JP2012032573A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color image forming device that can prevent defective secondary transfer due to dielectric breakdown of a recording sheet, and can ensure productivity in continuous printing and double sided printing.SOLUTION: An RC parallel circuit 50 in which a resistor 51 and a capacitor 43 are arranged in parallel is connected between a secondary transfer roller 9 and a second DC power source 45. A change-over switch 55 is arranged between the second DC power source and the RC parallel circuit 50. By bringing the change-over switch 55 into contact with a contact point 57a, a first circuit 47 is formed for applying a voltage to the secondary transfer roller 9 through the RC parallel circuit 50. A second circuit 48 for applying a voltage to the secondary transfer roller 9 not through the RC parallel circuit 50 is formed, by bringing the change-over switch 55 into contact with a contact point 57b.

Description

  The present invention relates to an intermediate transfer type color image forming apparatus that transfers a color image formed by sequentially transferring toner images of a plurality of colors onto an intermediate transfer member at a time onto a recording medium.

  Conventionally, various image forming apparatuses have been proposed. In the image forming apparatus, toner images of different colors are sequentially transferred (primary transfer) and superimposed on an endless intermediate transfer belt by a plurality of image forming units. There is an intermediate transfer type color image forming apparatus that forms a color image by transferring (secondary transfer) onto a recording medium (paper) at a time and fixing the recording medium (secondary transfer).

  In such a color image forming apparatus, the moisture content is reduced in a low-temperature and low-humidity environment or when the image on the first side is fixed on the both sides of the paper, after fixing the image on the second side. When printing a high density image on a resistive sheet, there was a phenomenon that a spot-like secondary transfer failure of about several hundred μm occurred. In particular, when a toner image of two or more colors has already been primarily transferred onto the intermediate transfer belt, a secondary transfer failure of the toner image closer to the surface of the belt that has been transferred first occurs, and hue unevenness tends to appear.

  In the electrostatic transfer method, the sheet is charged to form an electrostatic field between the toner and the sheet. When the resistance of the paper is high, the charge is difficult to escape from the paper, so that the amount of charge to be held increases and the electric field inside the paper becomes strong. As a result, it is considered that the dielectric breakdown of the paper occurs in a locally thin portion of the paper or a portion where charges are concentrated. Since the secondary transfer current flows intensively in the dielectric breakdown part, it is considered that a discharge to the toner on the intermediate transfer belt partially occurs and the toner is charged to a reverse polarity, and a patchy defective transfer image is generated. Therefore, such a patchy transfer failure becomes more prominent as the secondary transfer current is increased in addition to the increase in resistance of the sheet.

  Therefore, various methods for suppressing the spot-like secondary transfer failure have been proposed, and Patent Document 1 discloses a method of using a laminated transfer roller in which a high-resistance coating layer is provided on the surface of a conductive layer as the transfer roller. ing. Although electrostatic transfer uses the charge retention characteristics of paper, the paper conditions vary greatly depending on the market and environment, and are difficult to control. In this regard, as in Patent Document 1, by providing not only the sheet but also a transfer member with a charge holding capability that provides a transfer function, the transfer performance is stabilized by using a buffer against fluctuations in the charge held on the sheet. This is an effective approach.

  Patent Document 2 discloses a transfer apparatus in which an RC parallel circuit having a variable resistor and a capacitor is provided between a voltage applying unit and a transfer member. According to the method of Patent Document 2, it is possible to suppress a rapid change in the secondary transfer current without incurring the cost of a laminated roller.

JP-A-1-276182 JP-A-6-202502

  According to the method of Patent Document 1, since the coating layer has a relaxation time by laminating a high resistance coating layer on the surface of the transfer roller, the secondary transfer current is applied to the dielectric breakdown portion even if the paper causes dielectric breakdown. As a result, the toner on the intermediate transfer belt cannot be reversely charged and the secondary transfer can be performed uniformly. However, a transfer roller in which a plurality of materials having different characteristics in terms of electrical and shape are laminated requires much higher material costs and man-hours for manufacturing than a single-layer transfer roller.

