JP5511280B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that transfers a toner image from an intermediate transfer belt to a recording material in a secondary transfer portion using the transfer belt, and more particularly to a structure that improves the separation of the thin recording material from the transfer belt.

  An image forming apparatus for transferring a toner image onto a recording material carried on a transfer belt has been put into practical use (Patent Documents 1 and 2).

  Patent Document 1 discloses a recording material conveyance belt type image forming apparatus in which a toner image is transferred from a photosensitive drum and superimposed on a recording material carried on a transfer belt.

  In Patent Document 2, an image forming apparatus for transferring a toner image from an intermediate transfer belt to a recording material at a secondary transfer unit using a transfer belt has been put into practical use. Here, the transfer belt and the intermediate transfer belt are brought into contact with each other to form a contact portion continuous in the rotation direction of the intermediate transfer belt. A transfer voltage is applied to transfer the toner image to the downstream side of the first pressure mechanism (roller pair) that presses the transfer belt and the intermediate transfer belt to start the nip of the recording material. The pressure mechanism (electrode pair) is arranged.

  In these prior arts, by adhering the recording material to the transfer belt, it is possible to pass through the transfer portion stably without forming wrinkles or deformation even for a recording material with low rigidity such as a thin recording material or cloth. Can do. Then, after the toner image is transferred, the recording material can be forcibly separated from the intermediate transfer belt in a state where the recording material is adsorbed to the transfer belt.

JP 2004-133419 A JP 2008-139722 A

  The image forming apparatus disclosed in Patent Document 2 can efficiently separate the recording material from the intermediate transfer belt using the transfer belt, but after that, it is necessary to separate the recording material from the transfer belt. This is because, when the recording material cannot be separated from the transfer belt, the recording material rotates around the transfer belt and causes a jam.

  In general, the transfer belt mechanism can be optimized for separation of the recording material, such as by reducing the support roller diameter of the separation unit, so separating the recording material from the transfer belt is as much as separating from the intermediate transfer belt. Not difficult.

  However, in the case of a thin paper with extremely low rigidity and a resin film that is easily charged with static electricity, it is not easy to separate the recording material from the transfer belt, and the possibility of occurrence of jamming of the recording material increases.

  Therefore, it has been proposed to arrange a separation claw or corona neutralizer at the separation part from the transfer belt. However, wrinkles or deformation may occur in the recording material or unfixed toner images may be disturbed through contact or neutralization. there's a possibility that.

  An object of the present invention is to provide an image forming apparatus capable of easily separating a recording material from a transfer belt without damaging the surface of the recording material or a toner image even with a thin paper having low rigidity or a resin film which is easily charged with static electricity. .

The image forming apparatus of the present invention includes an intermediate transfer belt that carries a toner image and a transfer belt that carries a recording material, and the intermediate transfer belt and the transfer belt contact each other along the rotation direction of the intermediate transfer belt. The contact surface is formed. The intermediate transfer belt and the transfer belt are arranged on the inner side of the intermediate transfer belt and the inner side of the transfer belt so that the nip regions overlap each other, and the upstream portion of the contact surface in the rotational direction And the inner side of the intermediate transfer belt and the inner side of the intermediate transfer belt so that the nip regions of the first pressure electrode pair and the intermediate transfer belt and the transfer belt overlap with each other. A second pressurizing electrode pair that is disposed inside the transfer belt and pressurizes each other at the downstream side in the rotation direction of the contact surface and can apply an electric field, and the recording material has a rigidity. When the rigidity is lower than a predetermined rigidity, an electric field in a direction in which toner is transferred from the intermediate transfer belt to the recording material is applied to the first pressure electrode pair, and each of the second pressure electrode pairs is grounded. In the direction in which toner is transferred from the intermediate transfer belt to the recording material to the second pressurizing electrode pair when the recording material has a predetermined rigidity or higher. And an execution unit that executes a second transfer mode in which an electric field is applied and each of the first pressure electrode pairs is set to a ground potential.

  In the image forming apparatus of the present invention, by executing the first transfer mode, excess charges remaining after being injected when the toner image is transferred from the intermediate transfer belt on the upstream side of the continuous contact portion are continuously generated. It is possible to electrically cancel the downstream side of the contact portion. In a state where the recording material is sandwiched between the intermediate transfer belt and the transfer belt, both surfaces of the recording material are electrically connected to each other through the intermediate transfer belt and the transfer belt to induce the same potential.

  For this reason, the recording material does not retain an excessive amount of electric charge more than that necessary for attracting the toner, and the electrical adsorption force that prevents separation of the recording material and the transfer belt is reduced. Therefore, even a thin paper with low rigidity or a resin film that is easily charged with static electricity can easily separate the recording material from the transfer belt without damaging the surface of the recording material or the toner image.

1 is an explanatory diagram of a configuration of an image forming apparatus. FIG. 6 is an explanatory diagram of a charge state around a toner image before entering an upstream transfer unit. FIG. 6 is an explanatory diagram of a charge state around a toner image immediately after entering an upstream transfer unit. FIG. 6 is an explanatory diagram of charge movement accompanying transfer of a toner image. FIG. 6 is an explanatory diagram of a change in electrostatic attraction force between an intermediate transfer belt and a toner image accompanying transfer of a toner image. It is explanatory drawing of the effect of a separation nail. It is explanatory drawing of the effect of the corona static eliminator for isolation | separation. It is explanatory drawing of the static elimination in a downstream transfer part. FIG. 10 is an explanatory diagram of a configuration of an image forming apparatus in Embodiment 3. It is explanatory drawing of the 1st transfer mode and the 2nd transfer mode. 10 is a flowchart of control according to the third embodiment. It is explanatory drawing of the structure of the contact part in Example 4. FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the present invention, as long as the recording material is neutralized between the intermediate transfer belt and the transfer belt after the secondary transfer of the toner image, a part or all of the configuration of the embodiment is replaced with the alternative configuration. This embodiment can also be implemented.

  Therefore, any image forming apparatus that transfers a toner image from an intermediate transfer belt to a recording material carried on the transfer belt can be implemented without distinction between a tandem type and a one-drum type. In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about the general matter of the image forming apparatus shown by patent document 1, 2, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted.

