JP2011022343A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device separating a recording material from an intermediate transfer body having high resistance without affecting transfer of toner image adversely even if a blank part at a head of the recording material is narrow. <P>SOLUTION: An intermediate transfer body electrifying device 30 includes a tension roller 9 connected with ground potential and a bias roller 31 connected with power supply D30 and applies positive polarity voltages to the bias roller 31 to electrify a face to be overlapped on the blank part at a tip of the recording material P of an intermediate transfer belt 7 to have a polarity reverse to the electrifying polarity of the toner. In primary transfer parts TY, TM, TC, TK, voltage is applied to primary transfer rollers 11Y, 11M, 11C, 11K after a region overlapping on the blank part at the tip of the recording material P passes through them. Consequently, since the recording material passes through a secondary transfer part in an electrostatic charging state being favorable for separation of the recording material, curvature separation becomes easier than ever. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus for transferring a toner image to a recording material using a high resistance intermediate transfer member, and more particularly, to a structure that facilitates separation of a recording material after transfer using the high resistance of the intermediate transfer member and Regarding control.

An image forming apparatus of an intermediate transfer body type is widely used in which a toner image formed on a photosensitive drum is transferred to an intermediate transfer belt by a primary transfer unit and then transferred to a recording material by a secondary transfer unit (Patent Documents 1 and 2). ). In Patent Documents 1 and 2, an intermediate transfer belt having a volume resistance of 1 × 10 ⁸ to 10 ¹² [Ωcm] is used, but recently, a high resistance of 1 × 10 ¹³ to 10 ¹⁵ [Ωcm] is used. The use of an intermediate transfer belt is being studied.

  The intermediate transfer belt having a high resistance close to an insulator (dielectric) can keep the charged state long and stable when the toner image is transferred at the primary transfer portion. For this reason, the holding power of the transferred toner is increased, and even when the back surface comes into contact with a roller connected to the ground potential in the middle, the toner image scattering phenomenon like the intermediate transfer belt of intermediate resistance does not occur.

  However, since the electrostatic adhesion force with the recording material that has been in contact with the recording material that has been in contact with the recording material that has been charged for a long time increases, the recording material adheres to the intermediate transfer belt at the secondary transfer portion in the case of a thin recording material. May be difficult to separate.

  Japanese Patent Application Laid-Open No. 2004-260688 discloses control for improving the separation performance of the recording material with respect to the intermediate transfer member by adjusting the voltage application timing in the secondary transfer portion. By starting voltage application at the secondary transfer portion after the leading end portion of the recording material passes through the secondary transfer portion, the leading end portion of the recording material is prevented from being electrostatically attracted to the intermediate transfer belt.

JP 2004-317915 A JP 2000-137386 A Japanese Patent Laid-Open No. 10-240032

  The high-resistance intermediate transfer belt reaches the secondary transfer section while maintaining a high charged state at the primary transfer section, and therefore has a large electrostatic adsorption force to the recording material, and a voltage is applied from the outside at the secondary transfer section. The thin recording material cannot be separated sufficiently.

  Further, since the secondary transfer portion is 6 mm or more when a transfer roller is used and 10 mm or more when a corona charger is used, secondary transfer is started after waiting for the leading end region of the recording material to pass. Only the margin at the beginning of the recording material is insufficient. Alternatively, the secondary transfer starts in a state where the charging state of the recording material passing through the secondary transfer portion is not sufficiently stable, and density unevenness occurs in the head portion of the image.

  Therefore, it has been proposed to dispose the intermediate transfer belt by disposing a non-contact charge remover on the upstream side of the secondary transfer portion, but in this case as well, image density unevenness occurs when the charge removal area overlaps the toner image. appear. When static elimination is applied to the portion carrying the toner image, the toner may be scattered.

  An object of the present invention is to provide an image forming apparatus capable of separating a recording material from a high-resistance intermediate transfer member without affecting the transfer of the toner image even if the leading margin of the recording material is narrow. .

The image forming apparatus of the present invention includes an image carrier on which a toner image is formed, an intermediate transfer member having a volume resistance value of 1 × 10 ¹³ [Ωcm] or more, and pressing the intermediate transfer member to the image carrier. A transfer member to be contacted, a power source for applying a voltage to the transfer member so as to transfer a toner image from the image carrier to the intermediate transfer member, and a recording material superimposed on the intermediate transfer member to charge the intermediate member And a secondary transfer unit that transfers the toner image from the transfer member to the recording material. The secondary transfer unit has a charge opposite in polarity to the charge polarity of the toner in a region of the surface overlapping the recording material tip in the secondary transfer unit. The intermediate transfer member charging means for charging the intermediate transfer member before the toner image is transferred, and the transfer member after passing through the portion of the intermediate transfer member that overlaps the leading edge of the recording material. So that the voltage is applied And a control means for controlling the source, the.

  In the image forming apparatus of the present invention, a charged state that is convenient for separation of the recording material on the premise that the recording material is charged in the secondary transfer portion is imparted to the intermediate transfer body before the primary transfer portion. Then, in the primary transfer portion, the toner image is primarily transferred by applying a voltage avoiding a portion overlapping the head of the recording material so that the charged state is not impaired.

  For this reason, in the secondary transfer section, if transfer is performed by applying uniform conditions without switching the transfer conditions in the middle of the recording material, a charged state that is convenient for automatically separating the recording material is automatically The recording material is separated from the intermediate transfer member.

  Therefore, even if the leading margin of the recording material is narrow, the recording material can be separated from the high-resistance intermediate transfer member without affecting the transfer of the toner image.

1 is an explanatory diagram of a configuration of an image forming apparatus. It is explanatory drawing of a structure of an image formation part. FIG. 7 is an explanatory diagram of a relationship between a volume resistance value of an intermediate transfer belt and a charge decay rate. FIG. 6 is an explanatory diagram of a configuration of a toner image transfer unit. It is explanatory drawing of the relationship between the nip width in a primary transfer part, and transfer efficiency. FIG. 6 is an explanatory diagram of charge-up of an intermediate transfer belt in a primary transfer unit. FIG. 6 is an explanatory diagram of a recording material separation failure in a secondary transfer portion. 3 is a time chart of control in Embodiment 1. FIG. 4 is an explanatory diagram of recording material separation performance by control of Example 1; FIG. 6 is an explanatory diagram of an intermediate transfer member charging device according to a second embodiment. FIG. 6 is an explanatory diagram of an intermediate transfer member charging device according to a third embodiment. FIG. 10 is an explanatory diagram of a configuration of an image forming apparatus according to a fourth embodiment. 10 is a time chart of control of voltage application in Example 4. FIG. 10 is an explanatory diagram of an intermediate transfer member charging device according to a fifth embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention can be implemented in another embodiment in which a part or all of the configuration of the embodiment is replaced with the alternative configuration as long as the toner image is primarily transferred to the pre-charged intermediate transfer belt. .

  Therefore, any image forming apparatus that transfers a toner image from an intermediate transfer member to a recording material 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 documents 1-3, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted.

<Image forming apparatus>
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 yellow, magenta, cyan, and black image forming portions PY, PM, PC, and PK are arranged along an intermediate transfer belt 7. is there. The image forming units PY, PM, PC, and PK are each configured as a process unit and can be independently replaced.

  When a full-color image information signal is input to the control unit 90 from an external computer 300, the built-in image reader 200, or a facsimile, the control unit 90 activates the image forming apparatus 100. The control unit 90 forms an image on the recording material P by controlling the image forming units PY, PM, PC, and PK based on settings and operations through the operation panel 91. The control unit 90 controls the entire image forming apparatus 100, performs necessary image processing on the input full-color image information signal, and drives the image forming units PY, PM, PC, and PK at a predetermined timing of the image forming sequence. The exposure devices 3Y, 3M, 3C, and 3K output laser beams modulated in accordance with the image information signals of the respective colors subjected to image processing input from the control unit 90, and scan the photosensitive drums 1Y, 1M, 1C, and 1K. Exposure. As a result, electrostatic images corresponding to the scanning exposure patterns are formed on the photosensitive drums 1Y, 1M, 1C, and 1K, and the electrostatic images are developed into toner images of respective colors by the developing devices 4Y, 4M, 4C, and 4K.

