Image forming device

Abstract

To provide an image forming apparatus that can achieve both prevention of transfer failure from an intermediate transfer belt to a recording material, and prevention of toner adhesion from the intermediate transfer belt to a conveyance guide.SOLUTION: A pressing roller 25 pressing an intermediate transfer belt 5 to the outside, and a conveyance guide 26 having an upper guide 26a and a lower guide 26b electrically connected with each other and guiding a recording material S to a secondary transfer nip part T2, are electrically connected with each other. The conveyance guide 26 is grounded through a varistor 28. This can maintain the potential difference between the pressing roller 25 and the conveyance guide 26 within a range where part of toner on the intermediate transfer belt 5 is not attracted to the conveyance guide 26 at least while the recording material S passes through the secondary transfer nip part T2 without separating the conveyance guide 26 from the intermediate transfer belt 5. In this way, prevention of a failure of transfer of toner to the recording material S and prevention of adhesion of toner to the conveyance guide 26 can be achieved with a simple configuration.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus using electrophotographic technology, such as a printer, a copier, a facsimile, or a multifunction machine. [Background technology]

[0002] Conventionally, as an image forming apparatus for forming an image on a recording material, for example, an intermediate transfer type image forming apparatus has been used. In the intermediate transfer type image forming apparatus, a toner image formed on a photosensitive drum is transferred to an intermediate transfer belt in response to application of a primary transfer voltage. Thereafter, when a strong electric field is generated in a secondary transfer nip portion formed by a secondary transfer inner roller and a secondary transfer outer roller arranged to sandwich the intermediate transfer belt in response to application of a secondary transfer voltage, the toner image on the intermediate transfer belt is transferred to a recording material passing through the secondary transfer nip portion. Conventionally, in order to suppress transfer defects (such as transfer omissions and transfer dust) from the intermediate transfer belt to the recording material, a device has been proposed in which a pressing member is provided upstream of the secondary transfer nip portion in the rotation direction of the intermediate transfer belt (Patent Document 1). The pressing member is arranged inside the intermediate transfer belt like the secondary transfer inner roller, and presses the intermediate transfer belt from the inside to make it protrude outward (toward the secondary transfer outer roller), thereby making it easier to make the intermediate transfer belt and the recording material adhere to each other.

[0003] Incidentally, the recording material is guided to the secondary transfer nip portion while being posture-controlled by a conveying guide having an upper guide plate and a lower guide plate. In order to bring the intermediate transfer belt and the recording material into close contact with each other and to prevent the trailing end of the recording material from strongly contacting the intermediate transfer belt when it passes through the conveying guide, it is preferable to arrange the tip of the upper guide plate on the secondary transfer nip portion side close to the intermediate transfer belt side. However, since the conveying guide becomes charged when rubbed against the recording material, particularly in the case where the conveying guide is arranged close to the intermediate transfer belt side as described above, a part of the toner on the intermediate transfer belt is likely to scatter and adhere to the conveying guide, and as a result, the recording material may be stained with toner. Therefore, as in the device described in Patent Document 1, a voltage of a polarity opposite to the charged polarity of the toner is applied to a conductive pressing member to prevent the toner from adhering to the conveying guide from the intermediate transfer belt. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2004-061908 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the case of the configuration in which a voltage of the opposite polarity to that of the toner is applied to the pressing member as described above, in addition to applying the primary transfer voltage and the secondary transfer voltage, a voltage application means for applying a voltage to the pressing member is required, which makes the device complicated and large-sized, and increases the cost. Therefore, a device that can suppress transfer defects from the intermediate transfer belt to the recording material and suppress toner adhesion from the intermediate transfer belt to the conveying guide with a simple configuration has been desired, but such a device has not yet been proposed.

[0006] The present invention has been made in consideration of the above-mentioned problems, and aims to provide an image forming apparatus that can achieve both suppression of transfer defects from the intermediate transfer belt to the recording material and suppression of toner adhesion from the intermediate transfer belt to the transport guide with a simple configuration. [Means for solving the problem]