<Example 1>
FIG. 1 is an explanatory diagram of the configuration of the image forming apparatus. As shown in FIG. 1, the image forming apparatus 100 is a tandem intermediate transfer type full-color printer in which image forming portions PY, PM, PC, and PK of yellow, magenta, cyan, and black are arranged along an intermediate transfer belt 6. is there.

  In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 1Y and is primarily transferred to the intermediate transfer belt 6. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1M, and is primarily transferred onto the yellow toner image on the intermediate transfer belt 6. In the image forming units PC and PK, a cyan toner image and a black toner image are formed on the photosensitive drums 1C and 1K, respectively, and are sequentially transferred to the intermediate transfer belt 6 in order to be primarily transferred.

  The four-color toner images primarily transferred to the intermediate transfer belt 6 are conveyed to the upstream transfer portion N1, which is a secondary transfer portion, and are collectively secondary transferred to the recording material P carried on the transfer belt 24. The recording material P on which the four-color toner images are secondarily transferred is heated and pressed by the fixing device 13 to fix the toner images on the surface, and then discharged to the outside of the machine body.

  The recording material P drawn from the recording material cassette 14 based on the start signal is separated one by one by the separation roller 15 and sent to the registration roller 8. The registration roller 8 receives and waits for the recording material P in the stopped state, and sends the recording material P to the transfer belt 24 in synchronization with the toner image on the intermediate transfer belt 6.

  The recording material P conveyed by the registration roller 8 is adsorbed to the transfer belt 24 by being sandwiched between the transfer belt 24 and the adsorption roller 28 to which an adsorption bias is applied. The recording material P is fed by controlling the conveyance of the recording material P by the registration rollers 8 so that the leading edge of the toner image on the intermediate transfer belt 6 reaches the upstream transfer portion N1.

  The belt cleaning device 12 rubs the intermediate transfer belt 6 with a cleaning blade to collect the transfer residual toner remaining on the intermediate transfer belt 6 after passing through the contact portion T2 by escaping from the transfer to the recording material P.

  The fixing device 13 is a heat roller fixing device that rotates the fixing roller 13a and the pressure roller 13b while pressing them. A halogen lamp heater 13c is disposed inside the fixing roller 13a, and temperature control is performed to maintain the surface of the fixing roller 13a at a predetermined fixing temperature by controlling the voltage applied to the halogen lamp heater 13c. Yes. In the process in which the recording material P is introduced into the pressure contact portion between the fixing roller 13a and the pressure roller 13b and is nipped and conveyed, the toner of each color toner image on the recording material P is melted and mixed to fix the full color image.

  The image forming units PY, PM, PC, and PK are configured substantially the same except that the color of toner used in the developing devices 4Y, 4M, 4C, and 4K is different from yellow, magenta, cyan, and black. Hereinafter, the image forming unit PY will be described, and the other image forming units PM, PC, and PK will be described by replacing Y at the end of the reference numerals attached to the constituent members being described as M, C, and K. And

  In the image forming unit PY, a corona charger 2Y, an exposure device 3Y, a developing device 4Y, a primary transfer roller 5Y, and a cleaning device 11Y are arranged around the photosensitive drum 1Y. The photosensitive drum 1Y has a photosensitive layer having a negative charge polarity formed on the cylindrical outer peripheral surface of an aluminum cylinder, and rotates in the direction of arrow R1 at a process speed of 250 to 300 mm / sec. The corona charger 2Y irradiates the photosensitive drum 1Y with charged particles associated with corona discharge, and charges the surface of the photosensitive drum 1Y to a uniform dark potential VD having a negative polarity.

  The exposure device (laser beam scanner) 3Y scans a laser beam obtained by ON-OFF modulation of scanning line image data obtained by developing a yellow color separation image with a rotating mirror. When the surface potential of the photosensitive drum 1Y charged to the dark portion potential VD is exposed to light and decreases to the bright portion potential VL, an electrostatic image of an image is written on the photosensitive drum 1Y. The exposure device 3Y outputs laser light that is ON-OFF modulated in accordance with image information input from an external device such as an image scanner or a computer (not shown), and scan-exposes the charging processing surface on the photosensitive drum 1. . By this scanning exposure, an electrostatic latent image corresponding to target image information is formed on the surface of the photosensitive drum 1.

  The developing device 4Y charges a two-component developer containing yellow toner (non-magnetic) and carrier (magnetic) to be carried on the developing sleeve. By applying an oscillating voltage in which an AC voltage is superimposed on a negative DC voltage Vdc to the developing sleeve, the toner is transferred to a portion of the photosensitive drum 1Y having a relatively positive polarity and a bright portion potential VL, and an electrostatic image is obtained. Is reversely developed.

  The primary transfer roller 5 </ b> Y is pressed against the intermediate transfer belt 6 with a predetermined pressing force, and rotates following the rotation of the intermediate transfer belt 6. The primary transfer roller 5Y primarily transfers the toner image carried on the photosensitive drum 1Y to the intermediate transfer belt 6 by applying a positive DC voltage V1.

  The cleaning device 11Y slides a cleaning blade on the photosensitive drum 1Y to collect the transfer residual toner remaining on the photosensitive drum 1Y by escaping from the transfer to the intermediate transfer belt 6.

<Continuous contact part>
The intermediate transfer belt 6 is supported across the tension roller 22, the driving roller 20, and the opposing rollers 21, 23, and is driven by the driving roller 20 to rotate in the direction of arrow R2 at a process speed of 250 to 300 mm / sec. . The intermediate transfer belt 6 has a volume resistivity adjusted to 1 × 10 ⁹ to 1 × 10 ¹⁴ [Ω · cm] by containing an appropriate amount of carbon black as an antistatic agent in resins such as polyimide and polycarbonate, or various rubbers. ing. The intermediate transfer belt 6 has a length in the rotation direction of 1148 mm, a length in the width direction perpendicular to the rotation direction of 360 mm, and a thickness of 0.07 to 0.1 [mm].

