JP2010164636A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the breakdown of an insulating layer of a heater in contact with a fixing film without upsizing an apparatus. <P>SOLUTION: In an image forming apparatus, a capacitor 107 is connected between a pressure roller 104 and a frame ground 108. Thus, when excessive voltage noise is applied, an applied voltage is divided by the ratio of the capacity of the insulating layer of the heater, the capacity of the pressure roller 104 and the capacity of the capacitor, to suppress a voltage applied to both ends of each of the insulating layer and the pressure roller 104 to a dielectric strength voltage or below. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an image forming apparatus equipped with a heat fixing unit that heat-fixes a toner image formed on a recording material using a fixing film.

  As a fixing device mounted on an image forming apparatus such as an electrophotographic copying machine or a printer, a fixing device using a fixing film has been proposed. Hereinafter, a fixing device using a fixing film will be described with reference to FIG.

  Heater 101, heater glass 102, fixing film 103, pressure member (hereinafter referred to as pressure roller) 104, pressure roller shaft 105, conductive metal member (hereinafter referred to as frame ground) 108 connected to electrical ground , A resistor 106 connected between the pressure member and the frame ground, an AC power source 110, and a pressure roller bearing 111.

  In this fixing device, a heat-resistant film (fixing film) of a heating rotator as a heating member is brought into close contact with a heating element by a pressure roller, and the fixing film is slid and rotated to the heating element by a pressure roller to fix A recording paper carrying a toner image is introduced into a press nip (fixing nip) formed by a film and a pressure roller, and the recording paper is conveyed together with the fixing film and applied through the fixing film. The toner image is fixed as a permanent image on the recording paper by the heat from the heating element and the pressure applied at the fixing nip.

  The pressure roller 104 is formed by providing a heat-resistant and conductive rubber layer made of, for example, silicone sponge on the cored bar 105 and covering the surface with a release layer made of a PFA tube.

  For the fixing film 103, a release layer made of a fluororesin such as PTFE, PFA, FEP, etc. is formed on the surface of a base film such as polyimide, polyamideimide, PEEK, PES, etc. with a conductive primer layer as a coating or tube The composite layer film is made up of.

The resistor 106 connected between the pressure member and the frame ground is used to prevent dielectric breakdown of the insulating layer due to excessive voltage noise from the commercial power supply line and to reduce the noise level radiated from the power supply line. ing. When excessive voltage noise is applied to the power supply line, the voltage applied to the heater glass 102 can be lowered below the applied voltage, thereby preventing dielectric breakdown of the heater glass. Such a configuration is disclosed in Patent Document 1.
JP-A-6-51659

  However, in the above example, when the creepage and space distance between the pressure roller and the frame ground are not sufficient, high voltage due to excessive voltage noise such as lightning surge is applied to the pressure roller, and the pressure roller to the frame ground. There is a possibility of discharging. This discharge causes the pressure roller to have the same potential as the frame ground, and a high voltage due to excessive voltage noise such as lightning surge is applied to both ends of the insulating layer covering the heater and the insulating layer of the pressure roller. There is a possibility of dielectric breakdown. In order to avoid this dielectric breakdown, it is necessary to provide a sufficient creepage / space distance between the pressure roller and the frame ground, and this creepage / space distance has led to an increase in the size of the apparatus.

  Therefore, the present invention has been made to solve the above-described problems, and an object thereof is to realize a film heating type fixing device that achieves both the miniaturization of the device and the prevention of dielectric breakdown of the insulating layer.

  In order to solve the above problems, the present invention provides an image forming unit that forms a toner image on a recording material, a heater that generates heat when energized, an insulating layer that covers the resistor, and the insulating layer of the heater. A fixing unit comprising: a conductive fixing film that is in contact; and a pressure member that forms a fixing nip portion that sandwiches and conveys a recording material that carries a toner image together with the heater through the fixing film; An image forming apparatus having a metal member connected to ground, wherein a capacitor is connected between the pressure member and the metal member.

