Fixing apparatus and image forming apparatus

Abstract

Suppress core losses. [Solution] A fixing device for heating a recording material on which a toner image has been formed to fix the toner image to the recording material, comprising: a cylindrical rotating body having a conductive layer and extending in the longitudinal direction; a coil that forms an alternating magnetic field for heating the conductive layer; and a core disposed in a helical portion for inducing magnetic field lines of the alternating magnetic field, wherein the core is composed of a plurality of segmented cores aligned along the longitudinal direction, and the plurality of segmented cores are formed in a columnar shape with an aspect ratio of 1:5 or less between the maximum width in a cross section perpendicular to the longitudinal direction and the length in the longitudinal direction, and have compacted magnetic cores arranged so that the hard magnetization axis faces in the intersecting direction that intersects the longitudinal direction.

Description

Technical Field

[0001] The present invention relates to a fixing device for fixing a toner image on a recording material and an image forming apparatus including the same.

Background Art

[0002] Conventionally, an image forming apparatus including a fixing device using an electromagnetic induction heating method has been proposed (see Patent Document 1). In a fixing device using an electromagnetic induction heating method, an exciting coil is wound around the outer periphery of a magnetic core, and these magnetic core and exciting coil are inserted into a fixing film. Then, the magnetic resistance of the exciting coil is set to a predetermined value, and the magnetic core is divided into a plurality in the longitudinal direction.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the above-mentioned Patent Document 1, the magnetic core is made of sintered ferrite. However, for example, when the magnetic core is made of a powder compact magnetic core material, it has been desired to suppress core loss.

[0005] Therefore, an object of the present invention is to provide a fixing device with suppressed core loss and a form of an image forming apparatus including the same.

Means for Solving the Problems

[0006] One aspect of the present invention is a fixing device for heating a recording material on which a toner image has been formed to fix the toner image to the recording material, comprising: a cylindrical rotating body having a conductive layer and extending in the longitudinal direction; a coil for forming an alternating magnetic field for heating the conductive layer; and a core disposed inside the rotating body for inducing magnetic field lines of the alternating magnetic field, wherein the core is composed of a plurality of segmented cores aligned along the longitudinal direction, and the plurality of segmented cores are formed in a columnar shape with an aspect ratio of 1:5 or less between the maximum width in a cross section perpendicular to the longitudinal direction and the length in the longitudinal direction, and have compacted magnetic cores arranged such that the hard magnetization axis faces in an intersecting direction that intersects the longitudinal direction. [Effects of the Invention]

[0007] According to the present invention, core loss can be suppressed. [Brief explanation of the drawing]

[0008] [Figure 1] A schematic diagram showing the printer according to the first embodiment. [Figure 2] A schematic cross-sectional view showing the fixing device. [Figure 3] (a) is a diagram showing an example in which the conductive layer is provided on the entire surface of the fixing film 1, and (b) is a diagram showing an example in which the conductive layer is provided on a part of the surface of the fixing film. [Figure 4] A perspective view showing the fixing film, magnetic core, and excitation coil. [Figure 5] (a) is a graph showing the current waveform flowing through the excitation coil, and (b) is a graph of the BH curve showing the relationship between magnetic flux density and magnetic field corresponding to the current waveform flowing through the excitation coil. [Figure 6] (a) is a schematic diagram showing the excitation coil and magnetic field, and (b) is a graph showing the distribution of magnetic flux density along the central axis of the coil. [Figure 7] (a) is a schematic diagram showing an excitation coil with a magnetic core inserted and a magnetic field, and (b) is a graph showing the distribution of magnetic flux density along the central axis of the coil. [Figure 8](a) is a schematic diagram showing the arrangement of the circuit near the end of the magnetic core, and (b) is a graph showing the distribution of magnetic flux density along the central axis of the coil. [Figure 9] (a) is a schematic diagram showing the heating area of ​​the heating device and sheet according to this embodiment, and (b) is a graph showing the distribution of magnetic flux density along the coil's central axis. [Figure 10] (a) is a diagram showing the orientation of the hard magnetization axis when the divided core is compressed in the X direction, and (b) is a diagram showing the orientation of the hard magnetization axis when the divided core is compressed in the Y direction. [Figure 11] (a) is a diagram showing how eddy currents flow when a magnetic flux passes through compressed and insulated iron powder, and (b) is a diagram showing how eddy currents flow when a magnetic flux passes through iron powder compressed in a different direction than in Figure 11(a). (c) is a diagram showing how eddy currents flow when a magnetic flux passes through two adjacent iron powder particles. [Figure 12] A diagram showing the divided core viewed from a cross-sectional direction. [Figure 13] A diagram showing the direction of magnetic flux when multiple segmented cores are arranged side by side. [Figure 14] (a) is a cross-sectional view of the divided core according to the second embodiment, and (b) is a perspective view showing the divided core manufactured by compression. [Figure 15] (a) is a diagram showing how multiple divided cores are arranged so that the orientation of the cross-sections of adjacent divided cores is aligned, and (b) is a diagram showing how multiple divided cores are arranged so that the orientation of the cross-sections of adjacent divided cores is not aligned. [Modes for carrying out the invention]

[0009] <First Embodiment> [Image forming apparatus] The first embodiment will be described below with reference to the drawings. Figure 1 is a schematic diagram of the image forming apparatus 100 according to this embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic laser beam printer that forms monochrome toner images. An image forming apparatus includes printers, copiers, facsimile machines, and multifunction devices, and refers to a device that forms an image on a sheet used as a recording medium based on image information input from an external PC or image information read from a document. In addition to the main unit that has the image forming function, an image forming apparatus may be connected to auxiliary equipment such as optional feeders, image readers, and sheet processing devices, and the entire system with such auxiliary equipment connected is also a type of image forming apparatus.

[0010] As shown in Figure 1, the image forming apparatus 100 includes a sheet feeding unit 20 for feeding stacked sheets and an image forming unit 30 for forming images on the fed sheets. The image forming apparatus 100 also includes a fixing unit 40 for fixing the image transferred to the sheet and a pair of discharge rollers 111 that can discharge the sheet to a discharge tray 112.

