JP5247018B2 - Transfer device and image forming apparatus - Google Patents

Description

  The present invention relates to a transfer device for transferring a toner image to a recording material from a rotatable image carrier such as an electrophotographic photosensitive member or an electrostatic recording dielectric, or a rotatable intermediate transfer member such as an intermediate transfer belt or an intermediate transfer drum. About.

  The present invention also relates to an image forming apparatus such as an electrophotographic system or an electrostatic recording system such as a facsimile, a printer, or a copying machine having such a transfer device.

  Examples of the transfer device as described above include a bias roller transfer device and a corona transfer device. In the following description, it is assumed that the image carrier includes an intermediate transfer member.

  In the bias roller transfer device, a conductive / elastic roller is brought into contact with an image carrier as a transfer roller (contact transfer charging member) to form a transfer nip portion, and a recording material is introduced into the transfer nip portion to be nipped and conveyed. Then, while the recording material passes through the transfer nip portion, a toner image is transferred from the image carrier by applying a transfer bias having a predetermined potential opposite to the charging polarity of the toner from the power supply unit to the transfer roller. The recording material is mainly transferred by electrostatic transfer. Since the recording material that has exited the transfer nip is in electrostatic contact with the surface of the image carrier, it is separated from the surface of the image carrier by separation means such as a separation claw or a separation belt.

In the corona transfer device, a corona charger is disposed in contact with the image carrier in a non-contact manner, a recording material is introduced between the image carrier and the corona charger, and recording is performed by corona discharge of the corona charger. The back surface of the material is charged to a predetermined potential with a polarity opposite to the charging polarity of the toner. As a result, the recording material is electrostatically adhered to the surface of the image carrier, and the toner image is electrostatically transferred from the image carrier to the recording material. Again, since the recording material leaving the transfer portion is in close contact electrostatically to the surface of the image bearing member, it causes separation from the image bearing member surface in the separation claw and the separation belt or the like of the separating means.

There is also a transfer device using a recording material conveyance belt and a transfer charging member (Patent Document 1). The transfer apparatus includes a recording material conveyance belt that forms a transfer nip portion and an image carrier and sandwiches and conveys the recording material, and a transfer charging member that is disposed at the position of the transfer nip portion inside the belt. Have The transfer charging member is, for example, a transfer roller to which a transfer bias is applied. The toner image on the image carrier side is transferred to the recording material introduced into the transfer nip portion mainly by electrostatic transfer by the transfer charging member. Further, since the recording material is electrostatically held by the belt and conveyed, the conveyance property of the recording material in the transfer portion can be stabilized. The recording material that has exited the transfer nip is separated from the surface of the image carrier by the belt's recording material holding / conveying force because the adsorption force with the belt overcomes the adsorption force with the image carrier. Therefore, there is no separation means such as a separation claw or a separation belt.
JP-A-9-297475

  However, the transfer device using the recording material conveyance belt and the transfer charging member also 1) The surface property of the belt is deteriorated due to long-term use, so that the adsorbing force between the belt and the recording material is reduced at the transfer nip portion. 2) The adsorbing force between the image carrier and the recording material may increase due to an increase in potential after passing through the transfer nip portion of the belt. According to 1) and 2), a recording material that is particularly stiff, such as thin paper, is not attracted to the recording material conveyance belt after passing through the transfer nip portion, and may not be separated from the image carrier and may cause a separation failure. is there.

  The present invention has been made in view of the above technical problems. The purpose is to effectively prevent the separation failure of the recording material from the image carrier in the transfer device using the recording material conveying belt and the transfer charging member.

In order to achieve the above object, a typical configuration of a transfer apparatus according to the present invention includes: a rotatable image carrier; a transfer nip portion; and a belt that can be moved in a loop to sandwich and convey a recording material; and the belt a transfer charging roller disposed inside the position of the transfer nip portion, and a power supply unit for transferring electrostatically a toner image of the image carrier on the body by applying a transfer bias to said transfer charging roller to record material In the transfer device having the above, the belt is disposed in parallel to the transfer charging roller in the vicinity of the transfer charging roller inside the belt portion on the downstream side in the belt moving direction from the transfer nip portion and in parallel with the belt portion. , characterized by perforated electrodes plates to regulate the belt potential by applying the transfer bias.

Further, a typical configuration of the image forming apparatus according to the present invention for achieving the above object includes a rotatable image carrier, a toner image forming unit for forming a toner image on the image carrier, And a transfer unit that transfers a toner image on the image carrier onto a recording material. The transfer unit is capable of circular movement that forms a transfer nip portion with the image carrier and sandwiches and conveys the recording material. A transfer belt disposed inside the belt at a position of the transfer nip portion, and a transfer bias is applied to the transfer charge roller to statically transfer the toner image on the image carrier to the recording material. a power supply unit for electrostatic transfer, in a non-contact state with respect to the belt portion in the vicinity of the inner by the transfer charging roller downstream side in the movement direction of the belt portion of the belt from the transfer nip portion, and disposed parallel to , the transfer bus It characterized by perforated electrodes plates to regulate the belt potential by applying the astigmatism.

In order to achieve the above object, another representative configuration of the image forming apparatus according to the present invention includes a rotatable image carrier, and a toner image forming unit that forms a toner image on the image carrier. An image forming apparatus comprising: a primary transfer unit that transfers a toner image on the image bearing member to an intermediate transfer member that circulates; and a secondary transfer unit that transfers the toner image on the intermediate transfer member to a recording material. The secondary transfer means is disposed at a position of the transfer nip portion inside the belt, and a belt that is capable of rotating around the intermediate transfer member and forming a secondary transfer nip portion to sandwich and convey the recording material. A transfer charging roller, a power supply unit that applies a transfer bias to the transfer charging roller to electrostatically transfer the toner image on the image carrier onto the recording material, and a moving direction of the belt from the secondary transfer nip unit. Of the belt part on the downstream side Characterized in that the chromatic state of non-contact with said belt portion in the vicinity of the transfer charging roller on the side, and arranged in parallel to the electrode plate for regulating the belt potential by applying the transfer bias And

  According to the present invention, it is possible to avoid an increase in potential after passing through the transfer nip portion of the recording material conveyance belt, that is, an excessive increase in electric field strength downstream of the transfer nip portion. Accordingly, it is possible to effectively prevent the poor separation of the recording material that is particularly stiff, such as thin paper, from the image carrier or the intermediate transfer member.