  In addition, when a relaxation time is provided at the time of applying a transfer current using an RC parallel circuit as in Patent Document 2, a certain amount of time is required even when the transfer current condition is intentionally changed. For example, during secondary transfer, a bias having a polarity opposite to that of the toner is applied to the secondary transfer roller to transfer the toner image on the intermediate transfer belt to the paper. In order to prevent the toner from adhering to the secondary transfer roller and to collect the toner adhering to the secondary transfer roller to the intermediate transfer belt side, a bias having the same polarity as the toner is often applied to the secondary transfer roller. If it takes time to switch the bias, the productivity during continuous printing is affected.

  SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a color image forming apparatus that can prevent a secondary transfer failure due to dielectric breakdown of a sheet and can secure productivity at the time of continuous printing or double-sided printing.

  In order to achieve the above object, the present invention provides one or more image carriers, an endless intermediate transfer member in which a plurality of color toner images formed on the surface of the image carrier are sequentially laminated, and the intermediate transfer One or more primary transfer means for forming a full-color image by primarily transferring a plurality of color toner images, which are pressed against the image carrier and sandwiched between them and developed on the image carrier, onto the intermediate transfer member; A plurality of primary transfer voltage applying means that are connected to each of the primary transfer means and apply a voltage to the primary transfer means to flow a predetermined primary transfer current, and are laminated on the intermediate transfer body by the primary transfer means. A secondary transfer unit that transfers the toner images of a plurality of colors onto the recording medium at a time, and a secondary transfer unit that is connected to the secondary transfer unit and applies a voltage to the secondary transfer unit to flow a predetermined secondary transfer current. And an image forming apparatus including a next transfer voltage applying unit. An RC parallel circuit in which at least one resistor and at least one capacitor are arranged in parallel between the secondary transfer unit and the secondary transfer voltage application unit, and via the RC parallel circuit A first circuit that connects the secondary transfer unit and the secondary transfer voltage application unit, and a second circuit that connects the secondary transfer unit and the secondary transfer voltage application unit without going through the RC parallel circuit; And a control means for controlling the switching timing of the changeover switch.

  According to the present invention, in the image forming apparatus having the above-described configuration, the relaxation time of the capacitor constituting the RC parallel circuit is determined by the nip width of the secondary transfer nip portion formed by the intermediate transfer member and the secondary transfer unit. It is characterized in that it is more than the time for the recording medium to move.

  In the image forming apparatus having the above-described configuration, the secondary transfer voltage applying unit may secondarily transfer the image on the first surface after performing the secondary transfer in the double-sided printing mode in which printing is performed on both sides of the recording medium. Until the secondary transfer, a secondary transfer voltage having the same polarity as the toner is applied to the secondary transfer unit, and the control unit performs secondary transfer when the image on the second side is secondarily transferred to the recording medium. The transfer voltage is switched from the same polarity as that of toner to the opposite polarity of toner, and at the same time, the changeover switch is operated to switch from the second circuit to the first circuit.

  According to the present invention, in the image forming apparatus having the above-described configuration, the switching from the second circuit to the first circuit by the change-over switch is a secondary transfer nip formed by the intermediate transfer member and the secondary transfer unit. It is characterized in that it is performed more than the relaxation time of the RC parallel circuit before the timing when the tip of the recording medium passes through the part.

  Further, according to the present invention, in the image forming apparatus having the above-described configuration, a humidity detection unit that detects humidity inside and / or outside of the apparatus is provided, and the control unit is provided only when a detection result of the humidity detection unit falls below a predetermined value. Switching from the second circuit to the first circuit is performed.

  According to the first configuration of the present invention, by applying a secondary transfer voltage via the first circuit in a low-humidity environment where dielectric breakdown is likely to occur or during secondary transfer on the second surface during double-sided printing, It is possible to suppress the occurrence of spot-like secondary transfer failure by suppressing the discharge to the toner on the intermediate transfer member due to the transfer current flowing into the dielectric breakdown portion of the recording medium. In addition, during secondary transfer in high-humidity environments or in single-sided continuous printing, the secondary transfer voltage is applied via the second circuit to eliminate the waiting time until the transfer current becomes stable and maintain productivity. Can do.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the relaxation time of the capacitor configuring the RC parallel circuit is formed by the intermediate transfer member and the secondary transfer unit. By setting the nip width of the secondary transfer nip portion to be longer than the time required for the recording medium to move, discharge to the toner on the intermediate transfer member due to the transfer current flowing into the dielectric breakdown portion of the recording medium can be reliably prevented. it can.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the secondary transfer voltage is applied via the second circuit at the time of the secondary transfer of the first surface during duplex printing. By applying the voltage, the secondary transfer voltage can be quickly switched from the same polarity as the toner to the opposite polarity of the toner to maintain productivity. Also, during secondary transfer on the second side, a secondary transfer voltage is applied via the first circuit to suppress discharge to the toner on the intermediate transfer member due to transfer current flowing into the dielectric breakdown portion of the recording medium. can do.