  The transfer belt 24 (recording material carrier) is stretched by a stretching roller 25, a driving roller 26, and a tension roller 27, and is driven by the driving roller 26 to rotate in the direction of arrow R3 at 250 to 300 mm / sec. The driving roller 26 also serves as a separation roller that forms a curved surface for bending the recording material P by bending the transfer belt 24 at an acute angle.

The transfer belt 24 is prepared by adding an appropriate amount of carbon black as an antistatic agent to resins such as polyimide and polycarbonate, or various rubbers, and adjusting the volume resistivity to 1 × 10 ⁹ to 1 × 10 ¹⁴ [Ω · cm]. Yes. The transfer belt 24 has a length in the rotation direction of 352 mm, a length in the width direction perpendicular to the rotation direction of 376 mm, and a thickness of 0.07 to 0.1 [mm].

  The suction roller 28 is configured by a pair of an inner roller 28 a disposed inside the transfer belt 24 and an outer roller 28 b disposed outside. When the recording material P supplied to the transfer belt 24 is nipped and conveyed by the suction roller 28, a suction bias whose constant current is controlled at −15 to −30 μA by the suction bias power supply 30 is applied to the inner roller 28 b. As a result, the recording material P is electrostatically attracted to the transfer belt 24.

The inner roller 28 b is a rubber roller having an outer diameter of φ18 mm in which an outer periphery of a core metal connected to the suction bias power supply 30 is covered with an elastic layer of ion conductive solid rubber (NBR rubber). The resistance value was 1 × 10 ⁵ to 1 × 10 ⁶ [Ω] measured by applying 50 V in a normal temperature and normal humidity environment [N / N: 23 ° C., 50% RH].

The outer roller 28a is a fur brush roller having an outer diameter of 18 mm in which conductive nylon fibers having a bristle length of 5 mm are planted on the outer periphery of a core metal having a diameter of 8 mm, and enters the transfer belt 24 by 1.5 to 2 mm. . The resistance value was 1 × 10 ⁵ to 1 × 10 ⁶ Ω when measured by applying 100 V in a normal temperature and normal humidity environment [N / N: 23 ° C., 50% RH].

The intermediate transfer belt 6 carrying the toner image and the transfer belt 24 carrying the recording material P are in contact with each other to form a contact portion T2 (transfer nip region) having a length L continuous in the rotation direction of the intermediate transfer belt 6. . The opposing roller 21, the secondary transfer roller 9, and the high-voltage power source 41 constitute a first pressure electrode pair that pressurizes the upstream transfer portion N1 that is the upstream portion of the continuous contact portion T2 and upstream. The transfer voltage can be applied by the high-voltage power supply 41 on the side. The opposing roller 23 and the secondary transfer roller 10 constitute a second pressure electrode pair , and pressurize the downstream transfer portion N2 that is the downstream portion of the continuous contact portion T2. Further, a transfer voltage can be applied to the downstream transfer portion N2 by the high-voltage power source 42 on the downstream side. The secondary transfer roller 9 corresponds to the first transfer roller, and the secondary transfer roller 10 corresponds to the second transfer roller.

  High voltage power supplies 41 and 42, which are examples of power supply means, can output variable transfer voltages to the secondary transfer rollers 9 and 10 that press the inner surface of the transfer belt 24, respectively, and are connected to the ground potential. Is also possible. The opposing rollers 21 and 23 are connected to the ground potential.

Each of the secondary transfer rollers 9 and 10 has an outer diameter of 24 mm by covering the outer peripheral surface of the core metal with an elastic layer of ion conductive foamed rubber (NBR rubber), and has a surface roughness Rz = 6.0-12. 0.0 (μm). The resistance value was 1 × 10 ⁵ to 1 × 10 ⁷ Ω when measured by applying 2 kV in a normal temperature and normal humidity environment [N / N: 23 ° C., 50% RH].

In the image forming apparatus 100, the first transfer mode (thin paper mode) is set. In the first transfer mode, the high voltage power supply 41 applies a transfer voltage to the first pressure electrode pair (9), and the high voltage power supply 42 connects the second pressure electrode pair (10, 23) to the ground potential. To do. In the first transfer mode, the high-voltage power supply 41 causes a current of +30 to +40 μA to flow through the upstream secondary transfer roller 9 to transfer the toner image on the intermediate transfer belt 6 to the recording material P. At this time, the high-voltage power supply 42 removes the electric charge of the transfer belt 24 by dropping the secondary transfer roller 10 on the downstream side to the ground potential similarly to the counter roller 23.

  In the image forming apparatus 100, a second transfer mode (thick paper mode) can also be set. In the second transfer mode, the secondary transfer roller 9 on the upstream side is dropped to the ground potential so that a large electric field is not formed between the intermediate transfer belt 6 and the recording material P on the upstream side of the upstream transfer portion N1. The electric field is regulated. At this time, the high-voltage power source 42 causes a current of +30 to +40 μA to flow through the secondary transfer roller 10 on the downstream side to transfer the toner image on the intermediate transfer belt 6 to the recording material P.

  Thereafter, the recording material P onto which the toner image has been transferred passes through the continuous contact portion T 2, is separated from the intermediate transfer belt 6, is conveyed to the driving roller 26, and is transferred to the transfer belt 24 on the curved surface along the driving roller 26. Is separated from the curvature. At this time, the recording material P is separated from the transfer belt 24 using the separation claw 29 at the position of the driving roller 26 and sent to the fixing device 13.

<Electrostatic adsorption of recording material and transfer belt>
FIG. 2 is an explanatory diagram of a charge state around the toner image before entering the upstream transfer portion. FIG. 3 is an explanatory diagram of the charge state around the toner image immediately after entering the upstream transfer portion. FIG. 4 is an explanatory diagram of charge transfer accompanying the transfer of the toner image. FIG. 5 is an explanatory diagram of changes in the electrostatic attracting force between the intermediate transfer belt and the toner image accompanying the transfer of the toner image. FIG. 6 is an explanatory view of the effect of the separation claw. FIG. 7 is an explanatory view of the effect of the corona static eliminator for separation.

