JP5214093B2 - Image forming apparatus - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus applied as a copying machine, a facsimile machine, or a printer, and more particularly to a transfer means that functions when transferring a toner image formed on an image carrier onto a transfer material. The present invention relates to improvement of voltage application.

  The image forming apparatus supplies toner to an electrostatic latent image formed on the peripheral surface of a photosensitive drum as an image carrier based on image information read or sent to form a toner image. Is transferred to a sheet as a transfer material via a transfer means. In general, the transfer unit includes a transfer belt that is synchronously driven with the photosensitive drum in a state in which the sheet conveyance surface faces the circumferential surface of the photosensitive drum, and a circumferential surface that faces the circumferential surface of the photosensitive drum via the transfer belt. The transfer roller is arranged, and a power source for applying a voltage to the transfer roller is provided.

  When the sheet is conveyed by the transfer belt to the photosensitive drum that rotates in synchronization with the rotation of the transfer belt, a voltage having a polarity opposite to that of the toner is applied from the power source to the transfer roller. The toner image on the peripheral surface of the photosensitive drum is peeled off by charging with a polarity opposite to that of the toner image on the paper sheet, whereby the toner image is transferred onto the paper sheet. The paper after the transfer process is carried into the fixing device around the transfer belt, and after being subjected to the fixing process by heating, it is discharged out of the apparatus.

  At this time, a voltage having the same polarity as the polarity of the toner is applied from the power source to the transfer roller when the sheet is removed from the photosensitive drum. This is because the toner adhering to the transfer belt after completion of the transfer process is returned to the peripheral surface of the photosensitive drum by applying a bias of the same polarity, thereby cleaning the surface of the transfer belt. The residual toner trapped on the peripheral surface of the photosensitive drum is removed and collected by a cleaning device provided on the downstream side in the rotation direction.

An image forming apparatus provided with such a transfer unit is described in, for example, Patent Document 1.
Japanese Patent No. 3508088

  By the way, when the voltage control as described above is executed, the voltage of the transfer roller suddenly changes from the reverse polarity to the toner to the same polarity as the toner at the moment when the trailing edge of the paper is removed from the photosensitive drum. As a result, the potential on the peripheral surface of the photosensitive drum also suddenly fluctuates, and this impact causes residual toner remaining on the peripheral surface of the photosensitive drum and paper that is electrostatically attracted to the peripheral surface of the photosensitive drum. There is a problem in that the toner or the like adhering to the member for peeling (peeling claw) is blown off and falls onto the paper to contaminate the paper.

  Incidentally, Patent Document 1 does not list such a problem that occurs at the time of switching the polarity as a problem to be solved. Therefore, there is no description about a solution for such a problem.

  The present invention has been made in view of such a situation, and has transfer means configured to be able to change the polarity of a voltage applied to a transfer member in a state where toner scattering does not occur. An image forming apparatus is provided.

The invention according to claim 1 is an image comprising an image carrier on which a toner image is formed on the peripheral surface, and a transfer means for transferring the toner image of the image carrier rotating about the axis to a transfer material. In the forming apparatus, the transfer unit includes a voltage applying member that applies a voltage to the transfer material in a nip state in which the transfer material is sandwiched between the peripheral surface of the image carrier and a power source that supplies the voltage to the voltage applying member. And voltage control means for controlling the polarity and voltage value of the voltage supplied from the power source to the voltage application member. The voltage control means is opposite to the toner with respect to the voltage application member when the transfer material is in the nip state. A forward bias application period in which a voltage of polarity is applied, a reverse bias application period in which a voltage having the same polarity as the toner is applied to the voltage application member when the transfer material is not in the nip state, and a region in which the transfer material is released from the nip state. To the voltage application member Set the conversion period for switching the voltage to be pressurized from the opposite polarity to the same polarity, stepwise changing the voltage value in the conversion period, and, to a zero potential during the conversion period from the forward bias application period The voltage is indirectly controlled by controlling the current value during the period, and the voltage is directly controlled by controlling the voltage value during the period from the zero potential state during the conversion period to the reverse bias application period. It is characterized by performing control.

  According to this configuration, the toner image formed on the image carrier is transferred to the image carrier by the transfer material being conveyed, and the image carrier is rotated while being nipped by the image carrier and the voltage application member. In response to this, it is transferred to a transfer material. When the transfer material is in the nip state, a voltage having a polarity opposite to that of the toner is applied to the voltage application member during the transfer, so that the toner image carried on the image carrier is transferred to the transfer material by electrostatic attraction force. As a result, the transfer process is reliably performed on the transfer material. Also, when the transfer process to the transfer material is completed and the transfer material is no longer in the nip state, a voltage having the same polarity as the toner is applied from the power source to the voltage application member in this state, so that it is near the image carrier. The scattered toner is electrostatically attracted to the image carrier, and this prevents the inside of the apparatus from being contaminated with the scattered toner, and the inconvenience that the scattered toner adheres to the transfer material and causes transfer failure. The

  In particular, when switching the voltage application to the voltage application member from the moment when the transfer material is removed from the nip state, the switching from the reverse polarity voltage (bias) application to the same polarity voltage (bias) application is performed stepwise. Therefore, it is possible to reliably prevent the occurrence of inconveniences such as toner scattering caused by the impact when the switching is suddenly performed and the transfer image defect due to the scattered toner adhering to the transfer material.

