JP6439507B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an image forming apparatus that forms a toner image on a sheet having an uneven surface such as embossed paper.

  In an image forming apparatus such as an electrophotographic printer or a copying machine, a toner image is formed on an image carrier such as a photosensitive member, the formed toner image is transferred onto a sheet, and then heated and pressurized and fixed. Thus, a sheet on which a toner image is formed is obtained.

  In recent years, the use of copying machines and printers has expanded, and not only plain paper with a smooth surface, but also various types of paper such as embossed paper with an embossed surface have been used.

  In a paper with large irregularities such as embossed paper, there is a problem that the toner transferability is insufficient in the concave portion, and the uniformity of the image is lowered.

  In the image forming apparatus disclosed in Patent Document 1, transferability is improved by applying a transfer bias in which an AC voltage is superimposed on a DC voltage to a transfer unit.

  Further, in the image forming apparatuses disclosed in Patent Document 1 and Patent Document 2, a charge roller having a polarity opposite to that of the toner is applied to the transfer surface of the embossed paper by a charging roll before transfer. Uniformity is improved.

  Further, Patent Document 3 discloses an image forming apparatus in which a charge having the same polarity as that of toner is previously applied only to a convex portion of a sheet by a pre-transfer charge applying unit such as an electrode on the embossed sheet. As a result, a transfer electric field that is relatively stronger than the convex portion can be formed in the secondary transfer nip relative to the concave portion, and by setting a secondary transfer bias optimized for the concave portion, the entire surface of the paper can be transferred. The image uniformity is improved.

JP 2006-267486 A JP 2008-185890 A JP 2008-262085 A

  However, if a transfer bias obtained by superimposing an AC voltage on a DC voltage is applied to the transfer portion of Patent Document 1, the concave portion of the paper can be improved. However, in the convex portion of the blank area where there is no toner image, AC The surface is excessively charged by the transfer bias on which the voltage is superimposed. As a result, surrounding toner is attracted and image dust occurs.

  Further, in the technique of applying a charge having a polarity opposite to that of the toner to the transfer surface of the paper as disclosed in Patent Documents 1 and 2, if the amount of charge applied to the embossed paper by the charging roll is insufficient, the embossed paper As a result, the toner is not transferred well to the concave portion of the image, and the uniformity of the image becomes insufficient. On the other hand, when a larger amount of charge is applied to the embossed paper, toner transferability can be expected to be improved with respect to the toner transferability in the concave portion. As a result, discharge to a new toner and a phenomenon of attracting surrounding toner occur, and new transfer defects such as so-called transfer repelling and dusting occur.

  Further, the technique of Patent Document 3 makes it difficult to cause a transfer defect due to discharge to toner at a convex portion by applying a charge for canceling the transfer electric field to the convex portion of the paper. This technique achieves an improvement in transferability by increasing the secondary transfer bias when attention is paid to the recesses. However, for paper with large irregularities such as embossed paper, even if the secondary transfer bias is increased, there is a limit to the improvement in transferability, and conversely due to the discharge to the toner before the transferability of the recesses is sufficiently improved. Therefore, there is a problem that improvement in transferability of the recesses and improvement in transfer failure due to electric discharge are not compatible.

  The present invention has been made in view of the above circumstances, and achieves both improvement in transferability at the concave portion and improvement in transfer failure due to discharge at the convex portion, with respect to paper having large irregularities such as embossed paper. An object is to provide an image forming apparatus.

  The above object of the present invention is achieved by the following means.

(1) an image forming unit that forms a toner image on the image carrier;
A transport unit for transporting paper;
A pre-transfer charging unit that includes a brush-shaped charging member and applies a charge opposite to the normal charging polarity of the toner to the paper conveyed by the conveying unit by the brush-shaped charging member;
A part of the electric charge applied to the surface of the paper by the pre-transfer charging unit on the downstream side of the pre-transfer charging unit in the paper conveyance direction, and a part of the electric charge corresponding to the unevenness of the paper surface is removed. A static neutralizer to perform,
A toner image formed on the image carrier by the transfer electric field formed by applying a voltage having a polarity opposite to the normal charging polarity of the toner on the downstream side of the sheet conveyance direction from the charge eliminating unit is applied to the sheet. A transfer section to be transferred;
An image forming apparatus.

  (2) The brush fiber of the brush-shaped charging member is in contact with the paper, and the fiber diameter of the brush fiber is smaller than the diameter of the concave and convex portions on the surface of the paper used in the image forming apparatus. Image forming apparatus.

  (3) The image forming apparatus according to (1) or (2), wherein the brush fibers of the brush-shaped charging member are in contact with the paper, and the fiber diameter of the brush fibers is 40 μm or less.

  (4) The image forming apparatus according to any one of (1) to (3), wherein the brush-shaped charging member is a brush roller that is rotated by a driving source.

  (5) The image formation according to (4), wherein the rotation direction of the brush roller is set so that the conveyance direction of the paper and the movement direction of the outer periphery of the brush roller are opposite to each other at the contact position with the paper. apparatus.

  (6) The image forming apparatus according to (4) or (5), further including a flicker member that comes into contact with a tip of the brush fiber of the brush roller and restores the fall of the brush fiber.

  (7) The image forming apparatus according to any one of (1) to (3), wherein the brush-shaped charging member is a brush bar and slides on a surface of a sheet on which the brush fibers of the brush bar are conveyed. .

