JP2017138568A - Fixation device and image formation apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent electrostatic offset by suppressing a charge amount accumulated in a heating member.SOLUTION: A fixation device 18 of a printer 1 includes: a heat roller 24 which has a core bar 24a and an insulation layer 24b arranged around the core bar 24a; a pressure roller 25 which is brought into pressure contact with the heat roller 24 and forms a fixation nip N between the heat roller 24 and itself; and a first function member 27 which is connected to a portion between the core bar 24a and the ground G and blocks the current flowing between the ground G and the heat roller 24 in the first direction where the charge in the reverse polarity with respect to the toner is accumulated in the insulation layer 24b when the potential difference between the core bar 24a and the surface of the insulation layer 24b in the heat roller 24 in passage of the transfer sheet does not reach the first threshold voltage.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a fixing device that melts and fixes unfixed toner on a transfer material onto the transfer material, and an image forming apparatus such as a copying machine, a printer, and a facsimile machine including the fixing device.

  2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus such as a printer or a copying machine includes a heating roller type or film heating type fixing device that fixes a toner image onto a transfer material such as paper. The heating roller type fixing device has a simple configuration and can realize high speed, and the film heating type fixing device can realize energy saving. The heating member and the pressure member provided in the fixing device have a surface material made of fluororesin or the like, thereby ensuring the releasability of the transfer material with respect to the heating member and the pressure member. Since the fluororesin is insulative and has a property that it is difficult to attenuate once charged, an electrostatic offset may occur when applied to a heating member or a pressure member. Therefore, when this insulating fluororesin is applied to a heating member or a pressure member, the heating member and the pressure member are grounded by grounding the heating member or the pressure member via a functional member such as a Zener diode. Some of them stabilize the potential difference. For example, when an image is formed on the transfer material with positively charged toner, a Zener diode is provided by connecting a cathode on the heating member side and an anode on the ground side in order to suppress negative charging of the heating member. .

  For example, in the fixing device disclosed in Patent Document 1, a resistor and a constant voltage diode are connected in series between a fixing roller (heating member) and a ground, and between the pressure roller (pressing member) and the ground. Also, a resistor and a constant voltage diode are connected in series. When the toner carried on the transfer material is charged with a negative charge, the constant voltage diode connected to the fixing roller is provided with the anode connected to the fixing roller, and is connected to the pressure roller. The constant voltage diode is provided with the cathode connected to the pressure roller side.

Japanese Patent Laid-Open No. 2001-100578

  As described above, since the insulating layer constituting the surface material of the heating member has a property that the charged electric charge is accumulated without losing, the cored bar and the insulating layer of the heating member may behave like a capacitor. . When using a positively charged toner, the transfer roller that transfers the toner image to the transfer material applies a negative charge to the transfer material, so that the negative charge is charged from the negatively charged transfer material to the insulating layer of the heating member. Is done. In particular, when the transfer material is water-containing paper, the resistance of the paper is low, so the negative charge applied to the heating member is large. Since the negative charge is no longer applied to the transfer material after the trailing edge of the transfer material has passed through the transfer roller, the electrostatic adsorption force between the toner and the transfer material is reduced. At this time, when the negative charge charged in the insulating layer of the heating member is discharged, the positively charged toner may adhere to the surface of the heating member and generate an electrostatic offset. Therefore, it is necessary to reduce the amount of charge accumulated in the heating member to prevent electrostatic offset.

  An object of the present invention is to prevent electrostatic offset by suppressing the amount of charge accumulated in a heating member.

  The fixing device according to the present invention includes a heating member having a cored bar and an insulating layer provided around the cored bar, and a pressure member that presses the heating member and forms a fixing nip between the heating member and the heating member. The toner is connected between the cored bar and the ground, and the potential difference between the cored bar and the surface of the insulating layer in the heating member passing through the transfer material does not reach the first threshold voltage. And a first functional member that cuts off a current flowing between the ground and the heating member in a first direction in which charges having a polarity opposite to that of the insulating layer are accumulated in the insulating layer.

  According to such a configuration, in the fixing device, by blocking the current flowing between the ground and the heating member during the sheet passing, the potential difference between the core metal and the surface of the insulating layer can be suppressed to a low level. The amount of negative or positive charge accumulated on the surface of the insulating layer is suppressed to suppress the charge amount of the heating member. Therefore, even when the supply of negative or positive transfer bias to the transfer material passing through the heating member is interrupted and the electrostatic adsorption force between the positive or negative toner and the transfer material, particularly water-containing paper, is reduced. Since the negative charge or positive discharge of the heating member can be suppressed, electrostatic offset can be prevented.

  The first functional member is a Zener diode. When positively charged toner is transferred to the transfer material, the anode of the Zener diode is connected to the core metal side, and the cathode of the Zener diode is When negatively charged toner is transferred to the transfer material connected to the ground side, the anode of the Zener diode is connected to the ground side, and the cathode of the Zener diode is connected to the core metal side. Good.

  By adopting such a configuration, the first functional member can be realized with a simple configuration.

  The first function member has the first threshold voltage under a condition in which a transfer bias having a polarity opposite to that of toner and an absolute value of 1 KV is applied to the transfer material before entering the fixing nip. The absolute value of the same polarity as that of the toner may be set to 450 V or more and 2 KV or less.