  According to a second aspect of the present invention, in the first aspect of the invention, the voltage applying member is a transfer roller that applies a voltage to the transfer material in contact with or non-contact with the image carrier. It is.

  According to this configuration, since the voltage is applied from the transfer roller to the transfer material that has reached the image carrier, the voltage application state of the transfer material becomes uniform and stable.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, a transfer belt for conveying a transfer material is interposed between the transfer roller and the image carrier. Is.

  According to this configuration, the transfer material reaches the image carrier by being conveyed by the transfer belt, and a voltage from the transfer roller is applied via the transfer belt in a stable conveyance state.

  Incidentally, in the present invention, the voltage control for the voltage application member by the voltage control means is used as a concept including both a method of directly controlling the voltage value and a method of controlling the voltage value indirectly through the current value. ing.

  According to the first aspect of the present invention, since the power source is controlled so as to change the voltage application of the reverse polarity stepwise toward the voltage application of the same polarity from the moment when the transfer material leaves the nip state, When switching is performed, it is possible to reliably prevent the occurrence of inconveniences such as toner scattering caused by the impact and transfer failure due to the scattered toner adhering to the transfer material.

  According to the second aspect of the present invention, since voltage is applied from the transfer roller to the transfer material that has reached the image carrier, the voltage application state of the transfer material can be made uniform and stable.

  According to the invention of claim 3, since the transfer material reaches the image carrier by being conveyed by the transfer belt, the voltage from the transfer roller is applied via the transfer belt in a stable conveyance state with respect to the transfer material. Can be applied.

  FIG. 1 is a front cross-sectional view illustrating an embodiment of the internal structure of the image forming apparatus according to the present invention. As shown in FIG. 1, the image forming apparatus 10 includes a cylindrical photosensitive drum (image carrier) 20 and a charging device disposed clockwise along the peripheral surface of the photosensitive drum 20. The apparatus has a basic configuration including an apparatus 11, an exposure apparatus 12, a developing apparatus 13, a transfer unit 30, a cleaning apparatus 14, and a charge removal apparatus 15.

  In the present embodiment, the charging device 11 is disposed immediately above the photosensitive drum 20, the exposure device 12 is disposed slightly on the right side of the charging device 11, and the right side of the photosensitive drum 20 in FIG. The developing device 13 is disposed opposite to the part. In addition, the transfer unit 30 is provided at a position directly below the photosensitive drum 20, the cleaning device 14 is disposed to face the left side of the photosensitive drum 20 in FIG. 1, and the charge removal device 15 is connected to the cleaning device 14. It is provided between the charging device 11.

  The photosensitive drum 20 is formed by laminating a photoconductive amorphous layer mainly composed of silicon (Si) on the peripheral surface thereof. The photosensitive drum 20 is formed to be rotatable about a drum shaft 21 concentric with the photosensitive drum 20, and in the example shown in FIG. 1, it is formed by exposure from the exposure device 12 on the peripheral surface while rotating clockwise. A toner image is formed by supplying toner from the developing device 13 to the electrostatic latent image. The toner image formed on the peripheral surface of the photosensitive drum 20 is transferred to a predetermined sheet (transfer material) P by the transfer unit 30 as the photosensitive drum 20 rotates in the clockwise direction around the drum shaft 21. .

  The charging device 11 applies, for example, corona discharge to the peripheral surface of the photosensitive drum 20 prior to exposure by the exposure device 12, thereby forming uniform charges of a predetermined polarity on the peripheral surface. In the present embodiment, the peripheral surface of the photosensitive drum 20 is uniformly charged with a positive charge by the charging device 11.

  The exposure device 12 is a peripheral surface of the photosensitive drum 20 on which a uniform charge is formed by a laser beam having intensity based on image information read by a document reading device (not shown) or transmitted from another device. Is irradiated. By doing so, an electrostatic latent image is formed on the peripheral surface of the photosensitive drum 20. Then, the peripheral surface of the photosensitive drum 20 where the positive charges are uniformly formed has the potential of the portion irradiated with the laser beam lowered from about several hundred V to about several V, and the peripheral surface of the photosensitive drum 20 An electrostatic latent image is formed.

  The developing device 13 supplies toner to the peripheral surface of the photosensitive drum 20 on which the electrostatic latent image is formed, thereby forming a toner image on the peripheral surface, and is loaded into a predetermined developing device main body 131. The toner thus supplied is supplied from the peripheral surface to the peripheral surface of the photosensitive drum 20 by the rotation of the developing roller 132 around the axis.

  As a result, a toner image along the electrostatic latent image is formed on the peripheral surface of the photosensitive drum 20. In the present embodiment, since a positive charge is applied to the toner in advance, the toner supplied to the peripheral surface of the photosensitive drum 20 repels a high potential positively charged portion other than the electrostatic latent image. Thus, a positively charged toner image is formed on the peripheral surface of the photosensitive drum 20 by being attached to an electrostatic latent image having a low potential of about several hundred volts to about several volts.

  The transfer means 30 is for transferring a toner image formed on the peripheral surface of the photosensitive drum 20 rotating around the drum shaft 21 to the paper P, which will be described in detail later.

