Fixing apparatus and image forming apparatus

Abstract

To reduce the occurrence of electrostatic offset with a simple configuration.SOLUTION: A fixing device heats an image on a recording material by a fixing member while conveying the recording material holding it between the fixing member and a pressure member at a nip part, thereby fixing the image to the recording material, and the fixing device further comprises a circuit unit that has a contact member in contact with the fixing member and is configured to remove electric charges from the fixing member. A surface layer of the pressure member is electrically connected with the circuit unit through the surface layer of the fixing member. When the surface resistivity of the surface layer of the pressure member is defined as X(Ω / sq), and the surface resistivity of the surface layer of the fixing member as Y(Ω / sq), the following are satisfied. 4.0≤logX≤13.0, 5.0≤logY≤14.0, logY≥13.0-logX, and logY≤23.0-logX.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a fixing device that fixes an image onto a recording material, and an image forming apparatus that forms an image on a recording material. [Background Art]

[0002] An electrophotographic image forming apparatus forms an image on a recording material using toner as a developer, and then fixes the image onto the recording material by a fixing device. A heat-fixing type fixing device fixes an image by heating the image on the recording material while nipping and conveying the recording material between a fixing member and a pressure member. In this type of fixing device, when the pressure member becomes charged due to frictional electrification or the like, the toner on the recording material receives a repulsive force and adheres to the fixing member, and after the fixing member rotates once, the toner adheres to the recording material, which may cause toner smearing on the recording material. Such an image defect that occurs when the electrostatic force acting on the toner in the nip portion of the fixing device is out of an appropriate range is known as electrostatic offset.

[0003] Patent Document 1 discloses a technique for suppressing electrostatic offset by providing a conductive layer having conductivity on a fixing film, applying a voltage by bringing a power feeding brush into contact with the conductive layer exposed at a longitudinal end of the film, and grounding a pressure roller facing the fixing film. Patent Document 2 discloses a technique for suppressing electrostatic offset by bringing the conductive layer of the fixing film into contact with a conductive rubber ring provided at an end portion of the pressure roller, and grounding a core metal of the pressure roller. [Background Art] [Patent Literature]

[0004] [Patent Document 1] Japanese Patent Laid-Open No. 2002-132072 [Patent Document 2] Japanese Patent Laid-Open No. 2009-042303 [Summary of the Invention] [Problem to be Solved by the Invention]

[0005] Due to the demand for miniaturization and cost reduction of image forming equipment, there was a need to reduce the occurrence of electrostatic offset with a simpler configuration.

[0006] Therefore, the present invention aims to provide a configuration that can reduce the occurrence of electrostatic offset with a simple structure. [Means for solving the problem]

[0007] One aspect of the present invention comprises a rotatable fixing member, a rotatable pressurizing member that contacts the fixing member at the nip portion, and a heating means for heating the fixing member. A bearing that rotatably supports the pressurizing member, and a frame that supports the bearing, A fixing device comprising, wherein, in the nip portion, the fixing device heats the image on the recording material with the fixing member to fix it to the recording material while the recording material is sandwiched and transported between the fixing member and the pressing member, further comprising a circuit section having a contact member that contacts the fixing member and configured to remove charge from the fixing member, wherein the surface layer of the pressing member is electrically connected to the circuit section via the surface layer of the fixing member, and when the surface resistivity of the surface layer of the pressing member is X (Ω / □) and the surface resistivity of the surface layer of the fixing member is Y (Ω / □), then 4.0 ≤ logX ≤ 13.0, 5.0 ≤ logY ≤ 14.0, logY ≥ 13.0 - logX, and logY ≤ 23.0 - logX. Furthermore, the resistance value of the bearing in the circuit from the pressurizing member through the bearing to the frame is greater than the resistance value of the fixing member from the nip portion to the contact member. This fixing device is characterized by the following features. [Effects of the Invention]

[0008] According to the present invention, the occurrence of electrostatic offset can be reduced with a simple configuration. [Brief explanation of the drawing]

[0009] [Figure 1] A schematic diagram of an image forming apparatus according to the first embodiment. [Figure 2] A schematic diagram showing a cross-section of the fixing device according to the first embodiment. [Figure 3] A schematic diagram of the fixing device according to the first embodiment. [Figure 4] A diagram showing the layer structure of the fixing film according to the first embodiment. [Figure 5] A diagram showing the static elimination configuration of the pressure roller and fixing film according to the first embodiment. [Figure 6] Surface of a pressure roller and surface of a fixing film according to the first embodiment. [Figure 7] A diagram illustrating the manner in which electrostatic offset occurs. [Figure 8] A schematic diagram of a fixing device according to a modified example 1 of the second embodiment. [Figure 9] A schematic diagram of a fixing device according to a modified example 2 of the second embodiment. [Modes for carrying out the invention]

[0010] The embodiments relating to this disclosure will be described below with reference to the drawings.

[0011] ≪First Embodiment≫ The configuration of the image forming apparatus and fixing apparatus according to the first embodiment will be described below. Note that "image forming apparatus" refers to a device that forms an image on a recording material, such as a single-function printer, copier, or multifunction printer.

[0012] Figure 1 is a schematic diagram showing a printer 100 as an image forming apparatus according to this embodiment. The printer 100 is a monochrome laser beam printer that forms an image on a recording material P by an electrophotographic process based on image information received from an external source. A variety of sheet materials of different sizes and materials can be used as the recording material P (recording medium), including paper such as plain paper and cardboard, plastic film, cloth, sheet materials with surface treatments such as coated paper, and sheet materials of special shapes such as envelopes and index paper.

[0013] A printer 100 comprises an image forming section 101 that forms an image (toner image) on a recording material P using toner as a developer, a fixing device 6 that fixes the image onto the recording material P, and a conveyance mechanism for the recording material P. The image forming section 101 includes a photosensitive drum 1 as an image bearing member, a charging device 2, an exposure device 33, a developing device 4, a transfer roller 5, and a cleaning device 7.

[0014] The photosensitive drum 1 is an electrophotographic photosensitive member in which a photosensitive layer is formed of an organic photoconductor or the like on the outer peripheral portion of a cylindrical substrate. During image formation, the photosensitive drum 1 is rotationally driven at a predetermined peripheral speed in the direction of arrow r1 in the figure. The charging device 2 is, for example, a contact charging type charging roller. The charging device 2 uniformly charges the surface of the photosensitive drum 1 to a predetermined polarity and potential by applying a voltage from an electric circuit (not shown). The charging device 2 of the present embodiment charges the surface of the photosensitive drum 1 to a surface potential of -700V with reference to 0V, which is the reference potential (frame ground) of the printer 100. Although details of the reference potential will be described later, potential values and voltage values described below are expressed with reference to the reference potential of 0V.

[0015] The exposure device 33 is a laser beam scanner. A video controller 31 of the printer 100 converts image information received together with an image formation execution instruction from the outside into image formation information, and transmits the image formation information to a control section 32 of the exposure device 33. The control section 32 drives the exposure device 33 based on the image formation information, and causes the exposure device 33 to output a laser beam L. The laser beam L is irradiated onto the surface of the photosensitive drum 1 to discharge electric charges in the exposed portion, whereby an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 1. In the present embodiment, the output of the laser beam L is adjusted such that the surface potential (bright portion potential) of the exposed portion of the photosensitive drum 1 is -200V.

[0016] The developing device 4 develops an electrostatic latent image on the photosensitive drum 1 into a toner image by carrying toner as a developer on a developer carrier such as a developing roller and supplying the toner to the photosensitive drum 1. In the present embodiment, a toner having a negative charging polarity is used as the developer. Further, a voltage of -400V is applied to the developing roller by an electric circuit (not shown). Accordingly, toner does not adhere to an unexposed area (area having a surface potential of -700V) on the surface of the photosensitive drum 1, and toner adheres to an exposed area (area having a surface potential of -200V). The toner image developed by the developing device 4 is carried on the photosensitive drum 1 and travels toward a transfer portion formed between the photosensitive drum 1 and the transfer roller 5.

[0017] In parallel with the formation of the toner image by the image forming unit 101, the recording material P stored in a cassette C provided at a lower portion of the printer 100 is fed one sheet at a time by a feeding roller 10. The conveyance timing of the recording material P to the transfer portion is adjusted based on the timing at which a sensor 8 detects the leading edge of the recording material P. Accordingly, position alignment of an image relative to the recording material P in a sheet conveyance direction (sub-scanning direction during image formation) is performed.

[0018] The transfer roller 5 serving as a transfer device is applied with a positive voltage by an electric circuit (not shown). Accordingly, the toner image is transferred from the photosensitive drum 1 to the recording material P, and an unfixed image is formed on the recording material P. The value of the voltage applied to the transfer roller 5 varies depending on the usage environment of the printer 100 and the electric resistance of the recording material P, and is generally in a range from +0.5kV to +3.0kV. Transfer residual toner remaining on the surface of the photosensitive drum 1 after passing through the transfer portion is removed by a cleaning device 7, and the surface of the photosensitive drum 1 again becomes in a state suitable for processes after charging.