  Exemplary embodiments of the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. Absent. Further, the materials, shapes, etc. of the members once described in the following description are the same as those in the first description unless otherwise described.

(1) Image Forming Unit FIG. 1 is a schematic configuration diagram of an example of an image forming apparatus using a transfer device according to the present invention as a secondary transfer device.

  This image forming apparatus is a four-drum type (tandem type / in-line type) electrophotographic full-color image forming apparatus using an intermediate transfer belt.

This image forming apparatus has first to fourth image forming portions Y, M, C, and K arranged in series in order from left to right on the drawing. Each of these image forming units Y, M, C, and K is a laser scanning exposure type electrophotographic process mechanism having the same configuration, and is a rotatable drum type electrophotographic photosensitive member as a first image carrier. It has a body (hereinafter referred to as a drum) 1. Then, it has a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 5, a drum cleaner 6 and the like which are process means acting on the drum 1. However, yellow toner as a developer is used for the developing device 4 of the first image forming unit Y, magenta toner is used for the developing device 4 of the second image forming unit M, and the developing device 4 of the third image forming unit C. Is stored with cyan toner, and the developing device 4 of the fourth image forming unit K stores black toner. Each drum 1 is formed, for example, by forming a film of an organic photoreceptor (OPC) as a main component on an aluminum cylinder.

Further, it has an endless and flexible intermediate transfer belt ( intermediate transfer body that can be moved around: hereinafter referred to as a belt) 7 as a second image carrier. The belt 7 is wound around three parallel rollers, that is, a driving roller 8, a driven roller 9, and a secondary transfer counter roller (secondary transfer inner roller) 10, and is tensioned by a tension roller 11. is there. The driving roller 8 is on the fourth image forming unit K side, the driven roller 9 is on the first image forming unit Y side, and the secondary transfer counter roller 10 is between the driving roller 8 and the driven roller 9 by both rollers. Is also disposed at a lower position. The upper surface of the belt portion between the driving roller 8 and the driven roller 9 is brought into contact with the lower surface of the drum 1 of each image forming portion Y, M, C, K. The primary transfer rollers (primary transfer means) 5 of each of the image forming units Y, M, C, and K are disposed inside the belt portion between the driving roller 8 and the driven roller 9. Is pressed against the lower surface of the drum 1. The contact portions between the drum 1 and the belt 7 of each of the image forming portions Y, M, C, and K are primary transfer nip portions A1. A belt cleaner 12 for cleaning the outer surface of the belt 7 is disposed outside the belt winding portion of the driven roller 9.

The intermediate transfer belt 7 in this embodiment has a three-layer configuration of a base layer, an elastic layer formed on the surface of the base material, and a surface layer formed on the surface of the elastic layer. The base layer is based on a polyimide resin film having a thickness of 85 μm, and carbon black is dispersed to obtain a surface resistivity of 1 × 10 ¹² Ω / □ and a volume resistivity of 1 × 10 ^9.5 Ω · cm. The resistance was adjusted as follows. Moreover, the elastic modulus of the base material was adjusted to be 1.5 to 8 GPa. The elastic layer is made of fluorine rubber. The elastic layer has a volume resistivity of ^109.5 Ωcm and a thickness of 250 μm. The elastic modulus of the elastic layer was adjusted to be 0.1 to 1000 MPa. The surface layer is preferably made of a material with high toner releasability. In this embodiment, the surface layer is made of PFA, and has a volume resistivity of 10 ¹² Ωcm and a thickness of 15 μm. The elastic modulus of the surface layer was adjusted to be 0.01 to 1 GPa. In this example, the elastic modulus was defined as the tensile elastic modulus. The tensile modulus test method conforms to JIS K7161, and the test piece conforms to JIS K7262. As test conditions, the measurement was performed using a desktop material testing machine STA-1225 manufactured by Orientec Co., Ltd., a crosshead speed of 100 mm / min, a test piece width of 5 mm, a test piece length of 100 mm, 23 ° C., 50 Performed in a% Rh environment.

Each primary transfer roller 5 was a sponge roller provided with a foamed rubber layer made of a steel cored bar made of foamed EPDM as a base material. The resistance of the roller was adjusted by dispersing an ion conductive resistance adjusting agent so that the resistance was 10 ^6.5 Ω (when 100 V was applied) in an environment of 23 ° C. and 50% Rh.

  A secondary transfer device 13 is disposed outside the belt winding portion of the secondary transfer counter roller 10. The recording material conveyance belt 14 of the secondary transfer device 13 and the intermediate transfer belt 7 are brought into contact with each other to form a secondary transfer nip portion A2. The secondary transfer device 13 will be described in detail in the next item (2).

  A registration roller pair 18 is disposed upstream of the secondary transfer device 13 in the recording material conveyance direction. A fixing device 21 is disposed downstream of the secondary transfer device 13 in the recording material conveyance direction.

The operation for forming a full-color image is as follows. The first to fourth image forming units Y, M, C, and K are sequentially driven by a control unit (not shown) in accordance with a predetermined control timing of image formation. As a result, each drum 1 is rotationally driven in the counterclockwise direction indicated by the arrow at the same predetermined speed. The intermediate transfer belt 7 is also rotated by the driving roller 8 in the clockwise direction indicated by the arrow at the same speed as the rotational speed of the drum 1. In synchronization with this drive, the primary charging roller 2 uniformly charges the surface of the drum 1 to a predetermined polarity and potential. In this embodiment, the drum 1 is uniformly charged to a predetermined negative polarity potential. The charged surface of the drum 1 is image-exposed by the exposure device 3. In this embodiment, the exposure apparatus 3 is a laser scanner, which outputs a laser beam modulated in accordance with a time-series electric digital image information signal input from a control unit (not shown) to scan the charged surface of the drum 1. Exposure. Thereby, an electrostatic image (electrostatic latent image) corresponding to the scanning exposure pattern is formed on the drum surface. The formed electrostatic image is developed as a toner image by the developing device 4. In this embodiment, the primary charging roller 2, the exposure device 3, and the developing device 4 are toner image forming means for forming a toner image on the drum 1 (on the image carrier).

