JP3968342B2 - Double-sided transfer device and image forming apparatus - Google Patents

Description

  The present invention relates to a double-sided transfer device and an image forming apparatus that transfer a toner image onto both sides of a transfer body such as transfer paper by a so-called one-pass method.

  Conventionally, a so-called switchback method and a one-pass method are known as methods for transferring an image onto both surfaces of a recording medium such as transfer paper. In the switchback method, first, a toner image is transferred onto one surface of a transfer member that transfers a toner image on an image carrier such as a photosensitive member to a transfer member through the transfer member. Then, after the transfer body is inverted and switched back to the transfer means, the toner image is transferred to the other surface. On the other hand, in the one-pass method, as a transfer unit, a transfer process is performed on the transfer body from both the front and back sides of the transfer body, thereby forming images on both sides without switching back the transfer body. The one-pass method is superior to the switchback method in the following points. That is, it is possible to avoid an increase in cost by providing a complicated mechanism for switchback and an increase in image formation time due to switchback.

  When performing such one-pass double-sided transfer, how to avoid reverse transfer is an important factor. The reverse transfer referred to here is a phenomenon in which the toner image once transferred to the transfer body is reversely transferred from the transfer body to a pressing body such as a pressing roller in contact therewith. This reverse transfer occurs when an electrostatic force is applied to the toner image on the surface of the transfer body in the direction opposite to the direction of pressing toward the surface. Specifically, for example, after negatively charged first toner image is electrostatically transferred to the front surface of the transfer paper, another negative second toner image is electrostatically transferred to the back surface of the transfer paper. Suppose that. In the previous transfer, it is necessary to apply an electrostatic force toward the front surface of the transfer paper to the first toner image positioned on the front surface side of the transfer paper. On the other hand, in the latter transfer, it is necessary to apply an electrostatic force toward the back surface of the transfer paper to the second toner image positioned on the back surface side of the transfer paper. During the subsequent transfer, an electrostatic force that separates from the front surface is applied to the first toner image that has been transferred to the front surface of the transfer paper. The electrostatic force causes the first toner image to be reversely transferred from the front surface to a roller or the like that contacts the first toner image.

  In view of this, Japanese Patent Application Laid-Open No. H10-228667 proposes an image forming apparatus that transfers a toner image onto both sides of a transfer sheet using a pair of photosensitive members for individually forming toner images having different charging polarities. In this image forming apparatus, a transfer sheet is sandwiched between transfer positions that rotate while a pair of photoreceptors carrying toner images having different polarities are in contact with each other. It is assumed that an electric field that applies an electrostatic force in a direction toward the transfer paper is formed on one toner image that is negatively charged while being in contact with one surface of the transfer paper at this transfer position. Then, the electrostatic force in the direction toward the transfer paper can be applied to the other toner image that is positively charged on the opposite surface. Therefore, one-pass double-sided transfer can be realized without reversely transferring the toner image to the photoconductor. However, in such a configuration, a pair of photosensitive members, an optical writing unit, a developing unit, and the like for forming toner images having different polarities must be provided. For this reason, there is a problem that the number of types of parts is much larger and the maintainability is worse than that using only the toner having the same charge polarity (hereinafter, referred to as problem 1). Further, it is necessary to distinguish and manage toners that are different in charging polarity but have almost the same color appearance, which further deteriorates maintainability.

  On the other hand, there is known an image forming apparatus that transfers a toner image charged with the same polarity as the toner image onto a transfer paper having a toner image transferred on one surface thereof by a corona charger outside the transfer nip. (For example, those described in Patent Document 2). In this image forming apparatus, first, a first toner image is electrostatically transferred from the photoconductor to the intermediate transfer belt at a transfer nip where the photoconductor and the intermediate transfer belt come into contact with each other. Next, the timing at which the first toner image enters the transfer nip again along with the endless movement of the intermediate transfer belt is measured, and the transfer paper is fed into the transfer nip while forming the second toner image on the photoconductor. Then, in the transfer nip, the second toner image on the photoconductor, the transfer paper, and the first toner image on the intermediate transfer belt are sandwiched, and the second toner image on the photoconductor is transferred to the first of the transfer paper. Electrostatic transfer to two sides. At this time, since the electrostatic force that moves the first toner image on the intermediate transfer belt toward the belt is applied, it cannot be electrostatically transferred to the first surface of the transfer paper. The transfer paper that has passed through the transfer nip is separated from the surface of the photosensitive member, and is then conveyed along with the endless movement of the belt while being held on the surface of the intermediate transfer belt. Then, the corona charger is sent to a subsequent transfer section that faces the surface of the intermediate transfer belt with a predetermined gap. In the latter transfer section, the first toner image sandwiched between the transfer paper and the intermediate transfer belt is transferred from the belt to the first surface of the transfer paper by applying a charge to the second surface of the transfer paper by the corona charger. Electrostatic transfer is performed on one surface. In such a configuration, since two toner images are formed using only toners having the same charge polarity, one-pass double-sided transfer can be realized while eliminating the above-described deterioration in maintainability. However, there is a problem (hereinafter referred to as defect 2) that the second toner image on the second surface of the transfer paper is disturbed by an impact during charging in the subsequent transfer unit.

JP-A-2-259670 JP-A-1-209470

  As a result of earnest research to eliminate all of the various problems described so far, the present inventors have developed a new image forming apparatus that performs one-pass double-sided transfer by an electrostatic extrusion method (repulsive force method). It came. This image forming apparatus can electrostatically transfer two toner images charged to the same polarity to a transfer sheet in the transfer nip. Specifically, in this image forming apparatus, two intermediate transfer belts, a first belt and a second belt, are used, and both belts are moved in an endless manner to form a transfer nip. The toner images having the same polarity are intermediately transferred to both belts, and then fed to the transfer nip. In the vicinity of the entrance of the transfer nip, both belts are sandwiched between a first belt front support roller supporting the first belt and a second belt front support roller supporting the second belt. Further, transfer paper is sandwiched between both belts. Then, the toner image on the surface of one of the belts is electrostatically transferred to one of the surfaces of the transfer paper by a transfer electric field formed between the two rollers. In contrast, in the vicinity of the exit of the transfer nip, both belts are sandwiched between the first belt rear stage support roller supporting the first belt and the second belt rear stage support roller supporting the second belt. ing. Then, the other toner image is electrostatically transferred to the other surface of the transfer paper by the transfer electric field formed between both rollers. This electrostatic transfer is performed by an electrostatic extrusion method. Rather than the general electrostatic attraction method in which the toner image is electrostatically attracted toward the transfer bias roller to which the transfer bias is applied, the toner image is electrostatically pushed out from the transfer bias roller side toward the transfer paper. It is. At this time, theoretically, reverse transfer of the toner image is caused. Of the two rear support rollers, the electric field that electrostatically pushes one toner image toward the transfer paper from the one that functions as the transfer bias roller causes the other toner image to move statically from the transfer paper surface toward the other roller. This is because they are pushed out electrically. However, according to the experiments by the present inventors, the reason is not clear. However, when the second-stage transfer performed near the exit of the transfer nip is performed by the electrostatic extrusion method, one toner is not reversely transferred to the other toner image. It was possible to transfer the image. As long as the post-transfer is the electrostatic extrusion method, the pre-transfer performed near the entrance of the transfer nip may be either the electrostatic attraction method or the electrostatic extrusion method.