  According to the fourth configuration of the present invention, in the image forming apparatus having the third configuration, the switching from the second circuit to the first circuit by the switch is formed by the intermediate transfer member and the secondary transfer unit. The secondary transfer voltage is completely switched to the opposite polarity to the toner and recorded in a stable state by performing the secondary transfer nip portion before the timing at which the leading edge of the recording medium passes before the relaxation time of the RC parallel circuit. The medium can be passed.

  According to the fifth configuration of the invention, in the image forming apparatus having the third or fourth configuration, the detection result of the humidity detection unit that detects the humidity inside and / or outside of the apparatus falls below a predetermined value. By switching from the second circuit to the first circuit only in the case, in a high humidity environment, there is no waiting time when switching from the voltage having the same polarity as the toner to the voltage having the opposite polarity to the toner. The productivity can be further improved.

1 is a schematic cross-sectional view illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. Partial enlarged view of the periphery of the image forming portion Pa in FIG. Schematic sectional view showing an example of a control path of the image forming apparatus of the present invention 1 is a schematic diagram showing a configuration around an intermediate transfer belt in an image forming apparatus of the present invention. 6 is a graph showing an example of switching timing of the secondary transfer voltage in the image forming apparatus of the present invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a tandem color image forming apparatus of the present invention. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and yellow, cyan, magenta, and the like are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  These image forming portions Pa to Pd are provided with photosensitive drums 1a, 1b, 1c and 1d on which visible images (toner images) of the respective colors are formed, and on these photosensitive drums 1a to 1d. After the toner images formed on the toner image are sequentially transferred (primary transfer) onto the intermediate transfer belt 8 that moves adjacent to each image forming portion while rotating clockwise in FIG. 1 by a driving means (not shown). Then, the image is transferred (secondary transfer) at once onto the paper P as an example of the recording medium by the secondary transfer roller 9, and further fixed on the paper P by the fixing unit 7, and then discharged from the main body of the image forming apparatus 100. It is the composition which becomes. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The paper P onto which the toner image is transferred is housed in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. Further, a belt cleaning device 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed upstream of the image forming portion Pa.

  Next, the image forming units Pa to Pd will be described. There are charging devices 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d, which are rotatably arranged. An exposure unit 4 that forms an electrostatic latent image by exposing the toner, and developing devices 3a, 3b, 3c, and 3d that develop the electrostatic latent images formed on the photosensitive drums 1a to 1d to form toner images, Cleaning devices 5a, 5b, 5c and 5d for removing the developer (toner) remaining on the photosensitive drums 1a to 1d are provided.

  When image data is input from a host device such as a personal computer (hereinafter referred to as a personal computer) connected to the image forming apparatus 100, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging devices 2a to 2d. Then, the exposure unit 4 irradiates the surfaces of the photosensitive drums 1a to 1d with light, and electrostatic latent images corresponding to the image signals are formed on the photosensitive drums 1a to 1d. Each of the developing devices 3a to 3d includes a developing roller (developer carrying member) arranged to face the photosensitive drums 1a to 1d, and toners of respective colors of yellow, cyan, magenta, and black are supplied by a replenishing device (not shown). A predetermined amount is filled. This toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing devices 3a to 3d, and electrostatically adheres to the electrostatic latent image formed by exposure from the exposure unit 4. A toner image is formed.