  As an electrostatic image forming method, a background exposure method in which an electrostatic image is formed by exposing the surface of a charged drum corresponding to a background portion of image information, and conversely, exposure is performed in a static manner corresponding to the image information portion. There is an image exposure method for forming an electric image. The development of electrostatic images in the case of the background exposure method is a normal development method that develops portions other than the background portion, and the development of electrostatic images in the case of the image exposure method is a reversal that develops non-exposed portions. A development method is used. In this embodiment, a combination of an image exposure method and a reversal development method is used.

  Through the electrophotographic process as described above, in the first image forming unit Y, a yellow toner image corresponding to the yellow component image of the color separation component images of the full-color original image is formed on the drum 1 surface. In the second image forming unit M, a magenta toner image corresponding to the magenta component image, in the third image forming unit C, a cyan toner image corresponding to the cyan component image, and in the fourth image forming unit K, black. A black toner image corresponding to the component image is formed at a predetermined control timing.

Then, in the primary transfer nip portion A1 of the first image forming portion Y, the yellow toner image formed on the drum 1 is primarily transferred onto the belt 7 that is rotationally driven (circulated) . Next, in the primary transfer nip portion A1 of the second image forming portion M, the magenta toner image formed on the drum 1 is superimposed on the yellow toner image on the belt 7 and primarily transferred. Further, similarly, the magenta toner image and the black toner image are sequentially primary-transferred onto the belt 7 at the primary transfer nips A1 of the third image forming unit C and the fourth image forming unit K, respectively. . In other words, four color toner images of yellow, magenta, cyan, and black are sequentially superimposed and transferred onto the belt 7 in a superimposed manner (multiple) to form a full-color unfixed toner image.

  In each primary transfer nip portion A1, for primary transfer of the toner image from the drum 1 to the belt 7, a predetermined primary transfer bias is applied to the primary transfer roller 5 from a primary transfer power supply unit (not shown). Thus, the toner image is electrostatically transferred from the drum 1 to the belt 7. In this embodiment, the primary transfer bias is a DC voltage having a predetermined potential and a positive polarity opposite to the negative polarity that is the charging polarity of the toner.

  In each image forming unit Y, M, C, and K, the surface of the drum 1 after passing through the primary transfer nip is cleaned by the drum cleaner 6 after removal of the primary transfer residual toner, and repeatedly used for image formation. Is done.

The full-color unfixed toner image synthesized and formed on the belt 7 as described above is conveyed by the subsequent rotation of the belt 7, and the recording material is conveyed between the belt 7 and the secondary transfer device (secondary transfer means) 13. A secondary transfer nip portion (secondary transfer nip portion) A2 which is a contact portion with the belt 14 is reached.

On the other hand, a sheet-like recording material (transfer material) P as a recording medium is separated and fed by a control unit (not shown) from a paper feeding mechanism unit (not shown) at a predetermined control timing, and the leading end of the recording material P Is received by the nip portion of the registration roller pair 18 which has stopped rotating at that time. Thereby, the recording material P is corrected in skew. The recording material P is fed again by the registration roller pair 18 that is rotationally driven at a predetermined control timing, guided by the guide member 19, and conveyed to the secondary transfer nip portion A2. That is, the print start position of the recording material P coincides with the secondary transfer nip portion A2 at the timing when the front end of the full-color unfixed toner image formed on the belt 7 reaches the secondary transfer nip portion A2. The rotation start of the registration roller pair 18 is controlled. Then, in the process in which the recording material P is nipped and conveyed through the secondary transfer nip A2, the charging polarity of the toner from the secondary transfer power supply unit HV (FIG. 2) to the secondary transfer roller 15 of the secondary transfer device 13. A transfer bias having a predetermined potential is applied. In the present embodiment, the secondary transfer bias is a positive polarity having a positive polarity opposite to the negative polarity, which is the charging polarity of the toner, and a DC voltage having a predetermined potential. As a result, the full-color unfixed toner image on the belt 7 (the toner image on the intermediate transfer member) is secondarily transferred to the recording material P all at once.

  The recording material P that has exited the secondary transfer nip A2 is electrostatically held on the recording material conveyance belt 14 and separated from the surface of the intermediate transfer belt 7, and the fixing device 21 is rotated by the subsequent rotation of the recording material conveyance belt 14. And separated from the recording material conveying belt 14, guided by the guide member 20, and introduced into the fixing device 21.

The fixing device 21 in this embodiment is a heat-pressure fixing device having a roller pair of a heat roller and a pressure roller as a basic constituent member, and the recording material P is introduced into a fixing nip portion which is a pressure contact portion of the roller pair. By being nipped and conveyed, it receives heat and pressure. As a result, the toner of each color toner image is melted and mixed to be fixed on the surface of the recording material as a full color print image (fixed image formation), and the full color print is discharged outside the apparatus.

  The surface of the intermediate transfer belt 7 after separation of the recording material is cleaned by the removal of the secondary transfer residual toner by the belt cleaner 12 to prepare for the next image forming process.

(2) Secondary transfer device 13
FIG. 2 is an enlarged view of a portion of the secondary transfer device 13 in FIG. This transfer device 13 is an apparatus using an endless and flexible recording material conveying belt (hereinafter referred to as a secondary transfer belt) 14 and a secondary transfer roller (transfer charging roller) 15 as a transfer charging member 15. .

The secondary transfer belt 14 is a semiconductive dielectric. In this embodiment, the belt 14 is formed of a fluorine-based rubber elastic layer, has a volume resistivity of 10 ¹⁰ Ωcm, and has a thickness of 300 μm. The elastic modulus of the elastic layer was adjusted to be 10 to 100 MPa.