[0007] An image forming apparatus according to an embodiment of the present invention includes an image forming unit that forms a toner image, an endless belt that rotates while carrying a toner image, an inner roller that contacts an inner peripheral surface of the belt and forms a transfer nip portion where the toner image on the belt is transferred from the belt to a recording material, an outer member that contacts an outer peripheral surface of the belt and forms the transfer nip portion in cooperation with the inner roller, a pressing member that is provided upstream of the inner roller in a rotation direction of the belt and presses the belt from the inside, and a guide member that is provided upstream of the transfer nip portion in the rotation direction of the belt and guides the recording material toward the transfer nip portion. The guide member is configured to be grounded via a varistor. and a connecting member that electrically connects the guide member and the pressing member so that the guide member and the pressing member are at the same potential or the potential difference between the guide member and the pressing member is 100 V or less. Effect of the Invention

[0008] According to the present invention, a rotating toner image is carried on the Rube To suppress transfer failure from the inkjet to the recording material. , Be Ruto? The guide guides the recording material from the transfer nip to the transfer nip. Ido Department To materials It is possible to achieve both of the above-mentioned advantages with a simple configuration. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic diagram showing a configuration of an image forming apparatus according to an embodiment of the present invention. [Diagram 2] 5 is a schematic diagram showing the arrangement and electrical connection relationship of a pressure roller and a transport guide. [Diagram 3]6 is a graph showing the relationship between the potential difference between the pressure roller and the transport guide, and the toner adhering to the transport guide. [Figure 4] FIG. [Diagram 5] FIG. 13 is a schematic diagram showing a conventional example in which the pressure roller and the transport guide are not electrically connected. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [First embodiment] First, the configuration of the image forming apparatus of this embodiment will be described with reference to Fig. 1. The image forming apparatus 100 shown in Fig. 1 is an intermediate transfer type full-color printer equipped with yellow, magenta, cyan, and black image forming units PY, PM, PC, and PK along an intermediate transfer belt 5. In the following description, unless otherwise specified, upstream (upstream side) and downstream (downstream side) refer to the upstream (upstream side in the rotation direction) and downstream (downstream side in the rotation direction) in the rotation direction (direction of arrow R2) of the intermediate transfer belt 5.

[0011] The image forming apparatus 100 forms a toner image on a recording material S in response to an image signal from an original reading device (not shown) connected to the main body of the apparatus or an external device (not shown) such as a personal computer. The recording material S may be various types of sheet materials such as plain paper, thick paper, rough paper, textured paper, coated paper, plastic film, cloth, etc. The recording material S is stored in one or more cassettes 12.

[0012] The image forming apparatus 100 has image forming units PY, PM, PC, and PK that form yellow, magenta, cyan, and black images, respectively. In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 1Y and transferred (primary transfer) to the intermediate transfer belt 5. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1M and transferred to the intermediate transfer belt 5 in a superimposed state. In the image forming units PC and PK, a cyan toner image and a black toner image are formed on the photosensitive drums 1C and 1K, respectively, and transferred to the intermediate transfer belt 5 in a superimposed state. These image forming units PY, PM, PC, and PK are configured almost the same except that the colors of the toner contained in the two-component developer used in the developing devices 4Y, 4M, 4C, and 4K are different, that is, yellow, magenta, cyan, and black. Therefore, in the following, the yellow image forming unit PY will be described as a representative, and the description of the other image forming units PM, PC, and PK will be omitted.

[0013] The image forming section PY is mainly composed of a photosensitive drum 1Y, a charging device 2Y, an exposure device 3Y, a developing device 4Y, a photosensitive drum cleaner 7Y, etc. The photosensitive drum 1Y is composed of an aluminum cylinder having an outer diameter of 30 mm, for example, coated with an OPC (organic photoconductor) as a photosensitive layer on its outer peripheral surface. The photosensitive drum 1Y is driven to rotate in the direction of the arrow R1. The surface of the photosensitive drum 1Y is uniformly charged in advance by the charging device 2Y, and then an electrostatic latent image is formed on the surface by the exposure device 3Y, which is driven based on an image signal.

[0014] The electrostatic latent image formed on the photosensitive drum 1Y is developed into a toner image by the developing device 4Y using a two-component developer containing a non-magnetic toner and a magnetic carrier. The developing device 4Y supplies the toner to the photosensitive drum 1Y to develop the electrostatic latent image into a toner image. That is, in the developing device 4Y, the developing sleeve 42 arranged on the surface of the photosensitive drum 1Y with a small gap therebetween is rotated in the counter direction of the photosensitive drum 1Y. As a result, the toner of the two-component developer contained in the container 41 is charged and conveyed to the portion facing the photosensitive drum 1Y by the developing sleeve 42. In the case of this embodiment, the toner is negatively charged (normally charged). Then, in response to an oscillating voltage in which an AC voltage is superimposed on a DC voltage being applied to the developing sleeve 42, the negatively charged toner is transferred to the exposed portion of the photosensitive drum 1Y, and the electrostatic latent image is reversely developed, and a toner image is formed on the photosensitive drum 1Y. Incidentally, toner to be supplied to the developing device 4Y is accommodated in a toner supply container 8Y, and the toner is supplied from the toner supply container 8Y to the developing device 4Y.