  Then, the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d are arranged by the primary transfer rollers 6a to 6d disposed so as to be in pressure contact with the photosensitive drums 1a to 1d with the intermediate transfer belt 8 interposed therebetween. An electric field is applied at a predetermined transfer voltage during the interval (primary transfer portion), and the toner images of yellow, magenta, cyan and black on the photosensitive drums 1 a to 1 d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, in preparation for the subsequent formation of a new electrostatic latent image, the toner remaining on the surfaces of the photosensitive drums 1a to 1d after the primary transfer is removed by the cleaning devices 5a to 5d.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 rotates clockwise in FIG. 1 as the drive roller 11 is rotated by a belt drive motor (not shown). When the rotation starts, the sheet P is conveyed from the registration roller 13b to the nip portion (secondary transfer nip portion) between the secondary transfer roller 9 and the intermediate transfer belt 8 provided close to the intermediate transfer belt 8 at a predetermined timing. Then, the full color toner image is secondarily transferred from the intermediate transfer belt 8 onto the paper P at the nip portion. Hereinafter, the full color toner image is abbreviated as a toner image. The paper P on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet P, and a predetermined full-color image is formed. It is formed. The paper P on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when forming images on both sides of the paper P, a part of the paper P that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the paper P is distributed to the paper transport path 18 by the branching section 14 by rotating the discharge roller 15 in the reverse direction, and is transported again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the paper P on which the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  FIG. 2 is an enlarged view of the vicinity of the image forming portion Pa in FIG. Since the image forming units Pb to Pd have basically the same configuration, description thereof is omitted. Around the photosensitive drum 1a, a charging device 2a, a developing device 3a, and a cleaning device 5a are arranged along the drum rotation direction (counterclockwise in FIG. 2), and the primary transfer roller 6a is sandwiched between the intermediate transfer belt 8. Is arranged. Further, a belt cleaning device 19 including a belt cleaning roller 19a facing the driven roller 10 with the intermediate transfer belt 8 interposed therebetween is disposed on the upstream side in the rotation direction of the intermediate transfer belt 8 with respect to the photosensitive drum 1a.

  The charging device 2 a includes a charging roller 21 that contacts the photosensitive drum 1 a and applies a charging bias to the drum surface, and a charging cleaning roller 23 for cleaning the charging roller 21. The developing device 3a is a touch-down developing type having two agitating and conveying screws 25, a magnetic roller 27, and a developing roller 29. A developing bias having the same polarity (positive) as the toner is applied to the developing roller 29. Toner flies over the drum surface.

The cleaning device 5 a includes a rubbing roller 30, a cleaning blade 31, and a collection screw 33. The rubbing roller 30 is pressed against the photosensitive drum 1a with a predetermined pressure, and is driven to rotate in the same direction on the contact surface with the photosensitive drum 1a by a driving means (not shown). The speed is controlled faster than the peripheral speed of 1a (1.2 times here). Examples of the rubbing roller 30 include a structure in which a foam layer made of EPDM rubber and having an Asuka C hardness of 55 ° is formed as a roller body around a metal shaft. The material of the roller body is not limited to EPDM rubber, but may be rubber or foam rubber body of other materials, and those having an Asuka C hardness in the range of 10 to 90 ° are preferably used.

  A cleaning blade 31 is fixed in contact with the photosensitive drum 1a on the surface of the photosensitive drum 1a downstream of the contact surface with the rubbing roller 30 in the rotation direction. As the cleaning blade 31, for example, a polyurethane rubber blade having a JIS hardness of 78 ° is used, and is attached at a predetermined angle with respect to the tangential direction of the photosensitive member at the contact point. The material, hardness, dimensions, the amount of biting into the photosensitive drum 1a, the pressure contact force, and the like of the cleaning blade 31 are appropriately set according to the specifications of the photosensitive drum 1a.

  Residual toner removed from the surface of the photosensitive drum 1a by the rubbing roller 30 and the cleaning blade 31 is discharged to the outside of the cleaning device 5a (see FIG. 2) as the recovery screw 33 rotates. As the toner used in the present invention, silica, titanium oxide, strontium titanate, alumina or the like is embedded in the toner particle surface as an abrasive and held so as to partially protrude on the surface. Those that are electrostatically attached to the surface are used.