  As shown in FIG. 2A, a negatively charged toner image is placed on the intermediate transfer belt 6 before being transferred to the recording material P. The broken line portion is enlarged and shown in FIG. As shown in FIG. 2B, there are three negative charges that schematically represent the toner image on the intermediate transfer belt 6. On the other hand, as a charge that generates a force that attracts the toner image to the intermediate transfer belt 6, the surface of the intermediate transfer belt 6 has a positive injection charge for three toners and the charge of the toner image. There is a mirror charge for three toners formed on the surface. The positive injection charge for three toners is the charge supplied to transfer the toner image from the photosensitive drum 1Y to the intermediate transfer belt 6 in the primary transfer portion, and is the same amount as the negative charge amount of the toner image. ing.

  It should be noted that the amount of the mirror charge that has come out here is merely for illustrative purposes, and the ratio of the mirror charge to the charge of the toner image may be different in an actual machine. is there.

  As shown in FIG. 3A, the toner image on the intermediate transfer belt 6 and the recording material P are then conveyed to the upstream transfer portion N1 and nipped. The broken line portion is enlarged and shown in FIG. As shown in FIG. 3B, when a bias for transferring the toner image to the recording material P is applied to the secondary transfer roller 9 at the upstream transfer portion N1, the toner is transferred to the recording material P through the transfer belt 24. A positive injection charge having a polarity opposite to that is supplied. At this time, since the counter roller 21 is dropped to the ground potential, the same amount of negative injection charge as that supplied by the upstream secondary transfer roller 9 is supplied to the intermediate transfer belt 6. It is formed on the opposing roller 21.

  As shown in FIG. 4A, the toner image on the intermediate transfer belt 6 is adsorbed on the intermediate transfer belt 6 by both the injected charge and the mirror charge supplied at the primary transfer portion. For this reason, as shown in FIG. 4B, the toner image adsorbed on the intermediate transfer belt 6 does not cancel out both the injected charge and the mirror charge supplied by the primary transfer unit, and then the intermediate transfer belt 6. The recording material P is transferred to the recording material P side.

  First, in the state before transfer shown in FIG. 4A, there are three negative charges schematically showing the toner image. Each of these toners has an injection charge and a mirror charge. In this state, when a positive charge for two toners is supplied by the secondary transfer roller 9 on the upstream side, a negative charge for the same amount of two toners is supplied from the counter roller 21. Due to the two negative charges, the positive injection and the mirror charge that attracted the toner image onto the intermediate transfer belt 6 are not canceled electrostatically, and as shown in FIG. One toner is transferred from the intermediate transfer belt 6 to the recording material P.

  In this way, since the positive injection charge from the secondary transfer roller 9 is supplied to the recording material P, the amount of charge affecting the intermediate transfer belt 6 changes, so the amount of reflected charge on the intermediate transfer belt 6 is changed. Occurs.

  As shown in FIG. 5A, when the positive injection charge is not supplied to the recording material P side, the amount of charge affecting the surface of the intermediate transfer belt 6 is Q1. When a positive injection charge is supplied to the recording material P side in order to transfer the toner image from the intermediate transfer belt 6 to the recording material P side, as shown in FIG. The amount of charge affecting the surface of the film is reduced to Q2. In the state where the positive injection charge is supplied to the recording material P side, there is a charge that cancels the charge amount of the toner from the recording material P side, so the charge amount Q2 that affects the surface of the intermediate transfer belt 6 is , Smaller than the original Q1.

  As shown in FIG. 3B, when the toner image on the intermediate transfer belt 6 is completely transferred to the recording material P side, the toner image is not carried on the recording material P and returned to the intermediate transfer belt 6 again. Thus, the injection charge more than necessary for the transfer is supplied to the recording material P. On the intermediate transfer belt 6, there are three negative charges schematically showing the toner image, and on the recording material P, there are positive injected charges corresponding to six toners.

  The negative injection charge supplied from the opposing roller 21 is formed on the surface of the intermediate transfer belt 6 in such a way as to balance the negative charge of the toner image with the positive injection charge supplied excessively. Three toners are present.

  As described above, when the positive charge amount is excessively supplied to the recording material P, the electrostatic adsorption force between the recording material P and the transfer belt 24 is increased more than necessary. For this reason, the curvature separation of the recording material P from the transfer belt 24 in the driving roller 26 after passing through the continuous contact portion T2 is disadvantageous.

As shown in FIG. 6A, if the recording material P has a high rigidity, the recording material P electrostatically attracted to the transfer belt 24 is separated in curvature by the curved surface of the driving roller 26 of the transfer belt 24. The separation can be assisted by hooking the recording material P onto the separation claw 29. However, as shown in FIG. 6B, if the rigidity of the recording material P is extremely low, the curvature of the leading edge does not separate on the curved surface of the drive roller 26, and therefore the recording material P adheres to the transfer belt 24. The separation claw 29 cannot assist the separation. For example, in the case of thin paper having a basis weight of 40 g / m ² or less, the rigidity of the recording material P becomes extremely low, and separation from the transfer belt 24 becomes difficult even if the separation claw 29 is provided.

  For this reason, considering the separability from the transfer belt 24, the excessive charge shown in FIG. 3B is removed from the recording material P, and the electrostatic adsorption force between the recording material P and the transfer belt 24 is lowered. It is desirable to do it.

  Therefore, as shown in FIG. 7A, it is proposed to provide a separation corona static eliminator 35 (or a separation assisting means such as a static elimination needle) on the surface of the recording material P on which unfixed toner is placed. It was done. By irradiating charged particles from the corona static eliminator 35 for separation and removing the charge from the recording material P that has passed through the contact portion T2 while being adsorbed to the transfer belt 24, the static of the recording material P with respect to the transfer belt 24 is reduced. Reduce electroadsorption power. By reducing the electrostatic attracting force between the transfer belt 24 and the recording material P, even at the recording material P having low rigidity, the leading edge of the recording material P is separated by the curved surface of the driving roller 26. Even if the leading edge is separated by curvature, the effect of preventing re-adsorption by the separation claw 29 can be expected, so that the recording material P can be reliably separated from the transfer belt 24.