  By adopting such a configuration, the first functional member is configured to have a first threshold voltage that is sufficiently larger than the potential difference between the cored bar of the heating member during sheet passing and the surface of the insulating layer. In addition, when a sheet with a large amount of charge accumulated passes through the heating member, the first functional member is heated even when the negative or positive charge amount accumulated on the surface of the insulating layer suddenly increases. Leak discharge can be prevented by avoiding an excessive potential difference in the member.

  The fixing device described above is connected between the cored bar and the ground, and when the potential difference between the cored bar and the surface of the insulating layer in the heating member during sheet passing does not reach the second threshold voltage. A second functional member that cuts off a current that flows between the ground and the heating member in a second direction opposite to the first direction, and a connection between the cored bar and the ground is the first function. It is preferable to further include a switching unit that switches between connection via a member and connection via the second functional member.

  By adopting such a configuration, when it is easy to be charged by a large amount of negative charge or positive charge, the connection via the first functional member is used, while electrostatic by frictional charging between the sheet and the surface of the heating member is used. When there is a concern about the offset, connection via the second functional member can be used.

  The second functional member is constituted by a Zener diode, and when positively charged toner is transferred to the transfer material, the anode of the Zener diode is connected to the ground side, and the cathode of the Zener diode is When negatively charged toner is transferred to the core metal side and transferred to the core metal, the anode of the Zener diode is connected to the core metal side and the cathode of the Zener diode is connected to the ground side. Good.

  By adopting such a configuration, the second functional member can be realized with a simple configuration.

  The switch further includes a housing that houses the heating member and the pressure member, and the switching unit connects the first metal member to the ground when the external humidity of the housing is equal to or higher than a threshold humidity. On the other hand, when the external humidity is lower than the threshold humidity, the connection between the core metal and the ground may be switched to the connection via the second functional member.

  By adopting such a configuration, in the case of water-containing paper that is easily charged by a large amount of negative charge or positive charge due to low resistance, the connection via the first functional member is used while the transfer material having a low moisture content is used. In this case, there is concern about electrostatic offset due to frictional charging between the paper and the surface of the heating member, rather than a negative charge or a positive charge flowing into the surface of the insulating layer of the heating member. By using this, it is possible to configure so that the surface of the heating member is not charged with respect to the ground.

  The water content detection device further detects a water content state of the transfer material, and the switching unit is configured to connect the cored bar and the ground via the first functional member according to the water content state of the transfer material. Or you may switch to the connection via the said 2nd functional member.

  The moisture content detection device may be a moisture sensor that is provided at a position before entering the fixing nip in the transfer material conveyance path, and detects a moisture content as the moisture content of the transfer material. When the moisture content of the transfer material is equal to or higher than a predetermined moisture content threshold, the connection between the core metal and the ground is switched to the connection via the first functional member, while the moisture content of the transfer material is a predetermined moisture content. When the ratio is less than the threshold value, the connection between the metal core and the ground may be switched to the connection via the second functional member.

  By adopting such a configuration, it is possible to accurately determine whether or not the transfer paper is water-containing paper, and appropriately switch the connection between the core metal and the ground.

  A current detection unit that detects a current flowing between the ground and the heating member in the first direction; and the switching unit detects the current when the current detection unit detects a current. The connection between the metal core and the ground is switched to the connection via the second functional member when the current detection unit does not detect the current, while the connection is switched to the connection via the first functional member. Good.

  By adopting such a configuration, it is possible to reliably prevent unnecessary charge accumulation on the insulating layer of the heating member.

  The image forming apparatus of the present invention includes any one of the fixing devices described above.

  According to the present invention, it is possible to prevent electrostatic offset by suppressing the amount of charge accumulated in the heating member.

1 is a cross-sectional view illustrating a printer according to an embodiment of the present invention. 1 is a cross-sectional view illustrating a fixing device according to an embodiment of the present invention. 1 is a block diagram illustrating a printer control system according to an embodiment of the present invention. FIG. In a fixing device, a graph showing a relationship between a Zener voltage of a Zener diode between a core metal of a heating roller and a ground, a potential difference between the core metal of the heating roller and an insulating layer surface, and a current value flowing from the ground to the heating roller. It is. 5 is a table showing an example of a relationship between a Zener voltage of a Zener diode between a core metal of a heating roller and a ground and an electrostatic offset in the fixing device. It is sectional drawing which shows the fixing device which concerns on other embodiment of this invention. 6 is a table showing another example of the relationship between the Zener voltage of the Zener diode between the cored bar of the heating roller and the ground and the electrostatic offset in the fixing device. It is sectional drawing which shows the fixing device which concerns on other embodiment of this invention.

  First, the overall configuration of a printer 1 (image forming apparatus) according to an embodiment of the present invention will be described with reference to FIG. Hereinafter, for convenience of explanation, the front side of the sheet in FIG.

  The printer 1 includes a box-shaped printer main body 2. A paper feed cassette 3 for storing paper (transfer material) is accommodated in the lower part of the printer main body 2, and a paper discharge tray 4 is provided on the upper surface of the printer main body 2. An upper cover 5 is attached to the upper surface of the printer main body 2 so as to be openable and closable on the side of the paper discharge tray 4, and a toner container 6 is stored below the upper cover 5.

