It is explanatory drawing which shows the detail of the structure around the secondary transfer part of the image forming apparatus shown in FIG. (A) is an explanatory diagram showing a configuration example 1 of a switch used in the image forming apparatus according to the first embodiment, (b) is an explanatory diagram showing the same configuration example 2, and (c) shows the same configuration example 3. It is explanatory drawing. (A) is an explanatory diagram showing an image forming example 1 on a low resistance paper by the image forming apparatus according to the first embodiment, (b) is an explanatory diagram showing the same image forming example 2, and (c) is shown in FIG. It is explanatory drawing which shows an example of a discriminator. It is a flowchart which shows the paper type image formation sequence used in the image forming apparatus which concerns on Embodiment 1. FIG. The transfer operation process for low-resistance paper in the secondary transfer unit by the image forming apparatus according to the first embodiment is schematically shown. (A) is before the paper rear end front stage guide chute passes, and (b) is the paper rear end front stage. After passing through the guide chute, (c) is an explanatory diagram showing each state at the time of passing through the secondary transfer area at the rear end of the paper. The transfer operation process for low-resistance paper in the secondary transfer unit by the image forming apparatus according to the first aspect of the comparison is schematically shown. After passing through the guide chute, (c) is an explanatory diagram showing each state at the time of passing through the secondary transfer area at the rear end of the paper. (A) is an explanatory diagram schematically showing the flow of the transfer current in the transfer operation process for the low resistance paper by the image forming apparatus according to the first embodiment, and (b) is the low resistance by the image forming apparatus according to the comparative embodiment 1. It is explanatory drawing which shows typically the flow of the transfer current of the transfer operation process with respect to the paper. It is explanatory drawing which shows the main part of the image forming apparatus which concerns on Embodiment 2. (A) is an explanatory diagram showing a configuration example 1 of a switch used in the image forming apparatus according to the second embodiment, and (b) is an explanatory diagram showing the same configuration example 2. It is a flowchart which shows the paper type image formation sequence used in the image forming apparatus which concerns on Embodiment 2. (A) is an explanatory diagram showing a transfer operation process for high-resistance paper by the image forming apparatus according to the second embodiment, and (b) is an explanatory diagram showing a transfer operation process for the high-resistance paper by the image forming apparatus according to the comparative embodiment 1. It is a figure. (A) is an explanatory diagram showing an example of a transfer current measurement circuit in the secondary transfer unit when low resistance paper passes through the transfer area of the secondary transfer unit in Example 1, and (b) is an explanatory diagram of the measurement circuit example. It is explanatory drawing which shows the change example of the transfer current, and the influence of image formation on a low resistance paper. (A) is an explanatory diagram showing an example of a transfer current measurement circuit in the secondary transfer section when low resistance paper passes through the transfer area of the secondary transfer section in Comparative Example 1, and (b) is an explanatory diagram of the measurement circuit example. It is explanatory drawing which shows the change example of the transfer current, and the influence of image formation on a low resistance paper.

(1) Outline of the embodiment FIG. 1 shows an outline of the embodiment of the image forming apparatus to which the present invention is applied.
In the figure, the image forming apparatus has an image holding body 1 that holds and moves the image G, and a transfer member 2a that is arranged in contact with the image holding surface of the image holding body 1, and the image holding body 1 and the image holding body 1. The recording medium S is sandwiched between the transfer member 2a and conveyed, and the transfer electric field is applied to the transfer region TR between the image holder 1 and the transfer member 2a to record the image G held by the image holder 1. A transfer device 2 for electrostatically transferring to the medium S, a guide member 3 provided in a state of being grounded to the inlet side of the transfer area TR of the transfer device 2 and guiding the recording medium S toward the transfer area TR, and a transfer area TR. The discriminator 4 for discriminating the type of the recording medium S traveling toward the ground and the recording medium S discriminated by the discriminator 4 have a conductive layer having a surface resistance value equal to or less than a predetermined value or along the surface of the medium base material. Under the low resistance condition, the resistance value is lower than the resistance value of _{the high resistance grounding (grounding via the high resistance RH} ) predetermined as the grounding condition of the guide member 3, and the low resistance grounding (via the _{low resistance RL).} It is provided with a switch 5 for switching to grounding).
In FIG. 1, the transfer device 2 installs the facing member 2b on the back surface of the image holder 1 facing the transfer member 2a, and applies a transfer voltage to the facing member 2b from the transfer power supply 2c to transfer the transfer region TR. It forms a transfer electric field for transferring the image G to the image G. Further, reference numeral 7 is a control device that sends out a control signal for switching the switching device 5 based on the discrimination signal from the discriminator 4.

In such technical means, the present embodiment is intended to improve the transfer performance of the low-resistance recording medium S, and the type of the recording medium S may be appropriately selected, but mainly. It is effective in adding a recording medium S having a low resistance such as metallic paper as a transfer target.
In this example, as long as the image holder 1 holds the image G, it includes not only the intermediate transfer body of the intermediate transfer method but also the photoconductor and the dielectric of the direct transfer method.
Further, although the guide member 3 is shown in FIG. 1 as being divided into a first guide member 3a and a second guide member 3b, the present invention is not limited to this, and the guide member 3 is not divided into two. It may be an embodiment divided into three or more. Here, among the guide members 3, the guide member (second guide member 3b in this example) close to the transfer region TR is, for example, a recording medium S having a high surface resistance value (low resistance recording medium). Even if plain paper with a surface resistance value of ^{about 10 10} to ¹² Ω is used other than S), the water content of the recording medium S becomes high in a high humidity environment, so that the transfer current flowing through the transfer region TR is a guide member. In order to prevent unnecessary leakage through 3 (second guide member 3b in this example), a structure in which the guide member 3 (second guide member 3b in this example) is grounded with high resistance is often adopted. Regarding the guide member (first guide member 3a in this example) located away from the transfer area TR, the surface resistance of the recording medium S increases by the distance from the transfer area TR, and the triboelectric charge during traveling is increased. From the viewpoint of suppressing the accumulation of electric charge due to the above, a structure with low resistance grounding is often adopted.

Further, the discriminator 4 is not limited to the mode of detecting and discriminating the surface resistance value of the running recording medium S, but may also be a mode in which the type of the recording medium S is sorted by a sorter or the like and discriminated based on this.
Then, as a discrimination condition by the discriminator 4, under the condition that the recording medium S has a low resistance, in addition to the one having a surface resistance value or less determined in advance, an embodiment having a conductive layer along the surface of the medium base material (corresponding to so-called metallic paper). ) Was included. The latter is described separately because the resistance value itself measured by the surface resistance measurement method in accordance with the JIS standard does not fall below the threshold level, but it is substantially when a high voltage such as a transfer voltage is applied. Due to the existence of something that acts as a low resistance.
Further, the switching device 5 may be switched manually or automatically. Further, the low resistance grounding may be via a resistance value lower than the predetermined resistance value of the high resistance grounding, and as will be described later, the aspect of the resistance value 0 is also included. The guide member 3 is usually paired to regulate the transport path of the recording medium S, but may be grounded to the paired guide member 3 via a resistor. It also includes an embodiment in which a part of the guide member 3 of the configuration is brought into contact with the guide member 3 and one of the guide members 3 is grounded via a resistor.