  However, the new image forming apparatus having such a configuration has a new problem that it is difficult to separate the transfer paper after passing through the transfer nip from the belt. This problem will be described in detail with reference to FIG. 1, which is a schematic diagram showing an example of a transfer nip that realizes one-pass double-side transfer by an electrostatic extrusion method. In the figure, a first belt 9 as an intermediate transfer belt is endlessly moved counterclockwise in the figure while being supported by a first nip inlet roller 12, a first nip outlet roller 13, and the like. The second belt 31 as an intermediate transfer belt is endlessly moved clockwise in the drawing while being supported by the second nip entrance roller 33, the second nip exit roller 34, and the like. The transfer nip is in contact with the first belt extension portion from the first nip entrance roller 12 to the first nip exit roller 13 and the second belt extension portion from the second nip entrance roller 33 to the second nip exit roller 34. Is formed by. Prior to each of the first belt 9 and the second belt 31 entering the transfer nip, a toner image made of negative polarity toner is transferred onto the front surface in a region not shown. Then, in synchronization with the toner images entering the transfer nip, the transfer paper P is sent to the transfer nip by a registration roller (not shown) and is sandwiched between both belts.

  At the transfer nip entrance, a transfer electric field is formed between the first nip entrance roller 12 to which a negative polarity transfer bias is applied and the grounded second nip entrance roller 33. As a result, the toner image on the surface of the first belt 9 is electrostatically transferred to the first surface (the left surface in the figure), which is one surface of the transfer paper P, by the electrostatic extrusion method. On the other hand, at the transfer nip exit, a post-transfer electric field is formed between the second nip exit roller 34 to which a negative transfer bias is applied and the grounded first nip exit roller 13. As a result, the toner image on the surface of the second belt 31 is electrostatically transferred to the second surface (the right surface in the drawing) which is the other surface of the transfer paper P by the electrostatic extrusion method.

  In the illustrated example, the transfer paper P having the toner image transferred on both sides in this way needs to be separated from both belts immediately after passing through the transfer nip. However, the transfer paper P after passing through the transfer nip is wound around the second belt 31 and cannot be separated well. In such a case, if it is single-sided transfer, it is possible to avoid winding by providing a separation claw for separating the transfer paper P from the second belt 31, but in the case of double-sided transfer, The unfixed image is disturbed by the contact. Therefore, it cannot respond with a separation nail.

  Therefore, the present inventors have conducted further research and found the following. That is, the transfer sheet P is electrostatically attracted to the one of the first belt 9 and the second belt 31 that is backed up by a roller having a smaller electric resistance at the preceding transfer section in the transfer nip. It is considered that the back-up to a roller having a small electric resistance makes the surface potential of the belt easier to attract the transfer paper P. Then, the transfer paper P once electrostatically attracted by the preceding transfer unit remains electrostatically adsorbed on the belt attracted by the preceding transfer unit, regardless of the state of the electric field in the subsequent transfer unit thereafter. This is probably because once the electrostatic adsorption is performed, the adsorption state cannot be solved unless a fairly strong electric field is applied.

  In the illustrated example, the metal second nip entrance roller 33 has a lower electrical resistance than the first nip entrance roller 12 having a surface layer made of an elastic material. The transfer paper P is wound around the second belt 31. As a test, when the second nip entrance roller 33 is a rubber roller having a relatively high electric resistance and the first nip entrance roller 12 is a metal roller, the transfer paper P after passing through the transfer nip is wound around the first belt 31. It was.

  In addition, the present inventors can satisfactorily separate the transfer paper P from both belts without providing a separation claw by adjusting the curvature (1 / R) of the movement trajectory of both belts after passing through the transfer nip. I also found that I can do it. Specifically, in the illustrated example, a second nip exit roller 34 having a considerably small diameter is used, and the curvature (1 / R) of the moving track of the second belt 31 after passing through the nip is set to the first belt 9. I tried to make it much larger than the ones. As a result, the transfer paper P was successfully separated from the second belt 31 while avoiding the wrapping of the transfer paper P around the second belt 31 after passing through the nip. However, if the first nip exit roller 13 having a considerably large diameter is used, the transfer paper P is wound around the first belt 9 this time. From these experimental results, by making the curvature (1 / R) of the belt that causes the winding of the transfer paper P due to electrostatic adsorption larger than the curvature (1 / R) of the belt that does not cause the transfer paper P, Was found to be well separated.

  The present invention has been made in view of the above background, and an object thereof is to provide the following double-sided transfer device and an image forming apparatus including the same. That is, a double-sided transfer device and an image forming apparatus that can satisfactorily separate the transfer body after passing through the post-transfer portion from the belt without using the separation claw while avoiding the above-described problems 1 and 2. .

In order to achieve the above object, the invention of claim 1 is directed to a first nip inlet roller for supporting an endlessly moving first belt from the belt rear surface side, and a first nip outlet for supporting the first belt from the belt rear surface side. A roller, a second nip entrance roller for supporting the endlessly moving second belt from the back side of the belt, a second nip exit roller for supporting the second belt from the back side of the belt , and toner on the first nip entrance roller. Means for applying a transfer bias having the same polarity as the charging polarity, and means for applying a transfer bias having the same polarity as the charging polarity of the toner to the second nip exit roller, and the first belt formed by the first nip entrance roller. A pre-stage transfer portion is formed by contact between the support portion and the second belt support portion by the second nip entrance roller, and the first belt support portion by the first nip exit roller and the second belt A rear transfer portion is formed by contact with the second belt support portion by the nip exit roller, and one of the transfer members sandwiching the toner image carried on the front surface of the first belt between the front transfer portions. After the transfer to the surface, the toner image carried on the front surface of the second belt is transferred to the other surface of the transfer body sandwiched between the subsequent transfer portions, and the toner image is transferred to both surfaces of the transfer body. in the double-sided transfer device for electrostatically transferring, as an upper Symbol second nip entrance roller, the electrically grounded and the first nip inlet roller used as a metal which exhibits low electrical resistance than Rutotomoni, the transfer bias as the first nip entrance roller applied, using what surface layer made of a material containing a resistance adjusting agent in insulating elastic material is coated, the front transfer for the upper Symbol second belts The transfer body is electrostatically adsorbed by the As the first nip exit rollers, with use those electrically grounded and the metal which exhibits low electrical resistance than the second nip exit rollers, as the second nip exit rollers transfer bias is applied, By using a surface layer made of a material containing a resistance adjusting agent in an insulating elastic material, having a smaller diameter than the first nip exit roller and having a diameter of less than 20 [mm] , Of the first belt and the second belt that are separated from each other after passing through the rear transfer section, the second belt is caused to move with a larger curvature (1 / R) than the first belt. It is characterized by doing so .
Also, the invention of claim 2, in both transfer device 請 Motomeko 1, as the first belt and the second belt, as the surface roughness Rz of each front surface is 2 to 3 [[mu] m] It is characterized by using.
The invention according to claim 3 is the double-sided transfer device according to claim 1 or 2 , wherein the first belt and the second belt each have a surface resistance of 10 ⁵ to 10 ¹² [Ω / □]. It is characterized by this.
According to a fourth aspect of the present invention, an image is carried on both sides of a transfer body using an image carrier carrying a toner image and a double-side transfer means for transferring a visible image on the image carrier onto both sides of the transfer body. In the image forming apparatus for forming the image forming apparatus, the double-sided transfer device according to any one of claims 1 to 3 is used as the double-sided transfer unit.

In these inventions, the first belt and the second belt are brought into contact with each other to form transfer nips for the front transfer unit and the rear transfer unit, and the transfer nip for the rear transfer unit uses an electrostatic extrusion method. Transcription. In such a configuration, two toner images that are charged with the same polarity without forming two toner images with toners having different polarities or corona charging the toner images are transferred to the transfer nip where both belts abut. Can be electrostatically transferred to the opposite surfaces of the transfer paper. Therefore, it is possible to transfer the toner image onto both surfaces of the transfer body while avoiding deterioration in maintainability (fault 1) due to the use of toners having different polarities and disturbance of the toner image due to the impact of corona charge (fault 2). it can.
Further, in these inventions, the curvature of the movement locus of the second belt, around which the transfer body is to be wound, out of the first belt and the second belt, which are separated from each other after passing through the rear transfer portion, is expressed by the first belt. By making it larger than the curvature of the movement trajectory, the transfer body after passing through the latter transfer section can be satisfactorily separated from the former belt without providing a separation claw.
Therefore, in these inventions, it is possible to satisfactorily separate the transfer body after passing through the rear transfer portion from the belt without using the separation claw while avoiding the above-described problems 1 and 2.