  An exposure unit 7 composed of a laser scanning unit (LSU) is disposed below the discharge tray 4 above the printer body 2, and an image forming unit 8 is provided below the exposure unit 7. The image forming unit 8 is rotatably provided with a photosensitive drum 10 as an image carrier. Around the photosensitive drum 10, a charger 11, a developing device 12, a transfer roller 13, and a cleaning device 14 are provided. Are arranged along the rotation direction of the photosensitive drum 10 (see arrow X in FIG. 1).

  A paper transport path 15 is provided inside the printer body 2. A paper feed unit 16 is provided at the upstream end of the conveyance path 15, and a transfer unit 17 configured by the photosensitive drum 10 and the transfer roller 13 is provided at a middle portion of the conveyance path 15. Is provided with a fixing device 18, and a paper discharge unit 20 is provided at the downstream end of the conveyance path 15. A reverse path 21 for double-sided printing is provided below the transport path 15. In this embodiment, the transfer unit 17 applies a negative bias from the print back side of the paper by the transfer roller 13 so that the positively charged toner is electrically transferred onto the paper when passing through the paper. Composed. Alternatively, in another embodiment, the transfer unit 17 applies a positive bias from the print back side of the sheet by the transfer roller 13 so that the negatively charged toner is electrically transferred onto the sheet when passing through the sheet. Configured.

  Next, an image forming operation of the printer 1 having such a configuration will be described.

  When the printer 1 is turned on, various parameters are initialized, and initial setting such as temperature setting of the fixing device 18 is executed. When image data is input from a computer or the like connected to the printer 1 and an instruction to start printing is given, an image forming operation is executed as follows.

  First, after the surface of the photosensitive drum 10 is charged by the charger 11, exposure corresponding to the image data is performed on the photosensitive drum 10 by laser light from the exposure device 7 (see the two-dot chain line P in FIG. 1). The electrostatic latent image is formed on the surface of the photosensitive drum 10. Next, the electrostatic latent image is developed by the developing device 12 into a toner image with toner.

  On the other hand, the sheet taken out from the sheet cassette 3 by the sheet feeding unit 16 is conveyed to the transfer unit 17 in synchronization with the above-described image forming operation, and the toner image on the photosensitive drum 10 is transferred to the sheet by the transfer unit 17. Is transcribed. The sheet on which the toner image has been transferred is conveyed downstream in the conveyance path 15 and enters the fixing device 18 where the toner image is fixed on the sheet. The paper on which the toner image is fixed is discharged from the paper discharge unit 20 to the paper discharge tray 4. The toner remaining on the photosensitive drum 10 is collected by the cleaning device 14.

  Next, the fixing device 18 will be described with reference to FIGS.

  2 and the like, the fixing device 18 includes a housing 23, a heating roller 24 (heating member), a pressure roller 25 (pressure member), a heater 26 (heat source), and a first functional member 27. And a second functional member 28 and a switching unit 30. In the present embodiment, the configuration in which the fixing device 18 includes the first functional member 27 and the second functional member 28 will be described. However, the fixing device 18 may be configured to include only the first functional member 27.

  The housing 23 is formed in a substantially box shape that is long in the front-rear direction, and includes a paper feed port 31 that is opened at a substantially vertical center of the right surface and a paper discharge port 32 that is opened at a substantially vertical center of the left surface. The conveyance path 15 passes through the housing 23 via the paper feed port 31 and the paper discharge port 32, and the paper introduced into the housing 23 via the paper feed port 31 passes through the paper discharge port 32. It is discharged from the housing 23. Further, in the housing 23, the heating roller 24 and the pressure roller 25 are respectively arranged on the upper side and the lower side with the conveyance path 15 interposed therebetween.

  The heating roller 24 is formed in a cylindrical shape that is long in the front-rear direction, and is rotatably supported in the housing 23. The rotation axis direction of the heating roller 24 is the front-rear direction.

  The heating roller 24 includes, for example, a cylindrical cored bar 24a and an insulating layer 24b provided around the cored bar 24a. The cored bar 24a is formed of a metal such as aluminum or iron. The cored bar 24 a is connected to the ground G via the first functional member 27 and the second functional member 28. The insulating layer 24b is made of a fluororesin such as PFA having an insulating property. In the present embodiment, negative charge is accumulated on the surface of the insulating layer 24b by contact with the transfer material in which negative charge is accumulated, and the insulating layer 24b is positive on the core metal 24a side with respect to the surface of the insulating layer 24b. Become sexual. Alternatively, in another embodiment, the surface of the insulating layer 24b accumulates positive charges by contact with a transfer material in which positive charges are accumulated, and the insulating layer 24b has a surface of the insulating layer 24b on the core metal 24a side. To the negative polarity side. Thereby, the heating roller 24 behaves like a capacitor that accumulates charges by having a potential difference between the core metal 24a and the surface of the insulating layer 24b. The insulating layer 24b may be made of only a fluororesin, but it is made of an insulating silicon rubber or the like, and is formed of an elastic layer provided around the metal core 24a and an insulating PFA or the like. The release layer may be composed of a fluororesin and cover the elastic layer.