  According to the present invention, when an excessive voltage noise is applied by connecting a capacitor between the pressure member and the metal member, the applied voltage is determined by the capacitance of the insulating layer of the heater and the capacitance of the pressure member. The voltage is divided by the capacitance ratio of the capacitor, and the voltage applied to both ends of each of the insulating layer and the pressure member is suppressed to the withstand voltage or less. Therefore, each dielectric breakdown can be avoided. In addition, since the potential applied to the pressure member is lowered, the creepage / space distance for preventing the metal member from being discharged can be shortened, and the apparatus can be miniaturized.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 shows the configuration of an image forming apparatus equipped with a fixing device (fixing unit). Only one sheet of recording paper loaded on the paper feed cassette 201 is sent out from the paper feed cassette 201 by the pickup roller 202 and conveyed toward the registration roller 204 by the paper feed roller 203. Further, the recording paper is conveyed to the process cartridge 205 by the registration roller 204 at a predetermined timing. The process cartridge 205 is integrally composed of a charging unit 206, a developing roller 207 as a developing unit, a cleaner 208 as a cleaning unit, and a photosensitive drum 209 as an electrophotographic photosensitive member, and is a known electrophotographic process. Through this series of processes, an unfixed toner image is formed on the recording paper.

  The surface of the photosensitive drum 209 is uniformly charged by the charging unit 206, and then image exposure based on the image signal is performed by the scanner unit 211 which is an image exposure unit. Laser light emitted from the laser diode 212 in the scanner unit 211 passes through the rotating polygon mirror 213 and the reflecting mirror 214 in the main scanning direction, and is scanned in the sub scanning direction by the rotation of the photosensitive drum 209. A two-dimensional latent image is formed on the surface. The latent image on the photosensitive drum 209 is visualized as a toner image by the developing roller 207, and the toner image is transferred by the transfer roller 210 onto the recording paper conveyed from the registration roller 204. A portion where a toner image is formed on such a recording material is referred to as an image forming portion.

  Subsequently, when the recording paper to which the toner image has been transferred is conveyed to the fixing device 215, the recording paper is heated and pressurized, and the unfixed toner image on the recording paper is fixed to the recording paper. The recording paper is further discharged out of the image forming apparatus main body by the intermediate paper discharge roller 216 and paper discharge roller 217, and a series of printing operations is completed.

  FIG. 2 is a sectional view of the fixing device. 101 is a heater on which a resistor and heater glass (insulating layer) 102 covering the resistor are formed on a ceramic substrate, 103 is a fixing sleeve (conductive fixing film), and 104 is a conductive core (shaft). Reference numeral 105 denotes a pressure roller having a rubber layer, 107 denotes a capacitor, 108 denotes a frame ground (metal member), 110 denotes an AC power source, 111 denotes a pressure roller bearing, and 106 denotes a resistor that reinforces insulation. The heater 101 is in contact with the inner surface of the fixing film 103, and the pressure roller 104 forms a fixing nip portion that sandwiches and conveys a recording material carrying a toner image together with the heater 101 via the fixing film 103. The insulating layer 102 of the heater 101 is in contact with the inner surface of the fixing film 103. When the resistor of the heater 101 is energized (power is supplied), the resistor generates heat. The capacitor 107 is disposed in the fixing unit. The frame ground 108 is connected to an electrical ground (earth) of the image forming apparatus.

  In this embodiment, the pressure roller shaft, which is the shaft of the pressure member, is connected to the frame ground via a capacitor. The capacitor is stored in a pressure member bearing 111 made of a mold member, and is grounded to the frame ground 108 by a spring 114. The distance calculated from the creepage / space distance described later is taken between the frame ground pressure roller and the frame ground pressure roller. With this configuration, even if excessive voltage noise is applied to the AC power supply, the pressure is divided by the ratio of the heater glass capacity, the pressure member capacity, and the capacitor capacity. Such a potential can be lowered. Therefore, the creepage / space distance for preventing discharge from the pressure member to the frame ground can be shortened. In addition, the current caused by the application of lightning surge (hereinafter referred to as surge current) has a high resistance to enhance insulation, and the impedance of the capacitor is lower when comparing the impedance of the capacitor and the impedance of the resistor. It becomes easy to flow to the frame ground via the capacitor 107 and it becomes difficult to discharge.

  The method of determining the capacitor capacity, the pressure member, and the creepage / space distance of the frame ground in this embodiment will be described with reference to FIGS. FIG. 3 is a diagram showing the configuration of the fixing device in an electrical equivalent circuit. FIG. 4 is a diagram illustrating the determination method using a flowchart.