  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 7. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1YM, and is primarily transferred onto the yellow toner image on the intermediate transfer belt 7. In the image forming units PC and PK, a cyan toner image and a black toner image are formed on the photosensitive drums 1YC and 1K, respectively, and are sequentially transferred to the intermediate transfer belt 7 in order to be primarily transferred.

  The four-color toner images primarily transferred to the intermediate transfer belt 7 are conveyed to the secondary transfer portion T2 and collectively transferred to the recording material P. The recording material P on which the four-color toner images are secondarily transferred is heated and pressed by the fixing device 22 to fix the toner images on the surface, and then is discharged to the outside of the machine body.

  The recording material P drawn from the recording material cassette 14 by the pickup roller 15 is separated one by one by the separation roller 16 and sent to the registration roller 19. The registration roller 19 receives and waits for the recording material P in a stopped state, and sends the recording material P to the secondary transfer portion T2 in time with the toner image on the intermediate transfer belt 7.

  The intermediate transfer belt 7 is supported around a tension roller 9, a driving roller 8, and a counter roller 10, and is driven by the driving roller 8 to rotate in the direction of arrow R2 at a process speed of 300 mm / sec.

  The belt cleaning device 13 rubs the cleaning blade 13a against the intermediate transfer belt 7 to remove the transfer residual toner remaining on the intermediate transfer belt 7 through the secondary transfer portion T2 by escaping from the transfer to the recording material P. It collects in the container 13c. The transfer residual toner and the like collected in the cleaner container 13c are conveyed by the conveying screw 13b, guided to the end in the longitudinal direction of the belt cleaning device 13, and discharged to a collection tank (not shown). The belt cleaning device 13 is disposed on the outer peripheral surface side of the intermediate transfer belt 7 with the intermediate transfer belt 7 sandwiched between the opposite roller 17. The cleaning blade 13a is in contact with the intermediate transfer belt 7 at a predetermined angle and pressure.

  The fixing device 22 is a heat roller fixing device that rotates a fixing roller 22a and a pressure roller 22b that are rotationally driven in the direction of an arrow while pressing them. A heater 22c such as a halogen lamp is disposed inside the fixing roller 22a, and temperature control is performed to maintain the surface of the fixing roller 22a at a predetermined fixing temperature by controlling the voltage applied to the heater 22c. ing.

  The recording material P is introduced into a fixing portion which is a pressure contact portion between the fixing roller 22a and the pressure roller 22b and is nipped and conveyed. As a result, the toner of each color toner image on the recording material P is melted and mixed and fixed as a full-color image on the surface of the recording material P to form a full-color print.

<Image forming unit>
FIG. 2 is an explanatory diagram of the configuration of the image forming unit.

  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

  As shown in FIG. 2, in the image forming unit PY, a charging roller 2Y, an exposure device 3Y, a developing device 4Y, a primary transfer roller 11Y, and a cleaning device 5Y are arranged around the photosensitive drum 1Y.

  The photosensitive drum 1Y, which is an example of an image carrier, has an OPC photosensitive layer having a negative polarity charged on the cylindrical outer peripheral surface of an aluminum cylinder, and rotates in the direction of arrow R1 at a process speed of 300 mm / sec. .

  The charging roller 2Y is applied with an oscillating voltage obtained by superimposing an AC voltage on a DC voltage from the power supply D3, and charges the surface of the photosensitive drum 1Y to a uniform negative potential VD. The charging roller 2Y includes a conductive metal core 2a, and a low-resistance elastic layer 2b and a medium-resistance elastic layer 2c that are sequentially formed around the metal core 2a in a roller shape.

  The charging roller 2 is disposed substantially parallel to the photosensitive drum 1Y by rotatably supporting both ends of the cored bar 2a with bearing members. The bearing members at both ends are attached to the photosensitive drum 1Y by spring members (not shown). It is energized towards. As a result, the charging roller 2Y comes into pressure contact with the photosensitive drum 1Y with a predetermined pressing force, and rotates following the rotation of the photosensitive drum 1Y.

  The exposure device 3Y scans with a rotating mirror a laser beam obtained by ON-OFF modulation of scanning line image data obtained by developing a yellow separation color image. 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 apparatus 3Y includes a semiconductor laser, a rotating polygon mirror, an fθ lens, a reflecting mirror, and the like.

  The developing device 4Y charges a two-component developer obtained by mixing a yellow non-magnetic toner and a magnetic carrier and carries it on the developing sleeve 4b. The power source D4 applies an oscillating voltage obtained by superimposing an AC voltage to the negative DC voltage Vdc to the developing sleeve 4b, and transfers the toner to the portion of the light portion potential VL of the photosensitive drum 1Y having a relatively positive polarity. .

  The primary transfer roller 11 </ b> Y is applied with a positive DC voltage V <b> 1 from the power source DY to primarily transfer the toner image carried on the photosensitive drum 1 </ b> Y to the intermediate transfer belt 7. Bearing members at both ends of the primary transfer roller 11Y are urged toward the photosensitive drum 1Y by spring members (not shown). Thereby, the primary transfer roller 11Y sandwiches the intermediate transfer belt 7 between the photosensitive drum 1Y and forms a primary transfer portion TY between the photosensitive drum 1Y and the intermediate transfer belt 7. The primary transfer roller 11 </ b> Y is pressed against the intermediate transfer belt 7 with a predetermined pressing force, and rotates in accordance with the rotation of the intermediate transfer belt 7.

  The cleaning device 5Y slides the cleaning blade 5a 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 7 in the cleaner container 5c. Transfer residual toner or the like collected in the cleaner container 5c is conveyed by the conveying screw 5b, guided to the longitudinal end of the cleaning device 5Y, and discharged to a collection tank (not shown). The cleaning blade 5a is in contact with the photosensitive drum 1Y at a predetermined angle and pressure.

<Developing device>
In the developing device 4Y, a stirring / conveying screw 4f is disposed on the bottom side in the developing container 4a containing the two-component developer 4e. The two-component developer 4e in the developing container 4a is a mixture of a non-magnetic toner and a magnetic carrier and is conveyed while being stirred by the stirring and conveying screw 4f. A polymerized toner produced by a suspension polymerization method was used as the toner. The toner is frictionally charged to a negative polarity having the same polarity as the charging polarity of the photosensitive drum 1Y by rubbing with the carrier.

  The developing sleeve 4b as a developer carrying member is rotatably arranged with a part of the outer peripheral surface thereof exposed to the outside from the developing container 4a. The developing sleeve 4b is disposed in close proximity to the photosensitive drum 1Y while maintaining a predetermined closest distance (S-Dgap) from the photosensitive drum 1Y. The developing sleeve 4b is rotationally driven in a direction opposite to the rotational advance direction of the photosensitive drum 1Y at a portion facing the photosensitive drum 1Y.

  In the non-magnetic developing sleeve 4b, a magnet roller 4c is inserted in a non-rotating manner. Due to the magnetic force of the magnet roller 4c in the developing sleeve 4b, a part of the two-component developer 4e in the developing container 4a is adsorbed and held on the outer peripheral surface of the developing sleeve 4b as a magnetic brush layer.

  A regulating blade 4d is provided to face the developing sleeve 4b. The magnetic brush layer that is rotated and conveyed with the rotation of the developing sleeve 4b is layered into a predetermined thin layer by the developer regulating blade 4d, and the photosensitive drum 1Y is formed in the developing portion (the portion opposite the photosensitive drum 1Y and the developing sleeve 4b). It touches the surface and rubs moderately.

  An oscillating voltage obtained by superimposing an AC voltage on the DC voltage Vdc is applied from the power source D4 to the developing sleeve 4b. As a result, the toner in the two-component developer 4e conveyed to the portion facing the photosensitive drum 1Y is selectively attached to the surface of the photosensitive drum 1Y corresponding to the electrostatic image by the electric field of the DC voltage Vdc. The electrostatic image is developed as a toner image. Toner adheres to the exposed portion of the surface of the photosensitive drum 1Y, and the electrostatic image is reversely developed.

  The developer thin layer on the developing sleeve 4b that has passed through the portion facing the photosensitive drum 1Y is returned to the developer reservoir in the developing container 4a with the subsequent rotation of the developing sleeve 4b.