  The surface of the photosensitive drum 1a is polished by the residual toner containing the abrasive by rotating the rubbing roller 30 with a speed difference with respect to the photosensitive drum 1a, and the drum is formed by the rubbing roller 30 and the cleaning blade 31. Remove surface moisture and contaminants along with residual toner.

  Next, the control path of the image forming apparatus of the present invention will be described. FIG. 3 is a block diagram showing an example of a control path used in the image forming apparatus of the present invention. It should be noted that since various control of each part of the apparatus is performed when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, here, a portion of the control path that is necessary for the implementation of the present invention will be mainly described.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 that is a read-only storage unit, a random access memory (RAM) 93 that is a read / write storage unit, A plurality of (two in this case) that transmit a control signal to each device in the temporary storage unit 94, the counter 95, and the image forming apparatus 100 for storing image data and the like, and receive an input signal from the operation unit 50. The I / F (interface) 96 is provided. Further, the control unit 90 can be arranged at an arbitrary location inside the apparatus main body.

  The ROM 92 stores a control program for the image forming apparatus 100, data necessary for control, and the like that are not changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for controlling the image forming apparatus 100, and the like. The counter 95 adds up the number of printed sheets and counts it.

  In addition, the control unit 90 transmits a control signal from the CPU 91 to the respective units and apparatuses in the image forming apparatus 100 through the I / F 96. In addition, a signal indicating the state and an input signal are transmitted from each part or device to the CPU 91 through the I / F 96. For example, the image forming units Pa to Pd, the exposure unit 4, the primary transfer rollers 6 a to 6 d, the intermediate transfer belt 8, the secondary transfer roller 9, the image input unit 40, and the bias are included in each part and device controlled by the control unit 90. Examples include the control circuit 41, the operation unit 60, and the like.

  The image input unit 40 is a receiving unit that receives image data transmitted from the personal computer or the like to the image forming apparatus 100. The image signal input from the image input unit 40 is converted into a digital signal and then sent to the temporary storage unit 94.

  The bias control circuit 41 is connected to the charging bias power source 42, the developing bias power source 43, the first DC power sources 44 a to 44 d, the second DC power source 45, and the belt cleaning bias power source 46, and these signals are output by an output signal from the control unit 90. Each power source is operated, and according to a control signal from the bias control circuit 41, each of these power sources is charged by the charging bias power source 42 to the charging roller 21 in the chargers 2a to 2d and the developing bias power source 43 by the developing device 3a. To 3d, the first DC power supply 44a to 44d to the primary transfer rollers 6a to 6d, the second DC power supply 45 to the secondary transfer roller 9, and the belt cleaning bias power supply 46 to belt cleaning. A predetermined bias is applied to each of the rollers 19a.

  The RC parallel circuit 50 is connected between the secondary transfer roller 9 and the second DC power supply 45, and applies a secondary transfer voltage to the secondary transfer roller 9 via the RC parallel circuit 50. 47 and a second circuit 48 for applying a secondary transfer voltage to the secondary transfer roller 9 without passing through the RC parallel circuit 50. The detailed configuration and operation of the RC parallel circuit 50 will be described later.

  The operation unit 60 is provided with a liquid crystal display unit 61 and LEDs 62 indicating various states. The user operates the stop / clear button of the operation unit 60 to stop image formation, and operates the reset button to operate the image. Various settings of the forming apparatus 100 are set to a default state. The liquid crystal display unit 61 displays the state of the image forming apparatus 100, and displays the image forming status and the number of copies to be printed. Various settings of the image forming apparatus 100 are performed from a printer driver of a personal computer.

  The external humidity sensor 97 detects humidity outside the apparatus, and is installed near the intake duct (not shown) on the side of the paper cassette 16 in FIG. It can also be installed in other locations where external humidity can be detected accurately. The external humidity sensor 97 always detects the humidity outside the apparatus.

  FIG. 4 is a schematic diagram showing a configuration around the intermediate transfer belt in the image forming apparatus of the present invention. Portions common to FIGS. 1 and 2 are denoted by the same reference numerals, and description thereof is omitted. As shown in FIG. 4, first DC power supplies 44a to 44d for applying a primary transfer voltage are connected to the primary transfer rollers 6a to 6d. The secondary transfer roller 9 is connected to a second DC power supply 45 for applying a secondary transfer voltage.