[0015] Then, in response to the application of a primary transfer voltage to a primary transfer roller 6Y disposed across the image forming unit PY and the intermediate transfer belt 5, the toner image formed on the photosensitive drum 1Y is transferred (primary transfer) to the intermediate transfer belt 5 at a primary transfer nip T1Y. The primary transfer roller 6Y is pressed against the intermediate transfer belt 5 to form a primary transfer nip T1Y between the photosensitive drum 1Y and the intermediate transfer belt 5. A DC voltage of a polarity opposite to the charge polarity of the toner (positive polarity in this case) is applied to the primary transfer roller 6Y, so that the toner image (negative polarity) on the photosensitive drum 1Y is transferred to the intermediate transfer belt 5. The primary transfer roller 6Y abuts against the intermediate transfer belt 5 with a total pressure of, for example, 1.5 kgf, and rotates following the intermediate transfer belt 5. In this embodiment, the primary transfer roller 6Y is a cylindrical member formed of, for example, metal, with an electrical resistance of 5.0×10 6 The specimen was covered with a conductive elastic member having a resistance of 1.0 Ω / cm and a thickness of 1.0 mm.

[0016] Primary transfer residual toner remaining on the photosensitive drum 1Y after transfer is removed by a photosensitive drum cleaner 7Y. The photosensitive drum cleaner 7Y rubs a cleaning blade (not shown) made of, for example, polyurethane against the photosensitive drum 1Y.

[0017] The intermediate transfer belt 5 as an image bearing belt is an endless belt member that rotates in contact with the photosensitive drums 1Y to 1K. The intermediate transfer belt 5 is stretched by a tension roller 21, a drive roller 22, and a secondary transfer inner roller 23, and is driven in the direction of an arrow R2 by the drive roller 22. The tension roller 21, the drive roller 22, and the secondary transfer inner roller 23 are in contact with the inner peripheral surface of the intermediate transfer belt 5. The tension roller 21 as a stretching roller is a metallic roller, and is provided upstream of the secondary transfer inner roller 23 as an inner roller in the rotation direction of the intermediate transfer belt 5, and biases the intermediate transfer belt 5 from the inside to the outside by a tension spring (not shown). The drive roller 22 is a rubber roller having a conductive rubber layer as a surface layer on a metallic core metal. In this embodiment, the tension roller 21 is a roller made of metal and has a diameter of 1.0×10 3 ~1.0×10 5 The driving roller 22 used has an electrical resistance of 1.0Ω (when 100V is applied), and the core of the driving roller 22 is electrically grounded.

[0018] The image forming process for each color, which is performed in parallel by the image forming units PY to PK, is performed at a timing that sequentially superimposes the toner image of the upstream color that has been primarily transferred to the intermediate transfer belt 5. As a result, a full-color toner image is finally formed on the intermediate transfer belt 5 and is conveyed to the secondary transfer nip portion T2. ​​Note that the secondary transfer residual toner after passing through the secondary transfer nip portion T2 is removed from the intermediate transfer belt 5 by a belt cleaner 16.

[0019] The recording material S is taken out one by one from the cassette 12 by a feed roller 13 and transported to a registration roller 15 by a plurality of transport rollers 14. The toner image formed on the intermediate transfer belt 5 is transported to a secondary transfer nip portion T2 and secondarily transferred onto the recording material S. The secondary transfer nip portion T2 is a transfer nip portion formed by a secondary transfer inner roller 23 and a secondary transfer outer roller 24. The registration roller 15 sends out the recording material S toward the secondary transfer nip portion T2 in synchronization with the toner image formed on the intermediate transfer belt 5.