Hereinafter, as an image forming apparatus to which the present invention is applied, an embodiment of an electrophotographic printer (hereinafter simply referred to as a printer) will be described.
First, the basic configuration of the printer will be described. FIG. 2 is a schematic configuration diagram of the printer, showing the printer from the front direction. In the figure, the printer 100 includes four process cartridges 6Y, M, C, and K for generating toner images of yellow, magenta, cyan, and black (hereinafter referred to as Y, M, C, and K). Yes. These use Y, M, C, and K toners of different colors as the image forming material, but the other configurations are the same and are replaced when the lifetime is reached. Taking a process cartridge 6Y for generating a Y toner image as an example, as shown in FIG. 3, a drum-shaped photosensitive member 1Y as an image carrier, a drum cleaning device 2Y, a charge eliminating device 3Y, a charging device 4Y, and a developing device 5Y. Etc. In the photoreceptor 1Y, an aluminum cylinder having a diameter of 30 to 100 [mm] is covered with a surface layer of an organic semiconductor that is a photoconductive substance. An amorphous silicon surface layer may be coated. Further, it may be a belt shape instead of a drum shape. The charging device 4Y uniformly charges the surface of the photoreceptor 1Y that is rotated clockwise in the drawing by a driving unit (not shown). The uniformly charged surface of the photoreceptor 1 </ b> Y is exposed and scanned by the laser beam L to carry a Y electrostatic latent image. The Y electrostatic latent image is developed into a Y toner image by the developing device 5Y using Y toner. Then, primary transfer is performed on a first belt 9 described later. The drum cleaning device 2Y removes the toner remaining on the surface of the photoreceptor 1Y after the intermediate transfer process. Further, the charge removal device 3Y removes the residual charge of the photoreceptor 1Y after cleaning. By this charge removal, the surface of the photoreceptor 1Y is initialized and prepared for the next image formation. In the other process cartridges 6M, C, and K, M, C, and K toner images are similarly formed on the photoreceptors 1M, C, and K, and are primarily transferred onto the first belt 9. The developing device may use a two-component developer containing toner and a magnetic carrier, or may use only toner powder. Further, the toner may be a pulverized toner manufactured by a pulverization method or a toner formed in a spherical shape, and a toner having a particle size of about 6 [μm] is preferable.

  In FIG. 2, the exposure device 7 is disposed above the process cartridges 6Y, 6M, 6C, and 6K, and the image data processing device E1 is disposed on the left side in the drawing. . The image data processing device E1 generates an exposure scanning control signal based on an image information signal sent from a personal computer or the like and sends it to the exposure device 7. The exposure device 7 serving as a latent image forming unit irradiates each photoconductor in the process cartridges 6Y, 6M, 6C, and 6K with a laser beam L emitted based on the exposure scanning control signal. Electrostatic latent images for Y, M, C, and K are formed on the photoreceptors 1Y, 1M, 1C, and 1K exposed by this irradiation. The exposure device 7 irradiates the photoconductor with a laser beam (L) emitted from a light source through a plurality of optical lenses and mirrors while scanning with a polygon mirror rotated by a motor. Instead of the exposure apparatus 7 having such a configuration, an exposure unit that irradiates LED light from the LED array may be employed. In addition, a sealing member (not shown) is provided on the casing of the exposure device 7 in order to prevent contamination of internal parts due to the toner falling from the photosensitive members 1Y, 1M, 1C, and 1K disposed above the exposure device 7. Provided.

  On the lower side of the exposure apparatus 7 in the figure, a first paper cassette 50 having a first paper feed roller 50a and a second paper cassette 51 having a second paper feed roller 51a are arranged so as to overlap in the vertical direction. It is installed. Each of these cassettes is housed in a state of a transfer paper bundle in which a plurality of transfer papers P as transfer bodies are stacked.

  On the right side of the two sheet feeding cassettes (50, 51) in the figure, a sheet feeding path 42 extending in the vertical direction in the figure is disposed, and a pair of registration rollers 53 is provided near the ends thereof. . The first paper feed roller 50a and the second paper feed roller 51a are in contact with the first transfer paper P of the transfer paper bundle accommodated in the first paper cassette 50 and the second paper cassette 51, respectively. Yes. Then, the uppermost transfer paper P is sent out toward the paper feed path 52 by being driven to rotate by a driving means (not shown). The fed transfer paper P is sandwiched between the rollers of the registration roller pair 53 in the paper feed path 52. The registration roller pair 53 rotates both rollers in the forward direction so as to sandwich the transfer paper P, but once the transfer member is sandwiched between the rollers, the rotation of both rollers is temporarily stopped. Then, rotation is resumed at an appropriate timing, and the transfer paper P is sent out toward a two-stage transfer nip described later. That is, the registration roller pair 53 functions as a timing roller pair.

  The electrostatic latent images for Y, M, C, and K formed on the respective photoreceptors 1Y, 1M, 1C, and 1K undergo a primary transfer process by the first transfer unit 8 of the double-side transfer device. The first transfer unit 8 is arranged below the process cartridges 6Y, 6M, 6C, and 6K in the drawing, and includes a first belt 9 that is an intermediate transfer belt, and four primary transfer rollers 10Y, 10M, and 10C. , K, the first cleaning device 11 and the like. Also provided are a first nip inlet roller 12, a first nip outlet roller 13, a first cleaning backup roller 14, a driving roller 15, a tension roller 16, and the like. The first belt 9 is endlessly moved counterclockwise in the figure by the rotational drive of the driving roller 15 while being supported by the plurality of rollers. The four primary transfer rollers 10Y, 10M, 10C, and 10K serving as transfer bias rollers sandwich the first belt 9 thus moved endlessly between the photoreceptors 1Y, 1M, 1C, 1K, and perform primary transfer. A nip is formed. Then, upon receiving a power supply (not shown), a primary transfer bias having a polarity (for example, plus) opposite to that of the toner is applied to the back surface (loop inner peripheral surface) of the first belt 9.

  The first belt 9 sequentially passes through the primary transfer nips for Y, M, C, and K along with the endless movement thereof. At each primary transfer nip, Y, M, C, and K toner images on the photoreceptors 1Y, 1M, 1C, and 1K are primarily transferred in an overlapping manner by the action of nip pressure and primary transfer bias. As a result, a four-color superimposed toner image (hereinafter referred to as a four-color toner image) is formed on the first belt 9.

  In addition to the first transfer unit 8, the double-side transfer device has a second transfer unit 30 disposed on the right side in the drawing. The second transfer unit 30 includes a second belt 31, a second cleaning device 32, a second nip inlet roller 33, a second nip outlet roller 34, a separation roller 35, a driving roller 36, a second cleaning backup roller 37, and the like. ing. The second belt 31 is endlessly moved clockwise in the figure by the rotational driving of the driving roller 36 while being supported by the plurality of rollers.

  The first belt extension portion from the first nip entrance roller 12 to the first nip exit roller 13 of the first transfer unit 8 is in wide contact with the second belt 31 of the second transfer unit 30 to form a two-stage transfer nip. ing. In the two-stage transfer nip, the first belt 9 and the second belt 31 move their surfaces in the same direction. The visible four-color toner image formed on the first belt 9 is transferred onto both surfaces of the transfer paper P at the two-stage transfer nip for the reason described later.

  The transfer residual toner that has not been secondarily transferred to the second belt 31 or the transfer paper P is attached to the first belt 9 after passing through the two-step transfer nip. This is cleaned by the first cleaning device 11. Specifically, the first belt 9 includes a first cleaning device 11 disposed so as to abut on the outer surface (front surface) side of the loop, and a first cleaning backup disposed on the inner surface side of the loop. It is sandwiched between the rollers 14. Then, the transfer residual toner on the front surface is mechanically or electrostatically collected by the first cleaning device 11 and cleaned. Instead of the four primary transfer rollers 10Y, 10M, 10C, and 10K of the bias application method, a charger method that discharges from the electrodes may be used.