[0011] When an image forming command is output to the image forming apparatus 100, the image forming process by the image forming unit 30 is started based on image information input from an external computer or the like connected to the image forming apparatus 100. The image forming unit 30 includes a process unit 31, a laser scanner 103, and a transfer roller 108. The process unit 31 includes a photosensitive drum 101 that rotates in the direction of the arrow, a charging roller 102, a developing device 104, and a cleaning device 110 arranged along the photosensitive drum 101. The developing device 104 has a developing roller 104a that supplies toner to the photosensitive drum 101. The process unit 31 may be configured as a cartridge and detachably attached to the main body of the image forming apparatus 100.

[0012] The laser scanner 103 irradiates laser light toward the photosensitive drum 101 based on the input image information. At this time, the photosensitive drum 101 is pre-charged by the charging roller 102, and an electrostatic latent image is formed on the photosensitive drum 101 when irradiated with the laser light. Then, this electrostatic latent image is developed by the developing roller 104a, and a monochrome toner image is formed on the photosensitive drum 101.

[0013] In parallel with the above-described image forming process, the sheet P is fed from the sheet feeding unit 20. The sheet feeding unit 20 has a cassette 105 that is detachably supported on the apparatus main body of the image forming apparatus 100, and the cassette 105 supports the sheet P. The sheet P supported by the cassette 105 is fed by the pickup roller 106. Note that the sheet P includes paper such as paper and envelopes, plastic films such as overhead projector sheets (OHP), cloth, and the like.

[0014] The skew of the sheet P fed by the pickup roller 106 is corrected by the registration roller pair 107. The sheet P is conveyed at a predetermined conveyance timing by the registration roller pair 107 toward the transfer nip 108T formed by the photosensitive drum 101 and the transfer roller 108. Then, the toner image on the photosensitive drum 101 is transferred onto the sheet P by the electrostatic negative bias applied to the transfer roller 108. The residual toner remaining on the photosensitive drum 101 is recovered by the cleaning device 110.

[0015] The sheet P onto which the toner image has been transferred is given a predetermined heat and pressure by the fixing device 40, and the toner is melted and fixed (fixed). The sheet P that has passed through the fixing device 40 is discharged to the discharge tray 112 by the discharge roller pair 111.

[0016] [Fixing device] Next, the configuration of the fixing device 40 will be described using Figures 2 to 5(b). Figure 2 is a schematic cross-sectional view showing the fixing device 40. Figure 3(a) shows an example in which the conductive layer 1a is provided on the entire surface of the fixing film 1, and Figure 3(b) shows an example in which the conductive layer 1a is provided on a part of the surface of the fixing film 1. Figure 4 is a perspective view showing the fixing film 1, magnetic core 2, and excitation coil 3.

[0017] As shown in Figure 2, the fixing device 40 includes a fixing film 1 as a cylindrical rotating body, a magnetic core 2, an excitation coil 3, a film guide 9 as a nip-forming member that contacts the inner surface of the fixing film 1, and a pressure roller 7 as a counter member. The fixing film 1, magnetic core 2, excitation coil 3, and film guide 9 constitute a heating device 45 that heats the sheet P. The pressure roller 7, together with the film guide 9 via the fixing film 1, forms a fixing nip N. The fixing device 40 heats and pressurizes the sheet P, which carries the toner image T, in the fixing nip N, thereby fixing the toner image T to the sheet P.

[0018] The film guide 9 is pressed against the pressure roller 7 by a bearing and biasing part (not shown) with a total pressure of approximately 50N to 100N (approximately 5kgf to approximately 10kgf), sandwiching the fixing film 1 between them. The pressure roller 7 is rotated in the direction of the arrow by a drive source (not shown), and a rotational force acts on the fixing film 1 due to the frictional force at the fixing nip N, causing the fixing film 1 to rotate in accordance with the pressure roller 7. The film guide 9 also has the function of guiding the inner surface of the fixing film 1 and is made of a heat-resistant resin such as polyphenylene sulfide (PPS).

[0019] The fixing film 1 has a metallic conductive layer 1a (base layer) with a diameter (outer diameter) of 10 to 100 mm, an elastic layer 1b formed on the outside of the conductive layer 1a, and a surface layer 1c (release layer) formed on the outside of the elastic layer 1b. The fixing film 1 is flexible. The conductive layer 1a may be provided over the entire surface of the fixing film 1 as shown in Figure 3(a), or it may be provided in a ring shape on a part of the surface of the fixing film 1 as shown in Figure 3(b). When a high-frequency current flows through the excitation coil 3, which will be described later, an alternating magnetic field is formed, and the conductive layer 1a generates heat through electromagnetic heating induction.

[0020] As shown in Figures 2 and 4, a temperature sensing member 4 is positioned near the surface of the fixing film 1, and the temperature sensing member 4 is provided to detect the surface temperature of the fixing film 1. In this embodiment, the temperature sensing member 4 is composed of a non-contact type thermistor.

[0021] Furthermore, a magnetic core 2 and an excitation coil 3 are arranged inside the fixing film 1. A high-frequency converter 5 is connected to the excitation coil 3, and the high-frequency converter 5 supplies a high-frequency current to the excitation coil 3 via power supply contacts 3a and 3b. In Japan, the operating frequency for electromagnetic induction heating is set in the range of 20.05 kHz to 100 kHz according to the Radio Law Enforcement Regulations. Also, since a lower frequency of the current supplied to the power supply components increases the losses of the switching elements, it is desirable for the frequency of the current supplied by the high-frequency converter 5 to the excitation coil 3 to be higher.

[0022] A control circuit 6 is electrically connected to the high-frequency converter 5 and the temperature sensing member 4. The control circuit 6 controls the high-frequency converter 5 based on the temperature detected by the temperature sensing member 4. In this embodiment, the control circuit 6 performs frequency modulation control on the high-frequency converter 5 in the operating frequency band range of 50 kHz to 100 kHz. As a result, the fixing film 1 is heated by electromagnetic induction so that the surface temperature reaches a predetermined target temperature (approximately 150°C to 200°C).