According to the present embodiment, when the discriminator 4 determines that the recording medium S has a low resistance, the switch 5 is a guide member 3 (a second guide member 3b close to the transfer region TR in FIG. 1). Switch from high resistance ground to low resistance ground. At this time, when the low-resistance recording medium S passing through the guide member 3 reaches the transfer region TR, a transfer electric field due to the transfer voltage from the transfer power supply 2c acts on the transfer region TR, but the transfer member 2a side. Since the low-resistance recording medium S and the low-resistance grounding resistance of the guide member 3 are smaller than the system resistance, the transfer current due to the transfer electric field does not flow to the transfer member 2a side, and the low-resistance recording medium S is energized. The current flows from the guide member 3 (second guide member 3b in FIG. 1) to the low resistance ground. Therefore, from the time when the front end of the recording medium S reaches the transfer area TR until the rear end of the recording medium S passes through the guide member 3 (the second guide member 3b in FIG. 1), the transfer area TR is set. The transfer current is stably supplied.
On the other hand, assuming, for example, a situation in which the low resistance recording medium S reaches the transfer area TR in a mode in which the second guide member 3b close to the transfer area TR of the guide member 3 remains grounded with high resistance, it is low. When the resistance recording medium S passes through the guide member 3 (the first guide member 3a in FIG. 1) located at a position away from the transfer region TR, the transfer current of the transfer region TR energizes the low resistance recording medium S. As a path, the current flows from the first guide member 3a to the low resistance ground, but when the rear end of the recording medium S passes through the first guide member 3a, the transfer current in the transfer region TR becomes the low resistance recording medium S. Is used as an energization path, and flows from the second guide member 3b to the energization path to the high resistance ground. At this time, due to the difference in grounding conditions between the first guide member 3a and the second guide member 3b, the transfer current in the transfer region TR changes at the stage when the low-resistance recording medium S passes through the first guide member 3a. Therefore, there is a concern that transfer defects are likely to occur near the rear end of the recording medium S.

Next, a typical aspect or a preferable aspect of the image forming apparatus according to the present embodiment will be described.
First, as a preferred embodiment of the switch 5, there is an embodiment in which a high resistance grounding using a high resistance larger than the resistance value of the transfer member 2a is adopted. In this example, assuming that a high-resistance recording medium S having a high surface resistance value (corresponding to a medium other than the low-resistance recording medium S) is used, the recording medium S is between the guide member 3 and the transfer region TR. Even if it is arranged so as to straddle, the transfer current of the transfer region TR rarely flows to the guide member 3 side using the recording medium S as an energization path, but for example, a high resistance grounding having a resistance value of the transfer member 2a or less is adopted. Assuming that, since it cannot be said that the system resistance is larger on the guide member 3 side than on the transfer member 2a side, a part of the transfer current of the transfer region TR is on the guide member 3 side via the recording medium S. However, in this embodiment, such leakage of the transfer current is less likely to occur, which is preferable.

Further, another preferred embodiment of the switch 5 is an embodiment in which a low resistance grounding having a resistance value of 0 is adopted. In this example, when the recording medium S having a low resistance is used, the transfer current in the transfer region TR is sufficiently secured because the system resistance due to the low resistance grounding can be suppressed.
Further, as another preferred embodiment of the switch 5, the switch 5 has a high resistance under the condition that the discriminator 4 discriminates a recording medium S having a high insulation value equal to or higher than a predetermined resistance value as the type of the recording medium S. An embodiment of switching to bias grounding via static elimination bias Bd instead of grounding can be mentioned. In this example, under the condition of the recording medium S having high insulation, bias grounding is added instead of high resistance grounding. Here, in the embodiment in which the paired guide member 3 (the second guide member 3b in this example) is bias-grounded, it is necessary to apply the static elimination bias Bd to one side and ground the other side, so that they do not contact each other. It may be arranged or a switch mechanism for releasing the contact state may be provided.

Further, as the automatic switching method of the switching device 5, the grounding condition may be automatically switched based on the discrimination signal of the type of the recording medium S by the discriminator 4.
Further, as a simple configuration example of the switch 5, a variable resistor whose resistance value changes may be used.
Furthermore, as a preferred embodiment of the discriminator 4, there is one that discriminates the type of the recording medium S based on the detection signal from the detector that detects the surface resistance of the traveling recording medium S.
Further, as a typical embodiment of the guide member 3, a first guide member 3a provided at a portion distant from the transfer area TR, and a second guide member 3a provided between the first guide member 3a and the transfer area TR. It has a guide member 3b, and a switch 5 for switching the grounding condition is added to the second guide member 3b, and the first guide member 3a is set from the resistance value of the high resistance grounding of the second guide member 3b. There is also an embodiment of low resistance grounding. In this example, in the embodiment in which the guide member 3 is divided into two elements, a switch 5 is added to the second guide member 3b side, and the first guide member 3a is grounded with low resistance.

Further, as a preferred embodiment of the transfer device 2, transfer is performed under the condition that the recording medium S determined by the discriminator 4 has a surface resistance value or less determined in advance or has a conductive layer along the surface of the medium substrate. An embodiment having a constant current control unit that controls the transfer current supplied to the region TR to a constant current can be mentioned. In this example, when the recording medium S has a low resistance, in addition to switching the grounding condition of the guide member 3, the transfer current supplied to the transfer region TR is controlled by a constant current. Therefore, in this example, when the low-resistance recording medium S passes through the guide member 3, the transfer current of the transfer region TR flows to the transfer member 2a side, so that the system resistance of the transfer region TR changes. A desired transfer electric field is secured by controlling the current.

Further, mainly, when the measures for improving the transfer performance of the highly insulating recording medium S are planned, the following aspects may be used.
That is, in this embodiment, the image holder 1 that holds and moves the image and the transfer member 2a that is arranged in contact with the image holding surface of the image holder 1 are provided, and the image holder 1 and the transfer member 2a are provided. The recording medium S is sandwiched and conveyed, and the transfer electric field is applied to the transfer region TR between the image holder 1 and the transfer member 2a to statically transfer the image held by the image holder 1 to the recording medium S. The transfer device 2 for electric transfer, the guide member 3 provided in a state of being grounded to the inlet side of the transfer area TR of the transfer device 2 and guiding the recording medium S toward the transfer area TR, and traveling toward the transfer area TR. Under the condition that the discriminator 4 for discriminating the type of the recording medium S to be used and the recording medium S discriminated by the discriminator 4 are high-insulation recording media having a predetermined surface resistance value or more, the guide member 3 As the grounding condition, it may be provided with a switch 5 for switching to bias grounding via static electricity elimination bias Bd instead of high resistance grounding determined in advance.

◎ Embodiment 1
Hereinafter, the present invention will be described in more detail based on the embodiments shown in the accompanying drawings.
FIG. 2 shows the overall configuration of the image forming apparatus according to the first embodiment.
-Overall configuration of image forming device-
In the figure, the image forming apparatus 20 forms a plurality of color components (white # 1, yellow, magenta, cyan, black, white # 2 in the present embodiment) images in the image forming apparatus housing 21. An image forming unit 22 (specifically, 22a to 22f), a belt-shaped intermediate transfer body 30 that sequentially transfers (primary transfer) each color component image formed by each image forming unit 22, and an intermediate transfer body 30. A secondary transfer device (batch transfer device) 50 that secondary transfers (batch transfer) each color component image transferred above to a paper as a recording medium, and a fixing device 70 that fixes the secondary transferred image on the paper. And a paper transport system 80 for transporting the paper to the secondary transfer area. In this example, white # 1 and white # 2 use white materials of exactly the same color, but different white materials are used depending on whether they are located in the lower layer or the upper layer of the other color component images on the paper. Of course, the one used may be used. Further, for example, a transparent color material may be used instead of one of white # 1.