  Replenishment toner is accommodated in the toner replenishing chamber BY. In order to maintain the toner concentration of the two-component developer 4e in the developing container 4a within a substantially constant range, the toner concentration of the two-component developer 4e in the developing container 4a is detected by an optical toner concentration sensor (not shown). The The control unit 90 controls the rotation amount of the toner supply roller 4h in the toner supply chamber BY according to the detection information, and supplies the toner for supply in the toner supply chamber BY to the two-component developer 4e in the developing container 4a. Replenish. The replenished toner is agitated and mixed with the two-component developer 4e by the agitating and conveying screw 4f.

Here, an AC voltage having a peak-to-peak voltage Vpp = 1.2 kV is superimposed on the DC voltage Vdc = −450 V on the charging roller 2Y so that the dark portion potential VD of the photosensitive drum 1Y in the developing unit has a negative polarity of −400V. An oscillating voltage is applied. Then, the laser beam intensity of the exposure device 3 is set so that the light portion potential VL of the photosensitive drum 1Y in the developing portion becomes −100 V having a negative polarity. The developing sleeve 4b is applied with an oscillating voltage obtained by superimposing a rectangular wave AC voltage with a peak-to-peak voltage Vpp = 1.5 kV on the DC voltage Vdc = −300V. Through such a process, the charge amount of the toner developed on the photosensitive drum 1Y is about −30 μC / g of negative polarity, and the applied amount of toner for one color when the maximum density Dmax is formed is 0.50 mg / cm ^2. It became.

<Intermediate transfer belt>
FIG. 3 is an explanatory diagram of the relationship between the volume resistance value of the intermediate transfer belt and the charge decay rate.

  As shown in FIG. 1, an intermediate transfer unit 6 is disposed below the image forming portions PY, PM, PC, and PK. The intermediate transfer unit 6 can be exchanged as an intermediate transfer body in an assembled state where the endless and flexible intermediate transfer belt 7 is assembled to the drive roller 8, the tension roller 9, and the counter roller 10. The tension roller 9 is disposed on the image forming unit PY side, the driving roller 8 is disposed on the image forming unit PK side, and the opposing roller 10 is disposed below the tension roller 9 and the driving roller 8. The intermediate transfer belt 7 is applied with a certain tension by a tension roller 9, and the drive roller 8, the tension roller 9, and the opposing roller 10 are all connected to the ground potential.

  The intermediate transfer belt 7 is made of a dielectric resin such as PI, PC, PET, PVDF, and is a solid layer of dielectric resin, a single layer belt of an elastic layer, or a composite layer belt including such a layer. It is.

Here, a polyimide (PI) resin having a belt circumferential length L = 900 mm and a thickness t = 100 μm is employed, the relative dielectric constant is 3.5, and the volume resistance value is 1 × 10 ¹⁵ Ωcm. The volume resistivity was measured in an environment of 23 ° C. and 50% RH using a JIS-K6911 method-compliant probe at an applied voltage of 100 V and an applied time of 60 seconds. However, other materials and thicknesses may be used as long as the volume resistance value ≧ 1 × 10 ¹³ Ωcm. The reason why the resistance value of the intermediate transfer belt 7 is selected in this way is as follows.

The intermediate transfer belt is generally equivalent to a parallel circuit of a capacitance C and a resistance R. The amount of charge Q remaining on the intermediate transfer belt after time t seconds is expressed by the following equation, where the initial charge amount Q0, the dielectric constant ε of the intermediate transfer belt, and the volume resistance value ρ.
Q = Q0 × E (−t / C × ρ) (1)

Here, when the area of the intermediate transfer belt is S and the thickness is d, the time constant τ = C × ρ is expressed by the following equation.
C × ρ = ε × S / d × ρ (2)

Table 1 shows the results of measuring the relative dielectric constant by changing the volume resistance value of the intermediate transfer belt of polyimide (PI) resin in 7 stages from 1 × 10 ⁹ to 10 ¹⁵ . The volume resistance value was measured using an electrometer R8340A manufactured by Advantest Corporation. The measurement was performed under the conditions of an applied voltage of 100 V and an applied time of 60 sec using a JIS-K6911 method-compliant probe in a 23 ° C./50% RH environment. The relative dielectric constant was measured under the following conditions using a 4284A PRCION LCR METER manufactured by Hewlett-Packard Co., and a 16451B dielectric measurement electrode (electrode C).

Measurement method: electrode contact method described in the instruction manual for the electrode for measuring the electrode 16451B Sample dimensions: thin film guard electrode outer diameter = 56 mm, thin film guard electrode inner diameter = 51 mm, thin film main electrode diameter = 50 mm, sample thickness = 15 to 50 μm
Thin-film electrode formation method: E1030 Mild sputtering (manufactured by Hitachi Science Systems Co., Ltd.) Deposition measurement conditions with Pt-PD electrode: 1 Vpp, 100 Hz
Measurement environment: 23 ° C / 60%

  As shown in Table 1, since the intermediate transfer belt having a seven-stage volume resistance value has the same area S and thickness d, the intermediate transfer after t seconds from the dielectric constant ε and volume resistance value ρ of the belt. The charge decay rate of the belt can be calculated. In the image forming apparatus 100 of FIG. 1, the intermediate transfer belt 7 has the peripheral length L = 900 mm and the process speed SP = 300 mm / sec. The attenuation rate from the initial Q0 of the charge amount Q after rotating once can be calculated. The result is shown in FIG.

As shown in FIG. 3, when the charge attenuation rate is calculated considering only the self-attenuation amount of the intermediate transfer belt 7, if the volume resistance value ρ is 1 × 10 ¹³ [Ωcm] or more, the charge attenuation is almost in one rotation. It shows a tendency to stop.

Then, in the image forming apparatus 100, an experiment of toner scattering generated in the intermediate transfer belt 7 was performed using the intermediate transfer belt in each stage shown in Table 1. Then, when the volume resistance value ρ was 1 × 10 ¹³ [Ωcm] or more, the effect of suppressing toner scattering was noticeable. This is because the intermediate transfer belt 7 having a volume resistance value ρ of 1 × 10 ¹³ [Ωcm] or more is less likely to cause self-charge attenuation, and in addition to the drive roller 8 when passing through the grounded drive roller 8. This is probably because charge transfer is less likely to occur.

Therefore, in order to avoid high-quality image formation while avoiding toner scattering, the volume resistance value ρ is desirably 1 × 10 ¹³ [Ωcm] or more. Further, as will be described later, the volume resistance value ρ is 1 × 10 ¹³ [Ωcm] or more in order to charge the intermediate transfer belt 7 before 3/4 rotation in contact with the recording material to enhance the separation performance of the recording material. It is desirable to be.

<Primary transfer section>
FIG. 4 is an explanatory diagram of the configuration of the toner image transfer portion. FIG. 5 is an explanatory diagram of the relationship between the nip width and the transfer efficiency in the primary transfer portion. FIG. 6 is an explanatory diagram of charge-up of the intermediate transfer belt in the primary transfer portion. In FIG. 4, (a) is a primary transfer portion, and (b) is a secondary transfer portion.

  As shown in FIG. 2, a primary transfer roller 11 </ b> Y as a transfer member is disposed inside the intermediate transfer belt 7. The primary transfer roller 11Y is a conductive elastic roller in which an elastic layer 11b is formed in a roller shape on the outer peripheral surface of a conductive metal core 11a made of metal such as SUS or aluminum. The primary transfer roller 11Y is disposed substantially parallel to the photosensitive drum 1Y, with both ends of the core metal 11a being rotatably supported by bearing members. The primary transfer roller 11Y comes into pressure contact with the inner surface of the intermediate transfer belt 7 by urging the bearing members at both ends toward the photosensitive drum 1Y by spring members (not shown). Then, a constant current controlled voltage having a polarity opposite to the charging polarity of the toner is applied from the power source DY to the metal core 11a.

  The primary transfer roller 11Y has an outer diameter of 16 mm, of which the core bar 11a has a diameter of 8 mm and the elastic layer 11b has a thickness of 4 mm. The elastic layer 11b is made of a conductive urethane sponge in which carbon particles are dispersed to impart conductivity, and has a roller hardness (ASKER C) of 30 °.