  The cleaning device 14 is for removing the toner remaining on the peripheral surface of the photosensitive drum 20 after the transfer process to the paper P and cleaning it. The cleaning device 14 includes a box-shaped cleaning device main body 141, a blade 142 attached to the cleaning device main body 141, and a recovery screw 143. The blade 142 is disposed so as to face the peripheral surface of the photosensitive drum 20, and a tip edge thereof is in contact with the peripheral surface, whereby residual toner is removed by rotating the photosensitive drum 20 in the clockwise direction. It is designed to be scraped off from the surrounding surface. The residual toner thus scraped off is collected in a collection tank (not shown) outside the cleaning apparatus main body 141 via a collection screw 143.

  A separation claw 16 for peeling the paper P adsorbed on the peripheral surface of the photosensitive drum 20 is provided immediately below the cleaning device 14. Such a separation claw 16 is provided because the sheet P that has passed through the nip portion N (the upper surface of the sheet P is negatively charged) is electrostatically surrounded by the peripheral surface of the photosensitive drum 20 (of the photosensitive drum 20). This is to assist the separation of the transfer material adsorbed on the peripheral surface is positively charged).

  The separation claw 16 is usually in contact with the peripheral surface of the photoconductive drum 20 at the tip edge, but a slight gap may be provided with respect to the peripheral surface of the photoconductive drum 20. By providing a slight gap, it is possible to pass the residual toner remaining on the peripheral surface of the photosensitive drum 20, and the residual toner on the peripheral surface of the photosensitive drum 20 is scraped off and scattered by the separation claw 16. Such an inconvenience is prevented. The residual toner that has entered the separation claw 16 is removed from the peripheral surface of the photosensitive drum 20 by the blade 142 of the cleaning device 14.

  The static eliminator 15 is for erasing charges formed by irradiating the peripheral surface of the photosensitive drum 20 cleaned by the cleaning device 14 with predetermined light. The peripheral surface of the photosensitive drum 20 from which the charge has been erased is charged by the charging device 11 for the next electrostatic latent image formation.

  According to the image forming apparatus 10 configured as described above, in a state where the photosensitive drum 20 is rotated clockwise around the drum shaft 21, first, a predetermined charge is uniformly applied to the peripheral surface by the charging device 11. Then, light from the exposure device 12 is irradiated onto the peripheral surface of the photosensitive drum 20 to which the uniform charge is applied, whereby an electrostatic latent image is formed on the peripheral surface. Subsequently, toner is supplied from the developing device 13 to the peripheral surface of the photosensitive drum 20 on which the electrostatic latent image is formed, so that a toner image is formed on the peripheral surface. It is transferred to the paper P that has been transported from a paper storage section (not shown). The transfer-processed paper P is subjected to a fixing process by heating in an unillustrated fixing device, and then discharged to the outside of the image forming apparatus 10.

  On the other hand, the residual toner remaining on the peripheral surface of the photosensitive drum 20 after the transfer process on the paper P is removed by the blade 142 of the cleaning device 14. Then, the peripheral surface of the cleaned photosensitive drum 20 is irradiated with light from the static eliminator 15 to erase the charge on the peripheral surface. The peripheral surface of the photosensitive drum 20 after the charge is erased reaches the charging device 11 again, and is used for the next image transfer process.

  In the present embodiment, the transfer unit 30 includes a transfer roller (voltage applying member) 31 disposed below the photoconductive drum 20 so as to face the photoconductive drum 20, and the transfer roller 31 and the photoconductive drum 20. And a high-voltage power supply 33 that applies a high voltage to the transfer roller 31 to apply a charge to the transfer roller 31.

  As the high-voltage power supply 33, a commercial power supply may be used as it is, or a so-called power supply unit that performs a predetermined transformation process after temporarily receiving power from the commercial power supply may be employed. Incidentally, a power supply unit is employed in this embodiment.

  The transfer roller 31 is rotatably supported around a roller shaft 311 disposed in parallel with the drum shaft 21. The transfer roller 31 is formed of a rubber material such as EPDM (ethylene propylene diene copolymer) and carbon, and a predetermined charge is formed on the peripheral surface when a voltage is applied from a high voltage power source 33. To be able to be.

  A coil spring 312 that urges the transfer roller 31 upward is attached to the roller shaft 311, and the peripheral surface of the transfer roller 31 is moved through the transfer belt 32 by the urging force of the coil spring 312. It is in a state of being pressed against the peripheral surface. A nip portion N that nips the paper P is formed between the transfer belt 32 and the peripheral surface of the photosensitive drum 20.

  Incidentally, in the present invention, the nip means that the paper P is positioned between the photosensitive drum 20 and the transfer roller 31 (in this embodiment, the transfer belt 32 is interposed therebetween). Therefore, when the paper P is pressed and sandwiched between the photosensitive drum 20 and the transfer roller 31 via the transfer belt 32, there is a certain gap between the paper P and the transfer belt 32. It is a concept that includes both cases.

  The roller shaft 311 is made of a metal material. The roller shaft 311 is supplied with electric power from the high-voltage power source 33, thereby forming a predetermined charge on the peripheral surface of the transfer roller 31.