-Image forming part-
In the present embodiment, each image forming unit 22 (22a to 22f) has a drum-shaped photoconductor 23, and around each photoconductor 23, a toner or a transfer roll or the like on which the photoconductor 23 is charged is charged. The charging device 24, the exposure device 25 such as a laser scanning device in which the electrostatic latent image is written on the charged photoconductor 23, and the electrostatic latent image written on the photosensitive member 23 are developed with each color component toner. The developing device 26, the primary transfer device 27 such as a transfer roll in which the toner image on the photoconductor 23 is transferred to the intermediate transfer body 30, and the photoconductor cleaning device 28 from which the residual toner on the photoconductor 23 is removed are respectively arranged. It is a thing.

Further, the intermediate transfer body 30 is hung on a plurality of (three in the present embodiment) tensioning rolls 31 to 33, for example, a driving roll in which the tensioning roll 31 is driven by a drive motor (not shown). And is designed to circulate and move with the drive roll. Further, an intermediate transfer body cleaning device 35 for removing residual toner on the intermediate transfer body 30 after the secondary transfer is provided between the tension rolls 31 and 33.

-Secondary transfer device (batch transfer device)-
Further, in the secondary transfer device (batch transfer device) 50, as shown in FIGS. 2 and 3, the transfer transfer belt 53 is mounted on a plurality of (for example, two) tension rolls 52 (specifically, 52a and 52b). The stretched belt transfer module 51 is arranged so as to be in contact with the surface of the intermediate transfer body 30.
Here, the transfer transfer belt 53 is ^{a semi-conductive belt having a volume resistivity of 10 6} to ¹² Ω · cm using a material such as chloroprene, and one of the tension rolls 52a is configured as an elastic transfer roll 55. The elastic transfer roll 55 is pressure-welded to the intermediate transfer body 30 via the transfer transfer belt 53 in the secondary transfer area (collective transfer area) TR, and the tension roll 33 of the intermediate transfer body 30 is opposed to the elastic transfer roll 55. The facing rolls 56 forming the electrodes are arranged so as to face each other, and a transport path for the paper S is formed from the position of one tensioning roll 52a to the position of the other tensioning roll 52b.

Further, in this example, the elastic transfer roll 55 has a structure in which an elastic layer in which urethane foam rubber, EPDM, carbon black, or the like is mixed is coated around a metal shaft. In this example, each of the tension rolls 52 (52a, 52b) of the belt transfer module 51 is grounded to prevent the transfer transfer belt 53 from being charged. Further, considering the peelability of the paper S at the downstream end of the transfer transfer belt 53, it is effective to make the downstream side tension roll 52b smaller in diameter than the upstream side tension roll 52a.
_{Further, the transfer bias VTR} from the transfer power source 60 is applied to the facing roll 56 (also used as the tension roll 33 in this example) via the conductive feeding roll 57, and the elastic transfer roll 55 and the facing roll 56 are applied. A predetermined transfer electric field is formed between them.
In this example, a constant current control circuit 61 is connected in series between the transfer power supply 60 and the power supply roll 57, and a selection switch 62 is provided between the constant current control circuit 61 and the transfer power supply 60. Therefore, either the energization path on the constant current control circuit 61 side or the detour path bypassing the constant current control circuit 61 is selected.

-Fixing device-
As shown in FIG. 2, the fixing device 70 is pressure-welded and arranged to face the heat-fixing roll 71, which is a driveable and rotatable heat-fixing roll 71, which is arranged in contact with the image holding surface side of the paper, and is heat-fixed. It has a pressure fixing roll 72 that rotates following the roll 71, and the image held on the paper is passed through the transfer region between the two fixing rolls 71 and 72, and the image is heat-pressed and fixed. be.

-Paper transport system-
Further, as shown in FIGS. 2 and 3, the paper transport system 80 has a plurality of stages (two stages in this example) of paper supply containers 81 and 82, and is supplied from any of the paper supply containers 81 and 82. The paper S is transferred from the vertical transport path 83 extending in the substantially vertical direction to the secondary transfer region TR via the horizontal transport path 84 extending in the substantially horizontal direction, and then the paper S on which the transferred image is held is transferred to the transport belt. It reaches the fixing portion by the fixing device 70 via 85, and discharges the paper to the paper ejection receiver 86 provided on the side of the image forming apparatus housing 21.
Further, the paper transport system 80 has a reversible branch transport path 87 that branches downward from a portion of the horizontal transport path 84 located on the downstream side in the paper transport direction of the fixing device 70, and the branch transport path The paper S inverted in 87 is returned from the vertical transport path 83 to the horizontal transport path 84 via the return transport path 88, the image is transferred to the back surface of the paper S in the secondary transfer region TR, and the image is transferred through the fixing device 70. It is designed to be ejected to the paper ejection receiver 86.
Further, in the paper transport system 80, in addition to the alignment roll 90 for aligning the paper S and supplying it to the secondary transfer region TR, an appropriate number of transport rolls 91 are provided in each transport path 83, 84, 87, 88. Has been done.
Furthermore, a manual paper feeder 95 capable of supplying manual paper toward the horizontal transport path 84 is provided on the opposite side of the paper discharge receiver 86 of the image forming apparatus housing 21.

-Guide chute-
Further, on the inlet side of the secondary transfer area TR of the horizontal transport path 84, a plurality of (two in this example) guide chutes 92 and 93 for guiding the paper S passing through the alignment roll 90 to the secondary transfer area TR are provided. It is provided. In this example, the guide chute 92 located in the front stage away from the secondary transfer region TR guides the paper S that has passed through the alignment roll 90 to the inlet of the guide chute 93 located in the rear stage, and guides the paper S to the entrance of the guide chute 93 located in the rear stage. Guides the paper S toward the secondary transfer region TR, and the front guide chute 92 and the rear guide chute 93 are arranged in different inclined postures.
Further, the front-stage guide chute 92 arranged at a position away from the secondary transfer area TR is directly grounded, and the rear-stage guide chute 93 arranged at a position close to the secondary transfer area TR is grounded via the switch 96. The conditions can be switched.

<Switcher>
In this example, as shown in FIG. 4, the switch 96 switches the grounding condition according to the paper type, and is _{grounded via a high resistance RH} and a high resistance grounding via a _{low resistance RL.} The low resistance grounding to be grounded is switched and selected by the changeover switch 97.
In this example, the high resistance _RH is selected to have a resistance value higher than the resistance value of the belt transfer module 51, and the low resistance grounding has a direct grounding structure in which the _{low resistance RL is 0.}
Here, specific configuration examples 1-1 to 1-3 of the switch 96 will be described with reference to FIG.
● Configuration example 1-1:
In the configuration example 1-1 of the switch 96, as shown in FIG. 4A, a part of the paired chute members 93a and 93b constituting the rear guide chute 93 is brought into contact with each other to switch to one chute member 93a. The element as the device 96 is connected, the contact A side of the changeover switch 97 is grounded with high resistance, and the contact B side is grounded with low resistance.
● Configuration example 1-2:
In the configuration example 1-2 of the switch 96, as shown in FIG. 4 (b), the paired chute members 93a and 93b constituting the rear guide chute 93 are arranged in a non-contact manner, and the chute members 93a and 93b have both chute members 93a and 93b. The elements of the switch 96 (specifically, 96a and 96b) are connected to each other so that the contact A side of the changeover switch 97 is grounded with high resistance and the contact B side is grounded with low resistance. The changeover switch 97 of the changer 96 (96a, 96b) switches contacts A and B in synchronization with each other.
● Configuration example 1-3:
In the configuration example 1-3 of the switch 96, as shown in FIG. 4 (c), a part of the paired chute members 93a and 93b constituting the rear guide chute 93 is brought into contact with each other to switch to one chute member 93a. A variable resistor 98 was connected instead of the changeover switch 97 as an element of the device 96, and a high resistance _RH or a low resistance _RL was generated by the variable resistor 98 so that high resistance grounding or low resistance grounding was performed. It is a thing.