  The resistance value of the primary transfer roller 11Y was about 106Ω (23 ° C./50% RH). This resistance value is obtained by bringing the primary transfer roller 11Y into contact with a grounded metal roller with a load of 4.9 N (500 gf), applying a voltage of 500 V to the metal core 11a, and rotating at a peripheral speed of 50 mm / sec. It was obtained by measuring the current of the current state.

  It should be noted that other conductive elastic materials such as NBR, hydrin, and EPDM may be used as the material of the elastic layer 11b. Also, a roller hardness (ASKER C) of about 20 to 40 ° and a resistance value of about 105 to 108 may be used.

  As shown in FIG. 4A, the primary transfer roller 11Y sandwiches the intermediate transfer belt 7 between the photosensitive drum 1Y and forms a primary transfer portion TY between the photosensitive drum 1Y and the intermediate transfer belt 7. To do. The pressing force of the primary transfer roller 11Y against the photosensitive drum 1Y is set so that the nip width N1 of the primary transfer portion TY formed between the primary transfer roller 11Y and the intermediate transfer belt 7 is 4.0 mm.

  As shown in FIG. 1, the primary transfer rollers 11 </ b> Y, 11 </ b> M, 11 </ b> C, and 11 </ b> K are arranged in parallel to each other on the inner surface of the intermediate transfer belt 7 that is stretched between the drive roller 8 and the tension roller 9. . The primary transfer rollers 11Y, 11M, 11C, and 11K sandwich the intermediate transfer belt 7 between the photosensitive drums 1Y, 1M, 1C, and 1K of the image forming portions PY, PM, PC, and PK, and transfer the primary transfer portions TY, TM, TC, TK are formed.

  In the primary transfer portions TY, TM, TC, and TK of the respective colors, yellow, magenta, cyan, and black toner images are sequentially primary-transferred from the photosensitive drums 1Y, 1M, 1C, and 1K to the intermediate transfer belt 7.

  In the image forming apparatus 100, the voltage applied to the primary transfer rollers 11Y, 11M, 11C, and 11K is subjected to constant current control. The power supplies DY, DM, DC, and DK are variable to the primary transfer rollers 11Y, 11M, 11C, and 11K so that positive current of +18 μA, which is the same current value, flows to the primary transfer portions TY, TM, TC, and TK. Output voltage.

  As shown in FIG. 5, since the intermediate transfer belt 7 has a high resistance and a high charge retention characteristic, a charge-up occurs every time the intermediate transfer belt 7 passes through the primary transfer portions TY, TM, TC, and TK. The surface potential of the transfer belt 7 rises stepwise. FIG. 5 shows the potential on the toner image carrying surface side of the intermediate transfer belt 7.

  For this reason, in the case of the high-resistance intermediate transfer belt 7, it is difficult to perform constant voltage control on the voltage applied to the primary transfer rollers 11Y, 11M, 11C, and 11K. In constant voltage control in which the resistance value of the primary transfer roller is periodically detected to set a constant voltage at which a predetermined necessary current is obtained, the necessary current is stably secured when the potential of the intermediate transfer belt 7 increases. Because it is difficult. Therefore, in Example 1, the voltage applied to the primary transfer rollers 11Y, 11M, 11C, and 11K is subjected to constant current control.

<Secondary transfer section>
As shown in FIG. 4B, the secondary transfer roller 12 is disposed outside the belt winding portion of the counter roller 10. As shown in FIG. 1, the secondary transfer roller 12 has an elastic layer 12b made of conductive (medium resistance) EPDM foam material having a thickness of 6 mm on the outer periphery of a conductive core 12a having a diameter of 12 mm. The hardness (ASKER C) is 30 °.

  The secondary transfer roller 12 is arranged in parallel with the opposing roller 10 with both ends of the cored bar 12a being rotatably supported by the bearing member, and the bearing members at both ends are arranged on the opposing roller 10 by a spring member (not shown). It is energized towards. As a result, the intermediate transfer belt 7 is sandwiched between the secondary transfer roller 12 and the opposing roller 10, and a secondary transfer portion T <b> 2 is formed between the intermediate transfer belt 7 and the secondary transfer roller 12. The pressing force of the secondary transfer roller 12 against the opposing roller 10 is selected so that the nip width N2 of the secondary transfer portion T2 is 6.0 mm. Then, a constant current controlled voltage having a polarity opposite to the toner charging polarity is applied from the power source D2 to the cored bar 12a.

The resistance value of the secondary transfer roller 12 was about 1 × 10 ⁸ Ω (23 ° C./50% RH). This resistance value is determined by bringing the secondary transfer roller 12 into contact with a grounded metal roller with a load of 4.9 N (500 g weight), rotating it at a peripheral speed of 50 mm / sec, and applying a voltage of 2000 V to the cored bar 12a. And measured from the current value.

  It should be noted that other conductive elastic materials such as NBR, hydrin, and urethane may be used as the material of the elastic layer 12b.

  Here, the reason why the nip width of the primary transfer portion TY is N1 = 4.0 mm and the nip width of the secondary transfer portion T2 is N2 = 6.0 mm will be described.

  As shown in FIG. 5, the constant voltage Vconst was applied to the primary transfer roller 11Y, and the process speed was changed to high speed VP = 300 mm / sec, medium speed VP = 200 mm / sec, and low speed VP = 100 mm / sec. At this time, the transfer efficiency η changes according to the nip width N1 formed at the primary transfer portion TY.

  The case where the process speed is low will be described as an example. As the nip width N1 gradually increases from zero, the transfer efficiency η increases (area η1). Next, after passing through the region η2 before and after the transfer efficiency η is maximized, the transfer efficiency η decreases (region η3).

  In order to transfer the toner image formed on the photosensitive drum 1Y to the intermediate transfer belt 7, it is necessary to apply a sufficient charge to the inner surface of the intermediate transfer belt 7 through the primary transfer roller 11Y to attract the toner image. is there. At this time, if the nip width N1 is too narrow, charge application to the intermediate transfer belt 7 becomes insufficient, and the tendency of the region η1 is exhibited. If the nip width is too wide, the amount of charge applied to the intermediate transfer belt 7 becomes excessive, and the charge polarity of the toner is reversed and retransferred to the photosensitive drum 1Y. For this reason, in the image forming apparatus 100, the nip width N1 is selected in the region η2 where the transfer efficiency η peaks, and in the first embodiment, the nip width N1 = 4.0 mm.

Further, the nip width N2 formed at the secondary transfer portion T2 is also selected in the same manner as the nip width N1 of the primary transfer portion TY. In Example 1, the optimum nip width N2 was 6.0 mm. . The reason why the nip width N1 of the secondary transfer portion T2 is longer than the nip width N1 of the primary transfer portion TY is that the amount of toner transferred is larger than that of the primary transfer portion T1. Further, the following setting conditions are selected in order to stably convey the recording material without impairing the transferability of the toner image to various recording materials P.
Resistance value of secondary transfer roller 12> Resistance value of primary transfer roller 11 Outer diameter of secondary transfer roller 12> Outer diameter of primary transfer roller 11

  In the process in which the recording material P is nipped and conveyed across the secondary transfer portion T2, a DC voltage V2 having a polarity opposite to the toner charging polarity is applied to the secondary transfer roller 12 from the power source D2. In the image forming apparatus 100, constant current control is also used for the voltage applied to the secondary transfer roller 12. The power supply D2 variably outputs a voltage to be applied to the secondary transfer roller 12 so that positive polarity +36 μA flows to the secondary transfer portion T2. The reason why the constant current control is selected is the same as the reason for selection in the primary transfer portions TY, TM, TC, and TK described above. Since the intermediate transfer belt 7 arriving at the secondary transfer portion T2 is charged up and the surface potential is not stable, the constant voltage control causes the voltage (transfer contrast) to be applied to the intermediate transfer belt 7 to be excessive. This is because it is difficult to apply without lack.