  The transfer belt 32 is formed of an endless belt, and is between a drive roller 321 provided on the downstream side (left side in FIG. 1) of the transfer roller 31 and a driven roller 322 provided on the upstream side of the transfer roller 31. It is hung around. In the present embodiment, the transfer belt 32 is based on NBR (nitrile butadiene rubber (acrylonitrile butadiene copolymer rubber)) having a resistance value set to a predetermined value, and the surface thereof has good adsorbability to the paper P. In addition, in order to ensure good releasability with respect to the toner, it is formed by coating a urethane binder in which PTFE (polytetrafluoroethylene (tetrafluoroethylene resin)) is dispersed.

  The drive roller 321 is concentrically and integrally connected to the drive shaft of the drive motor 34 and is driven to rotate by the drive of the drive motor 34. Then, by driving the drive motor 34, the drive roller 321 rotates counterclockwise around the axis, so that the transfer belt 32 rotates synchronously with the photosensitive drum 20 between the drive roller 321 and the driven roller 322. It is supposed to be. Therefore, when the paper P is conveyed by the transfer belt 32 and reaches the nip portion N, the transfer belt 32 that circulates in the nip portion N and the peripheral surface of the photosensitive drum 20 that rotates about the drum shaft 21. The toner image formed on the peripheral surface of the photosensitive drum 20 is transferred onto the sheet P by the electrostatic action of the transfer roller 31.

  The high-voltage power supply 33 applies a voltage having a polarity opposite to that of the toner to the transfer roller 31 when the paper P is positioned at the nip portion N, while the transfer roller 31 when the paper P is not present at the nip portion N. In contrast, a voltage having the same polarity as that of the toner is applied. In the present embodiment, a negative voltage is applied to the transfer roller 31 from the high-voltage power supply 33 via the roller shaft 311 in a state where the paper P is positioned at the nip portion N, while the paper P is not positioned at the nip portion N. In this state, a positive voltage is applied to the transfer roller 31 from the high voltage power supply 33 via the roller shaft 311.

  Incidentally, the negative voltage is applied to the transfer roller 31 when the paper P is positioned at the nip N. The transfer roller 31 is positively applied to the peripheral surface of the photosensitive drum 20 during the transfer process when the paper P is positioned at the nip N. This is because the electrostatically charged toner image is electrostatically attracted to the upper surface of the paper P. In addition, when a positive voltage is applied to the transfer roller 31 in a state where the paper P is not present in the nip portion N, the positively charged toner scattered on the transfer belt 32 or floating in the space is electrostatically discharged. This is because the toner is repelled from the surface of the transfer belt 32 and directed toward the peripheral surface of the photosensitive drum 20, thereby capturing the scattered toner on the peripheral surface of the photosensitive drum 20.

  2A and 2B are partially enlarged explanatory views for explaining the charge formation state of each member in the nip portion N. FIG. 2A is a state when the forward bias is applied when the paper P exists in the nip portion N. ) Shows a state when a reverse bias is applied when the paper P does not exist in the nip portion N. The forward bias application means that the voltage on the peripheral surface of the photosensitive drum 20 (a positive charge is formed on the peripheral surface of the photosensitive drum 20 as a whole) and a voltage having a reverse polarity are applied to the roller shaft 311. The reverse bias application is a state in which a voltage having the same polarity as the voltage on the peripheral surface of the photosensitive drum 20 is applied to the roller shaft 311.

  First, as shown in FIG. 2A, when a forward bias is applied while the paper P is present in the nip portion N, a negative voltage “−” is applied from the high voltage power source 33 to the roller shaft 311. As a result, a positive charge “+” is formed at the inner peripheral surface position in close contact with the roller shaft 311 of the transfer roller 31 made of a semiconductive material, and a negative charge “−” is formed at the outer peripheral surface position. Accordingly, in the transfer belt 32 whose back side is in contact with the outer peripheral surface of the transfer roller 31, a positive charge “+” is formed on the back side and a negative charge “−” is formed on the front side. It has become.

  On the other hand, the position of the peripheral surface formed of the amorphous silicon photosensitive layer of the photosensitive drum 20 has a positive charge “+” formed as a whole, but the toner formed by the toner having the positive charge “+”. The portion where the image T (therefore, the toner image T is positively charged) is in an uncharged state in which no charge is present when the exposure device 12 irradiates the laser beam.

  Therefore, as shown in FIG. 2A, when the toner image T reaches the nip portion N, the toner image T having a positive charge is electrostatically adsorbed on the surface of the paper P having a negative charge. Thus, the transfer process for the paper P is performed.

  Next, as shown in FIG. 2B, when a reverse bias is applied when the paper P does not exist in the nip portion N, a positive voltage “+” is applied from the high voltage power source 33 to the roller shaft 311. Then, a negative charge “−” is formed at the inner peripheral surface position in close contact with the roller shaft 311 of the transfer roller 31 made of a semiconductor, and a positive charge “+” is formed at the outer peripheral surface position. As a result, a negative charge “−” is formed on the back surface side of the transfer belt 32 in contact with the outer peripheral surface of the transfer roller 31, and a positive charge “+” is formed on the front surface side. .