  However, the separation assisting means for removing charges by discharge such as the separation corona static eliminator 35 (or the static elimination needle) may cause an unfixed toner image to be disturbed by the discharge and cause an image defect. When the toner and the charge of the transfer belt 24 are locally lost due to the discharge, the toner image is not carried on the transfer belt 24 only by that portion, and toner rearrangement occurs such as scattering on the transfer belt 24. If the separation corona static eliminator 35 performs static elimination with a gap between the toner image and the surface of the toner image, the charge carrying the toner image or the charge of the toner itself may randomly disappear due to the discharge phenomenon. The toner image is disturbed and the image quality of the output image is impaired. This phenomenon has been a major problem in transporting thin paper by the transfer belt 24.

  For this reason, the first embodiment takes the form of contact-type gentle static elimination at the downstream transfer portion N2 where the surface of the recording material P and the intermediate transfer belt 6 are still in contact. That is, in the first transfer mode, a transfer bias is applied to the secondary transfer roller 9 on the upstream side, and the charge is removed while the recording material P is in contact with the intermediate transfer belt 6 by the secondary transfer roller 10 on the downstream side. . As a result, the separability at the driving roller 26 can be improved without disturbing the toner image.

<Static removal of transfer belt and recording material>
FIG. 8 is an explanatory diagram of static elimination at the downstream transfer portion. As shown in FIG. 8A, the second pressure electrode pair (10, 23) presses the downstream portion of the continuous contact portion T2 and intermediate transfer through the opposite surface sandwiching the recording material P. The belt 6 and the transfer belt 24 can be electrically connected.

  In the downstream transfer portion N2, excess charges that generate an excessive electrostatic attraction force between the transfer belt 24 and the recording material P are discharged through both the transfer belt 24 and the recording material P. In the downstream transfer portion N2, the secondary transfer roller 10 and the counter roller 23 on the downstream side are connected to the ground potential. As a result, excess charges on the transfer belt 24 and the intermediate transfer belt 6 are removed through the secondary transfer roller 10 and the opposing roller 23. By removing the excess charge from the transfer belt 24 and the intermediate transfer belt 6, the excess charge accumulated in a capacitor manner on the nipped recording material P is efficiently removed through the transfer belt 24 and the intermediate transfer belt 6. Go.

  Here, negative charges are removed from the intermediate transfer belt 6 through the opposing roller 23. However, the charge to be removed is limited to the negative injection charge supplied from the opposing roller 21 when the transfer voltage is applied by the secondary transfer roller 9 on the upstream side, and the toner image is transferred from the recording material P to the intermediate transfer belt. There is no forced charge transfer enough to pull back to 6 and retransfer. Comparing the negative charge of the toner image with the negative injection charge supplied from the opposing roller 21, the negative charge of the toner image has a much higher mobility than the negative injection charge due to the weight of the toner itself. Because it is small.

  In other words, the positive charge of the recording material P attracts the negative charge of the low-mobility toner image and escapes charge removal, so that the toner image remains carried on the recording material P. Then, what is removed through the secondary transfer roller 10 and the counter roller 23 is an excess portion of the charge remaining on the transfer belt 24, the recording material P, and the intermediate transfer belt 6 excluding necessary charges. Almost fits. The negative charge that moves in the direction of the opposing roller 23 is limited to that of the induced negative charge, and the toner image remains carried on the recording material P. For this reason, it is possible to remove the electrostatic adsorption force of the excess portion of the recording material P with respect to the transfer belt 24 without affecting the transfer state of the toner image.

  As shown in FIG. 8B, the downstream transfer portion N2 in the neutralized state schematically shows three toner images on the recording material P, has three negative charges, and is weak in the intermediate transfer belt 6. There is a mirror charge corresponding to one toner. In the recording material P, there is a positive charge for four toners as a positive injection charge for attracting the toner image and preventing the toner from being carried onto the intermediate transfer belt 6 by the mirror charge. . The electrostatic adsorption force between the recording material P and the transfer belt 24 is the minimum generated by plus and minus of four toner images, and the upstream transfer portion N1 shown in FIG. It becomes smaller than the electrostatic attraction force in.

  According to the first transfer mode of the first exemplary embodiment, the recording material P is connected to the intermediate transfer belt 6 even after the transfer voltage is applied by the secondary transfer roller 9 on the upstream side and the toner image is transferred to the recording material P. Nipping with the transfer belt 24 continues. In this state, the secondary transfer roller 10 and the counter roller 23 on the downstream side are short-circuited and connected to the ground potential, whereby the recording material P and the transfer belt 24 are neutralized. Since the charge is removed in a state where the toner image of the recording material P is nipped, excessive charge between the recording material P and the transfer belt 24 can be removed without causing a disturbance of the toner image due to a discharge phenomenon. As a result, the electrostatic adhesive force between the recording material P and the transfer belt 24 is lowered to improve the separation property of the recording material P with respect to the transfer belt 24, and the occurrence of image defects during the process of discharging while discharging is prevented. Can be compatible.

<Example 2>
In the second embodiment, process speed control is added to the control of the first embodiment. By reducing the process speed, the effect of neutralizing the recording material P through the continuous contact portion T2 is enhanced, but the productivity of the image forming apparatus 100 is decreased. For this reason, in Example 2, the process speed is reduced when the basis weight of the recording material P is lower than a predetermined criterion.