[0020] <Transport guide> A conveying guide 26 is provided at the destination where the registration roller 15 delivers the recording material S, with an upper guide 26a and a lower guide 26b arranged to face each other, and the recording material S is delivered between the upper guide 26a and the lower guide 26b of the conveying guide 26. The conveying guide 26 regulates the moving direction of the recording material S delivered from the feed roller 13 by the upper guide 26a as a second guide member and the lower guide 26b as a first guide member, and guides the recording material S to the secondary transfer nip portion T2. ​​The upper guide 26a and the lower guide 26b are arranged in two upper and lower stages on the outer side of the intermediate transfer belt 5 (the surface (toner image carrying surface) side of the intermediate transfer belt 5 that carries the toner image) with a gap therebetween from the intermediate transfer belt 5 along the conveying direction of the recording material S (the direction of the arrow R3). The upper guide 26a is arranged between the lower guide 26b and the intermediate transfer belt 5.

[0021] The upper guide 26a arranged on the side closer to the intermediate transfer belt 5 guides the second side (the side onto which the toner image has been transferred most recently) of the recording material S while restricting the recording material S from moving toward the intermediate transfer belt 5. On the other hand, the lower guide 26b arranged on the side farther from the intermediate transfer belt 5 guides the first side (the side opposite to the side onto which the toner image has been transferred most recently) of the recording material S while restricting the recording material S from moving away from the intermediate transfer belt 5. In this embodiment, the upper guide 26a and the lower guide 26b are formed into plate shapes using a conductive metal such as SUS (stainless steel), and as described later, the upper guide 26a and the lower guide 26b are electrically connected (see FIG. 2 described later).

[0022] The secondary transfer outer roller 24 as a nip forming member is disposed so as to sandwich the secondary transfer inner roller 23 and the intermediate transfer belt 5, and forms a secondary transfer nip portion T2 for transferring a toner image from the intermediate transfer belt 5 to the recording material S. In this embodiment, the toner image on the intermediate transfer belt 5 is transferred (secondarily transferred) to the recording material S nipped and conveyed at the secondary transfer nip portion T2 in response to the secondary transfer voltage being applied to the secondary transfer inner roller 23 by the secondary transfer high voltage power source E as a voltage application means. That is, while the secondary transfer outer roller 24 is grounded (potential 0V), the secondary transfer high voltage power source E applies a negative voltage (secondary transfer voltage, for example, -4000V) that is the same polarity as the charging polarity of the toner to the secondary transfer inner roller 23, thereby generating a transfer electric field in the secondary transfer nip portion T2. ​​In response to this transfer electric field, the negative polarity toner images of yellow, magenta, cyan, and black carried on the intermediate transfer belt 5 are transferred to the recording material S.

[0023] The recording material S onto which the toner image has been transferred at the secondary transfer nip T2 is transported towards the fixing device 9. In the fixing device 9, the fixing roller 9a and the pressure roller 9b come into contact with each other to form the fixing nip T3, and the toner image is fixed to the recording material S at the fixing nip T3. In the fixing device 9, the pressure roller 9b is pressed against the fixing roller 9a, which is heated by, for example, a lamp or a heater (not shown), by a biasing mechanism (not shown), to form the fixing nip T3. Heat and pressure are applied to the recording material S as it passes through the fixing nip T3, and the toner image is fixed to the recording material S. Thereafter, the recording material S is discharged outside the main body of the image forming apparatus 100.

[0024] Next, the intermediate transfer belt 5, the secondary transfer inner roller 23, the secondary transfer outer roller 24, and the pressure roller 25 will be described. The intermediate transfer belt 5 is made of a resin having a single layer or multilayer structure, and has a thickness of 45 to 100 μm, a Young's modulus of 1.0 GPa or more, a surface resistivity of 1.0×10 9 ~1.0×10 13 Ω / □”, volume resistivity “1.0×10 7 ~1.0×10 12In this embodiment, the intermediate transfer belt 5 is formed with a surface resistivity of 1.0×10 11 Ω / □”, volume resistivity “1.0×10 9 The electrical resistance was adjusted to 100 Ω·cm.

[0025] The secondary transfer inner roller 23 is a solid roller with a conductive rubber layer formed on its surface. The outer diameter of the secondary transfer inner roller 23 is, for example, "φ20 mm". On the other hand, the secondary transfer outer roller 24 is a sponge roller, with a conductive sponge rubber surface layer (sponge layer) formed on a core metal such as SUS (stainless steel). The outer diameter of the secondary transfer outer roller 24 is, for example, "φ24 mm", in which case the diameter of the core metal is "φ12 mm" and the thickness of the sponge layer is "6 mm". The electrical resistance of the secondary transfer outer roller 24 is, for example, "5.0×10 7 The hardness of the sponge layer is adjusted to, for example, Asker C hardness of 30 degrees. Such an outer secondary transfer roller 24 is biased toward the inner secondary transfer roller 23 by springs (not shown) at both ends in the direction of the rotation axis, and presses the inner secondary transfer roller 23 with, for example, a total pressure of 6.5 kgf.