[0023] The magnetic core 2, acting as the core, is cylindrical in shape and is positioned approximately in the center of the fixing film 1 at a fixed part (not shown). The magnetic core 2 guides the magnetic field lines (magnetic flux) of the alternating magnetic field generated by the excitation coil 3 into the fixing film 1, forming a path for the magnetic field lines (magnetic path). The material of this magnetic core 2 is preferably a ferromagnetic material composed of a material with low hysteresis loss and high relative permeability, such as calcined ferrite, ferrite resin, amorphous alloy, or permalloy, which are oxides or alloys with high magnetic permeability.

[0024] Figure 5(a) is a graph showing the current waveform flowing through the excitation coil 3, and Figure 5(b) is a graph of a BH curve showing the relationship between magnetic flux density and magnetic field corresponding to the current waveform flowing through the excitation coil 3. The vertical axis of Figure 5(a) and the vertical axis of Figure 5(b) correspond, indicating that if the current value in Figure 5(a) falls within the upper and lower limits of the magnetic flux density in Figure 5(b), it can be used without saturation.

[0025] The excitation coil 3, acting as a coil, is supplied with the above-mentioned high-frequency current by the high-frequency converter 5. Because the current waveform flowing through the excitation coil 3 differs depending on the drive method employed in the high-frequency converter 5, it is necessary to select a magnetic core made of a material suitable for that drive method. In Figure 5(a), the solid line shows the current waveform when the high-frequency converter 5 is driven by the first drive method DM1, and the dashed line shows the current waveform when the high-frequency converter 5 is driven by the second drive method DM2. Furthermore, in Figure 5(b), the solid line shows the case where the first magnetic core C1 is applied, and the dashed line shows the case where the second magnetic core C2 is applied. These first drive method DM1 and second drive method DM2, and the first magnetic core C1 and second magnetic core C2, are all different from each other.

[0026] The solid line in Figure 5(a), which shows the first drive system DM1, represents the current waveform centered around 0A for the current flowing through the excitation coil 3. As can be seen from Figures 5(a) and (b), both the first magnetic core C1 and the second magnetic core C2 can be utilized in regions where the magnetic flux density is not saturated. Generally, the first drive system DM1 includes circuit configurations such as the full-bridge inverter system.

[0027] The dashed line (current waveform) in Figure 5(a), which shows the second drive system DM2, is always a positive value (current), and the waveform is like the current waveform of the first drive system DM1 offset upwards. As shown in Figures 5(a) and 5(b), when the second magnetic core C2 is selected in the second drive system DM2, it can be used in the region where the magnetic flux density is not saturated. On the other hand, when the first magnetic core C1 is selected in the second drive system DM2, the limit of the magnetic flux density, i.e., the saturation magnetic flux density, exists within the range of magnetic flux changes corresponding to changes in drive current, so the first magnetic core C1 becomes saturated. Therefore, it is difficult to use the first magnetic core C1 in the second drive system DM2. The second drive system DM2 generally has circuit configurations such as the active clamp method.

[0028] Generally, the second drive system DM2 can reduce the number of components that make up the circuit compared to the first drive system DM1, making it possible to miniaturize the circuit size. In this embodiment, the excitation coil 3 is driven by the second drive system DM2. Therefore, the excitation coil 3 needs to use the second magnetic core C2, which has a relatively large magnetic flux density, rather than the first magnetic core C1, which has a low magnetic flux density.

[0029] Furthermore, if the magnetic permeability is low, the loss in the magnetic core will increase, potentially causing heat generation and preventing the rated temperature from being met. Therefore, it is preferable for the magnetic core material to have as high a magnetic permeability as possible and a high saturation magnetic flux density. Generally, magnetic cores made by compressing iron powder have a higher saturation magnetic flux density than magnetic cores formed from ferrite or the like. For this reason, it is preferable to use a magnetic core made from compressed iron powder, as described later, for magnetic core 2.

[0030] Furthermore, when a high-frequency AC current in the 21kHz to 100kHz range is passed through the excitation coil 3, the ferrite loss is small around 20kHz to 50kHz, but it has been confirmed that the magnetic core of the powdered magnetic core is small in the 50kHz to 100kHz range. In this embodiment, since 50kHz to 100kHz is used, a powdered magnetic core with low loss is preferred. Based on the above, in this embodiment, a second magnetic core C2 is used for the magnetic core 2, and the high-frequency converter 5 is driven by the second drive method DM2.

[0031] The magnetic core 2 should preferably have the largest possible cross-sectional area while still being able to be housed in the hollow portion of the fixing film 1. In this embodiment, the diameter of the magnetic core 2 is 5 mm to 20 mm, and the length L1 (see Figure 4) of the magnetic core 2 in the longitudinal direction LD is 10 to 100 mm. The shape of the magnetic core 2 is not limited to a cylindrical shape; it may also be a prism shape, etc. Furthermore, by arranging 10 to 30 magnetic cores 2 in the longitudinal direction LD to form a magnetic path, the durability of the magnetic core 2 against impacts, etc., is improved compared to a configuration in which a single magnetic core with a length equivalent to 10 to 30 cores is provided. The longitudinal direction LD of the magnetic core 2 is parallel to the longitudinal direction of the fixing film 1, and the fixing film 1 can be said to extend in the longitudinal direction LD. The longitudinal direction LD is also parallel to the generatrix direction of the fixing film 1, and the longitudinal direction LD can be rephrased as the generatrix direction of the fixing film 1.

[0032] The excitation coil 3 is formed by spirally winding a copper wire with a diameter of 1 to 3 mm, coated with, for example, heat-resistant polyamide-imide, around the magnetic core 2 for approximately 10 to 100 turns. The excitation coil 3 has a spiral portion 3c wound around a coil central axis AX1 that extends parallel to the longitudinal direction LD, and the magnetic core 2 is positioned within the spiral portion 3c. Depending on the insulation design, the excitation coil 3 may be formed from enameled wire, flat wire, etc., in addition to wire coated with heat-resistant polyamide-imide. In this embodiment, the number of turns of the excitation coil 3 is 20 to 50. Since the excitation coil 3 is wound around the magnetic core 2 in a direction intersecting the longitudinal direction LD, when a high-frequency current is passed through this excitation coil 3, an alternating magnetic field can be generated in a direction parallel to the longitudinal direction LD.