  Above the second transfer unit 30 in the figure, a fixing device 56 as a fixing unit is disposed. The fixing device 56 has two fixing rollers 56a and 56b that contact each other while rotating in the forward direction to form a fixing nip. Each of the fixing rollers 56a and 56b has heating means such as a halogen lamp (not shown), and heats the transfer paper P sandwiched between the fixing nips from both sides. By this heating, the full color images respectively formed on both sides of the transfer paper P are fixed on the transfer paper P. After the fixing, the transfer paper P is reversed along the reversing guide member 57 and then discharged to the outside through the paper discharge roller pair 58. And it is stacked on the stack part 55 formed on the upper surface of the housing of the printer body.

  The surface temperatures of the two fixing rollers 56a and 56b are respectively detected by temperature detection means (not shown). This temperature detection means transmits to the control part E2 arrange | positioned in the uppermost part in the housing of a printer main body. Based on the detection result of the surface temperature sent from the temperature detection means, the control unit E2 performs ON / OFF control of the power supply to the heat generating means of the fixing rollers 56a, b, and the surface temperature of the fixing rollers 56a, b. Is maintained within a certain range (target range). However, when the single-sided print mode in which an image is formed only on one side of the transfer paper P is executed, the image can be fixed with a smaller amount of heat than in the double-sided print mode. This is because an image is formed only on one side, and therefore the amount of toner on the transfer paper P is smaller than that on both sides. Therefore, in the single-sided printing mode, energy saving can be achieved by lowering the target range of the surface temperature than in the double-sided printing mode. In addition, since a single color image has a smaller amount of toner than a full color image, it is possible to save energy by switching the target range of the surface temperature according to the difference between the single color print mode and the full color print mode. .

  Transfer residual toner adheres to the front surface of the first belt 9 after passing through the above-described two-stage transfer nip. This is removed from the front surface of the first belt 9 at the cleaning position where the first belt 9 is sandwiched between the first cleaning device 11 and the first cleaning backup roller 14.

  In FIG. 3 described above, the surface of the photosensitive member 1Y after passing through the Y primary transfer nip where the photosensitive member 1Y and the first belt 9 abut is cleaned by the drum cleaning device 2Y. . Then, after being neutralized by the static eliminator 3Y, it is uniformly charged by the charging device 4Y.

  A bottle container 54 is disposed above the first transfer unit 8 in the drawing. In the bottle container 54, toner bottles BY, BM, BC, BK containing toner to be supplied to the developing devices in the process cartridges (6Y, M, C, K) are stored.

Next, a characteristic configuration of the printer will be described.
FIG. 4 is a schematic diagram showing the two-stage transfer nip and the surrounding configuration. In the figure, the second nip entrance roller 33 of the second transfer unit 30 sandwiches the first belt 9 and the second belt 31 between the first nip entrance roller 12 of the first transfer unit 8. The two-stage transfer nip region where the two belts are sandwiched in this way is the front-stage transfer portion. In the preceding transfer section, the solid first nip entrance roller 12 made of metal is grounded. Further, a secondary transfer bias having a positive polarity is applied to the second nip entrance roller 33 in which a metal core is covered with an elastic material such as rubber. By this application, a transfer electric field that electrostatically attracts a negative polarity toner image from the first nip entrance roller 12 side toward the second nip entrance roller 33 side is formed in the preceding transfer portion.

  On the other hand, the second nip outlet roller 34 of the second transfer unit 30 sandwiches the first belt 9 and the second belt 31 between the first nip outlet roller 13 of the first transfer unit 8. The two-stage transfer nip region in which both belts are sandwiched in this way is the latter-stage transfer portion. In such a subsequent transfer section, the solid first nip exit roller 13 made of metal is grounded. Further, a negative polarity tertiary transfer bias is applied to the second nip exit roller 34 in which a metal core is covered with an elastic material such as rubber. By this application, a transfer electric field that electrostatically pushes the toner image from the second nip exit roller 34 side toward the first nip exit roller 13 side is formed in the rear transfer portion.

  The registration roller pair 53 sends out the transfer paper P sandwiched between the rollers toward the two-stage transfer nip shown in the timing at which the transfer paper P can be brought into close contact with the four-color toner image primarily transferred onto the first belt 9. . However, when this four-color toner image is a second toner image to be transferred to the second surface (the surface facing upward on the stack portion 55 described later) which is the other surface of the transfer paper P, the transfer is performed. Paper P is not sent out. Therefore, at this time, the second toner image on the first belt 9 is secondarily transferred onto the second belt 31 under the action of the nip pressure and the transfer electric field at the preceding transfer portion of the two-step transfer nip. The second belt 31 onto which the second toner image has been secondarily transferred in this manner enters the subsequent transfer section along with its endless movement. At this time, the application of the tertiary transfer bias to the second nip exit roller 34 is not performed. Therefore, the second toner image on the second belt 31 passes through the two-step transfer nip while being held on the surface of the second belt 31 as it is. As the second belt 31 moves endlessly, it returns to the two-step transfer nip again.

  On the other hand, the registration roller pair 53 is a first toner image on which the four-color toner image on the first belt 9 is to be transferred to the first surface of the transfer paper P (the surface facing downward on the stack unit 40). In this case, the transfer paper P is sent out in synchronization with the first toner image. Therefore, the first toner image is secondarily transferred to the first surface, which is one surface of the transfer paper P, at the front transfer portion of the two-step transfer nip, and becomes a full color image combined with the white color of the transfer paper P. At this time, the second toner image previously transferred to the second belt 31 is brought into close contact with the second surface of the transfer paper P that has been sent to the preceding transfer portion. However, since the second toner image is attracted to the second belt 31 side by the action of the transfer electric field of the front transfer portion, it is in close contact with the second surface of the transfer paper P, but is tertiary transferred there. is not.

  As described above, in the front transfer portion, the second nip entrance roller 33 to which the secondary transfer bias is applied is an elastic roller having a rubber surface layer, whereas the grounded first nip entrance is provided. The roller 12 is a solid metal roller. In such a configuration, the electrical resistance of the first nip entrance roller 12 is much smaller than the electrical resistance of the second nip entrance roller 33. Then, in the preceding transfer section, the transfer paper P is electrostatically attracted to the surface of the first belt 9 supported by the first nip entrance roller 12 having the smaller electrical resistance. The electric resistance referred to here indicates not the volume specific resistance but the electric resistance value of the entire roller.

  The transfer paper P electrostatically attracted to the first belt 9 at the front transfer portion enters the rear transfer portion as the both belts move endlessly. At this time, the above-described tertiary transfer bias is applied to the second nip exit roller 34. Then, the second toner image on the surface of the second belt 31 is electrostatically pushed out toward the transfer paper P from the second nip exit roller 34 side, which is a transfer bias member, by the electrostatic extrusion method. Tertiary transfer is performed on the second surface. At this time, an electrostatic force acting from the transfer paper P side to the first belt 9 side acts on the first toner image secondarily transferred to the first surface of the transfer paper P at the front transfer unit. According to the experiment, there was no reverse transfer to the first belt 9.

  The transfer sheet P that is electrostatically attracted to the surface of the second belt 31 at the front transfer portion tries to remain electrostatically attracted to the second belt 31 even if it is affected by the transfer electric field of the rear transfer portion. Then, as it is, there is a possibility that the second belt 31 may be wound after passing through the two-step transfer nip. Therefore, in this printer, as shown in the drawing, the curvature of the movement locus of the first belt 9 out of the first belt 9 and the second belt 31 that have passed through the rear transfer portion and separated from each other is expressed by the second belt 31. It is much larger than the curvature of the movement trajectory. Then, the transfer paper P after passing through the rear transfer portion cannot follow the sudden change in the moving direction of the first belt 9 and is separated from the surface of the first belt 9 by its own waist strength. . Then, after passing through the post-transfer section, it is conveyed from the lower side to the upper side in the figure together with the second belt 31 while being electrostatically adsorbed and held on the surface of the second belt 31 that hardly changes the direction of the movement locus. .