  The recording material P that has exited the secondary transfer portion T <b> 2 is separated from the surface of the intermediate transfer belt 7 by the rigidity of the recording material P on the curved surface of the intermediate transfer belt 7 along the outer peripheral surface of the opposing roller 10. The recording material P separated from the intermediate transfer belt 7 is guided by the guide member and introduced into the fixing device 22.

<Comparative example>
FIG. 7 is an explanatory diagram of recording material separation failure in the secondary transfer portion.

  In recent years, the image forming apparatus of the intermediate transfer type has improved the image quality performance regardless of the one-drum type or the tandem type, but the technical problem of image scattering remains.

  As shown in FIG. 1, when the intermediate transfer belt 7 carrying a toner image moves from the primary transfer portion TY to the secondary transfer portion T2, the electrostatic holding force of the toner image by the intermediate transfer belt 7 is weakened. The toner scatters in the peripheral part of the eyelid that originally has no image. When such a scattering phenomenon occurs, the sharpness of the image is lost.

The two main causes of the scattering phenomenon are as follows. First, in the intermediate transfer belt 7 in the middle resistance region where the conventional volume resistance value ρ is 1 × 10 ⁸ to 10 ¹² [Ωcm], the charge applied to the intermediate transfer belt 7 is naturally extinguished within a few seconds. It has a self-damping characteristic. Second, when the intermediate transfer belt 7 moves from the primary transfer portion TY to the secondary transfer portion T2, it passes through the grounded driving roller 8. At this time, a sudden charge movement from the intermediate transfer belt 7 toward the drive roller 8 occurs.

As a method for suppressing such scattering phenomenon, it is known to increase the resistance of the intermediate transfer belt 7 to a volume resistance value of 1 × 10 ¹³ [Ωcm] or more. This is because the high-resistance intermediate transfer belt 7 is less likely to cause self-charge attenuation, and it is difficult for charge transfer to the drive roller 8 to occur when passing through the grounded drive roller 8.

  However, the high resistance intermediate transfer belt 7 has a problem of poor separation of the recording material P due to the high resistance. Due to the charge retention characteristics, the electrostatic attracting force with the recording material P is increased in the secondary transfer portion T2, and the separation of the recording material P from the intermediate transfer belt 7 is likely to occur.

  FIG. 7A schematically shows charges carried by the recording material P and the intermediate transfer belt 7 in the secondary transfer portion T2 when the intermediate transfer belt 7 having a medium resistance is mounted on the image forming apparatus 100. FIG. Represents. B in the drawing indicates the leading edge margin (non-image portion) I of the recording material P and the image portion. In the primary transfer portions TY, TM, TC, and TK, the primary transfer rollers 11Y, 11M, 11C, and 11K are included in the intermediate transfer belt 7 while the outer surface of the intermediate transfer belt 7 is in contact with the photosensitive drum (≈ground potential). A positive voltage is applied to the side surface.

  For this reason, in the primary transfer portions TY, TM, TC, and TK, the front and rear surfaces of the intermediate transfer belt 7 are applied in the same manner as when a positive voltage is applied to the other electrode of the capacitor having one electrode connected to the ground potential. Charge of reverse polarity is charged up. The inner side surface of the intermediate transfer belt 7 is charged up with a positive charge, and the outer side surface is charged up with a negative charge.

  However, during the movement from the primary transfer portion TK to the secondary transfer portion T2, the charge charged up between the front and back surfaces of the intermediate transfer belt 7 is brought to the ground potential through the conductivity of the intermediate transfer belt 7 and the driving roller 8. Lost by contact. As a result, the leading edge blank portion of the recording material arrives at the secondary transfer portion T2 in a state where the charge is lost except for the image portion I where the charge is continuously held by the mirroring force of the toner image.

  In this case, in the process of passing through the secondary transfer portion T2, the recording material P comes into contact with the secondary transfer roller 12 to which a positive voltage is applied, and a positive charge is charged on the back side surface of the leading edge blank portion. The For this reason, the recording material P is prevented from being separated by the positively charged charge and the reflection force generated on the intermediate transfer belt 7. However, since this force is not so great, the recording material P is separated in curvature by the curved surface of the intermediate transfer belt 7.

  On the other hand, FIG. 7B similarly shows the case where the high-resistance intermediate transfer belt 7 is mounted on the image forming apparatus 100. When a positive voltage is applied to the inner surface of the intermediate transfer belt 7 at the primary transfer portions TY, TM, TC, TK, the intermediate transfer belt 7 has a positive charge on the inner surface as in the case of the intermediate resistance. The battery is charged up so as to have a negative charge on the outer surface.

However, since the intermediate transfer belt 7 has a high resistance, the charge charged up to the intermediate transfer belt 7 is not lost during the movement from the primary transfer portion TK to the secondary transfer portion T2, and passes through the primary transfer portion TK. The secondary charging portion T2 arrives at the charged state. When the volume resistance value of the intermediate transfer belt 7 is 1 × 10 ¹⁴ or more, it exhibits an electric behavior similar to that of an insulator (no free electrons), so that electricity sufficiently flows in the thickness direction of the intermediate transfer belt 7. Absent.

  For this reason, the surface of the intermediate transfer belt 7 in contact with the leading edge margin of the recording material passes through the secondary transfer portion T2 in a negatively charged state and is charged to the positive polarity by the secondary transfer roller 12. Is superimposed. In this case, since a strong electrostatic adsorption force acts between the outer surface (−) of the intermediate transfer belt 7 and the leading edge margin (+) of the recording material, the recording material P is a curved surface of the intermediate transfer belt 7. Curvature separation is difficult.

  As described above, since the intermediate transfer belt 7 having a high resistance has a higher electrostatic adsorption force between the intermediate transfer belt 7 and the recording material P at the leading edge margin B than the intermediate transfer belt having a medium resistance, the secondary transfer portion T2 is used. Therefore, the separation failure of the recording material P is likely to occur. The phenomenon that the leading edge margin of the recording material is adsorbed without peeling off from the intermediate transfer belt 7 may cause jamming of the recording material in the image forming apparatus 100.

  Therefore, as shown in FIG. 7B, it has been proposed to start voltage application from the power source D2 to the secondary transfer roller 12 after the leading edge margin B of the recording material has passed through the secondary transfer portion T2. . In this case, the leading edge margin portion of the recording material is not charged to the positive polarity, and the leading edge margin portion B of the intermediate transfer belt 7 and the recording material P when superimposed on the negatively charged surface of the intermediate transfer belt 7. The electrostatic attraction force between them becomes small. However, when the recording material is superimposed on the negatively charged surface of the intermediate transfer belt 7, an electrostatic attraction force acts between the intermediate transfer belt 7 and the leading edge margin of the recording material P due to the reflection force of the recording material. Therefore, it does not lead to a fundamental solution.

  Further, as shown in FIG. 7 (a), it has been proposed that the portion of the intermediate transfer belt 7 that overlaps the leading edge margin of the recording material P is neutralized by a corona charger disposed in front of the secondary transfer portion T2. . However, in the image forming apparatus 100, since the intermediate transfer belt 7 has a high resistance, the amount of charge repeatedly applied to the intermediate transfer belt 7 in the primary transfer portions TY, TM, TC, TK and the secondary transfer portion T2 is reduced. It is difficult to control to zero.

  Further, as shown in FIG. 7B, the leading margin portion of the recording material P is reversed by reversing the polarity of the voltage applied to the secondary transfer roller 12 between the leading margin portion B and the image portion I of the recording material P. It has been proposed to charge B negatively. By charging the recording material P to negative polarity, the leading edge margin B of the recording material P repels against the outer surface of the intermediate transfer belt 7 charged with negative polarity, so that the separation property of the recording material P is improved. Because it increases. However, as shown in FIG. 4B, since the nip width N2 of the secondary transfer portion T2 is 6 mm, when a stable voltage is applied to the image portion I of the recording material P, the leading edge margin B is 10 mm. It became clear that it was necessary. It has been found that when the leading edge margin B is 8 mm or less, the effect of switching the voltage applied to the secondary transfer roller 12 affects the image area I and the image quality is degraded.

  That is, the power supply D2 can output both positive and negative polarities, and the primary transfer portion T1 applies a voltage of the same polarity to both the leading margin B and the image portion I of the recording material P to perform primary transfer of the toner image. To do. Thereafter, a voltage having a polarity opposite to that of the image portion I is applied to the leading margin B of the recording material P in the secondary transfer portion T2.