  In S100, the capacitance of the capacitor is Cx, the resistance value of the resistor is R, the lightning surge voltage (maximum voltage) is Vsurge, and the dielectric strength of the heater glass is Va. In addition, the electrostatic capacity of the heater glass (the electrostatic capacity between the resistor on the ceramic substrate and the fixing film) is Ch, and the electrostatic capacity of the pressure roller (the electrostatic capacity between the fixing nip portion and the pressure roller shaft). ) Is Cr, the respective impedances are Zh and Zr, and the combined impedance of the parallel capacitor and resistor is Zx. Overvoltage noise Vsurge is applied to point A.

In S101, Zh, Zr, Zx are
Zh = 1 / jωCh
Zr = 1 / jωCr (1)
Zx = R // (1 / jωCx)
If Zh, Zr, and Zx make the combined impedance Z,
Z = Zh + Zr + Zx = 1 / jωCh + 1 / jωCr + R // (1 / jωCx) (2)
It becomes.

In S102, the noise current Isurge is expressed as Isurage = Vsurge / Z, so the voltage applied to the heater glass (voltage applied between the resistor on the ceramic substrate and the fixing film) Vh is
Vh = Isurge × Zh
= Vsurge × Zh / Z
= Vsurge × Zh / (1 / jωCh + 1 / jωCr + R // (1 / jωCx)) (3)
It is represented by

In S103, since the surge voltage has a high frequency, the impedance of the resistor and the impedance of the capacitor have a relationship of R >> 1 / jωCx, and R // (1 / jωCx) ≈1 / jωCx.
From this, Vh obtained by equation (3) is
Vh≈Vsurge × Zh / (1 / jωCh + 1 / jωCr + 1 / jωCx)
= Vsurge × CxCr / (ChCx + CxCr + CrCh) (4)
Can be approximated.

In S104, in order to prevent dielectric breakdown, when a lightning surge voltage (maximum voltage) Vsurge is applied between the resistor on the ceramic substrate and the electrical ground, the voltage Vh applied to the heater glass is not insulated. It is necessary to make the breakdown voltage Va or less. Therefore,
Vh <Va (5)
It is necessary to satisfy.

From the formulas (5) and (6), Cx that does not cause dielectric breakdown of the heater glass is obtained.
In S105, by obtaining the capacitance Cx of this capacitor, the potential difference between the capacitor electrodes, that is, the potential Vx of the pressure member is
Vx = Vsurge × ChCr / (ChCx + CxCr + CrCh) (6)
Is required.

  In S106, the creepage / space distance X with respect to the frame ground for preventing discharge from the pressure member to the frame ground is determined by this Vx.

  In this embodiment, the dielectric strength voltage Va of the heater glass is 1.5 kV, whereas the excessive voltage noise Vsurge is 6 kV, the heater glass has a capacitance Ch of 1000 pF, the pressure roller has a capacitance Cr of 700 pF, and a resistance. Since R is 10 MΩ, the capacitance Cx of the capacitor was determined to be 100 pF. As a result, Vx was 6 kV in the prior art, whereas Vx could be 5 kV in this example. Therefore, the potential applied to the pressure roller can be suppressed more than before, the creepage / space distance between the pressure roller and the frame ground can be shortened, and the creepage / space distance can be determined with high accuracy, enabling downsizing of the device. it can.

  The configuration of the second embodiment other than the fixing device is the same as that of the first embodiment, and the same components are denoted by the same reference numerals and description thereof is omitted. The configuration of the fixing device will be described with reference to FIG. 5 which is a sectional view of the fixing device.

  This embodiment is different from the first embodiment in that a resistor is not connected between the pressure member and the frame ground. Therefore, the present embodiment is characterized in that the pressure roller shaft, which is the shaft of the pressure member, is installed on the frame ground via a capacitor. Even in such a configuration, as in the first embodiment, even when excessive voltage noise is applied to the AC power source, the potential applied to the pressure member can be lowered, and therefore the creeping surface for preventing discharge from the pressure member to the frame ground.・ Space distance can be shortened. Further, since the surge current has a small impedance with respect to the frame ground in the pressure member, the surge current tends to flow to the frame ground via the capacitor 107 and is difficult to discharge.

  The method of determining the capacitor capacity, the pressure member, and the creepage / space distance of the frame ground of this embodiment will be described with reference to FIGS. FIG. 6 is a diagram illustrating the configuration of the fixing device in an electrical equivalent circuit. FIG. 7 is a diagram illustrating the determination method using a flowchart.