  The image forming apparatus according to the present invention applies a charge to a paper having unevenness such as an embossed paper by a pre-transfer charging unit provided with a brush-like charging member, and a part of the applied charge. Then, a part of the electric charge corresponding to the unevenness on the surface of the paper is removed by the charge removing portion, and then the toner is transferred onto the paper by the transfer portion. By adopting such a configuration, it is possible to form a high-quality image on paper with unevenness, since it is possible to achieve both improved transferability at the recesses and improved transfer failure due to discharge at the protrusions. It becomes.

1 is a diagram illustrating an entire image forming apparatus A. FIG. 2 is a block diagram of an image forming apparatus A. FIG. FIG. 2 is a schematic diagram illustrating a configuration around a secondary transfer unit 30 in the first embodiment. It is a schematic diagram explaining the transfer defect in a comparative example. It is a figure which shows the state of the paper S in nip N1-N3. It is a schematic diagram which shows the structure of the secondary transfer part 30 periphery in a comparative example. It is a schematic diagram explaining the transfer defect in a comparative example. 6 is a table showing a comparison result of process speed and transfer performance in the comparative example and the first embodiment. It is a schematic diagram explaining the definition of the minimum concave diameter Ds of the recessed part of the paper surface. It is a table | surface which shows the evaluation result of the transfer performance by the combination of the minimum concave diameter Ds and the fiber diameter Df. It is a figure which shows the pre-transfer charging part 80 in 2nd Embodiment. It is a schematic diagram which shows the structure around the secondary transfer part 30 in 3rd Embodiment. It is a figure which shows the structure of a part of charging part 80, 80c before transfer in a modification.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

(Image forming device)
The image forming apparatus according to this embodiment will be described with reference to FIGS. FIG. 1 is a diagram illustrating the entire image forming apparatus A, and FIG. 2 is a block diagram of the image forming apparatus A.

  The image forming apparatus A is referred to as a tandem type color image forming apparatus. A plurality of sets of image forming units 10Y, 10M, 10C, and 10K, an intermediate transfer belt 6, a sheet feeding / conveying unit 20, a secondary transfer unit 30, and a fixing unit. The apparatus 40 includes an operation display unit 60, a scanner 70, and the like.

  In the specification of the present application, the constituent elements are collectively indicated by reference numerals with alphabetic suffixes omitted, and the individual constituent elements are indicated by Y (yellow), M (magenta), C (cyan), This is indicated by a reference symbol with a K (black) suffix.

  The image forming units 10Y, 10M, 10C, and 10K form toner images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Each image forming unit 10 has a strip electrode 2, an exposure unit 3, a developing unit 4 and a cleaning unit 5 disposed around a drum-shaped photoconductor 1 as an image carrier (for M, C, and K). Some reference signs are omitted).

  The photoreceptor 1 is composed of an organic photoreceptor in which a photosensitive layer made of a resin containing an organic photoconductor is formed on the outer peripheral surface of a drum-shaped metal substrate, and the width direction of the sheet S to be conveyed (see FIG. 1 is a direction perpendicular to the paper surface and extends in the following (also referred to as “axial direction”). Examples of the resin constituting the photosensitive layer include polycarbonate.

  The developing unit 4 includes a two-component developer composed of a toner having a small particle diameter of different colors of yellow (Y), magenta (M), cyan (C), and black (K) and a carrier. The two-component developer is composed of a carrier in which ferrite is used as a core and an insulating resin is coated around the toner, and a toner in which polyester is a main material and a colorant such as pigment or carbon black, a charge control agent, silica, titanium oxide, or the like is added Become. The carrier has a particle diameter of 10 to 50 μm, a saturation magnetization of 10 to 80 emu / g, the toner has a particle diameter of 4 to 10 μm, the toner has a negative charging characteristic, and the average charge amount is −20 to −60 μC / g. As the two-component developer, a mixture of these carrier and toner so that the toner concentration is 4 to 10% by mass is used.

  The belt-like intermediate transfer belt 6 is rotatably supported by a plurality of rollers. As the intermediate transfer belt 6 that functions as an image carrier, a polyimide semiconductor belt having a thickness of 80 μm and a volume resistivity of 8 to 11 LOGΩ · cm using polyimide as a material is used. The volume resistivity is measured according to a double ring electrode method (IEC 60093, ASTM D257, JIS K6911, JIS K6271).

  The toner images of the respective colors formed on the respective photoreceptors 1 from the image forming units 10Y, 10M, 10C, and 10K are sequentially transferred onto the rotating intermediate transfer belt 6 by the primary transfer roller 7 (primary transfer). A synthesized color image is formed. On the other hand, the residual toner is removed from each photoconductor 1 after image transfer by the cleaning roller of the cleaning unit 5 of each color.

  The sheet feeding / conveying unit 20 includes sheet storage units 291, 292, 293, a first sheet feeding unit 21, a sheet feeding roller 22, a registration roller 23, a duplex conveying path 24, and a sheet discharging roller 25. The registration roller 23 is connected to a clutch (not shown) and a drive motor. By performing stop and rotation control, the registration roller 23 is synchronized with the timing at which the toner image formed on the intermediate transfer belt 6 reaches the transfer nip N3. Thus, the sheet S is conveyed so that the sheet S reaches the transfer nip N3.