[0019] The recording material P that has passed through the transfer portion is conveyed to a fixing device 6. The fixing device 6 fixes the image onto the recording material P by heating the image on the recording material P while nipping and conveying the recording material P between a pair of rotating members. Details of the fixing device 6 will be described later. The recording material P that has passed through the fixing device 6 is discharged to the outside of the apparatus by a pair of discharge rollers 9, and is stacked as a product on a discharge tray provided on an upper surface portion of the printer 100.

[0020] (Fusing device) The configuration of the fixing device 6 in this embodiment will now be described. Figure 2 is a schematic diagram showing the cross-sectional configuration of the fixing device 6. Figure 3 is a schematic diagram showing the longitudinal arrangement of the main components of the fixing device 6.

[0021] As shown in Figures 2 and 3, the fixing device 6 includes a film unit 18 (film assembly) and a pressure roller 17. The film unit 18 consists of a fixing film 13, a heater 11, a heater holder 12, and an end flange 14.

[0022] The fixing film 13 is an example of a rotatable fixing member (first rotating body) that contacts the image surface (the surface carrying the unfixed toner T) of the recording material P. The pressure roller 17 is an example of a rotatable pressure member (second rotating body, opposing member) that contacts the surface of the recording material P opposite to the image surface. The heater 11 is an example of a heating means for heating the fixing member in order to fix an image to the recording material P.

[0023] The heater 11 is supported on the lower surface (the fixing nip side) of the heater holder 12, which serves as a holding member. The heater 11 and heater holder 12 are arranged in the internal space of the cylindrical fixing film 13. The end flanges 14 are attached to both longitudinal ends of the heater holder 12. The end flanges 14 have a support portion that supports the heater holder 12 inside the fixing film 13, and flange portions that extend from the support portion in a flange shape to restrict both ends of the fixing film 13.

[0024] The pressure roller 17 is in contact with the nip forming unit, which consists of the heater 11 and the heater holder 12, with the fixing film 13 in between. The end flange 14 is biased toward the pressure roller 17 by the pressure spring 15. As a result, a fixing nip N of a predetermined width is formed between the film unit 18 and the pressure roller 17. In the fixing nip N, the pressure applied by the pressure spring 15 causes the fixing film 13 to be in close contact with the heater 11 and the pressure roller 17.

[0025] Furthermore, the nip forming unit is not limited to one in which the heater 11 is in contact with the inner surface of the fixing film 13. For example, a thin plate or sheet material with high thermal conductivity may be placed between the heater 11 and the fixing film 13 so that the heat from the heater 11 is transferred to the fixing film 13 via the thin plate or sheet material.

[0026] In the following description, "longitudinal direction of the fixing device 6" or simply "longitudinal direction" refers to the direction of the rotation axis of the pressure roller 17. The longitudinal direction of the fixing device 6 can be rephrased as the direction of the generatrix of the fixing film 13, or the sheet width direction perpendicular to the sheet conveying direction at the fixing nip N.

[0027] In this embodiment, a lubricant such as heat-resistant grease is applied between the heater 11 and the inner surface of the fixing film 13, resulting in a low-friction state. Therefore, the frictional force between the surfaces of the fixing film 13 and the pressure roller 17 is greater. Consequently, when the pressure roller 17 is rotated in the direction of arrow r17, the fixing film 13 rotates in the direction of arrow r13 relative to the heater holder 12, while remaining in close contact with the heater 11.

[0028] The pressure roller 17 has a core metal 17c, which will be described later, held by a bearing 16, and its movement other than rotation is restricted. A lubricant is applied to the bearing 16 to reduce frictional resistance when the pressure roller 17 rotates.

[0029] The heater 11 has, for example, a highly insulating elongated ceramic substrate such as alumina (aluminum oxide) or AlN (aluminum nitride), or a heat-resistant resin substrate such as polyimide, PPS, or liquid crystal polymer. The heater 11 is formed by sequentially forming a heating element on the surface of this substrate, which is printed with a heat-generating paste layer such as Ag / Pd (silver palladium), RuO2, or Ta2N, and a glass coating layer to protect and insulate the heating element. In this embodiment, a heater 11 is used in which an Ag / Pd heat-generating paste layer and a glass coating layer are formed on an alumina substrate.

[0030] A power supply terminal electrically connected to the heating element is provided at the longitudinal end of the heater 11. When the connector of the power supply circuit provided on the printer 100 is connected to the power supply terminal, power is supplied to the heating element from the power supply circuit, and the heating element is heated by Joule heating. A temperature sensing element such as a thermistor is placed on the back of the heater 11 (the side opposite to the fuser nip N) to detect the temperature of the heater 11. The control unit of the printer 100 can maintain the temperature of the heater 11 and the fuser nip N at the target temperature by appropriately controlling the duty cycle and wavenumber of the voltage applied to the heating element in response to the signal from this temperature sensing element.

[0031] The heater holder 12 supports the heater 11, generates pressure on the fixing nip N, and has the function of reducing heat loss from the heater 11 to the opposite side of the fixing nip N (thermal insulation). The heater holder 12 is made of a material that has rigidity, heat resistance, and thermal insulation properties. Suitable materials to achieve these properties include, for example, liquid crystal polymer, phenolic resin, PPS, and PEEK. In this embodiment, liquid crystal polymer is used as the material for the heater holder 12.

[0032] The pressure roller 17 has a metal core 17c made of stainless steel, free-cutting steel (SUM), or aluminum, and an elastic layer 17b formed on the outer circumference of the core 17c from heat-resistant rubber such as silicone rubber or fluororubber, or from foamed silicone rubber. Furthermore, to improve the release properties and wear resistance of the roller surface, the pressure roller 17 has a surface layer 17a made of fluororesin such as PFA, PTFE, or FEP, or a mixture thereof, covering the elastic layer 17b. The core 17c of the pressure roller 17 is held by the bearing 16 as described above. In this embodiment, a pressure roller 17 with an outer diameter of 25 mm is used, which has an aluminum core 17c, a silicone rubber elastic layer 17b, and a PFA surface layer 17a.

[0033] Figure 4 shows the layer structure of the fixing film 13. The fixing film 13 is required to have heat resistance to withstand the heat from the heater 11, release properties to prevent unfixed toner images from welding to the surface, and robustness and surface strength to withstand the passage of the recording material P through the fixing nip N and contact with the pressure roller 17 without damage. These properties can be achieved with a single material, but multiple materials can also be used to share the functions.

[0034] For example, as shown in Figure 4, in order to achieve heat resistance and robustness, the base layer 13c of the fixing film 13 is a thin film tube made of a metal such as stainless steel, Al, Ni, Cu, or Zn, or a heat-resistant resin such as polyimide or polyamide. The surface layer 13a formed on the outer circumference of the base layer 13c is a fluororesin that has release properties and surface strength.

[0035] The base layer 13c is a thin film approximately 200 μm or less in thickness, formed in a cylindrical shape. In this embodiment, a 75 μm polyimide tube is used as the base layer 13c. The fluororesin used for the surface layer 13a is selected from fluororesins such as PFA, PTFE, FEP, ETFE, CTFE, or PVDF, or mixtures of these fluororesins. In this embodiment, a 10 μm PFA film formed by coating the outer periphery of the base layer 13c is used as the surface layer 13a.

[0036] Furthermore, in order to structurally stabilize and integrate the base layer 13c and the surface layer 13a, the fixing film 13 of this embodiment includes an adhesive layer 13b with a thickness of 5 μm between the above layers, which is made of a mixture of polyimide resin and fluororesin.

[0037] The operation of the fixing device 6 will now be explained. During image formation, the pressure roller 17 is driven to rotate in the direction of arrow r17 in Figure 2, and the fixing film 13 follows the pressure roller 17 and rotates in the direction of arrow r13. Also, by energizing the heater 11, the heater 11 is heated to a predetermined target temperature. In this state, the recording material P carrying the unfixed toner T is transported in the transport direction p0 in Figure 2. Then, the fixing device 6 transports the recording material P by sandwiching it between the fixing film 13 and the pressure roller 17 at the fixing nip N, and heats the unfixed toner T on the recording material P with the fixing film 13 heated by the heat from the heater 11 (non-radiative heat). As a result, the unfixed toner T melts, and the image is fixed to the recording material P.

[0038] This film heating method uses a fixing film 13 with a very small heat capacity to heat the image, offering advantages such as quick start-up and energy efficiency.

[0039] As shown in Figure 3, the length of the longitudinal fixing nip N is set to a length that exceeds the maximum width w of the recording material P that the printer 100 can image. Therefore, regardless of the size of the recording material P, even while the recording material P is passing through the fixing nip N, the surfaces of the fixing film 13 and the pressure roller 17 remain in contact at the first end region v1 and the second end region v2 outside the area through which the recording material P passes. Hereinafter, the longitudinal range through which the recording material P of maximum width w passes will be referred to as the "paper passage area".