  In S200, the capacitance of the capacitor is Cx, the lightning surge voltage (maximum voltage) is Vsurge, and the insulation withstand voltage of the heater glass is Va. The capacitances of the heater glass and the pressure roller are Ch and Cr, respectively, the impedances are Zh and Zr, and the combined impedance of the parallel capacitor and resistor is Zx. Overvoltage noise Vsurge is applied to point A.

In S201, Zh, Zr, Zx are
Zh = 1 / jωCh
Zr = 1 / jωCr (7)
Zx = 1 / jωCx
If Zh, Zr, and Zx make the combined impedance Z,
Z = Zh + Zr + Zx = 1 / jωCh + 1 / jωCr + 1 / jωCx (8)
It becomes.

In S202, the noise current Isurge is expressed as Isurage = Vsurge / Z, so the voltage Vh applied to the heater glass is
Vh = Isurge × Zh
= Vsurge × Zh / Z
= Vsurge × Zh / (1 / jωCh + 1 / jωCr + 1 / jωCx)
= Vsurge × CxCr / (ChCx + CxCr + CrCh) (9)
It is represented by

In S203, in order not to cause dielectric breakdown, the voltage Vh applied to the heater glass needs to be equal to or lower than its own withstand voltage.
Vh <Va (10)
It is necessary to satisfy.

  From the formulas (9) and (10), Cx that does not cause dielectric breakdown of the heater glass is obtained.

In S204, by obtaining the capacitance Cx of this capacitor, the potential difference between the capacitor electrodes, that is, the potential Vx of the pressure member is
Vx = Vsurge × ChCr / (ChCx + CxCr + CrCh) (11)
Is required.

  In S205, the creepage / space distance X with respect to the frame ground so as not to discharge from the pressure member to the frame ground is determined by this Vx.

  In this embodiment, the dielectric strength voltage Va of the heater glass is 1.5 kV, whereas the excessive voltage noise Vsurge is 6 kV, the heater glass has a capacitance Ch of 1000 pF, and the pressure roller has a capacitance Cr of 700 pF. Therefore, the capacitance Cx of the capacitor was determined to be 100 pF as in Example 1. Similar to Example 1, Vx was 6 kV in the prior art, whereas Vx could be 5 kV in this example. Therefore, even in the configuration as in the second embodiment, the potential applied to the pressure roller is suppressed more than before, the creepage / space distance between the pressure roller and the frame ground is shortened, and the creepage / space distance is accurately determined. Therefore, the apparatus can be downsized.

1 is a configuration diagram of a printer in the present invention. FIG. 1 is a cross-sectional view of a fixing device in Embodiment 1. FIG. FIG. 3 is an equivalent circuit diagram of the fixing device according to the first exemplary embodiment. 3 is a flowchart in the first embodiment. 6 is a cross-sectional view of a fixing device in Embodiment 2. FIG. 6 is an equivalent circuit diagram of a fixing device in Embodiment 2. FIG. 3 is a flowchart in the first embodiment. It is sectional drawing of the fixing device in a prior art example.

DESCRIPTION OF SYMBOLS 101 Resistor 102 Heater glass 103 Fixing film 104 Pressure roller 105 Pressure roller shaft 106 Resistance which strengthens insulation 107 Capacitor 108 Frame ground 109 Recording material 110 AC power supply 111 Pressure roller bearing 114 Spring

Claims (4)

An image forming unit for forming a toner image on a recording material;
A heater having a resistor that generates heat when energized, an insulating layer covering the resistor, a conductive fixing film in contact with the insulating layer of the heater, and a recording that carries a toner image together with the heater through the fixing film A pressure member that forms a fixing nip portion that sandwiches and conveys the material, and a fixing unit.
A metal member connected to the electrical ground of the device;
In an image forming apparatus having
An image forming apparatus, wherein a capacitor is connected between the pressure member and the metal member.   The image forming apparatus according to claim 1, wherein the capacitor is connected in parallel with a resistor connected between the pressure member and the metal member.   When a maximum voltage Vsurge is applied between the resistor and electrical ground, the capacitance Cx of the capacitor, the capacitance Ch between the resistor and the fixing film, the fixing nip portion of the pressure member, Voltage Vh applied between the resistor and the fixing film obtained by dividing the maximum voltage in inverse proportion to the ratio of the capacitance Cr to the axis of the pressing member (= Vsurge × CxCr / (ChCx + CxCr + CrCh)) The image forming apparatus according to claim 1, wherein is less than a withstand voltage Va of the insulating layer.   The image forming apparatus according to claim 1, wherein the capacitor is provided in the fixing unit.