[0033] Furthermore, the excitation coil 3 does not need to be directly wound around the magnetic core 2. The spiral portion 3c of the excitation coil 3 should be positioned inside the fixing film 1 such that the coil's central axis AX1 is parallel to the longitudinal direction LD, and the magnetic core 2 should be positioned within the spiral portion 3c. For example, the fixing film 1 may have a bobbin in which the excitation coil 3 is wound spirally, and the magnetic core 2 may be positioned inside that bobbin.

[0034] Furthermore, in terms of the heating principle, the heating efficiency is highest when the coil center axis AX1 of the excitation coil 3 is parallel to the longitudinal direction LD. However, if the parallelism of the coil center axis AX1 with respect to the longitudinal direction LD is deviated, the amount of magnetic flux passing parallel to the circuit decreases slightly, and the heating efficiency decreases accordingly. However, if the tilt is only a few degrees, there is no practical problem. In other words, the coil center axis AX1 of the helical portion 3c of the excitation coil 3 does not necessarily have to be parallel to the longitudinal direction LD; it is sufficient if it extends in a direction along the longitudinal direction LD.

[0035] [The principle of heat generation in fixing films] First, let's explain the shape of the magnetic field lines. For this explanation, we will use the magnetic field shape of a typical air-core solenoid coil. Figure 6(a) is a schematic diagram of the excitation coil 3 (for improved visibility, the number of turns in Figure 6(a) has been reduced and the shape simplified) and the magnetic field as an air-core solenoid coil. The excitation coil 3 has a finite length and a gap Δd, through which a high-frequency current flows. The direction of these magnetic field lines is the moment when the current is increasing in the direction of arrow I. The magnetic field lines mostly pass through the center of the excitation coil 3, leaking through the gap Δd and connecting around the outer circumference. Figure 6(b) is a graph showing the distribution of magnetic flux density at the coil's central axis AX1. As shown by curve B1 in the graph, it is highest at the center 0 and lowest at the ends of the excitation coil 3. This is because magnetic field lines (e.g., L1) leak from the gap Δd of the excitation coil 3. This also forms a circumferential magnetic field L2 around the excitation coil 3. The magnetic field L2 circumferentially around this coil can be said to pass through a path that is undesirable for efficiently generating heat in the fixing film 1.

[0036] Figure 7(a) is a schematic diagram showing the excitation coil 3 with the magnetic core 2 inserted and the magnetic field. Also, Figure 7(a), like Figure 6(a), shows the moment when the current is increasing in the direction of arrow I. The magnetic core 2 functions as a member that guides the magnetic field lines generated by the excitation coil 3 into its interior and forms a magnetic path. The magnetic core 2 in the first embodiment is not annular, but cylindrical in shape with ends in the longitudinal direction LD. Therefore, the majority of the magnetic field lines concentrate and pass through the magnetic path in the center of the excitation coil 3, and then diffuse at the ends in the longitudinal direction LD of the magnetic core 2, forming an open magnetic path. Compared with the air-core solenoid coil in Figure 6(a), the excitation coil 3 with the magnetic core 2 inserted as shown in Figure 7(a) has significantly reduced leakage of magnetic field lines in the gap Δd of the excitation coil 3, and the magnetic field lines emanating from both poles form an open magnetic path that connects far away on the outer circumference. Figure 7(b) is a graph showing the distribution of magnetic flux density at the coil's central axis AX1. The magnetic flux density of the excitation coil 3 through which the magnetic core 2 is inserted shows less attenuation of the magnetic flux density at the ends of the excitation coil 3 compared to curve B1 in Figure 6(b), as shown by curve B2 on the graph, resulting in a shape close to a trapezoid.

[0037] The heating principle of the fixing film 1 follows Faraday's law. Faraday's law states that "when the magnetic field in a circuit is changed, an induced electromotive force is generated that attempts to drive an electric current through the circuit, and this induced electromotive force is proportional to the time change of the magnetic flux perpendicular to the circuit." Consider the case where a circuit 10 with a larger diameter than the excitation coil 3 and magnetic core 2 is placed near the end of the magnetic core 2 inside the excitation coil 3 shown in Figure 8(a), and a high-frequency alternating current is passed through the excitation coil 3. When a high-frequency alternating current is passed through the excitation coil 3, an alternating magnetic field (a magnetic field whose magnitude and direction repeatedly change over time) is formed around the excitation coil 3. At that time, the induced electromotive force generated in the circuit 10 follows equation (1) below, and according to Faraday's law, it is proportional to the time change of the magnetic flux perpendicular to the circuit 10.

number

[0038] In other words, when a high-frequency alternating current is passed through the excitation coil 3 to generate an alternating magnetic field, if the time change of the vertical component of the magnetic field lines becomes large, the induced electromotive force generated in circuit 10 will increase, and a current will flow in circuit 10 in a direction that cancels out the change in magnetic flux. That is, when a current flows in circuit 10 as a result of generating an alternating magnetic field, the change in magnetic flux is canceled out, resulting in a magnetic field line shape different from that when a static magnetic field is formed. Furthermore, this induced electromotive force V tends to increase as the frequency of the alternating current increases (i.e., Δt is small).

[0039] Therefore, the electromotive force that can be generated with a given amount of magnetic flux differs greatly depending on whether a low-frequency AC current of 50-60 Hz is passed through the excitation coil or a high-frequency AC current of 21 kHz-100 kHz is passed through the excitation coil. By increasing the frequency of the AC current, a high electromotive force can be generated even with a small amount of magnetic flux. Therefore, increasing the frequency of the AC current is very effective when a large amount of heat is to be generated in a small fixing device, because a large amount of heat can be generated in a magnetic core with a small cross-sectional area. This is similar to how transformers can be miniaturized by increasing the frequency of the AC current. For example, a transformer used in the low-frequency band (50-60 Hz) requires a large magnetic flux Φ because Δt is large, and therefore requires a large cross-sectional area of ​​the magnetic core. In contrast, a transformer used in the high-frequency band (kHz) can reduce the magnetic flux Φ because Δt is small, and the cross-sectional area of ​​the magnetic core 2 can be designed to be small.