[0026] <Pressure roller> A pressing roller 25 is provided as a pressing member on the inside of the intermediate transfer belt 5, upstream of the inner secondary transfer roller 23 and downstream of the tension roller 21. The pressing roller 25 is a conductive metal roller formed such that the length in the width direction intersecting the rotation direction of the intermediate transfer belt 5 can be in contact with the entire width of the intermediate transfer belt 5. The pressing roller 25 is provided to ensure that the contact length between the recording material S and the intermediate transfer belt 5 is as long as possible on the upstream side of the secondary transfer nip portion T2. ​​This is because if the recording material S does not contact the intermediate transfer belt 5 with a certain length on the upstream side of the secondary transfer nip portion T2, a gap is formed between the recording material S and the intermediate transfer belt 5, and transfer defects such as transfer omissions due to abnormal discharge are likely to occur at that location.

[0027] Next, the arrangement of the pressure roller 25 and the conveying guide 26 will be described with reference to FIG. 2. The pressure roller 25 is arranged in a vertical position (a vertical position in FIG. 2) where a gap is unlikely to occur between the recording material S entering the secondary transfer nip portion T2 and the intermediate transfer belt 5. The intermediate transfer belt 5 is pressed by the pressure roller 25, so that a tension surface formed between the secondary transfer inner roller 23 and the tension roller 21 is extended outward from a virtual tension surface when the pressure roller 25 is not present. In other words, the intermediate transfer belt 5 is extended outward from a common tangent line F that contacts both the secondary transfer inner roller 23 and the tension roller 21 on the side where each of the secondary transfer inner roller 23 and the tension roller 21 stretches the intermediate transfer belt 5. For example, the pressure roller 25 is preferably arranged so that the intermediate transfer belt 5 is extended outward from the common tangent line F by "1.0 to 3.0 mm".

[0028] Moreover, the pressure roller 25 is disposed at a predetermined lateral position (a position in the left-right direction in FIG. 2) so as not to impede the driving of the secondary transfer inner roller 23 and the secondary transfer outer roller 24. For example, the pressure roller 25 is preferably disposed so as to come into contact with the intermediate transfer belt 5 at a position "3 to 15 mm" upstream from the entrance of the secondary transfer nip portion T2.

[0029] On the other hand, the conveying guide 26 is preferably disposed so as to be located upstream of the intermediate transfer belt 5 from the most downstream position Q where the upper guide 26a presses the intermediate transfer belt 5. This is because if the recording material S is not in close contact with the intermediate transfer belt 5 for a certain length upstream of the secondary transfer nip T2, a gap will be formed between the recording material S and the intermediate transfer belt 5, and transfer defects such as transfer omissions due to abnormal discharge will occur at that location. In order to prevent this transfer defect, it is preferable to guide the recording material S so as to follow the intermediate transfer belt 5 for, for example, 5 to 10 mm or more upstream of the secondary transfer nip T2. For this reason, the conveying guide 26 is preferably disposed so that the upper guide 26a is located upstream of the intermediate transfer belt 5 from the most downstream position P.

[0030] 2, the outer secondary transfer roller 24 may be shifted (offset) upstream relative to the inner secondary transfer roller 23. This is preferable because it ensures a longer contact length between the recording material S and the intermediate transfer belt 5 on the upstream side of the secondary transfer nip T2. The shift amount of the outer secondary transfer roller 24 relative to the inner secondary transfer roller 23 is, for example, 3 mm.

[0031] Next, a description will be given of the electrical connection between the pressure roller 25 and the transport guide 26 in this embodiment. In Fig. 2, the dotted lines in the drawing indicate the electrical connection.

[0032] As described above, the pressure roller 25 is a metal roller member, and the upper guide 26a and the lower guide 26b are metal plate-shaped members. In this embodiment, the pressure roller 25, the upper guide 26a, and the lower guide 26b, which have conductivity, are electrically connected to each other by, for example, a metal conductive member or a copper wire so that they are maintained at the same potential. In the example shown in FIG. 2, the upper guide 26a and the lower guide 26b are electrically connected to each other in the conveying guide 26, and the pressure roller 25 is electrically connected to the upper guide 26a. The lower guide 26b is grounded to earth via, for example, a varistor 28 of "2.4 kV".