  The pressure roller 25 is formed in a cylindrical shape that is long in the front-rear direction, and is rotatably supported in the housing 23. The rotation axis direction of the pressure roller 25 is the front-rear direction. The pressure roller 25 is pressed against the outer peripheral surface of the heating roller 24, so that a fixing nip N is formed between the heating roller 24 and the pressure roller 25. A driving gear (not shown) is attached to the rear end portion of the pressure roller 25. The pressure roller 25 is connected to a pressure roller drive source 43 (see FIG. 3) such as a motor via the drive gear, and is rotationally driven by the pressure roller drive source 43.

  The pressure roller 25 includes, for example, a cylindrical cored bar 25a, an elastic layer 25b provided around the cored bar 25a, and a release layer 25c that covers the elastic layer 25b. The cored bar 25 a of the pressure roller 25 is made of, for example, a metal such as SUS or aluminum, and the cored bar 25 a serves as a rotation axis of the pressure roller 25. The elastic layer 25b of the pressure roller 25 is formed of, for example, silicon rubber, and the release layer 25c of the pressure roller 25 is formed of, for example, a fluorine resin such as PFA.

  The heater 26 is a heat source configured by, for example, a halogen heater or a ceramic heater, and is provided in the internal space of the heating roller 24. The heater 26 is configured to generate heat by power supplied from a power source and to heat the heating roller 24.

  The first functional member 27 is connected between the cored bar 24a of the heating roller 24 and the ground G (grounding). The first functional member 27 accumulates charges having a polarity opposite to that of the toner in the insulating layer 24b when the potential difference between the core metal 24a of the heating roller 24 and the surface of the insulating layer 24b does not reach the first threshold voltage. The current flowing between the ground G and the heating roller 24 in the first direction is configured to be cut off. In the case of the present embodiment in which positively charged toner is transferred to the transfer material, the first functional member 27 allows a current from the heating roller 24 to the ground G, and at the same time a cored bar 24a of the heating roller 24 during paper passing. When the potential difference between the surface of the insulating layer 24b and the surface of the insulating layer 24b does not reach the first threshold voltage, the current from the ground G to the heating roller 24 is cut off, while the cored bar 24a of the heating roller 24 and the surface of the insulating layer 24b during paper passing Is configured to allow a current from the ground G to the heating roller 24 when the potential difference between the first and second voltages is equal to or higher than the first threshold voltage. Alternatively, in another embodiment in which negatively charged toner is transferred to a transfer material, the first functional member 27 allows current from the ground G to the heating roller 24 and also allows the heating roller 24 to pass through the sheet. When the potential difference between the cored bar 24a and the surface of the insulating layer 24b does not reach the first threshold voltage, the current from the heating roller 24 to the ground G is interrupted, while the cored bar 24a of the heating roller 24 and the insulating layer during paper passing The electric current from the heating roller 24 to the ground G is allowed when the potential difference from the surface of 24b is equal to or higher than the first threshold voltage.

  The first threshold voltage of the first functional member 27 is, for example, a condition in which a transfer bias having a reverse polarity of the toner and an absolute value of 1 KV is applied to the paper before entering the fixing nip N by the transfer roller 13. Therefore, the absolute value is set to 450 V or more and 2 KV or less with the same polarity as the toner. The first functional member 27 is constituted by, for example, a Zener diode having the first threshold voltage as a Zener voltage. In the present embodiment in which positively charged toner is transferred to a transfer material, the anode A is a cored bar 24a. And the cathode K is connected to the ground G side. Alternatively, the first functional member 27 has an anode A connected to the ground G side and a cathode K connected to the cored bar 24a side in other embodiments in which negatively charged toner is transferred to the transfer material. The

  The second functional member 28 is connected between the cored bar 24a of the heating roller 24 and the ground G (grounding). The second functional member 28 has a second direction opposite to the first direction when the potential difference between the cored bar 24a of the heating roller 24 and the surface of the insulating layer 24b does not reach the second threshold voltage. In FIG. 5, the current flowing between the ground G and the heating roller 24 is cut off. In the case of this embodiment in which positively charged toner is transferred to the transfer material, the second functional member 28 allows current from the ground G to the heating roller 24 and at the same time a cored bar 24a of the heating roller 24 during paper passing. And when the potential difference between the surface of the insulating layer 24b and the surface of the insulating layer 24b does not reach the second threshold voltage, the current from the heating roller 24 to the ground G is interrupted, while Is configured to allow the current from the heating roller 24 to the ground G when the potential difference between and the second threshold voltage is equal to or higher than the second threshold voltage. Alternatively, in another embodiment in which negatively charged toner is transferred to the transfer material, the second functional member 28 allows current from the heating roller 24 to the ground G and also allows the heating roller 24 to pass through the sheet. When the potential difference between the cored bar 24a and the surface of the insulating layer 24b does not reach the second threshold voltage, the current from the ground G to the heating roller 24 is cut off, while the cored bar 24a of the heating roller 24 and the insulating layer are passing through the sheet. When the potential difference from the surface of 24b is equal to or higher than the second threshold voltage, the current from the ground G to the heating roller 24 is allowed.