  The belt 14 is stretched between a secondary transfer roller (secondary transfer outer roller) 15 as a transfer charging member and a belt stretching roller 16 arranged in parallel therewith. The secondary transfer roller 15 is brought into pressure contact with the secondary transfer counter roller 10 with a predetermined pressing force with the intermediate transfer belt 7 and the recording material transport belt 14 sandwiched therebetween. A contact portion between the intermediate transfer belt 7 and the secondary transfer belt 14 is a secondary transfer nip portion A2. The belt stretching roller 16 is positioned downstream of the secondary transfer roller 15 in the recording material conveyance direction.

In this embodiment, the secondary transfer roller 15 is a sponge roller in which a foamed rubber layer based on foamed NBR (nitrile butadiene rubber) is provided on a steel core. The resistance of the roller was adjusted by dispersing an ion conductive resistance adjusting agent so as to be 10 ^7.5 Ω (when 2 kV was applied) in an environment of 23 ° C. and 50% Rh. Further, the hardness of the roller was adjusted to 48 degrees in Asker C hardness. A predetermined secondary transfer bias is applied to the secondary transfer roller 15 from the secondary transfer power supply unit HV at a predetermined control timing.

  In this embodiment, the belt stretching roller 16 is a SUS roller and is grounded.

  The secondary transfer belt 14 rotates at the same speed as the rotation speed of the intermediate transfer belt 7 in the counterclockwise direction of the arrow by rotating the secondary transfer roller 15 counterclockwise as indicated by the arrow by a drive mechanism (not shown). Is done. The belt stretching roller 16 rotates following the rotation of the belt 14. Note that the secondary transfer belt 14 may be configured to be rotated by the rotation of the belt 7 by the frictional force with the intermediate transfer belt 7 in the secondary transfer nip portion A2.

Further, inside the belt portion (belt portion between the rollers 15 and 16) on the downstream side in the moving direction of the secondary transfer belt 14 from the secondary transfer nip portion A2, there is no contact with the belt 14 and parallel. An electrode member (potential regulating member : electrode plate ) 17 is disposed on the substrate. In this embodiment, the electrode member 17 has a SUS plate shape with a thickness of 1 mm, a recording material conveyance direction length of 40 mm, and a width of 330 mm, and is approximately 2 mm from the inner surface of the belt 14 with respect to the belt 14. They are arranged in parallel.

  In this embodiment, the same voltage as the secondary transfer bias voltage applied to the secondary transfer roller 15 is applied as an electrode bias to the electrode member 17 from the secondary transfer power supply unit HV to the secondary transfer roller 15. It is applied while the next transfer bias voltage is being applied.

  Transfer of the toner image from the intermediate transfer belt 7 to the recording material P by the secondary transfer device 13 and conveyance of the recording material P by the secondary transfer belt 14 are illustrated in FIGS. explain.

In the secondary transfer nip portion A2, by applying a secondary transfer bias voltage at a location sandwiched between the secondary transfer roller 15 and the secondary transfer counter roller 10, the recording material P is transferred from the intermediate transfer belt 7 to the recording material P. The charged toner T is electrostatically transferred. At this time, as shown in FIG. 3, a secondary transfer electric field E _2tr is applied between the secondary transfer roller 15 and the secondary transfer counter roller 10. Before the secondary transfer, the negatively charged toner T on the intermediate transfer belt 7 is electrostatically carried by the positive charge for carrying the toner injected into the belt 7. In this state, when the secondary transfer electric field E _2tr is applied by reaching the secondary transfer nip A 2, positive charges are injected from the secondary transfer roller 15 to the semiconductive secondary transfer belt 14. . In the recording material P of the insulator, to the secondary transfer electric field _{E 2tr,} the spontaneous polarization _{E P} occurs. In the interior of the recording material P, the spontaneous polarization E _P, positive charge is induced on the toner carrying surface of the recording material P, the surface in contact with the secondary transfer belt 14 negative charge is induced, the amount of charge The amount is equal to the amount of positive charge injected into the secondary transfer belt 14. Further, the negatively charged toner T on the intermediate transfer belt 7 is electrostatically attracted by the positive charge induced on the toner carrying surface of the recording material P. As a result, the toner image is electrostatically transferred from the intermediate transfer belt 7 to the recording material P at the secondary transfer nip portion A2.

  At this time, after the secondary transfer, the positive charge for carrying the toner existing in the intermediate transfer belt 7 is left in the belt 7 as the toner runs out. A part is attenuated toward the secondary transfer counter roller 10 in accordance with the secondary transfer electric field, but all the positive charges are not absorbed by the secondary transfer counter roller 10, and a part remains in the intermediate transfer belt 7. It is considered a thing.

  In the contact surface, when a positive charge and a negative charge are present on the opposite surfaces, it is considered that an electrostatic adsorption force acts in addition to the non-electrostatic adhesion force. When passing through the secondary transfer nip portion A2, as shown in FIG. 3, there is an adsorption force Fp between the recording material P and the secondary transfer belt 14, and between the recording material P and the intermediate transfer belt 7. It is considered that the attracting force Fi acts on each of them, and electrostatic adhesion force is included in each.

  If the suction force Fp is greater than the suction force Fi at the secondary transfer nip portion A2, the recording material P to which the toner T has been transferred exits the intermediate transfer belt 7 when exiting the secondary transfer nip portion A2. Separate from. It is considered that the toner is attracted to the secondary transfer belt 14 and conveyed to the fixing device 21 by the rotation of the belt 14.

  On the contrary, when the above-mentioned suction force Fp is smaller than the above-mentioned suction force Fi, when the recording material P transferred with the toner T exits the secondary transfer nip portion A2, the recording material P becomes the intermediate transfer belt. 7 is considered not to separate.

  The following is considered as a case where such a recording material separation failure may occur.