-Paper type-
Examples of the paper S that can be used in this example include ^{plain paper having a surface resistance of 10 10} to ¹² Ω / □ and low resistance paper Sm having a lower surface resistance than plain paper.
Here, as a typical embodiment of the low resistance paper Sm, for example, as shown in FIG. 5A, a metal layer 101 such as aluminum is laminated on a base material layer 100 made of a paper base material, and the metal layer is laminated. There is a so-called metallic paper in which 101 is covered with a surface layer 102 made of a synthetic resin such as PET. In some cases, an adhesive layer made of PET or the like is provided between the base material layer 100 and the metal layer 101.
Although this kind of metallic sheet is also predetermined surface resistance value (e.g., 10 ⁷ ~10 ⁸ Ω / □) or less ones, as a metallic sheet equipped for example the surface layer 102 of the high-resistance material, JIS Standards Although the resistance value itself measured by the surface resistance measuring method according to the above is not lower than the threshold level, there are some that act as substantially low resistance when _{the transfer voltage VTR is applied.}
It is also possible to directly form a color image made of, for example, YMCK (yellow, magenta, cyan, black) on the metallic paper as this kind of low resistance paper Sm, but as shown in FIG. 5 (a), for example. , thereby forming a white image G _W of a background image by white (white) W by using the image forming unit 22f shown on metallic paper in FIG. 2, for example, the image forming unit 22b shown in FIG. 2 on a white image G _W ~ 22e may be used _{to form a color image G YMCK} by YMCK, or as shown in FIG. 5B, for example, the image forming portions 22b to 22e shown in FIG. 2 are used on metallic paper. In addition to forming the color image G _{YMCK by the existing YMCK, the white image GW by the white W} may be formed on the color image G _YMCK by using the image forming unit 22a shown in FIG.
Examples of the low resistance paper Sm include black paper containing a conductive material such as carbon black.

-A configuration example of a discriminator-
In this example, as shown in FIG. 3, a discriminator 110 for discriminating the paper type is provided in a part of the vertical transport path 83 or the horizontal transport path 84 of the paper transport system 80. In this discriminator 110, for example, as shown in FIG. 5C, paired configuration discriminating rolls 111 and 112 are arranged side by side along the transport direction of the paper S, and the paired configuration is located on the upstream side of the paper S in the transport direction. A discrimination power supply 113 is connected to one of the discrimination rolls 111, and the other is grounded via a resistor 114, between one of the paired discrimination rolls 112 located on the downstream side in the transport direction of the paper S and the ground. Is provided with an ammeter 115. The discriminant rolls 111 and 112 may also be used as the paper S transport member (alignment roll 90 or transport roll 91), or may be provided separately from the transport member.

In this example, assuming that plain paper (including high-resistance paper other than low-resistance paper) is used as the paper S, the surface resistance of the plain paper is large to some extent, and therefore, between the pairing rolls 111 and 112. Even if the plain paper is arranged so as to straddle the paper, the discriminant current from the discriminant power supply 113 flows so as to cross the discriminant roll 111 of the pair configuration as shown by the dotted line in FIG. There is almost nothing that reaches the current meter 115 on the discrimination roll 112 side.
On the other hand, assuming that low-resistance paper such as metallic paper is used as the paper S, the surface resistance of the low-resistance paper is smaller than that of plain paper. When the sheets are arranged so as to straddle each other, a part of the discrimination current from the discrimination power supply 113 flows so as to cross the discrimination roll 111 of the pair configuration as shown by the solid line in FIG. 5 (c), and the discrimination is performed. The remaining current travels through the paper S and reaches the current meter 115 on the discrimination roll 112 side, and the surface resistance of the paper S is calculated by the measured current measured by the current meter 115 and the applied voltage of the discrimination power supply 113 to calculate the paper. The species is determined.
In this example, the discriminator 110 discriminates the paper type by measuring the surface resistance of the paper S being conveyed. For example, based on a designated signal when the user specifies the paper type to be used. It may be designed to determine the paper type.

-Drive control system of image forming device-
In the present embodiment, as shown in FIG. 3, reference numeral 120 is a control device for controlling the image formation process of the image forming apparatus, and the control device 120 is a microcomputer including a CPU, ROM, RAM, and an input / output interface. It consists of switch signals such as a start switch (not shown), a mode selection switch for selecting an image drawing mode via an input / output interface, various sensor signals, and a paper discrimination signal from a discriminator 110 for discriminating the paper type. The various input signals of the above are taken in, the image formation control program (see FIG. 6) stored in advance in the ROM is executed by the CPU, and after the control signal for the drive control target is generated, each drive control target (switch 96, selection). A control signal is sent to the switch 62).

-Operation of image forming device-
Assuming that paper S having different surface resistances is mixedly used in the image forming apparatus shown in FIG. 2, as shown in FIG. 6, the image forming apparatus is operated by turning on a start switch (not shown). Printing (image processing) is started.
At this time, the paper S is supplied from either the paper supply containers 81, 82 or the manual paper feeder 95, and is conveyed toward the secondary transfer region TR via a predetermined transfer path, but is conveyed to the secondary transfer region TR. The surface resistance of the paper S is measured by the discriminator 110 (paper type discriminating process) during the transport before reaching.
The control device 120 determines whether or not the paper S is low resistance paper based on the discrimination result of the discriminator 110, and in the case of low resistance paper, the switch 96 switches the post-stage guide chute 93 to low resistance grounding. At the same time, the constant current control circuit 61 is selected by the selection switch 62 to enable constant current control.
On the other hand, when the control device 120 determines that the paper S is not a low resistance paper, the control device 120 switches and selects the paper S so as to maintain a high resistance grounding with respect to the rear guide chute 93 by the switch 96. The selection switch 62 is in a state of selecting a detour route that bypasses the constant current control circuit 61.
After that, when the paper S reaches the secondary transfer region TR, the image G formed by each image forming unit 22 (22a to 22f) and first transferred to the intermediate transfer body 30 is secondarily transferred to the paper S. After being fixed by the fixing device 70, the paper is discharged to the paper ejection receiver 86, and a series of printing (image drawing processing) is completed.