  A plurality of sheets S can be stored in the sheet storage units 291, 292, and 293, and the stored sheets S are fed by the first sheet feeding unit 21 and conveyed by the sheet feeding roller 22 and the registration roller 23. It is conveyed to the secondary transfer unit 30 downstream in the direction.

  The conveyed paper is conveyed to the secondary transfer unit 30 through the pre-transfer charging unit 80 and the pre-transfer charge eliminating unit 90, and is formed on the intermediate transfer belt 6 in the transfer nip N3 of the secondary transfer unit 30. Is transferred onto the sheet S (secondary transfer).

  By applying heat and pressure to the sheet S on which the color image is transferred in the fixing device 40, the color image (or toner image) is fixed on the sheet S. Thereafter, the paper is discharged from a paper discharge roller 25 provided in the paper discharge conveyance path, and placed on a paper discharge tray outside the apparatus.

  On the other hand, after the color image is transferred onto the paper S by the secondary transfer unit 30, the residual toner is removed by the belt cleaning unit 61 from the intermediate transfer belt 6 that has separated the curvature of the paper S.

  When images are formed on both sides of the paper S, the image formed on the first side of the paper S is fixed, and then the paper S is branched from the paper discharge conveyance path by the branch plate and introduced into the double-side conveyance path 24. Then, the paper is reversed and then conveyed again through the paper feed roller 22 and the like. A toner image formed on the intermediate transfer belt 6 is transferred onto the second surface of the sheet S by the image forming units 10Y, 10M, 10C, and 10K, and is heated and pressed by the fixing device 40, and then discharged. The paper roller 25 discharges the apparatus.

  FIG. 2 is a control block diagram illustrating a hardware configuration of the image forming apparatus A. In the drawing, the main parts necessary for explaining the operation of the present embodiment are mainly described, and other known parts of the image forming apparatus are omitted. The image forming apparatus A includes an image forming unit 10, a paper feeding / conveying unit 20, a control unit 50, an operation display unit 60, a scanner 70, and high-voltage power supplies 101 and 102.

  The control unit 50 includes a CPU 51 and a memory 52. The memory 52 includes a RAM, a ROM, an HDD, and the like. The ROM stores programs and data for various controls. The RAM is used as a work area for the CPU 51 and temporarily stores programs and data necessary for the CPU 51 to execute control of the image forming apparatus A. Then, the CPU 51 executes control of the image forming apparatus A based on the program and data expanded in the RAM.

  The operation display unit 60 is, for example, a touch sensor superimposed on a display surface of an LCD (liquid crystal display), and receives information from a user and displays information. In the present embodiment, the user can set information on sheets stored in the sheet storage units 291, 292, and 293 using the operation display unit 60. As the paper information, information on special paper such as coated paper, embossed paper, rough paper and the like can be set in addition to the paper thickness information such as thick paper, thin paper, or the amount of paper. The set information is stored in the memory 52.

  The scanner 70 reads an original and generates image data. A document placed on a document table of the scanner 70 or a document conveyed to a reading position by an ADF (not shown) is scanned and exposed by an optical system of a document image scanning exposure apparatus and read by a line image sensor. It is. The analog signal photoelectrically converted by the line image sensor is subjected to analog processing, A / D conversion, shading correction, image compression processing, and the like in the control unit 50, and then input to the exposure units 3Y, 3M, 3C, and 3K. .

  The high-voltage power supplies 101 and 102 apply voltages to the pre-transfer charging unit 80 and the secondary transfer unit 30, which will be described later.

  The control unit 50 controls the outputs of the high-voltage power supplies 101 and 102 in accordance with the timing when the paper S is conveyed to the nips N1 and N3. In particular, among the paper storage units 291 to 293, a paper storage unit that is set to store paper with large unevenness, such as embossed paper, is selected. The power supply 101 is controlled to output a bias voltage to the pre-transfer charging unit 80.

<First Embodiment>
FIG. 3 is a schematic diagram showing the periphery of the secondary transfer unit 30 in the first embodiment. As shown in FIG. 3, the registration roller 23, the pre-transfer charging unit 80, the pre-transfer charge eliminating unit 90, and the secondary transfer unit 30 are arranged in order from the upstream side along the conveyance path (indicated by a one-dot broken line) of the paper S. ing.

(Secondary transfer part)
The secondary transfer unit 30 includes a counter roller 31 disposed on the inner peripheral surface side of the intermediate transfer belt 6 and a secondary transfer roller 32, and a high-voltage power source 102 connects the secondary transfer roller 32 and the counter roller 31. In the meantime, a voltage for forming a transfer electric field is applied to the secondary transfer roller 32. The voltage to be applied is a voltage having a polarity opposite to the normal charging polarity (minus polarity) of the toner.

  Both the opposing roller 31 and the secondary transfer roller 32 are NBR (Nitrile Butadiene Rubber), a rubber hardness of 71 ° (Asker-C), and a volume resistance value of 5 to 10 LOGΩ. The outer diameters of the opposing roller 31 and the secondary transfer roller 32 are φ30 mm and φ38 mm straight, respectively.

  At least one of the opposing roller 31 and the secondary transfer roller 32 is urged with a pressing force of 80 N with respect to the other via the intermediate transfer belt 6. The axial length of the transfer nip N3 formed by both rollers is 340 mm. Note that the positions and lengths in the axial direction of nips N2 and N3 formed by the pre-transfer charging unit 80 and the pre-transfer charge eliminating unit 90 described below are set to be equal to those of the transfer nip N3.