  In FIG. 2 described above, the second belt 31 after passing through the rear transfer portion follows a substantially straight movement path. However, when the second belt 31 reaches the support position by the separation roller 35, the separation belt 35 is more than a half of its circumference. Move so that it wraps around the part. Then, the movement path which has been traced in a straight line from the lower side to the upper side in the drawing is almost reversed. In addition, since the separation roller 35 having a small diameter of about 10 [mm] is used, the second belt 31 approaching the support position by the separation roller 35 abruptly changes the curvature of the moving path. become. Then, the transfer paper P that has been electrostatically attracted to the second belt 31 until then cannot follow the sudden change in curvature, and is separated from the second belt 31 by the strength of its own waist, A straight movement path is followed in the same manner as the previous movement path. Then, it is delivered to the fixing device 56 disposed above the second transfer unit 30.

  The second belt 31 after passing through the rear transfer portion of the two-step transfer nip sequentially passes through the support position by the separation roller 35 and the drive roller 36 and then reaches the support position by the second cleaning backup roller 37. Here, the second belt 31 is sandwiched between the second cleaning backup roller 37 and the second cleaning device 32, and the transfer residual toner on the surface is mechanically or electrostatically cleaned. However, if the second cleaning device 32 is always in contact with the second belt 31, the second toner image secondarily transferred onto the second belt 31 is also cleaned. Therefore, the second cleaning device 32 is brought into contact with and separated from the second belt 31 by being swung by a swing mechanism (not shown). Then, at least while the second toner image passes through the cleaning position, the second toner image is separated from the second belt 31 to avoid cleaning the second toner image.

  As described above, the second toner image transferred onto the second surface of the transfer paper P is formed prior to the first toner image transferred onto the first surface. Then, after secondary transfer from the first belt 9 to the second belt 31 at the two-stage transfer nip, tertiary transfer is performed on the second surface of the transfer paper P at the subsequent-stage transfer unit. The second surface is a surface facing upward in the stack portion 55. Therefore, the transfer sheets P stacked on the stack unit 55 are sequentially stacked with the second toner image formed in the front direction facing up and the first toner image formed thereafter facing up. Go. In this printer 100, in order to align the page numbers of the transfer sheets P stacked in this order in order from the smallest, the images having two page numbers that are odd and even are consecutive. Formed as a toner image. For example, the second page image is formed as the second toner image prior to the first page image. As a result, even when documents of several pages are continuously output, the page numbers can be aligned in order from the bottom in the stack unit 55. However, when executing the single-sided print mode in which an image is formed only on the first surface of the transfer paper P, the images are formed in order from the image with the smallest page number, and each image is secondarily transferred onto the first surface of the transfer paper P. . Thus, even in the single-sided printing mode, the page numbers can be aligned in order from the bottom in the stack unit 55.

  In the four photoconductors 1Y, 1M, 1C, and 1K, each color toner image formed for the first toner image is formed as a non-mirror image (hereinafter referred to as a normal image). This is because each formed color toner image changes into a mirror image and a normal image in the process of reaching the transfer paper P through two transfer processes of primary transfer and secondary transfer. By forming a normal image on each photoconductor, a normal image is formed on the first surface of the transfer paper P. On the other hand, since each color toner image formed for the second toner image is subjected to the third transfer, the transfer process is increased once more than the first toner image. Therefore, it is formed as a mirror image on each photoconductor. As a result, a normal image, a mirror image, and a normal image change every transfer, and a normal image can be formed on the second surface of the transfer paper P.

  In the printer having the above-described configuration, the curvature of the movement locus of the first belt 9 to which the transfer paper P is to be wound out of the first belt 9 and the second belt 31 that have passed through the rear transfer portion and are separated from each other. By making this smaller than the curvature of the movement locus of the second belt 31, the transfer paper P can be satisfactorily separated from the first belt 31 without providing a separation claw. Further, in the process of transporting the transfer paper P from the two-stage transfer nip to the downstream side in the belt moving direction, the toner image is transferred from both sides of the transfer paper P by the double-side transfer device. Therefore, image formation by the one-pass method can be performed on both surfaces of the transfer paper P. Further, since the transfer paper P is not directly brought into contact with the photoconductor (1Y, M, C, K) as the image carrier, it is possible to suppress the adhesion of paper powder to the photoconductor. As in the printer 100, a plurality of image carriers such as photoconductors are arranged side by side, and a visible image formed by each is continuously superimposed and transferred to form a superimposed image such as a multicolor image. The method is called tandem method. In contrast, after forming a visible image on one image carrier and transferring it to the intermediate transfer member, the visible image is formed again on the image carrier and transferred onto the visible image on the intermediate transfer member. There is also a method of forming a superimposed image by repeating the process. In this method, the steps of visible image formation and transfer must be repeated. On the other hand, in the tandem method, a plurality of visible images to be superimposed and transferred can be formed almost simultaneously on the corresponding image carriers, so that the image forming time can be greatly shortened.

Each of the second nip entrance roller 33 and the second nip exit roller 34 in which a metal core is coated with an elastic material such as rubber has a surface resistance in the range of 10 ⁵ to 10 ¹² [Ω / □]. In addition, an appropriate amount of resistance adjusting agent is contained in the insulating elastic material. Further, the first nip entrance roller 12 that is a first belt front stage support member made of a solid metal roller and the second nip exit roller 13 that is a first belt rear stage support member both exhibit excellent conductivity. In such a configuration, an appropriate amount of transfer current is passed from the elastic roller side to which the transfer bias is applied to the grounded metal roller side in both the front transfer unit and the rear transfer unit to obtain good transferability. be able to.

  In each of the first belt 9 and the second belt 31, a thin surface layer made of a material capable of exhibiting good surface properties and appropriate electric resistance is coated on the surface of a thick base belt. Thereby, the surface roughness Rz of both the front surfaces of both belts is adjusted to 2 to 3 [μm]. In such a configuration, the surface roughness Rz of the belt is very small, so that the toner is prevented from being caught due to the rough surface of the transfer paper, and the separation of the transfer paper P from the belt when passing through the subsequent transfer section is improved. Improve. Therefore, the transfer paper P after passing through the post-transfer portion can be more favorably separated from both belts. Since it is difficult to process a belt having a surface roughness Rz of 1 [μm] or less with the current processing technology, if the surface roughness Rz of both belts is 1 [μm] or less, 2 [μm]. Compared with the above case, the manufacturing cost of the belt is greatly increased. Further, according to the experiments by the present inventors, when the surface roughness Rz is more than 3 [μm], the separation property of the transfer paper P from the belt is rapidly deteriorated.

In addition, the first belt 9 and the second belt 31 are each coated with a surface layer made of a material that exhibits an appropriate electric resistance by dispersing the resistance adjusting agent on the surface of the belt base. The surface resistance is in the range of 10 ⁵ to 10 ¹² [Ω / □]. With such a configuration, an optimal transfer current for realizing secondary transfer or tertiary transfer from the belt to the transfer paper P can be obtained, and the toner image can be satisfactorily transferred onto both surfaces of the transfer paper P. In addition, as a material of a surface layer, a urethane rubber can be illustrated, for example.