  Accordingly, the peripheral surface of the positive photosensitive drum 20 and the surface of the transfer belt 32 are in a state of electrostatically repelling each other. In this state, for example, the transfer belt 32 in the vicinity of the nip portion N is used. In the case where the positively charged scattered toner T1 is present, the scattered toner T1 adheres to the peripheral surface of the photosensitive drum 20 due to a potential difference between the transfer belt 32 and the photosensitive drum 20 or electrostatically. . Specifically, when a reverse bias of about +1400 V is applied to the transfer belt 32, the photosensitive drum 20 is charged to about +500 V, and the toner is attracted to the peripheral surface of the photosensitive drum 20 having a relatively low potential. It is done.

  FIG. 3 is a timing chart showing voltage application over time with respect to the transfer roller 31 in the first embodiment. As shown in this timing chart, in the transfer processing period b from when the leading edge of the paper P reaches the nip portion N until the trailing edge of the paper P passes through the nip portion N, the roller shaft A negative voltage is applied to 311. As a result, the surface side of the paper P is negatively charged (see FIG. 2A), so that the positively charged toner image on the surface of the photosensitive drum 20 is electrostatically adsorbed on the surface of the paper P. It has become. Further, the post-transfer potential on the peripheral surface of the photosensitive drum 20 at this time is affected by the application of a negative voltage to the transfer, and therefore the potential is lowered to about tens of volts or less.

  In the non-transfer processing period a in which the transfer process is not performed on the paper P (indicated as “paper gap” in FIG. 3), a positive voltage is applied from the high-voltage power supply 33 to the roller shaft 311. Yes. As a result, the surface side of the transfer belt 32 is positively charged (see (b) of FIG. 2), and thus the scattered toner T1 on the positively charged transfer belt 32 is caused by the potential difference between the transfer belt 32 and the photosensitive drum 20. Alternatively, it electrostatically adheres to the photosensitive drum 20 and is collected by the cleaning device 14. Further, the post-transfer potential on the peripheral surface of the photosensitive drum 20 at this time is affected by the application of a negative voltage to the transfer, so that the potential is raised to about several hundred volts.

  In the present embodiment, the voltage value applied from the high-voltage power source 33 to the roller shaft 311 of the transfer roller 31 immediately after the trailing edge of the paper P is removed from the nip portion N, that is, immediately after the transfer processing period b ends. Is switched from a negative voltage to a positive voltage, the voltage value is increased stepwise as shown by the non-transfer processing period a in FIG.

  The reason for this is that when the voltage is suddenly changed from a predetermined low voltage to a predetermined high voltage on the transfer roller 31, the voltage on the peripheral surface of the photosensitive drum 20 also changes rapidly following this. For this reason, untransferred toner on the peripheral surface of the photosensitive drum 20 is scattered, or sudden voltage fluctuations on the peripheral surface of the photosensitive drum 20 impact the separation claw 16, and the toner adhering to the separation claw 16. May scatter and fall and contaminate the surface of the paper P, but the impact applied by the stepwise increase in the voltage applied to the transfer roller 31 is reduced. This is because the toner adhering to 16 is prevented from scattering.

  FIG. 4 is a timing chart showing the voltage application to the transfer roller 31 in the second embodiment over time. In the second embodiment, as shown in FIG. 4, the high voltage power supply 33 applies the roller shaft 311 immediately before the leading edge of the paper P reaches the nip portion N, that is, immediately before the non-transfer processing period a ends. The voltage value is decreased stepwise from a preset positive voltage toward a preset negative voltage. The timing of voltage application is controlled so that the leading edge of the paper P reaches the nip portion N when the applied voltage to the roller shaft 311 reaches a predetermined negative voltage.

  According to the timing of switching between positive and negative of the applied voltage to the transfer roller 31 of the second embodiment, the transfer roller 31 is lowered stepwise from the predetermined positive voltage toward the predetermined negative voltage, so that a sudden voltage drop is avoided. The same effect as the first embodiment can be obtained.

  FIG. 5 is a timing chart showing the voltage application to the transfer roller 31 in the third embodiment over time. As shown in FIG. 5, the third embodiment is a combination of the first embodiment and the second embodiment, and immediately before the end of the non-transfer processing period a and immediately before the end of the transfer processing period b. In both cases, the voltage applied to the roller shaft 311 from the high-voltage power supply 33 is switched stepwise.

  According to the timing of switching between positive and negative of the voltage applied to the transfer roller 31 of the third embodiment, when the leading edge of the paper P reaches the nip portion N (that is, immediately before the non-transfer processing period a ends) and the paper P The change in the applied voltage to the transfer roller 31 becomes gradual both at the time when the rear end deviates from the nip portion N (that is, immediately before the transfer process period b ends), and from the non-transfer process period a to the transfer process period b. It is possible to prevent the scattered toner T1 from falling on the paper P both at the time of transfer and at the time of transfer from the transfer process period b to the non-transfer process period a.

  Next, voltage application control to the transfer roller 31 will be described with reference to FIG. 6 by taking the third embodiment (see FIG. 5) as an example. FIG. 6 is a block diagram illustrating an embodiment of voltage application control for the transfer roller 31. In order to perform voltage control on the transfer roller 31, a control unit 40 including a microcomputer is provided at an appropriate position of the image forming apparatus 10. The control unit 40 includes a central processing unit (CPU) 41 that is a central processing unit, a ROM 42 that is a read-only storage device for storing programs, invariant set numerical values, and the like, and temporary data that is generated during the calculation. A basic configuration including a RAM (Random Access Memory) 43 for storing various numerical values. The RAM 43 is provided with a timer 431 for measuring time.