[0040] In order to generate an induced electromotive force in circuit 10 with high efficiency using an alternating magnetic field, it is necessary to design the circuit 10 so that a larger proportion of the perpendicular component of the magnetic field lines passes through it. However, in an alternating magnetic field, the effects of the demagnetizing field when an induced electromotive force is generated in the coil must also be considered, making the phenomenon complex. To design the fixing device 40 of this embodiment, it is possible to proceed with the design using a simpler physical model by discussing the shape of the magnetic field lines in a static magnetic field state where no induced electromotive force is generated. In other words, by optimizing the shape of the magnetic field lines in a static magnetic field, it is possible to design a fixing device 40 that generates an induced electromotive force with high efficiency in an alternating magnetic field.

[0041] Figure 8(b) is a graph showing the distribution of magnetic flux density at the coil's central axis AX1. Considering the case where a DC current is passed through the excitation coil 3 to form a static magnetic field (a magnetic field that does not fluctuate over time), when the circuit 10 is placed at position X2, the magnetic flux perpendicular to the circuit 10 increases as shown by curve B2, compared to the magnetic flux when the circuit 10 is placed at position X1 in Figure 8(a). At position X2 in Figure 8(a), almost all of the magnetic field lines bound to the magnetic core 2 are contained within the circuit 10, and in the stable region M in the positive X-axis direction from position X2, the magnetic flux perpendicular to the circuit 10 saturates and reaches its maximum. The same can be said for the opposite end, and Figure 9(a) is a schematic diagram showing the image heating region ZL of the heating device 45 and sheet P in this embodiment. Figure 9(b) is a graph showing the distribution of magnetic flux density at the coil's central axis AX1 in Figure 9(a). As shown in the magnetic flux density distribution in Figure 9(b), the stable region M from position X2 to the opposite end X3 is stable because the magnetic flux density passing vertically through the circuit 10 is saturated. As shown in Figure 9(a), this stable region M is located within the region where the magnetic core 2 is located.

[0042] As shown in Figure 9(a), in this embodiment, the magnetic field line configuration allows the fixing film 1 to be covered in the region from position X2 to position X3 when a static magnetic field is formed. The shape of the magnetic field lines is designed so that the magnetic flux passes outside the fixing film 1 from magnetic pole NP at one end of the magnetic core 2 to magnetic pole SP at the other end. Then, the image on the sheet P, which is the recording material, is heated using the stable region M.

[0043] Therefore, in the first embodiment, the length of the longitudinal LD ​​of the magnetic core 2 for forming the magnetic path must be longer than the maximum image heating region ZL of the sheet P. A more preferable configuration is to make the length of the longitudinal LD ​​of both the magnetic core 2 and the excitation coil 3 longer than the maximum image heating region ZL. This makes it possible to heat the toner image on the sheet P uniformly to the edges. Also, the length of the longitudinal LD ​​of the fixing film 1 must be longer than the maximum image heating region ZL. In this embodiment, when the solenoid magnetic field shown in Figure 9(a) is formed, the two magnetic poles NP and SP must extend outward in the longitudinal direction of the magnetic core 2 beyond the maximum image heating region ZL. This makes it possible to generate uniform heat within the range of the image heating region ZL.

[0044] In this embodiment, the magnetic core 2 is longer than the helical portion 3c of the excitation coil 3 in the longitudinal direction LD, and protrudes from both ends of the helical portion 3c. Furthermore, the magnetic core 2 and the helical portion 3c are arranged to span the entire stable region M, which serves as the heating region of the fixing film 1, in the longitudinal direction LD. That is, the magnetic core 2 and the helical portion 3c protrude outward from both end faces of the fixing film 1 in the longitudinal direction LD. This makes it possible to stabilize the amount of heat generated across the entire longitudinal direction LD of the fixing film 1.

[0045] [Manufacturing method and arrangement method for segmented cores] Next, the manufacturing method and arrangement method of the multiple segmented cores 121 constituting the magnetic core 2 will be described using Figures 10(a) to 13. Figure 10(a) shows the orientation of the hard magnetization axis when the segmented core 121 is compressed in the X direction, and Figure 10(b) shows the orientation of the hard magnetization axis when the segmented core 121 is compressed in the Y direction.

[0046] The magnetic core 2 of this embodiment has a plurality of segmented cores 121. Each segmented core 121 is a powdered magnetic core, manufactured by placing insulating coated iron powder of about 20 μm into a mold, compressing it, and annealing it. In other words, the segmented core 121, which is a powdered magnetic core, is a powdered body made of insulating coated iron powder. By constructing the magnetic core 2 from a plurality of segmented cores 121, the magnetic core 2 becomes less susceptible to damage from external impacts and its durability can be improved compared to when the magnetic core 2 is constructed as a single, undivided component. In this embodiment, the plurality of segmented cores 121 are arranged without gaps between them, but this is not the only option. For example, the magnetic core 2 may be constructed by arranging the plurality of segmented cores 121 with small gaps between them and integrating these segmented cores 121 with a holder.

[0047] In Figures 10(a) and 10(b), the iron powder used to manufacture the segmented core 121 is schematically shown as iron powder 120. The iron powder 120 and segmented core 121 before compression are shown by dashed lines, and the iron powder 120 and segmented core 121 after compression are shown by solid lines. The segmented core 121 shown in Figure 10(a) is formed in a cylindrical shape extending in the X direction. Therefore, when the segmented core 121 is compressed from the top of the figure (downstream side in the X direction) as shown in Figure 10(a), the segmented core 121 is compressed in the X direction, and the iron powder 120 is crushed in the X direction from a circular shape to an elliptical shape. In the following, the diameter of the circular cross-section of the segmented core 121 is denoted as diameter N1, and the height of the segmented core 121, i.e., the length of the generatrix, is denoted as length N2. Diameter N1 is the maximum width in the cross-section perpendicular to the longitudinal direction LD of the segmented core 121.