  The second threshold voltage of the second functional member 28 is, for example, a condition in which a transfer bias having a reverse polarity of toner and an absolute value of 1 KV is applied to the paper before entering the fixing nip N by the transfer roller 13. Therefore, the absolute value is set to 450 V or more and 2 KV or less with the same polarity as the toner. The second functional member 28 is constituted by, for example, a Zener diode having the second threshold voltage as a Zener voltage. In the present embodiment in which positively charged toner is transferred to a transfer material, the anode A is on the ground G side. And the cathode K is connected to the cored bar 24a side. Alternatively, in another embodiment in which negatively charged toner is transferred to the transfer material, the second functional member 28 has the anode A connected to the cored bar 24a and the cathode K connected to the ground G side. The

  The switching unit 30 includes a switching element such as a relay, and is provided closer to the core metal 24 a of the heating roller 24 than the first functional member 27 and the second functional member 28. The switching unit 30 switches the connection between the cored bar 24 a and the ground G between a connection via the first functional member 27 and a connection via the second functional member 28 according to a predetermined switching condition. For example, when the external humidity of the housing 23 is equal to or higher than a predetermined threshold humidity, the switching unit 30 switches the connection between the metal core 24a and the ground G to the connection via the first functional member 27, while the external humidity is a predetermined threshold value. When the humidity is lower, the connection between the cored bar 24 a and the ground G is switched to the connection via the second functional member 28. The threshold humidity is set to 80%, for example, considering that the external humidity of the housing 23 at the normal time is about 60%.

  Next, the control system of the printer 1 will be described with reference to FIG.

  The printer 1 is provided with a control unit 40 composed of a CPU or the like. The control unit 40 is connected to a storage unit 41 configured by a storage device such as a ROM and a RAM, and the control unit 40 controls each unit of the printer 1 based on a control program and control data stored in the storage unit 41. Is configured to control.

  The control unit 40 is connected to a transfer bias power source 42 that supplies a transfer bias to the transfer roller 13, and the transfer bias power source 42 supplies a transfer bias to the transfer roller 13 based on a signal from the control unit 40.

  The control unit 40 is connected to a pressure roller drive source 43 such as a motor that drives the pressure roller 25 of the fixing device 18, and the pressure roller drive source 43 is added based on a signal from the control unit 40. The pressure roller 25 is rotated. When the pressure roller 25 rotates in this way, the heating roller 24 that is in pressure contact with the pressure roller 25 is driven to rotate in the opposite direction to the pressure roller 25.

  The control unit 40 is connected to the heater 26 of the fixing device 18. The heater 26 generates heat when electric power is supplied to the heater 26 based on a signal from the control unit 40.

  The control unit 40 is connected to the humidity detection unit 44. The humidity detector 44 is provided outside the housing 23 of the fixing device 18 and detects the in-machine humidity of the printer body 2 (external humidity of the housing 23).

  The control unit 40 is connected to the switching unit 30 of the fixing device 18. Then, the control unit 40 controls the switching unit 30 as described above based on the external humidity of the housing 23 detected by the humidity detection unit 44.

  Next, in the present embodiment in which positively charged toner is transferred to the transfer material, from the first threshold voltage of the first functional member 27, the potential difference between the core metal 24a of the heating roller 24 and the surface of the insulating layer 24b, and the ground G A relationship with the value of the current flowing to the heating roller 24 will be described with reference to FIG.

  In FIG. 4, when the core metal 24a of the heating roller 24 and the ground G are directly connected without using the first functional member 27 (GND), a zener voltage set to 150V, 300V, 450V, and 600V (first The potential difference between the cored bar 24a of the heating roller 24 and the surface of the insulating layer 24b and the current value flowing from the ground G to the heating roller 24 detected when using a Zener diode (first functional member 27) having a threshold voltage). Indicates. This potential difference is obtained by measuring the potential between the cored bar 24a of the heating roller 24 and the surface of the insulating layer 24b while passing the paper under the condition that the transfer bias supplied to the transfer roller 13 is set to -1 KV. The charge amount of the heating roller 24 can be calculated based on this potential difference. As for this current value, the current value flowing from the ground G to the heating roller 24 was measured with an ammeter simultaneously with the above potential measurement. Since the negative charge is easily accumulated on the surface of the insulating layer 24b by the paper in which the negative charge is accumulated during the sheet passing, the more the current flows from the ground G to the heating roller 24, the more the insulating layer 24b Negative charges accumulate on the surface, and the charge amount of the heating roller 24 tends to increase. Thus, there is a correlation between the charge amount of the heating roller 24 and the current value, and the charge amount increases as the current flows. In the case of other embodiments in which negatively charged toner is transferred to a transfer material, the polarity of charge and the direction of current flow are opposite to those in the above embodiment, but in the same way, the charge amount of the heating roller 24 There is a correlation with the current value.