[0033] The pressure roller 25 may be electrically connected to the lower guide 26b instead of the upper guide 26a, or may be electrically connected to both the upper guide 26a and the lower guide 26b. Also, the upper guide 26a may be grounded via the varistor 28 instead of the lower guide 26b.

[0034] As described above, in the present embodiment shown in FIG. 2, the lower guide 26b is grounded via the varistor 28 of, for example, "2.4 kV". If the varistor 28 is not provided, when the recording material S passes through the secondary transfer nip portion T2 in a state where the secondary transfer voltage is applied to the secondary transfer inner roller 23, a part of the transfer current is likely to flow through the conveying guide 26 that contacts the recording material S. If a part of the transfer current flows to the conveying guide 26 side, a transfer current sufficient to transfer the toner will not flow to the secondary transfer nip portion T2. ​​In order to prevent this, the varistor 28 is provided. That is, the varistor 28 has a characteristic that the current does not flow until a predetermined voltage is reached. Therefore, when the varistor 28 is provided, it is possible to make it difficult for a part of the transfer current to flow to the conveying guide 26 side compared to the case where the varistor 28 is simply grounded. In this way, the varistor 28 as an electric resistance member is provided so that a transfer current sufficient to transfer the toner flows to the secondary transfer nip portion T2 when the secondary transfer voltage is applied to the secondary transfer inner roller 23.

[0035] Although the lower guide 26b is grounded via the varistor 28, this is not limiting, and the upper guide 26a may be grounded via the varistor 28. Also, instead of the varistor 28, a resistor, a Zener diode, or the like may be used.

[0036] 3 shows the relationship between the potential difference between the pressure roller 25 and the transport guide 26 and the toner adhering to the transport guide 26. In detail, the results of an experiment were shown in which, with toner carried on the intermediate transfer belt 5, a voltage was applied from an external power source to each of the pressure roller 25 and the transport guide 26, and the density of the toner adhering to the tip of the upper guide 26a was measured for each potential difference ΔV. However, unlike this embodiment, this experiment used one in which the pressure roller 25 and the transport guide 26 were not electrically connected. Note that the toner density referred to here is the density measured by an optical reflection density meter, and is calculated from the ratio of the amount of light irradiated to the amount of light reflected.

[0037] 3, the concentration of toner adhering to the tip of the upper guide 26a increases as the potential difference ΔV between the pressure roller 25 and the transport guide 26 increases. This experimental result shows that in order to make it difficult for toner to adhere to the tip of the upper guide 26a, it is advisable to restrict the potential difference between the pressure roller 25 and the transport guide 26 to a range of, for example, "-100V to 100V."

[0038] Here, a conventional example is shown in Fig. 5. In the conventional example shown in Fig. 5, the same components as those in this embodiment shown in Fig. 2 are given the same reference numerals, and the description will be simplified or omitted. The conventional example shown in Fig. 5 is the same as this embodiment in that the upper guide 26a and the lower guide 26b are electrically connected, and the lower guide 26b is grounded via the varistor 28, but is different from this embodiment in that the pressure roller 25 and the transport guide 26 are not electrically connected. In addition, the pressure roller 25 is grounded separately from the transport guide 26, so its potential is "0V".

[0039] In the case of the conventional example, when the conveying guide 26 is charged by rubbing against the recording material S, a potential difference occurs between the pressure roller 25 and the conveying guide 26, and the potential difference is very large. Therefore, a part of the toner charged from the intermediate transfer belt 5 due to the potential difference is attracted to and adheres to the conveying guide 26. If this happens, when the recording material S is subsequently guided by the conveying guide 26, the toner adhered to the conveying guide 26 is transferred to the recording material S, causing toner stains. Such toner stains become more noticeable the closer the distance between the conveying guide 26 and the intermediate transfer belt 5 is. In order to prevent this, it is possible to separate the conveying guide 26 from the intermediate transfer belt 5. However, if the conveying guide 26 is disposed away from the intermediate transfer belt 5, the recording material S cannot be brought into contact with the intermediate transfer belt 5 for a certain length, even though the intermediate transfer belt 5 is pressed against the pressure roller 56, and transfer defects are likely to occur. Therefore, it is difficult to dispose the conveying guide 26 away from the intermediate transfer belt 5.