-Secondary transfer operation process-
<Plain paper>
Now, when the paper S is plain paper, as shown in FIGS. 3 and 6, the constant current control circuit 61 is not selected, so that the transfer voltage V _TR by the transfer power supply 60 is applied to the secondary transfer region TR. , A transfer current flows on the belt transfer module 51 side.
In this state, the paper S reaches the secondary transfer region TR via the guide chute 92 and 93, and the image G on the intermediate transfer body 30 is secondarily transferred to the paper S in the secondary transfer region TR. At this time, even if the paper S is in contact with the guide chutes 92 and 93 while the paper S passes through the secondary transfer region TR, the surface resistance of the paper S is high to some extent, so that the paper S is transferred in the secondary transfer region TR. A part of the current does not leak through the energization path leading to the grounding of the guide chutes 92 and 93 with the paper S as the energization path, and the transfer operation with respect to the paper S in the secondary transfer region TR is stably performed, and the paper S is No trouble such as a decrease in image density occurs in some parts.

<Low resistance paper>
Next, a case where the paper S is a low resistance paper (for example, metallic paper) Sm will be described.
In this case, as shown in FIGS. 3, 6 and 7 (a), the rear guide chute 93 is grounded with low resistance (directly grounded in this example), and constant current control is performed by the constant current control circuit 61. To.
_{Therefore, the transfer voltage VTR} by the transfer power supply 60 is applied to the secondary transfer region TR via the constant current control circuit 61.
In this state, since the rear guide chute 93 is grounded with low resistance (directly grounded in this example), the resistance component from the rear guide chute 93 to the ground is substantially 0, which is compared with the system resistance of the belt transfer module 51. Small enough.
Assuming that the rear end of the low resistance paper Sm is before passing through the front guide chute 92, as shown in FIG. 7A, the low resistance paper S has the secondary transfer region TR and the front guide chute 92. However, since the front guide chute 92 and the rear guide chute 93 are both grounded with low resistance (directly grounded in this example), the transfer current I flowing in the secondary transfer region TR is _As shown by the solid line in FIG. 7A, the TR flows from the rear guide chute 93, which is closer to the secondary transfer region TR than the front guide chute 92, to the ground by using the low resistance paper Sm as the current path. Therefore, in the secondary transfer region TR, a transfer electric field toward the low resistance paper Sm side acts on the image G of the intermediate transfer body 30, and a stable secondary transfer operation is performed.

Next, when the low-resistance paper Sm passes through the front-stage guide chute 92, the rear end of the low-resistance paper Sm passes through the rear-stage guide chute 93 while in contact with the low-resistance paper Sm, as shown in FIG. 7B. Sm is arranged so as to straddle between the secondary transfer region TR and the post-stage guide chute 93.
In this case, the subsequent guide chute 93 flowing from the fact that the low-resistance ground, the transfer current I _TR of the secondary transfer region TR and to the conduction path to ground from the rear stage guide chute 93 a low-resistance sheet Sm as conduction path. In this state, the transfer current I _{TR in the} secondary transfer region TR does not change between before and after the passage of the front guide chute 92, and flows from the rear guide chute 93 to the grounding path. In the secondary transfer region TR, a stable secondary transfer operation is continuously performed.
In particular, in this example, since the transfer current I _TR passing through the constant current control circuit 61 is supplied to the secondary transfer region TR, the transfer current I _TR of the secondary transfer region TR is maintained at a constant current.

Here, FIG. 9A defines the impedance of each element around the secondary transfer region TR of the present embodiment as follows, and schematically shows the equivalent circuit thereof.
Z _{BUR + ITB} : Impedance of opposed roll 56 _{+ intermediate transfer body Z BTB + DR} : Impidance of belt transfer module 51 (transfer transfer belt 53 + elastic transfer roll 55) Z _{toner: Impidance of} toner Z base layer: Base layer of low resistance paper Sm 100 Impidance Z Metal Layer: Impedance of Metal Layer 101 of Low Resistance Paper Sm Z Surface Layer: Impedance of Surface 102 of Low Resistance Paper Sm Z _{Cute (f) : Impedance Z Cute (r)} of Front Guide Chute 92: Rear Guide Chute 93 Impidance In FIG. 9A, V _TR indicates the transfer voltage and I _TR indicates the transfer current, respectively.

In the equivalent circuit shown in the figure, when the transfer voltage V _TR is applied to the secondary transfer region TR, the transfer current I _TR does not flow to the belt transfer module 51 side, and as shown in FIG. 9A, it is low. The metal layer 101 of the resistance sheet Sm is used as an energization path, and the current flows through the path leading to the grounding of the subsequent guide chute 93. Here, the transfer current I _{TR is determined by the transfer voltage V TR and the impedance (Z BUR + ITB} ) of the counter roll 56 and the intermediate transfer body 30 as shown in FIG. 9 (a).
Therefore, through the even positioned straddling between the low-resistance sheet Sm is the second transfer region TR and the guide chute 92 (93), the transfer current I _TR is low resistance sheet Sm and subsequent guide chute 93 Then, since it flows into the path leading to the grounding of the post-stage guide chute 93, the transfer current _ITR flowing through the secondary transfer region TR is maintained in a stable state. Thus, for example, even to print halftone images of uniform density on substantially the entire area of the low-resistance sheet Sm, density step of transferring image due to unevenness of the transfer current I _TR by the second transfer region TR is suppressed.

After that, when the rear end of the low resistance paper Sm passes through the rear guide chute 93 and passes through the secondary transfer region TR, the low resistance paper Sm passes through the secondary transfer region TR as shown in FIG. 7 (c). From the state of being arranged straddling between the rear guide chute 93 and the rear guide chute 93, the state changes from the state of being separated from the rear guide chute 93 and passing through the secondary transfer region TR, and the transfer of the secondary transfer region TR is performed. The current _ITR flows to the belt transfer module 51 side. At this time, since the system resistance of the secondary transfer region TR changes, the transfer current _ITR tries to change. However, in the present embodiment, the constant current control by the constant current control circuit 61 is performed. Even if the system resistance of the secondary transfer region TR changes, the transfer current _ITR maintains a constant current, and a stable secondary transfer operation is performed.
Therefore, in the present embodiment, for example, even if a halftone image having a uniform density is printed over substantially the entire area of the low-resistance paper Sm, there is no concern that a density step will occur in the transferred image at the rear end of the low-resistance paper Sm.
In this embodiment, the embodiment of the constant current control circuit 61 is shown, but even if the constant current control circuit 61 is not implemented, the post-stage guide chute 93 and the secondary transfer region TR are shown. if the distance to set as short as possible between the low resistance of the sheet Sm trailing edge of the transfer current I _TR due to that the system resistance of the secondary transfer region TR exits the subsequent guiding chute 93 to change Since the change region is a very small part of the rear end of the low resistance paper Sm, the transfer defect region due to the change of the transfer _{current ITR is suppressed to a narrow range.}

◎ Comparison form 1
Next, in evaluating the performance based on the configuration around the secondary transfer region TR according to the present embodiment, the performance based on the configuration around the secondary transfer region TR according to the comparative embodiment 1 will be described.
As shown in FIG. 8A, the basic configuration around the secondary transfer region TR according to the first embodiment is substantially the same as that of the first embodiment, but unlike the first embodiment, the metallic paper or the like is used. Even when the low resistance paper Sm is used, the grounding condition of the front guide chute 92 is low resistance grounding (direct grounding in this example), and the grounding condition of the rear guide chute 93 is high resistance 94 (embodiment). It remains grounded (high resistance grounded) via (equivalent to high resistance _{RH of 1), and further, a constant voltage transfer voltage VTR} by the transfer power supply 60 is sent to the power supply roll 57 without performing constant current control. It is designed to be applied. The components similar to those of the first embodiment are designated by the same reference numerals as those of the first embodiment, and detailed description thereof will be omitted here.