  For example, there may be a case where the surface of the secondary transfer belt 14 is finely scratched by long-term use, and the contact area between the belt 14 and the recording material P becomes small. The belt 14 may be rotationally driven with paper dust or the like attached thereto, and may be rubbed and damaged by being pressed at the secondary transfer nip portion A2. Alternatively, at the moment when the leading edge of the recording material P is nipped by the secondary transfer nip portion A2, the pressure at the secondary transfer nip portion concentrates on the leading edge portion of the paper, so that there is a possibility that the belt 14 may be damaged.

  When the surface of the belt 14 is scratched as described above, the contact area between the belt 14 and the recording material P decreases, and the adsorption force Fp between the recording material P and the belt 14 decreases. There is a possibility that a recording material separation defect occurs in the next transfer nip portion A2.

  On the other hand, regarding the suction force Fi acting between the recording material P and the intermediate transfer belt 7, the electrostatic suction force depends on the electric field strength between the secondary transfer belt 14 and the intermediate transfer belt 7.

As shown in FIG. 4, after passing through the secondary transfer nip A2 , the secondary transfer belt 14 is separated from the secondary transfer roller 15 and has an air gap α. In the secondary transfer, the positive charge amount injected into the belt 14 gives a charge amount corresponding to the total toner charge amount corresponding to the maximum density in the image forming apparatus. However, there are few scenes in which an image having the maximum density is taken on the entire surface, and in most cases, an excess positive charge larger than the charge amount of the transfer toner is applied to the belt 14. The recording material conveyance belt 14 to which an excessive charge is applied is separated from the secondary transfer roller 15, so that an ideal air gap α is formed between the recording material conveyance belt 14 and the secondary transfer roller 15 controlled to a predetermined potential. It is considered that a positive charge is placed in the air through the capacitor. In such a case, the capacitance of the air capacitor decreases as the gap length increases, and the potential difference between the air capacitors increases when a certain amount of charge exists. Therefore, after the belt 14 to which an excessive positive charge is applied passes through the secondary transfer nip portion A2 , the potential increases in the positive direction.

  At this time, as shown in the solid line graph of FIG. 5, the potential difference between the secondary transfer belt 14 and the secondary transfer roller 15 increases.

However, since the air gap between the secondary transfer belt 14 and the secondary transfer counter roller 10 or between the secondary transfer roller 15 and the intermediate transfer belt 7 also becomes large, there is an electric field strength as shown by a one-dot chain line graph in FIG. It is considered to have a local maximum at the position. This electric field strength is considered to be larger than the secondary transfer electric field _E2tr. Depending on the configuration downstream of the secondary transfer nip portion A in the recording material conveyance direction, if the electric field strength becomes too large, peeling discharge occurs. In addition, image defects may occur.

  With respect to the attractive force Fi acting between the recording material P and the intermediate transfer belt 7, the electrostatic attractive force downstream of the secondary transfer nip portion is the electric field acting between the secondary transfer belt 14 and the secondary transfer roller 15. Since it is proportional to the strength, it is greatest on the downstream side of the secondary transfer nip portion.

  Accordingly, if the suction force Fi acting between the recording material P and the intermediate transfer belt 7 on the downstream side of the secondary transfer nip portion becomes larger than the suction force Fp, the secondary transfer belt 14 is temporarily once in the secondary transfer nip. The recording material P electrostatically attracted to the toner is drawn toward the belt 7. For this reason, there is a possibility that a separation failure of the recording material from the intermediate transfer belt 7 may occur.

  As described above, the surface property of the secondary transfer belt 14 deteriorates due to long-term use, so that the adsorptive power between the secondary transfer belt 14 and the recording material P is reduced in the secondary transfer nip portion. As the potential of the secondary transfer belt 14 after passing through the secondary transfer nip increases, the suction force between the belt 14 and the recording material increases. In these cases, particularly in the case of the recording material P having a low stiffness such as thin paper, the recording material P is not attracted to the secondary transfer belt 14 after passing through the secondary transfer nip portion and is not separated from the intermediate transfer belt 7. There is a fear.

  The electrode member 17 disposed in the secondary transfer device 13 of FIGS. 1 and 2 is separated from the secondary transfer roller 15 by the subsequent rotation of the secondary transfer belt 14 to which excess charge is applied in the secondary transfer nip portion A2. This suppresses the phenomenon that the belt potential increases.

  This is because the electrode member 17 set so as to have the same electric potential as the secondary transfer roller 15 is arranged in parallel along the belt 14 so that the distance from the excess charge is kept constant. This is because it becomes possible.

  As shown in the solid line graph of FIG. 5, the potential increases as the potential of the secondary transfer belt 14 moves away from the secondary transfer roller 15 because the distance of the belt 14 from the secondary transfer roller 15 continues to increase. caused by. That is, as the distance of the belt 14 from the secondary transfer roller 15 increases, the air gap α between the belt 14 and the secondary transfer roller 15 electrically acts as a capacitor. For this reason, the longer the gap length, the smaller the capacitance, and the voltage across the capacitor holding a certain amount of charge increases.

  In contrast, the electrode member 17 having the same potential as that of the secondary transfer roller 15 serving as a charge transfer member is disposed in parallel with a predetermined distance from the belt 14 in a non-contact state, so that the belt 14 has an electrode. The distance to the member 17 does not increase. Therefore, the potential difference from the electrode member 17 or the secondary transfer roller 15 does not increase.

Due to the presence of the electrode member 17, the potential difference between the electrode member and the secondary transfer belt is a predetermined potential difference derived from the charge amount of the secondary transfer belt and the capacitor capacity formed by the air gap between the belt and the electrode member. Stipulated in At this time, as shown by the solid line graph in FIG. 5 , the potential difference between the secondary transfer belt and the secondary transfer roller increases immediately after passing through the secondary transfer nip, but the secondary transfer becomes closer to the electrode member 17. The potential of the belt is held with a predetermined potential difference from the potential applied to the electrode member. Accordingly, the potential difference from the electrode member 17 or the secondary transfer roller 15 does not increase.