  As shown in FIG. 4, when the first functional member 27 is not used (GND) or when the first functional member 27 having the first threshold voltage of 150 V or 300 V is used, it is insulated from the core metal 24 a of the heating roller 24. Since the first threshold voltage of the first functional member 27 is not set sufficiently large with respect to the potential difference from the surface of the layer 24b, a current flows from the ground G to the heating roller 24 and the charge amount of the heating roller 24 increases. . However, when the first functional member 27 having the first threshold voltage of 450V or 600V is used, the first threshold value of the first functional member 27 with respect to the potential difference between the core metal 24a of the heating roller 24 and the surface of the insulating layer 24b. Since the voltage is set sufficiently high, it can be seen that no current flows through the heating roller 24 and the potential difference between the cored bar 24a of the heating roller 24 and the surface of the insulating layer 24b is suppressed to about -45V. In the case of other embodiments in which negatively charged toner is transferred to a transfer material, the polarity of charge and the direction of current flow are opposite to those in the above embodiment, but in the same manner, the first threshold voltage having the first threshold voltage is used. The one functional member 27 can suppress a potential difference between the cored bar 24a of the heating roller 24 and the surface of the insulating layer 24b.

  Next, the relationship between the first threshold voltage of the first functional member 27 and the electrostatic offset in this embodiment in which the positively charged toner is transferred onto the transfer material will be described with reference to FIG.

  In FIG. 5, the first threshold value set to 150 V, 300 V, 450 V, 600 V, and 900 V when the core metal 24 a of the heating roller 24 and the ground G are directly connected without using the first functional member 27 (GND). The state of the electrostatic offset which arises in the case where the Zener diode as the 1st functional member 27 which has a voltage is used is shown. Here, a sheet of HAMMERMILL Tidal MP with a moisture content of 9% is used under the condition that the transfer bias supplied to the transfer roller 13 is set to −1 KV. The first functional member 27 of the Zener diode is provided with the anode A connected to the cored bar 24a and the cathode K connected to the ground G side.

  As shown in FIG. 5, it is understood that no electrostatic offset occurs when the first functional member 27 having the first threshold voltage of 450 V or higher is used. In the case of other embodiments in which negatively charged toner is transferred to the transfer material, the electrostatic offset can be similarly suppressed by the first functional member 27 having the first threshold voltage.

  According to the present embodiment, as described above, the fixing device 18 of the printer 1 (image forming apparatus) includes the cored bar 24a and the insulating layer 24b provided around the cored bar 24a, and has a negative polarity or a positive polarity. A negative or positive charge is accumulated on the surface of the insulating layer 24a by contact with the transfer material in which the charges are accumulated, whereby a heating roller 24 (heating member) that accumulates charges in the insulating layer 24b, and a heating roller 24 Is connected between the pressure roller 25 (pressure member) that forms a fixing nip N with the heating roller 24 and the cored bar 24a and the ground G, and the transfer paper is passed through the sheet. When the potential difference between the cored bar 24a and the surface of the insulating layer 24b in the heating roller 24 does not reach the first threshold voltage, heating with the ground G in the first direction in which charges having a polarity opposite to that of the toner are accumulated in the insulating layer 24b. roller It comprises a first functional component 27 to cut off the current flowing between the 4.

  Accordingly, by blocking the current flowing between the ground G and the heating roller 24 during the sheet passing, the potential difference between the cored bar 24a and the surface of the insulating layer 24b can be kept low, and the insulating layer 24b is separated from the paper. The charge amount of the heating roller 24 is suppressed by suppressing the negative charge amount or the positive charge amount accumulated on the surface. Therefore, when supply of the negative or positive transfer bias to the transfer material passing through the heating roller 24 is interrupted, the electrostatic adsorption force between the positive or negative toner and the transfer material, particularly water-containing paper, is reduced. However, since the negative charge or positive discharge of the heating roller 24 can be suppressed, electrostatic offset can be prevented.

  Further, according to the present embodiment, the first functional member 27 is constituted by a Zener diode. When the positively charged toner is transferred to the transfer material, the first functional member 27 has the Zener diode anode A connected to the cored bar 24a side and the Zener diode cathode K connected to the ground G side. Is done. Alternatively, when negatively charged toner is transferred to the transfer material, the first functional member 27 has the Zener diode anode A connected to the ground G side and the Zener diode cathode K connected to the cored bar 24a side. Is done.

  Thereby, the 1st functional member 27 is realizable with a simple structure.

  Further, according to the present embodiment, the first functional member 27 is subjected to a condition in which a transfer bias having a polarity opposite to that of the toner and having an absolute value of 1 KV is applied to the transfer material before entering the fixing nip N. The first threshold voltage is set to the same polarity as the toner and the absolute value is 450 V or more and 2 KV or less.

  Accordingly, the first functional member 27 is configured to have a first threshold voltage sufficiently larger than the potential difference between the core metal 24a of the heating roller 24 and the surface of the insulating layer 24b during paper passing. In addition, even when a sheet of a large amount of charge accumulated passes through the heating roller 24, the first functional member 27 even when the negative or positive charge amount accumulated on the surface of the insulating layer 24b suddenly increases. Can avoid the occurrence of an excessive potential difference in the heating roller 24 and prevent leakage discharge.

  Further, according to the present embodiment, the fixing device 18 is connected between the cored bar 24a and the ground G, and the potential difference between the cored bar 24a of the heating roller 24 and the surface of the insulating layer 24b during the sheet passing is also increased. When the second threshold voltage is not reached, the second functional member 28 that blocks the current flowing between the ground G and the heating roller 24 in the second direction opposite to the first direction, the cored bar 24a, and the ground G And a switching unit 30 that switches the connection between the first functional member 27 and the second functional member 28.