In the image forming apparatus 100 shown in FIG. 1, in order to sufficiently remove the excessive charge of the recording material P, it is necessary to sufficiently remove the charge of the transfer belt 24 electrostatically attracting the recording material P. is there. When the dielectric constant of the material of the transfer belt 24 is ε and the volume low efficiency is ρ, the discharge time constant τ of the transfer belt 24 is expressed by the following equation.
τb = ε × ρ (1)

Here, the dielectric constant of the transfer belt is εb (F / m), and the volume resistivity of the transfer belt is ρb (Ωm). Further, the distance from the contact position (N1) of the secondary transfer roller 9 to the transfer belt 24 to the contact position (N2) of the secondary transfer roller 10 is L (mm), and the rotational speed of the transfer belt 24 (= recording material P). The transport speed = process speed) is V (mm / s). At this time, in order to sufficiently remove the charge on the transfer belt 24, it is desirable that the time τ to pass the distance L at the process speed V is larger than the time constant τb of the discharge in the equation (1).
εb × ρb ≦ L / V Expression (2)

The transfer belt 24 used in the image forming apparatus (100) has a volume resistivity of 1 × 10 ⁹ to 10 ¹⁰ ΩCm and a relative dielectric constant of 10 to 100, and is located between the upstream transfer portion N1 and the downstream transfer portion N2. The distance L is 40 mm. The volumetric low efficiency and the relative dielectric constant were measured using an impedance analyzer manufactured by Solartron.

  The process speed of the image forming apparatus (100) is 250 to 300 mm / sec, and the relationship of Expression (2) is satisfied even when the fastest process speed is 300 mm / sec. For this reason, it is possible to remove excess charges on the transfer belt 24 while the recording material P moves from the upstream transfer portion N1 to the downstream transfer portion N2.

  By the way, in the formula (2), the charge removal of the recording material P between the upstream transfer portion N1 and the downstream transfer portion N2 is considered based on the transfer belt 24. However, in general, the recording material P has a volume resistivity higher than that of the transfer belt 24 and is less likely to be neutralized than the transfer belt 24. For this reason, in order to more effectively eliminate the charge of the recording material P, the time τ for passing the distance L between the upstream transfer portion N1 and the downstream transfer portion N2 at the process speed V is the time constant of the recording material P. It is desirable that it be larger than τp.

That is, when the dielectric constant of the recording material P is εp (F / m) and the volume resistivity of the recording material P is ρp (Ωm) , considering the discharge time constant of the recording material P, the following relational expression is satisfied. It is desirable.
εp × ρp ≦ L / V Expression (3)

Since the above equations (2) and (3) depend on the process speed V, in the control of the second embodiment, the process speed V is reduced during thin paper printing. That is, when the basis weight of the recording material P is 52 g / m ² or more, the process speed V is set to 300 mm / sec. When the basis weight is 52 g / m ² or more, the process speed V is reduced to 100 mm / sec. Let

As a result, although the relative dielectric constant of the general recording material P is 3, it is possible to sufficiently remove the charge up to the recording material P whose volume resistivity is as high as 1.2 × 10 ¹² Ωcm.

  According to the control of the second embodiment, the excess charge applied to the transfer belt 24 can be sufficiently removed between the upstream transfer portion N1 and the downstream transfer portion N2, so that the recording material P can be separated from the transfer belt 24. Can be improved. In addition, since the excess charge applied to the recording material P can be sufficiently removed between the upstream transfer portion N1 and the downstream transfer portion N2, the separation property of the recording material P with respect to the transfer belt 24 can be further improved. .

  Further, as shown in FIG. 1, since the secondary transfer rollers 9 and 10 each have an outer diameter of 24 mm and the distance L is 40 mm, the opposing distance G of the secondary transfer rollers 9 and 10 is 16 mm. The transfer voltage applied to the secondary transfer roller 9 is about 2 to 4 kV, and the maximum voltage Vg is 4 kV. When a voltage is applied to the secondary transfer rollers 9 and 10, the facing distance G (mm) between the electrodes where no leakage phenomenon occurs is empirically equal to or greater than the applied voltage (kV) × 3.

For this reason, if the relationship of the following equation is satisfied and the potential difference necessary for preventing the leakage phenomenon is 3 mm or more, leakage due to discharge does not occur between the secondary transfer rollers 9 and 10. If the distance L is about 40 mm, the transfer voltage is about several kV and is more than the distance affected by the discharge, so that discharge occurs at the opposing distance G of the secondary transfer rollers 9 and 10 to generate power noise. There is nothing to do.
Vg × 3 ≦ G (4)

  In the second embodiment as well, excess charge between the transfer belt 24 and the recording material P is removed in a state where the recording material P is nipped between the intermediate transfer belt 6 and the transfer belt 24 at the downstream transfer portion N2 of the continuous contact portion T2. . Therefore, it is possible to improve the separability of the recording material P with respect to the transfer belt 24 without disturbing the toner image due to discharge.

<Example 3>
FIG. 9 is an explanatory diagram of the configuration of the image forming apparatus according to the third embodiment. FIG. 10 is an explanatory diagram of the first transfer mode and the second transfer mode. FIG. 11 is a flowchart of control according to the third embodiment. In the first embodiment, the first transfer mode is set in the image forming apparatus. On the other hand, in the second embodiment, the image forming apparatus detects information (basis weight) related to the rigidity of the recording material. The first transfer mode is applied to a recording material whose rigidity is lower than a predetermined determination criterion, while the second transfer mode is applied to a recording material whose rigidity is higher than a predetermined determination criterion. . Since the configuration of the image forming apparatus according to the second embodiment is basically the same as that of the first embodiment, in FIG. 9, the same reference numerals as those in FIG. Omitted.

As shown in FIG. 9, the control means (101) controls the power supply means (41, 42) based on the output of the acquisition means (102). An operation panel 102 is attached to the image forming apparatus 100A, and the user designates the basis weight of the recording material P to be printed. The control unit 101 executes switching between the first transfer mode and the second transfer mode according to the basis weight of the recording material P designated by the user through the operation panel 102.

As shown in FIG. 10A, when the basis weight of the recording material P is less than 52 g / m ² , the control unit 101 executes the first transfer mode (thin paper mode). In the first transfer mode, the high voltage power supply 41 applies a transfer voltage to the first pressurizing means (9), and the high voltage power supply 42 connects the second pressurizing means (10, 23) to the ground potential. The high voltage power supply 41 causes a current of +30 to +40 μA to flow through the secondary transfer roller 9 on the upstream side to transfer the toner image on the intermediate transfer belt 6 to the recording material P. The high-voltage power supply 42 removes the electric charge of the transfer belt 24 by dropping the downstream secondary transfer roller 10 to the ground potential in the same manner as the counter roller 23.