[0040] (Control configuration for the charging potential of the pressure roller) Next, the control configuration of the charge potential of the fixing device 6 in this embodiment will be described. Figure 5 is a schematic diagram illustrating the configuration for controlling the charge potential of the fixing film 13 and the pressure roller 17 according to this embodiment.

[0041] As shown in Figure 5, the fixing device 6 has a contact member 21 that contacts the surface of the fixing film 13. The contact member 21 is preferably made of a highly flexible conductive sheet or conductive brush so that it can deform to follow the fixing film 13 without damaging the surface of the fixing film 13. In this embodiment, a conductive brush is used as the contact member 21.

[0042] In order to reduce contamination of the contact member 21 by paper dust, toner, etc., it is desirable that the contact position of the contact member 21 in the longitudinal direction of the fixing device 6 be outside the paper feeding area.

[0043] The contact member 21 is connected to the electrical ground 23 of the printer 100 via the power supply circuit 22. The contact member 21 and the power supply circuit 22 constitute a circuit configured to remove charge from the fixing film 13. The electrical ground 23 of the printer 100 is the part of the printer 100 that has a reference potential of 0V. The electrical ground 23 serves as the ground for each electrical circuit, such as the electrical circuit for executing the process of the image forming unit 101, the electrical circuit that supplies power to the motor that drives the rotating parts such as the photosensitive drum 1 and the transport rollers, and the electrical circuit for supplying power to the heater 11. If the frame that forms the housing of the printer 100 is made of metal, the frame can be used as the electrical ground 23 of the printer 100.

[0044] By connecting the contact member 21, which is in contact with the surface of the fixing film 13, to the electrical ground 23 of the printer 100 via the current supply circuit 22, the surface charge of the fixing film 13 can be transferred to the electrical ground 23. This controls the charge potential of the fixing film 13 when the fixing device 6 is in use (during image formation).

[0045] On the other hand, in this embodiment, the pressure roller 17 is not provided with a contact member 21 or a configuration equivalent to the current supply circuit 22. In this embodiment, as will be explained below, the charge potential of the pressure roller 17 when the fixing device 6 is in use is controlled by the contact between the surfaces of the pressure roller 17 and the fixing film 13.

[0046] The surface of the pressure roller 17 is electrically connected to the surface of the fixing film 13 at the fixing nip N, and the surface charge of the pressure roller 17 is configured to flow to the electrical ground 23 via the fixing film 13, contact member 21 and current-carrying circuit 22. The surface charge of the pressure roller 17 is configured to not flow substantially to any other paths (for example, a path to the bearing 16 via the elastic layer 17b and core metal 17c).

[0047] Specifically, in this embodiment, the resistance value of the path from the surface of the pressure roller 17 to the electrical ground via the bearing of the pressure roller 17 (path 1) is much greater than the resistance value of the path from the surface of the pressure roller 17 to the electrical ground 23 via the fixing film 13 (path 2). Path 1 is the path from the surface of the pressure roller 17, through the elastic layer 17b of the pressure roller 17, the core metal 17c, and the bearing of the pressure roller 17, to the frame of the fixing device 6, which serves as the electrical ground. The combined resistance of path 1 is, for example, 10 times or more the combined resistance of path 2.

[0048] One way to increase the resistance value of the above-mentioned path 1 is to use a high-resistance bearing for the pressure roller 17, for example. As an example, a bearing is used in which the resistance value from the inner surface of the bearing (the mating part with the core metal 17c) to the outer surface of the bearing (the mating part with the fixing device frame) is greater than the resistance value of the surface layer 13a of the fixing film 13 from the fixing nip N to the contact member 21. The above resistance value of the bearing can be measured, for example, with a digital insulation resistance meter MY600 (manufactured by Yokogawa Measuring Instruments Co., Ltd.).

[0049] As a way to increase the resistance of path 1, it is also possible to make the elastic layer 17b of the pressure roller 17 highly resistant. However, it should be noted that in that case, the elastic layer 17b may act as a capacitor and affect the potential at the fixing nip N. If the bearing of the pressure roller 17 is configured to be highly resistant as described above, then, for example, a conductive material such as carbon black can be dispersed in the elastic layer 17b to reduce its resistance and eliminate the capacitor-like behavior of the elastic layer 17b.

[0050] With the above configuration, in this embodiment, the surface charge of the pressure roller 17 is discharged via the surface of the fixing film 13, which is connected to the electrical ground 23 by the contact member 21. In other words, the surface charge of the pressure roller 17 is discharged by allowing the charge generated on the surface of the pressure roller 17 due to triboelectric charging to flow to the electrical ground 23 via the surface of the fixing film 13, the contact member 21, and the current supply circuit 22. That is, in this embodiment, the surface of the pressure roller 17, the surface of the fixing film 13, the contact member 21, the current supply circuit 22, and the electrical ground 23 substantially constitute a series circuit.

[0051] Furthermore, the electrical connection between the surface of the pressure roller 17 and the surface of the fixing film 13 in the fixing nip N refers to the physical contact between these surfaces. Specifically, during periods when the recording material P is not passing through the fixing nip N, the surfaces of the pressure roller 17 and the fixing film 13 are in contact with each other throughout the entire fixing nip N, thereby creating an electrical connection between these surfaces. During periods when the recording material P is passing through the fixing nip N, the surfaces of the pressure roller 17 and the fixing film 13 are in contact with each other in the first end region v1 and the second end region v2 (Figure 3) outside the paper-feeding area of ​​the fixing nip N, thereby creating an electrical connection between these surfaces.

[0052] According to the above configuration, there is no need to provide a rubber ring on the core metal 17c of the pressure roller 17, as described in Patent Document 2, as a configuration for removing surface charge from the pressure roller 17. Therefore, it is possible to avoid contact failures due to elastic deformation of the rubber ring and contact failures due to plastic deformation of the rubber ring due to long-term use, which are concerns when a rubber ring is attached to the core metal 17c. In other words, in this embodiment, the surfaces of the pressure roller 17 and the fixing film 13 are always in contact with each other, so the electrical connection between the pressure roller 17 and the electrical ground 23, which serves as a path for removing surface charge from the pressure roller 17, can be more stably secured.

[0053] (Surface resistivity of the pressure roller surface and the fixing film surface) Here, we will explain the setting of conditions for the surface layers 17a and 13a of the pressure roller 17 and the fixing film 13 in order to control the charging potential of the surface of the pressure roller 17 within an appropriate range.

[0054] In this embodiment, both the surface layer 17a forming the surface of the pressure roller 17 and the surface layer 13a forming the surface of the fixing film 13 are made of fluororesin (specifically PFA). The surface resistivity (also called sheet resistance) of these surface layers 17a and 13b can be adjusted by adding a conductive filler to the resin base material. Examples of conductive fillers include carbon black, carbon nanotubes, and particles of metal or metal oxide.

[0055] Hereinafter, the surface resistivity of the surface of the pressure roller 17 (surface resistivity of the surface layer 17a) will be denoted as X (Ω / □), and the surface resistivity of the surface of the fixing film 13 (surface resistivity of the surface layer 13a) will be denoted as Y (Ω / □). In this embodiment, the surface resistivity X and Y are configured to satisfy all of the following equations (1) to (4). Figure 6 illustrates the regions determined by equations (1) to (4) (shaded regions and dot pattern regions). 4.0 ≤ logX ≤ 13.0 ···(1) 5.0 ≤ logY ≤ 14.0 ···(2) logY≧13.0-logX ···(3) logY ≤ 23.0 - logX ... (4)

[0056] Surface resistivity will be measured using a Hi-Lester UX-MCP-HT800 (manufactured by Nitto Denko Analytech Co., Ltd.) and a ring probe UR-SS-MCP-HTP15, with a set voltage of 1V to 1000V and a measurement time of 30 seconds.

[0057] The reason for setting the surface resistivity X and Y of the pressure roller 17 surface and the fixing film 13 surface within the above range will be explained using Figure 6. In Figure 6, the horizontal axis shows the surface resistivity X of the pressure roller 17 surface on a common logarithmic scale, and the vertical axis shows the surface resistivity Y of the fixing film 13 surface on a common logarithmic scale. For example, if Y = 1.0E + 4 (Ω / □), the value on the horizontal axis is 4.0.

[0058] (Setting of X and Y values ​​that are independent of the combination of surface resistivity X and Y) To explain why the ranges of surface resistivity X and Y on the surface of the pressure roller 17 and the fixing film 13 are determined by the above formulas (1) to (4), we will first explain the numerical settings for surface resistivity X and Y when the combination of surface resistivity X and Y is not considered. In this case, the preferred ranges for surface resistivity X and Y are expressed by the following formulas (5) and (6) (shaded area in Figure 6). 6.0 ≤ logX ≤ 11.0 ···(5) 7.0 ≤ logY ≤ 12.0 ···(6)

[0059] The surface resistivity X of the pressure roller 17 surface is preferably 1.0E+11Ω / □ or less (logX is 11.0 or less). The surface resistivity Y of the fixing film 13 surface is preferably 1.0E+12Ω / □ or less (logY is 12.0 or less). If the surface resistivity X and Y are below the above values, the charge on the surface of the pressure roller 17 can be removed via the fixing film 13, thereby suppressing the generation of electrostatic offset due to overcharging of the surface of the pressure roller 17.