  However, this case has the following problems. In the secondary transfer portion T2, a nip width N2 of 6.0 mm is provided between the intermediate transfer belt 7 and the secondary transfer roller 12. Therefore, it is difficult to place a charge having a desired polarity and charge amount on a narrow area of about 3.0 to 5.0 mm corresponding to the leading edge margin B of the recording material P while switching the polarity of the voltage. . Further, when a voltage having a reverse polarity to the transfer is applied to the secondary transfer roller 12 during the continuous image forming operation, the fog toner on the intermediate transfer belt 7 accumulates on the secondary transfer roller 12. For this reason, the secondary transfer roller 12 is soiled, and the adverse effect of the back soiling of the recording material P also occurs.

  Therefore, in Example 1, the surface where the intermediate transfer member charging means (30, 9) overlaps the recording material leading end (B) of the intermediate transfer member (7) is transferred to the intermediate transfer member before the toner image is transferred. The toner is charged to a polarity opposite to that of the toner. Then, the control means (90) is configured to supply power (DY, DM, DC) so that a voltage is applied to the transfer members (11Y, 11M, 11C, 11K) after the region overlapping the recording material front end portion of the intermediate transfer member has passed. , DK).

  Thereby, in the image forming apparatus 100 including the high-resistance intermediate transfer belt, the separation failure of the recording material P from the intermediate transfer belt 7 can be effectively suppressed with a simple configuration, and stable recording material transportability can be obtained. It is done.

<Example 1>
FIG. 8 is a time chart of the control of the first embodiment. FIG. 9 is an explanatory diagram of the recording material separation performance under the control of the first embodiment.

  As shown in FIG. 1, an intermediate transfer member charging device 30 is disposed downstream of the belt cleaning device 13 in the rotation direction of the intermediate transfer belt 7.

  The intermediate transfer member charging device 30 is configured by sandwiching the intermediate transfer belt 7 between a bias roller 31 disposed on the toner image carrying surface side of the intermediate transfer belt 7 and a tension roller 9 that contacts the inner surface of the intermediate transfer belt 7. The The tension roller 9 is connected to the ground potential, and the bias roller 31 is connected to the power source D30. In other words, the intermediate transfer member charging means (30) has an intermediate transfer member between the grounded rotating member (9) that contacts the opposite surface of the intermediate transfer member (7) to which the toner image is transferred and the grounded rotating member. And a charging member (31) arranged so as to sandwich the battery. The ground rotating body (9) is connected to the ground potential, and the charging member (30) is applied with a voltage having the same polarity as the toner charging polarity from the charging power source (D30).

  The power source D30 supplies a positive direct current high voltage to the bias roller 31 so that a positive charge having a polarity opposite to the charge polarity of the toner can be charged on the toner image carrying surface of the intermediate transfer belt 7 before the toner image is primarily transferred. Is applied.

  In the first embodiment, the bias roller 31 is a stainless steel hollow roller having an outer diameter of φ10 mm and a longitudinal dimension L3 of 350 mm. Both ends of the bias roller 31 are rotatably supported by a bearing member (not shown), and the bearing members at both ends are biased toward the intermediate transfer belt 7 by a spring member (not shown). As a result, the bias roller 31 contacts the intermediate transfer belt 7 and rotates.

The bias roller 31 may be made of aluminum, or an elastic layer coating roller having a surface coated with a low resistance elastic layer (1 × 10 ⁴ Ω or less) made of a material such as urethane, hydrin, or EPDM may be used. I do not care. However, when an elastic layer coating roller is used, the roller hardness (ASKER C: load 1 kg) is preferably 80 ° or more.

  As shown in FIG. 8, energization to the bias roller 30 and the primary transfer roller 11Y (11M, 11C, 11K) is controlled. FIG. 8 shows the ON / OFF timing of voltage application in each main part corresponding to the moving position of the image transferred to the recording material P.

  A positive constant voltage is applied to the bias roller 31 by a power source D30 at a position corresponding to the leading margin of the recording material P of the intermediate transfer belt 7. As a result, a positive charge is applied to the toner image carrying surface of the intermediate transfer belt 7. That is, the intermediate transfer member charging unit (31) uniformly charges the position from the outside of the area overlapping the leading edge margin B of the surface overlapping the recording material P to the position outside the area overlapping the recording material rear end. .

  Subsequently, in order to transfer the toner image to the image portion I, a positive-constant-current-controlled voltage is transferred by the power source DY from the position where the primary transfer roller 11Y has passed through the leading edge margin B of the intermediate transfer belt 7. Applied to the roller 11Y. Similarly, the primary transfer rollers 11 </ b> M, 11 </ b> C, and 11 </ b> K are also applied with a constant current controlled voltage from a position that passes through the leading edge margin B of the intermediate transfer belt 7.

  Thereby, the charge-up by the primary transfer rollers 11Y, 11M, 11C, and 11K does not act on the portion of the toner carrying surface of the intermediate transfer belt 7 that overlaps the leading edge margin B of the recording material P. When passing through the primary transfer roller 11K, the part of the toner carrying surface of the intermediate transfer belt 7 that overlaps the leading margin of the recording material P is in contact with the bias roller 31 and remains positively charged. .

  As shown in FIG. 9, the charge on the intermediate transfer belt 7 which is a high resistance belt reaches the secondary transfer portion T2 while maintaining the charged state due to its charge retention characteristics. FIG. 9 shows the state of charge applied to the intermediate transfer belt 7 and the recording material P that pass through the secondary transfer portion T2 in an overlapping manner.

  A positive voltage with constant current control is applied to the secondary transfer roller 12 from the leading edge of the leading margin B of the recording material P by a power source D2. In the leading edge margin B of the recording material P, positive charge applied by the bias roller 31 is present on the toner image carrying surface of the intermediate transfer belt 7 facing the recording material P. Then, the recording material P comes into contact with the secondary transfer roller 12 to which a positive voltage is applied, and is charged positively. As a result, the intermediate transfer belt 7 and the recording material P are electrically repelled and less charge is transferred, and the recording material P is stably separated from the intermediate transfer belt 7.

  Further, the value of the DC voltage applied by the bias roller 31 is such that the amount of charge applied to the toner image carrying surface side of the intermediate transfer belt 7 from the secondary transfer roller 12 at the secondary transfer portion T2. It is selected so as to be equal to the amount of charge applied to the recording material P. That is, the charging power source (D30) supplies a voltage that can charge up the charge per unit area corresponding to the charge per unit area of the toner image having the maximum toner amount carried on the intermediate transfer body (7). Apply to.

In Embodiment 1, the toner charge amount Q / per unit area that can be transferred from the intermediate transfer belt 7 to the recording material P when the positive transfer current 36 μA is supplied to the secondary transfer roller 12 in the secondary transfer portion T2. S is −0.30 × 10 ⁻³ [C / m ² ].

For this reason, in the secondary transfer portion T2, the charge amount Q / S per unit area Q / S = + 0.30 on the back surface of the toner image carrying surface of the recording material P is balanced with the toner charge amount Q / S per unit area. It is considered that × 10 ⁻³ [C / m ² ] exists. Therefore, the charge D / 30 is applied to the bias roller 31 by the power source D30 so that the charge amount per unit area Q / S = + 0.30 × 10 ⁻³ [C / m ² ] is applied to the toner image carrying surface side of the intermediate transfer belt 7. A positive constant voltage value is set.