At this time, the potential applied to the electrode member is assumed to have the same polarity as the transfer bias and the same absolute value . When a bias having a polarity opposite to the transfer bias is applied, the positive charge for carrying the toner applied to the secondary transfer belt in the secondary transfer process is attracted toward the electrode member. As a result, the toner carrying force is reduced and image defects may occur. On the other hand, if a potential having the same polarity as the transfer bias and a larger absolute value is applied to the electrode member, the above-described effect of suppressing the potential increase of the secondary transfer belt by the electrode member is lost. Therefore, the electrode bias to the electrode member 17, the transfer bias of the same polarity to be applied to the secondary transfer roller 15, and the absolute value of the same accident are preferred. The electrode member 17 regulates the belt potential by applying an electrode bias.

  As a result, the potential increase of the secondary transfer belt 14 can be suppressed. On the other hand, since the distance between the secondary transfer belt 14 and the intermediate transfer belt 7 is increased by the rotation, the electric field strength at the downstream portion of the secondary transfer nip portion between the secondary transfer belt 14 and the intermediate transfer belt 7 is small. Become. As a result, since the electrostatic attraction force between the intermediate transfer belt 7 and the recording material P becomes smaller, the attraction force Fi becomes relatively smaller than the attraction force Fp and is always attracted during the durability of the apparatus. It becomes possible to maintain the state of force Fp> adsorption force Fi. Therefore, even in the case of the recording material P having a low stiffness such as thin paper, the separation failure of the recording material P from the intermediate transfer belt 7 can be effectively avoided.

  In the present embodiment, the image forming apparatus that forms toner images of four colors has been described. However, the number of image forming units (image forming units) is not limited to four.

  FIG. 6 is a schematic configuration diagram of the image forming apparatus of the present embodiment. This image forming apparatus is a one-drum electrophotographic full-color image forming apparatus using an intermediate transfer belt. That is, four color toner images of yellow, magenta, cyan, and black are sequentially superimposed on one drum 1 that is rotationally driven at a predetermined speed in the counterclockwise direction of the arrow to form an unfixed toner image of a full color image. To form a composite. The composite toner image is collectively transferred onto the intermediate transfer belt 7 at the primary transfer nip portion A1, and further secondary transferred onto the recording material P at the secondary transfer nip portion A2, and the recording material P is transferred. It is introduced into the fixing device 21.

  The developing device 4 is a rotary developing device in which four developing devices of a yellow toner developing device 4Y, a magenta toner developing device 4M, a cyan toner developing device 4C, and a black toner developing device 4K are mounted on a rotatable support 41. By rotating the support 41 sequentially in a clockwise direction indicated by an arrow at a predetermined rotation angle, the developing devices 4Y, 4M, 4C, and 4K are sequentially switched to a position facing the drum 1 (developing position).

  The primary transfer roller 5 has a first position indicated by a solid line where the intermediate transfer belt 7 is pressed against the surface of the drum 1 to form the primary transfer nip portion A1, and the intermediate transfer belt 7 is moved away from the drum 1 to the surface of the drum 1. Is switched to a second position indicated by a two-dot chain line separated from the first position. Reference numerals 22 and 23 denote belt stretching rollers disposed on the upstream side and the downstream side, respectively, in the intermediate transfer belt moving direction with the primary transfer roller 5 in the middle.

  The drum cleaner 6 is switched and moved between a first position indicated by a solid line acting on the drum 1 and a second position (non-acting position) indicated by a two-dot chain line separated from the drum 1.

  In a state where the primary transfer roller 5 and the drum cleaner 6 are switched to the second position, the color separation component image of the full-color original image is applied to the surface of the rotating drum 1 by the charging roller 2, the exposure device 3, and the developing device 4Y. A yellow toner image corresponding to the yellow component image is formed. The exposure apparatus 3 is a laser scanner, similar to the image forming apparatus of the first embodiment, and uses a combination of an image exposure method and a reversal development method.

  A magenta toner image corresponding to the magenta component image in the color separation component image of the full-color original image is formed on the yellow toner image on the drum 1 by the charging roller 2, the exposure device 3, and the developing device 4M.

  Further, a cyan toner corresponding to a cyan component image in a color separation component image of a full-color original image is superposed on the yellow toner image + magenta toner image on the drum 1 by a charging roller 2, an exposure device 3, and a developing device 4C. An image is formed.

  Then, the yellow toner image + magenta toner image + cyan toner image on the drum 1 is superimposed on the black component image of the color separation component image of the full-color original image by the charging roller 2, the exposure device 3, and the developing device 4K. A corresponding black toner image is formed.

  In this way, four color toner images of yellow, magenta, cyan, and black are sequentially superimposed on one rotating drum 1 in a predetermined manner, and a full-color unfixed toner image is synthesized and formed.

  Next, the primary transfer roller 5 is switched and moved to the first position at a predetermined control timing, and a predetermined primary transfer bias is applied to the primary transfer roller 5. As a result, in the primary transfer nip portion A1, the full-color unfixed toner images on the drum 1 surface side are collectively transferred sequentially from the rotating drum 1 surface side to the rotating intermediate transfer belt 7 side. Further, the drum cleaner 6 is switched to the first position to remove the primary transfer residual toner on the drum 1.

  The full-color unfixed toner image on the belt 7 is conveyed by the subsequent rotation of the belt 7 and reaches the secondary transfer nip portion A2 which is a contact portion between the belt 7 and the secondary transfer belt 14 of the secondary transfer device 13. . Then, the full-color unfixed toner image on the belt 7 is secondarily transferred to the recording material P at once as in the image forming apparatus of the first embodiment. The recording material P that has exited the secondary transfer nip A2 is electrostatically held on the secondary transfer belt 14 and separated from the surface of the intermediate transfer belt 7, and the belt 14 continues to rotate toward the fixing device 21. It is conveyed, separated from the belt 14, guided by the guide member 20, and introduced into the fixing device 21.