[0040] In view of the above, in this embodiment, as described above, the pressure roller 25 is electrically connected to the conveying guide 26, the lower guide 26b electrically connected to the upper guide 26a is grounded via the varistor 28. This makes it possible to maintain the potential difference between the pressure roller 25 and the conveying guide 26 within a range in which a part of the toner is not attracted to the conveying guide 26 at least while the recording material S passes through the secondary transfer nip portion T2, even if the conveying guide 26 is not disposed away from the intermediate transfer belt 5. In this way, it is possible to prevent the toner from adhering to the conveying guide 26. As described above, in this embodiment, it is possible to achieve both of preventing poor transfer of the toner from the intermediate transfer belt 8 to the recording material S and preventing the toner from adhering to the conveying guide 26 from the intermediate transfer belt 8 with a simple configuration.

[0041] [Second embodiment] Next, a second embodiment will be described with reference to Fig. 4. In the second embodiment shown in Fig. 4, the same components as those in the first embodiment shown in Fig. 2 are denoted by the same reference numerals, and the description thereof will be simplified or omitted.

[0042] In the first embodiment described above, the pressing roller 25 is provided to press the intermediate transfer belt 5 from the inside to make it protrude outward (toward the secondary transfer outer roller), but this is not limited to this. As shown in FIG. 4, instead of the pressing roller 25, a pressing plate 251 formed in a plate shape from a conductive metal or the like may be used. Alternatively, although not shown, a pressing member formed in a plate shape from a conductive metal or the like and a pressing member having a PET resin sheet attached to the upstream side of the plate member may be used. Even in this case, according to this embodiment, even if static electricity (frictional charging) is generated in the PET resin sheet due to friction as the recording material S passes through, it is advantageous because a part of the toner is not attracted from the intermediate transfer belt 5 to the conveying guide 26. Also, a resin plate formed in a plate shape from an insulating resin may be used, the surface of which abuts against the inner peripheral surface of the intermediate transfer belt 5 is coated with a conductive resin or the like, and the coated part of this resin plate and the conveying guide 26 may be electrically connected.

[0043] Furthermore, a pressure roller 27 serving as a second pressing member for pressing the intermediate transfer belt 5 from the inside to the outside may be provided upstream of a pressure plate 251 (pressure roller 25 in the first embodiment (see FIG. 2)) serving as a first pressing member. The pressure roller 27 is provided upstream of the secondary transfer nip portion T2 to bring the recording material S into closer contact with the intermediate transfer belt 5. In the example shown in FIG. 4, the pressure roller 27 is connected to the pressure plate 251, and the pressure plate 251 is electrically connected to the transport guide 26. Alternatively, the pressure roller 27 connected to the pressure plate 251 may be electrically connected to the transport guide 26.

[0044] As shown in FIG. 4, when electrically connecting the pressing plate 251 (pressing roller 25 in the first embodiment (see FIG. 2)) or the pressing roller 27 to the transport guide 26, it is preferable to connect them via a resistor 29 having a smaller resistance than the varistor 28. In the example shown in FIG. 4, the resistor 29 is provided between the transport guide 26 and the pressing plate 251. By providing the resistor 29, the potential on the side of the pressing plate 251 or the pressing roller 27 can be maintained higher than the potential of the transport guide 26, within a potential difference range in which a portion of the toner from the intermediate transfer belt 5 is not attracted to the transport guide 26.

[0045] [Other embodiments] The first and second embodiments described above are not limited to being applied to a configuration in which a toner having a negative charging characteristic is used and a negative secondary transfer voltage is applied to the secondary transfer inner roller 23. For example, they may be applied to a configuration in which a toner having a negative charging characteristic is used and a positive secondary transfer voltage is applied to the secondary transfer outer roller 24. In the case of a configuration in which a positive secondary transfer voltage is applied to the secondary transfer outer roller 24 and a negative secondary transfer voltage is applied to the secondary transfer inner roller 23, the toner polarity may be reversed to the positive polarity in the process in which the developed toner is discharged by primary transfer or the like. When positive toner is generated, the toner is likely to adhere to the transport guide 26 due to the relationship between the potential of the pressing roller 25 and the potential of the transport guide 26. Therefore, the amount of toner adhering to the transport guide 26 can be reduced in the configuration in which a positive secondary transfer voltage is applied to the secondary transfer outer roller 24. Even in the case of a configuration in which a positive polarity secondary transfer voltage is applied to such a secondary transfer outer roller 24, by suppressing the potential difference between the pressure roller 25 and the transport guide 26 to a range of, for example, "-100V to 100V", it is possible to make it difficult for the positive polarity toner to move along with the negative polarity toner.