  The CPU 41 includes a voltage switching determination unit 411 that determines voltage switching, and a control signal output unit 412 that outputs a control signal to the high-voltage power supply 33 based on the determination result of the voltage switching determination unit 411.

The voltage switching determination unit 411 determines the state of the paper P, such as whether or not the paper P has reached the nip N and whether the transfer process for the paper P has been completed. In order to make such a determination, light provided with, for example, a light emitting element and a light receiving element for detecting the leading edge and the trailing edge of the sheet P as shown in FIG. provided paper sensor 44 of sensor, detection signals the paper sensor 44 is detected, it is inputted one by one to the voltage switching threshold by 411.

  When the leading edge detection signal indicating that the leading edge of the paper P is detected is input from the paper sensor 44, the voltage switching determination unit 411 reaches the nip portion N based on the conveyance speed of the paper P. A time that is a preset time before the time to be calculated is calculated, and whether or not this time has arrived is determined based on time measurement information from the timer 431. Incidentally, the time required to reach the position slightly upstream (position on the upstream side) of the nip portion N after the leading edge of the paper P is detected by the paper sensor 44 is stored in the ROM 42.

  When the sheet P reaches a position slightly ahead of the preset nip portion N, the voltage switching determination unit 411 changes the predetermined voltage from the predetermined positive voltage to the predetermined negative voltage toward the control signal output unit 412. A command signal is output so as to output a control signal for dropping the voltage stepwise at a predetermined time pitch.

  Therefore, the control signal output unit 412 that has received this command signal outputs a control signal for stepping down the voltage toward the high-voltage power supply 33, whereby the transfer roller 31 has a potential as shown in FIG. It changes in steps from a predetermined positive voltage to a predetermined negative voltage.

  On the other hand, when the paper sensor 44 detects the rear edge of the paper P, the rear edge detection signal from the paper sensor 44 is input to the voltage switching determination unit 411. The voltage switching determination unit 411 to which the trailing edge detection signal is input calculates the time when the trailing edge of the paper P reaches the nip portion N based on the conveyance speed of the paper P stored in the ROM 42. Whether or not the time has come is discriminated based on the timing information from the timer 431. Incidentally, the time required to reach the position slightly upstream (position on the upstream side) of the nip portion N after the trailing edge of the sheet P is detected by the sheet sensor 44 is stored in the ROM 42.

  When the trailing edge of the paper P reaches the nip portion N, the voltage switching determination unit 411 directs the high voltage power source 33 toward the control signal output unit 412 for a predetermined time from a predetermined negative voltage to a predetermined positive voltage. A command signal is output so as to output a control signal for increasing the voltage stepwise at a pitch.

  Therefore, the control signal output unit 412 that has received this command signal outputs a control signal for increasing the voltage stepwise toward the high-voltage power supply 33, whereby the transfer roller 31 has a potential as shown in FIG. It changes in steps from a predetermined negative voltage to a predetermined positive voltage.

  As described above in detail, the image forming apparatus 10 according to the first embodiment of the present invention includes the photosensitive drum 20 on which the toner image is formed on the peripheral surface, and the photosensitive drum 20 that is rotatable about the axis. Transfer means 30 for transferring the toner image onto the paper P. The transfer means 30 is a nip that sandwiches the paper P with the peripheral surface of the photosensitive drum 20 via the transfer belt 32 in the nip portion N. A transfer roller 31 as a voltage application member for applying a voltage to the paper P in a state, and a high-voltage power supply 33 for supplying a voltage to the transfer roller 31. The high-voltage power supply 33 is a transfer roller when the paper P is in a nip state. While a voltage having a polarity opposite to that of the toner is applied to the toner 31, a voltage having the same polarity as that of the toner is applied to the transfer roller 31 when the paper P is not in the nip state, and from the moment when the paper P is released from the nip state. The voltage application polarity are controlled in order to stepwise change towards the applied voltage of the same polarity.

  According to such a configuration of the first embodiment, the toner image formed on the photosensitive drum 20 is conveyed and reaches the photosensitive drum 20, and the transfer belt 32 is formed by the photosensitive drum 20 and the transfer roller 31. The toner image is transferred onto the paper P in accordance with the rotation of the photosensitive drum 20 while being nipped through the sheet. During this transfer, the transfer roller 31 is applied with a voltage having a polarity opposite to that of the toner from the high-voltage power supply 33 when the paper P is in the nip state, so that the toner image carried on the photosensitive drum 20 is electrostatically attracted. As a result, the sheet P is peeled off toward the sheet P, whereby the transfer process for the sheet P is reliably performed.

  Further, when the transfer process to the paper P is completed and the paper P is no longer in the nip state, the photosensitive drum 20 has a slightly reverse polarity, and in this state, the same polarity as the toner is applied to the transfer roller 31 from the high voltage power source 33. Therefore, the toner scattered in the vicinity of the photoconductive drum 20 is attracted to the photoconductive drum 20 by the potential difference between the transfer belt 32 and the photoconductive drum 20 or electrostatically. It is possible to prevent inconveniences such as the inside being contaminated with the scattered toner or the scattered toner adhering to the paper P to cause a transfer failure.