[0060] "Electrostatic offset due to overcharging of the surface of the pressure roller 17" refers to an electrostatic offset that occurs when the surface of the pressure roller 17 becomes charged with the same polarity as the normal charging polarity of the toner due to friction with the fixing film 13 or recording material, and the amount of that charge exceeds the allowable range.

[0061] Point UU in Figure 6 represents the case where X = 1.0E + 11Ω / □ and Y = 1.0E + 12Ω / □. In this case, although charge accumulates on the surface of the pressure roller 17, it does not become overcharged, and the occurrence of electrostatic offset is suppressed.

[0062] Point UU corresponds to the upper limit (upper limit of X) of the ease with which charge can accumulate on the surface of the pressure roller 17, when using a fixing film 13 surface with minimum static elimination capability (upper limit of Y), so that the surface of the pressure roller 17 does not become overcharged. Therefore, if X is increased from point UU (moving to the right in Figure 6) or Y is increased from point UU (moving upward in Figure 6), the charge accumulation on the surface of the pressure roller 17 exceeds the static elimination capability of the fixing film 13. As a result, an electrostatic offset occurs due to overcharging of the surface of the pressure roller 17.

[0063] The surface resistivity X of the pressure roller 17 surface is preferably 1.0E+6Ω / □ or less (logX is 6.0 or more). The surface resistivity Y of the fixing film 13 surface is preferably 1.0E+7Ω / □ or more (logY is 7.0 or more). If the surface resistivity X and Y are above the above values, the occurrence of electrostatic offset due to charge outflow from the surface of the pressure roller 17 can be suppressed.

[0064] "Electrostatic offset due to charge outflow from the surface of the pressure roller 17" is the following phenomenon: In the electrophotographic process, a voltage with the opposite polarity to the normal charging polarity of the toner is applied to the transfer device, such as the transfer roller 5. When a toner image is transferred to a recording material with high electrical resistance, such as a recording material left in an environment with a relative humidity of less than 40%, the recording material becomes polarized on its front and back sides, with the side onto which the image is transferred (image side) having the same polarity as the toner, and the side opposite the image side (non-image side) having the opposite polarity of the toner. If the surface resistivity X of the surface of the pressure roller 17 is extremely low, when the polarized recording material passes through the fixing nip N, the charge from the non-image side of the recording material flows out from the surface of the pressure roller 17 to the surface of the fixing film 13. As a result, the amount of charge on the non-image side of the recording material decreases, and the electrostatic force that constrains the toner to the recording material decreases, making electrostatic offset more likely to occur.

[0065] Point DD in Figure 6 shows the case where X = 1.0E + 6Ω / □ and Y = 1.0E + 7Ω / □. In this case, although a certain amount of charge flows out from the surface of the pressure roller 17, excessive charge outflow is avoided, and therefore the occurrence of electrostatic offset is suppressed.

[0066] Point DD corresponds to the lower limit of the surface resistivity X of the pressure roller 17 surface in which no electrostatic offset due to charge outflow occurs when using the fixing film 13 surface which has a relatively easy current flow (lower limit of Y). Therefore, if X is reduced from point DD (moving to the left in Figure 6) or Y is reduced from point DD (moving downwards in Figure 6), the charge on the surface of the pressure roller 17 becomes more likely to outflow excessively through the fixing film 13. As a result, an electrostatic offset due to charge outflow from the surface of the pressure roller 17 may occur.

[0067] (Setting of X and Y values ​​considering the combination of surface resistivity X and Y) Incidentally, in this embodiment, the surface of the pressure roller 17 and the surface of the fixing film 13 are connected in series. Therefore, it was found that even when the surface resistivity X and Y are outside the range determined by the above formulas (5) and (6), the occurrence of electrostatic offset can be suppressed depending on the combination of surface resistivity X and Y.

[0068] For example, at point ex (logX=12.0, logY=6.0) in Figure 6, the surface resistivity X of the pressure roller 17 surface is greater than the upper limit of equation (5), and the surface resistivity Y of the fixing film 13 surface is less than the lower limit of equation (6). In this case, although the large value of X makes it easy for charge to accumulate on the surface of the pressure roller 17, the small value of Y gives the fixing film 13 surface high static discharge capability, thus maintaining a balance for the entire series circuit including the pressure roller 17 and the fixing film 13. Therefore, even at point ex, the occurrence of electrostatic offset due to overcharging of the surface of the pressure roller 17 can be suppressed.

[0069] Thus, we investigated combinations of surface resistivity X and Y that can suppress the occurrence of electrostatic offset even when the surface resistivity X and Y are outside the range determined by the above equations (5) and (6). As a result, we found that the occurrence of electrostatic offset can be sufficiently reduced in the region determined by the above equations (1) to (4) (the dot pattern region in Figure 6).

[0070] In other words, according to this embodiment, by considering the combination of surface resistivity X and Y, it is possible to sufficiently reduce the occurrence of electrostatic offset over a wider area (dot pattern area) compared to when X and Y are evaluated individually (shaded area in Figure 6). Therefore, while suppressing the occurrence of image defects due to electrostatic offset, it is possible to improve the design freedom of the fixing device regarding the material and structure of the pressure roller 17 or fixing film 13.

[0071] Specifically, according to formula (1), the surface resistivity X of the pressure roller 17 can be a value greater than the upper limit of formula (5) or less than the lower limit of formula (5). Also, according to formula (2), the surface resistivity Y of the fixing film 13 can be a value greater than the upper limit of formula (6) or less than the lower limit of formula (6).

[0072] However, if the values ​​of X and Y are outside the range given by formula (1) or (2), it is difficult to avoid electrostatic offset even when considering combinations of X and Y. For example, if the surface resistivity X exceeds 1.0E+13Ω / □, electrostatic offset is likely to occur even if the surface resistivity Y is reduced (lower right of eUeD in Figure 6). This is because the amount of charge on the surface of the pressure roller 17 easily becomes excessive, and the static charge cannot be completely removed even if the surface resistivity Y of the fixing film 13 surface is reduced. Similarly, for other combinations of X and Y that are outside the range given by formula (1) or (2), if either X or Y becomes larger or smaller than the allowable range, electrostatic offset due to overcharging of the surface of the pressure roller 17 or electrostatic offset due to charge outflow will become apparent.

[0073] Furthermore, even within the range of equations (1) and (2), when the surface resistivity X and Y are combined with relatively large values ​​(upper right of point UU in Figure 6), electrostatic offset due to overcharging of the surface of the pressure roller 17 is likely to become apparent. Also, even within the range of equations (1) and (2), when the surface resistivity X and Y are combined with relatively small values ​​(lower left of point DD in Figure 6), electrostatic offset due to charge outflow is likely to become apparent.

[0074] Therefore, using equations (3) and (4), we decided to eliminate combinations of large surface resistivity values ​​X and Y, and combinations of small surface resistivity values ​​X and Y.

[0075] To summarize, as shown in Figure 6, the setting conditions for the surface resistivity X and Y of the fixing film 13 surface and the pressure roller 17 surface in this embodiment are defined within the range of a hexagon tilted to the left.

[0076] In Figure 6, the allowable range of X and Y in this embodiment (dot pattern area) is wider than the allowable range of X and Y when the combination of surface resistivity X and Y is not considered (shaded area), by the amount of areas A and B. Area A is the area that satisfies logX > 11 or logY < 7 in addition to equations (1) to (4). Area B is the area that satisfies logX < 6 or logY > 12 in addition to equations (1) to (4).

[0077] (Numerical examples of surface resistivity X and Y and their effects) The relationship between the surface resistivity X and Y described above, and the electrostatic charge and electrostatic offset on the surface of each component will now be explained.

[0078] During the period before or after the recording material P enters the fixing nip N (while the pressure roller 17 is rotating and the recording material is not passing through), the fixing film 13 and the pressure roller 17 rub against each other in direct contact over the entire longitudinal direction of the fixing nip N. As a result, the surfaces of the fixing film 13 and the pressure roller 17 become triboelectrically charged, and this triboelectric charging is particularly likely to occur in environments with relative humidity below 40%. If the surface (surface layer 17a) of the pressure roller 17 is formed of a fluororesin such as PFA, the surface of the pressure roller 17 becomes negatively charged (i.e., to the same polarity as the normal charging polarity of the toner) due to triboelectric charging.

[0079] As described above, in this embodiment, a substantially series circuit is formed with the surface of the pressure roller 17 as the starting point and the electrical ground 23 as the ending point. Since the surface of the pressure roller 17 is on the starting point side of the surface of the fixing film 13, it usually retains more negative polarity charge than the surface of the fixing film 13, and the negative polarity potential is also larger. Therefore, if the amount of charge on the surface of the pressure roller 17 due to triboelectric charging becomes excessive, a repulsive force acts on the negatively charged unfixed toner T on the recording material P, causing an electrostatic offset.