  An RC parallel circuit 50 in which a resistor 51 and a capacitor (capacitor) 53 are arranged in parallel is connected between the secondary transfer roller 9 and the second DC power supply 45. A changeover switch 55 is provided between the second DC power supply 45 and the RC parallel circuit 50. A control unit 90 is connected to the changeover switch 55 and is selectively connected to the contact 57a or the contact 57b based on a control signal transmitted from the control unit 90. That is, the changeover switch 55 forms the first circuit 47 (see FIG. 3) by contacting the contact 57a, and forms the second circuit 48 (see FIG. 3) by contacting the contact 57b.

FIG. 5 is a graph showing an example of switching timing of the secondary transfer voltage in the image forming apparatus of the present invention. With reference to FIGS. 1 to 4, a method for controlling the secondary transfer voltage when performing double-sided printing with the image forming apparatus of the present invention will be described in detail with reference to FIG. 5. For example, in the color image forming apparatus 100 configured as shown in FIG. 4, the secondary transfer roller 9 has a resistance value of 10 ⁷ when an Asuka C hardness of 40 ° and a voltage of 1 kV are applied around a stainless steel shaft having a diameter of 14 mm. A foamed rubber roller having a diameter of 24 mm and a length of 300 mm formed by forming a foamed rubber layer of Ω is used, and the photoreceptor drums 1a to 1d are amorphous silicon photoreceptors having a diameter of 30 mm.

  The linear speeds of the photosensitive drums 1a to 1d and the intermediate transfer belt 8 are set to 200 mm / sec, and a secondary transfer voltage (−40 μA constant current control) having a polarity (negative) opposite to that of the toner is applied during the secondary transfer. A secondary transfer voltage (constant voltage control of +500 V) having the same polarity (positive) as that of the toner is applied between the sheets.

  First, the toner images primarily transferred from the photosensitive drums 1a to 1d onto the intermediate transfer belt 8 by the primary transfer rollers 6a to 6d are secondarily transferred to the first surface of the sheet. At this time, the changeover switch 55 is in contact with the contact 57b, and a secondary transfer voltage having a reverse polarity (negative) to the toner is applied from the second DC power supply 45 to the secondary transfer roller 9 through the second circuit 48. .

  After the printing on the first side is completed, the paper protrudes from the discharge roller 15 to the outside of the apparatus, and is then switched back by rotating the discharge roller 15 in the reverse direction. Is temporarily stopped at the registration roller pair 12b. Then, the sheet is conveyed again from the registration roller pair 12b toward the secondary transfer roller 9 in accordance with the timing at which the second-side image formed on the intermediate transfer belt 8 moves to the secondary transfer nip portion. During this time, a secondary transfer voltage having the same polarity (positive) as that of the toner is applied to the secondary transfer roller 9 from the second DC power supply 45 through the second circuit 48.

  Application of a secondary transfer voltage having a polarity opposite to that of the toner is started 150 msec before the sheet enters the secondary transfer nip (indicated by T1 in FIG. 5). At the same time, a control signal is transmitted from the control unit 90 to the changeover switch 55, and the circuit from the second DC power supply 45 to the secondary transfer roller 9 is brought into contact with the changeover switch 55 from the contact 57 b to the contact 57 a. To the first circuit 47. As a result, part of the charge passing through the first circuit 47 is accumulated in the capacitor 53, so that the secondary transfer voltage gradually reaches a predetermined value (indicated by hatching in FIG. 5).

  After the secondary transfer voltage reaches a predetermined value, the second-side toner image primarily transferred onto the intermediate transfer belt 8 is secondarily transferred to the second side of the sheet. At this time, since the secondary transfer current flows through the first circuit 47, even if the insulation breakdown of the paper occurs and the secondary transfer current flows intensively into the insulation breakdown portion, the buffer function of the RC parallel circuit 50 As a result, the discharge of the toner on the intermediate transfer belt 8 is suppressed.