(Charging part before transfer)
The pre-transfer charging unit 80 includes a brush roller 81 and a counter roller 82.

The brush roller 81 is a brush-like charging member composed of a core metal 811 and brush fibers 812. The outer diameter of the brush roller 81 is φ20 mm. The brush fibers 812 are planted on the peripheral surface of the cored bar 811 and extend from the surface of the cored bar 811 in the normal direction. As the brush fiber 812, nylon, polyester, acrylic, rayon, or the like can be used, and the resistance can be adjusted by adding a conductive material such as carbon black into the fiber. Each brush fiber 812 has a hair length of 1.0 to 5.0 mm, a fineness of 2 to 10 d (denier), and a yarn resistance of 7.5 to 11.5 logΩ. The flocking density of the brush fibers 812 is 120 to 430 kF / inch ² . A fineness of 2 to 10 d (denier) corresponds to a brush fiber diameter of φ10 to 40 μm.

  As the opposing roller 82, a metal roller made of SUS304 may be used, or the secondary transfer roller 32 may be diverted.

  The distance between the shaft centers of the brush roller 81 and the opposing roller 82 is set to be equal to or shorter than the sum of the outer diameters of the two. In the nip N <b> 2 formed by the brush roller 81 and the counter roller 82, the brush fiber 812 comes into contact with the paper S to be conveyed. The brush roller 81 is connected to a drive motor and rotates. As the rotation direction, for example, as shown in FIG. That is, the conveyance direction of the paper S and the movement direction of the brush fibers 812 are set to be opposite in the nip N1. The speed ratio θ of the outer peripheral moving speed is preferably in the range of 0.5 to 2.0, and more preferably θ = 1.0 where the paper transport speed and the outer peripheral moving speed of the brush roller 81 are equal. When the speed ratio θ is larger, the charging performance to the paper S is improved, but on the other hand, the paper transportability is deteriorated. In this embodiment, the outer peripheral surface of the brush roller 81 is the paper transport speed at the nip N1. Therefore, the speed ratio θ is 1.0.

  A voltage having a polarity opposite to the normal charging polarity (minus polarity) of the toner is applied to the brush roller 81 by the high voltage power source 101. The applied voltage is preferably about +3.0 to +6.0 kV, more preferably +4.0 kV. As the applied voltage is increased, the charging performance is improved, but on the other hand, the sheet conveying performance is deteriorated. Therefore, in the present embodiment, the applied voltage is set to +4.0 kV.

(Pre-transfer neutralization unit)
The pre-transfer charge removal unit 90 includes a charge removal upper roller 91 and a charge removal lower roller 92. The static elimination upper roller 91 and the static elimination lower roller 92 are both metal rollers made of SUS304 or the like. The outer diameter is 20 to 40 mm, respectively. These rollers are GND connected. By transporting the paper S charged by the pre-transfer charging unit 80 through the nip N2 constituted by the static elimination upper roller 91 and the static elimination lower roller 92, a part of the charge applied to the surface of the paper S is removed. .

  The static elimination upper roller 91 is configured by a counter roller 31 having a diameter of 30 mm and a roller whose outer peripheral surface is coated with the same material as the belt of the intermediate transfer belt 6, and the static elimination lower roller 92 is configured by a secondary transfer roller 32 having a diameter of 38 mm. Thus, they may be urged with a pressing force that gives the same load as that of the secondary transfer unit 30. As a result, the area where the secondary transfer belt 6 is in direct contact with the area where the static elimination upper roller 91 is in direct contact with each other on the surface of the paper having a predetermined unevenness or more. That is, sufficient transfer performance can be ensured in the transfer nip N3 where the secondary transfer belt 6 is in direct contact with the convex portion or shallow concave portion of the surface of the paper S without applying a charge by pre-transfer charging. The pre-transfer charge removal unit 90 can selectively remove only unnecessary charges existing in the convex and shallow concave regions on the surface of the paper S.

  Further, the brush roller 81 may be applied with a voltage instead of the GND connection. In this case, a high-voltage power supply different from the high-voltage power supply 101 is provided, and thereby a voltage having the same polarity as the normal charging polarity of the toner is applied to the static elimination upper roller 91. The applied voltage is preferably -3.5 kV.

(Issues and effects)
Next, referring to FIG. 4 to FIG. 7, when a toner image is transferred to a sheet S with large unevenness according to the present embodiment, a charge is applied to the surface using a brush, and a part of the applied charge is applied. The effect by the structure to remove is demonstrated.

  FIG. 4 is a schematic diagram for explaining the transfer failure in the comparative example. FIG. 4A shows the uneven state on the surface of the embossed paper. As an example of the embossed paper, there is Rezak 66 (trade name) of Special Paper Manufacturing Co., Ltd., and in this case, the height of the unevenness is a maximum of 100 to 150 μm. This is sufficiently larger than the toner particle size (4 to 10 μm). In the transfer nip N3, on a sheet with large unevenness such as embossed paper, the toner can come into contact with the surface of the paper S with respect to the convex part of the paper S, but the toner is in the concave part of the paper S. A gap will be formed without contacting.