[0080] Here, in Figure 6, in the region below equation (4), the surface resistivity X and Y of the pressure roller 17 surface and the fixing film 13 surface do not become excessively large compared to the upper region, so the generation of charge due to triboelectric charging becomes relatively gentle. In addition, in the region below equation (4) in Figure 6, charge movement within the pressure roller 17 surface and the fixing film 13 surface is relatively easier compared to the upper region. Therefore, the charge generated on the surface of the pressure roller 17 due to triboelectric charging can be moved to the surface of the fixing film 13 via the contact area between the pressure roller 17 and the fixing film 13, and then smoothly moved to the contact area between the fixing film 13 and the contact member 21.

[0081] Therefore, in the region below equation (4) in Figure 6, the charge on the surface of the pressure roller 17 can be removed by the series circuit to a sufficient extent to reduce the occurrence of electrostatic offset due to overcharging of the surface of the pressure roller 17.

[0082] However, even in the region below equation (4) in Figure 6, it is difficult to avoid electrostatic offset if the values ​​of X and Y are outside the range of equation (1) or (2). For example, if the surface resistivity X of the pressure roller 17 surface exceeds 1.0E+13Ω / □, electrostatic offset is likely to occur even if the surface resistivity Y is reduced. This is because the amount of charge on the surface of the pressure roller 17 surface easily becomes excessive, and static electricity cannot be completely eliminated even if the surface resistivity Y of the fixing film 13 surface is reduced. Similarly, if the surface resistivity Y of the fixing film 13 surface exceeds 1.0E+14Ω / □, electrostatic offset is likely to occur even if the surface resistivity X is reduced. This is because the static electricity elimination ability of the fixing film 13 surface on the pressure roller 17 surface is significantly insufficient.

[0083] In Figure 6, in the region above equation (3), charge transfer within the surface of the pressure roller 17 and the surface of the fixing film 13 is relatively gentler compared to the region below. Therefore, when recording materials with high electrical resistance and polarization on both sides, such as recording materials left in an environment with a relative humidity of less than 40%, pass through the fixing nip N, the outflow of charge from the non-image side of the recording material from the surface of the pressure roller 17 to the surface of the fixing film 13 can be reduced. As a result, the outflow of charge from the non-image side of the recording material through the surface of the pressure roller 17 reduces the electrostatic force that constrains the toner to the recording material, thereby suppressing the occurrence of electrostatic offset.

[0084] However, even in the region above equation (3) in Figure 6, it is difficult to avoid electrostatic offset if the values ​​of X and Y are outside the range of equation (1) or (2). For example, if the surface resistivity X of the pressure roller 17 surface is less than 1.0E+4Ω / □, even if the surface resistivity Y is increased, the charge outflow from the non-image surface of the recording material cannot be sufficiently reduced, and electrostatic offset is likely to occur. Also, if the surface resistivity Y of the fixing film 13 surface is less than 1.0E+5Ω / □, even if the surface resistivity X is increased, the charge transfer at the contact point between the fixing film 13 and the pressure roller 17 may become too active. Therefore, the charge outflow from the non-image surface of the recording material cannot be sufficiently reduced, and electrostatic offset may occur.

[0085] However, the above explanation does not necessarily apply when the surface resistivity X is very large and the surface resistivity Y is very small (lower right of point eUeD in Figure 6), or when the surface resistivity X is very small and the surface resistivity Y is very large (upper left of point eDeU in Figure 6). In these cases, however, it becomes difficult to control the amount of charge on the surface of the pressure roller 17 or the amount of charge on the non-image surface of the recording material, which may result in electrostatic offset.

[0086] As described above, in this embodiment, a series circuit is configured by connecting the surface of the pressure roller 17, the surface of the fixing film 13, the contact member 21, the current supply circuit 22, and the electrical ground 23 in series. By setting the surface resistivity X and Y of the surface of the pressure roller 17 and the surface of the fixing film 13 within a predetermined range determined by formulas (1) to (4), the amount of charge on the surface of the pressure roller 17 and the non-image surface of the recording material can be controlled to a degree that sufficiently reduces the occurrence of electrostatic offset.

[0087] In other words, according to the fixing device of this embodiment, the occurrence of electrostatic offset can be reduced with a simple configuration.

[0088] (Test results) This section describes the evaluation results of electrostatic offset when performing continuous image formation operations (continuous printing) using the printer 100 of this embodiment. The recording material P used was A4 size paper, Office70 (manufactured by Canon, basis weight 70 g / m²). 2 The A4 size paper was used in the test after being left in a 15°C, 10%RH environment for 48 hours after opening the package. The printing environment was also under the same environmental conditions. The printing conditions were 50 ppm throughput, and 50 pages were continuously printed on only one side of the recording material P. The charge amount of the toner at this time was -20 μC / g, and the voltage applied to the transfer roller 5 was +1.5 kV.

[0089] To evaluate the electrostatic offset, an image like the one shown in Figure 7 was used. Specifically, a character image (TI) was printed on the leading edge of the recording material P in the transport direction, over a length (13i) corresponding to one full turn of the fixing film 13. The degree of toner smudges (TJ) appearing in the solid white area (non-image-forming area) corresponding to one full turn of the fixing film 13 (13i) behind the area where the character image (TI) was printed was then evaluated. When electrostatic offset occurs, as shown in Figure 7, toner smudges (TJ) corresponding to the character image appear at a position shifted by a length corresponding to one full turn of the fixing film 13 from the original character image (TI).

[0090] In addition, the surface potentials of the fixing film 13 and the pressure roller 17 during printing were measured. The measurement method used was a combination of a surface potential meter Model 347 and a measuring probe Model 555P-1 (both manufactured by Advanced Energy Co., Ltd.).

[0091] The surface resistivity X and Y of the pressure roller 17 surface and the fixing film 13 surface for each evaluated embodiment are shown in Table 1. Example 1-1 corresponds to point CC (X=8.5, Y=9.5) in Figure 6. Example 1-2 corresponds to point meU (X=9.0, Y=14.0) in Figure 6. Examples 1-3 correspond to point eUm (X=13.0, Y=10.0) in Figure 6. Examples 1-4 correspond to point eUeD (X=13.0, Y=5.0) in Figure 6. Examples 1-5 correspond to point meD (X=8.0, Y=5.0) in Figure 6. Examples 1-6 correspond to point eDm (X=4.0, Y=9.0) in Figure 6. Examples 1-7 correspond to point eDeU (X=4.0, Y=14.0) in Figure 6. Examples 1-8 correspond to point DU (X=6.0, Y=12.0) in Figure 6. Examples 1-9 correspond to point UD (X=11.0, Y=7.0) in Figure 6.

[0092] Furthermore, as a comparative example, evaluation tests similar to those in each of the above examples were performed for configurations where the combination of surface resistivity X and Y of the pressure roller 17 surface and the fixing film 13 surface falls outside the range determined by formulas (1) to (4). Comparative Example 1-1 corresponds to point Z1 (X=12.0, Y=13.0) in Figure 6. Comparative Example 1-2 corresponds to point Z2 (X=13.5, Y=4.5) in Figure 6. Comparative Example 1-3 corresponds to point Z3 (X=5.0, Y=6.0) in Figure 6. Comparative Example 1-4 corresponds to point Z4 (X=3.5, Y=14.5) in Figure 6.

[0093] [Table 1]

[0094] The symbols for electrostatic offset image (TJ) evaluation in the table are as follows: A: No electrostatic offset (TJ) occurred. B: Electrostatic offset (TJ) is present (at a level that cannot be detected without magnified observation). C: Electrostatic offset (TJ) is present (at a level easily recognizable at a glance). D: Electrostatic offset (TJ) is present (at a level that is recognizable as text).

[0095] As shown in Table 1, no electrostatic offset (TJ) occurred in Example 1-1. In Example 1-1, the surface potential of the pressure roller 17 during passage of the recording material was -300V, and the surface potential of the fixing film 13 was -200V. Although the potential difference was negative (-100V) for the pressure roller 17 relative to the fixing film 13, the charge on the surface of the pressure roller 17 was not strong enough to create a repulsive force that would move the unfixed toner T, and therefore no electrostatic offset occurred. Furthermore, it is thought that the charge outflow from the non-image surface of the recording material was not significant enough to cause an electrostatic offset due to charge outflow.

[0096] In Examples 1-2, electrostatic offset (TJ) occurred, but the level was extremely minor and did not pose a problem in practical use. In Examples 1-2, the difference in surface potential between the pressure roller 17 and the fixing film 13 while the recording material was passing through was slightly larger than in Examples 1-1, as the pressure roller 17 had a negative polarity relative to the fixing film 13 (-150V). Therefore, it is thought that a very small portion of the unfixed toner T was repelled by the charge on the surface of the pressure roller 17 and adhered to the fixing film 13, resulting in a minor electrostatic offset.