Now, assuming that the rear end of the low resistance paper Sm has not passed through the front guide chute 92 as shown in FIG. 8A, the low resistance paper Sm is secondary as in the first embodiment. It is arranged so as to straddle between the transfer area TR and the front-stage guide chute 92.
At this time, since the front guiding chute 92 are low-resistance ground (ground in this example directly), the transfer current I _TR of the secondary transfer region TR reaches the ground from the preceding guide chute 92 a low-resistance sheet Sm as conduction path It will flow along the route. Therefore, the transfer current I _TR of the secondary transfer region TR stably flows from the opposed roll 56 to the low resistance paper Sm side via the intermediate transfer body 30, so that a stable secondary transfer operation can be performed in the secondary transfer region TR. Will be done.

Next, when the low-resistance paper Sm passes through the front-stage guide chute 92, as shown in FIG. 8B, the rear end of the low-resistance paper Sm passes through the rear-stage guide chute 93 while in contact with the low-resistance paper. Sm is arranged so as to straddle between the secondary transfer region TR and the post-stage guide chute 93.
At this time, unlike the front guide chute 92, the rear guide chute 93 is grounded with high resistance, so that the grounding conditions between the guide chute 92 and 93 with which the low resistance paper Sm comes into contact change, but the guide chute There is a concern that the _{transfer current ITR} may change in the secondary transfer region TR due to the difference in grounding conditions between 92 and 93.
Here, FIG. 9B shows an equivalent circuit of each element around the secondary transfer region TR in the first form of comparison. The impedance of each element in the figure is described in the same manner as that defined in FIG. 9 (a).
In the figure, the transfer voltage _{V TR} is applied to the secondary transfer region TR, for example when the impedance _{Z BTB + DR} belt transfer module 51 is higher than the impedance _{Z RH} by high resistance _{R H} of the subsequent guide chute 93, In the secondary transfer region TR, as shown by the solid line in FIG. 9B, the transfer current _ITR from the transfer power supply 60 flows from the subsequent guide chute 93 to the ground with the low resistance paper Sm as the energization path. .. Further, when the impedance _{Z BTB + DR} belt transfer module 51 is lower than the impedance _{Z RH} by high resistance _{R H} of the subsequent guide chute 93, the secondary transfer region TR, the transfer current I _'TR from the transfer power supply 60, As shown by a virtual line in FIG. 9B, the current flows from the elastic transfer roll 55 of the belt transfer module 51 to the grounding path. However, in any case, due to the difference in the resistance conditions between the guide chute 92 and 93, or the difference in the resistance conditions between the front guide chute 92 and the belt transfer module 51, the transfer current ITR in the secondary transfer region _TR May change. Therefore, for example, when printing a halftone image having a uniform density over substantially the entire area of the low-resistance paper Sm, a region between the front guide chute 92 and the secondary transfer region TR in the vicinity of the rear end of the low-resistance paper Sm. The density difference of the transferred image due to the unevenness of the transfer current _{ITR is likely to occur.}

After that, when the rear end of the low resistance paper Sm passes through the secondary transfer region TR, as shown in FIG. 8C, the rear end of the low resistance paper Sm is separated from the rear guide chute 93. In the next transfer region TR, the transfer current _ITR from the transfer power source 60 flows from the elastic transfer roll 55 of the belt transfer module 51 to the current path to the ground. At this time, if there is a difference between the impedance _{Z RH} by high resistance _{R H} leading to the ground of the impedance _{Z BTB + DR} and the rear guide chute 93 of the belt transfer module 51 further has a transfer current _{I TR} in the secondary transfer region TR Therefore, for example, when printing a halftone image having a uniform density over substantially the entire area of the low resistance paper Sm, the space between the rear guide chute 93 and the secondary transfer area TR in the vicinity of the rear end of the low resistance paper Sm. also it tends to occur density step of transferring image due to unevenness of the transfer current I _TR in the secondary transfer region TR.

◎ Embodiment 2
-Key parts around the secondary transfer part-
FIG. 10 shows a main part around the secondary transfer portion of the image forming apparatus according to the second embodiment.
In the figure, the basic configuration around the secondary transfer unit of the image forming apparatus 20 is substantially the same as that of the first embodiment, but the switching device 96 for switching the grounding condition of the rear guide chute 93 is the first embodiment. Is different from. The components similar to those of the first embodiment are designated by the same reference numerals as those of the first embodiment, and detailed description thereof will be omitted here.
In the present embodiment, the switch 96 has a high resistance ground (contact A) grounded via a _{high resistance RH and a low resistance RL} (resistance value 0 in this example), substantially similar to the first embodiment. The low resistance grounding (contact B) that is grounded via the above is switched and selected by the changeover switch 97, but unlike the first embodiment, the bias grounding that is grounded via a predetermined static elimination bias Bd is further selected. (Contact C) is switched and selected by the changeover switch 97.
In this example, the high resistance _RH is selected to have a resistance value higher than the resistance value of the belt transfer module 51, and the low resistance grounding has a direct grounding structure in which the _{low resistance RL is 0.} Further, the static elimination bias Bd is appropriate as long as it eliminates static electricity so as to smooth out charging unevenness during running, which is likely to occur in high-insulation paper ^{having a surface resistance of 10 13 Ω or more (for example, plastic film) among high-resistance papers.} It does not matter if it is selected, and in this example, a negative electrode DC bias of about 200 to 2000 V is selected.

Here, specific configuration examples 2-1 and 2-2 of the switch 96 will be described with reference to FIG.
● Configuration example 2-1:
In the configuration example 2-1 of the switch 96, as shown in FIG. 11A, a part of the paired chute members 93a and 93b constituting the rear guide chute 93 is brought into contact with each other to switch to one chute member 93a. The element as the device 96 is connected, the contact A of the changeover switch 97 is grounded with high resistance, the contact B is grounded with low resistance, and the contact C is grounded with bias. Then, in this example, the first open / close switch 131 is interposed in the connection path between the paired chute members 93a and 93b, and the other chute member 93b is grounded via the second open / close switch 132. As shown below, the open / close switches 131 and 132 are opened and closed in conjunction with the contact position of the changeover switch 97.
Changeover switch 97 First open / close switch 131 Second open / close switch 132
High resistance grounding on / off Direct grounding on / off Bias grounding off On In other words, when the switch 96 selects high resistance grounding or direct grounding for one chute member 93a, the chute members 93a and 93b of the pair configuration are partially contacted. The other chute member 93b may be brought into a non-grounded state, and when the switch 96 selects bias grounding for one chute member 93a, the chute members 93a and 93b of the pair configuration are arranged in a non-contact manner. Further, the other chute member 93b may be directly grounded.