  At the position of the transfer nip N3, the convex portion on the surface of the paper comes into contact with the toner on the intermediate transfer belt 6, so that the transferability is good. However, the concave portion is not in contact with the toner, so no pressure acts on the toner. The transfer is performed only by the force acting on the toner by the transfer electric field formed between the intermediate transfer belt 6 and the sheet S.

  As shown in FIG. 4B, since the transfer electric field itself formed in the concave portion where the gap is generated is weak, the toner is not transferred to the concave portion of the paper S but remains on the intermediate transfer belt 6. . In this case, since the amount of toner to be transferred is reduced in the concave portion of the paper, the density is lighter than that of the periphery thereof, resulting in a defective image that is white.

  When the value of the DC component of the transfer bias applied to the secondary transfer roller 32 is increased so that the transfer of the recess is appropriate, or when the transfer bias with the AC component superimposed is used, the transfer of the recess is performed. Sex improves. On the other hand, the protrusions on the surface of the paper are excessively charged, causing local discharge to the toner and a phenomenon of attracting the surrounding toner by the charge, resulting in transfer defects such as transfer repelling and dust.

  Hereinafter, the effect of this embodiment for such a problem will be described. FIG. 5 is a diagram illustrating the state of the sheet S in each nip illustrated in FIG. 3, and FIGS. 5A to 5C correspond to the nips N1, N2, and N3, respectively.

  As shown in FIG. 5A, the sheet S conveyed to the nip N1 is charged by the pre-transfer charging unit 80 to which a positive DC voltage is applied. By using the brush roller 81, an improvement in charging performance for the paper can be expected. In particular, the brush fiber 812 having a smaller fiber diameter than the unevenness of the paper S is used so that the tip of the brush fiber 812 follows the fine unevenness of the paper S, so that the brush fiber 812 also contacts the bottom of the recess. Can be made. Further, even when the process speed (the conveyance speed of the paper S) becomes higher, the tip of the brush fiber 812 sufficiently follows the unevenness of the paper S, so that a high level of charging performance can be maintained. This is superior to the case where an elastic roller such as a foamed sponge roller or a rubber roller is used instead of the brush roller (details will be described later). Further, by using the brush fiber 812, the tip thereof is in point contact with the surface of the paper S, so that there are many discharge points, and the electric charge can be imparted to the paper S with higher performance than the elastic roller. .

  As shown in FIG. 5B, a part of the charge applied to the surface of the sheet S is removed from the sheet S conveyed to the nip N2 by the pre-transfer charge eliminating unit 90 connected to the GND. Specifically, the sheet S charged by the pre-transfer charging unit 80 at the nip N1 is conveyed to the nip N2. And the static elimination upper roller 91 of the pre-transfer static elimination part 90 selectively neutralizes only the unnecessary electric charge which exists in this area | region by directly contacting the area | region of the convex part and shallow recessed part of the conveyed paper S surface. .

  In the sheet S that passes through the nips N1 and N2 and is conveyed to the transfer nip N3, positive charges remain at the bottom of the concave portion on the surface. As a result, as shown in FIG. 5C, the toner is easily transferred to the bottom of the recess, and the toner can be uniformly transferred to the entire surface of the sheet S including the recess.

  6 to 8 are a schematic diagram illustrating a configuration around the secondary transfer unit 30 in a comparative example, and a schematic diagram illustrating a transfer defect. Unlike the first embodiment shown in FIG. 3, the comparative example shown in FIG. 6 uses an elastic roller 88 instead of the brush roller 81. The configurations of the pre-transfer charge eliminating unit 90, the secondary transfer unit 30, and the like other than the pre-transfer charging unit 80b are the same.

  As the elastic roller 88 of the pre-transfer charging unit 80b, a roller having NBR foaming, rubber hardness of 45 ° (Asker-C), φ38 mm, and a resistance value of 6.5 LOGΩ is used. The lower charging roller 89 is diverted from the secondary transfer roller 32 and has the same dimensions, materials, and physical properties as the secondary transfer roller 32.

  By using the pre-transfer charging unit 80b having such a configuration according to the comparative example, an effect equivalent to that of the pre-transfer charging unit 80 according to the first embodiment can be obtained under certain specific conditions. FIGS. 7A and 7B are diagrams illustrating the state of the sheet S at the nip N1b of the pre-transfer charging unit 80b according to the comparative example. Of these, FIG. 7A shows the state of the sheet S at a low to medium speed (for example, 300 mm / sec or less), and FIG. 7B shows a case where the process speed is high (for example, 500 mm / sec or more). The state of the paper S is shown.

  The elastic layer of the elastic roller 88 also has viscosity and has a time dependency on the deformation speed. As the process speed increases, the influence of viscosity increases, and there is a risk that the unevenness of the elastic layer can be sufficiently ensured. As shown in FIG. 7A, if the speed is low to medium, the elastic layer of the elastic roller 88 follows and deforms along the concave and convex shape of the paper S, so that the bottom of the concave portion of the paper S is sufficient. Can be charged. On the other hand, when the process speed is high, the elastic layer of the elastic roller 88 cannot be deformed following the shape of the irregular shape of the paper S as shown in FIG. In this case, charging to the bottom of the concave portion of the paper S becomes insufficient.

  When charging by the pre-transfer charging unit 81b at the nip N1b is insufficient, a positive charge is not applied to the bottom of the recess in the sheet S conveyed to the transfer nip N3 on the downstream side. As a result, as shown in FIG. 7C, the transfer using only the transfer electric field results in insufficient toner transfer to the concave portion of the paper S, and remains on the intermediate transfer belt 6 as untransferred toner.