  The belt base of the second belt 31 is made of a material having excellent heat resistance such as polyimide or polyamide. This is for the reason explained below. That is, the transfer paper P conveyed while being held by the second belt 31 is sent to the fixing device 56 for toner image fixing processing. Since the fixing device 56 emits a high temperature of about 140 [° C.], the second belt 31 is heated by the heat from the fixing device 56 when the distance between the fixing device 56 and the second belt 31 is short. End up. If the belt base of the second belt 31 is inferior in heat resistance, it is deteriorated by heating and causes various problems. In order to suppress such a problem, it is conceivable to dispose the second belt 31 and the fixing device 56 sufficiently apart. However, when the second belt 31 and the fixing device 56 are arranged apart from each other, the transfer paper P that is freely bent due to the influence of gravity cannot be directly transferred from the belt to the fixing device 56. It is necessary to provide a paper guide member between them. The transfer paper P held by the second belt 31 carries a toner image not only on the front surface but also on the back surface, and the toner image on the back surface is rubbed by the paper guide member and is disturbed. Therefore, in this printer, the second belt 31 is made heat-resistant as a whole by using polyimide or polyamide as the material of the belt base material. In such a configuration, the second belt 31 and the fixing device 56 are disposed close to each other so that the paper guide member can be omitted, and the second belt 31 is heated by the fixing device 56 while avoiding the disturbance of the toner image. Various problems caused by doing so can be suppressed. In the embodiments and modifications described later, the first belt 9 also needs to be close to the fixing device 56 in the same manner as the second belt 31. Therefore, the material of the belt base of the first belt 9 is also resistant to heat. Uses excellent polyimide and polyamide.

  About the thickness of the 1st belt 9 or the 2nd belt 31, it is desirable to adjust in the range of 50-600 [micrometers]. This is for the reason explained below. That is, if the thickness is less than 50 [μm], the intermediate transfer belt 16 becomes brittle and easily broken. Further, when the thickness is larger than 600 [μm], the thickness of the belt base made of a resin material exhibiting elasticity is excessively increased and is freely deformed in the thickness direction in the nip. It becomes difficult to deform. As a result, it is difficult to expand the roller contact area due to the deformation of the elastic roller. By setting the thickness to 600 [μm] or less, the roller contact area can be expanded by deformation of the elastic roller, and a long pressing time for the toner image can be secured. And good secondary transfer property and tertiary transfer property can be obtained.

Next, a printer according to an example in which a more characteristic configuration is added to the printer according to the embodiment will be described.
FIG. 5 is a schematic configuration diagram illustrating the printer according to the present embodiment. In this printer, the transfer paper P that has passed through the two-stage transfer nip where the first belt 9 and the second belt 31 abut is separated from both belts without being held on the surface of the second belt 31 and is fixed to the fixing device 56. Is to be handed over. The first nip exit roller 13 of the first transfer unit 8 and the second nip exit roller 34 of the second transfer unit 30 function as separation rollers that separate the transfer paper P from both belts.

  FIG. 6 is a schematic diagram showing a two-step transfer nip and a surrounding configuration of the printer according to the present embodiment. In the front transfer portion of the two-step transfer nip in the figure, the first nip entrance roller 12 as the first belt front support member has a structure in which an elastic layer made of an elastic material is coated on a metal core. A negative transfer secondary transfer bias is applied. Further, the second nip entrance roller 33 as the second belt front stage support member is made of a solid metal roller and is electrically grounded. In such a pre-stage transfer unit, the first toner image and the second toner image on the first belt are electrostatically pushed from the first nip entrance roller 12 side toward the second nip entrance roller 33 side, The image is transferred to the first surface of the transfer paper P and the second belt 31. Further, the transfer paper P is electrostatically attracted to the front surface of the second belt 31 supported by the second nip entrance roller 33 which is a roller having a smaller electric resistance among the two rollers.

  In the rear transfer portion of the two-stage transfer nip, the first nip outlet roller 13 as the first belt rear stage support member is made of a solid metal roller and is electrically grounded. Further, the second nip exit roller 34 as the second belt rear stage support member has a configuration in which a metal core is covered with an elastic layer made of an elastic material, and a negative third transfer bias is applied. . In such a rear transfer unit, the second toner image on the second belt is electrostatically pushed from the second nip exit roller 34 side toward the first nip exit roller 13 side, and the second transfer image of the transfer paper P is transferred. Transferred to two sides.

  In the rear transfer section, the first belt 9 and the second belt 31 are circumferential surfaces of the first nip outlet roller 13 and the second nip outlet roller 34 so that the movement paths are changed by an angle of approximately 90 [°], respectively. Is greatly wrapped around. The second nip outlet roller 34 that supports the second belt 31 to which the transfer paper P is electrostatically attracted is a roller having a smaller diameter than the first nip outlet roller 13. In such a configuration, the transfer sheet P that has been electrostatically attracted to the second belt 31 cannot move following the rapid change in the moving path of the second belt 31 immediately after passing through the subsequent transfer section. It separates from the surface of the second belt 31 by its own waist strength. Here, if the curvature of the moving path of the first belt 9 immediately after passing through the rear transfer portion is somewhat small, the transfer paper P separated from the surface of the second belt 31 is electrostatically attracted to the surface of the first belt 9. End up. Therefore, in this printer, the first nip exit roller 13 has a diameter larger than that of the first nip exit roller 34 but a relatively small diameter capable of exhibiting a certain degree of curvature. Therefore, the transfer paper P immediately after passing through the rear transfer nip can be satisfactorily separated without being wound around both belts.

  As described above, in this printer, due to the difference in the diameters of the first nip exit roller 13 and the second nip exit roller 34, the first belt 9 and the second belt 31 after passing through the rear transfer portion respectively. The posture that makes the curvature of the movement trajectory different is taken. In such a configuration, the transfer sheet P immediately after passing through the rear transfer portion can be separated from both belts almost simultaneously due to the difference in the diameters of both rollers, and can be sent to the next fixing step.

  In the printer, the latter roller having the smaller diameter of the first nip exit roller 13 and the second nip exit roller 34 is configured as follows. That is, the first nip entrance roller 12 and the second nip entrance roller 33 are rollers that support the same belt (second belt 31) as the latter roller having the smaller electrical resistance. In this configuration, the transfer sheet P that has been electrostatically attracted to the second belt 31 supported by the second transfer nip 33, which is the latter roller, due to the small electrical resistance in the former transfer section. Can be separated from the second belt 31 as follows. That is, in the latter transfer section, the curvature of the movement locus of the second belt 31 is made larger than that of the first belt 9 due to the small diameter of the second nip exit roller 34, and the transfer paper P is removed from the second belt 31. To separate.

  The transfer paper P separated from both belts is transferred to the fixing belt 56c supported by the fixing roller 56a and the support roller 56d of the fixing device 56 as shown in FIG. Then, the two fixing rollers 56a and 56b enter the fixing nip where they are in contact with each other via the fixing belt 56c, and the full-color image is fixed on both sides.

The inventors made a prototype of the printer tester having the above-described configuration, and tested the winding of the transfer paper P around the first belt 9 after passing through the rear transfer portion. Specifically, as the second nip outlet roller 34, a roller having a diameter of 25 [mm] is used, and the diameter of the first nip outlet roller 13 is changed in seven steps from 25 [mm] to 18 [mm] The paper winding around the first belt 9 was examined. The results are shown in Table 1 below.

  As shown in Table 1, it was found that if the first nip exit roller 13 having a diameter of 20 mm or less is used, the transfer paper P can be prevented from being wrapped around the first belt 9.

Next, the inventors tested the winding of the transfer paper P around the second belt 31 after passing through the post-transfer portion. Specifically, as the first nip exit roller 13, one having a diameter of 20 [mm], which is a size that does not wind the transfer paper P around the first belt 9, was used. Then, the winding of the paper around the first belt 9 was examined while changing the diameter of the second nip exit roller 34 from 25 [mm] to 18 [mm] in seven steps. The results are shown in Table 2 below.