  Thereby, when it is easy to be charged by a large amount of negative charge or positive charge, while using the connection via the first functional member 27, there is a concern about electrostatic offset due to frictional charging between the paper and the surface of the heating roller 24. In some cases, connection via the second functional member 28 can be used.

  Further, according to the present embodiment, the second functional member 28 is constituted by a Zener diode. When the positively charged toner is transferred to the transfer material, the second functional member 28 has the Zener diode anode A connected to the ground G side and the Zener diode cathode K connected to the cored bar 24a side. Is done. Alternatively, when negatively charged toner is transferred to the transfer material, the second functional member 28 has the Zener diode anode A connected to the cored bar 24a side and the Zener diode cathode K connected to the ground G side. Is done.

  Thereby, the 2nd functional member 28 is realizable with a simple structure.

  In addition, according to the present embodiment, the fixing device 18 further includes the housing 23 that houses the heating roller 24 and the pressure roller 25. When the external humidity of the housing 23 is equal to or higher than the threshold humidity, the switching unit 30 switches the connection between the cored bar 24a and the ground G to the connection via the first functional member 27, while the external humidity is less than the threshold humidity. In addition, the connection between the cored bar 24 a and the ground G is switched to the connection via the second functional member 28.

  Thereby, the connection via the first functional member 27 is used in the case of water-containing paper that is easily charged by a large amount of negative charge or positive charge due to low resistance, while in the case of a transfer material having a low water content, a heating roller is used. Since there is a concern about electrostatic offset due to frictional charging between the sheet and the surface of the heating roller 24 instead of the negative charge or the positive charge flowing into the surface of the insulating layer 24b, the connection via the second functional member 28 is used. By doing so, the surface of the heating roller 24 can be configured not to be charged with respect to the ground G.

  In the above-described embodiment, the example of determining whether the external humidity of the housing 23 is equal to or higher than the threshold humidity has been described as the switching condition of the switching unit 30, but the switching condition of the switching unit 30 is not limited to this example. For example, in still another embodiment, the fixing device 18 may be configured to further include a moisture content detection device that detects the moisture content of the transfer material. The control unit 40 is connected to the water content detection device and controls the switching unit 30 based on the detection result of the water content detection device. The switching unit 30 switches the connection between the cored bar 24a and the ground G to the connection via the first functional member 27 or the connection via the second functional member 28 according to the moisture content of the transfer material.

  Specifically, as shown in FIG. 6, the moisture content detection device is provided at a position before entering the fixing nip N in the transfer material conveyance path 15 and detects moisture content as the moisture content of the transfer material. A sensor 45 is provided. The switching unit 30 switches the connection between the cored bar 24a and the ground G to the connection via the first functional member 27 when the moisture content of the transfer material is equal to or higher than a predetermined moisture content threshold (for example, 5%). When the moisture content of the transfer material is less than the predetermined moisture content threshold, the connection between the cored bar 24a and the ground G is switched to the connection via the second functional member 28. The moisture sensor 45 may be configured to, for example, pass a current through the transfer material and detect a current flowing through the transfer material, and calculate the moisture content of the transfer material based on the detection result. A sensor having another configuration capable of detecting the rate may be used.

  Thereby, it is possible to accurately determine whether the transfer paper is water-containing paper, and to appropriately switch the connection between the cored bar 24a and the ground G.

  For example, when a sheet having a relatively high moisture content of 9% is used, as described with reference to FIG. 5, the first functional member 27 of the Zener diode having the first threshold voltage of 450 V or more is used. When the anode A is connected to the core 24a side and the cathode K is connected to the ground G side, no electrostatic offset occurs.

  On the other hand, the relationship between the first threshold voltage of the first functional member 27 and the electrostatic offset when using a sheet having a moisture content of 5% will be described with reference to FIG.

  In FIG. 7, the first threshold value set to 150 V, 300 V, 450 V, 600 V, and 900 V when the core bar 24 a of the heating roller 24 and the ground G are directly connected without using the first functional member 27 (GND). The state of the electrostatic offset which arises in the case where the Zener diode as the 1st functional member 27 which has a voltage is used is shown. Here, a paper with a moisture content of 5% of Ledger size of HAMMERMILL Tidal MP is used under the condition that the transfer bias supplied to the transfer roller 13 is set to −1 KV. The first functional member 27 of the Zener diode is provided with the anode A connected to the ground G side and the cathode K connected to the core metal 24a side.

  As shown in FIG. 7, when a sheet having a relatively low moisture content of 5% is used, electrostatic offset occurs when the first functional member 27 having the first threshold voltage of 150 V or more is used. I understand that I don't. In the case of a sheet with a low water content, there is a concern about electrostatic offset due to frictional charging between the sheet and the surface of the heating roller 24 rather than a negative charge or a positive charge flowing into the insulating layer 24 b of the heating roller 24. Therefore, when the moisture content of the transfer material is less than a predetermined moisture content threshold value, the connection between the metal core 24a and the ground G is second in order not to charge the surface of the heating roller 24 with respect to the ground G. The connection is switched through the functional member 28, that is, the cathode K is connected to the core 24a side and the anode A is connected to the ground G side.