FIG. 8 is a table showing a comparison result of the process speed and the transfer performance in the comparative example (FIG. 6) and the first embodiment (FIG. 3 and the like), and was evaluated according to the following criteria.
○: Quality can be ensured by applying voltage 3-5 kV to pre-transfer charging units 80, 80b. Δ: Quality can be secured by adjusting applied voltage to pre-transfer charging units 80, 80b. Quality cannot be ensured by adjusting the applied voltage. For this evaluation, using Rezak 66 (basis weight 150 g) as the evaluation paper, the applied voltage to the pre-transfer charging units 80 and 80b is 3 to 5 kV, and the pre-transfer neutralizing unit 90 is applied. The voltage was performed under the conditions of GND connection.

  As shown in FIG. 8, in the comparative example using the elastic roller 88, the transfer performance decreases as the process speed increases. This is because the elastic layer does not sufficiently follow the unevenness of the paper S. On the other hand, in the first embodiment using the brush roller 81, such a problem does not occur, and sufficient transfer performance can be ensured even if the process speed is high.

(Fiber diameter Df and the diameter Ds of the recess on the paper surface)
Next, a preferable range of the fiber diameter Df will be described with reference to FIGS. FIG. 9 is a schematic diagram illustrating the definition of the minimum concave diameter Ds of the concave portion on the paper surface.

  FIG. 9A is a diagram showing the surface of the paper S having large irregularities, and arbitrary 10 points were taken as measurement points, and the profile of the surface shape was observed as shown in FIG. 9B with a laser microscope. . The smallest concave diameter is extracted from the diameters of the plurality of concave portions included in each measurement point (indicated by “Ds1 to Ds3” in the figure), and the average value of the smallest concave diameters extracted from each of the ten measurement points is obtained. The minimum concave diameter Ds of the concave portion was defined.

FIG. 10 is a table showing the evaluation results of the transfer performance by the combination of the minimum concave diameter Ds and the fiber diameter Df of the brush fiber 812, and the evaluation was performed according to the following criteria.
○: Quality can be secured by applying voltage 3-5 kV to the pre-transfer charging unit 80 Δ: Quality can be secured by adjusting the voltage applied to the pre-transfer charging unit 80 ×: Also by adjustment of the voltage applied to the pre-transfer charging unit 80 Quality can not be ensured. For this evaluation, using Rezak 66 (basis weight 150 g) as an evaluation paper, the process speed is 300 mm / sec, the applied voltage of the pre-transfer charging unit 80 is 3-5 kV, and the applied voltage of the pre-transfer charge removing unit 90 is GND. Performed under connection conditions.

  As shown in FIG. 10, when the fiber diameter Df is 20 μm and 40 μm, all the sheets S having the minimum concave diameter Ds of 50 to 120 μm exhibit satisfactory transfer performance. On the other hand, when the fiber diameter Df is 60 μm, the transfer performance is Δ for the paper S having the minimum concave diameter Ds of 50 μm, and when the fiber diameter Df is 100 μm, the transfer performance is Δ for the paper S having the minimum concave diameter 80 μm. The transfer performance of the paper S having the minimum concave diameter of 50 μm is x.

  Therefore, it is preferable that the fiber diameter Df of the brush fiber 812 is smaller than the diameter of the concave and convex portions on the surface of the paper to be used, and it is particularly preferable that the fiber diameter Df is smaller than the minimum concave diameter Ds. In general embossed paper, the minimum concave diameter Ds is about 50 μm, so that the preferable fiber diameter Df is 60 μm or less, and it is more preferably 40 μm or less because it is unnecessary to adjust the applied voltage. By selecting brush fibers having such a fineness (denier) as the fiber diameter Df, it is possible to ensure sufficient transfer performance for many embossed papers.

  As described above, according to the present embodiment, the precharged charging unit 80 including the brush roller 81 serving as the brush-like charging member applies a charge to a paper having unevenness such as an embossed paper, and the applied charge. A part of the electric charge corresponding to the unevenness on the surface of the paper is removed by the pre-transfer charge eliminating unit 90, and then the toner is transferred onto the paper by the transfer unit 30. By adopting such a configuration, it is possible to form a high-quality image because it is possible to achieve both improvement in transferability at the concave portion and improvement in transfer failure due to electric discharge at the convex portion even for paper with unevenness. Become. In particular, by setting the fiber diameter Df of the brush fiber 812 to be smaller than the diameter of the concave portion of the paper with unevenness, the brush fiber 812 follows the unevenness of the paper surface, so that sufficient charging performance can be achieved even in a wide range of process speeds. Can be secured. As a result, it is possible to form a high-quality image that achieves both improvement in transferability at the concave portions and improvement in transfer failure due to discharge at the convex portions on a sheet having large irregularities.

(Second Embodiment)
FIG. 11 is a diagram illustrating a pre-transfer charging unit 80 which is a main part of the image forming apparatus A according to the second embodiment. In the pre-transfer charging unit 80 shown in the figure, a flicker member 83 is provided. The configuration other than the flicker member 83 is the same as that of the first embodiment, and is not shown. The flicker member 83 shown in the figure is in contact with the tip of the brush fiber 812 of the rotating brush roller 81 and has an effect of returning the tip of the brush fiber 812 to return the fallen fiber of the brush fiber 812.