[0097] In Examples 1-3 and 1-4, electrostatic offset (TJ) occurred, but the level was extremely minor and not problematic in practical use. In Examples 1-3 and 1-4, the difference in surface potential between the pressure roller 17 and the fixing film 13 while the recording material was passing through was slightly larger than in Example 1-1, with the pressure roller 17 being negatively polarized relative to the fixing film 13 (-150V). Therefore, it is thought that a small portion of the unfixed toner T was repelled by the charge on the surface of the pressure roller 17 and adhered to the fixing film 13, resulting in a minor electrostatic offset.

[0098] In Examples 1-5, electrostatic offset (TJ) occurred, but the level was extremely minor and did not pose a problem in practical use. In Examples 1-5, the difference in surface potential between the pressure roller 17 and the fixing film 13 while the recording material was passing through was such that the pressure roller 17 was negatively polarized relative to the fixing film 13, and this value (-70V) was slightly smaller than that in Example 1-1. Therefore, it is thought that a small amount of unfixed toner T adhered to the fixing film 13 due to some charge leakage from the non-image surface of the recording material, resulting in a minor electrostatic offset.

[0099] In Examples 1-6 and 1-7, electrostatic offset (TJ) occurred, but the level was extremely minor and not problematic in practical use. In Examples 1-6 and 1-7, the difference in surface potential between the pressure roller 17 and the fixing film 13 while the recording material was passing through was such that the pressure roller 17 was negatively polarized relative to the fixing film 13, and the values ​​(-80V, -50V) were slightly smaller than those in Example 1-1. Therefore, it is thought that a small amount of unfixed toner T adhered to the fixing film 13 due to some charge leakage from the non-image surface of the recording material, resulting in a minor electrostatic offset.

[0100] In Examples 1-8 and 1-9, no electrostatic offset (TJ) occurred. In Examples 1-8 and 1-9, the difference in surface potential between the pressure roller 17 and the fixing film 13 during passage of the recording material was such that the pressure roller 17 was negatively polarized relative to the fixing film 13, and the potential difference values ​​(-90V, -110V) were similar to those in Example 1-1. Therefore, it is considered that the charge on the surface of the pressure roller 17 did not generate a repulsive force sufficient to move the unfixed toner T, and that the charge from the non-image surface of the recording material did not flow out to such an extent that electrostatic offset due to charge outflow would become a problem.

[0101] In Comparative Example 1-1, an electrostatic offset (TJ) at a level recognizable as text occurred. In Comparative Example 1-1, the difference in surface potential between the pressure roller 17 and the fixing film 13 while the recording material was passing through was such that the pressure roller 17 had a negative polarity relative to the fixing film 13, and this value (-1300V) was extremely large compared to Example 1-1. Therefore, it is considered that the unfixed toner T received a strong repulsive force from the charge on the surface of the pressure roller 17, causing the electrostatic offset.

[0102] In Comparative Example 1-2, a noticeable electrostatic offset (TJ) occurred. In Comparative Example 1-2, the difference in surface potential between the pressure roller 17 and the fixing film 13 during passage of the recording material was such that the pressure roller 17 was negatively polarized relative to the fixing film 13. This value (-700V) was extremely large compared to Example 1-1, although not as large as in Comparative Example 1-1. Therefore, it is considered that the unfixed toner T was subjected to a strong repulsive force from the charge on the surface of the pressure roller 17, resulting in the electrostatic offset.

[0103] In Comparative Examples 1-3, an electrostatic offset (TJ) at a level recognizable as text occurred. In Comparative Examples 1-3, the difference in surface potential between the pressure roller 17 and the fixing film 13 while the recording material was passing through was such that the pressure roller 17 was negatively polarized relative to the fixing film 13, and this value (-5V) was extremely small compared to Example 1-1. Therefore, it is thought that a large amount of charge flowed out from the non-image surface of the recording material, causing unfixed toner T to adhere to the fixing film 13, resulting in the electrostatic offset.

[0104] In Comparative Examples 1-4, a noticeable electrostatic offset (TJ) occurred. In Comparative Examples 1-4, the difference in surface potential between the pressure roller 17 and the fixing film 13 during the passage of the recording material was such that the pressure roller 17 was negatively polarized relative to the fixing film 13. This value (-20V) was extremely small compared to Example 1-1, although not as small as in Comparative Example 1-3. Therefore, it is thought that a large amount of charge flowed out from the non-image surface of the recording material, causing unfixed toner T to adhere to the fixing film 13 and resulting in the electrostatic offset.

[0105] Thus, according to the configuration of this embodiment, the occurrence of electrostatic offset can be reduced with a simple configuration.

[0106] Furthermore, as demonstrated in Examples 1-1, 1-8, and 1-9, where electrostatic offset is particularly effectively reduced, the shaded region in Figure 6 (equations (5) and (6)) is preferable in terms of suppressing electrostatic offset. On the other hand, by utilizing region A (logX>11 or logY<7) and region B (logX<6 or logY>12) in Figure 6, it is possible to reduce electrostatic offset to a practically acceptable level while improving the design flexibility of the fixing device.

[0107] (modified version) In the printer 100 of the first embodiment, as shown in Figure 5, the contact member 21 is directly connected to the electrical ground 23 by a current-carrying circuit 22 made of a conductor (wire). Alternatively, an electrical circuit composed of a circuit element or multiple circuit elements may be provided in the middle of the current-carrying circuit 22 to actively control the potential of the surface of the fixing film 13 and the surface of the pressure roller 17. Specifically, by providing current limiting means such as an electrical resistor or varistor, or current rectifier means such as a diode in the current-carrying circuit 22, the potential of the surface of the fixing film 13 and the surface of the pressure roller 17 can be controlled. Furthermore, multiple current control means and rectifier means may be arranged in series or parallel. In addition, a voltage application circuit may be provided in the current-carrying circuit 22 to apply a voltage to the contact member 21 and maintain it at a predetermined potential to control the potential of the surface of the fixing film 13 and the surface of the pressure roller 17.

[0108] 《Second Embodiment》 The configuration of the fixing device and printer according to the second embodiment will now be described. In this embodiment, at least one layer of the fixing film 13 other than the surface layer is a low-resistivity layer with a lower surface resistivity than the surface layer. Hereinafter, elements denoted by the same reference numerals as in the first embodiment have substantially the same configuration and operation as those described in the first embodiment, and the description will focus on the parts that differ from the first embodiment.

[0109] As shown in Figure 4, the fixing film 13 of the first embodiment has a surface layer 13a, an adhesive layer 13b, and a base layer 13c. In this embodiment, either the adhesive layer 13b or the base layer 13c, or both, are formed from a material with a lower surface resistivity than the surface layer 13a to form a low-resistivity layer.

[0110] The low-resistance layer can be one in which conductivity is imparted by adding conductive fillers such as carbon black, carbon nanotubes, or metals or metal oxides to the base resin. Alternatively, the low-resistance layer may be formed by creating a base layer 13c of a conductor such as a metal.

[0111] The surface resistivity of the low-resistivity layer shall be lower than the surface resistivity of the surface layer 13a. Furthermore, the surface resistivity of the low-resistivity layer shall be 1.0E+5Ω / □ or less.

[0112] The surface resistivity of the adhesive layer 13b can be measured using a Rolester GX-MCP-T700 (manufactured by Nitto Denko Analytech Co., Ltd.) and a PSP probe-MCP-TP06P, with an applied current of 0.1 μA or 1 μA and a measurement time of 30 seconds.

[0113] As described above, by providing a low-resistance layer inside the surface layer 13a, potential unevenness within the fixing film 13 can be reduced even when performing continuous printing for long periods of time, thereby reducing the occurrence of electrostatic offset due to potential unevenness.

[0114] Details are described below. In the first embodiment, as shown in Figures 3 and 5, the contact member 21 is positioned to contact the surface of the fixing film 13 outside the paper feeding area. Here, of the ends of the pressure roller 17 located outside the paper feeding area, the end region opposite to the contact member 21 is designated as the first end region v1, and the end region on the same side as the contact member 21 is designated as the second end region v2. In this case, the charge on the surface of the pressure roller 17 generated in the second end region v2 is transferred to the fixing film 13, and then moves across the fixing film 13 in the longitudinal direction to the contact member 21.

[0115] Therefore, in a configuration where the contact member 21 is in contact with one end of the fixing film 13, the charge on the surface of the pressure roller 17 in the end region opposite to the contact member 21 (in this case, the first end region v1) is less likely to be discharged. In the first embodiment, by setting the surface resistivity of the fixing film 13 surface to a smaller value, the potential unevenness (discharge unevenness of the pressure roller 17) within the fixing film 13 can be reduced.