  In particular, when switching the voltage application to the transfer roller 31 from the moment when the sheet P is removed from the nip state, the switching from the reverse polarity voltage application to the same polarity voltage application is performed in stages, so the switching is abrupt. When performed, it is possible to reliably prevent the occurrence of inconveniences such as toner scattering caused by the impact and transfer failure due to the scattered toner adhering to the paper P.

  The image forming apparatus 10 according to the second embodiment of the present invention includes the same photosensitive drum 20, transfer means 30, and high-voltage power supply 33 as those of the first embodiment. The voltage control by is different from the first embodiment. That is, the high voltage power supply 33 applies a voltage having the opposite polarity to the toner to the transfer roller 31 when the paper P is in the nip state, and applies a voltage having the same polarity as the toner to the transfer roller 31 when the paper P is not in the nip state. At the same time, immediately before the sheet P enters the nip state, the voltage application with the same polarity is controlled to change stepwise toward the voltage application with the opposite polarity.

  According to such a configuration of the second embodiment, it is possible to obtain the same effects as those of the first aspect with respect to voltage application to the transfer roller 31 during the transfer process and voltage application to the transfer roller 31 other than during the transfer process. .

  In particular, when switching the voltage application to the transfer roller 31 from the moment when the paper P reaches the nip portion N, the switching from the same polarity voltage application to the reverse polarity voltage application is performed stepwise, so that the switching is abrupt. The occurrence of inconveniences such as transfer failure due to the scattering of toner caused by the impact and the scattered toner adhering to the paper P can be reliably prevented.

  Furthermore, according to the image forming apparatus 10 of the third embodiment of the present invention, the voltage control of the high-voltage power supply 33 by the control unit 40 is a combination of the first embodiment and the second embodiment. Therefore, it is possible to reliably prevent inconveniences such as toner scattering due to an impact due to a sudden voltage fluctuation both when the paper P is removed from the nip portion N and when the paper P enters the nip portion N.

  In each of the above embodiments, since the transfer roller 31 is employed as the voltage application member, the sheet P that has reached the photosensitive drum 20 is transferred between the photosensitive drum 20 and the transfer roller 31. As a result, a voltage application state to the paper P can be made uniform and stable as compared with, for example, that by corona discharge.

  In each of the above embodiments, since the transfer belt 32 that conveys the paper P is interposed between the transfer roller 31 and the photosensitive drum 20, the paper P is conveyed by the transfer belt. A voltage from the transfer roller 31 is applied to the photosensitive drum 20 through the transfer belt in a stable conveyance state.

  The present invention is not limited to the above embodiment, and includes the following contents.

  (1) In the above embodiment, the transfer belt 32 is interposed between the photosensitive drum 20 and the transfer roller 31, and the voltage from the high-voltage power supply 33 is applied to the paper P conveyed by the transfer belt 32. Is applied to the sheet P via the transfer roller 31 and the transfer belt 32, but the sheet P is directly introduced between the transfer roller 31 and the photosensitive drum 20 without using the transfer belt 32 in particular. You may make it do.

  (2) In the above embodiment, the case where the transfer material is the paper P has been described as an example, but the present invention is not limited to the transfer material being the paper P, and the transfer roller 31 is not limited thereto. It may have a role as a transfer material. In this case, since the toner image formed on the peripheral surface of the photosensitive drum 20 is once transferred to the peripheral surface of the transfer belt 32, the toner image transferred to the peripheral surface of the transfer belt 32 is used. Further, a structure for transferring to the paper P may be adopted.

  (3) In the above embodiment, the transfer roller 31 is used to apply a voltage to the paper P that has reached the nip portion N via the transfer belt 32. However, in the present invention, the transfer roller 31 is used. However, the transfer roller 31 may be replaced with a brush, a blade, or the like.

  (4) In the above embodiment, the image forming apparatus 10 for so-called monotone printing in which only one photosensitive drum 20 is employed has been described as an example. However, in the present invention, the image forming apparatus 10 is monotone. It is not limited to the one for printing, and may be for so-called color printing in which a plurality of photosensitive drums 20 are arranged along the transfer belt 32 for each toner color. .

  (5) In the first embodiment, the voltage value applied to the transfer roller 31 is changed stepwise starting from the moment when the paper P leaves the nip portion N. The starting point of the stepwise change in application is not limited to the moment when the paper P leaves the nip N, and a predetermined time before and after the time when the paper P leaves the nip N depending on the situation. You may employ | adopt as a starting point of a voltage change.

  (6) In the second embodiment described above, the voltage value applied to the transfer roller 31 is changed stepwise starting just before the paper P enters the nip portion N. The starting point of the stepwise change in application is not limited to the time immediately before the paper P enters the nip portion N, and a predetermined time before and after the time immediately before the paper P enters the nip portion N depending on the situation. May be adopted as the starting point of the voltage change.

An example of stepwise voltage control that is actually employed will be described with reference to FIG. FIG. 7 is a graph showing a time-dependent change in applied voltage to the roller shaft 311 from the instant t ₀ when the paper P exits from the nip portion N, when moving from the forward bias application period to the reverse bias application period.