  The secondary transfer device 13 has the same configuration as the secondary transfer device 13 in the image forming apparatus of the first embodiment, and can obtain the same effects as the secondary transfer device 13 of the first embodiment. That is, the electrode member 17 having the same potential as that of the secondary transfer roller 15 serving as a charge transfer member is disposed in parallel with a predetermined distance from the secondary transfer belt 14 serving as a recording material conveying belt in a non-contact state. Thus, the distance between the belt 14 and the electrode member 17 does not increase. Therefore, the potential difference from the electrode member 17 or the secondary transfer roller 15 does not increase. Thereby, the potential increase of the belt 14 can be suppressed. On the other hand, since the distance between the secondary transfer belt 14 and the intermediate transfer belt 7 is increased by the rotation, the electric field strength at the downstream portion of the secondary transfer nip portion between the secondary transfer belt 14 and the intermediate transfer belt 7 is small. Become. As a result, since the electrostatic attraction force between the intermediate transfer belt 7 and the recording material P becomes smaller, the attraction force Fi becomes relatively smaller than the attraction force Fp and is always attracted during the durability of the apparatus. It becomes possible to maintain the state of force Fp> adsorption force Fi. Therefore, even in the case of the recording material P having a low stiffness such as thin paper, the separation failure of the recording material P from the intermediate transfer belt 7 can be effectively avoided.

The electrode bias for the electrode member 17 has the same polarity as the transfer bias applied to the secondary transfer roller 15 and has the same absolute value .

  FIG. 7 is a schematic configuration diagram of the image forming apparatus of this embodiment. This image forming apparatus is a monochrome (monocolor) electrophotographic image forming apparatus.

  An unfixed black toner image is formed by the charging roller 2, the exposure device 3, and the developing device 4K on the drum 1 that is rotationally driven in the clockwise direction indicated by the arrow. The toner image reaches the transfer nip portion A which is a contact portion between the drum 1 and a recording material conveyance belt (hereinafter referred to as a transfer belt) 14 of the transfer device 5A by the subsequent rotation of the drum 1. Then, the unfixed toner image on the drum 1 is transferred to the recording material P. The recording material P exiting the transfer nip A is electrostatically held on the transfer belt 14 and separated from the surface of the drum 1, and is conveyed toward the fixing device 21 by the subsequent rotation of the belt 14. Separated, guided by the guide member 20 and introduced into the fixing device 21.

  The transfer device 5A has the same configuration as the secondary transfer device 13 in the image forming apparatus of the first or second embodiment, and can obtain the same effect. That is, the electrode member 17 having the same potential as that of the transfer roller 15 as a charging transfer member is arranged in parallel with a predetermined distance from the belt 14 in a non-contact state. The distance does not increase. Therefore, the potential difference from the electrode member 17 or the transfer roller 15 does not increase. Thereby, an increase in potential of the transfer belt 14 can be suppressed. On the other hand, since the distance between the transfer belt 14 and the drum 1 is increased by the rotation, the electric field strength in the downstream portion of the transfer nip portion between the belt 14 and the drum 1 is reduced. As a result, since the electrostatic attraction force between the drum 1 and the recording material P becomes smaller, the attraction force Fi (here, the attraction force between the drum 1 and the recording material P) is relatively larger than the attraction force Fp. Get smaller. As a result, during the durability of the apparatus, it is possible to always maintain the state of the adsorption force Fp> the adsorption force Fi. Therefore, even in the case of the recording material P having a low stiffness such as thin paper, it is possible to effectively avoid the separation failure of the recording material P from the drum 1.

The electrode bias for the electrode member 17 has the same polarity as the transfer bias applied to the transfer roller 15 and the same absolute value .

  8A, 8B, and 8C are modifications of the transfer device 5A in FIG. (A) Suspends an endless transfer belt 14 between first and second belt stretching rollers 16a and 16b on the upstream side and the downstream side in the belt moving direction from the transfer nip portion A. It is. Then, a transfer charging roller 15 as a transfer charging member is disposed between the rollers 16a and 16b and pressed against the drum 1 with the belt 14 interposed therebetween, so that the transfer nip between the drum 1 and the belt 14 is reached. Part A is formed. Further, on the inner side of the belt portion (belt portion between the rollers 15 and 16b) on the downstream side in the moving direction of the belt 14 from the transfer nip portion A, the electrode member (potential) is in non-contact with the belt 14 and in parallel. A restricting member 17 is provided. A transfer bias voltage having a predetermined polarity and potential is applied to the transfer charging roller 15 from a transfer power supply HV. An electrode bias is applied to the electrode member 17. The electrode bias is a bias having the same polarity as the transfer bias applied to the transfer roller 15 and having the same absolute value or smaller than the transfer bias. Or it is 0V.

(B) shows a configuration in which the transfer charging roller 15 as a transfer charging member is changed to a transfer charging blade 15a in the charging device 5A of (a), and (c) shows a configuration in which it is changed to a corona charger 15b. . (B) and (c) are reference examples.

The secondary transfer device 13 in the image forming apparatus of Example 1 or 2 may be a transfer device 5A configured as (a) of FIG. 8.

  FIG. 9 is a schematic configuration diagram of the image forming apparatus of the present embodiment. This image forming apparatus is the same as the electrophotographic full-color image forming apparatus according to the first embodiment. The intermediate transfer belt 7 is an endless recording material conveying belt (hereinafter referred to as a transfer belt) 14 and a transfer roller 15 as a transfer charging member. The configuration is changed to the transfer device 5A used.

The transfer belt 14 of the transfer device 5A is suspended and stretched between a belt stretching roller 16b on the first image forming unit Y side and a belt stretching roller 16a on the fourth image forming unit K side. The upper surface of the belt portion between the rollers 6a and 6b and the lower surface of the drum 1 of each image forming portion Y, M, C, and K are brought into contact with each other to form a transfer nip portion A. In each transfer nip portion A, a transfer roller 15 as a transfer charging member is disposed on the inner side of the belt in contact with the inner surface of the belt. In addition, an electrode member (potential regulating member) 17 is disposed in parallel with the belt 14 in a non-contact state and in parallel with the belt portion on the downstream side in the moving direction of the belt 14 from each transfer nip A. It is. A transfer bias voltage having a predetermined polarity and potential is applied to each transfer roller 15 from a transfer power supply HV. An electrode bias is applied to each electrode member 17. The electrode bias for each electrode member 17 has the same polarity as the transfer bias applied to the corresponding transfer roller 15 and the same absolute value .