  As shown in Table 2, when the diameter of the second nip exit roller 34 is equal to or greater than that of the first nip exit roller 13, the transfer paper P is wound around the second belt 31 that is electrostatically attracted. However, it has been found that if the diameter is made smaller than that of the first nip exit roller, it is possible to avoid reverse adsorption to the first belt 9 without winding around the second belt 31. From these results, the roller having the smaller diameter (second nip exit roller 34 in the illustrated example), that is, the nip exit roller that supports the belt for electrostatically attracting the transfer paper P at the preceding transfer unit, is It is considered that the diameter needs to be less than 20 [mm].

  FIG. 7 is a schematic diagram showing the two-stage transfer nip and the surrounding configuration in the first modified example of the printer. In the first transfer unit of the first modified apparatus, the first nip entrance roller 12 is made of a solid metal roller and is electrically grounded. The second nip entrance roller 33 has a structure in which a metal core is covered with an elastic layer made of an elastic material, and a positive polarity secondary transfer bias is applied thereto. In such a pre-stage transfer unit, the first toner image and the second toner image on the first belt are electrostatically attracted from the first nip entrance roller 12 side toward the second nip entrance roller 33 side, The image is transferred to the first surface of the transfer paper P and the second belt 31. Further, the transfer paper P is electrostatically attracted to the front surface of the first belt 9 supported by the first nip entrance roller 12 which is the roller having the smaller electrical resistance among the two rollers.

  On the other hand, in the latter transfer section, the first nip exit roller 13 is made of a solid metal roller and is electrically grounded. The second nip exit roller 34 has a configuration in which a metal core is covered with an elastic layer made of an elastic material, and a negative polarity tertiary transfer bias is applied. In such a rear transfer unit, the second toner image on the second belt is electrostatically pushed from the second nip exit roller 34 side toward the first nip exit roller 13 side, and the second transfer image of the transfer paper P is transferred. Transferred to two sides.

  In the rear transfer section, the first belt 9 and the second belt 31 are circumferential surfaces of the first nip outlet roller 13 and the second nip outlet roller 34 so that the movement paths are changed by an angle of approximately 90 [°], respectively. Is greatly wrapped around. The first nip exit roller 13 that supports the first belt 9 on which the transfer paper P is electrostatically attracted is a roller having a smaller diameter than the second nip exit roller 34. In such a configuration, the transfer sheet P that has been electrostatically attracted to the first belt 9 cannot move following a sudden change in the moving path of the first belt 9 immediately after passing through the subsequent transfer section. It separates from the surface of the first belt 9 by its own waist strength. And it can isolate | separate favorably, without winding around both belts.

  FIG. 8 is a schematic diagram illustrating the two-step transfer nip and the surrounding configuration in the second modified example of the printer according to the embodiment. In the front transfer portion of the second modified apparatus, the first nip entrance roller 12 is made of a solid metal roller and is electrically grounded. The second nip entrance roller 33 has a configuration in which a metal core is covered with an elastic layer made of an elastic material, and a negative polarity tertiary transfer bias is applied. In such a front transfer unit, the second toner image on the second belt 31 is electrostatically pushed from the second nip entrance roller 33 side toward the first nip entrance roller 12 side, and the transfer paper P is transferred. Tertiary transfer is performed on the second surface. Further, the transfer paper P is electrostatically attracted to the front surface of the first belt 9 supported by the first nip entrance roller 12 which is the roller having the smaller electrical resistance among the two rollers.

  On the other hand, in the latter transfer portion, the first nip exit roller 13 is configured such that a metal core is covered with an elastic layer made of an elastic material, and a negative transfer secondary transfer bias is applied. The second nip exit roller 34 is made of a solid metal roller and is electrically grounded. In such a rear transfer portion, the first toner image and the second toner image on the first belt 9 are electrostatically pushed from the first nip outlet roller 13 side toward the second nip outlet roller 34 side. Then, the toner image is secondarily transferred to the first surface of the transfer paper P or the second belt 31.

  In the rear transfer section, the first belt 9 and the second belt 31 are circumferential surfaces of the first nip outlet roller 13 and the second nip outlet roller 34 so that the movement paths are changed by an angle of approximately 90 [°], respectively. Is greatly wrapped around. The first nip exit roller 13 that supports the first belt 9 on which the transfer paper P is electrostatically attracted is a roller having a smaller diameter than the second nip exit roller 34. In such a configuration, the transfer sheet P that has been electrostatically attracted to the first belt 9 cannot move following a sudden change in the moving path of the first belt 9 immediately after passing through the subsequent transfer section. It separates from the surface of the first belt 9 by its own waist strength. And it can isolate | separate favorably, without winding around both belts.

  FIG. 9 is a schematic configuration diagram illustrating a third modified example of the printer according to the embodiment. In the third modified apparatus, the first transfer unit 8 and the second transfer unit 30 support the first belt 9 and the second belt 31 in a horizontally long shape, respectively. Then, the first belt 9 and the second belt 31 disposed below in the drawing are brought into contact with each other at a horizontally long supporting portion to form a two-step transfer nip.

  The printer includes two process cartridges for each color. One for the first system and one for the second system. Among these, the process cartridges 6Y, 6M, 6C, and 6K for the first system contact the photoreceptors 1Y, 1M, 1C, and 1K with the first belt 9 at the upper part of the first transfer unit 8 in the drawing. It is arranged to make it. Then, Y, M, C, and K toner images are formed on the first belt 9 for primary transfer. The first toner image, which is a four-color toner image obtained on the first belt 9 by the superimposing primary transfer, is transferred to the first belt 9 and the first belt 9 along with the endless movement of the first belt 9 in the clockwise direction in the drawing. 2 is sent to a two-stage transfer nip where the belt 31 abuts.

  On the other hand, the process cartridges 40Y, 40M, 40C, and 40K for the second system are configured so that the photoreceptors 41Y, 41M, 41C, and 41K come into contact with the second belt 31 below the second transfer unit 30, respectively. It is arranged. Then, Y, M, C, and K toner images are formed on the second belt 31 for primary transfer. The second toner image, which is a four-color toner image obtained on the second belt 31 by the superimposing primary transfer, is transferred to the above-described two-stage transfer nip as the second belt 31 moves in the counterclockwise direction in FIG. Sent. The optical writing on the photosensitive bodies 41Y, M, C, and K of the second system process cartridges 40Y, M, C, and K is performed by the second system exposure apparatus 77. The exposure device 77 is disposed below the process cartridges 40Y, M, C, and K for the second system in the drawing. On the other hand, the exposure device 7 for the first system is disposed above the process cartridges 6Y, 6M, 6C, and 6K for the first system in the drawing.

  In this printer, a printer unit 100 for printing an image on a transfer paper P and a paper feed unit 101 as a transfer body storage means for storing the transfer paper P supplied to the printer unit are configured separately. ing. The paper feeding unit 101 rotates the paper feeding roller 92 pressed against the uppermost transfer paper P in the bundle of transfer paper accommodated therein at a predetermined timing, thereby transferring the transfer paper P to the printer unit. It feeds out toward 90 paper feed paths 88. The fed transfer paper P passes through a plurality of conveying roller pairs and is then sandwiched between rollers of a registration roller pair 53 disposed near the end of the paper feed path 88. Then, the toner is fed toward the two-stage transfer nip so as to be synchronized with the first toner image on the first belt 9 of the first transfer unit 8 and the second toner image on the second belt 31 of the second transfer unit 30. It is.

  In the third modified apparatus, the two-step transfer nip and its surroundings are the same as those shown in any of FIGS. 6, 7, and 8. Therefore, the transfer paper P after passing through the rear transfer portion is separated well from both belts and delivered to the fixing device 56.

  Up to this point, an example has been described in which the transfer bias is applied to the elastic roller of the pair of rollers facing each other via the belt at the front transfer unit and the rear transfer unit. However, the transfer bias is applied to a metal roller having a low electrical resistance. May be.