[0046] The pressure roller 25 (pressing plate 251) may be provided so that the pressure amount applied to the intermediate transfer belt 5 can be changed according to the type of recording material S. For example, in the case of a recording material S having a high bending stiffness, it is preferable to apply a larger pressure amount than in the case of a recording material S having a low bending stiffness. As a recording material S having a high bending stiffness, a material with a basis weight of 200 [g / m 2 For example, if the basis weight of recording material S is 280 g / m 2 The change in the pressing amount (for example, 0.9 mm) when the basis weight is "250 g / m 2 " is larger than the change in the pressure amount (e.g., 0.7 mm). When the pressure amount of the pressure roller 25 is changed, the gap between the intermediate transfer belt 5 and the transport guide 26 (particularly the upper guide 26a) may change. In the case of this embodiment, it is advantageous because toner is less likely to adhere to the transport guide 26 even if the gap between the intermediate transfer belt 5 and the transport guide 26 becomes narrow.

[0047] In the above-described first and second embodiments, the secondary transfer nip T2 is formed using the outer secondary transfer roller 24, but the present invention is not limited to this. For example, the secondary transfer nip T2 may be formed using an endless secondary transfer belt stretched by a plurality of tension rollers. [Explanation of symbols]

[0048] 5...image bearing belt (intermediate transfer belt), 21...tension roller, 23...inner roller (secondary transfer inner roller), 24...nip forming member (secondary transfer outer roller), 25...pressure member (first pressure member, pressure roller), 26a...second guide member (upper guide), 26b...first guide member (lower guide), 27...second pressure member (pressure roller), 28...electrical resistance member (varistor), 29...another electric resistance member (resistor), 100...image forming apparatus, 251...pressure member (first pressure member, pressure plate), E...voltage application means (secondary transfer high voltage power source), S...recording material, T2...transfer nip portion (secondary transfer nip portion)

Claims

1. an image forming unit that forms a toner image; an endless belt that rotates while carrying a toner image; an inner roller that contacts the inner circumferential surface of the belt and forms a transfer nip portion where a toner image on the belt is transferred from the belt to a recording material; an outer member that contacts an outer peripheral surface of the belt and cooperates with the inner roller to form the transfer nip; a pressing member provided upstream of the inner roller in a rotation direction of the belt and pressing the belt from the inside; a guide member that is disposed upstream of the transfer nip portion in the rotation direction and that guides a recording material toward the transfer nip portion, the guide member being configured to be grounded via a varistor; a connection member that electrically connects the guide member and the pressing member so that the guide member and the pressing member have the same potential or a potential difference between the guide member and the pressing member is 100 V or less.

1. An image forming apparatus comprising:

2. the guide member includes a first guide member that guides a first surface of the recording material, and a second guide member that is disposed between the first guide member and the belt and that guides a second surface of the recording material opposite to the first surface, the connecting member electrically connects the second guide member and the pressing member such that the second guide member and the pressing member have the same potential.

2. The image forming apparatus according to claim 1,

3. a power source for applying a voltage of the same polarity as that of the toner to the inner roller; 2. The image forming apparatus according to claim 1,

4. an upstream roller disposed upstream of the pressing member in the rotation direction; the connecting member electrically connects the pressing member and the guide member via the upstream roller so that the guide member and the upstream roller have the same potential as the pressing member or a potential difference between the guide member and the upstream roller and the pressing member is 100 V or less.

2. The image forming apparatus according to claim 1,

5. The pressing member is in a sheet form.

2. The image forming apparatus according to claim 1,

6. The pressing member is a roller.

2. The image forming apparatus according to claim 1,

7. The outer member is a belt.

2. The image forming apparatus according to claim 1,

8. The outer member is a roller.

2. The image forming apparatus according to claim 1,

9. the connecting member is a first connecting member, an upstream roller disposed upstream of the pressing member in the rotation direction; a second connection member that electrically connects the pressing member and the upstream roller so that the pressing member and the upstream roller have the same potential; 5. The image forming apparatus according to claim 4.

10. The pressing member is adjacent to the inner roller in the rotation direction.

2. The image forming apparatus according to claim 1,

11. The pressing member faces the guide member across the belt.

2. The image forming apparatus according to claim 1,

Images