  In actual voltage control, as shown in FIG. 7, a current of −100 μA is supplied to the roller shaft 311 during the forward bias application period, while a voltage of 1.4 kV is applied to the roller shaft 311 during the reverse bias application period. It is to be applied. Between the forward bias application period (the transfer process period b (see FIGS. 3 to 5)) and the reverse bias application period (the non-transfer process period a (see FIGS. 3 to 5)), the roller shaft A conversion period in which the voltage applied to 311 is increased stepwise is set, and the voltage is increased stepwise during this conversion period.

  In the voltage control in this example, the voltage is indirectly controlled by controlling the current value until the potential during the forward bias application period and the conversion period becomes “0”, while the conversion period A method in which the voltage value itself is directly controlled until the reverse bias application period is reached after the medium potential is “0” is employed. Such control is adopted as a result of an effect confirmation test using an actual machine, as a result of indirectly measuring the voltage through the current value until the potential during the forward bias application period and the conversion period becomes “0”. This is because it has been confirmed that a reliable toner scattering prevention effect can be obtained by performing the control, although the reason is not clear.

Specifically, at the moment (time t ₀ ) when the trailing edge of the paper P comes out of the nip portion N, it enters the conversion period. First, the current value supplied to the roller shaft 311 by current control is from −100 μA. is reduced to -67Myuei, after this state is continued until time _{t 1, the} current of -33μA is supplied to the time _{t 2.} At time t ₂ , the supply of current is stopped, and thereby no voltage is applied to the transfer roller 31.

After the state where the voltage is not applied is continued until time t _3, turn the roller shaft 311 to time t ₄ the voltage of 467V is applied from the voltage control, the voltage of 933V is applied at time t _4, the time t _{When 5} is reached, a reverse bias application period is entered and a voltage of 1400 V is applied to the roller shaft 311.

The time between each time (t _i -t _i-1 (i = 1 to 5)) is set to 15 ms. Accordingly, the total time of the conversion period is 75 ms (15 ms × 5).

  By performing such voltage control, there is no toner smearing at a rate of about one out of several tens of sheets on the paper P after the transfer process at the nip N, and the present invention is excellent. It was proved to have an effect.

1 is an explanatory front sectional view for explaining an embodiment of an internal structure of an image forming apparatus according to the present invention. FIG. 4 is a partially enlarged explanatory view for explaining a charge formation state of each member in a nip portion, where (A) is a state when a forward bias is applied when a sheet exists in the nip portion, and (B) is a state where the sheet is a nip portion. The states at the time of reverse bias application not existing in FIG. 3 is a timing chart showing the voltage application to the transfer roller in the first embodiment over time. 10 is a timing chart showing the voltage application to the transfer roller in the second embodiment over time. 10 is a timing chart showing the voltage application to the transfer roller in the third embodiment over time. It is a block diagram which shows one Embodiment of the voltage application control with respect to a transfer roller. It is a graph when changing from the forward bias application period to the reverse bias application period, showing the change over time of the applied voltage to the roller shaft from the moment when the sheet comes out of the nip portion.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 11 Charging apparatus 12 Exposure apparatus 13 Developing apparatus 131 Developing apparatus main body 132 Developing roller 14 Cleaning apparatus 141 Cleaning apparatus main body 142 Blade 143 Collecting screw 15 Neutralizing apparatus 16 Separation claw 20 Photosensitive drum (image carrier)
21 Drum shaft 30 Transfer means 31 Transfer roller (voltage applying member)
311 Roller shaft 312 Coil spring 32 Transfer belt 321 Drive roller 33 High voltage power supply 40 Control unit 41 CPU
411 Voltage switching determination unit 412 Control signal output unit 42 ROM
43 RAM
431 Timer 44 Paper sensor N Nip part P Paper (transfer material)
T toner image T1 scattering toner

Claims (3)

An image forming apparatus comprising: an image carrier on which a toner image is formed on a peripheral surface; and a transfer unit that transfers a toner image of the image carrier rotating about an axis to a transfer material.
The transfer means includes a voltage applying member that applies a voltage to the transfer material in a nip state in which the transfer material is sandwiched between the peripheral surface of the image carrier, a power source that supplies the voltage to the voltage applying member, and a voltage from the power source. Voltage control means for controlling the polarity and voltage value of the voltage supplied to the application member,
The voltage control means includes
A forward bias application period in which a voltage having a polarity opposite to that of the toner is applied to the voltage application member when the transfer material is in the nip state, and a voltage having the same polarity as that of the toner is applied to the voltage application member when the transfer material is not in the nip state. A reverse bias application period and a conversion period for switching the voltage applied to the voltage application member from the reverse polarity to the same polarity in the vicinity of the transfer material leaving the nip state are set.
In the conversion period, the voltage value is changed stepwise, and voltage control is indirectly performed by controlling the current value during the period from the forward bias application period to the zero potential during the conversion period, An image forming apparatus , wherein voltage control is performed directly by controlling a voltage value during a period from the zero potential state during the conversion period to the reverse bias application period .
  The image forming apparatus according to claim 1, wherein the voltage application member is a transfer roller that applies a voltage to the transfer material in contact with or non-contact with the image carrier.   The image forming apparatus according to claim 1, wherein a transfer belt for conveying a transfer material is interposed between the transfer roller and the image carrier.

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