Let S3 be the area where the intermediate transfer belt 7 is in contact with the bias roller 31 and receive charge injection, and Q3 will be the amount of charge applied to the area S3 of the intermediate transfer belt 7. Further, the potential difference between the front and back surfaces of the intermediate transfer belt 7 is V3, the thickness of the intermediate transfer belt 7 is d, the dielectric constant of the intermediate transfer belt 7 is ε, and the capacitance of the intermediate transfer belt 7 is C. At this time, the following relationship is generally established.
C = ε × S3 / d (3)
V3 = Q3 / C (4)
Q3 = ε × S3 / d × V3 (5)
V3 = (Q3 × d) / (ε × S3) (6)

Here, S3 is obtained by the following equation when the discharge nip width between the bias roller 30 and the intermediate transfer belt 7 is N3 and the longitudinal dimension of the bias roller 30 is L3.
S3 = N3 × L3

  Then, when a high sensitivity small camera was installed inside the image forming apparatus 100 and the discharge nip width N3 was measured, the discharge nip width N3 was about 0.50 mm. The high-sensitivity small camera was installed so as to visualize the vicinity of the facing portion between the bias roller 30 and the tension roller 9 that sandwiched the intermediate transfer belt 7. The discharge light formed between the bias roller 30 and the intermediate transfer belt 7 when a constant voltage was applied to the bias roller 30 was observed.

  By substituting known numerical values including the discharge nip width N3 = 0.50 mm and the longitudinal dimension L3 = 350 mm of the bias roller 30 into the equation (6), the potential difference V3 to be applied to the intermediate transfer belt 7 is calculated. Is done.

  The voltage value V30 to be applied to the bias roller 30 is a voltage value obtained by adding the discharge start voltage Vdis between the bias roller 30 and the intermediate transfer belt 7 to the potential difference V3.

From the calculation result of the formula (6), V3≈1000V was calculated, and since the discharge start voltage Vdis≈800V, the voltage value V30 was calculated as follows.
V30 = V3 + Vdis = 1000V + 800V = 1800V

<Experiment 1>
Recording at the secondary transfer portion T2 when Npi fine paper [basis weight 52 g / m ² , Gurley stiffness 0.25 mN] is used as the recording material P, and the voltage applied to the bias roller 30 is changed up and down around 1800V. The separability of the material was examined. The results are shown in Table 2.

  In the separation results shown in Table 2, ○ indicates that the separation and transportability is good and there is no paper jam. Means. When the applied voltages were 1800 V and 2200 V, the adsorption of the leading end portion of the recording material P to the intermediate transfer belt 7 was not observed at all on the first and second surfaces, and the separability was good. When the applied voltage was 1000 V, the first side was Δ level and the second side during duplex printing was X level. When the applied voltage was 1400 V or 2600 V, a Δ level separation failure was observed on the second side during duplex printing.

  Here, in the secondary transfer portion T2, the amount of charge applied to the toner non-carrying surface side of the recording material P by the secondary transfer roller 12 is QP, and the surface overlaps the leading edge margin of the recording material P in the intermediate transfer belt 7. The amount of charge imparted to Q is QB. At this time, when the applied voltage to the bias roller 30 is 1800 V to 2200 V, QP≈QB, so that charge transfer between the recording material P and the intermediate transfer belt 7 is very small, and the separation property is good. It is thought that there was. On the other hand, when the applied voltage to the bias roller 30 is 1000V to 1400V, QP> QB, and when 2600V, QP <QB. For this reason, charge is transferred between the recording material P and the intermediate transfer belt 7, and it is considered that an attractive force is generated between the recording material P and the intermediate transfer belt 7, resulting in poor separation.

  The reason why the separation on the second side of double-sided printing is worse than that on the first side is due to the effect of curling / waving of the recording material P caused by passing through the fixing device 22 and the decrease in the amount of moisture. This is considered because the recording material P is easily charged.

  From the results of the separation confirmation experiment shown in Table 2, it was found that 2000 V is optimal as the constant voltage value applied to the bias roller 31.

<Example 2>
FIG. 10 is an explanatory diagram of the intermediate transfer member charging device according to the second embodiment.

  As shown in FIG. 1, in the first embodiment, an intermediate transfer member charging device 30 is configured by sandwiching an intermediate transfer belt 7 between a bias roller 31 connected to a power source D30 and a tension roller 9 connected to a ground potential. .

  On the other hand, in the second embodiment, the intermediate transfer body charging device 30B is configured by sandwiching the intermediate transfer belt 7 between the bias roller 31 connected to the ground potential and the tension roller 9 connected to the power source D30B.

  As shown in FIG. 8, the power source D30B is configured to bias up the positive charge having the opposite polarity to the toner charge onto the toner image carrying surface of the intermediate transfer belt 7 before the toner image is primarily transferred. A negative DC high voltage is applied to the. The capacitor having the intermediate transfer belt 7 as a dielectric is charged as a transient characteristic, so that a negative charge is charged on the inner surface of the intermediate transfer belt 7 and a positive electrode is applied to the toner image carrying surface of the intermediate transfer belt 7. Sex charge is charged up.

  As a voltage value to be applied to the tension roller 9, it is appropriate to apply a constant voltage of -2000 V having a reverse polarity and the same absolute value as in the first embodiment. The operation and control timing for full color image formation including the configuration of the image forming apparatus 100B and energization control to the primary transfer roller are the same as those of the image forming apparatus 100 of the first embodiment.

<Example 3>
FIG. 11 is an explanatory diagram of the intermediate transfer member charging device according to the third embodiment.

  The intermediate transfer member charging device 30 of Example 1 applied a DC voltage (constant voltage) to the bias roller 31. In contrast, the intermediate transfer member charging device 30 </ b> C according to the third embodiment applies a vibration voltage to the bias roller 31 to increase the charging efficiency of the intermediate transfer belt 7. The power source D <b> 30 </ b> C applies an oscillating voltage obtained by superimposing a sin waveform AC voltage (constant voltage) to a positive DC voltage (constant voltage) to the bias roller 31.

  As a voltage value applied to the bias roller 31, it is appropriate to apply a constant DC voltage of +2000 V as in the first embodiment.

  The value of the AC voltage to be superimposed on the DC voltage is not affected by the residual charge history in the previous process of the intermediate transfer belt 7, and after passing through the bias roller 31, the charge density applied to the intermediate transfer belt 7 is the same. It is desirable to set it to be uniform. When the charge state of the intermediate transfer belt 7 was examined by changing the amplitude of the AC voltage with a frequency of 2 kHz superimposed on the DC voltage 2000 V, the charge density applied to the intermediate transfer belt 7 was found if the amplitude Vpp ≧ 0.9 kV. Was uniform.

  Thus, in the third embodiment, an AC voltage with an amplitude Vpp of 1.0 kV and a frequency of 2 kHz is superimposed on a 2000 V DC voltage with respect to the bias roller 31.

  The operation and control timing for full color image formation including the configuration of the image forming apparatus 100C and energization control to the primary transfer roller are the same as those of the image forming apparatus 100 of the first embodiment. As a result, as shown in FIG. 9, charges are present on the intermediate transfer belt 7 at the secondary transfer portion T2. The leading edge margin B of the recording material P and the image portion I have potential steps due to the difference in the polarity of the stored charges. This potential difference may cause image streaks (sub-scanning streaks) in the next image forming process.

  In the third embodiment, the intermediate transfer member charging device 30C is a charge eliminating device that eliminates such a potential difference and makes the charged state uniform. In other words, the intermediate transfer member charging means (30C) also serves as a means for discharging the charge charged up in the area where the previous toner image is transferred on the intermediate transfer body (7) before the toner image is transferred. . By applying an AV + DC oscillating voltage to the bias roller 31, the potential step remaining on the intermediate transfer belt 7 after the secondary transfer can be suppressed more effectively than in the first and second embodiments.

<Example 4>
FIG. 12 is an explanatory diagram of a configuration of the image forming apparatus according to the fourth embodiment. FIG. 13 is a time chart of control of each voltage application in the fourth embodiment.

  As illustrated in FIG. 12, the image forming apparatus 100 </ b> D according to the fourth embodiment does not include the independent dedicated intermediate transfer member charging device as in the first to third embodiments. Since the configuration other than having the dedicated intermediate transfer member charging device is the same as in the first to third embodiments, the same reference numerals as those in FIG. Description is omitted.

  In the fourth embodiment, the existing secondary transfer portion T2 also serves as an intermediate transfer body charging device. By controlling the voltage value and voltage application timing applied to the secondary transfer roller 12, the same as in the first to third embodiments. A charging pattern for the intermediate transfer belt 7 is formed. In the secondary transfer portion T2, a charge having a polarity opposite to that of the toner is applied to a portion of the intermediate transfer belt 7 that overlaps the leading edge margin B of the recording material on the toner image carrying surface.