<Thick paper mode>
Upstream of the upstream transfer portion N1, image defects due to discharge may be a problem when a strong electric field is applied in a state where a gap exists between the recording material P and the intermediate transfer belt 6. Image defects due to this electric discharge are more likely to occur as the recording material P is thicker (basis weight is larger). This is because the larger the thickness of the recording material P, the higher the impedance in the thickness direction of the recording material P, and a larger transfer voltage is applied to flow the current necessary for transfer to the upstream transfer portion N1. As a result, the electric field between the intermediate transfer belt 6 and the recording material P increases in the gap upstream of the upstream transfer portion N1, and discharge is likely to occur.

  When this discharge is received, the charged polarity (here, negative polarity) of the toner image is reversed. When the charging polarity is reversed and the upstream transfer portion N1 is passed and a transfer voltage having a polarity opposite to the normal charging polarity is applied to the transfer roller 9, the toner with the reversed polarity is not transferred to the recording material P. The trace becomes an abnormal image. In particular, the smaller the toner amount per unit area on the intermediate transfer belt 6, that is, the lower the image density, the smaller the toner charge amount at that portion, so that the polarity is easily reversed, and the abnormal image is easily manifested as an abnormal image. .

  Further, even if the thick paper with wrinkles (particularly the thick paper curled upward in the transport direction) is nipped by the suction roller 28 and sucked onto the transfer belt 24, a part of the paper is lifted from the transfer belt 24. Even if it is sandwiched between the transfer belt 24 and the intermediate transfer belt 6 immediately before the upstream transfer portion N1, there is a case where the bending of the recording material P cannot be suppressed by the tension of the intermediate transfer belt 6 and the transfer belt 24. As a result, there is a case where the recording material P does not follow the intermediate transfer belt 6, and a discharge occurs between the toner image and the recording material P upstream of the upstream transfer portion N1, and the toner image at the location exposed by the discharge is exposed. In some cases, the polarity was reversed.

  Even if the transfer voltage having the opposite polarity to the normal toner charging polarity is applied to the transfer roller 9 through the upstream transfer portion N1 in this state, the toner whose polarity is reversed is not transferred to the recording material P. The traces caused image defects.

Therefore, in Example 3, as shown in FIG. 10B, the control unit 101 executes the second transfer mode (thick paper mode) when the basis weight of the recording material P is 52 g / m 2 or more. In the second transfer mode, the high voltage power sources 41 and 42 connect the first pressure electrode pair (9, 21) to the ground potential and apply a transfer voltage to the second pressure electrode pair (10, 23). To do. The secondary transfer roller 9 on the upstream side is dropped to the ground potential, and the electric field is regulated so that a large electric field is not formed between the intermediate transfer belt 6 and the recording material P on the upstream side of the upstream transfer portion N1. The high voltage power source 42 causes a current of +30 to +40 μA to flow through the secondary transfer roller 10 on the downstream side to transfer the toner image on the intermediate transfer belt 6 to the recording material P.

  Thereafter, the recording material P onto which the toner image has been transferred passes through the continuous contact portion T 2, is separated from the intermediate transfer belt 6, is conveyed to the driving roller 26, and is transferred from the transfer belt 24 on the curved surface along the driving roller 26. The curvature is separated. At this time, the recording material P is separated from the transfer belt 24 using the separation claw 29 at the position of the driving roller 26 and sent to the fixing device 13.

  As shown in FIG. 11 with reference to FIG. 9, the control unit 101 branches to which of the secondary transfer rollers 9 and 10 the transfer voltage for transferring the toner image on the intermediate transfer belt 6 is applied. Switching is made at the point (S15).

  When the start of image formation is instructed, the control unit 101 acquires image information (S11) and determines the paper type selected by the user (S12). The image information may be transmitted as image forming job data from an external terminal or may be read and created by a document reading unit (not shown). The paper type may be attached to the image forming job data or may be set through the operation panel 102.

  The control unit 101 controls the image forming units PY, PM, PC, and PK to create each color toner image (S13), and performs primary transfer to the intermediate transfer belt 6 to form a multicolor toner image (S14).

The control unit 101 can select and execute a first transfer mode (thin paper mode) and a second transfer mode (thick paper mode). The first transfer mode (thin paper mode) can be executed for plain paper. When the basis weight of the recording material P is less than 52 g / m ² (Yes in S15), the first transfer mode (S15 to S19) described in the first embodiment is performed in order to improve the separation property of the recording material with respect to the transfer belt 24. Run. However, when the basis weight of the recording material P is 52 g / m ² or more, the second transfer mode (S21 to S23) is executed.

When the basis weight of the recording material P is larger than 52 g / m ² , the separation property from the transfer belt 24 is not a problem because the rigidity of the recording material P is large. Since the transfer voltage when transferring the toner image to the recording material P becomes larger than that, the discharge easily occurs between the recording material P and the intermediate transfer belt 6 upstream of the upstream transfer portion N1. When the toner image before being transferred to the recording material P is exposed to electric discharge, the charged charge is reversed in a part of the toner image, thereby causing a transfer failure such as image omission and lowering the quality of the output image.

  Therefore, when the recording material P has high rigidity, the secondary transfer roller 9 and the counter roller 21 are lowered to the ground potential as shown in FIG. Then, the nip of the recording material P is started in a state where the electric field in the gap between the recording material P and the intermediate transfer belt 6 is weakened, and the toner image on the intermediate transfer belt 6 is brought into contact with the recording material P.

  Then, after the toner image on the intermediate transfer belt 6 is superimposed on the recording material P at the upstream transfer portion N1, a toner image is transferred from the intermediate transfer belt 6 to the recording material P by applying a transfer voltage at the downstream transfer portion N2. Transcript. As a result, it is possible to prevent an image problem on the upstream side of the continuous contact portion T2, which is a problem when printing on thick paper. For a thick recording material P that has sufficient rigidity and can be easily separated from the transfer belt 24, the intermediate transfer belt 6 and the transfer belt 24, which have been neutralized by the secondary transfer roller 9 and the counter roller 21 on the upstream side, are disposed between the transfer belt 24 and the transfer belt 24. The recording material P is conveyed in the nipped state. Thereafter, the toner image is transferred to the recording material P by the secondary transfer roller 10 and the counter roller 21 on the downstream side. For this reason, it is possible to prevent image defects upstream of the upstream transfer portion N1, which is a problem when transferring a toner image to the thick recording material P.