[0116] On the other hand, when continuous printing is performed for a long period of time, the period during which the fixing film 13 and the pressure roller 17 are in contact over the entire area of ​​the fixing nip N becomes shorter. Therefore, the transfer of charge from the pressure roller 17 to the fixing film 13 mainly occurs in the first end region v1 and the second end region v2. In this case, since there is a lower limit to the surface resistivity of the surface of the fixing film 13 as mentioned above, it is possible that the charge on the surface of the pressure roller 17 cannot be completely removed in the first end region v1, resulting in an electrostatic offset.

[0117] Furthermore, when an additional cassette is connected to the printer 100 to increase the number of recording materials that can be fed, or when a very large number of images are output in continuous printing, the electrostatic offset caused by the potential unevenness of the fixing film 13 is more likely to become apparent. Also, when the surface resistivity of the pressure roller 17 surface and the fixing film 13 surface is relatively large (for example, in the range of logX≧6, logY≧12, or when the sum of logX and logY is close to 23), the electrostatic offset caused by the potential unevenness of the fixing film 13 is more likely to become apparent.

[0118] As a countermeasure against potential unevenness in the fixing film 13, it is conceivable to make an additional contact member 21 in contact with the fixing film 13 on the first end region v1 side, thereby removing charge at both longitudinal ends of the fixing film 13. However, this method complicates the configuration of the device.

[0119] Therefore, in this embodiment, at least one layer on the inside of the surface of the fixing film 13 is made into a low-resistance layer. Below, as an example of the configuration of this embodiment, a configuration in which the adhesive layer 13b is a low-resistance layer will be described.

[0120] First, let's conceptually explain the movement of electric charge. When the adhesive layer 13b is a low-resistance layer, the surface charge of the fixing film 13 itself and the charge received from the pressure roller 17 move in the thickness direction within the surface layer 13a of the fixing film 13 and reach the adhesive layer 13b. The charge then moves further within the adhesive layer 13b in the circumferential and longitudinal directions and flows towards the contact member 21.

[0121] The surface resistivity of the adhesive layer 13b is lower than the lower limit of the surface resistivity Y of the fixing film 13 surface (1.0E+5Ω / □). Therefore, charge can be transferred more quickly from the surface layer 13a of the fixing film 13 toward the contact member 21 within the adhesive layer 13b. In addition, the dispersion of charge within the adhesive layer 13b eliminates uneven charging on the surface of the fixing film 13. On the other hand, if the surface resistivity of the adhesive layer 13b is set to 1.0E+5Ω / □ or higher, the difference with the surface resistivity of the surface layer 13a becomes smaller, and the above effect decreases.

[0122] The charge that has moved within the adhesive layer 13b to the position of the contact member 21 moves in the thickness direction within the surface layer 13a and reaches the contact member 21. The subsequent flow of charge is the same as in the first embodiment.

[0123] As a modification 1, as shown in Figure 8, an exposed portion may be provided at the position of the contact member 21, where the surface layer 13a of the fixing film 13 is not provided, and the adhesive layer 13b is exposed. In the exposed portion, the adhesive layer 13b is exposed to the surface (outer surface) of the fixing film 13. When an exposed portion of the adhesive layer 13b is provided, the charge can be directly removed from the adhesive layer 13b by bringing the contact member 21 into contact with the exposed portion.

[0124] The exposed portion described above is the end on one side in the longitudinal direction of the fixing film 13. This prevents the adhesive layer 13b from coming into contact with the pressure roller 17 and the recording material P. If the adhesive layer 13b, which has a low surface resistivity, comes into contact with the pressure roller 17 or the recording material P, the expected potential relationship may not be achieved, potentially causing image defects such as electrostatic offset. Furthermore, since the exposed portion is not protected by the surface layer 13a, its performance in terms of release properties and abrasion resistance is inferior to that of the portion with the surface layer 13a, so it is preferable not to allow any object other than the contact member 21 to come into contact with it.

[0125] In this embodiment, the surface resistivity of the fixing film 13 surface is set within the same range as in the first embodiment. Therefore, even if the resistance of the adhesive layer 13b is lowered, the problem of charge outflow from the non-image surface of the recording material P via the surface of the pressure roller 17 can be avoided.

[0126] As a second modification, the adhesive layer 13b and the base layer 13c may be low-resistance layers. In this configuration, similar to the case where only the adhesive layer 13b is a low-resistance layer as described above, the contact member 21 may be brought into contact with the surface layer 13a, or the contact member 21 may be brought into contact with the base layer 13c which is partially exposed from the surface layer 13a. The effect of reducing potential unevenness in the fixing film 13 is the same as in this embodiment.

[0127] Furthermore, in the modified example 2 described above, the contact member 21 may be brought into contact with the inner surface of the cylindrical fixing film 13, as shown in Figure 9. Note that in Figure 9, a portion of the fixing film 13 is shown transparently (dashed line) to illustrate the contact member 21 located in the internal space of the fixing film 13.

[0128] In configurations where the contact member 21 is in contact with the surface or exposed portion of the low-resistance layer of the fixing film 13 (Figures 5 and 8), it is desirable to avoid contamination of the contact member 21 by paper dust or toner, and contact of the low-resistance layer with the pressure roller 17 or recording material P. For this reason, in the aforementioned example, the fixing film 13 was made longer in the longitudinal direction, and the contact member 21 was in contact with the vicinity of the longitudinal end of the fixing film 13. In the configuration of Figure 9, such a configuration is not necessary, which is advantageous in terms of manufacturing costs for parts and miniaturization of the device. In addition, since the contact portion between the contact member 21 and the fixing film 13 is hidden inside the fixing film 13, the possibility of mechanical damage to the contact member 21 can be reduced even when the fixing device is operating in a manner different from its intended operation. An operation different from the fixing device's intended operation is, for example, the operation when a jam occurs in the recording material and the recording material is pulled out from the fixing nip N.

[0129] (Test results) This section describes the evaluation results of the electrostatic offset when performing continuous image formation operations (continuous printing) using the printer 100 of this embodiment. The printing conditions were a throughput of 50 ppm, and 1000 pages were continuously printed on only one side of the recording material P. The electrostatic offset (TJ) was evaluated by dividing the recording material P into three regions in the width direction (longitudinal direction of the fixing film 13): the central region, the first end region v1, and the second end region v2, and evaluating each divided region. The rest of the evaluation method is the same as that described in the first embodiment.

[0130] In each configuration example evaluated, the surface resistivity X of the pressure roller 17 surface was set to 1.0E+11Ω / □, and the surface resistivity Y of the fixing film 13 surface was set to 1.0E+12Ω / □ (corresponding to point UU in Figure 6).

[0131] In Example 2-1, the surface resistivity of the adhesive layer 13b of the fixing film 13 was set to 1.0E+5Ω / □. In Example 2-1, the surface resistivity of the adhesive layer 13b of the fixing film 13 was set to 1.0E+7Ω / □. In both examples, the contact member 21 was brought into contact with the exposed portion of the adhesive layer 13b at the longitudinal end of the fixing film 13.

[0132] Furthermore, as Comparative Example 2, the surface resistivity X and Y were the same as in Examples 2-1 and 2-2, but the fixing film 13 was not provided with a low-resistivity layer, and the contact member 21 was in contact with the surface of the fixing film 13. This was evaluated in the same manner as in the examples. Note that the symbols for the electrostatic offset image (TJ) evaluation in the table are the same as in Table 1.

[0133] [Table 2]

[0134] First, let's explain Comparative Example 2. In Comparative Example 2, in the central part of the recording material P and the second end region v2, even under the conditions of this test, which involves continuous printing of 1000 pages, only an extremely slight electrostatic offset occurred that did not pose a practical problem. On the other hand, in the first end region v1, which is farther from the contact member 21, an electrostatic offset that was easily noticeable occurred.

[0135] In contrast, in Example 2-1, only a very slight electrostatic offset occurred throughout the entire width of the recording material P. In Example 2-2, the tendency for electrostatic offset to occur was the same as in Comparative Example 2.

[0136] In Example 2-1, the reduction in electrostatic offset in the first edge region v1 is thought to be due to the provision of the adhesive layer 13b as a low-resistivity layer, which reduced the potential unevenness of the fixing film 13 during continuous printing. On the other hand, in Example 2-2 and Comparative Example 2, the surface resistivity of the adhesive layer 13b was higher than in Example 2-1, which is thought to be why the potential unevenness of the fixing film 13 during continuous printing could not be sufficiently reduced.

[0137] In this embodiment, the fixing film 13 has a multilayer structure, and at least one layer inside the surface layer of the fixing film 13 is a low-resistance layer. The surface resistivity of the low-resistance layer is 1.0E+5Ω / □ or less. With this configuration, the occurrence of electrostatic offset can be reduced even under conditions such as continuous printing over a long period of time.

[0138] (Other embodiments) In the embodiment described above, a film heating method was illustrated using a fixing film 13 as the fixing member, a pressure roller 17 as the pressure member, and a heater 11 as the heating means. Alternatively, for example, a rigid cylindrical roller (fixing roller) may be used as the fixing member. Furthermore, when an endless film or belt is used as the fixing member or pressure member, it may be configured by stretching it over multiple rollers. In addition, for example, a halogen lamp that emits radiant heat may be used as the heating means.