[0048] Furthermore, in Figure 10(b), the segmented core 121 is compressed in the Y direction, that is, from the circumferential surface side of the cylindrical segmented core 121. As a result, in Figure 10(b), the segmented core 121 is compressed so that the diameter N1 of its circular cross-section becomes smaller, and the iron powder 120 is crushed in the Y direction from a perfect circle to an elliptical shape.

[0049] When the aspect ratio (diameter N1:length N2) of the compressed divided core 121 is 1:5 or less, as shown in Figure 10(a), it is easier to process the divided core 121 by compressing it in the longitudinal direction (direction of length N2; X direction). On the other hand, when the aspect ratio (diameter N1:length N2) of the compressed divided core 121 exceeds 1:5, as shown in Figure 10(b), it is easier to process the divided core 121 by compressing it in the radial direction (direction of diameter N1; Y direction).

[0050] Here, the easy magnetization axis refers to the direction in which magnetic flux easily passes through a magnetic material, and the hard magnetization axis refers to the direction in which magnetic flux difficult to pass through a magnetic material. Similar to the relationship between resistance, current, and heat generation in an electrical circuit, if the amount of current (magnetic flux) passing through is the same, a higher resistance (difficulty in passing magnetic flux) will result in greater heat generation. Therefore, if the direction in which the magnetic flux passes through the divided core 121 is the same as the hard magnetization axis of the divided core 121, the amount of heat generated in the divided core 121 will be greater compared to the easy magnetization axis. In other words, the loss of the divided core 121 will be greater. To suppress core loss, it is preferable to position the divided core 121 so that the magnetic flux flows in a direction other than the hard magnetization axis, and furthermore, it is preferable that the magnetic flux flows through the divided core 121 in the same direction as the easy magnetization axis.

[0051] There are three possible reasons why the easy magnetization axis and hard magnetization axis are formed. These three reasons are representative examples, and the easy and hard magnetization axes can also be formed for other reasons.

[0052] The first reason is the effect of the saturation magnetostriction density. When σ is the magnitude of the stress when compressed and λ is the saturation magnetostriction constant, the magnitude of the uniaxial anisotropy energy K is expressed by the following equation.

number

[0053] Uniaxial anisotropy energy is an energy that represents the degree of energy stability of the magnetization direction in a magnetic material with respect to a specific axis (the easy magnetization axis). It is the energy required for a magnetic material to be magnetized in the direction of the easy magnetization axis. Therefore, it can be said that increasing the compressive stress σ or using a material with a large saturation magnetostriction constant λ makes it easier to align the easy magnetization axes. This is equivalent to making it easier to align the hard magnetization axes.

[0054] Furthermore, when the saturation magnetostriction constant λ is positive, the hard magnetization axis is formed in the same direction as the compression direction of the divided core 121, and when the saturation magnetostriction constant λ is negative, the hard magnetization axis is formed perpendicular to the compression direction. In this embodiment, the iron powder has a saturation magnetostriction constant λ that is positive, so the hard magnetization axis is formed in the same direction as the compression direction.

[0055] The second reason is the effect of eddy currents due to the crushing of the iron powder 120 particles. Figure 11(a) shows how eddy currents flow when a magnetic flux passes through compressed and insulating coated iron powder 120, and Figure 11(b) shows how eddy currents flow when a magnetic flux passes through iron powder 120 compressed in a different direction than in Figure 11(a). The iron powder 120 shown in Figure 11(a) has a larger cross-sectional area in the direction of magnetic flux passage compared to the iron powder 120 shown in Figure 11(b). Therefore, the iron powder 120 shown in Figure 11(a) generates more eddy currents than the iron powder 120 shown in Figure 11(b).

[0056] When eddy currents flow, magnetic flux is generated in a direction that opposes the passing magnetic flux (opposite to the dashed arrow in Figure 11(a)). In other words, as many eddy currents are generated, the passing magnetic flux becomes less able to flow, and a hard magnetization axis is formed in the same direction as the compression direction.

[0057] The third reason is the effect of eddy currents due to contact between particles. Figure 11(c) shows the cross-section of iron powder and the flow of eddy currents when magnetic flux passes through it, when two adjacent particles come into contact and the insulating coating peels off during the manufacturing of a segmented core. When adjacent iron powder particles come into contact and the insulating coating peels off, the insulation between the particles is lost and they become one large particle. Although not shown, even if the insulation between particles peels off and only point contact occurs, if many particles come into contact and form an electrically closed-loop circuit, eddy currents will be generated by the magnetic flux passing through the closed-loop.

[0058] Similar to the second reason, when eddy currents are generated, magnetic flux is generated in a direction that opposes the magnetic flux, resulting in eddy current losses and a decrease in apparent permeability, so the compression direction of the divided core 121 becomes the hard magnetization axis. As mentioned above, the larger the cross-sectional area of ​​the iron powder perpendicular to the direction of magnetic flux passage, or the larger the area of ​​the electrically closed loop, the more eddy currents flow and the more difficult it becomes for magnetic flux to flow.

[0059] As shown above for the three main reasons, in this embodiment, a magnetization hard axis is generated in the same direction as the compression direction of the divided core 121.

[0060] In other words, in the compression direction shown in Figure 10(a), the hard magnetization axis is oriented in the X direction. When multiple cylindrical segmented cores 121 are arranged in the longitudinal direction LD to form a magnetic core 2, the magnetic flux passes in the direction of the coil central axis AX1 and in the longitudinal direction LD of the fixing film 1. As a result, the direction of magnetic flux passage becomes the hard magnetization axis, making it difficult for the magnetic flux to pass through the magnetic core 2, and increasing the amount of heat generated in the magnetic core 2.