● Configuration example 2-2:
In the configuration example 2-2 of the switch 96, as shown in FIG. 11B, the paired chute members 93a and 93b constituting the rear guide chute 93 are arranged in a non-contact manner, and the chute member 93a has a first position. Connect the switch 96a of No. 1, ground the contact A side of the first changeover switch 133 with high resistance, ground the contact B side with low resistance (direct ground in this example), and ground the contact C side with bias. A second switch 96b is connected to the other chute member, the contact D side of the second changeover switch 134 is grounded with high resistance, and the contact E side is grounded with low resistance (direct grounding in this example). ), And as shown below, the second changeover switch 134 is switched and selected in conjunction with the first changeover switch 133.
First changeover switch 133 Second changeover switch 134
High resistance ground High resistance ground
Direct grounding Direct grounding
Bias grounding Direct grounding

-Operation of image forming device-
Assuming that paper S having different surface resistances is mixedly used in the image forming apparatus shown in FIG. 10, as shown in FIG. 12, the image forming apparatus is operated by turning on a start switch (not shown). Printing (image processing) is started.
At this time, the paper S is supplied from either the paper supply containers 81, 82 or the manual paper feeder 95, and is conveyed toward the secondary transfer region TR via a predetermined transfer path, but is conveyed to the secondary transfer region TR. The surface resistance of the paper S is measured by the discriminator 110 (paper type discriminating process) during the transport before reaching.
The control device 120 determines whether or not the paper S is low resistance paper based on the discrimination result of the discriminator 110, and in the case of low resistance paper, the switch 96 switches the post-stage guide chute 93 to low resistance grounding. At the same time, the constant current control circuit 61 is selected by the selection switch 62 to enable constant current control.

On the other hand, when the control device 120 determines that the paper S is not a low resistance paper, the control device 120 determines whether the high resistance paper has a surface resistance of 10 ¹³ Ω / □ or more and is a highly insulating paper. Occasionally, the static elimination bias Bd is applied to the rear guide chute 93 by switching and selecting bias grounding for the rear guide chute 93 by the switch 96. Further, when the paper S is a high-resistance paper other than the high-insulation paper, the switch 96 switches and selects the paper S so as to maintain the high-resistance grounding with respect to the rear guide chute 93. When the paper S is a high resistance paper, the selection switch 62 is in a state of selecting a detour route that bypasses the constant current control circuit 61.
After that, when the paper S reaches the secondary transfer region TR, the image G formed by each image forming unit 22 (22a to 22f) and first transferred to the intermediate transfer body 30 is secondarily transferred to the paper S. After being fixed by the fixing device 70, the paper is discharged to the paper ejection receiver 86, and a series of printing (image drawing processing) is completed.

-Secondary transfer operation process-
Now, when the paper S is plain paper or low resistance paper, as shown in FIGS. 10 and 12, a secondary transfer operation substantially similar to that of the first embodiment is performed.
<Highly insulating paper>
Next, when the paper S is a high-resistance paper, especially the highly insulating paper St, a transfer voltage V _TR by the transfer power supply 60 is applied to the secondary transfer region TR as shown in FIG. 13 (a). _{Then, a transfer current ITR} flows on the belt transfer module 51 side. Further, the rear guide chute 93 is bias-grounded by the switch 96, and the static elimination bias Bd is applied to the rear guide chute 93.
At this time, the highly insulating paper St is locally charged due to rubbing against a guide member, a transport member, or the like during traveling to the secondary transfer region TR, and reaches the secondary transfer region TR. There is a concern that uneven charging may occur on the highly insulating paper St. However, since the highly insulating paper St moves while in contact with the post-stage guide chute 93 to which the static elimination bias Bd is applied, as shown in FIG. 13A, the charging potential Vp of the highly insulating paper St is the static elimination bias Bd. After being leveled to a substantially uniform charge level, the secondary transfer region TR is reached. Therefore, there is little concern that transfer defects due to uneven charging of the highly insulating paper St will occur during the secondary transfer operation in the secondary transfer region TR.
In particular, when a highly conductive image-forming material (for example, white toner) is used, the electrostatic adhesion force of the white toner to the intermediate transfer member 30 is weak in the first place, so that the highly insulating paper St has uneven charging. Although there is a concern that it may easily lead to uneven transfer of white toner, there is no concern as described above in the present embodiment.

◎ Comparison form 1
Next, in evaluating the performance based on the configuration around the secondary transfer region TR according to the present embodiment, the performance based on the configuration around the secondary transfer region TR according to the comparative embodiment 1 will be described.
As shown in FIG. 13B, the basic configuration around the secondary transfer region TR according to the first aspect of the comparison is different from the second embodiment, and the grounding condition of the front guide chute 92 is different from that of the second embodiment. Low resistance grounding (direct grounding in this example), the grounding condition of the post-stage guide chute 93 remains high resistance grounding, and a constant voltage transfer voltage _VTR is applied by the transfer power supply 60. Is.
At this time, assuming that the highly insulating paper St reaches the secondary transfer region TR via the alignment roll 90 and the guide chute 92, 93, the highly insulating paper St located immediately before the secondary transfer region TR As a result of investigating the charging potential Vp, there is a concern that the charging unevenness caused by rubbing during running may remain as it is on the highly insulating paper St, and the charging unevenness of the highly insulating paper St may easily lead to the transfer unevenness as it is. found. That is, it is understood that even if the highly insulating paper St passes through the post-stage guide chute 93 which is grounded with high resistance, the charge unevenness is hardly eliminated.

◎ Example 1
The first embodiment shows an example in which the image forming apparatus according to the first embodiment is embodied and a low resistance paper Sm such as a metallic paper is used.
In this embodiment, as shown in FIG. 5C, the discriminator 110 for discriminating the paper type monitors the current flowing through the ammeter 115 when the discriminating voltage is applied from the discriminating power supply 113, and has a certain threshold value. The low resistance paper Sm may be determined under the condition that the current exceeds the above. For example, when 130 μA is applied as the discrimination current by the discrimination power supply 113 and a low resistance paper Sm such as metallic paper is passed, 60 μA is detected as the monitor current of the ammeter 115, which is almost half, and in the case of plain paper. Assuming that it is less than 30 μA, it is possible to discriminate the low resistance paper Sm by selecting 30 μA as the threshold value of the ammeter 115.

Further, in this embodiment, under the condition that the low-resistance paper Sm passes through the secondary transfer region TR, as shown in FIG. 14A, the post-stage guide chute 93 is directly switched to ground and selected, and further, the transfer power supply 60 is selected. The transfer voltage _VTR from the above is applied to the feeding roll 57 via the constant current control circuit 61.
According to this embodiment, the monitor current of the ammeter 140 is measured by connecting the ammeter 140 in series to the transfer power supply 60 and the constant current control circuit 61, and the solid line is shown in FIG. 14 (b). as shown, although the change in the transfer voltage V _TR in the vicinity of the rear end of the low-resistance sheet Sm is seen, as indicated by a dotted line in FIG. 14 (b), the transfer current I _TR corresponding to the monitor current of approximately -100μA It was confirmed that the current was constant.
As a result, even if the low-resistance paper Sm passes through the secondary transfer region TR, the transfer electric field in the secondary transfer region TR is stable, so that a halftone image G having a uniform density is formed on the low-resistance paper Sm. A halftone image G with a uniform density is formed in the image of the low-resistance paper Sm even when the image-drawing process is performed under the image-forming conditions, and a density step is observed near the rear end of the low-resistance paper Sm. There wasn't.