  The flicker member 83 in the second embodiment is a rod made of a roller-like rigid material having a circular cross section extending in the direction of the rotation axis of the brush roller 81, and is fixedly disposed at a position where the brush fibers 812 are in contact with each other. Has been. An example of the flicker member 83 is a φ8 mm rod made of a metal (SUS304) material. Further, the flicker member 83 is connected to the high-voltage power supply 101 common to the brush roller 81, but may be connected to GND via a high-resistance resistor. The flicker member 83 is not limited to such a configuration as long as it has a function of repelling the tip side of the brush fiber 812. A plate-like member having a rectangular cross section may be applied as the flicker member. As another example, if it is a roller-shaped member, it can be driven by a drive motor, and the brush roller 81 can be rotated in the same direction or in the opposite direction with respect to the rotation direction of the brush roller 81, or the rotation speed can be set together with the rotation direction. You may make it the structure which can be varied.

  Further, the hair fall-back recovery process by the flicker member 83 may be executed at a predetermined timing (at the time of the startup process when the main body power switch is turned on, every predetermined period, etc.). An example of the fall-over process is to rotate the normally stopped flicker member 83 at a predetermined timing by the drive motor described above for a predetermined time. As another example, a high voltage power source different from the high voltage power source 101 may be provided so that an electric field can be formed between the flicker member 83 and the brush roller 81. After such a configuration, an electric field that raises the brush fibers 812 may be formed between the flicker member 83 and the cored bar 811 of the brush roller 81 as a fall-over process at a predetermined timing.

  The effects of the first embodiment can also be obtained in the second embodiment having such a configuration. In addition to this effect, in the second embodiment, by using the flicker member 83, the charging performance of the brush roller 81 can be maintained at a high level for a long period of time.

  In particular, when the elastic roller 88 as in the comparative example of FIG. 6 is used, when small-sized paper (for example, “postcard” size) is used continuously, the elastic roller 88 is elastic at a position corresponding to the edge portion of the paper. The roller 88 is worn, and the charging performance at that portion is reduced. Thereafter, when a large size (for example, “A4” size) paper with large unevenness is used, the pre-transfer charging becomes insufficient at the worn portion, and the transferability to the concave portion of the paper is deteriorated. In this regard, by using the brush roller 81 as in the second embodiment, the wear at the edge portion of the small size paper does not occur, and even if the hair falls down at that position, the flicker member is used. The brush fibers 812 can be easily raised. This is excellent in that the problem that occurs in the comparative example does not occur.

(Third embodiment)
FIG. 12 is a diagram illustrating a main part of the image forming apparatus A according to the third embodiment. In the drawing, only the configuration around the secondary transfer unit 30 is shown, and the other configurations are the same as those of the image forming apparatus A according to the first embodiment.

  As shown in the figure, the pre-transfer charging unit 80c includes a brush bar 85 as a brush-shaped charging member. The brush bar 85 includes a main body portion 851 made of a conductive material such as metal and brush fibers 852 planted on the main body portion 851. The main body 851 is a member having a rectangular cross section and extending in an axial direction orthogonal to the transport direction of the paper S. The brush fiber 852 has the same material, shape, and flocking density as the brush fiber 812. The pre-transfer charging unit 80c further includes a shielding plate 84, an elastic member 86 formed of a spring or the like, and a contact pressure variable mechanism 87 formed of an eccentric cam or the like. A voltage is applied to the main body 851 by the high voltage power supply 101. The output voltage of the high-voltage power supply 101 is equivalent to that in the first embodiment. The shielding plate 84 is made of an insulator and houses the elastic member 86 and the brush bar 85, and the brush bar 85 restricts movement in a direction other than the direction perpendicular to the paper surface of the paper (other than the vertical direction in FIG. 12). Further, the shielding plate 84 is not electrically connected to the conveyance guide plate 26.

  The brush bar 85 is urged by the elastic member 86 toward the lower conveyance path with a predetermined pressure. The brush fiber 852 slides on the paper S that is transported along the transport path. A claw-like stop claw 841 protrudes from the inner surface side of the shielding plate 84 and comes into contact with the lower surface of the main body portion 851. Thereby, the downward movement of the brush bar 85 is limited to the lower limit position. The lower limit position is set so that the tip of the brush fiber 852 is in contact with the surface of the paper S being conveyed. The contact pressure variable mechanism 87 can change the urging force of the elastic member 86 to the brush bar 85 by rotating a predetermined angle by a drive source (not shown 9). Accordingly, for example, the biasing force may be increased when a paper having unevenness such as embossed paper is used, and the biasing force may be released when other paper is used.

  The pre-transfer neutralization unit 90b includes a sheet neutralization plate 95 made of a conductive elastic member such as a thin metal plate. The sheet neutralizing plate 95 is a rectangular plate extending in the axial direction, and one end in the width direction is attached to the shielding plate 84, and the other end is a free end and is movable by springiness. As shown in FIG. 13, the other end is disposed so as to straddle the sheet conveyance path, and contacts the surface of the sheet conveyed through the conveyance path. The sheet neutralizing plate 95 is GND-connected, and removes a part of the charge applied to the surface of the sheet by contacting the sheet S being conveyed. A high-voltage power supply different from the high-voltage power supply 101 may be provided for the sheet neutralizing plate 95 so that a voltage having the same polarity as the normal charging polarity of the toner may be applied to the sheet neutralizing plate 95.