  As shown in FIG. 13, the ON / OFF timing corresponding to the recording material P of each main part in the image forming apparatus 100D is controlled. The secondary transfer roller 12 has a timing at which a voltage whose constant current is controlled to a positive constant current of 36 μA by the power source D2 is preceded by one turn of the intermediate transfer belt 7 from a position corresponding to the leading edge of the recording material P on the intermediate transfer belt 7. Applied. As a result, a positive charge is applied to the toner image carrying surface side of the intermediate transfer belt 7.

  Subsequently, as described in the first embodiment, the primary transfer rollers 11Y, 11M, 11C, and 11K are subjected to constant current control to 18 μA by the power sources DY, DM, DC, and DK from the timing when the leading edge margin B is passed. A voltage is applied.

  As a result, a positive charge remains on the toner carrying surface of the intermediate transfer belt 7 after passing through the primary transfer roller 11K in a portion corresponding to the leading margin B of the recording material P, and an image portion (toner image region) ) A negative charge is applied to the portion corresponding to I. Since the intermediate transfer belt 7 has high charge retention characteristics, the intermediate transfer belt 7 reaches the secondary transfer portion T2 while maintaining such a charged state.

  The secondary transfer roller 12 is applied with a constant current controlled voltage of 36 μA from the leading edge of the recording material P by the power source D2, as in the first to third embodiments. As a result, in the secondary transfer portion T2, as shown in FIG. 9, the charge present on the intermediate transfer belt 7 and the charge applied to the recording material P repel each other, and the secondary shown in FIG. The recording material P easily performs curvature separation on the curved surface of the intermediate transfer belt 7 on the downstream side of the transfer portion T2.

  In the first to fourth embodiments, the tandem type image forming apparatus has been described as an example. However, the present invention is also applicable to a one-drum type image forming apparatus by providing a separation mechanism for the bias roller 31, the belt cleaning device 13, and the secondary transfer portion T2. The invention can be implemented. However, the control of the fourth embodiment is preferably performed under the condition that the image formation is completed within the circumferential surface of the intermediate transfer belt 7. When an image is formed by rotating the intermediate transfer belt 7 a plurality of times as in the one-drum type, a charge having a desired polarity and charge amount is provided at a specific portion of the intermediate transfer belt 7 corresponding to the leading edge margin of the recording material P. It is because it is difficult to give.

<Example 5>
FIG. 14 is an explanatory diagram of the intermediate transfer member charging device according to the fifth embodiment.

  As shown in FIG. 14, a corona charger 31a and a potential sensor 32a are arranged so as to face the toner image carrying surface of the intermediate transfer belt 7. The control circuit 35a controls the output of the corona charger 31a so as to cancel the toner image carrying surface charging unevenness pattern measured using the potential sensor 32a and form the charging pattern described in the first embodiment. Further, a corona charger 31b and a potential sensor 32b are arranged to face the inner side surface of the intermediate transfer belt 7. The control circuit 35b controls the output of the corona charger 31b so as to cancel the charging unevenness pattern on the inner surface measured using the potential sensor 32b and form the charging pattern described in the first embodiment.

<Experiment 2>
Recording in the secondary transfer portion T2 is performed using the conventional image forming apparatus in which a high-resistance intermediate transfer belt is installed (see FIG. 12) and the five types of image forming apparatuses 100, 100B, 100C, and 100D in the first to fourth embodiments. The separation and transportability of the material P was compared. The recording material P was made into the following four types of A4 size, and 100 double-sided images were continuously formed for each paper type, and the separation / conveyance of the recording material at the secondary transfer portion was observed. Table 3 shows the results of evaluation of separation / conveyance as in Experiment 1.

1. CLC80 paper (basis weight 80g / m ² , Gurley stiffness 0.85mN)
2. EN100 paper (basis weight 64g / m ² , Gurley stiffness 0.57mN)
3. OK Prince fine paper (basis weight 52g / m ² , Gurley stiffness 0.35mmN)
4). Npi fine paper (basis weight 52g / m ² , Gurley stiffness 0.25mN)

  As shown in Table 3, when a high-resistance intermediate transfer belt is installed in an image forming apparatus according to the prior art, good separation and transportability is confirmed for thick and highly rigid CLC80 paper. It was a level. However, the EN100 paper, which is plain paper, has a Δ level separation / conveyance on the first side, and the separation / conveyability on the second side is x level. The thin and low-rigidity OK Prince quality paper and Npi quality paper were frequently jammed.

  On the other hand, in the case of the image forming apparatuses of Examples 1 to 4, the first and second surfaces of all four paper types were stably separated and conveyed, and the separation / conveyability was ◯ level.

  In Examples 1 to 4, in the secondary transfer portion, a charge having a polarity opposite to that of the toner exists in a portion corresponding to the leading edge margin of the recording material on the toner image carrying surface side of the intermediate transfer belt, and the toner of the recording material is not carried On the surface side, there is a charge having a polarity opposite to that of the toner. For this reason, the force of electrostatically attracting the intermediate transfer belt and the recording material is suppressed, and the separation failure of the recording material with respect to the intermediate transfer belt can be effectively suppressed, and stable paper transportability can be obtained even for thin paper.

Therefore, in the secondary transfer portion of the image forming apparatus provided with the high-resistance intermediate transfer belt, the force that electrostatically attracts the intermediate transfer belt and the recording material is eliminated, and the separation failure of the recording material can be effectively suppressed. In addition, stable paper transportability can be realized even with thin paper of 52 g / m ² .

1Y, 1M, 1C, 1K Photosensitive drums 2Y, 2M, 2C, 2K Charging rollers 3Y, 3M, 3C, 3K Exposure devices 4Y, 4M, 4C, 4K Development devices
7 Intermediate transfer belt 8 Drive roller 9 Tension roller 10 Opposing rollers 11Y, 11M, 11C, 11K Primary transfer roller 12 Secondary transfer roller 13 Belt cleaning device 30 Intermediate transfer member charging device 31 Bias rollers DY, DM, DC, DK, D2, D30 Power supply P Recording material

Claims (7)

An image carrier on which a toner image is formed, an intermediate transfer member having a volume resistance value of 1 × 10 ¹³ [Ωcm] or more, a transfer member that presses the intermediate transfer member to contact the image carrier, and A power source for applying a voltage to the transfer member so as to transfer a toner image from the image carrier to the intermediate transfer member, and a recording material superimposed on the intermediate transfer member to charge the toner image from the intermediate transfer member to the recording material In an image forming apparatus provided with a secondary transfer unit that transfers
Intermediate transfer for charging the intermediate transfer body before the toner image is transferred so that a charge having a polarity opposite to that of the toner is charged in a region of a surface overlapping the leading end of the recording material in the secondary transfer portion Body charging means;
An image forming apparatus comprising: a control unit that controls the power supply so that a voltage is applied to the transfer member after a portion of the intermediate transfer member that overlaps the leading end of the recording material passes.   The image forming apparatus according to claim 1, wherein the intermediate transfer member charging unit uniformly charges from a position outside the area of the overlapping surface to a position outside the area overlapping the trailing edge of the recording material. .   The intermediate transfer body charging means is provided between the grounded rotating body and the grounded rotating body that is in contact with the opposite surface of the intermediate transfer body to which the toner image is transferred and connected to the ground potential. The image forming apparatus according to claim 1, further comprising: a charging member disposed so as to sandwich the charging member; and a charging power source that applies a voltage having the same polarity as a charging polarity of toner to the charging member.   The charging power source applies to the charging member a voltage capable of charging up a charge per unit area corresponding to a charge per unit area of a toner image having a maximum toner amount carried on the intermediate transfer member. The image forming apparatus according to claim 3.   5. The image forming apparatus according to claim 3, wherein the charging power source applies a voltage obtained by superimposing an AC voltage on a DC voltage to the charging member.   The intermediate transfer member charging unit also serves as a unit that neutralizes charges charged up in a region where the previous toner image is transferred on the intermediate transfer member before the toner image is transferred. Item 6. The image forming apparatus according to any one of Items 1 to 5.   4. The image forming apparatus according to claim 1, wherein the power source applies a constant current controlled voltage to the charging member. 5.

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