  After the secondary transfer on the second side is completed and the sheet passes through the secondary transfer nip (indicated by T2 in FIG. 5), the secondary transfer voltage is returned to the same polarity as the toner, and the control signal from the control unit 90 Based on this, the switch 55 is brought into contact with the contact 57a to the contact 57b, and the circuit from the second DC power supply 45 to the secondary transfer roller 9 is returned from the first circuit 47 to the second circuit 48.

  Applying the secondary transfer voltage via the RC parallel circuit 50 at the time of the secondary transfer to the second side of the paper whose water content has decreased and became high resistance by the fixing process of the first side image in the above procedure. Thus, it is possible to suppress the discharge of the toner on the intermediate transfer belt 8 due to the transfer current flowing into the dielectric breakdown portion of the sheet. Therefore, it is possible to effectively suppress the occurrence of patchy secondary transfer failure.

  In addition, at the time of secondary transfer to the first side of the paper whose moisture content has not decreased, a secondary transfer voltage is applied without going through the RC parallel circuit 50, so that a voltage having the same polarity as that of the toner is changed from a voltage having the same polarity as that of the toner. There is no waiting time when switching to voltage. Therefore, for example, when continuous printing is performed only on one side of a sheet, secondary transfer can be performed without reducing productivity.

  The relaxation time of the RC parallel circuit 50 becomes longer as the capacitance of the capacitor 53 constituting the RC parallel circuit 50 increases. In addition, as the resistance value of the resistor 51 increases, the current flowing to the capacitor 53 side increases, so the relaxation time becomes longer. However, when the partial pressure of the resistor 51 and the capacitor 53 increases, leakage and noise to the peripheral circuit occur. The cost of the substrate also increases. On the other hand, when the capacitance of the capacitor 53 and the resistance value of the resistor 51 are reduced, the relaxation time is shortened, and the fluctuation of the transfer current cannot be sufficiently suppressed.

  Further, since the nip width of the secondary transfer nip portion is about 2 to 6 mm and the sheet moves at 200 mm / sec, it is equal to or longer than the time (10 to 30 msec) for which the sheet moves a distance corresponding to the nip width. If the relaxation time is present, it is considered that the transfer current can be prevented from flowing due to the dielectric breakdown of the paper. In the above embodiment, the resistance value of the resistor 51 is 10 MΩ, the capacitance of the capacitor 53 is 4700 pF, and the relaxation time is set to 47 msec.

  Further, since the secondary transfer failure due to the insulation breakdown of the paper is likely to occur in a low humidity environment, the control unit 90 applies the secondary transfer voltage only when the humidity detected by the external humidity sensor 97 falls below a predetermined value. You may make it control to switch a path | route to a 1st circuit. In this way, in a high-humidity environment, no waiting time occurs when switching from a voltage having the same polarity as that of the toner to a voltage having a polarity opposite to that of the toner, so that the productivity during double-sided printing can be further improved. It is also possible to provide an in-machine humidity sensor that detects the humidity inside the apparatus in place of the out-of-machine humidity sensor 97, and determine whether or not control is necessary based on the detected humidity of the in-machine humidity sensor.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, the RC parallel circuit 50 in which one resistor 51 and one capacitor 53 are arranged in parallel is used. However, the RC parallel circuit 50 may be configured using a plurality of resistors 51 or a plurality of capacitors 53. it can.

  In the above embodiment, the tandem color printer as shown in FIG. 1 has been described as an example of the image forming apparatus of the present invention. However, the present invention is not limited to this. For example, one photoconductor Four developing devices are arranged around the drum, a four-color image is sequentially formed on the photosensitive drum, and a single drum type that is superimposed on the intermediate transfer belt, or a rotary that sequentially switches the developing device facing the photosensitive drum The present invention can be applied to various color image forming apparatuses of an intermediate transfer system such as a color printer of the type, or a tandem type, a single drum type and a rotary type color copying machine.

  The resistance value of the resistor 51, the capacitance of the capacitor 53, the switching timing of the secondary transfer voltage (T1 in FIG. 5), the set value of the secondary transfer voltage (current), and the like are merely examples, and the process What is necessary is just to set suitably according to the specifications of an image forming apparatus, such as speed, and a use environment.