  The recording material P is guided from the registration roller pair 18 to the guide member 19 and fed onto the belt 14 and conveyed toward the transfer nip portion A of the fourth image forming unit. The black toner image formed on the drum 1 of the fourth image forming unit K is transferred. The recording material P is adsorbed and held on the belt 14 and then sequentially conveyed to the transfer nip A of the third to first image forming units C, M, and Y. The recording material P is further subjected to sequential superposition transfer of a cyan toner image, a magenta toner image, and a yellow toner image formed on each drum 1 of each image forming unit C, M, and Y. As a result, a four-color full-color undecided brown toner image is synthesized and formed on the recording material P conveyed by the belt 14. The recording material P is conveyed toward the fixing device 21 by the subsequent rotation of the belt 14, separated from the belt 14, guided by the guide member 20, and introduced into the fixing device 21.

  In each of the image forming units Y, M, C, and K, an electrode member 17 having the same potential as that of the transfer roller 15 serving as a charge transfer member is disposed in parallel with a predetermined distance from the belt 14 in a non-contact state. Thus, the distance between the belt 14 and the electrode member 17 does not increase. For this reason, the potential difference between the electrode member 17 and the transfer roller 15 does not increase in each of the image forming portions Y, M, C, and K. Accordingly, the potential increase of the belt 14 can be suppressed in each of the image forming units Y, M, C, and K. On the other hand, since the distance between the belt 14 and the drum 1 is increased by the rotation, the electric field strength in the downstream portion of the transfer nip portion between the belt 14 and the drum 1 in each of the image forming portions Y, M, C, and K is Get smaller. As a result, since the electrostatic attraction force between the drum 1 and the recording material P becomes smaller, the attraction force Fi (here, the attraction force between the drum 1 and the recording material P) is relatively larger than the attraction force Fp. Get smaller. As a result, during the durability of the apparatus, it is possible to always maintain the state of the suction force Fp> the suction force Fi in each of the image forming units Y, M, C, and K. Therefore, even in the case of the recording material P having a low stiffness such as thin paper, the separation failure of the recording material P from the drum 1 can be effectively avoided in each of the image forming portions Y, M, C, and K.

1 is a schematic configuration diagram of an image forming apparatus in Embodiment 1. FIG. FIG. 2 is an enlarged view of a secondary transfer device portion of the image forming apparatus of FIG. 1. FIG. 4 is a schematic diagram for explaining an attractive force acting on a recording material in a secondary transfer unit. FIG. 6 is a schematic diagram for explaining an attractive force acting on a recording material downstream of a secondary transfer unit. FIG. 6 is a diagram for explaining a potential of a secondary transfer belt (recording material conveyance belt) downstream of the secondary transfer unit and an electric field strength acting between the belt and the intermediate transfer belt. 6 is a schematic configuration diagram of an image forming apparatus in Embodiment 2. FIG. 6 is a schematic configuration diagram of an image forming apparatus in Embodiment 3. FIG. (A), (b), (c) is a schematic block diagram of the modification of the transfer device in the image forming apparatus of FIG. 6 is a schematic configuration diagram of an image forming apparatus in Embodiment 4. FIG.

Explanation of symbols

  1. ・ Image carrier, 7. ・ Intermediate transfer belt, A ・ A1 ・ A2 ・ ・ Primary transfer nip or secondary transfer nip, 5A ・ 13 ・ ・ Transfer device or secondary transfer device, 5.15 ・ ・ Transfer Charging member (primary transfer roller or secondary transfer roller), 14... Recording material conveying belt, 17... Electrode member, HV.

Claims (4)

A rotatable image carrier and a belt that can move around to form a transfer nip portion and sandwich and convey the recording material;
A transfer charging roller disposed at a position of the transfer nip inside the belt;
A power supply unit for transferring electrostatically a toner image on the transfer charging roller a transfer bias is applied to the image bearing member to a record member,
In a transfer device having
Wherein in a non-contact state with respect to the belt portion in the vicinity of the transfer charging roller inside the belt portion of the downstream side in the movement direction of the belt from the transfer nip portion, and disposed parallel to, applying the transfer bias a transfer device, characterized in that the perforated electrode plate for regulating the belt potential by. A rotatable image carrier;
Toner image forming means for forming a toner image on the image carrier;
Transfer means for transferring a toner image on the image carrier to a recording material;
In an image forming apparatus having
The transfer means includes a belt that is capable of rotating around the image carrier and forms a transfer nip portion for nipping and transporting a recording material, and a transfer charging roller disposed at the position of the transfer nip portion inside the belt. A power supply unit that applies a transfer bias to the transfer charging roller to electrostatically transfer the toner image on the image bearing member to the recording material; and an inner side of the belt portion on the downstream side in the moving direction of the belt from the transfer nip portion. in a non-contact state with respect to the belt portion in the vicinity of the transfer charging roller and parallel disposed, and characterized by the perforated electrode plates to regulate the belt potential by applying the transfer bias Image forming apparatus. A rotatable image carrier;
Toner image forming means for forming a toner image on the image carrier;
A primary transfer means for transferring the toner image on the image carrier to an intermediate transfer member that circulates;
Secondary transfer means for transferring the toner image on the intermediate transfer member to a recording material;
In an image forming apparatus having
The secondary transfer means is disposed at a position of the transfer nip portion inside the belt, and a belt that is capable of circular movement that forms a secondary transfer nip portion with the intermediate transfer member and sandwiches and conveys the recording material. A transfer charging roller; a power supply unit that applies a transfer bias to the transfer charging roller to electrostatically transfer the toner image on the image bearing member to the recording material; and a belt moving downstream from the secondary transfer nip unit. in a state of non-contact with said belt portion in the vicinity of the transfer charging roller inside the belt portion of the side and parallel disposed, the electrode plate for regulating the belt potential by applying the transfer bias beamed image forming apparatus characterized by.   4. The image forming apparatus according to claim 1, wherein a thickness of the electrode plate is smaller than a distance between the electrode plate and a back surface of the belt. 5.