  In the third embodiment having such a configuration, a charge is applied to a paper having unevenness such as an embossed paper by a pre-transfer charging unit 80c including a brush bar 85 as a brush-like charging member, and the application is performed. A part of the electric charge corresponding to the unevenness on the surface of the paper is removed by the pre-transfer charge eliminating unit 90b, and then the toner is transferred onto the paper by the transfer unit 30. The effect of the first embodiment can also be obtained in the third embodiment. Furthermore, in addition to this effect, in the third embodiment, the conveyance guide plate 26, the pre-transfer charging unit 80c, and the pre-transfer charge eliminating unit 90b can be integrated. By integrating, the device can be miniaturized. In addition, there is an effect that the degree of freedom of design related to the arrangement of these members is increased.

(Modification)
FIG. 13 is a diagram illustrating a partial configuration of the pre-transfer charging units 80 and 80c according to the modification. As shown in the figure, each brush fiber 812b, 852b is subjected to an oblique hair treatment by an oblique hair process.

  As shown in FIG. 13A, the tip of the brush fiber 812 b faces the upstream side in the rotation direction of the brush roller 81 by the oblique hair treatment. As shown in FIG. 13B, the tip of the brush fiber 852b is directed to the upstream side in the paper conveyance direction by the oblique hair treatment.

  By applying the oblique hair treatment in such a direction, the tips of the brush fibers 812b and 852b are directed to the conveyed paper S, so that the tips of the brush fibers are more likely to come into contact with the concave portion of the paper. This further improves the charging performance.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to these embodiment. In particular, the configuration described in each embodiment may be variously combined.

  In this embodiment, the tandem color image forming apparatus using the intermediate transfer belt is illustrated, but the present invention is not limited to this. In the present invention, the intermediate transfer belt is used like a monochrome image forming apparatus or an image forming apparatus that sequentially transfers images of each color formed on a plurality of photosensitive members onto a sheet conveyed on a secondary transfer belt. You may apply to the image forming apparatus of the system which transfers the toner image formed in the photoreceptor to a paper directly, without using.

  Also, depending on the user, the presence or absence of paper with large unevenness, the diameter and depth of the unevenness varies, and the brush conditions (length, fiber diameter) and speed difference of the brush-like charging member depend on the paper used. θ and the voltage applied by the high voltage power source 101 may be changed.

  In addition, the present invention is defined by the contents described in the claims, and various modifications are possible.

A image forming apparatus,
S paper,
10 Image forming unit,
1 Photoconductor,
2 band electrodes,
3 exposure part,
4 Development section,
5 Cleaning section,
6 Intermediate transfer belt (image carrier),
7 Primary transfer part,
20 paper feeding and conveying section,
23 Registration roller,
30 Secondary transfer part,
31 Opposing roller,
32 secondary transfer roller,
40 fixing device,
50 control unit,
51 CPU,
52 memory,
60 operation display section,
70 scanner,
80, 80b, 80c pre-transfer charging unit,
81 brush roller,
811 Core,
812 brush fiber,
82 opposite roller,
83 Flicker member,
88 elastic roller,
89 Opposing roller,
84 Shield plate,
85 brush bars,
851 body part,
852 brush fibers,
86 elastic members,
87 Contact pressure variable mechanism,
90, 90b
91 Static elimination upper roller,
92 Roller under static elimination,
95 Paper neutralization board,
101, 102 High voltage power supply.

Claims (7)

An image forming unit for forming a toner image on the image carrier;
A transport unit for transporting paper;
A pre-transfer charging unit that includes a brush-shaped charging member and applies a charge opposite to the normal charging polarity of the toner to the paper conveyed by the conveying unit by the brush-shaped charging member;
A part of the electric charge applied to the surface of the paper by the pre-transfer charging unit on the downstream side of the pre-transfer charging unit in the paper conveyance direction, and a part of the electric charge corresponding to the unevenness of the paper surface is removed. A static neutralizer to perform,
A toner image formed on the image carrier by the transfer electric field formed by applying a voltage having a polarity opposite to the normal charging polarity of the toner on the downstream side of the sheet conveyance direction from the charge eliminating unit is applied to the sheet. A transfer section to be transferred;
An image forming apparatus.   The image forming apparatus according to claim 1, wherein the brush fibers of the brush-shaped charging member are in contact with the paper, and the fiber diameter of the brush fibers is smaller than the diameter of the concave and convex portions on the surface of the paper used in the image forming apparatus. .   The image forming apparatus according to claim 1, wherein the brush fibers of the brush-shaped charging member are in contact with the paper, and the fiber diameter of the brush fibers is 40 μm or less.   The image forming apparatus according to claim 1, wherein the brush-like charging member is a brush roller that is rotated by a driving source.   5. The image forming apparatus according to claim 4, wherein the rotation direction of the brush roller is set so that the conveyance direction of the paper and the movement direction of the outer periphery of the brush roller are opposite to each other at a contact position with the paper.   The image forming apparatus according to claim 4, further comprising a flicker member that abuts against a tip of the brush fiber of the brush roller to restore the fall of the brush fiber.   The image forming apparatus according to claim 1, wherein the brush-shaped charging member is a brush bar, and slides on a surface of a sheet on which brush fibers of the brush bar are conveyed.