[0139] Furthermore, in the above-described embodiment, a direct transfer type image forming unit 101 was exemplified as the image forming means. Alternatively, an intermediate transfer type image forming unit may be used, in which the toner image formed on the image carrier (photosensitive drum 1) is first transferred to an intermediate transfer body such as an intermediate transfer belt, and then secondarily transferred from the intermediate transfer body to the recording material. In addition, the image forming unit may be configured to form a color image using multiple image carriers and multiple colors of toner.

[0140] (Summary of this disclosure) This disclosure includes at least the following components:

[0141] (Composition 1) A rotatable fixing member, A rotatable pressurizing member that contacts the fixing member at the nip portion, A heating means for heating the fixing member, A fixing device comprising, wherein the fixing device heats the image on the recording material by the fixing member while the recording material is being transported by being sandwiched between the fixing member and the pressurizing member in the nip portion, The circuit further comprises a contact member that contacts the fixing member and is configured to remove charge from the fixing member, The surface layer of the pressurizing member is electrically connected to the circuit section via the surface layer of the fixing member. Let X be the surface resistivity of the surface layer of the pressurizing member. When the surface resistivity of the surface layer of the fixing member is Y(Ω / □), 4.0 ≤ logX ≤ 13.0, 5.0 ≤ logY ≤ 14.0, logY ≥ 13.0 - logX, and, logY ≤ 23.0 - logX, A fixing device characterized by the following:

[0142] (Configuration 2) The contact member is in contact with the outer surface of the surface layer of the fixing member. The fixing device according to configuration 1, characterized by the above.

[0143] (Composition 3) The fixing member further has a low-resistance layer formed inside the surface layer of the fixing member, The surface resistivity of the low-resistivity layer is 1.0E+5Ω / □ or less, and is smaller than the surface resistivity Y of the surface layer of the fixing member. The fixing device according to configuration 1, characterized by the above.

[0144] (Composition 4) The contact member is in contact with the low-resistance layer. The fixing device according to configuration 3, characterized by the above.

[0145] (Composition 5) The low-resistance layer has an exposed portion on its outer surface that is exposed from the surface layer. The contact member is in contact with the exposed portion. The fixing device according to configuration 4, characterized by the features described above.

[0146] (Composition 6) The fixing member is cylindrical, The low-resistance layer constitutes the inner surface of the fixing member, The contact member is in contact with the inner surface of the fixing member. The fixing device according to configuration 4, characterized by the features described above.

[0147] (Composition 7) The contact member is provided outside the region through which the recording material passes the nip portion in the rotational axis direction of the pressurizing member, and only at one end of the fixing member in the rotational axis direction. A fixing device according to any one of configurations 3 to 6, characterized by the above.

[0148] (Composition 8) The fixing member has a base layer formed inside the surface layer and an adhesive layer that bonds the base layer and the surface layer. The low-resistance layer is either the base layer or the adhesive layer, or both. A fixing device according to any one of configurations 3 to 7, characterized by the above.

[0149] (Composition 9) 7.0 ≤ logY ≤ 12.0, and, 6.0 ≤ logX ≤ 11.0, A fixing device according to any one of configurations 1 to 8, characterized in that it is the same as the one described above.

[0150] (Composition 10) logX > 11 or logY < 7, A fixing device according to any one of configurations 1 to 8, characterized in that it is the same as the one described above.

[0151] (Composition 11) logX < 6 or logY > 12, A fixing device according to any one of configurations 1 to 8, characterized in that it is the same as the one described above.

[0152] (Composition 12) A bearing that rotatably supports the aforementioned pressurizing member, The frame that supports the bearing, It further possesses, In the circuit from the pressurizing member to the frame via the bearing, the resistance value of the bearing is greater than the resistance value of the fixing member from the nip portion to the contact member. A fixing device according to any one of configurations 1 to 11, characterized by the features described above.

[0153] (Composition 13) The circuit section has a conductor that connects the contact member and the electrical ground of the fixing device. A fixing device according to any one of configurations 1 to 12, characterized by the features described above.

[0154] (Composition 14) The circuit section is composed of circuit elements arranged in the middle of the path connecting the contact member and the electrical ground of the fixing device, and further includes current limiting means for limiting the current flowing from the contact member to the electrical ground, or rectifying means for rectifying the current flowing from the contact member to the electrical ground. The fixing device according to configuration 13, characterized in that...

[0155] (Composition 15) The circuit section includes a voltage application circuit that applies a voltage to maintain the contact member at a predetermined potential. A fixing device according to any one of configurations 1 to 12, characterized by the features described above.

[0156] (Composition 16) The fixing member is a tubular film, The heating means is a heater placed in the internal space of the film, The pressing member is a roller that faces the heater with the film in between, The fixing device heats the image on the recording material at the nip portion using the film heated by the non-radiative heat emitted by the heater. A fixing device according to any one of configurations 1 to 15, characterized by the features described herein.

[0157] (Composition 17) An image forming means that forms an image on a recording material using toner, A fixing device according to any one of configurations 1 to 16, which fixes the image formed by the image forming means onto the recording material, An image forming apparatus characterized by comprising: [Explanation of Symbols]

[0158] 11…Heating means (heater) / 13…Fixing member (fixing film) / 17…Pressure member (pressure roller) / 21…Contact member / 22…Circuit section (electrical circuit)

Claims

1. A rotatable fixing member, A rotatable pressurizing member that contacts the fixing member at the nip portion, A heating means for heating the fixing member, A bearing that rotatably supports the aforementioned pressurizing member, The frame that supports the bearing, A fixing device comprising, wherein the fixing device heats the image on the recording material by the fixing member while the recording material is being transported by being sandwiched between the fixing member and the pressurizing member in the nip portion, The circuit further comprises a contact member that contacts the fixing member and is configured to remove charge from the fixing member, The surface layer of the pressurizing member is electrically connected to the circuit section via the surface layer of the fixing member. Let X be the surface resistivity of the surface layer of the pressurizing member, When the surface resistivity of the surface layer of the fixing member is Y (Ω / □), 4.0 ≤ logX ≤ 13.0, 5.0 ≤ logY ≤ 14.0, logY ≥ 13.0 - logX, and, logY≦23.0−logX, And, In the circuit from the pressurizing member to the frame via the bearing, the resistance value of the bearing is greater than the resistance value of the fixing member from the nip portion to the contact member. A fixing device characterized by the following features.

2. The contact member is in contact with the outer surface of the surface layer of the fixing member. The fixing device according to feature 1.

3. The fixing member further has a low-resistance layer formed inside the surface layer of the fixing member, The surface resistivity of the low-resistivity layer is 1.0E + 5Ω / □ or less, and is smaller than the surface resistivity Y of the surface layer of the fixing member. The fixing device according to feature 1.

4. The contact member is in contact with the low-resistance layer. The fixing device according to feature 3.

5. The low-resistance layer has an exposed portion on its outer surface that is exposed from the surface layer. The contact member is in contact with the exposed portion. The fixing device according to feature 4.

6. The fixing member is cylindrical, The low-resistance layer constitutes the inner surface of the fixing member, The contact member is in contact with the inner surface of the fixing member. The fixing device according to feature 4.

7. The contact member is provided outside the region through which the recording material passes the nip portion in the rotational axis direction of the pressurizing member, and only at one end of the fixing member in the rotational axis direction. The fixing device according to feature 3.

8. The fixing member has a base layer formed inside the surface layer and an adhesive layer that bonds the base layer and the surface layer. The low-resistance layer is either the base layer or the adhesive layer, or both. The fixing device according to feature 3.

9. 7.0 ≤ logY ≤ 12.0, and, 6.0 ≤ logX ≤ 11.0, The fixing device according to claim 1, characterized in that it is the same as the one described in claim 1.

10. logX > 11 or logY < 7, The fixing device according to claim 1, characterized in that it is the same as the one described in claim 1.

11. logX < 6 or logY > 12, The fixing device according to claim 1, characterized in that it is the same as the one described in claim 1.

12. The circuit section has a conductor that connects the contact member and the electrical ground of the fixing device. The fixing device according to feature 1.

13. The circuit section is composed of circuit elements arranged in the middle of the path connecting the contact member and the electrical ground of the fixing device, and further includes current limiting means for limiting the current flowing from the contact member to the electrical ground, or rectifying means for rectifying the current flowing from the contact member to the electrical ground. The fixing device according to feature 12.

14. The circuit section includes a voltage application circuit that applies a voltage to maintain the contact member at a predetermined potential. The fixing device according to feature 1.

15. The fixing member is a tubular film, The heating means is a heater placed in the internal space of the film, The pressing member is a roller that faces the heater with the film in between, The fixing device heats the image on the recording material at the nip portion using the film heated by the heater. The fixing device according to feature 1.

16. An image forming means that forms an image on a recording material using toner, A fixing device according to any one of claims 1 to 15, which fixes an image formed by the image forming means onto the recording material, An image forming apparatus characterized by comprising:

Images