  The present invention is applicable to a color image forming apparatus using an intermediate transfer system, and at least one resistor and at least one capacitor are arranged in parallel between a secondary transfer unit and a secondary transfer voltage application unit. The arranged RC parallel circuit, the first circuit for connecting the secondary transfer means and the secondary transfer voltage application means via the RC parallel circuit, and the secondary transfer means and the secondary transfer voltage without going through the RC parallel circuit A change-over switch that switches between the second circuit that connects the application means and a control means that controls the change-over timing of the change-over switch are provided.

  By utilizing the present invention, the discharge onto the toner on the intermediate transfer member is caused by the transfer current flowing into the dielectric breakdown part of the paper in the low-humidity environment where dielectric breakdown is likely to occur or during the secondary transfer on the second side during double-sided printing. It is possible to effectively suppress the occurrence of patchy secondary transfer failure.

1a to 1d Photosensitive drum (image carrier)
2a to 2d Charging device 3a to 3d Developing device 4 Exposure unit 5a to 5d Cleaning device 6a to 6d Primary transfer roller (primary transfer means)
8 Intermediate transfer belt (intermediate transfer member)
9 Secondary transfer roller (secondary transfer means)
19 Belt cleaning device 44a-44d 1st DC power supply (primary transfer voltage application means)
45 Second DC power supply (secondary transfer voltage applying means)
47 1st circuit 48 2nd circuit 50 RC parallel circuit 51 Resistor 53 Capacitor 55 Changeover switch 57a, 57b Contact 90 Control part (control means)
97 External humidity sensor (humidity detection means)
100 Image forming apparatus Pa to Pd Image forming unit

Claims (5)

One or more image carriers, and an endless intermediate transfer member on which a plurality of color toner images formed on the surface of the image carrier are sequentially laminated;
One or more primary forming a full-color image by primarily transferring a plurality of color toner images, which are pressed against the image carrier and sandwiched between the intermediate transfer members and developed on the image carrier, onto the intermediate transfer member Transcription means;
A plurality of primary transfer voltage applying means connected to each of the primary transfer means and applying a voltage to the primary transfer means to flow a predetermined primary transfer current;
Secondary transfer means for transferring toner images of a plurality of colors laminated on the intermediate transfer member by the primary transfer means at a time onto a recording medium;
A secondary transfer voltage applying unit connected to the secondary transfer unit, and applying a voltage to the secondary transfer unit to flow a predetermined secondary transfer current;
In an image forming apparatus comprising:
An RC parallel circuit in which at least one resistor and at least one capacitor are arranged in parallel between the secondary transfer unit and the secondary transfer voltage application unit, and the second via the RC parallel circuit Switching between a first circuit that connects the secondary transfer means and the secondary transfer voltage application means and a second circuit that connects the secondary transfer means and the secondary transfer voltage application means without going through the RC parallel circuit An image forming apparatus comprising: a changeover switch; and control means for controlling a changeover timing of the changeover switch.   The relaxation time of the capacitor constituting the RC parallel circuit is longer than the time required for the recording medium to move the nip width of the secondary transfer nip formed by the intermediate transfer member and the secondary transfer means. The image forming apparatus according to claim 1. The secondary transfer voltage application unit is configured to perform the secondary transfer unit during a period from secondary transfer of the image on the first side to secondary transfer of the image on the second side in a duplex printing mode in which printing is performed on both sides of the recording medium. A secondary transfer voltage having the same polarity as the toner is applied to
The control means switches the secondary transfer voltage from the same polarity as the toner to the reverse polarity of the toner when the image on the second side is secondarily transferred to the recording medium, and at the same time operates the changeover switch from the second circuit. The image forming apparatus according to claim 1, wherein switching to the first circuit is performed.   Switching from the second circuit to the first circuit by the changeover switch is performed in parallel with the RC in comparison with the timing at which the recording medium passes through the secondary transfer nip formed by the intermediate transfer member and the secondary transfer means. The image forming apparatus according to claim 3, wherein the image forming apparatus is performed before the circuit relaxation time.   Humidity detection means for detecting the humidity inside and / or outside of the apparatus is provided, and the control means switches from the second circuit to the first circuit only when the detection result of the humidity detection means falls below a predetermined value. The image forming apparatus according to claim 3, wherein the image forming apparatus is performed.