  As described above, in the printers according to the embodiments, examples, and modifications, in the rear transfer unit, the first nip outlet roller 13 as the first belt rear stage support member and the second nip outlet roller as the second belt rear stage support member. 34, the toner image is electrostatically pushed out from the roller to which the transfer bias is applied toward the roller to which the transfer bias is not applied, and is transferred onto one surface of the transfer paper P. In such a configuration, it is possible to avoid reverse transfer of any toner image while transferring two toner images charged to the same polarity to the opposite surface of the transfer paper P at the transfer nip.

  In the printer according to the embodiment, the curvature of the movement locus of the first belt 9 out of the first belt 9 and the second belt 31 that have passed through the rear transfer portion and separated from each other is determined as the movement of the second belt 31. It is larger than the curvature of the trajectory. Then, the transfer paper P after passing through the latter transfer section is separated from the first belt 9 while being held on the surface of the second belt 31 and conveyed toward the fixing process which is the next process. In such a configuration, even when it is necessary to dispose the fixing device at a position relatively distant from the two-stage transfer nip due to layout restrictions, the transfer paper P after passing through the two-stage transfer nip is transferred to the second belt 31. It can be held on the surface and conveyed to the fixing device.

  Further, in the printer according to the embodiment and each modified example device, the first nip exit roller 13 and the second nip exit roller 34 having different diameters are used, and the first belt 9 is different due to the difference in the diameters of both rollers. The second belt 31 and the second belt 31 are each configured to have different curvatures of the movement trajectory after passing through the rear transfer portion. In such a configuration, the transfer sheet P immediately after passing through the rear transfer portion can be separated from both belts almost simultaneously due to the difference in the diameters of both rollers, and can be sent to the next fixing step.

  In the printer according to the embodiment and each modified example device, the latter roller having the smaller diameter of the first nip outlet roller 13 and the second nip outlet roller 34 is connected to the first nip inlet roller 12. Of the second nip entrance roller 33, this is a roller that supports the same belt (second belt 31) as the latter roller having the smaller electrical resistance. In this configuration, the transfer sheet P that has been electrostatically attracted to the second belt 31 supported by the second transfer nip 33, which is the latter roller, due to the small electrical resistance in the former transfer section. Can be separated from the second belt 31 as follows. That is, in the latter transfer section, the curvature of the movement locus of the second belt 31 is made larger than that of the first belt 9 due to the small diameter of the second nip exit roller 34, and the transfer paper P is removed from the second belt 31. To separate.

  Further, in the printer and each modified example device according to the embodiment, the smaller diameter of the first nip outlet roller 13 and the second nip outlet roller 34 is set to less than 20 [mm]. In such a configuration, the transfer sheet P immediately after passing through the rear transfer portion, the belt that has been electrostatically attracted by the front transfer portion, and the nip exit roller having a diameter of less than 20 mm immediately after passing through the rear transfer portion. The transfer paper P can be reliably separated from the belt by moving it along a large curvature.

  In the printers of the embodiments, examples, and modifications, the first nip entrance roller 12, the second nip entrance roller 33, the first nip exit roller 13, and the second nip exit roller 34 all have surfaces. A material made of a conductive material is used. With such a configuration, the toner image can be satisfactorily transferred onto the transfer paper P by causing a transfer current to be generated from one of the paired rollers toward the other at the front transfer portion and the rear transfer portion.

  In the printers of the embodiments, examples, and modifications, the first belt 9 and the second belt 31 that have a surface roughness Rz of 2 to 3 [μm] are used. Yes. In such a configuration, the transfer paper P after passing through the post-transfer portion can be more favorably separated from both belts for the reasons described above.

In the printers of the embodiments, examples, and modifications, the first belt 9 and the second belt 31 each having a surface resistance of 10 ⁵ to 10 ¹² [Ω / □] are used. In such a configuration, for the reasons described above, an optimal transfer current for realizing secondary transfer or tertiary transfer from the belt to the transfer paper P is obtained, and the toner image is satisfactorily transferred onto both sides of the transfer paper P. Can do.

FIG. 3 is a schematic diagram showing a transfer nip and its peripheral configuration of an image forming apparatus under development by the inventors. 1 is a schematic configuration diagram of a printer according to an embodiment. FIG. 3 is an enlarged configuration diagram illustrating a process unit for Y of the printer. FIG. 2 is a schematic diagram showing a two-stage transfer nip of the printer and its surrounding configuration. 1 is a schematic configuration diagram illustrating a printer according to an embodiment. FIG. 2 is a schematic diagram showing a two-stage transfer nip of the printer and its surrounding configuration. The schematic diagram which shows the 2 step | paragraph transfer nip and its surrounding structure in a 1st modification apparatus. The schematic diagram which shows the 2 step | paragraph transfer nip and its surrounding structure in a 2nd modification apparatus. The schematic block diagram which shows a 3rd modification apparatus.

Explanation of symbols

1Y, M, C, K photoconductor (image carrier)
8 First transfer unit (part of double-sided transfer device)
9 First belt 12 First nip entrance roller (first belt front stage support member)
13 First nip outlet roller (first belt rear stage support roller)
30 Second transfer unit (part of double-sided transfer device)
31 Second belt 33 Second nip entrance roller (second belt upstream support roller)
34 Second nip exit roller (second belt rear stage support roller)
41Y, M, C, K photoconductor (image carrier)
P Transfer paper (transfer body)

Claims (4)

A first nip entrance roller that supports the first belt that moves endlessly from the back side of the belt, a first nip exit roller that supports the first belt from the back side of the belt, and a second belt that moves endlessly from the back side of the belt A second nip entrance roller, a second nip exit roller for supporting the second belt from the back side of the belt , means for applying a transfer bias having the same polarity as the toner charging polarity to the first nip entrance roller, Means for applying a transfer bias having the same polarity as the charging polarity of the toner to the second nip exit roller, and a first belt support location by the first nip entrance roller and a second belt support by the second nip entrance roller. A pre-stage transfer portion is formed by contact with the portion, and a first belt support location by the first nip exit roller and a second belt support location by the second nip exit roller A second transfer portion is formed by contact, and a toner image carried on the front surface of the first belt is transferred to one surface of a transfer member sandwiched between the first transfer portions, and then the second belt In a double-sided transfer device that transfers a toner image carried on a surface to the other surface of the transfer body sandwiched between the subsequent transfer portions, and electrostatically transfers the toner image to both surfaces of the transfer body ,
As above Symbol second nip entrance roller, the first nip entrance roller electrically grounded and the first nip inlet roller used as a metal which exhibits low electrical resistance than Rutotomoni, transfer bias is applied as, using what surface layer made of a material containing a resistance adjusting agent in the elastic material of the insulation was coated, electrostatically the transfer member in the pre-stage transfer unit for the upper Symbol second belts adsorption Let
As the first nip exit roller, a metal roller that is electrically grounded and exhibits a lower electrical resistance than the second nip exit roller, and as the second nip exit roller to which a transfer bias is applied, By using a surface layer made of a material containing a resistance adjusting agent in an insulating elastic material, having a smaller diameter than the first nip exit roller and having a diameter of less than 20 [mm] , Of the first belt and the second belt that are separated from each other after passing through the rear transfer section, the second belt is caused to move with a larger curvature (1 / R) than the first belt. A double-sided transfer device characterized by the above . In both transfer device 請 Motomeko 1,
A double-sided transfer device, wherein the first belt and the second belt each have a surface roughness Rz of 2 to 3 [μm]. The double-sided transfer device according to claim 1 or 2 ,
A double-sided transfer device, wherein the first belt and the second belt each have a surface resistance of 10 ⁵ to 10 ¹² [Ω / □]. In an image forming apparatus that forms an image on both sides of a transfer body using an image carrier that carries a toner image and a double-side transfer unit that transfers a visible image on the image carrier to both sides of the transfer body.
As the double-sided transfer means, the image forming apparatus characterized by using either of the double-sided transfer apparatus according to claim 1 to 3.

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