[0061] On the other hand, in the case of the compression direction shown in Figure 10(b), the hard magnetization axis is oriented in the Y direction. That is, the hard magnetization axis is oriented perpendicular to the direction of the magnetic flux passing through the magnetic core 2. From the above, in order to suppress core loss and heat generation in the magnetic core 2, it is necessary to compress the divided core 121 in the compression direction shown in Figure 10(b) and make the hard magnetization axis perpendicular to the direction of magnetic flux passage.

[0062] As mentioned above, when the aspect ratio (diameter N1:length N2) of the segmented core is 1:5 or less, under normal manufacturing methods, the segmented core is formed in the compression direction shown in Figure 10(a) for ease of processing. Therefore, if multiple segmented cores are arranged in the longitudinal direction LD, the magnetization hard axis will be oriented in the direction of magnetic flux passage.

[0063] Therefore, in this embodiment, a substantially cylindrical (columnar) divided core 121 is manufactured as shown below. Figure 12 is a view of the divided core 121 from the cross-sectional direction. As shown in Figure 12, in this embodiment, the divided core 121 is compression molded in the direction of the arrows in the figure using two molds 131 having semicircular recesses. As a result, the cross-section 130 of the divided core 121 becomes circular, and even when the aspect ratio (diameter N1:length N2) is 1:5 or less, the orientation of the hard magnetization axis is in the Y direction.

[0064] Figure 13 shows the direction in which the magnetic flux passes when multiple segmented cores 121 are arranged in a row. The magnetic flux passes in the X direction shown in Figure 13. In this embodiment, multiple segmented cores 121 manufactured by the method described in Figure 12 are aligned in the X direction parallel to the longitudinal direction LD to form a magnetic core 2. At this time, the hard magnetization axis of each segmented core 121 is oriented in the Y direction, which is an orthogonal direction perpendicular to the longitudinal direction LD. On the other hand, since the direction in which the magnetic flux passes is the X direction, the arrangement is such that the hard magnetization axis is perpendicular to the direction in which the magnetic flux passes, which can suppress core loss and heat generation in the magnetic core 2. In other words, a more efficient fixing device 40 can be realized with suppressed core loss and heat generation in the magnetic core 2. Furthermore, since the magnetic core 2 is made of compacted magnetic core, the saturation magnetic flux density can be improved compared to when it is made of fired ferrite or the like.

[0065] <Second Embodiment> Next, a second embodiment of the present invention will be described. The second embodiment is constructed by changing the shape of the divided core of the first embodiment. For this reason, components similar to those of the first embodiment will not be shown in the illustration, or will be described using the same reference numerals in the illustration.

[0066] Figure 14(a) is a cross-sectional view of the divided core 221 according to the second embodiment, and Figure 14(b) is a perspective view showing the divided core 221 manufactured by compression. In this embodiment, a divided core 221 with a substantially rectangular prism shape is manufactured as follows. As shown in Figure 14(a), in this embodiment, the divided core 221 is compression molded in the direction of the arrow in the figure using two molds 231 having rectangular recesses. As a result, as shown in Figure 14(b), the cross-section 230 of the divided core 221 becomes rectangular, and even when the aspect ratio (maximum width N3: length N4) is 1:5 or less, the orientation of the hard magnetization axis is in the Y direction. The maximum width N3 is the maximum width in the cross-section perpendicular to the longitudinal direction LD of the divided core 221.

[0067] Figures 15(a) and 15(b) show the direction of magnetic flux when multiple segmented cores 221 are arranged in a row. In Figure 15(a), multiple segmented cores 221 are arranged so that the orientation of the cross-sections 230 of adjacent segmented cores 221 is aligned. In Figure 15(b), multiple segmented cores 221 are arranged so that the orientation of the cross-sections 230 of adjacent segmented cores 221 is offset by 90°.

[0068] The magnetic flux passes in the X direction as shown in Figures 15(a) and 15(b). Multiple segmented cores 221 manufactured using the method described in Figure 14(a) are arranged in the X direction to form a magnetic core 202. In this case, the hard magnetization axis of each segmented core 221 is oriented in the Y direction. On the other hand, since the direction in which the magnetic flux passes is the X direction, the arrangement is such that the hard magnetization axis is perpendicular to the direction in which the magnetic flux passes, thereby suppressing core loss and heat generation in the magnetic core 202. In other words, a more efficient fixing device 40 can be realized with reduced core loss and heat generation in the magnetic core 202.

[0069] Furthermore, arranging the multiple divided cores 221 in the configuration shown in Figure 15(a) allows magnetic flux to pass more easily through the magnetic core 202 than arranging them in the configuration shown in Figure 15(b), thereby reducing core loss. For this reason, the magnetic core 202 shown in Figure 15(b) is more preferable.

[0070] Furthermore, when arranging multiple segmented cores 221 side by side, the segmented cores 221 are arranged in a holder (or cover) to prevent movement. In this case, if the segmented cores 221 are arranged with their cross-sections 230 not aligned as shown in Figure 15(b), a gap may form between the segmented cores 221 and the holder, potentially causing damage to the magnetic core 202 due to vibration, thus reducing durability.

[0071] Therefore, as shown in Figure 15(a), multiple segmented cores 221 are arranged in a line so that their cross-sections 230 are aligned, and the holder is molded to match the shape of these multiple segmented cores 221, thereby reducing the gap between each segmented core 221 and the holder. This makes the magnetic core 202 less prone to vibration, improving the durability of the magnetic core 202.

[0072] <Other Embodiments> In all of the embodiments described above, the multiple segmented cores forming the magnetic core each had the same configuration, but this is not limited to that. That is, at least one of the segmented cores forming the magnetic core should be arranged so that its magnetization hard axis is oriented in a direction intersecting the longitudinal direction LD. In other words, some of the multiple segmented cores forming the magnetic core may be arranged so that their magnetization hard axes are oriented in the longitudinal direction LD.