  In recent years, with the expansion of applications of image forming apparatuses, it has been required to support recording materials P having a wide range of rigidity such as thin paper, thin cloth, resin film, and thick paper. However, thin paper with low rigidity cannot be stably transferred to the recording material P until the secondary transfer process, because the leading edge of the recording material is deformed or delayed due to the curling condition of the leading edge of the recording material or the friction with the guide. There is a case. For this reason, this is dealt with by using a technique in which the recording material P is electrostatically attracted to a known recording material conveyance body (transfer belt) such as Patent Document 1 and passed through the transfer portion. However, the thin paper has a problem of separation of the recording material with respect to the transfer belt as described in the first embodiment, and the thick paper has a problem of image defect due to discharge before transfer as described in the third embodiment. On the other hand, according to the control of the third embodiment, an image forming apparatus that can stabilize the transportability and the image quality can be provided for a wide range of recording materials P from ultrathin paper to ultrathick paper.

<Example 4>
FIG. 12 is an explanatory diagram of the configuration of the contact portion in the fourth embodiment. The fourth embodiment is a modification of the configuration of a very small part of the contact portion T2 of the first embodiment. Therefore, in FIG. 12, the same reference numerals as those in FIG.

  As shown in FIG. 12A, the secondary transfer roller 10 of the downstream transfer portion N2 in the first embodiment may be replaced with a neutralization electrode 10B. The static elimination electrode 10B is a metal member having an external shape and springiness. A conductive rubber blade conventionally used as a transfer blade may be used.

  As shown in FIG. 12B, the upstream transfer portion N1 in the first embodiment is configured by a resistance roller having a resistive elastic layer as a counter roller 21 that presses the inner surface of the intermediate transfer belt 6. May be. By applying a negative transfer voltage from the high voltage power supply 41B to the opposing roller 21, a transfer electric field is formed between the counter roller 21 and the secondary transfer electrode 9B connected to the ground potential. The secondary transfer electrode 9B is a metal member having an external shape and springiness. A conductive rubber blade conventionally used as a transfer blade may be used.

1Y, 1M, 1C, 1K Photosensitive drum 2Y, 2M, 2C, 2K Corona charger 3Y, 3M, 3C, 3K Exposure device 4Y, 4M, 4C, 4K Development device 5Y, 5M, 5C, 5K Primary transfer roller 6 Intermediate transfer Belt, 8 Registration roller, 9, 10 Secondary transfer roller 20 Driving roller, 21, 23 Opposing roller, 22 Tension roller 24 Transfer belt, 25 Stretching roller, 26 Driving roller (separation roller)
27 Tension roller, 28 Adsorption roller, 29 Separation claw 30 Adsorption bias power supply, 41, 42 High voltage power supply P Recording material, N1 upstream transfer portion, N2 downstream transfer portion, T2 Continuous contact portion

Claims (4)

An image forming apparatus comprising an intermediate transfer belt for carrying a toner image and a transfer belt for carrying a recording material, wherein the intermediate transfer belt and the transfer belt form a contact surface along the rotation direction of the intermediate transfer belt Because
The intermediate transfer belt and the transfer belt are arranged on the inner side of the intermediate transfer belt and the inner side of the transfer belt, respectively, so that the nip regions overlap each other. A first pressure electrode pair capable of pressurizing between and applying an electric field;
The intermediate transfer belt and the transfer belt are arranged on the inner side of the intermediate transfer belt and the inner side of the transfer belt, respectively, so that the nip regions overlap with each other. A second pressurizing electrode pair capable of pressurizing between and applying an electric field,
When the rigidity of the recording material is lower than a predetermined rigidity, an electric field in a direction in which toner is transferred from the intermediate transfer belt to the recording material is applied to the first pressure electrode pair and the second pressure electrode pair. In the first transfer mode in which each is set to the ground potential, when the rigidity of the recording material is equal to or higher than a predetermined rigidity, toner is transferred from the intermediate transfer belt to the recording material on the second pressure electrode pair. An image forming apparatus comprising: an execution unit that applies an electric field in a transfer direction and executes a second transfer mode in which each of the first pressure electrode pairs is set to a ground potential . The first pressure electrode pair includes a first transfer roller that presses an inner surface of the transfer belt to contact the intermediate transfer belt, and presses an inner surface of the intermediate transfer belt to the transfer belt. A first opposing roller to be contacted,
The second pressure electrode pair includes a second transfer roller that presses the inner surface of the transfer belt to contact the intermediate transfer belt, and presses the inner surface of the intermediate transfer belt to the transfer belt. A second opposing roller to be brought into contact with,
The dielectric constant of the transfer belt is εb (F / m), the volume resistivity of the transfer belt is ρb (Ωm), the rotation speed of the transfer belt is V (mm / s), and the rotation direction of the transfer belt When the distance from the contact position with the first transfer roller along the line to the contact position with the second transfer roller is L (mm),
εb × ρb ≤ L / V
The image forming apparatus according to claim 1 , wherein the image forming apparatus is an image forming apparatus. The dielectric constant of the recording material is εp (F / m), the volume resistivity of the recording material is ρp (Ωm), and the conveyance speed of the recording material is V (mm / s), along the rotation direction of the transfer belt. When the distance from the contact position with the first transfer roller to the contact position with the second transfer roller is L (mm),
εp × ρp ≤ L / V
The image forming apparatus according to claim 1 or 2, characterized in that. The first transfer roller and the second transfer roller are disposed along the inner surface of the transfer belt, and the distance between the opposing surfaces of the first transfer roller and the second transfer roller is G (mm). When the maximum potential difference applied to the second pressure electrode pair in the second transfer mode is Vg (kV),
Vg x 3 ≤ G
The image forming apparatus according to any one of claims 1 to 3, characterized in that.

Images