◎ Comparative example 1
Comparative Example 1 embodies the image forming apparatus (see FIGS. 8A to 8C) according to the first form of comparison, and as shown in FIG. 15A, the rear guide chute 93 has a high resistance. It is grounded (high resistance grounding) via _{94 (equivalent to the high resistance RH} of the present embodiment _{), and the transfer voltage VTR} from the transfer power supply 60 is applied to the feeding roll 57. In FIG. 15A, the same components as those in the first form of comparison are designated by the same reference numerals, and detailed description thereof will be omitted here.
In Comparative Example 1, an ammeter 141 is connected between the elastic transfer roll 55 of the belt transfer module 51 and the ground, a transfer voltage V _TR by a transfer power source 60 is applied to the secondary transfer region TR, and low resistance paper is used. The image forming process was performed under the image forming conditions for forming a halftone image having a uniform density in Sm, and the current flowing through the ammeter 141 was monitored while the low resistance paper Sm passed through the secondary transfer region TR. It is a thing.
In Comparative Example 1, as shown in FIG. 15A, when the rear end of the low resistance paper Sm passes through the secondary transfer region TR in the situation before passing through the front guide chute 92, the secondary transfer is performed. transfer current I _TR of frequency TR flows through a current path to ground from the preceding guide chute 92 through the low-resistance sheet Sm, monitor the current flowing through the ammeter 141 of the belt transfer module 51 side to maintain the state of substantially zero.

At this time, the second transfer region TR, the transfer current because I _TR is flowing from counter roll 56 and through the intermediate transfer member 30 to the low-resistance sheet Sm, the image G on the intermediate transfer member 30 a low-resistance sheet Sm side A stable secondary transfer operation is carried out by the action of the transfer electric field toward.
After that, when the rear end of the low resistance paper Sm passes through the front guide chute 92, the low resistance paper Sm comes into contact with the rear guide chute 93. At this time, since the grounding conditions (direct grounding / high resistance grounding) of the guide chute 92 and 93 are different, the transfer current _ITR flowing in the secondary transfer region TR is grounded from the subsequent guide chute 93 using the low resistance paper Sm as an energization path. Although it flows through the energization path leading to it, the transfer current _ITR tends to change. As shown by the dotted line in FIG. 15B, this is abrupt from the state where the monitor current of the ammeter 141 was approximately 0 level near the rear end of the low resistance paper Sm (about -20 μA in this example). It is also understood from the fact that it has changed to. In this state, a high possibility that the transfer current I _TR in the secondary transfer region TR is changed, the distance from the vicinity of the rear end (front guide chute 92 of the low-resistance sheet Sm up to the second transfer region TR In (corresponding to), transfer defects are likely to occur, and there is a concern that a density step Gd may occur in the uniform density halftone image G.

1 ... Image holder, 2 ... Transfer device, 2a ... Transfer member, 2b ... Opposing member, 2c ... Transfer power supply, 3 ... Guide member, 3a ... First guide member, 3b ... Second guide member, 4 ... Discrimination Instrument, 5 ... Switch, 7 ... Control device, TR ... Transfer area, _RH ... High resistance, _RL ... Low resistance, Bd ... Static elimination bias, G ... Image, S ... Recording medium

Claims (11)

An image holder that holds and moves an image, and an image holder that holds and moves the image.
It has a transfer member arranged in contact with the image holding surface of the image holder, and the recording medium is sandwiched and conveyed between the image holder and the transfer member, and the image holding surface of the image holder By applying a transfer voltage from the opposite side , a transfer electric field is applied to the transfer region between the image holder and the transfer member to electrostatically transfer the image held by the image holder to the recording medium. Transfer device and
A guide member provided on the inlet side of the transfer area of the transfer device in a grounded state and guiding the recording medium toward the transfer area, and a guide member.
A discriminator that discriminates the type of recording medium traveling toward the transfer region, and
Under the condition that the recording medium determined by the discriminator has a predetermined surface resistance value or less or a low resistance condition having a conductive layer along the surface of the medium base material, a high resistance predetermined as a grounding condition of the guide member is obtained. A switch that switches to low resistance grounding with a resistance value lower than the grounding resistance value, and
An image forming apparatus characterized by being equipped with. In the image forming apparatus according to claim 1,
The switching device is an image forming apparatus characterized by adopting a high resistance grounding using a high resistance larger than the resistance value of the transfer member. In the image forming apparatus according to claim 2,
The switching device is an image forming apparatus including a low resistance ground having a resistance value of 0. In the image forming apparatus according to any one of claims 1 to 3.
Under the condition that the discriminator discriminates a recording medium having a high insulation value equal to or higher than a predetermined resistance value as the type of recording medium, the switch should switch to bias grounding via static elimination bias instead of the high resistance grounding. An image forming apparatus characterized by. In the image forming apparatus according to any one of claims 1 to 4.
The switching device is an image forming apparatus characterized in that the grounding condition is automatically switched based on the discrimination signal of the type of the recording medium by the discriminator. In the image forming apparatus according to any one of claims 1 to 5.
The switching device is an image forming apparatus characterized by using a variable resistor whose resistance value changes. In the image forming apparatus according to any one of claims 1 to 6.
The discriminator is an image forming apparatus characterized in that it discriminates the type of a recording medium based on a detection signal from a detector that detects the surface resistance of the traveling recording medium. An image holder that holds and moves an image, and an image holder that holds and moves the image.
The image holder has a transfer member arranged in contact with the image holding surface of the image holder, and the image holder and the transfer member sandwich and convey the recording medium, and the image holder and the transfer member A transfer device that electrostatically transfers an image held by the image holder to the recording medium by applying a transfer electric field to the transfer region between the two.
A guide member provided on the inlet side of the transfer area of the transfer device in a grounded state and guiding the recording medium toward the transfer area, and a guide member.
A discriminator that discriminates the type of recording medium traveling toward the transfer region, and
Under the condition that the recording medium determined by the discriminator has a predetermined surface resistance value or less or a low resistance condition having a conductive layer along the surface of the medium base material, a high resistance predetermined as a grounding condition of the guide member is obtained. A switch that switches to low resistance grounding with a resistance value lower than the grounding resistance value, and
Equipped with
The guide member includes a first guide member provided at a site distant from the transfer zone, and a second guide member provided between said first guide member and said transfer zone, said first with the second guide member adds the switch which is switched ground conditions, characterized in that said first guide member in the high-resistance low resistance ground than the resistance value of the grounding of the second guide member Image forming device. In the image forming apparatus according to any one of claims 1 to 8.
In the transfer device, the transfer is supplied to the transfer region under the condition that the recording medium determined by the discriminator has a surface resistance value or less determined in advance or has a conductive layer along the surface of the medium substrate. An image forming apparatus having a constant current control unit that controls a current to a constant current. In the image forming apparatus according to any one of claims 1 to 9.
The image retainer is an intermediate transfer body that intermediately transfers and retains an image on the image forming retainer before being transferred to a recording medium, and the transfer device transfers an image on the intermediate transfer body to a recording medium. An image forming apparatus characterized in that it is to be transferred. An image holder that holds and moves an image, and an image holder that holds and moves the image.
It has a transfer member arranged in contact with the image holding surface of the image holder, and the recording medium is sandwiched and conveyed between the image holder and the transfer member, and the image holding surface of the image holder is conveyed. By applying a transfer voltage from the opposite side to the image holder, a transfer electric field is applied to the transfer region between the image holder and the transfer member, and the image held by the image holder is electrostatically transferred to the recording medium. With a transfer device to make
A guide member provided on the inlet side of the transfer area of the transfer device in a grounded state and guiding the recording medium toward the transfer area, and a guide member.
A discriminator that discriminates the type of recording medium traveling toward the transfer region, and
Under the condition that the recording medium determined by the discriminator is a recording medium having a high insulation property equal to or higher than a predetermined surface resistance value, the static elimination bias is replaced with the predetermined high resistance grounding as the grounding condition of the guide member. An image forming apparatus characterized by being equipped with a switch for switching to bias grounding via.

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