[0073] Furthermore, in all of the embodiments described above, the helical portion 3c of the excitation coil 3 was located inside the fixing film 1, but this is not limited to that. For example, the helical portion 3c of the excitation coil 3 may be located outside the fixing film 1. In other words, the excitation coil 3 may be located anywhere as long as it can form an alternating magnetic field to generate heat for the conductive layer 1a of the fixing film 1.

[0074] Furthermore, in all of the embodiments described above, each segmented core 121, 221 was arranged so that its magnetization hard axis was oriented in the Y direction perpendicular to the longitudinal direction LD, but this is not limited to this. For example, each segmented core 121, 221 may be arranged so that its magnetization hard axis is oriented in a direction intersecting the longitudinal direction LD.

[0075] Furthermore, although the fixing film 1 was composed of a thin film in all of the above-described forms, it is not limited to this. For example, the fixing film 1 may be replaced with a belt that is thicker than the film.

[0076] Furthermore, in all of the embodiments described above, the magnetic core 2,202 was composed of divided cores 121,221 aligned in the longitudinal direction LD, but is not limited to this. For example, the alignment direction of the divided cores 121,221 may be along the longitudinal direction LD.

[0077] Furthermore, the disclosure of this embodiment includes the following configuration examples and method examples. (Composition 1) In a fixing device that heats a recording material on which a toner image has been formed to fix the toner image to the recording material, A cylindrical rotating body having a conductive layer and extending in the longitudinal direction, A coil that forms an alternating magnetic field for heating the conductive layer, A core is disposed inside the rotating body for inducing the magnetic field lines of the alternating magnetic field, Equipped with, The core is composed of a plurality of segmented cores aligned along the longitudinal direction, The plurality of segmented cores are formed in a columnar shape with an aspect ratio of 1:5 or less between the maximum width in a cross section perpendicular to the longitudinal direction and the length in the longitudinal direction, and each has a compacted magnetic core arranged such that the hard magnetization axis is oriented in the intersecting direction that crosses the longitudinal direction. A fixing device characterized by the following features. (Configuration 2) Each of the plurality of divided cores is composed of the compacted magnetic core. The fixing device according to configuration 1, characterized by the above. (Composition 3) The compacted magnetic core is formed in a cylindrical shape with a circular cross-section perpendicular to the longitudinal direction. A fixing device according to configuration 1 or 2, characterized by the above. (Composition 4) The compacted magnetic core is formed in a rectangular prism shape with a rectangular cross-section perpendicular to the longitudinal direction. A fixing device according to configuration 1 or 2, characterized by the above. (Composition 5) The coil is positioned inside the rotating body and has a helical portion wound around a coil central axis extending along the longitudinal direction. A fixing device according to any one of configurations 1 to 4, characterized by the above. (Composition 6) The core is longer than the helical portion in the longitudinal direction and protrudes from both ends of the helical portion. The fixing device according to configuration 5, characterized by the features described herein. (Composition 7) The core and the helical portion are arranged so as to span the entire heating region of the rotating body in the longitudinal direction. The fixing device according to configuration 6, characterized by the features described above. (Composition 8) The aforementioned powder core is a powder compact made of iron powder with an insulating coating. A fixing device according to any one of configurations 1 to 7, characterized by the above. (Composition 9) The rotating body is a film. A fixing device according to any one of configurations 1 to 8, characterized by the above. (Composition 10) The system further includes a pressure roller that contacts the rotating body and forms a fixing nip section for heating and pressurizing the recording material. A fixing device according to any one of configurations 1 to 9, characterized by the above. (Composition 11) An image forming unit that forms a toner image on the recording material, A fixing device comprising any one of configurations 1 to 10, An image forming apparatus characterized by the following features. [Explanation of Symbols]

[0078] 1: Rotating body (fixing film) / 1a: Conductive layer / 2, 202: Core (magnetic core) / 3: Coil (excitation coil) / 3c: Helical section / 40: Fixing device / 100: Image forming apparatus / 121, 221: Divided core / AX1: Coil central axis / LD: Longitudinal direction / N1: Maximum width (diameter) / N2: Length / P: Recording material (sheet) / Y: Intersecting direction (direction)

Claims

1. In a fixing device that heats a recording material on which a toner image has been formed to fix the toner image to the recording material, A cylindrical rotating body having a conductive layer and extending in the longitudinal direction, A coil that forms an alternating magnetic field for heating the conductive layer, A core is disposed inside the rotating body for inducing the magnetic field lines of the alternating magnetic field, Equipped with, The core is composed of a plurality of segmented cores aligned along the longitudinal direction, The plurality of segmented cores are formed in a columnar shape with an aspect ratio of 1:5 or less between the maximum width in a cross section perpendicular to the longitudinal direction and the length in the longitudinal direction, and each has a compacted magnetic core arranged such that its hard magnetization axis is oriented in the intersecting direction that crosses the longitudinal direction. A fixing device characterized by the following features.

2. Each of the plurality of divided cores is composed of the compacted magnetic core. The fixing device according to feature 1.

3. The compacted magnetic core is formed in a cylindrical shape with a circular cross-section perpendicular to the longitudinal direction. The fixing device according to feature 1.

4. The compacted magnetic core is formed in a rectangular prism shape with a rectangular cross-section perpendicular to the longitudinal direction. The fixing device according to feature 1.

5. The coil is positioned inside the rotating body and has a helical portion wound around a coil central axis extending along the longitudinal direction. The fixing device according to feature 1.

6. The core is longer than the helical portion in the longitudinal direction and protrudes from both ends of the helical portion. The fixing device according to feature 5.

7. The core and the helical portion are arranged so as to span the entire heating region of the rotating body in the longitudinal direction. The fixing device according to feature 6.

8. The aforementioned powder core is a powder compact made of iron powder with an insulating coating. The fixing device according to feature 1.

9. The rotating body is a film. The fixing device according to feature 1.

10. The system further includes a pressure roller that contacts the rotating body and forms a fixing nip section for heating and pressurizing the recording material. The fixing device according to feature 1.

11. An image forming unit that forms a toner image on the recording material, A fixing device according to any one of claims 1 to 10, comprising An image forming apparatus characterized by the following features.

Images