  Alternatively, in still another embodiment, as illustrated in FIG. 8, the fixing device 18 further includes a current detection unit 46 that detects a current flowing between the ground G and the heating roller 24 in the first direction. You may comprise. The control unit 40 is connected to the current detection unit 46 and controls the switching unit 30 based on the detection result by the current detection unit 46. The switching unit 30 switches the connection between the cored bar 24a and the ground G to the connection via the first functional member 27 when the current detection unit 46 detects a current, while the current detection unit 46 does not detect the current. The connection between the cored bar 24a and the ground G is switched to the connection via the second functional member G. Note that the switching unit 30 may determine whether or not the current detection unit 46 has detected a current in the first direction that is equal to or greater than a predetermined current threshold as a predetermined switching condition.

  Thereby, it is possible to reliably prevent accumulation of unnecessary charges on the insulating layer 24b of the heating roller 24.

  In the present embodiment, the case where the heating roller 24 is used as a heating member has been described. However, in another different embodiment, a fixing belt may be used as a heating member. In the present embodiment, the case where the pressure roller 25 is used as the pressure member has been described. However, in another different embodiment, a pressure belt may be used as the pressure member.

  In this embodiment, the case where the configuration of the present invention is applied to the printer 1 has been described. However, in another different embodiment, the configuration of the present invention is applied to other image forming apparatuses such as a copying machine, a facsimile machine, and a multifunction machine. It is also possible.

1 Printer (image forming device)
8 Image forming unit 15 Conveyance path 18 Fixing device 23 Housing 24 Heating roller (heating member)
25 Pressure roller (pressure member)
26 Heater (heat source)
27 First functional member 28 Second functional member 30 Switching unit 40 Control unit 41 Storage unit 42 Transfer bias power source 43 Pressure roller drive source 44 Humidity detection unit 45 Moisture sensor 46 Current detection unit

Claims (10)

A heating member having a cored bar and an insulating layer provided around the cored bar;
A pressure member that presses against the heating member to form a fixing nip with the heating member;
When the potential difference between the core metal and the surface of the insulating layer in the heating member passing through the transfer material is not reached the first threshold voltage while being connected between the core metal and the ground, the toner and A fixing device comprising: a first functional member that cuts off a current that flows between the ground and the heating member in a first direction in which charges of opposite polarity are accumulated in the insulating layer.   The first functional member is a Zener diode. When positively charged toner is transferred to the transfer material, the anode of the Zener diode is connected to the core metal side, and the cathode of the Zener diode is When negatively charged toner is transferred to the transfer material connected to the ground side, the anode of the Zener diode is connected to the ground side, and the cathode of the Zener diode is connected to the core metal side. The fixing device according to claim 1.   The first function member has the first threshold voltage under a condition in which a transfer bias having a polarity opposite to that of toner and an absolute value of 1 KV is applied to the transfer material before entering the fixing nip. The fixing device according to claim 1, wherein the fixing device has the same polarity as the toner and an absolute value of 450 V or more and 2 KV or less. When the potential difference between the core metal and the surface of the insulating layer in the heating member during paper passing does not reach the second threshold voltage while being connected between the core metal and the ground, the first direction and Is a second functional member that cuts off a current flowing between the ground and the heating member in the second direction opposite to the first direction;
The switching part which switches the connection between the metal core and the ground to the connection via the first functional member and the connection via the second functional member, further comprising: 4. The fixing device according to any one of 3 above.   The second functional member is constituted by a Zener diode, and when positively charged toner is transferred to the transfer material, the anode of the Zener diode is connected to the ground side, and the cathode of the Zener diode is When negatively charged toner is transferred to the core metal side and transferred to the core metal, the anode of the Zener diode is connected to the core metal side and the cathode of the Zener diode is connected to the ground side. The fixing device according to claim 4. A housing for accommodating the heating member and the pressure member;
When the external humidity of the housing is equal to or higher than a threshold humidity, the switching unit switches the connection between the metal core and the ground to a connection via the first functional member, while the external humidity is less than the threshold humidity. The fixing device according to claim 4, wherein the connection between the metal core and the ground is switched to a connection via the second functional member. A water content detection device for detecting the water content of the transfer material;
The switching unit switches a connection between the cored bar and the ground to a connection via the first functional member or a connection via the second functional member according to a water content state of the transfer material. The fixing device according to claim 4 or 5. The moisture content detection device is a moisture sensor that is provided at a position before entering the fixing nip in the transfer material conveyance path and detects the moisture content as the moisture content of the transfer material,
The switching unit switches the connection between the cored bar and the ground to the connection through the first functional member when the moisture content of the transfer material is equal to or higher than a predetermined moisture content threshold, while the moisture content of the transfer material 8. The fixing device according to claim 7, wherein the connection between the cored bar and the ground is switched to a connection through the second functional member when the water content is less than a predetermined moisture content threshold value. A current detection unit for detecting a current flowing between the ground and the heating member in the first direction;
The switching unit switches the connection between the metal core and the ground to the connection through the first functional member when the current detection unit detects a current, while the current detection unit does not detect a current. The fixing device according to claim 4, wherein the connection between the cored bar and the ground is switched to a connection through the second functional member.   An image forming apparatus comprising the fixing device according to claim 1.

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